Co-reporter:Richard M. Crooks;Nevena Ostojic
Langmuir September 27, 2016 Volume 32(Issue 38) pp:9727-9735
Publication Date(Web):2017-2-22
DOI:10.1021/acs.langmuir.6b02578
We report that ultraviolet/ozone (UV/O3) treatment can be used to remove sixth-generation, hydroxyl-terminated poly(amidoamine) (PAMAM) dendrimers from dendrimer-encapsulated Pt nanoparticles (Pt DENs) previously immobilized onto a pyrolyzed photoresist film (PPF) electrode. Results from X-ray photoelectron spectroscopy, scanning transmission electron microscopy, and electrochemical experiments indicate that removal of the dendrimer proceeds without changes to the size, shape, or electrocatalytic properties of the encapsulated nanoparticles. The UV/O3 treatment did not damage the PPF electrode. The electrocatalytic properties of the DENs before and after removal of the dendrimer were nearly identical.
Co-reporter:Long Luo, Janis Timoshenko, Aliya S. Lapp, Anatoly I. Frenkel, and Richard M. Crooks
Langmuir October 31, 2017 Volume 33(Issue 43) pp:12434-12434
Publication Date(Web):October 9, 2017
DOI:10.1021/acs.langmuir.7b02857
We report the structural characterization of 1–2 nm Rh and RhAu alloy dendrimer-encapsulated nanoparticles (DENs) prepared by chemical reduction with NaBH4. In contrast to previously reported results, in situ and ex situ X-ray absorption spectroscopic experiments indicate that only a fraction of the Rh3+ present in the precursors are reduced by NaBH4. Additional structural analysis of RhAu alloy DENs using extended X-ray absorption fine structure spectroscopy leads to a model in which there is significant segregation of Rh and Au within the nanoparticles. In Rh-rich alloy DENs, Au atoms are segregated on the nanoparticle surface.
Co-reporter:Jamie A. Trindell, Jan Clausmeyer, and Richard M. Crooks
Journal of the American Chemical Society November 15, 2017 Volume 139(Issue 45) pp:16161-16161
Publication Date(Web):November 3, 2017
DOI:10.1021/jacs.7b06775
In this paper, we show that Au nanoparticles (AuNPs) stabilized with either citrate or by low-generation dendrimers rapidly grow during electrocatalytic reduction of CO2. For example, citrate-stabilized AuNPs and AuNPs encapsulated within sixth-generation, hydroxyl-terminated, poly(amidoamine) dendrimers (G6-OH DENs) having diameters of ∼2 nm grow substantially in size (to 6–7 nm) and polydispersity during just 15 min of electrolysis at −0.80 V (vs RHE). This degree of instability makes it impossible to correlate the structure of AuNPs determined prior to electrocatalysis to their catalytic function. In contrast to the G6-OH dendrimer, the higher generation G8-OH analogue stabilizes AuNPs under the same conditions that lead to instability of the other two materials. More specifically, G8-OH DENs having an initial size of 1.7 ± 0.3 nm increase to only 2.2 ± 0.5 nm during electrolysis in 0.10 M NaHCO3 at −0.80 V (vs RHE). Even when the electrolysis is carried out at −1.20 V, the higher-generation dendrimer stabilizes encapsulated AuNPs. This is presumably due to the compactness of the periphery of the G8-OH dendrimer. Although the G8-OH dendrimer nearly eliminates AuNP growth, the surface of the AuNP is still accessible for electrocatalytic reactions. The smaller, more stable G8-OH DENs strongly favor formation of H2 over CO. Some previous reports have suggested that AuNPs in the ∼2 nm size range yield primarily CO, but we believe these findings are a consequence of the growth of the AuNPs during catalysis and do not reflect the true function of ∼2 nm AuNPs.
Co-reporter:Alma D. Castañeda, Nicholas J. Brenes, Aditya Kondajji, and Richard M. Crooks
Journal of the American Chemical Society June 7, 2017 Volume 139(Issue 22) pp:7657-7657
Publication Date(Web):May 24, 2017
DOI:10.1021/jacs.7b03648
Here we report a sensing scheme for detection of microRNA (miRNA) using electrocatalytic amplification (ECA). ECA is a method in which nanoparticles (NPs) that are catalytic for a specific electrochemical reaction collide with an inert electrode surface. Each collision results in a detectable current transient. In the present article, we show that this general approach can be extended to detection of miRNA. Specifically, PtNPs are modified with a single-strand DNA (ssDNA) shell that is complementary to the miRNA target. Next, the ssDNA:miRNA conjugate is formed, which passivates the PtNP surface. In the presence of an enzyme called duplex specific nuclease (DSN), however, a fraction of the surface-bound DNA is removed thereby exposing some of the PtNP surface. In other words, the electrocatalytic properties of the PtNPs are reactivated only if miRNA complementary to ssDNA is present. This methodology resolves a number of problems that have rendered ECA ineffective for biosensing applications. Moreover, the results suggest that the underlying chemistry is broadly applicable to nucleic acid sensing.
Co-reporter:Xiang Li, Long Luo, and Richard M. Crooks
Analytical Chemistry April 4, 2017 Volume 89(Issue 7) pp:4294-4294
Publication Date(Web):March 17, 2017
DOI:10.1021/acs.analchem.7b00365
We describe the design and characteristics of a paper-based analytical device for analyte concentration enrichment. The device, called a hybrid paper-based analytical device (hyPAD), uses faradaic electrochemistry to create an ion depletion zone (IDZ), and hence a local electric field, within a nitrocellulose flow channel. Charged analytes are concentrated near the IDZ when their electrophoretic and electroosmotic velocities balance. This process is called faradaic ion concentration polarization. The hyPAD is simple to construct and uses only low-cost materials. The hyPAD can be tuned for optimal performance by adjusting the applied voltage or changing the electrode design. Moreover, the throughput of hyPAD is 2 orders of magnitude higher than that of conventional, micron-scale microfluidic devices. The hyPAD is able to concentrate a range of analytes, including small molecules, DNA, proteins, and nanoparticles, in the range of 200–500-fold within 5 min.
Co-reporter:Long Luo, Zhiyao Duan, Hao Li, Joohoon Kim, Graeme Henkelman, and Richard M. Crooks
Journal of the American Chemical Society April 19, 2017 Volume 139(Issue 15) pp:5538-5538
Publication Date(Web):April 7, 2017
DOI:10.1021/jacs.7b01653
In this paper, we show that PtAu and PdAu random alloy dendrimer-encapsulated nanoparticles with an average size of ∼1.6 nm have different catalytic activity trends for allyl alcohol hydrogenation. Specifically, PtAu nanoparticles exhibit a linear increase in activity with increasing Pt content, whereas PdAu dendrimer-encapsulated nanoparticles show a maximum activity at a Pd content of ∼60%. Both experimental and theoretical results suggest that this contrasting behavior is caused by differences in the strength of H binding on the PtAu and PdAu alloy surfaces. The results have significant implications for predicting the catalytic performance of bimetallic nanoparticles on the basis of density functional theory calculations.
Co-reporter:Eunsoo Yoon;Collin D. Davies;Tim A. Hooper
Lab on a Chip (2001-Present) 2017 vol. 17(Issue 14) pp:2491-2499
Publication Date(Web):2017/07/11
DOI:10.1039/C7LC00455A
In this article we report a microelectrochemical system that is able to partially desalinate water. The underlying principles are similar to previous reports in which a local electric field resists passage of ions. However, in the present case, no membrane is required and, most interestingly, much of the power for desalination originates from light rather than electricity. This could greatly increase the power efficiency for desalination. The device is based on a TiO2 photoanode coupled to a Pt cathode. Illumination of the photoanode drives faradaic reactions at the cathode that lead to an ion depletion zone. The resulting local electric field limits transport of charged species. In situ conductivity and fluorescence measurements demonstrate the effectiveness of the device.
Co-reporter:Nevena Ostojic; James H. Thorpe
Journal of the American Chemical Society 2016 Volume 138(Issue 21) pp:6829-6837
Publication Date(Web):May 18, 2016
DOI:10.1021/jacs.6b03149
Electrocatalytic oxygen reduction at carbon electrodes fully passivated by Al2O3 is reported. Specifically, pyrolyzed polymer film (PPF) electrodes were prepared and then coated with pinhole-free Al2O3 layers ranging in thickness from 2.5 to 5.7 nm. All of these ultrathin oxide film thicknesses completely passivated the PPF electrodes, resulting in no faradaic current for either inner-sphere or outer-sphere electrochemical reactions. The electrodes could, however, be reactivated by immobilizing Pt dendrimer-encapsulated nanoparticles (DENs), containing an average of 55 atoms each, on the oxide surface. These PPF/Al2O3/Pt DEN electrodes were completely stable under a variety of electrochemical and solution conditions, and they are active for simple electron-transfer reactions and for more complex electrocatalytic processes. This approach for preparing well-defined oxide electrodes opens the door to a better understanding of the effect of oxide supports on reactions electrocatalyzed by metal nanoparticles.
Co-reporter:Paul R. DeGregory, Yi-Ju Tsai, Karen Scida, Ian Richards and Richard M. Crooks
Analyst 2016 vol. 141(Issue 5) pp:1734-1744
Publication Date(Web):29 Jan 2016
DOI:10.1039/C5AN02386F
We report a paper-based assay platform for the detection of the kidney disease marker Trefoil Factor 3 (TFF3) in human urine. The sensor is based on a quantitative metalloimmunoassay that can determine TFF3 concentrations via electrochemical detection of environmentally stable silver nanoparticle (AgNP) labels attached to magnetic microbeads via a TFF3 immunosandwich. The paper electroanalytical device incorporates two preconcentration steps that make it possible to detect concentrations of TFF3 in human urine at the low end of the target TFF3 concentration range (0.03–7.0 μg mL−1). Importantly, the paper device provides a level of accuracy for TFF3 determination in human urine equivalent to that of a commercial kit. The paper sensor has a dynamic range of ∼2.5 orders of magnitude, only requires a simple, one-step incubation protocol, and is fast, requiring only 10 min to complete. The cost of the materials at the prototypic laboratory scale, excluding reagents, is just US$0.42.
Co-reporter:Long Luo, Liang Zhang, Zhiyao Duan, Aliya S. Lapp, Graeme Henkelman, and Richard M. Crooks
ACS Nano 2016 Volume 10(Issue 9) pp:8760
Publication Date(Web):September 1, 2016
DOI:10.1021/acsnano.6b04448
In this paper, we show that the onset potential for CO oxidation electrocatalyzed by ∼2 nm dendrimer-encapsulated Pt nanoparticles (Pt DENs) is shifted negative by ∼300 mV in the presence of a small percentage (<2%) of Cu surface atoms. Theory and experiments suggest that the catalytic enhancement arises from a cocatalytic Langmuir–Hinshelwood mechanism in which the small number of Cu atoms selectively adsorb OH, thereby facilitating reaction with CO adsorbed to the dominant Pt surface. Theory suggests that these Cu atoms are present primarily on the (100) facets of the Pt DENs.Keywords: bimetallic nanoparticle; CO electro-oxidation; density functional theory; isolated Cu
Co-reporter:Josephine C. Cunningham, Molly R. Kogan, Yi-Ju Tsai, Long Luo, Ian Richards, and Richard M. Crooks
ACS Sensors 2016 Volume 1(Issue 1) pp:40
Publication Date(Web):October 6, 2015
DOI:10.1021/acssensors.5b00051
Here we report a three-dimensional paper fluidic device configured for electrochemical detection of biomolecules labeled with silver nanoparticles (AgNPs). This new sensor, which we call a NoSlip, represents a major improvement of our previously reported oSlip system. Specifically, detection of AgNPs in the NoSlip is based on galvanic exchange rather than a chemical oxidant (bleach or MnO4– in the oSlip). Galvanic exchange is implemented by depositing a very small amount of gold onto the working electrode. Once the AgNP labels are brought into the proximity of the electrode through the use of magnetic force, a fraction of the Au0 is electrochemically oxidized to Au3+. The Au3+ reacts with the AgNPs to form Ag+ and Au0. The Ag+ is then detected by anodic stripping voltammetry. This new methodology resolves three shortcomings of the oSlip while simultaneously simplifying the basic sensor form factor. First, the NoSlip resolves an oxidant instability issue because of the inherent stability of the Au0 coating on the electrode that is used to electrogenerate the oxidant (Au3+). Additionally, Au3+ is a milder oxidizing agent than bleach or MnO4–, so it does not attack the major components of the NoSlip. Finally, the NoSlip eliminates the need for a slip layer because the oxidant (Au3+) is electrogenerated on demand. The NoSlip is able to detect AgNP labels down to concentrations as low as 2.1 pM, the time to result is ∼7 min, and the cost at the laboratory scale, not including application-specific reagents, is $0.30.Keywords: anodic stripping voltammetry; electrochemical sensor; galvanic exchange; paper analytical device; silver nanoparticles
Co-reporter:Dr. Richard M. Crooks
ChemElectroChem 2016 Volume 3( Issue 3) pp:357-359
Publication Date(Web):
DOI:10.1002/celc.201500549
Abstract
This is a tutorial on the basic concepts underlying bipolar electrochemistry. It is intended as an accompaniment to ChemElectroChem’s special issue on this topic. The guest editors of this special issue think it is worthwhile to introduce bipolar electrochemistry to a broader (electrochemistry) audience by explaining the similarities and differences between bipolar electrochemistry and normal electrochemical methods. The work presented herein has been adapted from: S. E. Fosdick, K. N. Knust; K. Scida; R. M. Crooks “Bipolar Electrochemistry” Angew. Chem. Int. Ed. 2013, 52, 10438–10456 (DOI: 10.1002/anie.201300947); “Bipolare Elektrochemie” Angew. Chem. 2013, 125, 10632–10651 (DOI: 10.1002/ange.201300947).
Co-reporter:Rachel M. Anderson, David F. Yancey, Liang Zhang, Samuel T. Chill, Graeme Henkelman, and Richard M. Crooks
Accounts of Chemical Research 2015 Volume 48(Issue 5) pp:1351
Publication Date(Web):May 4, 2015
DOI:10.1021/acs.accounts.5b00125
The objective of the research described in this Account is the development of high-throughput computational-based screening methods for discovery of catalyst candidates and subsequent experimental validation using appropriate catalytic nanoparticles. Dendrimer-encapsulated nanoparticles (DENs), which are well-defined 1–2 nm diameter metal nanoparticles, fulfill the role of model electrocatalysts.Effective comparison of theory and experiment requires that the theoretical and experimental models map onto one another perfectly. We use novel synthetic methods, advanced characterization techniques, and density functional theory (DFT) calculations to approach this ideal. For example, well-defined core@shell DENs can be synthesized by electrochemical underpotential deposition (UPD), and the observed deposition potentials can be compared to those calculated by DFT. Theory is also used to learn more about structure than can be determined by analytical characterization alone. For example, density functional theory molecular dynamics (DFT-MD) was used to show that the core@shell configuration of Au@Pt DENs undergoes a surface reconstruction that dramatically affects its electrocatalytic properties. A separate Pd@Pt DENs study also revealed reorganization, in this case a core–shell inversion to a Pt@Pd structure. Understanding these types of structural changes is critical to building correlations between structure and catalytic function.Indeed, the second principal focus of the work described here is correlating structure and catalytic function through the combined use of theory and experiment. For example, the Au@Pt DENs system described earlier is used for the oxygen reduction reaction (ORR) as well as for the electro-oxidation of formic acid. The surface reorganization predicted by theory enhances our understanding of the catalytic measurements. In the case of formic acid oxidation, the deformed nanoparticle structure leads to reduced CO binding energy and therefore improved oxidation activity. The final catalytic study we present is an instance of theory correctly predicting (in advance of the experiments) the structure of an effective DEN electrocatalyst. Specifically, DFT was used to determine the optimal composition of the alloy-core in AuPd@Pt DENs for the ORR. This prediction was subsequently confirmed experimentally. This study highlights the major theme of our research: the progression of using theory to rationalize experimental results to the more advanced goal of using theory to predict catalyst function a priori. We still have a long way to go before theory will be the principal means of catalyst discovery, but this Account begins to shed some light on the path that may lead in that direction.
Co-reporter:Jason J. Yoo, Joohoon Kim and Richard M. Crooks
Chemical Science 2015 vol. 6(Issue 11) pp:6665-6671
Publication Date(Web):29 Jul 2015
DOI:10.1039/C5SC02259B
Here, we report on the electrochemical detection of individual collisions between a conjugate consisting of silver nanoparticles (AgNPs) linked to conductive magnetic microbeads (cMμBs) via DNA hybridization and a magnetized electrode. The important result is that the presence of the magnetic field increases the flux of the conjugate to the electrode surface, and this in turn increases the collision frequency and improves the limit of detection (20 aM). In addition, the magnitude of the charge associated with the collisions is greatly enhanced in the presence of the magnetic field. The integration of DNA into the detection protocol potentially provides a means for using electrochemical collisions for applications in biological and chemical sensing.
Co-reporter:Xiang Li, Long Luo and Richard M. Crooks
Lab on a Chip 2015 vol. 15(Issue 20) pp:4090-4098
Publication Date(Web):04 Sep 2015
DOI:10.1039/C5LC00875A
We present a new paper-based isotachophoresis (ITP) device design for focusing DNA samples having lengths ranging from 23 to at least 1517 bp. DNA is concentrated by more than two orders of magnitude within 4 min. The key component of this device is a 2 mm-long, 2 mm-wide circular paper channel formed by concertina folding a paper strip and aligning the circular paper zones on each layer. Due to the short channel length, a high electric field of ~16 kV m−1 is easily generated in the paper channel using two 9 V batteries. The multilayer architecture also enables convenient reclamation and analysis of the sample after ITP focusing by simply opening the origami paper and cutting out the desired layers. We profiled the electric field in the origami paper channel during ITP experiments using a nonfocusing fluorescent tracer. The result showed that focusing relied on formation and subsequent movement of a sharp electric field boundary between the leading and trailing electrolyte.
Co-reporter:Josephine C. Cunningham, Karen Scida, Molly R. Kogan, Bo Wang, Andrew D. Ellington and Richard M. Crooks
Lab on a Chip 2015 vol. 15(Issue 18) pp:3707-3715
Publication Date(Web):30 Jul 2015
DOI:10.1039/C5LC00731C
We report a paper-based assay platform for detection of ricin a chain. The paper platform is assembled by simple origami paper folding. The sensor is based on quantitative, electrochemical detection of silver nanoparticle labels linked to a magnetic microbead support via a ricin immunosandwich. Importantly, ricin was detected at concentrations as low as 34 pM. Additionally, the assay is robust, even in the presence of 100-fold excess hoax materials. Finally, the device is easily remediated after use by incineration. The cost of the device, not including reagents, is just $0.30. The total assay time, including formation of the immunosandwich, is 9.5 min.
Co-reporter:Xiang Li, Karen Scida, and Richard M. Crooks
Analytical Chemistry 2015 Volume 87(Issue 17) pp:9009
Publication Date(Web):August 10, 2015
DOI:10.1021/acs.analchem.5b02210
Here we show that a simple paper-based electrochemical sensor, fabricated by paper folding, is able to detect a 30-base nucleotide sequence characteristic of DNA from the hepatitis B virus (HBV) with a detection limit of 85 pM. This device is based on design principles we have reported previously for detecting proteins via a metalloimmunoassay. It has four desirable attributes. First, its design combines simple origami (paper folding) assembly, the open structure of a hollow-channel paper analytical device to accommodate micrometer-scale particles, and a convenient slip layer for timing incubation steps. Second, two stages of amplification are achieved: silver nanoparticle labels provide a maximum amplification factor of 250 000 and magnetic microbeads, which are mobile solid-phase supports for the capture probes, are concentrated at a detection electrode and provide an additional ∼25-fold amplification. Third, there are no enzymes or antibodies used in the assay, thereby increasing its speed, stability, and robustness. Fourth, only a single sample incubation step is required before detection is initiated.
Co-reporter:Alma D. Castañeda, Timothy M. Alligrant, James A. Loussaert, and Richard M. Crooks
Langmuir 2015 Volume 31(Issue 2) pp:876-885
Publication Date(Web):January 8, 2015
DOI:10.1021/la5043124
We report electrochemical catalytic amplification of individual collisions between ∼57 nm diameter Pt nanoparticles (Pt NPs) and 12.5 μm diameter Au ultramicroelectrodes modified with passivating, electrostatically assembled polyelectrolyte multilayer (PEM) films prepared by the layer-by-layer deposition method. Two key findings are reported. First, despite the thicknesses of the insulating PEM films, which range up to 5 nm, electrons are able to tunnel from the Pt NPs to the electrode resulting in electrocatalytic N2H4 oxidation at the PEM film–solution interface. These single-particle measurements are in accord with prior reports showing that the electrochemical activity of passive PEM films can be reactivated by adsorption of metallic NPs. Second, it is possible to control the frequency of the collisions by manipulating the net electrostatic charge present on the outer surface of the PEM thin film. These results, which demonstrate that chemistry can be used to control electrocatalytic amplification, set the stage for future sensing applications.
Co-reporter:Rohit Bhandari, Rachel M. Anderson, Shannon Stauffer, Anthony G. Dylla, Graeme Henkelman, Keith J. Stevenson, and Richard M. Crooks
Langmuir 2015 Volume 31(Issue 23) pp:6570-6576
Publication Date(Web):June 3, 2015
DOI:10.1021/acs.langmuir.5b01383
The synthesis and characterization of Sn nanoparticles in organic solvents using sixth-generation dendrimers modified on their periphery with hydrophobic groups as stabilizers are reported. Sn2+:dendrimer ratios of 147 and 225 were employed for the synthesis, corresponding to formation of Sn147 and Sn225 dendrimer-stabilized nanoparticles (DSNs). Transmission electron microscopy analysis indicated the presence of ultrasmall Sn nanoparticles having an average size of 3.0–5.0 nm. X-ray absorption spectroscopy suggested the presence of Sn nanoparticles with only partially oxidized surfaces. Cyclic voltammetry studies of the Sn DSNs for Li alloying/dealloying reactions demonstrated good reversibility. Control experiments carried out in the absence of DSNs clearly indicated that these ultrasmall Sn DSNs react directly with Li to form SnLi alloys.
Co-reporter:Timothy M. Alligrant, Radhika Dasari, Keith J. Stevenson, and Richard M. Crooks
Langmuir 2015 Volume 31(Issue 42) pp:11724-11733
Publication Date(Web):October 12, 2015
DOI:10.1021/acs.langmuir.5b02620
Here we report on the effect of DNA modification on individual collisions between Pt nanoparticles (PtNPs) and ultramicroelectrode (UME) surfaces. These results extend recent reports of electrocatalytic amplification (ECA) arising from collisions between naked surfaces, and they are motivated by our interest in using ECA for low-level biosensing applications. In the present case, we studied collisions between naked PtNPs and DNA-modified Au and Hg UMEs and also collisions between DNA-modified PtNPs and naked Au and Hg UMEs. In all cases, the sensing reaction is the catalytic oxidation of N2H4. The presence of ssDNA (5-mer or 25-mer) immobilized on the UME surface has little effect on the magnitude or frequency of ECA signals, regardless of whether the electrode is Au or Hg. In contrast, when DNA is immobilized on the PtNPs and the electrodes are naked, clear trends emerge. Specifically, as the surface concentration of ssDNA on the PtNP surface increases, the magnitude and frequency of the current transients decrease. This trend is most apparent for the longer 25-mer. We interpret these results as follows. When ssDNA is immobilized at high concentration on the PtNPs, the surface sites on the NP required for electrocatalytic N2H4 oxidation are blocked. This leads to lower and fewer ECA signals. In contrast, naked PtNPs are able to transfer electrons to UMEs having sparse coatings of ssDNA.
Co-reporter:Long Luo; Liang Zhang; Graeme Henkelman
The Journal of Physical Chemistry Letters 2015 Volume 6(Issue 13) pp:2562-2568
Publication Date(Web):June 19, 2015
DOI:10.1021/acs.jpclett.5b00985
A theoretical and experimental study of the electrocatalytic oxidation of CO on PdxAu140–x@Pt dendrimer-encapsulated nanoparticle (DEN) catalysts is presented. These nanoparticles are comprised of a core having an average of 140 atoms and a Pt monolayer shell. The CO oxidation activity trend exhibits an unusual koppa shape as the number of Pd atoms in the core is varied from 0 to 140. Calculations based on density functional theory suggest that the koppa-shaped trend is driven primarily by structural changes that affect the CO binding energy on the surface. Specifically, a pure Au core leads to deformation of the Pt shell and a compression of the Pt lattice. In contrast, Pd, from the pure Pd cores, tends to segregate on the DEN surface, forming an inverted configuration having Pt within the core and Pd in the shell. With a small addition of Au, however, the alloy PdAu cores stabilize the core@shell structures by preventing Au and Pd from escaping to the particle surface.
Co-reporter:Christophe Renault ; Morgan J. Anderson
Journal of the American Chemical Society 2014 Volume 136(Issue 12) pp:4616-4623
Publication Date(Web):March 17, 2014
DOI:10.1021/ja4118544
In the present article we provide a detailed analysis of fundamental electrochemical processes in a new class of paper-based analytical devices (PADs) having hollow channels (HCs). Voltammetry and amperometry were applied under flow and no flow conditions yielding reproducible electrochemical signals that can be described by classical electrochemical theory as well as finite-element simulations. The results shown here provide new and quantitative insights into the flow within HC-PADs. The interesting new result is that despite their remarkable simplicity these HC-PADs exhibit electrochemical and hydrodynamic behavior similar to that of traditional microelectrochemical devices.
Co-reporter:Stephen E. Fosdick, Sean P. Berglund, C. Buddie Mullins, and Richard M. Crooks
ACS Catalysis 2014 Volume 4(Issue 5) pp:1332
Publication Date(Web):March 31, 2014
DOI:10.1021/cs500168t
Here, we report the development of a parallel electrocatalyst screening platform for the hydrogen evolution reaction (HER) using bipolar electrodes (BPEs). Electrocatalyst candidates are subjected to screening in a N2-purged bipolar electrochemical cell where a pair of driving electrodes produce an electric field in the electrolyte solution. The HER occurring at the BPE cathodes is electrically coupled to the electrodissolution of an array of Cr microbands present at the BPE anodes. The readout of this device is simple, where the species that dissolve the most Cr microbands are identified as the most promising electrocatalyst candidates for further evaluation. We demonstrate the utility of this technique by comparing several bi- and trimetallic systems involving Co, Fe, Ni, Mo, and W, which are compared directly with pure Pt. Of all the compositions tested, Ni8–Mo2 is demonstrated to be the most active for the HER in a neutral electrolyte solution.Keywords: electrocatalysis; electrochemistry; high-throughput screening
Co-reporter:Karen Scida, Josephine C. Cunningham, Christophe Renault, Ian Richards, and Richard M. Crooks
Analytical Chemistry 2014 Volume 86(Issue 13) pp:6501
Publication Date(Web):June 11, 2014
DOI:10.1021/ac501004a
We report a new type of paper analytical device that provides quantitative electrochemical output and detects concentrations as low as 767 fM. The model analyte is labeled with silver nanoparticles (AgNPs), which provide 250 000-fold amplification. AgNPs eliminate the need for enzymatic amplification, thereby improving device stability and response time. The use of magnetic beads to preconcentrate the AgNPs at the detection electrode further improves sensitivity. Response time is improved by incorporation of a hollow channel, which increases the flow rate in the device by a factor of 7 and facilitates the use of magnetic beads. A key reaction necessary for label detection is made possible by the presence of a slip layer, a fluidic switch that can be actuated by manually slipping a piece of paper. The design of the device is versatile and should be useful for detection of proteins, nucleic acids, and microbes.
Co-reporter:Josephine C. Cunningham, Nicholas J. Brenes, and Richard M. Crooks
Analytical Chemistry 2014 Volume 86(Issue 12) pp:6166
Publication Date(Web):May 28, 2014
DOI:10.1021/ac501438y
Here, we report a strategy for the design of an inexpensive paper analytical device (PAD) for quantitative detection of oligonucleotides and proteins. Detection is based on the principle of target-induced conformational switching of an aptamer linked to an electrochemical label. This simple and robust method is well matched to the equally simple and robust characteristics of the PAD platform. The demonstrated limits of detection for DNA and thrombin are 30 nM and 16 nM, respectively, and the device-to-device reproducibility is better than ±10%. The PAD has a shelf life of at least 4 weeks, involves little user intervention, and requires a sample volume of just 20 μL.
Co-reporter:Stephen E. Fosdick, Morgan J. Anderson, Christophe Renault, Paul R. DeGregory, James A. Loussaert, and Richard M. Crooks
Analytical Chemistry 2014 Volume 86(Issue 7) pp:3659
Publication Date(Web):March 13, 2014
DOI:10.1021/ac5004294
Here, we report the use of microwire and mesh working electrodes in paper analytical devices fabricated by origami paper folding (oPADs). The important new result is that Au wires and carbon fibers having diameters ranging from micrometers to tens of micrometers can be incorporated into oPADs and that their electrochemical characteristics are consistent with the results of finite element simulations. These electrodes are fully compatible with both hollow channels and paper channels filled with cellulose fibers, and they are easier to incorporate than typical screen-printed carbon electrodes. The results also demonstrate that the Au electrodes can be cleaned prior to device fabrication using aggressive treatments and that they can be easily surface modified using standard thiol-based chemistry.
Co-reporter:Jason J. Yoo, Morgan J. Anderson, Timothy M. Alligrant, and Richard M. Crooks
Analytical Chemistry 2014 Volume 86(Issue 9) pp:4302
Publication Date(Web):April 18, 2014
DOI:10.1021/ac404093c
We report electrochemical detection of collisions between individual magnetic microbeads, present at subattomolar concentrations, and electrode surfaces. This limit of detection is 4 orders of magnitude lower than has been reported previously, and it is enabled by using a magnetic field to preconcentrate the microbeads prior to detection in a microfluidic electrochemical cell. Importantly, the frequency of collisions between the microbeads and the electrode is not compromised by the low concentration of microbeads. These findings represent an unusual case of detecting individual electrochemical events at very low analyte concentration. In addition to experiments supporting these claims, finite-element simulations provide additional insights into the nature of the interactions between flowing microbeads and their influence on electrochemical processes.
Co-reporter:Morgan J. Anderson and Richard M. Crooks
Analytical Chemistry 2014 Volume 86(Issue 19) pp:9962
Publication Date(Web):September 26, 2014
DOI:10.1021/ac502869j
Here we report on the development of a high-efficiency, dual channel-electrode (DCE) generation-collection system and its application for interrogating redox-active surface-adsorbed thin films. DCE systems consist of two electrodes configured on the base of a microfluidic channel. Under laminar flow conditions, a redox reaction can be driven on the upstream generator electrode, and the products carried by convection to the downstream collector electrode where the reverse redox reaction occurs. One significant outcome of this report is that simple fabrication techniques can be used to prepare DCE systems that have collection efficiencies of up to 97%. This level of efficiency makes it possible to quantitatively measure the charge associated with redox-active thin films interposed between the generator and collector electrodes. This is important, because it provides a means for interrogating species that are not in sufficiently close proximity to an electrode to enable direct electron transfer or electroactive films adsorbed to insulating surfaces. Here, the method is demonstrated by comparing results from this indirect surface interrogation method, using Fe(CN)63– as the redox probe, and direct electroreduction of Au oxide thin films. These experimental results are further compared to finite-element simulations.
Co-reporter:Long Luo, Xiang Li, and Richard M. Crooks
Analytical Chemistry 2014 Volume 86(Issue 24) pp:12390
Publication Date(Web):December 2, 2014
DOI:10.1021/ac503976c
We present an origami paper-based electrophoretic device (oPAD-Ep) that achieves rapid (∼5 min) separation of fluorescent molecules and proteins. Due to the innovative design, the required driving voltage is just ∼10 V, which is more than 10 times lower than that used for conventional electrophoresis. The oPAD-Ep uses multiple, thin (180 μm/layer) folded paper layers as the supporting medium for electrophoresis. This approach significantly shortens the distance between the anode and cathode, and this, in turn, accounts for the high electric field (>1 kV/m) that can be achieved even with a low applied voltage. The multilayer design of the oPAD-Ep enables convenient sample introduction by use of a slip layer as well as easy product analysis and reclamation after electrophoresis by unfolding the origami paper and cutting out desired layers. We demonstrate the use of oPAD-Ep for simple separation of proteins in bovine serum, which illustrates its potential applications for point-of-care diagnostic testing.
Co-reporter:Christophe Renault, Jessica Koehne, Antonio J. Ricco, and Richard M. Crooks
Langmuir 2014 Volume 30(Issue 23) pp:7030-7036
Publication Date(Web):2017-2-22
DOI:10.1021/la501212b
In this paper we describe a method for three-dimensional wax patterning of microfluidic paper-based analytical devices (μPADs). The method is rooted in the fundamental details of wax transport in paper and provides a simple way to fabricate complex channel architectures such as hemichannels and fully enclosed channels. We show that three-dimensional μPADs can be fabricated with half as much paper by using hemichannels rather than ordinary open channels. We also provide evidence that fully enclosed channels are efficiently isolated from the exterior environment, decreasing contamination risks, simplifying the handling of the device, and slowing evaporation of solvents.
Co-reporter:Rachel M. Anderson, David F. Yancey, James A. Loussaert, and Richard M. Crooks
Langmuir 2014 Volume 30(Issue 49) pp:15009-15015
Publication Date(Web):2017-2-22
DOI:10.1021/la503956h
Here we outline a new method for synthesizing fully reduced Pt dendrimer-encapsulated nanoparticles (DENs). This is achieved by first synthesizing Cu DENs of the appropriate size through sequential dendrimer loading and reduction steps, and then galvanically exchanging the zerovalent Cu DENs for Pt. The properties of Pt DENs having an average of 55, 140, and 225 atoms prepared by direct chemical reduction and by galvanic exchange are compared. Data obtained by UV–vis spectroscopy, X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, and high-resolution electron microscopy confirm only the presence of fully reduced Pt DENs when synthesized by galvanic exchange, while chemical reduction leads to a mixture of reduced DENs and unreduced precursor. These results are significant because Pt DENs are good models for developing a better understanding of the effects of finite size on catalytic reactions. Until now, however, the results of such studies have been complicated by a heterogeneous mixture of Pt catalysts.
Co-reporter:James A. Loussaert, Stephen E. Fosdick, and Richard M. Crooks
Langmuir 2014 Volume 30(Issue 45) pp:13707-13715
Publication Date(Web):November 5, 2014
DOI:10.1021/la503232m
Here we report on the electrochemical properties of carbon electrodes coated with thin layers of Al2O3 and SnO2. These oxide films were deposited using atomic layer deposition (ALD) and range in thickness from 1 to 6 nm. Electrochemical experiments show that the thinnest oxide layers contain defects that penetrate to the underlying carbon electrode. However, oxygenation of the carbon surface prior to ALD increases the surface concentration of nucleation sites for oxide growth and suppresses the defect density. Films of Al2O3 just ∼3–4 nm in thickness are free of pinholes. Slightly thicker coatings of SnO2 are required for equivalent passivation. Both Al2O3 and SnO2 films are stable in both neutral and acidic electrolytes even after repeated voltammetric scanning. The results reported here open up the possibility of studying the effect of oxide supports on electrocatalytic reactions.
Co-reporter:Timothy M. Alligrant, Morgan J. Anderson, Radhika Dasari, Keith J. Stevenson, and Richard M. Crooks
Langmuir 2014 Volume 30(Issue 44) pp:13462-13469
Publication Date(Web):2017-2-22
DOI:10.1021/la503628h
We report on the effect of convection on electrochemically active collisions between individual Pt nanoparticles (PtNPs) and Hg and Au electrodes. Compared to standard electrochemical cells utilizing Hg and Au ultramicroelectrodes (UMEs) used in previous studies of electrocatalytic amplification, microelectrochemical devices offer two major advantages. First, the PtNP limit of detection (0.084 pM) is ∼8 times lower than the lowest concentration measured using UMEs. Second, convection enhances the mass transfer of PtNPs to the electrode surface, which enhances the collision frequency from ∼0.02 pM–1 s–1 on UMEs to ∼0.07 pM–1 s–1 in microelectrochemical devices. We also show that the size of PtNPs can be measured in flowing systems using data from collision experiments and then validate this finding using multiphysics simulations.
Co-reporter:Ravikumar Iyyamperumal ; Liang Zhang ; Graeme Henkelman
Journal of the American Chemical Society 2013 Volume 135(Issue 15) pp:5521-5524
Publication Date(Web):April 8, 2013
DOI:10.1021/ja4010305
We report electrocatalytic oxidation of formic acid using monometallic and bimetallic dendrimer-encapsulated nanoparticles (DENs). The results indicate that the Au147@Pt DENs exhibit better electrocatalytic activity and low CO formation. Theoretical calculations attribute the observed activity to the deformation of nanoparticle structure, slow dehydration of formic acid, and weak binding of CO on Au147@Pt surface. Subsequent experiments confirmed the theoretical predictions.
Co-reporter:Stephen E. Fosdick ; Morgan J. Anderson ; Elizabeth G. Nettleton
Journal of the American Chemical Society 2013 Volume 135(Issue 16) pp:5994-5997
Publication Date(Web):April 16, 2013
DOI:10.1021/ja401864k
Optical tracking of collisions between insulating microbeads and an ultramicroelectrode surface are correlated to electrochemical measurements and 3D simulations. The experiments are based on partial blocking of the electrode surface by the beads. Results obtained using these three methods provide details regarding the radial distribution of landing locations, the extent of current blockage, collision frequency, motion of beads on the electrode surface following collisions, and aggregation behavior both prior to collisions and afterward on the electrode surface.
Co-reporter:David F. Yancey, Samuel T. Chill, Liang Zhang, Anatoly I. Frenkel, Graeme Henkelman and Richard M. Crooks
Chemical Science 2013 vol. 4(Issue 7) pp:2912-2921
Publication Date(Web):13 May 2013
DOI:10.1039/C3SC50614B
In this paper we present a new methodology for the analysis of 1–2 nm nanoparticles using extended X-ray absorption fine structure (EXAFS) spectroscopy. Different numbers of thiols were introduced onto the surfaces of dendrimer-encapsulated Au nanoparticles, consisting of an average of 147 atoms, to systematically tune the nanoparticle disorder. An analogous system was investigated using density functional theory molecular dynamics (DFT-MD) simulations to produce theoretical EXAFS signals that could be directly compared to the experimental results. Validation of the theoretical results by comparing to experiment allows us to infer previously unknown details of structure and dynamics of the nanoparticles. Additionally, the structural information that is learned from theoretical studies can be compared with traditional EXAFS fitting results to identify and rationalize any errors in the experimental fit. This study demonstrates that DFT-MD simulations accurately depict complex experimental systems in which we have control over nanoparticle disorder, and shows the advantages of using a combined experimental/theoretical approach over standard EXAFS fitting methodologies for determining the structural parameters of metallic nanoparticles.
Co-reporter:Karen Scida, Bingling Li, Andrew D. Ellington, and Richard M. Crooks
Analytical Chemistry 2013 Volume 85(Issue 20) pp:9713
Publication Date(Web):September 26, 2013
DOI:10.1021/ac402118a
We demonstrate the hybridization-induced fluorescence detection of DNA on an origami-based paper analytical device (oPAD). The paper substrate was patterned by wax printing and controlled heating to construct hydrophilic channels and hydrophobic barriers in a three-dimensional fashion. A competitive assay was developed where the analyte, a single-stranded DNA (ssDNA), and a quencher-labeled ssDNA competed for hybridization with a fluorophore-labeled ssDNA probe. Upon hybridization of the analyte with the fluorophore-labeled ssDNA, a linear response of fluorescence vs analyte concentration was observed with an extrapolated limit of detection <5 nM and a sensitivity relative standard deviation as low as 3%. The oPAD setup was also tested against OR/AND logic gates, proving to be successful in both detection systems.
Co-reporter:Karen Scida, Eoin Sheridan and Richard M. Crooks
Lab on a Chip 2013 vol. 13(Issue 12) pp:2292-2299
Publication Date(Web):08 May 2013
DOI:10.1039/C3LC50321F
A method for controlling enrichment, separation, and delivery of analytes into different secondary microchannels using simple microfluidic architecture is described. The approach, which is based on bipolar electrochemistry, requires only easily fabricated electrodes and a low-voltage DC power supply: no pumps or valves are necessary. Upon application of a voltage between two driving electrodes, passive bipolar electrodes (BPEs) are activated that result in formation of a local electric field gradient. This gradient leads to separation and enrichment of a pair of fluorescent analytes within a primary microfluidic channel. Subsequently, other passive BPEs can be activated to deliver the enriched tracers to separate secondary microchannels. The principles and performance underpinning the method are described.
Co-reporter:Hong Liu and Richard M. Crooks
Lab on a Chip 2013 vol. 13(Issue 7) pp:1364-1370
Publication Date(Web):06 Feb 2013
DOI:10.1039/C3LC41263F
Here we report a method for highly reproducible chronoamperometric analysis of the contents of microdroplets. Aqueous microdroplets having volumes on the order of 1 nL and separated by a fluorocarbon solvent are generated within a microfluidic device using a T-shaped junction. The key finding is that stable and reproducible quasi-steady-state currents are observed if the electrochemical measurements are made in a narrowed segment of a microchannel. Under these conditions, the microdroplets are stretched, here by a factor of 10, leading to desirable intradroplet mass transfer characteristics. Microdroplet frequencies up to 0.67 s−1 are accessible using this method. The quasi-steady-state currents resulting from chronoamperometric analysis of microdroplets containing 1.0 mM Ru(NH3)63+ have relative standard deviations of just 1.8% and 2.8% at flow rates of 30 nL min−1 and 60 nL min−1, respectively. Importantly, the design of the microelectrochemical device ensures direct contact between intradroplet redox molecules and the electrode surface. That is, the fluorocarbon between microdroplets does interfere with inner-sphere electrocatalytic processes such as the oxygen reduction reaction. Finite-element simulations are presented that are in accord with the experimental findings.
Co-reporter:Timothy M. Alligrant, Elizabeth G. Nettleton and Richard M. Crooks
Lab on a Chip 2013 vol. 13(Issue 3) pp:349-354
Publication Date(Web):09 Nov 2012
DOI:10.1039/C2LC40993C
We report on real-time electrochemical detection of individual DNA hybridization events at an electrode surface. The experiment is carried out in a microelectrochemical device configured with a working electrode modified with single-stranded DNA probe molecules. When a complementary DNA strand labelled with a catalyst hybridizes to the probe, an easily detectable electrocatalytic current is observed. In the experiments reported here, the catalyst is a platinum nanoparticle and the current arises from electrocatalytic oxidation of hydrazine. Two types of current transients are observed: short bursts and longer-lived steps. At low concentrations of hydrazine, the average size of the current transients is proportional to the amount of hydrazine present, but at higher concentrations the hydrazine oxidation reaction interferes with hybridization.
Co-reporter:Hong Liu and Richard M. Crooks
Analytical Chemistry 2013 Volume 85(Issue 3) pp:1834
Publication Date(Web):December 20, 2012
DOI:10.1021/ac3032228
We report a potentiometric method for measuring the hemoglobin A1c (HbA1c, glycated hemoglobin) concentration, hemoglobin (Hb) concentration, and percent HbA1c (%HbA1c) in human blood hemolysate. The %HbA1c is important for diagnosis and management of diabetes mellitus. Alizarin red s (ARS) is used as a redox indicator. Phenylboronic acid (PBA) binds to both ARS and HbA1c via diol–boronic acid complexation. The binding of PBA to ARS shifts its redox potential negatively. However, when HbA1c competes with ARS for PBA binding, the solution potential shifts positively. This shift is linked to the HbA1c concentration. The concentration of Hb is determined by allowing it to react with Fe(CN)63–. The potential shift arising from the reduction of Fe(CN)63– by Hb is proportional to the logarithm of the Hb concentration. The results obtained for %HbA1c in human blood hemolysate are in good agreement with those determined using a reference method.
Co-reporter:Stephen E. Fosdick, Sean P. Berglund, C. Buddie Mullins, and Richard M. Crooks
Analytical Chemistry 2013 Volume 85(Issue 4) pp:2493
Publication Date(Web):February 6, 2013
DOI:10.1021/ac303581b
Here we report simultaneous screening of bimetallic electrocatalyst candidates for the oxygen reduction reaction (ORR) using bipolar electrochemistry. The analysis is carried out by dispensing different bimetallic precursor compositions onto the cathodic poles of an array of bipolar electrodes (BPEs) and then heating them in a reducing atmosphere to yield the catalyst candidates. Because BPEs do not require a direct electrical connection for activation, up to 33 electrocatalysts can be screened simultaneously by applying a voltage to the electrolyte solution in which the BPE array is immersed. The screening of the electrocatalyst candidates can be achieved in about 10 min. The current required to drive the ORR arises from oxidation of Cr microbands present at the anodic poles of the BPEs. Therefore, the most effective electrocatalysts result in oxidation (dissolution) of the most microbands, and simply counting the microbands remaining at the end of the screen provides information about the onset potential required to reduce oxygen. Here, we evaluated three Pd–M (M = Au, Co, W) bimetallic electrocatalysts. In principle, arbitrarily large libraries of electrocatalysts can be screened using this approach.
Co-reporter:Hong Liu, Xiang Li, and Richard M. Crooks
Analytical Chemistry 2013 Volume 85(Issue 9) pp:4263
Publication Date(Web):April 15, 2013
DOI:10.1021/ac4008623
We report a paper analytical device (PAD) that is based on the SlipChip concept. This SlipPAD enables robust, high-throughput, multiplexed sensing while maintaining the extreme simplicity of paper-based analysis. The SlipPAD is comprised of two wax-patterned paper fluidic layers. By slipping one layer relative to the other, solutions wick simultaneously into a large array of sensing reservoirs or sequentially into a large array of channels to carry out homogeneous or heterogeneous assays, respectively. The applicability of the device to high-throughput multiplex chemical analysis is demonstrated by colorimetric and fluorescent assays.
Co-reporter:Christophe Renault, Xiang Li, Stephen E. Fosdick, and Richard M. Crooks
Analytical Chemistry 2013 Volume 85(Issue 16) pp:7976
Publication Date(Web):August 9, 2013
DOI:10.1021/ac401786h
We present a microfluidic paper analytical device (μPAD) that relies on flow in hollow channels, rather than through a cellulose network, to transport fluids. The flow rate in hollow channels is 7 times higher than in regular paper channels and can be conveniently controlled from 0 to several mm/s by balancing capillary and pressure forces. More importantly, the pressure of a single drop of liquid (∼0.2 mbar) is sufficient to induce fast pressure-driven flow, making hollow channels suitable for point of care diagnostics. We demonstrate their utility for simple colorimetric glucose and BSA assays in which the time for liquid transport is reduced by a factor of 4 compared to normal cellulose channels.
Co-reporter:Stephen E. Fosdick;Kyle N. Knust;Karen Scida ; Richard M. Crooks
Angewandte Chemie 2013 Volume 125( Issue 40) pp:10632-10651
Publication Date(Web):
DOI:10.1002/ange.201300947
Abstract
Eine bipolare Elektrode (BPE) ist ein elektrisch leitfähiges Objekt, an dessen äußeren Enden elektrochemische Reaktionen ablaufen, ohne dass ein direkter Ohm’scher Kontakt vorhanden sein muss. Es reicht, an eine Elektrolytlösung mit eingetauchter BPE eine Spannung anzulegen, und ab einer bestimmten Potentialdifferenz zwischen BPE und Lösung laufen Oxidations- und Reduktionsreaktionen ab. Meist reicht eine einzige Gleichstromquelle oder sogar nur eine Batterie aus, um selbst sehr große Elektrodenanordnungen mit Spannung zu versorgen. Somit können neue Materialen für vielfältige Anwendungen kabellos elektrolytisch hergestellt und auf ihre Eigenschaften hin überprüft werden. Auch bipolar-elektrochemische Anwendungen mit mobilen, sich frei in Lösung bewegenden Elektroden, die Mikroschwimmer genannt werden, sind möglich.
Co-reporter:Stephen E. Fosdick;Kyle N. Knust;Karen Scida ; Richard M. Crooks
Angewandte Chemie International Edition 2013 Volume 52( Issue 40) pp:10438-10456
Publication Date(Web):
DOI:10.1002/anie.201300947
Abstract
A bipolar electrode (BPE) is an electrically conductive material that promotes electrochemical reactions at its extremities (poles) even in the absence of a direct ohmic contact. More specifically, when sufficient voltage is applied to an electrolyte solution in which a BPE is immersed, the potential difference between the BPE and the solution drives oxidation and reduction reactions. Because no direct electrical connection is required to activate redox reactions, large arrays of electrodes can be controlled with just a single DC power supply or even a battery. The wireless aspect of BPEs also makes it possible to electrosynthesize and screen novel materials for a wide variety of applications. Finally, bipolar electrochemistry enables mobile electrodes, dubbed microswimmers, that are able to move freely in solution.
Co-reporter:Rachel M. Anderson, Liang Zhang, James A. Loussaert, Anatoly I. Frenkel, Graeme Henkelman, and Richard M. Crooks
ACS Nano 2013 Volume 7(Issue 10) pp:9345
Publication Date(Web):October 2, 2013
DOI:10.1021/nn4040348
Bimetallic PdPt dendrimer-encapsulated nanoparticles (DENs) having sizes of about 2 nm were synthesized by a homogeneous route that involved (1) formation of a Pd core, (2) deposition of a Cu shell onto the Pd core in the presence of H2 gas, and (3) galvanic exchange of Pt for the Cu shell. Under these conditions, a Pd@Pt core@shell DEN is anticipated, but detailed characterization by in-situ extended X-ray absorption fine structure (EXAFS) spectroscopy and other analytical methods indicate that the metals invert to yield a Pt-rich core with primarily Pd in the shell. The experimental findings correlate well with density functional theoretical (DFT) calculations. Theory suggests that the increased disorder associated with <∼2 nm diameter nanoparticles, along with the relatively large number of edge and corner sites, drives the structural rearrangement. This type of rearrangement is not observed on larger nanoparticles or in bulk metals.Keywords: core@shell; dendrimer-encapsulated nanoparticles; DFT; EXAFS; inversion; PdPt
Co-reporter:Liang Zhang, Ravikumar Iyyamperumal, David F. Yancey, Richard M. Crooks, and Graeme Henkelman
ACS Nano 2013 Volume 7(Issue 10) pp:9168
Publication Date(Web):September 16, 2013
DOI:10.1021/nn403788a
We report that the oxygen binding energy of alloy-core@Pt nanoparticles can be linearly tuned by varying the alloy-core composition. Using this tuning mechanism, we are able to predict optimal compositions for different alloy-core@Pt nanoparticles. Subsequent electrochemical measurements of ORR activities of AuPd@Pt dendrimer-encapsulated nanoparticles (DENs) are in a good agreement with the theoretical prediction that the peak of activity is achieved for a 28% Au/72% Pd alloy core supporting a Pt shell. Importantly, these findings represent an unusual case of first-principles theory leading to nearly perfect agreement with experimental results.Keywords: core−shell nanoparticles; oxygen reduction; platinum
Co-reporter:Emily V. Carino ; Hyun You Kim ; Graeme Henkelman
Journal of the American Chemical Society 2012 Volume 134(Issue 9) pp:4153-4162
Publication Date(Web):February 22, 2012
DOI:10.1021/ja209115e
The voltammetry of Cu underpotential deposition (UPD) onto Pt dendrimer-encapsulated nanoparticles (DENs) containing an average of 147 Pt atoms (Pt147) is correlated to density functional theory (DFT) calculations. Specifically, the voltammetric peak positions are in good agreement with the calculated energies for Cu deposition and stripping on the Pt(100) and Pt(111) facets of the DENs. Partial Cu shells on Pt147 are more stable on the Pt(100) facets, compared to the Pt(111) facets, and therefore, Cu UPD occurs on the 4-fold hollow sites of Pt(100) first. Finally, the structures of Pt DENs having full and partial monolayers of Cu were characterized in situ by X-ray absorption spectroscopy (XAS). The results of XAS studies are also in good agreement with the DFT-optimized models.
Co-reporter:David F. Yancey, Liang Zhang, Richard M. Crooks and Graeme Henkelman
Chemical Science 2012 vol. 3(Issue 4) pp:1033-1040
Publication Date(Web):30 Jan 2012
DOI:10.1039/C2SC00971D
In this paper we report the electrochemical synthesis of core@shell dendrimer-encapsulated nanoparticles (DENs) consisting of cores containing 147 Au atoms (Au147) and Pt shells having ∼54 or ∼102 atoms (Au147@Ptn (n = 54 or 102)). The significance of this work arises from the correlation of the experimentally determined structural and electrocatalytic properties of these particles with density functional theory (DFT) calculations. Specifically, we describe an experimental and theoretical study of Pb underpotential deposition (UPD) on Au147 DENs, the structure of both Au147@Pbn and Au147@Ptn DENs, and the activity of these DENs for the oxygen reduction reaction (ORR). DFT calculations show that Pb binding is stronger on the (100) facets of Au as compared to (111), and the calculated deposition and stripping potentials are consistent with those measured experimentally. Galvanic exchange is used to replace the surface Pb atoms with Pt, and a surface distortion is found for Au147@Ptn particles using molecular dynamics simulations in which the Pt-covered (100) facets shear into (111) diamond structures. DFT calculations of oxygen binding show that the distorted surfaces are the most active for the ORR, and that their activity is similar regardless of the Pt coverage. These calculations are consistent with rotating ring-disk voltammetry measurements.
Co-reporter:Hong Liu and Richard M. Crooks
Analytical Chemistry 2012 Volume 84(Issue 5) pp:2528-2532
Publication Date(Web):February 22, 2012
DOI:10.1021/ac203457h
We report a battery-powered, microelectrochemical sensing platform that reports its output using an electrochromic display. The platform is fabricated based on paper fluidics and uses a Prussian blue spot electrodeposited on an indium-doped tin oxide thin film as the electrochromic indicator. The integrated metal/air battery powers both the electrochemical sensor and the electrochromic read-out, which are in electrical contact via a paper reservoir. The sample activates the battery and the presence of analyte in the sample initiates the color change of the Prussian blue spot. The entire system is assembled on the lab bench, without the need for cleanroom facilities. The applicability of the device to point-of-care sensing is demonstrated by qualitative detection of 0.1 mM glucose and H2O2 in artificial urine samples.
Co-reporter:Ioana Dumitrescu, David F. Yancey and Richard M. Crooks
Lab on a Chip 2012 vol. 12(Issue 5) pp:986-993
Publication Date(Web):26 Jan 2012
DOI:10.1039/C2LC21181E
In this paper we introduce a microelectrochemical cell configured for generation-collection experiments and designed primarily for examining the kinetics of electrocatalysts. The heart of the device consists of two, closely spaced, pyrolyzed photoresist microband electrodes enclosed within a microchannel. The cell is suitable for evaluating the efficiency of electrocatalysts under an unprecedented range of conditions. Specifically, compared to the gold-standard rotating ring-disk electrode (RRDE), this device offers four major advantages. First, collection efficiencies of 97% are easily achieved, compared to values of 20–37% that are characteristic of RRDEs. Second, mass transfer coefficients of 0.5 cm s−1 are accessible for typical redox species, which is significantly higher than RRDEs (up to 0.01 cm s−1). Third, we show that the device can operate effectively at temperatures up to 70 °C, which is important for measuring electrochemical kinetics that are relevant to fuel cell catalysts. Finally, much less catalyst and much smaller volumes of electrolyte solution are required to make kinetic measurements using the microelectrochemical device compared to the RRDE. Here, we present the simple procedure used to fabricate the device, fundamental electroanalytical characterization, and electrocatalytic measurements relevant to the oxygen reduction reaction.
Co-reporter:Eoin Sheridan, Dzmitry Hlushkou, Kyle N. Knust, Ulrich Tallarek, and Richard M. Crooks
Analytical Chemistry 2012 Volume 84(Issue 17) pp:7393
Publication Date(Web):August 14, 2012
DOI:10.1021/ac301101b
We have previously demonstrated up to 5 × 105-fold enrichment of anionic analytes in a microchannel using a technique called bipolar electrode focusing (BEF). Here, we demonstrate that BEF can also be used to enrich a cationic fluorescent tracer. The important point is that chemical modification of the microchannel walls enables reversal of the electroosmotic flow (EOF), enabling cations, instead of anions, to be enriched via an electric field gradient focusing mechanism. Reversal of the EOF has significant consequences on the formation and shape of the region of the buffer solution depleted of charge carriers (depletion zone). Electric field measurements and numerical simulations are used to elucidate the factors influencing the depletion zone. This information is used to understand and control the location and shape of the depletion zone, which in turn influences the stability and concentration of the enriched band.
Co-reporter:Byoung-Yong Chang, Kwok-Fan Chow, John A. Crooks, François Mavré and Richard M. Crooks
Analyst 2012 vol. 137(Issue 12) pp:2827-2833
Publication Date(Web):11 May 2012
DOI:10.1039/C2AN35382B
We report a two-channel microelectrochemical sensor that communicates between separate sensing and reporting microchannels via one or more bipolar electrodes (BPEs). Depending on the contents of each microchannel and the voltage applied across the BPE, faradaic reactions may be activated simultaneously in both channels. As presently configured, one end of the BPE is designated as the sensing pole and the other as the reporting pole. When the sensing pole is activated by a target, electrogenerated chemiluminescence (ECL) is emitted at the reporting pole. Compared to previously reported single-channel BPE sensors, the key advantage of the multichannel architecture reported here is physical separation of the ECL reporting cocktail and the solution containing the target. This prevents chemical interference between the two channels.
Co-reporter:V. Sue Myers, Anatoly I. Frenkel, and Richard M. Crooks
Langmuir 2012 Volume 28(Issue 2) pp:1596-1603
Publication Date(Web):January 5, 2012
DOI:10.1021/la203756z
In situ electrochemical extended X-ray absorption fine structure (EXAFS) was used to evaluate the structure of Pt dendrimer-encapsulated nanoparticles (DENs) during the oxygen reduction reaction (ORR). The DENs contained an average of just 225 atoms each. The results indicate that the Pt coordination number (CN) decreases when the electrode potential is moved to positive values. The results are interpreted in terms of an ordered core, disordered shell model. The structure of the DENs is not significantly impacted by the presence of dioxygen, but other electrogenerated species may have a significant impact on nanoparticle structure.
Co-reporter:Ioana Dumitrescu
PNAS 2012 Volume 109 (Issue 29 ) pp:
Publication Date(Web):2012-07-17
DOI:10.1073/pnas.1201370109
Here we report on the effect of the mass transfer rate (kt) on the oxygen reduction reaction (ORR) catalyzed by Pt dendrimer-encapsulated nanoparticles (DENs) comprised of 147 and
55 atoms (Pt147 and Pt55). The experiments were carried out using a dual-electrode microelectrochemical device, which enables the study of the ORR
under high kt conditions with simultaneous detection of H2O2. At low kt (0.02 to 0.12 cm s-1) the effective number of electrons involved in ORR, neff, is 3.7 for Pt147 and 3.4 for Pt55. As kt is increased, the mass-transfer-limited current for the ORR becomes significantly lower than the value predicted by the Levich
equation for a 4-electron process regardless of catalyst size. However, the percentage of H2O2 detected remains constant, such that neff barely changes over the entire kt range explored (0.02 cm s-1). This suggests that mass transfer does not affect neff, which has implications for the mechanism of the ORR on Pt nanoparticles. Interestingly, there is a significant difference
in neff for the two sizes of Pt DENs (neff = 3.7 and 3.5 for Pt147 and Pt55, respectively) that cannot be assigned to mass transfer effects and that we therefore attribute to a particle size effect.
Co-reporter:Hong Liu;Dr. Yu Xiang; Yi Lu; Richard M. Crooks
Angewandte Chemie 2012 Volume 124( Issue 28) pp:7031-7034
Publication Date(Web):
DOI:10.1002/ange.201202929
Co-reporter:Hong Liu;Dr. Yu Xiang; Yi Lu; Richard M. Crooks
Angewandte Chemie International Edition 2012 Volume 51( Issue 28) pp:6925-6928
Publication Date(Web):
DOI:10.1002/anie.201202929
Co-reporter:Stephen E. Fosdick
Journal of the American Chemical Society 2011 Volume 134(Issue 2) pp:863-866
Publication Date(Web):December 20, 2011
DOI:10.1021/ja210354m
We report a method for rapid screening of arrays of electrocatalyst candidates. The approach is based on simultaneous activation of the oxygen reduction reaction (ORR) and Ag electrodissolution at the cathodic and anodic poles, respectively, of bipolar electrodes (BPEs). Because the electrochemical activity of the two poles is directly coupled via the BPE, the extent of Ag electrodissolution is directly related to the ORR activity. The screening process lasts ∼12 min. Because Ag dissolution provides a permanent record of catalyst activity, the screening results can be determined by simple optical microscopy after the electrochemical experiment. The method has the potential to provide quantitative information about electrocatalyst activity.
Co-reporter:Ioana Dumitrescu ; Robbyn K. Anand ; Stephen E. Fosdick
Journal of the American Chemical Society 2011 Volume 133(Issue 13) pp:4687-4689
Publication Date(Web):March 15, 2011
DOI:10.1021/ja111050h
Here we report that pressure-driven flow alone (no external electrical energy) can be used to drive faradaic electrochemical reactions in microchannels with charged walls. Specifically, we show that solution flow can generate streaming potentials on the order of volts and that this is sufficient to carry out reactions on the anodic and cathodic poles of a bipolar electrode (BPE). The existence of faradaic reactions is proven by electrodissolution of Ag from the anodic end of the BPE.
Co-reporter:Hong Liu
Journal of the American Chemical Society 2011 Volume 133(Issue 44) pp:17564-17566
Publication Date(Web):October 17, 2011
DOI:10.1021/ja2071779
We report a method, based on the principles of origami (paper folding), for fabricating three-dimensional (3-D) paper microfluidic devices. The entire 3-D device is fabricated on a single sheet of flat paper in a single photolithographic step. It is assembled by simply folding the paper by hand. Following analysis, the device can be unfolded to reveal each layer. The applicability of the device to chemical analysis is demonstrated by colorimetric and fluorescence assays using multilayer microfluidic networks.
Co-reporter:V. Sue Myers, Michael G. Weir, Emily V. Carino, David F. Yancey, Surojit Pande and Richard M. Crooks
Chemical Science 2011 vol. 2(Issue 9) pp:1632-1646
Publication Date(Web):27 Jun 2011
DOI:10.1039/C1SC00256B
In this article we describe the synthesis, characterization, and applications of dendrimer-encapsulated nanoparticles (DENs). These materials are synthesized using a template approach in which metal ions are extracted into the interior of dendrimers and then subsequently reduced chemically to yield nearly size-monodisperse particles having diameters in the 1–2 nm range. Monometallic, bimetallic (alloy and core@shell), and semiconductor nanoparticles have been prepared by this route. The dendrimer component of these composites serves not only as a template for preparing the nanoparticle replica, but also as a stabilizer for the nanoparticle. In this perspective, we report on progress in the synthesis, characterization, and applications of these materials since our last review in 2005. Significant advances in the synthesis of core@shell DENs, characterization, and applications to homogeneous and heterogeneous catalysis (including electrocatalysis) are emphasized.
Co-reporter:Brian A. Zaccheo and Richard M. Crooks
Analytical Chemistry 2011 Volume 83(Issue 4) pp:1185
Publication Date(Web):January 19, 2011
DOI:10.1021/ac103115z
Here, we report a device for the detection of the proteolytic enzyme trypsin, which is a biomarker for pancreatitis. The sensor is self-powered, easy to use, and signals the presence of trypsin via a light-emitting diode (LED) that is visible to the unaided eye. Assay time is ∼3 h, and the limit of detection is 0.5 μg/mL, which is within the range required for detection of trypsin at levels signaling acute pancreatitis. The sensing mechanism relies on trypsin digestion of a gelled protein layer. Partial digestion of the protein layer permits hydroxide penetration and subsequent etching of an underlying Al membrane. Degradation of both the protein and Al layers exposes an underlying Mg anode and closes an electrochemical circuit that produces ∼2.2 V. This is sufficient voltage to illuminate the LED. A logarithmic relationship is observed between the time required for LED illumination and trypsin concentration. The device is equally effective for trypsin dissolved in buffer or serum media.
Co-reporter:Robbyn K. Anand, Eoin Sheridan, Kyle N. Knust, and Richard M. Crooks
Analytical Chemistry 2011 Volume 83(Issue 6) pp:2351
Publication Date(Web):February 25, 2011
DOI:10.1021/ac103302j
Bipolar electrode (BPE) focusing locally enriches charged analytes in a microchannel along an electric field gradient that opposes a counter-flow. This electric field gradient forms at the boundary of an ion depletion zone generated by the BPE. Here, we demonstrate concentration enrichment of a fluorescent tracer by up to 500 000-fold. The use of a dual-channel microfluidic configuration, composed of two microchannels electrochemically connected by a BPE, enhances the rate of enrichment (up to 71-fold/s). Faradaic reactions at the ends of the BPE generate ion depletion and enrichment zones in the two, separated channels. This type of device is equivalent to previously reported micro/nanochannel junction arrangements used for ion concentration polarization, but it is experimentally more flexible and much simpler to construct.
Co-reporter:Eoin Sheridan, Dzmitry Hlushkou, Robbyn K. Anand, Derek R. Laws, Ulrich Tallarek, and Richard M. Crooks
Analytical Chemistry 2011 Volume 83(Issue 17) pp:6746
Publication Date(Web):August 4, 2011
DOI:10.1021/ac201402n
We show that a label-free electrochemical method can be used to monitor the position of an enriched analyte band during bipolar electrode focusing in a microfluidic device. The method relies on formation of a depleted buffer cation region, which is responsible for concentration enrichment of the charged analyte. However, this depletion region also leads to an increase in the local electric field in the solution near a bipolar electrode (BPE), and this in turn results in enhanced faradaic reactions (oxidation and reduction of water) at the BPE. Therefore, it is possible to detect the presence of the concentrated analyte band by measuring the current passing through the BPE used for concentration enrichment, or the concentrated band can be detected at a secondary BPE dedicated to that purpose. Both experiments and simulations are presented that fully elucidate the underlying phenomenon responsible for these observations.
Co-reporter:Robbyn K. Anand, Eoin Sheridan, Dzmitry Hlushkou, Ulrich Tallarek and Richard M. Crooks
Lab on a Chip 2011 vol. 11(Issue 3) pp:518-527
Publication Date(Web):30 Nov 2010
DOI:10.1039/C0LC00351D
Bipolar electrode (BPE) focusing is a developing technique for enrichment and separation of charged analytes in a microfluidic channel. The technique employs a bipolar electrode that initiates faradaic processes that subsequently lead to formation of an ion depletion zone. The electric field gradient resulting from this depletion zone focuses ions on the basis of their individual electrophoretic mobilities. The nature of the gradient is of primary importance to the performance of the technique. Here, we report dynamic measurements of the electric field gradient showing that it is stable over time and that its axial position in the microchannel is directly correlated to the location of an enriched tracer band. The position of the gradient can be tuned with pressure-driven flow. We also show that a steeper electric field gradient decreases the breadth of the enriched tracer band and therefore enhances the enrichment process. The slope of the gradient can be tuned by altering the buffer concentration: higher concentrations result in a steeper gradient. Coating the channel with the neutral block co-polymer Pluronic also results in enhanced enrichment.
Co-reporter:Eoin Sheridan, Kyle N. Knust and Richard M. Crooks
Analyst 2011 vol. 136(Issue 20) pp:4134-4137
Publication Date(Web):25 Aug 2011
DOI:10.1039/C1AN15510E
We report a method for removing ions from aqueous solutions without the use of a membrane. The approach, which we call bipolar electrode depletion (BED), is based on the formation of an asymmetric electric field profile in a microchannel containing a bipolar electrode (BPE). The asymmetric field arises from local increases in conductivity caused by faradaic reactions at the BPE. We show how the asymmetric field can be used to deplete anions from a microchannel via a combination of electrophoresis and electroosmosis. We also apply this approach to filter an anionic species from a mixture of charged and neutral species being transported through a microchannel via electroosmosis. This technique could be utilized for desalination or filtration of any species possessing a net charge (e.g. heavy-metals, bacteria, proteins, or functionalized-nanoparticles).
Co-reporter:Surojit Pande, Michael G. Weir, Brian A. Zaccheo and Richard M. Crooks
New Journal of Chemistry 2011 vol. 35(Issue 10) pp:2054-2060
Publication Date(Web):11 May 2011
DOI:10.1039/C1NJ20083F
In this report we present the synthesis and characterization of Pt and Pd dendrimer-encapsulated nanoparticles (DENs) using the method of galvanic exchange. Sixth-generation hydroxyl-terminated poly(amidoamine) dendrimers were used to prepare Cu DENs composed of 55 atoms. In the presence of either PtCl42− or PdCl42−, the less noble Cu DENs oxidize to Cu2+ leaving behind an equal-sized DEN of Pt or Pd, respectively. DENs prepared by direct reduction with BH4−, which is the common synthetic route, and those prepared by galvanic exchange have the same composition, structure, and properties as judged by UV-vis spectroscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and electrochemical methods. However, the galvanic exchange synthesis is much faster (3 h vs. 96 h), and the yield of reduced DENs is significantly higher (nearly 100% in the case of galvanic exchange).
Co-reporter:Surojit Pande and Richard M. Crooks
Langmuir 2011 Volume 27(Issue 15) pp:9609-9613
Publication Date(Web):June 29, 2011
DOI:10.1021/la201882t
We report a UV–vis spectroscopic study of four different types of poly(amidoamine) dendrimers. The results indicate that the degree of protonation of the interior tertiary amines of these dendrimers correlates directly to an absorption band with λmax in the range of 280–285 nm. Specifically, at low pH, the tertiary amines are protonated and the 280–285 nm band is absent. However, at elevated pH, when these groups are deprotonated, this band appears. Similar results were obtained for a simple model compound. The dependence of the 280–285 nm band on the chemical state of the tertiary amines of the dendrimers was confirmed by complexing them with Pd2+ and Pt2+. In this case the band disappears, and it only reappears when the metal ions are decomplexed following reduction with BH4–. Finally, filtration experiments showed that the absorption band between 280−285 nm arises exclusively from intact, or nearly intact, dendrimers rather than low-molecular-weight fragments.
Co-reporter:Brian A. Zaccheo and Richard M. Crooks
Langmuir 2011 Volume 27(Issue 18) pp:11591-11596
Publication Date(Web):August 16, 2011
DOI:10.1021/la202405t
Here, we report that a conductive Au@Ag2O nanoparticle structure significantly enhances the stability of alkaline phosphatase (AlkP) in the presence of the inhibitors urea and l-phenylalanine (Phe). The enzyme/nanoparticle construct is prepared by associating the enzyme with citrate-capped Au particles, and then adding Ag+. UV–vis and XPS spectroscopy and transmission electron microscopy confirm the core@shell structure. AlkP activity was quantified in the presence and absence of the two inhibitors using a time-resolved colorimetric assay. The results indicate that 21% of the initial active AlkP is incorporated into the nanoparticle structure. More importantly, however, the Au@Ag2O core@shell host reduces the inhibitory effect of urea and Phe by factors ranging from 3 to 12, depending on the inhibitor and its concentration, compared to the wild-type enzyme.
Co-reporter:Emily V. Carino and Richard M. Crooks
Langmuir 2011 Volume 27(Issue 7) pp:4227-4235
Publication Date(Web):March 8, 2011
DOI:10.1021/la2001915
Dendrimer-encapsulated nanoparticles (DENs) containing averages of 55, 147, and 225 Pt atoms immobilized on glassy carbon electrodes served as the electroactive surface for the underpotential deposition (UPD) of a Cu monolayer. This results in formation of core@shell (Pt@Cu) DENs. Evidence for this conclusion comes from cyclic voltammetry, which shows that the Pt core DENs catalyze the hydrogen evolution reaction before Cu UPD, but that after Cu UPD this reaction is inhibited. Results obtained by in situ electrochemical X-ray absorption spectroscopy (XAS) confirm this finding.
Co-reporter:Christina H. Wales, Jacob Berger, Samuel Blass, Richard M. Crooks, and Neer Asherie
Langmuir 2011 Volume 27(Issue 7) pp:4104-4109
Publication Date(Web):March 1, 2011
DOI:10.1021/la1050095
Platinum dendrimer-encapsulated nanoparticles (DENs) containing an average 147 atoms were prepared within sixth-generation, hydroxyl-terminated poly(amidoamine) dendrimers (G6-OH). The hydrodynamic radii (Rh) of the dendrimer/nanoparticle composites (DNCs) were determined by quasi-elastic light scattering (QLS) at high (pH ∼10) and neutral pH for various salt concentrations and identities. At high pH, the size of the DNC (Rh ∼4 nm) is close to that of the empty dendrimer. At neutral pH, the size of the DNC approximately doubles (Rh ∼8 nm) whereas that of the empty dendrimer remains unchanged. Changes in ionic strength also alter the size of the DNCs. The increase in size of the DNC is likely due to electrostatic interactions involving the metal nanoparticle.
Co-reporter:Kwok-Fan Chow ; Byoung-Yong Chang ; Brian A. Zaccheo ; François Mavré
Journal of the American Chemical Society 2010 Volume 132(Issue 27) pp:9228-9229
Publication Date(Web):June 17, 2010
DOI:10.1021/ja103715u
Here we report a new type of sensing platform that is based on electrodissolution of a metallic bipolar electrode (BPE). When the target DNA binds to the capture probe at the cathodic pole of the BPE, it triggers the oxidation and dissolution of Ag metal present at the anodic pole. The loss of Ag is easily detectable with the naked eye or a magnifying glass and provides a permanent record of the electrochemical history of the electrode. More importantly, the decrease in the length of the BPE can be directly correlated to the number of electrons passing through the BPE and hence to the sensing reaction at the cathode.
Co-reporter:Stephen E. Fosdick ; John A. Crooks ; Byoung-Yong Chang
Journal of the American Chemical Society 2010 Volume 132(Issue 27) pp:9226-9227
Publication Date(Web):June 17, 2010
DOI:10.1021/ja103667y
This paper introduces the concept of two-dimensional bipolar electrochemistry and discusses its principle of operation. The interesting new result is that electrochemical reactions can be localized at particular locations on the perimeter of a two-dimensional bipolar electrode (2D-BPE), configured at the intersection of two orthogonal microfluidic channels, by controlling the electric field within the contacting electrolyte solution. Experimentally determined maps of the electric field in the vicinity of the 2D-BPEs are in semiquantitative agreement with finite element simulations.
Co-reporter:Byoung-Yong Chang ; John A. Crooks ; Kwok-Fan Chow ; François Mavré
Journal of the American Chemical Society 2010 Volume 132(Issue 43) pp:15404-15409
Publication Date(Web):October 13, 2010
DOI:10.1021/ja107095z
Here we report a simple design philosophy, based on the principles of bipolar electrochemistry, for the operation of microelectrochemical integrated circuits. The inputs for these systems are simple voltage sources, but because they do not require much power they could be activated by chemical or biological reactions. Device output is an optical signal arising from electrogenerated chemiluminescence. Individual microelectrochemical logic gates are described first, and then multiple logic circuits are integrated into a single microfluidic channel to yield an integrated circuit that can perform parallel logic functions. AND, OR, NOR, and NAND gates are described. Eventually, systems such as those described here could provide on-chip data processing functions for lab-on-a-chip devices.
Co-reporter:David F. Yancey ; Emily V. Carino
Journal of the American Chemical Society 2010 Volume 132(Issue 32) pp:10988-10989
Publication Date(Web):July 26, 2010
DOI:10.1021/ja104677z
Dendrimer-encapsulated Au nanoparticles comprised of an average of 147 atoms were synthesized and immobilized on a glassy carbon electrode. A one-atom-thick shell of Cu was added to the Au core by electrochemical underpotential deposition, and then this shell was replaced with Pt by galvanic exchange. The results indicate that this synthetic approach leads to well-defined core/shell nanoparticles <2 nm in diameter. The rates of oxygen reduction at the Au@Pt electrocatalysts were compared to Pt-only and Au-only, 147-atom dendrimer-encapsulated nanoparticles.
Co-reporter:Byoung-Yong Chang, François Mavré, Kwok-Fan Chow, John A. Crooks and Richard M. Crooks
Analytical Chemistry 2010 Volume 82(Issue 12) pp:5317
Publication Date(Web):May 27, 2010
DOI:10.1021/ac100846v
In this paper, we report a new electroanalytical technique we call snapshot voltammetry. This method combines the properties of bipolar electrodes with electrogenerated chemiluminescence (ECL) to provide a means for recording optical voltammograms in a single micrograph. In essence, the information in a snapshot voltammogram is contained in the spatial domain rather than in the time domain, which is the case for conventional voltammetry. The use of a triangle-shaped bipolar electrode stabilizes the interfacial potential difference along its length. Basic electrochemical parameters extracted from snapshot voltammograms are in good agreement with those obtained by conventional voltammetry. Although not explicitly demonstrated in this paper, this method offers the possibility of using arrays of bipolar electrodes to obtain numerous snapshot voltammograms simultaneously.
Co-reporter:M. Antonia Herrero, Javier Guerra, V. Sue Myers, M. Victoria Gómez, Richard M. Crooks and Maurizio Prato
ACS Nano 2010 Volume 4(Issue 2) pp:905
Publication Date(Web):January 29, 2010
DOI:10.1021/nn901729d
In this paper, we report the functionalization of the surface of multiwalled carbon nanotubes (MWNTs) with Au dendrimer encapsulated nanoparticles (DENs). The results show that, when pristine MWNTs having hydrophobic surfaces are exposed to DENs, the dendrimers aggregate on the MWNT surface. However, when the MWNTs are oxidized in acid prior to exposure to DENs, well-dispersed submonolayer coverages of Au nanoparticles are observed on the MWNT surface. This suggests that acid-induced debundling of the nanotubes is an essential prerequisite for attachment of nearly monodisperse DENs. Electron microscopy and NMR spectroscopy confirm that the structures of the DENs and dendrimers are retained after immobilization on the surface of acid-functionalized MWNTs.Keywords: carbon nanotubes; dendrimer; gold nanoparticles; nanomaterials
Co-reporter:Arther T. Gates, Elizabeth G. Nettleton, V. Sue Myers and Richard M. Crooks
Langmuir 2010 Volume 26(Issue 15) pp:12994-12999
Publication Date(Web):June 30, 2010
DOI:10.1021/la102214q
We report the synthesis and characterization of NiSn dendrimer-encapsulated nanoparticles (DENs) with sizes in the range of ∼1.2 nm. These types of materials have potential applications in energy storage, and particles in the 1−3 nm size range are particularly attractive for this use. The NiSn DENs described here contain an average of 147 atoms and are encapsulated within hydrophobic, sixth-generation poly(amidoamine) dendrimers. DENs prepared using four different Ni/Sn ratios, along with monometallic Ni and Sn DENs, are described. To prevent oxidation, the synthesis was carried out under dry conditions in toluene. These bimetallic DENs were characterized by UV−vis spectroscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy. The results indicate that the compositions of the nanoparticles reflect the ratio of Ni2+ and Sn2+ used in the first step of the synthesis; the NiSn nanoparticles remain encapsulated within the dendrimers, and when dry they have a degree of stability even after a short exposure to air.
Co-reporter:Michael G. Weir, Marc R. Knecht, Anatoly I. Frenkel and Richard M. Crooks
Langmuir 2010 Volume 26(Issue 2) pp:1137-1146
Publication Date(Web):October 19, 2009
DOI:10.1021/la902233h
PdAu dendrimer-encapsulated nanoparticles (DENs) were prepared via sequential reduction of the component metals. When Au is reduced onto 55-atom, preformed Pd DEN cores, analysis by UV−vis spectroscopy, electron microscopy, and extended X-ray absorption fine structure (EXAFS) spectroscopy leads to a model consistent with inversion of the two metals. That is, Au migrates into the core and Pd resides on the surface. However, when Pd is reduced onto a 55-atom Au core, the expected Au core−Pd shell structure results. In this latter case, the EXAFS analysis suggests partial oxidation of the relatively thick Pd shell. When the DENs are extracted from their protective dendrimer stabilizers by alkylthiols, the resulting monolayer-protected clusters retain their original Au core−Pd shell structures. The structural analysis is consistent with a study of nanoparticle-catalyzed conversion of resazurin to resorufin. The key conclusion from this work is that correlation of structure to catalytic function for very small, bimetallic nanoparticles requires detailed information about atomic configuration.
Co-reporter:Michael G. Weir;V. Sue Myers; Anatoly I. Frenkel; Richard M. Crooks
ChemPhysChem 2010 Volume 11( Issue 13) pp:2942-2950
Publication Date(Web):
DOI:10.1002/cphc.201000452
Abstract
We report an in situ X-ray absorption-fine structure (XAFS) spectroscopic analysis of ∼1.8 nm Pt dendrimer-encapsulated nanoparticles (DENs) during electrocatalytic oxidation of CO. The results indicate that Pt nanoparticles encapsulated within poly(amidoamine) (PAMAM) dendrimers and immobilized on a carbon electrode retain their electrocatalytic activity and are structurally stable for extended periods during CO oxidation. This is a significant finding, because nanoparticles in this size range are good experimental models for comparison to first-principles calculations if they remain stable.
Co-reporter:M. Victoria Gomez ; Javier Guerra ; V. Sue Myers ; Richard M. Crooks ;Aldrik H. Velders
Journal of the American Chemical Society 2009 Volume 131(Issue 41) pp:14634-14635
Publication Date(Web):September 28, 2009
DOI:10.1021/ja9065442
High-resolution solution 1H NMR spectroscopy has been used to characterize the size of Pd dendrimer-encapsulated nanoparticles (DENs). The Pd nanoparticles measured by this technique contain 55, 147, 200, or 250 atoms, and they are encapsulated within sixth-generation, hydroxyl-terminated poly(amidoamine) PAMAM dendrimers (G6-OH). Detailed analysis of the NMR data shows that signals arising from the innermost protons of G6-OH(Pdn) decrease significantly as the size of the encapsulated nanoparticles increase. A mathematical correlation between this decrease in the integral value and the theoretical number of Pd atoms in the nanoparticle is extracted. It enables the elucidation of the size of Pd DENs by 1H NMR spectroscopy. NMR pulse-field gradient spin−echo experiments demonstrate that G6-OH with and without DENs have identical hydrodynamic radii, which excludes the presence of dendrimer/nanoparticle aggregates.
Co-reporter:Kwok-Fan Chow ; François Mavré ; John A. Crooks ; Byoung-Yong Chang
Journal of the American Chemical Society 2009 Volume 131(Issue 24) pp:8364-8365
Publication Date(Web):May 28, 2009
DOI:10.1021/ja902683f
We report a microelectrochemical array composed of 1000 individual bipolar electrodes that are controlled with just two driving electrodes and a simple power supply. The system is configured so that faradaic processes occurring at the cathode end of each electrode are correlated to light emission via electrogenerated chemiluminescence (ECL) at the anode end. This makes it possible to read out the state of each electrode simultaneously. The significant advance is that the electrode array is fabricated on a glass microscope slide and is operated in a simple electrochemical cell. This eliminates the need for microfluidic channels, provides a fabrication route to arbitrarily large electrode arrays, and will make it possible to place sensing chemistries onto each electrode using a robotic spotter.
Co-reporter:Sue V. Myers, Anatoly I. Frenkel and Richard M. Crooks
Chemistry of Materials 2009 Volume 21(Issue 20) pp:4824
Publication Date(Web):September 24, 2009
DOI:10.1021/cm901378x
The synthesis and characterization of PdCu bimetallic nanoparticles and Pd and Cu monometallic nanoparticles, consisting of an average of ∼64 atoms, is described. The bimetallic nanoparticles were prepared by cocomplexation of Pd2+ and Cu2+ to interior functional groups of a sixth-generation poly(amidoamine) dendrimer template, followed by chemical reduction to yield dendrimer-encapsulated nanoparticles (DENs). Extended X-ray absorption fine structure (EXAFS) spectroscopy indicates that the particles have an alloy structure. TEM studies indicate particle diameters of 1.2−1.3 nm. This is a rare example of a stable nanoparticle in this size range that consists of one reactive metal and one substantially more noble metal. Such materials are predicted by first-principles theory to have interesting catalytic properties.
Co-reporter:Brian A. Zaccheo and Richard M. Crooks
Analytical Chemistry 2009 Volume 81(Issue 14) pp:5757
Publication Date(Web):June 18, 2009
DOI:10.1021/ac900585g
This paper reports a simple DNA sensor having a detection limit of about 24 oligonucleotides and that operates without the need for PCR amplification. The sensor platform is based on an interdigitated array (IDA) of electrodes. The electrodes are modified with DNA capture probes, which are complementary to an analog for the Epstein−Barr genome, and then exposed to an alkaline phosphatase-labeled target. The enzyme catalyzes the formation of L-ascorbic acid, which reduces Ag+ in solution to yield conductive Ag filaments that span the gap between the electrodes of the IDA. Resistance measurements, made with an inexpensive, hand-held multimeter, signal the presence of the target. The sensor response is insensitive to the presence of a large excess of non-complementary DNA sequences.
Co-reporter:François Mavré, Kwok-Fan Chow, Eoin Sheridan, Byoung-Yong Chang, John A. Crooks and Richard M. Crooks
Analytical Chemistry 2009 Volume 81(Issue 15) pp:6218
Publication Date(Web):July 2, 2009
DOI:10.1021/ac900744p
Bipolar electrodes are potentially useful for a variety of sensing applications, but their implementation has been hampered by an inability to easily monitor the current through such electrodes. However, current can be indirectly determined using electrogenerated chemiluminescence (ECL) as a reporting mechanism. This paper provides a detailed theoretical analysis of ECL reporting at bipolar electrodes. In addition, experiments are described that confirm the theory. Finally, we correlate ECL intensity directly to current through the use of split bipolar electrodes. The results indicate that the lowest current that can be indirectly detected through ECL reporting is ∼32 μA/cm2, which corresponds to a reporting sensitivity of ∼7200 counts/nA in the present experimental system.
Co-reporter:Derek R. Laws, Dzmitry Hlushkou, Robbyn K. Perdue, Ulrich Tallarek and Richard M. Crooks
Analytical Chemistry 2009 Volume 81(Issue 21) pp:8923
Publication Date(Web):September 30, 2009
DOI:10.1021/ac901545y
A method for simultaneously concentrating and separating analytes in a buffer-filled microfluidic channel is reported. The approach is based on modulation of the local electric field within the channel and the corresponding opposition of electrophoretic and electroosmotic flow (EOF) velocities. Dye molecules having different electrophoretic mobilities are focused at different locations within the channel where concentration takes place. At least three species, all small dye molecules, can be simultaneously concentrated and separated, with localized enrichment factors up to ∼600 achieved within 400 s. The enrichment zones affect the electric field profile, as evidenced by significant differences in focusing of single versus multiple analytes. The EOF could be modulated by modifying the channel walls with an appropriate polymer, and this had the effect of increasing both the enrichment factors and resolution of the separation. Numerical simulations provide insights into the underlying fundamental principles for the experimental findings.
Co-reporter:Robbyn K. Perdue, Derek R. Laws, Dzmitry Hlushkou, Ulrich Tallarek and Richard M. Crooks
Analytical Chemistry 2009 Volume 81(Issue 24) pp:10149
Publication Date(Web):November 18, 2009
DOI:10.1021/ac901913r
Bipolar electrode focusing at discontinuous bipolar electrodes (BPEs) provides new insight into the faradaic current and electric field characteristics associated with the technique and allows for the controlled transport of a focused anionic tracer in a microfluidic channel. The findings corroborate our previously reported simulation results, which describe the formation of an extended electric field gradient leading to concentration enrichment. This gradient has been attributed to the passage of faradaic current through a BPE affixed to the floor of the microchannel. Our results demonstrate that the onset of faradaic current is coincident with the onset of concentration enrichment. Utilizing an array of microband electrodes, the tracer may be passed from one stationary position to another by rapidly relocating the BPE. However, the tracer movement is limited to one direction, confirming that the electrophoretic velocity of the analyte exceeds the electroosmosis-driven bulk fluid flow velocity at only the cathodic edge of the BPE.
Co-reporter:Emily V. Carino, Marc R. Knecht and Richard M. Crooks
Langmuir 2009 Volume 25(Issue 17) pp:10279-10284
Publication Date(Web):May 13, 2009
DOI:10.1021/la9011108
The stability of Pd dendrimer-encapsulated nanoparticles (DENs) in air-, N2-, and H2-saturated aqueous solutions is reported. The DENs consisted of an average of 147 atoms per sixth-generation, poly(amidoamine) dendrimer. Elemental analysis and UV−vis spectroscopy indicate that there is substantial oxidation of the Pd DENs in the air-saturated solution, less oxidation in the N2-saturated solution, and no detectable oxidation when the DENs are in contact with H2. Additionally, the stability improves when the DEN solutions are purified by dialysis to remove Pd2+-complexing ligands such as chloride. For the air- and N2-saturated solutions, most of the oxidized Pd recomplexes to the interiors of the dendrimers, and a lesser percentage escapes into the surrounding solution. The propensity of Pd DENs to oxidize so easily is a likely consequence of their small size and high surface energy.
Co-reporter:Marc R. Knecht, Michael G. Weir, V. Sue Myers, William D. Pyrz, Heechang Ye, Valeri Petkov, Douglas J. Buttrey, Anatoly I. Frenkel and Richard M. Crooks
Chemistry of Materials 2008 Volume 20(Issue 16) pp:5218
Publication Date(Web):July 30, 2008
DOI:10.1021/cm8004198
In this article, we provide a detailed description of the synthesis and properties of Pt dendrimer-encapsulated nanoparticles (DENs) prepared using sixth-generation, hydroxyl-terminated, poly(amidoamine) (PAMAM) dendrimers (G6−OH) and three different PtCl42−/G6−OH ratios: 55, 147, and 240. Results obtained from UV−vis spectroscopy, X-ray photoelectron spectroscopy, electron microscopy, X-ray absorption spectroscopy, and high-energy X-ray diffraction show that only a fraction of the Pt2+/dendrimer precursors are reduced by BH4− and that the reduction process is highly heterogeneous. That is, after reduction each Pt2+/dendrimer precursor complex is either fully reduced, to yield a DEN having a size and structure consistent with the original PtCl42−/dendrimer ratio used for the synthesis, or the precursor is not reduced at all. This result is consistent with an autocatalytic process that entails slow formation of a nascent catalytic Pt seed within the dendrimer, followed by rapid, catalytic reduction of nearby Pt2+ ions. Details concerning the formation of the Pt2+/dendrimer precursor are also discussed.
Co-reporter:Marc R. Knecht, Michael G. Weir, Anatoly I. Frenkel and Richard M. Crooks
Chemistry of Materials 2008 Volume 20(Issue 3) pp:1019
Publication Date(Web):October 4, 2007
DOI:10.1021/cm0717817
Here we present evidence for an oxidation-driven structural conversion of quasi-alloy PdAu dendrimer-encapsulated nanoparticles (DENs) to a Au-core/Pd-shell configuration. The initial quasialloy was prepared by co-complexation of PdCl42− and AuCl4− within a sixth-generation, poly(amidoamine) dendrimer template followed by chemical reduction. Exposure to air resulted in selective reoxidation of the Pd atoms and subsequent re-reduction led to deposition of a Pd-rich shell on the surface of the remaining Au core. The core/shell nanoparticles were extracted as monolayer-protected clusters (MPCs) from within the dendrimer templates using dodecanethiol. The resulting materials were characterized by UV−vis spectroscopy, transmission electron microscopy, and extended X-ray absorption fine structure (EXAFS) spectroscopy.
Co-reporter:Marc R. Knecht and Richard M. Crooks
New Journal of Chemistry 2007 vol. 31(Issue 7) pp:1349-1353
Publication Date(Web):18 May 2007
DOI:10.1039/B616471B
The synthesis, characterization and magnetic properties of Fe nanoparticles containing an average of 55 and 147 atoms are described. The nanoparticles are prepared using dendrimer templates, and therefore they are nearly size-monodisperse. In the absence of oxygen and water, the Fe nanoparticles are stable, but they decompose quickly when exposed to air. Magnetic analysis indicates that the 55-atom nanoparticles are superparamagnetic, but the 147-atom materials undergo a transition to ferromagnetic at 6 K. Both materials exhibit suppression of the magnetic saturation compared to bulk Fe.
Co-reporter:V. Sue Myers, Michael G. Weir, Emily V. Carino, David F. Yancey, Surojit Pande and Richard M. Crooks
Chemical Science (2010-Present) 2011 - vol. 2(Issue 9) pp:NaN1646-1646
Publication Date(Web):2011/06/27
DOI:10.1039/C1SC00256B
In this article we describe the synthesis, characterization, and applications of dendrimer-encapsulated nanoparticles (DENs). These materials are synthesized using a template approach in which metal ions are extracted into the interior of dendrimers and then subsequently reduced chemically to yield nearly size-monodisperse particles having diameters in the 1–2 nm range. Monometallic, bimetallic (alloy and core@shell), and semiconductor nanoparticles have been prepared by this route. The dendrimer component of these composites serves not only as a template for preparing the nanoparticle replica, but also as a stabilizer for the nanoparticle. In this perspective, we report on progress in the synthesis, characterization, and applications of these materials since our last review in 2005. Significant advances in the synthesis of core@shell DENs, characterization, and applications to homogeneous and heterogeneous catalysis (including electrocatalysis) are emphasized.
Co-reporter:Alma D. Castañeda, Donald A. Robinson, Keith J. Stevenson and Richard M. Crooks
Chemical Science (2010-Present) 2016 - vol. 7(Issue 10) pp:NaN6457-6457
Publication Date(Web):2016/07/01
DOI:10.1039/C6SC02165D
We report a new and general approach that will be useful for adapting the method of electrocatalytic amplification (ECA) to biosensing applications. In ECA, individual collisions of catalytic nanoparticles with a noncatalytic electrode surface lead to bursts of current. In the work described here, the current arises from catalytic electrooxidation of N2H4 at the surface of platinum nanoparticles (PtNPs). The problem with using ECA for biosensing applications heretofore, is that it is necessary to immobilize a receptor, such as DNA (as in the case here) or an antibody on the PtNP surface. This inactivates the colliding NP, however, and leads to very small collision signatures. In the present article, we show that single-stranded DNA (ssDNA) present on the PtNP surface can be detected by selectively removing a fraction of the ssDNA using the enzyme Exonuclease I (Exo I). About half of the current associated with collisions of naked PtNPs can be recovered from fully passivated PtNPs after exposure to Exo I. Experiments carried out using both Au and Hg ultramicroelectrodes reveal some mechanistic aspects of the collision process before and after treatment of the ssDNA-modified PtNPs with Exo I.
Co-reporter:Jason J. Yoo, Joohoon Kim and Richard M. Crooks
Chemical Science (2010-Present) 2015 - vol. 6(Issue 11) pp:NaN6671-6671
Publication Date(Web):2015/07/29
DOI:10.1039/C5SC02259B
Here, we report on the electrochemical detection of individual collisions between a conjugate consisting of silver nanoparticles (AgNPs) linked to conductive magnetic microbeads (cMμBs) via DNA hybridization and a magnetized electrode. The important result is that the presence of the magnetic field increases the flux of the conjugate to the electrode surface, and this in turn increases the collision frequency and improves the limit of detection (20 aM). In addition, the magnitude of the charge associated with the collisions is greatly enhanced in the presence of the magnetic field. The integration of DNA into the detection protocol potentially provides a means for using electrochemical collisions for applications in biological and chemical sensing.
Co-reporter:David F. Yancey, Liang Zhang, Richard M. Crooks and Graeme Henkelman
Chemical Science (2010-Present) 2012 - vol. 3(Issue 4) pp:NaN1040-1040
Publication Date(Web):2012/01/30
DOI:10.1039/C2SC00971D
In this paper we report the electrochemical synthesis of core@shell dendrimer-encapsulated nanoparticles (DENs) consisting of cores containing 147 Au atoms (Au147) and Pt shells having ∼54 or ∼102 atoms (Au147@Ptn (n = 54 or 102)). The significance of this work arises from the correlation of the experimentally determined structural and electrocatalytic properties of these particles with density functional theory (DFT) calculations. Specifically, we describe an experimental and theoretical study of Pb underpotential deposition (UPD) on Au147 DENs, the structure of both Au147@Pbn and Au147@Ptn DENs, and the activity of these DENs for the oxygen reduction reaction (ORR). DFT calculations show that Pb binding is stronger on the (100) facets of Au as compared to (111), and the calculated deposition and stripping potentials are consistent with those measured experimentally. Galvanic exchange is used to replace the surface Pb atoms with Pt, and a surface distortion is found for Au147@Ptn particles using molecular dynamics simulations in which the Pt-covered (100) facets shear into (111) diamond structures. DFT calculations of oxygen binding show that the distorted surfaces are the most active for the ORR, and that their activity is similar regardless of the Pt coverage. These calculations are consistent with rotating ring-disk voltammetry measurements.
Co-reporter:David F. Yancey, Samuel T. Chill, Liang Zhang, Anatoly I. Frenkel, Graeme Henkelman and Richard M. Crooks
Chemical Science (2010-Present) 2013 - vol. 4(Issue 7) pp:NaN2921-2921
Publication Date(Web):2013/05/13
DOI:10.1039/C3SC50614B
In this paper we present a new methodology for the analysis of 1–2 nm nanoparticles using extended X-ray absorption fine structure (EXAFS) spectroscopy. Different numbers of thiols were introduced onto the surfaces of dendrimer-encapsulated Au nanoparticles, consisting of an average of 147 atoms, to systematically tune the nanoparticle disorder. An analogous system was investigated using density functional theory molecular dynamics (DFT-MD) simulations to produce theoretical EXAFS signals that could be directly compared to the experimental results. Validation of the theoretical results by comparing to experiment allows us to infer previously unknown details of structure and dynamics of the nanoparticles. Additionally, the structural information that is learned from theoretical studies can be compared with traditional EXAFS fitting results to identify and rationalize any errors in the experimental fit. This study demonstrates that DFT-MD simulations accurately depict complex experimental systems in which we have control over nanoparticle disorder, and shows the advantages of using a combined experimental/theoretical approach over standard EXAFS fitting methodologies for determining the structural parameters of metallic nanoparticles.
Co-reporter:Zhiyao Duan, Yuanyuan Li, Janis Timoshenko, Samuel T. Chill, Rachel M. Anderson, David F. Yancey, Anatoly I. Frenkel, Richard M. Crooks and Graeme Henkelman
Catalysis Science & Technology (2011-Present) 2016 - vol. 6(Issue 18) pp:NaN6885-6885
Publication Date(Web):2016/05/26
DOI:10.1039/C6CY00559D
In this study, we present a framework for characterizing the structural and thermal properties of small nanoparticle catalysts by combining precise synthesis, extended X-ray absorption fine structure (EXAFS) spectroscopy, and density functional theory (DFT) calculations. We demonstrate the capability of this approach by characterizing the atomic structure and vibrational dynamics of Au147. With the combination of EXAFS spectroscopy and DFT, the synthesized Au147 nanoparticles are determined to have an icosahedral structure. A decrease in the Einstein temperature of the Au147 particles compared to their bulk value was observed and interpreted in terms of softer vibration modes of surface bonds.
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