Co-reporter:Wei Xu, Kaiyu Fu, and Paul W. Bohn
ACS Sensors July 28, 2017 Volume 2(Issue 7) pp:1020-1020
Publication Date(Web):July 4, 2017
DOI:10.1021/acssensors.7b00292
Biosensors based on converting electrochemical signals into optical readouts are attractive candidates as low-cost, high-throughput sensor platforms. Here, we described a closed bipolar electrode (CBE)-based two-cell electrochromic device for sensing multiple metabolites, using the simultaneous detection of lactate, glucose, and uric acid as a model system. In the two-cell configuration, an analytical cell contains a redox mediator combined with a specific oxidase, e.g., lactate oxidase, glucose oxidase, or uricase, to form an electrochemical mediator–electrocatalyst pair that supports redox cycling. A closed bipolar electrode couples the electron transfer event in the analytical cell to an electrochromic reaction in a separate reporter cell, such that the magnitude of the color change is related to the concentration of metabolites in the analytical cell. To demonstrate multiplex operation, the CBE-based electrochromic detector is modified by integrating three sets of detection chemistries into a single device, in which simultaneous determination of glucose, lactate, and uric acid is demonstrated. Device sensitivity can be tuned by using reporter cells with different volumes. Furthermore, the analytical cell of this device can be fabricated as a disposable, paper-based carbon electrode without any pretreatment, demonstrating the potential to screen phenotypes that require multiple biomarkers in a point-of-care format.Keywords: biosensors; bipolar electrode; colorimetric detection; multiplex sensing; paper analytical device;
Co-reporter:Kaiyu Fu and Paul W. Bohn
ACS Applied Materials & Interfaces July 26, 2017 Volume 9(Issue 29) pp:24908-24908
Publication Date(Web):June 29, 2017
DOI:10.1021/acsami.7b06794
The ability to design, fabricate, and manipulate materials at the nanoscale is fundamental to the quest to develop technologies to assemble nanometer-scale pieces into larger-scale components and materials, thereby transferring unique nanometer-scale properties to macroscopic objects. In this work, we describe a new approach to the fabrication of highly ordered, ultrahigh density nanochannel arrays that employs nanosphere lithography to template the graphoepitaxy of polystyrene–polydimethylsiloxane, diblock copolymers. By optimizing the well-controlled solvent vapor annealing, overcoating conditions, and the subsequent reactive ion etching processes, silica nanochannel (SNC) arrays with areal densities, ρA, approaching 1000 elements μm–2, are obtained over macroscopic scales. The integrity and functionality of the SNC arrays was tested by using them as permselective ion barriers to nanopore-confined disk electrodes. The nanochannels allow cations to pass to the disk electrode but reject anions, as demonstrated by cyclic voltammetry. This ion gating behavior can be reversed from cation-permselective to anion-permselective by chemically inverting the surface charge from negative to positive. Furthermore, the conformal SNC array structures obtained could easily be lifted, detached, and transferred to another substrate, preserving the hierarchical organization while transferring the nanostructure-derived properties to a different substrate. These results demonstrate how nanoscale behavior can be replicated over macroscale distances, using electrochemical analysis as a model.Keywords: block copolymers; electrical double-layer effect; graphoepitaxy; ion accumulation; nanochannel arrays; nanosphere lithography; permselectivity;
Co-reporter:Jiayun Hu
Journal of Analysis and Testing 2017 Volume 1( Issue 1) pp:
Publication Date(Web):2017 January
DOI:10.1007/s41664-017-0002-z
Bacterial sensing is important for understanding the numerous roles bacteria play in nature and in technology, understanding and managing bacterial populations, detecting pathogenic bacterial infections, and preventing the outbreak of illness. Current analytical challenges in bacterial sensing center on the dilemma of rapidly acquiring quantitative information about bacteria with high detection efficiency, sensitivity, and specificity, while operating within a reasonable budget and optimizing the use of ancillary tools, such as multivariate statistics. This review starts from a general description of bacterial sensing methods and challenges, and then focuses on bacterial characterization using optical methods including Raman spectroscopy and imaging, infrared spectroscopy, fluorescence spectroscopy and imaging, and plasmonics, including both extended and localized surface plasmon resonance spectroscopy. The advantages and drawbacks of each method in relation to the others are discussed, as are their applications. A particularly promising direction in bacterial sensing lies in combining multiple approaches to achieve multiplex analysis, and examples where this has been achieved are highlighted.
Co-reporter:Kaiyu Fu;Donghoon Han;Chaoxiong Ma
Nanoscale (2009-Present) 2017 vol. 9(Issue 16) pp:5164-5171
Publication Date(Web):2017/04/20
DOI:10.1039/C7NR00206H
Surface charge characteristics and the electrical double layer (EDL) effect govern the transport of ions into and out of nanopores, producing a permselective concentration polarization, which dominates the electrochemical response of nanoelectrodes in solutions of low ionic strength. In this study, highly ordered, zero-dimensional nanopore electrode arrays (NEAs), with each nanopore presenting a pair of recessed electrodes, were fabricated to couple EDL effects with redox cycling, thereby achieving electrochemical detection with improved sensitivity and selectivity. These NEAs exhibit current amplification as high as 55-fold due to the redox cycling effect, which can be further increased by ∼500-fold upon the removal of the supporting electrolyte. The effect of nanopore geometry, which is a key factor determining the magnitude of the EDL effect, is fully characterized, as is the effect of the magnitude and sign of the charge of the redox-active species. The observed changes in limiting current with the concentration of the supporting electrolyte confirm the accumulation of cations and repulsion of anions in NEAs presenting negative surface charge. Exploiting this principle, dopamine was selectively determined in the presence of a 3000-fold excess of ascorbic acid within the NEA.
Co-reporter:Donghoon Han;Garrison M. Crouch;Kaiyu Fu;Lawrence P. Zaino III;Paul W Bohn
Chemical Science (2010-Present) 2017 vol. 8(Issue 8) pp:5345-5355
Publication Date(Web):2017/07/24
DOI:10.1039/C7SC02250F
The ability of zero-mode waveguides (ZMW) to guide light into subwavelength-diameter nanoapertures has been exploited for studying electron transfer dynamics in zeptoliter-volume nanopores under single-molecule occupancy conditions. In this work, we report the spectroelectrochemical detection of individual molecules of the redox-active, fluorogenic molecule flavin mononucleotide (FMN) freely diffusing in solution. Our approach is based on an array of nanopore-confined recessed dual ring electrodes, wherein repeated reduction and oxidation of a single molecule at two closely spaced annular working electrodes yields amplified electrochemical signals. We have articulated these structures with an optically transparent bottom, so that the nanopores are bifunctional, exhibiting both nanophotonic and nanoelectrochemical behaviors allowing the coupling between electron transfer and fluorescence dynamics to be studied under redox cycling conditions. We also investigated the electric field intensity in electrochemical ZMWs (E-ZMW) through finite-element simulations, and the amplification of fluorescence by redox cycling agrees well with predictions based on optical confinement effects inside the E-ZMW. Proof-of-principle experiments are conducted showing that electrochemical and fluorescence signals may be correlated to reveal single molecule fluctuations in the array population. Cross-correlation of single molecule fluctuations in amperometric response and single photon emission provides unequivocal evidence of single molecule sensitivity.
Co-reporter:Lawrence P. Zaino III, William R. A. Wichert, Garrison M. Crouch, and Paul W. Bohn
Analytical Chemistry 2016 Volume 88(Issue 8) pp:4200
Publication Date(Web):April 5, 2016
DOI:10.1021/acs.analchem.6b00399
The ability to perform electrochemistry in the presence of large voltages and electric field magnitudes without concern for the local potential has many possible applications in micro/nanofluidic assays and in capillary electrophoresis. Traditionally, electrochemistry in the presence of significant external electric fields has been dominated by end-channel detection for capillary and microchip electrophoresis detection. We describe novel instrumentation for potentiostatically controlled voltammetry that can be applied in the presence of high external voltages and electric fields. Cyclic voltammetry is demonstrated without significant shifts in the half-wave potential at working electrodes at local potentials of up to ∼1500 V and field strengths of up to 3000 V/cm, using a standard Ag/AgCl reference electrode.
Co-reporter:William R. A. Wichert, Donghoon Han and Paul W. Bohn
Lab on a Chip 2016 vol. 16(Issue 5) pp:877-883
Publication Date(Web):12 Jan 2016
DOI:10.1039/C5LC01413A
The effects of molecular confinement and crowding on enzyme kinetics were studied at length scales and under conditions similar to those found in biological cells. These experiments were carried out using a nanofluidic network of channels constituting a nanofluidic gradient mixer, providing the basis for measuring multiple experimental conditions simultaneously. The 100 nm × 40 μm nanochannels were wet etched directly into borosilicate glass, then annealed and characterized with fluorescein emission prior to kinetic measurements. The nanofluidic gradient mixer was then used to measure the kinetics of the conversion of the horseradish peroxidase (HRP)-catalyzed conversion of non-fluorescent Amplex Red (AR) to the fluorescent product resorufin in the presence of hydrogen peroxide (H2O2). The design of the gradient mixer allows reaction kinetics to be studied under multiple (five) unique solution compositions in a single experiment. To characterize the efficiency of the device the effects of confinement on HRP-catalyzed AR conversion kinetics were studied by varying the starting ratio of AR:H2O2. Equimolar concentrations of Amplex Red and H2O2 yielded the highest reaction rates followed by 2:1, 1:2, 5:1, and finally 1:5 [AR]:[H2O2]. Under all conditions, initial reaction velocities were decreased by excess H2O2. Crowding effects on kinetics were studied by increasing solution viscosity in the nanochannels in the range 1.0–1.6 cP with sucrose. Increasing the solution viscosities in these confined geometries decreases the initial reaction velocity at the highest concentration from 3.79 μM min−1 at 1.00 cP to 0.192 μM min−1 at 1.59 cP. Variations in reaction velocity are interpreted in the context of models for HRP catalysis and for molecular crowding.
Co-reporter:Sneha Polisetti, Amber N. Bible, Jennifer L. Morrell-Falvey and Paul W. Bohn
Analyst 2016 vol. 141(Issue 7) pp:2175-2182
Publication Date(Web):29 Feb 2016
DOI:10.1039/C6AN00080K
Chemical imaging of plant-bacteria co-cultures makes it possible to characterize bacterial populations and behaviors and their interactions with proximal organisms, under conditions closest to the environment in the rhizosphere. Here Raman micro-spectroscopy and confocal Raman imaging are used as minimally invasive probes to study the rhizosphere bacterial isolate, Pantoea sp. YR343, and its co-culture with model plant Arabidopsis thaliana by combining enhanced Raman spectroscopies with electron microscopy and principal component analysis (PCA). The presence of carotenoid pigments in the wild type Pantoea sp. YR343 was characterized using resonance Raman scattering, which was also used to confirm successful disruption of the crtB gene in an engineered carotenoid mutant strain. Other components of the Pantoea sp. YR343 cells were imaged in the presence of resonantly enhanced pigments using a combination of surface enhanced Raman imaging and PCA. Pantoea sp. YR343 cells decorated with Ag colloid synthesized ex situ gave spectra dominated by carotenoid scattering, whereas colloids synthesized in situ produced spectral signatures characteristic of flavins in the cell membrane. Scanning electron microscopy (SEM) of whole cells and transmission electron microscopy (TEM) images of thinly sliced cross-sections were used to assess structural integrity of the coated cells and to establish the origin of spectral signatures based on the position of Ag nanoparticles in the cells. Raman imaging was also used to characterize senescent green Arabidopsis thaliana plant roots inoculated with Pantoea sp. YR343, and PCA was used to distinguish spectral contributions from plant and bacterial cells, thereby establishing the potential of Raman imaging to visualize the distribution of rhizobacteria on plant roots.
Co-reporter:Wei Xu;Dr. Chaoxiong Ma; Paul W. Bohn
ChemElectroChem 2016 Volume 3( Issue 3) pp:422-428
Publication Date(Web):
DOI:10.1002/celc.201500366
Abstract
Electrochemical reactions occurring at the opposite ends of bipolar electrodes (BPEs) are necessarily coupled, enabling electron transfer events at one end to be read out optically, for example, by coupling to fluorogenic reactions at the other end. To explore the potential of this technique for studying multiple redox events, arrays of parallel BPE interdigitated electrode arrays (IDEAs) were fabricated and integrated with separate analytical and reporter microfluidic channels, respectively, in a closed BPE configuration. The apparatus was initially evaluated employing Fe(CN)63/4− in the analytical channel coupled to weakly emissive resazurin and strongly emissive resorufin as the fluorogenic redox reporter pair. The device was then used to investigate a proton-coupled electron transfer reaction, hydroquinone (QH2) oxidation, in structures with an integrated pH modulation electrode (PME). A pH-sensitive dye, fluorescein, was co-introduced into the analytical channel to monitor PME modulation of solution pH, and its coupling to QH2 oxidation, thereby permitting changes in solution pH, and consequently QH2 oxidation rate, to be monitored directly in the analytical channel and compared to the fluorescence in the reporter channel. In addition, diffusion of OH− generated at the PME produced a spatial pH profile that was visualized via fluorescein emission, and, because the oxidation of QH2 at each BPE is strongly dependent on the local pH, via the coupled fluorogenic reaction at the opposite pole of the corresponding BPE digit in the reporter channel. Thus, BPE IDEAs support the coupling of independent redox reactions and the use of fluorescence imaging to explore a diverse set of spatially varying electrochemical phenomena realized in a variety of electrochemical geometries.
Co-reporter:Chaoxiong Ma, Wei Xu, William R. A. Wichert, and Paul W. Bohn
ACS Nano 2016 Volume 10(Issue 3) pp:3658
Publication Date(Web):February 24, 2016
DOI:10.1021/acsnano.6b00049
Ion permselectivity can lead to accumulation in zero-dimensional nanopores, producing a significant increase in ion concentration, an effect which may be combined with unscreened ion migration to improve sensitivity in electrochemical measurements, as demonstrated by the enormous current amplification (∼2000-fold) previously observed in nanopore electrode arrays (NEA) in the absence of supporting electrolyte. Ionic strength is a key experimental factor that governs the magnitude of the additional current amplification (AFad) beyond simple redox cycling through both ion accumulation and ion migration effects. Separate contributions from ion accumulation and ion migration to the overall AFad were identified by studying NEAs with varying geometries, with larger AFad values being achieved in NEAs with smaller pores. In addition, larger AFad values were observed for Ru(NH3)63/2+ than for ferrocenium/ferrocene (Fc+/Fc) in aqueous solution, indicating that coupling efficiency in redox cycling can significantly affect AFad. While charged species are required to observe migration effects or ion accumulation, poising the top electrode at an oxidizing potential converts neutral species to cations, which can then exhibit current amplification similar to starting with the cation. The electrical double layer effect was also demonstrated for Fc/Fc+ in acetonitrile and 1,2-dichloroethane, producing AFad up to 100× at low ionic strength. The pronounced AFad effects demonstrate the advantage of coupling redox cycling with ion accumulation and migration effects for ultrasensitive electrochemical measurements.Keywords: current amplification; electrical double layer effect; ion accumulation; ion migration; nanopore electrode array; redox cycling
Co-reporter:Lawrence P. Zaino III;Chaoxiong Ma
Microchimica Acta 2016 Volume 183( Issue 3) pp:1019-1032
Publication Date(Web):2016 March
DOI:10.1007/s00604-015-1701-7
This review (with 116 refs.) addresses recent developments in nanoelectrode arrays and ensembles with particular attention to nanopore-enabled arrays and ensembles. Nanoelectrode-based arrays exhibit unique mass transport and ion transfer properties, which can be exploited for electroanalytical measurements with enhanced figures-of-merit with respect to microscale and larger components. Following an introduction into the topic, we cover (a) methods for fabrication of solid-state nanopore electrodes, (b) chemical and biochemical sensors, (c) nanochannel arrays with embedded nanoelectrodes; (d) recessed nanodisk electrode arrays; (e) redox cycling in nanopore electrode arrays, (f) finally discuss novel nanoarrays for electrochemistry, and then give a future outlook. A wide variety of nanoelectrode array-based chemical and biochemical sensors properties are discussed in addition to faradaic, ion transfer and spectroelectrochemical applications.
Co-reporter:Donghoon Han, Lawrence P. Zaino III, Kaiyu Fu, and Paul W. Bohn
The Journal of Physical Chemistry C 2016 Volume 120(Issue 37) pp:20634-20641
Publication Date(Web):May 18, 2016
DOI:10.1021/acs.jpcc.6b01287
A redox cycling geometry based on an array of nanopore-confined recessed dual-ring electrodes (RDREs) has been devised to amplify electrochemical signals and enhance the sensitivity of electroanalytical measurements. The RDRE arrays were fabricated using layer-by-layer deposition followed by focused ion beam milling. A characteristic feature of the nanoscale dual-ring geometry is that electrochemical reactions occurring at the bottom-ring electrode can be tuned by modulating the potential at the top-ring electrode. Thus, the resulting device was operated in generator–collector mode by holding the top-ring electrodes at a constant potential and performing cyclic voltammetry by sweeping the bottom-ring potential in aqueous Fe(CN)63–/4–. The enhanced (∼23×) limiting current, achieved by cycling the redox couple between top- and bottom-ring electrodes with high collection efficiency, was compared with that obtained in the absence of self-induced redox cycling (SIRC). Measured shifts in Fe(CN)63–/4– concentration distributions were found to be in excellent agreement with finite-element simulations. The SIRC effect in the RDRE array was also characterized by electrochemical experiments before and after oxygen plasma treatment. The plasma-treated RDRE array exhibited a significant signal amplification, with the faradaic current being augmented by a factor of ∼65 as a result of efficient redox cycling of electroactive species in the nanopores. The amplification factor of the devices was optimized by controlling the interpore distance, with larger pore density arrays exhibiting larger amplification factors.
Co-reporter:Chaoxiong Ma, Lawrence P. Zaino III and Paul W. Bohn
Chemical Science 2015 vol. 6(Issue 5) pp:3173-3179
Publication Date(Web):25 Mar 2015
DOI:10.1039/C5SC00433K
We present a new configuration for coupling fluorescence microscopy and voltammetry using self-induced redox cycling for ultrasensitive electrochemical measurements. An array of nanopores, each supporting a recessed disk electrode separated by 100 nm in depth from a planar multiscale bipolar top electrode, was fabricated using multilayer deposition, nanosphere lithography, and reactive-ion etching. Self-induced redox cycling was induced on the disk electrode producing ∼30× current amplification, which was independently confirmed by measuring induced electrogenerated chemiluminescence from Ru(bpy)32/3+/tri-n-propylamine on the floating bipolar electrode. In this design, redox cycling occurs between the recessed disk and the top planar portion of a macroscopic thin film bipolar electrode in each nanopore. Electron transfer also occurs on a remote (mm-distance) portion of the planar bipolar electrode to maintain electroneutrality. This couples the electrochemical reactions of the target redox pair in the nanopore array with a reporter, such as a potential-switchable fluorescent indicator, in the cell at the distal end of the bipolar electrode. Oxidation or reduction of reversible analytes on the disk electrodes were accompanied by reduction or oxidation, respectively, on the nanopore portion of the bipolar electrode and then monitored by the accompanying oxidation of dihydroresorufin or reduction of resorufin at the remote end of the bipolar electrode, respectively. In both cases, changes in fluorescence intensity were triggered by the reaction of the target couple on the disk electrode, while recovery was largely governed by diffusion of the fluorescent indicator. Reduction of 1 nM of Ru(NH3)63+ on the nanoelectrode array was detected by monitoring the fluorescence intensity of resorufin, demonstrating high sensitivity fluorescence-mediated electrochemical sensing coupled to self-induced redox cycling.
Co-reporter:Nameera F. Baig, Sage J. B. Dunham, Nydia Morales-Soto, Joshua D. Shrout, Jonathan V. Sweedler and Paul W. Bohn
Analyst 2015 vol. 140(Issue 19) pp:6544-6552
Publication Date(Web):24 Aug 2015
DOI:10.1039/C5AN01149C
Two label-free molecular imaging techniques, confocal Raman microscopy (CRM) and secondary ion mass spectrometry (SIMS), are combined for in situ characterization of the spatiotemporal distributions of quinolone metabolites and signaling molecules in communities of the pathogenic bacterium Pseudomonas aeruginosa. Dramatic molecular differences are observed between planktonic and biofilm modes of growth for these bacteria. We observe patterned aggregation and a high abundance of N-oxide quinolines in early biofilms and swarm zones of P. aeruginosa, while the concentrations of these secreted components in planktonic cells and agar plate colonies are below CRM and SIMS detection limits. CRM, in conjunction with principal component analysis (PCA) is used to distinguish between the two co-localized isomeric analyte pairs 4-hydroxy-2-heptylquinoline-N-oxide (HQNO)/2-heptyl-3-hydroxyquinolone (PQS) and 4-hydroxy-2-nonylquinoline-N-oxide (NQNO)/2-nonyl-hydroxyquinolone (C9-PQS) based on differences in their vibrational fingerprints, illustrating how the technique can be used to guide tandem-MS and tandem-MS imaging analysis. Because N-oxide quinolines are ubiquitous and expressed early in biofilms, these analytes may be fundamentally important for early biofilm formation and the growth and organization of P. aeruginosa microbial communities. This study underscores the advantages of using multimodal molecular imaging to study complex biological systems.
Co-reporter:Nicholas M. Contento
Microfluidics and Nanofluidics 2015 Volume 18( Issue 1) pp:131-140
Publication Date(Web):2015 January
DOI:10.1007/s10404-014-1424-9
While electrochemical methods are well suited for lab-on-a-chip applications, reliably coupling multiple, electrode-controlled processes in a single microfluidic channel remains a considerable challenge, because the electric fields driving electrokinetic flow make it difficult to establish a precisely known potential at the working electrode(s). The challenge of coupling electrochemical detection with microchip electrophoresis is well known; however, the problem is general, arising in other multielectrode arrangements with applications in enhanced detection and chemical processing. Here, we study the effects of induced electric fields on voltammetric behavior in a microchannel containing multiple in-channel electrodes, using a Fe(CN)63/4− model system. When an electric field is induced by applying a cathodic potential at one in-channel electrode, the half-wave potential (E1/2) for the oxidation of ferrocyanide at an adjacent electrode shifts to more negative potentials. The E1/2 value depends linearly on the electric field current at a separate in-channel electrode. The observed shift in E1/2 is quantitatively described by a model, which accounts for the change in solution potential caused by the iR drop along the length of the microchannel. The model, which reliably captures changes in electrode location and solution conductivity, apportions the electric field potential between iR drop and electrochemical potential components, enabling the study of microchannel electric field magnitudes at low applied potentials. In the system studied, the iR component of the electric field potential increases exponentially with applied current before reaching an asymptotic value near 80 % of the total applied potential. The methods described will aid in the development and interpretation of future microchip electrochemistry methods, particularly those that benefit from the coupling of electrokinetic and electrochemical phenomena at low voltages.
Co-reporter:Wei Xu;Erick Foster;Chaoxiong Ma
Microfluidics and Nanofluidics 2015 Volume 19( Issue 5) pp:1181-1189
Publication Date(Web):2015 November
DOI:10.1007/s10404-015-1636-7
In situ generation of reagents and their subsequent use downstream presents new opportunities to amplify the utility of nanofluidic devices by exploiting the confined geometry to address mass transport limitations on reaction kinetics and efficiency. Oxygen, an inherently valuable reactant, can be produced from electrolysis of water, a process that can be conveniently integrated within a nanofluidic system. Here, we construct and characterize a nanofluidic device consisting of a planar microband electrode embedded within a nanochannel for in situ electrochemical generation and optical monitoring of O2. Fluorescein, a dye with a pH-sensitive emission intensity, was used to monitor the spatiotemporal characteristics of the oxidation of H2O, using the co-produced H+. Application of anodic potentials at the nanochannel-embedded electrode results in a decrease in fluorescence intensity, which reflects the decreasing solution pH. A combination of fluorescence intensity and chronoamperometric response was used to quantitatively determine proton generation, and the H+/O2 stoichiometry was then used to determine the concentration of the O2 in the channel. Comparison of the experimental results to finite element simulations validates the use of fluorescein emission intensity to spectroscopically determine the local oxygen concentration in the nanochannel. By varying the applied potential, spatially averaged O2 concentrations ranging from 0.13 to 0.41 mM were generated. The results demonstrate a convenient route to in situ modulation of the dissolved O2 level in a nanofluidic device and the use of an optical probe to monitor its spatial and temporal distribution under flow conditions.
Co-reporter:Dorothy R. Ahlf, Rachel N. Masyuko, Amanda B. Hummon and Paul W. Bohn
Analyst 2014 vol. 139(Issue 18) pp:4578-4585
Publication Date(Web):24 Jun 2014
DOI:10.1039/C4AN00826J
A novel method of correlated imaging, combining confocal Raman microscopy (CRM) and matrix-assisted laser desorption ionization (MALDI) mass spectrometry imaging (MSI) was developed in order to investigate the structural and chemical diversity inherent in three-dimensional (3D) cell cultures. These 3D spheroidal cell cultures are high throughput in vitro model systems that recapitulate some of the chemical and physiological gradients characteristic of tissues. As a result, they are ideal for testing new imaging approaches due to the native diversity of cellular phenotypes found within a single culture. Individually, confocal Raman microscopy (CRM) and mass spectrometry imaging (MSI) produce different kinds of chemical information. CRM imaging reveals differences in cellular integrity and protein secretion across a typical near-equatorial transverse slice, while MSI shows localization of small molecules to discrete regions of the spheroid section. Correlating information obtained from these disparate imaging methods begins with an external fiducial mask, added to the spheroidal samples to orient image acquisition on the two orthogonal platforms. Rather than combine the images directly, principal component analysis is used to reveal the most chemically-informative elements, which are then combined using digital image correlation. Using this approach, relationships between the principal components of each method are visualized so that they may be compared on commensurate spatial length scales.
Co-reporter:Rachel N. Masyuko, Eric J. Lanni, Callan M. Driscoll, Joshua D. Shrout, Jonathan V. Sweedler and Paul W. Bohn
Analyst 2014 vol. 139(Issue 22) pp:6058-6060
Publication Date(Web):29 Sep 2014
DOI:10.1039/C4AN90079K
Correction for ‘Spatial organization of Pseudomonas aeruginosa biofilms probed by combined matrix-assisted laser desorption ionization mass spectrometry and confocal Raman microscopy’ by Rachel N. Masyuko et al., Analyst, 2014, DOI: 10.1039/c4an00435c.
Co-reporter:Tai-Wei Hwang and Paul W. Bohn
ACS Nano 2014 Volume 8(Issue 2) pp:1718
Publication Date(Web):January 13, 2014
DOI:10.1021/nn406098u
The effect of electrochemical potential on the behavior of electrochemically deposited Au–Ag–Au bimetallic atomic scale junctions (ASJs) is addressed here. A common strategy for ASJ production begins with overgrown nanojunctions and uses electromigration to back-thin the junction. Here, these steps are carried out with the entire junction under electrochemical potential control, and the relationship between junction stability and applied potential is characterized. The control of electrochemical potential provides a reliable method of regulating the size of nanojunctions. In general, more anodic potentials decrease junction stability and increase the rate at which conductance decays. Conductance behavior under these labile conditions is principally determined by Ag oxidation potential, electrochemical potential-induced surface stress, and the nature of the adsorbate. Junctions fabricated at more cathodic potentials experience only slight changes in conductance, likely due to surface atom diffusion and stress-induced structural rearrangement. Electrochemical potential also plays a significant role in determining adsorption–desorption kinetics of surface pyridine at steady state at Au–Ag–Au ASJs, as revealed through fluctuation spectroscopy. Average cutoff frequencies increase at more anodic potentials, as does the width of the cutoff frequency distribution measured over 80 independent runs. Three reversible reactions—pyridine adsorption, Ag atom desorption, and Ag-pyridine complex dissolution—can occur on the surface, and the combination of the three can explain the observed results.Keywords: adsorption kinetics; atomic scale junctions; electrochemical potential; fluctuation spectroscopy; nanowire; pyridine
Co-reporter:Lawrence P. Zaino III;Dr. Nicholas M. Contento;Dr. Sean P. Branagan;Dr. Paul W. Bohn
ChemElectroChem 2014 Volume 1( Issue 9) pp:1570-1576
Publication Date(Web):
DOI:10.1002/celc.201402111
Abstract
Convective mass transport is achieved in nanopore arrays containing embedded annular nanoband electrodes (EANEs) by using both two- and three-electrode systems. In the two-electrode configuration, the potential drop between the EANE and a counter/quasi-reference electrode (CE/QRE) controls the electro-osmotic flow (EOF). EOF enhances the rate of electron-transfer reactions at an EANE array. Three-electrode configurations, in which the EANE is placed between the CE/QRE and a second working electrode (WE2), are also studied. In the three-electrode configuration, the position and magnitude of the current peaks are dependent on the WE2 potential. These shifts, which are characteristic of strong coupling between the EOF caused by WE2 and the faradaic processes monitored at the EANE, are explained by a combination of EOF and the concentration of analyte in the nanochannel. EOF is dictated collectively by the EANE and the WE2, because the potential at both electrodes is modified by substantial iR drops in the high-resistance nanopores.
Co-reporter:Tai-Wei Hwang ; Sean P. Branagan
Journal of the American Chemical Society 2013 Volume 135(Issue 11) pp:4522-4528
Publication Date(Web):February 22, 2013
DOI:10.1021/ja400567j
The chemical noise contained in conductance fluctuations resulting from adsorption and desorption of pyridine at Au–Ag–Au bimetallic atom-scale junctions (ASJs) exhibiting ballistic electron transport is studied using fluctuation spectroscopy. ASJs are fabricated by electrochemical Ag deposition in a Au nanogap to produce a high-conductance Ag quantum wire, followed by electromigration-induced thinning in pyridine solution to create stable ASJs. The conductance behavior of the resulting ASJs is analyzed by sequential autocorrelation and Fourier transform of the current–time data to yield the power spectral density (PSD). In these experiments the PSDs from Ag ASJs in pyridine exhibit two main frequency regions: 1/f noise originating from resistance fluctuations of the junction itself at low frequencies, and a Lorentzian noise component arising from molecular adsorption/desorption fluctuations at higher frequencies. The characteristic cutoff frequency of the Lorentzian noise component determines the relaxation time of molecular fluctuations, which, in turn, is sensitive to the kinetics of the adsorption/desorption process. The kinetics are found to depend on concentration and on the adsorption binding energy. The junction size (<5G0), on the other hand, does not affect the kinetics, as the cutoff frequency remains unchanged. Concentration-dependent adsorption free energies are interpreted as arising from a distribution of binding energies, N(Eb), on the Ag ASJ. Other observations, such as long lifetime ASJs and two-level fluctuations in conductance, provide additional evidence for the integral role of the adsorbate in determining ASJ reorganization dynamics.
Co-reporter:Bei Nie, Barrett K. Duan, and Paul W. Bohn
ACS Applied Materials & Interfaces 2013 Volume 5(Issue 13) pp:6208
Publication Date(Web):June 13, 2013
DOI:10.1021/am401132s
Three-dimensional nanoporous gallium nitride(PGaN) produced by metal-assisted electroless etching is chemically embedded with silver nanoparticles via electroless deposition, forming a metallized semiconductor membrane with large surface area and nanoscale metal features. A new application utilizing the unique chemical and morphological features of these composite nanostructures is described here, laser induced desorption-ionization(LDI) of biomolecules(e.g., cholesterol and nucleotides) for direct mass analysis, without use of additional organic matrix. Although PGaN itself is a poor matrix for direct LDI mass spectrometry, the combination of Ag and PGaN greatly improves ion signals relative to PGaN or Ag nanostructure surfaces alone. This behavior is attributed to the combination of strong UV absorption, enhanced surface area, and favorable thermal properties of PGaN. Importantly, Ag-PGaN is shown to facilitate the formation of Ag adduct ions in some cases, for example adenine, where adducts are not observed from either porous anodic aluminum membranes or surfaces presenting Ag nanoparticles in isolation. Nanopore-embedded Ag nanostructures serve a dual role: as cationization agents and to assist thermal desorption under UV laser irradiation. The results reported here suggest that the combination of Ag nanostructures embedded in PGaN has the capacity for high quality matrix-free LDI mass analysis.Keywords: gallium nitride; laser desorption−ionization; mass spectrometry; nanoparticles; porous semiconductors;
Co-reporter:Chaoxiong Ma, Nicholas M. Contento, Larry R. Gibson II, and Paul W. Bohn
Analytical Chemistry 2013 Volume 85(Issue 20) pp:9882
Publication Date(Web):September 6, 2013
DOI:10.1021/ac402417w
Arrays of recessed ring–disk (RRD) electrodes with nanoscale spacing fabricated by multilayer deposition, nanosphere lithography, and multistep reactive ion etching were incorporated into nanofluidic channels. These arrays, which characteristically exhibit redox cycling leading to current amplification during cyclic voltammetry, can selectively analyze electroactive species based on differences in redox reversibility, redox potential, or both. Using Ru(NH3)63+ and ascorbic acid (AA) as model reversible and irreversible redox species, the selectivity for electrochemical measurement of Ru(NH3)63+ against a background of AA improves from ∼10, for an array operated in a fluidically unconstrained geometry, to ∼70 for an array integrated within nanofluidic channels. RRD arrays were also used for the detection of dopamine in the presence of AA by cyclic voltammetry. A linear response ranging from 100 nM to 1 mM with a detection limit of 20 nM was obtained for dopamine alone without nanofluidic confinement. In nanochannel-confined arrays, AA was depleted by holding the ring electrodes at +0.5 V versus Ag/AgCl, allowing interference-free determination of dopamine at the disk electrodes in the presence of a 100-fold excess of AA. For selective detection of electrochemically reversible interfering species on an RRD array without nanochannel confinement, a ring potential can be chosen such that one species exhibits exclusively cathodic (anodic) current, allowing the other species to be determined from its anodic (cathodic) current. This approach for selective detection is demonstrated in a mixture of Ru(NH3)63+ and Fe(CN)63–, which have resolved redox potentials. The same principle was successfully applied to differentiate species with overlapping redox potentials, such as dopamine/Fe(CN)63– and ferrocenemethanol/Fe(CN)64–.
Co-reporter:Rachel Masyuko, Eric J. Lanni, Jonathan V. Sweedler and Paul W. Bohn
Analyst 2013 vol. 138(Issue 7) pp:1924-1939
Publication Date(Web):29 Jan 2013
DOI:10.1039/C3AN36416J
Correlated chemical imaging is an emerging strategy for acquisition of images by combining information from multiplexed measurement platforms to track, visualize, and interpret in situ changes in the structure, organization, and activities of interesting chemical systems, frequently spanning multiple decades in space and time. Acquiring and correlating information from complementary imaging experiments has the potential to expose complex chemical behavior in ways that are simply not available from single methods applied in isolation, thereby greatly amplifying the information gathering power of imaging experiments. However, in order to correlate image information across platforms, a number of issues must be addressed. First, signals are obtained from disparate experiments with fundamentally different figures of merit, including pixel size, spatial resolution, dynamic range, and acquisition rates. In addition, images are often acquired on different instruments in different locations, so the sample must be registered spatially so that the same area of the sample landscape is addressed. The signals acquired must be correlated in both spatial and temporal domains, and the resulting information has to be presented in a way that is readily understood. These requirements pose special challenges for image cross-correlation that go well beyond those posed in single technique imaging approaches. The special opportunities and challenges that attend correlated imaging are explored by specific reference to correlated mass spectrometric and Raman imaging, a topic of substantial and growing interest.
Co-reporter:Chaoxiong Ma, Nicholas M. Contento, Larry R. Gibson II, and Paul W. Bohn
ACS Nano 2013 Volume 7(Issue 6) pp:5483
Publication Date(Web):May 21, 2013
DOI:10.1021/nn401542x
An array of nanoscale-recessed ring-disk electrodes was fabricated using layer-by-layer deposition, nanosphere lithography, and a multistep reactive ion etching process. The resulting device was operated in generator–collector mode by holding the ring electrodes at a constant potential and performing cyclic voltammetry by sweeping the disk potential in Fe(CN)63–/4– solutions. Steady-state response and enhanced (∼10×) limiting current were achieved by cycling the redox couple between ring and disk electrodes with high transfer/collection efficiency. The collector (ring) electrode, which is held at a constant potential, exhibits a much smaller charging current than the generator (disk), and it is relatively insensitive to scan rate. A characteristic feature of the nanoscale ring–disk geometry is that the electrochemical reaction occurring at the disk electrodes can be tuned by modulating the potential at the ring electrodes. Measured shifts in Fe(CN)63–/4– concentration profiles were found to be in excellent agreement with finite element method simulations. The main performance metric, the amplification factor, was optimized for arrays containing small diameter pores (r < 250 nm) with minimum electrode spacing and high pore density. Finally, integration of the fabricated array within a nanochannel produced up to 50-fold current amplification as well as enhanced selectivity, demonstrating the compatibility of the device with lab-on-a-chip architectures.Keywords: current amplification; generator−collector; nanoelectrode array; nanofluidic; recessed ring-disk electrode; redox cycling
Co-reporter:Barrett K. Duan, Paul W. Bohn
Sensors and Actuators A: Physical 2013 Volume 194() pp:220-227
Publication Date(Web):1 May 2013
DOI:10.1016/j.sna.2013.01.026
The interaction between CO and Pt can potentially be used to generate gas-sensing signals at Pt/GaN interfaces. Here, the CO response is compared to that generated by H2, and its dependence on Pt morphology is investigated. Similar to the H2 response, exposure to CO reduces the resistance of Pt/GaN structures. On dense, pinhole-free Pt films, the magnitude of the CO resistance change increases with decreasing Pt film thickness and with increasing Pt surface area, whereas discontinuous Pt films exhibit an increasing resistance change with film thickness. These results indicate that CO sensing is greatly affected by the Pt film morphology and the interaction between adsorbed CO and the charged Pt/GaN interface. A model is proposed in which surface charge induced by the adsorption of CO on Pt affects the interfacial polarization at the Pt/GaN interface. Simulations corroborate the proposed model. Transient resistance behavior is correlated with pre-adsorbed O species on the Pt surface and CO oxidation at atmospheric pressure, leading to a decrease of the Schottky barrier height and the resistance of the structure.
Co-reporter:Sean P. Branagan ; Nicholas M. Contento
Journal of the American Chemical Society 2012 Volume 134(Issue 20) pp:8617-8624
Publication Date(Web):April 16, 2012
DOI:10.1021/ja3017158
Electroosmotic flow (EOF) is used to enhance the delivery of Fe(CN)64–/Fe(CN)63– to an annular nanoband electrode embedded in a nanocapillary array membrane, as a route to high efficiency electrochemical conversions. Multilayer Au/polymer/Au/polymer membranes are perforated with 102–103 cylindrical nanochannels by focused ion beam (FIB) milling and subsequently sandwiched between two axially separated microchannels, producing a structure in which transport and electron transfer reactions are tightly coupled. The middle Au layer, which contacts the fluid only at the center of each nanochannel, serves as a working electrode to form an array of embedded annular nanoband electrodes (EANEs), at which sufficient overpotential drives highly efficient electrochemical processes. Simultaneously, the electric field established between the EANE and the QRE (>103 V cm–1) drives electro-osmotic flow (EOF) in the nanochannels, improving reagent delivery rate. EOF is found to enhance the steady-state current by >10× over a comparable structure without convective transport. Similarly, the conversion efficiency is improved by approximately 10-fold compared to a comparable microfluidic structure. Experimental data agree with finite element simulations, further illustrating the unique electrochemical and transport behavior of these nanoscale embedded electrode arrays. Optimizing the present structure may be useful for combinatorial processing of on-chip sample delivery with electrochemical conversion; a proof of concept experiment, involving the generation of dissolved hydrogen in situ via electrolysis, is described.
Co-reporter:Barrett K. Duan, Jingying Zhang, and Paul W. Bohn
Analytical Chemistry 2012 Volume 84(Issue 1) pp:2
Publication Date(Web):August 12, 2011
DOI:10.1021/ac201240w
Co-reporter:Xuewen Geng, Barrett K. Duan, Dane A. Grismer, Liancheng Zhao, Paul W. Bohn
Electrochemistry Communications 2012 Volume 19() pp:39-42
Publication Date(Web):June 2012
DOI:10.1016/j.elecom.2012.03.011
Metal-assisted chemical etching (MacEtch) is a top-down liquid semiconductor processing technology applied here to realize highly monodisperse collections of GaN nanowires. Subjecting n-type GaN wafers to AgNO3/HF simultaneously deposits Ag nanoparticle catalysts and initiates the MacEtch process. By varying the solution composition, concentration and etch time under UV illumination, different GaN nanostructures are produced. GaN nanowires form initially on a supporting framework of porous GaN, which can be removed upon prolonged etching, leaving cones of monodisperse nanowires. These results suggest a mechanism in which areas surrounding Ag particles etch faster than areas directly underneath the catalyst and the formation of a localized galvanic cell and associated exothermic production of soluble GaF2(OH).Highlights► Metal-assisted chemical etching is a top-down approach to nanowire production. ► AgNO3/HF simultaneously deposits Ag catalysts and initiates the MacEtch. ► Monodisperse nanowires form initially on a supporting framework of porous GaN. ► Etching is fast directly under the metallic catalyst particles and slow to the side.
Co-reporter:Sean P. Branagan and Paul W. Bohn
Analyst 2012 vol. 137(Issue 17) pp:3932-3939
Publication Date(Web):30 May 2012
DOI:10.1039/C2AN35488H
Thin Au films, patterned by focused ion beam (FIB) milling to contain an array of subwavelength nanopores, exhibit enhanced optical transmission (EOT) via front–back resonance coupling. The films also serve as working electrodes capable of controlling the local potential, allowing electrochemical processes to be monitored using wavevector-resolved spectral mapping. The precise value of the surface plasmon resonance (SPR) wavevector can be extracted from the enhanced optical transmission signal and correlated with several distinct classes of electrochemical processes: double layer reorganization, faradaic adsorption/desorption, heterogeneous electron transfer, and anion adsorption. Specifically, the protonation/deprotonation reaction of an adsorbed monolayer of 4-mercaptobenzoic acid, the adsorption/desorption reaction of dodecanethiol to Au, the solution-phase reaction of ferri–ferrocyanide, and sulfate adsorption/desorption are investigated. A simple model is presented that encompasses both the EOT signal and electrochemical processes and produces semi-quantitative agreement with the SPR spectral wavevector mapping observed experimentally.
Co-reporter:Xuewen Geng, Zhe Qi, Meicheng Li, Barrett K. Duan, Liancheng Zhao, Paul W. Bohn
Solar Energy Materials and Solar Cells 2012 103() pp: 98-107
Publication Date(Web):
DOI:10.1016/j.solmat.2012.04.020
Co-reporter:Nicholas M. Contento, Sean P. Branagan and Paul W. Bohn
Lab on a Chip 2011 vol. 11(Issue 21) pp:3634-3641
Publication Date(Web):13 Sep 2011
DOI:10.1039/C1LC20570F
In situ generation of reactive species within confined geometries, such as nanopores or nanochannels is of significant interest in overcoming mass transport limitations in chemical reactivity. Solvent electrolysis is a simple process that can readily be coupled to nanochannels for the electrochemical generation of reactive species, such as H2. Here the production of hydrogen-rich liquid volumes within nanofluidic structures, without bubble nucleation or nanochannel occlusion, is explored both experimentally and by modeling. Devices comprised of multiple horizontal nanochannels intersecting planar working and quasi-reference electrodes were constructed and used to study the effects of confinement and reduced working volume on the electrochemical reduction of H2O to H2 and OH−. H2 production in the nanochannel-embedded electrode reactor output was monitored by fluorescence emission of fluorescein, which exhibits a pH-dependent emission intensity. Initially, the fluorescein solution was buffered to pH 6.0 prior to stepping the potential cathodic of E0′ for the generation of OH− and H2. Because the electrochemical products are obtained in a 2:1 stoichiometry, local measurements of pH during and after the cathodic potential steps can be converted into H2 production rates. Independent experimental estimates of the local H2 concentration were then obtained from the spatiotemporal fluorescence behavior and current measurements, and these were compared with finite element simulations accounting for electrolysis and subsequent convection and diffusion within the confined geometry. Local dissolved H2 concentrations were correlated to partial pressures through Henry's Law and values as large as 8.3 atm were obtained at the most negative potential steps. The downstream availability of electrolytically produced H2 in nanochannels is evaluated in terms of its possible use as a downstream reducing reagent. The results obtained here indicate that H2 can easily reach saturation concentrations at modest overpotentials.
Co-reporter:Xuewen Geng;Meicheng Li;Liancheng Zhao
Journal of Electronic Materials 2011 Volume 40( Issue 12) pp:
Publication Date(Web):2011 December
DOI:10.1007/s11664-011-1771-1
Metal-assisted chemical etching (MacEtch) of semiconductor materials in HF/H2O2 solution using noble-metal particles as catalysts has gained much attention in the past few years due to its unique properties. In this work, nanoscale Ag particles were deposited on (100) and (111) surfaces of polished p-Si wafers through the silver-mirror reaction. Subsequently these wafers were etched in 1:1:1 (v:v:v) HF(49%):H2O2(30%):EtOH solution at ambient temperature and pressure for 12 h, producing a number of different quasi-ordered silicon micro/nanostructures. The resulting surface-modified wafers exhibited mixed micro- and nanostructures that are an inherent feature of the etch process; for example, steps appear on the sidewalls of crystallographically defined nanopores, because the catalytic Ag nanoparticles are convected as they transit the developing pore during the etching process. The resulting materials exhibited much reduced reflectivity, reaching a maximum of 3.7× reduction near 330 nm, which renders them of interest in potential applications such as back-reflector templates for deposition of thin-film solar cell materials.
Co-reporter:Lindsay C. C. Elliott, Moussa Barhoum, Joel M. Harris, and Paul W. Bohn
Langmuir 2011 Volume 27(Issue 17) pp:11037-11043
Publication Date(Web):July 19, 2011
DOI:10.1021/la201753v
Spatial and temporal heterogeneities in expanded and collapsed surface bound poly(N-isopropylacrylamide), pNIPAAm, films are studied by single molecule tracking (SMT) experiments. Tracking data are analyzed using both radius of gyration (Rg) evolution and confinement level calculations to elucidate the range of behaviors displayed by single Rhodamine6G (R6G) molecules. Confined diffusion that is dictated by the free volume within surface tethered chains is observed with considerable dispersion among individual R6G molecules. Thus, the distribution of probe behavior reflects nanometer-scale information about the behavior of the probe–polymer system at temperatures above (T > TLCST) and below (T < TLCST) the lower critical solution temperature (LCST). In this context, confinement-level analysis and Rg evolution both show a larger degree of confinement of the probe in pNIPAAm at T > TLCST. Temperature-dependent changes in confinement are evidenced at T > TLCST by a higher percentage of confined steps, longer periods of confined events, and smaller area of confined zones, as well as a shift in the overall distribution of Rg evolution paths and final Rg distributions.
Co-reporter:Tai-Wei Hwang and Paul W. Bohn
ACS Nano 2011 Volume 5(Issue 10) pp:8434
Publication Date(Web):September 17, 2011
DOI:10.1021/nn203404k
Atom-scale junctions (ASJs) exhibit quantum conductance behavior and have potential both for fundamental studies of adsorbate-mediated conductance in mesoscopic conductors and as chemical sensors. Electrochemically fabricated ASJs, in particular, show the stability needed for molecular detection applications. However, achieving physically robust ASJs at high yield is a challenge because it is difficult to control the direction and kinetics of metal deposition. In this work, a novel electrochemical approach is reported, in which Au–Ag–Au bimetallic ASJs are reproducibly fabricated from an initially prepared Au nanogap by sequential overgrowth and self-limited thinning. Applying a potential across specially prepared Au nanoelectrodes in the presence of aqueous Ag(I) leads to preferential galvanic reactions resulting in the deposition of Ag and the formation of an atom-scale junction between the electrodes. An external resistor is added in series with the ASJ to control self-termination, and adjusting solution chemical potential (concentration) is used to mediate self-thinning of junctions. The result is long-lived, mechanically stable ASJs that, unlike previous constructions, are stable in flowing solution, as well as to changes in solution media. These bimetallic ASJs exhibit a number of behaviors characteristic of quantum structures, including long-lived fractional conductance states, that are interpreted to arise from two or more quantized ASJs in series.Keywords: atom-scale junction; bimetallic; electrochemical nanofabrication; fractional conductance; nanowire; self-termination
Co-reporter:Aigars Piruska, Maojun Gong, Jonathan V. Sweedler and Paul W. Bohn
Chemical Society Reviews 2010 vol. 39(Issue 3) pp:1060-1072
Publication Date(Web):23 Oct 2009
DOI:10.1039/B900409M
Nanofluidic architectures and devices have already had a major impact on forefront problems in chemical analysis, especially those involving mass-limited samples. This critical review begins with a discussion of the fundamental flow physics that distinguishes nanoscale structures from their larger microscale analogs, especially the concentration polarization that develops at nanofluidic/microfluidic interfaces. Chemical manipulations in nanopores include nanopore-mediated separations, microsensors, especially resistive-pulse sensing of biomacromolecules, fluidic circuit analogs and single molecule measurements. Coupling nanofluidic structures to three-dimensional microfluidic networks is especially powerful and results in applications in sample preconcentration, nanofluidic injection/collection and fast diffusive mixing (160 references).
Co-reporter:Aigars Piruska, Sean P. Branagan, Alexandra B. Minnis, Zhen Wang, Donald M. Cropek, Jonathan V. Sweedler and Paul W. Bohn
Lab on a Chip 2010 vol. 10(Issue 10) pp:1237-1244
Publication Date(Web):16 Feb 2010
DOI:10.1039/B924164G
The introduction of metallic elements into microfluidic devices that support electrokinetic transport creates several fundamental issues relative to the high conductivity of the metal, which can act as a current shunt, causing profound effects on the transport process. Here we examine the use of Au-coated nanocapillary array membranes (Au NCAMs) as electrically addressable fluid control elements in multi-layer microfluidic architectures. Three alternative methods for fluid injection across Au NCAMs are presented: electrokinetic injection across NCAMs with Au coated on one side (asymmetric NCAM), electrokinetic injection across NCAMs with an embedded Au layer (symmetric NCAM), and field-free electroosmotic flow (EOF) pumping across either type of Au NCAM. Injection efficiency across asymmetric NCAMs depends on the orientation of the asymmetric membrane relative to the driving potential. Efficient injections are enabled when the Au coating is on the receiving side of the membrane, however, some distortion of the injected volume element is observed, especially with large injection potentials. These results for asymmetric membranes agree qualitatively with two-dimensional numerical simulations of injections across a single slit pore, which suggest that the direction-selective transport behavior is related to electrophoretic transport of the anionic fluorescein probe. Reproducible, high quality injections are also achieved in symmetric Au NCAMs having an embedded gold nanoband region within the nanopores. Nanoband Au NCAMs are excellent candidates for a range of applications, including high efficiency electrochemical sensing, electrochemically catalyzed conversion or pretreatment and label free sensing utilizing extraordinary optical transmission. EOF pumping could be an alternative to electrokinetic injections in some applications, however, this approach is only useful for relatively large pore sizes (>400 nm) and presents considerably worse sample spreading via Taylor dispersion.
Co-reporter:Zhen Li, Li-Qiang Chu, Jonathan V. Sweedler and Paul W. Bohn
Analytical Chemistry 2010 Volume 82(Issue 7) pp:2608
Publication Date(Web):March 5, 2010
DOI:10.1021/ac100026r
A detailed chemical and structural understanding of pre-enzymatic processing of lignocellulosic materials (LCMs) is a key objective in the development of renewable energy. Efficient rendering of biomass components into fermentable substrates for conversion into biofuel feedstocks would benefit greatly from the development of new technologies to provide high-quality, spatially resolved chemical information about LCMs during the various processing states. In an effort to realize this important goal, spatially correlated confocal Raman and mass spectrometric images allow the extraction of three-dimensional information from the perennial grass, Miscanthus x giganteus. An optical microscopy-based landmark registry scheme was developed that allows samples to be transferred between laboratories at different institutions, while retaining the capability to access the same physical regions of the samples. Subsequent to higher resolution imaging via confocal Raman microscopy and secondary ion mass spectrometry (SIMS), laser desorption-ionization mass spectrometry was used to place these regions within the overall sample architecture. Excellent sample registry was evident in the highly correlated Raman and SIMS images. In addition, the correlation of vibrational Raman scattering with mass spectra from specific spatial locations allowed confirmation of the assignment of intracellular globular structures to hemicellulose-rich lignin complexes, an assignment which could only be made tentatively from either image alone.
Co-reporter:Barrett K. Duan and Paul W. Bohn
Analyst 2010 vol. 135(Issue 5) pp:902-907
Publication Date(Web):06 Apr 2010
DOI:10.1039/B926182F
A unique hydrogen sensor structure based on Pt-decorated porous gallium nitride (PGaN) was fabricated by a two-step process consisting of metal-assisted electroless etching to produce PGaN with highly anisotropic pores followed by electroless deposition of Pt in the pores from an ammoniacal PtCl62− solution. The Pt-decorated PGaN structure contains 50–100 nm diameter nanopores which are 400 nm to 1 µm deep and filled with Pt islands. Both electroless etching and deposition steps are done in solution and allow for large-scale production. An AC four-point probe conductivity measurement was implemented at f = 1 kHz, a frequency where the impedance of Pt–PGaN is nearly entirely resistive, and the change in conductance upon H2 exposure was measured for three sample types: PGaN with a surface sputtered layer of Pt only; unetched GaN (CGaN) with both sputtered and electrolessly deposited Pt; and PGaN with both sputtered and electrolessly deposited Pt. The hydrogen sensing performance of the Pt-filled PGaN sensor was more than an order of magnitude better than either of the other two sample types under all experimental conditions, an observation attributed to the significant increase in Pt–GaN interfacial area in the electrolessly decorated PGaN samples, exhibiting a response to H2 concentrations as low as 1 ppm. The conductance changes are ascribed to adsorption-induced changes in interfacial polarization that produce changes in band bending and thus to the width of the space charge region near the Pt–GaN interface.
Co-reporter:Ping Shi and Paul W. Bohn
ACS Nano 2010 Volume 4(Issue 5) pp:2946
Publication Date(Web):April 15, 2010
DOI:10.1021/nn1003716
Metallic atom-scale junctions (ASJs) are interesting fundamentally because they support ballistic transport, characterized by conduction quantized in units of G0 = 2e2/h. They are also of potential practical interest since ASJ conductance is extraordinarily sensitive to molecular adsorption. Monometallic Au ASJs were previously fabricated electrochemically using an I−/I3− medium and a unique open working electrode configuration to produce slow electrodeposition or electrodissolution, resulting in reproducible ASJs with limiting conductance <5 G0. Here, bimetallic Au−Cu−Au and Au−Ag−Au ASJ structures are obtained by electrochemical deposition/dissolution of Cu and Ag in K2SO4 supporting electrolyte. The ASJs are fabricated in Si3N4-protected Au nanogaps obtained by focused ion beam milling, a protocol which yields repeatable and reproducible Au−Cu−Au or Au−Ag−Au ASJs without damaging the Au nanogap substrates. While Au−Ag−Au ASJs are relatively stable (hours) at open circuit potential in the supporting electrolyte, Au−Cu−Au ASJs exhibit spontaneous restructuring dynamics, characterized by monotonic, stepwise decreases in conductance under the same conditions. However, the Au−Cu−Au ASJs can be stabilized by applying sufficiently negative potentials. Hydrogen adsorption and shifts in the Fermi level are possible reasons for the enhanced stability of Au−Cu−Au structures at large negative overpotentials. In light of these observations, it is possible to integrate ASJs in microfluidic devices as renewable, nanostructured sensing elements for chemical detection.Keywords: atom-scale junction; conductance quantization; electrochemical nanofabrication; nanocontact; nanowire
Co-reporter:Zhen Wang, Travis L. King, Sean P. Branagan and Paul W. Bohn
Analyst 2009 vol. 134(Issue 5) pp:851-859
Publication Date(Web):27 Feb 2009
DOI:10.1039/B815590A
Horseradish peroxidase (HRP) was immobilized on the planar surfaces and inside the cylindrical nanopores of nanocapillary array membranes (NCAMs) to study how the enzyme-catalyzed oxidation of a fluorigenic substrate, Amplex Red (AR), to fluorescent resorufin by hydrogen peroxide is influenced by confinement. Because AR was also found to be converted to resorufin photolytically at high laser fluences, a modified laser-induced fluorescence protocol was developed to characterize the enzyme-catalyzed reaction in the absence of interference from the photolytic reaction. Surface-immobilized HRP was studied in two environments: bound to the surface of a microfluidic channel, and bound to the interior of cylindrical nanopores in NCAMs connecting crossed microfluidic channels. HRP was immobilized through reaction of solvent-accessible primary amines with the epoxy group of the methyl methacrylate–glycidyl methacrylate copolymer synthesized in either planar or annular geometries to construct the test structures for enzymatic activity. HRP immobilized on planar surfaces shows high activity (∼10 µM min−1) meaning that the copolymer membrane exhibits good potential for immobilizing the enzyme, especially since active structures are obtained in a one-step reaction. HRP was also immobilized inside nanopores via physisorption. Enzymatic reactions inside the nanopores were characterized and compared to finite element simulations of a modified Eley–Rideal mechanism to bracket the value of the overall rate constant for the confined enzyme. Reaction velocities were estimated to be ∼10-fold higher in the nanopores than for the same enzyme bound to a planar microfluidic surface.
Co-reporter:Aigars Piruska, Sean Branagan, Donald M. Cropek, Jonathan V. Sweedler and Paul W. Bohn
Lab on a Chip 2008 vol. 8(Issue 10) pp:1625-1631
Publication Date(Web):08 Aug 2008
DOI:10.1039/B805768K
Electrokinetically driven fluid transport was evaluated within three-dimensional hybrid nanofluidic–microfluidic devices incorporating Au-coated nanocapillary array membranes (NCAMs). Gold NCAMs, prepared by electroless gold deposition on polymeric track-etched membranes, were susceptible to gas bubble formation if the interfacial potential difference exceeded ∼2 V along the length of the gold region. Gold membranes were etched to yield 250 μm wide coated regions that overlap the intersection of two orthogonal microfluidic channels in order to minimize gas evolution. The kinetics of electrolysis of water at the opposing ends of the gold region was modeled and found to be in satisfactory agreement with experimental measurements of the onset of gas bubble formation. Conditions to achieve electrokinetic injection across Au-coated NCAMs were identified, with significant reproducible injections being possible for NCAMs modified with this relatively thin gold stripe. Continuous gold films led to suppressed injections and to a variety of ion enrichment/depletion effects in the microfluidic source channel. The suppression of injections was understood through finite element modeling which revealed the presence of a significant electrophoretic velocity component in opposition to electroosmotic flow at the edge of the Au-dielectric regions.
Co-reporter:Enid N. Gatimu, Xiaozhong Jin, Narayan Aluru and Paul W. Bohn
The Journal of Physical Chemistry C 2008 Volume 112(Issue 49) pp:19242-19247
Publication Date(Web):November 16, 2008
DOI:10.1021/jp806257d
Ionic transport in nanopores is dependent on the nature of the electrical communication between the pores and the surrounding environment. A particularly useful fluidic device structure uses nanopores in nanocapillary array membranes (NCAMs) as electrically switchable valves between vertically separated microfluidic channels. In the off-state, the gate isolates the fluidic environments in the microchannels, but when the appropriate forward-bias voltage is applied, it selectively allows ions and analytes to move between the microchannels. However, the populations of species in the microfluidic channels are perturbed from their steady-state values due to ion accumulation and depletion effects. Experiments conducted here characterize the electrical conduction along the length of a microfluidic channel, and laser-induced fluorescence probes the formation of a high- and low-concentration regions of fluorescent dye before and after application of forward- and reverse-bias voltage pulses in both small (a = 10 nm) and large (a = 100 nm) pore NCAMs. In all cases, switching from injection (transport across the NCAM) to microfluidic flow (transport only in the microfluidic channel) results in a multiphasic current recovery profile, signifying the presence of ion accumulation and depletion regions at the microfluidic−nanofluidic boundary, that is, in the region adjacent to the NCAM. The behavior is consistent with a model in which a volume of altered ion concentration is created at the microfluidic−nanofluidic boundary upon injection. Switching back to microfluidic flow causes this altered conductivity region to be swept from the microfluidic channel, re-establishing the steady state conduction properties.
Co-reporter:Ping Shi and Paul W. Bohn
ACS Nano 2008 Volume 2(Issue 8) pp:1581
Publication Date(Web):July 25, 2008
DOI:10.1021/nn8002955
Metallic atom-scale junctions (ASJs) constitute the natural limit of nanowires, in which the limiting region of conduction is only a few atoms wide. They are of interest because they exhibit ballistic conduction and their conductance is extraordinarily sensitive to molecular adsorption. However, identifying robust and regenerable mechanisms for their production is a challenge. Gold ASJs have been fabricated electrochemically on silicon using an iodide-containing medium to control the kinetics. Extremely slow electrodeposition or electrodissolution rates were achieved and used to reliably produce ASJs with limiting conductance <5 G0. Starting from a photolithographically fabricated, Si3N4-protected micrometer-scale Au bridge between two contact electrodes, a nanometer-scale gap was prepared by focused ion beam milling. The opposing Au faces of this construct were then used in an open-circuit working electrode configuration to produce Au ASJs, either directly or by first overgrowing a thicker Au nanowire and electrothinning it back to an ASJ. Gold ASJs produced by either approach exhibit good stability—in some cases being stable over hours at 300 K—and quantized conductance properties. The influence of deposition/dissolution potential and supporting electrolyte on the stability of ASJs are considered.Keywords: atom-scale junction; electrodeposition; electrodissolution; metal nanowire; nanofabrication
Co-reporter:Chaoxiong Ma ; Nicholas M. Contento
Journal of the American Chemical Society () pp:
Publication Date(Web):May 7, 2014
DOI:10.1021/ja502052s
In canonical electrochemical experiments, a high-concentration background electrolyte is used, carrying the vast majority of current between macroscopic electrodes, thus minimizing the contribution of electromigration transport of the redox-active species being studied. In contrast, here large current enhancements are achieved in the absence of supporting electrolyte during cyclic voltammetry at a recessed ring-disk nanoelectrode array (RRDE) by taking advantage of the redox cycling effect in combination with ion enrichment and an unshielded ion migration contribution to mass transport. Three distinct transport regimes are observed for the limiting current as a function of the concentration of redox species, Ru(NH3)62+/3+, revealed through the strong dependence of ion transport on ionic strength. Behavior at low analyte concentrations is especially interesting. In the absence of supporting electrolyte, ions accumulate in the nanopores, resulting in significantly increased current amplification compared to redox cycling in the presence of supporting electrolyte. Current enhancements as large as 100-fold arising from ion enrichment and ion migration effects add to the ∼20-fold enhancement due to redox cycling, producing a total current amplification as large as 2000-fold compared to a single microelectrode of the same total area, making these RRDE arrays interesting for electrochemical processing and analysis.
Co-reporter:Donghoon Han, Garrison M. Crouch, Kaiyu Fu, Lawrence P. Zaino III and Paul W Bohn
Chemical Science (2010-Present) 2017 - vol. 8(Issue 8) pp:NaN5355-5355
Publication Date(Web):2017/06/19
DOI:10.1039/C7SC02250F
The ability of zero-mode waveguides (ZMW) to guide light into subwavelength-diameter nanoapertures has been exploited for studying electron transfer dynamics in zeptoliter-volume nanopores under single-molecule occupancy conditions. In this work, we report the spectroelectrochemical detection of individual molecules of the redox-active, fluorogenic molecule flavin mononucleotide (FMN) freely diffusing in solution. Our approach is based on an array of nanopore-confined recessed dual ring electrodes, wherein repeated reduction and oxidation of a single molecule at two closely spaced annular working electrodes yields amplified electrochemical signals. We have articulated these structures with an optically transparent bottom, so that the nanopores are bifunctional, exhibiting both nanophotonic and nanoelectrochemical behaviors allowing the coupling between electron transfer and fluorescence dynamics to be studied under redox cycling conditions. We also investigated the electric field intensity in electrochemical ZMWs (E-ZMW) through finite-element simulations, and the amplification of fluorescence by redox cycling agrees well with predictions based on optical confinement effects inside the E-ZMW. Proof-of-principle experiments are conducted showing that electrochemical and fluorescence signals may be correlated to reveal single molecule fluctuations in the array population. Cross-correlation of single molecule fluctuations in amperometric response and single photon emission provides unequivocal evidence of single molecule sensitivity.
Co-reporter:Aigars Piruska, Maojun Gong, Jonathan V. Sweedler and Paul W. Bohn
Chemical Society Reviews 2010 - vol. 39(Issue 3) pp:NaN1072-1072
Publication Date(Web):2009/10/23
DOI:10.1039/B900409M
Nanofluidic architectures and devices have already had a major impact on forefront problems in chemical analysis, especially those involving mass-limited samples. This critical review begins with a discussion of the fundamental flow physics that distinguishes nanoscale structures from their larger microscale analogs, especially the concentration polarization that develops at nanofluidic/microfluidic interfaces. Chemical manipulations in nanopores include nanopore-mediated separations, microsensors, especially resistive-pulse sensing of biomacromolecules, fluidic circuit analogs and single molecule measurements. Coupling nanofluidic structures to three-dimensional microfluidic networks is especially powerful and results in applications in sample preconcentration, nanofluidic injection/collection and fast diffusive mixing (160 references).
Co-reporter:Chaoxiong Ma, Lawrence P. Zaino III and Paul W. Bohn
Chemical Science (2010-Present) 2015 - vol. 6(Issue 5) pp:NaN3179-3179
Publication Date(Web):2015/03/25
DOI:10.1039/C5SC00433K
We present a new configuration for coupling fluorescence microscopy and voltammetry using self-induced redox cycling for ultrasensitive electrochemical measurements. An array of nanopores, each supporting a recessed disk electrode separated by 100 nm in depth from a planar multiscale bipolar top electrode, was fabricated using multilayer deposition, nanosphere lithography, and reactive-ion etching. Self-induced redox cycling was induced on the disk electrode producing ∼30× current amplification, which was independently confirmed by measuring induced electrogenerated chemiluminescence from Ru(bpy)32/3+/tri-n-propylamine on the floating bipolar electrode. In this design, redox cycling occurs between the recessed disk and the top planar portion of a macroscopic thin film bipolar electrode in each nanopore. Electron transfer also occurs on a remote (mm-distance) portion of the planar bipolar electrode to maintain electroneutrality. This couples the electrochemical reactions of the target redox pair in the nanopore array with a reporter, such as a potential-switchable fluorescent indicator, in the cell at the distal end of the bipolar electrode. Oxidation or reduction of reversible analytes on the disk electrodes were accompanied by reduction or oxidation, respectively, on the nanopore portion of the bipolar electrode and then monitored by the accompanying oxidation of dihydroresorufin or reduction of resorufin at the remote end of the bipolar electrode, respectively. In both cases, changes in fluorescence intensity were triggered by the reaction of the target couple on the disk electrode, while recovery was largely governed by diffusion of the fluorescent indicator. Reduction of 1 nM of Ru(NH3)63+ on the nanoelectrode array was detected by monitoring the fluorescence intensity of resorufin, demonstrating high sensitivity fluorescence-mediated electrochemical sensing coupled to self-induced redox cycling.
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