Co-reporter:Mian Rong Lee, Hiang Kwee Lee, Yijie Yang, Charlynn Sher Lin Koh, Chee Leng Lay, Yih Hong Lee, In Yee Phang, and Xing Yi Ling
ACS Applied Materials & Interfaces November 15, 2017 Volume 9(Issue 45) pp:39584-39584
Publication Date(Web):October 11, 2017
DOI:10.1021/acsami.7b11649
We demonstrate a one-step precise direct metal writing of well-defined and densely packed gold nanoparticle (AuNP) patterns with tunable physical and optical properties. We achieve this by using two-photon lithography on a Au precursor comprising poly(vinylpyrrolidone) (PVP) and ethylene glycol (EG), where EG promotes higher reduction rates of Au(III) salt via polyol reduction. Hence, clusters of monodisperse AuNP are generated along raster scanning of the laser, forming high-particle-density, well-defined structures. By varying the PVP concentration, we tune the AuNP size from 27.3 to 65.0 nm and the density from 172 to 965 particles/μm2, corresponding to a surface roughness of 12.9 to 67.1 nm, which is important for surface-based applications such as surface-enhanced Raman scattering (SERS). We find that the microstructures exhibit an SERS enhancement factor of >105 and demonstrate remote writing of well-defined Au microstructures within a microfluidic channel for the SERS detection of gaseous molecules. We showcase in situ SERS monitoring of gaseous 4-methylbenzenethiol and real-time detection of multiple small gaseous species with no specific affinity to Au. This one-step, laser-induced fabrication of AuNP microstructures ignites a plethora of possibilities to position desired patterns directly onto or within most surfaces for the future creation of multifunctional lab-on-a-chip devices.Keywords: direct metal writing; gas analyte sensing; gold nanoparticles; surface-enhanced Raman scattering; two-photon lithography;
Co-reporter:Yijie Yang, Yih Hong Lee, Chee Leng Lay, and Xing Yi Ling
Chemistry of Materials July 25, 2017 Volume 29(Issue 14) pp:6137-6137
Publication Date(Web):July 3, 2017
DOI:10.1021/acs.chemmater.7b02211
In surface chemistry-directed nanoparticle self-assembly, it remains challenging to continuously modulate nanoparticle behavior at the oil/water interface without replacing surface functionality or particle morphology. Here, we utilize solvent-tunable molecular-level polymer conformation changes to achieve “multiple metacrystals using one nanoparticle with one chemical functionality”. We use Ag nanocubes functionalized with a mixed monolayer of thiol-terminated poly(ethylene glycol) (PEG) and hexadecanethiol (C16). We continuously modulate PEG conformation from swollen to coiled states by decreasing solvent polarity, whereas C16 promotes nanocube dispersion in organic carrier solvents. Such PEG conformation changes drive Ag nanocubes to adopt tilted, standing, and planar configurations at the oil/water interface, with their interfacial positions changing from halfway across the interface to almost immersed within the oil phase. We also identify four specific polarities which enable Ag nanocubes to assemble into large-area metacrystals with linear, hexagonal, and square close-packed lattices. Our work establishes an innovative strategy to achieve robust tunability of nanoparticle interfacial behavior and unprecedented modulation of metacrystal structure.
Co-reporter:Hiang Kwee Lee, Yih Hong Lee, Joseph V. Morabito, Yejing Liu, Charlynn Sher Lin Koh, In Yee Phang, Srikanth Pedireddy, Xuemei Han, Lien-Yang Chou, Chia-Kuang Tsung, and Xing Yi Ling
Journal of the American Chemical Society August 23, 2017 Volume 139(Issue 33) pp:11513-11513
Publication Date(Web):July 25, 2017
DOI:10.1021/jacs.7b04936
We demonstrate a molecular-level observation of driving CO2 molecules into a quasi-condensed phase on the solid surface of metal nanoparticles (NP) under ambient conditions of 1 bar and 298 K. This is achieved via a CO2 accumulation in the interface between a metal–organic framework (MOF) and a metal NP surface formed by coating NPs with a MOF. Using real-time surface-enhanced Raman scattering spectroscopy, a >18-fold enhancement of surface coverage of CO2 is observed at the interface. The high surface concentration leads CO2 molecules to be in close proximity with the probe molecules on the metal surface (4-methylbenzenethiol), and transforms CO2 molecules into a bent conformation without the formation of chemical bonds. Such linear-to-bent transition of CO2 is unprecedented at ambient conditions in the absence of chemical bond formation, and is commonly observed only in pressurized systems (>105 bar). The molecular-level observation of a quasi-condensed phase induced by MOF coating could impact the future design of hybrid materials in diverse applications, including catalytic CO2 conversion and ambient solid–gas operation.
Co-reporter:Hiang Kwee Lee, Yih Hong Lee, Gia Chuong Phan-Quang, Xuemei Han, Charlynn Sher Lin Koh, and Xing Yi Ling
Chemistry of Materials August 22, 2017 Volume 29(Issue 16) pp:6563-6563
Publication Date(Web):July 19, 2017
DOI:10.1021/acs.chemmater.7b02227
Particle-assembled fluid–fluid interfaces give rise to soft substrate-less platforms with wide-ranging applications, including remote and on-demand manipulation, optical modulation, catalysis, and multiphase and multiplex sensing, as well as in situ reaction kinetics elucidation. Notably, these soft platforms are easy to fabricate and can exhibit long-range order, both of which are challenging to achieve using traditional solid-based substrates. In this perspective, we provide an overview of the latest research in the fabrication and applications of these soft platforms. We begin with a brief discussion on the formation mechanism of two- and three-dimensional substrate-less platforms, followed by highlighting the unique properties of these platforms. We also discuss the application of these particle-assembled interfaces to three specific research areas, including dynamic tuning of optical properties, multiplex molecular sensing, and small-volume reaction modulation and kinetics monitoring. We end our perspective with an outlook on the promising research frontiers that can be achieved using these soft substrate-less platforms.
Co-reporter:Xuemei Han, Hiang Kwee Lee, Yih Hong LeeXing Yi Ling
The Journal of Physical Chemistry Letters 2017 Volume 8(Issue 1) pp:243-249
Publication Date(Web):December 15, 2016
DOI:10.1021/acs.jpclett.6b02743
The ability of an artificial microdroplet to mimic the rotational behaviors of living systems is crucial for dynamic mass transportation but remains challenging to date. Herein, we report dynamic microdroplet rotation using a liquid marble (RLM) and achieve precise control over mass transportation and distribution in a three-dimensional (3D) microdroplet. RLM rotates synchronously with an external magnetic field, creating circular hydrodynamic flow and an outward centrifugal force. Such spin-induced phenomena direct a spiral movement of entrapped molecules and accelerate their diffusion and homogenization in the entire liquid. Moreover, we demonstrate the rotation rate-controlled (between 0 and 1300 rpm) modulation of shell-catalyzed reaction kinetics from 0.13 to 0.62 min–1. The directed acceleration of reactants toward a catalytically active shell surface is 3-fold faster than conventional stir bar-based convective flow. RLM as an efficient magnetohydrodynamics transducer will be valuable for dynamical control over mass transportation in microdroplet-based chemical, biological, and biomedical studies.
Co-reporter:Gia Chuong Phan-Quang;Elizabeth Hui Zi Wee;Fengling Yang;Hiang Kwee Lee;Dr. In Yee Phang;Xiaotong Feng; Ramón A. Alvarez-Puebla; Xing Yi Ling
Angewandte Chemie 2017 Volume 129(Issue 20) pp:5657-5661
Publication Date(Web):2017/05/08
DOI:10.1002/ange.201702374
Abstract3D plasmonic colloidosomes are superior SERS sensors owing to their high sensitivity and excellent tolerance to laser misalignment. Herein, we incorporate plasmonic colloidosomes in a microfluidic channel for online SERS detection. Our method resolves the poor signal reproducibility and inter-sample contamination in the existing online SERS platforms. Our flow system offers rapid and continuous online detection of 20 samples in less than 5 min with excellent signal reproducibility. The isolated colloidosomes prevent cross-sample and channel contamination, allowing accurate quantification of samples over a concentration range of five orders of magnitude. Our system demonstrates high-resolution multiplex detection with fully preserved signal and Raman features of individual analytes in a mixture. High-throughput multi-assay analysis is performed, which highlights that our system is capable of rapid identification and quantification of a sequence of samples containing various analytes and concentrations.
Co-reporter:Gia Chuong Phan-Quang;Elizabeth Hui Zi Wee;Fengling Yang;Hiang Kwee Lee;Dr. In Yee Phang;Xiaotong Feng; Ramón A. Alvarez-Puebla; Xing Yi Ling
Angewandte Chemie International Edition 2017 Volume 56(Issue 20) pp:5565-5569
Publication Date(Web):2017/05/08
DOI:10.1002/anie.201702374
Abstract3D plasmonic colloidosomes are superior SERS sensors owing to their high sensitivity and excellent tolerance to laser misalignment. Herein, we incorporate plasmonic colloidosomes in a microfluidic channel for online SERS detection. Our method resolves the poor signal reproducibility and inter-sample contamination in the existing online SERS platforms. Our flow system offers rapid and continuous online detection of 20 samples in less than 5 min with excellent signal reproducibility. The isolated colloidosomes prevent cross-sample and channel contamination, allowing accurate quantification of samples over a concentration range of five orders of magnitude. Our system demonstrates high-resolution multiplex detection with fully preserved signal and Raman features of individual analytes in a mixture. High-throughput multi-assay analysis is performed, which highlights that our system is capable of rapid identification and quantification of a sequence of samples containing various analytes and concentrations.
Co-reporter:Charlynn Sher Lin Koh;Hiang Kwee Lee;Gia Chuong Phan-Quang;Dr. Xuemei Han;Mian Rong Lee;Zhe Yang; Xing Yi Ling
Angewandte Chemie International Edition 2017 Volume 56(Issue 30) pp:8813-8817
Publication Date(Web):2017/07/17
DOI:10.1002/anie.201704433
AbstractLiquid marbles are emergent microreactors owing to their isolated environment and the flexibility of materials used. Plasmonic liquid marbles (PLMs) are demonstrated as the smallest spectroelectrochemical microliter-scale reactor for concurrent spectro- and electrochemical analyses. The three-dimensional Ag shell of PLMs are exploited as a bifunctional surface-enhanced Raman scattering (SERS) platform and working electrode for redox process modulation. The combination of SERS and electrochemistry (EC) capabilities enables in situ molecular read-out of transient electrochemical species, and elucidate the potential-dependent and multi-step reaction dynamics. The 3D configuration of our PLM-based EC-SERS system exhibits 2-fold and 10-fold superior electrochemical and SERS performance than conventional 2D platforms. The rich molecular-level electrochemical insights and excellent EC-SERS capabilities offered by our 3D spectroelectrochemical system are pertinent in charge transfer processes.
Co-reporter:Charlynn Sher Lin Koh;Hiang Kwee Lee;Gia Chuong Phan-Quang;Dr. Xuemei Han;Mian Rong Lee;Zhe Yang; Xing Yi Ling
Angewandte Chemie 2017 Volume 129(Issue 30) pp:8939-8943
Publication Date(Web):2017/07/17
DOI:10.1002/ange.201704433
AbstractLiquid marbles are emergent microreactors owing to their isolated environment and the flexibility of materials used. Plasmonic liquid marbles (PLMs) are demonstrated as the smallest spectroelectrochemical microliter-scale reactor for concurrent spectro- and electrochemical analyses. The three-dimensional Ag shell of PLMs are exploited as a bifunctional surface-enhanced Raman scattering (SERS) platform and working electrode for redox process modulation. The combination of SERS and electrochemistry (EC) capabilities enables in situ molecular read-out of transient electrochemical species, and elucidate the potential-dependent and multi-step reaction dynamics. The 3D configuration of our PLM-based EC-SERS system exhibits 2-fold and 10-fold superior electrochemical and SERS performance than conventional 2D platforms. The rich molecular-level electrochemical insights and excellent EC-SERS capabilities offered by our 3D spectroelectrochemical system are pertinent in charge transfer processes.
Co-reporter:Wenxiong Shi;Yih Hong Lee;Shuzhou Li
Nanoscale (2009-Present) 2017 vol. 9(Issue 31) pp:11239-11248
Publication Date(Web):2017/08/10
DOI:10.1039/C7NR02194A
Shape-controlled polyhedral particles and their assembled structures have important applications in plasmonics and biosensing, but the interfacial configurations that will critically determine their resultant assembled structures are not well-understood. Hence, a reliable theory is desirable to predict the position and orientation of a polyhedron at the vicinity of a liquid/liquid interface. Here we demonstrate that the free energy change theory can quantitatively predict the position and orientation of an isolated octahedral nanoparticle at a liquid/liquid interface, whose vertices and facets can play crucial roles in biosensing. We focus on two limiting orientations of an octahedral nanoparticle, vertex up and facet up. Our proposed theory indicates that the surface wettability (hydrophilic/hydrophobic ratio) of the nanoparticle determines its most stable position and the preferred orientation at a water/oil interface. The surface wettability of an octahedron is adjusted from extremely hydrophobic to extremely hydrophilic by changing the amount of charge on the Ag surface in molecular dynamics (MD) simulations. The MD simulations results are in excellent agreement with our theoretical prediction for an Ag octahedral nanoparticle at a hexane/water interface. Our proposed theory bridges the gap between molecular-level simulations and equilibrium configurations of polyhedral nanoparticles in experiments, where insights from nanoparticle intrinsic wettability details can be used to predict macroscopic superlattice formation experimentally. This work advances our ability to precisely predict the final structures of the polyhedral nanoparticle assemblies at a liquid/liquid interface.
Co-reporter:Wei Gao;Yih Hong Lee;Ruibin Jiang;Jianfang Wang;Tianxi Liu
Advanced Materials 2016 Volume 28( Issue 4) pp:701-706
Publication Date(Web):
DOI:10.1002/adma.201503905
Co-reporter:Yijie Yang, Yih Hong Lee, In Yee Phang, Ruibin Jiang, Howard Yi Fan Sim, Jianfang Wang, and Xing Yi Ling
Nano Letters 2016 Volume 16(Issue 6) pp:3872-3878
Publication Date(Web):May 20, 2016
DOI:10.1021/acs.nanolett.6b01388
Current plasmonic metasurfaces of nanocubes are limited to planar configurations, restricting the ability to create tailored local electromagnetic fields. Here, we report a new chemical strategy to achieve tunable metasurfaces with nonplanar nanocube orientations, creating novel lattice-dependent field localization patterns. We manipulate the interfacial behaviors of Ag nanocubes by controlling the ratio of hydrophilic/hydrophobic molecules added in a binary thiol mixture during the surface functionalization step. The nanocube orientation at an oil/water interface can consequently be continuously tuned from planar to tilted and standing configurations, leading to the organization of Ag nanocubes into three unique large-area metacrystals, including square close-packed, linear, and hexagonal lattices. In particular, the linear and hexagonal metacrystals are unusual open lattices comprising nonplanar nanocubes, creating unique local electromagnetic field distribution patterns. Large-area “hot hexagons” with significant delocalization of hot spots form in the hexagonal metacrystal. With a lowest packing density of 24%, the hexagonal metacrystal generates nearly 350-fold stronger surface-enhanced Raman scattering as compared to the other denser-packing metacrystals, demonstrating the importance of achieving control over the geometrical and spatial orientation of the nanocubes in the metacrystals.
Co-reporter:Zhe Yang, Srikanth Pedireddy, Hiang Kwee Lee, Yejing Liu, Weng Weei Tjiu, In Yee Phang, and Xing Yi Ling
Chemistry of Materials 2016 Volume 28(Issue 14) pp:5080
Publication Date(Web):June 21, 2016
DOI:10.1021/acs.chemmater.6b01925
Bicontinuous nanoporous gold (NPG) is a high-performance catalyst characterized by its excellent electrochemical stability and immense active surface area with high electrolyte accessibility. However, the intrinsic catalytic activity of NPG is still lower compared to other metals (such as Pt), thus impeding its applicability in a commercial catalytic system. Herein, we incorporate secondary Pt metal with inherently strong catalytic activities into a zero-dimensional (0D) nanoporous gold bowl (NPGB) to develop Pt-NPGB bimetallic catalyst. Our strategy effectively exploits the highly accessible surface area of NPGB and the manipulative d-band electronic structure brought about by the synergistic intermetallic interaction for enhanced catalytic performance and durability. Deposition of Pt on the NPGB catalyst directly modulates its d-band electronic structure, with the electronic energy of Pt-NPGBs tunable between −3.93 to −4.24, approximating that of chemically resistant gold (−4.35 eV). This is vital to weaken the binding strength between Pt active sites and intermediate poisoning species. Together with the high Pt electrochemical active surface area (ECSA) of 17.1 mA/μgPt facilitated by NPGB cocatalyst, such synergistic effect enables the superior performance of Pt-NPGB hybrids over commercial Pt/C in methanol oxidation reaction (MOR), where an 11-fold and 227-fold better catalytic activity and durability are demonstrated even after an extended duration of 3600 s. Our study is therefore the first demonstration of NPGB on the exploitation of precisely modulated synergistic effect at the electronic level to control and boost catalytic performance. Furthermore, the chemically inert NPGB possesses an intrinsically higher gold surface area and electrolyte accessibility unique to 0D nanoparticle, hence empowering it as an immensely attractive cocatalytic platform extendable to a wide range of secondary metals. This is important to promote the catalytic performance for diverse electrochemical applications, especially in the field of energy, synthetic chemistry, and also environmental toxin degradation.
Co-reporter:Y. Liu, Y. H. Lee, Q. Zhang, Y. Cui and X. Y. Ling
Journal of Materials Chemistry A 2016 vol. 4(Issue 19) pp:4312-4319
Publication Date(Web):29 Mar 2016
DOI:10.1039/C6TC00682E
A major challenge in information security and the development of an anti-counterfeiting platform is to encode multiple identification features on a single platform where these features can be decoded with no interference. Here, we demonstrate a progressively complex anti-counterfeiting platform using a multiplex fabrication strategy. This multiplex strategy enabled us to realize a spatially selective encapsulation of dye molecules within an Ag nanopillar array embedding covert molecular information which was revealed using fluorescence, surface-enhanced Raman scattering (SERS), and signal intensities. A total of five identification layers were used to authenticate products in our nanopillar platform. Moreover, two spectroscopic techniques were required to fully decode the various covert layers encoded within the same nanopillar array, thereby greatly enhancing the security of the information. Hyperspectral imaging was used to precisely generate unique SERS fingerprints of molecules encapsulated in each nanopillar. This feature combined with the high ∼17000 pillars per inch (ppi) information density of the platform make its use extremely effective against counterfeiting and forgery. In summary, our encoding platform enables high security, large information density and low-error decoding.
Co-reporter:Xuemei Han, Hiang Kwee Lee, Wei Chun Lim, Yih Hong Lee, Gia Chuong Phan-Quang, In Yee Phang, and Xing Yi Ling
ACS Applied Materials & Interfaces 2016 Volume 8(Issue 36) pp:23941
Publication Date(Web):August 22, 2016
DOI:10.1021/acsami.6b07766
Liquid marble offers an attractive droplet manipulation approach by isolating microdroplet in a nonstick encapsulating shell formed via the spontaneous coating of hydrophobic particles onto the liquid surface. While liquid marble prepared using magnetic nanoparticles enables precise spatiotemporal actuation of microdroplets, these manipulations are generally limited to simple and linear spatial maneuver of microdroplets. Herein, we demonstrate the unique and three-dimensional spinning of microliter-sized liquid marble (LM) and its subsequent dual applications as (1) the world’s smallest centrifuge and (2) a miniature and localized viscometer. Our LM is responsive to an applied rotating magnetic field, with its spinning speed programmable between 0 and 1300 rpm. This spinning generates an unprecedented centrifugal force of >2g in a LM of ∼1 mm radius. Such centrifugal force facilitates an outward and radial hydrodynamic flow in the enclosed microdroplet, enabling LM to serve as a microcentrifuge for the sedimentation of nanoparticles with >85% separation efficiency. Furthermore, we apply spinning LM as an ultrasensitive spin-to-viscosity transducer to quantify the viscosity of the external suspended liquid in the relative viscosity (η/ηwater) range of 1–70 using ≤1 mL liquid sample. Collectively, the ensemble of benefits offered by spinning LM creates enormous opportunities in the development of multifunctional micromagneto-mechanical devices as promising surface-sensitive microsensor, miniature centrifugal pump, and even microreactor with directed heat and mass transfer mechanism.Keywords: centrifuge; liquid marble; magnetic field; miniature viscometer; rotation; spinning
Co-reporter:Chee Leng Lay, Yih Hong Lee, Mian Rong Lee, In Yee Phang, and Xing Yi Ling
ACS Applied Materials & Interfaces 2016 Volume 8(Issue 12) pp:8145
Publication Date(Web):March 14, 2016
DOI:10.1021/acsami.6b02306
The physical properties of aqueous-based stimuli-responsive photoresists are crucial in fabricating microstructures with high structural integrity and uniform responsiveness during two-photon lithography. Here, we quantitatively investigate how various components within bovine serum albumin (BSA) photoresists affect our ability to achieve BSA microstructures with consistent stimuli-responsive properties over areas exceeding 104 μm2. We unveil a relationship between BSA concentration and dynamic viscosity, establishing a threshold viscosity to achieve robust BSA microstructures. We also demonstrate the addition of an inert polymer to the photoresist as viscosity enhancer. A set of systematically optimized processing parameters is derived for the construction of dynamic BSA microstructures. The optimized BSA photoresists and processing parameters enable us to extend the two-dimensional (2D) microstructures to three-dimensional (3D) ones, culminating in arrays of micropillars with aspect ratio > 10. Our findings foster the development of liquid stimuli-responsive photoresists to build multifunctional complex 3D geometries for applications such as bioimplantable devices or adaptive photonic systems.Keywords: bovine serum albumin; free-standing microstructures; high aspect ratio; two-photon lithography; uniform stimuli responsiveness
Co-reporter:Yih Hong Lee, Hiang Kwee Lee, Jonathan Yong Chew Ho, Yijie Yang and Xing Yi Ling
Analyst 2016 vol. 141(Issue 17) pp:5107-5112
Publication Date(Web):04 Jul 2016
DOI:10.1039/C6AN01239F
Current substrate-less SERS platforms are limited to uncontrolled aggregation of plasmonic nanoparticles or quasi-crystalline arrays of spherical nanoparticles, with no study on how the lattice structures formed by nanoparticle self-assembly affect their detection capabilities. Here, we organize Ag octahedral building blocks into two large-area plasmonic metacrystals at the oil/water interface, and investigate their in situ SERS sensing capabilities. Amphiphilic octahedra assemble into a hexagonal close-packed metacrystal, while hydrophobic octahedra assemble into an open square metacrystal. The lower packing density square metacrystal gives rise to much stronger SERS enhancement than the denser packing hexagonal metacrystal, arising from the larger areas of plasmonic hotspots within the square metacrystal at the excitation wavelength. We further demonstrate the ability of the square metacrystal to achieve quantitative ultratrace detection of analytes from both the aqueous and organic phases. Detection limits are at the nano-molar levels, with analytical enhancement factors reaching 108. In addition, multiplex detection across both phases can be achieved in situ without any loss of signal quantitation.
Co-reporter:Wei Fan;Miao Yue-E;Xingyi Ling;Tianxi Liu
Chinese Journal of Chemistry 2016 Volume 34( Issue 1) pp:73-81
Publication Date(Web):
DOI:10.1002/cjoc.201500585
Abstract
Surface-enhanced Raman spectroscopy (SERS) as a powerful analytical tool has gained extensive attention. Despite of many efforts in the design of SERS substrates, it remains a great challenge for creating a universal substrate with long-term stability and reproducible SERS signals. In this work, Ag nanocubes and graphene oxide (GO) suspension were mixed to form a stable solution and further vacuum filtrated to obtain a free-standing hybrid paper. The Ag/GO hybrid papers exhibit excellent SERS activity because of the synergistic effect of Ag nanocubes and GO sheets. GO sheets can act as both SERS enhancement substrate and framework for supporting Ag nanocubes. Moreover, GO sheets can protect Ag nanoparticles from oxidation under ambient condition for prolonged life time SERS substrate. Furthermore, we demonstrate the use of the free-standing and flexible Ag/GO hybrid paper to enable direct, real-time and reliable detection of trace amounts of analytes in aqueous systems. This novel SERS substrate is expected to be applied in real-time analysis and expands the flexibility of SERS for useful applications in the materials and life science.
Co-reporter:Gia Chuong Phan-Quang;Hiang Kwee Lee; Xing Yi Ling
Angewandte Chemie 2016 Volume 128( Issue 29) pp:8444-8448
Publication Date(Web):
DOI:10.1002/ange.201602565
Abstract
Miniaturized liquid–liquid interfacial reactors offer enhanced surface area and rapid confinement of compounds of opposite solubility, yet they are unable to provide in situ reaction monitoring at a molecular level at the interface. A picoreactor operative at the liquid–liquid interface is described, comprising plasmonic colloidosomes containing Ag octahedra strategically assembled at the water-in-decane emulsion interface. The plasmonic colloidosomes isolate ultrasmall amounts of solutions (<200 pL), allowing parallel monitoring of multiple reactions simultaneously. Using the surface-enhanced Raman spectroscopy (SERS) technique, in situ monitoring of the interfacial protonation of dimethyl yellow (p-dimethylaminoazobenzene (DY)) is performed, revealing an apparent rate constant of 0.09 min−1 for the first-order reaction. The presence of isomeric products with similar physical properties is resolved, which would otherwise be indiscernible by other analytical methods.
Co-reporter:Gia Chuong Phan-Quang;Hiang Kwee Lee; Xing Yi Ling
Angewandte Chemie International Edition 2016 Volume 55( Issue 29) pp:8304-8308
Publication Date(Web):
DOI:10.1002/anie.201602565
Abstract
Miniaturized liquid–liquid interfacial reactors offer enhanced surface area and rapid confinement of compounds of opposite solubility, yet they are unable to provide in situ reaction monitoring at a molecular level at the interface. A picoreactor operative at the liquid–liquid interface is described, comprising plasmonic colloidosomes containing Ag octahedra strategically assembled at the water-in-decane emulsion interface. The plasmonic colloidosomes isolate ultrasmall amounts of solutions (<200 pL), allowing parallel monitoring of multiple reactions simultaneously. Using the surface-enhanced Raman spectroscopy (SERS) technique, in situ monitoring of the interfacial protonation of dimethyl yellow (p-dimethylaminoazobenzene (DY)) is performed, revealing an apparent rate constant of 0.09 min−1 for the first-order reaction. The presence of isomeric products with similar physical properties is resolved, which would otherwise be indiscernible by other analytical methods.
Co-reporter:Xuemei Han; Hiang Kwee Lee; Yih Hong Lee; Wei Hao; Yejing Liu; In Yee Phang; Shuzhou Li
The Journal of Physical Chemistry Letters 2016 Volume 7(Issue 8) pp:1501-1506
Publication Date(Web):April 6, 2016
DOI:10.1021/acs.jpclett.6b00501
Current microscale tracking of chemical kinetics is limited to destructive ex situ methods. Here we utilize Ag nanocube-based plasmonic liquid marble (PLM) microreactor for in situ molecular-level identification of reaction dynamics. We exploit the ultrasensitive surface-enhanced Raman scattering (SERS) capability imparted by the plasmonic shell to unravel the mechanism and kinetics of aryl-diazonium surface grafting reaction in situ, using just a 2-μL reaction droplet. This reaction is a robust approach to generate covalently functionalized metallic surfaces, yet its kinetics remain unknown to date. Experiments and simulations jointly uncover a two-step sequential grafting process. An initial Langmuir chemisorption of sulfonicbenzene diazonium (dSB) salt onto Ag surfaces forms an intermediate sulfonicbenzene monolayer (Ag–SB), followed by subsequent autocatalytic multilayer growth of Ag–SB3. Kinetic rate constants reveal 19-fold faster chemisorption than multilayer growth. Our ability to precisely decipher molecular-level reaction dynamics creates opportunities to develop more efficient processes in synthetic chemistry and nanotechnology.
Co-reporter:Wee Shern Chew, Srikanth Pedireddy, Yih Hong Lee, Weng Weei Tjiu, Yejing Liu, Zhe Yang, and Xing Yi Ling
Chemistry of Materials 2015 Volume 27(Issue 22) pp:7827
Publication Date(Web):November 2, 2015
DOI:10.1021/acs.chemmater.5b03870
Current synthesis of gold nanoframes has only demonstrated morphological control over wall thickness and wall length. Here, we demonstrate the ability to control the nanoscale porosity of these nanoframes, using a templated seed-mediated approach. The porosity on these nanoporous gold nanoframes (NGNs) is tuned by controlling the crystallite size of Au nanoparticles deposited on the AgCl templates. The yield of the NGNs approaches 100%. Despite its minimalist architectural construction, the NGNs are mechanically robust, retaining its morphology even after multiple centrifugation and sonication rounds. We further highlight that decreasing the porosity on the NGN leads to improved surface-enhanced Raman scattering (SERS) enhancement. Increasing the constituent Au crystallite size decreases the porosity, but increases the surface roughness of NGN, hence leading to greater SERS enhancement. The introduction of porosity in a gold nanoframe structure through our synthesis method is novel and generic, suggesting the extendibility of our method to other types of templates.
Co-reporter:Yan Cui, In Yee Phang, Yih Hong Lee, Mian Rong Lee, Qi Zhang and Xing Yi Ling
Chemical Communications 2015 vol. 51(Issue 25) pp:5363-5366
Publication Date(Web):24 Nov 2014
DOI:10.1039/C4CC08596E
We demonstrate a multiplex plasmonic anti-counterfeiting platform by embedding multiple molecules on a single substrate with superior nanometer scale spectral and spatial resolution. The encoded specific molecular information can be read out only using Raman spectroscopy. Multiplexing increases the complexity of these plasmonic security labels without compromising the spectral resolution.
Co-reporter:Gia Chuong Phan-Quang;Hiang Kwee Lee;Dr. In Yee Phang; Xing Yi Ling
Angewandte Chemie 2015 Volume 127( Issue 33) pp:9827-9831
Publication Date(Web):
DOI:10.1002/ange.201504027
Abstract
Colloidosomes are robust microcapsules attractive for molecular sensing because of their characteristic micron size, large specific surface area, and dual-phase stability. However, current colloidosome sensors are limited to qualitative fluorogenic receptor-based detection, which restrict their applicability to a narrow range of molecules. Here, we introduce plasmonic colloidosome constructed from Ag nanocubes as an emulsion-based 3D SERS platform. The colloidosomes exhibit excellent mechanical robustness, flexible size tunability, versatility to merge, and ultrasensitivity in SERS quantitation of food/industrial toxins down to sub-femtomole levels. Using just 0.5 μL of sample volumes, our plasmonic colloidosomes exhibit >3000-fold higher SERS sensitivity over conventional suspension platform. Notably, we demonstrate the first high-throughput multiplex molecular sensing across multiple liquid phases.
Co-reporter:Gia Chuong Phan-Quang;Hiang Kwee Lee;Dr. In Yee Phang; Xing Yi Ling
Angewandte Chemie International Edition 2015 Volume 54( Issue 33) pp:9691-9695
Publication Date(Web):
DOI:10.1002/anie.201504027
Abstract
Colloidosomes are robust microcapsules attractive for molecular sensing because of their characteristic micron size, large specific surface area, and dual-phase stability. However, current colloidosome sensors are limited to qualitative fluorogenic receptor-based detection, which restrict their applicability to a narrow range of molecules. Here, we introduce plasmonic colloidosome constructed from Ag nanocubes as an emulsion-based 3D SERS platform. The colloidosomes exhibit excellent mechanical robustness, flexible size tunability, versatility to merge, and ultrasensitivity in SERS quantitation of food/industrial toxins down to sub-femtomole levels. Using just 0.5 μL of sample volumes, our plasmonic colloidosomes exhibit >3000-fold higher SERS sensitivity over conventional suspension platform. Notably, we demonstrate the first high-throughput multiplex molecular sensing across multiple liquid phases.
Co-reporter:Chee Leng Lay, Mian Rong Lee, Hiang Kwee Lee, In Yee Phang, and Xing Yi Ling
ACS Nano 2015 Volume 9(Issue 10) pp:9708
Publication Date(Web):September 15, 2015
DOI:10.1021/acsnano.5b04300
Two-dimensional (2D) geometrical shape-shifting is prevalent in nature, but remains challenging in man-made “smart” materials, which are typically limited to single-direction responses. Here, we fabricate geometrical shape-shifting bovine serum albumin (BSA) microstructures to achieve circle-to-polygon and polygon-to-circle geometrical transformations. In addition, transformative two-dimensional microstructure arrays are demonstrated by the ensemble of these responsive microstructures to confer structure-to-function properties. The design strategy of our geometrical shape-shifting microstructures focuses on embedding precisely positioned rigid skeletal frames within responsive BSA matrices to direct their anisotropic swelling under pH stimulus. This is achieved using layer-by-layer two photon lithography, which is a direct laser writing technique capable of rendering spatial resolution in the sub-micrometer length scale. By controlling the shape, orientation and number of the embedded skeletal frames, we have demonstrated well-defined arc-to-corner and corner-to-arc transformations, which are essential for dynamic circle-to-polygon and polygon-to-circle shape-shifting, respectively. We further fabricate our shape-shifting microstructures in periodic arrays to experimentally demonstrate the first transformative 2D patterned arrays. Such versatile array configuration transformations give rise to structure-to-physical properties, including array porosity and pore shape, which are crucial for the development of on-demand multifunctional “smart” materials, especially in the field of photonics and microfluidics.Keywords: bovine serum albumin; geometrical shape-shifting; stimuli-responsive; transformative array configurations; two-photon lithography;
Co-reporter:Joel Ming Rui Tan ; Yih Hong Lee ; Srikanth Pedireddy ; Tom Baikie ; Xing Yi Ling ;Lydia Helena Wong
Journal of the American Chemical Society 2014 Volume 136(Issue 18) pp:6684-6692
Publication Date(Web):April 4, 2014
DOI:10.1021/ja501786s
Single-phase Cu2ZnSnS4 (CZTS) is an essential prerequisite toward a high-efficiency thin-film solar cell device. Herein, the selective phase formation of single-phase CZTS nanoparticles by ligand control is reported. Surface-enhanced Raman scattering (SERS) spectroscopy is demonstrated for the first time as a characterization tool for nanoparticles to differentiate the mixed compositional phase (e.g., CZTS, CTS, and ZnS), which cannot be distinguished by X-ray diffraction. Due to the superior selectivity and sensitivity of SERS, the growth mechanism of CZTS nanoparticle formation by hot injection is revealed to involve three growth steps. First, it starts with nucleation of Cu2–xS nanoparticles, followed by diffusion of Sn4+ into Cu2–xS nanoparticles to form the Cu3SnS4 (CTS) phase and diffusion of Zn2+ into CTS nanoparticles to form the CZTS phase. In addition, it is revealed that single-phase CZTS nanoparticles can be obtained via balancing the rate of CTS phase formation and diffusion of Zn2+ into the CTS phase. We demonstrate that this balance can be achieved by 1 mL of thiol with Cu(OAc)2, Sn(OAc)4, and Zn(acac)2 metal salts to synthesize the CZTS phase without the presence of a detectable binary/ternary phase with SERS.
Co-reporter:Wei Fan, Yih Hong Lee, Srikanth Pedireddy, Qi Zhang, Tianxi Liu and Xing Yi Ling
Nanoscale 2014 vol. 6(Issue 9) pp:4843-4851
Publication Date(Web):20 Feb 2014
DOI:10.1039/C3NR06316J
Graphene oxide (GO) is an emerging material for surface-enhanced Raman scattering (SERS) due to its strong chemical enhancement. Studying the SERS performance of plasmonic nanoparticle/GO hybrid materials at the single particle level is crucial for direct probing of the chemical effect of GO on plasmonic nanoparticles. In this work, we integrate GO and shape-controlled Ag nanoparticles to create hybrid nanomaterials, and the chemical enhancement arising from GO is investigated using single-particle SERS measurements. Ag nanoparticle@GO hybrid nanostructures are prepared by assembling Ag nanoparticles, including spheres, cubes and octahedra with GO sheets. The SERS behaviors of the hybrid nanostructures are characterized, and 2–3 times enhanced SERS intensities are detected from the Ag nanoparticle@GO hybrid nanostructures as compared to pure Ag nanoparticles. Furthermore, we probe the mechanism of SERS enhancement in the hybrid nanostructures by changing the surface coverage of GO on Ag octahedra, by using reduced GO in place of GO as well as by using probe molecules of different electronegativities. This hybrid system is an excellent candidate for single-particle SERS sensors. Sub-nanomolar levels of aromatic molecules are detected using a single Ag/GO hybrid nanomaterial. This as-prepared GO and shape-controlled Ag nanoparticle hybrid is capable of serving as a high performance SERS platform, providing new opportunities for efficient chemical and biological sensing applications.
Co-reporter:Yue-E Miao, Hiang Kwee Lee, Wee Shern Chew, In Yee Phang, Tianxi Liu and Xing Yi Ling
Chemical Communications 2014 vol. 50(Issue 44) pp:5923-5926
Publication Date(Web):15 Apr 2014
DOI:10.1039/C4CC01949K
Ag nanowire-based catalytic liquid marbles are fabricated as miniature reactors, which demonstrate highly efficient, support-free and rate-controllable heterogeneous degradation of methylene blue, with catalytic efficiency close to 100%. Our miniature catalytic liquid marbles are essential for reactions involving highly toxic/hazardous or costly reactants, where small volume preliminary reactions are preferred.
Co-reporter:Xing Li, Hiang Kwee Lee, In Yee Phang, Choon Keong Lee, and Xing Yi Ling
Analytical Chemistry 2014 Volume 86(Issue 20) pp:10437
Publication Date(Web):September 17, 2014
DOI:10.1021/ac502955w
The ultratrace detection and quantification of toxins in both water and organic liquids remains a challenge due to the random spreading and dilution of liquids on substrate-based sensors, especially for organic liquids with low surface tension. Herein, we fabricate a superhydrophobic-oleophobic (SHP-OP) 3D Ag nanowire mesh-like surface-enhanced Raman scattering (SERS) platform to overcome the random spreading issue, demonstrating ultratrace toxin sensing in both water and organic liquid. Our SHP-OP SERS platform is able to concentrate analyte solutions in water and toluene to 100-fold and 8-fold smaller areas, respectively, as compared to its omniphilic counterparts. The synergy of analyte-concentrating ability and intense SERS-enhancing properties on our SHP-OP SERS platform enables quantitative and ultratrace detection of melamine and Sudan I down to 0.1 fmol in water and toluene, respectively, using just 1 μL of analyte solution. These detection limits are 103-fold lower than the regulatory limits, clearly indicating our SHP-OP SERS platform as an appealing universal ultratrace toxin sensor. The ultratrace detection of spiked melamine in liquid milk down to 100 fmol also highlights the suitability of our SHP-OP SERS platform for the sensing of food toxins in real samples.
Co-reporter:Joel Ming Rui Tan, Justina Jiexin Ruan, Hiang Kwee Lee, In Yee Phang and Xing Yi Ling
Physical Chemistry Chemical Physics 2014 vol. 16(Issue 48) pp:26983-26990
Publication Date(Web):31 Oct 2014
DOI:10.1039/C4CP03679D
An analytical platform with an ultratrace detection limit in the atto-molar (aM) concentration range is vital for forensic, industrial and environmental sectors that handle scarce/highly toxic samples. Superhydrophobic surface-enhanced Raman scattering (SERS) platforms serve as ideal platforms to enhance detection sensitivity by reducing the random spreading of aqueous solution. However, the fabrication of superhydrophobic SERS platforms is generally limited due to the use of sophisticated and expensive protocols and/or suffers structural and signal inconsistency. Herein, we demonstrate a high-throughput fabrication of a stable and uniform superhydrophobic SERS platform for ultratrace molecular sensing. Large-area box-like micropatterns of the polymeric surface are first fabricated using capillary force lithography (CFL). Subsequently, plasmonic properties are incorporated into the patterned surfaces by decorating with Ag nanocubes using the Langmuir–Schaefer technique. To create a stable superhydrophobic SERS platform, an additional 25 nm Ag film is coated over the Ag nanocube-decorated patterned template followed by chemical functionalization with perfluorodecanethiol. Our resulting superhydrophobic SERS platform demonstrates excellent water-repellency with a static contact angle of 165° ± 9° and a consequent analyte concentration factor of 59-fold, as compared to its hydrophilic counterpart. By combining the analyte concentration effect of superhydrophobic surfaces with the intense electromagnetic “hot spots” of Ag nanocubes, our superhydrophobic SERS platform achieves an ultra-low detection limit of 10−17 M (10 aM) for rhodamine 6G using just 4 μL of analyte solutions, corresponding to an analytical SERS enhancement factor of 1013. Our fabrication protocol demonstrates a simple, cost- and time-effective approach for the large-scale fabrication of a superhydrophobic SERS platform for ultratrace molecular detection.
Co-reporter:Yan Cui, In Yee Phang, Ravi S. Hegde, Yih Hong Lee, and Xing Yi Ling
ACS Photonics 2014 Volume 1(Issue 7) pp:631
Publication Date(Web):June 4, 2014
DOI:10.1021/ph5001154
The surface-enhanced Raman scattering (SERS) spectrum exhibits huge potential as an alternative data storage element. Using plasmonic nanostructures as the physical building blocks where probe molecules are adsorbed, their corresponding structural and SERS information are stored within a finite volume of plasmonic nanostructures. However, the current SERS development is hampered by the difficulty in fabricating quantitative and homogeneous SERS platforms. Here, we introduce the concept of “plasmonic molecular data storage” using SERS intensity as the basic data storage element (or digit). SERS signal is quantitatively tunable by manipulating the orientation (hence the localized surface plasmon modes) of the respective nanowire nanostructures to achieve multiple-digit SERS intensity data storage. We address the reproducibility problem by fabricating homogeneous plasmonic nanowire structures using two-photon lithography and thermal evaporation. Silver (Ag) nanowires of different orientations carrying different digits of molecular information can be combined to form sophisticated 2D geometrical structures, such as geometrical patterns, letters in the alphabet, and complex tessellated reptiles to impart multiple-digit-per-microstructure data storage. In particular, a 7-digit SERS information storage system has been achieved by tuning the Ag nanowires’ orientation from 0° to 90° at 15° intervals. Spatial data, especially the coordinates and topology, brought about by the predefined Ag nanowire structures create an additional level of information to the plasmonic data storage system. Using 1 byte (8 binary digits) as the basis of comparison, our 7-digit platform is able to store 22 500-fold denser information than the binary system. In addition, our plasmonic nanowire data storage system also provides unique physical morphology and chemical Raman information. It is analogous to optical data storage, but it acquires richer multidimensional information and exhibits higher spectral resolution than the broader-band response of conventional optical spectroscopic techniques.Keywords: gray-scale Raman image; multiple-digit molecular data storage; polarization-dependent surface-enhanced Raman spectroscopy (SERS); silver nanowire
Co-reporter:Hiang Kwee Lee;Dr. Yih Hong Lee;Dr. In Yee Phang;Jiaqi Wei;Yue-E Miao; Tianxi Liu; Xing Yi Ling
Angewandte Chemie International Edition 2014 Volume 53( Issue 20) pp:5054-5058
Publication Date(Web):
DOI:10.1002/anie.201401026
Abstract
Inspired by aphids, liquid marbles have been studied extensively and have found application as isolated microreactors, as micropumps, and in sensing. However, current liquid-marble-based sensing methodologies are limited to qualitative colorimetry-based detection. Herein we describe the fabrication of a plasmonic liquid marble as a substrate-less analytical platform which, when coupled with ultrasensitive SERS, enables simultaneous multiplex quantification and the identification of ultratrace analytes across separate phases. Our plasmonic liquid marble demonstrates excellent mechanical stability and is suitable for the quantitative examination of ultratrace analytes, with detection limits as low as 0.3 fmol, which corresponds to an analytical enhancement factor of 5×108. The results of our simultaneous detection scheme based on plasmonic liquid marbles and an aqueous–solid–organic interface quantitatively tally with those found for the individual detection of methylene blue and coumarin.
Co-reporter:Hiang Kwee Lee;Dr. Yih Hong Lee;Dr. In Yee Phang;Jiaqi Wei;Yue-E Miao; Tianxi Liu; Xing Yi Ling
Angewandte Chemie 2014 Volume 126( Issue 20) pp:5154-5158
Publication Date(Web):
DOI:10.1002/ange.201401026
Abstract
Inspired by aphids, liquid marbles have been studied extensively and have found application as isolated microreactors, as micropumps, and in sensing. However, current liquid-marble-based sensing methodologies are limited to qualitative colorimetry-based detection. Herein we describe the fabrication of a plasmonic liquid marble as a substrate-less analytical platform which, when coupled with ultrasensitive SERS, enables simultaneous multiplex quantification and the identification of ultratrace analytes across separate phases. Our plasmonic liquid marble demonstrates excellent mechanical stability and is suitable for the quantitative examination of ultratrace analytes, with detection limits as low as 0.3 fmol, which corresponds to an analytical enhancement factor of 5×108. The results of our simultaneous detection scheme based on plasmonic liquid marbles and an aqueous–solid–organic interface quantitatively tally with those found for the individual detection of methylene blue and coumarin.
Co-reporter:Yih Hong Lee, Choon Keong Lee, Baorui Tan, Joel Ming Rui Tan, In Yee Phang and Xing Yi Ling
Nanoscale 2013 vol. 5(Issue 14) pp:6404-6412
Publication Date(Web):10 May 2013
DOI:10.1039/C3NR00981E
Interfacial self-assembly of nanoparticles is capable of creating large-area close-packed structures for a variety of applications. However, monolayers of hydrophilic cetyltrimethylammonium bromide (CTAB)-coated Au nanoparticles are challenging to assemble via interfacial self-assembly. This report presents a facile and scalable process to fabricate large-area monolayer films of ultrathin CTAB-coated Au nanoprisms at the air–water interface using the Langmuir–Schaefer technique. This is first achieved by a one-step functionalization of Au nanoprisms with poly(vinylpyrrolidone) (PVP). PVP functionalization is completed within a short time without loss of nanoprisms due to aggregation. Uniform and near close-packed monolayers of the Au nanoprisms formed over large areas (∼1 cm2) at the air–water interface can be transferred to substrates with different wettabilities. The inter-prism gaps are tuned qualitatively through the introduction of dodecanethiol and oleylamine. The morphological integrity of the nanoprisms is maintained throughout the entire assembly process, without truncation of the nanoprism tips. The near close-packed arrangement of the nanoprism monolayers generates large numbers of hot spots in the 2D arrays in the tip-to-tip and edge-to-edge inter-particle regions, giving rise to strong surface-enhanced Raman scattering (SERS) signals. When deposited on an Au mirror film, additional hotspots are created in the 3rd dimension in the gaps between the 2D nanoprism monolayers and the Au film. SERS enhancement factors reaching 104 for non-resonant probe molecules are achieved.
Co-reporter:Hiang Kwee Lee, Yih Hong Lee, Qi Zhang, In Yee Phang, Joel Ming Rui Tan, Yan Cui, and Xing Yi Ling
ACS Applied Materials & Interfaces 2013 Volume 5(Issue 21) pp:11409
Publication Date(Web):October 17, 2013
DOI:10.1021/am403655g
An analytical platform suitable for trace detection using a small volume of analyte is pertinent to the field of toxin detection and criminology. Plasmonic nanostructures provide surface-enhanced Raman scattering (SERS) that can potentially achieve trace toxins and/or molecules detection. However, the detection of highly diluted, small volume samples remains a challenge. Here, we fabricate a superhydrophobic SERS platform by assembling Ag nanocubes that support strong surface plasmon and chemical functionalization for trace detection with sample volume of just 1 μL. Our strategy integrates the intense electromagnetic field confinement generated by Ag nanocubes with a superhydrophobic surface capable of analyte concentration to lower the molecular detection limit. Single crystalline Ag nanocubes are assembled using the Langmuir-Blodgett technique to create surface roughness. To create a stable superhydrophobic SERS platform, an additional 25 nm Ag coating is evaporated over the Ag nanocubes to “weld” the Ag nanocubes onto the substrate followed by chemical functionalization with perfluorodecanethiol. The resulting substrate has an advancing contact angle of 169° ± 5°. Our superhydrophobic platform confines analyte molecules within a small area and prevents the random spreading of molecules. An analyte concentrating factor of 14-fold is attained, as compared to a hydrophilic surface. Consequently, the detection limit of our superhydrophobic SERS substrate reaches 10–16 M (100 aM) for rhodamine 6G using 1 μL analyte solutions. An analytical SERS enhancement factor of 1011 is achieved. Our protocol is a general method that provides a simple, cost-effective approach to develop a stable and uniform superhydrophobic SERS platform for trace molecular sensing.Keywords: plasmonic; silver nanocube; superhydrophobic; surface-enhanced Raman spectroscopy; trace molecular detection;
Co-reporter:Miaosi Chen, In Yee Phang, Mian Rong Lee, Joel Kwang Wei Yang, and Xing Yi Ling
Langmuir 2013 Volume 29(Issue 23) pp:7061-7069
Publication Date(Web):May 5, 2013
DOI:10.1021/la4012108
The surface-enhanced Raman scattering (SERS) “hot spots” are highly localized regions of enhanced electromagnetic field within a SERS substrate that dominate the overall SERS intensity. This results in inhomogeneous distribution of SERS intensity in a SERS substrate, thus limiting their application as reproducible and ultrasensitive sensing platforms. Here, we address this challenge by fabricating Ag nanowires into three-dimensional (3D) woodpile-like platforms via layer-by-layer Langmuir–Blodgett assembly. We focus on promoting strong electromagnetic coupling between parallel and vertically stacked Ag nanowire pairs within the woodpile structure to achieve a high density of “hot spots” across the entire 3D SERS substrates. Raman mapping (x–y plane) demonstrates that all of the 3D Ag nanowire arrays exhibit a homogeneous SERS Raman intensity over a large area, whereas their monolayer counterpart experiences >50% of zero and/or an undetectable SERS signal. The SERS enhancement factor increases from 3.1 × 103 to 2.6 × 104, as the assembled Ag nanowire layer increases from monolayer to three layers, respectively. We attribute the homogeneous SERS signal to the high density of “hot spots” arising from the vertical and lateral gaps within the woodpile layers. The SERS signals plateau off when the number of layers increase from three to five, which can be attributed to limited laser penetration depth. The assembled multilayered silver nanowires demonstrate a larger SERS depth cross section and angle-independent SERS intensity, making such woodpile 3D SERS substrate more reliable and versatile for future sensing applications.
Co-reporter:Srikanth Pedireddy, Anran Li, Michel Bosman, In Yee Phang, Shuzhou Li, and Xing Yi Ling
The Journal of Physical Chemistry C 2013 Volume 117(Issue 32) pp:16640-16649
Publication Date(Web):July 23, 2013
DOI:10.1021/jp4063077
Spiky nanoparticles exhibit higher overall plasmonic excitation cross sections than their nonspiky peers. In this work, we demonstrate a two-step seed-mediated growth method to synthesize a new class of spiky Ag–Au octahedral nanoparticles with the aid of a high molecular weight poly(vinylpyrrolidone) polymer. The length of the nanospikes can be controlled from 10 to 130 nm with sharp tips by varying the amount of gold precursor added and the injection rates. Spatially resolved electron energy-loss spectroscopy (EELS) study and finite-difference time-domain (FDTD) simulations on individual spiky Ag–Au nanoparticles illustrate multipolar plasmonic responses. While the octahedral core retains its intrinsic plasmon response, the spike exhibits a hybridized dipolar surface plasmon resonance at lower energy. With increasing spike length from 50 to 130 nm, the surface plasmon of the spike can be tuned from 1.16 to 0.78 eV. The electric field at the spike region increases rapidly with increasing spike length, with a 104 field enhancement achieved at the tips of 130-nm spike. The results highlight that it is important to synthesize long spikes (>50 nm) on nanoparticles to achieve strong electric field enhancement. A hypothesis for the formation of sharp spikes is proposed based on our studies using X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and high resolution transmission electron microscopy (TEM).
Co-reporter:Qi Zhang, Yih Hong Lee, In Yee Phang, Srikanth Pedireddy, Weng Weei Tjiu, and Xing Yi Ling
Langmuir 2013 Volume 29(Issue 41) pp:12844-12851
Publication Date(Web):September 18, 2013
DOI:10.1021/la403067h
We demonstrate the creation of Ag-based bimetallic platonic Janus nanostructures by confining galvanic replacement reaction at a nanoscale interface on highly symmetrical nanostructures such as Ag nanocubes and nanooctahedra using reactive microcontact printing (μCP). The extent of galvanic replacement reaction can be controlled kinetically to derive Janus nanostructures with Au nanodots deposited on either one or multiple facets of Ag nanocubes. The selective deposition of Au dots on a single facet of Ag nanocubes breaks the cubic symmetry and brings about unique and anisotropic plasmonic responses. High-resolution cathodoluminescence hyperspectral imaging of single Janus nanocube demonstrates that surface plasmon resonances corresponding to Au and Ag can be excited at different spots on one Janus nanocube. In addition, we demonstrate the fabrication of alternating Janus/non-Janus segments on 2D Ag nanowires by using a line-patterned polydimethylsiloxane (PDMS) stamp for galvanic replacement. Aside from Au, Pt and Pd can also be selectively deposited onto Ag nanocubes. These Janus nanostructures may find important applications in the field of plasmon-enhanced catalysis.
Co-reporter:Madeline Shuhua Goh, Yih Hong Lee, Srikanth Pedireddy, In Yee Phang, Weng Weei Tjiu, Joel Min Rui Tan, and Xing Yi Ling
Langmuir 2012 Volume 28(Issue 40) pp:14441-14449
Publication Date(Web):September 12, 2012
DOI:10.1021/la302795r
The effective number of surface-enhanced Raman spectroscopy (SERS) active hot spots on plasmonic nanostructures is the most crucial factor in ensuring high sensitivity in SERS sensing platform. Here we demonstrate a chemical etching method to increase the surface roughness of one-dimensional Ag nanowires, targeted at creating more SERS active hot spots along Ag nanowire’s longitudinal axis for increased SERS detection sensitivity. Silver nanowires were first synthesized by the conventional polyol method and then subjected to chemical etching by NH4OH and H2O2 mixture. The surfaces of silver nanowires were anisotropically etched off to create miniature “beads on a string” features with increased surface roughness while their crystallinity was preserved. Mapping of single-nanowire SERS measurements showed that the chemical etching method has overcome the limitation of conventional one-dimensional Ag nanowires with limited SERS active area at the tips to produce etched Ag nanowires with an increase in Raman hot spots and polarization-independent SERS signals across tens of micrometers length scale.
Co-reporter:Joel Ming Rui Tan, Justina Jiexin Ruan, Hiang Kwee Lee, In Yee Phang and Xing Yi Ling
Physical Chemistry Chemical Physics 2014 - vol. 16(Issue 48) pp:
Publication Date(Web):
DOI:10.1039/C4CP03679D
Co-reporter:Yue-E Miao, Hiang Kwee Lee, Wee Shern Chew, In Yee Phang, Tianxi Liu and Xing Yi Ling
Chemical Communications 2014 - vol. 50(Issue 44) pp:NaN5926-5926
Publication Date(Web):2014/04/15
DOI:10.1039/C4CC01949K
Ag nanowire-based catalytic liquid marbles are fabricated as miniature reactors, which demonstrate highly efficient, support-free and rate-controllable heterogeneous degradation of methylene blue, with catalytic efficiency close to 100%. Our miniature catalytic liquid marbles are essential for reactions involving highly toxic/hazardous or costly reactants, where small volume preliminary reactions are preferred.
Co-reporter:Yan Cui, In Yee Phang, Yih Hong Lee, Mian Rong Lee, Qi Zhang and Xing Yi Ling
Chemical Communications 2015 - vol. 51(Issue 25) pp:NaN5366-5366
Publication Date(Web):2014/11/24
DOI:10.1039/C4CC08596E
We demonstrate a multiplex plasmonic anti-counterfeiting platform by embedding multiple molecules on a single substrate with superior nanometer scale spectral and spatial resolution. The encoded specific molecular information can be read out only using Raman spectroscopy. Multiplexing increases the complexity of these plasmonic security labels without compromising the spectral resolution.
Co-reporter:Y. Liu, Y. H. Lee, Q. Zhang, Y. Cui and X. Y. Ling
Journal of Materials Chemistry A 2016 - vol. 4(Issue 19) pp:NaN4319-4319
Publication Date(Web):2016/03/29
DOI:10.1039/C6TC00682E
A major challenge in information security and the development of an anti-counterfeiting platform is to encode multiple identification features on a single platform where these features can be decoded with no interference. Here, we demonstrate a progressively complex anti-counterfeiting platform using a multiplex fabrication strategy. This multiplex strategy enabled us to realize a spatially selective encapsulation of dye molecules within an Ag nanopillar array embedding covert molecular information which was revealed using fluorescence, surface-enhanced Raman scattering (SERS), and signal intensities. A total of five identification layers were used to authenticate products in our nanopillar platform. Moreover, two spectroscopic techniques were required to fully decode the various covert layers encoded within the same nanopillar array, thereby greatly enhancing the security of the information. Hyperspectral imaging was used to precisely generate unique SERS fingerprints of molecules encapsulated in each nanopillar. This feature combined with the high ∼17000 pillars per inch (ppi) information density of the platform make its use extremely effective against counterfeiting and forgery. In summary, our encoding platform enables high security, large information density and low-error decoding.
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