single-nanoparticle electrochemical sers microscopy and spectroscopy measurements

?Analyzing the Heterogeneous Performance of Single Nanoparticles with Electrochemical Methods

Electrochemistry is an important measurement science for studying charge transfer, charge storage, mass transport, etc., with applications in diverse disciplines, including biology, chemistry, physics, and engineering1,2,3,4,5,6,7. Conventionally, electrochemistry involves measurements over an ensemble &#8212;&#160;a&#160;large collection of single entities such as molecules, crystalline domains, nanoparticles, and surface sites. However, understanding how such single entities contribute to ensemble-averaged responses is key for bringing forth new fundamental and mechanistic understandings in chemistry and related fields because of the heterogeneity of electrode surfaces in complex electrochemical environments8,9. For example, ensemble reduction has revealed site-specific reduction/oxidation potentials10, the formation of intermediates and minor catalysis&#160;products11, site-specific reaction kinetics12,13, and charge carrier dynamics14,15. Reducing ensemble averaging is particularly important in improving our understanding beyond model systems to applied systems, such as biological cells, electrocatalysis, and batteries, in which extensive heterogeneity is often found16,17,18,19,20,21,22.
In the past decade or so, there has been an emergence of techniques to study single-entity electrochemistry1,2,9,10,11,12. These electrochemical measurements have provided the capabilities to measure small electrical and ionic currents in several systems and revealed new fundamental chemical and physical characteristics23,24,25,26,27,28. However, electrochemical measurements do not provide information about the identity or structure of molecules or intermediates at the electrode surface29,30,31,32. Chemical information at the electrode-electrolyte interface is central to understanding electrochemical reactions. Interfacial chemical knowledge is typically obtained by coupling electrochemistry with spectroscopy31,32. Vibrational spectroscopy, such as Raman scattering, is well-suited to provide&#160;complementary chemical information on charge transfer and related events in electrochemical systems that predominately utilize, but are not limited to, aqueous solvents30. Coupled with microscopy, Raman scattering spectroscopy provides spatial resolution down to the diffraction limit of light33,34. Diffraction presents a limitation, however, because nanoparticles and active surface sites are smaller in length than optical diffraction limits, which, thus, precludes the study of individual entities35.
Surface-enhanced Raman scattering (SERS) has been demonstrated to be a powerful tool in studying interfacial chemistry in electrochemical reactions20,30,36,37,38. In addition to providing the vibrational modes of reactant molecules, solvent molecules, additives and the surface chemistries of electrodes, SERS provides a signal that is localized to the surface of materials that support collective surface electron oscillations, known as localized surface plasmon resonances. The excitation of plasmon resonances leads to the concentration of electromagnetic radiation at the surface of the metal, thus increasing both the flux of light to and the Raman scattering from surface adsorbates. Nanostructured noble metals such as Ag and Au are commonly used plasmonic materials because they support visible light plasmon resonances, which are desirable for detecting emission with highly sensitive and efficient charge-coupled devices. Although the largest enhancements in SERS come from aggregates of nanoparticles39,40, a new SERS substrate has been developed that allows SERS measurements from individual nanoparticles: gap-mode SERS substrate&#160;(Figure 1)41,42. In gap-mode SERS substrates, a metallic mirror is fabricated and coated with an analyte. Next, nanoparticles are dispersed over the substrate. When irradiated with circularly polarized laser light, a dipolar plasmon resonance formed by the coupling of the nanoparticle and substrate is excited, which enables SERS measurements on single nanoparticles. SERS emission is coupled to the dipolar plasmon resonance43,44,45, which is oriented along the optical axis. With the parallel alignment of the radiating electric dipole and collection optics, only high-angle emission is collected, thus forming distinct donut-shaped emission patterns46,47,48,49 and allowing the identification of single nanoparticles. Aggregates of nanoparticles on the substrate contain radiating dipoles that are not parallel to the optical axis50. In this latter case, low-angle and high-angle emissions are collected and form solid emission patterns46.
Here, we describe a protocol for fabricating gap-mode SERS substrates and a procedure to employ them as working electrodes to monitor electrochemical redox events on single Ag nanoparticles using SERS. Importantly, the protocol using gap-mode SERS substrates allows for the unambiguous identification of single nanoparticles by SERS imaging, which is a key challenge for current methodologies in single nanoparticle electrochemistry. As a model system, we demonstrate the use of SERS to provide a readout of the electrochemical reduction and oxidation of Nile Blue A (NB) on a single Ag nanoparticle driven by a scanning or stepped potential (i.e., cyclic voltammetry, chronoamperometry). NB undergoes a multi-proton, multi-electron reduction/oxidation reaction in which its electronic structure is modulated out of/in resonance with the excitation source, which provides a contrast in the corresponding SERS spectra10,51,52. The protocol described here is also applicable to non-resonant redox-active molecules and electrochemical techniques, which may be pertinent to applications such as electrocatalysis.

Author Spotlight: Tracking Electrochemistry on Single Nanoparticles with Surface-Enhanced Raman Scattering Spectroscopy and Microscopy  

Tracking Electrochemistry on Single Nanoparticles with Surface-Enhanced Raman Scattering Spectroscopy and Microscopy

A common theme in our research is using light matter interactions and electrical energy to measure, drive, and control interfacial chemical transformations at the nanoscale. In particular, we seek to understand how local environments and reaction intermediates affect selectivity in electric catalysis. Catalytic transformations are traditionally evaluated with ensemble average measurements of the products and the catalyst attributes.The frontier challenges to produce chemical products selectively include reducing this measurement averaging while maintaining the measurement sensitivity, so that we can better understand the actions of individual molecules in the reactive sites of the catalyst. Electrochemical techniques alone do not provide any chemical information on species that form and transform at the electrode surface. Our protocol enables electrochemical measurements at a single nanoparticle using vibrational spectroscopy as a readout, enabling correlations between electrochemical processes and molecular changes.Moving forward, our laboratory will continue to push the boundaries of measurement resolution and space and time to understand chemical and material processes at the single-molecule level and at the timescale of chemical reactions.

Watch this Scientific Journal Video about Single-Nanoparticle Electrochemical SERS Microscopy and Spectroscopy Measurements at JoVE.com

Single-Nanoparticle Electrochemical SERS Microscopy and Spectroscopy Measurements  

Place the electrochemical cell prepared with the gap mode SERS substrate on the stage of an inverted optical microscope. Secure the substrate to the microscope stage by taping the edges to prevent movement during spectro electrochemical measurements due to the tension of the wires connecting the cell to the potentiostat. Place the silver-silver chloride reference electrode into the home-built stand and fix its position by tightening the screw on the electrode holder stand.Attach the reference electrode to the potentiostat's reference electrode alligator clip. Then attach the platinum wire counter electrode to potentiostat's counter electrode alligator clip. Finally, clip the copper wire attached to the silver film to the potentiostat's working electrode alligator clip.Insert the platinum wire and alligator clip into the electrode holder and tighten the screw to secure its position. Place the electrode holder over the electrochemical cell to insert the electrodes. Then turn on the 642-nanometer laser and adjust the power to 500 microwatts.Next, add a drop of immersion oil to the objective lens. Then, move the focus knob to raise the objective until the oil contacts the bottom of the substrate. Focus the laser onto the surface of the gap mode SERS substrate.After removing one of the eye pieces from the microscope, insert the adapter in its place. Change the mode to video on the camera application and zoom in as much as possible. Scan the gap mode SERS substrate by moving the microscope stage to search for an isolated donut-shaped SERS emission pattern.Once the donut-shaped emission pattern is located, move the light diverter lever of the microscope to direct the admitted light to the spectrometer. Set the grading position to 1000 wave numbers to detect Stokes shifted ramen scattering from 400 to 1600 wave number region. Keeping the laser light focused on the donut-shaped emission pattern, add 3 milliliters of a 0.1-molar phosphate buffer of pH 5 into the electrochemical cell.In the potentiostat's software prepare a cyclic voltammogram experiment with at least three cycles from 0 to minus 0.6 volts versus silver-silver chloride and a scan rate of 50 millivolts per second. Then, run the simultaneous cyclic voltammetry and SERS experiments. Finally, move the light diverter lever, so that the light is directed to the phone camera and start recording a video while running the cyclic voltammetry experiment.Individual silver nanoparticles on the silver thin film can be unambiguously identified by a donut-shaped emission pattern in contrast to a solid emission pattern produced by nanoparticle dimers, trimers, or multimers. The SERS cyclic voltammograms were measured for single nanoparticles, and the Nile blue molecules in and around the gap between the silver nanoparticle and the silver film were electrochemically reduced. Spectroelectrochemical measurements were done with the same applied potential range.Electrochemical modulation of the Nile blue SERS spectrum by stepping the potential causes the peak intensity at the 592 wave number region to decrease with time due to the reduction of the Nile blue molecules. The magnitude of the electric bias altered the reduction kinetics, as evidenced by the decay of the area under the 592 wave number region peak.

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Research

JoVE Journal

Chemistry

181 Views.  University of Louisville. Place the electrochemical cell prepared with the gap mode SERS substrate on the stage of an inverted optical microscope. Secure the substrate to the microscope stage by taping the edges to prevent movement during spectro electrochemical measurements due to the tension of the wires connecting the cell to the potentiostat. Place the silver-silver chloride reference electrode into the home-built stand and fix its position by tightening the screw on the electrode holder stand.Attach the re...