gap-mode surface-enhanced raman scattering (sers) substrate preparation

?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 Gap-Mode Surface-Enhanced Raman Scattering SERS Substrate Preparation at JoVE.com

Gap-Mode Surface-Enhanced Raman Scattering (SERS) Substrate Preparation  

To begin, deposit copper and silver onto the cleaned cover slips using the electron beam thin film deposition system following standard procedures as recommended by the manufacturer. For copper deposition, increase the emission current gradually at 10 milliamperes per minute until the sensor reads a deposition rate close to 10 angstroms per second. Once the desired deposition rate is achieved, close the shutter and set the platen position to zero degrees.Open the shutter to start the deposition process and monitor the thickness on the display of the deposition sensor. Close the shutter when the desired thickness for copper is reached. Then rotate the crucible holder using the knob to direct the beam towards the crucible containing silver pellets, and perform deposition as demonstrated.Next, add 500 microliters of a 50 micromolar Nile Blue solution onto the surface of the silver thin film. After 15 minutes, rinse the silver thin film thoroughly with ultrapure water to remove weakly absorbed Nile Blue molecules. Finally, dry the silver thin film with nitrogen gas.Drop cast 500 microliters of a 100-fold dilution of the silver nanoparticle colloid onto the same region of the silver thin film drop cast with Nile Blue Solution. After 20 minutes, rinse the gap mode surface-enhanced Raman scattering, or SERS substrate, with ultrapure water. Then dry the substrate using nitrogen gas.The ultraviolet visible spectrum of the good silver thin film demonstrates that the film is partially transparent for the visible portion of the electromagnetic spectrum. A representative AFM image of the good substrate is shown here. The variation in the height of the silver thin film substrate is represented by the line profile, demonstrating the uniformity and smoothness of the film.The SEM image of silver nanoparticles drop cast and air dried on a silicon wafer showed an average diameter of approximately 79.2 nanometers.

gap-mode-surface-enhanced-raman-scattering-sers-substrate-preparation

Research

JoVE Journal

Chemistry

253 ビュー.  University of Louisville. まず、メーカーが推奨する標準的な手順に従って、電子ビーム薄膜堆積システムを使用して、洗浄したカバースリップに銅と銀を堆積させます。銅蒸着の場合は、センサーが毎秒10オングストロームに近い堆積速度を読み取るまで、毎分10ミリアンペアで放射電流を徐々に増やします。希望の堆積速度が達成されたら、シャッターを閉じてプラテン位置を0度に設定します...