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

Summary

The protocol describes how to monitor electrochemical events on single nanoparticles using surface-enhanced Raman scattering spectroscopy and imaging.

Abstract

Studying electrochemical reactions on single nanoparticles is important to understand the heterogeneous performance of individual nanoparticles. This nanoscale heterogeneity remains hidden during the ensemble-averaged characterization of nanoparticles. Electrochemical techniques have been developed to measure currents from single nanoparticles but do not provide information about the structure and identity of the molecules that undergo reactions at the electrode surface. Optical techniques such as surface-enhanced Raman scattering (SERS) microscopy and spectroscopy can detect electrochemical events on individual nanoparticles while simultaneously providing information on the vibrational modes of electrode surface species. In this paper, a protocol to track the electrochemical oxidation-reduction of Nile Blue (NB) on single Ag nanoparticles using SERS microscopy and spectroscopy is demonstrated. First, a detailed protocol for fabricating Ag nanoparticles on a smooth and semi-transparent Ag film is described. A dipolar plasmon mode aligned along the optical axis is formed between a single Ag nanoparticle and Ag film. The SERS emission from NB fixed between the nanoparticle and the film is coupled into the plasmon mode, and the high-angle emission is collected by a microscope objective to form a donut-shaped emission pattern. These donut-shaped SERS emission patterns allow for the unambiguous identification of single nanoparticles on the substrate, from which the SERS spectra can be collected. In this work, a method for employing the SERS substrate as a working electrode in an electrochemical cell compatible with an inverted optical microscope is provided. Finally, tracking the electrochemical oxidation-reduction of NB molecules on an individual Ag nanoparticle is shown. The setup and the protocol described here can be modified to study various electrochemical reactions on individual nanoparticles.

Introduction

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 engineering^{1,2,3,4,5,6,7}. Conventionally, electrochemistry involves measurements over an ensemble — a large collection of single entities such as molecules, crystalline domains, nanoparticles, and surface sites. However,....

Protocol

1. Gap-mode SERS substrate preparation

1. Clean No. 1 coverslips (see Table of Materials) using an acetone and water wash, as described below. Perform this step in a cleanroom to ensure that no debris or other unwanted matter is deposited onto the coverslips.
   1. Place the coverslips in a slide rack. Use tweezers when moving the coverslips/substrates. Place the slide rack in a glass container, and fill it with acetone.
      CAUTION: Acetone is highly flammable and has potential negative health effects. Handle it in a well-ventilated area using gloves, goggles, and a mask.
   2. Adjust the power control of the ....

Results

Figure 2A shows Ag thin film substrates prepared using an electron beam metal deposition system. The "good" substrate shown in Figure 2A has a homogenous coverage of Ag metal over the glass coverslip, while the "bad" substrate has a non-uniform coverage of Ag. The ultraviolet-visible spectrum of the "good" Ag thin film is shown in Figure 2B, which demonstrates that the film is partially transparent for the vi.......

Discussion

Depositing Cu and Ag thin metal films on clean coverslips is vital to ensure that the final film has a roughness no greater than two to four atomic layers (or a root mean square roughness less than or equal to around 0.7 nm). Dust, scratches, and debris present on the coverslip prior to metal deposition are common issues that prevent the fabrication of the smooth film required to produce donut-shaped emission patterns. Hence, it is recommended to sonicate the coverslips in different solvents before the metal deposition a.......

Materials

Name	Company	Catalog Number	Comments
Acetone, microelectronic grade	J. T. Baker	9005-05
Adjustable pipette, Eppendorf Reference 2 5000 mL	Eppendorf	4924000100
Analytical Balance, AB54-S/FACT	Metter Toledo	N.A.
Atomic Force Microscope, Easy scan 2	Nanosurf	N.A.
AXXIS Electron Beam Thin Film Deposition System	Kurt J. Lesker	N.A.
Cary 60 UV-Vis Spectrophotometer	Agilent	N.A.
Conductive epoxy, two part	Electron Microscopy Sciences	12642-14
Copper pellets, 99.99% pure	Kurt J. Lesker	EVMCU40EXE
Copper wire, bare, 18 AWG	VWR	66248-040
Crucible, Graphite E-Beam	Kurt J. Lesker	EVCEB-23
Diamond Scriber	Ted Pella	54484
EMCCD Camera, ProEM HS: 1024BX3	Teledyne Princeton Instruments	N.A.
Epoxy, Clear	Gorilla Glue	N.A.
Glass Tube Cutter	Wheeler-Rex	69012
Glass Tube, Borossilicate (OD 0.75", ID 0.62", L 12")	McMaster-Carr	8729K45
Immersion oil, Type-F	Olympus	IMMOIL-F30CC
Inverted Microscope, IX73	Olympus	N.A.
Laser, Excelsior One 642 nm Free space	Spectra-Physics	N.A.
LightField	Teledyne Princeton Instruments	N.A.
MATLAB 2022b	MathWorks	N.A.
Micro cover glass (coverslips), 24×60 mm No. 1	VWR	48404-455
Microscope Smartphone Camera Adapter	qhma	QHMC017A-S01
Nile Blue A, pure	Acros Organics	415690100
Nitrogen, Ultra Pure, Compressed	Specialty Gases	N.A.
Objective, UPLanXApo 100× Oil Immersion	Olympus	14-910
Polyimide Film, Kapton	3M	16089-4
Potassium Phosphate Monobasic	VWR	P285
Potentiostat, 660E	CH Instruments	N.A.
Pt wire	Alfa Aesar	10956-BS
Scanning Electron Microscope, Apreo C SEM	Thermo Fischer Scientific	N.A.
Si wafer	Ted Pella	16006
Silver nanoparticles (nanospheres), NanoXact 0.02 mg/mL in 2 mM citrate	nanoComposix	AGCN60
Silver pellets, 99.99% pure	Kurt J. Lesker	EVMAG40EXE-A
Slide Rack, Wash-N-Dry	Diversified Biotech	WSDR-2000
Smartphone, iPhone 13 mini	Apple	N.A.
Sodium Phosphate Dibasic Heptahydrate	VWR	0348
Spectrometer, IsoPlane SCT320	Teledyne Princeton Instruments	N.A.
Tissue Wipers, Light-duty	VWR	82003-820
Tweezers, KS-04	Kaisi Hardware	N.A.
Utrasonic Generator, sweepSONIK	Blackstone-NEY Ultrasonics	809379
Water Ultrapurifier, Sartorius Arium mini	Sartorius	N.A.