This work was supported by start-up funds from the University of Louisville and funding from Oak Ridge Associated Universities through a Ralph E. Powe Junior Faculty Enhancement Award. The authors thank Dr. Ki-Hyun Cho for creating the image in Figure 1. The metal deposition and SEM were performed at the Micro/Nano Technology Center at the University of Louisville.

﻿Analyzing the Heterogeneous Performance of Single Nanoparticles with Electrochemical Methods

The authors declare that they have no competing financial interests.

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...

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.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>Acetone, microelectronic grade</td>
			<td>J. T. Baker</td>
			<td>9005-05</td>
			<td></td>
		</tr>
		<tr>
			<td>Adjustable pipette, Eppendorf Reference 2 5000 mL</td>
			<td>Eppendorf</td>
			<td>4924000100</td>
			<td></td>
		</tr>
		<tr>
			<td>Analytical Balance, AB54-S/FACT</td>
			<td>Metter Toledo</td>
			<td>N.A.</td>
			<td></td>
		</tr>
		<tr>
			<td>Atomic Force Microscope, Easy scan 2</td>
			<td>Nanosurf</td>
			<td>N.A.</td>
			<td></td>
		</tr>
		<tr>
			<td>AXXIS Electron Beam Thin Film Deposition System</td>
			<td>Kurt J. Lesker</td>
			<td>N.A.</td>
			<td></td>
		</tr>
		<tr>
			<td>Cary 60 UV-Vis Spectrophotometer</td>
			<td>Agilent</td>
			<td>N.A.</td>
			<td></td>
		</tr>
		<tr>
			<td>Conductive epoxy, two part</td>
			<td>Electron Microscopy Sciences</td>
			<td>12642-14</td>
			<td></td>
		</tr>
		<tr>
			<td>Copper pellets, 99.99% pure</td>
			<td>Kurt J. Lesker</td>
			<td>EVMCU40EXE</td>
			<td></td>
		</tr>
		<tr>
			<td>Copper wire, bare, 18 AWG</td>
			<td>VWR</td>
			<td>66248-040</td>
			<td></td>
		</tr>
		<tr>
			<td>Crucible, Graphite E-Beam</td>
			<td>Kurt J. Lesker</td>
			<td>EVCEB-23</td>
			<td></td>
		</tr>
		<tr>
			<td>Diamond Scriber</td>
			<td>Ted Pella</td>
			<td>54484</td>
			<td></td>
		</tr>
		<tr>
			<td>EMCCD Camera, ProEM HS: 1024BX3</td>
			<td>Teledyne Princeton Instruments</td>
			<td>N.A.</td>
			<td></td>
		</tr>
		<tr>
			<td>Epoxy, Clear</td>
			<td>Gorilla Glue</td>
			<td>N.A.</td>
			<td></td>
		</tr>
		<tr>
			<td>Glass Tube Cutter</td>
			<td>Wheeler-Rex</td>
			<td>69012</td>
			<td></td>
		</tr>
		<tr>
			<td>Glass Tube, Borossilicate (OD 0.75&#34;, ID 0.62&#34;, L 12&#34;)</td>
			<td>McMaster-Carr</td>
			<td>8729K45</td>
			<td></td>
		</tr>
		<tr>
			<td>Immersion oil, Type-F</td>
			<td>Olympus</td>
			<td>IMMOIL-F30CC</td>
			<td></td>
		</tr>
		<tr>
			<td>Inverted Microscope, IX73</td>
			<td>Olympus</td>
			<td>N.A.</td>
			<td></td>
		</tr>
		<tr>
			<td>Laser, Excelsior One 642 nm Free space</td>
			<td>Spectra-Physics</td>
			<td>N.A.</td>
			<td></td>
		</tr>
		<tr>
			<td>LightField</td>
			<td>Teledyne Princeton Instruments</td>
			<td>N.A.</td>
			<td></td>
		</tr>
		<tr>
			<td>MATLAB 2022b</td>
			<td>MathWorks</td>
			<td>N.A.</td>
			<td></td>
		</tr>
		<tr>
			<td>Micro cover glass (coverslips), 24&#215;60 mm No. 1</td>
			<td>VWR</td>
			<td>48404-455</td>
			<td></td>
		</tr>
		<tr>
			<td>Microscope Smartphone Camera Adapter</td>
			<td>qhma</td>
			<td>QHMC017A-S01</td>
			<td></td>
		</tr>
		<tr>
			<td>Nile Blue A, pure</td>
			<td>Acros Organics</td>
			<td>415690100</td>
			<td></td>
		</tr>
		<tr>
			<td>Nitrogen, Ultra Pure, Compressed</td>
			<td>Specialty Gases</td>
			<td>N.A.</td>
			<td></td>
		</tr>
		<tr>
			<td>Objective, UPLanXApo 100&#215; Oil Immersion</td>
			<td>Olympus</td>
			<td>14-910</td>
			<td></td>
		</tr>
		<tr>
			<td>Polyimide Film, Kapton</td>
			<td>3M</td>
			<td>16089-4</td>
			<td></td>
		</tr>
		<tr>
			<td>Potassium Phosphate Monobasic</td>
			<td>VWR</td>
			<td>P285</td>
			<td></td>
		</tr>
		<tr>
			<td>Potentiostat, 660E&#160;</td>
			<td>CH Instruments</td>
			<td>N.A.</td>
			<td></td>
		</tr>
		<tr>
			<td>Pt wire</td>
			<td>Alfa Aesar</td>
			<td>10956-BS</td>
			<td></td>
		</tr>
		<tr>
			<td>Scanning Electron Microscope, Apreo C SEM</td>
			<td>Thermo Fischer Scientific</td>
			<td>N.A.</td>
			<td></td>
		</tr>
		<tr>
			<td>Si wafer</td>
			<td>Ted Pella</td>
			<td>16006</td>
			<td></td>
		</tr>
		<tr>
			<td>Silver nanoparticles (nanospheres), NanoXact 0.02 mg/mL in 2 mM citrate</td>
			<td>nanoComposix</td>
			<td>AGCN60</td>
			<td></td>
		</tr>
		<tr>
			<td>Silver pellets, 99.99% pure</td>
			<td>Kurt J. Lesker</td>
			<td>EVMAG40EXE-A</td>
			<td></td>
		</tr>
		<tr>
			<td>Slide Rack, Wash-N-Dry</td>
			<td>Diversified Biotech</td>
			<td>WSDR-2000</td>
			<td></td>
		</tr>
		<tr>
			<td>Smartphone, iPhone 13 mini</td>
			<td>Apple</td>
			<td>N.A.</td>
			<td></td>
		</tr>
		<tr>
			<td>Sodium Phosphate Dibasic Heptahydrate</td>
			<td>VWR</td>
			<td>0348</td>
			<td></td>
		</tr>
		<tr>
			<td>Spectrometer, IsoPlane SCT320</td>
			<td>Teledyne Princeton Instruments</td>
			<td>N.A.</td>
			<td></td>
		</tr>
		<tr>
			<td>Tissue Wipers, Light-duty&#160;</td>
			<td>VWR</td>
			<td>82003-820</td>
			<td></td>
		</tr>
		<tr>
			<td>Tweezers, KS-04</td>
			<td>Kaisi Hardware</td>
			<td>N.A.</td>
			<td></td>
		</tr>
		<tr>
			<td>Utrasonic Generator, sweepSONIK</td>
			<td>Blackstone-NEY Ultrasonics</td>
			<td>809379</td>
			<td></td>
		</tr>
		<tr>
			<td>Water Ultrapurifier, Sartorius Arium mini</td>
			<td>Sartorius</td>
			<td>N.A.</td>
			<td></td>
		</tr>
	</tbody>
</table>

tracking electrochemistry on single nanoparticles with surface-enhanced raman scattering spectroscopy and microscopy

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.

Figure 2A shows Ag thin film substrates prepared using an electron beam metal deposition system. The &#34;good&#34; substrate shown in Figure 2A has a homogenous coverage of Ag metal over the glass coverslip, while the &#34;bad&#34; substrate has a non-uniform coverage of Ag. The ultraviolet-visible spectrum of the &#34;good&#34; Ag thin film is shown in Figure 2B, which demonstrates that the film is partially transparent for the vi...

Watch this Scientific Journal Video about Tracking Electrochemistry on Single Nanoparticles with Surface-Enhanced Raman Scattering Spectroscopy and Microscopy at JoVE.com

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

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

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

Studying electrochemical reactions on single nanoparticles is important to understand the heterogeneous performance of individual nanoparticles. This ...

tracking-electrochemistry-on-single-nanoparticles-with-surface

Universidad San Sebastian

Research

JoVE Journal

Chemistry

2.0K Views.  University of Louisville. The protocol describes how to monitor electrochemical events on single nanoparticles using surface-enhanced Raman scattering spectroscopy and imaging.

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

A Filter-based Surface Enhanced Raman Spectroscopic Assay for Rapid Detection of Chemical Contaminants

We demonstrate a method to fabricate highly sensitive surface-enhanced Raman spectroscopic (SERS) substrates using a filter syringe system that can be applied to the detection of various chemical contaminants. Silver nanoparticles (Ag NPs) are synthesized via reduction of silver nitrate by sodium citrate. Then the NPs are aggregated by sodium chloride to form nanoclusters that could be trapped in the pores of the filter membrane. A syringe is connected to the filter holder, with a filter membrane inside. By loading the nanoclusters into the syringe and passing through the membrane, the liquid goes through the membrane but not the nanoclusters, forming a SERS-active membrane. When testing the analyte, the liquid sample is loaded into the syringe and flowed through the Ag NPs coated membrane. The analyte binds and concentrates on the Ag NPs coated membrane. Then the membrane is detached from the filter holder, air dried and measured by a Raman instrument. Here we present the study of the volume effect of Ag NPs and sample on the detection sensitivity as well as the detection of 10 ppb ferbam and 1 ppm ampicillin using the developed assay.

A procedure for fabrication and performing the filter-based surface enhanced Raman spectroscopic (SERS) assay for the detection of chemical contaminants (i.e., pesticide ferbam and antibiotic ampicillin) is presented.

We demonstrate a method to fabricate highly sensitive surface-enhanced Raman spectroscopic (SERS) substrates using a filter syringe system that can be ...

Functionalization of Single-walled Carbon Nanotubes with Thermo-reversible Block Copolymers and Characterization by Small-angle Neutron Scattering

We demonstrate a protocol for single-walled carbon nanotube functionalization using thermo-sensitive PEO-PPO-PEO triblock copolymers in an aqueous solution. In a carbon nanotube/PEO105-PPO70-PEO105 (poloxamer 407) aqueous solution, the amphiphilic poloxamer 407 adsorbs onto the carbon nanotube surfaces and self-assembles into continuous layers, driven by intermolecular interactions between constituent molecules. The addition of 5-methylsalicylic acid changes the self-assembled structure from spherical-micellar to a cylindrical morphology. The fabricated poloxamer 407/carbon nanotube hybrid particles exhibit thermo-responsive structural features so that the density and thickness of poloxamer 407 layers are also reversibly controllable by varying temperature. The detailed structural properties of the poloxamer 407/carbon nanotube particles in suspension can be characterized by small-angle neutron scattering experiments and model fit analyses. The distinct curve shapes of the scattering intensities depending on temperature control or addition of aromatic additives are well described by a modified core-shell cylinder model consisting of a carbon nanotube core cylinder, a hydrophobic shell, and a hydrated polymer layer. This method can provide a simple but efficient way for the fabrication and in-situ characterization of carbon nanotube-based nano particles with a structure-tunable encapsulation.

A method for the functionalization of carbon nanotubes with structure-tunable polymeric encapsulation layers and structural characterization using small-angle neutron scattering is presented.

We demonstrate a protocol for single-walled carbon nanotube functionalization using thermo-sensitive PEO-PPO-PEO triblock copolymers in an aqueous ...

Liquid-cell Transmission Electron Microscopy for Tracking Self-assembly of Nanoparticles

Drying a nanoparticle dispersion is a versatile way to create self-assembled structures of nanoparticles, but the mechanism of this process is not fully understood. We have traced the trajectories of individual nanoparticles using liquid-cell transmission electron microscopy (TEM) to investigate the mechanism of the assembly process. Herein, we present the protocols used for liquid-cell TEM studies of the self-assembly mechanism. First, we introduce the detailed synthetic protocols used to produce uniformly sized platinum and lead selenide nanoparticles. Next, we present the microfabrication processes used to produce liquid cells with silicon nitride or silicon windows and then describe the loading and imaging procedures of the liquid-cell TEM technique. Several notes are included to provide helpful tips for the entire process, including how to manage the fragile cell windows. The individual motions of nanoparticles tracked by liquid-cell TEM revealed that changes in the solvent boundaries caused by evaporation affected the self-assembly process of nanoparticles. The solvent boundaries drove nanoparticles to primarily form amorphous aggregates, followed by flattening of the aggregates to produce a 2-dimensional (2D) self-assembled structure. These behaviors are also observed for different nanoparticle types and different liquid-cell compositions.

Here we introduce experimental protocols for the real-time observation of a self-assembly process using liquid-cell transmission electron microscopy.

Drying a nanoparticle dispersion is a versatile way to create self-assembled structures of nanoparticles, but the mechanism of this process is not fully ...

Observation and Analysis of Blinking Surface-enhanced Raman Scattering

From a single molecule at a silver nanoaggregate junction, blinking surface-enhanced Raman scattering (SERS) is observed. Here, a protocol is presented on how to prepare the SERS-active silver nanoaggregate, record a video of certain blinking spots in the microscopic image, and analyze the blinking statistics. In this analysis, a power law reproduces the probability distributions for bright events relative to their duration. The probability distributions for dark events are fitted by a power law with an exponential function. The parameters of the power law represent molecular behavior in both bright and dark states. The random walk model and the speed of the molecule across the entire silver surface can be estimated. It is difficult to estimate even when using averages, autocorrelation functions, and super-resolution SERS imaging. In the future, power law analyses should be combined with spectral imaging, because the origins of blinking cannot be confirmed by this analysis method alone.

This protocol describes the analysis of blinking surface-enhanced Raman scattering due to the random walk of a single molecule on a silver surface using power laws.

From a single molecule at a silver nanoaggregate junction, blinking surface-enhanced Raman scattering (SERS) is observed. Here, a protocol is presented on ...

Probing the Structure and Dynamics of Interfacial Water with Scanning Tunneling Microscopy and Spectroscopy

Water/solid interfaces are ubiquitous and play a key role in many environmental, biophysical, and technological processes. Resolving the internal structure and probing the hydrogen-bond (H-bond) dynamics of the water molecules adsorbed on solid surfaces are fundamental issues of water science, which remains a great challenge owing to the light mass and small size of hydrogen. Scanning tunneling microscopy (STM) is a promising tool for attacking these problems, thanks to its capabilities of sub-&#197;ngstr&#246;m spatial resolution, single-bond vibrational sensitivity, and atomic/molecular manipulation. The designed experimental system consists of a Cl-terminated tip and a sample fabricated by dosing water molecules in situ onto the Au(111)-supported NaCl(001) surfaces. The insulating NaCl films electronically decouple the water from the metal substrates, so the intrinsic frontier orbitals of water molecules are preserved. The Cl-tip facilitates the manipulation of the single water molecules, as well as gating the orbitals of water to the proximity of Fermi level (EF) via tip-water coupling. This paper outlines the detailed methods of submolecular resolution imaging, molecular/atomic manipulation, and single-bond vibrational spectroscopy of interfacial water. These studies open up a new route for investigating the H-bonded systems at the atomic scale.

Here, we present a protocol to investigate the structure and dynamics of interfacial water at the atomic scale, in terms of submolecular resolution imaging, molecular manipulation, and single-bond vibrational spectroscopy.

Water/solid interfaces are ubiquitous and play a key role in many environmental, biophysical, and technological processes. Resolving the internal ...

High Resolution Physical Characterization of Single Metallic Nanoparticles

Individual molecules can be detected and characterized by measuring the degree by which they reduce the ionic current flowing through a single nanometer-scale pore. The signal is characteristic of the molecule&#39;s physicochemical properties and its interactions with the pore. We demonstrate that the nanopore formed by the bacterial protein exotoxin Staphylococcus aureus alpha hemolysin (&#945;HL) can detect polyoxometalates (POMs, anionic metal oxygen clusters), at the single molecule limit. Moreover, multiple degradation products of 12-phosphotungstic acid POM (PTA, H3PW12O40) in solution are simultaneously measured. The single molecule sensitivity of the nanopore method allows for POMs to be characterized at significantly lower concentrations than required for nuclear magnetic resonance (NMR) spectroscopy. This technique could serve as a new tool for chemists to study the molecular properties of polyoxometalates or other metallic clusters, to better understand POM synthetic processes, and possibly improve their yield. Hypothetically, the location of a given atom, or the rotation of a fragment in the molecule, and the metal oxidation state could be investigated with this method. In addition, this new technique has the advantage of allowing the real-time monitoring of molecules in solution.&#160;

Here, we present a protocol to detect discrete metal oxygen clusters, polyoxometalates (POMs), at the single molecule limit using a biological nanopore-based electronic platform. The method provides a complementary approach to traditional analytical chemistry tools used in the study of these molecules.

Individual molecules can be detected and characterized by measuring the degree by which they reduce the ionic current flowing through a single ...

Performing Spectroscopy on Plasmonic Nanoparticles with Transmission-Based Nomarski-Type Differential Interference Contrast Microscopy

Differential interference contrast (DIC) microscopy is a powerful imaging tool that is most commonly employed for imaging microscale objects using visible-range light. The purpose of this protocol is to detail a proven method for preparing plasmonic nanoparticle samples and performing single particle spectroscopy on them with DIC microscopy. Several important steps must be followed carefully in order to perform repeatable spectroscopy experiments. First, landmarks can be etched into the sample substrate, which aids in locating the sample surface and in tracking the region of interest during experiments. Next, the substrate must be properly cleaned of debris and contaminants that can otherwise hinder or obscure examination of the sample. Once a sample is properly prepared, the optical path of the microscope must be aligned, using Kohler Illumination. With a standard Nomarski style DIC microscope, rotation of the sample may be necessary, particularly when the plasmonic nanoparticles exhibit orientation-dependent optical properties. Because DIC microscopy has two inherent orthogonal polarization fields, the wavelength-dependent DIC contrast pattern reveals the orientation of rod-shaped plasmonic nanoparticles. Finally, data acquisition and data analyses must be carefully performed. It is common to represent DIC-based spectroscopy data as a contrast value, but it is also possible to present it as intensity data. In this demonstration of DIC for single particle spectroscopy, the focus is on spherical and rod-shaped gold nanoparticles.

The goal of this protocol is to detail a proven approach for the preparation of plasmonic nanoparticle samples and for performing single particle spectroscopy on them with differential interference contrast (DIC) microscopy.

Differential interference contrast (DIC) microscopy is a powerful imaging tool that is most commonly employed for imaging microscale objects using ...

In situ FTIR Spectroscopy as a Tool for Investigation of Gas/Solid Interaction: Water-Enhanced CO2 Adsorption in UiO-66 Metal-Organic Framework

In situ infrared spectroscopy is an inexpensive, highly sensitive, and selective valuable tool to investigate the interaction of polycrystalline solids with adsorbates. Vibrational spectra provide information on the chemical nature of adsorbed species and their structure. Thus, they are very useful for obtaining molecular level understanding of surface species. The IR spectrum of the sample itself gives some direct information about the material. General conclusions can be drawn concerning hydroxyl groups, some stable surface species and impurities. However, the spectrum of the sample is &#34;blind&#34; with respect to the presence of coordinatively unsaturated ions and gives rather poor information about the acidity of surface hydroxyls, species decisive for the adsorption and catalytic properties of the materials. Furthermore, no discrimination between bulk and surface species can be made. These problems are solved by the use of probe molecules, substances that interact specifically with the surface; the alteration of some spectral features of these molecules as a result of adsorption provides valuable information about the nature, properties, location, concentration, etc., of the surface sites.
The experimental protocol for in-situ IR studies of gas/sample interaction includes preparation of a sample pellet, activation of the material, initial spectral characterization through the analysis of the background spectra, characterization by probe molecules, and study of the interaction with a particular set of gas mixtures. In this paper we investigate a zirconium terephthalate metal organic framework, Zr6O4(OH)4(BDC)6 (BDC = benzene-1,4-dicarboxylate), namely UiO-66 (UiO refers to University of Oslo). The acid sites of the UiO-66 sample are determined by using CO and CD3CN as molecular probes. Furthermore, we have demonstrated that CO2 is adsorbed on basic sites exposed on dehydroxylated UiO-66. Introduction of water to the system produces hydroxyl groups acting as additional CO2 adsorption sites. As a result, CO2 adsorption capacity of the sample is strongly enhanced.

In situ FTIR Spectroscopy as a Tool for Investigation of Gas/Solid Interaction: Water-Enhanced CO2 Adsorption in UiO-66 Metal-Organic Framework

The use of FTIR spectroscopy for investigation of the surface properties of polycrystalline solids is described. Preparation of sample pellets, activation procedures, characterization with probe molecules and model studies of CO2 adsorption are discussed.

In situ infrared spectroscopy is an inexpensive, highly sensitive, and selective valuable tool to investigate the interaction of polycrystalline solids ...

Atomic Force Microscopy Combined with Infrared Spectroscopy as a Tool to Probe Single Bacterium Chemistry

Atomic Force Microscopy-Infrared Spectroscopy (AFM-IR) is a novel combinatory technique, enabling simultaneous characterization of physical properties and chemical composition of sample with nanoscale resolution. By combining AFM with IR, the spatial resolution limitation of conventional IR is overcome, enabling a resolution of 20&#8211;100 nm to be achieved. This opens the door for a broad array of new applications of IR toward probing samples smaller than several micrometers, previously unachievable by means of conventional IR microscopy. AFM-IR is eminently suited for bacterial research, providing both spectral and spatial information at the single cell and intracellular level. The increasing global health concerns and unfavorable future prediction regarding bacterial infections, and especially, rapid development of antimicrobial resistance, has created an urgent need for a research tool capable of phenotypic probing at the single cell and subcellular level. AFM-IR offers the potential to address this need, by enabling detail characterization of chemical composition of a single bacterium. Here, we provide a complete protocol for sample preparation and data acquisition of single spectra and mapping modality, for the application of AFM-IR toward bacterial studies.

Atomic Force Microscopy-Infrared Spectroscopy (AFM-IR) provides a powerful platform for bacterial studies, enabling to achieve nanoscale resolution. Both, mapping of subcellular changes (e.g., upon cell division) as well as comparative studies of chemical composition (e.g., arising from drug resistance) can be conducted at a single cell level in bacteria.

Atomic Force Microscopy-Infrared Spectroscopy (AFM-IR) is a novel combinatory technique, enabling simultaneous characterization of physical properties and ...

Direct Comparison of Hyperspectral Stimulated Raman Scattering and Coherent Anti-Stokes Raman Scattering Microscopy for Chemical Imaging

Stimulated Raman scattering (SRS) and coherent anti-Stokes Raman scattering (CARS) microscopy are the most widely used coherent Raman scattering imaging technologies. Hyperspectral SRS and CARS imaging offer Raman spectral information at every pixel, which enables better separation of different chemical compositions. Although both techniques require two excitation lasers, their signal detection schemes and spectral properties are quite different. The goal of this protocol is to perform both hyperspectral SRS and CARS imaging on a single platform and compare the two microscopy techniques for imaging different biological samples. The spectral focusing method is employed to acquire spectral information using femtosecond lasers. By using standard chemical samples, the sensitivity, spatial resolution, and spectral resolution of SRS and CARS in the same excitation conditions (i.e., power at the sample, pixel dwell time, objective lens, pulse energy) are compared. The imaging contrasts of CARS and SRS for biological samples are juxtaposed and compared. The direct comparison of CARS and SRS performances would allow for optimal selection of the modality for chemical imaging.

This paper directly compares the resolution, sensitivity, and imaging contrasts of stimulated Raman scattering (SRS) and coherent anti-Stokes Raman scattering (CARS) integrated into the same microscope platform. The results show that CARS has a better spatial resolution, SRS gives better contrasts and spectral resolution, and both methods have similar sensitivity.

Stimulated Raman scattering (SRS) and coherent anti-Stokes Raman scattering (CARS) microscopy are the most widely used coherent Raman scattering imaging ...