Influence of Charged Self-Assembled Monolayers on Single Nanoparticle Collision

Deciphering Complex Electrochemical Reaction Dynamics and Interactions of Single Nanoentities via Evanescent Scattering Microscopy

Electrochemical reactions at the nanoscale are governed by intricate surface interactions, yet existing imaging techniques often lack the surface sensitivity and throughput needed to resolve these dynamics clearly. Here, we introduce electrochemical evanescent scattering microscopy (EC-ESM), a high-throughput, surface-sensitive imaging technique that enables real-time tracking of single-nanoentity electrochemistry with high resolution. Using EC-ESM, we monitored the motion and dissolution dynamics of silver nanoparticles and identified a clear relationship between nanoparticle velocity and electron transfer rates. The high throughput of EC-ESM not only ensures statistical reliability but also allows the detection of rare electron transfer events in molecularly modified AgNPs. Additionally, EC-ESM's high resolution enabled direct imaging of both single and interacting silver nanowires, revealing diverse dissolution behaviors that provide insights into structural and surface properties. We envision EC-ESM as a powerful platform for advancing nanoscale electrochemical research and interfacial charge transfer studies.

Stochastic Impact Electrochemistry of Alkanethiolate-Functionalized Silver Nanoparticles

This study uses single-impact experiments to explore how the nanoparticles' surface chemistry influences their redox activity. 20 and 40 nm-sized silver nanoparticles are functionalized with alkanethiol ligands of various chain lengths (n = 3, 6, 8, and 11) and moieties (carboxyl ─COOH / hydroxyl ─OH), and the critical role of the particle shell is systematically examined. Short COOH-terminated ligands enable efficient charge transfer, resulting in higher impact rates and fast, high-amplitude transients. Even elevated potentials fail to overcome tunneling barriers for ligand lengths of n ≥ 6 and risk oxidizing the electrode, forming an insulating layer. Electrostatic interactions play a key role in governing reaction dynamics. In general, particles with a COOH-group exhibit higher impact rates and current amplitudes in KCl than those with an OH-group. This effect is more pronounced for 40 nm-sized particles; although, they rarely oxidize completely. The influence of electrolyte composition-concentration, pH, and a biologically relevant electrolyte-reveals that its impact on the redox activity can be as critical as that of the particle shell, with both determining particle adsorption and electron tunneling. These findings provide insights into the complex interdependencies at the electrode-particle-electrolyte interface, aiding the design of custom redox-active (silver) nanoparticles for ultrasensitive electrochemical sensing.

Measuring the Effects of Tunable Alkanethiol Monolayers on the Adsorption and Collision Dynamics of Platinum Nanoparticles

Platinum nanoparticles (PtNPs) catalyze the Hydrogen Evolution Reaction upon colliding at a catalytically inactive electrode surface when sufficient potential is applied, and in the presence of adequate hydrogen ion concentration. Here, we investigated nanoscale interactions of PtNPs at alkanethiol modified gold electrode surfaces and examined the effects of monolayer hydrophilicity/hydrophobicity on single particle collision dynamics. After colliding with and adsorbing onto the modified electrode surface, PtNPs generate measurable cathodic current arising from the reduction of hydrogen. Each single particle collision is indicated by a spike-step or spike of current in the current time trace. The shape, frequency, and size of these current steps are dependent on the terminal chemistry of the alkanethiol covalently bound to the electrode surface. Using the collisional frequency as a function of PtNP concentration, we determined the rate of particle adsorption,k ads , to be 2.23 × 10-6 cm/s and 8.85 × 10-6 cm/s for -CH3 and -OH terminated surfaces, respectively. Electrodes modified with a mixture of alkanethiols (-CH3/-OH) exhibited collision frequencies that scale linearly with the ratio of hydrophilicity of the alkanethiol immobilized on the electrode surface. The results indicate the dependence of intermolecular effects on PtNP collision dynamics at the electrode surface, with hydrophobic-dominating surfaces having the least observed collisions. This study provides insights into the influence of surface chemistry on single nanoparticle interactions, which could advance the designs of biosensors and more efficient nanocatalysts by offering a deeper understanding of the interfacial mechanism of PtNPs on modified electrode surfaces.

Impact electrochemistry for biosensing: advances and future directions

Impact electrochemistry allows for the investigation of the properties of single entities, ranging from nanoparticles (NPs) to soft bio-particles. It has introduced a novel dimension in the field of biological analysis, enhancing researchers' ability to comprehend biological heterogeneity and offering a new avenue for developing novel diagnostic devices for quantifying biological analytes. This review aims to summarize the recent advancements in impact electrochemistry-based biosensing over the past two to three years and provide insights into the future directions of this field.

Controlling the Collision Type and Frequency of Single Pt Nanoparticles at Chemically Modified Gold Electrodes

Studying the electrochemical response of single nanoparticles at an electrode surface gives insight into the dynamic and stochastic processes that occur at the electrode interface. Herein, we investigated single platinum nanoparticle collision dynamics and type (elastic vs inelastic) at gold electrode surfaces modified with self-assembled monolayers (SAMs) of varying terminal chemistries. Collision events are measured via the faradaic current from catalytic reactions at the Pt surface. By changing the terminal, solution-facing group of a thiolate monolayer, we observed the effect of hydrophobicity at the solution-electrode interface on single-particle collisions by employing either a hydrophobic -CH3 terminal group (1-hexanethiol), a hydrophilic -OH terminal group (6-mercaptohexanol), or an equimolar mixture of the two. Changes in the terminal group lead to alterations in collision-induced current magnitude, collisional frequency, and the distinct shape of the collision event current transient. The effects of the terminal group of the SAM were probed by measuring quantitative differences in the events monitored through both the hydrogen evolution reaction (HER) and hydrazine oxidation. In both cases, a platinum nanoparticle (PtNP) favors adsorption to bare and hydrophilic surfaces but demonstrates elastic collision behavior when it collides with a hydrophobic surface. In the case of a mixed monolayer, distinct characteristics of hydrophobic and hydrophilic surfaces are observed. We report how single nanoparticle collisions can reveal nanoscale surface heterogeneity and can be used to manipulate the nature of single-particle interactions on an electrode surface by functionalized self-assembled monolayers.

Impact of Surface Chemistry on Emulsion-Electrode Interactions and Electron-Transfer Kinetics in the Single-Entity Electrochemistry

Single-entity electrochemistry (SEE) provides powerful means to track a single particle, single cell, and even single molecule from the nano to microscale. The electrode serves as not only the detector of collision but also the surface supplier in SEE, and the fundamental understanding of the electrode surface chemistry on the dynamic particle-electrode interactions and electrochemical responses of a single particle still remains unexplored, particularly for soft particles. Herein, dynamic interactions of microemulsions and the interaction-controlled electron-transfer (ET) kinetics are studied employing SEE and fluorescence spectroscopy. The o/w-type nitrobenzene emulsions were prepared with the surfactant-type room temperature ionic liquids (RTILs). Biased the electrode potential for the reduction of 7,7,8,8-tetracyanoquinodimethane within emulsions, it is surprising to see the distinct collision current signals on the carbon fiber ultramicroelectrode (C UME) and Au ultramicroelectrode (Au UME) in the late stage of chronoamperometric measurements. Theoretical understanding was made to determine the ET kinetics behind the disparate current signals. It is believed that the electrode surface chemistry, i.e., the surface energy, has a great influence on the dynamic emulsion-electrode interactions and ET kinetics. On the hydrophilic surface of Au UME, emulsions tend to decompose/detach from the electrode surface immediately after colliding. In contrast, on the lipophilic surface of C UME with lower surface energy, a layer of oil phase accumulated by the coalescence of emulsions and the migration of the precedent colliding emulsions, which would serve as a barrier to block ET via tunneling as manifested by the gradual slowdown of ET rate and the reduced collision frequency in the late stage of measurement. The impacts of the emulsion size and amphiphilicity of RTILs on the C UME-emulsion interactions and ET kinetics were also investigated.