Single-Particle Spectroelectrochemistry: Electrochemical Approaches for Tuning Chemical Interfaces and Plasmon Damping in Single Gold Nanorods

In Situ Photoreversible Tuning of Chemical Interface Damping in Single Gold Nanorods Through Cucurbit[8]uril-Based Host-Guest Interactions

Chemical interface damping (CID) is a recently proposed plasmon-damping pathway based on the interfacial hot-electron transfer from metal to adsorbate molecules. However, the in situ reversible tuning of CID in single gold nanorods (AuNRs) has remained a considerable challenge. In this study, we used total internal reflection scattering microscopy and spectroscopy to investigate the CID induced by p-aminoazobenzene (p-AAB), which has fast photoisomerization characteristics, attached to single AuNRs. We demonstrated the in situ reversible tuning of CID in single AuNRs by switching between ultraviolet (UV, 365 nm) and visible (vis, 465 nm) irradiation to induce photoresponsive structural conversions between the cis and trans forms of p-AAB in ethanol, leading to different lowest unoccupied molecular orbital (LUMO) energies for both forms. The localized surface plasmon resonance (LSPR) line width was wide under vis irradiation but narrow under UV irradiation, indicating that hot electrons are more efficiently transferred to trans-p-AAB with a low LUMO energy level. We further investigated the in situ photoreversible tuning of CID by manipulating supramolecular host-guest interactions between cucurbit[8]uril (CB[8]) and p-AAB in the single AuNRs. Additionally, real-time in situ reversible tuning of CID in single AuNRs was achieved through photonic switching of the cis-trans forms of p-AAB inside CB[8]. The LSPR line width was narrow under vis irradiation but gradually widened under UV irradiation before narrowing again upon returning to vis irradiation, unlike the case with p-AAB only. These results can be ascribed to the fact that cis-p-AAB completely encapsulated within CB[8] in water is thermodynamically more favorable than trans-p-AAB. Therefore, we have discovered a new strategy for tuning the CID by performing p-AAB photoisomerization and adjusting the wavelength of incident light in single AuNRs. In addition, this study demonstrates that CID can be effectively applied to the development of biosensors to detect guest molecules and their structural changes inside the cavity of CB[8] in single AuNRs.

Direct Excitation Transfer in Plasmonic Metal-Chalcopyrite-Hybrids: Insights from Single Particle Line Shape Analysis

Hybrid nanomaterials containing both noble metal and semiconductor building blocks provide an engineerable platform for realizing direct or indirect charge and energy transfer for enhanced plasmonic photoconversion and photocatalysis. In this work, silver nanoparticles (AgNPs) and chalcopyrite (CuFeS2) nanocrystals (NCs) are combined into a AgNP@CuFeS2 hybrid structure comprising NCs embedded in a self-assembled lipid coating around the AgNP core. In AgNP@CuFeS2 hybrid structures, both metallic and semiconductor NCs support quasistatic resonances. To characterize the interactions between these resonances and their effect on potential charge and energy transfer, direct interfacial excitation transfer between the AgNP core and surrounding CuFeS2 NCs is probed through single particle line shape analysis and supporting electromagnetic simulations. These studies reveal that CuFeS2 NCs localized in the evanescent field of the central AgNP induce a broadening of the metal NP line shape that peaks when an energetic match between the AgNP and CuFeS2 NC resonances maximizes direct energy transfer. Dimers of AgNPs whose resonances exhibit poor energetic overlap with the CuFeS2 NC quasistatic resonance yield much weaker line shape broadening in a control experiment, corroborating the existence of resonant energy transfer in the AgNP@CuFeS2 hybrid. Resonant coupling between the metallic and semiconductor building blocks in the investigated hybrid architecture provides a mechanism for utilizing the large optical cross-section of the central AgNP to enhance the generation of reactive charge carriers in the surrounding semiconductor NCs for potential applications in photocatalysis.

Tuning of chemical interface damping in single gold nanorods through pH-dependent host-guest interactions using cucurbit[6]uril

Chemical interface damping (CID) in gold nanorods (AuNRs) arises from direct hot electron transfer from Au to adsorbed molecules. Despite recent studies on CID, its tunability in single AuNRs remains challenging. Herein, we present a method for in situ control of CID in single AuNRs using pH-dependent host-guest supramolecular interactions. We employ cucurbit[6]uril (CB[6]), a well-known host molecule capable of encapsulating and releasing guest molecules, along with bis(3-aminopropyl)amine (BAPA) as guest molecules forming a complex with CB[6] (CB[6]-BAPA). CID is induced by attaching the CB[6]-BAPA complex on AuNR surfaces through a strong Au-amine interaction. In addition, in situ tuning of CID is achieved by releasing CB[6] from the complex using a NaOH solution. Successful CB[6]-BAPA complex formation, their attachment onto AuNRs, and CB[6] release from the complex are confirmed through changes in the localized surface plasmon resonance (LSPR) peak and LSPR linewidth, alongside mass analysis. Therefore, this study offers a new method for in situ CID tuning using CB[6]-based pH-sensitive host-guest interactions in individual AuNRs. This study can be further used in CB[6]-based photochemical processes and biosensing studies.

Single-Particle Spectroelectrochemistry: Promoting the Electrocatalytic Activity of Gold Nanorods via Oxygen Plasma Treatment without Structural Deformation

Understanding of the electrocatalytic activity enhancement in gold nanoparticles is still limited. Herein, we present the effect of the oxygen plasma treatment on the electrochemical activity of gold nanorods (AuNRs). Oxygen plasma treatment resulted in the blueshift and line width narrowing of the localized surface plasmon resonance (LSPR) spectra obtained from individual AuNRs immobilized on an indium tin oxide (ITO) surface. These changes can be attributed to increases in the surface charges of the AuNRs. The formation of a Au-ITO heterojunction provided structural stability to the immobilized AuNRs regardless of the duration of oxygen plasma exposure. The electrocatalytic oxidation of hydrogen peroxide (H2O2) was induced by increases in the free-electron densities on the surfaces of these AuNRs owing to oxygen plasma treatment, and Au did not dissolve under the experimental conditions. However, the potential-dependent LSPR spectra of the individual AuNRs showed similar patterns of LSPR behavior, irrespective of the duration of oxygen plasma treatment and the concentration of H2O2. Therefore, this study based on single-particle spectroelectrochemistry and cyclic voltammetry improves the understanding of the role of oxygen plasma treatment in promoting the catalytic activity of structurally stable AuNRs immobilized on an ITO surface.

Tuning Chemical Interface Damping: Competition between Surface Damping Pathways in Amalgamated Gold Nanorods Coated with Mesoporous Silica Shells

The mechanism of mercury (Hg) amalgamation in gold nanorods coated with a mesoporous silica shell (AuNRs@mSiO2) and the effect of chemical treatments on the localized surface plasmon resonance (LSPR) spectral changes in single amalgamated AuNRs@mSiO2 remains unclear. In this study, we investigated Hg amalgamation and inward Hg diffusion in single AuNRs@mSiO2 without structural deformation via dark-field scattering spectroscopy and X-ray photoelectron spectroscopy. Then, we investigated the chemisorption of thiol molecules on single amalgamated AuNRs@Hg-mSiO2. Unlike previous studies on single AuNRs, the thiolation on single AuNRs@Hg-mSiO2 resulted in a redshift and line width narrowing of the LSPR peak within 1 h. To determine the chemical effect, we investigated the competition between two surface damping pathways: metal interface damping (MID) and chemical interface damping (CID). When we exposed amalgamated AuNRs@Hg-mSiO2 to 1-alkanethiols with three different carbon chain lengths for 1 h, we observed an increase in the line width broadening with longer chain lengths owing to enhanced CID, demonstrating the tunability of CID and LSPR properties upon chemical treatments. We also investigated the competition between the two surface damping pathways as a function of the time-dependent Au-Hg surface properties in AuNRs@Hg-mSiO2. The 24-h Hg treatment resulted in increased line width broadening compared to the 1-h treatment for the same thiols, which was attributed to the predominance of CID. This was in contrast to the predominance of MID under the 1-h treatment, which formed a core-shell structure. Therefore, this study provides new insights into the Hg amalgamation process, the effect of chemical treatments, competition between surface decay pathways, and LSPR control in AuNRs@mSiO2.