In situ reversible tuning of chemical interface damping in single gold nanorod-based recyclable platforms through manipulation of supramolecular host-guest interactions

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.

Emerging sensing platforms based on Cucurbit[n]uril functionalized gold nanoparticles and electrodes

Cucurbit[n]urils (CB[n]s, n = 5-8, 10, and 14), synthetic macrocycles with unique host-guest properties, have triggered increasing research interest in recent years. Gold nanoparticles (Au NPs) and electrodes stand out as exceptional substrates for sensing due to their remarkable physicochemical characteristics. Coupling the CB[n]s with Au NPs and electrodes has enabled the development of emerging sensing platforms for various promising applications. However, monitoring the behavior of analytes at the single-molecule level is currently one of the most challenging topics in the field of CB[n]-based sensing. Constructing supramolecular junctions in a sensing platform provides an ideal structure for single-molecule analysis, which can provide insights for a fundamental understanding of supramolecular interactions and chemical reactions and guide the design of sensing applications. This feature article outlines the progress in the preparation of the CB[n] functionalized Au NPs and Au electrodes, as well as the construction and application of supramolecular junctions in sensing platforms, based on the methods of recognition tunneling (RT), surface-enhanced Raman spectroscopy (SERS), single-molecule force spectroscopy (SMFS), and electrochemical sensing (ECS). A brief perspective on the future development of and challenges in CB[n] mediated sensing platforms is also covered.

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.

In situ reversible tuning of chemical interface damping in mesoporous silica-coated gold nanorods via direct adsorption and removal of thiol

Chemical interface damping (CID) is a recently proposed plasmon decay channel in gold nanoparticles. However, thus far, a very limited number of studies have focused on controlling CID in single gold nanoparticles. Herein, we describe a new simple method for reversible tuning of CID in single gold nanorods coated with a mesoporous silica shell (AuNRs@mSiO2). We used 1-alkanethiols with two different carbon chain lengths (1-butanethiol and 1-decanethiol) as adsorbates to induce CID. In addition, NaBH4 solution was used to remove the attached thiol from the AuNR surface. We confirmed the adsorption and removal of 1-alkanethiols on single AuNRs@mSiO2 and the corresponding changes in localized surface plasmon resonance (LSPR) peak wavelengths and linewidths. Furthermore, we investigated the effect of immersion time in NaBH4 solution on thiol removal from AuNRs@mSiO2. Therefore, the LSPR properties and CID can be controlled, thereby paving the way for in situ reversible tuning of CID by repeated adsorption and desorption of thiol molecules on single AuNRs@mSiO2.

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

The strong adsorption of thiol molecules on gold nanorods (AuNRs) results in localized surface plasmon resonance (LSPR) energy loss via chemical interface damping (CID). This study investigated the CID effect induced by thiophenol (TP) adsorption on single AuNRs and the in situ tuning of LSPR properties and chemical interfaces through electrochemical potential manipulation. The potential-dependent LSPR spectrum of bare AuNRs exhibited redshifts and line width broadening owing to the characteristics of capacitive charging, Au oxidation, and oxidation dissolution. However, TP passivation provided stability to the AuNRs from oxidation in an electrochemical environment. Electrochemical potentials induced electron donation and withdrawal, causing changes in the Fermi level of AuNRs at the Au-TP interface, thereby controlling the LSPR spectrum. Additionally, the desorption of TP molecules from the Au surface was electrochemically achieved at the anodic potentials further away from the capacitive charging region, which can be used to tune chemical interfaces and the CID process in single AuNRs.

Directing Energy Flow in Core-Shell Nanostructures for Efficient Plasmon-Enhanced Electrocatalysis

Conjugating plasmonic metals with catalytically active materials with controlled configurations can harness their light energy harvesting ability in catalysis. Herein, we present a well-defined core-shell nanostructure composed of an octahedral Au nanocrystal core and a PdPt alloy shell as a bifunctional energy conversion platform for plasmon-enhanced electrocatalysis. The prepared Au@PdPt core-shell nanostructures exhibited significant enhancements in electrocatalytic activity for methanol oxidation and oxygen reduction reactions under visible-light irradiation. Our experimental and computational studies revealed that the electronic hybridization of Pd and Pt allows the alloy material to have a large imaginary dielectric function, which can efficiently induce the shell-biased distribution of plasmon energy upon illumination and, hence, its relaxation at the catalytically active region to promote electrocatalysis.

Self-assembly of β-cyclodextrin-pillar[5]arene molecules into supramolecular nanoassemblies: morphology control by stimulus responsiveness and host-guest interactions

Macrocyclic molecules have attracted considerable attention as new functional materials owing to their unique pore size structure and excellent host-guest properties. With the development of macrocyclic compounds, the properties of mono-modified macrocyclic materials can be improved by incorporating pillar[n]arene or cyclodextrin derivatives through bridge bonds. Herein, we report the self-assembly of amphiphilic di-macrocyclic host molecules (H1-2) based on β-cyclodextrin and pillar[5]arene units linked by azophenyl or biphenyl groups. In a H₂O/DMSO (19 : 1, v/v) mixed polar solvent, an amphiphile H1 with an azophenyl group self-assembled into unique nanorings and exhibited an obvious photoresponsive colour change. This photochromic behaviour makes H1 suitable for application in carbon paper materials on which arbitrary patterns can be erased and rewritten. The amphiphile H2, with a biphenyl unit, self-assembled into spherical micelles. These differences indicate that various linker units lead to changes in the intermolecular and hydrophilic-hydrophobic interactions. In a CHCl₃/DMSO (19 : 1, v/v) mixed low-polarity solvent, the amphiphile H1 self-assembled into fibrous aggregates, whereas the molecule H2 assembled into unique nanoring aggregates. In this CHCl₃/DMSO mixed solvent system, small nanosheet aggregates were formed by the addition of a guest molecule (G) composed of tetraphenylethene and hexanenitrile groups. With prolonged aggregation time, the small sheet aggregates further aggregated into cross-linked nanoribbons and eventually formed large nanosheet aggregates. The data reveal that the morphology of H1-2 can be controlled by tuning the intermolecular interactions of the molecules via the formation of host-guest complexes. Moreover, the polyhydroxy cyclodextrin unit on H1-2 can be strongly adsorbed on the stationary phase in column chromatography via multiple hydrogen bonds, and the singly modified pillar[5]arenes can be successfully separated by host-guest interactions.

Effects of Amine Linkers with Different Carbon Chain Lengths at Guanine-Rich Polynucleotides on Chemical Interface Damping in Single Gold Nanorods

DNA-functionalized gold nanoparticles (AuNPs) are used for various bioapplications, such as biosensor development and drug delivery. Nevertheless, no study has reported the effect of polynucleotide chains on chemical interface damping (CID), the most recently proposed plasmon damping pathway in single AuNPs. In this study, we conducted total internal reflection scattering measurements of gold nanorods (AuNRs) to reveal the CID effect induced by amine (NH₂)-linked polynucleotides (or DNA) with guanine-rich sequences through the interaction between nitrogen and Au surfaces. Additionally, we elucidated the effect of a linear hydrocarbon chain length between NH₂ and DNA (NH₂-C_n-DNA, n = 6, 12, 18, 24) on spectral changes in single AuNRs. The localized surface plasmon resonance (LSPR) linewidth increased with an increasing number of linear carbon, from 6 to 24, due to the increase in van der Waals forces. Second, the effect of the direction (5' or 3' ends) of DNA attachment to the AuNR surfaces on LSPR spectral changes was investigated, and there was no significant difference in LSPR wavelength and full linewidth at half-maximum shifts caused by the DNA attachment directions (5' or 3' ends). Third, guanine-rich DNA can fold into four-stranded secondary structures called G-quadruplexes (GQs). We demonstrated the effect of linear carbon chain length, between NH₂ and GQs, on CID in single AuNRs. Lastly, a label-free detection of DNA hybridization events on single AuNRs was demonstrated for sensing applications. Thus, we provide an insight into the effect of amine-functionalized guanine-rich DNA with different carbon chains on LSPR spectral changes, including CID in single AuNRs.

Single-Particle Study on Hg Amalgamation Mechanism and Slow Inward Diffusion in Mesoporous Silica-Coated Gold Nanorods without Structural Deformation

This paper presents the structural and spectral variations of individual mesoporous silica-coated gold nanorods (AuNRs@mSiO₂) compared to bare AuNRs upon Hg-Au amalgamation. First, the aspect ratio of AuNRs@mSiO₂ exposed to Hg solutions was unchanged because the deformation related to the cores of AuNR was suppressed by the silica shell. Second, dark-field microscopy and spectroscopy revealed a blue shift of the localized surface plasmon resonance (LSPR) wavelength peak and strong plasmon damping in the individual AuNRs@mSiO₂ scattering spectra, exposed to Hg solutions. Furthermore, we investigated time-dependent adsorption kinetics and spectral changes during the formation of Au-Hg amalgam in single AuNRs@mSiO₂ over a long time frame without any disturbance from the structural deformation. The inward Hg diffusion into the AuNR core caused a gradual red shift and line width narrowing of the LSPR peak when AuNRs@mSiO₂ were withdrawn from Hg solution. Thus, this paper provides new insights into the relationship among amalgamation process, morphological change, the role of silica shell, Hg inward diffusion, LSPR peak, and line width at the single-particle level.

Single-particle study: effects of mercury amalgamation on morphological and spectral changes in anisotropic gold nanorods

This study investigated the amalgamation of gold nanorods (AuNRs) exposed to Hg(II) solution and its effects on structural and spectral changes in single AuNRs using scanning electron microscopy and total internal reflection scattering microscopy. First, Hg adsorption on AuNR surfaces formed AuNRs@Hg core-shell structures. Afterwards, they transformed to AuNRs@AuHg alloy shell structures in air due to the slow inward diffusion of Hg over time. The aspect ratio (AR) of the AuNRs@AuHg formed by the amalgamation was significantly decreased compared to that of bare AuNRs. Furthermore, the Hg coating on AuNRs induced a dramatic blue shift of the localized surface plasmon resonance (LSPR) peak and linewidth broadening, followed by a red shift and linewidth narrowing of the LSPR peak due to inward diffusion of Hg into the AuNR core. Finally, we investigated the effects of oxygen plasma treatment on the structural changes of AuNRs@AuHg and found that their AR was a decreasing function of the plasma treatment time. More notably, a major structural change was observed 5 min after the plasma treatment. Therefore, fundamental information on the relationship among amalgamation process, plasma treatment time, structural change, and LSPR peak and linewidth is provided at the single-particle level.

Single-particle spectroelectrochemistry: electrochemical tuning of plasmonic properties via mercury amalgamation in mesoporous silica coated gold nanorods without structural deformation

This paper presented the possibility of the in situ tuning of the LSPR properties of AuNRs@mSiO2 by Hg deposition via electrochemical potential manipulations without the disturbance of the structural variations of AuNR cores.