Localized surface plasmon resonance inflection points for improved detection of chemisorption of 1-alkanethiols under total internal reflection scattering microscopy.

Plasmonic gold nanoparticles are widely used in localized surface plasmon resonance (LSPR) sensing. When target molecules adsorb to the nanoparticles, they induce a shift in the LSPR scattering spectrum. In conventional LSPR sensing, this shift is monitored at the maximum of the LSPR scattering peak. Herein, we describe the sensitivity of detecting chemisorption of 1-alkanethiols with different chain lengths (1-butanethiol and 1-haxanethiol) on single gold nanorods (AuNRs) of fixed diameter (25 nm) and three different aspect ratios under a total internal reflection scattering microscope. For single AuNRs of all sizes, the inflection point (IF) at the long-wavelength side (or low-energy side) of the LSPR scattering peak showed higher detection sensitivity than the traditionally used peak maximum. The improved sensitivity can be ascribed to the shape change of the LSPR peak when the local refractive index is increased by chemisorption. Our results demonstrate the usefulness of tracking the curvature shapes by monitoring the homogeneous LSPR IF at the red side of the scattering spectrum of single AuNRs.

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

Recently, chemical interface damping (CID) has been proposed as a new plasmon damping pathway based on interfacial hot-electron transfer from metal to adsorbate molecules. It has been considered essential, owing to its potential implications in efficient photochemical processes and sensing experiments. However, thus far, studies focusing on controlling CID in single gold nanoparticles have been very limited, and in situ reversible tuning has remained a considerable challenge. In these scanning electron microscopy-correlated dark-field spectroscopic measurements and density functional theory calculations, cucurbit[7]uril (CB[7])-based host-guest supramolecular interactions were employed to examine and control the CID process using monoamine-functionalized CB[7] (CB[7]-NH2) attached to single gold nanorods (AuNRs). In situ tuning of CID through the CB[7]-oxaliplatin complexation, which can result in the variation of the chemical nature and electronic properties of adsorbates, was presented. In addition, in situ tuning of CID was demonstrated through the competitive release of the oxaliplatin guest from the oxaliplatin@CB[7] complex, which was then replaced by a competitor guest of spermine in sufficient amounts. Furthermore, nuclear magnetic resonance experiments confirmed that the release of the guest is the consequence of adding salt (NaCl). Thus, in situ reversible tuning of CID in single AuNRs was achieved through successive steps of encapsulation and release of the guest on the same AuNR in a flow cell. Finally, single CB[7]-NH2@AuNRs were presented as a recyclable platform for CID investigations after the complete release of guest molecules from their host-guest inclusion complexes. Therefore, this study has paved a new route to achieve in situ reversible tuning of CID in the same AuNR and to investigate the CID process using CB-based host-guest chemistry with various guest molecules in single AuNRs for efficient hot-electron photochemistry and biosensing applications.

Enhanced detection sensitivity of the chemisorption of pyridine and biotinylated proteins at localized surface plasmon resonance inflection points in single gold nanorods.

Plasmonic gold nanoparticles have been widely used for localized surface plasmon resonance (LSPR) sensing. Herein, we investigate the enhanced sensitivity for the detection of the chemisorption of pyridine and biotinylated bovine serum albumin (BSA) proteins, which are important molecules widely used in biological studies, at the inflection points (IFs) of the LSPR scattering spectra of single gold nanorods (AuNRs). The results showed that the homogeneous LSPR IFs located at the long wavelength side (or low energy side) of the LSPR scattering peak exhibited the highest sensitivity for the detection of chemical adsorption with respect to the counterpart LSPR peak maxima. The increased sensitivity can be attributed to the shape change of the LSPR peak when the local refractive index is increased by chemisorption. Furthermore, real-time monitoring of molecular binding events on single AuNRs was performed after introducing pyridine in water, and an improved efficiency of the sensors was observed at the LSPR IFs to detect target molecules in single AuNRs. Therefore, we present the significance of tracking curvature shapes through homogeneous LSPR IFs close to the resonance energy upon chemical adsorption of pyridine and BSA-biotin, rather than tracking their counterpart LSPR maximum peak shifts, for AuNRs.

Effect of chemisorbed thiophenols with an electron donating group on surface-enhanced Raman scattering of gold nanorods.

Surface-enhanced Raman scattering (SERS) is a powerful technique to amplify the weak Raman scattering intensity by molecules chemisorbed on a metallic surface. Herein, we present the interfacial electronic effect of para-substituted aromatic thiophenols (TPs) with an electron donating group (EDG) on SERS of anisotropic gold nanorods (AuNRs) under resonant conditions. Probe molecules with an EDG showed great SERS enhancement in AuNRs at the resonant excitation wavelength. We found that the SERS enhancement with an EDG is caused by the formation of aggregates through intermolecular interactions among probe molecules, such as dimerization with hydrogen bonding via an amino group (-NH2) of p-aminothiophenol (p-ATP) and hydroxyl group (-OH) of p-mercaptophenol (p-MP), resulting in hot-spots between AuNRs. Furthermore, SERS having a stronger EDG (-NH2, p-ATP) with the Hammett constant of -0.66 exhibited greater enhancement than p-MP having hydroxyl (-OH) groups with the Hammett constant of -0.37. We found that the greater enhancement is ascribed to the temporary formation of a positively charged electron withdrawing group (-NH3+) in p-ATP, unlike p-MP, via the interaction of the lone pair of the amino group (-NH2) with ethanol. Therefore, this investigation provides new insightful experimental observations on SERS enhancement of probe molecules with an EDG.

Influence of shell thickness on the refractive index sensitivity of localized surface plasmon resonance inflection points in silver-coated gold nanorods.

Silver-coated gold nanorods (Ag@AuNRs) have great potential for biological and chemical sensing, because of their sensitive spectral response to the local environment and unique localized surface plasmon resonance (LSPR) properties. Herein, we investigated the sensitivities of single Ag@AuNRs with different shell thickness with respect to variations in the refractive index (RI) of the surrounding environment. Single Ag@AuNRs with thick shell thickness showed higher RI sensitivity than single Ag@AuNRs with thin shell thickness, which demonstrates the improvement of LSPR sensor due to increase in silver layer thickness. Furthermore, we investigated homogeneous LSPR scattering inflection points (IFs) to better understand the RI sensitivity of single Ag@AuNRs with different Ag shell thickness. The LSPR IFs showed higher RI sensitivity when compared with the frequency shifts of counterpart LSPR peaks observed for single Ag@AuNRs. Finally, single Ag@AuNRs with thick Ag shell demonstrated improved RI sensitivity when compared with single AuNRs with thin Ag shell in the homogeneous LSPR IFs.