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diffusion into the AuNR core caused a gradual red shift and line width narrowing of the LSPR peak when AuNRs@mSiO₂ were withdrawn from Hg solution.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.

Mesoporous silica shell-coated single gold nanorods as multifunctional orientation probes in dynamic biological environments

Mesoporous silica shell-coated gold nanorods (AuNRs@mSiO₂) can be employed as promising multifunctional orientation probes in biological studies owing to their anisotropic optical properties, enhanced stability, excellent biocompatibility, etc. In this study, the optical properties of single AuNRs@mSiO₂ are characterized under dark-field and differential interference contrast (DIC) microscopy. Furthermore, we presented polarization-dependent, periodic DIC images and intensities of single AuNRs@mSiO₂ at their localized surface plasmon resonance wavelength and investigated their use as multifunctional orientation probes in dynamic biological environments. Moreover, the real-time rotational motions of the AuNRs@mSiO₂ on the HeLa cell membranes were tracked with millisecond temporal resolution. Overall, AuNRs@mSiO₂ demonstrated their capacity to act as multifunctional optical probes owing to the combined effect of the Au core, which can serve as an orientation probe and a local heat generator for phototherapy, and the mesoporous silica shell, which can be used as a reservoir of chemotherapeutics owing to its excellent loading capacity.

We presented polarization-dependent, periodic DIC images and intensities of single AuNRs@mSiO₂ at their LSPR wavelength and investigated their use as multifunctional orientation probes in biological environments.

Influence of oxygen plasma treatment on structural and spectral changes in silica-coated gold nanorods studied using total internal reflection microscopy and spectroscopy

This paper shows how oxygen plasma treatment affects the structural, localized surface plasmon resonance (LSPR) spectral, and spatial orientation changes in single gold nanorods coated with a mesoporous silica shell (AuNRs@SiO2) in comparison with bare AuNRs with the same aspect ratio (AR). Single AuNRs@SiO2 subjected to different plasma treatment times were characterized using scanning electron microscopy and total internal reflection scattering (TIRS) microscopy and spectroscopy. The AR of the single AuNRs without a silica shell was decreased by structural deformation, while their LSPR linewidth was increased with increasing plasma treatment time. In contrast, single AuNRs@SiO2 showed much higher structural and spectral stability due to the silica shell under the energetic plasma treatment. Furthermore, there was no noticeable variation in the three-dimensional (3D) orientations of single AuNR cores in the silica shell before and after the plasma treatment. The results support that no significant structural and spectral changes occur in single AuNRs@SiO2 and that the silica coating enhances the stability of AuNR cores against oxygen plasma treatment. Therefore, fundamental information on the relationship among plasma treatment time, structural change, LSPR damping, and defocused orientation patterns is provided at the single-particle level.

Single-particle study: effects of oxygen plasma treatment on structural and spectral changes of anisotropic gold nanorods

Oxygen plasma treatment is a common method for removing the surfactant capping material from gold nanoparticles, improving their functionalization and lowering their cytotoxicity for biological studies. This single-particle study investigates the effects of oxygen plasma treatment on the structural and localized surface plasmon resonance (LSPR) spectral variations of anisotropic gold nanorods (AuNRs). Single AuNRs subjected to different plasma treatment times were characterized by scanning electron microscopy and dark-field microscopy. The AuNR length was a gradually decreasing function of plasma treatment time. After 120 s of plasma treatment, the aspect ratio of the AuNRs was reduced by a major structural deformation. Furthermore, increasing the plasma treatment time gradually broadened the LSPR linewidth of the single AuNRs. This trend was attributed to the decreased aspect ratio and the increased plasmon damping. These results provide vital and fundamental information on the relationship among plasma treatment time, structural change, and LSPR damping at the single-particle level.

Heat generation by branched Au/Pd nanocrystals: influence of morphology and composition

Bimetallic gold-palladium particles were originally proposed as catalysts with tunable reaction rates. Following the development of synthesis routes that offer better control on the morphology and composition of the particles, novel optical sensing functionalities were more recently proposed. Since temperature is a fundamental parameter that interplays with every other proposed application, we studied the light-to-heat conversion ability of Au/Pd bimetallic nanoparticles with a regular octapodal shape. Both compositional (Au-to-Pd ratio) and structural (diagonal tip-to-tip distance and tip width) characteristics were screened and found to be essential control parameters to promote light absorption and efficient conversion into heat. Electromagnetic simulations reveal that the Pd content, and specifically its distribution inside the branched particle geometry, has a profound impact on the optical properties and is an essential criterion for efficient heating. Notably, the optical and photothermal responses are shown to remain stable throughout extended illumination, with no noticeable structural changes to the branched nanocrystals due to heat generation.

Single gold bipyramids with sharp tips as sensitive single particle orientation sensors in biological studies

Plasmonic gold bipyramids (AuBPs) with sharp tips are promising orientation probes in biological studies because of their anisotropic shape, strong electric field enhancement at the tips, and convenient manipulation into other shapes. Herein, we elucidate the optical properties of single AuBPs at their localized surface plasmon resonance (LSPR) wavelengths using dark-field (DF) microscopy and differential interference contrast (DIC) microscopy and test their use as orientation probes in a dynamic biological environment. Characteristic scattering field distributions together with a simulation study allowed us to achieve the high-throughput determination of the 3D orientation of single AuBPs within a single frame using defocused DF microscopy. We further present the polarization-dependent, periodic DIC images and intensities of single AuBPs at their LSPR wavelengths with high sensitivity. Finally, we successfully tracked the real-time rotational motions of transferrin-modified AuBPs on live cell membranes using DIC microscopy. Therefore, these results support the use of single AuBPs as sensitive orientation probes in dynamic biological studies using DIC microscopy.