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.

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.

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

This paper elucidates the mercury (Hg) amalgamation induced by electrochemical reduction on gold nanorods coated with mesoporous silica shell (AuNRs@mSiO2) using single-particle spectroelectrochemistry. First, the silica shell significantly enhanced the structural stability of AuNR cores after Hg amalgamation with the application of linear sweep voltages (LSVs). Thus, we were able to focus on the spectral changes of AuNRs@mSiO2 induced by the deposition of Hg without the disturbance of structural deformation, which also strongly affects localized surface plasmon resonance (LSPR) properties. Second, following the application of LSVs in the presence of Hg2+, a remarkable blueshift of the LSPR peak was observed, caused by the lowering of the work function due to the Hg adsorption, donating electron density to Au. Furthermore, the LSPR linewidth also dramatically increased after the Hg deposition with LSV. Lastly, the evolution of the Hg amalgamation process was directly observed by monitoring real-time LSPR peaks and LSPR linewidth shifts of a single AuNRs@mSiO2 in the Hg solution according to the application of the electrochemical potential. Moreover, the results showed 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.

Tuning plasmonic properties by promoting the inward Hg diffusion via oxygen plasma treatment in gold nanorods coated with a mesoporous silica shell.

This paper describes a new strategy using oxygen plasma treatment to tune the localized surface plasmon resonance (LSPR) of gold nanorods coated with a mesoporous silica shell (AuNRs@mSiO2) exposed to mercury (Hg) solution. Dark-field (DF) microscopy and spectroscopy were used to reveal the effects of oxygen plasma treatment on the structural and LSPR spectral changes of single amalgamated AuNRs@mSiO2. Due to the structural stability provided by the mesoporous silica shell, the amalgamated AuNRs@mSiO2 exposed to oxygen plasma treatment showed no morphological transformation. Furthermore, real-time monitoring of single AuNRs@mSiO2 showed slow Hg inward diffusion into AuNR cores in air over a long timeframe (24 h), as revealed by the redshift and linewidth narrowing of the LSPR peak. However, the oxygen plasma treatment resulted in faster inward diffusion of Hg into AuNR cores within 20 min, almost 60 times faster than the amalgamated AuNRs@mSiO2 untreated with oxygen plasma. Thus, oxygen plasma treatment was further presented as a new effective approach to control the LSPR properties by promoting the inward Hg diffusion in amalgamated AuNRs@mSiO2 without structural deformation.

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.

Influence of oxygen plasma treatment on structural and spectral changes in gold nanorods immobilized on indium tin oxide surfaces.

Oxygen plasma treatment is commonly used to sterilize gold nanoparticles by removing chemical contaminants from their surface while simultaneously inducing surface activation and functionalization of nanoparticles for biological, electrocatalytic, or electrochemical studies. In this study, we investigate the influence of oxygen plasma treatment on structural and localized surface plasmon resonance (LSPR) spectral changes of anisotropic gold nanorods (AuNRs) immobilized on an indium tin oxide (ITO) glass substrate. Unlike AuNRs deposited on a glass slide, no noticeable structural change or deformation of AuNRs on ITO was observed while increasing the oxygen plasma treatment time. This result indicates that ITO provides structural stability to AuNRs immobilized on its surface. Additionally, single-particle scattering spectra of AuNRs showed the broadening of LSPR linewidth within 60 s of oxygen plasma treatment as a result of the plasmon energy loss contributed from plasmon damping to ITO due to the removal of capping material from the AuNR surface. Nevertheless, an increase in the surface charge on the AuNR surface was observed by narrowing the LSPR linewidth after 180 s of plasma treatment. The electrochemical study of AuNRs immobilized on ITO electrodes revealed the surface activation and functionalization of AuNRs by increasing plasma treatment. Hence, in this study, a significant understanding of oxygen plasma treatment on AuNRs immobilized on ITO surfaces is provided.

Bioactive Phytochemicals from Salix pseudolasiogyne Twigs: Anti-Adipogenic Effect of 2'-O-Acetylsalicortin in 3T3-L1 Cells.

Salix pseudolasiogyne (Salicaceae) is a willow tree and has been used as a medicinal herb in Korea to treat pain and fever. As a part of an ongoing study to identify bioactive natural products, potential anti-adipogenic compounds were investigated using the ethanol (EtOH) extract of S. pseudolasiogyne twigs. Phytochemical investigation of the EtOH extracts using liquid chromatography-mass spectrometry (LC/MS) led to the separation of two compounds, oregonin (1) and 2'-O-acetylsalicortin (2). The structures of the isolates were identified using nuclear magnetic resonance spectroscopy and LC/MS analysis. To the best of our knowledge, it is the first report identifying oregonin (1) in twigs of S. pseudolasiogyne. Here, we found that the isolated compounds, oregonin (1) and 2'-O-acetylsalicortin (2), showed anti-adipogenic effects during 3T3-L1 cell differentiation. Notably, 2'-O-acetylsalicortin (2), at a concentration of 50 µM, significantly suppressed lipid accumulation. Moreover, the mRNA and protein levels of lipogenic and adipogenic transcription factors were reduced in 2'-O-acetylsalicortin (2)-treated 3T3-L1 cells. Taken together, these results indicate that 2'-O-acetylsalicortin (2), isolated from S. pseudolasiogyne twigs, has the potential to be applied as a therapeutic agent to effectively control adipocyte differentiation, a critical stage in the progression of obesity.

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.

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.

Characterizing the non-paraxial Talbot effect of two-dimensional periodic arrays of plasmonic gold nanodisks by differential interference contrast microscopy.

Exploiting the working principle of conventional differential interference contrast (DIC) microscopy, we experimentally investigate the non-paraxial Talbot effect of two-dimensional periodic arrays of gold nanodisks (AuNDs) with a periodicity ao comparable to the excitation wavelength λ. In the non-paraxial regime, strongly contrasting self-image patterns at the Talbot and fractional Talbot distances appeared perpendicularly to the AuND array. The experimental self-image distances were comparable to the calculated non-paraxial Talbot distances at two excitation wavelengths (540 nm and 600 nm). Beyond the paraxial limit, Talbot distances were observed at positions smaller than the paraxial Talbot distance.

Single-particle spectroscopy and defocused imaging of anisotropic gold nanorods by total internal reflection scattering microscopy.

Total internal reflection scattering (TIRS) microscopy is based on evanescent field illumination at the interface. Compared to conventional dark-field (DF) microscopy, TIRS microscopy has been rarely applied to the spectroscopic studies of plasmonic nanoparticles. Furthermore, there has been no detailed correlation study on the characteristic optical properties of single gold nanorods (AuNRs) obtained by DF and TIRS microscopy. Herein, through a single-particle correlation study, we compare the spectroscopic and defocusing properties of single AuNRs obtained by DF and TIRS microscopy, which have different illumination geometries. Compared to DF microscopy, TIRS microscopy yielded almost identical single-particle scattering spectra and localized surface plasmon resonance (LSPR) linewidth for the same in-focus AuNRs. However, TIRS microscopy, which is based on evanescent field illumination at the interface, provided a higher signal-to-noise ratio in the defocused image of the same AuNRs compared to DF microscopy. Furthermore, the heavily reduced background noise clarified the defocused scattering patterns of TIRS microscopy, which provided more detailed and accurate angular information than that obtained by conventional DF microscopy.

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.

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.

Tuning Chemical Interface Damping: Interfacial Electronic Effects of Adsorbate Molecules and Sharp Tips of Single Gold Bipyramids.

The optimization of the localized surface plasmon resonance (LSPR)-decaying channels of hot-electrons is essential for efficient optical and photochemical processes. Understanding and having the ability to control chemical interface damping (CID) channel contributions will bring about new possibilities for tuning the efficiency of plasmonic hot-electron energy transfer in artificial devices. In this scanning electron microscopy-correlated dark-field scattering study, the CID was controlled by focusing on the electronic nature of disubstituted benzene rings acting as adsorbates, as well as the effects of sharp tips on gold bipyramids (AuBPs) with similar aspect ratios to those of gold nanorods. The results showed that the sharp tips on single AuBPs, as well as the electronic effects of the adsorbate molecules, increase the interfacial contact between the nanoparticles and adsorbate molecules. Electron withdrawing groups (EWGs) on the adsorbates induce larger homogeneous LSPR line widths compared to those of electron donating groups (EDGs). Depending on the location (ortho, meta, and para) of the EDG, the effect of benzene rings with an EDG, which was considered to be induced by sulfur atoms bound to the nanoparticle surface, is weakened by the back transfer of electrons facilitated by the difference in the availability of the electrons of the EDG. Therefore, this study reports that the CID in the LSPR total decay channels can be tuned by controlling the electron withdrawing and electron donating features of adsorbate molecules with the surface topology of metal.

Polarization- and wavelength-dependent defocused scattering imaging of single gold nanostars with multiple long branches.

The scattering properties of gold nanostars (AuNSs) are not completely understood. Furthermore, there have been no studies on the dark-field (DF) scattering patterns of a AuNS in defocused DF microscopy. Herein, we demonstrate the polarization- and wavelength-dependent defocused scattering properties at the localized surface plasmon resonance (LSPR) wavelengths of single AuNSs with multiple long branches protruding from their surfaces. The defocused scattering intensities of single AuNSs at two LSPR excitation wavelengths changed periodically as a function of the rotational angle of a polarizer, whereas the doughnut-shaped scattering pattern remained unaltered. Furthermore, the characteristic doughnut-shaped defocused scattering pattern enabled the resolution of the spatial field distributions of single dipoles on the same AuNS surface at two LSPR wavelengths. Finally, we tracked the real-time rotational dynamics of a AuNS rotating on a glass slide using defocused microscopy. These results provide a deeper understanding of the defocused scattering properties of single AuNSs with multiple sharp, long branches randomly protruding from their surfaces.

Influence of the capping material on pyridine-induced chemical interface damping in single gold nanorods.

Chemical interface damping (CID) is one of the plasmon decay processes that occur in gold nanoparticles. With the aim of exploring new functional groups that can induce CID as an alternative to thiol groups, we performed dark-field (DF) scattering studies of gold nanorods (AuNRs) using pyridine as adsorbate. We found that the adsorption of pyridine molecules on single AuNRs though nitrogen-gold interactions leads to an increase of the localized surface plasmon resonance (LSPR) linewidth. However, pyridine molecules were not adsorbed effectively on AuNR surfaces having a capping reagent. This study allows us to gain insight into the effect and role of the capping reagent in pyridine-induced CID. Furthermore, pyridine was revealed to induce a strong CID through the interaction of the nitrogen atom with the Au surface, provided the capping material was previously removed from the AuNRs by oxygen plasma treatment. Finally, we demonstrated that CID could be used to sense pyridine and its derivatives in real-time.

Optical Super-Resolution Imaging of Surface Reactions.

Optical super-resolution imaging has gained momentum in investigations of heterogeneous and homogeneous chemical reactions at the single-molecule level. Thanks to its exceptional spatial resolution and ability to monitor dynamic systems, much detailed information on single-molecule reaction/adsorption processes and single-particle catalytic processes has been revealed, including chemical kinetics and reaction dynamics; active-site distributions on single-particle surfaces; and size-, shape-, and facet-dependent catalytic activities of individual nanocatalysts. In this review, we provide an overview of recent advances in super-resolution chemical imaging of surface reactions.

Single-particle correlation study: chemical interface damping induced by biotinylated proteins with sulfur in plasmonic gold nanorods.

Plasmonic gold nanoparticles can be an efficient source of hot electrons that can transfer to adsorbed molecules for photochemistry, followed by broadening of the homogeneous localized surface plasmon resonance (LSPR) linewidth. Although chemical interface damping (CID) is one of the main decay processes, it is the most poorly understood damping mechanism in gold nanoparticles. Herein, to better understand CID and to find new functional groups that can induce CID as an alternative to thiol groups (-SH, sulfhydryl groups), we carried out scanning electron microscopy (SEM) correlated dark-field (DF) scattering studies of gold nanorods (AuNRs) at the single-particle level. We found that biotin with sulfur can lead to strong plasmon damping in single AuNRs. We chose biotin in this study because it is widely used as a linker that can selectively bind to streptavidin in many biological sensing experiments. We further demonstrated the possibility of real-time detection of biological molecules, specifically biotinylated BSA proteins, by means of CID in single AuNRs. Therefore, this study allows us to gain a deeper insight into how adsorbate molecules with sulfur affect CID, which is an important step toward developing a CID-based LSPR biosensor to detect real biological molecules having sulfur or thiol groups without interference from the medium dielectric constant in single AuNRs.

Smart SERS Hot Spots: Single Molecules Can Be Positioned in a Plasmonic Nanojunction Using Host-Guest Chemistry.

Single-molecule surface-enhanced Raman spectroscopy (SERS) offers new opportunities for exploring the complex chemical and biological processes that cannot be easily probed using ensemble techniques. However, the ability to place the single molecule of interest reliably within a hot spot, to enable its analysis at the single-molecule level, remains challenging. Here we describe a novel strategy for locating and securing a single target analyte in a SERS hot spot at a plasmonic nanojunction. The "smart" hot spot was generated by employing a thiol-functionalized cucurbit[6]uril (CB[6]) as a molecular spacer linking a silver nanoparticle to a metal substrate. This approach also permits one to study molecules chemically reluctant to enter the hot spot, by conjugating them to a moiety, such as spermine, that has a high affinity for CB[6]. The hot spot can accommodate at most a few, and often only a single, analyte molecule. Bianalyte experiments revealed that one can reproducibly treat the SERS substrate such that 96% of the hot spots contain a single analyte molecule. Furthermore, by utilizing a series of molecules each consisting of spermine bound to perylene bisimide, a bright SERS molecule, with polymethylene linkers of varying lengths, the SERS intensity as a function of distance from the center of the hot spot could be measured. The SERS enhancement was found to decrease as 1 over the square of the distance from the center of the hot spot, and the single-molecule SERS cross sections were found to increase with AgNP diameter.