Ultrafast and Efficient Transport of Hot Plasmonic Electrons by Graphene for Pt Free, Highly Efficient Visible-Light Responsive Photocatalyst

We report that reduced graphene-coated gold nanoparticles (r-GO-AuNPs) are excellent visible-light-responsive photocatalysts for the photoconversion of CO₂ into formic acid (HCOOH). The wavelength-dependent quantum and chemical yields of HCOOH shows a significant contribution of plasmon-induced hot electrons for CO₂ photoconversion. Furthermore, the presence and reduced state of the graphene layers are critical parameters for the efficient CO₂ photoconversion because of the electron mobility of graphene. With an excellent selectivity toward HCOOH (>90%), the quantum yield of HCOOH using r-GO-AuNPs is 1.52%, superior to that of Pt-coated AuNPs (quantum yield: 1.14%). This indicates that r-GO is a viable alternative to platinum metal. The excellent colloidal stability and photocatalytic stability of r-GO-AuNPs enables CO₂ photoconversion under more desirable reaction conditions. These results highlight the role of reduced graphene layers as highly efficient electron acceptors and transporters to facilitate the use of hot electrons for plasmonic photocatalysts. The femtosecond transient spectroscopic analysis also shows 8.7 times higher transport efficiency of hot plasmonic electrons in r-GO-AuNPs compared with AuNPs

Glutathione Dimerization-Based Plasmonic Nanoswitch for Biodetection of Reactive Oxygen and Nitrogen Species

Reactive oxygen and nitrogen species (ROS and RNS) are continuously produced in the cellular systems and are controlled by several antioxidant mechanisms. Here, we developed a straightforward, sensitive, and quantitative assay for the colorimetric and spectroscopic detection of various ROS and RNS such as H₂O₂, ·OH, ^–OCl, NO<b>·</b>, and O₂^– using glutathione-modified gold nanoparticles (GSH-AuNPs). A basic principle here is that the GSHs on the AuNP surface can be readily detached <i>via</i> the formation of glutathione disulfides upon the addition of ROS and RNS, and destabilized particles can aggregate to generate the plasmonic couplings between plasmonic AuNPs that trigger the red shift in UV–vis spectrum and solution color change. For nonradical species such as H₂O₂, this process can be more efficiently achieved by converting them into radical species <i>via</i> the Fenton reaction. Using this strategy, we were able to rapidly and quantitatively distinguish among cancerous and normal cells based on ROS and RNS production

High Resolution Live Cell Raman Imaging Using Subcellular Organelle-Targeting SERS-Sensitive Gold Nanoparticles with Highly Narrow Intra-Nanogap

We report a method to achieve high speed and high resolution live cell Raman images using small spherical gold nanoparticles with highly narrow intra-nanogap structures responding to NIR excitation (785 nm) and high-speed confocal Raman microscopy. The three different Raman-active molecules placed in the narrow intra-nanogap showed a strong and uniform Raman intensity in solution even under transient exposure time (10 ms) and low input power of incident laser (200 μW), which lead to obtain high-resolution single cell image within 30 s without inducing significant cell damage. The high resolution Raman image showed the distributions of gold nanoparticles for their targeted sites such as cytoplasm, mitochondria, or nucleus. The high speed Raman-based live cell imaging allowed us to monitor rapidly changing cell morphologies during cell death induced by the addition of highly toxic KCN solution to cells. These results strongly suggest that the use of SERS-active nanoparticle can greatly improve the current temporal resolution and image quality of Raman-based cell images enough to obtain the detailed cell dynamics and/or the responses of cells to potential drug molecules

Thiolated DNA-Based Chemistry and Control in the Structure and Optical Properties of Plasmonic Nanoparticles with Ultrasmall Interior Nanogap

The design, synthesis and control of plasmonic nanostructures, especially with ultrasmall plasmonically coupled nanogap (∼1 nm or smaller), are of significant interest and importance in chemistry, nanoscience, materials science, optics and nanobiotechnology. Here, we studied and established the thiolated DNA-based synthetic principles and methods in forming and controlling Au core-nanogap-Au shell structures [Au-nanobridged nanogap particles (Au-NNPs)] with various interior nanogap and Au shell structures. We found that differences in the binding affinities and modes among four different bases to Au core, DNA sequence, DNA grafting density and chemical reagents alter Au shell growth mechanism and interior nanogap-forming process on thiolated DNA-modified Au core. Importantly, poly A or poly C sequence creates a wider interior nanogap with a smoother Au shell, while poly T sequence results in a narrower interstitial interior gap with rougher Au shell, and on the basis of the electromagnetic field calculation and experimental results, we unraveled the relationships between the width of the interior plasmonic nanogap, Au shell structure, electromagnetic field and surface-enhanced Raman scattering. These principles and findings shown in this paper offer the fundamental basis for the thiolated DNA-based chemistry in forming and controlling metal nanostructures with ∼1 nm plasmonic gap and insight in the optical properties of the plasmonic NNPs, and these plasmonic nanogap structures are useful as strong and controllable optical signal-generating nanoprobes

Tuning and Maximizing the Single-Molecule Surface-Enhanced Raman Scattering from DNA-Tethered Nanodumbbells

We extensively study the relationships between single-molecule surface-enhanced Raman scattering (SMSERS) intensity, enhancement factor (EF) distribution over many particles, interparticle distance, particle size/shape/composition and excitation laser wavelength using the single-particle AFM-correlated Raman measurement method and theoretical calculations. Two different single-DNA-tethered Au–Ag core–shell nanodumbbell (GSND) designs with an engineerable nanogap were used in this study: the GSND-I with various interparticle nanogaps from ∼4.8 nm to <1 nm or with no gap and the GSND-II with the fixed interparticle gap size and varying particle size from a 23–30 nm pair to a 50–60 nm pair. From the GSND-I, we learned that synthesizing a <1 nm gap is a key to obtain strong SMSERS signals with a narrow EF value distribution. Importantly, in the case of the GSND-I with <1 nm interparticle gap, an EF value of as high as 5.9 × 10¹³ (average value = 1.8 × 10¹³) was obtained and the EF values of analyzed particles were narrowly distributed between 1.9 × 10¹² and 5.9 × 10¹³. In the case of the GSND-II probes, a combination of >50 nm Au cores and 514.5 nm laser wavelength that matches well with Ag shell generated stronger SMSERS signals with a more narrow EF distribution than <50 nm Au cores with 514.5 nm laser or the GSND-II structures with 632.8 nm laser. Our results show the usefulness and flexibility of these GSND structures in studying and obtaining SMSERS structures with a narrow distribution of high EF values and that the GSNDs with < 1 nm are promising SERS probes with highly sensitive and quantitative detection capability when optimally designed

Amplified Photoacoustic Performance and Enhanced Photothermal Stability of Reduced Graphene Oxide Coated Gold Nanorods for Sensitive Photoacoustic Imaging

We report a strongly amplified photoacoustic (PA) performance of the new functional hybrid material composed of reduced graphene oxide and gold nanorods. Due to the excellent NIR light absorption properties of the reduced graphene oxide coated gold nanorods (r-GO-AuNRs) and highly efficient heat transfer process through the reduced graphene oxide layer, r-GO-AuNRs exhibit excellent photothermal stability and significantly higher photoacoustic amplitudes than those of bare-AuNRs, nonreduced graphene oxide coated AuNRs (GO-AuNRs), or silica-coated AuNR, as demonstrated in both <i>in vitro</i> and <i>in vivo</i> systems. The linear response of PA amplitude from reduced state controlled GO on AuNR indicates the critical role of GO for a strong photothermal effect of r-GO-AuNRs. Theoretical studies with finite-element-method lab-based simulation reveal that a 4 times higher magnitude of the enhanced electromagnetic field around r-GO-AuNRs can be generated compared with bare AuNRs or GO-AuNRs. Furthermore, the r-GO-AuNRs are expected to be a promising deep-tissue imaging probe because of extraordinarily high PA amplitudes in the 4–11 MHz operating frequency of an ultrasound transducer. Therefore, the r-GO-AuNRs can be a useful imaging probe for highly sensitive photoacoustic images and NIR sensitive therapeutics based on a strong photothermal effect

Plasmonic Effect of Gold Nanostars in Highly Efficient Organic and Perovskite Solar Cells

Herein, a novel strategy is presented for enhancing light absorption by incorporating gold nanostars (Au NSs) into both the active layer of organic solar cells (OSCs) and the rear-contact hole transport layer of perovskite solar cells (PSCs). We demonstrate that the power conversion efficiencies of OSCs and PSCs with embedded Au NSs are improved by 6 and 14%, respectively. We find that pegylated Au NSs are greatly dispersable in a chlorobenzene solvent, which enabled complete blending of Au NSs with the active layer. The plasmonic contributions and accelerated charge transfer are believed to improve the short-circuit current density and the fill factor. This study demonstrates the roles of plasmonic nanoparticles in the improved optical absorption, where the improvement in OSCs was attributed to surface plasmon resonance (SPR) and in PSCs was attributed to both SPR and the backscattering effect. Additionally, devices including Au NSs exhibited a better charge separation/transfer, reduced charge recombination rate, and efficient charge transport. This work provides a comprehensive understanding of the roles of plasmonic Au NS particles in OSCs and PSCs, including an insightful approach for the further development of high-performance optoelectronic devices