Broadside Nanoantennas Made of Single Silver Nanorods

Metal-Organic Framework-Enabled Trapping of Volatile Organic Compounds into Plasmonic Nanogaps for Surface-Enhanced Raman Scattering Detection

Utilizing electromagnetic hotspots within plasmonic nanogaps is a promising approach to create ultrasensitive surface-enhanced Raman scattering (SERS) substrates. However, it is difficult for many molecules to get positioned in such nanogaps. Metal-organic frameworks (MOFs) are commonly used to absorb and concentrate diverse molecules. Herein, we combine these two strategies by introducing MOFs into plasmon-coupled nanogaps, which has so far remained experimentally challenging. Ultrasensitive SERS substrates are fabricated through the construction of nanoparticle-on-mirror structures, where Au nanocrystals are encapsulated with a zeolitic imidazolate framework-8 (ZIF-8) shell and then coupled to a gold film. The ZIF-8 shell, as a spacer that separates the Au nanocrystal and the Au film, can be adjusted in thickness over a wide range, which allows the electric field enhancement and plasmon resonance wavelength to be varied. By trapping Raman-active molecules within the ZIF-8 shell, we show that our plasmon-coupled structures exhibit a superior SERS detection performance. A range of volatile organic compounds at the concentrations of 10-2 mg m-3 can be detected sensitively and reliably. Our study therefore offers an attractive route for synergistically combining plasmonic electric field enhancement and MOF-enabled molecular enrichment to design and create SERS substrates for ultrasensitive detection.

Celebrating 60 Years of The Chinese University of Hong Kong: Research Highlights in Nanoscience and Nanotechnology

Recent breakthroughs and advances in nanoscience and nanotechnology have profoundly impacted young-generation education, accelerated knowledge transfer to enhance the quality of life, and improved environmental and economic sustainability. The Chinese University of Hong Kong (CUHK), a globally recognized education and research institute, has played a crucial role in promoting major strategic research directions in nanoscience, including translational biomedicine and information and automation technology, as well as environment and sustainability. To celebrate the 60th Anniversary of CUHK, we present this Virtual Issue that showcases the cutting-edge research at CUHK published in ACS Nano.

Research Progress in Surface-Enhanced Infrared Absorption Spectroscopy: From Performance Optimization, Sensing Applications, to System Integration

Infrared absorption spectroscopy is an effective tool for the detection and identification of molecules. However, its application is limited by the low infrared absorption cross-section of the molecule, resulting in low sensitivity and a poor signal-to-noise ratio. Surface-Enhanced Infrared Absorption (SEIRA) spectroscopy is a breakthrough technique that exploits the field-enhancing properties of periodic nanostructures to amplify the vibrational signals of trace molecules. The fascinating properties of SEIRA technology have aroused great interest, driving diverse sensing applications. In this review, we first discuss three ways for SEIRA performance optimization, including material selection, sensitivity enhancement, and bandwidth improvement. Subsequently, we discuss the potential applications of SEIRA technology in fields such as biomedicine and environmental monitoring. In recent years, we have ushered in a new era characterized by the Internet of Things, sensor networks, and wearable devices. These new demands spurred the pursuit of miniaturized and consolidated infrared spectroscopy systems and chips. In addition, the rise of machine learning has injected new vitality into SEIRA, bringing smart device design and data analysis to the foreground. The final section of this review explores the anticipated trajectory that SEIRA technology might take, highlighting future trends and possibilities.

Rebuildable Silver Nanoparticles Employed as Seeds for Synthesis of Pure Silver Nanopillars with Hexagonal Cross-Sections under Room Temperature

Silver nanopillars with strong plasmonic effects are used for localized electromagnetic field enhancement and regulation and have wide potential applications in sensing, bioimaging, and surface-enhanced spectroscopy. Normally, the controlled synthesis of silver nanopillars is mainly achieved using heterometallic nanoparticles, including Au nanobipyramids and Pd decahedra, as seeds for inducing nanostructure growth. However, the seed materials are usually doped in silver nanopillar products. Herein, the synthesis of pure silver nanopillars with hexagonal cross-sections is achieved by employing rebuildable silver nanoparticles as seeds. An environmentally friendly, stable, and reproducible synthetic route for obtaining silver nanopillars is proposed using sodium dodecyl sulfate as the surface stabilizer. Furthermore, the seed particles induce the formation of regular structures at different temperatures, and, specifically, room temperature is beneficial for the growth of nanopillars. The availability of silver nanoparticle seeds using sodium alginate as a carrier at different temperatures was verified. A reproducible method was developed to synthesize pure silver nanopillars from silver nanoparticles at room temperature, which can provide a strategy for designing plasmonic nanostructures for chemical and biological applications.

Investigation of Plasmonic-Enhanced Solar Photothermal Effect of Au NR@PVDF Micro-/Nanofilms

Gold nanospheres (Au NSs) and gold nanorods (Au NRs) are traditional noble metal plasmonic nanomaterials. Particularly, Au NRs with tunable longitudinal plasmon resonance from the visible to the near-infrared (NIR) range were suitable for highly efficient photothermal applications due to the extended light-receiving range. In this work, we synthesized Au NRs and Au NSs of similar volumes and subsequently developed them into Au NR/poly(vinylidene fluoride) (PVDF) and Au NS/PVDF nanofilms, both of which exhibited excellent solar photothermal performance evaluated by solar photothermal experiments. We found that the Au NR/PVDF nanofilm showed a higher solar photothermal performance than the Au NS/PVDF nanofilm. Through detailed analysis, such as morphological characterization, optical measurement, and finite element method (FEM) modeling, we found that the plasmonic coupling effects inside the aggregated Au NR nanoclusters contributed to the spectral blue shifts and intensified the photothermal performance. As compared to Au NS/PVDF nanofilms, the Au NR/PVDF nanofilm exhibited a higher efficient light-to-heat conversion rate because of the extended light-receiving range and high absorbance, as a result of the strong plasmonic interactions inside nanoclusters, which was further validated by monochromatic laser photothermal experiments and FEM simulations. Our work proved that the Au NRs have huge potential for plasmonic solar photothermal applications and are envisioned for novel plasmonic applications.

Mode-dependent energy exchange between near- and far-field through silicon-supported single silver nanorods

Optical antenna effects endow plasmonic nanoparticles with the capability to enhance and control various types of light-matter interaction. Most reported plasmonic systems can be regarded as single-channel nanoantennas, which rely only on a bright dipole plasmon mode for energy exchange between near- and far-field. Herein we demonstrate a dual-channel plasmonic system that can separate the excitation and emission processes into two energy exchange pathways mediated by the different plasmon modes, offering a higher degree of freedom for the manipulation of light-matter interaction. Our system, consisting of high-aspect-ratio Ag nanorods and Si substrates, can support a series of bright and dark plasmon modes with distinct near- and far-field properties and generate relatively intensive local field enhancement in the gap region. As a proof-of-principle, we take plasmon-enhanced fluorescence of dye molecules as an example to reveal the energy exchange mechanism in the dual-channel plasmonic system. Such a system is potentially also useful for manipulating other types of light-matter interaction. Our work represents a step toward the utilization of a broader class of plasmon resonance for the development of optical antennas and various on-chip nanophotonic components.

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.

Navigation of Silver/Carbon Nanoantennas in Organic Fluids Explored by a Two-Wave Mixing

Within this work are analyzed third-order nonlinear optical properties with a potential influence on the dynamic mechanics exhibited by metal/carbon nanofluids. The nanofluids were integrated by multiwall carbon nanotubes decorated with Ag nanoparticles suspended in ethanol or in acetone. Optical third-order nonlinearities were experimentally explored by vectorial two-wave mixing experiments with a Nd-YAG laser system emitting nanosecond pulses at a 532 nm wavelength. An optically induced birefringence in the metal/organic samples seems to be responsible for a significant modification in density and compressibility modulus in the nanosystems. The measured nonlinear refractive index was associated with a thermal process together with changes in density, compressibility modulus and speed of sound in the samples. Nanofluid diffusivity was studied to characterize the dynamic concentration gradients related to the precipitation of nanostructures in the liquid solutions. The evolution of the nanoparticle density suspended in the nanofluids was considered as a temporal-resolved probabilistic system. It is stated that the incorporation of Ag nanoparticles in carbon nanotubes produces strong mechanical changes in carbon-based nanofluids. According to numerical simulations and optical evaluations, immediate applications for developing dynamic nanoantennas optical logic gates and quantum-controlled metal/carbon systems can be contemplated.

Evolution of near- and far-field optical properties of Au bipyramids upon epitaxial deposition of Ag

Bimetallic plasmonic nanostructures provide composition and spatial distribution of the individual components in the nanostructure in addition to overall size and morphology as degrees of freedom for tuning near- and far-field optical responses. AgAuAg nanorods (NRs) generated through epitaxial deposition of Ag on the tips of Au bipyramids (BPs) are an important bimetallic model system whose longitudinal dipolar plasmon mode first shows a spectral blue-shift upon initial deposition of Ag on the Au BP tips followed by a red-shift after additional deposition of Ag. Here, we quantify the relative contributions from morphological and compositional effects to the far-field spectral shift of the longitudinal and vertical dipolar plasmon modes during the initial deposition of Ag and compare the near-field in Ag and AgAuAg NRs with lengths between L = 130 nm-280 nm under whitelight illumination through electromagnetic simulations. Subsequently, we experimentally characterize the near-field around AgAuAg NRs with lengths between L = 88.1-749.0 nm at a constant excitation wavelength of 1064 nm on a silicon (Si) support through scattering type near-field scanning microscopy (sNSOM). We detect Fabry-Perot resonance-like higher order multipolar plasmon resonances whose order and near-field pattern depends on the length and composition of the NRs as well as the refractive index of the ambient medium. We find that under oblique illumination higher order multipolar modes with an even symmetry dominate on the high refractive index Si substrate due to strong electromagnetic interactions between the NR and the substrate.

Gold Nanobipyramids: An Emerging and Versatile Type of Plasmonic Nanoparticles

Gold nanobipyramids (Au NBPs) and gold nanorods (Au NRs) are two types of elongated plasmonic nanoparticles with their longitudinal dipolar plasmon wavelengths synthetically tunable from the visible region to the near-infrared region. Both have highly polarization-dependent absorption and scattering cross sections because of their anisotropic geometries. In terms of their differences, each Au NBP has five equally angularly separated twinning planes that are aligned parallel to the length direction, while the most common Au NRs are single-crystalline. As a result, Au NBPs possess two sharp end tips, while Au NRs have rounded or flat ends, resulting in very different plasmonic properties. In general, Au NBPs exhibit larger local electric field enhancements, larger optical cross sections, narrower line widths, better shape and size uniformity, and higher refractive index sensitivity than Au NRs. With the recent development of reliable methods for the growth of Au NBPs with high purity and uniformity, Au NBPs have been attracting much interest for the investigation of their intriguing plasmonic properties and applications. In this Account, we provide a concise introduction to Au NBPs, including their fascinating plasmonic properties, wet-chemistry growth methods, plasmonic applications, and structure-directing function. The synthesis of uniform Au NBPs with variable sizes is of vital importance to control their plasmonic properties. In the synthesis part, we summarize the recent developments on the synthesis of Au NBPs, with a focus on the role of seeds in the seed-mediated growth of pentatwinned Au NBPs and methods to improve their number purity. The excellent plasmonic properties of Au NBPs make them promising candidates for numerous applications. To further explore the largely improved functionalities of Au NBPs, different types of Au-NBP-based hybrid nanostructures have been prepared. They exhibit synergistic interactions between Au NBPs and the other components. We highlight the widespread plasmonic applications of Au NBPs and Au-NBP-based hybrid nanostructures in the fields of spectroscopy, photocatalysis, sensing, switching, and biomedical technologies. We next turn to the structure-directing function of Au NBPs to demonstrate the Au-NBP-directed growth of metal nanostructures and their applications. The structure-directing function is enabled by the unique pentatwinned crystalline structure of Au NBPs. Finally, we conclude with remarks on the future perspectives and research directions on Au NBPs as well as the remaining challenges. We hope that this Account will act as a platform to offer fascinating opportunities and stimulate fast-growing research on the various aspects of Au NBPs.

Colour routing with single silver nanorods

Elongated plasmonic nanoparticles have been extensively explored over the past two decades. However, in comparison with the dipolar plasmon mode that has attracted the most interest, much less attention has been paid to multipolar plasmon modes because they are usually thought to be "dark modes", which are unable to interact with far-field light efficiently. Herein, we report on an intriguing far-field scattering phenomenon, colour routing, based on longitudinal multipolar plasmon modes supported by high-aspect-ratio single Ag nanorods. Taking advantage of the distinct far-field behaviours of the odd and even multipolar plasmon modes, we demonstrate two types of colour routing, where the incident white light can be scattered into several beams with different colours as well as different propagation directions. Because of the narrow linewidths of the longitudinal multipolar plasmon modes, there is little spectral overlap between the adjacent peaks, giving rise to outstanding colour selectivity. Our experimental results and theoretical model provide a simple yet effective picture for understanding the far-field behaviour of the longitudinal multipolar plasmon modes and the resultant colour routing phenomenon. Moreover, the outstanding colour routing capability of the high-aspect-ratio Ag nanorods enables nanoscale optical components with simple geometries for controlling the propagation of light below the diffraction limit of light.

The localized surface plasmon resonance induced edge effect of gold regular hexagonal nanoplates for reaction progress monitoring

The LSPR induced edge effect of Au regular nanoplates gradually disappears accompanied with the change of doughnut-shaped far-field scattering patterns (DNSPs) into scattering solid spots with the reaction proceeding with ferric iron in a thiourea (TU) solution.