Reproducible Ultrahigh SERS Enhancement in Single Deterministic Hotspots Using Nanosphere-Plane Antennas Under Radially Polarized Excitation

A review of recent advances in the use of complex metal nanostructures for biomedical applications from diagnosis to treatment

Complex metal nanostructures represent an exceptional category of materials characterized by distinct morphologies and physicochemical properties. Nanostructures with shape anisotropies, such as nanorods, nanostars, nanocages, and nanoprisms, are particularly appealing due to their tunable surface plasmon resonances, controllable surface chemistries, and effective targeting capabilities. These complex nanostructures can absorb light in the near-infrared, enabling noteworthy applications in nanomedicine, molecular imaging, and biology. The engineering of targeting abilities through surface modifications involving ligands, antibodies, peptides, and other agents potentiates their effects. Recent years have witnessed the development of innovative structures with diverse compositions, expanding their applications in biomedicine. These applications encompass targeted imaging, surface-enhanced Raman spectroscopy, near-infrared II imaging, catalytic therapy, photothermal therapy, and cancer treatment. This review seeks to provide the nanomedicine community with a thorough and informative overview of the evolving landscape of complex metal nanoparticle research, with a specific emphasis on their roles in imaging, cancer therapy, infectious diseases, and biofilm treatment. This article is categorized under: Diagnostic Tools > In Vivo Nanodiagnostics and Imaging Therapeutic Approaches and Drug Discovery > Nanomedicine for Infectious Disease Diagnostic Tools > Diagnostic Nanodevices.

Mode-Specific Coupling of Nanoparticle-on-Mirror Cavities with Cylindrical Vector Beams

Nanocavities formed by ultrathin metallic gaps permit the reproducible engineering and enhancement of light-matter interaction, with mode volumes reaching the smallest values allowed by quantum mechanics. While the enhanced vacuum field in metallic nanogaps has been firmly evidenced, fewer experimental reports have examined the far-field to near-field input coupling under strongly focused laser beam. Here, we experimentally demonstrate selective excitation of nanocavity modes controlled by the polarization and frequency of the laser beam. We reveal mode selectivity by recording confocal maps of Raman scattering excited by cylindrical vector beams, which are compared to the known excitation near-field patterns. Our measurements reveal the transverse vs longitudinal polarization of the excited antenna mode and how the input coupling rate depends on laser wavelength. The method introduced here is easily applicable to other experimental scenarios, and our results help connect far-field with near-field parameters in quantitative models of nanocavity-enhanced phenomena.

Microscopic Understanding of Reaction Rates Observed in Plasmon Chemistry of Nanoparticle-Ligand Systems

Surface-enhanced Raman scattering (SERS) is an effective and widely used technique to study chemical reactions induced or catalyzed by plasmonic substrates, since the experimental setup allows us to trigger and track the reaction simultaneously and identify the products. However, on substrates with plasmonic hotspots, the total signal mainly originates from these nanoscopic volumes with high reactivity and the information about the overall consumption remains obscure in SERS measurements. This has important implications; for example, the apparent reaction order in SERS measurements does not correlate with the real reaction order, whereas the apparent reaction rates are proportional to the real reaction rates as demonstrated by finite-difference time-domain (FDTD) simulations. We determined the electric field enhancement distribution of a gold nanoparticle (AuNP) monolayer and calculated the SERS intensities in light-driven reactions in an adsorbed self-assembled molecular monolayer on the AuNP surface. Accordingly, even if a high conversion is observed in SERS due to the high reactivity in the hotspots, most of the adsorbed molecules on the AuNP surface remain unreacted. The theoretical findings are compared with the hot-electron-induced dehalogenation of 4-bromothiophenol, indicating a time dependency of the hot-carrier concentration in plasmon-mediated reactions. To fit the kinetics of plasmon-mediated reactions in plasmonic hotspots, fractal-like kinetics are well suited to account for the inhomogeneity of reactive sites on the substrates, whereas also modified standard kinetics model allows equally well fits. The outcomes of this study are on the one hand essential to derive a mechanistic understanding of reactions on plasmonic substrates by SERS measurements and on the other hand to drive plasmonic reactions with high local precision and facilitate the engineering of chemistry on a nanoscale.

Scanning probe microscopy by localized surface plasmon resonance at fiber taper tips

Plasmonic antenna probes have been widely investigated for detecting electrical permittivity changes on the nanometer scale by employing high-sensitivity localized surface plasmon resonance (LSPR). Although it is intuitive to integrate such a probe onto an atomic force microscope (AFM) to add one more measurable quantity to the family of scanning probe microscopy techniques, the strong scattering background of the AFM tip overwhelms the LSPR scattering signal. To solve this problem, we combined evanescent coupling, polarization and spatial filtering, confocal spectroscopy, and numerical methods to extract clean LSPR spectra from a gold nanosphere-antenna probe attached to the tip of a fiber taper. By mounting the fiber taper on a custom quartz-tuning-fork SPM, we achieved high-quality nanometer-scale imaging of gold nanospheres on glass slides by mapping the LSPR wavelength shift. In addition, we reported an LSPR wavelength shift enhancement by more complicated probe designs and the consequent promise for higher-sensitivity microscopy. Our optical system and spectral processing method provide an effective solution to the long-standing quest for LSPR scanning microscopy.

Synthesis and SERS application of gold and iron oxide functionalized bacterial cellulose nanocrystals (Au@Fe3O4@BCNCs)

Bacterial cellulose nanocrystals (BCNCs) are biocompatible cellulose nanomaterials that can host guest nanoparticles to form hybrid nanocomposites with a wide range of applications. Herein, we report the synthesis of a hybrid nanocomposite that consists of plasmonic gold nanoparticles (AuNPs) and superparamagnetic iron oxide (Fe3O4) nanoparticles supported on BCNCs. As a proof of concept, the hybrid nanocomposites were employed to isolate and detect malachite green isothiocyanate (MGITC) via magnetic separation and surface-enhanced Raman scattering (SERS). Different initial gold precursor (Au3+) concentrations altered the size and morphology of the AuNPs formed on the nanocomposites. The use of 5 and 10 mM Au3+ led to a heterogenous mix of spherical and nanoplate AuNPs with increased SERS enhancements, as compared to the more uniform AuNPs formed using 1 mM Au3+. Rapid and sensitive detection of MGITC at concentrations as low as 10-10 M was achieved. The SERS intensity of the normalized Raman peak at 1175 cm-1 exhibited a log-linear relationship for MGITC concentrations between 2 × 10-10 and 2 × 10-5 M for Au@Fe3O4@BCNCs. These results suggest the potential of these hybrid nanocomposites for application in a broad range of analyte detection strategies.

Linear scattering off a dynamically controlled nanosphere-mirror plasmonic antenna on a fiber taper

A quartz-tuning-fork shear-force microscope was used to demonstrate the gap size dependency of the resonance frequency for a nanosphere-mirror plasmonic antenna. The nanosphere was mounted at the end of a fiber taper scanning probe. A semi-transparent silicon film mirror was used to couple evanescent fields from incident light with the plasmonic antenna using an inverted optical microscope. The plasmon resonance spectra were acquired with a 0.4 nm-step gap size tuning resolution, and were confirmed by finite-difference time-domain simulations. The proposed technique provides a dynamic approach to tuning and detecting distance-dependent localized surface plasmon resonance with a sub-nanometer step resolution.

Molecular Affinity Agents for Intrinsic Surface-Enhanced Raman Scattering (SERS) Sensors

Research at the interface of synthetic materials, biochemistry, and analytical techniques has enabled sensing platforms for applications across many research communities. Herein we review the materials used as affinity agents to create surface-enhanced Raman spectroscopy (SERS) sensors. Our scope includes those affinity agents (antibody, aptamer, small molecule, and polymer) that facilitate the intrinsic detection of targets relevant to biology, medicine, national security, environmental protection, and food safety. We begin with an overview of the analytical technique (SERS) and considerations for its application as a sensor. We subsequently describe four classes of affinity agents, giving a brief overview on affinity, production, attachment chemistry, and first uses with SERS. Additionally, we review the SERS features of the affinity agents, and the analytes detected by intrinsic SERS with that affinity agent class. We conclude with remarks on affinity agent selection for intrinsic SERS sensing platforms.

Design of Ag nanorods for sensitivity and thermal stability of surface-enhanced Raman scattering

The technology of surface-enhanced Raman scattering (SERS) has found many applications and may find more if it can possess both sensitivity and thermal stability. This paper reports a rational design of Ag nanorods to simultaneously achieve two competing goals: the sensitivity and the thermal stability of SERS substrates. The Ag nanorods are designed and synthesized using physical vapor deposition under the condition of glancing angle incidence. The working pressure of the vacuum chamber is controlled so the mean free path of depositing atoms is comparable to the dimension of the chamber, so as to grow Ag nanorods with small diameter, and small but clear separation for optimal SERS sensitivity. Such Ag nanorods are further capped with Al₂O₃ on their top surfaces to reduce the diffusion-induced coarsening at high temperatures, and thereby to improve the thermal stability for SERS detections. Meanwhile, since the side surfaces of Ag nanorods are not coated with oxides in this approach, the SERS sensitivity is largely preserved while good thermal stability is achieved.

Semi-quantitative analysis of multiple chemical mixtures in solution at trace level by surface-enhanced Raman Scattering

Surface-enhanced Raman scattering (SERS) technology combines with chemometric method of principal component analysis (PCA) was used to calculate the composition of chemical mixtures in solution. We reported here that there exists composition discrepancy between molecules in solution and molecules adsorbed on Ag@Al₂O₃ nanorods substrates due to difference in adsorption kinetics of each component. We proposed here a way to calculate the adsorption kinetics factor for each component using a standard sample as the reference, with which one could correct the predictions given by PCA. We demonstrate the validity of this approach in estimating the compositions of mixtures with two, three and four components of 1, 4-Benzenedithiol, 2-Naphthalenethiol, 4-Mercaptobenzoic acid, and 4-Mercaptopyridine molecules, with acceptable errors. Furthermore, a general formula applied to more complex mixtures was proposed to calculate compositions in solution.

Rapid detection of hexamethylenetetramine based on the substrate UC@SiO2@Au@Ag using SERS

UC@SiO₂@Au@Ag was employed for the first time as a SERS substrate to detect HMT.

Electromagnetic theories of surface-enhanced Raman spectroscopy

A fundamental theoretical understanding of SERS, and SERS hotspots, leads to new design principles for SERS substrates and new applications in nanomaterials and chemical analysis.