Plasmonic refractive index sensing using strongly coupled metal nanoantennas: nonlocal limitations

Potential-Modulated Surface-Enhanced Raman Spectroscopy of Tolmetin at Gold Nanoparticle Film Functionalized Polarizable Liquid-Liquid Interfaces

An aqueous colloidal suspension of gold nanoparticles (AuNPs) may be condensed into a thin fractal film at the polarizable liquid-liquid interface formed between two immiscible electrolyte solutions upon injection of millimolar concentrations of sodium chloride to the aqueous phase. By adjusting the interfacial polarization conditions (negative, intermediate, and positive open-circuit potentials), the morphology of the film is modified, resulting in unique surface plasmon properties of the film, which enable in situ surface-enhanced Raman spectroscopy (SERS). Intense SERS signals are observed at the polarizable liquid-liquid interface when micromolar concentrations of tolmetin, a nonsteroidal anti-inflammatory drug, are entrapped in the AuNP fractal film. The change in the signal intensity, averaged over multiple spectra, with respect to the concentration of tolmetin, depends on the polarization conditions and suggests the presence of chemical-induced damping effects on the surface plasmons of the gold film.

Investigation of Performance Parameters of Spherical Gold Nanoparticles in Localized Surface Plasmon Resonance Biosensing

In this paper, we present numerical and experimental results on Localized Surface Plasmon Resonance (LSPR) refractive index (RI) sensitivity, Figure of Merit (FoM), and penetration depth (dp) dependence on spherical gold nanoparticles (AuNPs) size, and the effects of AuNP dimer interparticle distance (ds) studied numerically. These parameters were calculated and observed for d = 20, 40, 60, 80, and 100 nm diameter spherical AuNPs. Our investigation shows d = 60 nm AuNPs give the best FoM. The AuNP dimer interparticle distance can significantly influence the RI sensitivity. Therefore, the effect of distances between pairs of d = 20 nm and 60 nm AuNPs is shown. We discuss the importance of penetration depth information for AuNPs functionalized with aptamers for biosensing in the context of aptamer size.

Highly Stable Low-Strain Flexible Sensors Based on Gold Nanoparticles/Silica Nanohelices

Flexible strain sensors based on nanoparticle (NP) arrays show great potential for future applications such as electronic skin, flexible touchscreens, healthcare sensors, and robotics. However, even though these sensors can exhibit high sensitivity, they are usually not very stable under mechanical cycling and often exhibit large hysteresis, making them unsuitable for practical applications. In this work, strain sensors based on silica nanohelix (NH) arrays grafted with gold nanoparticles (AuNPs) can overcome these critical aspects. These 10 nm AuNPs are functionalized with mercaptopropionic acid (MPA) and different ratios of thiol-polyethylene glycol-carboxylic acid (HS-PEG7-COOH) to optimize the colloidal stability of the resulting NH@AuNPs nanocomposite suspensions, control their aggregation state, and tune the thickness of the insulating layer. They are then grafted covalently onto the surface of the NHs by chemical coupling. These nanomaterials exhibit a well-defined arrangement of AuNPs, which follows the helicity of the silica template. The modified NHs are then aligned by dielectrophoresis (DEP) between interdigitated electrodes on a flexible substrate. The flexibility, stability, and especially sensitivity of these sensors are then characterized by electromechanical measurements and scanning electron microscopy observations. These strain sensors based on NH@AuNPs nanocomposites are much more stable than those containing only nanoparticles and exhibit significantly reduced hysteresis and high sensitivity at very slight strains. They can retain their sensitivity even after 2 million consecutive cycles with virtually unchanged responsiveness. These improved performances come from their mechanical stability and the use of nanohelices as stable mechanical templates.

Point-of-Care Biosensing of Urinary Tract Infections Employing Optoplasmonic Surfaces Embedded with Metal Nanotwins

We report the synthesis of gold nanotwins (Au NTs) on a solid and transparent glass substrate which in turn has been employed for the selective optoplasmonic detection of Escherichia coli (EC) bacteria in human urine for the point-of-care diagnosis of urinary tract infections (UTIs). As compared to the single nanoparticle systems (Au NPs), the Au NTs show an enriched localized surface plasmon resonance (LSPR) due to the enhancement of the electric field under electromagnetic irradiation, e.g., photon, which helps in improving the limits of detection. For this purpose, initially a simple glass surface has been coated with Au NPs, with the help of the linker 3-aminopropyl-triethoxysilane - APTES. The surface has been linked further with another Au NP with the help of the 1,10-alkane-dithiol linker with two thiol ends, which eventually leads to the development of the optoplasmonic surface with Au NTs and an enhanced LSPR response. Subsequently, the EC specific aptamer has been chemically immobilized on the surface of Au NTs with the blocking of free sites via bovine serum albumin (BSA). Remarkably, Raman spectroscopy unfolds a 7-fold increase in the peak intensities with the Au NTs on the glass surface as compared to the surface coated with isolated Au NPs. The enhancement in the LSPR response of glass substrates coated with Au NTs and the EC specific aptamer has been further utilized for the selective and sensitive detection of UTIs. The results have been verified with the help of UV-visible spectroscopy to establish the utility of the proposed sensing methodology. An extensive interference study with other bacterial species unveils the selectivity and specificity of the proposed optoplasmonic sensors toward EC with a detection range of 5 × 10³ to 10⁷ CFU/mL. Intuitively, the method is more versatile in a sense that the sensor can be made specific to any other pathogens by simply changing the design of the aptamer. Finally, a low-cost, portable, and point-of-care optoplasmonic transduction setup is designed with a laser light illumination source, a sample holder, and a sensitive photodetector for the detection of UTIs in human urine.

Toward high-performance refractive index sensor using single Au nanoplate-on-mirror nanocavity

Refractive index sensors based on the localized surface plasmon resonance (LSPR) have emerged as powerful tools in various chemosensing and biosensing applications. However, owing to their limited decay length and strong radiation damping, LSPR sensors always suffer from low sensitivity and small figure of merit (FOM). Here, we fabricate a plasmonic nanocavity sensor consisting of a hexagonal Au nanoplate positioned over an ultrasmooth Au film. The strong coupling between the nanoplate and the lower metal film allows for the formation of a plasmonic gap mode that enhances the interaction of the local field with the ambient glycerol solution to increase the sensitivity. Meanwhile, the plasmonic gap mode has a trait of an antiphase charge oscillation in the gap region, imparting a strongly reduced radiative damping and a subsequently promoted FOM. The performance of our proposed refractive index sensor is further boosted by decreasing the gap size of the nanocavity, yielding an outstanding FOM of 11.2 RIU^-1 that is the highest yet reported for LSPR sensing in a single nanostructure.

Synthesis, Assembly, Optical Properties, and Sensing Applications of Plasmonic Gap Nanostructures

Plasmonic gap nanostructures (PGNs) have been extensively investigated mainly because of their strongly enhanced optical responses, which stem from the high intensity of the localized field in the nanogap. The recently developed methods for the preparation of versatile nanogap structures open new avenues for the exploration of unprecedented optical properties and development of sensing applications relying on the amplification of various optical signals. However, the reproducible and controlled preparation of highly uniform plasmonic nanogaps and the prediction, understanding, and control of their optical properties, especially for nanogaps in the nanometer or sub-nanometer range, remain challenging. This is because subtle changes in the nanogap significantly affect the plasmonic response and are of paramount importance to the desired optical performance and further applications. Here, recent advances in the synthesis, assembly, and fabrication strategies, prediction and control of optical properties, and sensing applications of PGNs are discussed, and perspectives toward addressing these challenging issues and the future research directions are presented.

Computational investigation of the plasmonic properties of TiN, ZrN, and HfN nanoparticles: the role of particle size, medium, and surface oxidation

Group 4 transition metal nitride (TMN) nanoparticles (NPs) display strong plasmonic responses in the visible and near-infrared regimes, exhibit high melting points and significant chemical stability, and thus are potential earth-abundant alternatives to Au and Ag based plasmonic applications. However, a detailed understanding of the relationship between TMN NP physical properties and plasmonic response is required to maximize their utility. In this study, the localized surface plasmon resonance (LSPR) frequency, bandwidth, and extinction of titanium nitride (TiN), zirconium nitride (ZrN), and hafnium nitride (HfN) NPs were examined as a function of the particle size, surface oxidation, and refractive index of the surrounding medium using finite element method (FEM). A linear redshift in the LSPR frequency and a linear increase in the associated full width at half maximum (FWHM) was observed with increasing the particle size, oxidation layer thickness, and medium refractive index. We show that the effect of surface oxidation on plasmonic properties of TMN NPs is strongly size-dependent with a significant LSPR redshift, intensity reduction, and broadening in small NPs compared with larger NPs. Furthermore, the performance and efficiency of HfN, ZrN, and TiN, as well as Au NPs for narrowband (photothermal therapy, PTT) and broadband (solar energy conversion) applications, was investigated in detail. The results indicate that narrowband and broadband photothermal performance of NPs strongly depend on the particle size, surface properties, and in case of narrowband absorption, excitation wavelength.

New hybridization coupling mechanism and enhanced sensitivity in a Cu2-xS@Au nanoparticle dimer

To improve the refractive index sensitivity of a localized surface plasmon resonance (LSPR) sensor, we employ a new interparticle hybridization plasmon coupled resonance in a semiconductor-metal (Cu_2-xS@Au) core-shell nanoparticle dimer (SMCSND), where the refractive index sensitivity can be improved by the generation of a tunable dual-band absorption spectrum at visible and near-infrared wavelengths. Owing to two LSPRs in different wavelength regions supported by the metal shell and semiconductor core, for the first time, we theoretically demonstrate that the new interparticle hybridization plasmon coupled mechanism in semiconductor-metal core-shell nanoparticle dimer depends not only on interparticle separation gap, but also on the nanoparticle shell thickness t. Electromagnetic model analysis reveals that there are two plasmon modes (Mode A and Mode C) associated with the interparticle hybridization plasmon coupled resonance, where the Mode C shows high sensitivity and figure of merit (FoM) to changes in the background dielectric medium. The tunability of the induced interparticle hybridization plasmon coupled resonance with different the separation distance and shell thickness can change the sensitivity and FoM of LSPR sensor in the visible to near-infrared region, which has broad application prospects.

Nano-colorimetrically determined refractive index variation with ultra-high chromatic resolution

We develop a front-to-end solution where the shift of chromaticity from scattering of plasmonic nanoparticles is used as the reporter for nano-environmental refractive index variation. By co-projecting possible power combinations of RGB LEDs and digitized color grid density of CCD with various luminance onto the CIE 1931 chromaticity diagram, optimum condition for nanoenvironment sensing can be achieved. The highest resolution for local refractive index change is 0.0021 per distinguishable color, which is higher than that of a typical handheld spectrometer by 4.8 times. This result shows great potential in simplifying nano-environment sensing instruments and is particularly useful for multi-point dynamical process.

Anomalously Large Spectral Shifts near the Quantum Tunnelling Limit in Plasmonic Rulers with Subatomic Resolution

The resonance wavelength of a coupled plasmonic system is extremely sensitive to the distance between its metallic surfaces, resulting in "plasmon rulers". We explore this behavior in the subnanometer regime using self-assembled monolayers of bis-phthalocyanine molecules in a nanoparticle-on-mirror (NPoM) construct. These allow unprecedented subangstrom control over spacer thickness via choice of metal center, in a gap-size regime at the quantum-mechanical limit of plasmonic enhancement. A dramatic shift in the coupled plasmon resonance is observed as the gap size is varied from 0.39 to 0.41 nm. Existing theoretical models are unable to account for the observed spectral tuning, which requires inclusion of the quantum-classical interface, emphasizing the need for new treatments of light at the subnanoscale.

Manipulating the confinement of electromagnetic field in size-specific gold nanoparticles dimers and trimers

Confinement of the electromagnetic field in gold nanoparticle dimers and trimers with variations in the interparticle distances and angles has been calculated.

Study of volume and surface plasmons in small silicon–hydrogen nanoclusters using the GW method

Numerical calculations of surface and volume plasma excitations in silicon–hydrogen nanoclusters in the range Si₁₀–Si₆₀ and Si₃H₈–Si₆₄H₅₆ (size range 4–13.5 Å) are performed within the GW approximation.