Enhancing Surface Sensing Sensitivity of Metallic Nanostructures using Blue-Shifted Surface Plasmon Mode and Fano Resonance

Optical Sensing Using Hybrid Multilayer Grating Metasurfaces with Customized Spectral Response

Customized metasurfaces allow for controlling optical responses in photonic and optoelectronic devices over a broad band. For sensing applications, the spectral response of an optical device can be narrowed to a few nanometers, which enhances its capabilities to detect environmental changes that shift the spectral transmission or reflection. These nanophotonic elements are key for the new generation of plasmonic optical sensors with custom responses and custom modes of operation. In our design, the metallic top electrode of a hydrogenated amorphous silicon thin-film solar cell is combined with a metasurface fabricated as a hybrid dielectric multilayer grating. This arrangement generates a plasmonic resonance on top of the active layer of the cell, which enhances the optoelectronic response of the system over a very narrow spectral band. Then, the solar cell becomes a sensor with a response that is highly dependent on the optical properties of the medium on top of it. The maximum sensitivity and figure of merit (FOM) are SB = 36,707 (mA/W)/RIU and ≈167 RIU-1, respectively, for the 560 nm wavelength using TE polarization. The optical response and the high sensing performance of this device make it suitable for detecting very tiny changes in gas media. This is of great importance for monitoring air quality and thecomposition of gases in closed atmospheres.

Comprehensive field pattern analysis for tailoring of reflectance in a hybrid subwavelength plasmonic grating refractive index sensor and its potential for noninvasive salivary glucose monitoring

A compact hybrid two-dimensional plasmonic subwavelength grating composed of gold and semiconductor ZnS is proposed. By implementing the finite-difference time-domain numerical technique, detailed field pattern analysis and reflectance characteristics of the grating structure are comprehensively investigated, tailored, and improved. An unfamiliar phenomenon of exponential decrease in resonance wavelength with an increase in groove width is observed, validated, and empirically modeled. This confirms that the reflectance resonance dip is because of the surface plasmon resonance in the grating structure, unlike the resonance dip obtained in the diffraction grating because of the Fabry-Perot resonance. A rigorous sensitivity analysis is performed for both generalized bulk and surface analyte detection. The surface sensitivity is observed to be 100.5 nm/RIU at dip 1 for 10-nm-surface analyte thickness. The bulk sensitivity for dip 1 and dip 2 was 104.3 nm/RIU and 800 nm/RIU, respectively. The refractive index range variation of dip 1 for the surface analyte is correlated with the refractive index of the blood by using the linear refractive index model and Gladstone-Dale law for blood. A linear regression analysis correlating blood glucose and salivary glucose with a surface analyte is used. The proposed sensor is observed to be promising for noninvasive salivary glucose monitoring with high surface sensitivity of 1.104 nm/mg/dl with a compact footprint of about 0.5µm×0.2µm in x-z dimensions.

Direct detection of virus-like particles using color images of plasmonic nanostructures

We propose a measurement method for sensitive and label-free detections of virus-like particles (VLPs) using color images of nanoplasmonic sensing chips. The nanoplasmonic chip consists of 5×5 gold nanoslit arrays and the gold surface is modified with specific antibodies for spike protein. The resonant wavelength of the 430-nm-period gold nanoslit arrays underwater environment is about 570 nm which falls between the green and red bands of the color CCD. The captured VLPs by the specific antibodies shift the plasmonic resonance of the gold nanoslits. It results in an increased brightness of green pixels and decreased brightness of red pixels. The image contrast signals of (green - red) / (red + green) show good linearity with the surface particle density. The experimental tests show the image contrast method can detect 100-nm polystyrene particles with a surface density smaller than 2 particles/µm². We demonstrate the application for direct detection of SARS-CoV-2 VLPs using a simple scanner platform. A detection limit smaller than 1 pg/mL with a detection time less than 30 minutes can be achieved.

Screening anti-metastasis drugs by cell adhesion-induced color change in a biochip

Metastasis is a frequent complication of cancer and accounts for more than 60% of patients' mortality. Despite technological advancements, treatment options are still limited. Ion channels participate in the regulation of cell adhesion, whilst the regulation of cell adhesion further controls metastasis formation. However, to develop a new ion channel inhibitor targeting metastasis takes tremendous effort and resources; therefore, drug repurposing is an emerging strategy in oncology. In previous studies, we have developed a metal-based nanoslit surface plasmon resonance (SPR) platform to examine the influence of drugs on the cell adhesion process. In this work, we developed a scanner-based cell adhesion kinetic examination (CAKE) system that is capable of monitoring the cell adhesion process by measuring color changes of SPR biosensors. The system's performance was demonstrated by screening the anti-metastasis ability of compounds from a commercial ion-channel inhibitor library. Out of the 274 compounds from the inhibitor library, zinc pyrithione (ZPT) and terfenadine were demonstrated to influence CL1-5 cell adhesion. The cell responses to the two compounds were then compared with those by traditional cell adhesion assays where similar behavior was observed. Further investigation of the two compounds using wound healing and transwell assays was performed and inhibitions of both cell migration and invasion by the two compounds were also observed. The results indicate that ZPT and terfenadine are potential candidates for anti-metastasis drugs. Our work has demonstrated the label-free drug screening ability of our CAKE system for finding potential drugs for cancer treatment.

Coupled plasmonic systems: controlling the plasmon dynamics and spectral modulations for molecular detection

This review describes recent studies on coupled plasmonic systems for controlling plasmon dynamics and molecular detection using spectral modulations. The plasmon dephasing time can be controlled by weak and strong coupling regimes between the plasmonic nanostructures or localized surface plasmon resonances (LSPRs) and the other optical modes such as microcavities. The modal coupling induces near-field enhancement by extending the plasmon dephasing time to increase the near-field enhancement at certain wavelengths resulting in the enhancement of molecular detection. On the other hand, the interaction between LSPR and molecular excited or vibrational states also modulates the resonance spectrum, which can also be used for detecting a small number of molecules with a subtle change in the spectrum. The spectral modulation is induced by weak and strong couplings between LSPRs and the electronic or vibrational states of molecules, and this method is sensitive enough to measure a single molecule.

Conformational Stability of Poly (N-Isopropylacrylamide) Anchored on the Surface of Gold Nanoparticles

To verify the temperature sensitive failure of poly (N-isopropylacrylamide) (PNIPAM) anchored on the surface of gold nanoparticles (AuNPs), the UV-Vis spectra with temperature variations of the following aqueous solutions respectively containing AuNPs-PNIPAM, Au-PNIPAM/PNIPAM, PNIPAM, in different media (including salt, ethanol, HCl and cetyltrimethylammoniumbromide (CTAB)), were systematically determined. The results indicated that the UV-Vis spectrum of AuNPs-PNIPAM suspension hardly changed even above the Lower Critical Solution Temperature (LCST) of PNIPAM, but that of Au-PNIPAM/PNIPAM sharply increased only in absorbance intensity. A possible mechanism of the failed temperature sensitivity of PNIPAM anchored on the surface of AuNPs was proposed. Being different from free PNIPAM molecules, a strong interaction exists among PNIPAM molecules anchored on the surface of AuNPs, restraining the change in conformation of PNIPAM. The temperature sensitivity of Au-PNIPAM/PNIPAM originates from the free PNIPAM molecules rather than the anchored PNIPAM one. The changing electrostatic interaction could effectively regulate the aggregation behavior of AuNPs-PNIPAM and enhance its sensitivity to temperature.

LMP1 gene detection using a capped gold nanowire array surface plasmon resonance sensor in a microfluidic chip

A new detection device by using SPR nanowire array chip and a microfluidics system was developed. A simple, low-cost and reproducible SPR nanowire chip with a visible light source displayed real-time detection capability.

Superstructure of silver crystals in a caged framework for plasmonic inverse sensing

Lowering the limit of detection is key to the design of sensors. Conventional transducers generate a signal proportional to the concentration of the molecule, but low concentrations are still difficult to detect with confidence. Here we present an inverse sensor that induces a signal that is larger when the target molecule is less concentrated. Each sensor consists of a micro-pot reactor with an inner volume for storing the reactants and the cage walls, over which many silver hotspots are spread, working as optical antenna to produce amplification of the signal. The aim is to attain inverse sensitivity during the enzymatic reaction, where reduction of the silver occurs. We demonstrate the sensitivity and robustness of this approach by detecting glucose concentrations down to 10^-12 M.

High-Density Plasmonic Nanoparticle Arrays Deposited on Nanoporous Anodic Alumina Templates for Optical Sensor Applications

This study demonstrates a new, robust, and accessible deposition technique of metal nanoparticle arrays (NPAs), which uses nanoporous anodic alumina (NAA) as a template for capillary force-assisted convective colloid (40, 60, and 80 nm diameter Au) assembly. The NPA density and nanoparticle size can be independently tuned by the anodization conditions and colloid synthesis protocols. This enables production of non-touching variable-density NPAs with controllable gaps in the 20-60 nm range. The NPA nearest neighbor center distance in the present study was fixed to 100 nm by the choice of anodization protocol. The obtained Au NPAs have the resonant scattering maxima in the visible spectral range, with a refractometric sensitivity, which can be tuned by the variation of the array density. The thickness of the NAA layer in an Aluminum-NAA-NPA multilayer system enables further tuning of the resonance frequency and optimization for use with specific molecules, e.g., to avoid absorption bands. Applicability of the mentioned multilayers for colorimetric refractive index (RI) sensing is demonstrated. Their use as Surface-Enhanced Raman Scattering (SERS) substrates is tested using hemoglobin as a biological probe molecule.