Sensitivity enhancement of a conventional gold grating assisted surface plasmon resonance sensor by using a bimetallic configuration

Advances in Nanoplasmonic Biosensors: Optimizing Performance for Exosome Detection Applications

The development of sensitive and specific exosome detection tools is essential because they are believed to provide specific information that is important for early detection, screening, diagnosis, and monitoring of cancer. Among the many detection tools, surface-plasmon resonance (SPR) biosensors are analytical devices that offer advantages in sensitivity and detection speed, thereby making the sample-analysis process faster and more accurate. In addition, the penetration depth of the SPR biosensor, which is <300 nm, is comparable to the size of the exosome, making the SPR biosensor ideal for use in exosome research. On the other hand, another type of nanoplasmonic sensor, namely a localized surface-plasmon resonance (LSPR) biosensor, has a shorter penetration depth of around 6 nm. Structural optimization through the addition of supporting layers and gap control between particles is needed to strengthen the surface-plasmon field. This paper summarizes the progress of the development of SPR and LSPR biosensors for detecting exosomes. Techniques in signal amplification from two sensors will be discussed. There are three main parts to this paper. The first two parts will focus on reviewing the working principles of each sensor and introducing several methods that can be used to isolate exosomes. This article will close by explaining the various sensor systems that have been developed and the optimizations carried out to obtain sensors with better performance. To illustrate the performance improvements in each sensor system discussed, the parameters highlighted include the detection limit, dynamic range, and sensitivity.

Optimizing Stability and Performance of Silver-Based Grating Structures for Surface Plasmon Resonance Sensors

The use of surface plasmon resonance sensors allows for the fabrication of highly sensitive, label-free analytical devices. This contribution reports on a grating coupler to enable surface plasmon resonance studies using silver on silicon oxide technology to build long-term stable plasmonic structures for biological molecule sensing. The structural parameters were simulated and the corresponding simulation model was optimized based on the experimental results to improve its reliability. Based on the model, optimized grating nanostructures were fabricated on an oxidized silicon wafer with different structural parameters and characterized using a dedicated optical setup and scanning electron microscopy. The combined theoretical and experimental results show that the most relevant refractive index range for biological samples from 1.32-1.46 may conveniently be covered with a highest sensitivity of 128.85°/RIU.

Theoretical and experimental study of a highly sensitive SPR biosensor based on Au grating and Au film coupling structure

A high-sensitivity surface plasmon resonance (SPR) sensor based on the coupling of Au grating and Au film is investigated through simulations and experiments. The SPR sensor is designed by using a hybrid method composed of genetic algorithm (GA) and rigorous coupled wave analysis (RCWA). The numerical results indicate the sensor has an angular sensitivity of 397.3°/RIU (refractive index unit), which is approximately 2.81 times higher than the conventional Au-based sensor and it is verified by experiments. Theoretical analysis, by finite-difference time-domain (FDTD) method, demonstrates the co-coupling between surface plasmon polaritons (SPPs) propagating on the surface of Au film and localized surface plasmons (LSPs) in the Au grating nanostructure, improving the sensitivity of the SPR sensor. According to the optimized structural parameters, the proposed sensor is fabricated using e-beam lithography and magnetron sputtering. In addition, the proposed sensor is very sensitive to the detection of small molecules. The limit of detection (LOD) for okadaic acid (OA) is 0.72 ng/mL based on an indirect competitive inhibition method, which is approximately 38 times lower than the conventional Au sensor. Such a high-sensitivity SPR biosensor has potential in the applications of immunoassays and clinical diagnosis.

Breaking the Symmetry of a Metal-Insulator-Metal-Based Resonator for Sensing Applications

This article designed a novel multi-mode plasmonic sensor based on a metal-insulator-metal waveguide side-coupled to a circular-shaped resonator containing an air path in the resonator. The electromagnet field distributions and transmittance spectra are investigated using finite element method-based simulations. Simulation results show that an air path in the resonator's core would impact the transmittance spectrum of SPPs. Besides, the air path is crucial in offering efficient coupling and generating multiple plasmon modes in the sensor system. The proposed structure has the advantage of multi-channel, and its sensitivity, figure of merit, and dipping strength can reach 2800 nm/RIU, 333.3 1/RIU, and 86.97%, respectively. The achieved plasmonic sensor can also apply for lab-on-chip in biochemical analysis for detecting the existence or nonappearance of diabetes through the human glucose concentration in urine.

Self-referenced refractive index sensor based on double-dips method with bimetal-dielectric and double-groove grating

Self-referenced refractive index sensors allow more accurate measurements and reduce the influence of extraneous factors. This work proposed a high-sensitivity, self-referenced surface plasmon resonance sensor with Na grating and Au-ZnS composite grating. When Transverse Magnetic-polarized light is incident into the prism, three surface plasmon resonances are excited at the interface of Na-MgF₂ grating and Au-ZnS grating. The first one is treated as the reference angle, the second and third are forward and backward surface plasmon resonance, respectively. Using the angular modulation, the single-dip sensitivities are 329.41 deg/RIU and 788.24 deg/RIU in the range of 1.330-1.347. To further improve the performance of the sensor, the double-dips method is adopted, and the average sensitivity in the range of 1.330-1.347 is 1117.65 deg/RIU, while the maximum reaches 4390 deg/RIU. Due to high sensitivity, a good figure of merit can be obtained even with a larger full width at half maximum of 3.30°. This proposed sensor provides potential application in the research of biomolecular detection and chemical testing.

On the performance of a tunable grating-based high sensitivity unidirectional plasmonic sensor

Optical biosensing is currently an intensively active research area, with an increasing demand of highly selective, sensitivity-enhanced and low-cost devices where different plasmonic approaches have been developed. In this work we propose a tunable optimized grating-based gold metasurface that can act both as a high sensitivity sensor device (up to 1500 nm/RIU) and as an unidirectional plasmon source. The theory behind surface plasmon polariton generation is recalled to thoroughly understand the influence that every parameter of the grating source has on the performance of the proposed device. The results and conclusions discussed here offer a key step toward the design of biosensors based on excitation of surface plasmons polaritons by grating-based structures or in the process of creating new nanophotonic circuit devices.

Sensitivity enhancement of an SPR biosensor with a graphene and blue phosphorene/transition metal dichalcogenides hybrid nanostructure

A novel surface plasmon resonance (SPR) biosensor based on Ag-Au bimetallic films with a hybrid structure of blue phosphorene (BlueP)/transition metal dichalcogenides (TMDCs) and graphene is presented. In order to improve the sensitivity, the thickness of silver and gold films is optimized to achieve minimum reflectivity and an adequate level of sensitivity; further, sensitivity for the monolayer BlueP/MoS₂ and graphene structure is enhanced by 19.73%, with respect to a traditional sensor. Besides, the effect of layers of different Blue/TMDCs heterostructures to the sensitivity of the SPR biosensor is investigated, and the highest sensitivity with 335.4°/RIU for the bilayer BlueP/WS₂ is obtained. Furthermore, distributions of the electric field and the changes of resonance angle to the refractive index of the sensing medium and prism in the visible regime are illustrated at optimal configuration. In virtue of highly sensitive characteristics, the proposed sensor structure will be a much better option to be employed for further biological detection.

Gold-aluminum-based surface plasmon resonance sensor with a high quality factor and figure of merit for the detection of hemoglobin

A gold (Au) and aluminum (Al)-based surface plasmon resonance (SPR) sensor, with a high-quality factor and figure of merit, for the detection of hemoglobin concentration in human blood is proposed. The sensing performance of a bimetallic Au grating over an Al film-based sensor depends on the thickness of the Al film. The sensor's performance is closely analyzed in terms of well-defined performance parameters: sensitivity, FWHM, and depth of SPR dips in order to optimize the thickness of Al film. With optimized thickness, we could achieve quality factor and figure of merit values of 286.2  RIU^-1 and 0.97  deg^-1, respectively. Al is chemically unstable, and, to address the oxidation problem, we analyzed the sensor with an ultrathin protective Au layer between the Au grating and Al film. We show that the introduction of an ultrathin Au layer gives stability to the sensor without much affecting the performance of the sensor. The obtained values of quality factor and figure of merit of the proposed sensor are 245.2  RIU^-1 and 0.86  deg^-1, respectively. We also discuss the potential of hemoglobin detection of the proposed sensor in the near-infrared region. Reported sensitivity of the sensor is 0.62°/dgL^-1.