Plasmonic Nanostructure Biosensors: A Review

Plasmonic nanostructure biosensors based on metal are a powerful tool in the biosensing field. Surface plasmon resonance (SPR) can be classified into localized surface plasmon resonance (LSPR) and propagating surface plasmon polariton (PSPP), based on the transmission mode. Initially, the physical principles of LSPR and PSPP are elaborated. In what follows, the recent development of the biosensors related to SPR principle is summarized. For clarity, they are categorized into three groups according to the sensing principle: (i) inherent resonance-based biosensors, which are sensitive to the refractive index changes of the surroundings; (ii) plasmon nanoruler biosensors in which the distances of the nanostructure can be changed by biomolecules at the nanoscale; and (iii) surface-enhanced Raman scattering biosensors in which the nanostructure serves as an amplifier for Raman scattering signals. Moreover, the advanced application of single-molecule detection is discussed in terms of metal nanoparticle and nanopore structures. The review concludes by providing perspectives on the future development of plasmonic nanostructure biosensors.

Membrane-Based In Situ Mid-Infrared Spectroscopic Ellipsometry: A Study on the Membrane Affinity of Polylactide-co-glycolide Nanoparticulate Systems

Mid-infrared (IR) ellipsometry of thin films and molecule layers at solid–liquid interfaces has been a challenge because of the absorption of light in water. It has been usually overcome by using configurations utilizing illumination through the solid substrate. However, the access to the solid–liquid interface in a broad spectral range is also challenging due to the limited transparency of most structural materials in the IR wavelength range. In this work, we propose a concept of a microfabricated analysis cell based on an IR-transparent Si membrane with advantages of a robust design, flexible adaptation to existing equipment, small volume, multiple-angle capabilities, broad wavelength range, and opportunities of multilayer applications for adjusted ranges of high sensitivity. The chamber was prepared by 3D micromachining technology utilizing deep reactive ion etching of a silicon-on-insulator wafer and bonded to a polydimethylsiloxane microfluidic injection system resulting in a cell volume of approximately 50 μL. The mechanical stability of the 2 and 5 μm-thick membranes was tested using different “backbone” reinforcement structures. It was proved that the 5 μm-thick membranes are stable at lateral cell sizes of 5 mm by 20 mm. The cell provides good intensity and adjustment capabilities on the stage of a commercial mid-IR ellipsometer. The membrane configuration also provides optical access to the sensing interfaces at a broad range of incident angles, which is a significant advantage in many potential sensing structure configurations, such as plasmonic, multilayer, 2D, or metamaterial applications.

Plasmonic sensor with high figure of merit based on differential polarization spectra of elliptical nanohole array

Using the difference of the polarization transmission spectra of elliptical nanohole arrays (ENAs), the figure of merit (FOM) of the sensor performance of ENA can be significantly improved, and is inversely proportional to the measurement resolution. By optimizing the aspect ratio of the elliptical holes, Ag thickness, substrate-effect, and adhesive layer, the sensitivity, FOM, and relative sensitivity of the ENA can be improved to be 775 nm RIU^-1, 705 RIU^-1, and 70.23%, respectively, with an excellent linear dependence on the change of refractive index. Such a high-performance sensor also can be used in monitoring the molecule adsorption and RNA hybridization, revealing a highly localized near-field enhancement. This will benefit the sensing of surface-specific binding events in biologic detection and medical diagnosis.

Confined surface plasmon sensors based on strongly coupled disk-in-volcano arrays

Disk-in-volcano arrays are reported to greatly enhance the sensing performance due to strong coupling in the nanogaps between the nanovolcanos and nanodisks. The designed structure, which is composed of a nanovolcano array film and a disk in each cavity, is fabricated by a simple and efficient colloidal lithography method. By tuning structural parameters, the disk-in-volcano arrays show greatly enhanced resonances in the nanogaps formed by the disks and the inner wall of the volcanos. Therefore they respond to the surrounding environment with a sensitivity as high as 977 nm per RIU and with excellent linear dependence on the refraction index. Moreover, through mastering the fabrication process, biological sensing can be easily confined to the cavities of the nanovolcanos. The local responsivity has the advantages of maximum surface plasmon energy density in the nanogaps, reducing the sensing background and saving expensive reagents. The disk-in-volcano arrays also possess great potential in applications of optical and electrical trapping and single-molecule analysis, because they enable establishment of electric fields across the gaps.

Periodic metallic nanostructures as plasmonic chemical sensors

Periodic plasmonic nanostructures are being widely studied, optimized, and developed to produce a new generation of low-cost and efficient chemical sensors and biosensors. The extensive variety of nanostructures, interrogation approaches, and setups makes a direct comparison of the reported performance from different sensing platforms a challenging exercise. In this feature Article, the most common parameters used for the evaluation of plasmonic nanostructures will be reviewed, with particular focus on the advances in periodic plasmonic nanostructures. Recent progress in the fabrication methods that allow for the high-volume production of periodic plasmonic sensors at low cost will be described, together with an assessment of the state of the art in terms of periodic structures employed for chemical sensing.

New trends in instrumental design for surface plasmon resonance-based biosensors

Surface plasmon resonance (SPR)-based biosensing is one of the most advanced label free, real time detection technologies. Numerous research groups with divergent scientific backgrounds have investigated the application of SPR biosensors and studied the fundamental aspects of surface plasmon polaritons that led to new, related instrumentation. As a result, this field continues to be at the forefront of evolving sensing technology. This review emphasizes the new developments in the field of SPR-related instrumentation including optical platforms, chips design, nanoscale approach and new materials. The current tendencies in SPR-based biosensing are identified and the future direction of SPR biosensor technology is broadly discussed.

Simultaneous time-resolved measurement of the reaction rates and the refractive index of photopolymerization processes

We explore the use of imaging surface plasmon resonance (iSPR) to simultaneously measure the refractive index and reaction rates of the commercially available Ormocore photosensitive resist during photopolymerization. To this end, we adapted a commercially available iSPR device. We demonstrate good accuracy in the measurement of the refractive index determined independently of the thickness of the polymerized film. Furthermore, we demonstrate that the refractive index is proportional to the degree of cure (double bond conversion) of the resist. This allows the determination of the reaction rates of the polymerization processes, which show reasonable agreement with photodifferential scanning calorimetry measurements.

Immobilisation of IgG onto gold surfaces and its interaction with anti-IgG studied by surface plasmon resonance

The optical excitation of surface plasmon resonance (SPR) at a metal dielectric interface has been used to study the binding of immunoglobulin G (IgG) to gold and anti-IgG to immobilised IgG layers. In these studies both a monoclonal mouse and polyclonal sheep IgG were used as receptor layers for anti-IgG. The kinetics of binding were investigated by monitoring the reflectivity of light at an angle close to plasmon resonance. Both the initial rate of change and final reflectivity were measured during and after protein binding. The amount of protein bound to the surface was found to be less for the monoclonal mouse IgG compared to the polyclonal sheep IgG, these two IgG nominally being of the same dimensions and molecular weight. Further, anti-IgG binding produced greater changes in reflectivity than the initial IgG layers. By fitting the full angle-dependent reflectivity data to the Fresnel equation the effective protein layer thicknesses of IgG and anti-IgG as a function of concentration were determined. Differences in the effective thickness of the bound layer for the two IgG was observed, the mouse IgG having a thinner effective thickness compared with the sheep IgG. The limitations of direct binding of protein to metal surfaces in SPR biosensor applications are discussed.