Nano-materials for Photonic and Sensor Applications

Surface Plasmon Resonance (SPR) Spectroscopy is a technique to detect the local refractive index changes within the proximity of a metal surface. It has become the leading method for label-free detection. SPR sensors available today are based on spectral, angular, and phase detections.

For spectral and angular SPR, the change in surface plasmon polariton (SPP) in response to the changes in refractive index, resonant wavelength shift or angle can be used as a signature for detecting the presence of the target analytes by scanning either the wavelength at a fixed angle or the angle at a fixed wavelength. On the other hand, phase-based SPR relies on measuring the phase difference between p- and s-outgoing fields to determine the refractive index change.

Working principle:

SPR sensors are based on an optical phenomenon. The collective coherent oscillations of free electrons in the conduction band of metal are first excited by the interactive electromagnetic field at a metal/dielectric interface and create charge density oscillations known as surface plasmon polaritons (SPPs).

Two main types of SPR sensors:

Propagating SPR (PSPR) sensors:

• Usually excited on continuous metal thin films through prism couplers or grating.
• PSP resonance can propagate up to hundreds of micrometres.

Principle of Propogating SPR sensors

Localized SPR (LSPR) sensors:

• It is a non-propagating surface plasmon.
• Can be excited on nanostructured metal surfaces.
• LSP resonance can be tuned by size, shape and composition.

J. Zhao, X.Y. Zhang, et al., Localized surface plasmon resonance biosensors.

Principle of Localized SPR sensors

LSPR Sensor

The main challenge for all electrical, mechanical and optical sensors is to detect low molecular weight (less than 400 Da) chemical and biological analytes under extremely dilute conditions. Surface plasmon resonance sensors are the most commonly used optical sensors due to their unique ability for real-time monitoring the molecular binding events. However, their sensitivities are insufficient to detect trace amounts of small molecular weight molecules such as cancer biomarkers, hormones, antibiotics, insecticides, and explosive materials for early-stage disease diagnosis, food quality control, environmental monitoring, and homeland security protection. With the development of advanced nanotechnology, nanomaterials-enhanced SPR sensors have been developed and used as effective tools to sense molecules up to amol concentration.

Recent Developments:

Functionalized magnetic nanoparticles (MNPs) like the Fe3O4 nanoparticles have been developed for signal amplification. Advantages of using MNPs are that the lower cost of production and the step for initial receptor immobilization onto the SPR sensing film is not needed. Graphene-modified SPR sensing substrates are coated with graphene layers on gold or silver thin films to induce larger SPR signal changes compared to those with bare-metal thin films. It is used to detect highly selective adsorption of aromatic ring structure molecules (e.g., DNA and TNT).