Sensing Voltage Dynamics with Differential Intensity Surface Plasmon Resonance Systems

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Project Description

This project is directed at researching the capacity of surface plasmon resonance systems for label-free detection of electrical signals that are generated by excitable cells. Electrical signals are important in communication and control in biological systems. Therefore, accurate and reliable measurements of these signals at the cell level provide a valuable tool for physiological and pharmacological investigations. Unlike the popular fluorescent and micro-electrode techniques, surface plasmon resonance is a label-free, non-invasive way to measure localised signals at the cell-sensor interface.
Surface plasmon resonance (SPR) sensors are conventionally used to detect molecular interactions at the metal-dielectric interface. Additionally, they demonstrated the ability to detect the externally-applied voltage, which characterises them as electrodes with optical readout. As the project is motivated by resolving weak signals associated with dynamic processes, it aims to (i) estimate the limit of voltage detection of SPR using theoretical and experimental approaches and (ii) investigate the response time of the sensors to demonstrate the ability of the technique to resolve the transient electrical signals.

Voltage Sensing System

To explore the voltage sensitivity of surface plasmon resonance systems, a voltage sensing system has been designed and tested, which combines SPR sensing configuration with an electrochemical unit. The optical configuration is used to excite surface plasmons at the metal-electrolyte interface while the electrochemical unit is used to control the potential at this interface. The applied potential is recorded with the optical system and the electrochemical system simultaneously.


The SPR sensing system is designed based on differential intensity detection using a bi-cell photodiode. In this system, a 633 nm linearly polarized laser beam is focused to a line onto the metal surface, passing through a hemicylindrical prism. As the incident angle of the beam is greater than the critical angle, the focused beam undergoes a total internal reflection. The reflected beam is collimated and detected with a pixelated camera or a bi-cell detector. The reflected beam features a drop of the intensity at the resonance angle and this resonance position is tracked by balancing the bi-cell detector. The outputs of the bi-cell detector (A and B) are processed to calculate (A-B)/(A+B) which is correlated to the resonance position. Below is an example of monitoring voltage-induced resonance shift using the bi-cell detection scheme.