Plasmonic-based impedance microspectroscopy of cells and biomolecules  

Session

Poster Session Three

Authors

Dr Sidahmed Abayzeed (1)

Affiliations

1. Optics and Photonics Research Group, Faculty of Engineering, University of Nottingham

Keywords

Impedance microspectroscopy - label-free optical microscopy - plasmonics  

Abstract text

This poster presents a novel optical imaging technique that is capable of mapping electrical impedance with submicrometre  diffraction-limited resolution. The method presented is based on the widely used surface plasmon resonance imaging; an optical microscopy technology with an emerging application in label-free and non-scanning imaging of voltage. High resolution probing of electrical impedance is extremely important. For instance, it provides a way of studying electrical properties of living cells that use electrical signalling to communicate and regulate a wide range of physiological processes.

Measurements of electrical current and impedance are performed using plasmonic sensors that provide electrode surfaces with optical readout. They are highly sensitive to external voltage. The principle of the technique is based on surface plasmon resonance that occurs due to the optical excitation of collective oscillations of free electrons at the metal dielectric interface. Externally applied voltage alters the density of free electrons in metal surface and therefore changes the resonance position of the sensor. Gold thin film is commonly used to construct the sensing structure where sample under study is deposited or cultured in case of living cells. The metallic thin film is also used as a working electrode in combination with a proximal reference electrode. To perform impedance spectroscopy, a small alternating voltage is applied to the gold surface, against a reference electrode, resulting in a current flow across the electrolyte solution and the sample under the test. The real-time change in resonance position of the sensor is monitored that is directly proportional to the charge modulation. Voltage-modulated surface charge density varies spatially depending on the impedance of the object.

One of the major challenges of impedance imaging is that measurements are distorted by the crosstalk from the optical properties of the sample resulting in an inaccurate impedance mapping. Here, a correction method is presented that provides an effective way of separating the effect of optical properties and therefore produces quantitative measurements of dynamic electrical signals. The method is illustrated by mapping the impedance of Bovine Albumin Serum (BSA) patterns deposited on the gold film. BSA patterns were fabricated on the gold surface using micro-contact printing. Impedance microspectroscopy was performed by sweeping voltage between 1 and 100 Hz while computing impedance and current maps for each frequency. Microscopic  impedance spectra were analysed using least square fitting to construct the equivalent electrical network of the sample. Within this frequency range, the surface capacitance has a dominant effect. Contrast in capacitance between the BSA and the gold background is observed. This is expected since the protein deposits have a lower static permittivity compared to the water molecules that constitute the dielectric properties of the double layer capacitor of the gold electrolyte interface. The technique shows a remarkable sensitivity being able to detect current as low as 0.1pA on a 0.5 micrometre scale. These findings demonstrate that this new impedance imaging technique is capable of revealing microscopic electrical properties with promising applications in studying cells and biomolecules. 

References

Abayzeed, Sidahmed A. "Plasmonic-based impedance microspectroscopy of optically heterogeneous samples." Biomedical Optics Express 11.11 (2020): 6168-6180.