The Role of Anion Adsorption in the Effect of Electrode Potential on Surface Plasmon Resonance Response

Adsorption of lysozyme on gold surfaces in the presence of an external electric potential

Adsorbed protein films consist of essential building blocks of many biotechnological and biomedical devices. The electrostatic potential may significantly modulate the protein behaviour on surfaces, affecting their structure and biological activity. In this study, lysozyme was used to investigate the effects of applied electric potentials on adsorption and the protein structure. The pH and the surface charge determine the amount and secondary structure of adsorbed lysozyme on a gold surface. In-situ measurements using polarization modulation infrared reflection absorption spectroscopy indicated that the concentration of both the adsorbed anions and the lysozyme led to conformational changes in the protein film, which was demonstrated by a greater amount of aggregated β-sheets in films fabricated at net positive charges of the Au electrode (E_ads > E_pzc). The changes in secondary structure involved two parallel processes. One comprised changes in the hydration/hydrogen-bond network at helices, leading to diverse helical structures: α-, 3₁₀- and/or π-helices. In the second process β-turns, β-sheets, and random coils displayed an ability to form aggregated β-sheet structures. The study illuminates the understanding of electrical potential-dependent changes involved in the protein misfolding process.

Plasmonic-based impedance microspectroscopy of optically heterogeneous samples

A robust impedance microscopy technique is presented. This optical tool enables high resolution imaging of electrical properties with promising biophysical applications. The underlying principle is that surface plasmon resonance (SPR) sensors are able to measure perturbations of surface charge density and therefore can be used to compute the impedance of surface-adhered cells. However, the ability to perform reliable quantitative impedance imaging is affected by the optical heterogeneity of the cell-sensor interface. To address this issue, a novel method for quantitative time-resolved resonance angle tracking is developed and applied to correct for the effect of the optical properties. To demonstrate the capability of this technique, impedance microspectroscopy of bovine serum albumin (BSA) patterns was performed enabling measurements of capacitance with submicroscopic resolution. The work presented offers an impedance microspectroscopy method that will create new avenues in studying the electrical properties of single cells and biomolecules as well as bio-electrical currents.

Electrochemical impedance spectroscopy of single Au nanorods

Monochromatic dark-field microscopy coupled with high-frequency potential modulation leads to non-faradaic electrochemical impedance spectroscopy of single Au nanorods.

We propose monochromatic dark-field imaging microscopy (DFM) to measure the non-faradaic electrochemical impedance spectroscopy (EIS) of single Au nanorods (AuNRs). DFM was utilized to monitor the plasmonic scattering of monochromatic incident light by surface-immobilized individual AuNRs. When modulating the surface potential at a certain frequency, non-faradaic charging and discharging of AuNRs altered their electron density, leading to periodical fluctuations in the scattering intensity. Analysis of the amplitude and phase of the optical intensity fluctuation as a function of modulation frequency resulted in the EIS of single AuNRs. High-frequency (>100 Hz) modulation allowed us to differentiate the intrinsic charging effect from other contributions such as the periodic migration and accumulation of counterions in the surrounding medium, because the latter occurred at a longer timescale. As a result, single nanoparticle EIS led to the surface capacitance of single AuNRs being closer to the theoretical value. Since interfacial capacitance has been proven sensitive to molecular interactions, the present work also offers a new platform for single nanoparticle sensing by measuring the single nanoparticle capacitance.