Plasmonic Electricity II: The Effect of Particle Size, Solvent Permittivity, Applied Voltage, and Temperature on Fluorophore-Induced Plasmonic Current

Abstract

In a recent paper, our laboratory has shown that fluorophores in close proximity to a non-continuous metal nanoparticle film can induce a detectable electrical current in the film. This current was found directly proportional to the fluorophore extinction coefficient and concentration when excited with p-polarized light. This finding threatens to change the way we both think about and use fluorescence spectroscopy as no longer do we have to use and are limited by traditional photodetectors and associated optics to collect and measure fluorescence signatures. This approach holds potential to significantly simplify fluorescence-based instrumentation. In this paper, we significantly expand upon our previous findings and show that plasmonic current is a function of the nanoparticle size and spacing in the film, which is explained by the concentric sphere model for nanoparticle capacitance. We also demonstrate the dependence of plasmonic current on the relative permittivity of the solvent, and that in an excess of salt, the fluorophore-induced current is significantly greater than the background current. This paves the way for downstream plasmonic assays in a variety of biological media. In addition, we have measured plasmonic current as a function of both applied bias voltage and temperature, allowing for the optimization of the fluorophore-induced plasmonic current. These findings allow for not only a better understanding of plasmonic current but also its optimization as it relates to fluorescence-based detection.

Fluorophore-Induced Plasmonic Current: Generation-Based Detection of Singlet Oxygen

“…As described in recent work by our laboratory, this electrical current is dependent on a number of factors including not only nanoparticle size and spacing in the film but also solvent permittivity, applied voltage, temperature, and the fluorophore extinction coefficient. , In our studies of extinction coefficient effects, an increase in the current was observed with increasing fluorophore extinction coefficient . This is thought to be due to the high extinction fluorophore absorbing more radiative energy from the far-field light (laser or LED) and bringing more energy into the near-electric field of the nanoparticles.…”

“…SEM of vapor deposited films was performed using a Nova NanoSEM 450 with secondary electron imaging, with representative film surface images displayed in previous work . All dry silver nanoparticle island films were characterized as electrically noncontinuous, displaying zero measurable current under zero applied bias voltage …”

“…12 All dry silver nanoparticle island films were characterized as electrically noncontinuous, displaying zero measurable current under zero applied bias voltage. 22 Instrumentation. Fluorescence and absorption measurements of rose bengal and methylene blue (Sigma-Aldrich) solutions were performed in a quartz cuvette.…”

“…Previous work from our laboratory has demonstrated the effect of particle size and spacing in the fluorophore-induced current. Here, it was found that larger and more closely spaced particles lead to an increase in particle capacitance and a decrease in the columbic gap required for electron transport, providing for increased current generation with fluorophore excitation . In addition, changing the particle size will cause a shift in the plasmon resonance frequency of the particle; however, this effect on the current is currently thought to be minor compared to the change in particle capacitance with changing particle size.…”

“…Here, it was found that larger and more closely spaced particles lead to an increase in particle capacitance and a decrease in the columbic gap required for electron transport, providing for increased current generation with fluorophore excitation. 22 In addition, changing the particle size will cause a shift in the plasmon resonance frequency of the particle; however, this effect on the current is currently thought to be minor compared to the change in particle capacitance with changing particle size. The strength of direct coupling between metal nanoparticles and far-field excitation light decreases as nanoparticles grow in size toward the wavelength of incoming radiation.…”