Direct Detection and Quantification of Aqueous Proteins via a Fluorescent Probe Through the Use of Fluorophore-Induced Plasmonic Current

Abstract

We report on the recent advancements in the sensing of proteins, both directly and with the use of a fluorescent probe, through the use of Fluorophore-Induced Plasmonic Current (FIPC). FIPC are a phenomenon where a fluorophore or excited state species is in close proximity to a plasmonically active metal nanoparticle film (MNF), and the excited state is able to couple to the particle, ultimately leading to enhanced spectroscopic properties. This phenomenon is similar to the well-reported metal-enhanced fluorescence (MEF) phenomenon, wherein the coupled complex produces an enhanced fluorescence emission and a shorter lifetime. However, if the particles themselves are sufficiently spaced and oriented, an induced current can transfer from each discreet particle to the next, creating a detectable current across the film. This detectable current has a magnitude that is proportional to the fluorescent properties of the species that produced it, and can be affected by the polarization of the excitation source; the spacing and size of the particles on the film; the overlap between the spectral properties of the film and the species; as well as externally applied voltages and currents. In this study, we examined whether it is possible to detect protein species, directly due to both their intrinsic fluorescent and absorptive properties, and how that compares to commercially available protein detection probes, in a similar manner to prior work by our group addressing analyte detection via turn-on fluorescent probes. This FIPC-based detection technique is a novel method that has not been used for the detection of proteins, and the use of this method could expand the dynamic sensing range of first-pass testing, while overcoming some of the physical limitations on the upper limit of detection of both absorption spectroscopy and fluorescence emission spectroscopy. Our experiments sought to highlight the selectivity of FIPC-based detection relative to both fluorescence and absorption spectroscopy, as well as its sensitivity when working with protein analytes. We examined the effects of protein concentration, intrinsic fluorescent properties, and turn-on probes, as well as how these techniques compare to traditional analytical techniques used today.