Deep UV plasmonic enhancement of single protein autofluorescence in zero-mode waveguides

TL;DR

This paper demonstrates the use of aluminum plasmonics in zero-mode waveguides to significantly enhance UV autofluorescence signals from single proteins, enabling label-free detection and characterization at physiological concentrations.

Contribution

It introduces a novel approach for UV plasmonic enhancement of single protein autofluorescence using zero-mode waveguides, advancing label-free single-molecule detection.

Findings

Enhanced UV autofluorescence of single proteins observed

Quantitative measurements of protein concentration and size achieved

Operation at micromolar concentrations demonstrated

Abstract

Single molecule detection provides detailed information about molecular structures and functions, but it generally requires the presence of a fluorescent marker which can interfere with the activity of the target molecule or complicate the sample production. Detecting a single protein with its natural UV autofluorescence is an attractive approach to avoid all the issues related to fluorescence labelling. However, the UV autofluorescence signal from a single protein is generally extremely weak. Here, we use aluminum plasmonics to enhance the tryptophan autofluorescence emission of single proteins in the UV range. Zero-mode waveguides nanoapertures enable observing the UV fluorescence of single label-free beta-galactosidase proteins with increased brightness, microsecond transit times and operation at micromolar concentrations. We demonstrate quantitative measurements of the local concentration, diffusion coefficient and hydrodynamic radius of the label-free protein over a broad range of zero-mode waveguide diameters. While the plasmonic fluorescence enhancement has generated a tremendous interest in the visible and near-infrared parts of the spectrum, this work pushes further the limits of plasmonic-enhanced single molecule detection into the UV range and constitutes a major step forward in our ability to interrogate single proteins in their native state at physiological concentrations.