Label-Free Optical Differentiation of Single Diffusing Amino Acids at Picomolar Concentrations

Label-free single-molecule detection schemes can provide key molecular information. Here, we deploy the elevated light-matter interaction in locked high-finesse fiber-based Fabry-Pérot microcavities (FFPCs) to push label-free detection to the single-amino acid limit, even to the smallest amino acid, glycine. Varying the sensitivity of the locked FFPC allows differentiation of subsets of amino acids at the single-molecule level, with examples including the differentiation of a small peptide from tryptophan, tryptophan from histidine, histidine from alanine, and phosphorylated threonine from threonine. Molecular detection events are visible as neuron-like signal bursts of decreased FFPC transmission, which can be replicated by simulations that include non-linear coupling between optical and thermal parameters of the locked FFPC. Importantly, differentiation was achieved in solution at pM concentrations, making the detection process suitable for integration with single-molecule processive-protein degradation. Thus, this combination of sensitivity and differentiating ability makes FFPCs a promising new photonic technology element for protein sequencing.