DNA-modified gold nanoparticles (AuNPs) are useful signal-reporters for detecting diverse molecules through various hybridization- and enzyme-based assays. However, their performance is heavily dependent on the probe DNA surface coverage, which can influence both target binding and enzymatic processing of the bound probes. Current methods used to adjust the surface coverage of DNA-modified AuNPs require the production of multiple batches of AuNPs under different conditions, which is costly and laborious. We here develop a single-step assay utilizing dithiothreitol (DTT) to fine-tune the surface coverage of DNA-modified AuNPs. DTT is superior to the commonly used surface diluent, mercaptohexanol, as it is less volatile, allowing for the rapid and reproducible controlling of surface coverage on AuNPs with only micromolar concentrations of DTT. Upon adsorption, DTT forms a dense monolayer on gold surfaces, which provides antifouling capabilities. Furthermore, surface-bound DTT adopts a cyclic conformation, which reorients DNA probes into an upright position and provides ample space to promote DNA hybridization, aptamer assembly, and nuclease digestion. We demonstrate the effects of surface coverage on AuNP-based sensors using DTT-regulated DNA-modified AuNPs. We then use these AuNPs to visually detect DNA and cocaine in colorimetric assays based on enzyme-mediated AuNP aggregation. We determine that DTT-regulated AuNPs with lower surface coverage achieve shorter reaction times and lower detection limits relative to those for assays using untreated AuNPs or DTT-regulated AuNPs with high surface coverage. Additionally, we demonstrate that our DTT-regulated AuNPs can perform cocaine detection in 50% urine without any significant matrix effects. We believe that DTT regulation of surface coverage can be broadly employed for optimizing DNA-modified AuNP performance for use in biosensors as well as drug delivery and therapeutic applications.
Keywords: DNA detection; DNA surface coverage; cocaine detection; dithiothreitol; enzyme-assisted target recycling; gold nanoparticles.