Rapid, Surfactant-Free, and Quantitative Functionalization of Gold Nanoparticles with Thiolated DNA under Physiological pH and Its Application in Molecular Beacon-Based Biosensor.

The controlled attachment of thiolated DNA to gold nanoparticles (AuNPs) dictates many applications. This is typically achieved by either "aging-salting" processes or low-pH method, where either Na+ or H+ is used to minimize charge repulsion and facilitate attachment of thiolated DNA onto AuNPs. However, the "aging-salting" process takes a long time, and is prone to aggregation when used with larger AuNPs. Surfactants are needed to precoat and thereby enhance the stability of AuNPs. The low-pH method can disrupt the structural integrity of DNAs. We report here an oligoethylene glycol (OEG) spacer-assisted method that enables quantitative and instantaneous attachment at physiological pH without the need for surfactants. The method is based on our finding that an uncharged OEG spacer as short as six EG units can effectively shield against repulsion between AuNPs and DNAs, substantially enhancing both the adsorption kinetics and thermodynamics of thiolated DNAs. We applied this to thiolated DNAs of various lengths and thiol modification positions and to large AuNPs. Importantly, our method also allows for the direct immobilization of thiolated molecular beacons (MB), and avoids particle aggregation due to intermolecular hydrogen bonding. The prepared MB-AuNPs were successfully used for the fluorescent detection of target DNA at nanomolar concentrations. The OEG spacer appears to offer a highly effective parameter for tuning DNA adsorption kinetics and thermodynamics besides pH and salt, providing a novel means for highly controllable and versatile functionalization of AuNPs.