An ultraefficient cap-exchange protocol (UCEP) that can convert hydrophobic quantum dots (QDs) into stable, biocompatible, and aggregation-free water-dispersed ones at a ligand:QD molar ratio (LQMR) as low as 500, some 20-200-fold less than most literature methods, has been developed. The UCEP works conveniently with air-stable lipoic acid (LA)-based ligands by exploiting tris(2-carboxylethyl phosphine)-based rapid in situ reduction. The resulting QDs are compact (hydrodynamic radius, Rh, < 4.5 nm) and bright (retaining > 90% of original fluorescence), resist nonspecific adsorption of proteins, and display good stability in biological buffers even with high salt content (e.g., 2 M NaCl). These advantageous properties make them well suited for cellular imaging and ratiometric biosensing applications. The QDs prepared by UCEP using dihydrolipoic acid (DHLA)-zwitterion ligand can be readily conjugated with octa-histidine (His8)-tagged antibody mimetic proteins (known as Affimers). These QDs allow rapid, ratiometric detection of the Affimer target protein down to 10 pM via a QD-sensitized Förster resonance energy transfer (FRET) readout signal. Moreover, compact biotinylated QDs can be readily prepared by UCEP in a facile, one-step process. The resulting QDs have been further employed for ratiometric detection of protein, exemplified by neutravidin, down to 5 pM, as well as for fluorescence imaging of target cancer cells.
Keywords: Förster resonance energy transfer; cap exchange; cell imaging; fluorescence; quantum dot; ratiometric sensing; ultraefficiency.