Towards quantum dot and FRET-based optical DNA biosensor technology: Surface chemistry and photoluminescence of CdSe/ZnS and Si quantum dots

Quantum dots (QDs) are nanostructures that are highly attractive to optical biosensing. We have developed a nucleic acid biosensing strategy based on the use of quantum dots as energy donors in FRET. One of the challenges in such an approach is avoiding the non-specific adsorption of oligonucleotides. In this report, we describe our efforts to develop poly(ethylene glycol) (PEG)-based hydrophilic surface chemistry and hexanethiol based hydrophobic surface chemistry to alleviate non-specific adsorption. With respect to the former, it was found that the PEG surface chemistry strongly quenched the band-edge luminescence of CdSe/ZnS QDs and yielded significant band-gap luminescence. Furthermore, the PEG chemistry proved ineffective in preventing adsorption. With respect to hexanethiol capped CdSe/ZnS QDs, it was found that good QD luminescence was retained in organic solvent but was quenched in aqueous solution. The use of hydrophobic hexanethiol QDs in aqueous solution required the immobilization of QDs. To achieve this, we used thiol modified biotin and avidin coated fused silica optical fibers. Despite the quenching of the QDs, minimal adsorption was observed suggesting the methodology has good potential. In addition, we describe the development of a one-pot method for both the synthesis and capping of silicon QDs. Our approach also allows versatile post-synthetic modification of the silicon QD capping to produce a variety of functional groups. Silicon QDs are of interest in biosensing due to their biocompatibility and much lower toxicity compared to II-VI semiconductors.