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.
Immobilization of single-stranded DNA onto fused silica and glass surfaces has been widely used for preparation of fiber optic sensors and DNA microarrays. Fundamental investigations of the orientation, motion and hybridization behavior of immobilized ssDNA is important to understand the design sensing devices. Using computational methods to evaluate molecular dynamics w have simulated a solid SiO2-linker-ssDNA system and studied the conformations assumed by such immobilized material under different physical constraints. We have also evaluated the molecular dynamics of fluorescent intercalating dyes that are attached to the ssDNA by tethers of various lengths, with the goal of preparing a label on ssDNA that will transducer hybridization. Periodic boundary conditions were applied to examine packing properties and nearest neighbor interactions of adjacent ssDNA molecules. Molecular dynamics have been performed at room temperature as well as on heated systems. Solvent effects were taken into account and the modeling assumed the presence of an aqueous environment. The results suggest the various conformations and orientations that immobilized ssDNA can assume, and indicate that the orientation is generally far more that is perpendicular to the interface. Exploration of tether lengths indicates the minimum lengths that are required for a fluorescent label to operate efficiently as an intercalating agent.
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