SiO2 beads with quantum dots: Preparation and stability investigation for bioapplications

SiO2 beads with quantum dots (QDs) including hydrophilic CdTe or hydrophobic CdSe/ZnS were prepared through sol–gel procedures. These beads contained single or multiple QDs by controlling the ligand exchange and the QD assembly during incorporation. Namely, SiO2 beads with CdTe QDs retained high photoluminescence (PL) efficiencies up to 40% when thioglycolic acid was partially replaced by 3-mercaptopropyltrimethoxysilane. Multiple CdSe/ZnS QDs were assembled into a SiO2 bead (∼7 QDs in each bead) through surface silanization using tetraethyl orthosilicate instead of organic ammine. The stability of CdTe QDs and luminescent SiO2 beads was investigated by measuring the degradation of PL intensity against the concentration of phosphate-buffered saline (PBS) buffer solutions. The Stern–Volmer quenching constant Ksv of QDs depended strongly on the properties of the QDs and the thickness and porous type of SiO2 shell. Red- and green-emitting CdTe QDs exhibited Ksv of 2.02 × 10−2 and 8.93 × 10−2 mM−1, respectively. The II-type pores (cylindrical pores) in the SiO2 shell resulted in quick degradation while the IV-type pores (ink-bottle-shaped pores) led the QDs a high stability. Because both two kinds of QDs in SiO2 beads revealed high PL efficiency and stability in buffer solution, these luminescent beads should be utilizable for bioapplications.