Water-soluble off-on spin-labeled quantum-dots conjugate

A study was conducted to demonstrate a facile and reagentless method for generating a water-soluble off-on-spin-labeled quantum dot (QD) conjugate. It was found that the quenched fluorescence of TGA-CdTe QDs can be restored, when the paramagnetic properties of the nitroxide moiety were eliminated. The study also investigated the effect of the nitroxide radical on the fluorescence of TGA-CdTe QDs in aqueous solution. The investigation revealed that the fluorescence of TGA-CdTe QDs were quenched by 4-amino-2,2,6, 6-tetramethylpiperidine-N-oxide radical free radical (4-AP). It was also found that 4-AP, with a pKb was protonated to its cationic conjugate acid and thioglycolic acid, with a first pKa of 3.55, formed a negatively charged layer on the surface of the TGA-CdTe QDs. The results show that self-assembling water-soluble thioglycolic acid capped CdTe QDs (TGA-CdTe QDs) with the 4-AP yielded a weak fluorescent spin-labeled QD conjugate in aqueous solution.