Quantum dots for off-on biological labeling

Summary form only given. Quantum dots (QDs) possess discrete energy states within the conduction band (CB) and valence band (VB), which may decay radiatively or non-radiatively via a number of different processes. The addition to a colloidal solution of molecules with a reduction potential less negative than the QD CB (or an oxidation potential less positive than the QD VB) will cause a transfer of the electron (or hole) from the photoexcited state of the QD to the added molecule. Direct chemical conjugation of the molecule enhances this effect. The fluorescence is quenched in this process, as the electron/hole pair is no longer available to combine radiatively. Many experiments have been done using electron acceptors, which have been shown to quench fluorescence of QDs and used to determine the distribution and depths of intra-gap electron traps. Many fewer studies have been done with hole acceptors and the presence and distribution of hole traps remains essentially unknown. However, many useful biological molecules are hole acceptors, among them the DNA purine bases and the two redox-active amino acids, tyrosine and tryptophan. The redox potentials of the purine bases are very close to the band edges of QDs in neutral pH aqueous solution, and quenching is only seen with green- and blue-fluorescent QDs, not with yellow or red. Quenching is not reversed by exposure to ambient light for 1 wk or to a 100 W Hg lamp for 1 hr. Effects are independent of pH, except when pH shifts the redox potential above or below the band edge.