Controlled Functionalization of Water-Soluble Semiconductor Quantum Dots for Bioconjugation

A one-step procedure using carbodiimide condensation was proposed for controlled functionalization of watersoluble semiconductor quantum dots (QDs) with azide groups. The changes of functional group surface density and ζ-potential could be controlled during modifi cation of anionic and analogous zwitterionic polymer-encapsulated CdSe/ZnS core–shell QDs. Ethanolamine-O-sulfate, aminotetraethylene glycol (H2N–TEG–OH), and aminotetraethylene glycol azide (H2N–TEG–N3) were used as modifiers. Functionalization was investigated by studying the conjugation kinetics of organic dye JOE to QDs using strain-promoted azide-alkyne cycloaddition (SPAAC). The reactivity of QDs in SPAAC was nearly proportional to the relative density of surface azides. The QD ζ-potential correlated with the ratio of modifiers. Azides on the QD surface could be easily transformed under mild conditions into primary amines using tris(2-carboxyethyl)phosphine as a reducing agent. The applicability of surface-functionalized QDs for bioconjugation was demonstrated via conjugation to fluorescent-dye-labeled bovine serum albumin.