Synthesis of monovalent quantum dots

DMYTRO YUSHCHENKO, ANA RUIZ, GUILLERMO MENÉNDEZ, DONNA J. ARNDT-JOVIN, ELIZABETH A. JARES-ERIJMAN, THOMAS M. JOVIN

Santiago de Compostela, España

Simposio; Fluoromag Symposium, Nanodots & Diagnostics; 2009

Fluoromag group

We described earlier a method for the synthesis of monovalent reactive quantum dots (QDs)1. This procedure exploited the procedure of Parak and coworkers2 for coupling (5-10 kDa) diamino polyethyleneglycol (PEG) to carboxyl QDs and subsequently separating mono-amino PEG QDs by gel electrophoresis. Following modification of the amino group with coenzyme A (CoA), the monoreactive QDs were shown to react enzymatically with an acyl carrier protein (ACP) tag fused to the EGF receptor on living cells. Several problems were encountered with this synthesis scheme. The purification procedure resulted in poor yields and aggregation of the QDs. In addition, electrophoresis reduced the quantum yield of the QD products. We are investigating alternative strategies incorporating (i) facile, rapid, high yield purification steps based on bulk separation techniques, e.g. magnetic microspheres, decantation; (ii) gentle conditions favoring the retention of key QD properties; (iii) recycling of valuable reagents such as QDs and modified microspheres; and (iv) generation of products with universal reactivities. The particular technique featured here is based on the magnetic separation of intermediate products. Dynabead 280 superparamagnetic particles (MPs), composed of ferromagnetic cores surrounded by highly cross-linked polystyrene with surface carboxyl groups, serve as the matrix for synthesis. A limited subset of the surface COOH of the MPs is modified with an organic linker carrying a cleavable disulfide linkage and a terminal biotin. Commercial QDs (Invitrogen) conjugated with streptavidin are added, leading to single QD/biotin interactions with the MPs. The latter are purified by magnetic separation and monothiol QDs are released by reduction of the disulfide bond in the linker. The resulting monovalent QDs will be used to create a variety of specialized new nanoprobes. (i) Single reactive reagents. Example: coupling to CoA allowing the covalent linkage of single QDs to individual cell surface proteins expressing the ACP tag. (ii) Ratiometric biosensors. Example; coupling two monovalent QDs of different colors, one of which is modified by an environment (e.g. pH) sensitive FRET probe3; and (iii) Multiplexing biosensors. Example: coupling of gold or magnetic nanodots to the QDs.  The current status of the synthetic procedures and applications of the products will be presented.