Surface-engineered quantum dots for the labeling of hydrophobic microdomains in bacterial biofilms

Quantum dots (QDs) nanoprobes are emerging as alternatives to small-molecule fluorescent probes in biomedical technology. This paper reports an efficient and rapid method of producing highly dispersed and stable CdSe-core QDs with a hydrophobic gradient. Amphiphilic core/shell CdSe/ZnS QDs were prepared by ligand exchange at the surface of lipophilic CdSe/ZnS QDs using the dihydrolipoic acid (DHLA) dithiol ligand linked to leucine or phenylalanine amino acids. Contact angle relaxations on a hydrophobic surface and surface tension measurements indicated that aqueous dispersions of CdSe/ZnS@DHLA-Leu or CdSe/ZnS@DHLA-Phe QDs exhibit increased hydrophobicity compared to CdSe-core QDs capped by the hydrophilic 3-mercaptopropionic acid (MPA) ligand. We found that the surface functional groups and the ligand density at the periphery of these QDs significantly dictated their interactions with a complex biological matrix called biofilm. Using fluorescence confocal microscopy and an autocorrelation function (semi-variogram), we demonstrated that MPA-capped QDs were homogeneously associated to the biopolymers, while amphiphilic CdSe/ZnS@DHLA-Leu or CdSe/ZnS@DHLA-Phe QDs were specifically confined allowing identification of hydrophobic microdomains of the biofilms. Results obtained clearly point out that the final destination of QDs in biofilms can properly be controlled by an appropriate design of surface ligands.