QD as a bifunctional cell-surface marker for both fluorescence and atomic force microscopy

Fluorescent quantum dots (QDs) are a new class of fluorescent label and have been extensively used in cell imaging. Streptavidin-conjugated QDs have a diameter of ca. 10–15 nm; therefore when used as probes to label cell-surface biomolecules, they can provide contrast enhancement under atomic force microscopy (AFM) and allow specific proteins to be distinguished from the background. In addition, the size and fluorescent properties potentially make them as probes in correlative fluorescence microscopy (FM) and AFM. In this study, we tested the feasibility of using QD-streptavidin conjugates as probes to label wheat germ agglutinin (WGA) receptors on the membrane of human red blood cells (RBCs) and simultaneously obtain fluorescence and AFM images. The results show that the distribution of QDs labeled on human RBCs was non-uniform and that the number of labeled QDs on different erythrocytes varied significantly, which perhaps indicates different ages of the erythrocytes. Thus, QDs may be employed as bifunctional cell-surface markers for both FM and AFM to quantitatively investigate the distribution and expression of membrane proteins or receptors on cell surface.