Selim et al. Nanoscale Research Letters 2011, 6:528 http://www.nanoscalereslett.com/content/6/1/528 Page 7 of 9 Figure 6 Optical microscopic images of human fibroblasts cultured for 6 and 24 h. In a (a) polystyrene culture dish and in the presence of (b) CSNPs, (c) PCSNPs, and (d) ICSNPs. Original magnification is ×200. cellular markers that target surface receptors expressed on the cell surface without being internalized. Receptors are highly regulated cell surface proteins that mediate specific interactions between cells and their extracellular milieu, and they are generally localized to the plasma membrane [36]. Based on this explanation, it could be said that the immobilization of biomolecules onto the surface of Q-dots can suppress their toxicity. In this study, PEG and insulin in combination reduced the cytotoxicity of cells. Conclusions Insulin was immobilized onto the surface of mercaptoa- cetic acid-coated cadmium sulfide nanoparticles (CSNPs), and confirmation of insulin immobilization was carried out by FT-IR and ESCA. Size distribution of Figure 7 Fluorescence microscopic images of Live/Dead dye-stained fibroblasts cultured for 2 days. In a (a) polystyrene culture dish and in the presence of (b) CSNPs, (c) PCSNPs, and (d) ICSNPs. Live and dead cells were stained and visualized in green and red, respectively, under a fluorescence microscope.