Selim et al. Nanoscale Research Letters 2011, 6:528 http://www.nanoscalereslett.com/content/6/1/528 N A N O E X P R E S S Open Access Reduced cytotoxicity of insulin-immobilized CdS quantum dots using PEG as a spacer KM Kamruzzaman Selim1, Zhi-Cai Xing1, Moon-Jeong Choi2, Yongmin Chang3, Haiqing Guo4 and Inn-Kyu Kang1* Abstract Cytotoxicity is a severe problem for cadmium sulfide nanoparticles (CSNPs) in biological systems. In this study, mercaptoacetic acid-coated CSNPs, typical semiconductor Q-dots, were synthesized in aqueous medium by the arrested precipitation method. Then, amino-terminated polyethylene glycol (PEG) was conjugated to the surface of CSNPs (PCSNPs) in order to introduce amino groups to the surface. Finally, insulin was immobilized on the surface of PCSNPs (ICSNPs) to reduce cytotoxicity as well as to enhance cell compatibility. The presence of insulin on the surface of ICSNPs was confirmed by observing infrared absorptions of amide I and II. The mean diameter of ICSNPs as determined by dynamic light scattering was about 38 nm. Human fibroblasts were cultured in the absence and presence of cadmium sulfide nanoparticles to evaluate cytotoxicity and cell compatibility. The results showed that the cytotoxicity of insulin-immobilized cadmium sulfide nanoparticles was significantly suppressed by usage of PEG as a spacer. In addition, cell proliferation was highly facilitated by the addition of ICSNPs. The ICSNPs used in this study will be potentials to be used in bio-imaging applications. Keywords: nanoparticles, immobilization, polyethylene glycol, insulin, cytotoxicity Introduction Recently, quantum dots [CdS, CdSe, ZnS, CdTe, etc.] (Q-dots) have attracted tremendous interest as lumines- cent probes in biological and medical researches due to their unique optical and chemical properties [1]. Com- pared with traditional dyes and fluorescent proteins used as imaging probes, Q-dots have several advantages, such as tunable emission from visible to infrared wave- lengths, broader excitation spectra, high quantum yield of fluorescence, strong brightness, photostability, and high resistance to photobleaching [2,3]. However, the potential applications of Q-dots in biology and medicine have been limited due to their cytotoxic effects [4]. Q-dots contain toxic components such as cadmium (from cadmium chalcogenide-based Q-dots) or lead (from lead chalcogenide-based Q-dots). Cd2+ and Pb2+ can be released from Q-dots, which would kill the cells [5]. Therefore, to enhance stability, the surface modifica- tion of Q-dots is required. For example, biomedical applications require high-quality water soluble and non- * Correspondence: ikkang@knu.ac.kr 1Department of Polymer Science and Engineering, Kyungpook National University, Daegu 702-701, South Korea Full list of author information is available at the end of the article toxic Q-dots. So far, numerous surface modifications of Q-dots have been explored, including the attachment of mercaptoacetic acid [6], mercaptopropionic acid [7], mercaptobenzoic acid [8], and biocompatible and che- mically functionalizable inorganic shells, such as silica or zinc sulfide [9]. All of these coatings can ensure the water solubility of Q-dots, but they are unable to enhance biocompatibility. Therefore, further coating with suitable water-soluble organic ligand/biomolecules is necessary to enhance the biocompatibility of Q-dots. To this end, Q-dots have been covalently linked with biorecognition molecules such as biotin [10], folic acid [11], peptides [12], bovine serum albumin [13], transfer- rin [14], antibodies [15], and DNA [16]. Polyethylene glycol (PEG) and its derivatives have been widely used as biomedical materials, such as drug delivery matrices and scaffolds for tissue engineering, due to their hydrophilicity, high solubility in aqueous and organic solvents, excellent biocompatibility, lack of toxicity and immunogenicity, and ease of excretion from living organisms. Among PEG derivatives, the most important one is amino-terminated PEG [17]. On the other hand, insulin, which reduces blood glucose levels, is often used for treating diabetic patients. However, © 2011 Selim et al; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.