Selim et al. Nanoscale Research Letters 2011, 6:528 http://www.nanoscalereslett.com/content/6/1/528 well with those of bare CdS (Figure 1b). The spectrum of CSNPs was further compared with the data of JCPDS file no. 10-454 and was in agreement with that of pure cubic-phase CdS, without signals from CdCl2, NaOH, or other precursor compounds. The three peaks observed in Figure 1a at 2 θ values of 26.439°, 43.862°, and 51.389° were found to correspond to the three crystal planes of (111), (220), and (311), indicating that the CSNPs were in cubic phase [24]. Again, the diffraction peaks of CSNPs were somewhat broad compared to those of bare CdS. This broadness was due to reduced particle size and surface defects [25]. Small-sized CSNPs possess a higher surface defect density due to a high surface-to-volume ratio [26]. Moreover, CSNPs possess higher negative zeta potential (ξ = -66.65 mV), indicat- ing excellent stability of the colloidal nanocrystal [27]. The surface modification of CSNPs with insulin was confirmed by FT-IR as displayed in Figure 2. For CSNPs (Figure 2a), two distinctive bands were observed at 1, 559 and 1, 375 cm-1, which originated from the asym- metric and symmetric stretching motion of carboxylate ion (-COO-) [28]. These findings clearly indicate the for- mation of a co-ordinate bond between the oxygen atom of mercaptoacetic acid and Cd+2. No free carboxylic acid band at 1, 730 to 1700 cm-1 due to C=O stretching is observed in capped nanoparticles [29]. The introduc- tion of PEG onto the surface of CSNPs was confirmed by the characteristic peak at 1, 575 cm-1, which can be attributed to a -CH2 bending vibration (Figure 2b). Besides, a peak at 1, 106 cm-1 indicated an ether bond (-C-O-) of PEG. Some other peaks were observed at positions of 2, 972, 1, 455, and 1, 375 cm-1, which origi- nated from the PEG chain. This implies that the basic structure of PEG did not change, except for the conver- sion of a terminal group [17]. After reaction of PEG- immobilized CSNPs (PCSNPs) with insulin, two new Page 4 of 9 peaks at positions around 1, 648 and 1, 540 cm-1 were observed based on -CO-NH- (amide I) and -CO-NH- (amide II) bands, respectively [20] (Figure 2c). These results indicate that insulin was successfully immobilized onto the surface of PCSNPs. Immobilization of insulin onto the surface of CSNPs was further confirmed by ESCA. The chemical composi- tions of CSNPs, PCSNPs, and ICSNPs, as calculated from the ESCA survey scan spectra, are shown in Table 1. In the case of PCSNPs, the oxygen content (22.09%) and carbon content (62.09%) were higher in comparison to those of CSNPs (oxygen content, 17.35% and carbon con- tent, 30.04%). Furthermore, one new element such as nitrogen (1.59%) was observed on the surface of PCSNPs, indicating the successful immobilization of PEG onto the surface of CSNPs. In the case of ICSNPs, nitrogen con- tent increased from 1.59% to 2.72% and oxygen content increased from 22.09% to 35.81%, indicating the success- ful immobilization of insulin onto the surface of PCSNPs. TEM images of CSNPs and ICSNPs are shown in Figure 3. It was observed that CSNPs had spherical morpholo- gies with an average diameter of ca. 4.5 nm. Due to the small dimensions and high surface energy of the particles, it was easy for them to aggregate as seen in Figure 3a. On the other hand, immobilization of insulin conferred a spherical morphology, thereby reducing the aggregation of particles. The average diameters of the ICSNPs were 13 nm as shown in Figure 3b. Larger diameters and lower aggregation of particles may have resulted from the immobilization of insulin and PEG onto the surface of CSNPs. Figure 4 shows the typical size and size distribu- tion of synthesized CSNPs (Figure 4a) and ICSNPs (Fig- ure 4b) as measured by DLS. The average size of CSNPs as determined by DLS was ca. 21 nm. On the other Figure 1 XRD patterns of (a) mercaptoacetic acid-coated CSNPs and (b) bare CdS. Figure 2 FT-IR spectra of (a) mercaptoacetic acid-coated CSNPs, (b) PCSNPs, and (c) ICSNPs.