Figure 7.1a illustrates the nanostructure of the Cu(0)-pHEMA hybrid where the Cu(0) nanoparticles are relatively homogeneously distributed in the pHEMA matrix and show a relatively uniform size of about 2~5 nm in diameter, as shown in Fig. 7.1b, a high resolution image of Cu(0) nano particles in pHEMA matrix. Fig. 7.1c is a selected area electron diffraction pattern, which confirmed that the Cu(0) nanoparticles are poorly crystallized metallic phase, which is due to its relatively small crystallite size. The dimensional restriction of the primary Cu(0) nanoparticles, which according to the work of Huang et al.
[187], should exhibit quantum confinement effect, where the electrons may behave in a wave-like rather than a particle-like manner.
Fig. 7.1 TEM photographs of Cu(0)-pHEMA hybrid showing (a) an orderly packing configuration of Cu(0) nanoparticles distributed in the pHEMA matrix, (b) High resolution image of Cu(0) nano particle in pHEMA matrix where the nanopaticle has a size of about 3 nm in average, and (c) selected area electron diffraction pattern of Cu(0) nano particle, indicated the Cu(0) a poorly crystalline (or amorphous) structure.
Although no direct evidence supports this argument at present, it is reasonably believed that the Cu(0) nanoparticles in the pHEMA matrix should behave as a semi-conductor, rather than that of bulk copper.
The highly-order arrangement of the Cu(0) nanoparticles, having a relatively constant particle-to-particle distance of 8-10 nm, within the matrix suggests to resulting from a possible mechanism evolved upon the synthesis of the Cu(II)-pHEMA hybrid. Upon the hybrid formation, the Cu(II) ions can be anchored by the unpaired electron of O from either the COOR group of the HEMA monomers or H2O in reaction mixture to form coordination bonds, via for instance, coupling reaction, as schematically shown in Fig. 7.2b. The Cu(II) ions are immobilized in the network structure of the HEMA upon polymerization and remained in place while subjecting to chemical reduction, resulting in the formation of amorphous metallic Cu(0) nanoparticles. This argument has also partially self-evidenced from the nanostructural development as illustrated in Fig. 7.1a where no fractal aggregation of the Cu(0) nanoparticles was observed in the resulting hybrid. In other words, the orderly packed configuration of the Cu(0) nanoparticles in the matrix is, as aforementioned, suggesting that the molecular network structure in the pHEMA matrix is acting as spaciously confined space for Cu(0) formation, as schematically elucidated in Fig. 7.2a. It also proposed an interaction between the Cu(II) ions and hydroxyl groups of pHEMA. Once the Cu(0) nanoparticles being nucleated upon chemical reduction and grew, it evolved in a confined nanometric space defined by the dimension of the pHEMA network (as a nano-cage), rather than agglomerated to form fractal aggregates. This may then result in a regular arrangement of the Cu(0) nanoparticles in the well-defined nanostructural network.
Such a highly uniformly distributed Cu(0) nanoparticles in the pHEMA ensures a nanometric-scale uniformity on the hybrid surface upon contacting with blood-clotting proteins, such as fibrinogen, from un-desirable adsorption.
Cu2+
Fig. 7.2 A schematic drawing for the proposed synthesis scheme of Cu(0)-pHEMA hybrid, where (a) the Cu(II) were localized and chemically reduced by N2H4 in-situ to metallic Cu(0) nanoparticles and (b) the Cu(II) ions were assumed to be coupled with the unpaired O of COOR groups along the pHEMA chain network, forming a regularly arranged nanostructure, as illustrated in Fig.
7.1a.
As mentioned earlier [133, 134], copper, in either metallic form or chemical complex, should play a critical role in catalytic nitric oxide generation for improved blood compatibility. It is important to realize the chemical composition of the Cu nanoparticles in-situ prepared and its possible oxidative state of the copper developed in the hybrids since both factors may also determine its electrochemical interaction to protein adsorption or platelet adhesion/aggregation. An XPS analysis on the pristine synthesis of Cu(0)-pHEMA hybrid before and after immersion in PBS is shown in Fig. 7.3 (a) and (b), respectively, where in Fig. 7.3a, the Cu 2p3/2 NPs spectrum of pristine synthesis Cu(0)-pHEMA hybrid shows a peak at 932.79 ± 0.2 eV which is mainly attributed to Cu(0), while a much weaker feature, lying at 933.8 ± 0.2 eV, is attributed to a small amount of CuO. A corresponding fraction of the copper species of respective concentration can be correlated with the area under the spectrum and this determines the metallic form of the Cu(0) to have 83.5% and CuO has 16.5% in the resulting Cu(0)-pHEMA hybrid. This indicates that the copper developed in the Cu(0)-pHEMA hybrid is a mixture of mainly metallic Cu(0) and a small fraction of CuO, after in-situ chemical reduction. The presence of small amount of divalent CuO suggests an incomplete reduction reaction of Cu(II) ions in the matrix. This Cu(II), i.e., CuO, may locate in the inner region of the metallic Cu(0) nanoparticles since the reduction reaction is considered to be a diffusion-controlled process. However, after immersing in PBS for 24 h, the resulting spectra, Fig. 7.3b, indicate that most of the metallic Cu(0) turned into ionic CuO, i.e., Cu(0) has a concentration of 21.1% and CuO, 78.9%, as a result of oxidation. This finding is indicative of the tendency of an electron-giving characteristic of the amorphous metallic Cu nanoparticles in the matrix. The presence of metallic copper nanoparticles, according to a recent report [185], should provide redox ability for thromboresistance.
945 940 935 930 925 920
Fig. 7.3 The Cu 2p3/2 peaks of XPS spectra of (a) as-synthesized Cu(0)-pHEMA hybrid and (b) after immersion in PBS for 24 h. Showing that a metallic Cu(0) was mainly characterized for the as-synthesized hybrid, however, it turned into Cu(II) after PBS immersion, suggesting the Cu(0) nanoparticles being oxidized to copper oxide, Cu(II).
945 940 935 930 925 920
0