The surface electrochemical behaviors of Cu(0)-pHEMA hybrid were analyzed by cyclic voltammetry (CV) method and alternating current (AC) impedance method. Cyclic voltammogram is presented in Fig. 8.1 for the Cu/pHEMA hybrid in phosphate-buffered saline solution over the potential range of -0.7 to 0.7V at a scan rate of 50mVs-1 at 25oC.
Fig. 8.1 Cyclic voltammograms of pHEMA and Cu(0)-pHEMA in the phosphate-buffered saline solution, v=50mVs-1.
As shown in Fig. 8.1, the Cu(0)-pHEMA hybrid exhibits a higher oxidation current at -310mV when the crystallinity, shown in chapter 6, of the in-situ-formed nano Cu particles is increased. The oxidation peak appeared at a potential of anodic peak is related to the formation of surface-bonded Cu(I) hydrous oxide([Cu+•nH2O]ads) and active Cu(II), respectively [198, 199]. The Cu atoms at an active site probably have a range of energies [200]
and some of these, temporarily present (due to energy fluctuations) in a very low lattice coordination state, may be sufficiently active to be oxidized at ca.-0.3V to the Cu (I) or Cu(II) state. The Cu(I) hydrous oxide and active Cu(II) play a major role in the oxidation on the
-600 -400 -200 0 200 400 600
-3 -2 -1 0 1 2
Hybrid-10 Hybrid-05 Hybrid-01 pHEMA
Current Den sity ( mA/ c m
2)
Potential vs Ag/AgCl (mV)
surface of hybrid at the potential range with respect to the formation of copper oxide. This particular material is easily oxidized and has a higher possibility of absorption hydroxyl group and release electrons in the reaction surface. In addition, from the alternating current (AC) impedance measurement, it can be observed that the incorporation of nano copper particles into pHEMA matrix increases the interface interaction between the solid film and aqueous solution. The AC impedance method was used to calculate the specific capacitance (Cspec) and the charge transfer resistance (RCT) of the Cu(0)-pHEMA hybrid. The spectra (i.e., Nyquist plot) for pHEMA and Cu(0)-pHEMA hybrids are presented in Fig. 8.2.
Fig. 8.2 Nyquist diagram of selected Cu(0)-pHEMA hybrid showing various values of time constant.
At high frequency, the spectral feature represents a limiting diffusion process and at low frequency it represents purely capacitive characteristic. For pHEMA, there is no copper nano particle within the polymer matrix, which indicates purely capacitive behavior. On the other hand, the AC spectrum for the Cu(0)-pHEMA hybrids displays a semicircle profile and is indicative of an electric double-layer capacitor behavior. The charge transfer resistances (RCT)
pHEMA Hybrid-01 Hybrid-10
0 3000 6000 9000 12000 15000 0
2000 4000 6000 8000 10000 12000
-Z
im(ohm /c m
2)
Z
re(ohm/cm
2)
the content of nano copper particle in polymer matrix increased, the redox ability of copper raised and promote the oxidation current shown in cyclic voltammograms, where the time constant (τ =CspecRs) of Hybrid-01 and Hybrid-10 which is about 0.098 and 0.133, respectively was obtained. A material with a higher value of time constant suggests that the probability of thrombosis or platelet adhesion is minimized. However, the copper oxide played a n-type role in the hybrid, it can be seen that the surface charge of the hybrids showed more negative with an increasing incorporation of the copper nanoparticle, to a value of as high as -24.6 mV as given in previous study. Accordingly, surface with increasing negative charge should exhibit improved anti-blood clotting behavior; together with redox ability of Cu nanoparticles reported earlier, it further reinforces the argument that the hybrids currently prepared should have improved thromboresistant property.
Nitric oxide has been widely recognized as a potent vasodilator and inhibitor of platelet adhesion and activation [201-203]. Because it has very short lifetime in blood [204] due to its reactivity with various blood components [205], a more abundant (i.e., micromolar concentrations) and stable form of NO in blood are S-nitroso adducts with thiol groups (RSNOs) [206], such as S-nitrosoglutathione (GSNO) [207-209]. In order to investigate the NO production from Cu(0)-pHEMA hybrid, the redox properties and the mechanism of copper mediated nitrosoglutathione (GSNO) reduction to nitric oxide (NO) were performed in aqueous system. Fig. 8.3 illustrates the typical current responses to different concentrations of nitrosoglutathione reduced by the Hybrid-10 at an operation voltage of -1.1V (vs. Ag/AgCl) at 25oC. As shown in Fig. 8.3, the reduction currents of hybrid varied with the concentration of nitrosoglutathione. With increasing levels of nitrosoglutathione (GSNO), the amount of NO generation increased. The reduction current of nitrosoglutathione into NO was linear with the nitrosoglutathione concentration as shown in Fig 8.3 (b).
Fig. 8.3 (a) Amperometric responses of NO-generating from Hybrid-10. (b) The correlation between nitrosoglutathione (GSNO) and reduction current.
The reduction current of nitrosoglutathione for Composite-05 micro, Composite-05 nano (synthesis in chapter 6), Hybrid-2, and Hybrid-4 (synthesis in chapter 6) have also been tested.
As shown in Fig. 8.4, for the same copper content (about 0.3~0.5 wt %), the in-situ Cu(0)-pHEMA hybrid has higher reduction current than the composite. It might be due to the higher surface area of the in-situ hybrid.
Fig. 8.4 Amperometric responses of NO-generating from Composite-05 micro, Composite-05 nano, Hybrid-2, and Hybrid-4 with 20mM GSNO.
Baseline
NO: 2.5ppm 0.000 0.005 0.010 0.015 0.020 0.025 0.030
0
940 938 936 934 932 930 928 926 -200
0 200 400 600 800
Count
(
a. u.)
Binding Energy (eV) 1
2
3
An XPS analysis of Cu(II)-pHEMA hybrid after redox with GSNO was shown in Fig.
8.5., where the Cu 2p3/2 NPs spectrum of the Cu(II)-pHEMA hybrid shows a peak at 933.8
± 0.3 eV which is mainly attributed to CuO, while two weaker feature, lying at 932.3 ± 0.2 eV and 934.7 ± 0.2 eV are attributed to a small amount of Cu2O and Cu(OH)2, respectively.
A corresponding fraction of the copper species of respective concentration can be correlated with the area under the spectrum and this determines the fraction of the CuO was 77.1%, Cu2O was 10.3%, and Cu(OH)2 was 12.6% in the resulting Cu(II)-pHEMA hybrid.
Fig. 8.5 The Cu 2p3/2 peaks of XPS spectra of Cu(II)-pHEMA hybrid after redox reaction with GSNO.
# Position (eV)
Intensity FWHM (eV)
Area Area(%) Component
1 932.3 196.38 2.8 382.8 10.3% Cu2O
2 933.8 623.51 1.7 3347.6 77.1% CuO
3 934.7 293.66 1.2 546.5 12.6% Cu(OH)2
This indicates that the copper oxidized into copper oxide developed in the Cu(II)-pHEMA hybrid is a mixture of mainly CuO and a small fraction of Cu2O and Cu(OH)2, after electrochemical oxidation. The presence of mainly divalent CuO imparts a negative surface charge to the hybrid (present in chapter 7) which may further prevent the occurrence of the oxidative reaction of fibrinogen as schematically shown in Fig. 8.6, leading to an improved anti-thrombosis character of the hybrid composites.
Fig. 8.6 Scheme of the electrochemical reaction for Cu(0)-pHEMA hybrid.
8.4 Summary
An electrochemical activated Cu(0)-pHEMA hybrid was successfully fabricated by in-situ UV-light polymerization and chemical reduction process. The nano copper particles were uniformly dispersed in pHEMA matrix to form a Cu(0)-pHEMA hybrid and alternated the electrochemical properties of the polymer matrix. The particle size in pHEMA matrix would be increased by broadening the network space via controlling the monomer concentration. The oxidation reaction of nano copper particles in pHEMA to form copper oxide was enhanced with the content of the copper nano-particles and followed that the time constant of the Cu(0)-pHEMA hybrid was decreased.
OH
-Cu + 2OH- Æ CuO + 2H2O + 2e