# Deposition of Cell Culture Coatings Using a Cold Plasma Deposition Method
## 3 Results
### Characterization of the Plasma Discharge
The other model inputs, i.e., the diffusion coefficient of the probe in solution and its formal potential, E\({}^{o^{\prime}}\), were extracted from the experimental cyclic voltammograms recorded at slow scan rates so that the responses were not influenced by the rate of heterogeneous electron transfer. In all cases, the peak currents were within 3% of the experimental values and the anodic, E\({}_{\text{pa}}\), and cathodic, E\({}_{\text{pc}}\), peak potentials were reproduced to within 5 mV. As shown in Table 2, the rate of heterogeneous electron transfer depends significantly on the identity of the probe used. Cyclic voltammograms were taken for each of the electroactive probes at the gold electrode modified with a rat tail collagen film, using the wet chemical deposition method. The effect of film formation on the general properties of the voltammetric response depends markedly on the identity of the probe. For example, for ferrocyanide at a scan rate of 500 mV/s, the difference between E\({}_{\text{pa}}\) and E\({}_{\text{pc}}\), i.e., \(\Delta\)E\({}_{\text{p}}\), increases by more than 180 mV following collagen deposition. The CVs obtained at the collagen modified electrodes were also fitted to a classical diffusion-controlled model and the best fits are shown as open circles in Figure 10. In fitting these voltammograms, reproducing the experimental peak potentials is achieved by varying k\({}^{o^{\prime}}\) and in all cases satisfactory agreement between experimental and model peak potentials is obtained. Significantly, while k\({}^{o^{\prime}}\) for the ferrocene methanol and ruthenium hexamine agrees with the bare electrode value to within experimental error, the electron transfer rate constant for ferrocyanide is approximately 20 times lower at the collagen modified surface. This result does not arise because of differences in mass transport between bare and coated electrodes and suggests that electron transfer occurs through the nonconducting collagen layer. The impeding of electron transfer could arise due to electrostatic repulsion of the negatively charged probe by the negatively charged collagen layer or physical blocking by a continuous, nonporous film. For ferrocene methanol and ruthenium hexamine, the model accurately reproduces the general features of the voltammogram as well as the peak potentials. In contrast, for ferrocyanide, the experimental currents away from the peak potentials deviate from those predicted by the model indicating that the collagen film changes the nature of mass transport to the electrode surface. However, to reproduce the experimental peak currents for all three probes, a normalization factor was required. This factor reflects changes in the area available for electron transfer as well as changes in the interfacial concentration of the probe, e.g., repulsion of ferrocyanide and preconcentration of ruthenium hexamine. For both the ferrocyanide and ferrocene methanol, the normalization factors are less than unity, 0.80 and 0.70, respectively, i.e., the experimental peak currents are lower than predicted. This result suggests that the effective electrode area and/or the interfacial concentration of the probes are lower compared to the bare electrode. In contrast, the factor for ruthenium hexamine is 1.1 suggesting that the positively charged probe preconcentrates in the negatively charged collagen film and, based on the indistinguishable bare and coated k\({}^{o^{\prime}}\) values, undergoes facile electron transfer.