Impuestos obligatorios

As the modification of GC with graphene has thus far shown similar electrochemical properties to that seen at the bare electrode, and in some cases impeded the electrochemical response, the influence of the substrate on the peak current and peak separation was investigated. In this study, 5 µL casts of the sonicated graphene sample were compared at GC, basal plane highly orientated pyrolytic graphite (HOPG) and platinum (Pt) electrodes.

3.2.2.2.1 The Electrochemical Properties of HOPG modified with Sonicated Graphene

The use of basal plane HOPG as a comparative substrate was investigated as it is known to have poor electron transfer properties.213 This facilitated testing of the electrochemical properties of graphene without interference from the underlying substrate. In Figure 3.15 (a) and (b) results obtained by cycling bare glassy carbon and basal plane HOPG in the ferricyanide solution are compared to those modified with sonicated graphene. It is evident in Figure 3.15 (b) that redox cycling of the [Fe(CN)6]3-/[Fe(CN)6]4- couple is inefficient at the bare

96 basal plane HOPG electrode. The slow electron transfer was identified by a broad ΔEp of > 600 mV which is characteristic of this substrate when it is virtually free from defects.213, 214 In Figure 3.15 (b), the slow electron transfer observed at the bare basal plane HOPG substrate was increased upon modification with sonicated graphene and exhibited a ΔEp of 0.185 V (n=3). The large difference between this peak separation and that observed for the analogous experiment carried out at GCE (0.088 V) suggested that there was some interaction between the GC substrate and the graphene or that the electron transfer properties of GC contributed to the results observed in Section 3.3.2.1.

The increase in Ip for the oxidation of K3Fe(CN)6 to 0.0048 mA observed with modification of the basal plane HOPG with sonicated graphene was significant in comparison to the bare substrate, due to the low levels of defects achieved during preparation of the substrate. The peak currents monitored at the GCE modified with sonicated graphene were notably larger; at 0.0156 mA. These values were obtained by removal of the background capacitive currents and therefore indicated that there was some interaction between the graphene and the glassy carbon electrode that enhanced the electronic properties of the modified electrode. This could be explained by the structure of GC as it is reported to have fullerene–like structures and many edge-plane sites,219 which would likely interact with graphene via π-π interactions. It could also indicate that some electroactivity occurred at the GC substrate. As no composite or filler materials were used, it was likely that some of the GC substrate was left uncovered and contributed to the reaction.

97 (a)

(b)

Figure 3.15: Cyclic voltammograms of (a) glassy carbon and (b) basal plane

graphite electrodes comparing — bare surfaces and — surfaces modified with

5 µL sonicated graphene recorded in 1.00 × 10-3 M K3Fe(CN)6 with 0.05 M KCl and 0.05 M KH2PO4 as a supporting electrolyte system, at a scan rate of 100 mV s-1. -0.04 -0.03 -0.02 -0.01 0.00 0.01 0.02 0.03 -0.40 -0.20 0.00 0.20 0.40 0.60 Cu rr en t / mA Potential / V vs. SCE -0.04 -0.03 -0.02 -0.01 0.00 0.01 0.02 0.03 -0.40 -0.20 0.00 0.20 0.40 0.60 Cu rr en t / mA Potential / V vs. SCE

98

3.2.2.2.2 The Electrochemical Properties of Pt modified with

Sonicated Graphene

To further investigate the possibility of carbon-based interactions between the defects of glassy carbon and graphene, a platinum substrate was investigated as an electrode material. Figure 3.16 displays results similar to those in Figure 3.15 (a), whereby both the bare substrate and the substrate modified with 5 µL sonicated graphene were compared for their electrochemical properties. It appears from these voltammograms that the addition of graphene to the platinum surface increased the faradaic current response in comparison to the bare electrode. However, the anodic peak currents observed at the modified platinum electrode were substantially lower than the currents observed at GC modified with sonicated graphene (0.016 mA), which supported observations that some interaction between the GC substrate and the sonicated graphene enhanced the amount of analyte oxidised at the modified electrode. Also, as the redox behaviour of K3Fe(CN)6 is known to be largely affected by defects at the electrode surface such as edge-plane sites, it is a possibility that any uncoated parts of the GCE reacted directly with the K3Fe(CN)6.

The findings outlined above support observations of the contribution of GC to the observed electronic properties of graphene and suggested that there were electrostatic interactions between the graphene and the glassy carbon substrate.57 Similar results have been reported by Borowiec et al.44 in comparing MWCNTs cast on Pt, Au and GC electrodes for the oxidation of ketoconazole. They found that GCEs modified with MWCNTs provided the highest current response for ketoconazole detection in comparison to Au and Pt modified electrodes. A decrease in the peak separation from 0.076 V at the bare platinum to 0.066 V was observed as a result of modification with sonicated graphene. This indicated that electron transfer at the platinum modified electrode was faster than that observed at the modified GC substrate (0.088 V); however the bare platinum was also superior to GC in this respect. This observation further highlights the contribution of the substrate to the

99 electrochemical response of the modified electrodes to the redox behaviour of the ferricyanide couple.

Figure 3.16: Cyclic voltammograms comparing the redox couple of 1.00 × 10-3 M [Fe(CN)6]3-/[Fe(CN)6]4- with 0.05 M KCl and 0.05 M KH2PO4 as a

supporting electrolytes, at — bare Pt electrode and — Pt electrode modified with 5 µL sonicated graphene.