Enhancement of heterogeneous electron transfer dynamics tuning single-walled carbon nanotube forest height and density

Electrochemical sensors are growing in number and importance. Surface modifications could enhance charge transfer properties occurring at the interfaces and carbon nanoassemblies is one of the most used strategy to improve sensitivity to measurements. However, well defined protocols of surface modification are needed in order to fabricate electrochemically effective nanostructured sensors. Therefore, we aim at investigating the electrochemical properties of single-walled carbon nanotube (SWCNT) forests as a function of height and nanotube surface density. Height of the forests is accurately controlled tuning the oxidation temperatures in the range of 293–313 K of SWCNTs. The surface density of carbon nanotubes was adjusted developing cysteamine/2-mercaptoethanol (CYS/ME) self-assembled monolayers (SAMs) on gold surfaces at different ratios (1:0, 1:3, 1:10, 1:100, 0:1). Apparent electron transfer rate was analyzed with electrochemical impedance spectroscopy (EIS) and experimental data show that transfer rate constant, kapp, increases from 1 × 10−4 cm/s to 6 × 10−4 cm/s rising oxidation temperatures (i.e. lowering forest height); therefore forests with reduced height show higher electron transfer rate without significant difference in electrodic reversibility. On the other hand, tuning SWCNT surface density, forests obtained with no ME show optimal Δpeak value of 0.087 ± 0.015 V and highest kapp value of 9.15 × 10−3 cm/s. Surprisingly, electrochemical surface area analysis shows that samples with lower amount of cysteamine have an active surface area three times bigger than samples with 1:3 CYS/ME ratio. Low electrochemical efficiency associated with high active surface may be related to unwanted SWCNT bundles adsorbed on the surface for 1:10 and 1:100 CYS/ME ratio samples as confirmed by AFM morphological characterization. Further investigation shows that a transition from a semi-infinite planar diffusion mechanism to a radial diffusion one takes place when SAMs with low chemical affinity to nanotubes are used. Wettability analysis confirms the robustness of the surface chemical modification during the forest development. Altogether these results show that optimal electrochemical properties of carbon modified electrodes require an accurate control of forest fabrication in terms of carbon nanotube structural assembling.