## 3 Results and Discussion
The pink line corresponding to 2 mM Ru(NH\({}_{3}\))6\({}^{2+/3+}\) in 0.5 M aqueous KCl has been replotted as an inset to make it visible. valently bonded to the metal surface. In addition, a physiadsorbed HD-CNT cross-section to Cu metal was also characterized, but the results were significantly inconsistent. The covalently bonded to Cu and Pt and Ag paste-connected CNTs displayed very similar CV characteristics, suggesting good electrical contact between the CNTs and metals. The contact effectiveness with the metal surface was evaluated using cyclic voltammetry and the electroactive surface area, as determined using the Randles-Sevcik equation [79], which was similar to the geometrical surface area. To determine the heterogeneous electron transfer rates (\(k^{\circ}\), cm s\({}^{-1}\)), cyclic voltammetry experiments were performed in 2 mM of Ru(NH\({}_{3}\))\({}_{6}\)\({}^{2+/3+}\) with 0.5 M KCl as a supporting electrolyte in distilled water at scan rates of 10-20 mV s\({}^{-1}\). As can be seen in Figure 5, the covalently bonded HD-CNTs displayed a sigmoidal steady state limiting current with a magnitude of \(\sim\)17 nA. These are typical characteristics of hemispherical diffusion at a reduced diameter of microelectrodes. The steady state behavior of both redox species at a scan rate of 10 mV s\({}^{-1}\) was determined in a similar manner to our previous work, in which CNTs were connected with Ag paint [58]. The peak current response increased as the scan rate increased, further confirming that radial diffusion occurred at the electrode-electrolyte interface [58]. In addition, the electrode response was evaluated at increasing potentials. The electrodes generated reproducible cyclic voltammetry responses in the potential range from +1 V to \(-\)1.25 V. Furthermore, an \(E_{1/4}-E_{3/4}\) wave potential difference of 59 mV was observed for the open-ended CNTs connected through covalent bonds to the metal. Theoretical studies and a few experimental reports have shown the enormous potential of open-ended CNTs connected to metals [50; 55; 58; 80]. Electrical resistivity of 9 \(\times\) 10\({}^{-7}\)\(\Omega\) m was reported for CNTs connected to Au, which is two orders of magnitude higher than that achieved using simple contact, conducting almost as much as Cu might [46]. To evaluate the impact of covalent bonding HD-CNT to metal, the electrical resistances of the covalently bonded CNT cross-section electrodes were determined using a sensitive picoamp meter. The covalently bonded CNT cross-sections were compared to connections made using colloidal Ag paint and simple pressure contact (physis adsorbed HD-CNTs). In all cases, sealing with epoxy was applied in a similar manner to all covalently bonded HD-CNTs. The average resistance measured for covalently bonded CNTs was 305 \(\Omega\), whereas that for colloidal Ag-connected CNTs was 151 \(\Omega\), and the pressure contact samples gave inconsistent measurements throughout five different samples. The difference, although minimal, was attributed to the nanoscale roughness of the metal surfaces and required optimization in order to guarantee that all dangling carbons at the circumference of the open ends were bonded. Although our results are not close to the theoretical published results, to the best of our knowledge, this is the first attempt to covalently bond open-ended HD-CNTs to metal substrates. Finally, the sensing capabilities of the CNTs covalently bonded to metal surfaces were demonstrated by detecting Pb\({}^{2+}\) ions in an aqueous solution (Figure 6). The fabricated CNT-Cu bonded electrodes were able to detect low concentrations of Pb\({}^{2+}\), and a linear relationship was observed between the Pb\({}^{2+}\) concentration and the peak current response (R\({}^{2}\) = 0.9973). These results are extremely important because they confirm the possibility of using CNTs bonded to a Cu metal surface via wet chemistry functionalization to develop electrochemical sensors for heavy metals.