## 5 Biosensor Application of Electrodeposited Composite Nanostructures
### Composite Electrodes Containing Carbon Nanomaterials  
A composite nanostructured layer containing copper nanoparticles, multiwalled carbon nanotubes (MWCNT), and chitosan was formed on gold wire electrodes as a support for a biosensor based on sarcosine oxidase [109]. Carboxylated MWCNT (c-MWCNT) dispersed in 3:1 H\({}_{2}\)SO\({}_{4}\):HNO\({}_{3}\) were mixed with an aqueous solution of EDC and NHS of pH 6.0. Chitosan was electropolymerized onto the gold electrode by applying 20 cycles from \(-\)1.1 to 0.1 V (versus Ag/AgCl). A suspension of copper nanoparticles (average size: 50 nm) and c-MWCNT was then electrodeposited by applying 20 cycles of \(-\)1.1 to 0.1 V at 20 mV sec\({}^{-1}\). Sarcosine oxidase was immobilized onto the activated composite electrodes and used as an amperometric sensor for sarcosine, which was noted as present in urine and blood during prostate cancer, with a detection limit of 0.1 pM. A biosensor for the detection of DNA hybridization based on electrochemiluminescence was constructed from a complex composite structure [110]. Carbon nanotubes were first deposited onto a GCE. Au/Pt nanoparticles were electrodeposited at \(-\)0.2 V for 200 s in a solution of 1.0 mM of HAuCl\({}_{4}\) and 1.0 mM of H\({}_{2}\)PtCl\({}_{6}\). Then, a capture DNA probe was assembled onto the surface, including the use of mercaptohexanol to promote improved probe orientation. After hybridization with the target DNA, a DNA-modified nanosheet of graphite-like carbon nitride nanosheets (g-C\({}_{3}\)N\({}_{4}\)) modified with gold nanoparticles was bound to the duplex. The assembly generated a electrochemiluminescence signal that was detected while scanning the potential from 0 to \(-\)1.5 V for 10 cycles at 100 mV sec\({}^{-1}\). A linear range from 0.04 to 5.0 \(\times\) 10\({}^{4}\) fM and a detection limit of 0.018 fM were observed. The detection of DNA spiked into serum samples was also reported. A complex nanocomposite electrode was formed by one-step electrodeposition in order to prepare a highly sensitive first-generation glucose sensor [111]. As a first step, graphene oxide was dispersed, and H\({}_{2}\)PtCl\({}_{4}\) was added, followed by formic acid; then, after 12 days, a dispersion of reduced graphene oxide (rGO) and platinum nanoparticles was obtained at pH 4, and then drop case onto the gold electrode. Subsequently, electrodeposition was carried out from a solution of 5.0 mg mL\({}^{-1}\) of glucose oxidase, 84 mM of the ionic liquid imidazolium alkoxysilane, 0.5% chitosan, and 30 mM of benzoquinone by conducting cyclic voltammetry scans between 0.7 V and \(-\)0.8 V (versus SCE). The electrodes were characterized by SEM (S4800, Hitachi, Tokyo, Japan), FTIR (Nexus 670, Nicolet, Madison, WI, USA) and Raman spectroscopy (Invia, Renishaw, Wotton-under-Edge, UK). SEM showed rGO sheets with dispersed platinum nanoparticles. The amperometric response to H\({}_{2}\)O\({}_{2}\) was evaluated and found to be maximal at 0.55 V and for films formed on the gold electrode by 10 potential cycles. The sensor was found to have a very low detection limit for glucose of 0.02 uM and a linear range from 0.002 to 5.5 mM. An impedimetric sensor for tumor necrosis factor alpha (TNF-\(\alpha\)) was constructed based on the electrodeposition of reduced graphene oxide and then gold nanoparticles onto ITO electrodes [112] The rGO was electrodeposited from a 1 mg mL\({}^{-1}\) dispersion by cyclic voltammetry conducted between zero and \(-\)1.6 V (versus Ag/AgCl) at 25 mV sec\({}^{-1}\). Subsequently, gold nanoparticles were electrodeposited from 0.5 mM of HAuCl\({}_{4}\) at \(-\)0.9 V for 30 s. Examination by atomic force microscopy (PSIA XE-100, Park Systems, Seoul, South Korea) showed that the rGO only covered surfaces that showed sheet-like structures of 37-nm thickness, the electrodeposition of only gold nanoparticles showed particles about 170 nm in size, and the composite of rGO and gold nanoparticles showed aggregated structures on the surface.