## 5 Biosensor Application of Electrodeposited Composite Nanostructures
### Composite Electrodes Containing Carbon Nanomaterials
The combination of these different nanomaterials was designed to enhance the response to H\({}_{2}\)O\({}_{2}\) that was formed from the reaction sequence in which acetylcholinesterasecholinesterase converts acetylcholine to thiocholine, and choline oxidase then converts the thiocholine and H\({}_{2}\)O\({}_{2}\), which will partake in the luminol reaction sequence that ultimately yields luminescence. Using acetylthiocholine chloride as the substrate, detection of the ECL signal upon scanning the potential continuously between 0.2 and 0.8 V at 100 mV sec\({}^{-1}\) gave a sensor with a detection limit of 0.022 uM and a linear range from 0.0067 to 921 uM. An electrochemical sensor for the organophosphate pesticide Phoxim was developed based on a composite electrode of reduced graphene oxide and Au nanoparticles formed on a GCE [116]. Reduced graphene oxide was first electrodeposited by 500 cycles of pulsed electrodeposition in which the potential was stepped between \(-\)1.3 V and 0.6 V (versus Ag/AgCl) and held for two seconds, and then stepped back and held for two seconds. Then, the electrodeposition of gold nanoparticles was carried out for 60 s at \(-\)0.2 V in a solution of 1% HAuCl\({}_{4}\) in 0.5 M of H\({}_{2}\)SO\({}_{4}\). SEM (SUPRA 55, Carl Zeiss AG,Oberkochen, Germany) showed flake-like and wrinkled shapes for the rGO sheets covered by Au nanoparticles with an average diameter of 35 nm. Proxim could be reduced on this modified electrode with a peak current near -0.69 V. Using DPV, a detection limit of 0.003 uM was found for phoxim. A biosensor for a segment of DNA from hepatitis B virus (HBV) was made based on gold nanoparticles electrodeposited onto a GCE modified with both graphene and carboxylated multiwalled carbon nanotubes (c-MWCNT) [117]. A suspension of graphene oxide and c-MWCNT was first drop cast onto a GCE; then, a potential sweep from zero V to \(-1.5\) V (versus Ag/AgCl) was used to reduce graphene oxide to graphene. Then, gold nanoparticles were formed on the surface by electrodeposition from 2.9 mM of HAuCl\({}_{4}\) at \(-0.2\) V for times varied from 200 to 1000 s. An electrodeposition time of 900 s was found to optimize the surface area. A thiolated hairpin DNA probe was assembled onto the gold nanoparticles. SEM (JSM-6330F, JEOL Co. Ltd., Beijing, China) showed the surface of graphene wrapped with the CMWCNT, and then the Au nanoparticles in the size range of 100 to 180 nm. The capture of the target DNA opened the hairpin and allowed the formation of a quadruplex with hemin that served as a DNAzyme to catalyze the H\({}_{2}\)O\({}_{2}\)-mediated oxidation of 3,3\({}^{\prime}\),5,5\({}^{\prime}\)-tetramethylbenzidine (TMB). Differential pulse voltammograms of the reduction of TMB gave a detection limit for the DNA segment, a specific sequence of the HBV X gene of 0.5 pM, and a linear range from 10 pM to 10 nM. A composite nanostructured electrode was used to immobilize uricase and act as an amperometric sensor for uric acid [118]. Silver nanoparticles were first electrodeposited onto a gold rod electrode at \(-0.40\) V (versus Ag/AgCl) for 30 s from a solution of one mM of AgNO\({}_{3}\) in 0.2 M of KNO\({}_{3}\). A membrane of chitosan and multiwalled carbon nanotubes was then electrospun onto the electrode. Using glutaraldehyde activation, uricase was immobilized through the amino groups of chitosan. Then, the enzyme electrode was used in a flow injection system. The reaction of uricase on uric acid consumes O\({}_{2}\) and produces allantoin, H\({}_{2}\)O\({}_{2}\), and CO\({}_{2}\).The electrode catalyzed the electroreduction of O\({}_{2}\), and the addition of uric acid resulted in a reduction of the reduction current.