## 5 Biosensor Application of Electrodeposited Composite Nanostructures
### Composite Electrodes Containing Carbon Nanomaterials
The change in reduction current was linear over the range one to 400 uM of uric acid, and the detection limit was one uM when the sensor was operated at the optimal potential of \(-0.35\) V.  
Composite nanostructures of MWCNT and clusters of gold nanoparticles were formed on GCE and used to make an amperometric H\({}_{2}\)O\({}_{2}\) sensor based on immobilized hemoglobin [119]. A dispersion of 0.1 mg cm\({}^{-3}\) of MWCNT in toluene was drop cast onto a GCE, and then subjected to potential cycling from 1.8 V to \(-0.4\) V (versus Ag/AgCl) for 10 min in 0.5 M of K\({}_{2}\)SO\({}_{4}\) to introduce oxidized functional groups on defects in the MWCNT. Next, gold oxide complexes were formed on these sites by pulsing the potential from 0.8 V to 1.5 V, applying different pulse widths in a solution of 0.1 M of HAuCl\({}_{4}\) in 0.1 M of K\({}_{2}\)SO\({}_{4}\). The gold oxide clusters were converted to clusters of Au nanoparticles by cycling the potential from 1.2 V to \(-0.4\) V in 0.1 of H\({}_{2}\)SO\({}_{4}\). Hemoglobin was immobilized from an 80-uM solution by cycling the potential from \(-0.15\) V to 1.15 V at 100 mV sec\({}^{-1}\) for 30 cycles. To complete the electrode modification, Nafion was solution cast over the surface. SEM (S-4800, Hitachi, Tokyo, Japan) revealed the occurrence of distinct clusters of Au nanoparticles along MWCNT. The pulse width that was used in the first step of the preparation influenced the nanoparticle size; 0.0001-second pulses resulted in the formation of one nm of nuclei, which were converted into five-nm Au nanoparticles. Meanwhile, 0.0005-second pulses resulted in formation of two-nm nuclei that were converted into 10-nm Au nanoparticles, and 0.001-second pulses resulted in formation of five-nm nuclei that were converted into 50-nm Au nanoparticles. The direct electrochemistry of hemoglobin was observed, and a detection limit of 0.2 uM and linear range from 0.5 to 1000 uM was obtained from H\({}_{2}\)O\({}_{2}\). A carbon ionic liquid electrode was used for the electrodeposition of Au nanoparticles for the immobilization of myoglobin to make a direct electron transfer amperometric sensor for trichloroacetic acid or H\({}_{2}\)O\({}_{2}\)[120]. The compound 1-hexylpyridinium hexafluorophosphate and graphite powder was mixed into a paste, packed into a glass tube, and connected by a copper wire. Graphene sheets were electrodeposited from graphene oxide dispersion at \(-1.3\) V (versus SCE) for 600 s. The Aunanoparticles were electrodeposited at \(-0.4\) V for 300 s from a solution of five mM of HAuCl4 and 0.5 M of KNO3. Myoglobin was adsorbed, and then covered by a layer of Nafion. For H2O2, a detection limit of 0.66 uM and a linear range from 1.98 to 238.2 uM was found, and for trichloroacetic acid, a detection limit of 0.13 mM and a linear range from 0.4 to 20 mM was found. Gold nanoparticles were electrodeposited onto GCE, but first they were surface modified with reduced graphene oxide sheets [121]. The electrodeposition at \(-0.2\) V (versus Ag/AgCl) from 0.3 mM of HAuCl4 produced Au nanoparticles decorating the petal-shaped graphene nanosize flakes. The decoration with Au nanoparticles doubled the effective surface area, which was determined using the Randles-Sevcik equation analysis and the K4 Fe(CN)6 redox probe. The gold surfaces were modified with 2-(3,4-dihydroxy phenyl) benzothiazzole, which promoted the electrocatalytic detection of levodopa, uric acid, and folic acid, whose peaks in square wave voltammetry sweeps were separated so that all three could be detected simultaneously. A dispersion of exfoliated graphene oxide sheets and 0.5 mM of HAuCl4 was electrodeposited onto a GCE by scanning the potential (versus Ag/AgCl) to form a film of graphene sheets decorated with nanoparticles [122]. This electrode was found useful for the analysis of CH3 Hg+, and the maximum response was for the electrode formed by five cycles of deposition.