## 2 Biosensor Application of Electrodeposited Nanoparticles
### Deposition at a Constant Potential for a Fixed Time Period
Electrodeposition including CTAB and HRP gave a deposited layer of aggregated nanoparticles, which was attributed to the electrostatic interaction between CTAB and HRP. The amperometric response to H\({}_{2}\)O\({}_{2}\) was greatest at pH 7.0 for the 50-minute deposition time and an operating potential of \(-0.3\) V. Cyclic voltammetry (CV) data indicated that multilayers of HRP were formed, as the peak current was 23 times the value that was expected for monolayer coverage. Direct electron transfer between HRP and gold was found, and the electrode could detect H\({}_{2}\)O\({}_{2}\) (detection limit = 0.23 uM, linear range 0.5 to 105 uM), t-butyl hydroperoxide (detection limit = 8.6 uM, linear  
Figure 1: (a) SEM images of gold nanoparticles formed on glassy carbon electrode (GCE) by electrodeposition at \(-0.2\) V and 220 s from three mM of HAuCl\({}_{4}\) and 0.5 M of H\({}_{2}\)SO\({}_{4}\). (b) SEM image after surface modification with 3-mercaptoethanolpropionic acid and conjugation to tubulin. Reproduced with permission from ref. [13], Copyright 2009, Springer. range 10 to 1000 \(\upmu\)M), and cumene hydroperoxide (detection limit = 3.6 \(\upmu\)M, linear range 10 to 750 \(\upmu\)M). A biosensor for the antibiotic tetracycline was constructed by immobilized thiolated aptamer onto gold nanoparticles directly electrodeposited onto GCE [17]. The gold nanoparticles were electrodeposited at \(-\)0.2 V (versus saturated calomel electrode, SCE) from 3 mM of HAuCl4 for 200 s. The result was coverage of the surface with irregularly shaped nanoparticles. Methylene blue redox indicator bound to the aptamer was displaced by tetracycline, resulting in a decrease in the redox current. A detection limit of 0.42 \(\times\) 10\({}^{-11}\) M was found, as was good recovery and the detection of tetracycline spiked into milk at concentrations of 5 \(\times\) 10\({}^{-9}\) M, 5 \(\times\) 10\({}^{-7}\) M, and 5 \(\times\) 10\({}^{-5}\) M.  
Gold nanoparticles electrodeposited onto gold electrodes have also been used as supports for biosensor development. An electrochemical sensor for protein casein kinase II (CK2) activity was constructed using gold nanoparticles electrodeposited onto a gold electrode [18]. The electrodeposition was carried out at \(-\)0.2 V (versus Ag/AgCl) for 250 s in a solution of three mM of HAuCl4 containing 0.1 M of KNO3. A substrate peptide (CDDDDDSDDDA) for carboxypeptidase Y (CPY) was immobilized onto the gold nanoparticles. Phosphorylation of the substrate peptide hindered the CPY activity, and the response of the [Fe(CN)6]3\({}^{-/4-}\) redox probe did not increase as much when a phosphorylated substrate peptide was exposed to CPY as when a non-phosphorylated substrate peptide was exposed to CPY. The scheme was used to determine an IC50 value of 39.77 nM for ellagic acid, which is an inhibitor of CK2. An electrodeposited film resembling nanoporous gold, but not requiring a dealloying process, was formed by the electrodeposition of gold nanoparticles onto a Si surface onto which five nm of Ti and 43 nm of Au had been deposited by magnetron sputter coating [19]. Electrodeposition from 10 mM of HAuCl4 solution containing polyethylene glycol (PEG, 200 MW) at -1.3 V (versus Ag/AgCl) for 30 s resulted in a 150-nm thick film of interconnected Au nanoparticles presenting pores of 30-nm diameter. The electrode had a detection limit of 100 nM for the oxidation of dopamine in the presence of 10 \(\upmu\)M of ascorbic acid and 1 mM of uric acid, as determined using the peak currents from differential pulse voltammetry (DPV) scans. The electrodeposition of gold onto gold-coated glass slides by the application of two sequential fixed potentials was found to result in a surface morphology resembling a pile of brick-like shapes [20].