## 2 Biosensor Application of Electrodeposited Nanoparticles
### Electrodeposition during Potential Sweeps or Cycles
platinum nanoparticles were electrodeposited onto a seven-um diameter carbon fiber electrode and used as a support for HRP [24]. The Pt nanoparticles were electrodeposited by cycling the potential between 100 mV and -250 mV (versus SCE) at 15 mV sec\({}^{-1}\) in N\({}_{2}\) saturated 4.7 mM of H\({}_{2}\)PtCl\({}_{6}\) in 0.50 M of H\({}_{2}\)SO\({}_{4}\). After the deposition of a layer of chitosan, and activation with glutaraldehyde HRP was immobilized. The direct electron transfer of HRP was observed. A detection limit of 0.35 uM and a linear range from 0.64 uM to 3.6 mM was observed for H\({}_{2}\)O\({}_{2}\). The electrodeposition of gold nanoparticles onto a gold electrode was used to make an RNA aptasensor for cyclic adenosine monophosphate (cAMP) [25]. The gold nanoparticles were electrodeposited by scanning from \(-\)0.2 V to \(-\)1.2 V (versus Ag/AgCl) in 2 mM of HAuCl\({}_{4}\) in 0.1 M of KCl. The thiolated RNA aptamer was immobilized, and mercaptohexanol was applied as both a blocker and spacer. Using Faradaic impedance spectroscopy with the [Fe(CN)\({}_{6}\)]\({}^{3-/4-}\) redox probe, a limit of detection of 50 pM for cAMP was achieved. An electrochemical immunosensor for _Salmonella pullorum_ was made using gold nanoparticles electrodeposited onto screen-printed carbon electrodes (SPCE) [26]. The nanoparticles were electrodeposited onto SPCE by cycling the potential between \(-\)1.5 V and 0.5 V (versus Ag/AgCl) at 25 mV sec\({}^{-1}\) in a 100-uM HAuCl\({}_{4}\). The surfaces were then modified by the deposition of an anti-S. _pullorum_ antibody conjugated to HRP along with an ionic liquid. In the presence of optimized concentrations of H\({}_{2}\)O\({}_{2}\) and thionine, the current due to enzyme activity was amplified. The reduction in current upon the binding of bacteria to the surface was used as the response, and a detection limit of 548 CFU mL\({}^{-1}\) (CFU, colony forming units) was obtained. Indium tin oxide (ITO)-coated glass is a highly attractive substrate for electrodeposition, as it is optically transparent and useful for sensors based on localized surface plasmon resonance, and is highly versatile, as has recently been reviewed [27]. Gold nanoparticles were electrodeposited onto ITO-coated glass and used as a support for HRP to make an amperometric sensor for H\({}_{2}\)O\({}_{2}\)[28]. Electrodeposition was carried out by cycling the potential between \(-\)0.4 V and \(-\)1.3 V (versus SCE) at 50 mV sec\({}^{-1}\) for 20 cycles in a solution of two mM of K[Au(CN)\({}_{2}\)] in pH 8 phosphate buffer. SEM (S-4700, Hitachi, Tokyo, Japan) revealed a high coverage of gold nanoparticles of near 25 nm in size and of low size dispersion. Since the substrate is transparent, absorbance spectroscopy showed the presence of a plasmon absorption peak at 540 nm. The gold nanoparticles were modified by cysteine, and then, HRP was immobilized by adsorption. Direct electron transfer to the enzyme was observed. The detection limit for H\({}_{2}\)O\({}_{2}\) was two uM, with a linear range from 8 uM to 30 mM. The direct electrodeposition of gold nanoparticles onto an ITO-coated glass electrode was used to make a sensor for morphine [29]. The deposition was done using CV between \(-\)0.3 V and \(-\)1.15 V for 60 cycles at 50 mV sec\({}^{-1}\) from a solution of 0.12 M of K[Au(CN)\({}_{2}\)] in 0.02 M of phosphate buffer at 30 \(\pm\) 1 \({}^{\circ}\)C. The nanoparticles formed were 25 to 30 nm and quasi-spherical in shape. Morphine was oxidized readily, and a detection limit of 0.21 uM and a linear range for 0.8 to 16 uM were found. Twinned Au nanoparticle structures were formed by electrodeposition onto ITO-coated glass electrodes and used as localized surface plasmon resonance (LSPR) sensors [30]. Deposition to produce twinned structures was done at 60 \({}^{\circ}\)C and pH 2 by applying 20 cycles from 0.3 V to \(-\)0.7 V (versus SCE) at 50 mV sec\({}^{-1}\) from 0.1 mM of HAuCl\({}_{4}\) and 0.1 M of KCl.