## 2 Biosensor Application of Electrodeposited Nanoparticles
### Electrodeposition during Potential Sweeps or Cycles
For comparison, isolated Au nanoparticles were electrodeposited onto ITO-coated glass at 30 \({}^{\circ}\)C. The twinned Au nanoparticles were 60 nm in size and appeared in chains of two or more with an average aggregation number of 2.3. The twinned Au particles showed two plasmon absorbance peaks at 540 nm and 705 nm, which were assigned as longitudinal and transverse, similar to what is seen for gold nanorods [31]. The 705-nm peak was highly sensitive to refractive index change with a shift of 245 nm RIU\({}^{-1}\). When modified by goat anti-IgG (Immunoglobulin G) monoclonal antibody, the binding of monoclonal IgG from a 500-ng mL\({}^{-1}\) solution caused a 21-nm red shift as compared to only a five-nm red shift for isolated Au nanoparticles. Additional study of these twinned structures showed that the longitudinal band emerged at 10 cycles, and grew in prominence at 20 and 40 cycles; the deposition at 20 \({}^{\circ}\)C did not result in a longitudinal band, and hence no twinned structures; and that the lower potential limit of \(-\)0.7 V was required for the appearance of the longitudinal band [32]. The size of the particles increasedgradually with the number of deposition cycles, from 45 \(\pm\) 7-nm at 10 cycles, and reaching 80 \(\pm\) 8 nm at 40 cycles. Gold nanoparticles electrodeposited onto ITO-coated glass electrodes were subsequently coated by a thin silver shell to improve their plasmon absorbance refractive index sensitivity [33]. Gold nanoparticles were electrodeposited by applying 60 cycles in 0.1 mM of HAuCl\({}_{4}\) at pH 2 between 0.3 V and \(-\)0.5 V at 50 mV sec\({}^{-1}\), and then, a silver shell was electrodeposited by applying 20 cycles between 0-0.3 V in 0.02 mM of AgNO\({}_{3}\). The addition of a 0.7-nm silver shell was optimal for increasing the refractive index sensitivity of the structure, and resulted in a five-nm blue-shift in the absorbance peak. The average size of the deposited Au particles increased from 72 \(\pm\) 12 nm to 89 \(\pm\) 13 nm if 200 cycles of Ag deposition were applied. The complimentary approach of electrodepositing Ag nanoparticles onto ITO glass and then electrodepositing a 1.3-nm gold shell also enhanced the plasmon absorbance refractive index response, from 123 nm RIU\({}^{-1}\) for Ag nanoparticles to 230 nm RIU\({}^{-1}\) for the silver nanoparticles covered with a gold shell [34]. When Au nanoparticles were electrodeposited onto ITO glassware, it was found that the larger ones with an average size of 130 nm had the highest plasmon absorbance peak wavelength response to a refractive index change of 187 nm RIU\({}^{-1}\)[21]. The plasmon absorbance peak red shifted with increasing nanoparticle size, which was controlled by varying the number of cycles of potential between 0.3 V and \(-\)0.5 V (versus SCE) from 10 to 20 to 40 cycles at 50 mV sec\({}^{-1}\). Cyclic voltammetry as an electrodeposition method was noted as superior to single potential step methods for controlling nucleation and growth rates. Au nanoparticles electrodeposited on ITO glass were found to show the electrocatalytic oxidation of glucose [35]. The deposition was carried out from 0.12 mM of K[Au(CN)\({}_{2}\)] in 0.05 M of pH 7.4 phosphate buffer by cycling the potential from \(-\)0.3 V to lower limits of either \(-\)1.15 V, \(-\)1.20 V, or \(-\)1.25 V (versus SCE) at 50 mV sec\({}^{-1}\). The particle size increased with a more negative lower limit, from 20 to 30 nm at \(-\)1.15 V to 35 to 45 nm at \(-\)1.20 V and to 50 to 60 nm at \(-\)1.25 V. A linear range of 0.004 to 0.50 mM for glucose was achieved in 0.01 M of NaOH when the applied potential was 0 V.