## 2 Biosensor Application of Electrodeposited Nanoparticles
### Electrodeposition during Potential Sweeps or Cycles  
In many recent applications, gold nanoparticles were electrodeposited during potential sweeps or multiple CV cycles. It has been proposed that the use of potential sweeps and cycles provides better control over particle nucleation and growth, and results in a more regularly spaced deposition of particles that are closer to uniformly shaped [21]. The potential limits, scan rate, and number of cycles can all be adjusted in this approach in addition to the variables of choice of substrate and solution composition. Figure 2 shows the SEM (S-4700, Hitachi, Tokyo, Japan) micrographs of gold nanoparticles of different sizes deposited onto indium tin oxide (ITO) glass using potential cycling. The plasmon absorbance spectra for these deposited nanoparticles are also shown. Three different preparations of nanoparticles were supported. Electrodeposition by 20 cycles at 50 mV sec\({}^{-1}\) between 0.3 V and \(-\)0.5 V (versus SCE) resulted in particles with an average size of 90 nm from a solution of one mM of HAuCl4 at pH 2. The addition of 0.05 M of KCl to the solution increased the average particle size under these conditions to 90 nm. Larger particles with an average size of 137 nm were electrodeposited by applying 80 cycles at 0.05 mV sec\({}^{-1}\) from 0.3 V to \(-\)0.85 V from one mM of HAuCl4 at pH 2. The refractive index sensitivity for the nanoparticles were 45 nm RIU\({}^{-1}\), 137 nm RIU\({}^{-1}\), and 187 nm RIU\({}^{-1}\) for the 35-nm, 90-nm, and 137-nm particles, respectively. In comparison to the deposited particles shown in Figure 1, which were electrodeposited at a constant potential, it can be seen that the particles in Figure 2 have more regular spacing, fewer indications of neighboring particles near contact, and more uniform size. In another example, gold nanoparticles were formed on GCE by sweeping the potential in 10 mM of HAuCl\({}_{4}\) from 1.1 V to \(-\)0.1 V (versus Ag/AgCl) for a number of scans ranging from 5 to 35 [22]. The coverage of gold nanoparticles on the surface was observed to increase until a full monolayer of nanoparticles was seen for 25 cycles, and beyond that, multilayers began to form. After 25 cycles, a roughness factor of 183.6 \(\pm\) 1.2 was determined using [Fe(CN)\({}_{6}\)]\({}^{3-/4-}\) as a diffusing redox probe, with more cycles making the roughness factor drop significantly. A mixed SAM of 4-aminothiophen (4-APh) and 4-mercaptoethanol acid (4-MBA) was formed. The enzyme _Corynascus thermophilus_ cellobiose dehydrogenase, which is the C291Y mutant that was designed for enhanced sensitivity to glucose and lower cross-reactivity with maltose, was immobilized by cross-linking with glutaraldehyde. The result was a selective and stable (over 20 days) third generation (direct electron transfer) biosensor for glucose with a detection limit of 6.2 uM and a linear range from 0.02 to 30 mM. Gold nanoparticles electrodeposited onto GCE were used as a support for a DNA-based electrochemiluminescence (ECL) sensor for Hg\({}^{2+}\)[23]. The Au nanoparticles were electrodeposited for five seconds after a potential step from 1.1 V to \(-\)1.0 V (versus Ag/AgCl) in solutions of HAuCl\({}_{4}\) in the range of 40 to 140 uM in 0.5 M of KNO\({}_{3}\). The surface density of Au nanoparticles increased with the concentration of the HAuCl\({}_{4}\) solution, and the maximal ECL was found when a concentration of 100 uM was used. The distance between ferrocene as a quencher and [Ru(bpy)\({}_{3}\)]\({}^{2+}\) as a reporter would increase greatly upon Hg\({}^{2+}\) binding, which caused the DNA hairpin structure to open up and increased the ECL signal. A detection limit of 0.1 nM for Hg\({}^{2+}\) was found. In another example,  
Figure 2: Representative SEM images of GNPs (gold nanoparticles) on an indium tin oxide (ITO) surface at an average size of (**A**) 35 nm, (**B**) 90 nm, and (**C**) 130 nm. (**D**) UV-VIS absorption spectra of the GNPs deposited on an ITO surface. Reproduced with permission from ref. [21], Copyright 2009, Elsevier.