## 4 Biosensor Application of Ramified or Dendritic Electrodeposited Structures
### Nanostructures Described as 'Flower-Like', Irregular, of Unusual Morphology  
A nanostructure formed on gold electrodes by first electrodeposition gold nanocrystals followed by electrodeposition platinum nanoflowers provided an amperometric H\({}_{2}\)O\({}_{2}\) sensor of good sensitivity operating at a low potential [84]. Gold nanocrystals that were formed by electrodeposition at \(-\)3.0 V (versus Ag/AgCl) in solution of 0.01 M of HAuCl\({}_{4}\) and 2.5 M of NH\({}_{4}\)Cl for 120 s were created by using the hydrogen bubbles formed at this highly negative potential as a template [85]. The resulting nanocrystals were examined by SEM (Merlin, Carl Zeiss AG, Oberkochen, Germany) and showed a honeycomb-like structure of large pores of \(\sim\)50 \(\upmu\)m whose walls were an agglomeration of small gold nanoparticles that formed in the interstitial spaces between the hydrogen bubbles, which then detach and leave behind a porous structure. In order to achieve these structures, it was noted that a low concentration of HAuCl\({}_{4}\) was required, because otherwise, dendrites would form. The platinum nanoflowers were formed on the gold nanocrystals by electrodeposition in 25 mM of H\({}_{2}\)PtCl\({}_{6}\) and 50 mM of H\({}_{2}\)SO\({}_{4}\) at \(-\)1 V for 90 s, while platinum nanospheres would form on the gold nanocorrals if electrodeposition took place in a solution of 25 mM of K\({}_{2}\)PtCl\({}_{4}\) and 50 mM of H\({}_{2}\)SO\({}_{4}\) at \(-\)0.2 V for 200 s. The gold nanocorrals decorated with platinum nanoflowers had the superior sensitivity for the oxidation of H\({}_{2}\)O\({}_{2}\) at a potential of 0.15 V. Glucose oxidase was immobilized by cross-linking with glutaraldehyde to make a first-generation glucose biosensor with a linear range of 0.01 to 2.0 mM and good sensitivity. Figure 8 shows SEM images of gold nanocrystals alone and decorated with platinum nanoflowers and platinum nanospheres. Caulflower-shaped gold nanoparticles were formed on SPCE and used to make an electrochemical immunosensor [86]. A solution of 0.25 mM of Au(III) was used, and the potential that was applied to the SPCE was swept from 0.8 V to 0 V (versus Ag/AgCl) over up to 15 cycles. The well-separated caulflower-shaped nanoparticles grew on edges on the carbon surface and increased in size with the number of cycles, and were shown in the range of 15 to 61 nm. Then, a ferrocene-conjugated lipoic acid derivative was immobilized onto the gold surfaces. Then, antibodies against human IgG were immobilized and covered by bovine serum albumin. The binding of antigen resulted in an increase in peak current from the oxidation of the ferrocene units, which was attributed to a conformational change upon binding. The immobilization of HRP for the detection of H\({}_{2}\)O\({}_{2}\) oxidation was also reported. Electrodeposition under different conditions onto boron-doped diamond electrodes was used to produce three distinct morphologies that resulted in different electrochemical behavior for immobilized hemoglobin [87]. Electrodeposition from two mM of HAuCl\({}_{4}\) at 0.5 V (versus Ag/AgCl) resulted in  
Figure 8: SEM images of (**a**) Au nanocrystals, (**b**) Au nanocrystals with Pt nanoflowers (-1 V for 90 s from tetravalent Pt), and (**c**) with Pt nanospheres (-0.2 V for 200 s from divalent Pt). Reproduced with permission from ref. [84], Copyright 2017, Elsevier. flower-like deposits 100 to 150 nm in size and with XRD patterns that had a dominant Au(111) peak in addition to Au(200) and Au(220) peaks. In contrast, electrodeposition at the same potential from 0.2 mM of HAuCl4 gave spherical nanoparticles that were 70 to 100 nm in size for which the Au(111) XRD peak was smaller than in the flower-like structures. Changing the deposition potential to \(-\)0.1 V with 2 mM of HAuCl4 resulted in convex nanoparticles with a grain size of 150 to 200 nm, and yet less intense Au(111) XRD peaks.