## 4 Biosensor Application of Ramified or Dendritic Electrodeposited Structures
### Nanostructures Described as 'Flower-Like', Irregular, of Unusual Morphology
The toluidine blue redox probe that binds preferentially to dsDNA was used together with DPV, and a detection limit of 3.1 \(\times\) 10\({}^{-20}\) M and a log-linear range of 1.0 \(\times\) 10\({}^{-19}\) M - 1.0 \(\times\) 10\({}^{-12}\) M for the complementary strand were achieved. The anisotropic Au nanoparticles were also used as support for a DNA sensor for _Leishman infantum_[93]. In contrast, the use of the polyamine spermidine as a structure-directing agent resulted in the formation of gold nanoleaves on a gold electrode surface for modification by a thiolated DNA probe for detection of _Leishmania major_ using differential pulse voltammetry and the methylene blue redox probe [94]. A detection limit of 1.8 \(\times\) 10\({}^{-20}\) M was found for a synthetic complimentary strand, and a detection limit of 0.07 ng mL\({}^{-1}\) was found for the genomic DNA from the protozoa. Using cadaverine (1,5-pentanediammine) as a structure-directing agent, hairbrush-like gold nanostructures were formed by electrodeposition and used as a platform for an aptamer-based sensor for a prostate-specific antigen [95]. The deposition onto a gold electrode was carried out from 500 mM of H\({}_{2}\)SO\({}_{4}\), 150 mM of cadaverine, and 20 mM of HAuCl\({}_{4}\) at zero V (versus Ag/AgCl) for 600 s. The surface that was produced consisted of hairbrush-like rods that were made of 30 to 150-nm spindles. The thiolated aptamer against prostate specific antigen (PSA) was immobilized along with 6-mercapto-1-hexanol. The binding of PSA to the aptamer resulted in a change in its folding, bringing the redox probe methylene blue closer to the electrode surface and resulting in an increase in peak current in DPV. A linear range for PSA detection from 0.125 to 125 ng mL\({}^{-1}\), and a detection limit of 0.04 ng mL\({}^{-1}\) was achieved. Fractal tree-like nanostructures formed by electrodeposition onto ITO-coated glass were first modified by a polyelectrolyte layer that was formed by alternating the deposition of anionic polystrene sulfonate and poly(diallydimethylammonium chloride), and then used as the support for a sensor for human apolipoprotein E4, which is of diagnostic value for [PERSON]'s disease [96]. The electrodeposition was done for 1800 s from 10 mg mL\({}^{-1}\) of HAuCl\({}_{4}\) in 0.5 M of H\({}_{2}\)SO\({}_{4}\) at \(-\)1800 mV (versus Ag/AgCl). The fractal shapes were five to 10 \(\upmu\)m in length, had an overall height of 15 to 26 \(\upmu\)m, and were composed of many \(\sim\)10-nm nanoparticles. Antibodies against the APOE-4 isoform were immobilized on the gold surfaces by adsorption. The APOE-4 protein would bind between the surface-immobilized antibodies and antibodies conjugated to HRP. Detection, using the addition of H\({}_{2}\)O\({}_{2}\) and hydroquinone as a mediator, gave a detection limit of 0.3 ng mL\({}^{-1}\). Fractal-shaped gold nanostructures with high ramification were found to be more efficient at capturing cancer cells than less ramified structures [97]. The structures were deposited onto ITO-coated glass that was precoated with a layer of poly(acrylic acid) and poly(ethylenimine) at either \(-\)0.1 V, \(-\)0.15 V, or \(-\)0.3 V (versus Ag/AgCl) for 180 min or 30 min from one mg mL\({}^{-1}\) of HAuCl\({}_{4}\). The most ramified structures were produced at \(-\)0.3 V and 30 min. The structures were about five \(\upmu\)m in size, and were characterized by a fractal dimension by image analysis. The surfaces were modified with a thiol-PEG-biotin derivative to which streptavidin would bind, and then, a biotinylated antibody against EpCam as well. Epcam is overexpressed on the surface of cancer cells, and the most ramified gold structures had the highest efficiency regarding the capture of MCF7 breast cancer cells. The electrodeposition of gold nanospikes onto Au-coated Si wafers resulted in substrates with great anti-bacterial efficiency [98].