## 4 Biosensor Application of Ramified or Dendritic Electrodeposited Structures
### Electrodeposition of Single Ramified Nanostructures Through Apertures  
Electrodeposition through microscale apertures enables the possibility of creating arrays of individually addressable nanostructures. The study of individual nanostructures allows insight into the relation between their size and morphology and their analytical response. The electrodeposition of nanostructured gold microdeposits onto a gold surface through 10-\(\upmu\)m apertures produced using photolithography was studied as a function of gold ion concentration, solution viscosity, and deposition potential in order to determine the conditions that would lead to an optimal response for the electrochemical detection of DNA hybridization (duplex formation) [99]. It was reasoned that the larger areas of structures that protruded out into solution would increase the frequency of biomolecular encounters and also make the transport of redox probes more efficient by radial diffusion. The quantity of gold that was deposited was kept constant across all of the structures by adjusting the deposition time. The concentration of HAuCl\({}_{4}\) was varied from one to 500 mM in 0.5 M of HCl. Using 50 mM of HAuCl\({}_{4}\), the potential was varied from 500 mV to \(-\)700 mV. Viscosity was varied by preparing the solution to be up to 75% glycerol, with electrodeposition carried out at zero V and in 50 mM of HAuCl\({}_{4}\). Depending on the conditions, the electrodeposited structures were classified as roughened hemispheres, twinned leaves, pentagonal needles, or amorphous growth. As the concentration of HAuCl\({}_{4}\) increased, the morphology of the electrodeposited structures evolved from rounded but with distinct lobes to leaf-like and covered with geometric patterns, and then to needles that became smoother at high concentrations. An increase in the viscosity of the solution resulted in more compact structures. Variation of the electrodeposition potential resulted in an evolution in morphology from hemispherical to needle-like to spiky to flaky. Then, the structures were surface modified with a thiolated DNA strand, and hybridization was monitored using methylene blue as a redox probe. The rate of DNA hybridization was fastest on the finer, spiky nanostructures. Nanostructured microelectrode arrays of Pd were fabricated by electrodeposition and used as DNA sensors [100; 101]. The single nanostructures were grown onto gold surfaces exposed through 500-nm apertures produced using photolithography on a SiO\({}_{2}\) layer on a gold-coated silicon chip. The parameters of H\({}_{2}\)PdCl\({}_{4}\) concentration, electrolyte solution, deposition potential, and time were varied to produce structures of about 10 \(\upmu\)m in size, which were classified either as smooth, moderately rough (nanostructured on a 100 to 300-nm length scale), or finely rough (nanostructured on a 20 to 50-nm length scale). The structures were surface modified by a thiolated peptide nucleic acid that was chosen for its neutral charge, which enhanced the response of the reporter system redox probes. The binding of a complimentary 20-mer DNA strand was monitored using an electrocatalytic reporter system consisting of 10 \(\upmu\)M of [Ru(NH\({}_{3}\))\({}_{6}\)]\({}^{3+}\) and four \(\upmu\)M of [Fe(CN)\({}_{6}\)]\({}^{3-}\). The electrode array had eight individually addressable nanostructures exhibiting ideal microelectrode behaviors. The finely rough nanostructures had the lowest detection limit of 10 aM for the complimentary DNA strand with a log-linear range from one to 100 aM. The hybridization kinetics were fastest on the finely rough nanostructures. A change in the Pd particle structure from smooth to nanostructured reduced the detection limit for 20-mer DNA from 100 fM to one fM [102]. The Pd nanostructured microelectrode arrays were also modified with peptide nucleic acids and applied to the detection of gene fusion sequences that are unique to prostate cancer and were taken from prostate cancer tissue [103].