## 4 Biosensor Application of Ramified or Dendritic Electrodeposited Structures
### Electrodeposition of Single Ramified Nanostructures Through Apertures
A study comparing the hybridization efficiency and ssDNA surface coverage on the extent of the nanostructuring of the Pd surface found that the most nanostructured surface had the highest hybridization efficiency, and that the hybridization efficiency on a spherical surface was very low [104]. Figure 9 shows SEM images of the individual palladium nanostructures, which were classified as smooth, moderately nanostructured, and highly nanostructured. Nanostructured microelectrode arrays of gold were fabricated in a similar manner and used to detect mRNA in the form of a gene fusion from chronic myeloid leukemia [105]. The apertures through which the electrodeposition was done were five \(\upmu\)m in size, and 100-\(\upmu\)m spiky and dendritic structures were formed. Electrodeposition was conducted at 0 V for 175 s from 20 mM of HAuCl\({}_{4}\) in 0.5 M of HCl. The surfaces were modified by a thiolated peptide nucleic acid probe and 6-mercapto-1-hexanol. Using the catalytic reporter system of 10 \(\upmu\)M of [Ru(NH\({}_{3}\))\({}_{6}\)]\({}^{3+}\) and four \(\upmu\)M of Fe(CN)\({}_{6}\)\({}^{3-}\), it was possible to detect the mRNA from as few as 10 lysed cancer cells using DPV. Nanostructured microelectrode arrays were used to make biosensors for the protein antigen CA-125, which is a biomarker for ovarian cancer [106]. Gold nanostructures of different sizes (eight \(\upmu\)m, 30 \(\upmu\)m, and 100 \(\upmu\)m) were prepared on microchips through five-\(\upmu\)m apertures by electrodeposition. The 100-\(\upmu\)m structures were formed by electrodeposition at zero V for 200 s from 20 mM of HAuCl4 in 0.5 M of HCl, the 30-\(\upmu\)m structures were formed by electrodeposition from this solution at 150 mV for 200 s, and the eight-\(\upmu\)m structures were formed by chronoamperometry at 30 nA for 50 s. The eight-\(\upmu\)m structures had a fuzzy shape, the 30-\(\upmu\)m structures were spikey in appearance, and the 100-\(\upmu\)m ones were of a more dendritic nature. The gold surfaces were modified by cystamine, followed by activation with glutaraldehyde for immobilization of the anti-CA 125 antibody. The blocking of the redox current from a solution of 2.5 mM of K3[Fe(CN)6] and 2.5 mM of K4[Fe(CN)6] upon binding of CA-125 to the antibody was used for detection. The eight-\(\upmu\)m size structures enabled the detection of as low as 0.1 U mL\({}^{-1}\) of CA-125 where 0.1 U = 500 fM, while the 30-\(\upmu\)m structures allowed the detection of one U mL, and the 100-\(\upmu\)m structures allowed the detection of 10 U mL\({}^{-1}\). Si chips that were patterned with Au working, reference, and auxiliary electrodes were modified with gold nanostructures, also through five-\(\upmu\)m apertures, and used to detect DNA from bacteria [107]. The nanostructures were electrodeposited at -700 mV for 10 s from 20 mM of HAuCl4 in 0.5 M of HCl. Then, the nanostructures were modified by peptide nucleic acid. The device contained a special lysis chamber, allowing for the lysis of bacteria, followed by detection down to a level of one CFU per \(\upmu\)L. The total time for analysis, from lysis to detection of the bacterial DNA, was less than 30 min [108]. The use of gold instead of palladium was needed to produce structures extending 100 \(\upmu\)m into the solution.