## 4 Biosensor Application of Ramified or Dendritic Electrodeposited Structures
### Dendritic-Like Nanostructures
The copper dendrites were formed by electrodeposition from 0.15 M of CuCl\({}_{2}\) and 0.75 M of H\({}_{2}\)SO\({}_{4}\) at \(-\)0.5 V (versus Ag/AgCl) for five minutes. The primary stems of the dendrite were 10 \(\upmu\)m in length with branches 0.1 to 2.0 \(\upmu\)m in length and 0.3 \(\upmu\)m in diameter. Nickel hydroxide was deposited using chronopotentiometry at a current of \(-\)0.1 mA cm\({}^{-2}\) in a solution of 0.1 M of Ni(NO\({}_{3}\))\({}_{2}\) and 0.1 M of KNO\({}_{3}\) for 10 min. Glucose detection was carried out at a potential of 0.6 V in 0.1 M of NaOH. A detection limit of 0.24 \(\upmu\)M and a linear range from one \(\upmu\)M to 4.5 mM were obtained, and the analysis both worked well for glucose spiked into human urine samples, and was resistant to interferents such as urea, aspartic acid, ascorbic acid, dopamine, and uric acid. Dendritic gold nanostructures formed by electrodeposition were used as the basis of a sensor for superoxide [76]. At a constant current of \(-\)1.0 mA and for fixed times of either 300 s, 500 s, or 700 s, gold was electrodeposited onto a GCE from a solution of five mM of HAuCl\({}_{4}\) in one M of H\({}_{2}\)SO\({}_{4}\). For a 700-second deposition, the surface was covered with dendritic gold structures. Using the gold oxide stripping method, the surface area of 2.21 \(\pm\) 0.01 cm\({}^{2}\) was found, which was 31.6 times greater than the geometric surface area for the GCE of 0.07 cm\({}^{2}\). Then, the gold structures were surface modified with cysteine for the electrostatic immobilization of superoxide dismutase. Using the Laviron analysis [45] based on peak current versus scan rate data from CV, a heterogeneous electron transfer coefficient of 10 \(\pm\) 0.1 sec\({}^{-1}\) was obtained, as well as a transfer coefficient of \(\alpha\) = 0.5. Direct electron transfer was seen between the enzyme and gold surface. Both the reduction and oxidation peak currents for the immobilized enzyme increased upon the addition of superoxide. For a potential of 300 mV (versus Ag/AgCl), a linear range from 0.05 to 440 \(\upmu\)M was found, and at \(-\)200 mV, a linear range from 0.01 to 540 \(\upmu\)M was found, along with a detection limit of 2.1 nM. Figure 7 shows SEM (JSM 7600, JEOL Ltd., Tokyo, Japan) images of these dendritic structures at different deposition times, along with an EDS (energy dispersive spectroscopy) spectrum confirming their gold composition. Gold nanostructures were formed on the surface of a gold disc electrode by electrodeposition at \(-\)1.0 V (versus SCE) from 2.8 mM of HAuCl\({}_{4}\) in 0.1 M of HClO\({}_{4}\) for 600 s to form a support for an amperometric H\({}_{2}\)O\({}_{2}\) sensor using immobilized hemoglobin [77]. At the -1.0 V potential, hydrogen bubbles that formed on the surface served as a dynamic template, resulting in a surface covered by networks of fine dendritic structures with macroporous openings. The roughness factor as determined by the oxide stripping method in 0.5 M of H\({}_{2}\)SO\({}_{4}\) was 47.7. The gold surfaces were then modified by a SAM of 3-mercaptoethanol-propylphosphonic acid and hemoglobin immobilized by adsorption. The immobilized hemoglobin showed direct electron transfer and could be used as an amperometric sensor for H\({}_{2}\)O\({}_{2}\) operating at \(-\)0.3 V with a detection limit of 25 nM and a linear range of 78 nM to 91 \(\upmu\)M.  
Dendritic gold micro/nanostructures were formed by electrodeposition onto GCE and used as a non-enzymatic sensor for glucose [78]. The deposition time, deposition potential, and HAuCl\({}_{4}\) concentration were varied, and the effect of these on the deposit morphology examined. At 3600 s and 10 mM of HAuCl\({}_{4}\), deposition at 0.3 V (versus Ag/AgCl) produced uniform dispersed flower-like and tetrahedral shapes made of nanoparticles that were three \(\upmu\)m in size. The morphology changed to denser flower-like structures at 0.1 V, to smaller dendritic clusters at -0.1 V, and to denser and longer dendrites at \(-0.3\) V that were microns in length and had feathery branches.