## 3 Biosensor Application of Nanostructures Electrodeposited onto a Template
### Other Templates for Electrodeposition  
The electrodeposition of palladium inside a nanoporous gold (np-Au) electrode was used to create an electrochemical biosensor for the neurotransmitter dopamine [61]. Nanoporous gold provides an  
Figure 6: Schematic drawing of (**a**) synthesis of AuNTsA (Gold nanotubes array) and (**b**) fabrication of the DNA hybridization biosensor based on AuNTsA electrodes. (WE–working electrode, RE–reference electrode, CE–counter electrode). Reproduced with permission from ref. [58], Copyright 2016, Elsevier. interconnected and continuous structure of high conductivity. The np-Au covered a 0.2-mm diameter gold wire electrode with inter-ligament gaps of about 200 nm, and was then placed in a solution of five mM of sodium dodecyl sulfate and 2.5 mM of PdCl\({}_{2}\), and the potential was cycled between 0.2 V and 1.2 V (versus SCE) to form a near 20-nm thick Pd coating inside the np-Au. The np-Au/Pd electrode was found to be sensitive to dopamine oxidation as detected using differential pulse voltammetry with a resulting sensitivity of 1.19 \(\upmu\)A \(\upmu\)M\({}^{-1}\), a detection limit of one \(\upmu\)M, and a detection range of one to 220 \(\upmu\)M. The electrode was found to be stable for up to 40 repetitions of use, and the dopamine signal was not affected significantly by potential interferents such as ascorbic acid, uric acid, and others. A complex nanostructured zinc oxide electrode coating based on a phospholipid template was used to incorporate the enzymes alcohol dehydrogenase or glucose oxidase to make amperometric sensors for ethanol or glucose operating with direct electron transfer [62]. The phospholipid 1,2-diplamitoyl-sn-glycero-3-phosphate (DPGP) was used to form lamellar bilayer structures for templating the formation of lamellar mesoporous ZnO upon electrodeposition. DPGP at 0.02 wt% was dissolved in 25 wt% of N-methyl-2-pyrrolidone (NMPD). Electrodeposition was carried out from the 0.02-M Zn(NO\({}_{3}\))\({}_{2}\) solution with DPGP and NMPD, and also in the presence of enzyme at a four to one weight ratio of DPGP/enzyme at \(-\)0.5 V (versus SCE) for 60 min onto gold plates. Electrodeposition without enzyme resulted in a morphology of cross-linked flakes under SEM (S-4800, Hitachi, Tokyo, Japan). Higher magnification transmission electron microscopy (TEM) (Tecnai G2 F20 S-Twin, FEI Co., Hillsboro, OR, USA) showed that the flakes were lamellar stacks with a 0.436-nm repeat distance. X-ray diffraction confirmed a 0.437-nm repeating lamellar phase of ZnO formed between DPGP bilayers. Upon the addition of enzyme, the film changed to a smoother flat morphology, for a 200-nm thick film, and TEM revealed arrays of 2.5 to 3.5-nm nanopores. The use of a lipid template was proposed for providing a biocompatible environment, allowing successful electrodeposition with protein. The enzyme-incorporating films showed a high level of direct electron transfer. For ethanol, a detection limit of 2.1 \(\upmu\)M was found, and for glucose, the detection limit was 3.6 \(\upmu\)M.  
Diatoms, the silica shell of microalgae, were electrodeposited together with gold onto screen-printed carbon electrodes at a range of potential, time, and HAuCl\({}_{4}\) concentration [63]. Diatoms and other biominerals provide inorganic templates that already present porous microstructures requiring no complex fabrication. Dendritic gold microstructures were observed on deposition at \(-\)1.0 V, but when electrodeposition was carried out at \(-\)0.5 V, mostly 15 to 20-\(\upmu\)m gold microspheres were observed. If the HAuCl\({}_{4}\) concentration was lowered from 0.60 M to 0.15 M and electrodeposition carried out at \(-\)0.5 V, leafy morphologies were seen. The diatoms were immobilized intact and mixed in with the gold microstructures. The modified electrode was found to be electroatalytically active for the oxidation of H\({}_{2}\)O\({}_{2}\).