## 3 Biosensor Application of Nanostructures Electrodeposited onto a Template
### Electrodeposition through Aluminum Oxide and other Templates  
Aluminum anodized in polyprotic acid will generate aluminum oxide with pores parallel to the surface whose diameter depends on the applied voltage and can range from 10 to 450 nm [50]. Each pore is part of a hexagonal domain of aluminum oxide [51]. The mechanism of pore formation has been described, and involves breaking Al-O bonds and the release of Al\({}^{3+}\)[52]. Gold nanotube arrays or gold nanowire arrays were formed by galvanostatic (current controlled) electrodeposition into an anodic aluminum oxide template with a 200-nm pore size and applied to non-enzymatic glucose sensing [53]. The aluminum oxide templates were sputter coated with a 300-nm thick gold layer. Electrodeposition was carried out at a constant current of 0.1 mA cm\({}^{-2}\) in an electroplating bath containing HAuCl\({}_{4}\), Na\({}_{2}\)SO\({}_{3}\), ethylenediaminetetraacetic acid (EDTA), K\({}_{2}\)HPO\({}_{4}\), and CoSO\({}_{4}\). Electrodeposition at pH 9 resulted in nanowires that were two \(\upmu\)m in length, but adjustment to pH 11 using KOH resulted in the formation of nanotubes with 50-nm openings. After etching away the aluminum oxide template, the arrays were affixed to a GCE using Nafion. The gold nanotube arrays when used at 0.25 V gave higher sensitivity than the nanowire arrays for glucose oxidation, and achieved a detection limit of 2.1 \(\upmu\)M and a linear range from five \(\upmu\)M to 16.4 mM. In contrast, the nanowire arrays gave a detection limit of 12.8 \(\upmu\)M and a linear range from 20 \(\upmu\)M to 15.2 mM. The authors noted that the nanotube arrays had a higher active surface area than the nanowire arrays by a factor of almost two. The approach of electrodeposition onto aluminum oxide templates can in principle allow for variation in nanowire or nanotube size, since the pore diameter and spacing in this type of template can be varied during its preparation. Electrodeposition was used to create gold nanowire arrays as a support for an enzymatic glucose sensor in a flow-injection system [54]. Anodic aluminum oxide templates with a thickness of 16 \(\upmu\)m and 80-nm pores were used as templates. The gold wire nanoarrays were fixed onto a three-mm diameter gold electrode using conductive carbon tape. The enzymes glucose oxidase and HRP were cross-linked on the gold surfaces in the presence of bovine serum albumin (BSA). The composition of enzymes and of BSA was optimized along with the pump time and sampling speed. The optimal working potential was found to be 0.05 V (versus Ag/AgCl), and a detection limit of one \(\upmu\)M was achieved with a linear range from five to 1000 \(\upmu\)M. The response of the electrode declined only 4% over 10 days, and 93% of the response remained after the determination of 40 samples. The regular spacing and uniformity of size of the nanowires provided a structure into which the cross-linked enzymes and proteins could become coated between and around the nanowires, as can be seen in Figure 5, which shows the nanowire array before and after protein modification. The author pointed out the distinct advantage of this arrangement for facilitating electron transfer between the nanowires and the active enzyme sites. Figure 4: SEM images of Au nanorings created from one-\(\upmu\)m colloids. (**a**) Low magnification image of a domain of ordered nanorings. (**b**) Increased magnification of nanorings with outer radius = 330 \(\pm\) 5 nm and width = 135 \(\pm\) 4 nm, and (**c**) the highest magnification of a single ring. Reproduced with permission from ref. [49], Copyright 2013, American Chemical Society. A highly ordered array of Pt nanotubules was produced by electrodeposition into a porous anodic alumina template, and found to be effective as a non-enzymatic glucose sensor [55].