## 3 Biosensor Application of Nanostructures Electrodeposited onto a Template
### Electrodeposition through Aluminum Oxide and other Templates
The porous alumina membrane was first modified with 3-aminopropyltrimethoxysilane, and then Pt was electrodeposited from 8.23 mM of K\({}_{2}\)PtCl\({}_{6}\) in 30 g L\({}^{-1}\) of boric acid at \(-\)0.1 V (versus SCE) until a desired amount of charge had passed. Immersion in 3 M of NaOH removed the alumina template and left behind Pt nanotubules that were three \(\upmu\)m in length, had a 70 \(\pm\) 10-nm inner diameter and had a 150 \(\pm\) 10-nm outer diameter. The roughness factor of the surface was found to be 758. The amperometric detection of glucose at 0.4 V gave a detection limit of 1.0 \(\upmu\)M and a linear range from two to 14 mM. The electrodeposition of Au through an alumina nanomask covering an ITO-coated glass surface was used to create an ensemble of hemispherical gold nanoparticles [56]. The alumina mask was prepared by the chemical solution deposition of block copolymer and aluminum chloride followed by drying and heating [57]. The holes in the nanomask were 8.6-nm deep and 34-nm in diameter, and their surface density was 300 \(\upmu\)m\({}^{-2}\). Electrodeposition was carried out at \(-\)0.5 V (versus Ag/AgCl) for 10 s. The nanomask allowed improved control of nanoparticle size and inter-particle spacing. It was also noted that the use of the nanomask improved the thermal stability of the ensemble of particles, preventing coalescence at temperatures as high as 500 \({}^{\circ}\)C. The nanoparticles showed a plasmon absorbance peak at 559 nm in air that red shifted when exposed to liquid media of higher refractive index, which indicated that the nanoparticles were useful for application as localized surface plasmon resonance biosensors. Gold nanotube arrays were also prepared by electrodeposition to make a biosensor for detecting DNA from mycobacterium tuberculosis [58]. A polycarbonate membrane with 200-nm pores was sputter coated on one side with a gold layer. Subsequently, Au was electrodeposited inside the membrane at \(-\)1.0 V (versus Ag/AgCl) from a gold plating solution. The average length of the supported vertical nanotubes was 1.5 \(\upmu\)m, and they were 200 nm in diameter. The authors noted that nanotube arrays produced by electrodeposition are less prone to deformation or bunching of nanotubes as compared to those produced by physical or chemical vapor deposition. The gold-coated membrane was then placed on a gold electrode using conducting paste, and an O-ring was used to position the electrochemical cell. The polycarbonate template was dissolved using dichloromethane. A thiolated probe DNA sequence was immobilized on the gold surface. Detection of the complimentary DNA strand from the bacteria was achieved using the methylene blue redox probe and DPV. A detection limit of 0.05 ng uL\({}^{-1}\) was obtained. The same authors had previously demonstrated the application of  
Figure 5: Top-view of the SEM images of Au NWAs (gold nanowire arrays) without modification (**A**) and modified with GLA–BSA–GO–HRP (GLA = glutaraldehydehyde, BSA = bovine serum albumin, GOx = glucose oxidase, HRP = horseradish peroxidase) (**B**). Reproduced with permission from ref. [54], Copyright 2014, Electrochemical Society. similarly prepared gold nanowire arrays in the electrochemical detection of DNA hybridization [59]. Figure 6 shows a schematic of the process that was used to create these gold nanotube array sensors. In related work [60], a sensor for DNA from human papillomavirus (HPV) was developed by electrodeposition onto polycarbonate membranes also presenting pores in the 150 to 200-nm size range, randomly arranged. The inner pores were surface modified with 3-aminopropyltrimethoxysilane. A 20-nm layer of gold was sputter coated onto one side of the template, and short nickel nanotubes were electrodeposited at \(-900\) mV (versus Ag/AgCl) from a solution containing 0.8 M of NiSO\({}_{4}\) in 0.3 M of H\({}_{3}\)BO\({}_{3}\). The short nickel nanotubes served to protect the backside of the membrane.