ZnO has been extensively used for chemical and biological sensing due to its thermal stability under usual operating conditions, as well as excellent biomimetic properties combined with high electron communication features which makes it an attractive actuator for the so-called third generation biosensors [27�C33]. On the other hand, ZNRs have unique advantages in immobilizing enzymes that retain their bioactivity due to the desirable microenvironment and the direct electron transfer between the enzyme’s active sites and the electrode [34�C37], in addition to the relatively larger surface-to-volume ratios compared to their thin film and bulk material counterparts. It is known that the sensing mechanism of ZnO is of the surface controlled type, in which the grain sizes, surface states, and oxygen adsorption quantities all play important roles in its sensitivity [6,31,38�C42].

2.?Experimental2.1. The Fabrication of ZnO NRs on the Gold Coated Glass SubstrateWe have fabricated the sensor electrodes utilizing glass substrates coated with gold then followed by the growth of ZNRs. All processing steps for the preparation of the present sensor electrodes are as follows: the first step was cleaning of the substrate, where the glass substrates were sonicated in an ultrasonic bath for about 10 min in isopropanol and acetone, respectively. Then these substrates were cleaned with deionized water and lastly they were dried with an air gun. Then these glass substrates were affixed into the vacuum chamber of an evaporator instrument (Satis CR 725, Zurich, Switzerland).

After this an adhesive layer of 20 nm of titanium was evaporated on the substrates and then a 100 nm thickness layer of gold thin film was evaporated.Well-aligned ZNRs have been grown by the low temperature ACG method. This could be described as a two-step process: spin-coating a ZnO seed layer on the substrate followed by the growth of the nanorods. In the first step, the ZnO seed precursor was prepared from zinc acetate dihydrate in methanol under basic conditions as described in [43]. The solution was then spin-coated on the substrate in the first step at 1,500 rpm for 10 s and the second step at 3,000 rpm for 20 s. The second step, growth of the ZNRs, involved a hydrothermal process [44] where the substrates coated with ZnO seeds were introduced horizontally and upside-down into a 0.

025 M equimolar solution Brefeldin_A of hexamethylenetetramine and zinc nitrate hexahydrate and then kept in a preheated electric oven at 90 ��C for 4�C6 h. Before the substrates were placed into the solution, a small part of the gold coated glass was covered in order to be used as a contact pad for the electrochemical measurements. Finally, the samples were rinsed several times in deionized water to remove any residual salt on the surface of nanostructures and then they were dried with an air gun.