Development of a highly selective and sensitive sensor for the determination of ascorbic acid (AA) using gold nanoparticles (AuNPs)-polypyrrole (PPy) composite modified titanium dioxide (TiO2) is reported in this work. TiO2 nanotubes were fabricated by anodisation of titanium foil in 0.15M NH4F in an aqueous solution of glycerol (90% v/v). Electropolymerisation of pyrrole and deposition of AuNPs on to the TiO2 nanotube array electrode were carried out by cyclic voltammetry (CV). Electrochemical characterization of the sensor was performed by CV and electrochemical impedance spectroscopy (EIS). The morphology of the electrode was studied after every step of modification using field emission scanning electron microscope (FESEM) and atomic force microscope (AFM). The sensor was tested for AA and other biomolecules in phosphate buffered saline solution (PBS) of pH 7 using CV, differential pulse voltammetry (DPV) and amperometry. The sensor exhibited very high sensitivity of 63.912 μA mM−1 cm−2 and excellent selectivity to AA in the presence of other biomolecules such as uric acid (UA), dopamine (DA), glucose and paraacetaminophen (PA). It also showed very good linearity (R = 0.9995) over a wide range (1 µM to 5 mM) of detection. The sensor was tested for AA in lemon and found its concentration to be 339 mg ml-1.
Key Words: Titanium dioxide nanotube arrays, Gold nanoparticles, Polypyrrole, Ascorbic acid sensor
近十年来，纳米颗粒、纳米管、纳米线和纳米棒等纳米结构的制备、表征和应用，由于其特殊的性质。在一维体系结构中，纳米管阵列有一个更高的比纳米线的表面面积，由于封闭的中空结构的额外的表面积[ 1 ]。TiO2纳米管，其中最近的一次在本组资料的补充，是一种氧化物半导体材料，广泛用于探索各种应用包括气体传感器[2-9]，染料敏化太阳能电池[ ]和[ 19 ]传感器10-15。TiO2纳米管可以用电化学腐蚀从氟化物电解质[ 20 ]容易制作。
虽然不同的纳米材料用于AA的定量测定电化学传感器和生物传感器的制备，纳米金的使用引起了由于其良好的导电性的独特的性质，如一些研究者的关注，有用的电催化性能和生物相容性，[ 26 ]。据认为，金纳米粒子的催化活性来源于量子尺寸是由于体积比和表面上的[ 27特殊结合位点存在大面]。几种策略采用固定金纳米粒子在电极基板包括电[ 28 ]和固定通过共价键或静电相互作用的自组装单分子膜（SAMs）合适的官能团[ 22 ]终止，29-31。个人和同时测定AA纳摩尔UA用放大的，柠檬酸盐稳定的电流法[ 32研究的金纳米颗粒自组装单分子层修饰金电极二巯基-1,3,4-噻二唑]。AA在DA的存在是在多壁碳纳米管的二氧化硅网络利用纳米金纳米修饰电极测定采用DPV [ 33 ]。
催化效率，因此对电流传感器的电流响应是电极表面积高度相关。不同的方法被用来增加电极表面积，如纳米碳纤维【34,35】电极改性碳纳米管[ 36-38 ]利用纳米多孔电极[ 39,40 ]。近年来TiO2纳米管阵列已经由于准备的几个研究人员的兴趣，缓解高定位、大面积、高均匀性、以及良好的生物相容性[ 2 ]。一个共同的银铂纳米粒子装饰的二氧化钛纳米管阵列显示，显示一九倍的催化活性增加相比，铂电极[ 42 ]。最近，基于二氧化钛纳米管的葡萄糖生物传感器具有良好的选择性和灵敏度，采用辣根过氧化物酶[ 45 ]和葡萄糖氧化酶[ 46 ]。金纳米粒子修饰的TiO2纳米管阵列生物传感器用于过氧化氢[ 16 ]和Pt纳米粒子修饰的TiO2纳米管阵列电极的测定采用葡萄糖氧化酶的固定化，用于葡萄糖[ 47 ]的安培检测。最近，王等人。[ 48 ]报道葡萄糖非酶安培检测TiO2纳米管阵列复合镍电极的使用。
众所周知，导电聚合物（CPS）有电活动对各种基材，包括离子和有机化合物[ 49 ]。在所有的CPS，聚吡咯（PPy）发现在生物传感器中的应用的广泛使用，因为它具有相对稳定的导电性能和可生物相容性条件下合成的电] [ 50-56。导电聚合物通常被认为是有用的矩阵的固定化的分散的贵金属催化剂。由于导电聚合物的相对高的导电性，它是可能的穿梭的电子通过电极和分散的金属颗粒之间的聚合物链，其中发生的电催化反应。因此，一个高效的催化作用可以实现对这些复合材料的表面。最近的趋势是向金属纳米粒子的聚吡咯复合材料的发展为生物传感器中的应用[装置]。在AA的存在下，肾上腺素和尿酸测定的生物传感器的制备金纳米团簇的电化学沉积超薄咯膜[ 57 ]。微电位血红蛋白免疫传感器基于PPy AuNP复合修饰电极的电化学合成报道[ 58 ]。对有机磷农药生物传感器是利用金纳米粒子的聚吡咯纳米线复合膜修饰玻碳电极上的固定化乙酰胆碱酯酶的发展[ 59 ]。
Last decade has witnessed the preparation, characterization and application of various nanostructures like nanoparticles, nanotubes, nanowires and nanorods due to their special properties. Among the one-dimensional architectures, nanotube arrays have a higher surface area than nanowires due to the additional surface area enclosed inside the hollow structure . TiO2 nanotube, one of the recent additions in this group of materials, is an oxide semiconducting material which is explored extensively for various applications including gas sensor [2-9], dye sensitized solar cell [10-15] and biosensor [16-19]. TiO2 nanotubes can be easily fabricated by electrochemical anodisation from fluoride electrolytes .
Although different nanomaterials were employed for the fabrication of electrochemical sensors and biosensors for the quantitative determination of AA, the use of gold nanoparticles attracted the attention of several researchers due to their unique properties such as good conductivity, useful electrocatalytic behaviour and biocompatibility [21-26]. It is believed that the catalytic activity of AuNPs originates from the quantum scale dimension and is attributed to the large surface-to-volume ratio and the existence of special binding sites on their surface . Several strategies were employed to immobilize AuNPs on the electrode substrate which includes electrodeposition  and immobilization through covalent or electrostatic interactions with the self-assembled monolayers (SAMs) terminated with suitable functional groups [22, 29-31]. Individual and simultaneous determination of nanomolar UA and AA using enlarged, citrate-stabilized AuNPs self-assembled 2,5-dimercapto-1,3,4-thiadiazole monolayer modified Au electrode was studied by amperometric method . AA in the presence of DA was determined at multiwalled carbon nanotube-silica network-gold nanoparticles based nanohybrid modified electrode using DPV .
The catalytic efficiency and hence the current response of the amperometric sensor are always highly related to the electrode surface area. Various methods were used to increase the electrode surface area, such as the modification of electrodes with carbon nanofibers [34,35], carbon nanotubes [36-38] and using nanoporous electrodes [39,40]. In recent years TiO2 nanotube arrays have drawn interest of several researchers due to the ease of preparation, high orientation, large surface area, high uniformity, and excellent biocompatibility [41-45]. A Co-Ag-Pt nanoparticle-decorated TiO2 nanotube array was found to show a nine fold increase in catalytic activity when compared to platinum electrode . Recently, TiO2 nanotube based glucose biosensors of good selectivity and sensitivity were made using horse radish peroxidase  and glucose oxidase . AuNP modified TiO2 nanotube array biosensors were adopted for the determination of hydrogen peroxide  and Pt-Au nanoparticles modified TiO2 nanotube array electrodes were used for the immobilisation of glucose oxidase and used for the amperometric detection of glucose . Very recently, Wang et al. reported the use of TiO2 nanotube array-Ni composite electrodes for non-enzymatic amperometric detection of glucose .
It is well known that conducting polymers (CPs) have electrocatalytic activities towards various substrates, including ions and organic compounds . Amongst all CPs, polypyrrole (PPy) finds extensive use for biosensor application, since it has relatively stable electrical conductivity and can be electro synthesized under biocompatible conditions [50-56]. Conducting polymers are often considered to be useful matrices for the immobilization of the dispersed noble metal catalysts. Because of a relatively high electric conductivity of conducting polymers, it is possible to shuttle the electrons through polymer chains between the electrode and dispersed metal particles, where the electrocatalytic reaction occurs. Thus, an efficient electrocatalysis can be achieved on the surface of these composite materials. Recent trend is towards the development of metal nanoparticles-PPy composites for biosensor applications [57-59]. Biosensor for the determination of epinephrine and uric acid in the presence of AA was fabricated by electrochemical deposition of gold nanoclusters on ultrathin overoxidized polypyrrole film . A micro-potentiometric hemoglobin immunosensor based on electrochemically synthesized PPy-AuNP composite modified electrode was reported . Biosensor for organophosphate pesticides was developed using AuNP-PPy nanowires composite film and immobilized acetylcholinesterase on glassy carbon electrode .
In the present work, a highly sensitive and selective sensor for the detection of ascorbic acid was developed by the electropolymerisation of pyrrole on TiO2 nanotube arrays followed by the electrodeposition of AuNPs. The sensor was characterized for its morphological and electrochemical characteristics. It was then tested with AA and other biomolecules and very promising results were obtained.