TiO2 nanostructured array for optical ultrasensitive biosensing

Climate changes and the recent pandemic crisis are both evidences of strong changes occurring in the environment that urgently have to be faced, striking a balance between the impact of human productive and everyday activities and the preservation of the environment in order to guarantee human health well-being. This goal could be achieved through the implementation of direct and decisive actions, accompanied by constant and careful monitoring of the air quality. This emergent scenario needs a technological advancement in the field of sensors for atmospheric pollutants, with the aim of monitoring with the highest sensitivity and accuracy the type and the amount of chemical and biological pollutants. In particular, as SARS-CoV-2 pandemic [1] clearly showed, monitoring the dispersion of pathogens is a challenging but crucial and necessary approach to prevent the spread of a pandemic and to guarantee health safety, in particular in close public indoor spaces and in working sites [2]. Driven by the motivation to monitor airborne pathogens, we propose here the development of a portable, real-time, and label-free device for optical biosensing, capable to adsorb specific biological pollutants efficiently and selectively; a device suitable to identify and quantify different chemical and biological pollutants through multi-messenger high-sensitivity vibrational spectroscopy. This multidisciplinary approach is innovative with respect to existing, conventional monitoring techniques (ELISA, PCR, RT-PCR, etc). It offers many advantages and, in particular, the possibility to overcome the limitations of currently used biological and chemical essays in terms of costs, labour time, selectivity and rapidity. In addition, being based on the recognition of characteristic molecular vibrational modes, the interrogability with IR radiation ensures that the detection procedure is unique for all pathogens. Here, we briefly illustrate the steps towards the development of an infrared TiO2 nanostructured array. We considered TiO2 nanotubes (NTs) with a diameter between 50 and 100 nm and a height of ~2 µm fabricated on Ti metallic foil by means of electrochemical anodization. This structure allows to increase the surface area available for the pathogen concentration. In order to increase the NTs sensor selectivity, we chemically modified the surface of TiO2 NTs to efficiently bind the airborne pathogen [3]. Nanotubes (NTs) were biofunctionalized following a five steps protocol. First, the array surface was treated with piranha solution, generating ordered OH groups on the surface. Then, NTs were silanized with a solution of APT (3-aminopropyltriethoxysilane) in toluene and thermally treated in order to polymerize the silane. After that, the nanostructured array is treated for the bioconjugation step with a solution of BS3 in PBS, so that N-hydroxysulfosuccinimide (NHS) ester reacts (through SN2) with primary amines of silanized surface forming stable amine bonds and releasing an NHS group. Finally, a testing protein (BSA) was diluted in PBS and dropped on the sensor platform. The process of biofunctionalization has been monitored and validated step by step by using optical methods, including Infrared [4] and UV-Vis spectroscopy. In conclusion, our experimental results show that a TiO2 NTs array can be used as a versatile platform for sensing of biochemical molecules interrogable with optical methods. Moreover, IR spectroscopy has been proven to be an effective tool, able to recognize and monitor pathogens. This work paves the way for the analysis of other different nanostructured platforms and optimized biofunctionalization process, for the development of an efficient optical biosensor for airborne pathogens.