Zinc Oxide Nanostructure Based Noninvasive and Continuous Sweat pH Sensing and Monitoring Platform
Authors: Alam Fahmida , Forouzanfar Shahrzad , Karabiyik Mustafa , Akter Naznin , Matos Randy , Pala Nezih 
Published: February 2020

We report a wearable, continuous and noninvasive monitoring of sweat pH platform. Recently there has been a surge in research to develop of highly stable and sensitive pH sensors for several biochemical and biological processes. pH is all a dynamic parameter in a comprehensive range of applications as monitoring of pH in sweat provides information about the body’s metabolic activity, as well as physiological responses. With the expansion of nano technology, pH sensors based on nanomaterials have been investigated by researchers. Among numerous metal oxide-based semiconductive nanomaterials, Zinc oxide (ZnO) has been considered as an attractive candidate for noninvasive wearable health-monitoring applications. Particularly, two-dimensional Zinc-oxide Nanoflakes (2D ZnO-NFs) attains technical interest for many biomedical applications. ZnO has a large direct energy band gap of 3.37eV with wurtzite structure and a large vibration in the binding energy of 60 meV. ZnO is an amphoteric oxide which reacts with H+ and OH- ions in pH solutions because of absorption by creating oriented dipoles or surface bonds. This absorption rate is proportional to the concentration of the free holes or electrons at the NFs surface. High surface-to-volume ratio of ZnO-NFS reduces the diffusion space of the analyte to the electrode surface, hence enhancing the signal-to noise ratio and increasing the sensitivity and reaction time. Thermal stability, biocompatibility, biosafety, high catalytic efficacy, chemical stability in functional environments, electrochemical activity, electron communication features, mechanical strength, low toxicity make it promising as a continuous pH sensing platform. The large surface area of nanoflakes makes them excellent candidates for pH sensing. Sonochemically synthesized ZnO-NFs are 20 nm thick and they have the average lateral dimensions of 5 um× 5 um. Using single step sonochemical technique, 2D ZnO-NFs are synthesized on gold deposited flexible polyethylene terephthalate (PET) substrates. Flexible polyethylene terephthalate (PET) substrate is used as it has achieved good reputation for wearables because of its thermal stability, excellent dielectric properties, intrinsic elasticity, hydrophobicity, structural resiliency against repeated bending forces, low coefficient of thermal expansion and compatibility with roll-to-roll fabrication processes for low price and scalable manufacturing. Sonochemical process permits synthetization of nanostructures (NSs) on a large variety of substrates (including flexible ones) in an atmospheric conditions (room temp, atmospheric pressure). This method offers simplification of fabrication procedure and improved long-term stability through large scale nanostructures synthesis. We choose this technique as it is fast, low-priced, catalyst free, complementary metal-oxide-semiconductor (CMOS) responsive and environmentally friendly. In comparison to other hydrothermal methods and the like well oriented NF synthesis with reaction rates up to 100 times higher can be achieved with the sonochemical method over a wide range of substrates. Selective area electron diffraction (SAED) characterization and high resolution transmission electron microscopy (HRTEM) images of ZnO-NFs ensure that ZnO-NFs have wurtzite structure and they are oriented in [0001] direction with a = 3.25 Å and c = 5.21 Å and they are single crystalline. The area of the working electrode is 5 mm ´ 5 mm. Using electrochemical studies and with the potential range of -0.6V – 0.6V at a scan rate of 50mVs-1, the specific pH measurements technique between reference electrode of Ag/AgCl and working electrode of ZnO-NFs have been explored. The designed pH sensor shows the sensitivity of 48.8 mV/pH/cm2 with a short response time of about 4.5s. The standard pH buffer solutions are applied to measure the performance of the pH sensor. The sensitivity of this sensor is determined in pH buffer solutions of the pH range from 4 to 10. A commercial pH glass-electrode sensor is used before every test to validate the pH value of the pH buffer solutions.The small form factor and biocompatibility of ZnO-NFs make this wearable, sweat based pH sensor a robust candidate for wearable and continuous pH monitoring device.