Self-cleaning cellulose-based devices: towards sustainable piezotronics?

Cellulose-based devices are affordable analytical platforms enabling the fabrication of sensors with electrochemical signal transduction. A still poorly explored feature of these sensors is the integration of self-cleaning and antibacterial properties, for improving their range of application in sensor technology (e.g. wearability, thermal stability) [1]. This work shows composites films formed by nanostructured ZnO (n-ZnO) with cellulose acetate (CA) showing photocatalytic properties and the release of zinc ions triggered by simulated solar light. The piezoelectric properties of n-ZnO are well known and have permitted the onset of piezotronics. Although considered safe, n-ZnO, especially in the form of nanoparticles, could generate reactive oxygen species (ROS) and leach zinc ions under dark conditions induce cytotoxicity [2]. Herein, n-ZnO are synthesized as micrometric flower-shaped grains dispersed in CA (2 mg/mL in ethyl acetate), leading to self-standing micrometre thick flexible films. The n-ZnO induced a decrease of the hydrophilicity compared to pristine CA, and a thermal stabilization effect on CA due to the n-ZnO. The photocatalytic activities of the films under simulated solar light are analysed by the degradation of methylene blue dye. The photocatalytic activity is proportional to the n-ZnO loading up to the 10% w/w, reaching a value of 0.030 ± 0.002 min-1, above which higher n-ZnO loading is not beneficial due to the higher hydrophobicity of the composite films. The reusability and repeatability of the photocatalytic process are investigated. The leaching of ionic species and zinc ions are analysed by electrochemical impedance spectroscopy and anodic stripping voltammetry conducted on miniaturized sensors. The higher the n-ZnO filler concentration, the lower is the solution bulk impedance, as a result of zinc ionic species leaching in solution. Solar light induces an approximately 2.5-fold increase of the zinc ion concentration leached in solution, highlighting a possible mechanism of light-triggered ZnO degradation (Figure 1).