3D Electrically Conductive, In Situ Assembled VOC Sensor Incorporating Ti₃C₂T_X MXene and Graphene Nanoribbon (GnR)

Detection of exhaled volatile organic compounds (VOCs) have long been critical in breathalyzing as a non-invasive diagnosis method. In this regard, development of wearable electrochemical sensors that can accurately monitor exhaled VOCS with various concentrations is essentially important for therapeutic applications. In this study, a novel flexible, highly sensitive, fast responding VOC sensors have been designed and fabricated that can be incorporated into breath analysis devices. Firstly, Ti₃C₂T_X MXene was chosen and synthesized to utilize its high electrical conductivity (5000 S/m) and abundant -O, -OH and -F terminal groups inherently grafted on its surface area. Next, MXene was hybridized with graphene oxide nanoribbon (GOnR), synthesized by chemical unzipping of multi-walled carbon nanotubes (MWCNT). The hybridized compounds were then thermally annealed in an Ar atmosphere at 700 ^oC, to reduce the GOnR to graphene nanoribbon (GnR) in presence of MXene and benefitting from its large aspect ratio (500), bridging effect and facile orientation. The redox reaction during the annealing process, transferred the oxygen groups from GOnR to MXene, further significantly increasing MXene’s reactive epoxy groups. Secondly, a novel fabrication technique was developed by combining coaxial electrospinning of styrene-butadiene-styrene (SBS)@cetrylammoniumbromide (CTAB) core@shell membranes and simultaneous electrospraying of MXene and GnR hybrids. The formation of electropositive CTAB shell of SBS membranes electrostatically attracted the electronegative nanohybrids, which enabled in-situ formation of a 3-D electroactive network. The manipulation of MXene’s terminal groups and significant increase in the more active oxygen groups, resulted in up to 10-fold enhanced ammonia, acetone, ethanol and methanol response compared to the neat MXene and GnR sensors. The in-situ selective deposition of the fibers led to a 3-D active and conductive structure with enhanced specific surface area of 148% and 125% compared to the neat SBS and conventionally coated SBS and XYZ directions conductivity of 12.21 S/m, 11.75 S/m, 10 S/m, respectively. Thanks to these enhancements, the sensors demonstrated expanded working range of 40 ppb-6000 ppm attributed. The sensors also exhibited mechanical stability for up to 600 deformation cycles which is required for integration into wearable devices and environmental stability when exposed to 80% relative humidity (RH).