Ratiometric fluorescent sensor based on TPU-PVP coaxial nanofibers for monitoring trace ammonia in breath

Fluorescent sensors with low detection limits and fast responses have caused significant attention for their application in medical diagnoses. Herein, we described the construction of ratiometric nanofibers (NFs) for sensing breath ammonia (NH3), which was an important biomarker for Helicobacter pylori (Hp) affection, by integrating of a 'turn on' fluorescent probe of fluorescein (FLU, green emission) and a 'turn off' fluorescent probe of difluoroboron-curcumin analog (BFC, red emission). Such NFs were constructed by coaxial electrospinning using thermoplastic polyurethane (TPU) with FLU as the shells and polyvinyl pyrrolidone (PVP) with BFC as the cores. The core/shell structures of NFs provided independent channels for the chosen fluorophores and enabled the sensor with low humid interferences, high mechanical properties and outstanding NH3 sensing performance. This nanofibers-based sensor showed superior sensitivity with an NH3 theoretical detection limit of ∼7.16 ppb, quick response time of ∼2.2 s, high selectivity and reversibility for NH3, and excellent stability in various conditions, especially under high humid conditions. Furthermore, because of the use of two probes with reverse sensing trends in the red and green emitting windows, the ratiometric sensor showed clear color changes when tracing NH3, which was visualized by eyes and cameras easily. Preliminary tests for Hp infection subjects showed that the NH3 changing levels before and after urea-pill ingestion were much higher than those healthy subjects. Thus, an appropriate combination of polymeric matrixes with NH3-sensitive probes in the form of dual-emission coaxial NFs as the ratiometric sensor is expected to be used as a simple approach/tool for monitoring trace NH3 concentrations and broadening the sensing design strategy.