Ultrasensitive Exhaled Gas Detection via Evanescent Wave-Excited Fiber SERS Sensor Assembled with Silver Nanocubes

Exhaled breath analysis offers a non-invasive route for metabolic monitoring and disease screening, but its practical implementation requires sensing platforms that combine high sensitivity, robustness, and simplicity. Here, we report an evanescent wave-excited fiber-optic surface-enhanced Raman scattering (SERS) sensor based on silver nanocubes (Ag NCs) assembled onto a fiber taper waist (FTW), and the design is further extended to an Ag/graphene oxide (GO) hybrid interface for enhanced gas detection. Finite element and finite-difference time-domain simulations were employed to optimize the FTW geometry and Ag NC dimensions for efficient evanescent-field excitation and plasmonic enhancement. The fabricated FTW-SERS probe achieved a minimum detectable concentration of 10−9 M for crystal violet, together with good linearity and a relative standard deviation below 5%. For gas sensing, ethanol and acetone vapors were detected down to 50 ppm using the Ag NC-based FTW-SERS probe. After introducing a 0.3 mg/mL GO functional layer, the minimum detectable concentrations of both analytes were further reduced to 25 ppm. In addition, proof-of-concept monitoring of exhaled ethanol after alcohol consumption revealed dynamic spectral changes consistent with ethanol metabolism. These results demonstrate the potential of evanescent wave-excited FTW-SERS probes for compact and sensitive breath-analysis applications.