Nanohybrid optical electronic nose based on zinc oxide/red carbon dots for ammonia sensing at room temperature using surface plasmon resonance on a smartphone platform.

This study reports the development of an optical electronic nose sensor designed for the highly sensitive detection of ammonia vapor at room temperature, utilizing zinc oxide/red-emitting carbon dot (ZnO/R-CD) nanohybrid films. The detection mechanism is based on the monitoring of refractive index changes in the ZnO/R-CD films upon exposure to ammonia vapor, employing the surface plasmon resonance (SPR) technique integrated with a smartphone platform. The results showed that a ZnO film thickness of 200 nm provided the highest sensitivity. In addition, under blue light illumination, the sensing response was nine times greater than that of unilluminated film. The proposed sensor also demonstrated the ability to distinguish between five volatile organic compounds (VOCs), including ammonia, ethanol, methanol, isopropanol, and acetone, by using five different illumination colors on the smartphone display. Among these, ammonia produced the highest sensing response, approximately six times greater than those of the other VOCs. A linear response was observed over an ammonia concentration of 4-33 ppm, with a sensitivity of 1.406 %/ppm. The limit of detection, calculated from the standard deviation of the noise signal, was 0.079 ppm. Furthermore, the sensor exhibited rapid response and recovery times (6 s/54 s) and maintained excellent stability over a period of 45 days. These findings underscore the sensor's potential for practical applications involving the detection of ammonia vapor, and particularly for monitoring spoiled milk.