Label-free carbon dots from black sesame seeds for real-time detection of ammonia vapor via optical electronic nose and density functional theory calculation

Synthesis of carbon dots from natural precursors and their use in novel sensing applications currently gain much attention in the carbon dot research community because of its practicality and scaling-up possibility. Herein, carbon dots were synthesized from black sesame seeds via hydrothermal method and used as a dual-mode probe for the detection of ammonia in vapor and solution phases. The size of carbon dots was around 7.6 nm, and they exhibited blue-color emission under UV excitation in solution with a quantum yield of 2%. For the first time, the obtained carbon dots as a sensing layer were then used in optical electronic nose for the real-time, room-temperature detection of ammonia vapor and several volatile organic compounds. Our carbon dots in optical electronic nose could sense ammonia vapor generated from ammonia aqueous solutions with a detection limit of 0.8%v/v. Furthermore, they could also differentiate ammonia in various mixtures, containing methanol and water and from ethylenediamine and triethylamine. The density functional theory calculations were used to confirm the change in optical absorbance of carbon dots when exposed to solvent vapors. The computational results supported the experimental findings well in which the carbon dots and ammonia formed the most stable complex among all vapor molecules computed. A paper-based device was also constructed to prove the practicality and versatility of our carbon dot-based ammonia sensors. In summary, solid-state optical absorption sensing via electronic nose has proved to be a real-time, convenient approach for the detection of ammonia vapor. This work also demonstrates experimentally and computationally that the carbon dots are unique nanomaterials and the ammonia sensors based on carbon dots will be useful in various fields.