Chromium (VI) detection by microbial carbon dots: Microwave synthesis and mechanistic study

Functionalized carbon dots (CDs) derived from Citrobacter freundii bacterial cells were used for selective detection of Cr(VI). A microwave-heating-based green synthesis approach is adopted to produce functionalized CDs from C. freundii bacterial cells (CF-CDs). The reaction was carried out in a 500 W microwave digester at 200°C for 20 min. The supernatant was filtered with a 0.2 µm filter and highly monodisperse CDs were obtained. Inherent functionalization of CF-CDs with nitrogen and oxygen-containing functional groups made them extremely selective toward Cr(VI) with a lower limit of detection of ~1.7 ppm. More importantly, CF-CDs could distinguish between Cr(VI) and Cr(III), which is highly desirable for practical applications. The fabricated sensor had a dual linear response range between 0 and 50 µM and 50-250 µM. The synthesized CDs were inherently functionalized which made them highly selective for Cr(VI) detection. CF-CDs also possess high stability over long storage period. This study reports the facile synthesis and characterization of a highly selective sensor for Cr(VI). As opposed to similar CDs sensors reported for Cr(VI), based on inner-filter effect, CF-CDs exhibit a strong, specific interaction toward Cr(VI), indicated by the large binding constant.