Blue Carbon Dot-Based Portable Smartphone Platform for Visualization of Copper(II)

The acidic soil where tea grows makes the accumulation of copper ions (Cu2+) easy. Long-term drinking of tea infusions and tea beverages will cause abnormal accumulation of copper in the human body and endanger human health. Herein, we designed a visual portable smartphone platform based on ratiometric fluorescent nanoprobes for the rapid, real-time, and sensitive detection of Cu2+ in tea and tea beverages. We synthesized the ratiometric fluorescent nanoprobe formed by blue carbon dots (BCDs), which are used as internal standards, and a sensing reagent (diethyl-[5-(2-1,4,7,10-tetraaza-cyclododec-1-yl-acetylamino)-benzo[α]phenoxazin-9-ylidene]-ammonium (cyclen-NB)) for Cu2+. With an excitation wavelength of 330 nm, the emission wavelengths of cyclen-NB and BCDs are 674 and 451 nm, respectively. Cyclen-NB effectively captures Cu2+ to form a metal complex via a ligand-to-metal charge transfer (LMCT), resulting in obvious color changes from red to blue-violet. The designed ratiometric fluorescent nanoprobe has high sensitivity, good selectivity, and strong anti-interference ability, and the detection limit can be as low as 1.37 nM. Furthermore, a visual portable sensing platform was constructed based on the ratiometric fluorescent nanoprobe in combination with smartphones. The color recognition software of the smartphone can convert the color information of the photos taken by the mobile phone into digital data for analysis; and the detection limit is as low as 4.83 nM, and the yield rate is 86.79–111.20% (RSD ≤ 4.95). Therefore, the method reported in this manuscript can provide a convenient method for rapid on-site and visual inspection of tea and tea beverages for food safety.