Two birds, one stone: long-wavelength carbon dots enables ratiometric detection of ciprofloxacin and Co 2+ for smartphone, logic gate, and imaging applications

Environmental pollution poses a significant threat to human health and sustainable development, highlighting the need for rapid and sensitive contaminant detection. Here, we present a long-wavelength ratiometric fluorescent carbon dots to detect ciprofloxacin (CIP) and cobalt ion (Co ²⁺ ). The carbon dots (D-CDs) exhibiting outstanding dual emission at 445 nm and 662 nm are successfully synthesized through a one-pot hydrothermal approach, with methylene blue serving as the sole precursor. Interestingly, the blue fluorescence at 445 nm is significantly enhanced since the effect of formation of hydrogen bonds and charge transfer between D-CDs and CIP, while the 662 nm emission remains unchanged, yielding a ratiometric fluorescence response (F _445nm /F _662nm ) across a range of 0.048 − 3.58 nM, with a detection limit of 16.7 pM. Additionally, the fluorescence of CDs/CIP can be efficaciously ratiometric restored by right of a particular reaction of Co ²⁺ with CIP, achieving ratio fluorescence quantitative assay of Co ²⁺ (LOD = 14.7 nM). Density functional theory (DFT) calculations has been used to illustrate the potential interaction mechanisms, which shows strong agreement with the experimental results. Notably, a smartphone-integrated colorimetric test strip enables on-site monitoring of CIP and Co ²⁺ , expanding environmental applications, which has been demonstrated by effectively detection of CIP in milk and river water. Further exploration has been conducted by developing a logic gate sensor which harnesses the activated cascade effect to serve as an intelligent probe for monitoring trace levels of CIP and Co ²⁺ . Furthermore, D-CDs are applied in cellular imaging, demonstrating their strong potential in sensing, bioimaging, and environmental analysis.