Red emitting nitrogen-doped carbon dots for fluorescence and colorimetric dual-mode detection of Cu2+ and biological sensing

N-CDs preparation and Cu2+ detection. [Display omitted] •A new excitation-independent red-emitting carbon dot is prepared.•A method for detecting Cu2+ based on the smartphone was developed, and real-time detection of Cu2+ was achieved by analyzing RGB values, realizing dual-mode detection of Cu2+ by fluorescence and colorimetry.•The synthesized carbon dots can not only enter the cytoplasm, but also enter the nucleus, realizing the detection of Cu2+ in vitro and in vivo. Herein, red-emitting nitrogen-doped carbon dots (N-CDs) were prepared using p-phenylenediamine and polyethylene glycol 20,000 as precursors. The N-CDs are spherical and uniformly distributed with average particle sizes of 2.36 nm. The N-CDs display excitation-independent fluorescence behavior with the optimal excitation and the maximum emission wavelengths centered at 487 and 628 nm, respectively. Results indicated that as prepared CDs can provide a rapid and highly selective recognition of Cu2+ with the range of 45–70 µM and a detection (LOD) limit of 45.87 µM. A clear color change of the N-CDs solution (orange-red to black) can also be visualized by adding Cu2+. Moreover, a smartphone-based platform for the detection of Cu2+ was developed to detect Cu2+ in real time by analyzing RGB values, giving a linear range of 20–140 µM. and a LOD limit of 35.83 µM. In addition, it was applied for the detection of Cu2+ in tap water, the recoveries were 86.18 %-104.95 %, and the relative standard deviations were 1.64 %-2.43 %. The N-CDs can be used to detect Cu2+ in vitro and in vivo.