Haematococcus pluvialis residues derived deep red emissive carbon dots for ultrasensitive and selective Cu²⁺...

Accurate and reliable detection of trace copper ions (Cu²⁺) is of great importance due to their dual role as essential micronutrients and potential environmental toxins. Herein, we report the first synthesis of deep red emissive CDs (CD-HP) derived from Haematococcus pluvialis residues, followed by nitrogen-doping modification (CD-HPE series) to enhance ion selectivity. Among them, CD-HPE2 exhibits excitation-independent emission at 674 nm with a narrow full width at half maximum (FWHM) of 34 nm, rivaling heavy metal-based quantum dots while retaining biocompatibility. The sensor demonstrates outstanding sensing performance, achieving an ultralow detection limit of 3.03 nM and a linear response range of 10-100 nM. The sensor shows excellent selectivity for Cu²⁺ against potentially interfering ions and maintains stable fluorescence across pH 6-9 and salinity levels up to 50 ‰. Application to real seawater samples yielded recoveries of 94.10-106.45 %, confirming its reliability in complex matrices. Compared with recently reported biomass-derived CDs, CD-HPE2 offers unique optical properties, superior sensitivity, and strong anti-interference capability. This work highlights an eco-friendly strategy to upcycle microalgal residues into high-value nanosensors and establishes a robust platform for ultrasensitive Cu²⁺ detection in environmental analysis.