Haematococcus pluvialis residues derived deep red emissive carbon dots for ultrasensitive and selective Cu(2+) detection in seawater.

Accurate and reliable detection of trace copper ions (Cu) 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 Cu 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 Cu detection in environmental analysis.