Double-pulse laser-induced breakdown spectroscopy assisted by quantum dots for quantification of trace metals in...

Rapid quantification of hazardous trace metals in waste oil remains challenging because of the high viscosity and severe matrix effects of oily substrates. Here, we developed a quantum dot-enhanced double-pulse laser-induced breakdown spectroscopy (DP-QDELIBS) platform for ultrasensitive and matrix-tolerant metal analysis in waste oil. AgNC@AgAu_x core-shell quantum dots were used as an interfacial enhancement layer to improve laser energy coupling and initial plasma formation, and a second laser pulse was introduced to reheat the primary plasma and further amplify metal emission. Compared with conventional single-pulse LIBS, DP-QDELIBS achieved signal enhancement factors of up to 47 for Mg emission lines. Ablation morphology analysis indicated that the enhancement was associated with porous microstructures generated by intensified interfacial charge transfer and localized plasma expansion. Under optimized conditions, the method showed good linearity for representative metals, with coefficients of determination above 0.97 and limits of detection of 0.02 ppm for Mg, 0.06 ppm for Ca, 0.05 ppm for Cr, and 0.05 ppm for Ba. Analysis of real industrial waste-oil samples, including engine oils and brake fluids, showed strong agreement with inductively coupled plasma optical emission spectrometry (R² > 0.97). These results demonstrate the potential of DP-QDELIBS for rapid screening of hazardous metals in complex oily wastes and for waste-oil risk assessment and management.