Droplet Constraint by a Superhydrophobic-Superhydrophilic Hybrid Surface with a SiO₂ NP Coating for Determination of Heavy Metals Using LIBS

Nowadays, the detection of heavy metals is urgent, but traditional methods partially fulfill detection requirements (e.g., time-consuming and low-sensitivity). Laser-induced breakdown spectroscopy (LIBS) assisted with the dried droplet method (DDM) provides impressive performance in hazardous metal detection. However, the coffee ring effect in the droplet drying process causes inhomogeneous distribution of heavy metals, thus reducing the accuracy and sensitivity of LIBS. Herein, we developed a superhydrophobic-superhydrophilic hybrid (SSH) substrate to suppress the coffee ring effect for stable and sensitive LIBS analysis to trace heavy metals. The superhydrophobic surface with a micro/nanostructure was prepared by coating polydimethylsiloxane and silicon dioxide nanoparticles sequentially. Also, the superhydrophilic pits (diameter of 1 mm and depth of 45 μm) were fabricated onto the superhydrophobic surface by laser processing. Trace heavy metals barium (Ba) and cadmium (Cd) were analyzed using LIBS to evaluate the analytical performance (stability and sensitivity) of the SSH substrate. The results of Ba and Cd (Ba: RSD = 12.4%, LoD = 0.34 mg/L; Cd: RSD = 6.4%, LoD = 0.22 mg/L) demonstrated the coffee ring effect suppression, solute enrichment, and ultrasensitive heavy metal element detection. All in all, the SSH substrate has great potential and good application prospect in LIBS analysis for heavy metals.