Engineered liquid-repellent substrate for enhanced Laser-Induced Breakdown spectroscopy (LIBS) of liquid samples

The detection of the elemental composition of liquid samples having low concentrations of analytes is challenging. The direct LIBS analysis of a liquid sample often results in contamination of the optics due to the splashing of the sample caused by laser ablation, whereas the drop-coating method is prone to inducing a ring-like structure known as the “coffee-ring” of the analyte after the evaporation of the liquid content, leading to incorrect results arising from the choice of analytical site. In the current study, we are proposing the fabrication of a superhydrophobic metallic substrate using nanosecond laser ablation to develop a superhydrophobic polydimethylsiloxane (PDMS) substrate via soft lithography. Following the selection of copper as the substrate, the laser wavelength with the best coupling to the metallic surface was chosen, and the optimization of the focusing optics, along with the laser energy, was performed to achieve the desired substrate surface characteristics. The applicability of the replicated PDMS substrate in reducing the coffee-ring effect, as well as enhancing the signal due to the enrichment of the analyte in a confined area, is evaluated. Finally, the feasibility of the substrate in characterizing environmental and biological samples was demonstrated, with more than three times the signal intensity enhancement observed across multiple emission lines, namely 518.14 nm (Mg), 508.6 nm (Cd), and 393.36 nm (Ca), resulting in significantly higher sensitivity.