Use of porous silicon in underwater laser-induced breakdown spectroscopy for detecting lithium dissolved in a sodium chloride aqueous solution

Abstract

Underwater laser-induced breakdown spectroscopy (underwater LIBS) is gaining increasing attention as an on-site analytical technique available in water-confined environments, such as the deep sea. Use of a solid substrate in underwater LIBS is beneficial for detecting dissolved elements. In this study, we demonstrate the first use of porous silicon (Si) as a substrate for underwater LIBS, which is fabricated by metal-assisted etching (metal-assisted chemical etching) using silver nanoparticles. A long-nanosecond laser (30 ns) operating at a wavelength of 1064 nm and a low energy of 1.5 mJ was focused onto a substrate immersed in a 0.6 mol per L sodium chloride aqueous solution containing 2.5 ppm (mg L⁻¹) lithium (Li). By using porous Si instead of flat Si as the substrate, the spectral line intensity of Li was enhanced by 8.0 times. A linear calibration curve with a coefficient of determination of 0.999 was obtained using porous Si in the range of Li concentration from 0.5 to 10.0 ppm. This study suggests a new potential approach for utilizing porous Si and extends the applicability of LIBS to liquid analysis.