Microstructured substrate based on in situ growth of silver particles for copper determination in liquid samples by LIBS

The development of simple and efficient analytical methods is fundamental for controlling the levels of toxic metals in liquid samples. In the present study, it is proposed the fabrication of a microstructured substrate based on filter paper, using in situ growth method of silver nanoparticles (AgNPs) into silver microparticles (AgMPs) to enhance the LIBS emission signal in the determination of Cu(II) in different liquid samples. Initially, ascorbic acid, NaBH₄ and SnCl₂ were evaluated as reducing agents for Ag particle formation. An 8-fold improvement in the copper signal was obtained when SnCl₂ was used. The Ag particles obtained with the latter reducing agent were further grown, and the signal was enhanced up to 14-fold compared to the traditional LIBS method using unmodified filter paper. UV-Vis diffuse reflectance spectroscopy, X-ray diffraction measurements, and scanning electron microscopy (SEM) indicated efficient immobilization of both Ag nano- and microparticles on the microfibers of substrate. A calibration analytical curve for Cu(II) ions was obtained with a coefficient of determination (R²) of 0.99, a linear response from 0.2 to 1.0 mg L^-1, and LOD and LOQ of 34 and 114 μg L^-1, respectively. The signal enhancement achieved with the microstructured substrate fabricated using the in situ growth method can be attributed to the synergistic effect of the nanostructures present in the cellulosic microfibers and the surface enhancement caused by the dense layer of Ag microparticles. The developed analytical strategy was successfully applied to the determination of Cu(II) ions in fortified samples of river water, artificial urine, tea infusion, and canned tea, providing recoveries ranging from 85 to 112 %. These results indicate the effectiveness of a microstructured silver substrate for sensitive detection of metal ions at low concentrations in liquid samples.