Owing to the development of modern industries, environmental water-pollution issues have become increasingly prominent. In industrial production, waste water containing heavy metals such as Pb and Cr is discharged into rivers and lakes, which threatens human health. Therefore, the development of highly sensitive detection technologies for harmful heavy metals in water is necessary. Laser-induced breakdown spectroscopy (LIBS) has been widely acknowledged for its advantages, such as the non-requirement for sample pretreatment, full elemental-analysis capability, and non-contact operation. It has been widely applied in fields such as environmental monitoring, alloy analysis, coal and metallurgy, biomedical research, and space exploration. However, owing to the effects of liquid splashing and the quenching effect of water molecules on atomic radiation in plasma, LIBS presents a lower sensitivity and stability in the direct analysis of aqueous samples. The accuracy of LIBS in analyzing aqueous solutions can be improved by converting liquid-phase samples into solid-phase samples. Compared with the direct LIBS analysis of liquid-phase samples, surface-enhanced LIBS (SELIBS) requires only the deposition of a minute amount of sample on the surface of a solid substrate (such as aluminum, magnesium, or silicon) and solvent evaporation. The interaction between the laser pulse and metal substrate can effectively enhance the sample signal strength, thereby improving the detection sensitivity of the elements in the liquid sample. In this study, a small high-repetition-rate microchip laser was used to process the substrate surface to obtain the microstructures prior to SELIBS. Periodic dot-array microstructures were fabricated on brass-substrate surfaces. The effect of the periodic surface microstructure on the plasma temperature and electron density was investigated via SELIBS. The effect of the microstructure on the spectral intensities of Cr and Pb in aqueous solutions was investigated via LIBS analysis.

A microchip laser with a wavelength of 1064 nm, a pulse width of 750 ps, and a pulse repetition rate of 1 kHz was used to prepare periodic microstructures on the surface of a copper plate. The trigger of the microchip laser was synchronously controlled using a waveform generator. Different periodic surface microstructures were obtained by adjusting waveform-generator parameters. The horizontal and vertical distances of the lattice microstructures on the metal substrate surface were obtained by synchronously setting the movement speed of the two-dimensional platform and the cycle period of the waveform generator. The desired depth of the periodic microstructures was achieved by adjusting the cumulative number of laser pulses at the same position. An electro-optically Q-switched Nd∶YAG laser with a pulse repetition rate of 2 Hz and a wavelength of 1064 nm was used as excitation sources for SELIBS analysis. For the evaluation, a compact fiber-optic spectrometer coupled with an intensified charge-coupled device was used to record the spectra.

The signal intensity of plasma emitted from the brass substrate with a periodic microstructure enhanced significantly. Compared with using a smooth brass substrate, the abovementioned substrate copper shows higher atomic emission intensities at 510.47, 515.20, and 521.68 nm by 8.07, 9.09, and 7.71 times, respectively (Fig. 4). The effect of the depth of the periodic microstructure on the atomic emission intensities was investigated experimentally. When the depth of the periodic microstructure increases continuously, the spectral signal-enhancement factor of the substrate-material element changes accordingly (Fig. 5). To clarify the signal-enhancement mechanism of SELIBS based on a periodic microstructure, the plasma-temperature variations in SELIBS with and without a periodic microstructure were determined based on Boltzmann plots using various copper atomic lines. The plasma temperatures are 8914 K and 9840 K for SELIBS without and with the periodic microstructure, respectively (Fig 7), which correspond to a temperature increase by 926 K. The average electron density in SELIBS without a microstructure and with a periodic microstructure was evaluated based on the Stark broadening of the selected atomic emission line. The results show that compared with the case using a smooth substrate, the electron density in SELIBS with a periodic microstructure is significantly higher (Fig. 8). Calibration curves of Cr and Pb in aqueous solutions were generated using SELIBS with an optimized periodic microstructure. By adopting Cr I 425.38 nm and Pb I 405.74 nm analytical lines, the detection limits of Cr and Pb in aqueous solution were determined to be 106.3 ng/mL and 49.2 ng/mL, respectively (Fig. 10).

A low-cost and highly efficient method for the fabrication of periodic microstructures based on a microchip laser was proposed. This method is suitable for other applications that require the fabrication of micrometer-scale surface microstructures. The interaction surface area between the metal substrate and laser is significantly higher in SELIBS with a periodic microstructure. The signal enhancement is primarily due to an increase in the plasma temperature and the collision mechanism. The energy-utilization efficiency of ablation laser in SELIBS is higher than that in conventional SELIBS without a microstructure when the ablation laser is focused on the surface of the periodic-microstructure substrate. Owing to the periodic microstructure substrate, collisional processes during plasma interaction promote the ionization of species, thus affording significant signal enhancement. This technique significantly improves the signal intensity of SELIBS, as well as provides scientific significance for further improving the analytical sensitivity of SELIBS and achieving better analytical results. It is promising for sensitive and rapid elemental analyses under different water environments.