Laser-induced breakdown spectroscopy (LIBS) has such as no sample pretreatment, simultaneous multi-element detection, and rapid analysis. It is currently the only technique capable of direct in-situ detection of solid metal elements underwater. Although LIBS has been successfully applied underwater, it encounters challenges like weak characteristic radiation, severe spectral line broadening, and short signal lifetime due to the properties of water. Therefore, it is necessary to develop enhancement methods tailored for in-situ underwater LIBS detection. Previous studies have confirmed in the laboratory that solid substrate-assisted LIBS can effectively enhance spectral intensity. Based on this, we verify the feasibility of this enhancement method for underwater in-situ applications using a self-developed deep-sea LIBS system, tested in both shallow- and deep-sea environments.

Using the LIBSea Ⅱ system developed by Ocean University of China (OUC), we incorporate a solid substrate-assisted enhancement module. The system structure is shown in Figure 1. The module consists of an underwater stepper motor and a solid substrate target. The solid substrate target is placed on a substrate carrier device designed as a quarter-circle for ease of operation by robotic systems. Six solid targets are positioned equidistantly on the carrier device and secured with adhesive. In practice, the underwater stepper motor drives the substrate carrier in a reciprocating motion, rotating 90° each time, with the laser sequentially acting on the diagonal of the six square substrates. We test the system in a laboratory pool, in the shallow waters off Jiaozhou Bay, Qingdao, and in the South China Sea at a depth of 1503 m to validate the method.

In the laboratory validation, comparing the enhancement effects of silicon, zinc, copper and nickel substrates, silicon demonstrates the best performance and is thus used as the substrate material in subsequent tests. Six identical silicon substrates are fixed in the substrate carrier, and rotation is controlled by the underwater motor. The LIBS system operates continuously for 240 min. Figure 3 shows the seawater LIBS spectra assisted by the silicon substrate over time. The spectral intensities of Ca I (422.7 nm), Na I (588.9 nm, 589.6 nm), and K I (766.5 nm, 769.9 nm) are illustrated in Figure 4. The intensities of Ca, Na, and K decrease as working time increases. The spectral intensities remain relatively stable during the first 90 min of continuous operation but significantly decrease after 90 min, and the substrate no longer exhibits enhancement effects after 170 min of continuous use. In shallow-sea tests (Fig. 7), the spectral signals of Ca are enhanced, and the atomic spectral lines of Na and K were enhanced by more than 6 times, with Na (588.9 nm) enhanced by 6.6 times, Na (589.6 nm) by 6.2 times, K (766.4 nm) by 6.0 times, and K (769.9 nm) by 6.4 times. In deep-sea tests (Fig. 10), the spectral intensity is significantly enhanced with substrate assistance, showing a 5-fold enhancement for Na and K elements.

We verify the feasibility of solid substrate-assisted enhancement for underwater in-situ LIBS detection. A solid substrate enhancement module, consisting of an underwater stepper motor and a solid substrate target, is developed. The service life of the substrate is extended by motor rotation. After comparing different substrates in the laboratory, silicon is selected for its superior enhancement effect, which is most effective within 90 min of continuous operation. Beyond 90 min, enhancement sharply decreases due to surface damage. Shallow- and deep-sea trials confirm the feasibility of substrate-assisted in-situ detection, with more than 6-fold enhancement achieved in shallow seas using silicon substrates. At a depth of 1503 m in the deep sea, a 5-fold enhancement is obtained using an iron substrate, which outperforms the long-pulse enhancement method reported to date.