Surface-enhanced laser-induced breakdown spectroscopy (SE-LIBS) is an effective method for analyzing trace elements in liquid samples. In SE-LIBS, a solid substrate is typically required. It has been demonstrated that fabricating periodic microstructures on the substrate surface significantly enhances the analytical performance of SE-LIBS. However, creating these microstructures on each substrate individually is time-consuming. To develop a fast and convenient method for fabricating periodic microstructures on solid substrates, we explore a new method based on electroplating and coining. This method enables easy duplication of periodic microstructures across various substrates, reducing the cost of preparation and improving SE-LIBS analytical performance.

A commercially available sapphire plate with a 2 μm periodic microstructure is purchased and this microstructure is duplicated on a Ni plate using electroplating. The Ni plate then serves as a template to transfer the microstructure on a Sn plate through the coining method. The prepared Sn plate is subsequently used as the solid substrate for SE-LIBS to facilitate sensitive elemental analysis of liquid samples. Aqueous solution samples are applied to the Sn substrate and dried using heat. For SE-LIBS analysis, a Q-switched Nd∶YAG laser with electro-optical control is used as the excitation source. The laser’s wavelength, pulse width, and repetition rate are 1064 nm, 12 ns, and 5 Hz, respectively. The laser beam is focused on the sample surface using a plano-convex quartz lens (f =150 mm) to generate the sample plasma. During the experiment, the sample is mounted on a 2D platform that moves at a linear speed of 0.5 mm/s. The plasma emission is collected by the fiber entrance of a compact three-channel spectrometer (Avantes, AvaSpec-ULS2048-3-USB2) coupled with non-intensified CCD detectors. The spectrometer is externally triggered by DG535 and operated in LIBS mode with a 1.5 μs gate delay and 2.0 ms gate width. The spectrometer’s wavelength range is 200?550 nm, and five repeated measurements are averaged each time. The averaged spectral data are transferred to a computer for further analysis. The laser beam’s focus setting is optimized by monitoring the intensity of a Ni atomic line. The effect of the micro-structured surface on the intensity of Sn atomic lines is then studied. Finally, quantitative elemental analysis of Mn in aqueous solution is performed.

Aqueous solution samples with varying mass concentrations of Mn are prepared and analyzed using SE-LIBS with Sn substrates featuring periodic surface microstructures, as well as smooth Sn substrates without microstructures. Quantitative elemental analysis is conducted, and calibration curves for Mn are established. The detection limits of Mn are evaluated under the current experimental conditions for both types of substrates. When the laser energy is set at about 20 mJ, the limit of detection for Mn in solution is determined to be 0.136 mg/L. A 50% improvement in signal intensity is achieved with the microstructured Sn substrates compared to the smooth Sn substrates. The significance of this method for fabricating periodic microstructures on metal substrates is considerable. First, the process is simple and efficient, significantly reducing surface preparation time. Second, the Ni plates obtained through electroplating can be reused multiple times, thus lowering the experimental costs. Third, this method allows for batch processing of substrates used in SE-LIBS. While Sn plates are soft and easy to replicate microstructures on, Sn has numerous atomic and ionic lines in the UV region, which can introduce spectral interference when used in SE-LIBS analysis. A potential solution to this issue is to fabricate periodic microstructures on polymer substrates. This approach is currently being explored in our laboratory.

Periodic microstructures can be fabricated on metal surfaces using the coining method with an electroplated Ni plate, which duplicates the surface microstructure of an existing template. This provides a fast and convenient way to create microstructures on metal surfaces. Sn plates with 2 μm periodic microstructures are prepared and used as substrates in SE-LIBS for sensitive analysis of Mn in aqueous solutions, with a detection limit of 0.136 mg/L under the current experimental conditions. The Ni plates produced by this method can be reused multiple times, reducing costs. Therefore, the method for fabricating periodic microstructures on the metal surface described here is highly beneficial for SE-LIBS application, especially in the sensitive elemental analysis of liquid samples.