Nanoparticle-assisted metal–organic framework (MOF) enhanced laser-induced breakdown spectroscopy for the detection of heavy metal ions in liquid samples

Abstract

In recent years, the low sensitivity caused by plasma quenching has restricted the development of LIBS technology in practical applications. In order to improve the sensitivity of the LIBS analysis method, we proposed a nanoparticle-assisted metal–organic framework (MOF) enhanced laser-induced breakdown spectroscopy, which combines a metal–organic framework (MOF), Au nanoparticles (AuNPs), and a portable LIBS instrument. Common metal ions in water were selected as the research objects to be tested by the proposed method. First, the synthesized three-dimensional (3D) MOF, namely [Zn₂(L₂)]2DMA·3H₂O (complex 1) [H₂L = 2-(imidazole-1-yl)terephthalic acid], was used to capture metal ions. Second, complex 1 loaded with metal ions induced AuNPs to aggregate on its surface. Third, the samples were attached to the glass substrate with double-sided tape and analyzed by LIBS. As a result, the limits of detection (LODs) for Pb and Cr obtained in this work were 8.0 and 4.2 ng mL⁻¹, respectively, which were notably 8–10 times lower than those of the complex 1 enrichment method alone and 4 orders of magnitude lower than those of traditional LIBS methods. The significant improvement in LODs was also a new breakthrough for LIBS in liquid analysis. The great improvement in sensitivity can be mainly attributed to the unique microporous structure and the exposed metal binding sites of complex 1, which provide a huge space to accommodate metal ions. Also, the coupling action between laser electromagnetic fields and the surface plasmon of AuNPs greatly improved the laser ablation efficiency in the process of laser–matter interaction. Finally, the analysis of actual samples yielded good recoveries (92.8–99.0%) and reproducibility (lower than 11.9%), which shows the great potential of the combination of this method and portable LIBS in-field detection.