Research progress of solid phase extraction materials in the application of metal ion pretreatment

Monitoring of trace heavy metal pollutants released during industrial and agricultural processes is essential because of their widespread distribution in the environment and health hazards. Several techniques, including inductively coupled plasma-mass spectrometry (ICP-MS), inductively coupled plasma-optical emission spectrometry (ICP-OES), electrothermal atomic absorption (ETAAS), and flame atomic absorption spectrometry (FAAS), have been proposed for the determination of heavy metals in serum, plasma, whole blood, and food. All these techniques have earned robust recognition in the field of trace heavy metals and have many advantages such as multi-elemental analysis capability, large dynamic linear range, low detection limits, and high productivity. Nevertheless, most of the recommended techniques require digestion of the sample and extraction with an organic solvent for isolation of the metal ion from the sample solution prior to analysis. Despite improvements in the performance of modern analytical instruments, the direct determination of heavy metal ions in real samples is difficult because of their low concentration levels and matrix interference. Thus, extraction and clean-up steps are required for pre-concentration of the analyte, so that detection and elimination of the interfering matrix component are possible. Solid-phase extraction (SPE) is one of the popular metal ion pretreatment methods. The advantages of SPE include easy cartridge/column regeneration, high analytical frequency, and high preconcentration factors for sorbents with high adsorption capacities. On the other hand, when the analytes are extracted from a complex matrix such as serum and meat samples, large amounts of proteins from the samples can be retained on the sorbent surface, obstructing the binding sites on the sorbent and leading to poor precision and accuracy. The key to metal ion detection is the development of new SPE materials with high efficiency and enrichment factors as well as an effective pretreatment technology. Nanomaterials such as restricted-access carbon nanotubes, nanoadsorbents, nanoparticle carriers, and magnetic nanoparticles have shown great promise in advancing biomedical and environmental analysis because of the unique properties originating from their ultrafine dimensions. Nanomaterials can provide large specific surface areas and tunable functional groups to facilitate metal ion absorption. They could also possess superior optical properties and allow for high sensitivity in simple fluorescent or colorimetric detection methods. Owing to their excellent mechanical and chemical stability, polymer materials have been of great interest as adsorbents for the SPE of metal ions from solution. Moreover, a designed polymeric material can show triple functionality such as physical adsorption, chelate formation, and ion exchange for the target metal ions. A dual-functional nanomaterial-DNAzyme platform can simultaneously allow for the sensitive detection and effective removal of heavy metal ions in water. Thus, this platform can serve as a simple, cost-effective tool for rapid and accurate metal quantification in the determination of human metal exposure and inspection of environmental contamination. Furthermore, the new photocaged chelator can uncage and release the combined metal ions into an aqueous solution that is free of the other components of the matrix. In this manner, we can develop diagnostic tests for metal ions that are often difficult to detect using other methods. In this paper, the characteristics of new SPE materials, including nanomaterials, polymer materials, and functional materials as well as advances in their applications to the preparation of complex samples are summarized, and the direction for future development is proposed.