Solid-phase extraction (SPE) is an analytical technique designed for rapid and selective enrichment and purification of various compounds ranging from small molecules to biological macromolecules.
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In recent years, magnetic nanoparticles (MNPs) have found extensive use in SPE applications owing to the wide range of available surface chemistries, their very high specific surface area (and absorption capacity), and the ability to manipulate MNPs by applying external magnetic fields, resulting in a unique separation/purification performance.
Sample preparation is an important step in almost any analytical method used to characterize complex specimens, such as chemicals, novel chemicals, nanomaterials, foods, biological and environmental samples. Sample preparation enables enriching and isolating the target compounds for subsequent detailed characterization, thus enhancing the efficiency and throughput of the analytical techniques.
Faster, Cleaner, and Simpler Sample Preparation
Among the sample preparation techniques, separation and extraction are the most often used, in particular when analyzing samples with ultralow (trace) concentrations. Depending on the used extraction phase, the sample preparation procedure can be either solvent-based (relying on the analyte solubility in two different immiscible liquids) or adsorption-based extraction.
The adsorption-based extraction techniques include SPE, where the target analyte is separated from the sample bulk by adsorbing onto a solid stationary phase adsorbent. Compared to solvent-based extraction, the SPE is often preferred due to its simplicity, better repeatability, shorter extraction times, and applicability to a wider range of analytes.
Tailored Adsorbents for a Wide Range of Applications
The SPE method saw a rapid development throughout the 1990s as a sample preparation (pre-cleaning) stage for other analytical methods upstream, such as gas chromatography-mass spectrometry, high-performance liquid chromatography, and inductively coupled plasma atomic emission spectrometry. Such a staged approach enabled the analytical scientists to achieve very high enrichment factors and extremely low detection limits.
The properties of the adsorbent materials are of critical importance for the performance of the SPE technique. In recent years, the development of materials with high adsorption capacity and good selectivity has gained considerable interest from academic and industrial researchers. Various adsorbent materials have been developed, including ionic liquids, polymers, carbon-based materials, aerogels, metal-organic frameworks, and nanomaterials.
Magnetic Nanoparticles Become an Important Analytical Tool
The introduction of nanomaterials with magnetic properties as adsorbents in the early 2000s greatly enhanced the performance of the SPE method. The magnetic nanoparticles (MNPs) could be simply dispersed into a sample solution and efficiently recovered by applying an external magnetic field. Thus, the dispersive magnetic SPE bypasses some common issues with conventional packed SPE, such as adsorbent packing or clogging.
Among the wide variety of MNPs, iron oxide nanoparticles, such as magnetite (Fe2O3) and maghemite (γ-Fe3O4), have found extensive use in SPE owing to their high magnetic moments, small size, high surface area, biocompatibility, facile synthesis.
The MNPs are usually coated with different organic or inorganic materials. Silica is one of the most widely used coatings because of its chemical resistance and ease of surface functionalization. The rapid development of novel composite magnetic nanomaterials substantially widened the range of applications of the magnetic SPE method.
SPE Characterization of Food, Environmental and Biological Samples
In recent years, of particular interest is using MNPs in bioanalytical applications, where various inorganic and organic compounds are isolated from biological samples. Newly-developed magnetic graphene nanocomposites were used to isolate antibiotics (sulfonamide, fluoroquinolones, tetracycline, cephalosporin) and anti-inflammatory drugs (naproxen, diclofenac, ibuprofen) from samples such as wastewater, foods, and biologic fluids.
Researchers from China Pharmaceutical University in Nanjing pioneered the use of magnetic graphene oxide nanocomposite functionalized with β-cyclodextrin for the detection of carbamazepine, phenytoin, and diazepam in blood plasma samples.
Surfactant-Coated MNPs for Drug Detection
Surfactants are extensively used to extract and purify a large number of organic and inorganic compounds. Taking advantage of the high adsorption of some ionic surfactants like sodium dodecyl sulfate, cetyltrimethylammonium bromide, and cetylpyridinium chloride onto inorganic MNPs, several research groups developed a new type of surfactant-based adsorbent for magnetic SPE with high chemical stability and good magnetic separability.
Such surfactant-coated MNPs were successfully utilized for the rapid detection and isolation of both hydrophobic and hydrophilic drugs from urine samples.
Detection of Trace Elements by Magnetic SPE
The widespread use of metallic nanoparticles in various consumer products, agriculture, medicine, electronics, and the food industry gradually increases the concentration of metal ions in the environment. Even at very low concentrations, heavy metal ions are of great concern for the environment and pose risks to human health.
The ability to tailor the surface properties of the MNPs enabled several research groups to develop highly specific adsorbents and to employ high sensitivity SPE methods for the detection and quantification of metal ions, such as Au, Ag, Cd, Zn, Ni, Hg, and Pb in food samples, wastewater, and drinking water.
The diverse properties of MNPs warrant a promising future for their use as SPE adsorbents. Further modification of the existing MNPs and development of novel composite MNPs will ensure improved selectivity of the extraction process and reduced background signal caused by non-specific adsorption.
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References and Further Reading
Er, E. O., et al. (2020) Magnetic Nanoparticles Based Solid Phase Extraction Methods for the Determination of Trace Elements. Critical Reviews in Analytical Chemistry, Available at: https://doi.org/10.1080/10408347.2020.1797465
Hagarová, I. (2020) Magnetic Solid Phase Extraction as a Promising Technique for Fast Separation of Metallic Nanoparticles and Their Ionic Species: A Review of Recent Advances. Journal of Analytical Methods in Chemistry, 2020, 8847565. Available at: https://doi.org/10.1155/2020/8847565
Corps Ricardo, A. I., et al. (2020) Magnetic solid phase extraction as a valuable tool for elemental speciation analysis. Trends in Environmental Analytical Chemistry, 27, e00097. Available at: https://doi.org/10.1016/j.teac.2020.e00097