Dynamic Duo

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Can combining stir-bar sorptive extraction (SBSE) with molecularly imprinted polymers (MIPs) extend the practical applications of SBSE? The Column spoke to Antonio Martin-Esteban of the Department of Environment of the National Institute of Research and Technology in Agricultural and Food Science (INIA) in Madrid, Spain, to find out more. - Interview by Lewis Botcherby

Q. Why was stir-bar sorptive extraction (SBSE) developed and what benefits does combining the technique with molecularly imprinted polymers (MIPs) offer the analytical scientist?

Antonio Martín-Esteban: Stir-bar sorptive extraction (SBSE) was introduced in 1999 as an alternative, or at least complementary technique, to solid‑phase microextraction (SPME) with the aim of addressing the typical low capacity of fibres used in SPME (1). Since then, SBSE has been routinely used in analytical laboratories around the world in environmental, food, biomedical, and life sciences applications (2).

However, for years, only polydimethylsiloxane was available for SBSE commercially and a PDMS/ethylene glycol copolymer as a coating phase was only recently introduced to the market, which restricts the range of applications to the extraction of hydrophobic compounds from aqueous‑based liquid samples because of the apolar character of PDMS. 

Accordingly, the development of new magnetic stir-bars coated with more polar and selective sorbents has been an active area of research for the past 15 years (3). In parallel, MIPs have proven to be useful materials in different applications, especially when improved selectivity is required. MIPs are “tailor-made” materials with cavities in the polymeric matrix that are complementary in size, shape, and chemical functionality to the template molecule used during their synthesis. Thus, the imprinted polymer can selectively rebind a given analyte (the template) or closely related compounds under certain experimental conditions (4,5). It was clear to us that the combination of molecular imprinting and SBSE could be a powerful analytical tool. 

Q. When magnetic stir-bars were first coated with MIPs there were issues surrounding preparation of the method. Could you please explain why the preparation process was an issue?

AM: Our research group has been working for several years on the incorporation of MIPs to sample preparation techniques, including solid-phase extraction (SPE), solid-phase microextraction (SPME), and liquid‑phase microextraction (LPME), so SBSE was obviously of interest too.

We initially explored the preparation of MIP-coated glass magnetic stir-bars (6), because it appeared to be the simplest approach. However, we discovered that this was not so easy. One of the main drawbacks was the essential pre-treatment stage of the glass surface. This involved several tedious steps, including cleaning, etching, washing, NaOH treatment to increase the amount of available silanol groups, and finally silanization to allow the coating of MIP to the surface. The treated glass magnet stir-bar then had to be immersed in the polymerization mixture in a vertical position, which was rather tricky, at least for us, because it was difficult to obtain MIP magnetic stir-bars with the proper size and shape in a reproducible and reliable manner.

Q. The process you proposed in a recent publication (7) aims to circumvent these deficiencies through a simpler approach. Could you explain the theory behind its development? 

AM: As I mentioned earlier, one of our main research areas is the incorporation of MIP technology to provide selectivity for sample preparation. We were working on the preparation of core–shell magnetic particles to be used in magnetic SPE and suddenly the idea came to us. Would it be possible to prepare a MIP magnetic stir-bar just by trapping magnetic nanoparticles (NPs) inside a polymer monolith? Theoretically it would be, but we did not know whether magnetic NPs would provide enough magnetism to allow the free rotation of the stir-bar for stirring solvents and solutions. Thus, our first experiments consisted-without any previous optimization-of preparing a suspension of magnetic NPs in a typical polymerization mixture and then performing a classical bulk polymerization. After some trials we could finally see our magnetic monolith rotates on a magnetic stirrer. It was rather exciting!

Q. How well did the process work and were there any potential drawbacks?

AM: After those preliminary experiments, we started to optimize the different variables that could affect the performance of the new magnetic stir-bars in terms of both polymer morphology (that is, the number of magnetic NPs, components of polymerization mixture, and polymerization time) and binding-elution conditions of target analytes. The main problem was to find the right number of magnetic NPs to use. It had to be large enough to provide magnetism to the stir‑bars, but their presence must not affect the creation of binding sites. Besides, it was necessary to modify the magnetic NPs with oleic acid since “nude” NPs suffered deterioration by the use. Unfortunately, a reduction of the number of binding sites was observed with the presence of oleic acid, which negatively affected the capacity of the magnetic MIP stir-bars. 

Q. What advice would you give to scientists attempting to use this technique in their own research?

AM: The approach we proposed is rather simple and the preparation of magnetic MIP stir-bars can be performed in any laboratory equipped with basic instrumentation. Basic knowledge in polymer synthesis is requiredWe used commercial magnetic NPs, which deteriorated and led to dirty chromatograms. Therefore, I would recommend testing magnetic NPs first. It is possible that other magnetic NPs are more stable, and in that case, it would be unnecessary to use oleic acid, improving the rebinding abilities of the stir-bars. If someone finds such particles, please e-mail me!

Q. What are you currently working on in relation to imprinted stir-bars?

AM: We are synthesizing our own magnetic NPs following different experimental procedures reported in the literature (8) to make them more stable and provide improved magnetic characteristics. We are also testing different surface modifiers, such as poly(ethylene glycol) or poly(vinyl alcohol), to protect the magnetic NPs. In this manner, we expect to get more robust and stable magnetic MIP stir-bars with improved capacity and selectivity.

Q. What practical applications are there for this new technique?

AM: Our first work focused on the extraction of triazines from soil sample extracts (7), and so we are currently preparing new magnetic MIP stir-bars for the extraction of other contaminants from both environmental and food samples to demonstrate the general applicability of our approach.

References

1. E. Baltussen, P. Sandra, F. David, and C. Cramers, J. Microcolumn Sep.11, 737–747 (1999).
2. F. David and P. Sandra, J. Chromatogr. A 1152, 54–69 (2007).
3. J. Nogueira, Trac-Trends Anal. Chem.71, 214–223 (2015).
4. A. Mayes and K. Mosbach, Trac-Trends Anal. Chem.16, 321–332 (1997).
5. A. Martin-Esteban, Trac-Trends Anal. Chem.45, 169–181 (2013).
6. E. Turiel and A. Martin-Esteban, J. Sep. Sci.35, 2962–2969 (2012).
7. M. Díaz-Álvarez, E. Turiel, and A. Martín‑Esteban, J. Chromatogr. A1469, 1–7 (2016).
8. L. Chen and B. Li, Anal. Methods4, 2613–2621 (2012).

Antonio Martín‑Esteban is a Senior Researcher at the Department of Environment of the National Institute of Research and Technology in Agricultural and Food Science (INIA) in Madrid (Spain). He is author or co-author of over 70 publications, including original research papers, reviews, and book chapters. The current research of his group is focused on the development of improved sample preparation procedures to determine a wide range of contaminants both in food and in environmental samples by chromatographic techniques. One of his main areas of research deals with the synthesis and subsequent use of molecularly imprinted polymers in sample preparation.

E-mail:amartin@inia.es

Website:www.researchgate.net/profile/Antonio_Martin-Esteban