The Evolution of MIPs

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Raquel Garcia and Maria João Cabrita reveal the theory and practical applications of molecularly imprinted polymers (MIPs).

Q. When and why were molecularly imprinted polymers (MIPs) developed for the purpose of sample preparation?

A: The first molecularly imprinted polymer solid-phase extraction (MISPE) technique was described by Sellergren in 1994 for the determination of pentamidine in urine samples for the treatment of AIDs-related pneumonia.¹ The number of publications dealing with MIP technology and its use for sample preparation has subsequently increased.^2,3 The most common use is probably as a sorbent for solid-phase extraction (SPE), which is known as MISPE. The use of molecular imprinting for the development of new adsorbents for SPE is gaining more and more attention because the coupling of MIPs with SPE combines the advantages of both enhancement of template molecular recognition and traditional separation methods. MISPE technology is now available commercially for specific sample preparation applications, including pesticides (triazine), endocrine disruptors (Bisphenol A) and polycyclic aromatic hydrocarbons.

Q. Why were MIPs developed for sample preparation?

A: MIPs are prepared in such a way that one can achieve the formation of an artificially generated three-dimensional polymer network that has recognition sites complementary in size, shape, and spatial orientation of the target molecule. MIPs can be “tailor-made”, meaning that, hypothetically, we can design a MIP for the specific target analyte we want. They are highly selective and that is why these polymers are so suitable as sorbents for SPE. It is a very useful technique when the focus of an analysis is a specific compound that we need to extract from a matrix to analyze.

Q. What is the theory behind developing MIPs for solid-phase extraction and what possible advantages do they offer over current sample preparation techniques?

A: Generally, the analysis of complex matrices involves a previous step that comprises the isolation of the target compound from the whole matrix followed by their quantification using appropriate chromatographic techniques. In complex samples, this first task is particularly demanding because it is quite challenging to extract the target compounds without co-extraction of some interferent compounds, which could interfere in the quantification or eventually cause damage in the chromatographic system. Thus, the development of alternative sample preparation techniques that enable a selective extraction of the target molecule from the whole matrix is highly warranted.

The implementation of MIP-based sorbents for analytical purposes looks promising and aims to avoid all the drawbacks of common SPE sorbents, namely the lack of selectivity. The use of these “tailor-made” sorbents for SPE, based on mechanisms of molecular recognition, is an ingenious way to enhance selectivity in sample preparation methods. Other advantages of this type of polymeric material include the high stability in a broad range of pH-, pressure- and temperature ranges (<180 °C). In addition, MIPs can be reused several times without loss of the “memory effect”. Added together these features have contributed to the widespread use of MIPs in sample preparation.

Q. Is there any particular areas that MIPs are currently being used in?

A: MIPs can be used in several areas. Some examples are the use of MISPE in the selective extraction of analytes from biological, environmental,⁴ or food samples.⁵ We work mainly with food samples, particularly wine and olive oil, so we are developing MIPs to be used as MISPE for pesticides residues in olive oil samples. It is a challenging task because of the fatty characteristics of the olive oil matrix. When we first applied for funding there were no articles published dealing with olive oil samples and MISPE technology.

Even today there is only one other group working with olive oil, but in other food matrices, such as milk, fruits, eggs, vegetables, and beverages, there are several applications reported.⁵ Regarding food contaminants, there are two types of MISPE that have been widely described: triazine‑based and urea-based herbicides. MISPE of triazine is the oldest one reported in the literature.⁶ That is why we are devoting our attention to pesticides belonging to organophosphorus and pyrethroid classes, using deltamethrin and dimethoate as template molecules. And, of course, we are also working with triazine.

As well as being used as adsorbents for SPE, MIPs can be used as chromatographic stationary phases.⁷ They are also being explored for drug delivery, and seem promising in medical research. More recently there is a growing interest on the application of MIPs in sensing technology. In particular, their use as selective recognition elements in sensing platforms takes advantage of the fact that the synthesis of these polymers is fully compatible with lab‑on-a-chip and nanotechnology, constituting a reasonable alternative to biosensors, which avoids the limited operational and storage stability of the biomolecules.

Indeed, the inherent molecular recognition abilities of the imprinting materials plays a key role in the development of highly sensitive and selective analytical methods based on different transduction mechanisms. Nevertheless, some challenges still remain in this field mainly related to the difficulty of integrating the imprinting materials with the transducer as well as the transformation of the binding event into a measurable signal. The exploitation of new monomers with responsive functionalities and the introduction of new polymerization techniques are being attempted on the development of sensor devices for different analytes.

Q. Can you illustrate the advantages of MIPs with some practical examples from your group?

A: Since our research group focuses on food science and food technology, specifically in food matrices, such as wine and olive oil, we are currently developing selective MIPs to be used as SPE sorbents on the development of sample preparation methods that enable the isolation/pre‑concentration of pesticides residues from olive oil samples. This multidisciplinary work covers firstly the design, synthesis, and chemical characterization of the imprinting materials followed by evaluating their molecular recognition abilities and selectivity, as well as the implementation of the MISPE methodology to the olive oil matrices.^8–12

Owing to the complexity of this food matrix, namely the high fatty contents, this work is particularly challenging but the results are encouraging, enabling us to be optimistic about the possibility of evaluating pesticide residues with high selectivity and sensitivity in olive oils using the MISPE-based approach. The development of an MISPE methodology for the extraction of the latest generation of pesticides from olive oil and, subsequently, quantification of pesticide residues in this food matrix represents a versatile and promising approach that is being explored by our research team.

The implemented sample preparation method enables the trace analysis of pesticide residues, bringing new improvements for the analytical methods already in use in terms of cleanness of the extracts, thereby enabling the achievement of lower limits of detection (LOD) and limits of quantification (LOQ). Moreover, the regulations are increasingly stringent both in terms of the number of compounds to trace and low maximum residue limits (MRLs) established. This scenario points to the need for sensitive, selective, and effective techniques for pesticide testing in olive oil. The work developed by our research team fits these requirements, highlighting the huge potential of MISPE-based sample preparation methods for the trace analysis of pesticide residues in samples with high complexity.

Q. What is the future of MIPs in separation science?

A: The huge potential of MIPs in separation science are a result of their high affinity and selectivity for the target molecule. The development of MIP-based products with application in separation science demands further progress on the understanding of molecular imprinting technology by means of a molecular modelling approach for the design and efficient selection of new functional and cross-linker monomers. These new findings complemented by the advances of related techniques such as nanotechnology, microfabrication technology, support materials for surface-imprinted beads, membranes, and sensing transducer elements will contribute to the development of more advanced and functional materials for separation science applications.

Acknowledgements

FEDER, COMPETE and FCT (Foundation for Science and Technology) under the project PTDC/AGR-ALI/117544/2010 and UID/AGR/00115/2013.

References

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Raquel Garcia received her degree in Chemistry and her PhD in Chemistry from the University of Lisbon, Portugal, in 1998 and 2006, respectively. She is currently a postdoctoral fellow at REQUIMTE (Rede de Química e Tecnologia) - New University of Lisbon and ICAAM (Instituto de Ciências Agrárias e Ambientais Mediterrânicas) - University of Évora, Portugal. Her research interests are focused in food science applied to olive oil and wine matrices, encompassing the study of nutritional benefits and adulteration. Her main research activity is focused on the development of more advanced materials for food analysis based on molecular imprinting technology.

Maria João Cabrita studied Food Science at the Technical University of Lisbon where she obtained her degree in 1991 and her MSc in 1994. Her PhD was obtained at Évora University (ÉU) in 2005, and in 2016 she obtained the habilitation degree. She is now Assistant Professor at ÉU in Crop Science Department, researcher at ICAAM (Instituto de Ciências Agrárias e Ambientais Mediterrânicas), and head of the Laboratory of Enology at ÉU. She is involved in three main research areas: wine technology involving wine and wood chemistry; geographic and varietal origin of olive oils; and the development of molecularly imprinting polymers for pesticide residues in olive oil.