# Applications of molecularly imprinted polymers and perspectives for their use as food quality trackers

Laura Carballido, Thomas Karbowiak, Philippe Cayot, Massimiliano Gerometta, Nicolas Sok, Elias Bou-Maroun

► To cite this version:

Laura Carballido, Thomas Karbowiak, Philippe Cayot, Massimiliano Gerometta, Nicolas Sok, et al. Applications of molecularly imprinted polymers and perspectives for their use as food quality trackers. Chem, 2022, 8 (9), pp.2330-2341. 10.1016/j.chempr.2022.05.021. hal-03766398

HAL Id: hal-03766398

https://institut-agro-dijon.hal.science/hal-03766398v1

Submitted on 12 Jun 2025

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## Review

# Applications of molecularly imprinted polymers and perspectives for their use as food quality trackers

Laura Carballido,¹ Thomas Karbowiak,¹,* Philippe Cayot,¹ Massimiliano Gerometta,¹ Nicolas Sok,¹ and Elias Bou-Maroun¹

## SUMMARY

Molecularly imprinted polymers (MIPs), often referred to as "artificial" or "plastic" antibodies, are synthetic alternatives to natural receptors. Based on the "key-and-lock" principle, a template is imprinted into a polymer, which is then able to selectively bind to a target molecule. Over the past 50 years, MIPs have continued to attract attention in both academic research and commercial applications. MIPs have a wide range of applications in analytical chemistry, medicine, and pharmaceutics, as well as in the environment. This paper highlights the new perspectives offered by molecularly imprinting technology to food science. Food analysis is challenging because of the complexity of food matrices and the low concentration of analytes. MIPs are ideally suited to overcome both of these issues because of their high selectivity and sensitivity. Furthermore, MIPs could greatly benefit the entire food chain as a tool to ensure food safety, quality, and traceability, while reducing food waste.

## INTRODUCTION

The greatest challenge facing the food industry today is to promote the accessibility of affordable food while ensuring product safety, traceability, and nutritional quality—aiming to guarantee a balanced diet for all. Although food safety is a major concern for the agro-food industry, health scandals are recurrent, and they affect all types of products. A few examples of the scandals that have shaken the food industry in recent decades include the following: Minamata disease caused by mercury-infected fish in the 1950s, mad cow disease caused by the neurodegenerative Creutzfeldt-Jakob disorder in 1986 and 1996, bird flu in 2004, melamine found in milk in 2008, and the insecticide fipronil identified in eggs in 2017. The agro-food sector is always looking for better analytical tools to ensure the safety of its products.¹ In addition, reducing food waste remains one of today's greatest challenges to be overcome.

Food waste can be defined as food originally produced for human consumption that is lost at some point in the food chain.² According to the United Nations Food and Agriculture Organization (FAO), 1.3 billion tons of food is wasted each year worldwide; this represents one-third of the global production. Food waste is an environmental, economic, and a social issue. In addition to the consumption of natural resources for food production, manufacturing, packaging, and transport, the carbon footprint of food waste is as high as that of the third most polluting country in the world.³ The FAO estimates the economic loss of food waste at $750 billion per year. According to a 2019 study by the United Nations World

## THE BIGGER PICTURE

Challenges and opportunities:

* Detection of viruses or allergens in food products by imprinting proteins or their epitopes for the design of new MIPs

* Compliance with stringent regulations regarding food contact materials for commercial development of simple-to-use and portable MIP-based sensors for real-time and on-site detection and quantification of quality markers in food products. Example: design of intelligent packaging based on MIPs to monitor the dynamic shelf life of food

* Development of MIP-based solutions to simultaneously detect a variety of targets with high selectivity and sensitivity to provide a global contamination index of food products
Food Program, more than 800 million people, i.e., 1 in 9 people worldwide, suffer from hunger today.⁴ The need to feed more than 9 billion people by 2050 is forcing the food industry to find sustainable methods of production and consumption. Maximizing product shelf life and reducing waste are thus top priorities. Food waste occurs throughout the food chain from the producers to consumers. In developing countries, most losses occur at the beginning of the food chain because of storage and distribution limitations, whereas in medium- and high-income countries most food is wasted at the end of the food chain by the consumer because of high quality standards and health precautions.² The agro-food sector could thus greatly benefit from a reliable and easy-to-use tool capable of monitoring food quality all along the chain to ensure food safety and traceability and increase product shelf life while reducing waste. Hence, molecularly imprinted polymers (MIPs) could be a solution of choice.

MIPs AND THEIR APPLICATIONS

MIPs are synthetic alternatives to natural receptors. Based on the "key-and-lock" principle, a template, which is the molecule targeted or a structural analog, is imprinted into a polymer. In the three-step process for the synthesis of MIPs (Figure 1), a host-guest complex is first formed between the template and the monomer(s). Then, the cross-linker is copolymerized with the monomer around the previously formed complex. The final step is the removal of the template by solvent extraction. At the end of the process, the polymer material is able to bind to the target in a selective manner. As shown in Figure 1, the selectivity is the ability of the MIP to bind only to the target molecule compared with other compounds with a similar chemical structure.⁵ MIPs have functional groups located in cavities, offering a unique chemical and steric complementarity with the target molecule. Their synthesis can be conducted by either the so-called covalent or non-covalent approaches. This refers to either the chemical bonds or physical interactions governing the formation of the template-monomer complex prior to polymerization, which must be stable enough to withstand the polymerization step, but easily cleavable afterward. In the "cova-lent" approach,⁶⁻⁸ the template is chemically bound to the monomer(s); this offers the benefit of providing homogeneous binding cavities and strong chemical and physical stability for the synthesis. However, template removal occurs under drastic conditions, and the kinetics of target binding and release are relatively slow. In the widely used "non-covalent" approach,⁹ the template and monomer physically interact through attractive forces, such as hydrogen bonding, π-π stacking, or van der Waals forces. This route is easy to implement, and the template can be removed under mild conditions. Binding and release kinetics are fast. Finally, the "semi-cova-lent approach"¹⁰ integrates the two previously mentioned principles. Imprinting is based on chemical bonds, whereas binding to the target relies on physical interac-tions. This approach combines the respective advantages of the two previously mentioned ones.

Following the imprinting, polymerization can be achieved either by radical polymer-
ization or by a sol-gel process. The radical polymerization route requires a template,
one or more organic or inorganic functional monomers, a cross-linker, a polymeriza-
tion initiator, and a solvent. The solvent plays two major roles: solubilization of the
reactants and pore formation within the structure of the polymer material. Radical
polymerization is usually performed either thermally or photo-initiated. There are
different types of radical polymerization leading to polymers of various morphol-
ogies and performances. Bulk polymerization is the oldest and the simplest. The re-
sulting block polymers must be ground and sieved prior to use, affording particles of
irregular size and shape. Mass transfer is relatively slow because some binding

<footnote n="1">¹Univ. Bourgogne Franche-Comté, Institut Agro,
PAM UMR 02 102, 1 Esplanade Erasme, 21000
Dijon, France</footnote>

<footnote>*Correspondence:
thomas.karbowiak@agrosupdijon.fr
https://doi.org/10.1016/j.chempr.2022.05.021</footnote>
A Key-and-lock principle

<figure><img src="image_2.png" /></figure>

B

<figure><img src="image_3.png" /><figcaption>Figure 1. Schematic illustration of the synthesis route and operating principle of MIPs</figcaption></figure>

(A) Key-and-lock principle for the synthesis of a MIP.

(B) MIP selectivity is defined by the ability of the MIP to bind only with the target molecule compared with other compounds of a similar chemical structure. Binding cavities in the polymer provide chemical and steric complementarity with the target.

cavities remain trapped within the structure. Precipitation polymerization requires large amounts of solvent in which the growing polymer is insoluble and gradually precipitates. ¹¹ The resulting monodispersed spherical particles (ranging in size from nanometers to millimeters¹²,¹³) show better mass transfer properties due to the increase in the specific surface area. Although more difficult to realize, suspension and emulsion polymerizations lead to similar particles. *In situ* polymerization, e.g., electropolymerization,¹⁴ allows the formation of a MIP as a thin layer on the outer surface of a material by grafting. MIPs can also be obtained as a stand-alone film. Hence, the polymerization process strongly influences the morphology of the MIP and thus its final performance.

The sol-gel route, first used in the 1930s¹⁵ and developed in the 1950s,¹⁶⁻¹⁹ relies on the generation of a sol, a colloidal fluid containing the template and inorganic monomers, which is then transformed into a gel. Molecularly imprinted silica obtained by the sol-gel route shows better physical stability and higher selectivity, compared with a MIP synthesized by radical polymerization.²⁰ The reactants used are also less toxic.²¹ However, the sol-gel synthesis is conducted in an aqueous medium, which limits the choice of reactants, due to solubility limitations. This makes the sol-gel route challenging, hence less explored.

Among the synthesis parameters, the molar ratio between the different reactants is the most critical parameter for the polymer material's performance because it influences its morphology and its selectivity as well as the number of binding cavities and their
<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /><figcaption>Figure 2. Overview of the application areas with their related fields of science covered by developments based on MIPs</figcaption></figure>

accessibility. ¹¹ Other synthesis parameters are also crucial and hence require optimiza-
tion. For example, the selectivity of MIPs is higher when using UV polymerization
compared with thermally initiated polymerization. ²² The selectivity of MIPs is also
higher when binding experiments are conducted in the same solvent as that used for
the synthesis, ²³ a phenomenon known as the “memory effect” of the solvent.

Since the first publication by Wulff and Sarhan in 1972,²² MIPs have attracted a great
deal of attention within the scientific community,²⁴ leading to the development of appli-
cations in many fields from analytical chemistry to medicine and pharmaceutics, as well
as in the environment and food science, as displayed in Figure 2. Molecular recognition
is one of the most important features of natural receptors because it underlies many bio-
logical processes. Similar to antibody-antigen, enzyme-substrate, and hormone-recep-
tor pairs, MIPs can distinguish chiral molecules and were first used for this purpose.²⁵
This field of research is still active²⁶ because this property is very important in the phar-
maceutical and flavor industries. MIPs have the advantage over their biological analogs
<figure><img src="image_6.png" /></figure>

of being easy to synthesize at low cost. Furthermore, they benefit from being more sta-
ble over a wide variety of physical and chemical conditions. Therefore, MIPs are used as
biomimics in enzyme-linked immunosorbent assay<sup>27</sup> tests to investigate the mechanisms
occurring in natural systems,<sup>27</sup> as well as for bioimaging,<sup>28</sup> cell culture,<sup>29</sup> drug develop-
ment,<sup>30</sup> and cancer diagnosis<sup>31</sup> and therapy.<sup>32</sup> In addition, based on stimuli-responsive
MIPs, which have a switchable binding and release capacity in response to an external
physical or chemical stimulus (e.g., pH, temperature, light, and electric fields),<sup>33</sup> drug de-
livery can be optimized. Lastly, their reusability also makes MIPs particularly interesting
for catalysis.<sup>34</sup>

MIPs are also widely used as sensors. MIP sensors possess a transducer capable of translating the target-MIP interaction into a measurable signal. Here, the main categories include optical, electrochemical, mass-sensitive, thermal, and acoustic sensors. MIP sensors combine the advantages of their two components: the MIP recognition unit and the signal transducer. This leads to technologies with high stability, sensitivity, selectivity, fast detection, and miniaturization capacities.35 Optical sensors are based on chemiluminescence, fluorescence, surface plasmon resonance, surface-enhanced Raman scattering, or resonance light scattering. Electrochemical sensors rely on potentiometry, amperometry, conductometry, impedimetry, or voltammetry measurements. Recently, the first combination of MIPs with microfluidic devices was even proposed.36 Thus, the development of MIP-based sensors has increasingly been pursued over the last 10 years, a trend that is likely to continue.

In analytical chemistry, MIPs are used as stationary phase in flow analytical tech-
niques, including chromatography and electrophoresis. Sample preparation is a crit-
ical step to extract the analyte of interest from the matrix prior to analysis. However,
the process usually suffers from two major limitations: the lack of selectivity of the
sorbent (the so-called matrix effect) and the trace level of analytes. MIPs allow the
selective separation of the analyte from complex matrices and sensitive determina-
tion of its concentration.37 MIPs have been mainly designed for sample preparation
via solid-phase extraction (SPE), solid-phase microextraction (SPME), dispersive SPE
(d-SPE), magnetic SPE (MSPE), and stir bar sorptive extraction (SBSE).

EMERGING TRENDS IN THE USE OF MIPs TO ADDRESS FOOD ISSUES

The analysis of food samples is particularly challenging due to the complexity of the
matrices and the low concentration of analytes. 35 The presence of contaminants in
food products is usually determined by liquid or gas chromatography coupled
with mass spectroscopy, which requires the use of expensive facilities by highly
trained professionals as well as time-consuming and complex sample preparation.
MIPs have the advantage of being easily synthesized at low cost, and they are simple
to use. MIPs are synthetic receptors with high selectivity, and they have been shown
to be very sensitive materials as detection limits can be as low as the trace level
(down to ng/L levels). 38 Furthermore, they can also be tailor-made for any target
molecule and are chemically and physically very stable, which opens up a wide range
of applications. Moreover, MIP-based sensors tend to be portable, which facilitates
their rapid application in any product.

As shown in Figure 3, MIPs can be used as a powerful analytical tool for quality con-
trol in food science because they allow the detection, quantification, and even
extraction of many target compounds along the food chain, from agro-resources
to wastes. The targets can be the contaminants present in water and soil, such as
antibiotic residues<sup>39</sup> (e.g., chloramphenicol and tetracycline) and heavy metals<sup>40</sup>
<figure><img src="image_7.png" /><figcaption>**Figure 3. Examples of MIP targets along the food chain, from agro-resources to waste**</figcaption></figure>

MIPs can be used as tunable quality trackers for detection, quantification, and extraction of various contaminants along the food chain.

(Color: the part of the food chain involving MIP applications; bold: family of chemical compounds identified as MIP targets; italics: selected examples of MIP targets.)

(e.g., arsenic and mercury). MIPs can also be designed for the decontamination of food and beverages at each step of the food chain from production to consumption. Various molecules can be targeted, such as pesticides,⁴¹ like herbicides (e.g., atrazine and glyphosate) and insecticides (e.g., λ-cyhalothrin and diazinon); adulterants⁴² (e.g., melamine and ractopamine); mycotoxins⁴³ (e.g., aflatoxin B1 and ochratoxin A); substances migrating from packaging⁴⁴ (e.g., acrylamide and bisphenol A); allergens⁴⁵ (e.g., α-casein and genistein, an isoflavone from soy); and even food spoilage substances that can be dangerous in the event of human consumption (e.g., histamine).⁴⁶ Finally, MIPs can be used as analytical or treatment tools during the waste management process, such as for the extraction of dye residues⁴⁰ (e.g., Orange II, Sudan I) from food packaging to favor their recyclability.

Hence, opportunities are constantly emerging, leading to the development of new applications of MIPs in food science. MIP-based sensors are particularly interesting to ensure food safety and reduce food waste. In terms of food safety, MIPs could
indeed be interesting tools to fight food adulteration, prevent allergic reactions, ensure sustainable production methods, avoid antibiotic resistance, and protect health. As an example, to fight against food adulteration, a potentiometric sensor based on MIPs has been developed for the rapid detection of melamine in milk in as short a time as 16 s.⁴² In comparison, a classical chromatographic analysis takes several minutes, not including the time required for sample preparation. To prevent allergic reactions, an electrochemical MIP-based sensor has recently been proposed to selectively track the presence of the genistein isoflavone, a molecule identified as a marker for soy allergens.⁴⁵ This sensor allows in-field detection, and it has been tested on a selection of dishes offered by a restaurant. The results obtained within 5 min showed excellent selectivity. Even if the detection limit remains higher than that achievable by mass spectroscopy, it is still 100 times lower than the allergic threshold. In addition to the restrictions on pesticide use imposed by regulations, the increasingly systematic detection of pesticide residues in food products forces the producers to shift toward sustainable production methods to protect nature's biodiversity. A recently reported portable MIP-based electrochemical sensor allows real-time detection and quantification of glyphosate in beverages with a limit of detection of 2 µg/L (or 10 nM).⁴¹ The portability of such a sensor, a feature that cannot be offered by any currently used analytical tool, extends its applicability. However, to date, its efficiency has only been proven in orange juice and rice beverages spiked with glyphosate; it has not yet been tested in complex solid food matrices. To avoid antibiotic resistance, a sensor combining magnetic MIPs and surface plasmon resonance has been developed to detect tetracycline down to 1 ng/L (or 2 pM) in milk.³⁹ This detection limit is comparable to that obtained with chromatographic devices.⁴⁷ However, this sensor was not tested in contaminated milk but only in tetracycline-spiked milk. Such sensors, if made available to retailers and distributors, should assist in preventing the overuse, by farmers, of antibiotics in animal feeding. At the consumer level, to protect health by warning of mycotoxin contaminations, a MIP-based sensor has recently been developed to detect *Fusarium* in cereals using a smartphone,⁴³ therefore making it usable at home.

MIPs could also be reliable tools to reduce food waste, either by extending or by moni-
toring the shelf life of products. The decrease in organoleptic properties and nutritional
value as well as the potential generation of compounds that are toxic to humans during
food degradation processes are the main reasons for the need to monitor product qual-
ity throughout the whole food chain.⁴⁸ To address these food-related issues, active and
intelligent materials are being considered in recent and promising improvements in
food preservation. On the one hand, active materials can interact with food or its envi-
ronment through a controlled sorption or release of compounds to extend the shelf life
of a product (EC 450/2009). Such active materials could be advantageously used when
packaging with optimal barrier properties or modified atmosphere packaging are not
sufficient. A few MIPs have already been designed with preservatives that are gradually
released into the food to prevent spoilage, such as ferulic acid as a potent antioxidant to
prevent lipid oxidation⁴⁹ or allyl isothiocyanate as an antibacterial agent to preserve
meat.⁵⁰ On the other hand, intelligent materials can monitor food or its environment
and provide information to the user (EC 450/2009). Such intelligent packaging could
be of great value to monitor product quality throughout the food chain by indicating
the dynamic shelf life of the product to the retailer or consumer. The field of intelligent
packaging has attracted increasing interest in recent years with the development and
commercialization of time-temperature indicators, leakage indicators, cold chain break
indicators, or radio frequency identification tags. However, none of these innovations
exploit the analytical benefits of MIPs. Photoresponsive and fluorescent MIPs are ideally
suited for the design of intelligent packaging. These MIPs are obtained via monomers
possessing photosensitive chromophores (e.g., azobenzene, spiropyran) or fluoro-
phores (e.g., fluorescein, rhodamine), allowing the optical properties of the material
to be modified once the MIP-target interaction occurs. Obviously, the compromise be-
tween the benefits brought by the addition of such MIP-based sensors and their contri-
bution to overpackaging has to be carefully considered, as well as the end-of-life man-
agement of the material and its recycling.

TRACKING FOOD QUALITY WITH MIPs: CHALLENGES AND
SOLUTIONS

Although much research has been carried out in the field of MIPs over the last 50 years,
several technical and application aspects remain challenging, particularly regarding
their technological transfer to use them as food quality trackers, as shown in Table 1.

Technical challenges that must be overcome concern both the synthesis and the use of MIPs. The limited number of suitable commercially available monomers (especially al-koxysilane derivatives) restricts the number of MIPs that can be synthesized and hence the number of possible applications to track any target compound in food matrices. The organic synthesis of tailor-made monomers is an alternative, but this is often costly and time-consuming. Generally, synthesis optimization is a long process, particularly as there are many parameters that strongly influence MIP performance. To reduce the experimental time, scientists can rely on computer simulations⁵¹ and experimental design⁵² to support their choices in terms of reactants and synthesis parameters. The use of automatic reactors for the synthesis and testing of MIPs can also contribute to reducing experimental time.⁵³ Complete removal of the template at the final stage of MIP synthesis is also difficult to achieve. If it is not done properly, it will result in inac-cessible cavities and potential leakage of the template when using the MIP, which can thus compromise the results of binding experiments or the compliance as a food con-tact material. The use of a dummy template, which is a structural analog of the target compound, may solve the problem caused by template leakage. It can also be worth-while when the template to imprint is expensive or even dangerous, such as in the case of some contaminants of food products such as mycotoxins.⁵⁴ When the structure and/ or the conformation of the template can be modified due to radicals generated during radical polymerization, an altered molecule could be imprinted. Yet, controlling the exact imprinting of the conformation of a protein is essential when it is responsible for a food allergy. In this case, the sol-gel polymerization can be an alternative route. Another frequently reported problem is the binding of the target outside the cavities. In imprinted polymers, the cavities responsible for steric complementarity between the target and the polymer contain appropriately oriented binding sites that can interact chemically and physically with the target compound. However, some binding sites, such as functional groups from the monomer, can be randomly distributed in the polymer and can be responsible for physical interactions with the target outside the cavities. The cross-linker can also sometimes physically interact with the target through attractive forces and be responsible for binding the target outside the cav-ities. Optimization of the selectivity of MIPs is crucial to improve their performance, especially for their use as sensors in complex matrices such as food. Such optimization would, moreover, reduce the cost of resulting MIP-based sensors for future commer-cial applications. Of interest is the fact that it has already been proven that molecularly imprinted silicas are less affected by these unintended interactions,²⁰ mainly because the use of a mineral cross-linker limits the interactions with organic targets.

Reproducibility of the polymer synthesis, lack of homogeneity of the polymer, and
surface immobilization of the polymer material are major problems that hinder the
<table><caption>Table 1. Current challenges and potential solutions for using MIPs to track the quality of food</caption><thead><tr><th>Challenges</th><th>Potential solutions</th><th>Interest for food</th></tr></thead><tbody><tr><td><strong>Design</strong></td><td></td><td></td></tr><tr><td>Commercial availability of monomers</td><td>synthesis of monomers</td><td>- controlled toxicity of the MIP<br>- extension of targeted molecules</td></tr><tr><td>Optimization of synthesis parameters</td><td>- computational modeling<sup>51</sup><br>- experimental design<sup>52</sup><br>- use of automatic reactor<sup>53</sup></td><td>time saving</td></tr><tr><td>Complete template removal</td><td>- imprinting of a structural analog of the template<sup>54</sup></td><td>avoiding the migration of a toxic compound</td></tr><tr><td>Template leakage</td><td>- solid-phase synthesis<sup>55</sup></td><td>in contact with a food product</td></tr><tr><td>Imprinting of toxic/expensive template</td><td>imprinting of a structural analog of the template<sup>54</sup></td><td>imprinting of mycotoxins</td></tr><tr><td>Template degradation or isomerization<br>caused by radical polymerization</td><td>- imprinting of a structural analog of the template<sup>54</sup><br>- sol-gel synthesis to avoid radical polymerization<sup>19</sup></td><td>imprinting of proteins, antioxidants, vitamins</td></tr><tr><td>Accessibility of binding cavities</td><td>- core-shell particles<sup>56</sup><br>- solid-phase synthesis<sup>55</sup></td><td>improvement of the MIP performance</td></tr><tr><td><strong>Technology transfer</strong></td><td></td><td></td></tr><tr><td>Reproducibility of the synthesis</td><td>standardization of the synthesis method<sup>57</sup></td><td>development of simple-to-use and portable<br>sensors for real-time and on-site detection,<br>and quantification of target compounds in<br>food products</td></tr><tr><td>Lack of homogeneity of the MIP</td><td>- core-shell particles<sup>56</sup><br>- solid-phase synthesis<sup>55</sup></td><td></td></tr><tr><td>Surface immobilization of the MIP</td><td>- MIP films<br>- coating<br>- surface grafting<br>- electropolymerization<sup>13</sup></td><td></td></tr><tr><td><strong>Applications</strong></td><td></td><td></td></tr><tr><td>MIP synthesis and use in aqueous phase</td><td>- use of water-soluble monomers<sup>58</sup><br>- hydrophilic coating of the MIP<sup>59</sup></td><td>use of MIPs in beverages or in humid foods</td></tr><tr><td>Imprinting of proteins</td><td>epitope imprinting<sup>60</sup></td><td>allergen and virus tracking</td></tr><tr><td>Compliance as food contact material</td><td>- choice of low toxicity monomers<br>- use of silica polymers<sup>21</sup><br>- green chemistry approach: synthesis in food-grade<br>solvents or supercritical fluids<sup>61,62</sup></td><td>strong regulations for food contact</td></tr><tr><td>Simultaneous detection of various targets</td><td>- imprinting of a common structure of the targets<sup>63</sup><br>- mixing several MIPs<br>- imprinting of several targets in a single MIP<sup>64</sup></td><td>global contamination index of a food product<br>(e.g., pesticides, antibiotics)</td></tr></tbody></table>

technological transfer and development of commercial applications of MIP-based sensors. First, reproducibility of the synthesis is challenging due to the numerous parameters that influence the MIP performance and the resulting difficulty in controlling them all perfectly. A standardization of the synthesis protocol is then required.⁵⁷ Second, a lack of homogeneity of the MIP is usually due to a heterogeneous distribution of the binding cavities within the polymer. Such homogeneity is rather difficult to control during synthesis, especially when bulk polymerization is used. Synthesis of core-shell particles allows for better control of the distribution and access to binding cavities as they are located in the outer layer of the MIP, which favors high specific surface area and mass transfer.⁵⁶ Solid-phase synthesis, which relies on polymerization around a template molecule immobilized by grafting onto a surface, also improves accessibility to the binding cavities, compared with polymers obtained by bulk polymerization.⁵⁵ Third, MIPs are often synthesized as powders, which raises the question of their immobilization to use them as sensors. A homogeneous distribution of a powder on the surface of a sensor is rather difficult to achieve. The synthesis of MIPs directly in the form of stand-alone films is an interesting outcome, but it is very challenging to achieve. Indeed, MIP films are brittle when dried and can only be used at high relative humidity. Coating (e.g., spin- or drop-coating) of a MIP solution onto a surface can also be considered. Surface grafting and electropolymerization offer an interesting solution to directly synthesize the MIP on the surface of a material. Addressing these various challenges is required prior to the market launch of simple-to-use and portable MIP-based sensors for real-time and on-site detection and quantification of target compounds in food products.
Concerning application challenges, the analysis of food products represents, in many respects, an underestimated complexity. MIPs as selective materials have still to be improved for their use in aqueous phase, their ability to bind high-molecular-weight molecules, their compliance as food contact materials, and their ability to simultaneously detect several targets. First, the synthesis and use of MIPs in aqueous phase remain challenging for three main reasons. For the synthesis, most reactants are insoluble in water. Moreover, water involves hydrogen bonds between the solvent and the reactants, which can reduce the interactions between the template and the functional monomers during the synthesis and, once polymerization has taken place, during binding experiments. Finally, the performance of MIPs is low in aqueous media because they are usually hydrophobic. Some proposals have been made to overcome this issue by selecting monomers that are water soluble<sup>58</sup> or using hydrophilic coatings on the surface of MIPs.<sup>59</sup> This challenge, which has been addressed by the scientific community for many years, is important to overcome when dealing with the use of MIPs in humid foods and beverages.

Second, imprinting high-molecular-weight molecules, such as proteins, is another
difficulty due to their conformational flexibility, fragile nature, and insolubility in
most organic solvents.⁶⁵ This is an important hurdle to overcome, for example,
when targeting food allergens. To deal with this challenge, the imprinting of an
epitope (a peptide located on the surface of the targeted protein) could be sufficient
to achieve selective binding of the whole protein.⁶⁰ This method could also be
extended to the imprinting of viruses or bacteria that alter food products. However,
the native structure and conformation of the protein surface are not always known,
and an isolation process could alter it.

Third, MIPs made for food contact applications must comply with the related regulations (EC 1935/2004). The toxicity of a polymer is generally related to the potential release of the functional monomer unit. Therefore, the latter has to be carefully chosen, and its toxicity must be assessed at the outset. Imprinted polymers obtained by the sol-gel process seem to be particularly indicated for food applications because of their lower toxicity compared with the more classical MIPs resulting from radical polymerization.²¹ Moreover, for the application of MIPs in food, a green chemistry approach is highly encouraged to replace organic solvents in the synthesis.⁶¹ The alternative use of supercritical CO₂ could also be a solution, albeit challenging.⁶²

Lastly, the high selectivity of MIPs is one of their greatest advantages, but this also
limits their application. Indeed, food applications often require the simultaneous
detection of several target compounds. For example, the wine industry is seeking
a tool for monitoring multiresidues of pesticides rather than a single molecule.⁶⁶
Three potential solutions can be investigated to address this limitation. The first
one is the synthesis of a single MIP capable of binding multiple targets. This is
possible if all targets have a similar chemical structure. In this case, a common
part of the targets can be selected as a template. This solution has already proven
to be successful in the detection of antibiotics from the amphenicol group in shrimp
samples.⁶³ The second one relies on mixing several MIPs selective for different tar-
gets. However, the difference in reactivity of the different MIPs and their response in
the presence of interferents must be carefully evaluated. The third attractive alterna-
tive based on the imprinting of several targets in a single MIP also requires some
technical issues to be addressed, such as the solubility of the targets in the same sol-
vent, the control of the distribution of binding cavities in the resulting polymer, and
the difference in reactivity between the MIP and the targets. Because of the various
difficulties, this route has been less explored.⁶⁴
To conclude, molecularly imprinting technology is still quite a young field, but it has already led to significant developments in many different fields. Extending its use to food analysis offers great benefits, but several challenges remain. There are exciting perspectives to be explored in the coming years.

## ACKNOWLEDGMENTS

This work was supported by the French "Investissements d'Avenir" program, project ISITE-BFC (contract ANR-15-IDEX-0003).

## AUTHOR CONTRIBUTIONS

E.B.-M. and T.K. conceptualized the topic. L.C. wrote the original draft. L.C., T.K., P.C., M.G., N.S., and E.B.-M. contributed to the reviews. L.C. and T.K. designed the figures, and M.G. drew them. T.K. handled the funding acquisition.

## DECLARATION OF INTERESTS

The authors have a patent related to this work (FR2113403).

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