Elsevier

Analytica Chimica Acta

Volume 717, 2 March 2012, Pages 7-20
Analytica Chimica Acta

Review
Photopolymerization and photostructuring of molecularly imprinted polymers for sensor applications—A review

https://doi.org/10.1016/j.aca.2011.12.026Get rights and content

Abstract

Biosensors are already well established in modern analytical chemistry, and have become important tools for clinical diagnostics, environmental analysis, production monitoring, drug detection or screening. They are based on the specific molecular recognition of a target molecule by a biological receptor such as an antibody or an enzyme. Synthetic biomimetic receptors like molecularly imprinted polymers (MIPs) have been shown to be a potential alternative to biomolecules as recognition element for biosensing. Produced by a templating process at the molecular level, MIPs are capable of recognizing and binding target molecules with similar specificity and selectivity to their natural analogues. One of the main challenges in MIP sensor development is the miniaturization of MIP structures and their interfacing with the transducer or with a microchip. Photostructuring appears thereby as one of the most suitable methods for patterning MIPs at the micro and nano scale, directly on the transducer surface. In the present review, a general overview on MIPs in biosensing applications is given, and the photopolymerization and photopatterning of MIPs are particularly described.

Highlights

Molecularly imprinted polymers (MIPs) are synthetic receptors for sensors and biochips. ► The photopolymerization of MIPs allows for the spatially resolved synthesis of micro and nanostructures. ► Using photopolymerization MIPs can be easily patterned at transducer surfaces.

Introduction

Biological recognition elements such as enzymes or antibodies have been widely used over the years for the development of biosensors. However, although they are perfectly fitting with their natural targets, biological receptors are unstable when not in their native environment, they are not always easy to obtain for a given target, and can be difficult to fine-tune for a specific application. In this context, researchers have developed synthetic tailor-made receptors capable of selectively recognizing and binding target molecules with high affinity, but that are at the same time more stable, easier to produce, available at low costs, and easy to integrate into standard industrial fabrication processes. One of the most straightforward strategies to create such artificial receptors is molecular imprinting of synthetic polymers [1], [2], [3], [4], [5]. This technique is based on the co-polymerization of functional monomers and cross-linking monomers in the presence of an imprint molecule, also called the molecular template (the target molecule or a derivative thereof). Initially, the template and functional monomers form a complex. After co-polymerization the functional groups are “frozen” in a specific position by the cross-linked polymeric network. Subsequent removal of the template molecules leads to empty cavities in the polymer structure, which are complementary in size, shape and positioning of chemical groups to the template (Fig. 1). The molecularly imprinted polymer (MIP) thus has a molecular memory and is now able to specifically recognize and bind the target molecule. Polymethacrylate or polyvinyl type polymers obtained by free radical polymerization are most often used as matrix for molecular imprinting, although other organic polymers, and also sol–gel materials, are becoming increasingly popular.

Two approaches were developed for imprinting synthetic polymers at the molecular level, differing by the nature of the bonds formed between template and functional monomers prior to polymerization. The first procedure was introduced by Wulff in 1972 and consists in covalent coupling the monomers to the template prior to polymerization [3]. As an alternative, Mosbach and co-worker developed a non-covalent approach, in which the functional monomers form a complex with the target molecules by self-assembly [4]. A combination of both covalent and non-covalent methods can sometimes also be used [6]. Despite the fact that the use of weak non-covalent bonds results in association–dissociation equilibria with the template, yielding a heterogeneous population of binding sites in terms of the positioning of the binding groups, this strategy allows for a much larger choice of functional monomers and can be adapted to a wide range of templates. It is more similar to biological recognition processes since biomolecular interactions are most often of the non-covalent type. Being more straightforward in practice, non-covalent imprinting is by far the most widely used method for MIP fabrication. Molecular imprinting of synthetic polymers can be applied to a wide range of target molecules ranging from small organic molecules (pharmaceuticals, steroids, sugars, amino-acids, etc.) [7], [8], [9], [10] to peptides [11], [12], [13], and proteins [14], [15], [16]. However, imprinting macromolecules, such as proteins, in a synthetic matrix is not easy and still remains one of the major challenges in the area. MIPs are employed in a broad range of domains such as solid-phase extraction (SPE) [17], [18], controlled drug delivery [19], [20], affinity separation [21], immunoassays [22], chemical sensors [23], directed synthesis and catalysis [24], [25], [26] and others. While the use of MIPs as antibody mimics in immunoassays, thus as a diagnostic tool, has been proposed by Mosbach as early as 1993 [27], their application as therapeutics had never been shown. However, the group of Shea recently published outstanding results about the potential use of MIPs as therapeutic antibody mimics [12]. They demonstrated MIP nanoparticles to be capable of efficiently binding and neutralizing the cytotoxic peptide melittin in the bloodstream of mice, which is the first example of biocompatible MIPs for in vivo applications [28]. Similarly, in our group we have recently shown that MIP nanogels can be used as specific enzyme inhibitors [16], and thus are potential drug candidates.

Although affinity separation remains the most important application for molecularly imprinted synthetic polymers from a commercial point of view, the development of MIP-based sensors and biochips has raised increasing interest during the past decade [29]. This is due to the growing demand in clinical diagnostics, food analysis, environmental pollution measurements, production monitoring, or drug detection [23], [30].

In biosensors, upon binding of the analyte molecule to the recognition element, a chemical or physical signal is generated. The transducer, which is in close contact with the recognition element, will then transform this signal into a measurable output signal that can be correlated with the analyte concentration (Fig. 2). Biosensors are based on biological receptors (e.g. enzymes, DNA, antibodies) that are in most cases immobilized on the surface of the transducer [31], [32]. Although they exhibit high molecular affinity for the template, their use is limited because of their high fabrication costs and limited stability (pH, temperature, ionic strength, organic solvents and other additives), resulting in difficult handling and storage. Researchers are therefore trying to develop stable and low cost biomimetic synthetic receptors, such as molecularly imprinted polymers, which can be implemented as recognition elements in biosensors and biochips as substitutes of natural receptors. MIPs were combined to different types of transducers. Electrochemical sensors can translate electrochemical reactions of the analyte, or differences of the electrochemical properties of the system in the presence of the analyte, into an electrical signal [33]. Acoustic sensors can detect changes in the propagation velocity or in the frequency of an acoustic wave upon analyte binding [34], and calorimetric sensors measure the heat released upon a chemical reaction or a recognition phenomenon in which the analyte is involved [35]. Optical detection techniques, for example fluorescence or luminescence, were also used with MIPs for sensing applications [36], [37]. A recent trend goes toward the development of label-free optical sensors, in which the detection can be performed without the need of a specific property of the analyte, and without the use of labels. Changes in more general properties of the MIP like absorption, reflectivity, or refractive index can be used for analyte detection and quantification if they can be translated into a suitable output signal. A nice example is the work published by Wu et al. in 2008 who developed a label-free optical sensor based on molecularly imprinted photonic polymers [38]. They were able to detect traces of the herbicide atrazine at low concentrations in aqueous solution using a 3D-ordered interconnected macroporous inverse polymer opal. Atrazine adsorption into the binding sites resulted in a change in Bragg diffraction of the polymer characterized by a color modification (Fig. 3). Since this color change was visible with the naked eye, this is in fact an example where the recognition element acts at the same time as the transducer. Other examples of label-free optical sensors with MIPs are based on surface plasmon resonance [39], or on reflectometric interferometry [40].

The development of highly sensitive MIP-based sensors normally requires a perfect interfacing between the recognition element and the transducer. For that, either a preformed MIP is coated onto the surface of the transducer [40], [41], [42], [43], or the MIP is polymerized in situ on the surface of the transducer [40], [44]. One of the advantages of photopolymerization is the versatility of this method for spatially controlled in situ polymerization. For example, molecularly imprinted polymer thin films can be fabricated with good control of both film thickness (down to the nanometer range) and inner morphology (porosity, optical properties), and have found applications in the sensor area [45], [46]. Standard microfabrication techniques such as spin-coating have been used for the deposition of pre-polymerization formulations that after photopolymerization yield porous thin molecularly imprinted films [44], [47]. Another example is the generation of nanostructured films by nanomolding the photopolymerized MIP on a porous alumina substrate. This yields layers of parallel MIP nanofilaments of 50–200 nm in diameter and 0.5–5 μm in length [48]. Thin films can also be the starting point for the patterning of MIPs on surfaces, for example for the development of biochips. In fact, light can be used not only to initiate polymerization, but also to structure MIP films. During the past few years, polymer photostructuring resulted in a number of innovations and advances in the field of MIP-based sensor development. A number of more or less standard approaches and techniques have been adapted to MIP fabrication, as outlined in the following part of this review.

Section snippets

Photoinduced free-radical polymerization

As mentioned earlier, the vast majority of MIPs is synthesized by free radical polymerization (FRP). Highly flexible in terms of reaction conditions, FRP can be initiated either thermally or photochemically, and thus be carried out in a wide range of temperatures. It can be performed in bulk format or in solution, and is not water sensitive. Moreover, a large number of vinyl-based monomers are commercially available, carrying different types of functional groups. This makes FRP particularly

Photostructuring of molecularly imprinted polymers

For the development of sensors and biochips based on MIPs as recognition element, interfacing of the polymer with a substrate is required, often in the form of a pattern, which usually implies a resolution between a few micrometers and tens of nanometers. To achieve soft matter structuring at the micro scale, the different approaches commonly used can be classified in two groups, contact or non-contact techniques. Among the contact approaches, soft lithography, also called nanoimprint

Conclusions

Photopolymerization and optical structuring of molecularly imprinted polymers are well adapted for the development of miniaturized MIP-based microchips and microsensors. Compared to other initiation modes, photoinduced reactions have been shown to be particularly well-suited for non-covalent imprinting of synthetic polymers, since polymerization can be carried out at low temperature, resulting in a better imprinting efficiency. MIP films patterning by the use of optical methods is relatively

Acknowledgments

The authors gratefully acknowledge the French National Research Agency (ANR – Project HOLOSENSE, ANR-08-BLAN-0236-02), and the Regional Council of Picardy (France) as well as the European Union for funding of equipment under CPER 2007-2013.

Yannick Fuchs studied polymer chemistry at the European Engineering School of Chemistry, Polymers and Materials Science (ECPM) in Strasbourg, France, where he graduated in 2009 with a degree in chemistry engineering. The same year, he received his M.S. degree in polymer sciences from the Strasbourg University. Yannick is currently a Ph.D. student at Compiègne University of Technology (UTC), in the group of Prof. Karsten Haupt, and his main research interests are molecularly imprinted polymers,

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    Yannick Fuchs studied polymer chemistry at the European Engineering School of Chemistry, Polymers and Materials Science (ECPM) in Strasbourg, France, where he graduated in 2009 with a degree in chemistry engineering. The same year, he received his M.S. degree in polymer sciences from the Strasbourg University. Yannick is currently a Ph.D. student at Compiègne University of Technology (UTC), in the group of Prof. Karsten Haupt, and his main research interests are molecularly imprinted polymers, material nanostructuring and chemical sensors. In 2011, Yannick received a Young Scientist Award from the European Material Research Society in recognition of his work for Bioinspired and Biointegrated Materials as New Frontiers Nanomaterials.

    Dr Olivier Soppera is graduated from Ecole Normale Supérieures Cachan (Aggrégation de chimie in 1998). He completed his PhD in Polymer and Photochemistry (Université de Haute-Alsace) in 2003 and then went to Porto University-Portugal for a post-doctoral position (European Marie Curie Grant). He joined CNRS in 2004 as a Senior Researcher and he is now at Institut de Sciences des Matériaux de Mulhouse (IS2M – CNRS LRC 7228). He is heading a team of 6 permanent people devoted to develop photomaterials for applications in optics and nanotechnologies. His current research activities are focused on photomaterials for micro and nano-fabrication for applications in optics, photonics and biology. In particular, he developed optical setup and suitable materials for photofabrication in the DUV and the NIR wavelengths. Olivier Soppera received the Médaille de Bronze du CNRS in 2009 for his research work.

    Karsten Haupt studied Biochemistry at the University of Leipzig, Germany, where he received an MSc Degree in 1991. In 1994 he obtained his PhD in Bioengineering from Compiègne University of Technology, France. After a one-year lectureship at Compiègne University, he spent three years as a research fellow at Lund University, Sweden, where he worked on molecular imprinting with Klaus Mosbach. After returning to France he spent one year as a researcher at INSERM, Paris, before joining the University of Paris 12 as an associate professor in 1999. In 2003 he was appointed full professor of Nanobiotechnology at Compiègne University of Technology, France, where he is currently the Head of the Enzyme and Cell Engineering Institute (GEC – CNRS UMR 6022). Professor Haupt is also one of the founders and scientific advisor of the French company PolyIntell that commercializes molecularly imprinted polymer-based products for biomedical, food and environmental analysis. His present research interests include affinity technology, chemical and biosensors, molecularly imprinted polymers and synthetic receptors, biomimetic polymers and nanostructured materials.

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