Molecularly imprinted polymer sensor arrays
Introduction
The development of chemical sensors that can be tailored for specific analytes is important for many fields of study including environmental testing, chemical manufacturing, therapeutics, and organic synthesis [1, 2, 3]. The traditional approach to sensor design has been to develop individual sensing elements with high specificity for the analyte of interest. However, sensing is a demanding application analogous to finding a needle in a haystack. The analyte of interest is usually present at very low concentrations in a complex matrix of similar compounds. Thus, even sensors based on highly selective enzymes and antibodies are susceptible to cross-reactivity and false-positive responses [4]. Recently, an alternative strategy based on arrays of multiple sensing elements has been employed, which can identify analytes based on their unique response patterns. Examples of sensor arrays are found in biological systems, most notably the senses of smell and taste. Artificial sensor arrays have also been built that can identify explosives, floral aromas, and biogenic amines [5, 6]. The primary advantage of the array approach is that it can take individual sensing elements with poor selectivities and high cross-reactivities and produce sensors that have high levels of selectivity and discrimination. For example, early artificial sensor arrays were assembled from simple synthetic polymers that did not possess any inherent recognition abilities. Despite these limitations these polymer arrays were still effective sensors and were even commercialized as electronic noses or tongues [7].
More recently, sensor arrays based on synthetic receptors have yielded even higher levels of accuracy and have the advantage that they can be targeted to specific analytes [8, 9, 10]. However, a major challenge to the sensor array approach has been synthesizing sufficient numbers of recognition elements with unique selectivity patterns. The preparation of individual biological or synthetic molecular receptors is a time and resource intensive process and the synthesis of an array of receptors only multiplies the difficulty of this task. One solution is to use molecularly imprinted polymers (MIPs) as the recognition elements in sensor arrays. MIPs are crosslinked polymers that are formed in the presence of a template molecule [11, 12]. Removal of the template creates binding cavities with affinity and selectivity for the template molecule. This templated imprinting process enables the rapid preparation of an array of polymers with different binding selectivities via the use of different templates in the imprinting process (Figure 1). MIPs also possess attractive material properties. Like other synthetic polymers, MIPs can be rapidly prepared and possess excellent thermal, chemical, and mechanical stabilities [13]. Thus, the use of MIPs in sensor arrays has the potential to greatly speed up the development process. MIP sensor arrays also have the advantage of being able to be tailored to specific analytes via the molecular imprinting process. This review will examine the advantages and challenges of building sensor arrays from MIPs.
Section snippets
Introduction to MIPs
The enantioselective l-phenylalanine anilide (l-PAA) imprinted polymer shown in Figure 1 will be used to introduce the molecular imprinting process [14, 15]. The primary advantage of MIPs is their ease and low cost of synthesis in comparison to other methods of preparing materials with tailored recognition properties (Table 1). For example, the l-PAA MIP was formed in one step from commercially available monomers: methacrylic acid (MAA) and ethylene glycol dimethacrylate (EGDMA). The mixture of
Introduction to MIP sensors
The ease of synthesis and the attractive material properties of MIPs have facilitated their use in a wide range of applications that require molecular recognition including sensing [30, 31]. MIPs are well suited for sensing applications as they are insoluble polymers and thus do not require additional immobilization steps that are usually necessary for enzyme, antibody, and synthetic molecular receptor based sensors. An early MIP pseudoimmunoassay developed by Andersson et al. demonstrated the
MIP sensor arrays
The use of imprinted materials in sensor arrays is still relatively new, as there are only about 10 examples in the literature. These examples span a wide range of different imprinted materials, including both organic polymers and inorganic polymers such as sol–gels and self-assembled monolayers. Sensor arrays also utilized an assortment of different signaling mechanisms including fluorescent competition assays, electrical capacitance, UV-vis, and QCM. Four examples were chosen to highlight the
Conclusion
The four selected examples highlight the potential of MIP arrays in sensing applications. The imprinting process is an efficient, versatile, and inexpensive way to generate multiple recognition surfaces with different selectivity patterns. The array format offers solutions to many of the problems observed with previous MIP sensors while also embracing many of the advantageous qualities of MIPs. The imprinting approach falls within the extremes of simply forming random polymers and carefully
Acknowledgement
This material is based upon work supported by the National Science Foundation under Grant No. (CBET 0828897).
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