Colorimetric biomimetic sensor systems based on molecularly imprinted polymer membranes for highly-selective detection of phenol in environmental samples

Molecularly Imprinted Polymer-Based Biomimetic Systems for Sensing Environmental Contaminants, Biomarkers, and Bioimaging Applications

Molecularly imprinted polymers (MIPs), a biomimetic artificial receptor system inspired by the human body's antibody-antigen reactions, have gained significant attraction in the area of sensor development applications, especially in the areas of medical, pharmaceutical, food quality control, and the environment. MIPs are found to enhance the sensitivity and specificity of typical optical and electrochemical sensors severalfold with their precise binding to the analytes of choice. In this review, different polymerization chemistries, strategies used in the synthesis of MIPs, and various factors influencing the imprinting parameters to achieve high-performing MIPs are explained in depth. This review also highlights the recent developments in the field, such as MIP-based nanocomposites through nanoscale imprinting, MIP-based thin layers through surface imprinting, and other latest advancements in the sensor field. Furthermore, the role of MIPs in enhancing the sensitivity and specificity of sensors, especially optical and electrochemical sensors, is elaborated. In the later part of the review, applications of MIP-based optical and electrochemical sensors for the detection of biomarkers, enzymes, bacteria, viruses, and various emerging micropollutants like pharmaceutical drugs, pesticides, and heavy metal ions are discussed in detail. Finally, MIP's role in bioimaging applications is elucidated with a critical assessment of the future research directions for MIP-based biomimetic systems.

Rationally designed molecularly imprinted polymer membranes as antibody and enzyme mimics in analytical biotechnology

The paper is a self-review of works on development of new approaches to formation of mimics of receptor and catalytic sites of biological macromolecules in the structure of highly cross-linked polymer membranes and thin films. The general strategy for formation of the binding sites in molecularly imprinted polymer (MIP) membranes and thin films was described. A selective recognition of a number of food toxins, endocrine disruptors and metabolites is based on the results of computational modeling data for the prediction and optimization of their structure. A strategy proposed for the design of the artificial binding sites in MIP membranes was supported by the research performed by the authors on development of a number of the MIP membrane-based affinity and catalytic biosensors for selective and sensitive measurement (detection limits 0.3-100 nM) of the target analytes. Novel versatile approaches aimed at improving sensitivity of the developed biosensor systems were discussed.

Molecularly Imprinted Polymers (MIPs) in Sensors for Environmental and Biomedical Applications: A Review

Molecular imprinted polymers are custom made materials with specific recognition sites for a target molecule. Their specificity and the variety of materials and physical shapes in which they can be fabricated make them ideal components for sensing platforms. Despite their excellent properties, MIP-based sensors have rarely left the academic laboratory environment. This work presents a comprehensive review of recent reports in the environmental and biomedical fields, with a focus on electrochemical and optical signaling mechanisms. The discussion aims to identify knowledge gaps that hinder the translation of MIP-based technology from research laboratories to commercialization.

The Selectivity of Molecularly Imprinted Polymers

The general claim about novel molecularly imprinted polymers is that they are selective for their template or for another target compound. This claim is usually proved by some kind of experiment, in which a performance parameter of the imprinted polymer is shown to be better towards its template than towards interferents. A closer look at such experiments shows, however, that different experiments may differ substantially in what they tell about the same imprinted polymer's selectivity. Following a short general discussion of selectivity concepts, the selectivity of imprinted polymers is analyzed in batch adsorption, binding assays, chromatography, solid phase extraction, sensors, membranes, and catalysts. A number of examples show the problems arising with each type of application. Suggestions for practical method design are provided.

Assessment of cross-reactivity in a tailor-made molecularly imprinted polymer for phenolic compounds using four adsorption isotherm models

Cross-reactivity is an important feature of molecularly imprinted polymers (MIPs), and is central to successful use of a pseudo-template in molecular imprinting. The adsorption and cross-reactivity of a molecularly imprinted polymer (MIP) designed for recognition of phenols from water was assessed using four different isotherm models (Langmuir (LI), Freundlich (FI), Langmuir-Freundlich (L-FI), and Brunauer, Emmett, and Teller (BET)). The L-FI model succeeded in explaining the cross-reactivity behavior through the total number of binding sites, the affinity constants and heterogeneity indices of the small phenols (phenol (ph), 2-methylphenol (2-MP), 3-methylphenol (3-MP), 2-chlorophenol (2-CP), 2,4-dimethylphenol (DMP), 2,4-dichlorophenol (DCP), 4-chloro-3-methylphenol (CMP)) with evidence that the phenols compete for binding sites based on their hydrophobicity as well as π-π, π-σ and dipole-dipole intermolecular forces. The recognition of the large phenols (2,4,6-trichlorophenol (TCP), pentachlorophenol (PCP), 4-teroctylphenol (4-OP), 4-nonylphenol (4-NP), which have much higher binding affinities than the smaller phenolic compounds, was explained with the BET isotherm model that predicts that multiple layers adsorb to the adsorbed monolayer. The adsorption behavior with MIPs is also shown to be superior to corresponding non-imprinted polymers and applicability of MIPs for trace analysis is highlighted.

Sensor Based on Molecularly Imprinted Polymer Membranes and Smartphone for Detection of Fusarium Contamination in Cereals

The combination of the generic mobile technology and inherent stability, versatility and cost-effectiveness of the synthetic receptors allows producing optical sensors for potentially any analyte of interest, and, therefore, to qualify as a platform technology for a fast routine analysis of a large number of contaminated samples. To support this statement, we present here a novel miniature sensor based on a combination of molecularly imprinted polymer (MIP) membranes and a smartphone, which could be used for the point-of-care detection of an important food contaminant, oestrogen-like toxin zearalenone associated with Fusarium contamination of cereals. The detection is based on registration of natural fluorescence of zearalenone using a digital smartphone camera after it binds to the sensor recognition element. The recorded image is further processed using a mobile application. It shows here a first example of the zearalenone-specific MIP membranes synthesised in situ using "dummy template"-based approach with cyclododecyl 2, 4-dihydroxybenzoate as the template and 1-allylpiperazine as a functional monomer. The novel smartphone sensor system based on optimized MIP membranes provides zearalenone detection in cereal samples within the range of 1-10 µg mL^-1 demonstrating a detection limit of 1 µg mL^-1 in a direct sensing mode. In order to reach the level of sensitivity required for practical application, a competitive sensing mode is also developed. It is based on application of a highly-fluorescent structural analogue of zearalenone (2-[(pyrene-l-carbonyl) amino]ethyl 2,4-dihydroxybenzoate) which is capable to compete with the target mycotoxin for the binding to zearalenone-selective sites in the membrane's structure. The competitive mode increases 100 times the sensor's sensitivity and allows detecting zearalenone at 10 ng mL^-1. The linear dynamic range in this case comprised 10-100 ng mL^-1. The sensor system is tested and found effective for zearalenone detection in maize, wheat and rye flour samples both spiked and naturally contaminated. The developed MIP membrane-based smartphone sensor system is an example of a novel, inexpensive tool for food quality analysis, which is portable and can be used for the "field" measurements and easily translated into the practice.

Development of a smartphone-based biomimetic sensor for aflatoxin B1 detection using molecularly imprinted polymer membranes

A novel smartphone-based optical biomimetic sensor based on free-standing molecularly imprinted polymer (MIP) membranes was developed for rapid and sensitive point-of-care detection of aflatoxin B1. The developed MIP membranes were capable of selective recognition of the target analyte and, at the same time, of generation of a fluorimetric sensor response, which could be registered using the camera of a smartphone and analysed using image analysis. The developed system provides a possibility of synchronous detection of aflatoxin B1 in 96 channels. UV irradiation of aflatoxin B1, selectively bound by the MIP membranes from the analysed samples, initiated fluorescence of aflatoxin B1 with intensity directly proportional to its concentration. The composition of the MIP membranes used as a recognition element was optimised taking into account data of computational modelling. Two functional monomers (2-acrylamido-2-methyl-1-propansulfonic acid and acrylamide) were identified as optimal for the formation of aflatoxin B1-selective binding sites in the structure of the MIP membranes. Working characteristics of the smartphone-based sensor system were also estimated. The influence of pH and of buffer and NaCl concentrations on the smartphone-based sensor responses were studied. High selectivity of the developed sensor system towards aflatoxin B1 was confirmed in experiments with the close structural analogue of the target analyte - aflatoxin G2, and ochratoxin A. The detection limit for aflatoxin B1 using the smartphone-based sensor systems was found to be 20 ng mL^-1 for the sensor based on MIP membranes synthesised with acrylamide as a functional monomer. The storage stability of the recognition elements of the developed sensors was estimated as one year when stored at 22 °C. The possibility to detect the aflatoxin B1 in contaminated food samples was shown. The MIP-membrane-based sensor system provided a convenient point-of-care approach in food safety testing.

One Binder to Bind Them All

High quality binders, such as antibodies, are of critical importance for chemical sensing applications. With synthetic alternatives, such as molecularly imprinted polymers (MIPs), less sensor development time and higher stability of the binder can be achieved. In this feature paper, I will discuss the impact of synthetic binders from an industrial perspective and I will challenge the molecular imprinting community on the next step to leapfrog the current status quo of MIPs for (bio)sensing. Equally important, but often neglected as an effective chemical sensor, is a good match of transducer and MIP coating for a respective application. To demonstrate an application-driven development, a biosensing use case with surface-imprinted layers on piezoacoustic sensors is reported. Depending on the electrode pattern for the transducer, the strong mechanical coupling of the analyte with the MIP layer coated device allows the adoption of the sensitivity from cell mass to cell viability with complete reversibility.

From porous gold nanocups to porous nanospheres and solid particles--a new synthetic approach

We report a versatile approach for the synthesis of porous gold nanocups, porous gold nanospheres and solid gold nanoparticles. Gold nanocups are formed by the slow reduction of gold salt (HAuCl4⋅3H2O) using aminoantipyrene (AAP) as a reducing agent. Adding polyvinylpyrrolidone (PVP) to the gold salt followed by reduction with AAP resulted in the formation of porous gold nanospheres. Microwave irradiation of both of these porous gold particles resulted in the formation of slightly smaller but solid gold particles. All these nanoparticles are thoroughly characterized by UV-visible spectroscopy, scanning electron microscopy (SEM), high resolution transmission electron microscopy (HR-TEM) and bright-field tomography. Due to the larger size, porous nature, low density and higher surface area, these nanomaterials may have interesting applications in catalysis, drug delivery, phototherapy and sensing.