Simultaneous Chronoamperometry and Piezoelectric Microgravimetry Determination of Nitroaromatic Explosives Using Molecularly Imprinted Thiophene Polymers

Molecularly Imprinted Polymers for Chemical Sensing: A Tutorial Review

The field of molecularly imprinted polymer (MIP)-based chemosensors has been experiencing constant growth for several decades. Since the beginning, their continuous development has been driven by the need for simple devices with optimum selectivity for the detection of various compounds in fields such as medical diagnosis, environmental and industrial monitoring, food and toxicological analysis, and, more recently, the detection of traces of explosives or their precursors. This review presents an overview of the main research efforts made so far for the development of MIP-based chemosensors, critically discusses the pros and cons, and gives perspectives for further developments in this field.

Molecularly Imprinted Polymer-Based Sensors for Priority Pollutants

Globally, there is growing concern about the health risks of water and air pollution. The U.S. Environmental Protection Agency (EPA) has developed a list of priority pollutants containing 129 different chemical compounds. All of these chemicals are of significant interest due to their serious health and safety issues. Permanent exposure to some concentrations of these chemicals can cause severe and irrecoverable health effects, which can be easily prevented by their early identification. Molecularly imprinted polymers (MIPs) offer great potential for selective adsorption of chemicals from water and air samples. These selective artificial bio(mimetic) receptors are promising candidates for modification of sensors, especially disposable sensors, due to their low-cost, long-term stability, ease of engineering, simplicity of production and their applicability for a wide range of targets. Herein, innovative strategies used to develop MIP-based sensors for EPA priority pollutants will be reviewed.

Protein Determination with Molecularly Imprinted Polymer Recognition Combined with Birefringence Liquid Crystal Detection

Liquid crystal-based sensors offer the advantage of high sensitivity at a low cost. However, they often lack selectivity altogether or require costly and unstable biomaterials to impart this selectivity. To incur this selectivity, we herein integrated a molecularly imprinted polymer (MIP) film recognition unit with a liquid crystal (LC) in an optical cell transducer. We tested the resulting chemosensor for protein determination. We examined two different LCs, each with a different optical birefringence. That way, we revealed the influence of that parameter on the sensitivity of the (human serum albumin)-templated (MIP-HSA) LC chemosensor. The response of this chemosensor with the (MIP-HSA)-recognizing film was linear from 2.2 to 15.2 µM HSA, with a limit of detection of 2.2 µM. These values are sufficient to use the devised chemosensor for HSA determination in biological samples. Importantly, the imprinting factor (IF) of this chemosensor was appreciable, reaching IF = 3.7. This IF value indicated the predominant binding of the HSA through specific rather than nonspecific interactions with the MIP.

Polydopamine-based molecularly imprinted thin films for electro-chemical sensing of nitro-explosives in aqueous solutions

A sensitive electrochemical sensor was developed for the detection of nitro-explosives in aqueous solutions based on thin molecularly imprinted polydopamine films. Dopamine was identified in silico, based on DFT (density functional theory) calculations with the ωB97X-D/6-31G* basis set, as the best functional monomer and electropolymerized via cyclic voltammetry (CV) in the presence of carboxylic acid-based structural analogues ('dummy' templates) for two model nitro-explosives: TNT (2,4,6-trinitrotoluene) and RDX (Research Department eXplosive, 1,3,5-trinitroperhydro-1,3,5-triazine). This approach afforded a homogenous coverage of gold electrodes with imprinted films of tunable thickness. The electropolymerized molecularly imprinted polydopamine films allowed for a 10⁵-fold sensitivity improvement over a bare gold electrode based on tracking the redox peaks of the targets by CV. This improved sensitivity is ascribed to the ability of the MIP to concentrate its target in proximity to the transduction element. The MIP films showed reproducible binding in phosphate buffer (10 mM, pH 7.4), with a dynamic range from 0.1 nM to 10 nM for both TNT and RDX and an increased selectivity over closely related structural analogues.

Molecularly Imprinted Polymer Materials as Selective Recognition Sorbents for Explosives: A Review

Explosives are of significant interest to homeland security departments and forensic investigations. Fast, sensitive and selective detection of these chemicals is of great concern for security purposes as well as for triage and decontamination in contaminated areas. To this end, selective sorbents with fast binding kinetics and high binding capacity, either in combination with a sensor transducer or a sampling/sample-preparation method, are required. Molecularly imprinted polymers (MIPs) show promise as cost-effective and rugged artificial selective sorbents, which have a wide variety of applications. This manuscript reviews the innovative strategies developed in 57 manuscripts (published from 2006 to 2019) to use MIP materials for explosives. To the best of our knowledge, there are currently no commercially available MIP-modified sensors or sample preparation methods for explosives in the market. We believe that this review provides information to give insight into the future prospects and potential commercialization of such materials. We warn the readers of the hazards of working with explosives.

Rapid detection of TNP based on a commercial fluorescent probe

A simple, rapid and low cost sensing method for the fluorescent detection of TNP in 100% aqueous media was successfully developed based on a commercial probe. Under the non-covalent interactions between TNP with probe, a ratiometric output signal was achieved, which can be applied in real samples test.

Facile Fabrication of Surface-Imprinted Macroporous Films for Chemosensing of Human Chorionic Gonadotropin Hormone

We present an improved approach for the preparation of highly selective and homogeneous molecular cavities in molecularly imprinted polymers (MIPs) via the combination of surface imprinting and semi-covalent imprinting. Toward that, first, a colloidal crystal mold was prepared via the Langmuir-Blodgett (LB) technique. Then, human chorionic gonadotropin (hCG) template protein was immobilized on the colloidal crystal mold. Later, hCG derivatization with electroactive functional monomers via amide chemistry was performed. In a final step, optimized potentiostatic polymerization of 2,3'-bithiophene enabled depositing an MIP film as the macroporous structure. This synergistic strategy resulted in the formation of molecularly imprinted cavities exclusively on the internal surface of the macropores, which were accessible after dissolution of silica molds. The recognition of hCG by the macroporous MIP film was transduced with the help of electric transducers, namely, extended-gate field-effect transistors (EG-FET) and capacitive impedimetry (CI). These readout strategies offered the ability to create chemosensors for the label-free determination of the hCG hormone. Other than the simple confirmation of pregnancy, hCG assay is a common tool for the diagnosis and follow-up of ectopic pregnancy or trophoblast tumors. Concentration measurements with these EG-FET and CI-based devices allowed real-time measurements of hCG in the range of 0.8-50  and 0.17-2.0 fM, respectively, in 10 mM carbonate buffer (pH = 10). Moreover, the selectivity of chemosensors with respect to protein interferences was very high.

Molecularly imprinted electrospun nanofibers for adsorption of 2,4-dinitrotoluene in water

2,4-Dinitrotoluene molecularly imprinted nanofibers fabricated by a simple electrospinning technique show higher adsorption capacity and possess remarkable stability and reusability.

Aromatic Functionality of Target Proteins Influences Monomer Selection for Creating Artificial Antibodies on Plasmonic Biosensors

Natural antibodies used as biorecognition elements suffer from numerous shortcomings, such as limited chemical and environmental stability and cost. Artificial antibodies based on molecular imprinting are an attractive alternative to natural antibodies. We investigated the role of aromatic interactions in target recognition capabilities of artificial antibodies. Three proteins with different aromatic amino acid content were employed as model targets. Artificial antibodies were formed on nanostructures using combinations of silane monomers of varying aromatic functionality. We employed refractive index sensitivity of plasmonic nanostructures as a transduction platform for monitoring various steps in the imprinting process and to quantify the target recognition capabilities of the artificial antibodies. The sensitivity of the artificial antibodies with aromatic interactions exhibited a protein-dependent enhancement. Selectivity and sensitivity enhancement due to the presence of aromatic groups in imprinted polymer matrix was found to be higher for target proteins with higher aromatic amino acid content. Our results indicate that tailoring the monomer composition based on the amino acid content of the target protein can improve the sensitivity of plasmonic biosensors based on artificial antibodies without affecting the selectivity.

Molecularly Imprinted Polymer Chemosensor for Selective Determination of an N-Nitroso-l-proline Food Toxin

A molecularly imprinted polymer (MIP)-based chemosensor for the selective determination of a chosen toxin, N-nitroso-l-proline (Pro-NO), was devised and fabricated. By means of DFT, the structure of the pre-polymerization (functional monomer)-template complex was modeled. This complex was then potentiodynamically electropolymerized in the presence of cross-linking monomer to form a MIP-Pro-NO thin film. Next, the Pro-NO template was extracted from MIP-Pro-NO with 0.1 m NaOH. Piezoelectric microgravimetry (PM) on an electrochemical quartz crystal microbalance and electrochemical (differential pulse voltammetry (DPV) and electrochemical impedance spectroscopy (EIS)) techniques were used to transduce binding of Pro-NO to molecular cavities of the MIP-Pro-NO. With DPV and EIS chemosensing, the limits of detection (LODs) were about 80.9 and 36.9 nM Pro-NO, respectively; and the selectivity coefficients for urea, glucose, creatinine, and adrenalin interferences were 6.6, 13.2, 2.1, and 2.0, respectively, with DPV as well as 2.3, 2.0, 3.3, and 2.5, respectively, with EIS. With PM under flow injection analysis conditions, the LOD was 10 μm Pro-NO. The MIP-Pro-NO chemosensor detectability and selectivity with respect to interferences were sufficiently high to determine Pro-NO in protein-providing food products.

Study on Dicyandiamide-Imprinted Polymers with Computer-Aided Design

With the aid of theoretical calculations, a series of molecularly imprinted polymers (MIPs) were designed and prepared for the recognition of dicyandiamide (DCD) via precipitation polymerization using acetonitrile as the solvent at 333 K. On the basis of the long-range correction method of M062X/6-31G(d,p), we simulated the bonding sites, bonding situations, binding energies, imprinted molar ratios, and the mechanisms of interaction between DCD and the functional monomers. Among acrylamide (AM), N,N’-methylenebisacrylamide (MBA), itaconic acid (IA), and methacrylic acid (MAA), MAA was confirmed as the best functional monomer, because the strongest interaction (the maximum number of hydrogen bonds and the lowest binding energy) occurs between DCD and MAA, when the optimal molar ratios for DCD to the functional monomers were used, respectively. Additionally, pentaerythritol triacrylate (PETA) was confirmed to be the best cross-linker among divinylbenzene (DVB), ethylene glycol dimethacrylate (EGDMA), trimethylolpropane trimethylacrylate (TRIM), and PETA. This is due to the facts that the weakest interaction (the highest binding energy) occurs between PETA and DCD, and the strongest interaction (the lowest binding energy) occurs between PETA and MAA. Depending on the results of theoretical calculations, a series of MIPs were prepared. Among them, the ones prepared using DCD, MAA, and PETA as the template, the functional monomer, and the cross-linker, respectively, exhibited the highest adsorption capacity for DCD. The apparent maximum absorption quantity of DCD on the MIP was 17.45 mg/g.

Visual detection of 2,4,6-trinitrotolune by molecularly imprinted colloidal array photonic crystal

We developed a photonic crystal (PhC) sensor for the quantification of 2,4,6-trinitrotoluene (TNT) in solution. Monodisperse (210nm in diameter) molecularly imprinted colloidal particles (MICs) for TNT were prepared by the emulsion polymerization of methyl methacrylate and acrylamide in the presence of TNT as a template. The MICs were then self-assembled into close-packed opal PhC films. The adsorption capacity of the MICs for TNT was 64mg TNT/g. The diffraction from the PhC depended on the TNT concentration in a methanol/water (3/2, v/v) potassium dihydrogen phosphate buffer solution (pH=7.0, 30mM). The limit of detection (LOD) of the sensor was 1.03μg. The color of the molecularly imprinted colloidal array (MICA) changed from green to red with an 84nm diffraction red shift when the TNT concentration increased to 20mM. The sensor response time was 3min. The PhC sensor was selective for TNT compared to similar compounds such as 2,4,6-trinitrophenol, 2,4-dinitrotoluene, 2,6-dinitrotoluene, 2-nitromesitylene, 4-nitrotoluene, 2-nitrotoluene, 1,3-dinitrobenzene, methylbenzene, 4-nitrophenol, 2-nitroaniline, 3-aminophenol and 3-nitroaniline. The sensor showed high stability with little response change after three years storage. This sensor technology might be useful for the visual determination of TNT.

Photophysical Diversity of Water-Soluble Fluorescent Conjugated Polymers Induced by Surfactant Stabilizers for Rapid and Highly Selective Determination of 2,4,6-Trinitrotoluene Traces

The increasing application of fluorescence spectroscopy in development of reliable sensing platforms has triggered a lot of research interest for the synthesis of advanced fluorescent materials. Herein, we report a simple, low-cost strategy for the synthesis of a series of water-soluble conjugated polymer nanoparticles with diverse emission range using cationic (hexadecyltrimethylammonium bromide, CTAB), anionic (sodium dodecylbenzenesulfonate, SDBS), and nonionic (TX114) surfactants as the stabilizing agents. The role of surfactant type on the photophisical and sensing properties of resultant polymers has been investigated using dynamic light scattering (DLS), FT-IR, UV-vis, fluorescence, and energy dispersive X-ray (EDS) spectroscopies. The results show that the surface polarity, size, and spectroscopic and sensing properties of conjugated polymers could be well controlled by the proper selection of the stabilizer type. The fluorescent conjugated polymers exhibited fluorescence quenching toward nitroaromatic compounds. Further studies on the fluorescence properties of conjugated polymers revealed that the emission of the SDBS stabilized polymer, N-methylpolypyrrole-SDBS (NMPPY-SDBS), is strongly quenched by 2,4,6-trinitrotoluene molecule with a large Stern -Volmer constant of 59 526 M(-1) and an excellent detection limit of 100 nM. UV-vis and cyclic voltammetry measurements unveiled that fluorescence quenching occurs through a charge transfer mechanism between electron rich NMPPY-SDBS and electron deficient 2,4,6-trinitrotoluene molecules. Finally, the as-prepared conjugated polymer and approach were successfully applied to the determination of 2,4,6-trinitrotoluene in real water samples.

Advances in explosives analysis--part I: animal, chemical, ion, and mechanical methods

The number and capability of explosives detection and analysis methods have increased substantially since the publication of the Analytical and Bioanalytical Chemistry special issue devoted to Explosives Analysis (Moore and Goodpaster, Anal Bioanal Chem 395(2):245-246, 2009). Here we review and critically evaluate the latest (the past five years) important advances in explosives detection, with details of the improvements over previous methods, and suggest possible avenues towards further advances in, e.g., stand-off distance, detection limit, selectivity, and penetration through camouflage or packaging. The review consists of two parts. This part, Part I, reviews methods based on animals, chemicals (including colorimetry, molecularly imprinted polymers, electrochemistry, and immunochemistry), ions (both ion-mobility spectrometry and mass spectrometry), and mechanical devices. Part II will review methods based on photons, from very energetic photons including X-rays and gamma rays down to the terahertz range, and neutrons.

Selective electrochemical sensing of human serum albumin by semi-covalent molecular imprinting

We devised and prepared a conducting molecularly imprinted polymer (MIP) for human serum albumin (HSA) determination using semi-covalent imprinting. The bis(2,2'-bithien-5-yl)methane units constituted the MIP backbone. This MIP was deposited as a thin film on an Au electrode by oxidative potentiodynamic electropolymerization to fabricate an electrochemical chemosensor. The HSA template imprinting, and then its releasing from the MIP was confirmed by the differential pulse voltammetry (DPV), electrochemical impedance spectroscopy (EIS), XPS, and PM-IRRAS measurements as well as by AFM imaging. Semi-covalent imprinting provided a very well defined locations of recognition sites in the MIP molecular cavities. These sites populated the imprinted cavities or the MIP surface only. The DPV and EIS response of the MIP film coated electrode to the HSA analyte was linear in the range of 0.8 to 20 and 4 to 80 µg/mL HSA, respectively, with the limit of detection of 16.6 and 800 ng/mL, respectively. The impressively high imprinting factor reached, exceeding 20, strongly confirmed that semi-covalent imprinting resulted in formation of a large number of very well defined molecular cavities with high affinity to the HSA molecules. The MIP selectivity against low-(molecular weight) interferences, common for physiological fluids, such as blood and urea, was very high. There was no response to the presence of these interferences at concentrations encountered in the samples analyzed. Moreover, the chemosensor selectivity to the myoglobin and cytochrome c interferences was excellent while that to lysozyme was slightly lower but still high. The chemosensor was useful for determination of abnormal HSA concentration in a control blood serum.

Molecularly imprinted polymers as recognition materials for electronic tongues

For over three decades now, molecularly imprinted polymers (MIPs) have successfully been used for selective chemical sensing because the shape and size of their imprinted molecular cavities perfectly matched those of the target analyte molecules. Moreover, orientation of recognizing sites of these cavities corresponded to those of the binding sites of the template molecules. In contrast, electronic tongue (e-tongue) is usually an array of low-affinity recognition units. Its selectivity is based on recognition pattern or multivariate analysis. Merging these two sensing devices led to a synergetic hybrid sensor, an MIP based e-tongue. Fabrication of these e-tongues permitted simultaneous sensing and discriminating several analytes in complex solutions of many components so that these arrays compensated for limitation in cross-reactivity of MIPs. Apparently, analytical signals generated by MIP-based e-tongues, compared to those of ordinary sensor arrays, were more reliable where a unique pattern or 'fingerprint' for each analyte was generated. Additionally, several transduction platforms (from spectroscopic to electrochemical) engaged in constructing MIP-based e-tongues, found their broad and flexible applications. The present review critically evaluates achievements in recent developments of the MIP based e-tongues for chemosensing.

Aggregates of a hydrazono-sulfonamide adduct as picric acid sensors

A sensitive and selective sensor for picric acid was developed using aggregates of a novel hydrazono-sulfonamide adduct.

Fullerene derived molecularly imprinted polymer for chemosensing of adenosine-5'-triphosphate (ATP)

For molecular imprinting of oxidatively electroactive analytes by electropolymerization, we used herein reductively electroactive functional monomers. As a proof of concept, we applied C60 fullerene adducts as such for the first time. For that, we derivatized C60 to bear either an uracil or an amide, or a carboxy addend for recognition of the adenosine-5'-triphosphate (ATP) oxidizable analyte with the ATP-templated molecularly imprinted polymer (MIP-ATP). Accordingly, the ATP complex with all of the functional monomers formed in solution was potentiodynamically electropolymerized to deposit an MIP-ATP film either on an Au electrode of the quartz crystal resonator or on a Pt disk electrode for the piezoelectric microgravimetry (PM) or capacitive impedimetry (CI) determination of ATP, respectively, under the flow-injection analysis (FIA) conditions. The apparent imprinting factor for ATP was ∼4.0. After extraction of the ATP template, analytical performance of the resulting chemosensors, including detectability, sensitivity, and selectivity, was characterized. The limit of detection was 0.3 and 0.03mM ATP for the PM and CI chemosensor, respectively. The MIP-ATP film discriminated structural analogues of ATP quite well. The Langmuir, Freundlich, and Langmuir-Freundlich isotherms were fitted to the experimental data of the ATP sorption and sorption stability constants appeared to be nearly independent of the adopted sorption model.

Selective fluorescence sensing of polynitroaromatic explosives using triaminophenylbenzene scaffolds

C₃-Symmetric 1,3,5-tris(4′-aminophenyl)benzene has been employed as a selective fluorescence chemosensor for polynitroaromatic compounds.