Mahavir Prasad Tiwari

Molecularly imprinted polymer based enantioselective sensing devices: a review.

Chiral recognition is the fundamental property of many biological molecules and is a quite important field in pharmaceutical analysis because of the pharmacologically different activities of enantiomers in living systems. Enantio-differentiating signal of the sensor requires specific interaction between the chiral compounds (one or a mixture of enantiomers) in question and the selector. This type of interaction is controlled normally by at least three binding centers, whose mutual arrangement and interacting characteristics with one of the enantiomers effectively control the selectivity of recognition. Molecular imprinting technology provides a unique opportunity for the creation of three-dimensional cavities with tailored recognition properties. Over the past decade, this field has expanded considerably across the variety of disciplines, leading to novel transduction approaches and many potential applications. The state-of-art of molecularly imprinted polymer-based chiral recognition might set an exotic trend toward the development of chiral sensors. The objective of this review is to provide comprehensive knowledge and information to all researchers who are interested in exploiting molecular imprinting technology toward the rational design of chiral sensors operating on different transduction principles, ranging from electrochemical to piezoelectric, being used for the detection of chiral compounds as they pose significant impact on the understanding of the origin of life and all processes that occur in living organisms.

Development of a highly sensitive and selective hyphenated technique (molecularly imprinted micro-solid phase extraction fiber-molecularly imprinted polymer fiber sensor) for ultratrace analysis of folic acid.

A simple polymerization strategy is reported in this work which allows molecularly imprinted polymeric fiber (monolith) fabrication for direct use in sensing devices. This is advantageous for achieving higher degree of enrichment of target analyte (folic acid) from the complex matrices of real samples, without any surface fouling, cross-reactivity, and non-specific (false-positive) contributions. In order to measure serum folic acid at ultratrace level to detect spina bifida, a neural tube defect in mother, and other acute cases of proteomic diseases, the hyphenation between molecularly imprinted micro-solid phase extraction fiber and a complementary molecularly imprinted polymer-carbon composite fiber sensor has been found quite efficient. The primitive diagnosis of many chronic diseases is feasible by estimating folic acid as biomarker, with the detection limit as low as 0.0036 ng mL(-1) (relative standard deviation=0.13%, signal/noise=3) in human blood serum.

Enatioselective quantitative separation of d- and l-thyroxine by molecularly imprinted micro-solid phase extraction silver fiber coupled with complementary molecularly imprinted polymer-sensor

Thyroxine is a known disease biomarker which demands a highly sensitive and selective technique to measure ultratrace level with enantiodifferentiation of its optical isomers (d- and l-), in real samples. In this work, an approach of hyphenation between molecularly imprinted micro-solid phase extraction and a complementary molecularly imprinted polymer-sensor was adopted for enantioseparation, preconcentration, and analysis of d- and l-thyroxine. In both techniques, the same imprinted polymer, coated on a vinyl functionalized self-assembled monolayer modified silver wire, was used as the respective extraction fiber as well as sensor material. This combination enabled enhanced preconcentration of test analyte substantially so as to achieve the stringent limit [limit of detection: 0.0084 ng mL(-1), RSD=0.81%, S/N=3 (d-thyroxine); 0.0087 ng mL(-1), RSD=0.63%, S/N=3 (l-thyroxine)] of clinical detection of thyroid-related diseases, without any problems of non-specific false-positive contribution and cross-reactivity.

Doubly imprinted polymer nanofilm-modified electrochemical sensor for ultra-trace simultaneous analysis of glyphosate and glufosinate

A rapid, selective, and sensitive double-template imprinted polymer nanofilm-modified pencil graphite electrode was fabricated for the simultaneous analysis of phosphorus-containing amino acid-type herbicides (glyphosate and glufosinate) in soil and human serum samples. Since both herbicides respond overlapped oxidation peaks and only glyphosate is prone to nitrosation, n-nitroso glyphosate and glufosinate were used as templates for obtaining the well-resolved quantitative differential pulse anodic stripping voltammetric peaks on the proposed sensor. Toward sensor fabrication, a nano-structured polymer film was first grown directly on the electrode via initial immobilization of gold nanoparticles at its surface. This was followed by linking of monomeric (N-methacryloyl-l-cysteine) molecules through S-Au bonds. Subsequently, these molecules were subjected to free radical polymerization, in the presence of templates, cross linker, initiator, and multiwalled carbon nanotubes as pre-polymer mixture. The modified sensor observed wide linear ranges (3.98-176.23 ng mL(-1) and 0.54-3.96 ng mL(-1)) of simultaneous analysis with detection limits as low as 0.35 and 0.19 ng mL(-1) (S/N=3) for glyphosate and glufosinate, respectively, in aqueous samples. The respective oxidation peak potentials of both analytes were found to be substantially apart by 265 mV. This enabled the simultaneous determination of one target in the presence of other, without any cross reactivity, interferences, and false-positives, in real samples.

Metal ion mediated imprinting for electrochemical enantioselective sensing of L-histidine at trace level.

Enantioselective trace level sensing of l-histidine (limit of detection, 1.980 ngm L(-1), S/N=3) was feasible with the use of a typical, reproducible, and rugged complex imprinted polymer-based pencil graphite electrode, in aqueous samples. In the present instance, the Cu(2+) ion-mediated imprinting of l-histidine in an molecularly imprinted polymer motif actually helped upbringing electrocatalytic activity to respond an enhanced differential pulse anodic stripping voltammetric oxidation peak of l-histidine, without any cross-reactivity and false-positive, in real samples. The proposed sensor could be considered suitable for the practical applications in biomarking histedinemia, a disease associated with L-histidine metabolic disorders, in clinical settings.

Enantioselective recognition of d- and l-tryptophan by imprinted polymer-carbon composite fiber sensor

Electrochemical sensors demonstrating enantioselectivity to tryptophan enantiomers, with high selectivity and sensitivity, were fabricated by the use of a monolithic fiber of molecularly imprinted polymer-carbon composite. The recognition mechanism and performance of these sensors were evaluated by differential pulse anodic stripping voltammetry. The sensor imprinted for l-tryptophan not only discriminated the target from its analogues and other amino acids but also responded specifically in racemic mixture in aqueous, biological, and pharmaceutical samples. The binding kinetics of L-tryptophan was also established with the help of anodic stripping cyclic voltammetry and chronocoulometry. The detection limit for L-tryptophan was as low as 0.24 ng mL(-1) (signal/noise=3) which is appropriate for biomarking diseases, caused by an acute tryptophan-depletion, in clinical setting.

Quantum dots-multiwalled carbon nanotubes nanoconjugate-modified pencil graphite electrode for ultratrace analysis of hemoglobin in dilute human blood samples.

A novel molecularly imprinted polymer, selective for human hemoglobin, was immobilized on the surface of CdS quantum dots-multiwalled carbon nanotubes nanoconjugate-modified pencil graphite electrode. The fabricated sensor was found to be water-compatible and biologically benign, since the molecular imprinting was exclusively carried out in water, without any protein denaturation and electrode fouling. Notably, the pencil graphite electrode modified with merely a nanoconjugate matrix might involve the onset possibilities of electrode passivation and protein denaturation. However, a polymer coating onto the nanoconjugate obviated such obstacle while evaluating human hemoglobin in an aqueous environment (pH 4.2). The quantification of the hemoglobin in the dilute whole blood samples varied in the linear range 27.8-444.0 ng mL(-1); and the detection limit was obtained as 6.73 ng mL(-1) (S/N=3), without any cross-reactivity and false-positives. The proposed sensor can be used as a cost effective sensor for hemoglobin, in clinical settings.

Highly sensitive and selective hyphenated technique (molecularly imprinted polymer solid-phase microextraction–molecularly imprinted polymer sensor) for ultra trace analysis of aspartic acid enantiomers

The present work is related to combination of molecularly imprinted solid-phase microextraction and complementary molecularly imprinted polymer-sensor. The molecularly imprinted polymer grafted on titanium dioxide modified silica fiber was used for microextraction, while the same polymer immobilized on multiwalled carbon nanotubes/titanium dioxide modified pencil graphite electrode served as a detection tool. In both cases, the surface initiated polymerization was found to be advantageous to obtain a nanometer thin imprinted film. The modified silica fiber exhibited high adsorption capacity and enantioselective diffusion of aspartic acid isomers into respective molecular cavities. This combination enabled double preconcentrations of d- and l-aspartic acid that helped sensing both isomers in real samples, without any cross-selectivity and matrix complications. Taking into account 6×10(4)-fold dilution of serum and 2×10(3)-fold dilution of cerebrospinal fluid required by the proposed method, the limit of detection for l-aspartic acid is 0.031ngmL(-1). Also, taking into account 50-fold dilution required by the proposed method, the limit of detection for d-aspartic acid is 0.031ngmL(-1) in cerebrospinal fluid.

Layer-by-layer assembled molecularly imprinted polymer modified silver electrode for enantioselective detection of D- and L-thyroxine.

The present work describes a new, simple, and easy method for the generation of stable molecularly imprinted sites in polymeric film, combining self-assembled monolayer and Layer-by-layer approaches through thermal cross-linking of the layered structures, onto the surface of silver electrode. Modified silver electrodes demonstrate enantiodifferentiation and sensitive (detection limits 0.0060 ng mL(-1) for L- and 0.0062 for D-thyroxine) determination of d- and l-thyroxine with the help of differential pulse anodic stripping voltammetric technique. The binding kinetics of thyroxine was explored using anodic stripping cyclic voltammetry and chronocoulometry. The sensor was also validated for D- and L-thyroxine determinations in biological and pharmaceutical samples.

Enantioselective separation and electrochemical sensing of d- and l-tryptophan at ultratrace level using molecularly imprinted micro-solid phase extraction fiber coupled with complementary molecularly imprinted polymer-fiber sensor

Highly efficient enantioselective separation and quantitative recoveries of D- and L-tryptophan in aqueous and real samples can be achieved, with a monolithic molecularly imprinted polymeric fiber that serves both for micro-solid phase extraction and ultratrace sensing, without any false-positive (non-specific) contribution and cross-reactivity, in the range of 0.15-30.00 ng mL(-1) with detection limit as low as 0.0261 ng mL(-1) (relative standard deviation=0.64%, signal/noise=3). The proposed method combining molecularly imprinted micro-solid phase extraction fiber and a complementary molecularly imprinted polymer-carbon composite fiber sensor is proven to be useful for clinical diagnosis of stress-related diseases caused by acute tryptophan depletion.