Highly selective and sensitive molecularly imprinted sensors for the electrochemical assay of quercetin in methanol extracts of Rubus sanctus and Fragaria vesca

Quercetin (QUE) is a powerful antioxidant and one of the common phenolic compounds found in plants, vegetables, and fruits, which has shown many pharmacological activities. The complex nature of the matrix in which QUE is found and its importance and potential uses in diverse applications force the researchers to develop selective and sensitive sensors. In the present work, a novel molecularly imprinted polymer (MIP)-based electrochemical sensor was fabricated for the selective and sensitive determination of the QUE in plant extracts and food supplements. Tryptophan methacrylate (TrpMA) was chosen as the functional monomer, whereas the photopolymerization (PP) method was applied using a glassy carbon electrode (GCE). Electrochemical and morphological characterizations of the developed sensor (TrpMA@QUE/MIP-GCE) were performed using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and scanning electron microscopy (SEM). The linear range of the developed sensor was determined to be in the range of 1.0-25 pM, while the limit of detection (LOD) was calculated to be 0.235 pM. In conclusion, The TrpMA@QUE/MIP-GCE sensor might be classified as a promising platform for selective and sensitive determination of QUE not only in plant extracts but also in commercial food supplements because of its reliability, reproducibility, repeatability, stability, and fast response time.

A smartphone-based colorimetric assay using Cu-tannic acid nanosheets (Cu-TA NShs) as a laccase-mimicking nanozyme for visual detection of quercetin in vegetables

The interaction of Cu-tannic acid nanosheets (Cu-TA NShs) as nanozyme in a surfactant solution of CTAB under relatively acidic conditions is shown to exhibit a catalytic effect on quercetin (Qur). This catalytic property of Cu-TA NShs, which mimics laccase enzyme with many advantages, has been applied to developing a selective colorimetric sensor for the determination of trace amounts of Qur in vegetable samples. This strategy presents a desirable linear relationship between the absorbance signal intensity and the concentrations of Qur from 0.350 to 32.09 µM with a detection limit (LOD) of 0.064 µM (S/N = 3). The feasibility of the proposed portable colorimetric sensor for in situ analysis of the real samples has been validated with the high-performance liquid chromatography (HPLC) method as reference method, and two-tailed test (t test) statistical analysis certifies good agreement between the results. This enzyme-free and sensitive naked-eye sensor with the smartphone-based color map is promising to provide technical support for the rapid and visual detection of Qur in vegetables.

The Electrochemical Detection of Ochratoxin A in Apple Juice via MnCO3 Nanostructures Incorporated into Carbon Fibers Containing a Molecularly Imprinting Polymer

A novel electrochemical sensor based on MnCO3 nanostructures incorporated into carbon fibers (MnCO3NS/CF), including a molecularly imprinting polymer (MIP), was developed for the determination of Ochratoxin A (OTA). In this study, a sensitive and selective sensor design for OTA detection was successfully performed by utilizing the selectivity and catalysis properties of MIP and the synthesized MnCO3NS/CF material at the same time. MnCO3 nanostructures incorporated into carbon fibers were first characterized by using various analytical techniques. The sensor revealed a linearity towards OTA in the range of 1.0 × 10-11-1.0 × 10-9 mol L-1 with a detection limit (LOD) of 2.0 × 10-12 mol L-1. The improved electrochemical signal strategy was achieved by high electrical conductivity on the electrode surface, providing fast electron transportation. In particular, the analysis process could be finished in less than 5.0 min without complex and expensive equipment. Lastly, the molecular imprinted electrochemical sensor also revealed superior stability, repeatability and reproducibility.

Advances in Molecular Imprinting Technology for the Extraction and Detection of Quercetin in Plants

Quercetin is a kind of flavonoid compound, which has antioxidative, anti-aging and anti-cancer effects, so it is of great importance to study the efficient extraction and highly sensitive detection of quercetin. Molecular imprinting technology has remarkable selectivity and resistance to complex matrix interference, which is often used for extracting quercetin. The methods of molecular imprinted solid phase extraction, molecularly imprinted microsphere extraction, molecularly imprinted electrochemical sensor recognition and molecularly imprinted composite material extraction of quercetin from plant samples were discussed in detail. This review provides valuable information on efficient and sensitive methods for separating and purifying quercetin in plants. It also provides a technical reference for further investigation of the separation and analysis of active ingredients in natural products.

The development of an electropolymerized, molecularly imprinted polymer (MIP) sensor for insulin determination using single-drop analysis

An electrochemical sensor for the detection of insulin in a single drop (50 μL) was developed based on the concept of molecularly imprinted polymers (MIP). The synthetic MIP receptors were assembled on a screen-printed carbon electrode (SPCE) by the electropolymerization of pyrrole (Py) in the presence of insulin (the protein template) using cyclic voltammetry. After electropolymerization, insulin was removed from the formed polypyrrole (Ppy) matrix to create imprinting cavities for the subsequent analysis of the insulin analyte in test samples. The surface characterization, before and after each electrosynthesis step of the MIP sensors, was performed using atomic force microscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The performance of the developed MIP-SPCE sensor was evaluated using a single drop of solution containing K₃Fe(CN)₆ and the square-wave voltammetry technique. The MIP-SPCE showed a linear concentration range of 20.0-70.0 pM (R² = 0.9991), a limit of detection of 1.9 pM, and a limit of quantification of 6.2 pM. The rapid response time to the protein target and the portability of the developed sensor, which is considered a disposable MIP-based system, make this MIP-SPCE sensor a promising candidate for point-of-care applications. In addition, the MIP-SPCE sensor was successfully used to detect insulin in a pharmaceutical sample. The sensor was deemed to be accurate (the average recovery was 108.46%) and precise (the relative standard deviation was 7.23%).

Development of a Redox-Label-Doped Molecularly Imprinted Polymer on β-Cyclodextrin/Reduced Graphene Oxide for Electrochemical Detection of a Stress Biomarker

Cortisol is a major stress biomarker involved in the regulation of metabolic and immune responses. Readily accessible assays with sufficient quantitative and temporal resolution can assist in prevention, early diagnosis, and management of chronic diseases. Whereas conventional assays are costly in terms of time, labor, and capital, an electrochemical approach offers the possibility of miniaturization and detection at the point-of-care. Here, we investigate the biosensor application of molecularly imprinted polypyrrole (PPy) doped with hexacyanoferrate (HCF) and coupled to reduced graphene oxide functionalized with β-cyclodextrin (β-CD). β-CD provides an inclusion site for lipophilic cortisol and was electrochemically grafted simultaneous with reduction of GO. Next, PPy was electrochemically deposited in presence of cortisol template with HCF dopant ions serving as intrinsic redox probe. Thus, the sensor response was evaluated via changes of redox peak current in cyclic voltammetry and demonstrated a broad logarithmic detection range (5 pg/mL to 5000 ng/mL, R ² = 0.995), with a sensitivity of 8.809 μA log^-1 (ng/mL) cm^-2 and LOD of 19.3 pM. The sensor was shown to be specific toward cortisol in reference to salivary cortisol concentration in saliva over structural analogues. The sensor was exhibited to determine cortisol in artificial saliva at normal and elevated levels. The good performance and facile electrochemical fabrication of this antibody- and external label-free interface are promising for the development of affordable point-of-care biosensors.

A Disposable Saliva Electrochemical MIP-Based Biosensor for Detection of the Stress Biomarker α-Amylase in Point-of-Care Applications

The design and synthesis of artificial receptors based on molecular imprinting (MI) technology for the development of a new MIP-based biosensor for detection of the stress biomarker α-amylase in human saliva in point-of-care (PoC) applications is described in this work. The portable electrochemical devices for monitoring α-amylase consists of cost-effective and disposable gold screen-printed electrodes (AuSPEs). To build the electrochemical device, the template biomolecule was firstly immobilized directly over the working area of the gold chip previously activated with a self-assembled monolayer (SAM) of cysteamine (CA). Then, pyrrole (Py) monomer was selected as building block of a polymeric network prepared by CV electropolymerization. After the electropolymerization process, the enzyme was removed from the polymer film in order to build the specific recognition sites for the target enzyme. The MIP biosensor showed a very wide linear concentration range (between 3.0 × 10−4 to 0.60 mg mL−1 in buffer solution and between 3.0 × 10−4 to 3.0 × 10−2 mg mL−1 in human saliva) and low detection levels were achieved (LOD < 3.0 × 10−4 mg mL−1) using square wave voltammetry (SWV) as the electroanalytical technique.

Gut microbiota derived trimethylamine N-oxide (TMAO) detection through molecularly imprinted polymer based sensor

Trimethylamine N-oxide (TMAO), a microbiota-derived metabolite has been implicated in human health and disease. Its early detection in body fluids has been presumed to be significant in understanding the pathogenesis and treatment of many diseases. Hence, the development of reliable and rapid technologies for TMAO detection may augment our understanding of pathogenesis and diagnosis of diseases that TMAO has implicated. The present work is the first report on the development of a molecularly imprinted polymer (MIP) based electrochemical sensor for sensitive and selective detection of TMAO in body fluids. The MIP developed was based on the polypyrrole (PPy), which was synthesized via chemical oxidation polymerization method, with and without the presence of TMAO. The MIP, NIP and the non-sonicated polymer (PPy-TMAO) were separately deposited electrophoretically onto the hydrolyzed indium tin oxide (ITO) coated glasses. The chemical, morphological, and electrochemical behavior of MIP, non-imprinted polymer (NIP), and PPy-TMAO were characterized using Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), and electrochemical techniques. The detection response was recorded using differential pulse voltammetry (DPV), which revealed a decrease in the peak current with the increase in concentration of TMAO. The MIP sensor showed a dynamic detection range of 1-15 ppm with a sensitivity of 2.47 µA mL ppm^-1 cm^-2. The developed sensor is easy to construct and operate and is also highly selective to detect TMAO in body fluids such as urine. The present research provides a basis for innovative strategies to develop sensors based on MIP to detect other metabolites derived from gut microbiota that are implicated in human health and diseases.

Preparation and applications of electrochemical chemosensors based on carbon-nanomaterial-modified molecularly imprinted polymers

The past few decades have witnessed a rapid development in electrochemical chemosensors (ECCSs). The integration of carbon nanomaterials (CNMs) and molecularly imprinted polymers (MIPs) has endowed ECCSs with high selectivity and sensitivity toward target detection. Due to the integrated merits of MIPs and CNMs, CNM-modified MIPs as ECCSs have been widely reported and have excellent detection applications. This review systematically summarized the general categories, preparation strategies, and applications of ECCSs based on CNM-modified MIPs. The categories include CNM-modified MIPs often hybridized with various materials and CNM-encapsulated or CNM-combined imprinting silica and polymers on working electrodes or other substrates. The preparation strategies include the polymerization of MIPs on CNM-modified substrates, co-polymerization of MIPs and CNMs on substrates, drop-casting of MIPs on CNM-modified substrates, self-assembly of CNMs/MIP complexes on substrates, and so forth. We discussed the in situ polymerization, electro-polymerization, and engineering structures of CNM-modified MIPs. With regard to potential applications, we elaborated the detection mechanisms, signal transducer modes, target types, and electrochemical sensing of targets in real samples. In addition, this review discussed the present status, challenges, and prospects of CNM-modified MIP-based ECCSs. This comprehensive review is desirable for scientists from broad research fields and can promote the further development of MIP-based functional materials, CNM-based hybrid materials, advanced composites, and hybrid materials.

Molecularly imprinted polymer SPE sensor for analysis of CA-125 on serum

Considering the high incidence level and mortality rate of ovarian cancer, particularly among the European female population, the carbohydrate antigen 125 (CA-125) was selected as the protein target for this study for the development of a MIP-based biosensor. This work presents the development of molecular imprinting polymers (MIPs) on gold electrode surface for CA-125 biomarker recognition. The preparation of the CA-125 imprinting was obtained by electropolymerization of pyrrole (Py) monomer in a gold electrode using cyclic voltammetry (CV) in order to obtain highly selective materials with great molecular recognition capability. The quantification of CA-125 biomarker was made through the comparison of two methods: electrochemical (square wave voltammetry -SWV) and optical transduction (surface plasmon resonance -SPR). SWV has been widely used in biological molecules analysis since it is a fast and sensitive technique. In turn, SPR is a non-destructive optical technique that provides high-quality analytical data of CA-125 biomarker interactions with MIP. Several analytical parameters, such as sensitivity, linear response interval, and detection limit were determined to proceed to the performance evaluation of the electrochemical and optical transduction used in the development of the CA-125 biosensor. The biosensor based in the electrochemical transduction was the one that presented the best analytical parameters, yielding a good selectivity and a detection limit (LOD) of 0.01 U/mL, providing a linear concentration range between 0.01 and 500 U/mL. This electrochemical biosensor was selected for the study and it was successfully applied in the CA-125 analysis in artificial serum samples with recovery rates ranging from 91 to 105% with an average relative error of 5.8%.

Quercetin spectrofluorometric quantification in aqueous media using different surfactants as fluorescence promoters

Quercetin spectrofluorometric quantification was carried out in aqueous media (pH 7) using micelles of surfactants, namely: CTAB, SDS and TX100 as promoters of quercetin fluorescence, having a critical micelle concentration, CMC, of: 0.94 ± 0.03, 7.7 ± 0.6 and 0.18 ± 0.3 mM, respectively, measured through UV-Vis spectrophotometry. The thermodynamic binding constant, K, for the quercetin-surfactants' micelles supramolecular complex was estimated to be (log(K/M⁻¹)) = 2.87 ± 0.02, 2.78 ± 0.04 and 2.80 ± 0.04, respectively, from fluorescence spectrophotometry. With the aid of these parameters it was possible to construct species distribution diagrams and choose the experimental conditions where quercetin can be quantified in aqueous media from fluorescence measurements. The best lowest limit of detection (0.244 ± 0.092) μM was achieved with CTAB micelles while the best sensitivity (2.919 ± 0.054) M⁻¹ corresponded to SDS.

Quercetin spectrofluorometric quantification was carried out in aqueous media using micelles of surfactants, namely: CTAB, SDS and TX100 as promoters of quercetin fluorescence and stability.