Polymerization Parameters Influencing the QCM Response Characteristics of BSA MIP

Fast, sensitive and selective simultaneous determination of paraquat and glyphosate herbicides in water samples using a compact electrochemical sensor

We report a new ready-to-use sensor for simultaneous determination of paraquat (PQ) and glyphosate (GLY) based on a graphite screen-printed electrode modified with a dual-molecularly imprinted polymer coated on a mesoporous silica-platinum core. Amino-mesoporous silica nanoparticles (MSN-NH2) were first synthesized by a simple co-condensation method using tetraethyl orthosilicate and 3-aminopropyltrimethoxysilane. PtNPs were then decorated on the surface of MSN-NH2 by chemical reduction. Finally, the dual-MIP was successfully coated on the MSN-PtNP core. This 3D-surface-imprinting strategy enhances the conductivity and monodispersity of the MSN-PtNPs@d-MIP. Quantitative analysis was performed by differential pulse voltammetry with an oxidation current appearing at -0.95 V for PQ and +0.97 V for GLY. The dual-MIP sensor shows good linear calibration curves in the range of 0.025-500 μM for both analytes with detection limits of 3.1 nM and 4.0 nM for PQ and GLY, respectively. The dual-MIP sensor shows high selectivity and specificity, attributed to the increased affinity of the imprinted cavities formed on the polymer film for the target PQ and GLY molecules. The proposed dual-MIP sensor was successfully applied to detect PQ and GLY concentrations simultaneously in water samples. The ready-to-use dual-MIP sensor is well suited for water-quality control and on-site applications without sophisticated instrumentation.

Thin Film Plastic Antibody-Based Microplate Assay for Human Serum Albumin Determination

Herein we demonstrate molecularly imprinted polymers (MIP) as plastic antibodies for a microplate-based assay. As the most abundant plasma protein, human serum albumin (HSA) was selected as the target analyte model. Thin film MIP was synthesized by the surface molecular imprinting approach using HSA as the template. The optimized polymer consisted of acrylic acid (AA) and N-vinylpyrrolidone (VP) in a 2:3 (w/w) ratio, crosslinked with N,N'-(1,2-dihydroxyethylene) bisacrylamide (DHEBA) and then coated on the microplate well. The binding of MIP toward the bound HSA was achieved via the Bradford reaction. The assay revealed a dynamic detection range toward HSA standards in the clinically relevant 1-10 g/dL range, with a 0.01 g/dL detection limit. HSA-MIP showed minimal interference from other serum protein components: γ-globulin had 11% of the HSA response, α-globulin of high-density lipoprotein had 9%, and β-globulin of low-density lipoprotein had 7%. The analytical accuracy of the assay was 89-106% at the 95% confidence interval, with precision at 4-9%. The MIP-coated microplate was stored for 2 months at room temperature without losing its binding ability. The results suggest that the thin film plastic antibody system can be successfully applied to analytical/pseudoimmunological HSA determinations in clinical applications.

Quartz crystal microbalance-based biosensors as rapid diagnostic devices for infectious diseases

Infectious diseases are the ever-present threats to public health and the global economy. Accurate and timely diagnosis is crucial to impede the progression of a disease and break the chain of transmission. Conventional diagnostic techniques are typically time-consuming and costly, making them inefficient for early diagnosis of infections and inconvenient for use at the point of care. Developments of sensitive, rapid, and affordable diagnostic methods are necessary to improve the clinical management of infectious diseases. Quartz crystal microbalance (QCM) systems have emerged as a robust biosensing platform due to their label-free mechanism, which allows the detection and quantification of a wide range of biomolecules. The high sensitivity and short detection time offered by QCM-based biosensors are attractive for the early detection of infections and the routine monitoring of disease progression. Herein, the strategies employed in QCM-based biosensors for the detection of infectious diseases are extensively reviewed, with a focus on prevalent diseases for which improved diagnostic techniques are in high demand. The challenges to the clinical application of QCM-based biosensors are highlighted, along with an outline of the future scope of research in QCM-based diagnostics.

Biomimetic sensors targeting oxidized-low-density lipoprotein with molecularly imprinted polymers

Oxidized-low-density lipoprotein (oxLDL) is well-recognized as an actual patho-atherogenic lipoprotein: elevated serum concentration of oxLDL increases the risk for developing atherosclerosis, leading to coronary artery disease (CAD). Herein, we report an approach for sensing oxLDL directly in serum with molecularly imprinted polymer (MIP) thin films on quartz crystal microbalance (QCM). The resulting MIP sensors show low cross-reaction toward low-density lipoprotein (LDL) and high-density lipoprotein (HDL): signals are around one magnitude smaller. Very-low-density lipoprotein (VLDL) and human serum albumin (HSA) do not lead to any significant sensor response. The sensor allowed for accurately assessing oxLDL over the detection range of 86-5600 μg dL^-1, which covers the clinically relevant concentrations. The sensor determines oxLDL with recovery accuracy of 92-107% and a precision of 1-8% coefficient variation. Compared with commercially available oxLDL ELISA test kit our sensor reveals similar characteristics obtaining a correlation coefficient of 0.98. However, the sensors have rapid response times of 10 min compared to 210 min of ELISA, which demonstrates their efficiency in assessing this sensitive atherogenic biomarker for CAD diagnostics.

A compact device for simultaneous dielectric spectroscopy and microgravimetric analysis under controlled humidity

Water plays a key role in the functioning of natural and synthetic molecular systems. Despite several hydration studies, different techniques are employed individually for monitoring different physical features such as kinetics, dynamics, and absorption. This study describes a compact hydration cell that enables simultaneous dielectric relaxation spectroscopy (DRS) and mass loss/uptake measurements in thin organic layers under controlled humidity conditions and in a wide temperature range. This approach enabled us to correlate the physical quantities obtained during the same experiment by complementary techniques. To demonstrate the performance of this device, a 200 nm thick poly(methyl methacrylate) (PMMA) layer was measured at various relative humidity levels (0%-75%), temperatures (25-75 °C), and frequencies (DRS: 0.1 Hz-1 MHz) to study how hydration and dehydration processes affect its molecular dynamics. The results show the capability of this setup to study the changes in the PMMA film regarding the kinetics and molecular dynamics upon variation of the water content.

Combined Layer/Particle Approaches in Surface Molecular Imprinting of Proteins: Signal Enhancement and Competition

Herein we report novel approaches to the molecular imprinting of proteins utilizing templates sizing around 10 nm and some 100 nm. The first step comprised synthesizing nanoparticles of molecularly imprinted polymers (MIP) towards bovine serum albumin (BSA) and characterizing them according to size and binding capacity. In a second step, they were utilized as templates. Quartz crystal microbalances (QCM) coated with MIP thin films based on BSA MIP nanoparticles lead to a two-fold increase in sensor responses, compared with the case of directly using the protein as the template. This also established that individual BSA molecules exhibit different "epitopes" for molecular imprinting on their outer surfaces. In light of this knowledge, a possible MIP-based biomimetic assay format was tested by exposing QCM coated with BSA MIP thin films to mixtures of BSA and imprinted and non-imprinted polymer (NIP) nanoparticles. At high protein concentrations (1000 ppm) measurements revealed aggregation behavior, i.e., BSA binding MIP NP onto the MIP surface. This increased sensor responses by more than 30% during proof of concept measurements. At lower a BSA concentration (500 ppm), thin films and particles revealed competitive behavior.

High-density lipoprotein sensor based on molecularly imprinted polymer

Decreased blood level of high-density lipoprotein (HDL) is one of the essential criteria in diagnosing metabolic syndrome associated with the development of atherosclerosis and coronary heart disease. Herein, we report the synthesis of a molecularly imprinted polymer (MIP) that selectively binds HDL, namely, HDL-MIP, and thus serves as an artificial, biomimetic sensor layer. The optimized polymer contains methacrylic acid and N-vinylpyrrolidone in the ratio of 2:3, cross-linked with ethylene glycol dimethacrylate. On 10 MHz dual electrode quartz crystal microbalances (QCM), such HDL-MIP revealed dynamic detection range toward HDL standards in the clinically relevant ranges of 2–250 mg/dL HDL cholesterol (HDL-C) in 10 mM phosphate-buffered saline (PBS, pH = 7.4) without significant interference: low-density lipoprotein (LDL) yields 5% of the HDL signal, and both very-low-density lipoprotein (VLDL) and human serum albumin (HSA) yield 0%. The sensor reveals recovery rates between 94 and 104% at 95% confidence interval with precision of 2.3–7.7% and shows appreciable correlation (R² = 0.97) with enzymatic colorimetric assay, the standard in clinical tests. In contrast to the latter, it achieves rapid results (10 min) during one-step analysis without the need for sample preparation.

Graphical Abstract

Molecular Imprinting Technology in Quartz Crystal Microbalance (QCM) Sensors

Molecularly imprinted polymers (MIPs) as artificial antibodies have received considerable scientific attention in the past years in the field of (bio)sensors since they have unique features that distinguish them from natural antibodies such as robustness, multiple binding sites, low cost, facile preparation and high stability under extreme operation conditions (higher pH and temperature values, etc.). On the other hand, the Quartz Crystal Microbalance (QCM) is an analytical tool based on the measurement of small mass changes on the sensor surface. QCM sensors are practical and convenient monitoring tools because of their specificity, sensitivity, high accuracy, stability and reproducibility. QCM devices are highly suitable for converting the recognition process achieved using MIP-based memories into a sensor signal. Therefore, the combination of a QCM and MIPs as synthetic receptors enhances the sensitivity through MIP process-based multiplexed binding sites using size, 3D-shape and chemical function having molecular memories of the prepared sensor system toward the target compound to be detected. This review aims to highlight and summarize the recent progress and studies in the field of (bio)sensor systems based on QCMs combined with molecular imprinting technology.

Molecularly Imprinted Polymer Nanoparticles for Formaldehyde Sensing with QCM

Herein, we report on molecularly imprinted polymers (MIPs) for detecting formaldehyde vapors in air streams. A copolymer thin film consisting of styrene, methacrylic acid, and ethylene glycol dimethacrylate on quartz crystal microbalance (QCM) yielded a detection limit of 500 ppb formaldehyde in dry air. Surprisingly, these MIPs showed specific behavior when tested against a range of volatile organic compounds (VOCs), such as acetaldehyde, methanol, formic acid, and dichloromethane. Despite thus being a suitable receptor in principle, the MIPs were not useful for measurements at 50% humidity due to surface saturation by water. This was overcome by introducing primary amino groups into the polymer via allyl amine and by changing the coating morphology from thin film to nanoparticles. This led to the same limit of detection (500 ppb) and selectivity as before, but at the real-life conditions of 50% relative humidity.

Dopaminergic receptor-ligand binding assays based on molecularly imprinted polymers on quartz crystal microbalance sensors

Molecularly imprinted polymers (MIPs) have been successfully applied as selective materials for assessing the binding activity of agonist and antagonist of dopamine D1 receptor (D1R) by using quartz crystal microbalance (QCM). In this study, D1R derived from rat hypothalamus was used as a template and thus self-organized on stamps. Those were pressed into an oligomer film consisting of acrylic acid: N-vinylpyrrolidone: N,N'-(1,2-dihydroxyethylene) bis-acrylamide in a ratio of 2:3:12 spin coated onto a dual electrode QCM. Such we obtained one D1R-MIP-QCM electrode, whereas the other electrode carried the non-imprinted control polymer (NIP) that had remained untreated. Successful imprinting of D1R was confirmed by AFM. The polymer can re-incorporate D1R leading to frequency responses of 100-1200Hz in a concentration range of 5.9-47.2µM. In a further step such frequency changes proved inherently useful for examining the binding properties of test ligands to D1R. The resulting mass-sensitive measurements revealed Kd of dopamine∙HCl, haloperidol, and (+)-SCH23390 at 0.874, 25.6, and 0.004nM, respectively. These results correlate well with the values determined in radio ligand binding assays. Our experiments revealed that D1R-MIP sensors are useful for estimating the strength of ligand binding to the active single site. Therefore, we have developed a biomimetic surface imprinting strategy for QCM studies of D1R-ligand binding and presented a new method to ligand binding assay for D1R.

Low-Density Lipoprotein Sensor Based on Molecularly Imprinted Polymer

Increased level of low-density lipoprotein (LDL) strongly correlates with incidence of coronary heart disease. We synthesized novel molecularly imprinted polymers (MIP) as biomimetic specific receptors to establish rapid analysis of LDL levels. For that purpose the ratios of monomers acrylic acid (AA), methacrylic acid (MAA), and N-vinylpyrrolidone (VP), respectively, were screened on 10 MHz dual-electrode quartz crystal microbalances (QCM). Mixing MAA and VP in the ratio 3:2 (m/m) revealed linear sensor characteristic to LDL cholesterol (LDL-C) from 4 to 400 mg/dL or 0.10-10.34 mmol/L in 100 mM phosphate-buffered saline (PBS) without significant interference: high-density lipoprotein (HDL) yields 4-6% of the LDL signal, very-low-density-lipoprotein (VLDL) yields 1-3%, and human serum albumin (HSA) yields 0-2%. The LDL-MIP sensor reveals analytical accuracy of 95-96% at the 95% confidence interval with precision at 6-15%, respectively. Human serum diluted 1:2 with PBS buffer was analyzed by LDL-MIP sensors to demonstrate applicability to real-life samples. The sensor responses are excellently correlated to the results of the standard technique, namely, a homogeneous enzymatic assay (R(2) = 0.97). This demonstrates that the system can be successfully applied to human serum samples for determining LDL concentrations.