An Insulin Molecularly Imprinted Electrochemical Sensor Based on Epitope Imprinting

Specific features of epitope-MIPs and whole-protein MIPs as illustrated for AFP and RBD of SARS-CoV-2

Molecularly imprinted polymer (MIP) nanofilms for alpha-fetoprotein (AFP) and the receptor binding domain (RBD) of the spike protein of SARS-CoV-2 using either a peptide (epitope-MIP) or the whole protein (protein-MIP) as the template were prepared by electropolymerization of scopoletin. Conducting atomic force microscopy revealed after template removal and electrochemical deposition of gold a larger surface density of imprinted cavities for the epitope-imprinted polymers than when using the whole protein as template. However, comparable affinities towards the respective target protein (AFP and RBD) were obtained for both types of MIPs as expressed by the KD values in the lower nanomolar range. On the other hand, while the cross reactivity of both protein-MIPs towards human serum albumin (HSA) amounts to around 50% in the saturation region, the nonspecific binding to the respective epitope-MIPs is as low as that for the non-imprinted polymer (NIP). This effect might be caused by the different sizes of the imprinted cavities. Thus, in addition to the lower costs the reduced nonspecific binding is an advantage of epitope-imprinted polymers for the recognition of proteins.

Immunosensors for Autoimmune-Disease-Related Biomarkers: A Literature Review

Immunosensors are a special class of biosensors that employ specific antibodies for biorecognition of the target analyte. Immunosensors that target disease biomarkers may be exploited as tools for disease diagnosis and/or follow-up, offering several advantages over conventional analytical techniques, such as rapid and easy analysis of patients' samples at the point-of-care. Autoimmune diseases have been increasingly prevalent worldwide in recent years, while the COVID-19 pandemic has also been associated with autoimmunity. Consequently, demand for tools enabling the early and reliable diagnosis of autoimmune diseases is expected to increase in the near future. To this end, interest in immunosensors targeting autoimmune disease biomarkers, mainly, various autoantibodies and specific pro-inflammatory proteins (e.g., specific cytokines), has been rekindled. This review article presents most of the immunosensors proposed to date as potential tools for the diagnosis of various autoimmune diseases, such as type 1 diabetes, rheumatoid arthritis, and multiple sclerosis. The signal transduction and the immunoassay principles of each immunosensor have been suitably classified and are briefly presented along with certain sensor elements, e.g., special nano-sized materials used in the construction of the immunosensing surface. The main concluding remarks are presented and future perspectives of the field are also briefly discussed.

Electrochemical sensor based on molecularly imprinted polymer cryogel and multiwalled carbon nanotubes for direct insulin detection

Insulin is the polypeptide hormone that regulates blood glucose levels. It is used as an indicator of both types of diabetes. An electrochemical insulin sensor was developed using a gold electrode modified with carboxylated multiwalled carbon nanotubes (f-MWCNTs) and molecularly imprinted polymer (MIP) cryogel. The MIP provided specific recognition sites for insulin, while the macropores of the cryogel promoted the mass transfer of insulin to the recognition sites. The f-MWCNTs increased the effective surface area and conductivity of the sensor and also reduced the potential required to oxidize insulin. Insulin oxidation was directly measured in a flow system using square wave voltammetry. This MIP cryogel/f-MWCNTs sensor provided a linear range of 0.050-1.40 pM with a very low limit of detection (LOD) of 33 fM. The sensor exhibited high selectivity and long-term stability over 10 weeks of dry storage at room temperature. The results of insulin determination in human serum using the sensor compared well with the results of the Elecsys insulin assay. The developed MIP sensor offers a promising alternative for the diagnosis and treatment of diabetes.

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%).

Epitope imprinted polymeric materials: application in electrochemical detection of disease biomarkers

Epitope imprinting is a promising method for creating specialized recognition sites that resemble natural biorecognition elements. Epitope-imprinted materials have gained a lot of attention recently in a variety of fields, including bioanalysis, drug delivery, and clinical therapy. The vast applications of epitope imprinted polymers are due to the flexibility in choosing monomers, the simplicity in obtaining templates, specificity toward targets, and resistance to harsh environments along with being cost effective in nature. The "epitope imprinting technique," which uses only a tiny subunit of the target as the template during imprinting, offers a way around various drawbacks inherent to biomacromolecule systems i.e., traditional molecular imprinting techniques with regards to the large size of proteins, such as the size, complexity, accessibility, and conformational flexibility of the template. Electrochemical based sensors are proven to be promising tool for the quick, real-time monitoring of biomarkers. This review unravels epitope imprinting techniques, approaches, and strategies and highlights the applicability of these techniques for the electrochemical quantification of biomarkers for timely disease monitoring. In addition, some challenges are discussed along with future prospective developments.

How an ACE2 mimicking epitope-MIP nanofilm recognizes template-related peptides and the receptor binding domain of SARS-CoV-2

Here we aim to gain a mechanistic understanding of the formation of epitope-imprinted polymer nanofilms using a non-terminal peptide sequence, i.e. the peptide GFNCYFP (G485 to P491) of the SARS-CoV-2 receptor binding domain (RBD). This epitope is chemisorbed on the gold surface through the central cysteine 488 followed by the electrosynthesis of a ∼5 nm thick polyscopoletin film around the surface confined templates. The interaction of peptides and the parent RBD and spike protein with the imprinted polyscopoletin nanofilm was followed by electrochemical redox marker gating, surface enhanced infrared absorption spectroscopy and conductive AFM. Because the use of non-terminal epitopes is especially intricate, here we characterize the binding pockets through their interaction with 5 peptides rationally derived from the template sequence, i.e. implementing central single amino acid mismatch as well as elongations and truncations at its C- and N- termini. Already a single amino acid mismatch, i.e. the central Cys488 substituted by a serine, results in ca. 15-fold lower affinity. Further truncation of the peptides to tetrapeptide (EGFN) and hexapeptide (YFPLQS) results also in a significantly lower affinity. We concluded that the affinity towards the different peptides is mainly determined by the four amino acid motif CYFP present in the sequence of the template peptide. A higher affinity than that for the peptides is found for the parent proteins RBD and spike protein, which seems to be due to out of cavity effects caused by their larger footprint on the nanofilm surface.

Electrochemical sensing of macromolecules based on molecularly imprinted polymers: challenges, successful strategies, and opportunities

Looking at the literature focused on molecularly imprinted polymers (MIPs) for protein, it soon becomes apparent that a remarkable increase in scientific interest and exploration of new applications has been recorded in the last several years, from 42 documents in 2011 to 128 just 10 years later, in 2021 (Scopus, December 2021). Such a rapid threefold increase in the number of works in this field is evidence that the imprinting of macromolecules no longer represents a distant dream of optimistic imprinters, as it was perceived until only a few years ago, but is rapidly becoming an ever more promising and reliable technology, due to the significant achievements in the field. The present critical review aims to summarize some of them, evidencing the aspects that have contributed to the success of the most widely used strategies in the field. At the same time, limitations and drawbacks of less frequently used approaches are critically discussed. Particular focus is given to the use of a MIP for protein in the assembly of electrochemical sensors. Sensor design indeed represents one of the most active application fields of imprinting technology, with electrochemical MIP sensors providing the broadest spectrum of protein analytes among the different sensor configurations.

Clinical Evaluation of a Novel Insulin Immunosensor

BACKGROUND

The estimation of available active insulin remains a limitation of automated insulin delivery systems. Currently, insulin pumps calculate active insulin using mathematical decay curves, while quantitative measurements of insulin would explicitly provide person-specific PK insulin dynamics to assess remaining active insulin more accurately, permitting more effective glucose control.

METHODS

We performed the first clinical evaluation of an insulin immunosensor chip, providing near real-time measurements of insulin levels. In this study, we sought to determine the accuracy of the novel insulin sensor and assess its therapeutic risk and benefit by presenting a new tool developed to indicate the potential therapeutic consequences arising from inaccurate insulin measurements.

RESULTS

Nine adult participants with type-1 diabetes completed the study. The change from baseline in immunosensor-measured insulin levels was compared with values obtained by standard enzyme-linked immunosorbant assay (ELISA) after preprandial injection of insulin. The point-of-care quantification of insulin levels revealed similar temporal trends as those from the laboratory insulin ELISA. The results showed that 70% of the paired immunosensor-reference values were concordant, which suggests that the patient could take action safely based on insulin concentration obtained by the novel sensor.

CONCLUSIONS

This proposed technology and preliminary feasibility evaluation show encouraging results for near real-time evaluation of insulin levels, with the potential to improve diabetes management. Real-time measurements of insulin provide person-specific insulin dynamics that could be used to make more informed decisions regarding insulin dosing, thus helping to prevent hypoglycemia and improve diabetes outcomes.

Development of a Novel Insulin Sensor for Clinical Decision-Making

BACKGROUND

Clinical decision support systems that incorporate information from frequent insulin measurements to enhance individualized diabetes management remain an unmet goal. The development of a disposable insulin strip for fast decentralized point-of-care detection replacing the current centralized lab-based methods used in clinical practice would be highly desirable to improve the establishment of individual insulin absorption patterns and algorithm modeling processes.

METHODS

We carried out the development and optimization of a novel decentralized disposable insulin electrochemical sensor focusing on obtaining high analytical and operational performance toward achieving a true point-of-care insulin testing device for clinical on-site application.

RESULTS

Our novel insulin immunosensor demonstrated an attractive performance and efficient user-friendly operation by providing high sensitivity capability to detect endogenous and analog insulin with a limit of detection of 30.2 pM (4.3 µiU/mL), rapid time-to-result, stability toward remote site application, and scalable low-cost fabrication with an estimated cost-of-goods for disposable consumables of below $5, capable of near real-time insulin detection in a microliter (≤10 µL) sample droplet of undiluted serum within 30 minutes.

CONCLUSIONS

The results obtained in the optimization and characterization of our novel insulin sensor illustrate its suitability for its potential application in remote clinical environments for frequent insulin monitoring. Future work will test the insulin sensor in a clinical research setting to assess its efficacy in individuals with type 1 diabetes.

Artificial Biomimetic Electrochemical Assemblies

Rapid, selective, and cost-effective detection and determination of clinically relevant biomolecule analytes for a better understanding of biological and physiological functions are becoming increasingly prominent. In this regard, biosensors represent a powerful tool to meet these requirements. Recent decades have seen biosensors gaining popularity due to their ability to design sensor platforms that are selective to determine target analytes. Naturally generated receptor units have a high affinity for their targets, which provides the selectivity of a device. However, such receptors are subject to instability under harsh environmental conditions and have consequently low durability. By applying principles of supramolecular chemistry, molecularly imprinted polymers (MIPs) can successfully replace natural receptors to circumvent these shortcomings. This review summarizes the recent achievements and analytical applications of electrosynthesized MIPs, in particular, for the detection of protein-based biomarkers. The scope of this review also includes the background behind electrochemical readouts and the origin of the gate effect in MIP-based biosensors.

Amino Acids, Peptides, and Proteins: Implications for Nanotechnological Applications in Biosensing and Drug/Gene Delivery

Over various scientific fields in biochemistry, amino acids have been highlighted in research works. Protein, peptide- and amino acid-based drug delivery systems have proficiently transformed nanotechnology via immense flexibility in their features for attaching various drug molecules and biodegradable polymers. In this regard, novel nanostructures including carbon nanotubes, electrospun carbon nanofibers, gold nanoislands, and metal-based nanoparticles have been introduced as nanosensors for accurate detection of these organic compounds. These nanostructures can bind the biological receptor to the sensor surface and increase the surface area of the working electrode, significantly enhancing the biosensor performance. Interestingly, protein-based nanocarriers have also emerged as useful drug and gene delivery platforms. This is important since, despite recent advancements, there are still biological barriers and other obstacles limiting gene and drug delivery efficacy. Currently available strategies for gene therapy are not cost-effective, and they do not deliver the genetic cargo effectively to target sites. With rapid advancements in nanotechnology, novel gene delivery systems are introduced as nonviral vectors such as protein, peptide, and amino acid-based nanostructures. These nano-based delivery platforms can be tailored into functional transformation using proteins and peptides ligands based nanocarriers, usually overexpressed in the specified diseases. The purpose of this review is to shed light on traditional and nanotechnology-based methods to detect amino acids, peptides, and proteins. Furthermore, new insights into the potential of amino protein-based nanoassemblies for targeted drug delivery or gene transfer are presented.

Epitope-imprinted polymers: Design principles of synthetic binding partners for natural biomacromolecules

Molecular imprinting (MI) has been explored as an increasingly viable tool for molecular recognition in various fields. However, imprinting of biologically relevant molecules like proteins is severely hampered by several problems. Inspired by natural antibodies, the use of epitopes as imprinting templates has been explored to circumvent those limitations, offering lower costs and greater versatility. Here, we review the latest innovations in this technology, as well as different applications where MI polymers (MIPs) have been used to target biomolecules of interest. We discuss the several steps in MI, from the choice of epitope and functional monomers to the different production methods and possible applications. We also critically explore how MIP performance can be assessed by various parameters. Last, we present perspectives on future breakthroughs and advances, offering insights into how MI techniques can be expanded to new fields such as tissue engineering.

Functional Hybrid Micro/Nanoentities Promote Agro-Food Safety Inspection

The rapid development of nanomaterials has provided a good theoretical basis and technical support to solve the problems of food safety inspection. The combination of functionalized composite nanomaterials and well-known detection methods is gradually applied to detect hazardous substances, such as chemical residues and toxins, in agricultural food products. This review concentrates on the latest agro-food safety inspection techniques and methodologies constructed with the assistance of new hybrid micro/nanoentities, such as molecular imprinting polymers integrated with quantum dots (MIPs@QDs), molecular imprinting polymers integrated with upconversion luminescent nanoparticles (MIPs@UCNPs), upconversion luminescent nanoparticles combined with metal-organic frameworks (UCNPs@MOFs), magnetic metal-organic frameworks (MOFs@Fe₃O₄), magnetic covalent-organic frameworks (Fe₃O₄@COFs), covalent-organic frameworks doped with quantum dots (COFs@QDs), nanobody-involved immunoassay for fast inspection, etc. The presented summary and discussion favor a relevant outlook for further integrating various disciplines, like material science, nanotechnology, and analytical methodology, for addressing new challenges that emerge in agro-food research fields.

“Out of Pocket” Protein Binding—A Dilemma of Epitope Imprinted Polymers Revealed for Human Hemoglobin

The epitope imprinting approach applies exposed peptides as templates to synthesize Molecularly Imprinted Polymers (MIPs) for the recognition of the parent protein. While generally the template protein binding to such MIPs is considered to occur via the epitope-shaped cavities, unspecific interactions of the analyte with non-imprinted polymer as well as the detection method used may add to the complexity and interpretation of the target rebinding. To get new insights on the effects governing the rebinding of analytes, we electrosynthesized two epitope-imprinted polymers using the N-terminal pentapeptide VHLTP-amide of human hemoglobin (HbA) as the template. MIPs were prepared either by single-step electrosynthesis of scopoletin/pentapeptide mixtures or electropolymerization was performed after chemisorption of the cysteine extended VHLTP peptide. Rebinding of the target peptide and the parent HbA protein to the MIP nanofilms was quantified by square wave voltammetry using a redox probe gating, surface enhanced infrared absorption spectroscopy, and atomic force microscopy. While binding of the pentapeptide shows large influence of the amino acid sequence, all three methods revealed strong non-specific binding of HbA to both polyscopoletin-based MIPs with even higher affinities than the target peptides.

Epitope-imprinted polydopamine electrochemical sensor for ovalbumin detection

A novel, sensitive and selective electrochemical sensor based on epitope-imprinted polydopamine (PDA) was developed for ovalbumin (OVA) detection. Molecularly imprinted polydopamine was synthesized on an AuNP-coated screen-printed carbon electrode (SPCE) via electropolymerization in the presence of OVA IgE-binding epitope as the template. Key process parameters including template concentration, electropolymerization cycle, pH, time required for template removal and rebinding were optimized. Electrochemical detection of OVA was performed by differential pulse voltammetry (DPV) in 5 mM K₃Fe(CN)₆ and 0.1 M KCl as the supporting electrolyte. Under optimized conditions, the sensor demonstrated excellent sensitivity toward OVA with linear range from 23.25 to 232.50 nM (1 to 10 ppm), limit of detection (LOD) of 10.76 nM (0.46 ppm), and limit of quantification (LOQ) of 35.87 nM (1.54 ppm). The sensor also exhibited good selectivity against other proteins such as human serum albumin (HSA), bovine serum albumin (BSA), and lysozyme (LYZ). OVA in wine samples was detected with RSD of 5.63-10.82%, and recovery percentage of 104.74-105.96%. The developed method can be easily adapted to detect other allergic proteins in the food supply chain.

Epitope-imprinted polymers: applications in protein recognition and separation

Molecularly imprinted polymers (MIPs) have evolved as promising platforms for specific recognition of proteins. However, molecular imprinting of the whole protein molecule is complicated by its large size, conformational instability, and structural complexity. These inherent limitations can be overcome by using epitope imprinting. Significant breakthroughs in the synthesis and application of epitope-imprinted polymers (EIPs) have been achieved and reported. This review highlights recent advances in epitope imprinting, from the selection of epitope peptide sequences and functional monomers to the methods applied in polymerization and template removal. Technological innovations in detection and extraction of proteins by EIPs are also provided.

Epitope imprinting of alpha-synuclein for sensing in Parkinson's brain organoid culture medium

Parkinson's disease (PD) is a progressive nervous system disorder that affects movement, whose early signs may be mild and unnoticed. α-Synuclein has been identified as the major component of Lewy bodies and Lewy neurites, which are the characteristic proteinaceous deposits that are the hallmarks of PD. In this work, three alpha-synuclein peptides were synthesized as templates for the molecular imprinting of conductive polymers to enable recognition of alpha-synuclein via ultrasensitive electrochemical measurements. The peptide sequences encompassed specific residues where mutations are known to accelerate PD (though the target sequences, in this study, were wild-type.) The different peptide targets were all successfully imprinted, but with differing imprinting effectiveness, probably owing to differences in target carboxylic acids (which can bind to the aniline (AN) m-aminobenzenesulfonic acid (MSAN) MIP polymers.) Composition of the imprinted polymer, (the mole proportions of AN and MSAN), and the concentrations and sequences of imprinted peptide templates were optimized by measuring the electrochemical responses to target peptides. The imprinted electrode can detect alpha-synuclein at fg/mL levels, and was therefore used to measure alpha-synuclein in the culture medium of human brain organoids generated from normal and idiopathic PD patients.

Design and fabrication of a smart sensor using in silico epitope mapping and electro-responsive imprinted polymer nanoparticles for determination of insulin levels in human plasma

A robust and highly specific sensor based on electroactive molecularly imprinted polymer nanoparticles (nanoMIP) was developed. The nanoMIP tagged with a redox probe, combines both recognition and reporting capabilities. The developed nanoMIP replaces enzyme-mediator pairs used in traditional biosensors thus, offering enhanced molecular recognition for insulin, improving performance in complex biological samples, and yielding high stability. Also, most of existing sensors show poor performance after storage. To improve costs of the logistics and avoid the need of cold storage in the chain supply, we developed an alternative to biorecognition system that relies on nanoMIP. NanoMIP were computationally designed using "in-silico" insulin epitope mapping and synthesized by solid phase polymerisation. The characterisation of the polymer nanoparticles was performed by transmission electron microscopy (TEM), dynamic light scattering (DLS), Fourier-transform Infrared (FT-IR) and surface plasmon resonance (SPR). The electrochemical sensor was developed by chemical immobilisation of the nanoMIP on screen printed platinum electrodes. The insulin sensor displayed satisfactory performances and reproducible results (RSD = 4.2%; n = 30) using differential pulse voltammetry (DPV) in the clinically relevant concentration range from 50 to 2000 pM. The developed nanoMIP offers the advantage of large number of specific recognition sites with tailored geometry, as the resultant, the sensor showed high sensitivity and selectivity to insulin with a limit of detection (LOD) of 26 and 81 fM in buffer and human plasma, respectively, confirming the practical application for point of care monitoring. Moreover, the nanoMIP showed adequate storage stability of 168 days, demonstrating the robustness of sensor for several rounds of insulin analysis.