An electrochemical chlorpyrifos aptasensor based on the use of a glassy carbon electrode modified with an electropolymerized aptamer-imprinted polymer and gold nanorods

A molecularly imprinted electrochemical sensor MIP/Cu-MOF/rGO/AuNPs/GCE for highly sensitive detection of electroneutral organophosphorus pesticide residues

A novel electrochemical sensor, MIP/Cu-MOF/rGO/AuNPs/GCE, was developed by depositing gold nanoparticles, coating Cu-MOF/GO on the surface of glassy carbon electrode (GCE) before electroreducing graphene oxide (GO) to rGO and covering molecularly imprinted membrane by electropolymerization for highly sensitive detection of electroneutral organophosphorus pesticide residues in agricultural product. Cyclic voltammetry, differential pulse voltametry, scanning electron microscopy, energy-dispersive spectroscopy, and atomic force microscopy were used to characterize the imprinted sensor. Several key factors such as chitosan concentration, suspension volume, pH of polymerization solution, and polymerization scanning rate during preparation of the imprinted sensor were optimized in detail. When electroneutral phosmet was used as a template, the linear range of MIP/Cu-MOF/rGO/AuNPs/GCE for detecting phosmet was 1.00 × 10-14-5.00 × 10-7 mol/L with the limit of detection of 7.20 × 10-15 mol/L at working potentials of - 0.2 to 0.6 V. The selectivity, reproducibility, and repeatability of MIP/Cu-MOF/rGO/AuNPs/GCE were all acceptable. The recoveries of this method for determining phosmet in real samples ranged from 94.2 to 106.5%. The MIP/Cu-MOF/rGO/AuNPs/GCE sensor could be applied to detect electroneutral pesticide residues in organisms and agricultural products.

Recent Advancements in Molecularly Imprinted Polymers Based Aptasensors: Critical Role of Nanomaterials for the Efficient Food Safety Analysis

Biosensors are being studied extensively for their ability to detect and analyze molecules. There has been a growing interest in combining molecular imprinted polymers (MIPs) and aptamers to create hybrid recognition elements that offer advantages such as target binding, sensitivity, selectivity, and stability. These hybrid elements have been successfully used in identifying a wide range of analytes in food samples. However, the application of MIP-based aptasensors in different sensing approaches is still challenging due to the low conductivity of MIPs-aptamers and limited adsorption capacity of MIPs. To address these limitations, researchers have been exploring the use of nanomaterials (NMs) to design efficient multiple-recognition systems that exploit the synergies between aptamers and MIPs. These hybrid systems can enhance the sensitivity and selectivity of MIP-based aptasensors in quantifying analytical samples. This review provides a comprehensive overview of recent advancements in the field of MIP-based aptasensors. It also introduces technologies that combine MIPs and aptamers to achieve higher sensitivity and selectivity in quantifying analytical samples. The review also highlights potential future trends and practical approaches that can be employed to address the limitations of MIP-based aptasensors, including the use of new NMs, the development of new fabrication techniques, and the integration of MIP-based aptasensors with other analytical tools.

Research progress on preparation methods and sensing applications of molecularly imprinted polymer-aptamer dual recognition elements

The aptamer (Apt) and the molecularly imprinted polymer (MIP), as effective substitutes for antibodies, have received widespread attention from researchers because of their creation. However, the low stability of Apt in harsh detection environment and the poor specificity of MIP have hindered their development. Therefore, some researchers have attempted to combine MIP with Apt to explore whether the effect of "1 + 1 > 2" can be achieved. Since its first report in 2013, MIP-Apt dual recognition elements have become a highly focused research direction in the fields of biology and chemistry. MIP-Apt dual recognition elements not only possess the high specificity of Apt and the high stability of MIP in harsh detection environment, but also have high sensitivity and affinity. They have been successfully applied in medical diagnosis, food safety, and environmental monitoring fields. This article provides a systematic overview of three preparation methods for MIP-Apt dual recognition elements and their application in eight different types of sensors. It also provides effective insights into the problems and development directions faced by MIP-Apt dual recognition elements.

Recent Advances in Aptasensors for Rapid Pesticide Residues Detection

Pesticides are applied widely to increase agricultural output and quality, however, this practice results in residual issues that not only harm the environment but also put people and animals' lives and health at risk. As a result, it is critical to find pesticide residues in a variety of sources, including crops, water supplies, and soil. Aptamers are more flexible in their synthesis and modification, have a high level of specificity, are inexpensive, and have good stability compared to conventional detection methods. They have therefore attracted a lot of interest in the industry. This study reviews the most recent aptasensor advancements in the detection of pesticide residues. Firstly, aptamers specifically binding to many pesticides are summarized. Secondly, the combination of aptasensors with colorimetric, fluorescent, surface enhanced Raman spectroscopy (SERS), resonance Light Scattering (RLS), chemiluminescence (CL), electrochemical, and electrochemiluminescence (ECL) technologies are systematically introduced, and their advantages and disadvantages are expounded. Importantly, the aptasensors for the detection of various pesticides (organochlorine, organophosphorus, neonicotinoids, carbamates, and pyrethroids) that have been developed so far are systematically analyzed and discussed. Finally, the furture prospects and challenges of the aptasensors are highlighted. It is expected to offer suggestions for the later creation of novel, highly effective and sensitive aptasensors for the detection of pesticide residues.

Molecularly imprinted sensor based on poly-o-phenylenediamine-hydroquinone polymer for β-amyloid-42 detection

A sensitive and selective molecularly imprinted polymer (MIP) sensor was developed for the determination of amyloid-β (1-42) (Aβ₄₂). The glassy carbon electrode (GCE) was successively modified with electrochemical reduction graphene oxide (ERG) and poly(thionine-methylene blue) (PTH-MB). The MIPs were synthesized by electropolymerization with Aβ₄₂ as a template and o-phenylenediamine (o-PD) and hydroquinone (HQ) as functional monomers. Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), chronoamperometry (CC), and differential pulse voltammetry (DPV) were used to study the preparation process of the MIP sensor. The preparation conditions of the sensor were investigated in detail. In optimal experimental conditions, the response current of the sensor was linear in the range of 0.12-10 μg mL^-1 with a detection limit of 0.018 ng mL^-1. The MIP-based sensor successfully detected Aβ₄₂ in commercial fetal bovine serum (cFBS) and artificial cerebrospinal fluid (aCSF).

Selection and electrochemical-sensor application of an DNA-aptamer for methyl parathion detection

An aptamer-based electrochemical sensor for methyl parathion (MP) detection is herein reported. The modified magnetic beads-systematic evolution of ligands by enrichment (MB-SELEX) was used to select the MP aptamer. After 14 rounds of selection, the aptamer (MPapta-6) with high affinity for MP was obtained, and its dissociation constant (K_d) was 39.66 ± 14.73 μM. Using the MPapta-6, the ultra-sensitive electrochemical sensor based on PLL-BP and AuNPs was constructed. The linear range of MP was 1-10⁵ pM and detection limit (LOD) was as low as 0.49 pM. In addition, the application of the sensor in water samples was verified, and the recovery rate was 96.6%-103.5%. The results from this study showed that this strategy could be applied in practical detection.

Picomolar or beyond Limit of Detection Using Molecularly Imprinted Polymer-Based Electrochemical Sensors: A Review

Over the last decades, molecularly imprinted polymers (MIPs) have emerged as selective synthetic receptors that have a selective binding site for specific analytes/target molecules. MIPs are synthetic analogues to the natural biological antigen-antibody system. Owing to the advantages they exhibit, such as high stability, simple synthetic procedure, and cost-effectiveness, MIPs have been widely used as receptors/sensors for the detection and monitoring of a variety of analytes. Moreover, integrating electrochemical sensors with MIPs offers a promising approach and demonstrates greater potential over traditional MIPs. In this review, we have compiled the methods and techniques for the production of MIP-based electrochemical sensors along with the applications of reported MIP sensors for a variety of analytes. A comprehensive in-depth analysis of recent trends reported on picomolar (pM/10^-12 M)) and beyond picomolar concentration LOD (≥pM) achieved using MIPs sensors is reported. Finally, we discuss the challenges faced and put forward future perspectives along with our conclusion.

A comprehensive review on electrochemical and optical aptasensors for organophosphorus pesticides

There has been a rise in pesticide use as a result of the growing industrialization of agriculture. Organophosphorus pesticides have been widely applied as agricultural and domestic pest control agents for nearly five decades, and they remain as health and environmental hazards in water supplies, vegetables, fruits, and processed foods causing serious foodborne illness. Thus, the rapid and reliable detection of these harmful organophosphorus toxins with excellent sensitivity and selectivity is of utmost importance. Aptasensors are biosensors based on aptamers, which exhibit exceptional recognition capability for a variety of targets. Aptasensors offer numerous advantages over conventional approaches, including increased sensitivity, selectivity, design flexibility, and cost-effectiveness. As a result, interest in developing aptasensors continues to expand. This paper discusses the historical and modern advancements of aptasensors through the use of nanotechnology to enhance the signal, resulting in high sensitivity and detection accuracy. More importantly, this review summarizes the principles and strategies underlying different organophosphorus aptasensors, including electrochemical, electrochemiluminescent, fluorescent, and colorimetric ones.

Molecularly Imprinting–Aptamer Techniques and Their Applications in Molecular Recognition

Molecular imprinting–aptamer techniques exhibit the advantages of molecular imprinting and aptamer technology. Hybrids of molecularly imprinted polymer–aptamer (MIP–aptamer) prepared by this technique have higher stability, binding affinity and superior selectivity than conventional molecularly imprinted polymers or aptamers. In recent years, molecular imprinting–aptamer technologies have attracted considerable interest for the selective recognition of target molecules in complex sample matrices and have been used in molecular recognition such as antibiotics, proteins, viruses and pesticides. This review introduced the development of molecular imprinting–aptamer-combining technologies and summarized the mechanism of MIP–aptamer formation. Meanwhile, we discussed the challenges in preparing MIP–aptamer. Finally, we summarized the application of MIP–aptamer to the molecular recognition in disease diagnosis, environmental analysis, food safety and other fields.

Recent advances in electrochemical and optical sensing of the organophosphate chlorpyrifos: a review

Chlorpyrifos (CP) is one of the most popular organophosphorus pesticides that is commonly used in agricultural and nonagricultural environments to combat pests. However, several concerns regarding contamination due to the unmitigated use of chlorpyrifos have come up over recent years. This has popularized research on various techniques for chlorpyrifos detection. Since conventional methods do not enable smooth detection, the recent trends of chlorpyrifos detection have shifted toward electrochemical and optical sensing techniques that offer higher sensitivity and selectivity. The objective of this review is to provide a brief overview of some of the important and innovative contributions in the field of electrochemical and optical sensing of chlorpyrifos with a primary focus on the comparative advantages and shortcomings of these techniques. This review paper will help to offer better perspectives for research in organophosphorus pesticide detection in the future.

Electrochemical biosensor with aptamer/porous platinum nanoparticle on round-type micro-gap electrode for saxitoxin detection in fresh water

Cyanotoxins are toxins produced by cyanobacteria; they negatively impact water resources used by humans and disrupt ecosystems worldwide. Among cyanotoxins, saxitoxin (STX) is a small molecule that causes paralysis in humans and contamination in freshwater resources. To monitor low concentration of STX levels, a sensitive and high fidelity detection system is required. In this study, a round-type micro-gap electrode (RMGE) was fabricated that provides the high signal fidelity for STX detection in real freshwater sample. The RMGE has the 15 pairs of identical electrode wire length between gap that gives the high signal fidelity. In addition, the sensitivity for STX detection was improved by introducing the porous platinum nanoparticle (pPtNP) that enahced the electrochemical sensitivity and the STX aptamer was used as the bioprobe. An electrochemical measurement method (square wave voltammetry (SWV) and electrochemical impedance spectroscopy (EIS)) was introduced to construct STX biosensor. To evaluate the biosensor performance, the limit of detection (LOD) and selectivity test were performed on real freshwater samples. The biosensor demonstrated high selectivity even in freshwater samples over a wide linear concentration range of 10 pg/mL to 1 μg/mL and a detection limit of 4.669 pg/mL. These results suggest that the designed biosensor shows a wide range of possibilities for the detection of toxicants in freshwater that provide the new direction to the biosensor electrode design.

An Aptasensor Based on a Flexible Screen-Printed Silver Electrode for the Rapid Detection of Chlorpyrifos

In this work, we propose a novel disposable flexible and screen-printed electrochemical aptamer-based sensor (aptasensor) for the rapid detection of chlorpyrifos (CPF). To optimize the process, various characterization procedures were employed, including Fourier transform infrared spectroscopy (FT-IR), electrochemical impedance spectroscopy (EIS), and cyclic voltammetry (CV). Initially, the aptasensor was optimized in terms of electrolyte pH, aptamer concentration, and incubation time for chlorpyrifos. Under optimal conditions, the aptasensor showed a wide linear range from 1 to 105 ng/mL with a calculated limit of detection as low as 0.097 ng/mL and sensitivity of 600.9 µA/ng. Additionally, the selectivity of the aptasensor was assessed by identifying any interference from other pesticides, which were found to be negligible (with a maximum standard deviation of 0.31 mA). Further, the stability of the sample was assessed over time, where the reported device showed high stability over a period of two weeks at 4 °C. As the last step, the ability of the aptasensor to detect chlorpyrifos in actual samples was evaluated by testing it on banana and grape extracts. As a result, the device demonstrated sufficient recovery rates, which indicate that it can find application in the food industry.

Recent Advancements in the Technologies Detecting Food Spoiling Agents

To match the current life-style, there is a huge demand and market for the processed food whose manufacturing requires multiple steps. The mounting demand increases the pressure on the producers and the regulatory bodies to provide sensitive, facile, and cost-effective methods to safeguard consumers' health. In the multistep process of food processing, there are several chances that the food-spoiling microbes or contaminants could enter the supply chain. In this contest, there is a dire necessity to comprehend, implement, and monitor the levels of contaminants by utilizing various available methods, such as single-cell droplet microfluidic system, DNA biosensor, nanobiosensor, smartphone-based biosensor, aptasensor, and DNA microarray-based methods. The current review focuses on the advancements in these methods for the detection of food-borne contaminants and pathogens.

Dual-recognition colorimetric sensing of thrombin based on surface-imprinted aptamer-Fe3O4

Thrombin plays an essential role in blood coagulation and some physiological and pathological processes. The convenient, rapid, sensitive, and specific detection of thrombin is of great significance in clinical research and diagnosis. Herein, surface molecularly imprinted polymer (MIP) was modified on aptamer-functionalized Fe3O4 nanoparticles (MIP-aptamer-Fe3O4 NP) for thrombin colorimetric assay by taking advantage of the peroxidase-like activity of Fe3O4 NP. With the adsorption of thrombin into imprinted cavities, the exposed surface area of Fe3O4 NP decreased, causing a decrease in its peroxidase-like activity toward 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of H2O2. On the other hand, the reductive amino acids on the thrombin surface also impeded the oxidation of TMB. Both phenomena caused the light blue color of the sensing solution. Thus, a specifically sensitive colorimetric approach for the visual detection of thrombin was proposed with a linear range and limit of detection of 108.1 pmol L-1-2.7 × 10-5 mol L-1 and 27.8 pmol L-1, respectively. Moreover, due to the double recognition elements of MIP and aptamer, the prepared MIP-aptamer-Fe3O4 NP showed higher selectivity to thrombin than that based on only one recognition element. It is worth noting that no special property (e.g. electrochemical or fluorescence activity) of the template was required in this work. Thus, more template molecules can be easily, selectively, and sensitively detected based on the proposed MIP-aptamer-mimic enzyme colorimetric sensing strategy.

Molecularly Imprinted Polymers Combined with Electrochemical Sensors for Food Contaminants Analysis

Detection of relevant contaminants using screening approaches is a key issue to ensure food safety and respect for the regulatory limits established. Electrochemical sensors present several advantages such as rapidity; ease of use; possibility of on-site analysis and low cost. The lack of selectivity for electrochemical sensors working in complex samples as food may be overcome by coupling them with molecularly imprinted polymers (MIPs). MIPs are synthetic materials that mimic biological receptors and are produced by the polymerization of functional monomers in presence of a target analyte. This paper critically reviews and discusses the recent progress in MIP-based electrochemical sensors for food safety. A brief introduction on MIPs and electrochemical sensors is given; followed by a discussion of the recent achievements for various MIPs-based electrochemical sensors for food contaminants analysis. Both electropolymerization and chemical synthesis of MIP-based electrochemical sensing are discussed as well as the relevant applications of MIPs used in sample preparation and then coupled to electrochemical analysis. Future perspectives and challenges have been eventually given.

A highly sensitive acetylcholinesterase electrochemical biosensor based on Au-Tb alloy nanospheres for determining organophosphate pesticides

Accurately detect the residues of organophosphate pesticides (OPs) in food and environment is critical to our daily lives. In this study, we developed a novel acetylcholinesterase (AChE) biosensor based on Au-Tb alloy nanospheres (NSs) for rapid and sensitive detection of OPs for the first time. Au-Tb alloy NSs that with good conductivity and biocompatibility were produced with a mild hydrothermal. Under optimal conditions, the AChE biosensor was obtained by a simple assembly process, with a big linear range (10^-13-10^-7M) and the limit of detection was 2.51 × 10^-14M for the determination of methyl parathion. Moreover, the determination of methyl parathion with the prepared biosensor presented a high sensitivity, outstanding repeatability and superior stability compared with other reported biosensors. Through the determination of tap water and Yanming lake samples, it was proved that the modified biosensor with satisfactory recoveries (96.76%-108.6%), and are realizable in the determination of OPs in real samples.

A facile colorimetric aptasensor for low-cost chlorpyrifos detection utilizing gold nanoparticle aggregation induced by polyethyleneimine

A colorimetric aptasensor for chlorpyrifos detection utilizing the localized surface plasmon resonance (LSPR) of gold nanoparticle (AuNP) aggregates coupling with a specific aptamer and cationic polyethyleneimine (PEI) has been developed. The measurement principle is based on a remarkable characteristic of AuNPs that can change their colors under the aggregation and dispersion conditions, which enables a sensitive colorimetric detection. In the absence of chlorpyrifos, negatively charged phosphate backbones of the aptamer potentially interact with the cationic PEI, resulting in the red color appearance of the dispersed AuNPs, whereas, in the presence of chlorpyrifos, the aptamer binds explicitly to chlorpyrifos, consequently releasing cationic PEI. Uninteracted PEI induces AuNP aggregation, causing a color change from red to blue that can be observed through the naked eye. Under the optimized conditions, 6 nM PEI, 10 nM aptamer, and a pH buffer of 7.5, the colorimetric aptasensor gives a linear response in the range of 20-300 ng mL-1 with a low detection limit of 7.4 ng mL-1. The developed method has been successfully applied to complex sample analysis. The accuracy and precision of chlorpyrifos quantification in spiked samples, including tap water, pomelo, and longan samples, are in the acceptable criteria of method validation, indicating that the developed aptasensor can be utilized as an alternative analytical tool for chlorpyrifos determination in complex samples. This aptasensor provides advantages such as a simple procedure, low cost, short analysis time, and involving uncomplicated instruments. Moreover, it offers high sensitivity, selectivity, and stability.

Electrochemical aptasensor based on Mo2C/Mo2N and gold nanoparticles for determination of chlorpyrifos

Two-dimensional Mo₂C/Mo₂N composites were synthesized by high temperature ball milling and used as support materials for fabricating a chlorpyrifos (CPF) aptasensor. Gold nanoparticles (Au NPs) were electrodeposited on the surface of a Mo₂C/Mo₂N-modified electrode to connect with the ferrocene (Fc) probe via Au-S bonds. The Fc probe can hybridize with the aptamer probe to form a double-stranded structure. The addition of CPF made the double strands melt and the Fc probe approached the surface of the electrode, thereby resulting in amplification of the electrochemical response. The current response of the aptasensor for detecting CPF in solutions linearly varied from 0 to 400 ng mL^-1 (with a maximum at 0.98 V vs. Ag/AgCl). The Au NPs/Mo₂C/Mo₂N composites exhibited satisfactory electrochemical behavior due to their excellent electrical conductivity and large surface area. This ultrasensitive aptasensor showed a low limit of detection of 0.036 ng mL^-1. It was applied to determine CPF in real samples with acceptable recoveries from 94.7 to 116.7%, and the relative standard deviation was from 2.57 to 7.08%.Graphical abstract Schematic diagram of the manufacturing process of the aptasensor. Electrochemical aptasensor based on Mo₂C/Mo₂N/Au NP composites show excellent performance in detecting CPF.

Recent Advances in Aptamer-Based Biosensors for Global Health Applications

Since aptamers were first reported in the early 2000s, research on their use for the detection of health-relevant analytical targets has exploded. This review article provides a brief overview of the most recent developments in the field of aptamer-based biosensors for global health applications. The review provides a description of general aptasensing principles and follows up with examples of recent reports of diagnostics-related applications. These applications include detection of proteins and small molecules, circulating cancer cells, whole-cell pathogens, extracellular vesicles, and tissue diagnostics. The review also discusses the main challenges that this growing technology faces in the quest of bringing these new devices from the laboratory to the market.

An enzyme-free electrochemiluminescence sensing probe based on ternary nanocomposite for ultrasensitive determination of chlorpyrifos

Herein, an innovative enzyme free electrochemiluminescence chlorpyrifos sensor was reported based on the incorporation of ternary nanocomposite (ruthenium nanobeads/silver nanoparticles/graphene oxide) on the surface of glassy carbon electrode. The silver nanoparticles as a robust signal enhancing agent were well deposited on the modified graphene oxide/glassy carbon electrode and exhibited supreme electrocatalytic proficiency. Then, Ru(bipy)₃²⁺ species in bead-like nanoparticles were uniformly anchored on the surface of the modified electrode with the help of chitosan, as a good crosslinking agent. Boron nitride quantum dots as a new coreactant species showed the excellent proficiency for signal enhancement of the fabricated electrode. The fabricated electrode was successfully used as an ultra-sensitive sensor for trace analysis of chlorpyrifos by ECL technique. The wide linear range and the low detection limit were obtained from 5.0 × 10^-15 to 4.2 × 10^-9 M and 6.5 × 10^-16 M, respectively.

Pesticide Aptasensors-State of the Art and Perspectives

Contamination by pesticides in the food chain and the environment is a worldwide problem that needs to be actively monitored to ensure safety. Unfortunately, standard pesticide analysis based on mass spectrometry takes a lot of time, money and effort. Thus, simple, reliable, cost-effective and field applicable methods for pesticide detection have been actively developed. One of the most promising technologies is an aptamer-based biosensor or so-called aptasensor. It utilizes aptamers, short single-stranded DNAs or RNAs, as pesticide recognition elements to integrate with various innovative biosensing technologies for specific and sensitive detection of pesticide residues. Several platforms for aptasensors have been dynamically established, such as colorimetry, fluorometry, electrochemistry, electrochemiluminescence (ECL) and so forth. Each platform has both advantages and disadvantages depending on the purpose of use and readiness of technology. For example, colorimetric-based aptasensors are more affordable than others because of the simplicity of fabrication and resource requirements. Electrochemical-based aptasensors have mainly shown better sensitivity than others with exceedingly low detection limits. This paper critically reviews the progression of pesticide aptasensors throughout the development process, including the selection, characterization and modification of aptamers, the conceptual frameworks of integrating aptamers and biosensors, the ASSURED (affordable, sensitive, specific, user-friendly, rapid and robust, equipment-free and deliverable to end users) criteria of different platforms and the future outlook.

Recent Advances of Conducting Polymers and Their Composites for Electrochemical Biosensing Applications

Conducting polymers (CPs) have been at the center of research owing to their metal-like electrochemical properties and polymer-like dispersion nature. CPs and their composites serve as ideal functional materials for diversified biomedical applications like drug delivery, tissue engineering, and diagnostics. There have also been numerous biosensing platforms based on polyaniline (PANI), polypyrrole (PPY), polythiophene (PTP), and their composites. Based on their unique properties and extensive use in biosensing matrices, updated information on novel CPs and their role is appealing. This review focuses on the properties and performance of biosensing matrices based on CPs reported in the last three years. The salient features of CPs like PANI, PPY, PTP, and their composites with nanoparticles, carbon materials, etc. are outlined along with respective examples. A description of mediator conjugated biosensor designs and enzymeless CPs based glucose sensing has also been included. The future research trends with required improvements to improve the analytical performance of CP-biosensing devices have also been addressed.

Electrochemical Affinity Biosensors Based on Selected Nanostructures for Food and Environmental Monitoring

The excellent capabilities demonstrated over the last few years by electrochemical affinity biosensors should be largely attributed to their coupling with particular nanostructures including dendrimers, DNA-based nanoskeletons, molecular imprinted polymers, metal-organic frameworks, nanozymes and magnetic and mesoporous silica nanoparticles. This review article aims to give, by highlighting representative methods reported in the last 5 years, an updated and general overview of the main improvements that the use of such well-ordered nanomaterials as electrode modifiers or advanced labels confer to electrochemical affinity biosensors in terms of sensitivity, selectivity, stability, conductivity and biocompatibility focused on food and environmental applications, less covered in the literature than clinics. A wide variety of bioreceptors (antibodies, DNAs, aptamers, lectins, mast cells, DNAzymes), affinity reactions (single, sandwich, competitive and displacement) and detection strategies (label-free or label-based using mainly natural but also artificial enzymes), whose performance is substantially improved when used in conjunction with nanostructured systems, are critically discussed together with the great diversity of molecular targets that nanostructured affinity biosensors are able to quantify using quite simple protocols in a wide variety of matrices and with the sensitivity required by legislation. The large number of possibilities and the versatility of these approaches, the main challenges to face in order to achieve other pursued capabilities (development of antifouling, continuous operation, wash-, calibration- and reagents-free devices, regulatory or Association of Official Analytical Chemists, AOAC, approval) and decisive future actions to achieve the commercialization and acceptance of these devices in our daily routine are also noted at the end.

Magnetically controlled colorimetric aptasensor for chlorpyrifos based on copper-based metal-organic framework nanoparticles with peroxidase mimetic property

The fabrication of a magnetically controlled colorimetric aptasensor for chlorpyrifos is reported. The aptasensor was fabricated by the attachment of the colorimetric labels onto the magnetic carrier due to the hybridization reaction between the complementary DNA and aptamer. Chlorpyrifos detection was realized by monitoring the color changes of the TMB/H₂O₂ solution before and after incubation of the aptasensor with chlorpyrifos via exposure to external magnetic force. The color change was monitored at 650 nm by UV-Vis spectrophotometer. Under the optimal conditions, this magnetically controlled Cu-MOF-based aptasensor showed a detection limit of 4.4 ng/mL with a linear range of 0-1250 ng/mL. The colorimetric aptasensor displayed high selectivity for chlorpyrifos toward other interfering pesticides. The aptasensor was successfully applied for the spiked test of chlorpyrifos in fruits and vegetable samples with good recovery, which were in agreement with data obtained by GC-MS analysis. This magnetically controlled Cu-MOF-based sensing strategy not only leads to development of efficient and facile phase separation, but also expands the MOF's target scope from H₂O₂ or glucose to pesticides. Graphical abstract.

Molecularly Imprinted Electrochemical Sensors: Analytical and Pharmaceutical Applications Based on Ortho-Phenylenediamine Polymerization

Background:

The molecular imprinting technique has been applied in many fields including separation, artificial antibody mimics, catalysis, sensing studies, and drug delivery. The reasons for the popularity of this technique among the researchers are high selectivity due to the cavities that are formed on the polymer surface for the specific analyte, high robustness, high durability under extreme conditions and low cost. When these advantages are combined with the advantages of electrochemical methods such as rapid response time, ease of use, cheapness and miniaturizability, Molecularly Imprinted Polymer (MIP) based electrochemical sensors turn out to be a widely-preferred sensing tool.

Objective:

This article provides the reader with information on MIP-based electrochemical sensors and reviews the applications of the MIP sensors prepared by electropolymerization of orthophenylenediamine, a monomer whose mechanical and chemical stability is very high.

Results and Conclusion:

The literature survey summarized in this review shows that cyclic voltammetry is the most widely preferred electrochemical technique for electropolymerization of o-PD. The media chosen is generally acetate or phosphate buffers with different pH values. Although there are numerous solvents used for template removal, generally methanol and NaOH have been chosen.

Electrochemical aptasensors based on the gold nanostructures

Electrochemical aptasensors as novel diagnostic tools have attracted sufficient research interest in biomedical sciences. In this review, recent leading trends about gold (Au) nanostructures based electrochemical aptasensors have been collected, reviewed, and compared. Here, the considered electrochemical aptasensors were categorized based on the analytes and diagnostic techniques. Pharmaceutical analytes and biomolecules were reviewed in a separate section consisting of a variety of antibiotics, analgesics, and other biomolecules. Various aptasensors have also measured toxins, ions, and hazardous chemicals, and the findings of them have also been reviewed. Many aptasensors have been designed to detect different disease biomarkers that will play an essential role in the future of early diagnosis of diseases. Pathogen microorganisms have been considered as the analyte in several designed electrochemical aptasensors in recent researches, and their results have been reviewed and discussed as another section. Important aspects considered in the review of the mentioned aptasensors were the type of analyte, features of the aptamer as the biorecognition element, type of Au nanostructures, diagnostic technique, diagnostic mechanism, detection range and the limit of detection (LOD). In the last section, an in-depth analysis has been provided based on the crucial features of all included aptasensors.

A competitive immunoassay for electrochemical impedimetric determination of chlorpyrifos using a nanogold-modified glassy carbon electrode based on enzymatic biocatalytic precipitation

A direct competitive impedimetric immunoassay for chlorpyrifos (CPF) was developed that is based on the specific affinity of immunoassay and the enzymatic biocatalytic precipitation amplification strategy. The CPF antibody (anti-CPF) was anchored onto an electro-deposited nanogold modified glassy carbon electrode surface by adsorption of the Au-NH₂ bond and Au-SH bond. This improved the electrode reactivity and the loading amount of anti-CPF. Abundant horseradish peroxidase (HRP) and bovine serum albumin-CPF (BSA-CPF) were anchored onto spherical gold nanoparticles (AuNPs, 16 ± 2 nm) to form HRP-AuNP-BSA-CPF (analyte competitor). CPF determination was achieved when the competitive immunoassay occurred between CPF and analyte competitor with anti-CPF. In the presence of H₂O₂ and 4-chloro-1-naphthol, an enzyme-mediated biocatalytic precipitation process was triggered and produced an insoluble 4-chloro-1(4H)-naphthalenone. This insoluble substance increased the Faradaic impedance of the base electrode. The impedimetric signal was determined at the formal potential of 220 mV and alternating voltage of 10 mV. This signal decreased with increasing concentration of CPF over a linear range of 1.0 × 10^-3 ng mL^-1~10 ng mL^-1 with a detection limit of 0.070 pg mL^-1. The immunoassay has been tested for determination of chlorpyrifos in complex matrices such as artificially spiked vegetables with recoveries in the range 85 to 110%. The relative standard deviations were less than 7.5%. Graphical abstractSchematic representation of electrochemical impedimetric immunoassay for chlorpyrifos determination before enzymatic biocatalytic precipitation (BCP, red line) process and after BCP process (blue line).

A sensitive electrochemical sensor modified with multi-walled carbon nanotubes doped molecularly imprinted silica nanospheres for detecting chlorpyrifos

A highly sensitive and convenient electrochemical sensor, based on surface molecularly imprinted polymers and multiwalled carbon nanotubes, was successfully developed to detect chlorpyrifos in real samples. In order to solve the problems like uneven shapes, poor size accessibility, and low imprinting capacity, the layer of the molecularly imprinted polymer was prepared on the surface of silica nanospheres. Moreover, the doping of multiwalled carbon nanotubes greatly improved the electrical properties of developed sensor. Under the optimal conductions, the electrochemical response of the sensor is linearly proportional to the concentration of chlorpyrifos in the range of 5.0 × 10^-12 -5.0 × 10^-8  mol/L with a low detection limit of 8.1 × 10^-13  mol/L. The prepared sensor exhibited multiple advantages such as low cost, simple preparation, convenient use, excellent selectivity, and good reproducibility. Finally, the prepared sensor was successfully used to detect chlorpyrifos in vegetable and fruit.

An imprinted polymeric matrix containing DNA for electrochemical sensing of 2,4-dichlorophenoxyacetic acid

The authors describe an electrochemical method for the determination of herbicide 2,4-dichlorophenoxyacetic acid (2,4-D). It is based on the use of a molecularly imprinted polymer (MIP) and of dsDNA as a bio-specific substance. The modified electrode was prepared by electropolymerization of ortho-phenylenediamine (oPD) in the presence of DNA and of 2,4-D (the template). The imprinted MIP was placed on a pencil graphite electrode (PGE) modified with chitosan and multiwalled carbon nanotubes (MWCNTs). The template was removed with 0.4 M NaOH. The interaction of DNA with 2,4-D leads to its adsorption on the electrode, and this increases the sensitivity and selectivity of the method. After rebinding 2,4-D, the decrease in the peak current of oxidation of iron(II) acting as an electrochemical redox probe was measured by differential pulse voltammetry (DPV). The current, typically measured at around 0.5 V, increases linearly in the 0.01 to 10 pM 2,4-D concentration range, and the detection limit is 4.0 fM. The method is highly selective for 2,4-D. The modified electrode was applied to quantify 2,4-D in spiked environmental water and soil samples and gave absolute recoveries varying from 91.5 to 109.0%. Graphical abstractSchematic representation of the fabrication of an electrochemical sensor for determination of 2,4-dichlorophenoxyacetic acid (2,4-D). Initially, the electrode was modified with chitosan and MWCNTs and then a composite was formed on it consisting of ortho-phenylenediamine (oPD), DNA and 2,4-D.

Screen printed carbon electrode sensor with thiol graphene quantum dots and gold nanoparticles for voltammetric determination of solatol

This work, a highly selective and sensitive sensor is described for voltammetric determination of the sotalol (SOT). The dual actions of sotalol lead to reductions in the automaticity of myocardial cells and in conduction through the atrioventricular node. Drug analysis has an extensive impact on public health. The molecularly imprinted sensor was constructed by modifying a screen printed carbon electrode (SPCE) with thiol graphene quantum dots (GQD-SH) and gold nanoparticles (AuNPs). Under optimal conditions the nanotools has a dynamic range that covers the 0.1-250 μM SOT concentration range, and the detection limit is 0.035 μM. This is lower than any of the previously reported methods. The MIP-sensor also exhibited excellent selectivity, good stability and adequate reproducibility for the detection of the SOT over its structural analogs. The prepared sensor was successfully applied to the measurement of SOT in various real samples including tablet and human blood serum.

Recent advances of molecularly imprinted polymer-based sensors in the detection of food safety hazard factors

With increasing economic globalization, food safety is becoming the most serious concern in the food production and distribution system. Food safety hazard factors (FSHFs) can be categorized into chemical hazards, biological hazards and physical hazards, with the detection of the former two having fascinated interdisciplinary research areas spanning chemistry, material science and biological science. Molecularly imprinted polymer (MIP) -based sensors overcome many limitations of traditional detection methods and provide opportunities for efficient, sensitive and low-cost detection using smart miniaturized equipment. With highly specific molecular recognition capacity and high stability in harsh chemical and physical conditions, MIPs have been used in sensing platforms such as electrochemical, optical and mass-sensitive sensors as promising alternatives to bio-receptors for food analysis. In this systemic review, we summarize recent advances of MIPs and MIP-based sensors, such as popular monomers, usual polymerization strategies, fresh modification materials and advanced sensing mechanisms. The applications of MIP-based sensors in FSHF detection are discussed according to sensing mechanisms, including electrochemistry, optics and mass-sensitivity. Finally, future perspectives and challenges are discussed.

Electrostatic assembly of gold nanoparticles on black phosphorus nanosheets for electrochemical aptasensing of patulin

An aptamer based impedimetric assay for the mycotoxin patulin (PAT) is described. A glassy carbon electrode (GCE) was modified with black phosphorus nanosheets (BP NSs) and modified with PAT aptamer by electrostatic attraction. Detection is based on the variations of electron transfer resistance at the modified electrode surface. This assay can detect PAT over a linear range that extends from 1.0 nM to 1.0 μM with a 0.3 nM detection limit. To improve the performance of the sensor, the BP NS-GCE was further modified with gold nanoparticles and then with thiolated PAT aptamer. This modified electrode, operated at an applied potential of 0.18 V (vs. Ag/AgCl), has a wider linear range (0.1 nM to 10.0 μM) and a lower detection limits (0.03 nM). Both assays were successfully applied to the analysis of (spiked) genuine food samples. Graphical abstract Black phosphorus nanosheets (BP NSs) were used to fabricate an aptamer based assay for patulin. To further improve the performance of the electrode, gold nanoparticles (AuNP) were placed on the surface of black phosphorus nanosheets (AuNP-BP NSs) by electrostatic attraction for patulin aptasensing.

Fluorescent aptasensing of chlorpyrifos based on the assembly of cationic conjugated polymer-aggregated gold nanoparticles and luminescent metal–organic frameworks

A fluorescent aptasensor of chlorpyrifos based on terbium(iii) based on metal–organic frameworks and PDDA-aggregated-gold nanoparticles.