Electrochemical Aptasensor Based on Poly(Neutral Red) and Carboxylated Pillar[5]arene for Sensitive Determination of Aflatoxin M1

Novel Sensor Approaches of Aflatoxins Determination in Food and Beverage Samples

The rapid quantification of toxins in food and beverage products has become a significant issue in overcoming and preventing many life-threatening diseases. Aflatoxin-contaminated food is one of the reasons for primary liver cancer and induces some tumors and cancer types. Advancements in biosensors technology have brought out different analysis methods. Therefore, the sensing performance has been improved for agricultural and beverage industries or food control processes. Nanomaterials are widely used for the enhancement of sensing performance. The enzymes, molecularly imprinted polymers (MIP), antibodies, and aptamers can be used as biorecognition elements. The transducer part of the biosensor can be selected, such as optical, electrochemical, and mass-based. This review explains the classification of major types of aflatoxins, the importance of nanomaterials, electrochemical, optical biosensors, and QCM and their applications for the determination of aflatoxins.

CRISPR-Cas12a-based colorimetric aptasensor for aflatoxin M1 detection based on oxidase-mimicking activity of flower-like MnO2 nanozymes

Given the highly mutagenic and carcinogenic nature of Aflatoxin M1 (AFM1), the quantity assessment of AFM1 residues in milk and dairy products is necessary to maintain consumer health and food safety. Herein, CRISPR-Cas12a-based colorimetric aptasensor was developed using the catalytic activity of flower-like nanozymes of MnO2 and trans-cleavage property of CRISPR-Cas12a system to quantitatively detect AFM1. The basis of the developed colorimetric aptasensor relies on whether or not the CRISPR-Cas12a system is activated, as well as the contrast in oxidase-mimicking capability exhibited by flower-like MnO2 nanozymes when AFM1 is absent or present. When AFM1 is not present in the sample, single-stranded DNA (ssDNA) is degraded by the activated CRISPR-Cas12a, and the solution turns into yellow due to the catalytic activity of the nanozymes. While, in the attendance of AFM1, ssDNA degradation does not occur due to the inactivation of the CRISPR-Cas12a. Therefore, with the adsorption of the ssDNA on the MnO2 nanozymes, their catalytic activity decreases, and the solution color becomes pale yellow due to less oxidation of the chromogenic substrate. In this aptasensor, the relative absorbance changes increased linearly from 6 to 160 ng L-1, and the detection limit was 2.1 ng L-1. The developed aptasensor displays a selective detection performance and a practical application for quantitative analysis of AFM1 in milk samples. The results of the introduced aptasensor open up the way to design other selective and sensitive aptasensors for the detection of other mycotoxins by substitution of the used sequences.

The Ultrasensitive Detection of Aflatoxin M1 Using Gold Nanoparticles Modified Electrode with Fe3+ as a Probe

The increasing incidence of diseases caused by highly carcinogenic aflatoxin M₁ (AFM₁) in food demands a simple, fast, and cost-effective detection technique capable of sensitively monitoring AFM₁. Recent works predominantly focus on the electrochemical aptamer-based biosensor, which still faces challenges and high costs in experimentally identifying an efficient candidate aptamer. However, the direct electrochemical detection of AFM₁ has been scarcely reported thus far. In this study, we observed a significant influence on the electrochemical signals of ferric ions at a gold nanoparticle-modified glassy carbon electrode (AuNPs/GCE) by adding varying amounts of AFM₁. Utilizing ferricyanide as a sensitive indicator of AFM₁, we have introduced a novel approach for detecting AFM₁, achieving an unprecedentedly low detection limit of 1.6 × 10^-21 g/L. Through monitoring the fluorescence quenching of AFM₁ with Fe³⁺ addition, the interaction between them has been identified at a ratio of 1:936. Transient fluorescence analysis reveals that the fluorescence quenching process is predominantly static. It is interesting that the application of iron chelator diethylenetriaminepentaacetic acid (DTPA) cannot prevent the interaction between AFM₁ and Fe³⁺. With a particle size distribution analysis, it is suggested that a combination of AFM₁ and Fe³⁺ occurs and forms a polymer-like aggregate. Nonetheless, the mutual reaction mechanism between AFM₁ and Fe³⁺ remains unexplained and urgently necessitates unveiling. Finally, the developed sensor is successfully applied for the AFM₁ test in real samples, fully meeting the detection requirements for milk.

Selective and Sensitive Electrochemical Sensor for Aflatoxin M1 with a Molybdenum Disulfide Quantum Dot/Metal-Organic Framework Nanocomposite

Aflatoxins are the hepatotoxic secondary metabolites which are highly carcinogenic and known to cause several adverse effects on human health. The present study reports a simple, sensitive, and novel electrochemical sensor for aflatoxin M1 (AFM1). The sensor has been fabricated by modifying the screen-printed carbon electrodes with a functional nanocomposite of molybdenum disulfide (MoS₂) quantum dots (QDs) and a zirconium-based metal-organic framework (MOF), that is, UiO-66-NH₂. The MoS₂/UiO-66-modified electrodes were decorated with the AFM1-specific monoclonal antibodies and then investigated for the electrochemical detection of AFM1. Based on the electrochemical impedance spectroscopy analysis, it was possible to detect AFM1 in the concentration range of 0.2-10 ng mL^-1 with a limit of detection of 0.06 ng mL^-1. The realization of an excellent sensing performance can be attributed to the electroactivity of MoS₂ QDs and the large surface to volume area achieved by the addition of the MOF. The presence of UiO-66-NH₂ is also useful to attain readily available amine functionality for the robust interfacing of antibodies. The performance of the developed sensor has also been validated by detecting AFM1 in the spiked milk samples.

Monitoring of Aflatoxin M1 in milk using a novel electrochemicalaptasensorbased on reduced graphene oxide and gold nanoparticles

Accurate and rapid detection of Aflatoxins as one of the most hazardous compounds in foodstuffs is very important. In this study, a label-free electrochemical aptasensor was developed to identify aflatoxin M1 using a reduced graphene oxide (rGO) and gold nanoparticles (AuNPs)-based pencil graphite electrode (PGE). The morphological characteristics of the electrode surface were investigated using SEM and rGO functional groups were confirmed by FTIR. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) techniques were used to characterize various stages of electrode modification. In order to optimize the impedimetric response of the aptasensor, aptamer immobilization time, aptamer concentration, and binding aflatoxin M1 with aptamer time were optimized. Under optimal conditions, the linear concentration range of 0.5-800 ng/L and limit of detection (LOD) of 0.3 ng/L were obtained for aflatoxin M1 by measuring the resistance charge transfer data. Finally, the fabricated aptasensor was successfully used to measure AFM1 compared to HPLC method.

Recent Achievements in Electrochemical and Surface Plasmon Resonance Aptasensors for Mycotoxins Detection

Mycotoxins are secondary metabolites of fungi that contaminate agriculture products. Their release in the environment can cause severe damage to human health. Aptasensors are compact analytical devices that are intended for the fast and reliable detection of various species able to specifically interact with aptamers attached to the transducer surface. In this review, assembly of electrochemical and surface plasmon resonance (SPR) aptasensors are considered with emphasis on the mechanism of signal generation. Moreover, the properties of mycotoxins and the aptamers selected for their recognition are briefly considered. The analytical performance of biosensors developed within last three years makes it possible to determine mycotoxin residues in water and agriculture/food products on the levels below their maximal admissible concentrations. Requirements for the development of sample treatment and future trends in aptasensors are also discussed.

A novel colorimetric aptasensor for ultrasensitive detection of aflatoxin M1 based on the combination of CRISPR-Cas12a, rolling circle amplification and catalytic activity of gold nanoparticles

Colorimetric approaches have received noticeable attention among sensing methods in view of simplicity and watching the color change of sample by the naked eyes. However, developing colorimetric sensing methods which show high sensitivity is still problematic. Herein, based on CRISPR-Cas12a, rolling circle amplification (RCA) and catalytic activity of gold nanoparticles (AuNPs), a colorimetric aptasensor was introduced for highly sensitive detection of aflatoxin M₁ (AFM₁). In the presence of AFM₁, the CRISPR-Cas12a is inactivated and large single-stranded DNA (ssDNA) structures are formed on the surface of AuNPs following the addition of T4 DNA ligase and phi29 DNA polymerase. So, the sample color remains yellow after addition of 4-nitrophenol. However, no huge DNA structure is observed on the surface of AuNPs in the absence of target because of activation of CRISPR-Cas12a and digestion of primer. So, the color of sample switches to colorless. The results indicated that the biosensor had high selectivity toward AFM₁ and the approach achieved a detection limit as low as 0.05 ng/L. In addition, it could sensitively identify AFM₁ in the spiked milk samples. Overall, this approach is highly sensitive and does not require sophisticated equipment. Therefore, it maintains promising potential for other mycotoxins detection in real samples by simply replacing the applied sequences.

Encapsulation of hemin in Fe-based metal-organic frameworks and its application for the direct determination of aflatoxin M1

A label-free electrochemical aptasensor was constructed for the sensitive and selective determination of AFM1. For preparation of the aptasensor, the AFM1 aptamer was immobilised on the surface of a glassy carbon electrode modified with hemin encapsulated in Fe-based metal-organic frameworks (hemin@Fe-MIL-101). The morphology and the structure of Fe-MIL-101 and hemin@Fe-MIL-101 were evaluated by scanning electron microscopy, Fourier-transform infrared spectroscopy, X-ray powder diffraction and Brunauer-Emmett-Teller-N2 sorption methods. Electrochemical impedance spectroscopy and cyclic voltammetry were performed to monitor the fabrication process of the electrochemical aptasensor. The electrochemical reduction current of hemin encapsulated in Fe-MIL-101 serves as a signal for the quantitative determination of AFM1. Differential pulse voltammetry was done to determine the AFM1 concentration in the linear range of 1.0×10-1-100.0 ng/ml. The detection limit of AFM1 was estimated to be 4.6×10-2 ng/ml. Finally, the fabricated aptasensor was applied to determine AFM1 in raw and boiled milk samples.

Towards Universal Stimuli-Responsive Drug Delivery Systems: Pillar[5]arenes Synthesis and Self-Assembly into Nanocontainers with Tetrazole Polymers

In this work, we have proposed a novel universal stimulus-sensitive nanosized polymer system based on decasubstituted macrocyclic structures-pillar[5]arenes and tetrazole-containing polymers. Decasubstituted pillar[5]arenes containing a large, good leaving tosylate, and phthalimide groups were first synthesized and characterized. Pillar[5]arenes containing primary and tertiary amino groups, capable of interacting with tetrazole-containing polymers, were obtained with high yield by removing the tosylate and phthalimide protection. According to the fluorescence spectroscopy data, a dramatic fluorescence enhancement in the pillar[5]arene/fluorescein/polymer system was observed with decreasing pH from neutral (pH = 7) to acidic (pH = 5). This indicates the destruction of associates and the release of the dye at a pH close to 5. The presented results open a broad range of opportunities for the development of new universal stimulus-sensitive drug delivery systems containing macrocycles and nontoxic tetrazole-based polymers.

Pillararene-based self-assemblies for electrochemical biosensors

The ingenious design and synthesis of novel macrocycles bring out renewed vigor of supramolecular chemistry in the past decade. As an intriguing class of macrocycles, pillararene and pillararene-based functional materials that are constructed through the noncovalent bond self-assembly approach have been undergoing a rapid growth, benefiting from their unique structures and physiochemical properties. This review elaborates recent significant advances of electrochemical studies based on pillararene systems. Fundamental electrochemical behavior of pillar[n]arene[m]quinone and pillararene-based self-assemblies as well as their applications in electrochemical biosensors are highlighted. In addition, the advantages and functions of pillararene self-assembly systems resulted from the unique molecular architectures are analyzed. Finally, current challenges and future development tendency in this burgeoning field are discussed from the viewpoint of both fundamental research and applications. Overall, this review not only manifests the main development vein of pillararene-based electrochemical systems, but also conquers a solid foundation for their further bioelectrochemical applications.

Towards new nanoporous biomaterials: self-assembly of sulfopillar[5]arenes with vitamin D3 into supramolecular polymers

Novel water-soluble, deca-substituted pillar[5]arenes containing thiasulfate and thiacarboxylate fragments were synthesized and characterized. UV-vis, 2D 1H-1H NOESY and DOSY NMR spectroscopy revealed the ability of pillar[5]arenes containing thiasulfate fragments to form an inclusion complex with cholecalciferol (vitamin D3) in a 1 : 2 ratio (lg Kass = 2.2). Using DLS and SEM it was found that upon concentration and/or evaporation of the solvent, the supramolecular polymer (pillar[5]arene/vitamin D3 (1 : 2)) forms a porous material with an average wall diameter of 53 nm. It was shown that the supramolecular polymer is stable during photolysis by UV radiation (k1 = 1.7 × 10-5 s-1).

Recent Advances in the Aptamer-Based Electrochemical Biosensors for Detecting Aflatoxin B1 and Its Pertinent Metabolite Aflatoxin M1

The notable toxicological impacts of aflatoxin B1 (AFB1) and its main metabolite, aflatoxin M1 (AFM1), on human being health make the evaluation of food quality highly significant. Due to the toxicity of those metabolites-even very low content in foodstuffs-it is crucial to design a sensitive and reliable procedure for their detection. Electrochemical aptamer-based biosensors are considered the most encouraging option, based on multi-placed analysis, rapid response, high sensitivity and specificity. The present review specifically emphasizes the potential utilization of the electrochemical aptasensors for determining the AFM1 and AFB1 with different electrodes.

A review: Recent advances in ultrasensitive and highly specific recognition aptasensors with various detection strategies

One of the most studied topics in analytical chemistry and physics is to develop bio-sensors. Aptamers are small single-stranded RNA or DNA oligonucleotides (5-25 kDa), which have advantages in comparison to their antibodies such as physicochemical stability and high binding specificity. They are able to integrate with proteins or small molecules, including intact viral particles, plant lectins, gene-regulation factor, growth factors, antibodies and enzymes. The aptamers have reportedly shown some unique characteristics, including long shelf-life, simple modification to provide covalent bonds to material surfaces, minor batch variation, cost-effectiveness and slight denaturation susceptibility. These features led important efforts toward the development of aptamer-based sensors, known as apta-sensors classified into optical, electrical and mass-sensitive based on the signal transduction mode. This review provided a number of current advancements in selecting, development criteria, and aptamers application with the focus on the effect of apta-sensors, specifically for disease-associated analyses. The review concentrated on the current reports of apta-sensors that are used for evaluating different food and environmental pollutants.

Nanoparticles based on the zwitterionic pillar[5]arene and Ag+: synthesis, self-assembly and cytotoxicity in the human lung cancer cell line A549

For the first time, stable pillar[5]arene/Ag+ nanoparticles, consisting of water-soluble pillar[5]arene containing γ-sulfobetaine fragments and Ag+ ions without Ag-Ag bonds, were synthesized and characterized. The pillar[5]arene/Ag+ (ratio 1:10) nanoparticles obtained were cubic with a rib length of 100 nm and are less cytotoxic than Ag+ ions. The survival of the A549 model cells in the presence of pillar[5]arene/Ag+ (1:10) nanoparticles at a concentration of 30 and 40 μM was 76% and 55%, while in the absence of pillar[5]arene, the cell survival for free Ag+ ions at the same concentration was 30% and 10%, respectively. The results can be used to create new antibacterial materials and 2D biomedical coatings.

A review on recent developments in optical and electrochemical aptamer-based assays for mycotoxins using advanced nanomaterials

This review (with 163 refs) covers the recent developments of nanomaterial-based optical and electrochemical sensors for mycotoxins. The review starts with a brief discussion on occurrence, distribution, toxicity of mycotoxins and the legislations in monitoring their levels. It further outlines the research methods, various recognition matrices and the strategies involved in the development of highly sensitive and selective sensor systems. It also points out the salient features and importance of aptasensors in the detection of mycotoxins along with the different immobilization methods of aptamers. The review meticulously discusses the performance of different optical and electrochemical sensors fabricated using aptamers coupled with nanomaterials (CNT, graphene, metal nanoparticles and metal oxide nanoparticles). The review addresses the limitations in the current developments as well as the future challenges involved in the successful construction of aptasensors with the functionalized nanomaterials. Graphical abstract Recent developments in nanomaterial based aptasensors for mycotoxins are summarized. Specifically, the efficiency of the nanomaterial coupled aptasensors (such as CNT, graphene, metal nanoparticles and metal oxide nanoparticles) in optical and electrochemical methods are discussed.

An Electrochemical DNA Biosensor for Carcinogenicity of Anticancer Compounds Based on Competition between Methylene Blue and Oligonucleotides

A toxicity electrochemical DNA biosensor has been constructed for the detection of carcinogens using 24 base guanine DNA rich single stranded DNA, and methylene blue (MB) as the electroactive indicator. This amine terminated ssDNA was immobilized onto silica nanospheres and deposited on gold nanoparticle modified carbon-paste screen printed electrodes (SPEs). The modified SPE was initially exposed to a carcinogen, followed by immersion in methylene blue for an optimized duration. The biosensor response was measured using differential pulse voltammetry. The performance of the biosensor was identified on several anti-cancer compounds. The toxicity DNA biosensor demonstrated a linear response range to the cadmium chloride from 0.0005 ppm to 0.01 ppm (R² = 0.928) with a limit of detection at 0.0004 ppm. The biosensor also exhibited its versatility to screen the carcinogenicity of potential anti-cancer compounds.

DNA-Polylactide Modified Biosensor for Electrochemical Determination of the DNA-Drugs and Aptamer-Aflatoxin M1 Interactions

DNA sensors were assembled by consecutive deposition of thiacalix[4]arenes bearing oligolactic fragments, poly(ethylene imine), and DNA onto the glassy carbon electrode. The assembling of the layers was monitored with scanning electron microscopy, cyclic voltammetry and electrochemical impedance spectroscopy. The configuration of the thiacalix[4]arene core determined self-assembling of the polymeric species to the nano/micro particles with a size of 70–350 nm. Depending on the granulation, the coatings show the accumulation of a variety of DNA quantities, charges, and internal pore volumes. These parameters were used to optimize the DNA sensors based on these coatings. Thus, doxorubicin was determined to have limits of detection of 0.01 nM (cone configuration), 0.05 nM (partial cone configuration), and 0.10 nM (1,3-alternate configuration of the macrocycle core). Substitution of native DNA with aptamer specific to aflatoxin M1 resulted in the detection of the toxin in the range of 20 to 200 ng/L (limit of detection 5 ng/L). The aptasensor was tested in spiked milk samples and showed a recovery of 80 and 85% for 20 and 50 ng/L of the aflatoxin M1, respectively.

Electrochemical Immuno- and Aptasensors for Mycotoxin Determination

Modern analysis of food and feed is mostly focused on development of fast and reliable portable devices intended for field applications. In this review, electrochemical biosensors based on immunological reactions and aptamers are considered in the determination of mycotoxins as one of most common contaminants able to negatively affect human health. The characteristics of biosensors are considered from the point of view of general principles of bioreceptor implementation and signal transduction providing sub-nanomolar detection limits of mycotoxins. Moreover, the modern trends of bioreceptor selection and modification are discussed as well as future trends of biosensor development for mycotoxin determination are considered.

Label-Free and Redox Markers-Based Electrochemical Aptasensors for Aflatoxin M1 Detection

We performed a comparative analysis of the sensitivity of aptamer-based biosensors for detection mycotoxin aflatoxin M₁ (AFM1) depending on the method of immobilization of DNA aptamers and method of the detection. Label-free electrochemical impedance spectroscopy (EIS) and differential pulse voltammetry (DPV) for ferrocene labeled neutravidin layers were used for this purpose. Amino-modified DNA aptamers have been immobilized at the surface of polyamidoamine dendrimers (PAMAM) of fourth generation (G4) or biotin-modified aptamers were immobilized at the neutravidin layer chemisorbed at gold surface. In the first case the limit of detection (LOD) has been determined as 8.47 ng/L. In the second approach the LOD was similar 8.62 ng/L, which is below of allowable limits of AFM1 in milk and milk products. The aptasensors were validated in a spiked milk samples with good recovery better than 78%. Comparative analysis of the sensitivity of immuno- and aptasensors was also performed and showed comparable sensitivity.

Electrochemical DNA Sensor Based on Carbon Black-Poly(Neutral Red) Composite for Detection of Oxidative DNA Damage

Voltammetric DNA sensor has been proposed on the platform of glassy carbon electrode covered with carbon black with adsorbed pillar[5]arene molecules. Electropolymerization of Neutral Red performed in the presence of native or oxidatively damaged DNA resulted in formation of hybrid material which activity depended on the DNA conditions. The assembling of the surface layer was confirmed by scanning electron microscopy and electrochemical impedance spectroscopy. The influence of DNA and pillar[5]arene on redox activity of polymeric dye was investigated and a significant increase of the peak currents was found for DNA damaged by reactive oxygen species generated by Cu²⁺/H₂O₂ mixture. Pillar[5]arene improves the electron exchange conditions and increases the response and its reproducibility. The applicability of the DNA sensor developed was shown on the example of ascorbic acid as antioxidant. It decreases the current in the concentration range from 1.0 μM to 1.0 mM. The possibility to detect antioxidant activity was qualitatively confirmed by testing tera infusion. The DNA sensor developed can find application in testing of carcinogenic species and searching for new antitumor drugs.

Label-Free Impedance Sensing of Aflatoxin B₁ with Polyaniline Nanofibers/Au Nanoparticle Electrode Array

Aflatoxin B1 (AFB₁) is produced by the Aspergillus flavus and Aspergillus parasiticus group of fungi which is most hepatotoxic and hepatocarcinogenic and occurs as a contaminant in a variety of foods. AFB₁ is mutagenic, teratogenic, and causes immunosuppression in animals and is mostly found in peanuts, corn, and food grains. Therefore, novel methodologies of sensitive and expedient strategy are often required to detect mycotoxins at the lowest level. Herein, we report an electrochemical impedance sensor that selectively detects AFB₁ at the lowest level by utilizing polyaniline nanofibers (PANI) coated with gold (Au) nanoparticles composite based indium tin oxide (ITO) disk electrodes. The Au-PANI nanocomposites were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) spectroscopy, and electrochemical impedance spectroscopy (EIS). The composite electrode exhibited a 14-fold decrement in |Z|1Hz in comparison with the bare electrode. The Au-PANI acted as an effective sensing platform having high surface area, electrochemical conductivity, and biocompatibility which enabled greater loading deposits of capture antibodies. As a result, the presence of AFB₁ was screened with high sensitivity and stability by monitoring the changes in impedance magnitude (|Z|) in the presence of a standard iron probe which was target specific and proportional to logarithmic AFB₁ concentrations (CAFB₁). The sensor exhibits a linear range 0.1 to 100 ng/mL with a detection limit (3) of 0.05 ng/mL and possesses good reproducibility and high selectivity against another fungal mycotoxin, Ochratoxin A (OTA). With regard to the practicability, the proposed sensor was successfully applied to spiked corn samples and proved excellent potential for AFB₁ detection and development of point-of-care (POC) disease sensing applications.