A gold-based nanobeacon probe for fluorescence sensing of organophosphorus pesticides

A photoelectrochemical aptasensor for omethoate determination based on a photocatalysis of CeO2@MnO2 heterojunction for glucose oxidation

In the present work, we developed a photoelectrochemical aptasensor to determine omethoate (OMT) based on the dual signal amplification of CeO2@MnO2 photocatalysis for glucose oxidation and exonuclease I-assisted cyclic catalytic hydrolysis. CeO2@MnO2 heterojunction material prepared by hydrothermal method was linked with captured DNA (cDNA) and then assembled on the ITO conductive glass to form ITO/CeO2@MnO2-cDNA, which exhibited significant photocurrent response and good photocatalytic performance for glucose oxidation under visible light irradiation, providing the feasibility for sensitive determining OMT. After binding with the aptamer of OMT (apt), the formation of rigid double stranded cDNA/apt kept CeO2@MnO2 away from ITO surface, which ensured a low photocurrent background for the constructed ITO/CeO2@MnO2-cDNA/apt aptasensor. In the presence of target OMT, the restoration of the cDNA hairpin structure and the exonuclease I-assisted cyclic catalytic hydrolysis led to the generation and amplification of measurement photocurrent signals, and allowed the aptasensor to have an ideal quantitative range of 0.01-10.0 nM and low detection limit of 0.0027 nM. Moreover, the aptasensor has been applied for selective determination of OMT in real samples with good precision of the relative standard deviation less than 6.2 % and good accuracy of the recoveries from 93 % to 108 %. What's more, the aptasensor can be used for other target determination only by replacing the captured DNA and corresponding aptamer.

Aptamer recognition-promoted hybridization chain reaction for amplified label-free and enzyme-free fluorescence analysis of pesticide

Development of high-performance fluorescence sensors for pesticide is highly urgent but remains a grand challenge. It is due to that most of known fluorescence sensors detect pesticides based on enzyme-inhibited strategy, which requires high-price cholinesterase, suffers from serious interference of reductive materials, and can't difference pesticides with each other; the known aptamer-based fluorescence ones entail tool enzymes or nanomaterials to transducer/amplify the signal and demand signalers to be tagged in nucleic acid, which are expensive and intricate. Herein, we develop a novel aptamer-based fluorescence system for label-free, enzyme-free and highly sensitive detection of pesticide (profenofos) based on target-initiated hybridization chain reaction (HCR)-assisted signal amplification and specific intercalation of N-methylmesoporphyrin IX (NMM) in G-quadruplex DNA. Hairpin probe ON1 recognizes profenofos to generate profenofos@ON1 complex, which switches the HCR to yield multiple G-quadruplex DNA, consequently making large numbers of NMM be locked. In comparison with profenofos absence, a sharply improved fluorescence signal was recorded and it was dependent on profenofos dose. Hence, label-free, enzyme-free and highly sensitive detection of profenofos is achieved with limit of detection of 0.085 nM, which compared favorably with or superior to those of known fluorescence methods. Furthermore, the present method was applied to determine the profenofos residue in rice with agreeable result, and will provide more valuable information for guaranteeing the pesticide-related food safety.

Dual-ratiometric aptasensor for simultaneous detection of malathion and profenofos based on hairpin tetrahedral DNA nanostructures

Due to the diversification and complexity of organophosphorus pesticide residues brings great challenges to the detection work. Therefore, we developed a dual-ratiometric electrochemical aptasensor that could detect malathion (MAL) and profenofos (PRO) simultaneously. In this study, metal ions, hairpin-tetrahedral DNA nanostructures (HP-TDN) and nanocomposites were used as signal tracers, sensing framework and signal amplification strategy respectively to develop the aptasensor. Thionine (Thi) labeled HP-TDN (HP-TDNThi) provided specific binding sites for assembling Pb2+ labeled MAL aptamer (Pb2+-APT1) and Cd2+ labeled PRO aptamer (Cd2+-APT2). When the target pesticides were present, Pb2+-APT1 and Cd2+-APT2 were dissociated from the hairpin complementary strand of HP-TDNThi, resulting in reduced oxidation currents of Pb²⁺ (I_Pb²⁺) and Cd²⁺ (I_Cd²⁺), respectively, while the oxidation currents of Thi (I_Thi) remained unchanged. Thus, I_Pb²⁺/I_Thi and I_Cd²⁺/I_Thi oxidation current ratios were used to quantify MAL and PRO, respectively. In addition, the gold nanoparticles (AuNPs) encapsulated in the zeolitic imidazolate framework (ZIF-8) nanocomposites (Au@ZIF-8) greatly increased the catch of HP-TDN, thereby amplifying the detection signal. The rigid three-dimensional structure of HP-TDN could reduce the steric hindrance effect on the electrode surface, which could greatly improve the recognition efficiency of the aptasensor for the pesticide. Under the optimal conditions, the detection limits of the HP-TDN aptasensor for MAL and PRO were 4.3 pg mL-1 and 13.3 pg mL-1, respectively. Our work proposed a new approach to fabricating a high-performance aptasensor for simultaneous detection of multiple organophosphorus pesticides, opening a new avenue for the development of simultaneous detection sensors in the field of food safety and environmental monitoring.

Integration of Metallic Nanomaterials and Recognition Elements for the Specifically Monitoring of Pesticides in Electrochemical Sensing

Although all countries have been controlling the excessive use of pesticides, incidents of pesticide residues still existed. Electrochemical biosensors are extensively applied detection techniques to monitor pesticides with the help of different types of biorecognition components mainly including, antibodies, aptamers, enzymes (i.e., acetylcholinesterase, organophosphorus hydrolase, etc.), and synthetic molecularly imprinted polymers. Besides, the electrode materials mainly affected the sensitivity of electrochemical biosensors. Metallic nanomaterials with various structures and excellent electrical conductivity were desirable choice to construct electrochemical platforms to achieve the detection with high sensitivity and good specificity toward the target. This work reviewed the developed metallic materials including monometallic nanoparticles, bimetallic nanomaterials, metal atoms, metal oxides, metal molybdates, metal-organic frameworks, MXene, etc. Integration of recognition elements endowed the electrode materials with higher specificity toward the target pesticide. Besides, future challenges of metallic nanomaterials-based electrochemical biosensors for the detection of pesticides are also discussed and described.

Recent Advances in Nanoparticle-Based Optical Sensors for Detection of Pesticide Residues in Soil

The excessive and unreasonable use of pesticides has adversely affected the environment and human health. The soil, one of the most critical natural resources supporting human survival and development, accumulates large amounts of pesticide residues. Compared to traditional spectrophotometry analytical methods, nanoparticle-based sensors stand out for their simplicity of operation as well as their high sensitivity and low detection limits. In this review, we focus primarily on the functions that various nanoparticles have and how they can be used to detect various pesticide residues in soil. A detailed discussion was conducted on the properties of nanoparticles, including their color changeability, Raman enhancement, fluorescence enhancement and quenching, and catalysis. We have also systematically reviewed the methodology for detecting insecticides, herbicides, and fungicides in soil by using nanoparticles.

Advanced visual sensing techniques for on-site detection of pesticide residue in water environments

Pesticide usage has increased to fulfil agricultural demand. Pesticides such as organophosphorus pesticides (OPPs) are ubiquitous in world food production. Their widespread usage has unavoidable detrimental consequences for humans, wildlife, water, and soil environments. Hence, the development of more convenient and efficient pesticide residue (PR) detection methods is of paramount importance. Visual detecting approaches have acquired a lot of interest among different sensing systems due to inherent advantages in terms of simplicity, speed, sensitivity, and eco-friendliness. Furthermore, various detections have been proven to enable real-life PR surveillance in environment water. Fluorometric (FL), colourimetric (CL), and enzyme-inhibition (EI) techniques have emerged as viable options. These sensing technologies do not need complex operating processes or specialist equipment, and the simple colour change allows for visual monitoring of the sensing result. Visual sensing techniques for on-site detection of PR in water environments are discussed in this paper. This paper further reviews prior research on the integration of CL, FL, and EI-based techniques with nanoparticles (NPs), quantum dots (QDs), and metal-organic frameworks (MOFs). Smartphone detection technologies for PRs are also reviewed. Finally, conventional methods and nanoparticle (NPs) based strategies for the detection of PRs are compared.

Ultrasensitive electrochemiluminescence aptasensor based on ABEI reduced silver nanoparticles for the detection of profenofos

An ultrasensitive electrochemiluminescence (ECL) aptasensor for detection of profenofos was constructed by the reducibility and chemiluminescence property of N-(aminobutyl)-N-(ethylisoluminol) (ABEI). ABEI was used to reduce silver nitrate (AgNO₃) to silver nanoparticles (AgNPs), which could be adsorbed on the lattice of graphene oxide (GO) to form ABEI-AgNPs-GO complex. This compound could achieve excellent luminescence. The aptamer (Apt) modified (5') by sulfhydryl groups could be immobilized on AgNPs to capture profenofos. When profenofos was present, the ECL signal of the aptasensor would be weakened. To further demonstrate the successful construction of the aptasensor, cyclic voltammetry tests were performed on an electrochemical workstation and an ECL analyzer, respectively. The standard curve and specificity experiment both showed that the sensor had the advantages of low limit of detection (LOD) and good specificity. Under the optimal conditions, the aptasensor had a good linear response for profenofos in the range of 1 × 10^-1-1 × 10⁴ ng/mL. It also had a LOD of 6.7 × 10^-2 ng/mL and a correlation coefficient (R²) of 0.9991. The aptasensor had been successfully applied to the detection of profenofos in vegetables. The recovery range of the proposed ECL aptasensor was 98 % ~ 107.4 %.

Novel colorimetric aptasensor based on MOF-derived materials and its applications for organophosphorus pesticides determination

For the visual detection of four organophosphorus pesticides (OPs), a colorimetric aptasensor was developed based on aptamer-mediated bimetallic metal-organic frameworks (MOFs) nano-polymers. Fe-Co magnetic nanoparticles (MNPs) and Fe-N-C nanozymes were prepared based on pyrolytic reaction, and were labeled with broad spectrum aptamers and complementary chains of organophosphorus pesticides respectively. The hybridization of aptamers and complementary chains led to the formation of nano-polymers. In the presence of target pesticides, they competed with complementary chains for aptamers on Fe-Co MNPs, resulting in a large number of Fe-N-C nanozymes signal labels being released into the supernatant. Fe-N-C nanozymes showed similar activity to peroxidase and catalyzed the 3,3',5,5'-tetramethylbenzidine-hydrogen peroxide (TMB-H₂O₂) color system to turn the solution blue-green under mild conditions. The magnetic probes had good selectivity and sensitivity, and were easily separated by magnetic absorption. The sensor functioned well under optimal conditions, demonstrating good stability and specificity for four pesticides: phorate, profenofos, isocarbophos and omethoate, and the detection limits of four pesticides were as low as 0.16 ng/mL, 0.16 ng/mL, 0.03 ng/mL and 1.6 ng/mL respectively, and the recovery rate of OPs residue in vegetable samples was satisfactory. The work described here provided a simple, rapid and sensitive way to construct a biosensor.

Fluorescence Assay for Detecting Four Organophosphorus Pesticides Using Fluorescently Labeled Aptamer

In this work, we reported a rapid and sensitive fluorescence assay in homogenous solution for detecting organophosphorus pesticides by using tetramethylrhodamine (TAMRA)-labeled aptamer and its complementary DNA (cDNA) with extended guanine (G) bases. The hybridization of cDNA and aptamer drew TAMRA close to repeated G bases, then the fluorescence of TAMRA was quenched by G bases due to the photoinduced electron transfer (PET). Upon introducing the pesticide target, the aptamer bound to pesticide instead of cDNA because of the competition between pesticide and cDNA. Thus, the TAMRA departed from G bases, resulting in fluorescence recovery of TAMRA. Under optimal conditions, the limits of detection for phorate, profenofos, isocarbophos, and omethoate were 0.333, 0.167, 0.267, and 0.333 µg/L, respectively. The method was also used in the analysis of profenofos in vegetables. Our fluorescence design was simple, rapid, and highly sensitive, which provided a means for monitoring the safety of agricultural products.

Progress in Nanomaterials-Based Enzyme and Aptamer Biosensor for the Detection of Organophosphorus Pesticides

With the improvement of people's safety awareness, the requirement of pesticide detection is gradually increasing, and many new detection methods toward Organophosphorus pesticide (OPs) has been further developed and applied. Nanomaterials-based biosensors have played an important role in the trace detection of OPs. This article mainly introduces the detection principle of enzymes and aptamers as the identification element of biosensors. Various nanomaterials (i.e., metals and metal oxides, carbon nanotubes, graphene and graphene oxide, quantum dots, metal organic frameworks, molecular imprinted polymers, etc.) possess their unique properties and play different roles in the enzyme and aptamer-based biosensors toward OPs: (a) to produce the optical or electrochemical signal; (b) as a carrier to load the enzyme or aptamer; (c) to enhance the signal response. Besides, the intelligent portable devices provide the possibility to realize the onsite and real-time detection. The limitations of some nanomaterials and the future development are discussed. Finally, the future of enzyme and aptamer-based biosensors has prospected.

β-Cyclodextrin Polymer-Based Host-Guest Interaction and Fluorescence Enhancement of Pyrene for Sensitive Isocarbophos Detection

The extensive use of organophosphorus pesticides in agriculture poses a high risk to human health and has boosted the demands for developing sensitive monitoring methods. Herein, we developed a facile and sensitive method for isocarbophos detection based on the remarkable fluorescence enhancement of pyrene during host-guest interaction of β-cyclodextrin polymer (β-CDP) and pyrene. The 3'-pyrene-labeled isocarbophos aptamer could be cleaved by exonuclease I to obtain free pyrene that was tagged on mononucleotides, which could enter the hydrophobic cavity of β-CDP, resulting in a prominent fluorescence enhancement. While the target isocarbophos was added, aptamer could undergo a conformational change into a hairpin complex, which prevented the cleavage and host-guest interaction because of the steric hindrance, leading to a weak fluorescence. The isocarbophos has been sensitively and selectively analyzed by detecting the system fluorescence intensity with a detection limit as low as 1.2 μg/L. In addition, we have verified the ability of our proposed method in real sample detection from fruit extract.

Aptamer-binding zirconium-based metal-organic framework composites prepared by two conjunction approaches with enhanced bio-sensing for detecting isocarbophos

A novel label-free and enzyme-free detection strategy has been developed for the electrochemical biosensor detection of isocarbophos (ICP) using UiO-66-NH₂ and aptamer as the signal transducers. In this work, the ICP aptamers were attached to UiO-66-NH₂ through physical mixing and chemical combination methods. In the presence of ICP, the aptamers could undergo conformational change and bind to them, which prevent the electron transfer to the surface of electrode. By comparing the two conjunction approaches of aptasensors, these proposed strategies could selectively and sensitively detect ICP with a detection limit of 6 ng mL^-1 (20.74 nM) and 0.9 ng mL^-1 (3.11 nM). Furthermore, we have also demonstrated the capability of this strategy in the detection of ICP in real samples from vegetable and fruit extract, indicating the potential application of this strategy in food safety issues.

A novel electrochemiluminescence aptasensor based on copper-gold bimetallic nanoparticles and its applications

In this work, a novel electrochemiluminescence (ECL) aptasensor was structured for the detection of four organophosphorus pesticides (OPs). Firstly, multi-walled carbon nanotubes (MWCNTs) were used to create a favorable loading interface for the fixation of tris (2, 2'-bipyridyl) ruthenium (II) (Ru (bpy)₃²⁺). At the same time, copper (core)-gold (shell) bimetallic nanoparticles (Cu@Au NPs) were synthesized in the aqueous phase for the sensor construction. Gold nanoparticles (Au NPs) could promote the electrochemiluminescence intensity of Ru (bpy)₃²⁺ with high efficiency by catalyzing the oxidation process of tri-n-propylamine (TPrA). Compared with the Au NPs, Cu@Au NPs increased the solid loading of Au NPs by virtue of the large specific surface area of copper nanoparticles (Cu NPs), which could further improve the sensitivity of aptasensor. When OPs were added, the ECL intensity was significantly reduced, and the concentration of OPs could be detected through the ECL intensity. Under the optimum conditions, the aptasensor had a wider dynamic range and ultra-low detection limit for the detection of four pesticides: profenofos, isocarbophos, phorate, and omethoate, and their detection limits were 3 × 10^-4 ng/mL, 3 × 10^-4 ng/mL, 3 × 10^-3 ng/mL, and 3 × 10^-2 ng/mL respectively (S/N = 3). The aptasensor had the merits of good stability, reproducibility, and specificity, and had a favorable recovery rate in detecting OPs residues in vegetables. This work provided an effective method for the construction of a simple, rapid, and sensitive biosensor.

Dual-readout performance of Eu3+-doped nanoceria as a phosphatase mimic for degradation and detection of organophosphate

Eu³⁺-Doped nanoceria (Eu:CeO₂) with self-integrated catalytic and luminescence sensing functions was synthesized by a simple and gentle one-pot method to build a dual-readout nanozyme platform for organophosphate compound (OPC) sensing in this work. The catalytic degradation of the model substrate of OPC, p-nitrophenyl phosphate (p-NPP), by as-prepared Eu:CeO₂ can be completed in 2 min with little influence of temperature and pH values, highlighting the advantages of Eu:CeO₂ as an artificial enzyme for dephosphorylation. Most importantly, the characteristic red emission of Eu³⁺ (592 nm) from Eu:CeO₂ can be quenched by p-NPP, accompanied by a color change from colorless to yellow. Based on this, linear ranges of 4-50 μM with a detection limit of 3.3 μM and 1-20 μM with a detection limit of 0.6 μM for p-NPP were obtained by colorimetric and fluorescence methods, respectively. Furthermore, the fluorescence strategy was effectively applied to the determination of ethyl para-nitrophenyl (EPN), one of the most commonly used pesticides, with a detection limit of 5.86 μM. The proposed strategy was also successfully applied to the assay of p-NPP and EPN in real water samples, showing great application prospects in detecting OPC in the environment.

Sulphur-doped graphene quantum dot based fluorescent turn-on aptasensor for selective and ultrasensitive detection of omethoate

Development of selective, ultra-sensitive, rapid and facile methods for the detection of chemical residues of toxic pesticides and hazardous chemicals are quite important in food safety, environmental monitoring and safeguarding public health. Herein, we presented a fluorescent turn-on aptasensor based on sulphur-doped graphene quantum dot (S-GQD) utilizing specific recognition and binding property of aptamer for the ultra-sensitive and selective detection of omethoate (OM) which is a systemic organophosphorus pesticide. The detection method is based on tuning aggregation-disaggregation mechanism of S-GQD by way of conformational alteration of the recognition probe. Fluorescence 'turn-on' process includes aggregation-induced quenching of S-GQD with aptamer via S-GQD-aptamer complex formation and its subsequent fluorescence recovery with the addition of OM by structural switching of S-GQD-aptamer complex to aptamer-omethoate complex. The reported 'switch-on' aptasensor has exhibited a low limit of detection of 0.001 ppm with high selectivity for OM over other pesticides.

Recent Advances on Detection of Insecticides Using Optical Sensors

Insecticides are enormously important to industry requirements and market demands in agriculture. Despite their usefulness, these insecticides can pose a dangerous risk to the safety of food, environment and all living things through various mechanisms of action. Concern about the environmental impact of repeated use of insecticides has prompted many researchers to develop rapid, economical, uncomplicated and user-friendly analytical method for the detection of insecticides. In this regards, optical sensors are considered as favorable methods for insecticides analysis because of their special features including rapid detection time, low cost, easy to use and high selectivity and sensitivity. In this review, current progresses of incorporation between recognition elements and optical sensors for insecticide detection are discussed and evaluated well, by categorizing it based on insecticide chemical classes, including the range of detection and limit of detection. Additionally, this review aims to provide powerful insights to researchers for the future development of optical sensors in the detection of insecticides.

Recent developments in non-enzymatic (bio)sensors for detection of pesticide residues: Focusing on antibody, aptamer and molecularly imprinted polymer

The utilization of pesticides has been increased in recent years due to population growth and increasing urbanization. The constant use of pesticides has resulted in contamination of the environment and agricultural products with serious human health concerns associated with their use. Therefore, detection and quantification of pesticides by sensitive and selective methods is highly required in food safety management. Traditional detection methods cannot realize highly sensitive, selective and on-site detection, which limits their application. (Bio)sensors and (bio)assays are emerging tools with unique properties such as rapid, sensitive, efficient and portable detection. Among them, enzyme-based biosensors have been widely developed and some have even been commercialized. However, they suffer from some limitations such as instability and low reproducibility that originate from the nature of enzyme. Non-enzymatic (bio)sensors overcome the current limitations of enzyme-based detection methods and provide great potential for efficient, highly sensitive and low-cost detection assays using smart and miniaturized devices. In this study, we provide an overview of recent advances and new trends in optical and electrochemical non-enzymatic (bio)sensors for the detection of pesticides by focusing on antibody, aptamer and molecularly imprinted polymer (MIP) as recognition elements. Performance, advantages and drawbacks of the developed (bio)sensors are discussed well. The main advantage these recognition elements is their stability over an extended period of time compared to the enzymes. Furthermore, the combination of nanomaterials in these (bio)sensors can significantly improve their performance.

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.

Broad-spectrum electrochemical immunosensor based on one-step electrodeposition of AuNP-Abs and Prussian blue nanocomposite for organophosphorus pesticide detection

Broad-spectrum antibodies can effectively recognize substances with similar structures and have broad application prospects in field rapid detection. In this study, broad-spectrum antibodies (Abs) against organophosphorus pesticides (OPs) were used as sensitive recognition elements, which could effectively recognize most OPs. Gold nanoparticles (AuNPs) have good biocompatibility. It combined with Abs to form a gold-labeled probe (AuNPs-Abs), which enhances the effective binding of antibodies to nanomaterials. Prussian blue (PB) was added to electrodeposition solution to enhance the conductivity, resulting in superior electrochemical performance. The AuNP-Abs-PB composite film was prepared by electrodeposition on the electrode surface to improve the anti-interference ability and stability of the immunosensor. Under the optimal experimental conditions, the immunosensor had a wide detection range (IC₂₀-IC₈₀: 1.82 × 10^-3-3.29 × 10⁴ ng/mL) and high sensitivity. Most importantly, it was simple to be prepared and could be used to detect multiple OPs.

Selective determination of non-organophosphorus insecticide using DNA aptamer-based single-use biosensors

In the present study, we developed a disposable aptamer-based biosensor for rapid, sensitive, and reliable detection of acetamiprid (ACE). To improve the sensitivity of the aptasensor, poly-5-amino-2-mercapto-1,3,4-thiadiazole [P(AMT)] and gold nanoparticles (AuNPs) were progressively electrodeposited on the screen-printed electrode (SPE) surface by using cyclic voltammetry (CV) technique. For the determination of ACE, thiol-modified primary aptamer (Apt1) was selected by using the SELEX method and immobilized on the surface of the P(AMT) and AuNPs-modified SPE (SPE/P(AMT)/AuNPs) via AuS bonding. Then, the surface-bound aptamer was incubated with ACE for 45 Min. After that, the biotin-labeled aptamer 2 (Apt2) was interacted with the ACE, then the enzyme-labeled step was performed. In this step, alkaline phosphatase (ALP) was bound to the surface through the interaction between Apt2 labeled with biotin and streptavidin (strep)-ALP conjugate. The determination of ACE was achieved by measuring the oxidation signal of α-naphthol, which is formed on the electrode surface through the interaction of ALP with α-naphthyl phosphate. The working range of the developed aptasensor was determined as 5 × 10^-12 -5 × 10^-10  mol L^-1 with a low limit of detection (1.5 pmol L^-1 ). It was also found that the proposed aptasensor possessed great advantages such as low cost, good selectivity, and good reproducibility.

Voltammetric determination of organophosphorus pesticides using a hairpin aptamer immobilized in a graphene oxide-chitosan composite

An aptasensor is described for electrochemical determination of organophosphorus pesticides (OPPs), specifically of profenofos, phorate, isocarbophos, and omethoate. The method uses a hairpin aptamer as signalling donor. Its 5' and 3' ends were modified with amino groups and the redox probe ferrocene (Fc), respectively. A nanocomposite consisting of graphene oxide and chitosan (GO-chit) was used to immobilize the aptamer via formation of an amide link. Its good conductivity facilitates monitoring of the electrochemical responses. Upon addition of an OPP, it will be bound by the aptamer. This results in an opening of the hairpin structure. Thus, Fc is shifted away from the surface of the electrode. As a result, the impedance increases and the redox signal of Fc decreases. The electrochemical performance, binding capacity and response of the aptasensor for profenofos, phorate, isocarbophos and omethoate were studied. The limits of detection are as low as 0.01, 0.1, 0.01 and 0.1 nM, respectively. Graphical abstract Schematic representation of an electrochemical aptasensor prepared by immobilizing ferrocene (Fc) labeled hairpin aptamer (HP) on the surface of graphene oxide-chitosan (GO-chit) modified electrode, and its application to the determination of organophosphorus pesticides (OPPs) by voltammetry.

Dual-modal aptasensor for the detection of isocarbophos in vegetables

An aptamer-based colorimetric-phosphorescent assay was developed for the detection of isocarbophos. The colorimetric assay relied on the aggregation of gold nanoparticles (AuNPs) caused by the competitive binding of aptamer between isocarbophos and AuNPs in the presence of a high salt concentration. The further addition of persistent luminescence nanorods (PLNRs) into the system showed the phosphorescence sensitively proportional to the concentration of isocarbophos, due to the inner filter effect between PLNRs and AuNPs. The assay showed good linearity within 50-500 μg/L and 5-160 μg/L, and limit of detection of 7.1 μg/L and 0.54 μg/L in colorimetry and phosphorescence mode, respectively. The feasibility of this approach for food analysis was demonstrated with the sensitive and selective determination of isocarbophos residues in vegetables.

Aptasensors for pesticide detection

Pesticide contamination has become one of the most serious problems of public health in the world, due to their wide application in agriculture industry to guarantee the crop yield and quality. The detection of pesticide residues plays an important role in food safety management and environment protection. However, the conventional detection methodologies cannot realize highly sensitive, selective and on-site detection, which limits their applications. Aptamers are short single-stranded oligonucleotides (RNA or DNA) selected by SELEX method, which can selectively bind to their targets with high affinity. Compared with the commonly used antibodies or enzymes in designing biosensors, aptamers exhibit better stability, low molecular weight, easy modification and low cost, and were regarded as excellent candidates for developing aptasensors for pesticide detection. In this review, application of aptamers for pesticide detection was reviewed. Firstly, aptamers specifically bind to various pesticides were first summarized. Secondly, the progresses and highlights of developing aptasensors for highly-sensitive and selective detection of pesticide residues were systematically provided. Finally, the present challenges and future perspectives for developing novel highly-effective aptasensor for the detection of pesticide residues were discussed.

Fluorometric determination of pesticides and organophosphates using nanoceria as a phosphatase mimic and an inner filter effect on carbon nanodots

Nanoceria with a remarkable phosphatase mimicking activity was synthesized and used to catalyze the hydrolysis of phosphate esters in pH 10 solution. The catalytic effect of nanoceria was firstly investigated by selecting p-nitrophenyl phosphate as a model substrate. The pH value, incubation temperature, reaction time, and concentration of nanoceria were optimized. The catalytic effect was then confirmed by using methyl-paraoxon as a substrate. The p-nitrophenol anion released by the enzyme mimic is yellow and exerts an inner filter effect on the fluorescence of the carbon dots (with excitation/emission maxima at 400/520 nm). Response to methyl-paraoxon is linear in the 1.125-26.25 μmol L^-1 concentration range. The method was applied to the determination of pesticides in spiked Panax quinquefolius and water samples. Recoveries ranged from 85 to 103% (n = 3). The technique is rapid, reliable, and can be used for on-site detection of pesticides and organophosphates. Graphical abstract Schematic presentation of a fluorometric technique for the detection of organophosphate compound and pesticide using nanoceria as a phosphatase mimic and an inner filter effect on the blue fluorescence of carbon dots (with excitation/emission maxima at 400/520 nm).

Advances in aptasensors for the detection of food contaminants

Food safety is a global health objective, and foodborne diseases represent a major crisis in health. Techniques that are simple and suitable for fast screening to detect and identify pathogenic factors in the food chain are vital to ensure food safety. At present, a variety of analytical methods have been reported for the detection of pathogenic agents. Whereas the sensitivity of detection and quantification are still important challenges, we expect major advances from new assay formats and synthetic bio-recognition elements, such as aptamers. Owing to the specific folding capability of aptamers in the presence of an analyte, aptasensors have substantially and successfully been exploited for the detection of a wide range of small and large molecules (e.g., toxins, antibiotics, heavy metals, bacteria, viruses) at very low concentrations. Here, we review the use of aptasensors for the development of highly sensitive and affordable detection tools for food analysis.

Critical Review: DNA Aptasensors, Are They Ready for Monitoring Organic Pollutants in Natural and Treated Water Sources?

There is a growing need to monitor anthropogenic organic contaminants detected in water sources. DNA aptamers are synthetic single-stranded oligonucleotides, selected to bind to target contaminants with favorable selectivity and sensitivity. These aptamers can be functionalized and are used with a variety of sensing platforms to develop sensors, or aptasensors. In this critical review, we (1) identify the state-of-the-art in DNA aptamer selection, (2) evaluate target and aptamer properties that make for sensitive and selective binding and sensing, (3) determine strengths and weaknesses of alternative sensing platforms, and (4) assess the potential for aptasensors to quantify environmentally relevant concentrations of organic contaminants in water. Among a suite of target and aptamer properties, binding affinity is either directly (e.g., organic carbon partition coefficient) or inversely (e.g., polar surface area) correlated to properties that indicate greater target hydrophobicity results in the strongest binding aptamers, and binding affinity is correlated to aptasensor limits of detection. Electrochemical-based aptasensors show the greatest sensitivity, which is similar to ELISA-based methods. Only a handful of aptasensors can detect organic pollutants at environmentally relevant concentrations, and interference from structurally similar analogs commonly present in natural waters is a yet-to-be overcome challenge. These findings lead to recommendations to improve aptasensor performance.

Aptamer-based microcantilever-array biosensor for profenofos detection

Profenofos, a highly poisonous organophosphorus pesticide, has been widely used in agricultural production. These pesticide residues have seriously influenced food security and threatened human health, and new methods with high sensitivity are greatly needed to detect profenofos. Here, we developed an aptamer-based microcantilever-array sensor operated in stress mode to detect profenofos, with advantages of being a label-free, highly sensitive, one-step immobilization method capable of quantitative and real-time detection. The microcantilevers were functionalized with a profenofos-specific aptamer (SS2-55), and then the specific binding of profenofos to aptamer induced a deflection of the microcantilever, which was monitored using an optical method in a real-time manner. The microcantilever deflection showed a positive relationship with profenofos concentration, and the detection limit was low to 1.3 ng mL^-1 (3.5 nM) for profenofos, which was much lower than other aptamer-based detection methods. The selectivity of the sensor was verified with another organophosphorus pesticide. Additionally, we successfully detected profenofos dissolved in vegetable-soak solution. Our results showed that this aptamer-based microcantilever-array sensor is a convenient and label-free method for detecting profenofos in small amounts and has great potential for food-security applications.

Homogeneous electrochemical aptasensor based on a dual amplification strategy for sensitive detection of profenofos residues

A homogeneous type of electrochemical aptasensor was designed based upon the principle of target-induced and tool enzyme-assisted signal amplification, which was employed for the detection of profenofos residues.

A highly sensitive impedimetric aptasensor for the selective detection of acetamiprid and atrazine based on microwires formed by platinum nanoparticles

A novel impedimetric biosensor was developed for the detection of the two extensively used pesticides, acetamiprid and atrazine. By employing the sputtering and e-beam lithography techniques, platinum nanoparticles (Pt NPs) were deposited in a bridge-like arrangement, in between interdigitated electrodes (IDEs). The resulting Pt NP microwires were chemically functionalized to allow the covalent immobilization of aptamers against the two target analytes onto the sensor surfaces. The biosensing platform facilitated charge transfer through the microwire-bridged IDEs, while upon analyte binding to the immobilized aptamers electron transfer was hindered, resulting in an increase of the electrochemical cell's impedance. The combination of Pt NPs microwires and aptamers allowed the sensitive and highly selective detection of acetamiprid with a linear range of response in the range of 10pM to 100nM with a limit of detection (LoD) at 1pM, and of atrazine with a linear range of responses from 100pM to 1μM and a LoD at 10pM respectively. Its performance was tested against a number of other commonly used pesticides as well as in real water samples.

Highly sensitive GQDs-MnO2 based assay with turn-on fluorescence for monitoring cerebrospinal acetylcholinesterase fluctuation: A biomarker for organophosphorus pesticides poisoning and management

In this study, we demonstrated an assay with turn-on fluorescence for monitoring cerebrospinal acetylcholinesterase (AChE) fluctuation as a biomarker for organophosphorus pesticides (OPs) poisoning and management based on single layer MnO₂ nanosheets with graphene quantum dots (GQDs) as signal readout. Initially, the fluorescence of GQDs was quenched by MnO₂ nanosheets mainly due to the inner filter effect (IFE). However, with the presence of reductive thiocholine (TCh), the enzymatic product, hydrolyzed from acetylthiocholine (ATCh) by AChE, the redox reaction between MnO₂ and TCh occurred, leading to the destruction of the MnO₂ nanosheets, and thereby IFE was diminished gradually. As a consequence, the turn-on fluorescence of GQDs with the changes in the spectrum of the dispersion constituted a new mechanism for sensing of cerebrospinal AChE. With the method developed here, we could monitor cerebrospinal AChE fluctuation of rats exposed to OPs before and after therapy, and could thereby open up the pathway to a new sensing platform for better understanding the mechanism of brain dysfunctions associate with OPs poisoning.

Fluorescence Determination of Omethoate Based on a Dual Strategy for Improving Sensitivity

Omethoate is a frequently used organophosphorus pesticide, and the establishment of a sensitive, selective, and simple method to determine omethoate is very important for food safety. In this paper, a dual strategy was applied to improve the detection sensitivity of omethoate. In the first strategy, graphene quantum dots (GQDs) were doped with nitrogen to increase the fluorescence quantum yield to 30%. By coupling N-GQDs with omethoate aptamer, an N-GQDs-aptamer probe was synthesized. The fluorescence of the N-GQDs-aptamer probe was turned off by graphene oxide (GO), but recovered by omethoate. Based on this principle, the fluorescence method for detecting omethoate was established with a detection limit of 0.041 nM. To further improve the detection sensitivity, the fluorescence polarization analysis method was applied as another strategy based on the polarization signal of GQDs. The detection limit was decreased to 0.029 pM by using the fluorescence polarization method. The detection limits in this paper were lower than those in other reports. The imaging of omethoate on plant leaves showed that the probe could be used for visual semiquantitative determination of omethoate.