Oligonucleotides and pesticide regulated peroxidase catalytic activity of hemin for colorimetric detection of isocarbophos in vegetables by naked eyes

Ultrasensitive Fluorescent Microsensors Based on Aptamers Modified with SYBR Green I for Visual Quantitative Detection of Organophosphate Pesticides

Organophosphate pesticides (OPs) are widely utilized in agricultural production, and the residues threaten public health and environmental safety due to their toxicity. Herein, a novel and simple DNA aptamer-based sensor has been fabricated for the rapid, visual, and quantitative detection of profenofos and isocarbophos. The proposed DNA aptamers with a G-quadruplex spatial structure could be recognized by SYBR Green I (SG-I), resulting in strong green fluorescence emitted by SG-I. The DNA aptamers exhibit a higher specific binding ability to target OP molecules through aromatic ring stacking, disrupting the interaction between SG-I and DNA aptamers to induce green fluorescence quenching. Meanwhile, the fluorescence wavelength of G-quadruplex fluorescence emission peaks changes, accompanied by an obvious fluorescence variation from green to blue. SG-I-modified aptasensor without any additive reference fluorescence units for use in multicolor fluorescence assay for selective monitoring of OPs was first developed. The developed aptasensor provides a favorable linear range from 0 to 200 nM, with a low detection limit of 2.48 and 3.01 nM for profenofos and isocarbophos, respectively. Moreover, it offers high selectivity and stability in real sample detection with high recoveries. Then, a self-designed portable smartphone sensing platform was successfully used for quantitative result outputs, demonstrating experience in designing a neotype sensing strategy for point-of-care pesticide monitoring.

Constructing graphene oxide/Au nanoparticle cellulose membranes for SERS detection of mixed pesticide residues in edible chrysanthemum

Edible chrysanthemum is widely cultivated and used as an important ingredient of medicine, tea and multifunctional food. During the planting of chrysanthemum, pesticides are extensively used for preventing plant diseases and insect pests. To ensure the food safety of edible chrysanthemum, rapid detection methods are urgently needed for on-site inspection. In this study, a graphene oxide/Au nanoparticle (GO/Au NP) cellulose substrate was prepared through layer-by-layer assembly of GO and Au NPs on a mixed cellulose ester membrane. Surface-enhanced Raman spectroscopy (SERS) detection of four types of organophosphorus and organosulfur pesticides was achieved by filtering the extracting solution through the substrate and analysing SERS spectra. Qualitative and semi-quantitative detection of fenthion, phoxim, isocarbophos and thiram was accomplished with the detection limits of 38.01, 8.13, 48.97 and 8.74 ng mL-1, respectively. A spiking experiment further demonstrated the feasibility of this method for rapid and on-site detection of mixed pesticides in chrysanthemum. This study provides a new approach for rapid detection of multiple hazardous substances in flowering and herbal plants.

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.

Enzyme-targeted near-infrared fluorescent probe for organophosphorus pesticide residue detection

Pesticide residues significantly affect food safety and harm human health. In this work, a series of near-infrared fluorescent probes were designed and developed by acylating the hydroxyl group of the hemicyanine skeleton with a quenching moiety for monitoring the presence of organophosphorus pesticides in food and live cells. The carboxylic ester bond on the probe was hydrolyzed catalytically in the presence of carboxylesterase and thereby the fluorophore was released with near-infrared emission. Notably, the proposed probe 1 exhibited excellent sensitivity against organophosphorus based on the carboxylesterase inhibition mechanism and the detection limit for isocarbophos achieved 0.1734 μg/L in the fresh vegetable sample. More importantly, probe 1 allowed for situ visualization of organophosphorus in live cells and bacteria, meaning great potential for tracking the organophosphorus in biological systems. Consequently, this study presents a promising strategy for tracking pesticide residues in food and biological systems.

Facile colorimetric smartphone-based biosensor for rapid detection of organophosphorus pesticides residues in environment using the aptamer-enhanced oxidase activity of octahedral Ag2O particles

The long-term and excessive use of organophosphorus pesticides (OPs) leads to hazardous residues in the environment, which threatens human health to a considerable extent. Colorimetric methods can quickly and readily identify pesticide residue, but they still have various challenges in accuracy and stability. Herein, a smartphone-assisted and non-enzymatic colorimetric biosensor was constructed for rapid monitoring of multiple OPs based on the enhanced effect of aptamer on the catalytic ability of octahedral Ag₂O. It was demonstrated that the aptamer sequence could enhance the affinity of colloidal Ag₂O to chromogenic substrates, and accelerate the generation of oxygen radicals such as superoxide radical (·O₂^-) and singlet oxygen (¹O₂) from dissolved oxygen, thereby significantly increasing the oxidase activity of octahedral Ag₂O. The color change of the solution can be easily converted to the corresponding RGB values by a smartphone for quantitative and rapid detection of multiple OPs. Hence, the smartphone-based and visual biosensor of multiple OPs was acquired with limit of detection of 10 μg L^-1, 28 μg L^-1 and 40 μg L^-1 for isocarbophos, profenofos and omethoate, respectively. The colorimetric biosensor also exhibited good recoveries in several environmental and biological samples, showing that it may have broad application prospects for detecting OPs residues.

G-quadruplex-deficient precursor hairpin probes for ultra-low background dual-mode detection of miRNAs

Design of oligonucleotide probe-based isothermal amplification with the ability to identify miRNA biomarkers is crucial for molecular diagnostics. Herein, we engineered a miRNA-21 responsive G-quadruplex-deficient precursor hairpin probe (PHP) to achieve dual-mode detection of fluorescent signal and colorimetric signal. Due to lack of complete G-quadruplex sequence, PHP becomes shorter in length, lower background signal and less interference. Based on the polymerase-driven amplification mechanism, in the presence of miRNAs, two simultaneous amplification reaction processes will occur in PHP: miRNA-based amplification process and endogenous amplification process along the 3' end. Due to the positional difference between the starting points of the two amplification processes, the orderly and efficient occurrence of the two amplification processes can be achieved. Based on an interesting concept, PHP can achieve high detection performance with only simple amplification cycles. In such a way, the detection limits for fluorescence and colorimetry were 2.93 fM and 8.81 fM, which would cover most of clinical qualitative and quantitative needs. Thus, the accurate quantitative and visual miRNA detection technology based on PHP is beneficial to carry out extensive disease screening and treatment monitoring in various complex occasions.

SERS- and absorbance-based catalytic assay for determination of isocarbophos using aptamer-modified FeMOF nanozyme and in situ generated silver nanoparticles

A new Fe metal-organic framework-loaded liquid crystal 4-octoxybenzoic acid (FeMOF@OCTB) nanosol was synthesized using 1,3,5-phthalic acid, ferrous sulfate, and OCTB as precursors. The FeMOF@OCTB exhibits good stability and strong catalytic effect for the polyethylene glycol 400-Ag (I) indicator reaction, which was evaluated rapidly by the slope procedure. The generated silver nanoparticles have a strong surface-enhanced Raman scattering (SERS) effect and a surface plasmon resonance absorption (Abs) peak at 420 nm. This new bimodal nanosilver indicator reaction was coupled with the isocarbophos (IPS)-aptamer (Apt) reaction. A FeMOF@OCTB nanocatalytic amplified-SERS/Abs bimodal Apt assay for IPS was established. The SERS assay can detect IPS in the concentration range 0.02-1.2 nM, with a detection limit of 0.010 nM. It has been applied to the determination of IPS in rice samples. The relative standard deviation was 4.4-5.8%, and the recovery was 97.7-104%. An Ag nanosol plasmon SERS/Abs dimode aptamer assay was fabricated for trace isocarbophos, based on highly catalysis MOF@OCTB nanoenzyme.

Sequence Effect on the Activity of DNAzyme with Covalently Attached Hemin and Their Potential Bioanalytical Application

In this work we investigated the effect of a DNA oligonucleotide sequence on the activity of a DNAzyme with covalently attached hemin. For this purpose, we synthesized seven DNA-hemin conjugates. All DNA-hemin conjugates as well as DNA/hemin complexes were characterized using circular dichroism, determination of melting temperatures and pKa of hemin. We observed that hemin conjugation in most cases led to the formation of parallel G-quadruplexes in the presence of potassium and increased thermal stability of all studied systems. Although the activity of DNA-hemin conjugates depended on the sequence used, the highest activity was observed for the DNA-hemin conjugate based on a human telomeric sequence. We used this DNAzyme for development of "sandwich" assay for detection of DNA sequence. For this assay, we used electric chip which could conduct electricity after silver deposition catalyzed by DNAzyme. This method was proved to be selective towards DNA oligonucleotides with mismatches and could be used for the detection of the target. To prove the versatility of our DNAzyme probe we also performed experiments with streptavidin-coated microplates. Our research proved that DNAzyme with covalently attached hemin can be used successfully in the development of heterogeneous assays.

Target-receptive structural switching of ssDNA as selective and sensitive biosensor for subsequent detection of toxic Pb2+ and organophosphorus pesticide

A structural switching in Single-stranded DNA (ssDNA) fluorescence biosensor for quick turn-on/off detection of Pb²⁺ ions and pesticide was reported. The design strategy of Hex-labelled ssDNA consists of two types of aptamer probe, G-rich base pair sequence forms G-quadruplex confirmation with Pb²⁺ ions. While other part of base pair sequence exhibits affinity to fold isocarbophos pesticide. MoS₂ nanosheets were identified as quick quencher of Hex fluorescence intensity via Vander-Waals interaction and its significance was compared with other nanomaterials. This sensing mechanism proposes a specific affinity of GA-rich ssDNA with Pb²⁺ to form G-quadruplex via G-Pb²⁺-G sequences. Consequently, ssDNA relived from MoS₂ nanosheets to restore the fluorescence intensity (turn-on). Subsequent addition of pesticide shows stronger affinity towards unfolded aptamer probe to form a random coil like structure. This causes Hex-labelled 5' end closer to the G-quadruplex connected at the 3' end of ssDNA resulting in a remarkable fluorescence quenching (turn-off) owing to PET process. Moreover, the sensing probe (Hex-labelled GA-rich ssDNA) was recycled by introducing acetylcholinesterase enzyme and thiocoline into the reaction mixture. The detection limits of Pb²⁺ and isocarbophos pesticide was estimated to be 0.6 nM and 0.018 μg/L respectively. Moreover, this study reveals a high sensitivity and selectivity towards target molecules in environmental samples.

Recent advancements in the detection of organophosphate pesticides: a review

Organophosphorus pesticides (OPPs) are generally utilized for the protection of crops from pests. Because the use of OPPs in various agricultural operations has expanded dramatically, precise monitoring of their concentration levels has become the critical issue, which will help in the protection of ecological systems and food supply. However, the World Health Organization (WHO) has classified them as extremely dangerous chemical compounds. Taking their immense use and toxicity into consideration, the development of easy, rapid and highly sensitive techniques is necessary. Despite the fact that there are numerous conventional ways for detecting OPPs, the development of portable sensors is required to make routine analysis considerably more convenient. Some of these advanced techniques include colorimetric sensors, fluorescence sensors, molecular imprinted polymer-based sensors, and surface plasmon resonance-based sensors. This review article specifically focuses on the colorimetric, fluorescence and electrochemical sensors. In this article, the sensing strategies of these developed sensors, analytical conditions and their respective limit of detection are compiled.

Colorimetric detection of chlorpyrifos in peach based on cobalt-graphene nanohybrid with excellent oxidase-like activity and reusability

Cobalt nanocrystal has been widely used as nano-enzyme for sensing and catalysis due to its high stability and low cost, but poor catalytic activity limits its applications in bioanalysis. The study reports one strategy for synthesis of cobalt-graphene nanohybrid. Histidine-functionalized graphene quantum dot (His-GQD) was bound to graphene sheet via π-π stacking and then combined with cobalt ions in the presence of cetyltrimethylammonium chloride to form stable complex and finally reduced under nitrogen to obtain Co-His-GQD-G. The as-synthesized nanohybrid offers well-defined three-dimensional structure and quasi-superparamagnetism. The cobalt nanoparticles were well dispersed on graphene sheets. The unique structure improves oxidase-like activity of Co-His-GQD-G. Further, Co-His-GQD-G was used as the nanozyme for colorimetric detection of chlorpyrifos. Co-His-GQD-G catalyzes oxidization of 3,3',5,5'-tetramethylbenzidine into blue product. Thiocholine produced by hydrolysis of acetylthiocholine under catalysis of acetylcholinesterase inhibits catalytic activity of Co-His-GQD-G and leads to a reduced oxidization rate. Chlorpyrifos inhibits activity of acetylcholinesterase and brings an enhanced absorbance of blue product. The absorbance at 652 nm linearly increases with increasing chlorpyrifos concentration in the range of 2-20 ng mL^-1 with detection limit of 0.57 ng mL^-1 (S/N = 3). The method was successfully applied in determination of chlorpyrifos in peach by preparing Co-His-GQD-G magnetic gel sheet.

G-quadruplex DNAzyme as peroxidase mimetic in a colorimetric biosensor for ultrasensitive and selective detection of trace tetracyclines in foods

The colorimetric method can determine the initial results even by the naked eyes, but its main challenge for antibiotics detection in food at present is the relatively low sensitivity. Herein, an ultrasensitive colorimetric biosensor based on G-quadruplex DNAzyme was firstly proposed for the rapid detection of trace tetracycline antibiotics like tetracycline, oxytetracycline, chlortetracycline and doxycycline. DNAzyme composed of hemin and G-quadruplex has peroxidase-like activity, and tetracyclines can combine with hemin to form a stable complex and reduce catalytic activity, making the color of solution changes from yellow to green. The limits of detection (LOD) of the proposed colorimetric biosensor for tetracyclines is determined as low as 3.1 nM, which is lower than most of the other colorimetric methods for antibiotics detection. Moreover, the average recovery range of tetracyclines in actual samples is from 89% to 99%, indicating that such strategy may has bright application prospects for tetracyclines detection in foods.

Colorimetric Aptasensor for Testosterone Detection Based on Aggregation of Gold Nanoparticles Induced by Cationic Surfactant

This paper proposes a colorimetric aptasensor for the detection of testosterone (TES) in environmental water, using TES-specific aptamer (apT5) as a sensing probe, gold nanoparticles (AuNPs) as indicator, and hexadecyltrimethylammonium bromide (CTAB) as inducer, respectively. Based on competition between TES and CTAB for apT5, the aptamer can form an aptamer–TES complex, leaving CTAB free to aggregate AuNPs in the presence of TES. Dispersed and aggregated AuNPs have different absorption wavelengths and the signal of absorption intensity is associated with the concentration of TES, so TES can be detected quantitatively based on the signal absorption intensity. This sensitive aptasensor for TES detection has a wide linear range (R=0.998) from 1.91–800nM and a limit of detection (LOD) of 1.91nM. In addition, this aptasensor has high selectivity over some interferents. The method detects TES in tap water samples with recoveries in the range of 98.9–102.6% (RSD ≤ 7.35%). This biosensor presents a good and potential application to rapidly detect TES in actual environmental water samples.

Adsorption enhanced the oxidase-mimicking catalytic activity of octahedral-shape Mn3O4 nanoparticles as a novel colorimetric chemosensor for ultrasensitive and selective detection of arsenic

Several researches have reported that Mn₃O₄ nanoparticles (NPs) could be used as adsorbent to remove arsenic from aqueous solution. However, we found that Mn₃O₄ NPs can not only adsorb arsenic, but also enhance the catalytic activity of Mn₃O₄ NPS, which enable us to establish a new method for the determination of arsenic. Herein, the adsorption of arsenic changes surface morphology of octahedral Mn₃O₄ NPs and further release Mn²⁺ to generate sufficient active sites, which enhances their oxidase-mimicking catalytic activity. Consequently, the solution changes to yellow and displays a characteristic absorption peak at 450 nm. This property enables us to construct a novel colorimetric chemosensor for arsenic detection. The limit of detection (LOD) of such colorimetric chemosensor for arsenic detection was determined as 1.32 μg⋅L^-1, which is lower than the threshold recommended by WHO. The chemosensor allows arsenic to be determined visually at the concentrations as low as 10 μg⋅L^-1, and displays excellent selectivity against other metal ions. Moreover, the chemosensor was successfully validated by analyzing several actual environmental and biological samples, indicating the excellent prospect of octahedral Mn₃O₄ NPs in the application of arsenic detection and removal.