1 Background

Pesticides are commonly employed in modern agriculture to control weeds and pests, regulate and promote plant growth, and enhance food production [90]. Crop disease management is applied to avoid destructive losses in agriculture and subsequently to satisfy the demands of a growing world population [155]. Raw and processed horticultural crops such as fruits and vegetables enrich nutritional intake and human health [20, 22, 131]. However, pesticide residues on fruit and vegetables hold serious health implications.

According to a report published in 2009, pathogens, insects, and weeds cause crop (losses of 13, 14, and 13%, respectively [143]). Regarding crop management worldwide, herbicides are used mostly (44%), followed by fungicides and bactericides (27%), insecticides (22%), and various others (7%) [113]. Excessive use or abuse of pesticides results in residues in food, which can threaten human health [66, 128, 65]. Pesticide residues contain hazardous compounds that even at extremely low concentrations have negative effects on human health and the environment. As a result, effective residue detection methods were designed for food security monitoring and public health safeguards [19, 25].

Instrumental detection techniques, which include high-performance liquid chromatography (HPLC), gas chromatography (GC), and chromatographic methods linked with mass spectrometry (MS) detectors, are commonly used to determine pesticide residues [6]. These techniques give precise qualitative and quantitative information about the residues. However, extensive sample pre-treatment limits the use of these techniques, because highly trained technicians and expensive equipment are needed [17]. Rapid approaches to determine pesticides are comparatively simple and include electrochemical techniques, spectroscopic analyses, and immunoassays. Although rapid methods lack the accuracy and precision of traditional analytical techniques, they can be employed as supplementary pre-screening procedures. Novel analytic methods for the quick, low cost, reliable, and selective detection of pesticides are therefore in demand [140].

The application of chemical agents not only increased agricultural productivity in short period, but also increased chemical toxicity in the air, water, and soil over time [112]. If chemicals are applied at incorrect times and products are harvested before the end of the pre-harvest interval, large concentrations of pesticides will remain in the product, which is dangerous to human health [159]. Residues on products must not exceed international regulatory maximum residue limits (MRLs) [180]. The concentration of pesticides in crops must be taken into account to maintain public health and protect the environment.

Developing and implementing pesticide alternatives must be cost-effective, environmentally safe, and produce rapid results. Thus, this review focuses on nanomaterials that are used as agricultural promoters and highlights the importance of nanomaterials in detecting pesticide residues [152]. Nanotechnology has many advantages over traditional methods, including high sensitivity and reducing energy consumption [47, 48]. Many nanomaterials such as nanoparticles, nanotubes, and nanocomposites can be utilized for the detection, degradation, and removal of pesticides [1]. This review discusses methods to detect pesticide residues on horticultural crops, explaining their advantages and disadvantages, including conventional, rapid test paper and immunoassay methods.

2 Main text

2.1 Conventional methods in pesticide detection

Hercegová et al. [77] demonstrated that there are several traditional analytical techniques for analysing pesticides and their products. These comprise flame ionization detection, diode array, electrochemical detection, gas chromatography (GC) with electron capture detection, fluorescence, and ultraviolet liquid chromatography (LC), all of which lack selectivity. The most common techniques are mass spectrometry (MS) merged with gas and/or liquid chromatography. At low detection limits, these methods have high sensitivity and selectivity, but are limited because they use sophisticated, time-consuming, and expensive equipment, which require skill to operate [155].

2.1.1 Gas Chromatography

Gas chromatography is one of the main analytical strategies utilized in food analysis to identify and quantify pesticide residues in complex matrices. Petsas and Vagı [142] demonstrated that GC differentiates between the pesticides based on their volatilities and thermal stability [45]. Gas chromatography can be combined with different detection methods and depends on the category of pesticides being quantified [100]. For example, methods such as mass selective detection (MSD), flame ionization detection (FID), nitrogen–phosphorous detection (NPD), and flame photometric detection (FPD) are used to determine pesticide residues in cereal samples [69]. The detectors provide selectivity and sensitivity for a particular pesticide. Electron capture detectors (ECDs) are notably used to for halogenated compounds such as organochlorine pesticides [14, 105]. The NPD is sensitive for organophosphate and nitrogenous pesticides ([105]), and the FPD for sulphur and phosphorus pesticides [14, 71]. The techniques are limited for pesticides such as N-methyl carbamate, because they are either maintained on the chromatographic column or decomposed to their phenols. Furthermore, derivatization methods can limit sensitivity and applicability of fragrant carbamates ECD, which include initial hydrolysis to the equivalent phenols or amines and reaction with halogen-rich reagents [130].

2.1.2 Liquid chromatography

Liquid chromatography (LC) is used to detect pesticide residues for limited categories of compounds or single compounds and for which there were no appropriate GC conditions. Esquinas-Requena et al. [55] demonstrated that the original detectors used for LC methods were the UV or diode array detectors (DAD). These methods are generally accurate and effective, but necessitate the use of costly instruments, specialized staff, and have lengthy procedures [61]. To develop both selectivity and sensitivity, effective coupling between LC separation can be done with MS (LC-MS and LC-MS/MS) to improve the determination pesticide residues and their transformation products in complex matrices such as food [161]. Currently, high-resolution MS (HRMS) and tandem MS (MS/MS) are widely used. Celeiro et al. [34] showed that the usual MS analysers used in food analysis are quadrupole(Q), triple quadrupole (QqQ), time of flight (TOF), hybrid quadrupole ion trap (QTrap), and Orbitrap [34, 70].

2.1.3 High-performance LC (HPLC)

High-performance LC is widely utilized and can be combined with many detectors. It can be used with DAD and/or UV to determine organophosphorus and triazines in various matrices. The HPLC uses a pump to promote the movement of the mobile phase (s) and analyte across the column and includes a detector to allow keeping time for the analyte [175]. Many factors affect the analyte keeping time and depends on the extent to which it interacts with the stationary, the solvent composition, and the mobile phase flow rate. Reversed-phase liquid chromatography (RPLC) is a type of HPLC that is most approved, due to its capability to perform successful separation of polar to apolar pesticides with good performance and detection that cannot be directly applied to GC [79]. Table 1 summarizes conventional methods such as GC, HPLC, infrared spectroscopy, MS, and spectrophotometry, which are expensive and time-consuming, and indicates the need to develop simple and rapid methods.

Table 1 Conventional methods to detect pesticides
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2.1.4 Detection of pesticide residues through multivariate analysis and VIS /NIR spectroscopy

Using chromatographic methods has several disadvantages such as a complex evaluation procedure, long detection cycle, and lagged nature of detection results. So, it is important to develop fast, reliable techniques [53, 138, 139]. Near-infrared (NIR) spectroscopy is a convenient technique used in quantitative and qualitative analysis in fields such as medicine, agriculture, and chemistry. Ultraviolet visible–NIR spectroscopy can be used to predict soil composition and pesticide absorption [88, 103]. Near-infrared spectroscopy (12900–4000 cm−1) is categorized within NIR reflectance spectroscopy and NIR transmission spectroscopy. The NIR can be non-dispersive (filter-based instrumentation) and dispersive and can use Fourier transform-based instrumentation. This method is cheap, safe, simple, environmentally friendly, avoids using organic solvents, and does not need sample preparation as does chromatographic methods [87, 156]. Models and regressions (partial least squares discriminant analysis (PLS-DA)) and models (partial least squares (PLS)) are used for quantitative determination of total nitrogen (TN) and organic carbon (OC) in soil [162]. Table 2 lists some NIR spectroscopic techniques appropriate for pesticide calculation.

Table 2 Near-infrared (NIR) spectroscopy applications for the determination of pesticides
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3 Rapid detection technology of pesticides

3.1 Paper chromatography

Paper chromatography refers to an analytical approach that separates coloured chemicals or substances on chromatographic paper. This technique is extensively implemented to separate complex mixtures of carbohydrates, steroids, amino acids, peptides, amino acids, purines, simple organic compounds, and inorganic ions [41, 89]. It was the only technique available for the isolation and detection of pesticide residues before the development of GC and thin-layer chromatography (TLC) [58]. Currently, GC is used, because of its sensitivity and simultaneous quantitative estimation capabilities. However, paper chromatography is still implemented to validate non-specific gas chromatographic results [69]. However, TLC is increasingly replacing paper chromatography in pesticide residue research due to its enhanced resolution and shorter development time [158].

3.2 Paper chromatography in pesticide detection

Yang et al. [189] and Getz [189, 64] extensively explored various applications of paper chromatography for pesticide identification. Pesticide isolation, detection, and identification from cleaned tissue extracts have been the primary applications of paper chromatography. Pesticide residue separation depends on paper chromatography, especially insect tissue extracts to separate insecticides from their metabolites, and organophosphate residues from their lipid content. Paper chromatography has also been used to separate pesticides from plant waxes. Pesticide residues were quantified by measuring the spot size on a paper chromatogram [91].

3.3 Paper chromatography techniques

Paper chromatography cannot be applied to water-insoluble materials such as chlorinated hydrocarbons and organophosphate-based pesticides, which limits its scope [123, 164]. Zweig and Archer used paper chromatography to isolate and detect sevin and 1-naphthol in wine [120], and the same method has also been used to detect herbicides. For example, Mitchell used the method to determine monuron and 3-amino-1,2,4-triazole [126, 127], and Anliker et al. separated phosphamidon and its metabolites [195]. In pesticide residue studies, the most common method is reverse-phase chromatography where the paper only supports the immobile solvent. The compounds of interest are separated through the partition between the immobile solvent and the mobile solvent, which passes through it. Stationary phases are usually made of vegetable or mineral oils, silicone, propylene glycol, and dimethylformamide. This method is suitable for isolating compounds with a very low water solubility, such as chlorinated hydrocarbons and organophosphorus (OP) pesticides [28, 179]. Paper chromatography is used to separate pesticide residues without chemically altering the chromatographic paper. However, fibreglass papers have been used for reverse-phase chromatography of OP compounds [91]. McKinley, Colovic and their colleagues have also used acetylated papers to separate organophosphorus pesticides in reverse-phase chromatography [42].

3.4 Advantages of paper chromatography

The ease of the procedure, low cost, and the ease of altering conditions are all advantages of paper chromatography. In paper chromatography, many characteristics, including the paper, immobile phase, developing solvents, development direction, duration, and detection method, can be changed quickly and effortlessly [24]. This aspect is vital when developing procedures for samples under specific circumstances. If the sample is processed after chromatographic separation, but before quantitative analysis, then paper chromatography has a significant advantage over TLC, because the paper can be processed along with the sample [24]. It is occasionally necessary to elute the separate parts from the thin-layer chromatogram in TLC before processing. As paper chromatography has been used in pesticide studies for several years, it is more familiar than newer chromatographic techniques. Due to this familiarity, interpreting paper chromatographic findings is more straightforward and done with more confidence [24, 43].

3.5 Mechanism of rapid test paper technology in pesticide detection

An enzyme inhibitor is a molecule that binds to an enzyme’s active site, thereby reducing its activity and preventing the substrate from binding. This prevents the development of enzyme–substrate complexes, the catalysis of reactions and reduces product formation. Enzyme inhibition-based methods have been used to detect organophosphates and carbamates [21, 32]. Guo et al. created a rapid test strip for visual pesticide identification by inactivating the enzyme, its substrate, and a chromogenic agent on a paper matrix [73]. This method is portable, fast, easy to use, and inexpensive. It has a shelf life of 2 months when stored at four degrees Celsius; however, the enzyme activity is lower if stored at room temperature for 2 months [73]. Increasing colour resolution improves visual pesticide residue identification accuracy, because human optic cells are especially sensitive to wavelengths on the blue-green spectrum. Sun et al. [169] developed a dual-film screening strip made of fibreglass and polyester fibre-containing acetylcholinesterase (AChE) and indoxyl acetate. The strip was able to soak up and liberate all of the AChE or indoxyl acetate (Fig. 1).

Fig. 1
figure 1

Schematic illustration of the dual-film visual screening card: A panorama, B decomposition schematic, and C longitudinal section

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4 Colorimetric analysis

Colorimetric analysis is a commonly used technique in paper-based analysis. Colorimetric analysis has several advantages, including high effectivity, operation ease, and increased stability [149]. Screening tools that are rapid, simple, inexpensive, and detectable by the naked eye are designed for the high-throughput screening of pesticides [39]. In colorimetric analysis, the sample solution is inserted into a test zone via capillary action. The sample then reacts with a colour reagent, and the colour changes. The colour formation or colour change is used to perform qualitative and quantitative studies of the pesticide from the test result [149].

4.1 Colour signals

Colour signals can be collected in two ways: (1) Smartphones, single-lens reflex cameras, and inexpensive desktop scanners can be used to directly image the result, after which quantitative analysis can be performed by specific software. (2) A spectrophotometer can be used to measure the absorbance at a specific wavelength, giving an accurate quantitative result [149].

A paper-based microfluidic chip can measure de-oxy-nivalenol (DON-Chip) in animal feed and food rapidly and at a low cost. The DON-Chip combines a colorimetric immunoassay with gold nanoparticles (AuNPs) and a paper microfluidic apparatus. The AuNPs act as signal indicators. As shown in Fig. 2, a new ratiometric analysis technique proposed for the analysis of DON performed well and successfully detected compounds in 12 minutes [149].

Fig. 2
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Schematic illustration of DON-Chip production and operation

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4.2 Surface-enhanced Raman spectroscopy (SERS) swabs to detect pesticides in vegetables and fruits on-site

The feasibility of using silver nanoparticles–graphene oxide (Ag NP/GO) surface-enhanced Raman spectroscopy (SERS) swabs was tested on-site in fruits and vegetables with and without spiked pesticide [112]. The peels of vegetables and fruits were cut into 1 cm2 squares. Following this, 10 mL of pesticide solutions of varying concentrations was added to the peels [112]. Ten mL ethanol was added onto the AgNP/GO paper before applying Raman measurements to improve contact with the area of analysis and pesticide adsorption [112]. The square-shaped peel was blotted with the paper for 3 seconds. The Ag NP/GO paper was placed on a glass slide for SERS analysis after the strips dried out [112] (Fig. 3).

Fig. 3
figure 3

Illustration of the screen-printing production process for SERS swabs its application to detect pesticides in fruits and vegetables

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4.3 Examples of rapid paper use in pesticide detection

Blažková et al. developed a strip-based immunoassay that quickly detects thiabendazole in fruit juice. The immunoassay depends on the interactions between thiabendazole–ovalbumin and thiabendazole. C-nanoparticles are combined with anti-thiabendazole to create a detection complex after which thiabendazole can be visualized [27]. If thiabendazole is absent, the detection complex will bind to thiabendazole–ovalbumin to produce a black band [27]. If thiabendazole is present, a portion of the detection complex will be neutralized. The test line’s colour intensity is inversely correlated with the thiabendazole concentration [27]. There is a similar test card similar, which can detect carbaryl [80]. The test card relies on the capture of carbon NPs by inactivated antibodies on the test zone, giving a black band.

Free carbaryl binds to immobilized antibodies, which stops the interaction of carbon NPs with the inactivated antibodies. The colour intensity of the test band is inversely proportional to the sample’s carbaryl concentration [80]. A competitive immunoassay dipstick based on AuNPs was developed as a rapid test for dichlorodiphenyltrichloroethane (DDT). The DDT pesticide is harmful, because it does not break down easily in the environment, and causes nervous system damage and complications in animal reproduction [106]. The gold nanoparticles are coupled with anti-DDT antibodies, after that the immune-complex solution is added to nitrocellulose membrane cards, which contain free DDT and the antigen. The free DDT then competitively inhibits the antigen at the AuNPs binding site. In the absence of free DDT, the card will show the red colour of the AuNPs. The red colour intensity decreases with an increase in free DDT concentration [106].

4.4 A Highly sensitive immunoassay of pesticide

The previous methods have disadvantages such as costly apparatuses needed, the long time needed for analysis, and the necessity of professional staff and not being safe to the environment. As a result, using of immunoassay-based antigen-antibody is widely used. [44] demonstrated that the immunoassay is an analytical method for detecting different substances using antigen-antibody specific binding reactions. There are two types of immunoassay: (i) labelled immunoassays and (ii) unlabelled immunoassays. For example, labelled immunoassays involve the bio-barcode immunoassay, enzyme-linked immunoassay (ELISA), fluorescence immunoassay (FIA), etc., while unlabelled immunoassays comprise immunoelectrophoresis and immunodiffusion. Advantages of immunoassay include simplicity, low cost, high sensitivity, and its ability to identify multiple types of pesticides (veterinary drugs, bio-toxins, heavy metals, etc.) or different types of small molecules at the same time. Cui et al. [44] demonstrated the main method of multiple residual detections which includes bio-barcode assay immunoassay, ELISA, FIA, etc. There are pesticides such as fenpropathrin, decamethrin, λ-cyhalothrin parathion, methyl parathion, fenitrothion organothiophosphate pesticides, organophosphate pesticides, and chlorpyrifos fenthion analysed and detected by ELISA and CLIA [44], 170.

5 Types of test paper technology utilized in rapid detection of pesticides

5.1 Nanoparticle markers

Nanoparticle markers have recently been developed and are known as ultrafine particles or nano-dust. The particles have a diameter of less than one nanometre (typically between 1 and 100 nm) [47]. Concerning light, heat, and susceptibility to magnetic fields, the particles are different from ordinary particles and have a broad specific surface region [23]. Nanoparticles are analytically important with various applications [47, 48]. Chemical nanoparticles, colloidal gold, lanthanides, quantum dots, magnetic nanoparticles, and carbon nanotubes are only a few of the nanoparticle markers that have been included in test strip processes [194].

Organic nanoparticles with strong optical properties, such as fluorescein isothiocyanate (FITC) nanoparticles, were one of the first to be used in the test strip process [8]. The primary amines of proteins are provided by the FITC nanoparticles, resulting in the ideal dye-protein conjugate [121]. The addition of fluorescein in the compound can be used to determine the presence of proteins. However, this approach has low sensitivity and photochemical stability since it relies too heavily on the chemiluminescent properties and lacks the effect of inorganic nanoparticles, which may regulate wavelength. As a result, new markers are increasingly being developed.

Colloidal gold, called a gold sol, is a multiphase uneven system created by the electrostatic repulsion between gold particles in water [118]. A system will ingest biological macromolecules without disrupting biological function and emit colours varying from green to red to purple. This allows for use as markers to differentiate between macromolecules such as proteins, polysaccharides, nucleic acids, and hormones. The test strip colloidal gold marker is the oldest and most thoroughly researched technique [140]. It has been used to test for aflatoxins in food. Many compounds such as the gold marker test strips, which are used for the identification of veterinary drug residues and pesticides, have been commercialized and include vibrio parahaemolyticus, Sudan red, and MicroRNA [182]. The gold colloidal-based immuno-dip strip was used to detect Sudan red I residue in tomato sauce and chilli powder samples quickly, with a limit of detection of 10 ng/g [72].

Lanthanide elements are a group of intermediate elements with atomic numbers ranging from 57 to 71 in the periodic table. A group of fluorescent up-conversion phosphor particles are produced by combining two related lanthanide ions as “light absorber” and “emitter” and incorporating them into ceramic particles that serve as “primary substrates”. Hong et al. [81] produced a strip that is stable for 10 days at 37 degrees Celsius with 10.3 per cent using up-conversion phosphor particles as markers. Its sensitivity and quantitative findings are equivalent to those of a traditional immunology assay. The traditional enzyme-linked immunosorbent assay (ELISA) is used for antibody detection with a linearity fitting coefficient of determination (R2) between 0.93 and 0.99. As a result, using lanthanide elements in test strips provides ideal detection limit and stability [81], resulting in rapid application development.

Quantum dots, also classified as fluorescence semiconductor nanoparticles, have compounds and nanoparticles of Si and related elements, as well as primary groups II-IV (e.g. CdSe) and III-V (e.g. InP). Particle diameters range from 1 to 10 nanometres. Since they mimic tiny dots, they are named after quantum dot. A core–shell standardized quantum dot is currently the most widely used since it not only has strong photochemical stability, but also a high luminescence quantum yield (30–50%). Quantum dot application in test strip markers is still under investigation, but its viability has been documented. With a minimal test line of 1–2 nm, Petryayeva and Algar [141] were able to complete the quantitative detection of protease in 5 minutes.

Nano-magnetic particles, also known as super-paramagnetic particles, are a relatively new kind of nanomaterial. Their super-paramagnetic feature, large specific surface area, and compact particle size incorporate the aspects of magnetic particles and nanomaterials. Magnetic materials (e.g. iron oxide) that act as a stable phase carrier are common markers. When active groups are added to the layers of magnetic substances, a coupled reaction between the magnetic materials and biological molecules including enzymes and antibodies occurs. The research material can be detected easily and quantitatively in this manner [63]. Fisher et al. [57] developed an immunomagnetic lateral flow system that allowed the identification of Bacillus anthracis spores in 10 mL dairy samples (n = 38) at a concentration of 5 × 105 CFU mL¯1, resulting in a 60-fold increase in sensitivity over standard strip methods. However, since magnetic particles are vulnerable to aggregation during the chromatographic process, few records of nano-magnetic particles used in test strip markers exist.

Carbon nanotubes, called buckytubes, are quantum substances with a topological shape resembling a twisted hexagonal grid structure of graphite. Carbon nanotubes have quantum effects similar to ordinary nanoparticles, have a large surface area, and have high conductivity and high mechanical power. Its distinct black colour makes it easier to identify qualitatively or semi-quantitatively with the naked eye. A nucleic material lateral flow method was defined for detecting listeria infection [136], with a low visual level of 0.1 ng of the labelled amplicon. The PCR solution is specifically applied to the strip, and the presence of clear amplicons is shown by the appearance of a grey/black line mediated by carbon nanoparticles (maximum time of 15 min). However, removing the carbon graphite and amorphous carbon debris mixed in the carbon nanotubes is technically challenging.

5.2 Paper in microfluidics

As described by Whiteside in 2006 [26], microfluidics is the modern science of devices that manage and control small quantities of fluid (10−9 L). Fluidic channels of hundreds to tens of micrometres in diameter are used. Because of variation in length, use of small amounts of samples and reagents, and rapid isolation and detection with high resolution and sensitivity [2], microfluidics experienced exponential growth with significant impacts in analytical chemistry. Glass, silicon, and polymers such as polydimethylsiloxane (PDMS) were used in early microfluidic studies. Even though microfluidic systems miniaturize traditional approaches for precise isolation and identification, they have disadvantages such as the cost of substrate materials and the need for power and fluid transfer instruments [157].

Paper is an attractive substrate medium to synthesize microfluidic devices [122]. Paper has many benefits as a low-cost diagnostic tool, which has been extensively explored: It can be printed quickly, coated, and impregnated; the cellulose structure is consistent with proteins and biomolecules, widely available and environmentally friendly since it can be disposed of by incineration [99]. The cellulose membrane network of microfluidic paper-based analytical devices (µPADs) uses paper as the primary substrate to provide instrument-free liquid transport through capillary action. Paper has a large surface area, a volume ratio that improves detection limits for colorimetric assays, and has the capacity to store chemical components in their active state within the paper fibre network. While µPADs lack the high resolution and sensitivity of silicon, glass, or plastic-based instruments, their implementation is suitable for point-of-need monitoring. The µPADs can be used in inexpensive research for constant testing, especially in less developed countries where complex instrumentation, analytical laboratories, and experts are scarce. As a result, µPADs have emerged as an appealing alternative to highly sophisticated instrumentation in analytical applications for food and water monitoring [137]. Much research studies have been conducted on the construction and deployment of µPADs for water and food protection and quality control and include fabrication techniques of µPADs and appropriate detection methods for quantitative and qualitative analyses [137].

5.3 Molecular imprinted polymer grafted paper-based multi-disc micro-disc plate (MIP method)

Pesticides have been used in agriculture for many years and have made a major contribution to food safety and productivity. However, these compounds harm human well-being [33]. Wang et al. [183] created a paper-based molecular imprinted polymer-grafted multi-disc micro-disc plate (MIP) for 2,4-dichlorophenoxyacetic acid CL detection (2,4-D).The MIP method had been considered as an option for immunoassay, which depends on antibodies. There are, however, significant disadvantages such as antibody hydrolysis and instability during manufacture and transport.

Tobacco peroxidase (TOP)-labelled 2,4-D molecularly imprinted on a polymer-grafted unit was used in an indirect comparative assay. The luminol–TOP–H2O2 CL system produced enzyme-catalysed chemiluminescence emission with a limit of detection of 1.0 pM [182, 183]. Liu et al. [111] created a simple paper-based luminol–H2O2 chemiluminescence for the identification of dichlorvos (DDV). Gilbert-López et al. [67] formalized a µPAD chemiluminescence assay to detect DDV in fruits and vegetables using paper chromatography, and the separation was completed in 12 minutes using 100 mL of developing reagent. The technique was successfully applied to identify trace DDV on cucumber, onion, and cabbage using a spiking method (3.6 ng mL−1 detection maximum). Liu et al. [111] proposed another MIP method for chemiluminescence detection of DDV using a paper-based instrument with a molecularly imprinted polymer. The detection limit was 0.8 ng mL−1, and the procedure worked well on cucumber and tomato. A paper-based colorimetric technique for identifying organophosphate and carbamate pollutants has also been demonstrated. Wang et al. [183] created a system based on the inhibition of organophosphate (methomyl) and carbamate (profenophos) pesticides and used acetylcholinesterase (AChE) to degrade acetylcholine molecules into choline and acetic acid. The degree of AChE inhibition suggested pesticide toxicity, making AChE a typical bioevaluator for the presence of organophosphates and carbamates [111].

5.4 Smartphone-based detection

The use of smartphones is growing, along with the number of µPAD techniques that combine tablets or smartphones for measurements [166]. Chaiyo et al. [35] established a µPAD sensor and a mobile application for on-site colorimetric identification of organophosphate pesticides (paraoxon and malathion) based on the pesticides' inhibition of immobilized AChE. The enzyme AChE hydrolyses the substrate in the absence of pesticides, and the colourless indoxyl acetate substrate is converted to an indigo-coloured substance. The colour strength decreases with rising pesticide concentration due to AChE inhibition. The colour intensity is evaluated using an image analysis algorithm on a smartphone, resulting in real-time monitoring and mapping of water quality. The tool can detect pesticide concentrations as low as 10 nM, as demonstrated by a colour shift in the µPAD [134]. Zhang et al. [191] focused on the use of nanoceria-coated µPAD for colorimetric organophosphate pesticide detection using enzyme inhibition assay with AChE and choline oxidase. In the existence of pesticides, AChE activity is prevented, resulting in no or limited H2O2 production and, as a result, less yellow colour formation of the nanoceria. (The colour production process is depicted in Fig. 4.) The assay could detect methyl-paraoxon and chlorpyrifos-oxon with detection limits of 18 ng mL−1 and 5.3 ng mL−1, respectively. The procedure was applied successfully for methyl-paraoxon identification on spiked cabbage and dried green mussel, with 95% recovery values for both samples.

Fig. 4
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The colour production process in nanoceria-coated µPAD colorimetric organophosphate pesticide detection

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5.5 Paper-based visual detection

The design of a mobile (CdTe) paper-based sensor for identifying carbamate pesticides was continued. The nano-ZnTPyP concentration on the paper chip was increased to ensure the high sensitivity of the sensor. The same concentration (10 g L−1) of metolcarb was tested using a CdTe-based paper sensor with different concentrations of nano-ZnTPyP (17.04, 17.85, 18.75, 19.73, and 20.83 mol L−1). A low concentration of nano-ZnTPyP was not beneficial to visual identification, whereas a high concentration of nano-ZnTPyP was detrimental to eventual fluorescence recovery. The nano-ZnTPyP concentration of 17.85 mol L−1 had the most noticeable variations in colour change between quenching and regeneration, which was useful for visual recognition. As a result, the nano-ZnTPyP concentration for the paper-based sensor was determined to be 17.85 mol/L. The paper chips were then used thereafter to identify carbamate pesticides. As shown in Fig. 5, various concentrations of metolcarb (1–20 g L−1) were applied to the paper chips. As the metolcarb concentrations rose, the colour of the paper changed from dark green to yellow-green, then to pale green, and eventually to green, allowing for the quick visual identification of pesticides. Images were taken with a camera under 365 nm ultraviolet analyser, and the colour RGB values were collected and simulated by a computer programme, revealing the same pattern of colour shifts on the paper. Carbofuran and carbaryl were detected visually and quantitatively by the paper-based sensor under the same conditions. It was stated that the production of a novel nano-zinc 5, 10, 15, 20-tetra(4-pyridyl)-21H-23H-porphine (nano-ZnTPyP)-CdTe-based paper sensor could successfully detect carbamate pesticides in a quick, highly sensitive, highly precise, and on-site manner [148].

Fig. 5
figure 5

Change in paper colour with rising of the metolcarb concentrations using a CdTe-nano-ZnTPyP-based paper sensor

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6 Recent studies in rapid test paper technology

Pesticide enzymatic and immunoassay test kits have been produced. Enzyme-based test kits can indicate organophosphate (OP) and carbamate (CM) pesticides in water, ground, vegetables, fruits, and other ecological samples. Also, to evaluate enzyme activity before and after pesticide exposure, techniques such as fluorimetry, amperometry, spectrophotometry, potentiometry, and thermometry are used [96]. There are several recent studies in rapid test paper technology, as summarized in Table 3.

Table 3 Advantages and disadvantages of the techniques used for pesticide detection in horticulture crops
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7 Conclusions

The usage of pesticides can lead to toxic residues in horticulture crops, which, if consumed, can lead to decreased immunity, splenomegaly, renal failure, hepatitis, respiratory disorders, and cancer in humans. As a result, safe and practical strategies for detecting these residues in horticultural crops and monitoring food security is essential. Each of the discussed methods can be used in a certain situation, and the variety of methods enable detection of different types of pesticides in the environment. For instance, conventional methods, such as ECDs, are effective for organochlorine pesticides detection, while NPD, LC, HPLC, and NIR are suitable for organophosphate and nitrogenous pesticides, single compounds, organophosphates/triazines, and prediction of soil composition/pesticide absorption, respectively. Recently, rapid detection technologies have proved a great success, such as TLC that has enhanced resolution and shorter development time. Paper chromatography has a significant advantage over TLC, because the paper can be processed along with the sample. Colorimetric analysis and SERS that combine nano-markers, tablets, or smartphones for measurements are more effective to detect pesticides on horticulture crops on-site. Interestingly, the highly sensitive immunoassay, which offers the advantages of being low cost, specific, and sensitive, allows it to be integrated into many detection fields to accurately detect pesticides (Table 4).

Table 4 Summary of recent studies in rapid test paper technology
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