Functionalized gold nanoparticles for sensing of pesticides: A review.

Pesticides are a family of non-biodegradable chemical compounds which widely used in agriculture to control pests and increase yield production. However, overuse or abuse of pesticides and their metabolites may cause potential toxicity for the environment as well as human health and all other living organisms, even at deficient concentrations. Consequently, the development of sensors for monitoring these compounds is significant. Recently, nanoparticles-based sensors have been extensively employed as a potential alternative or complementary analytical tool to conventional detection methods for pesticides. Among them, gold nanoparticles (AuNPs) owing to their unique optical properties have been developed as smart sensors with high selectivity, sensitivity, simplicity, and portability. These comprehensive reviews have summarized various studies performed based on different detection strategies, i.e., colorimetric, fluorescence, surface-enhanced Raman scattering, and electrochemical, using AuNPs as sensing probes for pesticide analysis in various matrices. Additionally, the current challenges and future trends for developing novel AuNPs-based sensors for the detection of pesticides are also discussed.

1. Introduction

Pesticides are a large and heterogeneous group of heavily employed chemicals in modern agriculture for protecting crops, controlling insect pests, and improving productivity [1]. In terms of their chemical structures, pesticides can be classified into four leading groups, including organochlorines, organophosphorus, carbamate, chlorophenols, and synthetic pyrethroids [2]. If concerned with pesticides’ function, they can be categorized into five types that include insecticides, herbicides, fungicides, rodenticide, and nematocides [3-5]. Many toxic pesticides have been extensively used in agriculture due to their low cost and high effectiveness [1,2]. However, the overuse, abuse, and misuse of pesticides lead to their residues and metabolites’ release, causing the adverse effect on the ecological system and human health [6]. Even exposure to very low pesticide levels could be highly involved in food and water safety [7]. Additionally, some of the pesticides, identified as endocrine disruption, have been implicated with neurotoxicity, genotoxicity, mutagenicity, and carcinogens [8-10]. Based on the consideration of toxic pesticides’ threats to human health and the environment, it is urgent to develop sensitive, selective, low-cost, and affordable analytical methods to monitor their level in real-world samples.

Current analytical methods used to determine pesticides and their metabolites mainly include three parts of quick, easy, cheap, effective, rugged and safe (QuEChERS) extraction, separation techniques (gas chromatography and high-performance liquid chromatography), and mass spectrometry (e.g., quadrupole ion trap and time-of-flight instruments) [11,12]. Although offering satisfactory sensitivity and excellent separation efficiency, most mass spectrometry-related approaches still suffer from time-consuming sample preparation, sophisticated instrumentation, and required skilled operators [13]. More importantly, these methods are hard to transform into a portable device for on-site detection. Recently, ligand-capped gold nanoparticles (AuNPs) with a size range from 1 to 100 nm have attracted tremendous interests since they provide high molar extinction coefficients (>108 M−1 cm−1), size-dependent optical properties, strong Rayleigh scattering, and easy functionalization. These features enable ligand-capped AuNPs to be well-suited for the practical applications in photonics [14-16], electronics [17,18], biosensing [18-20], bioimaging [21,22], and nanomedicine [23-26]. In addition to these applications mentioned above, ligand-capped AuNPs also serve as a sensing platform for colorimetric, fluorometric, and electrochemical detection of a broad range of analytes from metal ions and small biomolecules to protein and DNA macromolecules. The sensing mechanism of ligand-capped AuNPs can be divided into three strategies: (1) a target analyte induces the aggregation of ligand-capped AuNPs via the coordination interaction between two ligands and a target analyte; (2) a target analyte triggers the removal of a colloidal stabilizer from the nanoparticle surface via the formation of Au–S bonds, resulting in the salt-mediated assembly of ligand-capped gold nanoparticles; (3) a target analyte drives the liberation of a recognition molecule from the nanoparticle’s surface through the complexation reaction. The aggregation of ligand-capped AuNPs can promote a redshift in surface Plasmon resonance, a restoration in fluorescence peak, and an enhancement in Raman scattering. Taking these advantages, researchers have developed different kinds of ligand-capped AuNPs for probing pesticides in real-world samples. In this review, we present a comprehensive discussion associated with the recent advancements in the use of unmodified and functionalized AuNPs to detect pesticides. To the author’s knowledge, there is no systematic review covering such topics. We especially emphasize integrating the colorimetric, fluorescence, surface-enhanced Raman scattering (SERS), and electrochemical strategies (Fig. 1) with ligand-capped AuNPs for the determination of pesticides.

Fig. 1

Schematic illustration for various AuNPs-based sensing strategies for the pesticide detection.

2. Synthesis of AuNPs and their modification

Gold has different oxidation states that include Au+3 (auric), Au+1 (aurous), and atomic Au0. Since AuNPs consist of numerous Au0, the synthetic procedure involves the reduction of Au+3 or Au+1 to Au0 with a reducing agent. Examples of reducing agents include citric acid [18,27,28], ascorbic acid [29,30], sodium borohydride [31], oxalic acids [32,33], sulfites [34,35], and hydrogen peroxide [27,36]. At the beginning of the process synthesis, chloroauric acid (HAuCl4) is frequently employed as a precursor chemical in the preparation of the AuNPs, and it can be purchased from chemical companies or obtained from the reaction of aqua regia and gold foil. Also, capping ligands are needed in a precursor solution to prevent the formed AuNPs from aggregation and further growth during the synthetic process. The capping ligands enable the AuNPs to have sufficient electrostatic repulsion, steric hindrance, or both to be dispersed in an aqueous solution [37-39]. Thiol-terminated small molecules [37,40], trisodium citrate [41,42], surfactants [43,44], polymers [38,45], phosphate-containing nucleotides [46,47], and phosphorus ligands [48] are exemplified by capping ligands. It is interesting to emphasize that several capping ligands serve as a reducing agent and a stabilizing agent [49]. After mixing HAuCl4 with a weak reducing agent in the presence of capping ligands, the resultant solution is commonly treated with heating [50], microwave irradiation [51], and UV light exposure [52]. Once substituting a weak reducing agent with a strong one, the reaction simply proceeds at ambient temperature [53,54]. UV–visible and X-ray spectroscopies have been implemented for monitoring the nucleation and growth of the AuNPs [55]. Electron microscopic and optical spectroscopic techniques can provide detailed information associated with the morphology, compositions, valence state, and optical properties of the AuNPs [56,57]. Moreover, single-particle inductively coupled plasma mass spectrometry (ICP-MS) has been adopted for accurate quantification of the molar and particle number concentrations of the AuNPs [58,59]. The measurement of zeta potential of the AuNPs can help the investigators to assess whether they are present in the dispersed state [60]. The purification of the AuNPs can be performed by filtration, centrifugation, dialysis, and separation techniques [61]. A more detailed characterization of the AuNPs has been reviewed in the recent literature [62].

Faraday has pioneered the development of the methodology for producing 5 nm-sized AuNPs from the reduction of HAuCl4 in the presence of the phosphorus-ether solution [63]. During the synthetic process, the mixed solution’s color varied from brown, grey, purple, to deep red, which can be easily visualized by the naked eye. This breakthrough opens the door to the synthesis of stable AuNPs and motivates the researchers to design new synthetic routes. The Turkevich method is one of the most common ones used for the reproducible and simple synthesis of approximately 10–50 nm AuNPs with a spherical shape [27]. The formed citrate-capped AuNPs and their surface modification are intensively implemented as a colorimetric reporter to sense organophosphate pesticides. It is worth emphasizing that the preparation of citrate-capped gold nanoparticles is straightforward and simple without high-cost dedicated equipment and a great experience. Besides, the cost for 60 mL of 2.57 nM citrate-capped gold nanoparticles is only ca. US$ 0.61. In comparison to the Turkevich method, the AuNPs produced from the seed-mediated growth method [64], Brust-Schiffrin approach [31], electrochemical techniques [65-67], ionic liquid-related preparation [68-70], and biological molecule-mediated synthesis [71-73] are rarely utilized as a platform for the detection of organophosphate pesticides. Fig. 2 reveals the above-mentioned synthetic methods. Accordingly, in the review article, we focus on the detailed discussion of the Turkevich method. Briefly, the Turkevich method involves injecting trisodium citrate into a boiling solution of HAuCl4 under reflux and vigorous stirring. Immediately, rapid nucleation occurs in the initial stage, followed by the aggregation of the nuclei. Next, the formed bigger particles undergo slow diffusion growth by reducing gold ion precursor and further coalescence. Finally, the resultant particles undergo rapid growth to form a fixed size of the AuNPs. The as-prepared citrate-capped AuNPs exhibit the surface plasmon band (SPR) in the visible regions. The mechanism of gold nanoparticle formation mentioned above has been demonstrated using small-angle X-ray scattering (SAXS) and X-ray absorption near-edge spectroscopy (XANES) employing synchrotron radiation [74]. The particle size and size distribution of the AuNPs were reported to be heavily connected with the ratio of HAuCl4 to sodium citrate [75,76], solution pH [77], and synthetic temperature [78]. Although the Turkevich method provides highly reproducible and straightforward for spherical particles’ production, the resultant citrate-capped AuNPs are easy to aggregate in a high-ionic-strength solution. Thus, it is required to modify the surface of citrate-capped AuNPs through the ligand-change reaction or stabilizers’ adsorption. Examples of colloidal stabilizers include DNA aptamer [79,80], proteins [81,82], neutral surfactants [83,84], adenosine triphosphate [85], cysteine [86], lipoic acid [87], p-nitroaniline dithiocarbamate [88]. Among the stabilizers and capping ligands mentioned above, the DNA aptamers-, adenosine triphosphate-, cysteine-, lipoic acid- and p-nitroaniline dithiocarbamate-modified AuNPs have been utilized for colorimetric assay of organophosphate pesticides that will be discussed in the following section. In addition to using citrate as a reducing and stabilizing agent, D’Souza et al. reported the preparation of ascorbic acid-capped AuNPs through the hydrothermal reduction of HAuCl4 with ascorbic acid at 100 °C for 10 min [89]. The SPR band of the ascorbic acid-capped AuNPs was centered at 525 nm with a particle size of 14.5 nm. Park et al. incubated heparin with HAuCl4 at 60 °C for 20 h, resulting in 20 nm-sized AuNPs with the SPR band of 524 nm [90]. Likewise, heparin behaved as a reducing and stabilized ligand for the synthesis of the AuNPs. Barman et al. synthesized highly stable AuNPs through trichloroacetic acid-triggered reduction of HAuCl4 in the presence of cetyl-trimethylammonium bromide (CTAB) [91]. The formed CTAB-capped AuNPs kept dispersed for more than six months. The ascorbic acid-, heparin-and CTAB-capped AuNPs mentioned above have been demonstrated to be powerful for sensing organophosphate pesticides. The sensing procedure will be discussed in the next section.

Fig. 2

Summary of the classical methods for the synthesis of AuNPs with different size, shape, and capping ligands.

3. Detection strategies for pesticides based on gold nanoparticles

3.1. Colorimetric assays

The AuNP-related colorimetric assay is a powerful method for the naked-eye detection of numerous analytes due to their large molar extinction coefficient [92]. The localized surface Plasmon resonance wavelength of AuNPs is highly susceptible to their aggregation degree and particle size. The presence of the target analyte induces the change in color and surface plasmon resonance wavelength of AuNPs as a consequence of either the conversion from dispersed to aggregated nanoparticles or the disassembly of aggregated nanoparticles, as shown in Fig. 3 [93]. Based on this sensing mechanism, AuNP-based colorimetric sensors have been well-developed for pesticide detection and showed their advantages of rapidity, simplicity, convenience, cost-effectiveness, and visualization by naked-eyes summarized in Table 1. Xu et al. [94] found that acetamiprid molecules were capable of directly attached on the surface of citrate-capped AuNPs, leading to the nanoparticle aggregation and color change. The visualized color change of citrate-capped AuNPs from red to blue was well-suited for determining the concentration of acetamiprid in an aqueous solution. This method was successfully applied to detect acetamiprid in vegetables. Aptamers are short single-stranded oligonucleotides (RNA or DNA) with a specific binding affinity towards their target analytes, and they developed through systematic evolution of ligands by exponential enrichment method [95-97]. Weerathunge et al. [98] demonstrated an aptamer-nanozyme for fast, highly selective, and sensitive detection of acetamiprid based on the inhibition of the peroxidase-like activity of AuNPs. Pesticides-specific aptamer modified on AuNPs also has been developed as apatsensors for colorimetric detection of phorate [99], omethoate [80], malathion [100] and acetamiprid [101] with high selectivity and sensitivity. Dong et al. [102] developed a dual strategy for the fluorescent and visual detection of cyanazine based on the quantum dots (QDs)-AuNPs sensing system. In the presence of cyanazine can induce the aggregation of AuNPs with a color change from red to blue. Fahimi-Kashani et al. [103] demonstrated a colorimetric sensor array for monitoring five organophosphate pesticides based on aggregation behaviors of citrate-AuNPs at different pH values and ionic strengths, as shown in Fig. 4. Rana et al. [104] explored ligand exchange reactions on citrate-AuNPs with a similar concept for colorimetric detection of acephate, phenthoate, profenofos, acetamiprid, chloronitrile, and cartap in water and vegetable samples. Baek et al. [105] designed a portable system for the detection of tebuconazole based on the aggregation of citrate-AuNPs. The colorimetric sensors based on the aggregation of citrate-AuNPs was also used for the detection of carbendazim [106] and chlorothalonil [107]. Wu et al. [108] showed a colorimetric method for parathion analysis based on the enzymatic hydrolysis reaction of acetylcholinesterase (AChE) and the dissolution of AuNPs in Au3+-cetyl-trimethylammonium bromide solutions. The activity of AChE is inhibited in the presence of parathion to induce the decreases in both the concentration and size of the AuNPs with noticeable color changes from red to light pink or red to colorless. By loading AuNPs on a cellulose paper, a dipstick could be used for the colorimetric detection of parathion by naked eyes with a limit of detection of 35 ppb. A combined analytical method by utilizing dispersive liquid-liquid microextraction and AuNPs as a colorimetric sensor based on in situ formation of AuNPs in carbon tetrachloride as an organic phase was developed for the detection of zineb [109] and ziram [110]. Modified small molecules on the surface of AuNPs can improve their selectivity and sensitivity. Thus, Kang et al. [111] have demonstrated pymetrozine-induced aggregation of melamine-AuNPs as a colorimetric sensor. The addition of pymetrozine caused the aggregation of melamine-AuNPs, indicating a color change from red to blue. Wang et al. [112] modified AuNPs with 2-mercapto-6-nitrobenzothiazole as colorimetric probes for the detection of deltamethrin. Tu et al. [113] employed cysteamine-modified AuNPs to probe glyphosate in aqueous solution. The presence of glyphosate induced the aggregation of AuNPs accompanied with the color change from red to blue. The glyphosate spiked spinach, apple, and corn leaves were visualized by the naked eye. Kailasa et al. [114] developed 6-aza-2-thiothymine functionalized AuNPs to probe pencycuron. The sensing mechanism involves the hydrogen bonding, π-π, and van der Waal interactions between pencycuron and 6-aza-2-thiothymine functionalized AuNPs, leading to a change in color from red to blue. The designed AuNPs-based colorimetric sensor has been applied for quantitative determination of pencycuron in rice, potato, cabbage, and water samples. According to the above discussions, label-free AuNPs are sensitive to change in ionic strength and pH of the buffer and other inorganic and organic analytes with high affinity to AuNPs. The labeling AuNPs provide more interactions between target analytes and probe labeled AuNPs with high specificity and sensitivity.

Fig. 3

Typical strategies of colorimetric detection mechanism with gold nanoparticles. Reproduced with permission from Ref. [93].

Table 1

AuNPs based colorimetric sensors for pesticides detection.

Pesticides	Probes	Linear ranges	LODs	Matrix	Ref.
Acetamiprid	Citrate-AuNPs	0.66–6.6 μM6.6–66 μM	0.044 μM	Green vegetables, Eggplant, Cucumber	[94]
Acetamiprid	Apt-AuNPs	0.1–10 μg/mL	1.8 μg/mL	–	[98]
Phorate	Apt-AuNPs	0.01 nM–1.3 μM	0.01 nM.	Apple	[99]
Omethoate	Apt-AuNPs	0.1–10 μM	0.1 μM	Soil	[80]
Malathion	Apt-AuNPs	0.01–0.75 nM	1.94 pM	Tap water, Lake water, Apple	[100]
Acetamiprid	Apt-AuNPs	10–160 μg/mL	1.02 μg/mL	Tomato, Wastewater	[101]
Cyanazine	QDs-AuNPs	2.0–9.0 μM	0.2201 μM	Tap water, River water, Cabbage	[102]
Azinphos-methyl	Citrate-AuNPs	80–400 ng/mL	75 ng/mL	Rice, Paddy water	[103]
Chlorpyrifos		12–80 ng/mL	118 ng/mL
Fenamiphos		80–400 ng/mL	75 ng/mL
Pirimiphos-methyl		40–800 ng/mL	30 ng/mL
Phosalone		40–320 ng/mL	37 ng/mL
Acephate	Citrate-AuNPs	10–900 μM	0.346 μM	Tap water,	[104]
Phenthoate		0.01–1.50 μM	3.0 nM	Canal water,
Profenofos		1.0–200 μM	0.6 μM	River water,
Acetamiprid		0.001–0.15 μM	0.624 nM	Cabbage,
Chloronitrile		1.0–1000 μM	0.375 μM	Tomato,
Cartap		0.05–1.50 μM	17 nM	Potato
Tebuconazole	Citrate-AuNPs	0–1.0 μg/mL	52.0 ng/mL	–	[105]
Carbendazim	Citrate-AuNPs	10–600 ng/mL	3.4 ng/mL	Cabbage, Apple	[106]
Chlorothalonil	Citrate-AuNPs	5–100 ng/mL	3.6 ng/mL	Cucumber	[107]
Parathion	AChE-AuNPs-Au3+-CTAB	15–65 ng/mL, 140–1000 ng/mL	0.7 ng/mL (2.4 nM)	Apple washing solution, Tap water, Sea water	[108]
Zineb	AuNPs	0.0008–0.020 μg/mL	0.00055 μg/mL	River water, Tap water, Well water, soil	[109]
Ziram	AuNPs	0.12–2.52 ng/mL	0.06 ng/mL	Well water, River water, Soil, Potato, Carrot, Wheat, Paddy soil	[110]
Pymetrozine	Melamine-AuNPs	10–1000 nM	10 nM	Tap water, Lake water, Green tea, Apple juice	[111]
Deltamethrin	2-mercapto-6-nitrobenzothiazole-AuNPs	0.005 – 1 μM	0.005 μM	Cherry, Mini tomato	[112]
Glyphosate	Cysteamine-AuNPs	0.001–1000 μg/mL	0.026 μg/mL	Spinach leaf, Corn leaf, Apple peel	[113]
Pencycuron	6-aza-2-thiothymine-AuNPs	2.5–100 μM	0.42 μM	Water, Rice, Potato, Cabbage	[114]

AChE: acetylcholinesterase; Apt: aptamer; CTAB: cetyltrimethylammonium bromide; QD: quantum dots.

Fig. 4

Diagrams of colorimetric sensor array and detection principle of Organophosphorus pesticides based on unmodified AuNPs. Reproduced with permission from Ref. [103].

3.2. Fluorescence assays

Fluorescence-based sensors are performed mainly based on the quenching (turn-off) and enhancement (turn-on) of fluorescence intensity, or the fluorescence resonance energy transfer (FRET) [115]. AuNPs can act as highly efficient fluorescence quenchers, owing to the high molar extinction coefficients and broad adsorption spectrum overlapping [116]. The interactions between fluorescent molecules and AuNPs cause the change of the fluorescence intensity. Thus, AuNPs can be utilized as excellent fluorescence quenchers for FRET- and IFE-based assays for the determination of pesticides, as shown in Table 2. Nebu et al. [117] demonstrated that fluorescein-capped AuNPs were sensitive for fluorescence turn-on detection of fenitrothion. As shown in Fig. 5, the fluorescence of fluorescein was quenched by AuNPs and recovered in the presence of fenitrothion. The approach was successfully demonstrated on a paper strip with the detection limit in the nanomolar range. Hung et al. [118] developed a rhodamine B (RB)-functionalized AuNPs as fluorescence “turn-on” probe for the determination of dimethoate. The sensing mechanism is based on the emission spectrum of RB significantly overlaps with the absorption spectrum of AuNPs. In the presence of dimethoate, the fluorescence was recovered owing to the competitive adsorption between RB molecules and dimethoate on the surface of AuNPs. The approach has been applied to detect dimethoate in water and fruit samples with satisfactory recoveries. Tseng et al. [119] used a similar strategy to detect thiodicarb in water and food samples. Su et al. [120] used carbendazim-specific aptamer as a sensing probe, AuNPs, and RB as the indicator to develop an aptasensor for the detection of carbendazim. The aptamer can specifically be combined with carbendazim to form a stable complex and desorbed from the surface of AuNPs, induced the aggregation of AuNPs by NaCl. Bahreyni et al. [121] developed a fluorometric aptasensor to detect acetamiprid based on the use of an aptamer against acetamiprid, different complementary strands, and AuNPs. Except for organic dyes, upconversion nanoparticles, quantum dots, and carbon dots have been proved to be efficient fluorescence donors in recent years. You et al. [122] constructed a competitive immunoassay for the detection of midacloprid by using AuNPs as an absorber for the fluorescence of upconversion nanoparticles through inner filter effect. Yang et al. [123] demonstrated a colorimetric and fluorometric method based on the principle of target-triggered structure switch of aptamers, salt-induced AuNPs aggregation, and signal amplification from upconversion nanoparticles for the detection of acetamiprid. Liu et al. [124] developed a fluorescence turn-on method base on the luminescence resonance energy transfer between upconversion nanoparticles, and AuNPs for the detection of cyano-containing pesticides (acetamiprid, fenpropathrin, and chlorothalonil). The approach also has been applied to detect acetamiprid in Lanceolata, Angelica dahurica and Astragalus. Dong et al. [102] developed a dual strategy for the fluorescent and visual detection of cyanazine based on the quantum dots (QDs)-AuNPs sensing system. The fluorescence of CdTe QDs was remarkably quenched by AuNPs via the inner filter effect (IFE). Upon addition of cyanazine can adsorb on the surface of AuNPs to induce the aggregation of AuNP, and the weakened IFE recovered the fluorescence intensity of CdTe QDs. Guo et al. [125] reported a fluorescent assay for acetamiprid based on the specific binding of aptamers and the inner filter effect of AuNPs on the fluorescence of CdTe quantum dots. On the other hand, carbon dots characterize excellent aqueous solubility, resistance to photobleaching, bright fluorescence, low toxicity and good biocompatibility and act as good alternative to organic dyes. Wang et al. [126] reported a fluorescent aptasensor based on the IFE between AuNPs and carbon dots for the detection of acetamiprid. Upon adding acetamiprid, the fluorescence intensity can be recovered and proportioned to the concentration of acetamiprid. Qin et al. [127] developed an aptasensor based on the different aggregation states of AuNPs to quench the fluorescent carbon dots to detect acetamiprid. The assay has been used to detect acetamiprid in rice field water samples. Korram et al. [128]. Constructed a FRET sensing probe using carbon quantum dots and AuNPs for the detection of paraoxon, malathion, methamidophos, and carbaryl. The fluorescence of carbon quantum dots was quenched in the presence of AuNPs and recovered by the addition of acetylthiocholine iodide and AChE. By evaluating the inhibition effect on the activity of acetylthiocholine and fluorescence intensity, the concentration of pesticides could be quantified. Chen et al. [129] demonstrated a sensing system consisted of aggregation-induced emission (AIE) NPs, AuNPs, and AChE for the detection of paraoxon. The sensing platform was proved to have a wider linear range from 0.8 to 60 ng/mL with a LOD at 0.38 ng/mL.

Table 2

AuNPs based fluorometric sensors for pesticide detection.

Pesticides	Probes	Linear ranges	LODs	Matrix	Ref.
Fenitrothion	Fluorescein-AuNPs	–	6.05 nM9.41 nM7.84 nM	Well water, Tap water, River water	[117]
Dimethoate	RB-AuNPs	0.005–1.0 μg/mL	0.004 μg/mL	Water, Fruit, Rice, Vegetable, Tea	[118]
Thiodicarb	RB-AuNPs	0.1–10.0 μg/mL	0.08 μg/mL	Water, Fruit, Rice	[119]
Carbendazim	RB-Apt-AuNPs	2.33–800 nM	2.33 nM	Water	[120]
Acetamiprid	Apt, Apt-AuNPs	5–50 nM	2.8 nM	Tap water	[121]
Imidaclothiz	UNCPs, AuNPs	2.1–171.2 ng/mL	2.1 ng/mL	Paddy water, Soil, Ear, Rice, Apple, Tomato, Pakchoi, Cabbage	[122]
Acetamiprid	UNCPs, Apt-AuNPs	0.025–1 μM	0.36 nM0.00008 μg/mL	Celery leaves, Green tea	[123]
Acetamiprid	UCNPs, Apt-AuNPs	0.10–100 ng/mL	0.015 ng/mL	Herbal medicine	[124]
Fenpropathrin		1.0–100 ng/mL	0.24 ng/mL
Chlorothalonil		0.10–50 ng/mL	0.011 ng/mL
Cyanazine	QDs, AuNPs	0.5–9 μM	0.1578 μM	–	[102]
Acetamiprid	QDs, Apt-AuNPs	0.05–1.0 μM	7.29 nM	Lettuce, Pakchoi, Cauliflower, Pamphrey	[125]
Acetamiprid	CDs, Apt-AuNPs	5–100 ng/mL	1.08 ng/mL	Vegetable	[126]
Acetamiprid	CDs, Apt-AuNPs	7.8 nM–1.4 μM	1.5 nM	Water	[127]
Paraoxon	CQDs, AuNPs	0.16–5 nM	50 pM	Tap	[128]
Malathion		10–500 nM	0.1 nM	Water, River water,
Methamidophos		10–500 nM	0.12 nM	Apple juice
Carbaryl		10–666 nM	0.13 nM
Paraoxon	PTDNP, AuNPs	0.8–60 ng/mL	0.38 ng/mL	Lake water, Cabbage	[129]

Apt: aptamer; CDs: carbon dots; CQDs: carbon quantum dots; PTDNP: aggregation-induced emission amphiphilic polymers nano-particles; RB: rhodamine B; QDs: quantum dots; UCNPs: upconversion nanoparticles.

Fig. 5

The schematic representation of the mechanism of quenching of fluorescein by AuNPs and turn-on response of AuNPs quenched fluorescein in the presence of fenitrothion. Reproduced with permission from Ref. [117].

3.3. Surface-enhanced Raman scattering assays

Surface-enhanced Raman scattering (SERS) is based on a significant enhancement of Raman signal, typically up to 6 orders of magnitude, from a rough metal surface owing to the electromagnetic and chemical enhancement [130]. Nanostructured surfaces of gold, silver, or copper are the most common SERS-active substrates. SERS provides the characteristic fingerprint of desirable analytes with high sensitivity at nanomolar to picomolar concentration, even at the single-molecule level [131-133]. Thus, SERS has become a powerful analytical method for detecting pesticides because of its high sensitivity, good selectivity, low cost, and rapidity [134,135].

Recently, Xu et al. reviewed the development of the SERS technique for pesticide detection in food with the advantages of high sensitivity, reproducibility, selectivity, and low-cost [136]. The liquid samples can be directly detected for pesticide residues by SERS (Fig. 6a); the pesticide residues on the surface of a solid or solid-liquid mixture can be detected by SERS (Fig. 6b); the pesticide residues inside solid sample also can be detected by SERS with an extraction process (Fig. 6c). Thus, the following are recent achievements in fabricating appropriate SERS-active substrates for pesticide detection. In recent year, several pesticides such as omethoate [137], chlorpyrifos [137], acetamiprid [138], clothianidin [138], phosmet [138], thiram [138], paraquat [139], carbendazim [140], pyrimethanil [141] was determined by SERS with easy-to-prepare AuNPs as shown in Table 3. Fernandes et al. [142] used dendrimer stabilized anisotropic AuNPs as SERS probes for the detection of sodium diethyldithiocarbamate, thiram, and paraquat in water. Tan et al. [143]. Demonstrated that osmium carbonyl-clusters binging on the AuNPs as a SERS probe to enhance CO stretching vibration signal and avoid the interference of biomolecule. The probe has been used for highly sensitive detection of glyphosate in spiked beer. Hong et al. [144] used a suction method to fabricate a SERS substrate via the immobilization of AuNPs on an ultrafiltration membrane. The thiabendazole standard solution and orange peel extract can be concentrated on the substrate and analyzed by portable Raman spectrometry. Luo et al. [145] performed AuNPs were in situ synthesized on pseudo-paper films and used as a SERS substrate for the analysis thiram and parathion methyl with 3.02 × 106 enhancement. Yaseen et al. [146] demonstrated a simultaneously detect multi-class pesticides (thiacloprid, profenofos and oxamyl) in peach with SERS on silver-coated AuNPs with 26 nm Au core size and 6 nm Ag shell thickness. Mesoporous silica-supported orderly-spaced AuNPs also used as the SERS substrate for the detection of 2,4-D, pymetrozine and thiamethoxam in food samples [147]. Kim et al. [148] fabricated a low-cost and flexible SERS substrate based on cellulose nanofiber/AuNPs nanocomposites via vacuum-assisted filtration. The SERS substrate can effectively detect thiram and tricyclazole with 4.5 × 109 enhancement and LODs were down to 1 pM.

Fig. 6

Pesticide residues detection in liquid (a), on the surface (b), or inside solid foods by SERS. Reproduced with permission from Ref. [136].

Table 3

AuNPs based sensors for pesticide detection by SERS.

Pesticides	Substrates	Linear ranges	LODs	Matrix	Ref.
Omethoate	AuNPs	51.2 – 263 g/L	1.63 mg/cm2,	Apple	[137]
Chlorpyrifos			2.64 mg/cm2
Acetamiprid, Clothianidin, Imidacloprid, Thiamethoxam, Carbophenothion, Chlorpyrifos, Coumaphos, Malathion, Phosalone, Phosmet, Profenofos, Diphenylamine, Fludioxonil, Thiabendazole, Thiram, Carbofuran, Methomyl, Permethrin, Transfluthrin, Trichlorfon, DEET	AuNPs	–	0.001 – 1 ppm	–	[138]
Paraquat	AuNPs	0.2 – 10 μg/L	–	Apple juice	[139]
Carbendazim	AuNPs	0 – 10 ppm	–	Oolong tea	[140]
pyrimethanil	AuNPs	0 – 40 mg/kg	4.74 ppm	Pome fruit	[141]
Diethyldithiocarbamate	Dendirmer-AuNPs	–	10 nM	–	[142]
Thiram
Paraquat
Glyphosate	10OsCO-Au NPs	0 – 0.1 ppm	0.1 ppb	Beer	[143]
Thiabendazole	immobilization of AuNPs on UF membrane	0.001 – 100 ppm	0.01 μg/mL (standard)	Orange extract	[144]
			0.125 ppm (orange extract)
Thiram	AuNPs on PPFs	10 – 100000 ng/cm2	1.1 ng/cm2	Apple peels	[145]
Parathion methyl
Thiacloprid, Profenofos, Oxamyl	Au@Ag NPs	–	0.01 mg/L (thiacloprid)	Peach extract	[146]
			0.001 mg/L (profenofos)
			0.001 mg/L (oxamyl)
2,4-D, Pymetrozine, Thiamethoxam	well-ordered AuNPs@MSF	0.01 – 100 ng/mL	0.79 pg/mL.	Tap water, Apple, Milk	[147]
		0.1 – 1000 ng/mL	1.04 pg/mL
		0.1 – 1000 ng/mL	1.21 pg/mL
Thiram, Tricyclazole	CNF/AuNP nanocomposites	–	1 pM (0.3 ppt)	Apple peel, Plant leaf	[148]
			10 pM (2.4 ppt)

10OsCO: organometallic osmium carbonyl clusters; CNF: cellulose nanofiber; DEET: N,N-diethyl-meta-toluamide; MSF: mesoporous silica film; PPFs: pseudo-paper films; UF: Ultrafiltration.

3.4. Electrochemical assays

Electrochemical sensors, because of their portability, rapidity, low-cost, high sensitivity and selectivity, have become one of the most effective analytical methods [149-151]. Electrochemical sensors can be basically divided into potentiometric, amperometric and conductometric methods according to the property of obtained response [152,153]. In recent years, AuNPs have been used for electrochemical sensors for the detection of pesticides as shown in Table 4. Several AuNPs based electrochemical biosensors have been described in the literature by using AChE enzyme [154,155]. Dong et al. [154] developed an AChE sensor for the detection of malathion and parathion methyl in cabbage and water samples based on a film of AuNPs/three-dimensional graphene. Hua et al. [154] demonstrated a disposable amperometric sensor for parathion ethyl determination based on a screen-printed carbon electrode consisted of AChE immobilized onto the surface of multiwalled carbon nanotubes, chitson and AuNPs. Jiang et al. [156] demonstrated to construct stable covalently attached multilayer films by using layer-by-layer self-assembly of diazo-resins, AuNPs, and AChE. The films were immobilized on the surface of a p-aminobenzesulfonic acid-modified glassy carbon electrode and used for quantitative detection of malathion and parathion methyl. Rodrigues et al. [157] performed the fabrication of layer-by-layer films composed of reduced graphene oxide and AuNPs for the detection of parathion methyl (Fig. 7). The differential pulse voltammetry was applied on the layer-by-layer films modified on indium oxide electrode, in the presence of AuNPs, a wider linear range was achieved between 0.5 and 60 ppm, with LOD of 0.770 ppm for parathion methyl. Singh et al. have synthesized the AuNPs/neutral red-BSA-functionalized graphene nanocomposite and immobilized on Nafion modified glassy carbon electrode [158]. The prepared electrode exhibits higher electrocatalytic ability towards methyl parathion with the enlarged current. A sensitive voltammetric sensor for methyl parathion and parathion was developed by Lu et al. [159] based on reduced graphene oxide and hollow AuNPs immobilized on a glassy carbon electrode. In addition, the molecularly imprinted electrochemical sensors have also used to detect simazine [160] and tebuconazole [161] with AuNPs modified electrodes. Recently, Liu et al. reviewed the development of aptasensor for pesticide detection [162]. Thus, aptasensors based on specific aptamers modified on the AuNPs have been used for the detection of diazinon [163], carbendazim [164] and malathion [165]. Electrochemical immunosensor based on AuNPs have been used for the detection of chlorpyrifos [166] and imidacloprid [167] with ultrahigh sensitivity.

Table 4

AuNPs based electrochemical sensors for pesticide detection.

Pesticides	Probes	Linear ranges	LODs	Matrix	Ref.
Malathion	AChE/Naflon/AuNPs/rGO/GCE	0.0001–1 ng/mL	0.0278 pg/mL (0.084 pM)	Tap water, Mineral water, Chinese cabbage	[154]
Methyl parathion			0.0217 pg/mL (0.0824 pM)
Paraoxon-ethyl	AChE/MWCNTs-CS/AuNPs/SPCE	0.01–10 μg/mL	0.03 μg/mL	Spinach	[155]
		10–100 μg/mL
Malathion	GCE/P-ABSA/DAR/AuNPs/DAR/AChE	0.003–30 pM	0.0016 pM	Tap water, Well water, Chinese cabbage	[156]
Methyl parathion		0.0038–38 pM	0.0022 pM
Methyl parathion	ITO/(GPDDA/GPSS)10	0.95–152 μM (0.25–40 ppm)	0.859 μM (0.226 ppm)	Tap water, Soil, Cabbage	[157]
	ITO/(GPDDA/GPSS)1(AuNP/GPSS)10	1.90–228 μM (0.5–60 ppm)	2.930 μM (0.770 ppm)
Methyl parathion	AuNPs/NR-BSA-graphene/Naflon/GCE	0.02–0.153 μM	6 nM	Soil, Water, Potato juice	[158]
		0.153–1.36 μM
Methyl parathion	HAuNPs/rGO/GCE	0.3–10 μM	0.12 μM		[159]
Parathion		0.11–50 μM	23 nM
Simazine	MIP/ATP@AuNPs/ATP/Au electrode	0.03–140 μM	0.012 μM	Tap water, River water, soil	[160]
Tebuconazole	MIP/Au-PB/SH-G/AuNPs/GCE	50 nM–40 mM	12.5 nM	Cucumber, Green vegetable, Strawberry	[161]
Diazinon	Apt/AuNPs/SPGE	0.0304–304 ng/mL	0.005 ng/mL	Rat plasma	[163]
Carbendazim	MCH/Apt/AuNPs/1-AP-CNHs/GCE	0.001–1.0 ng/mL	0.5 pg/mL	Lettuce, Orange juice	[164]
Malathion	Apt/MCH/CP/AuNPs/PDA/GCE	0.5–650 pg/mL	0.5 pg/mL	Cauliflower, Cabbage	[165]
Chlorpyrifos	FTO-AuNPs-chlAb	1 fM - 1 μM	10 fM	Apple, Pomegranate, Cabbage	[166]
Imidacloprid	AuNPs-SPCE	50–10000 pM	22 pM	Tap water, Watermelon, Tomato	[167]

AP-CNHs: 1-aminopyrene modified carbon nanohorns; AChE: acetylcholinesterase; Apt: aptamer; ATP: o-aminothiophenol; AuNPs/rGO: gold nanoparticles/three-dimensional graphene; BSA: bovine serum albumin; chlAb: anti-chlorpyrifos antibodies; CP: capture probe; CS: chitosan; DAR: diazo-resins; FTO: fluorine doped tin-oxide; GCE: glassy carbon electrode; HAuNPs: hollow gold nanoparticles; ITO: indium tin oxide; MCH: 6-mercapto-1-hexanol; MIP: molecularly imprinted polymer; MWCNT: multiwalled carbon nanotube; NR: neutral red; P-ABSA: p-aminobenzenesulfonic acid; PB: Prussian blue; PDA: polydopamine; SH-G: thiol graphene; SPCE: screen-printed carbon electrode.

Fig. 7

Procedures to modify ITO electrodes with (GPPDA/GPSS)10 and (GPDDA/GPSS)1(AuNP/GPSS)10 LbL films (assembly): immersion time was 15 min for both GPDDA and GPSS, and 6 min for AuNP. Reproduced with permission from Ref. [157].

4. Conclusions and future trends

This review has given a brief overview about recent advances of AuNPs-based colorimetric, fluorescence, SERS, and electrochemical sensors for the determination of pesticides in environmental samples. In contrast to conventional chromatographic and mass spectrometry-based methods, AuNPs have been shown to be a very useful, alternative tool for pesticide sensing owing to their unique optical property, facile synthesis, easy surface functionalization, and satisfactory biocompatibility. The selectivity of the AuNP-based probe toward a specific pesticide has been effectively improved by modifying the nanoparticle surface with aptamer and integrating the enzyme-substrate system. However, these reported AuNP-based probes still suffer from some limitations, including the adsorption of non-target molecules on the nanoparticle surface, uncontrolled nanoparticle aggregation in a high-ionic-strength solution, and poor selectivity in complex matrices. Although the modification of AuNPs with neutral surfactants, such as Tween 20 and fluorosurfactant, enhances their colloidal stability in a high-ionic-strength solution, these modifiers provide an adverse effect on selectivity toward a specific pesticide. Additionally, the above-discussed detection methods (i.e., colorimetric, fluorescence, SERS, and electrochemical spectroscopy) are unable to simultaneously sense multiple pesticides with similar structural properties. Therefore, enrichment or separation of the target pesticides would be inevitable prior to using the AuNP-based probe. It is suggested that the integration of the AuNP-based probe with a recognition element-modified platform could be a promising way to allow the detection of target analytes despite of other possible interfering compounds. Examples of recognition elements include aptamer, antibodies, and molecularly imprinted polymers. Also, the integration of the novel technologies such as handheld devices, microfluidic or paper chips and portable test strips with sensing system may promise a bright future for on-site application. The recognition event can be delivered immediately to the servers through wire-less networking and miniaturized device such as smart phone. The applications of AuNPs-based sensors may lead to a new generation in real-time and onsite monitoring pesticides.