Development of a Selective Spectrophotometric Method for Deltamethrin Using Silver Nanoparticles

Abstract

The present work proposes a spectrophotometric method for deltamethrin determination using silver nanoparticles (AgNPs). The AgNPs are spherical with a diameter of~11 nm and a negative surface charge with zeta potential ranging from −4.1 mV (pH 2) to −48 mV (pH 10). The AgNP colloidal system showed greater stability at higher pH values and for a molar ratio of 6 between the sodium borohydride and silver nitrate in the synthesis. This is because the borate ions from the oxidation of borohydride are present on the surface of the nanoparticles, promoting an electrostatic repulsion between them which keeps them dispersed. The method was validated, obtaining satisfactory results of veracity and precision, and the limits of detection and quantification were 0.17 and 0.51 mg L⁻¹, respectively. The method was selective for deltamethrin compared to the compounds cypermethrin, endosulfan, thiamethoxam, atrazine, chlorpyrifos and parathion. Deltamethrin promotes the formation of dendritic silver nanostructures, changing the color of the system. The results demonstrate the development of a reliable and selective method for the detection of deltamethrin using AgNPs.

1. Introduction

The use of pesticides to restrain the proliferation of weeds, pests and diseases in agriculture has increased considerably in recent years. However, due to their excessive and irregular use, residual pesticides have become an environmental problem. Due to the mobility and persistence of pesticides in the environment, they can be detected in different places from those where they were initially applied, such as agroecological soils [1], rainwater [2], rivers [3] and subtropical reservoirs [4]. In addition, they can accumulate in fish tissues [3,5,6], leading to contamination in humans and animals, if ingested [3,7].

Synthetic pyrethroids are a class of insecticides that are highly effective and have low toxicity in mammals and birds [8,9]; however, they have high toxicity for ectothermic vertebrates, especially for fish [9]. These characteristics, along with the fact that organochlorines and organophosphates are banned, has caused the use of pyrethroids to increase significantly [8,9,10]. Deltamethrin, one of Type II broad-spectrum synthetic pyrethroids, is used as an insecticide and an acaricide for pest control in agriculture, in urban sites, in nurseries, and in homes for the control of human disease vectors [8,9,10]. However, the excessive and widespread use of deltamethrin can pose a threat to the affected biota. Deltamethrin has residual activity in some insects and surfaces, causing it to persist in the environment [10]. In addition, several studies with untargeted organisms have documented that exposure to deltamethrin causes toxic effects, including neurotoxicity [11], the cardiotoxic effect [9], genotoxicity [12] and reproductive toxicity [13]. Therefore, the detection of deltamethrin residues is necessary.

The determination of deltamethrin in the environment can be performed by different analytical methods, including fluorescence spectroscopy [14] and UV–visible and near-infrared spectroscopy [15]. In addition, well-established techniques can be used, such as gas chromatography [16,17] and high-performance liquid chromatography [18,19]. However, these methods make use of sophisticated equipment, which, in general, are expensive and require trained technicians to operate them. In this context, colorimetric sensors employing silver nanoparticles appear as a promising tool and can be used in the detection of different compounds [20]. These detection methods involving colorimetric sensors are simple, rapid and have a lower cost than the usual analytical methods, in addition to presenting the possibility of in situ analysis [21].

Colloidal suspensions of noble metal nanoparticles, such as gold and silver, have attracted significant global research interest due to their resistance to corrosion and oxidation as well as their notable optical properties [22,23,24,25,26,27,28]. These properties include surface plasmon resonance (SPR), which is responsible for the plasmonic resonance band in the UV–Vis spectrum and results from the behavior of conduction electrons. This phenomenon imparts sensor capabilities to the colloidal suspensions of these nanoparticles [23,25]. However, due to the higher cost associated with synthesizing gold nanoparticles, silver nanoparticles (AgNPs), also known as silver nanosensors, are becoming increasingly prominent [23,27,28,29,30]. AgNPs offer a range of advantageous properties, including antibacterial [28,31,32,33], antiviral [31,32] and antifungal [31,32] activity, good conductivity [28], catalytic effects [27] amd sensor properties [22,23], among others [21]. The aggregation state of the particles is altered when in contact with some substances, causing color change of the system [34].

AgNPs have been used to detect different compounds, such as pesticides [20,35,36], mercury [37], hydrogen peroxide [38] and fluoroquinolones [22], with promising results. However, there are no reports in the literature about deltamethrin determination by AgNPs. Given the above, the aim of this work was to develop a spectrophotometric method for deltamethrin determination by AgNPs. In addition, parameters that interfere with the stability of AgNPs were evaluated.

2. Materials and Methods

2.1. Reagents and Solutions

The chemical reagents used were deltamethrin (Sigma-Aldrich, San Luis, MO, USA, 99%), silver nitrate (AgNO₃) (Neon, São Paulo, Brazil, 99.8%), sodium borohydride (NaBH₄) (Neon, São Paulo, Brazil, 98.0%), acetonitrile (ACN) (Sigma-Aldrich, San Luis, MO, USA, 99.8%), methanol (Alphatec, São José dos Pinhais, Brazil, 99.8), ethanol (LS Chemicals, Mumbai, India, 99.7%), propanol (Nuclear, São Paulo, Brazil, 99.9%) and acetone (Neon, São Paulo, Brazil, 99.6%).

A stock solution of deltamethrin in ACN was prepared (500 mg L⁻¹). From the stock solution, working solutions were prepared using water type 1, obtained by a Milli-Q system (Millipore Corporation, Bedford, MA, USA). All solutions were stored under refrigeration at 4 °C.

2.2. AgNP Synthesis

The initial synthesis of the AgNPs was based on the methodology proposed by Moraes et al. [22]: succinctly, 75.00 mL of NaBH₄ solution (2.0 × 103 µmol L⁻¹) was immersed in an ice bath for approximately 15 min (T ≅ 2 °C), followed by the addition of 25.00 mL of AgNO₃ solution (1.0 × 10³ µmol L⁻¹), with a flow rate of 0.15 mL s⁻¹ under constant agitation.

To optimize the colloidal system of the AgNPs, the influence of the molar ratio between sodium borohydride and silver nitrate ${\mathrm{n}}_{{\mathrm{NaBH}}_4}/{\mathrm{n}}_{{\mathrm{AgNO}}_3})$ used in the synthesis of the AgNPs was evaluated. In general, 25.00 mL of AgNO₃ (1.0 mmol L⁻¹) was added with a flow of 0.15 mL s⁻¹ in 75.00 mL of NaBH₄ solution with initial concentrations of 1.5, 1.8, 2.0, 2.2 and 2.5 mmol L⁻¹. These were immersed in an ice bath (T ≅ 2 °C) for 15 min followed by the addition of AgNO₃. Therefore, the molar ratios ${\mathrm{n}}_{{\mathrm{NaBH}}_4}/{\mathrm{n}}_{{\mathrm{AgNO}}_3}$ were 4.5, 5.4, 6.0, 6.6 and 7.5, respectively.

Another important factor to be studied in the stabilization of AgNPs is the presence of a co-solvent with less polarity than water which will be added to the system due to the low solubility of deltamethrin. For this purpose, the protic solvents (methanol, ethanol and propanol) and aprotic solvents (acetonitrile and acetone) were added separately to the suspension of the AgNPs (250 µmol L⁻¹) in the proportion 1:4 v/v (co-solvents:suspension). In all systems, after the addition of the co-solvent the system was quickly homogenized and analyzed using an absorption spectrophotometer in the UV–Vis region, with the SPR band monitored at 400 nm.

The kinetic behavior of the AgNP suspension aggregation was assessed in the presence of acetonitrile (ACN), both with and without deltamethrin. The ACN was added to the AgNP suspension (250 µmol L⁻¹) at a volume ratio of 1:2.7 (ACN). The final concentrations of deltamethrin and AgNPs were 2.5 mg L⁻¹ and 93 µmol L⁻¹, respectively. For this study, the SPR band was monitored for 20 min at 400 nm and the half-height band displacement.

The suspensions of the obtained AgNPs were analyzed using an absorption spectrophotometer in the UV–Vis region (Thermo Scientific, Inc., Waltham, MA, USA, Evolution Array model), using a quartz cuvette with a 1 cm optical path. The absorption spectra of the suspensions were obtained in the scanning mode (200 to 1100 nm), and the monitoring of the surface plasmon resonance band (SPR) was carried out at a wavelength of 400 nm [22].

2.3. Characterization of AgNPs

The morphological analysis of the AgNPs was performed using a Transmission Electron Microscope (TEM) (Tecnai G2–12—SpiritBiotwin FEI model—120 kV). The AgNP diameter measurements were performed by the ImageJ program.

The AgNP size was also determined by Dynamic Light Scattering (DLS), performed on the Brookhaven Co. instrument (Holtsville, NY, USA), system BI-200SM, HeNe laser with a wavelength of 632.8 nm and power of 75 mW), with a scattering angle set at 20°. AgNP suspensions were filtered (PES or Nylon membrane filters, 0.22 µm porosity) five times before use.

The Zeta Potential (ZP) of the AgNPs was performed using Zetasizer NanoZS, (Malvern Instruments, England, UK, model Zen 3600), in which the measurements were performed in suspensions at different pH values (2 to 10). The pH adjustment was performed with solutions of sulfuric acid (0.1 mol L⁻¹) and sodium hydroxide (0.1 mol L⁻¹).

2.4. Method Validation of Deltamethrin Determination by AgNPs

The method of validation of deltamethrin determination by AgNPs was carried out according to the parameters required by INMETRO [39,40]. The limit of detection (LoD), limit of quantification (LoQ), veracity (n = 3), repeatability (n = 3), intermediate precision (n = 9) and selectivity parameters were determined. Veracity and repeatability were assessed at two levels of deltamethrin concentration (2.50 and 7.50 mg L⁻¹), which were determined by the recovery method, with three replicates at each level. The intermediate precision was evaluated on three different days, under the same working conditions. The linear working range of the analytical curve was between 1.25 and 12.50 mg L⁻¹.

System absorbance measurements were performed using a spectrophotometer in the UV–Vis region, monitoring the 400 nm wavelength. The AgNP suspension (at a final concentration of 93 µmol L⁻¹) was mixed with the deltamethrin solution, homogenized and then analyzed by UV–Vis spectrophotometry after 15 s. The SPR band was monitored at a 400 nm wavelength.

To assess the selectivity of the AgNPs, the compounds deltamethrin, cypermethrin, endosulfan, thiamethoxam, atrazine, chlorpyrifos and parathion were added to the suspensions of AgNPs to obtain final concentrations of 2.5 mg L⁻¹ and 93 µmol L⁻¹ mmol L⁻¹, for pesticides and suspensions of AgNPs, respectively.

2.5. Application of the Method in Authentic Samples

The method developed for deltamethrin determination was applied in water samples obtained from a Water Treatment Plant (WTP). The water sample displayed a pH = 6.7 (Lab1000 model mPA-210) and a conductance = 106 µS cm⁻¹ (Digimed model, São Paulo, Brazil, DM-32 and conductivity cell Digimed, São Paulo, Brazil, DMC-010M). The ultrapure water (Type 1) used in the method validation displayed a pH = 7.6 and a conductance = 0.9 µS cm⁻¹. The analytical curves were constructed by matrix-matched calibration wherein the water (WTP) was fortified with deltamethrin solutions in the range of 1.25 to 12.5 mg L⁻¹. The other parameters were determined as previously described.

3. Results and Discussion

3.1. Optimization of the Synthesis of AgNPs

The AgNP suspension displayed an intense yellow color (Figure S1), with a single band in the UV–Vis region attributed to the excitation of surface plasmons of the AgNPs [41], with a maximum wavelength between 395–400 nm. The images of the AgNPs obtained by TEM are shown in Figure 1. It can be observed that the AgNPs have a spherical shape (Figure 1A), with a size of 11.4 ± 3.4 nm, and form clusters (Figure 1B).

Similar results were observed by Melo et al. [25], who obtained AgNPs with a diameter of 20 ± 5 nm and plasmonic resonance bands with a maximum wavelength of 396 ± 5 nm. Haider and Mehdi [42], synthesized AgNPs with a diameter of 26.0 ± 1.2 nm and a maximum wavelength of absorption of 391 ± 2 nm.

Measurements of the zeta potential of AgNPs at different pH values are shown in Figure S2. It is possible to observe that all potentials were negative and showed an increase in absolute value from −4.1 ± 0.5 mV to −48 ± 3 mV with increasing pH from 2 to 10, respectively. Those results indicate that AgNPs are more stable in high pH systems because, for a suspension to be considered stable, it is necessary that its absolute ZP value be greater than 30 mV [43]. The increased stability of AgNPs at higher pH is due to a greater electrostatic repulsion between the nanoparticles since the increase in pH promotes an increase in anion density in the AgNPs’ electrical double-layer mainly due to borate B(OH)⁻₄. According to Vanysek [44], the B(OH)⁻₄ undergoes an oxidoreduction process, forming the borate ion and promoting Ag⁺ and water reduction (Equations (1)–(3)) [45], which is favored as pH increases. These results are similar to those obtained in other works [28,42,43,44,45].

B(OH)⁻_{4 (aq)} + 4H₂O_(l) + 8e⁻ ⇌ BH⁻_4(aq) +8OH⁻ _(aq)       E⁰ = −1.24

(1)

Ag⁺ _(aq) + 1e⁻ ⇌ Ag_(s)   E⁰ = +0.80 V

(2)

2 H₂O _(l) + 2e⁻ ⇌ H_{2 (g)+} + 2OH⁻ _(aq)     E⁰ = −0.83 V

(3)

In addition to the pH, the concentration of borate ions present in the system is also a factor that is fundamental for the stabilization of AgNPs. According to Song et al. [28], the stability of AgNPs is influenced by the ratio of NaBH₄ and AgNO₃ concentrations. Thus, for different molar ratios of these substances, the AgNP stability time was monitored as can be seen in

Table 1. Stability time of AgNPs after synthesis. Experimental conditions: [AgNO₃]_initial: 1.0 mmol L⁻¹; [NaBH₄]_initial: 1.5, 1.8, 2.0, 2.2 and 2.5 mmol L⁻¹; volume of NaBH₄ = 75.00 mL, volume of AgNO₃ = 25.00 mL.

Molar Ratios ${\mathbf{n}}_{{\mathbf{NaBH}}_4}/{\mathbf{n}}_{{\mathbf{AgNO}}_3}$	Stability Time of AgNPs after Synthesis/min
4.5	~5 min
5.4	~20 min
6.0	Stable
6.6	~40 min
7.5	~40 min

. The suspension images obtained, as well as the UV–Vis molecular absorption spectra, can be seen in Figure S3.

It can be seen in Table 1 that only AgNPs with a molar ratio ${\mathrm{n}}_{{\mathrm{NaBH}}_4}/{\mathrm{n}}_{{\mathrm{AgNO}}_3}$= 6 remained stable. This result is also evidenced by the yellow color of the suspension and the UV–Vis spectrum (Figure S3), in which the SPR band was observed with no apparent displacements. The other suspensions with different molar ratios were unstable, displaying different colors as well as a decrease in the absorbance intensity of the SPR band. In addition, a bathochromic shift for a molar ratio = 5.4 and a slight hypsochromic shift for molar ratios of 4.5, 6.6 and 7.5 were also observed, which also showed an elevation of the baseline associated with light scattering. All these changes observed in the UV–Vis spectra for AgNPs occur due to the agglomeration of the nanoparticles.

Since borate ions contribute to the stabilization of the AgNPs, the greater the amount of NaBH₄ added to the system, the greater the stability of the AGNPs, which is evidenced when using a molar ratio of ${\mathrm{n}}_{{\mathrm{NaBH}}_4}/{\mathrm{n}}_{{\mathrm{AgNO}}_3}$= 6, as the system was stable for up to six months. However, for molar ratios of ${\mathrm{n}}_{{\mathrm{NaBH}}_4}/{\mathrm{n}}_{{\mathrm{AgNO}}_3}$ 6.6 and 7.5, the system remained stable only for 40 min. This instability may be due to the increased ionic strength of the system, which promotes a decrease in the thickness of the electrical double-layer which causes a decrease in the electrostatic repulsion between AgNPs, facilitating its aggregation [28].

These results show that the amounts of NaBH₄ and AgNO₃ are a major factor in the stability of AgNPs, wherein the ideal molar ratio under these synthesis conditions was found to be equal to 6. In other works, changes in the UV–Vis spectrum of AgNPs were also observed for different molar ratios of ${\mathrm{n}}_{{\mathrm{NaBH}}_4}/{\mathrm{n}}_{{\mathrm{AgNO}}_3}$ [46,47]. Other stabilizers were evaluated, such as sodium dodecyl sulfate, glutathione, sodium citrate, ethylenediaminetetraacetic acid (EDTA), and polyvinylpyrrolidone, but they interfered with the detection of deltamethrin.

Once the AgNP synthesis parameters were established, the behavior of the colloidal system was evaluated in the presence of certain solvents, as can be seen in Figure S4. In the presence of methanol or propanol (Figure S4A), it can be observed a slight bathochromic shift of the SPR band, accompanied by a decrease in absorbance at 400 nm. For ethanol, this effect was more pronounced. In the presence of acetone (Figure S4B), a decrease in the SPR band intensity was observed as well as a bathochromic shift of this band. For ACN (Figure S4B), there was only a small decrease in the SPR band intensity. Thus, ACN was selected because it has less aggregation effect on silver nanoparticles and because it is a solvent commonly used in the preparation of solutions for analysis due to its lower toxicity compared to other solvents, such as methanol, for example.

The SPR band was monitored over time for the AgNP suspension only in the presence of ACN (AgNPs–ACN) (Figure S5A). A small bathochromic shift in the SPR band of AgNPs–ACN and a reduction in absorbance at 400 nm can be observed (Figure S5A), which suggests that AgNPs aggregate over time. These changes in the SPR band are more evident when observing the absorbance decrease and the half-height shift of the isosbestic point over time (Figure S5B). The half-height displacement is determined by the wavelength whose absorbance value intersects half of the isosbestic point absorbance (insert of Figure S5A) [22].

DLS measurements were also performed to evaluate the agglomeration of AgNPs–ACN for 20 h. In all correlograms, two different decays were observed, indicating two distinct populations of nanoparticles in the suspension, with hydrodynamic diameters of ~0.2 nm and ~45 nm initially, and increasing to ~0.3 nm and 80 nm after 20 h, showing the agglomeration of AgNPs (Figure S6A,B).

3.2. Deltamethrin Interaction with AgNPs

The deltamethrin solution was added to the suspension of AgNPs and the monitoring of the SPR band over time is shown in Figure 2.

It was observed that, upon adding deltamethrin to the AgNPs suspension, there was a time-dependent bathochromic shift in the SPR band, along with a decrease in absorbance at 400 nm (Figure 2A). The bathochromic shift is better evidenced in the insert of Figure 2A, where the half-height displacement of the peaks is shown. This displacement has a more prominent behavior than that observed for the AgNP suspension in the presence of ACN without deltamethrin (Figure S5). These changes are more evident when observing Figure 2B, in which the absorbance reduction (400 nm) and half-height displacement of SPR are shown as a function of time. These results show that there is a tendency for AgNPs to agglomerate in the presence of deltamethrin, which allows this pesticide to be determined by AgNPs.

DLS measurements were performed to evaluate the agglomeration kinetics of AgNPs–ACN in the presence of deltamethrin, and the results are shown in Figure 3A. It can be observed that the normalized intensity correlation function has two characteristic decays, indicating two size distributions in the suspension. Figure 3B shows the time evolution of the mean hydrodynamic diameter of the two populations. The size of the smaller particles remains constant at ~0.30 nm over the whole time. However, the larger particles increase from 31 nm to ~35 nm after 22.5 h. The system reaches equilibrium after five hours, but the measurements still exhibit variation. These results show that there is an increase in AgNP agglomeration in the presence of deltamethrin, as observed by the changing absorption spectra.

TEM analyzes were also performed for AgNPs originally dispersed in ACN without deltamethrin (Figure 4A,B). The images are shown in Figure 4. It can be seen that AgNPs have a spherical shape with a size of 15.3 ± 6.8 nm and form more aggregates than those observed in Figure 1. However, in the presence of deltamethrin (Figure 4C,D), the AgNPs form aggregates with dendritic shapes, evidenced by more electrodense points in the image. This observed aspect of agglomeration possibly comes from diffusion-limited aggregation, which consists of a model used to describe dendritic growths.

According to Gentile et al. [47], the diffusion-limited aggregation phenomenon is based on the idea that, in very fast chemical reactions, the dynamics of aggregation are determined by diffusion. For this reason, the displacement and trajectory direction of the metal ions in the solution are disordered as per Brownian motion. This disordered movement of the particles intensifies the agglomeration, causing the formation of dendrites when the particles dry. Thus, there are two elementary characteristics of this aggregation model: (1) free suspended particles perform random movements, and (2) the aggregation of the particles occurring by simple contact [30,46,48]. From this perspective, when AgNPs come into contact with deltamethrin, the kinetics of the reaction will occur quickly due to the disordered movement of the particles, which favors the aggregation of the particles by limited diffusion, leading to the formation of dendrites.

3.3. Method Validation of Deltamethrin Determination Using Silver Nanoparticles (AgNPs)

The method of deltamethrin determination by AgNPs has been validated and the results are shown in

Table 2. Veracity, repeatability and intermediate precision for deltamethrin determination method by AgNPs.

Concentration/ (mg L−1)	Repeatability		Intermediate Precision
	R *%	CV **%	R *%	CV **%
2.50	83.1	16.7	102.7	16.6
7.50	114.9	2.8	110.7	9.6

* R = Recovery. ** CV = Coefficient of variation.

. The linear working range was 2.5 to 25 µmol L⁻¹, presenting analytical curves with coefficient of determination values close to the unit (Table S1), and indicating the good quality of the adjustments. The LoD and LoQ were equal to 0.17 and 0.51 mg L⁻¹, respectively. The method showed good veracity since the recoveries were within the acceptable limit established by INMETRO, 80–20% [39]. The method showed good repeatability, with values below 20%. The intermediate precision was evaluated at the same levels, showing satisfactory results: recovery between 80–120%, and coefficient of variation less than 20%.

The method’s selectivity was evaluated by applying AgNPs to pesticides in common use: cypermethrin, atrazine, endosulfan, chlorpyrifos, thiamethoxam and parathion. The results are shown in Figure 5, in which it is possible to observe that the suspension of AgNPs changes its color only in the presence of deltamethrin, indicating good selectivity of the method. It is suggested that future studies include theoretical calculations to elucidate the aggregation mechanisms specific to different chemical compounds in relation to AgNPs.

3.4. Application in Real Samples

Matrix-matched calibration was constructed to minimize the effect caused by the presence of ions in the water of the WTP. The quality of the adjustments was confirmed by the determination of coefficient values (R²) greater than 0.9 (Table S1) and by the residuals of the adjustment, which are randomly distributed around zero, showing no tendency.

The comparison of curve sensitivity using Type 1 water (0.010 ± 0.002) L mg⁻¹ and WTP water (0.012 ± 0.003) L mg⁻¹ was performed to observe the presence or absence of matrix effects. Thus, the F-test was performed to assess the mean of variances and the Student’s t-test was performed to assess the mean of the slopes (sensitivity). Based on the results presented in Table S2, it was possible to conclude that, for the F-test, the tabulated value is greater than the calculated value, indicating that the variances do not differ significantly. From the result of the F-test, the Student’s t-test was carried out, and it is possible to conclude that the tabulated value is also greater than the calculated value. Thus, the averages of the slopes do not differ significantly. Based on these results, it is concluded that there was no matrix effect.

LoD and LoQ were equal to 0.35 and 1.07 mg L⁻¹, respectively. The method showed good veracity, with recoveries within the range of 80–120% (

Table 3. Veracity, repeatability and intermediate precision for deltamethrin determination method by AgNPs in authentic samples.

Concentration/ (mg L−1)	Repeatability		Intermediate Precision
	R *%	CV **%	R *%	CV **%
2.50	121.2	8.7	108.7	12.6
7.50	107.4	3.3	111.9	3.8

* R = Recovery; ** CV = Coefficient of variation.

), except for the first level (121.2%). The repeatability was satisfactory, with a coefficient of variation less than 20%. The intermediate precision was also considered satisfactory, with recovery values between 80–120% and a coefficient of variation less than 20%. The parameters of merit, veracity, repeatability, intermediate precision, LoD and LoQ were determined, as pre-established by INMETRO [39].

The method proposed in this work was compared with other methods developed to detect deltamethrin and other pyrethroid pesticides, as shown in

Table 4. Comparison between LoD and LoQ parameters of conventional analytical methods for detecting deltamethrin and the proposed analytical method.

Analytical Method	Matrix	LoD and LoQ	Reference
Proposed analytical method	Water	LD—0.17 mg L−1 LQ—0.51 mg L−1	Present work
Photochemically-induced fluorescence	Natural water	LoD = 2.9 µg L−1 LoQ = 8.9 µg L−1	[14]
Micro-liquid-liquid extraction and GC-FID *	Water	LD—0.2 µg L−1 LQ—0.8 µg L−1	[16]
Molecularly imprinted solid phase extraction and GC-ECD **	Sea water	LoD—28.0 ng L−1 LoQ—93.5 ng L−1	[17]
Solid-phase extraction and HPLC-MS ***	Sea water	LD—0.3 µg L−1 LQ—0.8 µg L−1	[18]
Solid-phase extraction and HPLC-UV	Water	LoD—4.3 ng L−1	[19]

* Gas Chromatography (GC) with Flame Ionization Detector (FID); ** Electron-Capture Detector (ECD); *** High Performance Liquid Chromatography (HPLC) coupled Mass Spectroscopy (MS).

. The method presented has the great advantage of the possibility of determining deltamethrin in situ, which gives it more practicality and less cost when compared to other methods that use more sophisticated equipment.

4. Conclusions

In this work, a simple, cheap and reliable method of determination of deltamethrin by silver nanoparticles was developed and validated. The molar ratio of ${\mathrm{n}}_{{\mathrm{NaBH}}_4}/{\mathrm{n}}_{{\mathrm{AgNO}}_3}$ used in the synthesis of AgNPs was a primordial factor in obtaining a more stable colloidal system, which, in this work, was equal to 6. The pH of the system was also shown to be a parameter that influences the stability of AgNPs, which can be improved by increasing the pH of the system. Regarding the selectivity of the system, the AgNPs were selective for deltamethrin compared to the compounds cypermethrin, endosulfan, thiamethoxam, atrazine, chlorpyrifos and parathion. In view of the results obtained, it was possible to verify that the analytical method for detecting deltamethrin was validated with satisfactory parameters, which allows deltamethrin to be detected in authentic water samples.