Pesticide-induced photoluminescence quenching of ultra-small Eu³⁺-activated phosphate and vanadate nanoparticles

1. Introduction

Lanthanide (Ln³⁺ = Eu³⁺, Sm³⁺, Dy³⁺) doped nanomaterials have become irreplaceable over the years in many fields such as displays, solar cells, medicine, lasers, security markers and optical fibers. The constant development of nanotechnology and new Ln³⁺-doped materials resulted in new ways for their utilization, for instance, temperature sensing via luminescence or chemical sensing for detection of a variety of different analytes such as heavy metal ions, small molecules, DNA, H₂O₂, O₂, glucose, or pesticides [[1], [2], [3], [4], [5], [6], [7], [8], [9]].

Pesticides are used in the elimination and control of animal or plant diseases and pests, such as fungi weeds and others that can cause crop damage. Pesticides can be classified in more than 15 different categories based on their chemical structure, among which organophosphates, organochlorides, nitrophenol derivatives and arsenic compounds are considered to be the most dangerous. They are potentially toxic to humans, with acute and chronic health effects such as nausea, vomiting, internal hemorrhage, damage to the central nervous systems, and etc [10,11]. The United Nations Population Division estimated that there will be 9.7 billion people on Earth by the year 2050, and it is evident that expansion of food production will be necessary. It can be expected in the future that pesticides will be used continuously and globally to raise the bar when it comes to food production [12,13]. Their worldwide use and toxicity are the reasons that pesticides are in the group of environmental pollutants, and therefore their application must be strictly regulated and controlled.

Conventional detection techniques such as the ultra-performance liquid chromatography (UPLC), liquid chromatography (HPLC), gas chromatography (GC), gas chromatography coupled with mass spectrometry (GC/MS) and enzyme-linked immunosorbent assay (ELISA) have very low detection limits and the high reproducibility. However, they are expensive and involve a multi-step sample preparation, the use of large volumes of solvents, and work of well-trained operators [[14], [15], [16], [17], [18]]. To address the aforementioned drawbacks, optical methods like luminescence, Raman scattering and colorimetry are considered promising. Among them, luminescence techniques are non-destructive, quick and low cost, sensitive even in single molecule detection, and require simplified sample preparation procedures. The detection and quantification of analytes using luminescence can be based on various reading schemes: energy transfer, excited state lifetime, emission anisotropy or polarization, fluorescence, or phosphorescence emission intensity. The simplest method is to measure a change in the probe luminescence intensity that can be either quenched or enhanced in response to different concentrations of analytes, while the excited state lifetime measurements could be used to evaluate the luminescence quenching mechanism [19,20].

Traditionally, luminescent organic dyes were used as luminescent sensing probes. Nowadays, luminescent nanoparticles (NPs), such as carbon dots (CDs), semiconductor quantum dots (QDs), and nanophosphors are investigated for chemical sensing via luminescence. Regarding the pesticide detection, a number of reports on the use of QDs, doped QDs, graphene QDs (GQDs) and other functional nanomaterials have been published recently [[21], [22], [23], [24], [25], [26], [27], [28], [29], [30]].

As a result of the optical properties coming from f-f transitions of Ln³⁺ ions (sharp emission, well defined bands, large Stokes shift), temperature and chemical stability, lanthanide activated nanophosphors are emerging as promising candidates for the luminescence sensing. To date, emissions of Ln³⁺ nanophosphors have been used for the detection of heavy metal ions, H₂O₂, etc., however, the luminescence response in the presence of pesticides has not been extensively investigated [6,[31], [32], [33], [34], [35], [36]]. Here, the aim was to demonstrate that extremely large surface-to-volume ratio of phosphor ultra-small NPs in the presence of pesticide facilitates the quenching of lanthanides’ emissions [6].

In this work, pesticide-induced photoluminescence quenching of two types of Eu³⁺-activated colloidal materials: (1) phosphate-based (EuPO₄ and Eu³⁺-activated REPO₄ (RE³⁺ = La³⁺, Dy³⁺, Er³⁺)) and (2) vanadate-based (EuVO₄ and Eu³⁺-doped GdVO₄) was investigated. Materials were selected because of high thermal, chemical and photo-stability, biocompatibility and easy activation with rare earth ions at a wide range of concentrations without significantly affecting the lattice structure [37,38]. In addition, the selected colloids can be heavily doped with rare-earth ions without concentration quenching [38]. Then, the presence of Eu³⁺ ions in high concentrations ensures a strong luminescent signal, and eases the detection of different quenchers. Different combinations of Eu³⁺/Ln³⁺ ions (Ln³⁺= La³⁺, Dy³⁺, Er³⁺) were selected to study if the quenching of Eu³⁺ emission intensity in synthesized phosphate host materials would change. Pesticides selected for experiments (2,4-D, MCPA, Glyphosate) are among the most commonly used conventional pesticides in the home and garden market sector in 2012 according to the United States Environmental Protection Agency (EPA) [39]. In addition, the nature of the emission quenching and detection limits were determined in the presence of selected commercial pesticides.

2. Experimental

2.1. Materials and reagents

High-purity (99.99%) rare earth nitrates (La(NO₃)₃ × 6H₂O, Dy(NO₃)₃ × 5H₂O, Er(NO₃)₃ × 5H₂O, Gd(NO₃)₃ × 6H₂O, Eu(NO₃)₃ × 6H₂O) and analytical grade reagents: ammonium hydrogen phosphate (NH₄)₂HPO₄, ammonium vanadate (NH₄VO₃), sodium citrate (Na₃C₆H₅O₇ × 2H₂O), 2,4-Dichlorophenoxyacetic acid (2,4-D), 4-Chloro-2-methylphenoxyacetic acid (MCPA) and N-(phosphonomethyl)glycine (Glyphosate), were used as starting materials without further purification. Additionally, the milli-Q deionized water with electrical resistivity of 18.2 MΩ cm^-1 was used as solvent.

2.2. Synthesis of Eu³⁺- activated phosphate colloidal nanoparticles

Four colloidal samples: EuPO₄, La_0.5Eu_0.5PO₄, Dy_0.5Eu_0.5PO₄, and Er_0.5Eu_0.5PO₄ NPs were prepared according to the method described earlier by our group [6,38]. All precursor solutions were prepared in water with concentration of 0.025 M. In this procedure, the solution of trisodium citrate (25 mL) was added drop by drop to the mixture of Eu(NO₃)₃×6H₂O and La(NO₃)₃×6H₂O/Dy(NO₃)₃×5H₂O/Er(NO₃)₃×5H₂O solutions (total volume was 20 mL) at a stoichiometric ratio (50 mol% of Eu³⁺ ions to La³⁺/Dy³⁺/Er³⁺ ions or 100 mol% of Eu³⁺ ions for synthesis of stoichiometric EuPO₄) at room temperature. The addition of trisodium citrate is followed by a white precipitate indicating that an Eu³⁺-(La³⁺/Dy³⁺/Er³⁺)-Cit^3- -complex is formed. After vigorous stirring and slow addition of (NH₄)₂HPO₄ solution (10 mL), the white precipitate was completely dissolved and solutions become transparent. The stirring was continued for 1 h at room temperature, and the respective phosphates were obtained via dialysis for 24 h, which was needed to remove excess ions. The measured pH value of all colloidal phosphates was ∼6. The obtained colloidal NPs (concentration of NPs ∼ 3 mg/mL) were used for the study of their luminescent properties, while samples in the powder form (were obtained by evaporation of aqueous colloidal solutions and drying at 70 °C for 20 h were used for the structural and morphological characterization.

2.3. Synthesis of Eu³⁺- activated vanadate colloidal nanoparticles

Two colloidal samples: EuVO₄ and Gd_0.75Eu_0.25VO₄ were synthesized in accordance with the method described earlier [2,40,41]. The concentration of each precursor solution in water was 0.05 M. An aqueous solution of trisodium citrate (15 mL) was added drop-wise to the mixture (total volume was 20 mL) of Eu(NO₃)₃ × 6H₂O and Gd(NO₃)₃ × 6H₂O in stoichiometric ratio (25 mol% of Eu³⁺ ions to Gd³⁺ ions for synthesis of Gd_0.75Eu_0.25VO₄ or 100 mol% of Eu³⁺ ions for synthesis of stoichiometric EuVO₄) at room temperature. After vigorous stirring and slow addition of NH₄VO₃ dissolved in 0.15 M NaOH (15 mL), the white precipitate of the Gd³⁺(Eu³⁺)-citrate complex was completely dissolved and the corresponding vanadete-based nanostructures were slowly grown. Then, as-prepared colloidal solutions were additionaly stirred and heated at 60 °C for 1 h, and finally dialisied against water for 24 h. The obtained colloidal NPs (concentration of NPs ∼ 6 mg/mL) were used for luminescent properties studies, while samples in powder form were used for the structural and morphological characterization.

2.4. Sample preparation for photoluminescence measurements

Photoluminescence (PL) quenching of Eu³⁺ ions emission intensity in phosphate-based EuPO₄, La_0.5Eu_0.5PO₄, Dy_0.5Eu_0.5PO₄ and Er_0.5Eu_0.5PO₄ samples was studied upon addition of the 2,4 D-acid or MCPA pesticides. Additionally, for vanadate-based NPs (Gd_0.75Eu_0.25VO₄ and EuVO₄), PL quenching was studied with one more pesticide, the Glyphosate. This pesticide has not been used in the experiments with phosphate-based colloidal systems due to matrix emission intensity enhancement in the presence of phosphor ions, which are constituent of the used analyte. All obtained colloidal solutions were diluted from their initial concentration to the concentration of 0.2 mg/mL (the total volume of 2 mL). The concentration of the stock solutions of pesticides used in this experiment was 10^-4 M. The pesticide stock solutions were added in different volumes to colloidal NPs to adjust the final concentrations. All emission spectra of synthesized NPs were recorded over the range of 580-630 nm under 393 nm excitation radiation at room temperature (5 min after the addition of the pesticide solution, the time needed for the stabilization of emission intensity) [6].

3. Characterization methods and instrumentation

Crystal structures of powders were studied by X-ray diffractometer (XRD) (Rigaku SmartLab, Cu-K_{α1, 2} radiation, λ =0.1540 nm) at room temperature. All measurements were recorded over the 2θ range of 10°‒90°, with a step size of 0.01° and a counting time of 1°/min. Morphology of samples were obtained by transmission electron microscopy (TEM, Tecnai G20, FEI), operated at an accelerating voltage of 200 kV. Photoluminescence properties (PL) were studied utilizing spectrofluorometric system (Fluorolog-3 Model FL3-221, Horiba JobinYvon) with a 450 W xenon lamp as an excitation source for steady-state emission measurements.

4. Results and discussion

4.1. Structural and microstructural properties of Eu³⁺-activated phosphate and vanadate colloidal nanoparticles

XRD patterns of phosphate-based EuPO₄ and RE_0.5Eu_0.5PO₄ (RE = La, Dy, Er) ultra-small NPs along with the reference data for monoclinic monazite structure of bulk EuPO₄ (card COD No. 9001652) and RE_0.5Eu_0.5PO₄ (card COD No. 9001647) samples with the space group of P121/n1 are shown in Fig. S1 (Supporting Information). Because of the same valance and similar ionic radii between Eu³⁺ and RE³⁺ = La³⁺, Dy³⁺, Er³⁺ (ionic radii: La³⁺ =1.216 Å, Dy³⁺ =1.083 Å, Er³⁺ =1.062 Å, Eu³⁺ =1.12 Å, for the IX coordination), Eu³⁺ ions were successfully incorporated into the REPO₄ host lattice [42]. The X-ray patterns show broad diffraction peaks, and this broadening can be due to an amorphous and/or nano-size particles. Additionally, the rietveld refinement method (obtained using the PDXL2 software) was used to calculate the crystallite size and other structural parameters of EuPO₄ and Eu³⁺ activated REPO₄ NPs. The analysis results are summarized in Table S1 (Supporting Information). Also, XRD patterns of vanadate-based ultra-small NPs (EuVO₄ and Gd_0.75Eu_0.25VO₄), along with the reference data for the single tetragonal zircon-type phase (ICDD card No. 01-086-0996) with the space group of I41/amd are shown in Fig. S2, while Table S2 (Supporting Information) contains the crystallite size and structural parameters calculated by the Rietveld refinement of experimental data.

All synthesized NPs have the same morphology and representative TEM images of the Eu³⁺-activated LaPO₄ and GdVO₄ nanoparticles are shown in Fig. 1. Fig. 1(a) and (b) (phosphate based NPs) shows round NPs of approximately 2.1 nm in diameter (when measured from edge to edge) and a narrow size distribution (standard deviation, σ_d =0.3 nm), while Fig. 1(c) and (d) (vanadate based NPs) shows ultra-small crystallites of approximately 2.4 nm in diameter (when measured from edge to edge) and a narrow size distribution (standard deviation, σ_d =0.5 nm). The mean particle size for both systems, Eu³⁺ activated REPO₄/GdVO₄, roughly equals the crystallite size calculated using XRD, which suggests that each particle consists of a single crystallite. In already discussed ultra-small NPs, the surface-to-volume ratio is extremely large (2.86 × 10⁹ m^-1 for La_0.5Eu_0.5PO₄ NPs and 2.50 × 10⁹ m^-1 for Gd_0.75Eu_0.25VO₄ NPs), i.e., the percentage of Eu³⁺ dopant ions located on the surface of the particle is 92% and 94% for phosphate- and vanadate-based NPs, respectively. It is very important to note that the ions located on the surface of particles are strongly exposed to the influence of ligands [6,43].

4.2. Photoluminescent properties

To study the effect of matrix on the quenching of Eu³⁺ emission surrounded by pesticide molecules, the stoichiometric EuPO₄ and EuVO₄ and four different Eu³⁺ activated samples (ErPO₄, LaPO₄, DyPO₄, and GdVO₄) as well as three different pesticides (2,4-D, MCPA and Glyphosate) were investigated.

4.2.1. Photoluminescence quenching of Eu³⁺- activated phosphate NPs in the presence of 2,4-D and MCPA pesticides

The photoluminescence spectra of all synthesized samples, in the measured spectral range (580-630 nm) under 393 nm excitation, show two intensive characteristic red emission peaks of Eu³⁺ ions originating from the ⁵D₀ → ⁷F₁ magnetic-dipole electronic transition (at ∼590 nm) and the hypersensitive ⁵D₀ → ⁷F₂ electric-dipole electronic transition (at ∼615 nm). It is well known that the ⁵D₀→⁷F₂ transition is sensitive to the local symmetry around Eu³⁺ ions, and is strongly affected by the presence of chemical analytes in the particles’ surface vicinity [18].

Fig. 2(a) shows PL spectra of the colloidal EuPO₄ NPs with different concentrations (0‒0.175 mM) of 2,4-D pesticide. One can observe that PL intensity of Eu³⁺ emission decreases with the increase in 2,4-D pesticide concentration in the measured colloidal solution. The 2,4-D pesticide concentration of 0.0875 mM quenched the emission intensity by 50%, while the largest concentration (0.175 mM) of the 2,4-D reduced the emission intensity by 70%. The PL spectra of colloidal Er_0.5Eu_0.5PO₄, La_0.5Eu_0.5PO₄ and Dy_0.5Eu_0.5PO₄ in the presence of different concentrations of 2,4-D pesticide are shown in Fig. S3 (Supporting Information). The obtained results on Eu³⁺ activated REPO₄ systems show that there is a similar decrease in luminescence intensity of Eu³⁺ emission upon the addition of 2,4-D pesticide.

Fig. 2(b) presents the typical Stern-Volmer plots of experimental data (integrated emission intensity vs. quencher concentration) for different concentrations of 2,4-D pesticide. This linear relation was found for all synthesized phosphate-based NPs according to the Stern-Volmer equation [19]:

A₀/A = 1 + K_sv [Q], (1)

where A₀ is the integrated emission intensity (over the 580 to 630 nm range) in the absence of quencher, A is the integrated emission intensity in the presence of the pesticide of concentration [Q], and K_sv is the Stern-Volmer quenching constant. A good linear relationship (R², in Table 1) with no deviation from the linear fit indicates that all NPs were equally accessible to the quencher.

The determined values of the K_sv, as the slope (S) from linear fitting data using Eq. (1) for Eu³⁺ PL quenching in the four different phosphates-based structures, are given in Table 1.

The limit of detection (LOD) was calculated using the 3ϭ/S formula, where ϭ represents the standard deviation of blank measurements and S is the slope of the calibration curve (Table 1). The LOD value of 0.7 μM was obtained for EuPO₄ and Dy_0.5Eu_0.5PO₄, while in the case of La_0.5Eu_0.5PO₄ and Er_0.5Eu_0.5PO₄ the LOD has slightly larger value of 1 μM.

Fig. 3(a) shows the PL spectra of the colloidal Er_0.5Eu_0.5PO₄ NPs with different concentrations (0-0.175 mM) of MCPA pesticide, while the PL spectra of other three phosphate-based (EuPO₄, La_0.5Eu_0.5PO₄ and Dy_0.5Eu_0.5PO₄) NPs, are shown in the Fig. S4 (Supporting Information).

One can observe that PL intensity of Eu³⁺ ions decreases with the increase of the MCPA pesticide concentration. The MCPA concentration of 1 mM quenched emission intensity by 50%, while the largest concentration of MCPA (1.75 mM), reduced emission intensity by ∼65%.

Fig. 3(b) presents linear fits of the experimental data (integrated emission intensity vs. quencher concentration) to the modified Stern-Volmer equation for all synthesized phosphate-based NPs, EuPO₄ and Eu³⁺ activated REPO₄. A good linear relationship (R² of 0.990 for all systems, Table 2) with no deviation from the linear fit, indicates that only dynamic quenching occurred in this concentration range of MCPA pesticide, and that all NPs were equally accessible to the quencher. The calculated K_sv for EuPO₄ and Eu³⁺ activated REPO₄ are presented in Table 2.

The LOD of 10 μM is the same for all Eu³⁺ activated REPO₄ and EuPO₄ NPs, indicating that there is no significant difference in the sensitivity of these systems towards MCPA. The comparison of experimental results indicates that luminescence quenching in EuPO₄ and Eu³⁺ activated REPO₄ systems is more evident with 2,4-D pesticide compared to MCPA. There is significant decrease in photoluminescent intensity of Eu³⁺ ions upon addition of 0.0125 mM in the case of 2,4-D, while in the case of MCPA, the concentration needed to produce the similar effect was 20 times larger, 0.25 mM. The LOD values for 2,4-D pesticide compared to MCPA are 10 times lower, as presented in Table 1, Table 2. Additionally, the presented results show that different combinations of Eu³⁺/Ln³⁺ ions (Ln³⁺= La³⁺, Dy³⁺, Er³⁺) in synthesized phosphate host materials do not change the trend in quenching of Eu³⁺ emission, and, consequently, LODs are in the same pesticide concentration range.

4.2.2. Photoluminescence quenching of Eu³⁺-activated vanadate NPs in the presence of Glyphosate, 2,4-D and MCPA pesticides

EuVO₄ and Gd_0.75Eu_0.25VO₄ PL spectra were recorded in the 580-630 nm range under 393 nm excitation. Fig. 4(a) shows the quenching of the PL intensity of the Gd_0.75Eu_0.25VO₄ colloidal NPs upon addition of different concentrations (0-1.75 mM) of Glyphosate pesticide. One can observe that PL intensity decreases with an increase in Glyphosate pesticide. The Glyphosate pesticide concentration of 0.75 mM quenched the emission intensity by 50%, while the largest concentration of Glyphosate (2 mM) reduced the emission intensity by 70%. The obtained results for fluorescence quenching for EuVO₄ NPs show that there is a similar decrease in luminescence intensity of Eu³⁺ ions upon the addition of Glyphosate, Fig. S5 (Supporting Information).

Fig. 4(b) presents linear fits of experimental data (the integrated emission intensity vs. quencher concentration) to the modified Stern-Volmer equation for EuVO₄ and Gd_0.75Eu_0.25VO₄ NPs. A good linear relationship (R², see Table 3) with no deviation from the linear fit, indicates that only dynamic quenching occurred in this concentration range of Glyphosate pesticide, and that all NPs were equally accessible to the quencher.

Results of the quenching of Eu³⁺ emission in vanadate-based colloids upon addition of MCPA and 2,4-D pesticide are presented in Table 3. The corresponding emission spectra and Stern-Volmer plots are shown in Figs. S6 and S7, respectively (Supporting Information). The calculated K_sv and LOD for EuVO₄ and Gd_0.75Eu_0.25VO₄ NPs for Glyphosate/MCPA/2,4-D are presented in Table 3.

LOD values for all pesticides are slightly lower for EuVO₄ compared to Gd_0.75Eu_0.25VO₄ NPs indicating that there is no significant difference between the performances of these compounds. The lowest LOD is obtained for EuVO₄/2,4-D pesticide combination (9 μM). In both phosphate- and vanadate-based colloidal NPs, the luminescence quenching is more pronounced with the 2,4-D pesticide. By taking into the consideration the lowest LOD of 9 μM for the 2,4-D pesticide in vanadate-based colloids, and the lowest LOD of 0.7 μM in phosphate-based colloids, one can observe that LOD is about 13 times lower for Eu³⁺ activated REPO_4, making them more suitable for the pesticide detection compared to vanadate based nanoparticles.

4.2.3. Mechanism of Eu³⁺ emission quenching by pesticides

The absorption spectra of pesticides are measured (Fig. S8 in Supporting Information) to observe if there is any spectral overlap with the Eu³⁺ emission. In contrast to the metal-induced PL quenching, where the decrease in europium photoluminescence occurs because of the considerable spectral overlap between metal absorption and Eu³⁺ emission [44], there is no spectral overlap in this case. Therefore, one may exclude the energy transfer between donor-acceptor pairs as the quenching mechanism. Two quenching schemes are possible. In the first one, the emission quenching could be explained on account of the pesticide-Eu³⁺ electrostatic interaction on the NPs surface. Once pesticides are added to the colloidal solution, they may act as ligands that interact with europium ions via O atoms of a carboxylic group and inactivate Eu³⁺ emission. The ligand interaction scheme is presented in Fig. 5. Glyphosate (N-(phosphonomethyl)glycine), depending on the pH value of the solution, can exist in water in several forms. Deprotonation of the phosphate oxygen is associated with a measured 5.57 pKa value that was more recently estimated to be 5.69 in zero ionic strength solutions [45]. In the studied phosphate and vanadate colloids, the pH was 6-7, meaning that glyphosate predominantly exists in a di-anion form with a great affinity for the cationic Eu³⁺ center [45]. MCPA (4-Chloro-2-methyl-phenoxyacetic acid) and 2,4 D (2,4-Chlorophenoxyacetic acid) consist of aromatic rings which are strongly influenced by a nature of the bounded groups. Beside the polar carboxylic groups in both pesticides, the MCPA has one and the 2,4-D has two additional chloride groups (see Supporting Information Fig. S8). The chloride on the aromatic ring acts as an electron-withdrawing group, and affects the ring by reducing overall electron density in the system. Consequently, the presence of electron-withdrawing chloride group makes carboxylic group more partially negative becoming a potentially strong acceptor in interactions with Eu³⁺ ions [46].

The second quenching scheme may be related to the hard-soft acid-base (HSAB) interaction. Here, the observed trend in the NPs fluorescence quenching is 2,4-D > MCPA > glyphosate. The more intense quenching is generally observed for the chloride substituted aromatic pesticides, and this can be explained by the HSAB theory. Soft acids interact more strongly with soft bases, while hard acids interact more strongly with hard bases [[47], [48], [49]]. The Eu³⁺ ions on NP surface act as the hard acids, chloride ions act as hard bases, while the phosphor acts as a soft base. This could be the potential reason why chloride substituted aromatic pesticides lead to higher emission quenching compared to glyphosate pesticide. The presence of two electron-withdrawing chloride groups makes 2,4-D the strongest quencher of the Eu³⁺ photoluminescence among investigated pesticides. Further work is needed to confirm these mechanisms and to quantify their contributions to emission quenching.

5. Conclusion

Luminescent Eu³⁺-activated phosphate and vanadate NPs were successfully prepared by the precipitation method using citrate ions as surface ligands. Due to ultra-small dimensions (the average diameter of particles is approximately 2 nm) and the high surface-to-volume ratio of NPs, a large number of the Eu³⁺ ions are located on the surface of particles (92% and 94%, respectively). All luminescent colloids showed similar spectral features with two intensive characteristic red emission peaks of Eu³⁺ ion centered at ∼590 nm and at ∼615 nm. The integrated emission intensities of all NPs were reduced upon addition of different milimolar concentrations of glyphosate/MCPA/2,4-D pesticides. In all systems, the luminescence quenching was more pronounced with the 2,4-D pesticide compared to the MCPA and glyphosate. The lowest LOD of 0.7 μM was obtained with EuPO₄ and Dy_0.5Eu_0.5PO₄ in the presence of 2, 4-D pesticides, which suggests that these two systems could be systems of choice for the luminescence chemical sensing of pesticides. However, the selectivity remains a challenge, and it may be overcome by the further surface functionalization of NPs. Finally, one may conclude that Eu³⁺ activated ultra-small nanophosphors could potentially be used for the detection of pesticides in the environmental and biomedical fields.

Acknowledgments

Miroslav D. Dramićanin thanks the support from the National Recruitment Program of High-end Foreign Experts (Grant No. GDT20185200479) offered by the Chongqing University of Posts and Telecommunications (CQUPT), China. Jovana Periša, Željka Antić, Jelena Papan, Dragana Jovanović and Miroslav D. Dramićanin acknowledge the financial support of the Ministry of Education, Science and Technological Development of the Republic of Serbia (Nos. 45020 and 172056). Authors thank Dr. Krisjanis Smits for his support with the TEM.

Appendix A. Supplementary data

Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jmst.2019.09.004.

---