CHARACTERIZATION OF NEW SYNTHESIZED Fe 2 O 3 NANOPARTICLES AND THEIR APPLICATION AS DETECTION SIGNAL AMPLIFIERS IN HERBICIDE BENTAZONE ELECTROANALYTICAL DETERMINATION

The iron oxide nanoparticles (Fe2O3 NPs) were synthesized from two different iron salts by solid-state synthesis method. The synthesized powder of Fe2O3 NPs is soluble in water, and the colloidal dispersion was characterized by TEM, FTIR, UV-Vis spectroscopy and zeta potential measurements. Obtained NPs are spherical in shape with narrow particle size distribution and an average diameter of 3 nm. Further, the possible application of Fe2O3 NPs was proposed, due to significant electroanalytical signal amplification in the determination of herbicide bentazone in natural river water.


INTRODUCTION
Nanotechnology  is the field that has attracted attention over the past decade among many researches, because of multitude possibilities of nanoparticles (NPs) synthesis and/or their surface modification.Because of their small size, NPs have a large surface area-to-volume ratio, and due to that, they have enhanced surface chemistry.There are numerous studies and different methods of inorganic nanoparticles synthesis, both for metal and for metal oxides NPs, with different size, shape and surface coverage (Nguyen et al., 2006;Abdulwahab et al., 2014;Liao et al., 2010;Tang et al., 2006).
In this work, we report the synthesis of iron(III) oxide NPs by simple solid-state chemical reaction method, at room temperature, and in the air.The solid-state chemical reaction method presents a mechanochemical process where the chemical reaction occurs during grinding solid precursors.Application of this method provides a high yield of NPs.For Fe 2 O 3 NPs synthesis two different iron salts were used as precursors.
As NPs and nanomaterials were recognized as excellent carriers and optical or electrical signal tags in sensing of biomolecules (Lei & Ju, 2012), the main idea of this work, besides obtaining monodisperse colloid dispersion, was also investigation of possibility of improvement in electroanalytical determination of pesticides, as serious environmental pollutants, due to amplification effect of NPs.Hence, previously synthesized Fe 2 O 3 NPs obtained from different salts were used to enhance voltammetric response in determination of herbicide bentazone, one of the most frequently used herbicides in Europe, recently studied on boron doped diamond electrode by our research group (Jevtić et al., 2018).

Reagents and standard solutions
and potassium hydroxide (KOH), from Aldrich, were used as received.Stock solution of bentazone, 3-Isopropyl-1H-2,1,3benzothiadiazin-4(3H)-one 2,2-dioxide (in further text presents with abbreviation BZ), 1x10 -3 M was prepared in methanol.This compound was the product of Sigma -Aldrich.Britton Robinson buffer (BR) was made from acetic, boric, and phosphoric acid (all 0.2 M) adjusted by sodium hydroxide to pH value 4. Water Purified Millipore Mili-Q with a resistivity of 18 MΩ was used for washing the reaction mixture.

Apparatus and measurements
Transmission electron microscopy (TEM) was used to estimate the shape and the average size of Fe 2 O 3 NPs.TEM measurements were carried out using FEI Talos F200X at an operating voltage of 200 kV.
Absorption spectra of iron oxide colloidal dispersion were measured by Perkin Elmer Lambda 35 UV -Vis spectrophotometer using the quartz cuvette with 1cm path length.
FTIR spectra were measured by NICOLET I5 ATR spectrometer.
Zeta potential (ZP) measurements by laser Doppler electrophoresis (LDE) were performed using a Zeta-sizer Nano ZS with 633 nm He-Ne laser (Malvern Instruments, UK).Data were analyzed by the Zetasizer Software Version 6.20 (Malvern Instruments, UK).
Electrochemical measurements were performed at PalmSens 3 potentiostat/galvanostat/impedance analyzer with PSTrace software (PalmSens BV, Netherlands).Three electrode cell was consisted from CH Instruments reference Ag/AgCl electrode and counter Pt electrode, while boron-doped diamond electrode, BDDE, (Windsor Scientific Ltd, Slough, Berkshire, United Kingdom) was used as a working electrode for bentazone determination.DPV parameters were set as in our previous work (Jevtić et al., 2018).Differential pulse voltammetry (DPV) measurements were performed in solutions made as it follows: 2 ml of water from Serbian river Ibar was spiked with 1 ml of a colloidal suspension of synthesized Fe 2 O 3 NPs (1.5 mg/10 ml) and proper volume of a standard solution of bentazone, and then fulfilled to 10 ml with BR buffer of pH 4.

Synthesis of iron oxide nanoparticles.
Iron oxide NPs can be prepared by the solid-state method, via the chemical reaction between ferrous iron (Fe 2+ ) and/or ferric iron (Fe 3+ ) with the alkaline.In this work, we used two different iron salts, FeSO 4 •7H 2 O and FeCl 3 •6H 2 O, in the molar ratio 1 : 2.5 (0.002 mol and 0.005mol).Iron salts were mixed in a mortar and ground for 30 min in air at room temperature; the yellow paste was obtained.The particle size and polydispersity of NPs can be tuned up by various factors (Ali et al., 2016), one of them is an ionic strength, so KCl is added in the reaction mixture to provide small NPs (Li et al., 2002).In yellow paste 0.02 mol of KOH was added to the mortar and grinded for the next 30 min.During grinding, yellow paste became dark brown to black and dry.The reaction mixture then was washed several times with purified water until SO 4 2-and Cl -ions could not be detected.The advantage of this method is its simplicity, low cost, and possibility to be done in every chemistry lab with simple chemical reagents.The chemical reaction goes on by the following equations: In further work NPs were used as a powder or as a colloidal dispersion.
TEM characterization.The morphology and size of Fe 2 O 3 NPs were determined from the diluted colloidal dispersion by TEM measurements.TEM micrograph along with particle size distribution are given in Fig. 1.TEM measurements show that Fe 2 O 3 NPs are spherical in shape with the average diameter of the particles of 2.9±0.07 nm.The mean particle size was obtained by fitting the TEM data with Gaussian distribution function (Fig. 1, inset).
Spectroscopic study.The absorption spectra of iron oxide colloidal dispersion contain broad absorption band around the area of 300 -400 nm (Fig. 2.).Obtained data are in agreement with previously reported spectra for iron oxide colloids (Sayed and Polshettiwar, 2015).Further, colloidal dispersion was left to dry in the air, at room temperature.Obtained powder of Fe 2 O 3 NPs was than characterized by FTIR measurements.In FTIR spectrum, Fig. 3, the observed bands at 601 and 888 cm -1 are assigned to Fe -O stretching vibrations.The bands at 1640 and 3280 cm -1 are from bending and stretching vibrations of bonds in the molecule of water, which is present in a small amount in Fe 2 O 3 NPs powder.Electrochemistry of bentazone at BDDE in the presence of Fe 2 O 3 NPs.Due to previously reported iron nanoparticles application as effective adsorbent material for wastewater and groundwater treatment of toxic contaminants (heavy metals, dyes etc.) (Chen et al., 2008;Saif et al., 2016), river water samples which contain different concentrations of herbicide bentazone were spiked with the synthesized and characterized Fe 2 O 3 NPs.The content of BZ in river water samples were examined with previously reported electroanalytical method (Jevtić et al., 2018).Instead of the expected reduction of the voltammetric signal, it was found that the presence of Fe 2 O 3 NPs in solution leads to a significant increase of DPV detection signal (Fig. 4).DPV voltammograms of river water samples without Fe 2 O 3 NPs were also given in Fig. 4 for comparison.As it can be seen, the peak potential for samples with Fe 2 O 3 NPs was shifted to higher potentials probably due to interactions of molecules of BZ with the surface of Fe 2 O 3 NPs.The main limitation of electroanalytical determination of bentazone represents instability of cation radical of BZ, its dimerization, and adsorption at the electrode (Manuela Garrido et al., 1998;Rahemi et al., 2013).The amplification of the detection signal may occur from the stabilization of cation radicals of bentazone due to interactions with Fe 2 O 3 NPs.Nature of those interactions will be a subject of our further research.
Fig. 5 presents DPV voltammograms of BZ in the presence of Fe 2 O 3 NPs in solution, recorded in the range of 5 to 90 μM of BZ, under the same experimental conditions and working DPV parameters as in our previous work (Jevtić et al., 2018).Hence, the next task could be finding optimal experimental conditions and working parameters to achieve more sensitive determination of bentazone in presence of Fe 2 O 3 NPs in solution, or even to employ Fe 2 O 3 NPs as a modifier on the surface of the electrode.

CONCLUSION
Iron(III) oxide NPs were sucssesfuly synthesized by solidstate chemical reaction method from two different iron salts.The morphology characteristics show fine structured NPs, spherical in shape with a narrow distribution and an average diameter of ~3 nm.In electrochemical studies, Fe 2 O 3 NPs show the amplifying effect on the voltammetric determination of herbicide bentazone at BDDE, offering interesting and promising results in the application of these NPs in sensing of toxic materials and environmental pollutants.

Figure 3 .
Figure 3. FTIR spectra of iron oxide NPs powder.Zeta potential measurements.The charge of Fe 2 O 3 NPs was determined by zeta potential measurements and shows that the surfaces of NPs are positively charged (+10 mV).Electrochemistry of bentazone at BDDE in the presence of Fe 2 O 3 NPs.Due to previously reported iron nanoparticles application as effective adsorbent material for wastewater and groundwater treatment of toxic contaminants (heavy metals, dyes etc.)(Chen et al., 2008;Saif et al., 2016), river water samples which contain different concentrations of herbicide bentazone were spiked with the synthesized and characterized Fe 2 O 3 NPs.The content of BZ in river water samples were examined with previously reported electroanalytical method(Jevtić et al., 2018).Instead of the expected reduction of the voltammetric signal, it was found that the presence of Fe 2 O 3 NPs in solution leads to a significant increase of DPV detection signal (Fig.4).DPV voltammograms of river water samples without Fe 2 O 3 NPs were also given in Fig.4for comparison.As it can be seen, the peak potential for samples with Fe 2 O 3 NPs was shifted to higher potentials probably due to interactions of molecules of BZ with the surface of Fe 2 O 3 NPs.The main limitation of electroanalytical determination of bentazone represents instability of cation radical of BZ, its dimerization, and adsorption at the electrode(Manuela Garrido et al., 1998;Rahemi et al., 2013).The amplification of the detection signal may occur from the stabilization of cation radicals of bentazone due to interactions with Fe 2 O 3 NPs.Nature of those interactions will be a subject of our further research.Fig.5presentsDPV voltammograms of BZ in the presence of Fe 2 O 3 NPs in solution, recorded in the range of 5 to 90 μM of BZ, under the same experimental conditions and working DPV parameters as in our previous work(Jevtić et al., 2018).

Figure 4 .
Figure 4. DPV voltammograms of different concentrations of BZ without and with Fe 2 O 3 NPs in solution.

Figure 5 .
Figure 5. DPV voltammograms of different concentrations ofBZ (5, 7, 10, 15, 30, 50, 60, 70, 90 μM)  in BR buffer pH 4 at BDDE; Inset: Corresponding calibration curve.The linear dependence expressed with the regression equation: I (µA) = 1.660 + 0.119 × C (µM); R = 0.993 was presented in the inserted graph in the same figure.The amplification of DPV signal of BZ in presence of Fe 2 O 3 NPs also resulted in lowering of limit of detection (LOD) to the value of 0.09 μM.Hence, the next task could be finding optimal experimental conditions and working parameters to achieve more sensitive determination of bentazone in presence of Fe 2 O 3 NPs in solution, or even to employ Fe 2 O 3 NPs as a modifier on the surface of the electrode.