Nanoparticles are being applied in environmental remediation with great success (reference). These nanoparticles have high reactivity due to their nanoscale size. Equally important, it is their enormous flexibility for applications "in situ " [1,2]. An important application in this direction is the removal of organic and inorganic contaminants from aqueous solutions, as far as we know, little research has investigated for soil remediation; the nanoparticles ZVI can be used to remove pyrene in the soil [3-6]. The preparation of the nanoparticles has been done using conventional chemicals such as sodium borohydride to reduce the metallic ions [6,7]. However, this reagent can be toxic for microorganisms (search for the reference). In the past years, some researchers have developed new methods for synthesizing iron nanoparticles, replacing the dangerous and expensive reagents with extracts of plants and fruits such as the balm of lemon (Melissa officinalis ), parsley (Petroselinum crispum), sorghum bran (Sorghum spp. ), coffee and green tea [8-11].

The extracts of these plants and fruits contain molecules that carry alcoholic functional groups, mainly phenolic type and can be exploited for the reduction and the formation of stable complexes with iron [12]. In this context, this study focuses on the development of an economic, simple and friendly synthesis of iron nanoparticles. In this work, we use extracts of fruit cherries of capuli (Prunus serotina ) and mortiño (Vaccinium floribundum ) as reducing and stabilizing agents of the iron oxide and zero-valent iron nanoparticles. Polyphenols of the extracts when mixed with aqueous solutions of iron chloride reduce iron cations and make nanoparticles of zero-valent iron and iron oxides and also stabilize them.

Materials

Mortiño and capuli extracts used independently as a source of polyphenols were bought in a retail market of Quito. Iron chloride hexahydrate analytical grade (N 99.0%, Merck., Germany), Folin Ciocalteu's reagent (Sigma Aldrich, Switzerland), anhydrous sodium carbonate (Sigma Aldrich, Switzerland) and gallic acid (Sigma Aldrich, China) were purchased to a Spectrocrom, a local dealer.

Preparation of extracts

Fruit cherries were selected from those that had optimal conditions. Cleaning of cherries was performed with water and with distilled water for several times. For ethanolic extraction (96% EtOH) capuli and mortiño fruits were used in 3:1 ratio (w/v). The content was soaked during 48 h and then the pulp of cherries was filtered three times to minimize particle size through filter paper of 1 mm diameter and Whatman paper (0.22 μm). The resulting solution was concentrated in a rotary evaporator (Buchi-850) to remove the ethanol.

Chemical analysis of the extracts

The total content of polyphenols (CTP) was determined as equivalent gallic acid (EAG) following the Folin-Ciocalteu method (ISO 14502-1). With this methodology, the extract reacts with the aqueous solutions of FeCl₃ solutions resulting blackish brown discoloration and black, indicating the formation of nanostructures that are characteristic of zero-valent iron nanoparticles and iron oxides.

Synthesis of iron nanoparticles

It was prepared a solution of 0.5M FeCl₃.6H₂O in a 500 mL flask connected with a nitrogen gas line. Then, the fruit extract fixed at pH of 10 for V. floribundum and pH 12 for P. serotina was dropped slowly keeping a ratio of 10:1 (w/v) and the content was agitated in an orbital shaker at 100 rpm. The formation of a black precipitate in the flask is an indication of the growth of Fe (0) nanoparticles. Ultimately, nanoparticles were dried by evaporation using a hot plate (Freed Electric) and the resulting solid was stored for further analyses.

Characterization of iron nanoparticles

Size distribution of nanoparticles was performed using a submicron particle analyzer HORIBA LB-550. Mean and size distribution was obtained with software. Morphological analysis of iron was done with a Transmission Electron Microscope and images were digitally recorded (Tecnai G2 Spirit TWIN, FEI, Netherlands). Mineral composition of the nanoparticles were performed with X-rays diffractometer (EMPYREAN, PANalytical) with θ-2θ configuration (generatordetector) wherein a copper disc that emits X-ray at a wavelength of λ = 1.54 Ao. FTIR attenuated total reflectance spectra were recorded on a Spectra Two IR spectrometer (Perkin Elmer, USA) to detect the functional groups of the extracts as well as the nanoparticles prepared with the extracts.

Synthesis of nanoparticles

When synthesis was completed, the aqueous solutions showed a blackish-brown and black colors for mortiño (V. floribundum ) and capuli (P. serotina ) extracts that are related to the formation of iron oxides and zero-valent iron nanoparticles, respectively. The resulting nanoparticles are similar to those reported [13]. Extracts of P. serotina and V. floribundum are a great source of natural antioxidants because they contain polyphenols at concentrations of 1494 ± 385 (mg GAE/ 100g sample FW) and 2167 ± 835 (mg GAE/100g sample FW), respectively [14]. The mechanism of formation and stabilization of the iron nanoparticles might be associated to the -OH and -COOH groups contained in the biomolecules of polyphenols extracted from the cherries.

Characterization of nanoparticles

TEM images of nanoparticles prepared with P. serotina and V. floribundum are shown in Figure 1.

Figure 1: TEM micrographs: (a) nZVI 0.1M P. serotina and (b) of 0.1M nZVI V. floribundum ; ratio 10:1 (v/v).

According to statistical calculations, NZVIs prepared with mortiño extract showed diameter of 13.18 ± 5.89; fractal dimension of 1.52 ± 0.14; roughness of 0.92 ± 0.91 and roundness of 0.85 ± 0.14. While dimensions of nZVIs synthesized with capuli extract are in diameter of 11.88 ± 7.93, fractal dimension of 1.50 ± 0.10, roughness of 0.68 ± 0.16 and roundness of 0.97 ± 0.03 (Figure 1a and 1b).

The mineral structure of nanoparticles was further characterized by XRD. Peaks in the diagram were identified as iron oxides and zerovalent iron, respectively (Figures 2a and 2b). Peaks in the FTIR spectra in the range 1800 cm^-1 for both the fruit extract and the nanoparticles solutions are attributed to functional groups of polyphenols. While peaks approximately at 1000 cm^-1, vibration of CO which it is not shown in the sample of nanoparticles (Figure 3).

Figure 2: DRX: (a) P. serotina with 0.1M nZVI and (b) 0.1M nZVI with V. floribundum in 10: 1 (v / v).

Figure 3: FTIR: (a) P. serotina with 0.1M nZVI and (b) 0.1M nZVI with V. floribundum in 10: 1 (v / v).

Two raw materials, including fruit extracts capuli (Prunus serotina ) and mortiño (Vaccinium floribundum ), were selected, based on their antioxidant content, and evaluated as reducing agents for the production of iron nanoparticles suspensions. The structure of polyphenols collaborates in the synthesis and stability of nanoparticles.

In regard to the morphological features are similar nanoparticles both fruit extracts. The synthesis of nanoparticles of zero-valent iron and iron oxides is strongly influenced by the amount of extracts of capuli and mortiño. Time is a factor that has a significant influence on the formation of nanoparticles. The synthesized nanoparticles have a morphology preferably spheroidal and diameter of 13.18 ± 5.89 for Zero-valent iron nanoparticles with mortiño extract (V. floribundum ), and for capuli extract (P. serotina ), were: diameter of 11.88 ± 7.93. Increasing the concentration of the precursor salt the amount of the synthesized nanoparticles increases.

Biological synthesis may be an efficient, inexpensive and environmentally friendly alternative, in the production of metal nanoparticles.

Authors acknowledge the assistance of Eng. Andrea Vaca for their help with TEM imaging, data processing software and XRD analysis. In addition, we thank the University of the Armed Forces ESPE for funding for the project 2015-PIC-004.