HIGHLY SELECTIVE AND SENSITIVE DETERMINATION OF Cr6+ (nM) IN GELATIN CAPSULE USING AgF/Ag2WO4 NANOCOMPOSITE

HIGHLY SELECTIVE AND SENSITIVE DETERMINATION OF Cr⁶⁺ (nM) IN GELATIN CAPSULE USING AgF/Ag₂WO₄ NANOCOMPOSITE

Ramya Arumugam and Prakash Periakaruppan ^*

Department of Chemistry, Thiagarajar College, Madurai - 625009, Tamil Nadu, India.

ABSTRACT: The present work deals with a novel synthesis of AgF/Ag₂WO₄ nanocomposite by co-precipitation method. The morphology and size of the synthesized AgF/Ag₂WO₄ nanocomposite were confirmed by Ultraviolet-Visible (UV-Vis) spectroscopy, Fourier Transform-Infrared (FT-IR) spectroscopy, Transition Electron Microscopy (TEM) and X-Ray Diffraction (XRD) analyses. The performance of the nanocomposite was successfully evaluated for Cr⁶⁺ detection in a gelatin capsule, which indicated that this convenient and sensitive material offered great promise for onsite environmental monitoring of Cr⁶⁺. Control experiments with the addition of over 10 other metal ions (Na⁺, K⁺, Mg²⁺, Fe²⁺, Hg²⁺, Ca²⁺, Cu²⁺, Ni²⁺, Mn²⁺, Zn²⁺) did not result in a distinct change in the colour or in the spectrum of the suspension which indicated that these ions did not interfere in the colorimetric determination of Cr⁶⁺ in gelatin capsule. The detection concentration of Cr (VI) ranged from 0.5 mg to 1.0 mg, and the detection limit was 2 nM.

AgF/Ag₂WO₄  Nanocomposite, Colorimetric sensor, Gelatin capsule, Chromium

INTRODUCTION: High soluble and toxic content with the carcinogenic effect of hexavalent chromium Cr⁶⁺ has been consistently found to be associated with an elevated incidence of respiratory cancers and other adverse health consequences. Chromium commonly exists in nature with two stable oxidation states, Cr³⁺ and Cr⁶⁺, whereas Cr⁶⁺ is highly water soluble and toxic, Cr (III) is much less soluble in water and less toxic to humans in the absence of complexing ligands ^1-3. Many industries produce this carcinogenic pollutant including leather tanning, cement, textile, printers and stainless steel welding ^4-7.

There are several methods used for the detection of Cr⁶⁺ such as atomic emission spectrometry, atomic absorption spectrometry, electrochemical method, and UV-vis spectrophotometry, X-ray fluorescence, solid phase extraction and solvent extraction ^8-15.

Among all the above-said methods, they are considered as a simple and low-cost, fast and portable and easy to carried out. Such a cost-effective monitoring tool for the detection of Cr⁶⁺ in gelatin capsule is of essential relevance, particularly in sensitive environments like pharmaceuticals, groundwater and industrial wastewater effluents. For the last few decades, pharmaceutical products have been growing rapidly in the global market. This development is highly expected to a greater extent because of the growth of a huge population and efficient health treatments ^16-21. Generally, gelatin capsules are administered to heal. The chromium content has been checked in various medicines such as Cipmox 250, OMEGA B-Complex capsule, N-Cycline-250 and Doxylab. Among them, the chromium content is particularly more in N-Cycline-250 medicinal capsule. Many research literature has reported the evaluation of drug composition in pharmaceuticals, but no work has been carried out for the detection of a trace amount of chromium in empty gelatin capsules.

In the last few decades, the nanomaterials have been significant due to their various physical and chemical properties. Due to fascinating electronic, optical, magnetic, chemical and biological properties, the bimetallic nanomaterials exhibit new bifunctional or synergic effects ^22-23. One of the transition metal compounds, tungstates possess a combination of covalent, ionic and metallic bonding and form an essential class of functional materials. Tungstates render important properties like ferroelectricity, conductivity, and photo-luminescence which have attracted significant attention among the scientific community due to its unique symmetry dependent and spontaneous polarization properties ^24-26.

Among the tungstates, silver tungstate is emerging as a new material which shows high potential applications in enormous fields such as sanitary ceramics as an antibacterial agent, plasmonics, high-temperature tribological applications as solid-state lubricants, the catalyst for water splitting and dye degradation, electrochemical sensor, antibacterial property, luminescence host ^27-35, etc. Various methods are involved during the preparation of tungsten oxides such as sol-gel method, chemical precipitation method and microwave irradiation method ^36-38. Among them, chemical precipitation method plays a vital role due to its simple operation, less consumption of time and purity of the material compared to other methods. Ag and F is the best candidate as a dopant for Ag₂WO₄. AgF nanoparticles were formed due to the diffusion of Ag⁺ which reacted with the adsorbed F^-. These nanoparticles are low cost, stable and nontoxic.

The present work, reports the synthesis of novel AgF/Ag₂WO₄ nanocomposite. The synthesized nanocomposite has been characterized by UV-visible spectroscopy, FT-IR spectroscopy, XRD, HR-TEM, and EDX analyze. To the best of our knowledge, this is the first ever reported study which probes a highly selective and sensitive detection of Cr⁶⁺ in gelatin capsule using AgF/Ag₂WO₄ nanocomposite.

Experimental: All the reagents used in this work were analytical grade and used without further purification. All chemicals such as silver nitrate, sodium fluoride and sodium tungstate (Na₂WO₄.2H₂O) were purchased from Sigma Aldrich, India. The commercial gelatin capsules were purchased from Apollo Pharmacy, Madurai, India. Deionized water (DI) was used throughout the work.

Preparation of AgF: 10 mmol AgNO₃ and 2.5 mmol NaF were each separately dissolved in 20 ml of DI water. NaF solution was stirred constantly for 10 min and then added dropwise to AgNO₃ solution with constant stirring for 3 h and there obtained a light grey colored suspension, which was centrifuged to remove the precipitate by washing twice with water and ethanol and dried in an oven at 333 K for 24 h.

Preparation of Ag₂WO₄: The Ag2WO4 was prepared by a precipitation process at room temperature. In a typical procedure, 0.5 mM of Na₂WO₄ was dissolved in 20 ml of DI water to form a clear solution, followed by a dropwise addition of 20 ml of AgNO₃ aqueous solution (0.025 M). The solution turned white, indicating the formation of Ag₂WO₄. The precipitate formed was continuously stirred for 1 h and collected by centrifugation. After washing the as-prepared Ag₂WO₄ three times with DI water and ethanol, it was dried at 60 °C for 6 h.

Preparation of AgF/Ag₂WO₄ Nanocomposite: In an experimental procedure, 0.023 g of the as-prepared Ag₂WO₄ was added into 15 mL of DI water with the assistance of ultrasonication. 0.023g of prepared AgF was added into Ag₂WO₄ solution. After continuously stirred for 2 h at room temperature in the dark, the products were collected by centrifugation.

The obtained products were washed with DI water and ethanol three times each and dried at 60 °C for 6 h. Scheme 1 illustrates the formation of the AgF/Ag₂WO₄ nanocomposite and its sensing of Cr⁶⁺ in gelatin capsules.

SCHEME 1: SYNTHESIS OF AgF/Ag₂WO₄ NANOCOMPOSITE AND ITS SENSING PROPERTIES OF Cr⁶⁺ IN GELATIN CAPSULE

Instrumentation: UV-visible spectra (Jasco V-560 model) of synthesized AgF/Ag₂WO₄ nano-composite was recorded between the range 200-800nm. The FT-IR spectral measurement was recorded using a KBr disc on a JASCO FT-IR 460 Plus spectrophotometer which was collected at a spatial resolution of 4 cm^-1 in the transmission mode between 4000-400 cm^-1. XRD analysis was carried out using an X-ray diffraction unit using Cu Ka radiation (λ = 1.5418Aº) on a JEOL JDX 8030 X-ray diffract meter. The size and morphology of synthesized AgF/Ag₂WO₄ nanocomposite were examined using transmission electron microscopy (HR-TEM, JEOL JEM 2100 model) instruments. An Energy Dispersive X-ray (EDX) spectrometer was used for elemental analysis which was attached to the transmission electron microscope. All experiments were carried out at room temperature.

RESULTS AND DISCUSSION:

UV-Vis Spectroscopy: The UV–vis spectra of the samples AgF, Ag₂WO₄ and AgF/Ag₂WO₄ are depicted in Fig. 1. The UV-vis absorption spectrum of AgF sample exhibits a peak at 427 nm with absorption in the visible region. The spectrum for Ag₂WO₄ sample has an absorption maximum at 420 nm. The peak at 422 nm confirms the formation of AgF/Ag₂WO₄ nanocomposite ³⁹.

FIG. 1: UV-VISIBLE SPECTRA OF SYNTHESIZED AgF (A), Ag₂WO₄ (B) AND AgF/Ag₂WO₄ (C) NANOCOMPOSITE

FT-IR Spectroscopy: Fig. 2 shows the FT-IR spectra of AgF, Ag₂WO₄, and AgF/Ag₂WO₄ nanocomposite.  In both the samples Ag₂WO₄ and AgF/Ag₂WO₄, FT-IR spectra show the absorption band at 3399 cm^-1 and 1350 cm^-1 which indicates that the presence of -OH stretching and its corresponding H-OH vibration appears at 1650 cm^-1. The peak appears at 2300 cm^-1 assigned C-H stretching vibration. The strong vibrational peak appears at 856 cm^-1 for both the samples Ag₂WO₄ and AgF/Ag₂WO₄ which is ascribed to asymmetric stretching vibrations of the O-W-O indicates the characteristic of tetrahedral tungstate ⁴⁰. Although a broad absorption peak appears at 746-825 cm^-1 for the synthesized AgF/Ag₂WO₄ nanocomposite which can also be observed due to the existence of WO₄^2- anions ⁴¹.

FIG. 2: FT-IR SPECTRUM OF AgF, Ag₂WO₄, AgF/Ag₂WO₄ NANOCOMPOSITE

XRD Measurement: Fig. 3 explains the diffraction peaks for the crystalline structure of as-prepared AgF/Ag₂WO₄ nanocomposite with the corresponding 2θ values and crystal planes. The diffraction peaks of AgF sample (JCPDS No 3-890) denote the crystalline structure of AgF/Ag₂WO₄. The characteristics peaks of AgF at 29.2°, 38.9°, 56.1°, 70.3° are gradually observed and well matched with the (111), (200), (220), (222) plane of the face-centered cubic structure. The main characteristic peaks of Ag₂WO₄ at 13.9°,15.7°, 16.7°, 19.5°, 26.7°, 32.9°, 33.8°, 37.4°, and 40.8° can be indexed to the (020), (121), (022), (220), (200), (040), (242), (060) and (224), crystal planes of Ag₂WO₄ (JCPDS 34-0061) with orthorhombic phase. The extra peaks on composite particles compared to AgF at 2θ values of 31.7°, 45.4° and 56.4° corresponding to the crystal planes (111), (200) and (220)  indicate the presence of Ag metal (JCPDS card no.04-0783) ⁴². Furthermore, the diffraction peak of the AgF/ Ag₂WO₄ has been shifted to a small degree compared to AgF which indicates that there is an effective interaction between AgF and Ag₂WO₄ in AgF/Ag₂WO₄ nanocomposite. The size of the synthesized AgF/Ag₂WO₄ nanocomposite was calculated using Debye Scherrer’s equation and found to be 10 nm.

FIG. 3: XRD MEASUREMENT OF AgF, Ag₂WO_4, AgF/Ag₂WO₄ NANOCOMPOSITE

TEM: The surface morphology, size, and shape of the AgF/Ag₂WO₄ nanocomposite were characterized by HR-TEM images.

Fig. 4(a-d) displays the HR-TEM images with different magnifications of the synthesized AgF/Ag₂WO₄ nanocomposite. Fig. 5(a) presents the selected-area electron diffraction (SAED) patterns which describes the concentric diffraction bright spots corresponding to the presence of (020), (121), (111), (220), (200), (242), (200) and (222)  planes of the face-centered cubic (fcc) AgF/ Ag₂WO₄ nanocomposite. The EDX spectrum as shown in Fig. 5b for the AgF/Ag₂WO₄ nanocomposite indicates the presence of major elements such as Ag, F, W, and O. Fig. 5(c) shows the histogram analysis for finding the average size of synthesized AgF/Ag₂WO₄ nanocomposite and found to be 10 nm which agrees well with the size confirmed using Sherrer’s equation.

Colorimetric Sensing of Cr⁶⁺ in Gelatin Capsules using AgF/Ag₂WO₄ Nanocomposite Selectivity Studies: The synthesized AgF/Ag₂WO₄ nanocomposite was tested for the detection of Cr⁶⁺ in gelatin capsules over other environmental metal cations by adding different heavy metal cations such as Na⁺, K⁺, Mg²⁺, Fe²⁺, Hg²⁺, Ca²⁺, Cu²⁺, Ni²⁺, Mn²⁺ and Zn²⁺ to the AgF/Ag₂WO₄ nanocomposite solution and color changes were recorded.

Fig. 6 represents the photographic image of AgF/Ag₂WO₄ nanocomposite with different heavy metal cations. There is no change in color observed when other heavy metal ions were added to the composite solution except for Cr⁶⁺ in gelatin capsule solution.

FIG. 4 (A-D): TEM IMAGES OF DIFFERENT MAGNIFICATIONS OF AgF/Ag₂WO₄ NANOCOMPOSITE

FIG. 5: (A) SAED PATTERN OF AgF/Ag₂WO₄ NANOCOMPOSITE (B) EDX SPECTRUM OF AgF/Ag₂WO₄ NANOCOMPOSITE (C) HISTOGRAM OF PARTICLE SIZE DISTRIBUTION OF AgF/Ag₂WO₄ NANOCOMPOSITE

Adding different heavy metal cations to AgF/Ag₂WO₄ nanocomposite has been recorded by UV-vis absorption spectra that were shown in Fig. 7(a). A few differences in selectivity histogram of AgF/Ag₂WO₄ nanocomposite for Cr⁶⁺ in gelatin capsule are shown in Fig. 7(b).

It shows that the change in the absorption peak is high selectivity. It is confirmed from the selectivity results that the proposed colorimetric sensor can detect specifically Cr⁶⁺ in a gelatin capsule.

FIG. 6: PHOTOGRAPHIC IMAGE OF AgF/Ag₂WO₄ NANOCOMPOSITE WITHIN DIFFERENT HEAVY METAL CATIONS

FIG. 7: (A) UV-VISIBLE SPECTRA OF AgF/Ag₂WO₄ NANOCOMPOSITE SOLUTION WITH DIFFERENT TRANSITION-METAL IONS. (B) THE COLORIMETRIC RESPONSE OF AgF/Ag₂WO₄ NANOCOMPOSITE TO VARIOUS METAL CATIONS

Sensitivity Studies: To detect the sensing range of this sensor, sensitivity studies were carried out. For these investigations, the prepared solution of 1mM of AgF/Ag₂WO₄ nanocomposite was diluted with distilled water. Then, 1.5 mL of AgF/Ag₂WO₄ nanocomposite was mixed with 1 mL of various amounts (0.5 mg - 1.0 mg) of Cr⁶⁺ in gelatin capsule solution. When Cr⁶⁺ in gelatin capsule solution is added to AgF/Ag₂WO₄ nano-composite, a significant color change is observed from yellow to colorless. The color changes are observed due to the redox reaction between silver and chromium. The photographic image of AgF/Ag₂WO₄ nanocomposite in the presence of a different quantity of Cr⁶⁺ in gelatin capsule solution was depicted in Fig. 8.

FIG. 8(A-F): PHOTOGRAPHIC IMAGES OF AgF/Ag₂WO₄ NANOCOMPOSITE WITH VARIOUS QUANTITIES (0.5 mg - 1.0 mg) OF Cr⁶⁺ IN GELATIN CAPSULE

Further, it was proved by using UV-vis spectroscopy by measuring the SPR intensity of AgF/Ag₂WO₄ nanocomposite. The UV-vis spectra of AgF/Ag₂WO₄ nanocomposite in the presence of different concentrations of Cr⁶⁺ are seen from Fig. 9(a). The absorbance intensity of AgF/Ag₂WO₄ nanocomposite gradually decreases while increasing the concentration of Cr⁶⁺ from 0.5 mg - 1.0 mg.

The absorbance values of AgF/Ag₂WO₄ nano-composite versus the concentration of Cr⁶⁺ was plotted and shown in Fig. 9(b). It shows a linear relationship (R² = 0.9988) between the SPR absorbance of AgF/Ag₂WO₄ nanocomposite at 441 nm and Cr⁶⁺ at concentration from 0.5 mg to 1.0 mg. The lowest limit of detection can be calculated as:

LOD = 3S/b

Where S is the standard deviation of the lowest concentration of Cr⁶⁺ and b is the slope of the calibration curve. The synthesized AgF/Ag₂WO₄ nanocomposite is found to sense Cr⁶⁺ with the limit of detection (LOD) of 2 nM. The HR-TEM images obtained for AgF/Ag₂WO₄ nanocomposite before and after the addition of Cr ⁶⁺ in gelatin capsule are shown in Fig. 10. As seen, while well-defined AgF/Ag₂WO₄ nanocomposite appears before addition Fig. 10a, it disappears after the addition of Cr ⁶⁺ in gelatin capsule Fig. 10b. Fig. 10c presents the elemental analysis of the AgF/Ag₂WO₄ nanocomposite using EDX which confirms the existence of Ag, W, and O. Fig. 10d confirms the existence of Ag, W, O, and Cr after sensing Cr⁶⁺ in the gelatin capsule. The sensitivity of this colorimetric sensor has been compared to those of earlier studies using nanoparticles sensors as shown in Table 1 which shows a superior limit of detection for the proposed sensor ^43-47.

Real Sample Analysis: An application of AgF/Ag₂WO₄ as a colorimetric sensor in the analysis of gelatin capsule spiked with Cr⁶⁺ was made. The detected concentration of Cr⁶⁺ was 2 nM, close to that of the added Cr⁶⁺ (0.5 - 1.0 mg). The recovery was 84%. These results indicate that AgF/Ag₂WO₄ has good accuracy in monitoring Cr⁶⁺ in gelatin capsules.

FIG. 9: A) UV-VISIBLE ABSORPTION SPECTRA OF AgF/Ag₂WO₄ NANOCOMPOSITE AFTER THE ADDITION OF VARIOUS QUANTITIES (0.5 mg - 1.0 mg) of Cr ⁶⁺ IN GELATIN CAPSULE (A-F) B) PLOT OF ABSORBANCE INTENSITY VERSUS Cr

FIG. 10: TEM IMAGE OF AgF/Ag₂WO₄ NANOCOMPOSITE BEFORE (A) AND AFTER (B) ADDITION OF Cr IN GELATIN CAPSULE AND EDX SPECTRA OF AgF/Ag₂WO₄ NANOCOMPOSITE BEFORE (C) AND AFTER (D) ADDING Cr IN GELATIN CAPSULE

TABLE 1: COMPARISON OF COLORIMETRIC SENSORS PROPOSED FOR DETECTION OF Cr⁶⁺ IN THE LITERATURE

CONCLUSION: In summary, we have reported here a facile and novel synthesis of AgF/Ag₂WO₄ nanocomposite. The synthesized AgF/Ag₂WO₄ nanocomposite has been employed as a simple, environmentally friendly and low-cost colorimetric sensor for the detection of Cr⁶⁺ in gelatin capsules. The proposed sensor shows high sensitivity, with a significant change in the color of the solution from yellow to colorless in the presence of a small amount of Cr⁶⁺ in gelatin capsules. There is no obvious interference from other heavy metal ions, and therefore the sensor is highly selective for Cr ⁶⁺ in gelatin capsules. The reported probe has the lowest detection limit (2 nM).

ACKNOWLEDGEMENT: The authors thank the management of Thiagarajar College for providing lab facilities.

CONFLICT OF INTEREST: There is no conflict of interest.

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    How to cite this article:

    Arumugam R and Periakaruppan P: Highly selective and sensitive determination of Cr⁶⁺ (nM) in gelatin capsule using AgF/Ag₂WO₄ nanocomposite. Int J Pharm Sci & Res 2019; 10(3): 1439-47. doi: 10.13040/IJPSR.0975-8232.10(3).1439-47.

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