Efficient Optical and UV-Vis Chemosensor Based on Chromo Probes-Polymeric Nanocomposite Hybrid for Selective Recognition of Fluoride Ions.

A novel colorimetric sensor based on the TiO2/poly(acrylamide-co-methylene bis acrylamide-co-2-(3-(4-nitro-phenyl)thioureido)ethyl methacrylate) nanocomposite was synthesized via a surface modification strategy; methacryloxypropyltrimethoxysilane was used to provide reactive vinyl groups on the surface of TiO2 nanoparticles for the successful surface polymerization of Am (acrylamide), MBA (methylenbisacrylamide), and NPhM (2-(3-(4-nitrophenyl)thioureido)ethyl methacrylate) components. The successful preparation of nanocomposites was confirmed using Fourier transform infrared, 1H NMR, 13C NMR, scanning electron microscopy, transmission electron microscopy, thermogravimetry analysis, and X-ray diffraction methods, and the sensing ability of the probe toward fluoride ions was investigated using naked-eye detection and UV-vis measurement. The interaction of the prepared polymeric nanocomposite with fluoride ions elicited a significant visible change in color from pale yellow to orange and was further affirmed by a clean interconversion of the two absorption bands at 330 and 485 nm. The selective binding ability of the polymeric nanocomposite towards fluoride over other anions, such as I-, Cl-, Br-, AcO-, H2PO4 -, and H2SO4 - was further explored; the prepared chemosensor could detect fluoride ions in acetonitrile with a detection limit of 3 μM.

Introduction

In recent decades, the field of identifying and sensing anions has attracted significant attention because of their detrimental effects on environmental and biological processes. Fluoride ions, in comparison with other anions, have been studied more extensively because it plays an important role in biology, medicine, and environmental sciences.[1,2] Also, fluoride ions are ubiquitous in the health of teeth and osteoporosis treatment,[3,4] and they show unique properties such as ease of absorbability and slow release in human body which are among several reasons responsible for attracting great attention.[5] The excessive concentration of fluoride ions could cause dental fluorosis and severe kidney and gastric problems.[6−8] In addition, chronic bone disease fluorosis is related to the high levels of fluoride ions in drinking water.[9] Further, the Environmental Protection Agency (US EPA) set 4 mg/L (∼0.21 mM) for fluoride ions as its drinking water standard. Furthermore, fluoride ions have many military usages, including refining uranium and detecting nerve agents for instance sarin and soman.[10]

Over the past decade, several analytical methods have been applied to measure fluoride ions sensitively and selectively; electrochemical methods, mass spectrometry, fluorescent methods, colorimetric methods, and as such are examples of these techniques.[10] Among these analytical procedures, optical methods[11−21] are considered as powerful tools because of their low cost, simplicity, ease of use, and quick response time.[22,23] These methods are mainly based on the N–H proton transfer from the donor unit to the fluoride ions, which induces N–H proton–deprotonation.[24] In this protocol, the most common hydrogen donators are moieties such as amine,[25] pyrrole,[26] amide,[27] urea,[28−30] and thiourea.[31,32] These probes are often neutral in nature and contain acidic hydrogen (−NH or −OH) that binds to fluoride ions through hydrogen bonding and causes a colorimetric response.[3,33] In this arena, nanomaterials that exhibit optical responses, for example, fluorescence, chemiluminescence, and colorimetric hold great promises for fluoride ions sensing.[34] Among a wide variety of nanomaterials deployed for fluoride ion detection, include several oxides namely V2O5, MoO3, TiO2, silica, and zirconium dioxide. Cost-effective TiO2 nanoparticles (NPs) are especially important due to their chemical stability in varied environments, durability, nontoxicity, and high corrosion resistance properties; however, drawbacks associated with their instability in aqueous environments and large aggregate formation have prompted many researchers to modify their structures with polymers.[35] To the best of our knowledge, there has not been any report for the modification of TiO2 NPs with a polymer toward selective fluoride ion detection. The modified polymeric shell surrounding TiO2 NPs provides improvement in stability as well as the detectability toward fluoride anions.[36,37] They exhibit a higher degree of sensitivity and selectivity over small molecules because of their repeating structure that may give rise to effects such as multivalency or cooperativity in the context of supramolecular interaction. However, the most notable differences between small molecule and polymeric-based sensors stem from the ability of the latter to gain signal amplification, to provide high levels of sensitivity in terms of detecting fluoride anion.[38]

In this work, a novel polymeric nanocomposite was used as a highly selective and sensitive colorimetric sensor for the detection of ultratrace levels of hazardous (fluoride) ions in organic solutions. First, to obtain the polymeric shell on the surface of the TiO2 NPs, acrylamide (AM), 2-(3-(4-nitrophenyl)thioureido)ethyl methacrylate (NPhM), and methylenbisacrylamide (MBA) were used as a monomer, comonomer-probe, and cross-linker, respectively; MBA was used as a cross-linker to bestow a three-dimensional structure to reach maximal interaction between the polymeric shell and fluoride ions. In this process, the changes in color from the pale yellow to orange were observed upon the chemosensor interaction with fluoride ions (graphical abstract and Scheme ). The obtained trace levels of detection could be attributed to the repeating structure of the polymer as well as high surface area of the ensuing polymer–nanocomposite material.

Scheme 1

Chemosensors Application

Results and Discussion

Characterization

Fourier Transform Infrared Analysis

Fourier transform infrared (FT-IR) spectra of the TiO2 NPs, TiO2–MAPTMS nanocomposite, 3-(4-nitro-phenyl)thioureido, NPhM, and TiO2/poly(acrylamide-co-methylene bis acrylamide-co-2-(3-(4-nitro-phenyl)thioureido)ethyl methacrylate) [TiO2/poly(Am-co-MBA-co-NPhM)] nanocomposite are shown in Figure . The broad bands below 800,[40] 1623, and 3228 cm–1 were ascribed to the vibration of Ti–O, OH bending vibrations of surface absorbed water molecules, and OH groups of TiO2 NPs,[41,42] respectively (Figure a). The spectra of modified TiO2 NPs with methacryloxypropyltrimethoxysilane (MAPTMS) (Figure b) exhibited a band in the region of 1715 cm–1 which was assigned to the ester functional groups of MAPTMS. The successful synthesis of 1-(2-hydroxyethyl)3-(4-nitro-phenyl)thioureido (Figure c) was confirmed with the appearance of bands at 1322 cm–1 (C=S),[43] 1496, 1582 cm–1 (NO2), 1636 cm–1 (C=C), 2947–2998 cm–1 ([−(CH)−]), and 3215 cm–1 (−OH groups). The spectra of NPhM (Figure d) exhibited a band in the region of 1700 cm–1 that was attributed to conjugated ester group. The successful polymerization of amidic monomers and NPhM on the TiO2–MAPTMS nanocomposite (Figure e) was confirmed by the appearance of new peaks at around 1657 and 1736 cm–1 which were associated with amidic groups of Am and MBA and ester groups of NPhM, respectively.

Figure 1

FT-IR spectra of TiO2 NPs (a), TiO2–MAPTMS nanocomposites (b), 3-(4-nitro-phenyl)thioureido (c), NPhM (d), and TiO2/poly(Am-co-MBA-co-NPhM) nanocomposites (e).

Figure 2

XRD pattern of TiO2 NPs, TiO2/poly(Am-co-MBA-co-NPhM) nanocomposites.

X-ray Diffraction Analysis

X-ray powder diffraction is a powerful tool for the investigation of crystallographic structure of the NPs and nanocomposites. The TiO2 NPs show well-defined X-ray diffraction (XRD) patterns corresponding to reflections in the anatase phase (the characteristic peaks at 2θ = 25.35, 37.77, 47.83, 54.22, 62.92, 75.40).[44] Despite loading organic moieties onto the TiO2 NPs surface, typical Bragg diffraction peaks were retained, thus affirming the stability of TiO2 NPs under the employed condition of the polymerization reaction. Moreover, a broad diffraction peak at 2θ = 10°–30° corresponding to the scattering of amorphous polymeric shell was observed (Figure ).

Thermal Analysis

In Figure , the thermogravimetry analysis (TGA) curve is indicated to evaluate the thermal stability of the synthesized compounds and the percentages of the organic component in the TiO2–MAPTMS and TiO2/poly(Am-co-MBA-co-NPhM) nanocomposites. As can be seen from Figure , the thermogram for TiO2 NPs showed that these NPs are stable over 25–1000 °C. The weight loss within the temperature range of 550–700 °C in Figure b is believed to determine the percentage of conjugated MAPTMS on the TiO2 NPs surface. In contrast, the TGA curve for TiO2/poly(Am-co-MBA-co-NPhM) nanocomposites showed two weight loss steps. The first stage below 200 °C possibly is due to the elimination of moisture in the sample. The major weight loss from 250–750 °C (approximately 72%) is the step that could be attributed to the percentage of polymeric shell in the final nanocomposite (Figure c).

Figure 3

TGA curves of TiO2 NPs, TiO2–MAPTMS nanocomposites, and TiO2/poly(Am-co-MBA-co-NPhM) nanocomposites.

Scanning Electron Microscopy Images

The uniform spherical morphology of TiO2 NPs and the TiO2/poly(Am-co-MBA-co-NPhM) nanocomposite is evident from Figure a,b. The scanning electron microscopy (SEM) observation indicates the thickness of the ensuing product is about 20 nm (Figure a) and 120 nm (Figure b), respectively. Clearly, the product is roughly spherical in geometry and has a smooth surface morphology with quite a very narrow size distribution. The results showed that grafting MAPTMS onto TiO2 NPs and polymerization of monomers, as a shell on NPs, were successfully accomplished.

Figure 4

SEM images of TiO2 NPs (a) and TiO2/poly(Am-co-MBA-co-NPhM) nanocomposite (b).

Transmission Electron Microscopy Images

Polymer grafting on TiO2 NPs and the effect of polymeric shell on the size of nanocomposite were investigated using transmission electron microscopy (TEM) analysis. Figure shows TEM images of pristine TiO2 and polymeric nanocomposite which confirms that the grafting of the polymeric shell on the TiO2 NPs surface has taken place. The mean size of TiO2 NPs and polymeric shell on surface of TiO2 NPs are 20 and 120 nm approximately, respectively.

Figure 5

TEM images of TiO2 NPs (a) and TiO2/poly(Am-co-MBA-co-NPhM) nanocomposite (b).

Naked-Eye Colorimetric Detection

Naked-eye detection of fluoride ions was performed using a colorimetric sensor in an organic medium of acetonitrile (ACN) at a low concentration level of about (3 μM). The visual detection of fluoride ions was carried out in a wide concentration range of these ions (3–600 μM), and with increasing the concentration of fluoride ions, the color changed from pale yellow to orange. The obtained results demonstrated that the features of the TiO2/poly(Am-co-MBA-co-NPhM) nanocomposite optical sensor compared to the other spectroscopic methods of ion detection, are simplicity and high level of sensitivity. The described method is based on selective interactions between the hydrogen-bond donor moiety and fluoride ions (creating a hydrogen bonding between the proton of chemosensor and fluoride ions), thus eliminating the requirement of sophisticated instruments.

Addition of excess tetrabutylammonium fluoride, a base known to promote a second deprotonation of 4-nitrophenyl thiourea, resulted in the formation of an orange color attributable to the monoanion (Figure ).

Figure 6

Chemosensor color changes with different concentrations of fluoride ion (3–600 μM).

Investigating Chemosensors Anion Interaction

The visual response of the chemosensor to various anions such as F–, Br–, Cl–, I–, HSO4–, H2PO4–, and AcO– was investigated using colorimetric analysis. By adding aforementioned anions to receptor solutions of chemosensor with a 2:1 ratio, a significant color change from pale yellow to orange was immediately observed exclusively in the presence of fluoride ions. On the other hand, the addition of OAc2 ions similarly led to color transformation from pale yellow to orange. Therefore, the proposed method suggested a clear technique to detect fluoride and acetate ions via a naked-eye route. The change in color may be due to the deprotonation of NH protons in the ligand using ions, which affects the electronic properties of the chromophore and ensures a strong color alteration (Figure ).

Figure 7

Colorimetric response of chemosensor in the presence of various anions; color changes in ACN solution (150 μM).

UV–Visible Investigation of Chemosensor

Regarding the results of colorimetric analysis, UV–visible spectroscopy studies were performed upon interactions with acetate and fluoride ions at 150 μM concentration level. Adding fluoride ions to a chemosensor in the organic medium showed a new peak at a longer wavelength of 485 nm, while the addition of other ions did not exhibit any change in the chemosensor spectrum, as shown in Figure . The F– ion showed the highest absorbance in the 485 nm region, because of its alkaline nature in organic solvents, which combines with itself as [HF2]−, leading to the deprotonation of hydrogen bonding agents, that in most cases is accompanied by color changes. After fluoride, acetate ions showed the highest changes in absorption which is because two oxygen groups of acetate, due to their proper spatial structure, can interact with both NH groups of thiourea and as a result cause changes in absorption.

Figure 8

Absorption spectra of chemosensor upon the addition of 2 equiv of fluoride ions, Cl–, Br–, I–, H2PO4–, HSO4–, and AcO– ions in ACN solution (150 μM).

Titration of F– Ions

It was further observed that gradual addition of a standard solution of F– ions (TBAF) to a 150 μM solution of chemosensor in ACN, resulted in progressive increase in the absorption band at 480 nm in the UV–visible spectra with a simultaneous decrease in the absorption band at 330 nm. This was accompanied by the appearance of an isosbestic point at 389 nm (Figure ).

Figure 9

UV–visible titration spectra of chemosensor (150 μM) with 0.5–4 equiv of TBAF in ACN solution.

When fluoride ion concentration increased to the level of 1 equiv with respect to the thiourea moiety, a color change from pale yellow to orange was discerned in the probe solution, and this color became deeper on further addition of fluoride ions with concomitant increase in the intensity of absorption band at 485 nm. The observed colorimetric changes in probe solutions reached its limiting value with the addition of 2 equiv of fluoride ions.

A more detailed investigation, obtained from UV–visible titration results, also indicates that the limiting value in the λmax centered at 485 nm is achieved using 2 equiv of fluoride ions. This observation could be rationalized on the premise that the initial addition of fluoride ions establishes a hydrogen bond interaction between these ions and NH groups proton of probe through 1:1 hydrogen-bonded adduct formation. If further addition of fluoride ions beyond 1 equiv continues, the first hydrogen bonded fluoride ions cooperate to bind the second ones which leads to the formation of 1:2 hydrogen bonds between the probe and fluoride ions. Subsequently, in this ratio the deprotonation occurring at concentration levels higher than 2 equiv of fluoride ions, causes the formation of a more stable bi fluoride [HF2]− species.[45,46] Experimental observations indicate that the appearance of a new absorption band at higher wavelengths (485 nm) during probe–fluoride interaction could be either due to the formation of hydrogen bonds with the NH groups of chemosensor or its deprotonation by fluoride ions. Both these events would result in a visual color change possibly through efficient intramolecular charge transfer.[47]

Conclusions

In summary, a new simple and easy-to-use colorimetric sensor based on a hybrid of organic and inorganic material comprising 3-(4-nitro-phenyl)thioureido moieties has been designed and synthesized through free-radical polymerization. Its selectively results in the recognition of F– ions among other anions which was achieved through the color transformation from pale yellow to orange, as well as UV–visible spectroscopy with the appearance of a new absorption peak at a longer wavelength of 485 nm. The fluoride ion was detected through the formation of hydrogen bonds between NH groups of nanocomposites and F– ions, followed by the deprotonation process of thiourea groups of the chemosensor at concentration levels beyond 2 equiv of F– ions. The detection limit of colorimetric sensor is 3 μM for fluoride ions which indicates its high level of sensitivity.

Experimental Section

Reagents and Materials

1-Isothiocyanato-4-nitrobenzene, ethanolamine, triethylamine (TEA), methacryloyl chloride, ammonium(II) sulfate, MAPTMS, titanium(IV) chloride, ammonium hydroxide (25–30%), acrylamide (AM), MBA, azobisisobutyronitrile (AIBN), tetra-n-butylammonium F–, Cl–, Br–, I–, H2PO4–, HSO4–, AcO–, and CN–, dimethyl sulfoxide (DMSO), 2-ethylhexanol, tetrahydrofuran (THF), dichloromethane (DCM), ACN, sodium bicarbonate, magnesium sulfate, hexane, ethyl acetate, and ethanol were all purchased from reputable companies. They all were of analytical grade and used as received without further pretreatment.

TiO2 NPs Synthesis

In order to synthesize TiO2 NPs, a solution of ammonium sulfate (1.5 mol L–1) and titanium(IV) chloride (0.75 mol L–1) was used which was heated up to 75 °C and then was kept at this constant temperature for 90 min. Under high-speed stirring, in the next step, ammonium hydroxide (2.5 mol L–1) was added to the above solution (at pH = 7). The insoluble sediment ensued as the precipitated product which was then filtrated and washed with a mixture of water/ethanol and dried at 60 °C. The temperature used to calcine the sample was 450 °C (4 h), and then, it was set aside to cool down slowly at ambient temperature.[39]

TiO2–MAPTMS Nanocomposite Preparation

Half a gram of TiO2 NPs was dissolved in 20 mL of toluene to prepare TiO2–MAPTMS NPs. The mixture was placed in an ultrasonic bath for 15 min followed by the addition of 1.5 mL of MAPTMS, then atmospheric nitrogen was applied to keep the solution for 24 h at room temperature while being continuously stirred. The final product was separated by centrifuging, and toluene was used to wash it twice; then, TiO2–MAPTMS nanocomposite was dried under vacuum at 60 °C for 24 h.

Monomer Synthesis

1-(2-Hydroxyethyl)-3-(4-nitro-phenyl)thioureido Synthesis

In a flask, 1 g (5.55 mmol, 1 equiv) of 1-iso-thiocyanato-4-nitrobenzene was dissolved in 20 mL of THF solution. Subsequently, 369 μL (6.10 mmol, 1.1 equiv) of ethanolamine was added dropwise to the stirring solution, and the reaction was kept at ambient temperature for 12 h. At the end of the reaction, the solvent and unreacted ethanolamine were separated by a rotary evaporator affording a pale-yellow solid (95% yield) (Figures S1 and S2 in the Supporting Information).

NPhM Synthesis

In a flask, 1.43 g (5.92 mmol, 1 equiv) of product was dissolved in 25 mL of methylene dichloride (DCM) and was then added to a well-stirred mixture of 998.4 μL (7.10 mmol, 1.2 equiv) of dry TEA. At 0 °C, 582 μL (5.92 mmol, 1 equiv) of methacryloyl chloride, in 20 mL dried DCM, was added dropwise to the reaction solution during a period of 30 min (Figures S3 and S4 in the Supporting Information).

Polymerization Step

To cover the surface of vinyl-modified TiO2 NPs with a polymeric layer of monomers AM (0.5 g), MBA (1.2 g), NPhM (0.5 g), and vinyl-modified TiO2 (0.95 g) were all stirred in a solution of 17 mL DMSO/2-ethyl-1-hexanol (with 2:5 ratio) at ambient temperature for 1 h. Subsequently, AIBN (0.15 g) was added under the nitrogen gas environment for 15 min and then refluxed for 5 h at 80 °C. The synthesized polymer was obtained by removal of the solvent and then washed three times with ethanol and water. Finally, it was dried at 50 °C for 24 h (Scheme ).

Scheme 2

Chemosensors Synthesis Stages

Absorption Measurement and Colorimetric Detection of Fluoride Anions by a Chemosensor

In order to measure the detection limit of fluoride anion by chemosensor, the various concentrations (1, 3, 36, 75, 150, 300, 600 μM) were prepared in 2 mL of ACN. Then, each of the prepared solution of anion fluoride was added to a 20 mg of sensor, and the detection limit of anion fluoride was sensed by 3 μM sensor.

Ion-Selective Sensing System

In order to evaluate the selectivity of chemosensor for fluoride anions, each of the anions F–, Cl–, Br–, I–, HSO4–, H2PO4–, and OAc– (solution of 150 μm in 2 mL) was prepared. Each of the prepared solutions was separately added to 20 mg of chemosensor, and the color changes of chemosensor were examined by naked eye detection (Figure ) and a UV–vis spectrophotometer (Figure ).

Instruments

Optical sensor spectroscopy measurements were performed using a solid-state 2100-UV-vis Shimadzu spectrometer. SEM is an effective method for investigating the morphological characteristics of NPs; thus, it was evaluated using Philips XL-30 SEM. TEM images of TiO2 NPs and polymeric nanocomposites were obtained using a Philips CM-300 microscope. TGA and studying thermal stability of the synthesized materials were carried out using TGA/DTA BAHR: STA503. Patterns of XRD were obtained using Siemens D5000 diffractometer, and a 2θ scan was done in the range of 2θ = 10°–80° with monochromatic Cu Kα (λ = 1.54060 A). The FT-IR measurements were performed using a BOMEM MB-Series FT-IR spectrometer in the form of KBr pellets. On a Bruker, 300 and 500 advance instruments in CDCl3 and DMSO; δ in ppm, J in Hz, H and 13C NMR spectra were obtained.