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Sunlight Induced Preparation of Functionalized Gold Nanoparticles as Recyclable Colorimetric Dual Sensor for Aluminium and Fluoride in Water Anshu Kumar, Madhuri Bhatt, Gaurav Vyas, Shreya Bhatt, and Parimal Paul ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 04 May 2017 Downloaded from http://pubs.acs.org on May 4, 2017

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ACS Applied Materials & Interfaces

Sunlight Induced Preparation of Functionalized Gold Nanoparticles as Recyclable Colorimetric Dual Sensor for Aluminium and Fluoride in Water Anshu Kumar,ab Madhuri Bhatt,ab Gaurav Vyas,ab Shreya Bhatt ab and Parimal Paul*ab a

Analytical Division and Centralized Instrument Facility, CSIR-Central Salt and Marine

Chemicals Research Institute, G. B. Marg, Bhavnagar 364 002, India b

Academy of Scientific and Innovative Research, Central Salt and Marine Chemicals

Research Institute, G.B. Marg, Bhavnagar 364 002, India

Abstract: Sunlight induced a simple green route has been developed for the synthesis of polyacrylate functionalized gold nanoparticles (PAA-AuNPs), in which poly-acrylic acid functions as reducing as well as stabilizing agent. This material has been characterized on the basis of spectroscopic and microscopic studies, it exhibited selective colorimetric detection of Al3+ in aqueous media and the Al3+ induced aggregated PAA-AuNPs exhibited detection of F- with sharp colour change and high selectivity and sensitivity out of a large number of metal ions and anions tested. The mechanistic study revealed that for Al3+, the colour change is due to shift of the SPR band because of the Al3+ induced aggregation of PAA-AuNPs., whereas for F-, the reverse colour change (blue to red) with returning of SPR band to its original position is due to dispersion of aggregated PAA-AuNPs, as F- removes Al3+ from the aggregated species by complex formation. Only concentration dependent fluoride ion can prevent Al3+ for making aggregation of PAA-AuNPs. The method is successfully used for the detection of Fin water collected from various sources by spiking method, in toothpaste of different brands by direct method. The solid Al3+-PAA-AuNPs was isolated, adsorbed on ZIF@8 (Zeolitic

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Imidazolate Framework) and also on cotton strip and applied as solid sensing material for detection of F- in aqueous media. Keywords: Gold nanoparticles, sunlight induced preparation, colorimetric sensor, fluoride, alluminium, recyclable nanosensor. 1. INTRODUCTION The development of highly selective and sensitive simple analytical methods for detection of trace amount of fluoride in water is of significant interest because of their direct impacts on the human health and plays an important role in environmental allied aspects.1 Fluoride also plays important role in biological and medical processes and in military applications. As medical application, it used to added to tooth paste, tap water as a medicine against tooth decay, and osteoporosis.2,3 For military application such as refinement of uranium in nuclear weapons and military nerve gas, fluoride is used.4,5 On the other side, excessive consumption of fluoride can cause diseases such as osteoporosis, urolithiasis, fluorosis, neurological, metabolic dysfunctions and even cancer.6-8 Groundwater used to contaminate with fluoride due to dissolution of natural minerals in the rocks and soils with which water interacts, it may be beneficial if it’s concentration is within the permissible limit, however higher concentration creates a major problem in safe drinking water supply. This problem is growing in day by day, as noted by a number of surveys that have been undertaken to assess the groundwater quality.9 The U.S. Environmental Protection Agency (EPA) has set a maximum contaminant level (MCL) of 4.0 mg L−1 (4 ppm) in drinking water.10 However, the Indian agency recommend an acceptable fluoride concentration of 1.0 mg L-1.11 At present, fluoride contaminated water which is above this level regularly drink by many people all over the world due to lack of awareness and simple method of its detection. For detection of fluoride, standard methods such as ion-selective electrode and ion


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chromatography has been recommended by the World Health Organization.12-15 However, both the techniques are difficult to promote for practical use in villages and remote areas. As alternative methods, colorimetric detection using various sensing materials is growing rapidly.16 Among the colorimetric based sensing materials, metal nanoparticles have attracted attention due to their extremely high visible-region extinction coefficients (1 × 108 to 1 × 1010 M-1 cm-1), which is several orders of magnitude higher than that of organic dyes.17 Among them, both gold (AuNPs) and silver nanoparticles (AgNPs) are of particular interest as the detection method based on the monitoring of the color change, either by nacked eyes or by UV-vis spectral change due to the aggregation/corrosion, is simple and rapid, due to unique surface plasmon resonance (SPR).18-23 AuNPs based sensor has been widely applied in the colorimetric detection of heavy metal ions and bioactive molecules but not many reports are there for anions.24-27 There are few reports on colorimetric detection of anions such as iodide and cyanide using modified/hybrid nanoparticles such as CN−/Ag+-Au NPs/MCEM,28 AuNPs-membrane modified cellulose,29 Au@Ag core/shell nanoparticles,30 citrate-stabilized core/shell Cu@Au nanoparticles.31 Kalluri and co-workers used dithiothreitol (DTT), and cysteine (Cys) modified gold-nanoparticle for dynamic light scattering (DLS) assay for selective detection of arsenic in groundwater.32 For colorimtric and fluorometric detection of fluoride, a few AuNPs functionalized/decorated by various receptors have also been reported.33-38 In the present study, water despercible poly-acrylic acid (PAA) functionalized AuNPs, which functions as highly selective colorimeric sensor for Al3+ as well as F- in water out of a large number of metal ions and anions tested is reported. There are some reports, where functionalized AuNPs function as colorimetric sensor for Al3+ due to metal induced aggregation of nanoparticles.39-43 In the present study, PAA-AuNPs did not show any colour change when F- was added, however when it is added into the Al3+ induced agrregated 3 ACS Paragon Plus Environment


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solution, rapid colour change was observed, which was not noted for other anions tested. PAA-AuNPs, therefore functions as dual colorimetric sensor for Al3+ and F-, it can also be used as reversible and recycleable sensing material for both Al3+ and F-. The entire functions are summarized and submitted as ESI (Scheme S-1). Herein we report solar aided synthesis of PAA-AuNPs in water and its application as colorimetric dual sensor for Al3+ and F- and its recyclibility. 2. EXPERIMENTAL SECTION 2.1. Materials. Hydrogen tetrachloroaurate (HAuCl4·3H2O), poly-acrylic acid (PAA), HEPES buffer and sodium phosphate monobasic were purchased from Sigma-Aldrich. All metal perchlorate salts were purchased from Alfa Aesar. Tetrabutylammonium (TBA) salts of chloride, bromide, iodide, sulphate, acetate, nitrate, nitrile, thiocyanide, sodium azide, and PCl3- were purchased from Alfa Aesar and sodium fluoride and aluminium sulphate were purchased from CDH-INDIA and used without further purification. 2.2 Methods. UV−vis absorption spectra were recorded on a Varian model CARY 500 spectrophotometer. Transmission electron microscopy (TEM) measurements were conducted on a JEOL, model JEM 2100 transmission electron microscope, elemental analysis by EDX (INCA, Japan) and powder XRD was recorded on a model Empyrean powder XRD (Cu-Kα radiation) supplied by PanAnalytical, Netherland. 2.3. Preparation of Polyacrylate functionalized gold nanoparticle (PAA-AuNPs). Polyacrylic acid stabilized gold nanoparticles were prepared by the poly-acrylic acid mediated reduction of HAuCl4 with exposure to solar radiation. In a typical procedure, the mixture of an aqueous solution of 0.3 mM HAuCl4 (100 mL) and (0.05-0.5) % (w/v) poly-acrylic acid solution was kept at sunlight for 120 min. During exposure to solar radiation, the colloidal solution changed to purple-black in 5 min and then turned to red after 120 min, which in turn results in the characteristic SPR absorbance at 530 nm representative of gold nanoparticles.44 4 ACS Paragon Plus Environment

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For the purification of PAA-AuNPs, following sonication, the PAA functionalized AuNPs solution was centrifuged at 15,000 rpm for 15 minutes and the supernatant was carefully removed and resultant pellets of PAA-AuNPs, deposited at the bottom of the centrifuge tube, was re-dispersed in water. The particle concentration of PAA-AuNPs (ca.11.25 nM) was determined according to Beer’s law using the molar extinction coefficient of ca. 1 × 108 M1

cm-1 at 530 nm for AuNPs of 7 nm diameter, as found in TEM image (discussed later).

2.4. Detection of Al3+. Typically, 10 mL of PAA-AuNPs suspension was diluted with 30 mL of deionised water to give a total volume of 40 mL as a stock solution for the detection of Al3+. To evaluate the sensitivity toward Al3+, different concentrations of Al3+ (0-300 µM) were added to the solution of PAA-AuNPs suspension and the mixed solutions were equilibrated for 10 min before spectral measurement. To investigate the selectivity to Al3+, the same amount of PAA-AuNPs solution but with different metal ion solutions were mixed, equilibrated for 10 min and then UV-vis spectra of the resultant solutions were recorded. 2.5. Detection of fluoride. Typically, 10 mL of PAA-AuNPs suspension was diluted with 30 mL of deionised water to give a total volume of 40 mL as a stock solution for the detection of fluoride. The sensitivity of PAA-AuNPs suspension to fluoride was further optimized with Al3+. Different amount of fluoride (0-500 µM) and optimized amount (140 µM) of Al3+ was added into 2 mL of the above PAA-AuNPs suspension. The colour change was noted and the UV-vis spectra were recorded. The masking kinetics of PAA-AuNPs in presence of fluoride and Al3+ was obtained by the measurements of UV-vis spectra at the interval of 1 min. To investigate the selectivity towards fluoride, the same amount of PAA-AuNPs solution but with different anion solutions were mixed and equilibrated for 10 min before UV-vis spectra were recorded using spectrophotometer.



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Figure 1. (a) Schematic presentation for synthesis of PAA-AuNPs under sunlight (b) TEM and Uv-vis spectra of PAA-AuNPs (inserted image). 3. RESULT AND DISCUSSION 3.1. Synthesis and characterization of PAA-AuNPs. The simple greenery route followed for the synthesis of poly-acrylic acid stabilized gold nanoparticles is shown in Figure 1(a) and optimized concentration of poly-acrylic acid for synthesis of PAA-AuNPs is shown in Figure S-1 and S-2 (SI). 3.2. Stability of PAA-AuNPs. The stability of the PAA-AuNPs in aqueous media with variation in pH has been studied. The UV-vis spectra of PAA-AuNPs in aqueous media was recorded at various pH in the range of 2.0 to 8.0 and the corresponding zeta potential (ζ) was also measured. A plot of the ratio of absorbance at 630 to 530 nm and the corresponding zeta potential, plotted as a function of pH, is shown in Figure S-3 (SI). Colloidal particles having ζ value higher than +30 mV or lower than -30 mV are generally regarded as stable because of the strong electrostatic repulsion between the particles, which prevents them from aggregation.45 The plot in Figure S-3 (SI) suggests that pH higher than 6.0 were conducive to stable PAA-AuNPs in solution, whereas at a pH of 5.0 and lower PAA-AuNPs possessed low


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ζ values, in which range the carboxylic groups of PAA may exist in its neutral form, resulting in surface interaction destabilizing dispersed PAA-AuNPs particles in solution. 3.3. Optimization of the responsive condition of PAA-AuNPs toward Al3+ and fluoride in solution. Since the PAA-AuNPs are stable in the pH range higher than 6.0, therefore detection of fluoride was carried out at pH 7.5. In presence of Al3+ (100 µM), PAA-AuNPs displayed a red-shift of the absorption maxima at pH 7.5 (Figure S-4) due to aggregation, which continued with time (Figure 2a). A plot of the ratio of the absorbance intensity of the growing band to the original band, A630/A530, as a function of time exhibited that the ratio reached to a

Figure 2. Time-course measurements of (a) UV-vis spectra of PAA-AuNPs with Al3+ (100.0 µM), photographic image (inserted) and (b) A630/A530 for PAA-AuNPs after the addition of Al3+ (100.0 µM), photographic image (inserted) (c) ) UV-vis spectra aggregated PAA-AuNPs in presence of fluoride (200 µM) and (d1) A630/A530 for aggregated Al3+ (100.0 µM)@PAAAuNPs after the addition of fluoride (200 µM) and (d2) PAA-AuNPs with F- (200.0 µM) solution in presence of Al3+ (100.0 µM). 7 ACS Paragon Plus Environment


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maximum value after about 10 min and remained almost constant after that till the measurement was continued (15 min, Figure 2b). The optimum condition for the detection of Al3+ was therefore set at pH 7.5 with 10 min incubation time. For detection of fluoride, 200.0 µM solution of the anion was added into the aggregated PAA-AuNPs solution and the UV-vis spectral change with time was recorded for 15 min (Figure 2c). Then the ratio of the absorbance intensity, A630/A530, as a function of time was plotted (Figure 2d1) and the plot suggests that the intensity ratio reached to a minimum value after about 10 min and then it remained almost constant till the end of the measurement (15 min), indicating that 10 min incubation time is sufficient for fluoride to complete its interaction with the Al3+ induced aggregated PAA-AuNPs particles. Another experiment was carried out in which F- (200.0 µM) was added first into PAA-AuNPs and then Al3+ (100.0 µM) was added and UV-vis spectra of the resulting solution was recorded with time for 15 min. There was no change in absorption spectra, and the plot A630/A530 as a function of time (Figure 2d2) indicates that no Al3+ induced aggregation

of PAA-AuNPs takes place in presence of fluoride ion. Therefore, pH 7.5 and incubation time of 10 min was chosen as experimental conditions for all detection study. 3.4. Selectivity study of PAA-AuNPs towards different metal ions The selectivity of PAA-AuNPs towards various metal ions and anions (Hg2+, Cd2+, Fe3+, Cu2+, Zn2+, Pb2+, Co2+, Ni2+, Ca2+, Mg2+, Cr3+, Al3+, Sr2+, Ba2+, F-, Cl-, Br-, I-, CN-, SO4-, AcO-, NO3, NO2-, SCN- , N3-, [Cr2O7]2-, PCl3- and H2PO4-) has been examined by recording UV-vis spectra of the aqueous dispersion of PAA-AuNPs nanoparticles upon addition of the solutions of the metal ions and anions (30.0 µM) at 7.5 pH with 10 min incubation time. The UV-vis spectral change (Figure 3a), corresponding colour of the solution (Figure 3b), and the plot of A630/A530 for different ions (Figure 3c) are shown in Figure 3. It may be noted that only Al3+ induced colour change of PAA-AuNPs from red to purple with substantial enhancement of A630/A530 value and for other metal ions, no significant changes is noted. This spectral and


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Figure 3. (a) the UV-vis spectral changes of PAA-AuNPs upon addition of metal ions and anions, (b) corresponding colour changes, (c) plot of A630/A530 for different metal ions and anions (30.0 µM), and (d) TEM images of PAA-AuNPs solution in absence (left) and presence of Al3+ (inserted photographic image of respectively solution. colour change for Al3+ is due to metal induced aggregation of PAA-AuNPs, as evident from TEM images, shown in Figure 3d. The PAA-AuNPs, therefore selectively recognise Al3+ out of a large number of metal ions and anions tested. 3.5. Determination of limit of detection (LOD) for Al3+ To evaluate the limit of detection (LOD), the UV-vis spectra of the solution of PAA-AuNPs was recorded with incremental addition of Al3+ with concentrations ranging from (0 to 300) 9 ACS Paragon Plus Environment


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(a)

(b)

Figure 4. (a) UV-vis spectra of PAA-AuNPs suspension in the presence of different Al3+ concentrations: 0-300 µM and photographic image of color change of PAA-AuNPs suspension in the presence of different Al3+ concentrations: 0-300 µM (Inserted) and (b) calibration curve of A630/A530 vs. Al3+ concentration: 0-300 µM in the PAA-AuNPs suspension. The linear dependence of A630/A530 on the concentration of Al3+ is also shown (inserted). µM. The spectral change, plot of A630/A530 as a function of concentration of Al3+ and corresponding colour changes are shown in Figure 4. From Figure 4b, it appears that the enhancement of A630/A530 is linear in the concentration range 50 to 150 µM (R2 = 0.992) with lower detection limit (LOD) of 2 µM. However, visually the colour change can be detected by bare eye from 8 µM. 3.6. Detection of fluoride Fluoride was detected with the aid of PAA-AuNPs and Al3+. A series of solutions were prepared by adding 100 µM of 14 different anions (F-, Cl-, Br-, I-, CN-, SO4-, AcO-, NO3-, NO2-, SCN- , N3-, [Cr2O7]2-, PCl3-, H2PO4-) into the aqueous dispersion of PAA-AuNPs and then 80 µM Al3+ solution was added into all of these solutions. The UV-vis spectra of the resulting solutions were recorded (Figure 5b), the values of the A630/A530 for different anions


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Figure 5. Graphical mechanism (a), UV-visible absorption of PAA-AuNPs in presence of different kinds of anions (100 µM) solution treated with Al3+ (80 µM) (b), Value of A630/A530 (c), and photographic images (d) of PAA-AuNPs in the presence of different anions (100.0 µM) solution treated with Al3+ (80 µM) at pH 7.5. were plotted (Figure 5c), corresponding colours were noted (Figure 5d) and all these results are displayed in Figure 5. From the figures, it is clear that for F-, there is no spectral and colour change, obviously no enhancement of A630/A530 is noted. For other anions, distinct changes in UV-vis spectra as well as colour of the solution and substantial enhancement of A630/A530 are noted. The results indicate that F- prevents Al3+ induced aggregation of PAAAuNPs, whereas aggregation and colour change took place for all other anions. It has been also investigated by TEM analysis, the TEM images and the corresponding colour of PAAAuNPs solutions recorded in presence of Al3+ (Figure 6a) as well as Al3+ and F- (Figure 6b),



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Figure 6. TEM images of PAA-AuNPs solution in the (a) presence of Al3+ (80.0 µM) and (b) presence of Al3+ ions (80.0 µM) treated with F- (100 µM) at pH 7.0 with inserted photographic image. shown in Figure 6, clearly exhibited that aggregation and colour change occurred in presence of only Al3+. 3.7. Determination of limit of detection (LOD) for FFor the detection of LOD for F-, initially optimized concentration of Al3+ (140 µM ) was added into the PAA-AuNPs and after incubation time of 10 min, the UV-vis spectrum was recorded. Then F- of different concentrations (0-500 µM) was added into the aggregated Al3+PAA-AuNPs solution and after 10 min incubation time UV-vis spectra of the resulting solutions were recorded. The process of anti-aggregation of Al3+-PAA-AuNPs induced by fluoride was evident from the spectral change (Figure 7b), the λmax at 560 nm of the aggregated solution started moving towards 530 nm, the λmax due to dispersed PAA-AuNPs and it was also evident from gradual colour change of the solution (Figure 7c). For quantification, the ratio of A630/A530 was plotted against the concentration of fluoride (0-500 µM) and from the plot (Figure 7c), it is clear that the value of A630/A530 started decreasing when the concentration of F- is 18 µM, which is considered as the limit of detection (LOD) and visually the colour change can be detected by bare eye from 100 µM. It is also observed that in the concentration range 30-200 µM of F-, the decrease in A630/A530 values are almost 12 ACS Paragon Plus Environment

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Figure 7. (a) Graphical mechanism, (b) UV-vis absorption spectra of PAA-AuNPs before and after addition of different concentrations of F- treated Al3+ (140 µM), (c) responses of A630/A530 as a function of the concentration of different fluoride ion, respectively and (d) photographic image for naked eye detection of fluoride. linear (R2=0.9915), therefore quantification of F- can be done if its concentration is within this range. The LOD values of some recent reports for Al3+ and F- determined by colorimetric method using functionalized AuNPs are summarized in Table 1. The LOD value for F- of the present study is comparable or better than some of the reports and for Al3+, the LOD value of the present study is close to a number of reports. 3.8. Regeneration of PAA-AuNPs after use for F- detection It is known that Al3+ binds F- strongly forming stable complex, in this case upon addition of F-



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Table 1. Recently reported LOD values for Al3+ and F- of some of the systems determined by colorimetric method using functionalized AuNPs are summarized. Method

AuNPs Functionalized with F-

LOD of F-/ Al3+

Solvent

Ref.

Colorimetric

Thioglucose-

20 mM

Water

33

Colorimetric

agglomeration

120 µM

water

34

Colorimetric

AuNPs

10 µM

Organic 35 solvent

SR-PRET

Brightening AuNPs Al3+

0.072 nM

Water

38

Colorimetric

Citrate

1 µM

Water

39

Colorimetric

Pyridoxal derivative

0.51 µM

Water

40

Colorimetric

Ionic liquid

1 µM

Water

41

Colorimetric

Schiff base

0.29 µM

Water

42

Fluorometric

Silver−Gold Alloy NCs

0.8 µM

Water

43

Colorimetric

PAA

Al3+=2 µM F-=18 µM

Water

This Work

into the aggregated Al3+-PAA-AuNPs, the F- ion in presence of Na+ (from NaF) forms Na3AlF6 complex (characterized by XRD, discussed below), which remains in solution regenerating PAA-AuNPs.46 The regenerated PAA-AuNPs were then separated by centrifugation; the supernatant liquid containing Na3AlF6 was separated by decanting leaving the red coloured solid of PAA-AuNPs at the bottom of the centrifuge tube. The PAA-AuNPs thus obtained was dispersed in aqueous media for further use as sensing material and this process was repeated for three cycles, as shown in Figure 8. 3.9. Mechanistic study A schematic presentation of the mechanism of the sensing and regeneration process of PAA14 ACS Paragon Plus Environment

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Figure 8. (A) Scheme of regeneration of PAA-AuNPs and (B) Responses of A630/A530 as a function of the number of regeneration performance of PAA-AuNPs is shown. AuNPs is shown in Figure 9. The AuNPs were functionalized and stabilized with polyacrylic acid, which during reduction of gold has converted to polyacrylate and absorbed onto the surface of the nanoparticles (Figure 9a). After anchoring onto the gold surface, the acid groups interact with Al3+, as shown in Figure 9b, leading to the aggregation of PAA-AuNPs and thereby colour change of the solution to blue due to shifting of SPR band. After addition of F-, the anion has taken away Al3+ forming complex leading to the dispersion of PAAAuNPs, which can be reused as sensing material (Figure 9c). The F- complex formed (Na3AlF6) in the process was isolated and its powder XRD pattern was recorded and the diffractogram is compared with that of AuNPs and Na3AlF6@PAA-AuNPs (Figure 10a) and the pattern exhibits characteristic peaks for PAA-AuNPs and Na3AlF6. The (111), (200), and (220) planes of cubic gold (JPCD: 00-001-1172) and (220 and 004), (211 and -211), (112), (110), (101) and (111 and -111) plane of mmonoclinic Na3AlF6 (JPCD: 04-014-3569) are visible in the Na3AlF6@PAA-AuNPs diffraction pattern. The formation of Al3+ induced aggregation of PAA-AuNPs and Na3AlF6 is further confirmed by TEM and EDX analysis.



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Figure 9. Schematic mechanism by PAA-AuNPs (a), for sensing Al3+ (b), regeneration of sensor (c), sensing of F- (d).

Figure 10. (a) Powder XRD spectra of Na3AlF6@AuNPs, (b) TEM image of aggregated Al3+PAA-AuNPs solution, (c) EDX of aggregated Al3+-PAA-AuNPs solution, (d) TEM image of after addition fluoride aggregated Al3+-PAA-AuNPs solution and (e) EDX of after addition fluoride aggregated Al3+-PAA-AuNPs solution. 16 ACS Paragon Plus Environment

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The TEM images, shown in Figure 10(b) and (c), clearly exhibited the formation of aggregation and dispersion of PAA-AuNPs in presence of Al3+ before and after addition of F-, respectively and the EDX analysis of the same material confirmed the presence of Al3+ and F-, which supports the mechanism. This was also confirmed by HR-TEM analysis, which showed highly crystalline Na3AlF6 with d-spacing of ca.0.19, 0.23, 0.34, 0.38, and 0.42 nm (Figure 11a) corresponding to the (220 and 004), (211 and -211), (111), (110), and (101) planes of Na3AlF6, calculated using the Bragg equation. This was also confirmed by electron diffraction (111), (211), (112) and (220), as shown in Figure 11b.

Figure 11. HRTEM images of Na3AlF6, isolated from Na3AlF6@AuNPs: (a) lattice fringe pattern (b) Electron diffraction pattern. 4. APPLICATIONS The UV-vis data recorded after addition of standard solution of F- of different concentrations. In a typical experiment, Al3+ induced aggregated PAA-AuNPs samples of ground water were interfered by spiked with standard solutions containing various concentrations of fluoride (0– 500 µM) and UV-vis spectra of the solutions were recorded after 10 min incubation time (Figure S-5a, SI), then the values of A630/A530 was ploted as a function of concentration of F-, which leads to a straight line (R2 = 0.9927), as shown as inset of Figure S-5a (SI) and colour indentification as a function of concentration of F-, as shown in Figure S-5b (SI). 4.1. Estimation of fluoride in real samples, water and toothpaste 17 ACS Paragon Plus Environment


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As real sample, water was collected from the tap in the laboratory, river, drinking water bottle (commercial) and ground water source and these water samples were spiked with a standard solution (1 mM) of fluoride. The concentration of F- in these solutions were then estimated by ion selective electrode (ISE) and by PAA-AuNPs, the method reported here (Figure S-6). The results obtained (Table 2) are in good agreement with the added amount, which validates the method developed. The method was further applied to estimate F- in toothpaste of different

Table 2. Determination of concentration of F- by spike method using present procedure and and their comparisons with the results obtained by direct method using ion selective electrode (ISE) Sr.

Sample

No.

1 2 3 4

Spike

Results

Result obtained using

concentration

using ISE present mathoda/ (µM)

of F- (µM)

(µM)

a

n=3

Tap Water 191

190

190.4 ± 0.2

50

49.4

50.1 ± 0.35

139

138.6

138.2 ± 0.5

88

87.3

87.8 ± 0.25

River Water Drinking Water Ground Water

brands. In a typical method, 30 mg of toothpaste was taken in 100 mL of water and stirred for 5 h at pH 8 and after that the the solution was centrifuged with 15,000 rpm for 20 minutes and then the supernant was collected for analysis. The F- content in the solutions were then directly mesured by the PAA-AuNPs based method developed using the calibration line Table 3. Fluoride content in four commercially available toothpastes and one mouthwash determined by the present method and that determined by ion selective electrode.


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Sr.

Toothpaste & Liquid Result obtained using Result obtained using

No.

Mouthwash Sample

ISE method (µM)

present mathod (µM)

1 Market toothpaste 1

186.2

186.8 ± 0.3

Market toothpaste2

201

202 ± 0.5

Market toothpaste 3

192.5

192 ± 0.46

Market toothpaste 4

190.5

191 ± 0.25

252

253.9 ± 0.95

2 3 4 5 Market liquid Mouthwash 5

generated and the results obtained is presented in Table 3 together with the results measured by ion-selective electrode (ISE) for verification. The results are in good agreement.

4.2. Application as solid sensor for detection of fluoride in water The blue coloured aggregated solution of PAA-AuNPs was adsorbed on the surface of the white ZIF@8, a zeolite based porous material, a blue colour ZIF@8-Al3+- PAA-AuNPs solid thus formed was isolated by centrifugation and the solid obtained is shown in Figure 12(a). When this blue colour solid of ZIF@8-Al3+-PAA-AuNPs was added into an aqueous solution of F- (140 µM) with stirring, the colour of the solid changed from blue to red (Figure 12b) as PAA-AuNPs was regenarated due to the formation of Al3+ and F- complex, which confirmed the presence of F- in solution. The blue solid has also been used to make cotton strip for testing of F-. The blue solid material spreaded on cotton strip/cotton buds and then ground water spiked with different



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Figure 12. (a) Al3+PAA-Au(0) are coated on the surface of ZIF@8 blue colour solid sensor for fluoride. (b) Blue colour solid sensor of ZIF@8-Al3+-PAA-AuNPs changed its colour red in presence of F- (140 µM). amount of fluoride was added onto the cotton strip or cotton buds was dipped into the solutions of F- and in both the cases the colour of the cotton strips/buds changed to red gradually with increasing the concentration of F-, as may be seen in the Figure S-7 (SI).

5. CONCLUSIONS A simple green route has been developed for synthesis of polyacrylate functionalized gold nanoparticles (PAA-AuNPs) in one step process. This PAA-AuNPs exhibited colorimetric detection of Al3+ in aqueous media with high selectivity and sensitivity out of a large number of metal ions and anions tested. Further, the Al3+ induced aggregated PAA-AuNPs selectively detects fluoride with sharp colour change at neutral pH. Therefore, the new material prepared functions as dual colorimetric sensor for Al3+ and F- in aqueous media and it is recyclable. Extensive spectroscopic and microscopic studies were carried out for characterization of this material, for sensing study and to establish mechanism of the sensing process for both the 20 ACS Paragon Plus Environment

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ions. Mechanistic study revealed that selective detection of Al3+ is triggered by Al3+ induced aggregation of PAA-AuNPs, which leads to blue shift of SPR band causing sharp colour change from red to blue. The reverse colour change upon addition of F- is because of the dispersion of aggregated PAA-AuNPs due to removal of Al3+ from the aggregated species by complexation with F-. The sensing material thus developed has been used for detection of fluoride in water collected from various sources and the results obtained are in good agreement with the calculated values. The blue coloured aggregated species, Al3+-PAAAuNPs, adsorbed in ZIF@8 and also on cotton ball, is also applied as solid sensing material for detection of F- in aqueous media. AUTHOR INFORMATION Corresponding Author *

E-mail: [email protected]

ACKNOWLEDGEMENTS CSIR-CSMCRI publication number is 028/2017. A. K. gratefully acknowledges UGC for awarding Research Fellowship. Financial support in the form of network project (CSC 0134) from CSIR, AD&CIF for analytical support and CSIR-CSMCRI for infrastructure facility are gratefully acknowledged. We thank G. R. Bhadu, Dr. D. N. Srivastava, Laiya Riddhi P. for recording TEM images and XRD data, respectively.

Supporting Information The supporting Information is available free of charge on the ACS Publications website at DOI: ………. Schematic illustration of the summary of work presented, colour change at different stages of the synthesis of AuNPs under sunlight, Uv-vis spectra of AuNPs with variation of polyacrylic acid concentration, pH-dependent changes of the absorption spectra and Zeta potential, UV21 ACS Paragon Plus Environment


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vis changes of PAA-AuNPs as a function of pH in presence of Al3+ and as a function of Fconcentration, determination of fluoride in unknown solution, and the cotton ‘test buds’ for the colorimetric detection of fluoride


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