Colorimetric and paper-based detection of lead using PVA capped silver nanoparticles: Experimental and theoretical approach

Microchemical Journal 150 (2019) 104156

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Colorimetric and paper-based detection of lead using PVA capped silver nanoparticles: Experimental and theoretical approach

T

Kamlesh Shrivasa, , Bhuneshwari Sahua, Manas Kanti Deba, , Santosh Singh Thakurb, Sushama Sahua, Ramsingh Kurreya, Tushar Kanta, Tarun Kumar Patlea, Rajendra Jangdec ⁎

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a

School of Studies in Chemistry, Pt. Ravishankar Shukla University, Raipur CG-492010, India Department of Chemistry, Guru Ghasidas Vishwavidyalaya, Koni, Bilaspur CG-495009, India c University Institute of Pharmacy, Pt. Ravishankar Shukla University, Raipur CG-492010, India b

ARTICLE INFO

ABSTRACT

Keywords: Silver nanoparticles PVA Colorimetric probe Paper based analytical devices Lead Water

We report a plasmonic colorimetric sensing strategy using polyvinyl alcohol (PVA) modified silver nanoparticles (AgNPs) and paper-based analytical devices (PADs) for selective detection of lead (Pb). Method is based on the measurement of red shift of localized surface plasmon resonance (LSPR) absorption band of AgNPs/PVA in visible region after the addition of Pb(II) using UV–Vis spectrophotometry and color intensity of PADs was recorded with Smartphone followed by the processing in ImageJ software. The mechanism of color change and red shift (∆λ) of LSPR band from 410 nm to 550 nm is due to the interaction of Pb(II) ions towards the PVA through strong ion-dipole interaction perturbing the stability AgNPs which further directed the aggregation of particles. The density functional theory (DFT) using Gaussian (C.01) program assisted by experimental data was used to elucidate the plausible mechanism for selective detection of analyte. The calibration curve gave a good linearity in the range of 20–1000 μgL−1 with limit of detection (LOD) of 8 μgL−1 by colorimetry and 50–1000 μgL−1 with LOD value of 20 μgL−1 using PADs. In addition, the results obtained with UV–Vis and PADs were compared with ICP-AES for quantitative determination of Pb(II) in different water samples. The advantages of using AgNPs/PVA as plasmonic colorimetric probe and PADs found to be simple, low cost and selective for determination of lead from surface water and industrial waste water samples.

1. Introduction Lead (Pb) is a bluish-gray metal found in the earth crust. The anthropogenic activities such as burning of fossil fuels, leaded gasoline and mining release a large quantity of waste into the environment that containing lead. Its widespread use causes the extensive contamination of soil, water, vegetation and foods [1–3]. Lead has a number of side effects on human beings such as disruption of nervous system, depression, learning disability, negative reproductive effects, such as damage of sperm and chance of miscarriage and premature birth [4,5]. Thus, the determination of lead is necessary to avoid the entry of this chemical in to environmental samples. There are several analytical techniques such as inductively coupled plasma-atomic emission spectrometry (ICP-AES) [6], graphite furnaceatomic absorption spectrometry (GF-AAS) [7], ICP-mass spectrometry (MS) [8], atomic fluorescence spectrometry (AFS) [9], X-ray fluorescence (XRF) [10], cyclic voltammetry (CV) [11] and UV–Vis spectrophotometry [12–14] have been reported for determination of lead in

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variety of samples. The sophisticated instruments like ICP-MS, GF-AAS, ICP-MS, AFS, XRF and CV are found to expensive, tedious and time consuming for preparation of sample to determine the lead from complex sample matrixes. UV–Vis is simple and rapid technique for determination of lead though the selectivity of the method is poor due to the use of chromophoric reagents. Therefore, an alternative method is required that should be simple, selective, label-free and low cost for determination of lead from different types of samples. Recently, noble metal nanoparticles (NPs) such as silver (Ag), gold (Au), copper (Cu) have been widely exploited in the field of analytical chemistry as chemical probes or sensing probes for detection of variety of analytes in environmental, food and pharmaceutical samples. This is due to the distinct optical property of AgNPs (yellow), AuNPs (pink) and CuNPs (red) in aqueous solution showing the specific localized surface plasmon resonance (LSPR) absorption band in UV–Vis. LSPR is a specific property of noble metal NPs that is related to conduction of free electrons when visible light interacts with it and dependent on the size of NPs. However, the introduction of analyte into the NPs solution

Corresponding authors. E-mail addresses: [email protected] (K. Shrivas), [email protected] (M.K. Deb).

https://doi.org/10.1016/j.microc.2019.104156 Received 11 May 2019; Received in revised form 31 July 2019; Accepted 1 August 2019 Available online 01 August 2019 0026-265X/ © 2019 Elsevier B.V. All rights reserved.

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cause the aggregation of particles and results the color change and shift of LSPR band to longer wavelength in the visible region. LSPR based probe are susceptible to detect the extremely low concentration of analytes which present in different type of samples [15,16]. Therefore, noble metals NPs are used as colorimetric probes for detection of variety of chemical substances such as vitamins [17], amino acids [18], proteins [19], pesticides [20–22], ascorbic acid [23], nucleic acids [24] and cationic surfactants [25]. In addition, we have demonstrated the use of laurylsulphate capped AuNPs for colorimetric detection of arsenic from contaminated water. This sensing is based on the color change of AuNPs from pink to blue and red shift from 520 nm to 730 nm due to the replacement of capping groups from the surface of NPs causing the aggregation followed by the red shift of LSPR band [26]. In another work, we showed the determination of chromium from surface water, industrial waste water and vegetable samples using tartaric acid capped AgNPs as a sensing probe where the mechanism of sensing is based on coordination complex between chromium ions and tartaric acid present on the surface of NPs [27]. Recently, malonate capped AuNPs is exploited for determination of Ba(II) and Ni(II) from river, pond and tap water samples using gold nanoparticles as a chemical probe. Method for determination of these metals is based on the measurement of signal intensity of LSPR band of aggregated NPs in the presence of analyte due to the coordination complex between Ba(II) and Ni(II) with carboxylate ions of malonate capped AuNPs [28]. Thus, we endeavored to develop a new simple method for selective determination of lead in different type of water samples by taking the account of silver and gold NPs as colorimetric probes for detection of number of analytes. In recent times, the paper-based analytical devices (PADs) have been attracted the scientists and researchers around the world because of its simplicity, low cost and small amount of sample is required for monitoring the contaminates in food, pollutants in environment and clinical diagnosis. Here, the paper based devices are generally coupled with commercial electronic devices (e.g. cell phone and scanner, etc.) to read out the response of chemical reaction occurred between analytes and chromophoric reagents. The paper based devices offered a high surface ratio and high optical contrast for colorimetric detection of variety of chemical substances [29–32]. In this context, Ratnarathorn et al. reported the paper based detection of copper using AgNPs as a sensing probe based on the aggregation of NPs on paper substrate. This is due to the strong affinity of binding between homocysteine and dithiothreitol of NPs followed by the shift in LSPR band in visible region [30]. Yakoh et al. developed a portable paper device by fabricating the silver nanoprism on paper substrate and used for detection of chloride from environmental samples integrated with Smartphone to determine the colorimetric response of analyte in samples [31]. Chen group demonstrated the use of low cost Smartphone based diagnostic for monitoring the conditions of food status using paper based colorimetric probe array. This is based on color change of dye fabricated on paper substrate after the release of volatile organic compounds from food samples. For readout, Smartphone with a camera is used to capture the images of dye deposited on paper substrate to determine the status of food condition [32,33]. In present work, simple wet chemical method was used for synthesis of AgNPs by the reduction of silver nitrate (AgNO3) using sodium borohydride (NaBH4) as a reducing agent and polyvinyl alcohol (PVA) was used as a capping agent. The pH of sample solution, reaction time and concentration of NPs that affected the detection of Pb(II) from samples were optimized. The analytical parameters, like linearity range, precision and accuracy and limit of detection were evaluated for validation of newly developed method. Consequently, Whatman filter paper fabricated with AgNPs/PVA was used for quantitative determination of Pb(II) using ImageJ software and compared the results of UV–Vis analysis. Finally, the AgNPs/PVA was used as a plasmonic colorimetric probe and PADs for determination of Pb(II) from surface water and industrial waste water samples.

2. Experimental section 2.1. Materials and chemical reagents All chemicals used were of analytical grade reagents. All metal salts, polyvinyl alcohol (PVA), silver nitrate (AgNO3) and sodium borohydride (NaBH4) were obtained from HiMedia (Mumbai, India) and S.D. Fine Chemical Ltd. (Mumbai, India). Stock standards solutions of different metal ions were prepared from dissolving the appropriate of amount their salts in 10 mL of millipore water. The pH of the sample solution was maintained using 0.1 M NaOH and 0.1 M HCl solution. Glass microfiber Whatman filters paper (GF/A) was used to fabricate AgNPs/PVA for selective detection Pb(II). 2.2. Sampling of surface water and industrial waste water for determination of lead The water samples from different sources such as river water, pond water and industrial waste water collected from Raipur city, Chhattisgarh, India. The samples were collected in polyethylene bottles in the month of January 2018. These samples were filtered by use of Whatman filter paper-42 and stored in refrigerator at 5 °C until the analysis. 2.3. Synthesis of AgNs/PVA AgNPs/PVA was prepared by reduction of AgNO3 using NaBH4 as a reducing agent and polyvinyl alcohol (PVA) used as a capping agent. Briefly, solution mixture of 2 mL PVA (1%) and 20 mL of ice-chilled NaBH4 (2 mM) was taken in conical flask and stirred constantly for 30 min in ice bath. Afterwards, AgNO3 solution (1 mM) was added drop-wise into the stirred solution mixture. The appearance of colorless to bright yellow of solution showing the formation of PVA capped AgNPs. The synthesized AgNPs/PVA was found to be stable up to 3 months without sign of any aggregation which could be used as a plasmonic colorimetric probe and PADs. 2.4. Procedure for determination of lead using plasmonic colorimetric and PADs An aliquot of standard solution of lead (20–1000 μgL−1) or filtered water samples were taken into a glass bottle containing 1.0 mL of AgNPs/PVA and total volume of solution was made with 3 mL of millipore water. Afterwards, the solution mixture was kept at room temperature for prescribed reaction time. The color change of sample solution from yellow to reddish yellow was obtained. The color change of solution mixture can be observed by naked eyes and signal intensity of solution mixture was monitored using UV–Vis spectrometer in the range of 200–800 nm. For PADs determinations, glass fiber paper was punched into small size of circular diameter (0.5 cm) and stacked on paper substrate having the blue colored wax of hydrophobic surface. A 5 μL of AgNPs/PVA was deposited on each test zones of paper with micropipette followed by the addition of 5 μL different concentrations of Pb(II) from 50 to 1000 μgL−1. The color developed on paper was recorded using Smartphone and imported to ImageJ software to measure the mean color intensity of deposited NPs containing analyte. The freshly prepared PADs were used for analysis of lead and the stability of device was checked by the analysis the sample for seven days on paper substrate without change in the results. However, the paper devices should be stored in dark box before the use in order to prevent the oxidation of fabricated silver NPs from the surface of paper substrate. 2.5. Apparatus UV–visible spectrometer, type Carry-60 (Agilent Technologies, USA) 2

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with 1 cm quartz cell was employed for measurement of LSPR absorption of AgNPs/PVA solution. UV probe software was used for absorbance measurements. Smartphone (MI Xiaomi, Max Prime) was used to record the image of paper substrate after the deposition of lead and AgNPs and further processed with ImageJ software for quantitative analysis of analyte. The transmission electron microscope (TEM) was used for determination of size and image of AgNPs/PVA in presence and absence of analyte at an accelerating voltage of 100 kV. Fourier transform-infra red spectrometer (FTIR) of type-nicolet-10 (Thermo Scientific, USA) was used for acquiring the IR spectra of pure PVA, AgNPs/PVA and AgNPs/PVA with Pb(II). Gaussian 09 (C.01) program with B3LYP method density functional theory (DFT) with LANL2DZ basis set was demonstrated for optimization and interaction between PVA and Pb(II) as well as with AgNPs. Smartphone was used to capture the color intensity of deposited NPs and analytes on paper substrate. The JPEG image obtained by phone was transferred to ImageJ software (National Institute of Health, USA) for measurement of mean color intensity of developed paper.

S3(a) to (g). The value of ∆A was increased with increasing the concentration from 50 to 500 μM and after there was no significant enhance in ∆A value was observed. Thus, 500 μM concentrations of AgNPs were further exploited for detection of lead. Finally, all the experiments were performed by keeping the sample solution for 5 min of reaction time at pH 7.0 using 500 μM concentration of NPs for optimum detection of Pb(II) from sample solution. 3.3. Sensing mechanism for selective detection of lead using AgNPs/PVA as a plasmonic colorimetric probe and PADs AgNPs capped with PVA showed a well dispersion of particles in aqueous solution (yellow color) with a sharp LSPR absorption band at around 410 nm, shown in Fig. 2(a). The determination of LSPR absorption peak at 410 nm confirming the size of NPs was found around in the range of 10–40 nm [21,27]. However, the addition of Pb(II) into the NPs solution caused the color change from yellow to reddish yellow as well as red shift from 410 to 550 nm due to the aggregation of particles as shown in Fig. 2(b). The aggregation of NPs cause the decrease in distance among the particles followed by strong enhancement of localized electric field which produce a red shift in the LSPR band at around 550 nm in visible region, shown in Fig. 2(b). In addition, the TEM measurement was also performed to verify the actual size of NPs before and after addition of Pb(II). Fig. 2(c) shows the TEM image of AgNPs/ PVA without addition of lead ions displaying the monodispersity in aqueous solution. Fig. 2(d) shows the TEM image of agglomerated NPs in the presence of analyte and average size of the NPs was found several folds higher than that of dispersed NPs. In addition, the DLS measurements were carried out to determine the size distribution of NPs in aqueous solution before and after the addition of analyte. The results are shown in Fig. 2(e) and (f). From the results, the size distribution of NPs in absence and presence of analyte in aqueous solution was found to be ± 22.5 nm and 90 ± 3.5 nm, respectively. The result acquired with DLS measurement was found identical to results acquired with UV–Vis and TEM analyses. Hence, the monodispersed NPs exhibited yellow color and the addition of analyte into it caused the aggregation followed by the color change and red shift of LSPR absorption. This phenomenon was exploited for selective detection of Pb(II) from sample solution using plasmonic colorimetry and PADs. Thus, the selective detection of Pb(II) is illustrated based on the red shift of LSPR absorption band of AgNPs/PVA after the introduction of analyte by UV–Vis, TEM and DLS analyses. The red shift of band is due to the decrease in inter-particle distance after the agglomeration of NPs compared to monodispersed particles in aqueous solution. This resulted the selective interaction (ion-dipole) of Pb (II) metal ions with oxygen of polyvinyl alcohol (calculated bond distance by DFT Pb-O = 2.500 A°) present on the surface of NPs. The strong ion-dipole interaction of lead ion (calculated Energy = −623.2176 a.u.) with polyvinyl alcohol perturbed the stability of AgNPs/PVA and triggered the aggregation of NPs [34]. Thomas research group also proposed a similar consequence for formation of non-bonding type of supramolecular complex formation of Pb(II) ion with a bifunctional gallic acid leading to a strong interparticle interaction of gold and silver NPs [35]. In, addition, we performed the FTIR measurement to verify the complexation between PVA and Pb(II) using AgNPs as a plasmonic colorimetric probe. For this, the FTIR spectra of pure PVA, AgNPs/PVA and AgNPs/PVA with Pb(II) was recorded. The results are given in Fig. 2(g) where the broad band observed between 3550 and 3300 cm−1 corresponded to OeH stretching from the intermolecular and intramolecular hydrogen bonds. The band observed around at 3000 and 2800 cm−1 associated with CeH stretching of CH2 groups and bands at around 1600 cm−1 are due the CeO stretching from PVA molecule [36]. The shift of band towards the higher wave number showed the modification of AgNPs with PVA molecules. However, the solution mixture of AgNPs/PVA with Pb(II) exhibited the decrease and appearance of broader band than pure PVA and AgNPs/PVA demonstrated the binding of analyte with PVA

3. Result and discussion 3.1. Assay for selective detection of lead using plasmonic colorimetric probe and PADs AgNPs functionalized with PVA was tested for colorimetric detection of different metal ions such as Pb(II), Ca(II), Ba(II), Na(I), Cu(II), Ni (II), Hg(II), Co(II), K(I), Zn(II), Fe(III), As(III), Cd(II) and Cr(VI) at the concentration of 200 μgL−1. For this, metal ions with AgNPs/PVA was taken into a 5-mL glass vial in the ratio of 1:1 and kept for 5 min of reaction time. The results are shown in Fig. 1A{(a) to (q)}. The AgNPs/ PVA solution with different metal ions did not show any color change and red shift of LSPR band at 410 nm and remained bright yellow color after the addition of these metal ions, shown in Fig. 1(c) to (q). The addition of Pb(II) into the NPs displayed the color change from yellow to reddish yellow as well as red shift of LSPR band of NPs from 410 nm to 550 nm in the visible region, shown in Fig. 1(b). This unambiguous color change and appearance of a new LSPR band in the visible region after adding of Pb(II) metal ions to the NPs could be used for sensing of target element from sample solution. Consequently, we performed the experiments by depositing the different metal ions (200 μgL−1) on the paper substrate fabricated with AgNPs/PVA for selective detection of Pb(II), shown in Fig. 1B{1(a) to (q)}. The NPs containing Pb(II) ions (Fig. 1(b) only exhibited the color change from yellow to reddish yellow and no color change was observed with other metal ions (Fig. 1(c) to (q). The same color change of NPs was obtained using UV–Vis analysis. Thus, AgNPs/PVA can be also used as a paper based analytical devices (PADs) for selective detection of lead from sample solution. 3.2. Optimization for detection of lead using AgNPs/PVA The parameters such as reaction time, pH and concentration of AgNPs were optimized for better detection of Pb(II) from sample solution by monitoring the ∆A (difference in absorbance at 410 nm and 550 nm) using AgNPs/PVA as a plasmonic colorimetric probe. First, pH of solution was optimized by introducing the different pH solution (3.0, 5.0, 7.0, 9.0 and 11.0) containing Pb(II) ions followed by the addition of NPs. The results are given in Fig. S1(a) to (e). The solution containing pH 7.0 showed the maximum ∆A and further this pH was used for maintaining the pH of sample. Next, reaction time was optimized by keeping the solution mixture of NPs and Pb(II) for different reaction time at room temperature, shown S2(a) to S2(j). There was no significant effect of reaction time though the 5 min of reaction was found good for detection of Pb(II). In addition, the concentration of NPs was also optimized by recording the ∆A at different concentrations of NPs (50, 100, 200, 300, 400, 500 and 600 μM). The results are given in Fig. 3

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(a) (b) (c) (d) (e) (f) (g) (h) (i) (j) (k) (l) (m) (n) (o) (p) (q)

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Fig. 1. (A) Photographs of glass vials containing solution mixtures of different metal ions (200 μL−1) and AgNPs/PVA with pH 7.0 for 5 min reaction time at room temperature and their respective UV–Visible spectra; (B) Photographs of paper strips containing solution mixtures of different metal ions (200 μL−1) and AgNPs/PVA with pH 7.0 for 5 min reaction time at room temperature.

molecules. Further, the theoretical study was also performed via density functional theory (DFT) calculations using Gaussian 09 (C.01) program with B3LYP method and LANL2DZ basis set [37,38] in order to show interactions of PVA with AgNPs as well as with Pb(II) ion. The data obtained for coordinates, bond parameters and energy values is given in supporting information (Table S1 to S4 and Fig. S4). The optimized structure and charge distribution of each atom of Ag20 nanoclusters and PVA (five monomeric units) are shown in Fig. 3. The silver nanoclusters of Ag20 were taken as a model to display the bond length of AgeAg (min. 2.6085A° to max. 2.6687 A°), bond angle of Ag-Ag-Ag (min. 59.2240° to max. 178.8004°) and dihedral angle of Ag-Ag-Ag-Ag (min. 91.7570° to max. -178.3095° (Fig. 3a). The total energy was calculated to be −2915.9919 a.u. (Table S1). The charge distribution in Ag20 is shown in Fig. 3(b) where 12 atoms showed the positive and 8 atoms showed the negative charge. The optimized structure with charge distribution of PVA molecules is also shown in Fig. 3(c). In Ag20 only 4 positively charged atoms present in outer edge, integration with negative charged oxygen of PVA is shown for clarity (Fig. 3d). The internuclear distance of AgeO was calculated to be 2.7740 A° with Ag-OC bond angle 89.8082°. When fully charged cations Pb(II) ions approached close proximity to PVA the strong ion-dipole interaction occurred and thus breaking of the many AgeO interactions and stabilizing a new PbeO moiety proceeded with internuclear distance of 2.5000 A°, shown in Fig. 3(d). Hence, the formation of PbeO entity in aqueous solution caused the aggregation pursued by the red shift of LSPR band of AgNPs in the visible region. The presence of other metal ions was

found less significant and showed almost no selective interaction with PVA and AgNPs. The schematic plausible sensing mechanism for detection of Pb(II) is designed based on experimental and theoretical calculation. AgNPs capped with PVA in aqueous solution exhibited the yellow color due to the monodispersity of particles achieved by long carbon chain of PVA molecules. The long carbon chain of PVA prevented the NPs from aggregation due to the steric hindrance, shown in Fig. 4(a). Thus, the LSPR absorption band NPs capped with PVA was observed around at 410 nm in the visible region. However, the addition of Pb(II) into the NPs showed the color change from yellow to reddish yellow because of agglomeration of particles caused due to the removal of PVA molecules from surface of NPs, shown in Fig. 4(b). This is due to the strong iondipole interaction between PVA molecules and Pb(II) ions as well as the red shift of LSPR band was observed from 410 to 550 nm in the visible region. The change in color intensity and red shift is found proportional to the concentration of analyte, which is then used for colorimetric sensing and PADs analysis of Pb(II) from sample solution. 3.4. Assay for analytical evaluation for plasmonic colorimetry and PADs for determination of lead The effectiveness of using AgNPs/PVA as a plasmonic colorimetric probe was determined by evaluating the linearity range, precision, limit of detection (LOD) and stability for determination of lead at the optimized conditions. Linearity range was predicted by spiking a different concentration of lead (20, 100, 300, 500, 700 and 1000 μgL−1) in 4

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Fig. 2. (a) Glass vial containing an aqueous solution of monodispersed AgNPs/PVA along with LSPR band at 410 nm, (b) Glass vial containing AgNPs/PVA with Pb(II) showing aggregation of particles in aqueous solution along with LSPR band at 550 nm, (c) TEM images of AgNPs/PVA before addition of Pb(II) and (d) TEM images of Pb(II) after the addition of Pb(II); (e) DLS measurement of AgNPs/PVA, (f) DLS measurement after the addition of Pb(II) ions into the AgNPs/PVA; (g) FTIR spectra of pure of pure PVA, PVA capped AgNPs and NPs with Pb(II). 5

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Fig. 3. (a) Optimized geometry of Ag20 nanoclusters calculated by DFT-B3LYP with basis set LANL2DZ (Energy = −2915.99193032 a.u.) (b) Atomic charge distribution of Ag20 nanoclusters each atom considering NBO type ranging from −0.408 to 0.408 and (c) Optimized geometry of polyvinyl alcohol (PVA) calculated by DFT-B3LYP with basis set 6-31G and (Energy = −770.3550 a.u.) along with charge distribution of each atom of PVA considering NBO type ranging from −0.651 to 0.651); (d) Interaction of Ag20 nanoclusters with PVA calculated by DFT-B3LYP with basis set LANL2DZ (Energy = −3532.33380712 a.u.) (e) Interaction of PVA with Pb(II) calculated by DFT-B3LYP with basis set LANL2DZ (Energy = −623.21764078 a.u.)

separate glass vials containing a 1 mL of AgNPs/PVA and the total volume was diluted to 3 mL with Millipore water. The results are shown in Fig. 5. The value of LSPR absorption ratio obtained at 410 nm and 550 nm was used to determine the linear least square equation for

quantitative determination of lead. A good linear line was observed in the range of 20–1000 μgL−1 with correlation of estimation (r2) 0.978. The value of LOD was computed by taking three times of standard deviations (3 × σ) at minimum quantity of analyte spiked into a NPs 6

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PVA

Fig. 4. Plausible mechanism for the detection of Pb(II) ion by AgNP-PVA. (a) Glass vial containing yellow color dispersed PVA capped AgNPs along with paper strip (b) glass vial containing reddish yellow aggregated AgNPs along with paper strip. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

solution that could give a change in visible region and value of LOD acquired was 8 μgL−1. The precision is most important parameter to know the reproducibility of newly developed method. Thus, the precision of the method assessed by calculating the relative standard deviation (RSD) of six analyses of 200 μgL−1 Pb(II). The result is given in S5. A good RSD value of 3.4% showing the precision and stability of NPs as a plasmonic colorimetric probe for colorimetric detection of Pb (II) from the sample solution. Consequently, the calibration curve was prepared by depositing the different standard solution of Pb(II) on circular paper fabricated with AgNPs/PA. The image acquired with Smartphone was processed using ImageJ software to determine the color intensity for different concentration of Pb(II). The calibration curve was drawn between different concentrations of lead from 50 to 1000 μgL−1 against the respective mean color intensity, shown in Fig. 6. Good linear range (50–1000 μgL−1) was obtained for determination of lead. The linear least square equation (y = 0.0732x + 43.037) was used to determine the lead in real samples. The LOD obtained using PADs was 20 μgL−1. Thus, paper based detection of lead found to be simple and economic and can be applied at the sample source. 3.5. Real samples analysis of lead using plasmonic colorimetry and PADs The reliability of newly developed method was tested by quantitative determination of Pb(II) in pond water, tube well, river water and industrial waste water samples. For this, 1 mL of water samples was mixed with 1 mL of AgNPs/PVA while maintaining the pH of sample for 5 min of reaction time and the total volume of sample solution was maintained to 3 mL with Millipore water. The ∆A (difference in absorbance at 410 nm and 550 nm) was used to determine the concentration of Pb(II) present in water samples using linear least square equation ( y=0.0004x+0.0844). The concentration of lead present in surface water and industrial waste water samples was found in the range of 130–1055 μgL−1, given in Table 1. The same samples were analyzed with PADs by depositing the 5 μL of filtered water samples on test zone of paper containing AgNPs/PA, and color developed was processed with Smartphone followed by analysis in ImageJ software to determine the color intensity of analyte.

Fig. 5. Glass vial containing different concentration (20, 100, 300, 500, 700 and 1000 μgL−1 Pb(II)) with AgNPs/PVA along with their respective UV–Vis spectra and calibration curve for determination of Pb(II) in water sample.

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solution containing a 1 mL of AgNPs/PVA and total volume was made up to 3 mL with millipore water. The results are shown in Fig. 7. The yellow color bar diagram represents the AgNPs/PVA, brown color bar diagram corresponds to AgNPs/PVA and Pb(II), light green bar diagram stands for AgNPs/PVA and different diverse substances and dark yellow color bar diagram presents AgNPs/PVA, Pb(II) and different diverse substances. There was no color change and red shift of LSPR band in visible region demonstrating the selectivity of using AgNPs/PVA as a colorimetric probe for detection of Pb(II) from complex water samples such as industrial waste, pond and river water samples. In addition, the effect of interferences on determination of lead using PADs was investigated at the optimized conditions. The tolerance limit of metal ions and anions is given in Table S5 (Supplementary material) showing that these diverse substances did not affect the determination of lead from water samples. Further, the colored metal ions of concentration of 450 mgL−1 for Fe(III), Cu(II): 800 mgL−1 for Ni(II), Mn(II), Co(II) and 850 mgL−1 for Cr(IV) did not showed the color change of silver NPs from yellow to reddish yellow illustrating the selective detection of lead from complex matrix samples.

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(f) 3.7. Comparison for determination of lead compared to other reported methods The potentiality of the developed was determined by comparing the linearity range and LOD for determination of lead with other methods reported in the literatures [35,39–41], given in Table 2. The present method showed a good linear range and LOD values compared to most of the reported methods for determination of Pb(II). AgNPs capped with gallic acid [35] and AuNPs capped with citrate ions [41] showed a better sensitivity though the chance of interference was found more. However, the synthesis of AgNPs/PVA was found to be single step process, simple and cost effective compared to other methods for detection of Pb(II).

100 gL-1 300 gL-1 700 gL-1 1000 gL-1

Fig. 6. Circular filter strip fabricated with AgNPs/PVA along with deposition of different concentration of Pb(II): (a) AgNPs/PVA (blank), (b) 50 μgL−1, (c) 100 μgL−1 (d) 300 μgL−1 (e) 700 μgL−1and (f) 1000 μgL−1.

The linear least square equation (Y = 0.0732× + 43.037) was used to determine the concentration of Pb(II) in samples, given in Table 1. The results obtained with plasmonic colorimetry and PADs were validated by analyzing with ICP-AES. The results are given in Table 1. The comparable results were acquired with both developed methods and ICP-AES. Therefore, the AgNPs/PVA was successfully used for determination of Pb(II) in different type of water samples in plasmonic colorimetry and PADs.

4. Conclusions The present work demonstrated the integration of plasmonic colorimetric sensing and paper based analytical devices for onsite monitoring of lead in environmental samples. A new sensing mechanism is proposed for selective detection of Pb(II) based on leaching of PVA from the surface of AgNPs due to the strong ion-dipole interaction between analyte and PVA molecules with breaking of AgeO interactions and stabilizing a new PbeO moiety. This phenomenon perturbed the stability of AgNPs and causing the aggregation followed by the color change from yellow to reddish yellow and red shift of LSPR absorption band of AgNPs in UV–Vis. The principle of sensing mechanism is confirmed by experiments as well as through theoretical calculation using Gaussian software (9.0) with DFT. The data obtained with UV–Vis measurements were validated with paper based detection of lead using AgNPs/PVA as a chemical probe. Finally, we successfully exploited the AgNPs in plasmonic colorimetry and PADs for selective detection of Pb

3.6. Specificity of AgNPs-based chemical probe for detection of Pb(II) in the presence complex matrices Metal ions such as Na(I), K(I), Mg(II) Ca(II), Ba(II), Fe(III), Mn(II), Al(III), Cu(II), Co(II), Ni(II), Zn(II), Cd(II), Cr(VI), Hg(II), Pb(II) and As (III), and anions such as Cl−, SO42−, CO3−, NO3− and PO43− that may be present in water samples were tested using AgNPs/PVA as a colorimetric probe at the optimized conditions. For this, diverse substances with tolerance concentration limit of Mg(II), Ca(II), Ba(II), Hg(II), NO3− (600 mgL−1); Ni(II), Mn(II), As(III), Al(III), Co(II), SO42− (700 mgL−1); Pb(II), Cu(II), Cd(II), Fe(III), PO43− (400 mgL−1) and Na (I), K(I), Cr(IV), Zn(II), Cl−, CO3– (800 mgL−1) were spiked into

Table 1 Application for determination of Pb(II) in water samples using AgNPs as a plasmonic colorimetric probe and PADs and compared with results of ICP-AES. Samples/sources

AgNPs-colorimetry Pb, μgL−1

RSD, %

PADs Pb, μgL−1

RSD, %

ICP-AES Pb, μgL−1

RSD, %

Industrial waste-1, Urla Steel plant waste, Urla Pond water, Jarwai River water, Kumhari Industrial waste-2, Sarora Industrial waste-3, Sarora Pond water, Raipur Industrial waste, Bhanpuri Tube well water/Raipur

130 140 ND* 160 1020 185 ND* 1055 ND*

2.5 3.1 – 3.8 3.1 2.9 – 3.1 –

127 145 ND* 156 1012 188 ND* 1049 ND*

3.5 2.9 – 3.7 4.2 3.4 – 4.5 –

133 137 ND* 151 1022 180 ND* 1058 ND*

4.5 4.0 – 2.9 4.0 3.7 – 3.4 –

ND* Not detected with present method. 8

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K. Shrivas, et al.

Fig. 7. Effect of diverse substances on determination of Pb(II) in water samples exploiting AgNPs/PVA as a plasmonic colorimetric probe for 5 min of reaction time, 500 μM concentration of NPs at pH 7.0. The light yellow color bar diagram, brown color bar diagram, light green bar diagram and dark yellow color bar diagram present the signal response of AgNPs/PVA, AgNPs/PVA + Pb(II), AgNPs/PVA + different diverse substances and AgNPs/PVA + Pb(II) + different diverse substances, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Table 2 Comparison of the proposed method with other reported NPs-based plasmonic colorimetric methods for the determination of lead. Methods

AuNPs/gallic acid AuNPs/DNAzyme AuNPs/Papain AuNPs/citrate AgNPs/PVA Paper-AgNPs/PVA

Pb(II)

Samples

Linearity range

LOD, nM

10–1000 nM 0.5–1000 μM 0–40 μM 2.5 nM–10 μM 96 nM–4.8 μM 241 nM–4.8 μM

10 500 200 0.8 38 96

[7]

Ref. [8]

Drinking water Water Water Water and soil Surface water and industrial waste

35 39 40 41 Present method

[9] [10] [11]

(II) from surface water and industrial waste water samples.

[12]

Declaration of competing interest

[13]

There are no conflicts to declare.

[14]

Acknowledgements

[15]

We would like to thank the Science and Engineering Research Board (SERB), New Delhi for awarding Extra Mural Research Project to Kamlesh Kumar Shrivas (File No.: EMR/2016/005813). Financial assistance from DST-FIST [No.-SR/FST/CSI-259/2014(c)] and UGC-SAP [No.-F-540/7/DRS-II/2016 (SAPeI)] are also gratefully acknowledged.

[16] [17]

[18]

Appendix A. Supplementary data

[19]

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.microc.2019.104156.

[20]

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[21]

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