Cost-effective synthesis of bifunctional silver nanoparticles for simultaneous colorimetric detection of Al(III) and disinfection

Accepted Manuscript Title: Cost-Effective Synthesis of Bifunctional Silver Nanoparticles for Simultaneous Colorimetric Detection of Al(III) And Disinfection Authors: Ritu Painuli, Priyanka Joshi, Dinesh Kumar PII: DOI: Reference:

S0925-4005(18)31040-2 https://doi.org/10.1016/j.snb.2018.05.131 SNB 24787

To appear in:

Sensors and Actuators B

Received date: Revised date: Accepted date:

12-2-2018 5-5-2018 23-5-2018

Please cite this article as: Ritu Painuli, Priyanka Joshi, Dinesh Kumar, Cost-Effective Synthesis of Bifunctional Silver Nanoparticles for Simultaneous Colorimetric Detection of Al(III) And Disinfection, Sensors and Actuators B: Chemical https://doi.org/10.1016/j.snb.2018.05.131 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Cost-Effective Synthesis of Bifunctional Silver Nanoparticles for Simultaneous Colorimetric Detection of Al(III) And Disinfection Ritu Painuli1, Priyanka Joshi1, and Dinesh Kumar2 1

Department of Chemistry, Banasthali University, Rajasthan 304022, India

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School of Chemical Science, Central University of Gujarat, Gandhinagar 382030, India

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Highlights

● The bifunctional Jamun stabilized silver nanoparticles (J-AgNPs) were utilized for the

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simultaneous colorimetric detection of Al(III) as well as disinfection of the synthetic and groundwater samples.

● The lowest limit of detection for the proposed method is 0.01 ppm, which falls within the permissible limit set by Environmental Protection Agency (USEPA), that is, 50

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ppm.

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● The synthesized J-AgNPs also displayed effective antibacterial activity against the Gram−positive bacteria and Gram−negative bacteria.

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● The estimated cost of the prepared nanosensor is US $0.03, thus making it extremely

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affordable in the areas with limited financial resources. Abstract

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The functionalization of nanoparticles (NPs) by utilizing biologically important substances through a green route is a novel aspect of the design of a colorimetric sensor. In this work, we

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use the bifunctional Jamun (Syzygium cumini) stabilized silver nanoparticles (J−AgNPs) for simultaneous colorimetric detection of Al(III) and the disinfection of synthetic and real contaminated groundwater samples. The J−AgNPs were prepared under natural sunlight

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irradiation by utilizing Jamun leaves extract (JLE) as both the reducing agent and the stabilizer. The lower detection limit for Al(III) with this method is 0.01 ppm (S/N=3), which falls within the permissible limits set by the Environmental Protection Agency (USEPA), that is, 50 ppm.

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The proposed method also displays the excellent antibacterial activity with the value of 45 µg/mL (IC50) for S. aureus bacteria and 25 µg/mL (IC50) for E. coli bacteria. The estimated total cost of the prepared nanosensor (100 mL) is US $ 0.03, which makes the present method cost-effective in areas with resource-limited settings. As far as we know, this is the first study to use natural sunlight as a source of energy for the generation of J−AgNPs in an aqueous medium, making the process environmentally friendly, attractive and efficient. Keywords: Jamun leaves extract, silver nanoparticles, Al(III) detection, antibacterial activity 1

Introduction Heavy metal ion contamination poses a severe menace to the environment and human society [1]. Some light metals are also extremely noxious and harmful to human health, even at trace level concentrations [2]. Amid these metal ions, aluminum is the third most ubiquitous element in the earth’s crust accounting for about 8% of its mass. It widely subsists in the environment due to acidic rain and is toxic to biological activities [3]. The prevalent utilization of aluminum

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in our day to day life, such as aluminum foil, vessels, trays, water treatment, and in many industrial activities including the manufacturing of cars and computers, often exposes mankind

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to this metal [4]. Extreme exposure of aluminum to the human body might alter the structures

of cells, damage cellular energy transfer processes, and metabolism [5]. The cellular toxicity of aluminum has been related to numerous diseases, such as Parkinson's disease [6], Alzheimer’s disease [7], and dialysis encephalopathy [8]. To avoid grave health problems,

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caused by the uptake of aluminum, the World Health Organization (WHO) has recommended

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an acceptable dietary uptake of aluminum in the human body at 7 mg per kg body weight per

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week [9]. Therefore, developing a convenient and economical assay for the trace level detection of Al(III) is extremely imperative.

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The traditional, well−defined techniques used for the trace level detection of aluminum include atomic absorption spectroscopy (AAS) [10], inductively coupled plasma atomic emission

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spectroscopy (ICP/AES) [11], inductively coupled plasma−mass spectroscopy (ICP−MS) [12], electrochemical methods [13] etc. It is apparent that these methods are inconvenient for

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aluminum analyses due to the complicated sample preparation, expensive sophisticated instrumentation and so on. To circumvent these challenges, colorimetric NPs based methods seem to be a promising substitute for metal ion detection because of their many advantages,

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such as result visualization, high sensitivity, cost-effectiveness, and easy operation [14]. The NPs of noble metals, such as AgNPs and AuNPs have drawn considerable interest as they display strong absorption of electromagnetic waves in the visible region due to surface plasmon

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resonance (SPR), highly chemical inertness, stable dispersions etc [15]. Many experimental methods have been reported for the detection of Al(III) ion, such as various fluorescent chemosensors [16-19], triazole-ether functionalized AuNPs [20], 5−Mercaptomethyltetrazole capped AuNPs [21], and ionic liquid coated AgNPs [22]. Hitherto, discussed methods are highly sensitive and selective but possess various drawbacks, i.e. the poor aqueous solubility of fluorophores, utilize toxic chemicals, non-polar solvents, and synthetic additives or capping agents, thus precluding their applications [23,24]. Therefore, the quest for the development of 2

a clean, eco−friendly, and reliable method for the synthesis of NPs requires researchers to adopt “green” chemistry and bioprocesses [25,26]. Synthesis of metal nanoparticles using sunlight and stabilizing them by biocompatible capping agent provide great enhancement over other existing methods. Sunlight is an external stimulus utilized to control the switchable nature of NPs, a thermal energy alternate. This irradiation is deliberated to be the largest source of carbon-neutral renewable energy- nontoxic and nonpolluting [27]. Along with the toxic metal ions, potable water is often polluted with contaminants, such as E.

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coli, S. aureus. The harmful strains of these two Gram-positive and Gram-negative bacteria ingested into the human body by various means can cause harmful diseases for instance anemia,

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kidney failure, skin infections etc. [28]. Consequently, the removal or inactivation of these

contaminants is a prime matter of the concern for researchers. The traditional disinfection methods, like chlorination and ozonations, have some limitations [29]. As a result, metal nanoparticles have gained a lot of interest as antibacterial agents. To the best of our knowledge,

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this is the first ever study for the sensing of the Al(III) ions as well as the disinfectant activity

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in the aqueous systems simultaneously by utilizing the JLE.

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Recently, our research group has also reported the label-free colorimetric sensor for the on-site detection of the Al(III) ions in aqueous systems [30]. In this study, we report a very effective,

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economical sunlight-induced protocol for the synthesis of AgNPs. JLE acts as a green multifunctional agent at the room temperature. This method is easy to implement, eco-friendly

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and does not require any sophisticated instruments. The total cost of the prepared nanosensor (100 mL) is US $0.03, making it financially viable. The nanomaterial developed in the present

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study performed the dual role i.e. as a sensor for the sensing of the Al(III) ions and as a disinfectant for the Gram-positive and Gram-negative bacteria. In comparison with existing materials and methods for Al(III) detection, this approach opens a unique, environmentally

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friendly as well as the cost-effective technique for the efficient detection of Al(III) as well as an effective antibacterial agent.

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Experimental

Chemical and materials Fresh leaves from Jamun plant were obtained from Banasthali University Campus, Rajasthan, India. Silver nitrate (AgNO3) was procured from Sigma-Aldrich. Salts of different metals were purchased from Merck Pvt. Ltd and Sigma-Aldrich and were used as received. The glassware were washed with aqua-regia and rinsed well with Milli−Q water prior to use. Preparation of J−AgNPs 3

Fresh Jamun leaves were cleaned with Milli−Q water and cut into fine pieces. The small pieces were then dried in an oven. 3 g dried leaves were crushed into fine powder by mortar pestle and moved into a beaker with 100 mL Milli−Q water. The extract was prepared by boiling the leaves at approximately 37 ℃ for 4 h. The prepared extract was kept for cooling, filtered and then kept in the refrigerator before its use in the synthesis of J−AgNPs. To synthesize the J−AgNPs, a 50 mL solution of silver nitrate (0.001M) was added to various concentrations of JLE (50−1200 µL). The resulting mixture was then kept undisturbed under

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ambient sunlight radiation for 13 min. The solution changed from colorless to yellowish brown. This solution was used as a detection probe, which was further characterized and used for the

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colorimetric detection of aluminum.

Quantitative estimation of chemical constituents presents in Jamun leaves extract (JLE) To identify the active multifunctional biomolecules presents in the JLE that are responsible for the synthesis as well as for the stabilization of AgNPs, we estimated its total phenolic content

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along with the total flavonoid content. The total phenolic content was estimated by the

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MacDonald method [31]. The total flavonoid content was estimated by the aluminum chloride

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method [32]. The estimation of total phenolic contents and flavonoid contents in the Jamun leaves extract were carried out in triplicates and the results were averaged (See Supporting

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information and Fig.S1).

Colorimetric detection of Al(III) ions

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For the colorimetric detection of Al(III) ions, 300 µL aqueous solutions of Al(III) of different concentrations were added to a 400 µL of J−AgNPs solution. The mixtures were kept at room

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temperature and then characterized by approaches viz. colorimetric and spectrometry. Antibacterial activity of the J−AgNPs for the E. coli and S. aureus E. coli strain and S. aureus strain was cultured in Luria−Bertani broth (LB broth) for 24 h at

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37±1 ℃. To observe the concentration of synthesized nanoparticles at which 50% of the bacterial growth is prevented (IC50), the samples of different concentrations of J−AgNPs were treated with a bacterial sample. To perform this, a sequence of culture tubes comprising 25 mL

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of LB broth was organized. To each culture tube, 200 µL of the bacterial solution and different concentrations of J−AgNPs ranging from 10 to 100 µg/mL was added. The culture tubes were then incubated for 24 h at 37±1 ℃ to grow the bacteria. All these experiments were accomplished in triplicate. The bacteria concentration in every single incubated culture tubes was determined by the spectrophotometer, and then the IC50 was estimated. Statistical analysis 4

The statistical analysis was carried out by utilizing SPSSv16.0 (Predictive Analytics Software). The analytical determination was made in triplicate. All the experimental data are expressed as a mean ± standard deviation. To estimate the statistical significance, a student’s t-test was implemented. Furthermore, p values were computed by means of the t-test and were considered as significant at 0.05. Well diffusion method

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To perform a well diffusion method, agar plates were arranged in triplicate with similarly bored

wells. Consequently, 40 µg/mL and 45 µg/mL J−AgNPs were added for the E. coli strain while

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25 µg/mL and 20 µg/mL J−AgNPs were added for the S. aureus strain after the single drop of

methylene blue dye. Finally, the plates were incubated for 24 h at 37±1 ℃ for the growth of the zone of inhibition.

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Instruments

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UV−Visible spectra were recorded using a Lab India 3000+ UV-vis spectrophotometer containing double beam in each compartment with a 1 cm path length quartz cell cuvette. The

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morphology of the J−AgNPs surface was determined by means of high-resolution transmission

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electron microscopy (HRTEM) operated at 200 kV on TENAI instrument. Zeta potential and dynamic light scattering (DLS) studies were carried out on a Nano series–ZS 90, Malvern

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instrument. To investigate the functional group involved in the synthesis of J−AgNPs and detection of Al(III), Fourier transforms infrared spectral studies were done by using FTIR

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spectrometer (Shimadzu). To perform this, the samples were dried at room−temperature and mixed with KBR to form a pellet. Then the FTIR spectra were recorded in the range between 4000 to 400 cm−1. Thermo K Alpha XPS instrument has been utilized for determining the

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surface elemental composition of the samples. Cyclic voltammogram (CV) was recorded using a computer-controlled 400A electrochemical analyzer using Pt electrode as the reference electrode. SERS spectra were recorded with a thermo scientific instrument equipped with Ar

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laser (λ¼532 nm) and a charge-coupled device (CCD) as the detector. Samples for Raman spectroscopy were prepared on gold-coated slides. Application of the sensor To confirm the practical applicability of developed probe, tap water samples were collected from our research laboratory and used as such without filtration. Results and discussions

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The UV-vis absorption spectra of the JLE and J-AgNPs are demonstrated in Fig. 1. The JAgNPs were prepared at two different conditions i.e. at room temperature and in the presence of sunlight. The UV-vis spectra of JLE shows a narrow peak at about 280 nm (Fig. 1 (a)) while the J-AgNPs prepared at room temperature showed a peak at 426 nm (Fig. 1(b)). Absorption spectra of the J−AgNPs under sunlight irradiation method showed a characteristic and intense SPR peak at 412 nm (Fig. 1(c)). The peak at 280 nm nearly disappeared from the SPR spectrum of J−AgNPs solution which confirmed that whole plant extract has been employed in the

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reduction of Ag+ ions into the AgNPs. The narrow peak at 412 nm indicates the synthesized

nanoparticles could be of small size and monodispersed in nature. The results were further

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supported by the HRTEM and FESEM images of the J−AgNPs prepared by these two methods

(S2A & B). Inset of Fig.1 shows the UV-vis spectra of freshly prepared J−AgNPs and after 40 days. Any significant change was not observed in both peak position and peak intensity, which shows the long-term stability of J−AgNPs. Ultraviolet rays from the sun generate a free radical

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that has the tendency to react with other compounds. Therefore, the unstable free radical will

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readily reduce AgI to Ag0, and the generated atoms nucleate with further growth to form NPs

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[33,34] (Scheme 1). Thus, considering the minimum time required for AgNPs synthesis, the sunlight irradiation method has proven to be more efficient for sensitive metal ions detection

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over room temperature method. To check the reproducibility of the proposed method, we performed our experiment in different batches of the synthesis. Fig. S3A exhibits the SPR

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spectra of J−AgNPs synthesized in different batches. The SPR spectra overlapped at 412 nm with same intensities. This confirms the good synthetic reproducibility of our proposed method.

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The synthetic reproducibility was further confirmed by HRTEM images of J−AgNPs (Fig. S3B). The histograms representing the synthetic reproducibility with respect to size (Inset of Fig. S3B). To achieve the effective preparation of nanoparticles, other factors viz. the amount

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of extract, irradiation time, and pH effect were thoroughly evaluated.

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Fig. 1(a) UV-vis spectra of fresh JLE, J−AgNPs prepared by heating method (b), and J−AgNPs

AgNPs after 40 days. Amount of Jamun leaves extract

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prepared under ambient sunlight irradiation (c), inset of the figure shows the stability of J-

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The effect of the amount of JLE on the synthesis of J−AgNPs was demonstrated by adding various volumes of JLE ranging from 50 to 1200 µL to the AgNO3 solution under sunlight

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radiation as shown in Fig. S4(A). The intensity of SPR peak gradually increased with the increasing the volume of JLE from 50 to 800 µL. On further increasing the volume from 1000

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to 1200 µL, peak intensity was found to decrease. However, peak broadening was also observed, probably due to the formation of a secondary layer, which decreases the electron density upon the thin layer of the J−AgNPs. The findings were well supported by the change in the color of the solution (Inset of Fig. S4(A)). The full width at half maxima (FWHM) of the

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corresponding peaks determines the dispersity of nanoparticles, where a large FWHM attributed to peak broadening due to increased polydispersity of nanoparticles [35]. The increase in FWHM values from 94 to 176 nm, may be due to increasing in the size of AgNPs (Fig. S5(A & B), Table S1). Hence, 800 µL of the JLE was selected as the optimum volume for the synthesis of J−AgNPs.

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Time-dependent spectra of J−AgNPs To determine the reaction time, the kinetics of preparation of AgNPs was observed at various time intervals from 1 to 15 min by using a 1×10−3 M AgNO3 solution with 800 µL of JLE under sunlight radiation. At the initial stage, no distinctive peak was observed (Fig. S4(B)). After a min, peak started to appear, which became sharpest after 13 min. Accordingly, the solution

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gradually changed from faint yellow to reddish brown as shown in the inset of Fig. S4(B). Thereafter, no significant changes in the peak position and in the peak intensity were observed when the sunlight radiation was extended up to 15 min, indicating the completion of the

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reaction. It confirms that 13 min is the optimal time for the synthesis of J−AgNPs. pH effect

The pH effect on the stability of J−AgNPs was determined on the basis of changes in the

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position and in the intensity of SPR peak. The effect of pH was studied in the range of 6−11 (Fig. S4(C)). The intensity of peak increased significantly from pH 6 to 7.0. With the further

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increase, the pH the intensity of SPR peak was decreased. This might be due to the

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destabilization of J−AgNPs. Hence, pH 7.0 was chosen as the optimum pH for the stabilization

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of J−AgNPs. The major advantage of this study is that the developed nanosensor will work on the neutral water pH. FTIR studies

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The FTIR spectroscopic study of the JLE and the J−AgNPs was performed to understand the involvement of the active biomolecules of JLE in the synthesis of J−AgNPs and their

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stabilization. The FTIR spectrum of JLE shows characteristic peaks at 3393 cm −1, 2926 cm−1, 1617 cm−1, 1373 cm−1, and 1036 cm−1, which may be associated with –OH, C−H, CH=CH2,

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C–O stretching of the phenolic group (Fig. 2(a)). Thus, the data reveal the presence of polyphenolic compounds in the extract. After the synthesis of the J−AgNPs the intensity of the peak at 3393 cm−1 was decreased, while the slight shifting in the peaks from their positions 2926 cm−1 to 2890 cm−1, 1617 cm−1 to 1570 cm−1 was observed, (Fig. 2(b)), which could be

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due to the reduction of the Ag+ ions by the phenol groups into the Ag atom. Thus, these results suggest that the phenolic compounds present in the JLE may act as reducing agent and stabilizing agent. The presence of tannins and other phytochemical molecules could be responsible for the conversion of silver ions into AgNPs [36]. For example, polyphenolic compounds are known to undergo photooxidation easily in presence of sunlight that would reduce Ag+ into Ag0 [37]. 8

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Fig. 2 FTIR spectra of (a) JLE, (b) J−AgNPs, and (c) J−AgNPs with Al(III).

Scheme 1 Synthesis of the silver nanoparticles in the sunlight irritation.

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Scheme 2. Synthesis of J−AgNPs and its sensing ability for the detection of Al(III) ions.

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Cyclic voltammetry studies

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The cyclic voltammogram shows the electrochemical activities of J−AgNPs. To determine the electrochemical behavior of the NPs, Pt electrode was used as the reference electrode along

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with the three runs in the range of the potential −1.5 to 1.5 V at 100 mV s −1 scan rate. Inset of Fig. S6 shows the well−defined cathodic and anodic peaks of the JLE at −0.249 V and −0.622

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V. The negative potential shows the reducing capacity of the JLE [38]. Fig. S6 depicts the interaction of different volumes of the JLE ranging from 300 to 800 µL with the bare silver nitrate. Fig. S6 shows that on increasing the concentration of the JLE, the cathodic peak current

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was increased. The corresponding peak current represents the reduction of the Ag+/Ag0 [39].

Antibacterial activity of the synthesized nanoparticles

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On the growth of the Gram-negative and Gram-positive bacteria viz. E. coli and S. aureus, the concentration effect of the J−AgNPs was predicted by measuring the optical density at 600 nm. The data were then matched with the standard antibiotics (Fig. 3, S7). t-test analysis presented p>0.05 that at higher J−AgNPs concentrations, no noteworthy difference from the control was indicated. A concentration of 45 µg/mL of the J−AgNPs prevented the growth of E. coli bacteria by 50% while 25 µg/mL concentration of the J−AgNPs prevented the growth of S. aureus bacteria by 50%. Moreover, the IC50 value for the J−AgNPs was comparable to that of 10

the standard antibiotics. Furthermore, the zone of inhibition (ZOI) developed by the J−AgNPs was 14 mm for the E. coli bacteria and 19 mm for the S. aureus bacteria (Fig. S8 & S9). The ZOI of J−AgNPs was comparable to those of standard antibiotic streptomycin 16 mm for the E. coli bacteria, 21 mm for the S. aureus bacteria, signifying the efficiency of the J−AgNPs as a bactericidal agent. The t-test demonstrates that there is no noteworthy difference between the ZOI developed by the antibiotics and the J−AgNPs. The greater antibacterial activity of the J−AgNPs for the S. aureus than that against E. coli is owing to the different cell structures of

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both between Gram-negative bacteria and Gram-positive bacteria. The outer membrane of Gram-negative bacteria, such as E. coli was predominantly covered with a layer of special

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membrane named lipopolysaccharide (LPS) molecules, which was not found in the outer membrane of Gram-positive bacteria and possibly provided an effective resistive barrier

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against J−AgNPs [40].

Fig. 3 UV-vis spectrophotometric calculation for the determination of the IC50 value of the J−AgNPs. E. coli, and S. aureus (mean standard deviation of three ± independent variables).

Colorimetric detection of Al(III) ions

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Selectivity study of the detection probe To estimate the selectivity of J−AgNPs towards different metal ions, 300 µL of each metal ion was added to the as-prepared J−AgNPs. Upon the addition of various environmentally relevant metal ions namely, Ba(II), Ca(II), Cd(II), Cr(III), Cr(VI), Hg(II), Co(II), Mn(II), Ni(II), Pb(II) and Zn(II), Mg(II), Fe(II), Cu(II) to the AgNPs solution, no significant change in the SPR spectra of probe solutions was observed. Though, a noteworthy change in the SPR band intensity was observed, when Al(III) was added as shown in Fig. 4(A) and Scheme 2. The

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selectivity of the J−AgNPs towards Al(III) was further estimated by the plot of the absorption intensity ratio (A447/412) against the concentration of different metal ions (10−100 ppm) (Fig.

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4(B)). The Al(III) induced value of A447/412 was considerably greater than that of remaining metal ions, which can be employed to demonstrate the distinctive interaction of Al(III) with

J−AgNPs (Scheme 2). The hypsochromic shift and augmentation in the value of A447/412 are due to the interaction between Al(III) and J−AgNPs. This can be further confirmed by a change

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in color of the nanoparticles solution from yellowish brownish to yellow and can be easily

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observed by the naked eye as shown in Fig. 4(C). However, there is no color change of

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nanoparticles solution after the addition of other environmentally relevant metal ions (Fig. 4(D)). Additionally, the selectivity of the synthesized probe for the Al(III) ions was further

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confirmed in the synthetic water samples. We prepared two different types of synthetic water samples possessing all the aforementioned metal ions (Sample a & b). The difference between

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sample a and b was the presence of additional Al(III) ions. Sample a possesses all the abovementioned metal ions except Al(III) ions. After that, the prepared water samples a and b were

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treated with the J−AgNPs. It is quite evident from the inset of Fig. 4(D) that J−AgNPs detected Al(III) ions in sample b even in the presence of other environmental relevant metal ions and gave negative results with the sample a. The results confirm that the present method is highly

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selective for Al(III) ions even in the presence of other metal ions. However, the method was found to give false positive results when the concentration of other metal ions was higher than

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3 ppm in synthetic water samples.

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Al(III) (C), and Optical image of the detection systems incubated with mixture of Al(III) and other ions; Inset shows the response of J−AgNPs in synthetic water samples (D). Sensitivity study of the detection probe To estimate the appropriate detection limit of the prepared nanosensor, various concentrations of Al(III) ions were added into the J−AgNPs. The colorimetric results display that with an increase in the concentration of Al(III) from 0.001 to 10 ppm, a continual color change from yellowish brown to colorless was observed as shown in Fig. 5(A). The change in the peak

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intensity of UV-vis spectra of the detection probe after the interaction with different

concentrations of Al(III) ions can be seen from Fig. 5(B). A gradual decrease in the SPR

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intensity along with line broadening and shift towards longer wavelength as a function of

Al(III) concentration was observed. Fig. 5(C) demonstrates that there is a linear relationship between absorption intensity changes and the concentrations of Al (III) over a range of 0.1–10 ppm with a linear correlation value (R2) of 0.998. The lowest detection limit of the synthesized

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probe is 0.01 ppm making it appropriate for the quantitative detection of Al(III) ions in the

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aqueous systems. The sensing reproducibility was further estimated by performing 8 replicate

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experiments for 0 ppm (blank) and 0.01 ppm of Al(III) (Fig. S10). The value of A447/A412 nm for 0.01 ppm Al(III) is considerably similar for all the replicates, which confirms their sensing

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reproducibility [41]. Furthermore, the value of A447/A412 obtained for 0.01 ppm of Al(III) ions was higher than blank, but there is no difference in the A447/A412 value between 0.009 ppm and

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blank which further confirms the LOD is 0.01 ppm (S/N = 3) [42]. Under identical conditions, HRTEM analysis was also carried out in the presence and absence

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of Al(III) ions. Fig. 6(A) shows that the J−AgNPs were apparently monodispersed, and ~12−30 nm in average diameter which underwent aggregation upon the addition of Al(III) (Fig. 6B). The histogram depicts the mean size of the synthesized particles to be around 20 nm (Fig. 6C).

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Fig. S11A & B shows DLS data which indicate that the average diameter of particles in the absence of Al(III) ions is 70 nm, which increases simultaneously up to 130 nm with the addition of Al(III) ions. The DLS records the higher values as the light is scattered from the core particle

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as well as layer on the surface of the NPs while the HRTEM measured only the metallic particle core due to this reason there is the difference in the data obtained from DLS and HRTEM. It was already observed from the experimental section that the JLE contains flavonoids and polyphenolic contents [43]. The phenolic hydroxyl constituents present in the JLE have affinity towards Al(III) ions. Al(III) is a hard acid and it has a strong tendency to coordinate with hard bases such as O and N atom [44,45]. Therefore, it can easily coordinate with the O atom present in the polyphenols. Further, a change in the zeta potential from −26 mV to 13 mV confirmed 15

aggregation of J−AgNPs in the presence of Al(III) ions. The aggregation of the J-AgNPs after the interaction with the Al(III) ions was further confirmed by the XPS analysis. Fig. 7 shows the wide scan spectrum of all the respective elements present in the synthesized AgNPs, whereas after the interaction an extra element i.e. Al(III) was also observed with the AgNPs. Thus, confirming the interaction of Al(III) with the J-AgNPs. The signal at the binding energies of 367.55 eV and 373.60 eV corresponds to the 3d5/2 and 3d3/2 orbits of Ag0 (metallic silver) before the addition of the Al(III) ions. Changes in the

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binding energies of Ag 3d peaks at 367.58 eV and 373.62 eV was observed after the addition

of the Al(III) ions (Fig. 7(A&B)) [46]. Thus, indicating the formation of silver oxide (AgO)

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after reaction of the AgNPs with Al(III) ions. Strong peaks located at about 72.9 eV and 74.6

eV in Al 2p spectrum corresponded to Al(OH)3 and Al metal ion, respectively [47]. These peaks confirm the binding of Al(III) on the surface of J-AgNPs (Fig. 7(C)). The C 1s spectrum can be deconstructed into three components at about

283.68, 284.84, and 288.39 eV

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corresponds to the C-C, carbon in C-O, and the C=O. A change was noticed in the binding

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energy after the addition of the Al(III) ions into 284.15 eV, 285.01, and 289.08 eV respectively (Fig. 7(D&E)). The three peaks at 530.13 eV, 531.70 eV and 530.85 eV for the O 1s signal of

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the J-AgNPs corresponds to the O group of the hydroxyl group in JLE, –C=O group, and M-

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OH. After the Al(III) interaction with the J-AgNPs, a shift towards lower binding energy 530.24 eV, 532.81 eV, 531.85 eV respectively was observed (Fig. 7(F&G). The binding energy

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changes are mainly attributed to the formation of the Al-O bonds between J-AgNPs and Al(III)

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ions (Table S2 depicts binding energy of J–AgNPs before and after interactions with Al(III)).

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ion concentrations (0.001−10 ppm) (B), linear calibration plot for the various concentrations

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of Al(III) ions (C).

Fig. 6 HRTEM images of J−AgNPs (A) before, (B) after interaction with Al(III) ions, (C)

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Histogram depicting the mean size of the synthesized nanoparticles.

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Fig. 7 XPS spectra of (A)Ag 3d before (B) after, (C) Al 2p, (D) C 1s before (E) after, (F) O 1s before (G) after the addition of Al(III) ions. SERS study on the interaction of Al(III) ions with J−AgNPs The interaction between the J−AgNPs and the Al(III) ions was further explored by using Raman spectroscopy. The SERS spectrum of the J−AgNPs is shown in Fig. S12. The 20

sensitivity of the AgNPs for the SERS is very less; therefore, no noticeable signal was observed. To improve the response of AgNPs for SERS, it was obligatory to take advantage of the additional boost up in intensity provided by a molecular resonance, and therefore dye molecule Rhodamine 6G (R6G) was utilized. Thus, the SERS spectra were observed at laser power 0.05% and at λ¼ = 532 nm. In the SERS spectrum of R6G molecule various peaks were observed with low intensity at 1648 cm−1,1575 cm−1, 1539 cm−1, 1507 cm−1, 1423 cm−1, 1389 cm−1,1361 cm−1, 1310 cm−1, 1119 cm−1, 771 cm−1, and 612 cm−1 as shown in curve a Fig. S12.

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The band at 771 cm−1 is assigned to the out of plane bending of the hydrogen atoms of the

xanthene skeleton, whereas the peak at 612 cm−1 is ascribed to in−plane bending of the

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xanthene ring [48]. For the signal enhancement, the AgNPs surface was coated with the solution of 10−4 M R6G and a drop of mixture placed on the gold−coated−plate. After drying the sample for 12 h, spectra were taken. With AgNPs there was an enhancement of R6G molecule signal as shown in curve b, Fig. S12 [49]. As the various concentration of the Al(III) ions in the range

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from 0.01 to 10 ppm was added on these substrates; a decrease in the signal of the gaps with

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the R6G molecule as the function of the Al(III) ions concentration was observed (curve c-f,

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Fig. S12). The similar observations were noticed during the study of the variation of the intensity of SPR peaks for AgNPs.

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Effect of the size on the sensing of J−AgNPs

To determine the effect of the size of the synthesized particles on the sensing response, we

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considered particle size 12 nm and 48 nm. The sensitivity of the AgNPs was increased from 0.01 to 0.005 ppm when the size of AgNPs was increased from 12 to 48 nm. The sensitivity of

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the sensor is mainly governed by shape, orientation, interparticle distance, size and distribution of aggregates (number of particles). With the increase in the particle size, distribution of aggregates increases, which results in a decrease of the interparticle distance, which in turn

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enhances the sensitivity by plasmon coupling of nanoparticles [50]. Another reason for increased sensitivity could be due to increasing radiative damping or retardation [51]. Fig. 8(A) depicts the UV-vis spectra of J−AgNPs (48 nm) with Al(III) ions. With the increase in the concentration

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of the Al(III) ions, a decrease in the peak intensity was observed. Accordingly, the color of the particles changes from green-brown to light yellow Fig. 8(B). The aggregation of nanoparticles was further confirmed by HRTEM analysis (Fig. 8(C)). Though the largely sized nanoparticles are more sensitive, their stability is lower, and this limits their application.

21

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Fig. 8(A) UV-vis spectra, an optical image of J−AgNPs (25 nm) in the presence of Al(III) (B), and HRTEM image showing the aggregation of particles in the presence of Al(III) ions (C).

Practical Application

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The field application of the synthesized J−AgNPs was tested in real water samples. The samples were spiked with a known concentration of Al(III) and added to AgNPs solution. The responses were then examined by using UV-vis spectroscopy. Fig. S13A displays the colorimetric response of the J−AgNPs in tap water with Al(III). With the increase in the concentration of the Al(III) ions, there was a linear increase in the response of the detection probe. The response of the detection probe was somewhat comparable to tap water as well as 22

for the Milli−Q water. The detection system displays linearity with R2 values of 0.999 for Al(III) ions, at small concentrations (Fig. S13B). The sensitivity of this colorimetric method is compared to reported methods [20, 21, 22, 45,52,53]. The limit of detection of the present method is lower or comparable to the other reported methods for Al(III) ion (Table 1). Thus, the present method used biosynthesized AgNPs is specific, green and accurate and could be applied to detect Al(III) in real water samples.

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Table 1. The comparison of reported colorimetric methods with present work in terms of

Method

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reducing and stabilizing agent, pH, and LOD.

Sensing

Reducing Stabilizing Real

LOD

probe

agent

(ppm)

agent

water

Reference

analysis AuNPs

NaBH4

Triazole

NaBH4

MMT

Water and 0.01429 21 human

NaBH4 _

Chalcone

Ionic

urine specimen Vermicelli 0.026

22

Lake

and 0.030

52

Liquid _

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Fluorescent

AuNPs

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Colorimetric

20

A

AuNPs

M

Colorimetric

0.0004

N

ether

Seawater

U

Colorimetric

based

tap water

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organic

nanoparticles

Colorimetric

AuNPs

Citrate

Citrate

Water

0.026

53

Colorimetric

1_ H

_

_

Abiotic

0.016

54

0.01

This

and living

A

and

fluorescent Colorimetric AgNPs

cells JLE

JLE

Tap water

Cost analysis of present nanosensor

23

work

The materials required for the synthesis of the nanosensor were easily available and includes AgNO3, easily available and inexpensive Jamun leaves. The estimated cost for nanosensor (100 mL) is estimated to be US $0.03. This proves that the cost of developed nanosensor is most affordable in the areas with resource-limited settings. For the field level applicability of the prepared nanosensor, a synthetic water sample was prepared with the known concentration of the Al(III) ions. The 0.1 mL of the prepared nanosensor is sensed 100 mL of the synthetic water sample. And the nanosensor did not give any response with the water sample having any other

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A

N

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metal ions (Scheme 3.)

Scheme 3. Diagrammatic representation of the field level capability of the prepared

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nanosensor

Conclusion

We have developed a cost-effective and greener method for the synthesis of AgNPs using

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Jamun leaves extract in an aqueous medium under ambient solar energy. The size of the AgNPs was altered by changing the concentrations of capping agent, reaction time and pH of the reaction medium. For the stabilization of the synthesized AgNPs, the Jamun leaves extract behaved as the reducing as well as the capping agent while the stability of the synthesized particles lasts up to 40 days. The synthesized J−AgNPs were employed as colorimetric nanosensors for selective detection Al(III) over the other heavy metal ions in aqueous medium. 24

Due to the obvious changes in solution color, Al(III) could be detected by the naked eye with a visual detection limit of approximately 0.01 ppm. When the simultaneous presence of other environmentally relevant metal ions is higher than 3 ppm, the sensor gives false positive results. The proposed method has numerous advantages over other existing technique for the Al(III) ion detection that it does not require any modification, complicated instrumentation and is costeffective which simplifies the operation and reduces associated costs. The prepared nanosensor also displayed effective antibacterial activity against E. coli and S. aureus strains. This method

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provides a new economical, rapid, and simple route for the potential applications in environmental water sample analysis.

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Notes The authors declare no competing financial interest

Acknowledgments

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We gratefully acknowledge support from the Ministry of Human Resource Development,

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Department of Higher Education, Government of India under the Scheme of Establishment of Centre of Excellence for Training and Research in Frontier Areas of Science and Technology

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(FAST), vide letter No, F. No. 5−5/201 4−TS. Vll. The authors also thank AIIMS, New Delhi,

A

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and University of Rajasthan, Jaipur, for providing the HRTEM facility.

25

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Author Biographies Dr. Dinesh Kumar

Dr. Dinesh Kumar is an academician and researcher of international recognition in School of Chemical Sciences at the Central University of Gujarat, Gandhinagar. He has joined

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as an Associate Professor in the School of Chemical Sciences at the Central University of Gujarat, Gandhinagar in

2017. Prior to Joining the Central University of Gujarat, he

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taught at the Banasthali University, Rajasthan as an Assistant

Professor and as an Associate Professor. Dr. Kumar has obtained his master and Ph.D. degrees in Chemistry from the Department of Chemistry, University of

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Rajasthan, Jaipur in 2002 and 2006, respectively. Dr. Kumar has received many

the

development

of

capped

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national and international awards and fellowships. His research interest focuses on nanoparticles,

core-shell,

metal

oxide-based

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nanoadsorbents and nanosensors for the removal and sense of health hazardous

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inorganic toxicants like fluoride and heavy metal ions from aqueous media. To boot, his research interests also focus on the synthesis of supramolecular metal complexes,

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metal chelates and searching their biological effectiveness. He has authored and coauthored more than 70 publications in journals of international repute, one book, more than three dozen book chapters, and 70 presentations/talks at national/international

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conferences. He is a member of many reputed academic societies, and he invited an as subject expert in many selection committees. He has supervised thirteen Ph.D.

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students, and four students are working for their Ph.D. degree under his supervision. Dr. Kumar has completed several major research projects, which had awarded from

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different govt funding agencies.

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Dr. Priyanka Joshi Dr. Priyanka Joshi received her Ph.D. degree in 2018 from the Banasthali University on for colorimetric sensing of heavy metal ions in aqueous media. She completed her M.Sc. degree from Banasthali University, Rajasthan. She has published 4 papers in peer reviewed international Journals like ACS Sustainable Engineering and Chemistry and 3 book

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chapters in very reputed publishers like Elsevier. Her current research interest focuses on the development of colorimetric nanosensors for waste

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water remediation.

Ms. Ritu Painuli

M

A

N

U

Ritu Painuli is pursuing her Ph.D. degree in the Department of Chemistry at Banasthali University, Rajasthan. She received her M.Sc. degree in Chemistry from Banasthali University in 2015. She has authored and co-authored in 2 publications in peer reviewed international Journals. She has written 4 book chapters in renowned publishers like Elsevier. Her current research interest is on development of colorimetric

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nanosensors for the detection of heavy metal ions from different water resources.

32