Green synthesis of silver nanoparticle for the selective and sensitive colorimetric detection of mercury (II) ion

Accepted Manuscript Green synthesis of silver nanoparticle for the selective and sensitive colorimetric detection of mercury (II) ion

Vijay Kumar, Devendra K. Singh, Sweta Mohan, Daraksha Bano, Ravi Kumar Gundampati, Syed Hadi Hasan PII: DOI: Reference:

S1011-1344(16)31059-4 doi: 10.1016/j.jphotobiol.2017.01.022 JPB 10723

To appear in:

Journal of Photochemistry & Photobiology, B: Biology

Received date: Revised date: Accepted date:

18 November 2016 24 December 2016 26 January 2017

Please cite this article as: Vijay Kumar, Devendra K. Singh, Sweta Mohan, Daraksha Bano, Ravi Kumar Gundampati, Syed Hadi Hasan , Green synthesis of silver nanoparticle for the selective and sensitive colorimetric detection of mercury (II) ion. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Jpb(2017), doi: 10.1016/j.jphotobiol.2017.01.022

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ACCEPTED MANUSCRIPT Green synthesis of silver nanoparticle for the selective and sensitive colorimetric detection of mercury (II) ion. Vijay Kumara, Devendra K. Singha,

Sweta Mohana, Daraksha Banoa, Ravi Kumar

Gundampatib, Syed Hadi Hasana* Nano Material Research Laboratory, Department of Chemistry, Indian Institute of Technology

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(BHU), Varanasi - 221005, U.P., India.

Department of Chemistry and Biochemistry, University of Arkansas, Fayetteville, AR-72701,

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b

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USA.

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*Corresponding author’s details:

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E-mail; [email protected], [email protected]

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Mobile No.: +91 9839089919

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Phone No.: +91-542-6702861

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ACCEPTED MANUSCRIPT Highlights:  Instant biosynthesis of AgNPs using an aqueous extract of Murraya koenigii.  AgNPs was synthesized without external energy supply and instrumental support.  30 min exposure time, 4 mM AgNO3 and 2% AEM were found to be optimum for the synthesis of AgNPs.

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 The synthesized AgNPs showed selective and sensitive colorimetric detection towards

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Hg2+.

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ACCEPTED MANUSCRIPT Abstract An ecofriendly and zero cost approach has been developed for the photoinduced synthesis of more stable AgNPs using an aqueous extract of Murraya koenigii (AEM) as a reducing and stabilizing agent. The exposed reaction mixture of AEM and AgNO3 to sunlight turned dark brown which primarily confirmed the biosynthesis of AgNPs. The biosynthesis was

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monitored by UV–vis spectroscopy which exhibited a sharp SPR band at 430 nm after 30 min of

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sunlight exposure. The optimum conditions for biosynthesis of AgNPs were 30 minute of sunlight exposure, 2.0% (v/v) of AEM inoculum dose and 4.0 mM AgNO3 concentration. TEM

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analysis confirmed the presence of spherical AgNPs with average size 8.6 nm. The crystalline nature of the AgNPs was confirmed by XRD analysis where the Bragg’s diffraction pattern at

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(111), (200), (220) and (311) corresponded to face centered cubic crystal lattice of metallic

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silver. The surface texture was analyzed by AFM analysis where the average roughness of the

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synthesized AgNPs was found 1.8 nm. FTIR analysis was recorded between 4000-400 cm-1 which confirmed the involvement of various functional groups in the synthesis of AgNPs. On the

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basis of the linear relationship between SPR band intensity and different concentration of Hg2+, the synthesized AgNPs can be used for colorimetric detection of Hg2+ with a linear range from 50

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nm to 500 µM. Based on experimental findings, an oxidation-reduction mechanism between

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AgNPs and Hg2+ was also proposed. Keywords: AEM; AgNPs; SPR; Colorimetric detection; Hg2+

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ACCEPTED MANUSCRIPT 1. Introduction Nanobiotechnology is an emerging field of science and technology for the development, improvement, and utility of nanostructures [1]. Since last two decades the green synthesis of silver nanoparticles (AgNPs) has received a considerable attention of the researchers due to growing need in several fields; such as, catalysis [2], biosensing [3], antibacterial [4], antioxidant

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[5] antiviral [6] and anticancer [7], wound healing [8], medicine [9] etc. The principles of green

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chemistry have played a prominent role in nanobiotechnology as it employed the involvement of green and eco-friendly route using algae [10], fungi [11] and plants [3] for the synthesis of silver

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nanoparticles. Currently, green synthesis is most preferred as it avoids the sophisticated instrumentations, technical expertise and excessive use of toxic chemicals hazardous to the

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environment [12-18].

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At present, plant extracts mediated green synthesis is more advantageous over other

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biological systems as it eliminates the need of culture, aseptic condition and maintenance [2-3]. In addition to this, the plant extract mediated synthesis is user-friendly, economical and less

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biohazard process. It contains various phytochemicals such as terpenoids, flavonoids, phenol

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derivatives and plant enzymes like hydrogenases, and reductases which act as both reducing and capping agent which reduce the metal salts and deters the aggregation of the nanoparticles [4-5].

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Hence; currently, the plant extracts are being used extensively as a reducing agent for the synthesis of AgNPs [19]. Recently, several plants like Rosmarinus officinalis [20], Coffea arabica [21], Crataegus douglasii [22], Skimmia laureola [23], etc. have been reported for the green synthesis of AgNPs (Table 1). The photoinduced green synthesis of AgNPs using plant extract is proved to be more economic efficient and eco-friendly where the visible light increases rate of biosynthesis. There

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ACCEPTED MANUSCRIPT are several articles which have been published for the biosynthesis of AgNPs using sunlight induced route. In our previous study, we have reported the photoinduced synthesis of silver nanoparticles using an aqueous extract of Erigeron bonariensis [2], Croton bonplandianum [3], Euphorbia hirta [4], Polyalthia longifolia [5], Salvinia molesta [24] and Xanthium strumarium [25] through sunlight induced route. The plant Murraya koenigii belongs to family Rutaceae,

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commonly called as curry leaf. Murraya koenigii is a native of, Sri Lanka, India, and other South

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Asian countries which is used traditionally as antiemetic, antidiarrhoeal, and blood purifier. Phytochemical analysis of leaves revealed the presence of carbohydrate, tannin, alkaloid, steroid,

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triterpenoid, flavonoids, saponins, sugars, and protein [26, 27].

The main objective of the present study is photoinduced, one pot and green synthesis of

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stable AgNPs using an aqueous leaf extract of M. koenigii. It was also tried to avoid the

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utilization of any toxic chemical, technical expertise and sophisticated instrumentation in the

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synthesis of AgNPs which might make it eco-friendly, economical and nonhazardous. Thus parameters affecting the synthesis of the AgNPs would also be optimized and the optimum

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different metal ions.

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AgNPs thus obtained would be applied for the investigation of sensing potential towards of

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2. Materials and Method

In the current study, silver nitrate (AgNO3), the only chemical used for the synthesis of AgNPs was of analytical grade having high purity and sourced from Merck, India. The standard stock solution (1 mM) of metal ions (Fe3+, Fe2+, Pb2+, Cd2+, Co2+, Al3+, As3+, As5+, Cu2+, Hg2+, Cr6+) were prepared with ultrapure water from the respective metal salts (FeCl 3, Cl2Fe.4H2O, Pb(NO3)2, Cd(NO3)2.4H2O, Co(NO3)2.6H2O, Al(NO3)3, As2O3, Na2HAsO4.7H2O, CuCl2.2H2O, Hg(NO3)2.H2O, K2Cr2O7. Fresh leaves of M. koenigii were collected from campus area of Indian

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ACCEPTED MANUSCRIPT Institute of Technology (Banaras Hindu University), Varanasi, India and were further processed in the lab to prepare leaf extract. 2.1 Preparation of leaf extract The aqueous leaf extract of M. koenigii was made by washing the fresh leaves several

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times with deionized water to remove dust and other adhering impurities. After that, the leaves

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were air dried under shade to eradicate the moisture completely. The dried leaves were cut into

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fine pieces, and 25g of it was boiled for 10 min in 100 mL of deionized water. After that, the

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aqueous extract of M. koenigii (AEM) was collected and filtered through Whatman filter paper No. 1. The prepared AEM was stored as a stock solution at 4°C and used within 3 days (Figure

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S1).

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2.2 Biosynthesis of AgNPs

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For the biosynthesis of AgNPs 1% (v/v) of AEM inoculum dose was added into 100 mL of 1 mM AgNO3 solution and exposed to bright sunlight. The pH of the reaction mixture was

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neutral. The temperature of the ambient environment in bright sunlight and solar intensity of

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incident sunlight radiation were 29°C and 63600 lx. The color of the reaction mixture exposed to bright sunlight changed from colorless to reddish-brown instantaneously. This change in color

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confirmed the synthesis of AgNPs. The synthesis of AgNPs was regularly monitored using UV– visible spectroscopy. For the comparison purpose, the same experiment was also performed in dark condition by keeping it in a black colored closed vessel where the temperature of the dark condition and light intensities were 24°C and 0 lx respectively. The reaction mixture kept in the dark did not exhibit any sharp SPR band as well as a significant change in color up to 6 hrs which clearly indicated the photo catalytic action of sunlight on synthesis of AgNPs. Therefore, further all the AgNPs synthesis experiments were performed in bright sunlight to optimize the 6

ACCEPTED MANUSCRIPT other process variables using one factor at a time approach. The process variables like duration of sunlight exposure, AEM inoculum dose, and AgNO3 concentration were screened for optimization purpose in the range from 0-30 min, 1.0%-6.0% (v/v) and from 1.0 mM to 6.0 mM respectively. After the completion of reaction at these optimized conditions the synthesized AgNPs were purified by centrifuging at 15000 rpm for 15 min and subsequently re-dispersed in

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deionized water to eliminate the water soluble biological molecules and other secondary

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metabolites. This process was repeated four times and after vacuum drying the final mass of AgNPs was collected.

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2.3 Characterization of AgNPs.

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The optical property of AgNPs was studied UV–Visible spectrophotometer (Evolution 201, Thermo Scientific) in the range of 300 to 800 nm. The involvement of several functional groups the

synthesis

of

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investigated

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Fourier

transform

infrared

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spectrophotometer (Perkin Elmer Spectrum 100) the range of 4000–400 cm-1. The crystalline nature of AgNPs was determined using X-ray Diffractometer (Rigaku Miniflex II) having Cu Kα

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radiation source and Ni filter in the range of 20° to 80° at a scanning rate of 6° min−1. The

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surface texture was analyzed by Atomic Force Microscopy (AFM) the using NT-MDT working in the contact mode. The AFM images have been processed using NOVA software. The

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morphology and size of AgNPs were initially determined by Field Emission Scanning Electron Microscopy equipped with energy-dispersive X-ray analysis (SEM-EDX, Hitachi H-7100). EDX analysis confirmed the elemental composition and purity of powdered sample. The morphology and size of the biosynthesized AgNPs were further analyzed by Transmission Electron Microscopy (TEM) carried out on TECNAI 20 G2-electron microscope operated at accelerating voltage 200 kV. The Selected Area Electron Diffraction (SAED) demonstrated the concentric diffraction rings which also confirmed the crystallinity of synthesized AgNPs. 7

ACCEPTED MANUSCRIPT 2.4 Detection of metal ions For the colorimetric detection of metal ion, 1 mL of AgNPs solution was used as a primary testing sample. Then the effect of various metal ions on the intensity of the SPR band of AgNPs was investigated by adding 100 µL of particular metal ion and the corresponding UV–vis

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absorption spectra were recorded. The maximum quenching in the SPR band intensity of AgNPs

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and disappearance of color was observed only in the presence of Hg2+. Therefore, different

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range by employing the quenching in AgNPs SPR spectra.

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concentration ranges 50 nM to 500 µM for Hg2+ was used to determine the linear detectable

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3. Results and Discussion 3.1. Primary confirmation

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The surface plasmon resonance (SPR) produced through collective oscillations of free

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conduction electrons caused to change in color of the reaction mixture from watery to reddish brown [28]. This change in color is the primary confirmation of biosynthesis of AgNPs. The

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reaction mixture exposed to bright sunlight exhibited an instant change in color within few min

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of AEM inoculation into AgNO3 solution which showed the rapid synthesis of AgNPs whereas the reaction mixture kept at dark condition failed to attain the characteristics brown color even

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after 6 hours of incubation (Fig. S2). This time lag clearly indicated that the potent biosynthesis of AgNPs was much faster in bright sunlight as compared to dark condition. The UV–visible absorption spectroscopy is a novel tool to ascertain the formation AgNPs. The effect of bright sunlight exposure on the synthesis of AgNPs, color change and the pattern of SPR band was studied by withdrawing the samples from the reaction mixture at a regular time interval of 5 min and screened between 300 to 800 nm through UV-visible spectroscopy. It was found that after 25 min of bright sunlight exposure, the screened samples 8

ACCEPTED MANUSCRIPT showed an SPR band at 426 nm which attributed to the characteristic surface plasmon resonance of AgNPs having λmax values in the range of 400-500 nm [29]. It has been investigated that the nature, surrounding media, shape and size of the nanoparticles governs the SPR absorbance [30]. A single SPR band favors the growth of spherical AgNPs whereas two or more SPR bands favor the variation in shape of the AgNPs [25]. In the present study, the reaction mixtures showed

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single SPR band which indicated the presence of spherical shaped biosynthesized AgNPs which

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was further confirmed by FESEM and HRTEM analysis. 3.2. Optimization

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3.2.1. Sunlight exposure

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The experiments were conducted to optimize the exposure time and its effect on the synthesis of AgNPs by keeping the reaction mixture in bright sunlight. It was found that the

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reaction mixtures exposed to bright sunlight showed an instant color change within few seconds.

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The effect of sunlight exposure time on the biosynthesis of AgNPs was optimized by screening the reaction mixtures at each 5 min of the interval where the UV-visible spectra at 441 nm after 5

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min of sunlight exposure exhibited characteristics of spherical AgNPs [31]. The screened

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samples at each 5 min of the interval from 1% to 3% AEM showed single and sharp SPR band whereas from 4% to 6 % AEM the single and sharp SPR band changed to broader SPR band with

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a shoulder peak at higher wavelength (Fig. 1). The presence of single and sharp SPR band indicated the presence of monodispersed AgNPs while the broader SPR band with shoulder peak at a longer wavelength advocated the presence of polydispersed AgNPs [25]. Figure S3 clearly corroborated that while using 1% AEM, the SPR band intensity increased gradually up to 30 min which indicated that the biosynthesis process of AgNPs also increased while further no increase in the intensity of SPR band confirmed the completion of the biosynthesis process [32]. During biosynthesis process, neither blue nor red shift was observed. The reaction volume kept in a 9

ACCEPTED MANUSCRIPT closed vessel in the dark exhibited no characteristic SPR band up to 6 hrs which showed that the reduction of silver ion in the current system was slower because of the absence of sunlight (Fig. S2). On the basis of these results 30 min was considered as optimum exposure time for the

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photoinduced biosynthesis of AgNPs and it was fixed for further experiments. The control

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solution of AEM and AgNO3 neither developed the characteristic reddish brown colors nor did

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they display the characteristic SPR band on exposure to sunlight. These results indicated that

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abiotic reduction of AgNO3 did not occur under the reaction conditions that were used (Fig. S3). 3.2.2. AEM Inoculum dose

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The quantity of the reductant in the reaction mixture was optimized by varying AEM

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inoculum dose (v/v) from 1.0% to 6.0% while keeping other parameters constant at 30 min of sunlight exposure time and 1mM AgNO3 concentration. The pattern of SPR band intensity and

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respective color change of the reaction mixture on increasing the AEM inoculum dose is shown

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in Figure 1. It was found that that the color of the reaction became darker at each screening time interval from 5 to 30 min of each volume of the AEM from 1% to 6% which indicated the

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increased synthesis of AgNPs [33, 34]. The screening of the reaction mixture of each inoculum

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doses from 5 to 30 min showed that while using 1% AEM inoculum dose the single SPR bands were broader and less intense which supported that the AgNPs were fewer and larger in size [25]. Further, on increasing the AEM dose up to 2.0 % AEM inoculum dose the intensity and sharpness of single SPR band increased up to 30 min exposure time which indicated the increased synthesis of AgNPs in larger number [35]. The presence of single SPR band indicated that the particles were spherical in shape which was confirmed by FESEM and HRTEM images also.

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ACCEPTED MANUSCRIPT Further, on increasing the AEM inoculum dose from 3.0% to 6.0% the sharpness of the single SPR band decreased and started to flattened accordingly with increasing inoculum dose as well as sunlight exposure time. It was also observed that SPR band of 4%, 5% and 6% AEM developed a small hump towards longer wavelength at 489 nm, 493 nm, and 489 nm. The decreasing of the sharpness and broadening of the SPR band towards larger wavelength indicated

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that at higher AEM dose anisotropic larger size AgNPs were synthesized with lesser number due

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to agglomeration [25]. 3.2.3. AgNO3 concentration

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The synthesis of AgNPs was also optimized using various concentration of AgNO3 from

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1.0 mM to 6.0 mM at 2% AEM inoculum dose and 30 min of sunlight exposure time. Figure 2 represented the SPR bands of AgNPs of different AgNO3 concentrations i.e. 1.0 mM, 2.0 mM,

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3.0 mM, 4.0 mM, 5.0 mM and 6.0 mM at 439 nm, 446 nm, 452 nm, 451 nm, 451 nm and 451 nm

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respectively after 30 min of sunlight exposure. It was found that as the concentration of AgNO 3 increased the color of reaction mixture darkens gradually from 5 to 30 min. Since the color of

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reaction mixture directly related to the size of nanoparticles thus it is clear that AgNO3

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concentration governs the particle size distribution [36]. The SPR band of AgNPs showed a distinct pattern with increasing concentration of AgNO3. Figure 2 represented that the intensity

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and sharpness of the SPR band of each AgNO3 concentration increased up to 4 mM at each time interval and then decreased. The increase in SPR band intensity up to 4 mM at each screening time interval indicated that as AgNO3 concentration increased the synthesis of AgNPs also increased [29]. The simultaneous shifting of SPR bands with the advancement of time as well as AgNO3 concentration was also observed. With the proceeding of time from 5 to 30 min, the SPR band of 1.0 mM remained at the same point. Whereas SPR band of 2.0 mM, 3.0 mM, 4.0 mM, 5.0 mM and 6.0 mM shifted towards higher wavelength from 441 nm to 446 nm, 441 nm to 452 11

ACCEPTED MANUSCRIPT nm, 443 nm to 451, 443 nm to 451 and 441 to 451 nm respectively. On increasing the AgNO3 concentration from 1.0 mM to 6.0 mM after 30 min of sunlight exposure the SPR band red shifted from 439 nm to 451 nm respectively. This red shifting of SPR band of each AgNO3 concentration with each screening time indicated the enhancement of AgNPs size which might

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be due to agglomeration [37].

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Thus, to get smaller and more AgNPs with controlled growth and smaller size, 4.0 mM

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AgNO3 concentration at 2.0% AEM and 30 min exposure time were chosen as optimal for the

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current study. 3.3. Mechanism involving AgNPs formation

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Currently, the photoinduced route using aqueous extracts of leaves are widely used for

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the synthesis of AgNPs. Such a green route has evolved as an economically efficient, environment-friendly and much potent to synthesize AgNPs without instrumentation.

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phytochemicals present in AEM such as carbohydrate, tannin, and flavonoids act as an effective

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reducing agent while proteins and some other phyto-chemicals act as a capping (stabilizing)

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agent for AgNPs synthesis [4, 38].

The phytochemical constituents of M. koenigii, photocatalytic effect of sunlight, and

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FTIR analysis of AEM and synthesized AgNPs inspired us to propose the possible synthesis pathway mechanism. We have proposed a stepwise mechanism of AgNPs synthesis. When AEM was added into the AgNO3 solution, the OH group of polyphenolic compound (tannin and flavonoid) present in AEM bound to Ag+ and formed the Ag+/AEM complex [5]. The rapid change in the color clearly revealed that the synthesis of AgNPs was driven by the photocatalytic effect of sunlight. In the first step, the Ag+/AEM complex got photo-activated and produced the hydrated electrons by debonding of OH group of AEM after absorbing the photons of light. The 12

ACCEPTED MANUSCRIPT hydrated electrons thus generated in the first step reduced Ag+ to Ag0 in the second step. In the third step, the Ag0 nucleated to form nanoclusters which were followed by the fourth step where the formation of AgNPs occurred by the aggregation of nanoclusters (Scheme 1). 3.4. Characterization

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After initial confirmation of biosynthesis through UV-visible spectrophotometer further

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the optimized AgNPs obtained at 30 min exposure time, 4 mM AgNO3 concentration and 2.0%

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of AEM inoculums dose was characterized through FESEM, EDX, HRTEM, SAED, XRD,

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FTIR and AFM analysis.

The FESEM analysis was carried out by drop casting technique. For this, a single drop of

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AgNPs colloid was casted over a small rectangular piece of aluminum foil and dried under table

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lamp for 8 hr. The FESEM images thus obtained confirmed the presence of smaller and isotropic AgNPs which is shown in Figure 3A. The Energy dispersive X-ray detector (EDX) equipped

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with FESEM showed a prominent spectral signal in the silver region (Ag) approximately at 3

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keV which confirmed the elemental composition of AgNPs (Fig. 3B). The presence of characteristics spectral signals for carbon and oxygen in the EDX spectrum corresponded to the

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presence of biological materials like proteins, carbohydrates and other biomolecules of AEC

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present in the vicinity of AgNPs. The HRTEM analysis was carried out to investigate the detailed size and shape of the synthesized AgNPs. For the HRTEM analysis, a drop of colloidal AgNPs was placed on the carbon coated copper grid, dried under table lamp and loaded onto a specimen holder. The HRTEM images of AgNPs at different magnifications are shown in Figure 4 A to C which revealed the abundance of AgNPs ranging from 2 to 22 nm in samples. The shape of the AgNPs was spherical that was in agreement with the single SPR band obtained from the optimized 13

ACCEPTED MANUSCRIPT reaction mixture of UV-visible spectroscopy. Figure 4 D showed the selected area electron diffraction (SAED) pattern of AgNPs. The typical SAED pattern with bright circular rings confirmed that the synthesized AgNPs were crystalline in nature. The size distribution histogram of AgNPs corresponding to HRTEM images represented that maximum AgNPs were in the

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range of 7 nm to 9 nm having an average size distribution of 8.6 nm (inset Fig. 4C).

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XRD analysis determined the crystalline nature of the synthesized AgNPs, and data were

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collected in the angular range 10° ≤ 2θ ≤ 80°. Figure 5 represented the typical XRD pattern of AEM synthesized AgNPs where the diffraction peaks observed at 2θ were 38.08°, 44.12°, 64.14°

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and 77.1°. The Peaks were well matched with standard diffraction data with those reported for

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silver by the Joint Committee on Powder Diffraction Standards (JCPDS) file no: 040783 and attributed to (111), (200), (220) and (311) Bragg reflections respectively. These Bragg

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reflections corresponded to the crystalline planes of the face-centered cubic (fcc) crystal lattice

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of metallic silver. The average estimated crystallite size of the AgNPs was in the range of 12.5

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nm. These results were consistent with the sizes of AgNPs obtained from the TEM analysis. FTIR study confirmed the involvement of various functional groups of reducing and

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stabilizing agent present in the AEM. Figure 6 represents the FTIR spectra of both dried AEM

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and powdered AgNPs where the FTIR spectra of AEM was taken as a control to confirm the involvement of functional groups in the synthesis of AgNPs. The FTIR spectra of AEM showed the absorbance band at 3442 cm-1, 2076 cm-1, 1637 cm-1, 1328 cm-1 and 669 cm-1. The FTIR band at 3442 cm-1 was responsible for OH stretching [39]. According to the proposed mechanism, the enol form present in tannins and flavonoids changed to the quinonoid form after reduction which caused the shifting of the –OH group peak from 3442 cm-1 to 3436 cm-1 [40]. The band present at 1637 cm-1 is a characteristic of amino acids containing NH2 groups of the

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ACCEPTED MANUSCRIPT protein present in AEM which were utilized in the stabilization of AgNPs [41]. The band present at 1328 cm-1, represented the polyphenolic groups of tannin and flavonoids present in AEM. These bands completely vanished in the FTIR spectra of the AgNPs showing that the -OH groups were the main group responsible for the synthesis of AgNPs.

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The AFM technique was used to study the surface texture of biosynthesized AgNPs. For

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the AFM analysis, a single drop of colloidal AgNPs was casted over cover slip and dried under

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table lamp for two hrs. The 2D and 3D topographical view of AgNPs of sampling area 2 µm × 2 µm are represented in Figure 7 A and B respectively. It was found that the average roughness of

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the sample was 1.8 nm. (Fig. 7 C). The maximum profile peak height and valley depths were 3.5

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nm and 5.0 nm respectively.

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3.5. Detection of metal ions

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The AgNPs obtained at the optimum conditions (30 min exposure time, 2 % AEM dose and 4 mM AgNO3 concentration) was applied for the colorimetric detection of metal ions. The

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metal ion detection ability of AEM synthesized AgNPs was studied separately for each metal ions (Fe3+, Fe2+, Pb2+, Cd2+, Co2+, Zn2+, Ni2+, Al3+, As3+, As5+, Cu2+, Hg2+, Mn2+). A fixed

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concentration of 100 μL of a 500 μM metal ion solutions were added to the 1 mL of the

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optimized AgNPs solution (4 mM AgNO3). UV-visible spectroscopy monitored the change in intensity of the SPR band. When the solutions of the different metal ion were added to the AgNPs solution, it was notified that Cu2+, Cr+6, Fe3+, Pb2+, Hg2+, Al3+, As3+, As5+, Cd2+, Co2+, Fe2+, metal ions did not cause a change in color of the solution and showed a slight change in SPR band intensity. Whereas, when Hg2+ salt was added, the solution color of the AgNPs changed from dark brown to transparent (Fig. 8A). In addition to this, the SPR band intensity for Hg2+ completely vanished which corroborated the high selectivity and sensitivity towards Hg2+ 15

ACCEPTED MANUSCRIPT metal ion (Fig. 9A). Figure 9B showed the change in absorbance after adding different metal ion solution into the AgNPs solution. Further, the quantitative detection study of Hg2+ metal ion was carried out by adding the different concentrations ranging from 50 nM to 500 µM into the AgNPs solution at similar

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experimental conditions. When different concentration of Hg2+ were added to the AgNPs

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solution, it was observed that the color of the solution kept on changing from dark brown to light

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brown and finally transparent (Fig. 8B). In addition to this, SPR band shifted towards the blue region and completely vanished accordingly to the increased concentration of the Hg2+ (Fig. 9C).

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The linear relationship between the SPR band intensity and Hg2+ ion concentration within the

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range of 50 nM to 500 µM is shown in Figure 9D. The fitting line can be expressed as ΔA= 9.13 [Hg2+] + 0.0125 (ΔA=A0-A, where A0 and A represented the intensity of the SPR band in the

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absence and presence of Hg2+ respectively, [Hg2+] refers to the concentration of Hg2+) having

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3.6. Mechanism of detection

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linear regression coefficient (R2) of 0.952.

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Usually, the colorimetric sensing of transition metal ions is carried out using fluorescent nanoparticles which is caused by the strong interaction resulting into quenching effect [42]. Non-

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fluorescent AgNPs have also revealed the colorimetric detection of Hg2+ on the basis of alloy formation and anti-aggregation mechanism [43]. In the current study, we have proposed the mechanism of the colorimetric sensing of Hg2+ on the basis of the disappearance of the color of the AgNPs solution, diminishing of SPR band and the obtained TEM image after addition of Hg2+ into AgNPs solution. When Hg2+ was added into the AgNPs solution, it was found that the dark brown color disappeared and the SPR band intensity diminished with minor shifting towards blue region which suggested the reduction in the amount of AgNPs in the solution with a 16

ACCEPTED MANUSCRIPT change in the size. After the addition of Hg2+ into the AgNPs solution, the presence or absence of the AgNPs was further investigated by TEM and UV-visible spectrum (Fig. S4). It is clear from the Figure S4 that Hg2+ added solution did not reveal the UV-visible spectra. On the basis of these results, we have proposed an oxidation-reduction mechanism for the colorimetric detection of AgNPs (Scheme 2). As per this mechanism, when Hg2+ is added into the AgNPs solution, an

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oxidation-reduction reaction occurs between Ag0 and Hg2+ ions, where the AgNPs get oxidized

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to Ag+ and Hg2+ ions reduced to Hg0 atom [44]. This oxidation of AgNPs to Ag+ was further confirmed by TEM analysis which did not reveal the presence of AgNPs (Fig. S4).

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4. Conclusion:

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In the present study a very simple, one step photoinduced green route was developed for the synthesis of stable and spherical shaped AgNPs. The developed photoinduced route was

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found to be very rapid and efficient to initiate AgNPs synthesis within seconds without any

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instrumental support or external energy supply like heating and stirring. Thus synthesized AgNPs was optimized using one factor at a time approach, and the optimum conditions were

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found to be 30 min of sunlight exposure time, 2.0% (v/v) of AEM inoculum dose and 4 mM

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AgNO3 concentration. Most of the AgNPs were spherical and ranged between 2 nm to 22 nm having an average size distribution of 8.6 nm. Tannin and flavonoid were concluded as possible

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reducing agents. Thus obtained AgNPs colloid was used for the selective colorimetric detection of Hg2+ which showed a linear relationship between SPR band intensity and different concentration of Hg2+ (50 nm to 500 µM). Based on experimental findings, an oxidationreduction mechanism of Hg2+ detection was also proposed. Acknowledgements

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ACCEPTED MANUSCRIPT Financial assistance to VK, DKS, SM and DB in the form of a research fellowship from MHRD New Delhi, Government of India is gratefully acknowledged. The authors are thankful to

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the Central Instrument Facility Centre IIT (BHU) Varanasi for providing research facilities.

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ACCEPTED MANUSCRIPT References 1. M.S. Akhtar, J. Panwar, Y.S. Yun, Biogenic synthesis of metallic nanoparticles by plant extracts, ACS Sustainable Chem. Eng.1 (2013) 591-602. 2. V. Kumar, D. K. Singh, S. Mohan, S. H Hasan, Photo-induced biosynthesis of silver nanoparticles using aqueous extract of Erigeron bonariensis and its catalytic activity against

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ACCEPTED MANUSCRIPT 8. P. Pourali, B. Yahyaei, Biological production of silver nanoparticles by soil isolated bacteria and preliminary study of their cytotoxicity and cutaneous wound healing efficiency in rat. J.Trace Elements Med. Biol. 34 (2016) 22-31. 9. M. Shaalan, M. Saleh, M. El-Mahdy, M. El-Matbouli, Recent progress in applications of nanoparticles in fish medicine: A review. Nanomed. Nanotechnol. Biol. Med. 12 (2016) 701-

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14. C. Jayaseelan, R. Ramkumar, A.A. Rahuman, P. Perumal, Green synthesis of gold nanoparticles using seed aqueous extract of Abelmoschus esculentus and its antifungal activity, Indus. Crop. Prod. 45 (2013) 423– 429. 15. J.C. Chen, Z.H. Lin, X.X. Ma, Evidence of the production of silver nanoparticles via pretreatment of Phoma sp.3.2883 with silver nitrate, Lett. Appl. Microbiol. 37 (2003) 105108.

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ACCEPTED MANUSCRIPT 16. N. Muniyappan, N.S. Nagarajan, Green synthesis of silver nanoparticles with Dalbergia spinosa leaves and their applications in biological and catalytic activities, Process Biochem. 49 (2014) 1054–1061. 17. P.P.N.V. Kumar, S.V.N. Pammi, P. Kollu, K.V.V. Satyanarayana, U. Shameem,

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18. J.Y. Song, B.S. Kim, Rapid biological synthesis of silver nanoparticles using plant leaf

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19. A. Saxena, R.M. Tripathi, F. Zafar, P. Singh, Green synthesis of silver nanoparticles using aqueous solution of Ficus benghalensis leaf extract and characterization of their antibacterial

AN

activity, Mat. Let. 67 (2012) 91–94.

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20. M. Ghaedi, M. Yousefinejad, M. Safarpoor, H. Z. Khafri, M. K. Purkait, Rosmarinus

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officinalis leaf extract mediated green synthesis of silver nanoparticles and investigation of its antimicrobial properties, J. Ind. Eng. Chem. 31(2015)167-172.

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21. V. Dhand, L. Soumya, S. Bharadwaj, S. Chakra, D. Bhatt, B. Sreedhar, Green synthesis of silver nanoparticles using Coffea arabica seed extract and its antibacterial activity, Mat. Sci.

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22. M. Ghaffari-Moghaddam, R. Hadi-Dabanlou, Plant mediated green synthesis and antibacterial activity of silver nanoparticles using Crataegus douglasii fruit extract, J. Ind. Eng. Chem. 20 (2014) 739-744. 23. M. J. Ahmed, G. Murtaza, A. Mehmood, T. M. Bhatti, Green synthesis of silver nanoparticles using leaves extract of Skimmia laureola: Characterization and antibacterial activity, Mat. Let. 153 (2015) 10-13.

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ACCEPTED MANUSCRIPT 24. D. K. Verma, S. H. Hasan, R. M. Banik, Photo-catalyzed and phyto-mediated rapid green synthesis of silver nanoparticles using herbal extract of Salvinia molesta and its antimicrobial efficacy, J. Photochem. Photobiol. B 155 (2016) 51-59. 25. V. Kumar, R. K. Gundampati, D. K. Singh, M. V. Jagannadham, S. Sundar, S. H. Hasan, Photo-induced rapid biosynthesis of silver nanoparticle using aqueous extract of Xanthium

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strumarium and its antibacterial and antileishmanial activity, J. Indus. Eng. Chem. 37 (2016)

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Biological activity and phytochemical analysis of three Indonesian medicinal plants, Murraya koenigii, Syzygium polyanthum and Zingiber purpurea, J. Acupuncture merid. 4

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27. M.S. Argal, S. Kumar, H.S. Choudhary, R.M. Thakkar, S.K. Verma, C. Seniya, The efficacy

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of Murraya koenigii leaf extract on some bacterial and a fungal strain by disc diffusion method. J. Chem. Pharm. Res. 3 (2011) 697-704.

(1996) 788-800.

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28. P. Mulvaney, Surface plasmon spectroscopy of nanosized metal particles, Langmuir 12

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29. M. Saravanan, A. Nanda, Extracellular synthesis of silver bionanoparticles from Aspergillus clavatus and its antimicrobial activity against MRSA and MRSE, Colloid Surf. B 77 (2010)

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214-218.

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ACCEPTED MANUSCRIPT 40. K. Kathiresan, S. Manivannan, M.A. Nabeel, B. Dhivya, Studies on silver nanoparticles synthesized by a marine fungus, Penicillium fellutanum isolated from coastal mangrove sediment, Colloids surf. B 71 (2009) 133-137. 41. T.Y. Suman, S.R.R. Rajshree, R. Ramkumar, C. Rajthilak, P. Perumal, The Green synthesis of gold nanoparticles using an aqueous root extract of Morinda citrifolia L, Spectrochim Acta

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44. M. Annadhasan, T. Muthukumarasamyvel, V. R. Sankar Babu, N. Rajendiran, Green

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synthesized silver and gold nanoparticles for colorimetric detection of Hg2+, Pb2+, and Mn2+

AC

CE

PT

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in aqueous medium, ACS Sustainable Chem. Eng. 2 (2014) 887-896.

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ACCEPTED MANUSCRIPT Scheme Captions: Scheme 1. Schematic representation of AgNPs synthesis via two probable reduction routes using flavonoid and tannin where the enol form of ‘n’ number of flavonoid and tannin present in AEM formed the complex with Ag+. Further, after photoactivation, the ‘n’ number of electron thus generated reduced the n[Ag+] to n[Ag0] which was followed by nucleation, clusters formation, and AgNPs growth.

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Scheme 2. Schematic representation of oxidation-reduction process of colorimetric detection of Hg2+.

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Figure Captions:

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Figure 1. UV–visible spectra of AgNPs using different AEM inoculum dose from 1% to 6% and the corresponding increase in intensity and gradual change in color with the increase of time from 5 to 30 min (conditions; 1 mM AgNO3 concentration and 5 to 30 min sunlight exposure time).

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Figure 2. UV–visible spectra of AgNPs using different AgNO3 concentration from 1 mM to 6 mM and the corresponding increase in intensity and gradual color change with the proceeding of time (conditions; 2.0% AEM inoculum dose and 5 to 30 min sunlight exposure time).

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Figure 3. FESEM images of colloidal AgNPs synthesized by 2.0 % AEM, 4 mM AgNO3 and 30 min of sunlight exposure (A), EDX spectrum of AgNPs (B).

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Figure 4. HRTEM images of optimized AgNPs at different magnifications (A-C), SAED patterns of crystalline AgNPs (D) AgNPs histogram showing size distribution (inset C).

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Figure 5. X-ray diffraction patterns of AgNPs. Figure 6. Typical FTIR spectra of AEM and AgNPs.

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Figure 7. AFM images of biosynthesized AgNPs showing 2D view (A), 3D view (B) and Roughness Profile (C). Figure 8. Pattern of color change of AgNPs solution (A) after addition of different metal ions including Cu2+, Cr6+, Fe3+, Hg2+, Pb2+, Al3+, As3+, As5+, Cd2+, Co2+ and Fe2+ (A), after addition of different concentrations of Hg2+ (50 nm to 500 µM). Figure 9. Change in UV-visible spectra of AgNPs with the addition of 100 µL (500 µM) solution of different metal ions including Cu2+, Cr6+, Fe3+, Hg2+, Pb2+, Al3+, As3+, As5+, Cd2+, Co2+ and Fe2+ (A), Selectivity of AgNPs towards Hg2+ (B), UV-visible spectra of AgNPs with the addition different concentration of Hg2+ ranging from 50 25

ACCEPTED MANUSCRIPT nM to 500 µM (C), Linear relationship between the change in UV-visible absorbance and Hg2+ concentration (D).

Figure S1. Murraya koenigii leaves (A), aqueous extract of Murraya koenigii (B). Figure S2. UV-visible spectra of AgNPs synthesis in sunlight and dark condition.

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Figure S3. UV-visible spectra of AgNPs recorded in bright sunlight at 5, 10, 15, 20, 25 and 30 min (from 5 to 30 minute) showing the establishment of equilibrium.

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Figure S4. HRTEM images of AgNPs and corresponding UV-visible spectra without and with Hg2+. Table Caption:

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Table 1: Table showing the list of different plants for the green synthesis of silver nanoparticles.

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ACCEPTED MANUSCRIPT Schemes: Scheme 1

Photoactivation Ist Probable Reduction Route HO

nAg+

OH

T

O

O

O-Ag+

O OH O

OH

O

O

OH

O

OH

OH

O

OH

O

O

OH OH

O OH

OH OH

OH

OH

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OH OH

Ag0

n

O-Ag+ O

OH OH

OH OH

Ag0

OH

O O

OH

Ag0

OH

O O

Reduction

O

OH

n

Ag0

Ag0

Ag0

OH

n

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(Tannin)

OH

nAg+

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O

Ag0

O-Ag+ O-Ag+

O O O

OH O

O

O

OH

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OH

n

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O O

Ag0

OH

(Flavonoid)

Reduction

OH

OH OH

OH

OH

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O OH O

AEM

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HO

SUN

O-Ag+

OH

IInd Probable Reduction Route

Ag0

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Ag0

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0 0 0 0 Ag Ag Ag 0 Ag Ag 0 0 0 0 Ag 0 Ag 0 Ag Ag Ag Ag 0 Ag0 0 0Ag0Ag Ag0 Ag Ag0Ag 0 0 Ag00AgAgAg0 Ag0Ag0 Ag Ag0Ag0AgAg 0 00 Ag0 Ag0 0 Ag 0 0 0 Ag Ag Ag 0 0 Ag Ag 0 Ag 0 Ag AgAgAg 0 0

AgNPs

Ag0 Ag0

Ag0 Ag0 Ag0 0

Ag0Ag 0 Ag Ag0 Ag0Ag0 0 0 AgAgAg 0 0 Ag0Ag 0 Ag Ag0 Ag0Ag0 0 0 AgAg Ag 0

Clustures formation

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Ag0

Ag0

Nucleation

ACCEPTED MANUSCRIPT Scheme 2

Hg2+ 2+ Hg2+ Hg

0

Hg2+

Ag0

oxidised

Ag0Ag 0 0 Ag00Ag 0 0Ag0 Hg2+ Hg2+Ag Ag Ag 0 Ag AgAg0 Ag+

reduced Hg2+

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+ Ag+ Hg0 Ag

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Hg0

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Ag+

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Ag0Ag 0 0 Ag Ag00Ag 0 0 0 Ag Ag Ag 0 Ag AgAg0

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Hg2+

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ACCEPTED MANUSCRIPT

Part used

Extract type

UV-vis

Shape

Size

Activity

Stabilit y

Refer ence

Murraya koenigii

Leaves

Aqueous

425

Spherical

8.6 nm

Hg2+ detection

4 month

Curre nt Study

Erigeron bonariensis

Leaves

Aqueous

435

Spherical

13 nm

Catalytic

7 days

[2]

Croton bonplandianum

Leaves

Aqueous

428 nm

Spherical

19.4 nm

Fe3+ detection, Antibacterial and Antioxidant

9 month

[3]

Euphorbia hirta

Leaves

Aqueous

425 nm

Spherical

15.4 nm

Antibacterial and Hydrogen peroxide detection

-

[4]

Polyalthia longifolia

Leaves

Aqueous

450 nm

irregular

13.4 nm

Antioxidant

7 days

[5]

Aegle marmelos

Leaves

Aqueous

422 nm

Spherical

60 nm

-

Dalbergia spinosa

Leaves

Aqueous

439 nm

Spherical

18 ± 4 nm.

Antibacterial and Catalytic

Boerhaavia diffusa

Leaves

Aqueous

418 nm

Spherical

25

Antibacterial

[17]

Platanus orientalis

Leaves

Aqueous

cubical

15-500 nm

-

[18]

Diopyros kaki

Leaves

Aqueous

430 nm

cubical

15-500 nm

-

[18]

Ginko biloba

Leaves

Aqueous

430 nm

cubical

15-500 nm

-

[18]

Magnolia kobus

Leaves

Aqueous

430 nm

cubical

15-500 nm

-

[18]

[19]

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430 nm

T

Plant

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Table 1

[13]

-

[16]

Ficus benghalensis

Leaves

Aqueous

410 nm

Spherical

16 nm

Antibacterial

Rosmarinus officinalis

Leaves

Aqueous

450 nm

Spherical

29 nm

Antibacterial

-

[20]

Coffea arabica

Seed

Hydroalcoholic

459 nm

Spherical

10-40 nm

Antibacterial

-

[21]

Crataegus douglasii

Fruit

Aqueous

425 nm

Spherical

29.28 nm

Antibacterial

-

[22]

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ACCEPTED MANUSCRIPT Skimmia laureola

Leaves

Aqueous

460 nm

Spherical and

38 nm

Antibacterial

-

[23]

Hexagonal Salvinia molesta

Leaves

Aqueous

425 nm

Spherical

12.46 nm

Antibacterial

7 days

[24]

Xanthium strumarium

Leaves

Aqueous

436 nm

Spherical

18 nm

Antibacterial and antileishmanial

2 days

[25]

Artocarpus heterophyllus

Seed

Aqueous

410

Irregular

10.78 nm

Antibacterial

Mimusops elengi

Leaves

Aqueous

434 nm

Spherical

55-83 nm

Antibacterial

Jatropha curcas

Seed

Aqueous

425 nm

Spherical

20-30 nm

Citrus lemon

Juice

Aqueous

Spherical

<50 nm

Morinda citrifolia

Root

Aqueous

Triangular and Spherical

12-38 nm

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30 days

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540 nm

[30] [32] [35]

-

-

[39]

-

-

[42]