soymida febrifuga aqueous root extract maneuvered silver … · 2018-11-12 · world scientific...

Available online at www.worldscientificnews.com

( Received 14 October 2018; Accepted 03 November 2018; Date of Publication 04 November 2018 )

WSN 114 (2018) 84-105 EISSN 2392-2192

Soymida febrifuga aqueous root extract maneuvered

silver nanoparticles as mercury nanosensor and potential microbicide

T. Sowmyya1,* and G. Vijaya Lakshmi2

1Forensic Science Unit, Department of Chemistry, University College of Science, Osmania University, Hyderabad - 500007, Telangana, India

2Department of Chemistry, Osmania University College for Women, Koti, Hyderabad - 500095, Telangana, India

*E-mail address: [email protected]

ABSTRACT

The present communication reports a rapid, uncomplicated, sustainable and facile method of eco-

friendly synthesis of silver nanoparticles (AgNPs). The pressing need for the development of benign,

profitable and eco-friendly alternative routes has inspired researchers to explore plant extracts as safer

replacements to hazardous chemicals. In the present study, a benign method of synthesis of AgNPs using

Soymida febrifuga aqueous root extract has been developed. The characterization studies of synthesized

AgNPs revealed spherical morphology and crystalline nature of AgNPs. The average particle size was

21.81 nm. The synthesized AgNPs were employed as mercury nanosensor for the selective and sensitive

detection of toxic mercury ions in water and soil samples. The AgNPs showed a marked visual color

change and change in surface plasmon resonance band on interaction with mercury ions. The greater

selectivity of AgNPs towards mercury ions was observed. The limit of detection of mercury by 100 μL

of colloidal AgNPs was found to be 2×10-4 M visually and 1.332×10-5 M spectrophotometrically in

water samples and ×-4 M visually and 22.3×10-5 M spectrophotometrically in soil samples. The

method makes use of a small quantity of AgNPs for detection of mercury in water and soil samples. The

method proposed in the present study provides a rapid, selective and sensitive method for detection of

mercury ions in environmental water and soil samples. The synthesized AgNPs were also used as

effective microbicidal agents. The microbicidal potential of the synthesized AgNPs was checked against

two gram positive and gram negative bacterial strains.

http://www.worldscientificnews.com/

World Scientific News 114 (2018) 84-105

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Keywords: Green synthesis, Silver nanoparticles, Soymida febrifuga, Characterization, Mercury

nanosensor, Microbicide

1. INTRODUCTION

Nanomaterials are evincing prodigious utilization in the realm of science and technology.

They are exhibiting unprecedented applications in diverse products such as cosmetics, paints,

packaging materials, biomedical products, biosensors and water filters. Synthesis of

nanomaterials is a propitious sector which is extending swiftly. Many routes of synthesis have

been devised for the production of nanomaterials. But, regrettably, most of the methods

developed till today make use of pernicious organic solvents which render nanoparticles

unendurable for human use. Green and sustainable methods with higher efficiency than the

conventional methods of nanoparticle synthesis are the need of the hour. These methods involve

the use of natural and non-toxic substances for synthesis diminishing the use of hazardous

chemicals.

Silver nanoparticles (AgNPs) are the most important metal nanoparticles exhibiting

remarkable properties such as antimicrobial activity [1], catalytic activity [2], antioxidant

activity [3] and anticancer activity [4]. Silver has been known to be an antimicrobial agent since

ancient times [5]. Nanosilver exhibits a notable antimicrobial activity due to increased surface

area and a considerable decrease in toxicity to human beings [6]. These prospective properties

of AgNPs necessitate the need for the development of novel and safer substitute methods of

synthesis which are cost effective and eco-friendly. AgNPs have been synthesized using

physical methods [7] and chemical methods [8]. With the advent of green chemistry, there was

an upsurge of use of green synthesis procedures to engineer AgNPs. Plant-mediated synthesis

is considered most efficient of the methods available as these methods are cost beneficial, make

use of innocuous and eco-friendly solvents, can be scalable for large scale production of AgNPs

and does not require complex reaction conditions to be maintained during synthesis. Many

plants have been exploited for the synthesis of AgNPs as evident from literature. Some of the

examples of plant-mediated synthesis of AgNPs involve the use of aqueous root extracts of

Catharanthus roseus [9], Delphinium denudatum [10] and Rheum palmatum [11]. Spherical

shaped crystalline silver nanoparticles of size 35 – 55 nm, ≤ 85 nm and 121 ± 2 nm were

synthesized using aqueous root extracts of Catharanthus roseus root extract [9], Delphinium

denudatum [10] and Rheum palmatum [11], respectively. The present study uses the aqueous

root extract of Soymida febrifuga in the reduction and stabilization of AgNPs of size 21.81 nm

which is smaller than the AgNPs synthesized from the earlier reported results [9–11]. The use

of curcumin (turmeric) has also been employed for the green synthesis of AgNPs [12].

AgNPs are also known to act as potential sensors in the detection of heavy metals and

other chemical compounds [12]. Mercury is a heavy metal which is potentially detrimental to

human health and is a hazardous environmental pollutant. The World Health Organization

considered mercury as one of the top ten chemicals or group of chemicals of major public health

concern. The presence of mercury in massive quantities in environmental samples such as soil

and water samples is leading to bioaccumulation of mercury in the ecological food chain. This

leads to a devastating effect on human population and other living organisms.


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Hence, a need to identify sensors for monitoring, sensing and detecting mercury in

environmental samples is much-needed technology today. Despite the availability of several

instrumental techniques, they have limitations. The conventional methods have poor selectivity

for mercury, involve the use of a complicated procedure, require complex and expensive

instruments and are not suitable for on field testing. Novel methods which can overcome the

limitations of the conventional sensing methods need to be devised for screening, sensing and

quantifying the mercury present in a sample. Being tiny in size, AgNPs have an enhanced

surface area which can be exploited in the detection of mercury present in environmental

samples.

The unique properties exhibited by AgNPs may be attributed to their small size

morphology. The AgNPs synthesized from aqueous root extract of Soymida febrifuga being

smaller in size when compared to their counterparts synthesized from various other plant

sources [9–11] can offer a large surface area ending up as a potential mercury sensor.

In the present study, the use of aqueous root extract of Soymida febrifuga, an Indian

medicinal plant belonging to the family, Meliaceae has been reported. The use of Soymida

febrifuga root has not yet been exploited for AgNP synthesis. The method developed makes

use of very small amounts of the plant material and silver nitrate for the effective synthesis of

AgNPs. The method is simple, fast, cost-effective and environment-friendly. To obtain

particulars about size, shape and morphology of the synthesized AgNPs, the characterization of

AgNPs was accomplished employing assorted instruments such as UV-Visible spectroscopy,

Fourier Transform Infrared Spectroscopy (FTIR), X-Ray Diffractometer (XRD) and

Transmission Electron Microscope (TEM). The synthesized AgNPs have been employed as a

simple and selective nanosensor for rapid sensing of mercury present in water and soil samples.

The colorimetric sensing of mercury using AgNPs is simple, cost-effective, quick,

environmentally benign and highly active. The presence of divalent mercury in water and soil

samples can be detected both visually and spectrophotometrically. The synthesized AgNPs

were evaluated for their antibacterial efficacy against two gram positive and two gram negative

bacterial strains.

2. EXPERIMENTAL

2. 1. Materials

Silver Nitrate (AgNO3) was purchased from Sigma-Aldrich, India. Sodium sulphate

(Na2SO4), Potassium chloride (KCl), Barium chloride (BaCl2·2H2O), anhydrous Calcium

chloride (CaCl2), Magnesium sulphate heptahydrate (MgSO4·7H2O), Cobalt (II) acetate

tetrahydrate (Co(CH3COO)2·4H2O), Copper sulphate (CuSO4), Manganese sulphate

monohydrate (MnSO4.H2O), Mercuric sulphate (HgSO4), Lead nitrate (Pb(NO3)2) and Zinc

acetate dihydrate (Zn(CH3COO)2·2H2O) were purchased from SD Fine Chemicals Ltd.,

Mumbai, India.

Nutrient agar was procured from HiMedia Laboratories, Mumbai, India. Double-distilled

water was used for the experiments.

2. 2. Collection of plant material

Healthy roots of Soymida febrifuga were collected from the Nallamala forest area,

Nagarkurnool district, Telangana, India. The roots were thoroughly washed with tap water


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followed by double distilled water several times in order to remove the dust particles and were

shade-dried to remove the residual moisture. The roots were finely chopped into small pieces

and then powdered using a conventional blender. The dried root powder was then sieved and

stored in amber colored airtight container for further use.

2. 3. Preparation of aqueous root extract of Soymida febrifuga

The aqueous root extract of Soymida febrifuga used to reduce silver ions to silver

nanoparticles was prepared by adding 0.1g of dried root powder to 100 mL of double distilled

water and the contents were heated at 60 °C for 15 minutes. The extract was cooled to room

temperature and filtered through Whattman No. 1 filter paper. The filtered extract was stored

in refrigeration for further use in the synthesis of AgNPs.

2. 4. Synthesis of AgNPs

The filtered aqueous root extract of Soymida febrifuga was treated with 1 mM silver

nitrate solution in the molar ratio of 1:2 respectively. The mixture was heated in a water bath at

100 °C for 15 minutes. A color change of the solution from an orange color to reddish brown

color demonstrated the formation of AgNPs. The experiment was repeated thrice for

confirmation of AgNP formation.

2. 5. Characterization of AgNPs

The scan of optical absorbance between 200 – 800 nm with a Shimadzu double beam UV-

Visible spectrophotometer UV-2600 was performed to investigate the reduction of silver ions

to AgNPs by the aqueous root extract of Soymida febrifuga. SHIMADZU IR Prestige21 Fourier

Transform Infrared spectrophotometer was used to obtain the infrared spectra of the synthesized

AgNPs and the root extract using KBr pellet method. FTIR spectra were recorded from wave

number of 250 – 4000 cm-1. X’pert Pro X-Ray diffractometer [Pan-alytical B. V., The

Netherlands] operating at 40 kV and a current of 30 mA at a scan rate of 0.388 min-1 using

CuKα radiation [nm] over a 2 range of 20-80° with a step size of 0.02° was used for

recording the diffraction pattern of the synthesized AgNPs. The particle size and surface

morphology were confirmed using the Transmission Electron Microscope [TEM]. The

ultrasonicated nanoparticle suspension was cast onto a carbon-coated copper grid and the

sample was allowed to dry at room temperature. The grid with the AgNPs was then loaded into

the TEM holder. The TEM holder was then loaded into a Tecnai G2 (FEI, The Netherland)

TEM operating at a voltage of 200 kV.

2. 6. Nanosensing of mercury (Hg2+) in water and soil samples using AgNPs

The nanosensing of mercury (Hg2+) in water and soil samples was done using Soymida

febrifuga aqueous root extract primed AgNPs as colorimetric probes and nanosensor. To

evaluate the propensity of AgNPs in detection of metal ions, eleven disparate metal salts, i.e.,

Sodium sulphate (Na2SO4), Potassium chloride (KCl), Barium chloride (BaCl2·2H2O),

anhydrous Calcium chloride (CaCl2), Magnesium sulphate heptahydrate (MgSO4·7H2O),

Cobalt (II) acetate tetrahydrate (Co(CH3COO)2·4H2O), Copper sulphate (CuSO4), Manganese

sulphate monohydrate (MnSO4·H2O), Mercuric sulphate (HgSO4), Lead nitrate (Pb(NO3)2) and

Zinc acetate dihydrate (Zn(CH3COO)2·2H2O) have been used. For the nanosensing of mercury

(Hg2+), 100 μL (0.01 mg/L) of the synthesized AgNP colloidal solution was added to 10 mM


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stock solutions of different metal ions, shaken well and allowed to stand for 5 minutes of

interaction to assess the selectivity of the synthesized AgNPs towards mercury ions (Hg2+). The

UV-Visible absorption spectra of the resulting solutions were measured. In another set of

experiment, various concentrations (0.001 μM to 10000 μM) of mercury (II) ions were

subjected to the colloidal suspension of AgNPs synthesized from aqueous root extract of

Soymida febrifuga, shaken well and allowed to stand for interaction of 5 minutes to determine

the sensitivity or limit of detection of Hg2+ by the synthesized AgNPs. The UV-Visible spectra

of the resulting solutions were recorded.

2. 7. Preparation of water and soil samples

To demonstrate the practical applicability of AgNPs as colorimetric probes and potential

nanosensor, water and soil samples collected from Osmania University, Hyderabad were spiked

with different metal ion solutions and different concentrations of mercury (II) ion solutions as

described in the general protocol given above. To determine the selectivity and sensitivity of

the synthesized AgNPs in the detection of mercury (II) ions, the water samples were directly

exposed to colloidal AgNPs and the metal ions in the soil samples were extracted using warm

water, filtered through Whattman filter paper and then subjected to colloidal AgNPs.

2. 8. Antibacterial activity

Soymida febrifuga aqueous root extract primed AgNPs were experimented with four

human disease-causing bacteria to examine the microbicidal effectualness of the synthesized

AgNPs. Disc diffusion method was used to evaluate the microbicidal activity of the synthesized

AgNPs against two gram positive bacterial strains (Bacillus subtilis and Staphylococcus aureus)

and two gram negative bacterial strains (Escherichia coli and Pseudomonas putrida) in

Mueller-Hinton agar plates. The bacterial cultures were incubated for 24 hours at 37 °C on

nutrient agar medium followed by storage under refrigeration at 4 °C. The bacterial strains were

then inoculated and grown on Mueller Hinton agar plates to obtain an equable bacterial growth.

Sterile filter paper discs were cut and saturated with different concentrations of 5 μL (0.54

μg/L), 10 μL (1.08 μg/L), 15 μL (1.62 μg/L) and 20 μL (2.16 μg/L) of the colloidal AgNP

solution. The sterile discs saturated with AgNPs were then placed on the petriplates with

equable bacterial growth. A standard antibiotic, Ampicillin [10 μg/10 mL] was used as a

positive control. The plates were then incubated for 24 hours at 37 °C. After incubation, the

diameter of the zones of inhibition was measured.

3. RESULTS AND DISCUSSION

3. 1. Visual observation

The successful synthesis of AgNPs from the aqueous root extract of Soymida febrifuga

was primarily confirmed by the visual color change from orangish red color to dark brown color

exponentially with time as evident from the inset image in Figure 1. This phenomenon may be

an outcome of excitation of the SPR during the synthesis of AgNPs [13]. Similar results were

reported in the synthesis of AgNPs using aqueous leaf extract of Azadirachta indica where the

colour of the solution changed from yellow to reddish brown colour [14].


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Figure 1. UV-visible spectra of Soymida febrifuga aqueous root extract, AgNPs and AgNO3.

Inset 1: Soymida febrifuga root; Inset 2: Visual colour change

3. 2. UV-Visible spectral analysis

UV-Visible spectroscopy has been a widely used technique in the characterization of the

synthesized AgNPs. The UV-Visible spectra as shown in Figure 1 have shown a single strong

characteristic absorption peak at 421 nm. This absorption peak corresponds to the Surface

Plasmon Resonance (SPR) peak of AgNPs. The formation of a single narrow SPR band at

shorter wavelengths indicates the formation of small sized spherical AgNPs without

agglomeration. Similar results were reported in AgNPs synthesized from agricultural waste

Annona squamosa peel extract [15].

3. 3. Fourier Transform Infrared Spectroscopy

FTIR spectra of Soymida febrifuga aqueous root extract and AgNPs thus synthesized were

recorded as clearly seen in Figure 2. The bands at 3415 and 3342 cm-1 in FTIR spectrum of

Soymida febrifuga aqueous root extract correspond to the stretching vibrations of OH bond

indicating the presence of phenol. The bands observed in the region of 2926 cm-1 and 1739 cm-

1 correspond to the stretching vibration of C-H bond in aromatic compound [16] and to the

stretching vibration of C=O carbonyl group respectively. The peaks at 1616 cm-1, 1516 cm-1

and 1438 cm-1 may be attributed to the stretching vibrations of C=C of the phenyl rings of

flavonoids [17]. The peak at 1319 cm-1 may be assigned to the C-H bond in an aromatic


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hydrocarbon [18]. The FTIR peaks at 1261 cm-1, 1107 cm-1 and 1033 cm-1 correspond to

stretching vibration of the C-O bond, an ester group and to the stretching C-OH vibrations of

carbohydrates and glucosyl moieties respectively. The FTIR spectrum of aqueous root extract

of Soymida febrifuga revealed the presence of flavonoids. The decrease in peak intensities and

shift of the peaks indicated the participation of the OH groups in the reduction of silver ions to

nanosilver and the participation of flavonoids as stabilizing and capping agents in the AgNP

synthesis.

Figure 2. FTIR spectra of aqueous root extract of Soymida febrifuga and AgNPs

3. 3. 1. Probable mechanism of AgNP formation

The change in peak intensities and peak positions as evident from the FTIR spectra clearly

indicate the participation of flavonoids in reduction and stabilization of the synthesized AgNPs.

Flavonoids are a group of polyphenolic compounds which actively chelate and reduce silver

ions into nanosilver. The tautomeric transformation of flavonoids from enol-form to keto-form

may release a hydrogen atom that can reduce the metal ions to form nanoparticles. The probable

mechanism of nanoparticle formation can be explained in stages of initiation of nanoparticle

formation or nucleation and further aggregation [19]. The mechanism of AgNP formation

primed by the aqueous root extract of Soymida febrifuga has been explained in Scheme 1.


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Scheme 1. Probable mechanism of AgNP formation and stabilization by flavonoids present in

aqueous root extract of Soymida febrifuga

3. 4. X-Ray Diffraction analysis

Figure 3. XRD pattern of AgNPs synthesized from Soymida febrifuga aqueous root extract

The X-Ray diffraction pattern obtained for dried AgNPs is as shown in Figure 3. The

indexing process of the diffraction pattern obtained was done and the Miller indices (hkl) have

been assigned to each peak corresponding to AgNPs. The Table 1 shows the peak indexing of

the AgNPs synthesized from aqueous root extract of Soymida febrifuga. The dcalculated and

dobserved values were also in close agreement with each other as seen from the Table 1. The XRD


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pattern of the synthesized AgNPs displayed the presence of intense peaks at 2 values of 37.69°,

45.81°, 64.00° and 77.04° representing the (111), (200), (220) and (311) lattice plane reflections

of face centered cubic lattice structure of silver respectively. These diffraction angles and d-

spacing values of peaks corresponding to fcc structure of silver in AgNPs were compared with

the standard powder diffraction card of Joint Committee on Powder Diffraction Standards

(JCPDS) File No.: 04-0783 corresponding to silver and were found to be in agreement [16].

The details of the same have been depicted in Table 2. The XRD analysis confirmed the

formation of FCC AgNPs. The peak broadening of diffraction peaks indicated the small

crystallite size of AgNPs [20]. The high intensity peak for FCC materials is generally [111]

reflection which is seen in the sample along with other peaks whose intensities have reflected

the high degree of crystallinity of the AgNPs synthesized from the aqueous root extract of

Soymida febrifuga [20]. This feature adds to the special antibacterial properties of AgNPs [21].

Table 1. Peak indexing with dcalculated and dobserved values and crystallite sizes with lattice

strain values of Soymida febrifuga aqueous root extract primed AgNPs

S. No. 2μ dcalculated

(Å)

dobserved

(Å) hkl

FWHM

(β) in

radians

Crystallite

size D (nm)

Lattice

parameter

a (Å)

Lattice

strain

1. 37.6954 2.38390 2.38640 111 0.0055 27.85 4.129 0.0040

2. 45.8150 1.97821 1.98060 200 0.0082 19.08 3.956 0.0049

3. 64.0017 1.45302 1.45479 220 0.0082 20.72 4.109 0.0033

4. 77.0434 1.23633 1.23681 311 0.0201 9.2 4.100 0.0063

From the XRD analysis, the average particle size has been calculated using the Debye-

Scherrer’s formula as seen in Table 1. The average crystallite size of the synthesized AgNPs

calculated using the Debye Scherrer’s formula was found to be 19 nm. The crystallite size

values were found to be within the range of the particle size obtained by TEM analysis. The

synthesized AgNPs were found to be smaller in size than AgNPs synthesized from leaf extracts

of C. circinalis, F. amplissima, C. benghalensis and L. nodiflora [22]. The lattice constant ‘a’

depicts the unit cell edge of fcc silver crystal. Theoretically the lattice constant ‘a’ value for

silver is calculated to be 4.07Å. Experimentally the lattice constant values have been depicted

in the Table 1. The experimental lattice constant ‘a’ calculated from the various Bragg

reflections of AgNPs was found to be 4.0735 ± 0.117Å. The theoretical and experimental lattice

constant ‘a’ values were found to be in close agreement with each other and also with the lattice

constant value from the JCPDS File No. 04-0783 which was found to be 4.086Å. The XRD

parameters of the synthesized AgNPs are reported in Table 3. Unassigned peaks at 2

diffraction angles of 27.32°, 31.76°, 43.77° and 54.43° may imply the presence of biological

capping by phytochemicals of the aqueous root extract of Soymida febrifuga on the surface of

AgNPs [23]. The diffraction peak at 31.76° indexed with [101] plane in biosynthesized AgNPs

was observed by Amutha et al. [24] and Marslin et al. [25]. Similar results have been observed

in the AgNPs synthesized from aqueous root extract of Soymida febrifuga.


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Table 2. Comparison of the experimental diffraction angle and d-spacing with the standard

JCPDS Silver File No. 04-0783

S. No. hkl Soymida febrifuga root extract

primed AgNPs Standard JCPDS Silver: 04-0783

Experimental

diffraction angle d-spacing (Å)

Standard

diffraction

angle

d-spacing (Å)

1. 111 37.6954 2.38640 38.117 2.35900

2. 200 45.8150 1.98060 44.279 2.04400

3. 220 64.0017 1.45479 64.428 1.44500

4. 311 77.0434 1.23681 77.475 1.23100

Table 3. XRD parameters of Soymida febrifuga aqueous root extract mediated AgNPs

PARAMETER Structure Average

crystallite size Bond angle

Lattice

parameters Volume

VALUES FCC 19 nm α = β = γ = 90° a = b = c =

4.0735 ± 0.117Å 67.59 Å3

3. 5. Transmission Electron Microscope [TEM]

The shape and size of the resultant AgNPs were elucidated with the TEM analysis as

evident from the Figure 4a. The morphology of the AgNPs synthesized from the aqueous root

extract of Soymida febrifuga was predominantly spherical with a smooth surface. The TEM

micrographs have displayed that the edges of the particles were lighter than the centers

suggesting the capping of the AgNPs by biomolecules of the plant extract [26]. The particles

were well dispersed. Similar results were reported by Das et al., 2015 [27]. The histogram

showing the particle size distribution of AgNPs is shown in Figure 5. This suggests that a

maximum number of particles were in the size range of 20 - 25 nm. The average particle size

was found to be 21.81 nm. The particle size obtained is smaller than the AgNPs synthesized

from aqueous root extracts of Catharanthus roseus root extract [9], Delphinium denudatum [10]

and Rheum palmatum [11].

The TEM study confirmed that the AgNPs were not agglomerated and were well-

dispersed. Figure 4b depicts the selected area electron diffraction (SAED) pattern of AgNPs

synthesized from aqueous root extract of Soymida febrifuga. The AgNPs were crystalline in

nature as evident from the SAED pattern. The diffraction rings were assigned lattice planes

similar to the diffraction peaks obtained from XRD analysis. Mollick et al., 2015 reported

similar findings [4].


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Figure 4. TEM images of AgNPs synthesized from aqueous root extract of Soymida

febrifuga: (a) AgNPs at magnification of 20 nm (b) SAED pattern of synthesized AgNPs

Figure 5. Histogram showing the particle size distribution of AgNPs

3. 6. Nanosensing of mercury (Hg+2) in water and soil samples using AgNPs

3. 6. 1. Selectivity of AgNPs towards mercury (II) ions in water and soil samples

The AgNPs synthesized from the aqueous root extract of Soymida febrifuga were found

to possess properties of a colorimetric probe and a nanosensor in the detection of mercury (II)

ions in environmental water and soil samples. On addition of colloidal solution of AgNPs as

small as 100 μL (0.01 mg/L) to solutions of different metal ions, a distinct color change was

noted only in the case of mercury (II) ions. In the case of mercury, the color of the AgNPs


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disappeared whereas no specific color changes were displayed in case of other metal ions.

However, peak broadening and peak shift were observed for other metal ions. This showed the

selectivity of AgNPs to mercury (II) ions.

Figure 6. Selectivity of AgNPs for mercury (Hg2+) ions in (a) Water samples (b) Soil samples

Figure 7. Change in absorbance of colloidal AgNPs in the presence of metal ions

The mechanism involved is dependent on the electrochemical differences between the

two metal ions, Ag+ and Hg2+. The standard reduction potential of Ag+/Ag and Hg2+/Hg is 0.80

V and 0.85 V respectively. This shows the occurrence of redox reaction between nanosilver and

mercury (II) ions [28]. The elemental mercury then formed an amalgamation with the silver

particles resulting in the disappearance of the SPR band of AgNPs. The other metal ions

exhibited redox potentials lower than that of silver which resulted in very slight variations in

the SPR band of AgNPs [29]. The synthesized AgNPs were found to be practical probes or


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sensors in selective detection of mercury in environmental samples. The selectivity of AgNPs

towards mercury (II) ions in water and soil samples is shown in Figure 6a and Figure 6b

respectively. The synthesized AgNPs showed largest absorbance difference for Hg2+ ions as

seen in Figure 7a and 7b respectively for water and soil samples. The disappearance of the

AgNP peak and the large absorbance difference seen for Hg2+ ions makes the synthesized

AgNPs a selective mercury nanosensor. This provides a positive indicator for qualitative

detection of mercury ions leading further to quantitative applications.

3. 6. 2. Sensitivity of AgNPs towards mercury (II) ions in water and soil samples

Figure 8. Sensitivity of AgNPs towards different concentrations of Hg+2 ions in water

samples

The minimum detection limit of mercury (II) ions by AgNPs for visual and

spectrophotometric detection of mercury was also determined by using various concentrations

of Hg (II) ions in both water and soil samples. In the water samples, the color of the solution

gradually disappeared and the absorbance of the SPR band gradually decreased with the

increase in the concentration of Hg (II) ions in solution as seen in Figure 8a. The limit of

detection or sensitivity of the present method visually is 200 μM of mercury (II) ions in water

samples as seen in Figure 8b.

The efficacy of AgNPs to quantitatively detect Hg2+ ions was demonstrated by adding

colloidal AgNPs to different concentrations of Hg2+ ions and calculating the change in

absorbance intensity monitored by UV-Visible spectrophotometry. A linear graph of change in

absorbance intensity versus Hg2+ ion concentration in micro molar (μM) in water samples was

plotted to calculate the limit of detection spectrophotometrically as seen in Figure 10a. The

linear regression coefficient (R2) was 0.992 with a detection limit of 13.32 μM


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Figure 9. Sensitivity of AgNPs towards different concentrations of Hg2+ ions in soil samples

Figure 10. Linear plot of difference in absorbance intensity and concentration of Hg2+ ions in

(a) water samples (b) soil samples

A similar result was obtained while assessing the limit of detection for mercury in soil

samples. There was a change in SPR band with a change in concentration of mercury (II) ions

as evident from Figure 9a. The limit of detection of mercury (II) in soil samples by the present

method visually is 400 μM as seen from Figure 9b. A linear graph of change in absorbance

intensity versus Hg2+ ion concentration in micro molar (μM) in soil samples was plotted to

calculate the limit of detection spectrophotometrically as seen in Figure 10b. The linear

regression coefficient (R2) was 0.9604 with a detection limit of 223.3 μM


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Table 4. LOD values of Hg (II) ions in water and soil samples both visually and

spectrophotometrically.

Hg (II) Visual LOD UV-visible spectrophotometric LOD

Water samples 2 × 10-4 M 1.332 x 10-5 M

Soil samples 4 × 10-4 M 22.33 x 10-5 M

The detection limit value of 1.332×10-5M of mercury ions in water samples by the

synthesized AgNPs was found to be more efficient than the detection limit of Hg (II) ions by

AgNPs synthesized from fresh water apple fruits which displayed a detection limit of 8.5×10-5

M [30]. The detection limit of Hg (II) ions by the present method is close to the method used

by Shiva Prasad et al., 2017 [31].

This clearly indicates the practical applicability of synthesized AgNPs for visual and

spectrophotometric detection of mercury (II) ions present in environmental water and soil

samples. The method makes use of a minimum quantity of 100 μL (0.01 mg/L) AgNPs for the

detection of Hg (II) ions in both water and soil samples. The method can be scaled up

considerably to improve the sensitivity and limit of detection of Hg (II) ions. A considerable

increase in the concentration of AgNPs can further reduce the limit of detection of Hg (II) ions

in both water and soil samples.

3. 7. Antimicrobial activity

The AgNPs synthesized from the aqueous root extract of Soymida febrifuga exhibited

outstanding microbicidal activity when explored against four infectious microorganisms. Disc

diffusion method was used for evaluating the antibacterial efficacy of synthesized AgNPs

against two gram positive strains, Bacillus subtilis and Staphylococcus aureus and two gram

negative strains, Pseudomonas putrida and Escherichia coli.

The zones of inhibition were observed when the synthesized AgNPs were tested against

the four pathogenic bacterial strains as seen from Figure 11. The zones of inhibition were

limpid. The measured diameters of the zones of inhibition are represented in the Figure. 12 and

Table 5.

The AgNPs synthesized from aqueous root extract of Soymida febrifuga evinced efficient

microbicidal activity in comparison with the standard antibiotic, Ampicillin. The microbicidal

activity of the AgNPs can be attributed to the small size of AgNPs and a large surface to volume

ratio. The AgNPs manifested a dose-dependent antibacterial activity against all four disease-

causing pathogenic microorganisms.

Dose-dependent antibacterial efficacy was also seen in AgNPs synthesized from Urtica

dioica Linn. leaves [16]. In contrast to the standard antibiotic, Ampicillin, the antibacterial

effectualness was more for AgNPs in the case of gram positive bacterial strains than gram

negative bacterial strains.


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Figure 11. Microbicidal potential of AgNPs synthesized from aqueous root extract of

Soymida febrifuga (AMP) Ampicillin control (1) 5 μL AgNPs (2) 10 μL AgNPs

(3) 15 μL AgNPs (4) 20 μL AgNPs

Figure 12. Bar diagram showing microbicidal activity of AgNPs


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Table 5. Antibacterial activity of the synthesized AgNPs from aqueous root extract of

Soymida febrifuga against some human pathogens

S. No. Bacterial

strains

Zone of

inhibition in

mm for 5 µL of

AgNPs (0.54

μg/mL)

Zone of

inhibition in mm

for 10 µL of

AgNPs

(1.08 μg/mL)

Zone of

inhibition in mm

for 15 µL of

AgNPs

(1.62 μg/mL)

Zone of

inhibition in mm

for 20 µL of

AgNPs

(2.16 μg/mL)

Ampicillin

(10 µg/

10 μL)

1 Bacillus subtilis 9 11 16 18 7

2 Escherichia coli 13 14 15 15 13

3 Pseudomonas

putrida 15 16 18 18 14

4 Staphylococcus

aureus 15 17 20 22 7

The mechanism of the microbicidal efficacy of AgNPs has not been clearly elucidated till

date. Nevertheless, sundry theories have been proposed by various researchers to describe the

antimicrobial activity of AgNPs [32–34].

The nano size of the AgNPs enhances the surface area of the particles making the AgNPs

more susceptible to penetration into the bacterial cell. With the increase in the concentration of

AgNPs, there is a substantial increase in the surface area for attachment to the bacterial cell

wall resulting in inhibition of the bacterial growth and reproduction. The Soymida febrifuga

aqueous root extract primed AgNPs displayed more effective bactericidal property against gram

positive bacterial strains in comparison to the gram negative bacterial strains. This can be

attributed to the structural organization of the bacterial cell wall. The gram positive bacteria

have a plasma membrane as their cell wall and do not possess the bacterial outer membrane.

In contrast, the gram negative bacteria have an extra outer membrane made up of

lipopolysaccharides and proteins as a part of their cell wall. The structural differences in the

cell wall composition of the gram positive and gram negative bacterial strains are the result of

differential antibacterial activity shown by AgNPs. The AgNPs interact quickly with the naked

peptides on the cell wall of gram positive strains and penetrate readily through the cell wall to

inhibit the bacterial growth [35]. However, as the outer wall of the gram negative bacterial

strain is constructed with a thick lipopolysaccharide layer, it provides an effective barrier and

prevents the swift entry of AgNPs through the membrane [36]. Another fact which explains the

effective microbicidal activity of AgNPs towards gram positive strains in comparison to gram

negative bacterial strains is the fact that the cell walls of gram positive bacteria bind large

quantities of metals than the gram negative bacteria [37]. Similar results were reported by Niska

et al., 2016, Umadevi et al., 2011 and Abalkhil et al., 2017.

A concentration as low as 5μL (0.54 μg/mL) of colloidal AgNPs synthesized from

aqueous root extract of Soymida febrifuga has exhibited effective microbicidal activity against

gram positive and gram negative bacterial strains. The antibacterial activity reported by Dhand


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et al. showed effective detrimental effect at a low concentration of 0.2675 mg/L of AgNPs [38].

A concentration of AgNPs as low as 0.54 μg/L synthesized from Soymida febrifuga aqueous

fruit extract exhibited effective antibacterial activity [39]. The antibacterial activity reported

from same concentrations of AgNPs synthesized from aqueous root extract of Soymida

febrifuga exhibited greater zones of inhibition when compared to the AgNPs synthesized from

aqueous fruit extract of Soymida febrifuga. However, the antibacterial activity was less for

Escherichia coli [39]. The AgNPs thus produced are cost-efficient, eco-friendly, non-toxic and

effective microbicidal agents which can be used for the treatment of human pathogenic bacteria

and biofilms as well.

4. CONCLUSIONS

An uncomplicated, lucrative and environmentally benign method of AgNP synthesis

maneuvered by aqueous root extract of Soymida febrifuga was explained in the present study.

For the first time, Soymida febrifuga aqueous root extract has been used for the synthesis of

AgNPs. The functional biomolecules, flavonoids present in the aqueous root extract of Soymida

febrifuga participated in the reduction and stabilization of AgNPs. The prepared AgNPs were

found to be spherical in shape with an average particle size of 21.81 nm. The AgNPs were well

scattered. The synthesized AgNPs were checked for their practical application as a colorimetric

nanosensor or probe in the detection of mercury (II) ions in environmental water and soil

samples. The biosynthesized AgNPs in a very small volume of 100 μL (0.01 mg/L) showed

selective and sensitive detection of mercury (II) ions. The present method can be employed for

detection of toxic mercury (II) ions present in environmental samples. This can help in the

detection and further prevention of mercury pollution in the environment. The AgNPs with

concentration as low as 0.54 μg/mL have shown excellent microbicidal properties against gram

positive and gram negative bacterial strains. The current study is environment centric and

sustainable method which makes use of a minimum quantity of plant material (0.1% of plant

extract) and precursor salt (1 mM AgNO3) for the synthesis of stable AgNPs. The method can

be adopted for the colorimetric sensing of mercury (II) ions in environmental samples all over

the world. This can help reduce mercury pollution in the environment which is utmost

hazardous to living organisms.

Acknowledgement

The authors thank DMRL, Kanchanbagh, Hyderabad and CFRD, Osmania University, Hyderabad for providing

the experimental facilities.

Biography

Dr. T. Sowmyya is an Assistant Professor in Forensic Science, Department of Chemistry, Osmania University,

Hyderabad. She has a teaching experience of 7 years. Her core subjects are Forensic Chemistry and Forensic DNA

Fingerprinting. Her area of research interest is nanotechnology. She has published several research papers and

review articles in journals of repute. She has presented research papers in various international and national

conferences where she has been awarded best paper presenter award. She is a member of various research

organizations.


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Dr. G. Vijaya Lakshmi is an Assistant Professor in Department of Chemistry, Osmania University College for

Women, Hyderabad. Her area of specilaization is Physical Chemistry. Her areas of research interest are

nanotechnology and chemical kinetics. She has published several research papers in journals of repute and

presented papers in international and national conferences.

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