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Journal of Molecular Liquids xxx (2014) xxx–xxx

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Journal of Molecular Liquids

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Silver nano-disks: Synthesis, encapsulation, and role of watersoluble starch

Ommer Bashir, Zaheer Khan ⁎Nanoscience Research Laboratory, Department of Chemistry, Jamia Millia Islamia (Central University), New Delhi 110025, India

⁎ Corresponding author.E-mail address: [email protected] (Z. Khan).

http://dx.doi.org/10.1016/j.molliq.2014.09.0410167-7322/© 2014 Elsevier B.V. All rights reserved.

Please cite this article as: O. Bashir, Z. Khan, Sdx.doi.org/10.1016/j.molliq.2014.09.041

a b s t r a c t
a r t i c l e i n f o
Article history:Received 26 May 2014Received in revised form 11 August 2014Accepted 23 September 2014Available online xxxx

Keywords:Nano-disksMorphologyCapping agentCTAB

Silver nano-disks have been synthesized by using starch as reducing- and stabilizing agent for the first time. UV–visible spectroscopy, transmission electron microscopy (TEM) and iodometric techniques were used to monitorthe quantitative growth, andmorphology of Ag-nanoparticles. The polyhedral nano-diskswith an average diam-eter of 500 nm contain a large number of truncated triangular nanoplates, spherical, quantum dots, and irregu-larly shaped nanosilver were formed with starch. The silver nanodisks with an average diameter 500 nmcontain a large number of truncated triangular and spherical nanoplates and quantum dots, and irregularlyshaped nanosilver were formedwith starch. The cetyltrimethylammonium bromide (CTAB) markedly enhancedthe reaction path and changed themorphology (size, shape and distribution). This environmental friendlymeth-od of biological silver nanoparticle production provides the use of starch as a capping agent.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Nanoparticles are of great scientific interest as they are, in fact, abridge between bulk materials and atomic or molecular structures. Abulk material should have constant physical properties regardless ofits size, but at the nano-scale size-dependent properties are often ob-served [1,2]. Nanoparticles often possess unexpected optical propertiesas they are small enough to confine their electrons and produce quan-tum effects [3]. Over the last decades silver nanoparticles have foundapplications in catalysis, optics, electronics and other areas due totheir unique size-dependent optical, electrical and magnetic proper-ties [4]. Currently most of the applications of silver nanoparticles areas antifungal agents in biotechnology and bioengineering, textile engi-neering, water treatment, and silver-based consumer products [5]. Sil-ver nanoparticles have gained more attention owing to their broadspectrum of antimicrobial activity and low cost of manufacturing.There is also an effort to incorporate silver nanoparticles into a widerange of medical devices, including but not limited to bone cement, sur-gical instruments, surgical masks, andwound dressings. Silver nanopar-ticles have been used as a cathode in a silver-oxide battery. There areseveral wet chemical methods for the synthesis of silver nanoparticles.Typically, they involve the reduction of a silver salt with a reducingagent like sodium borohydride in the presence of suitable colloidal sta-bilizers. Bakshi et al. [6] used bovine serum albumin protein as areducing-and stabilizing agent to prepare conjugated gold nanoparti-cles and explore their applications as drug delivery vehicles in systemiccirculation. These workers also reported that the sulfur-, oxygen- and

ilver nano-disks: Synthesis, en

nitrogen-bearing groups mitigate the high surface energy of the nano-particles during their reduction. Lee and his co-workers [7] developeda facile method to prepare a magnetic-silver nano-composite whichpossesses a high antimicrobial activity against the model microbesEscherichia coli and Bacillus subtilis. The hydroxyl groups on the celluloseare reported to help in stabilizing the particles. Polydopamine-coatedmagnetic-bacterial cellulose contains multifunctional groups, whichacts as a reducing agent for in situ preparation of reusable antibacterialAg-nanocomposites [8]. A novel wet chemistry method used to createsilver nanoparticles took advantage of D-glucose as a reducing sugarand starch as the stabilizer and also cellulosemolecular chain is appliedto employ the reducing and stabilizing features of cellulose to synthe-size nanosilver [9].

Sulfur-, oxygen-, and nitrogen-containing strong andweak reducingagents have been used with polymers, surfactants, lipids, proteins,starch and cellulose as stabilizing agents [10]. Starch is a biodegradablenatural polymer of α-D-glucose produced by many plants as a source ofstored energy. Starch can be separated into two fractions—amylose andamylopectin. Among natural and synthetic polymers, the use of starch(fully biodegradable, consists 10–20% amylose forms a colloidal disper-sion in hot water and amylopectin 80–90% completely water insoluble).The structure of amylose consists of long polymer chains of glucoseunits connected by an alpha acetal linkage. Amylose in starch is respon-sible for the formation of a deep blue color in the presence of iodine–po-tassium iodide reagents. The iodinemolecule slips inside of the amylosecoil. If starch amylose is not present, then the color will stay orange oryellow. Starch amylopectin does not give the color, nor does cellulose,nor do disaccharides such as sucrose in sugar.

In the present study, starch coated silver nano-disks and/or nano-particles were synthesized and characterized by UV/Vis spectroscopy.

capsulation, and role ofwater soluble starch, J.Mol. Liq. (2014), http://

http://dx.doi.org/10.1016/j.molliq.2014.09.041

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400 450 500 550 600 650 7000.0

0.1

0.2

0.3

0.4

0.5

Abs

orba

nce

Wavelength (nm)

Fig. 1. Spectra of starch capped Ag-nanoparticles at different [CTAB]. Reaction con-ditions: [Ag+] = 25.0 × 10−4 mol dm−3, [CTAB] = 0.0 (▼), 2.5 (▲), 10.0 (●) and12.5 × 10−4 mol dm−3 (■), [PS] = 5.0 (▲, ●, ■) and 10.0 cm3 (▼).

2 O. Bashir, Z. Khan / Journal of Molecular Liquids xxx (2014) xxx–xxx

This paper intends to give a clear overview of starch-capped silvernanoparticles preparation, characterization, morphology, and shape-controlling activity of CTAB. Eco friendly starch coated silver nanoparti-cle could provide a simple chemical-reductionmethod for the synthesisof silver nano-disks and prevent the spreading and persistence of end-odontic infections.

2. Experimental

2.1. Materials and preparation of solutions

Deionized double distilled, CO2 andO2 freewater used as the solventfor the preparation of stock solutions of all reagents. All glassware waswashed with aqua regia (3:1 HCl and HNO3), rinsed with water, anddrying before use. Silver nitrate (AgNO3, Merck India, 99.99%), CTAB(99%, Fluka), potassium iodide (KI, Merck India, 99%) and iodine (I2,Merck India, 99.9%) were used as received. Potatoes were purchasedfrom a local market near to the campus of Jamia Millia Islamia, NewDelhi. The 0.1 N KI–I2 reagents was prepared by dissolving the requiredamounts in required water, stored in an amber colored glass bottle andused to detect the presence of starch in the potato water extract. Iodineis not very soluble inwater; therefore the iodine reagent ismade by dis-solving iodine in water in the presence of KI. This makes a deep bluecomplex with starch and/or triiodide ion and iodine molecules. Thetriiodide ion slips into the coil of the starch causing an intense blue-black color.

2.2. Preparation of potato starch aqueous extracts (PSE)

The 15.0 g of potato was rinsed with deionized water, chopped intofine pieces, soaked in 250ml water, heated for 30 min on water bath at60 °C, allowed to cool, stand for 24 h at room temperature, and the su-pernatant was filteredwithWhatman paper No. 1. The filtrate was usedto the reduction of Ag+ ions into Ag0. The resulting PSE containsmainlywater soluble starch which has been confirmed by the addition ofiodine-potassium iodide reagents (wide infra). In order to see the pres-ence of reducing sugars (monosaccharides) in the PSE, spot tests werenegative with PSE and Benedict reagent indicating that PSE does notcontain any monosaccharides.

2.3. Green synthesis and characterization of Ag-nanoparticle using PSE

In the present studies, an aqueous PSEwas used as reducing agent tothe bio-reduction of Ag+ ions into Ag0. To synthesize Ag-nanoparticles,PSE from 2.0 cm3 to 10.0 cm3 were added in an aqueous 10.0 cm3 of0.01 mol dm−3 of AgNO3 solution. As the reaction proceeds, the color-less reaction mixture turned yellow to orange, indicating the formationof nanoparticles [11]. Preliminary observations showed that the appear-ance of color was very fast and reaction completed within ca. 40 min atroom temperature. In all experiments, PSE was added in the required[Ag+]. UV–visible Spectrophotometer (model UV-260 Shimadzu) with1 cm light path quartz curette was used to monitor the progress of thereaction under different experimental conditions. The transmissionelectron microscopy (TEM) images were obtained on a JEOL, JEM-1011; Japan, transmission electron microscope operating at 160 kV.The samples for TEM were prepared by drop-casting one drop of theprepared silver sols onto carbon-coated copper grids and then dryingin air.

2.4. Experimental evidence to the complete reduction and kineticmeasurements

It has been well known that the morphology (shape, size and thesize distribution) of the silver nanoparticles depend on the experimen-tal conditions such as [reactants], reduction potentials of reactants,[stabilizers], temperature, reaction time, order of mixing and PH.

Please cite this article as: O. Bashir, Z. Khan, Silver nano-disks: Synthesis, endx.doi.org/10.1016/j.molliq.2014.09.041

Therefore, in order to establish the role of [Ag+], [CTAB], [extract],and reaction time, a series of experimentswere performed under differ-ent experimental conditions. In a typical experiment, 5.0 cm3

(0.01 mol dm3) NaCl was added in a reaction mixture containing PSEextract (10.0 cm3) + AgNO3 (10.0 cm3 of 0.01 mol dm−3) + CTAB5.0 cm3 of (0.01 mol dm3) after appearance of the perfect transparentyellow orange color silver sol. We did not observe the white precipitateand/ or turbidity of AgCl indicating the complete reduction of Ag+ ionsto Ag0. It was also observed that atmospheric oxygen and or nitrogengashas no significant effect on thenucleation and growth of silver nano-particles. These observations are in good agreement to our previous re-sult regarding the formation of silver nanoparticles by using ascorbicacid and CTAB as reducing and stabilizing agents, respectively [12].

The kinetic measurements were carried out in a three neckedreaction vessel fittedwith a double surface condenser to check evapora-tion by adding the required concentrations of AgNO3, CTAB and water(for dilution maintained). The progress of the reaction was followedspectrophotometrically by adding the required concentrations of leavesextract. The absorbance of the appearance of yellowish-brown color solwas measured at 475 nm at definite time intervals. Apparent rate con-stants were calculated from the initial part of the slopes of the plots ofln (a / (1− a)) versus time by a fixed timemethod (vide infra). The re-sults were reproducible to within ± 5% with average linear regressioncoefficient.

3. Results and discussion

3.1. Morphology of Ag-nanoparticles

In thefirst set of experiments, a solution of PSE (3 cm3)was added toa AgNO3 solution (0.01 mol dm−3; 10.0 cm3, total vol. 40 cm3). Reduc-tion of the Ag+ ion to Ag0 by PSE extracts could be followed by colorchange. Fig. 1 show theUV–vis spectra recorded from the reactionmedi-um as a function of different experimental conditions. It is interesting tonote that a broad band begins to develop in the whole visible region in-stead of a peak very slowly (Fig. 1;▼). It has been established that CTABhas strong influence on the morphology of silver and gold nanoparticles[13–16]. To increase the nucleation rates, the effect of shape directing[CTAB] (from 2.5 × 10−4 to 15.0 × 10−4 mol dm−3) was studied at dif-ferent [PSE] and fixed [Ag+] = 25.0 × 10−4 mol dm−3. The spectra ofresulting colored silver sols are given in Fig. 1. The most interesting fea-tures of the present observations are the fast appearance of color with




Table 1Impacts of [starch], [CTAB], and [Ag+] on themorphology andkinetics of Ag-nanoparticles.

[Starch](cm3)

104 [CTAB](mol dm−3)

104 [Ag+](mol dm−3)

Morphology 104 kobs(s−1)

2.0 0.0 5.0 No color3.0 0.0 25.0 No color5.0 No color10.0 Orange color; no SRP band 0.953.0 2.5 25.0 Orange; broad band 1.1

5.0 Orange; broad band 1.010.0 Orange; broad band 0.98

5.0 2.5 25.0 Bright orange; broad shoulder 1.35.0 Bright orange; sharp SRP band 0.987.5 Dark orange; broad shoulder 1.3

10.0 Dark orange; broad shoulder 0.9812.5 Brown golden; broad shoulder 1.015.0 Brown golden; broad shoulder 1.117.5 Yellowish turbidity

10.0 0.0 35.0 Yellowish turbidity50.0 Yellowish precipitate75.0 Yellowish precipitate

A

B

C

Fig. 2. TEM images and selected area electron diffraction ring patterns of silver nano-disks.

3O. Bashir, Z. Khan / Journal of Molecular Liquids xxx (2014) xxx–xxx

increasing [CTAB] (Table 1). The formation of perfect transparent yellow-orange color was observed in [CTAB] N 2.5 × 10−4 mol dm−3. As the[CTAB] increases, absorbance increases and a well defined peak devel-oped at 475 nm (Fig. 1; ■). Such type of CTAB behavior on the growth-kinetics, shape of the spectra, stability and color of silver nanoparticlesmight be due to the solubilization and incorporation of the amylose(water soluble constituent of PSE) in the Stern- and Palisade-layers ofcationic micelles of CTAB through electrostatic and/or hydrophobicinteractions [17].

Figs. 2 and 3 shows TEM images for Ag-nanoparticles prepared inabsence and presence of CTAB. It is to be noted that two features areapparent from these Figs. First, quantum dots along with truncated tri-angular polyhedral nano-disks, and spherical polydispersed particleswere observed (Fig. 2A for quantum dots with nano-disks and B forspherical nanoparticles).

The nano-disks could be formed by the dissolution of the corneratoms of truncated triangular nanoplates. Fig. 2 shows the TEM imagesof silver nano-disks having length 151nmandwidth 146 nm.On carefulobservation of TEM images, a thin shell (layer = 3 to 4 nm) of amylose(main constituents of potato extract) is seen around the silver nano-disks. The capping is prominent on each particle and the same mayalso be responsible for interparticle binding. Each silver nano-disk is agroup of several truncated triangular nanoplates (indicated by a redcircle in Fig. 2A: large fraction of triangles having round corners).

Second, the morphology has been abruptly changed in presence ofCTAB. The polyhedral nano-disks were not seen in Fig. 3A and resultingnanoparticles are poly-dispersed and irregular shaped. We can see thatparticles are large (size = ca. 49 nm) and widely dispersed (Fig. 3A). Incomparison with the Ag-nanoparticles in water, the mean diameter ofthe nanoparticles is smaller (ca. 10 nm; Fig. 2B) as well as with a widedistribution, indicating the shape-directing role of CTAB for the aniso-tropic growth [8,9]. The selected area electron diffraction ring patternsof silver nanoparticles were also recorded to see crystalline nature ofthe pure nanoparticles. These results are summarized Figs. 2C and 3Bin absence and presence of CTAB, respectively. The diameter of polyhe-dral nano-disks was ca. 500 nm. Fig. 2A (red circle) clearly shows thatthe nano-disks were formed by the dissolution of the corner atoms oftruncated triangular nanoplates. The formation of nano-disks was re-ported by Chen et al. [18]. These results (band at 475 nm) are in goodagreement with the optical extinction observations these investigators.Inspection of TEM images (Fig. 2A) clearly indicates that the quantumdots adsorbed onto the surface of silver nano-disks and a thin shelllayer (size = 3 to 4 nm) is also covered the nano-disks, which mightbe due to the ion-pair formation between the lone-pairs of –OH groups


of amylose and positive surface of nano-disks. As results, silver nano-disks were capped by starch and it acted as capping agents in additionto reducing agents. In Fig. 2 a white spots were observed, showingthat these silver nano-disks are surrounded by double-layer of PS.

Electrostatic, hydrophobic, and hydrogen bonding are the main fac-tors involved in the rate enhancement of a micelles-assisted bimolecu-lar reaction. Micelles incorporate and/or solubilize the reactants withinits small volume decrease the surface area of the reactants and affect re-action rates rather than by changing the solvent properties ofwater [19]. The frequency ofmolecular collisions increases as a consequence of theclose association of the two reacting species at the micellar interface.Amylose incorporates into the Stern-layer of CTAB micelles. Micellarsurfaces are water-rich, presence of Ag+ ions in the Stern-layer cannot




A

B

Fig. 3. TEM images and selected area electron diffraction ring patterns of silver nanoparti-cles in presence of CTAB.

0 50 100 150 2000.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

Abs

orba

nce

Time (min)

nucleation

growth

Fig. 4. Reaction–time curves to the formation of starch capped Ag-nanoparticles. Reactionconditions: [Ag+] = 25.0 × 10−4 mol dm−3, [CTAB] = 2.5 (■), 5.0 (▲), 12.5 (●) and 2.5(▼), 12.5 × 10−4 mol dm−3 (Y) for [PS]= 5.0 (■,▲,●) and 3.0 cm3 (▼,Y), respectively.

O Oo

o o H

OH

OHHO

OHHO

HOHn

+ Ag+

(Amylose unit)

Amylose unit-Ag+ .CH2OH CHOH

....


be ruled out completely. Catalytic role of CTAB micelles suggests thatthe reduction of Ag+ ions by solubilized –OH groups of amylose occurin the junctural region of palisade–Stern layers. The poly-dispersedmorphology (Fig. 3A) of Ag-nanoparticles is due to the incorporationof amylose helix and Ag+ ions in different micelles and ion-pair forma-tion with CTAB other aggregates (monomers, dimmers, trimmers, etc.).Segregation deactivates the reactants because Ag+ in one micelle can-not react with amylose in other. The reaction can take place onlywhen both reactants are located inside the same surfactant cavity,whichmust also contain growing nuclei [17]. In micelle-mediated reac-tions, it is not possible to precisely locate the exact site of the reactionbut, at least, localization of the reactants can be considered.

Amylose unit-Ag+O O

O

+ H+ + Ag0

(Radical)

Radical + Ag+

k1

fast O OCH2OH CHO

O

+ Ag0 + H+

(product)Ag0 + Ag+ Ag2

+fast

Ag2+ + Ag2

+ Ag42+

Amylose + Ag42+ Amylose stabilized Ag- nanoparticlesadsorption

(orange color)

fast

Scheme 1. Reduction of Ag+ ions by potato-starch amylose.

3.2. Effects of [reactants] and [CTAB] on the nucleation and growthprocesses

To elucidate the influence of [starch], [Ag+], [CTAB] and reactiontime on the formation of Ag-nanoparticles, a series of experimentswere conducted under different conditions. Fig. 4 shows the progress(reaction–time curves) of the reaction at different concentrations ofCTAB and starch. The appearance of color and values of reaction ratesare summarized in Table 1 which indicates that [CTAB] has no signifi-cant effect on the nucleation and growth process but color of the solschanged with [CTAB]. The yellowish-turbidity appeared instead ofprefect transparent orange-color silver sols at higher [Ag+]. A watersoluble constituent of starch, amylose, is a spiral polymer made up ofD-glucose units. In amylose, the 1-carbon on one glucose molecule islinked to the 4-carbon on the next glucose molecule (α(1 → 4)


bonds) [20]. D-Glucose exists in equilibrium between α- and β-pyranose forms with free aldehyde form as intermediates. Out ofthese anomers, β-form has been considered as the reactive species ofD-glucose. The reduction of Ag+ ions into Ag0 with 6-carbon of glucoseunit of amylose can proceed due to the complex formation betweenAg+ and lone-pair of –OH group. However, the –CH2OH group reducedAg+ tometal Ag at room temperature and one type of glucose moleculeconverted to another that contains a –CHO group (Scheme 1).

In Scheme 1, amylose-Ag+ complex under goes one-electronoxidation-reduction transfer mechanism (rate-determining step) leads




5O. Bashir, Z. Khan / Journal of Molecular Liquids xxx (2014) xxx–xxx

to the formation of radical and Ag0. Acrylonitrile was used to detect thein situ generation of free radicals in the reactionmixture. White precip-itate [21] appeared slowly as the reaction proceeded. Controlled exper-iments with starch did not show formation of a precipitate. After theslow step, the complexation and dimerization of the formed Ag0

atoms with Ag+ ions and 2 Ag2+ ions yield orange-color silver sol, re-spectively [22,23]. The agglomeration number of Ag-nanoparticles(NAg) has been calculated by using the Eq. (1) [24]:

NAg ¼ 4=3ð Þ ∏ R3ρ NA M−1 ð1Þ

where R, NA, ρ and M are the radius of particle, avogadro number, den-sity, and atomicweight of silver. The agglomeration numbers calculatedby using the Eq. (1) is also plotted versus diameter of Ag-nanoparticlesare depicted graphically in Fig. 5 in the form of agglomeration number–diameter profiles. The diameter of Ag-nanoparticles increases with in-creasing the agglomeration number exponentially, which could be dueto the adsorption of large number of quantum dots onto the surface ofmetallic silver nano-disks (Fig. 2A) [25].

Fig. 5 clearly demonstrate the aggregation of small nanoparticleswhich leads to the formation of silver nano-disks Amylose (water solu-ble constituents of PSE), acted as a reducing cum capping agent andthere is no need to use high pressure, temperature and toxic chemicals.

3.3. Encapsulation studies

In order to see insight into the encapsulation of Ag-nanoparticles inthe helix structure of amylose, the intensity of the blue iodine–starchcomplex has been determined. In a typical experiment, an adequateamount of PSE is treated with KI–I2 reagent. The first measurement isdone by the dilution of a proper amount of water soluble solution ofstarchpurchased from theMerck Indiawithwater and after adding a re-quired amount of 0.1 N iodine–iodide solution. The appearance of blue-black suggests the presence of amylose in the authentic sample ofstarch. The secondmeasurement is donewith the PSE solution. Interest-ingly,we did not observed the blue-black color formationwith PSE (nei-ther iodine element alone nor iodide ions alone will give the color),indicating the presence of small amount of water soluble amylose inthe PSE but the presence of amylose cannot be ruled out. Therefore,our studied are limited and we did not confidently stated about theencapsulations of silver nano-disks into the helix of amylose.

0 10 20 30 40 50 60 70 80 900

2000000

4000000

6000000

8000000

10000000

12000000

14000000

16000000

Agg

lom

erat

ion

Num

ber

Diameter (nm)

Fig. 5. Plot of agglomerization number versus diameter of Ag-nanoparticles.


4. Conclusions

For the first time, we reported the use of potato-starch as a reducingand capping agent in the synthesis of silver nano-disks and have provid-ed insights into anisotropic particle formation in this system. The sizesof anisotropic Ag-nano-disks are higher in solutions than in presenceof CTAB. TEM images show the ca. 4 nm ring of bilayer around the silvernano-disks (Figs. 2 and 3). The mechanism does not implicate the in-volvement of surfactant micelles in controlling the shape anisotropy offcc metallic nano-disks. The average particle size could be controlledfrom quantum dots to 500 nm by changing the [CTAB] and reactiontime. In the present study, PSE is able to synthesis larger nanoparticlethan the natural source of starch due to the slow nucleation step.

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