synthesis and surface chemistry of nano silver particles

Silver nano particles of the size 40–80 nm are formed in the process of oxidation of glucose to gluconic acid by amine in the presence of silver nitrate, and the gluconic acid caps the n

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Synthesis and surface chemistry of nano silver particles

International Advanced Research Center for Powder Metallurgy and New Materials, Balapur PO, Hyderabad 500005, India

a r t i c l e i n f o

Article history:

Received 29 September 2008

Accepted 5 May 2009

Available online 23 May 2009

Keywords:

Nano silver

Gluconic acid

Diethyl amine

Chemical synthesis

a b s t r a c t

In this report, we present a simple wet chemical route to synthesize nano-sized silver particles, and their surface properties are discussed in detail Silver nano particles of the size 40–80 nm are formed in the process of oxidation of glucose to gluconic acid by amine in the presence of silver nitrate, and the gluconic acid caps the nano silver particle The presence of gluconic acid on the surface of nano silver particles was conﬁrmed by XPS and FTIR studies As the nano silver particle is encapsulated by gluconic acid, there was

no surface oxidation, as conﬁrmed by XPS studies The nano silver particles have also been studied for their formation, structure, morphology and size using UV–Visible spectroscopy, XRD and SEM Further, the antibacterial properties of these nano particles show promising results for E Coli The inﬂuence of the alkaline medium towards the particle size and yield was also studied by measuring the pH of the reaction for DEA, NaOH and Na2CO3

1 Introduction

Metal nano particles have attracted a great deal of attention in

recent years due to their optical, physical and chemical properties

that differentiates them from bulk material properties Hence they

ﬁnd wide application in various ﬁelds like catalysis, photonics,

optoelectronics, information storage, antibacterial applications,

etc Silver powders, having ultra ﬁne and uniformly distributed

particle size, are of considerable use in the electronics industry

as thick ﬁlm conductors in integrated circuits due to their unique

properties such as high electrical and thermal conductivity, high

resistance to oxidation Recently a method was perfected for using

nano silver particles in inkjet printing that could see circuits get

even smaller and cheaper[1] Apart from electronic applications,

it has been known for centuries that silver has bactericidal

proper-ties Silver is a safe and effective bactericidal metal because it is

non-toxic to animal cells and highly toxic to bacteria such as

Escherchia coli (E coli) and Staphylococcus aureas[2,3] Silver based

compounds have been used in recent years to prevent bacterial

growth in applications such as burn care[4] Silver doped polymer

fabrics, catheters and polyurethane are well known for their

anti-bacterial functionality [2,5] Colloidal silver [6,7], nano silver

coated fabric[8], nano silver metal oxide granules[9]and nano

sil-ver coated ceramic materials[10]are used for antibacterial

appli-cations Nano silver in the form of powders as well as suspensions,

due to the high surface to volume ratios, has been used in the

above said applications as it enables the loading of small

quanti-ties of silver and thus makes the product cost effective

Research-ers around the world are trying to produce nanosized mono dispersed powders by different methods Pluym et al prepared nano silver powders by the technique of spray pyrolysis, with the production at a rate of 1–2 g/h[11], but the silver particles tend to be agglomerated, irregular shaped, and hollow due to sol-vent evaporation

There are several aqueous based chemical methods[12–23] re-ported in the literature to produce nano silver While most of these reports deal with inorganic bases such as NaOH and Na2CO3to con-trol the pH to above 9, the contamination of silver with metals ions will cause limitations in speciﬁc applications such as electronics, and hence organic bases are required in the nano silver synthesis,

as reported by Hsu and co-worker[23] The same group used or-ganic bases such as triethyl amine and pyridine for contamina-tion-free silver nanoparticles, useful for micro-interconnects, that cure at relatively lower temperatures Chaki et al used a non-aque-ous single-phase preparation of Ag clusters from silver benzoate using triethylamine as the organic base[12] In both the organic base related studies, they have used external capping agents for stabilizing the powders or suspensions In all the cases, the stabil-ity and antibacterial activstabil-ity are dependent on the surface chemis-try of the silver produced For example, Kvítek et al.[24] have reported a SDS stabilized nano silver suspension to have better dis-persion and antibacterial action The best condition for high perfor-mance of nano silver, for either electronic or antibacterial applications, is that the particles should contain minimum or no capping agents so that the maximum surface is available for anti-bacterial action or sintering at low temperatures Apart from the contamination issues, we have also found that the yield of the nano silver in the reduction reaction depends on the strength of the base

* Corresponding author Fax: +91 40 24442699.

E-mail address: tata@arci.res.in (T.N Rao).

Contents lists available atScienceDirect

Polyhedron

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / p o l y

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In the present work, we have synthesised silver nanoparticles

by an aqueous chemical method with an organic base and with

no external capping agents, and carried out surface chemistry

stud-ies and compared the propertstud-ies, such as stability and yield, with

those obtained with inorganic bases, NaOH and Na2CO3 We made

an attempt to synthesize nano silver colloids using a lower

concen-tration of the reactants that could be stable for a few weeks

with-out any surfactant Nano silver colloids prepared in the absence of

surfactant using inorganic bases such as NaOH and Na2CO3settle

down immediately Hence we tried to explore the possibility of

using an organic base, diethyl amine, that provides a good yield and low particle size in the absence of a protective agent It is inter-esting to explore the surface chemistry of the nano silver particles that confirms the capping by gluconic acid, which prevents the oxi-dation/sulfidisation of the nano silver particle as confirmed by XPS studies The suspensions made in this work using diethyl amine were found to be stable for nine months The merits of this method are that it is a room temperature process with glucose as a reduc-ing agent, which also modifies the surface after undergoreduc-ing oxida-tion to stabilize the suspension without the requirement of other

Table 1

Details of the reaction parameters used in the silver nanoparticles synthesis, corresponding size of the particles measured from the SEM micrographs and k max observed for nano silver suspensions.

S No Concentration of

AgNO 3 (mM)

Concentration of glucose (mM)

Concentration of DEA (mM)

k max (nm) Powder/suspension Particle size from

SEM (±10 nm)

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external capping or stabilizing agent The resulting nano silver

ex-hibit high antibacterial activity and is also useful for electronic

interconnecting applications

2 Experimental

2.1 Preparation of the nano silver suspension and powders

Analytical grade of silver nitrate (Ultraﬁne Chemicals), glucose,

diethyl amine, sodium carbonate and sodium hydroxide

(Quali-gens) were used as starting materials The different concentrations

of aqueous solutions of silver nitrate and glucose solutions were

mixed together and stirred to obtain a homogeneous solution An

aqueous solution of diethyl amine (DEA) of a pre-decided molarity was added to it quickly and stirred vigorously Similar experiments were done with sodium carbonate and sodium hydroxide The

col-or of the solution changed to black, brown and then ﬁnally a light green colored precipitate was obtained for higher concentrations S5–S10 After decanting and washing repeatedly 2 to 3 times with distilled water, the precipitate was collected and dried in air at 50–

60 °C The ﬁnal pale green colored dried powder was subjected to further characterization and antibacterial tests The details of the reaction parameters for DEA are given inTable 1 For the samples S1–S4 low concentrations of reactants were used and suspensions were obtained which were stable for a few weeks when prepared using DEA in contrast to NaOH and Na2CO3, which settled immediately

Fig 2 UV–Visible absorption spectra for silver nanoparticles for samples S1, S2, S3 and S4.

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To understand the exact role of glucose, the reactions were also

performed without addition of glucose, keeping all the other

reac-tion parameters the same In these reacreac-tions, a black colored

pow-der was collected as the end product When the reactions were

done without amine, the rate of reaction was found to be very slow

and after 24 h the color of the solution changed to light brown

Sil-ver nano particles thus prepared were in the form of either stable

suspensions or powders, depending on the reactant concentrations

as mentioned inTable 1

2.2 Characterization of the nano silver powders

The powders obtained under the various conditions were

char-acterized using different techniques The particles were tested for

their optical absorption property using a PELambda 650,

Perkin–El-mer UV–Visible spectrometer, to ensure the formation of nano

sil-ver The suspensions were tested as prepared The powdered

samples were prepared by dispersing them in water using stirring

and sonication The powdered samples were subjected to X-ray

dif-fraction (XRD) studies on a Bruker AXS D8 Advanced XRD to

under-stand their structure The morphology, size and shape of the

particles were obtained using an Hitachi Scanning Electron

Micro-scope equipped with EDAX A drop of nano silver suspension or

dispersed nano silver powders in water was placed on the carbon tape on an aluminum stub and dried Fourier Transform Infra Red (FTIR) spectroscopy was performed on a THERMO Nicolet Nexus

740 spectrometer The powdered samples were mixed with KBr and pelletized at 1000 psi pressure X-ray photoelectron spectros-copy (XPS) measurements were obtained on a KRATOS-AXIS 165 instrument equipped with dual aluminum–magnesium anodes using Mg Karadiation The X-ray power supply was run at 15 kV and 5 mA The pressure of the analysis chamber during the scan was 109Torr The peak positions were based on calibration with re-spect to the C 1s peak at 284.6 eV The obtained XPS re-spectra were ﬁtted using a non-linear square method with the convolution of Lorentzian and Gaussian functions after polynomial background subtraction from the raw data

2.3 Antibacterial test

Nutrient agar was poured on two disposable sterilized Petri dishes and was allowed to solidify To understand the antibacterial activity of the nano silver powder, 0.1 gm of nano silver powder was mixed with 1 ml of E coli bacterial water containing

170 CFU/ml One milliliter of the E coli bacterial water containing

170 CFU/ml was streaked on one agar plate and 1 ml of the bacte-rial water mixed with nano silver powder was streaked on the other plate and it was spread uniformly Plates were incubated at

37 °C for 24 h Growth of the colonies of bacteria was observed

A zone of inhibition test was done on the nano silver coated plastic The nano silver powder was ultrasonicated in acetone for

5 min and it was spin-coated on the plastic at 500 rpm Nutrient agar was poured on two Petri dishes The nano silver powder coated plastic and uncoated plastic were placed in the two Petri dishes and then the Nutrient agar was allowed to solidify 1 ml of

170 cfu/ml concentrated E coli bacterial water was streaked on the two Petri dishes The plates were incubated at 37 °C for 24 h and then the zone of inhibition was observed

3 Results and discussion

The silver mirror test is commonly used to detect the presence

of an aldehyde group in an organic compound Here excess

ammo-Fig 4a FTIR spectra for (i) silver nitrate (ii) glucose (iii) gluconic acid (iv) silver Fig 4b Enlarged FTIR spectra for (i) gluconic acid and (ii) resolved spectrum of

1

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nia and silver nitrate are reacted in the presence of the aldehyde

which oxidizes to form carboxylic acid and release silver, which

deposits on the glass container wall to give silver mirror The

gen-eral reaction can be written as:

RCHO þ 2AgðNH3Þ2OH ! RCOONH4þ 2Ag þ 3NH3þ H2O ð1Þ

Following this reaction, the process was modiﬁed to get silver

nano particles Glucose was used as an aldehyde and an amine

was used in the place of ammonia The proposed reaction

mecha-nism for this reaction is very similar to that of the silver mirror test

When the amine is dissolved in water, it withdraws hydrogen ions

by leaving hydroxyl ions in solution The hydrated amine ion

fur-ther reacts with silver nitrate to form a complex of silver The

remaining hydroxyl ions oxidize the aldehyde group here in

glu-cose to form gluconic acid and an electron is released in the

pro-cess This electron reduces the silver complex to get metallic

silver The DEA acts as a catalyst The reactions can be written as

follows:

Agþ

R ¼ ðCHOHÞ4CH2OH

Some reactions were carried out in the absence of glucose to

conﬁrm its exact role in the silver formation It was observed that

the absence of glucose in the reaction produces silver oxide (Ag2O)

instead of metallic silver Here the silver complex formed with

amine in presence of hydroxyl ions produces silver hydroxide

which converts into silver oxide as:

Although glucose is a known reducing agent, its activity on sil-ver nitrate was observed to be sil-very weak in the absence of amine, and both reactants remain unreacted for many hours Yan et al., modiﬁed a fabric by using a similar process and deposited silver

by heating the fabric[8] To investigate the effect of the concentra-tion of the precursors towards the particle size of the nano silver, different studies were performed by varying the concentrations

of glucose, silver nitrate and diethyl amine, as mentioned inTable

1 The particle size was measured from SEM images and also from XRD data by calculating the average particle size using the Scherror formula, and both were found to be in agreement

The AgNO3:glucose molar ratio was changed from 1:0.5 to 1:2 The concentration of glucose did not show much inﬂuence on the particle size, but was found to be very essential to form metallic silver nano particles, without which an oxide is formed as shown

in reaction (4), indicating the role of glucose as a reducing agent The concentration of silver nitrate is one of the most important factors that decide the particle size As the concentration of silver nitrate is increased from 3 to 300 mM, correspondingly the particle size increases from 40 to 630 nm and grows towards bulk silver The concentration of DEA was found to be another size controlling factor In the case of S2–S4 (suspensions) as DEA the concentration

is decreased from 115 to 23 mM, the particle size increases from 40

to 100 nm, as mentioned inTable 1 In the case of S5–S7 (powders) the increasing concentration of DEA shows a decrease in the aver-age particle size from 110 to 63 nm

The scanning electron micrograph and particle size distribution for the samples S2, S3 and S4 are shown inFig 1a–c Nearly spher-ical particles are seen with a maximum number of particles in the size range of 40–50 nm, 50–70 nm and 80–100 nm for samples S2, S3 and S4, respectively The sharp boundaries of the particles clearly show non-agglomeration of particles even after drying, indicating that some factor is controlling and maintaining the nano

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size of the silver particles.Fig 1d shows a typical micrograph for a

powder sample (S7) used for XPS and FTIR studies Here the

parti-cles in the size range of 60–70 nm are seen to be somewhat

agglomerated as at some places necking between the particles is

observed This is obvious for nano powders prepared using higher

concentrations of precursors and dried in air at 50–60 °C

The change in the particle size also reﬂected in the optical

absorption studies The kmax values observed in the UV–Visible

absorption studies for samples S1, S2, S3 and S4 are depicted in

Fig 2 The samples S1 and S2 show a blue shift in kmax, which is

at 432 and 426 nm, respectively The decrease in the silver nitrate

concentration has resulted in a smaller particle size The sample S3

and S4 were prepared with smaller concentrations of DEA as

com-pared to S2 and show kmaxat 433 and 464 nm, respectively, as

shown inFig 2 It clearly shows a red shift in kmaxfrom S2 to S4

with the decreasing concentration of DEA, and a decrease in the

FWHM with an increase in concentration of DEA The powder

sam-ples did not show any clear absorption peaks in UV–Visible range

This may be due to the agglomeration of the particles, observed in

the SEM

Typical XRD spectra are given inFig 3a and b for silver and

sil-ver oxide, respectively The powdered silsil-ver nanoparticles show a

cubic structure showing peaks at 2h: 38.1 (1 1 1), 44.2 (2 0 0),

64.4 (2 2 0) and 77.472(3 1 1), which match exactly with the

stan-dard data.Fig 3b shows the spectra for particles prepared in the

absence of glucose It shows the cubic phase of Ag2O, which is in

good agreement with the literature[25]

It was very interesting to understand the formation of nano

par-ticles with the change in reactant concentrations This was possible

only when the particles were studied by FTIR, to understand the

re-mains of the organic molecules, if any, after completion of the

reac-tion The surface properties of the nano silver particles, studied

using XPS, throw some light about their size controlling factors

Fig 4ai–v shows the FTIR spectra recorded for silver nitrate, glu-cose, gluconic acid, silver nanoparticles and silver oxide nanoparti-cles, respectively The attempt made here is to do only a qualitative study The IR spectrum for the silver nanoparticles (Fig 4aiv) shows various peaks The peaks at 3423 and 1605 cm1are very broad and strong, and can be assigned to the hydroxyl groups

[25], either from glucose/gluconic acid, from adsorbed moisture

or both A prominent and very sharp peak is observed at

1384 cm1which was concluded to be due to the nitrate ions when compared with the FTIR spectrum for silver nitrate, as shown in

Fig 4ai Some other smaller peaks present at 1740, 1400, 1250,

1100 and 1039 cm1were also observed These peaks are absent

in the silver oxide IR spectrum (v), which shows major peaks at

3380, 1651 and 1382 cm1, and are due to the OH group[26], ad-sorbed moisture and nitrate impurities, respectively As explained

in reaction (4), the gluconic acid is produced by oxidation of glu-cose and it may be present in the sample Hence these IR peaks were also compared with the glucose and gluconic acid peaks The smaller peaks in nano silver do not match with those of glu-cose (Fig 4aii) but they match with many features in gluconic acid The FTIR spectrum of gluconic acid (i) and the resolved spectrum for nano silver powder (ii) was compared in the wave number re-gion from 900 to 1900 cm1, as depicted inFig 4b The gluconic acid shows peaks at 1740, 1638, 1412, 1230, 1100, 1036 and

875 cm1 Almost all these peaks match with those for the nano sil-ver sample Thus a small amount of gluconic acid remains in the sample even after repetitive washings of the silver precipitate

Fig 5ai shows the core-level spectrum for C 1s for the nano sil-ver powder The broad peak can be ﬁtted to four different compo-nents at binding energies 289.7, 288.1, 286.5 and 284.6 eV The peak at 284.6 eV is mostly due to the carbon present on the sample surface due to handling[27] This peak was observed in all sam-ples While discussing the other two components, it is necessary

to consider the gluconic acid molecule, the presence of which

Table 2 Comparison of sodium carbonate, diethyl amine and sodium hydroxide.

pK b Yield (%) Particle size (nm)

4 5 6 7 8 9

10

NaOH Amine

Na 2 CO 3

Time in seconds

Fig 6 Photograph of antibacterial test (a) control (b) control with nano silver particles (c) control with uncoated plastic and (d) control with nano silver coated

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was observed in the FTIR studies In one gluconic acid molecule,

depending on the functional groups attached, there are two types

of carbons in the atomic ratio 1:5, as numbered below

The carbon in carboxylic group shows a binding energy at

289 ± 2 eV [28] The component at 288.1 eV is due to C-1, which

is a carboxylic group Here the shift is observed towards a lower

binding energy This is in good agreement with the literature, where

carboxylic groups attached to a metal particle like Co, Cu show a

binding energy shift to 288.1 eV[29] Also the standard data

avail-able for silver acetate shows a XPS peak for carbon at 288.1 eV

[30,31] It has been shown that the carboxylic acid group binds to silver surface in the case where citrate is used as the reducing agent

In this case, it was shown that two carboxylic groups of the citrate bind to the Ag surface, having the third group normal to the surface, which make the Ag particle charged, and that is responsible for the electrostatic stability[32] However in the present case only one carboxylic acid is available on gluconic acid that is likely to bind with the Ag surface The capping of the silver particle controls the grain growth but does not provide charge to the particles The peak

at 286.5 eV is due to C-2 (the carbon with OH groups)[33,34] The intensities of the corresponding XPS peaks are fairly in agreement with the atomic ratio of these carbons in the molecule A very small component at 289.7 eV may be due to some non-bonded gluconic acid The C 1s spectrum was also recorded for silver oxide powders where only two components were observed, at 284.5 and 286.5 eV,

as shown inFig 5aii The former peak is due to a carbon impurity The latter peak was unexpected and may be due to some unknown carbon contamination, as no C–O bond is possible in this sample

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The core-level spectrum for Ag 3d, shown inFig 5bi, is ﬁtted in

two components, one is major and the second is very small in the%

concentration ratio of (96:4) The peak at 368.2 eV is clearly due to

metallic silver [25,35] A surprising small peak shifted to lower

binding energy at 365.8 eV may be due to the surface silver atoms

surrounded by the six carbon chain of gluconic acid molecules

Un-like in the silver nanoparticles, silver oxide nanoparticles show

only one peak at a binding energy of 367.4 eV, which is a

character-istic for Ag2O, as shown inFig 5bii

Fig 5c–i shows the core-level spectra for O 1s in the case of

sil-ver nanoparticles It shows a broad peak which can be

deconvo-luted into three peaks at 535.0, 533.4 and 531.7 eV The ﬁrst

peak can be assigned to the adsorbed water[36] The component

at 533.4 can be correlated to either C–O or C–OH present in the

gluconic acid molecules[37–39] The peak at 531.7 eV was

ob-served in all samples, and can be attributed to adsorbed oxygen

on the surface as an impurity in the form of either atomic oxygen

or in the hydroxyl form[40] Some of its contribution can also be

due to organometallic oxygen Hoof et al studied Co and Cu bonded

with organic molecules and found the binding energy of O 1s at

531.6 eV[29] Here if the silver atoms on the surface of the

nano-particles are bonded with carboxylic ions, the oxygen from the

car-boxylic group may show a binding energy at 531.7 eV The peculiar

peak of silver oxide at 529.6 eV, as shown inFig 5cii, is observed in

the case of the silver oxide nanoparticles, but is absent in the silver

nanoparticles Other than the oxide peak, the silver oxide

nanopar-ticles show two components at 533.3 and 531.7 eV, which can be

attributed to hydroxyl groups which may be present on the surface

of the oxide particles and surface adsorbed oxygen, respectively

This clearly tells us that the surface of the silver nanoparticles does

not have any oxide layer as it is protected by gluconic acid

molecules

The reducing ability of aldehyde is dependant on the pH As

al-ready mentioned, if only glucose is added the reduction rate is very

slow at room temperature because of the low pH We tried to

ex-plore the inﬂuence of different alkaline medium towards the

parti-cle size and yield of nano silver powder The pKbvalues, yield and

particle size of nano silver powder for NaOH, Na2CO3and DEA are

listed inTable 2 The changes in the pH of the solution for NaOH,

Na2CO3and DEA are shown inFig 6

The yield of the nano silver powder was found in increase with a

decrease in the pKbvalue The lower the pKbvalue, the stronger is

the basicity of the alkaline medium Under highly alkaline

condi-tions, the aldehyde functional group in glucose gets converted to

a ketone[41] As ketones are weak reducing agents when

com-pared with aldehydes, the yield of the nano silver precom-pared by

so-dium hydroxide (pKb= 0.2) is less when compared with DEA and

Na2CO3[14,42] The reaction mechanism is the same in each case

Gluconic acid formed as a by-product in the reaction acts as a

cap-ping agent This imparts a negative charge on the surface, which

was conﬁrmed by immediate adsorption of negatively charged

sil-ver particles on the anion exchange resin Amberlite IRA 400[43]

From all these results, the inference that can be drawn is like

this As mentioned in Table 1, an increase in concentration of

DEA reduces the size of nano silver particle As discussed earlier,

the amine in aqueous solution forms a hydrated ion and hydroxy

ions are left in the solution which in turn oxidizes glucose to form

gluconic acid If the concentration of amine is greater then the

number of hydroxy ions are present is greater and the amount of

gluconic acid formed also increases These gluconic acid molecules

present on the surface of the nano particles may act as a particle

size controlling factor So in turn the amine concentration is

deter-mining the size of the particles for a particular set of

concentra-tions of silver nitrate and glucose, as mentioned inTable 1(S2–

S4) It is very easy to understand the increase in the size of particles

with the concentration of AgNO, due to the increase in the ratio of

available gluconic acid to silver particles In the given set of condi-tions inTable 1, by increasing the concentration of glucose, the size remains unaffected as the glucose to gluconic acid conversion in the presence of amine is a size controlling factor and not just the glucose concentration Just by changing the molar ratio of reac-tants, we can get a stable nano silver suspension or powder The reactions for samples S5–S10 can be scaled up to a total vol-ume of 5 liters, which can yield 45–50 gm of nano silver powder which is quiet a substantial yield

The powders of nano silver were tested for antibacterial proper-ties on E coli The effect of the silver nano particles on E coli is shown in the photograph inFig 6for sample S7 The number of

E coli has been reduced by a considerable number because of the antibacterial action of the nano silver particles, whereas growth was observed in the control, as shown inFig 7a and b, respectively The zone of inhibition observed for the nano silver powder coated plastic conﬁrms the antibacterial action of the nano silver particles, whereas there was no zone of inhibition for the uncoated plastic, as shown in Fig 7c and d, respectively These observations clearly indicate the antibacterial properties of nano silver It is interesting

to observe that, although the nanoparticles contain traces of glu-conic acid on the surface, as is evident from the XPS analysis, the surface of the nano silver particle is still available for efﬁcient anti-bacterial activity

4 Conclusions

The present work reveals a simple, cost effective wet chemical synthetic route to form nano silver powders Silver nano particles are formed in a simple oxidation–reduction reaction in diethyl amine, glucose and silver nitrate A blue shift in kmax observed for an increase in the diethyl amine concentration in samples S2–S4 was found to be the size controlling factor Detailed surface analysis studies using XPS and FTIR revealed the presence of glu-conic acid that encapsulates the nano silver particle The negative charge on the surface due to the presence of gluconic acid was con-ﬁrmed by anion exchange resin The antibacterial studies show very high activity against E coli This process of producing large scale silver nano powders with highly efﬁcient antibacterial prop-erties is certainly going to increase its commercial value and widen the application range

Acknowledgement

Authors would like to thank Dr B Sreedhar, IICT, Hyderabad for providing the XPS and FTIR facility

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