A 'Green' Chitosan Silver Nanoparticle Composite as a Heterogeneous as Well as Micro-heterogeneous...

8/12/2019 A 'Green' Chitosan Silver Nanoparticle Composite as a Heterogeneous as Well as Micro-heterogeneous Catalyst

1/10


2/10

IOP PUBLISHING NANOTECHNOLOGY

Nanotechnology19(2008) 015603 (9pp) doi:10.1088/0957-4484/19/01/015603

A green chitosansilver nanoparticlecomposite as a heterogeneous as well asmicro-heterogeneous catalyst

A Murugadoss1 and Arun Chattopadhyay1,2

1 Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781 039, India2 Centre for Nanotechnology, Indian Institute of Technology Guwahati, Guwahati 781 039,

India

E-mail:[email protected]

Received 3 July 2007, in final form 8 October 2007

Published 29 November 2007

Online atstacks.iop.org/Nano/19/015603

Abstract

In this paper, we report on the catalytic activity of a new metal nanoparticlepolymer composite

consisting of Ag nanoparticles (NPs) and environmentally friendly (green) chitosan. The

polymer (chitosan) not only acted as the reducing agent for the metal ions, but also stabilized

the product NPs by anchoring them. The majority of the particles produced in this way had

sizes less than 5 nm. The catalytic activity of the composite was investigated photometrically by

monitoring the reduction of 4-nitrophenol (4NP) in the presence of excess NaBH4in water,

under both heterogeneous and micro-heterogeneous conditions. The reaction was first order

with respect to the concentration of 4NP. We also observed that the apparent rate constant, kapp,

for the reaction was linearly dependent on the amount of Ag NPs present in the composite.Moreover, the turn-over frequency (TOF) of the catalyst was found to be

(1.5 0.3) 103 s1, when the reaction was carried out under heterogeneous conditions. The

Ag NPs in the composite retained their catalytic activities even after using them for ten cycles.

Our observations also suggest that the catalytic efficiency under micro-heterogeneous

conditions is much higher than under heterogeneous conditions. Thus the composite we have

represents an ideal case of an environmentally friendly and stable catalyst, which works under

heterogeneous as well as micro-heterogeneous conditions with the advantage of nanoscopic

particles as the catalyst.

S Supplementary data are available from stacks.iop.org/Nano/19/015603

1. Introduction

The future of sustainable nanoscale science and technology not

only depends on overcoming knowledge barriers in developing

newer principles, but also on appropriately addressing the

environmental and societal concerns, especially when large-

scale material production is involved. The primary challenges

in this regard are the maximization of usage of environmentally

friendly materials and integration of the principles of green

chemistry, as far as chemistry is involved, in the generation

of nanoscale-based products. Among the many products that

have been developed, polymermetal nanoparticle composites

seem to receive special attention because of their applicationpotential in sensors [1], actuators [2], electrodes for fuel

cells [3], supported micro-heterogeneous and heterogeneouscatalyses [4], and in optical and optoelectronic devices [5].

Also, advantages of mechanical, optical, thermal and electrical

properties of the polymer and optical, electrical and catalytic

properties of the metal NPs can be suitably integrated to

achieve superior performance. For example, we have for the

first time synthesized Au nanoparticlepolyaniline composite

in a single reaction condition where the composite product had

higher electrical conductivity in comparison to the polymer

alone [6]. We have subsequently developed methods of

synthesis of a polymermetal composite nanoparticle and a

polymerTiO2 nanoparticle thin film composite with superior

photocatalytic properties [7]. In addition, we have alsodemonstrated that an Au NPpolyaniline composite on a resin

0957-4484/08/015603+09$30.00 2008 IOP Publishing Ltd Printed in the UK1
http://dx.doi.org/10.1088/0957-4484/19/01/015603mailto:[email protected]://stacks.iop.org/Nano/19/015603http://stacks.iop.org/Nano/19/015603http://stacks.iop.org/Nano/19/015603http://stacks.iop.org/Nano/19/015603mailto:[email protected]://dx.doi.org/10.1088/0957-4484/19/01/015603


3/10

Nanotechnology 19(2008) 015603 A Murugadoss and A Chattopadhyay

bead could act as a catalyst for the oxidation of glucose with

simultaneous colorimetric detection of the product [8]. Several

other groups have subsequently developed various synthetic

schemes for the generation of metal NPpolymer composites

for appropriate applications. For example, Ballauff and

co-workers [9] have synthesized several metal NPpolymer

composites for catalysis with incorporation of thermosensitiveactivity as one of the major advantages. On the other hand,

Gronnowet al[10]have used an expanded starch supported Pd

catalyst with high activity for Suzuki, Heck and Sonogashira

reactions. Among the metal NPs, Ag NPs generated under

various conditions are known to be efficient catalysts for the

reduction of organic dyes [11] and also for the reduction of

4-nitrophenol (4NP) to 4-aminophenol (4AP) [12]. However,

incorporation of Ag NPs into polymer, especially biopolymer,

has not received much attention, possibly because of the

experimental difficulty in the syntheses of composites.

Chitosan, a novel biopolymer, is a product of deacetylation

of chitin, which is the second most abundant natural

polymer in the world after cellulose. The biocompatibilityand antibacterial properties of chitosan and its being an

environmentally friendly polyelectrolyte makes it appealing

for several applications such as water treatment [13], novel

fibers for textiles [14], photographic paper[15], biodegradable

films [16], biomedical devices and microcapsule implants for

controlled release in drug delivery [17]. Thus a composite

of Ag NPs and chitosan would ideally be suited for various

commercial applications, especially if generated under benign

conditions. Although there are methods that are available for

the synthesis of Pd NPchitosan composite, currently there

is no simple method of synthesis of an Ag NPchitosan

composite.

Herein we report the development of a mild one-step

method of Ag NPchitosan composite, by using chitosan as the

reducing agent, at 95 C and in the presence of aqueous sodium

hydroxide. The NPs produced in the medium get attached

to the chitosan and are stable for weeks without noticeable

change in properties. The NPs produced were small, with sizes

mostly below 5 nm, and were found to have high catalytic

activity. Also, the amount of Ag NPs produced could be

easily controlled by controlling the amount of starting material

(AgNO3), without significant variation in sizes. We have

studied the catalytic reduction of 4NP to 4AP by the composite

using NaBH4 as the reducing agent, under both heterogeneous

and micro-heterogeneous conditions. When the heterogeneouscatalytic reaction was carried out in the presence of excess

NaBH4, it was observed to follow pseudo first-order kinetics

and the apparent rate constant depended linearly on the

concentration of Ag NPs present in the composite. The turn-

over frequency (TOF) was observed to be (1.5 0.3)

103 s1, when the reaction was carried out in heterogeneous

conditions. Interestingly, we observed that the composite

could be used for catalyzing the same reaction at least for ten

cycles without significant loss of its activity. Transmission

electron microscopy (TEM) evidence indicated that Ag NPs

were present and sizes were retained even after several cycles

of catalytic reductions. Further, when the reaction was carried

out in micro-heterogeneous conditions the rate constant wasmuch higher.

2. Experimental details

2.1. Materials

Silver nitrate (AgNO3, 99.5%; Merck), acetic acid (glacial,

99100%; Merck), chitosan of high molecular weight (75%

deacetylated; Sigma-Aldrich), sodium borohydride (NaBH4,

95%; Merck), sodium hydroxide (NaOH, 98%; Merck) and4-nitrophenol (95%; Merck) were used as received without

further purification. Milli-Q grade water was used in all

experiments.

2.2. Synthesis of the chitosanAg nanoparticle composite

A 100 ml beaker containing 50 ml of Milli-Q water was kept in

an oil bath at a temperature 9395 C with constant magnetic

stirring. This was followed by addition of 0.1 g of chitosan

into the beaker. A freshly prepared 1 ml solution of 20 mM

AgNO3 was then added to this mixture, which was followed

by addition of 100

l of 0.3 M NaOH. The pH of the solutionwas measured to be 10.4. The resultant solution turned yellow

in about a minute, indicating the formation of Ag NPs in the

medium. The reaction was allowed to continue for additional

10 min. Finally, powdered yellow colored solid appeared at the

bottom of the beaker. The beaker was then taken out from the

oil bath and cooled to room temperature. This was followed

by separation and washing of the powder with Milli-Q water

five times. The solid was then air dried and kept for further

use. The filtrate was treated with concentrated HCl to check

for any excess of Ag+ present in the medium; the result was

negative. In other words, all of the Ag+ ions had reacted in the

production of Ag NPs.

In order to prepare chitosanAg NPs composites withvarying amount of Ag, the concentration of AgNO3 was

appropriately varied, keeping all other conditions the same.

2.3. Catalysis study

2.3.1. Heterogeneous conditions. Investigations of the

catalytic activity of chitosanAg NP composites with different

Ag content were pursued in the following way, using the

reduction of 4NP to 4AP by NaBH4 as a model reaction. In

a 3 ml quartz cuvette, a given amount of chitosanAg NP

composite powder was added and this was followed by the

addition of 2.5 ml of 10 mM NaBH4

solution. Subsequently,

25 l of 10.0 mM of 4NP was added to the mixture and the

cuvette was shaken vigorously for mixing. The cuvette was

then placed in a UVvisible spectrophotometer to monitor the

time-dependent absorption at 400 nm at room temperature (23

24 C).

2.3.2. Micro-heterogeneous conditions. 40 mg of Ag NP

chitosan composite was dissolved in 20 ml 0.1% (V/V) acetic

acid in water. The corresponding solution of the pH was found

to be 4.3. The pH of the solution was then adjusted to around

6.3 by the addition of NaOH solution. This stock solution was

used for further catalysis studies under micro-heterogeneous

conditions. The resultant solution was also used for physicalcharacterization studies of the composite. For catalysis studies,

2


4/10


5/10


Figure 2.(a) and (b): transmission electron micrographs of the chitosanAg NP composite; (c) is the particle size distribution histogram asobtained from figure2(a), and (d) is the selected area electron diffraction pattern corresponding to a particle.

with increase in absorption intensity. This implies that when

the concentration of silver ions was increased, the loading ofthe silver content also increased on the chitosan. However,

increasing the number of Ag+ ions led possibly to an increase

in the number of NPs but not their size. It is also possible that

their size decreased with the loading. This could be due to

different nucleation mechanisms operating at lower and higher

Ag+ concentrations attached to chitosan.

It is important to mention here that the excess amine and

hydroxyl groups present in the chitosan polymer support the

nucleation as well as stabilization of the Ag NPs formed in the

process. In other words, the Ag NPs adsorbed on the surface

of the polymer are prevented from further aggregation. In

order to investigate the size distribution of the Ag NPs on the

surface of the chitosan polymer, TEM studies were pursued.

Typical TEM micrographs of drop cast composite are shown

in figures 2(a) and (b), and the corresponding particle size

histogram, corresponding to the micrograph 2(a), is shown

in figure2(c). As is clear from the figures, small NPs were

formed and deposited all over the polymer. The majority

of the particles formed had sizes less than 5 nm. It is also

interesting to note that the particles were rather uniform in

size. The selected area diffraction (SAD) pattern of the NPs

(figures 2(d)) consisted of a hexagonal structure of the (111)

face of single-crystalline Ag NPs. Also, these results correlated

well with the dynamic light scattering study of the composite

particles. The dynamic light scattering (DLS) measurementsof the composite, dissolved in 0.1% (V/V) acetic acid

in water, also indicated the formation of small particles

with nearly uniform particle size distribution. A typicalparticle size histogram from DLS measurements is shown

in the electronic supplementary information (ESI; figure S3

(available at stacks.iop.org/Nano/19/015603). Clearly, more

than 90% of the particles formed were of sizes in the range 2

4 nm, with the majority of particles formed being about 3.0 nm

in diameter. Thus the present method could produce stable,

small sized Ag NPs in the form of chitosanAg NP composite

with uniform and narrow particle size distributions.

Also important to mention here is the observation that

when chitosanAg NP composite was dissolved in 0.1% (V/V)

of acetic acid in water and kept for a couple of months,

no discernible precipitation could be observed and there was

no change in the UVvisible spectrum, indicating that these

particles were also stable in solution form when present with

the polymer as composite. The ease of synthesis of chitosan

Ag NP composite and the stability of the NPs in the composite

provide opportunities for applications, especially with respect

to the catalysis of important chemical reactions. This is

especially advantageous where the NPs act as the catalyst and

the adherence of the NPs to the bulk and insoluble polymer

helps in recovery and thus repeated use. In order to test the

efficacy of the present composite in catalyzing heterogeneous

chemical reactions, we have studied the reduction of 4NP

to 4AP by NaBH4 in the presence of the composite. The

reactions have been monitored spectrophotometrically. Also,important to mention here is the use of excess NaBH4 so that

4
http://stacks.iop.org/Nano/19/015603http://stacks.iop.org/Nano/19/015603


6/10


Table 1. ChitosanAg NP composites used in the catalysis studies and corresponding Ag NPs present in the reaction mixture. For thesynthesis of the composite, 0.1 g of chitosan and 0.4 mM AgNO 3in 50 ml solution were used. Different amounts of the composite were takenfrom the powder generated from this reaction medium.

Sl. No.

Amount of chitosanAg NPcomposites in unit volume ofcatalysis reaction mixture

(g l1)

Corresponding amount of Agpresent in the composite inthe catalysis reaction mixture

(mg l1)

Concentration of Ag inthe composite in thecatalysis reactionmixture

(103 mol dm3)

1 0.40 21 0.192 1.20 63 0.583 1.60 84 0.774 2.00 105 0.975 2.84 149 1.336 3.24 170 1.577 3.60 189 1.758 4.40 231 2.179 5.20 273 2.53

Figure 3.UVvisible spectra of solution of 4NP measured atdifferent time intervals in the presence of chitosanAg NP compositeparticles. The reaction conditions were as follows: [4NP]

= 0.1 mM dm3, [NaBH4] = 10 mM dm3, amount of chitosanAgNP composite powder = 2.1 mg in 2.5 ml (i.e. the concentration ofAg was 0.4 mM).

the kinetics of the reaction could be followed as a function of

concentrations of Ag NPs. An aqueous solution of 4NP shows

a distinct spectral profile with an absorption maximum at

317 nm, which shifts to 400 nm in the presence of NaBH4due

to the formation of the 4-nitrophenolate ion[20]. The anionic

species remain stable for weeks in the absence of any other

reagent. However, in the presence of the solid chitosanAg NP

composite the peak at 400 nm gradually decreased with time,along with the appearance of a new peak at 290 nm, which is

known to be due to absorption of 4-amino phenol [20b].

A typical time-dependent catalytic reduction of 4NP as

followed spectrophotometrically is shown in figure 3. As

is clear from the figure, under the reaction conditions, the

reduction took place in about 25 min. Also important to note

is the presence of an isosbestic point at 320 nm (figure 3)

indicating the formation of the product 4AP as a result of

reduction from 4NP. We further pursued the investigations with

respect to the rate of the reaction as a function of concentration

of the catalyst (Ag NPs) in the composite.

This was achieved by varying the amount of composite

while keeping the ratio of chitosan to AgNO3 the same, as

indicated in table 1. Since our observations indicated the

Figure 4. Plot of the absorbance atmaxof 4NP versus time fordifferent amounts of chitosanAg NP composite per unit volume.Corresponding concentrations of Ag in heterogeneous condition aregiven in table1.The reaction conditions were as follows: [4NP]= 0.1 mM dm3 and [NaBH4] = 10 mM dm

3.

absence of Ag+ in the medium after the formation of the

composite, it is possibly safe to assume that all the Ag+

ions present in the medium resulted in the formation of NPs

in the composite. Also, since the particle size distribution

studies indicated that the NPs formed were nearly uniform

in size and spherical in nature, we have used the method

adopted by Pikeramenou et al [21] to calculate the number

of particles present in each amount of composite used for

catalysis. Also, the same method was used to calculate the

number of surface atoms present in the NPs, which was further

used for the calculations of turn-over frequency (TOF). The

details of the method and calculations are included in the

ESI. For each concentration of the composite (and thereby

Ag NPs) the conversion of the 4NP to 4AP was studied

by following the time-dependent absorption changes of 4NP

spectrophotometrically, as before. Since the absorbance of

4NP is proportional to its concentration in the medium, the

ratio of absorbance at timet(A)to that att= 0 (t0), i.e. A/A0,

could be used as the ratio of concentrations of 4NP at timet to that at t0 (i.e. A/A0 = c/c0). The concentrations of

5


7/10


4NP and NaBH4 (used in excess) were kept constant for all

the reactions. The amount of chitosanAg NP composite used

in each of these reactions is indicated in table 1. Since the

concentration of NaBH4used in the reactions was in far excess

of that of 4NP, the reaction can be considered as a pseudo first-

order reaction. The rate of the reaction is then proportional to

the concentration of 4NP alone, and can be written as follows.

dct

dt ct. (1)

Here, ctis the concentration of 4NP at time t. We observed

that the rate of the reaction increased with the increase in the

amount of chitosanAg NP composite used for the reactions.

A plot of the normalized concentration change as a

function of time for the reduction of 4NP for various amounts

of chitosanAg NP composite (and thus Ag NPs) is shown

in figure 4. As is clear from the figure, the longest time

taken for the completion of the reaction was 32 min, when the

concentration of the Ag used was 0.19 mM. On the other hand,

the reaction was over in about 5 min when the concentration

was 2.53 mM. Thus the catalytic reaction was sensitive to

the amount of Ag NPs present in the composite. It is also

interesting to note that when the concentration of the catalyst

was low (0.19 mM) there was significant induction period

(about 13 min) in the reaction, whereas the reaction started

immediately when the concentration was high (2.53 mM). At

the intermediate concentrations the induction period was also

in between the above values, and it decreased with the increase

in the concentration of the catalyst. The induction periods

and their reduction with the concentration of the catalysts have

been observed also by previous researchers working with metal

NPs and have been attributed to the activation of the catalystpresent in the reaction mixture [20b,22]. In view of the above

observations, equation(1) can be rewritten as

dct

dt= kappct, (2)

where ctis the concentration of p-nitrophenol at time t, and

kapp is the apparent rate constant, which is a function of the

concentration of the chitosanAg NPs used. A plot of the

apparent rate constant with the concentration of the catalyst, as

obtained from the ln(c/c0)versus time plot using the data from

figure4,is shown in figure5(a). As is clear from the figure, the

apparent rate constant of the reaction is linearly proportional tothe amount of catalyst used for the reaction. Thus equation (2)

can be written as

dct

dt= kappct= k1cAgNPct= k

1WchiAgct, (3)

where cAgNPs is the concentration of Ag present in the

composite as per table1;WchiAgis the weight of the chitosan

Ag NP composite present in a unit volume of the system, which

we observed to increase linearly with the amount of Ag+ in the

parent salt that was used to generate the composite (figure5).

k1 and k1 are constants. Since the reaction carried out was

heterogeneous and the same volume of reaction mixture was

used for all the reactions, the weight (WchiAg) used hereincan be assumed to be equivalent to the concentration of Ag

Figure 5. The plot of rate constantkappobtained from figure4. Plot(a) represents the rate constantkappas a function of the amount of

chitosanAg NP composite normalized to the unit volume of thesystem and plot (b) shows the rate constant variation withconcentration of Ag.

NPs (cAgNPs). We have calculated the rate constant (k1)

of the reaction in the present system from the slope of the

graph in figure5(b). The rate constant (k1) was found to be

1.06 mol1 dm3 s1.

An important parameter indicating the efficiency of a

catalyst is the turn-over frequency (TOF). Since the surface

atoms of the catalyst are involved in the catalysis, the efficiency

is related to the number of surface atoms present in the catalyst.

And since the number of surface atoms present in NPs is high

compared to the volume the efficiency is expected to be higher

for NPs in comparison to the bulk catalyst, especially when the

surface to volume ratio is the overriding determining factor.

The TOF is defined as

TOF =M

Ns t, (4)

whereMis the number of molecules (4NP here) reacting in the

presence of catalyst in timetto produce the product, and Ns is

the number of surface atoms of the catalyst that is involved in

the reaction. In calculating the TOF we have assumed that the

average particles size of Ag NP was 4 nm, which was obtainedfrom the TEM picture. Since the amount of Ag as well as

6


8/10


Table 2. Calculation of the number of surface atoms and TOF (turn-over frequency) with induction period and without induction period. Thevalues in the rows, i.e. number of Ag NPs, correspond to the amount of chitosanAg NP composite as in table 1.

Sl. No.

No. of Ag NPs

(NNPs) 1014

No. of surfaceatoms(NS) 10

17

Time (s)includinginduction period

Time (s)excludinginduction period

TOF(s1) 103

includinginduction period

TOF(s1) 103

excludinginduction period

1 0.60 0.37 1920 1110 2.10 3.6

2 1.80 1.12 1404 804 0.95 1.63 2.40 1.49 984 594 1.00 1.74 3.01 1.87 750 432 1.00 1.85 4.27 2.66 576 378 0.98 1.56 4.87 3.03 510 354 0.97 1.47 5.41 3.33 438 348 1.00 1.38 6.61 4.12 396 306 0.92 1.29 7.82 4.85 330 240 0.94 1.3

Figure 6.Reusability of the composite catalyst as demonstrated byseveral cycles of reactions catalyzed by the same catalyst. The plot

indicates changes in the visible absorption spectra of 4NP due to thereduction to 4AP, as a function of time, for several cycles of use ofthe composite catalyst. The numbers in the right box indicate thecycle number in the catalysis.

the number of Ag NPs were known, the number of surface

Ag atoms present for each amount of composite was then

calculated. Also, the number of reactant molecules (4NP) used

for each case was fixed at 1.50 1017. Hence the TOF was

calculated for each of the reactions carried out with different

amount of catalyst. Table2 lists the amount of Ag NPs used,

the total time taken and the TOF for each reaction. The average

value of TOF was found to be (0.970.02)103 s1. Thesecalculations were performed using the total time as observed,

which included the induction periods for lower concentrations

of Ag NPs (and the lowest concentration data were not

considered). However, if we subtract the induction time period

and discount the first data (with very low concentration of

catalyst) then an average value of(1.5 0.3) 103 s1 was

found for the TOF. This value is reasonable for a nanocatalyst

such as small Ag NPs, and is consistent with the literature

value. One may point out that since Ag NPs are supported

on the chitosan the exact number of surface atoms may not be

exactly the same as the calculated value. However, as indicated

in table2,the consistency of the TOF value at different Ag NP

concentrations suggests that the effect of the support if any may

be minimal in terms of error in the calculation.

We were further interested in finding out the reusability

of the catalyst. For this we carried out a set of reactions, in

sequence, where the ratio of the amount of catalyst (chitosan

Ag NP composite) and the reactant (4NP) were kept nearly

the same. For example, 10 mg (the concentration of Ag was1.9 mM) of the composite was taken in a cuvette with the

addition of 2.5 ml of a solution containing 4NP and NaBH4as before (see the caption of figure 3). Once the reactions

were over (as observed by the absorbance decay of 4NP), the

composite was recovered by decantation of the liquid (using

a micropipette). This was followed by washing of the solid

composite with Milli-Q grade water and decantation of the

liquid again. Then 2.5 ml of the same reaction mixture was

added to the cuvette. The absorbance decay of 4NP with time

was followed as before. The completion of the reaction was

taken as the completion of the second cycle. Similarly, the

cycles were pursued eight more times to have ten cycles ofreaction using the same catalyst. The results are shown in

figure6. As is clear from the figure, the catalytic activity was

retained quite well, even up to the tenth time. For example,

whereas the reaction was completed in 516 s in the first cycle,

it took 1716 s to be completed in the fifth cycle, while the

same reaction took 1075 s in the eighth cycle. The apparent

difference between the progressive cycles could be due to the

loss of catalyst during handling in between two cycles. Also,

oxidation of Ag NPs and subsequent reduction by NaBH4,

while in the composite (in the reaction medium), may alter the

average size of the particle, which may also be the reason for

observed differences between cycles. This means that the AgNPs were stable in the composite and retained their activity.

That the catalytic Ag NPs were stable and remained attached to

chitosan polymer was evident from the TEM measurements of

the catalyst after four cycles of catalytic reactions. The typical

TEM pictures shown in the ESI (figure S3(a) available at

stacks.iop.org/Nano/19/015603 ) clearly indicate the presence

of small Ag NPs in the composite after the fourth cycle

of operation. The SAED patterns (figure S3(b) available at

stacks.iop.org/Nano/19/015603 ) corresponding to the NPs also

corroborate the presence of Ag NPs. These results show that

metal NPs on the surface of the chitosan were not getting

leached significantly during the course of the reaction. This

possibly indicates that the strong affinity of chitosan for AgNPs stabilizes the NPs on the surface and that might prevent

7
http://stacks.iop.org/Nano/19/015603http://stacks.iop.org/Nano/19/015603http://stacks.iop.org/Nano/19/015603http://stacks.iop.org/Nano/19/015603


9/10


Figure 7.Plot of the absorbance atmaxof 4NP versus time, in thepresence of different amounts of chitosanAg NP composite per unitvolume. The reaction was conducted under micro-heterogeneousconditions. The reaction conditions were as follows: [4NP]= 0.1 mM dm3, [NaBH4] = 10 mM dm

3.

the metal from leaching during the catalytic reaction. Thus we

have an efficient, workable, easily recoverable Ag NP catalyst

with reasonably high TOF. This is important in terms of value

of a catalyst in industrial applications.

Finally, we were further interested in finding the potential

of the composite as a catalyst working under micro-

heterogeneous conditions for increased efficiency. This would

be even more advantageous if the reaction conditions were

such that the composite would be dispersible in the reaction

mixture at a certain pH and the reaction could be carried out

without the catalyst getting precipitated out of the solution.

Fortuitously, the Ag NPchitosan composite was dispersiblein the reaction mixture (aqueous solution) up to a pH of 6.3,

and the reduction of 4NP to 4AP could be carried out under

this condition using NaBH4 as the reducing agent afterwards.

The results of the catalysis of the conversion of 4NP to 4AP

by the composite in the presence of NaBH4 are shown in

figure7. As is evident from the figure, the reduction of 4NP

under micro-heterogeneous conditions was much faster than

that while using the catalyst under heterogeneous conditions

(figure 4). Also important to note is that the concentrations

of catalyst needed to carry out the same reaction (with

similar concentrations of reactants) were much less when

carried out in micro-heterogeneous conditions in comparison

to heterogeneous conditions. For example, while the catalyst

concentrations needed under heterogeneous condition were on

the order of several g l1, the same reaction could be achieved

in similar times using catalyst concentrations of mg l1. This

indicates that the reaction when carried out using the composite

as the catalyst under micro-heterogeneous conditions is much

more efficient than under heterogeneous conditions.

That the reaction is faster in micro-heterogeneous

conditions can be explained using the concentration of metal

NPs present in the medium, where the rate will directly depend

on the available surface area of the NPs in the medium. In that

case the rate of the reaction can be expressed as

dct

dt= kappct= k2 Sct, (5)

Figure 8. Plots of the rate constantkappobtained from figure7(reaction under micro-heterogeneous conditions). Plot (a) represents

the rate constantkappas function of the amount of chitosanAg NPscomposite and plot (b) shows the rate constant variation withconcentration of Ag normalized to the unit volume of the reactionsystem under micro-heterogeneous conditions.

wherec tis the concentration of 4NP at time t, k2 is the rate

constant, and Sis the total available surface area of the NPs

in the medium (normalized to unit volume of the system).

The validity of this approximation can be made from figure 7,

where it can be observed that increase in the concentration

of the chitosanAg NP composite leads to a rapid increase

in the rate of the reaction. Also, the values of rate constants

in figure 8 support that the apparent rate constant kapp isproportional to the total surface area of the NPs as well as

the concentration of chitosanAg NP composite used in the

system. Further, the amount of chitosanAg NP composite

used in the micro-heterogeneous reaction conditions is much

smaller than that in heterogeneous conditions. From the above

observations, it can be concluded that the rate of the reaction in

the micro-heterogeneous conditions, which can be carried out

with less amount of catalyst, is much faster than that under

heterogeneous conditions. Finally, our observations suggest

that the catalytic activity of chitosanAg NP composite that

we have prepared is much higher in comparison to results

available in the literature. For example, Pradhan et al [12]

have used Ag NPs with an average size of about 25 nm tocatalyze 4NP to 4AP using NaBH4 and have obtained a rate

8


10/10


constant kapp = 2.3 103 s1 with 5.0 106 mol l1

Ag catalysts at 20 C. The rate constant when normalized

with the surface area of the catalyst was calculated to be

k= 3.78 107 s1 m2 l. On the other hand, Lu et al[22]

synthesized Ag NP composite with a thermosensitive polymer,

having average diameter of the Ag NPs between 8 and 10 nm,

to catalyze the above reaction. They obtained a rate constantk= 5.2102 s1 m2 l, higher than that obtained by Pradhan

et al [12]. In our case, with the sizes of the Ag NPs (in the

composite) much smaller, the rate constant was calculated from

figure8(b) to bek2 = 1.5 101 s1 m2 l, which is higher

than the literature reports. However, considering the sizes of

the NPs in the present system and also the catalyst working in

micro-heterogeneous conditions, this increase in rate constant

is not surprising. The present set of results demonstrate

that the conditions of reaction (micro-heterogeneous versus

heterogeneous) and sizes of the particle (catalyst) are crucial

in catalysis involving metal NPs.

4. Conclusion

We have developed a simple synthetic scheme for the

generation of stable chitosanAg NP composite with high

catalytic activity. The NPs generated in the present method

were of uniform and narrow particle size, with typical size

less than 5 nm. The composite was tested for heterogeneous

catalysis, with Ag NPs being the active catalyst and chitosan

as the support. The catalytic efficiency for the reduction of

4NP to 4AP for the composite was high, with a TOF value

of(1.5 0.3) 103 s1. The small particle size and thus

high number of surface atoms makes the composite a rather

appealing material for catalysis and other applications. Further,the additional advantage of being able to use the catalyst under

micro-heterogeneous conditions makes its usefulness rather

appealing. To the best of our knowledge, there exists no

other report of chitosan being used as a reducing agent for

the production of Ag NPs as well as their stabilizer, although

reports exist of functional NPs that were separately attached

to chitosan for catalysis. The synthetic method, which we

have developed in the present study, might be useful for

further development of green NP synthesis in other situations.

Since chitosan is biocompatible, the composite can be useful

for biological applications, which we are currently pursuing.

Since chitosan can be made in the form of beads, fibers, andmembranes, the application potential of the composite could

become even more versatile.

Acknowledgments

We thank the Department of Science and Technology,

Government of India, for financial assistance (DST Nos

SR/S5/NM-01/2005 and 2/2/2005-S.F.).

References

[1] He H, Zhu J, Tao N J, Nagahara L A, Amlani I andTsui R 2001J. Am. Chem. Soc. 1237730

Stucky G D 2001Nature410885[2] Jager E, Ingalis O and Lundstrom L 2000Science2882335[3] Bashyam R and Zelenay P 2006Nature44363[4] Tsunoyama H, Sakurai H, Negishi Y and Tsuguda T 2005

J. Am. Chem. Soc. 109692Zhang J, Xu S and Kumacheva E 2004 J. Am. Chem. Soc.

1267908Calo V, Nacci A, Monopoli A, Fornaro A, Sabbatini L,

Cioffi N and Ditaranto N 2004 Organometallics22 5154Vincent T and Guibal E 2003Langmuir19 8475

[5] Sih B S and Wolf O M 2005Chem. Commun.20053375Novak P J and Feldheim L D 2000 J. Am. Chem. Soc.

1223979Atay T, Song J-H and Nurmikko V A 2004 Nano Lett.

41627[6] Sarma T K, Chowdhury D, Paul A and Chattopadhyay A 2002

Chem Commun.20021048Sarma T K and Chattopadhyay A 2004 J. Phys. Chem.A

1087837

[7] Chowdhury D, Paul A and Chattopadhyay A 2005Langmuir214123

[8] Majumdar G, Goswami M, Sarma T K, Paul A andChattopadhyay A 2005Langmuir21 1663

[9] Lu Y, Mei Y, Drechsler M and Ballauff M 2006Angew.Chem. Int. Edn45813

[10] Gronnow M J, Luque R, Macquarrie D J and Clark H J 2005Green Chem.7552

[11] Jana R N, Sau K T and Pal T 1999J. Phys. Chem.B103115Jiang J-Z, Liu Y-C and Sun W-L 2005 J. Phys. Chem.B

1091730[12] Pradhan N, Pal A and Pal T 2002Colloids Surf.A 196247[13] Kobayashi S, Toshitsugu k and Shin-ichiro S 1996J. Am.

Chem. Soc.11813113[14] Shin Y, Yoo D I and Jang J 1999J. Appl. Polym. Sci. 742911

[15] Shigehiro H, Yasuo K, Shuntaro K, Testsuo T, Hisaya T,Kakuko M and Takeshi Y 1991 Biochem. Syst. Ecol.19379

Domare A and Roberts G A F 1998Advances in ChitinScienceed J Andre (Keelung: National Taiwan OceanUniversity)

[16] Dhanikula A B and Panchangnula R 2004AAPS J.6 1Payne F G and Raghavan S R 2007 Soft Matter3 1

[17] Roy K, Mao H Q, Huang S K and Leong K W 1999NatureMed.5387

Bartkowiak A and Hunkeler D 1999 Chem. Mater.11 2486Suzuki T, Mizushima Y, Umeda T and Ryo O 1999J. Biosci.

Bioeng.88194[18] Yi H, Wu L-Q, Bentley W E, Ghodssi R, Rubloff G W,

Culver J N and Payne G F 2005 Biomacromolecules62881

[19] Hardy J J E, Hubert S, Macquarrie D J and Wison A J 2004Green Chem.653

Vincent T and Guibal E 2002Ind. Eng. Res. Chem. 415158Han H S, Jiang S N, Huang M and Jiang J J 1996 Polym. Adv.

Technol.7704[20a] Pradhan N, Pal A and Pal T 2001Langmuir17 1800[20b] Panigrahi S, Basu S, Praharaj S, Pande S, Jana S, Pal A,

Ghosh K S and Pal T 2007J. Phys. Chem.C 1114596[21] Lewis J D, Day M T, MacPherson V J and

Pikeramenou Z 2006Chem. Commun.20061433[22] Lu Y and Ballauff M 2006J. Phys. Chem.B 1103930

9
http://dx.doi.org/10.1021/ja016264ihttp://dx.doi.org/10.1021/ja016264ihttp://dx.doi.org/10.1038/35073733http://dx.doi.org/10.1038/35073733http://dx.doi.org/10.1126/science.288.5475.2335http://dx.doi.org/10.1126/science.288.5475.2335http://dx.doi.org/10.1038/nature05118http://dx.doi.org/10.1038/nature05118http://dx.doi.org/10.1021/ja031523khttp://dx.doi.org/10.1021/ja031523khttp://dx.doi.org/10.1021/om049586ehttp://dx.doi.org/10.1021/om049586ehttp://dx.doi.org/10.1021/la034364rhttp://dx.doi.org/10.1021/la034364rhttp://dx.doi.org/10.1039/b501448dhttp://dx.doi.org/10.1039/b501448dhttp://dx.doi.org/10.1021/ja000477ahttp://dx.doi.org/10.1021/ja000477ahttp://dx.doi.org/10.1021/nl049215nhttp://dx.doi.org/10.1021/nl049215nhttp://dx.doi.org/10.1039/b201014chttp://dx.doi.org/10.1039/b201014chttp://dx.doi.org/10.1021/jp049348xhttp://dx.doi.org/10.1021/jp049348xhttp://dx.doi.org/10.1021/la0475425http://dx.doi.org/10.1021/la0475425http://dx.doi.org/10.1021/la047440ehttp://dx.doi.org/10.1021/la047440ehttp://dx.doi.org/10.1002/anie.200502731http://dx.doi.org/10.1002/anie.200502731http://dx.doi.org/10.1039/b501130bhttp://dx.doi.org/10.1039/b501130bhttp://dx.doi.org/10.1021/jp982731fhttp://dx.doi.org/10.1021/jp982731fhttp://dx.doi.org/10.1021/jp046032ghttp://dx.doi.org/10.1021/jp046032ghttp://dx.doi.org/10.1016/S0927-7757(01)01040-8http://dx.doi.org/10.1016/S0927-7757(01)01040-8http://dx.doi.org/10.1021/ja963011uhttp://dx.doi.org/10.1021/ja963011uhttp://dx.doi.org/10.1016/0305-1978(91)90054-4http://dx.doi.org/10.1016/0305-1978(91)90054-4http://dx.doi.org/10.1208/aapsj060327http://dx.doi.org/10.1208/aapsj060327http://dx.doi.org/10.1039/b613872ahttp://dx.doi.org/10.1039/b613872ahttp://dx.doi.org/10.1038/7385http://dx.doi.org/10.1038/7385http://dx.doi.org/10.1021/cm9910456http://dx.doi.org/10.1021/cm9910456http://dx.doi.org/10.1016/S1389-1723(99)80201-1http://dx.doi.org/10.1016/S1389-1723(99)80201-1http://dx.doi.org/10.1021/bm050410lhttp://dx.doi.org/10.1021/bm050410lhttp://dx.doi.org/10.1039/b312145nhttp://dx.doi.org/10.1039/b312145nhttp://dx.doi.org/10.1021/ie0201462http://dx.doi.org/10.1021/ie0201462http://dx.doi.org/10.1002/(SICI)1099-1581(199608)7:8%3C704::AID-PAT567%3E3.0.CO;2-3http://dx.doi.org/10.1002/(SICI)1099-1581(199608)7:8%3C704::AID-PAT567%3E3.0.CO;2-3http://dx.doi.org/10.1021/la000862dhttp://dx.doi.org/10.1021/la000862dhttp://dx.doi.org/10.1021/jp067554uhttp://dx.doi.org/10.1021/jp067554uhttp://dx.doi.org/10.1039/b518091khttp://dx.doi.org/10.1039/b518091khttp://dx.doi.org/10.1021/jp057149nhttp://dx.doi.org/10.1021/jp057149nhttp://dx.doi.org/10.1021/jp057149nhttp://dx.doi.org/10.1039/b518091khttp://dx.doi.org/10.1021/jp067554uhttp://dx.doi.org/10.1021/la000862dhttp://dx.doi.org/10.1002/(SICI)1099-1581(199608)7:8%3C704::AID-PAT567%3E3.0.CO;2-3http://dx.doi.org/10.1021/ie0201462http://dx.doi.org/10.1039/b312145nhttp://dx.doi.org/10.1021/bm050410lhttp://dx.doi.org/10.1016/S1389-1723(99)80201-1http://dx.doi.org/10.1021/cm9910456http://dx.doi.org/10.1038/7385http://dx.doi.org/10.1039/b613872ahttp://dx.doi.org/10.1208/aapsj060327http://dx.doi.org/10.1016/0305-1978(91)90054-4http://dx.doi.org/10.1021/ja963011uhttp://dx.doi.org/10.1016/S0927-7757(01)01040-8http://dx.doi.org/10.1021/jp046032ghttp://dx.doi.org/10.1021/jp982731fhttp://dx.doi.org/10.1039/b501130bhttp://dx.doi.org/10.1002/anie.200502731http://dx.doi.org/10.1021/la047440ehttp://dx.doi.org/10.1021/la0475425http://dx.doi.org/10.1021/jp049348xhttp://dx.doi.org/10.1039/b201014chttp://dx.doi.org/10.1021/nl049215nhttp://dx.doi.org/10.1021/ja000477ahttp://dx.doi.org/10.1039/b501448dhttp://dx.doi.org/10.1021/la034364rhttp://dx.doi.org/10.1021/om049586ehttp://dx.doi.org/10.1021/ja031523khttp://dx.doi.org/10.1038/nature05118http://dx.doi.org/10.1126/science.288.5475.2335http://dx.doi.org/10.1038/35073733http://dx.doi.org/10.1021/ja016264i

A 'Green' Chitosan Silver Nanoparticle Composite as a Heterogeneous as Well as Micro-heterogeneous...

Documents

Transcript of A 'Green' Chitosan Silver Nanoparticle Composite as a Heterogeneous as Well as Micro-heterogeneous...