Formation of gold nanoparticles upon chitosan leading to formation and collapse of gels

This journal is©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2014 New J. Chem., 2014, 38, 63--69 | 63

Cite this: NewJ.Chem., 2014,

38, 63

Formation of gold nanoparticles upon chitosanleading to formation and collapse of gels†

Radha Perumal Ramasamy‡*a and Shihabudheen M. Maliyekkal‡b

In this article we have demonstrated the formation of chitosan–gold hydro gels that collapse in time.

The mechanism for the formation and collapse in time is proposed. The gels were characterized using

scanning electron microscopy (SEM), attenuated total reflectance Fourier transform infrared spectro-

scopy (ATR-FTIR), rheometry and X-ray diffraction (XRD). The formation of the gels is hypothesised to be

due to the electrostatic interaction between the AuCl4� and the NH3

+ in the protonated chitosan. The

collapse of the gel is attributed to the reduction of Au(III) to Au nanoparticles on the chitosan surface.

The formation and the subsequent collapse of the gel were unique to gold under the experimental

conditions. A model is proposed for the formation and the collapse of the gel. We coin the term ‘‘self

collapsing gels’’ for these kinds of gels. The gels were tested for their abilities to be used for the

encapsulation of drugs using insulin. This self collapsing nature of the gels combined with

biocompatibility may benefit drug delivery research as encapsulants for the controlled release of drugs.

1. Introduction

Biopolymers are an important class of polymeric materials thathave attracted the attention of the scientific community greatly.Chitosan is a biopolymer and is derived from chitin, which is thesecond most abundantly available biopolymer in nature.1 Chitosanand its derivatives have found extensive biomedical applicationssince they are biocompatible and biodegradable.2,3 Chitosan is alsoshown to have an antibacterial property.4 Some of its applicationsinclude heavy metal chelation, wound healing, cell immobilizationin wastewater treatment and as an additive in food andcosmetics.5,6 The properties of chitosan solutions depend uponvarious parameters that include concentration, temperature,pH, ionic strength, type of acid and acetyl groups present.7

In addition to these properties, chitosan is reported to havea good ability to bind with other polymers and inorganicmaterials and thus form composite materials.8 Typically, suchhybrid materials show the properties of the individual buildingblocks along with synergetic effects, due to the interactionbetween the individual components.9 The possible multifunc-tional properties of these kinds of composites attracted wideattention recently. Gold nanoparticles (Au NPs) are extensivelyused to create composites with chitosan and its derivatives.

Many applications of gel–chitosan composites have beenreported. These include surface-enhanced Raman scattering,10

electrochemical sensors,11 molecular imaging12 and catalysis.13

A green synthetic approach for making gold nanoparticles isnew and useful for the environment.14 Recently, Hortiguelaet al.15 reported an interesting synthetic approach that inducedchitosan gelation upon the simultaneous formation of AuNPs.They reported that prolonged thermal treatment at 80 1Cpromoted the hydrogels to become a viscous liquid.

In this article we report a single-step approach for the formationof a chitosan–gold hydrogel and the collapse of the gel in time withthe formation of chitosan–gold nanoparticle composites at roomtemperature (30� 2 1C). The studies were done as a function of theAuCl4

� concentration, pH, temperature and reaction time. Impor-tantly, we report for the first time the formation and the collapse ofa gold–chitosan hybrid gel with time and therefore we coin theterm ‘‘self collapsing gels’’ for these classes of gels that collapse dueto the chemical content. One of the possible applications of the gelthat will collapse at specific times may include the encapsulation ofa drug and its controlled delivery. The as-synthesized gels are alsofound to be sensitive to external stimuli like pH, gold concentrationand heat. Such responsive hybrid-microgels may be interesting fora variety of applications including catalysis, optical, sensors, smartactuators and transport media.

2. Experimental section2.1. Reagents

Chitosan powder and acetic acid (99.9% purity) were purchasedfrom SRL Pvt. Ltd, Mumbai, India. Chloroauric acid powder

a Department of Applied Science and Technology, Anna University,

Chennai – 600 025, India. E-mail: [email protected],

[email protected] School of Mechanical and Building Sciences, VIT University, Chennai Campus,

Chennai – 600 127, India. E-mail: [email protected]

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3nj00603d‡ Both the authors have contributed equally.

Received (in Montpellier, France)6th June 2013,Accepted 27th September 2013

DOI: 10.1039/c3nj00603d

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64 | New J. Chem., 2014, 38, 63--69 This journal is©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2014

(HAuCl4�3H2O) with 49% Au was purchased from the CDHCompany Pvt. Ltd, India. Double distilled water was usedthroughout the synthesis process. Human insulin (biphaseisophane insulin injection I.P. 40 IU ml�1) was purchased fromTorrent Pharmaceutical Ltd, India.

2.2. Preparation of chitosan gold gels

In this work, 1% (w/v) of chitosan powder (i.e.) 2 g/200 ml ofchitosan and 1.5% (w/v) of acetic acid (i.e.) 3 g/200 ml weredissolved in 200 ml of double distilled water. The solution wasstirred using a magnetic stirrer and heated at 60 1C for 60 minuntil the chitosan powder was completely dissolved and a semi-transparent thick chitosan solution was obtained. To thissolution, appropriate amounts of 1 M HAuCl4�3H2O were addedto have 1–5 mM HAuCl4.

2.3. Characterization

UV-visible absorption spectra were recorded using a Perkin-ElmerLambda 25 spectrophotometer in the range 200–1100 nm. A FEIQuanta FEG 200 was used for taking the high resolution scanningelectron microscope (HRSEM) images. The samples were spot-castupon ITO substrates and dried under ambient conditions. Theinstrument RheoPlus (Anton Paar) was used to measure theviscosity of the samples. The viscosity was measured at 25 1Cand at a shear rate of 10 s�1. The X-ray diffraction (XRD) patternsof the sample were collected using a Bruker AXS, D8 Discoverdiffractometer. The wavelength used was 1.54184 Å. The sampleswere scanned in the 2y range from 10 to 901. All the peaks were

assigned and compared with the database published by the JointCommittee on Powder Diffraction Standards (JCPDS).

3. Results and discussions3.1. Formation of gels

Fig. 1A shows the effect of concentrations of HAuCl4 (1–5 mM)on the formation of the gold–chitosan hydrogel at room tem-perature. A direct correlation was observed between the initialconcentration of the HAuCl4 and the rate of gel formation. Theaddition of the gold precursor also resulted in color changes ofthe solution with time. For 1 mM HAuCl4, it started as yellow,then became wine red in 24 h and finally became dark brown.Samples containing 2 mM HAuCl4 also showed a similar trendas that of 1 mM HAuCl4. This indicates the formation of goldnanoparticles. Very recently, Boufi et al.,17 have proved theability of chitosan to reduce Au(III) to Au NPs in absence ofany external chemical reducing agents. They observed a colorchange of a AuCl4

� chitosan solution from yellowish to redwhen exposed to UV irradiation and attributed the color changeto the formation of Au NPs. Huang and Yang16 also reported asimilar color change from pale yellow-to red after heating aAuCl4

� chitosan solution at 80 1C for 2 h and UV-irradiation for15 min. Our study showed the color change and formation ofAu NPs at room temperature (30 � 2 1C) in the absence of anyexternal physical and chemical agents. Interestingly, uponincreasing the concentration of HAuCl4 to 3 mM and above,the formation of a gel was observed and the rate of formation

Fig. 1 (A) Photographic images showing the time dependent response of chitosan–HAuCl4 complex. (1) 1 mM HAuCl4 (2) 2 mM HAuCl4 (3) 3 mM HAuCl4(4) 4 mM HAuCl4 and (5) 5 mM HAuCl4. (B) UV-vis spectra of chitosan–gold (C) change in viscosity of the gels (1 wt% chitosan + 5 mM HAuCl4) with timeas a function of NaOH concentration.

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was found to be concentration dependent. It was also observedthat as time progressed, the gels broke and became a viscousliquid again. The gels with 3, 4 and 5 mM HAuCl4 concentra-tions collapsed after 2, 3 and 4 days, respectively. Unlikeprevious reports, the observed gel formation and the subse-quent collapse of the gel due to the formation of Au NPs mightbe due to the difference in the concentration of the reactantsemployed.

3.1. UV-vis measurements

The composite was characterized using a UV-vis spectrophoto-meter. Fig. 1B shows the UV-vis spectra of the chitosan solu-tions with various concentrations of HAuCl4, measured afterthe collapse of the gel. A broad absorption peak was observed ataround 531 nm for the chitosan solution with 1 mM HAuCl4.This peak corresponds to the excitation of the plasmon reso-nance band for gold NPs. It is also evident from the figure thatas the concentration of the HAuCl4 increased, both the positionof the peak (red-shift) and its absorbance increased linearly(Fig. S1 of ESI†). The progressive red-shift observed in theposition of the absorbance can be attributed to the increasein the size of the gold nanoparticles. This observation is inagreement with the color change observed with the increase inthe gold precursor concentration. The data reveal the formationof the gold nanoparticles on the chitosan template and the sizeof the gold particles formed are a function of the spikedconcentration of HAuCl4. The results also confirm the inherentreducing ability of chitosan to reduce Au(III) to Au NPs in theabsence of any external physical or chemical reducing agents.

3.2. Effect of pH and heat

Experiments were also conducted to know the effect of externalstimuli like pH and temperature on the gel formation. The pHof the solution was varied using a 0.1 M NaOH solution.A predetermined volume of 1 M HAuCl4 was added to 5 mLof 1 wt% chitosan, to have a final concentration of 5 mMHAuCl4 and stirred for 2 minutes. To these, appropriateamounts of the NaOH were added to have concentrations of50, 100 and 200 mM and stirred for 2 minutes. The pH was inthe acidic range for all the samples. The pH for the 1 wt% chitosansolution, chitosan + 5 mM HAuCl4 and chitosan + 5 mM HAuCl4 +100 mM NaOH were 4, 3.3 and 4.7, respectively.

The gel responded well to pH variations and the gel becameharder with an increase in the concentration of the NaOH. Thisphenomenon was further confirmed by measuring the viscosityof the gels containing 5 mM HAuCl4 as a function of theconcentration of NaOH added. The samples were also analyzedfor their viscosities as a function of time and the results areshown in Fig. 1C. From the figure, it was observed that for asolution with 50 mM NaOH, the viscosity increased to amaximum in two days and then decreased. For 100 to 200 mMNaOH, the viscosity increased rapidly and reached a maximumvalue in B1 h. The reduction in viscosity was observed withtime. This indicates that the addition of NaOH acceleratesthe formation of the gel. To know the effect of heat, a chitosan–gold gel having 5 mM HAuCl4 was placed on a hot plate. It was

observed that the color of the gel exposed to the hot plategradually changed from yellow to red. In time the entire gelbecame red in color and it collapsed. These results are shown inFig. S2 of the ESI.† This indicates that heat accelerates theformation of the gold nanoparticles and subsequently the gelcollapses due to reduction of the gold.

3.3. SEM measurements

Fig. 2 shows the SEM images of the gel (after collapsing) madeusing 5 mM HAuCl4. The EDAX images show that Au, Cl, C, Nand O are distributed uniformly in the gel. The presence of Snin the EDAX image is due to the ITO substrate. Fig. 3A, B and Cshow the HRSEM images for the parent chitosan, a chitosan–gold gel formed using 5 mM HAuCl4 and a chitosan–gold gelformed using 5 mM HAuCl4 and 100 mM NaOH, respectively.The samples were obtained by drying the solutions after the gelhad collapsed. It was observed that chitosan did not show anyorderly features (Fig. 3A). The chitosan gel formed using 5 mMHAuCl4, had spherical gold nanoparticles of size 71 � 37 nm(Fig. 3B) while the chitosan gel formed using 5 mM HAuCl4 and100 mM NaOH had gold nanoparticles of size 50 � 9 nm(Fig. 3C). In the gel prepared in the presence of 100 mM NaOH,most of the particles were spherical, while some where trian-gular and rod shaped. The study also reveals that the gel issensitive to external stimuli and small variations in the synthesisprotocol can lead to a hybrid gel with different morphologies.This may help in making gels with different propertiesand hence different applications. The EDAX spectrum forthe gel formed using 5 mM HAuCl4 is shown in Fig. 3D.

Fig. 2 SEM image of a chitosan–gold complex (1 wt% chitosan + 5 mMHAuCl4). Corresponding EDAX mappings for various elements present inthe complex are shown below.

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The corresponding spectra of the gel formed using 5 mMHAuCl4 and 100 mM NaOH are shown in Fig. S3 of the ESI.†To confirm if the particle size increased with the increase in theconcentration of HAuCl4 (as indicated by UV-vis measure-ments), SEM images of chitosan with 1 mM HAuCl4 were also

taken (Fig. S4 of ESI†). The size of the nanoparticles formedusing 1 mM HAuCL4 was 7.3 � 1.6 nm, while the chitosan gelformed using 5 mM HAuCl4, had spherical gold nanoparticlesof size 71 � 37 nm (Fig. 3B). These results are in agreementwith the UV-vis measurements and confirm that the size ofthe nanoparticles increases as the concentration of HAuCl4

increases.

3.4. FTIR measurements

The samples were analyzed using ATR-FTIR (Fig. 4A).The spectra of the gel sample (chitosan–gold gel made using5 mM HAuCl4) were obtained at different time intervals (2–28 h)from the time of formation of the gels. Peaks were observed at3227, 2927, 2886, 1635, 1550, 1406, 1340, 1153, 1064, 1018 and897 cm�1, respectively for both chitosan and the chitosan gelsformed using 5 mM HAuCl4. Here, 3100–3500 cm�1 are due toOH and NH, 2927–2886 cm�1 are due to CH, 1635 cm�1 is dueto the coupling of CO and NH, 1550 cm�1 is due to NH and1406 cm�1 is due to the coupling of CN and NH.18 The positionof the transmittance peaks for the protonated chitosan and thegel did not show any change between the time interval 2–28 h(Fig. 4A). Since the UV-vis measurements did not show anyabsorbance peak corresponding to that of the Au nanoparticles(531 nm) during this time interval (2–28 h), this indicates thatthe formation of the gel is due to electrostatic interactionsbetween Au(III) and the chitosan. To further understand theeffect of the formation of the nanoparticles upon the gels, FTIRspectra for the gel made using 5 mM HAuCl4 were taken at two

Fig. 3 (A) HRSEM image of protonated chitosan. (B) HRSEM image of thecomposite containing 1 wt% chitosan and 5 mM HAuCl4. (C) HRSEM imageof the composite containing 1 wt% chitosan, 5 mM HAuCl4 and 100 mMNaOH. (D) EDAX spectrum of the composite containing 1 wt% chitosanand 5 mM HAuCl4.

Fig. 4 (A) ATR-IR spectra of the chitosan composite (1 wt% chitosan + 5 mM HAuCl4). Traces are shifted vertically for clarity. (B) XRD pattern of chitosan,(C) XRD pattern of the composite (1 wt% chitosan + 5 mM HAuCl4) and (D) XRD pattern of the composite containing 1 wt% chitosan, 5 mM HAuCl4 and100 mM NaOH.

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different times (1) 6 h (the gel had freshly formed) and (2) 120 h(the gel had completely collapsed). The results are shown inFig. S5 of the ESI.† For chitosan, the amide I, amide II and amideIII bands are located at 1656, 1593 and 1373 cm�1, respectively.19

From Fig. S5, ESI,† it was observed that the absorbance foramide II and amide III were suppressed for the time equal to120 h. Also, the position of the amide III peak was shifted from1395 (for 6 h) to 1385 (for 120 h). This indicates that theformation of the gold nanoparticles affects the amide bands ofthe chitosan. This can lead to a weakening of the binding linksand thereby result in the collapse of the gels.

3.5. XRD analysis

The XRD patterns of the as-synthesized samples were alsorecorded and the data are shown in Fig. 4B, C and D. Thesamples for the analyses were obtained by drying the solutionsafter the gel had collapsed. The XRD of the gels formed using5 mM HAuCl4 showed peaks for Au (2y equal to 38.21, 44.51,64.671 and 77.541). The position and the intensity of the peakscorresponding to Au matched with the reported value(PCPDFWIN-01-1172). The gel formed using 5 mM HAuCl4 +100 mM NaOH also showed peaks for Au (for 2y equal to 38.21,44.51, 64.671 and 77.541) (PCPDFWIN-01-1172). The peakscorresponding to NaCl (27.4, 31.8, 53.8, 56.6, 66.2, 73.3 and75.3) (PCPDFWIN-01-0993) and NaOH (11.7, 22.8, 28.7, 32.5and 36.5) (PCPDFWIN-75-0642) were also observed (Fig. 4). Thepresence of planes corresponding to Au further confirms theinherent ability of chitosan to reduce Au(III) to Au(0).

3.6. Estimation of ratio of gold(III) and gold NPs

The role of the ratio Au(III)/AuNPs in the stability of the gel wasfound by the following technique. Chitosan–1 mM HAuCl4 andchitosan–3 mM HAuCl4 were prepared. The UV-vis absorbancewas measured as a function of time for both the samples. Theresults are shown in Fig. 5. It was observed that the chitosan–1 mM HAuCl4 formed a liquid, while chitosan–3 mM HAuCl4

formed a gel. At 48 h, the chitosan–3 mM HAuCl4 had collapsed

and become a liquid. Interestingly, the intensity of the absorbancemaxima for the chitosan–3 mM HAuCl4 was comparable to thatof the chitosan–1 mM HAuCl4. Also, the area under the peak(500–750 nm) for the chitosan–3 mM HAuCl4 was nearly the sameas that of the area under the peak (450–700 nm) for the chitosan–1 mM HAuCl4. Hence, the amount of gold as nanoparticles wasthe same in the chitosan–1 mM HAuCl4 and in the chitosan–3 mM HAuCl4 collapsed gel. The absorbance spectra for thechitosan–1 mM HAuCl4 solution reached maximum in 2 days,indicating that all the gold was reduced as nanoparticles. Thisindicates that the amount of nanoparticles in the chitosan–3 mMHAuCl4 at the time of the collapse was 1 mm. Hence, theremaining 2 mM was Au(III). This indicates that the ratio ofAu(III) and AuNPs is 2 : 1. When the ratio becomes less than2 : 1, the gel collapses and becomes a liquid. These results showthat out of 3 Au(III) ions, even if one of them is reduced to Au(0)then the gel will collapse.

3.7. Small angle neutron scattering studies

In a different study involving small angle neutron scattering (SANS),one of the authors (Ramasamy) has shown that the radius of gyrationfor chitosan was found to be 6.0 � 0.5 nm, by fitting a model of aGaussian polymer (communicated elsewhere). Assuming that in agel, the radius of gyration is nearly the same as in solution (we canmake this assumption as no change was observed in the volume ofthe chitosan–gold solution after it became a gel) and that thechitosan molecules do not cross each other, we can fill the gel spacewith boxes of dimensions 12 � 12 � 12 nm3, with each box havingone chitosan unit. The number of chitosan molecules per cm3 isthen B5.8 � 1017/cm�3. For the same volume the number of Au(III)in 3 mM HAuCl4 is B18.0 � 1017/cm�3. This shows that in achitosan–3 mM HAuCl4 gel the ratio of chitosan to Au(III) is 1 : 3.1.This indicates that 3 Au(III) ions are associated with a chitosanmolecule for it to be a gel. Both the analysis involving UV-vis andSANS show that for the gel to form, three Au(III) ions are required perchitosan molecule and that when one out of the three Au(III) ionsgets reduced, then the gel will collapse. This is a collective phenom-enon that involves many particles. Hence, the reduced Au(0)particles form gold NPs, with chitosan as their nucleating sites.

3.8. Dependence upon concentration of chitosan and otheradditives

It was also observed that the formation of the gel dependedupon the concentration of the chitosan solution. To furtherprove this, to two different concentrations of a chitosan (0.25%and 1%) solution in 1.5% acetic acid, HAuCl4 was added to havefinal concentration of 5 mM. The results show that 0.25% ofchitosan solution did not form gels (Fig. S6 of the ESI†) and acritical minimum concentration of chitosan was need for theformation of the gel. The studies further reveal the responsivenature of the gel to external conditions. The studies show thatthe formation of gel, the rate of gel formation, viscosity of thegel, the rate of formation of nanoparticles and collapse ofthe gel are largely dependent on the reaction environment, likethe reactant concentration, pH and temperature. This responsivenature of the composite could be the reason for the absence of

Fig. 5 UV-vis spectra for (A) chitosan–3 mM HAuCl4 at 24 h (B) chitosan–3 mM HAuCl4 at 36 h (diluted by two) (C) chitosan–3 mM HAuCl4 at 48 h(diluted by two) and (D) chitosan–1 mM HAuCl4 at 48 h (diluted by two).The area under the peaks for (C) and (D) are the same. This indicates thatthe amount of Au as nanoparticles in (C) and (D) are the same.

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gel formation and the formation of gold NPs under ambientconditions, reported in previous studies by Boufi et al., 201317

and Huang and Yang, 2004.16

In order to test if other chemicals (instead of gold) couldalso yield chitosan gels, various chemicals were added in equalamounts (5 mM). The chemicals added include acetic acid,NaOH, NiCl2, ZnCl2, FeCl3, and AgNO3. The results reveal aninteresting phenomenon which is specific to gold. The gelformation and subsequent collapse of the gel was found to beunique to HAuCl4 under the experimental conditions studied.

3.9. Application in drug delivery

To check if the chitosan–gold self collapsing gels can be used fordrug delivery, the following studies were performed. Human insulin(biphase isophane insulin injection I.P. 40 IU ml�1) did not showany characteristic peaks, however the absorbance values decreasedwith increasing wavelength and the absorbance value at any parti-cular wavelength was linearly proportional to the concentration. Theabsorbance at 400 nm was used to calibrate the dependence of theabsorbance upon concentration of the insulin. 1 ml of humaninsulin was included in 5 ml of the chitosan–5 mM HAuCl4. Oncea gel was formed, the gel was placed inside a beaker containing100 ml of water. The absorbance of the supernatant was measuredas a function of time (Fig. S7A of ESI†). The absorbance value for400 nm was noted. The absorbance of the insulin source (1 mlhuman insulin dissolved in 100 ml water) is also shown in the figurefor reference. From the absorbance profile in Fig. S7A, ESI,† it wasobserved that 25%, 30% and 35% of the insulin had been releasedfrom the gel into the solution for times equal to 24 h, 48 h and 72 h,respectively. At 96 h, the gel had completely collapsed and theabsorbance value became more than that of the insulin source. Itis therefore clear that the insulin was gradually and slowly released,until the gel completely collapsed.

In another experiment, 10 mg of methyl orange (MO) which is acommon dye, was included in 5 ml of the chitosan–5 mM HAuCl4.Once a gel was formed, the gel was placed inside a beakercontaining 100 ml of water. MO has an absorption maximum at450 nm. Fig. S7A and S7B of the ESI† show the dependence of theabsorbance of the supernatant upon time. The absorbance at450 nm was noted. The absorbance of the methyl orange source(20 mg dissolved in 100 ml water) is also shown in the figure forreference. From Fig. S7B, ESI† it was observed that 17%, 20% and26% of the MO had been released from the gel into the solution fortimes equal to 24 h, 48 h and 72 h, respectively. These experimentsclearly indicate that chitosan–gold gels can be used to encapsulatedrugs and release them in a controlled way.

3.10. Model for formation of the self collapsing gels

Fig. 6 shows the model for the formation of the self collapsinggels when 1, 3 and 5 mM HAuCl4 are added to a solutioncontaining 1 wt% chitosan, with respect to time.

24 h. For times less than 24 h, chitosan solution containing1 mM HAuCL4 forms gold nanoparticles, due to reduction ofgold upon chitosan. The amount of gold is not sufficient to formgels as the ratio of the gold concentration to chitosan is less than1 : 3.1 (from section 3.7. Small angle neutron scattering studies).

For times less than 24 h, the chitosan solution containing 3 mMHAuCL4 and 5 mM HAuCL4 forms gels. This is due to thepresence of Au(III) ions (dark small circles). Also, the ratio ofgold concentration to chitosan is more than 1 : 3.1.

48 h. In 48 h, the chitosan solution containing 1 mMHAuCL4 forms larger gold nanoparticles due to the furtherreduction of gold and it still remains as liquid. The chitosansolution containing 3 mM HAuCL4 collapses and becomes aliquid, since the ratio of Au(III) and Au (NPs) becomes less than2 : 1 (from section 3.6. Estimation of ratio of gold(III) and goldNPs). The chitosan solution containing 5 mM HAuCL4 remainsas a gel, as the ratio of Au(III) and Au (NPs) is greater than 2 : 1.

72 h. In 72 h, the chitosan solution containing 1 mMHAuCL4 attains a saturation in the particle size. The chitosansolution containing 3 mM HAuCL4 has larger AuNPs, while thechitosan solution containing 5 mM HAuCL4 collapses as theratio of Au(III) and Au (NPs) becomes less than 2 : 1 due tothe reduction of the gold upon chitosan.

These indicate that, the electrostatic interaction between the(AuCl4)� and (NH3)+ on the protonated chitosan favors the formationof chitosan–gold–chitosan networks, leading to the formation ofgels. In time, as more gold is reduced on the chitosan, the chitosan–gold–chitosan links breaks, leading to the collapse of the network.

4. Conclusions

Many studies have been reported explaining the gold–chitosaninteraction. Several authors demonstrated the ability of chitosan

Fig. 6 Model for the interaction between chitosan and HAuCl4.

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to bind with gold ions and generate NPs. Huang et al.14 andWang et al.20 reported that chitosan can act as both a metalstabilizing and a reducing agent. However, the reduction of goldwas achieved by an external reducing agent, or heating, or by UVirradiation. It was hypothesized that the radical formed during theUV irradiation might act as the electron donor for the reduction.Another hypothesis says that the amino group on the chitosan actsas a nucleating site on which NPs grow using the hydroxyl groupsas reducing agents. Our study supports the latter mechanism forNP formation, because Au(III) reduction was achieved without theinfluence of external chemical or physical agents.

This article describes the unique property of HAuCl4 to crosslink chitosan and form a gel. The mechanism of formation ofthe hybrid gel and the self collapse with time is proposed. Theformation of the gel is due to an electrostatic interactionbetween the gold precursor and the protonated chitosan. Thecollapse of the hybrid gel is attributed to the reduction of Au(III) andthe subsequent formation of gold nanoparticles on the chitosantemplate. The formation and self collapse of the hybrid gel is foundto be a function of external stimuli like pH, temperature andreactants concentrations. The ability of the chitosan–gel to releasedrugs has been demonstrated using insulin. These kinds ofgels may therefore have potential applications in materials forcontrolled drug delivery, sensors, catalysis, etc.

Acknowledgements

The authors gratefully thank Prof. Abhijit P. Deshpande of thePolymer Engineering Laboratory, Chemical EngineeringDepartment, Indian Institute of Technology Madras (IITM),India, for providing the facility for making the rheology measure-ments. The authors gratefully thank Prof. T. Pradeep of the DSTunit of Nanoscience, Department of Chemistry and SophisticatedAnalytical Instrument Facility, IITM, India, for providing the facilityfor making the UV-vis and SEM measurements. Prof. Ligy Philip,from the Department of Civil Engineering of IITM is acknowledgedfor providing the ATR-IR facility. This research was made possibledue to funding provided by the BRNS (DAE) India Sanction No.2011/37C/21/BRNS.

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Formation of gold nanoparticles upon chitosan leading to formation and collapse of gels

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