Synthesis of MS (M = Zn, Cd, and Pb)–chitosan nanocomposite film via a simulating biomineralization method

Synthesis of MS (M = Zn, Cd,and Pb)–ChitosanNanocomposite Film Via aSimulating BiomineralizationMethod

SHAN WANG, YI HUANG, MINYAN ZHENG, YONGSHENG WEI,SIPING HUANG, YUANZI GUSchool of Chemistry and Chemical Engineering, Xianyang Normal University, Xianyang, 712000Shaanxi Province, People’s Republic of China

Received: October 9, 2010Accepted: January 23, 2011

ABSTRACT: MS nanoparticles were prepared with chitosan (CS) as thesynthesis template at normal temperature by simulating biomineralizationmethod. ZnS , PbS , and CdS CS nanocomposites as model were prepared bysimulating biomineralization method of corresponding twain valent ions intonanoparticles in the presence of CS. The structural and morphological features ofMS (M = Zn, Cd, and Pb)–CS were investigated by scanning electron microscopy.Optical characteristics of nanocomposites were studied by the approaches ofultraviolet–visible spectroscopy and fluorescence, from which the conclusionsshow that the CS matrix offers limited conditions for ZnS to grow, the precursorconcentration can also affect the formed particle size, and thus the particle sizecan be controlled in the CS matrix by these two means. The differential scanningcalorimetery result indicated a strong and uniform interaction between CS andnanoparticles. The experimental results indicated that all the nanoparticlesaggregated into nanopartical and CS as a template played an important role inthe formation of nanopartical. In addition, a possible structure-controlling

Correspondence to: Shan Wang; e-mail: [email protected].

Advances in Polymer Technology, Vol. 30, No. 4, 269–275 (2011)C© 2011 Wiley Periodicals, Inc.

SYNTHESIS OF MS (M = Zn, Cd, and Pb)–CHITOSAN NANOCOMPOSITE FILM

mechanism was proposed. C© 2011 Wiley Periodicals, Inc. Adv Polym Techn 30:269–275, 2011; View this article online at wileyonlinelibrary.com. DOI10.1002/adv.20222

KEY WORDS: Biomineralization, Chitosan, Synthesis

Introduction

I n recent years, fabrication of semiconductornanoparticles in solid polymer matrixes is at-

tracting more and more attention because thecombination of semiconductor nanoparticle andpolymer provides a simple route to stable and pro-cessable material integrating the promising prop-erties. Polymers are considered as a good choicefor host materials, as they can be designed fortheir physical properties, such as long-term sta-bility and possess flexible reprocessability. In-teresting optical properties such as fluorescence,electroluminescence, and optical nonlinearity havealready been observed in these composites. Numer-ous studies have been performed on the prepara-tion of new organic/inorganic hybrid materials andnanocomposites.1 Polymers are considered as a goodhost material for semiconductor,2−6 and nanopar-ticles exhibit unique optical and electrical proper-ties. The interaction between nanoparticles and chro-mophores has lately awakened the interest of thescientific community to fully develop hybrid sys-tems (metal–chromophores) with potential applica-tions in the biomedical field, among others.7

The nanocomposites possess interesting electri-cal, optical, and magnetic properties, usually supe-rior to polymer or inorganic species.8−10 Recently,much attention has been paid to chitosan (CS) filmas a potential polysaccharide resource.11−14 Huanget al.15 found cross-linked needle-like crystal struc-tures and snowflake-like structures in a protein–CSfilms casting from a solution of different proteinsand CS due to the interactions between the pro-teins and CS. The morphology of CS films cast fromformic acid solution was studied by Dong et al.16

and spherulites were observed in these films; themorphology varied with the concentration of CS.

In this study, MS CS nanocomposites, ZnS, PbS,and CdS CS nanocomposites, as model were pre-pared by simulating biomineralization method ofcorresponding twain valent ions into nanoparti-cles in the presence of CS. In addition to the opti-cal and other properties of these nanocomposites,

the morphology of nanocomposite films was alsoinvestigated.

Experimental

MATERIALS AND INSTRUMENT

Chitosan (degrees of deacetylation 95%) waspurchased from the Yuhuan Biology Engineering,Zhejiang, People’s Republic of China. Zinc nitratehexahydrate [Zn(NO3)2·4H2O], cadmium nitratetetrahydrate [(Cd(NO3)2·4H2O], lead nitratetetrahy-drate [Pb(NO3)2·4H2O], acetic acid, thioacetamide(TAA, CH3CSNH2), and ethanol (CH3CH2OH) wereof analytical reagent grade quality and all obtainedfrom Shanghai Sanpu Chemical Co., Ltd. (People’sRepublic of China), and used without further pu-rification. Distilled water used in the experimentwas double distilled without further purification.Thermal analytical experiments were performed us-ing a differential scanning calorimeter with an Ana-lytik Jena AG Q100 (DSC; TA Instruments, Alzenau,Germany) operated in the conventional differentialscanning calorimetery (DSC) mode at a heating rateof 10◦C/min to simultaneously determine the cor-relation between temperature and weight loss in anitrogen atmosphere. Scanning electron microscopy(SEM) was performed with a JSM-6380 microscopeat an accelerating voltage of 200 keV and with aHitachi Model H-800 transmission electron micro-scope (H-800; Hitachi, Tokyo, Japan) at an acceler-ating voltage of 200 keV. The samples were coatedwith a thin layer of gold before measurement. Flu-orescence measurements were performed at roomtemperature with a RF-5301PC fluorescence spec-trometer (Shimadzu Instruments Inc., Tokyo, Japan).

METHOD OF SYNTHESIS

Chitosan–MS nanocomposite film was preparedusing simulating biomineralization method as fol-lows: CS with degree of deacetylation 95% was pu-rified before use. CS dilute acetate solution withconcentration of 2% (w/v) was prepared. Under the

270 Advances in Polymer Technology DOI 10.1002/adv


50 100 150 200 250 300

-30

-20

-10

0

b

a

Hea

t cap

acity

(J/

(g°C

))

Temperature (°C)

FIGURE 1. DSC spectra of chitosan and ZnS–chitosan film (a, chitosan; b, chitosan–ZnS film).

protection of N2 with stirring, the solution was caston clean glass slides and CS film was formed. Then,complexes with Zn2+ ions through the reaction withzinc acetate salts for 12 h, which was afterward driedin vacuum. After drying, the film was immersed infresh TAA solution (pH 10) and the process was re-peated five times. Then, the film was washed usingsaturated NaCl solution and dried under N2 to geta smooth transparent white nanocomposite film. Fi-nally, the film was dried at 60◦C for 4 h under atmo-spheric condition and collected for further charac-terization.

Results and Discussion

DSC ANALYSIS

The DSC (Fig. 1) curve shows that the tempera-ture of CS endothermic peak at about 100◦C is at-tributed to the volatilization of residual water andorganic solvent. The exothermic peak observed at272◦C is related to the decomposition of the poly-mer. The DSC curve shows that the volatilizationof residual water and organic solvent takes place atabout 112◦C, the temperature of CS ZnS endother-mic peak. This phenomenon was observed in dif-ferent polymer composites and was interpreted as

reflecting the glass transition of the polymer chainsthat were less mobile than free polymer, for thereis a substantial contribution from macromolecules.17

Obviously, this result indicates a strong and uniforminteraction between CS and nanoparticles.

ULTRAVIOLET–VISIBLE SPECTRUMOF ZnS

Ultraviolet–visible is used to characterize opticalabsorption properties of semiconductor powder ma-terials. Figure 2 shows the absorption spectra of theZnS CS composite films prepared with different Znconcentration for the same reaction time. As shownin Fig. 2, the ZnS particles exhibited an absorptionedge ranging from 323 to 293 nm, corresponding tothe absorption edge of semiconductor material. It isclear that with C[Zn] increasing, the absorption on-set shifted to a higher wave number, indicating anincrease in particle size. This change is expected be-cause at higher Zn concentration, the formed ZnSseeds became fairly close to one another and thisshorter interparticle distance facilitated the parti-cle growth. We may conclude from the above thatthe CS matrix offers limited conditions for ZnS togrow; the precursor concentration can also affectthe formed particle size, thus we can control theparticle size in the CS matrix by these two means.

Advances in Polymer Technology DOI 10.1002/adv 271


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0.0

0.5

1.0

1.5

2.0

2.5

1-6A

bsor

banc

e (I

)

Wavelength (nm)

FIGURE 2. Ultraviolet–visible spectrum of CS–ZnS nanocomposite films prepared at various concentrations (excited atthe maximum excitation wavelength of each film) C(Zn2+)/mol·L−1: 1–1.3 × 10−5, 2–2.5 × 10−5, 3–5.0 × 10−5,4–1.0 × 10−4, 5–9.0 × 10−4, and 6–4.5 × 10−3.

SEM OBSERVATIONS OF ZnS

Figure 3a shows the SEM micrographs of CS ZnSnanocomposite film, which provide direct evidenceof the formation of true nanocomposites. The im-age shows that the ZnS particles were almost welldispersed in the CS matrix. The average size of theZnS nanocomposites was about 70 nm. The poly-mer chains might have been bridged by their con-nection to the same nanoparticle, and the multiplic-

ity of such bridged chains and particles could leadto particle clusters.18 The nanoparticles were clearlydispersed in the CS matrix on a nanoscale, confirm-ing the formation of a nanocomposite. Some spher-ical particles with the mean diameters of 70 nmcould be found (see Fig. 3b). These spherical par-ticles should be ascribed to the aggregation ofsmall ZnS nanoparticles during the formation of thenanocomposites.

FIGURE 3. (a) SEM of CS ZnS film and (b) transmission electron microscopy of CS ZnS film.



MSOH

OH

NH2

OHNH2

NH2

OHNH2

M2+ TAA,pH=10

Recycled five times

M2+OH

OH

NH2

OHNH2

NH2

OHNH2

M2+

M2+

M2+

OH

OH

NH2

OHNH2

NH2

OHNH2

MS

MS

MS

OH

OH

NH2

OHNH2

NH2

OHNH2

MSOH

OH

NH2

OHNH2

NH2

OHNH2

M2+ TAA,pH=10

Recycled five times

M2+OH

OH

NH2

OHNH2

NH2

OHNH2

M2+

M2+

M2+

OH

OH

NH2

OHNH2

NH2

OHNH2

OH

OH

NH2

OHNH2

NH2

OHNH2

OH

OH

NH2

OHNH2

NH2

OHNH2

M2+ TAA,pH=10

Recycled five times

M2+OH

OH

NH2

OHNH2

NH2

OHNH2

M2+

M2+

M2+

M2+OH

OH

NH2

OHNH2

NH2

OHNH2

OH

OH

NH2

OHNH2

NH2

OHNH2

M2+

M2+

M2+

OH

OH

NH2

OHNH2

NH2

OHNH2

OH

OH

NH2

OHNH2

NH2

OHNH2

MS

MS

MS

OH

OH

NH2

OHNH2

NH2

OHNH2

OH

OH

NH2

OHNH2

NH2

OHNH2

OH

OH

NH2

OHNH2

NH2

OHNH2

FIGURE 4. The scheme synthesis of ZnS CS nanocomposite hybrids.

THE FORMATION MECHANISM OF MSNANOPARTICALS IN CS

In MS nanoparticals growth, CS plays an im-portant role. The proportion of CS between acety-lated and nonacetylated residues is responsible forthe balance between hydrophilic and hydrophobicinteractions.19 Shi et al.20 and Chen et al.21 haveconcluded that CS molecular chains can form somehydrophobic microdomains because of their self-aggregation behavior. Li et al.22 reported that thestructure of CS chains is transformed from stretchedchains into coils and further transformed into inter-twisted coils with hydrophobic microdomains com-ing into being in the intertwisted coils with furtherincrease in concentration during the aggregationprocess, and the macromolecule structure of inter-twisted coils becomes compact and the movement ofthe macromolecule is restricted. It is possible to forma dispersion of CS nanopartical. TAA decomposesslowly in aqueous solution and releases S2− ions ho-mogeneously to give nanomaterials.23 At the sametime, because the S2− ions are supplied smoothly,the MS nucleation and growth are well controlled.It is clearly seen that all the generated nanoparti-cles aggregate into CS film. Figure 4 describes thescheme of the formation of MS nanoparticals. WhenM2+ ions were added, they can coordinate with hy-drophilic groups NH2 and OH in the CS chain,the outer M2+ ions concentration is more than the

inner. At the same time, S2− ions released from TAAfirst access the outer surface of CS, then diffuse intothe inner surface because of the hindrance betweenCS chains. Furthermore, S2− ions first react with M2+

in the outer interface that covers the surface of CSfilm, and make it more difficult for H2S moleculeto access the inner surface. So, the dissolved ionscan recrystallize on larger crystallites in the outerinterface.24

THE TEMPLATE EFFECT TO MAKE CdS,PbS NANOPARTICAL IN CS

We substituted Zn2+ for Cd2+, Pb2+, preparedCdS, PbS nanopartical, in the same way to provethe template effect of CS. SEM and fluorescencestudies confirmed the nanocrystalline CdS phaseformation. Figures 5 and 6 are SEM and fluores-cence measurement observations for CdS CS, re-spectively. And from Fig. 5, it can be clearly seenthat the nanoparticals appeared in the film, and theirsize was about 75 nm. In the fluorescence emissionspectra (Fig. 6) of CS CdS nanocomposite films pre-pared at various cadmium concentrations (excited atthe maximum excitation wavelength of each film),the emission wavelength of CdS particles exhib-ited from 383 to 485 nm, corresponding to semi-conductor material for different concentration ofCd2+(C(Cd2+)/mol·L−1: 1–1.3 × 10−5, 2–2.5 × 10−5,



1 μm

FIGURE 5. SEM of CdS CS nanocomposite hybrids.

3–5.0 × 10−5, 4–1.0 × 10−4, 5–9.0 × 10−4, and 6–4.5 ×10−3). Figure 6 indicated that they are uniform CdSnanoparticals with the average size of 75 nm (thecorresponding emission wavelength is 383 nm). Itverified the conclusion of SEM. Moreover, substitut-ing Zn2+ for Pb2+, Fig. 7 shows SEM observations forPbS. The method, which synthesizes CdS, PbS, andZnS nanoparticals, proves the template effect of CStoo. At the same time, Na2S is used instead of TAAto perform the preparation, and it is found that thenanoparticles are not uniform at all, which furtherindicated that TAA plays a indispensable role in thefabrication of uniform nanopartical.

300 350 400 450 500 550 600 650 700 750

0.0

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0.9

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1.1

E (nm)

6

5

4

3

2

1

I (a.

u.)

FIGURE 6. Fluorescence emission spectra of CS CdSnanocomposite films prepared at various cadmiumconcentrations (excited at the maximum excitationwavelength of each film) C(Cd2+)/mol·L−1:1–1.3 × 10−5, 2–2.5 × 10−5, 3–5.0 × 10−5,4–1.0 × 10−4, 5–9.0 × 10−4, and 6–4.5 × 10−3).

FIGURE 7. SEM of CdS CS nanocomposite hybrids.

Conclusions

Chitosan was sccessfully used as a template forthe synthesis of ZnS, CdS, and PbS nanoparticals bysimulating biomineralization method. CS as a tem-plate plays an important role in the formation ofnanopartical and S2− ions homogeneously releasedfrom the hydrolysis of TAA play another importantrole in the formation of nanopartical. It provides aneasy approach to control the size growth and dis-tribution of the MS nanocrystals. Such a template-directed synthesis method is expected to synthesizeother MS films.

Acknowledgments

The authors are grateful for the financial supportof Shaanxi Province, People’s Republic of China’sScholarship Council through grant numbers 2010JM2009 and 2010JK902.

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Synthesis of MS (M = Zn, Cd, and Pb)–chitosan nanocomposite film via a simulating biomineralization method