Ab

Sa

b

a

ARRAA

KCZFNT

1

mispavspnZogrmbcZiti

XT

0d

Materials Science and Engineering B 176 (2011) 873– 877

Contents lists available at ScienceDirect

Materials Science and Engineering B

jou rna l h om epage: www.elsev ier .com/ locate /mseb

new fluorescent film sensor for Pb(II) ions developed by simulatingio-mineralization process synthesizing of ZnS/CS nanocomposite

han Wanga,b,∗, Demei Yub, Guangwen Wub, Jinchan Guoa, Cao Leia

School of Chemistry and Chemical Engineering, Xianyang Normal University, Xianyang, 712000, Shaanxi Province, PR ChinaDepartment of Applied Chemistry, Xi’an Jiaotong University, Xi’an, 710049, Shaanxi Province, PR China

r t i c l e i n f o

rticle history:eceived 21 September 2010eceived in revised form 29 March 2011ccepted 1 May 2011vailable online 18 May 2011

a b s t r a c t

Chitosan/zinc sulfide (CS/ZnS) nano-composite films have been prepared by simulating bio-mineralization process. Factors affecting the hydrothermal stability and fluorescence properties of thefilms have been studied. Furthermore, the sensing properties of nano-composite films to lead ions havebeen systematically investigated. SEM and TEM observations showed that the size of ZnS particles is70 nm, and the particles are evenly distributed within the CS films. The fluorescence emission of the

eywords:hitosannSluorescenceano-composite materials

nano-composite films indicates that the sizes of real fluorescing ZnS particles are less than 20 nm. Thissuggests that ZnS particles observed via SEM and TEM may be aggregates of smaller ZnS particles, andthe smaller particles may be separated by the organics. The fluorescence emission (363 nm) of the nano-composite films is very sensitive to the presence of Pb ions. C(Pb

2+) increased from 0 to 664.2 mg L−1

increases the emission dramatically. The emission is hardly affected by common ions in water, excepts ma

hin film for the iron ions. The film

. Introduction

Recently, fabrication of inorganic nanoparticles in solid polymeratrices has attracted much attention because the combination of

norganic nanoparticles and polymer provides a simple route fortable and processable materials, which integrates the promisingroperties of the both components [1]. Polymers are considereds good host materials because they can be designed to yield aariety of bulk physical properties, normally exhibit long-termtability, and possess flexible reprocessability. Interesting opticalroperties, such as fluorescence, electroluminescence, and opticalonlinearity, have been observed in organic–inorganic composites.inc sulfide (ZnS) is a semiconductor material, which has a band gapf 3.70 eV. It is an ideal material for studies on variation in energyap due to the reduction of physical dimensions into the nanoscaleange [2]. A study on the growth reaction of ZnS nanoparticles in theicroemulsion system using UV–vis absorption spectroscopy had

een done by Fakis et al. [3]. They reported that the growing pro-ess of ZnS nanoparticles can be well-described by the power laws.hang et al. [4] reported the successful preparation of ZnS nanorods

n a lamellar liquid crystal template. Quantum dots (QDs), such ashose made of ZnS, have been used widely as fluorescent probesn the biological research field. Good results have been achieved

∗ Corresponding author at: School of Chemistry and Chemical Engineering,ianyang Normal University, Xianyang, 712000, Shaanxi Province, PR China.el.: +86 29 33720935.

E-mail address: shanwang2005@163.com (S. Wang).

921-5107/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.mseb.2011.05.002

y be developed as excellent sensing films for Pb ions in water.© 2011 Elsevier B.V. All rights reserved.

in molecular and cell imaging [5], nucleic acid research [6,7], drugscreening and drug discovery [8–10]. QDs have narrow emissionband width, symmetrical and tunable profiles according to theirsize and material composition, excellent photostability, and broadabsorption spectra, all of which make them the best choice forfluorescent probes [11].

Monitoring of heavy metals in the environment has been a greatconcern over the last decade [12]. Since Pb2+ ion is one of themost toxic metal ions when present at certain concentrations inthe environment, the study of selective signaling of Pb2+ becomesnecessary.

A simple approach to incorporate ZnS nanoparticles into a chi-tosan (CS), (Fig. 1) network is reported in this paper. The uniquemicrostructure formed in the CS network due to the clustering ofchelating groups, such as NH2, was used as a confined medium tosynthesize nanosized particles. The resultant ZnS–chitosan hybridcomposite was a homogeneous transparent film. The nanoparti-cles morphology was investigated, and fluorescence properties ofthe composites at 363 nm wavelength were characterized. The flu-orescence emission of the nano-composite films was tested for itssensitivity to Pb ions.

2. Materials and methods

2.1. Materials

CS (Mw = 750,000 g/mol) with a 95% degree of deacetylation wassupplied by Aldrich and used as received. All other materials were

874 S. Wang et al. / Materials Science and En

oa

2

NKanarS6Maops

2

csfw

Fig. 1. The chemical formula of Chitosan.

f analytical reagent (AR) grade and commercially available usedfter purifying.

.2. Characterizations

Fourier transform infrared (FTIR) spectroscopy was done with aICOLET-NEXUS670 spectrometer. The samples were ground withBr crystals, and pressed into a flake for IR measurement. Thermalnalysis experiments were performed using a differential scan-ing calorimeter (DSC) operated in the conventional DSC mode at

heating rate of 10 ◦C/min to determine simultaneously the cor-elation of temperature and weight loss in a nitrogen atmosphere.canning electron microscopy (SEM) was performed with a JSM-380 microscope at an accelerating voltage of 200 kV and a Hitachiodel H-800 transmission electron microscope (TEM) at acceler-

ting voltage of 200 keV. The samples were coated with a thin layerf gold before measurement. Fluorescence measurements wereerformed at room temperature with a RF-5301PC fluorescencepectrometer (Shimadzu Instruments Inc, Japan).

.3. Activation of quartz plates

A clean quartz plate (0.9 cm × 3 cm) was placed into a con-

entrated H2SO4 solution containing 5 wt% K2CrO4 [13,14] .Theolution was heated to 100 ◦C and maintained at this temperatureor 10 min. The plate was taken out of the solution and rinsed withater. After the treatment, the plate was expected to be covered

Fig. 2. The scheme of CS/

gineering B 176 (2011) 873– 877

with a smooth thin layer of water and was free of dust and otherimpurities. The oxidizing treatment should be repeated if the platedid not pass the test. The activated quartz plate was dried at 100 ◦Cin a dust-free oven for 1 h, cooled to room temperature, and keptin a desiccator.

2.4. Preparation of CS/ZnS nano-composite films

A CS/ZnS nano-composite film was prepared using a biomineralmethod as follows: CS (Mv = 750,000) was dissolved in a dilutedacetate solution at concentration of 2% (w/v). Under the protectionof N2 and with stirring for 12 h, CS formed complexes with Zn2+

ions through a reaction with zinc acetate salts (1.0 × 10−4 mol L−1).The solution was cast on clean glass slides, which were then driedunder vacuum. After drying, the film was immersed for 24 h in freshthioacetamide (TAA) solution with pH 10. With the formation ofZnS QDs, the color of the films was transformed from transparent towhitish. The film was washed using a saturated NaCl solution. Then,it was dried in N2 to get a smooth, translucent, and whitish nano-composite film. A schematic diagram of the synthesis of CS/ZnSnano-composite hybrids is shown in Fig. 2.

2.5. Sensing and reversibility of the response of the film to Pb2+

ions

In the experiment, the film was allowed to adhere to one innerside of a quartz cell with a volume of ca. 3.5 cm3. Then, solventwith a volume of 2.5 cm3 was added into the cell. Finally, the spec-tra were recorded when the fluorescence intensity became stableafter the injection of Pb2+ solution into the cell. The reversibilityof the film to Pb2+ was examined using a standard method. Thefilm was exposed to an aqueous solution of the analyte, and thenthe maximum emission intensity of the film was recorded. Thiswas followed by adding the proper amount of Pb2+ to the solution.Finally, the emission intensity of the film was measured 5 times

every 6 min. After the measurements, the film was washed withpure water several times. The measurement was repeated 5 timeswith the same concentration of the analyte. To test the reversibil-ity of the film sensor for Pb2+, the film was alternatively exposed

ZnS film synthesis.

S. Wang et al. / Materials Science and Engineering B 176 (2011) 873– 877 875

Fig. 3. FTIR of chitosan (a) and chitosan/ZnS film (b).

tcfip

3.3. Microstructure of ZnS/CS film

Fig. 4. DSC of chitosan (a) and ZnS/chitosan film (b).

o a solution of Pb2+ and pure water, and corresponding fluores-

ence emissions were measured. After each measurement of thelm immersed in salt solution, it was washed several times withure water.

Fig. 5. SEM (a) and TEM

Fig. 6. XRD of ZnS/chitosan film.

3. Results and discussion

3.1. FTIR Spectral data of CS film and ZnS/CS film

Fig. 3 shows the infrared absorption spectra of the CS film andanother film containing ZnS. The characteristic absorption peak forthe NH2 group of CS shifted from 1554 cm−1 to 1598 cm−1 afterit coordinated with Zn2+ and subsequently with S2−. The changein frequency means that coordinate bonds formed between NH2and ZnS. The Zn2+ ions in the ZnCl2 solution were considered tohave formed strong complex bonds with the NH2 groups in CS. Inaddition, a number of Zn2+ ions in solution possibly agglomeratedthe complex Zn2+ ions in the CS molecules to form metal ion groupsvia adsorption [15]. The coordinated Zn2+ or agglomerated Zn2+

ions in the CS film reacted with S2− ions in the TAA solution, formingZnS clusters that stuck to the CS film.

3.2. The DSC analysis CS film and ZnS/CS film

The DSC curve (Fig. 4) shows a CS endothermic peak at 100 ◦C,which was attributed to volatilization of residual water and organicsolvent. The exothermic peak observed at 272 ◦C was related todecomposition of the polymer. The CS/ZnS endothermic peak at112 ◦C was the volatilization of residual water and organic solvent.And the exothermic peak observed at 275 ◦C indicated a strong anduniform interaction between CS and nanoparticles.

Fig. 5a shows the SEM micrographs of CS/ZnS nano-compositefilm, which provide direct evidence of formation of true nano-

of CS/ZnS film (b).

876 S. Wang et al. / Materials Science and Engineering B 176 (2011) 873– 877

F0

cwchTgedbo[mp7cd

3

cp

FC

ig. 7. Fluorescence of the film at the various concentrations of Pb2+ (CPb2+/mg L−1:

; 73.8; 134.0; 171.0; 228.0; 369.0; 442.8; 516.6; 619.0; 664.2).

omposites. The images confirmed that ZnS particles were almostell-dispersed in the CS matrix. The average size of ZnS nano-

omposite was about 70 nm. The molecular structure of chitosanad highly regular structure and was soluble in acid environment.he orderly sited –NH2 groups provided a major contribution to theood dispersion of ZnS nanocrystals. And the molecular chain andlectrostatic repulsion could prevent aggregation of nanoparticlesuring their growth. The polymer chains might have been bridgedy their connection to the same nanoparticle, and the multiplicityf such bridged chains and particles could lead to particle clusters.16] Nearly monodisperse particles were obtained dispersed in CS

atrix at nanoscale, confirming formation of nanoparticle on theolymer film. Some spherical particles with the mean diameter of0 nm could be found by TEM (see Fig. 5b). These spherical parti-les should be ascribed to aggregation of small ZnS nanoparticlesuring the formation of the nanocomposite.

.4. The X-ray diffraction

The X-ray diffraction (XRD) patterns of the CS/ZnS nano-omposite film are depicted in Fig. 6. It is clear that all the diffractioneaks in the spectrum are analogous to hexagonal würtzite struc-

ig. 8. Dependence of the fluorescence intensity (�ex = 297 nm; �em = 363 nm) of theS/ZnS film on Pb2+ concentration.

Fig. 9. Fluorescence of the film in the various concentrations of Fe3+ (CFe3+/mg L−1:

39.6; 79.1; 118.4; 157.7; 196.8; 235.7; 274.6; 313.3; 351.9; 390.3).

ture, the crystal lattice constants a = 0. 382, c = 0. 626 are the samewith the card (No: 36-1450).

3.5. Sensing properties to Pb2+ ions

Fig. 7 shows the fluorescence emission spectra of the film asa function of Pb2+ concentration. Clearly, the emission of the filmincreased significantly with increasing Pb2+ concentration [C(Pb

2+)

increased from 0 to 664.2 mg L−1]. Fig. 8 shows the dependenceof the fluorescence intensity (�ex = 297 nm; �em = 363 nm) of theCS/ZnS film on Pb2+ concentration. The correlation is representedby Eq. (1):

I = 0.4167c − 22.587(R2 = 0.9919) (1)

Generally speaking, the increase in fluorescence intensity couldlast as long as 8 h. Considering that some minerals exist in tap water,such ions as Cu2+, Mg2+, Na+, Al3+, CH3COO−, K+, Fe3+, chloride,

sulphate, nitrate, etc., were employed to test the films’ fluorescencesensitivity. The results show that the common ions of all the addedsalts, i.e., Cu2+, Mg2+, Na+, Al3+, CH3COO−, K+, NO3

−, chloride, and

Fig. 10. Reversibility of the film sensor for Pb2+.

and En

si

3

tmlsm

4

pansptoiimw

[[

[[

S. Wang et al. / Materials Science

ulphate, did not affect the fluorescence behavior. Only Fe3+ wasnvolved in an inhibition effect (Fig. 9).

.6. Reversibility of the film’s response to Pb2+ ions

The results are shown in Fig. 9. The response of the film tohe same concentrations of Pb2+ was fully reversible. Further-

ore, the time needed to reach the response equilibrium wasess than 6 min, which is relatively fast. The fluorescence emis-ion of the film was found to be fully restored through thisethod (Fig. 10).

. Conclusion

Nanosized semiconductor ZnS particles were successfully pre-ared within a CS film matrix, which proved effective in preventingggregation and controlling the growth of ZnS nanocrystals. Theew film was developed for Pb2+ sensing. The fluorescence emis-ion of the film was sensitive to the presence of Pb2+ in an aqueoushase. Among various commonly dissolved ions, Pb2+ was proveno be the more efficient and sensitive to the fluorescence emissionf the film. This exceptional result was attributed to the hindrance

nduced by ZnS particles. Considering the sensitivity, reversibil-ty, and fast response of the present film towards Pb2+, the film

ay have the potential for applications in Pb2+ monitoring inater.

[[[

gineering B 176 (2011) 873– 877 877

Acknowledgments

The authors are grateful to the financial support given by theShaanxi Province, the Scholarship Council of the People’s Repub-lic of China (grant 11JK0513), and the Scholarship Council of theXianyang Normal University (grant 08XSYK109).

References

[1] J. Pyun, K. Matyjaszewski, Chem. Mater. 13 (2001) 3436–3442.[2] Y.H. Ni, X. Ge, Z.C. Zhang, Mater. Sci. Eng. B 130 (2006) 61–65.[3] M. Fakis, F. Zacharatos, V. Gianneta, P. Persephonis, V. Giannetas, A.G. Nas-

siopoulou, Mater. Sci. Eng. B 165 (3) (2009) 252–255.[4] D.B. Zhang, L.M. Qi, H.M. Cheng, J.M. Ma, J. Colloid Interface Sci. 246 (2002)

413–419.[5] M.Y. Han, X.H. Gao, J.Z. Su, S.M. Nie, Nat. Biotechnol. 19 (2001) 631–635.[6] H.X. Xu, M.Y. Sha, E.Y. Wong, J. Uphoff, Y.Z. Xu, J.A. Treadway, A. Truong, E.

O’Brien, S. Asquith, M. Stubbins, N.K. Spurr, E.H. Lai, W. Mahoney, Nucleic AcidsRes. 31 (2003) e43.

[7] H.C. Yeh, Y.P. Ho, T.H. Wang, Nanomed. Nanotechnol. Biol. Med. 1 (2005)115–121.

[8] S.K. Sahoo, V. Labhasetwar, Drug Discov. Today 8 (24) (2003) 1112–1120.[9] M. Yokoyama, J. Artif. Organs 8 (2005) 77–84.10] M. Ozkan, Drug Discov. Today 9 (24) (2004) 1065–1071.11] M. Bruchez, M. Moronne, P. Gin, S. Weiss, A.P. Alivisatos, Science 281 (1998)

2013–2016.12] A.K. Krishna, N.N. Murthy, P.K. Govil, Atomic Spectrosc. 28 (6) (2007) 202–214.13] Y. Ohyabu, T. Adegawa, T. Yoshioka, T. Ikoma, Mater. Sci. Eng. B 165 (3) (2010)

204–207.14] C.H. Chiang, H. Ishida, J.L. Koening, J. Colloid Interface Sci. 74 (1980) 396–400.15] X.L. Guan, X.Y. Liu, Z.X. Su, J. Appl. Polym. Sci. 104 (6) (2007) 3960–3966.16] R. Premachandran, S. Banerjee, V.T. John, G.L. McPherson, J.A. Akkara, D.L.

Kaplan, Chem. Mater. 9 (1997) 1342–1347.