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Materials Chemistry and Physics 105 (2007) 315–319

Electrostatic layer-by-layer self-assembly of PAMAM–CdSnanocomposites on MF microspheres

Fei Guo, Yihua Zhu ∗, Xiaoling Yang, Chunzhong LiKey Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering,

East China University of Science and Technology, Shanghai 200237, China

Received 30 September 2005; received in revised form 28 February 2007; accepted 29 April 2007

bstract

PAMAM–CdS nanocomposites with their photoluminescence property were prepared in methanol solution and the ratio of Cd2+ and PAMAMas adjusted to investigate its effect on the optical property of the PAMAM–CdS nanocomposites. The formed PAMAM–CdS nanocompositesere adsorbed onto the melamine formaldehyde (MF) microspheres by electrostatic interaction to form microspheres with their photoluminescence

roperty. Electrostatic layer-by-layer assembly of the PAMAM–CdS nanocomposites using poly(sodium 4-styrenesulfonate) (PSS) as the oppositelyharged polyelectrolyte leading to MF microspheres with their photoluminescence intensity is reported. The formed composite microspheres cane applied in the fields such as biological assays. 2007 Elsevier B.V. All rights reserved.

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eywords: CdS; Nanocomposites; Electrostatic self-assembly; Photoluminesce

. Introduction

Recently, semiconductor nanoparticles have attracted greatttention because of their unique quantum effect [1,2]. CdSanoparticles show excellent application in the field of sensornd biological assays because of their eminent physical, chemi-al and photoelectric properties [3]. Now there are many reportsn fabrication of CdS nanoparticles stabilized with functionalolymers in different solvent [4,5]. Sooklal et al. [6] had usedolyamidoamine (PAMAM) dendrimers as a template to prepareAMAM–CdS nanocomposites with stable photoluminescenceroperty in water and methanol solvent. Wu et al. [7] synthesizedAMAM–CdS nanocomposites from methanolic Cd2+ and S2−ith amine-terminated polyamidoamine dendrimers of gener-

tion 8 (G8NH2) as stabilizers. By controlling the preparationonditions, nanoparticles with diameters <2 nm can be obtainedith a narrow size distribution.Ideally, dendrimers are perfect monodisperse macro-

olecules with a regular and highly branched three-dimensionalrchitecture. PAMAM dendrimers of varying generations haveifferent numbers of tertiary nitrogen and primary amino groups.

∗ Corresponding author. Tel.: +86 21 64252022; fax: +86 21 64250624.E-mail address: yhzhu@ecust.edu.cn (Y. Zhu).

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254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2007.04.077

AMAM dendrimers

s a kind of familiar dendrimer, PAMAM have been appliedn many fields, such as medicinal applications [8], catalysts9] and fabrication of nanoparticles [10]. It is a new tech-ology developed rapidly of late years that makes compositeicrospheres by adsorbing polyelectrolyte and nanoparticles

lternately on latex bead by electrostatic interaction [11–13]. Weabricated the core–shell latex spheres with CdS/polyelectrolyteomposite multilayers using electrostatic layer-by-layer (LbL)elf-assembly technique [14]. PAMAM has many positive (full-eneration) and negative (half-generation) charges on its surfacehich make it easy to adsorb on surface of oppositely charged

ubstrates and microspheres by electrostatic forces [15,16]. He etl. [17] utilized PAMAM for preparation of Au-nanoparticles,hich were deposited on substrates by electrostatic layer-by-

ayer self-assembly. Sooklal et al. [6] prepared thin films ofhe CdS/dendrimer nanocomposites by casting solutions ontorosted microscope slides followed by solvent evaporation andormed film exhibited luminescent properties similar to those ofhe parent solutions. However, at present we have not seen anyelative report that PAMAM–CdS nanocomposites are adsorbedn the surface of latex beads.

In this paper, the PAMAM–CdS nanocomposites areabricated in methanol solvent and the composite micro-pheres with luminescent properties are prepared by adsorbingAMAM–CdS nanocomposites and polyelectrolyte alternately

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16 F. Guo et al. / Materials Chemis

n the surface of melamine formaldehyde (MF) microspheres.he size of prepared CdS using PAMAM dendrimers as tem-lates is about 2–3 nm, which is less than that we prepared inur previous work [14]. The formed fluorescent microspheresave a potential application in biological assays.

. Experimental

.1. Materials

Ethylenediamine (EDA) (analytical), sodium sulfide (analytical), anhydrousethanol, cadmium nitrate were purchased from Sinopharm Chemical Reagento. Ltd.; methyl acrylate was purchased from Alfa Aesar; poly(sodium 4-

tyrenesulfonate) (PSS, molecular weight 70,000) was purchased from Acrosrganics, and poly(allylamine hydrochloride) (PAH, molecular weight 70,000)as purchased from Aldrich. All materials except EDA were used without fur-

her purification. Melamine formaldehyde (MF) microspheres, with the diameterf about 2 �m, were prepared by dispersed polymerization [18].

.2. Synthesis procedure of PAMAM dendrimer

According to literature [19,20], PAMAM was prepared by a divergentynthesis method using the reagent excess method starting from EDA by consec-tive Michael addition and ester amidation reaction. General procedure for theynthesis of ester-terminated (half-generation) PAMAM dendrimers is that full-eneration dendrimer (EDA in the very first step) reacted with a slight excess ofethyl acrylate (∼10%) in methanol at 0 ◦C for several days. Amino-generation

full-generation) PAMAM is synthesized by reacting half-generation PAMAMith excess EDA in methanol at 0 ◦C for several days. The excess EDA, methyl

crylate and solvent were removed in rotating evaporator under vacuum.

.3. Fabrication of PAMAM–CdS nanocomposites in methanol

Cadmium nitrate dissolved in methanol was added to 4.0 generationAMAM methanol under magnetic stirring in ice bath and the mol ratio ofd2+ to PAMAM are 5:1 or 10:1. After 20 min, sodium sulfide dissolved inethanol was added and the mol ratio of Cd2+ to S2− is 1:1. Twenty minutes

ater the product was kept at −10 ◦C for further use.

.4. Fabrication of MF/(PE3/PAMAM–CdS/PE/PAMAM–CdS)n

Prior to the deposition of the PAMAM–CdS nanocomposites layer, therimer three polyelectrolyte layers (PSS/PAH/PSS, PE3) were formed bylternate adsorption of PAH and PSS (1 mg mL−1 containing 0.30 M NaCl)nto 20 mg of positively charged MF microspheres. The adsorption time ofolyelectrolyte was 20 min. After each adsorption step, the excess polyelec-rolyte was removed by three cycles of centrifugation/wash cycles/dispersion.he PAMAM–CdS nanocomposites were deposited by adding 5 mL ofAMAM–CdS (the ratio of PAMAM to CdS is 1:10) nanocomposites whichere dissolved in methanol and 0.6 mL of 0.67 g mL−1 of NaCl aqueous solution

nto 20 mg of PE3-coated MF microspheres, after stirring 45 min and removingxcess nanocomposites by three cycles of centrifugation/wash cycles/dispersion,wo following layers of PSS/PAMAM–CdS were coated on MF micro-pheres onto which PE3/PAMAM–CdS had deposited. Subsequently depositingther PE3 (PSS/PAH/PSS) layers as outlined above. The processes wereepeated until the desired number of PAMAM–CdS nanocomposites layers wasormed.

.5. Characterization

UV–vis absorption spectra were taken on a Shimadzu UV-2102 PC spec-rometer. VARIAN Cary 500 UV–vis Spectrometer was used to test the UV–viseflectance absorption properties of composite microspheres. The fluorescencepectra were recorded on a Shimadzu RF-5301 PC spectrophotometer. Malvern000HS ZETASIZER was used to test the zeta potential of the composites dur-

Foc

d Physics 105 (2007) 315–319

ng the process of self-assembly. TEM images were taken on JEOL JEM-100X and the acceleration voltage is 100 kV.

. Results and discussion

The most remarkable feature of the PAMAM–CdS nanocom-osites is their optical properties. Absorbance measurementsndicate that methanol solutions of dendrimer do not absorbight at wavelengths greater than about 250 nm (Fig. 1a). Onhe other hand, methanol solutions of PAMAM–CdS nanocom-osites show significant absorption of UV light (Fig. 1a) butlmost none in the visible region of the spectrum (400–750 nm).hese results consist with those of report [6]. Two kinds ofAMAM–CdS nanocomposites almost have same absorptiondge values of 350 nm and the size of these CdS clusters is 2.4 nmccording to Brus’ effective mass model [21]. The UV–vis

ig. 1. UV–vis spectra of PAMAM/Cd2+ and PAMAM–CdS with different ratiof PAMAM to CdS (a) and UV–vis reflectance spectra of MF microspheresoated with two layers of PAMAM–CdS nanocomposites (b).

F. Guo et al. / Materials Chemistry and Physics 105 (2007) 315–319 317

Fig. 2. Photoluminescence of PAMAM–CdS nanocomposites prepared inM

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eOH solutions. Fig. 3. Zeta potential as a function of layer number for PSS/(PAH orPAMAM–CdS) multilayers on the charged MF microspheres. The odd layernumbers correspond to PSS adsorption and the even layer numbers to PAH andPAMAM–CdS deposition.

ig. 4. TEM images of cross-section of MF microspheres coated with three layers (a) and six layers (c) of PAMAM–CdS nanocomposites and their higher magnificationmages (b and d).

3 try and Physics 105 (2007) 315–319

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pectrum. During the self-assembly process. It can be observedhat as the layers of PAMAM–CdS nanocomposites coatingn MF microspheres increase, the color of MF microspheresecome yellow gradually, which indicates PAMAM–CdS weredsorbed on the surface of MF microspheres layer-by-layer. Thisaximum absorbance at 360 nm should originate from the CdS

anoparticles, even though it has a 60 nm red shift in comparisonith the parent methanol solutions. The reason for this red shifterhaps is that due to the size increase of CdS nanoparticles, thebsorption band shifts to longer wavelengths [6,17].

Upon excitation at 320 nm, the methanol solutions ofAMAM–CdS nanocomposites show a photoluminescence peakFig. 2) with emission maximum at about 450 nm. The absorp-ion (Fig. 1) and photoluminescence (Fig. 2) spectra indicate thatAMAM–CdS nanocomposites with different ratio of Cd2+ toAMAM had same peak position, but the absorption and lumi-escence intensity enhance as the ratio of Cd2+ to PAMAMncreases.

Fig. 3 shows the dependence of zeta potentials on step-ise growth of polyelectrolytes including PSS, PAH and

AMAM–CdS nanocomposites on MF microspheres. Threerimer films (PSS/PAH/PSS) provide both a uniform and neg-tively charged outer surface to facilitate the adsorption of theuantum dots [22]. The PAMAM–CdS nanocomposites electro-tatically interact with PSS through the protonated amino groups–NH3

+) groups on the surface of the PAMAM–CdS nanocom-osites [17,16] and the anion groups (–SO3

−) of negativelyharged PSS, which is similar to the electrostatic interactionf PAH and PSS [23].

The formation of the multilayer films on the MF micro-pheres was visualized by cross-section transmission electronicroscopy (TEM). Fig. 4 shows cross-section TEM micro-

raphs of MF microspheres coated with three (Fig. 4a and b)nd six (Fig. 4c and d) layers of PAMAM–CdS nanocompos-tes. TEM images show that the size of CdS nanoparticles onhe composite microspheres is about 3 nm, and as the num-er of layers of CdS nanoparticles increases from three to six,he thickness of the multilayer shell is increased and the CdSanoparticles on the composite microspheres can be observedn the TEM images more clearly (Fig. 4d). The results sug-est that the PAMAM–CdS nanocomposites can be adsorbed onF microspheres layer-by-layer successfully, and the amount

f CdS deposited on each composite microsphere rises with theayer increasing gradually. However, TEM photographs showhat coating of PAMAM–CdS nanocomposites on microsphereay have caused some aggregation of CdS clusters, possibly

ltering the composite spheres’ absorption properties (whichs shown in Fig. 1) compared to those of the parent methanololutions.

Further evidence for the formation of PE/PAMAM–CdSultilayer films on colloids was provided by absorption and

uminescence spectroscopy. The PL properties of MF micro-pheres coated with CdS–PAMAM were investigated at the

xed concentration of coated MF microspheres. Fig. 5 shows

he PL emission spectra of the nanocomposites with a differentumber of PAMAM–CdS layers up to six layers. The emissionands at about 450 nm were demonstrated. The PL intensity was

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ig. 5. Photoluminescence of MF microsphere coated with two layers, fourayers and six layers of PAMAM–CdS nanocomposites.

nhanced with a corresponding increase of the thickness of poly-lectrolyte interlayers. The observed increase of the PL intensityf PAMAM–CdS versus the corresponding deposition numberndicated a stepwise and uniform assembling process.

. Conclusions

PAMAM–CdS nanocomposites were prepared in methanoly adjusting the ratio of CdS to PAMAM. UV–vis and photolu-inescence spectra of two samples indicate that PAMAM–CdS

anocomposites with the higher ratio (10:1) of CdS to PAMAMave higher absorption and luminescence intensity. The testesults, such as zeta potentials, TEM, photoluminescence andV–vis indicate that the formed PAMAM–CdS nanocomposites

an be adsorbed on the MF microspheres and the photolu-inescence intensity of the formed composite microspheres

an be changed by adjusting layers of coated PAMAM–CdSanocomposites on MF microspheres. Adsorption of PAMAMn the outer layer of composite microspheres makes theurface-modification of microspheres with –NH2 groups andhe as-prepared composite microspheres can bond with someiological molecules in further research.

cknowledgments

This work was supported by the National Natural Sci-nce Foundation of China (20236020, 20676038), the Nationaligh Technology Research and Development Program of China

2006AA03Z358), the Special Projects for Key Laboratoriesn Shanghai (05DZ22302, 06DZ22008), and the 973 Program2004CB719500).

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