Materials Letters 65 (2011) 2345–2347

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Materials Letters

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Microstructure and hemocompatibility of neodymium doped zinc oxide thin films

Z.Y. Huang a, P. Luo a, M. Chen a, S.R. Pan b, D.H. Chen a,⁎a State Key Laboratory of Optoelectronic Materials and Technologies, Sun Yat-Sen University, Guangzhou 510275, Chinab Artificial Heart Lab, the 1st Affiliate Hospital of Sun Yat-Sen University, Guangzhou 510080, China

⁎ Corresponding author. Tel./fax: +86 20 84113398.E-mail address: stscdh@mail.sysu.edu.cn (D.H. Chen

0167-577X/$ – see front matter © 2011 Elsevier B.V. Adoi:10.1016/j.matlet.2011.05.031

a b s t r a c t

a r t i c l e i n f o

Article history:Received 29 November 2010Accepted 7 May 2011Available online 12 May 2011

Keywords:SputteringNeodymiumSurface energyHuman fibrinogenBiomaterials

By the radio frequency magnetron sputtering, both un-doped and neodymium (Nd) doped ZnO thin filmswere grown on Si (100) substrates. The microstructure of the films has preferred c-axis growth orientationconfirmed by the X-ray diffraction spectra. The crystallite size of ZnO crystallite decreases with the increasingof Nd concentration. We show that the hemocompatibility can be improved by a suitable Nd doping in theZnO thin films. It is verified by both calculations on surface energy and interfacial tension of the sample, andexperiments on the wettability, the proteins adsorption, and the platelets adhesion.

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© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Tissue compatibility and blood compatibility are two factors ofmedical devices implanted into the human body. In particular, themelioration of blood compatibility is significantly important for theblood contacting devices. Recently, Zinc oxide (ZnO) is widely used inbiomedical field [1,2], solar cells [3], piezoelectric transducers, lightemitting devices [4], and gas sensor [5,6], due to its predominantproperties such as wide bandgap (~3.3 eV) and large exciton bindingenergy (~60 meV). We previously studied the influence on surfacemicrostructure and hemocompatibility of un-doped ZnO thin films bychanging annealing temperature and radio frequency power [7,8].

On the other hand, doping modification is an effective method toimprove biocompatible performance. Rare-earth doping has beenpaid tremendous attentions in medicine applications for its excellentproperties of low toxicity, anticoagulant, antineoplastic, immuno-modulatoryandantiseptic [9,10]. For example, the rare-earth compoundshave as fast efficiency as heparin for anticoagulant. Furthermore, theanticoagulant using light rare earth elements (e.g. neodymium)hasmoresignificant effect than using heavy ones.

In this paper, we deposited the neodymium (Nd) doped ZnO thinfilmby the radio frequencymagnetron sputtering (RFMS). Sampleswithdifferent doping concentration have been synthesized to study variousproperties, such asmicrostructure, surfaceproperties andhemocompat-ibility. We find that the doped structure is a good candidate to improvehemocompatibility.

2. Experimental details

We employed the RFMS to synthesize samples on the Si (100)substrates by using the ceramic targets of pure ZnO (99.99%) and Nd-doped ZnO (with Ndmolar concentration of 1%, 2% and 3%). After beingevacuated to a base pressure of 3×10−3 Pa, the deposition was carriedout at 4.0 Pa of argon (99.99% purity) pressure. The substrate was notintentionally heated during the deposition. The sputtering power was100Wand the deposition timewas 15 min. The thicknesses of undopedand 1 at.%, 2 at.%, 3 at.% Nd-doped ZnO thin films are about 250, 300,310, and 290 nm, respectively.

The structures of ZnO thin films were characterized by the X-raydiffractometer (D/MAX 2200 VPC, RIGAKU, Jap.) with a Cu Kα X-raysource (wavelength of 1.54 Å) working at 40 kV and 20 mA. Thewettability behavior and surface properties were determined by thecontact angle goniometer (SL2008). The test solutions were deionizedwater and formamide.

Experiments of proteins adsorption and platelets adhesion wereperformed to evaluate hemocompatibility of the Nd-doped ZnO thinfilms. The adsorption behavior of plasma proteins (human fibrinogen(HFG) and human serum albumin (HSA)) was investigated byenzyme-linked immunosorbent assay (ELISA) [11]. The procedurescan be found in our previous paper [8]. Optical density (OD) of eachwell was measured at a wavelength of 450 nm using an enzyme-labeling instrument (MK3). OD (also called absorbance) is used todescribe the attenuating performance of the solution for a given wave-length. The higher optical density, the higher concentrations of plasmaproteins adsorption are. In the experiment of platelets adhesion, thewhole bloodwas centrifuged to achieve platelet-rich plasma (PRP)withthe concentration of 2000 cells/μl. All the samples were saturated inPRP and centrifugalized at 3500 r/min for 1 min. After incubation, the

Fig. 1. (a) XRD patterns of ZnO thin films with different Nd doping; SEM images(×100,000) of (b) undoped ZnO; (c) 1 at.%; (d) 2 at.%; (e) 3 at.% Nd doped ZnO thin films.

Fig. 2. (a) Optical density of adsorbed HFG and HSA on the surface of different samples.(b) Density of platelets adhered on the surface of the Nd doped ZnO thin films withvarious concentrations.

2346 Z.Y. Huang et al. / Materials Letters 65 (2011) 2345–2347

samples were fixed in 2.0% glutaraldehyde for 1 h, and washed anddehydrated in a graded ethanol. The quantity,morphology, aggregation,and pseudopodium of the platelets adhering on the sample surfaceswere observed using SEM after the samples critical-point dried.

3. Results and discussions

3.1. Structure properties

XRD patterns show that all the samples are poly-crystalline,belonging to hexagonal wurzite structure with highly preferred c-axisorientation (Fig. 1). Intensity of (002) peak reduces when the Ndconcentration increases, indicating that the doping can deteriorate thec-axis preferential orientation. The (002) peak shifts towards lowerdiffraction angle in the larger doping, as ZnO lattice expands along thec-axis when the bigger ions are doped [12]. As the Nd concentrationincreases, the full-width at half maximum (FWHM) of the (002) peakbecomes broader, and the crystallite size becomes smaller (see Table 1).This is consistentwith the SEM images as illustrated in Fig. 1(b)–(e). Thecrystallite size affects on the surface properties of the films that will bediscussed below. The weaker texture ZnO with the c-axis is observedwhen the dopedNd ions substitute into Zn sites, indicating the restraintof crystallite growth.

3.2. Surface properties and blood compatibility

Table 1 shows that both undoped and doped ZnO films arehydrophilic since their contact angles with deionized water (θwater)exceed 90°. The angle reaches maximum for 1 at.% Nd doping sample,and slightly decreases as the Nd concentration increases. The surfaceparameters of test solutions and blood components are found in Ref. [8].The surface energy and the interfacial tension between ZnO surface and

Table 1FWHM and crystallite size from XRD, contact angles and surface properties of ZnO films.

Samples FWHM Crystallite size (nm) θwater

(deg) (calculated by Scherrer equation)

Nd-0% 0.322 25.792 96.09±0.68Nd-1% 0.329 25.248 113.20±2.01Nd-2% 0.674 12.320 111.80±2.28Nd-3% 0.702 11.837 105.76±1.52

blood components are calculated by using the following expressions

cosθ = −1 + 2ffiffiffiffiffiffiffiffiffiffiffiγdsγd

L

q= γs + 2

ffiffiffiffiffiffiffiffiffiffiffiγpsγ

pL

q= γs and γs, blood− components=

[(γblood− componentsd )1/2−(γs

d)1/2]2+[(γblood− componentsp )1/2−(γs

p)1/2]2,respectively [13]. It is shown that both surface energy and interfacialtensionsdecrease in theNddoping samples. They reachminimum in the2 at.% Nd doping sample, except the interfacial tension between ZnOsurface and HSA. It indicates that the Nd atoms enable to reduce surfaceenergy and weaken interfacial interaction with blood components.

Results of OD measurement are shown in Fig. 2(a). After Nd-doping,both the amount of HFG and HSA decrease, and the amount of HSA islarger than that of HFG. It implies that fewer plasma proteins adsorb onthedoped sample surface, andmoreHSA is adsorbed thanHFG. Statisticalnumber of platelets reduces in the doped samples (Fig. 2(b)). Most ofplatelets in the doped samples (Fig. 3(b)–(d)) have better quality thanthe undoped one (Fig. 3(a)). For example, the platelets remain round andhave no pseudopod in Fig. 3(b)–(d).

Platelets adhesion and activation on the surface of biomaterial arethe most essential characteristics to determine the hemocompatibility

Surface energy (dyn/cm) Interfacial tension (dyn/cm)

γs=γsd+γsp γs−blood γs−HSA γs−HFG

44.66 45.17 32.62 40.8212.15 26.42 29.11 32.3912.12 24.95 27.92 30.9722.57 30.92 27.47 32.70

Fig. 3. High magnification SEM images (×3000) of platelets adhered to (a) undoped ZnO; (b) 1 at.%; (c) 2 at.%; (d) 3 at.% Nd doped ZnO thin films.

2347Z.Y. Huang et al. / Materials Letters 65 (2011) 2345–2347

of blood contacting materials. From the contact angle results, thehydrophobic surface of Nd-doped films benefits to reduce the surfaceenergy and the interfacial tension (see Table 1). The weaker interfacialtension, the fewer proteins are adsorbed. So the amounts of proteins(HFG and HSA) adsorbed on the surface decrease in the doped samples.A layer of plasma proteins including HFG and HSA are first adhered onthe biomaterial surface when the biomaterials contact with the blood.The adsorption of plasma proteins may affect the adhesion and defor-mation of platelets [14]. But the influence on the adsorption of HFG andHSA is different. The adsorption of HFG will promote the adhesionand activation of the platelets, whereas the adsorption of HSA doesoppositely. Moreover, the fewer platelets adhesion and deformation,the better hemocompatibility biomaterials possess. Therefore, the Nd-doped ZnO thin films benefit to the hemocompatibility due to thedecreasing of the HFG adsorption (see Fig. 2(a)). In addition, rare earthelements have obvious anticoagulant effect because of the antagonisticeffect of rare earth ions on Ca2+ ions. The Ca2+ is one of the key factorsto turn prothrombin into thrombin, which would promote clotting. Sosuitable rare earth doping leads to imbalance of Ca2+ and inhibition ofcoagulation [15].

4. Conclusion

The undoped ZnO and Nd-doped ZnO thin films were prepared bythe RFMS. The samples with a preferential orientation of (002) arecharacterized by the XRD spectra. The intensity of (002) peak and thegrowthof nanoparticles are restraineddue to theNddoping, resulting inthe change of wettability on ZnO surface. Results from the proteinsadsorption and the platelets adhesion experiment reveal that a suitableNd concentration can improve the blood compatibility. The surfaceof Nd-doped films prefers to adsorb HSA than HFG, and there is fewer

platelets adhered and deformed on the Nd-dopedfilms.We believe thatthe excellenthemocompatibility ofNd-dopedZnOthinfilms attribute tothe hydrophobic surface of ZnO and the anticoagulant property of therare earth element.

Acknowledgments

This work is supported by the National Natural Science Foundationof China under Grant Nos. 81071264 and 30770588, the projectScience and Technology of Guangdong Province (1012210400369,C010515) and the project Science and Technology of Gungzhou City(2010planD00011).

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