AgNPs

Applied Surface Science 257 (2011) 6799–6803

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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc

The preparation and antibacterial effects of dopa-cotton/AgNPs Hong Xu ∗ , Xue Shi, Hui Ma, Yihang Lv, Linping Zhang, Zhiping Mao ∗ Key Lab of Science and Technology of Eco-textile, Ministry of Education, College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, Shanghai 201620, China

a r t i c l e

i n f o

Article history: Received 24 November 2010 Received in revised form 15 February 2011 Accepted 26 February 2011 Available online 4 March 2011 Keywords: Dopamine Silver nanoparticles Antibacterial activity

a b s t r a c t Silver nanoparticles (AgNPs) have been known to have powerful antibacterial activity. In this paper, in situ generation of AgNPs on the surface of dopamine modified cotton fabrics (dopa-cotton/AgNPs) in aqueous solution under room temperature is presented. X-ray photoelectron spectroscopy (XPS) and field emission scanning electron microscope (FE-SEM) were used to analyze the surface chemical composition and the morphology of the modified cotton fabrics, respectively. The results indicated that the surface of cotton fabrics was successfully coated with polydopamine and AgNPs. The cotton fabrics with AgNPs showed durable antibacterial activity. © 2011 Elsevier B.V. All rights reserved.

1. Introduction In the past decades, considerable attention has been directed towards the efficient, non-toxic and durable antibacterial finishing of different materials [1–3]. The minimization of microbial growth on the materials was important since the microbes were harmful to not only the material itself but also the user. Among the various antibacterial materials, silver ion (Ag+ ) has been on the central concern because it shows strong inhibitory and bactericidal effects as well as a broad spectrum of antibacterial activities [4]. Silver in its uncharged state (AgNPs) was also found to possess antibacterial activities [2–9]. Although the mechanism of this action is still inexplicit, it is widely believed that AgNPs can interact with the constituents of the outer membrane of bacteria, cause structural change and degradation [10–12]. In recent years, various chemical and physical methods have been employed to prepare AgNPs in different materials. For example, the plasma sprayed technology was utilized to prepare the nano-silver coating on the titanium substrate [13]. Akhavan and co-workers prepared the antibacterial Ag–SiO2 thin films and Ag/a–TiO2 nanocomposite thin films with high silver concentration by sol–gel methods [2,9]. The AgNPs were immobilized in pore-arrays of the SU-8 polymeric matrix by the optical interference lithography method made the samples with high antibacterial activity [7]. These researches lay the foundation for the further study.

Dopamine (3,4-dihydroxyphenethylamine), a kind of lowmolecular-weight catecholamine mimics as the adhesive protein, could spontaneously self-polymerize on various inorganic or organic materials surface and form uniformly films under mild conditions [14–17]. Furthermore, the catechol groups in dopamine could perform as metal reducing agents because they have redox active [14,18]. In this paper, a facile method for binding AgNPs on cotton fabrics was reported. The cotton fabrics were modified with dopamine at first and then treated with AgNO3 in an aqueous medium under room temperature. Silver ions (Ag+ ) were reduced to AgNPs and distributed homogeneously on the surface of dopamine modified cotton fabrics. The cotton fabrics with AgNPs showed a highly antibacterial activity even after washing 30 times. 2. Experimental 2.1. Materials Bleached cotton fabrics (pristine cotton fabrics) supplied by Esquel Group (Gaoming, China) was cleaned by diethyl ether anhydrous before used. Dopamine hydrochloride was purchased from Webb Chemical Co., Ltd. (Guangzhou), which was imported from Acros Organics USA. Silver nitrate, Tris (hydroxymethyl) aminomethane (Tris) and hydrochloric acid (HCl) were purchased from Sinopharm Chemical Reagent Co., Ltd. All chemicals were used without further purification. 2.2. The formation of polydopamine membrane on the cotton fabrics

∗ Corresponding authors at: No. 2999 North Renmin Road, Songjiang District, Shanghai 201620, China. Tel.: +86 21 67792720; fax: +86 21 67792707. E-mail addresses: [email protected] (H. Xu), [email protected] (Z. Mao). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.02.129

Dopamine solution with a concentration of 0.2 mol/L was prepared by dissolving dopamine in Tris buffer solution. Then the pH

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H. Xu et al. / Applied Surface Science 257 (2011) 6799–6803

value of the solution was regulated to a typical marine environment (pH = 8.5). The pristine cotton fabrics were dipped directly into dopamine solution with stirring for 24 h at room temperature and then taken out and rinsed with deionized water until the solution turned to clear. The cotton fabrics treated by dopamine would be called as dopa-cotton fabrics in the subsequent discussion.

Table 1 Surface chemical composition of cotton and dopa-cotton fabrics determined by XPS.

C 1s(at.%) N 1s(at.%) O 1s(at.%) N/C O/C

Cotton

Dopa-cotton

Dopamine (theoretic)

61.8 0.8 36.5 0.013 0.590

73.1 7.2 18.9 0.098 0.259

72.7 9.1 18.2 0.125 0.250

2.3. The loading of AgNPs on the surface of dopa-cotton fabrics The dopa-cotton fabrics were immersed into silver nitrate solution (0.29 mol/L) at room temperature for 8 h. Then the samples were rinsed with flowing deionized water for 10 min and dried at 40 ◦ C in the vacuum oven for 6 h. The obtained samples were designated as dopa-cotton/AgNPs fabrics in the subsequent discussion. 2.4. X-ray photoelectron spectroscopy (XPS) XPS measurements were carried out on a Kratos AXIS Ultra DLD X-ray photoelectron spectrometer with an Al K␣ X-ray source (1486.6 Ev photons). Binding energies were calibrated using the containment carbon (C 1s = 284.8 eV). 2.5. Field emission scanning electron microscope (FE-SEM) The surface morphologies of the pristine cotton fabrics, dopacotton fabrics and dopa-cotton/AgNPs fabrics were investigated by a Hitachi S-4800 field emission scanning electron microscopy (FESEM) at an accelerating voltage of 10 kV. 2.6. Inductively coupled plasma optical emission spectrometry (ICP-OES) The total content of silver in the samples was determined by ICPOES used the prodigy spectrometer (Teledyne Leeman Labs, USA). The wavelength ranged from 165 nm to 800 nm. 2.7. Washing fastness testing The washing fastness testing of dopa-cotton/AgNPs fabrics was performed according to ISO105-C10:2006 standard using the SW12A Launder-O-Meter Standard Instrument. The liquid ration was 50:1 and the washing temperature was 40 ◦ C. The samples were washed for 30 min in a solution of soap with a concentration of 5 g/L for one cycle, rinsed with deionized water and dried at 40 ◦ C in the vacuum oven for 6 h. 2.8. Antibacterial activity The antibacterial efficiencies of treated samples were quantitatively estimated using gram-negative bacteria Escherichia coli according to AATCC 100-2004 standard. Before the microbiological experimentation, all glass wares and samples were sterilized by autoclaving at 121 ◦ C for 15 min. Two pieces of circular finished fabrics with the diameter of 4.8 cm were put into a 250 ml Erlenmeyer flask and inoculated with 1.0 ml of a nutrient broth culture containing 1–2 × 105 CFU (CFU-colony forming units) of bacteria. The original cotton fabrics were used as a control. After incubation at 37 ◦ C for 24 h, the bacteria were eluted from the fabrics by shaking them vigorously in 100 ml sterilized water for 1 min. After made serial dilutions with sterilized water, the bacterial suspension were spread on nutrient agar plate and incubated at 37 ◦ C for 24 h. Then the number of bacteria forming units (CFU) was counted. And the reported data were the average value of three parallel runs.

The percentage of microbe reduction (R%) was calculated using the following equation: R=

B − A B

× 100%



where A (CFU) is the number of microbial colonies of the finished fabrics after inoculation 24 h, and B (CFU) is the number of microbial colonies of the finished fabrics which was immediately eluted after inoculation (at “0 contact time). 3. Results and discussion 3.1. The fabrication of multifunctional polydopamine film on cotton fabrics Fig. 1 shows the images of pristine and dopamine modified cotton fabrics. It could be seen the color of the pristine fabrics turned from white to dark brown after it was treated by dopamine. According to reports, the catechol groups in dopamine were redox active, and were easily oxidized in the alkaline conditions to form quinone structures. The quinone structures could further react with amines and other catechol or quinone to form an adherent polydopamine film [14–17]. In order to confirm that polydopamine film has been formed on the cotton fabrics surface, XPS was used to determine the surface chemical composition of cotton fabrics (Fig. 2) and the data were listed in Table 1. Fig. 2b shows the essential marker of nitrogen 1 s (N 1s) (400 eV) peaks of dopa-cotton fabrics, however, no peaks corresponding to nitrogen were observed for the pristine cotton fabrics (Fig. 2a), which indicated the nitrogen atom was derived from dopamine on the surface of dopa-cotton fabrics. The data in Table 1 indicated that a nitrogen-to-carbon signal ratio (N/C = 0.098) of dopa-cotton fabrics was slightly lower than the theoretical value for dopamine (N/C = 0.125). It could be the carbon atom belonged to the cotton fabrics were detected by XPS. And the O/C ratio (0.259) of the dopacotton fabrics was in good agreement with the theoretical value for dopamine (O/C = 0.250). It was suggested that the surface of dopacotton fabrics were covered with polydopamine membrane and the thickness of the film approximated to the detection deepness of XPS technique which was about 3 nm for the organic matrix. The surface morphology of the samples was further characterized by FE-SEM (Fig. 3). As shown in Fig. 3b, the surface of cotton fabrics was coated by striated polydopamine membranes, sulting in the possibility of further in situ reduction and immobilization the silver particles [19]. 3.2. The in-situ reduction and binding of AgNPs on the surface of dopa-cotton fabrics XPS was performed to determine the chemical state of silver on the surface of dopa-cotton/AgNPs fabrics. Fig. 4 presents the high-resolution XPS spectrum of Ag(3d) core level in the dopacotton/AgNPs fabrics before and after washed. The Ag(3d5/2 ) and Ag(3d3/2 ) peaks were found at bind energies of 368.0 eV and 374.0 eV shown in Fig. 4a respectively, meanwhile the bind ener-

H. Xu et al. / Applied Surface Science 257 (2011) 6799–6803

6801

1000

800

600

400

O 2s

O 2s

N 1s

Intensity(cps)

C 1s N 1s

1200

O 1s

B

Intensity(cps)

A

C 1s

O 1s

Fig. 1. Images of the cotton fabrics: (a) pristine cotton fabrics and (b) dopa-cotton fabrics.

200

Binding Energy(eV)

0

1000

800

600

400

200

0

Binding Enerey(eV)

Fig. 2. XPS spectra of (a) pristine cotton fabrics and (b) dopa-cotton fabrics.

gies of 368.2 eV and 374.2 eV in Fig. 4b. Moreover, the slitting of the 3d doublet of Ag was 6.0 eV as shown in Fig. 4a and b, which indicated the formation of metallic silver [7,20,21] on the surface of dopa-cotton fabrics. To further study the chemical state of the silver particles, a detailed deconvolution of the Ag(3d) peak was analyzed. For Fig. 4a, the binding energy of the Ag(3d5/2 ) core level for Ag, Ag2 O and AgO was 368.7 eV, 368.1 eV and 367.8 eV, respectively. Hence, the Ag(3d5/2 ) peak was deconvoluted into three Gaussian components with identical full width at half maximum(FWHM). Based on the deconvolution analysis, it was found that about 75.2% of the silver atoms on the surface of dopa-cotton fabrics were in the Ag0 (metallic) state, while about 6.7% and 18.1% of the silver atoms

were in the Ag+ and Ag2+ chemical states, respectively. It could be seen from Fig. 4b, the binding energy of the Ag(3d5/2 ) core level for Ag, Ag2 O and AgO was 368.6 eV, 368.2 eV and 367.9 eV, respectively. It could conclude that there was 76.3% of the Ag0 (metallic) state, while about 8.3% and 15.4% of the silver atoms were in the Ag+ and Ag2+ chemical states, respectively. Compared with Fig. 4a, the content of Ag0 and Ag+ increased slightly in Fig. 4b, but with a subtle reduced content of Ag2+ . It was confirmed that the wash process caused no considerable change in the chemical state of the silver particles. The possible Ag+ in situ reduction mechanism for polydopamine was shown in Scheme 1. The catechol groups in the polydopamine

Fig. 3. FE-SEM images of (a) the pristine cotton fabrics (b) dopa-cotton fabrics.

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H. Xu et al. / Applied Surface Science 257 (2011) 6799–6803

6.0 eV

6.0 eV

B

Intensity (arb.unit)

Intensity (arb.unit)

A

360

365

370

375

380

385

360

365

370

375

380

385

Binding Energy (eV)

Binding Energy (eV)

Fig. 4. Deconvolution of Ag(3d) XPS peak of the dopa-cotton/AgNPs fabrics (a) unwashed (b) washed 30 times.

Fig. 5. FE-SEM images of the dopa-cotton/AgNPs fabrics (a) unwashed (b) washed 30 times.

film first chelated with Ag+ , and then in situ reduced the Ag+ to Ag0 . The catechol groups were simultaneously oxidized to the quinone structures. The catechol and quinone structures were in equilibrium state in aqueous media, and the equilibrium shifts toward to the catechol group at neutral and acid condition [22], which ensured the reduction reaction was continuously performed without addition of an exogenous reducing agent in the silver nitrate solution. According to the report [23], the Ag0 could be bond on the N-site and O-site in polydopamine film and formed seed precursor. The seed Ag0 can grow to nanoparticles through the atom by atom growth with the reduction of Ag+ . The surface morphology of the dopa-cotton/AgNPs fabrics was observed by FE-SEM shown in Fig. 5. It could be seen from Fig. 5a that there existed some monodispersed silver nanoparticles as well as some aggregated silver particles with diameter of 100–300 nm in the striated polydopamine membranes on the surface of dopacotton fabrics. However, when the samples were washed after 30 times, it could be noticed that the aggregated silver particles Polydopamine OH

Metal chelation +

H2N

Polydopamine H O

Ag+

H2N

OH

e lin ka al nno

Polydopamine

-si In

O

were washed away and only some silver nanopartiles with diameter of 80–90 nm were left as shown in Fig. 5b. It could confirm that polydopamine membranes owned strong metal-binding ability and adhesion property to the monodispersed silver nanopartiles [14,15,18]. Fig. 6a shows the total content of silver in the dopacotton/AgNPs fabrics and Fig. 6b represents the mass concentration of silver on the surface of dopa-cotton/AgNPs fabrics determined by XPS analysis with the depth of investigation about 3 nm. It could be

Ag+

OH

tu

c du re

n tio

+ Ag H2N

O

Scheme 1. Hypothetical complexation and in situ reduction reaction pathway for dopamine.

Fig. 6. The variation of the silver concentration of the samples after washing (a) the total silver content characterized by ICP-OES. Inset: calibration curve of the ICP-OES spectra (b) the mass concentration of silver on the surface of dopa-cotton/AgNPs washed with different times detected by XPS.

H. Xu et al. / Applied Surface Science 257 (2011) 6799–6803

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Table 2 Reduction percent of bacteria E. coli. (R%) according to the AATCC 100-2004 standard method. Samples

Average of viable E. coli count after “0  contact time (CFU)

Average of viable E. coli count after 24 incubation (CFU)

Original cotton fabrics Unwashed dopa-cotton fabric Unwashed dopa-cotton/AgNPs fabrics Dopa-cotton/AgNPs fabrics washed 30 times

1.6 × 103 3.7 × 102 3.5 × 102 3.8 × 102

1.1 × 103 2.6 × 102 <1 <1

seen from Fig. 6a that the content of silver decreased sharply during washing process in former several times, however, the content of silver decreased slowly and sustained steady after washing five times. This trend of releasing was corresponded to the results given by Akhavan and co-workers [2,7,9]. Even when the samples were washed 30 times, the content of silver remained high up to 15 mg/g. For researching on the mass concentration of the samples, the similar elution pattern was also observed. The mass concentration of silver on the surface of dopa-cotton fabrics decreased dramatically during washed former five times, and then slowly decreased. Combination with the images of FE-SEM shown in Fig. 5, it was confirmed that some silver particles with large size or aggregation just adsorbed onto the surface of cotton fabrics were easily washed away, and the monodispersed AgNPs which located in the polydopamine membranes were firmly interlocked and hardly washed away, which could be attributed to the strong metal-binding ability and adhesion of the polydopamine membrane [14,15,18]. 3.3. Antibacterial activity The bactericidal effects of the samples were investigated against E. coli, as shown in Table 2. To determine the antibacterial activity, the growth of E. coli was performed in the different cotton fabrics. The dopa-cotton fabrics showed a weak antibacterial activity with the rate of reduction only 29.7%. However, the growth of E. coli was completely inhibited by the silver loaded on dopacotton fabrics. The reduction percent of E. coli seeded on the dopa-cotton/AgNPs was 99.99% after 24 h incubation. Even it was washed 30 times, the reduction percent of E. coli were also up to 99.99%. It was clearly suggested that dopa-cotton/AgNPs fabrics owned the durable antibacterial activity. Early reports [24,25] showed that AgNPs (where silver was present in the Ag0 form) also contained microscale concentrations of Ag+ ions, and they have confirmed that Ag0 and Ag+ both contribute to the antibacterial activity. In this paper, the calculated ratio of Ag0 to Ag+ was 11.3:1 according to XPS analysis. Hence, we believed that the silver ions and metallic silver both synergistically contribute to the antibacterial activity, in agreement with the earlier reports. 4. Conclusions The polydopamine film could be formed on the surface of cotton fabrics after treated with dopamine in the Tris buffer solution. The catechol groups in the polydopamine film could chelate with Ag+ and in situ reduce the Ag+ to Ag0 without adding any further reducing agent. AgNPs can be generated and distributed homogeneously on the surface of dopamine modified cotton fabrics. The cotton

R (%)

29.7 99.99 99.99

fabrics coated with AgNPs shows excellent durable antibacterial activity. Acknowledgement This work was supported by grants from the National Natural Science Foundation of China (Grant no. 21074021), Shanghai Municipal Natural Science Foundation (no. 09ZR1401600), National High Technology Research and Development Program of China (863 Program) (no. 2007AA03Z336) and the Program for New Century Excellent Talents in University (no. NCET-07-0174). References [1] R. Czajka, Fibers and Textile in Eastern Europe 13 (2005) 13–15. [2] O. Akhavan, E. Ghaderi, Current Applied Physics 9 (2009) 1381–1385. [3] M. Kawashita, S. Toda, H.M. Kim, et al., Journal of Biomedical Materials Research-Part A 66 (2003) 266–274. [4] R.L. Davies, S.F. Etris, Catalysis Today 36 (1997) 107–114. [5] J.R. Morones, J.L. Elechiguerra, A. Camacho, K. Holt, J.B. Kouri, J.T. Ramirez, M.J. Yacaman, Nanotechnology 16 (2005) 2346–2353. [6] I. Sondi, B. Salopek-Sondi, Journal of Colloid and Interface Science 275 (2004) 177–182. [7] O. Akhavan, M. Abdolahad, R. Asadi, Journal of Physics D: Applied Physics 42 (2009) 135416–135423. [8] A. Balamurugan, G. Balosssier, S. Pina, Dental Materials 24 (2008) 1343–1351. [9] O. Akhavan, Journal of Colloid and Interface Science 336 (2009) 117–124. [10] S.K. Gogoi, P. Gopinath, A. Paul, A. Ramesh, S.S. Ghosh, A. Chattopadhyay, Langmuir 22 (2006) 9322–9328. [11] J.S. Kim, E. Kuk, K.N. Yu, J.H. Kim, S.J. Park, H.J. Lee, et al., Nanomedicine, Nanotechnolgy, Biology Medicine 3 (2007) 95–101. [12] C.N. Lok, C.M. Ho, R. Chen, Q.Y. He, W.Y. Yu, H. Sun, P.K.H. Tam, J.F. Chiu, C.M. Che, Journal of Biological Inorganic Chemistry 12 (2007) 527–534. [13] B. Li, X.Y. Liu, F.H. Meng, J. Chang, C.X. Ding, Materials Chemistry and Physics 118 (2009) 99–104. [14] H. Lee, S.M. Dellatore, W.M. Miller, P.B. Messersmith, Science 19 (2007) 318–426. [15] M. Yu, J. Hwang, T.J. Deming, Journal of the American Chemical Society 121 (1999) 5825–5826. [16] Z.Y. Xi, Y.Y. Xu, L.P. Zhu, Y. Wang, B.K. Zhu, Journal of Membrane Science 327 (2009) 244–253. [17] H. Lee, N.F. Scherer, P.B. Messersmith, Proceedings of the National Academy of Sciences of the United States of America 103 (2006) 12999–13003. [18] H. Lee, Y.H. Lee, A.R. Statz, J. Rho, T.G. Park, P.B. Messersmith, Advanced Materials 20 (2008) 1619–1623. [19] Y. Liao, B. Cao, W.C. Wang, L.Q. Zhang, D.Z. Wu, R.G. Jin, Applied Surface Science 255 (2009) 8207–8212. [20] S.W. Han, Y.S. Kim, K. Kim, Journal of Colloid and Interface Science 208 (1998) 272–278. [21] C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, G.E. Mullenberg, Handbook of X-Ray Photoelectro Spectroscopy, Physical Electronics Division, PerkinElmeration Corporation, 1979. [22] H. Lee, J. Rho, P.B. Messersmith, Advanced Materials 21 (2009) 431–434. [23] C.H. Liu, H.Y. He, R. Pandey, S. Hussain, S.P. Kama, The Journal of Physical Chemistry 112 (2008) 15256–15259. [24] A. Kumar, P.K. Vemula, p.m. Ajayan, G. John, Nature Materials 7 (2008) 236–241. [25] J.R. Morones, J.L. Elechiguerra, A. Camacho, K. Holt, et al., Nanotechnology 16 (2006) 2346–2353.