- Email: [email protected]
The significant adhesion enhancement of Ag–polytetrafluoroethylene antibacterial coatings by using of molecular bridge
Accepted Manuscript Title: The significant adhesion enhancement of Agpolytetrafluoroethylene antibacterial coatings by using of molecular bridge Author: Ruijie Guo Guangda Yin Xiaojuan Sha Qi Zhao Liqiao Wei Huifang Wang PII: DOI: Reference:
S0169-4332(15)00447-X http://dx.doi.org/doi:10.1016/j.apsusc.2015.02.131 APSUSC 29811
To appear in:
APSUSC
Received date: Revised date: Accepted date:
20-10-2014 3-2-2015 22-2-2015
Please cite this article as: R. Guo, G. Yin, X. Sha, Q.Z. Liqiao Wei, H. Wang, The significant adhesion enhancement of Ag- polytetrafluoroethylene antibacterial coatings by using of molecular bridge, Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.02.131 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
The significant adhesion enhancement of Ag- polytetrafluoroethylene antibacterial coatings by using of molecular bridge Ruijie Guo*11 Guangda Yin1, Xiaojuan Sha1 Qi Zhao2 Liqiao Wei1 Huifang Wang1 (1Key Laboratory of Interface Science and Engineering in Advanced Materials (Taiyuan University of
ip t
Technology), Ministry of Education, Taiyuan 030024, China 2 Division of Mechanical Engineering and Mechatronics, University of Dundee, Dundee, DD1 4HN,
cr
UK)
Abstract
us
Weak adhesion between the metal-based antibacterial coatings and polymer substrates
an
limits their clinical applications, surface modification is an effective way to solve this intrinsic problem. In this study, UV irradiation was employed to activate the inert silicon rubber
M
substrates, and the grafting of coupling agent (3-Mercaptopropyl) trimethoxy silane into the UV-irradiated substrates generated reactive surface containing –SH groups. During electroless
ed
plating S which has lone pair electrons anchored Ag+ and produced antibacterial coatings with
pt
improved adhesion. The grafting of (3-Mercaptopropyl) trimethoxy silane into silicon rubber
Ac ce
was verified by X-ray photoelectron spectroscopy (XPS). The adhesion was tested by American Society of Testing Materials (ASTM D 3359-02). Surface elements content and distribution were observed and analyzed by X-ray energy disperse spectroscopy (EDS). The antibacterial performance was characterized by inhibition halo test and shake flash method. The results showed that the as-prepared composite Ag-polytetrafluoroethylene coatings possessed remarkably enhanced adhesion and superior antibacterial activity. Key words: Surface modification, coatings, adhesion, Ag-PTFE, antibacterial 1. Introduction 1
*Corresponding author. Tel: +86 13835189092, Fax: +86 351 6010311 Email: [email protected] 1
Page 1 of 18
Silicon rubber is a kind of commonly used biomaterial in biomedical devices and implants due to its flexibility, low toxicity, physiological inertness and biocompatibility [1]. However, microbes adhered to it and formed biofilm easily because of its special surface properties, and biomaterial-centered infections happened [2]. Once the biofilm formed, it is
ip t
very hard, or sometimes impossible, to remove it by washing or antibiotic treatment [3].
cr
Bacterial infections associated with implanted devices pose a significant threat to patients and a serious challenge to clinicians [4-6]. Antibacterial coatings deposited on the polymer surface
us
have been proved to be an effective way to discourage bacterial infections. As for the
an
antibacterial agent, it has long been agreed that silver has inherent antibacterial property, and the Ag coatings deposited on polymer surface could serve as an antibacterial layer to
M
minimize infection [7]. Among the coating technologies, electroless plating is a cost-effective
ed
alternative since no special equipment is required compared with other possible routes [8, 9]. Silicon rubber is lack of reactive groups, so it is difficult to bond firmly with metal-base
pt
antibacterial layer. The inertness often connects the applications of silicon rubber to the risk of
Ac ce
adhesion failure, leading to constrained clinical practice. Hence surface modification strategy becomes indispensable with the purpose to activate the inert surface and improve adhesion. UV irradiation is believed to be an effective way to modify polymer surface and generate a large number of active groups. One of the most important characteristics of UV irradiation is that it acts only on a thin surface layer, whereas the sample remains unchanged and the modified material keeps its chemical and mechanical properties [10]. As the newly generated active groups (such as hydroxy, peroxide and hydroperoxide ) are readily to disappear soon after the irradiation, silane coupling agents (3-mercaptopropyl) trimethoxysilane (MPTS) are employed to act as “molecular bridge” in the interface of substrates and metal antibacterial 2
Page 2 of 18
layer [11]. MPTS is a multifunctional organic molecule which is made of a thiol group (-SH) and a trimethoxysilane function (–Si(OCH3)3) [12]. The -SH group is designed to react with Ag+ and form a strong sulfur-Ag (S-Ag) interaction [13, 14], the trimethoxysilane function (-Si(OCH3)3) has grafting ability on active surface and cross-links through the formation of
ip t
siloxane bonds, forming a Si-O-Si network [12,15]. Such an organic molecular is accordingly
cr
expected to improve adhesion performances. MPTS was widely used in the aspect of modifying metal material interface [12, 14, 16], but relatively rare applications in the polymer
us
modification were reported.
an
Besides, polytetrafluoroethylene (PTFE) nanoparticles were incorporated in this study as PTFE has inherent non-stick properties due to its low surface energy and friction coefficient,
M
and so initial bacterial adhesion to a substratum surface is inhibited and biofilm formation is
[17,18,19].
ed
prevented, resulting in remarkable bacterial resistance as has been demonstrated before
pt
In this paper, the silicon rubber matrix was modified firstly by UV irradiation followed
Ac ce
by the grafting of coupling agent MPTS. This modification method relies on the “anchor” of Ag+ by S in MPTS. S has lone pairs of electrons for donation, it captured Ag+ which has unoccupied orbital, and so Ag-PTFE antibacterial composite coatings are covalently bonded onto silicon rubber, leading to dramatically improved metal/polymer adhesion (as shown in Scheme 1). In our previous study, we demonstrated that metal-based coatings were firmly bonded onto polymer substrates through coupling agent 3-amino propyl triethoxy silane (γ-APTS), and the adhesion up to 4B was obtained [20]. In this work, a more effective “molecular bridge” MPTS was chosen and presented significantly improved adhesion up to 5B, the highest level. S in MPTS has smaller electronegativity (2.58) than that of N in γ-APTS 3
Page 3 of 18
(3.04), that is, S-Ag was stronger than N-Ag, leading to dramatically enhanced adhesion. Our study showed that the “anchor” played key role with respect to the adhesion improvement, and thiol proved
more effective than amino. 2. Experimental
ip t
2.1 Materials
cr
All the chemical reagents were analytically pure and used as received. MPTS (98.0%) and PTFE emulsion (60%) with particle size in the range of 0.05-0.5 μm were obtained from
us
Aldrich. Silver nitrate (AgNO3, 99.8%), glucose (C6H12O6, 99%) and xylose (C4H6O6, 99%)
an
were purchased from Tianjin Delan Fine Chemical Factory. Ethanol absolute (CH3CH2OH, 99.7%), acetone (C3H6O, 99.5%), ammonia solution (NH3·H2O, 25%), Hydrochloric acid
M
(37.5%) and sodium hydroxide (NaOH, 96%) were supplied by Tianjin Chemical Reagent Co.
Ltd.
ed
Inc. Silicon rubber was provided by Jinan Chensheng Medical Silicon Rubber Product Co.
pt
2.2 Surface modification of silicon rubber
Ac ce
The silicon rubber sheets with of 10 mm×10 mm×0.5 mm (for inhibition halo test, disc
shaped sheets with diameter of 10 mm) were chosen as substrates in our experiments, and were firstly cleaned with deionized water and acetone to remove smut, dirt and vegetable oil from the surface. They were then treated by using dilute HCl solution (1M) and NaOH solution (0.125M), followed by ultrasonic cleaning in alcohol and drying. Then the rubbers were placed into UV-lamp chamber and subjected to UV irradiation from both sides for about 15 minutes, followed by the immersion of the treated sheets into MPTS solution for 1 hour in order to bond the MPTS onto silicon rubber surface and create the reactive surface. The MPTS solution was a mixture of 10 ml of MPTS and 1300 ml of alcohol. Then the sheets 4
Page 4 of 18
were taken out of the solution and washed successively with acetone, alcohol and deionized water to remove the adsorbed coupling agent molecules completely. The modified samples were at last dried in ambient conditions. 2.3 Electroless plating of Ag-PTFE antibacterial composite coatings
ip t
The coatings were deposited on modified silicon rubber surface using electroless plating
cr
method under dark condition. The plating procedure was the same as reported before [17]. The silver ion solution was a mixture of AgNO3 (2.3g/l), NaOH (1.6g/l) and NH3·H2O
us
(50ml/l), reducing agent solution was made up of C6H12O6 (1.8g/l) and C4H6O6 (0.2g/l). PTFE
an
emulsion (10 ml/l) was added into reducing agent solution and stirred for 30 minutes to get a uniform mixture. The pre-treated silicon rubber sheets were then placed into the reaction vials
M
containing mixed silver ion solution and reducing agent solution, and kept standing for half an
dried in ambient conditions.
pt
2.4 Characterization
ed
hour. After plating, samples were taken out of the vial and rinsed with deionized water and
Ac ce
X-ray photoelectron spectroscopy (XPS) was performed to analyze the surface composition and chemical states by a PHI 1600 ESCA System under high vacuum environment of less than 5×10−9 torr. Spectra were produced with an Al anode at 250 W. The adhesion of the coating to the substrates was evaluated by a tape test according to ASTM D 3359-02 standard [21]. A sharp knife and tempered steel rule graduated in 0.5 mm for measuring individual cutes were used, and the cuts were made manually. A grid of parallel lines was made by the knife on the sample surface, and then cleaned by a brush. Afterward the tape was attached to the grid, then removed, the surface was finally observed with a lens for detecting coating damage. The adhesion was rated in accordance with scale illustrated in 5
Page 5 of 18
ASTM D 3395-02. Scanning electron microscopy (SEM) was carried out to reveal the composition and their distribution within the coatings on a FEI-quanta-200F equipped with energy dispersive X-ray spectrometer (EDS). The acceleration voltage was 30kV. Antibacterial behavior of the Ag-PTFE composite coatings was characterized by the
ip t
inhibition halo test toward two bacterial, Gram negative (Escherichia Coli) and Gram positive
cr
(Staphylococcus aureus) in accordance to standards of the National Committee for Clinical Laboratory (NCCLS) [22] and shake flask method. For inhibition halo test, the test bacteria
us
were cultured at 37°C by shaking in a nutrient broth (NB) overnight. The bacterial suspension
an
was diluted in NB to 1-5×107 cells/ml of test bacterial suspension. The inhibition halo test was performed as follows. The diluted bacteria solution was placed on NB agar culture
M
medium and the Ag-PTFE composite coatings deposited on modified silicon rubber were then
ed
placed on the NB agar. After 24h of cultivation, the width of inhibitory halo around the coatings was observed. For the shake flash test, the Ag-PTFE coatings were immersed in a 40
pt
ml PBS solution containing 400 μl of 1-5×107 cells/ml bacterial suspension. After the samples
Ac ce
were incubated for 8h at 37°C, the number of viable bacteria was counted. 3. Results and discussions
3.1 Coating characterization The surface analysis before and after grafting of coupling agent MPTS was carried out
by XPS (Fig.1). The silicon rubber substrates exhibited intense peaks of O1s, C1s and Si2p (Fig.1A), located at 533, 285 and 104eV, respectively. For the modified sample, the spectrum revealed the occurrence of the thiol characteristic doublet (S2p) at around 164 eV besides O1s, C1s and Si2p peaks (Fig. 1B and C). S2p peak attributed to MPTS which reacted with UV initiated –OH on modified silicon rubber, and cross-linked laterally through the formation of 6
Page 6 of 18
siloxane bonds forming a Si-O-Si network after the hydrolysis of methoxy groups. The results evidenced the grafting of MPTS onto the top surface of UV-irradiated silicon rubber. The EDS analysis of the Ag-PTFE composite coatings deposited on silicon rubber substrates confirmed the presence of Ag and F which revealed the coexistence of both Ag and
ip t
PTFE (Fig.2A). Due to the EDS volume resolution, S attributed to coupling agent MPTS was
cr
also detected and presented further testimony of the grafting of MPTS onto the substrates. And the EDS mapping showed that F was distributed uniformly, indicating the uniform
us
distribution of PTFE particles within coatings (Fig. 2B). The presence of S in the mapping
an
coincided with the EDS elemental analysis result, and it was also distributed uniformly (Fig.
3.2 Adhesion characterization
M
2D).
ed
The adhesion between the coatings and substrates was measured by tape test (ASTM D3359-02 standard [21]), and reported in Fig. 3. It could be concluded that coatings on
pt
modified substrates (Fig. 3B) presented well adhesion to the polymer. For silicon rubber
Ac ce
without modification, the coatings desquamated from the substrates after tape test and the adhesion was rated 1B. After the substrates were modified with UV irradiation and MPTS grafting, the coating displayed remarkably improved adhesion, the tape test did not induce any detectable damage on the coated samples and no sign of coating detachment was visible on the tape. Hence, the sample could be classified as 5B with 0% of damage according to the corresponding standard [21]. In our previous study, the tape test for coatings modified by UV and γ-APTS showed that small flakes were detached at intersection and less than 5% of the area was affected, the adhesion was rated 4B [20]. It was believed that the “anchor” played key role in increasing
adhesion. The results proved that the bridge role of MPTS attributed to the adhesion 7
Page 7 of 18
enhancement. The U V irradiation provided more reactive surface favoring the formation of MPTS self-assembly monolayer, and the Ag+ captured by S was accordingly bonded onto polymer surface firmly. 3.3 Antibacterial activity
ip t
The antibacterial activity of the prepared sample toward several bacterial species, both
cr
Gram positive (S.aureus) and Gram negative (E.coli), was evaluated through the inhibition halo test and the results are reported in Fig. 4. No halo zones are detected for silicon rubber
us
substrates, and for coatings with or without modification, both S.aureus and E.coli had clear
an
halo zones in the culture medium. In fact, although modification enhanced the adhesion of coatings dramatically, the halo test didn’t present significant changes as coatings on substrates with and without
antibacterial performance.
After being modified, silicon rubber maintained the superior
ed
adhesion and they didn’t peel off readily.
M
modifications had the same antibacterial agents. But modification endowed the coatings with better
pt
The antibacterial activity of the coatings against E.coli was also examined by shake flash
Ac ce
method. The presence of the original silicon rubber resulted in no clear decrease in bacteria numbers. But the coated samples were quite effective in reducing the bacteria count, after cultured 18-24 hours, the bacteria count was almost zero, meaning the antibacterial efficiency of about 99.99%.
4. Conclusion
UV irradiation is an effective way to activate the inert silicon rubber surface, and the grafting of silane coupling agent MPTS produces a reactive surface containing thiol group (-SH) which “anchor” Ag+ during electroless plating and so remarkably enhance the adhesion. 8
Page 8 of 18
MPTS acts like a bridge linking both substrates and coatings. Compared with amino in γ-APTS, thiol in MPTS displays intensive bond with Ag+, and stronger S-Ag than N-Ag led to significantly improved adhesion. In addition, the as-prepared coatings also have superior antimicrobial activity against both S.aureus and E.coli. With excellent adhesion and satisfied
ip t
antibacterial performance, implants plated with the composite Ag-PTFE coatings will be
cr
promising candidates in the clinical applications. Besides, the simple and cost-efficient
us
modification methods facilitate the industrialization accordingly. Acknowledgements
an
The authors acknowledge the financial support of the Program of International Science and Technology cooperation (2011DFA90830) and Natural Science Foundation of Shanxi,
References
ed
Scientist Office- CZB-4/441.
M
China (2012021008-4). The authors also acknowledge support by Scottish Executive Chief
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[1] X. Ding, C. Y., T. P. Lim, L. Yang, A. C. Engler, J. L. Hedrick, Y. Yang, Antibacterial and antifouling catheter coatings using surface grafted PEG-b-cationic polycarbonate diblock copolymer, Biomaterials 33 (2012) 6593-6603. [2] I. Fundeanua, H. C. van der Mei, A. J. Schouten, H. J. Busscher, Polyacrylamide brush coatings preventing microbial adhesion to silicone rubber, Colloid Surf. B-Biointerfaces 64 (2008) 297-301. [3] P. Kaali, E. Strömber, R. E. Aune, G. Czél, D. Momcilovic, S. Karlsson, Antimicrobial properties of Ag loaded zeolite polyester polyurethane and silicone rubber and long-term properties after exposure to in-vitro ageing, Polym. Degrad. Stabil. 95 (2010) 1456-1465. [4] R. O. Darouiche, Treatment of infections associated with surgical implants, N. Engl. J. Med. 350 (2004) 1422-1429. [5] R.O. Darouiche, Device-associated infections: a macroproblem that starts with microadherence, Clin. Infect. Dis. 33 (2001) 1567-1572. [6] B. Foxman, Epidemiology of urinary tract infections: incidence, morbidity, and economic costs, Am. J. Med. 113 (2002) 5-13. 9
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[7] I. Sawada, R. Fachrul, T. Ito, Y. Ohmukai, T. Maruyama, H. Matsuyama, Development of a hydrophilic polymer membrane containing silver nanoparticles with both organic antifouling and antibacterial properties, J. Membr. Sci. 387-388 (2012) 1- 6. [8] D.P. Dowling, K. Donnelly, M.L. McConnell, R. Eloy, M.N. Arnaud, Deposition of anti-bacterial silver coatings on polymeric substrates, Thin Solid Films 398-399 (2001) 602-606.
ip t
[9] C. Balagna, S. Perero, S. Ferraris, M. Miola, G. Fucale, C. Manfredotti, A. Battiato, D. Santella, E. Verne, E. Vittone, M. Ferraris, Antibacterial coating on polymer for space application, Mater. Chem. Phys. 135 (2012) 714-722.
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[10] P. Alvesa, S. Pintoa, J. Kaiserb, A. Bruininkb, H. C. de Sousaa, M.H. Gil, Surface grafting of a thermoplastic polyurethane with methacrylic acid by previous plasma surface activation and by ultraviolet irradiation to reduce cell adhesion, Colloid Surf. B-Biointerfaces 82 (2011) 371-377.
an
[11] A. Xing, Y. Gao , J. Yin, G. Ren, H. Liu, M. Ma, Preparation and atomic oxygen erosion resistance of silica film formed on silicon rubber by sol-gel method, Appl. Surf. Sci. 256 (2010) 6133-6138.
ed
M
[12] P. Alvesa, S. Pintoa, J. Kaiserb, A. Bruininkb, H. C. de Sousaa, M.H. Gil, Surface grafting of a thermoplastic polyurethane with methacrylic acid by previous plasma surface activation and by ultraviolet irradiation to reduce cell adhesion, Colloid Surf. B-Biointerfaces 82 (2011) 371-377.
pt
[13] F. Sinapi, T. Issakova, J. Delhalle, Z. Mekhalif, Study of (3-mercaptopropyl) trimethoxysilane reactivity on zinc: Comparison with organothiol and organosilane thin films, Thin Solid Films 515 (2007) 6833-6843.
Ac ce
[14] F. Sinapi, J. Delhalle, Z. Mekhalif, XPS and electrochemical evaluation of two-dimensional organic films obtained by chemical modification of self-assembled monolayers of (3-mercaptopropyl) trimethoxysilane on copper surfaces, Mater. Sci. Eng. C-Mater. Biol. Appl. 22 (2002) 345-353. [15] P. Pallavicini, A. Taglietti, G. Dacarro, Y. A. Diaz-Fernandez, M. Galli, P. Grisoli, M. Patrini, G. S. De Magistris, R. Zanoni, Self-assembled monolayers of silver nanoparticles firmly grafted on glass surfaces: Low Ag+ release for an efficient antibacterial activity, J. Colloid Interface Sci. 350 (2010) 110-116. [16] A. Baraket, N. Zine, M. Lee, J. Bausells, N. J. Renault, F. Bessueille, N. Yaakoubi, A. Errachid, Development of a flexible microfluidic system based on a simple and reproducible sealing process between polymers and poly(dimethylsiloxane), Microelectron. Eng. 27 (2013)1-7. [17] P. N. Floriano, O. Schlieben, E. E. Doomes, I. Klein, J. Janssen, J. Hormes, E.D. 10
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Poliakoff, R. L. McCarley, A grazing incidence surface X-ray absorption fine structure (GIXAFS) study of alkanethiols adsorbed on Au, Ag, and Cu, Chem. Phys. Lett. 321 (2000) 175-181. [18] Q. Zhao, Y. Liu, C. Wang. Development and evaluation of electroless Ag-PTFE composite coatings with anti-microbial and anti-corrosion properties, Appl. Surf. Sci. 252 (2005) 1620-1627.
ip t
[19] Q. Zhao, Effect of surface free energy of graded Ni-P-PTFE coatings on bacterial adhesion, Surf. Coat. Technol. 185 (2004) 199-204.
us
cr
[20] Ruijie Guo, Guangda Yin, Xiaojuan Sha, Liqiao Wei, Qi Zhao, Effect of surface modification on the adhesion enhancement of electrolessly deposited Ag-PTFE antibacterial composite coatings to polymer substrates, Mater. Lett. 143(2015) 256-260 [21] ASTM D3359-02 Standard Test Methods for Measuring Adhesion by Tape Test.
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pt
ed
M
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[22] NCCLS M2-A9, Performance Standard for Antimicrobial Disk Susceptibility Tests, Approved Standard, ninth ed., 2006.
11
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*Highlights (for review)
Highlights: The more effective coupling agent is employed to modify surface;
S-Ag displays more intensive bond strength than that of N-Ag;
The coatings possess the highest level of adhesion;
Ac
ce pt
ed
M
an
us
cr
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Page 12 of 18
Figure
4
B
6 4
O 2s
2
0
0
800
600
400
200
0
1,000
800
Binding Energy (eV)
950
C
Energy :164.380
850
C/S
400
200
0
us
900
800
700 650
165
164
163
162
161
M
166
an
750
600 167
600
Binding Energy(e/V)
cr
1,000
ip t
2
S 2p Si 2s Si 2p O 2s
4
Atomic % C 1s :51.9 O 1s :26.4 Si2p :21.3 S 2p : 0.4
O KLL
8
C 1s
O 1s
10
Si 2s Si 2p
6
O KLL
160
159
Binding Energy (e/V)
ce pt
ed
Fig. 1
Ac
C/S
8
× 10
C 1s
Atomic % C 1s :50.5 O 1s :26.0 Si2p :23.4
A 10
12
O 1s
×104
C/S
12
Page 13 of 18
Figure
A
B
ip t
b
D
Ac
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Fig. 2
an
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cr
C
Page 14 of 18
Figure
A
D
an
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C
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B
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Fig. 3
Page 15 of 18
Figure
A
10mm
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B
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10mm
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Fig. 4
Page 16 of 18
Figure captions
Scheme1. Schematic representation of UV modification mechanism and the preparation of antibacterial coatings covalently bonded onto substrates surface.
ip t
Fig.1 XPS survey spectra of (A) silicon rubber substrates and (B)
cr
substrates after grafting of coupling agent MPTS. The S2p peak in (B)
us
was amplified and shown in (C).
Fig.2 EDS analysis of Ag-PTFE composite coatings deposited on silicon
an
rubber substrates (A), and EDS mapping for F (B), Ag (C) and S (D).
M
Fig.3 Sample surface before (A, B) and after (C, D) the tape test, (A)
ed
coatings on substrates without modification, (B) coatings on modified substrates, (C) after tape test for sample (A) and after tape test for sample
ce pt
(B).
Fig. 4 Inhibition halo tests against S.aureus (A) and E.coli (B). (a)
Ac
coatings on unmodified substrates and (b) coatings on modified substrates.
Page 17 of 18
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Scheme 1
Page 18 of 18
S0169-4332(15)00447-X http://dx.doi.org/doi:10.1016/j.apsusc.2015.02.131 APSUSC 29811
To appear in:
APSUSC
Received date: Revised date: Accepted date:
20-10-2014 3-2-2015 22-2-2015
Please cite this article as: R. Guo, G. Yin, X. Sha, Q.Z. Liqiao Wei, H. Wang, The significant adhesion enhancement of Ag- polytetrafluoroethylene antibacterial coatings by using of molecular bridge, Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.02.131 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
The significant adhesion enhancement of Ag- polytetrafluoroethylene antibacterial coatings by using of molecular bridge Ruijie Guo*11 Guangda Yin1, Xiaojuan Sha1 Qi Zhao2 Liqiao Wei1 Huifang Wang1 (1Key Laboratory of Interface Science and Engineering in Advanced Materials (Taiyuan University of
ip t
Technology), Ministry of Education, Taiyuan 030024, China 2 Division of Mechanical Engineering and Mechatronics, University of Dundee, Dundee, DD1 4HN,
cr
UK)
Abstract
us
Weak adhesion between the metal-based antibacterial coatings and polymer substrates
an
limits their clinical applications, surface modification is an effective way to solve this intrinsic problem. In this study, UV irradiation was employed to activate the inert silicon rubber
M
substrates, and the grafting of coupling agent (3-Mercaptopropyl) trimethoxy silane into the UV-irradiated substrates generated reactive surface containing –SH groups. During electroless
ed
plating S which has lone pair electrons anchored Ag+ and produced antibacterial coatings with
pt
improved adhesion. The grafting of (3-Mercaptopropyl) trimethoxy silane into silicon rubber
Ac ce
was verified by X-ray photoelectron spectroscopy (XPS). The adhesion was tested by American Society of Testing Materials (ASTM D 3359-02). Surface elements content and distribution were observed and analyzed by X-ray energy disperse spectroscopy (EDS). The antibacterial performance was characterized by inhibition halo test and shake flash method. The results showed that the as-prepared composite Ag-polytetrafluoroethylene coatings possessed remarkably enhanced adhesion and superior antibacterial activity. Key words: Surface modification, coatings, adhesion, Ag-PTFE, antibacterial 1. Introduction 1
*Corresponding author. Tel: +86 13835189092, Fax: +86 351 6010311 Email: [email protected] 1
Page 1 of 18
Silicon rubber is a kind of commonly used biomaterial in biomedical devices and implants due to its flexibility, low toxicity, physiological inertness and biocompatibility [1]. However, microbes adhered to it and formed biofilm easily because of its special surface properties, and biomaterial-centered infections happened [2]. Once the biofilm formed, it is
ip t
very hard, or sometimes impossible, to remove it by washing or antibiotic treatment [3].
cr
Bacterial infections associated with implanted devices pose a significant threat to patients and a serious challenge to clinicians [4-6]. Antibacterial coatings deposited on the polymer surface
us
have been proved to be an effective way to discourage bacterial infections. As for the
an
antibacterial agent, it has long been agreed that silver has inherent antibacterial property, and the Ag coatings deposited on polymer surface could serve as an antibacterial layer to
M
minimize infection [7]. Among the coating technologies, electroless plating is a cost-effective
ed
alternative since no special equipment is required compared with other possible routes [8, 9]. Silicon rubber is lack of reactive groups, so it is difficult to bond firmly with metal-base
pt
antibacterial layer. The inertness often connects the applications of silicon rubber to the risk of
Ac ce
adhesion failure, leading to constrained clinical practice. Hence surface modification strategy becomes indispensable with the purpose to activate the inert surface and improve adhesion. UV irradiation is believed to be an effective way to modify polymer surface and generate a large number of active groups. One of the most important characteristics of UV irradiation is that it acts only on a thin surface layer, whereas the sample remains unchanged and the modified material keeps its chemical and mechanical properties [10]. As the newly generated active groups (such as hydroxy, peroxide and hydroperoxide ) are readily to disappear soon after the irradiation, silane coupling agents (3-mercaptopropyl) trimethoxysilane (MPTS) are employed to act as “molecular bridge” in the interface of substrates and metal antibacterial 2
Page 2 of 18
layer [11]. MPTS is a multifunctional organic molecule which is made of a thiol group (-SH) and a trimethoxysilane function (–Si(OCH3)3) [12]. The -SH group is designed to react with Ag+ and form a strong sulfur-Ag (S-Ag) interaction [13, 14], the trimethoxysilane function (-Si(OCH3)3) has grafting ability on active surface and cross-links through the formation of
ip t
siloxane bonds, forming a Si-O-Si network [12,15]. Such an organic molecular is accordingly
cr
expected to improve adhesion performances. MPTS was widely used in the aspect of modifying metal material interface [12, 14, 16], but relatively rare applications in the polymer
us
modification were reported.
an
Besides, polytetrafluoroethylene (PTFE) nanoparticles were incorporated in this study as PTFE has inherent non-stick properties due to its low surface energy and friction coefficient,
M
and so initial bacterial adhesion to a substratum surface is inhibited and biofilm formation is
[17,18,19].
ed
prevented, resulting in remarkable bacterial resistance as has been demonstrated before
pt
In this paper, the silicon rubber matrix was modified firstly by UV irradiation followed
Ac ce
by the grafting of coupling agent MPTS. This modification method relies on the “anchor” of Ag+ by S in MPTS. S has lone pairs of electrons for donation, it captured Ag+ which has unoccupied orbital, and so Ag-PTFE antibacterial composite coatings are covalently bonded onto silicon rubber, leading to dramatically improved metal/polymer adhesion (as shown in Scheme 1). In our previous study, we demonstrated that metal-based coatings were firmly bonded onto polymer substrates through coupling agent 3-amino propyl triethoxy silane (γ-APTS), and the adhesion up to 4B was obtained [20]. In this work, a more effective “molecular bridge” MPTS was chosen and presented significantly improved adhesion up to 5B, the highest level. S in MPTS has smaller electronegativity (2.58) than that of N in γ-APTS 3
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(3.04), that is, S-Ag was stronger than N-Ag, leading to dramatically enhanced adhesion. Our study showed that the “anchor” played key role with respect to the adhesion improvement, and thiol proved
more effective than amino. 2. Experimental
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2.1 Materials
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All the chemical reagents were analytically pure and used as received. MPTS (98.0%) and PTFE emulsion (60%) with particle size in the range of 0.05-0.5 μm were obtained from
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Aldrich. Silver nitrate (AgNO3, 99.8%), glucose (C6H12O6, 99%) and xylose (C4H6O6, 99%)
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were purchased from Tianjin Delan Fine Chemical Factory. Ethanol absolute (CH3CH2OH, 99.7%), acetone (C3H6O, 99.5%), ammonia solution (NH3·H2O, 25%), Hydrochloric acid
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(37.5%) and sodium hydroxide (NaOH, 96%) were supplied by Tianjin Chemical Reagent Co.
Ltd.
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Inc. Silicon rubber was provided by Jinan Chensheng Medical Silicon Rubber Product Co.
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2.2 Surface modification of silicon rubber
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The silicon rubber sheets with of 10 mm×10 mm×0.5 mm (for inhibition halo test, disc
shaped sheets with diameter of 10 mm) were chosen as substrates in our experiments, and were firstly cleaned with deionized water and acetone to remove smut, dirt and vegetable oil from the surface. They were then treated by using dilute HCl solution (1M) and NaOH solution (0.125M), followed by ultrasonic cleaning in alcohol and drying. Then the rubbers were placed into UV-lamp chamber and subjected to UV irradiation from both sides for about 15 minutes, followed by the immersion of the treated sheets into MPTS solution for 1 hour in order to bond the MPTS onto silicon rubber surface and create the reactive surface. The MPTS solution was a mixture of 10 ml of MPTS and 1300 ml of alcohol. Then the sheets 4
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were taken out of the solution and washed successively with acetone, alcohol and deionized water to remove the adsorbed coupling agent molecules completely. The modified samples were at last dried in ambient conditions. 2.3 Electroless plating of Ag-PTFE antibacterial composite coatings
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The coatings were deposited on modified silicon rubber surface using electroless plating
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method under dark condition. The plating procedure was the same as reported before [17]. The silver ion solution was a mixture of AgNO3 (2.3g/l), NaOH (1.6g/l) and NH3·H2O
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(50ml/l), reducing agent solution was made up of C6H12O6 (1.8g/l) and C4H6O6 (0.2g/l). PTFE
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emulsion (10 ml/l) was added into reducing agent solution and stirred for 30 minutes to get a uniform mixture. The pre-treated silicon rubber sheets were then placed into the reaction vials
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containing mixed silver ion solution and reducing agent solution, and kept standing for half an
dried in ambient conditions.
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2.4 Characterization
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hour. After plating, samples were taken out of the vial and rinsed with deionized water and
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X-ray photoelectron spectroscopy (XPS) was performed to analyze the surface composition and chemical states by a PHI 1600 ESCA System under high vacuum environment of less than 5×10−9 torr. Spectra were produced with an Al anode at 250 W. The adhesion of the coating to the substrates was evaluated by a tape test according to ASTM D 3359-02 standard [21]. A sharp knife and tempered steel rule graduated in 0.5 mm for measuring individual cutes were used, and the cuts were made manually. A grid of parallel lines was made by the knife on the sample surface, and then cleaned by a brush. Afterward the tape was attached to the grid, then removed, the surface was finally observed with a lens for detecting coating damage. The adhesion was rated in accordance with scale illustrated in 5
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ASTM D 3395-02. Scanning electron microscopy (SEM) was carried out to reveal the composition and their distribution within the coatings on a FEI-quanta-200F equipped with energy dispersive X-ray spectrometer (EDS). The acceleration voltage was 30kV. Antibacterial behavior of the Ag-PTFE composite coatings was characterized by the
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inhibition halo test toward two bacterial, Gram negative (Escherichia Coli) and Gram positive
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(Staphylococcus aureus) in accordance to standards of the National Committee for Clinical Laboratory (NCCLS) [22] and shake flask method. For inhibition halo test, the test bacteria
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were cultured at 37°C by shaking in a nutrient broth (NB) overnight. The bacterial suspension
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was diluted in NB to 1-5×107 cells/ml of test bacterial suspension. The inhibition halo test was performed as follows. The diluted bacteria solution was placed on NB agar culture
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medium and the Ag-PTFE composite coatings deposited on modified silicon rubber were then
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placed on the NB agar. After 24h of cultivation, the width of inhibitory halo around the coatings was observed. For the shake flash test, the Ag-PTFE coatings were immersed in a 40
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ml PBS solution containing 400 μl of 1-5×107 cells/ml bacterial suspension. After the samples
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were incubated for 8h at 37°C, the number of viable bacteria was counted. 3. Results and discussions
3.1 Coating characterization The surface analysis before and after grafting of coupling agent MPTS was carried out
by XPS (Fig.1). The silicon rubber substrates exhibited intense peaks of O1s, C1s and Si2p (Fig.1A), located at 533, 285 and 104eV, respectively. For the modified sample, the spectrum revealed the occurrence of the thiol characteristic doublet (S2p) at around 164 eV besides O1s, C1s and Si2p peaks (Fig. 1B and C). S2p peak attributed to MPTS which reacted with UV initiated –OH on modified silicon rubber, and cross-linked laterally through the formation of 6
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siloxane bonds forming a Si-O-Si network after the hydrolysis of methoxy groups. The results evidenced the grafting of MPTS onto the top surface of UV-irradiated silicon rubber. The EDS analysis of the Ag-PTFE composite coatings deposited on silicon rubber substrates confirmed the presence of Ag and F which revealed the coexistence of both Ag and
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PTFE (Fig.2A). Due to the EDS volume resolution, S attributed to coupling agent MPTS was
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also detected and presented further testimony of the grafting of MPTS onto the substrates. And the EDS mapping showed that F was distributed uniformly, indicating the uniform
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distribution of PTFE particles within coatings (Fig. 2B). The presence of S in the mapping
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coincided with the EDS elemental analysis result, and it was also distributed uniformly (Fig.
3.2 Adhesion characterization
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2D).
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The adhesion between the coatings and substrates was measured by tape test (ASTM D3359-02 standard [21]), and reported in Fig. 3. It could be concluded that coatings on
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modified substrates (Fig. 3B) presented well adhesion to the polymer. For silicon rubber
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without modification, the coatings desquamated from the substrates after tape test and the adhesion was rated 1B. After the substrates were modified with UV irradiation and MPTS grafting, the coating displayed remarkably improved adhesion, the tape test did not induce any detectable damage on the coated samples and no sign of coating detachment was visible on the tape. Hence, the sample could be classified as 5B with 0% of damage according to the corresponding standard [21]. In our previous study, the tape test for coatings modified by UV and γ-APTS showed that small flakes were detached at intersection and less than 5% of the area was affected, the adhesion was rated 4B [20]. It was believed that the “anchor” played key role in increasing
adhesion. The results proved that the bridge role of MPTS attributed to the adhesion 7
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enhancement. The U V irradiation provided more reactive surface favoring the formation of MPTS self-assembly monolayer, and the Ag+ captured by S was accordingly bonded onto polymer surface firmly. 3.3 Antibacterial activity
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The antibacterial activity of the prepared sample toward several bacterial species, both
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Gram positive (S.aureus) and Gram negative (E.coli), was evaluated through the inhibition halo test and the results are reported in Fig. 4. No halo zones are detected for silicon rubber
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substrates, and for coatings with or without modification, both S.aureus and E.coli had clear
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halo zones in the culture medium. In fact, although modification enhanced the adhesion of coatings dramatically, the halo test didn’t present significant changes as coatings on substrates with and without
antibacterial performance.
After being modified, silicon rubber maintained the superior
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adhesion and they didn’t peel off readily.
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modifications had the same antibacterial agents. But modification endowed the coatings with better
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The antibacterial activity of the coatings against E.coli was also examined by shake flash
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method. The presence of the original silicon rubber resulted in no clear decrease in bacteria numbers. But the coated samples were quite effective in reducing the bacteria count, after cultured 18-24 hours, the bacteria count was almost zero, meaning the antibacterial efficiency of about 99.99%.
4. Conclusion
UV irradiation is an effective way to activate the inert silicon rubber surface, and the grafting of silane coupling agent MPTS produces a reactive surface containing thiol group (-SH) which “anchor” Ag+ during electroless plating and so remarkably enhance the adhesion. 8
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MPTS acts like a bridge linking both substrates and coatings. Compared with amino in γ-APTS, thiol in MPTS displays intensive bond with Ag+, and stronger S-Ag than N-Ag led to significantly improved adhesion. In addition, the as-prepared coatings also have superior antimicrobial activity against both S.aureus and E.coli. With excellent adhesion and satisfied
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antibacterial performance, implants plated with the composite Ag-PTFE coatings will be
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promising candidates in the clinical applications. Besides, the simple and cost-efficient
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modification methods facilitate the industrialization accordingly. Acknowledgements
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The authors acknowledge the financial support of the Program of International Science and Technology cooperation (2011DFA90830) and Natural Science Foundation of Shanxi,
References
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Scientist Office- CZB-4/441.
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China (2012021008-4). The authors also acknowledge support by Scottish Executive Chief
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[7] I. Sawada, R. Fachrul, T. Ito, Y. Ohmukai, T. Maruyama, H. Matsuyama, Development of a hydrophilic polymer membrane containing silver nanoparticles with both organic antifouling and antibacterial properties, J. Membr. Sci. 387-388 (2012) 1- 6. [8] D.P. Dowling, K. Donnelly, M.L. McConnell, R. Eloy, M.N. Arnaud, Deposition of anti-bacterial silver coatings on polymeric substrates, Thin Solid Films 398-399 (2001) 602-606.
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[11] A. Xing, Y. Gao , J. Yin, G. Ren, H. Liu, M. Ma, Preparation and atomic oxygen erosion resistance of silica film formed on silicon rubber by sol-gel method, Appl. Surf. Sci. 256 (2010) 6133-6138.
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[12] P. Alvesa, S. Pintoa, J. Kaiserb, A. Bruininkb, H. C. de Sousaa, M.H. Gil, Surface grafting of a thermoplastic polyurethane with methacrylic acid by previous plasma surface activation and by ultraviolet irradiation to reduce cell adhesion, Colloid Surf. B-Biointerfaces 82 (2011) 371-377.
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[13] F. Sinapi, T. Issakova, J. Delhalle, Z. Mekhalif, Study of (3-mercaptopropyl) trimethoxysilane reactivity on zinc: Comparison with organothiol and organosilane thin films, Thin Solid Films 515 (2007) 6833-6843.
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[14] F. Sinapi, J. Delhalle, Z. Mekhalif, XPS and electrochemical evaluation of two-dimensional organic films obtained by chemical modification of self-assembled monolayers of (3-mercaptopropyl) trimethoxysilane on copper surfaces, Mater. Sci. Eng. C-Mater. Biol. Appl. 22 (2002) 345-353. [15] P. Pallavicini, A. Taglietti, G. Dacarro, Y. A. Diaz-Fernandez, M. Galli, P. Grisoli, M. Patrini, G. S. De Magistris, R. Zanoni, Self-assembled monolayers of silver nanoparticles firmly grafted on glass surfaces: Low Ag+ release for an efficient antibacterial activity, J. Colloid Interface Sci. 350 (2010) 110-116. [16] A. Baraket, N. Zine, M. Lee, J. Bausells, N. J. Renault, F. Bessueille, N. Yaakoubi, A. Errachid, Development of a flexible microfluidic system based on a simple and reproducible sealing process between polymers and poly(dimethylsiloxane), Microelectron. Eng. 27 (2013)1-7. [17] P. N. Floriano, O. Schlieben, E. E. Doomes, I. Klein, J. Janssen, J. Hormes, E.D. 10
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Poliakoff, R. L. McCarley, A grazing incidence surface X-ray absorption fine structure (GIXAFS) study of alkanethiols adsorbed on Au, Ag, and Cu, Chem. Phys. Lett. 321 (2000) 175-181. [18] Q. Zhao, Y. Liu, C. Wang. Development and evaluation of electroless Ag-PTFE composite coatings with anti-microbial and anti-corrosion properties, Appl. Surf. Sci. 252 (2005) 1620-1627.
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[19] Q. Zhao, Effect of surface free energy of graded Ni-P-PTFE coatings on bacterial adhesion, Surf. Coat. Technol. 185 (2004) 199-204.
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[20] Ruijie Guo, Guangda Yin, Xiaojuan Sha, Liqiao Wei, Qi Zhao, Effect of surface modification on the adhesion enhancement of electrolessly deposited Ag-PTFE antibacterial composite coatings to polymer substrates, Mater. Lett. 143(2015) 256-260 [21] ASTM D3359-02 Standard Test Methods for Measuring Adhesion by Tape Test.
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[22] NCCLS M2-A9, Performance Standard for Antimicrobial Disk Susceptibility Tests, Approved Standard, ninth ed., 2006.
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*Highlights (for review)
Highlights: The more effective coupling agent is employed to modify surface;
S-Ag displays more intensive bond strength than that of N-Ag;
The coatings possess the highest level of adhesion;
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Figure
4
B
6 4
O 2s
2
0
0
800
600
400
200
0
1,000
800
Binding Energy (eV)
950
C
Energy :164.380
850
C/S
400
200
0
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900
800
700 650
165
164
163
162
161
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166
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750
600 167
600
Binding Energy(e/V)
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1,000
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2
S 2p Si 2s Si 2p O 2s
4
Atomic % C 1s :51.9 O 1s :26.4 Si2p :21.3 S 2p : 0.4
O KLL
8
C 1s
O 1s
10
Si 2s Si 2p
6
O KLL
160
159
Binding Energy (e/V)
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ed
Fig. 1
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C/S
8
× 10
C 1s
Atomic % C 1s :50.5 O 1s :26.0 Si2p :23.4
A 10
12
O 1s
×104
C/S
12
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Figure
A
B
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b
D
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Fig. 2
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C
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Figure
A
D
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C
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B
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Fig. 3
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Figure
A
10mm
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B
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10mm
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Fig. 4
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Figure captions
Scheme1. Schematic representation of UV modification mechanism and the preparation of antibacterial coatings covalently bonded onto substrates surface.
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Fig.1 XPS survey spectra of (A) silicon rubber substrates and (B)
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substrates after grafting of coupling agent MPTS. The S2p peak in (B)
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was amplified and shown in (C).
Fig.2 EDS analysis of Ag-PTFE composite coatings deposited on silicon
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rubber substrates (A), and EDS mapping for F (B), Ag (C) and S (D).
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Fig.3 Sample surface before (A, B) and after (C, D) the tape test, (A)
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coatings on substrates without modification, (B) coatings on modified substrates, (C) after tape test for sample (A) and after tape test for sample
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(B).
Fig. 4 Inhibition halo tests against S.aureus (A) and E.coli (B). (a)
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coatings on unmodified substrates and (b) coatings on modified substrates.
Page 17 of 18
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Scheme 1
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