Durable antimicrobial finishing of cellulose with QSA silicone by supercritical adsorption

Applied Surface Science 264 (2013) 171–175

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Durable antimicrobial finishing of cellulose with QSA silicone by supercritical adsorption Yong Chen a,∗ , Mengqi Niu b , Shu Yuan b , Hongni Teng a a b

Department of Applied Chemistry, College of Chemical & Environmental Engineering, Shandong University of Science and Technology, Qingdao 266590, PR China Department of Chemical Engineering, College of Chemical & Environmental Engineering, Shandong University of Science and Technology, Qingdao 266590, PR China

a r t i c l e

i n f o

Article history: Received 4 August 2012 Received in revised form 28 September 2012 Accepted 29 September 2012 Available online 16 October 2012 Keywords: QAS silicone Antibacterial coating Supercritical adsorption

a b s t r a c t This study demonstrated a generic and simple approach to generate durable antibacterial ability on cellulose without using covalently bonding tethering groups that limit the structure design. CO2 -philic silicone with quaternary ammonium salt (QAS) pendants was synthesized through hydrosilylation reaction of poly(methylhydrosiloxane) (PMHS) and 2-(dimethylamino)ethyl acrylate in the presence of platinumbased catalyst and subsequent quaternization with 1-bromohexane. The resultant QAS silicone was deposited onto cellulose by adsorption from supercritical CO2 (scCO2 ) to provide potent biocidal activities against Staphylococcus aureus and Escherichia coli. Presented data also showed that the antibacterial layer was very stable toward washing and UV irradiation owning to the low surface tension and relatively high bond energy of the backbone of silicone. This procedure is applicable to substrates of other shape and chemistry. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Well controlled and characterized modification is a necessary precondition to generate desired interfacial properties. Therefore, surface modification and functionalization are critical to numerous applications [1]. For instance, antibacterial coating surfaces of substrates is an important way to prevent infections [2–4]. Chemicals with QAS groups have been known to inhibit the growth of many microorganisms by interaction with the cell membrane, which leads to protein inactivation. In addition, QAS groups cause a loss of the replication ability of microorganisms through influencing their DNA. As a result, microorganisms are killed due to the lost of biological functions related to the cell membrane and DNA including transport, product and proliferation [5]. Therefore, QAS compounds are widely used biocides that have been incorporated into ordinary materials to achieve biocidal functions [6–13]. The current approaches to attach antibacterial agents containing QAS groups to materials can be classified into physical and chemical methods. Physical method uses weak forces including hydrogen bonding, physisorption, and electrostatic attraction to absorb antibacterial agents to substrates [14]. This modification of materials is therefore restricted because of the short-term effectiveness due to the release of the antimicrobial agents [15,16]. Chemical method employs covalent bonds to tether QAS compounds to surface reactive groups of substrates through chemical

∗ Corresponding author. E-mail address: [email protected] (Y. Chen). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.09.165

reactions. Although chemical tethering provides durable biocidal function, the number of chemical reaction steps involved to generate biocidal surfaces increases the difficulty in processing [17]. Also, it is not always that surfaces of substrates have or can be generated required functional groups for desired covalent conjugations. In addition, both physical and chemical modifications are conventionally performed in liquid solvents that cause contamination and occasional decomposition of materials [18]. It is therefore highly desirable to develop a “universal” coating method that is generally applicable to many polymers and can be readily adapted to current manufacturing technologies. Self-assembly of polymers with functional group can form various structures to achieve multiple applications. Here the term functional group is used generically to indicate any chemical moiety that brings about a desired function [19–24]. When coated on materials, functional polymers order at surfaces and generate desirable interfacial properties [25,26]. The procedure produced durable surface properties yet without the need for covalently bonding the functional polymers to the substrates. Part of the functional polymer anchors into materials to form stable interpenetration, while the functional groups order at surfaces of substrates to provide required interfacial properties. The functional polymers are easily spin-coated onto substrates. However, the primary shortcoming of spin-coating is the difficulty in recycling residual solvent leading to environmental and safety concerns. In addition, spin-coating can only create thin films on small and flat substrates. It has been shown functional polymers could be coated onto substrates of arbitrary shape by adsorption from supercritical fluids the main component of which is CO2 [27,28]. This modification method uses

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nonflammable, nontoxic, and inexpensive CO2 as an alternative to conventional solvents. The added benefits of the scCO2 coating process include the dissolving power of scCO2 is simply adjusted by varying its pressure and temperature, the critical point of scCO2 is readily accessible, and excess functional polymer precipitates out after deposition and is recycled without complex recovery steps. In the example presented here, antibacterial QAS silicone was synthesized and coated on cellulose to offer durable biocidal activities by adsorption from scCO2 . Cellulose is used as our research substrate because it is extensively used in everyday life and scientific research. The use of QAS silicone as the functional polymer is because silicones are highly soluble in scCO2 [29,30], hydrophobic to provide stable biocidal function toward washing, widely employed in industry due to their desirable properties including high gas permeability, low toxicity, and excellent stability. This coating procedure is applicable to materials of other chemistry and shape since it has no such special requirements for substrates. 2. Experimental 2.1. Materials Poly(methylhydrosiloxane) (Mn: 3.1 × 103 , Mw/Mn = 2.5, hydrogen content = 1.55 wt%) was purchased from Cantochina. Ethanol, sodium thiosulfate, phosphate buffered saline, and toluene were purchased from Jinke Company. 2-(dimethylamino)ethyl acrylate and 1-bromohexane were purchased from Haorong Chemical Reagent Co., Ltd. Platinum(0)-1,3-divinyl-1,1,3,3tetramethyldisiloxane complex was purchased from SSMMM Co., Ltd. Both Escherichia coli and Staphylococcus aureus were purchased from Guangzhou Industry Microbe Test Center. 2.2. Synthesis of QAS silicone The hydrosilylation reaction was carried out using standard Schlenk techniques. The PMHS and 2-(dimethylamino)ethyl acrylate were placed into the reaction vessel in the mole ratio of 1:1.5 (related to Si–H group) and dissolved in dried and degassed toluene. Then, platinum-based catalyst was added in such an amount to meet the ratio of 10−5 mol per 1 mol Si–H. The Schlenk tube was sealed and degassed by three pump-thaw cycles, and then filled with nitrogen to prevent the system from oxidation. Under continuous stirring, the Schlenk tube was placed in a thermostat bath that was heated to 110 ◦ C and subjected to reflux for 48 h. Solvent and unreacted 2-(dimethylamino)ethyl acrylate were removed on a rotary evaporator after reaction to yield a slight yellow liquid. The conversion of the hydrosilylation reaction was calculated to be 60 wt%. The antibacterial precursor was quaternaried with 1bromohexane at 70 ◦ C for two hours to get the antibacterial QAS silicone. The yield of quaternization was estimated to be 71% by measuring the bromine content with Volhard titration method [31]. The overall synthesis procedure is schematically shown in Fig. 1.

can be coated on cotton to achieve different biocidal efficiency by varying its solubility that is controlled by the operation pressure and temperature. However, the primary focus of this research is to test previously mentioned advantages of the coating procedure and the biocidal polysiloxane, so the detailed biocidal efficiency as a function of solubility of QAS silicone was not studied.

2.4. Biocidal function assessment All of the samples were dipped in ethanol/water (70/30, v/v) for five seconds and dried in a sterile hood before antimicrobial testing. No colonies were observed after that, so the ethanol treatment was enough to remove or kill microbes. Gram-negative E. coli and Grampositive S. aureus were used to challenge the antibacterial functions of the QAS silicone coated samples according to a “sandwich test” [32]. Briefly, E. coli and S. aureus were grown in broth solutions for 24 h at 37 ◦ C. The bacteria were harvested by centrifuge, washed with phosphate buffered saline, and then resuspended in PBS to densities of 108 –109 CFU/mL. A total of 100 ␮L of the freshly prepared bacterial suspension was added to the middle of two pieces of QAS silicone coated samples (2.5 cm2 per swatch). After a certain period of contact time, the swatches were transferred into 10 mL of sterilized sodium thiosulfate solution (0.03 wt%) and vortexed for 2 min. The viable cell concentrations were measured using the serial dilution and spread plate technique. Uncoated cotton swatches were tested under the same conditions as controls.

2.5. Washing stability test AATCC 61-1996 method (Test 2A procedure) was used to evaluate durability of the coatings toward repeated washing cycles. In this method, 2.54 × 5.08 cm cotton swatches were subjected to repeated laundry washings inside stainless steel canisters in which 50 stainless steel balls were added with 150 mL of 0.15% AATCC detergent water solution at 49 ◦ C. One washing cycle is equivalent to five machine washings. After washing, samples were rinsed with distilled water three times and allowed to dry at ambient temperature. The samples with antibacterial coating were then titrated to determine the density of QAS group by measuring the bromine content with Volhard titration method [31].

2.6. UV light stability test UV light stability of the QAS silicone coating on cotton swatches was measured using UV-light produced by a Q8 model Accelerated Weathering Tester (Hongzhan Group) (20 ◦ C, 40 W, 25% relative humidity, 340 nm). After exposure to UV irradiation for a specific time, the density of QAS was determined using Volhard titration method to evaluate the UV stability of the coating layer.

2.3. Coating procedure

2.7. Instrumentation

QAS silicone and a magnetic stir were placed into a glass vial to avoid direct contact with cotton swatches. The glass vial and cotton swatches were placed inside a high pressure cell with a design pressure of 30 MPa and an internal volume of 100 ml. The whole system was flushed the air out with CO2 for 1 min and then was charged with CO2 to the 25 MPa and maintained at that pressure and 50 ◦ C for three hours before releasing the scCO2 . The experimental time was selected because the coating results did not change measurably after an incubation time of two hours. The temperature was chosen because silicones have maximum solubility at this temperature [29]. It is naturally expected that other amount of QAS silicone

X-ray photoelectron spectroscopy (XPS) spectra were recorded with a PHI 5300 spectrometer equipped with an electron energy analyzer, a multichannel detector, and an Al K␣ monochromator radiation source. The working pressure of the instrument was maintained below 1 × 10−6 Pa and a low energy electron flood gun was used to neutralize surface charging. Spectra were obtained at 45◦ takeoff angle with respect to the plane of the holder surface using an analyzer pass energy of 93.9 eV and a binding energy (BE) resolution of 0.8 eV. A Nicolet Magna IR-560 Fourier Transform Infrared Spectrometer was used for IR measurements.

Y. Chen et al. / Applied Surface Science 264 (2013) 171–175

CH3

CH3 Si

O m

H

C O O

Catalyst

Si

173

CH3

Br(CH2)5CH3 O

Si

O

m

m

C O

(CH2)2N(CH3)2

C O

O

O Br

(CH2)2N(CH3)2

(CH2)2N(CH3)2 (CH2)5CH3

Fig. 1. Schematic procedure for synthesis of silicone with QAS pendants.

Cotton fibers before and after coating in scCO2 were sputter coated with gold and then subjected to image analysis using a KYKY-2800B scanning electron microscope (SEM). 3. Results and discussion The synthesis of the antibacterial QAS silicone was characterized by FT-IR study. IR spectra of PMHS, antibacterial precursor produced by hydrosilylation, and QAS silicone synthesized by quaternization the antibacterial precursor with 1-bromohexane are shown in Fig. 2. Wherein, IR spectrum of PMHS shows strong peaks at 2168 and 844 cm−1 for Si–H stretching and bending vibrations, respectively. After hydrosilylation reaction these two peaks decreases significantly due to the hydrosilylation reaction between Si–H groups of PMHS with C C groups of 2-(dimethylamino)ethyl acrylate as shown in Fig. 2(b). In addition, Fig. 2(b) shows typical absorption bands at 2821 and 2771 cm−1 that are assigned to asymmetric and symmetric stretches of the N (CH3 )2 . In addition, in the carbonyl region a signal from the C O stretching vibration is found at 1735 cm−1 . Following quaternization the stretching and deformation vibrations of C H at around 2980 and 1461 cm−1 strengthen caused by the introducing of the hexyl chain as shown in Fig. 2(c). Besides, the specific absorptions of N (CH3 )2 stretches at 2821 and 2771 cm−1 have disappeared, indicating the conversion of the tertiary amines to quaternary ammonium compounds. Synthesized QAS silicone was then coated on cellulose by adsorption from scCO2 . Successful coating of QAS silicone on cotton swatches is ascertained by comparing the XPS spectra of the cotton surfaces before and after the scCO2 adsorption process as shown in Fig. 3. The XPS survey spectrum of coated cotton exhibits new peaks besides C1s and O1s that associate with different elements

originating from the QAS silicone. The peaks are assigned to Si 2p at around 101 eV, Si 2s at about 153 eV, Br 3p at about 182 eV, Br 3d at about 70 eV, and N1s at around 402 eV, respectively. However, these peaks are not present in the spectrum of the original cotton samples that are composed of only carbon and oxygen. Therefore, XPS scans show that the antibacterial silicone is coated onto the cotton fibers successfully. The density of QAS group was estimated to be 3.3 × 1017 cm−2 by measuring the bromine content with Volhard titration method. It is expected that different thicknesses of QAS silicone layers can be created by varying the operation parameters such as temperature and pressure. Thicker coating is desirable for antibacterial function since it is more resistant to washing, UV irradiation, and leaching process. It has been shown that different thicknesses of antibacterial layers in the range from several to over 100 nm could be generated by methods including grafting from, grafting onto, and surface-initiated ATRP [9–11]. In this study, the thickness of the coating was about 48 nm by using the following equation: h=

d(W1 − W0 )C 4W0 Q

where h is the thickness of the coating layer, d is the diameter of a fiber, W1 and W0 are weights of the fiber before and after coating, respectively, C (1.55 g/cm3 ) and Q (1.0 g/cm3 ) are densities of cellulose and QAS silicone, respectively. Fig. 4 shows SEM images of cotton fibers before and after coating process. The coated fiber was coarser than the coated one, indicating the existence of QAS silicone coating. Functionality of the coating layer is critical to the applications of substrates. The antibacterial efficacies of control samples (uncoated cotton swatches) and antibacterial samples (QAS silicone coated

4000

300

C1s

O1s

(c) 250 200

1461

Intensity (CPS)

Transmittance (%)

3000 2980 (b) 150 2821

2771 1735

100 (a)

N1s

(a) 2000

Br3p

Si2p

Br3d

1000

50

(b)

2168 0 4000

Si2s

844 3500

3000

2500

2000

1500

1000

500

-1

Wavenumbers (cm ) Fig. 2. FT-IR spectra of: (a) PMHS, (b) antibacterial precursor and (c) OAS silicone.

0 1000

800

600

400

200

0

Binding Energy (eV) Fig. 3. XPS spectra of QAS silicone coated cotton (a) and original cotton (b).

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Table 1 Antibacterial efficacies of the QAS silicone coated and control cotton swatches (2.5 cm × 2.5 cm) against S. aureus and E. coli. Material

Contact time (min)

Log reduction of E. coli

Log reduction of S. aureus

Control samples

30 60

0.91 1.05

0.85 1.20

Coated samples

30 60

5.21 7.14

5.85 7.17

S. aureus and E. coli at an inoculum population of 1.5 × 107 and 1.4 × 107 CFU, respectively.

25

12 10

20

8

15

6 10 4 5

2 0 0.8

1.2

1.6

2.0

2.4

2.8 17

3.2

UV Irradiation Time (h)

Number of Washing Cycles

cotton swatches) were tested by challenging with S. aureus and E. coli. It was observed that QAS silicone coated swatches provided complete inactivation of S. aureus within 60 min of contact time. However, the control sample showed only a 1.2 log reduction after 60 min contact time, most likely due to the natural death and the adhesion of the bacteria to the cotton surface. A similar result was obtained for E. coli. Control samples exhibited a limited degree of bacterial reduction, whereas the QAS silicone coated swatches

0 3.6

2

Density of QAS (10 /cm ) Fig. 5. Remained QAS as a function of washing cycles () and UV irradiation (䊉), respectively.

provided to a total kill in 1 h. The results of the biocidal activities for both control and antibacterial samples are summarized in Table 1. The final characterization of this research is to evaluate the sustainability of the coatings toward the most likely damages that are washing and UV irradiation. QAS groups can be transformed into tertiary amines and some chemical bonds of QAS silicone can undergo decomposition during the washing and UV irradiation. The silicone coating in this study should be sustainable theoretically, since the backbone of the QAS silicone is composed of repeat Si O Si that has a relatively high bond energy to relatively withstand UV irradiation and a very low surface tension to ensure resistance to washing process. Repeated washing cycles and UV irradiation were performed to evaluate the hypothesis. After designed experiments, samples with antibacterial coating were rinsed with distilled water three times, allowed to dry at ambient temperature, and then measured the density of QAS by titration to evaluate the stability of the coating. The loss of QAS increases with washing cycles and irradiation time is shown in Fig. 5. However, QAS coatings via the scCO2 deposition technique exhibited excellent stability and durability: all of the samples would still be antimicrobial after 50 machine washings or 24 h irradiation since they all still had more than 5 × 1015 QAS/cm2 remaining [10]. Table 2 Antibacterial efficacies of the QAS silicone coated cotton samples (2.5 cm × 2.5 cm) after washing and UVA irradiation against S. aureus and E. coli.

Fig. 4. SEM images of uncoated cotton fiber (a) and QAS silicone coated cotton fiber (b).

Material

Minimum contact time for a total kill of E. coli

Minimum contact time for a total kill of S. aureus

Coated Samples after 50 machine washing Coated Samples after 24 h UV irradiation

120 min

90 min

105 min

90 min

S. aureus and E. coli at an inoculum population of 9.0 × 106 and 1.0 × 107 CFU, respectively.

Y. Chen et al. / Applied Surface Science 264 (2013) 171–175

175

quaternization. Subsequent experiments showed that the coating provided potent biocidal functions and was stable toward repeated washing cycles and UV irradiation. The relatively benign scCO2 can deliver the dissolved CO2 -philic antibacterial polymer to any outer or inner surfaces including tiny pores where liquid solutions cannot arrive due to the capillary force. Acknowledgements This project is sponsored by doctoral start-up fund of Shandong University of Science and Technology under contract number 011130226 and by SRF for ROCS, SEM. References

Fig. 6. Morphologies of (a) coated sample after 50 machine washings and (b) coated sample after 24 h UVA irradiation.

The actual antibacterial efficiencies are listed in Table 2. Various antibacterial abilities can be obtained by changing the solubility of the biocidal silicone in scCO2 . The morphologies of the coating surface at the ends of washing and UV irradiation are shown in Fig. 6. Several fibers were shown in one image to best reflect the uneven damage of the coating due to the roughness of the cotton swathes. It was observed that more coating was lost from some fibers that were smoother. 4. Conclusions It was demonstrated that stable antibacterial layer could be formed by an easy and efficient scCO2 adsorption method without the need for the covalent tethering groups. CO2 -philic QAS silicone was synthesized for this purpose by reactions of hydrosilylation and

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