Characterization and Mechanism for the Protection of Photolytic

Jan 29, 2016 - Department of Chemistry, University of Miami, Coral Gables, Florida 33146, United States .... according to the standard AATCC Test Meth...
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Characterization and Mechanism for the Protection of Photolytic Decomposition of N‑Halamine Siloxane Coatings by Titanium Dioxide Ying Liu,† Jing Li,† Lin Li,† Stuart McFarland,‡ Xuehong Ren,*,† Orlando Acevedo,*,‡ and T. S. Huang§ †

Key Laboratory of Eco-textiles of Ministry of Education, College of Textiles and Clothing, Jiangnan University, Wuxi 214122, Jiangsu, China ‡ Department of Chemistry, University of Miami, Coral Gables, Florida 33146, United States § Department of Poultry Science, Auburn University, Auburn, Alabama 36849, United States S Supporting Information *

ABSTRACT: N-Halamine antibacterial materials have superior inactivation activities due to oxidative chlorine species. However, N−Cl bonds and bonds between N-halamine and substrates often decompose rapidly under UV irradiation, leading to unrecoverable loss of antimicrobial activity. In this study, titanium dioxide was covalently bonded onto Nhalamine siloxane poly[5,5-dimethyl-3-(3′triethoxysilylpropyl)hydantoin] (PSPH) via a sol−gel process. Experimental testing of the chlorinated cotton fabrics treated with TiO2/PSPH demonstrated that the residual oxidative chlorine in cotton-TiO2/PSPH-Cl was still effective for inactivating bacteria after 50 washing cycles and under UV light irradiation for 24 h. Quantum mechanical calculations found that TiO2 improves the UV stability of the PSPH-Cl system by increasing the activation barrier of the C−Si scission reaction responsible for the loss of the biocidal hydantoin moiety. SEM, XPS and FTIR spectra were used to characterize the coated cotton samples. Cotton-TiO2/PSPH-Cl samples exhibited good antibacterial activity against Staphylococcus aureus (ATCC 6538) and Escherichia coli O157:H7 (ATCC 43895). The storage stability and washing stability of treated cotton fabrics were also investigated. KEYWORDS: N-halamine siloxane, titanium dioxide, UV stability, antibacterial, inorganic/organic hybrid, DFT calculations

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INTRODUCTION Cross contamination of harmful bacteria and pathogenic viruses continues to be a serious worldwide health concern. Cellulosic materials are easily contaminated with bacteria and other pathogenic microorganisms at suitable temperatures and humidity and, as such, cellulosic materials with antibacterial property are highly desirable. N-Halamine,1−3 metal oxides and metal ions,4,5 and quaternary ammonium salts6,7 have been widely used to inhibit the breed and spread of pathogenic organisms. Among all of these mentioned biocides, N-halamine compounds have effective antibacterial efficacy against a wide spectrum of Gram-positive and Gram-negative bacteria, e.g., 6− 7 logs of bacteria could be totally inactivated within 30 min.8,9 In addition, the antimicrobial activity of N-halamines can be recharged, and it has been reported that some kinds of amide N-halamine compounds are nontoxic and not irritant to rabbit skin.10 However, the N−Cl bonds in siloxane compounds can readily decompose under UV light irradiation, resulting in the cleavage of N-halamine functional groups from substrates that leads to unrecoverable loss of antibacterial activity.11,12 Previous efforts have been undertaken to improve UV stability of N-halamine compounds in order to extend the life of © XXXX American Chemical Society

the antibacterial materials. For example, Sandstrom and Sun reported that N-halamine compounds with an aromatic structure could absorb UV light and inhibit the decomposition of N−Cl bonds.13 However, these aromatic compounds could not bear the UV light stability test conducted in a weathering chamber, and the lost chlorine could not be restored. Simple modifications of N-halamine compounds failed to enhance substantially the UV stability of these compounds. Research efforts in our lab focus upon improving UV stability of Nhalamine compounds via the incorporation of the UV absorption agent titanium dioxide. Nanotitania particles were introduced to solution of N-halamine diol and coated onto cotton with the help of 1,2,3,4-butanetetracarboxylic acid (BTCA) via a pad-dry-cure process. The UV stability of Nhalamine diol increased by 14%, whereas the tensile strength decreased. 14 Poly[5,5-dimethyl-3-(3′-triethoxysilylpropyl)hydantoin] (PSPH) and commercial titania nanoparticles were coated onto cotton in a one-bath process, and N−Cl Received: December 23, 2015 Accepted: January 21, 2016

A

DOI: 10.1021/acsami.5b12601 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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After refluxing for 5 h, PSPH was obtained by the removal of ethanol and residual water. Coating Procedures. Nanotitanium dioxide sol−gel solution was prepared according to the method reported by others.17,18 Briefly, 0.02 mol of TIP and 0.01 mol of triethylamine were adequately mixed in 50 mL of isopropyl alcohol, and vigorously stirred for 2−3 min under nitrogen protection. This solution was added into 50 mL of isopropyl alcohol containing a certain amount of deionized water and hydrochloric acid. The mixture was vigorously stirred for 30 min under nitrogen protection. A transparent titanium dioxide sol−gel solution was obtained after the reaction completed. The solution was stable under storage for 2 h. The particle size of the formed titanium dioxide was 30−40 nm. 3%−5% PSPH and equimolar deionized water were added to the above-formed titanium dioxide sol−gel solution, and the solution was vigorously stirred to ensure adequate dissolution of PSPH. Then, the cotton sample was soaked in the bath for 15 min, and cured at 95 °C for 1 h, finally boiled in deionized water and further dried at 45 °C for 1 h. The cotton sample treated with both PSPH and titanium dioxide was denoted as cotton-TiO2/PSPH. To determine the effect of titanium dioxide on UV light stability, antibacterial stability, and washing stability of treated samples, cotton swatches treated with PSPH and TiO2 alone were also prepared according to the abovementioned method and referred to cotton-PSPH and cotton-TiO2, respectively. Chlorination and Analytical Titration. N-Halamine precursors can obtain rapid antibacterial activity via chlorination reactions.19 Commercial antiformin solution (with 5.2% sodium hypochlorite) was diluted 10 times, followed by an adjustment of the final pH to 7 using diluted sulfuric acid. This solution served as the chlorination solution for the treated samples. In this study, cotton-TiO2/PSPH and cottonPSPH samples were soaked in 100 mL of the above-mentioned solution for 1 h. The chlorinated cotton swatches were washed with deionized water 3 times and dried at 45 °C to remove all free chlorine absorbed on the surface of cotton. The chlorinated cotton-TiO2/ PSPH and cotton-PSPH samples were denoted as cotton-TiO2/PSPHCl and cotton-PSPH-Cl, respectively. Chlorine loading of cotton-TiO2/PSPH-Cl and cotton-PSPH-Cl samples were detected by iodometric/thiosulfate titration method, which is widely used in titrating oxidative chlorine on fabrics.19,20 Approximately 0.1 g of chlorinated samples was cut into small pieces and then added to a beaker containing 20 mL of water and 0.5 g of KI. The mixture was stirred at room temperature for 10 min. Sodium thiosulfate solution (0.001 N) was used in the titration. Three replicates were produced for each sample, and the averages were reported. The oxidative Cl wt % of cotton-TiO2/PSPH-Cl and cottonPSPH-Cl were determined according to the following equation:

bonds on cotton samples showed better UV stability compared to that treated with PSPH alone. However, the usefulness of the one-bath process methodology is limited for practical applications of antibacterial materials.15 In the current study, titanium dioxide particles were produced via a sol−gel process in order to obtain antibacterial cellulosic materials with good UV stability and to improve the possibility of industrial applications. The produced titanium dioxide particles and poly[5,5-dimethyl-3-(3′triethoxysilylpropyl)hydantoin] (PSPH) were covalently bonded onto cotton fabrics’ surface via a pad-dry-cure process. XPS spectra indicated that covalent bonds could be formed between titanium dioxide particles and PSPH. The treated cotton fabrics were characterized by SEM and FTIR. UV light stability and antibacterial property against Staphylococcus aureus (ATCC 6538) and Escherichia coli O157:H7 (ATCC 43895) were evaluated. The storage stability and washing stability of the treated fabrics were also gauged. Calculations were performed in order to elucidate the origin of the enhanced UV stability derived from the addition of TiO2 to the Nhalamine complex.

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EXPERIMENTAL SECTION

Materials. Bleached cotton fabrics (133 × 72/40S × 40S) were obtained from Zhejiang Guandong Dyeing and Printing Company, China. 5,5-Dimethylhydantoin was purchased from Hebei Yaguang Chemical Company, China. Tetraisopropyl titanate (TIP) and γchloropropyltriethoxysilane were provided by J&K Scientific Ltd., Shanghai, China. Other chemicals were obtained from Sinopharm Chemical Reagent Company, Ltd. All of the chemical reagents used in this research were used as received with no further purification. The bacteria used in antibacterial testing were Staphylococcus aureus (S. aureus) ATCC 6538 and Escherichia coli O157:H7 (E. coli O157:H7) ATCC 43895 (American type culture collection, Rockville, Medical department), and trypticase soy agar was purchased from Difco laboratories, Detroit, MI. Instruments. Surface morphology of the treated fabrics was characterized by an SU-1510 filed-emission scanning electron microscopy (SEM, Hitachi, Tokyo, Japan) instrument. An ESCALAB 250 Xi (Thermo Scientific, USA) was used to detect X-ray photoelectron spectroscopy (XPS) of the treated samples. Fourier transform infrared (FT-IR) spectra were illustrated by a NEXUS 470 spectrometer (Nicolet Instrument Corporation, Madison). UV light stabilities of the treated samples were measured in an Accelerated Weathering Tester (Darong Textiles Instrument Company, Ltd., Zhejiang, China). Bending properties were tested in a Kawabata Evaluation System for Fabric (KES-FB AUTO-A, KATOH, Japan). UV−vis spectra were tested in a Textile UPF Tester (Cary 50, Luozhong Textile Science Company, China). Synthesis of Poly[5,5-dimethyl-3-(3′-triethoxysilylpropyl)hydantoin] (PSPH). The polymeric N-halamine precursor PSPH was synthesized according to the method reported.16 In brief, equal amounts of 5,5-dimethylhydantoin and sodium hydroxide were mixed in a 250 mL flat-bottomed flask, and refluxing in ethanol for 10 min. The formed organic salt was isolated under vacuum and dried for 2 days at 45 °C. Then the organic salt produced was dissolved in 150 mL of N,N-dimethylformamide (DMF), following with the adding of equimolar amount of γ-chloropropyltriethoxysilane. The mixture was stirred vigorously at 95 °C for 8−16 h. NaCl and DMF were removed from the mixtures by filtration and evaporation under vacuum, respectively. 5,5-Dimethyl-3-(3′-triethoxysilylpropyl)hydantoin (SPH) was obtained after purification. The formation of polymeric SPH was completed by hydrolysis and copolymerization reactions of SPH under acidic condition. Briefly, a certain amount of SPH, ethanol and water (Vethanol:Vwater = 1:2) were added to a 100 mL stand-up flask, and pH of the solution was adjusted to 3.5−5.5 by diluted hydrochloric acid.

Cl% =

35.5 × (VS − VO) × N × 100 2 × WS

where N is the normality of the used Na2S2O3 solution (equiv. L−1), VS and VO are the final and the initial volumes (L) of Na2S2O3, respectively, and WS is the weight (g) of tested swatches. UV Light Stability Testing. UV light stability of cotton-TiO2/ PSPH-Cl and cotton-PSPH-Cl were investigated according to ASTM D4587 standards in the Accelerated Weathering Tester. The tested samples were placed in the chamber with UV light (315−400 nm, 0.89 W/m2, 60 °C). After exposed to the UV light for specific time, the tested samples were removed from the chamber and the chlorine loading were determined according to the method mentioned in the Analytical Titration section. Antibacterial Testing. A modified AATCC 100-2004 Test Method was used to evaluate antibacterial activities of the initial treated samples and the treated samples after 12 h of UV exposure. Gram-positive S. aureus (ATCC 6538) and Gram-negative E. coli O157:H7 (ATCC 43895) were used in this experiment. A single bacteria colony was incubated in trypticase soy broth at 37 °C 1 day before the experiment. 3 mL of the incubated bacteria colonies was transferred to tube, centrifuged at 1500 rpm for 15 min, and washed B

DOI: 10.1021/acsami.5b12601 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. SEM images of cotton (A), cotton-TiO2 (B), cotton-TiO2/PSPH (C), and cotton-TiO2/PSPH after 50 washings (D). twice with phosphate buffer solution. Bacterial stock suspension was made by resuspending bacterial pellet into 5 mL of phosphate buffer solution. 25 μL bacteria suspension was added to center of fabric swatches (2.54 cm2), and a second swatch was covered on surface of the former one. Sterile weights were used to guarantee complete contact between tested samples and bacteria. The tested samples were quenched with 5 mL of sodium thiosulfate (0.02 N) after certain contacting interval. Then the quenched solution was diluted with phosphate buffer solution, inoculated onto trypticase soy agar and incubated at 37 °C for 24 h. The number of bacterial colonies on trypticase soy agar was used to determine the antibacterial activity of each tested sample. Washing Stability Testing. The washing stabilities of the treated samples were investigated in a Launder-Ometer according to the standard AATCC Test Method 61-1996. The treated samples (1 in ×2 in) were placed in stainless steel canisters, which contained AATCC detergent water solution and 50 stainless steel balls. The washing procedure was completed at 42 rpm, at 49 °C for 45 min. One washing cycle is equivalent to 5 machine washings. After a certain washing cycles, the samples were washed with distilled water and dried at 45 °C for 1 h. The restored chlorine loading of rechlorinated samples was determined by titration in order to determine the effect of TiO2 on washing stability of PSPH. In this research, the washed and rechlorinated cotton-TiO2/PSPH-Cl samples were also subjected to UV light in order to evaluate the washing stability of TiO2. Computational Methods. Unrestricted density functional theory (DFT) calculations at the UB3LYP/6-311++G(2d,p) theory level were used to characterize the transition structures and ground states for the chlorinated TiO2-model compounds in vacuum using Gaussian 09. The complete computational details are provided in the Supporting Information (S1 and S2). The DFT calculations were used for geometry optimizations and computations of vibrational frequencies, which confirmed all stationary points as either minima or transition structures, and provided thermodynamic corrections. All calculations were performed on computers located at the Alabama Supercomputer Center.

The aggregation of TiO2 and TiO2/PSPH particles made the surface of modified fabrics rougher than that of unmodified cotton fabrics. However, bending property experiments determined that both the bending rigidity (B) value and the bending moment (2HB) value of cotton treated with TiO2/ PSPH are smaller than that of unmodified cotton (Table 1). Table 1. Results of Bending Property Experimentsa bending rigidity (B, gf cm2/cm)

a

bending moment (2HB, gf cm/cm)

samples

warp

weft

warp

weft

cotton cotton-TiO2/PSPH cotton-TiO2/PSPH-Cl

0.2006 0.1240 0.1586

0.0520 0.0208 0.0303

0.1438 0.0847 0.0838

0.0371 0.0176 0.0246

The data are automatically recorded by the evaluation system.

This signifies that cotton fabrics treated with TiO2/PSPH became softer and recovered easier from bending than untreated cotton due to the formation of covalent bonds.24 The thickness of coating is difficult to assess because of the uneven surface of cotton fabrics. Still, after 50 washes no obvious delamination could be detected (Figure 1D). To determine chemical composition and chemical binding state of the cotton fabrics modified with TiO2/PSPH, global range high-resolution XPS was utilized, and the XPS spectra of unmodified cotton, cotton-TiO2/PSPH, and Si 2p core level are illustrated in Figure 2. The main elements detected on the surface of unmodified cotton samples were oxygen and carbon with binding energies of 531.3 and 285.7 eV, respectively, which belong to glucose rings on cellulose macromolecular chains.25 Cotton fabrics were modified by TiO2/PSPH with new peaks observed at binding energies of 464.6, 458.7, 398.8, 152.4, and 102.1 eV, which is consistent with the binding energy of Ti 2p1/2, Ti 2p3/2, N 1s, Si 2s, and Si 2p, respectively.14,26 Thus, TiO2 and PSPH have been successfully coated onto the surface of cotton fabrics via a sol−gel process. The titanium and silicon atomic content on the surface of modified cotton calculated from XPS peaks are 6.66% and 4.37%, respectively. To illustrate further the formation of chemical bond between TiO2 and N-halamine precursor PSPH, Si 2p core-level spectra of cotton-TiO2/PSPH are shown in Figure 2B. The Si 2p region of cotton-TiO2/PSPH was decomposed into three contributions, appearing at 103.2, 102.4, and 101.9 eV, which are attributed to chemical species of Si− O−Si, Si−O−Ti, and Si−O−C components, respectively.15,27,28 The binding energy of Si−O−Ti was higher than that of Si−O−C because the greater electronegativity of Ti increases the effective positive charge of Si.29 The results showed that connection between TiO2 and PSPH had been established through ether bondings, and the Si−O−Ti binding was not detected in the control sample of cotton-PSPH (Figure 2C). Additionally, the mass fraction of Si−O−Ti in cotton-

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RESULT AND DISCUSSION Characterization of Cotton Fabrics Treated with TiO2 and TiO2/PSPH via Sol−Gel Process. The synthesized PSPH is soluble in isopropyl alcohol under vigorous stirring and can be coated onto cotton when cured at 95 °C for 1 h due to the formation of ether bonds between −OH groups of cotton and −OH groups of hydrolyzed N-halamine PSPH precursor.12 Si−O−Ti bonds can also be formed through condensation reaction of −OH groups on surface of TiO2 and siloxane compounds by the sol−gel method as reported previously.21−23 SEM images were used to investigate the surface morphology of TiO2 and TiO2/PSPH modified cotton fabrics (Figure 1). It can be seen that unmodified cotton presented a smooth surface with some parallel ravines (Figure 1A). The parallel ravines were covered by the deposition of TiO2 particles when the fabrics treated with TiO2 sol−gel solution. Cracks could be observed in Figure 1B due to the existence of thick deposition layers. After treatment with TiO2/PSPH, a uniform coating was formed on the surface of treated cotton fabrics (Figure 1C). C

DOI: 10.1021/acsami.5b12601 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 2. XPS spectra of (A) survey spectra of unmodified cotton (a) and cotton-TiO2/PSPH (b), (B) Si 2p core-level spectra of cotton-TiO2/ PSPH and (C) Si 2p core-level spectra of cotton-PSPH.

TiO2/PSPH was calculated from the result of Si 2p curve fitting of XPS spectra with value of 39.5%, which indicates that a certain amount of PSPH had been chemically bond by TiO2 in the sol−gel process. FTIR spectra of cotton, cotton-TiO2, cotton-TiO2/PSPH, and cotton-TiO2/PSPH-Cl are presented in Figure 3. A new

Table 2. UV Stability of Cotton-PSPH-Cl and Cotton-TiO2/ PSPH-Cl chlorine loading (wt %) time (h) 0 1 2 4 8 12 24 48 rechlorination (48 h)

cotton-PSPH-Cl 0.26 0.14 0.12 0.06 0.03 0.01 0.00 0.00 0.13

± ± ± ± ± ±

0.01 0.01 0.01 0.01 0.01 0.01

± 0.02

cotton-TiO2/PSPH-Cl 0.30 0.23 0.22 0.20 0.17 0.15 0.10 0.06 0.25

± ± ± ± ± ± ± ± ±

0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.02 0.01

loading of 0.06 wt % is still effective for inactivating bacteria.33,34 From the above results, it is clear that the introduction of TiO2 could significantly improve the UV stability of N-halamine siloxane PSPH. Kocer et al. studied the mechanism of photolytic decomposition of N-halamine siloxane using a joint experimental and computational approach.35 They reported the Nchlorinated coatings slowly decomposed upon UVA irradiation, whereas the unhalogenated coatings did not. The decomposition mechanism of chlorinated siloxane coatings was explained by an intramolecular photorearrangement reaction followed by cleavage of an Si−alkyl bond resulting in the loss of hydantoin units from the surface of the coated materials. The chlorine loss caused by the scission reaction could not be restored.35 In this study, to determine the influence of TiO2 on chemical structure of PSPH, rehalogenation of samples irradiated for 48 h was conducted. It can be seen that after rechlorination, about 84% of oxidative chlorine was regained for cotton-TiO2/PSPH-Cl, compared with 50% for cotton-PSPHCl samples. We hypothesize that TiO2 may impair the scission reaction caused by the dissociated oxidative Cl atoms under UV irradiation, thus protecting the chemical structure of PSPH. However, additional factors likely contribute to the TiO2 derived UV stability in textiles,36 including a high refractive index that allows for effective reflection and scattering of UVrays37,38 and the ability to absorb UV radiation via favorable semiconductive properities.39 Lower transmissivity of UV light for treated samples was detected via UV−vis spectra, which was related to scattering and absorption of UV light (Supporting Information, Figure S1). The mechanism for this protecting effect is further investigated below.

Figure 3. FTIR spectra of (A) cotton, (B) cotton-TiO2, (C) cottonTiO2/PBA, and (D) cotton-TiO2/PBA-Cl.

peak was detected at 1698 cm−1 (Figure 3C) and assigned to carbonyl stretching vibration of hydantoin rings of PSPH; the peak was not observed in the spectra of unmodified cotton (Figure 3A) and cotton-TiO2 (Figure 3B).30 After chlorination, the carbonyl vibrational peak shifted to 1713 cm−1 (Figure 3D) due to the electron withdrawing effect of chlorine atoms in cotton-TiO2/PSPH-Cl.31 However, the vibration of Si−O−Ti at approximately 940 cm−1 was not observed due to overlap of strong vibrational bands at 920−1180 cm−1 of C−O and C−C vibration modes of cellulose backbones.15,32 UV Light Stability Testing. The UV light stabilities of cotton-PSPH-Cl and cotton-TiO2/PSPH-Cl samples were tested, and the results are presented in Table 2. The oxidative chlorine loading of cotton-PSPH-Cl decreased sharply by 54% over the initial 2 h from 0.26% to 0.12%. Whereas, the chlorine loading of cotton-TiO2/PSPH-Cl decreased by only 27% within 2 h of UV light irradiation. After 12 h of irradiation, almost all of the chlorine was lost from samples of cotton-PSPH-Cl, whereas cotton-TiO2/PSPH-Cl retained 50% of the chlorine. The chlorine content of cotton-TiO2/PSPH-Cl samples decreased slowly with the extension of irradiation time, reaching at 0.06 wt % within 48 h of irradiation. Chlorine D

DOI: 10.1021/acsami.5b12601 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Scheme 1. Molecular Interactions among PSPH, TiO2 and Cellulose Substrate

Computational Evidence of TiO2 Induced Stability. Density functional theory (DFT) calculations were performed to elucidate the role of TiO2 (Ti−O−Si, Ti−O−Ti) in enhancing resistance to UV degradation in the cotton-TiO2/ PSPH-Cl systems. Molecular interactions among these are shown in Scheme 1, where red, green, and blue parts are related to washing fastness, UV resistance, and antibacterial function, respectively. Kocer et al. previously reported a dissociation mechanism for a 7-chloro-5,5-dimethylsilylpropylhydantoin mimic (MMSi-Cl) that involves (1) a homolytic bond cleavage between N−Cl (antibacterial function), followed by (2) a 1,5hydrogen atom transfer in the MMSi radical, then (3) the Cl radical binds to the carbon radical in the beta position to the silicon atom, and finally (4) the loss of antimicrobial efficacy occurs when the chlorine is simultaneously transferred to the silicon atom of the siloxane surface coating accompanied by cleavage of the alkyl chain (Scheme 2).35

Figure 4. Calculated transition structure for the cleavage of a TiO2/ PSPH model system using UB3LYP/6-311++(2d,p). Distances given in angstroms.

Table 3. Activation Enthalpies (kcal/mol) for the 7-Chloro5,5-dimethylsiloxane Model Transition Structures with and without TiO2, TiO2-MMSi-Cl and MMSi-Cl, Respectively, from UB3LYP/6-311++G (2d, p)

Scheme 2. Dissociation Mechanism of a 7-Chloro-5,5dimethylsilylpropylhydantoin Mimic (MMSi-Cl)

ΔH

‡

TiO2-MMSi-Cl

MMSi-Cl35

34.8

33.9

the calculations support the current experimental evidence that the bound TiO2 makes the photolytic decomposition less favorable. Antibacterial Testing. Initial cotton-PSPH, cotton-TiO2/ PSPH, cotton-PSPH-Cl samples with chlorine loading of 0.17 wt %, and cotton-TiO2/PSPH-Cl with chlorine loading of 0.19 wt % were challenged with S. aureus and E. coli O157:H7. Untreated cotton, cotton-PSPH, and cotton-TiO2/PSPH were used as control. The antibacterial results are shown in Table 4. It can be seen that both of the two control samples caused relatively low bacteria reduction within 30 min due to adhesion of bacteria onto cotton fabrics surface as reported previously.42 Cotton-PSPH-Cl with 0.17 wt % oxidative chlorine could inactivate all S. aureus and E. coli O157:H7 within 10 and 30 min, respectively. The difference in inactiving rates was attributed to the different shape and surface chemical compositions of these two bacteria.43 For cotton-TiO2/ PSPH-Cl with chlorine loading of 0.19 wt %, all of the bacteria were inactivated within 30 and 10 min for S. aureus and E. coli O157:H7, respectively. It can be seen that the addition of TiO2 in the coating did not significantly effect the inactivation rate of siloxane N-halamine PSPH because TiO2 could present antibacterial activity only under UV irradiation.44 The antibacterial activity of samples after exposure to UV irradiation for 12 h was also tested, and the results are given in Table 5.

To quantify the protective effect of TiO2 on the C−Si scission reaction, the UB3LYP/6-311++G(2d,p) level of theory was used to compute the transition state for the beta position cleavage of a TiO2-7-chloro-5,5-dimethylsilylpropylhydantoin model (TiO2-MMSi-Cl), where the covalent ether linkages to the cellulose surface are truncated with hydrogen atoms and the titanium is bonded to three hydroxyl groups to maintain titanium’s tetrahedral geometry. This DFT methodology has provided accurate geometries and energies in multiple studies of N-chlorohydantoin systems.35,40,41 The reaction occurred in a concerted fashion with the Cl−C bond breaking at 2.71 Å, the Cl−Si bond forming at 2.38 Å, and the Si−C bond breaking at 2.12 Å (Figure 4). The geometries are similar to that of the MMSi-Cl transition structure lacking TiO2 with making/ breaking bond distances of 2.71, 2.42, and 2.17 Å, respectively.35 The computed activation enthalpy for the final cleavage of the Si-alkyl bond in the presence of TiO2 is 34.8 kcal/mol compared to 33.9 kcal/mol for MMSi-Cl lacking TiO2 (Table 3). That energy difference correlates to an approximate 4.6-fold decrease in the rate of C−Si scission at 25 °C. As such, E

DOI: 10.1021/acsami.5b12601 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Table 4. Biocidal Results of the Initial Coated Cotton against S. aureus and E. coli O157:H7 log reduction sample cotton-PSPH cotton-TiO2/PSPH cotton-PSPH-Cl(0.17 wt % Cl+)

cotton-TiO2/PSPH-Cl(0.19 wt % Cl+)

a

contact time (min)

S. aureusa

E. coli O157:H7b

30 30 5 10 30 5 10 30

0.11 0.07 2.40 5.97 5.97 1.18 2.52 5.97

0.35 0.04 0.36 1.51 6.01 1.45 6.01 6.01

Inoculum concentration was 9.33 × 105 CFU/sample. bInoculum concentration was 1.10 × 106 CFU/sample.

Table 5. Biocidal Results of Cotton Samples after UV Exposed for 12 h log reduction sample

contact time (min)

S. aureus

30 30 30 5 10 30 5 10 30

1.48 0.68 0.89 0.91 1.50 1.80 4.22 6.04 6.04

cotton cotton-PSPH cotton-TiO2/PSPH cotton-PSPH-Cl (0 wt % Cl+)

cotton-TiO2/PSPH-Cl (0.11 wt % Cl+)

a

a

E. coli O157:H7b 0.05 0.03 1.54 0.65 0.15 0.50 6.32 6.32 6.32

Inoculum concentration was 1.10 × 106 CFU/sample. bInoculum concentration was 2.10 × 106 CFU/sample.

cotton-TiO2/PSPH-Cl. The loss chlorine was caused by cleavage of N−Cl bonds, which could be completely restored by the rechlorination reaction mentioned in the UV light stability testing section. The high retention rate is related to low dissociation constant of amide groups.20 Washing Stability Testing. The washing stability of cotton-PSPH-Cl has been previously studied by Worley.45 After 50 washing cycles, about 32.5% of oxidative chlorine was retained, and 37.5% of chlorine could be restored after rehalogenization. In this research, to investigate the influence of TiO2 on washing stability of PSPH, cotton-TiO2/PSPH-Cl with chlorine loading of 0.50 wt % was conducted under AATCC standard washing method as described in the Experimental Section. The results are shown in Table 6. It can be seen that the chlorine loading of the rechlorinated cotton-TiO2/PSPH-Cl was restored 40.0% after 50 washing cycles, which was very close to that of cotton-PSPH-Cl (37.5%), indicating that TiO2 has very small effect on the washing stability of PSPH. To determine further the stability of TiO2 introduced via the sol−gel process toward washing, the rechlorinated cottonTiO2/PSPH-Cl was washed for 5, 10, 25, and 50 cycles and samples were challenged with UA irradiation; the results are shown in Table 6. After irradiating for 24 h, the chlorine loadings of cotton-TiO2/PSPH-Cl washed for 5, 10, 25, and 50 cycles samples were 0.07, 0.05, 0.05, and 0.05 wt %, respectively. All of these residual oxidative chlorines were effective in inactivating bacteria. After rechlorination of samples irradiated for 24 h, the chlorine contents of cotton-TiO2/ PSPH-Cl washed for 5, 10, 25, and 50 cycles were restored 61.5%, 63.9%, 70%, and 70%, respectively. As the restored chlorine did not decrease with the increase of washing cycles, the introduced TiO2 could reasonably withstand the vigorous washing challenge. The small difference in recovery chlorine

After UV irradiation, the chlorine loading of cotton-PSPH-Cl and cotton-TiO2/PSPH-Cl decreased to 0 and 0.11 wt %, respectively. The majority of the antibacterial property associated with the cotton-PSPH-Cl samples was lost due to N−Cl decomposition. In contrast, the remaining active chlorine loading on the cotton-TiO2/PSPH-Cl samples after 12 h of UV exposure was still sufficient to inactivate bacteria. Storage Stability. Figure 5 shows the storage stability of cotton-PSPH-Cl and cotton-TiO2/PSPH-Cl samples in the

Figure 5. Storage stability of (a) cotton-PSPH-Cl and (b) cottonTiO2/PSPH-Cl.

dark at ambient temperature. After 40 days, 78% of the oxidative chlorine of cotton-PSPH-Cl was retained, decreasing from an initial 0.14 to 0.11 wt %. For cotton-TiO2/PSPH-Cl, 81% of the chlorine was retained. There was no significant difference for storage stability between cotton-PSPH-Cl and F

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ACS Applied Materials & Interfaces Table 6. Washing Stability of Cotton-TiO2/PSPH-Cla chlorine loadings (wt %) 5b

irradiation time (h) 0 1 4 8 12 24 rechlorination a

0.39 0.38 0.22 0.17 0.10 0.07 0.24

± ± ± ± ± ± ±

10b 0.01 0.01 0.01 0.00 0.01 0.01 0.01

0.36 0.30 0.17 0.13 0.09 0.05 0.23

± ± ± ± ± ± ±

25b

0.01 0.00 0.01 0.01 0.02 0.00 0.00

0.30 0.26 0.15 0.14 0.08 0.05 0.21

± ± ± ± ± ± ±

50b

0.02 0.01 0.02 0.01 0.01 0.00 0.02

0.20 0.15 0.11 0.11 0.07 0.05 0.14

± ± ± ± ± ± ±

0.01 0.01 0.02 0.01 0.01 0.01 0.01

Initial chlorine loading of cotton-TiO2/PSPH-Cl was 0.50 wt %. bMachine washing cycles.

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ratios might be due to the uneven coating of TiO2 on cotton samples. The excellent washing durability and UV stability of cotton-TiO2/PSPH-Cl potentailly allows for long life usage in industrial applications.

Corresponding Authors

*Tel: +86-051085912007. Fax: +86-051085912009. Email: [email protected] (Xuehong Ren). *E-mail: [email protected] (Orlando Acevedo).

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CONCLUSIONS Titanium dioxide was covalently bonded onto N-halamine siloxane precursor PSPH, and the chlorinated cotton fabrics treated with TiO2/PSPH showed excellent UV light stability. After 48 h of UV irradiation, the residual oxidative chlorine in cotton-TiO2/PSPH-Cl was 0.05 wt %, which was still effective in killing bacteria. After rechlorination, approximately 84% of the oxidative chlorine was regained for cotton-TiO2/PSPH-Cl, compared with 50% for cotton-PSPH-Cl samples. Washing stability testing indicated that the introduction of titanium dioxide had no effect on washing fastness of PSPH, and the covalently bonded titanium dioxide could suffer 50 cycles of machine washing. Even after 50 washing cycles and irradiation under UV light for 24 h, the residual oxidative chlorine in cotton-TiO2/PSPH-Cl also retained 0.05 wt %. Cotton-TiO2/ PSPH-Cl with chlorine loading of 0.19 wt % showed good antibacterial activity by inactiving 100% of S. aureus (ATCC 6538) and E. coli O157:H7 (ATCC 43895) within 30 and 10 min, respectively. The decomposition mechanism of chlorinated siloxane coatings has been reported to involve an intramolecular photorearrangement reaction followed by the cleavage of a Si−alkyl bond that resulted in loss of biocidal hydantoin units from the surface of the coated materials. Calculations using UB3LYP/6-311++G(2d,p) on a model TiO2/PSPH-Cl system found the inclusion of TiO2 yielded a ΔH‡ of 34.8 kcal/mol for the C−Si bond cleavage reaction compared to 33.9 kcal/mol for the PSPH-Cl model lacking TiO2. The enhanced Si−alkyl bond strength suggests that the electronic structure of the system may be altered directly by the titanium and may, in part, explain the amplified protection of the siloxane coatings from UVA photodegradation. In summary, antibacterial cotton fabrics treated TiO2/PSPH via a sol−gel process have great potential in industrial application with potent UV stability and washing stability.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support was provided by the Project for Jiangsu Scientific and Technological Innovation Team, the Scientific Research Foundation for Returned Overseas Chinese Scholars, Ministry of Education, China, and the Graduate Student Innovation Plan of Jiangsu Province of China (KYLX-1142). Gratitude is also expressed to the Alabama Supercomputer Center.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b12601. Additional complete computational details, Gaussian 09 reference, and UV−vis spectra of untreated cotton and cotton-TiO2/PSPH-Cl (PDF). G

DOI: 10.1021/acsami.5b12601 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acsami.5b12601 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX