A novel antimicrobial photosensitizer: hydroxyethyl Michler’s ketone

Abstract

A new photoactive antimicrobial agent was synthesized by introducing a reactive functional group (–CH₂CH₂OH) to 4,4′-bis(dimethylamino)benzophenone (Michler’s ketone) that could enhance the fixation of benzophenone derivatives to cotton fabric. Cotton fabrics containing organic photosensitizers were investigated to demonstrate antimicrobial properties in the application to protective and germ-free clothing. Photosensitizers such as benzophenone, Michler’s ketone and synthesized (4-(dimethylamino)phenyl)(4-((2-hydroxyethyl)(methyl)amino)phenyl)methanone (MK-EtOH) were applied on cotton fabrics through a pad–dry–cure method. The introduction of the reactive functional groups (–CH₂CH₂OH) enhanced the fixation of benzophenone derivatives to cotton fabrics as confirmed by Fourier transform infrared spectroscopy analysis and methylene blue staining method. No significant reductions of tensile strength and thermal degradation were observed with cotton fabrics treated with MK-EtOH. The scanning electron microscope morphology of the treated cotton showed that organic photosensitizers were evenly distributed and diffused to cotton fibers. After ultraviolet irradiation (365 nm), the cotton fabrics treated with synthesized MK-EtOH photosensitizer provided more effective and durable antimicrobial properties than Michler’s ketone and benzophenone, even after multiple washing.

Keywords

Antimicrobial benzophenone derivatives cotton Michler’s ketone photosensitizer

The emergent social attention toward the protection of human health from toxic chemicals, contagious disease, and germs has continuously stimulated research on antimicrobial and detoxifying agents, and their applications,^1–3 as well as promoted the development of polymers and nanotechnology.⁴ The most commonly used antimicrobial agents are silver-based,^5,6 halamine compounds,⁷ and quaternary ammonium.^8,9 They are usually attached to the surfaces of substance (or embedded in the polymeric substance) and released slowly so that they can directly react with germs by oxidation–reduction mechanisms. Photoactive organic antimicrobial agents that produce radicals through ultraviolet (UV) irradiation have been designated as a potential agent for the self-decontamination of clothing because of their durability and re-chargeability.^10,11

Among various antimicrobial agents, organic photoactive chromophoric groups have been actively studied to improve performance properties and durability as well as fixation to polymeric materials.^12,13 Generally, the carbonyl group of benzophenone (BP) generates free radicals by UV irradiation. The excited triplet structure can be easily quenched with oxygen and it extracts a hydrogen atom from any active source to form a ketyl radical; subsequently, harmful materials in contact with the ketyl group are oxidized and destroyed.^12,13 Fabrics containing a BP chromophoric group would easily be excited and generate radicals when activated by UV light; in addition, polymer materials incorporating BP have shown an extended lifetime of radical activity. BP structures have traditionally been used as an UV absorbent due to their rechargeable mechanism qualities and the human nontoxicity of BP.^14,15 The surface modification and functionalization of textile materials with BP derivatives have been extensively examined and discussed.¹⁶ BP derivatives are incorporated into polymeric materials by physical trapping (through film formation or electrospinning)^17–19 and by the chemical modification of the substrate.^20,21 Cotton cellulose was also grafted with hydroxybenzophenone and 1,2,3,4-butanetetracarboxylic acid (BTCA) as a crosslinker.^22,23 The surface grafting process through a functional group is a useful technique with various advantages. Owing to the formation of strong covalent bonds between the grafted chains and fiber surfaces, it is an easy and controllable application that does not change the bulk properties of the materials.¹⁶

This study looks for a more active organic photosensitizer that can be applied directly to cotton fiber for the development of highly active and durable self-decontamination textiles. We synthesized a highly effective BP derivative that can be strongly activated by UV irradiation and is chemically more reactive with cellulose than hydroxyl benzophenones by introducing the –CH₂CH₂OH group to MK; subsequently, there was a highly efficient grafting on the surface of cotton fabrics. Cotton fabrics were treated with synthesized 4-(dimethylamino)phenyl)(4-((2-hydroxyethyl)(methyl)amino)phenyl)methanone (MK-EtOH, BP, and MK by a pad–dry–cure method. Photosensitizers such as BP and MK were selected from preceding research¹⁸ for comparison. Photosensitizers were dissolved with acetone in order to measure ultraviolet/visual (UV/vis) absorbance. The morphology of cotton fabrics was confirmed with a scanning electron microscope (SEM) and the color change of materials was detected with a CIE L*a*b* colorimetric system. Thermal gravimetric analysis (TGA) and Fourier transform infrared (FTIR) were used to evaluate thermal stability and chemical bonding. Lastly, an antimicrobial test (JIS Z 2801) evaluated the antimicrobial activity of treated cotton fabrics.

Experimental details

Materials and methods

Bleached and desized cotton fabrics (78 × 76 in²) which were purchased from KATRI (Korea Apparel Testing and Research Institute, Seoul, Korea). Organic photosensitizers such as BP (99% pure) and MK (98% pure) were purchased from Sigma-Aldrich. Dichloromethane, fluorobenzene, p-nitrobenzoyl chloride, AlCl_3, tin(II) chloride dehydrate, ethylacetate, methyl iodide, K₂CO_3, dimethylformamide, BTCA, and sodium hypophosphite hydrate were also purchased from Sigma-Aldrich. Acetone was used as the solvent for the photosensitizer. All the chemicals were of reagent grade and used without further purification.

Preparation

In order to incorporate organic photosensitizers, cotton fabrics were treated by the pad–dry–cure process. The cotton fabrics, cut in sizes of around 30 cm × 30 cm, were immersed in a finishing solution containing the designated concentration (0.1 M) of BP, MK, MK-EtOH, 0.1 M of BTCA, and 0.1 M of sodium hypophosphite hydrate. The fabrics were padded through a laboratory mangle to have a wet pick-up rate around 100%. Then the treated fabrics were wet-fixed by putting them in plastic zipper bags and storing them in a convection oven for 30 min at 85℃. Then, the fabrics were cured at 160℃ for 3 min, followed by washing with distilled water and air-drying in a conditioning room (25 ± 1℃, 65 ± 2% relative humidity (RH)) for 24 h.

Characterization

UV-vis spectra of each material were measured with a single-beam Agilent 8453 UV-vis spectrometer. SEM (COXEM, CX-100 S, Korea) was employed to study the effect of photosensitizer treatment on topological characteristics of the treated cotton fabrics. The color measurements of treated cotton fabrics were made with a color spectrophotometer (Scinco, Color Mate). Color changes of cotton fabrics according to each photosensitizer were reported with a CIE L*a*b* colorimetric system. The reference illuminant was D65 (standard daylight) and observer angle was diffuse/8° (illumination/measurement). This used Target (USRS-99-010 AS-01158-060) material and the wavelength ranged from 400 nm to 700 nm. An average value was taken from the five measurements.

An FTIR spectroscope (Jasco, FT/IR-6300, Japan) with an attached ATR was used to analyze the treated cotton fabrics in the spectral region 4000–600 cm⁻¹ with 54 scans at 4 cm⁻¹ resolution. The fixation of BP derivatives within the treated cotton was also analyzed by the methylene blue (MB) staining technique. The staining bath was made with 0.1 g/l dye and 1 g/l ammonia at a 50:1 liquor ratio. Then fabrics were stained at 25℃ for 20 min. After the staining process, the reflectance of the fabrics was measured by Minolta Color Eye CM-512M3 (Japan) and the K/S value was calculated according to the Kubelka–Munk equation.²⁴

K / S = (1 - R)^{2} / 2 R

where R is the reflectance of the sample at the wavelength of maximum absorption.

Tensile strengths of the cotton fabrics were tested according to ASTM D5035-11 (cut strip method) by using a UTM machine (Type H10KS, Hounsfield, USA) with a 20 mm/min crosshead speed and 760 mm graph distance. At least three replications were carried out. TGA (Mettler, USA) was used to study thermal characteristics of pristine and treated cottons.

Antibacterial properties of treated cotton fabrics were measured for Staphylococcus aureus (ATCC 6583P) and Escherichia coli (ATCC 8739) according to a modified testing method for antibacterial activity of the fabrics (JIS Z2801). All fabrics (5 × 5 cm) were inoculated by bacteria with an initial concentration of 2.5 × 10⁵ colony forming units/ml (CFU/ml) in a Petri dish. The diluted microbial solution (0.4 ml) was spread evenly on the cotton fabrics. Then the inoculated cotton fabrics were immediately exposed to two UV-A light lamps (365 nm) with 4 W and 8 W for 2 h in a container. The cotton fabrics were then immersed in 20 ml of quenching solution (saline solution) and 1 ml of the solution was transferred to a test tube with 9 ml of the saline solution in order to dilute the microbes. The 1 ml of cell suspension was dropped onto a new Petri dish with a growth medium of trypticase soy agar. The plates were incubated at 37℃ for 24 h and the number of colonies was determined through a colony counter. The reduction of bacteria was calculated as follows:¹⁸

Reductionofbacteria (%) = [(M_{b} - M_{c}) / M_{b}] \times 100

where M_b is the average number of bacteria in a reference sample after incubation for 24 h and M_c is the average number of bacteria in an antimicrobial sample after incubation for 24 h. If the value of the average number of bacteria in a reference sample immediately after the vaccination of testing bacteria is lower than that of M_b, the measurement of the sample is confirmed to fail by natural reduction due to the external environment.

Results and Discussion

Synthesis of MK-EtOH

In order to enhance the fixation of BP derivatives to cotton fabrics, a new photoactive antimicrobial agent was synthesized by introducing an active functional group (–N–CH₂CH₂OH) to MK, which turned out to be the most effective photoactive antimicrobial agent in a previous study^18,19 MK-EtOH was synthesized from p-nitrobenzoyl chloride as the starting material, as described in the synthesis shown in Scheme 1.

(4-Fluorophenyl)(4-nitrophenyl)methanone

A methylene chloride (100 ml) containing fluorobenzene (9.0 ml, 96 mmol) and p-nitrobenzoyl chloride (8.9 g, 48 mmol) was stirred for 5 min at room temperature. Aluminum chloride (10.6 g, 80 mmol) was added to this solution, and stirred for one hour under N₂. For reaction quenching, ice-cold water was poured into the flask and the organic layer was concentrated in the evaporator. The aqueous solution was washed with ethyl acetate and the organic layer was dried over MgSO₄. The solid residue was filtered and the filtrated solution was concentrated in the evaporator. The crude product was recrystallized from ethyl acetate to afford (4-fluorophenyl)(4-nitrophenyl)methanone (a) (8.9 g, 77%). ¹H NMR (300 MHz, CDCl₃): 8.37 (2H, d, J = 9 Hz), 7.95 (2H, d, J = 9 Hz), 7.86 (2H, m), 7.43 (2H, t, J = 9 Hz).

(4-Aminophenyl)(4-fluorophenyl)methanone

(4-Fluorophenyl)(4-nitrophenyl)methanone (7 g, 28 mmol) was dissolved in methanol (100 ml) to which tin(II) chloride dehydrate (18.9 g, 84 mmol) was added and the mixture was refluxed for 12 h. After the reaction finished, the mixture solution was carefully adjusted to pH 8 using NaOH 1 M solution and then extracted with ethylacetate (50 ml, three times). The solid residue was filtered and the filtrated solution was concentrated in an evaporator to afford (4-aminophenyl)(4-fluorophenyl)methanone (b) (5.5 g, 89%). ¹H NMR (300 MHz, DMSO-d6): 7.68 (2H, dd, J = 9 Hz, 7 Hz), 7.51 (2H, d, J = 7 Hz), 7.33 (2H, J = 9 Hz), 6.59 (2H, d, J = 9 Hz), 6.20 (2H, s).

(4-(Dimethylamino)phenyl)(4-fluorophenyl)methanone

A mixture of (4-aminophenyl)(4-fluorophenyl)methanone (4.5 g, 21 mmol), methyl iodide (3.9 ml, 63 mmol), and potassium carbonate (8.7 g, 63 mmol) in dry dimethylformamide (40 ml) was heated to 60℃ for 24 h. The reaction mixture was poured into ice water (200 ml). The precipitate was filtered, washed with water several times, and dried. This crude product was subjected to column chromatography using ethyl acetate/hexane (1:8) and afforded (4-(dimethylamino)phenyl)(4-fluorophenyl)methanone (c) (3.8 g, 74 %). ¹H NMR (300 MHz, DMSO-d6): 7.73-7.62 (4H, m), 7.34 (2H, tt, J = 2 Hz, 9 Hz), 6.76 (2H, dt, J = 2 Hz, 9 Hz), 3.04 (6H, s).

(4-(Dimethylamino)phenyl)(4-((2-hydroxyethyl)(methyl)amino)phenyl)methanone

(4-(Dimethylamino)phenyl)(4-fluorophenyl)methanone (2 g, 8.2 mmol) was dissolved in dimethyl sulfoxide (20 ml), 2-(methylamino)ethanol (2 ml, 24.7 mmol) was added and the mixture refluxed for 10 h. The reaction solution was extracted with ethyl acetate (50 ml, three times). The MgSO₄ was filtered and the filtrated solution was concentrated in the evaporator. The precipitate was filtered off, dried, dissolved in chloroform (10 ml), and dropped into excess hexane (200 ml). The precipitate was filtered off and dried to give (4-(dimethylamino)phenyl)(4-((2-hydroxyethyl)(methylamino)phenyl)methanone (MK-ETOH) (d) (1.87 g, 76 %). ¹H NMR (300 MHz, DMSO-d6): 7.60–7.55 (4H, m), 6.75 (4H, d, J = 9 Hz), 4.76 (1H, t, J = 5 Hz), 3.59–3.47 (4H, m), 3.02–2.99 (4H, m) (Figure 1).

Scheme 1.

Preparation of (4-(dimethylamino)phenyl)(4-((2-hydroxyethyl)(methyl)amino)phenyl)methanone) (MK-EtOH).

Figure 1.

¹H NMR spectrum of (4-(dimethylamino)phenyl)(4-((2-hydroxyethyl)(methyl-amino)phenyl)methanone) (MK-EtOH).

Optical properties of organic photosensitizers

Figure 2 shows the UV/vis spectra of BP, MK, and MK-EtOH selected as organic photosensitizers. The materials were dissolved in an acetone to measure the absorbance that ranged from 300 nm to 450 nm. The maximum peaks of UV/vis absorbance appeared strongly at 326 nm (BP), 349 nm (MK), and 350 nm (MK-EtOH). The long absorption tail of ketone extending beyond 320 nm indicated that the light of this wavelength induced n–π*transitions. The intensity of the UV absorbance curve at the wavelength of UVA (365 nm) light used for the antimicrobial test showed similar values with MK and MK-EtOH. This greater intensity of UV absorbance peaks for MK and MK-EtOH (Figure 2(b)) than that of BP (Figure 2(a)) demonstrates the hyperchromic effect of substituents such as bis(N,N-dimethylamine) and (N-2-hydroxyethyl-N-methylamine).

Figure 2.

UV/vis spectra of photosensitizers BP, MK, and MK-EtOH in solvent (acetone at a concentration of 0.02 mmol).

Preparation of benzophenone chromophoric group incorporated cotton fabrics

Crosslinking cotton cellulose with polycarboxylic acids is well known since the effective polycarboxylic acids contain three or more carboxylic groups that are capable of forming five- or six-member cyclic anhydride rings.^25,26 Therefore, BTCA could serve as a crosslinking unit to connect MK-EtOH to the cellulosic substrate through esterification when cellulose fabrics were treated with a solution containing synthesized MK-EtOH, BTCA, and sodium hypophosphite via a pad–dry–cure method. The reaction of polycarboxylic acid with cellulosic polymers occurs through the esterification of hydroxyl groups; however, not all the carboxylic acids are able to react with the cellulosic substrate. The application of MK-EtOH to cotton by surface grafting that was characterized by FTIR and is compared to other antimicrobial agents (BP and MK) in Figure 3.

Figure 3.

FTIR spectra of pristine cotton (a) and cotton fabrics treated by various photosensitizers—BP (b), MK (c), and MK-EtOH, (d) from 500 to 4000 cm⁻¹.

Spectral analyses of the treated cotton fabrics indicated that no strong ester carbonyl peak appeared at around the 1700 ∼ 1750 cm⁻¹ region. This suggested that the presence of these photosensitizers tended to hinder direct esterification between BTCA and cotton, showing a minimum attachment of BTCA. However, low intensity peaks for ketone carbonyl were shown at 1642 cm⁻¹ in the treated fabrics (Figure 3(b) to (d)). The location of the peak was somewhat lower in energy compared with that of a normal ketone carbonyl because of conjugation with two aromatic groups in BP and its derivatives. The low intensity of this carbonyl peak was believed to be due to low fixation of BP derivatives to cotton. Nevertheless, in the spectra of the cotton fabrics treated by BP, MK, and MK-EtOH, a shoulder at 3019 cm⁻¹ from the stretching vibration of sp²-hybridized C–H bonds and a small peak at 1600 cm⁻¹ from the aromatic ring stretch substantiated the fixation of these compounds.

In addition, the most significant differences in the MK-EtOH treated cotton from other treated or pristine fabrics were the presence of peaks at 1258 cm⁻¹ for C–N stretching vibration and 799 cm⁻¹ for out-of-plane bending peak for aromatic rings. In addition, a group of small peaks appeared at around 2850 cm⁻¹ in the MK-EtOH treated cotton, probably due to possible amine salt (–R₃NH⁺) formation within MK-EtOH. This revealed that an introduction of reactive functional groups (–CH₂CH₂OH) enhanced the fixation of BP derivatives to cotton fabrics.

Cationic MB staining has often been used to estimate negatively charged functional groups within the molecules.²⁴ Negative charges within the treated cotton fabrics could come from two functional groups: free carboxyl groups from attached BTCA molecules and ketone carbonyl groups within the photosensitizers. Both functional groups were expected to be negatively charged in alkaline condition. Since the same amounts of BTCA and photosensitizer were employed in all the systems, the greater cationic MB staining could suggest the greater fixation of the photosensitizer and BTCA. Therefore, the results clearly demonstrated that the K/S value of cotton fabric treated by MK-EtOH and stained by MB was considerably greater than those of pristine and cotton fabrics treated by BP and MK (Figure 4).

Figure 4.

K/S values of cotton fabrics treated by various photosensitizers and stained by MB.

Antimicrobial properties

Table 1 presents the antimicrobial properties of cotton fabrics that contain various photosensitizers. The antimicrobial properties of cotton fabrics that contain MK and MK-EtOH are superior to those that contain conventionally used BP. Scheme 2 shows that the chromophoric groups (carbonyl groups) of organic photosensitizers undergo light excitation from n to π* triplet states through UVA irradiation. The triplet tends to abstract atoms from any active hydrogen donors, such as weak C–H bonds from polymers that can be quenched by oxygen and humidity to form a hydroperoxyl radical or ketyl radical.^13,27 Biological agents and toxic materials often serve as sources of active hydrogen that result in an antimicrobial effect; subsequently, fibers containing a photosensitizer would be easily excited to the active radicals that can decompose bacteria when activated by UV light. The antimicrobial effects of treated cotton fabric for S. aureus were superior to those of E. coli. The antimicrobial activity increased after exposure to UV irradiation. The overall results indicated that MK and MK-EtOH provided superior antimicrobial properties to those of BP. In addition, MK-EtOH provided a durable antimicrobial property after multiple washing and indicated a better fixation to the substrate.

Table 1.

Antimicrobial test results (reduction of bacteria (%)) of cotton fabrics treated with BP, MK, and MK-EtOH (0.1 M) after UVA irradiation for 2 h, respectively

	Staphylococcus aureus			Escherichia coli
	No UV	UV irradiation for 2 h	After washing (5 cycles)*	No UV	UV irradiation for 2 h	After washing (5 cycles)^a
Pristine cotton	32.8	49.8	-	26.2	38.3	–
BP	49.9	92.6	89.5	32.1	85.3	61.7
MK	84.6	99.9	99.9	77.3	99.9	88.5
MK-EtOH	89.5	99.9	99.9	75.6	99.9	95.9

Samples were exposed to UV for 2 h after washing five times.

Scheme 2.

A proposed antimicrobial mechanism of MK-EtOH.

Strength and thermal properties

As shown in Figure 5, tensile strength retentions of cotton fabrics treated by various photosensitizers did not change much from that of pristine cotton regardless of photosensitizers used. This was somewhat unexpected since the pad–dry–curing process of cotton with BTCA and acidic catalyst generally decreased cotton strengths. A probable reason for little reduction in the strength was the use of relatively low temperature curing (160℃) compared to 180℃, which was the common BTCA curing temperature. This resulted in a low fixation of BTCA along with photosensitizers, as previously indicated by no strong carbonyl peak in the FTIR spectra (Figure 3). A long drying time (30 min) did not adversely influence the strength either.

Figure 5.

Retention of tensile strength of cotton fabrics that contain photosensitizers BP, MK, and MK-EtOH.

The thermal stability of cotton fabrics treated with various antimicrobial agents by the pad–dry–cure method was measured with TGA and the results are shown in Figure 6(a). A differential graph of the TGA, as shown in Figure 6(b), indicates that the maximum decomposing temperature of pristine cotton was 303.18℃ and those of treated cottons with BP, MK, and MK-EtOH were 316.26℃, 320.14℃, and 270.45℃, respectively. Figure 6 and Table 2 show that critical degradation temperature and thermodegradation residue (%) of the treated cotton fabrics are not significantly affected by physical inclusion of BP and MK. A slight increase in the critical degradation temperature with BP and MK compared with that of the pristine cotton is probably due to the heat resistance character of the aromatic ring originating from BP chromophoric groups. However, the critical degradation temperature of MK-EtOH/BTCA-treated cotton decreased and percentage residue at 450℃ increased. This indicates that the introduction of BP chromophoric groups to cotton fabrics increases the carbonization rate of the treated fabrics under thermodegradation, and the decomposition of cellulosic substrate is catalyzed by the low acidity of BTCA and the presence of acid catalyst (sodium hypophosphite) at an elevated temperature.²²

Figure 6.

TGA (a) and DTG (b) measurements of cotton fabrics that contain photosensitizers BP, MK, and MK-EtOH.

Table 2.

TGA data of cotton fabrics treated with BP, MK, and MK-EtOH

Weight residue (%) (at 450℃)	Critical degradation temperature (℃)
Pristine cotton	13.09	303.18
BP (0.1 M)	11.45	316.26
MK (0.1 M)	12.04	320.14
MK-EtOH (0.1 M)	17.24	270.45

Surface and color properties

Figure 7 shows the morphology of cotton fabrics that contain organic photosensitizers. The SEM morphology of the treated cotton showed that organic photosensitizers were evenly distributed and diffused to the cotton fibers. Overall structure defects of cotton do not appear in Figure 7.

Figure 7.

Morphology of pristine cotton and treated cotton with various photosensitizers: BP, MK, and MK-EtOH.

The color change of cottons that contain organic photosensitizers are presented in Table 3. The lightness (L*) of cotton that contains BP shows similar high values; however, the lightness of cotton fabric treated by MK and MK-EtOH is slightly lower than that of other materials. All samples show negative a* values and positive b* values that indicate a light yellow–greenish color; especially, the ΔE value of the MK-EtOH-treated cotton is greater than that of the others, indicating a difference in functional group and fixation level of the photosensitizer in cotton fabrics treated by MK and MK-EtOH. However, the color of all materials looked white because the value L* of the samples is close to 100.

Table 3.

CIE L*a*b* colorimetric system of pristine cotton and cotton treated with BP, MK, and MK-EtOH

L^*	a^*	b^*	ΔE
Pristine cotton	98.561	−0.203	0.114	–
BP	98.138	−0.416	0.215	0.484
MK	97.736	−2.554	4.372	4.933
MK-EtOH	97.381	−4.017	9.327	10.965

Conclusions

To enhance the fixation of BP derivatives to cotton fabrics, a new photoactive antimicrobial agent was synthesized by introducing an active functional group (–CH₂CH₂OH) to 4,4′-bis(dimethylamino)benzophenone (MK). The 4-(dimethylamino)phenyl)(4-((2-hydroxyethyl)(methylamino)phenyl)methanone) (MK-EtOH) was successfully synthesized from p-nitrobenzoyl chloride as a starting material and confirmed by NMR. The optical properties of organic photosensitizers indicate that active UV absorbance ranges are different; BP (325 ∼ 340 nm), MK (325–400 nm), and MK-EtOH (300 nm and 330–390 nm). The intensity of the UV absorbance curve at the wavelength of UVA (365 nm) light used for the antimicrobial test showed a higher value with MK-EtOH and MK as compared to BP.

Photosensitizers such as BP, MK, and the synthesized MK-EtOH were applied to cotton fabrics by a pad–dry–cure method to demonstrate the antimicrobial properties in the application of material for protective clothing. Antimicrobial activity increased after exposure to UV irradiation. The overall results indicated that MK and MK-EtOH provided superior antimicrobial properties to BP. In addition, MK-EtOH provided durable antimicrobial activity after multiple washing and indicated better fixation to the substrate. It was confirmed through FTIR analysis, in which the MK-EtOH-treated cotton showed the presence of a 1258 cm⁻¹ peak for C–N stretching vibration and 799 cm⁻¹ peak for out-of-plane bending for aromatic rings. The MB staining method was also used to substantiate greater fixation of MK-EtOH. The SEM morphology of the treated cotton showed that organic photosensitizers were evenly distributed and diffused to cotton fibers. Furthermore, the tensile strengths of cotton fabrics treated by various photosensitizers were not significantly reduced from that of pristine cotton regardless of photosensitizers used. There is a slight reduction of thermal stability and yellowish color changes; however, it was proved that the synthesized MK-EtOH is a suitable photoactive antimicrobial agent for cotton fabric with a strong potential to protect human health from pollutants and bacteria that can have useful applications for clothing, industrial materials, and medical applications.

Footnotes

Funding

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (Project Nos. 2012-0002165 and 2012-047656).

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