Preparation and Characterization of Silver-Loaded Hemp Fibers with ...

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Sorption properties of hemp fibers were improved by non-selective oxidation ... Obtained silver-loaded hemp fibers show antimicrobial activity against tested.
Fibers and Polymers 2014, Vol.15, No.1, 57-64

DOI 10.1007/s12221-014-0057-7

Preparation and Characterization of Silver-Loaded Hemp Fibers with Antimicrobial Activity Mirjana M. Kostic*, Jovana Z. Milanovic, Marija V. Baljak, Katarina Mihajlovski, and Ana D. Kramar Faculty of Technology and Metallurgy, University of Belgrade, 11000 Belgrade, Serbia (Received February 20, 2013; Revised May 31, 2013; Accepted June 8, 2013) Abstract: The objective of this research was to impart antimicrobial properties to hemp fibers by incorporation of silver ions in hemp fibers by chemisorption. Sorption properties of hemp fibers were improved by non-selective oxidation using hydrogen peroxide and potassium permanganate. The optimal conditions for silver ions sorption by hemp fibers were determined by changing sorption conditions: pH value and concentration of aqueous silver nitrate solution, as well as duration of sorption. The maximum sorption capacity of modified hemp fibers was 1.84 mmol of Ag+ ions per gram of fibers. Antimicrobial activity of silver-loaded hemp fibers against different pathogens: Staphylococcus aureus, Escherichia coli, and Candida albicans was evaluated in vitro. Obtained silver-loaded hemp fibers show antimicrobial activity against tested pathogens. Keywords: Hemp fibers, Hydrogen peroxide, Potassium permanganate, Silver ion sorption, Antimicrobial activity

Introduction

of different applications as far as sportswear or occupational clothing, medical and hygienic tasks, etc [1,6-8]. Various methods have been developed or are under development to impart antimicrobial activity to textiles, and several major classes of antimicrobial agents are used (i.e. metals and metal salts, quaternary ammonium compounds, triclosan, dyes, etc.). Depending on the particular active agent and fiber type, antimicrobial properties can be accomplished in textile by incorporation of volatile and non-volatile antimicrobial agents directly into fibers, coating or adsorbing antimicrobials onto fiber surfaces, immobilization of antimicrobials to fibers by ion or covalent linkages, and the use of fibers that are inherently antimicrobial (i.e. chitin and chitosan fibers) [3,7,9,10]. According to the literature [11], one of the most effective antimicrobial methods is based on the design of a coating that contains silver. Silver in various forms has a long history as an antimicrobial agent, being unique because of its inherent properties of high thermal stability and long-term activity. Positively charged silver ion has been one of the most versatile antimicrobial agents due to its intense antimicrobial properties and little toxicity to mammalian cells and tissue [12,13]. Silver has been proven effective in killing over 650 disease-causing organisms, and is active against gram-negative and gram-positive bacteria, as well as fungi, protozoa and certain viruses [14]. Furthermore, silver is one of a few antimicrobial agents which possess both antibacterial and antifungal activity, and bacteria are not able to develop their resistance to the silver, as in the case of antibiotics [2,13,15]. The objective of this research was to impart antimicrobial properties to hemp fibers in order to obtain high added value products based on hemp fibers and thus to tap new markets with the product like clothes for special medical conditions (i.e. skin diseases), footwear lining, or bedclothes for hospitals, etc. Hemp fibers were chosen among traditional textile raw

It has long been recognized that microorganisms can grow on textile substrate. In fact, conventional fibers and polymers not only do not show any resistance against micro-organisms and materials generated from their metabolism but also they are most common materials for accumulation and proliferation of micro-organisms into the surrounding environment. Clothing such as footwear, sportswear and underwear, hospital healthcare textiles (beddings, curtains, uniforms, towels, etc.), military garments, civil defense personnel protective garment are excellent substrate for a rapid multiplication of microorganisms, due to several factors such as suitable temperature and humidity, presence of dust, soil, spilled food and drinks stains, skin dead cells, sweat and oil secretions of skin glands, and also finishing materials on the textile surfaces. The growth of microorganisms on textiles inflicts a range of unwanted effects not only on the textile itself but also on the wearer. Furthermore, clothing is with the longest contact to the human skin and plays an important role especially in skin conditions like atopic dermatitis, hyperhidrosis, diabetic patients and aged skin [1-5]. One way to avoid the microbial degradation of textile fibers, limit the incidence of bacteria, reduce the formation of odor following the microbial degradation of perspiration, and as the most important protect users by avoiding the transfer and spread of pathogens, is the treatment of textiles with antimicrobial agents. Whilst in the past it was predominantly technical textiles which had antimicrobial finishes, in particular to protect against fungi, nowadays the number of applications of antimicrobial-finished textiles has increased dramatically, for instance, in outdoor textiles, air filters, automotive textiles, domestic home furnishings and medical textiles. Furthermore, textiles worn close to the body have been antimicrobial-finished and developed for a variety *Corresponding author: [email protected] 57

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materials, because of their specific properties, namely high absorbency and hygroscopicity, good thermal and electrostatic properties, protection against UV radiation and lack of any allergenic effect, that make them different from other fibers, and the fact that their excellent properties have not yet been fully exploited [16-18]. Moreover, there are just a few studies on the obtaining of antimicrobial hemp fibers: Racu et al. [19] grafted hemp fibers with beta-cyclodextrin derivatives, while Milanovic et al. [20] incorporated silver nano particles into previoisly selectively oxidized hemp fibers by 2,2,6,6-tetramethylpiperidine-1-oxy radical (TEMPO). In this paper, antimicrobial hemp fibers were prepared by non-selective oxidation, i.e., oxidation with peroxide and permanganate, followed by silver sorption from aqueous silver nitrate solution. A series of the peroxide and permanganate oxidations, under different conditions, were done in order to determine the most suitable experimental conditions for “the activation” of hemp fibers. The sodium hydroxide and silver ions uptake, as well as tensile strength, were used to assess the changes in hemp fibers due to the oxidation. The optimal conditions for silver ions sorption from aqueous silver nitrate solution by hemp fibers were determined by changing sorption conditions, and antimicrobial activity of silver-loaded hemp fibers against different pathogens: Staphylococcus aureus, Escherichia coli, and Candida albicans was evaluated in vitro, before and after 1 and 3 washing cycles.

Experimental Materials The fibers used in this investigation were good-quality water-retted hemp obtained from ITES Odzaci, Serbia. Chemical composition of used fibers are: water solubles 1.70 %, fats and waxes - 1.59 %, pectin - 1.55 %, a-cellulose - 76.12 %, lignin - 5.65 % and hemicellulose - 12.28 %. All chemicals were obtained from commercial sources and were p.a. grade. Methods Preparation of Oxidized Hemp Fibers Hemp fibers were modified by non-selective oxidation using hydrogen peroxide and potassium permanganate (3 % and 6 %), 1:50 liquor ratio, at room temperature and 45 oC, for different periods of time (10, 20 and 30 minutes), and pH 5, 7, 9 and 11, followed by washing with distilled water (in the case of permanganate also 2 % sodium bisulphite was used), and overnight drying in air. The pH of oxidizing agents solutions were adjusted by adding sodium hydroxide or acetic acid. Determination of NaOH Uptake The amount of cation exchange functions in hemp fibers was characterized by NaOH uptake, which was determined by the change in solution concentration from before to after

Mirjana M. Kostic et al.

sorption by fibers applying the volumetric method. Hemp fibres were treated with 0.01 M NaOH at room temperature for 1 hour, and after that solution was titrated with 0.01 M HCl in presence of an indicator. Determination of Mechanical Properties The tensile strength and elongation of single hemp fibers were determined as the average of at least ten measurements, on tester type AVK-Budapest (Hungary) with clamps spaced at 100 mm and with strain rate (bottom clamp rate) of 150 mm/min, by following the usual procedure described elsewhere [21]. Due to the variation in the hemp fibers fineness, as well as the fact that raw fibers stick into bundles, while oxidized fibers are mainly separated into single elemental cells (fibers), the tensile strength is expressed as tenacity - specific value related to fineness (force per unit fineness). For such purposes fineness of each single fiber was determined before tensile testing. Silver Ions Sorption by Oxidized Hemp Fibers Silver ions were incorporated into previously oxidized hemp fibers and control one by chemisorption under following conditions: fibers (0.1 g) were immersed in 100 ml of AgNO3 solution (0.001-0.1 mol/dm3), at pH 3.5-11, and shaken at room temperature for 15-300 min in the dark. The pH value of the initial AgNO3 solution was adjusted with HNO3 or NH4OH. The change in concentration of Ag+ after sorption was determined by KSCN titrations employing Fe(NH4)(SO4)2 as an indicator according to Volhard’s method [22]. Antimicrobial Activity Antimicrobial activity of modified hemp fibers was assessed using two test methods, agar diffusion test [23], which is semi-quantitative, and quantitative method based on microorganism counting [24]. Three test organisms: Staphylococcus aureus ATCC 25923, Escherichia coli ATCC 25922 and Candida albicans ATCC 24433 were used. The agar diffusion test consists in placement of 1×1 cm samples onto an agar support inoculated with tested micro organisms and, after 24 h incubation at 37 oC, measuring the width of the zone of inhibition (clear) or suppression (diffuse) of growth against the indicator organisms in comparison to a control sample. The method based on microorganism counting consists of the following procedure: each of the test microorganism (105-106 log N/ml) was inoculated into 9 ml sterile potassium hydrogen phosphate buffer solution (pH 7.2) at 37 oC for 24 h. Modified hemp fiber samples were added in solution and incubated at 37 oC for 24 h. Viable cells (log N/ml) were enumerated on TSA agar by pour plating 1 ml of serial dilutions of physiological solution followed by incubation at 37 oC for 48 h. The average values of the duplicates were converted to colony forming units per milliliter. The percentage reduction of test organism can be calculated by the following equation: R (%) = (A−B)×100/A

(1)

where A and B are the number of colonies per milliliter for

Silver-Loaded Hemp Fibers with Antimicrobial Activity

the control and silver-loaded hemp fibers test samples, respectively. Washing Durability of Antimicrobial Finishes Washing of silver loaded hemp fibers was performed according to standard ISO 105 - C01 [25]. The samples were washed in the bath containing 0.5 % standard soap. After 30 min of washing at 40 oC, samples were rinsed with distilled water for 1 min and then thoroughly rinsed with tap water for 1 min. After rinsing, the samples were dried at 40 oC. Antimicrobial activity was evaluated after 1 and 3 washing cycles.

Results Modification of Hemp Fibers The first stage of obtaining antimicrobial silver-loaded hemp fibers involves the formation of reactive functional groups by oxidation with non-selective oxidizing agents, hydrogen peroxide and potassium permanganate. Hydrogen peroxide and potassium permanganate are well known bleaching agents, capable of oxidizing low molecular impurities on natural fiber surfaces, thus obtaining not only greater whiteness but also eventual surface cleaning and oxidation of available functional groups of fibers [26-28]. Oxidation of hemp fibers was chosen because of both improving sorption properties due to the formation of more reactive functional groups (i.e. cation exchange functions) and fiber refinement by lignin removal. By changing the parameters of the oxidation (concentration of oxidizing agents, pH, reaction time and temperature) it is possible to obtain hemp fibers with a different amount of cation exchange functions (0.87 to 5.88 mmol Na+ per gram of fibers) which is shown in Figures 1-3. Amount of cation

Figure 1. NaOH uptake by oxidized hemp fibers vs. concentration and pH value of oxidizing agent solution.

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exchange functions into the fibers depends directly on the time of modification, pH value of solution, temperature, as well as concentration of oxidizing agents. Also, higher values were obtained in the case of permanganate oxidation. This is probably consequence of instability of H2O2 which goes under self scavenging and generates hydroperoxyl radical and this radical terminates reaction by reacting with hydroxyl radical and in the end producing water and oxygen [29-31]. Results presented in Figure 2 and Figure 3 clearly indicate that hydrogen peroxide reaches the highest activity in the first 10 min and with prolonged treatment time NaOH uptake is not changing significantly, while, in the case of permanganate, NaOH uptake is rising constantly during

Figure 2. NaOH uptake by oxidized hemp fibers vs. concentration of oxidizing agent solution and reaction time.

Figure 3. NaOH uptake by oxidized hemp fibers vs. temperature of oxidizing agent solution and reaction time.

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treatment and the highest uptake is obtained after 30 min of treatment, which is maximum treatment time used in this work. Changes in the structure of hemp fibers, caused by peroxide and permanganate oxidation, have significant influence on physical and mechanical properties of the oxidized fibers, such as fineness and tensile strength, that are important from the standpoint of practical applications. Technical (multicellular) hemp fibers are composed of elementary fiber bundles joined by the middle lamella, which is mostly build of the woody component - lignin, while the interfibrillar regions are fulfilled with hemicelluloses, and each elementary fiber can be considered as a network of ultrafine cellulose fibrils embedded in a matrix of hemicelluloses and lignin [17,32-34]. As a result of oxidative treatment and lignin removal, oxidized hemp fibers acquired a high level of divisibility and elementary fiber liberation, fiber fineness was reduced from 21.5 tex for unmodified up to 9.5 tex for modified sample (data for fiber fineness are not given). The oxidized fibers are soft in hand, unlike to unmodified fibers that are too coarse and stiff. Fibers modified under severe conditions (higher pH value and temperature of solution, as well as longer reaction time) became too fragile and their fineness was not determined. Since the oxidation of hemp fibers leads to changes in their chemical composition and fineness, changes in mechanical properties of oxidized fibers also are expected. The effect of peroxide and permanganate oxidation on fibers tenacity can be seen in Figure 4. In our experiment, the tenacity of all treated fibers was reduced, with higher decrease obtained for permanganate oxidation. Obtained decrease can be explained by lignin removal from middle lamella, that binding the elementary fibers and microfibrills [35], and cellulose depolimerization [36,37]. According to literature [36,37],

Figure 4. Tenacity of oxidized hemp fibers vs. pH value of oxidizing agent solution and reaction time.

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the cellulose chain is not shortened by oxidation, but has suffered from a modification that eases further attacks on the molecule, mostly due to preceding β-elimination reaction that is pronounce under alkaline pH conditions. Applied oxidation has no significant effect on the elongation of modified hemp fibers. Modification with hydrogen peroxide and potassium permanganate leads to a slight increase of elongation, from 5.04 % to 8.35 %. Hemp fibers with cation-exchange functionality obtained in this way, can be used for effective isolation of different cations from solutions and gasses. Functionalities within the fiber allow chemoterapeutic agents with basic properties to bond chemically with the fibers. Therefore, in the second stage of obtaining of antimicrobial hemp fibers, we used introduced cation-exchange functions as reactive “chemical hooks” for silver ions. Obtaining of Silver-loaded Hemp Fibers Sorption of metal ions by hemp fibers is not restricted to one sorption mechanism only, but comprises several mechanisms such as ion exchange, chelation, precipitation, sorption by physical forces and ion entrapment in inter- and intrafibrillar capillaries and spaces of structural lignin and polysaccharide networks. Therefore, sorption of metal ions by lignocellulosics is affected by several factors such as initial pH, initial metal ion concentration, contact time, temperature, fiber pretreatment, etc [38-40]. In this work the optimal conditions for silver ions sorption from aqueous silver nitrate solution by hemp fibers with improved chemisorption properties were determined by changing sorption conditions: pH value, duration of sorption and concentration of aqueous silver nitrate solution. The effect of the solution pH on the silver ions sorption by hemp

Figure 5. Effect of the pH value of the AgNO3 solution on the quantity of Ag+ sorbed by oxidized hemp fibers from 0.01 M AgNO3, for 240 min, at RT.

Silver-Loaded Hemp Fibers with Antimicrobial Activity

fibers oxidized with 3 % and 6 % permanganate solutions is illustrated in Figure 5. As seen in the figure, metal ions sorption was strongly dependent on the solution pH. The maximum Ag+ sorption occurred at the pH 9 because in the alkali environment, fibers pores are swelling, allowing solution to penetrate into the fibers thus enabling Ag+ ions to access internal cation exchange functions. Since these functions are progressively deprotonated at higher pH, ion exchange sorption is even more promoted. Decrease in Ag+ sorption above pH 9 can be explained by competition among NH4+ and Ag+ ions as well as increase in concentration of Ag(NH3)2+ relative to Ag+. In spite of these results, as the optimal pH value of the initial AgNO3 solution pH 5 (the second peak) was selected by taking into consideration the alkaline

Figure 6. Effect of the duration of sorption on the quantity of Ag+ sorbed by oxidized hemp fibers from 0.01 M AgNO3, pH 5, at RT.

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instability and alkaline depolymerization of oxidized celluloses [37,41]. Obtained results are in agreement with literature data [40]. The quantity of Ag+ ions sorbed by hemp fibers depends directly on the duration of sorption (results presented in Figure 6), as well as concentration of AgNO3 solutions (Figure 7). The results show an increase from 0.018 mmol/g for unmodified hemp fibers to 1.834 mmol/g for oxidized hemp fibers. Obtained results are much higher than the value found in literature [40] for sulphydryl hemp fibers (0.1 mmol/g), which proves that oxidation of hemp fibers is suitable for improving sorption properties of hemp fibers and besides other complicated methods can be avoided. Color of hemp fibers was changed after silver sorption from cream yellow for unmodified fibers to brown, as it is illustrated in Figure 7(right). The color strength depends on the silver content. A concentration of 0.01 M was chosen as optimal parameter for treatment of oxidized hemp fibers with AgNO3, despite the fact that maximum silver sorption per gram of fibers was obtained using 0.05 M solution (Figure 7). Silver ion exhaustion is the same for 0.01 and 0.05 M concentration of solution (data not shown), therefore lower concentration means lower chemical consumption and economically more justified process, together with satisfactory antimicrobial activity, which will be shown in the next section. Influence of oxidation conditions on silver ion uptake under chosen parameters of silver sorption (0.01 M AgNO3, pH 5, RT, 240 min) is presented in Figure 8. Fibers previously oxidized with KMnO4 show better sorption of silver compared to fibers oxidized with H2O2. Also, when fibers are oxidized with permanganate at 45 oC, pH of oxidizing solution does not have influence on quantity of silver ions sorbed by fibers. Overall, the best results in regard to silver sorption are obtained using fibers oxidized at pH 9 regardless of type

Figure 7. Effect of the concentration of the AgNO3 solution on the quantity of Ag+ sorbed by oxidized hemp fibers, for 240 min, at pH 5, RT (left); effect of the quantity of sorbed Ag+ on the hemp fibers color (right).

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Figure 8. The effects of oxidation conditions (temperature and pH value of the oxidizing agent solution) on the silver ions uptake by oxidized hemp fibers (sorption experiments: 0.01 M AgNO3, 240 min, RT, pH 5).

of oxidizing agent or temperature. This is again consequence of fibers swelling in alkali environment, which causes better penetration of oxidizing agent into internal structure of fibers and consequently introduction of larger amount of cation exchange functionalities available to interact with silver ions. Antimicrobial Activity of Silver-loaded Hemp Fibers The antimicrobial activity of the silver-loaded hemp fibers was examined against different pathogenes: Gram-negative bacteria strains - Escherichia coli, Gram-positive bacteria strains - Staphylococcus aureus, and yeast - Candida albicans. Results of agar diffusion test (Table 1) show that incorporation of silver ions in hemp fibers leads to the generation of antimicrobial materials having activity against a broad spectrum of microbes. Among tested microorganisms, the Gram-positive bacteria strains of Staphylococcus aureus are

Mirjana M. Kostic et al.

the most sensitive to silver-loaded hemp fibers, which can be more easily seen from the results of microorganism reduction presented in Table 2. The silver-loaded hemp fibers provide maximum microorganism reduction in the case of S. aureus (100 %), while in the case of E. coli and C. albicans only hemp fibers with silver content higher than 1.3 mmol/g and 1.5 mmol/g, respectively, reach the level of microbial reduction 99.9 %. Among tested micro-organisms, the yeast C. albicans is the least sensitive to silver-loaded hemp fibers. Furthermore, obtained antimicrobial activity of the silver-loaded hemp fibers is persistent after three standard washing cycles, data presented in Table 3. After the first washing, there is no changes in antimicrobial activity of the hemp fibers previously oxidized under more severe conditions (samples No 7 and 9, oxidized with 6 % of peroxide and permanganate, respectively), while a little decrease in the microbial reduction was observed Table 2. Microorganism reduction (R, %) of silver-loaded hemp fibers (for sample description see Table 1) The quantity of R (%) Sample Ag+ ions sorbed no. by hemp fibers S. aureus* E. coli** C. albicans*** (mmol/g) 1 0.018 100 97.6 99.2 2 0.060 100 99.5 99.5 3 0.129 100 92.8 78.3 4 0.271 100 99.7 99.5 5 1.521 100 99.9 99.9 6 0.074 100 95.2 95.6 7 1.388 100 99.9 99.5 8 0.313 100 98.1 99.5 9 1.834 100 99.9 100 *Initial number of microbial colonies: 3.7×106, **initial number of microbial colonies: 4.2×105, and ***initial number of microbial colonies: 2.3×105.

Table 1. Antimicrobial activity of the silver-loaded hemp fibers determined by the agar diffusion test The quantity of Ag+ ions sorbed by hemp fibers (mmol/g) 1 Unmodified hemp fibers + 0.01 AgNO3 0.018 2 3 % H2O2 + 0.01 AgNO3 0.060 3 3 % H2O2 + 0.1 AgNO3 0.129 0.271 4 3 % KMnO4 + 0.01 AgNO3 5 3 % KMnO4 + 0.1 AgNO3 1.521 6 6 % H2O2 + 0.01 AgNO3 0.074 1.388 7 6 % H2O2 + 0.1 AgNO3 8 6 % KMnO4 + 0.01 AgNO3 0.313 9 6 % KMnO4 + 0.1 AgNO3 1.834 *IC - inhibition in contact and **- zone of suppression. Sample no.

Sample description - hemp fibers oxidized 20 min, RT, pH7, with:

Width of the inhibition zone after 24 h (mm) S. aureus

E. coli

C. albicans

2.0 4.0 2.5 3.5 4.0 4.0 4.0 4.5 3.5

IC* 2.0 IC+11** 2.0 4.0 IC+9** 2.5 2.0 3.0

IC 2.5 IC 2.5 4.0 1.5 3.0 2.5 2.5

Silver-Loaded Hemp Fibers with Antimicrobial Activity Table 3. Washing durability of antimicrobial activity of the silverloaded hemp fibers: microorganism reduction (R, %) of silverloaded hemp fibers (for sample description see Table 1) The quantity R (%) Number of Ag+ ions Sample of sorbed by S. E. C. no. washing hemp fibers aureus* coli** albicans*** cycles (mmol/g) 1 88.9 87.6 84.3 1. 0.018 3 79.1 78.5 65.5 1 99.8 92.1 78.1 3. 0.129 3 93.8 82.7 74.1 1 100 99.9 99.9 5. 1.521 3 99.3 95.2 97.2 1 100 99.9 99.5 7. 1.388 3 99.9 98.1 99.2 1 100 99.9 100 9. 1.834 3 100 99.9 99.9 *Initial number of microbial colonies: 7×105, **initial number of microbial colonies: 2.2×107, and ***initial number of microbial colonies: 3.5×104.

for samples previously oxidized under mild condition (samples No 3 and 5, oxidized with 3 % of peroxide and permanganate, respectively). After the third washing, decrease in microbial reduction was observed for all samples. The strongest decrease in antimicrobial activity was observed for the hemp fibers treated with silver nitrate solution, without oxidative pretreatment; after the first washing cycle, sample shows microbial reduction below 89 %, and after the third washing level of microbial reduction decreases to 65.5 % in the case of C. albicans. There is no clear dose dependant antimicrobial activity but the quantity of bonded silver ions, in all cases, is enough to develop desirable antimicrobial activity in the hemp fibers. This can be explained by the fact that the silver ions act in a complex manner, i.e. silver binds to electron donor receptors, notably disulphide, amino, imidazole, carbonyl and phosphate residues on membranes leading to intracellular absorption. Inactivation of membrane-related enzymes results in denaturation of the bacterial cell envelope which ultimatly leads to the lethal effect on a cell [13]. This mode of antimicrobial action does not necessarily depend on quantity of silver ions, but rather on availability of silver in cellulose matrix and ability to reach and react with microorganisms cell. Moreover, according to Davis and Etris [42] silver ions does not attack microorganisms directly; it operates as a catalytic agent and it is not consumed in this process.

Conclusion This study confirms the possibility of obtaining biologically

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active silver-loaded hemp fibres using oxycellulose hemp fibers. Herein, the hemp fibers were first oxidized using nonselective oxidation agents, hydrogen peroxide and potassium permanganate, in order to improve their sorption properties, mainly by increasing cation exchange functions. Different oxidation conditions were used to produce larger amounts of the cation exchange functions. The amounts of the cation exchange functions in hemp fibres increased with increased oxidation time, pH value of solution, temperature and oxidant concentration. Also, higher values were obtained in the case of permanganate oxidation. The maximum sorption capacity of modified hemp fibers was 1.84 mmol of Ag+ ions per gram of fibers. Obtained silver-loaded hemp fibers show antimicrobial activity against tested pathogens (Staphylococcus aureus, Escherichia coli, and Candida albicans). These fibers indicate different activity against different micro-organisms, generally showing stronger bactericidal effects for gram-positive bacteria S. aureus than for gram-negative bacteria E. coli and can be used in manufacturing of specific textiles like bed sheets and shirts for e.g. the patients with atopic dermatitis or footwear lining.

Acknowledgements This study has been supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia (Project OI 172029). The authors also thank ITES Odzaci (Serbia) for supplying hemp fibers.

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