Introduction

Protection against microorganisms is an important functional property of textile fibres. Textiles with antimicrobial properties are used in various products with apparel, protective, technical, medical and hygienic applications. Different antimicrobial agents are used to achieve antimicrobial properties. Silver and silver-based compounds, especially in the form of nanosized particles, are some of the most important biocides used to impart antimicrobial properties to textile fibres through chemical modification processes [1–3]. Silver is a leaching antimicrobial agent [2, 4], the efficiency of which depends directly on the concentration of the silver ions and silver nanoparticles released from the textile fibres into the surroundings, where the ions and nanoparticles act as a poison to microorganisms. Accordingly, the biostatic activity of silver is obtained at concentrations higher than the minimum inhibitory concentration; in contrast, to achieve biocidal activity, the minimum lethal concentration should be exceeded. In addition to its antimicrobial properties, silver offers the additional advantage of not constituting a major risk to human health, especially in low concentrations [5–7].

The concentration of silver ions or nanoparticles released is directly affected by the concentration and the particle size of the silver-based antimicrobial agent present on the fibres. Specifically, the efficiency of the silver nanoparticles is believed to strongly increase with a reduction in size because the specific surface area of a particle increases as size decreases [8–13]. This effect allows small particles to interact with microorganisms while simultaneously enabling a significant increase in the concentration of silver cations released. Therefore, the chemical structure, as well as the morphological and topological properties of the fibres, both of which influence the silver adhesion ability and the sorption capacity, can be assumed to substantially affect the amount of silver particles adsorbed and the mode of adsorption. Consequently, the antimicrobial properties of the silver-modified fibres also would be modified.

The chemical modification of fibres with silver-based compounds has been researched intensively. Studies have been mainly focused on different application processes and antimicrobial activity of silver-based compounds [14–27]. The mode of the silver antimicrobial activity in relation to the minimum inhibitory and the minimum lethal concentrations against various types of microorganisms has been thoroughly investigated on the individual fibres [16–18, 21, 22], among which cotton predominates [11, 16, 17, 19, 21–25]. However, the relationship between the fibre structure and its antimicrobial activity has not been fully and systematically established. Moreover, to the best of our knowledge, the literature contains no reports regarding a universal process for the application of a silver-based compound with the ability to provide biocidal activity to both natural and synthetic fibres as well as their blends.

In this study, we show that the chemical structure and morphological properties of fibres directly affect the concentration of the adsorbed silver nanoparticles and that the mode by which the silver particles bind to the fibres substantially influences the antimicrobial activity of the fibres. Furthermore, the ultimate goal of this study is to introduce a novel antimicrobial application procedure that is effective for the chemical modification of different fabrics, irrespective of their chemical, morphological and construction properties. To this end, a two-step procedure that includes the pad-dry-cure method for the creation of a silica matrix on the fibre surfaces based on an inorganic–organic hybrid sol–gel precursor [reactive binder (RB)] followed by the in situ synthesis of AgCl particles on the RB-treated fibres was developed. We subsequently applied this procedure to eight fabric samples of different fibre compositions to create Ag–silica–fibre nanocomposites. The presence of the silica matrix on the fibres was expected to enhance the concentration of the adsorbed silver, which would be particularly significant in the case of hydrophobic synthetic fibres. Furthermore, we hypothesise that the incorporation of particles of Ag compounds into the Ag–silica–fibre nanocomposites would lead to bactericidal and fungicidal activities.

Experimental

Materials

Eight plain-weave woven fabric samples were used in the experiments. Fabric samples were composed of different natural and man-made fibres and their blends which are usually used in the apparel. The fabric sample codes, as well as their chemical compositions and construction parameters, are presented in Table 1. The final steps of fabric sample preparation included washing with distilled water, neutralising in a dilute CH3COOH solution, thoroughly rinsing with distilled water, squeeze-drying and then air-drying at room temperature.

Table 1 Fabric code (Fc), fibre composition and constructional parameters
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To achieve the antimicrobial finish, silver nitrate (AgNO3; Sigma-Aldrich) and sodium chloride (NaCl; Carlo Erba) of purity grade puriss. p.a. (dry substance ≥98.5 %) were used. A commercial water-borne Si- and Zr-based sol–gel precursor known as iSys MTX (CHT, Germany) was used as a reactive binder (RB) with Kollasol CDO (CHT, Germany), which is an anti-foaming and wetting agent. iSys MTX is an organic–inorganic hybrid material that includes different metal atoms and reactive organic groups. Detailed analysis of the chemical composition of RB was described in our previous papers [16, 21]. Kollasol CDO is a hydrophilic silicone surface-active agent mixed with higher alcohols. All solutions were prepared in double-distilled water.

Antimicrobial coating of the fabrics

The fabric samples were antimicrobially coated using a two-step procedure, according to our previous research [21]. In the first step, the samples were treated with iSys MTX and Kollasol CDO using the pad-dry-cure method. This method includes full immersion at room temperature, squeezing to the wet pickup of 80 %, drying at 120 °C and curing for 1 min at 150 °C to remove the non-flammable solvents with high boiling points. The wet pickup (wpu) was determined as follows:

$$ {\text{wpu}} = \frac{{m_{2} }}{{m_{1} }} \times 100\left( \% \right), $$
(1)

where m 2 is the mass of the solution applied and m 1 is the mass of the dry fabric sample [4].

After coating, the fabric samples were maintained for 7 days under standard atmospheric conditions (65 ± 2 % relative humidity and 20 ± 1 °C) for complete RB network formation due to the applied iSys MTX. In the second step, the in situ synthesis of AgCl particles on the RB-treated samples was performed as follows: the samples were immersed for 10 min in a AgNO3 solution with a concentration of either 0.10 or 0.50 mM with a liquor ratio of 50:1 and were left at room temperature with occasional stirring; the fabric sample was subsequently immersed in a NaCl solution of the same concentration under the same conditions. The consecutive immersion procedure was repeated twice. Afterwards, the Ag–silica-coated fabric samples were intensively rinsed with double-distilled water to remove the excess chemicals, and the samples were dried at room temperature.

For comparison, a one-step application procedure (1S) was applied to the fabrics, where only the in situ synthesis of AgCl particles was performed on the samples according to the second step of the 2S procedure. The fabric sample codes, the application procedures and the sol concentrations are listed in Table 2.

Table 2 Fabric sample codes and sol concentrations used for coating of fabric samples
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Washing procedure

The coated fabric samples were washed in an AATCC Atlas Launder-O-Meter standard instrument. The duration of the washing cycle was 30 min; the washing was performed at a temperature of 40 °C, and the washing cycles were conducted in a solution of SDC standard detergent at a concentration of 5 g/L, which resulted in a liquor ratio of 50:1. After being washed, the samples were rinsed in cold distilled water and then immersed in cold tap water for 10 min, squeezed and dried at room temperature.

Analyses and measurements

Scanning electron microscopy with energy-dispersive X-ray spectroscopy

Untreated and coated fabric samples were analysed using a JEOL JSM 5610 LV scanning electron microscope equipped with an Oxford-Link ISIS 300 EDXS system with an ultra-thin window Si(Li) detector. Prior to performing the scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDXS) analyses, we applied a 20-nm-thick carbon layer to each fabric sample to ensure sufficient electrical conductivity and to avoid charging effects. SEM micrographs were recorded using secondary electron (SE) and backscattered electron (BSE) imaging modes. BSE compositional contrast (Z-contrast) was applied to accentuate the differences between the added particles and the fibre matrix. Two parallel assessments were performed for each coated fabric sample, and the corresponding atomic concentration is reported as the mean value and corresponding standard deviation (SD).

Inductively coupled plasma mass spectroscopy

The concentration of Ag in the coated fabric bulk samples was determined by inductively coupled plasma mass spectroscopy (ICP-MS) using a Perkin Elmer SCIED Elan DRC spectrophotometer. Fabric samples (0.5 g) were prepared in a Milestone microwave system by acid decomposition using 65 % HNO3 and 30 % H2O2. Three measurements were taken for each sample, and the Ag concentrations are reported as the mean values and the SD.

Water uptake

The water uptake (WU) of a sample was measured by immersing a thoroughly dried, pre-weighed fabric sample in 300 mL of double-distilled water for 40 min to simulate the conditions present in the in situ synthesis of AgCl particles on the RB-treated samples. After being immersed, the fabric sample was gently removed, placed on filter paper to remove the surface water and reweighed to obtain the wet mass. WU was calculated as follows:

$$ {\text{WU}} = \frac{{m_{3} - m_{1} }}{{m_{1} }} \times 100\left( \% \right), $$
(2)

where m 3 is the mass of the wet fabric sample. Five measurements were taken for each sample, and the corresponding WU value for each is reported as the mean value and SD.

Antimicrobial activity

Antibacterial activity

The antibacterial activities of the untreated and coated fabric samples were estimated for the gram-negative bacterium Escherichia coli (ATCC 25922) and gram-positive bacterium Staphylococcus aureus (ATCC 25923) according to the ASTM E 2149-01 standard method. The analysis was performed by a certified laboratory. The fabric sample (approximately 1 g) was immersed in 50 mL of an inoculated buffer solution in a flask, which was then agitated using a wrist-action shaker. The bacterial concentration was approximately 105 CFU/mL. The time period for the exposure was 1 h. After the flasks were shaken, an aliquot of each suspension was placed on nutrient agar and incubated at 37 °C for 24 h. The number of colony-forming units (CFU) was counted, and the reduction of bacteria, R, was calculated as follows:

$$ R = \frac{B - A}{B} \times 100 \left( \% \right) $$
(3)

where A is the CFU for a flask containing a substrate after 1 h of contact time, and B is the CFU for a flask used to determine A before the addition of a substrate (time ‘0’). Two parallel assessments were performed for each fabric sample, and the mean value and SD were determined.

Antifungal activity

The antifungal activities of the untreated and coated fabric samples were estimated for the fungus Aspergillus niger (ATCC 6275) as follows. The fabric samples were sterilised by ironing. An aqueous spore suspension of A. niger was prepared at a concentration of 7.5–9 × 106/mL. The fabric samples (5 × 5 cm2) were placed on the cover of a Petri dish, and nine drops of spore suspension (10 μL) were dropped onto different locations of each fabric sample; the suspensions penetrated into the porous structure of the fabric samples. The bottom of a Petri dish coated with water agar (WA) was used as a cover to maintain a relatively high air humidity level in the Petri dish that contained the fabric sample. Thereafter, the Petri dishes with the fabric samples were incubated in the dark at 20 °C and 100 % relative humidity provided by WA. After incubation, the sporulation of A. niger in the area surrounding each spore-suspension drop was observed microscopically after 7, 14 and 35 days of incubation. The degree of sporulation was determined in percentage form as the ratio between the number of drops with fungus sporulation detected (denoted with a positive value: +9 is +100 %) and the number of drops without sporulation (denoted with a negative value: −9 is −100 %). Two parallel assessments were performed for each fabric sample, and the corresponding mean value and SD were determined.

Results and discussion

Characterisation of the fibre–silica–Ag composites

The creation of the fibre–silica–Ag composites after the two-step chemical modification procedure of the studied fabric samples was confirmed via SEM/BSE images (Fig. 1) and EDXS analyses (Fig. 2; Table 3). SEM/BSE images of the fabric samples revealed spherical particles of the silver compound distributed over all of the fibre surfaces, with particle sizes ranging from 30 to 200 nm. This result indicated that both smaller and larger particles of the silver compound had formed on all of the studied fibre surfaces (Fig. 1a–h).

Fig. 1
figure 1

SEM/BSE images of Fc–RB–Ag(B) fibre surface. Fc: a CO, b WO, c SE, d CV, e PA, f PES, g CO/PES and h WO/PES

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Fig. 2
figure 2

Representative EDX spectra acquired from particles’ coatings on the SE–RB–Ag(B) (a) and PA–RB–Ag(B) (b) samples

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Table 3 Elemental composition of the Fc–RB–Ag(B) fabric samples obtained by EDXS analysis (number in the bracket represents SD, n = 2)
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The EDXS analysis (Table 3) indicated the elemental composition of the fibre–silica–Ag composites. The Si-Kα and Zr-Lα lines belonged to the silica matrix created by the RB precursor during the process of condensation on the fibre surfaces, which was performed in the presence of an additional cross-linker based on a zirconium compound [16, 28]. The presence of both Ag-Lα and Cl-Kα lines proves that the consecutive immersion steps of fabric samples in AgNO3 and NaCl solutions resulted in the formation of AgCl particles. The C-Kα and O-Kα lines originated primarily from the carbon and oxygen present in the fabric samples as well as from the applied silica matrix. The C-Kα line also partially originated from the conductive carbon layer applied to the fabric samples prior to the SEM/EDXS analysis. The S-Kα lines originated from the sulphur in the wool fibres.

The influence of the silica matrix and fibre properties on the concentration of absorbed Ag

The measured bulk concentrations of Ag particles on the coated fabric samples (Table 4) demonstrate that the RB silica matrix, which formed on the surface of the studied fibres, increased the fibres’ capacity to adsorb AgCl particles, which further resulted in higher concentrations of adsorbed Ag compared with the same fibres investigated without RB. This beneficial effect was observed at both the 0.10 mM and 0.50 mM concentrations of the AgNO3 and NaCl solutions. The increased adsorption capacity of fibre–silica composites could be confirmed by the results from the literature [20] which reveal that the in situ synthesis of Ag nanoparticles on the silk fibres in the solution of silver ions of 1 mmol/L resulted in 1.2 times lower silver content in comparison with the SE-RB-Ag(B) sample where only 0.50 mM AgNO3 was used (Table 4). In contrast to this, the amount of the adsorbed silver on the silk fibres was very similar to that from the literature, if the silica matrix was not applied (the SE–Ag(A) sample).

Table 4 Concentration of Ag in the coated fabric samples using 0.10 mM (A) and 0.50 mM (B) concentrations of AgNO3 and NaCl solutions before (0 W) and after single washing cycle (1 W) determined with ICP–MS (concentration of Ag is presented as mean value and SD, n = 3)
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After the coated fabric samples were washed once, the concentration of the adsorbed Ag compound particles decreased for all of the investigated samples (Table 4). These results indicate that the Ag compound particles were physically embedded into the silica matrix and that the presence of the matrix did not inhibit their leaching. Further investigation of the results in Table 4 also reveals that the silica matrix did not contribute to the adsorption of Ag in the same manner on different fibres, which suggests that the Ag sorption ability also was significantly influenced by the fibre itself.

The concentration of Ag adsorbed onto the fabric samples increased in the order of PES < PA < CO/PES < CO < SE < CV < WO/PES < WO. Taking into account the properties of the studied fabric samples presented in Table 1, we expected that the chemical and morphological properties of the fibres, as well as the construction parameters of the woven fabric samples, would affect the amount of Ag adsorbed onto the fibres in addition to the mode of Ag adsorption. Although these parameters simultaneously influenced the behaviour of the fabric samples in the finishing solutions, further discussion of the relationship between the fibre properties and their antimicrobial activity is very complex.

The WO–RB–Ag sample absorbed the greatest amount of Ag compound particles. This phenomenon is directly affected by the fabric’s extreme absorption property, which is caused by the presence of a significant number of hydrophilic functional groups and by the amorphous molecular structure of the fibre [29, 30]. Wool fibre is composed of the protein keratin, which is composed of different amino acids that have the ability to provide binding sites for Ag+. Silver cations can react with the carboxyl (–COOH), hydroxyl (–OH) and thiol (–SH) groups, as well as with cysteine, on the WO fibres. Furthermore, because of the helical structure of polypeptide molecules and the bulky side chains on the polymers, wool is only approximately 30 % crystalline [30], which increases its sorption capacity. This statement is in agreement with the results in Fig. 3, where the WO fabric sample is shown to exhibit the greatest water uptake. We can imagine that while swelling in water, wool fibres are capable of absorbing a substantial amount of the AgNO3 and NaCl solutions, thereby resulting in a high concentration of absorbed Ag compound particles. The latter can adhere easily to the characteristic WO fibre scales (Fig. 1b). The amount of Ag compound particles absorbed was also enhanced by the significant thickness and volume of the WO fabric sample (Table 1).

Fig. 3
figure 3

WU of different untreated fabrics (error bar indicates SD, n = 5)

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The SE–RB–Ag sample absorbed less Ag compound particles than did the WO–RB–Ag sample. Silk is similar to wool in that it is a protein fibre composed of amino acids; however, silk and wool exhibit important structural differences between their chemical compositions and morphologies. The fibroin of the silk filament includes carboxyl (–COOH) and hydroxyl (–OH) groups as binding sites for Ag+ but does not contain sulphur [30, 31]. Furthermore, the polymeric molecules of silk are fully extended and more closely packed than those of wool, which results in a high degree of orientation and crystallinity in the silk filament. These morphological characteristics caused the lower water uptake of silk compared with wool (Fig. 3) and, consequently, the lower sorption capacity of silk for the AgNO3 and NaCl solutions.

Comparing synthetic polyamide and naturally occurring polyamides, which include the proteins in wool and silk fibres [32], we find that the PA–RB–Ag sample absorbed the lowest concentration of Ag compound particles (Table 4), which was accompanied by the lowest water uptake compared with wool and silk (Fig. 3). These results are reasonable: synthetic polyamides are linear molecules that include only carboxyl (–COOH) end-groups as the binding sites for Ag+, the concentration of which is approximately 0.09 mol/kg; this concentration is almost ten times lower than that of wool (0.8 to 0.9 mol/kg) [33]. Furthermore, polyamide fibres do not swell appreciably in water and, therefore, absorb only a small amount of water compared with natural protein fibres.

A comparison of the CO–RB–Ag and CV–RB–Ag samples reveals that, despite the presence of cotton and viscose rayon filaments that consist of cellulose with –OH groups as the binding sites for Ag+, the concentration of the absorbed Ag compound particles was more than 1.6 times higher for the CV–RB–Ag sample than for the CO–RB–Ag sample. This greater concentration was attributed to a lower degree of cellulose molecule polymerisation on the viscose rayon and to the lower degrees of orientation and alignment in the filament compared to those in cotton [30, 32]. Accordingly, the molecular structure of the viscose rayon was more amorphous (25–30 % proportion of crystallinity) than that of the cotton fibres (70–75 % proportion of crystallinity) [30, 34], which caused viscose to exhibit a higher sorption capacity compared to that of cotton (Fig. 3). In addition, a greater amount of the Ag compound particles embedded within the viscose. Furthermore, a significant concentration of the Ag absorbed onto the CV–RB–Ag samples was also influenced by the increased surface area of the viscose rayon, which was caused by the shape of the fibres with longitudinal lines (Fig. 1d) called striations and their serrated cross section.

The lowest concentration of absorbed Ag compound particles was observed for the PES–RB–Ag sample (Table 4). The high hydrophobicity and crystallinity (65–85 %) of the fibres [30, 32] hindered fibre access to the aqueous solution and, consequently, reduced Ag+ adsorption from the finishing bath. Since the moisture regain of the PES fibres is very low (0.4–0.5 %), the fibres exhibited the lowest water uptake (Fig. 3), which consequently permitted the formation of AgCl particles only on the surface of the PES fibres.

In the case of the WO/PES–RB–Ag and CO/PES–RB–Ag fabric samples, the concentration of the absorbed Ag compound particles was lower compared to that of the WO–RB–Ag and CO–RB–Ag samples and higher than that of the PES–RB–Ag sample, as expected. The presence of the polyester component in the fabric blends decreased the hydrophilicity of the sample and, consequently, their sorption capacity for the AgNO3 and NaCl solutions. However, the WO/PES sample absorbed a greater concentration of Ag than did the CO/PES sample, and this finding was attributed to the extremely high affinity of the wool fibres for the Ag compound particles.

Antimicrobial properties of the fibre–silica–Ag composites

The results of our investigation of the antimicrobial properties of the coated fabric samples are presented in Table 5 and in Fig. 4. As evident in Table 5, bactericidal activity with R values equal to 100 % against E. coli and S. aureus was observed for all of the studied fabric samples prepared in 0.10 mM solutions of AgNO3 and NaCl; the only exception was the WO–RB–Ag(A) sample, for which insufficient antibacterial activity was observed. Bactericidal activity against both bacteria was obtained with the WO–RB–Ag(B) sample, which contained a higher concentration of the Ag compound particles equalled to 1700 mg/kg. This result suggests that the antibacterial protection of these coated fabric samples can be controlled by the concentration of the AgNO3 and NaCl solutions used for preparation. It should be stressed that the excellent antibacterial protection of the WO–RB–Ag(B) sample was obtained at more than 4 times lower concentration of Ag compared with the literature [35]. This indicates high antibacterial efficiency of the in-situ synthesised AgCl particles in the silica matrix.

Table 5 Bacterial reduction, R, against bacteria E. coli and S. aureus using 0.10 mM (A) and 0.50 mM (B) concentrations of AgNO3 and NaCl solutions for different fabric samples
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Fig. 4
figure 4

Degree of sporulation of A. niger in terms of number of spots where the sporulation was present (positive value) and spots where the sporulation was not present (negative value) on coated fabric samples using 0.10 and 0.50 mM AgNO3 and NaCl solutions after 7, 14 and 35 days of incubation (error bar indicates SD, n = 2)

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The results of the antifungal activities of the coated fabric samples (Fig. 4) show the similar trends as in the case of the antibacterial activity. Specifically, fungicidal activity was exhibited by all samples treated in 0.10 mM solutions of AgNO3 and NaCl, resulting in the total inhibition of the growth of A. niger. In contrast, the 0.10 mM AgNO3 and NaCl solutions were too dilute to provide sufficient antifungal properties for the WO–RB–Ag(A) and WO/PES–RB–Ag(A) samples.

In addition, we found that the Ag concentration of 8.9 mg/kg was sufficient to preserve the biocidal concentration of the Ag compound particles released from the PES–RB–Ag sample, whereas a Ag concentration of 310 mg/kg did not provide a minimal inhibitory concentration of Ag in the case of the WO–RB–Ag sample. This observation indicates that the antimicrobial protection was directly affected not only by the concentration of the Ag compound particles within the fabric samples but also by the mode of their embedment. The insufficient antimicrobial protection of the wool fibres was most likely due to the chemical binding of Ag to the thiol groups of wool and to the formation of silver mercaptides, which hindered the release of silver cations from the WO fibres into the surroundings; this process is critical for wool fibre antimicrobial activity [24, 36]. Similar results were obtained for the antifungal protection of the WO/PES–RB–Ag(A) sample. For all of the investigated fabric samples excellent antibacterial and antifungal protective properties were observed, when the concentrations of the AgNO3 and NaCl solutions were increased to 0.50 mM. This concentration caused a complete growth reduction of E. coli and S. aureus as well as a complete reduction of sporulation of A. niger through the whole incubation period.

Conclusions

In this study, we demonstrate that the two-step antimicrobial finishing process that we previously developed was also appropriate for the chemical modification of all the studied natural and chemical textile substrates, irrespective of the chemical structure and morphology of the fibres and the construction parameters of the fabric. The important advantages of the process are:

  • The creation of a silica matrix on the fibre surfaces by the application of the RB precursor, which enhanced the fibres’ capacity for adsorbing the Ag compound particles, and

  • The biocidal concentration of the Ag compound particles embedded into the silica matrix, which was easily obtained by increasing the concentrations of the AgNO3 and NaCl solutions.

The results show that the chemical composition and morphology of the fibres directly influenced the concentration of the absorbed Ag compound particles, the mode of their binding to the functional groups of the fibres and, consequently, the degree of antimicrobial activity exhibited by the fibres. However, when appropriate concentrations of the AgNO3 and NaCl solutions were used, a sufficient amount of Ag compound particles needed to achieve biocidal activity was imbedded in each of the investigated fibres. Because of the simplicity of the preparation method of the novel fibre–silica–Ag composites, this process has the potential to be applied to a variety of applications.