Introduction

Nanoparticles and nanocomposites have very high surface area-to-volume ratio and therefore they can be used in a large number of industrial applications. Nanoparticles are employed to develop innovative environment-friendly chemical mechanical polishing slurries (Zhang et al. 2018, 2019), which are used in semiconductor and microelectronics industries. In addition, nanoparticles are also used to develop novel machining approaches and diamond wheels (Zhang et al. 2015, 2017, 2018). With the use of nanoparticles, nanoscale precision surfaces are achieved (Zhang et al. 2012a, b, 2013). These studies are a breakthrough and milestone contribution to the environment, effectively eliminate the pollution and contamination induced by traditional machining and manufacturing (Zhang et al. 2019).

The surface properties and functionalities can be enhanced by treating the surface with various NPs. The combination of various NPs may also have some synergistic effect and impart novel characteristics (Giannousi et al. 2015). Some of NPs may be toxic and cause not environmentally acceptable. By fixation of NPs on a biopolymer such as chitosan, it is possible to reduce the consumption of NPs and eliminate the aforementioned problems. Moreover, the inherent properties of biopolymers can also be enhanced and some new functionalities can be generated by the addition of NPs. As an example, the antimicrobial properties can be enhanced by combining metal/metal oxide with biopolymers (Peschel et al. 2011; Petkova et al. 2014).

Chitosan is the second natural polysaccharide after cellulose and is achieved from chitin (Kong et al. 2010). The use of chitosan in the various industry is developing due to its antimicrobial potential, biodegradability, and non-toxicity. However, the solubility, cationic nature, and antimicrobial properties of chitosan decrease by increasing pH above 7. To overcome this problem, chitosan can be chemically modified (Lim and Hudson 2013; Jayakumar et al. 2010; Rinaudo 2006; Zamani and Taherzadeh 2012; Enescu 2008; Sashiwa et al. 2002). Since, the free amino groups of chitosan are very suitable to produce strong complexes with metals/metal oxide, the combination of metal oxide NPs with chitosan is another approach to improve its antimicrobial efficiency (Kumar et al. 2012). The metal complexes can improve the stability and antimicrobial activity of chitosan 10–20-fold (Kyzas and Bikiaris 2015). For example, the complex of silver ion on chitosan backbone increased the stability and antibacterial activity of Ag+ by 20-fold compared to that of chitosan (Nayak et al. 2016), and also ZnO NPs on chitosan were slightly less effective than silver NPs (Li et al. 2010; Vaseeharan et al. 2015; Sanmugam et al. 2017; Kim et al. 2010; Moura et al. 2016; Li et al. 2010; de Godoi et al. 2013).

In this study the antimicrobial activities of treated wool yarns by Ch-NPs are investigated. First, Ch is treated with a series of inorganic salts (e.g., AgNO3, Cu2O and ZnO) and their combinations. Then, FTIR, SEM, DSC and DLS techniques are utilized for the characterization of Ch-NPs and the treated wool fibers. The objective of this study is to understand the antimicrobial activities and durability of the treated wool samples with Ch-NPs to shed more light on the topic of environmentally friendly wool finishing.

Materials and methods

Chitosan with molecular weight > 600 kD and degree of deacetylation 98% was supplied by Mahtani Chitosan PVT. LTD Chitin India. ZnO, AgNO3, and Cu2O were purchased from Merck (Germany), and the specification of wool yarn used in this study was 200 Tex/fourfold. Scouring of wool yarns was performed with nonionic detergent (Lotensol, Hansa), and the other materials were of analytical grade.

The functional groups of Ch-NPs were studied by a Nicolet NEXUS FTIR spectrometer (USA) in the area of 400–4000 cm−1. The tests were performed at 20 °C and in a relative humidity of 65%. The surface properties of synthesized Ch-NPs composites and the treated wool samples were studies by scanning electron microscope (SEM) Seron AIS 2300 at an acceleration voltage of 20 kV. The particle size of NPs was determined by the dynamic light scattering (DLS) technique via measuring the intensity of scattered light over time. Differential scanning calorimetry (DSC) was used to study the calorimetry behaviors of the Ch-NPs composites and the treated wool samples under constant nitrogen purge using a TA Instrument in the range of 0–350 °C with the scanning rate of 10 °C/min. Also, the tear strength of all wool samples was measured by the Instron 5566 electromechanical instrument.

Synthesis of Ch-NPs composite

Chitosan (5.0 g) was solubilized in 150 mL acetic acid solution (2% v/v) at 60 °C with continuous stirring. The freshly prepared aqueous solution of AgNO3, Cu2O, ZnO, or their mixtures (10 mM and 3 mL) was sonicated at 40 °C for 30 min and added to the chitosan solution following the recipe reported in (Aryabadie et al. 2015). The mixing was terminated after 2 h by adding NaOH (1%) and the precipitated Ch-NPs composite was collected by filtration. The particle size of the filtered fine powder was in the range of 0.2–0.4 mm. Figure 1 shows the preparation method of Ch-NPs composite schematically.

Fig. 1
figure 1

The synthesis procedure of Ch-NPs composite

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Treatment of wool yarns with Ch-NPs

Nonionic surfactant (5% o.w.f) was utilized for scouring of wool yarns at 50 °C for 30 min with the liquor ratio (L.R) of 1:40. Then, the samples were rinsed, and dried at room temperature.

The fine powder of Ch-NPs (1–10% o.w.f.) was dissolved in water (30 mL) containing 4% o.w.f. of citric acid and 4% sodium hypophosphite. Then, the scoured wool yarn (2 g) was immersed in different concentrations of Ch-NPs (1–10% o.w.f.) at 50 °C for 3 h. Afterward, the samples were dried at 80 °C for 5 min, and then cured at 120 °C for 5 min. Finally, the treated wool yarn was rinsed and dried at room temperature. The grafting procedure of prepared Ch-NPs on wool yarns is given in Fig. 2. The amount of Ch-NPs deposited on the samples was determined by the weighing gain analysis of wool yarns before and after treatment.

Fig. 2
figure 2

Grafting procedure of Ch-NPs on wool yarns

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Antimicrobial test

A quantitative evaluation of microbial reduction of treated samples with Ch-NPs was completed against Gram-positive S. aureus and Gram-negative E. coli bacteria according to the AATCC test method 100–2004. In summary, the samples were put into tubes and were autoclaved for 15 min at 121 °C. The suspension of bacteria on culture medium was prepared, and 100 µL of bacteria suspension (106 bacteria) was added into tubes, incubation was performed at 37 ± 2 °C for 18 h, and the reduction in the number of colonies on the treated and untreated samples was calculated by Eq. 1:

$${\text{Antimicrobial effect }}\left( \% \right) = \frac{A - B}{A} \times 100$$
(1)

where A is the number of bacteria recovered from the inoculated wool yarn and B is the number of colonies recovered from the inoculated treated wool yarns after 24 h.

Results and discussion

The interaction mechanism of chitosan with transition metal cations has been reviewed before (Pestov and Bratskaya 2016). There are two possible models of binding metal ions by chitosan: the “bridge” model describing the metal ion coordination with several amino groups belonging to different glucosamine units of the same or different polymeric chains (Fig. 3, structures 2 and 4) and the “pendant” model, in which the metal ion is coordinated with just one amino group (Fig. 3, structures 1 and 3). The most active coordination site for transition metal ions is the nitrogen atom of the chitosan amino group, while many authors investigated the possibility of participation of other chitosan functional fragments in complex formation using quantum chemistry methods-acetamide and hydroxyl groups as well as the oxygen atom of the glycoside bond (Pestov and Bratskaya 2016).

Fig. 3
figure 3

Possible interactions between metal ions and chitosan (Pestov and Bratskaya 2016)

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Characterization of Ch-NPs

As illustrated in Fig. 4, the FTIR spectrum of Ch exhibits strong peaks at around 1023 cm−1, 1081 cm−1, and 1375 cm−1, which are characteristic of a polysaccharide molecule (Sadeghi-Kiakhani et al. 2018; Giri Dev et al. 2009). Additionally the broadband of O–H stretching, and N–H stretching vibrations were observed in the region of 3200–3450 cm−1, and carbonyl groups (C=O) were apparent at 1650 cm−1 (Vaseeharan et al. 2015; Sanmugam et al. 2017). The adsorption and complex formation of NPs on chitosan surface follows the three-step model: (1) adsorption of NPS onto the external surface of chitosan, (2) surface adsorption via electrostatic interactions, van der Waals forces, and hydrogen bondings, (3) coordination and chelation between NPS and chitosan (Sanmugam et al. 2017; Sadeghi-Kiakhani et al. 2018). Decreasing the intensity of peaks at the frequencies 2925 and 2854 cm−1 corresponding to C–H groups, and disappearance of the peak in 1744 cm−1, and appearance of a new peak at 1485 cm−1 indicates NPs ions are involved in complexation with –NH2, –CONH, and –OH groups of Ch (Fig. 3). Also, the representative peak for NPS at 855 cm−1 becomes more visible, and differences are apparent in the range of low frequencies from 750 to 550 cm−1. These variations can exhibit the presence of NPs and their coordination with chitosan (Vaseeharan et al. 2015; Sanmugam et al. 2017).

Fig. 4
figure 4

FTIR spectra of synthesized Ch-NPS composite: a chitosan-independent NPs, b chitosan-mixture of NPs

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The distribution of NPS in the chitosan was investigated by SEM (Fig. 5). SEM micrographs clearly showed that the surface characteristic of chitosan modified by NPS and the cavities on the surface of chitosan is occupied by NPS (Govindan et al. 2012; Klaykruayat et al. 2010). Since chitosan molecules act as a reducing agent, even after drying the chitosan molecules do not allow NPS to get merge with each other.

Fig. 5
figure 5

SEM images of synthesized Ch-NPS composite: a Ch, b Ch-Ag, c Ch-Cu, d Ch-Zn, e Ch-Ag/Zn, f Ch-Ag/Cu, g Ch-Cu/Zn

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The glass transition (Tg) and thermal behavior of fibers were studied by DSC. Figure 6 shows the DSC thermograms of Ch and Ch-NPs. Due to the rigid crystalline nature and formation of strong inter/intra-molecular hydrogen bonding of Ch, Tg was not found. It has been reported that Tg of Ch could occur at the higher temperatures than its degradation (Aryabadie et al. 2015; Wazed et al. 2011). Ch thermogram exhibited a broad peak at 279 °C, which can be ascribed to the breakdown of amine groups of Ch (Aryabadie et al. 2015; Wazed et al. 2011). The treated samples indicated the shifting of the peak to the higher temperature (327–335 °C). This phenomenon presented the slow decomposition of new linkages between Ch and NPs with respect to control.

Fig. 6
figure 6

DSC spectra of a chitosan-independent NPs, b chitosan-mixture NPs

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Wool surface characterization

DLS study

Horiba SZ-100 particle size analyzer was used to measure the size of Ch-NPs. The average size obtained by the DLS distribution plot was around 8–15 nm (Fig. 7). The average particle sizes of the Ch-NPs are significantly diverse and the type of metal salts affects its size. Ch–Ag/Cu particles had the smallest average size distribution among other NPs. Generally, to reduce metal ions and form NPs, a reducing agent is used to produce free electrons. In this study, thiol groups of wool fiber are formed through the degradation of cysteine bonds of wool fibers under moisture and heat (Al-Karawi et al. 2011; An et al. 2011). So, it was concluded that the reduction of metal ions into metal nanoparticles is occurred via available groups in the protein chains of wool fibers and chitosan as a biopolymer without further reducing agents. Here, Ag+ and NO3− and ionic interactions are formed between metal positive ions and thiol groups with a negative charge. Thus, Ch-metal ions can be reformed to Ch-metal NPs with the available thiol groups in wool fibers and functional groups of chitosan.

Fig. 7
figure 7

DLS analyses of Ch-NPs a Ag, b Cu, c Zn, d Ag/Cu, e Ag/Zn, f Cu/Zn (Ch-NPs concentration = 5% o.w.f.)

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Weight gain analysis

The weight gain percent of treated samples was measured as a criterion for grafting the Ch-NPs on the wool yarns (Fig. 8). The results showed that the weight gain of the wool fibers treated by Ch-NPs increased somewhat linearly by the initial concentration of Ch-NPs. This can be explained by the fact that the number of adsorbate species increases by their initial concentration and there is a higher chance of adsorption and complexation on wool fibers statistically. While at the more concentrations of Ch-NPs > 5 (%o.w.f.), the active sites available for the interaction of Ch-NPs with wool yarn are inaccessible and relatively limited, so, the concentration of Ch-NPs 5% o.w.f. can be considered as an optimal amount for pretreatment of wool yarns with Ch-NPs.

Fig. 8
figure 8

The effect of Ch-NPs composite concentrations on weight gain of wool (T 50 °C, L.R 30:1, t 3 h)

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Ch-NPs can attach to polar and ionizable functional groups of wool macromolecules by electrostatic interactions, hydrogen bonding, van der Waals forces, and coordination (Fig. 9). The grafting of Ch-NPs on the textile substrates can be completed via physical adsorption or chemical bonding. Also, the presence of various active sites in the protein chains of the wool fibers can improve the Ch-NPs grafting on the wool, and consequently the formation of Ch-NPs aggregation is diminished. Citric acid can also promote the complex formation between Ch-NPs and wool fibers.

Fig. 9
figure 9

Treatment of Ch-NPs on wool yarns in the presence of CA (citric acid: cross-linking agent) and SHP (Sodium hypophosphite: catalyst)

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SEM analysis

SEM images of all samples were studied to detect the produced variations in surface properties of wool yarns (Fig. 10). The scales are very obviously realized in the surface of wool fibers. Results clearly exhibited that a smooth coating on the surface of wool yarn is provided by Ch-NPs. As a result, the surface samples became softer and smoother due to the presence of a uniform layer on the surface of wool yarn, so that the surface roughness of wool yarn was covered with this modification (Safapour et al. 2019; Sadeghi-Kiakhani et al. 2019a, b).

Fig. 10
figure 10

SEM images: a wool, b wool-Ch-Ag, c wool-Ch-Cu, d wool-Ch-Zn, e wool-Ch-Ag–Zn, f wool-Ch-Ag–Cu, g wool-Ch-Cu–Zn (Ch-NPs concentration = 5% o.w.f.)

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Antimicrobial activity of wool yarn

The antimicrobial activity of all the wool samples treated with various Ch-NPs was tested against E. coli, and S. aureus bacteria (Table 1). It was found that both chitosan and Ch-NPs significantly inhibited completely bacterial growth after 24 h incubation. Antimicrobial activity of chitosan against microorganisms is due to the formation of electrostatic interactions of anionic groups of microbial cell membranes with the positively charge of amine groups. So, the growth of the cells stops by disrupting nutrient intake to them (Sadeghi-Kiakhani et al. 2019a, b; Matyjas-Zgondek et al. 2008; Rezaie et al. 2017; Tang et al. 2014). Moreover, the results indicated that metal inorganic could increase the antimicrobial activity of chitosan, which can be attributed to synergistic antibacterial activities of both chitosan and NPs. The photocatalytic production of reactive oxygen species (ROS) is formed owing to the disruption of the bacterial cell membranes by NPs ions. Also, the formation of ROS finally results in the damage of bacterial structure and cell death (Sadeghi-Kiakhani et al. 2019a, b; Matyjas-Zgondek et al. 2008; Rezaie et al. 2017; Tang et al. 2014). The treated wool yarns with Ch-Ag had the highest antimicrobial activity among all of the used Ch-NPs. The concentration of Ch-NPs was an essential factor in antimicrobial activates of the tread samples, so that the higher inhibition of growth of bacteria is achieved by increasing the initial concentration of Ch-NPs. It was found that suitable antimicrobial activities were obtained at initial Ch-NPs concentration of 5% o.w.f., which was considered as the optimum concentration for treating the wool samples.

Table 1 Antimicrobial activity of all samples against E. coli and S. aureus
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The durability of the antimicrobial treatment of the samples was also tested after repeated washing cycles according to ISO 6330-1984 standard test method (Table 2). The strength of bonding between Ch-NPs and wool fibers is a detrimental parameter on the washing durability of the antimicrobial finishing. The results indicated that the samples treated with Ch-Ag exhibited a very good antimicrobial durability (over 92%) after 10 repeated washing cycles. The poorest result was achieved for the samples treated with Ch-Zn and the antimicrobial activity dropped to 43% after 10 repeated washing cycles. It is likely that the grafted Ch-NPs can be separated from the wool surface after repeated washing times which adversely affects the antimicrobial activity. When the samples are treated with the Ch-NPs using citric acid, the antimicrobial activity and their durability were improved. Indeed, without citric acid as a crosslinking agent, satisfactory antimicrobial activities cannot be obtained. That shows that citric acid acts as a crosslinking agent between the wool fibers and Ch-NPs.

Table 2 Antimicrobial activity to washing durability of all samples (Ch-NPs concentration = 5% o.w.f.)
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Conclusions

This study deals with an eco-friendly method for improving antimicrobial activities of Ch-NPs grafted on the wool fibers. The obtained results from FTIR, and SEM showed that the grafting of Ch-NPs was successfully performed. The distribution plot of DLS showed that the average size obtained by various Ch-NPs was around 8–15 nm, and Ch-metal ions can be converted to Ch-metal NPs with the accessible functional groups of chitosan and thiol groups in wool fibers. Furthermore, the antimicrobial activities of treated wool samples with Ch-NPs indicated that Ch-Ag among all of the used Ch-NPs could improve significantly the antimicrobial potential of chitosan, which can be attributed to synergistic antibacterial activities of both chitosan and NPs. Also, the results showed that the reduction growth of bacteria on the treated samples with Ch-Ag was above 92% after 10 repeated washing times.