Synthesis and characterization of an antimicrobial textile by hexagon silver nanoparticles with a new capping agent via the polyol process

Abstract

In this study, we developed hexagon silver nanoparticles (AgNPs) prepared by the polyol method in ethylene glycol media using aminopropyltrimethoxysilane (APTMS) as a capping agent for the first time. The presence of the primary amine group makes APTMS a great candidate as a capping agent for the synthesis of silver nanostructures, but it has not been used for this purpose until now. The synthesized silver nanostructures were characterized by transmission electron microscopy, Fourier transform infrared spectroscopy, differential scanning calorimetry and ultraviolet-visible spectroscopy. The molar ratio of APTMS to AgNO₃ varied from 0.5 to 6.8, which resulted in an average diameter of the obtained nanoparticles from 50 to 100 nm. The optimum morphology of hexagonally shaped AgNPs was obtained at a molar ratio of 2.2. The prepared AgNPs were evaluated for their antibacterial and antifungal properties. Antimicrobial tests showed that a cotton textile coated with colloidal amino functionalized AgNPs has excellent antimicrobial properties as an inhibitor of growth against the gram-positive bacteria Staphylococcus aureus, gram-negative bacteria, Escherichia coli and fungus Candida albicans, and therefore showing potential for real applications.

Keywords

silver nanoparticles polyol process antimicrobial textile aminopropyltrimethoxysilane

With the development of nanotechnology, different practical applications have emerged for colloidal metal nanoparticles. Metal nanostructures, such as gold, silver, zinc and copper nanoparticles, have become of special interest because of their potential applications in various fields. These metal nanostructures have recently been researched extensively due to their unique chemical, electrical, catalytic, biological and optical properties,^1–5 which are all dependent on their size, shape and crystallinity^6,7 and, thus, their wide range of potential applications.^1–10

Among metal nanoparticles, silver nanostructures have received much more attention due to their numerous applications in the catalysis,^{2,6,8,11–15} biomolecular detection and diagnostics,^2,6,8 therapeutics,² lithography,³ photography,^6,12 micro-electronics^2,3,6 and sensing fields,^2,3,6,8,11 as super magnets or conductors,^3,11–13 antistatic materials,¹¹ conductive inks and adhesives¹² and photonic and optoelectronic devices,^6,8,12,14 with prevalence in antimicrobial applications.^{3,10,11,15,16} In recent years, using nanoparticles with antimicrobial properties has become an extremely important issue in the textile industry. In the family of metal nanoparticles, silver nanostructures have high antimicrobial activity. Their antimicrobial ability is also higher compared to many silver compounds or bulk silver, and it was shown that they can easily be embedded in or be bound to various substrates for no release or the slow release of silver ions, which makes them more effective for long-term antimicrobial activity.^{1,3,5,10,16–20}

Since the antimicrobial properties of silver nanoparticles (AgNPs) are believed to be dependent on their size, morphology and surface chemistry, much research has been focused on preparing AgNPs with well-defined size and shape, using simple, reproducible procedures.^1,5,6,11,15 The antimicrobial efficiency of nanoparticles increases strongly with reduced size, due to a larger specific area that enables the increased release of cations from particle surfaces and, thus, increased biocidal activity.⁵ In addition, numerous reports suggest that shape also influences their antimicrobial activity, apparently due to interaction with the biomolecules.^16,21

There are many chemical and physical routes for preparing AgNPs, such as sonochemical, laser ablation, microwave, gamma rays, ultraviolet (UV) irradiation, electrochemical deposition, hydrothermal, wet chemical, sol–gel, solid or porous template techniques and ultrasonic spray pyrolysis,^{2–6,14,15,20,22–24} but several of the methods are complicated, need expensive equipment, are high energy demanding, use toxic chemicals or produce hazardous by-products.

The most common chemical methods, including the chemical reduction method, electrochemical techniques and physicochemical reduction, are used widely for the synthesis of Ag nanoparticles. The chemical reduction method, including polyol synthesis, is one of the most common ways to prepare colloidal metal nanoparticles, because it does not need an extremely high operational temperature, high pressure or vacuum or expensive equipment. Among the most important physical approaches are the evaporation–condensation and laser ablation methods. In physical methods, for the synthesis of AgNPs, a tube furnace is used that occupies a large space and consumes a huge amount of energy while raising the environmental temperature around the source material. It requires the use of power up to more than several kW, and needs a pre-heating time of several tens of minutes to attain a stable operating temperature. However, in the polyol chemical reduction method, instead of using an extra reduction agent, we used ethylene glycol (EG) as both the solvent and reduction agent, and only used an inexpensive oil bath heated up to 160℃ for about 1 h. The polyol method is feasible without utilizing a furnace or expensive and powerful heating instrument, which is used in the physical method to produce AgNPs.^25,26

Research is still focused on preparing reproducible, highly concentrated, shape-controlled nanoparticles with narrow size distribution,^4,11 as these parameters define specific properties and, thus, the use of the produced nanomaterials.^{6,9,12,13,15,22,23}

The polyol process is one of the most impressive approaches for the preparation of silver nanostructures, in which the polyol, usually EG, acts as a solvent and reducing agent in the reaction mixture. With the heating of EG, acetaldehyde with a reduction ability is produced and, so, in the polyol process, the reductant is produced constantly in situ. With changing the synthesis parameters, the nanostructures of different sizes and morphologies, such as nanorods, nanowires, nanocubes, nanoplates and others, can be obtained. The growth of nanoparticles and initial nucleation, which have a huge impact on the shape of the final particles, can be modified further by adding capping agents, of which polyvinylpyrrolidone (PVP) is commonly introduced as a shape-controlling agent and stabilizer. Lately, the addition of various inorganic salts, such as NaCl, KCl, KBr, CuCl₂ and FeCl₂, to the reaction mixture is believed to influence the initial nucleation due to seed oxidative etching and, thus, affecting the morphology of the produced particles.^{4,6,8,9,12–15,22,23}

Khalil-Abad and Yazdanshenas¹⁹ reported a facile process to prepare superhydrophobic antibacterial cotton, with AgNPs that were formed on cotton fibers by treatment with aqueous KOH and AgNO₃. Further modification of cotton textiles with octyltriethoxysilane led to hydrophobic surfaces with a water contact angle of 151°. The coated fabrics showed antibacterial activity toward gram-positive and gram-negative bacteria, due to the presence of AgNPs on the textile surface. Xue et al.²⁷ synthesized antibacterial AgNPs on cotton fibers by reduction of the [Ag(NH₃)₂]⁺ complex with glucose. Anand et al.²⁸ synthesized AgNPs with the aqueous extract of an indigenous South African plant, Ekebergia capensis. Zhao et al.¹⁵ produced cubic AgNPs via the polyol method with PVP as a capping agent. They investigated the effect of NH₄OH as the complexing agent on the size of AgNPs, and the results showed that the size of AgNPs decreased from 90 to 10 nm by increasing the pH of the solution. Dermenci et al.²⁴ prepared spherical and homogenous Ag/ZnO nanocomposites through single-step ultrasonic spray pyrolysis to investigate their photodegradation ability for Methylene Blue dye from textile wastewaters. They prepared Ag/ZnO nanocomposite aqueous solutions from different concentrations (0.05–0.2 M) and at different temperatures (700–900℃). The highest degradation and decolorization of Methylene Blue were 27% and 40%, respectively, obtained for a nanocomposite synthesized from a 0.2 M solution at 900℃, due to their dense structure and higher surface area.

In this study, we developed a synthesis of AgNPs with optimum hexagon morphology using aminopropyltrimethoxysilane (APTMS) as a capping agent via the polyol method. Hexagonally shaped AgNPs were obtained by controlling the reaction parameters, without the presence of inorganic salts or a protective atmosphere. The effect of APTMS on the particle size of AgNPs was studied intensively. In addition, the antimicrobial activity of synthesized hexagon nanostructures was determined. The obtained hexagon AgNPs showed excellent antibacterial and antifungal activities. These antimicrobial colloidal AgNPs can be used in the textile industry, and various applications where antimicrobial activity is desired.

Experimental details

Materials

EG (98%, Sigma-Aldrich), APTMS (97%, Sigma-Aldrich) and silver nitrate (AgNO₃) (>99%, Sigma-Aldrich) were used as received without further purification.

Synthesis of AgNPs

AgNPs were synthesized by the reduction of AgNO₃ with EG as a solvent and reducing agent in the presence of APTMS as a capping agent. A polyol synthesis involves heating of the polyol with a salt precursor and a capping agent to generate metal colloids.

In a typical procedure, 10 ml of EG in a glass reactor is heated up to 160℃ in an oil bath for 60 min, while stirring with a speed of 260 rpm. Afterwards, 5 ml of 0.1 M AgNO₃ solution in EG was added dropwise within 30 s to the heated EG solution at 160℃, resulting in the formation of slightly yellowish staining of the colloidal solution, indicating the formation of silver seeds. After 5 min, the reaction flask was removed from the oil bath, and was kept stirring at room temperature until it had cooled down completely. Then APTMS at different molar ratios between 0.5 and 6.8 was added dropwise to the 15 ml of prepared colloidal silver solution. The color of the colloidal silver solution changed to brown and, later, to a green–brown solution. The reaction was maintained with stirring at room temperature for a further 10 h. The AgNP solution was then purified by centrifugation for 20 min at 7500 rpm. Figure 1 shows the AgNP synthesis procedure flow chart and the proposed reaction of AgNP formation.

Figure 1.

Scheme of (a) silver nanoparticle (AgNP) synthesis procedure and the (b) proposed AgNP formation reaction. EG: ethylene glycol; APTMS: aminopropyltrimethoxysilane.

Characterization

The nanometric structure and morphology of AgNPs were observed on transmission electron microscopy (TEM) images obtained from a transmission electron microscope, JEOL JEM 2010-Fx, operated at an accelerating voltage of 200 kV. Samples were prepared by placing a small drop of each colloid on the intended copper grids coated with amorphous carbon film. For the confirmation of the attached APTMS capping agent to the silver seeds, characteristic organic and inorganic bands, such as -C-N, -C-H and Si-O-, were studied by Fourier transform infrared spectroscopy (FT-IR). Ultraviolet-visible spectroscopy (UV-Vis) absorption spectra of the colloidal silver solutions were obtained in the range of 328–828 nm using Agilent apparatus. Differential scanning calorimetry (DSC) apparatus was used to observe the changes during the heat treatment of the colloids up to 600℃ at a heating rate of 10℃/min in an air atmosphere with a BAHR-503 (Germany).

Antimicrobial activity

The antimicrobial properties of coated textiles were evaluated according to the ASTM E2149-13(a) standard test method for determining the antimicrobial activity under dynamic contact conditions. This test method evaluates the resistance of specimens treated with non-leaching antimicrobial agents to the growth of selected microbes, including E. coli, S. aureus and C. albicans, under dynamic contact conditions.^29,30 Antimicrobial tests were performed for gram-positive bacteria S. aureus (ATCC 25923, S. aureus), gram-negative bacteria E. coli (ATCC- 25922) and fungus C. albicans (ATCC 10231). Each test culture was diluted in a nutrient broth with a sterilized 0.3 mM phosphate buﬀer (KH₂PO₄, pH = 6.8) to obtain the final concentration of 1.5–3.0 × 10⁵ (CFU)/ml, which is the working bacterial dilution. The prepared textile samples (0.2–0.5 g) were transferred to a 250 ml Erlenmeyer flask containing 50 ml of the working bacterial dilution. All flasks were capped, and placed on a wrist-action shaker at maximum speed for 1 h. After a series of dilutions with the buﬀer solution, immediately after 1 h, 1 ml of the diluted solution from each flask was transferred to the nutrient agar. All the nutrient agar Petri dishes were allowed to incubate at 37℃ for 24 h, and then the colonies formed in each Petri dish were counted. The counted numbers of the formed colonies of triplicate Petri dishes for each sample were recorded, and the average value was converted to colony forming units per milliliter (CFU/ml) of buffer solution in the flask. The percent reduction is calculated according to formula (1) below, and results from the contact of the organisms with the sample. Antimicrobial activity is evaluated as a percentage of the reduction, and is considered as better at higher percentages^17,29–31

Reduction, % (CFU / ml) = (B - A) / B \times 100

(1)

where A is CFU/ml for the flask containing the treated sample after the specified contact time and B represents CFU/ml for the flask containing the untreated sample after the specified contact time.^29–31

Results and discussion

The effect of APTMS in polyol synthesis of AgNO₃

The polyol synthesis involves heating a polyol with a salt precursor and capping agent to produce metal nanoparticles.¹⁸ In this case of AgNP synthesis, EG, AgNO₃ and APTMS serve as the polyol, salt precursor and capping agent, respectively. One of the most important features that distinguishes the polyol process from the other methods is that the solvent also acts as a reducing agent and, therefore, an additional reducing agent is not needed.

In the polyol method, heating of the polyol EG up to 160℃ causes the EG to lose water and acetaldehyde is produced. Acetaldehyde acts as a reducing agent, and the reduction of silver ion (Ag⁺) to silver atom (Ag⁰) occurs within the first minute of the reaction. Silver atoms tend to form silver seeds or silver nuclei, whose structure fluctuates, depending upon their size and the available thermal energy. In other words, after the formation of silver nuclei, because of their small size and available thermal energy and depending on their energetic favorability, defects are formed or removed.^4,12–15,19

Most silver nuclei contain twin boundary defects because of their lower surface energy due to the defects. As the silver nuclei grow, they obtain different morphologies, such as singly twinned, single-crystal or multiple twinned seeds. This is the nucleation stage, which can be controlled thermodynamically or kinetically. After that, different seeds grow into the nanostructures with different shapes. The crystal growth of the silver nanostructures occurs as isotropic or anisotropic growth. The addition of capping agents that adsorb to the surface of silver seeds can be unselective or selective, resulting in the formation of quasi-spherical nanostructures and shape-controlled nanoparticles, respectively.^{5,12–15,20,32}

In this research work, APTMS was introduced as a capping agent to prevent aggregation, and as a structure-directing agent. APTMS has strong polar amino groups (−NH₂) with free electron pairs, which have affinity toward silver seeds and can form coordinative compounds with them. Polar groups, such as the amino group of an APTMS chain, can interact with silver metal ions on the surface of the nanoparticles and form an Ag-N bond (as shown in Figure 1(b)).

In order to investigate the effect of APTMS as a capping agent on the formation of AgNPs, several APTMS to AgNO₃ molar ratios were tried. The AgNO₃ concentration was fixed, while the APTMS/AgNO₃ molar ratio was varied from 0.5 to 6.8. Figure 2 shows the different coloring of the prepared colloidal AgNPs in different concentration of APTMS and colorless 0.1 M AgNO₃ solution for comparison.

Figure 2.

Sample images of (a) 0.1 M AgNO₃ solution and colloids with different molar ratios of aminopropyltrimethoxysilane/AgNO₃: (b) 0.5; (c) 1; (d) 2.2; (e) 3.4; (f) 4.4; (g) 6.8. (Color online only.)

After the reduction of silver ions by heated EG, silver seeds were formed in the solution, and the color of the solution started to change from colorless to slightly yellowish staining. When APTMS as a capping agent was introduced to the mixture of the reaction, the color of the solution changed gradually to the yellow and brown–green colors, which confirmed the formation of AgNPs in the reaction mixture. Figure 3 shows TEM images of AgNPs prepared by various APTMS to AgNO₃ molar ratios. TEM images confirmed the formation of nanocrystalline silver particles.

Figure 3.

Transmission electron microscopy images of the prepared silver nanoparticles at aminopropyltrimethoxysilane (APTMS)/AgNO₃ molar ratios of (a) 0.5, (b) 2.2, (c) 3.4, (d) 4.4 and (e) 6.8. (f) Electron diffraction patterns of hexagon nanostructures. (g) Higher magnification of APTMS/AgNO₃ = 2.2, which was used for the preparation of the antimicrobial cotton textile.

When the molar ratio of APTMS/AgNO₃ was increased from 2.2 to 4.4, we observed that the size of Ag nanoparticles decreased from 100 to 50 nm. From the potential point of view, the electrical potential of Ag⁺ to Ag⁰ is + 0.77 V, while the electrical potential of the silver-amine complex to Ag⁰ is decreased dramatically, hence the reduction of the Ag complex to Ag⁰ is more difficult than that of Ag⁺ to Ag⁰. As a result, the growth of silver grain can be prevented through a delay in the reduction reaction of the silver ion, but also after the formation of silver grains, APTMS as a capping agent can coat their surface and suppress the grain growth, leading to the formation of AgNPs of smaller size.^15,18

However, with a further increase of APTMS concentration, we found that, instead of even smaller AgNPs, silica particles were formed, which precipitated at the bottom of the glass reactor. The obtained white precipitate was dried and characterized by TEM and FT-IR. The TEM image (inset Figure 4(a)) showed the obtained silica particles, and connections between the particles can be observed. FT-IR spectra of these silica particles (Figure 4(b)) show an intensive peak at around 1100 cm⁻¹ that can be attributed to the Si-O-Si bond. The broad peak at around 3500 cm⁻¹ can be attributed to -OH or -NH₂ bonds. From these findings, we assume that, because of the high concentration of APTMS, hydrolysis of methoxy groups occurs, and Si-OH groups are produced. Then, self-condensation of the silanol groups causes the formation of Si-O-Si linkages. Figure 4(a) shows the proposed reaction of the silica particle formation. It can be attributed to the fact that at a high concentration of APTMS, the condensation reaction of the methoxy groups occurs, which leads to the formation of the Si-O-Si linkages. Adding a higher, and in this case excessive, amount of APTMS therefore acts as an inhibitor for the formation of smaller and more abundant AgNPs. Self-condensation of silanol groups hinders the attachment of -NH₂ groups in APTMS with the Ag seeds, which determines the capping ability of APTMS in the AgNP formation. As can be seen in the TEM image (Figure 3(e)) and the bright yellow color of the colloid (Figure 2(g)), after the centrifugation of the colloid solution and removal of the silica particles, a low concentration of small AgNPs remained in the solution.

Figure 4.

Proposed hydrolysis and self-condensation reaction of the aminopropyltrimethoxysilane (APTMS) groups (a) and transmission electron microscopy image (inset (a)) and Fourier transform infrared spectra (b) of the obtained white precipitate.

UV-Vis characterization

The formation of AgNPs can be observed by UV–Vis absorption spectra, as shown in Figure 5. Metal nanoparticles have free electrons, which lead to the combination of electron vibrations in resonance with the light wave and, hence, give characteristic surface plasmon resonance (SPR) absorption bands. However, the characteristic UV–Vis absorption band of silver nanostructures depends slightly on their size and shape.^13,22 AgNPs of small, nearly spherical morphologies have a characteristic sharp band at around 420 nm. In the sample of EG used as a blank, there was no surface plasmon peak. In the samples of synthesized AgNP colloids with the presence of APTMS as a capping agent, a broad surface plasmon peak was observed at around 425 nm, which confirms our assumption of the color change from colorless to the yellow solution already mentioned and shown in Figure 2. The intensity of the absorption peak increased (Figure 5) with the increasing molar ratio of APTMS/AgNO₃ in the second step of the polyol process and, at the same time, the color change of the solutions was observed, changing from light yellow to brownish-green (Figure 2).

Figure 5.

Ultraviolet-visible spectroscopy spectra of ethylene glycol (a) and aminopropyltrimethoxysilane/AgNO₃ colloids: (b) 0.5; (c) 1; (d) 2.2; (e) 3.4; (f) 4.4; (g) 6.8.

These characteristic color variations arise from the excitation of SPR in the metallic silver nanostructure. The peak changes indicate that the concentration of silver ions has decreased and AgNPs were generated.^12–15 As can be seen in Figure 5(g), at higher concentrations of APTMS, due to the self-condensation reaction of silane groups and the formation of silica particles in the reaction solution, the interactions of amino silane compounds with the silver seeds decreased, and the surface plasmon peak related to the formation of the AgNPs also decreased significantly.

FT-IR analysis

Figure 6 shows the infrared (IR) spectra of pure APTMS (a) and APTMS coated AgNPs in colloid (b). The spectra of pure APTMS (Figure 6(a)) show a characteristic double peak at 2936 and 2841 cm⁻¹ related to -C-H, while the appearance of sharp bands at 1079 and 809 cm⁻¹ is related to Si-O-Si and Si-O-C, respectively. The IR spectra also show weak peaks at 3354 cm⁻¹ that can be attributed to -NH₂, and at around 1600 cm⁻¹, attributed to -N-H. The spectra of APTMS capped AgNPs (Figure 6(b)) also show a characteristic double peak at around 2900 cm⁻¹ that is attributed to -C-H, a peak attributed to Si-O-Si at 1028 cm⁻¹ and a peak attributed to Si-O-C at 887 cm⁻¹. The characteristic peak related to -NH₂ at 3315 cm⁻¹ is of greater intensity compared to Figure 6(a), probably due to the -OH groups, which could be residues of the polyol used for the synthesis. A less noticeable characteristic peak at around 1600 cm⁻¹, attributed to -NH₂, as well as shifts of other peaks and changes in the fingerprint region, are indications of structural changes of APTMS due to its interactions with AgNPs.^15,33–35 Characteristic peaks of the APTMS structure observed on the FT-IR spectra of AgNP colloid confirm the presence of anchored amino silane groups on the AgNPs.^15,33–35

Figure 6.

Infrared spectra of (a) pure aminopropyltrimethoxysilane (APTMS) and (b) APTMS capped silver nanoparticles.

Thermal behavior

Figure 7 shows the DSC thermogram of APTMS capped AgNPs. The endothermic peak below 250℃ can be attributed to the evaporation of absorbed water and solvent and the decomposition of organic APTMS molecules adsorbed on the AgNPs surface, while the obvious exothermic peak from 420℃ to 500℃ can be attributed to the organic capping layer decomposition from APTMS attached to the AgNPs due to chemical interaction between the APTMS groups and AgNPs. Above 550℃ there is an exothermic peak, which continues even above 600℃, and can be related to the thermal decomposition of the silver core.^15,35 The thermal analysis results confirm that the AgNPs are coated with APTMS.

Figure 7.

Differential scanning calorimetry curve of silver nanoparticles. APTMS: aminopropyltrimethoxysilane.

The antimicrobial activity of AgNPs colloid coated on cotton textile

The antimicrobial activity of the substrate-bound antimicrobials depends on the direct contact of the microbes with the active antimicrobial agent. According to ASTM E2149-13a, the antimicrobial activity of the treated specimen is determined by shaking the samples with surface-bound materials and untreated control samples separately, in a concentrated microbial suspension for 1 h, after which the suspension is diluted and cultured on agar plates for 24 h, so that colonies of selected microbes can form and be counted.^17,29–31 As was shown in Equation (1), 100% reduction means that the number of microorganisms in 1 ml of the suspension has decreased by 100% due to the contact with the treated sample.

For the antimicrobial test, 0.2 g of raw cotton textile was coated with silver colloidal solution APTMS/AgNO₃ = 2.2 via the dip coating method. The samples were kept in AgNP solution for 24 h and then oven heated at 50℃ until they were completely dry. The weight percentage of coated AgNPs on the samples was determined from the dry weight difference of the sample before and after the coating. The weight percentage of AgNPs coated on the samples was calculated at around 8%, based on the dry weight of the textile sample, before and after the coating procedure (Figure 8).

Figure 8.

Coating steps: (a) untreated cotton fabric; (b) dip coating of the cotton fabric with the colloidal silver nanoparticle solution; (c) coated samples after drying.

Figure 9 shows cultured agar plates after the antimicrobial test (1 h contact time), where pictures (a)–(c) are inoculated solutions of selected testing cultures from flasks containing an untreated cotton sample and pictures (d)–(f) are inoculated solutions of selected testing cultures from flasks containing a cotton sample coated with AgNP colloid. The counted colonies that formed after the cultivation time of 24 h are listed in Table 1 for each selected testing organism.

Figure 9.

Cultured agar plates after the antimicrobial test: (a)–(c) inoculated solutions of selected testing cultures from flasks containing an untreated cotton sample; (d)–(f) inoculated solutions of selected testing cultures from flasks containing a cotton sample coated with silver nanoparticle (AgNP) colloid.

Table 1.

Counted colonies that formed after the cultivation time of 24 h for each selected testing organism and calculated % reduction according to ASTM E2149-13(a)

Colony count and calculated % reduction
		A Treated sample after 1 h	B Untreated sample after 1 h	% Reduction (B – A)/B × 100
Escherichia coli NCCB 100297 (equivalent to ATCC 25922)	0.1 ml orig	0	/
		0
		0
	0.1 ml 1:10	0	/
		0
		0
	0.1 ml 1:100	0	209
		0	148
		0	226
	CFU/ml	0	194,333	100%
Staphylococcus aureus NCCB 100294 (equivalent to ATCC 25923)	0.1 ml orig	1	/
		2
		0
	0.1 ml 1:10	0	/
		0
		0
	0.1 ml 1:100	0	119
		0	104
		0	114
	CFU/ml	10	112,333	100%
Candida albicans DSM 1386 (equivalent to ATCC 10231)	0.1 ml orig	270	/
		268
		248
	0.1 ml 1:10	30	/
		28
		14
	0.1 ml 1:100	2	64
		0	61
		0	69
	CFU/ml	2620	64,667	96%

Cellulose polymer in cotton fabric has hydroxyl functional groups, which can act as nucleophiles. The presence of APTMS in the reaction provides an alkali media for the AgNP colloids (pH 9.2), making the fiber surface negatively charged when soaked in such a colloid. Namely, under alkaline conditions, the hydroxyl groups of cellulose deprotonate and form cellulose–O⁻ groups. These deprotonated groups of cellulose are reactive, and can bind silver ions on the fiber surface, providing good stability of AgNPs on the cotton fibers.^18,19,27

Adjusting the pH of the solution to around 10 to achieve better control of the particle size in AgNP synthesis and better adsorption, or even binding of AgNPs, on cellulose fibers for antimicrobial activity has already been reported in other research works.^15,18,19,27 Adding aqueous ammonia to the AgNO₃ solution and adjusting the pH of the solution to around 10 caused Ag⁺ to react with ammonia and form an [Ag(NH₃)₂]⁺ complex. Under such conditions, these complexes had high affinity toward cotton fibers, and were adsorbed easily on their surface. Afterwards, they were dipped in glucose solution, which caused in situ reduction of the [Ag(NH₃)₂]⁺ into AgNPs on the surfaces of cotton fibers, and resulted in stable AgNP coating on these fibers, even after 10 cycles of rinsing simulation.^19,27

In this research, we used APTMS, whose primary amine groups have a great potential toward Ag⁺ (similar to NH₃) and can form a complex [Ag(H₂NR)₂]⁺ (R: -R'-Si-OR”₃). In addition, APTMS provides an alkaline medium (pH 9.2) of the synthesized colloids, thus creating appropriate conditions for the deprotonation of hydroxyl groups of cellulose polymers of cotton textile during the coating process (the soaking step in Figure 8(b)). Deprotonated cellulose has better adsorption of AgNPs on the fiber, presumably forming strong and stable O-Ag bonds.^15,18,19,27

As the results show, the percentage reductions for organisms S. aureus (gram-positive bacteria), E. coli (gram-negative bacteria) and C. albicans (fungus) were 100%, 100% and 96%, respectively, which proves the excellent antimicrobial properties of the AgNP coated cotton textile described in this work. The reduction of microbial growth for the AgNP treated samples can be related to the fact that AgNPs can act as a vehicle to release Ag⁺ on the bacterial cytoplasm and membrane, leading to a decrease of the surrounding pH and, hence, cell disruption in the organisms, which can affect bacterial growth.^10,16,21 Specific research on the spherical and hexagonal AgNPs revealed that the AgNPs adsorbed on the bacterial cell surface can cause a kind of depression on the cell surface. As a result, AgNPs can enter pathogens and combine with their protein groups, thereby killing the bacteria.³⁶

Conclusion

Colloidal AgNPs have been synthesized by the polyol method. APTMS was introduced as a capping agent in the synthesis of AgNPs for the first time. The influence of different concentrations of APTMS on the size and morphology of the synthesized AgNPs was investigated, and the obtained nanoparticles were characterized by TEM, UV-Vis, FT-IR and DSC. TEM images confirmed the nanocrystalline structures of synthesized AgNPs. AgNPs with hexagon morphology were obtained at molar ratio APTMS/AgNO₃ = 2.2. UV-Vis spectra of synthesized AgNPs showed characteristic SPR peaks between 425 and 445 nm. FT-IR and DSC analysis of synthesized AgNPs colloids confirmed organic residues of APTMS, which we assume is on the surface of the particles in the synthesized AgNPs colloids. In addition, we applied the optimum AgNP colloid (APTMS/AgNO₃ = 2.2) on the cotton fabric, and tested the antimicrobial activity of the prepared AgNP coated textile according to ASTM 2149-13a. The antimicrobial tests showed excellent inhibition of growth for E. coli, S. aureus and C. albicans. The prepared AgNPs could find their application in biomedical devices, healthcare products, packaging materials, air and water purification devices or any other materials where antimicrobial activity is desired.

Footnotes

Acknowledgements

Maedeh Ramezani thanks Majid & Parmisa Amirdehi for their kind support during this research work.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by Slovenian Research Agency (ARRS) through a Grant for Young Researches (No. 1000-16-0552) and RS and EU funds (European Regional Development Fund) through the project “MultiNanoTex” (KKIPP-75/2017).

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Synthesis and characterization of an antimicrobial textile by hexagon silver nanoparticles with a new capping agent via the polyol process

Abstract

Keywords

Experimental details

Materials

Synthesis of AgNPs

Characterization

Antimicrobial activity

Results and discussion

The effect of APTMS in polyol synthesis of AgNO3

UV-Vis characterization

FT-IR analysis

Thermal behavior

The antimicrobial activity of AgNPs colloid coated on cotton textile

Conclusion

Footnotes

Acknowledgements

Declaration of conflicting interests

Funding

References

The effect of APTMS in polyol synthesis of AgNO₃