Synthesis of silver nanoparticles in NMMO and their in situ doping into cellulose fibers

Spectroscopic studies were performed at 23 °C in quartz trays using an Evolution 201 UV–Vis spectrophotometer from Thermo Scientific Company.

The size distribution of the formed nanoparticles was determined by the dynamic light scattering (DLS) method using a Nicomp 380ZLS (Particle Sizing Systems, USA) including a laser with wavelength of 532 nm and power of 50 mW. The frequency of the photons measured by the autocorrelator was fixed at about 200 kHz, with time of 3 min for each measurement. A polymethylmethacrylate (PMMA) tray with dimensions of 40 × 10 × 10 mm³ was used for the measurements. The hydrodynamic diameter was calculated using the Stokes–Einstein equation, assuming measurement temperature of 298 K and viscosity of the continuous phase (water) of 0.891 mPa s.

Microstructural studies of the silver particles were conducted using an FEI Tecnai G2 20 X-TWIN equipped with an energy-dispersive anlaysis of X-rays (EDAX) microanalyzer.

Analysis of the elemental composition of the prepared fibers was performed by scanning electron microscopy (SEM) (LEO 435 VP, Zeiss + Leica), equipped with an energy-dispersive X-ray spectroscopy (EDS) microanalysis system from Rontec GmbH.

The chemical structure of the samples was examined by Fourier-transform infrared (FTIR) spectroscopy using a Thermo Scientific Nicolet 380 spectrometer. Spectra were recorded in the range of 60–4000 cm⁻¹ at resolution of 4 cm⁻¹.

A colloid solution was created by reduction of silver ions in the basic sample containing 50 % aqueous solution of NMMO, AgNO₃ at concentration of 2.7 mmol/L, and polyethylenimine (M _w = 2 kDa) at 8.1 mmol/L. In the UV–Vis spectrum of the colloid solution, a characteristic peak was observed at wavelength of 420 nm, indicating presence of silver nanoparticles (Fig. 2a).

According to the DLS results, the average size of the silver nanoparticles synthesized in the basic solution was about 4 nm (Fig. 2b).

During chemical synthesis of nanoparticles of various materials, including silver, stabilizers are used to prevent agglomeration, also affecting the size and shape of the resulting nanoparticles. During the synthesis of silver nanoparticles in aqueous solution, three different polymers, i.e., polyvinylpyrrolidone (PVP), polyethylene glycol (PEG), and polyethylenimine (PEI), were used as stabilizers. The aim of the first stage of the tests was to determine the effect of the kind of stabilizer and its molecular weight on the reduction process. The peak typical for silver was only observed when polyethylenimine was used as stabilizing agent (Fig. 3), occurring between 400 and 450 nm depending on the mean molecular weight of the PEI. According to Mie theory (1908), such a shift in the absorption signal is due to a change in the size and/or shape of the nanoparticles. In the case of high-molecular-weight polyethylenimine, the peak was very broad, revealing that the synthesized particles had large diameter. Meanwhile, in the spectra of the colloids stabilized using polyvinylpyrrolidone or polyethylene glycol, regardless of their molecular weight, no characteristic peak of silver nanoparticles was observed. In the presence of these polymers, large silver particles formed and quickly settled to the bottom of the measuring cuvette.

DLS measurements of the average size of the synthesized particles confirmed these observations and UV–Vis test results. Monodisperse silver nanoparticles were only synthesized in presence of polyethylenimine as stabilizer (Fig. 4), their size depending on the average molecular weight of the PEI. The particles synthesized in presence of PEI with average molecular weight of M _w = 750 kDa or M _w = 2 kDa measured 4 and 11 nm, respectively, while large particles exceeding 200 nm were synthesized in presence of high-molecular-weight branched polyethylenimine.

It is suggested that these changes in the resulting silver nanoparticles are related to the length of the polymeric chain, as effective nanoparticle stabilization is only achieved by polymers of appropriate length and macromolecular structure (Mikami et al. 2011). One end of the polymer binds with the nanoparticle surface, whereas the remaining part forms a loose layer around the nanoparticle. A double polymer layer is thereby formed, consisting of polymer functional groups chemically bound to the nanoparticle surface and polymer chains located on the outer layer. When this polymer layer has appropriate thickness, it can effectively protect the particles from agglomeration (Fig. 5a, b). However, polymers that are too long may become excessively entangled in solution, in turn resulting in gathering of particles and their agglomeration (Fig. 5c).

On the basis of these tests, it was concluded that silver nanoparticles were only created in solution of NMMO with polyethylenimine as stabilizer. For this reason, further tests were conducted using PEI with M _w = 2 kDa.

The presence of stabilizer on the nanoparticle surface was confirmed by HRTEM observations (Fig. 6), which clearly showed that silver nanoparticles synthesized in NMMO solution lay within polymeric micelles, protecting them from further growth or agglomeration.

Spinning solution for cellulose fiber production is prepared at elevated temperature. Therefore, the influence of temperature on the synthesis of silver nanoparticles was studied. During these tests, reduction of silver ions was conducted at ambient temperature as well as when heating the sample at 70 °C for 60 min. In both cases, the characteristic peak appeared in the UV–Vis spectrum after 24 h (Fig. 7a). Monodisperse colloid solution containing nanoparticles with mean size of 5 nm was also created at the elevated temperature (Fig. 7b).

At ambient temperature, larger nanoparticles (approximately 8 nm) were formed (Fig. 7). The smaller size of the nanoparticles obtained at higher temperature may be due to accelerated nucleation and creation of the nanoparticles themselves. As a result, more smaller nanoparticles were created in the solution compared with synthesis at ambient temperature.

The first aim here was to determine whether it is necessary to apply polyethylenimine in the synthesis. UV–Vis results showed that silver nanoparticles were created both in presence of and without stabilizing polymer. Reduction of silver ions occurred in both cases. However, the synthesis of nanoparticles was much slower in the solution without stabilizer, and the synthesized nanoparticles agglomerated quickly (Fig. 8a). Therefore, we conclude that polyethylenimine does not trigger silver ion reduction but acts as an effective stabilizer of the nanoparticle size. HRTEM results showed that nanoparticles of similar size were created in both presence and absence of polyethylenimine as stabilizer. However, the nanoparticles synthesized in presence of polyethylenimine did not exhibit agglomeration (Fig. 8b), in contrast to the nanoparticles synthesized without stabilizer (Fig. 8c).

The observed agglomeration of nanoparticles synthesized without stabilizing polymer is typical of nanomaterials. Due to their high surface energy, they have a tendency to agglomerate. For this reason, different stabilizing substances are commonly used in their synthesis (Kholoud et al. 2010).

The nanoparticle synthesis process was analyzed by observing the change in the absorption of the solution over time. At each time point, 0.5 mL of reaction mixture was diluted with deionized water to volume of 4 mL. The reaction was conducted at ambient temperature. Two absorption peaks were observed in the UV–Vis spectrum of colloids synthesized in presence of PEI (Fig. 9). The first, much weaker peak occurred at wavelength of 320 nm (Fig. 9a), while the other, much more intense peak occurred at wavelength of about 420 nm (Fig. 9b). The peak at 320 nm was observed within the first few dozen minutes of reduction. It then disappeared, and the other peak at wavelength of about 420 nm appeared. The peak intensity increased with the duration of the synthesis. In presence of PEI, maximum absorption was reached within 24 h.

Silver ion reduction also occurred in the case of synthesis in NMMO solution without stabilizer (PEI), but much more slowly. The peak typical of Ag nanoparticles was not observed in the spectrum after 24 h (Fig. 9c). Only after 48 h did this peak start to appear, and its intensity was still increasing after 100 h. These results confirm that PEI acts to stabilize the nanoparticles and accelerates the reaction leading to their creation.

During reaction, silver ions are reduced to their metallic form Ag⁰ (1); silver atoms then bond with silver ions to create dimer cations \( {\text{Ag}}_{2}^{ + } \) (2), which may also bond in pairs to create tetracations \( {\text{Ag}}_{4}^{2 + } \) (3). Further reactions of ions, atoms, dimer cations, and tetracations lead to creation of metallic silver nanoparticles.

The two peaks observed in the UV–Vis spectra (Fig. 9) are connected with the presence of silver in different forms. The first peak at wavelength of 310 nm is connected to presence of di- and tetracations or small nonmetallic clusters (Ag _n ). These molecules are created at the initial stage of nanoparticle synthesis (Fig. 10a). After approximately 1 h, a second peak starts to appear in the spectrum of the colloid created with PEI, confirming presence of silver nanoparticles (Fig. 10b). Its intensity increased considerably with prolonging synthesis time. After about 5 h, the peak at 310 nm entirely disappeared, indicating that the intermediate forms of silver had transformed into metallic nanoparticles (Mercado et al. 2002).

To confirm the above conclusions and observations, electrochemical tests were conducted on NMMO solutions containing only silver nitrate or silver nitrate and polyethylenimine (Fig. 11). The tests were carried out in a two-electrode system, measuring the changes in the potential of a silver-ion-selective electrode. According to the Nernst equation, the reference electrode potential depends on the concentration of silver ions (4) as follows:where E is the electrode potential, E ⁰ is the standard electrode potential, R is the gas constant with value of 8.314 J K⁻¹ mol⁻¹, T is temperature (K), n is the number of electrons exchanged in the reaction, and F is the Faraday constant with value of 96,485 C mol⁻¹.

A decrease of the reference electrode potential was observed after adding silver nitrate to the NMMO or NMMO with PEI solution. In the initial stage, in the solution without PEI, the potential remained at a constant level due to the fact that the reaction was very slow (Fig. 9c). In the case of synthesis in NMMO with PEI solution, there was a slight decrease in the potential, symptomatic of slow reduction of silver ions according to reactions (1–3). Afterwards, the electrode potential settled at a stable level of 250 mV. These results are consistent with the UV–Vis test results. On the basis of both measurements, it can be concluded that complete reduction of silver ions occurred within approximately 24 h in the solution containing PEI, whereas in absence of PEI, no silver ion reduction occurred in the solution within the first 50 h.

To describe the entire mechanism of nanoparticle synthesis, it is essential, in the first place, to determine the agent responsible for reducing the silver ions. It is known that reduction of silver ions may be triggered by ultraviolet electromagnetic radiation, elevated temperature, or a reducer. The reduction of silver ions in solution of N-methylmorpholine N-oxide is a typical oxidation–reduction process. The solvent is water containing 50 % NMMO by weight plus polyethylenimine as stabilizing agent. In such solutions, silver ion reduction is also triggered without a stabilizing agent. To define the mechanism of Ag nanoparticle synthesis, it is vital to determine which substance in the solution acts as a Ag⁺ ion reducer. It is common knowledge that NMMO is a very strong oxidant (Rosenau et al. 2001).

In the presented research, all reactions were conducted in the dark (with vessels wrapped in tin foil), so electromagnetic radiation could not be a reducing agent. Elevated temperature was also excluded, because the reduction process in NMMO solution occurred at both ambient and elevated temperature (70 °C). Therefore, the reducing agent must be a substance found in the solution. In nanoparticle synthesis, a stabilizing polymer may have a double function, acting as both stabilizer and reducing agent (Note et al. 2006). However, the results above show that polyethylenimine does not act as a reducer here, since silver ion reduction also occurred in NMMO solution without polyethylenimine.

As silver ion reduction in NMMO is a redox reaction, and factors such as temperature, light, and presence of polyethylenimine can be excluded, we investigated further to identify the reducing agent. The solution in which the silver nanoparticles were synthesized contained NMMO molecules, water, polyethylenimine, hydroxyl ions, nitrate ions, and silver ions undergoing reduction. Since solution pH is an important parameter influencing chemical reactions, changes in acidity were monitored during the synthesis (Lee et al. 2004). The solution acidity is a significant parameter for synthesis of nanoparticles, particularly due to the fact that the shape of polymer molecules depends not only on the structure of the polymer chain but also on the solution pH. It has been reported that, at high pH, PEI deprotonation occurs and its chain begins to roll up. At appropriate pH and PEI chain length, agglomeration of nanoparticles does not occur (Yuan and Li 2010).

The acidity of the aqueous NMMO solution (50 % by weight) was approximately 9.37. After adding silver nitrate to the solution, there was a sharp decline in pH to approximately 9.3 (Fig. 12a), due to dissociation and hydrolysis of dissolved silver nitrate.

The pH not only influences the behavior of the PEI chains but also affects the NMMO. Aqueous solution of NMMO is alkaline in nature, being a strong oxidant, and very labile in acidic environment. With decreasing pH, the protonation of NMMO significantly increases, leading to decomposition of NMMO into N-methylmorpholine (NMM) and morpholine (M), because the protonated form of NMMO is very labile (Jung et al. 1996). Although commercially available NMMO contains insignificant amounts of NMM and M, it further decomposes in presence of metals, negatively affecting spinning solution preparation. Therefore, elevated pH due to addition of PEI leads to stabilization of NMMO rather than its decomposition. The conductance of the colloids was tested during the 4 days of synthesis. In the case of synthesis conducted in presence of polyethylenimine, the solution conductance decreased in the first 24 h, then settled at a stable level of about 100 µS/cm (Fig. 13). This decrease in conductance was caused by silver ion reduction and synthesis of nanoparticles. The settling of the conductance at a stable level, similar to the conductance of pure NMMO, indicates that complete reduction of Ag⁺ ions had taken place. These results correspond to those of UV–Vis spectroscopy (Fig. 9) and the silver ion concentration measurements (Fig. 11), in which the potential also stabilized after 24 h. In the case of synthesis conducted without PEI, the solution conductance decreased continuously despite the fact that, in the initial stage, the number of Ag⁺ ions did not decrease in the chemical composition, as confirmed by the UV–Vis test results (Fig. 9c). The decrease in conductance may be connected with changes in NMMO triggered by metal ions (Rosenau et al. 2001). As mentioned above, NMMO is a labile compound, and in reaction with cellulose and/or heavy metals, decomposes to N-methylmorpholine, morpholine, carbon dioxide, and water as main products (Taeger et al. 1985). During such degradation, highly labile intermediate products such as methyl radical morpholinium, and N-(methylo)cation morpholinium are also created (Rosenau et al. 2001). It is commonly believed that, in the cellulose/NMMO system, degradation of NMMO is most effectively prevented by alkaline hydroxides and antioxidant stabilization (NaOH or PG, propyl gallate). Substances containing functional groups or surface-active materials can additionally trigger the degradation process (Wendler et al. 2005). Each of these additional characteristics, i.e., pH value, presence of functional groups (–COOH, –NH₂), surface structure, particle size, etc., could lead to potential interactions with main components of the NMMO/cellulose solution. Activation of the highly labile structure of the NMMO molecule is triggered by protonation, creating complexes with metals or O-alkylation. Loosening of the NMMO ring structure leads to creation of resonance-controlled nitroxyl radicals and permanent ring-degradation products. Our test results confirmed that polyethylenimine not only stabilized nanoparticles by preventing their agglomeration but also prevented NMMO degradation by avoiding its contact with the metal surface.

The mixture in which the silver nanoparticle synthesis was conducted contained Ag⁺, \( {\text{NO}}_{3}^{ - } \), and \( {\text{OH}}^{ - } \) ions, polyethylenimine, water molecules, and N-methylmorpholine N-oxide. It should be concluded that the silver, nitrogen, and hydrogen atoms are at their highest oxidation level, while oxygen atoms are at their lowest. This means that, in the solution, there are three substances which may act as an oxidizer: NMMO and Ag⁺ and \( {\text{NO}}_{3}^{ - } \) ions. The fact that the solution is alkaline (Fig. 12) whereas nitrate ion reduction occurs in acidic solution indicates that they will not participate in the studied reaction. The standard electrochemical potential for oxidation of hydroxyl ions (5) is 0.88 V, significantly lower than the NMMO potential (1.47 V). As NMMO is a strong oxidant and has higher potential in solution, reaction (6) will occur first: It is reported that, in the reaction of metal ions with NMMO, the ions act as a reducer and NMMO as an oxidant. However, these reactions do not occur when the metal ions are at their highest oxidation level, as is the case for silver here (Ferris et al. 1967). Perhydroxyl ions (\( {\text{HO}}_{2}^{ - } \)) created as a result of reaction (6) oxidize according to reaction (7), whose potential (−0.76 V) is lower than that of silver ions (0.799 V, 8). Hence, the created perhydroxyl ions may act as the silver ion (Ag⁺) reducer according to the final reaction (9). On the basis of the above test results and analysis of literature data, the authors conclude that perhydroxyl ions act as a reducer in silver nanoparticle synthesis in NMMO. Moreover, pH plays a significant role in this process, as appropriately high pH accelerates such synthesis. This explains the influence of polyethylenimine on the synthesis. Apart from stabilizing already formed nanoparticles, it also accelerates the reaction due to the increased solution pH, i.e., higher number of \( {\text{OH}}^{ - } \) ions, which accelerates reaction (6) and thereby \( {\text{Ag}}^{ + } \) reduction according to reaction (9).

SEM analysis of the cellulose fiber microstructure revealed a smooth surface (Fig. 14), even after modification. Such maintenance of the fiber smoothness was possible as the whole volume of the fiber was doped rather than just the surface. The appearance of the doped fibers was visibly different, changing color after doping. The pure cellulose fibers (Fig. 14c) were white, whereas the fibers with silver nanoparticles, depending on the kind of stabilizing polymer applied, were yellowish orange in color (Fig. 14a), or dark brown in case of synthesis without stabilizer (Fig. 14b). The color change in the fibers is due to the effect of surface plasmon resonance in the silver nanoparticles in the doped fibers. Fibers containing silver created without polyethylenimine addition were dark brown, because the size of the silver particles did not exceed 200 nm. Lack of stabilizer led to creation of much larger particles in the cellulose matrix. Additionally, analysis of the silver dispersion in the fibers (Fig. 14d) showed that the silver nanoparticles were uniformly dispersed throughout the entire fiber volume.

Tests of mechanical properties revealed that doping the fibers with silver nanoparticles led to a significant improvement of their mechanical properties (Table 1). Composite fibers could bear much higher loads and showed higher Young’s modulus. The size and uniform dispersion of the nanoparticles in the volume of the composite fibers led to the increased Young’s modulus, an effect commonly known as dispersion strengthening of composite materials. The endurance of cellulose fibers doped with silver nanoparticles obtained without polymeric stabilizer was 24.59 cN/tex, similar to previously achieved results falling in the range of 22.00–24.27 cN/tex (Smiechowicz et al. 2011). Fibers containing stabilized silver nanoparticles in their volume were characterized by higher endurance (29.73 cN/tex) in comparison with nondoped fibers (28.42 cN/tex). The higher mechanical endurance of these fibers is due to the small volume and good dispersion of the admixture in the fibers. It may also be due to improved fiber quality, because, as previously reported, PEI also acts as an NMMO stabilizer, preventing its degradation during fiber synthesis. These results confirm the need to apply additives to stabilize the size of nanoparticles synthesized in situ.

We first tested the formation of nanoparticles in NMMO solution. This is a very interesting research problem, because this process occurs without use of standard reducing agents such as ascorbic acid, but in presence of NMMO, a strong oxidant. The analysis above demonstrates that redox processes take place in the AgNO₃–NMMO system, leading to formation of perhydroxyl ions, which act as a reducer of silver ions. Such conclusions are novel, and verified based on a series of tests. This finding greatly contributes to the scientific achievements in this field, and may can help understand a number of phenomena that occur during nanoparticle doping processes.

We next synthesized silver nanoparticles in a system for fiber preparation. Cellulose dissolution was carried out in a closed reactor at elevated temperature and pressure. Under such conditions, it is not possible to control the progress of silver ion reduction. To examine indirectly the impact of cellulose on the mechanism of the observed reactions, we prepared fibers with and without PEI, considering the possibility that cellulose might stabilize the nanoparticles. These experiments showed that the resulting cellulose material failed to meet expectations, having dark-brown colour typical of microparticles (Fig. 14b) and poorer mechanical strength (Table 1), showing that it is indispensable to apply a stabilizer such as PEI. These results suggest that cellulose did not significantly affect the silver nanoparticle synthesis, in terms of either reduction or their stabilization. Additionally, Emam and El-Bisi (2014); Emam et al. (2014) presented a method for synthesis of silver nanoparticles using different types of cellulosic materials. It was demonstrated there that Ag⁺ ions are reduced to AgNPs, simultaneously oxidizing cellulose. Alcoholic and aldehydic groups in cellulose change to an oxidized form, e.g., carboxylic groups. These studies show that reduction of Ag⁺ ions affects Lyocell fibers, because they are spun using NMMO, a strong oxidant. These studies confirm the hypothesis that cellulose has an insignificant effect on the investigated reduction reaction of Ag⁺ ions. During fiber production, when silver ions are reduced, antioxidant and PEI are also added to the system, preventing cellulose degradation. Therefore, the elaboration of the exact mechanism of Ag nanoparticle synthesis in NMMO will help understand the phenomena taking place during in situ synthesis of nanoparticles in spinning solution.