Cellulose-Based Nanofibers Electrospun from Cuprammonium Solutions

doi:10.21203/rs.3.rs-3195822/v1

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Cellulose-Based Nanofibers Electrospun from Cuprammonium Solutions

https://doi.org/10.21203/rs.3.rs-3195822/v1

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The production of nanofibers based on cellulose has attracted considerable interest due to their remarkable biocompatibility and thermal and mechanical characteristics, rendering them increasingly popular for numerous biomass-based fibrous applications. The current research describes the electrospinning process of cellulose utilizing cuprammonium solutions. Polyethylene oxide (PEO) is also introduced to improve electrospinning and end material characteristics. The impact of the cellulose source, cellulose concentration, PEO molecular weight, and PEO concentration on spinnability and fiber morphology was systematically investigated. The analysis of membrane morphology and other associated characteristics was conducted through scanning electron microscopy with X-ray diffractometer and X-ray photoelectron spectroscopy techniques. A direct relationship exists between cellulose concentration and PEO molecular weight, resulting in an observed enhancement in fiber diameter. The nanofiber membranes demonstrate notable antibacterial characteristics for Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli) owing to copper nanoparticles due to cuprammonium solution. Hence, the nanofiber membranes exhibit promising potential for utilization in several domains, such as water treatment, food packaging, medical, and separation.

Cellulose

Electrospinning

Cuprammonium solution

Poly(ethylene oxide)

Molecular weight

Antimicrobial activity

The production of nanofibers based on cellulose has attracted considerable interest in the scientific community due to its distinctive characteristics and wide-ranging application possibilities in protective clothing, biomedical areas, filtration, separation, and mechanical areas (Dahiya 2006; Razali et al. 2018; Zahari et al. 2018). Electrospinning is a highly adaptable method that facilitates the formation of nanofibers with regulated morphology, an elevated volume-to-surface area ratio, and customize able mechanical characteristics (Adamu et al. 2021). Cellulose, the most prevalent natural organic polymer, possesses inherent benefits, such as its ability to be renewed, biodegraded, and biocompatible, rendering it a desirable material for producing nanofibers (Habibi 2014). Plants are considered the primary cellulose source, which may also be derived from several other sources, like animals, fungi and algae (Rojas 2016). Electrospinning can produce nonwoven cellulose membranes for various applications (Dizge et al. 2019; Ge et al. 2021; Li et al. 2023; Teixeira et al. 2020).

Cellulose demonstrates limited mobility and lacks solubility in typical organic solvents. The structural stiffness of cellulose is attributed to the substantial presence of inter- and intra-molecular hydrogen bonds linked to hydroxyl groups in the glycosidic rings, rendering it a highly robust polymer. Instead, it necessitates precise solvent blends that can effectively weaken hydrogen bonding, thereby inducing the separation of cellulose molecules (Hachaichi et al. 2021).

Solvents applied for the dissolution of cellulose are classified into two groups: derivatizing and non-derivatizing. Cellulose is subject to a chemical covalent modification process when it is derivatized with solvents, substituting some hydroxyl groups with ester, ether, or acetal groups. This transformation chemically modified it into a soluble derivative various solvent (El Seoud et al. 2019). Various cellulose molecules have been modified, including cellulose acetate, cellulose nitrate, cellulose xanthate, carbamate, and cellulose phosphate (Costa et al. 2019). Solvents that do not undergo derivatization participate solely in intermolecular interactions.

The non-derivatizing techniques rely on a physical alteration of intermolecular interactions instead of a chemical modification. Illustrations of such instances comprise cellulose-metal complexes encompassing a transition metal and an ammonium or amine constituent, such as cuprammonia, cadoxen, zincoxen and nioxam (Van De Ven and Godbout 2013). Various non-derivatizing techniques are available, including the utilization of aqueous alkalies such as cupriethylenediamine hydroxide, cuprammonium hydroxide, polyethylene glycol, thiourea, lithium hydroxide with urea, sodium hydroxide complexes with urea, concentrated molten inorganic salt hydrates and n-methylmorpholine n-oxide dimethyl, dimethylacetamide/lithium chloride, and sulfoxide/tetra-n-butylammonium fluoride (Sayyed et al. 2019). Furthermore, non-derivatizing techniques for cellulose dissolution involve the utilization of ionic liquids, which consist of diverse organic cation and inorganic anion derivatives derived from ammonium, imidazolium, phosphonium, and pyridinium (Sayyed et al. 2019; Xu et al. 2017a; Xu et al. 2017b). Illustrations of those compounds comprise 1-ethyl-3-methyl-imidazolium acetate and 1-butyl methyl-imidazolium chloride. The total number of conceivable ion combinations is 10¹² (Medronho et al. 2018).

Almost all solvents utilized in the processing of cellulose are intrinsically ecologically hazardous. Overuse of resources causes the buildup of toxic substances in the surrounding ecosystem. As mentioned earlier, the statement highlights that while cellulose-derived materials spanning from the macroscale to the nanoscale have the potential to be ecologically viable, the requisite chemical processes involved in their production may not possess similar attributes. The chemical process known as cuprammonium was first discovered by M.E. Schweizer, a Swiss chemist, in 1857. Schweizer observed that a composite of ammonium hydroxide and copper hydroxide exhibited the ability to dissolve cellulose. The utilization of this cuprammonium complex was employed in the production of cuprammonium rayon fibers (Liebert 2010).

The cuprammonium ion always pairs up with the hydroxyl groups at positions C₂ and C₃ of the glucopyranose units in the cellulose structure (Fig. 1). According to Saalwächter et al. (2000), this interaction reduces intermolecular hydrogen bonding within cellulose, thereby facilitating the separation of cellulose molecules. The presence of copper occurs mainly within the amorphous region, leading to a reorganization of the molecular arrangement of cellulose (Miyamoto et al. 1996). The dissolution mechanism has been thoroughly examined and confirmed by various researchers (Reeves 1951; Saalwächter and Burchard 2001; Saalwächter et al. 2000). According to Zhang et al. (2022), copper hydroxide laid on the surface of the fabric while ammonium hydroxide evaporated.

The formation of fibrous materials from electrospinning was not observed in pure cellulose solution, as it frequently resulted in the production of spherical nanoparticles (Qi et al. 2010). Using PEO, a synthetic polymer soluble in water, is a general practice in combination with cellulose for better electrospinnability and mechanical properties of the resultant nanofibers. The integration of PEO enhances the intermolecular interaction, resulting in better fiber formation and increased mechanical strength (Zaitoon and Lim 2020). Moreover, PEO decreases the diameter of fibers by affecting spinnability and jet stability in the electrospinning procedure (El Fawal 2020).

This research presents an in-depth investigation of the electrospinning process for producing nanofibers, utilizing a blended matrix consisting of cuprammonium cellulose and aqueous PEO solutions. Cellulose was previously electrospun with PEO in the derivative form like ethyl cellulose (Zaitoon and Lim 2020), Carboxymethyl cellulose (Shi et al. 2016), cellulose acetate (Zhang and Hsieh 2008), with different solvent systems and with non-derivative form in water like cellulose nanofiber (Yamagata et al. 2021), microfibrillated cellulose (Fortunato et al. 2012), Cellulose nanocrystal (Zhou et al. 2011), bacterial cellulose fibril (Park et al. 2007), however, based on the available information, there is a lack of existing research on this topic on the formation of cellulose nanofibers with different concentrations by utilizing the cuprammonium complex for dissolution in conjunction with PEO of varying molecular weights. The change in solution ratio, cellulose and PEO concentrations, cellulose source, and PEO molecular weight were examined and associated with the diameter of nanofibers. The development of cellulose nanofibers with cuprammonium solution allows for the production of a better candidate for water treatment, antimicrobial, and separation applications.

Materials

The reagents utilized in this study were ammonium hydroxide solution with a concentration range of 28–30%, copper (II) hydroxide, and PEO with molecular weights of 300, 500, and 1000 kDa, which were procured from Shanghai Aladdin, China. Cellulosic sources, namely cotton linters (degree of polymerization ≈ 1430) and needle wood (degree of polymerization ≈ 1185), were procured from Dalian Yangrun, China. The reagents utilized in this study were of analytical grade and obtained from reputable vendors in Qingdao, China. The experiments were conducted using deionized water.

Electrospinning of cellulose nanofibers

The electrospinning process for producing cellulose nanofibers was conducted per the methodology outlined by Dias et al. (2020). A cuprammonium solution was synthesized by mixing 2-weight percent copper hydroxide with ammonium hydroxide using a magnetic stirrer for 10 minutes. The cuprammonium solution was utilized to dissolve a mixture of cotton linters and needle wood fibers. The concentration of the fibers in the solution was maintained at a range of 2 to 5 weight percent. The mixture was subjected to magnetic stirring for 30 minutes. Distinct solutions of PEO in distilled water with concentrations varied from 1 to 2-weight percent were prepared individually and subsequently blended with cellulose cuprammonium solution in varying ratios. Subsequently, following a 15-minute mixing period, the blend solution was subjected to electrospinning at ambient temperature. This process utilized a needle with an internal diameter of 0.6 mm at a supply rate ranging from 1.67 to 16.67 µl/min and at applied voltages between 15 and 27 kV. The distance from the collector was maintained at 100 to 160 mm, while the relative humidity was kept within the 28 to 59% range. The electrospun nanofiber membranes are dried at 60°C for 2 hours.

Solution Casting

The preparation of cellulose cuprammonium solution involved using varying cellulose concentrations, and the resultant solutions were subjected to stirring for 24 hours at ambient temperature. The composite membrane films were fabricated via solution casting onto an Aluminum foil dish and subsequently dried for 24 hours at ambient temperature. The dish was dried in an drying oven, maintained at a temperature of 60°C, for 12 hours. After the solvent had evaporated, the solid membrane films were extracted from the dish.

Characterization

The nonwoven fabric's morphological characteristics were analyzed by scanning electron microscopy (SEM) employing a Phenom Pro apparatus manufactured in the Hitzacker, Germany. A nonwoven sample was affixed to conductive tape and subjected to gold spraying for 1 minute using an SBC-12 small ion sputtering instrument. Subsequently, the specimen was examined via SEM utilizing an electron acceleration potential of 10 kV. The measurement of fiber diameter was conducted using the NanoMeasurer 1.2.5 software. The X-ray diffractometer (XRD) patterns were recorded using a DX-2700 X-ray diffractometer equipped with Cu Kα radiation (40 kV, 40 mA). The specific range of 2θ examined was 5–30°, and the scanning rate was 5.0° min-1. Specimens included cotton linters and needle wood obtained from a supplier and regenerated cellulose nanofibers produced through the solution casting method using cuprammonium solution. X-ray photoelectron spectroscopy (XPS) was used to examine the bonding states of surface components within the sample. The experiments were conducted using a Thermo Fisher ESCALAB 250Xi spectrometer having an Al Kα X-ray source with an energy of 1486.6 eV.

Antibacterial Activity Assessment

The process of disc diffusion was used to evaluate the antibacterial efficiency of cuprammonium cellulose with PEO compound membrane against E. coli and S. aureus (Liu et al. 2019). In a short procedure, the bacterial suspensions, measuring 200 µl and having a final concentration of 1×10⁶ CFU/ml, were introduced onto the solid agar medium and evenly distributed using a glass spreading rod. Initially, the customized specimens with a diameter of approximately 25 mm were placed in Petri dishes cultivated with E. coli and S. aureus bacteria. Subsequently, the Petri dishes were incubated for 24 hours at 37°C. Following this, the area of inhibition zones surrounding each sample was determined using the ImageJ 1.46r software. The antibacterial experiments were conducted in triplicate.

Establishing an essential proportion of entry variables and focusing on a moderate quantity of chosen variables is imperative. In the forthcoming analysis, we will focus on the reciprocal interaction between cellulose sources, cellulose concentration, PEO concentration, PEO molecular weight, the viscosity of the cellulose cuprammonium solution, and the mean diameter of the resultant nanofibers. The parameters utilized in this study were based on prior research, which indicated that the characteristics of the solution have a more significant influence on the diameter and morphology of electrospun nanofibers made from zein (Neo et al. 2012), than other processing parameters (Neo et al. 2012), for zein fibers (Tan et al. 2005); for poly(L-lactid-co-caprolactone) and poly(L-lactic acid) fibers).

Table 1 presents the solution compositions of cellulose and PEO, as well as the fiber diameter and morphology. Table 2 displays the pertinent electrospinning process parameters for each of the samples. No Taylor cone was observed for cuprammonium cellulose solutions within the 1 to 5-weight percent under the influence of applied voltage. The electrospinning process resulted in a spherical droplet that increased in size with increasing concentration. Ultimately, the droplet was sprayed with minimal extension, which deviates from the typical electrospinning process. The observation of micro- and nanoparticles with a "tailed" morphology was noted upon an elevation in polymer concentration.

Nevertheless, it was observed that the solutions became excessively viscous for electrospinning as the cellulose concentrations were raised beyond 5-weight percent. Specifically, the pumping of solutions to the needle tip proved challenging, and the droplet at the needle tip exhibited minimal deformation despite exposure to a potent electric field. The electrospinning process is widely acknowledged to be influenced by the solution properties, including but not limited to surface tension, viscosity, and net charge density, which are crucial factors in the development of polymer fibers (Schubert 2019). The observed surface tension of the particles emitted through the electrospinning method may be ascribed to the configuration of the polymer chains and the intermolecular forces between these chains. An attempt was made to enhance the spinning efficiency by incorporating PEO, a hydrophilic polymer, into the cellulose cuprammonium solution. PEO was chosen due to its favorable towing characteristics and widespread utilization in electrospinning, attributed to its high viscosity even at low concentrations. Previous studies have reported successful electrospinning outcomes with mixed solutions of PEO and other biodegradable polymers (Chen et al. 2016; Gao et al. 2017; Nikbakht et al. 2020). PEO can facilitate the electrospinning of cellulose due to the cellulose hydroxyl groups and intermolecular interactions among the PEO ether groups. The anticipated outcome of this interaction is the enhancement of PEO-cellulose chain interactions and chain-chain attachment, which keeps stabilized polymer jet during electrospinning.

Table 1

Cellulose and PEO solution combinations and their fiber morphology
Sample ID	Cellulose Source	Cellulose Conc. (%) in Cupram. solution	PEO Molecu–lar weight (kDa)	PEO Conc. (%) in Aqueous solution	Solution Ratio (Cellulose + PEO)	Fiber Dia. (nm)	Fiber Morphology
S1	Cotton linters	2	300	2	1:1	145 ± 13	Fibers with spindle like beads
S2	Cotton linters	3	300	2	1:1	176 ± 24	Fibers with few spindle-like beads
S3	Cotton linters	4	300	2	1:1	195 ± 13	Beads with few fibers
S4	Cotton linters	5	300	2	1:1	230 ± 23	Fibers with few beads
S5	Cotton linters	2	500	2	1:1	130 ± 25	Spindle like Beads with few fibers
S6	Cotton linters	3	500	2	1:1	215 ± 11	Fibers with spindle like beads
S7	Cotton linters	4	500	2	1:1	245 ± 16	Fibers with spindle like beads
S8	Cotton linters	5	500	2	1:1	310 ± 23	Few fibers with beads
S9	Cotton linters	2	1000	1	4:1	330 ± 17	Fibers with few beads
S10	Cotton linters	3	1000	1.5	3:1	270 ± 24	Fiber with beads
S11	Cotton linters	4	1000	1	3:1	--	Electrospraying
S12	Needle wood	2	300	2	1:1	130 ± 23	Fibers with beads
S13	Needle wood	3	300	2	1:1	--	Electrospraying
S14	Needle wood	4	300	2	1:1	--	Electrospraying
S15	Needle wood	5	300	2	1:1	225 ± 23	Fibers with beads
S16	Needle wood	2	500	2	1:1	--	Electrospraying
S17	Needle wood	3	500	2	1:1	--	Electrospraying
S18	Needle wood	4	500	2	1:1	270 ± 34	Fibers with few beads
S19	Needle wood	5	500	2	1:1	--	Electrospraying
S20	Needle wood	2	1000	1	4:1	310 ± 28	Regular nanofibers
S21	Needle wood	3	1000	1.5	3:1	150 ± 34	Fiber with beads
S22	Needle wood	4	1000	1	3:1	350 ± 32	fibers with spindle like beads

Table 2

Electrospinning process parameters
Sample ID	F/Rate (µL/min)	Applied voltage (KV)	Distance (mm)	Temperature (°C)	RH (%)
S1	6.67	23.05	160	32	39
S2	6.67	23.07	160	32	38
S3	5.00	23.11	160	30	42
S4	8.34	24.63	160	29	43
S5	8.34	23.05	160	32	39
S6	8.34	23.05	160	31	39
S7	8.34	23.07	160	30	41
S8	8.34	25.10	160	30	42
S9	8.34	15.81	110	30	30
S10	16.67	27.24	110	20	31
S11	7.50	24.00	100	22	44
S12	5.83	21.78	130	22	44
S13	5.00	23.05	125	24	38
S14	6.67	22.85	125	23	30
S15	2.50	16.00	120	24	42
S16	3.34	20.48	120	22	44
S17	5.00	23.12	130	31	40
S18	5.00	21.11	125	22	36
S19	1.67	19.12	140	25	43
S20	8.34	15.80	110	20	28
S21	11.67	27.22	110	21	39
S22	16.67	22.10	150	27	48

Figure 2 shows the viscosity of both cellulose sources. Viscosity increases with the increased concentration of cotton linters and needle wood in cuprammonium solution. Cotton linters show high viscosity due to a high degree of polymerization (Hallac and Ragauskas 2011; Monshizadeh 2015). The viscosity of a solution exhibits a correlation with the level of entanglement observed among the polymer chain molecules contained within the solution (Hung et al. 2016). An elevation in the viscosity of a polymer is observed as a consequence of an augmented number of polymer molecules, which in turn leads to a rise in polymer chain entanglement (Venkatesh and Lee 2022).

The surface morphology of cotton linters concentrations (ranging from 2 to 5%) with varying molecular weights (300, 500, and 1000 kDa) of PEO (Fig. 3). SEM micrographs of cotton linters nanofibers (Fig. 3b, d and i) shows a continuous nanofiber with mean diameters of 176 ± 24, 230 ± 23 and 330 ± 17nm. Ohkawa (2015) successfully produced nanofibers from cotton and wood-pulp cellulose utilizing TFA as a solvent, which exhibited a reduced diameter range of 30–50 nm. Notably, when the cellulose concentration exceeds 5-weight percent, the electrospinning dispersion is often disrupted by the expulsion of large droplets from the spinneret. The observed phenomenon can be associated with the increased viscosity of the solution and the cellulose flocculation induced in the dispersion of the separation phases (Olsson et al. 2010).

Samples S3 and S8 exhibited cellulose concentrations of 4% and 5% and PEO molecular weights of 300 and 500 kDa, with a limited number of nanofibers present, most of which consisted of large blocks. Despite attempts to decrease the electric field, the creation of said blocks or beads remained inevitable. Noteworthy variations in the structural morphology and diameter of nanofibers were observed upon varying the molecular weight of the PEO. Samples S1 to S4, which contained cotton linters concentrations ranging from 2 to 5-weight percent and PEO with a molecular weight of 300 kDa, exhibited uniform nanofibers. In contrast, samples S5 to S8, which also had cotton linters concentrations in the same range but PEO with a molecular weight of 500 kDa, displayed irregular spindle-shaped beads with some fibers. The molecular weight of PEO notably influenced the spinnability of solutions. The observed variations in the morphology and spinnability of the nanofibers may be associated with alterations in surface tension, solution viscosity, and conductivity. The observed increase in bead size and quantity at higher PEO molecular weight can be attributed to a rise in viscosity and a decrease in the charge density taken by the jet (Deshawar et al. 2020).

From S9 to S11, cotton linters concentrations were 2 to 4% with 1000 kDa PEO having 1 to 1.5% PEO concentration, and the mixing ratio was 4:1 and 3:1; S9 showed continuous fibers, but S10 having fibers and beads and electrospraying behavior showed by S11. The observed phenomenon can be associated with the elevated concentration of cotton linters in S10 and S11. Nie et al. stated that adding a minute quantity of The incorporation of high molecular weight PEO with a concentration of 0.175% w/v and a molecular weight of 1000 kDa has demonstrated enhanced spinnability of a water-based solution of sodium alginate. However, the same effect was not observed with low molecular weight PEO, regardless of whether it was added in an immense amount (35% w/v) (Filip and Peer 2019).

Figure 4 represents diameter distribution curves depicting the size comparison of electrospun fibers from different cellulose percentages. The nanofibers were produced using cotton linters/PEO and varied in molecular weights and concentrations. Table 1 presents the documented diameters of nanofibers. The diameters of cotton linters/PEO with varying concentrations and molecular weights fall within the 100 to 350 nm range. The cotton linters comprising 2 to 5% of PEO 300, with a fiber diameter ranging from 145 ± 13 to 230 ± 23nm. The increase in fiber diameter observed in response to elevated concentrations of cotton linters is attributed to the higher cellulose content and increasing viscosity. Elevated viscosity levels generally result in an augmentation of fiber diameter (Angel et al. 2022; Gulzar et al. 2022). This is achieved by implementing higher polymer concentrations within the solution or utilizing polymers with larger molecular weights. Following the results of Jacobs et al. (2010), the average fiber diameter exhibits a comparable pattern whereby the mean values escalate with the augmentation of PEO concentration. Filip and Peer (2019) found a positive correlation between nanofibers' diameter and molecular weight and concentration.

Figure 5 illustrated the surface morphology of samples ranging from S12 to S22, which were prepared using 2–5% needle wood concentrations and various molecular weights (300, 500, and 1000 kDa) of PEO. SEM micrographs of needle wood nanofibers (Fig. 5a, g and i) showed continuous nanofibers with mean diameters of 130 ± 23, 270 ± 34 and 310 ± 28nm, respectively. The electrospinnability behavior of needle wood cellulose exhibited dissimilarities compared to cotton linters. The majority of compositions exhibited electrospraying as opposed to electrospinning. Regular nanofibers are observed solely in samples S12, S18, and S21. The lower viscosity of needle wood compared to cotton linters is the reason for this observation. Specifically, 2% needle wood's viscosity is equivalent to 2% cotton linters.

The fiber morphological characteristics of S12 and S20 are highly similar to those of S1 and S9 of cotton linters. The polymer molecular weight is a crucial determinant of its chain length, subsequently impacting the solution’s viscosity at a specific concentration. One of the necessary prerequisites for electrospinning, which results in the development of nanofibers, is the presence of a polymer with appropriate molecular weight (Han et al. 2022). A solution is characterized by low viscosity exhibits a correspondingly low viscoelastic force, rendering it insufficient to counterbalance the columbic and electrostatic repulsion forces that act upon the stretching of the jet. This phenomenon results in the partial fragmentation of the jet. The phenomenon of surface tension results in the aggregation of numerous solvent molecules within a solution, leading to the development of spherical beads owing to their cohesive forces. The increase in the concentration of a solution yields a proportional rise in its viscosity, thereby enhancing of its viscoelastic properties. Therefore, the partial fragmentation of the jet is inhibited. The augmentation of polymer chain entanglement as a function of solution concentration facilitates the dispersion of solvent molecules across the interlaced polymer chains, producing smooth fibers and enhancing fiber homogeneity (Mit-uppatham et al. 2004).

Figure 6 displays the diameter curves of needle wood/PEO nanofibers generated from diverse molecular weights at varying concentrations. Table-1 presents the documented diameters of nanofibers. The diameters of needle wood/PEO with varying concentrations and molecular weights fall within 100 to 382nm.

XRD was utilized to analyze the original materials and calculate the structural disparities in the regenerated solution-cast cellulose material (Fig. 7). It is noteworthy that the cotton linters and needle wood used in the current study were utilized in their original form, and the process of preparation was proprietary. The origin of ammonia cellulose can potentially be ascribed to the confidential pre-treatment method, as follows. Initially, the utilization of liquid ammonia for cellulose treatment facilitates the processing of cellulose fibers by reacting with hydroxyl groups, subsequently disrupting hydrogen bonds, thereby accelerating the swelling of cellulose fibers (Mittal et al. 2011; Wada et al. 2006). Additionally, it has been documented that those specific techniques employed in producing needle wood fibers involve a sequence of procedural stages, including mercerization via liquid ammonia and NaOH to separate lignin and other soluble constituents. Likewise, concerning cotton linters, the purification procedure encompasses mercerization via liquid ammonia and NaOH (Saapan et al. 1984).

The data presented in Fig. 7a and c indicates a significant peak at 2θ = 23.04° and 22.9° respectively, which serves as evidence of ammonia cellulose's existence. The phenomenon mentioned above is attributed to the diverse structures that ammonia cellulose can exhibit, which can range from ammonia cellulose I to ammonia cellulose III, owing to the pretreatment conditions (Mittal et al. 2011). According to Abou-Sekkina et al. (1986), the peaks are associated with ammonia cellulose. However, it is noteworthy that the corresponding XRD patterns exhibit a striking resemblance. Figure 7b and d demonstrate that the XRD patterns of cellulose casting films derived from cotton linters and needle wood, respectively, exhibit peaks at 11° and 23° evidence of cellulose (Zhang et al. 2022), while the peaks at 35° and 43° are the evidence of copper (Dong et al. 2016). The observed low signal-to-noise ratio in Fig. 7b and d suggested the lack crystalline structure. This phenomenon can be associated with the crystallization process in cotton linters and needle wood fibers occurring naturally during the stages of plant growth and subsequently maintained through processing over intense stretching. Including cuprammonium salts within the cellulose induces the disruption of the cellulose long-range crystalline arrangement, ultimately leading to the formation of amorphous cellulose. These research groups had previously presented a comparable effect (Bazilevsky et al. 2008; Sinha-Ray et al. 2011).

The XPS spectra of cuprammonium cellulose/PEO nanofibers membrane are given in Fig. 8. In a theoretical context, it has been observed that pure cellulose demonstrates the presence of two distinct peaks of carbon and oxygen in the full XPS spectra (Belgacem et al. 1995; Johansson et al. 2020). The XPS spectra of cellulose/PEO nanofiber displayed the three signals at around 284, 531 eV for carbon and oxygen and 931 eV that associated with the presence of copper atom (Mongioví et al. 2022).

The high-resolution XPS spectrum of C1s of cuprammonium cellulose/PEO nanofibers membrane showed C–O, (C–C/C–H), and C = O groups (Fig. 8b). The carbon atoms and/or hydrogen atoms (C-C/C-H) links to a carbon atom attach only to other arises mainly from cellulose (Johansson and Campbell 2004; Kamdem et al. 1991). The carbon atom is connected to a single non-carbonyl oxygen atom (C-OH), which is primarily derived from cellulose (Johansson et al. 2012; Kamdem et al. 1991). A carbon atom's peak is when it is linked to two non-carbonyl oxygen atoms or only one carbonyl oxygen (C = O, O-C-O) atom (Stark and Matuana 2004). The binding energy associated with the O1s orbital engaged in the -C = O moiety is estimated to be approximately 531.4 (Nzokou and Pascal Kamdem 2005). Figure 8c shows XPS peaks around 931 eV and 951 eV characteristics for Cu 2p_3/2 and Cu 2p_1/2, respectively. Copper had a characteristic 2p_3/2 binding energy at 931 eV, which separated from 2p_1/2 by 20 eV (Jiang et al. 2020; Mao et al. 2017), indicating the presence of Cu²⁺.

The disc diffusion method examined the antibacterial properties of cuprammonium cellulose with PEO membranes. The quantification of the growth of bacteria in the dishes was determined by measuring the area of the inhibition zone (IZA) (Fig. 9).

The diameters of the IZA of S. aureus and E. coli are reported as 10.7 ± 1.14 mm² and 6.47 ± 1.10 mm², respectively. The findings of this study clearly show that nanofiber membranes exhibit exceptional antibacterial properties. There is a possibility that copper ions are released into the surrounding environment, leading to the manifestation of strong bactericidal properties. Copper ions kill bacteria by making a copper-peptide complex, which helps make reactive oxygen species that kill cells (Booshehri et al. 2015).

The present research focused on the electrospinning process of cellulose in cuprammonium solution with concentrations ranging from 2 to 5-weight percent, along with PEO in an aqueous solvent of varying molecular weights (300, 500, and 1000 kDa) at concentrations of 1 to 2-weight percent. The spinning of cellulose cuprammonium solutions in isolation is not feasible. However, introducing aqueous PEO has notably impacted the fibres' spinnability, solution parameters, and resulting morphology. The cellulose source, concentration, PEO molecular weight, PEO concentration, and solution ratio all impacted the fiber morphology and spinnability of the electrospun nanofibers. Electrospinning cellulose-based nanofibers from a cuprammonium solution to make continuous nanofibers exhibiting a range of diameters varying from 130 nm to 382 nm is a possible and valuable method. PEO with a lower molecular weight and a higher molecular weight with a lower concentration in cuprammonium cellulose solution is suitable for smooth nanofiber formation. Fiber diameter increased for both cellulosic sources with the increase in cellulose concentration and PEO molecular weight. The needle wood has a lower viscosity than the cotton linters and relatively poor spinnability. In particular, this study contributes substantially to the fundamental understanding of cellulose-derived nanofibers from cuprammonium solutions produced through electrospinning. Additionally, it offers valuable insights into optimizing process parameters to attain specific fiber properties.

Furthermore, the XPS and XRD analyses revealed that copper exists in the nanofiber membranes as nanoparticles. Both cellulosic materials showed an amorphous nature after dissolution in cuprammonium solution. The findings suggested that the inclusion of copper significantly influenced the antibacterial activity of the membrane. The antibacterial activity of the fiber membrane was also assessed against S. aureus and E. coli. Therefore, the newly synthesized nanofiber membranes have significant potential for water treatment, biomedical research, food packaging, and highly efficient separation materials.

Acknowledgements This research was supported by a key project of State Key Laboratory of Bio-Fibers and Eco-Textiles of Qingdao university and major scientific and technological innovation projects of Shandong province.

Author contributions DI: Conceptualization, Investigation, Methodology, Formal analysis, Data curation, Writing – original draft. RZ: Methodology, Investigation. MIS: Supervision, Writing—review and editing. XN: Supervision, Conceptualization, Project administration, Funding acquisition, Writing – review & editing.

Funding This research was funded by a key project of State Key Laboratory of Bio-Fibers and Eco-Textiles of Qingdao university (RZ2000003348) and major scientific and technological innovation projects of Shandong province (2019JZZY020220).

Data availability and materials All data generated or analyzed during this study are included in this submitted article.

Conflict of interest The authors declare that they have no competing interests as defined by Springer, or other interests that might be perceived to influence the results and/or discussion reported in this paper.

Ethics approval All authors have understood and complied with the code of ethics, approved, and agreed to participate.

Consent for publication The manuscript is approved by all the authors for publication.

Ethical approval and consent to participate According to the guide for authors, I would like to declare on behalf of my co-author that this work described was an original comment that has not been published previously. All the authors listed have approved the manuscript that is enclosed.

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Cellulose-Based Nanofibers Electrospun from Cuprammonium Solutions

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