Introduction

Cellulose is the most abundant polysaccharide in nature and features a linear structure consisting of glucose monomers linked together through β-1,4-glycosidic bonds. As a renewable biopolymer, cellulose is nontoxic, biodegradable, and biocompatible (Rostami et al. 2019). However, cellulose itself does not have any antibacterial properties (Tavakolian et al. 2020) and is susceptible to microbial colonization and growth because of its porous structure and hygroscopicity (Cao et al. 2020), which severely limits its applications as a material, particularly in healthcare and food packaging (Cao et al. 2020).

Among the cellulose derivatives, dialdehyde cellulose (DAC) with unique structure and properties is an interesting material due to its wide application potential. Generally, DAC can be obtained through the oxidation of cellulose with NaIO4. In order to overcome the problem of low oxidation efficiency caused by high-crystallinity and tight structure of cellulose, Zhang et al. (2023) developed a new preparation method of DAC with low energy consumption and chemical consumption, that is, cellulose raw materials were pretreated with LiBr·3H2O to improve the accessibility of cellulose. As a result, the aldehyde group content of DAC was increased by around 30% and the production cost was reduced by 45%. Xu et al. (2023) comparatively studied the preparation methods of DAC using NaIO4 pre-oxidation and synchronous oxidation via choline chloride (ChCl)/urea-based deep eutectic solvent (DES), the results showed that the two methods could obtain different micro-morphology, yield and aldehyde group content, which was an effective preparation strategy of DAC. As a polymeric dialdehyde similar to glutaraldehyde, the DAC was able to combine with proteins and nucleic acids of microbes by crosslinking, which may contribute to its antimicrobial activity (Zhang et al. 2017). The research report showed that the antibacterial activity of DAC against Gram-positive bacteria methicillin-resistant S. aureus (MRSA) (Luo et al. 2021; Mou et al. 2017) and Gram-negative bacteria E. coli (Mayer et al. 2021) increased in vitro with the increase of aldehyde group content, and it had biological safety and good biocompatibility. Unfortunately, DAC had poor antibacterial stability. The antibacterial activity of DAC gradually decreased with the extension of time, and basically disappeared after 7 days (He et al. 2021). Since microbial contamination and the emergence of new bacterial diseases can both significantly impact human health and safety, there is an urgent demand for new materials with robust and long-term antibacterial properties.

Considering that cellulose is a versatile and ubiquitous polymer, the ability to endow it with antibacterial properties would have tremendous implications in many fields that utilize cellulose-based materials (Sun et al. 2020). Endowing cellulose with antibacterial properties requires the loading of antibacterial agents onto cellulose fibers (CFs) through physical adsorption (Zhang et al. 2022; Zmejkoski et al. 2022; Hu et al. 2022) and/or chemical grafting (Dong et al. 2022; Catel-Ferreira et al. 2015; Fan et al. 2022). However, the physical adsorption of antibacterial agents is much weaker than chemical grafting and typically leads to the uncontrolled release of the adsorbed bacteriostatic agents, which will reduce the bacteriostatic performance of the cellulose-based materials over time and increase the risk of contaminating objects in contact with the cellulose-based materials (Fabrega et al. 2011; Poças et al. 2010). Therefore, the chemical bonding of antibacterial agents onto CFs is much more advantageous (Liu et al. 2010), as the antibacterial agents will not be as easily released from the CFs as those that are adsorbed through non-covalent interactions, meaning the material will maintain the antibacterial properties for longer periods of time (Saini et al. 2016a, b). The maintenance of antibacterial properties is highly advantageous, especially for objects with which people often have close contact, such as banknotes, stamps and clothes. In addition, there is significant utility for materials with robust antibacterial properties in food packaging, medical, and sanitary products.

Contact-active antibacterial surfaces have been developed using various molecular strategies, including the use of antibacterial peptides (Weishaupt et al. 2020), cationic polymers (Park et al. 2006; Tiller et al. 2001), quaternary ammonium compounds (Isquith et al. 1972), silanes (Saini et al. 2017; Hassanpour et al. 2018; Saini et al. 2016a, b), metal salts (Ibrahim et al. 2006), and N-halamines (Demir et al. 2015; Wang et al. 2022; Gouda et al. 2008). Schiff base-formation reactions, which are condensation reactions between an aldehyde (or ketone) and an amine to form an imine (Jia et al. 2015), are the most common reactions to chemically graft bacteriostatic agents onto cellulose because of the mild reaction conditions. The preparation of Schiff base materials is based on the integration of two efficient reactions: selective oxidation of cellulose using sodium periodate yielding dialdehyde cellulose and subsequent Schiff base reaction with compounds containing primary amines. Several cellulose-based Schiff base materials prepared by grafting nisin (Wu et al. 2019), L-lysine (Zhang et al. 2020), chitosan (Hou et al. 2008; Han et al. 2009), ε-polylysine (He et al. 2021), and glycine (Xu et al 2020) with dialdehyde cellulose have demonstrated suitable antibacterial properties.

Cinnamic acid, a naturally occurring phenylpropanoid derivative isolated from a variety of different plants and microorganisms, has low toxicity and can be metabolized in organisms (Sova 2012). Its antibacterial, antioxidant, and anticancer properties (Natella et al. 1999) make the compound advantageous for a wide variety of uses in flavors, fragrances, food additives, and medicine (Lafay and Gil-Izquierdo 2007). The bacteriostatic mechanism of cinnamic acid involves the destruction of the bacterial membrane, causing the cells to die (Cai et al. 2019). Cinnamic acid has demonstrated antibacterial activity against E. coli, Salmonella enterica (Whitney et al. 2008), and Alicyclobacillus acidoterrestris (Cai et al. 2015). In addition, cinnamic acid derived from Capsicum annum extracts were found to inhibit Listeria monocytogenes, Bacillus cereus, S. aureus, and Salmonella typhimurium (Dorantes et al. 2000), with a minimum inhibitory concentration (MIC) against S. aureus of 125 μg/mL (Mascotti et al. 2010).

Over time, the antibacterial properties of cinnamic acid physically adsorbed on CFs tend to gradually weaken. Therefore, it is more advantageous to chemically bind cinnamic acid to CFs, which has not been previously reported. In this work, to prevent leaching, cinnamic acid was chemically grafted onto CFs by first oxidizing the CFs into dialdehyde cellulose fibers (DCFs) by sodium periodate, after which the DCFs was modified with 4-aminocinnamic acid by reacting the amine with the aldehyde groups on the DCFs to generate Schiff bases (Scheme 1). The resulting C-DCFs were then characterized by SEM, FTIR, 13C-NMR, XPS, XRD, and TGA. Following structural characterization, the antibacterial activities of the C-DCFs were inspected by the colony counting method. The preparation procedure of C-DCFs was performed in CFs suspension without any other solvent except water as the aqueous medium. Our approach may also offer promising opportunities for the preparation of other cellulose-based antibacterial materials, such as fabrics and textiles.

Scheme 1
scheme 1

Schematic diagram of 4-aminocinnamic acid grafted onto the DCFs

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Materials and methods

Materials

Bleached kraft wood pulp was obtained from Mudanjiang Hengfeng Paper Co., Ltd. (Heilongjiang, China). 4-aminocinnamic acid was obtained from Biosynth Carbosynth Co., Ltd. (United Kingdom). Sodium periodate (NaIO4) was purchased from Tianjin Yongda Chemical Reagent Co., Ltd. The bacterial culture medium was from Beijing Aoboxing Biotechnology Co., Ltd. S. aureus (ATCC25923-3) and E. coli (ATCC25922-3) were purchased from Qingdao Hi-Tech Park Haibo Biotechnology Co., Ltd. (Shandong, China).

Preparation of the DCFs

To prepare the DCFs, the wet bleached kraft wood pulp (20 g dry weight) was added to 750 mL of a 0.12 M NaIO4 solution in deionized water. The pH of the suspension was adjusted to 4.0 with 1.0 mol/L HCl, and the system was stirred at 500 rpm and 50 °C for 4 h (Hou et al. 2008). After the oxidation was completed, the reaction was terminated by adding excess ethylene glycol. The DCFs were filtered by suction filtration, washed with deionized water, and a portion of the sample was dried for analysis and testing.

The aldehyde group’s content of the DCFs was evaluated in accordance with the method reported by Zhang et al (2020). In this method, the aldehyde groups in the DCFs reacted with hydroxylamine hydrochloride, releasing hydrochloric acid that was neutralized with NaOH. Therefore, the volume of NaOH consumed was directly related to the aldehyde group content. The content of aldehyde groups in the DCFs was calculated to be 3.00 ± 0.03 mmol/g.

Preparation of the C-DCFs

Different amounts of 4-aminocinnamic acid were added to 70 mL of deionized water to prepare a series of solutions with different concentrations. The wet DCFs (1.0 g dry weight) were dispersed in 4-aminocinnamic acid solution, and the suspensions were continuously stirred for 2 h at 60 °C. Following, the modified DCFs (C-DCFs) were filtered by suction filtration, repeatedly washed with water to remove the free 4-aminocinnamic acid that was not adsorbed and dried in an oven at 60 °C for 6 h. The C-DCFs prepared with varying molar ratios of aldehydes to 4-aminocinnamic acid of 1:0.7, 1:1.0, and 1:1.3 were referred to herein as C-DCF1, C-DCF2, and C-DCF3, and their yields were 91.58%, 90.22%, and 88.03%, respectively.

SEM characterization

The DCFs and C-DCFs were sprayed with gold before observation under a JSM-7500F scanning electron microscope (JEOL). The micrographs of the DCFs and C-DCFs were observed at magnifications of 200 × and 1000 × , respectively.

FT-IR characterization

A Vector22 infrared spectrometer (Bruker) was used to acquire FTIR spectra of the CFs, DCFs, and C-DCFs over the spectral window of 5000–500 cm–1 (resolution: 1 cm–1, 16 scans). Potassium bromide (KBr) was mixed with ~ 20 mg of each of the dried samples, and the solid mixtures were pressed into tablets before analysis.

13C-NMR characterization

Solid-state 13C-NMR spectra of the C-DCFs were recorded on a Bruker 400 M spectrometer (Switzerland). The samples were spun at 10 kHz, the pre-scan delay was 6.5 μs, the rotor diameter was 4 mm, and the pulse program used for the acquisition was cp. The 13C-NMR spectrum (liquid-phase) of 4-aminocinnamic acid was recorded on a Bruker 500 M spectrometer in deuterated water.

XPS characterization

XPS spectra of the CFs, DCFs, and C-DCFs were acquired on a X-ray photoelectron spectrometer (ESCALAB 250Xi, Thermo Fisher Scientific). Each sample was scanned at 1.0 eV per step and at an analyzer transfer energy of 160 eV.

Calculation of the substitution degree of 4-aminocinnamic acid

The degree of substitution (DS) of 4-aminocinnamic acid was calculated on the basis of the elemental composition of N, as determined by XPS (Sun et al. 2020). Since there are two O atoms, nine C atoms, seven H atoms, and one N atom per molecule of 4-aminocinnamic acid, the C content is nine times that of N, so the formula used to calculate the C content is shown in Eq. (1) below:

$$\% {\text{C}}_{{{4} - {\text{aminocinnamic acid}}}} = \, \% {\text{N }} \times { 9}$$
(1)

Therefore, when the percentage of C atoms in 4-aminocinnamic acid is known, the % C in the anhydroglucose unit (AGU) was calculated using Eq. (2) below:

$$\% {\text{C}}_{{{\text{AGU}}}} = \, \% {\text{C}}_{{{\text{total}}}} - \, \% {\text{C}}_{{{4} - {\text{aminocinnamic acid}}}}$$
(2)

In one AGU, which contains six carbon atoms, there are two O atoms that can participate per oxidation reaction. Thus, the percentage of reactive O atoms per AGU was calculated using Eq. (3) below:

$$\% {\text{O}}_{{{\text{AGU}}}} = \, \% {\text{C}}_{{{\text{AGU}}}} \times { 2}/{6}$$
(3)

Finally, the DS of 4-aminocinnamic acid was calculated using Eq. (4) below:

$${\text{DS }} = \, \% {\text{N}}/\left( {\% {\text{O}}_{{{\text{AGU}}}} } \right)$$
(4)

XRD characterization

XRD spectra of the CFs, DCFs, and C-DCFs were acquired on a D/max2200VPC diffractometer (Rigaku, Japan). The samples were scanned over the range of 0–70° at a scan rate of 5°/min, a scanning step of 0.02° (current: 30 mA, voltage: 40 kV). The data were subtracted from a blank run (air) to remove the environmental background. The crystallinity index (CrI, %) (French 2014) was calculated using Eq. (5) below:

$$CrI\left( \% \right) = \frac{{I_{200} - I_{{{\text{am}}}} }}{{I_{200} }} \times 100$$
(5)

wherein I200 is the intensity of diffraction peak of (200) lattice plane, and Iam is the intensity of the minimum between the 110 and 200 peaks at about 18.5° attributed to amorphous cellulose.

TGA characterization

The CFs, DCFs, and C-DCF samples were measured using a TGA 5500 (TA Instruments, United States). Each sample (10 mg) was placed on a platinum crucible and heated from 50 to 600 °C at a heating rate of 10 °C/min under a pure nitrogen atmosphere with a flow rate of 50 mL/min.

Antibacterial assessment

The colony counting method was performed to quantitate the antibacterial activities of the C-DCFs (Sun et al. 2020). Each experiment was performed in triplicate. A two-step pre-cultivation process was used to prepare the bacterial inoculum suspension. All test bacteria were activated, diluted with a nutrient broth, and transferred to 0.03 mol/L PBS buffer, and the medium was mixed thoroughly and diluted to 105–106 CFU/mL with viable bacteria to inoculate each cellulose sample. Next, the diluted suspensions were sterilized at 121 °C and 103 kPa for 30 min in an autoclave. The samples (0.1 g) were dispersed into a mixture of the inoculum (1 mL) and 0.03 mol/L PBS buffer (25 mL) in a 50 mL triangular flask. The untreated cellulose was used as a blank control. The flasks were covered with bottle stoppers and incubated in a constant temperature shaker at 37 °C while shaking at 140 r/min for 24 h. Then, 1 mL of each test solution was removed from its respective flask, transferred to a test tube containing 9 mL of 0.03 mol/L PBS buffer, and mixed thoroughly. The solutions were serially diluted tenfold, after which 100 μL of the bacterial suspension at each dilution factor was removed from the test tubes to inoculate nutrient agar plates, which were incubated upside down at 37 °C for 18 h. A plate with a dilution factor between 30 and 300 CFU was chosen for counting. In addition, long-term antibacterial activity experiments were conducted, in which the C-DCFs were placed in an air environment. The antimicrobial ratios of the C-DCFs were tested for every 10 days.

The antibacterial ratio was calculated using Eq. (6) below:

$${\text{Antibacterial ratio }}\left( \% \right) \, = {\text{X}} - {\text{Y}}/{\text{X}} \times {1}00\%$$
(6)

wherein X and Y are the average numbers of viable bacteria in the flasks containing the blank sample and Schiff base sample, respectively, after incubating for 18 h.

The bacteria log reduction of the samples was calculated using Eq. (7) below:

$${\text{log reduction}}\, = \,{\text{log CFU T}}_{{{18}}} {\text{blank }}\left( {{\text{control}}} \right){-}{\text{log CFU T}}_{{{18}}} \left( {{\text{sample}}} \right)$$
(7)

In the standard dynamic shake flask method, at least a 1 log reduction in bacteria load is required to claim antibacterial properties (Saini et al. 2016a, b).

Results

Surface topography

Images of the macroscopic and microscopic morphologies of DCFs and C-DCF2 acquired by a digital camera and SEM are shown in Fig. 1. After the loading of 4-aminocinnamic acid onto the DCFs, the color of the DCFs underwent a significant change from milk white to dark, and the dark green color of C-DCF2 did not fade after repeated washing, indicating that the bacteriostatic agent had been chemically grafted onto the DCFs substrate (Fig. 1a, d). As determined by SEM, the morphology of DCFs was obviously bent and twisted, and the fibers were relatively complete and had no obvious fractures, while the morphology of C-DCF2 was relatively straight and had obvious fractures, which might have been attributed to the introduction of 4-aminocinnamic acid that changed the fiber morphology of the DCFs (Xu et al. 2020) (Fig. 1b, e). These differences in morphology were more pronounced when observed at higher magnifications (Fig. 1c, d).

Fig. 1
figure 1

Surface topography of the samples. a and d Photographs of DCFs and C-DCF2, respectively, b and c SEM images of DCFs, and e and f SEM images of C-DCF2

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Analysis of the FTIR

The FTIR spectra of CFs, DCFs, and the C-DCFs are shown in Fig. 2. The spectrum of the CFs featured two absorption bands at 3415 cm–1 (–OH stretching) and 2907 cm–1 (symmetric C–H vibrations) (Keshk and Haija 2011). In addition, the absorption band at 1040 cm–1 was ascribed to C − O stretching vibrations (Liu et al. 2011), and the absorption band at 1640 cm–1 was due to the absorbed water (Sun et al. 2015a, b). In the spectrum of the DCFs, a characteristic band at 1733 cm–1 was observed, which was assigned to C=O stretching vibrations (He et al. 2021), indicating that the aldehyde group had been formed. In the C-DCFs, the characteristic band at 1733 cm–1 disappeared, while new band at 1631 cm–1 appeared, which were derived from the C=N bonds of the imine group formed by the condensation between the aldehydes in the DCFs and 4-aminocinnamic acid (Song et al. 2022; Gadkari et al. 2019). The absorbance band at 1690 cm–1 corresponded to the C=O stretching vibrations of the carboxylic acid group in the 4-aminocinnamic acid, while the absorption bands at 1601 cm–1, 1514 cm–1, and 1428 cm–1 were ascribed to the skeletal vibrations of the benzene ring of cinnamic acid; furthermore, the absorption band at 1264 cm–1 corresponded to C − N bending vibrations (Keshk et al. 2015). Lastly, in the FTIR spectrum of the C-DCFs, the missing characteristic band at 1733 cm–1 and the appearance of the new absorption band at 1631 cm–1 confirmed that the imine bond had formed between the C=O group in the DCFs and the -NH2 group of 4-aminocinnamic acid.

Fig. 2
figure 2

FTIR spectra of the CFs, DCFs, and C-DCFs

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Analysis of the 13C NMR spectra

The liquid-phase 13C NMR spectrum of 4-aminocinnamic acid and the solid-state 13C NMR spectra of the DCF2 and the C-DCF2 are shown in Fig. 3. As shown in Fig. 3a, the C1 resonance of the carboxylic acid group appeared at 169.8 ppm, and the signals at 144.1 and 118.4 ppm were assigned to C2 and C7 of unsaturated double bond, respectively. The resonances at 134.3, 131.4, 129.6, and 123.3 ppm were assigned to C3, C4, C5, and C6 of benzene ring, respectively.

Fig. 3
figure 3

a Liquid-phase 13C-NMR spectrum of 4-aminocinnamic acid, b and c solid-state 13C-NMR spectra of DCF2 and C-DCF2, respectively

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After oxidation by periodate (Fig. 3b), the signals of the DCF2 detected at 105.2, 89.0, 74.9, 72.4, 71.9 and 65.3 ppm were assigned to C1’, C4’, C2’, C3’, C5’ and C6’ of anhydroglucose units (AGU), and the appearing signal at 95.7 ppm originated from hemiacetal structures between two aldehyde groups (Siller et al. 2015). In the solid-state 13C NMR spectrum of C-DCF2 (Fig. 3c), the signals were observed at 170.6, 161.6, 146.9, 128.5, 115.9, 105.0, 89.0, 74.1, 72.1, and 65.7 ppm. The signals at 170.6, 146.9, and 115.9 ppm were ascribed to the C1, C2, and C7 of 4-aminocinnamic acid, respectively, while the signals at 128.5 ppm represented the cluster of resonances of the C3, C4, C5, and C6 of 4-aminocinnamic acid. The signal at 105.0 ppm was ascribed to C1’ of the AGU, the signal at 65.7 ppm represented the C6’ (Davies et al. 2002) of the AGU, the signal peak at 89.0 ppm was associated to C4’ of the AGU, and the cluster of resonances between 70 and 81 ppm were ascribed to the C2’, C3’, and C5’ atoms of the AGU (Maunu et al. 2000). The signal representing the hemiacetal structure disappeared. Lastly, the signal at 161.6 ppm in the C-DCF2 spectrum was assigned to the C=N group (Zhang et al. 2020).

Analysis of the XPS

The XPS spectra of the CFs, DCFs, and C-DCFs are shown in Fig. 4, and the elemental atomic percentages of the materials are provided in Table 1. As shown in Fig. 4a, the XPS spectra of the CFs and DCFs featured characteristic peaks at 531.1 eV and 284.8 eV, indicating that both materials comprised only O and C elements, respectively. In the C-DCFs, a new signal peak appeared at 399.1 eV that corresponded to an N1s peak (Sun et al. 2020). The high-resolution C1s spectrum of the DCFs (Fig. 4b), as well as the high-resolution C1s (Fig. 4c) and N1s (Fig. 4d) spectra of the C-DCFs, provided information on chemical states of the atoms in the materials. The chemical shifts of the carbon (C1s) atoms in the DCFs could be divided into four categories (Gao et al. 2012): O–C=O, C=O (or O–C–O), C–O, and C–C, with peaks centered at 288.6, 287.8, 286.6, and 284.8 eV, respectively. In the high-resolution C1s spectrum of the C-DCFs, two additional peaks appeared at 287.3 and 286.7 eV, which were ascribed to C=N and C–N groups, respectively (Lin et al. 2008). The high-resolution N1s spectrum of the surface of the C-DCFs featured a peak at 398.9 eV, which was attributed to the = N– atom of the Schiff base (Lebrini et al. 2007; Bentiss et al. 1999). The peak corresponding to the N–H group was not observed in the high-resolution N1s spectra, which indicated that there were no free 4-aminocinnamic acid molecules in the modified fibers. As shown in Table 1, as the proportion of 4-aminocinnamic acid in the C-DCFs increased from 0.7:1 to 1.3:1, the atomic percentage of N increased from 4.23 to 6.10%. By calculation, the substitution degree of 4-aminocinnamic acid on the surface of the C-DCFs ranged from 0.33 to 1.12. These results indicated that 4-aminocinnamic acid was successfully grafted onto the DCFs.

Fig. 4
figure 4

a XPS spectra of the CFs, DCFs, and C-DCFs, b high-resolution C1s XPS spectra of the DCFs, c and d high-resolution C1s and N1s XPS spectra of the C-DCF2

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Table 1 Elemental composition of the CFs, DCFs, and C-DCFs determined by XPS
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Analysis of the XRD

The XRD patterns of the CFs, DCFs, and the C-DCFs are shown in Fig. 5. The XRD pattern of the CFs featured four diffraction peaks at 2θ = 14.9°,16.7°, 22.5°, and 34.5° (French 2014). The maximum peak at 22.5° was due to the (200) lattice plane of lattice structure of the cellulose I, and the two overlapping peaks at 14.9° and 16.7° were ascribed to the (1–10) and (110) lattice planes, respectively (French 2014; Park et al. 2010; Sun et al. 2015a, b). The CrI value of the CFs was calculated to be 60.76%. However, after oxidation, the peaks at 2θ = 14.9°,16.7° and 22.5° became much broader, and the CrI value of the DCFs dropped significantly to 47.06%. This was attributed to the oxidation of cellulose by NaIO4, which caused the glucose rings to open, thereby destroying the highly ordered structure of the cellulose molecules (Kim et al. 2000). Compared to the DCFs, the peaks at 2θ = 14.9°, 16.7°, and 22.5° in the XRD pattern of the C-DCFs became much broader, and the intensity become weaker. The CrI values of C-DCF1, C-DCF2, and C-DCF3 were 41.14%, 34.52%, and 31.04%, respectively. Compared to the DCFs, the C-DCFs had relatively low CrI values, indicating that the generation of the Schiff base linker between the DCFs and 4-aminocinnamic acid reduced the crystallinity of DCFs.

Fig. 5
figure 5

XRD patterns of the CFs, DCFs, and C-DCFs

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Thermogravimetric analysis (TGA)

To determine whether the grafting of 4-aminocinnamic acid onto the DCFs changed the thermal stability of the DCFs, the CFs, as well as the DCFs and C-DCFs, underwent TGA. As shown in Fig. 6, the initial mass loss of the CFs, the DCFs, and the C-DCF2 represented 5–10% of the total mass loss, which was ascribed to the loss of absorbed water (Siller et al. 2015). The CFs underwent a noticeable decomposition beginning at ~ 290 °C (Fig. 6a), and its corresponding DTG peak temperature was observed at around 340 °C (Fig. 6b), while the DCFs exhibited a more pronounced decomposition at lower temperature (245 °C) (Fig. 6a), and its corresponding DTG peak temperature was observed at around 300 °C (Fig. 6b). The difference in the temperature at which the CFs and DCFs underwent thermal decomposition might have been due to the highly orderly arrangement of the cellulose chains in the crystal structure of the CFs, which prevented the non-thermoplastic melting of cellulose (Visakh et al. 2010). However, after oxidation, the opening of the glucopyranose rings resulted in a reduction of the orderly packing of cellulose chains, thereby reducing the crystallinity of the DCFs and causing the decomposition temperature of the DCFs to shift to lower temperature. For the C-DCF2, a more significant thermal decomposition occurred at 210 °C (Fig. 6a) was observed. And its DTG peaks were observed at 240 °C and 330 °C (Fig. 6b), this may be caused by the thermal decomposition of cinnamic acid (Zhao et al. 2014) and cellulose with aldehyde group and C=N bond. Therefore, TGA and DTG analysis showed that the introduction of 4-aminocinnamic acid increased the thermal stability of cellulose materials.

Fig. 6
figure 6

TGA and DTG curves of the CFs, DCFs, and C-DCF2

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Antibacterial activity

The antibacterial ratios of the C-DCFs were also quantitatively measured using the colony counting method. As shown in Table 2, after the C-DCFs were exposed to bacterial suspensions of each organism for 18 h, the antibacterial ratios of the DCF2 against E. coli and S. aureus were 69.4% and 71.8%, respectively, and the antibacterial ratios of the C-DCFs against E. coli and S. aureus were greater than 99.0% and 99.6%, respectively. For the two bacteria, the log reduction numbers of the DCF2 were about 0.5, while the log reduction numbers of the C-DCFs were greater than 2.0. Compared with the DCF2, the C-DCFs exhibited better inhibitory effect against two tested bacteria, which is very important for the potential applications. The numbers of colonies grown in the culture dishes are shown in Fig. 7a, b. The numbers of bacterial colonies in the culture dishes containing the suspension treated with C-DCF2 at the same dilution level were significantly lower than in the dishes of the control group, indicating that the C-DCF2 materials more significantly inhibited the growth of the bacteria after contact with the bacterial suspension.

Table 2 Antibacterial ratios and parameters of C-DCFs against S. aureus and E. coli
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Fig. 7
figure 7

Antibacterial effect of the C-DCFs. Photos of the S. aureus control (a1) and C-DCF2 (a2); photos of the E. coli control (b1) and C-DCF2 (b2); long-term antibacterial activity of C-DCF2 (c)

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To evaluate the antibacterial durability of the C-DCFs, we exposed the samples to an air environment under ambient conditions for 2 months and measured the antibacterial activities of each C-DCFs sample at different time periods. As shown in Fig. 7c, the C-DCFs retained more than 99.0% of their antibacterial activity, even after two months, demonstrating their long-lasting antibacterial properties.

In Table 3, the antibacterial ratio was compared between the synthesized C-DCFs and other studies. As shown in Table 3, the antibacterial ratios of C-DCFs were markedly higher than the ratio of other chemically loaded materials; however, some materials prepared using physical impregnation methods were higher than the C-DCFs prepared in this study. The potent antibacterial properties of the C-DCFs might have been due to the combination of the imine groups and grafted cinnamic acid.

Table 3 Comparison of the antimicrobial effect between the modified cellulose materials and other known cellulose derivatives
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Conclusion

We propose a simple and green strategy to convert CFs into cellulose-based antibacterial materials by combining two efficient reactions: oxidation of CFs yielding DCFs as well as Schiff base reaction between DCFs and 4-aminocinnamic acid generating antibacterial CFs. This strategy is highly advantageous for large-scale industrial applications because the synthetic process is highly sustainable. It was verified by FTIR and XPS spectra that CFs was successfully grafted with 4-aminocinnamic acid. The C-DCFs materials demonstrated remarkable and stable antibacterial properties against S. aureus and E. coli, with antibacterial ratios greater than 99.0%. Moreover, the C-DCFs presented long-term antibacterial effectiveness, maintaining 99% antibacterial ratio after two months of exposure to the air environment. The long-term antibacterial performance of C-DCFs is mainly due to the fact that the chemically grafted 4-aminocinnamic acid is non-leaching and kills bacteria by contact. We anticipate that these modified cellulose materials will have extensive applications in the fields of daily contact products, healthcare, and food packaging.