A green, efficient and economical polypeptide—modified bamboo fiber and its application in glycopeptide antibiotics adsorption

Inspired by wet-cool/dry-hot cycle derived prebiotic reactions, a green and efficient dry–wet cycle modification method was proposed for the preparation of polypeptide-modified bamboo fiber (P-MBF). Functionalization of P-MBF was characterized by Fourier transform infrared spectroscopy, element analysis, X-ray photoelectron spectroscopy, X-ray diffraction, and scanning electron microscopy. Compared to raw bamboo fiber materials, P-MBF showed better dispersibility in aqueous solutions, and thus exhibited better adsorption performance for glycopeptide antibiotics in sewage treatment. With vancomycin as the target antibiotic, adsorption performance of P-MBF was studied in aqueous solution. The results showed that the adsorption reached equilibrium after 15 min, and adsorption kinetics and isotherms were well correlated with pseudo-second-order model (R2 = 0.999) and Langmuir model (R2 = 0.996), respectively. Investigation of thermodynamic parameters implied that the adsorption was a spontaneous exothermic process. The adsorption efficiencies in standard solution and drinking water were 97.83% and 91.32%, respectively. This study provides a green, efficient and economical modified bamboo fiber material for the removal of glycopeptide antibiotics from sewage samples.


Introduction
Bamboo fiber (BF) is a cellulose fiber extracted from naturally grown bamboo, and is the fifth most consumed natural fiber after cotton, hemp, wool and silk (Ashori 2008;Jhu et al. 2019). Compared with other natural fibers, BF has the advantages of fast growth and low cost. In addition, excellent mechanical properties and natural antibacterial properties of BF have made it widely used in medicine, textile, building materials and other fields (Aziz and Ansell 2004;Elschner and Heinze 2015;Chen et al. 2018). Generally, BF is composed of cellulose, hemicellulose and lignin, and the highest content in BF is cellulose (Mizera et al. 2016;Yan et al. 2016;Hu et al. 2021). Cellulose is a polysaccharide composed of D-glucose with β-1,4-glycosidic bonds. Due to the aggregated structural characteristics of natural cellulose, the existence of many hydrogen bonds and high crystallinity between molecules, it cannot be dissolved in water and general organic and inorganic solvents, and also lacks thermal plasticity (Mohammed et al. 2015;Pickering and Efendy 2016;Kaur et al. 2018). Additionally, poor chemical resistance and strength are extremely unfavorable to its molding, processing and application (Febrianto et al. 2010;Joubert et al. 2014;Song et al. 2016;Shen et al. 2022). Therefore, it is necessary to modify cellulose with new functional groups to expand its functions and applications (Hughes et al. 2011;Wang et al. 2019a, b;Dampanaboina et al. 2021;Peter 2021). Generally, cellulose has 3 hydroxyl groups per structural unit. Among these hydroxyl groups, C6-OH is the most active and is always used as the main reaction site in the modification process of cellulose (Islam et al. 2023;Abd-Elhamid et al. 2022).
As the basic building blocks of life, amino acids are ideal polymer monomers which could polymerize to peptides by amide reactions (Pyrgiotakis et al. 2018;Zhang et al. 2021). In the modification of cellulose, amino acids were bonded to the surface of cellulose by the esterification reaction. After then, the rest part of the amino acid molecules could extend to peptide chains by further reaction with free amino acid molecules (Barrett 1991;Bier 2003;Bealer et al. 2020). Thus, the peptide modified BF could be obtained. Although various peptides have been successfully used to modify cellulose, current modification methods still meet various drawbacks. For example, traditional cellulose modification methods such as silane coupling agent modification and peroxide modification always involve toxic and harmful reagents, strict reaction conditions, and cumbersome and complicated modification process Wang et al. 2019a, b;Onak et al. 2020).
In 2005, Forsythe et al. proved a reasonable conjecture that dry-wet cycle reactions can drive the polycondensation of amino acid monomers to form polypeptide chains. α-hydroxy acids and α-amino acids were reported to form peptide-oligomers with a combination of ester and amide linkages in wet-cool/ dry-hot cyclically driven prebiotic reaction, and the length of the peptide polymer chain increases with reaction time (Forsythe et al. 2015). As we know, the surface of cellulose is rich in hydroxyl groups. If an ester bond is formed between the carboxyl group in amino acid and surface hydroxyl group of cellulose, the covalent functionalization of cellulose with polypeptide chains could be achieved with the dry-wet cycle reactions. In follow-up studies, Ying et al. found that adding phosphate to the dry-wet cycle reaction system can promote the formation efficiency of peptides in further (Ying et al. 2018a, b). Compared with traditional cellulose modification methods, such dry-wet cycle method is not only simple to operate but also clean, efficient and economical, which is promised to be a green modification method for the preparation of polypeptide-modified bamboo fiber (P-MBF). Moreover, polypeptide modification of cellulose by dry-wet cycle method can improve its dispersibility and biocompatibility to a certain extent, and the presence of polypeptide chains will enable cellulose with the ability for glycopeptide antibiotics adsorption.
In this work, a green, simple and effective microwave dry-wet cycle modification method was proposed to covalently functionalize BF with polypeptide. Peptide contents on the cellulose surface was affected by the pH value of the reaction system. The grafting of polypeptide improves the dispersibility of BF obviously, and exhibited potential adsorption ability to glycopeptide antibiotics. With vancomycin as the target antibiotic, adsorption kinetics, adsorption isotherms and thermodynamics parameters were investigated. Finally, applications of P-MBF in real water samples showed great potential for the removal of glycopeptide antibiotics from sewage.

Materials
Sodium hydroxide (AR), sodium trimetaphosphate (P 3 m) (AR), glycine (AR), L-alanine (AR) and L-serine (AR) were purchased from Aladdin Chemical Co. Ltd (Shanghai, China). Methanol of chromatographically pure were obtained from SIMARK (Germany). BF was provided by the China National Bamboo Research Center, and all the standard chemicals used in this study were of analytical grade.

Polypeptide-modification of BF
The obtained 5 kinds of pretreated BFs were mixed with amino acids solution at a mass ratio of 1:1. The solution was consisted of three amino acids (Gly, L-Ala and L-Ser) with the molar ratio of the three amino acids was 4:3:1. Composition and proportion of the amino acid solution are prepared according to the amino acids composition in the side chain of silk protein (Gu et al. 2019). The pH of the solutions was adjusted to the same as that of the corresponding pretreated BFs. An appropriate amount of P 3 m catalyst was added subsequently. The mixture was put into microwave oven for 4-6 min and medium heat for 4-8 min to achieve a cycle of dry-wet reaction. 10-20 mL deionized water was added to the reaction system before each cycle reaction. After 8 cycles, P-MBF was obtained by centrifugation, precipitation washing (Wash to neutral) and vacuum drying. The specific reaction mechanism is shown in Fig. 1. In this process, five pretreated BFs (P-BF-5, P-BF-7, P-BF-9, P-BF-11 and P-BF-13) were modified at the corresponding pH and named as P-MBF-5, P-MBF-7, P-MBF-9, P-MBF-11 and P-MBF-13, respectively.

Adsorption experiments
The standard solution of vancomycin (pH = 7) was diluted in deionized water to the required concentrations for further experiments (50 mg L −1 for kinetics study, while 5, 10, 15, 20, 40 and 50 mg L −1 for isotherms study and 50 mg L −1 for thermodynamics study). Vancomycin solutions were mixed with a certain amount of pretreated BF (P-BF-9) and polypeptide modified BF (P-MBF-9) at a solid-liquid ratio of 10:1 (g/L), respectively. The mixed solution was then placed in a 4 mL centrifuge tube in an ultrasonic water bath with a power of 360 W and a vibration frequency of 28 kHz for homogeneous adsorption. Adsorption temperature for kinetic and isotherm studies was set at 50 °C and adsorption time varied from 1 to 30 min. For thermodynamic analysis, adsorption temperatures were set at 20 °C, 35 °C and 50 °C, respectively. Residual vancomycin contents after adsorption experiments were detected by high performance liquid chromatography (HPLC) analysis. The adsorption capacity and adsorption efficiency of glycopeptide were calculated according to the following equations: where C 0 (mg L −1 ) and C e (mg L −1 ) are the initial concentration and equilibrium concentration, respectively, V (mL) is the total volume of solution, and W (mg) is the weight of adsorbents.

Fourier transform infrared spectroscopy
Fourier transform infrared (FTIR) spectra of pretreated BF and P-MBFs (P-MBF-5, P-MBF-7, Synthesis mechanism of polypeptide-modified bamboo fiber. The blue wavy lines represent peptides that are produced after many cycles P-MBF-9, P-MBF-11 and P-MBF-13) was recorded by Nicolet 6700 FTIR spectrometer (USA). The weight of the acidic end-substrate from the resolution was 4 cm −1 , and the scans were performed 64 times.

Elemental analysis
Elemental analysis (EA) to evaluate the composition of BFs before and after modification was performed by a Vario EL III elemental analyzer (Germany). CHNS mode: the equal mass sample were burned in pure oxygen and converted into CO 2 , H 2 O and N 2 , which were separated by chromatographic column and tested for thermal conductivity the content of C, H, N in the sample.
X-ray photoelectron spectroscopy X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha, 12 kV, 6 mA) with a step size of 0.1 eV and an A1 Kα radiation were employed to determine the morphology and chemical characterization of pretreated BF and P-MBF-9. The test fiber sample was cut into approximately 5 mm × 5 mm sizes and glued onto conductive adhesives for XPS analysis.

Analysis of crystallinity by X-ray diffraction
The X-ray diffraction (XRD) curves of fiber samples were recorded in reflective mode (D8 ADVANCE, Bruker, Germany, 40 kV, 40 mA) at the scanning angle (2θ) ranging from 5° to 40° with a step size of 0.02° and a Cu Kα radiation source of 1.5406 Å. A blank was run before crystallinity analyses to eliminate the impact of the environmental background and the excess noise has been eliminate with over 2000 counts for the highest peak. The crystallization index (CI) was obtained with Eq. (3): where I 200 is the diffraction intensity of the 200 lattice plane at 2θ = 22.5°, and I am is the diffraction intensity of amorphous fraction at 2θ = 18.5° (Segal et al. 1959).

Characterization
In this work, BFs were pretreated and modified with polypeptide in different pH. To investigate the modification degree in different conditions, the obtained P-MBFs were characterized by FTIR and EA. In addition, physical and chemical properties of P-MBF obtained in the optimized condition were also compared with raw BFs and pretreated BFs by XPS, XRDand SEM. As FTIR results shown in Fig. 2, the characteristic peak of P-MBF appeared in the region of 1650 cm −1 was assigned to the typical amide bonds and amino groups (Ganim and Tokmakoff 2006). Absorption peak at 3418 cm −1 corresponded to the signal of hydroxyl groups. The significant peak enhancement in P-MBF at 1650 and 3418 cm −1 indicated that polypeptides were successfully grafted onto BF.
In addition, elemental analysis was also carried out to verify chemical compositions of BF before and after modification. The results are shown in Table 1. Compared with pretreated BF, the percentage of N, C and H in P-MBFs increased significantly. In particular, obvious increase of N element showed that the proposed method could graft peptides onto the surface of BF successfully. According to the content of N element, P-MBF-9 exhibited the highest graft efficiency, which may be due to the higher catalytic efficiency of P 3 m under weak alkaline conditions of pH 9 (Gan et al. 2022).
Based on the result of EA and FTIR, XPS analysis further confirmed the status of carbon, nitrogen and oxygen in pretreated BF and P-MBF-9 (Fig. 3). In C1s XPS spectra of pretreated BF (Fig. 3a), three peaks at 284.8, 286.3, 287.8 eV were attributed to C1(C-C/C-H), C2(C-O/C-O-H) and C3(O-C-O/ C=O), respectively. Compared with the C1s spectra of the pretreated BF, new fitting peaks in P-MBF appeared at 285.5 eV (Fig. 3b), which was assigned to C-N (Li et al. 2018) in grafted peptides. In N1s spectra, a weak peak at 397.1 eV was observed in pretreated BF (Fig. 3c). Theoretically, N element should not be observed in cellulose, the slight peak may resulted from nitrogen pollution in BF extraction. However, obvious nitrogen signal peaks in 398.6 eV (C-N) and 397.4 eV (C-N-C) (Peng et al. 2020) were observed in the P-MBF (Fig. 3d) sample, which also proved the successful grafting of peptides. In O1s spectra, peaks in pretreated BF were fitted into O1s1 (C-OH…O) and O1s2 (C-OH) (Fig. 3e). As the results in Fig. 4f, peak proportion of O1s2 in P-MBF decreased significantly, indicating the decrease of free hydroxyl groups on the surface of cellulose. The result could be attributed to the esterification between the peptides and the hydroxyl groups on the surface of cellulose. Based on the above discussion, we could assume that the polypeptide has been successfully grafted onto the surface of BF.
As the degree of crystallization is known to affect the tensile properties, adsorption properties, chemical reactivity properties and flexibility of celluloses, the P-MBF-9 was selected for XRD characterization in this experiment, and changes in crystal structure of BF before and after modification were explored.
The characterization results are shown in Fig. 4. It is found that the XRD patterns for the Pretreated BFand P-MBF-9 nanocomposites exhibited almost similarcharacteristic peaks at 2θ = 14.7°, 16.8°, 22.5°, and 34.5° which are assigned to the (1 1 0), (110), (200), and (004) crystal planes of cellulose I, respectively (Jiang and Hsieh 2013;French 2014). Compared with pretreated BF (CrI = 76.4%), the crystallinity of the P-MBF-9 (CrI = 74.7%) decreased to a certain extent, which suggested that the grafting of polypeptides interfered the orientation of macromolecular chains in the crystalline region of BF materials.
Since amino acids have been grafted onto the surface of BFs, morphological structures of pristine and modified BFs are expected to be different. Direct evidence for polypeptide grafting was investigated by SEM. Figure 5 showed the surface morphologies of the raw BF, pretreated BF and P-MBF-9. The raw BFs have a narrow size-distribution with average particle size of 7-10 μm. The surface of the materials was relatively clean and smooth (Fig. 5b). However, after being immersed in alkaline solutions, the pretreated BF is relatively loose, and the surface of the fibers is relatively rough with certain cracks (Fig. 5c,  d). It is speculated that the changes are caused by the rupture of hydrogen bonds in the cellulose. Compared with the pretreated BF, the surface of P-MBF-9 is covered with a layer of flocculent structure, which could be interpreted as the grafting of polypeptide chains (Fig. 5f). Therefore, SEM images further confirmed that the peptides had been successfully grafted to the surface of BFs. Another indication of successful polypeptide grafting was the increase dispersibility of BF, tested by ultrasonicating them in deionized water for 10 min. A sample suspension of 1.0 mg/mL was prepared and maintained for 20 min. Figure 6 showed the dispersion results of raw BF, pretreated BF and P-MBF-9 in water. At the initial dispersion, P-MBF-9 exhibited good dispersibility, while the raw BF and pretreated BF agglomerated and the dispersibilities were far less than that of P-MBF-9. After 20 min, P-MBF-9 were still partially dispersed in water, while raw BF and pretreated BF were precipitated completely. The results indicated that dispersibility of modified BFs in water was significantly improved, and it also proved the successful grafting of peptides on the surface of BFs.

Effect of polypeptides modification
The comparison of vancomycin adsorption efficiency of pretreated BF and P-MBF-9 were investigated by HPLC analysis. Chromatogram of 15 mg L −1 vancomycin standard solution and residual vancomycin after adsorption is shown in Fig. 7. The results showed that the pretreated BF have little adsorption ability for vancomycin, while the adsorption capacity of P-MBF-9 was improved significantly, which suggested that the dry-wet cycle peptide modification significantly improved the adsorption efficiency of BFs. The possible reason is that the dispersibility of the BF is improved, and the surface-modified polypeptide provides more interaction sites for vancomycin adsorption.

Effect of temperature on adsorption
Temperature is considered to be one of the most important factors affecting the adsorption efficiency. Adsorption experiments were carried out by dispersing 20 mg P-MBF-9 adsorbent in vancomycin solutions at three different temperatures (20, 35 and 50 °C). Concentration of residual vancomycin after adsorption were detected by HPLC. As the calculated adsorption efficiency shown in Table 2, adsorption capacity of the P-MBF-9 adsorbent increases slightly with the decrease of temperature, and the adsorption was proved to be an exothermic process.

Adsorption kinetics
The effect of adsorption time was investigated by using P-MBF-9 as adsorbent. Adsorption capacity within 30 min is shown in Fig. 8a. Vancomycin was adsorbed rapidly at the beginning and reached equilibrium within 15 min. In order to study the adsorption mechanism of vancomycin on P-MBF-9, three kinetic models were used to fit the adsorption results, including the intraparticle-diffusion model (Eq. (4)), the pseudo-first-order model (Eq. (5)) and the pseudosecond-order model (Eq. (6)). Intraparticle-diffusion model: Pseudo-first-order model: Pseudo-second-order model: (4) q t = K p t 0.5 + C (5) ln q e − q t = ln q e − K 1 t Fig. 4 XRD analyses of pretreated BF and P-MBF-9. The BF pattern was spaced to avoid overlap with the P-MBF-9 pattern by adding 50,000 counts  BF (a, b), pretreated BF (c, d) and P-MBF-9 (e, f) Evaluation of dispersion of P-MBF in deionized water where q t (mg g −1 ) and q e (mg g −1 ) are the amount of vancomycin adsorbed at t min and equilibrium, respectively. K p (mg g −1 min 0.5 ), K 1 (min −1 ) and K 2 (g (mg min) −1 ) are the rate constants of the intraparticle diffusion model, pseudo-first-order model and pseudo-second-order model, respectively. The parameters and plots of the fitting results were shown in Table 3 and Fig. 8b-d. The correlation coefficients (R 2 ) for intraparticle-diffusion model, the pseudo-first-order model and the second-order model was 0.664, 0.849 and 0.999, respectively. Therefore, the pseudo-second-order model was considered to be the best fitting model for the experimental kinetic data, indicating that chemisorption was the rate-limiting step in the adsorption of glycopeptide by P-MBF-9. According to the surface structure of P-MBF, modified polypeptides can provide more opportunities in adsorption process, and the chemisorption was achieved by electron sharing and electron transfer.

Adsorption isotherms
Adsorption isotherms are often used to describe the relationship between adsorption capacity and solution concentration. Generally, there are two commonly used adsorption isotherm models for adsorption equilibrium isotherm data research. The equations are expressed as follows: Langmuir equation: Freundlich equation: (7) q e = q max bC e 1 + bC e (8) q e = K F C 1 n e Fig. 8 Vancomycin adsorption kinetics by P-MBF-9 (a), intra-particle kinetics model (b), pseudo-first-order kinetics model (c) and pseudo-second-order kinetics model (d) where q max (mg g −1 ) is the maximum adsorption capacity of the adsorbent, b (L mg −1 )is the Langmuir adsorption equilibrium constant. K F (mg g −1 ) and n are the Freundlich constant related to adsorption and interaction between the adsorbate and adsorbent, respectively. The plots of the adsorption isotherms were presented in Fig. 9, and the calculated parameters were presented in Table 4. The Langmuir model had good description of the adsorption process with R 2 > 0.99. Such result showed that the adsorption of vancomycin by P-MBF mainly took place on surface with monolayer adsorption. It is worth mentioning that the maximum adsorption capacity of the adsorbent (q max in Langmuir equation) in 50 °C is 2.353 mg g −1 , which is relatively lower than other reported vancomycin adsorbents (Yuzuriha et al. 2020;Lorenzin et al. 2022). The result may be caused by the high degree of polymerization (DP) of BF materials (DP > 1000) (Yang et al. 2007) and high adsorption temperature. Therefore, adsorption efficiency of the proposed adsorbent willbe improved in our future work by reducing the DP of BFs and optimizing the adsorption conditions.

Thermodynamics parameters
In consideration of the results at different temperatures, vancomycin adsorption process is temperature dependent. To investigate the temperature effect on vancomycin adsorption, the standard Gibbs free energy change (ΔG) was calculated using the following equation: where R is the universal gas constant (8.314 J mol −1 K −1 ) and T is the adsorption temperature (K). KC is thermodynamic equilibrium constant defined as Eq. (10): According to the van't Hoff equation, the average entropy change (ΔS) and standard enthalpy change (ΔH) can also be calculated: The results of thermodynamic parameters are shown in Table 5. ΔG in negative values implied that the adsorption procedure occurred spontaneously. A negative ΔH implies that vancomycin adsorption is an exothermic process, which is consistent with the adsorption results at different temperatures.

Applications in water samples
As antibiotics in water samples are very harmful to humans and animals, purification and improvement of water quality is of great significance. Under the optimized condition, P-MBF-9 was utilized for vancomycin adsorption in 10 mg L −1 vancomycin standard solution and 10 mg L −1 spiked drinking water. Adsorption results were characterized by HPLC (Fig. 10). The adsorption efficiencies in standard solution and spiked drinking water were 97.83% and 91.32%, respectively (Table 6). Compared to standard solution, adsorption efficiencies in drinking water decreased slightly, which could be due to the interference of complex matrix in samples. However, the adsorption sites on the surface of the P-MBF-9 can still ensure the adsorption efficiency of more than 90%.

Conclusion
In this work, an efficient dry-wet cycle modification approach was utilized to achieve the green modification of polypeptides on the surface of BF. Surface modification with polypeptides not only improve the dispersion of BF in water, but also enabled them the ability to adsorb antibiotics in aqueous solutions. The pH of pretreatment and modification was proved to have a great influence on modification efficiency, and the best grafting effect was obtained at pH 9. The synthesized P-MBF-9 was used for vancomycin adsorption and reached adsorption equilibrium within 15 min. Adsorption kinetics and isotherms fitted well with the pseudo-second-order model (R 2 = 0.999) and the Langmuir model (R 2 = 0.996), respectively.. Thermodynamics parameters showed the adsorption process is endothermic and lower temperature was favorable for the adsorption process. Finally, adsorption efficiency of vancomycin in standard solution and actual water samples is more than 90%, indicating that the proposed P-MBF has great application prospects for the adsorption of vancomycin and other glycopeptide antibiotics in aqueous solutions.