1 Introduction

Graphene (G) is an sp2 hybridized two–dimensional single-layer plate of carbon atoms, in which carbon atoms are organized in a hexagonal pattern (Albertsen et al. 1980; Armakovic et al. 2016). Because of the intense interactions between carbon atoms in graphene, it has a very limited water solubility. Graphene oxide (GO) is one of graphene’s derivatives, having different oxygen functional groups on its surface such as hydroxyl (–OH), carbonyl (C = O), and carboxylic (–COO) groups, allowing it to interact with other materials more effectively. These groups enable GO to form covalent connections with a wide range of substrates, particularly polymers like chitosan (Bustos-Ramírez et al. 2013; Han et al. 2011). GO is more biocompatible than G and has a wider range of applications in medicine, which include but not limited to providing very strong and light composites (Vashist and Luong 2015; Kandjou et al. 2019), drug delivery (as a drug carrier) (Ghosh et al. 2010), tissue engineering to develop high-level special scaffolding (Olad et al. 2019), and biosensors (Wang et al. 2011).

2D G or GO sheets are projected to be a potential nanoscale filler for the next generation of nanocomposite materials due to their amazing mechanical properties and affordable costs. Some notable papers have covered the potential applications of GO–polymer nanocomposites, such as organic conductive films and heat-resistant materials (Zheng et al. 2014; Wang et al. 2012). Since GO may be dispersed at the individual sheet level in water, so if water is employed as a common solvent for both GO and the polymer matrix, a fully molecular-level dispersion of GO can be achieved. Furthermore, the epoxy groups in GO have shown preference to react with primary amine groups through addition, which has been widely employed to modify GO. Therefore, in addition to the hydrogen bonding between them, a new mixture of chitosan and GO can be formed by a specific interaction (Selatile et al. 2021). GO has been commonly used in biomedical field, such as polymeric-GO composites for tissue engineering and regeneration applications (Khan et al. 2022; Al-Arjan et al. 2022; Khan et al. 2021a, b), and for drug delivery (Khan et al. 2021b, c; Al-Arjan et al. 2020). GO has also proven to be a very promising and efficient material for supercapacitors applications (Tamang et al. 2023). To enhance the capacitance of reduced GO (rGO) for supercapacitor applications, it was coupled to amino hydroquinone dimethylether (Luo et al. 2023). GO was also introduced to enhance the performance of nickel oxide supercapacitor electrodes (BinSabt et al. 2023). rGO was also utilized to synthesize a novel high–performance charge storage Al2O3-rGO hybrid electrode (Ratha et al. 2023). A GO–Thioamide hybrid composite was developed as a high–performance supercapacitor electrode (Czepa et al. 2023). A GO/Co3O4 nanocomposite was also synthesized and presented as a highly efficient material for supercapacitor application (Veeresh et al. 2022).

Chitosan is a biodegradable natural biopolymer (Yang et al. 2007) that has emerged as a popular choice among scientists for research and development, as well as for environmental protection through the production of biodegradable products (Singh et al. 2013; Islam et al. 2013, 2012). Green biopolymer-based composites have a variety of important advantages, including being biodegradable, renewable, environmentally friendly, and long-lasting.

As a green biopolymer, chitosan is widely used in membranes, tissue engineering, drug delivery, wound dressing, and packaging materials owing to its high biocompatibility, antibacterial properties, biodegradability, and non-toxicity (Bansal et al. 2011; Antony et al. 2019; Ueno 2001; Morin-Crini et al. 2019). It is derived from chitin, the second most abundant natural biopolymer on earth after cellulose, and it has gained a lot of attention because of its low cost, low immunogenicity, and the fact that it is a natural green adsorbent with hydroxyl, amino, and carbonyl groups (Li et al. 2020). This cationic amino polysaccharide has multiple reactive sites for grafting, ionic contacts, insoluble ionic complexes with a variety of water-soluble anionic polymers, and metal ion adsorption (Saheed et al. 2021). Furthermore, chitosan is a stimulus-responsive polymer whose solubility may be adjusted by altering the pH value (Antony et al. 2019). The molecular weight and degree of deacetylation (DDA) of chitosan influence its unique features. Re-acetylation can diminish DDA, while acidic de-polymerization can reduce molecular weight (Desai et al. 2021). To enhance the mechanical properties of chitosan, different approaches such as nanoparticles inclusion, crosslinking, graft copolymerization, complexation, chemical changes, and mixing are used (de Oliveira et al. 2021; Shoueir et al. 2021). Using nano-fillers such as carbon nanotubes, clays, and other materials has already proven to be a viable solution to different problems (Sanusi et al. 2020). Carbon nanotubes are the best reinforcing agents, but their industrial use is limited due to their costly production methods and poor dispersibility (Mohd Nurazzi et al. 2021). Similar to GO, chitosan has also been one of the frequently used materials in the field of supercapacitors. Supercapacitor electrodes were fabricated from activated carbon derived from chitosan biomass (Abu et al. 2023). Chitosan was also used as a binder in the fabrication of polyaniline/multi-walled carbon nanotubes supercapacitors (Yesilyurt et al. 2023). Films of carboxylated chitosan-graft-poly (Vinyl-2-Pyrrolidone) were also synthesized, and electrochemical investigation revealed their potential as electrodes in supercapacitor devices (Zaghlool et al. 2023). Carboxylated chitosan matrix was also used with polyacrylamide and glycerol to fabricate a polymeric hydrogel electrolyte for solid-state supercapacitor application (Wang et al. 2023).

In this study, we present a straightforward and environmentally friendly method for manufacturing chitosan/GO composite films (Han et al. 2011) that involves integrating GO into the chitosan matrix with diluted acetic acid as the processing solvent. Density functional theory (DFT) computations have recently been widely employed to investigate the electrical and chemical characteristics of G, GO, and functionalized G (Hernández Rosas et al. 2011; Anota et al. 2013a). Furthermore, DFT simulations were utilized to investigate chitosan and functionalized chitosan (Juárez et al. 2013; Anota et al. 2013b; Rodríguez-Juárez et al. 2015) using several models, such as the B3LYP/6-31g(d) model (Juarez-Morales et al. 2017).

The aim of this study is to obtain computed vibrational spectra comparable to experimental ones for chitosan in order to identify the most suitable basis set. To achieve this goal, computations were performed using Hartree–Fock methods (HF) and B3LYP method of DFT (DFT:B3LYP) using a variety of basis sets, including 3-21g, 6-31g, 6-311g, LANL2DZ, LANL2MB and 3-21g**. The most suitable basis set will then be utilized to determine the best site for functionalization of chitosan’s chain (center or terminal). Finally, several functional groups such as OH, NH2, COOH, CH3, CHO, CN, SH and GO are introduced and examined for the functionalization of chitosan through the optimum site of interaction in order to investigate the impact of functionalization on the electronic properties of chitosan.

2 Material and methods

2.1 Materials

Chitosan (low molecular weight; M.W. 120 kDa) was purchased from ABCO Laboratories Eng. Ltd (Gillingham, England). GO was synthesized using graphite powder (particle size < 20 µm) from Fluke, Germany, and solvents such as H2SO4 (98%), H2O2 (30%), and HCl (33%) were all acquired from El-Nasr Pharmaceutical Company, Egypt. KMnO4 98% was acquired from Alfa Aesar, Germany.

2.2 Synthesis of GO

The Hummers method (Hummers 1958) was used to synthesize GO. 1 g of graphite was mixed with 35 ml of H2SO4 and 3 g of KMnO4, and then stirred for 1 h in an ice bath at temperatures 0–4 °C. 105 ml of H2O2 were carefully mixed with the solution for 1 h, and the mixture was heated up to approximately 100 °C. This mixture was then diluted with 280 ml of distilled water. Precipitated GO was washed with 2 M HCl, then with deionized water and dried.

2.3 Preparation of chitosan and chitosan-GO

For the preparation of chitosan and chitosan-GO films, 0.25 g of chitosan was dissolved in 100 ml of distilled water containing 2%v/v acetic acid with continuous stirring for 2 h at 70 °C until chitosan was completely dissolved. The solution was then doped with GO (20 wt%) and stirred again for approximately 2 h at 70 °C until a homogeneous solution was obtained. The solution was then cast in glass petri dishes and dried at room temperature for 5 days.

2.4 Fourier transform infrared spectroscopy

Attenuated total reflection (ATR) FTIR spectra were obtained using Vertex 70 FTIR spectrometer from Bruker Optik GmbH, Germany, equipped with diamond ATR crystal system in the spectral range of 4000–400 cm–1 with the resolution of 4 cm−1.

2.5 Calculation details

Energy optimization and vibrational calculations for the studied structures were conducted using both HF and DFT:B3LYP (Lieb and Simon 1977; Becke 1993; Lee et al. 1988; Vosko et al. 1980) methods, using various basis sets including 3-21g, 6-31g, 6-311g, LANL2DZ and LANL2MB. Calculations were performed with Gaussian 09 software (Frisch et al. 2010) at Molecular Spectroscopy and Modeling Unit, National Research Centre, Cairo, Egypt. In addition to vibrational spectra, the physical parameters of total dipole moment (TDM), HOMO–LUMO band gap energy (∆E), and molecular electrostatic potential (MESP) were also calculated.

3 Result and discussion

3.1 ATR-FTIR characterization of chitosan and chitosan–GO

The FTIR spectrum of chitosan is demonstrated in Fig. 1 and its characteristic FTIR bands and their assigment are demonstrated in Table 1 (Brudzyńska et al. 2023; Sulej et al. 2023; Ibrahim et al. 2011; Anandhavelu and Thambidurai 2011). The band at 3410 cm–1 is attributed to O–H coupled with N–H stretching vibrations. The band at 2920 cm–1 is assigned for C–H stretching. Carbonyl C = O (amide I) is centered at 1655 cm–1, while C–N stretching and N–H bending vibration (amide II) is centered at 1546 cm–1. The bands at 1420 and 1380 cm–1 are corresponding to CH2 and CH3 bending vibrations, respectively. C–O stretching vibrations are located at ⁓1150–1035 cm–1. Finally, the band at 895 cm–1 is ascribed to the C–N fingerprint band.

Fig. 1
figure 1

ATR-FTIR spectra of chitosan and chitosan–GO

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Table 1 Band assignment of FTIR result of Chitosan and Chitosan–GO
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Figure 1 and Table 1 demonstrate the characteristic FTIR bands of Chitosan–GO film. A broad stretching absorption band at 3430 cm–1 is attributed to OH groups of chitosan and GO. A new band appeared at 1620 cm–1 which is assigned to the carboxyl (COOH) group of GO, which appeared at a lower wavenumber as a result of hydrogen bonding between the NH2 group of chitosan and COOH group of GO. Finally, the band representing NH bending shifted to a higher wavenumber at 1557 cm–1, which reflects the interaction between chitosan and GO via the NH2 group (Samuel et al. 2019). Results confirmed the proper formation of Chitosan–GO composite. In a previous study (Ezzat et al. 2023), we have characterized the FTIR spectrum of pristine GO.

3.2 Theoretical IR result for chitosan

Three units of chitosan were initially optimized, and their vibrational spectra were calculated using HF and DFT:B3LYP at various basis sets as shown in Tables 2 and 3, respectively. The calculated vibrational spectra were compared with the experimental spectrum. The theoretical IR findings from several theories and basis sets were analyzed to identify the ideal basis set providing the best results in good agreement with the experimental data. The characteristic IR frequencies, which provide fingerprint information, have played a very important role in many research disciplines in chemistry. Therefore, the theoretical IR frequencies of chitosan calculated at different methods and basis sets, and corrected with an ideal scaling factor were evaluated by comparing them with the experimental results. Experimental and theoretical IR results presented in Tables 1 and 2 indicated that the spectrum calculated using DFT:B3LYP/3-21gmodel had the closest values to the experimental data. Consequently, and for higher accuracy, IR frequencies utilizing 3-21g** basis set were calculated to be compared with those obtained using 3-21g basis set. Several vibrational modes resulting from DFT: B3LYP/3-21g** calculations appeared to be identical to the experimental data. This result suggested that DFT:B3LYP/3-21g** can be used to investigate chitosan functionalized by various functional groups and GO to obtain theoretical results in closer agreement with the experimental ones. It must, however, be mentioned that B3LYP/3-21g** has some limitations such as lager bond and complexations energies than experiment for loosely bound systems, in addition to poor prediction of ionization potentials; however, despite these limitations, the B3LYP/3-21g** is quite sufficient for small molecular systems, such as chitosan, and can successfully predict the geometry and electronic structure of such systems (Riley et al. 2007; Zandler and D’Souza 2006).

Table 2 Computed IR (Scaled) approximated frequencies of chitosan using HF/3-21g, 6-31g, 6-311g, LANL2DZ and LANL2MB  models compared with experimental result
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Table 3 Computed IR (Scaled) approximated frequencies of Chitosan using DFT:B3LYP/3-21 g, 6-31 g, 6-311 g, LANL2DZ, LANL2MB and 3-21 g** models compared with experimental result
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It is, therefore, worth mentioning that functionlaized chitosan was also subjected to vibrational frequency caculations. Figure 2 presents some examples of the DFT:B3LYP/3-21g** calculated IR spectra for functionalized chitosan. The calculated IR spectra demonstrated positive frequencies which is an indication that the calculated frequencies are corresponding to energy minimum, which is a good indicator for the validity of the studied structures at this level of theory.

Fig. 2
figure 2figure 2

DFT:B3LYP/3-21g** calculted IR Spectra for functionalized Chitosan for (a) Chitosan-OH; (b) Chitosan-NH2; (c) Chitosan-COOH; (d) Chitosan-CH3; (e) Chitosan-CHO; and (f) Chitosan-CN

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3.3 Binding energy

Based on the theoretical IR results, the DFT:B3LYP/3-21g** was selected as the best model to study functionalized chitosan and to identify the optimal site for interaction (center or terminal). As shown in Fig. 3, chitosan was proposed to be functionalized with OH group once through a central unit and once through a terminal unit. Since chitosan’s functionalization takes place through the active site of NH2 group, the model hypothesized a complex interaction between the OH group and the active site of chitosan. The binding energy of the two proposed interactions was obtained in order to evaluate the most probable place of connection.

Fig. 3
figure 3

Optimized structure of chitosan functionalized with OH group through (a) Center and (b) Terminal

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The binding energies of chitosan functionalized with OH group through the two proposed site of interactions were calculated twice. The calculated total energy and binding energies are presented in Table 4. From the obtained data, Chitosan-OH-Center was found to have a larger positive binding energy value than that of Chitosan-OH-Terminal. Consequently, Chitosan-OH-Center had the lowest negative binding energy and is then the most suitable site for chitosan’s functionalization (Tavakol 2017).

Table 4 Calculated approximated total energy for chitosan and chitosan interacted with OH group through center and terminal
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3.4 Total dipole moment, HOMO–LUMO band gap and molecular electrostatic potential of functionalized chitosan

As shown in Fig. 4, OH, NH2, COOH, CH3, CHO, CN, SH and GO as functional groups were used to functionalize chitosan via the perfect site of chitosan’s functionalization identified by binding energy calculations. TDM, ∆E, and MESP were calculated for chitosan models with the specified functional groups and GO to evaluate the influence of functionalization on chitosan’s reactivity. Increased TDM together with decreased ∆E is considered a significant contributor demonstrating the enhancement of electronic properties and structural stability (Al-Fifi et al. 2014; Al-Bagawi et al. 2020). Table 5 demonstrates the variance in calculated ∆E and TDM values of all proposed models. TDM of functionalized chitosan increased from 4.071 Debye to 5.724, 5.245, 6.736, 5.207, 6.957, 8.701, 6.957 and 50.836 Debye for chitosan functionalized with OH, NH2, COOH, CH3, CHO, CN, SH and GO, respectively. On the other hand, ∆E of all functionalized chitosan structures decreased from 7.919 eV to 6.860, 7.183, 7.003, 5.207, 6.571, 7.571, 6.571 and 0.3023 eV for chitosan functionalized with OH, NH2, COOH, CH3, CHO, CN, SH and GO, respectively. These results clearly indicate that Chitosan-GO have the highest reactivity, owing to its highest TDM and lowest ∆E values, which also dedicate it to be the most stable structure.

Fig. 4
figure 4figure 4figure 4

Optimized structures and MESP of chitosan interacted with different functional groups through center as (a) Chitosan; (b) Chitosan-OH; (c) Chitosan-NH2; (d) Chitosan-COOH; (e) Chitosan-CH3; (f) Chitosan-CHO; (g) Chitosan-CN; (h) Chitosan-SH; and (i) Chitosan-GO calculated using DFT:B3LYP/3-21 g** model

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Table 5 DFT:B3LYP/3-21g** calculations to computed TDM (Debye) and ΔE (eV) for chitosan and functionalized chitosan interacted through terminal
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MESP is also an important descriptor for studying the reactivity of chemical interactions by mapping the electrostatic potential of a given structure based on the electron density formed by the nuclei and electrons in that structure (Ourhzif et al. 2021; Donglai et al. 2007). The MESP values are displayed as a color map on the molecule’s surface, with the colors arrayed from highest electron density area (red color) to lowest electron density one (blue color) (Ourhzif et al. 2021; Donglai et al. 2007; Mol et al. 2022). MESP colored maps of chitosan and functionalized chitosan with the different groups are illustrated in Fig. 4. The resulting maps confirmed that the NH2 group of chitosan is the most reactive site of chitosan molecule. The intensity of the red-colored areas on the polymer chain increased when chitosan was functionalized with different functional groups. All functional groups exerted a physical characteristic change on chitosan’s electronic properties and reactivity, especially Chitosan–GO which demonstrated significant improvement of the electronic properties and can, in turn, be used in electronic applications such as the development of electrodes for supercapacitors.

4 Conclusion

Experimental FTIR and molecular modeling calculations with different levels of theory and basis sets were conducted for chitosan and chitosan-GO composite. Experimental FTIR spectra of chitosan and chitosan-GO film verified the formation of chitosan-GO composite, and confirmed that the interaction is taking place through NH2 group of chitosan. The calculation of IR spectrum at both HF and DFT levels with different basis sets revealed that DFT:B3LYP/3-21g** is the best model, yielding several vibrational modes identical to the experimental FTIR data. Binding energy calculations for chitosan confirmed that the most probable site for chitosan’s interaction is through center units. DFT:B3LYP/3-21g** model was used to study the effect of the functionalization of chitosan by OH, NH2, COOH, CH3, CHO, CN, SH and GO functional groups, and the results confirmed that Chitosan-GO is the most reactive and stable structure based on the obtained TDM, ΔE and MESP values. The significant enhancement in the electronic properties of Chitosan-GO composite can promote it for potential electronic applications such as supercapacitor electrodes. Finally, it can also be concluded that molecular modeling is a suitable and very useful technique in studying both electronic and vibrational properties of raw and functionalized biopolymers such as chitosan.