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. 2026 Oct 21. Online ahead of print. doi: 10.1039/d6ra05799c

Functionalization of graphene oxide with cinnamic acid and lignin for enhanced pesticides adsorption for water purification

Sibel Hasanova a,b, Alakbar Huseynzada c,✉, Alibala Aliyev c, Ulviyya Hasanova d, Imamaddin Amiraslanov e,g, Carole Gardiennet f, Gunel Aliyeva g, Ilaha Hasanova h, Famil Chiraqov a, Eldar Gasimov i, Fuad Rzayev j, Zarema Gakhramanova b, Haji Vahid Akhundzada c,k,l,m
PMCID: PMC13686148  PMID: 42910331

Abstract

Pest control in modern agronomy leads to systematic pesticide usage that influences the water ecosystem and can lead to mutagenic and chronic diseases of diversity, emphasizing the growing demand for the creation of efficient adsorbents with the possibility of regeneration. That's why we herein report the application of the obtained graphene oxide-cinnamic acid and graphene oxide-lignin hybrid systems for water purification from pesticides. For this purpose, biodegradable polymer lignin (by-product of the pulp industry) and its phenolic structural precursor – cinnamic acid have been functionalized with graphene oxide nanolayers for improved adsorption properties. Modification of graphene oxide with lignin and cinnamic acid was performed through covalent functionalization using epichlorohydrin and acyl chloride moieties as crosslinking units, respectively. The structural changes and morphology of the synthesized hybrid nanocomposite systems were investigated by XRD, TEM, FTIR, ssNMR and XPS. BET surface area and pore size analyses were also conducted. Formation of several topologies in the obtained molecules was observed by TEM, whereas XRD determined restructuring of both modifications, likewise the introduction of the guest molecule in the host system. The highest removal efficiencies were achieved at pH 6, and performance remained close to optimal under near-neutral conditions. Regeneration studies showed that the adsorbents retained more than 90% of their initial removal efficiency after three adsorption–desorption cycles. Further, adsorption analyses on the mentioned pesticides were performed by the batch method, which revealed promising results in the case of both hybrid materials, with good performance in the case of the graphene oxide-lignin hybrid system.

Pest control in modern agronomy leads to systematic pesticide usage that influences the water ecosystem and can lead to mutagenic and chronic diseases, emphasizing the growing demand for the creation of efficient adsorbents. Generated with Google Gemini.graphic file with name d6ra05799c-ga.webp

1. Introduction

In modern agronomy, the use of pesticides and toxic chemicals for plant and crop protection represents an effective strategy against insects and pests, which triggers pollution in the water system. Effective chemicals tend to accumulate in soil and groundwater, changing their structure, having a toxic effect on biodiversity and damaging the ecosystem.1–3 In particular, neonicotinoid insecticides are highly soluble in water, penetrate into soil or tree tissue, and meanwhile reach non-target biota.4 Chemical agents can affect human health in the short term, while prolonged contact exposure may lead to carcinogenic risk and even have an influence on the brain and endocrine system.5

There are various methods of water purification, among which adsorption is the most promising one. It combines efficiency, environmental friendliness, and safety of the process by removing needless contamination.6,7 An effective sorbent should be multifunctional and satisfy several requirements, including simplicity, economic feasibility, regeneration capability, recyclability, and adsorption capacity. In addition, high specific surface area, porous structure, and mechanical stability also contribute to the efficiency in performance of the chosen material.8

Natural biodegradable materials such as chitosan, lignin, and cellulose are attracting much attention due to their high sorption capacity and good mechanical and regenerative properties. Due to the fact that polysaccharides lack phenolic structures, they demonstrate less effective sorption of aromatic organic pollutants in contrast to lignin.9,10 Despite its abundance and unique aromatic structure, lignin is still largely underused. Although lignin is produced on a large scale as a byproduct of the pulp and bio-processing industries, only about 2–5% of it is used for commercial value-added purposes, while the bulk is burned for energy recovery.11,12 This limited use has stimulated a growing interest in developing new approaches to enhance the value of lignin by creating functional materials. It should also be mentioned that the polymeric structure of lignin enforces the limits of sorption centers caused by the structural heterogeneity of the biopolymer.13 Therefore, the use of its structural precursor as a functionally flexible adsorbent allows studying the role of the phenolic unit in adsorption and comparing it with lignin. It is also known that cinnamic acid derivatives contribute to the formation of the lignin polymer as precursors of monolignols and participate in the lignification process. Furthermore, cinnamic acid and its derivatives impart reactivity in lignin, being embedded in a polymer network.14,15

Along with it, one of the popular materials that can be used for the preparation of adsorbents is graphene oxide (GO).16 Its building blocks, supplied with active sites, provide nanomaterials with unique optical, electrical, mechanical, and energy storage properties. Its carbon lattice is rich with numerous oxygen-containing groups (epoxy, carboxyl, and hydroxyl), which make it a suitable material for further functionalization.17–19 Because of its significant chemical and physical behavior, graphene oxide has shown enhanced results in pesticide adsorption applications.16 However, innovative research is no longer focused so much on the usage of pure GO, as on the creation of its based nanomaterials with improved properties as potential future applicants for water remediation. To achieve selective removal and acquire enhanced features, GO is frequently modified through different functionalization approaches. These modifications provide nanomaterials with a developed structure and multifunctional properties.20 Currently, synthesised GO magnetic composites, GO/MOF hybrids, GO biochar, GO polymer composites, as well as biopolymer materials are actively being investigated.21 Such modifications make it possible to increase the number of active sites, reduce the aggregation of graphene oxide sheets, and improve the removal efficiency.

Considering the above-mentioned, the performed study presents covalently attached graphene oxide-lignin and graphene oxide-cinnamic acid nanocomposites as promising adsorbent materials. We specifically selected two chemically different aromatic modifiers – the complex natural polymer lignin and the low molecular weight cinnamic acid – and compared how their chemical nature changes GO and its ability to remove different pesticides. Cinnamic acid was also chosen as an additional functionalizing agent to lignin because its specific low molecular weight and aromatic structure provide a more controlled source of π-conjugated and carboxylic functional compounds. Unlike structurally heterogeneous lignin, cinnamic acid promotes the functionalization of individual aromatic components and allows us to assess its effect on the adsorption of pesticides that differ in structure. It should also be mentioned that commercially available alkaline lignin (by-product of the pulp industry) was intentionally used in this research, rather than purified lignin, to promote the increasing value of this underutilized resource derived from biomass.

The conjugation of graphene oxide to lignin and cinnamic acid has been achieved using epichlorohydrin and acyl chloride approaches. The introduction of phenolic polymer and its structural precursor into the GO matrix was confirmed by solid-state NMR (ssNMR) and XPS, while structural and topological changes were characterized using XRD, TEM, and FTIR analyses. BET surface area and pore size analyses were also conducted. The adsorption of various pesticides was evaluated using batch experiments and appropriately described by isotherms and kinetic models, which revealed promising adsorption potency in the case of the lignin-graphene oxide nanocomposite.

2. Experimental

2.1. Materials and methods

2.1.1. General information

Purchased from a commercial supplier, all solvents and chemicals used in the experiment were of analytical grade and utilized without additional purification.

2.1.2. Transmission electron microscopy (TEM) analysis

TEM was used to analyse the morphological features of samples with a current of 80–120 kV using a JEOL device (Japan). The GO sample was prepared by dispersing it in water and ultrasonicated for 15 minutes to obtain a homogeneous suspension. A few microliters of the obtained suspension were taken and dropped onto a carbon-coated copper mesh (200 mesh) and allowed to dry at room temperature.

Ethanol was used as the solvent for GO@cinnamic acid sample preparation; the sample was sonicated using an ice bath for 15 minutes, and the resulting suspension was dropped onto a carbon-coated copper mesh (200 mesh) and allowed to dry at room temperature.

Lignin was prepared according to the method described in the literature.22 The GO@lignin sample was prepared analogously to the GO@cinnamic acid sample.

2.1.3. Powder X-ray diffraction (PXRD) analysis

All measurements were performed using a Bruker D2 Phaser diffractometer at room temperature. A copper X-ray source (Cu Kα radiation, λ = 1.5406 Å) operating at a voltage of 30 kV and a current of 10 mA was used. Diffraction patterns were recorded in the range from 5° to 80°.

2.1.4. Solid-state NMR experiments

Magic angle spinning solid-state NMR spectra were collected on a Bruker AVANCE III spectrometer operating at 300 MHz 1H resonance frequency (7.05 T) equipped with a 4 mm HXY triple-resonance probe, operating in double-resonance mode. 13C chemical shifts were calibrated using adamantane as an external ref. 23. Quantitative 13C single pulse excitation experiments were performed with a 4 µs 13C pulse and 30 s recycle delay to ensure reliable quantitation. 80 kHz SPINAL-64 1H decoupling was applied during a 20 ms acquisition, for a total measurement time of 13 to 16 h, corresponding to the accumulation of 1600 to 1920 transients.24

2.1.5. Sonication

All the synthesized samples containing GO have been sonicated (ultrasound treatment) using a Sonics Vibra Cell sonicator at the following parameters: pulse on – 30 seconds, pulse off – 2 seconds; power – 40%.

2.1.6. Fourier transform infrared (FTIR) analysis

FTIR spectra were carried out using a Bruker Alpha FTIR spectrometer equipped with a full attenuation reflection (ATR) probe on a zinc selenide crystal at room temperature in the wavenumber range from 400 to 4000 cm−1.

2.1.7. Adsorption experiments and UV-Vis quantification

The adsorption ability of the pristine GO and its functionalized derivatives was investigated using batch adsorption experiments, while the residual concentration of pesticides was determined using a Varian Cary 50 UV-Vis spectrophotometer.

Stock solutions of pesticides (trifluralin, acetamiprid, carbendazim, clothianidin and thiamethoxam) were prepared by dissolving 100 mg of each compound in 100 mL of water (trifluralin 1 : 1 ethanol–water system due to poor water solubility) (c = 1000 mg L−1). The working concentration range was between 1–60 mg L−1, depending on the pesticide.

To evaluate the adsorption capacity, 25 mg of adsorbent was added to 50 mL of pesticide solution in a 100 mL conical flask with a solid–liquid ratio of 0.5 g L−1. To obtain uniform distribution, the suspension was ultrasonicated for 10 minutes and stirred at 400 rpm at ambient temperature (25 ± 1 °C) under natural pH conditions.

For kinetic investigation, the initial concentration of pesticides was fixed (25 mg L−1 for trifluarin, 20 mg L−1 for carbendazim, and 10 mg L−1 for the remaining compounds), and aliquots were withdrawn at predetermined time intervals ranging from 1 to 360 minutes. The obtained samples were centrifuged at 4000 rpm for 10 minutes to separate the adsorbent, and the supernatant was collected.

Equilibrium adsorption studies were done at varying initial concentrations with constant adsorbent dosage and a contact time of 180 minutes, as determined by kinetic experiments to be sufficient.

The concentration of pesticides in supernatants was investigated by UV-Vis spectroscopy by measuring absorbance at their λmax (trifluarin λmax = 274 nm, carbendazim λmax = 285 nm, acetamiprid λmax = 245 nm, clothianidin λmax = 269 nm, thiamethoxam λmax = 251 nm).

The adsorption capacity at equilibrium (qe) and at time (qt) was calculated from the mass balance between the initial and remaining pesticide concentration in solution. All experiments were conducted in triplicate, and the obtained values represent the average values with a deviation below ±3%.

2.1.8. Effect of solution pH

The effect of initial solution pH on pesticide removal was studied over pH 3–9 under the conditions of the kinetic experiments (0.5 g L−1 adsorbent, 400 rpm, 25 ± 1 °C; C0 = 10 mg L−1 for acetamiprid, clothianidin and thiamethoxam, 20 mg L−1 for carbendazim and 25 mg L−1 for trifluralin). The pH of the pesticide solutions was adjusted with 0.1 M HCl or 0.1 M NaOH before the adsorbent was added. After the equilibrium contact time was determined from the kinetic experiments, the suspensions were centrifuged, and the residual pesticide concentrations were determined by UV-Vis using calibration curves recorded at the corresponding pH.

2.1.9. Regeneration and reusability

The reusability of pristine GO, cinnamic acid-GO, and lignin-GO was evaluated over three consecutive adsorption–desorption cycles for all five pesticides under the conditions of the kinetic experiments (0.5 g L−1 adsorbent, 400 rpm, 25 ± 1 °C; C0 = 10 mg L−1 for acetamiprid, clothianidin, and thiamethoxam, 20 mg L−1 for carbendazim, and 25 mg L−1 for trifluralin). After each adsorption step, the adsorbent was separated by centrifugation (4000 rpm, 10 min), and residual pesticide concentrations were determined by UV-Vis. The spent adsorbent was then regenerated by dispersing it in 20 mL ethanol and stirring for 60 min. It was then centrifuged, washed twice with deionized water, dried at 60 °C, and used in the next cycle. Removal efficiency in each cycle was calculated as described above.

2.1.10. X-ray photoelectron spectroscopy (XPS)

Samples were fixed onto carbon tape and analysed using a ThermoFisher Scientific Nexsa XPS instrument equipped with a monochromated, micro-focused, low-power Al Kα X-ray source. Data processing was performed using Avantage software. Spectra were fitted using the Smart background function for O 1s high-resolution spectrum and Tougaard background function for C 1s high-resolution spectrum, and binding energies were calibrated relative to the C 1s peak at 284.8 eV. Peak fitting was employed, focusing on the expected peaks' binding energies.

2.1.11. BET analyzes

N2 adsorption–desorption measurements were performed at 77 K using a Micromeritics TriStar II 3020 surface area and porosity analyser. Prior to analysis, the samples were subjected to degassing under vacuum to remove physically adsorbed species. The specific surface area was determined using the Brunauer–Emmett–Teller (BET) method,25 while the pore size distribution and pore volume were evaluated using the Barrett–Joyner–Halenda (BJH) method.26

2.2. Synthetic procedures

2.2.1. Graphene oxide synthesis

GO was obtained through the modified Hummers' method,27,28 which included dispersing graphite (10 g) in a mixture of sodium nitrate (5 g) and sulfuric acid (250 mL). The dispersion was carried out in an ice bath to control the low temperature.

Then, potassium permanganate (35 g) was gradually added to the mixture over 3 hours. Exact temperature control was maintained throughout the addition process to ensure that the reaction temperature did not exceed 20 °C. After the complete addition of potassium permanganate, the reaction mixture was vigorously stirred for 4 hours, during which the temperature was carefully increased to 35 °C. The reaction was then stopped by adding deionized water (450 mL) to the mixture. The unreacted potassium permanganate was neutralized by hydrogen peroxide (2500 mL of a 3% solution).

The resulting oxidized graphite was isolated by centrifugation. This was followed by thorough rinsing with distilled water until it reached a neutral pH, and this process usually took about 20 days to ensure complete removal of residual acids.

2.2.2. Covalent binding of graphene oxide with cinnamic acid

The synthesis was carried out in a 100 mL three-necked flask equipped with a reflux refrigerator, magnetic stirrer, and thermometer. Freshly distilled thionyl chloride (4.2 mL) has been added to the reaction flask in an inert atmosphere. 2.96 g of trans-cinnamic acid has been slowly added to the flask in small portions over 15 minutes. The reaction mixture was heated to 50 °C until the complete dissolution of the acid. After full dissolution, the reaction mixture was kept stirring at 80 °C for 2 hours. At the end of the reaction time, the residual thionyl chloride was removed from the reaction mixture by distillation in a rotary evaporator.29

Further, 1.4 g of GO was sonicated in 85 mL of DMFA for 30 minutes. The resulting solution of GO nanolayers was added to the flask containing cinnamic acid acyl chloride, followed by the addition of 3 mL of triethylamine. The formed reaction mixture was stirred for 24 h at 100 °C. Then, the formed solid solution was filtered, washed with dichloromethane and dried.

2.2.3. Covalent binding of graphene oxide with lignin

The reaction was performed at room temperature in a 200 mL single-neck flask equipped with a magnetic stirrer. Freshly sonicated graphene oxide (0.3 g) dispersed in 81 mL of deionized water was introduced into the flask with sodium hydroxide (7 g), urea (12 g) and epichlorohydrin (6 mL). The mixture was stirred for 20 min, after which 0.6 g of alkali lignin was added to the flask and continuously stirred for 48 h. The resulting mixture was then centrifuged (4000 rpm, 15 min), washed with ethanol, and dried.30

3. Results and discussion

3.1. PXRD

In order to determine the phase composition and crystallite size of the samples, PXRD analysis was performed. All the values of the interplane distance (d) for the corresponding diffraction peaks (2θ) were calculated according to Bragg's formula.31

The recorded diffractogram in Fig. 1A (broad single maximum at 2θ = 19.9) represents a typical lignin pattern, which characterizes its amorphous nature.32 The major diffraction sharp peak of the pristine GO (Fig. 1C) presents the characteristic intense peak at 2θ = 11.51 with the Miller index (001), proving the oxidation of graphite sheets with an interplanar distance equal to 7.68 Å.33 The XRD diffractogram of the obtained lignin modified with GO (Fig. 1B) reveals that the proportion of the ordered part increases after modification of the polymer by nanoparticles. In addition, the broad diffuse background characterizes the amorphous nature of lignin. Numerous sharp reflections in the range of 2θ = 18–70.75 are observed on the diffractogram, with the most intense peaks at 2θ = 28.94 and 33.76, which indicates the formation of a highly crystalline phase of the obtained polymer-grafted nanolayer. Furthermore, the main peak of GO is shifted to 2θ = 8.95, which confirms the incorporation of lignin fragments between the layers of graphene oxide and expansion of the distance spacing. Consequently, the covalent binding of lignin with GO leads to the formation of new ordered domains, indicating a reorganization of initial molecules.

Fig. 1. The XRD pattern of lignin (A), lignin modified with GO (B), GO (C).

Fig. 1

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The crystallite sizes and other structural parameters of the molecules, calculated using the Scherrer equation,34 are given in Table S1 (SI), whereas the calculated average number of graphene layers (N) is presented in Table S2 (SI).35 The increase in the size of crystallites calculated from the (001) reflection, i.e., the average number of graphene layers, indicates a structural alteration of the layered structure of the initial GO, as a result of multilayering.36

After modification of GO with cinnamic acid (Fig. 2A), the main peak of GO shifted to a lower angle equal to 2θ = 9.34, which corresponds to the irreversible inclusion of cinnamic acid between the graphene oxide's sheets, which increased the distance between them. The simultaneous increase in interlayer d-spacing to 9.47 Å (Table S3, SI) provides clear evidence of structural changes in the original molecular arrangement.

Fig. 2. XRD pattern of cinnamic acid (C), GO (B), cinnamic acid modified with GO (A).

Fig. 2

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The appearance of a broad diffuse band in the region of 19.25–25.60, instead of the sharp Bragg's reflection characteristic of the pure acid crystals (Fig. 2C), indicates the loss of long-range order and definitely the transition to the amorphous state.

3.2. FTIR

To obtain complete information about the synthesized molecules, FTIR investigations were used, the results of which are presented in Fig. S1 and S2 (SI).

The appearance of oxygen-containing groups on the graphene oxide surface was confirmed by a number of absorption bands shown in Fig. S1A (SI). A wide band at 3194 cm−1 indicates the presence of a large number of hydroxyl groups. The presence of bands at 1713 cm−1 and 1618 cm−1 corresponds to the stretching vibration of C Created by potrace 1.16, written by Peter Selinger 2001-2019 O and COO in carbonyl/carboxylic groups and aromatic fragments, respectively, while a peak at 1042 cm−1 refers to valence vibrations of C–O–C.37–39 These results confirm the oxidation of graphite sheets to graphene oxide.

As it can be seen from FTIR spectrum of lignin (Fig. S1B, SI), the low band at 2936 cm−1 corresponds to the valence vibrations of C–H aliphatic groups, which is used to evaluate the ratio of aliphatic and aromatic structures of lignin.40 Intense bands related to aromatic rings and deformation vibrations of C–H bonds are recorded in the region of 1591, 1511, 1450, and 1424 cm−1, respectively. A complex of bands in the range of 1265–1211 cm−1 corresponds to the fluctuations of C–O of a guaiacyl-based aromatic ring that is typical for natural biopolymer lignin, whereas the bands at 1372 cm−1 and 1124 cm−1 are the C–O deformation of the methoxy group of the syringyl ring.41,42

Looking at FTIR spectrum of the GO-lignin complex (Fig. S1C, SI), the shifting of a broad absorbance band of graphene oxide from 3194 to 3456 cm−1 indicates the participation of hydroxyl groups in the interaction between GO and lignin. A decrease in the intensity of peaks at 1450 cm−1 (deformation vibrations of the –CH2 and –CH3 groups) and 1265–1028 cm−1 (vibrations of the C–O–H and C–O–C groups) suggests the involvement of these groups in the modification of lignin using epichlorohydrin, which leads to the formation of the ether linkage. The intense band at 848 cm−1 indicates the formation of ether bonding as a result of the interaction of graphene oxide and lignin through epichlorohydrin linkage.38 The formation of ether bridges between the polymer and the nanomaterial leads to the formation of covalent binding between them.

The FTIR spectrum of the pure cinnamic acid is depicted in Fig. S2D (SI). The stretching vibration at 3000 cm−1 is related to the absorbance peak of unsaturated C–H vibrations, while the peak at 2500 cm−1 represents the saturated ones. The intense peak at 1670 cm−1 is ascribed to the carboxyl group in the molecule of cinnamic acid, whereas the narrow peaks at 1627 cm−1 and 1500 cm−1 are attributed to the presence of C Created by potrace 1.16, written by Peter Selinger 2001-2019 C bonding of the aromatic ring.43

The obtained nanocomposite based on cinnamic acid and graphene oxide is demonstrated in Fig. S2E (SI). A noticeable decrease in the absorption peak at 3194 cm−1 and a shift of the peak from 1670 cm−1 to 1644 cm−1, which represents the hydroxyl group of graphene oxide and the carboxyl and carbonyl groups of cinnamic acid, respectively, indicate a successful covalent functionalization of molecules. Importantly, the appearance of a novel absorbance band at 1713 cm−1 corresponds to the vibration of C Created by potrace 1.16, written by Peter Selinger 2001-2019 O groups, proving the formation of an ester linkage and confirming the successful binding of GO with cinnamic acid.44

3.3. XPS investigations

In order to determine the elemental composition of the synthesized nanocomposites, XPS investigations were performed. Comparison of the ratio of atomic % of O to C for functionalized and non-functionalized GO demonstrates that the O/C ratio decreases from 0.43 in pure GO to 0.34 in functionalized GO after functionalization. This may be related to the presence of cinnamic acid loaded on the surface of GO, since in cinnamic acid, the ratio of O/C is 0.3. The presence of an N peak is related to the absorption of DMF by GO. This is confirmed by the value of the binding energy of the high-resolution N 1s XPS peak. The conclusions regarding the interaction between GO and cinnamic acid were drawn from the high-resolution O 1s XPS spectrum of functionalized GO. It was reported that unfunctionalized GO has carbonyl, carboxyl, hydroxyl, and epoxy groups.39,45–48 It is expected that the carboxylic group of cinnamic acid will interact with the epoxy group of GO to give an ester and interact with the carboxylic group of GO to give an anhydride of carboxylic acid. Table 1 provides data on the concentration (at%) of different C and O atoms on the surface of functionalized GO after deconvolution of the high-resolution O 1s1S spectrum. As it can be seen from Table S4 (SI) and Fig. 3, although after the interaction of cinnamic acid with GO, we still have 8.34% of carboxylic groups and 0.78% of epoxy groups, 0.07% of anhydride groups ((C Created by potrace 1.16, written by Peter Selinger 2001-2019 O)–O*–(C Created by potrace 1.16, written by Peter Selinger 2001-2019 O)) and 0.1% of ester groups were also formed. This means that the carboxylic group on the surface of GO interacted with the carboxylic group of cinnamic acid. At the same time, the epoxy group of GO also interacted with the carboxylic group of cinnamic acid to form an ester, and the ratio of converted epoxy groups is much higher than that for carboxylic groups on the surface.

Table 1. XPS data.

Atom C O N S Cl
Atomic % of GO 66.12 28.67 2.52 2.69 —
Atomic % of cinnamic-acid-GO 71.8 24.45 2.64 0.16 0.95
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Fig. 3. High-resolution XPS spectrum of O 1s.

Fig. 3

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3.4. BET analyzes

A Micromeritics TriStar II 3020 surface area and porosity analyzer was used to quantify N2 adsorption–desorption at 77 K. The Brunauer–Emmett–Teller (BET) method25 was used to calculate the specific surface area, and the Barrett–Joyner–Halenda (BJH) method26 was used to assess the pore size distribution and pore volume.

Isotherm data were collected for each sample. BET model-based calculation of surface area and BJH model-based calculation of pore volume (Table 2) show that the surface area of lignin-modified graphene oxide (7.49 m2 g−1), as well as its pore volume (0.0061 cm3 g−1), dramatically decreased compared to those of pure graphene oxide (86 m2 g−1 and 0.047 cm3 g−1 correspondingly). This suggests that lignin was successfully attached to the surface of graphene oxide and exposed for further adsorption. On the other hand, average pore sizes did not increase dramatically, showing that lignin was adsorbed in small pores of graphene oxide and a small number of slightly larger pores were still available after functionalisation of graphene oxide with lignin. While in the case of graphene oxide functionalized with cinnamic acid we observe the same pattern, the residual BJH adsorption cumulative volume of pores is much larger, suggesting that smaller molecules of cinnamic acid pack in each graphene oxide pore in larger quantities, which leaves graphene oxide with more available pores for adsorption (Scheme 1). As far as residual pore sizes are concerned, the calculated BJH adsorption average pore width for graphene oxide functionalized with cinnamic acids is around the same value as the one calculated for lignin-functionalized graphene oxide.

Table 2. BET data of GO, lignin-GO and cinnamic acid-GO.

Sample BET surface area, m2 g−1 BJH adsorption cumulative volume of pores, cm3 g−1 BJH adsorption average pore width, nm
GO 86 ± 2 0.047 3.43
Lignin-GO 7.49 ± 0.07 0.0061 5.04
Cinnamic-acid-GO 14.27 ± 0.09 0.016 5.13
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Scheme 1. Illustration of the effect of lignin and cinnamic acid modifications on the accessible pores of GO.

Scheme 1

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3.5. Solid-state NMR

In order to prove the chemical bonding of GO to cinnamic acid and lignin, solid-state NMR investigations were also performed. As it can be seen from the 13C NMR spectra of graphene oxide (Fig. 4), the characteristic signals of C–O–C, C–O–H, O–C–O, and sp2-hybridised carbon are observed at 60, 70, 100, and 130 ppm, respectively, and coincide with those reported in the literature.49 Along with it, tiny signals of carboxylic and carbonyl groups are observed at 170 and 190 ppm, respectively. All these confirm the successful synthesis of graphene oxide.

Fig. 4. 13C spectra of pure graphene oxide (black), GO with cinnamic acid (red) and GO with lignin (green). Black arrows indicate typical chemical shifts for GO samples, as discussed in the text. Dotted lines corresponding to GO C–OH and C–O–C positions are drawn to guide the eye. Expected signals from pure cinnamic acid50 and pure lignin51 are marked by red and green arrows, respectively. Since lignin is a complex biopolymer, the corresponding spectral regions where lignin signals are expected are broad.

Fig. 4

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The spectrum of cinnamic acid bound to GO is given in Fig. 4. The intensity ratio between C–O–C and C–O–H peaks, marked by dotted lines in Fig. 4, clearly indicates the evolution of the relative quantities of the corresponding moieties. This points to a mechanism involving epoxy ring opening and ester bond formation. Additionally, the signals assigned to the carboxylic and anhydride groups exhibit a noticeable shift, suggesting that some parts of the carboxylic groups also interact with the cinnamoyl chloride, forming an anhydride group. The shift of the sp2 C Created by potrace 1.16, written by Peter Selinger 2001-2019 C region indicates a π–π stacking interaction between the aromatic structure of GO and cinnamic acid.

Upon modification of lignin with GO, a significant decrease in the intensity of the C–O–C and C–OH signals is observed, pointing to the opening of the GO epoxy ring and involvement of hydroxyl groups in the reaction, respectively. At the same time, the appearance of a narrow signal at 168 ppm, as well as a broad signal at 178 ppm, suggests the formation of an ester fragment in the grafted sample. Additionally, a shift of the sp2 C Created by potrace 1.16, written by Peter Selinger 2001-2019 C region is also observed, which may indicate the π–π interaction of aromatic fragments of lignin on the surface of GO.

3.6. TEM

According to the TEM results (Fig. 5a), ultrasonically exfoliated graphene oxide (GO) sheets exhibit an amorphous morphology. The wrinkles and folds observed on sheets indicate that the thin layers are flexible and capable of folding.27 TEM analysis revealed three different structural features that appear upon grafting GO with cinnamic acid. One of these is spherical nanoparticles with diameters of approximately 375.01 nm (SI Fig. S3(a) and Table S5), which are formed as a result of the attachment of graphene oxide with cinnamic acid. As clearly shown in Fig. 5b, the formation of the spherical nanoparticles is attributed to the introduction of additional hydrophobic interactions that induce the folding of graphene oxide layers. This leads to the formation of symmetric spherical structures, where cinnamic acid molecules are likely oriented towards the interior of the structure.52–54 Another feature is the presence of multiple layers that are attributed to the folding and stacking of the nanosheets, driven by the multiple interactions resulting from the grafting of the cinnamic acid and layers of GO.55

Fig. 5. TEM images of GO (a), GO modified with cinnamic acid (b), pure lignin (c), lignin modified with GO (d).

Fig. 5

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TEM images of cinnamic acid modified with graphene oxide also revealed another type of topological deformation, viz nanofibers. Ultrasonicated single layers tend to roll up tightly, forming one-dimensional nanofibers, as illustrated in Fig. 5b, with a diameter of approximately 58.32 nm (SI Fig. S3B and Table S5).56,57 Overall, graphene oxide itself plays the primary role in the formation of these nanofibers, whereas cinnamic acid-modified graphene oxide gives rise to spherical structures, as well as induces a noticeable thickening of the nanosheets.

TEM analysis demonstrates the clear differences in the morphological properties of pure lignin nanoparticles and graphene oxide-modified lignin nanocomposite. The pure lignin nanoparticles presented in Fig. 1C are mostly spherical or semi-spherical in shape. These shapes appear as dark aggregated domains, with a diameter of 30 nm (SI Fig. S3C).58 Although the sizes of the nanoparticles are relatively homogeneous, some particles are observed to coalesce with each other to form larger aggregates. The amorphous nature of lignin, the formation of hydrogen bonds, as well as π–π interactions between the molecules can explain this behavior.

In contrast, the TEM images of the lignin-GO nanocomposite shown in Fig. 5d reveal a more complex and layered morphology. The typical thin and twisted structures of modified GO can be seen in the obtained nanocomposite. The lignin is distributed on the surface and between the GO layers, forming wrinkled and locally rolled worm-like features. High-contrast regions indicate the multilayered stacking, forming a partially tubular and ribbon-like nanocomposite with a hybrid architecture, rather than a flat plate of GO or a sphere of lignin. This structure suggests the existence of strong interactions between lignin and GO, including covalent and hydrogen bonds, as well as electrostatic interactions. The thiol, phenolic, and hydroxyl groups of lignin interact with the oxygen-containing functional groups of GO, leading to the formation of a more stable and uniform nanostructure.

Thus, GO acts as a framework, inducing structural restructuring of lignin along its surface by non-covalent interactions. It promotes the formation of quasi-tubular interconnected assemblies and increases the specific surface area of the nanocomposite. Such fibrillar morphology, with a transverse diameter of 30 nm (SI Fig. S3D), has a partially hollow nature, which can improve the mechanical stability and functional properties of lignin particles.59–61

These TEM results demonstrate that the incorporation of lignin particles into the GO matrix not only changes the morphology but also leads to the formation of an organized, layered, and functional nanocomposite structure.

3.7. UV-Vis investigation of adsorption behavior

The adsorption characteristics of synthesized materials toward the selected pesticides were evaluated using UV-Vis spectroscopy by monitoring the temporal evolution of the normalized concentration (Ct/C0) and removal efficiency (%). The obtained results are presented in Fig. 6a–e.

Fig. 6. Adsorption kinetics of pesticides onto pristine-GO, cinnamic acid-GO and lignin-GO presented as normalized concentration (Ct/C0) versus time: (a) acetamiprid, (b) carbendazim, (c) clothianidin, (d) thiamethoxam and (e) trifluralin.

Fig. 6

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C t /C0 results (Fig. 6a–e) show a sharp decrease in relative concentration during the initial adsorption stage, followed by a gradual approach to equilibrium. This type of behavior indicates that at early stages, the adsorption process takes place through available active surface sites, but at later stages, it becomes controlled by slower diffusion and equilibrium limitations. Lignin-functionalized GO shows the lowest Ct/C0 values, confirming its high adsorption capabilities across all pesticides. Cinnamic-GO showed intermediate values, while pristine GO showed the lowest adsorption efficiency.

The enhanced ability of lignin-GO can be related to the presence of additional functional groups within the lignin structure, which is able to promote multiple interaction mechanisms, such as hydrogen bonding, π– π stacking, and possible electrostatic interactions.

The removal ability of the adsorbent also supports these findings. Within the first minutes of contact, a sharp increase in removal percentage was observed (Fig. S4a–e). The maximum removal efficiencies reached by lignin-GO were significantly higher than those of pristine GO, confirming the beneficial effect of surface modification.

The consistency between Ct/C0 and removal efficiency indicates the reliability of the adsorption process and the reproducibility of the experimental system. To validate the quantification of spectroscopic methods and provide additional insights into the adsorption mechanism, Ct (absolute concentration (mg L−1)) and absorbance at the characteristic λmax of each pesticide were also investigated (Fig. S5 and S6, SI).

Overall, the UV-Vis spectroscopy analyses demonstrate that surface functionalization of GO significantly increases adsorption performance, both in terms of adsorption rate and equilibrium capacity across all investigated pesticides.

3.8. Adsorption isotherm and kinetic analyses

The equilibrium adsorption values of pesticides on pristine and functionalized GO were investigated with the Langmuir isotherm model (Fig. 7a–e). The equilibrium concentrations for each pesticide (Ce) were calculated with UV-Vis spectroscopy based on corresponding calibration curves.

Fig. 7. Nonlinear Langmuir adsorption isotherms describing the uptake of representative pesticides on pristine and functionalized GO. Panels correspond to (a) acetamiprid, (b) carbendazim, (c) clothianidin, (d) thiamethoxam and (e) trifluralin. Discrete markers denote experimental adsorption data, whereas continuous lines indicate model fits.

Fig. 7

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The obtained experimental isotherms show a typical saturation profile with a sharp increase in adsorption capacity at low equilibrium concentrations followed by a plateau, proving progressive occupation of available adsorption sites. The equilibrium adsorption capacity (qe) increased from about 70 to 95 mg g−1 (system a), 157 to 178 mg g−1 (system b), 56 to 73 mg g−1 (system c), 53 to 69 mg g−1 (system d) and 175 to 190 mg g−1 (system e) when pristine GO was substituted by lignin-GO, depending on the pesticide investigated. Cinnamic-GO showed intermediate performance in all cases, with adsorption capacity reaching approximately 83, 168, 67, 64 and 184 mg g−1, respectively. The obtained data were fitted using the Langmuir equation:3.8.where qe (mg g−1) is the adsorption capacity at equilibrium, Ce(mg L−1) is the equilibrium concentration obtained from UV-Vis, qmax (mg g−1) is the maximum monolayer adsorption capacity, and KL (L mg−1) is the Langmuir constant.

The high value of correlation (R2 = 0.95–0.997) confirms that adsorption has a monolayer behavior on a relatively homogeneous surface. Moreover, functionalization of the surface significantly improves the adsorption ability of the adsorbent, reflected in increased qmax and KL values, indicating higher capacity and stronger interaction between adsorbent and adsorbate.

The kinetic investigations were done using time-dependent measurements with Ct values from UV-Vis absorbance data at each time interval (Fig. 8a–e).

Fig. 8. Kinetic adsorption profiles of selected pesticides on pristine and functionalized GO. (a) trifluralin, (b) acetamiprid, (c) carbendazim, (d) clothianidin and (e) thiamethoxam. Discrete markers denote experimental adsorption data, whereas continuous lines indicate kinetic model fits.

Fig. 8

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Sharp initial uptake followed by a plateau was observed from the adsorption profiles. While the initial stage was governed by surface adsorption-driven high concentration gradients, later stages were controlled by diffusion processes as active sites became occupied. Improved accessibility and active site interaction strength of functionalized adsorbents led to faster adsorption rates and higher equilibrium capacities compared to pristine GO.

Therefore, functionalization of both lignin and cinnamic acid enhanced pesticide adsorption as compared to pristine GO, with lignin-functionalized GO exhibiting the highest adsorption capacities and removal efficiencies. The improvement was more prominent for systems (a) and (d), with an increase in removal efficiency of about 24 and 19 percentage points, respectively, relative to pristine GO. These results confirm that the aromatic functionalization of GO provides additional adsorption sites as well as a stronger interaction with the pesticide molecules.

The more intricate and multipurpose structure of lignin is a plausible explanation for the improved adsorption efficiency of the lignin-modified GO as compared to the cinnamic acid-modified GO. Lignin has a three-dimensional aromatic polymeric structure rich in phenolic and aliphatic hydroxyl, methoxy, and carboxyl functionalities, in contrast to cinnamic acid, which has a single aromatic ring and a single carboxyl group. Therefore, adding lignin can provide more aromatic domains for π–π interactions and hydroxyl-containing sites for hydrogen bonding, while also increasing the density and variety of possible adsorption sites. Additionally, lignin's polymeric structure may help build a GO-based structure that is less aggregated and more stable, increasing the accessibility of adsorption sites. Previous research62,63 on GO-lignin composites has revealed that lignin-GO interactions can accelerate the creation of stable nanostructures and raise the effective concentration of oxygen-containing functional groups, resulting in increased adsorption performance. Therefore, rather than a single particular interaction, the improved adsorption seen for the lignin-modified GO is probably related to the combined chemical functionality and structural impacts of lignin.

3.8.1. Effect of solution pH

The influence of solution pH on pesticide removal is shown in Fig. 9. For all pesticides and adsorbents, removal efficiency increased from pH 3 to a maximum at pH 6 and then decreased towards pH 9. Lignin-GO was the most efficient adsorbent over the whole pH range, reaching 77.7% (acetamiprid), 82.7% (carbendazim), 58.0% (clothianidin), 55.9% (thiamethoxam) and 67.4% (trifluralin) at pH 6. Because acetamiprid, clothianidin, thiamethoxam and trifluralin do not ionize between pH 3 and 9, the observed trend mainly reflects changes in the surface chemistry of the adsorbents. At low pH, protonation of the carboxyl groups of GO reduces electrostatic repulsion between the sheets, promoting their aggregation and restacking and reducing the number of accessible adsorption sites. At high pH, the increasing negative charge density and hydration of the surface, together with partial deprotonation of phenolic hydroxyl groups in GO and lignin, weaken hydrogen bonding and π–π interactions with the pesticide molecules. Carbendazim was the most pH-sensitive pesticide, and its removal by pristine GO fell from 68.4% at pH 6 to 49.0% at pH 3. This is consistent with protonation of its benzimidazole nitrogen below pKa ≈ 4.2, which makes the molecule more hydrophilic and less prone to π–π stacking. In contrast, the hydrophobic, non-ionizable trifluralin was the least affected (removal at pH 3 and 9 remained 82–91% of the maximum), confirming that hydrophobic and π–π interactions dominate its adsorption. Removal efficiencies at pH 7 were within 4 percentage points of the optimum for all systems. This shows that the adsorbents perform close to their best in the near-neutral pH range typical of natural waters, and supports the use of natural pH in the kinetic, equilibrium and regeneration experiments.

Fig. 9. Effect of initial solution pH on the removal efficiency of (a) acetamiprid, (b) carbendazim, (c) clothianidin, (d) thiamethoxam and (e) trifluralin by pristine GO, cinnamic acid-GO and lignin-GO.

Fig. 9

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3.8.2. Regeneration and reusability

The removal efficiencies of the adsorbents over three adsorption–desorption cycles are shown in Fig. 10 and Table S6. All adsorbents showed a gradual and moderate decrease in performance. After the second cycle, lignin-GO, cinnamic acid-GO and pristine GO retained 95.1–96.7%, 95.3–98.4% and 95.8–97.9% of their initial removal efficiency, respectively. After the third cycle, these values were 90.2–93.9%, 94.4–96.6% and 94.4–95.6%. Lignin-GO remained the most efficient adsorbent for every pesticide throughout, with third-cycle removal efficiencies of 68.5% (acetamiprid), 73.2% (carbendazim), 53.3% (clothianidin), 50.1% (thiamethoxam) and 60.6% (trifluralin). At the same time, lignin-GO showed the largest relative loss, and its advantage over cinnamic acid-GO narrowed with cycling (from 12.2 to 8.2 percentage points for acetamiprid). This is consistent with the stronger multi-point π–π stacking and hydrogen bonding between the pesticides and the aromatic, hydroxyl-rich lignin domains. These interactions, which give lignin-GO its superior capacity, also make complete desorption harder and leave part of the active sites occupied after each regeneration step. The steady and moderate decline, with no abrupt loss of performance, indicates that the covalent GO-lignin and GO-cinnamic acid linkages withstand the regeneration conditions and supports the practical reusability of these hybrid adsorbents.

Fig. 10. Removal efficiency of (a) acetamiprid, (b) carbendazim, (c) clothianidin, (d) thiamethoxam and (e) trifluralin on pristine GO, cinnamic acid-GO and lignin-GO over three consecutive adsorption–desorption cycles.

Fig. 10

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The adsorption performance of lignin-GO was compared with that of previously reported adsorbents for the same pesticides (Table 3). For carbendazim, lignin-GO (178 mg g−1) outperformed activated carbon prepared from rape straw (111.3 mg g−1)68 as well as pristine and FeCl3-modified corn straw biochars (49.5 and 108.1 mg g−1, respectively).67 For the neonicotinoids, the capacity of lignin-GO toward acetamiprid (94 mg g−1) was higher than those of porous activated carbon from lignocellulosic waste (35.7 mg g−1),66 a CuO/rGO-modified plantain peel biochar (32.8 mg g−1)69 and commercial activated carbon (85.7 mg g−1).70 It was lower than that of a magnetic AC/Fe3O4 composite (192 mg g−1).71 The capacities of lignin-GO toward clothianidin (73 mg g−1) and thiamethoxam (69 mg g−1) were more than an order of magnitude higher than those of a magnetite-graphene oxide-β-cyclodextrin/MOF hybrid (2.88 mg g−1 for both)65 and exceeded that of magnetic nanomodified activated carbon toward thiamethoxam (13.4 mg g−1).72 In addition, lignin-GO removed 53.8% of thiamethoxam at 10 mg L−1, compared with about 50% for graphene oxide nanolayers.64 A boric acid-modified walnut shell biochar showed a higher thiamethoxam capacity (137.5 mg g−1),73 but its BET surface area (640.6 m2 g−1) is almost two orders of magnitude larger than that of lignin-GO (7.49 m2 g−1). This shows that the performance of lignin-GO derives mainly from its surface chemistry, namely aromatic, phenolic and hydroxyl functionalities that support π–π stacking and hydrogen bonding, rather than from its specific surface area. For trifluralin, quantitative capacity data are scarce.

Table 3. Comparison of pesticide adsorption by lignin-GO and other reported adsorbents.
Pesticide Adsorbent q (mg g−1) or removal Conditions Ref.
Acetamiprid Lignin-GO 94 Natural pH, 25 °C This work
Magnetic AC/Fe3O4 composite 192 — 71
Commercial activated carbon 85.7 — 70
Porous activated carbon from lignocellulosic waste 35.7 — 66
CuO/rGO-modified plantain peel biochar 32.8 pH 6, 25 °C 69
Magnetite-GO-β-cyclodextrin/MOF 2.96 100 mg L−1 65
Carbendazim Lignin-GO 178 Natural pH, 25 °C This work
Rape straw activated carbon (H3PO4, 600 °C) 111.3 pH 7, 20 °C 68
FeCl3-modified corn straw biochar 108.1 Room temperature 67
Pristine corn straw biochar (700 °C) 49.5 Room temperature 67
Clothianidin Lignin-GO 73 Natural pH, 25 °C This work
Magnetite-GO-β-cyclodextrin/MOF 2.88 100 mg L−1 65
Thiamethoxam Lignin-GO 69 (53.8% at 10 mg L−1) Natural pH, 25 °C This work
Boric acid-modified walnut shell biochar 137.5 25 °C 73
Magnetic nanomodified activated carbon 13.4 Sugarcane juice, pH 5.3–5.5 72
Magnetite-GO-β-cyclodextrin/MOF 2.88 100 mg L−1 65
Graphene oxide nanolayers ∼50% removal — 64
Trifluralin Lignin-GO 188 Natural pH, 25 °C This work
NH2-MIL-101(Cr)/PAN membrane 95.1% removal (20 mg L−1) Flow-through filtration 74
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An NH2-MIL-101(Cr)/PAN membrane removed 95.1% of trifluralin (20 mg L−1) in flow-through filtration,74 whereas lignin-GO showed an equilibrium uptake of 188 mg g−1 in batch mode. These comparisons should be regarded as approximate, because the cited studies used different initial concentrations, adsorbent doses, pH values and temperatures. Nevertheless, they show that lignin-GO, which is prepared from an inexpensive industrial by-product, is competitive with or superior to most reported adsorbents for these pesticides.

To summarize, literature data confirm that functionalization of GO and the lignin-containing adsorbents can enhance the adsorption performance toward organic molecules. It should be also underlined that without more direct mechanistic research, the suggested adsorption mechanisms should not be regarded as conclusive as they are dependent on the experimental results. Therefore, the present work provides a direct comparison of the pristine GO, GO modified with cinnamic acid and GO modified with lignin for pesticide adsorption, which makes it possible to evaluate the effect of aromatic functionalization on pesticide adsorption capacity. It ought to be emphasized as well that additional research is necessary to evaluate its efficacy in actual water matrices, regeneration and reusability, long-term stability, and possible environmental effects prior to the full establishment of its practical applicability.

4. Conclusions

Nanocomposites of graphene oxide with lignin (commercially accessible alkaline lignin (by-product of the pulp industry) was purposely used in this research, rather than purified lignin, to highlight the rising importance of this underused resource obtained from biomass) and cinnamic acid were successfully synthesized and their structures, as well as morphology, were analyzed by solid-state NMR, PXRD, TEM, FTIR, and XPS, which revealed the covalent nature of binding and allowed determination of the functional groups that participated in covalent interaction. Two chemically different aromatic modifiers – the complex natural polymer lignin and the low molecular weight cinnamic acid – were selected specifically to investigate the impact of their chemical nature on GO and its ability to remove different pesticides. Indeed, the BET and BJH analyses show that functionalization significantly modifies graphene oxide's porous structure. Along with it, TEM analysis revealed the formation of a rolled worm-like hybrid system, resulting from the formation of the GO framework and the restructuring of lignin. However, in the case of GO attached to cinnamic acid, the formation of three types of topology, including thickening of nanosheets, spherical structures and formation of rolled-up nanofibers, was observed. All adsorbents showed maximum pesticide removal at pH 6, with only minor losses at neutral pH, indicating their suitability for treating natural waters. Regeneration experiments showed that all adsorbents retained more than 90% of their initial removal efficiency after three adsorption–desorption cycles, with lignin-GO remaining the most efficient adsorbent throughout, confirming the reusability of the developed hybrid materials. Synthesized bio-based adsorbents were used for the removal of pesticides such as acetamiprid, trifluralin, thiamethoxam, clothianidin, and carbendazim, from contaminated aqueous solution, and a batch experiment demonstrated enhanced results in the case of the lignin-GO composite.

Author contributions

Sibel Hasanova: investigation, conceptualization, methodology, writing – original draft, writing – review & editing; Alakbar Huseynzada: investigation, conceptualization, methodology, writing – original draft, writing – review & editing; Alibala Aliyev: investigation, conceptualization, methodology, writing – original draft; writing – review & editing; Ulviyya Hasanova: supervision, project administration, funding acquisition, data curation, writing – review & editing; Imamaddin Amiraslanov: validation, formal analysis, writing – original draft; Carole Gardiennet: validation, formal analysis, writing – original draft; Gunel Aliyeva: formal analysis, vizualization, writing – original draft; Ilaha Hasanova: formal analysis, writing – original draft; Famil Chiraqov: formal analysis, writing – original draft; Eldar Gasimov: formal analysis, visualization, writing – original draft; Fuad Rzayev: formal analysis, visualization, writing – original draft; Zarema Gakhramanova: methodology, writing – original draft. Haji Vahid Akhundzada: writing – review & editing.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Supplementary Material

RA-OLF-D6RA05799C-s001

Acknowledgments

This work was supported by ERASMUS-JMO-2022-HEI-TCH-RSCH (GOS in EU – 101085674).

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d6ra05799c.

Notes and references

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Supplementary Materials

RA-OLF-D6RA05799C-s001
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Data Availability Statement

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d6ra05799c.

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