Biodegradable CMC/Zn/Ca/Cu cross-linked novel superabsorbent hydrogel bead for plant pathogen prevention and agricultural applications

Madhusmita Baruah^a, Rimjim Gogoi^a, Tanmoy Karak^b, Sangita Bharali^c, Anurag Kashyap^c and Jiban Saikia*^a
aDepartment of Chemistry, Dibrugarh University, Dibrugarh 786004, Assam, India. E-mail: jibansaikia@dibru.ac.in; Tel: +91-9706379107
^bDepartment of Soil Science, School of Agricultural Sciences, Nagaland University, Medziphema Campus-797106, Nagaland, India
^cDepartment of Plant Pathology, Assam Agricultural University, Jorhat 785013, Assam, India

First published on 4th August 2025

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1. Introduction

The issues associated with food production are expanding alarmingly with the increasing world population and urbanization. In response to global food demands, farmers are applying huge amounts of fertilizers to maximize the crop yield.^1,2 The soil continues to be polluted by petrochemical derivatives, crude oil, fertilizers, heavy metals and polycyclic aromatic hydrocarbons.^3,4 Nitrogen (N) fertilizers are rampantly used in agriculture for maximizing the yield.⁵ Only 20–30% of N fertilizers are effectively used by plants and the rest leach, potentially polluting the farmland soil and groundwater.⁶ Soil pollution has seriously endangered biodiversity and human health. Increasing the concentration of fertilizers in agricultural wastewater has led to widespread eutrophication in aquatic environments.⁷ After several decades of research, although organic and inorganic substances including zeolite, mesoporous hollow silica, and montmorillonite have been introduced to regulate the release of conventional chemical fertilizers, their extraction is a costly process.⁸ Therefore, there is an urgent need to find a cost-effective method for the replacement of traditional fertilizers.

Recently, slow-release fertilizer (SRF) systems made from nanocomposite hydrogels have attracted a lot of attention for their advantage of dosing and reduced poisoning.^9,10 Hydrogels are hydrophilic polymer materials that are cross-linked to form insoluble matrixes at physiological temperature, pressure, ionic strength and pH.^11,12 The hydrophilicity arises due to the presence of –NH₂, –COOH, –OH and –CONH₂ groups and the combined effect of capillary and osmotic pressures.¹³ Hydrogels can retain biological fluids or water and swell several times their dry volumes owing to their cross-linked network structure. The structure, chemistry and functionality of hydrogel systems can be manipulated, making them potential candidates in various applications including drug delivery, shape morphing, tissue engineering and agriculture and as soft actuators and artificial muscles.^14,15 Their aqueous environment permits the transportation of substances such as drugs and nutrients throughout its surrounding medium; however, their properties are reliant on the polymer hydrophilicity, polymer iconicity, type of recognition entity and cross-linking density.^16,17

Controllable methods for preparing hydrogels include radiation cross-linking, chemical cross-linking and physical cross-linking.¹⁸ The majority of hydrogel composites have low degradability.¹⁰ However, there is an emerging demand and expanding interest in the synthesis of cellulose-based hydrogels for soil water retention and as a nutrient release agent. One particular starting material that has been comprehensively studied for agricultural application is carboxymethyl cellulose (CMC). The CMC-based hydrogel can be extensively used for wound dressing, drug delivery, protein purification, crystallization of minerals, distillation and heavy metal capturing. CMC with many hydroxyl and carboxylic groups is considered a green, natural, inexpensive, nontoxic, biodegradable and biocompatible material.^19,20 Embedding inorganic components with a hydrogel matrix is an excellent strategy to overcome the inherent limitations of cellulose-based hydrogels.²¹ The high absorption ability, low cost, biodegradability, biocompatibility and non-toxicity of CMC make it an ideal cellulose derivative for preparing hydrogels.^22,23

Macronutrients and micronutrients are essential for plant growth and metabolism. Macronutrients such as nitrogen (N) and calcium (Ca) and micronutrients such as zinc (Zn) and copper (Cu) play a pivotal role in plant physiology.²⁴ N is known as a vital nutrient for crops to promote a healthier root system and to improve plant growth.²⁵ Ca participates in stimulus-response coupling and activates proteins such as phosphatases or phospholipases and kinases.^24,26 As essential plant nutrients, Zn and Cu participate in several physiological processes.^27,28 Micronutrients participate in plant metabolism, nitrogen transformation and protein synthesis and contribute to the development and structure formation of tissue. Their deficiency negatively affects plant growth.²⁹ The incidence of micronutrient deficiency in crops will be minimized by applying multi-nutrient precise fertilization in the vicinity of plants. The majority of micronutrients are co-granulated or co-delivered with a macronutrient fertilizer. Despite mounting evidence on the importance of hydrogel nanocomposite as an SRF system and the composites that might be a way to sustain plant growth, very limited information is available. On the other hand, plant pathogen is the major cause of food shortage. Tomato (Solanum lycopersicum L.) is one of the most popular crops and the second cultivated horticultural crop in the world. One-third of the global crops are lost due to insect injuries and pathogenic diseases.³⁰ Fungal infection of tomato can cause necrosis in the stem and later complete defoliation, which ultimately leads to death of the plant, accounting for up to 80% loss of tomato crop.³¹ Bacterial wilt caused by Ralstonia solanacearum is a serious soil-borne disease, and this bacterium causes wilt by infecting many plants, including tomato, through the roots and colonizing the stem of vascular tissue.³² Management of tomato production against this pathogen is crucial to maximize the crop's yield. Different approaches are developed to control this disease via the application of chemicals, soil disinfectants, antibiotics, organic amendments, and bacterial antagonists and the use of resistant varieties.³³ However, the hazardous effects of agrochemicals on the environment and the frequent breakdown of resistant varieties needed an alternative and sustainable disease management approach.^33,34 Bactericidal hydrogel nanocomposites could be exploited for such bacterial diseases.

Inspired by this background, we herein developed a facile strategy for encapsulation of plant growth ingredients in a hydrogel matrix made from carboxymethyl polymer via an ionic cross-linking mechanism that does not require any synthetic polymeric material. Herein, we report the synthesis of a multi-nutrient hydrogel by intermolecular cross-linking of CMC with ZnO and calcium chloride, followed by the addition of copper sulfate. The CMC/ZnO/Ca cross-linked beads (CMC@C) are soluble in water; therefore, they are further coated with copper by dipping the CMC@C beads in a copper sulfate solution. These beads are further loaded with a nitrate solution for slow N delivery. All the beads are thoroughly characterized by FTIR spectroscopy, SEM-EDX, XPS, XRD and rheological analysis. The performance of increased water retention capacity and synchronously slow release of nitrate has been realized. The major parameters that impact the swelling ratio of hydrogels were systematically evaluated. After degradation, the composite eventually releases plant nutrients such as Zn, Ca and Cu in the plant's available form. The nutrient delivery profile and effects of these hydrogels on soil and on the growth pattern of tomato plants (Solanum lycopersicum L.) were studied. Moreover, the in vitro antibacterial study was conducted to screen the antibacterial activity of the prepared hydrogel bead against tomato bacterial wilt pathogen Ralstonia solanacearum F1C1. These dual action beads could deliver nutrients and also act as bactericides for pathogen infection prevention in plants, thus promoting sustainability.

2. Experimental section

2.1 Materials

Sodium carboxymethyl cellulose (high viscosity) having the molecular formula [C₆H₇O₂(OH)_x(OCH₂COONa)_y]_n (n = approx. 500) was purchased from TCI (Tokyo, Japan). Zinc chloride hexahydrate (ZnCl₂·6H₂O, extrapure), sodium nitroprusside (extrapure, 98%) and dipotassium hydrogen phosphate (Na₂HPO₄, extrapure) were obtained from Finar (Ahmedabad, Gujrat, India). Potassium chloride and calcium chloride dihydrate (extrapure) were obtained from Loba Chemie (Mumbai, Maharashtra, India). Sodium chloride, potassium nitrate and copper sulfate pentahydrate were obtained from Rankem (India). N-(1-Napthyl)ethylenediamine dihydrochloride (NEDA; extrapure; AR, ACS, 98%), sodium hypochlorite solution (4–7% Cl), copper(II) sulfate pentahydrate (pure, 98%), sodium hydroxide (pure, 98%), and buffer capsules of pH 4, 7 and 9.2 were obtained from Sisco Research Laboratories Pvt. Ltd (Mumbai, India). De-ionized water was used as a solvent for fabricating the hydrogels.

2.2 Preparation of hydrogels

Hydrogel bead were prepared by a simple ionic cross-linking method, as briefly illustrated below. Initially, 4% CMC was added to 40 mL distilled water at room temperature and stirred until a viscous medium was obtained. Then, 10 mL zinc chloride solutions of different concentrations (shown in Table 1) were added slowly into the CMC solution. After stirring for 5 hours, solid sodium hydroxide (0.736 g) was added to the above-mentioned solution and stirred for another 4 hours. For the formation of homogeneous uniform spherical beads, the cross-linked CMC solution was drop-wise added into a calcium chloride solution using a syringe pump (at a rate of 100 mL per hour); meanwhile, the calcium chloride solution was stirred at a rate of 200 rpm. CMC/ZnO/Ca hydrogel bead (CMC@C) were allowed to stand for 1 hour in the calcium chloride solution to achieve the maximum cross-linking. The white color circular beads formed were filtered and washed with a suitable amount of ethanol to remove the unreacted chemical. CMC@C beads are not suitable for application as the cross-linked network breaks when it comes in contact with water. To overcome this inherent limitation, CMC@C was further soaked in a copper sulfate (2.5 g 50 mL⁻¹) solution for 12 hours. The calcium and copper cross-linked hydrogel bead (CMC@CC) were filtered and washed several times with distilled water. Finally, beads were dried for 6 hours in a vacuum desiccator and then oven-dried to carry out further analysis. Six different types of hydrogel bead were prepared, and their composition is shown in Table 1. Table 1 depicts the minimum concentrations of calcium and copper salts that are necessary for the formation of well-developed hydrogel bead. By lowering the concentration of both the reagents (Ca, and Cu) as mentioned in Table 1, well-developed hydrogel bead were not formed. Under optimized conditions, i.e. for the MH-4 sample (named as CMC@CC), the size of the hydrogel bead was found to be 0.3 cm in swollen form, as displayed in Fig. S1(b). From the digital images, it can be seen that the hydrogel bead have smooth surfaces in a swollen state, but after drying, the beads turn rough and the size is reduced to 0.2 cm (see Fig. S1(c)).

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2.3 Characterization

The morphological analysis of hydrogel was carried out using a scanning electron microscope (Jeol 6390LA/OXFORD XMX N). The images were observed at different magnifications at an accelerating voltage of 15 kV. The elemental analysis of hydrogels was performed by EDX. The FTIR spectra of the samples were studied using an FTIR spectrometer (Agilent Model No. Cary 630, United States; SL. No. MY20192018), within the range of 400–4000 cm⁻¹. XRD spectra were obtained using a Bruker D8 Advance instrument in the 2θ range of 2°–50° at a scan rate of 2° min⁻¹. The thermal stability of hydrogel bead was investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) (Make: Hitachi, Model: NEXTA STA300) at a heating rate of 10 °C min⁻¹ in a nitrogen atmosphere. To analyze the surface chemistry of materials, X-ray photoelectron spectroscopy (XPS) was performed using an Auger electron spectroscopy module (Model: PHI5000 Versa Prob II, FEI Inc.). Frequency sweep measurements were performed in a measuring mode at a frequencies ranging from 0.1 to 10 Hz, to determine the values of storage modulus (G′) and loss modulus (G′′). During the frequency sweep test, a rheometer with a parallel stainless-steel plate was applied to measure the value of the loss tangent.

2.4 Determination of the swelling ratio (SR)

The tea bag method is the most conventional method for measuring the swelling ratio of the hydrogel. A definite amount of hydrogel bead was placed into a tea bag, and the bag was immersed entirely in an excessive amount of swelling medium and allowed to swell until it reached an equilibrium state. Swelling data were collected at regular periods under ambient conditions. To remove surface water, the tea bag was hung for 15 minutes, followed by soft pressing the tea bag sample between the folds of filter paper. The same procedure was also analyzed for an empty bag. The swelling ratio of the hydrogel was measured using eqn (1):

	(1)

W_t is the weight of the tea bag after time t, W_d is the dry weight of hydrogel bead, and W_a is the weight of the tea bag. The sensitivity of the hydrogel bead was measured in distilled water, saline medium and also in different pH media.

2.5 Swelling kinetics

To study the effect of the composition of cross-linkers on the kinetics of water uptake of hydrogels, the swelling data were fitted into two kinetic models, namely, Fickian diffusion (eqn (2)) and second-order kinetic (eqn (3)) models, to analyze the experimental data. The Fickian diffusion equation is as follows:

	(2)

where Q_t is the degree of swelling ratio at time t, Q_eq is the equilibrium or maximum swelling ratio of the hydrogel, and k_p is the swelling rate constant. The second-order kinetic model is written as follows:

	(3)

where S_eq is the theoretical equilibrium swelling, the relationship K_is = K_sS_eq² is the initial swelling rate constant and K_s is the swelling rate constant.³⁵

2.6 Study the loading and release of fertilizers

Potassium nitrate was considered as a model fertilizer and loaded in the prepared hydrogel bead via the sorption mechanism. This was performed by immersing pre-weighed dry CMC@CC samples in a 0.1 M potassium nitrate solution and allowed to attain maximum swelling. The swollen samples were subsequently dried at 30 °C and stored for further analysis. The fertilizer-loaded sample was named NCMC@CC. The loading efficiency (LE) of nitrate was calculated using eqn (4):

	(4)

where W₀ is the initial amount of nitrate and W₁ is the amount of nitrate left in the solution.

The fertilizer release profile was measured under both water and soil conditions. To determine the fertilizer release profile, 2 g hydrogel composite beads were placed into a 250 mL volumetric flask containing distilled water. After a definite interval of time, 1 mL aliquots of corresponding media solutions were withdrawn, and 1 mL of the corresponding fresh sample was added to maintain the constant volume. The absorbance of the supernatant was read using a UV-visible spectrophotometer at 369 nm. The cumulative release rate of nitrate (C_r) was calculated using eqn (5):

	(5)

where n is the time of aliquot collection and 0 < i < n. C_n is the concentration of fertilizer in the release medium after the removal of the nth sample. V₀ is the total volume of release medium, whereas V_e is the volume of solution collected at time i, and M₀ is the mass of fertilizer in the hydrogel bead. The release kinetics of the fertilizer were also measured, and the kinetics equations are shown in Section S2.

2.7 Pot experiments and in vitro screening of antibacterial activities

Tomato (Solanum lycopersicum L.) was selected as a model plant to evaluate the effect of hydrogels on its growth. The seeds of the tomato were selected to be nearly of equal size. The seeds were sterilized with a 10% NaClO solution. To determine the effectiveness of the hydrogel, tomato seeds were allowed to grow under the following treatments: control (only soil); CMC@CC-soil and NCMC@CC-soil. Equal amounts of tomato seeds were placed in plastic boxes containing a definite amount of soil and the pot studies were conducted under normal laboratory conditions. Three replicates were carried out for each treatment. All the treatments were given equal amounts of tap water daily to ensure an adequate water supply. After 9 days of plantation, the plants were collected. The growth of plants was measured in terms of shoot and root length, as well as dry and fresh weight of biomass.

For the antibacterial studies, pre-identified preserved bacterial culture of R. solanacearum was procured from Assam Agricultural University, Jorhat, Assam, India. The antagonistic activity of the prepared hydrogel bead against R. solanacearum was evaluated by an agar diffusion method. The optimized hydrogel bead, i.e., CMC@CC, were tested for in vitro study for their ability to inhibit the growth of R. solanacearum. For this purpose, an initially fresh pure culture of the above-mentioned strain was inoculated on an agar plate containing hydrogel bead. A control sample was also prepared without the addition of hydrogel bead. The antibacterial activity was evaluated by measuring the inhibition zone observed around each of the disks after 48 hours of incubation. This was done three times for both the control sample and the hydrogel bead.

2.8 Statistical analysis of data

The plant study data were analyzed using one-way analysis of variance (ANOVA) followed by pairwise comparison using the least significant difference test (LSD). The LSD values were calculated at the percentage probability levels (p = 0.05) to determine the significance of differences between any two treatment averages. The significance of treatment variation vs. error variance was measured using the variance ratio.

3. Results and discussion

3.1 FTIR analysis

FTIR spectroscopy was used to relate the possible interactions in the hydrogel molecules. The FTIR spectra of pure CMC and the composite at each step of preparation are presented in Fig. 1. Major peaks are magnified in Fig. 1(b) and (c). A wide peak observed in all the samples at around (3000–3500) cm⁻¹ was due to the –OH stretching frequency, and a band at (1614–1625) cm⁻¹ was attributed to the CO stretching frequency of the CMC molecule. After the addition of zinc chloride, both –OH and CO bands of CMC/Zn suspension (CMC@Z) were shifted to a lower wavelength with a decrease in transmittance that that of pure CMC, indicating a possible interaction of hydroxyl and carbonyl groups with Zn²⁺ ions. The –OH stretching frequency in CMC@Z is extremely broad, indicating the chance of huge intermolecular hydrogen bonding of CMC with Zn²⁺ ions. Moreover, after the addition of sodium hydroxide, this –OH stretching vibration again shifted to higher transmittance and became less broad than that of CMC@Z.

Sodium hydroxide converts Zn²⁺ into ZnO nanoparticles, which was ascribed by the vibrational stretching frequency at 599 cm⁻¹, and due to this some intermolecular bonding was also destroyed. Furthermore, Fig. 1(d) depicts the FTIR spectra of CMC@CC, i.e., after cross-linking with calcium chloride and copper sulphate. Here, the characteristics vibration bands at 530 cm⁻¹ and 1420 cm⁻¹ were related to the stretching vibration of nano ZnO and the –CH₂ scissoring vibration of CMC molecules, respectively. Furthermore, it is noted that after the addition of calcium chloride and copper sulphate, both –OH and CO stretching vibrations again shifted to 3400 cm⁻¹ and 1625 cm⁻¹, respectively, suggesting the establishment of new non-covalent interactions with calcium and copper ions. The appearance of multiple peaks instead of the usual one broad band for asymmetric single stretching is indicative of different hydrogen bonding environments resulting from the interactions of calcium and copper ions with hydroxyl groups. The change in the FTIR spectra of CMC@CC proves that both calcium and copper can be successfully entrapped into the hydrogel network system. The FTIR spectra of KNO₃ and NCMC@CC are shown in (see Fig. S2). The peaks at 830 cm⁻¹ and 1380 cm⁻¹ correspond to the angular anti-symmetric deformation of O–N–O and the anti-symmetric stretching mode of free nitrate ion, respectively.³⁶ These peaks also appeared for NCMC@CC, which, in turn, confirmed the successful loading of nitrate in the hydrogel composite.

3.2 Morphological and rheology analysis

SEM measurements were carried out to analyze the surface morphology of the prepared hydrogel bead in both swollen and dry forms. The outer surface and cross-sectional morphology of CMC@C are portrayed in Fig. S3 and S4, respectively. The outer layer is associated with a rough and porous structure, which may be the result of the formation of a cross-linked structure with calcium ions. Fig. S3(d) and S4(d) represent the EDX spectra of the outer surface and the cross-section of CMC@C, which confirm the presence of all the characteristic elements. The cross-sectional morphology of CMC@C beads shows a distinct morphology compared with the outer surface. Fig. 2(a)–(c) and (g)–(i) show the top view of CMC@CC in both swollen and dry forms, respectively, at different magnifications. CMC@CC possesses a smooth surface in a swollen state and a rough surface with large wrinkles in a dry state, as shown in Fig. 2(a) and (g), respectively. Fig. 2(d)–(f) and (j)–(l) show the cross-sectional images of CMC@CC in both swollen and dry forms, respectively, at different magnifications. It can be seen that the cross-sectional images of CMC@CC show different morphologies and pore structures as compared to the outer surface. The EDX spectra of CMC@CC in both swollen and dry forms are shown in Fig. S5. The functionalized porous 3D network with nano ZnO will provide more adsorption sites in the hydrogel, which may be suitable for the immigration and emigration of fertilizers and hence enhance its efficiency towards sustainable agricultural applications. Similar in situ formation of nano ZnO in carboxymethyl cellulose-based hydrogel composites has also been reported by other researchers.^37,38 Moreover, rheological analysis was also performed for all the hydrogel bead, as shown in Fig. S6, and the rheological behavior is explained in (see Section S6).

3.3 Thermal analysis

To gain some knowledge about the thermal properties of nanocomposites, the thermal degradation behavior was examined and compared with the pure CMC. Thermogravimetry analysis of CMC@CC and NCMC@CC is exhibited in Fig. 3(a) and (b), respectively. The TGA, DTA and DSC thermograms of pure CMC are shown in Fig. S7.

The pure CMC sample experienced two stages of weight loss: the major decomposition started at (222–600) °C with a weight loss of ∼49% due to the inorganic moiety,³⁹ and a minor decomposition (14.5%) at <220 °C. Meanwhile, the degradation process of the hydrogel is divided into three distinct steps. The first-stage degradation occurred for both the samples at <150 °C, with ∼14.46% and ∼12.96% of overall weight loss for CMC@CC and NCMC@CC, respectively. This may be attributed to the elimination of absorbed water and the decomposition of low-molecular weight compounds. At this stage, the decomposition of side groups of polymers or copolymers and the destruction of cross-linking or non-cross-linking sites occur. The third stage degradation occurred at (281–600) °C with ∼20.46% weight loss for CMC@CC and at (265–600) °C with ∼19.80% weight loss for NCMC@CC. At 600 °C, ∼50.10% and ∼47.715% of CMC@CC and NCMC@CC are left, indicating that only half of the hydrogel was degraded at 600 °C. The increasing trend of decomposition temperature states that the thermal stability of CMC@CC is better than NCMC@CC. The exothermic peak appeared at ∼115 °C for CMC@CC, which is characteristic of the hydrogel. Moreover, this peak has shifted to ∼127 °C for NCMC@CC due to the formation of some non-covalent interactions between potassium nitrate and hydrogel bead. Furthermore, the DTA curves for both CMC@CC and NCMC@CC are shown in Fig. S8. Here, the obtained peaks demonstrated the characteristics of the hydrogel.

3.4 XRD analysis

To confirm the in situ formation of ZnO nanoparticles within the hydrogel matrix, XRD analysis was performed. The XRD pattern of pure CMC is shown in (see Fig. S9). CMC displays a large band at 22.6° corresponding to the (200) plane, indicating a relatively ordered structure characterized by the crystalline form of cellulose.⁴⁰ The XRD pattern of CMC@CC and NCMC@CC within the 2θ range of 2–80° is shown in Fig. 3(c). The (200) plane corresponds to CMC's remaining intake in both the hydrogels, and the peaks become sharper in hydrogel bead, resulting in higher perfection of the crystal lattice in this plane than in the CMC sample. We assign the peaks of CMC@CC at 2θ = 31°, 34°, 37°, 47°, 56°, 68° and 76° to the (100), (002), (101), (102), (110), (112) and (202) crystallographic planes of ZnO, indicating that the nano ZnO was formed in the polymer matrix. The characteristic peaks of nano ZnO in NCMC@CC are slightly shifted as compared to CMC@CC. This may arise due to the generation of some non-covalent interactions between CMC@CC with potassium nitrate. Moreover, the additional peaks that appeared at 2θ of 39.18, 47.17 and 32 could be assigned to the (200), (202) and (110) diffraction of CuO nanoparticles, respectively. During the loading of Cu²⁺ ions in the hydrogel matrix, some copper ions are converted into the CuO form.^41,42

3.5 XPS analysis

XPS measurements have been used to investigate the chemical state and to analyze the interactions between metal ions within the hydrogel matrix and with the fertilizer. The full-scan XPS survey of CMC@CC and NCMC@CC is shown in (see Fig. S10), which reveals the presence of C, O, Zn, Ca and Cu in the hydrogel matrix. Fig. 4(a) and (c) display the high-resolution survey scan of CMC@CC and NCMC@CC for C 1s, and it is deconvoluted into four peaks at 283.38/284.6 eV, 289.19/289.10, 285.76/285.84 and 287.79/287.80 corresponding to C–C, –O–CO, C–OH and CO, respectively. Fig. 4(b), (d) and (e) depict the high-resolution XPS spectra for Ca 2p, Zn 2p and Cu 2p, respectively. The peaks that appeared at 346.5 eV and 350.07 eV are in perfect agreement with the characteristics of Ca 2p_1/2 and Ca 2p_3/2, respectively.⁴³ Here, ZnO nanoparticles in the hydrogel matrix display Zn 2p peak positions at different binding energy values as compared to pure ZnO. For Zn 2p, two peaks appeared at 1021.04 and 1044.1 corresponding to Zn 2p_3/2 and Zn 2p_1/2, respectively, whereas in pure ZnO, they appeared at 1021.48 eV and 1044.58 eV for Zn 2p_3/2 and Zn p_1/2, respectively.^44,45 The characteristic peaks at 932.7 eV and 952.5 eV were attributed to Cu 2p_3/2 and Cu 2p_1/2, respectively.⁴⁶ The CuO peak was observed at 942.5 eV, and this peak is slightly shifted due to the interaction with the polymer matrix. A similar type of spectrum for CuO was also reported by Zhang et al.⁴⁷

3.6 Plausible mechanism of the formation of hydrogel bead

The formation of homogeneous circular hydrogel bead can be attributed to strong interaction via hydrogen bonds between carboxyl and hydroxyl groups with cross-linkers, and the predicted mechanism is schematically depicted in Fig. 5. The figure represents the plausible interactions based on characterization techniques and the available literature. The beads forming cross-linking agent calcium chloride and copper sulphate were assumed to act by complexation of the carboxylate anion of CMC by its divalent calcium and copper ions. When CMC/Zn/NaOH suspension (CMC@B) came into contact with calcium chloride, the cations immediately interacted with the anionic polyelectrolyte and formed circular beads on the drop surface. From the FTIR spectrum (Fig. 1), it is seen that after the addition of ZnCl₂, both –OH and CO, stretching frequencies have shifted, resulting in the formation of hydrogen bonding interactions of Zn²⁺ with –OH and CO groups, as shown in Fig. 5(ii). Moreover, after the addition of NaOH, both these stretching frequencies shifted, as at lower temperatures, the –OH ions break the hydrogen bonding interactions in cellulose molecules, and therefore, some interactions are broken.⁴⁸ The formation of ZnO was confirmed by FTIR spectroscopy and XRD analysis. It is assumed that the in situ formation of ZnO will push the hydrogel chain towards the bead formation. A similar type of interaction of CMC or alginate with Zn²⁺ was reported by Priyadarshi et al.,⁴⁹ Kumar et al.,⁵⁰ and Ekanayake et al.⁵¹ When CMC@C beads are dried and allowed to be studied in the water; then the beads are degraded. Therefore, after cross-linking with calcium, the beads are washed with ethanol; as water washing results in the disintegration of the beads. Washing beads are allowed to dip in a CuSO₄ solution and copper is cross-linked with the CMC chain via non-covalent interactions, resulting in the formation of well-developed CMC/ZnO/Ca/Cu cross-linked hydrogel bead, as portrayed in Fig. 5(v). The complexation and electrostatic interaction between the functional groups of the CMC chain and metal ions are responsible for the formation of stable cross-linked hydrogel bead.

3.7 Study of the swelling and deswelling properties of hydrogel nanocomposites

3.8 Biodegradability test

The biodegradation process can be comprised of mainly three stages of degradation. In the first phase of degradation, microorganisms participate in the degradation of bio-based materials of hydrogel and boost the degradation process. Moreover, the degradation stage was slow in the second phase due to increase in the water content of the hydrogels, which, in turn, restricted oxygen transfer to the hydrogel bead. It generates an anaerobic environment and, hence, slows the microbial growth. However, in the final stage, the hydrogel bead are converted into their simplest form, where complete degradation of the polymer backbone occurs.⁵⁸ Fig. S16(a) shows the biodegradation profile of both CMC@CC and NCMC@CC, and the degradation percentage was calculated using eqn (S5). Furthermore, Fig. S16(b) depicts the digital images of both the hydrogel bead after one month of application. According to the results obtained from the soil burial method, decomposition occurs very slowly in the initial stage. On the 10th day, the percentage of degradation for CMC@CC and NCMC@CC was found to be ∼98% and ∼97%, respectively. After one month, the degradation turned ∼94% and ∼96% for CMC@CC and NCMC@CC, respectively. The slow biodegradation profile confirms that the prepared beads are suitable for agricultural delivery. To date, we have found only limited literature on the application of CMC/Zn/Ca cross-linked hydrogels for agricultural purposes. Akalin et al. prepared zinc-loaded CMC/FeCl₃ and carrageenan/glutaldehyde cross-linked hydrogels and examined their effectiveness on the growth of wheatgrass.⁵⁹ Sharif et al. used CMC as a coating material to modify the controlled release behavior of a zinc/aluminium layered double hydroxide, quinclorac.⁶⁰ Our work offers distinct advantages over the existing studies. Here, micronutrients, i.e., Zn, Ca and Cu, are participating in the cross-linking process; therefore, these are released to the soil only after degradation.

3.9 Study of the cumulative release and release mechanism of hydrogels

3.10 In vitro antibacterial activity

The in vitro antibacterial properties of CMC@CC were tested against R. solanacearum. The inhibition zone under and around the tested sample for the bacterial growth was detected visually, and the zone of inhibition (ZOI) results of the application of CMC@CC are shown in Fig. 8(b). Growth inhibition of R. solanacearum can be observed for CMC@CC treatment, whereas in the control group, there is complete growth of bacteria within 48 hours (Fig. S18(a)). The measured individual inhibition zone diameter for each replicate is shown in Table S5, and their digital images are shown in Fig. S18(b). The average ZOI was found to be (13.2 ± 2.53)mm, and the size and consistency of this inhibition zone depend on the size of the beads. From the results, we revealed that R. solanacearum has been controlled successfully using our prepared hydrogel bead under in vitro conditions. Moreover, the visual changes were also observed in R. solanacearum after the application of CMC@CC, as characterized by a colour change from white to yellow. In the literature, various pesticide formulations are available to control the plant pathogen bacteria, R. solanacearum. Pradhanang et al. conducted a greenhouse experiment using plant essential oils, including thymol, palmarosa oil, and lemongrass, to suppress the bacterial population and minimize the wilt incidence of the tomato plant.³² Elsayed et al. examined the antagonistic behaviour of bacterial strains Bacillus velezensis (B63) and Pseudomonas fluorescens (P142) against this plant pathogen under both in vitro and in vivo conditions. These biocontrol agents under field conditions significantly decrease the bacterial densities in the rhizosphere and in tomato shoots.³³ Chen et al. fabricated a surfactant-stabilized Ag nanoparticle,⁶⁶ and Yang et al. used hydroxycoumarin as a promising antibacterial agent for this pathogen.⁶⁷ Ma et al. prepared a tetramycin-loaded pH-responsive adipic dihydrazide (ADH) and Ca²⁺ cross-linked oxidized sodium alginate-based gel as a carrier of pesticides.⁶⁸ Most of the existing formulations have some limitations. For example, for better performance, we must provide a suitable environment for bacteria, which itself is challenging for the viability and growth of the bacteria. Moreover, neither Ag nor tetramycin is essential for the growth of plants. This work aimed to minimize the limitations of pesticide administration to control this plant pathogen, and we believe that, as our prepared hydrogel bead contains only plant nutrients, it has application potential in the near future.

3.11 Study of the growth of Solanum lycopersicum L.

To investigate the practical application of these hydrogel bead as a package of nutrients, plant trials on Solanum lycopersicum L. were performed. The application of nanocomposites to soil ameliorates the adverse effects of chemical fertilizer on soil. Fig. 9(a) displays the digital images of plant growth in terms of plant height using different fertilization methods. Fig. 9(b) and (c) show the comparison of stem length, root length, plant height and plant biomass with the control. It was observed that tomato plants placed in soil treated with NCMC@CC showed a stem length of 80 mm, while control soil produced a stem length of 50.8 mm. The greatest plant height corresponding to NCMC@CC and CMC@CC were 89.8 mm and 71.0 mm, respectively, larger than that of the control (50 mm). The root length and dry weight of tomato plants treated with NCMC@CC were also increased as compared to the control. Potassium nitrate has a significant affect the growth process, and this difference was visible in NCMC@CC. It was evident that the application of NCMC@CC benefited plant growth more than that of CMC@CC. This detailed analysis was observed under sufficient water conditions. CMC@CC contains zinc, calcium and copper, which may be responsible for these early growth parameters. NCCMC@CC contains nitrate as a major fertilizer, which enhances plant growth. Moreover, the synergitic effect or positive interrelationship between macro- and micronutrients may also be responsible for the growth parameters. Therefore, the prepared hydrogel bead may have potential application prospects in agriculture to enhance crop productivity. Moreover, the physiological properties of the soil were also examined after collecting the tomato plants. The changes in pH and electrical conductivity of soil in comparison to the control are shown in Fig. 9(d). There is hardly a change in the above-mentioned parameters, indicating that our prepared nanocomposite hydrogel has maintained soil fertility throughout the trial periods. Moreover, one-way ANOVA followed by the LSD test was used to determine the statistical significance of different treatments, and is explained in Section S18.

4. Conclusion

The objective of the study was to develop a new superabsorbent material based on carboxymethyl cellulose, which could be further reinforced and cross-linked with calcium and copper, and loaded with nitrate to make it a multi-nutrient package for plants. The results obtained during experimental and mathematical analyses showed that 0.22 M of cross-linking agent (i.e., zinc chloride) is suitable for preparing the best hydrogel. From the FTIR spectra, XRD patterns and SEM images, we can confirm the in situ formation and distribution of nano ZnO within the hydrogel matrix. To analyze the nitrate release mechanism in water, we evaluated the fractional release data from the static release experiment by fitting the curve of fractional release using four mathematical models. Copper cross-linking is essential for enhancing the slow-release behavior of nutrients and the water retention properties of the hydrogel. The cross-linking ratio revealed a significant effect on the swelling ratio of hydrogels. The maximum swelling ratio under optimized conditions in distilled water was found to be 915% against MH-4 samples. The application of the prepared hydrogel composite shows the statistical significance of the growth pattern of tomato plants as compared to the control. In vitro study against R. Solanacearum of the prepared hydrogel bead reveals its antagonistic behavior in controlling the bacterial wilt disease of the tomato plant. These materials not only satisfy the demand of the economic and environmental sustainability market but also go a long way towards fulfilling future requirements to synthesize renewable biodegradable polymers in the nanotechnology concept. In addition, due to the combination of different macro- and micro-nutrients, the prepared hydrogel composites have emerged as one of the most promising polymeric materials for the efficient delivery of agrochemicals. The dual role of hydrogel bead, providing water and nutrients, while possessing bactericidal properties, makes them potential candidates for promoting agricultural sustainability.

Author contributions

Madhusmita Baruah: data curation, formal analysis, investigation, methodology, writing – original draft; Rimjim Gogoi: data curation, formal analysis, investigation, methodology, writing – original draft; Tanmoy Karak: data curation, formal analysis, investigation, methodology, writing – review and editing; Sangita Bharali: data curation, formal analysis; Anurag Kashyap: data curation, formal analysis; Jiban Saikia: conceptualization, funding acquisition, methodology, project administration, resources, validation, visualization, writing – review and editing.

Conflicts of interest

The authors declare no competing financial interest.

Data availability

The data supporting this article have been included as part of the SI.

Digital images of hydrogel bead; fertilizer release kinetics; FTIR spectra of KNO₃ and NCMC@CC; SEM micrograph and SEM-EDX of CMC@C; cross-sectional SEM micrograph and SEM-EDX of CMC@CC; SEM-EDX spectra of CMC@CC in both swollen and dry forms; rheological analysis; XRD of CMC; TGA, DSC and DTA thermogram of CMC; DTA of CMC, CMC@CC and NCMC@CC; full-scan XPS survey of CMC@CC and NCMC@CC; equilibrium swelling ratio of hydrogel bead; de-swelling property and repetitive swelling cycle of hydrogel bead; swelling kinetics graphs for the hydrogel using the Korsmeyer–Peppas model and second-order kinetic model; swelling kinetic parameters for the Fickian diffusion and second-order kinetic models; study of the stimuli responsive behavior of hydrogels; determination of SR (%) of CMC@CC in a saline medium (NaCl, KCl and CaCl₂) after 24 hours of swelling; comparison of the digital images of CMC@CC in different saline media; biodegradation study of hydrogel bead; release kinetics of nitrate in water (zero-order, first-order, Higuchi model and Korsmeyer–Peppas model); antibacterial activity and statistical analysis of plant growth. See DOI: https://doi.org/10.1039/d5tc01581b

Acknowledgements

The authors declare that financial support was received for the research, authorship and/or publication of this article. J. S. thanks DST-PURSE program [under project no. SR/PURSE/2022/143(C)], DBT [under project no. BT/PR36255/NNT/28/1728/2020] and DST SERB-SURE program [under project no. SUR/2022/001492 and under the DST-FIST (SRF/FST/CS-l/2020/152)] for the financial support. The authors are immensely grateful to the Dibrugarh University for providing all the infrastructural facility. The authors are also thankful to UGC, New Delhi, for Special Assistance Programme (UGC-SAP) to the Department of Chemistry, Dibrugarh University. Finally, we wish to record our thanks and gratitude to the anonymous reviewers for their valuable suggestions that helped us a lot in improving this manuscript.

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