Graphene quantum dot decorated magnetic graphene oxide filled polyvinyl alcohol hybrid hydrogel for removal of dye pollutants

Journal of Molecular Liquids 302 (2020) 112591

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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq

Graphene quantum dot decorated magnetic graphene oxide filled polyvinyl alcohol hybrid hydrogel for removal of dye pollutants Niladri Sarkar, Gyanaranjan Sahoo, Sarat K. Swain ⁎ Department of Chemistry, Veer Surendra Sai University of Technology, Burla, Sambalpur 768018, India

a r t i c l e

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Article history: Received 9 September 2019 Received in revised form 21 January 2020 Accepted 26 January 2020 Available online 28 January 2020 Keywords: Graphene quantum dot Nanocomposite hydrogel AFM Swelling Adsorption

a b s t r a c t Herein, a novel and simple coagulation route is adopted for quick preparation of graphene quantum dots (GQDs) integrated magnetic graphene oxide (GO/Fe3O4/GQD) by using ethanol-HCl as destabilizing agent. The ultra-high surface area of GQDs with layered graphene like structure makes them more sensitive to environmental changes as well as eco-friendly (cell viability ~ 86.7%) candidate for dye removal application. The as-prepared nanohybrid filler is then in situ incorporated within PEG-1500 induced macroporous polyvinyl alcohol/carboxymethyl cellulose hydrogel beads. The prepared PVA/CMC-B@GO/Fe3O4/GQD (L) nanocomposite hydrogel is characterized with SEM, TEM, XRD and FTIR. Surface topography and roughness of the nanocomposite hydrogel is evaluated through AFM microscopy, while pore distribution in the nanocomposite hydrogel is explored through BET adsorption isotherm and mercury porosimetry. Moreover, pH responsive swelling behavior of the nanocomposite hydrogel is well studied along with detail investigation towards removal of cationic pollutants (MB/RhB) with respect to contact time and solution temperature. © 2020 Elsevier B.V. All rights reserved.

1. Introduction Rapid growth in different industrial sectors such as paper printing [1], leather [2,3], textile [4] and cosmetic [5] results to an upswing in wastewater due to massive contamination of organic dyes. These organic pollutants not only hamper the aquatic lives, but also significantly contribute to various diseases like hemolysis, hypertension, respiratory disorders, jaundice, organ damage, tissue necrosis, etc. in humans. Therefore, huge research interest has been focused for removal of these toxic dyes. Several techniques, like ozonation [6], coagulation [7], electrochemical oxidation [8], photocatalysis [9] and adsorption [10] are applied for this purpose. Among all the techniques, adsorption is the most frequently used due to its high efficiency, economic viability, easiness of regeneration and insensitivity to toxic substances [11]. However, most of the low-cost adsorbents of dye pollutants like fly-ash, biochar, activated carbon and inorganic adsorbents are not biodegradable and may induce particle-contamination if it is not effectively separated from the medium after adsorption. Recently, ecofriendly hydrogels have gained significant attention as an efficient adsorbent of dye pollutants in waste-water management along with other important applications like drug delivery [12], wound⁎ Corresponding author. E-mail address: [email protected] (S.K. Swain).

https://doi.org/10.1016/j.molliq.2020.112591 0167-7322/© 2020 Elsevier B.V. All rights reserved.

healing [13], biosensing [14], and controlled delivery of micronutrients [15] in agriculture. It is because of their three dimensional cross-linked structures with hydrophilic pores which allow them to have high water-sorption and retention capacity [16]. Additional advantages are ultra-low surface friction, biocompatibility, biodegradability and ease of chemical and physical modification [17]. Low-cost polyvinyl alcohol (PVA) has been widely used as the raw material for preparing biocompatible hydrogels via physical [18] and/or chemical [19] cross-linking protocols. PVA based interpenetrating network (IPN) hydrogels with various biopolymers like sodium alginate [20], cellulose [21], gelatin [22], pectin [23], starch [24], and chitosan [25] are highly appreciated over freezethawed [26,27] and simple chemically cross-linked forms [28–30] for waste-water treatment. Incorporation of natural biopolymer, not only improves the hydrophilicity but also the network-stability, biocompatibility and biodegradability of PVA based hydrogels. Recently, Zhang et al. [31] synthesized yeast containing Polyvinyl alcohol/carboxymethyl cellulose hybrid hydrogel for degradation of methylene blue dye. Carboxymethyl cellulose (CMC) is a low-cost gel forming anionic polymer, obtained by carboxymethyl (-CH2-COOR) modification [32] on naturally abundant cellulose which provides a structural integrity with Al3+ [33] or Fe3+ [34] along with pH responsive high swelling capacity. It has huge applicability in drug delivery [35], enzyme immobilization [36], wound healing [37] and wastewater treatment [38]. In recent times, FI-Arnaouty et al. [39] synthesized poly (carboxymethyl cellulose-g-methacrylic acid/

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acrylamide) hydrogel using direct radiation copolymerization technique and efficiently used for adsorptive removal of acid blue dye and methyl green. Nowadays, nanocomposite hydrogels are known to offer much improved adsorption behavior due to incorporation of nanomaterials which have high aspect ratio and contributing to the surface functionality and cross-linking density of hydrogel structure. Carbon based fillers like carbon nanotubes, fullerenes and graphene analogues [40,41] are in the prime position of the list as compared to other adsorbents because of their excellent adsorption and regeneration efficiencies. Biocompatible graphene oxide (GO) has numerous oxygen functionalities like epoxide, carboxylic and hydroxyl groups along with their large surface area, high aqueous dispersibility and mechanical strength which makes them useful in dye adsorption/degradation [42–44]. Dong et al. [45] incorporated chemically reduced GO and magnetite (Fe3O4) nanoparticles in polyacrylamide hydrogel to effectively degrade (90%) rhodamine B solution within 60 min. On the other hand, Mahdavinia et al. [46] fabricated kappa-carrageenan/PVA/Fe3O4 nanocomposite hydrogel for adsorption of cationic dyes. Sahraei et al. [47] used modified gum tragacanth with GO and Fe3O4 for effective removal of crystal violet (94 mg/g) and congo red (101.4 mg/g) dyes. The incorporation of Fe3O4 nanostructures in hydrogel matrix is generally performed to achieve magnetically separable properties [48] along with improved mechanical properties as elucidate in earlier literature [49]. Nanostructural Fe3O4 is also reported to produce effective adsorbent in water remediation when combined with small-sized graphene quantum dots as reveled by Razmi and his co-workers [50]. Graphene quantum dots are the miniature of graphene oxide nanostructure with size b 10 nm, and therefore, having ultra-high surface area and unique photo-physical properties which makes them potential candidate in the field of biomedical application [51] and water remediation [52]. But the presence of Fe3O4 nanostructures in GO or GQD also occupies some of the free active sites. Addressing of such issue is very rare in the literature. Therefore, we are motivated to prepare graphene quantum dot (GQD) decorated GO/Fe3O4; a tri-component nanohybrid to achieve more active sites for dye molecules with improved surface area. To the best of our knowledge, for the first time we have prepared GQD decorated GO/Fe3O4 nanohybrid system via simple ethanol-HCl route. In present investigation, we have modified the PVA/CMC hybrid gel structure with our prepared GO/Fe 3 O 4 /GQD hybrid nanostructural system. In recent works, Dai and his co-workers [53] prepared the hybrid nanocomposite hydrogel of PVA/CMC with incorporation of GO and bentonite for removal of methylene blue dye. On the other hand, we have crucially controlled the porous morphology by using polyethylene glycol (PEG-1500) as porogen which in turn may enhance the sorption capacity and the response rate by reducing the transport resistance factors. Use of PEG as porogen has earned a huge attention for developing macroporosity in hydrogel structure [54–56] over other poreforming techniques like salt leaching [57], porogen [58], phase separation [59] and gas foaming [60]. The choice of biocompatible PEG-1500 as porogen is due to its excellent leaching behavior with warm water which in turn is related to its strong hydrophilicity and melting point (50 °C). This manuscript is mainly focuses on the design of a unique tricomponent filler, GO/Fe3O4/GQD via HCl:EtOH route and its subsequent incorporation in boric acid cross-linked PVA/CMC-B hydrogel matrix to improve the dye adsorption behavior. Moreover, in reducing the crosslinking density and improving the porous morphology in nanocomposite hydrogel, in situ incorporation and post synthesis leaching of PEG1500 is performed simply by washing the beads with warm water. The detail study on dye adsorption behavior along with its kinetic behavior is performed to elucidate the role of GO, GQD and Fe3O4 nanostructures in PVA/CMC-B hydrogel. Also the effect of PEG-1500 leaching in PVA/CMC-B@GO/Fe3O4/GQD (L) nanocomposite hydrogel is also investigated towards the adsorption of dye pollutants.

2. Experimental section 2.1. Materials 2.1.1. Carboxymethyl cellulose (Na-CMC) (DS N 0.4) powder (viscosity: ~1500 ± 4000 cPs@ 1%, aq., 20 °C) as well as PEG-1500 (Mw ~ 1400–1600, M. P ~ 42–48 °C; viscosity: 6–9 cPs) were obtained from CDH, Central drug house (P) Ltd., New Delhi. On the other hand, polyvinyl alcohol (PVA) with degree of hydrolysis 86–89%, was procured from Loba Chemie (India), while; orthoboric acid (H3BO3) [pH ~ 3.8–4 for 3.3% (aq.)] was purchased from CDH, Central drug house (P) Ltd., New Delhi. Citric acid monohydrate (N99%, pH ~ 1.8 at 50 g·L−1 at 20 °C) and graphite powder (particle size ~60 mesh) were obtained from Loba Chemie (India). Salts like FeCl3.6H2O (AR, 98%) and FeCl2.4H2O (AR, 99%), were procured from Loba Chemie, Mumbai, India. Other reagents, like NaOH, NH3, were of analytical grade and used as such. All solutions were prepared with double distilled water. 2.2. Synthesis of graphene oxide/Fe3O4/graphene quantum dot (GO/Fe3O4/ GQD) nanohybrid Graphene oxide/Fe3O4/graphene quantum dot (GO/Fe3O4/GQD) nanohybrid system was prepared by simple approach of mixing all the colloidal dispersions in aqueous phase and then destabilizing the mixed colloidal state by means of ethanol and hydrochloric acid (EtOH.HCl) as reported by Tripathi et al. [61]. Graphene oxide was synthesized by the modified Hummer's method reported earlier (S1) [62], while; GQD preparation was performed by earlier literature of Dong et al. (S2) [63]. GQD formation was confirmed through UV–visible, photoluminescence and life-time measurement [Fig. S1]. On the other hand, nano-Fe3O4 was synthesized through co-precipitation approach (S3) [64]. Initially, 50 mL of GO solution [0.02 g/20 mL] was mixed with 50 mL of nanostructural Fe3O4 dispersion (0.05 g) via ultrasonication for 1 h. Then 20 mL of the as-prepared GQD solution (Quantum yield: 12%) was added to the solution mixture and allowed for another 1 h ultrasonication to homogenize the solution. Afterward the mixed colloidal dispersion was made to destabilize with addition of ethanol and hydrochloric acid (EtOH.HCl)[5 mL of absolute ethanol (EtOH) (99.9%) and 2 mL of HCl (35%, 1 L = 1.16 kg)] to precipitate out the GO/Fe3O4/GQD nanohybrid from the water medium by means of centrifugation (8000 rpm). The total synthesis of GO/Fe3O4/GQD nanohybrid is shown in Scheme S1. The same synthetic protocol is adopted for the preparation of GO/Fe3O4 nanohybrid with exclusion of GQDs (aq.) step. 2.3. Synthesis of macroporous CMC/PVA@GO/Fe3O4/GQD nanocomposite hydrogels Porous carboxymethyl cellulose interpenetrated polyvinyl alcohol (CMC/PVA) based nanocomposite hydrogel was prepared with incorporation of ternary GO/Fe3O4/GQD nanohybrid filler during ionically crosslinking with boric acid (H3BO3). In present investigation, polyethylene glycol (PEG-1500) was used as pore forming reagent. In a typical procedure, first aqueous solutions of CMC, PVA as well as pore forming polymer, PEG were prepared as per their solubility criterion. 2.5 g of polyvinyl alcohol (PVA) powder was dissolved in 50 mL of double distilled water in hot condition (~90 °C) with vigorous stirring. After dissolution of PVA granules, solution was allowed to come at room temperature (~30 °C) for transparent PVA solution. CMC solution was prepared with dissolution of 0.25 g in 50 mL of double distilled with stirring for 30 min, while PEG-1500 solution was formed with 0.05 g in 50 mL of double distilled water. The final solution blend for bead was prepared with mixing of PVA, CMC and PEG solutions in the volume ratio of 10: 5: 1. The solution blend was then stirred well at 600 rpm at 30 °C for 1 h to assure interpenetration of each polymeric network.

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In next step, 10 mL of aqueous phase dispersed hybrid nanofillers (GO/ Fe3O4/GQD) of different weight percentages were added to the hybrid polymeric solution blend and then subjected to probe ultrasonication for 30 min for proper dispersion of nanostructural phase in interpenetrating hybrid polymeric network (IHPN) of PVA/CMC/PEG. Then the whole solution mixture was kept at refrigerator for 1 h for cooling (b25 °C). Afterward, 1 mL of the prepared solution mixture was taken in a 2 mL Eppendorf tube along with 1 mL of saturated H3BO3 solution followed by vigorous shaking to make the round shaped blackish beads of CMC/PVA/PEG@GO/Fe3O4/GQD nanocomposite hydrogel. To prepare porous CMC/PVA@GO/Fe3O4/GQD nanocomposite hydrogel from CMC/PVA/PEG@GO/Fe3O4/GQD nanocomposite hydrogel, extraction of pore forming PEG was performed through aqueous phase swelling of the dried CMC/PVA/PEG@GO/Fe3O4/GQD nanocomposite hydrogel beads at 50 °C for 5 h. Preparation of CMC/PVA @GO/Fe3O4 nanocomposite beads were performed through the same procedure with addition GO/Fe3O4instead of GO/Fe3O4/GQD nanostructural phase, whereas porous CMC/PVA hydrogel beads were prepared in same analogy without the addition of nanostructural phase. All hydrogels and nanocomposite hydrogels were produced as round shaped beads. The total synthesis of nanocomposite hydrogel is shown in Scheme 1. 2.4. Techniques used for characterization The surface microstructures of pure CMC, as-synthesized boric acid cross-linked PVA/CMC/PEG-B, PVA/CMC-B (L) hydrogels, as well as PVA/CMC-B@GO/Fe 3 O 4 (L) and PVA/CMC-B@GO/Fe 3 O 4 / GQD (L) nanocomposite hydrogels are explored by field emission scanning electron microscope (FESEM) from Jeol Ltd., Japan (JSM6700F).The distribution of nanostructural phases like GO, nanoFe 3O 4 and GQDs in their tri-component hybrid system, GO/Fe3 O4 / GQD was identified with Tec-nai 12, Phillips high resolution transmission electron microscope (HRTEM) operating at 120 kV. HRTEM analysis was also performed for PVA/CMC-B@GO/Fe 3 O 4 (L) and PVA/CMC-B@GO/Fe3O4/GQD (L) nanocomposite hydrogels to allocate the distribution and orientation of hybrid nanostructures like GO/Fe3O4 and GO/Fe 3O4/GQD, respectively. FTIR spectra of synthesized PVA/CMC/PEG-B@GO/Fe 3 O 4 /GQD and PVA/CMC-B@GO/

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Fe 3 O 4 /GQD (L) nanocomposite hydrogel along with their polymeric/nanostructural components were obtained through the FTIR spectrometer (model: Eco-ATR, Alpha, Bruker Optik GmbH, Ettlingen, Germany) equipped with an attenuated total reflectance (ATR) accessory was used to take the spectra of samples. FTIR spectra were recorded in dispersive mode over a range of 500–4000 cm −1 at a resolution of 4 cm −1 (16 scans/trace). X-ray diffraction (XRD) patterns of PVA/CMC-B@GO/Fe 3 O 4 /GQD (L) nanocomposite hydrogel with its constituents was compared through X-ray diffractometer (model: Smart Lab, Rigaku Corporation, Japan) operated at a voltage of 40 kV and current of 30 mA with a Cu-Kα1 (λ = 1.54 Å) radiation source. The thermogravimetric analysis of the prepared PVA/CMC-B@GO/Fe 3 O 4 /GQD (L) nanocomposite hydrogel was performed under nitrogen purge and heating cycle of 10°/min, using a TGA apparatus (model: DTG-60, Shimadzu Corporation, Japan). The surface topology of PVA/CMC/PEG-B@GO/Fe 3 O 4 /GQD and PVA/CMC-B@GO/Fe 3 O 4 / GQD (L) nanocomposite hydrogels were examined through atomic force microscopy (AFM) [Model: Pico plus 5500 ILM AFM with a piezo scanner having maximum range of 100 μm]. The rheological properties of PVA/CMC-B@GO/Fe 3 O 4 /GQD nanocomposite hydrogel along with its pure hydrogel phase were investigated using a rheometer (Anton Paar modular compact rheometer: MCR 102) with a parallel plate geometry. The diameter of the parallel plate is 40 mm. The rheometer was connected with a Peltier circular thermo cube for accurate control of temperature. A spacing of approximately 0.8 mm was used for all measurements. The storage (G/ ) and loss modulus (G //) of the samples were recorded in their linear viscoelastic region at a strain of γ = 2% and a frequency range of 0.1–100 rad·s −1 . The magnetic properties of pure nanoFe3O 4 and PVA/CMC-B@GO/Fe3O4 /GQD (L) nanocomposite hydrogel were determined by vibrating sample magnetometer (VSM, Lakeshore, Model 7300 series). All Absorption spectra regarding dye removal experiments were obtained using a Hitachi U-3010 double beam UV–Visible spectrophotometer. These results were averaged values of three runs with an error of ±4.0%. A digital pH meter ELICO (LI 120) was used to measure and adjust the pH of different solution. RIVOTEK incubator was used to maintain temperatures at different values in order to verify the temperature

Scheme 1. Synthesis of CMC/PVA-B@GO/Fe3O4/GQD (L) nanocomposite hydrogels.

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dependency of the adsorption process. The swelling and dye removal experiments were performed as per S4 and S5 of supporting information. 3. Results and discussion 3.1. Electron microscopic analysis As the present context deals with the two step synthetic approach; initially, the preparation of GO/Fe3O4/GQD nanohybrid filler and then, in situ incorporation of this filler to PVA based hybrid hydrogel, it is very essential to justify the proper formation of hybrid nanofillers by means of TEM in prior to examine the morphological changes of the prepared PVA based hybrid nanocomposite hydrogel. Although, we have used dilute acidic condition (pH ~ 4.5) in HCl:EtOH treatment, but it is mandate to confirm the structure of nano-Fe3O4 in the prepared GO/ Fe3O4 and GO/Fe3O4/GQD nanohybrids as nano-Fe3O4 are not stable in strong acidic environment. On the other hand, as both, GO and GQD have the almost same chemical structures, it is worthy to investigate the change in their orientation and attachment before and after the application of HCl:EtOH treatment. Fig. S2a shows the aqueous phase mixtures of nanostructural GO and GQDs at pH ~ 7.0 where, they are observed to exist as separate phase without any significant attachment in between them. Moreover, the as-prepared GQDs are also noticed to be in high dispersibility due to acquiring long distance in solution phase. This fact actually suggests the same surface polarity of GO and GQDs at pH ~ 7.0 which opposes each other to interact well and also to form the GO/GQD nanohybrid phase. The optimized condition for obtaining GO/GQD nanohybrid phase (HCl: EtOH treatment) is primarily verified with pure GO. As revealed in Fig. S2-b, pure GO at this optimized condition shows the rosette type pattern due to self-assembly in layer architecture. When this condition is applied to the aqueous phase mixture of GO and GQDs, small sized ensemble of GO/GQD nanohybrid is appeared which is found to

grow over the time (30 min, 6 h and 24 h) (Fig. S2-c, d & e) and acquire larger architecture like rosette pattern of GO with incorporating ultrasmall GQDs on its surface. As the distinct presence of GQDs is not clearly viewed in aqueous phase treatment, TEM analysis of GO/GQD nanohybrid is performed with chloroform after drying. As shown in Fig. S2-f, TEM micrograph of GO/GQD nanohybrid reveal the clear presence of GQDs on the layer structure of GO. Fig. 1a–c depict the TEM/HRTEM view of pure GO, Pure GQD and nano-Fe3O4, while the as-prepared GO/GQD, GO/Fe3O4 and GO/Fe3O4/ GQD nanohybrids are shown in Fig. 1d–f, respectively. As expected, TEM micrographs of nanohybrids are found to contain the GO entrapped nano-Fe3O4 for GO/Fe3O4 nanohybrid and GQD decorated nano-Fe3O4 containing GO sheets for GO/Fe3O4/GQD nanohybrid. This indicates the successful preparation of GO/Fe3O4/GQD nanohybrid in aqueous phase via HCl: EtOH treatment which is supposed to happen due to minimizing the repelling surface polarity between GO and GQDs with adequate protonation by HCl as well as formation of hydrogen bonding with the added ethanol as compared to water molecules. These promote the instability to the state of as-prepared colloidal GO, where GO sheets are in ionic equilibrium due to protonated and deprotonated structures in aqueous medium at pH ~ 7.0 and help to coagulate them from the aqueous medium. After ensuring the preparation of hybrid nanofillers (GO/Fe3O4 and GO/Fe3O4/GQD) via TEM, the effects of these nanostructural hybrid fillers to the morphology of PVA/CMC/PEG-B hydrogel structure is encountered via SEM. Moreover, the additional changes in the morphology of nanocomposite hydrogels due to leaching of PEG via swelling treatment are also crucially examined and interpreted. As displayed in Fig. 2a–c, the cross-sectional scanning electron micrographs of boric acid cross-linked PVA/CMC-B hydrogel bead depict the three dimensional porous morphology with varying pore geometry and sizes (100 nm to 1 μm). Moreover, the irregularly distributed porous architectures are observed to be inter-connected in nature and separated by thin fibrous polymeric chains of polyvinyl alcohol (PVA).

Fig. 1. TEM micrographs of (a) pure GO; (b) Pure GQDs; (c) Pure nano-Fe3O4; (d) GO/GQD after HCl:EtOH treatment (e) GO/Fe3O4 after HCl:EtOH treatment (f) GO/Fe3O4/GQD after HCl: EtOH treatment (N.B: All samples after drying are dispersed in chloroform to check TEM).

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Fig. 2. SEM micrographs of (a) PVA/CMC/PEG-B hydrogel; (b) Zoomed view of region (A) of PVA/CMC/PEG-B hydrogel; (c) Zoomed view of region (B) of PVA/CMC/PEG-B hydrogel; (d), (e) PVA/CMC-B (L) hydrogel after leaching of PEG and zone (C) of PVA/CMC-B (L) hydrogel with higher magnification; (f), (g) PVA/CMC/PEG–B@GO/Fe3O4 nanocomposite hydrogel with different magnifications: (h) PVA/CMC–B@GO/Fe3O4 (L) nanocomposite hydrogel after leaching of PEG; (i) PVA/CMC/PEG-B@GO/Fe3O4/GQD nanocomposite hydrogel; (j), (k), (l) PEG leached PVA/CMC-B@GO/Fe3O4/GQD (L) nanocomposite hydrogel of different regions.

On the other hand, used CMC in its pure state shows the spindle like morphology which is consistent with earlier literature by Klinpituksa et al. [65]. On a closer look to the zone, labeled as (A) in Fig. 2b, the surface of the pore wall is distinctly appeared as rough with disturbing the regular structural pattern of PVA due to covalent attachment of boric acid to PVA. In addition to this, effective physical entanglements of carboxymethyl cellulose (CMC) and PEG chins within boric acid linked PVA network can also be accounted for the surface roughness of the hybrid hydrogel. More interestingly, the cross-sectional surface of the hybrid PVA/CMC/PEG-B hydrogel bead is found to contain both, interconnected open pores (Fig. 2b) and closed pores [Fig. 2c; zoomed

zone as labeled (B)] which may be beneficial for their swelling capacity. After leaching of PEG from the hybrid network of PVA/CMC-B (L), the surface of the hydrogel is observed to develop the macro-porous architecture (Fig. 2d & e) along with more surface roughness as compared to PVA/CMC/PEG-B hydrogel bead due to random exclusion of PEG from the cross-linked hybrid network. The most important thing is observed for the surface of PVA/CMC/ PEG-B@GO/Fe3O4 nanocomposite hydrogel which eventually appeared as less porous with layer coverage of nano-Fe3O4 entrapped GO sheets (Fig. 2f), while its leached state [PVA/CMC-B@GO/Fe3O4 (L)] again shows the macroporous morphology with simultaneous existence of

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layered GO and nano-Fe3O4(Fig. 2g & h). Actually, the less porous morphology of PVA/CMC/PEG-B@GO/Fe3O4 nanocomposite hydrogel may be attributed from the effective cross-linking behavior of GO with hybrid polymeric matrix (PVA/CMC/PEG-B). This fact is found to be more dominant as observed from the SEM micrographs (Fig. 2j) of PVA/ CMC/PEG-B@GO/Fe3O4/GQD nanocomposite hydrogel due to synergistic contribution of layered GO and ultra-small sized GQDs in forming more compact structure with hybrid polymeric matrix (PVA/CMC/PEG). In the same manner, after leaching of PEG from PVA/CMC/PEG-B@GO/ Fe3O4/GQD nanocomposite hydrogel network, it also reveals the macroporous architecture (Fig. 2j–l) with presence of GO/Fe3O4/GQD hybrid nanostructures. Although the co-existence of GO and nano-Fe3O4 is prominent in hybrid nanostructure (GO/Fe3O4/GQD), but the distinct appearance of ultrathin GQD in the nanocomposite hydrogel is not clear which require further analysis with transmission electron microscopy (TEM). TEM micrographs (Fig. 3a & b) of PEG leached PVA/CMC-B@Fe3O4 (L) nanocomposite hydrogel exhibits the existence of light phases PVA chains along with spindle- shaped carboxymethyl cellulose (CMC). Moreover, the hybrid polymer entrapped nano- Fe3O4 with porous background is also observed in Fig. 3-b along with as desired crystalline

domain of nano-Fe3O4 (Fig. 3c). The selected area electron (SAED) diffraction pattern reveals all the crystal planes like (111), (220), (311), (400), (422) of pure nano-Fe3O4 even after their incorporation in PVA/ CMC-B hydrogel. On the other side, TEM micrographs (Fig. 3d & e) of PVA/CMC-B@ GO/Fe3O4 (L) nanocomposite hydrogel depicts the nano-Fe3O4 embedded GO sheets in a network arrangement which in turn indicates the successful incorporation of GO/Fe3O4 hybrid in porous hydrogel network and the result is well communicated with the SEM micrographs. Also the SAED pattern of PVA/CMC-B@GO/Fe3O4 (L) nanocomposite hydrogel reveals the dominated crystalline phases of nano-Fe3O4 (Fig. 3f) over GO. Moreover; the uniform distribution of GQDs in hydrogel matrix along with GO layer and Fe3O4 nanostructure is distinctly observed from the TEM images (Fig. 3-g and h) which clarify the exposure of GQD on the porous gel network, instead of being wrapped by the gel network. Like PVA/CMC-B@GO/Fe3O4 (L) nanocomposite hydrogel, PVA/ CMC-B@GO/Fe3O4/GQD (L) nanocomposite hydrogel also shows the as usual crystalline domains of nano-Fe3O4 along with other strong spots in SAED pattern (Fig. 3i), related to the strong interfacial interaction GO, Fe3O4 and GQDs.

Fig. 3. TEM micrographs of (a), (b) PVA/CMC-B/Fe3O4 (L) nanocomposite hydrogel with different magnifications; (c) selected area electron diffraction (SAED) pattern of PVA/CMC-B/Fe3O4 (L) nanocomposite hydrogel; (d), (e) PVA/CMC–B@GO/Fe3O4 (L) nanocomposite hydrogel of different areas along with different magnification; (f) selected area electron diffraction (SAED) pattern of PVA/CMC–B@GO/Fe3O4 (L) nanocomposite hydrogel; (g), (h) PVA/CMC-B@GO/Fe3O4/GQD (L) nanocomposite hydrogel of different regions and magnification; (i) selected area electron diffraction (SAED) pattern of PVA/CMC-B@GO/Fe3O4/GQD (L) nanocomposite hydrogel.

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3.2. FTIR analysis The structural bonding in boric acid cross-linked magnetic PVA/ CMC-B/Fe3O4 nanocomposite hydrogel as well as other nanocomposite hydrogel like PVA/CMC-B@GO/Fe3O4 and PVA/CMC-B@GO/Fe3O4/GQD are explored in terms of their FTIR spectra with detail comparison of their pure polymeric components (PVA, CMC, PEG) (Fig. 4a) and pure nanostructural filler (GO, GQD, Fe3O4) phases (Fig. 4b). Moreover, the change in chemical/physical bonding of PVA/CMC based hybrid hydrogel bead before and after the leaching procedure is also examined through the FTIR spectroscopy of PVA/CMC/PEG-B (Fig. 4a) and PVA/ CMC-B hydrogels. The prime components of as-prepared hybrid hydrogel bead are polyvinyl alcohol (PVA) and carboxymethyl cellulose (CMC). PVA in its pure form shows the strong existence of inter or intra-molecular hydrogen bonded –OH functionality (broad band@ῡ ~ 3379–3255 cm−1) [66,67] along with less predominant non-bonded –OH functionality (ῡ ~ 3662–3590 cm−1). In addition to this, FTIR spectrum of pure PVA demonstrates the presence of asymmetric vibrational mode of NCH2 stretching (ῡ ~ 2917 cm−1), NC=O stretching (ῡ ~ 1726 cm−1) of remaining acetate group, NCH2 bending (ῡ ~ 1422 cm−1), NCH2 wagging (ῡ ~ 1371 cm−1) [68], -C-H wagging (ῡ ~ 1241 cm−1), C\\O stretching (ῡ ~ 1081 cm−1) and C\\C stretching (ῡ ~ 839 cm−1). Most importantly, the distinct FTIR peak of pure PVA at 1142 cm−1 is used to confirm the atactic nature of semicrystalline PVA [69]. On the other hand, the other important component of PVA hybrid hydrogel bead, carboxymethyl cellulose (CMC) in its pure state displays the-OH functionality at 3292 cm−1, alkyl C\\H stretching (ῡ ~ 2903 cm−1) and bending (ῡ ~ 1316 cm−1) vibrations. As obvious the presence of carboxylate (-COO−) groups on CMC backbone is assigned through the FTIR peaks at 1579 cm−1 (asymmetric stretching) and 1411 cm−1 (symmetric stretching). The band at 1018 cm−1 is due to the asymmetric skeletal vibration of C-O-C bond (β-1,4-glycosidic bond) of CMC. We also incorporate another polymeric component into the PVA/CMC based hybrid hydrogel as pore forming agent which is polyethylene glycol (PEG) and shows various FTIR bands including O\\H stretching (3460 cm−1), C\\H stretching (2889 cm−1), CH2scissoring (1470 cm−1), C\\H bending (1341 cm−1), C\\O stretching (1097 cm−1) and –OH bending (957 cm−1). When we blended the above mentioned three polymeric components and allowed to cross-link with use of boric acid, the resultant hydrogel bead of PVA/CMC/PEG-B exhibits the FTIR spectrum with demonstrating the presence of inter/intra molecularly hydrogen bonded –OH groups at 3402 cm−1 which is red shifted as compared to PEG but blue shifted as compared to pure PVA and CMC. Therefore, strong interactions via hydrogen bonding with three polymeric

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components; PVA, CMC and PEG can be interpreted along with the reduction of oxygen functionality of PVA due to reaction with boric acid. The alkyl C\\H stretching vibration of PVA/CMC/PEG-B is noticed at 2866 cm−1 along with the FTIR bands at 1347 cm−1, 1243 cm−1, 1094 cm−1 and 943 cm−1, related to the characteristics IR modes of macromolecular PEG. Moreover, the FTIR spectrum of PVA/CMC/PEG-B (Fig. 4a) reveals the existence of NC=O group (ῡ ~ 1724 cm−1) of PVA along with the stretching vibration of COO– group of CMC. PVA/CMC/ PEG-B also shows the two new peaks at 1418 cm−1 and 1330 cm−1, related to the asymmetric stretching relaxation of B-O-C bond, while the FTIR band at 657 cm−1 is appeared due to O-B-O bending. This reflects the introduction of boronic structure to the hybrid gel network and formation of chemical linkages with –OH groups of PVA. This result is in accordance with the earlier reports during preparation of borax crosslinked PVA/nanocellulose hybrid foam by Hu et al. [70] and boric acid cross-linked PVA/GO-B nanocomposite hydrogel by Huang et al. [71]. The FTIR spectrum of PVA/CMC- B/Fe3O4 (L) represents the internal chemical structure of PVA/CMC-B/Fe3O4 hybrid hydrogel after the leaching of PEG macromolecules (Fig. 4c). As obvious, the distinct FTIR bands corresponding to the presence of PEG are not seen in the FTIR spectrum of PVA/CMC- B (L) hydrogel and indicate the effective removal of PEG from the hybrid hydrogel network with treatment of water medium for 5 h. But the presence of stretching vibration of NC= O group (ῡ ~ 1726 cm−1), asymmetric (ῡ ~ 1579 cm−1) and symmetric stretching mode (ῡ ~ 1411 cm−1) of COO– group as well as the presence of stretching vibration of Fe\\O bond (ῡ ~ 530 cm−1) confirm the borate cross-linked compact structure of nano- Fe3O4 incorporated PVA/CMC hydrogel bead. In PVA/CMC- B (L) hydrogel, the Fe\\O bond is red shifted as compared to pure nano-Fe3O4 (ῡ ~ 560 cm−1) due to strong interfacial adhesion between hybrid PVA/CMC polymer and nanoFe3O4. In a next note, the other two nanocomposite hydrogels, PVA/ CMC-B@GO/Fe3O4 and PVA/CMC-B@GO/Fe3O4/GQD have been prepared with incorporation of two hybrid fillers, GO/Fe3O4 and GO/ Fe3O4/GQD and therefore, require the FTIR analysis of the hybrid fillers (GO/Fe3O4 and GO/Fe3O4/GQD) first and then, these hybrid fillers incorporated nanocomposite hydrogel heads. In case of GO/Fe3O4 nanohybrid filler, the FTIR spectrum reveals the same characteristics peaks of GO without much alteration in the wavenumber along [Hbonded –OH groups (ῡ ~ 3424 cm−1), NC=O (ῡ ~ 1730 cm−1), -C-OH (ῡ ~ 1390 cm−1), alkoxy –C-O (ῡ ~ 1090 cm−1), epoxy C\\O (ῡ ~ 1269 cm−1), -C=C- (ῡ ~ 1634 cm−1), Stretching C\\H (ῡ ~ 2933 cm−1 for asymmetric and 2860 cm−1 for symmetric)] with the appearance of Fe\\O band which is found to be red shifted to 535 cm−1 as compared to pure phase of the nanostructural Fe3O4 due to coating effect. Interestingly, the broad band of pure GO in the region of 3300–3500 cm−1 becomes sharper in the GO/Fe3O4 nanohybrid due

Fig. 4. FTIR spectra of (a) CMC, PVA, PEG and PVA/CMC/PEG-B hydrogel; (b) GO, GQD, nano-Fe3O4, GO/Fe3O4 and GO/Fe3O4/GQD phases; (c)PVA/CMC-B/Fe3O4(L), PVA/CMC-B@GO/Fe3O4 (L) and PVA/CMC-B@GO/Fe3O4/GQD (L) nanocomposite hydrogel (L stands for the leaching of PEG).

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to the inhibition of inter-molecular hydrogen bonding between different GO layers as a result of effective incorporation of nano-Fe3O4 on to layer of graphene oxide. The citric acid (CA) derived graphene quantum dot shows the highly broad peak of inter/intra-molecular hydrogen bonded –OH groups at around ~3387 cm−1, while the asymmetric and symmetric –COO– groups of graphene quantum dot is prevailed with the appearance of FTIR bands at 1611 cm−1 and 1440 cm−1. Moreover, the as-obtained GQD shows the FTIR peak of NC=O group at 1733 cm−1 along with the existence of C-OH group (1176 cm−1) of unreacted citric acid (CA) residues. On the other hand, GO/Fe3O4/GQD tri-component nanohybrid filler, the FTIR band of –OH functionality becomes boarder due to the enhanced hydrogen bonding interactions with incorporation of smallsized GQDs on to the surface of nano-Fe3O4 trapped graphene oxide layers. The FTIR spectrum of GO/Fe3O4/GQD shows the simultaneous presence of GO and GQD with their characteristics peaks as well as red-shifted Fe\\O band (524 cm−1). In the FTIR spectrum of PVA/CMC-B@GO/Fe3O4 (L) nanocomposite hydrogel, the –OH group is appeared to be red shifted at 3328 cm−1 as compared to PVA/CMC-B/Fe3O4 (L) hydrogel and can be accounted as extensive hydrogen bonding between PVA/CMC hybrid polymeric matrix and graphene oxide. Moreover, the FTIR band for Fe\\O bond is also red shifted at 528 cm−1 for this nanocomposite hydrogel due to strong interaction of nano-Fe3O4 with GO and PVA/CMC matrix. The characteristics FTIR bands of GO are also prevailed with some overlapping in the FTIR spectrum of PVA/CMC-B@GO/Fe3O4 (L) nanocomposite hydrogel, but with low intensity due lower content of GO/Fe3O4 nanohybrid filler. For PVA/CMC-B@GO/Fe3O4/GQD (L) nanocomposite hydrogel, the –OH band is further red shifted to 3314 cm−1 because of the enhancement in oxygen functionality with incorporation of ultra-small sized GQDs as GO/Fe3O4/GQD hybrid filler. Moreover, the FTIR spectrum of PVA/CMCB@GO/Fe3O4/GQD (L) is found to contain the characteristics peaks of GQDs along with the red shifted Fe\\O bond at 526 cm−1. Therefore, the resultant final PVA/CMC-B@GO/Fe3O4/GQD (L) nanocomposite hydrogel exhibits the existence of PVA as boric acid cross-linked hydrogel structure along with CMC as interpenetrating network. Moreover, the overall analysis reveals the effective incorporation of GO, GQD and nano-Fe3O4 in the hybrid hydrogel structure. 3.3. XRD analysis X-ray diffraction pattern of as-synthesized PVA/CMC-B@GO/Fe3O4/ GQD (L) nanocomposite hydrogel is also explored in Fig. 5 and analyzed well by detail comparison with the XRD patterns of PVA/CMC-B and PVA/CMC/PEG-B hydrogel along with, PVA/CMC-B@GO/GQD

(L) nanocomposite hydrogel. Moreover, the XRD patterns of pure polymeric phases (PVA and CMC) and nanostructural filler components (GO, GQD, Fe3O4) are also interpreted to elucidate the role of hybrid nanofillers (GO/Fe3O4/GQD) in governing the crystalline behavior of PVA/CMC-B@GO/Fe3O4/GQD (L) nanocomposite hydrogel. As observed from Fig. 5a, pure PVA depicts the semicrystalline attitude with two diffraction peaks at 2θ values of 19.68° and 22.54°, related to the (101) and (200) crystallographic planes, respectively of orthorhombic lattice structure which is highly consistent with reported literature by Han et al. [72]. This type of semicrystalline behavior in PVA is basically developed due to intra/inter-molecular hydrogen bonding interactions. On the other hand, carboxymethyl cellulose (CMC) displays the cellulose type II supramolecular structure with assigning of a semicrystalline peak at 2θ value of 20.6° [73]. When these two polymeric components (PVA, CMC) are transformed into IPN structure of PVA/CMC-B hydrogel with reaction of boric acid, the resultant diffraction pattern shows a decrement in the intensity of (101) plane of PVA. This decrease in crystallinity of PVA-CMCB can be assigned as the small degree of crystallinity coupled with a large amount of diffuse scattering, related to the dominant presence of amorphous phase in the hydrogel structure. This in turn reflects the strong complexation between boric acid and PVA which destroys the organized structural pattern of PVA. As confirmed from the FTIR spectra, complexation between PVA and boric acid is happened due to covalent bond formation between –OH functionalities of PVA and boric acid. Similar type of result is also observed by Han et al. [72] during XRD analysis of cellulose nanoparticle incorporated PVA-borax hydrogel. However, PEG incorporated PVA/CMC-B (PVA/CMC/PEG-B) represents some short of crystalline phases of PEG along with the as usual expected broad diffraction pattern. As revealed by Fig. 5a, pure PEG like earlier report [74], shows major diffraction peaks at 2θ values of 19.31°, 23.32° and a few minor peaks at: 2θ values of 26.40°, 36.28°, 39.74° and 45.22°, related to its intense crystalline characteristics. The crystalline phases of PEG in PVA/CMC/PEG-B hydrogel are appeared with diffraction peaks at 2θ values of 15.9°, 25.49° and 26.75° due to effective interpenetration as well as different structural arrangement of PEG in hydrogel system. Interestingly observed that PVA/CMC-B@GO/GQD (L) and PVA/CMC-B@GO/Fe3O4/ GQD (L) nanocomposite hydrogels does not appear with the crystalline phases of PEG, indicating the complete removal of PEG-1500 from the hydrogel network to promote macro porosity. Moreover, in case of PVA/CMC-B@GO/GQD (L) nanocomposite hydrogel, the amorphous behavior of PVA/CMC-B hydrogel is appeared to be more prominent due to high cross-linking attitude of GO and ultra-small GQD with their oxygenous surface functionalities. As PVA/CMC-B@GO/Fe3O4/GQD

Fig. 5. XRD pattern of pure PVA, CMC and PEG-1500 (inset) (a); GO, Fe3O4/GQD (inset), nano-Fe3O4 and GO/Fe3O4/GQD (b); PVA/CMC-B, PVA/CMC/PEG-B, PVA/CMC-B@GO/GQD (L) and PVA/CMC-B@GO/Fe3O4/GQD (L) (c).

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(L) nanocomposite hydrogel is designed with incorporation of nanohybrid filler, GO/Fe3O4/GQD instead of separate incorporation of GO, GQD and nano-Fe3O4, it is very essential to investigate the crystalline behavior of GO/Fe3O4/GQD with respect to its pure nanostructural phases (GO, Fe3O4 and GQD) prior to the detail XRD analysis of the PVA/CMC-B@GO/Fe3O4/GQD (L) nanocomposite hydrogel. As illustrated in Fig. 5b, pure nano-Fe3O4 demonstrates the cubic spinel crystal structure with highly crystalline diffraction pattern (JCPDS:19-0629) where, distinguished peaks are appeared at 2θ values of 18.2°, 30.1°, 35.25°,43.18°,53.63°, 57.18°, 62.72°and 74.09° and related to the (111), (220), (311), (400), (422), (511), (400) and (533) crystal planes, respectively. When these nanostructural-Fe3O4 particles are dispersed in the as-prepared GO suspension, characterized with XRD peak at 2θ value of 10.1° (d spacing ~ 0.88 nm) along with GQD solution (broad diffraction pattern at 2θ ~ 27.2°) and then coagulated with dilute acid: ethanol treatment to get the GO/GQD/Fe3O4, the obtained XRD pattern of this nanohybrid reflects the characteristic crystalline phases of nano-Fe3O4 along with the disappearance of characteristic GO peak, indicating the exfoliated structure of GO sheets with entrapped nano-Fe3O4 and GQDs. As expected the XRD pattern of PVA/CMC-B@GO/Fe3O4/GQD (L) nanocomposite hydrogel (Fig. 5c) shows distinct amorphous behavior along with crystalline phases of GO/GQD/Fe3O4 nanohybrid and therefore, indicating the successful incorporation of exfoliate GO based Fe3O4/GQD nanohybrid into the hydrogel structure of PEG leached PVA/CMC-B hydrogel. The

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improvement in crystalline phases of porous nanocomposite hydrogel also leads to the significant enhancement of thermal stability [S6; Fig. S3]. 3.4. AFM analysis Atomic force microscopic (AFM) tool is used to evaluate the surface roughness of PVA-B based hydrogels [Fig. S4-a & c] and nanocomposite hydrogels [Fig. S5-a & c and Fig. 6-a & c]. Effect of other polymeric networks (CMC and PEG) to the surface roughness of PVA-B based hydrogels/nanocomposite hydrogel is also explored. Moreover, the influence of bi-nanostructural (GO/Fe3O4) and tri-nanostructural (GO/ Fe3O4/GQD) fillers are also interpreted to the topographical changes of PVA-B based nanocomposite hydrogels. In addition to this, the surface roughness characteristics of the final PEG leached PVA/CMC-B@GO/ Fe3O4 (L) and PVA/CMC-B@GO/Fe3O4/GQD (L) nanocomposite hydrogels are also analyzed in detail. As depicted in Fig. S4a, the surface topography of boric acid cross-linked PVA hydrogel (PVA\\B) bead shows the “trough and crest” pattern with the realization of shallow pores in covalently cross-linked hydrogel structure. The measured average roughness of PVA-B hydrogel is found to be 3.22 nm. The height distribution profile of as prepared PVA-B hydrogel (Fig. S4b) depicts the variation of height from −20 nm, negative regime to +12 nm, positive regime. The surface roughness is found to be significantly improved (~29.74 nm) (Fig. S4c and d) with formation of PVA/CMC/PEG-B

Fig. 6. 3-D AFM micrograph of (a) PVA/CMC/PEG-B@GO/Fe3O4/GQD nanocomposite hydrogel along with (b) height histogram profile; (c) 3-D AFM micrograph of PVA/CMC-B@GO/Fe3O4/ GQD (L) nanocomposite hydrogel along with (d) height histogram profile.

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hydrogel structure due simultaneous interpenetration of both, CMC and PEG chains into the boric acid linked PVA network. On the other hand, with incorporation of nanostructural GO/Fe3O4 filler into the PVA/CMC/PEG-B hydrogel, the surface topography is appeared to be highly rough with presence of more elevated peaks towards Z-direction. This fact is simply raised from the strong interfacial adhesion between nano-Fe3O4 incorporated GO and polymeric chains of PVA/CMC/PEG-B hydrogel network. Moreover, the height distribution profile of PVA/CMC/PEG-B@GO/Fe3O4 nanocomposite hydrogel (Fig. S5b) is found to be more inclined towards the positive regime (0 to +300 nm), confirming a significant decrease in porous morphology with layer coverage of GO/Fe3O4 hybrid nanostructures. The strong attachment of nano-Fe3O4 onto the surface of GO makes the ultimate contribution to make the surface more rough in comparison to PVA/CMC/ PEG-B hydrogel. The software processed average roughness for PVA/ CMC/PEG-B@GO/Fe3O4 nanocomposite hydrogel is obtained as 126.9 nm. Further; interesting fact is noticed for PEG leached PVA/ CMC-B@GO/Fe3O4 (L)nanocomposite hydrogel, where topographical view leads to the realization of void spaces (deep blue region) along with nanostructural roughness and therefore, confirming the formation of highly porous structure with leaching of PEG from the gel network. The average surface roughness of PVA/CMC-B@GO/Fe3O4 (L) nanocomposite hydrogel (Fig. S5c) is measured as 133.3 nm. This fact is also in accordance with the height distribution profile of PVA/ CMC-B@GO/Fe3O4 (L) nanocomposite hydrogel (Fig. S5d), where height profile is well distributed from the negative regime to positive regime (−400 nm to +400 nm). When we incorporated tri-nanostructural hybrid (GO/Fe3O4/GQD) into the hybrid hydrogel structure of PVA/CMC/ PEG-B bead, the surface roughness is observed to be more enhanced (~139.7 nm) (Fig. 6a) as compared to bi-nanostructural hybrid (GO/ Fe3O4) due to combined effect of ultra-small sized GQDs along with GO/Fe3O4 phase. This is more clarified in height histogram (Fig. 6b).

The well and uniform dispersion of ultrathin GQDs as nanohybrid filler (GO/Fe3O4/GQD) in porous hydrogel matrix may be accounted from the high surface roughness (~160.4 nm) of PVA/CMC-B@GO/ Fe3O4/GQD (L) nanocomposite hydrogel (Fig. 6c) as compared to PVA/ CMC-B@GO/Fe3O4 (L) nanocomposite hydrogel (~133.3 nm) (Fig. S5c). Moreover, the height histogram profile of PVA/CMC-B@GO/Fe3O4/GQD (L) nanocomposite hydrogel reveals a uniform distribution pattern of surface topography (Fig. 6d). Similar type of surface behavior is also observed in earlier report [75] with well distribution of layered nanostructure in polyvinyl alcohol hybrid hydrogel bead to promote high adsorption of methylene blue (MB) dyes. 3.5. Rheological behavior Rheological parameters like storage modulus (G′), loss modulus (G″) and complex viscosity (η*) of the gel materials (Fig. 7a, b & d) are very important to interpret their viscoelastic nature and gel strength. As depicted in Fig. 7a, the storage modulus (G′) of boric acid cross-linked polyvinyl alcohol/carboxymethyl cellulose/Polyethylene glycol (PVA/ CMC/PEG-B) hydrogel shows an increasing pattern with increase in angular frequency, ω (rad/s) at a strain of γ = 2% which is typical characteristic of chemical gel. The increase of G′ in the higher frequency zone is actually contributed by the additional intermolecular physical interactions between entangled carboxymethyl cellulose (CMC), polyethylene glycol (PEG-1500) and cross-linked PVA chains, because at lower frequency range (upto ω b 10 rad/s) transient physical interactions are dissociated as per the mentioned report by Narita et al. [76] during analysis of viscoelastic phenomenon physically, chemically and dual crosslinked PVA gels. But the frequency dependency behavior is not significant like boric acid cross-linked pure PVA gels. Moreover, the G′ values are always observed to be larger for PVA/CMC/PEG-B hydrogel as compared to G″ within the operational frequency range (0.1 to 100 rad/s)

Fig. 7. Storage modulus (a), loss modulus (b), loss factor (c), and complex viscocity (d) of PVA/CMC/PEG-B hydrogel, PVA/CMC/PEG-B@Fe3O4, PVA/CMC-B@Fe3O4 (L), PVA/CMC-B@GO/ Fe3O4 (L) and PVA/CMC-B@GO/Fe3O4/GQD (L) nanocomposite hydrogels.

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which indicates the significant elastic response as compared to viscous response. This in turn basically represents the efficient cross-linking of PVA chains with H3BO3 along with interpenetration of CMC and PEG chains in vigorous shacking condition and more correlating with the report by Zhang et al. [77] during analysis of polyoxypropylenetriol (PPG) and H3BO3 based co-crosslinked network of PVA. As compared to the earlier report by Yi Zhang and his research group [78] to prepare glycerol induced H3BO3 cross-linked PVA gels, our vigorous shaking condition is found to be more useful in getting enhanced cross-linkages through H3BO3. On the other side, the less frequency dependence of both the moduli, storage and loss modulus along with their combatively high values in case of in situ incorporated magnetite based PVA/ CMC/PEG-B hydrogel may be linked with the generation of structural integrity by nano-Fe 3 O4 domain. Surprisingly, the frequency dependency character is strongly introduced within the PVA/ CMC-B@Fe 3 O 4 nanocomposite hydrogel after leaching of macromolecular PEG from the network structure through swelling protocol. As the removal of PEG leads to micro-scale voids as observed from the SEM micrographs, it apparently reduces the intermolecular physical interactions in between the network polymers and therefore, offering less cross-linking density. Due to lowering of compact structure in PVA/CMC-B@Fe 3 O 4 (L) nanocomposite hydrogel, frequency dependency is raised. Such frequency dependent character is also observed for PVA/CMC-B@GO/Fe3O4 (L) and PVA/ CMC-B@GO/Fe 3 O 4 /GQD (L) nanocomposite hydrogels. But the value of G′ is observed to be increased more with incorporation of GO due to its physical cross-linking attitude along with high aspect ratio. Similar type of result is reported in earlier report by Meng et al. [79] in case of polyvinyl alcohol/N-[(trimethoxysilyl) propyl ethylene diamine–triactic acid]/GO (0.5 wt%) nanocomposite hydrogel. Significant increment in storage modulus is observed for the PVA/CMC-B@GO/Fe 3 O 4 /GQD (L) nanocomposite hydrogel, prepared with dual incorporation of nanostructural GO and ultrasmall GQDs. Such improvement may be attributed by the synergistic contribution of GO and GQD towards effective physical interaction (electrostatic interaction, hydrogen bonding) with chemically cross-linked hybrid PVA/CMC network. Loss factor (tan δ) is generally assigned as the ratio of G″ to G′ and considered as the useful parameter in illustrating the damping behavior of the hydrogel with representation of loosely formed network. As depicted in Fig. 7c, the loss factors (tanδ) of all boric acid cross-linked hydrogel and nanocomposite hydrogels are found to be very low which ultimately demonstrates the stable cross-linked hydrogel networks for all cases even after the removal of PEG-1500 macromolecules. In a true case of completely elastic material, the appearance of loss factor should be zero due to significant lowering in G″ value (G″ ~ 0). But the finite value of tanδ in all real cases may be raised due to occurrence of looped and dangling chains in a random manner. This type of structural attribute is known as elastic defects in hydrogel structure. From the frequency variation of complex viscosity, η* (Fig. 7d); it is observed that the complex viscosity of all hydrogel and nanocomposite hydrogel are initially appeared with very high values at low frequency domain and then falls rapidly. Decrement is found to be occurred more rapidly for PVA/CMC/PEG-B hydrogel as compared to other nanocomposite hydrogels like PVA/CMC/PEG-B@Fe3O4, PVA/CMC-B@Fe3O4 (L), PVA/CMC-B@GO/Fe3O4 (L) and PVA/CMC-B@GO/Fe3O4/GQD (L) nanocomposite hydrogels. This indicates the improved physical interaction in nanocomposite hydrogel structures with incorporation of nanostructural Fe3O4, GO and GQDs even after the leaching of PEG. The high linearity in logarithmic frequency variation of complex viscosity is the representation of chemically cross-linked hydrogels. The improved mechanical aspects in terms of rheological behavior of PVA/CMC-B@GO/ Fe3O4/GQD (L) nanocomposite hydrogel may be proposed for its practical application in harsh environment.

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3.6. Swelling ratio measurement Swelling measurement of the hydrogel materials is very important to interpret its applicability in aqueous based system. The swelling phenomenon of hydrogel is strongly linked with their surface functionality as well as cross-linking density. As shown in Fig. S6a, the swelling percentage of PVA/CMC/PEG-B hydrogel is found to achieve an equilibrium phase within 4 h (240 min) with swelling capacity of 54.2%. After an hour of the equilibrium time, the hydrogel surface is found to swell as well as dissolute to the immersing aqueous phase without net gain in swelling capacity and an erratic pattern is noticed. The fact can be attributed by less penetration of water dipoles to the core environment of the prepared hydrogel. Moreover, the fact can be physically realized by minutely observing the situation just after dropping the hydrogel bead to aqueous medium. In initial stage upto 2 min, the hydrogel beads are appeared to be floated on the surface of water indicating the significantly light weight with respect to the bead volume. Slight solubilization of boric acid cross-linked PVA/CMC-B hydrogel after long time immersion indicates the inefficiency of gel network to trap water molecules and well correlated with earlier report by Zhang et al. [80]. But interestingly, PVA/CMC-B(L) hydrogel beads (Fig. S6b) are found to be immediately (2 s) occupied at the bottom of the container just after dropping and shows an improved swelling capacity of ~93.9% within 2.5 h (150 min). Although the swelling capacity is found to lower with incorporations of nanostructural Fe3O4 [SE ~ 44.32%], nanostructural hybrids GO/Fe3O4 [SE ~ 38.49%] and GO/Fe3O4/GQDs [SE ~ 30.58%] to the hydrogel structure of PVA/CMC/PEG-B, but after leaching of PEG from the nanocomposite hydrogel, swelling behavior is comparatively enhanced [SE ~ 68.6%, SE ~ 114.2% and SE ~ 152.6% for leached samples containing Fe3O4, GO/Fe3O4 and GO/Fe3O4/GQD phases, respectively]. This enhancement is related to developed macro-porosity after leaching of PEG which improves the network dimension and thereby, swelling capacity. Moreover, with incorporation of nanostructural phase (Fe3O4/GO/GQD) the slight solubilization of loosely cross-linked PVA is totally restricted. As the principle network structure of the present hydrogel is comprised with the effective entanglement of CMC chains, pH-dependent swelling behavior is tested in pH ~ 10 (Fig. S6c & d) and pH ~ 3 (Fig. S6e & f). At pH ~ 10 (Fig. S6c & d), the swelling behavior of all hydrogel and nanocomposite hydrogels are found to highly improved due to favor of deprotonation in –COOH functionality of the network CMC chains as well as nanostructural GO and GQDs. Due to effective deprotonation, significant negative charges are developed within the hydrogel which undergo in repulsive interaction and promote network expansion to accommodate large amount of water dipoles. Moreover, as our raw PVA is not completely deacetylated (degree of hydrolysis is 86.5% to 89%), it contains some unhydrolyzed acetyl groups which frequently react in NaOH environment to generate negative charges and therefore, participating in pH responsive swelling behavior. This pH responsive character is in strong correlation with earlier report by Wis'niewska et al. [81] during analysis of surface properties of polyvinyl alcohol adsorbed alumina surface. Although the hydrogel and nanocomposite hydrogel are swelled very high in pH ~ 10, but their structural integrity is indeed maintained even in case of pure PVA/CMC/PEG-B and PVA/CMC-B (L) hydrogels. This fact may be linked with the etherification via self cross-linking of polyvinyl alcohol chains and subsequent removal of water in presence of basic medium. This fact relates the high applicability of our synthesized PVA/CMC-B@GO/Fe3O4/GQD nanocomposite hydrogel towards the removal of positively charged pollutants in basic environment. On the other side, swelling capacity of PVA/CMC/PEG-B and PVA/ CMC-B (L) hydrogels at pH ~ 3 (Fig. S6e & f), are found to have low value as compared to neutral environment due to protonation of acid groups of the network CMC chains. This protonation leads to the effective physical interaction like hydrogen bonding in between the network forming polymer chains and therefore offer highly compact structure

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for PVA/CMC/PEG-B hydrogel and restricting the encapsulation of large amount of water. Moreover, PVA dissolution is not happened for the present case. But with formation of macro-porous structure by PEG leaching process, degree of swelling is enhanced (Fig. S6f) even after protonation due to creation of more available space for accommodating water molecules. As compared to the hydrogel system, suppression in swelling capacity at pH ~ 3 is more significant for nanocomposite hydrogels due to large oxygenous functionality of layered GO and GQDs along with their large surface area. The obtained high swelling behavior in basic medium as compared to acidic medium is in high correlation with earlier report by Chaudhary et al. [82] during estimation of pH dependent swelling behavior of boric acid cross-linked polyvinyl alcohol/carboxymethyl agarose based hydrogel. 3.7. Dye removal studies In prior to study the dye adsorption behavior; biocompatibility and cell imaging behaviors of the as prepared GQD is studied [Fig. S7]. Most satisfactorily, the prepared GQD shows high biocompatibility as evidenced from MTT assay (cell viability ~ 86.7% at 100 μg/mL). Adsorption kinetic is very crucial to illustrate the progress and efficiency of the adsorption process. As depicted in Fig. 8a, b, d & e, the maximum dye (MB/RhB) adsorption is found to be facilitated within 4 h (240 min) and 2.5 h (150 min) of the initial contact between the dye solution [C0 ~ 20 mg/L, 200 mL] and 0.5 g of PVA/CMC/PEG-B@Fe3O4 and PVA/ CMC/PEG-B@Fe3O4 (L) hydrogel, respectively. The quick dye (MB/RhB) adsorption behavior of PVA/CMC/PEG-B (L) hydrogel is related with their macroporous structure due to leaching of PEG macromolecules from the network structure. This macroporous structure leads to the easy diffusion of dye molecules to the core framework of the hydrogel. Additionally, the equilibrium dye uptake time is observed to be shortened for PEG leached hydrogel (3 h) as compared to un-leached hydrogel (~5 h) and this time period is well matched with the swelling

results. The adsorption of cationic dyes (MB/RhB) to the PVA/CMC/PEGB hydrogel and PVA/CMC-B (L) hydrogel is basically operated via physical interactions such as electrostatic interaction and hydrogen bonding between positively charged dye molecules and –OH, –COOH functionality of the hydrogel network. The dye adsorption behavior is found to improve with incorporation of nanostructural GO due to high oxygenous surface functionality of GO and improved surface area of PVA/CMC/ PEG–B@GO nanocomposite hydrogel. However, at this GO content (2 wt% with respect to PVA), equilibrium time is observed to be increased to almost 5 h due to formation of more compact structures via physical cross-linkages. Interestingly, in case of PEG leached GO based nanocomposite hydrogel system, no alternation in equilibrium time is observed along with high adsorption capacity for both the cationic dyes (MB/RhB), indicating the high efficiency of the PEG leached macroporous PVA/CMC-B@GO/Fe3O4 (L) nanocomposite hydrogel towards dye removal application. Further, significant improvement in dye adsorption behavior (Fig. 8b & e) is noticed for PVA/CMC-B@GO/Fe3O4/GQD (L) nanocomposite hydrogel due to unique arrangement of GQD, GO and Fe3O4 in a sheet architecture. The surface coverage of GO/Fe3O4 with ultra-small GQD provides the highly improved surface area in the nanostructural phase along with availability of huge oxygen containing groups which may be considered for the improved removal performance. The positively charged dye molecules with aromatic ring structures undergo in strong electrostatic interaction, hydrogen bonding interaction and π-π interaction with GO and GQD of the GO/Fe3O4/GQD hybrid phase along with –OH, -COOH functionality of cross-linked hybrid network of PVA/CMC-B. As illustrated from the Fig. 8a, b, d and e, the variation of adsorption capacity of all hydrogel and nanocomposite hydrogel with contact time demonstrates two distinct phases. The initial phase documents the rapid adsorption of dye molecules with increasing contact time, revealing the high availability of active binding sites at the beginning of the

Fig. 8. (a) Adsorption capacity of PVA/CMC/PEG-B@Fe3O4, PVA/CMC/PEG-B@GO/Fe3O4 and PVA/CMC/PEG-B@GO/Fe3O4/GQD nanocomposite hydrogels towards the removal of methylene blue; (b) Adsorption capacity of PEG leached PVA/CMC -B@Fe3O4 (L), PVA/CMC-B@GO/Fe3O4 (L) and PVA/CMC-B@GO/Fe3O4/GQD(L) nanocomposite hydrogels towards the removal of methylene blue; (c) Variation of adsorption capacity of PVA/CMC-B@GO/Fe3O4/GQD(L) nanocomposite hydrogel (0.2 g) with different MB concentration; (d) Adsorption capacity of PVA/CMC/PEG-B@Fe3O4, PVA/CMC/PEG-B@GO/Fe3O4 and PVA/CMC/PEG-B@GO/Fe3O4/GQD nanocomposite hydrogels towards the removal of rhodamine-B; (e) Adsorption capacity of PVA/CMC-B@Fe3O4 (L), PVA/CMC-B@GO/Fe3O4 (L) and PVA/CMC-B@GO/Fe3O4/GQD(L) nanocomposite hydrogels towards the removal of rhodamine-B [Co ~ 20 mg/L with a adsorbent dosage of 0.2 g]; (f) Variation of adsorption capacity of PVA/CMC-B@GO/Fe3O4/GQD(L) nanocomposite hydrogel (0.2 g) with different RhB concentration.

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adsorption process [30]. On the other hand, the 2nd phase is ascribed by the relatively slow adsorption behavior due to initial occupancy during first phase and finally ended with reaching to the equilibrium phase. The adsorption kinetic of the prepared PVA/CMC-B@GO/Fe3O4/GQD (L) nanocomposite hydrogel are tested in dye (MB/RhB) solutions (Fig. 8c & f), having different initial concentration like 20 mg/L, 100 mg/L, 200 mg/L, 300 mg/L and 500 mg/L with adsorbent dosage of 0.2 g at pH ~ 7. As depicted in Fig. 8c (for MB) and f (for RhB), the adsorption capacity of PVA/CMC-B@GO/Fe3O4/GQD (L) nanocomposite hydrogel is found to increase with increasing the initial dye concentration and achieve improved adsorption capacity of 46.79 mg/g for MB and 44.89 mg/g for RhB solutions, having concentration of 300 mg/L. Further, increment in dye concentration does not lead to any increment in the value of adsorption capacity. This implies the achieving of maximum adsorption capacity values of PVA/CMC-B@GO/Fe3O4/GQD (L) nanocomposite hydrogel at 300 mg/L dye concentration. Increment in adsorption capacity up to 300 mg/L of dye concentration is directly linked with the improved probability of interaction between dye molecules and active surface functionality of the nanocomposite hydrogel. On the other hand, the adsorption capacity of PVA/CMC-B@GO/ Fe3O4/GQD (L) nanocomposite hydrogel is observed to be decreased with increase of adsorbent dosage (Fig. S8a) (0.1 g, 0.2 g, 0.3 g, 0.5 g) and the fact is assigned to the reduction of specific surface area with increasing the mass value of the adsorbent. But with respect to the percentage of dye removal (Fig. S8-b), the scenario is different. Increase in dye concentration leads to the reduction in percentage of dye removal, whereas increase in adsorbent dosage shows the improvement in percentage of dye removal. To further investigate the kinetic rate of the adsorption process towards MB/RhB dye, the obtained experimental data are fitted to four different kinetic models (Figs. S9 and S10) such as pseudo 1st order, pseudo 2nd order and Elovich kinetic models (S8). From the value of regression coefficients (R2) of different kinetic fitting, it is noticed that the present adsorption of MB/RhB onto the surface of PVA/CMC-B@GO/ Fe3O4/GQD (L) nanocomposite hydrogel is well-fitted to the pseudo 2nd order kinetic model with high R2 ~ 0.999 for MB and R2 ~ 0.998–0.999 for RhB (Tables S1 and S2) along with the close matching of experimental and as-obtained maximum adsorption capacity values. Therefore, the adsorption process may preferably have described as the strong interaction like chemisorptions and well matched with the earlier report by Gong et al. [83] during analysis of carboxymethyl starch/polyvinyl alcohol hybrid gel for removal of methylene blue dye from the aqueous medium. The poor fitting of Elovich model predominantly describe the strong physisorption behaviors of PVA/CMC-B@GO/Fe3O4/GQD (L) nanocomposite hydrogel. To check the effect of diffusion controlled aspects of the adsorption process, the obtained kinetic values are also fitted to intra-particle diffusion model (S8). Intra particle diffusion (IPD) model fitted to the experimental values of the present adsorption process shows two linear fitting with comparable values of regression coefficients [R2 ~ 0.900–0.999 for 1st step and R2 ~ 0.962–0.991 for 2nd step @MB; R2 ~ 0.979–0.999 for 1st step and R2 ~ 0.854–0.927 for 2nd step@RhB]. Moreover, the initial intercepts are close to the origin points, and therefore, the present adsorption may be linked to the diffusion controlled mechanism. Hence, the kinetic behavior of the prepared PVA/CMC-B@GO/Fe3O4/GQD (L) nanocomposite hydrogel towards the removal of cationic dyes (MB/RhB) can be best described with pseudo 2nd order kinetic model along with slight contribution from intra-particle diffusion model. 3.8. Adsorption isotherms of dye removal As the surface texture and nature of the active binding sites of the adsorbent are the guiding parameters for molecular arrangement of the dye molecules over the adsorbent surface, it is very essential to evaluate the nature of adsorption isotherm. Adsorption isotherm is basically a mathematical relation between the equilibrium adsorption capacity

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(qe) and equilibrium dye concentration (Ce). The obtained experimental values of qe and Cefor different initial dye concentration are fitted to four isotherm models (S9) like Langmuir, Freundlich, Elovich and Temkin (Fig. S11). From Table S3, it is observed that the regression coefficient (R2 ~ 0.99) of the fitted Langmuir model is high as compared to other adsorption models and therefore, represents a homogeneous surface with monolayer coverage. Similar type of observation is found in earlier literature by Kong et al. [84] during adsorption study of composite hydrogel bead towards removal of methylene blue dye from aqueous solution. 3.9. Temperature dependency of dye removal Solution temperature plays a crucial role in guiding the adsorption performance depending on the nature of operative interactions. Adsorption performance of PVA/CMC-B@GO/Fe3O4/GQD (L) nanocomposite hydrogel is performed at four different temperatures (298 K, 303 K, 308 K and 313 K) with a dye concentration of 50 mg/L (MB/RhB). As observed from Fig. S12, the adsorption capacity is found to increase with increase of solution temperature. This increase in adsorption capacity with increase of temperature is related to the improved rate of diffusion due to thermal activation. In order to explain the accurate fact of temperature dependency of the present adsorption process, enthalpy (ΔH) and entropy (ΔS) changes during adsorption of MB and RhB onto the surface of PVA/ CMC-B@GO/Fe3O4/GQD (L) nanocomposite hydrogel are thermodynamically evaluated as S10, related to the logarithmic of equilibrium constant with ΔH, ΔS and temperature (T). The variation of ln (qe/Ce) factor with reciprocal of temperature (T−1) in Kelvin scale provides the values of ΔH and ΔS from the slope and intercept of the well fitted straight line on the experimental data points. For calculation of ΔH and ΔS, slope is equalized to – ΔH/R; where intercept is equalized to ΔS/R. From the estimated values of ΔH and ΔS, free energy (ΔG) values of the present adsorption at a dye concentration of 50 mg/L are calculated as per S10 at different experimental temperatures (298 K, 303 K, 308 K and 313 K) and tabulated in Tables S4 and S5. As the calculated values of ΔH are 23.72 kJ/mol for MB and 26.03 kJ/mol for RhB, these describe the endothermic feature of the adsorption process. As discussed by Mahdavinia et al. [85] the ΔH value lower than 20 kJ/mol is basically ascribed due to van der Waals interaction, while ΔH N 20 kJ/mol is attributed from the electrostatic interactions between adsorbent and adsorbate. Therefore, as per the obtained ΔH values, present adsorption process is linked to the physisorption characteristics with strong electrostatic interactions. As the increase in temperature shows the negative response of ΔG, it depicts the spontaneity in the adsorption behavior with increase of solution temperature. Similar type of observation is reported in earlier report [86]. 3.10. pH sensitivity of dye removal The significance of solution pH [Fig. 9 (a, b) for MB removal and Fig. 9 (c, d) for RhB removal] in dye removal is basically raised from the presence of ionizable functionality in the network structure. From the earlier discussion on swelling behavior of PVA/CMC-B@GO/Fe3O4/GQD (L) nanocomposite hydrogel, it is clear that the synthesized nanocomposite hydrogel has an improved swelling effect in basic medium (pH ~ 10) due to relaxation of network structure. This relaxation behavior is developed through the intermolecular repulsive forces, caused by the deprotonation of –COOH functionality from the network chains as well as nanostructural phase. As excessive concentration of NaOH has a slight negative impact on the cationic dye adsorption performance due to screening effect of Na+ ions. Therefore, the dye removal performance of PVA/CMC-B@Fe3O4 (L), PVA/CMC-B@GO/Fe3O4 (L) and PVA/CMC-B@GO/Fe3O4/GQD (L) nanocomposite hydrogels are tested towards the removal of MB and RhB in a wide range of pH (pH ~ 3, pH ~ 4, pH ~ 5, pH ~ 7, pH ~ 8,

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Fig. 9. pH responsive dye adsorption capacity of PVA/CMC-B@Fe3O4 (L), PVA/CMC-B@GO/Fe3O4 (L) and PVA/CMC-B@GO/Fe3O4/GQD (L) nanocomposite hydrogel towards (a, b) MB and (c, d) RhB [(b) and (d) are the respective zoom version of lower pH region].

pH ~ 10, pH ~ 12). The highest dye adsorption capacity is found for PVA/ CMC-B@GO/Fe3O4/GQD (L) nanocomposite hydrogel at pH ~ 8, while very slight decrement in dye removal is observed at pH ~ 10. The low dye removal capacities of all nanocomposite hydrogels are at lower pH is related to the protonation of –COO_ functionality of polymeric as well as nanostructural phases. Very high value of adsorption capacity of the prepared PVA/CMC-B@GO/Fe3O4 (L) nanocomposite hydrogel for both the cationic dyes in basic medium (pH ~ 8 to 12) signifies their effective applicability for removal of cationic dyes. This fact may be well explained from Fig. S13 and S14. As shown in Fig. S13, asprepared nanohybrid filler, GO/Fe3O4/GQD shows the improved BET surface area value from 74.6 m2/g to 178 m2/g on preparation of GO to GO/Fe3O4/GQD nanohybrid. This improvement is related to the unique decoration of ultrathin GQDs on GO/Fe3O4 structure which is also in strong correlation with TEM image of nanohybrid filler. On the other hand, the N2-adsorption and desorption curves for PVA/CMC-B@GO/ Fe3O4 (L) nanocomposite hydrogel (Fig. S14), shows a non-uniform hysteresis pattern with similar nature of Type- V adsorption isotherm. Moreover, uplifted low pressure region is similar to that of Type-II pattern. Therefore, the present adsorption isotherm reveals the coexistence of macro- as well as mesopores/hydrophobic mesopores.

Hence, BET surface calculation is not sufficient to explain the observed phenomenon. A result from mercury porosimetry measurement is represented in Fig. S14-(b). Fig. S14-b indicates the heterogeneously developed macroporosity, ranging from 1 μm to 55 μm (pore diameter) which in turn relates the random exclusion of PEG-1500 from the GO/ Fe3O4/GQD nanohybrid incorporated PVA/CMC-B hydrogel network. Moreover, at pH ~ 4, slight contribution of PVA/CMC-B@Fe 3 O 4 (L), PVA/CMC-B@GO/Fe 3 O 4 (L) and PVA/CMC-B@GO/Fe 3 O 4 /GQD (L) nanocomposite hydrogels are observed towards the removal of methyl orange (MO) dye. Fig. S15 shows the adsorption behavior of PVA/CMC-B@Fe 3 O 4 (L), PVA/CMC-B@GO/Fe 3 O 4 (L) and PVA/ CMC-B@GO/Fe 3 O 4 /GQD (L) nanocomposite hydrogels at pH ~ 7.0 (Fig. S15a & c) and pH ~ 8 (Fig. S15d & f) and pH ~ 4.0 (Fig. S15g– i) towards the removal of methyl orange dye. In neutral and basic environment, all prepared nanocomposite hydrogel shows negligible contribution in removal of methyl orange dye. Similar type of result is reported in earlier literature [87]. The present work is compared with earlier literature in Table 1, which shows that introduction of GO/Fe3O4 nanohybrid in hydrogel matrix somehow reduces the MB adsorption capacity as compared to pure GO, but Fe3O4 gives the easy separation and mechanical stability,

Table 1 Comparison of MB-adsorption capacity with earlier literature based on GO; GO/Fe3O4 nanostructural phase. Adsorbent

Maximum adsorption capacity, qmax (mg/g)

Dye

Reference

Chitosan/graphene oxide/Fe3O4 Magnetic calcium alginate/graphene/Fe3O4 Poly allylamine hydrochloride/carboxylated graphene oxide/Fe3O4 Polyethylenimine (PEI)/GO Polyvinyl alcohol/carboxymethyl cellulose/graphene oxide/bentonite Polypyrrole/graphene oxide/Fe3O4 (pH 8) PVA/CMC-B@GO/Fe3O4/GQD (At pH 8)

30.01 51.6 35.95 325 mg/g 136.56 mg/g 323.2 ~1000 mg/g

Methylene blue Methylene blue Methylene blue Methylene blue Methylene blue Methylene blue Methylene blue

[88] [89] [90] [91] [53] [92] Present work

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therefore, it is essential for field application. However, such type of nanocomposite hydrogel shows improved performance with increasing pH (pH ~ 8) due to deprotonation followed by structural relaxation. But our synthesized PVA/CMC-B@GO/Fe3O4/GQD nanocomposite hydrogel due to porous morphology and GO/Fe3O4/GQD nanohybrid (synergetic contribution) phase shows outstanding adsorption capacity (~1000 mg/g) for MB dye at pH 8. 3.11. Study of magnetic behavior As illustrated in Fig. 10-I, the magnetic behavior of pure Fe3O4 NPs, PVA/CMC-B@Fe3O4 (L), PVA/CMC-B@GO/Fe3O4 (L) and PVA/CMC-B@ GO/Fe3O4/GQD (L) nanocomposite hydrogels are explored through vibrating sample magnetometer (VSM) at 300 K. As expected, the asprepared nano-Fe3O4 shows the superparamagnetic behavior instead of ferrimagnetism. This superparamagnetic feature is also observed to be persisted for all nanocomposite hydrogels like PVA/CMC-B@Fe3O4 (L), PVA/CMC-B@GO/Fe3O4 (L) and PVA/CMC-B@GO/Fe3O4/GQD (L) as observed from Fig. 10-I and III. Although the magnified version shows a very little coercivity around 20 Oe, it may be assumed to be superparamagnetic feature. Moreover, the magnetization vs temperature (M-T) curve (Fig. 10-IV) shows the bifurcation for zero field cooled (ZFC) and field cooled (FC) curve at ~269 K. The ZFC curve shows broad maximum starting around 102 K, indicating the superparamagnetic behavior of PVA/CMC-B@GO/Fe3O4/GQD (L) nanocomposite hydrogel. The strong temperature dependency of FC curve also reveals the superparamagnetic nature. The observed value of magnetic saturation is found to 45.80 emu·g−1 for nano-Fe3O4 which is found to be significantly reduced after incorporation into the gel network of boric acid cross-linked PVA/CMC-B hydrogel (20.55 emu·g−1). But the value is sufficient for the purpose of magnetic separation of the adsorbent.

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In case of PVA/CMC-B@GO/Fe3O4 (L) and PVA/CMC-B@GO/Fe3O4/ GQD (L) nanocomposite hydrogels the magnetic saturation values are obtained as 15.50 emu·g−1 and 11.91 emu·g−1, respectively. As compared to the PVA/CMC-B@Fe3O4 (L) nanocomposite hydrogel, saturation of magnetization is not significantly affected with nanostructural (GO, GQD) incorporation. The prepared PVA/CMC-B@GO/Fe3O4/GQD (L) nanocomposite hydrogel can be easily separated from the solution after dye adsorption. Fig. 10-II, represent the digital photograph, showing the magnetic response of the prepared PVA/CMC-B@GO/Fe3O4/GQD (L) nanocomposite hydrogel in presence and absence of hand held magnet. Our observation is in well accordance with earlier report by Por et al. [93] during analysis of polyvinyl alcohol/carboxymethyl starchg- poly(vinyl imidazole). 3.12. Reusability study Regeneration of the adsorbent from the dye adsorbed condition and its further implication to dye adsorption process is known as recyclability. Reusability of the adsorbent is a very important from the economical point of view. The dye adsorbed PVA/CMC-B@GO/Fe3O4/GQD (L) nanocomposite hydrogel is treated with ethanol/water, 0.1 (N) NaCl medium via ultrasonication for 4 h to remove the adsorbed MB and RhB molecules from the nanocomposite hydrogel surface. The prepared PVA/CMC-B@GO/Fe3O4/GQD (L) nanocomposite hydrogel offers 95% removal efficiency for MB and 89% removal efficiency for RhB even after four successful cycles of adsorption and desorption (Fig. S16). 4. Conclusion We have used graphene quantum dots (GQDs) and GO together with association of magnetite (Fe3O4) nanoparticles for the removal as

Fig. 10. Magnetic measurement (I) of nano-Fe3O4 along with PVA/CMC-B@Fe3O4 (L), PVA/CMC-B@GO/Fe3O4 (L) and PVA/CMC-B@GO/Fe3O4/GQD (L) nanocomposite hydrogels at 300 K; digital photograph (II) on magnetic behavior of PVA/CMC-B@GO/Fe3O4/GQD (L) nanocomposite hydrogel in presence or absence of hand held magnet; (III) Magnified version of image I; (IV) Magnetization-temperature (M-T) curve of PVA/CMC-B@GO/Fe3O4/GQD (L) nanocomposite hydrogel [FC refers field cooling, whereas ZFC refers zero field cooling].

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well as separation of dye pollutants. The tri-nanostructural assembly, GO/Fe3O4/GQD is successfully designed via simple HCl:EtOH route and confirmed through HRTEM analysis. Then, as-prepared GO/Fe3O4/GQD is incorporated within PEG-1500 induced macroporous boric acid cross-linked polyvinyl alcohol/carboxymethyl cellulose (PVA/CMC) hydrogel beads. Prepared nanocomposite hydrogel beads are morphologically and structurally characterized with FESEM, TEM, XRD and FTIR. In case of PVA/CMC-B@GO/Fe3O4 (L) nanocomposite hydrogel, the TEM micrographs show the nano-Fe3O4 embedded GO sheets in a network arrangement which in turn indicates the successful incorporation of GO/Fe3O4 hybrid nanostructures in PEG leached PVA/CMC-B hydrogel network. PVA/CMC/PEG-B also shows the two new peaks at 1418 cm−1 and 1330 cm−1, related to the asymmetric stretching relaxation of B-O-C bond, while the FTIR band at 657 cm−1 is appeared due to O-B-O bending. Complete removal of PEG from hybrid hydrogel and nanocomposite hydrogel structures are confirmed from the disappearance of characteristic FTIR and XRD peaks of PEG-1500. Rheological and swelling behaviors of PEG leached boronic acid cross-linked PVA hydrogel and nanocomposite hydrogel offers enhanced swelling behavior with quicker equilibrium time. As compared to neutral environment (pH ~ 7.0), PVA/CMC-B@GO/Fe3O4/GQD (L) nanocomposite hydrogel shows significant high swelling behavior in pH ~ 10 due to ionization of –COOH functionality of polymeric counterpart as well as nanostructural phase. The dye adsorption capacity of PVA/CMC-B@GO/ Fe3O4/GQD (L) nanocomposite hydrogel shows improved behavior towards removal of cationic pollutants (MB/RhB) as compared to PVA/ CMC-B@Fe3O4 (L)and PVA/CMC-B@GO/Fe3O4 (L) nanocomposite hydrogel, while this behavior is more significant in basic environment. The adsorption of MB/RhB onto the surface of PVA/CMC-B@GO/Fe3O4/ GQD (L) nanocomposite hydrogel follows the Langmuir adsorption isotherm, where it is well linked to pseudo secondorder kinetics. Slight increment in temperature reveals the increase in adsorption capacity which in turn reflects the spontaneity of adsorption process. CRediT authorship contribution statement Niladri Sarkar: Methodology, Investigation, Data curation. Gyanaranjan Sahoo: Formal analysis. Sarat K. Swain: Supervision, Conceptualization. Declaration of competing interest Authors declare that there is no conflict of interest for publishing this manuscript in the “Journal of Molecular Liquids”. Acknowledgements Authors express their thanks to the University Grant Commission (UGC), New Delhi, India for providing financial assistance to pursue doctoral degree of Niladri Sarkar through Rajiv Gandhi National Fellowship. Authors also thank to TEQIP-III of the Veer Surendra Sai University of Technology for partial support. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.molliq.2020.112591. References [1] T. Nandy, S. Shastry, P.P. Pathe, S.N. Kaul, Pre-treatment of currency printing ink wastewater through coagulation-flocculation process, Water Air Soil Pollut 148 (2003) 15–30. [2] F.M.M. Paschoal, M.A. Anderson, M.V.B. Zanoni, Simultaneous removal of chromium and leather dye from simulated tannery effluent by photo electrochemistry, J. Hazard. Mater. 166 (2009) 531–537.

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