CelloZIFPaper: Cellulose-ZIF Hybrid Paper for Heavy Metal Removal and Electrochemical Sensing
The processing of hierarchical porous zeolitic imidazolate frameworks (ZIF-8) into a cellulose paper using sheet former Rapid-Köthen (R.K.) is reported. The procedure is a promising route to overcome a significant bottleneck towards applying metal-organic frameworks (MOFs) in commercial products. ZIF-8 crystals were integrated into cellulose pulp (CP) or TEMPO (2,2,6,6tetramethylpiperidine-1-oxyl radical)-oxidized cellulose nanofibrils (TOCNF) following an insitu or ex-situ process; the materials were denoted as CelloZIFPaper_In Situ and CelloZIFPaper_Ex Situ. The materials were applied as adsorbents to remove heavy metals from water, with adsorption capacities of 66.2 -354.0 mg/g. CelloZIFPaper can also be used as a standalone working electrode for the selective sensing of toxic heavy metals, for instance, lead ions (Pb 2+ ), using electrochemical-based methods with a detection limit of 8 µM. The electrochemical measurements may advance 'Lab-on-CelloZIFPaper' technologies for label-free detection of Pb 2+ ions.
Introduction<!>Materials and Methods<!>Synthesis of Cellulose/ZIFs and processing into CelloZIFPaper<!>Characterizations<!>Metal adsorption<!>Electrochemical analysis<!>Characterization of Materials and CelloZIFPaper<!>Adsorption of heavy metals<!>Electrochemical-based sensing of heavy metal ions<!>Declaration of Competing Interest
<p>Porous materials [1,2] such as Metal-organic frameworks (MOFs) [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] and zeolitic imidazolate frameworks (ZIFs) are hybrid porous materials consisting of metal ions connected with organic linkers [21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38]. Processing MOFs using template materials gives rise to a new class of mixed materials that combines the advantages of both components [21][22][23][24][25][26]. In recent years, hybridizing biopolymers such as cellulose with MOFs has resulted in the development of cellulose-MOF functional materials, which opens up vast new advantages in terms of high performance, good processability, and good sustainability [26]. Cellulose-MOFs have thereby several advanced applications, for instance, drug delivery [40,41], carbon dioxide (CO2) adsorption [42], water remediation [43][44][45], thermal insulation, and fire retardancy [46]. Combining MOFs with cellulose offers significant processing flexibility, which is inherently limited for MOFs, using methods such as three-dimensional (3D) printing [40,47], freeze-drying [42], and sol-gel [48,49]. These methods can, among others, engineer cellulose-MOFs into scaffolds [40], elastic aerogels [46], foams [42],</p><p>or coatings [50]. However, some of these methods require the use of a cellulose template for the in-situ growth of MOFs [51], complicated multi-step procedures [52], and the use of solvents or conditions harmful to the environment [26]. Thus, it is vital to develop facile cellulose-MOF processing methods that can be used in larger than lab-scale applications while using environmentally friendly conditions.</p><p>Water contamination with heavy metals is a critical issue for modern societies. The Agency for Toxic Substances and Disease Registry and the U.S. Environmental Protection Agency (EPA) recently published the 2019 Priority List of Hazardous Substances that stated the most toxic elements including several metal ions, e.g., arsenic (As), cadmium (Cd), mercury (Hg), lead (Pb), and selenium (Se) [53]. In this context, methodologies to remove as well as sense/detect these ions are highly required to advance the water treatment. To that end, MOFs, among several other materials, offer the possibility for heavy metal removal via adsorption [26] as well as the detection of such contaminants using imaging-assisted sensing [54] and electrochemical and optical sensing [55].</p><p>This study reports the processing of ZIFs incorporated into cellulosic filter papers for the first time.</p><p>It demonstrates the use of cellulose as a more environmentally friendly alternative to replacing inorganic or synthetic polymer substrates for ZIFs (sub-class of MOFs)-based paper fabrication [20]. We report the manufacturing of composite flat sheets consisting of ZIF-8 (or metal azolate framework-4, MAF-4) and different grades of cellulose, denoted as CelloZIFPaper, using Rapid Köthen flat sheet former. Both in-situ and ex-situ procedures are reported for the synthesis of CelloZIFPaper hybrid materials using TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl radical)oxidized cellulose nanofibrils (TOCNF) as well as cellulose pulp (CP). The addition of CP facilitates the fabrication of flat sheets and allows the tuning of the structure of TOCNF-ZIF networks. The synthesis procedures are straightforward without needing a template or excessive harsh chemicals. They are mainly water-based and, in that respect, more environmentally friendly than other methods [52,56]. The CelloZIFPapers reported herein were evaluated for the adsorption and electrochemical sensing of heavy metal ions (Fe 3+ , Cu 2+ , Pb 2+ , Co 2+ , and Cd 2+ ). In addition, this study is the first example of 'lab-on-MOF' for the electrochemical sensing of toxic metal ions.</p><!><p>Spruce cellulose pulp (CP) was obtained from Domsjö Fabriker AB (Sweden) in a never-dried form (17 wt.%) and was dispersed in water (0.3 wt.%) with extensive mixing (Ultra Turrax (IKA) at 15000 rpm for one h). FeCl3•6H2O, CoCl2•6H2O, Zn(NO3)2•6H2O, CuSO4•5H2O, Pd(NO3)2, Cd(NO3)2•4H2O, Na2SO4, 2-methylimidazole (Hmim), nitric acid (68%), and triethylamine (TEA, anhydrous, ≥99%) were purchased from Sigma-Aldrich (Germany). TOCNF was prepared according to the previously reported method [57].</p><!><p>The in-situ synthesis procedure involved the addition of the chemical reagents directly in the CP and TOCNF dispersions, separately. More specifically, Zn(NO3)2 (6 g) was added to the CP dispersion (0.3 wt.%) with stirring for 30 min before the addition of TEA (20 mL, 99.5%). The dispersions were stirred for one hour until a white precipitate was observed. Then, a solution of Hmim (60 mL, 0.84 M) was added, and the mixture was stirred for one hour. The same procedure was followed for the TOCNF dispersion (0.3 wt.%). Finally, the dispersions of CP/ZIF-8 and TOCNF/ZIF-8 were mixed in ratios of 1:0.5, 1:1, 1:2, and 1:4 using a total volume of 500 mL.</p><p>The ex-situ procedure involved the synthesis of ZIF-8 using the TEA-assisted method [58]. ZIF-8 was added in different amounts, 1.5 g, 3 g, 6 g, and 12 g, to 1 L of CP/TOCNF dispersion (CP/TOCNF ratio of 1:30 (wt.:wt.)) yielding a weight ratio of 1:0.5, 1:1, 1:2, and 1:4 (cellulose: ZIF-8, wt.:wt.), respectively.</p><p>The cellulose/ZIFs papers were fabricated using a Rapid Köthen 5-Automatic sheet former (Paper Testing Instruments GMBH, Austria) with a diameter of 200 mm. The materials were denoted as CelloZIFPaper_In Situ and CelloZIFPaper_Ex Situ using in situ and ex situ synthesis procedures.</p><p>All mixtures, both in-situ and ex-situ, were mixed with Ultra Turrax for 30 min before sheet preparation to make them homogeneous. The total volume of the aqueous dispersions was 9 L to achieve long dewatering times and secure the homogeneity of the sheets. The dispersions were vacuum filtered through poly(vinylidene fluoride) filters (PVDF, pore diameter 0.65 μm, Merck) to minimize the material losses. The papers were dried at 93 °C for 10 min under the pressure of 1 bar.</p><!><p>X-ray diffraction (XRD) was carried out using a PANalytical X'PertPRO X-ray diffractometer (Cu Kα1 radiation) using step size, current, and tension voltage of 0.05°, 40 mA, and 45 kV, respectively. Fourier-transform infrared attenuated total reflection (FTIR-ATR) spectroscopy was performed using a Perkin Elmer Spectrum 2000 FT-IR spectrometer. The particle size and morphology of the formed liquid phases during the in-situ synthesis were characterized using transmission electron microscopy (TEM, JEM 2100, JEOL, Japan, accelerating voltage 200 kV).</p><p>The surface morphology and the cross-sections of the prepared papers were imaged using a scanning electron microscope (SEM, JSM-7000F, JEOL, accelerating voltage 2 kV). The loading of ZIF-8 onto CP and TOCNF was evaluated using energy-dispersive X-ray spectroscopy (EDX) with the same instrument. Nitrogen (N2) adsorption-desorption isotherms were measured at 77 K using a Micromeritics ASAP 2020 instrument (U.K.). They were used for the calculation of specific surface areas using the Brunauer-Emmett-Teller (BET) [59] and the Langmuir methods [60]. The t-plot method was used for the calculation of the external surface areas. The pore size distribution was evaluated via the non-local density functional theory (NLDFT) method using N2@77 on a carbon slit. Metal concentration was estimated using atomic absorption spectrophotometer (FAAS) (Buck scientific 210VGC). X-ray photoelectron (XPS) analysis for the CelloZIFPaper_In situ before and after metal adsorption was recorded using Thermo Fischer (Kalpha, Al Kα radiation, 1486.6 eV). FT-IR for the material before and after adsorption was recorded using the KBr pellet method (Nicolet spectrophotometer, model 6700). Thermogravimetric analysis (TGA) was performed under air using Perkin Elmer TGA 7 thermal analyzer apparatus (Shimadzu, Japan) at a heating and airflow rate of 10 o C/min, and 30 mL/min, respectively. The electrochemical measurements were recorded using CorrTest ® (CS350, Wuhan, China). Diffuse reflectance spectroscopy (DRS) measurements before and after metal ions adsorption were recorded using Evolution 220 spectrophotometers coupled with ISA-220 integrating sphere (Thermo Fisher Scientific, U.K.). ). The bandgap energies of ZIF-8, CP, and CelloZIFPaper before and after metal adsorption were calculated using Tauc's plot according to Equation 1 [61]:-</p><p>Eq.1</p><p>Where h, ν, α, A, n, and Eg were Planck constant, frequency, absorption coefficient, constant, n =2, and bandgap, respectively. The photon energy (hν, eV) was calculated using wavelength (nm) using the equation; hν = 1240/λ.</p><!><p>The CelloZIFPapers were investigated for adsorption of Fe 3+ , Co 2+ , Cu 2+ , Pb 2+ , and Cd 2+ from aqueous solutions. A 100 mg specimen of each sheet was added to the metal solutions (1000 ppm) and incubated for 12 h. The adsorption process was followed at a solution temperature of 23±2 oC. The remaining metal concentration of each solution was measured via metal analysis using flame atomic absorption spectroscopy (FAAS). The changes in pH for the Pb 2+ solution was recorded using a pH meter (Accumet® pH meter Model 810, Fischer Scientific).</p><p>The CelloZIFPaper_In Situ was investigated for the adsorption of Co 2+ , Cu 2+ , and Pb 2+ from aqueous solutions. A 100 mg specimen of CelloZIFPaper_in situ (1:4 wt. ZIF: TOCNF) was added to the metal solutions (20 mL, 1000 ppm) and incubated for 12 h. The desorption was performed via stirring with acetonitrile (100 mL) for 12 h. The adsorption was recorded as percentage referring to the original adsorbed value of each metal ion. The recyclability of CelloZIFPaper was assessed following the same procedure for three cycles.</p><p>The adsorption of different amounts of Pb 2+ was determined via soaking specimens (7 mg) of CelloZIFPaper into Pb 2+ solutions with the volumes of 5, 10, 20, 50, and 100 mL (1000 ppm), respectively. The materials were incubated for 12 h. The materials were digested into nitric acid (68%). The concentration of Pb 2+ metal ions was measured with FAAS.</p><!><p>Electrochemical experiments; cyclic voltammetry (CV), linear sweep voltammetry (LSV), potentiodynamic polarization test, chronoamperometry, and electrochemical impedance spectroscopy (EIS) were performed using a three-electrode cell including CelloZIFPaper (1×2 cm 2 ) as working electrode, a platinum mesh as a counter electrode, and Ag/AgCl electrode as a reference electrode. The measurements were performed in a Na2SO4 solution (0.1 M). Solutions of each metal ion (0.1 M) were dropwise applied onto the CelloZIFPaper in volumes of 0.1, 0.2, 0.3, 0.4, 0.5, 1, 2, and 3 mL. CV voltammograms were recorded at a potential scan rate of 50 mV/s from +0.6 to +0.2 V vs. Ag/AgCl. The EIS spectra were recorded at a potential of 0.2 V vs. Ag/AgCl in the frequency ranges from 100 kHz to 0.01 Hz with an amplitude perturbation potential of 10 mV.</p><!><p>The ex-situ and in-situ synthesis procedures are schematically represented in Fig. 1. The ex-situ synthesis procedure includes two steps: 1) TEA-assisted synthesis of ZIF-8 crystals and 2) synthesis of cellulose/ZIF-8 via different weight ratios. The in-situ synthesis procedure involves successive chemical reagents of Zn(NO3)2, TEA, and organic linker Hmim (Fig. 1). The dispersions were processed into sheets using a Rapid Köthen sheet former yielding sheets with diameters of 200 mm (Fig. 1). The sheets were characterized using XRD (Fig. S1), TEM (Fig. 2), XPS (Fig. 3), SEM (Fig. 4), FT-IR (Fig. S2), N2 adsorption-desorption isotherms (Fig. S3-S4), TGA (Fig. S6), CV (Fig. S7), LSV (Fig. S7), and EIS (Fig. S8). The phases formed during the synthesis were also characterized with XRD (Fig. S9), FT-IR (Fig. S10), and TEM (Fig. 3). XRD patterns for the in-situ synthesis procedure in the presence of CP and TOCNF confirm the successful synthesis of a pure phase of ZIF-8 crystals (Fig. S1a). This observation indicates no negative impact of CP or TOCNF on the crystal growth of ZIF-8 materials. XRD pattern for the white precipitate formed during the in-situ procedure stated the presence of zinc oxide (ZnO) and zinc hydroxide nitrate (Zn(OH)8(NO3)2•2H2O, JCPDS card 24-1460) (Fig. S8) [62]. XRD patterns of CP or TOCNF show peaks at 15.3°, 16.9°, 22.7°, and 34.9° corresponding to Miller indices of (1ī0), ( 110), (200), and (004), respectively [63]. After the crystal growth of ZIF-8, the prominent peak at 22.7° was preserved, indicating the presence of cellulose in the final products (Fig. S9).</p><p>The transformation of ZnO into ZIF-8 crystal is fast and requires 30 min [58]. TEM images of the observed phases indicate the presence of ZnO crystals with a particle size of 500 nm (Fig. 2a-b).</p><p>The interactions between the functional groups of CP and TOCNF, respectively, were characterized using FT-IR (Fig. S10). The FT-IR spectrum of CP shows characteristic peaks at 3300 cm −1 , 2900 cm −1 , 1638 cm −1 , 1306 cm −1 , and 890 cm −1 corresponding to stretching vibration of O-H, C-H, bending H-O-H, C-O, and C-O-C vibration for β, 1,4-glycosidic linkage, respectively (Fig. S10). A new peak, assigned to Zn-O, at 422-425 cm -1 was observed after adding TEA to cellulose/Zn 2+ (Fig. S10). The crystal growth of ZIF-8 materials in the presence of CP or TOCNF can be confirmed from the Zn-N bond at 420 cm -1 (Fig. S10). The signal at 420 cm -1 is preserved after the processing into CelloZIFPaper, indicating the presence of ZIF-8 crystals in the final product. The successful synthesis of ZIF-8 can be confirmed from the XRD pattern of the paper that shows the characteristic diffraction pattern for ZIF-8 crystals (Fig. S1). The diffraction patterns of the paper proceeding using the ex-situ synthesis procedure show firm diffraction peaks for ZIF-8 compared to the paper prepared via the in-situ procedure (Fig. S1). The chemical bonds between cellulose (e.g., CP and TOCNF) and ZIF-8 inside CelloZIFPaper_</p><p>In Situ and CelloZIFPaper_Ex Situ were characterized using XPS analysis (Fig. 3). The elemental survey for the materials verifies the presence of Zn, O, C, and N (Fig. 3a). The XPS analysis of O 2p for CelloZIFPaper_ In Situ shows peaks at a binding energy of 533.1 eV, 532.2 eV, and 531.4</p><p>eV corresponding to C-O, C=O, and Zn-O, respectively (Fig. 3b). Spectrum analysis of O 2p of CelloZIFPaper_Ex Situ shows only two peaks at the binding energy of 532.9 eV and 532.1 eV corresponding to C-O C=O, respectively (Fig. 3b). The data analysis indicates a stronger interaction between TOCNF and ZIF-8 crystals using the in-situ synthesis procedure than the exsitu procedure. This observation can be confirmed from the study of Zn 2p (Fig. 3c) as well. Both hybrid papers, i.e., CelloZIFPaper_In Situ and CelloZIFPaper_Ex Situ, show peaks at binding energies of 1045.3 eV and 1022.2 eV corresponding to Zn 2p1/2 and Zn 2p3/2, respectively (Fig. 3c). However, an extra peak corresponding to Zn-O was observed at a binding energy of 1024.4</p><p>eV for CelloZIFPaper_In Situ (Fig. 3c). XPS analysis indicates a stronger interaction between TOCNF and ZIF-8 crystals inside CelloZIFPaper_In Situ than CelloZIFPaper_Ex Situ. This was further sustained by observing the leaching of white particles upon touching the papers synthesized via ex-situ synthesis procedure, probably, due to loosening binding to cellulose particles. The crystal morphology and particle size in dispersion and CelloZIFPaper were evaluated using TEM (Fig. 2c-d) and SEM imaging (Fig. 4). The in-situ growth of ZIF-8 in CP gives rise to smaller particles (average crystal particle size of 40 nm) than the ZIF-8 crystals grown in the presence of TOCNF (average crystal particle size 250 nm, Fig. 2c-d). ZIF crystals can be in-situ synthesized on the cellulose fibrils of CP (Figure 1c). SEM images of CelloZIFPaper_In Situ show homogenous crystal distribution within the membrane (Fig. 4a-c). Additionally, the amount of ZIF-8 crystals increases with the increase of the loading of CP/TOCNF and ZIF-8 (i.e., higher ratio values) from 1:0.5 (Fig. 4d-f) to 1:4 (Fig. 4g-i). TEM and SEM images show mesoporous structure inside the ZIF-8 crystal, indicating that the synthesis procedure yields hierarchical porous ZIF-8.</p><p>Material porosity was determined using nitrogen adsorption-desorption isotherms (Fig. S2).</p><p>Specific surface areas (BET and Langmuir), external surface area, and pore volumes are summarized in Table 1. When the ex-situ synthesis procedure was applied, the surface area increased with ZIF-8 loading (Table 1). From the collected data, one can observe that both in-situ and ex-situ prepared papers with the lowest ratios (1:0.5) have the highest ZIF-8 loading (Table 1). The in-situ synthesis procedure yielded higher ZIF-8 loading (70 wt.%) compared to the exsitu synthesis procedure (59 wt.%) at the low ratio of 1:0.5 (Table 1). CelloZIFPaper was fabricated with a diameter of 200 mm and can be folded without significant damage (Figure 1c). CelloZIFPaperswere prepared via a-c) ex-situ procedure and d-i) in-situ procedure using cellulose: ZIF ratio of d-f) 1:0.5 and g-i) 1:4.</p><!><p>The CelloZIFPapers with the highest ZIF-8 content were evaluated for heavy metal adsorption capacity using aqueous solutions of Cd 2+ , Co 2+ , Cu 2+ , Fe 3+ , and Pb 2+ (Fig. 5). The metal adsorption using ZIF-8 is usually fast [64,65]. The adsorption equilibrium for Co 2+ ions can be achieved after 30 min (Fig. 5a). The fast adsorption can also be confirmed from the fast color change of the CelloZIFPaper from white to blue or purple after the soaking into Cu 2+ or Co 2+ ions solution, respectively, after 10 min. CelloZIFPaper_In Situ exhibited higher adsorption capacity than CelloZIFPaper_Ex Situ. The materials exhibited adsorption capacities of 143, 350, 354.0, 260.8, and 307.3 mg/g for Cd 2+ , Co 2+ , Cu 2+ , Fe 3+ , and Pb 2+ ions, respectively (Fig. 5b). On the other hand, CelloZIFPaper_Ex Situ showed adsorption capacities of 196.8, 300, 166.5, 66.2, and 87.2 mg/g for Cd 2+ , Co 2+ , Cu 2+ , Fe 3+ , and Pb 2+ , respectively (Fig. 5b). There is no significant Zn leaching during the adsorption (Fig. 5a). The adsorption of Pb 2+ using CelloZIFPaper_In Situ increased with the initial metal ion concentration (Fig. 5c). Several mechanisms for metal ion adsorption using ZIF8-based materials have been suggested [26,39,67]. The metal can be adsorbed using ZIF-8/TOCNF/CP via ion exchange (e.g., with a metal node of ZIF crystals) or coordination to the functional groups of cellulose (i.e., -O.H. or -COOH). In this work, pH changes (Figure S11), FT-IR (Figure S12), DRS (Fig. 5d), Tauc's plot (Figure S13) [61], XPS (Fig. 6), CV (Fig. S14a-b), and LSV (Fig. S14c) analyses were performed to elucidate the mechanism that controls these interactions. The Pb 2+ solution is acidic, i.e., pH=4 (Figure S11). There is an increase in pHduring the adsorption due to the removal of the metal ions (Figure S11). It was reported that ZIF-8 crystals provide a variety of active adsorption sites, including O-H and N-H groups, low-coordinated Zn atoms, and free Ngroups belonging to Hmim [68]. Cellulose and TOCNF have functional groups such as O-H and COO -, adsorb metal ions [45]. FT-IR spectra for CelloZIFPaper before and after adsorption of metal ions are reported (Figure S12). Tetrahedrally coordinated atoms such as Co and Cu can replace Zn in the frameworks. FT-IR spectrum of CelloZIFPaper shows a peak at 421 cm -1 corresponding to Zn-N (Figure S12). The FT-IR spectrum of Cu adsorbed CelloZIFPaper shows two peaks at 425 cm -1 and 430 cm -1 corresponding to Zn-N, and Cu-N, respectively (Figure S12). DRS spectra of the papers before and after metal adsorption indicate the change in the intensity of the UV-Vis absorption (Fig. 5b). This change can be attributed to the strong interaction between the adsorbed metal ions and the functional groups of the membranes that can be determined from Tauc's plot (Figure S13). The bandgap values of CelloZIFPaper before and after adsorption are tabulated in Table S1. The change in the bandgap of CelloZIFPaper reveals the presence of the adsorbed metal inside the crystal of ZIF-8. Thus, the white color of CelloZIFPaper turns purple and blue after the adsorption of Co 2+ and Cu 2+ , respectively (Figure 5e). Other metal ions, such as Pb 2+ or Cd 2+ , caused no dramatic change in the bandgap of CelloZIFPaper due to the low tendency for coordination inside ZIF-8 crystals (Table S1). The elemental survey for CelloZIFPaper_In Situ confirms the presence of C, N, Zn, and adsorbed metal species (Fig. 6a). XPS analysis of Pb@CelloZIFPaper_In Situ shows two peaks at binding energies of 143.9 eV and 138.9 eV corresponding to Pb 4f5/2 and Pb 4f7/2, respectively (Fig. 6b). The Pb 4f7/2 peak reveals three peaks at binding energies of 137.8 eV and 140.4 eV, which are associated with Pb-O, and Pb-O-C, indicating the coordination to the carboxylic group of TOCNF (Fig. 6b). The analysis of the Cd adsorbed membrane shows two peaks at binding energies of 405.6 eV and 412.3 eV, corresponding to Cd 3d5/2 and Cd 3d3/2 (Fig. 6c). At the same time, the spectrum of the CelloZIFPaper used for the adsorption of Fe 3+ shows peaks at 711.0 eV, and 724.3 eV, corresponding to Fe 2p3/2 and Fe 2p1/2, respectively (Fig. 6d). The Cu ion adsorbed paper analysis shows peaks at binding energies of 934.7 eV and 954.7 eV, corresponding to Cu 2p3/2 and Cu 2p1/2, respectively, with two satellite peaks at 943.8 eV and 962.9 eV (Fig. 6e). The analysis confirms that the oxidation state of Cu is 2+ indicating no reduction of Cu 2+ after adsorption (Fig. 6e). This observation suggests that the adsorption mechanism is different from the previously reported mechanism using only TOCNF, which causes the reduction of Cu 2+ and formation of Cu2O [69]. The ion-exchange mechanism for the adsorbed Cu 2+ can be investigated via the analysis of N 1s (Fig. 6f). The XPS spectrum of N 1s of ZIF-8/TOCNF/CP paper shows two peaks at binding energies of 399.4 eV and 400.7 eV corresponding to -C-N and -C=N in Hmim, respectively [70]. After adsorption tests with the Cu 2+ solution, the two peaks of N 1s for Hmim still exist, and an extra peak at 398.9 eV is observed, indicating the coordination between the nitrogen atom and Cu 2+ (Fig. 6f). A previous study reported a new peak at a binding energy of 403.4 eV due to ion exchange and coordination between Cu 2+ and ZIF-8 crystals [70]. This peak (at a binding energy of 403.4 eV) is not present in our case, indicating that the interactions are mainly electrostatic and coordination to N in Hmim. Both</p><p>CelloZIFPaper_In situ and CelloZIFPaper_Ex situ after adsorption of Co 2+ show a peak at a binding energy of 782.1 eV referring to Co2p3/2 (Fig. 6g-h). The analysis of the adsorbed metal ions indicates that the adsorption takes place via coordination to the carboxylic groups of TOCNF or via electrostatic interaction without any observation of an ion-exchange mechanism. The adsorption of tetrahedral ions such as Cu 2+ or Co 2+ can be coordinated to the functional groups of ZIF-8, including O-H and N-H groups, low-coordinated Zn atoms, and free Ngroups belonging to Hmim [68]. Thus, these metals render the recyclability of the materials difficult using the desorption via acetonitrile compared to other metal ions such as Pb 2+ (Figure 5f). MOFs-based materials are recognized as promising matrices for metal adsorption [71][72][73][74].</p><p>According to previous studies, ZIF-8 shows adequate metal ion adsorption (Table 2) [3,39,75].</p><p>However, this material lacks functional groups such as -N-H, -O-H, and -COOH that facilitate the binding to metal ions; hence, adsorption can occur via either ion exchange [70] or intraparticle diffusion [76]. The adsorption of metal ions using ZIF-8 as an adsorbent can be further improved via conjugation with other nanomaterials, for instance, graphene oxide (GO) [77]. Removal of Cu 2+ ions may occur via the coordination to the functional groups of GO without any observation of ion exchange with the Zn nodes of ZIF-8 crystals [77]. TOCNF and CP improved the metal ion adsorption compared to other materials (Figure 5b, Table 2). The preparation methods presented herein are fast and straightforward without needing a solvothermal procedure that requires a long reaction time (24 h) and high temperature (Table 2) [76]. A drawback of using ZIF sorbents in powder form is their tedious separation after the adsorption process. Thus, ZIF-8 was modified with magnetic nanoparticles to make their separation easier [78]. The use of our CelloZIFPaper allows the simple separation of the adsorbent without the need for any extra steps (Table 2). In a recent study, ZIF-8 nanoparticles were in-situ grown onto electrospun polyacrylonitrile (PAN) nanofibers membrane (ZIF-8/PAN NF) via hot pressing [79]. ZIF-8 crystals were also produced on an alumina hollow fiber membrane via the in-situ solvothermal method and the layer-by-layer method (Table 2) [80]. This membrane fabrication method requires silanization of alumina with 2% of 3-(aminopropyl)triethoxysilane (APTES) solution using toluene which is toxic and harmful to the environment (Table 2) [80]. In contrast, the membrane preparation methods proposed in our study are entirely water-mediated, rendering them simpler and more environmentally friendly than the one mentioned earlier. Additionally, the CelloZIFPaper exhibited higher adsorption capacity toward heavy metal ions than several other adsorbents (Table 2).</p><!><p>The adsorption performance of ions onto the CelloZIFPaper offers further use for electrochemical sensing (Fig. 7). Electrochemical measurements reveal significant changes in the current output of CelloZIFPaper upon metal adsorption (Figure S14). Thus, CelloZIFPaper can be used as an electrode for metal detection. The preparation of the electrode is simple. CelloZIFPaper, prepared both with the ex-situ and the in-situ procedure 1:0.5 and 1:4 ratio (selected as a reference due to their ZIF content), were cut into rectangular-shaped pieces (1×2 cm 2 ). Then, aqueous solutions of the metal ions (Co 2+ , Cu 2+ , Fe 3+ , Cd 2+ , and Pb 2+ ) were poured dropwise on the electrodes and dried before immersing them into an electrolyte solution (Fig. 7). The electrochemical measurements such as CV and LSV were conducted (Fig. 8). The CelloZIFPaper electrode shows a significant shift in both current outputs, i.e., the anodic and cathodic, after the adsorption of metal ions. The adsorbed Pb 2+ ions show the most remarkable current output change, revealing high selectivity. The high selectivity of the CelloZIFPaper electrode toward Pb 2+ is mainly due to the precipitation as insoluble PbSO4. Tetrahedral metal ions such as Cu 2+ and Co 2+ ions can coordinate with Zn metal ions inside ZIF frameworks. These interactions are further supported by the potentiodynamic polarization test (PD, Figure S15).</p><p>There is an apparent anodic and cathodic polarization after the adsorption of metal ions. The shift toward positive potential, i.e., anodic polarization, of CelloPaper after adding ZIF indicates the possibility of electrons withdrawing from the electrode. There is a significant cathodic polarization (the working electrode becomes more negative) after the adsorption of Co 2+ ions. These observations indicate that CelloZIFPaper can be used as a working electrode for electrochemicalbase sensing of metal ions (Fig. 8c). LSV measurements of CelloZIFPaper before and after metal ions deposition were recorded (Fig. 9). The output current of CelloZIFPaper shows changes upon interaction with different metals.</p><p>Based on the relative output current of the CelloZIFPaper electrode, there is high selectivity toward Pb 2+ compared to the other investigated metal ion species (Fig. 8d). Furthermore, there is a reduction peak at a potential of -0.47 V for Pb 2+ absent for other metal ions. These observations reveal that the CelloZIFPaper electrode can be used to detect Pb 2+ ions using electrochemicalbased sensing. Paper interaction with different concentrations of Pb 2+ was also recorded using LSV (Fig. 9). CelloZIFPaper electrodes show a limit of detection (LOD) of 40 µM and 8 µM for membranes synthesized via ex-situ (Fig. 9a) and in-situ (Fig. 9b) procedures, respectively.  3. The use of CelloZIFPaper as a working electrode is cheaper than conventional electrodes such as glassy carbon electrodes (GCE) [81] or gold (Au) electrodes (AE) for selective detection of Hg 2+ ions [82]. CelloZIFPaper-based electrodes exhibit good current stability using the chronoamperometry test (Figure S16). The presence of active sites such as nitrogen pyridinic-N and pyrrolic-N with two unpaired electrons acted as additional adsorption sites for metal ions [83]. CelloZIFPaper can be used directly without carbonizing ZIF-8 to ZnO@C or deposition of other nanomaterials [81]. It can be used directly without the need for binding or conductive materials. The electrode fabrication is simple and requires no tedious efforts [55]. In addition, the presence of cellulose functional groups (-OH and -COOH) in CelloZIFPaper enhances the metal adsorption making it a good candidate for the electrochemical sensing of, for instance, Pb 2+ (Table 3).</p><p>cyclodextrin (β-CD) [84] and chitosan [85]. ZIF-8@dimethylglyoxime/β-cyclodextrin/reduced graphene oxide (ZIF-8@DMG/β-CD/RGO) was reported for electrochemical sensing of Ni 2+ using differential pulse adsorptive stripping voltammetry (DP AdSV, Table 3) [84]. CelloZIFPaper was used without the need for GCE or chelating agents such as DMG, β-CD, or rGO [84]. Zn/Ni-ZIF-8/Carbon black (XC-72)/Nafion/GCE was used for the detection of Pb 2+ and Cu 2+ using differential pulse voltammetry (DPV, Table 3) [86]. It showed good selectivity towards both of the studied metal ions based on each species' reduction potential [86]. However, the electrode preparation is tedious and suffers from particle aggregation. ZIF8-derived Ag@Au core-shell nanoparticles (Ag@Au/ZIF-8) were modified with an aptasensor modifier and loaded into AE for successful selective detection of Hg 2+ [82]. However, electrode fabrication requires expensive chemical reagents. CelloZIFPaper is promising as a lab-on-paper application for label-free detection of heavy metal ions in aqueous solutions. CelloZIFPaper could potentially be used for related applications, for instance, the adsorption of volatile organic compounds and toxic gases.</p><p>acquisition. Aji Mathew: Conceptualization, methodology, validation, supervision, resources, review and edit of the manuscript, project administration, funding acquisition.</p><!><p>The authors declare no competing financial or any other sort of interest that may influence the work presented in this study.</p>