Conformal Functionalization of Cotton Fibers via Isoreticular Expansion of UiO-66 Metal-Organic Frameworks

We report on the growing of metal-organic frameworks that are isoreticular and isostructural to UiO-66, onto cotton fabrics via a solvothermal method. Four different metal-organic frameworks (MOFs) (UiO-66, UiO-66-NH2, UiO-66-NDC, and UiO-67) were chosen as a case study. The presence of the UiO-based MOFs was confirmed through X-ray diffraction and Scanning Electron Microscopy. We used thermogravimetric analysis to quantify the amount of the MOF loading, which ranged from 0.8% to 2.6% m/m. We also explored the role of ligand size, growth time, and reaction temperature on the conformal coating of cotton fibers with these Zr-based MOFs. Cotton fabrics coated with Zr-based MOFs can find applications as selective filters in aggressive environments due to their enhanced chemical and thermal stabilities.

The isoreticular concept originates from the Greek "iso" meaning "same" and from the Latin "reticulum'' meaning "net" or "network" [18], and refers to the expansion of a unit cell. In the particular case of UiO-66, isoreticular growth refers to an increase in the distances between Zr 6 O 4 (OH) 4 clusters, while keeping the connectivity between the metal oxide cornerstones and the ligands. Isoreticular chemistry is of great relevance, as it can dictate the pore size of the resulting MOF structure [19][20][21][22][23].
The first reported synthesis of UiO-66 and its isoreticular counterpart UiO-67 by Lillerud et al. was reported in 2008 [15]. Five years later, the isostructural UiO-66-NH 2 , in which terephthalic acid is replaced by 2-amino terephthalic acid, adding a -NH 2 moiety to the structure, was reported [24]. Reports on the synthesis of an intermediate member called UiO-66-NDC followed [25]. Ligand replacement can enhance hydrogen bonding [26] and catalytic activity [27] of the MOF. For example, substitution of terephthalic acid with 2,6-naphthalenedicarboxylic acid (NDC) or 4,4'-dibenzoic acid (BPDC) results in an enhanced π-π stacking and a larger pore size [28]. Figure 1a shows a schematic of the structure of UiO-66 with the Zirconium oxide polyhedron forming the cornerstones and each metal oxide cornerstone coordinating with twelve terephthalic ligands. Terephthalic acid can be substituted with different ligands, as highlighted in Figure 1b-e, and it has been reported that the size and chemistry of the ligands have a significant impact on the kinetics of crystal growth [29].
Coatings 2020, 10, x FOR PEER REVIEW 2 of 10 metal oxide cornerstone coordinating with twelve terephthalic ligands. Terephthalic acid can be substituted with different ligands, as highlighted in Figure 1b-e, and it has been reported that the size and chemistry of the ligands have a significant impact on the kinetics of crystal growth [29]. Cotton fabrics are versatile in their fabric pore size, are biodegradable, and offer a flexible yet mechanically strong scaffolding structure to support MOFs in various environments. Potential applications for coated fabrics depend on the functionalization (i.e. type of MOF). In the literature, applications for MOF-functionalized fabrics range from filters [17,30] to sensors [31] and photocatalysts [32].
In a prior work, we reported on the functionalization of cotton with UiO-66 using carboxylated cotton substrates [17]. In this manuscript, we focus on the isoreticular and isostructural growth of UiO-66 while bypassing the carboxylation step. We also show the effect of different reaction conditions on the properties of cotton fabrics functionalized with UiO-based MOFs. We grew the UiO-based structures without the aid of a binder, which was previously required to anchor Zr ions to textile substrates [33]. The optimized reaction pathways and reaction conditions are summarized in Figure 2 along with SEM images illustrating that the resulting cotton fibers are conformably coated with a thin layer of a UiO-based MOFs. Cotton fabrics are versatile in their fabric pore size, are biodegradable, and offer a flexible yet mechanically strong scaffolding structure to support MOFs in various environments. Potential applications for coated fabrics depend on the functionalization (i.e., type of MOF). In the literature, applications for MOF-functionalized fabrics range from filters [17,30] to sensors [31] and photocatalysts [32].
In a prior work, we reported on the functionalization of cotton with UiO-66 using carboxylated cotton substrates [17]. In this manuscript, we focus on the isoreticular and isostructural growth of UiO-66 while bypassing the carboxylation step. We also show the effect of different reaction conditions on the properties of cotton fabrics functionalized with UiO-based MOFs. We grew the UiO-based structures without the aid of a binder, which was previously required to anchor Zr ions to textile substrates [33]. The optimized reaction pathways and reaction conditions are summarized in Figure 2 along with SEM images illustrating that the resulting cotton fibers are conformably coated with a thin layer of a UiO-based MOFs.

Materials
Standardized cotton fabrics TIC-400 were obtained from Testfabrics, Inc. (West Pittston, PA, USA) and were cut into 2 × 2 cm 2 squares using a laser cutter. All chemicals were purchased from commercial sources and were used without further purification. Zirconium (IV), chloride (ZrCl4,

Methods
Cotton swatches were immersed in a scouring solution at 100 °C following a procedure reported in a previous manuscript [17]. Individual procedures for each one of the UiO-based structures are described below.

Cotton+UiO-66 and Cotton+UiO-66-NH2
ZrCl4 (0.55 mmol) was dissolved in a mixture of DMF:HCl (5mL:1mL). Using an ultrasonic bath, either H2BDC (0.75 mmol) or BDC-NH2 (0.75 mmol) were dissolved in 10 mL DMF. A scoured cotton swatch was added to the combined solution. The sealed containers were kept at 85 °C overnight and the functionalized fabrics were washed vigorously three times in DMF and deionized H2O before being dried and conditioned at room temperature.

Cotton+UiO-66-NDC and Cotton+UiO-67
ZrCl4 (0.55 mmol) was dissolved in a mixture of DMF:HCl (5mL:1mL). Either Naphthalene-2,6dicarboxylic acid or 4,4-dibenzoic acid (0.75 mmol) were dissolved in 10 mL of DMF. A scoured cotton swatch was added to the combined solutions. The sealed containers were kept at 120 °C for 24 h, and the functionalized fabrics were washed vigorously three times in DMF and deionized H2O before being dried and conditioned at room temperature for characterization.

Materials
Standardized cotton fabrics TIC-400 were obtained from Testfabrics, Inc. (West Pittston, PA, USA) and were cut into 2 × 2 cm 2 squares using a laser cutter. All chemicals were purchased from commercial sources and were used without further purification. Zirconium (IV), chloride (ZrCl 4

Methods
Cotton swatches were immersed in a scouring solution at 100 • C following a procedure reported in a previous manuscript [17]. Individual procedures for each one of the UiO-based structures are described below.

Cotton+UiO-66 and Cotton+UiO-66-NH 2
ZrCl 4 (0.55 mmol) was dissolved in a mixture of DMF:HCl (5mL:1mL). Using an ultrasonic bath, either H 2 BDC (0.75 mmol) or BDC-NH 2 (0.75 mmol) were dissolved in 10 mL DMF. A scoured cotton swatch was added to the combined solution. The sealed containers were kept at 85 • C overnight and the functionalized fabrics were washed vigorously three times in DMF and deionized H 2 O before being dried and conditioned at room temperature.

Cotton+UiO-66-NDC and Cotton+UiO-67
ZrCl 4 (0.55 mmol) was dissolved in a mixture of DMF:HCl (5 mL:1 mL). Either Naphthalene-2,6dicarboxylic acid or 4,4-dibenzoic acid (0.75 mmol) were dissolved in 10 mL of DMF. A scoured cotton swatch was added to the combined solutions. The sealed containers were kept at 120 • C for 24 h, and the functionalized fabrics were washed vigorously three times in DMF and deionized H 2 O before being dried and conditioned at room temperature for characterization.

Characterization of the Samples
X-ray diffraction (XRD) experiments were performed on a Bruker (Madison, WI, USA) D8 powder diffractometer with a step size of 0.04 • . Scanning electron microscopy (SEM) was performed on a LEO 1550 FESEM (Keck SEM, Maple Grove, MN, USA). Prior to imaging, the specimens were coated with a thin layer of carbon. Thermograms were obtained using a TGA 550 thermal analyzer (TA Instruments, New Castle, DE, USA, heating rate of 10 • C/min) in air.

Results
We bypassed the cellulose carboxylation step commonly reported as a necessary condition for growth MOF structures onto cotton fibers, showing that MOFs can be grown directly onto unmodified cotton fibers. We observed that longer ligands (naphthalene-2,6-dicarboxylic acid and 4,4-dibenzoic acid) required of higher temperatures and longer reaction times to grow on the surface of the cotton fabrics. At 85 • C and 12 h, UiO-66-NDC and UiO-67 did not precipitate and the cotton fibers showed no evidence of crystal growth on their surfaces. However, when the reaction conditions were modified according to Butova et al. [34], that is to say, the temperature was increased from 85 • C to 120 • C and the growth time is increased from 12 h to 24 h, UiO-based structures were observed on the surface of the fabrics. Ragon et al. showed that not only does the availability of the inorganic cluster highly influence the crystallization rate [35], but also the kinetics of MOF growth become slower when the size of the ligand is longer [29].
The growth of isoreticular UiO-based MOFs on the surface of cotton fibers was confirmed using X-ray diffraction as shown in Figure 3. The bottom trace in Figure 3, corresponding to the scoured cotton specimen, exhibits broad diffraction peaks at 15 • , 17 • , and 23 • , which are commonly assigned to cellulose I [36,37]. A clear superposition of the cellulose I diffraction peaks can be observed in the XRD spectra for all five specimens, indicating that the crystallinity of the cotton fibers was not modified during growth of the UiO-based MOFs. Furthermore, well defined diffraction peaks ascribed to Zirconium MOFs are observed at lower angles (between 5 • and 14 • ). The pattern of the specimens quantitatively agrees with those reported in the literature for powdered UiO-66 [15], UiO-66-NDC [38], and UiO-67 [24], and also agrees with PXRD simulations of the respective MOFs showed in dotted lines in Figure 3. UiO-66-NH 2 is isostructural with UiO-66 [24]; therefore, the diffraction peaks for cotton+UiO-66 and cotton+UiO-66-NH 2 show the same diffraction peak positions. UiO-66-NDC and UiO-67 exhibit larger cell parameters, which is confirmed by the shifting of the diffraction peaks to lower angles.
The SEM images shown in Figure 4 illustrate the conformal surface coverage of the cotton fibers for all specimens. Figure 4A shows how crystals of UiO-67 form clusters of various sizes. Images of the cotton+UiO-67 and the cotton+UiO-66-NDC specimens show a uniform MOF layer that covers the cotton, and on top of that layer, additional crystalline structures are seen ( Figure 4A-D). These node-like structures are also seen on the images of the cotton+UiO-66-NH 2 and the cotton+UiO-66 specimens ( Figure 4E-H).
SEM images taken at higher magnifications confirm the coverage of the cotton's surface ( Figure 4B,D,F,H). Cotton+UiO-66-NDC, cotton+UiO-66-NH 2, and cotton+UiO-66 specimens show a lower dispersion in particle size than the cotton+UiO-67 specimen. This observation can be explained on slower nucleation and growth kinetics. While MOFs with smaller ligands grow fast, the generation of nucleolus is instantaneous, and all these nucleoli grow simultaneously. Larger ligands hence generate a more polydisperse crystalline system [39]. Figure 5 shows the thermograms of the cotton fabrics coated with UiO-based MOF that were performed to quantify the amount of MOF growth onto each fabric. In the thermogram, a small weight loss between 30 and 100 • C, due to loss of water (3%-4%), is noted for all specimens. UiO-66-NH 2 exhibits a difference in the thermogram shape between 200 and 300 • C. This behavior was previously reported and could be related to the degradation of the amino group that is present only in this MOF [40]. Oxidative degradation of cellulose chains, resulting in combustion gases, is generally observed between 300 and 650 • C, and char pyrolysis occurs at temperatures higher than 400 • C [41,42]. At temperatures ranging between 300 and 350 • C, dehydration of Zr 6 O 4 (OH) 4 to Zr 6 O 6 occurs [24]. The thermogram of the cotton specimen shows a complete transformation into H 2 O and CO 2 leaving less than 0.2% solid residue, which is used as reference for further calculations. Weight measurements between 700 and 900 • C indicate total degradation of cellulose and of the UiO66-based MOFs. Values of the residual mass of ZrO 2 left after the TGA experiments are shown in Table 1. Table 1 presents the MOF formula for each specimen, the residue (% m/m), and the MOF loading. The residue column in Table 1 is the sum of the ash from the unmodified cotton specimen and the mass of ZrO 2 . The ZrO 2 % in the specimen is calculated by subtracting the mass of the ash residue obtained from the ash generated by the untreated cotton. The mass of ZrO 2 residue is used to calculate the loading of the MOF using the MOF formula and assuming a complete transformation of that structure into CO 2 , H 2 O, and ZrO 2 . The cotton+UiO-66-NH 2 specimen has the highest MOF loading, while the cotton specimen coated with UiO-67 exhibited the lowest weight residue, albeit being the heaviest of all MOFs used in this study.  The SEM images shown in Figure 4 illustrate the conformal surface coverage of the cotton fibers for all specimens. Figure 4A shows how crystals of UiO-67 form clusters of various sizes. Images of the cotton+UiO-67 and the cotton+UiO-66-NDC specimens show a uniform MOF layer that covers the cotton, and on top of that layer, additional crystalline structures are seen ( Figure 4A-D). These node-like structures are also seen on the images of the cotton+UiO-66-NH2 and the cotton+UiO-66 specimens ( Figure 4E-H).  SEM images taken at higher magnifications confirm the coverage of the cotton's surface ( Figure  4B,D,F,H). Cotton+UiO-66-NDC, cotton+UiO-66-NH2, and cotton+UiO-66 specimens show a lower dispersion in particle size than the cotton+UiO-67 specimen. This observation can be explained on slower nucleation and growth kinetics. While MOFs with smaller ligands grow fast, the generation of nucleolus is instantaneous, and all these nucleoli grow simultaneously. Larger ligands hence generate a more polydisperse crystalline system [39]. Figure 5 shows the thermograms of the cotton fabrics coated with UiO-based MOF that were performed to quantify the amount of MOF growth onto each fabric. In the thermogram, a small weight loss between 30 and 100 °C, due to loss of water (3%-4%), is noted for all specimens. UiO-66-NH2 exhibits a difference in the thermogram shape between 200 and 300 °C. This behavior was previously reported and could be related to the degradation of the amino group that is present only in this MOF [40]. Oxidative degradation of cellulose chains, resulting in combustion gases, is generally observed between 300 and 650 °C, and char pyrolysis occurs at temperatures higher than 400 °C [41,42]. At temperatures ranging between 300 and 350 °C, dehydration of Zr6O4(OH)4 to Zr6O6 occurs [24]. The thermogram of the cotton specimen shows a complete transformation into H2O and CO2 leaving less than 0.2% solid residue, which is used as reference for further calculations. Weight measurements between 700 and 900 °C indicate total degradation of cellulose and of the UiO66-based MOFs. Values of the residual mass of ZrO2 left after the TGA experiments are shown in Table 1.     Table 1 presents the MOF formula for each specimen, the residue (% m/m), and the MOF loading. The residue column in Table 1 is the sum of the ash from the unmodified cotton specimen and the mass of ZrO2. The ZrO2 % in the specimen is calculated by subtracting the mass of the ash residue obtained from the ash generated by the untreated cotton. The mass of ZrO2 residue is used to calculate the loading of the MOF using the MOF formula and assuming a complete transformation of that structure into CO2, H2O, and ZrO2. The cotton+UiO-66-NH2 specimen has the highest MOF loading,

Conclusions
Cotton fibers were successfully coated with an isoreticular Zr-MOF series, namely, UiO-66, UiO-66-NH 2 , UiO-66-NDC, and UiO-67, using a solvothermal method and without the need of prior functionalization of the cotton. Ligand solubility in DMF as well as particle formation kinetics appear to have the most impact in the growth of the UiO-based MOFs on the surface of cotton fibers. The growth of UiO-based MOFs was confirmed using XRD and SEM. The harsh washing of the specimens provides evidence that the coatings of UiO-based MOFs on the cotton fabrics are mechanically robust. The experimental results also offer evidence that solvothermal synthesis is an effective pathway for fabric functionalization and that with minor modifications in reaction time and reaction temperature, this method can be extended to an entire MOF family. Thermogravimetric analysis was used to determine the amount of loading on each fabric, which ranged from 0.8% to 2.6% We found that the larger the MOF structure, the lower the loading of the MOF on the fabric, which is in agreement with the need for longer growth times at higher temperatures to accelerate the kinetics and facilitate solubilization of the reagents. Further efforts are needed in order to understand whether the thickness of the MOF coating is dependent on the availability of starting materials, on the thermodynamics of crystal growth or on steric restrictions. The reported method opens the possibility for a vast range of applications for functional fibers operating in aqueous environments under varying pH values and relatively high temperatures.