Layer-by-layer decoration of MOFs on electrospun nanofibers

The design and fabrication of novel organic–inorganic nanocomposite membranes using metal–organic frameworks as building blocks have attracted numerous scientists. Here, HKUST-1 particles were decorated on crosslinked polymer nanofibers through a layer-by-layer method. The immersion sequence, the crosslinking and the number of the deposition cycles have a significant impact on the formation of the HKUST-1 decorated nanofibrous membranes. Moreover, it has been shown that such a membrane could be applied as a catalyst for visual detection of hydrogen peroxide.


Introduction
Over the past two decades, metal-organic frameworks (MOFs), which are built from metal-based nodes and organic linkers, have been recognized as an emerging new type of porous material, which holds great potential for applications in adsorption, separation, catalysis, energy storage and conversion, sensing and drug delivery. [1][2][3][4] The fascinating features such as highly tailorable nanostructures of the MOF materials have attracted considerable interest. However, because of their fragility, it is urgent to develop MOF composite materials to meet the requirements of practical devices.
An electrospun nanober membrane has acted as outstanding exible three-dimensional template for the assembly of various nanomaterials since it provides a high surface area, highly porous nanostructure and large surface area to volume ratio. [5][6][7] Since the pioneering work done by Hatton et al., recent emerging MOF-assembled electrospun nanobers have been designed and applied to many elds such as explosive detection, gas separation and light-emitting devices. [8][9][10][11] Generally, there are two different strategies to fabricate such composite material. The rst strategy is the assembly of ex situ synthesized MOFs in the nanobers by mixing these MOFs with polymers in the precursor solution.  For example, four kinds of MOFs were synthesized and mixed with three polymers such as polyacrylonitrile (PAN), polystyrene (PS), and poly(vinylpyrrolidone) (PVP) in solution by Wang et al. 31 Then the mixture was electrospun into nanobers, which showed impressive capability of air pollution control. Moreover, another group reported that different amounts of ZIF-8 MOF nanoparticles were incorporated into the poly(lactic acid) (PLA) to prepare the nanocomposite electrospun membrane. 25 The obtained PLA/ZIF-8 electrospun nanober membranes exhibited increased oil wettability and improved mechanical property than the pure PLA membrane. It should be noted that the composite membrane with imbedded MOFs is not good at accessing external chemicals because of the polymer barrier, though these MOFs are well protected by the surrounding polymers. The second strategy is the in situ growth of MOFs on the electrospun nanobers. 8,[41][42][43][44][45][46][47][48][49][50][51][52][53] Peterson and Parsons found that Zr-based MOF thin lms could be formed in situ on the TiO 2 coatings, which was deposited via atomic layer deposition onto polyamide-6 electrospun nanobers. 51 Another work presented by Ma and coworkers showed that bimetal ZIFs could directly grow on 2-methylimidazole/PAN electrospun nanober mats, which showed excellent electrocatalytic performance for oxygen reduction reaction aer the carbonization step. 52 Owing to the drawbacks such as time-consuming procedures and high cost of the existing methods, a facile and controllable method is urgently required for the design and fabrication of MOF nanocomposite membrane.
Here, HKUST-1, which is one of the most widely studied MOF materials, was used as model MOF to decorate poly-(acrylic acid)/poly(vinyl alcohol) (PAA/PVA) electrospun nano-bers using a layer-by-layer method. Before the decoration, a simple and fast crosslinking process was applied to generate water-stable nanobers with abundant active sites. The morphological evolution of the HKUST-1 decorated electrospun nanobers was systematically investigated. Furthermore, the MOF nanocomposite membrane was utilized for the rst time as a colorimetric platform for visual detection of hydrogen peroxide.

Preparation of PAA/PVA electrospun nanober membrane
PAA (M w ¼ 240 000, J&K Scientic) and PVA (PVA-2088, Shanghai Chenqi Chemical Science Co., Ltd) with weight ratio of 4 : 1 were dissolved in water by stirring for 12 h to form a 25 wt% transparent solution. In a typical electrospinning process, the PAA/PVA aqueous solution was pumped through a syringe pump at a ow rate of 0.3 mL h À1 . The applied voltage was kept at 15 kV, while the distance between the needle tip and the collector was 15 cm. The PAA/PVA electrospun nanober membrane was collected and then dried in a vacuum oven at room temperature. The obtained nanobers were crosslinked upon a simple heating treatment at 145 C for 15 min.

Layer-by-layer decoration of MOFs on electrospun nanobers
The growth of the HKUST-1 MOFs on the PAA/PVA electrospun nanobers was accomplished using a layer-by-layer method. Briey, each growth cycle consisted of the immersion of the nanobers in the 0.05 M copper(II) acetate (Cu(OAc) 2 , Sinopharm Chemical Reagent Co., Ltd) aqueous solution for 2 h and the immersion of the nanobers in the 0.05 M 1,3,5- benzenetricarboxylic acid (BTC, Sinopharm Chemical Reagent Co., Ltd) ethanol solution for 2 h. The membrane was rinsed with water and ethanol aer each immersion step. In order to control the whole growth process of dense MOFs-decorated nanober membrane, different growth cycles were performed.
The HKUST-1 powder sample was prepared by direct mixing 10 mL of 0.15 M Cu(OAc) 2 solution with 10 mL of 0.1 M BTC solution under stirring. The reaction was allowed to proceed for 24 h, and then the HKUST-1 particles were collected by centrifugation at 10 000 rpm for 10 min. Finally, the particles were washed with ethanol and dried in an oven.

Visual detection of hydrogen peroxide
1 mg of HKUST-1 decorated nanober membrane was added into 4 mL PBS buffer solution (pH ¼ 7), followed by adding 100 mL of 50 mM o-phenylenediamine (OPD, Sinopharm Chemical Reagent Co., Ltd) solution and 150 mL of 50 mM hydrogen peroxide (H 2 O 2 , Sinopharm Chemical Reagent Co., Ltd). The mixed solution was incubated at 75 C for 30 min. Then the optical spectrum of the solution was recorded using an UV-vis spectrophotometer (Unico UV-4802).

Characterization
The morphologies of the nanober membranes were observed using scanning electron microscopy (SEM, Hitachi SU8010). The X-ray diffraction (XRD) patterns were acquired from a DX-2700 X-ray diffractometer. The thermogravimetric (TG) measurements were performed on a TA Q 50 thermogravimetric analyzer. Fourier-transform infrared spectroscopy (FTIR) spectra were collected with a Nicolet IS50 spectrometer using an attenuated total reection module. The specic surface area of the nanober membranes were measured by a Micromeritics ASAP-2460 accelerated surface area and porosimetry system.

Results and discussion
It is worth to mention that the in situ growth strategy requires both stable nanobers as the template and numerous active groups (e.g., hydroxyl and carboxyl) to anchor the MOFs. PAA/ PVA nanobers were selected as template for the growth of HKUST-1 because they can provide lots of binding sites for the metal ions. However, the nanober membrane is soluble in aqueous solution owing to the usage of water-soluble polymers. A facile thermal treatment was introduced to crosslink the carboxylic acid of PAA and the hydroxyl groups of PVA, resulting in water-stable PAA/PVA nanobers. 54,55 As shown in Fig. 1a and b, the diameter of the crosslinked nanobers (about 250 nm) was a bit larger than that of the pristine nanobers (about 150 nm), while the three-dimensional porous feature of the membrane was retained. This nanobrous nanostructure benets the access of the metal ions and the ligands in the growth solution.
The deposition of HKUST-1 on the nanobers was accomplished by repeated dipping the nanober membrane rst in Cu(OAc) 2 solution and then in BTC solution. Aer one cycle of growth, a few typical octahedral HKUST-1 crystals were observed on the surface of the nanobers (Fig. 1c). More and more MOF Paper particles were decorated on the nanobers by increasing the deposition cycles (Fig. 1d and e), and nally a dense lm ful-lled with HKUST-1 particles was achieved aer eight cycles of growth (Fig. 1f). The size of the HKUST-1 crystal also increased with the increasing deposition cycles. Moreover, the color of the membrane changed from white to blue, implying the successful decoration of HKUST-1 (the insets of Fig. 1). It is worth mentioning that the layer-by-layer growth allows the formation of uniform MOF-decorated electrospun nanobers over a large area (e.g. Fig. 1e), which might facilitate the fabrication of large area devices.
The decoration of HKUST-1 on the nanobers was further analyzed using multiple techniques. The XRD pattern of the PAA/PVA nanobers shows an amorphous structure, while the patterns of the MOF decorated nanobers contain characteristic diffraction peaks of HKUST-1 (Fig. 2a), suggesting the formation of well-crystallized MOF structure. 14 Further conrmation of the formation of the nanocomposite membrane is provided by the FTIR spectra presented in Fig. 2b. Both the typical vibration bands of HKUST-1 located at 729, 759, 1370 and 1714 (carbonyl stretching, also showed in PAA/PVA nanobers) cm À1 and the vibration of -CH 2group of the polymer nanober located at 1246 cm À1 were clearly observed for all MOF-decorated nanober membranes. 46 What is more, the TGA tests suggest that the content of the residue increases with the increasing content of the decorated HKUST-1 (Fig. 2c). Since porous MOF materials were assembled on the electrospun nanobers, it is expected that the surface area would intensively increase. The Brunauer-Emmett-Teller (BET) surface area of the HKUST-1 decorated nanobers increased from 25.7 m 2 g À1 aer one cycle of growth to 227.7 m 2 g À1 aer eight cycles of growth, while the BET surface area of the PAA/PVA nanobers is 1.9 m 2 g À1 . It is believed that such MOF membrane could be applied to gas adsorption and separation.  In order to get more insight into the growth process, complementary experiments were performed. Fig. 3a shows that none HKUST-1 particles would form without the BTC ligand. When the nanobers were immersed into the ligand solution rst and then into the metal ion solution (Fig. 3b), there was no decoration of MOFs on the nanobers, indicating that the active sites on the PAA/PVA nanobers should bind the metal ion rst through electrostatic interaction or chelation. Moreover, the crosslinking process could efficiently affect the amount of the active groups since the active groups of the polymers take part in the crosslinking, resulting in less decoration of HKUST-1 on the nanobers with increasing crosslinking time (Fig. 3c and d).
Owing to its extreme simplicity and low cost, visual detection of target analyte, which is based on color changes observed by naked eyes, has received a lot of attention. It is recently reported that some MOFs show peroxidase-like catalytic activities, catalyzing the oxidation of o-phenylenediamine (OPD) with obvious color change of solution in the presence of hydrogen peroxide (H 2 O 2 ). 56,57 As a proof of concept, the visual detection of H 2 O 2 using HKUST-1 decorated nanober membrane as catalyst is demonstrated here. As shown in Fig. 4, no absorption was noticed for the solutions consisted of OPD and H 2 O 2 , while the solution consisted of OPD, H 2 O 2 and HKUST-1 decorated nano-ber membrane turned brownish yellow, which was eye-catching. The oxidation of OPD might be mediated by the Cu 2+ ions in HKUST-1 because the PAA/PVA nanober membrane did not show catalytic activity (Fig. 5a). The Cu 2+ ions might act as a peroxidase mimic to the oxidation of OPD, and they might originate from the dissolution of the HKUST-1 particles. It is notable that the dissolved oxygen may also act as oxidant, resulting in light yellow solution aer the addition of HKUST-1 decorated nanober membrane. 58 Nevertheless, a novel platform based on the peroxidase-like activity of the HKUST-1 decorated nanober membrane was successfully established for visual detection of H 2 O 2 . Moreover, this MOF-decorated nano-ber membrane could be readily recycled and reused for at least ve times (Fig. 5b), which is superior to other MOF powders. 56,57

Conclusions
HKUST-1 particles could be decorated on the surface of the crosslinked PAA/PVA nanobers via alternative immersion into metal ion solution and ligand solution. This controllable layerby-layer method is easy-to-process and cost-efficient. The surface area of the HKUST-1 decorated nanober membrane was 227.7 m 2 g À1 aer eight cycles of growth. Furthermore, the HKUST-1 decorated nanober membrane could be used for the visual detection of H 2 O 2 . It is believed that such designed MOFdecorated nanober membranes may have a promising future in many elds such as gas adsorption, hazardous removal and catalysis.

Conflicts of interest
There are no conicts to declare.