Fabrication of cellulose based superhydrophobic microspheres for the production of magnetically actuatable smart liquid marbles

Cellulose microspheres were fabricated on the basis of sol-gel transition using NaOH/urea/H2O as the solvent system. These microspheres had an average diameter of about 30 μm. Upon modification with Fe3O4 and poly (DOPAm-co-PFOEA), superhydrophobic magnetic cellulose microspheres were generated, which were analyzed by FTIR, TG, XRD, XPS and water contact angle tests. Magnetic cellulose microspheres contained approximately 15 wt% of Fe3O4. Poly(DOPAm-co-PFOEA)/Fe3O4/cellulose microspheres and had a low surface energy and a high water-repellency. These superhydrophobic microspheres were also converted into liquid marbles via an easily scalable process.


INTRODUCTION
The development of superhydrophobic materials with an extremely high water-repellency has gained great attention due to their interesting potential applications in terms of self-cleaning, anti-corrosion, fuel transport, and low dragging coatings. 1,2 These materials are essentially fabricated by combining material surface roughness with low surface energy material.
Magnetic cellulose microspheres can be fabricated on the basis of in-situ synthesis, and these materials have excellent adsorption capacity and controlled delivery on bovine serum albumin. 6 The filter paper coated by chitosan and beeswax has strong resistance to water vapor and grease. 11 Cellulose acetate can be generated by reacting cellulose with acetic anhydride, and its hydrophobicity can be improved by increasing the degree of substitution (DS); this cellulose derivative can also be formed into liquid marbles once water droplets are embedded into its matrix. 15 Interestingly, in one study, the grafting of methyl acrylate onto cellulose fibers via atom transfer radical polymerization (ATRP) was reported. 7 In this process, the hydrophobicity of cellulose fibers was significantly improved. For the grafted fibers with a degree of polymerization of 300, water contact angle was increased to 133°. Upon modification with SiO 2 nanoparticles and octyltrimethoxysilane, surface energy of the filter paper was further reduced, resulting in the generation of superhydrophobicity and superoleophilicity. 10 In our previous study, 5 we reported the concept of using superhydrophobic magnetic cellulose microspheres as building blocks for the fabrication liquid marbles, which had a remarkable stability in terms of liquid droplet transportation and manipulation. As a continuation of this study, the process for the fabrication of superhydrophobic magnetic cellulose microspheres and their morphological characteristics were investigated.

Materials
The raw cellulose was bamboo dissolving pulp, which was obtained from Sichuan Tianzhu Bamboo Resources Development Co. Ltd. (Sichuan, China). The gel permeation chromatographic (GPC) (Waters 1515) measurements showed that cellulose had a M w of 141 k and a M n of 56 k. Analytical-grade ethanol, ammonia, sodium hydroxide (NaOH), urea, ferric chloride, ferrous chloride, and other reagents were purchased from Aladdin Reagent (Shanghai) Co. Ltd. and used without further purification.

Fabrication of cellulose microspheres (CM)
11 g of cellulose was added to 200 mL of NaOH/urea/H 2 O (7:12:81 by weight) solution at -13 °C with stirring for 10 min to generate a homogenous cellulose solution. 100 mL of cellulose solution was added into 600 mL of the solution of Span-80 in paraffin oil (5 wt%) and stirred at 800 rpm for 5 h. The cellulose droplets were converted into solids, and CM was regenerated when the suspension was heated to 45 °C followed by sufficient washing with distilled water and ethanol.

Fabrication of magnetic cellulose microspheres (MCM)
0.15 g of FeCl 2 and 0.4 g of FeCl 3 (mole ratio FeCl 3 : FeCl 2 = 2:1) were dissolved in deionized water, and 1 g of CM was added to the solution by stirring under vacuum. 0.86 mL of ammonia (25 %) was added and stirred to allow for complete reaction. The MCM was obtained by washing the mixtures with water to remove excess ammonia, NH 4 Cl and impurities.

Fabrication of liquid marbles
Liquid marbles were prepared by shaking water droplets onto a layer of PMCM. 5 µL of water was rolled over the powders to form stable magnetic liquid marbles.

Characterizations
Water contact angle was tested with a contact angle instrument (DSA30, GER KRUSS) by placing water droplets on the surface of molded powders. Cellulose microspheres were observed with an optical microscope (XPL-60). Fourier transform infrared spectroscopic spectra of cellulose microspheres were recorded with a FTIR spectrometer (Bruker, VERTEX70) having a resolution of 4 cm -1 . Thermal gravimetric analysis of microspheres was conducted with a TG-DTA instrument (Netzsch STA 449F3). X-ray diffraction patterns were recorded with a MiniFlex-2 diffractometer. The data were collected by using the Cu-Kα radiation in the scanning range of 5° to 60° at a scanning speed of 5 °/min. Surface chemical composition of cellulose-based microspheres was tested by using an X-ray photoelectron spectrometer (XPS, ESCALAB 250 spectrometer) with Al Kα X-ray source.
Magnetic properties of PMCM were tested with a PPMS-9 vibrating sample magnetometer (Quantum Design, USA) at 300 K. Inset is the size distribution measured with a laser particle size analyzer.

RESULTS AND DISCUSSION
CM can be fabricated via a sol-gel transition process with NaOH/urea/H 2 O as the solvent system. 22 The generation of MCM can be achieved by CM modification with Fe 3 O 4.
6 In this study, MCM was modified with poly(DOPAm-co-PFOEA) for superhydrophobicity development. Figure 1 shows the optical microscopic image and size distribution of CM. CM exhibited a spherical shape, and its particle size distribution was tested with a laser particle size analyzer. CM had a diameter of 20-60 µm and an average particle diameter of 30 µm.
Cellulose, CM, MCM and PMCM were characterized by FTIR spectroscopy. As shown in Figure 2, the characteristic peaks of cellulose located at 1640 cm -1 and 898 cm -1 can be attributed to the stretching vibration of β-(1-4) glycosidic bond of gluconolactone. The absorption peak of CM at 3445 cm -1 shifted to a higher wavenumber compared with that of cellulose, possibly indicating enhanced intermolecular hydrogen bonding as a result of cellulose regeneration. 22 The characteristic absorption peak of the Fe-O bond of Fe 3 O 4 located at 587 cm -1 showed the presence of Fe 3 O 4 in MCM. 6 When compared to MCM, www.Bioresources-Bioproducts.com 112 new absorption peaks in PMCM appeared at 1739 cm -1 , 1576 cm -1 , 1203 cm -1 , which corresponds to stretching vibration of the C=O bonds, amide II and stretching vibration of the C-F bonds, respectively. These results indicated that poly(DOPAm-co-PFOEA) was chemically bound to MCM. Figure 3 shows the TG curves of cellulose, CM, MCM, and PMCM. Weight losses (3 wt.%, 80 wt.%, and 13 wt.%) of cellulose during first, second and third stages can be attributed to water evaporation (30- In order to verify the modification of CM with Fe 3 O 4 and poly(DOPAm-co-PFOEA), XPS spectra of unmodified/modified microspheres were tested. Figure 5A shows the wide scan spectra of cellulose, CM, MCM and PMCM. The wide scan spectra of cellulose and CM showed two main signals corresponding to C1s and O1s. In the spectrum of MCM, the presence of a typical peak with binding energies of 710 eV corresponds to Fe1s. Furthermore, the presence of poly(DOPAm-co-PFOEA) on the PMCM surface can be evidenced from the new peak (689 eV) associated with the fluorine species of poly(DOPAm-co-PFOEA). As shown in Figure 5B To evaluate the magnetic properties of PMCM, PMCM were measured with a vibrating sample magnetometer. Figure 6 shows magnetization as a function of applied magnetic field. The saturation magnetization obtained from the hysteresis loop was 9.4 emu/g, showing that PMCM had good magnetic properties. The small hysteresis loop and low coercivity showed that the magnetization of PMCM had a superparamagnetic behavior. To evaluate the hydrophobicity of PMCM, water contact angels were tested by placing water droplets on molded powders. The contact angle of CM was less than 10° ( Figure 7A), due to the hydrophilic nature of cellulose. Upon modification with poly(DOPAm-co-PFOEA), water contact angle of PMCM increased to 154.7° ( Figure 7B). These results indicated PFOEA moieties containing fluorinated units imparted cellulose-based microspheres a low surface energy, leading to superhydrophobicity development. Due to the low surface energy of PMCM, PMCM can encapsulate liquid droplets to form liquid marbles. Liquid marbles are droplets covered by hydrophobic micro-or nano-scaled particles. 26 Hydrophobic 5,[27][28][29][30][31][32][33] or Janus 34,35 micro-or nano-particles were generally used to fabricate liquid marbles. As a result of surface encapsulation with hydrophobic materials, the apparent surface energy of liquid decreases. Liquid marbles have attracted significant attention in miniature reactors, microfluidics, drug transport, gas sensing and water pollution detection, etc. [36][37][38][39][40][41][42][43] www.Bioresources-Bioproducts.com

CONCLUSION
The process for the fabrication of superhydrophobic magnetic cellulose-based microspheres and their morphological characteristics were investigated. On the surface of modified cellulose-based microspheres, DOPAm moieties were anchored to magnetic Fe 3 O 4 , while PFOEA moieties were coated onto the surface to impart low surface energy and excellent liquid repellency. These cellulose microspheres encapsulated water droplets to form stable liquid marbles, which may find use in microreactors, micro-pumps, gas and water pollution sensors, biological engineering, and biomedical applications.