Water-Insoluble Hydrophilic Electrospun Fibrous Mat of Cyclodextrin

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Water-Insoluble Hydrophilic Electrospun Fibrous Mat of Cyclodextrin-Epichlorohydrin Polymer as Highly Effective Sorbent Asli Celebioglu, Fuat Topuz, and Tamer Uyar ACS Appl. Polym. Mater., Just Accepted Manuscript • DOI: 10.1021/acsapm.8b00034 • Publication Date (Web): 29 Nov 2018 Downloaded from http://pubs.acs.org on December 2, 2018

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ACS Applied Polymer Materials

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Water-Insoluble Hydrophilic Electrospun Fibrous Mat

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of Cyclodextrin-Epichlorohydrin Polymer

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as Highly Effective Sorbent

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Asli Celebioglu, Fuat Topuz and Tamer Uyar*

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Institute of Materials Science & Nanotechnology, UNAM-National Nanotechnology Research

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Center, Bilkent University, 06800 Ankara, Turkey

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*

To whom correspondence should be addressed: Prof. T. Uyar ([email protected]) Phone: +90 (312) 290 8987

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ABSTRACT. The electrospinning of cyclodextrin (CD) functional fibers has emerged as a

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promising strategy to develop high performance fibers with great promising applications in

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many fields, including water treatment. Here, we report the fabrication of water-insoluble

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hydrophilic poly(cyclodextrin-epichlorohydrin) (poly(CD-ECH)) fibers by the electrospinning

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of an aqueous solution of hydroxypropyl--CD (HP--CD) and ECH, and their cross-linking

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through heat treatment. The viscosity and time-dependent oscillatory deformation tests

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revealed that the cross-linking reactions between CD and ECH are highly sensitive to

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temperature, and main cross-linking takes place after the heat-treatment of the electrospun

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mat, which was confirmed by FTIR and XPS analyses. The water contact-angle (WCA)

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measurement showed the hydrophilicity of the poly(CD-ECH) mat, which could maintain its

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fibrous structure in water. Furthermore, the poly(CD-ECH) fibers were stable in various

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organic solvents, i.e., acetonitrile, ethanol, methanol, trichloromethane, dimethylformamide

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and dimethyl sulfoxide. Thermal gravimetric analysis (TGA) showed the cross-linking

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increased thermal stability of the poly(CD-ECH) fibers compared to the pristine CD, while

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XRD analysis revealed the amorphous structure of the poly(CD-ECH) fibers. As a proof-of-

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concept study, the fiber and granule forms of the poly(CD-ECH) were exploited for the

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scavenging of an organic dye (i.e., phenolphthalein) and a polycyclic aromatic hydrocarbon

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(PAH) (i.e., phenanthrene) from their aqueous solutions, and rapid removal was observed for

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the poly(CD-ECH) fibers than its granule form. Given that the hydrophilic nature of the

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poly(CD-ECH) electrospun mat, this high scavenging performance can be ascribed to the

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presence of high active CD content in the poly(CD-ECH) fibers, along with their high specific

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surface area.

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Keywords.

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Cyclodextrin,

electrospinning,

nanofibers,

inclusion-complexation,

phenolphthalein, phenanthrene, water treatment

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INTRODUCTION

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Cyclodextrin (CD) is a cyclic oligomer of glucose with a unique torus-like shape with a

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central hydrophobic cavity interior and a hydrophilic exterior.1 CDs are inherently nontoxic2

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and edible molecules,3 and have been widely used as solubilization enhancers for poorly

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soluble drug molecules,4 drug carriers,5-6 food3 and tissue engineering7, and textile industry8.

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Likewise, materials derived from CDs have taken considerable interest in a wide spectrum of

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applications, in which each CD molecule can involve in the removal of water

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micropollutants.9-13 In one example, Alsbaiee et al. reported the synthesis of porous -CD

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polymers for the removal of various organic micropollutants, i.e., 2, 4-dichlorophenol, 2-

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naphthol, 1-napthylamine, metolachlor, bisphenol A, bisphenol S, ethinyl oestradiol,

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propranolol hydrochloride, from water.14 The authors used tetrafluoroterephthalonitrile as a

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linker for the production of porous CD polymers with a surface area as high as 263 m2 g-1,

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which allowed rapid removal of organic pollutants. The same research group used

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decafluorobiphenyl as a cross-linker of -CD moieties under nucleophilic aromatic

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substitution conditions and exploited the CD networks for the scavenging of environmentally

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persistent perfluorooctanoic acid (PFOA).15 The materials could sequester PFOA in a few

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hours from aqueous solutions. Likewise, epichlorohydrin (ECH) cross-linked CD polymers

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were exploited as sorbents for the removal of various types of pollutants.16-17 The sorption

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performance of such materials can be enhanced with increasing their surface areas. In this

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regard, the electrospinning of CD-functional nanofibers has been a focus of interest owing to

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their high specific surface area (i.e., more active CD content) and highly porous structure,

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which enable faster and efficient removal of organic micropollutants.

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The embedment of CD molecules via blending with a polymeric carrier is a simple route to

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produce CD-functional electrospun fibers. But, the possibility of leaching of CD molecules

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from the fiber matrix restricts their reuse18 unless they are chemically attached to the fibrous 3 ACS Paragon Plus Environment


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network19. Further, their embedment within the polymeric matrix can substantially block the

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CD cavity to able to form inclusion-complexation. Many studies have been reported using this

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strategy due to its simplicity. In this regard, Zhao et al. reported the preparation of CD-

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functional fibers using the mixed solution of poly(acrylic acid), citric acid and -CD. The

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electrospinning and thereafter, the heat-treatment of the resultant fibers led to CD-functional

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fibers, which were exploited for the removal of methylene blue with a maximum adsorption

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capacity of 826.45 mg/g.20 -CD was also loaded in the poly(ether sulfone) (PES) nanofibers

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and implemented for water depollution from hormone estradiol and pesticide chlorpyrifos.21

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The removal performance of the materials was compared with the CD-free PES fibers, and the

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results revealed an enhanced sorption capacity with the incorporation of 10 wt.% CD. CD was

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also used for the decoration of the fiber surface to enhance the sorption performance of fibers.

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In this regard, Kayaci et al. reported the surface modification of polyester nanofibers with CD

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motifs and exploited the fibers for the removal of phenanthrene.22 On the other hand, the use

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of electrospun fibers made by only CDs would be a rational strategy with enhanced

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performance for sorption applications.23-24 However, such fibers suffer from rapid dissolution

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in water by reason of their uncross-linked structure.25-26 To address this problem, in our recent

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study, we have reported the production of electrospun fibers of cross-linked CD polymers

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using a multivalent carboxylic acid (i.e., 1,2,3,4-butanetetracarboxylic acid) functional linker

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and their use in the efficient removal of methylene blue from water.27 Even though the

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presence of their high potential applications, yet, there is not much strategy in the fabrication

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of water-insoluble electrospun CD fibers. Therefore, there exists an increasing demand

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towards the production of the insoluble CD fibers while maintaining their hydrophilic

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structure for the snaring of pollutants from an aqueous environment through inclusion-

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complexation.


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The proportion of active CD content in the fiber can be explored by means of inclusion-

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complexation with phenolphthalein through UV-Vis measurements because of its color

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change from pink to colorless with the formation of lactonoid dianion.28 Moreover, it can be

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regarded as a water micropollutant due to its adverse health effects on animals. In this regard,

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Dunnick et al. reported the continuous administration of phenolphthalein to mice over two

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years using the concentrations of 0, 3 000, 6 000, and 12 000 ppm and observed multiple

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carcinogenic effects associated with phenolphthalein uptake.29 Furthermore, Artymowicz et

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al. reported phenolphthalein-induced toxic epidermal necrolysis in humans.30 Their findings

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suggests that that phenolphthalein should be included in the list of drug-induced toxic

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epidermal necrolysis (TEN). CD molecules can form ICs with phenolphthalein, and

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particularly, -CD has shown good complexation with phenolphthalein with a rate constant of

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3.7 x 10-7 M-1 s-1.31 Thus, the materials derived from -CD molecules would be rational

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platforms for the removal of phenolphthalein from water. In this regard, poly(CD-ECH) fibers

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can be used for the removal of phenanthrene, which is one of the most ubiquitous

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environmental micropollutants. Unlike most polycyclic aromatic hydrocarbons (PAHs), the

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water solubility of phenanthrene exceeds 1 ppm, and thus, it can be found in water at a larger

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extent than other PAH molecules. Furthermore, the half-lives of phenanthrene in soil and

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sediment were reported in the range of 16-126 days,32 and with time, phenanthrene can

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contaminate water sources. Therefore, timely clean-up of PAH molecules, such as

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phenanthrene from contaminated sites, particularly from water sources, is crucial. Like

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phenolphthalein, CD molecules can also form IC with phenanthrene since CD presents a

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suitable microcavity to able to host phenanthrene molecules.19, 22

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In this study, poly(CD-ECH) fibers were produced by the electrospinning of CD molecules

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in the presence of epichlorohydrin (ECH) as the cross-linker. Rheology was used to explore

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the flow behavior of the electrospinning solution and their cross-linking at 25 and 50 oC over 5 ACS Paragon Plus Environment


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time. Then, the electrospun fibrous mat was exposed to thermal treatment at 150 oC for the

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cross-linking of hydroxyl moieties of CD molecules with ECH. The stability of the

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electrospun poly(CD-ECH) fibers was explored in water and various organic solvents (i.e.,

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trichloromethane (TCM), dimethylformamide (DMF), dimethyl sulfoxide (DMSO), ethanol

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(EtOH), methanol (MeOH), and acetonitrile (ACN)). As a proof-of-concept study, the

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poly(CD-ECH) fibrous mats were used for the removal of phenolphthalein and phenanthrene

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from aqueous solutions, and their performance was compared with the granule form of the

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poly(CD-ECH).

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EXPERIMENTAL SECTION

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Materials and Methods. Hydroxypropyl--cyclodextrin (HP--CD, Cavasol® W7 HP))

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was obtained as a gift from Wacker Chemie GmbH, Germany. Acetonitrile (ACN, ≥99.5%),

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methanol (MeOH, ≥99.8%), ethanol (EtOH, ≥99.8%), dimethylformamide (DMF, ≥99.8%),

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dimethyl sulfoxide (DMSO, ≥99.9%), and trichloromethane (TCM, ≥99%) were purchased

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from Sigma Aldrich and used as received. Epichlorohydrin (ECH, 99%), phenolphthalein

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(ACS reagent) and phenanthrene (98%) were received from Sigma Aldrich. The distilled

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water was produced by a Millipore Milli-Q Ultrapure System.

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Electrospinning and Cross-linking of CD-ECH Fibers. HP--CD molecules were

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dissolved in an aqueous solution of NaOH (25% (w/v, g/mL)), and ECH (1:10 molar ratio of

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HP--CD/ECH, nECH/HP--CD=10) was slowly added. The solution was mixed at 50 oC for few

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minutes to increase viscosity due to partial cross-linking and thereafter, cooled down to room

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temperature. The solution was transferred into 3 mL syringes having sharped edged metallic

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needles of 0.6 mm inner diameter. The syringes were horizontally placed on a syringe pump,

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and the flow rate was set to 1 mL/h for the electrospinning. A high voltage power supply

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(Matsusada, AU series) was used to apply 15 kV. The fibers were deposited on a grounded


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stationary rectangular metal collector, which was covered by a piece of an aluminum foil. The

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distance between needle and collector set to 10 cm. The electrospinning was performed at ~24

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oC

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and kept at 150

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electrospinning of aqueous solutions of HP--CD and HP--CD/ECH was performed. After

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thermal treatment, poly-CD fiber webs were washed with water and ethanol, respectively to

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remove the unreacted reagents.

in an enclosed Plexiglas chamber. After the electrospinning, the fibrous mat put in an oven oC

for 5 h for the cross-linking reactions. As control fibers, the

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Synthesis of Poly(CD-ECH) Granules. As a control material, poly(CD-ECH) granules

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were produced using identical concentrations of the precursors: HP--CD (120% (w/v,

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g/mL)) was dissolved in an aqueous solution of NaOH (25% (w/v, g/mL)), and thereafter,

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ECH (nECH/HP--CD=10) was gradually added to the solution. The solution kept stirring at 50 oC

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for few minutes, and then, left in oven at 150 oC for 5 h for cross-linking reactions.

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Characterization. The morphology of the poly(CD-ECH) fibers and granules was explored

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by scanning electron microscopy (SEM, Quanta 200 FEG, FEI). Prior to SEM analysis, the

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samples were coated with 5 nm Au/Pd using a Gatan PECS 682 etching and coating system.

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The mean diameter of the fibers was calculated from the SEM images of the fibers with

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ImageJ software (US National Institutes of Health, USA) by analyzing 100 fibers for each

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sample. The rheological analysis of the samples was performed with an Anton Paar Physica

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MCR 301 rheometer. A cone-and-plate geometry was used with a gap size of 104 m and a

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diameter of 50 mm (angle of 1o) during the experiments. Flow tests were performed in the

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shear rate range of 10-3-103 s-1 at 50 oC. Afterwards, the solution was subjected to a dynamic

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oscillatory deformation test at 50 oC for 90 min at the constant shear of 0.1% and frequency of

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1 Hz. Fourier transform infrared spectroscopy (FTIR) analysis was performed using a Bruker-

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VERTEX 70 spectrometer system. The samples were mixed with KBr and pressed to obtain



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transparent pellets. The measurements were performed between 4000 and 400 cm-1 with a

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resolution of 4 cm-1. An X-ray photoelectron spectrometer (XPS) (Thermo Scientiﬁc) was

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used to study the chemical composition of the fiber surface. XPS was used by means of a

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ﬂood gun charge neutralizer system equipped with an Al K-α X-ray source (hυ=1486.6 eV).

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High-resolution spectra were recorded for the spectral regions of C1s at a pass energy of 50

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eV. The thermal analysis of the samples was performed using a thermogravimetric analyzer

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(TGA, Q500, TA Instruments). The measurements were performed between 25 and 500 oC

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with a heating rate of 20 oC under N2 atmosphere.

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Removal of Phenolphthalein from Aqueous Solutions. Removal tests were performed

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with a Varian Cary 5000 spectrophotometer. The measurements were performed in the

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wavelength range of 400-800 nm to monitor changes in the concentration of phenolphthalein

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during the treatment with the poly(CD-ECH) fibers and granules. In this regard, the fiber

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sample (0.1 g and 6 cm * 6 cm dimensions) and granules (0.1 g) were separately transferred

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into the bottom of quartz cuvettes containing phenolphthalein solution (0.4 mM) prepared

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using a buffer solution (pH= 11). The reaction was performed at room temperature (25±2 oC)

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by continuous stirring, and the spectra were collected in a fixed time interval up to 10 h.

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Removal of Phenanthrene from Aqueous Solutions. The removal tests of phenanthrene

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were performed using a HPLC system (Agilent 1200 Series). The separation of phenanthrene

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molecules was performed with a Zorbax Eclipse XDB-C18 column (150 mm × 4.6 mm, 5 µm

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particle size), and the concentration was detected at the wavelength of 254 nm. Acetonitrile

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(100%) was used as a mobile phase at the flow rate of 0.3 mL/min, and the injection volume

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was 10 L. Phenanthrene was dissolved in acetonitrile and afterwards, diluted in water to

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perform filtration measurements. The fiber sample (~0.1 g and 6 cm * 6 cm dimensions) and

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granules were treated with phenanthrene solutions (c= 1.8 ppm) at room temperature (25±2

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oC)

up to 27 h. The calibration curve of phenanthrene was prepared using the stock solutions 8 ACS Paragon Plus Environment

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with different concentrations: 1.8, 0.9, 0.45, and 0.23 μg/ml. The curve showed a linearity

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with R2≥0.99. The results were adapted to this calibration curve in terms of the peak area

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under curves. Moreover, the reusability of the poly(CD-ECH) fibrous mat was explored at

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room temperature (25±2 oC). In this regard, the poly(CD-ECH) mat was washed with

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acetonitrile for 10 min, which led to the removal of 80% of the adsorbed phenanthrene from

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the fibrous mats. The samples were retreated with phenanthrene solutions at room temperature

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(25±2 oC) for 27 h, and the sorption performance was determined as previously reported.

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RESULTS and DISCUSSION

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The cross-linking between HP--CD and ECH takes place over primary and secondary

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hydroxyl groups of HP--CD. A highly alkaline condition leads to the formation of alkoxide

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groups. Bifunctional ECH reacts with the formed alkoxide groups, giving rise to water-

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soluble CD macromolecules through polycondensation in highly branched structures.33 The

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reaction between the hydroxyl groups of CD molecules with the epoxide ring of ECH results

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in glyceryl bridges between neighboring CD molecules. However, ECH molecules can also

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react themselves under alkaline conditions, leading to the homopolymers of ECH. As ECH

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was added to the solution prior to the electrospinning, it may lead to changes on the solution

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properties with time. In this regard, the solution viscosity becomes a critical factor for the

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electrospinning of CD molecules due to their partial cross-linking with ECH. Further, the

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cross-linking reactions are highly sensitive to temperature and therefore, the cross-linking

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significantly rises with an increase in temperature. After the addition of ECH, the reaction

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was stirred for a short time at 50 oC, and thereafter, cooled down to room temperature. Figure

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1a shows the flow curve of the HP--CD solution containing ECH molecules (nECH/HP--

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CD=10).

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shear rate ( &) range of 10-3-103 Pa·s, where the solution showed a strong decrease in the 

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with increasing &. The solution did not reach to the infinite-shear viscosity (∞ range owing

The viscosity () of the electrospinning solution was explored by rheology in the



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to the occurrence of the partial cross-linking reactions between precursors at the onset of the

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measurement. While a dynamic oscillatory sweep test displays changes in shear moduli (i.e.,

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elastic (G’) and loss (G’’)) over time at 50 oC because of cross-linking reactions (Figure 1b).

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Initially, the loss modulus (G’’) was lower than the elastic modulus (G’), suggesting the sol-

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state of the mixture. With time, G’ rapidly increased and could led to a densely cross-linked

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network with a MPa level of G’, demonstrating the formation of a mechanically strong

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network. The cross-over of G’ and G’’ was observed at 18 min, and thereafter, both moduli

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drastically increased with time. This cross-over of both moduli can be considered as the

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gelation point. Thus, the incubation time at 50 oC is highly critical and can lead to a cross-

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linked network, which cannot be electrospun into fibers. The formation of a cross-linked

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network can also be seen over the photos of the sample before and after heat treatment. Figure

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1b (insets) shows the optical photos of the HP--CD/ECH solution before and after heat

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treatment, where a color change to yellowish was observed. Further, the CD solution

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transformed into a viscoelastic gel network at the inverted position. On the other hand, the

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electrospinning solution did not lead to a gel network at 25 oC in (Figure S1). This is highly

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critical since the electrospinning requires a steady-flow over time without any drastic changes

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on the solution viscosity in a given time. With that, the electrospinnability of the system does

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not significantly change over time throughout the electrospinning so that the proposed

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concept is straightforward and reproducible.

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Figure 1. Rheological properties of an alkaline solution of HP--CD molecules in the

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presence of ECH at 50 oC (nECH/HP--CD=10). (a) Viscosity () versus shear rate () and (b) the

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respective shear elastic (G’) and loss (G’’) moduli of the solution as a function of time (t).

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Inset photos show the CD solution before and after cross-linking at 50 oC for 90 min. cHP--CD=

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120% (w/v, g/mL), NaOH (25% (w/v, g/mL)) and ECH ([ECH/HP--CD]=10).

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When the viscosity of the electrospinning solution started to increase, the solution cooled

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down to room temperature to prevent further cross-linking reactions and thereafter,

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electrospun into fibers. The CD fibers were further kept in oven at 150 oC for 5 h for cross-

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linking reactions, which led to the formation of the cross-linked poly(CD-ECH) fibers. The

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optical photo of the poly(CD-ECH) fibers shows a free-standing electrospun mat whose SEM

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analysis revealed the formation of bead-free fibers (Figure 2b-c). The cross-linking

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mechanism was shown in Figure 2d, where neighboring CD molecules attached each other via

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glyceryl bridges. The mats were stable in water due to the occurrence of highly efficient

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cross-linking reactions between CD and ECH (Figure 2e), as revealed by rheological

250

characterization of the respective system. Further, water-contact angle (WCA) measurements

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revealed the spreading of a water droplet on the fiber surface due to hydrophilic structure of

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the fibrous mat owing to the hydrophilicity of both HP--CD and ECH and formed glyceryl

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bridges (Figure 2f).



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Figure 2. (a) An optical photo of the poly(CD-ECH) mat and inset photo (i) shows the

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folded fiber mat. (b) Scanning electron micrograph of CD fibers before (b) and after (c) cross-

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linking (cHP--CD= 120% (w/v, g/mL), NaOH (25% (w/v, g/mL)) and ECH ([ECH/HP--CD]=

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10). (d) Cross-linking mechanism between CD and ECH, and (e) the poly(CD-ECH) mat

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exposed to water, showing water-insoluble nature and structure stability of the poly(CD-ECH)

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mat in water. (f) An optical photo shows the spreading of a water droplet on the poly(CD-

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ECH) mat.

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The electrospraying of the HP--CD molecules took place at the CD concentration of 50%

263

(w/v, g/mL) and [ECH/CD] of 8 and 10, while increasing the concentration of HP--CD to 75

264

and 100% (w/v, g/mL) led to beaded fibers (Figure S2, Supporting Information). On the other

265

hand, with increasing the CD concentration to 120% (w/v, g/mL), the electrospinning of the

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HP--CD solution led to bead-free fibers with a mean diameter of 1.01 ±0.73 m, which is

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higher than the ECH-free HP--CD fibers, demonstrating that the partial cross-linking of HP-

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-CD molecules increased the viscosity and gave rise to the formation of larger fibers (Figure

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3a). On the other hand, mean size of the fibers prior to the cross-linking was calculated as 12 ACS Paragon Plus Environment

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1.06 ±0.53 m, suggesting that cross-linking did not induce significant changes on the fiber

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size (Figure 2b). The mean fiber diameter of the ECH-free CD system at the HP--CD

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concentration of 160% (w/v, g/mL) was previously reported as 0.75 m25 while it increased to

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1.01 ±0.73 m with the incorporation of ECH, suggesting that the occurrence of partial cross-

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linking prior to the electrospinning process. This was also seen over the concentration of HP-

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-CD needed to form bead-free fibers. Normally, the concentration of HP--CD should be

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over 160% (w/v, g/mL) in water form to obtain bead-free fibers. While it decreased to 120%

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(w/v, g/mL) when ECH was added to solution for the cross-linking. The partial cross-linking

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of HP--CD molecules prior to the electrospinning led to higher viscosity, and therefore, the

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lower concentration of HP--CDs was enough to produce bead-free fibers. After exposure to

280

water for 24 h, the fiber size remained nearly unchanged (Figure 3b).

281 282 283

Figure 3. Scanning electron micrographs of the cross-linked poly(CD-ECH) fibers before (a) and after (b) exposure to water for 24 h.

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The stability of the poly(CD-ECH) fibers was also tested in various organic solvents (i.e.,

285

methanol, ethanol, acetonitrile, DMF, TCM and DMSO) for 24 h. The SEM photos of the

286

respective fibers showed different fiber morphologies depending on the solvent treated



Page 14 of 27

287

(Figure 4). The poly(CD-ECH) fibers were stable in all solvents tested, and could preserve

288

their fibrous morphology. The thinnest fibers were observed after exposure to MeOH,

289

followed by EtOH, DMSO, DMF, and TCM, while the largest fibers were observed after

290

treatment with ACN. It is known that alcohols are bad solvents for CD systems and can lead

291

to the collapse of the fiber matrix. Thus, the smallest diameter in alcohols can be attributed to

292

their non-solvent character for CD molecules.

293 294

Figure 4. Scanning electron micrographs of the cross-linked poly(CD-ECH) fibers after

295

exposure to various solvents for 24 h. (a) TCM, (b) DMF, (c), DMSO, (d) EtOH, (e) MeOH

296

and (f) ACN.

297

The cross-linking of the HP--CD fibers with ECH was confirmed by XPS measurements

298

(Figure 5). The atomic composition of the HP--CD and poly(CD-ECH) fibers was shown in

299

Table S1 (Supporting Information). The XPS analysis of the poly(CD-ECH) fibers revealed

300

the presence of Na1s (4.27%) and Cl2p (3.28%) atoms at significant proportions. While no


Page 15 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


301

peaks related to these elements appeared for the pure HP--CD fibers since they were

302

electrospun from water in the absence of ECH. The deconvoluted C1s spectra show a broad

303

peak related to C atom, which can further be deconvoluted into three peaks as follows: the

304

peaks appeared at 284.8, 286, and 288 eV are respectively ascribed to C-C (or C-H), C-O and

305

O-C-O.34 The broader C peak centered at 286.4 eV was observed for the poly(CD-ECH)

306

fibers due to the formation of glyceryl bridges with ECH. On the other hand, the atomic

307

percentage of C-C drastically decreased from 10.16 to 3.6% with the incorporation of ECH,

308

which increased the ratio of C-O in overall composition from 45.6 to 50.45% by taking

309

account of C and O atoms (Table S1).

310 311

Figure 5. The deconvoluted C1s spectra of (a) HP--CD and (b) poly(CD-ECH) fibers.

312

The chemical analysis of the poly(CD-ECH) fibers was further explored by FT-IR analysis

313

(Figure 6a). The bands between 1035 and 1155 cm-1 are due to the stretching band of the

314

glycosidic links of CD molecules: the IR bands at 1155, 1082 and 1035 cm-1 can be attributed

315

to the stretching vibration of C-H and C-O bonds. Similar IR bands were also observed in

316

poly(CD-ECH) fibers. Here, the absorbance ratio (A1082/A1035) in this region of the IR

317

spectrum gives information about the degree of cross-linking and A1082/A1035 ratio of

318

poly(CD-ECH) fibers is 66% higher compared to HP--CD fibers. This ratio reveals the

319

partial cross-linking such that around 66% (approximately 66% of OH groups) of the primary 15 ACS Paragon Plus Environment


Page 16 of 27

320

OH groups of HP--CD transformed into secondary OH forming the cross-link junction.27 For

321

the poly(CD-ECH) fibers, the peak intensity at 1456 cm-1 due to the bending of methylene

322

groups of the formed glyceryl links drastically increased, while the peaks at 948 and 1143 cm-

323

1

324

presence of unreacted epoxy groups. The thermal analysis of the fibers was carried out using

325

TGA, which revealed the enhanced stability of HP--CD fibers after cross-linking with ECH

326

(Figure 6b). The pyrolysis of HP--CD molecules starts over 300 oC, while it increased to 372

327

from 347 oC due to thermal stabilization of the CD fibers with the cross-linking by ECH. It is

328

known that the cross-linking of molecules generally rises the degradation temperature, and

329

thus, an increase in the degradation temperature can be attributed to the presence of a densely

330

cross-linked CD network, which stabilized the CD structure on the course of heating.

decreased for the poly(CD-ECH) fibers. The typical absorption band at 850 cm-1 shows the

331 332 333

Figure 6. (a) FTIR and (b) TGA of the pure HP--CD fibers, uncross-linked HP-CD/ECH fibers and poly(CD-ECH) fibers.

334

The structural properties of the pure HP--CD fibers and poly(CD-ECH) fibers were also

335

explored by wide-angle XRD measurements (Figure S3, Supporting Information). Unlike

336

native CD molecules (,  and ), hydroxypropyl-substituted CD molecules are amorphous

337

and give a broad peak centered at 19o. Likewise, the XRD patterns of the pure HP--CD 16 ACS Paragon Plus Environment

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338

fibers and poly(CD-ECH) fibers displayed amorphous diffraction peaks. Interestingly, the

339

peak positions the HP--CD and poly(CD-ECH) fibers shifted to higher 2 and peak

340

intensities decreased with respect to the powder form of HP--CD (Figure S3). This shift and

341

decrease in the intensity after cross-linking may be attributed to the loss of structural

342

arrangement (i.e., an increase in the amorphous structure) of the HP--CD with heat

343

treatment.

344

The presence of active CD contents in the cross-linked (poly(CD-ECH) fibers was explored

345

using phenolphthalein over host-guest inclusion-complexation of accessible CD molecules.

346

Phenolphthalein is a model guest molecule, which is widely used for the exploration of active

347

CD contents in the CD-based materials.35 Furthermore, it is a dye molecule with some

348

toxicity.29 Thus, the removal of phenolphthalein from aqueous media is desired for water

349

treatment. It forms IC with CD molecules, which can easily be monitored through UV-Vis

350

measurements due to the change of its pink color to colorless, driven by the constrained

351

conformation of phenolphthalein in the CD cavity.28, 35 The removal performance was further

352

compared with the granule form of the poly(CD-ECH). Figure 7 shows the UV-Vis spectra of

353

phenolphthalein solutions exposed to the poly(CD-ECH) fibers and granules over time. The

354

maximum absorption intensity at 553 nm decreased with time, and after 120 min no peak

355

related to phenolphthalein was observable, suggesting about 80% phenolphthalein formed

356

inclusion complex with accessible CD molecules. The complete removal of phenolphthalein

357

was accomplished in 2h while 50% removal was reached in 25 min (Fig 7a-b), suggesting the

358

rapid removal of phenolphthalein molecules from water. On the other hand, the identical

359

concentration of phenolphthalein could be removed in 10 h using poly(CD-ECH) granules

360

(Figure 7b-d). The removal of phenolphthalein by the poly(CD-ECH) fibers was also shown

361

over the optical photos of the solutions over time (Figure 7b inset) with the decolorization of

362

the phenolphthalein solution. The faster removal of phenolphthalein by the poly(CD-ECH) 17 ACS Paragon Plus Environment


Page 18 of 27

363

fibers can be attributed to their smaller size and the presence of a highly porous structure. The

364

morphology of the poly(CD-ECH) fibers could be preserved after the sorption tests,

365

suggesting their high potential for water treatment applications from organic micropollutants

366

(Figure S4).

367 368

Figure 7. Filtration performance of the electrospun poly(CD-ECH) fibrous mat over

369

phenolphthalein. (a) UV-Vis spectra of phenolphthalein solution treated with the poly(CD-

370

ECH) fibers as a function of time and (b) decrease in the absorbance (λ=553 nm) of the

371

phenolphthalein solution with time. (c) UV-Vis spectra of a phenolphthalein solution treated

372

with the poly(CD-ECH) granules with time and (d) decrease in the absorbance intensity

373

(λ=553 nm) over time.

374

The electrospun poly(CD-ECH) fibrous mat was also used for the removal of phenanthrene

375

from aqueous solutions with time (Figure 8). The concentration of phenanthrene was

376

measured over time by HPLC. The stock solutions of phenanthrene was prepared at the 18 ACS Paragon Plus Environment

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377

maximum solubility limits (i.e., 1.8 g/mL) in water/ACN mixtures (59 mL/10 L). Within

378

the first 40 min, 50% removal of the phenanthrene was accomplished, while after 350 min the

379

concentration of phenanthrene decreased to 0.2 g/mL. The removal performance of

380

poly(CD-ECH) fibrous mat was very high and could scavenge 88% of initial phenanthrene

381

content. On the other hand, poly(CD-ECH) granules could reach to an equilibrium sorption

382

level at ~900 min. Both fibrous mat and granules exhibited similar sorption levels, but in

383

different rate. The use of poly(CD-ECH) fibrous mat fastened the sorption rate by nearly 3-

384

fold, which can be attributed to the smaller size of the fibers and their highly porous structure.

385

Despite showing a similar sorption capacity, there are drastic differences in the sorption rate. This can

386

be attributed to the enhanced surface area for the nanofiber form of the poly(CD-ECH), which boosts

387

the sorption rate. For many applications, rapid removal of such pollutants is highly desired.

388

Furthermore, unlike granule form of the poly(CD-ECH), nanofibers are easy to use and do not require

389

any specific container during their use. Furthermore, after washing with ACN to remove

390

phenanthrene molecules from the poly(CD-ECH) mat, the poly(CD-ECH) mat was retreated

391

with phenanthrene solution containing an identical phenanthrene concentration of the first

392

solution. The poly(CD-ECH) mat could remove 70% of phenanthrene after second use

393

(Figure S5). The performance loss in removal capacity can be attributed to the presence of

394

remained phenanthrene molecules in CD cavities, which could not be washed away easily

395

after the first use. The SEM analysis of the poly(CD-ECH) fibers and poly(CD-ECH)

396

granules after sorption tests revealed no significant changes in their morphology (Figure S6).

397

The sorption capacity for phenanthrene was found to be 944 g per gram poly(CD-ECH)

398

fibers. This is higher than many sorbent materials used for phenanthrene removal from water

399

(Table S1, Supporting Information). Although the presence of some sorbent systems with

400

better sorption values, they used alcohols as co-solvent to increase the solubility of

401

phenanthrene. However, among water-based systems, poly(CD-ECH) fibers showed a good



Page 20 of 27

402

sorption capacity, which is better than DNA-based nanogels (sorption capacity = 720 g/g)36

403

and CD-grafted cellulose acetate fibers (sorption capacity = 540 g/g)19 (Table S2).

404 405 406

Figure 8. Time-dependent removal of phenanthrene from aqueous solutions using poly(CDECH) fibers and granules.

407

The poly(CD-ECH) electrospun fibers are a novel type of materials with great promising

408

applications for water treatment. As we have shown here with the removal of phenolphthalein

409

and phenanthrene, they can be exploited as high-performance sorbent materials due to their

410

high specific surface area, along with a high active CD content. Given that their high stability

411

in aqueous and various organic solvent systems owing to their highly cross-linked network by

412

epichlorohydrin-based chemistry, these highly stable fibrous materials may find more

413

application area as highly effective sorbents in coming years. Unlike other methods used for

414

the cross-linking for CD molecules, epichlorohydrin-based cross-linking offers robust CD

415

networks, which are stable even in polar aprotic solvents, such as DMF. This allows their

416

reuse for sorption-based applications after removal of the adsorbed molecules with intense

417

washing with aprotic solvents.


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418 419

CONCLUSIONS

420

Insoluble poly(CD-ECH) fibers were produced by the electrospinning of the aqueous

421

solutions of HP--CD and ECH and their cross-linking by heat-treatment. The flow and

422

dynamic oscillatory deformation tests revealed partial cross-linking of CD molecules prior to

423

the electrospinning, however main cross-linking takes place after the heat-treatment of the

424

fibers at 150 oC. Due to highly efficient cross-linking reactions, poly(CD-ECH) fibers were

425

stable in water and various organic solvents (i.e., ACN, TCM, DMSO, DMF, MeOH and

426

EtOH) and could maintained their fibrous structure. The presence of high active CD content

427

in the fibers was revealed by removal tests of phenolphthalein and phenanthrene through

428

inclusion-complexation. The removal performance of poly(CD-ECH) fibers was compared

429

with poly(CD-ECH) granules, and the results showed that rapid removal of phenolphthalein

430

and phenanthrene by the fibers compared to the granule form of poly(CD-ECH) due to their

431

smaller size and high porous structure. The poly(CD-ECH) fibrous membrane with a high

432

specific surface area and structural stability in water and organic solvents are promising

433

materials in water treatment applications.

434

ASSOCIATED CONTENT

435

Supporting Information

436

The following files are available free of charge.

437

Dynamic oscillatory time sweep profile of an aqueous mixture of HP--CD and ECH. The

438

SEM images of the poly(CD-ECH) fibers and granules synthesized at various concentrations

439

and after their use in removal of phenolphthalein and phenanthrene. The atomic composition



Page 22 of 27

440

of the fibers obtained by XPS analysis. The reuse of poly(CD-ECH) fibers in the removal of

441

phenanthrene.

442

AUTHOR INFORMATION

443

Corresponding Author

444

* E-mail: [email protected] (T.U)

445

ORCID ID

446

F. T.: 0000-0002-9011-4495

447

T. U.: 0000-0002-3989-4481

448 449

Funding:

450

The Scientific and Technological Research Council of Turkey (TUBITAK, project number:

451

113Y348) is acknowledged for funding the research.

452

Author contributions:

453

A.C., F.T and T.U. conceived, built and carried out the experiment and analyzed the data. All

454

authors contributed to the preparation of the manuscript.

455

Notes:

456

The authors declare no competing financial interest.

457

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Table of Contents (ToC) Graphic

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Water-Insoluble Hydrophilic Electrospun Fibrous Mat of Cyclodextrin

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