Enamine approach for versatile and reversible functionalization on

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Enamine approach for versatile and reversible functionalization on cellulose related porous sponges Liduo Rong, Hongchen Liu, Bijia Wang, Zhiping Mao, Hong Xu, Linping Zhang, Yi Zhong, Jinying Yuan, and Xiaofeng Sui ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01369 • Publication Date (Web): 30 May 2018 Downloaded from http://pubs.acs.org on May 30, 2018

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ACS Sustainable Chemistry & Engineering

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Enamine approach for versatile and reversible functionalization on

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cellulose related porous sponges

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Liduo Rong†, Hongchen Liu†, Bijia Wang†, Zhiping Mao†, Hong Xu†, Linping Zhang†,

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Yi Zhong†, Jinying Yuan‡, Xiaofeng Sui†, *

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† Key Lab of Science & Technology of Eco-textile, Ministry of Education, College of

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Chemistry, Chemical Engineering and Biotechnology, Donghua University, No. 2999

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North Renmin Road, Shanghai 201620, People’s Republic of China.

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‡

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Key Lab of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Department of Chemistry, No.1, Tsinghua Yuan Road, Haidian District, Tsinghua University, Beijing, 100084, People’s Republic of China.

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*Corresponding authors:

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Address: No. 2999 North Renmin Road, Shanghai 201620, People’s Republic of

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China.

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Tel.: +86 21 67792605. Fax: +86 21 67792707.

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E-mail: [email protected]

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ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 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

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Abstract

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A readily modifiable cellulose sponge was prepared from cellulose

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acetoacetate (CAA). Facile post-modification with primary amino-containing

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modifiers such as octadecyl amine (ODA), cysteine (CYS), and L-glutamic acid

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(GLU) could be achieved demonstrating the ease of anchoring a broad selection of

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functional groups to the surface of the sponges. This post-modification process was

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systematically characterized by Fourier transform infrared spectroscopy, and X-ray

25

photoelectron spectroscopy, which confirmed the formation of the enamine bonds.

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Besides, the microstructures and mechanical properties of the sponges were well

27

preserved throughout the post-modification process. The enamine bonds, as one of the

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dynamic covalent bonds, were easily formed under the mild and neutral conditions

29

and broken exposure to a low pH stimulus. The enamine bonds were used to modify

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the CAA sponges, which can achieve the versatility and recycling of cellulose porous

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materials. Therefore, the resulting sponges could serve as a versatile precursor to a

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broad spectrum of multifunctional porous materials, paving a new way for

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constructing smart sponges through the post-modification strategy.

34 35 36 37

Key words: Cellulose acetoacetate; Post-modification; Enamine bond; Sponge; Reversible

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38

INTRODUCTION

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Functional cellulose porous materials (aerogels or sponges)1-2 have high porosity,

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large surface areas, and low density; hence, they can be good candidates for diverse

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applications in catalyst support, thermal insulation, adsorption, etc.3-13 Nevertheless,

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the performance of porous materials often depend chemical composition on the

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surface, and these functional cellulose sponges always have a high density of surface

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specific groups, e.g. alkyl groups,14 amino groups,9 thiol groups,15 to meet particular

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requirements. For example, hydrophobic materials could be potentially used to adsorb

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oil from water.16-17 The thiol group as the good heavy metal adsorbent18-19 and catalyst

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carrier20-21 has been widely studied. By regulating the pH, the carboxyl group could

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effectively adsorb the protein.22 Thus, it remains important to the optimize the surface

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properties of the cellulose sponge.

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To obtain functionalized cellulose sponges, two methods have been developed

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including either derivatizing the cellulose before preparing the sponge, or

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post-modification after sponge is formed.23 Xin et.al reported a hierarchically porous

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cellulose acetate (CA) as the starting monolith as a template-free fabricated,

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functionalized platform, which could be modified with typical reactive groups such as

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epoxy, primary amino, and aldehyde.24 However, the modification processes are

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irreversible and often demand the tedious chemical transformations. Laure and

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co-workers achieved reversible tethering of end-functionalized polymers onto

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catechol-based titanium platforms based on a reversible Diels−Alder (DA)

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cycloaddition reaction,25 which allowed recycle and reuse of the modifiers and the

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original materials. Therefore, it is desirable to develop a mild, reversible and versatile

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strategy for post-modification of the cellulose sponge to better preserve their delicate

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morphological features.

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The use of mussel-inspired polydopamine (PDA) chemistry to create a versatile

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nanoplatform capable of further modifications toward various applications has been

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achieved on ordered mesoporous carbons (OMCs).26 Inspired by the PDA-based

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platform, we constructed a CAA-based sponge reinforced by cellulose nanofibers 3



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(CNF) and demonstrated that it could be used as a universal precursor for

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multifunctional cellulose sponges via a post-modification strategy. Cellulose

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acetoacetate (CAA), as a water-soluble cellulose ester, has received extensive

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attention and research by both industry and academia.27-28 The acetoacetyl group is

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susceptible to a series of transformations via the active methylene and carbonyl

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carbons. A dynamic covalent enamine bond can be formed by reacting the ketone

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carbonyl carbon with a primary amino group under mild conditions,29-30 and it is

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reversibly cleaved, restoring the carbonyl upon exposure to a low pH stimulus. The

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transformations could be performed under mild conditions without catalysis; thus,

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CAA could be used as the starting materials to develop a mild, facile, and reversible

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strategy for the post-modification of cellulose sponge materials to better preserve their

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delicate morphological features. In this work, acetoacetate-containing cellulose

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sponges reinforced with CNF using APTES could be easily post-modified with the

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functional molecules containing the primary amino group due to the unreacted

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acetoacetyl groups on the sponge.

82 83

Scheme 1. (a) Reaction mechanism of the CAA sponge crosslinked with CNF and APTES; (b)

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Schematic illustration of the reversible surface-modified CAA sponge.

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EXPERIMENTAL SECTION

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Materials

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A cellulose nanofibers (CNF) suspension (solid content = 1 wt%) was provided by

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Tianjin Haojia Cellulose Co., Ltd. (China) and was subjected to high-pressure 4


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homogenization (APV-2000 Homogenizer, Germany) before use. The average CNF

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diameter was under 100 nm. The Wood pulp was supplied by Xinxiang Natural

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Chemical Co., Ltd. The viscosity-average degree of polymerization was estimated as

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870

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cupriethylenediamine

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1-allyl-3-methylimidazolium chloride (AMIMCl) was purchased from the Center for

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Green Chemistry and Catalysis (CGCC), LICP, CAS. Octadecyl amine (ODA),

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cysteine (CYS), L-glutamic acid (GLU), 3-aminopropyltriethoxysilane (APTES),

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hydrochloric acid (HCl), and tert-butyl acetoacetate (t-BAA, 99%) were supplied by

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Sinopharm Chemical Reagent Co., Ltd. All other chemicals were of analytical grade

99

and used without further purification.

100

according

to

measurements hydroxide

using solution

an

Ubbelohde

(CUEN).

The

viscometer ionic

in

liquid

Preparation of CAA

101

The synthesis of CAA (degree of substitution (DS) = 0.87, Figure S1) was carried

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out according to our previous reports.31 In brief, 2.0 g of cellulose was dispersed in 38

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g of 1-allyl-3-methylimidazolium chloride (AMIMCl) in a three-necked flask, and the

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mixture was heated to 95 °C and held there until the sample was dissolved completely.

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Dimethylformamide (DMF) (10.0 mL) was added to the solution as diluent,

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subsequently, the solution was heated to 110 °C, and tert-butyl acetoacetate (t-BAA,

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15.6 g, 98.7 mmol) was added dropwise under the protection of nitrogen with stirring

108

for 4 h. After the reaction, the products were precipitated using methanol, then washed

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and dried for use.

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Preparation of CAA sponges

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A CNF suspension (1 wt%) was sonicated for 1 h to achieve a thorough dispersion

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(TEM image of the CNF is shown in Figure. 1a). To 10.0 g CNF (1 wt%) suspension

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was added drop-wise a predetermined amount of 10 wt% hydrolyzed APTES solution

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(0, 0.1, 02, 0.3, 0.6 mL). The mixed suspension was magnetically stirred for 2 h at

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25 °C, followed by addition of 10 g 1 wt% CAA aqueous solution. Stirring was

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continued for 30 min. The suspensions were then kept still for gelatinization. After 2 h,

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the gel was subjected to a bottom-up quick-freeze with liquid nitrogen to yield ice 5



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gels, which were freeze-dried using a Labconco apparatus (FD5-3, Labconco USA) at

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-55 °C under vacuum. The resulting sponges were further dried and cured for 30 min

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at 110 °C. The sponges were coded CAA-n, where n% represented the mass ratio of

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APTES to CAA and CNF.

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The post-modification of CAA sponges with various amino compounds

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In 100 mL of ethanol, 0.4 g of ODA was dissolved at 40 °C. Separately, 0.4 g of

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CYS and GLU were dissolved in 100 mL each of an ethanol/water (1:1 v/v, pH = 8.5)

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solution at 40 °C, following which, 0.2 g each of the CAA-10 sponges were subjected

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separately to the above-mentioned different amino compound solutions. The CAA-10

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mixtures were magnetically stirred at 40 °C for 12 h. After modification, the samples

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were washed with ethanol and water to remove the physically adsorbed amino

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compounds on the surface of the sponge. Finally, the samples were freeze-dried. The

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samples were coded as CAA-ODA, CAA-CYS, and CAA-GLU.

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Reversible modification of CAA sponges with ODA, CYS, GLU

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The modified CAA sponges could be easily restored to the original sponges

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through the dynamic enamine bonds. Specifically, the CAA-ODA, CAA-CYS,

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CAA-GLU sponges were placed in an acidic ethanol/water solution (pH = 3.0) and

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the ensuing suspension was stirred at room temperature for 4 h. These experiments

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were also carried out with the neutral ethanol/water solution (pH = 7.0) as a control.

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Then, the samples were washed with ethanol and water to remove the physically

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adsorbed amino-molecules in the sponges. Finally, the samples were freeze-dried. The

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resulting sample could be repeatedly modified again with ODA, CYS, GLU according

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to the procedure in above mentioned section and coded as CAA-mODA, CAA-mCYS,

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CAA-mGLU where m represented the times of the cyclic modifications. The results

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of the reversible modification process were characterized by FTIR spectroscopy.

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Characterization

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1

H NMR spectra were acquired on an Avance 400 MHz (Bruker, USA)

145

spectrometer in the proton noise-decoupling mode with a standard 5 mm probe. The

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DS of CAA was calculated according to Equation (1). 6


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DS =

I × 7 (1) I × 3

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where I1 is the integration value of the methyls of acetoacetate and I2 is the integration

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value of the backbone of the anhydroglucose ring of CAA.27, 31

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Fourier transform infrared (FTIR) spectroscopy analyses were carried out using a

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PerkinElmer Spectrum Two (USA) equipped with an attenuated total reflectance

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(ATR) accessory. All spectra were recorded in the 4000–450 cm−1 range and at a

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resolution of 4 cm−1. The thermal properties of the samples were evaluated by

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thermogravimetric analysis (TGA) (NETZSCH 209F1, German). Under nitrogen

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purge, the samples were heated from 30 to 600 °C at a heating rate of 10 °C/min. The

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morphologies of the samples were observed using a Hitachi TM-1000 scanning

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electron microscope (Japan). The sponges before and after modification were

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analysed using X-ray photoelectron spectroscopy (XPS) (Thermo Fisher ESCALAB

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250Xi, USA) to gain further information about the surface composition. Before XPS

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analysis, all the samples were pressed into the uniform film to meet the test

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requirement. The mechanical properties of the sponges were evaluated on a universal

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testing machine (HY 940F, China) equipped with the special pressure sensing

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accessories (Transcell, BAB-20MT, three-digit accuracy). The testing procedures

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(Figure S2) were with a compression speed of 2 mm/min (sample diameter: 20 mm,

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scale distance 12 mm). The thickness recovery S was defined as the percentage of the

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original thickness according to Equation (2)32

166 167 168

S(%) = 100 −

(2)

where the εfinal is the strain at the final position when the force detected reached 0 N.

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The N2 adsorption-desorption isotherms and specific surface areas were

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characterized by a TriStar II 3020M analyser (Micrometrics instrument corp.). The

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porosities of the sponges were measured using a previously reported gravimetric

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method.33 The dynamic water contact angle (WCA) was measured with a DSA30

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contact angle analyzer (Kruss, German). 7



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RESULTS AND DISCUSSION

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Synthesis and characterization of the CAA sponges

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Scheme 1a illustrates the preparation of the CAA sponge via cross-condensation

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with APTES to form a physically stable gel with three-dimensional network structure.

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CNF were used as reinforced component (Figure S3). Subsequently, the CAA

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sponges were post-modified with the amino-containing octadecyl amine (ODA),

180

cysteine (CYS), or L-glutamic acid (GLU), as representative examples, to prove the

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feasibility of the post-modification strategy. The modification with amino compounds

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showed reversible pH responsiveness (Scheme 1b).

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As shown in Scheme 1a, the amount of APTES was controlled to ensure that the

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CAA sponges have a large number of unreacted acetoacetyl groups (Table S1), that

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could react with the functional molecules containing primary amino groups including

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ODA, CYS, and GLU in the post-modification process. The resulting sponge presents

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a white appearance and has low density (Figure 1b).

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Figure 1c shows the FTIR spectra of the CAA sponges containing different

189

amounts of APTES. With the increase of APTES, the double carbonyl-characteristic

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peaks of acetoacetyl at 1700−1760 cm−1 became weaker.30 The peaks at 1650 and

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1605 cm−1 could be ascribed to the enamine bond,29 and their intensity became

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stronger (Figure S4a). For the CAA sponge, the typical Si−O−Si bond-stretching

193

adsorption peaks from the siloxane compounds in the 1000–1130 cm−1 region

194

overlapped with the peaks for the C−O bonds of cellulose,34 while the peak at 800

195

cm−1 from the CAA sponges was assigned to the C−O−Si stretching vibration and

196

became increasingly prominent with the increase of APTES.35 As shown in Figure.

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1d, as more APTES was incorporated the compressive modular of the sponges

198

increased from 14 to 36 KPa. The major contributor to the enhancement of

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compressive strength is due to higher degree of crosslinking.

200

The thermal stability of the CAA sponges was also studied with TGA. The

201

samples presented a gradient change in thermostability as shown in Figure 1e. The 8


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maximum rate of thermal decomposition decreased with increasing the content of

203

APTES according to DTG (Figure S4b).36 However, the temperature of the DTG

204

peaks did not change significantly. Besides, the char residue mass fractions

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significantly increased to 32.0 % with increasing APTES ratios, which could be

206

ascribed to silicon and the low oxygen content in APTES, along with lower

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decomposition rates that contributed to the improved thermal stability of the

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crosslinked CAA sponges. As seen from the SEM images of the sponges (Figure S5),

209

little difference was observed in terms of morphology between samples containing

210

varying amounts of APTES. This was as expected since the crosslinking happened on

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the molecular level.

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Considering that there was a trade-off between reinforcement and available

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modification sites, the CAA-10 sponge which still had 1.45 mmol/g remnant

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acetoacetyl group (Table S1) and reasonably good mechanical strength was chosen

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for the post modification experiments.

216 217

Figure 1. (a) Transmission electron microscopy (TEM) image of CNF; (b) Photograph of a

218

round sample of the CAA sponge on the leaf; (c) FTIR spectra, (d) compressive stress-strain 9



219

curves, and (e) TGA of CAA sponges crosslinked with different APTES amounts.

220

Surface modification with ODA, CYS, and GLU

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The CAA sponges could be imparted with the new properties through the

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post-modification process. As shown in Scheme 1b, ODA, CYS, and GLU were used

223

to introduce different specific groups (alkyl, thiol, or carboxyl, respectively) and to

224

functionalize the CAA sponge. After surface modification and pH-cleavage reaction,

225

the sponges were always freeze-dried. The modified sponges were washed with

226

ethanol and subjected to air-drying and vacuum-drying. The results are included as

227

Figure S6 in the supporting information. As can be seen from Figure S6, both

228

air-dried and vacuum-dried samples suffered from deformation and shrinkage to some

229

extent, while the freeze-dried sample could maintain its original shape. Hence,

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freeze-drying was chosen to prepare the modified CAA sponges in this work.

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Figure 2a-f shows the FTIR spectra of the original CAA-10 sponge and the

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CAA-ODA, CAA-CYS, and CAA-GLU sponges. After post-modification with ODA,

233

CYS, and GLU, the intensity of the peak for the acetoacetyl groups at 1700−1760

234

cm−1 on the CAA-10 sponge became lower and even disappeared, and the peak of the

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enamine bonds at 1650 and 1605 cm−1 became stronger.29-30 These results clearly

236

demonstrate that the alkyl group, thiol group, and carboxyl group were successfully

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anchored on the skeleton surface of the CAA sponges. Besides, the peaks at

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2865−2930 cm−1 for the CAA-ODA sponge corresponded to the CH stretching

239

vibration.37 The peak for C=O in the CAA-CYS, and CAA-GLU sponges at

240

approximately 1655 cm−1 overlapped with the peak of the enamine bond as shown in

241

Figure 2d and 2f.

242

10


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243 244

Figure 2. FTIR spectra of the ODA(a), CYS(c), GLU(e) modified CAA sponges. FTIR spectra in

245

the range of peaks assignable to acetoacetyl groups and enamine bonds in the ODA(b), CYS(d),

246

GLU(f) modified CAA sponges.

247 248 249

Figure 3. SEM images of the CAA-10 sponges (a, b), CAA-ODA sponges (c, d), CAA-CYS sponges (e, f), and CAA-GLU sponges (g, h).

250

The morphologies and microstructures of the unmodified and modified sponges

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were observed using SEM (Figure 3). The micro-structures of the CAA-10 sponge

252

exhibited a network structure with pore sizes in the range of 20−60 µm. 11



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253

Freeze-casting offers a versatile approach to produce highly anisotropic porous

254

materials.38 When the CAA-10 gel was frozen inside molds, the first cores of ice were

255

formed, and the fibers were separated from frozen water. Then, large ice crystals grew

256

along the cooling/freezing direction.39 During this process, the cellulose fibers were

257

confined to the interstitial regions, which caused contraction and aggregation between

258

the ice crystals. After modification with ODA, CYS, and GLU, SEM images showed

259

that the morphological structure of the sponge did not change after treatment with

260

amino-containing modifiers.

261

To further illustrate the surface composition, XPS analyses of the CAA-10 and

262

post-modified sponges were conducted, the results for which are shown in Figure

263

4a-c. In the wide-scan spectra, the characteristic signals of C1s (285 eV), O1s (545

264

eV), N1s (399 eV), Si2s (153 eV), Si2p (100 eV), S2s (233 eV), and S2p (169 eV)

265

were all detected. As shown in Figure 4d-f, the high-resolution spectrum of the N1s

266

peaks can be resolved into peaks at 402.0 eV and 399.3 eV, attributed to the formation

267

of the enamine bond.40 Figure 4d-f clearly demonstrated that the N1s peaks

268

(attributed to enamine Nitrogen) of the amine-modified samples were more intense

269

compared to that of the original CAA-10, suggesting the amino-containing functional

270

molecules were fixed on the surface of the sponge by enamine bonds. The elemental

271

composition of the CAA-10 sponge and post-modified sponges were determined

272

using survey scans from XPS. Table 1 summarizes the elemental composition derived

273

from the XPS survey spectra for the CAA-10 sponge before and after

274

functionalization. The composition of surface elements changed significantly, which

275

confirmed that various functional molecules had been fixed on the surface of the

276

sponges. X-ray photoelectron spectroscopy (XPS) is the most commonly used

277

technique of surface analysis in the nanometer depth.41 After modification by ODA, it

278

was expected that the surface silicon atoms would be covered by the C18 carbon

279

chain of ODA, which could explain the observation that intensity of Si2s was greatly

280

reduced while that of C1s was enhanced.

12


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281 282

Figure 4. XPS wide-scan spectra of the CAA-10 sponge compared with the CAA-ODA,

283

CAA-CYS, and CAA-GLU sponges (a-c), and high-resolution N1s spectra of the CAA-10

284

compared with the CAA-ODA, CAA-CYS, CAA-GLU sponges (d-f).

285 286 287

Table 1. Percent elemental concentration for the CAA-10, CAA-ODA, CAA-CYS, and CAA-GLU sponges from XPS survey scan Samples

Element

CAA-10 CAA-ODA CAA-CYS CAA-GLU

Atomic composition (%) O1s N1s

C1s 59.08 73.21 54.98 56.29

35.04 21.3 36.14 37.68

2.06 3.76 3.75 3.42

Si2s

S2p

3.43 1.73 2.34 2.61

2.8 -

288 289

To illustrate influences of the post-modification process on the specific surface 13



Page 14 of 22

290

areas and hierarchical porous structures of the CAA-10 sponge, the nitrogen

291

adsorption/desorption isotherms for the CAA sponge and modified sponges are shown

292

in Figure 5a-c. The Brunauer–Emmett–Teller (BET) surface area of the CAA-10

293

sponge was calculated to be 9.076 m2/g. The lack of any distinct peak below 500 Å in

294

the pore size distribution calculated from the Barrett–Joyner–Halenda method (BJH)

295

indicates the lack of micro- or mesoporosity.42 The BET surface areas, bulk densities,

296

and porosities of all samples are summarized in Table 2. The modified sponges had a

297

trend towards a reduction in specific surface areas, and an increase in bulk densities.

298

This may be because during the post-modification and freeze-drying process, the

299

samples had a certain degree of contraction of the porous framework from

300

freeze-drying.

301 302 303 304

Figure 5. N2 adsorption-desorption isotherms of (a) CAA sponges compared with CAA-ODA sponges, (b) CAA-CYS sponges, and (c) CAA-GLU sponges; (d) The compressive stress-strain curves of modified CAA sponges

305 306

Table 2. Physical properties of the CAA-10, CAA-ODA, CAA-CYS, and CAA-GLU sponges 14


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Samples 2

BET surface area (m /g) Bulk density (mg/cm3) Porosity (%)

CAA-10

CAA-ODA

CAA-CYS

CAA-GLU

9.0760 14.3 95

8.3782 16.6 92

8.4368 16.7 91

7.3074 16.8 90

307

The compressive stress-strain curves for the CAA-10 sponge and the modified

308

sponges are shown in Figure 5d. The four sponges were compressed to 60% of their

309

original thickness. In general, the modified sponges had comparable compressive

310

stresses and Young’s moduli in comparison with the CAA-10 sponge，indicating the

311

post-modification had no detrimental effects on the mechanical properties of the

312

sponge. The reason for CAA-ODA sponge to have better recovery performance (70%)

313

could be due to the formation of new hydrogen bonding post-deformation was

314

hindered by the long carbon chains.

315

3.3 Reversible surface modification

316

During the post-modification process, the enamine bond was used to immobilize

317

functional molecules on the surface of the CAA sponges. The enamine bond, as a

318

dynamic covalent bond, could be reversibly cleaved and reformed upon exposure to

319

pH stimuli.29, 43 Therefore, this post-modification could be reversible. To verify this

320

assumption, the modified sponges were taken as an example. The FTIR spectra were

321

used to investigate the entire recyclable modification process (Figure 6c, Figure S7).

322

As the original material, the CAA-10 sponge was super-hydrophilic, and the

323

water droplet immediately permeated the sample (< 1s) and caused a little shrinkage

324

of the sponge. The sponge functionalized with ODA displayed durable hydrophobicity

325

with a water contact angle of approximately 145° as shown in Figure 6a. It is worth

326

noting that the reaction of hydrophobic modification not only occurred on the surface

327

of the sponge but also in its interior. This result could be ascribed to the highly porous

328

structure of the CAA sponge, which could promote the rapid diffusion of ODA, and

329

permeation through the sponge skeleton.44 As a proof of concept of the reversible

330

modification, the ODA-modified sponge was immersed in the acid ethanol/water

331

solution (pH = 3.0) for 4 h. The experiment was also carried out with the neutral

332

ethanol/water solution as a control. As shown in Figure 6a, after acid treatments, the 15



Page 16 of 22

333

surface of the CAA-ODA sponge was restored to the original hydrophilic

334

characteristics, suggesting the removal of the hydrophobic molecules from the

335

sponges. More importantly, this sponge could be cyclically post-modified with ODA.

336

As shown in Figure 6b, after four cycles, the contact angle remains above 140°. FTIR

337

spectra were used to trace the change of the characteristic peaks during the recycle

338

modification process. As shown in Figure 6c, Figure S7, the peaks of the enamine

339

and acetoacetyl groups transformed to one another throughout the cycle, confirming

340

the reversible nature of the dynamic covalent enamine bonds. In addition, CAA-ODA

341

sponge immersed in the neutral ethanol/water solution showed no change in FTIR

342

spectra indicating that the enamine bond was relatively stable under neutral conditions

343

(Figure 6d).

344

Specifically, the amine-modified sponges were subjected to pH-cleavage

345

followed by re-modification and the compressive stress of the samples prior to and

346

after the re-modification were compared (Figure S8). The results showed that, for all

347

three modified-amines, the change of compressive strength due to the re-modification

348

process were less than 5%. The porosity of all samples is approximately 90%, which

349

has no significant change compared with the CAA-10 sponge. In addition, the SEM

350

images of sponges before and after re-modification showed no obvious difference

351

(Figure S9). These results confirmed the practicability for cyclic use of the sponges.

16


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352 353 354 355 356

Figure 6. (a) Evolution of the CAA-10 and CAA-ODA sponge contact angles, (b) contact angles of the recycled CAA sponge, (c) FTIR spectra of the reversibly modified ODA, and (d) FTIR spectra of contrast samples.

CONCLUSIONS

357

In the present study, a functional CAA sponge was successfully prepared by

358

crosslinking CAA and CNF with APTES. Mechanical strength of up to 24 KPa and

359

thermal stability of up to 230 °C could be achieved with an APTES loading of 10%,

360

leaving sufficient unreacted acetoacetate groups to allow the facile post-modification.

361

The

362

amino-containing modifiers ODA, CYS, and GLU via the dynamic covalent enamine

363

bond, to yield cellulose sponges featuring hydrophobic long carbon chain, thiol, and

364

carboxyl moieties, respectively. After post-modification, the sponges still maintained

365

excellent three-dimensional microstructures. In addition, the pH-induced cleavage of

366

the modifiers demonstrated that the post-modification could be reversed. The CAA

367

sponge could be therefore a key intermediate in constructing multifunctional cellulose

368

sponges through a mild and general post-modification strategy.

sponge

was conveniently and efficiently

modified

by the

primary

17



Page 18 of 22

369 370 371

Supporting Information

372

Figure S1. 1H NMR spectra of CAA in DMSO-d6. Table S1. The theoretical content

373

of the functional groups on different CAA sponges. Figure S2. Sequential photos of

374

the sponge during the compression process. Figure S3. (a) FTIR spectra, (b)

375

compressive stress-strain curves of the CAA-10 sponge and the CAA-10-N sponge

376

without CNF. Figure S4. (a) FTIR spectra in the range of peaks assignable to

377

acetoacetyl groups and enamine bonds, (b) DTG of CAA sponges crosslinked with

378

different APTES amounts. Figure S5. SEM images of the CAA-0, CAA-5, CAA-10,

379

CAA-15, CAA-30 sponges. Figure S6. Photograph of a round samples of the sponges

380

before and after drying via different methods. Figure S7. FTIR spectra of the

381

reversibly modified CAA-CYS (a), CAA-GLU (b) sponges. Figure S8. The

382

compressive stress for the sponges in the cycling experiments at 60% strain. Figure

383

S9. SEM images of the regenerated CAA-10 sponges (a, b), CAA-2ODA sponges (c,

384

d), CAA-2CYS sponges (e, f), CAA-2GLU sponges (g, h).

385

ACKNOWLEDGEMENTS

386

This work was financially supported by the Fundamental Research Funds for the

387

Central Universities (No. 2232018A3-04 and No. 2232018-02), the National Key

388

R&D Program of China (No. 2016YFC0802802), and the Programme of Introducing

389

Talents of Discipline to Universities (No. 105-07-005735)

390

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Synopsis The CAA sponges could serve as a versatile and reversible precursor, paving a new way for constructing smart sponges through the post-modification strategy.

22


Enamine approach for versatile and reversible functionalization on

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