Fluorescent Multifunctional Polysaccharides for Sustainable

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Fluorescent multifunctional polysaccharides for sustainable supramolecular functionalization of fibers in water Olga Grigoray, Holger Wondraczek, Annett Pfeifer, Pedro Fardim, and Thomas Heinze ACS Sustainable Chem. Eng., Just Accepted Manuscript • Publication Date (Web): 26 Dec 2016 Downloaded from http://pubs.acs.org on December 26, 2016

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ACS Sustainable Chemistry & Engineering

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To ACS Sustainable Chemistry and Engineering

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Fluorescent multifunctional polysaccharides for

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sustainable supramolecular functionalization of

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fibers in water

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Olga Grigoray,1 Holger Wondraczek,2 Annett Pfeifer,2Pedro Fardim,1,3,* Thomas Heinze1,2,*

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1

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20500, Åbo, Finland

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2

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Macromolecular Chemistry, Friedrich Schiller University of Jena, Humboldtstraße 10, 07743

Laboratory of Fiber and Cellulose Technology, Åbo Akademi University, Porthansgatan 3, FI-

Center of Excellence for Polysaccharide Research, Institute of Organic Chemistry and

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Jena, Germany

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3

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2424, 3001 Leuven, Belgum

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KEYWORDS: multifunctional cellulose derivatives, naphthalimide group, polyelectrolyte,

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eucalyptus Kraft fibers, fluorescent fibers, authenticity indicator

Department of Chemical Engineering (CIT), University of Leuven, Celestijnenlaan 200f - box

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ACS Paragon Plus Environment

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ABSTRACT

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The paper describes the synthesis of multifunctional cellulose derivatives (MCD) containing a

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fluorescent- and a cationic moiety and their application in the functionalization of pulp fibers.

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The cellulose derivatives, namely N-(3-propanoic acid)- and N-(4-butanoic acid)-1,8-

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naphthalimide esters of cellulose, differed in the degree of substitution (DS) and by the aliphatic

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chain connecting naphthalimide photoactive groups to the polymer backbone. The derivatives

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were decorated with a cationic moiety, namely (3-carboxypropyl)trimethylammonium chloride.

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The fluorescent pulp fibers were prepared by direct self-assembly of the water soluble

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fluorescent MCDs on the fibers in water at room temperature. The results indicated that the

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adsorption was mainly driven by an ion exchange mechanism. UV-vis- and fluorescence

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spectroscopic studies showed that the adsorption yield of the fluorescent MCDs depended on the

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length of the aliphatic chain of the photoactive groups. Because of the adsorption, the modified

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pulp fibers gained fluorescence in the visible part of the spectrum. Under black light

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illumination, the modified fibers fluoresced, which made them visually distinguishable from the

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reference fibers. Thus, the fluorescent pulp fibers prepared in a simple way can be potentially

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used as an authenticity indicator in packaging materials.

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INTRODUCTION

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Supramolecular functionalization of pulp fibers by adsorption of bio-based polymers is a method

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that allows the introduction of novel properties to the fibers without impairing their bulk

35

properties. This type of functionalization is characterized by the formation of supramolecular

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assemblies which are created by association of biopolymers with pulp fiber constituents through

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non-covalent intermolecular interactions, such as hydrogen bonding, electrostatic or hydrophobic


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interactions.1 The adsorption of the polymers can be carried out at mild conditions in water,

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which simplifies the integration of this stage in the industrial processes, and preserves the

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structure of the fiber. Supramolecular functionalization of pulp fibers can be done using specially

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designed multifunctional cellulose derivatives (MCDs). Such cellulose derivatives are decorated

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with specific functional groups that endow new functionality to the fibers and ionic functional

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groups that impart solubility to the derivatives in water and facilitate interaction with the

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fibers.2,3 Depending on the target applications, different functional groups can be introduced into

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the structure of MCDs. For instance, Grigoray et al.4,5 used coumarin-type cellulose

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polyelectrolytes to prepare light-responsive pulp fibers/fibrous materials whose mechanical

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properties could be controlled by light. Vega et al.6 prepared functional fibers by adsorption of

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cellulose polyelectrolytes containing amino groups for efficient immobilization of enzymes.

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Thus, supramolecular functionalization of fibers gives an opportunity to improve the

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performance of the existing fibrous materials and to create novel bio-based materials, which, in

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turn, helps to valorise pulp fibers and broadens their application. In this study, supramolecular

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functionalization was applied to prepare fluorescent pulp fibers for application as authenticity

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indicators.

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Nowadays, counterfeiting causes serious problems for modern society, and large quantities of

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various fake products among pharmaceuticals, perfumes, clothes, videos, and software exist on

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the market.7 Industries manufacturing genuine commodities suffer huge profit losses and struggle

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to compete in the market with cheap, fake products. In addition, consumers pay excessive price

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for poor quality products that can also put their health and safety in danger.

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One of the approaches which manufacturers utilize to protect their goods from forgery is secure

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packaging. Such packaging is identifiable and contains specific features that are difficult to copy.


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Commonly, both overt (visible) and covert (invisible) features are applied to insure security of

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the packaging. Overt features are instantly recognized by visual perception, and they include

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holograms, color-shift inks, barcoding, and tamper-resistant tape. In contrast, covert features are

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impossible to notice without special equipment and conditions. Presence and location of such

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features in the package is highly confidential. Covert features include invisible printing (UV

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inks), UV fluorescing fibers, and micro-text printing.7–10

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Designing secure packaging requires consideration of its life cycle. To decrease negative impact

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on the environment, utilization of sustainable materials is a necessary measure. This brings

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paper-based packaging into wide use. Wood derived fibers (pulp fibers) being a main component

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in paper-based packaging could also serve as its security elements, for example in a form of

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fluorescent fibers. Such security fibers are invisible under daylight, which means that they are

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indistinguishable from other fibers. Under UV light exposure, the security fibers fluoresce, what

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makes them clearly visible.7,9,11

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To our knowledge, only few approaches have been proposed aiming at modification of wood

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derived pulp fibers for subsequent application as fluorescent security fibers. In one of the

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approaches it was proposed to treat pulp fibers with fluorescent whitening agents (FWAs)7 and in

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another one FWAs were loaded into the lumen of the fibers.11 In both cases, the modification

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steps were applied prior to mixing with neat fibers and formation of fibrous materials. However,

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FWAs used in such approaches suffer one significant disadvantage of low affinity to pulp fibers,

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which in turn requires addition of different fixing agents or salts.

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Thus, the objective of the current study was to produce fluorescent security pulp fibers via

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simple supramolecular modification in aqueous solution using specially-designed bio-based


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surface modifying agents. In the scope of the study, novel types of fluorescent MCDs (FMCDs)

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containing both cationic (3-carboxypropyl)trimethylammonium chloride ester and N-(3-

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propanoic acid)-1,8-naphthalimide or N-(4-butanoic acid)-1,8-naphthalimide moieties were

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synthesized, characterized and used to functionalize eucalyptus Kraft pulp fibers. Interaction of

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the synthesized derivatives with pulp fibers, the effect of modification on the optical properties

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of the fibers as well as their potential application are covered in this study.

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EXPERIMENTAL SECTION

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Materials

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Microcrystalline cellulose (Avicel® PH 101, Fluka, Neu-Ulm, Germany) used as starting

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material for the derivatives had a degree of polymerization (DP) of 135 calculated from the

93

intrinsic viscosity (ISO 5351). Drying of the cellulose and LiCl (Sigma-Aldrich, Deisenhofen,

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Germany) was done at 105 °C for 6 h in a vacuum over KOH before the usage. N-(3-propanoic

95

acid)-1,8-naphthalimide and N-(4-butanoic acid)-1,8-naphthalimide were synthesized according

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to Reger et al.12 Other chemicals and solvents were purchased from Sigma-Aldrich

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(Deisenhofen, Germany) and used without further treatment. Eucalyptus unrefined bleached

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Kraft pulp was obtained from Metsä Fiber (a former Botnia, Kaskö, Finland). Sodium

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bicarbonate and sodium chloride were purchased from J.T. Baker and Merck and used as

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received.

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Methods

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Synthesis of N-(3-propanoic acid)-1,8-naphthalimide esters of cellulose 2a-d, typical example

103

(same procedure for N-(4-butanoic acid)-1,8-naphthalimide esters of cellulose 3a-d)


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Cellulose (10 g, 61 mmol of AGU) was dissolved in N,N-dimethylacetamide (DMA)/LiCl

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(300 mL/18 g) according to Wondraczek et al.2 A solution of N-(3-propanoic acid)-1,8-

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naphthalimide (4.15 g, 15.25 mmol) and N,N-carbonyldiimidazole (CDI; 2.5 g, 15.25 mmol) was

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prepared using DMA (130 mL) as a solvent. The solution was kept at 70 °C until the gas

108

formation ceased, and then it was added to the cellulose solution. The mixture was left to react at

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70 °C for 20 h under stirring. The product of the reaction was precipitated in 2500 mL of 2-

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propanol, filtered, washed twice with 700 mL of 2-propanol and once with 700 mL of water. A

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pale yellow solid (9.8 g, 90%) was obtained by drying the product at 40 °C under a vacuum.

112

Elemental analysis (EA), [%] C: 46.78, H: 6.59, N: 1.19.

113

13

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(C6s), 73.47 (C2), 75.28 (C3, C5), 80.51 (C4), 102.95 (C1), 120.06 (C16), 122.41 (C11), 127.63

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(C15), 131.18 (C13, C14), 134.95 (C12), 136.75 (C10), 170,84 (C7).

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Perpropionylation

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A perpropionylation reaction of the cellulose esters was carried out in order to determine the DS

118

of the esters by means of 1H NMR spectroscopy.2 The success of the reaction was proven by the

119

absence of OH stretching in the FTIR spectra (Nicolet AVATAR 370 DTGS spectrometer, KBr

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technique) and by the presence of signals of propionate moieties in 1H NMR spectra of the

121

derivatives.

122

Synthesis of mixed N-(3-propanoic acid)-1,8-naphthalimide-(3-

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carboxypropyl)trimethylammonium chloride esters of cellulose 4a-d, typical example (same

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procedure for corresponding N-(4-butanoic acid)-1,8-naphthalimide derivatives 5a-d)

C NMR (100 MHz, DMSO-d6/LiCl): δ [ppm] = 21.88 (C9), 35.00 (C8), 60.37 (C6), 65.12


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The N-(3-propanoic acid)-1,8-naphthalimide ester of cellulose (4 g, 20 mmol) was dissolved in

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DMA/LiCl as described for cellulose. A solution of (3-carboxypropyl)trimethylammonium

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chloride (1.84 g, 10 mmol) and CDI (1.62 g, 10 mmol) was prepared by dissolution in DMA

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(50 mL). The solution was kept at 70 °C until the gas formation ceased, and then it was added to

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the dissolved cellulose ester. The reaction was carried out at 70 °C for 20 h under stirring. The

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product of the reaction was precipitated in 800 mL of acetone, filtered, washed thrice with

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300 mL of acetone and twice with 250 mL of ethanol. For purification, the product was dissolved

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in water, and dialysis against water was performed using a cellulose membrane (Spectra/Porr;

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MWCO = 3500 g/mol). A pale yellow solid (2.85 g) was obtained after lyophilization.

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EA, [%] C: 46.66, H: 6.38, N: 2.39, Cl: 3.39.

135

13

136

(C8), 53.04 (C21), 60.96 (C6s), 63,95 (C6), 65.31 (C20), 72.74 (C2), 73.70 (C3), 75.31 (C5),

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80.45 (C4), 100.01 (C1s), 103,20 (C1), 116.75 (C16), 122.58 (C11), 127.65 (C15), 131.86 (C13),

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134.82 (C14), 137.81 (C12), 163.86 (C10), 171.08 (C7), 172.03 (C17).

139

Characterization

140

Characterization of the derivatives was done as described elsewhere.2 Briefly, 13C NMR was

141

used to prove formation of covalent bond between the AGUs and the introduced functional

142

groups. DS of photoactive groups (DSPhoto) was determined by 1H NMR. NMR spectra were

143

recorded using a Bruker Avance 400 MHz spectrometer. Signals in the spectra were assigned

144

using two dimensional measurements (HSQC-DEPT, TOCSY) and by comparison with the

145

spectra of the carboxylic acids used for esterification (spectra not shown). DSPhoto was calculated

C NMR (100 MHz, DMSO-d6/LiCl): δ [ppm] = 18.48 (C19), 30.73 (C9), 32.73 (C18), 36.17


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from the spectral integrals of the naphthalimide moiety protons (7.5–9 ppm, Figure 1) and the

147

methyl group protons of the propionate moiety (1.0 ppm, Figure 1) according to Equation 1:

3 6

!"

#

$

(1)

148

149 150

Figure 1. 1H NMR spectrum of perpropionylated N-(3-propanoic acid)-1,8-naphthalimide ester

151

of cellulose (2a, DSPhoto 0.07)

152 153

In addition to the NMR, DSPhoto and DS of cationic groups (DSCat) were obtained using UV-vis

154

spectroscopy and elemental analysis.2 Elemental analysis was carried out with a CHNS 932

155

Analyzer (Leco) and chloride content was obtained with a procedure described elsewhere.13

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Naphthalimide substituents of FMCDs had a characteristic light absorption at λmax=334 nm

157

recorded by a UV-vis spectrophotometer. Their molar concentration (% , mol/L) was

158

calculated from calibration curves built with N-(3-propanoic acid)-1,8-naphthalimide and N-(4-

159

butanoic acid)-1,8-naphthalimide used as standards for corresponding cellulose esters.

160

Subsequently, the mass fraction of nitrogen originated from the photoactive substituents of the

161

derivatives (&' ) was calculated from the ratio of nitrogen mass concentration in the


8

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photoactive moieties ((' , g/L) to the total mass concentration of the derivatives (()* ,

163

g/L) used for UV-vis spectroscopic measurements: &'

% ∗ ,' ()*

(2)

164

The mass fraction of nitrogen of the cationic substituents (&' -) ) was obtained by subtracting

165

&' from the total amount of nitrogen in the derivative (&' )* ) obtained by elemental

166

analysis. The molar concentration of the cationic moieties (%-) , mol/L) was calculated as: %-)

167

()* ∗ &' -) ,'

The molar concentration of the RUs was obtained using the following formula: ()* ∗ &./0 ,./0

(4)

()* − ( − (-) ()*

(5)

%./0 168

Where &./0 was calculated as: &./0

()* − % ∗ , − , − %-) ∗ ,-) − , ,./0 DSPhoto and DSCat of the esters were found as: %./0

169

(3)

-)

2 2./0 2-) 2./0

(6)

(7) (8)

170 171

Kinetic studies of adsorption

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An amount of wet pulp (dry content around 30 %) corresponding to 200 mg of dry pulp was

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soaked in 10-5 M NaHCO3 solution for 30 min under agitation. The derivatives were dissolved in


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10-5 M NaHCO3 solution to obtain stock solutions with a concentration of 0.5 g/L. A certain

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volume of the derivative solution was added to the pulp suspension, so the dosage of the

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derivatives was 2 % (w/w) based on dry weight of fibers, and the final pulp consistency was

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0.6 % (based on total weight of the suspension). The adsorption was carried out at 25 ± 2 °C and

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270 rpm agitation speed using a multipoint agitation plate. The adsorption time was varied from

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10 min to 17 hours (overnight). After adsorption time elapsed, the suspensions were centrifuged

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at 3500 rpm for 5 min, and the supernatants were analyzed with a Shimadzu 2600 UV-vis

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spectrophotometer. The amounts of the remaining derivatives in the supernatants were found by

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using a calibration curve for the corresponding derivative and an absorbance maximum in the

183

vicinity of 343 nm for all derivatives. Subsequently, the adsorbed amounts were calculated by

184

subtracting the amount of the derivatives remained in the solution from the added amounts. The

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adsorption studies were done in duplicate.

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Adsorption studies at different electrolyte concentrations

187

Sodium chloride solutions of 10-2, 10-3, 10-4 and 10-5 M were used to dissolve the derivatives and

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to adjust consistency of the pulp suspension. NaCl was chosen as an electrolyte due to its low

189

effect on pH at all studied concentrations. The pHs of NaCl solutions at the applied

190

concentrations were 6.2±0.1. The reaction time of adsorption was 17 hours. Other parameters of

191

the experiment and the analysis of the supernatants were similar to ones in the kinetic studies.

192

Adsorption isotherms

193

Time for the adsorption at equilibrium (17 hours) was chosen based on the kinetic studies.

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Dosages of the derivatives varied from 0.1 % to 5 % (w/w, based on dry pulp). Other conditions

195

were the same as for the kinetic studies. The supernatants obtained after centrifuging the


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suspension were analyzed by a Perkin-Elmer LS50B spectrofluorometer. Before the analysis, the

197

supernatants as well as the stock solutions were diluted by 10-5 M NaHCO3 solution. Thus, the

198

concentrations of the samples were adjusted to be in the linear range of the response of emission

199

intensities to the concentrations of the derivative solutions. The samples were excited at their

200

absorbance maxima and emission spectra were collected. The emission maxima were found in

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the vicinity of 394 nm. The determination of the adsorbed amounts was done as described for the

202

kinetic studies.

203

Excitation spectra of the solutions

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The excitation spectra were recorded with the Perkin-Elmer LS50B spectrofluorometer used for

205

fluorescence measurements. The polymer was diluted to 0.8 mg/L concentration with 10-5 M

206

NaHCO3, and excitation spectra were monitored at the emission wavelengths of 390 nm and

207

496 nm, which corresponded to maxima in the emission spectra. The solutions were diluted to

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limit the possible effect of concentration on the excimer formation. On the other hand, the

209

concentration was kept high enough to avoid effects of artifacts in the spectra, e.g. stray light.

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Preparation of sheets from pulp fibers

211

To evaluate optical properties, hand-sheets were formed from the reference and modified fibers

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according to SCAN-CM 11:95. After the adsorption, the pulp suspensions were diluted 20-fold

213

by10-5 M NaHCO3 solution and transferred directly to the sheet former. The formed sheets were

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dried overnight in a conditioned room at 50 % relative humidity and 23 °C.

215

Optical properties of the fibers


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A Shimadzu 2600 spectrophotometer equipped with an integrated sphere ISR-2600 Plus was

217

used to collect UV-vis spectra of diffuse reflection from the fibers. Emission spectra were

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obtained using a Perkin-Elmer LS50B spectrofluorometer. The fibers were excited at a

219

wavelength of the UV absorbance maximum of the derivatives. Fluorescence values over a full

220

visible spectrum were calculated as a difference between two whiteness values measured with

221

and without UV component in the incident light (SCAN-G 5:03) using an Elrepho

222

spectrophotometer. The fluorescence data include the values from parallel samples. Visualization

223

of the fluorescent fibers was performed using an Olympus BX60 fluorescence microscope

224

coupled with a Nikon DS-Fi2 camera. The microscope was equipped with an excitation filter

225

(330-385 nm), a dichroic mirror (400 nm) and a barrier emission filter (420 nm). To simulate an

226

incorporation of the modified fibers in the fiber material (packaging), the templates were cut

227

from the sheets made of the modified fibers and placed on the top of the sheets made of the

228

reference fibers. A UV-black light bulb (15W, absorption band 320-415 nm) was used as a

229

source of a black (UVA) light to illuminate the samples.

230 231

RESULTS AND DISCUSSION

232

In order to get photoactive cellulose derivatives, esterification of the biopolymer was carried out.

233

As demonstrated recently, a very efficient acylation method of cellulose is the application of

234

carboxylic acids activated with CDI that allows the synthesis of MCDs decorated with

235

photoactive and cationic substituents.2

236

Esterification of cellulose with photoactive- and cationic carboxylic acids


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237

In the current study, novel FMCDs were synthesized in a two-step procedure (Figure 2). The

238

esterification of cellulose with N-(3-propanoic acid)-1,8-naphthalimide and N-(4-butanoic acid)-

239

1,8-naphthalimide dyes was done by the reaction of cellulose dissolved in N,N-

240

dimethylacetamide/LiCl with imidazolides of the photoactive carboxylic acids obtained in a

241

separate reaction.

242 243

Figure 2. Synthesis scheme of N-(3-propanoic acid)-1,8-naphthalimide- respectively N-(4-

244

butanoic acid)-1,8-naphthalimide esters of cellulose and the corresponding mixed naphthalimide

245

(3-carboxypropyl)trimethylammonium chloride esters of cellulose via in situ activation of N-(3-

246

propanoic acid)-1,8-naphthalimide, N-(4-butanoic acid)-1,8-naphthalimide, and (3-

247

carboxypropyl)trimethylammonium chloride (4) with N,N-carbonyldiimidazole (CDI) in N,N-

248

dimethylacetamide/LiCl (DMA/LiCl)

249


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Page 14 of 37

250

DSPhoto of the esters could be controlled to a certain extent by the molar ratio of AGU to

251

carboxylic acid imidazolide (Table 1). At comparable molar ratio, the DSPhoto of N-(3-propanoic

252

acid)-1,8-naphthalimide esters (2a-d) was slightly lower compared to the products containing N-

253

(4-butanoic acid)-1,8-naphthalimide esters (3a-d) that can be attributed to sterical reasons, i.e.

254

the different chain length of the linker separating the bulky naphthalimide moieties from the

255

polymer backbone. At molar ratios of AGU to carboxylic imidazolide of 1:0.75 and 1:1, it was

256

noticed that the DSPhoto reached its maximum for both naphthalimide carboxylic acid derivatives

257

(samples 2c,d and 3c,d, Table 1). Thus, the maximum DSPhoto of N-(3-propanoic acid)-1,8-

258

naphthalimide esters was about 0.20 and in the case of N-(4-butanoic acid)-1,8-naphthalimide

259

esters it was about 0.32. These DSPhoto were well sufficient to get photoactive polymers.

260 261

Table 1. Results of the synthesis of N-(3-propanoic acid)-1,8-naphthalimide- and N-(4-butanoic

262

acid)-1,8-naphthalimide esters of cellulose by activation of the carboxylic acids with N,N-

263

carbonyldiimidazole (CDI) carried out in N,N-dimethylacetamide/LiCl Molar ratio Carboxylic acid

N-(3-propanoic acid)-1,8naphthalimide

N-(4-butanoic acid)-1,8naphthalimide

AGUa CDI

carboxylic acid

Sample No. DSNMRb DSUVc

1

0.25

0.25

2a

0.07

0.07

1

0.50

0.50

2b

0.14

0.13

1

0.75

0.75

2c

0.20

0.20

1

1.00

1.00

2d

0.20

0.22

1

0.25

0.25

3a

0.11

0.12

1

0.50

0.50

3b

0.18

0.20

1

0.75

0.75

3c

0.30

0.32

1

1.00

1.00

3d

0.32

0.32

264

a

265

determined by means of UV-vis spectroscopy in DMA/LiCl.

Anhydroglucose unit; bDegree of substitution (DS) determined by means of 1H NMR spectroscopy after perpropionylation; bDS


14

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266 267

To introduce cationic moieties, the photoactive cellulose derivatives (2 and 3) were allowed to

268

react with imidazolide of (3-carboxypropyl)trimethylammonium chloride (Figure 2 and Table 2).

269

At a molar ratio of modified AGU to carboxylic acid imidazolide of 1:0.5, the obtained DSCat

270

was 0.3, and it was independent of the DSPhoto of the starting polymer. In addition, no

271

transesterification occurred, thus, DSPhoto remained almost constant during the esterification.

272 273

Table 2. Results of the synthesis of mixed naphthalimide-(3-carboxypropyl)trimethylammonium

274

chloride esters of cellulose by activation of (3-carboxypropyl)trimethylammonium chloride with

275

N,N-carbonyldiimidazole (CDI) carried out in N,N-dimethylacetamide/LiCl (molar ratio of

276

modified repeating unit/CDI/(3-carboxypropyl)trimethylammonium chloride = 1.00/0.50/0.50) Product

Starting sample No.a

Solubility DSPhotob DSCatb

No.

H2O

DMSO

2a

0.07

0.31

4a

+

+

2b

0.09

0.34

4b

+

+

2c

0.18

0.35

4c

-

+

2d

0.21

0.31

4d

-

+

3a

0.11

0.32

5a

+

+

3b

0.22

0.33

5b

+

+

3c

0.32

0.30

5c

-

+

3d

0.31

0.34

5d

-

+

277 278 279

a

280

13

281

as cationic substituents. In the spectra, it can be noticed that the peaks for positions C1-C5 of the

See Table 1; analysis.

b

Degree of substitution (DS) determined by means of a combination of UV-vis spectroscopy and elemental

C NMR spectra proved the covalent linkages formed between AGU and naphthalimide- as well


15


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Page 16 of 37

282

AGU were almost unaffected while the signal of the primary C atom (C6) peak was shifted to

283

lower field (C6s) by esterification (Figure 3).

284

At a certain ratio, the introduced cationic substituents facilitated solubility of the cellulose

285

derivatives in water (Table 2). However, the obtained derivatives became water insoluble above

286

certain DSPhoto, which was about 0.2 for 2 and 0.3 for 3.

287 288

A

B

C

289

Figure 3. 13C NMR spectra of: A) cellulose dissolved in DMSO-d6/tetrabutylammonium fluoride

290

(TBAF), B) N-(4-butanoic acid)-1,8-naphthalimide ester of cellulose (degree of substitution of

291

photoactive moiety, DSPhoto 0.11) in DMSO-d6, C) N-(4-butanoic acid)-1,8-naphthalimide-(3-


16

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292

carboxypropyl)trimethylammonium chloride ester of cellulose (DSPhoto 0.11, DSCat 0.32) in

293

DMSO-d6.

294 295

Supramolecular functionalization of pulp fibers

296

Synthesized FMCDs were used for modification of eucalyptus unrefined bleached Kraft pulp

297

fibers to introduce photoactive properties to the fibers. The fiber functionalization was performed

298

in a water-based system in one single step by addition of FMCD solutions to pulp fiber

299

suspensions. The adsorption was conducted at mild conditions, i.e. at room temperature under

300

agitation. During adsorption, cationic polyelectrolytes interact quickly with negatively-charged

301

fibers through an ion exchange mechanism driven by the entropy gain from release of counter-

302

ions.14 However, the type of polymer backbone as well as its side groups have strong influence

303

on the interaction with the fibers. Hence, the adsorption of synthesized derivatives was studied in

304

more detail.

305

Among the synthesized FMCDs (Table 2), only water soluble derivatives were chosen as

306

modifying agents. In the case of cationic cellulose esters containing butanoic naphthalimide

307

moiety, there were two types of the water soluble derivatives, they were designated as 5a

308

(DSPhoto = 0.11; DSCat = 0.32) and 5b (DSPhoto = 0.22; DSCat = 0.33). These derivatives were used

309

to study the effect of the DSPhoto on the adsorption onto pulp fibers and the optical properties of

310

the resulting modified fibers. In addition, adsorption of cationic cellulose ester containing

311

propanoic naphthalimide moiety (4a, DSPhoto = 0.07 and DSCat = 0.31) was studied in order to

312

investigate how the length of the aliphatic chain connecting the photoactive moieties to the


17


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Page 18 of 37

313

cellulose backbone affects the fiber modification, and the results were compared to those

314

obtained for the 5a derivative.

315

Adsorption kinetics

316

Kinetic studies were performed in order to investigate the rate of the adsorption process and,

317

also, to find time required to achieve the equilibrium of the adsorption reaction. For that purpose,

318

adsorption isotherms were obtained by addition of the derivative solution at 2 % (w/w) dosage to

319

the fibers and measuring residual concentration versus reaction time. The results of the kinetic

320

studies are summarized in Figure 4.

321 322

Figure 4. Kinetic studies of the adsorption of FMCDs onto pulp fibers: N-(3-propanoic acid)-

323

1,8-naphthalimide-(3-carboxypropyl)trimethylammonium chloride ester of cellulose (4a, DSPhoto

324

0.07, DSCat 0.31) and N-(4-butanoic acid)-1,8-naphthalimide-(3-

325

carboxypropyl)trimethylammonium chloride esters of cellulose (5a, DSPhoto 0.11, DSCat 0.32; 5b,

326

DSPhoto 0.22, DSCat 0.33 )

327


18

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328

The adsorption isotherms showed the same trend for all three derivatives and no differences in

329

the results were observed between the derivatives. In the beginning, adsorption was fast, and the

330

concentration of the derivatives in the solutions decreased steeply during the first 10 minutes. At

331

that point, the amounts of adsorbed 4a, 5a and 5b were 75, 70 and 62 % (w/w) based on the total

332

adsorbed amount. For the rest of the reaction time the adsorption was much slower. The rate of

333

the adsorption of the derivatives during specific time intervals calculated as a difference of the

334

adsorbed amounts to the time interval (∆ads/∆time) was 3.3–4 %/min during first 10 min, and it

335

drastically decreased for the following time interval, 10–60 min (∆ads10-60/∆time10-60

336

< 0.2 %/min). During next 120 min the adsorption rate (∆ads60-180/∆time60-180) was ≤ 0.04 %/min

337

and continued to decrease thereafter approaching the equilibrium. Therefore, it is not necessary

338

to conduct adsorption for a long period of time, and 10–60 min can be sufficient for the

339

modification of pulp fibers. The high initial adsorption rate was attributed to fast ion exchange

340

reactions between the accessible negatively charged groups of the fibers and the cationic

341

moieties of the derivatives.4 Subsequent decrease of the adsorption rate could be explained by

342

several factors. First, the fibers are porous material consisting of fibrils. Therefore, the applied

343

derivatives could penetrate into the inner layers of the fiber wall, and sufficient amount of time

344

was required for the macromolecules to diffuse into the fiber wall and to reach adsorption sites.15

345

Second, steric and electrostatic repulsion could take place between the layers of adsorbed

346

polymer chains and the polymer macromolecules approaching these layers.14 Generally, these

347

types of repulsion retard adsorption of polyelectrolytes on the external fiber surfaces as well as

348

they slow down the diffusion of the polymers into the fiber wall and thus adsorption onto the

349

internal surfaces. 14

350

Effect of electrolyte concentration


19


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Page 20 of 37

351

The type of interactions contributing to the adsorption of FMCDs onto the fibers was studied

352

using various electrolyte concentrations in the suspensions during the reactions. The time and the

353

concentration of the polymers were kept constant for all experiments.

354

The results of the adsorption as a function of salt concentration are shown in Figure 5. Increasing

355

electrolyte concentration led to a gradual increase in the adsorbed amounts for all three

356

derivatives. The ions of the added salt screened the charge of both the derivatives and the fibers,

357

which, in turn, caused a decrease in the electrostatic attraction between the cationic

358

polyelectrolytes and the cellulosic fibers.16 Thus, the enhanced adsorption at higher electrolyte

359

concentration could be explained by non-ionic interactions such as hydrophobic interactions.14

360

This type of interaction could occur between hydrophobic sites of cellulose chains of the fibers

361

and of the polymers.4 Such sites are C-H groups located on the axial position of the

362

glucopyranose ring.17 In addition, photoactive groups of the polymers, which are also

363

hydrophobic, could contribute to these interactions with the fibers.4 This is in agreement with the

364

studies published earlier4 where cationic cellulose derivatives without and with coumarin

365

photoactive moieties adsorbed irreversibly onto model surfaces bearing hydrophobic methyl

366

groups.

367

Another type of non-ionic interaction, i.e. hydrogen bonding, presumably played a minor role

368

during adsorption of FMCDs. This hypothesis is based on the same study,4 where it was shown

369

that no adsorption of the derivatives took place on the model surfaces with hydroxyl groups.

370

Apart from ionization, ionic strength has also an effect on the conformation of the

371

polyelectrolytes in the solution.18 At higher electrolyte concentrations, the conformation changes

372

from expanded to more compact due to decrease of the segment repulsions. As a result, the


20

Page 21 of 37

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373

adsorbed polymers occupy less area on the fibers and more free surfaces are available for the

374

adsorption.18 This could also have a positive impact on the adsorption along with non-ionic

375

interactions.

376 377

Figure 5. Effect of electrolyte concentration on the adsorption of FMCDs onto pulp fibers: N-(3-

378

propanoic acid)-1,8-naphthalimide-(3-carboxypropyl)trimethylammonium chloride ester of

379

cellulose (4a, DSPhoto 0.07, DSCat 0.31) and N-(4-butanoic acid)-1,8-naphthalimide-(3-

380


381


382 383

Independent of the ionic strength, the derivatives were strongly attached to the fibers. After re-

384

dispersion of the modified fibers in the buffers, the amount of derivatives detected in the solution

385

corresponded to less than 5 % (w/w) of the adsorbed amount. These amounts were attributed to

386

the polymers entrapped with the solutions that were not removed during the filtration of the

387

fibers after modification.

388

Equilibrium adsorption isotherms


21


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Page 22 of 37

389

The adsorption isotherms gave important information about the adsorption mechanism and an

390

effective dosage of the derivatives for the fiber modification. The adsorption was performed

391

during 17 hours to reach the reaction equilibrium, and at 10-5 M concentration of the buffer

392

solution to promote ionic interactions. The adsorption isotherms (Figure 6A) were obtained by

393

applying different dosages of the polymers to the fibers and measuring the amount of the

394

derivatives in solution before and after the reaction.

395

The adsorption isotherms of 5a and 5b derivatives reached a saturation plateau. The differences

396

in the adsorbed amounts for the derivatives were insignificant. Thus, the DSPhoto did not have any

397

impact on the adsorption of the derivatives and therefore, it can be concluded that the adsorption

398

was predominantly governed by the ion exchange mechanism. On the other hand, 4a derivative

399

adsorbed to a greater extent, especially, at higher applied dosages. In addition, the adsorption

400

isotherm of 4a derivative did not reach a clear saturation plateau. Such results could be explained

401

by differences in the structure of the photoactive moieties. 4a derivative had a shorter length of

402

alkyl chain connecting photoactive group with the cellulose backbone than the other derivatives,

403

and because of this, 4a macromolecules could take more favorable conformation for the

404

adsorption.


22

Page 23 of 37

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405 406

Figure 6. Adsorption isotherms of FMCDs onto pulp fibers: N-(3-propanoic acid)-1,8-

407

naphthalimide-(3-carboxypropyl)trimethylammonium chloride ester of cellulose (4a, DSPhoto

408


409


410


411 412

As it was already mentioned, the FMCDs interacted with the fibers mainly through an ion-

413

exchange mechanism. Analysis of the reference fibers by a methylene blue sorption method19


23


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Page 24 of 37

414

showed that the amount of the anionic groups in the fibers was equal to 89 µmol/g.4 However,

415

the amount of cationic charge adsorbed together with the derivatives was much lower than this

416

value. At the largest adsorbed amount of the derivatives, it corresponded to 14 µmol/g of cationic

417

groups for 5a and 5b, and 24 µmol/g for 4a. This means that the derivatives did not neutralize all

418

charged groups of the fibers. One possible explanation is that the adsorbed polymers sterically

419

blocked available charged groups of the fibers for the interactions with the derivatives in the

420

solution.4 Another explanation could be limited accessibility of the fiber walls.20

421

Figure 6B shows that for all the derivatives the adsorption was more favorable at lower

422

concentrations of the polymer in the solution or polymer dosage meaning that at lower applied

423

dosages a higher portion of the added amount was adsorbed.

424

Optical properties of FMCDs treated fibers

425

Interaction of the material with UV and visible light governs its visual appearance. Hence, the

426

ability of the fibers to absorb, emit and reflect the incident light determines the potential

427

application of the fibrous materials.

428

Light absorption

429

Light absorption by the modified and reference fibers was measured in the range of 200-700 nm

430

using a UV-vis spectrometer. Reference pulp fibers exhibited two absorbance peaks at 230 and

431

275 nm, originating from residual hexenuronic acid and lignin (Figure 7A).21 The spectra of 4a,

432

5a and 5b modified fibers looked similar to each other and had strong absorption at 340 nm

433

characteristic of naphthalimides.22 The light absorbance became higher as the amount of the


24

Page 25 of 37

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434

absorbed derivatives increased. Thus, the UV spectra qualitatively proved that the modification

435

reaction was successful and the fibers were endowed with new light absorption properties.

436 437

Figure 7. Optical properties of fibers treated with FMCDs at different dosages: absorption

438

spectra (A) and fluorescence spectra (B). Optical properties of solutions of FMCDs: emission

439

spectra (C) and excitation spectra (D). N-(3-propanoic acid)-1,8-naphthalimide-(3-

440

carboxypropyl)trimethylammonium chloride ester of cellulose (4a, DSPhoto 0.07, DSCat 0.31) and

441

N-(4-butanoic acid)-1,8-naphthalimide-(3-carboxypropyl)trimethylammonium chloride esters of

442

cellulose (5a, DSPhoto 0.11, DSCat 0.32; 5b, DSPhoto 0.22, DSCat 0.33 )

443 444


25


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Page 26 of 37

445

Fluorescent properties of the fibres

446

Fluorescent properties of the fibers were studied using a spectrofluorometer, and the emission

447

spectra were collected by irradiating the fibers with the light having the wavelength of the UV

448

absorbance maximum (Figure 7B). All fibers modified by FMCDs had an emission band with a

449

maximum at 393-398 nm which is typical of naphthalimides.22 However, the 5b treated fibers

450

had a second broad emission band located at longer wavelengths and overlapping with the first

451

one. This band could be attributed to the emission of photons by excimers.22 Generally, excimers

452

are dimeric structures formed by association of fluorophores located at sufficiently close

453

distance.23 To confirm the formation of the excimers the fluorescent properties of FMCDs

454

solutions were additionally studied. The emission spectrum of 4a solution showed one strong

455

band at 395 nm that originated from naphthalimide. However, in the case of 5a and 5b, the

456

spectra showed two separate emission bands, even at very low concentrations (below 5 mg/L).

457

The first band had maximum in the vicinity of 395 nm corresponding to monomer emission, and

458

the second maximum was at 482 nm for 5a and at 495 nm for 5b corresponding to excimer

459

emission (Figure 7C). The ratio of the excimer band maximum to that of monomer was 0.3 for

460

5a (concentration range of 0.8-10 mg/L) and 0.8-0.9 for 5b (concentration range of 0.4-5 mg/L).

461

The range of concentrations for 5a solutions was two times higher than for 5b in order to reach

462

similar concentration of photoactive groups in the solutions. The increase of the emission

463

originated from the excimer with increase of DSPhoto at the same concentration of photoactive

464

moieties in the solutions could be explained by intramolecular interactions of the fluorophores

465

attached to the same polymer chain. This explained the second strong emission band of the fibers

466

modified with 5b in the blue-green spectral range (Figure 7B).


26

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467

There are two types of excimers.23 Dynamic excimer is formed by association of monomer in an

468

electronic excited state with monomers in a ground state.23 Static excimers are formed by the

469

interaction of monomers in a ground state.23 To distinguish the type of excimer, the excitation

470

spectra were recorded for 5b solution and these are shown in Figure 7D. As can be seen, the

471

excitation spectrum collected at monomer emission maximum could not be superimposed with

472

the one collected at excimer emission maximum. From these spectra it was calculated that the

473

peak to valley ratio of the characteristic band in the monomer excitation spectrum was higher

474

than that in the excimer excitation spectrum. Another way to compare the excitation spectra is to

475

plot the intensity of normalized excitation spectra for excimer against of that for monomer.24

476

There was no linear correlation between the intensity of scans. These differences in the

477

excitation spectra are characteristic features for the so-called static excimers which formed by

478

the excitation of fluorophores interacting in the ground state.23

479

Visual appearance of the modified fibres

480

Figure 8 shows microscopic images of the modified fibers under white and UV light

481

illumination. In later case, the fibers were excited at 330-385 nm, and the image was obtained by

482

collecting emitted light at λ > 420 nm. As can be seen in Figure 8A, the modified fibers

483

fluoresced (glowed) under UV light exposure. By comparing the fluorescent image with one

484

obtained under white light (Figure 8B, bright field image), the same microstructural elements

485

(pits) and fiber morphology were observed indicating that the fibers were completely covered

486

with the polymer. It is important to mention that during imaging of untreated reference fibers

487

under the same conditions no clear fluorescence image could be obtained. Thus, the fibers

488

modified by the derivatives were clearly distinguishable from the reference fibers.


27


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Page 28 of 37

489

490 491 492

Figure 8.Visualization of fluorescent pulp fibers by epi-fluorescence microscope under exposure

493

of UV light (A) and white light (B). The fibers were modified with 5b and the dosage was 2 %

494

(w/w). N-(4-butanoic acid)-1,8-naphthalimide-(3-carboxypropyl)trimethylammonium chloride

495

ester of cellulose (5b, DSPhoto 0.22, DSCat 0.33).

496 497

An Elrepho spectrophotometer equipped with D65 illuminant was used to quantify the

498

fluorescence over the visible light wavelength range. The obtained values were used to compare

499

the fibers modified with different FMCDs. As shown in Figure 9, no significant differences in

500

the fluorescence were found between three types of the modified fibers. In particular, 5a and 5b

501

being adsorbed on the fibers to the same extent exhibited similar fluorescence despite the double


28

Page 29 of 37

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502

difference in the DSPhoto. Thus, all modified fibers have potential to be used as authenticity

503

indicators.

504 505

Figure 9. Fluorescence intensity of the reference and the fibers modified by FMCDs over the full

506

visual spectrum. N-(3-propanoic acid)-1,8-naphthalimide-(3-carboxypropyl)trimethylammonium

507

chloride ester of cellulose (4a, DSPhoto 0.07, DSCat 0.31) and N-(4-butanoic acid)-1,8-

508

naphthalimide-(3-carboxypropyl)trimethylammonium chloride ester of cellulose (5a, DSPhoto

509

0.11, DSCat 0.32; 5b, DSPhoto 0.22, DSCat 0.33 )

510 511

Figure 10 shows the image of hand-sheets made of reference and modified fibers under black

512

light which is commonly used to reveal counterfeiting. UV light caused glowing of the modified

513

fibers. The glowing effect was stronger at higher amounts of the adsorbed polymers. Visually,

514

the difference in the optical performance between the modified fibers under black light was in

515

the following order: 4a > 5a > 5b. This difference was explained by the formation of excimers in

516

the cases of 5a and 5b that gave additional emission band in the range of 440 nm-500 nm (blue-

517

green region). On other hand, the sheets made of reference and modified fibers looked similar

518

under daylight. Hence, prepared modified fibers could serve as invisible security fibers that


29


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Page 30 of 37

519

become visible when being exposed to UV light. One of the possible applications of such fibers

520

is in smart packaging. They can serve as authenticity indicator for packaging as well as for the

521

products. 0%

0.1%

0.5%

1%

3%

5%

4a

5a

5b 522 523

Figure 10. The picture of fiber hand-sheets under black light illumination. The quadrates and the

524

background made of FMCDs treated and reference fibers, respectively. N-(3-propanoic acid)-

525

1,8-naphthalimide-(3-carboxypropyl)trimethylammonium chloride ester of cellulose (4a, DSPhoto

526


527


528


529

CONCLUSION

530

In summary, the FMCDs containing both fluorescent and cationic functional moieties were

531

successfully synthesized, characterized and used as surface modifying agents for pulp fibers. The

532

FMCDs possessed high affinity to pulp fibers, and they adsorbed irreversibly onto the fibers

533

mainly via charge-directed self-assembly. DSPhoto of FMCDs did not have effect on the

534

adsorption of the derivatives. On the other hand, the adsorption was dependent on the length of

535

the aliphatic chain connecting the naphthalimide moiety to cellulose backbone, thus the


30

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536

derivatives with propanoic naphthalimide moiety exhibited better adsorption compared to the

537

derivatives with butanoic naphthalimide moiety.

538

As a result of the modification, the fibers became fluorescent and they emitted visible light under

539

UV light exposure. Prepared in such way modified pulp fibers can be potentially applied as

540

security features in objects made of fibrous materials, e.g. in packaging to validate its

541

authenticity. Even though all of the tested derivatives can be used for the preparation of

542

fluorescent pulp fibers, the derivative with the butanoic naphthalimide moiety and DSPhoto of

543

0.22 was superior to others as a modifying agent. The developed in this study concept is not

544

limited only to naphthalimide moieties, and as a future work FMCDs with other type of

545

fluorescent groups, having e.g. fluorescence in specific region, can be synthesized and tested as

546

surface modifying agents.

547 548


31


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549

AUTHOR INFORMATION

550

*Corresponding Author:

551

Pedro Fardim (E-mail: [email protected])

552

Thomas Heinze (E-mail: [email protected])

Page 32 of 37

553 554 555


32

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556

ACKNOWLEDGMENT

557

Finnish Funding Agency for Technology and Innovation (Tekes) and Tekniikan edistämissäätiö

558

(TES) foundation are acknowledged for the financial support of the work. Leonore Bretschneider

559

is kindly acknowledged for her help with the synthesis of the derivatives.

560


33


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561

REFERENCES

562

(1)

563 564

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Ariga, K.; Kunitake, T. Overview-What is Supramolecular Chemistry? In Supramolecular Chemistry – Fundamentals and Applications; Springer Berlin Heidelberg, 2006; pp 1–6.

(2)

Wondraczek, H.; Pfeifer, A.; Heinze, T. Water Soluble Photoactive Cellulose Derivatives:

565

Synthesis and Characterization of Mixed 2-[(4-methyl-2-oxo-2H-chromen-7-yl)oxy]acetic

566

acid–(3-carboxypropyl)trimethylammonium Chloride Esters of Cellulose. Cellulose 2012,

567

19 (4), 1327–1335.

568

(3)

Vega, B.; Wondraczek, H.; Bretschneider, L.; Näreoja, T.; Fardim, P.; Heinze, T.

569

Preparation of Reactive Fibre Interfaces Using Multifunctional Cellulose Derivatives.

570

Carbohydr. Polym. 2015, 132, 261–273.

571

(4)

Grigoray, O.; Wondraczek, H.; Heikkilä, E.; Fardim, P.; Heinze, T. Photoresponsive

572

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626

TABLE OF CONTENTS GRAPHIC

Cellulosic fibers

Fluorescent biopolymer

+

UV light, fiber glowing C E L L

U

L

O

S

E 800

I, [a.u.]

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


600

Fiber fluorescence Modified

400 200

Reference

0 350 400 450 500 550 600

nm

Secure packaging under UV light

627 628

SYNOPSIS:

629

Preparation of sustainable bio-based authenticity indicators via simple supramolecular

630

functionalization of pulp fibers in water using novel fluorescent multifunctional cellulose

631

derivatives

632


37

Fluorescent Multifunctional Polysaccharides for Sustainable

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