Fluorescent Multifunctional Polysaccharides for Sustainable

Research Article pubs.acs.org/journal/ascecg

Fluorescent Multifunctional Polysaccharides for Sustainable Supramolecular Functionalization of Fibers in Water Olga Grigoray,† Holger Wondraczek,‡ Annett Pfeifer,‡ Pedro Fardim,*,†,§ and Thomas Heinze*,†,‡ †

Laboratory of Fiber and Cellulose Technology, Åbo Akademi University, Porthansgatan 3, FI-20500, Åbo, Finland Center of Excellence for Polysaccharide Research, Institute of Organic Chemistry and Macromolecular Chemistry, Friedrich Schiller University of Jena, Humboldtstraße 10, 07743 Jena, Germany § Department of Chemical Engineering (CIT), University of Leuven, Celestijnenlaan 200f−box 2424, 3001 Leuven, Belgum ‡

ABSTRACT: The paper describes the synthesis of multifunctional cellulose derivatives (MCDs) containing a fluorescent and a cationic moiety and their application in the functionalization of pulp fibers. The cellulose derivatives, namely N-(3-propanoic acid)- and N-(4-butanoic acid)-1,8naphthalimide esters of cellulose, differed in the degree of substitution (DS) and by the aliphatic chain connecting naphthalimide photoactive groups to the polymer backbone. The derivatives were decorated with a cationic moiety, namely (3-carboxypropyl)trimethylammonium chloride. The fluorescent pulp fibers were prepared by direct self-assembly of the water-soluble fluorescent MCDs on the fibers in water at room temperature. The results indicated that the adsorption was mainly driven by an ion exchange mechanism. UV−vis and fluorescence spectroscopic studies showed that the adsorption yield of the fluorescent MCDs depended on the length of the aliphatic chain of the photoactive groups. Because of the adsorption, the modified pulp fibers gained fluorescence in the visible part of the spectrum. Under black light illumination, the modified fibers fluoresced, which made them visually distinguishable from the reference fibers. Thus, the fluorescent pulp fibers prepared in a simple way can be potentially used as an authenticity indicator in packaging materials. KEYWORDS: Multifunctional cellulose derivatives, Naphthalimide group, Polyelectrolyte, Eucalyptus Kraft fibers, Fluorescent fibers, Authenticity indicator

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controlled by light. Vega et al.6 prepared functional fibers by adsorption of cellulose polyelectrolytes containing amino groups for efficient immobilization of enzymes. Thus, supramolecular functionalization of fibers gives an opportunity to improve the performance of the existing fibrous materials and to create novel biobased materials, which, in turn, helps to valorise pulp fibers and broadens their application. In this study, supramolecular functionalization was applied to prepare fluorescent pulp fibers for application as authenticity indicators. Currently, counterfeiting causes serious problems for modern society, and large quantities of various fake products among pharmaceuticals, perfumes, clothes, videos, and software exist on the market.7 Industries manufacturing genuine commodities suffer huge profit losses and struggle to compete in the market with cheap, fake products. In addition, consumers pay excessive prices for poor quality products that can also put their health and safety in danger. One of the approaches which manufacturers utilize to protect their goods from forgery is secure packaging. Such packaging is

INTRODUCTION Supramolecular functionalization of pulp fibers by adsorption of biobased polymers is a method that allows the introduction of novel properties to the fibers without impairing their bulk properties. This type of functionalization is characterized by the formation of supramolecular assemblies which are created by association of biopolymers with pulp fiber constituents through noncovalent intermolecular interactions, such as hydrogen bonding and electrostatic or hydrophobic interactions.1 The adsorption of the polymers can be carried out at mild conditions in water, which simplifies the integration of this stage in the industrial processes and preserves the structure of the fiber. Supramolecular functionalization of pulp fibers can be done using specially designed multifunctional cellulose derivatives (MCDs). Such cellulose derivatives are decorated with specific functional groups that endow new functionality to the fibers and ionic functional groups that impart solubility to the derivatives in water and facilitate interaction with the fibers.2,3 Depending on the target applications, different functional groups can be introduced into the structure of MCDs. For instance, Grigoray et al.4,5 used coumarin-type cellulose polyelectrolytes to prepare light-responsive pulp fibers/fibrous materials whose mechanical properties could be © 2016 American Chemical Society

Received: October 21, 2016 Revised: December 21, 2016 Published: December 26, 2016 1794

DOI: 10.1021/acssuschemeng.6b02539 ACS Sustainable Chem. Eng. 2017, 5, 1794−1803

Research Article

ACS Sustainable Chemistry & Engineering

naphthalimide (4.15 g, 15.25 mmol) and N,N-carbonyldiimidazole (CDI; 2.5 g, 15.25 mmol) was prepared using DMA (130 mL) as a solvent. The solution was kept at 70 °C until the gas formation ceased, and then, it was added to the cellulose solution. The mixture was left to react at 70 °C for 20 h under stirring. The product of the reaction was precipitated in 2500 mL of 2-propanol, filtered, washed twice with 700 mL of 2-propanol and once with 700 mL of water. A pale yellow solid (9.8 g, 90%) was obtained by drying the product at 40 °C under a vacuum. Elemental analysis (EA), [%] C: 46.78, H: 6.59, N: 1.19. 13 C NMR (100 MHz, DMSO-d6/LiCl): δ [ppm] = 21.88 (C9), 35.00 (C8), 60.37 (C6), 65.12 (C6s), 73.47 (C2), 75.28 (C3, C5), 80.51 (C4), 102.95 (C1), 120.06 (C16), 122.41 (C11), 127.63 (C15), 131.18 (C13, C14), 134.95 (C12), 136.75 (C10), 170.84 (C7). Perpropionylation. A perpropionylation reaction of the cellulose esters was carried out in order to determine the DS of the esters by means of 1H NMR spectroscopy.2 The success of the reaction was proven by the absence of OH stretching in the FTIR spectra (Nicolet AVATAR 370 DTGS spectrometer, KBr technique) and by the presence of signals of propionate moieties in 1H NMR spectra of the derivatives. Synthesis of Mixed N-(3-Propanoic Acid)-1,8-naphthalimide-(3Carboxypropyl)trimethylammonium Chloride Esters of Cellulose 4a−d, Typical Example (Same Procedure for Corresponding N-(4Butanoic Acid)-1,8-naphthalimide Derivatives 5a−d). The N-(3propanoic acid)-1,8-naphthalimide ester of cellulose (4 g, 20 mmol) was dissolved in DMA/LiCl as described for cellulose. A solution of (3-carboxypropyl)trimethylammonium chloride (1.84 g, 10 mmol) and CDI (1.62 g, 10 mmol) was prepared by dissolution in DMA (50 mL). The solution was kept at 70 °C until the gas formation ceased, and then, it was added to the dissolved cellulose ester. The reaction was carried out at 70 °C for 20 h under stirring. The product of the reaction was precipitated in 800 mL of acetone, filtered, washed thrice with 300 mL of acetone and twice with 250 mL of ethanol. For purification, the product was dissolved in water, and dialysis against water was performed using a cellulose membrane (Spectra/Porr; MWCO = 3500 g/mol). A pale yellow solid (2.85 g) was obtained after lyophilization. EA, [%] C: 46.66, H: 6.38, N: 2.39, Cl: 3.39. 13 C NMR (100 MHz, DMSO-d6/LiCl): δ [ppm] = 18.48 (C19), 30.73 (C9), 32.73 (C18), 36.17 (C8), 53.04 (C21), 60.96 (C6s), 63.95 (C6), 65.31 (C20), 72.74 (C2), 73.70 (C3), 75.31 (C5), 80.45 (C4), 100.01 (C 1s), 103.20 (C1), 116.75 (C16), 122.58 (C11), 127.65 (C15), 131.86 (C13), 134.82 (C14), 137.81 (C12), 163.86 (C10), 171.08 (C7), 172.03 (C17). Characterization. Characterization of the derivatives was done as described elsewhere.2 Briefly, 13C NMR was used to prove formation of covalent bond between the AGUs and the introduced functional groups. DS of photoactive groups (DSphoto) was determined by 1H NMR. NMR spectra were recorded using a Bruker Avance 400 MHz spectrometer. Signals in the spectra were assigned using twodimensional measurements (HSQC-DEPT, TOCSY) and by comparison with the spectra of the carboxylic acids used for esterification (spectra not shown). DSphoto was calculated from the spectral integrals of the naphthalimide moiety protons (7.5−9 ppm, Figure 1) and the methyl group protons of the propionate moiety (1.0 ppm, Figure 1) according to eq 1:

identifiable and contains specific features that are difficult to copy. Commonly, both overt (visible) and covert (invisible) features are applied to ensure security of the packaging. Overt features are instantly recognized by visual perception, and they include holograms, color-shift inks, barcoding, and tamperresistant tape. In contrast, covert features are impossible to notice without special equipment and conditions. The presence and location of such features in the package are highly confidential. Covert features include invisible printing (UV inks), UV fluorescing fibers, and microtext printing.7−10 Designing secure packaging requires consideration of its life cycle. To decrease negative impact on the environment, utilization of sustainable materials is a necessary measure. This has brought paper-based packaging into wide use. Wood derived fibers (pulp fibers) are a main component in paperbased packaging which could also serve as a security element, for example in the form of fluorescent fibers. Such security fibers are invisible under daylight, which means that they are indistinguishable from other fibers. Under UV light exposure, the security fibers fluoresce, what makes them clearly visible.7,9,11 To our knowledge, only a few approaches have been proposed aiming at modification of wood derived pulp fibers for subsequent application as fluorescent security fibers. In one of the approaches it was proposed to treat pulp fibers with fluorescent whitening agents (FWAs),7 and in another one FWAs were loaded into the lumen of the fibers.11 In both cases, the modification steps were applied prior to mixing with neat fibers and formation of fibrous materials. However, FWAs used in such approaches suffer one significant disadvantage of low affinity to pulp fibers, which in turn requires the addition of different fixing agents or salts. Thus, the objective of the current study was to produce fluorescent security pulp fibers via simple supramolecular modification in aqueous solution using specially designed biobased surface modifying agents. In the scope of the study, novel types of fluorescent MCDs (FMCDs) containing both cationic (3-carboxypropyl)trimethylammonium chloride ester and N-(3-propanoic acid)-1,8-naphthalimide or N-(4-butanoic acid)-1,8-naphthalimide moieties were synthesized, characterized, and used to functionalize eucalyptus Kraft pulp fibers. The interaction of the synthesized derivatives with pulp fibers and the effect of modification on the optical properties of the fibers as well as their potential applications are covered in this study.

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EXPERIMENTAL SECTION

Materials. Microcrystalline cellulose (Avicel PH 101, Fluka, NeuUlm, Germany) used as starting material for the derivatives had a degree of polymerization (DP) of 135 calculated from the intrinsic viscosity (ISO 5351). Drying of the cellulose and LiCl (Sigma-Aldrich, Deisenhofen, Germany) was done at 105 °C for 6 h in a vacuum over KOH before the usage. N-(3-Propanoic acid)-1,8-naphthalimide and N-(4-butanoic acid)-1,8-naphthalimide were synthesized according to Reger et al.12 Other chemicals and solvents were purchased from Sigma-Aldrich (Deisenhofen, Germany) and used without further treatment. Eucalyptus unrefined bleached Kraft pulp was obtained from Metsä Fiber (a former Botnia, Kaskö, Finland). Sodium bicarbonate and sodium chloride were purchased from J.T. Baker and Merck and used as received. Methods. Synthesis of N-(3-Propanoic Acid)-1,8-naphthalimide Esters of Cellulose 2a−d, Typical Example (Same Procedure for N(4-Butanoic Acid)-1,8-naphthalimide Esters of Cellulose 3a−d). Cellulose (10 g, 61 mmol of AGU) was dissolved in N,Ndimethylacetamide (DMA)/LiCl (300 mL/18 g) according to the work of Wondraczek et al.2 A solution of N-(3-propanoic acid)-1,8-

DSphoto =

3(IH12 + 13 + 14 + 16 + 17 + 18)

(

6

(IH12 + 13 + 14 + 16 + 17 + 18) 6

+

IH21 3

)

(1)

In addition to the NMR, DSphoto and DS of cationic groups (DScat) were obtained using UV−vis spectroscopy and elemental analysis.2 Elemental analysis was carried out with a CHNS 932 Analyzer (Leco) and chloride content was obtained with a procedure described elsewhere.13 Naphthalimide substituents of FMCDs had a characteristic light absorption at λmax = 334 nm recorded by a UV−vis spectrophotometer. Their molar concentration (cphoto, mol/L) was calculated from calibration curves built with N-(3-propanoic acid)-1,8naphthalimide and N-(4-butanoic acid)-1,8-naphthalimide used as 1795


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time elapsed, the suspensions were centrifuged at 3500 rpm for 5 min, and the supernatants were analyzed with a Shimadzu 2600 UV−vis spectrophotometer. The amounts of the remaining derivatives in the supernatants were found by using a calibration curve for the corresponding derivative and an absorbance maximum in the vicinity of 343 nm for all derivatives. Subsequently, the adsorbed amounts were calculated by subtracting the amount of the derivatives remained in the solution from the added amounts. The adsorption studies were done in duplicate. Adsorption Studies at Different Electrolyte Concentrations. Sodium chloride solutions of 10−2, 10−3, 10−4, and 10−5 M were used to dissolve the derivatives and to adjust consistency of the pulp suspension. NaCl was chosen as an electrolyte due to its low effect on pH at all studied concentrations. The pHs of NaCl solutions at the applied concentrations were 6.2 ± 0.1. The reaction time of adsorption was 17 h. Other parameters of the experiment and the analysis of the supernatants were similar to ones in the kinetic studies. Adsorption Isotherms. Time for the adsorption at equilibrium (17 h) was chosen based on the kinetic studies. Dosages of the derivatives varied from 0.1% to 5% (w/w, based on dry pulp). Other conditions were the same as for the kinetic studies. The supernatants obtained after centrifuging the suspension were analyzed by a PerkinElmer LS50B spectrofluorometer. Before the analysis, the supernatants as well as the stock solutions were diluted by 10−5 M NaHCO3 solution. Thus, the concentrations of the samples were adjusted to be in the linear range of the response of emission intensities to the concentrations of the derivative solutions. The samples were excited at their absorbance maxima and emission spectra were collected. The emission maxima were found in the vicinity of 394 nm. The determination of the adsorbed amounts was done as described for the kinetic studies. Excitation Spectra of the Solutions. The excitation spectra were recorded with the PerkinElmer LS50B spectrofluorometer used for fluorescence measurements. The polymer was diluted to 0.8 mg/L concentration with 10−5 M NaHCO3, and excitation spectra were monitored at the emission wavelengths of 390 and 496 nm, which corresponded to maxima in the emission spectra. The solutions were diluted to limit the possible effect of concentration on the excimer formation. On the other hand, the concentration was kept high enough to avoid effects of artifacts in the spectra, e.g. stray light. Preparation of Sheets from Pulp Fibers. To evaluate optical properties, hand-sheets were formed from the reference and modified fibers according to SCAN-CM 11:95. After the adsorption, the pulp suspensions were diluted 20-fold by 10−5 M NaHCO3 solution and transferred directly to the sheet former. The formed sheets were dried overnight in a conditioned room at 50% relative humidity and 23 °C. Optical Properties of the Fibers. A Shimadzu 2600 spectrophotometer equipped with an integrated sphere ISR-2600 Plus was used to collect UV−vis spectra of diffuse reflection from the fibers. Emission spectra were obtained using a PerkinElmer LS50B spectrofluorometer. The fibers were excited at a wavelength of the UV absorbance maximum of the derivatives. Fluorescence values over a full visible spectrum were calculated as a difference between two whiteness values measured with and without UV component in the incident light (SCAN-G 5:03) using an Elrepho spectrophotometer. The fluorescence data include the values from parallel samples. Visualization of the fluorescent fibers was performed using an Olympus BX60 fluorescence microscope coupled with a Nikon DS-Fi2 camera. The microscope was equipped with an excitation filter (330−385 nm), a dichroic mirror (400 nm), and a barrier emission filter (420 nm). To simulate an incorporation of the modified fibers in the fiber material (packaging), the templates were cut from the sheets made of the modified fibers and placed on the top of the sheets made of the reference fibers. A UV-black light bulb (15W, absorption band 320− 415 nm) was used as a source of a black (UVA) light to illuminate the samples.

Figure 1. 1H NMR spectrum of perpropionylated N-(3-propanoic acid)-1,8-naphthalimide ester of cellulose (2a, DSphoto 0.07). standards for corresponding cellulose esters. Subsequently, the mass fraction of nitrogen originated from the photoactive substituents of the derivatives (wNphoto) was calculated from the ratio of nitrogen mass concentration in the photoactive moieties (ρNphoto, g/L) to the total mass concentration of the derivatives (ρtotal, g/L) used for UV−vis spectroscopic measurements:

wNphoto =

c photoMN ρtotal

(2)

The mass fraction of nitrogen of the cationic substituents (wNcat) was obtained by subtracting (wNphoto) from the total amount of nitrogen in the derivative (wNtotal) obtained by elemental analysis. The molar concentration of the cationic moieties (ccat, mol/L) was calculated as ρ wNcat ccat = total MN (3) The molar concentration of the RUs was obtained using the following formula: ρ wAGU cAGU = total MAGU (4) Where wAGU was calculated as ρtotal − ρphoto − ρcat wAGU = ρtotal

cAGU =

(5)

ρtotal − c photo(M photo − MH) − ccat(Mcat − MH) MAGU

(6)

DSphoto and DScat of the esters were found as DSphoto =

DScat =

C photo CAGU

Ccat CAGU

(7) (8)

Kinetic Studies of Adsorption. An amount of wet pulp (dry content around 30%) corresponding to 200 mg of dry pulp was soaked in 10−5 M NaHCO3 solution for 30 min under agitation. The derivatives were dissolved in 10−5 M NaHCO3 solution to obtain stock solutions with a concentration of 0.5 g/L. A certain volume of the derivative solution was added to the pulp suspension, so the dosage of the derivatives was 2% (w/w) based on dry weight of fibers, and the final pulp consistency was 0.6% (based on total weight of the suspension). The adsorption was carried out at 25 ± 2 °C and 270 rpm agitation speed using a multipoint agitation plate. The adsorption time was varied from 10 min to 17 h (overnight). After adsorption 1796


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Figure 2. Synthesis scheme of N-(3-propanoic acid)-1,8-naphthalimide and N-(4-butanoic acid)-1,8-naphthalimide esters of cellulose and the corresponding mixed naphthalimide (3-carboxypropyl)trimethylammonium chloride esters of cellulose via in situ activation of N-(3-propanoic acid)1,8-naphthalimide, N-(4-butanoic acid)-1,8-naphthalimide, and (3-carboxypropyl)trimethylammonium chloride (4) with N,N-carbonyldiimidazole (CDI) in N,N-dimethylacetamide/LiCl (DMA/LiCl).

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RESULTS AND DISCUSSION In order to get photoactive cellulose derivatives, esterification of the biopolymer was carried out. As demonstrated recently, a very efficient acylation method of cellulose is the application of carboxylic acids activated with CDI that allows the synthesis of MCDs decorated with photoactive and cationic substituents.2 Esterification of Cellulose with Photoactive and Cationic Carboxylic Acids. In the current study, novel FMCDs were synthesized in a two-step procedure (Figure 2). The esterification of cellulose with N-(3-propanoic acid)-1,8naphthalimide and N-(4-butanoic acid)-1,8-naphthalimide dyes was done by the reaction of cellulose dissolved in N,Ndimethylacetamide/LiCl with imidazolides of the photoactive carboxylic acids obtained in a separate reaction. DSphoto of the esters could be controlled to a certain extent by the molar ratio of AGU to carboxylic acid imidazolide (Table 1). At comparable molar ratio, the DSphoto of N-(3-

propanoic acid)-1,8-naphthalimide esters (2a−d) was slightly lower compared to the products containing N-(4-butanoic acid)-1,8-naphthalimide esters (3a−d) that can be attributed to sterical reasons, i.e., the different chain length of the linker separating the bulky naphthalimide moieties from the polymer backbone. At molar ratios of AGU to carboxylic imidazolide of 1:0.75 and 1:1, it was noticed that the DSphoto reached its maximum for both naphthalimide carboxylic acid derivatives (samples 2c,d and 3c,d, Table 1). Thus, the maximum DSphoto of N-(3-propanoic acid)-1,8-naphthalimide esters was about 0.20, and in the case of N-(4-butanoic acid)-1,8-naphthalimide esters, it was about 0.32. These DSphoto values were sufficient to get photoactive polymers. To introduce cationic moieties, the photoactive cellulose derivatives (2 and 3) were allowed to react with imidazolide of (3-carboxypropyl)trimethylammonium chloride (Figure 2 and Table 2). At a molar ratio of modified AGU to carboxylic acid imidazolide of 1:0.5, the obtained DScat was 0.3, and it was independent of the DSphoto of the starting polymer. In addition, no transesterification occurred, and thus, DSphoto remained almost constant during the esterification. 13 C NMR spectra proved the covalent linkages formed between AGU and naphthalimide as well as cationic substituents. In the spectra, it can be noticed that the peaks for positions C1−C5 of the AGU were almost unaffected while the signal of the primary C atom (C6) peak was shifted to lower field (C6s) by esterification (Figure 3). At a certain ratio, the introduced cationic substituents facilitated solubility of the cellulose derivatives in water (Table 2). However, the obtained derivatives became water insoluble above certain DSphoto, which was about 0.2 for 2 and 0.3 for 3. Supramolecular Functionalization of Pulp Fibers. Synthesized FMCDs were used for modification of eucalyptus unrefined bleached Kraft pulp fibers to introduce photoactive properties to the fibers. The fiber functionalization was performed in a water-based system in one single step by addition of FMCD solutions to pulp fiber suspensions. The adsorption was conducted at mild conditions, i.e. at room temperature under agitation. During adsorption, cationic

Table 1. Results of the Synthesis of N-(3-Propanoic Acid)1,8-naphthalimide and N-(4-Butanoic Acid)-1,8naphthalimide Esters of Cellulose by Activation of the Carboxylic Acids with N,N-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

sample

AGUa

CDI

carboxylic acid

1 1 1 1 1 1 1 1

0.25 0.50 0.75 1.00 0.25 0.50 0.75 1.00

0.25 0.50 0.75 1.00 0.25 0.50 0.75 1.00

no. 2a 2b 2c 2d 3a 3b 3c 3d

DSNMRb DSUVc 0.07 0.14 0.20 0.20 0.11 0.18 0.30 0.32

0.07 0.13 0.20 0.22 0.12 0.20 0.32 0.32

a

Anhydroglucose unit. bDegree of substitution (DS) determined by means of 1H NMR spectroscopy after perpropionylation. cDS determined by means of UV−vis spectroscopy in DMA/LiCl. 1797


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propanoic naphthalimide moiety (4a, DSphoto = 0.07 and DScat = 0.31) was studied in order to investigate how the length of the aliphatic chain connecting the photoactive moieties to the cellulose backbone affects the fiber modification, and the results were compared to those obtained for the 5a derivative. Adsorption Kinetics. Kinetic studies were performed in order to investigate the rate of the adsorption process and, also, to find time required to achieve the equilibrium of the adsorption reaction. For that purpose, adsorption isotherms were obtained by addition of the derivative solution at 2% (w/ w) dosage to the fibers and measuring residual concentration versus reaction time. The results of the kinetic studies are summarized in Figure 4. The adsorption isotherms showed the same trend for all three derivatives and no differences in the results were observed between the derivatives. In the beginning, adsorption was fast, and the concentration of the derivatives in the solutions decreased steeply during the first 10 min. At that point, the amounts of adsorbed 4a, 5a, and 5b were 75, 70 and 62% (w/ w) based on the total adsorbed amount. For the rest of the reaction time the adsorption was much slower. The rate of the adsorption of the derivatives during specific time intervals calculated as a difference of the adsorbed amounts to the time interval (Δads/Δtime) was 3.3−4%/min during first 10 min, and it drastically decreased for the following time interval, 10− 60 min (Δads10−60/Δtime10−60 < 0.2%/min). During next 120 min the adsorption rate (Δads60−180/Δtime60−180) was ≤0.04%/ min and continued to decrease thereafter approaching the equilibrium. Therefore, it is not necessary to conduct adsorption for a long period of time, and 10−60 min can be sufficient for the modification of pulp fibers. The high initial adsorption rate was attributed to fast ion exchange reactions between the accessible negatively charged groups of the fibers and the cationic moieties of the derivatives.4 Subsequent decrease of the adsorption rate could be explained by several factors. First, the fibers are porous material consisting of fibrils. Therefore, the applied derivatives could penetrate into the inner layers of the fiber wall, and sufficient amount of time was

Table 2. Results of the Synthesis of Mixed Naphthalimide(3-carboxypropyl)trimethylammonium Chloride Esters of Cellulose by Activation of (3Carboxypropyl)trimethylammonium Chloride with N,NCarbonyldiimidazole (CDI) Carried out in N,Ndimethylacetamide/LiCla product starting sample

solubility

no.b

DSphotoc

DScatc

no.

H2O

DMSO

2a 2b 2c 2d 3a 3b 3c 3d

0.07 0.09 0.18 0.21 0.11 0.22 0.32 0.31

0.31 0.34 0.35 0.31 0.32 0.33 0.30 0.34

4a 4b 4c 4d 5a 5b 5c 5d

+ + − − + + − −

+ + + + + + + +

a

Molar ratio of modified repeating unit/CDI/(3-carboxypropyl)trimethylammonium chloride = 1.00/0.50/0.50). bSee Table 1. c Degree of substitution (DS) determined by means of a combination of UV−vis spectroscopy and elemental analysis.

polyelectrolytes interact quickly with negatively charged fibers through an ion exchange mechanism driven by the entropy gain from release of counterions.14 However, the type of polymer backbone as well as its side groups have strong influence on the interaction with the fibers. Hence, the adsorption of synthesized derivatives was studied in more detail. Among the synthesized FMCDs (Table 2), only watersoluble derivatives were chosen as modifying agents. In the case of cationic cellulose esters containing butanoic naphthalimide moiety, there were two types of the water-soluble derivatives, they were designated as 5a (DSphoto = 0.11; DScat = 0.32) and 5b (DSphoto = 0.22; DScat = 0.33). These derivatives were used to study the effect of the DSphoto on the adsorption onto pulp fibers and the optical properties of the resulting modified fibers. In addition, adsorption of cationic cellulose ester containing

Figure 3. 13C NMR spectra of (A) cellulose dissolved in DMSO-d6/tetrabutylammonium fluoride (TBAF), (B) N-(4-butanoic acid)-1,8naphthalimide ester of cellulose (degree of substitution of photoactive moiety, DSphoto 0.11) in DMSO-d6, (C) N-(4-butanoic acid)-1,8naphthalimide-(3-carboxypropyl)trimethylammonium chloride ester of cellulose (DSphoto 0.11, DScat 0.32) in DMSO-d6. 1798


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Figure 4. Kinetic studies of the adsorption of FMCDs onto pulp fibers: N-(3-propanoic acid)-1,8-naphthalimide-(3-carboxypropyl)trimethylammonium chloride ester of cellulose (4a, DSphoto 0.07, DScat 0.31) and N-(4-butanoic acid)-1,8-naphthalimide-(3-carboxypropyl)trimethylammonium chloride esters of cellulose (5a, DSphoto 0.11, DScat 0.32; 5b, DSphoto 0.22, DScat 0.33).

Figure 5. Effect of electrolyte concentration on the adsorption of FMCDs onto pulp fibers: N-(3-propanoic acid)-1,8-naphthalimide-(3carboxypropyl)trimethylammonium chloride ester of cellulose (4a, DSphoto 0.07, DScat 0.31) and N-(4-butanoic acid)-1,8-naphthalimide(3-carboxypropyl)trimethylammonium chloride esters of cellulose (5a, DSphoto 0.11, DScat 0.32; 5b, DSphoto 0.22, DScat 0.33).

required for the macromolecules to diffuse into the fiber wall and to reach adsorption sites.15 Second, steric and electrostatic repulsion could take place between the layers of adsorbed polymer chains and the polymer macromolecules approaching these layers.14 Generally, these types of repulsion retard adsorption of polyelectrolytes on the external fiber surfaces as well as they slow down the diffusion of the polymers into the fiber wall and thus adsorption onto the internal surfaces.14 Effect of Electrolyte Concentration. The type of interactions contributing to the adsorption of FMCDs onto the fibers was studied using various electrolyte concentrations in the suspensions during the reactions. The time and the concentration of the polymers were kept constant for all experiments. The results of the adsorption as a function of salt concentration are shown in Figure 5. Increasing electrolyte concentration led to a gradual increase in the adsorbed amounts for all three derivatives. The ions of the added salt screened the charge of both the derivatives and the fibers, which, in turn, caused a decrease in the electrostatic attraction between the cationic polyelectrolytes and the cellulosic fibers.16 Thus, the enhanced adsorption at higher electrolyte concentration could be explained by nonionic interactions such as hydrophobic interactions.14 This type of interaction could occur between hydrophobic sites of cellulose chains of the fibers and of the polymers.4 Such sites are C−H groups located on the axial position of the glucopyranose ring.17 In addition, photoactive groups of the polymers, which are also hydrophobic, could contribute to these interactions with the fibers.4 This is in agreement with the studies published earlier4 where cationic cellulose derivatives without and with coumarin photoactive moieties adsorbed irreversibly onto model surfaces bearing hydrophobic methyl groups. Another type of nonionic interaction, i.e. hydrogen bonding, presumably played a minor role during adsorption of FMCDs. This hypothesis is based on the same study,4 where it was shown that no adsorption of the derivatives took place on the model surfaces with hydroxyl groups.

Apart from ionization, ionic strength has also an effect on the conformation of the polyelectrolytes in the solution.18 At higher electrolyte concentrations, the conformation changes from expanded to more compact due to decrease of the segment repulsions. As a result, the adsorbed polymers occupy less area on the fibers and more free surfaces are available for the adsorption.18 This could also have a positive impact on the adsorption along with nonionic interactions. Independent of the ionic strength, the derivatives were strongly attached to the fibers. After redispersion of the modified fibers in the buffers, the amount of derivatives detected in the solution corresponded to less than 5% (w/w) of the adsorbed amount. These amounts were attributed to the polymers entrapped with the solutions that were not removed during the filtration of the fibers after modification. Equilibrium Adsorption Isotherms. The adsorption isotherms gave important information about the adsorption mechanism and an effective dosage of the derivatives for the fiber modification. The adsorption was performed during 17 h to reach the reaction equilibrium, and at 10−5 M concentration of the buffer solution to promote ionic interactions. The adsorption isotherms (Figure 6A) were obtained by applying different dosages of the polymers to the fibers and measuring the amount of the derivatives in solution before and after the reaction. The adsorption isotherms of 5a and 5b derivatives reached a saturation plateau. The differences in the adsorbed amounts for the derivatives were insignificant. Thus, the DSphoto did not have any impact on the adsorption of the derivatives and therefore, it can be concluded that the adsorption was predominantly governed by the ion exchange mechanism. On the other hand, 4a derivative adsorbed to a greater extent, especially, at higher applied dosages. In addition, the adsorption isotherm of 4a derivative did not reach a clear saturation plateau. Such results could be explained by differences in the structure of the photoactive moieties. 4a derivative had a shorter length of alkyl chain connecting photoactive group with 1799


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the solution or polymer dosage meaning that at lower applied dosages a higher portion of the added amount was adsorbed. Optical Properties of FMCD Treated Fibers. Interaction of the material with UV and visible light governs its visual appearance. Hence, the ability of the fibers to absorb, emit, and reflect the incident light determines the potential application of the fibrous materials. Light Absorption. Light absorption by the modified and reference fibers was measured in the range of 200−700 nm using a UV−vis spectrometer. Reference pulp fibers exhibited two absorbance peaks at 230 and 275 nm, originating from residual hexenuronic acid and lignin (Figure 7A).21 The spectra of 4a, 5a, and 5b modified fibers looked similar to each other and had strong absorption at 340 nm characteristic of naphthalimides.22 The light absorbance became higher as the amount of the absorbed derivatives increased. Thus, the UV spectra qualitatively proved that the modification reaction was successful and the fibers were endowed with new light absorption properties. Fluorescent Properties of the Fibers. Fluorescent properties of the fibers were studied using a spectrofluorometer, and the emission spectra were collected by irradiating the fibers with the light having the wavelength of the UV absorbance maximum (Figure 7B). All fibers modified by FMCDs had an emission band with a maximum at 393−398 nm which is typical of naphthalimides.22 However, the 5b treated fibers had a second broad emission band located at longer wavelengths and overlapping with the first one. This band could be attributed to the emission of photons by excimers.22 Generally, excimers are dimeric structures formed by association of fluorophores located at sufficiently close distance.23 To confirm the formation of the excimers the fluorescent properties of FMCDs solutions were additionally studied. The emission spectrum of 4a solution showed one strong band at 395 nm that originated from naphthalimide. However, in the case of 5a and 5b, the spectra showed two separate emission bands, even at very low concentrations (below 5 mg/L). The first band had maximum in the vicinity of 395 nm corresponding to monomer emission, and the second maximum was at 482 nm for 5a and at 495 nm for 5b corresponding to excimer emission (Figure 7C). The ratio of the excimer band maximum to that of monomer was 0.3 for 5a (concentration range of 0.8−10 mg/ L) and 0.8−0.9 for 5b (concentration range of 0.4−5 mg/L). The range of concentrations for 5a solutions was two times higher than for 5b in order to reach similar concentration of photoactive groups in the solutions. The increase of the emission originated from the excimer with increase of DSphoto at the same concentration of photoactive moieties in the solutions could be explained by intramolecular interactions of the fluorophores attached to the same polymer chain. This explained the second strong emission band of the fibers modified with 5b in the blue−green spectral range (Figure 7B). There are two types of excimers.23 Dynamic excimer is formed by association of monomer in an electronic excited state with monomers in a ground state.23 Static excimers are formed by the interaction of monomers in a ground state.23 To distinguish the type of excimer, the excitation spectra were recorded for 5b solution and these are shown in Figure 7D. As can be seen, the excitation spectrum collected at monomer emission maximum could not be superimposed with the one collected at excimer emission maximum. From these spectra it was calculated that the peak to valley ratio of the characteristic band in the monomer excitation spectrum was higher than that

Figure 6. Adsorption isotherms of FMCDs onto pulp fibers: N-(3propanoic acid)-1,8-naphthalimide-(3-carboxypropyl)trimethylammonium chloride ester of cellulose (4a, DSphoto 0.07, DScat 0.31) and N-(4-butanoic acid)-1,8-naphthalimide-(3-carboxypropyl)trimethylammonium chloride esters of cellulose (5a, DSphoto 0.11, DScat 0.32; 5b, DSphoto 0.22, DScat 0.33).

the cellulose backbone than the other derivatives, and because of this, 4a macromolecules could take more favorable conformation for the adsorption. As it was already mentioned, the FMCDs interacted with the fibers mainly through an ion-exchange mechanism. Analysis of the reference fibers by a methylene blue sorption method19 showed that the amount of the anionic groups in the fibers was equal to 89 μmol/g.4 However, the amount of cationic charge adsorbed together with the derivatives was much lower than this value. At the largest adsorbed amount of the derivatives, it corresponded to 14 μmol/g of cationic groups for 5a and 5b, and 24 μmol/g for 4a. This means that the derivatives did not neutralize all charged groups of the fibers. One possible explanation is that the adsorbed polymers sterically blocked available charged groups of the fibers for the interactions with the derivatives in the solution.4 Another explanation could be limited accessibility of the fiber walls.20 Figure 6B shows that for all the derivatives the adsorption was more favorable at lower concentrations of the polymer in 1800


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Figure 7. Optical properties of fibers treated with FMCDs at different dosages: absorption spectra (A) and fluorescence spectra (B). Optical properties of solutions of FMCDs: emission spectra (C) and excitation spectra (D). N-(3-Propanoic acid)-1,8-naphthalimide-(3-carboxypropyl)trimethylammonium chloride ester of cellulose (4a, DSphoto 0.07, DScat 0.31) and N-(4-butanoic acid)-1,8-naphthalimide-(3-carboxypropyl)trimethylammonium chloride esters of cellulose (5a, DSphoto 0.11, DScat 0.32; 5b, DSphoto 0.22, DScat 0.33).

in the excimer excitation spectrum. Another way to compare the excitation spectra is to plot the intensity of normalized excitation spectra for excimer against of that for monomer.24 There was no linear correlation between the intensity of scans. These differences in the excitation spectra are characteristic features for the so-called static excimers which formed by the excitation of fluorophores interacting in the ground state.23 Visual Appearance of the Modified Fibers. Figure 8 shows microscopic images of the modified fibers under white and UV light illumination. In later case, the fibers were excited at 330− 385 nm, and the image was obtained by collecting emitted light at λ > 420 nm. As can be seen in Figure 8A, the modified fibers fluoresced (glowed) under UV light exposure. By comparing the fluorescent image with one obtained under white light (Figure 8B, bright field image), the same microstructural elements (pits) and fiber morphology were observed indicating that the fibers were completely covered with the polymer. It is important to mention that during imaging of untreated reference fibers under the same conditions no clear fluorescence image could be obtained. Thus, the fibers modified by the derivatives were clearly distinguishable from the reference fibers. An Elrepho spectrophotometer equipped with D65 illuminant was used to quantify the fluorescence over the visible light wavelength range. The obtained values were used to compare the fibers modified with different FMCDs. As shown in Figure 9, no significant differences in the fluorescence were found between three types of the modified fibers. In particular, 5a and 5b being adsorbed on the fibers to the same extent exhibited similar fluorescence despite the double difference in the DSphoto. Thus, all modified fibers have potential to be used as authenticity indicators. Figure 10 shows the image of hand-sheets made of reference and modified fibers under black light which is commonly used

Figure 8. Visualization of fluorescent pulp fibers by epi-fluorescence microscope under exposure of UV light (A) and white light (B). The fibers were modified with 5b and the dosage was 2% (w/w). N-(4Butanoic acid)-1,8-naphthalimide-(3-carboxypropyl)trimethylammonium chloride ester of cellulose (5b, DSphoto 0.22, DScat 0.33).

to reveal counterfeiting. UV light caused glowing of the modified fibers. The glowing effect was stronger at higher amounts of the adsorbed polymers. Visually, the difference in 1801


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they adsorbed irreversibly onto the fibers mainly via chargedirected self-assembly. DSphoto of FMCDs did not have effect on the adsorption of the derivatives. On the other hand, the adsorption was dependent on the length of the aliphatic chain connecting the naphthalimide moiety to cellulose backbone, thus the derivatives with propanoic naphthalimide moiety exhibited better adsorption compared to the derivatives with butanoic naphthalimide moiety. As a result of the modification, the fibers became fluorescent and they emitted visible light under UV light exposure. Prepared in such way modified pulp fibers can be potentially applied as security features in objects made of fibrous materials, e.g. in packaging to validate its authenticity. Even though all of the tested derivatives can be used for the preparation of fluorescent pulp fibers, the derivative with the butanoic naphthalimide moiety and DSphoto of 0.22 was superior to others as a modifying agent. The developed in this study concept is not limited only to naphthalimide moieties, and as a future work FMCDs with other type of fluorescent groups, having e.g. fluorescence in specific region, can be synthesized and tested as surface modifying agents.

Figure 9. Fluorescence intensity of the reference and the fibers modified by FMCDs over the full visual spectrum. N-(3-Propanoic acid)-1,8-naphthalimide-(3-carboxypropyl)trimethylammonium chloride ester of cellulose (4a, DSphoto 0.07, DScat 0.31) and N-(4-butanoic acid)-1,8-naphthalimide-(3-carboxypropyl)trimethylammonium chloride ester of cellulose (5a, DSphoto 0.11, DScat 0.32; 5b, DSphoto 0.22, DScat 0.33).

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: pfardim@abo.fi (P.F.). *E-mail: [email protected] (T.H.). ORCID

Pedro Fardim: 0000-0003-1545-3523 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Finnish Funding Agency for Technology and Innovation (Tekes) and Tekniikan edistämissäaẗ iö (TES) foundation are acknowledged for the financial support of the work. Leonore Bretschneider is kindly acknowledged for her help with the synthesis of the derivatives.

Figure 10. Picture of fiber hand-sheets under black light illumination. The quadrates and the background made of FMCDs treated and reference fibers, respectively. N-(3-Propanoic acid)-1,8-naphthalimide(3-carboxypropyl)trimethylammonium chloride ester of cellulose (4a, DSphoto 0.07, DScat 0.31) and N-(4-butanoic acid)-1,8-naphthalimide(3-carboxypropyl)trimethylammonium chloride esters of cellulose (5a, DSphoto 0.11, DScat 0.32; 5b, DSphoto 0.22, DScat 0.33).

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REFERENCES

(1) 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: Synthesis and Characterization of Mixed 2-[(4-methyl-2-oxo-2H-chromen-7-yl)oxy]acetic acid−(3carboxypropyl)trimethylammonium Chloride Esters of Cellulose. Cellulose 2012, 19 (4), 1327−1335. (3) Vega, B.; Wondraczek, H.; Bretschneider, L.; Näreoja, T.; Fardim, P.; Heinze, T. Preparation of Reactive Fibre Interfaces Using Multifunctional Cellulose Derivatives. Carbohydr. Polym. 2015, 132, 261−273. (4) Grigoray, O.; Wondraczek, H.; Heikkilä, E.; Fardim, P.; Heinze, T. Photoresponsive Cellulose Fibers by Surface Modification with Multifunctional Cellulose Derivatives. Carbohydr. Polym. 2014, 111, 280−287. (5) Grigoray, O.; Wondraczek, H.; Daus, S.; Kühnöl, K.; Latifi, S. K.; Saketi, P.; Fardim, P.; Kallio, P.; Heinze, T. Photocontrol of Mechanical Properties of Pulp Fibers and Fiber-to-Fiber Bonds via Self-Assembled Polysaccharide Derivatives. Macromol. Mater. Eng. 2015, 300 (3), 277−282. (6) Vega, B.; Fazeli, E.; Näreoja, T.; Trygg, J.; Hänninen, P.; Heinze, T.; Fardim, P. Advanced Cellulose Fibers for Efficient Immobilization of Enzymes. Biomacromolecules 2016, 17 (10), 3188−3197.

the optical performance between the modified fibers under black light was in the following order: 4a < 5a < 5b. This difference was explained by the formation of excimers in the cases of 5a and 5b that gave additional emission band in the range of 440−500 nm (blue−green region). On other hand, the sheets made of reference and modified fibers looked similar under daylight. Hence, prepared modified fibers could serve as invisible security fibers that become visible when being exposed to UV light. One of the possible applications of such fibers is in smart packaging. They can serve as authenticity indicator for packaging as well as for the products.

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CONCLUSION In summary, the FMCDs containing both fluorescent and cationic functional moieties were successfully synthesized, characterized, and used as surface modifying agents for pulp fibers. The FMCDs possessed high affinity to pulp fibers, and 1802


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