Grafting Nature-Inspired and Bio-Based Phenolic Esters onto Cellulose Nanocrystals Gives Biomaterials with Photostable Anti-UV Properties

Article abstract of DOI:10.1002/cssc.202002017

New nature-inspired and plant-derived p-hydroxycinnamate esters and p-hydroxycinnamate diesters provide excellent protection against UV radiation when incorporated into a matrix. Herein, an efficient and sustainable pathway is reported to graft these phenolic compounds onto cellulose nanocrystals (CNCs) via click-type copper-catalyzed azide/alkyne cycloaddition (CuAAC) reaction. The successful grafting of the phenolic esters on CNC surface was evidenced by a range of chemical analyses, and the degrees of substitution (DS) of the CNC were found to depend on the structure of the phenolic ester grafted. Moreover, aqueous suspensions of the phenolic ester-grafted CNCs not only strongly absorb in both the UVA and UVB regions, but they also exhibit average to very high photostability. Their wide spectrum UV-absorbing properties and their stability upon exposure to UV are highly influenced by the structure of the phenolic ester, particularly by the extra ester group in p-hydroxycinnamate diesters. These findings demonstrate that cellulose nanocrystals decorated with such plant-derived and nature-inspired phenolic esters are promising sustainable nanomaterials for anti-UV applications.

Full text of DOI:10.1002/cssc.202002017

Chemistry–Sustainability–Energy–Materials
Accepted Article
Title: Grafting Nature-Inspired and Bio-Based Phenolic Esters onto
Cellulose Nanocrystals Gives Biomaterials with Photostable Anti-
UV Properties
Authors: David J Mendoza, Louis M M Mouterde, Christine Browne,
Vikram S Raghuwanshi, George P Simon, Gil Garnier, and
Florent Allais
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: ChemSusChem 10.1002/cssc.202002017
Link to VoR: https://doi.org/10.1002/cssc.202002017
01/2020

10.1002/cssc.202002017
ChemSusChem
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Grafting Nature-Inspired and Bio-Based Phenolic Esters onto Cellulose
Nanocrystals Gives Biomaterials with Photostable Anti-UV Properties
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David Joram Mendoza, Louis M.M. Mouterde*, Christine Browne, Vikram S. Raghuwanshi,
George P. Simon, Gil Garnier* and Florent Allais*
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D. J. Mendoza, Dr. C. Browne, Dr. V. S. Raghuwanshi, Prof. G. Garnier, Prof. F. Allais
Bioresource Processing Research Institute of Australia (BioPRIA), Department of Chemical
Engineering, Monash University, Clayton, VIC 3800, Australia
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gil.garnier@monash.edu, florent.allais@agroparistech.fr
Dr. L. M. M. Mouterde, Prof. G. Garnier, Prof. F. Allais
URD Agro-Biotechnologies Industrielles (ABI), CEBB, AgroParisTech, 51110, Pomacle, France
louis.mouterde@agroparistech.fr, gil.garnier@monash.edu, florent.allais@agroparistech.fr
Prof. G. P. Simon
Department of Materials Science and Engineering, Monash University, Clayton, VIC 3800, Australia
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Keywords: cellulose nanocrystals, phenolic esters, p-hydroxycinnamate esters, click-
chemistry, Knoevenagel condensation, UV-absorbent
ABSTRACT
New nature-inspired and plant-derived p-hydroxycinnamate esters and p-hydroxycinnamate
diesters provide excellent protection against UV radiation when incorporated into a matrix.
Herein, we report an efficient and sustainable pathway to graft these phenolic compounds onto
cellulose nanocrystals via click-type copper-catalyzed azide/alkyne cycloaddition (CuAAC)
reaction. The successful grafting of the phenolic esters on CNC surface was evidenced by a
range of chemical analyses and the degree of substitutions (DS) of the CNC were found to
depend on the structure of the phenolic ester grafted. Moreover, not only aqueous suspensions
of the phenolic ester-grafted CNCs strongly absorb in both the UVA and UVB regions, but they
also exhibit average to very high photostability. Their large spectrum UV-absorbing properties
and their stability upon exposure to UV are highly influenced by the structure of the phenolic
ester, particularly by the extra ester group in p-hydroxycinnamate diesters. These findings
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10.1002/cssc.202002017
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undoubtedly demonstrate that cellulose nanocrystals decorated with such plant-derived and
nature-inspired phenolic esters are promising sustainable nanomaterials for anti-UV
applications.
39 1. INTRODUCTION
Excessive exposure to ultraviolet (UV) radiation can cause a myriad of detrimental
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effects to biological systems[1]. As a result, nature has evolved and developed photoprotection
mechanisms to dissipate the damaging energy from UV, primarily through the biosynthesis of
strongly absorbing UV chromophores. For instance, human skin expresses the production of
UV-absorbing melanin pigments (eumelanin, pheomelanin) upon overexposure to UV radiation
[2]. Plants, which need UV for numerous biological processes, can also suffer deleterious effects
from overexposure to UV radiation [3]. In response, plants produce sunscreens in epidermal
tissues via the phenylpropanoid pathway [2, 4]. These plant sunscreens are commonly phenolic
compounds like flavonoids or p-hydroxycinnamic derivatives which are often esterified with
other hydroxyacids or glycosylated [5]. These compounds display strong UV absorption due to
the conjugated nature of the aromatic ring and the adjacent carbon-carbon double bonds [6]. In
fact, some of these compounds are already being utilized in commercial sunscreens and
cosmetics as an organic sunscreen additive [7].
Recent trends in nanotechnology has driven researchers to functionalize different
nanomaterials with naturally-occurring phenolic compounds; primarily to impart the anti-UV,
antioxidant, anticancer, and other functional properties of these phenolic compounds [8]. Of
particular interest is nanocellulose ─ a class of nanostructured cellulosic materials that refer to
cellulose nanofibers (CNFs), cellulose nanocrystals (CNCs), and bacterial nanocellulose
(BNC). Nanocellulose is a renewable and biodegradable nanomaterial with excellent
mechanical properties, unique optical properties, and tunable surface chemistry [9]. A few
studies have reported functionalization of some phenolic moieties on cellulose nanocrystals as
UV-absorbent nanofillers in sunscreen and polymer films [10]. However, the molecules grafted
are limited to cinnamate derivatives, namely poly(cinnamoyloxyethyl) methacrylate and
cinnamoyl chloride.
Owing to their reactive hydroxyl groups, nanocellulose can easily be modified with a
myriad of functionalities. Indeed, reactions of nanocellulose such as oxidation [11], esterification
[15]
[12], amidation [13], carbamation [14], and etherification
are well-described in the literature.
Recently, the click-chemistry approach and particularly the Cu(I)-catalyzed azide–alkyne
cycloaddition (CuAAC) reaction have attracted significant attention in nanocellulose
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modification, due to its simplicity, stability, and efficiency [16]. Since its introduction by
Sharpless et al. [17] in 2001, click-chemistry reactions have diversified into many forms (such
as CUAAC [18]) and have become the most popular approach to synthesize carbohydrate
mimetics and their derivatives. The most common pathway is the pre-functionalization of
cellulose with azide groups followed by the copper-catalyzed cycloaddition reaction with the
terminal alkyne-bearing moiety. For example, azide-modified CNCs have been attached to
[17]
alkyne-bearing molecules such as polycaprolactone diol [19], P(NIPAAm-co-HEMA)
,
ferrocene [20], porphyrin [21], and ethynylbenzoxazine [22] for different applications. A variety of
[24]
functional nanocellulose based-materials such as drug delivery systems [23], hydrogels
,
[26]
[21]
dendrimers [25], nanocomposites
and photobactericidal
materials were successfully
fabricated using this approach. However, there are no reports of functionalization of
nanocellulose with phenolic compounds using the click-chemistry approach.
Here, we report a new and innovative synthetic route to decorate cellulose nanocrystals
with UV-absorbing phenolic esters using the CUAAC click-chemistry approach. We
synthesized a set of alkyne-modified p-hydroxycinnamate mono- and di-esters (coumarate,
ferulate, and sinapate) and grafted them on azide-bearing cellulose nanocrystals. The phenolic
ester-grafted CNCS were rigorously characterized by FTIR, elemental and XPS analyses to
confirm the successful grafting of the phenolic esters. The effect of the phenolic ester moiety
on the degree of substitution (DS) and the changes in morphology and surface properties of the
grafted CNCs were also shown in the study. We also highlighted that the CNCs grafted with
the phenolic esters strongly absorb UVA (315-400 nm) and UVB (280-315 nm) radiations, and
that the UV-absorbing properties and photostability of the grafted CNCs are highly dependent
on the structure of the phenolic ester attached (particularly the ester group in the acrylic double
bond). The novel phenolic esters and their CNC derivatives are not only obtained from
naturally-occurring phenolic precursors, but also that are prepared via green chemistry
approaches, i.e., sustainable proline-mediated Knoevenagel condensation reaction of phenolic
aldehydes and diethyl malonate in ethanol, and click-chemistry (atom economy and high
yielding reactions). This study ultimately aims to develop a new class of sustainable and
efficient UV-screening nanomaterials inspired from nature’s photoprotective mechanisms and
suitable for incorporation in to other material systems where the chemically-modified
nanocellulose nanocrystal acts as an aid to good dispersion as well as providing functional
properties.
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102 2. MATERIALS AND METHODS
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2.1. Materials
Sulfuric acid-hydrolyzed cellulose nanocrystals (98% dry powder, 0.8-0.9% sulfur) were
acquired from University of Maine, USA. Ferulic acid, coumaric acid, syringaldehyde, malonic
acid, aniline, copper (I) bromide (CuBr2), potassium carbonate (K2CO3), and p-toluenesulfonyl
chloride were purchased from Sigma-Aldrich. Absolute ethanol and N,N-dimethylformamide
(DMF) were acquired from Ajax FineChem. Propargyl bromide (80% wt.% in toluene) and
sodium azide (NaN3) were purchased from Acros Organics and ascorbic acid and triethylamine
were from Alfa Aesar. Diethyl malonate and pyridine were acquired from Tokyo Chemical
Industries (TCI). Sinapic acid was synthesized from syringaldehyde and malonic acid as
reported previously [27]. All other chemicals are analytical grade and used without further
purification.
2.2. Synthesis of phenolic esters and their alkyne derivatives
Monoethyl phenolic esters (ethyl coumarate, ethyl ferulate, ethyl sinapate) were synthesized
via acid-catalyzed esterification of their corresponding p-hydroxycinnamic acids (coumaric
acid, ferulic acid, sinapic acid) with ethanol (Figure S1) [28]. Monoethyl phenolic esters with
high purity and yield (~90%) were obtained.
Diethyl phenolic esters were synthesized via proline-mediated or microwave-assisted
Knoevenagel-Doebner condensation of the corresponding p-hydroxybenzaldehydes (4-
hydroxybenzaldehyde, vanillin, syringaldehyde) and diethyl malonate (Figure S2) [29]. Diethyl
phenolic esters with high purity and yield (~80-85%) were afforded.
The synthesis of alkyne derivatives was adapted from a previous study with some modifications
[30]
(Figure S3). Briefly, the corresponding phenolic ester (1 eq) was dissolved in 10 mL
anhydrous DMF and was cooled to 0 ˚C. K2CO3 (1 eq) and propargyl bromide (1.2 eq) were
then added and the reaction mixture was allowed to stir overnight at room temperature under
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nitrogen. Alkyne derivatives were obtained in high yield (70-85%). H and C NMR spectra
and assignments of the alkyne derivatives are shown in Figure S4-S13.
All reactions were monitored by thin-layer chromatography (TLC) using pre-coated aluminium
sheets (Silica gel 60 F254, Merck Germany) and spots were visualized under UV light.
Purification of the compounds was performed by flash chromatography (Puriflash 4100,
Interchim with pre-packed silica gel column) with cyclohexane/ethyl acetate (8:2) as eluent. 1H
NMR spectra of the compounds in (CD3)2CO (residual signal at δ = 2.05 ppm] were recorded
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at 300 MHz (Fourier 300 Bruker) at 20 °C. 13C NMR spectra of the compounds in (CD3)2CO
(residual signal at δ = 206.26 and 29.84 ppm) were recorded at 75 MHz at 20 °C.
2.3. Surface tosylation and azidation of cellulose nanocrystals
Surface tosylation and azidation reactions of CNCs were performed according to previous
studies, with some modifications [20b, 21]. Basically, CNCs (1.0 g, 6.17 mmol AGU) were
suspended in 20 mL pyridine and cooled to 10 ˚C. p-Toluenesulfonyl chloride (1.8 g, 10 mmol)
was slowly added and the reaction mixture was stirred at room temperature for 48 h. CNCs
were precipitated by the addition of 200 mL ethanol followed by centrifugation at 11,000 rpm
for 20 min. The particles were subsequently washed five times each with ethanol and water
and freeze-dried for 2 days. Surface tosylated CNCs (CNC-Tos) were obtained in a 94% yield
(0.94 g).
Approximately 0.9 g of CNC-Tos (1 eq) suspended in 50 mL DMF was reacted with 0.9 g (1
eq) of sodium azide. The reaction was stirred at 100 ˚C for 24 h. Similarly, CNCs were isolated
by the addition of 200 mL ethanol followed by repeated centrifugation (11,000 rpm, 20 mins)
and washing with ethanol and water. The particles resuspended in water were dialysed against
Milli-Q water for 5 days and freeze-dried for 2 days. Surface-azidized CNCs (CNC-N3) were
obtained in a 96% yield (0.86 g).
2.4. Click reaction of CNC-N3 with alkyne-modified phenolic esters
The copper (I)-catalyzed cycloaddition reaction of alkyne derivatives of phenolic esters with
[20b, 21, 31]
CNC-N3 was adapted from previous reports
with some modifications. Alkyne
derivatives (0.4 g, 1 eq) and CNC-N3 (0.4 g, 1 eq) were mixed in 40 mL DMF. A solution
containing CuBr2 (56 mg), ascorbic acid (160 mg), and triethylamine (400 µL) in DMF was
then slowly added and the reaction was allowed to proceed at 60 ˚C for 24 h. The crude product
was isolated by ethanol precipitation (200 mL) followed by repeated centrifugation (11,000
rpm, 20 mins) and washing with ethanol and water. The particles were dialysed against Milli-
Q water for 5 days and freeze-dried for 2 days. Approximately 0.4 g of the click product was
obtained (50% yield).
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2.5. Characterization of CNCs
FT-IR
Freeze-dried pristine CNCs and decorated CNCs were analysed with a Fourier Transform
Infrared (FTIR) spectrometer (Agilent Technologies Cary 630 FTIR) equipped with a diamond
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ATR accessory. Spectra in the range of 4000-500 cm-1 were recorded with a resolution of 4
cm-1 resolution by accumulating 32 scans.
Elemental Analysis and Degree of Substitution
Elemental analyses of pristine CNCs and decorated CNCs were performed using Thermo
Scientific FlashSmart CHNS analyser. The degree of substitution (DS) of CNC-N3 was directly
calculated from the %N. The DS of phenolic ester-modified CNCs were quantified based on
%C and calculated using (1) as previously reported[32]
:
ꢀ ×ꢁ ꢂ%ꢃ × ꢁ
!
"#$
177
ꢀꢁ =
(1)
ꢁ
×%ꢃ ꢂꢁ
!%
%
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where 6 x Mc is the carbon mass of one anhydroglucose unit (72.07), MAGU is the mass of
anhydroglucose unit (162.14), Mg is the mass of the grafted molecule and MCg is the carbon
mass of the grafted molecule.
Contact Angle
Water contact angle (WCA) measurements were performed using a contact angle machine
(Dataphysics, OCA35). Pristine CNCs and decorated CNCs were dispersed in water and
sonicated for 5 mins using an ultrasonic homogeniser at 19.5 kHz and 70% amplitude (ON/OFF,
5 s). The aqueous dispersions were then spin-coated on plasma-cleaned glass. The static WCA
was recorded by analysing images of three water droplets (3 µL).
Zeta potential
The zeta-potential of sonicated CNC aqueous dispersions were measured using a zeta potential
analyser (Brookhaven Nanobrook Omni). Measurements were performed five times for each
sample and the average is reported.
UV-Vis Spectroscopy
The absorbance spectra of aqueous dispersions of pristine CNCs and decorated CNCs (0.5 wt.
%) were generated at 300-800 nm using a UV-Vis spectrometer (Cary 60 UV-Vis Agilent
Technologies). For comparison, the absorbance spectra of the “free” phenolic esters (50 µM in
ethanol) were also recorded.
X-Ray Photoelectron Spectroscopy (XPS)
Surface chemical analyses of freeze-dried pristine CNCs and decorated CNCs were performed
using a Thermo Scientific Nexsa XPS spectrometer equipped with an Al Kα irradiation source.
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Survey scans were generated from -10 to 1350 eV with a scan step of 1 eV. High resolution
C1s and N1s spectra were also recorded from 279-298 eV and 392-410 eV, respectively. All
data processing and analyses were performed using the Thermo Avantage software.
Transmission Electron Microscopy (TEM)
TEM analysis was performed using a FEI Tecnai F20. Dilute aqueous suspensions (~0.001
wt.%) of pristine CNCs and decorated CNCs were dried on plasma-cleaned copper grids. The
samples were then stained with 2% uranyl acetate, air dried, and examined at 200 kV.
2.6. UV-Irradiation Experiments
Phenolic ester solutions (50 µM in ethanol) and their corresponding decorated CNCs (0.5 wt.%
in water) were loaded in quartz cuvettes and irradiated at 365 nm for 2 h using a 32-W UV-
lamp. The absorbance spectra of the samples were measured at different time intervals with a
UV-Vis spectrometer and the %retention was quantified using (2):
ꢁꢀ
% ꢀꢁꢂꢁꢃꢂꢄꢅꢃ = 100 − ꢀ
(2)
!
%
!
&
"#$
"#$
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ꢀ
!
&
"#$
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220
where ꢆꢂ
is the absorbance at λmax in a given irradiation time and ꢆꢂ ꢄ is the absorbance
"#$
ꢃ
"#$
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at λmax before UV-irradiation.
223 3. RESULTS
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3.1. Synthesis of phenolic ester-grafted CNCs
Grafting of phenolic esters onto cellulose nanocrystal surface was achieved by the
copper-catalyzed cycloaddition reaction (Scheme 1). The click reaction requires azide and
alkyne moieties to form a triazole ring. Here, we synthesized the alkyne analogues from mono-
and diethyl esters of p-hydroxycinnamates (Figure S3). The reaction involves the nucleophilic
substitution of propargyl bromide with the corresponding phenolic ester under basic conditions.
The terminal alkyne-modified phenolic esters were isolated in 70-85% yield. On the other hand,
we attached the azide functional group on CNCs via a 2-step process: activation of the primary
hydroxyl groups by tosylation, followed by a nucleophilic substitution of the tosylate with azide
groups (Scheme 1a). FTIR analysis of CNC-Tos (Figure S16) shows that the tosylation reaction
was successful, as evidenced by the formation of vibrational peaks assigned to the aromatic
ring (1592, 815 cm-1) and SO2 (1396 cm-1). It should be noted that the SO2 peak is also present
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in pristine CNC as a consequence of sulfuric acid hydrolysis. Displacement of the tosyl group
with azide groups was also confirmed by the appearance of a sharp stretching band at 2110 cm-
1
expected for the anti-symmetric azide stretching motion [33]. Successful substitution is also
supported by the concomitant loss of the aromatic peaks corresponding to tosyl groups.
The click reaction employed is a Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC)
reaction (Scheme 1b). The optimal results were achieved when CuBr2 in combination with
ascorbic acid and triethylamine were used. Successful click reaction between the alkyne and
CNC-N3 is evidenced in the FTIR spectrum of the phenolic ester-grafted CNC (CNC-EC)
(Figure 1). The appearance of a sharp peak at 1630 cm-1 provides clear evidence of the
formation of the triazole ring. A sharp stretching band at 1715 cm-1, attributed to the presence
of α,β-unsaturated esters, and vibrational peaks assigned to the aromatic ring (1585, 815 cm-1)
also confirm the successful grafting of the phenolic ester. However, not all azide groups were
completely converted to triazole groups as evidenced by the presence of the azide band at 2100
[20a, 21, 31]
cm-1 (~70% decrease in intensity). This corroborates previous findings
on CUAAC
reactions of azide-bearing CNCs, where a 50-70% reduction in azide intensity is observed.
a) Azidation of CNCs
O
S
OH
O
N3
O
TsCl, pyridine
48h, RT
O
O
NaN3
O
O
O
HO
O
HO
DMF, 100 oC, 24h
HO
OH
OH
OH
CO2Et
R3
b) Copper-catalysed click reaction
R2
R3
CO2Et
O
R1
R2
O
N
N3
N
R1 = OCH3, H
R2 = OCH3, H
R3 = CO2Et, H
O
N
CuBr2, ascorbic acid,TEA
DMF, 60 oC, 24 h
O
HO
+
O
OH
R1
O
HO
OH
c) Phenolic esters grafted
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
H3CO
H3CO
HO
HO
HO
HO
HO
HO
OCH3
OCH3
OCH3
OCH3
Diethyl
ferulate (DEF)
Diethyl
sinapate (DES)
Diethyl
coumarate (DEC)
Ethyl
sinapate (ES)
Ethyl
coumarate (EC)
Ethyl
ferulate (EF)
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Scheme 1. Synthesis of phenolic ester-grafted CNCs. (a) Surface tosylation and azidation of
CNCs. (b) Copper(I)-catalyzed cycloaddition reaction between CNC-N3 and phenolic ester
alkyne derivatives. (c) p-Hydroxycinnamate esters grafted onto CNCs.
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257 Figure 1. FTIR spectra of pristine, azide-modified (CNC-N3), and phenolic ester-grafted CNCs.
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X-ray photoelectron spectroscopy was used to investigate the changes in the surface
chemistry of CNCs after the click reaction. Survey scans of pristine CNC (Figure S17) show
carbon (287 eV) and oxygen (533 eV) as the major elements, as expected in cellulosic materials.
A residual peak at around 169 eV, attributed to the binding energy of sulfate groups, is also
present as a consequence of sulfuric acid hydrolysis. CNC-N3 and the phenolic-ester grafted
CNCs (in this case, CNC-DES) also show small peaks at around 400-405 eV which is attributed
to the presence of nitrogen-rich groups. This suggests that the azidation and cycloaddition
reactions may be successful. These results are supported by the decrease of the O/C and N/C
ratios of the CNCs after surface grafting (Table S1).
To confirm further the successful grafting of phenolic esters onto CNC surface, high
resolution N 1s and C 1s scans (Figure 2) were performed. The N1s spectrum of CNC-N3
shows two broad peaks at around 400-4005 eV which can be further deconvoluted into three
peaks. These peaks are attributed to three different N environments present in an azide group:
N+ azide, N azide and N- azide. However, after the click reaction, only one sharp peak at 401
eV was observed. This can be explained by the formation of the 1,2,3-triazole ring with three
closely related nitrogen environments, confirming the successful cycloaddition of the alkyne
with the azide group [34]. Figures 2c-d show the C 1s spectra of pristine and phenolic-ester
grafted CNCs. Both CNCs exhibit three characteristic peaks typical for cellulosic materials: C–
O (alcohol and ether groups) at 285 eV, O–C–O (acetal group) at 286 eV, and C–C (non-
oxidised carbon from hydrocarbon contamination) at 288 eV [32]. However, after the click
reaction, another small peak at around 289 eV (assigned to carboxylic groups (O–C=O))
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appeared. This can be associated with the ester groups present in the phenolic compounds
grafted on CNCs (Figure S12).
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Figure 2. High resolution N 1s spectra of (a) azide-modified and (b) phenolic-ester grafted
CNCs. High resolution C 1s spectra of (c) pristine and (d) phenolic-ester grafted
CNCs.
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Elemental analysis was used to calculate to degrees of substitution (DS) and surface
substitution (DSsurf) achieved after the azidation and click reactions (Table 1). DS values were
converted to their corresponding DSsurf values using the equation DS = RcDSsurf assuming a
CNC chain ratio (Rc) of 0.29 [35]. The chain ratio corresponds to the ratio of the surface chains
(57) to the total number of chains (201) within a CNC rectangular crystal with dimensions of
8.6 ×7.7 nm[21, 36]. The results show that the azidation reaction lead to a DS = 0.15 in agreement
with DS values reported previously [21, 31]. This corresponds to a DSsurf = 0.5 which is the
expected theoretical value for maximum surface substitution of primary hydroxymethyl groups
on CNC surface [20a]. Table 1 also shows the cycloaddition reaction afforded DSsurf values
ranging from 0.24-0.41 ─ corresponding to 48-82%conversion of the azide groups to their
triazole derivatives. This corroborates the reduction in the FTIR azide intensity reported
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300
301
302
303
304
305
306
307
308
309
310
previously. Incomplete grafting is expected as the reactions were carried out under
heterogeneous conditions. Further, the variation in DS of the grafted-CNCs can be explained
by the steric hindrance of the phenolic moiety attached. Indeed, grafting of ethyl coumarate
(EC) and ethyl ferulate (EF) on CNCs afforded DSsurf values of 0.41 and 0.34, respectively. The
addition of an extra methoxy group in the aromatic ring thus decreased the DSsurf by 17%.
Further, the addition of an extra ethyl ester group in the acrylic group (for example, CNC-EC
and CNC-DEC) also decreased DSsurf, however, only by 7%. Overall, these findings
undoubtedly demonstrate that the steric hindrance of the phenolic moiety - that is directly linked
to the presence of methoxy group and ethyl ester groups - impedes the efficiency of the
cycloaddition reaction.
Table 1. Degrees of substitution (DS) and surface substitution (DSsurf) of phenolic-ester grafted
cellulose nanocrystals.
CNC-EC
0.12
CNC-EF
0.10
CNC-ES
0.08
CNC-DEC
0.11
CNC-DEF
0.09
CNC-DES
0.07
DS
DSsurf 0.41
0.34
0.28
0.38
0.31
0.24
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
3.2. Surface properties and morphology of grafted CNCs
All phenolic ester-grafted CNCs could be dispersed in water via ultrasonication (0.5
wt.%). These grafted CNCs appear as opaque yellow suspensions, as shown in Figure 3a. The
dispersions remain stable over 6 months with no precipitation observed. Zeta potential
measurements (Figure 3b) confirm the stability of the grafted CNCs in water and the negative
surface charge was preserved after the click reaction. The high negative surface charge in the
grafted CNCs is attributed to the sulfate groups which were not completely removed after the
click reaction (as confirmed by elemental and XPS analyses). While surface polarity provides
the excellent hydrophilicity of CNCs, the negative surface charge rendered the phenolic ester-
grafted CNCs fully dispersible in water.
Figure 3c-d shows that the water contact angles (WCA) of the grafted CNCs (32.8 -
38.9˚) were higher than that of the pristine CNC (10.9˚). This shows that the phenolic ester-
grafted CNCs were less hydrophilic than pristine CNC and further support the successful
grafting of the phenolic esters on CNCs. The WCA of the grafted CNCs also increases with the
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328
329
330
331
molecular mass and hydrophobicity of the phenolic moiety grafted in line with expectation. It
is also noteworthy to mention that TEM analysis further reveals that the grafted CNCs retained
their nanorod morphology and high aspect ratio (Figure 4). Previous studies also show that the
morphology and nanocrystal structure of the particles were preserved after performing similar
click reactions with CNC [10b, 20a]
.
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333
Figure 3. (a) Aqueous dispersions of pristine and phenolic-ester grafted CNCs. (b) Zeta
potential of pristine and phenolic-ester grafted CNCs dispersed in water. (c) Water contact
angles (WCA) of pristine and phenolic-ester grafted CNCs. (d) Water droplets deposited on
glass coated with CNC and phenolic-ester grafted CNC dispersions.
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Figure 4. Transmission electron micrograph of (a) pristine and (b) phenolic-ester grafted
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CNCs.
3.3. UV-absorption properties and photostability of grafted CNCs
Figure 5 compares the UV-visible absorbance spectra of aqueous suspensions of CNC
and phenolic ester-grafted CNCs. As expected, the aqueous CNC suspensions did not display
appreciable absorption over the whole UV and visible ranges, indicating that CNCs alone do
not provide UV protection. On the other hand, the phenolic ester-grafted CNCs exhibit a broad
and significant UV absorption for UVA and/or UVB regions and negligible absorption (or high
transmittance) in the visible region. CNCs grafted with coumarate and sinapate esters (CNC-
EC, CNC-DEC, CNC-ES, CNC-DES) strongly absorb in the UVB region (280-315 nm), similar
to their “free” phenolic ester counterparts. Interestingly, ferulate ester-grafted CNCs (CNC-EF,
CNC-DEF) strongly absorb in both UVA (315-400 nm) and UVB regions, similar to ethyl and
diethyl ferulate. Table 2 shows that the addition of an identical ester moiety in the acrylic double
bond causes a hyperchromic or “red” shift on the λmax due to increase in π-system conjugation.
The grafting of the phenolic esters onto CNCs, however, caused a hypsochromic or “blue” shift,
most probably due to the change in electronic density of the aromatic ring resulting from the
etherification of the phenol moieties.
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Figure 5. UV-vis absorbance spectra of pristine CNCs and phenolic ester-grafted CNCs in
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water.
Table 2. λmax (nm) of “free” and CNC-grafted phenolic esters.
λmax (nm)
Phenolic Ester
Free
300
Grafted on CNC
Ethyl coumarate (EC)
Ethyl ferulate (EF)
295
295,325
305
295, 320
300
Ethyl sinapate (ES)
Diethyl coumarate (DEC)
Diethyl ferulate (DEF)
Diethyl sinapate (DES)
310
305
300, 330
335
300, 325
315
362
363
364
365
366
367
368
369
370
371
372
Further, the photostability of the grafted CNCs was quantified by irradiating their
aqueous suspensions at 365 nm for 2 h. The UV absorbance spectra of CNC-DES and other
phenolic ester-grafted CNCs at varying UV irradiation time are shown in Figures 6a and S19,
respectively. The retention of the grafted CNCs was recorded as a function of irradiation time
and plotted in Figure 6b. The absorbance of the grafted CNCs decreases with UV irradiation
time indicating possible degradation or rearrangement of the grafted moieties. CNCs grafted
with phenolic monoesters degrade exponentially with time whereas those with phenolic diesters
remain relatively photostable. After 2 h of irradiation at 365 nm, phenolic diester-grafted CNCs
(CNC-DEC, CNC-DEF, CNC-DES) retained about 80-90% of their original absorbance
whereas phenolic monoester-grafted CNCs only retained 40-67% (Figure 6c). Interestingly,
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these retention values do not significantly differ with their non-covalently attached
counterparts, indicating that grafting on CNCs, and thus etherification of the phenol moieties,
did not change their photostability. Surprisingly, the diffraction and diffusion of light by the
CNC particles had no effect. Horbury et al. [29a] also quantified the photostability of ethyl and
diethyl sinapates and found a decrease in absorbance of 16% and 3.3%, respectively after 2 h
of UV irradiation at solar intensities. The difference in photostability between our work and
that of Horbury et al.’s can be attributed to the difference in the wavelength and power of the
lamp and the solvent used.
381
382
Figure 6. Photostability of phenolic ester-grafted CNCs in water. Aqueous suspensions of the
grafted CNCs (0.5 wt%) were irradiated at 365 nm (32 W) for 2 h. (a) UV absorption spectra
of CNC-DES at varying UV irradiation time. (b) Retention of phenolic ester-grafted CNCs as
a function of UV irradiation time. (c) Retention of “free” phenolic esters in ethanol and phenolic
ester-grafted CNCs in water after UV-irradiation for 2 h. `
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388 4. DISCUSSION
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In this work, a new synthetic route to anchor p-hydroxycinnamate esters onto cellulose
nanocrystals (CNCs) is reported. We have adapted the classical copper-catalyzed azide/alkyne
cycloaddition (CuAAC) reaction as a “click-chemistry” approach to graft moieties on CNCs
because of its simplicity, ease, and efficiency [16, 37]. The straightforward synthetic route
developed herein (Scheme 1) requires pre-functionalization of the CNCs with an azido group
and the cinnamate esters with terminal alkyne moiety. The precursor phenolic esters were
synthesized from naturally-occurring p-hydroxycinnamic acids or p-hydroxybenzaldehydes
using fully-renewable and non-toxic reagents and solvents. The phenolic monoesters were
synthesized by simple reflux of phenolic acids in ethanol whereas the phenolic diesters were
synthesized by a green piperidine/pyridine-free Knoevenagel-Doebner condensation reaction
in ethanol [29b]. The subsequent Williamson etherification then provided the alkyne derivatives
in good yield and purity. The 2-step tosylation/azidation reaction employed to synthesize azide-
modified CNCs proved highly efficient; most of the available hydroxymethyl groups on CNC
surface have been substituted with azide groups (DSsur f= 0.5) [35]. The click reaction between
the phenolic esters etherified with a propargyl group and CNC-N3 was successful as revealed
by FTIR (Figure 1) and XPS analyses (Figure 2), albeit incomplete conversion was achieved.
The reaction afforded grafted CNCs with varying DS (Table 1) which can be attributed to the
“bulkiness” of the moiety attached (i.e. methoxy in the aromatic ring and ethyl ester group in
the acrylic group). The cycloaddition reaction, however efficient, uses DMF as a solvent to
dissolve the phenolic esters and promote CNC dispersion and swelling during the reaction.
Replacement of DMF with a benign solvent (ideally water) would lead to a more sustainable
and environmentally friendly synthetic route for the phenolic-ester grafted CNCs.
Ultimately, grafting phenolic esters on CNCs transformed the originally UV-inert
nanoparticles into strongly UV-absorbing materials (Figure 5). Not only do all of the grafted
CNCs absorb in the UVB region, but also, interestingly, CNC-EF and CNC-DEF exhibit two
characteristic λmax which cover both the UVA and UVB regions. Grafting of these phenolic
esters onto CNCs, however, resulted in a small hypsochromic or “blue” shift that may be
attributed to the etherification of the phenol group of the aromatic ring and/or the presence of
the triazole ring which induced a shift towards a shorter wavelength. The hypsochromic shift
observed is quite similar to the effect of methylation or glycosylation of some phenolic
compounds reported previously [38]. Further, the addition of an ethyl ester group in the acrylic
double bond causes a hyperchromic or “red” shift due to increase in π-system conjugation.
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453
454
Steady-state irradiation experiments show the photostability of the grafted CNCs and
their “free” counterparts (Figure 6). Surprisingly, grafting of these molecules onto CNC surface
did not significantly change their photostability (Figure 6c). Varying degree of photostability
were observed depending on the moiety attached on CNC. In general, most conjugated organic
molecules or sunscreens work by absorbing the destructive energy from UV, and dispersing it
to the surroundings in the form of heat [7]. The photostability of cinnamate derivatives, in
particular, is attributed to (1) the efficient trans-cis and cis-trans photoisomerization, resulting
in a photo-equilibrium between the isomers, and (2) the number of potential degradation and
rearrangements pathways that transform the aforementioned isomers into non-UV-absorbing
compounds. Coumaric acid undergoes a rapid photo-isomerization when excited around the 11
ππ* origin in polar solvents [39]. Horbury et al. [40] explored the photodynamics of ethyl ferulate
in cyclohexane and found that excitation of the molecule at the 11 ππ∗ and 21 ππ∗ states drives
trans-cis photoisomerization. Sinapate derivatives were also reported to undergo this
photoisomerization [28, 41]. In most cinnamate derivatives, UV irradiation results in the formation
of the cis isomer which generally absorbs less UV relative to the trans isomer [42]. Results
reported in Figure 6c show that the addition of an identical ethyl ester on the acrylic double
bond enhances the photostability of diethyl phenolic esters (both CNC-grafted and “free”). This
corroborates the study of Horbury et al. [29a] where diethyl sinapate was found to be significantly
more photostable relative to ethyl sinapate. However, due to the symmetrical structure of DES,
the authors were not able to determine if the photoisomerization occurs completely or is an
aborted photoisomerization and attributed the enhanced photostability of DES to the apparent
ground state bleach recovery. Nevertheless, this reversible isomerization is not the only reason
to account for the apparent photostability of free or grafted diethyl phenolic esters (both CNC-
grafted and “free”); indeed, Schrader et al. demonstrated that Octinoxate (trans-3-methylbutyl-
4-methoxycinnamate) and its cis-isomer can undergo photo- and oxygen-induced
rearrangements, resulting in non-conjugated compounds (A-G), thus reducing both the UV-
filter activity and the photostability of Octinoxate (Scheme 2) [29a]. Considering the
aforementioned discussion on the steric hindrance of the phenolic compounds, and more
precisely on the presence of methoxy substituents on the aromatic ring, from Scheme 2, it is
easy to understand that the presence of a substituent at the position 5 of the aromatic ring
(ferulates, EF) limits the formation of compound C. Similarly, having two substituents at the
positions 3 and 5 (sinapate, ES) prevents the formation of compounds A, B, C and D. These
considerations are in perfect accordance with the obtained results where sinapates are the most
photostable cinnamates (Figures 6b & 6c). Moreover, the higher photostability of DEC, DEF
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455
456
457
and DES can be explained by the extra ester moiety at the position a that prevents the formation
of all rearrangement compounds A-G, thus preserving the UV-absorbing activity of the cis-
trans isomers (Scheme 2).
5
RO
RO
RO2C
CO2R
CO2R
O2 (Air)
O
3
RO2C
α
H
H
M
A
J
O
R
CO2R
A
B
α
β
1
2
3
6
5
OR
OR
P
A
T
H
W
A
Y
trans-isomer
Radical pathway
4
Diels-Alder
OR
CO2R
OR
O
Ar
Ar
CO2R
OH
RO
hν
H
H
RO2C
O
H
RO
H
CO2R
CO2R
D
C
OR
cis-isomer
CO2R
OR
= Ar
Ar
Ar
CO2R
Ar
CO2R
Ar
Ar
CO2R
CO2R
CO2R
RO2C
Ar
E
F
G
458
459
Scheme 2. Isomerization and rearrangement pathways of coumarates (Adapted from Schrader
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
et al. [41]
)
The novel phenolic ester-grafted CNCs disclosed can be regarded as a next-generation
sunscreen and anti-UV materials with a range of applications. Aside from their strong
absorption in the UVA and UVB regions, these materials have high transmittance of visible
light indicating that this can be used as a non-whitening sunscreen. These grafted CNCs can
also be dispersed in water via ultrasonication and be used as anti-UV nanofillers for water-
based sunscreen formulations and hydrophilic polymer composites. Further, grafting these
phenolic esters preserve the original morphology and nanocrystal structure of the cellulose
nanocrystals. This is particularly important when taking advantage of the excellent mechanical
properties and optical of CNCs; for example, reinforcement in flexible or rigid polymeric films
such as polyvinylalcohol [10b, 26], polyvinyl chloride [10a], and polylactic acid [43]. Overall, this
study provides a new approach to functionalize cellulose nanocrystals with naturally-occurring
phenolic esters and develop promising, sustainable nanomaterials for anti-UV applications.
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475 5. CONCLUSION
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
We have reported a new synthetic route to graft phenolic esters onto cellulose
nanocrystals using the classical copper-catalyzed azide/alkyne cycloaddition (CuAAC)
reaction. The synthetic phenolic di- and mono-esters grafted were efficiently prepared from
naturally-occurring p-hydroxybenzaldehydes and p-hydroxycinnamic acids through sustainable
Knoevenagel condensation or ethanol esterification. FTIR and XPS analyses confirmed the
successful cycloaddition of azide-modified CNCs and phenolic ester alkyne analogues.
Elemental analysis showed that the degree of substitution achieved is dependent on the
“bulkiness” or steric effects of the phenolic ester moiety. Phenolic ester-grafted CNCs are less
hydrophilic than pristine CNCs but can be readily dispersed in water via ultrasonication. These
grafted CNCs show a strong absorption in both the UVA and the UVB regions while exhibiting
high transmittance in visible light. Grafting these phenolic esters on CNC, however, causes a
hypsochromic or blue shift on their λmax. Steady-state experiments revealed that the phenolic
esters remain photostable after the click reaction. Interestingly, CNCs grafted with diethyl
esters (CNC-DEC, CNC-DEF, CNC-DES) are significantly more stable than those with
monoethyl esters (CNC-EC, CNC-EF, CNC-ES). Aqueous suspensions of these grafted CNCs
are water-borne, non-whitening, and photostable. Our results show that cellulose nanocrystals
decorated with nature-inspired bio-based phenolic esters are promising sustainable
nanomaterials which can be used as multifunctional additives for anti-UV applications.
495 6. ACKNOWLEDGEMENT
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Funding from the Australian Research Centre-Industry Transformation Research Hub;
Processing Advanced Lignocellulosics (PALS) [grant number IH130100016] is gratefully
acknowledged. We also thank Dr. T. Williams of Monash Centre for Electron Microscopy
(MCEM) for his assistance in TEM analysis. Many thanks to J. Griffith and Y. Hora of Monash
X-Ray Platform (MXP) for the XPS analysis. D. Mendoza is grateful to URD ABI for hosting
him for ca. 4 months. The authors also thank Grand Reims and Région Grand Est for their
financial support.
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F. Allais, L. M. M. Mouterde, and G. Garnier conceived the project. F. Allais, L. M. M.
Mouterde, G. P. Simon, and G. Garnier supervised the project. F. Allais, L. M. M. Mouterde,
and D. J. Mendoza designed the synthesis. D. J. Mendoza performed the experiments and
characterization of the phenolic compounds. C. Browne, V. S. Raghuwanshi assisted in the
characterization of the cellulose nanocrystals and UV irradiation experiments. D. J. Mendoza
wrote the first draft of the manuscript. All the authors discussed the results and commented on
the manuscript.
595 9- Contributions
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The authors declare no conflict of interest.
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598 10- ORCID
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David Joram Mendoza 0000-0002-7959-0837
Louis M.M. Mouterde 0000-0002-4096-1629
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This article is protected by copyright. All rights reserved.

10.1002/cssc.202002017
ChemSusChem
601
Christine Browne 0000-0002-1867-8413
Vikram Raghuwanshi 0000-0001-9524-1314
George P. Simon 0000-0001-8832-1164
Gil Garnier 0000-0003-3512-0056
602
603
604
605
606
Florent Allais 0000-0003-4132-6210
607 11- TOC
608
609
610
611
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New nature-inspired and plant-derived p-hydroxycinnamate esters and p-hydroxycinnamate
diesters were successfully synthesized, derivatized and grafted onto cellulose nanocrystals
using azide-alkyne click-chemistry. Ultrasonication-mediated dispersion of the resulting
materials in water provided solutions exhibiting high anti-UV activities and great
photostability.
22
This article is protected by copyright. All rights reserved.