Photoresponsive cellulose paper as a molecular printboard for covalent printing

Article abstract of DOI:10.1039/c7tc01081h

This contribution unveils and defines the concept of covalent printing onto cellulose paper through the support of a simple and efficient strategy. A photoresponsive cellulose paper bearing disulfide functional groups was created as a molecular printboard for spatio-temporal writing through thiol-X ligation of functional inks upon light irradiation. This strategy using molecular inks in a modular way culminated with the creation of patterns with a high resolution. Hidden covalent writing to the naked eyes was developed through the use of coumarin-based inks while the use of colored inks containing a rhodamine backbone established an uncovered pH-responsive covalent printing, both strategies can potentially find applications in the formation of anti-counterfeiting devices.

Full text of DOI:10.1039/c7tc01081h

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Materials Chemistry C
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PAPER
Photoresponsive cellulose paper as a molecular
printboard for covalent printing†
Cite this:DOI: 10.1039/c7tc01081h
a
ab
J. Rull-Barrull,‡a M. d’Halluin, ‡a E. Le Grognec
and F.-X. Felpin
*
This contribution unveils and defines the concept of covalent printing onto cellulose paper through the
support of a simple and efficient strategy. A photoresponsive cellulose paper bearing disulfide functional
groups was created as a molecular printboard for spatio-temporal writing through thiol–X ligation of
functional inks upon light irradiation. This strategy using molecular inks in a modular way culminated
with the creation of patterns with a high resolution. Hidden covalent writing to the naked eyes was
developed through the use of coumarin-based inks while the use of colored inks containing a
rhodamine backbone established an uncovered pH-responsive covalent printing, both strategies can
potentially find applications in the formation of anti-counterfeiting devices.
Received 13th March 2017,
Accepted 18th April 2017
DOI: 10.1039/c7tc01081h
rsc.li/materials-c
This approach offers a number of benefits including excellent
robustness to pattern wash off in solvents and a high molecular
Introduction
Cellulose paper is one of the most fascinating materials modularity allowing the use of inks displaying various proper-
produced from renewable resources as it possesses appealing ties. However, covalent printing on cellulose paper remains a
properties such as low-cost, biocompatibility, high hydrophili- great challenge since it must meet several requirements of a
city for compatibility with aqueous conditions and excellent (i) facile covalent ligation of the ink to the paper surface
thermal and chemical stability.1,2 Cellulose paper consists of through click chemistry, (ii) spatial resolution, at least in the
densely entangled fibers which result from the supramolecular millimeter scale, (iii) temporal control, and (iv) modularity for
arrangement of linear chains of b (1-4) linked D-glucose units use in diverse applications. Moreover, the writing process must
through a dense hydrogen bond network.
(v) preserve the whiteness of the paper in non-patterned regions
Writing is certainly one of the most powerful media of and (vi) the covalent linkage between the cellulose paper and
human communication which is an essential complement to the ink must not ideally absorb in the range of 250–800 nm to
spoken languages. Since the Egyptian papyri, cellulose paper avoid signals interacting with the ink.
has been the main medium for writing and, in most daily life
Methods allowing the spatially resolved covalent functiona-
applications non-handwriting on paper mainly relies on the use lization of cellulose paper are scarce due to the fragility of paper
of inkjet printers. With inkjet printing technologies, patterns in solvents and the low chemical reactivity of surface hydroxyl
are mostly created by physisorbed polymers as inks3 while the use groups.9,10 For instance, Konig et al. described the Meerwein
¨
of nanoparticles,4 metal organic frameworks5 and therapeutic photo-catalyzed arylation of paper sheets functionalized with
drugs6,7 has also shown great promise. The use of small molecules coumarin units.11 The photo-patterning proceeds in DMSO
as inks can suffer from the drawback of limited stability since the using RuCl2(bpy)3 as the catalyst under visible light irradiation
ability of cellulose to retain adsorbed non-polymeric molecular at 455 nm. Ratner et al. reported the covalent immobilization
architectures upon contact with liquids can significantly vary, of biomolecules using the divinyl sulfone (DVS) chemistry
leading to degraded or even erased patterns.8
combined with inkjet printing to produce biomolecular patterns.12
In this work, we introduced the concept of covalent printing The group of Barner-Kowollik has produced pioneering contribu-
using small molecules as covalent inks, in contrast to current tions to this research line with the spatially-resolved ligation
printing technologies using adsorption processes of polymers. of proteins and polymers by photo-triggered nitrile imine-
mediated tetrazole–ene cycloaddition reactions.13,14 While
a
´
Universite de Nantes, UFR Sciences et Techniques, UMR CNRS 6230, CEISAM,
these relevant studies for covalent patterning of paper are
extremely efficient, none of these approaches are well suited
for covalent printing since they either use organometallic
catalysts staining the paper, use a linker (DVS) of low stability
or produce strongly fluorescent linkages (pyrazoline) in the
visible light.
`
2 rue de la Houssiniere, BP 92208, 44322 Nantes Cedex 3, France.
E-mail: fx.felpin@univ-nantes.fr; Fax: +33 251 125 40
b Institut Universitaire de France, 1 rue Descartes, 75231 Paris Cedex 05, France
† Electronic supplementary information (ESI) available: Experimental procedures,
compound characterization data. See DOI: 10.1039/c7tc01081h
‡ These authors contributed equally to the project.
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We recently reported the first approach allowing the writing was shaken overnight on an orbital agitator. The cellulose samples
and erasing of hidden optical data on cellulose paper.15 This were washed 3 times with 50 mL of EtOH and stored in EtOH.
strategy, based on the reversible light-mediated dimerization of
coumarin units covalently grafted onto the paper surface, represents
the most complex covalently anchored pattern described to date on
cellulose paper as the form of a QR-Code. While this approach
represents an important breakthrough for covalent writing on
paper, it suffers from low modularity since it is restricted to optical
data storage in the range of coumarin absorption (ca. 310–340 nm).
In the current contribution, we centered our effort on
developing the newly introduced concept of covalent printing,
addressing most drawbacks discussed above and using thiol–ene
and thiol–yne ligations. Advantages of these modular ligation
tools are obvious since they adhere to the main criteria of click
chemistry16 and the C–S–C bond linkage does not produce any
optical nor colorimetric signature. This study shows the power of
the thiol–ene and thiol–yne ligations in producing modular hidden
to the naked eyes or colorimetric covalent patterns onto cellulose
Synthesis of dithiodiglycolic acid
A round bottom flask was charged with 1% H2O2 solution
(54.3 mmol) and tetrabutylammonium iodide (TBAI, 0.54 mmol).
Thioglycolic acid (54.3 mmol) was added to the solution at room
temperature with vigorous stirring for 10 min. After the comple-
tion of the reaction (10 min), the crude was filtered and extracted
with ethyl acetate (3 Â 50 mL). Then the combined organic
phases were washed with water (2 Â 40 mL) and dried with
anhydrous MgSO4. The solvent was evaporated under reduced
pressure to obtain the corresponding disulfide as a white solid
(91% yield). Caution: the mixing of H2O2 and TBAI is highly
exothermic.
Grafting of dithiodiglycolic acid on cellulose paper
paper. Ultimately, we demonstrate how our approach can be used A pre-treated cellulose filter paper (approx. 145 mg, 0.90 mmol
for an unprecedented pH-responsive colorimetric covalent writing. glucose units) washed 2 times with 10 mL of toluene, was
immersed in dry toluene (20 mL). A catalytically amount of
p-TsOH (20 mg) was added to the reaction media and stirred for
5 minutes. Then, dithiodiglycolic acid (3 equiv. per glucose unit)
was added to the solution. The mixture was stirred for 16 h at
Experimental
Material and methods
reflux under a N2 atmosphere. The piece of paper was sequen-
All commercial solvents and reagents were used as received
tially washed with EtOH, MeOH, acetone and CH2Cl2 under
from Sigma-Aldrich, Fischer Scientific Ltd, Alfa Aesar. Whatmans
sonication. The sample was dried under vacuum and stored
under nitrogen.
grade 6 filter paper (42.5 mm +) was used as the cellulose source.
1H and 13C spectra, recorded at 400 MHz, 100 MHz, respectively,
were recorded on a Bruker Advance 400. Proton chemical shifts
were internally referenced to the residual proton resonance in
Synthesis of 7-allyloxycoumarin (1)
A solution of umbelliferone (5 mmol), K2CO3 (5 mmol) and allyl
bromide (5 mmol) in 50 mL acetone was refluxed for 5 h. After
filtration, 1 was collected and purified by recrystallization with
ethanol to give 1 (90%) as colourless crystals. 1H NMR (300 MHz,
DMSO) d 4.68 (app dt, 2H, J = 1.5, 5.3 Hz), 5.29 (app dq, 1H,
J = 1.5, 10.5 Hz), 5.42 (app, dq, 1H, J = 1.6, 17.3 Hz), 5.99–6.11
(m, 1H), 6.29 (d, 1H, J = 9.5 Hz), 6.96 (dd, 1H, J = 2.4, 8.5 Hz), 7.00
(d, 1H, J = 2.4 Hz), 7.62 (d, 1H, J = 8.8 Hz), 7.98 (d, 1H, J = 9.6 Hz);
13C NMR (75 MHz, DMSO) d 68.8, 101.4, 112.4, 112.5, 112.8, 118.1,
129.5, 132.9, 144.3, 155.3, 160.2, 161.3; MS (ESI) 225.1 [M + Na+].
DMSO (2.50 ppm). Carbon chemical shifts were internally refer-
enced to the deuterated solvent signals in DMSO (39.52 ppm).
Melting points were recorded on a Stuart Scientific 7SMP3
apparatus. FT-IR spectra were recorded on a Bruker Tensor 27
spectrometer by the ATR technique. HRMS spectra in the ESI
mode were recorded on a LTQ-Orbitrap (ThermoFisher Scientific)
at the ENV of Nantes. Scanning electron microscopy (SEM) images
were recorded using a JEOL 7600 F at the ‘‘Centre de micro-
´
´
caracterisation de l’IMN, Universite de Nantes’’. X-ray photo-
electron spectroscopy was performed on a ThermoFisher
Scientific K-ALPHA spectrometer used for disk surface analysis
with a monochromatized AlKa source (hn = 1486.6 eV) and a
200 micron spot size. A pressure of 10À7 Pa was maintained in
the chamber during the analysis. The full spectra (0–1150 eV)
were obtained at a constant pass energy of 200 eV and high
resolution spectra at a constant pass energy of 40 eV. Charge
neutralization was required for all insulating samples. High
resolution spectra were fitted and quantified using the AVANTAGE
software provided by ThermoFisher Scientific. UV-visible absorp-
tion spectra were recorded using a Varian Model Cary 5E spectro-
photometer, using an integrating sphere DRA 2500.
Synthesis of 7-propargyloxycoumarin (2)
Anhydrous potassium carbonate (3.41 mmol) was added to a
solution of umbelliferone (2.28 mmol) in CH3CN (25 mL) under
an inert atmosphere. Then, propargyl bromide (2.72 mmol) was
added to the solution and stirred for 12 h at 60 1C. After the
completion of the reaction, the solution was filtered and
concentrated under reduced pressure. The solid residue was
purified by recrystallization with hexane to give 2 (90%) as
colorless crystals. 1H NMR (300 MHz, DMSO) d 3.65 (t, 1H,
J = 2.4 Hz), 4.93 (d, 2H, J = 2.4 Hz), 6.31 (d, 1H, J = 9.5 Hz), 6.99
(dd, 1H, J = 2.5, 8.6 Hz), 7.05 (d, 1H, J = 2.4 Hz), 7.65 (d, 1H,
J = 8.6 Hz); 13C NMR (75 MHz, DMSO) d 56.1, 78.5, 78.9, 101.9,
General procedure for the pre-treatment of cellulose paper
Four pieces of cellulose filter papers were immersed in a freshly 112.8, 112.9, 122.9, 129.5, 144.2, 155.1, 160.1, 161.2; MS (ESI)
prepared aqueous solution of 10 wt% NaOH (300 mL). This mixture 223.0 [M + Na+].
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Synthesis of methacrylamide rhodamine B (4)17
was irradiated with blue light (20 W, lmax = 400 nm) for 150 min.
Finally, the piece of paper was sequentially washed with water,
MeOH, acetone and CH2Cl2 under sonication, dried under
vacuum and stored under nitrogen.
To a solution of aminoethyl rhodamine B 318 (1 mmol) and
triethylamine (2.4 mmol) in CH2Cl2 (3.5 mL) under nitrogen at
0 1C was slowly added over 1 hour a solution of methacryloyl
chloride (2 mmol) in CH2Cl2 (2 mL). The reaction mixture was
allowed to warm up to room temperature and was further
stirred for 18 h. The crude solution was washed with water
(3 Â 10 mL), saturated Na2CO3 solution (3 Â 10 mL), brine
(10 mL), dried over anhydrous MgSO4 and concentrated under
reduced pressure. The residue was quickly purified over a pad
of silica gel (AcOEt) to give an unstable purple solid (55% yield).
1H NMR (300 MHz, CDCl3) d 1.16 (t, 12H, J = 7.0 Hz), 1.98
(s, 3H), 3.03–3.08 (m, 2H), 3.33 (m, 10H, J = 7.1 Hz), 5.29–5.31
(m, 1H), 5.77 (br s, 1H), 6.26 (dm, 2H, J = 8.7 Hz), 6.38 (br s, 2H),
6.43 (d, 2H, J = 8.8 Hz), 7.06–7.09 (m, 1H), 7.42–7.48 (m, 2H),
Thiol–ene reaction on cellulose paper with diallyl rhodamine B (5)
A mixture of diallyl rhodamine B 5 (40 mg) and DMPA (5 mg) in
DMSO (500 mL) was added dropwise on cellulose paper grafted
with dithiodiglycolic acid (70 mg). Then the paper was irradiated
with blue light (20 W, lmax = 400 nm) for 150 min. Finally, the piece
of paper was sequentially washed with water, MeOH, acetone and
CH2Cl2 under sonication, dried under vacuum and stored under
nitrogen.
7.68 (br s, 1H), 7.88–7.94 (m, 1H); 13C NMR (75 MHz, CDCl3) d Results and discussion
12.7, 18.8, 40.1, 41.4, 44.5, 65.9, 97.9, 104.8, 108.4, 199.8, 122.9,
Free-radical thiol–ene and thiol–yne reactions, also referred
124.0, 128.3, 128.5, 130.5, 132.9, 139.8, 149.1, 153.4, 154.0,
168.3, 170.3; MS (ESI) 553.3 [M + H+].
to as thiol–X chemistry, involve the ligation of thiyl radicals
with alkenes and alkynes, respectively.20–22 These reactions
are modular ligation tools adhering to the criteria of click
chemistry and widely used for surface grafting23–26 but surpris-
ingly, they have never been used for surface patterning of
cellulose paper, though, isolated examples of functionalization
of cellulose using the thiol–ene ligation has been reported but
without considering spatial-control criteria.27–29 The covalent
immobilization of an ink via these click transformations can
follow two different strategies. The first one involves the
functionalization of pristine paper with alkene or alkyne groups
while the covalent ink bears thiol functions. The second
approach involves an inverted process expressing the thiyl
radical precursor on the surface of cellulose paper as opposed
to the alkene or alkyne moieties. While these strategies are in
principle synthetically two feasible and related options, anchoring
the photoactive component (i.e., the thiol group) onto the surface
of cellulose paper is certainly more judicious to avoid the loss of
spatial resolution through diffusion processes.
One of the limiting steps of free-radical thiol–X processes, is
related to the high S–H bond dissociation energy, in the range
of 85–90 kcal molÀ1 for alkyl thiols, requiring the use of either
light irradiation at high energetic wavelengths (l o 230 nm) or
radical initiators under heating. By contrast, the S–S bond
dissociation energy is ca. 20 kcal molÀ1 lower with respect to
the S–H bonds of the related structures, making disulfides
much more reactive precursors for thiyl radical generation.
Harsh printing conditions using either energetic wavelengths
or excessive heating could damage the paper structure and are
obviously unsuitable for covalent printing. Surprisingly, thiol–X
ligation with disulfides has been essentially limited to the use
of phenyl disulfide22,28 and its application to surface chemistry
has been overlooked.22,30
Synthesis of diallyl rhodamine B (5)19
Aminoethyl rhodamine B 318 (1 mmol) and Et3N (3.6 mL) were
added in chloroform (25 mL) under nitrogen and the resulting
mixture was stirred for 20 min. Then, 3-bromoprop-1-ene
(7.1 mmol) was added at room temperature and the reaction
mixture was further stirred for 12 h at 75 1C. After cooling to
room temperature, water (12 mL) was added to the crude
mixture. The organic layer was separated, dried over anhydrous
MgSO4 and concentrated under vacuum. The crude was pur-
ified by flash chromatography (10% EtOAc–petroleum ether) to
give the expected rhodamine derivative 5 (50%) as a pale orange
solid. 1H NMR (300 MHz, CDCl3) d 1.16 (t, 12H, J = 7.0 Hz),
2.22–2.27 (m, 2H), 2.94 (d, 4H, J = 6.5 Hz), 3.18–3.23 (m, 2H),
3.33 (q, 8H, J = 7.0 Hz), 4.98 (br s, 2H), 5.03 (br d, 2H, J = 5.6 Hz),
5.58–5.71 (m, 2H), 6.25 (dd, 2H, J = 2.6, 8.9 Hz), 6.37 (d, 2H,
J = 2.6 Hz), 6.41 (d, 2H, J = 8.9 Hz), 7.04–7.10 (m, 1H), 7.39–7.45
(m, 2H), 7.85–7.91 (m, 1H); 13C NMR (100 MHz, CDCl3) d 12.6,
37.9, 44.4, 50.6, 56.6, 64.9, 97.9, 105.8, 108.2, 117.4, 122.7, 123.8,
128.0, 129.0, 131.6, 132.2, 135.3, 148.8, 153.4, 153.5, 167.7;
MS (ESI) 565.4 [M + H+].
Thiol–ene reaction on cellulose paper with 7-allyloxycoumarin (1)
A mixture of 7-allyloxycoumarin 1 (30 mg) and 2,2-dimethoxy-2-
phenylacetophenone (DMPA, 5 mg) in DMSO (500 mL) was
added dropwise on cellulose paper grafted with dithiodiglycolic
acid (70 mg). Then the paper was irradiated with blue light
(20 W, lmax = 400 nm) for 150 min. Finally, the piece of paper
was sequentially washed with water, MeOH, acetone and CH2Cl2
under sonication, dried under vacuum and stored under nitrogen.
Thiol–yne reaction on cellulose paper with
7-propargyloxycoumarin (2)
With these considerations in mind, we wondered if the
covalent coating of cellulose paper with photoactive disulfide
A mixture of 7-propargyloxycoumarin 2 (30 mg) and DMPA reagents could enable room temperature ligations of inks
(5 mg) in DMSO (500 mL) was added dropwise on cellulose through light-triggered S–S bond cleavage, resulting in highly
paper grafted with dithiodiglycolic acid (70 mg). Then the paper reactive thiyl radicals trapped by alkenes or alkynes.
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Preparation of photoresponsive paper
is characteristic of residual adsorbed water in the cellulosic
material. The successful grafting is confirmed by the new band
at 1725 cmÀ1 attributed to the CQO stretching frequency of ester
groups (Fig. 1b). Importantly, when the background esterification
of cellulose paper was carried out in the absence of p-TsOH as the
catalyst, no trace of carbonyl bands was observed in the resulting
FTIR spectrum, suggesting that Cell-DiS does not hold physi-
sorbed dithiodiglycolic acid residues.
The analysis of the surface composition of the material by
X-ray photoelectron spectroscopy (XPS) gave complementary
insights to demonstrate the grafting of dithiodiglycolic acid.
While the full scan spectrum of the pristine paper shows only
carbon and oxygen elements, the spectrum of Cell-DiS displays
S2s and S2p peaks at 229.1 and 164.3 eV, respectively (Fig. 2a
and c). A close examination of the S2 p peak in the high
resolution spectrum shows closely spaced spin–orbit compo-
nents at 165.0 and 163.8 eV for S2p1/2 and S2p3/2, respectively,
with a spin–orbit splitting of 1.2 eV, characteristic of disulfide
functions (Fig. 2e). The high resolution spectrum of the C1s
region allows the determination of the valence state of the
carbon elements present at the surface of cellulose paper. While
cellulosic materials display a main peak at 286.4 eV accounting for
C–O bonds and two less intense peaks at 284.8 and 287.9 eV for
C–C and O–C–O bonds, respectively (Fig. 2b and d), the spectra of
the C1s region of Cell-DiS exhibit a new peak at a high binding
energy (289.1 eV), corresponding to the newly created ester linkage
(Fig. 2d).
The global covalent coating of cellulose paper with photoactive
disulfide agents was envisaged by esterification of the hydroxyl
functions of glucose units with dithiodiglycolic acid. The
chemical modification of paper, including the esterification
reaction, is usually hampered by the low reactivity of surface
hydroxyl groups due to the bulk nature of paper and the dense
hydrogen bond network. Therefore, prior to surface functiona-
lization, breaking hydrogen bonds is an essential step for
increasing both the surface area of the paper and the reactivity
of hydroxyl groups. The activation step was achieved by soaking
cellulose paper for 24 hours at room temperature in 10 wt%
NaOH aqueous solution.31–33 We initially accomplished the
esterification with dithiodiglycolic acid chloride in DMF with
pyridine as a base. While the successful coating was secured
by elemental analysis and Fourier Transform InfraRed (FTIR)
spectroscopy, a low degree of substitution (DS) was determined
(ca. 4%) and a yellow-stained surface, unsuitable for covalent
printing applications, was obtained. We therefore examined
alternative procedures and, along this work, discovered that a
simple acid-catalyzed esterification process using dithiodiglycolic
acid and p-toluenesulfonic acid as catalysts in toluene preserved
the whiteness of the paper and significantly increased the DS to
ca. 14% as determined by elemental analysis, meaning that an
average of one dithiodiglycolic acid unit was incorporated every 8
glucose units (Scheme 1).
The grafting of dithiodiglycolic acid onto cellulose paper,
giving the photoactive paper Cell-DiS, was followed by FTIR spectro-
scopy (Fig. 1). The spectra recorded for the pristine and grafted
papers show typical adsorption bands of cellulosic material.
The broad band in the region of 3100–3400 cmÀ1 accounts
for the stretching vibration of the O–H bonds embedded in
dense hydrogen bond networks while the bands at ca. 2900 and
1100 cmÀ1 are attributed to the stretching frequencies of C–H and
C–O bonds, respectively (Fig. 1a). The weak band at ca. 1640 cmÀ1
Conserving the physical integrity of the photoactive paper
Cell-DiS with respect to the pristine material is an important
requirement for covalent printing applications which was
assessed by scanning electron microscopy (SEM). As depicted
in Fig. 3, the covalent coating of cellulose paper does not affect
the fiber network, while the average diameter of fibers remains
in the range of 10–20 mM. Moreover, we mention that the
whiteness of the Cell-DiS is fully preserved with respect to the
Scheme 1 Preparation of the photoresponsive paper Cell-DiS.
Fig. 2 Survey scan spectra of (a) pristine paper, (c) Cell-DiS. High resolu-
tion C1s spectra of (b) pristine paper, (d) Cell-DiS. (e) High resolution S2p
spectra of Cell-DiS.
Fig. 1 FT-IR spectra of (a) pristine cellulose paper and (b) Cell-DiS.
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patterning of paper since it is colorless and absorbs in a large
range of wavelengths (ca. 300–400 nm). The impregnated paper
was covered with an aluminum photomask representing the
acronym of the University of Nantes (UN) and irradiated with a
20 W lamp emitting at 400 nm to achieve covalent printing
leading to Cell-TEC and Cell-TYC materials (Scheme 2) for
thiol–ene and thiol–yne printing of coumarins, respectively. When
DMPA was omitted for the covalent printing process, the patterns
were obtained with a lower contrast, suggesting a lower grafting
efficiency.
Fig. 3 SEM images of (a) pristine cellulose paper, (b) Cell-DiS.
The control of the light-triggered click ligation of coumarin
units in the predetermined area was initially followed by FTIR
and XPS spectroscopy but both analyses did not clearly evidence
the successful grafting. The FTIR spectrum of the modified surface
after the thiol–ene ligation, corresponding to the Cell-TEC mate-
original material, confirming the high potential of this newly
discovered photoactive paper for covalent printing.
Hidden covalent printing
In the first stage of this work, we examined both thiol–ene and rial, is very similar to that of the initial photoactive paper (Fig. S1a,
thiol–yne mediated hidden printing. Covalent printing on ESI†) while the FTIR spectrum of the Cell-TYC material obtained
paper, hidden to the naked eyes under visible light is of great via the thiol–yne ligation, shows a new small band in the area of
promise for anti-counterfeiting and tracking applications as it the double bonds suggesting the presence of coumarin units
allows storing robust and tamper-proof information without (Fig. S1b, ESI†).
affecting the physical appearance of the paper.15 To succeed
Similarly, the surface elemental composition characterized
with such a promising concept, we identified a number of by XPS for the patterned paper shows an increase of the carbon
essential requirements: (i) the ligation in a predetermined area element with respect to the unmodified photoactive paper but
of (ii) chromophores not absorbing in the visible light must use the C1s region does not clearly evidence the presence of coumarin
(iii) a linkage not absorbing light in the range of 250 to 800 nm units (Fig. S2, ESI†). These results can be easily explained by the
in order to avoid superposition of optical signals with the modest degree of substitution attainable (ca. 14% max) regarding
chromophore.
the initial DS of the photoactive disulfide, resulting in a very large
Coumarin was selected as a strongly fluorescent chromo- contribution of the glycosidic elements to both the spectral
phore, transparent in the visible light and absorbing in the signature in FTIR and atomic composition observed by XPS.
range of ca. 310–340 nm, for hidden covalent printing. To Fortunately, the success of the ligation was unambiguously
evaluate the applicability of the thiol-mediated click ligation, ascertained by a simple revelation of the covalent printing under
we prepared two derivatives of umbelliferone substituted by an a UV-lamp at 365 nm, showing the two capital letters of the
allyl34 or propargyl35 ether group at C7 (Scheme 2, compounds 1–2) University of Nantes (Scheme 2, photographs b and d). The
for the thiol–ene and thiol–yne reaction, respectively. The photo- pattern of the covalent printing shows a good resolution with
active paper Cell-DiS was impregnated with a few drops of a sharp lettering, suggesting that diffusion processes were negli-
solution of the fluorescent chromophore and 2,2-dimethoxy-2- gible at the millimetre scale, while the patterning remained
phenylacetophenone (DMPA) as a free-radical photoinitiator fully hidden to the naked eyes under visible light (Scheme 2,
in DMSO; DMPA is a very attractive photoinitiator for the photographs a and c).
Scheme 2 General strategy for the covalent printing of a hidden pattern on paper. (a) Photograph of Cell-TEC under visible light, (b) photograph of
Cell-TEC under a UV lamp at 365 nm, (c) photograph of Cell-TYC under visible light, (d) photograph of Cell-TYC under a UV lamp at 365 nm.
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prerequisites to succeed with the covalent printing using
colored responsive inks: (i) the ligation in a predetermined
area of (ii) a dye (iii) absorbing light in the visible domain should
be (iv) responsive to an external stimulus modifying its color. To
tackle this ambitious strategy, we considered that developing an
ink containing rhodamine backbones as responsive dyes could
fulfill our expectations. Indeed, rhodamines exist as a colorless
closed-ring isomer in equilibrium with a deep purple opened-
ring form, strongly absorbing visible light in the range of
510–560 nm. The equilibrium composition of the two isomers
can be accurately controlled by adjusting the concentration of
protons onto the material surface; the colored form is predomi-
nant at high proton concentration while under basic conditions
only the colorless closed ring isomer exists. We evaluated this
unprecedented pH-responsive covalent printing, using the
spatially controlled thiol–ene ligation of rhodamine derivatives
4 and 5, readily prepared in 2 steps from commercially available
rhodamine B (Scheme 3). Briefly, rhodamine B was treated with
ethylenediamine in refluxing EtOH to give the corresponding
rhodamine derivative 3 functionalized with a primary amino
group.18 The aminoethyl rhodamine B 3 was then further reacted
with either methacryloyl chloride or Et3N in CH2Cl2 to give
ink 4,17 or an excess of allylbromide and Et3N in CHCl3 leading
to the twice functionalized ink 5.19 We mentioned that the
thiol–yne ligation of dyes was also envisaged in preliminary
Fig. 4 UV-vis absorption spectra of Cell-TEC (green line), Cell-TYC (red
line) and Cell-DiS (violet line).
The successful ligation was also confirmed by UV-visible
spectroscopy with new absorbance bands at 326 and 324 nm for
Cell-TEC and Cell-TYC materials, respectively, characteristic of
the optical signature of coumarin units (Fig. 4). We mentioned
that covalent printing was also studied using a paper covered
with thioglycolic acid units,36,37 but the low contrast of the
patterns suggested a lower efficiency of the photo-triggered
thiol–X ligation from the thiol function with respect to dis-
ulfides as discussed above.
Colorimetric covalent printing
The great efficiency of Cell-DiS as photoactive paper for hidden studies, but lower grafting was obtained, with respect to the
covalent printing, prompted us to push further our investiga- thiol–ene ligation, and therefore, this strategy was not further
tions for the development of an unprecedented colorimetric explored. We initially studied the ligation of rhodamine 4,
covalent printing. As mentioned earlier, we consider that bearing a methacrylate linker for the covalent printing process.
covalent printing should find huge interest for the preparation Unfortunately, the deep purple pattern of the Cell-TERa mate-
of new anti-counterfeiting devices, though related strategies for rial observed upon removing the photomask becomes barely
catalysis38 and biomedical applications9,14 are also promising. discernable upon usual washings to remove adsorbed residues
In the frame of anti-counterfeiting, one of the main factors for as depicted in the photograph (a) of Scheme 4. We attributed
establishing the level of security of a technology lies with its this low pattern stability to a retro-Michael-type fragmentation
facility to be duplicated by counterfeiters. Therefore, we reasoned upon successive washings, producing the leaching of rhoda-
that developing a stimulus-responsive ink for tamper-proof mine moieties from the paper surface. In order to increase the
covalent printing should significantly increase the level of security pattern robustness and avoid this fragmentation process, we
of the technology with respect to current inkjet printing. It should prepared the diallyl rhodamine derivative 5. We anticipated
be mentioned that the development of functional inks for inkjet that compound 5 cannot lead to a retro-Michael-type fragmen-
printing on cellulose paper is also a recent active area of tation since double bonds are not conjugated with a carbonyl
innovation39–41 that, however, still suffers from the drawbacks group, while the presence of two allyl groups should consider-
of the inkjet printing technology discussed earlier (vide supra). ably increase the pattern robustness through dual anchoring
With these considerations in mind, we identified several on the paper surface. As expected, when the photoactive paper
Scheme 3 Preparation of rhodamine-based inks.
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Scheme 4 General strategy for the covalent printing of a colored pattern on paper. (a) Photograph of Cell-TERa (using rhodamine 4) under visible light,
(b) photograph of Cell-TERb (using rhodamine 5) under visible light.
Both of these observations confirmed the successful introduc-
tion of the dye onto the cellulosic material Cell-TERb.
The FTIR spectra of Cell-TERb also attested the presence of
the dye with new bands at 1516 and 1638 cmÀ1 corresponding
to the C–N stretching vibration in combination with N–H
bending (amide II) and the CQO stretching frequency (amide I),
respectively. Moreover, the bands at 1585 and 1611 cmÀ1 corre-
spond to the CQC stretching vibration of the aromatic structure of
the dye (Fig. S3, ESI†).
Fig. 5 UV-vis absorption spectra of Cell-DiS (violet line) and Cell-TERb
(green line).
The well-known pH-sensitivity of rhodamines was further
explored in the quest of developing a pH-responsive covalent writing
onto cellulose paper (Fig. 7). The photo-thiol–ene-mediated printing
of dye 5 and the subsequent washings with protic solvents lead to
the Cell-TERb material patterned with the deep purple UN capital
letters, strongly visible by the naked eye. Upon immersion of
Cell-TERb for a few seconds in a 1 M NaHCO3 aqueous
solution, the printed pattern completely discolors and becomes
almost indiscernible to the naked eye. Interestingly, the pattern
reverts back to the initial deep purple color upon immersion of
Cell-TERb in a 1 M HCl aqueous solution. These studies high-
light the colorimetric reversibility of the covalent printing from
deep purple to colourless upon modification of the proton
concentration at the surface of Cell-TERb as a means of
authenticating anti-counterfeiting applications. Importantly,
using NaOH instead of NaHCO3 for the alkaline treatment also
leads to pattern discoloration, but the initial deep purple colour
cannot be fully restored upon subsequent acid treatment. This
unexpected behaviour is attributed to the partial NaOH-mediated
Cell-DiS was permeated with a few drops of a solution of 5 in
DMSO in the presence of DMPA and illuminated for 1 hour with
a blue lamp emitting at 400 nm through the photomask, a deep
purple pattern was covalently anchored onto the surface of the
Cell-TERb material (Scheme 4, photograph b). The writing was
obtained with an impressive resolution, expressed by excellent
contrast and sharp lettering. The covalent grafting of the dye
was evidenced by the UV-vis spectra of the pattern showing the
characteristic broad band of the open form of the rhodamine
skeleton as the monomer at ca. 564 nm while the shoulder at
ca. 532 nm also suggested the coexistence of several aggregated
forms onto the paper surface (Fig. 5).42
The grafting of the dye was also demonstrated by the
analysis of the surface composition of the patterned paper by
XPS (Fig. 6). The full scan spectrum of the patterned area
showed the presence of the N1s peak at 399.7 eV, while the
high resolution spectra of the C1s region showed a significant
increase of the peak accounting for C–C bonds at 284.7 eV.
Fig. 6 (a) Survey scan spectrum and (b) high resolution C1s spectrum of
Cell-TERb.
Fig. 7 Photographs of the covalently printed pattern (Cell-TERb) under
(a) protic and (b) basic conditions.
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Journal of Materials Chemistry C
10 T. Tischer, T. K. Claus, K. K. Oehlenschlaeger, V. Trouillet,
saponification of the pattern, suggesting that the use of a less
aggressive base such as NaHCO3 is required to fully preserve the
integrity of covalent printing.
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Conclusions
In summary, we have unveiled the concept of covalent printing
as a very appealing strategy for the patterning of cellulose paper
with small molecules acting as inks covalently anchored to the
surface of the cellulosic material. The general strategy is based
on the use of a photoresponsive paper bearing disulfide func-
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thiol–X ligations of chromophoric small molecules as inks with
a powerful spatio-temporal control. The studies described in
this contribution led to the covalent printing of (i) hidden
patterns using coumarin units as inks and (ii) pH-responsive
colorimetric writing through the use of a rhodamine moiety as
a responsive dye. While the concept of covalent printing onto
cellulose paper is in its early stage and deserves improvement
for increasing its throughput, we believe that it will find a broad
range of applications for sensing, medicine, catalysis and anti-
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¨
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Acknowledgements
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MdH thanks the ‘‘Region des Pays de la Loire’’ and the CNRS for
the grant. FXF is the member of the ‘‘Institut Universitaire de
France, IUF.’’ Denis Loquet (University of Nantes) and François-
`
Xavier Lefevre (University of Nantes) are gratefully acknowledged
for elemental and SEM analyses, respectively. SEM analyses were
carried out at the ‘‘Institut des materiaux Jean Rouxel.’’
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