Lignin coating to quench photocatalytic activity of titanium dioxide nanoparticles for potential skin care applications

Article abstract of DOI:10.1039/c5ra05232g

Ultraviolet light can cause photodamage to the skin, such as sunburns and melanomas. TiO₂ is introduced in sunscreen formulations to reflect and scatter UV radiation. However, it can also photocatalyze the production of reactive species like O<

Full text of DOI:10.1039/c5ra05232g

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ARTICLE
Lignin coating to quench photocatalytic activity of titanium
dioxide nanoparticles for potential skincare applications
M. Morsella,a M. Giammatteo,b L. Arrizza,b L. Tonucci,c M. Bressana and N. d’Alessandroa*
Received 00th January 20xx,
Accepted 00th January 20xx
Ultraviolet light can cause photodamage to the skin, such as sunburn and melanomas. TiO2 is introduced in sunscreen
formulations to reflect and scatter UV radiation. However, it can also photocatalyze the production of reactive species like
O2.- and OH.. Here, we aimed to remove the photocatalytic activity of TiO2 (anatase and rutile), while preserving the UV
filter property. Anatase and rutile were modified through two preparative protocols. The first used HCl lignin precipitation
of ethylene glycol lignin solution in the presence of the cross-linker glutaraldehyde and anatase or rutile nanoparticles. The
second protocol used HNO3 lignin precipitation of lignin aqueous solution in the presence of anatase or rutile
nanoparticles. Both methodologies were performed at room temperature and ambient pressure in green media, with
vigorous mixing followed by 20-kHz sonication. The composite materials obtained were fully characterized by SEM, XRD
analysis and FT-IR spectroscopy, and their photostability, and photo and shielding activities were evaluated through
reference reactions: oxidation of 2-propanol, an ene-reaction conducted on an α,β-unsaturated carboxylic derivative and
photochemical transformation of o-nitrobenzaldehyde to o-nitrosobenzoic acid. Therefore, in the near future, industrial
use of these new clusters can help to minimize TiO2 phototoxicity in sunscreen formulations, while preserving the
sunscreen photoprotection activity.
DOI: 10.1039/x0xx00000x
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scientifically only from the second half of the last century, and
it is well recognized that sunscreens reduce most
photodamage to the skin, such as sunburns and melanomas,
Introduction
Research in chemistry has always tried to develop different
synthesis pathways and to use materials with the lowest
possible toxicity levels, in order to generate safer industrial
processes. Moreover, eco-friendly and one-pot procedures
carried out using innovative products to minimize
environmental impact are now being given high priority. For
this reason, the use of materials derived from raw plant
materials should be favored. This defines the modern concept
of sustainability, as was introduced by Anastas and Warner,
and as is commonly known as “green chemistry”.1-3
which are primarily caused by UV radiation.10-12
Today, as described in detail in a recent contribution by
“L’Oreal Research & Innovation”,13 there is a great need to
evaluate the entire sustainability of the productive cycle of a
good sunscreen product. The ideal sunscreen should,
therefore, provide higher efficacy with broad-spectrum
coverage,14,15 high photostability,16 good sensorial attributes,17
and last but not least, at an affordable cost to the consumer. A
variety of marketing strategies have led consumers to believe
that sunscreen lotions prevent skin damage, while allowing
gradual tanning. However, to protect people from serious skin
damage, sunscreens must include many more attributes, the
most important of which is that the absorbed energy must be
dissipated efficiently via photophysical and/or photochemical
pathways. This will thus avoid formation of harmful reactive
intermediates that can penetrate the skin and be transported
into the cells, where they can cause damage to DNA.
Photoprotection and its associated risks is an ancient problem
that was even carefully considered by our ancestors.4 The
effects of light on the skin can appear immediately or after a
long delay. In the first case, there can be reversible damage
(erythema), which can be treated pharmacologically. Instead,
the effects that typically occur over the long term can be much
more dangerous, such as photoaging, immunosuppression,
and carcinogenicity.5-7 Sunscreens8,9 began to be studied
Sun filters can be broadly classified into organic absorbers and
inorganic particulates.18-20 All of the available organic filters are
of synthetic origin, and they are mainly based on molecules
that effectively absorb the UVA and/or UVB radiation.21
However each group has advantages and disadvantages. For
example, cinnamates and diphenylmethanes, have good
sunscreen action against both UVA and UVB, but they are
either poorly resistant to water (cinnamates) or photounstable
(diphenylmethanes), with the formation of by-products that
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are not always safe for the skin. Other popular organics have full characterization and a series of tests to evaluate their
low molar absorbance coefficient in the UVA (e.g., p- degree of quenching of the photocatalytic activity of TiO2.
aminobenzoates, salicylates) or in the UVB region (e.g.,
benzophenones).22-24
Experimental Section
Inorganic particulates have been introduced into sunscreen
formulations as alternatives or in addition to organic filters, to
better reflect, scatter and/or absorb the electromagnetic
radiation. Among these inorganic particulates, inert
semiconductor metal oxides are the most widely used in
commercial sunscreens formulations, such as TiO2 and ZnO.25
These metal oxides have bandgaps at ~3 eV, which is within
the range of the UV photons in solar light (3.1-4.3 eV), and
thus there is a relatively high probability that electron hole
pairs are created. There are essentially two such de-excitation
mechanisms, as either a simple emission of photon of energy
equal to the semiconductor bandgap, or the involvement of
the chemical environment of the inorganic particle (i.e.,
absorbed water, organic moieties). This last can, however, lead
to the formation of dangerous species, such as radicals,26-28 or
electrophilic cytotoxic species, such as singlet oxygen.29,30
Therefore, inorganic filters should be treated before their use
in any cosmetic formulations.31,32 Such treatments might
include, for example, coating of the metal oxide core by adding
Materials
Alkali lignin, ethylene glycol, glutaraldehyde (25% aqueous
solution), TiO2 anatase (nanopowder; particle size, <25 nm),
TiO2 rutile (nanopowder; particle size, <100 nm), o-
nitrobenzaldehyde (oNBA) and tiglic acid (sodium salt) were
from Sigma-Aldrich (Italy). Hydrochloric acid (37%), nitric acid
(65%), sodium hydroxide pellets and acetone were from Carlo
Erba Reagente (Italy). 2-Propanol and 1-butanol were from
Fluka (Italy).
Instruments
NMR spectra were recorded using a Bruker Avance 300
spectrometer (7.05 Tesla) equipped with a high-resolution
multinuclear probe that was operated in the range of 30 MHz
to 300 MHz. The 1H spectra (1H NMR) were run in a 5-mm
NMR tube that contained a co-axial sealed glass capillary
(diameter, 1 mm) that was filled with 30 mM 3-(trimethylsilyl)-
2,2,3,3-tetradeutero propionic- acid (sodium salt) in D2O. The
free induction decay was acquired at 22 °C using an
appropriate pulse sequence (zgcppr; Bruker-made) with a
spectral width of -0.5 ppm to 12.5 ppm and with the
presaturation signal centered at 4.705 ppm (proton water
signal). A 90° excitation pulse (7.6 μs) and a 1-s relaxation
delay were used to collect 128 scans for each spectra acquired.
The UV visible spectra were recorded using a Jenway 6505
UV/Vis system. Before acquisition, the reaction mixtures were
centrifuged and diluted with water, directly into a 3-ml high-
precision quartz cell (made of Suprasil® quartz; Hellma), and
the spectra were acquired over the wavelength range of 200
nm to 600 nm.
a
free radical scavenger (e.g., an antioxidant), or by
passivating33,34 or doping the semiconductor surface with a
metal, like vanadium and manganese.35 Encapsulation
technologies have also been used, such as the addition of solid
lipid nanoparticles (NPs),36 zeolites,37 polymer hybrid
nanocapsules,38 inorganic oxides (which form hydrated oxides
that capture hydroxyl radicals; such as SiO2,39 ZrO2 or Al2O3),40
and more recently, graphitized carbon41 and carbon
nanotubes.42 Alternatively, the addition of antioxidants can
minimize the formation of free radicals while still providing
protection against UV irradiation, with a preference for
antioxidants of natural origin. 43
Based on these possibilities, we believe that modification of a
nanomaterial like TiO2 using biopolymers would be a good
strategy to attain biocompatibility and functionality of the NPs.
Lignin is a natural, heterogeneous, phenylpropanoid polymer
that is biosynthesized in plants to provide structural rigidity
and to prevent the hydrolysis of cellulose, thereby protecting
plants from external chemical and/or biological attack.44 Lignin
is currently available in very large quantities as a by-product of
the paper and related industries, and its full valorization still
represents an important challenge in biorefineries.45 In this
perspective, there are several emerging trends for the use of
lignin as feedstock to produce fine chemicals, such as vanillin
and other simple monoaromatic derivatives,46-48 or for new
materials to use in industrial catalysis, i.e. nanoaggregates
between lignin and metal NPs.49,50 Moreover, several studies in
the medical field have shown that lignin can act as an effective
free radical scavenger51 without inducing irritation to the eyes
and skin,52 thus strongly suggesting that it can be safely used in
cosmetic formulations, such as for topical applications.53
In the present study, we described two methods for the
preparation of TiO2 NPs coated with lignin, together with their
The gas chromatography (GC) apparatus (model 6890; Hewlett
Packard) was equipped with a split-splitless injector, a free
induction decay detector, and
a 5% diphenyl dimethyl
polysiloxane capillary column (HP-5 MS; length, 30 m; internal
diameter, 0.32 mm; film thickness, 0.25 μm). The acquisition
parameters for 2-propanol detection were: injector pressure,
35 kPa; injector temperature, 250 °C; detector temperature,
260 °C; initial temperature, 50 °C (1 min), then 20 °C min-1 up
to 140 °C (kept for 0 min), and 40 °C min-1 up to 250 °C (kept
for 5 min). The carrier gas was helium, while 1 μl of the diluted
1:10 solution was injected in split mode (split ratio, 1:15).
For the GC mass spectroscopy (GC-MS) analyses, a gas
chromatograph (Thermo Scientific Focus series) was coupled
to an ISQ mass-selective detector operated in electron-impact
mode at 70 eV. The GC-MS was equipped with a split-splitless
injection system and a TR-5 MS (cross-linked, 5% diphenyl
dimethyl polysiloxane) capillary column (length, 30 m; internal
diameter, 0.25 mm; film thickness, 0.25 μm film thickness)
(Thermo Scientific Inc.; Waltham, USA), with helium as the
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carrier gas. The acquisition parameters for the detection of Synthesis of TiO2–lignin clusters
volatile derivatives coming from lignin degradation and of
For the syntheses of TiO2–lignin clusters, we started from the
idea developed by Frangville,54 which precipitated, at acidic
pH, the simple lignin NPs, starting from a soluble alkaline
lignin. In our study, we added firstly TiO2 NPs and, after a
vigorous homogenization (using an ultraturrax homogenizer),
followed by sonication (using an ultrasonic probe operating at
20 kHz), the pH assisted precipitation of the TiO2-lignin clusters
was performed.
oNBA were: injections (1 μL) made in split mode (ratio, 1:10);
constant injector pressure for all analyses, 20 kPa; injector
temperature, 250 °C; source temperature, 250 °C; transfer line
temperature, 250 °C; mass range, 33-300 u; initial
temperature, 50 °C (1 min), then 10 °C min-1 up to 160 °C (kept
for 0 min), and 30 °C min-1 up to 250 °C (kept for 2.5 min).
A photochemical multiray apparatus (Helios Italquartz; Milano,
Italy) was used for photochemical reactions, which contained
10 UV lamps of 15-W power each that emitted light at around
313 nm or 365 nm. Transparent quartz vials (10 mL) were used
for irradiation of the suspensions.
In the methodology illustrated in Scheme 1-A, alkali lignin (0.1
g) in ethylene glycol (10 mL) was filtered under reduced
pressure (Buchner funnel) to remove insoluble impurities. The
stock solution was transferred to a 20-mL test-tube, and TiO2
powder (0.01 g, anatase/rutile phase) was added. The
suspension was stirred for 1 h, homogenized for 2 min, and
sonicated for 5 min. The mixture was cooled to 0 °C in an ice
bath, and HCl solution (4 ml, 1 M) was added drop-wise. The
suspension obtained was again homogenized for 2 min and
sonicated for 10 min. After transfer of the entire mixture into a
250-mL flask, aqueous glutaraldehyde solution (0.2% v/v, 30
mL) was added drop-wise under stirring, over 2 h.
The X-ray diffractometer (XRD) analysis was performed on a
Miniflex II Rigaku automated power XRD system (Cu Kα
radiation, 45 kV, 100 mA) (RINT 2500, Japan). The diffraction
data were recorded using continuous scanning at 3° min-1, with
0.010° steps.
The Fourier transform infrared (FT-IR) spectrometer (model
1600; Perkin Elmer) was operated with a spectral window of
4000 cm-1 to 400 cm-1.
An ultra-turrax T25 (Janke & Kunkel, IKA Labortechnik, 220 V,
600 W, 50/60 Hz, 24000 min-1) was used for the cluster
homogenization. An ultrasonic processor (Model VCX 400;
Sonics Ultrasonic Processor; 400 W power, CV 26 Probe; 120 V,
50/60 Hz, 15 amps; probe diameter, 1/8 inch) was used for the
cluster sonication. A stereomicroscope (stereozoom S8 APO;
Leica) equipped with a camera (EC3) was used to observe the
dried powder.
In the methodology illustrated in Scheme 1-B, alkali lignin (0.1
g) was added to 7 mL ultrapure water under stirring in a 20-mL
test-tube. NaOH (3 mL, 1 M) was added drop-wise, for the
lignin solubilisation step. TiO2 powder (0.01 g, anatase/rutile
phase) was added, and the dispersion was homogenized for 4
min, sonicated for 15 min, and cooled in an ice bath. The
mixture was diluted with 180 mL ultrapure water, and taken to
pH 2 by drop-wise addition of HNO3 (0.25 M) over 2 h.
In both of these experimental procedures, the lignin/TiO2
suspensions were centrifuged and the residues (clusters A, B
from methodologies in Scheme 1-A, 1-B, respectively) were
washed twice with ultrapure water and dried in an oven at 80
°C for 2 h.
Clusters characterization
Following sonication, the samples for TEM analysis were
prepared by placing 5 μL of a 10-fold diluted ethanolic solution
of the clusters onto 3 mm, 300 mesh, Cu grids (Agar Scientific
Ltd).
The powder for SEM analysis was dried, glued onto Al stubs,
and then sputter-coated with a thin gold film (20 mA, 45 s).
The sample was then embedded in epoxy resin (EpoxiCureTM
,
Buehler), and prior to section analysis, it was polished with
polishing cloths with diamond polishing suspensions of
decreasing size (9, 3, 1 μm). The SEM analysis of uncoated
sections was carried out under low-vacuum conditions.
The samples for XRD analysis were prepared by evaporation of
the solvent of an entire cluster suspension, with the resulting
powder placed on the glass plate of the instrument.
Scheme 1 Detailed protocol adopted for the preparation of the
lignin/TiO2 composite materials.
The transmission electron microscopy images (TEM; CM100;
Philips) were obtained using 100 kV as acceleration voltage,
and were recorded using a camera (Megaplus, Kodak).
The scanning electron microscopy (SEM) images were
performed using a Philips XL30CP equipped with microanalysis
(Oxford Inca Energy 250) with both secondary electrons (SE)
and back-scatter electrons (BSE) detectors.
The samples for FT-IR analysis were prepared by evaporation
of the solvent (water) from the cluster suspensions. The
resulting residue was milled with KBr (1% w/w) to form a
homogeneous powder, which was then compressed into a thin
disk that was suitable for the FT-IR analysis.
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Stability and photostability studies
Dried clusters were re-suspended in aqueous solutions at
room temperature at three different pHs: pH 7 (in phosphate
buffer); pH 2 (in 0.01 M HCl); and pH 13 (in 0.1 M NaOH). After
24 h, 8 days, 12 days, 30 days, these samples underwent
several analyses (i.e., 1H NMR, GC-MS, UV-vis) to determine
the integrity of the clusters through the detection of any short-
chain products that can arise from the degradation of lignin.
Furthermore, a cluster mixture re-suspended in water was
irradiated at 313 nm for 24 h, with the same analyses of their
integrity as indicated above, and together with further
instrumental analyses, including SEM and IR (data not shown).
Scheme
2
Photooxidation of 2-propanol. Conditions: TiO2
suspension in aqueous medium, room temperature, irradiation at
313 and 365 nm.
Scheme 3 Ene-reaction between tiglic acid (sodium salt) and
singlet oxygen. Conditions: TiO2 suspension in D2O, room
temperature, aerobic conditions, irradiation at 313 nm.
Photoactivity
The radical-mediated photooxidation of 2-propanol to
acetone55 (see Scheme 2) was carried out at 313 nm and 365
nm for 48 h. 2-Propanol aqueous solution (10 mL, 5 mM) in a
quartz tube and under stirring was irradiated in the presence
of a TiO2 suspension (1 g L-1, anatase/rutile). This was then
repeated with the addition of alkali lignin, clusters A and B (of
Scheme 1-A and 1-B, respectively), and the lignin/TiO2 mixture
obtained without any treatment, for both anatase and rutile
TiO2. The data were recorded under GC and 1H NMR.
Scheme 4 Photoconversion of
o-nitrobenzaldehyde to o-
nitrosobenzoic acid. Conditions: dichloromethane solution,
room temperature, irradiation at 365 nm.
Results and Discussion
Singlet oxygen photooxidation was determined using the ene
reaction performed in the presence of an unsaturated
electron-rich derivative (see Scheme 3).56 A solution of tiglic
acid sodium salt in D2O (10 mM) in a quartz tube (2 mL) was
irradiated at 313 nm under stirring for 6 h in the presence of
TiO2 (10 mg, anatase/rutile), to determine the hydroperoxide
production (see Scheme 3). Then, this was repeated with the
TiO2 photosensitizer substituted by alkali lignin alone (100 mg),
and clusters A and clusters B. The hydroperoxide levels were
quantified by 1H NMR.
With the aim being to coat TiO2 NPs with lignin, we initially
realized that simple mechanical mixing of a lignin solution with
TiO2 particles, will not generate a homogeneous coating, even
under various experimental conditions. This is because TiO2
quickly aggregates and separates out as a white solid at the
bottom of the lignin-containing solution.
Therefore, to make effective an “interaction” between lignin
and TiO2, a precipitation method needs to be used. Our
method consists of precipitation of lignin at appropriate pHs
(i.e., from alkaline to neutral-acidic) in presence of nano-TiO2;
furthermore, the possible presence of a crosslinker (i.e.,
glutaraldehyde) was taken into consideration.
To test the photoactivities of clusters after the photooxidation
of 2-propanol, we removed the solvent by evaporation and we
re-suspended the clusters in water; then we carried out again
the reaction of photooxidation of 2-propanol under the same
condition indicated above.
Several commercially available lignins were initially taken into
consideration, but only alkaline lignin, which is soluble at pH >
11, gave encouraging results in terms of lignin precipitation on
the titania particle surface as lignin/TiO2 clusters. We applied
this methodology to lignin solutions containing TiO2
suspensions, either in ethylene glycol in the presence of
glutaraldehyde cross-linker (Scheme 1-A) or in alkaline water
(without glutaraldehyde; Scheme 1-B).
Shielding activity
The photoconversion of oNBA to o-nitrosobenzoic acid was
used to determine the shielding activity of the prepared
samples (see Scheme 4).57 A methylene chloride solution of
The detailed pathways are illustrated in Scheme 1. Comparing
these two synthetic procedures, we noted that the initial dark
brown color of the solutions (which was more intense for the
aqueous solution; see Fig. S1 in the ESI) turned dark green
when following Scheme 1-A, likely due to cross-polymerization
of the crosslinking agent with lignin,58 and turned yellow-
orange when following Scheme 1-B. Fig. S2-S5 in the ESI show
light microscope images of the obtained materials. To achieve
a good mixing and to reduce the particles size of the
composite material, vortex homogenization was used,
followed by ultrasonic treatment (20 kHz).59 Phase separation
was never observed, even after ultracentrifugation.
oNBA (3 mM, 2.5 mL) in a 3-mL spectrophotometric cuvette
was irradiated for 1.5 h at 365 nm with a LOT 200 W LSH303/
303 27A lamp. The experiment was repeated inserting a
further cuvette containing the filters, such as TiO2
(anatase/rutile), alkali lignin, clusters A or clusters B, between
the light source and the cuvette containing the oNBA. The
residual amounts of reagent were determined by GC-MS
analysis.
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Characterization
possibly due to their irregular shapes (i.e., nonspherical), as
also demonstrated by SEM analysis (see below).
The characterization of the TiO2-lignin clusters particles was
performed by IR spectroscopy, XRD analysis and TEM and SEM.
The IR spectra showed a clear-cut change in the shape of the
broad band in the 500-800 cm-1 region, where the Ti-O-Ti
vibration is expected to fall (Fig. 1 and Fig. S6). This indicates a
modification of the structure of the inorganic moiety, likely
due to a strong interaction with the organic part of composite.
Moreover, in Scheme 1-B, where the strong oxidant HNO3
used induced a partial bleaching of the lignin, the resulting
yellow-orange materials showed new absorption peaks in the
IR spectra. Those at 2940, 2361, 1722, 1631 and 1384 cm-1
were attributable to the oxidized (carbonyl) forms of the
alcoholic moieties of lignin, and those at 1545 and 1339 cm-1
were attributable to nitro groups introduced in the lignin
skeleton.
Preliminary screening was carried out using light microscopy
and TEM. In the TiO2 samples of both anatase and rutile the
particles were nanometer size (20-40 nm for anatase and 80
100 nm for rutile), with a tendency to aggregation (see Fig. S9-
S12 in the ESI). In contrast, with lignin alone (see Fig. S13 in the
ESI) or in association with TiO2 (see Fig. S14 and S15 in the ESI),
the images were difficult to interpret, likely because of to the
thickness of the particles, too large to be resolved with any
accuracy.
Figure 1 Representative IR spectra of clusters A (left) and B
(right) for anatase (top) and rutile (bottom).
Figure 3 Representative SEM images of cluster A containing
anatase (top) and rutile (bottom) (5000x of magnification)
The SEM images of TiO2 alone (Fig. S16), by confirming the
data acquired by TEM, allowed the resolution of the structure
of the lignin, which was seen as empty spherical particles that
were micrometer size (50-100 ꢀm; Fig. S17). The TiO2-lignin
Figure 2 Representative XRD analyses of clusters A (left) and B
(right) for anatase (top) and rutile (bottom).
cluster particles had irregular shapes, irrespective of the
preparative method used: they exhibited a powdery aspect
when containing anatase TiO2, while with rutile TiO2 were
The XRD analyses showed that the initial crystalline phase of more angular. In all cases, these particles were strongly
TiO2 were preserved under these treatments, whereas no reduced in size (to 10-20 m) with respect to pure lignin and
ꢀ
other solid crystalline by-products were detected (Fig. 2, Fig. showed a tendency to aggregate that was comparable to that
S7 and S8 in the ESI). We applied the Sheerer formula to of the naked TiO2 particles (Fig. 3 and 4; Fig. S18-S21). Indeed,
determine the sizes of the particles, although not successfully, the ultrasound treatment, in favouring the reduction of the
sizes of lignin particles, also prevented aggregation of the TiO2
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NPs, which appeared to be homogeneously dispersed inside
the lignin matrix. All samples were polished and then
examined again by SEM. The polished samples images showed
high dispersion of the TiO2 NPs, with marked differences
between anatase and rutile TiO2. The anatase NPs, apparently
because of their smaller sizes, showed higher aggregation than
the rutile NPs and always appeared as white agglomerates
dispersed inside the lignin matrix; in contrast, rutile
nanoparticles looked like a homogeneously dispersed fine
powder that was difficult to see at low magnification. In short,
the visual impression of these SEM images (Fig. 5 and 6; Fig.
S22-S25) resembled “dark chocolate cake” (i.e., the lignin) that
contains “white chocolate chips” (i.e., the TiO2) for the anatase
samples, or “coconut flour” (i.e., the TiO2) for the rutile ones.
Microanalyses, carried out on the particles (see Fig. S26-S29 in
the ESI) or on their sections (see Fig. S30-S33 in the ESI),
confirmed the presence of the expected elements, i.e. carbon,
titanium and oxygen.
Figure 5. Representative SEM images of polished sample of
cluster A with anatase (top) and rutile (bottom) (2500x of
magnification)
Figure 4 Representative SEM images of cluster B containing
anatase (top) and rutile (bottom) (5000x of magnification)
Figure 6. Rappresentative SEM images of polished sample of
cluster B with anatase (top) and rutile (bottom) (2500x of
magnification)
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Stability and photostability studies
Tests of the stabilities of the composite materials were
performed for water suspensions, at acidic, neutral and
alkaline pHs. The presence in the supernatant of short chain
products that came from lignin degradation/modification (e.g.,
methanol, formic acid, acetic acid, vanillin, among others) was
used to determine the stability/photostability of the materials,
and also the potential quenching role of lignin as a sacrificial
protector.
100
75
50
25
0
Even after 30 days in the dark no degradation products were
detected for the A clusters under any of the experimental
conditions used; the IR spectra were superimposable to those
of the initial clusters. Instead, in the same conditions, the
supernatant of the materials obtained by following Scheme 1-
B, always turned light yellow, while containing small amounts
of degradation products (i.e., methanol, formic acid, vanillin), Figure 7. Quantification of residual 2-propanol after
more abundant at higher pH, in agreement with a previous photooxidation under 48 h of irradiation at 310 nm for TiO2
report on lignin nanoparticles.54
alone and with lignin, and for the clusters A and B (according
When the neutral water suspension of the clusters was to Scheme 1) for anatase and rutile TiO2, as indicated. Values
irradiated at 313 nm for 24 hours, both the SEM images and are mean of three replicates; percent errors are always < 10%
the IR spectra remained unchanged. For the Scheme 1-A
clusters, the supernatant remained always colourless,
although containing small amounts of methanol, while the
Scheme 1-B clusters resulted in a light yellow water phase,
with formation of methanol, formic acid, vanillin and acetic
acid to greater levels than those formed in the dark (see Fig.
S34 and S35 in the ESI).
90
80
70
60
50
Photoactivity
The photoactivity was evaluated using two reference
reactions: the photooxidation of 2-propanol to acetone55 and
the ene-reaction between the TiO2-photogenerated singlet
oxygen and tiglic acid (a standard electron-rich unsaturated
model derivative, which yields, as dominant reaction product,
2-carboxy-3-hydroperoxo-1-butene).56
Figure 8. Quantification as for Figure 7, and when the clusters
were re-cycled a second time, as indicated. Values are mean of
three replicates; percent errors are always < 10%
1
Once it had been ascertained by H NMR that TiO2 does not
physically retain the alcohol, an aqueous solution of 2-
propanol (5 mM) was irradiated under stirring at both 313 and
365 nm for 48 h in the presence of the materials under
examination. As shown by the time course for the irradiation
at 313 nm in Fig. S36 in the ESI (the data for irradiation at 365
nm were comparable; data not shown), 2-propanol remained
substantially unchanged (within a 10% approximation) in the
presence of the four examined clusters (i.e., for anatase and
rutile, and under Scheme 1-A and 1-B), all of which clearly
acting as inhibitors of the phooxidation reaction (Fig. 7). On
the contrary, the reaction mixtures containing anatase or rutile
alone showed, as expected, strong photocatalytic activity (by
being anatase more effective than rutile). Finally, for a
mechanical mixture of lignin and TiO2, only 45% of 2-propanol
remained unreacted upon irradiation, pointing to a partial
quenching of the photocatalytic activity.
100
90
80
70
60
50
Figure 9. Quantification of residual oNBA after 1.5 h irradiation
at 365 nm, without any filter, and with filters of TiO2 alone and
with lignin, and for the clusters A and B (according to Scheme
1) for anatase and rutile TiO2, as indicated. Values are mean of
three replicates; percent errors are always < 10%
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Of note, the crosslinking agent appears to be critical for the photogenerated electrons, which appears to occur via
stabilization of the composite material, especially when in the sacrificial chemical reactions that lead to the formation of
alkaline media, while it does not have a role in the quenching short chain molecules.
of the photocatalytic activity of TiO2. Finally here, and as As the sizes of these particles are important for the
shown in Fig. 8, comparable percentages of residual 2- acceptability of the cosmetic preparation (i.e., smaller particles
propanol were seen when neo-synthetized or re-cycled in the formulation will result in a more homogeneous and
clusters were used, for both anatase and rutile TiO2 phases.
natural skin color), a further evolution of this study can be
When tiglic acid (sodium salt) in D2O (10 mM) was irradiated at envisaged, with the aim to reduce the size of the particles.
313 nm in the presence of the various samples, small amounts Other improvements can be achieved by investigating other
of the expected 2-carboxy-3-hydroperoxo-1-butene were crosslinkers, which should include in particular those of natural
detected only with anatase and rutile TiO2 alone (10 % and 4 % origin, like genipin60 or olive leaf extract.61
conversions, respectively, after 6 h). With lignin and with the A small amount of inorganic sunscreen is required in the
clusters following Scheme 1-A and 1-B, no reaction was cosmetic formulations, therefore the dark colour of lignin
observed, thus indicating that singlet oxygen was not formed, should not be a problem. Anyway, modifying the lignin
or, if formed, in very small amounts, not of concern for structure by simple oxidation reactions, it is possible to bleach
significant oxidative effects (see Fig. S37 of the ESI).
partially the polymer overcoming this potential problem.
Moreover, the light brown colour might be used in other daily
cosmetics (e.g., the last novelty of cosmetics called BB Cream -
Blemish Balm Cream) to protect and to donate a light colour to
Shielding activity
The photo-protection activity was determined according to the the skin.
photoconversion of NBA to
-nitrosobenzoic acid.57 This was Finally, it would be intriguing to combine physical and chemical
achieved by irradiation of a quartz cuvette containing 3 mM photoprotection by functionalizing the lignin with a common
NBA in dichloromethane, using a UV lamp (200 W) at 365 nm. organic sunscreen chromophores.
o
o
o
This test was repeated in the presence of filters, which were
put into a second cuvette placed between the irradiation
Conflicts of interest
source and an
oNBA sample. After 1.5 h irradiation, 34% of the
oNBA was converted in the absence of any filters (Fig. 9),
The author declares no conflict of interest.
whereas with the alkali lignin filter, lower conversion was
observed (20%), likely because of the relatively low UV
absorption by the lignin (see Fig. S38 of the ESI). In contrast, in
the presence of the TiO2 (both anatase and rutile) suspensions
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
as filters, oNBA was almost completely preserved (indeed, TiO2
is a well known inorganic filter that effectively reflects UV
radiation) and exactly the same result was observed with the
clusters Scheme 1-A and 1-B as filters, for both the anatase
and rutile TiO2. Therefore, and notwithstanding the
micrometric size of the particles, photoprotection by these
clusters was more effective than lignin, by reaching the same
level of the naked TiO2.
Abbreviations
NPs
nanoparticles;
GC
gaschromatograph
GC-MS
XRD
gas chromatograph mass spectrometry
X-ray diffractometry
NMR
nuclear magnetic resonance
Conclusions
A simple and versatile procedure that can be used to prepare
TiO2-lignin composite materials was developed here. The
experimental conditions adopted were mild and the by-
products from the overall procedure (i.e., ethylene glycol,
glutaraldehyde) were soluble in water, and were thus
removable by simple water washing. After the full
characterization of the clusters from Scheme 1-A and 1-B, we
evaluated their photochemical properties. All the examined
clusters induced almost complete quenching of the
photocatalytic power of TiO2 and complete inhibition of the
production of singlet oxygen by TiO2 (when used under aerobic
conditions), whereas they retained the same photoprotection
activity as TiO2. We can assume here that the UV photons can
be absorbed by both lignin and TiO2, but when TiO2 is
activated, the lignin layer helps to dissipate the
Aknowledgements
We thank the “Consorzio di Ricerca per l’Innovazione
Tecnologica, la Qualità e la Sicurezza degli Alimenti S.C.R.L.”
(CIPE funding 20.12.04; DM 28497) and the “Ministero
dell’Istruzione, dell’Università e della Ricerca” (MIUR) for
financial support. MM thanks the “Scuola Superiore” of the ‘G.
d’Annunzio’ University for the scholarship relative to the PhD
programme (cycle XXVIII).
Notes and references
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