Preparation and application of solid-state upconversion materials based on sodium polyacrylate

Article abstract of DOI:10.1039/c9ra01027k

By loading a microemulsion containing both sensitizer and emitter into porous sodium polyacrylate (PAAS), a water-absorbent resin (WAR) upconversion (UC) material was fabricated for photocatalysis applications. This WAR UC material showed a highly efficient UC process in the ambient environment owing to its liquid/solid encapsulation structure. In the application measurement, the UC emission from WAR UC materials can excite the catalyst Pt/WO₃ to produce hydroxyl radicals, yielding 7-hydroxycoumarin by reacting with coumarin. In another case, since the band gap of ZnCdS matches the energy of UC emission, hole-electron pairs can be obtained under the UC irradiation and capture electrons from rhodamine B, leading to the degradation of rhodamine B. The maximum of the photocatalysis efficiency can be up to 97%. This work solves the oxygen quenching problem by preparing a triplet-triplet annihilation upconversion (TTA-UC) O/W microemulsion and loading it into PAAS WAR, and opens a new avenue to solid-state devices for TTA-UC. The applications of photocatalytic synthesis and photocatalytic degradation lay a foundation for future practical applications for TTA-UC materials.

Full text of DOI:10.1039/c9ra01027k

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PAPER
Preparation and application of solid-state
upconversion materials based on sodium
Cite this: RSC Adv., 2019, 9, 17691
polyacrylate†
*
Changqing Ye, Jinsuo Ma,
Pengju Han,
Shuoran Chen,
Ping Ding, Bin Sun
*
and Xiaomei Wang
By loading a microemulsion containing both sensitizer and emitter into porous sodium polyacrylate (PAAS),
a water-absorbent resin (WAR) upconversion (UC) material was fabricated for photocatalysis applications.
This WAR UC material showed a highly efficient UC process in the ambient environment owing to its
liquid/solid encapsulation structure. In the application measurement, the UC emission from WAR UC
materials can excite the catalyst Pt/WO3 to produce hydroxyl radicals, yielding 7-hydroxycoumarin by
reacting with coumarin. In another case, since the band gap of ZnCdS matches the energy of UC
emission, hole–electron pairs can be obtained under the UC irradiation and capture electrons from
rhodamine B, leading to the degradation of rhodamine B. The maximum of the photocatalysis efficiency
can be up to 97%. This work solves the oxygen quenching problem by preparing a triplet–triplet
annihilation upconversion (TTA-UC) O/W microemulsion and loading it into PAAS WAR, and opens a new
avenue to solid-state devices for TTA-UC. The applications of photocatalytic synthesis and
photocatalytic degradation lay a foundation for future practical applications for TTA-UC materials.
Received 8th February 2019
Accepted 18th May 2019
DOI: 10.1039/c9ra01027k
rsc.li/rsc-advances
singlet-excited state. This annihilator at the singlet-excited state
could ultimately go back to the ground state with UC
uorescence.23
1 Introduction
Upconversion is a phenomenon of lower-energy (longer wave-
length) photons converted into higher-energy photons (shorter
wavelength), which can be carried out by either two-photon
absorption excited by the illuminant with high power
density1–3 or triplet–triplet annihilation (TTA) excited with low
power density.4–7 Owing to the potential widespread practical
Researches of TTA-UC material were generally carried on
within solution system ever since proposed in 1962.24 Moreover,
the mechanism of TTA-UC is based on the triplet population,
triplet energy transfer and triplet annihilation process of
organic compounds, so the system should be isolated from
oxygen to avoid the quenching of triplet excitons.25–27 Inevitably,
necessity of anaerobic environment and solution system
signicantly restricts the practical application of TTA-UC.28
Thus, high-efficiency TTA-UC in the ambient environment has
attracted great interest and been explored by many researchers.
For example, sensitizers and annihilators were embedded into
solid polymer lms29,30 or nano particles,31 including ethyl-
eneoxide–epichlorohydrin copolymer32 with low glass transition
temperature or rigid polymethyl methacrylate.33 The encapsu-
lation of sensitizers and annihilators could produce anaerobic
environment for TTA-UC. However, the triplet–triplet energy
transfer and TTA processes of TTA-UC were somewhat suffo-
cated as the diffusion of sensitizers and annihilators restricted
by polymer encapsulation and in consequence leading to the
signicantly decreased quantum efficiency.34,35 So, compared
with organic solvents system, these strategies all involved
complex preparation process and low UC efficiency,36 which
made TTA-UC difficult for further applications.
applications such as in photovoltaics,8,9 photocatalysis10,11
/
photodegradation,12,13 photodynamic therapy of cancer14,15 and
displays,16 triplet–triplet annihilation upconversion (TTA-UC)
has attracted more attention.
A TTA-UC system mainly consists of a triplet sensitizer and
annihilator,17–19 and the UC mechanism can be divided into
several micro-channel processes as follows:20–22 Upon excitation
of the low-energy illuminant, the triplet sensitizer will achieve
its singlet state (1S*), followed by an intersystem crossing
process to reach the triplet state (3S*). Secondly, the energy will
be transferred to the annihilator to generate its triplet state
(3A*) via triplet–triplet energy transfer. Thirdly, two triplet state
annihilator will further annihilate by collision and populate the
Research Centre for Green Printing Nanophotonic Materials, Jiangsu Key Laboratory
for Environmental Functional Materials, Institute of Chemistry, Biology and
Materials Engineering, Suzhou University of Science and Technology, Suzhou
215009, P. R. China. E-mail: wxyhpj@163.com
For above concerns, we explored a new liquid/solid encap-
sulation strategy for developing high TTA-UC system. First, we
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra01027k
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utilized an o/w microemulsion which can act as an effective operated in a glovebox and the quantity of water and oxygen
medium for TTA-UC performance in ambient air environment were both below 10 ppm. Aer the microemulsion was well
to dissolve the sensitizer PdTPP and the annihilator DPA. Then adsorbed, the surface of PAAS WAR UC materials was dried by
the O/W microemulsion was further encapsulated into a water- nitrogen ow and then kept in atmospheric environment before
absorbent resin (WAR) of the porous sodium polyacrylate further tests and applications.
(PAAS) to fabricate solid-state UC material, which can realized
highly efficient solid-state TTA-UC in practical ambient envi- 2.3 Upconversion property measurements
ronment. This WAR solid-state UC material has been demon-
Excitation source for UC was a solid-state continuous laser
strated to be applied in photocatalytic synthesis of 7-
(Nd:YVO4 + KTP) with wavenumber of 532 nm and power
hydroxycoumarin and photocatalytic degradation of rhodamine
density of 60 mW cmÀ2. First, a mount of mixed toluene solu-
B, which could lay a foundation for future practical effective
tion contains PdTPP (0.12 mM) and DPA (36 mM) was put into
energy utilization applications for TTA-UC materials.
a quartz colorimetric ware and bubbling nitrogen by a N2-vent
needle for 40 min. Then the green laser (532 nm, 60 mW cmÀ2
)
irradiated at the solution in the quartz colorimetric ware and
the blue upconverted uorescence appeared.37 Spectrum of the
UC uorescence was recorded by spectral scanning colorimeter
(PR-655) with a 532 nm optical lter to get rid of the scattered
laser peak.
2 Experimental section
2.1 Materials
Palladium(II) tetraphenylporphyrin (PdTPP) was synthesized as
previously reported.5 Acrylic acid, sodium hydrate, isopropyl
alcohol, potassium peroxodisulfate and toluene were purchased
from Tianjin Bodi Chemical co. Tween-20, 9,10-diphenylan-
thracene (DPA), H2PtCl6$H2O and nano powder WO3 were
purchased from J&K Chemical technology co. Methanol,
coumarin and ethyl alcohol were purchased from Sinopharm
Reagent Co. All reagents were analytical reagents and used as
received without further purication. Deionized water was used
in the experiments throughout.
2.4 Photocatalytic synthesis experiment driven by WAR UC
materials
Preparation of photocatalyst Pt/WO3 was as follows: 130 mg
WO3 nano powder, 30 mL deionized water and 10 mL methanol
were put in a 100 mL beaker with ultrasonic treated for 30 min.
Then H2PtCl6$H2O (0.4 mL, 5 Â 10À2 mol LÀ1) was added in the
beaker with stirring. Aer evenly stirred, the beaker was pack-
aged by foil, irradiated under mercury lamp (500 W). The ob-
tained solid was then ltered, washed with deionized water and
absolute ethyl alcohol, dried and compounded as aqueous
solution (10 g LÀ1) for further experiment.
Application demonstration of photocatalytic synthesis
driven by WAR UC materials was as follows. 5 mL coumarin
aqueous solution (200 mg LÀ1), 0.6 mL prepared photocatalyst
Pt/WO3 (10 g LÀ1) were stirred evenly in a 100 mL beaker, and
then 1.5 g WAR UC materials was taken in the mixed solution.
Green laser (532 nm, 60 mW cmÀ2) irradiated directly at WAR
UC materials and the obtained blue UC uorescence could
irradiate at coumarin aqueous solution consequently. The
beaker was also packaged by foil to reduce the loss of UC uo-
rescence. The solution was extracted every 20 min to detect the
uorescence spectrum of 7-hydroxycoumarin. All the experi-
ments were carried out in ambient environment.
2.2 Preparation of WAR UC materials
An amount of 36 mL deionized water was added into a 150 mL
three-necked ask, and the ask was then bubbling nitrogen by
a N2-vent needle for 40 min. 10 mL Tween-20 was then added
into the ask with syringe and dissolved in deionized water with
magnetic stirring. Aer that, 4 mL mixed toluene solution
contained PdTPP (0.12 mM) and DPA (36 mM) was added into
the ask with syringe. The mixed solution was magnetic stirred
under nitrogen atmosphere to form a homogeneous micro-
emulsion. The microemulsion was static down to transparency
and sealed for further experiment.
PAAS WAR was prepared using acrylic acid as the monomer
and potassium persulfate as the initiator. 8 g NaOH and 20 mL
deionized water was added into a 250 mL three-necked ask.
When the NaOH was dissolved with magnetic stirring and the
solution cool down, 25 mL acrylic acid was slowly added in with
magnetic stirring in cold bath. Then 20 mL isopropyl alcohol
was added in and the solution was transferred to a water bath of
2.5 Photocatalytic degradation experiment driven by PAAS
WAR UC materials
70 ꢀC. Aer that, 5 mL potassium peroxodisulfate (40 mM) was ZnCdS powder was used as the photocatalyst candidates since
dropwise added through dropping funnel with constant pres- its absorption edges at 528–573 nm and the estimated band-
sure. Aer becoming viscous, the solution was transferred into gaps at 2.35–2.17 eV that could effectively absorb our green-to-
a 150 mL beaker and the reaction continued in a constant blÀue upconversion (436 nm, 2.84 eV) to form electron/hole
temperature oven of 50 ꢀC for 2 h. As the polymerization (e /h+) pairs,38 which made it be able to degrade rhodamine B.
nished, raw PAAS was obtained as a faint yellow solid. Before
Application demonstration of photocatalytic degradation
applied in the next section, PAAS solid was immersed in ethyl driven by WAR UC materials was as follows: 5 mL rhodamine B
alcohol for 2 h and then dried, placed in drying oven for further aqueous solution (10 mM), 25 mg ZnCdS powder were stirred
experiment.
10 g dried PAAS was put into the prepared UC O/W micro- taken in the mixed solution. Green laser (532 nm, 60 mW cmÀ2
emulsion and soaked for about 48 h. The whole process was irradiated directly at WAR UC materials and the obtained blue
evenly in a 100 mL beaker, and then 1.5 g WAR UC materials was
)
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UC uorescence could irradiate at rhodamine B aqueous solu- chain macromolecular polymer. The prepared PAAS resin was
tion consequently. The beaker was also packaged by foil to faint yellow solid gel (Fig. 2a). As shown in SEM image (Fig. 2b),
reduce the loss of UC uorescence. The solution was extracted porous microstructures can be seen bestrewed within the PAAS
every 20 min to detect the absorption uorescence spectra of resin, which made PAAS WAR excellent absorption perfor-
rhodamine B. All the experiments were carried out in ambient mance. The water absorbency was 400 mL gÀ1 (eqn (S8), ESI†).
environment.
Therefore, the microemulsion can be absorbed into PAAS resin
to form WAR UC materials.
Micro-droplets of PdTPP/DPA/toluene homogeneously
dispersed in PAAS resin along with O/W microemulsion as seen
in Fig. 2d. PdTPP and DPA molecules can move unrestrictedly as
in solution system, which made the energy transfer smoothly as
seen in Fig. 2c. Excited by laser of 532 nm, there revealed
obvious blue light path in WAR material, which showed a high
efficiency as in solution system with anaerobic treatment. All
the experiments were carried out in the ambient environment,
which revealed that WAR UC materials had solved the problems
of oxygen-quenching and low UC efficiency in solid-state
system.
Absorption and emission properties of PAAS matrix in range
of 300–800 nm were shown in Fig S1a and b (ESI†), and there
was no absorption and emission peaks showed up. The trans-
mittance of PAAS WAR/microemulsion system has been char-
acterized in Fig S1c.† The corrections have been applied to
estimate the effect of the light loss in PAAS WAR/microemulsion
system (details in ESI†). Relationship of TTA-UC intensity and
excitation light power density had been explored in Fig. 3. The
intensity of WAR UC materials enhanced as the increase of
excitation light power density (Fig. 3a). Fig. 3b showed loga-
rithmic plot of TTA-UC intensity versus excitation light power
3 Results and discussion
In this paper, PdTPP was used as the sensitizer and DPA as the
annihilator to constitute TTA-UC system. As metalloporphyrin
complex, PdTPP showed a Soret absorption band at 415 nm and
a Q-band at 510–550 nm (Fig. 1a), which determined the
wavelength of the exciting laser (532 nm). There were two
emission characteristic peaks as the uorescence at 610 nm and
phosphorescence at 710 nm, respectively (Fig. 1a).
Absorption of DPA appeared respectively at 324 nm, 341 nm,
359 nm and 378 nm that belonged to the vibration of anthra-
cene group of DPA (Fig. 1b). Excited by laser of 532 nm, emis-
sion characteristic peak showed at 400–450 nm, which revealed
DPA was an appropriate luminescent material.
The preparation process of PAAS WAR is as follows. Firstly,
sodium acrylate was prepared through neutralization reaction
of acrylic acid and sodium hydroxide. Secondly, sodium acrylate
monomers combined with free radicals that obtained by
thermal decomposition of the initiator potassium perox-
odisulfate to realize chain growth. Thirdly, isopropyl alcohol
was added as chain transfer agent while PAAS grew into long
Fig. 2 (a) Photograph of WAR UC materials; (b) microstructure of WAR
UC materials; (c) photograph of UC fluorescence of WAR materials; (d)
Fig. 1 Absorption and fluorescence spectra of PdTPP (a) and DPA (b) schematic diagram of UC microemulsion in microcellular of WAR UC
with solvent toluene at a concentration of 1 Â 10À6 mol LÀ1
.
materials.
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trigger the chemical synthesis, which shows potential applica-
tions in photovoltaics and photocatalysis. Here, the WAR UC
materials was rstly used to drive photocatalytic synthesis of
organic molecules in air. Photocatalytic synthesis is a tech-
nology that using illuminant as a chemical synthesis element to
substitute chemical reagent or synthesis condition, which
supplied an environmentally friendly way for conventional
chemical synthesis.42
In this paper, WAR UC uorescence was used as illuminant
for the photocatalytic synthesis of 7-hydroxycoumarin using
coumarin as raw material. Emission characteristic peaks of
coumarin and 7-hydroxycoumarin were at 375 nm and 460 nm
as shown in Fig. 4a. The initial concentration of coumarin is two
orders of magnitude larger than 7-hydroxycoumarin, which
made the spectra intensity of coumarin is nearly constant as
shown in Fig. 4a. There was no uorescence peak at 460 nm at
the beginning, while this uorescence peak increased as illu-
mination time increasing. This phenomenon demonstrated
that 7-hydroxycoumarin was generated though the photo-
catalytic synthesis, and its concentration in solution was
increasing as the illumination of WAR UC uorescence
continued. The photocatalytic synthesis of 7-hydroxycoumarin
Fig. 3 (a) Spectrum of UC intensity under different excitation light
power density of WAR UC materials; (b) logarithmic plots of upcon-
version intensity versus excitation light power density of WAR UC
materials.
density from 11.45 mW cmÀ2 to 30.08 mW cmÀ2, and the ob-
tained K1 value is 1.89 that closed to 2. The value represents two
photons absorption of the TTA-UC process and the maximum
efficiency has been reached. When the excitation light power
density were increased from 30.08 mW cmÀ2 to 60.3 mW cmÀ2
,
the obtained K2 value is 1.19 that closed to 1. The value of 1.19
suggests that the two photons absorption process has achieved
a saturated situation and efficiency attenuation of the TTA-UC
process may occur.39 Thus, the excitation power in our upcon-
version measurements should be limited below 30.08 mW
cmÀ2. The relationship of the WAR UC materials was the same
as UC in solution system, which indicated that the PAAS resin
could provide analogous condition for TTA-UC as solution
system. The efficiency of WAR UC under different temperature
has been studied in Fig. S2 (ESI†).
Conventional methods for the production of 7-hydrox-
ycoumarin may cause formation of by-products and introduces
corrosion problems. For these reasons, some attempts should
be taken to nd alternative environmentally benign and
heterogeneously catalyzed synthesis routes.40 The TTA-based
upconversion can be obtained at very low excitation energy
and even near-infrared light, which has a much higher pene-
tration depth through various media, notably biological
tissue.41 TTA-UC has the ability to transform the unused low
energy radiation into high energy photons that beyond energy
gaps of main commercial photocatalysts, and consequently
Fig.
4 (a) Fluorescence spectrum of 7-hydroxycoumarin under
different irradiation time; (b) conversion rate of coumarin under
different conditions.
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needed a short time to be stable at the beginning. Peaks at
413 nm and 435 nm were characteristic peaks of DPA. The vast
majority of the DPA was encapsulated into the PAAS resin along
with microemulsion. However, there remained small amount of
the DPA molecules in the outermost layer of PAAS resin, which
could diffuse into coumarin solution. Emission spectra of DPA
and 7-hydroxycoumarin tested using laser (365 nm) was shown
in Fig. S3 (ESI†). The spectra intensity of DPA at 420 nm came
closer to the constant aer 20 min, suggesting the concentra-
tion of DPA in solution had already balanced. And the effect of
DPA could be considered as the same because the spectra
intensity of DPA was constant all along the photocatalytic
synthesis of 7-hydroxycoumarin (Fig. 4a).
The mechanism of the photocatalytic synthesis was as
follow: Firstly, low energy green light was converted into blue
light of high energy that matched the band gap of Pt/WO3. Then
electrons of Pt/WO3 was excited and transferred from valence
band to conduction band. Secondly, the electrons on conduc-
tion band reacted with dissolved oxygen in solution and
generated superoxide anion and hydroxyl radical.43 The
hydroxyl radical on Pt/WO3 is mainly generated through the
reduction of O2. The charge carriers on conduction band reac-
À
ted with dissolved oxygen in solution and generated $O2À
radical. Then H2O2 was generated by the reaction between $O2
radical and H+. And H2O2 was reduced by electrons and splited
into hydroxyl radical and OHÀ. The electron of OHÀ can be
despoiled by electron hole and also transferred into hydroxyl
radical. The reaction equations (eqn (S13)–(S20)) are shown in
ESI.† Coumarin combined with the hydroxyl radical and con-
verted into 7-hydroxycoumarin.
Single factor groups of photocatalyst and UC uorescence
were established to determine the dominant factor. Under the
same condition, one group without WAR while the other
without Pt/WO3 (Fig. 4b). The ratio of uorescence intensity of
7-hydroxycoumarin over time and the initial intensity (I/I0) was
calculated as conversion rate. As seen in Fig. 4b, the conversion
rates of 7-hydroxycoumarin in the two single factor groups were
nearly constant. While only the two factors of photocatalyst and
UC uorescence coexisted, the conversion rates of 7-hydrox-
ycoumarin increased and was proportional to time. The effi-
ciency of photocatalytic synthesis of 7-hydroxycoumarin was
also obtained (eqn (S12), ESI†).
Another demo application of WAR UC materials in the
photocatalytic degradation were demonstrated. Rhodamine B is
a mature industrial dye that has been widely applied in
cosmetics and colored glass eld. However, rhodamine B is
carcinogenic and its industrial or domestic waste may be
threats for human health. Using the UC uorescence as illu-
minant, we tried to study the photocatalyst degradation
behavior of rhodamine B by WAR UC materials. Schematic
diagram of the photocatalytic degradation of rhodamine B was
shown in Fig. 5a. Semiconductor materials ZnCdS was used as
photocatalyst. Under the irradiation of 532 nm laser, PdTPP and
DPA emitted high-energy UC uorescence that exciting ZnCdS
to generate electric charge effect. Through the electric charge
effect, rhodamine B was gradually degraded.
Fig. 5 (a) Schematic diagram of photo-degradation test of WAR UC
materials/ZnCdS/rhodamine B; (b) absorption fluorescence spectra of
rhodamine B under different irradiation time; (c) photo-degradation
curves of rhodamine B under different conditions.
Absorption spectrum of rhodamine B solution varied over
time under the irradiation of WAR UC uorescence as shown in
Fig. 5b. Absorbance intensity of rhodamine B gradually
declined along with the irradiation of UC uorescence, which
means the concentration of rhodamine B declined gradually.
The initial absorbance of rhodamine B was 0.57, while the value
turned to 0.018 aer illumination for 10 min. The concentration
of rhodamine B had decreased for almost 97%, which indicated
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a remarkable photocatalytic degradation of rhodamine B
through WAR UC materials.
Acknowledgements
Single factor groups of photocatalyst and UC uorescence
were also established to determine the dominant factor. Under
the same condition, one group without WAR while the other
without ZnCdS, as shown in Fig. 5c. The relative concentration
of rhodamine B in the two single factor groups were nearly
constant. While only the two factors of photo catalyst and UC
uorescence coexisted, the relative concentration of rhodamine
B decreased distinctly over time. The degradation efficiency was
up to 97% that corresponded to the result of ultraviolet
absorption test. The mechanism of the degradation of rhoda-
mine B was as follows. Firstly, low energy exciting light was
converted into blue UC uorescence of high energy through
WAR. Secondly, the energy of blue UC uorescence beyond the
energy difference of valence band and conduction band of
ZnCdS, which generated electron holes and electrons. And these
charge carriers on conduction band reacted with dissolved
oxygen in solution and generated hydroxyl radical in the same
way of Pt/WO3. The reaction equations are shown in ESI.†
Thirdly, rhodamine B was gradually degraded into CO2 and H2O
by hydroxyl radical following a series of redox processes. The
remarkable photocatalytic degradation of rhodamine B through
WAR material demonstrated the potential in industrial appli-
cations of photocatalytic degradation eld.
The authors are grateful to National Natural Science Founda-
tion of China (Grant No. 51873145, 51603141), Natural Science
Foundation of Jiangsu Province-Excellent Youth Foundation
(BK20170065), Natural Science Foundation of Jiangsu Province
(BK20160358), Natural Science Foundation of the Higher
Education Institutions of Jiangsu Province (17KJA430016), Qing
Lan Project, 5th 333 High-level Talents Training Project of
Jiangsu Province (No. BRA2018340), Six Talent Summits Project
of Jiangsu Province (No. XCL-79) for the nancial supports.
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4 Conclusions
In this paper, water-absorbent resin (WAR) solid-state upcon-
version (UC) material based on triplet–triplet annihilation
mechanism was prepared by loading PdTPP/DPA as sensitizers
and annihilators, using polyporous sodium polyacrylate as
matrix. PdTPP/DPA was dissolved in toluene and prepared into
O/W microemulsion. Characterization of micro-morphology
revealed that porous microstructures of the PAAS resin absor-
bed the microemulsion to form WAR UC materials. UC property
showed that PAAS WAR UC materials revealed strong blue
emission under the simulation of a semiconductor laser
(532 nm, 60 mW cmÀ2), realizing highly efficient solid-state
TTA-UC in air. In the further application measurement, pho-
tocatalyst Pt/WO3 was excited by UC emission from WAR UC
materials to produce hydroxyl radicals, yielding 7-hydrox-
ycoumarin by reacted with coumarin. Similarly, the band gap of
ZnCdS matched the energy of UC emission, hole–electron pairs
were obtained under the UC irradiation and captured electrons
from rhodamine B, leading to the degradation of rhodamine B.
The maximum photocatalysis efficiency was up to 97%.
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This work solves the oxygen quenching problem and opens
a new avenue to solid-state devices for TTA-UC by the prepared
WAR UC materials. Furthermore, the applications of photo-
catalytic synthesis and photocatalytic degradation lay a foun-
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Conflicts of interest
There are no conicts to declare.
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