Luminescent PMMA Films and PMMA@SiO2 Nanoparticles with Embedded Ln3+ Complexes for Highly Sensitive Optical Thermometers in the Physiological Temperature Range**

Article abstract of DOI:10.1002/chem.202004951

In recent years, luminescent materials doped with Ln³⁺ ions have attracted much attention for their application as optical thermometers based on both downshifting and upconversion processes. This study presents research done on the development

Full text of DOI:10.1002/chem.202004951

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Organic–Inorganic Hybrid Composites |Hot Paper|
Luminescent PMMA Films and PMMA@SiO2 Nanoparticles with
Embedded Ln3+ Complexes for Highly Sensitive Optical
Thermometers in the Physiological Temperature Range**
Dimitrije Mara,*[a, b] Anna M. Kaczmarek,*[a] Flavia Artizzu,[a] Anatolii Abalymov,[c, d]
Andre G. Skirtach,[c] Kristof Van Hecke,[a] and Rik Van Deun[a]
Abstract: In recent years, luminescent materials doped with
Ln3+ ions have attracted much attention for their application
as optical thermometers based on both downshifting and
upconversion processes. This study presents research done
on the development of highly sensitive optical thermome-
ters in the physiological temperature range based on poly-
(methyl methacrylate) (PMMA) films doped with two series
of visible Ln3+ complexes (Ln3+ =Tb3+, Eu3+, and Sm3+) and
SiO2 nanoparticles (NPs) coated with these PMMA films. The
best performing PMMA film doped with Tb3+ and Eu3+ com-
plexes was the PMMA[TbEuL1tppo]1 film (L1 =4,4,4-trifluoro-
1-phenyl-1,3-butadionate; tppo=triphenylphosphine oxide),
which showed good temperature sensing of Sr =4.21%KÀ1
at 313 K, whereas for the PMMA films doped with Tb3+ and
Sm3+ complexes the best performing was the
PMMA[TbSmL2tppo]3 film (L2 =4,4,4-trifluoro-1-(4-chloro-
phenyl)-1,3-butadionate), with Sr =3.64%KÀ1 at 313 K. Addi-
tionally, SiO2 NPs coated with the best performing films from
each of the series of PMMA films (Tb–Eu and Tb–Sm) and
their temperature-sensing properties were studied in water,
showing excellent performance in the physiological temper-
ature range (PMMA[TbEuL1tppo]1@SiO2: Sr =3.84%8C at
208C; PMMA[TbSmL2tppo]3@SiO2: Sr =3.27%8C at 208C)
and the toxicity of these nanoparticles on human cells was
studied, showing that they were nontoxic.
Introduction
tive, metrology, electronics, cell biology, nanomedicine, and
biomedicine.[2–10] In biotechnological applications and biologi-
cal research, the reactivity and dynamics of the biomolecules
are correlated with temperature. Elevated temperature in an
organism can indicate that there is a physiological disorder,
such as illness.[7] For example, cancer cells show a higher tem-
perature than that of normal cells. The reason for this differ-
ence in temperature is the higher metabolism of cancer
cells.[11,12] One of the ways to measure temperature in cells is
to employ luminescent thermometry measurements because
they are noncontact, give a real-time response, and have preci-
sion at the molecular level.[13]
Temperature is one of the fundamental thermodynamic prop-
erties that can influence physical, chemical, or biological pro-
cesses.[1] The accurate and precise measurement of tempera-
ture is needed for different industrial and technological devel-
opments and scientific research, such as aerospace, automo-
[a] Dr. D. Mara, Prof. A. M. Kaczmarek, Dr. F. Artizzu, Prof. K. Van Hecke,
Prof. R. Van Deun
Department of Chemistry, Ghent University
Krijgslaan 281-S3, 9000 Ghent (Belgium)
E-mail: anna.kaczmarek@ugent.be
Organic dyes have been extensively used for this purpose,
yet they show certain drawbacks, such as short excited-state
lifetimes, small Stokes shifts, broad emission bands, photo-
bleaching, and their emission intensity depends on the local
environment.[7,14–18] Alternatively, lanthanide (Ln3+) ions have
large Stokes shifts, narrow and sharp emission bands, long ex-
cited-state lifetimes, and their luminescence properties are less
dependent on the local environment, and therefore, they are
very attractive for luminescent sensing applications.[19–23] Lan-
thanide emission is based on the parity-forbidden 4 f –4 f tran-
sitions, and direct excitation of Ln3+ is therefore an inefficient
process because of the low molar extinction coefficients. To
avoid this limitation, Ln3+ can form complexes with organic li-
gands, in which these ligands can act as sensitizers to increase
the absorption of light and transfer the absorbed energy to
[b] Dr. D. Mara
Department of Chemistry, KU Leuven
Celestijnenlaan 200D, 3001 Leuven (Belgium)
E-mail: dimitrije.mara@kuleuven.be
[c] A. Abalymov, Prof. A. G. Skirtach
Department of Biotechnology, Ghent University
Coupure Links 653, 9000 Ghent (Belgium)
[d] A. Abalymov
Educational Research Institute of Nanostructure and Biosystems
Saratov State University, Saratov 410012 (Russia)
[**] PMMA=poly(methyl methacrylate).
Supporting information and the ORCID identification number(s) for the
author(s) of this article can be found under:
https://doi.org/10.1002/chem.202004951. It contains details on the synthesis
of lanthanide b-diketonate complexes; crystal data and refinement details;
FTIR, ESI-MS, and TGA-DTA data; a discussion on the photoluminescence
properties; and a discussion on temperature-dependent samples.
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the lanthanide ions. Organic molecules, which can be em-
ployed as sensitizers, are, for example, heterocyclic, aromatic
carboxylates or b-diketonates as ligands.[24–26] The long excited-
state lifetimes of lanthanide probes versus the short excited-
state lifetimes of organic dyes present a clear advantage in
biological experiments: the fluorescence of a typical organic
molecule will be observable for about the same amount of
time as that of the autofluorescence of biomolecules, whereas
the emitted light of a Ln3+ probe is longer lived. Therefore, if
Ln3+ probes can be made to bind selectively to certain parts
of tumor cells, these can be very precisely located by using the
longer-lived lanthanide luminescence.
be doped to obtain ratiometric thermometers. In particular,
doping was carried out with two systems consisting of Tb3+
Eu3+ and Tb3+ –Sm3+ mixed-metal complexes, which were al-
ready proved to show promising temperature-sensing proper-
ties in the physiological range.[30] Although the Tb3+ –Eu3+
system is an already popular thermometer in the physiological
temperature range,[5,13,31–34] the Tb3+ –Sm3+ system is rarely
investigated, despite its promising potential, due to the diffi-
culty in obtaining an intense Sm3+ emission at temperatures
above room temperature.[35,36] Finally, to explore the potential
of these materials to be used as thermometers in biological
applications, SiO2 NPs coated with doped PMMA films were in-
vestigated.
–
Lanthanide molecular structures based on b-diketonate com-
plexes retain high color purity and quantum yields. b-Diketo-
nate ligands possess high energetic singlet and triplet levels,
which makes them suitable for exciting Ln3+. Furthermore,
they have a strong affinity to form stable complexes with Ln3+
because they are hard Lewis bases and Ln3+ are hard Lewis
acids. With Ln3+, they mostly form tris complexes, leaving two
vacant coordination sites, which can be occupied by water
molecules or by additional neutral ligands. This creates the op-
portunity to change the chemical properties just by changing
the coordination environment. Furthermore, these compounds
can be easily processed into complex matrices, such as organic
polymers, silica glasses, or other materials.[27]
Results and Discussion
Preparation and optimization of doped PMMA films
The optimized synthesis of the doped PMMA films is described
in the Experimental Section. The polymerization of PMMA and
the conditions for doping the lanthanide complexes into the
polymer were first optimized. In the first attempts, different
solvents were used to dissolve PMMA (acetonitrile and metha-
nol, pure acetonitrile, dichloromethane, and chloroform).
Chloroform was identified as the best solvent to codissolve
PMMA and the complexes. The second step of optimizing the
film preparation was finding the optimal ratio between the
two complexes [Ln1(L1(2))3(tppo)2] and [Ln2(L1(2))3(tppo)2] (Ln1 =
Tb3+; Ln2 =Eu3+ or Sm3+), which would give the desired lumi-
nescent properties (see the PL studies of complexes in the
Supporting Information) and would allow the final materials to
be used for temperature-sensing properties. Table 1 gives the
optimized ratios of Ln1 to Ln2 complexes in PMMA films. The
films were obtained by slow solvent evaporation at 308C in
Petri dishes with a diameter of 70 mm. Transparent films with
Ln3+ complexes were obtained (Figure 2). PL characterization
was carried out before the samples were investigated for tem-
Herein, we report on the synthesis, photoluminescence (PL),
and sensing properties of poly(methyl methacrylate) (PMMA)
films and SiO2 nanoparticles (NPs) coated with PMMA films
doped with luminescent tris-b-diketonate lanthanide com-
plexes (Figure 1). The lanthanide b-diketonate complexes with
L1 or L2 ligands were isolated as water-free molecules in which
the neutral tppo molecules completed the coordination
sphere. The complexes of Eu3+ and Tb3+ are new complexes
reported herein, whereas the complexes of Sm3+ were report-
ed in our previous study.[28] The best performing complexes,
selected on the basis of their photoluminescent properties and
emission quantum yields, were incorporated as dopants in
transparent and flexible PMMA films.[29] PMMA was chosen as a
homogenous material, in which two different complexes could
Table 1. Molar ratio of doped lanthanide complexes (Ln1 =Tb3+; Ln2 =
Eu3+, Sm3+) after optimization in PMMA films, with general formulas of
LnL1tppo and LnL2tppo.
Sample[a]
Ln1
Ln2
PMMA[TbEuL1tppo]1
PMMA[TbEuL1tppo]2
PMMA[TbEuL1tppo]3
PMMA[TbEuL2tppo]1
PMMA[TbEuL2tppo]2
PMMA[TbEuL2tppo]3
PMMA[TbSmL1tppo]1
PMMA[TbSmL1tppo]2
PMMA[TbSmL1tppo]3
PMMA[TbSmL2tppo]1
PMMA[TbSmL2tppo]2
PMMA[TbSmL2tppo]3
12
16
20
30
34
38
4
6
10
4
1
1
1
1
1
1
1
1
1
1
1
1
12
16
Figure 1. Schematic representation of the approach employed herein. From
the free complex to PMMA-coated SiO2 NPs; L1 =4,4,4-trifluoro-1-phenyl-1,3-
butadione, L2 =4,4,4-trifluoro-1-(4-chlorophenyl)-1,3-butadione; tppo=tri-
phenylphosphine oxide.
[a] The numbers 1–3 in labels represent the different doping ratios of the
Ln1 and Ln2 in PMMA films presented in this table. Each number corre-
sponds to an individual ratio from this table.
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Figure 2. Photographs of PMMA Ln3+-doped films: a) the transparent
PMMA[Ln1Ln2L1(2)tppo] film, b) PMMA[TbEuL1tppo]1 under a UV lamp with
excitation at 365 nm, and c) PMMA[TbSmL2tppo]3 under a UV lamp with ex-
citation at 365 nm.
perature-dependent luminescence sensing. A brief discussion
of the results obtained for the PL properties of the PMMA films
is given with Figures S32–S35 and Tables S3 and S4 in the Sup-
porting Information.
Figure 3. SEM and TEM images of SIO2 NPs coated with a PMMA-
[TbEuL1tppo]1 film: a) dispersion of the particles, b) magnification of the
SiO2 NPs by SEM, c) particles, and d) magnification of the surface of SiO2 NPs
showing PMMA coating by TEM.
PMMA-coated SiO2 NPs
Two different methods of coating the SiO2 NPs were investigat-
ed. As a model complex, EuL1tppo-doped PMMA was used.
The first method was based on the sonication of a suspension
of SiO2 NPs with PMMA for 30 min. The second method con-
sisted of vigorous stirring of a suspension of SiO2 NPs with
PMMA for 30 min; in both methods, chloroform was used as
the solvent. After sonication or stirring, the SiO2 NPs were sep-
arated from the solution by centrifugation and left for 1 d to
dry in air. Next, the NPs were washed once with a mixture of
methanol and water to remove all residuals of the synthesis.
The luminescent properties of coated SiO2 NPs were then stud-
ied, and the intensity of the emitted light provided an indica-
tion of the homogeneity and thickness of the coating layer. Re-
sults for the PMMA@SiO2 NPs samples showed that the second
method was more efficient in coating of the silica NPs. The stir-
ring process was further prolonged to 2 h to ensure complete
coating of the SiO2 NPs. SEM and TEM images of the PMMA@-
SiO2 NPs are shown in Figure 3. After the coating process,
there were no visible changes in the size and shape of the par-
ticles, ranging from 150 to 500 nm in diameter (Figures S19–
S22 in the Supporting Information). Upon higher magnifica-
tion, a thin coating with a thickness of about 7 nm can be ob-
served on the particle surface (see Figure 3d).
coupled for these type of thermometers, external thermal cali-
bration of the thermometric parameter is needed. The calibra-
tion process requires an independent measurement by using,
for example, a thermocouple or infrared camera, which allows
the intensity-to-temperature conversion. The dual-center-based
luminescent thermometers overcome these disadvantages by
possessing two emitting centers. In this type of thermometer,
the emitting center can be thermally coupled, but, in most
cases, they are not, and thus, external calibration of the ther-
mometric parameter is not necessary.[22] Herein, the ion pairs of
Tb3+ –Eu3+ and Tb3+ –Sm3+ as systems for dual-center ratio-
metric thermometers were chosen. To calculate the thermo-
metric parameter D, Equations (1) and (2) were used:[38]
I1
I2
1
D
D
D0
ꢀ
ꢁ
2
DE
1
aexp À
kB
T
7
7
in which I1 (Tb3+ =5D4! F5) and I2 (Eu3+ =5D0! F2 or Sm3+
=
5G5/2! H9/2) are integrated intensities of the transitions, D0 is D
(T=0 K), a presents the ratio between nonradiative (W0 at T=
0 K) and radiative (WR) rates, and DE is the activation energy
for the nonradiative pathway. The Sr value indicates the relative
change of the thermometric parameter per degree of the tem-
perature change (%KÀ1) and was calculated by using Equa-
tion (3):
6
Luminescent thermometry
Luminescent thermometers can be based on different princi-
ples, such as the measurement of luminescence lifetimes, shifts
of the emission peaks, or changes to the luminescence intensi-
ty of one or more emission peaks.[2,37] In this case, luminescent
thermometry based on emission intensity was chosen. There
are two types of ratiometric emission intensity thermometry
methods. The difference is based on the number of emission
centers present in the materials. Single-center ratiometric-
based luminescent thermometers are based on a single emis-
sion center, in which both emission peaks are generated from
two different excited levels, which are in thermal equilibrium
of the same emitting center. Because these levels are thermally
ꢂ
ꢃ
1 @D
D @T
Sr 100%
3
which is inversely proportional to the thermometric parameter
(D) and the variation of the thermometric parameter @D per
temperature variation (@T). The Sr value is used for a compari-
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son of thermometers composed of different materials and it is
independent of the nature of the thermometer. The maximal
value of the relative sensitivity for the measured temperature
range can be presented as Sm.
The performance and stability of the thermometer is esti-
mated by cycling the temperature in the given temperature in-
terval, ensuring that the thermometer reaches thermal equilib-
rium for each measurement. It can be described by Equa-
tion (4):
max jDc À Dij
4
R
1 À
Dc
in which Dc is the mean thermometric parameter extracted
from the calibration curve, and Di is the value of each thermo-
metric parameter measurement.[22]
The temperature uncertainty, dT, is an important parameter
in the assessment of the performance of the desired thermom-
eter. dT can be calculated by using Equation (5):
1 dD
Sr D
5
dT
Temperature-dependent luminescence of
PMMA[Ln1Ln2L1(2)tppo] films (Ln1 =Tb3+; Ln2 =Eu3+ or Sm3+
Figure 4. a) Emission map of the PMMA[TbEuL1tppo]1 film, as measured
over the 253–353 K range (step 10 K). b) Plot representing the calibration
curve for PMMA[TbEuL1tppo]1. The points show the experimental D parame-
ter and the solid line is the best fit of the experimental points obtained
from Equation (2). c) Plot showing the relative sensitivity, Sr, values at differ-
ent temperatures (253–353 K); the solid line is intended as a guide.
)
The temperature-dependent luminescence was measured in
the physiological temperature region, which was extended
down to 253 K (À208C) for the samples of this series because
of interest from fundamental studies of the biological, bio-
chemical, and physiological processes occurring in cells if they
have been frozen. During defrosting, there is the need to accu-
rately measure the temperature, while these physiological pro-
cesses are monitored.
that this sample has one of the highest Sr values over the
physiological temperature range reported, to date (Table 2).
For the films from the PMMA[TbEuL2tppo] series, the best re-
sults were obtained for the PMMA[TbEuL2tppo]3 sample. A de-
tailed discussion of the temperature-dependent luminescence
of this sample and the other two samples in the series is pre-
sented in the Supporting Information. For the PMMA-
[TbEuL2tppo]3 sample, the maximal value of the relative sensi-
tivity, Sr =4.04%KÀ1, was obtained at 313 K.
For the PMMA[TbEuL1(2)tppo] films, six samples were studied
for their temperature-dependent luminescence properties:
three with L1 and three with L2 ligands. Of these, the PMMA-
[TbEuL1tppo]1 sample is discussed in detail, whereas the other
two samples are discussed in detail in the Supporting Informa-
tion (Figures S36 and S37).
The integrated areas under the peaks used for these sam-
Similarly, six PMMA[TbSmL1(2)tppo] films were investigated
for their temperature-dependent luminescence properties and
ability to work as temperature sensors. For both ligands L1 and
L2, three samples were investigated.
5
7
ples are, for Tb3+, the D4! F5 transition peak, which is ob-
5
7
served between 533 and 558 nm, and for Eu3+, the D0! F2
transition peak, which is observed between 604 and 630 nm.
An emission map of PMMA[TbEuL1tppo]1, which is measured
in the temperature range from 253 to 353 K, is presented in
Figure 4a. The I543/I614 intensity ratio of the integrated areas
under the peaks for PMMA[TbEuL1tppo]1 was calculated (Fig-
ure 4b), for the indicated temperature range, over which the
sample showed monotonic behavior. The data points were
fitted with Equation (2), yielding DE=3537 cmÀ1 (R2 =0.9993).
The calculated energy difference between the emitting level of
Tb3+ (5D4 =20500 cmÀ1) and the emitting level of Eu3+ (5D0 =
17300 cmÀ1), which is DE=3200 cmÀ1. In Figure 4c, the rela-
tive sensitivity, Sr, is presented at different temperatures. The
maximum value of Sr =4.21%KÀ1 obtained at 313 K indicates
The integrated peaks used for these samples were at
7
543 nm (5D4! F5) for the Tb3+ ion, which was integrated over
the range between 533 and 558 nm, and the Sm3+ ion peak at
6
643 nm (5G5/2! H9/2), which was integrated over the range of
630–660 nm.
For the samples from the PMMA[TbSmL1tppo] series, of the
three investigated samples, the best results were obtained for
PMMA[TbSmL1tppo]1, which is discussed in detail, along with
the other two samples, in the Supporting Information. For the
PMMA[TbSmL1tppo]1 sample, a maximal relative sensitivity of
Sr =3.45%KÀ1 was obtained at 313 K.
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In the series of the samples with general formula
PMMA[TbSmL2tppo], the sample showing the best results is
PMMA[TbSmL2tppo]3 and is discussed herein, whereas the
other two samples are discussed in the Supporting Information
(Figures S44 and S45).
times. Although the results for samples based on the Tb3+
–
Sm3+ system show that their relative sensitivity is slightly
higher than those of similar systems with only one material,[36]
it has nevertheless shown higher sensitivity. A comparison of
our samples with different systems, such as hybrid systems or
organic dyes, shows similar or higher sensitivity, even up to
four times.
An emission map of the PMMA[TbSmL2tppo]3 sample, which
is measured over the temperature range from 253 to 343 K, is
presented in Figure 5a. The I543/I643 intensity ratio of the inte-
grated areas under the peaks for PMMA[TbSmL2tppo]3 was cal-
culated (Figure 5b) for the indicated temperature range and
showed monotonic behavior. Fitting of the data points was
performed by using Equation (2), yielding DE=3274 cmÀ1 (R2 =
0.9994). The calculated energy difference can be best matched
by the energy difference between the triplet level (T1 =
24000 cmÀ1) of the tppo ligand and the emitting level of Tb3+
(5D4 =20500 cmÀ1), which is DE=3500 cmÀ1. In Figure 5c, the
relative sensitivity, Sr, is presented, as calculated for different
temperatures. The maximum value of Sr =3.64%KÀ1 obtained
at 313 K indicates that this sample is a very good temperature
point in the physiological range. In Table 2, a comparison of
the materials reported herein with known materials from litera-
ture is presented.
To show the potential of the thermometer materials for real
applications, stability tests were carried out, in which the sam-
ples were subjected to three heating/cooling cycles. The stabil-
ity tests for the PMMA[TbEuL2tppo]3 and PMMA[TbSmL1tppo]1
samples are presented in Figure 6, showing a repeatability of
up to 95% for PMMA[TbEuL2tppo]3 and up to 97% for
PMMA[TbSmL1tppo]1. The temperature uncertainty for these
two compounds has been calculated and it is presented in
Supporting Information (Figure S46). The proposed energy-
transfer (ET) mechanisms based on results obtained for sam-
ples from both systems are presented in Figure 7.
Temperature sensing of PMMA-coated SiO2 NPs in water
For SiO2 NPs coated with PMMA films containing lanthanide
complexes, the temperature-sensing capability was studied in
water over the 5 (278) to 508C (323 K) temperature range. It is
known that water is an efficient quencher of the luminescence
properties of the lanthanide ions through vibrational quench-
The results for the Tb3+ –Eu3+ materials from this study
have shown very good relative sensitivities compared with
those of other Tb3+ –Eu3+ materials reported in the literature
(Table 2). They are much more sensitive, even up to seven
Table 2. A comparison of the sensitivity of the reported thermometers with the sensitivity of other luminescent thermometers over the physiological tem-
perature range. An overview of the materials, temperature range [K], and maximal relative sensitivity values (Sm) is given.
Material[a]
T range [K]
Sm [%KÀ1] ([K])
Ref.
[Eu0.53Tb0.47(tfac)8]Na2
[Tb0.90Sm0.10(tfac)8]Na2
Eu0.50Tb0.50DPA-PMO
Sm0.95Tb0.05DPA-PMO
BPy-PMO@Tb,Sm(acac)3_10_90
Tb0.92Eu0.08-HPIDC-OX
[Ln(ad)0.5(phth)(H2O)2] (Ln=5EuIII/10TbIII)
Tb@UiO-66 hybrid film
Eu@UiO-66 hybrid film
Ca8.98Mg1.5(PO4)7:x(Eu2+, Eu3+
[Tb0.97Eu0.03(L)(OX)(H2O)]
Eu0.05-Tb1.95-PDC
273–373
280–360
260–460
280–460
253–333
303–473
303–423
303–353
303–403
293–473
250–340
298–333
293–328
298–373
293–353
253–353
253–353
253–353
253–343
253–343
253–343
253–373
253–353
253–343
253–373
253–333
253–343
2.70 (353)
2.30 (360)
1.56 (360)
2.38 (340)
4.93 (253)
0.6 (473)
[30]
[30]
[35]
[35]
[36]
[39]
[40]
[41]
[41]
[42]
[43]
[44]
[45]
[46]
[47]
this work
this work
this work
this work
this work
this work
this work
this work
this work
this work
this work
this work
1.21 (303)
2.76 (303)
4.26 (363)
1.192 (383)
1.38 (340)
1.37 (333)
1.19 (313)
2.98 (373)
1.28 (293)
4.21 (313)
3.99 (313)
3.81 (313)
3.57 (293)
3.78 (303)
4.04 (313)
3.45 (313)
3.15 (313)
2.93 (313)
3.39 (333)
3.50 (323)
3.64 (313)
)
Tb0.8Eu0.2BPDA
(Tb/Eu-TPTZ)PMMA
ZJU-88Jꢀperylene
PMMA[TbEuL1tppo]1
PMMA[TbEuL1tppo]2
PMMA[TbEuL1tppo]3
PMMA[TbEuL2tppo]1
PMMA[TbEuL2tppo]2
PMMA[TbEuL2tppo]3
PMMA[TbSmL1tppo]1
PMMA[TbSmL1tppo]2
PMMA[TbSmL1tppo]3
PMMA[TbSmL2tppo]1
PMMA[TbSmL2tppo]2
PMMA[TbSmL2tppo]3
[a] tfac=1,1,1-trifluoroacetylacetone, DPA=pyridine dicarboxamide, PMO=periodic mesoporous organosilica, BPy=2,2’-bipyridyl, acac=acetylacetonate,
H3PIDC=2-pyridin-4-yl-4,5-imidazoledicarboxylic acid, OX=oxalate, ad=adipate, phth=phthalate, PDC=pyridine-2,6-dicarboxylate, H2BPDA=1,1’-biphen-
yl-3,3’-dicarboxylic acid, TPTZ=2,4,6-tris(2-pyridyl)-1,3,5-triazine.
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Figure 6. Temperature cycling tests for PMMA[TbEuL2tppo]3 (a) and
PMMA[TbSmL1tppo]1 (b) over the temperature range of 253 to 373 K. The
dashed lines are intended as a visual guide to the eye.
Figure 5. a) Emission map of the PMMA[TbSmL2tppo]3 film, measured over
the 253–343 K range (step 10 K). b) Plot representing the calibration curve
for PMMA[TbSmL2tppo]3. The points show the experimental D parameter
and the solid line is the best fit of experimental points from Equation (2).
c) Plot showing the relative sensitivity, Sr, values at different temperatures
(253–343 K); the solid line is intended as a guide to the eye.
ing.[27] For this reason, it is taken as a case study for these pre-
liminary measurements. Cells and the whole human body con-
sist of up to 70% water. However, the cytosol, in addition to
water, also contains salts, proteins, and other biomolecules,
which were not taken into consideration at this point of study.
This and in vitro temperature-sensing studies will be a part of
future investigations. As four coated PMMA films, two samples
with the best performing temperature-sensing properties from
the Tb3+ –Eu3+ system and two from the Tb3+ –Sm3+ system
and one of each from both systems were taken from the series
of the films, which consisted of sample derivatives with L1 and
L2 complexes. The integrated areas of the peaks, which were
used for calculations in both systems, are given in the last sec-
tion. The stability of the PMMA-coated SiO2 NPs was investigat-
ed after having been dispersed in water for one month. It was
observed that the emission intensity of the Ln3+ complexes
embedded in the PMMA film was reduced by about 20% rela-
tive to the initial measurements. A decrease in the emission in-
tensity could be due to partial diffusion of water molecules in
the PMMA polymer, which can then partially quench the Ln3+
emission.
Figure 7. Energy-level diagram presenting the mechanism of ET in
a) PMMA[TbEuL1(2)tppo] films and PMMA[TbEuL1(2)tppo]@SiO2 NPs, and
b) PMMA[TbSmL1(2)tppo] films and PMMA[TbSmL1(2)tppo]@SiO2 NPs. S0 and
S1 are the singlet ground level and singlet excited level, respectively; T1 is
the triplet level; ET is to the lanthanide ions from the ligands; BT is the
energy back transfer; DE represents the energy gap and is indicated in a) in
purple for both series of samples and in b) in purple for samples of
PMMA[TbSmL1tppo] and in dark blue for samples PMMA[TbSmL2tppo].
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An emission map of PMMA[TbEuL1tppo]1@SiO2, which was
measured over the temperature range from 5 to 508C, is pre-
sented in Figure 8a. The I543/I614 intensity ratio of the integrated
areas under the peaks for PMMA[TbEuL1tppo]1@SiO2 was cal-
culated (Figure 8b), for the mentioned temperature range,
over which the sample showed monotonic behavior. The data
points could be well fitted by using Equation (2), yielding DE=
3143 cmÀ1 (R2 =0.9936). The calculated energy difference can
be well matched with the energy difference between the emit-
ting level of Tb3+ 5D4 (20500 cmÀ1) and the emitting level of
Eu3+ 5D0 (17300 cmÀ1). In Figure 8c, the relative sensitivity, Sr,
calculated at different temperatures is presented. The maximal
value of Sr =3.84%8C obtained at 208C indicates that this
sample shows very good thermometric behavior, as highlight-
ed by a comparison with values reported in the literature
(Table 3).
An emission map of PMMA[TbSmL2tppo]3@SiO2, measured
over the temperature range from 5 to 508C, is presented in
Figure 9a. The I543/I643 intensity ratio of the integrated areas
under the peaks for PMMA[TbSmL2tppo]3@SiO2 has been cal-
culated (Figure 9b). The sample shows monotonic behavior
over the indicated temperature range. The data points could
be well fitted by using Equation (2), yielding DE=2533 cmÀ1
(R2 =0.9987). The calculated energy difference again matched
well with the energy difference between the emitting level of
5
Tb3+ 5D4 (20500 cmÀ1
)
and emitting level of Sm3+ G5/2
(17900 cmÀ1). In Figure 9c, the relative sensitivity, Sr, calculated
for the different temperatures is presented, showing a maxi-
mum value of Sr =3.27%8CÀ1 at 208C, which is among the
best performing thermometers in water for this system
(Table 3).
The results obtained for the samples herein show similar or
higher sensitivity to that of the examples from the literature,
except for one example,[51] which was measured in the solid
state. Our samples based on the Tb3+ –Eu3+ system are over
12 times more sensitive than that of samples based on the
same system, and the samples based on the Tb3+ –Sm3+
system are up to 1% more sensitive than that of the most sen-
sitive example presented to date.
Figure 8. a) Emission map of PMMA[TbEuL1tppo]1@SiO2 NPs measured over
the 5–508C range (step 58C). b) Plot representing the calibration curve for
PMMA[TbEuL1tppo]1@SiO2. The points show the experimental D parameter
and the solid line is the best fit of the experimental points by using Equa-
tion (2). c) Plot showing the relative sensitivity, Sr, values at different temper-
atures (5–508C); the solid line is intended as a guide to the eye.
Table 3. A comparison of the sensitivity of our thermometers with the sensitivity of other luminescent thermometers over the physiological temperature
range. An overview of the materials, temperature range [8C], and maximal relative sensitivity value (Sm) is provided.
Material[a]
T range [8C]
Sm [%8CÀ1] ([8C])
Medium
Ref.
BPy-PMO@Tb,Sm(acac)3_20_80
ZJU-88Jꢀperylene
5–50
2.72 (5)
solution
solution
solution
solution
tissue
solid state
solution
solution
solution
solution
solution
[36]
[47]
[48]
[49]
[50]
[51]
[52]
this work
this work
this work
this work
20–80
20–60
25–79
20–60
20–80
25–47
5–50
5–50
5–50
5–50
1.28 (20)
3.39 (60)
3.40 (35)
0.6 (60)
7.22 (80)
0.31 (45)
3.84 (20)
3.05 (20)
2.93 (15)
3.27 (20)
ZJU-28ꢀDPASD-1
[Tb(dipicCbz)3]3-
THA@EuNMOF@Fe/TA
Eu0.058Tb0.942BPT
Tb0.99Eu0.01(BDC)1.5(H2O)2
PMMA[TbEuL1tppo]1@SiO2
PMMA[TbEuL2tppo]3@SiO2
PMMA[TbSmL1tppo]1@SiO2
PMMA[TbSmL2tppo]3@SiO2
[a] dipicCbz=4-(9H-carbazol-9-yl)pyridine-2,6-dicarboxylato, NMOF=nanoscale metal–organic framework, TA =tannic acid, H3BPT=biphenyl-3,4’,5-tricar-
boxylate acid, BDC=1-4-benzendicarboxylate.
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Figure 10. Calculated temperature uncertainty, dT, for PMMA[T-
bEuL1tppo]1@SiO2 NPs (a) and PMMA[TbSmL2tppo]3@SiO2 NPs (b).
Toxicity tests
To fully assess the potential use of PMMA-coated SiO2 NPs for
biological applications, cytotoxicity tests were conducted on
living cells. Normal human dermal fibroblastic (NHDF) cells
were used for these experiments. The results of the viability
test demonstrated that the PMMA[TbEuL1tppo]1@SiO2 and
PMMA[TbSmL2tppo]3@SiO2 NPs samples are nontoxic
(Figure 11). At rather high concentrations of 1, 0.5, and 0.1 mg
per well, the particles have shown toxicity towards fibroblastic
cells. Cell viability at these concentration ranges from 20 to
40%, which shows a toxic effect. Disruption of cell adhesion
and morphology was indicated by round cell morphology at
concentrations of 1, 0.5, and 0.1 mg per well (Figure 11 b). At a
concentration of 0.05 mg per well, cells start to attach better, a
higher number of cells have a spindle-shaped morphology,
which is inherent in NHDF cells. ANOVA results showed a
strong difference between the control at concentrations of 1,
0.5, and 0.1 mg per well. Most likely, at these concentrations,
NPs tend to conglomerate, which can lead to mechanical pres-
sure on the cells and stop cell adhesion. At concentrations of
0.05 mg per well and lower, the cell viability is 75% or higher
(Figure 11 a), which is considered to be almost nontoxic.[53]
Figure 9. a) Emission map of PMMA[TbSmL2tppo]1@SiO2 NPs measured over
the 5–508C range (step 58C). b) Plot representing the calibration curve for
PMMA[TbSmL2tppo]1@SiO2. The points show the experimental D parameter
and the solid line is the best fit of the experimental points by using Equa-
tion (2). c) Plot showing the relative sensitivity, Sr, values at different temper-
atures (5–508C); the solid line is intended as a guide to the eye.
The temperature uncertainty, dT, calculated for the above-
mentioned samples remains below 1 K over the whole studied
temperature range, confirming the excellent behavior of these
thermometers (Figure 10). For the PMMA[TbEuL1tppo]1@SiO2
NPs (Figure 10a), the temperature uncertainty, dT, is more con-
sistent, and it is in the range from 0.015 to 0.038C, whereas for
the PMMA[TbSmL2tppo]3@SiO2 NPs (Figure 10b) dT is in the
range from 0.1 to 0.248C. The difference in dT comes from the
intensity of the emission peaks, which is used for ratiometric
temperature sensing in these two systems. In the physiological
temperature range, the emission intensity of the Sm3+ peak at
6
643 nm (5G5/2! H9/2) is significantly lower than that of the
7
emission intensity of the Eu3+ peak at 614 nm (5D0! F2),
Conclusion
which leads to a possibly higher uncertainty due to an increase
of the background noise during temperature readout for ther-
mometers based on the Tb3+ –Sm3+ system compared with
those based on the Tb3+ –Eu3+ system.
Thin PMMA films and PMMA-coated SiO2 NP thermometers
doped with tppo derivatives of lanthanide b-diketonate com-
plexes showed very good temperature-sensing properties over
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Experimental Section
Preparation of PMMA films doped with [Ln1(L1(2))3(tppo)2]
and [Ln2(L1(2))3(tppo)2] complexes (Ln1 =Tb3+; Ln2 =Eu3+
Sm3+
,
)
The PMMA films were prepared as follows: PMMA (100 mg) was
weighed and placed in a 25 mL vial. It was dissolved in chloroform
(10 mL) and stirred at room temperature for 15 min. Solutions of
complexes [Ln1(L1(2))3(tppo)2] and [Ln2(L1(2))3(tppo)2] (Ln1 =Tb3+ and
Ln2 =Eu3+ or Sm3+) were prepared by dissolving different ratios of
Ln1 and Ln2 complexes in chloroform (3 mL) with total concentra-
tions of complexes in the range of 1.67”10À3 to 1.67”
10À2 molLÀ1, depending of the ratio used for lanthanide com-
plexes, to obtain different intensity ratios of emission peaks of Ln1
and Ln2. The solutions of lanthanide complexes were added drop-
wise to the solution of PMMA and left to stir for 2 h. After this
time, the solution was transferred to a Petri dish and a heating
plate at 308C was used to evaporate chloroform and form clear
films.
Figure 11. a) Cell viability after exposure to different concentrations of
PMMA[TbEuL1tppo]1@SiO2 and PMMA[TbSmL2tppo]3@SiO2 NPs. An asterisk
(*) indicates significant differences from the control cell group. Statistical
analysis was performed by analysis of variance (ANOVA) followed by the
Tukey test (p<0.05). b) Fluorescence microscopy images of cells at different
SiO2-coated NP concentrations (cells are green). Scale bar: 250 mm.
Preparation of SiO2 NPs coated with PMMA[Ln1Ln2L1(2)tppo]
films
The silica NPs were synthesized by a procedure previously estab-
lished in our group.[54] The coating of the SiO2 NPs was done as fol-
lows: PMMA[Ln1Ln2L1(2)tppo] (50 mg) was dissolved in chloroform
(5 mL) and an excess of PMMA (40 mg) was added, and the solu-
tion was left to stir for 30 min at room temperature. At the same
time, SiO2 NPs (70 mg) were dispersed in chloroform (3 mL), and
after 30 min of stirring the SiO2 NP suspension was added to the
solution of PMMA[Ln1Ln2L1(2)tppo] and left to stir for 2 h. After-
wards, the suspension was transferred to a 15 mL centrifuge tube
and centrifuged for 5 min at 5000 rpm to separate the SiO2 NPs
from the rest of the solution of PMMA. The solution of PMMA was
decanted and the PMMA[Ln1Ln2L1(2)tppo]@SiO2 NPs were left to dry
in open air for 1 d. The next day, the NPs were washed once with a
mixture of methanol/water (1/1, v/v) to remove residual unreacted
PMMA. After washing and decanting of the rinsing solvent, the
NPs were left to dry in open air for 1 d.
Details of the characterization of the PMMA films doped with
[Ln1Ln2(L1(2))3(tppo)2] and the SiO2 NPs coated with PMMA
[Ln1Ln2L1(2)tppo] films can be found in the Supporting Information.
Deposition Numbers 1964128, 1964129, 1964130, 1964131, and
1964132 contain the supplementary crystallographic data for this
paper. These data are provided free of charge by the joint Cam-
bridge Crystallographic Data Centre and Fachinformationszentrum
Karlsruhe Access Structures service www.ccdc.cam.ac.uk/structures.
the physiological temperature range. The thermometers were
based on two ion-pair systems, Tb3+ –Eu3+ and Tb3+ –Sm3+
,
in which lanthanide ions were doped in the PMMA polymer in
different rations for a series of samples with L1 and L2 com-
plexes. The results obtained for the PMMA films in both sys-
tems showed promising results for this temperature range, for
which the Tb3+ –Eu3+ pair had Sr values in the range from
3.57 to 4.21%KÀ1, reaching the highest obtained value of Sr =
4.21%KÀ1 at 313 K for the PMMA[TbEuL1tppo]1 sample. For
the second system, the Tb3+ –Sm3+ pair, the Sr values were in
the range from 2.93 to 3.64%KÀ1, with the highest value of
Sr =3.64%KÀ1 at 313 K found for the PMMA[TbSmL2tppo]3
sample. The best performing samples, one from each series of
PMMA films, were used to coat the SiO2 NPs, and the results
showed that the relative sensitivity were similar to those in the
films, with only slightly lower Sr values. The value of Sr =
3.84%8CÀ1 at 208C was retrieved for the PMMA-
[TbEuL1tppo]1@SiO2
sample,
whereas
the
PMMA[TbSmL2tppo]3@SiO2 sample had
a
value of Sr =
3.27%8CÀ1 at 208C. The obtained results of relative sensitivity
reported for PMMA films are among the highest values for the
physiological temperature range in both the solid state and Acknowledgements
water, as coatings of SiO2 NPs. The promising results of the
toxicity tests give an opportunity for future work with in vitro
and in vivo measurements of temperature with PMMA-coated
SiO2 NPs.
D.M. and R.V.D. acknowledge the Ghent University Special Re-
search Fund (BOF) for Ph.D. grants (BOF15/24J/049). D.M. ac-
knowledges KU Leuven Postdoctoral Mandate Internal Funds
(PDM) for a postdoctoral fellowship (PDM/20/092). A.M.K. ac-
knowledges Ghent University for funding. F.A. acknowledges
the FWO (Research Foundation Flanders) and the European
Union’s Horizon 2020 program for a [Pegasus]2 Marie Skłodow-
ska-Curie fellowship “Nanocomposite materials for highly effi-
cient sensitized lanthanide emission”. K.V.H. thanks the Her-
cules Foundation (project AUGE/11/029 “3D-SPACE: 3D Struc-
tural Platform Aiming for Chemical Excellence”) and the Special
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Manuscript received: November 13, 2020
Revised manuscript received: January 20, 2021
Accepted manuscript online: January 21, 2021
Version of record online: March 11, 2021
Chem. Eur. J. 2021, 27, 6479 –6488
www.chemeurj.org
6488
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