A lanthanide-functionalized covalent triazine framework as a physiological molecular thermometer

Article abstract of DOI:10.1039/d1tc00999k

Crystalline covalent triazine frameworks (CTFs) with intrinsic porosity and high stability are an excellent platform for engineering luminescence properties, as their building blocks and guest ions are all important factors in light emission. Herein, a highly crystalline bipyridine-based CTF (Bipy-CTF) is synthesized under mild conditions. The controlled tethering of lanthanide ions (Ln = Eu3+ and Tb3+) onto Bipy-CTF combined with selective photoexcitation results in a ratiometric luminescent thermometer (LnCTF). This LnCTF thermometer exhibits an excellent linear response in the solid state over a wide range of temperatures (200-340 K), with a temperature uncertainty below 0.2% and very good reusability (up to 98.5% repeatability). Moreover, the suspended material in water shows a temperature sensitivity down to 253 K (-20 °C), which is a very important finding for monitoring the physiological processes within biological and biochemical systems during freezing/defrosting treatment with precise temperature measurements. We also studied and confirmed the low cytotoxicity of the LnCTF towards cells thereby opening prospects for future in vivo applications. This work thus highlights a new application of LnCTF materials as ratiometric luminescent molecular thermometers with excellent sensitivity in the physiological temperature range.

Full text of DOI:10.1039/d1tc00999k

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A lanthanide-functionalized covalent triazine
framework as a physiological molecular
Cite this: DOI: 10.1039/d1tc00999k
thermometer†
a
Parviz Gohari Derakhshandeh, ‡a Sara Abednatanzi,‡a Laurens Bourda,
f
Sasanka Dalapati,ab Anatolii Abalymov,c Maria Meledina,de Ying-Ya Liu,
c
a
Andre G. Skirtach,
Pascal Van Der Voort
Kristof Van Hecke,
Anna M. Kaczmarek *a and
a
*
Crystalline covalent triazine frameworks (CTFs) with intrinsic porosity and high stability are an excellent
platform for engineering luminescence properties, as their building blocks and guest ions are all important
factors in light emission. Herein, a highly crystalline bipyridine-based CTF (Bipy-CTF) is synthesized under
mild conditions. The controlled tethering of lanthanide ions (Ln = Eu3+ and Tb3+) onto Bipy-CTF combined
with selective photoexcitation results in a ratiometric luminescent thermometer (LnCTF). This LnCTF
thermometer exhibits an excellent linear response in the solid state over a wide range of temperatures
(200–340 K), with a temperature uncertainty below 0.2% and very good reusability (up to 98.5% repeatabil-
ity). Moreover, the suspended material in water shows a temperature sensitivity down to 253 K (ꢀ20 1C),
which is a very important finding for monitoring the physiological processes within biological and biochem-
ical systems during freezing/defrosting treatment with precise temperature measurements. We also studied
and confirmed the low cytotoxicity of the LnCTF towards cells thereby opening prospects for future in vivo
applications. This work thus highlights a new application of LnCTF materials as ratiometric luminescent
molecular thermometers with excellent sensitivity in the physiological temperature range.
Received 3rd March 2021,
Accepted 12th April 2021
DOI: 10.1039/d1tc00999k
rsc.li/materials-c
aromatic triazine linkages, were reported for the first time in
2008 by Thomas and Antonietti.15 They form a subclass of COFs
Introduction
Covalent organic frameworks (COFs) can be described as crystalline, and are especially interesting because of their high surface area,
ordered and porous organic polymers. They have huge potential due high nitrogen content and high physicochemical and thermal
to their crystalline structure, high porosity, and structural stability.16,17 Current strategies to design crystalline COFs involve
diversity.1,2 COFs have been designed for diverse applications reversible covalent bond formation, referred to as the ‘‘error-
including gas adsorption and separation, luminescence and correcting’’ reversible bond formation approach. On the downside,
sensing, catalysis, optoelectronics, drug delivery, water purifica- COFs synthesized by highly reversible bond formation often show
tion, CO2 capture, energy storage and energy conversion.3–14 poor stability. Although COFs based on imine linkages (CQN) are
Covalent triazine frameworks (CTFs), identified by their typical more robust than those with B–O bonds, they still suffer from
hydrolysis under acidic conditions.18,19 CTFs, on the other hand,
a Department of Chemistry, Ghent University, Krijgslaan 281-S3, Ghent 9000,
Belgium. E-mail: Anna.Kaczmarek@UGent.be, Pascal.VanDerVoort@UGent.be
b Department of Materials Science, School of Technology, Central University of
Tamil Nadu (CUTN), Thiruvarur-610005, Tamil Nadu, India
form highly conjugated and stable materials, but often show
moderate to poor crystallinity.
To date, a variety of approaches have been reported for the
preparation of CTFs, such as the cyclization of nitrile or amide
aromatic building blocks, using ZnCl2 (ionothermal), P2O5
(ionothermal), CF3SO3H and AlCl3 as catalysts.15,20–23 However,
the high temperature (400–500 1C) of the ionothermal synthetic
pathways leads to non-photoactive materials due to partial
carbonization. On the other hand, the other two catalytic
pathways using triflic acid or AlCl3 only show good results for
a limited number of building blocks and amorphous materials
are typically obtained. Recently, Cooper et al. developed a novel
c Department of Biotechnology, Faculty of Bioscience Engineering, Ghent University,
Ghent 9000, Belgium
d RWTH Aachen University, Central Facility for Electron Microscopy, D-52074,
Aachen, Germany
e
¨
¨
Forschungszentrum Julich GmbH, Ernst Ruska-Centre (ER-C 2), D-52425, Julich,
Germany
f State Key Laboratory of Fine Chemicals, Dalian University of Technology, 116024,
Dalian, P. R. China
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1tc00999k
‡ These authors contributed equally to this work.
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synthesis strategy involving the condensation reaction of aldehydes Bipy-CTF greatly improve the delocalization of p electrons to
and amidines to make CTFs under relatively mild conditions.24 This overcome ACQ. These features make Bipy-CTF an excellent
method provides a great opportunity to design novel porous candidate for photoluminescence applications.18,19
structures using a variety of building blocks.25–29 The resulting
In this work, we present a novel application of CTFs as photo-
materials are typically obtained as yellow or orange powders, luminescent materials. For this purpose, we designed a bipyridine-
retaining most of their photophysical properties, while still not based CTF via the polycondensation reaction of Bipy containing
being crystalline. Tan et al. found that slowing down the nucleation monomers and 1,4-benzenedimethanol. The nitrogen-rich
process through the in situ oxidation of alcohol monomers into material provides excellent docking sites for the immobilization
aldehydes during the polycondensation reaction results in highly of various guests, examined for two different lanthanide guests
crystalline CTFs.30 By introducing metal coordination sites, such as (Eu3+ and Tb3+). The Bipy sites not only act as anchoring points
in the case of 2,20-bipyridine-based CTF (Bipy-CTF), metal for lanthanides but also enable the excitation of Ln3+ ions
complexes can be tethered onto the CTFs. Such materials have through intramolecular energy transfer known as the ‘‘antenna
already shown great results in heterogeneous catalysis.31 effect’’. The obtained materials exhibit excellent temperature
Clearly, also photoactive CTFs can be synthesized in this way.
sensing properties in the physiological range. The Bipy-CTF
Temperature is a fundamental thermodynamic parameter, the before and after immobilization with lanthanides showed low
measurement of which is crucial in both science and industrial cytotoxicity to human fibroblastic cells even at high concentration
applications. Generally, traditional thermometers such as ther- and showed excellent temperature sensing properties, also in
mistors, thermocouples and liquid-filled thermometers are water. To the best of our knowledge, this is the first report on
not applicable at the submicrometer scale. Therefore, precise the applicability of LnCTFs as photoluminescent materials for
temperature measurement is still a challenging task in the physiological molecular thermometers.
fields of nanotechnology and nanomedicine, but it is essential for
biomedical applications, for example, intracellular release.32 This
Experimental section
has encouraged scientists to develop new temperature sensors to
Synthesis of 2,20-bipyridine-5,50-diamidine dihydrochloride
measure temperatures with micrometric and nanometric precision.
Luminescent thermometers are of great interest due to their
brilliant color visibility and temperature-dependent luminescence
features. High sensitivity, instant response, noninvasive functioning
and applicability in strong magnetic or electric fields are some of
the important benefits of luminescent thermometers compared to
other molecular thermometers. Trivalent lanthanides (Ln3+)
have attracted comprehensive attention and are often used in
luminescence applications due to their unique spectroscopic
characteristics, particularly, narrow band emission, long life-
time, and high quantum yield values.
The integration of lanthanide ions or complexes with organic
bridging linkers is a well-studied methodology to overcome the
low absorption coefficients of lanthanides. We and others have
prepared lanthanide-tethered metal–organic frameworks (MOFs),
coordination polymers (CPs) and periodic mesoporous organo-
silicas (PMOs).33–40 Still, some of these materials such as LnMOFs
and CPs suffer from low stability and low water dispersibility and
are cytotoxic. As COFs have a very low biological toxicity,41,42 the
immobilization of lanthanide (Ln) ions onto COFs can result in
excellent temperature sensors arising from their multiple lumines-
cence centers and highly tunable luminescence properties.43–45
However, most of the COFs are non-luminescent, because of
non-radiative deactivation arising from intramolecular bond
rotation and aggregation-caused quenching (ACQ) driven by
the p–p stacking in the structure. In this regard, the confine-
ment of intramolecular bond rotation and/or the construction
of relatively rigid frameworks is one approach to enhance the photo-
luminescence activity by minimizing non-radiative deactivation.
To a solution of 2,20-bipyridine-5,50-dicarbonitrile (0.412 g,
2.0 mmol) in 8 mL of THF, 8 mL of 1 M LiN(SiMe3)2 solution
was added dropwise over the time of 30 min at 0 1C. The
mixture was stirred at 25 1C for 24 h and then cooled to 0 1C.
The reaction was quenched by careful addition of 6 M HCl–
EtOH (8 mL) and the mixture was left overnight. The solid
sample was then filtered and washed with diethyl ether. Then,
the solid was recrystallized from the H2O–EtOH mixture. Yield:
(0.767 g, 96%). 1H NMR (400 MHz, DMSO-d6): d = 9.80 (s, 4H,
NH), 9.53 (s, 4H, NH), 9.18 (d, 2H, aromatic H), 8.63 (d, 2H,
aromatic H), 8.47 (d, 2H, aromatic H), 13C NMR (75 MHz, D2O)
d = 164.36, 158.19, 148.30, 138.15, 125.65, 122.81.
Synthesis of Bipy-CTF
1,4-Benzenedimethanol (0.069 mg, 0.5 mmol), 2,20-bipyridine-
5,50-diamidine dihydrochloride (0.311 g, 1.0 mmol), and
cesium carbonate (0.700 g, 2.2 mmol) were added to a DMSO
solution (15.0 mL) in a round-bottom flask (100 mL). The
mixture was heated at 100 1C for 24 h, then the temperature
was increased to 150 1C for 36 h. The precipitate of the yellow
suspension was washed with dilute HCl (1 M, 3 ꢁ 20 mL) to
remove residual cesium carbonate, and further washed with
water (3 ꢁ 30 mL), EtOH (3 ꢁ 30 mL), and THF (3 ꢁ 20 mL). The
filtered solid was freeze-dried for 24 h to yield Bipy-CTF as a
yellow powder (150 mg, 84% yield).
Synthesis of Ln@Bipy-CTF
This is the main reason that we targeted CTFs, as they have a rigid The samples were prepared by weighting of 0.005 mmol of the
framework built by triazine linkages and show minimal intra- Bipy-CTF and 0.005 mmol of Ln(bzac)3ꢂ2H2O (bzac = benzoyl-
molecular rotation by ‘exocyclic’ covalent bonds. Besides, the acetone) complexes and both powders were placed in a Pyrex
organic donor–acceptor pairs and crystalline structure of the tube. After the addition of dichloromethane (5 mL), the sealed
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tube was placed in an ultrasound bath for 10 minutes. Then,
The experimental and simulated structural X-ray diffraction
the tube was heated on a heating block at 45 1C for 24 h. (XRD) patterns of Bipy-CTF are shown in Fig. 1D and E. Powder
Afterwards, the filtered powder was washed with a tiny amount XRD was used to characterize the crystalline nature of the synthe-
of methanol and dried at 80 1C.
sized material. Bipy-CTF exhibits two major peaks, one is centered
at 5.51, which is assigned to the combination of the (110) and (200)
reflection and a broad peak at 26.81, indicative of the d-spacing
between layers or (001) reflection. Some smaller peaks matching
with the predicted Bragg peaks could also be observed in the
Results and discussion
The Bipy-CTF material was synthesized by the condensation of background-corrected pattern, namely, the combined (310) and
2,20-bipyridine-5,50-diamidine dihydrochloride with 1,4 benze- (220) reflections centered at 9.91 and the (420) reflection at 14.61.
nedimethanol (Fig. 1A and B). The successful formation of the For the Bipy-CTF, the eclipsed (AA) and staggered (AB) structures
triazine frameworks was confirmed by diffuse reflectance infra- were compared (Fig. 1D and Fig. S14, ESI†). The eclipsed stacking
red Fourier transform spectroscopy (DRIFTS) analysis, revealing the structure matches best with the theoretical XRD pattern and with
characteristic stretching vibrations from the triazine units at B1516 the experimental pattern. Subsequently, Pawley refinements were
and 1351 cmꢀ1 for Bipy-CTF (Fig. 1C). Also, the disappearance carried out for the eclipsed structure of Bipy-CTF and an optimized
of the intense aldehyde band (B1716 cmꢀ1) and broad O–H band structure was assigned to the Cmmm space group with unit cell
(B3300 cmꢀ1) is indicative of the complete consumption of the parameters of a = 30.95 Å, b = 20.79 Å and c = 3.50 Å with refined
monomers to form triazine linkages. The carbon and nitrogen parameters of Rp =1.51% and wRp = 2.22%. The diffractograms
contents obtained from elemental analysis are in close agreement show good crystallinity for the Bipy-CTF material, rarely observed
with the theoretical values (Table S1, ESI†).
for the previously reported CTFs.
Fig. 1 (A) Scheme showing the triazine framework synthesis through the in situ formation of aldehyde monomers by the controlled oxidation of alcohol
monomers and condensation with amidine, and (B) LnCTF (Ln = Eu, Tb) synthesis. (C) DRIFTS of Bipy-CTF, diamidine linker, terephthalaldehyde and
1,4-benzendimethanol. (D) Background corrected experimental (black) and Pawley refined (red) PXRD patterns of Bipy-CTF AA stacking, accompanied by
the difference plot (blue) and Bragg peak positions (green) of the model. (E) Top and side views of the Bipy-CTF structure; carbon is shown in grey,
nitrogen in blue, and hydrogens were omitted for clarity. (F) Nitrogen (N2) adsorption/desorption isotherms of Bipy-CTF (black) and EuTb@Bipy-CTF
(red). (G) Solid-state 13C CP/MAS NMR spectra of Bipy-CTF. (H) TGA data of Bipy-CTF (black) and EuTb@Bipy-CTF (red) measured under air atmosphere.
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The porosity of Bipy-CTF was determined by N2 sorption Eu3+ and Tb3+ complexes on the Bipy-CTF was confirmed by
measurements at 77 K. As shown in Fig. 1F, the Bipy-CTF DRIFTS analysis (Fig. S3, ESI†). In the DRIFTS spectra of the
displayed typical type I isotherm, indicative of a microporous EuTb@Bipy-CTF sample, intense bands at 1599 cmꢀ1 (CQO) and
material. The Bipy-CTF exhibited a Brunauer–Emmett–Teller 1569 cmꢀ1 (CQC) are observed, which are attributed to the
(BET) and Langmuir surface area of 439 and 635 m2 gꢀ1
respectively. The total pore volume is 0.23 cm3 gꢀ1 at P/P0
,
=
presence of lanthanide complexes in the framework. Moreover,
the bands around 2850–3060 cmꢀ1 are related to n(C–H) and are
0.98. As shown in Fig. 1G, the 13C CP/MAS solid-state NMR also present in the spectra of the EuTb@Bipy-CTF sample.
spectrum of Bipy-CTF exhibits five well-resolved signals at 121, Additionally, because of its coordination with the lanthanides,
130, 137.5, 149.9, and 157.5 ppm, which can be assigned to the CQN peak of the bipyridine linker shifts from 1685 to
the Bipy carbons. Furthermore, the appearance of a peak at 1675 cmꢀ1 in the EuTb@Bipy-CTF. In addition, the overall N2
169.8 ppm is characteristic of the carbon signal from the uptake of EuTb@Bipy-CTF decreased upon grafting, indicating
triazine rings which clearly confirms the formation of triazine the successful immobilization of lanthanides within the porous
units in the Bipy-CTF.24,30 The thermogravimetric analysis channel of the CTF framework (Fig. 1F). The BET and Langmuir
(TGA) reveals excellent thermal stability of the Bipy-CTF mate- surface area of EuTb@Bipy-CTF decreased notably to 50 and
rial with no weight loss up to B600 1C (Fig. 1H). Furthermore, 89 m2
g
ꢀ1, respectively, and the total pore volume reduced to
ꢀ1. The crystallinity of the hosting network is also
scanning electron microscopy (SEM) shows micrometer-sized 0.06 cm3
g
particles with no particular shape (Fig. S1, ESI†). Transmission clearly visualized by means of ADF-STEM (Fig. 2b). TEM-EDX
electron microscopic (TEM) and high resolution (HR)-TEM images analysis of LnCTF was carried out to check the presence of the Eu
were taken to further investigate the microscopic structure analysis and Tb elements in the Bipy-CTF after post-modification (Fig. S4,
of the Bipy-CTF. The layered and crystalline structure of Bipy-CTF ESI†). The observed results revealed that both Eu and Tb ions are
with an ordered lattice structure is visible in ADF-STEM and well distributed within the EuTb@Bipy-CTF skeletons. The optical
HR-TEM images (Fig. 2a and Fig. S2, ESI†).
absorption properties of the yellowish pristine Bipy-CTF sample
As mentioned above, the key to resolve the limitations of were measured by diffuse reflectance (DR)-UV-visible spectrophoto-
photoluminescent materials lies in the development of crystalline metry. It was observed that the material could absorb a wide range
porous materials with limited intramolecular rotation and a of visible light (Fig. S5, ESI†). The luminescence properties of
delocalized p electron system while the stability is still preserved. the pristine Bipy-CTF were first investigated at room tempera-
Accordingly, the crystalline, stable, porous Bipy-CTF material was ture (Fig. S6, ESI†). Upon excitation with UV light, the Bipy-CTF
applied to examine its potential application as a luminescent shows broad emission in the range of 450–750 nm, with a peak
molecular thermometer in the physiological temperature range. maximum centered at 575 nm. The luminescence properties of
The Bipy-CTF material comprises numerous N functionalities, the materials were also studied after grafting with lanthanide
including triazine and bipyridine edges that form ideal docking complexes. Fig. 3A demonstrates a picture of the four samples,
sites for grafting Eu3+ and Tb3+ complexes simultaneously via a Tb@Bipy-CTF (1), Eu@Bipy-CTF (2), 3%Eu,97%Tb@Bipy-CTF
post-modification procedure (Fig. 1B). The immobilization of (3) and 5%Eu,95%Tb@Bipy-CTF (4), upon exposing to a UV
Fig. 2 ADF-STEM image of (a) pristine Bipy-CTF and (b) Bipy-CTF after loading together with the corresponding FFT in the inset revealing the crystallinity
of the materials.
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Fig. 3 (A) Photograph taken when placing 1, 2, 3 and 4 (from left to right) samples under a UV lamp (302 nm excitation). Combined excitation–emission
spectra of (B) 1, (C) 2, (D) 3 and (E) 4 compounds. The labeled peaks have been assigned in the ESI† (Tables S2–S5).
lamp at 302 nm excitation wavelength. It can be observed that and is presented in Figs S10 and S11, ESI†). Fig. 4B represents the
the emitted colors are green, red and yellow, respectively (with plot that shows the calibration curve. The maximum relative tem-
different yellow undertones).
perature sensitivity, Sr, was determined to be 2.30% Kꢀ1 (240 K)
All four compounds show strong visible emission with the (Fig. 4C). For compound 4 also, the I549/I610 ratio of the integrated
characteristic f–f transitions of the lanthanides upon exciting areas was calculated (Tb3+: 530–565 and Eu3+: 605–630). The emis-
into the maximum of the broad linker band originating from sion map recorded at different temperatures (253–333 K) is pre-
the pristine CTF (250–400 nm) (Fig. 3). The assignment of sented in Fig. 4D. These data points could be well fitted using the
the labelled peaks in the spectra are presented in the ESI† equation of D (R2 = 0.985) yielding DE = 948 cmꢀ1 (Fig. 4E). Also, in
(Tables S2–S5). The decay times of the compounds are also the case of this material, the nonradiative deactivation energies
presented in the ESI† (Fig. S7–S9 and Table S6). In addition, we probably involve the ligand triplet level and the 5D4 level of Tb3+.
studied the temperature-dependent luminescence characteris- The plot, showing the calibration curve, is presented in Fig. 4E.
tics of two Eu–Tb co-doped compounds, as it is known that this The maximum value of Sr was determined to be 2.12% Kꢀ1
lanthanide pair can be used for developing optical thermo- (253 K) (Fig. 4F). The schematic representation of the energy
meters. Ratiometric thermometers are thermometers based on levels of the Tb3+ and Eu3+ and the singlet and triplet levels of
the intensity (wavelength) ratio of two transitions (Eu3+ and the ligands along with the possible ways of energy transfer from
Tb3+ in this case) that suffer from certain limitations such as Tb3+ to Eu3+ is given in Scheme S1 (ESI†). Both materials show
fluctuations of the excitation source. Here, the emission spectra good thermometric performance in the extended physiological
of the samples were recorded at various temperatures, from range compared to the previous reports (Table S7, ESI†). For
200 to 340 K (Fig. 4).
both compounds, the uncertainty was calculated and was
For compound 3, the I549/I610 ratio of the integrated areas determined to be very low, below 0.2%, which means that these
was calculated (Tb3+: 530–565 and Eu3+: 605–630) in the 200–340 K thermometers show very good performance in the solid state
temperature range (Fig. 4A). The obtained data points could be (Fig. 4G and H). To additionally prove that they can be reused
fitted excellently using the equation of D (R2 = 0.996) yielding for several measurement runs, compound 3 was tested for its
DE = 1203 cmꢀ1 (Fig. 4B). The nonradiative deactivation recyclability. It showed up to 98.5% repeatability (Fig. 4I).
energies might involve the ligand triplet level and the 5D4 level
of Tb3+ (the triplet level of the Bipy-CTF has been determined in the physiological temperature range, we further investigated
Inspired by the good performance of the LnCTF thermometers
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Fig. 4 (A) Emission map of 3 recorded between 200 and 340 K with a step size of 20 K. (B) Plot presenting the calibration curve of 3 when the equation
of D is employed. The points show the experimental delta parameters and the solid like presents the best fit of the experimental points. (C) Plot presenting
the relative sensitivity, Sr, values at varied temperatures (200–340 K) for compound 3. (D) Emission map of 4 recorded between 253 and 333 K with a step
size of 10 K. (E) Plot presenting the calibration curve of 4 when the equation of D is employed. The points show the experimental delta parameters and the
solid like presents the best fit of the experimental points. (F) Plot presenting the relative temperature sensitivity, Sr, values at varied temperatures
(253–333 K) for compound 4. The solid line is a guide to increase visibility. (G) Graph presenting the temperature uncertainty for compound 3 and (H)
compound 4. (I) Cycle test showing the reusability of the compound 3 thermometer material (98.5% repeatability).
their performance in water. Moreover, we performed cytotoxicity sample is solvatochromic and in water the ratio of Tb-to-Eu may
tests on human cells. Sample 3 was also selected for studying change compared to the powder, which influences the final
the thermometric properties in water. For the temperature- thermometric performance of the material. The temperature
dependent luminescence measurements, a suspension was uncertainty, dT, is plotted in Fig. 5D. The increase in dT is due to
prepared by adding 1 mg of the sample in 1 mL of distilled a weaker signal of the material when measured in water
water. The suspension demonstrated a light red emission color compared to the solid state. However, at maximum, it only
when illuminated by a UV lamp (302 nm). The thermometric reaches up to 0.3 K, which is a very good value. This additional
properties of the EuTb@Bipy-CTF compound were measured dataset indicates that the lanthanide-grafted CTF materials
over 283.15–323.15 K, with a step size of 5 K. The emission map show good performance in a colloidal water suspension, con-
of the material is presented in Fig. 5A. The data points are firming their potential application as biological thermometers.
well fitted by using the equation of D (R2 = 0.992) giving DE =
To completely evaluate the suitability of these materials for
1506 cmꢀ1 (Fig. 5B). The maximum relative sensitivity, Sr, was employing in biomedical systems, it is essential to investigate
calculated to be 2.70% Kꢀ1 (283.15 K) (Fig. 5C). Due to the their cytotoxicity towards living cells. Accordingly, we carried
stronger influence of signal-to-noise ratio on the measurements out experiments using normal human dermal fibroblastic cells
performed in water, we have included error bars on the plots of (NHDF cells). The viability tests of the cells demonstrated that
Delta and Sr. This result is somewhat higher than that reported the Bipy-CTF and Ln@Bipy-CTF materials are non-toxic at lower
for powder samples. This can be explained by the fact that the concentrations (Fig. 6A). The particle toxicity towards fibroblastic
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Fig. 5 (A) Emission map of sample 3 recorded between 283.15 and 323.15 K with a step size of 5 K in a water suspension. (B) Plot presenting the
calibration curve of compound 3 when the equation of D is employed. The points show the experimental delta parameters and the solid like presents the
best fit of the experimental points. (C) Plot presenting the relative sensitivity, Sr, values at varied temperatures (283.15–323.15 K) for compound 3 in water
suspension. (D) Graph presenting the temperature uncertainty for 3 measured in water suspension.
Fig. 6 (A) Graph presenting the cell viability (%) after exposure to different concentrations of the Bipy-CTF and compound 2 particles. An asterisk (*)
indicates significant differences from the control cell group. Statistical analysis was performed by ANOVA followed by the Tukey test (p o 0.05). (B)
Fluorescence microscopy images of the cells at different compound concentrations (cells are green). The scale bar is 250 mm.
cells occurred only at very high concentrations of 1 and the cells. The fluorescence images show the morphology of
0.5 mg per well for Bipy-CTF and Ln@Bipy-CTF, respectively, the cells after incubation with the particles (Fig. 6B). The fusi-
that might be due to the mechanical pressure of the particles on form morphology of the cells incubated with Bipy-CTF and
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Ln@BipyCTF at 0.1 mg mlꢀ1 and lower concentrations are Acknowledgements
comparable to control which fully correlates with the cell viability
PGD and SA gratefully acknowledge the financial support from
test. High stability, very good thermometric performance in the
solid-state and water and, nontoxicity to human cells make these
compounds very promising for use in biological systems.
the Research Foundation Flanders (FWO Vlaanderen, grant no.
G000117N). SA acknowledges the financial support from the
Ghent University BOF doctoral grant 01D04318. SD would like
to acknowledge the ‘‘European Union’s Horizon 2020 research
and innovation programme under the Marie Sklodowska-Curie
grant agreement No. 840011’’ for the funding support. AGS
acknowledges the support from FWO Flanders (I002620N,
G043219) and BOF UGent (01IO3618, BAS094-18). Y-YL would like
to acknowledge the International S&T Cooperation Program of
China (2016YFE0109800). MM wants to acknowledge the Verbund-
vorhaben iNEW: Inkubator Nachhaltige Elektrochemische
Conclusions
Here, we report the synthesis of a new bipyridine-based CTF,
where the bipyridine linker was purposefully introduced for the
selective docking of lanthanide ions. We successfully synthe-
sized a highly crystalline, porous and robust Bipy-CTF via the
in situ generation of an aldehyde intermediate. We applied a
polycondensation strategy by using a simple bipyridine amidine
monomer, which opens up immense possibilities of monomer
design for synthesizing new crystalline CTFs under mild conditions.
The organic donor–acceptor pairs and the ordered structure of the
conjugated Bipy-CTF moiety greatly improve the delocalization of p
electrons to overcome nonradiative energy dissipation. With these
collective advantages of crystalline and porous Bipy-CTF in hand,
docking various lanthanide ions not only enables luminescence
color tuning but also facilitates the excitation of lanthanide ions
through the antenna effect. The presence of bipyridine docking sites
within the porous channel wall of the Bipy-CTF allows an easy and
controlled grafting of lanthanide complexes, which opens up a new
avenue for designing LnCTF-based ratiometric luminescent thermo-
meters. The EuTb@Bipy-CTF thermometer exhibited an excellent
linear response over a wide range of temperatures from 200 to
340 K, with a temperature uncertainty below 0.2% and very good
reusability (up to 98.5% repeatability). Moreover, the thermometer
also exhibited excellent performance in the physiological tempera-
ture range when dispersed in water. High crystallinity and stability,
outstanding thermometric performance in the solid-state as well as
in water, and nontoxicity to human fibroblastic cells make this
LnCTF material a very promising candidate as a physiological
molecular thermometer.
¨
Wertschopfungsketten with the funding number 03SF0589A
(German Federal Ministry for Economic Affairs and Energy
BMWi). KVH acknowledges the Special Research Fund (BOF) –
UGent (project 01N03217) for funding. AA acknowledges the
support from the program ‘‘Global Education’’. AMK would like to
thank Ghent University for BOF funding (BOF/STA/202002/004).
The authors thank Mr Olivier Janssens for SEM measurements.
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Conflicts of interest
There are no conflicts to declare.
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