Exceptional oxygen sensing properties of new blue light-excitable highly luminescent europium(III) and gadolinium(III) complexes

Article abstract of DOI:10.1002/adfm.201401754

New europium(III) and gadolinium(III) complexes bearing 8-hydroxyphenalenone antenna combine efficient absorption in the blue part of the spectrum and strong emission in polymers at room temperature. The Eu(III) complexes show characteristic red luminescence whereas the Gd(III) dyes are strongly phosphorescent. The luminescence quantum yields are about 20% for the Eu(III) complexes and 50% for the Gd(III) dyes. In contrast to most state-of-the-art Eu(III) complexes the new dyes are quenched very efficiently by molecular oxygen. The luminescence decay times of the Gd(III) complexes exceed 1 ms which ensures exceptional sensitivity even in polymers of moderate oxygen permeability. These sensors are particularly suitable for trace oxygen sensing and may be good substitutes for Pd(II) porphyrins. The photophysical and sensing properties can be tuned by varying the nature of the fourth ligand. The narrow-band emission of the Eu(III) allows efficient elimination of the background light and autofluorescence and is also very attractive for use e.g., in multi-analyte sensors. The highly photostable indicators incorporated in nanoparticles are promising for imaging applications. Due to the straightforward preparation and low cost of starting materials the new dyes represent a promising alternative to the state-of-the-art oxygen indicators particularly for such applications as, e.g., food packaging.

Full text of DOI:10.1002/adfm.201401754

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Exceptional Oxygen Sensing Properties of New Blue
Light-Excitable Highly Luminescent Europium(III) and
Gadolinium(III) Complexes
Sergey M. Borisov,* Roland Fischer, Robert Saf, and Ingo Klimant
to be reliable analytical tools and repre-
New europium(III) and gadolinium(III) complexes bearing 8-hydroxyphenal-
enone antenna combine efficient absorption in the blue part of the spectrum
and strong emission in polymers at room temperature. The Eu(III) complexes
show characteristic red luminescence whereas the Gd(III) dyes are strongly
phosphorescent. The luminescence quantum yields are about 20% for the
Eu(III) complexes and 50% for the Gd(III) dyes. In contrast to most state-of-
the-art Eu(III) complexes the new dyes are quenched very efficiently by molec-
ular oxygen. The luminescence decay times of the Gd(III) complexes exceed
1 ms which ensures exceptional sensitivity even in polymers of moderate
oxygen permeability. These sensors are particularly suitable for trace oxygen
sensing and may be good substitutes for Pd(II) porphyrins. The photophysical
and sensing properties can be tuned by varying the nature of the fourth
ligand. The narrow-band emission of the Eu(III) allows efficient elimination of
the background light and autofluorescence and is also very attractive for use
e.g., in multi-analyte sensors. The highly photostable indicators incorporated
in nanoparticles are promising for imaging applications. Due to the straight-
forward preparation and low cost of starting materials the new dyes represent
a promising alternative to the state-of-the-art oxygen indicators particularly
for such applications as, e.g., food packaging.
sent a promising alternative to the Clark
electrode. The materials used in optical
oxygen sensors are quite established.[1–4]
Most oxygen sensors employ such wide-
spread indicators as ruthenium(II)
polypyridyl complexes,[5–7] platinum(II)
and palladium(II) porphyrins[8–11] and
their analogues.[12–14] Other lumines-
cent indicators are less popular which is
explained by their inferior photophysical
properties (low absorption coefficients
and luminescence quantum yields, short
luminescence decay times, poor photosta-
bility etc.)[3] or by complicated multi-step
synthesis. Such indicators as cyclometa-
lated iridium(III) and platinum(II) com-
plexes,[15–17] osmium(II),[18] rhenium(I)[19]
and copper(I)[20,21] complexes should also
be mentioned. Europium(III) complexes
represent a particularly interesting group
of luminescent compounds which are dis-
tinguished by their very large Stokes shifts,
long luminescence decay times (hundreds
of µs – few ms) and very narrow emis-
sion bands.[22] These complexes are widely
1. Introduction
applied as labels in various luminescence assays both as free
dyes bearing a functional group for conjugation and in the form
of dye-doped nanoparticles,[22,23] but application in optical sen-
sors and analyte-sensitive probes is comparably rare.[24–27] For
example, luminescent Eu(III) complexes were applied as optical
temperature probes,[27–31] pH indicators,[32–36] fl uoroionophores
for bicarbonate,[37,38] citrate[39] and lactate[40] and as hydrogen
peroxide[41] and nitrogen monooxide probes.[42] Several Eu(III)
complexes were applied as luminescent oxygen indicators.[43–48]
However, the sensitivity of these oxygen-sensing materials is
rather low (quenching at 100% O2 I0/I not exceeding 3-4) for
them to be suitable for measurements in physiologically rele-
vant conditions with good resolution. Moreover, UV light was
required for the excitation of all the indicators which is not
attractive for many reasons. The number of Eu(III) complexes
excitable in the visible part of electromagnetic spectrum is
very limited. The drawbacks include low luminescence bright-
ness,[37,41,49] poor chemical stability,[50] extremely laborious
multi-step synthesis[51] or rather poor photostability.[30,52]
The sensing properties of Gd(III) complexes remain mostly
unexplored. Despite the fact that these complexes are widely
Monitoring of oxygen is of high importance in numerous
fields of science and technology. Optical oxygen sensors proved
Dr. S. M. Borisov, Prof. I. Klimant
Institute of Analytical Chemistry
and Food Chemistry
Graz University of Technology
NAWI Graz
Stremayrgasse 9 8010, Graz, Austria
E-mail: sergey.borisov@tugraz.at
Prof. R. Fischer
Institute of Inorganic Chemistry
Graz University of Technology
NAWI Graz
Stremayrgasse 9 8010, Graz, Austria
Prof. R. Saf
Institute of Chemistry and Technology of Materials
Graz University of Technology
NAWI Graz
Stremayrgasse 9 8010, Graz, Austria
DOI: 10.1002/adfm.201401754
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used for determination of the triplet state energy of the respec-
tive ligands at 77K there are very few reports on luminescence
of the Gd(III) chelates at room temperature (RT). For example,
Strasser and Vogler reported on luminescence of UV light-
excitable chelates at RT[53]; we[54] and others[55] recently dem-
onstrated that some Gd(III) chelates also possess viable lumi-
nescence at RT. Much lower cost of europium and gadolinium
compared to the cost of platinum group metals can make
them attractive substitutes to the state-of-the-art indicators pro-
viding that the photophysical properties are more favorable and
quenching by oxygen is efficient. Additionally, Gd(III) may be
promising as materials for simultaneous oxygen and magnetic
resonance imaging, which is by far the most important applica-
tion of the non-emissive Gd(III) chelates.[56–58]
In this contribution we will present a series of new Eu(III)
and Gd(III) complexes which combine efficient excitation in
the blue part of the electromagnetic spectrum, strong emis-
sion and efficient quenching by molecular oxygen. It will be
shown that 9-hydroxy-1H-phenalen-1-one, in contrast to an
early report,[59] represents an efficient antenna for sensitizing
luminescence of the lanthanides under visible light and thus
represents an excellent choice for preparation of high-perfor-
mance optical materials. We will demonstrate the application of
the complexes in optical oxygen sensors and will highlight the
advantages over the state-of-the-art indicators.
(784 mg, 4 mmol) in 70 mL of ethanol containing 1 mL of 25%
aqueous ammonium hydroxide. The heating was continued for
5-20 min. The progress of the reaction was controlled via UV–
Vis spectroscopy and bathochromic shift of the absorption peaks
in the visible part of the spectrum was observed. The obtained
yellow precipitate was collected via centrifugation and was washed
3 times with water and once with water : methanol (1:1 v/v) mix-
ture. The crude product was used without further purification.
Yield: 760 mg (78 %) for Eu(HPhN)3; MS (EI): m/z [M]+ 736.0538
found, 736.0536 calc. Yield: 680 mg (69%) for Gd(HPhN)3; MS
(MALDI): m/z [MNa]+ 766.05 found, 766.05 calc.
Europium(III)/gadolinium(III) tris-[9-(hydroxy-kO)-1H-phe-
naleno-1-onato-kO]-1,10
-phenanthroline
(Eu(HPhN)3phen/
Gd(HPhN)3phen): The solution of 1,10-phenanthroline mono-
hydrate (19.8 mg , 0.1 mmol) in toluene (5 mL) was added drop-
wise to the hot dispersion of Eu(HPhN)3 (73.7 mg, 0.1 mmol)
or Gd(HPhN)3 (74.3 mg, 0.1 mmol), respectively, in hot toluene
(5 mL, 100 °C). The heating was continued for 1h, the resulted
solution was rapidly cooled to room temperature and centri-
fuged to remove the remaining sediment. The X-ray quality
crystals were grown by slow diffusion of hexane into the toluene
solution at room temperature. Eu(HPhN)3phen·1.25C7H8.
Yield: 28 mg (31%). MS(MALDI) m/z [M – 1HPhN]+ 723.0827
found, 723.0796 calc. Analysis for C51H29N2O6Eu·1.25C7H8:
C 69.61, H 3.43, N 2.88 found, C 69.48, H 3.81, N 2.71 calc.
Gd(HPhN)3phen·0.75C7H8. Yield: 35 mg (38%). MS(MALDI)
m/z [M – 1HPhN]+ 728.0800 found, 728.0830 calc. Analysis
for C51H29N2O6Gd·0.75C7H8: C 68.15, H 3.40, N 2.79 found, C
68.1, H 3.56, N 2.82 calc.
2. Experimental Section
2.1. Materials
Europium(III)/gadolinium(III)
tris-[9-(hydroxy-kO)-1H-
phenaleno-1-onato-kO]-4,7-diphenyl-1,10-phenanthroline
(Eu(HPhN)3dpp/Gd(HPhN)3dpp): The complexes were pre-
pared analogously to Eu(HPhN)3phen and Gd(HPhN)3phen
but 4,7-diphenyl-1,10-phenanthroline was used instead of
1,10-phenanthroline. Eu(HPhN)3dpp·1C7H8. Yield: 38 mg
(36%) of X-ray quality yellow crystals. MS(MALDI) m/z
[M – 1HPhN]+ 875.1367 found, 875.1425 calc. Analysis for
C63H37N2O6Eu·1C7H8: C 72.20, H 3.76, N 2.35 found, C 72.33,
H 3.9, N 2.41 calc. Gd(HPhN)3dpp·0.75C7H8. Yield: 29 mg
(27%) of X-ray quality yellow crystals. MS(MALDI) m/z [M –
1HPhN]+ 880.1404 found, 880.1459 calc. Analysis for C63H37
N2O6Gd·0.75C7H8: C 71.65, H 3.67, N 2.42 found, C 71.63, H
3.79, N 2.45 calc.
Gadolinium(III) nitrate hexahydrate, 4,7-diphenyl-1,10-phenan-
throline ( = dpp), 10-(2-benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7-
tetramethyl-1H,5H,11H-(1)benzopyropyrano(6,7-8-I,j)
quinolizin-11-one (coumarin C 545 T) and aqueous dispersion
of polystyrene-block-vinylpyrrolidone ( = PS-PVP) were obtained
from Aldrich, europium(III) chloride hexahydrate was from
ABCR, 1,10-phenanthroline monohydrate – from Merck, poly-
styrene (MW 250,000) – from Acros Organics. Polymethacry-
lonitrile ( = PMAN, MW = 20,000) was acquired from Scientific
Polymer Products Inc. (Ontario, NY, USA) and Eudragit Rl-100
from Evonic Industries (Essen, Germany). All the solvents were
supplied by Carl Roth (Germany). Poly(ethylene terephthalate)
(PET) support Melinex 505 was from Pütz (Taunusstein, Ger-
many). Nitrogen (99.999% purity) was obtained from Linde
Gas (Austria). Synthesis of the 9-hydroxy-1H-phenalen-1-one
( = HPhN) is described elsewhere.[60] 1,1′-(9,9-dimethyl-9H-xan-
thene-4,5-diyl)bis-1,1-diphenyl-phosphine oxide (DDXPO) was
prepared according to the literature procedure.[61]
Europium(III)/gadolinium(III)
phenaleno-1-onato-kO]-1,1′-(9,9-dimethyl-9H-xanthene-4,5-
diyl)bis-1,1-diphenyl-phosphine oxide (Eu(HPhN)3DDXPO/
tris-[9-(hydroxy-kO)-1H-
Gd(HPhN)3DDXPO): The complexes were prepared analo-
gously to Eu(HPhN)3phen and Gd(HPhN)3phen but DDXPO
was used instead of 1,10-phenanthroline. After addition of
DDXPO the hot toluene solution was filtered and the X-ray
quality crystals were grown by slowly cooling the solution
from 100 °C to room temperature (cooling rate 10 °C/h).
Yield of Eu(HPhN)3DDXPO: 54 mg (40%). MS(MALDI) m/z
[M – 1HPhN]+ 1153.195 found, 1153.194 calc. Analysis for
C78H53O9P2Eu: C 68.94, H 3.70 found, C 69.49, H 3.96 calc.
Yield of Gd(HPhN)3DDXPO: 46 mg (34%). MS(MALDI) m/z
[M – 1HPhN]+ 1158.2017 found, 1158.1976 calc. Analysis for
C78H53O9P2Gd: C 68.31, H 3.74, found, C 69.22, H 3.95. As
indicated by the elemental analysis some impurity may be
2.2. Synthesis
Europium(III)/gadolinium(III) tris-[9-(hydroxy-kO)-1H-phenaleno-
1-onato-kO] (Eu(HPhN)3/Gd(HPhN)3) : First, europium(III) chlo-
ride hexahydrate (488 mg, 1.32 mmol) or gadolinium(III) nitrate
hexahydrate (596 mg, 1.32 mmol), respectively, was dissolved in
the mixture of water:ethanol mixture (1:4 v/v, 10 mL). This solu-
tion was added dropwise to the warm solution (70 °C) of HPhN
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present in the complex; however the composition of the com-
plex were unambiguously confirmed by X-ray spectroscopy.
Preparation of the Oxygen Sensors: A lanthanide(III) complex
(1.5 mg) and polystyrene (200 mg) were dissolved in toluene
(1.8 g) and the resulted “cocktail” was knife-coated on the PET
support to result in about 2.5 µm-thin films after evaporation
of the solvent. The ratiometric sensors for RGB imaging were
prepared analogously but the “cocktail” additionally contained
0.3 mg of coumarin C545T.
Preparation of the Nanoparticles: Eu(HPhN)3dpp was incor-
porated into PS-PVP nanoparticles according to the previously
reported procedure.[62] The Rl-100 and PMAN beads were
obtained via the precipitation procedure[63] from the polymer
solutions in tetrahydrofuran:acetone (1:1 v/v). The concentra-
tion of the polymers in the solvent mixture was 0.2% wt. and
fast addition of water was performed. All nanoparticles con-
tained 1% wt. of the Eu(III) complex in respect to the polymer.
Photobleaching of the sensors was investigated using a high
power blue LED array (Germany, www.led-tech.de) operated
at 7.4 W input power; photon flux about 4000 µmol·s−1m−2 of
photons as estimated by a Li-250A Light meter from Li-COR.
The absorption changes were monitored on a Cary 50 UV–Vis
spectrophotometer.
RGB photographic imaging was performed with a Canon 5D
digital camera equipped with a 24–105 mm f/4L IS objective
from Canon. Excitation of the sensor foils was performed with
a 366-nm line of a mercury lamp. The photographic images
of the dispersions of the nanobeads were determined with the
same equipment; however a blue 470 nm LED (Roithner) was
used for the excitation. A plastic dark Amber filter from LEEfil-
ters was used in front of the objective.
2.4. Crystal Structure Determination
Suitable crystals for X-ray structural analyses were selected
and mounted on the tip of a glass fiber. Diffraction data
were collected at 100 K on a Bruker D8 Kappa diffractometer
equipped with a SMART APEX II CCD detector with Mo Kα
(λ = 0.71073Å) radiation. Data were integrated with SAINT[65]
and empirical methods as implemented in SADABS[66] were
used to correct for absorption effects. Structures were solved
with direct methods using SHELXS-97. SHELXL-97 was used
for refinement against all data by full-matrix least-squares
methods on F2.[67] All non-hydrogen atoms were refined with
anisotropic displacement parameters. Hydrogen atoms were
refined isotropically on calculated positions using the riding
model implemented in SHELXL-97. Tables containing crystal-
lographic data and refinement parameters are given in the Sup-
porting Information.
The crystal structures were visualized and the packing dia-
grams were generated using Mercury 3 visualization pro-
gramme.[68] The files CCDC 971370-97135 contain the supple-
mentary crystallographic data. These data can be obtained free
of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or
from the Cambridge Crystallographic Data Center, 12 Union
Road, Cambridge CB2 1EZ, UK; fax +44 1223 336033; e-mail
deposit@ ccdc.cam.ac.uk).
2.3. Measurements
Absorption spectra were measured on a Cary 50 UV–Vis spec-
trophotometer (Varian). Emission spectra were acquired on a
Fluorolog 3 fluorescence spectrometer from Horiba (Japan)
equipped with a NIR-sensitive photomultiplier R2658 from
Hamamatsu (Japan). All the spectra were corrected for the
sensitivity of the photomultiplier. The luminescence quantum
yields in frozen glasses (77K, toluene:tetrahydrofuran 4:6 v/v)
were determined relative to N,N´-bis-(2,6-diisopropylphenyl)-
1,6,7,12-tetra-(4-phenoxy)perylene-3,4,9,10-tetracarboxylic
acid diimide assuming Φ = 1 in toluene/tetrahydrofuran
glass at 77K (Φ = 0.96 in chloroform at room temperature).[64]
The absolute quantum yields for the sensors at room tem-
perature were measured with an integrating sphere from
Horiba. The sphere was thoroughly flushed with nitrogen
prior to the measurement. Luminescence decay times
were measured in a frequency domain with a two-phase
lock-in amplifier (SR830, Stanford Research Inc., USA)
equipped with a photomultiplier tube (H5701-02, Hama-
matsu, Japan). The excitation was performed with the light
of a 455-nm LED (Roithner Lasertechnik, Austria) filtered
through a BG-12 filter from Schott (Germany). The emis-
sion was recorded using a combination of an OG-580 filter
(Schott) and Calflex X filter from Linos. The modulation fre-
quencies were 2.5, 2 kHz and 1.5 kHz for Eu(HPhN)3phen,
Eu(HPhN)3dpp and Eu(HPhN)3DDXPO, respectively and 205,
180 and 145 Hz for the Gd(HPhN)3phen, Gd(HPhN)3dpp
and Gd(HPhN)3DDXPO, respectively. The intensity-based
quenching plots were obtained at the modulation frequen-
cies of 165 Hz and 21 Hz for the Eu(III) and Gd(III) com-
plexes, respectively. Gas calibration mixtures were obtained
from nitrogen and compressed air using a gas mixing device
from MKS (Andover, MA, USA). Temperature was controlled
using a cryostat ThermoHaake DC50 (Thermo Fisher Scien-
tific Inc). Luminescence decays at 77K were acquired on a
Fluorolog 3 fluorescence spectrometer using a SpectraLED
(λexc = 392 nm) and DeltaHubTM TCSPC unit from Horiba.
The size of the nanobeads was determined with a particle
size analyzer Zetasizer Nano ZS from Malvern.
3. Results and Discussion
Synthesis and Structure of the Complexes: The europium(III) and
gadolinium(III) complexes can be conveniently prepared in
only two steps starting from readily available 8-hydroxyphenale-
none (Scheme 1). This ligand can be synthesized in large quan-
tities from cinnamoyl chloride and 2-methoxynaphthalene.
The lanthanide salts used for complexes represent low cost
materials which compare favorably with the price of platinum
group metals used in most oxygen indicators. The intermediate
complexes europium(III) and gadolinium(III) tris-[9-(hydroxy-
kO)-1H-phenaleno-1-onato-kO] (Eu(HPhN)3 and Gd(HPhN)3,
respectively) are poorly soluble in common solvents and their
thorough purification was not performed. Instead, the crude
materials were introduced in the second step in which a qua-
ternary complex was formed (Scheme 1). The fourth ligand is
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The crystals of M(HPhN)3phen (M = Eu, Gd, both space
group P2(1)/c ) are isomorphous as are the two structures of
M(HPhN)3DDXPO (space group P-1). This is in contrast to
the structures of the complexes based on the dpp ligand. In
the latter case, the Eu(III) complex co-crystallizes in P-1 with
two molecules of toluene per unit cell. The Gd-sample, how-
ever, also crystallizes in P-1, but in a unit cell about twice as
large, where the asymmetric unit contains two molecules of the
Gd(III)-complex and three molecules of co-crystallized toluene.
All compounds studied crystallographically exhibit eight-
coordinate rare earth metal centers in the oxidation state +3.
The average M-O distance between the rare earth metal ions
and the oxygen atoms of the chelating HPhN ligands account
to 2.364 Å for Eu(HPhN)3phen and 2.362 Å Eu(HPhN)3dpp,
which is ca. 0.01 Å longer than in the respective Gd derivatives
(2.356 Å in Gd(HPhN)3phen and 2.350 Å in Gd(HPhN)3dpp).
These values agree very well with ionic radii of eight coordinate
Eu(III) (r = 1.066 Å) and Gd(III) (r = 1.053 Å).[69] In all com-
plexes studied, each of the coordinated HPhN ligands shows a
pair of essentially identical C-O distances of ca. 1.27 Å, which
is in between a C-O single bond (C(sp2)-O 1.35 Å) and a double
bond (C(sp2)-O 1.21 Å).[70] In all complexes, the rare earth metal
centers are located essentially in one plane with the ligand
backbones of the HPhN, phen and dpp substituents.
OMe
O
EuCl3/
AlCl3
0 - 150 °C
Gd(NO3)3
O
O
OH
O
3
Cl
M
HPhN
M(HPhN)3
n
e
h
p
+
p
p
d
+
+ DDXPO
O
O
N
O
O
N
3
O
O
3
3
M
M
M
O
N
N
O
O
P
P
M(HPhN)3phen
M(HPhN)3dpp
M = Eu(III), Gd(III)
M(HPhN)3DDXPO
Scheme 1. Synthesis of the new Eu(III) complexes.
used to saturate the coordination sites of the metal in order to
reduce the radiationless deactivation of the excited state and
enhance the solubility. The solubility in common organic sol-
vents indeed improves significantly. Due to the paramagnetic
character of the central atom the nuclear magnetic resonance
studies were not successful and the composition of the com-
plexes was confirmed by mass spectrometry and elemen-
tary analysis. All the complexes were isolated as X-ray quality
crystals. The X-ray spectroscopy unambiguously confirms the
structure of the complexes and allows identification of the coor-
dinated solvent molecules (Figure 1, Figure S1-S3).
The solid state structures of the complexes M(HPhN)3phen,
M(HPhN)3dpp and M(HPhN)3DDXPO (Eu, Gd) were studied
by means of single crystal X-ray diffraction structure analyses.
The molecular structures of the Eu(III) complexes are given in
Figure 1, the respective Gd(III) derivatives are shown in the
Supporting Information, Figure S1-S3. Selected bond distances
and angles for all complexes are given in Table S1. Table S2 in
the Supporting Information contains the crystallographic data
for all compounds.
Together with the almost identical M(III)-OHPhN distances,
this demonstrates the symmetrical bonding situation of the
ligands and the metal center. Furthermore, the O-M(III)-O angles
enclosed by the metal centers and the HPhN ligands show very
little deviation from an average of ca 70°, which originates from
the structural rigidity of the HPhN ligands. The coordination of
the phenanthroline ligands phen and dpp in M(HPhN)3phen
and M(HPhN)3dpp towards the central metal ions is again char-
acterized by a strictly coplanar motif and slightly longer M(III)-N
distances in the Eu(III) than the Gd(III) complexes. The even
more constrained nature of the phenanthroline-based ligands
is represented by acute N-M-N angles around 61°. In contrast,
the O-M(III)-O angles in the phosphine oxide based complexes
Eu(HPhN)3DDXPO and Gd(HPhN)3DDXPO are wider and
account to ca. 74°. The higher flexibility of the ligand allows
for a stronger interaction of the DDXPO ligand with the central
metal ions which, as will be shown below, is also represented by
bathochromic shifts in the UV–Vis spectra.
Figure 1. Thermal ellipsoid plots (50% probability) of the solid state molecular structures of the Eu(III) complexes. Hydrogen atoms have been omitted
for clarity.
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In comparison to the phen and dpp complexes, slightly
elongated, yet also almost equidistant M-O bond lengths
are observed in the phosphine oxide based complexes
M(HPhN)3DDXPO (Eu, Gd). Overall, the structural properties
correlate exceptionally well with results from optical spectros-
copy, where almost identical spectra were found likewise for the
pairs Eu(HPhN)3phen/Eu(HPhN)3dpp and Gd(HPhN)3phen/
Gd(HPhN)3dpp and which are notably different from the
respective DDXPO complexes.
Absorption Spectra: Similarly to the parent ligand all the com-
plexes feature an intense absorption band in the UV region
and two less intense bands in the blue part of the electromag-
netic spectrum (Figure 2A). The position of the bands in the
UV region is only minor affected by the complexation. On the
other hand, a pronounced bathochromic shift of the absorp-
tion bands (21-23 nm or 1030-1130 cm−1) located in the vis-
ible part of the spectrum is observed. The absorption spectra
of Eu(HPhN)3phen and Eu(HPhN)3dpp are virtually identical
(Figure 2A and Figure S4) but are slightly different from the
spectra of Eu(HPhN)3DDXPO. The same trend is observed
for the respective Gd(III) complexes (Figure 2C). Notably, the
absorption spectra of the Eu(III) complexes and the respective
Gd(III) chelates are fully identical (Figure 2A and 2C, Table 1).
The longest absorption band in the complexes shows excel-
lent overlap with the emission of a 465 nm blue LED which
is one of the brightest LEDs available. This is particularly
helpful for application of the dyes in compact sensing devices.
The molar absorption coefficients of 25,000-30,000 M−1cm−1
(Table 1) are comparable to those of the most common oxygen
indicators such as ruthenium(II) polypyridyl complexes and
metalloporphyrins (for the Q bands in the visible part of the
spectrum).[3]
Emission Spectra and Sensitization Mechanism: The
europium(III) complexes in deoxygenated solutions show
only very poor luminescence at room temperature (QYs about
1%). This is in good agreement with the results obtained by
Van Deun et al.[59] (quantum yields of 0.5% for the tetrahydro-
furan solution of Eu(HPhN)3). However, the complexes possess
very bright emission originating from the Eu(III) ion in frozen
glasses at 77K (Figure 2B) and, as will be demonstrated below,
in polymer matrices at room temperature. Enhancement of
luminescence in frozen solutions at low temperatures and in
highly viscous media is a rather common phenomenon. Poor
luminescence in solutions at room temperature is often attrib-
uted to efficient radiativeless deactivation of the excited states
involving e.g., solvent molecules and quenchers and is particu-
larly pronounced for the long-lived triplet state. The emission
of all the complexes is similar (Figure S5, Figure S6). Excita-
tion with the UV light (350 nm, Figure S7) reveals very efficient
energy transfer to the metal center since virtually no emission
from the ligand is observed. The HPhN ligand is fluorescent at
77K but extremely week phosphorescence at about 540 nm is
also observed (Figure 2B). Importantly, the excitation spectra of
the complexes are virtually identical to the absorption spectra
(Figure S8). The decay profiles in frozen solutions are ade-
quately described by the mono-exponential model (Figure S9)
and the calculated lifetimes are close to those obtained in the
frequency domain measurements (Table 1).
6
The P7/2 level of the Gd(III) (32105 cm−1)[71] is located well
above the triplet states of the ligands so that the metal affects
1.0
1.0
Eu(HPhN)3phen
Eu(HPhN)3phen
A
B
Eu(HPhN)3DDXPO
HPHN
0.8
0.8
HPHN
0.6
0.4
0.2
0.0
0.6
0.4
0.2
0.0
300
350
400
450
500
450
500
550
600
650
700
Wavelength, nm
Wavelength, nm
1.0
1.0
0.8
0.6
0.4
0.2
0.0
Gd(HPhN)3
Gd(HPhN)3DDXPO
Gd(HPhN)3phen
D
C
Gd(HPHN)3phen
0.8
0.6
0.4
0.2
0.0
Gd(HPHN)3DDXPO
300
350
400
450
500
450
500
550
600
650
700
Wavelength, nm
Wavelength, nm
Figure 2. Spectral properties of the Eu(III) and Gd(III) complexes and the HPhN ligand. A and C: absorption spectra in toluene at room temperature.
B and D: emission spectra in toluene:tetrahydrofuran (4:6 v/v) frozen glass (λexc = 460 for the Eu(III) and Gd(III) complexes and 400 nm for HPhN,
respectively).
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Table 1. Photophysical properties of the Eu(III) and Gd(III) complexes in solutions and in polystyrene.
max em,b) nm
442
Φc)
τ,b) µs
λ
max abs, nmd)
n.d.
λ
max em, nmd)
n.d.
Φd)
τ,d) µs
Compound
λ
max abs (ε), nm
λ
a)
(M−1·cm−1
)
HPhN
337 (10100);
352 (20000);
0.04
n.d.
n.d.
n.d.
416 (9400); 440
(10700)
Eu(HPhN)3 phen
Eu(HPhN)3 dpp
Eu(HPhN)3 DDXPO
Gd(HPhN)3 phen
Gd(HPhN)3 dpp
)Gd(HPhN)3 DDXPO
335 (39600);
350 (71500);
434 (32800); 461
(27500)e)
611f)
611f)
611f)
547
0.30
0.30
0.70
0.34
0.77
0.65
330g)
350g)
400g)
335; 351; 434; 461
335; 351; 434; 461
333; 348; 436; 463
336; 352; 435; 461
336; 351; 424; 461
333; 348; 435; 462
611f)
611f)
611f)
558
0.18
0.18
0.20
0.42
0.56
0.53
94g)
335 (45000);
350 (78100);
434 (34600); 461
(28800)e)
110g)
331 (35700); 347
(69000); 435
(35600); 463
(30000)
128g)
334 (32200); 350
(60000); 433
(27800); 460
(23600)e)
6300h) (52%) 1120
(48%)
1280g)
1350g)
2010g)
334 (39700);
350 (70300);
433 (32100); 460
(27300)e)
547
3400h) (38%) 840h)
(62%)
558
331 (32500); 346
(64800); 435
(33800); 463
(28400)
555
4100h) (44%) 860
(56 %)
563
a)in toluene at room temperature; b)in toluene at 77K; c)in toluene/tetrahydrofuran frozen glass at 77K; d)in polystyrene at 298K; e)calculated for the dye containing no
enclosed solvent molecules on basis of the elementary analysis data; f)the peak with the highest intensity; g)frequency domain measurement; h)time domain measurement,
Figure S11.
their photophysical properties solely by promoting inter-system
crossing due to the heavy atom effect and paramagnetism.
These properties of Gd(III) are helpful
for determination of the triplet state ener-
gies at low temperature. All the Gd(III)
complexes: Gd(HPhN)3, Gd(HPhN)3phen,
Gd(HPhN)3dpp and Gd(HPhN)3DDXPO
did not show any detectable fluorescence
in frozen solutions at 77K but very strong
phosphorescence (Figure 2D, Figure S10).
The emission spectra of Gd(HPhN)3phen
and Gd(HPhN)3dpp are very similar to the
spectrum of Gd(HPhN)3. The phospho-
rescence of Gd(HPhN)3DDXPO is slightly
shifted bathochromically (Figure 2D).
The energy of the T1 state of HPhN esti-
mated from the emission spectrum of
Gd(HPhN)3 is about 18,300 cm−1. The ener-
gies of the singlet and triplet states of dpp,
phen and DDXPO are significantly higher
DDXPO, Figure 3).[61] Thus, these ligands are very likely not to
be involved in the sensitization process if the excitation with
than those for the HPhN ligand (e.g., ES1
=
27,930 cm−1, ET1 = 21,830 cm−1 for phen;[72]
ES1 = 31,850 cm−1, ET1 = 23,470 cm−1 for
Figure 3. Jablonski diagram for the energy levels of Eu(III) and the respective ligands. The
energy levels for the dpp ligand are similar to those of phen and omitted for simplicity.
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visible light is performed. The energy of the S1 state of the
free HPhN ligand estimated from the fluorescence spectrum
at 77K is about 22,600 cm−1 and about 21,500 cm−1 in coordi-
nated state as calculated from the difference in position of the
absorption bands of the coordinated and free ligand. Thus,
the energy gap between S1 and T1 levels of the HPhN is only
3,200 cm−1 which is significantly smaller than for known vis-
ible light-excitable Eu(III) complexes showing bright lumines-
cence (e.g., 4,200 cm−1 for a dipyrazolyltriazine-based ligand[52]
and 5,000 cm−1 for a pi-extended acridone derivative).[51] Thus,
HPhN represents a rather unique antenna chromophore which
can be efficiently excited above 460 nm and yet enables efficient
sensitization of Eu(III) luminescence.
of the long-living ligand triplet states gives rise to a corre-
sponding strong decrease of the Eu(III) emission intensity and
decay time.
Planar optodes were prepared by incorporating the Eu(III)
complexes in polystyrene. Many other polymers can also be
used; however polystyrene represents one of the most common
materials and is popular for it high chemical stability, good
mechanical properties, adequate oxygen permeability and com-
patibility with most indicators. Therefore, polystyrene was the
matrix of choice in this work. The absorption spectra of the
complexes in polystyrene are virtually identical to those in
solution (Table 1). In contrast to the solutions of the dyes the
sensors are brightly luminescent at room temperature in the
absence of oxygen (Figure 4, insert). The luminescence spec-
trum corresponds to the emission of Eu(III) from 5D0 state
(Figure S12). The luminescence quantum yields for all the com-
plexes in polystyrene approach 20% (Table 1) which is extraordi-
nary high considering very long excitation wavelengths. Indeed,
the quantum yields are comparable to those of typical oxygen
indicators, e.g., ruthenium(II)-tris-4,7-diphenyl-1,10-phenan-
throline ( = Ru-dpp, Φ = 0.37),[74] platinum 5,10,15,20-tetrakis-
(2,3,4,5,6-pentafluorphenyl)-porphyrin ( = PtTFPP, Φ = 0.09),[75]
platinum octaethylporphyrin (Φ = 0.41)[76] and platinum(II)
octaethylporphyrin ketone (Φ = 0.12).[12]
Since the T1 level of HPhN in the complexes is located below
5
the D1 level of Eu3+ (E = 19,000 cm−1) only population of the
5D0 level (E = 17,500 cm−1) appears to be possible (Figure 3).
The energy gap between the T1 level of HPhN and the 5D0 level
of Eu3+ is very small (500-800 cm−1). This is expected to favor
thermally activated back energy transfer and reduce the effi-
ciency of sensitization. Nevertheless, the sensitization is sur-
prisingly efficient even at room temperature (albeit in rather
rigid polymer matrix) since the quantum yields of Eu3+ lumi-
nescence are about 20% (Table 1). A possible explanation for
this phenomenon is high rigidity of the HPhN ligand (com-
pared e.g., to β-diketones) which dramatically reduces non-
radiative deactivation of the excited triplet state. Indeed, the
related work demonstrates that the BF2-chelated HPhN pos-
sesses extraordinary long phosphorescence decay time of about
300 ms.[73] Thus, back energy transfer is unlikely to signifi-
cantly reduce the luminescence quantum yields and the main
The quenching by oxygen is indeed very efficient
(Figure 4). Similarly to most state-of-the-art oxygen sensors
the Stern-Volmer plots are non-linear. The non-linearity is
higher for the decay time plots compared to the intensity plots
(Figure 4B and 4C) which is a typical case for the state-of-the-
art oxygen sensors. The Stern-Volmer plots can be fitted by so-
called “two-site model” assuming localization of the dye in two
different environments:[77]
5
deactivation passway is the emission from D0 level of Eu(III).
However, the back energy transfer may be responsible for
extraordinary efficient quenching by molec-
ular oxygen which will be discussed below.
Oxygen-Sensing Properties of the Eu(III)
Complexes: The luminescence of virtually all
known Eu(III) complexes is barely affected
by molecular oxygen, which is due to rather
weak interaction of the partly filled f-orbitals
(shielded by the 4d and 5p electrons) with
the environment and quenchers. In fact,
optical oxygen sensors based on the lantha-
nide complexes show poor sensitivity and
quenching even at 100% O2 does not exceed
factor of 4.[43–48] As will be shown below,
the proximity of the T1 level of HPhN and
the 5D0 level of Eu3+ significantly improves
the oxygen-sensing capabilities due to back
energy transfer from the Eu3+ to the HPhN
ligand. In fact, the quenching of the T1 state
of the organic and metal-organic compounds
by molecular oxygen is typically very effi-
cient and is limited only by diffusion of the
interacting species. Because the levels 5D0
of the Eu(III) and T1 of the ligand are nearly
Figure 4. A-C: Calibration plots for Eu(HPhN)3dpp in polystyrene (excitation 465 nm LED);
A – lifetime plot, B and C – Stern-Volmer plots for the luminescence decay time and intensity,
respectively. D: comparison of the Stern-Volmer plots for the sensors based on the Eu(III) com-
plexes in polystyrene at 25 °C. Insert: photographic image of the planar optode under excitation
with 365 nm line of a UV lamp.
resonant (Figure 3), both forward and back
energy-transfer processes occur resulting in
a thermodynamic equilibrium between these
two states. Therefore, the oxygen quenching
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of Ir(CS)2acac and PtTFPP should also be considered. On the
contrary, about 80% of the total luminescence of the Eu(III)
complexes originates from the 5D0→7F2 transition (FWHM
∼ 4 nm). This very narrow emission is particularly valuable
design of multiplexing assays and multi-parameter sensors.
Evidently, when using the new Eu(III) complexes virtually the
whole spectral region is available for other indicators or labels.
Also, the new sensors can be used for measurements at bright
daylight. No optical isolation is necessary since the emission
of the indicator can be isolated with very narrow interference
filters. In the case of conventional indicators the photodetector
can be saturated in these conditions.
Oxygen-Sensing Properties of the Gd(III) Complexes: Simi-
larly to efficient emission in frozen organic solvents at 77K,
bright phosphorescence is also observed in anoxic condi-
tions at room temperature for the dyes embedded in poly-
mers such as polystyrene (Figure 6A(insert), Figure S13). The
emission bands are almost identical for Gd(HPhN)3phen and
Gd(HPhN)3dpp, but a small bathochromic shift is observed in
case of Gd(HPhN)3DDXPO. The emission quantum yields in
polystyrene are about 50% (Table 1) which is a very high value
for phosphorescent metal complexes. Such intense emission is
even more impressive considering that the phosphorescence
decay times are rather long (1.3-2 ms, Table 1). To the best of
our knowledge there are no phosphorescent metal complexes
combining such long decay times and high emission efficiency.
Thus, the Gd(III) complexes represent a new class of phospho-
rescent emitters which combine visible light excitation, very
strong emission and long decay times and do not rely on rather
expensive platinum group metals such as commonly used plat-
inum, palladium, iridium and ruthenium.
As expected from the long phosphorescence decay times
of the Gd(III) complexes, the emission of the polystyrene-
based materials is completely quenched in air (Figure 6).
The quenching efficiency is about 20-fold higher than for the
Eu(III) complexes. Notably, the bimolecular quenching con-
stants kq are similar for both complex types (Table S3 and S4).
In contrast to the Eu(III) complexes, the temperature depend-
ency of the τ0 is rather low (0.07–0.25%/K, Table S3). Again,
the Stern-Volmer plots show better linearity for the lumines-
cence intensity compared to the decay time measurements
(Figure 6B and 6C). In full analogy to the Eu(III) complexes
fine tuning of the oxygen sensitivity is possible by choosing
the appropriate complex (Figure 6D). It should be noted that
polystyrene represents only a model matrix chosen for easy
comparison with the existing sensors. Polymers with signifi-
cantly higher oxygen permeability such as ethylcellulose or
organically modified silica can be employed to obtain even
more sensitive sensors. Thus, the Gd(III) complexes are par-
ticularly suitable for designing highly sensitive trace oxygen
sensors[80–82] which attract increasing attention in various areas
of science and technology, for example for protection of corro-
sion,[83] tumour hypoxia imaging[84] or investigation of oxygen
minimum zones in oceans.[85,86] Paramagnetic character of the
Gd(III) complexes can pave the way to some new applications
such as simultaneous oxygen sensing and magnetic resonance
imaging. Of course, light absorption and scattering in tissues
should be considered and synthesis of NIR-emitting probes is
likely to be necessary for application in practice.
I
τ
f1
f2
=
=
+
(1)
I0 τ0 1+KS1V [O2 ] 1+KS2V [O2 ]
where f1 and f2 are the fractions of the total emission for each
environment, respectively (with f1 + f2 being 1), and KSV and
KSV are the Stern-Volmer constants for each component.
1
2
Although the equation is physically meaningful for the inten-
sity only, it can often be used to fit the decay time plots as well.
The Stern-Volmer plots are very similar for the temperature
range from 5 to 45 °C. This behavior may appear rather unu-
sual but is easily explained by rather high dependency of the
luminescence decay time τ0 on temperature (Figure 4A). For
example, in case of Eu(HPhN)3dpp/PS sensor the decay times
decrease from 134.2 µs at 5 °C to 81.6 µs at 45 °C. Thus, despite
very similar KSV1 values (0.29, 0.30 and 0.30 kPa−1 for 5, 25 and
45 °C, respectively; Table S3) the bimolecular quenching con-
stants kq = KSV/ τ0 are significantly different and increase with
temperature (2.1, 2.7 and 3.7 Pa−1·s−1, respectively).
The sensitivity of the sensors can be tuned by choosing
the Eu(III) complex (Figure 4D). The Stern-Volmer con-
stant is the lowest for Eu(HPhN)3phen (0.27 kPa−1 at 25 °C,
Table S3) and the highest for Eu(HPhN)3DDXPO (0.53 kPa−1)
and Eu(HPhN)3dpp occupies intermediate position. This
correlates well with the luminescence decay times in the
absence of oxygen (Table 1) which are 93, 110 and 128 µs for
Eu(HPhN)3phen, Eu(HPhN)3dpp and Eu(HPhN)3DDXPO,
respectively. The kq values for the sensors based on the new
Eu(III) complexes (Table S3) are very similar to those of the
common oxygen sensors based on PtTFPP (kq = 3.5 Pa−1s−1)[78]
or Ru-dpp in the same polymer (kq = 2.89 Pa−1s−1).[78] For com-
parison, the kq values for the state-of-the-art sensors based on
Eu(III) complexes are much lower (e.g., kq ∼ 0.04 Pa−1s−1[79] or
0.1 Pa−1s−1).[45]
Figure 5 compares the emission spectra of the sensors
based on the new Eu(III) chelates with those of the state-
of-the-art oxygen sensors. The complexes of iridium(III)
bis(benzothiazol-2-yl)-7-(diethylamino)-coumarin)(acetylaceto-
nate) ( = Ir(CS)2acac), Ru-dpp and PtTFPP emit in yellow-red
part of the spectrum. The emission of the indicators is rather
broad: FWHM is 32, 25 and 90 nm for Ir(CS)2acac, PtTFPP
and Ru-dpp respectively. The second less intense band in case
1.0
0.8
0.6
0.4
0.2
0.0
3
1
2
4
550
600
650
700
750
Wavelength, nm
Figure 5. Emission spectrum of Eu(HPhN)3 dpp in polystyrene (1) com-
pared to the emission spectra of Ir(CS)2acac (2), PtTFPP (3) and Ru-dpp
(4) in the same polymer.
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Figure 6. A-C: Calibration plots for Gd(HPhN)3dpp in polystyrene (excitation 465 nm LED); A – lifetime plot, B and C – Stern-Volmer plots for the lumi-
nescence decay time and intensity, respectively. D: comparison of the Stern-Volmer plots for the sensors based on the Gd(III) complexes in polystyrene
at 25 °C. Insert: photographic image of the planar optode under excitation with 365 nm line of a UV lamp.
Photostability of the Sensors: Photostability of the sensors
in air-saturated conditions was investigated by irradiating the
planar optodes with intense blue light (λmax ∼ 465 nm, photon
flux ∼ 4000 µmol·s−1m−2) and monitoring the absorption of the
dye. In case of all sensors the absorption gradually decreases
with time (Figure 7a) and no products absorbing in the vis-
ible part of the spectrum are formed. The photodegradation
rates are similar for all the Eu(III) complexes, photodegrada-
tion being the fastest for Eu(HPhN)3DDXPO (Figure 7b). The
photobleaching is only slightly faster than for the highly pho-
tostable Ru-dpp/PS sensors. In fact, only 13–17 % of the dyes
are bleached after 2 h of continuous irradiation. It should be
noted that the luminescence of Ru-dpp in PS is only minor
quenched at air equilibrated conditions (τ0/ τ ∼ 1.3). Since oxi-
dation by photosensitized singlet oxygen can be a predominant
bleaching mechanism very slow bleaching of Ru-dpp in PS can
be explained by low production of singlet oxygen. For compar-
ison, 15% of Ir(CS)2acac, which is also excitable with blue light,
is bleached after 2 min of irradiation.
The photodegradation of the Gd(III) complexes bearing
phen and dpp ligands is about 2-folds faster than for the corre-
sponding Eu(III)-complexes (Figure S14). Gd(HPhN)3DDXPO
appears to be significantly less photostable than the Eu(III)
and Gd(III) complexes but still much more photostable than
Ir(CS)2acac. Thus, it can be concluded here that the new sen-
sors possess good photostability which is an attractive feature
for prolong measurements or measurements at high intensity
(e.g., microscopy).
in (marine)biology (e.g., for imaging of oxygen distribution in
sediments, biofilms, microbial mats etc.)[88] and becomes
Time, min:
0
30
0.2
60
90
120
0.1
0.0
A
350
400
450
500
Wavelength, nm
Ru-dpp
100
90
80
70
60
50
40
30
20
Eu(HPhN)3DDXPO
Eu(HPhN)3phen
Eu(HPhN)3dpp
Ir(CS)2acac
B
0
20
40
60
80 100 120
Time, min
Figure 7. A: absorption spectra of Eu(HPhN)3DDXPO/polystyrene
during irradiation with a 465 nm high power LED array (photon flux ∼
4000 µmol·s−1m−2) under air equilibrated conditions; B: photodegrada-
tion profiles for the polystyrene sensors based on different oxygen indica-
tors under the same conditions.
Ratiometric Materials for Imaging Applications: Imaging of
oxygen distribution on surfaces is one of the very important
applications of oxygen sensors.[87] It is an established method
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increasingly important in other fields of science and technology.
Imaging of wound oxygenation on skin[89,90] and imaging of
total pressure via pO2 (pressure-sensitive paints)[91] can be men-
tioned. Recently, RGB imaging with digital cameras became
popular[92–95] most probably due to rather low cost and great
availability of the required equipment (such as consumer dig-
ital cameras). Although some oxygen sensors for RGB imaging
have been proposed it is often challenging to find a pair of dyes
(oxygen indicator and a reference dye) which nicely match the
color channels of the camera. Figure 8A shows that the emis-
sion of the Eu(III) complexes in polystyrene almost ideally
matches the spectral sensitivity of the red channel of a digital
camera. In order to obtain a system for ratiometric read-out
polystyrene was co-doped with a fluorescent coumarin dye (C
545T). This lipophilic dye is excitable with blue light and emits
in the green part of the spectrum. The luminescence intensity
in the green channel is not affected by pO2; the intensity in the
red channel decreases with increasing pO2 (Figure 8B). These
changes correlate well to the emission spectra of the ratiometric
sensor (Figure 8A). A non-linear Stern-Volmer plot is observed
(Figure 8C). As can be seen from Figure 8D this simple ratio-
metric sensor allows imaging of oxygen distribution with dig-
ital cameras.
Oxygen-Sensitive Nanoparticles: Microscopic imaging of
oxygen distribution is a promising method for analysis of
enzymatic activity, respiration of mammalian and microbial
cells, monitoring of oxygenation in cell cultures and tissues
and in the area of food and microbial safety to mention only
some of the important applications.[96] The new Eu(III) com-
plexes are attractive for microscopic imaging due to their very
narrow emission (which leaves virtually the whole spectral
region available for labels or other cellular probes) and excellent
photostability (since high light intensities are typically used).
Dye-doped nanoparticles are particularly suitable due to their
low toxicity, protection of the indicator from undesired inter-
ferences (quenchers) by the polymeric shell and the possibility
of modification of the nanoparticles surface which enables
e.g., cell penetration.[97] Nanosensors can be prepared by sev-
eral methods, including copolymerization with the monomers,
swelling or nanoprecipitation. The Eu(HPhN)3dpp complex
Figure 8. A: Emission spectra of the ratiometric sensor based on coumarin C545T and Eu(HPhN)3dpp in PS (λexc 460 nm) and the relative spectral sen-
sitivity for the green and red channels of the digital camera (dashed lines); B: photographic RGB images obtained with a CMOS camera and the images
for the red and green channels (λexc 365 nm); C: Stern-Volmer plot for the luminescence intensity ratio obtained from the photographic images; D: the
photographic image of the sensing foil in air with nitrogen stream in the middle of the foil and the respective false-color image of the pO2 distribution.
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was successfully incorporated in two types of oxygen-permeable
nanoparticles, namely neutral poly(styrene-block-vinylpyrro-
lidone) (PS-PVP)[98] and Rl-100, which is a poly(methyl meth-
acrylate) derivative bearing positively charged quaternary
ammonium groups.[99] These materials were demonstrated
to be suitable for extracellular[98] and intracellular measure-
ments,[99] respectively. In good agreement with the previous
work involving other oxygen indicators the size of the PS-PVP
the possibility of such applications we incorporated
Eu(HPhN)3dpp in gas-blocking polymethacrylonitrile (PMAN)
nanoparticles via precipitation method.[63] The nanoparticles
are virtually insensitive to oxygen (Figure 9c). Optimization
regarding the size of the nanobeads which are likely to be too
large for practical applications (Zav = 180 nm, PDI = 0.126) and
availability of the functional groups on their surface is neces-
sary but it is beyond the scope of this work. Incorporation of
the Gd(III) complexes inside the nanoparticles is of course also
possible. Such materials can be used e.g., for imaging at nearly
anoxic conditions.
beads is significantly larger than that of the Rl-100 beads (Zav
=
184 nm, PDI = 0.062 and Zav = 33 nm, PDI = 0.14 for PS-PVP
and Rl-100 particles, respectively). The particles show bright
luminescence under excitation with the blue light; the lumines-
cence is quenched by molecular oxygen (Figure 9a). Excitation
spectra (Figure 9b) indicate that the red emission originates
from the Eu(III) complex. The sensitivity of both materials is
similar (Figure 9c). The other Eu(III) complexes can be also
incorporated to fine-tune the sensitivity if necessary. It should
be mentioned that a careful consideration of the solvents is
required particularly when using the nanoprecipitation pro-
cedure (from dilute dye solutions) and polar coordinating sol-
vents (e.g., dimethylformamide) should be avoided to prevent
dissociation of the complexes.
4. Conclusions
A new class of visible light-excitable Eu(III) and Gd(III) com-
plexes is presented. The unique 8-hydroxyphenalenone antenna
ligand featuring a very small singlet-triplet energy gap enables
efficient excitation in the blue part of the electromagnetic spec-
trum. The Eu(III) complexes show rather strong luminescence
originating from the Eu(III) ion which is unprecedented for
such low energetic excitation. The Gd(III) complexes show
very strong room-temperature phosphorescence in polymers
in the absence of oxygen with quantum yields of about 50%
which places them among the strongest phosphorescent emit-
ters known. In contrast to the state-of-the-art Eu(III) complexes,
which luminescence is only minor affected by molecular
oxygen, the new dyes show the sensitivity comparable to that
The Eu(III) complexes are often used as inert luminescent
labels, also in the form of the nanoparticles.[22] Visible-light
excitable labels are advantageous because of less interference
for the biological systems and broad availability and lower cost
of the light sources. A good label should be inert to common
interferences including oxygen. Thus, in order to demonstrate
Figure 9. A: photographic images of the aqueous dispersion of the beads (stained with Eu(HPhN)3dpp) under ambient light illumination and in dark-
ness upon excitation with a 465 nm LED; B: excitation spectra of the dispersions of the beads (λem = 614 nm); C: oxygen sensitivity of the dye-doped
nanoparticles.
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wileyonlinelibrary.com
Adv. Funct. Mater. 2014,
DOI: 10.1002/adfm.201401754
2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
11

www.afm-journal.de
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DOI: 10.1002/adfm.201401754
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