A new indicator for potassium ions at physiological pH by using a macrocyclic luminescent metal complex

Article abstract of DOI:10.1002/chem.201202925

Anyone for a π-type sandwich? A luminescent indicator was developed based on the potassium-binding-induced conformational change of a macrocyclic terbium complex. The indicator adopts a random coil conformation in the absence of K⁺. In the presence of K⁺, it forms a π-type sandwich complex with the arene, resulting in an effective energy transfer and strong luminescence (see figure). Copyright

Full text of DOI:10.1002/chem.201202925

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DOI: 10.1002/chem.201202925
A New Indicator for Potassium Ions at Physiological pH by Using a
Macrocyclic Luminescent Metal Complex
Xi Yan,* Shasha Lv, and Rong Guo[a]
Potassium ions [potassium(I)] play an important role in
biological systems. They are not only involved in the mainte-
nance of extracellular osmolarity and proper pH balance,
but also associated with the regulation of blood pressure
and concentration of other ions in living cells, such as calci-
um and chloride ions, which are transported across the
plasma membrane.[1] Moreover, its importance for regulating
heartbeat has already been reported.[2] An unbalance of po-
tassium(I) concentration can lead to several human diseases,
such as stroke, seizures, hypertension, myasthenia and renal
disease.[3] Therefore, a delicate balance of potassium is deci-
sive between beneficial and harmful roles, and the sensitive
and selective detection of potassium(I) concentration in
human body fluids is crucial to biomedical diagnosis.
Although a number of potassium(I) probes have been pro-
posed, none of them present adequate sensitivity and selec-
tivity for practical imaging applications.[4] Selectively and ac-
curately measuring the extracellular concentration of potas-
sium(I) is challenging owing to the presence of the large
excess of sodium in the medium. In the blood, the normal
concentration of potassium(I) is 3.5–5.3 mm, whereas that of
sodium(I) is about 135–148 mm.[5] Obviously, an effective
probe for potassium(I) must possess great selectivity so as
not to be impacted by the large excess of sodium(I). To
meet this challenge, many methods[6] have been developed
for the exclusive detection of potassium(I) concentration, of
which fluorescence resonance energy transfer (FRET) is the
most common. Background luminescence from biological
media can also interfere with accurate detection of potas-
biological medium. In previous work we found that terbium
complexes, with extremely long luminescence lifetimes in
the millisecond range, were ideally suited for such applica-
tions. Herein we present a new terbium-based luminescent
sensor for the time-gated detection of potassium(I) concen-
tration with enhanced selectivity and sensitivity through
change of the conformation of terbium complex (Tb-L).
In our design, the Tb-L is based on sensitized lumines-
cence because of its considerable advantages over the use of
existing fluorescent probes for the detection of potassium(I)
concentration. Firstly, terbium ions are photochemically
inert because their f–f transitions are Laporte forbidden,
and thus they have negligible molar extinction coefficients.[7]
Secondly, the forbidden nature of the f–f transition also re-
sults in extremely long luminescence lifetimes, up to a milli-
second, which can be utilized to increase detection sensitivi-
ty by eliminating the short-lived background fluorescence of
biological samples by using a time delay set between the ex-
citation pulse and the detection window.[8] Lastly, owing to
the shielding effect of the outer electron shell, the emission
spectra of the terbium are line-like and insensitive to envi-
ronmental changes.[9] Although the terbium ion exhibits
weak luminescence in the aqueous solution because of the
negligible absorption, the problem can be overcome by
grafting an antenna onto the ligand complexing the metal.
The antenna absorbs energy from UV–visible radiation and
subsequently transfers it to the terbium ion upon irradiation.
The metal ion then emits its characteristic light.[10] For the
terbium ion, the distinctive luminescence range from 480 to
630 nm consists of four emission bands, assigned to the re-
ACHTUNGTRENNUNGsium(I) concentration; this results in much inconvenience in
5
7
clinical diagnosis. Time-gated luminescence imaging presents
an elegant solution to the problem of background lumines-
cence by setting a time delay between the excitation pulse
and luminescence detection; this allows the luminescence of
the media to decay before that of the probe is measured.
However, this technique requires chemical probes with lu-
minescence lifetimes significantly longer than that of the
spective transitions from D4 state to the ground state FJ
(J=3, 4, 5, 6).[11] In terms of antenna selection, benzophe-
none (BP) was chosen as the antenna since it was previously
proven to be an efficient sensitizer of terbium ion lumines-
cence.[12] A diaza-18-crown-6 receptor was also used to dis-
tinguish potassium(I) from other metal ions as reported
before.[13] However, the difference is that the high selectivity
and sensitivity of the Tb-L observed did not result only
from selective binding of potassium(I) by the diazacrown.
We reasoned that our probe could enhance selectivity and
sensitivity because of the cation–p interaction[14] that occurs
between the cation added and the arene in our Tb-L com-
plex.
[a] Prof. X. Yan, S. Lv, R. Guo
Department of Chemistry, Beijing Normal University
Beijing 100875 (P.R. China)
Fax : (+86)10-58802075
The design of our potassium probe Tb-L also depends on
the distance between the terbium ion and the antenna, BP.
Figure 1 outlines the assay process for detecting potas-
E-mail: yanxi@bnu.edu.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201202925.
Chem. Eur. J. 2013, 19, 465 – 468
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
465

Figure 2. Relative time-delayed luminescence of Tb-L as a function of
potassium(I) concentration. Excitation at 232 nm, emission at 545 nm,
time delay 0.2 ms, [Tb-L]=80 mm, T=258C. All measurements were ac-
quired in triplicate and are shown as an average. I and I0 are the lumines-
cence intensity of the system at 545 nm in the presence and absence of
potassium(I), respectively.
Figure 1. Chemical structure and mode of action of the potassium(I)
sensor Tb-L. The sensor is extended in the absence of potassium(I). The
flexible structure of the ligand results in an overall large separation be-
tween the Tb center and its sensitizing antenna, BP, resulting in weak Tb
luminescence. However, in the presence of potassium(I) the Tb-L binds
potassium(I) and forms a p-type sandwich complex with the arene by
cation–p interaction. The locking conformation causes the antenna to be
significantly closer to the Tb center. Consequently, the efficiency of
energy transfer from BP to Tb and the luminescence from the complex
are increased, which can be used to determine potassium(I) concentra-
tion in a complex biological medium.
in the clinically important range of 0–10 mm. The Tb-L
probe was synthesized according to Scheme 1.
To further investigate whether the different metal ions
that could be present inside cells and human body fluids,
such as sodium(I), lithium(I), cadmium(II), calcium(II),
magnesium(II), cobalt(II), mercury(II), lead(II), mangane-
se(II), iron(II), nickel(II), zinc(II) and copper(II), interfere
with potassium(I) detection, potassium(I)-binding-induced
luminescence changes were measured in the presence of an
equal amount of other metal ions (see the Supporting Infor-
mation). Firstly, the signal after the addition of other metal
ions (20 mm) to Tb-L was measured, followed by the addi-
tion of an equal amount of potassium(I) in the tested
system. The luminescence intensity change in response to
the other metals was shown by setting the 0 mm potas-
ACHTUNGTRENNUNGsium(I). In the absence of potassium(I), the complex Tb-L
adopts a random coil conformation in which the sensitizing
antenna BP is far away from the metal center because of
the flexibility of the probe; this results in weak Tb lumines-
cence. In the presence of potassium(I), complexation of po-
tassium(I) by the ring favors a cation–p interaction with the
aryl ether and induces the probe to change from a random
coil conformation to a cation–p-type sandwich conforma-
tion, which keeps the antenna BP in close spatial proximity
to the terbium center; this allows for the occurrence of sen-
sitized emission. Consequently, the resulting luminescence
from the probe is increased. As a result of the smaller size
of sodium ions its complexation by the ring does not enable
the cation–p interaction, and sensitized emission does not
occur. Therefore, the complex Tb-L shows much higher se-
lectivity toward potassium(I) over sodium(I) than the previ-
ously reported complexes.[15]
AHCTUNGTREGsNNUN ium(I) and 0 mm other metals sample at 0% and the 20 mm
potassium(I) sample at 100% in the system (Figure 3). So-
dium(I) at 20 mm, led to only a 3% change of the maximum
signal induced by potassium(I), whereas no significant
A time-gated titration curve of Tb-L with potassium
chloride in water is shown in Figure 2 (detailed measure-
ments are given in the Supporting Information). The time
delay was set at 0.2 ms; this ensures that any background lu-
minescence in the system is negligible. From Figure 2, it can
be indicated that the luminescence intensity of the Tb-L in-
creases significantly with increasing potassium(I) concentra-
tion. Addition of 12 mm potassium(I) results in a 19-fold in-
crease in Tb luminescence at 545 nm. The property is well-
suited for the determination of potassium(I) concentration
Figure 3. Selectivity of Tb-L towards potassium(I), sodium(I), lithium(I),
cadmium(II), calcium(II), magnesium(II), cobalt(II), mercury(II),
lead(II), manganese(II), iron(II), nickel(II), zinc(II), and copper(II). Tb-
L (50 mm) with 20 mm of the respective metal ions is shown by the left-
hand columns, and addition of 20 mm potassium(I) to the sample is
shown by the right-hand columns. Excitation at 232 nm, emission at
545 nm, time delay 0.2 ms, T=258C. All measurements were acquired in
triplicate and are shown as an average.
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Chem. Eur. J. 2013, 19, 465 – 468

A Macrocyclic Terbium Complex Indicator for Potassium Ions
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changes were induced by other metal ions tested. In all
cases, 100% of the signal was recovered; this shows that po-
tassium(I)-binding-induced luminescence was much stronger
compared to that of other metal ions tested, and that the
presence of other metal ions did not interfere with potas-
AHCTUNGTRENNUNG
selective binding of potassium(I) by the diaza-18-crown-6.
The selectivities of the lariat ether for potassium(I) over so-
dium(I) and calcium(II) in solution are barely five- to ten-
fold.[16] Tb derivatives of these ethers also demonstrate poor
selectivity (fourfold).[17] The greater selectivity observed for
Tb-L for potassium(I) over sodium(I) therefore has to in-
clude another cause. We postulate that the enhanced selec-
tivity observed is the result of the conformation of the com-
plex locked by potassium–p interactions, so that the antenna
is significantly closer to the Tb center.
In summary, we have developed a new terbium complex
as the potassium(I) probe for complicated biological sys-
tems. This probe presents high sensitivity towards potas-
AHCTUNGTREGsNNUN ium(I) with a 19-fold increase in luminescence intensity be-
tween 0 and 12 mm potassium(I). Moreover, Tb-L is highly
selective for potassium(I) with a more than 97-fold selectivi-
ty over sodium(I) and 99-fold selectivity over other cations
tested. We reason that the good selectivity of the Tb-L is
the result of potassium–p interaction. In this work, time-re-
solved measurements were utilized to solve the problem of
background luminescence. The presented design principle
can be applicable to many other chemical and biosensor de-
velopments, and allows highly selective and sensitive quanti-
tative analysis of potassium(I) concentration in human body
fluids at physiological pH.
Acknowledgements
The work was supported by the National Natural Science Foundation of
China (Grant No.: 20271008).
Keywords: biosensors · luminescence · potassium · terbium ·
time-resolved spectroscopy
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Scheme 1. Synthesis of Tb-L. Reagents and conditions: a) Boc2O, diox-
ane, 258C, 24 h; b) 4-(2-bromoethyl)phenol, Cs2CO3, MeCN, 658C, 24 h;
c) Cs2CO3, DMF, 258C, 1.5 h; d) TFA, CH2Cl2, 258C, 1 h; e) chloroacetyl
chloride, NEt3, CH2Cl2, 258C, 2.5 h; f) cyclen, Cs2CO3, MeCN, 608C, 8 h;
g) tert-butylbromoacetate, Cs2CO3, MeCN, 258C, 20 h; h) TFA, CH2Cl2,
258C, 16 h, i) TbCl3, NaOH, H2O, 80 8C, 16 h. Detailed experimental pro-
cedures are described in the Supporting Information.
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Received: August 16, 2012
Published online: December 7, 2012
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Chem. Eur. J. 2013, 19, 465 – 468