A highly selective luminescent sensor for the time-gated detection of potassium

Full text of DOI:10.1021/ja8077889

Published on Web 12/29/2008
A Highly Selective Luminescent Sensor for the Time-Gated Detection of
Potassium
Aurore Thibon and Vale´rie C. Pierre*
Department of Chemistry, UniVersity of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota, 55455
Received October 2, 2008; E-mail: pierre@umn.edu
The sensitive and selective detection of potassium is essential
to biomedical diagnosis since variations in serum and extracellular
potassium levels have been linked to hypertension, stroke, and
seizures.1 The difficulty in measuring accurately the extracellular
concentration of potassium stems from the large excess of sodium
present in the medium. In blood, the typical concentration of K+ is
3.5-5.3 mM, whereas that of Na+ is 135-148 mM.2 A practical
probe must therefore bind K+ with great selectivity so as not to be
impacted by the large excess of Na+ also present. Although a
number of luminescent K+ probes have been proposed, none present
adequatesensitivityandselectivityforpracticalimagingapplications.3-5
The most promising probes demonstrate sufficient selectivity versus
Na+ but minimal increase in signal at the mM K+ concentration
that has clinical relevance.4 Consequently, the background lumi-
nescence from the biological media may interfere with accurate
Figure 1. Chemical structure and mode of action of the K+ sensor Tb-1.
measurement of K+ concentration.
Time-gated luminescence imaging presents an elegant solution
to the problem of background luminescence by setting a time delay
between the excitation pulse and the luminescence detection, thereby
allowing the luminescence of the media to decay before measuring
that of the probe. This technique, however, requires chemical probes
with luminescence lifetimes significantly longer than that of the
biological medium. Lanthanide complexes, with extremely long
luminescence lifetimes in the millisecond range, are ideally suited
for such applications.6 Herein we present a luminescent sensor for
the time-gated detection of K+ with enhanced selectively, likely
resulting from the formation of a cation-π interaction.
the aryl ether, thereby locking the complex in a conformation where
the antenna is significantly closer to the Tb center. Consequently,
the efficiency of energy transfer from the azaxanthone to the Tb
and the resulting luminescence from the complex are increased.
Since complexation of Na+ by the ring does not enable the cation-π
interaction, it was predicted that Tb-1 would demonstrate greater
selectivity toward K+ over Na+ than the previously reported
complexes.5 Azaxanthone was chosen as the antenna since it was
previously demonstrated to be an efficient sensitizer of Tb lumi-
nescence.11
The selectivity of any probe is inherently limited by that of its
receptor. The poor sensitivity (detection in the M range) and
selectivity toward Na+ (4-fold) of the Tb-based K+ probes previ-
ously reported5 is consistent with the poor selectivity for K+ over
Na+ (5- to 10-fold) of their diaza 18-crown-6 receptors.7 We
reasoned that the selectivity of this class of probes could be
enhanced by use of the cation-π interaction.8 Gokel and co-workers
reported phenyl derivatives of diaza 18-crown-6 with apparent
selectivity for K+ over Na+ that results from a selective cation-π
interaction.9 As demonstrated by X-ray crystallography, the structure
of the receptor enables concomitant complexation of K+ by the
Lariat ether and sandwich π-type complexation with the arene. In
contrast, the crystallographic structure of the receptor bound to the
Na+ revealed that although Na+ can be complexed by the ring, its
smaller size sterically prevents formation of the cation-π interac-
tion. We reasoned that the selectivity of a sensor for K+ could be
significantly enhanced by use of this interaction.
The potassium sensor Tb-1 was synthesized according to Scheme
1. A time-gated titration of Tb-1 with potassium acetate in ethanol
is shown in Figure 2. The binding affinity of Tb-1 for K+ of 0.33(4)
µM-1 is well suited for the determination of K+ concentration in
the clinically important range of 0-10 mM. Moreover, the time
delay of 0.2 ms ensures that any background luminescence is
negligible. Under these conditions, the luminescence of Tb-1 still
The design of our potassium probe Tb-1 (Figure 1) also relies
on the sharp dependence of the lanthanide luminescence on the
distance separating the metal center from its sensitizing antenna.6,10
In our system, in the “off” state, the flexible structure of the ligand
results in an overall large separation between the Tb ion and its
sensitizing azaxanthone, resulting in weak Tb luminescence.
Complexation of K+ by the ring favors a cation-π interaction with
Figure 2. Relative time delayed luminescence of Tb-1 as a function of
K+ concentration (error bars represent standard deviation (s.d.), n ) 3).
Inset, luminescence spectra of Tb-1·K+ titration. Excitation at 332 nm,
emission at 545 nm, time delay 0.2 ms, [Tb-1] ) 50 µM, T ) 20 °C.
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434 J. AM. CHEM. SOC. 2009, 131, 434–435
10.1021/ja8077889 CCC: $40.75
2009 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1. Synthesis of Tb-1a
Figure 3. Selectivity of Tb-1 to various physiological cations. White bars
represent the time-delayed relative luminescence intensity after addition of
an excess of the appropriate cation (20 mM for NaOAc, LiOAc, Mg(OAc)2,
and Ca(NO3)2). Black bars represent the time-delayed relative luminescence
intensity after subsequent addition of 20 mM K+. Excitation at 332 nm,
emission at 545 nm, time delay 0.2 ms, [Tb-1] ) 50 µM, T ) 20 °C. Error
bars represent s.d., n ) 3.
with a 22 fold increase in luminescence intensity between 0 and
10 mM K+. Moreover, Tb-1 is highly selective for K+ over other
physiological cations, with a 93 fold selectivity over Na+.
Acknowledgment. This work was supported by the University
of Minnesota. We thank Hee-Yun Park for help with NMR
characterization.
Supporting Information Available: Detailed experimental proce-
dures and characterizations, excitation profile of Tb-1·K+ titration. This
material is available free of charge via the Internet at http://pubs.acs.org.
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Addition of 10 mM K+ results in a 22-fold increase in Tb
luminescence at 545 nm. Notably, the signal is stable over several
hours.
The selectivity of Tb-1 toward several physiological cations is
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was observed, respectively. Importantly, the subsequent addition
of 20 mM KOAc restores the 26-fold increase in luminescence,
demonstrating that the presence of competing cations does not affect
the determination of K+ concentration.
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of the lariat ether for K+ over Na+ and Ca2+ in anhydrous alcohol
are barely 5- to 10-fold.7 Tb derivatives of these ethers also
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selectivity observed is the result of this interaction.
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In conclusion, a terbium complex for the time-gated luminescence
detection of K+ is presented. Tb-1 demonstrates high sensitivity
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