C O M M U N I C A T I O N S
analyzer to measure potassium with a precision better than (0.06
mM (1 SD). From steps k to m, the sodium concentration is
increased from 0 to 200 mM, while potassium is held constant at
5 mM. The slope of sodium in the presence of potassium is
-0.047% signal change per millimolar of sodium. The slight
decrease in fluorescence intensity with increasing sodium can be
attributed to the displacement of some bound potassium ion by
sodium. The smaller sodium cation cannot fill the cryptand [223]
completely and is less efficient at interrupting PET quenching by
the arylamine lone pair electrons. In other words, the displacement
of bound potassium by sodium causes the net loss of sum total of
fluorescence enhancement, because binding of potassium gives
much higher fluorescence enhancement than binding of sodium. A
more detailed explanation of interference of sodium and other
cations can be found in the references.14 It is noteworthy that the
sensor gives very similar response to 5 mM potassium in the
presence and absence of 160 mM sodium (steps e and k). That is
one of the essential requirements for a potassium sensor to be
practically useful for measurements of potassium in blood or serum
with very minor or even no correction for sodium. No fluorescence
response to calcium (steps n and o) or pH (steps p and q) at fixed
sodium and potassium concentrations is observed. One can see from
the whole curve that stable signals are generally obtained in less
than 1 min and the fluorescence response is fully reversible.
In conclusion, we have described a new optical sensor suitable
for practical measurement of extracellular (serum or whole blood)
potassium. Sensor 1 responds rapidly and reversibly to changes in
potassium concentrations typical of whole blood samples. No
interferences from clinical concentrations of calcium or pH are
observed, and the sodium interference is very minor. Excitation
and emission occur in the visible light region. This new potassium
sensor is currently used in the Roche OPTI CCA, a commercially
available whole blood analyzer.
Figure 2. Excitation and emission spectra of a sensor disk exposed to
different potassium chloride solutions in TRIS-HEPES buffer at pH 7.40
(left, excitation spectra, fixed emission at 540 nm; right, emission spectra,
fixed excitation at 470 nm).
Figure 3. Response curve of 1 in TRIS-HEPES buffer at pH 7.40,
excitation at 470 nm, emission at 540 nm. Potassium, sodium, and calcium
concentrations are (K/Na/Ca, in mM): (a) 0/0/0; (b) 0/60/0; (c) 0/160/0;
(d) 2/160/0; (e) 5/160/0; (f) 10/160/0; (g) 25/160/0; (h) 50/160/0; (i) 100/
160/0; (j) 200/160/0; (k) 5/0/0; (l) 5/100/0; (m) 5/200/0; (n) 5/160/0; (o)
5/160/5; (p) 5/160/0, pH 6.8; (q) 5/160/0, pH 7.8. Insertion: Intensity of
emission as a function of log [K+].
Supporting Information Available: Experimental details for
syntheses and characterization of 1, sensor disk preparation, and
description of Roche OPTI CCA (PDF). This material is available free
The complete synthesis14 of potassium sensor 1 is illustrated in
Figure 1.
References
Figure 2 shows the fluorescence emission and excitation spectra
of a sensor disk exposed to increasing concentrations of potassium
chloride in pH 7.4 TRIS-HEPES buffer. The green fluorescence
intensity increases substantially with increasing concentrations of
potassium ion. As expected, binding of the cation to the triaza-
cryptand inhibits fluorescence quenching by the anisidine donor.
The fluorophore does not directly interact sterically or electronically
with the cation. As a result, the excitation and emission maxima
are nearly invariant with changing potassium concentrations. This
is characteristic of a PET sensor and contrasts internal charge
transfer (ICT) sensors in which the excitation maxima of a
fluorophore change upon binding of an analyte.
Figure 3 shows the dynamic response of a potassium sensor disk
to a series of sodium, potassium, and calcium containing buffers.
From steps a to c, the sodium concentration is raised from 0 to
160 mM (without any potassium present), and a small increase in
fluorescence is observed. The fluorescent response to potassium
in the presence of 160 mM sodium is shown in steps c-j. In the
clinically important concentration range between 2 and 10 mM
potassium, a 5.8% signal change per millimolar of potassium is
observed. This response allows the Roche OPTI CCA whole blood
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