Luminescence lifetime-based sensor for cyanide and related anions

Article abstract of DOI:10.1021/ja0259180

A new Ru(II) complex is described which serves as a luminescence lifetime-based sensor for fluoride and cyanide anions (K_F = 640 000 mol^-1, K_CN = 430 000 mol^-1). This chromophore displays observable changes in its UV-vis and steady-state luminescence spectra upon cyanide binding. Prior to cyanide addition, this complex exhibits a single-exponential lifetime (τ = 377 ± 20 ns). With increasing cyanide concentrations, the intensity decays are composed of two exponentials: long τ (320-370 ns) and short τ (13-17 ns). The average lifetimes shorten as a function of cyanide concentration since the fractional intensity shifts from an initial dominant long lifetime component to the short lifetime component. This work represents the first example of a direct method for the luminescence lifetime-based sensing of anions. Copyright

Full text of DOI:10.1021/ja0259180

Published on Web 05/03/2002
Luminescence Lifetime-Based Sensor for Cyanide and Related Anions
Pavel Anzenbacher, Jr.,* Daniel S. Tyson, Karolina Jurs´ıkova´, and Felix N. Castellano*
Department of Chemistry and Center for Photochemical Sciences, Bowling Green State UniVersity,
Bowling Green, Ohio 43403
Received February 13, 2002
Inorganic anions such as chloride, fluoride, and phosphate
continue to be extensively used as food additives, drugs, and
agricultural fertilizers. Cyanide has common industrial use as a raw
material in the production of organic chemicals and polymers such
as nitriles, nylon, and acrylic plastics.1 Cyanide is also critical to
the gold-extraction process.2 Thus, the widespread application of
cyanide raises a number of environmental concerns, particularly
in terms of its retention in leech circuits, recovery, and potential
for contamination.2 These issues necessitate the development of
sensor molecules that can adequately detect these anions in a variety
of settings.
Optical sensors for anions have recently received considerable
attention, focused mainly upon the supramolecular chemistry of
receptor design.3 The binding event is typically monitored by
changes in color, fluorescence intensity, or both from a chromophore
covalently attached to the receptor.4 Competitive noncovalent
chromophore-displacement methods have also been devised for the
optical sensing of anions.5 Regardless of sensor function, both
organic3a,4 and inorganic6 moieties have been employed as chro-
mophores in emission intensity-based sensors for inorganic anions.
Unfortunately, the performance accuracy of sensors based on
fluorescence intensity measurements can be limited by many factors
including excitation source intensity fluctuations, light loss in the
optical path, light scattering, and detector drift.7 Signal transduction
utilizing emission lifetimes (lifetime-based chemical sensing) is
particularly attractive, because it is unaffected by the sample’s
emission intensity, thereby circumventing many limitations of
intensity-based methods.7 In this contribution, we report a new anion
sensor based on a luminescent Ru(II) metal complex and 2,3-di-
(1H-2-pyrrolyl)quinoxaline receptor (1),8 representing the first
example of a direct luminescence lifetime-based sensor for anions.
Figure 1. Changes in the UV-vis spectrum of sensor 1 (9.04 µM in CH2-
Cl2/CH3CN (98:2)) upon the addition of cyanide (0-49.02 µM). (Inset)
Binding isotherm monitored by the absorption decrease at 465 nm.
The anion binding affinity of 1-3 was evaluated by monitoring
their respective UV-vis and steady-state emission properties as a
function of anion concentration. Time-resolved photoluminescence
decays were also measured for 1 to test its viability as a lifetime-
based sensor for anions. [Ru(bpy)3](PF6)2 (4) served as a parallel
control for all experiments that incorporated 1. Air-equilibrated
solutions were used for all measurements to appropriately simulate
real-world sensing conditions. Preliminary investigations revealed
that fluoride, chloride, dihydrogenphosphate, and cyanide, available
as tetra-n-butylammonium salts, would serve as substrates for 1-3
in CH2Cl2/CH3CN (98:2). An addition of fluoride, cyanide, and
(to a lesser extent) phosphate to 9 µM solutions of 1-3 caused
significant changes to the UV-vis and steady-state emission
properties in all three receptors. For brevity, only the spectroscopic
data for 1 obtained from the titration with cyanide will be discussed
in detail. These results are of particular importance, given the lack
of suitable emission-based cyanide sensors presently available. The
UV-vis and emission titrations of 1 and 3 with various halides
and polyatomic anions are reported in Supporting Information.
Figure 1 presents the changes in the UV-vis spectrum of 1 as
a function of CN- concentration. The low-lying MLCT absorption
and the high-energy π-π* ligand-based absorptions decrease mono-
tonically throughout the addition, with saturation observed toward
the end of the titration. A low-energy absorption feature concomi-
tantly grows in with increasing CN- concentration. These absorption
features are likely a result of anion binding with the metal complex
through the DPQ receptor. In control experiments with [Ru(bpy)3]2+
the addition of CN- did not have a marked effect on the absorption
spectrum at the low concentrations used in the titration.13
The complex 1 combines the luminescence characteristics of
Ru(II) diimine compounds possessing metal-to-ligand charge
transfer (MLCT) excited states9 with the anion recognition capabili-
ties of 2,3-di(1H-2-pyrrolyl)quinoxaline, DPQ (2).10 The 1,10-
phenanthroline-containing DPQ ligand (3) was prepared by con-
densation of 1,10-phenanthroline-5,6-diamine11 with 1,2-di(1H-2-
pyrrolyl)ethane-1,2-dione in 75% yield. The reaction of 3 with
12
Ru(bpy)2Cl2 followed by a treatment with NH4PF6 (aq) and
chromatography over Sephadex LH-20 afforded 1 in 54% yield.
Figure 2 presents the quenching of the steady-state photolumi-
nescence spectrum of 1 measured throughout the cyanide titration.
In all luminescence experiments, the excitation wavelength corre-
* To whom correspondence should be addressed. E-mail: pavel@
bgnet.bgsu.edu. E-mail: castell@bgnet.bgsu.edu
9
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J. AM. CHEM. SOC. 2002, 124, 6232-6233
10.1021/ja0259180 CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
([CN-] ) 6.20 µM). At higher CN- concentrations, the intensity
decays exhibited complex kinetics that adequately fit a sum of two
exponentials. Both exponential components remained effectively
constant, ranging from 320 to 370 ns (long τ) and 13 to 17 ns (short
τ). The short component lies near our IRF, and thus these lifetimes
may in fact be shorter. Importantly, the average lifetimes shorten
as a function of the CN- concentration since the fractional intensity
shifts from an initial dominant long lifetime component to the short
lifetime component as the CN- concentration increases, Figure 3
inset. These data suggest that there are at least two distinct
luminescent species, consisting of anion-bound 1 (short τ) and free
1, the sum of which results in the observed lifetime quenching in
Figure 3. Fortunately, the shift in fractional intensity makes 1 a
suitable lifetime-based sensor for anions.
Figure 2. Changes in photoluminescence intensity of sensor 1 (9.04 µM
in CH2Cl2/CH3CN (98:2)) upon the addition of cyanide (0-38.79 µM).
(Inset) Binding isotherm monitored by the integrated luminescence intensity.
λex ) 493 nm (isosbestic point in UV-vis spectra).
In summary, we have prepared the first sensor based on the direct
observation of anion-induced luminescence lifetime changes. The
proof-of-concept experiments described here suggest that metallo-
complexes such as 1 open up the possibility for producing novel
materials and approaches to anion sensing.
Table 1. Affinity Constantsa for Compounds 1, 2,10b and 3 (mol-1
and Anionic Substrates in 2% Acetonitrile in Dichloromethane at
22 °C
)
anion
Ru2+ sensor (1)
DPQ (2)
ligand (3)
Acknowledgment. This work was supported by BGSU (TIE
Grant 038/0569 to P.A. and F.N.C.). The support from Kraft Foods
to P.A., McMaster Endowment (Hammond Fellowship to D.S.T.),
the support from PRF (ACS-PRF 36156-G6 to F.N.C.), the NSF
(CAREER Award CHE-0134782 to F.N.C.) is acknowledged.
F-
640 000
428 000
1 700
18 200
220
50
68 000
3 450
< 500
1 620
CN-
Cl-
-
H2PO4
14 000
60
a Anions used in this assay were in the form of their tetrabutylammonium
salts. Fits were performed using single reciprocal plots and 1:1 stoichiometry
being obtained from Job plots.
Supporting Information Available: Experimental procedures for
the preparation of 1 and 3, Job plots and titration results (PDF). This
material is available free of charge via the Internet at http://pubs.acs.org.
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Figure 3. Changes in the time-resolved PL decay of sensor 1 (9.04 µM)
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(Inset) Shifts in fractional intensity of the two lifetime components obtained
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