ChemComm
Communication
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Fluorescence titration of 2 with other anions was carried out
in order to determine the selectivity (Fig. 4). From these
titrations it is clear that only the addition of fluoride ions
enhances pyrene excimer formation, since while other anions
(BrÀ, IÀ, OHÀ, BF4À, HSO4À, H2PO4À and PF6À) show enhance-
ment of monomer fluorescence, they only result in minimal
enhancement of the excimer emission is observed.
From these observations it is clear that 2 exhibits high selectivity
for fluoride anions over the other anions employed in this study. The
fluorescence titration of 1 and 3 with other anions was also per-
formed in order to determine their selectivity (Fig. S6, ESI†), which
indicated that compounds 1 and 3 were not fluoride selective.
It is essential for the fluorescence sensors to work in the presence
of water if they are ever to be used to detect environmental levels of
fluoride. In order to demonstrate the practical utility of sensor 2, we
investigated its ability to detect fluoride anion in aqueous solutions.
Therefore, an aqueous solution (1 mL) of NaF was vigorously shaken
for 2 min with a dichloromethane solution of 2 (50 mM, 2 mL). On
standing the organic phase separated and displayed strong fluores-
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intensity exhibits a good linearity that can be expressed as I =
0.77 + 4.27C[FÀ] (R2 = 0.972) over a fluoride anion concentrations
range of 0.1–1.5 ppm. Therefore, a limit of detection of 0.1 ppm can
be achieved. Consequently, sensor 2 can be used for fluoride
detection in water at the sub-ppm range.
In summary, we have prepared a simple fluorescent turn-on
sensor 2 that is selective for fluoride ions. Both fluorescence and
mass spectrometry data indicate that 2 coordinates fluoride
anion as a 1 : 2 complex. Addition of fluoride anions to the
sensor gives a significant excimer emission through pÀp stack-
ing interactions. This biphasic experiment indicates that sensor
2 can be used for the quantification and detection of fluoride
anions in water. Sensor 2 can detect fluoride in water between
20 B.-G. Zhang, P. Cai, C.-Y. Duan, R. Miao, L.-G. Zhu, T. Niitsu and
H. Inoue, Chem. Commun., 2004, 2206–2207.
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organization (WHO) guidelines for detecting the level of fluoride
30 K. M. K. Swamy, Y. J. Lee, H. N. Lee, J. Chun, Y. Kim, S.-J. Kim and
J. Yoon, J. Org. Chem., 2006, 71, 8626–8628.
sanitation_health/dwq/chemicals/fluoride.pdf) We are currently
calibrating the system for interferences and ionic strength in
order to facilitate the deployment of the sensor as a tool for the
detection and quantification of fluoride anions in environmental
samples and the production of a simple visual warning system.
T.D.J, T.N. and K.S. thank the Royal Society for funding
through an International Joint Project. We would also like to
thank the Catalysis and Sensing for our Environment (CASE)
network for facilitating collaboration.
31 For our previous results on using pyridinium sensors for sensing see
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32 For our recent reviews on boronic acids as sensors see
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c
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This journal is The Royal Society of Chemistry 2013