A novel reaction-based, chromogenic and "turn-on" fluorescent chemodosimeter for fluoride detection

Article abstract of DOI:10.1039/c0nj00937g

A reactivity based fluoride sensing method exploiting both its nucleophilic and basic character has been developed. This novel dual mode strategy offers both a visible "naked-eye" and a "fluorescence-on" detection. Upon nucleophilic addition of F^-

Full text of DOI:10.1039/c0nj00937g

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LETTER
A novel reaction-based, chromogenic and ‘‘turn-on’’ fluorescent
chemodosimeter for fluoride detectionw
´ ´
Clement Padie and Kirsten Zeitler*
Received (in Gainesville, FL, USA) 27th November 2010, Accepted 24th February 2011
DOI: 10.1039/c0nj00937g
A reactivity based fluoride sensing method exploiting both its
nucleophilic and basic character has been developed. This novel
dual mode strategy offers both a visible ‘‘naked-eye’’ and a
‘‘fluorescence-on’’ detection. Upon nucleophilic addition of FÀ to
fluorochromic maleimides this approach allows for selective
detection of fluoride from other (critical) basic anions, such as
acetate, hydroxide or cyanide.
strong limitations for water containing samples.6 Using covalent
bonding as an alternative approach allows for improved
performance. Many methods exploiting the strong interaction
between fluoride and boron or the high affinity of fluoride to
silicium for complexation, respectively, deprotection approaches
are well known.7,8
Despite substantial progress interferences caused by other basic
anions such as hydroxide, phosphates, acetate or cyanide still
present an additional challenge. Therefore, the development of
new fluoride chemosensors and visualisation methods is highly
desirable.
Fluoride ions are ubiquitously present in our environment. As
they play an essential role in a broad range of industrial, chemical
and biological processes their detection and the design and
development of efficient methods capable of sensing fluoride ions
has attracted considerable interest.1 At low concentration fluoride
as a trace element shows beneficial effects for dental health and in
the treatment of osteoporosis in the human body; however, at
high concentration, skeletal fluorosis will occur, which in severe
cases can even lead to death; the inhibition of certain enzymatic
functions is also well documented.2 Due to this biased nature of
fluoride and in order to prevent such disorder, the World Health
Organization recommends monitoring the fluoride levels in local
water supplies and stipulates the development of new methods for
the quantitative analysis of fluoride.3
Ion chromatography and ion-selective electrode methods
allow for fast screening of small samples that do not need any
special preparation. However, in the presence of other ions, at
low concentration of fluoride or in samples with high ionic
strength fluoride detection can prove problematic. Additionally,
these methodologies require specific equipment and a laboratory
as well as chemical background knowledge.3,4 Thus, due to their
simplicity, low cost and reliability, spectrophotometrical methods
have received increasing attention and different sensors
transforming chemical information (fluoride concentration) into
analytically useful signals (UV-visible absorption, fluorescence
intensity etc.) have been reported.1,5 However, the development
of sensing methods based on electrostatic interactions
(coordinating receptors) suffers from the propensity of fluoride
to form strong hydrogen bonds with protic solvents causing
Maleimides are a class of well known Michael acceptor systems
that are widely used as building blocks in polymers providing
excellent thermal stability, good chemical resistance and out-
standing mechanical properties of the corresponding materials.
It has been reported that imidazole catalyzes the formation of a
dark red coloured, short polymer with N-ethylmaleimide.9
Additionally, based on the known loss of fluorescence of suitable
fluorochromes that are covalently introduced at the imide position
of maleimides, the obtained composite molecules can be used as
sensors for different nucleophiles.10 Nucleophiles such as thiols
(e.g. cysteine residues in proteins) or amino groups in biological
Institut fur Organische Chemie, Universitat Regensburg,
¨
¨
Universitatsstr. 31, D-93053 Regensburg, Germany.
¨
E-mail: kirsten.zeitler@chemie.uni-regensburg.de
w Electronic supplementary information (ESI) available: Synthetic
procedures and analytic data. See DOI: 10.1039/c0nj00937g
c
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systems can undergo Michael addition to such a,b-unsaturated
imides to generate a highly fluorescent, substituted succinimide.10b,c
Considering the pKa values in DMSO of imidazole (pKa = 18)
and fluoride (pKa = 15)11 as well as their relative nucleophilicity
together with a previous report of Fry et al. on the dimerisation of
methyl crotonate in the presence of TBAT (tetrabutylammonium
difluorotriphenylsilicate) as a fluoride source,12 we hypothesized on
the possibility to use fluoride as an initiator for such a colorimetric
reaction and to introduce maleimides as novel signalling agents for
fluoride.
characteristic colour vanishes due to the interruption of electronic
delocalization of the former cross-conjugated system (see ESIw,
Fig. S3). The deeply red colour which is developing during the
reaction allows for an easy ‘‘naked eye’’ qualitative visualisation of
the presence of fluoride in the reaction media.
Encouraged by this result we envisaged the design and
synthesis of a simple fluorogenic maleimide to both increase
the sensitivity of our rapid sensor and to concurrently allow
for a quantitative determination of the fluoride concentration.
Benzothiazoles are commonly used fluorochromes that are
conveniently synthesized from the corresponding o-aminothio-
phenols and carboxylic acids. Their non-fluorescent character
if containing squaraine or maleimide moieties and restoration
of the strong fluorescence upon the formation of adducts and
the inherent electronic changes has found widespread
applications.10a,15
Herein we report a reactivity-based, dual mode visible and
fluorescent, selective fluoride sensing strategy with a simple
benzothiazole substituted maleimide as fluorochrome as a first
successful application of this approach.13
As a proof of concept, we started our investigations with
N-methyl maleimide (NMM) as a model compound. Upon
addition of 10 mol% tetrabutylammonium fluoride (TBAF) to
a solution of NMM in THF a pink precipitate is immediately
formed (see ESIw, Fig. S3) and after one hour, no more NMM
could be detected in the mixture by TLC. An isolated sample of
the precipitate revealed the ‘‘consumption’’ of the double bond as
shown in 1H NMR by disappearance of the unsaturated
maleimide protons (singlet signal at 6.71 ppm) and the missing
of the characteristic signal at 134.2 ppm in 13C NMR (see ESIw,
Fig. S1). This result was also corroborated by FT IR by
vanishing of the bands at 3095 cmÀ1 and 1598 cmÀ1, which are
characteristic of the Csp2–H and CQC stretching vibrations (see
ESIw, Fig. S2). Mass spectrometry revealed the presence of short
oligomers (dimer and trimer formation). Interestingly, the
resulting polymer showed no incorporation of fluoride as proven
by both elemental analysis and 19F NMR.
Starting from simple and inexpensive building blocks our
fluorogenic maleimide, 2-(4-aminophenyl)-benzothiazole
based maleimide 1, was synthesized in very good yields over
two steps. First, 2-aminothiophenol was condensed with
4-amino benzoic acid in the presence of polyphosphoric acid
as a dehydrating agent. The maleimide moiety was then
introduced via amide formation of the free amine with maleic
anhydride followed by ring closure with acetic anhydride as
solvent and activating agent (Scheme 2).
Upon addition of a solution of TBAF to a solution of
compound 1 in DMSO the mixture turns dark red immediately
and a precipitate appears (Fig. 2B) in a similar way as
observed in the case of simple NMM (See ESIw, Fig. S3).
Additionally, this easily visible effect of the addition of
We propose a ‘‘Baylis Hillman type’’ pathway (Scheme 1) for
the observed chromogenic oligomerisation reaction. Similar to
phosphines or tertiary amines, such as DABCO, fluoride can be
considered as a nucleophilic catalyst and thus should be able to
add on the a,b-unsaturated system to form an enolate that could
initiate the polymerisation reaction by nucleophilic addition to
another maleimide molecule. The termination of the reaction could
then occur upon a proton shift followed by elimination of the
fluoride as similarly observed in the case of nucleophilic additions
to cross conjugated dienones bearing b-fluorine substituents.14
Deprotonation of the amide’s a-position by the basic fluoride
might cause a further bathochromic effect of the already
described red coloured conjugated enolate observed during the
anionic polymerisation. Upon reprotonation of the polymer, the
Scheme
2 Synthesis of 2-(4-maleimidophenyl)benzothiazole (1).
Reagents and conditions: (i) polyphosphoric acid, 195 1C, 12 h,
86%; (ii) (a) maleic anhydride, CHCl3/THF, (b) sodium acetate, acetic
anhydride, reflux, 4 h, 89%.
Scheme 1 Proposed mechanism for the polymerisation induced by fluoride.
c
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The intensity response of the dye is proportionally related to
the amount of fluoride present in the solution. The detection
limit was determined to be smaller than c = 50 mmol LÀ1
(r1 ppm) and is therefore comparable to other state-of-
the-art methods.16
Having established a reliable quantitative fluoride sensing
system we next examined the role of the fluoride counterion.
Cations ensuring good solubility of their corresponding fluoride
salt in organic solvents (e.g. DMSO) such as potassium, caesium
and tetrabutylammonium gave a satisfactory analytical answer
while poorly soluble salts (sodium or ammonium) showed low to
no response (see Fig. 2C) as expected.17
With respect to the proposed reaction based sensing
mechanism we presumed that it could also be possible
to determine the fluoride content of aqueous solutions.
Therefore, we examined the analytical signal for reaction
media containing up to 10% (v/v) water. We were pleased to
find that our sensing system tolerates up to 5% water in the
solvent (see ESIw, Fig. S10).
Fig. 1 Fluorescence intensity for titration of fluorochromic sensor 1
(c = 3.9 Â 10À5 mol LÀ1) with TBAF (0 r c r 13.0 Â 10À4 mol LÀ1
)
in DMSO (lexcit = 365 nm).
Apart from quantification of fluoride the selectivity
of a sensing system is regarded as a further feature of similar
importance. As mentioned above a major common drawback
is the interference with other anions present in typical
analyte solutions. To further challenge the reliability of our
fluoride detection system we added various different anions to
our fluorogenic compound. With respect to the proposed
mode of action only strongly basic anionic resp. nucleophilic
species can induce a response. Therefore, our system allows
for the discrimination of fluoride from other halogenides
such as chloride, bromide or iodide and additionally from
non-nucleophilic fluorine containing anions such as hexa-
fluorophosphate or tetrafluoroborate (see Fig. 2C). Importantly,
no significant signal could be observed in the presence
of acetate and hydrogen sulfate ions. Albeit
a direct
differentiation between hydroxide (Fig. 2C, column c) and
fluoride by a single point measurement of the fluorescence at
412 nm was not possible, the two species can unambiguously
be discriminated based on their different fluorescence
maxima (Fig. 2, inset) by recording a complete fluorescence
spectrum.18
Furthermore, our dual mode sensor also offers a distinct
differentiation of fluoride and cyanide. In contrast to fluoride,
nucleophilic, but less basic cyanide (pKa = 13)11 does not
cause any visible colour change (Fig. 2B, vial d), potentially
due to the missing deprotonation event being responsible for
the observed red colouring.
However, after longer incubation times19 a significant
fluorescence increase could also be detected (Fig. 2C,
column b in lower bar chart).
Fig. 2 (A) Fluorescence intensity at 412 nm (lexcit. 365 nm) of 1
(3.9 Â 10À5 mol LÀ1) in DMSO after addition of 10 equivalents of (a)
TBAF, (b) CsF, (c) NaF, (d) KF, (e) NH4F. (B) Picture of fluorogenic
maleimide in a solution of DMSO (a) before and 15 min after addition
of 10 equivalents of (b) TBAF (c) TBAOH (d) KCN. (C) Fluorescence
intensity at 412 nm (lexcit 365 nm) of 1 (3.9 Â 10À5 mol LÀ1) in DMSO
after addition of 10 equivalents of (a) TBAF, (b) KCN, (c) TBAOH,
(d) TBABr, (e) TBAI, (f) TBAOTs, (g) TBAPF6, (h) TBABF4,
(i) TBAHSO4, (j) NaOAc, (k) TBACl.19
In summary, we have developed a novel quantitative
reaction-based approach for fluoride sensing. This sensitive
methodology allows for discriminating fluoride from other
halogenides. Its dual mode character (visible ‘‘naked-eye’’ and
fluorescence) enables a distinct differentiation of potentially
problematic other basic anions like acetate, hydroxide and
cyanide from fluoride often present in real life samples. We
believe that the present work offers new opportunities for
future reactivity-based sensing systems. Further studies are
ongoing in our laboratory.
fluoride causes a dramatic, ca. 10-fold increase of fluorescence
intensity (Fig. 1). After 20 minutes the reaction is complete
and allows for an operationally simple quantification of the
fluoride content based on the fluorescence intensity (Fig. 1).
c
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125.5, 126.0, 126.5, 128.2, 132.8, 133.6, 134.4, 135.2, 154.2, 166.8,
169.1. HRMS (EI) calcd for C17H10N2O2S [M+]: 306.0463 found
306.0468.
Experimental
General considerations
NMR spectra were recorded on a Bruker Avance 300
(300.13 MHz) spectrometer using the solvent peak as an
internal reference (CDCl3: d H 7.26; d C: 77.0). Multiplicities
are indicated, s (singlet), d (doublet), t (triplet), m (multiplet);
coupling constants (J) are in Hertz (Hz).
Acknowledgements
Generous support from the EU (IEF Marie-Curie Fellowship
for C. P.) is gratefully acknowledged.
Mass spectra (MS ESI) were recorded with a Finnigan MAT
95 or Varian MAT 311A. All reactions were monitored by
thin-layer chromatography (TLC) using Merck silica gel plates
60 F254; visualization was accomplished with UV light and/or
staining with appropriate stains (anisaldehyde, vaniline).
Standard chromatography procedures were followed (particle
size 63–200 mm). Fluorescence spectra were recorded on a Varian
cary eclipse fluorescence spectrometer at 25 1C. All chemicals
were obtained from commercial sources and were used as
received. Solvents were distilled prior to use.
Notes and references
1 M. Cametti and K. Rissanen, Chem. Commun., 2009, 2809.
2 E. Gazzano, L. Bergandi, C. Riganti, E. Aldieri, S. Doublier,
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3 J. Fawell, K. Bailey, J. Chilton, E. Dahi, L. Fewtrell and
Y. Magara, in Fluoride in Drinking Water, World Health Organi-
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4 (a) J. A. Arancibia, A. Rullo, A. C. Olivieri, S. Di Nezio, M. Pistonesi,
A. Lista and B. S. F. Band, Anal. Chim. Acta, 2004, 512, 157; (b) R. De
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references therein; (c) M. Neal, C. Neal, H. Wikham and S. Harman,
Hydrol. Earth Syst. Sci., 2007, 11, 294.
Synthesis of the fluorochrome substituted maleimide
Synthesis of 2-(4-aminophenyl)-benzothiazole (2). A well
stirred mixture of o-aminothiophenol (1.4 mL, 13 mmol),
p-aminobenzoic acid (2.0 g, 13 mmol) and 20 g of polyphosphoric
acid is heated to 195 1C overnight. The mixture is then poured into
150 mL of ice-cold water and stirred at room temperature for
20 min. The solid is recovered by filtration and washed twice with
20 mL of water. The solid is then poured into 100 mL of a
saturated solution of NaHCO3 and the mixture is stirred at room
temperature during 10 min. The solid is collected by filtration,
washed twice with 30 mL of water and pressed as dry as possible.
The resulting greenish yellow solid is further dried under reduced
pressure over phosphorus pentoxide (2.55 g, yield 86%).
1H NMR (d6 DMSO): d = 5.91 (s, 2H), 6.67 (d, J = 8.5 Hz,
2H), 7.33 (t, J = 7.7 Hz, 1H), 7.45 (t, J = 7.7 Hz, 1H), 7.77
(d, J = 8.5 Hz, 2H), 7.90 (d, J = 8.1 Hz, 1H), 8.01 (d, J = 8.1 Hz,
1H). 13C NMR (d6 DMSO): d = 113.8, 120.0, 121.4, 121.8,
124.5, 126.4, 128.7, 133.3, 151.8, 153.3, 168.6. HRMS (EI)
calcd for C13H10N2S [M+ꢀ]: 226.0565 found 226.0564.
5 For selected recent reviews, see: (a) C. S. Wade, A. E. J. Broomsgrove,
S. Aldridge and F. P. Gabbai, Chem. Rev., 2010, 110, 3958;
(b) T. W. Hudnall, C.-W. Chiu and F. P. Gabbai, Acc. Chem. Res.,
2009, 42, 388; (c) R. Vilar, Eur. J. Inorg. Chem., 2008, 357; (d) J. Perez
and L. Riera, Chem. Commun., 2008, 533; (e) P. A. Gale, S. E.
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6 P. D. Beer and P. A. Gale, Angew. Chem., Int. Ed., 2001, 40, 486.
7 For a general review on reaction-based sensing systems, see:
D.-G. Cho and J. L. Sessler, Chem. Soc. Rev., 2009, 38, 1647.
8 Selected recent examples: (a) O. A. Bozdemir, F. Sozmen,
O. Buyukcakir, R. Guliyev, Y. Cakmak and E. U. Akkaya, Org.
Lett., 2010, 12, 1400; (b) S. Y. Kim, J. Park, M. Koh, S. B. Park
and J.-I. Hong, Chem. Commun., 2009, 4735; (c) R. Hu, J. Feng,
D. Hu, S. Wang, S. Li, Y. Li and G. Yang, Angew. Chem., Int. Ed.,
2010, 49, 4915 and references cited therein.
9 D. G. Smyth, A. Nagamatsu and J. S. Fruton, J. Am. Chem. Soc.,
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10 For the use of fluorochrome modified maleimides as probes for C–C
bond formation by creation of fluorescent succinimide, see:
(a) F. Tanaka, R. Thayumanavan and C. F. Barbas III, J. Am. Chem.
Soc., 2003, 125, 8523. For the addition of thiols to maleimides bearing
fluorochromes, see: (b) Y. Kanaoka, Angew. Chem., Int. Ed. Engl., 1977,
16, 137; (c) S. Girouard, M. H. Houle, A. Grandbois, J. W. Keilor and
S. W. Michnick, J. Am. Chem. Soc., 2005, 127, 559.
Synthesis of 2-(4-maleimidophenyl)-benzothiazole (1). To a
solution of 2 (2.0 g, 8.85 mmol) in 75 mL of THF is added at
0 1C a solution of maleic anhydride (1.0 g, 10.2 mmol) in 100 mL
of chloroform. The mixture is allowed to stir at room
temperature overnight. The solid is collected by filtration
and washed twice with 50 mL of THF. The isolated solid is
dried under reduced pressure and then poured into 50 mL of
acetic anhydride and the mixture is refluxed for 4 h. Sub-
sequently, the mixture is poured into 150 mL of water. The
solid is collected by filtration and then dissolved in ethyl
acetate. The organic layer is washed twice with 100 mL of a
saturated solution of NaHCO3 and once with 100 mL of brine,
dried over Na2SO4, filtered and evaporated under reduced
pressure to give a greenish solid that is purified by column
chromatography (SiO2, eluent: dichloromethane 100%).
Subsequent evaporation of the pure fractions (Rf = 0.75) yields
the title compound as a bright yellow solid (2.0 g, yield 74%).
1H NMR (CDCl3): d = 6.90 (s, 2H), 7.40 (t, J = 7.6 Hz, 1H),
7.53 (m, 3H), 7.92 (d, J = 8.1 Hz, 1H), 8.09 (d, J = 8.1 Hz, 1H),
8.20 (d, J = 9.0 Hz, 2H). 13C NMR (CDCl3): d = 121.7, 123.4,
11 F. G. Bordwell, Acc. Chem. Res., 1988, 21, 456.
12 (a) J. X. Xuan and A. J. Fry, Tetrahedron Lett., 2001, 42, 3275;
(b) E. DiMauro and A. J. Fry, Tetrahedron Lett., 1999, 40, 7945.
13 For
a recent sensing approach using basic anion addition
(FÀ, AcOÀ, CNÀ, H2PO4À) to a chromofluorescent acridinium
salt, see: Y.-C. Lin and C.-T. Chen, Org. Lett., 2009, 11, 4858.
14 J. Ichikawa, M. Fujiwara, S. Miyazaki, M. Ikemoto, T. Okauchi
and T. Minami, Org. Lett., 2001, 3, 2345.
15 (a) G.-S. Jiao, A. Loudet, H. B. Lee, S. Kalinin, L. B.-A. Johansson
and K. Burgess, Tetrahedron, 2003, 59, 3109; (b) O. S. Wolfbeis and
H. Marhold, Monatsh. Chem., 1983, 114, 599.
16 (a) X. Jiang, M. C. Vieweger, J. C. Bollinger, B. Dragnea and
D. Lee, Org. Lett., 2007, 9, 3579; (b) K. K. Upadhyaya,
R. K. Mishraa, V. Kumara and P. K. R. Chowdhury, Talanta,
2010, 82, 312; (c) see ref. 8c.
17 Attempts to increase the response of those salts by in situ exchange
of the cation for tetrabutylammonium or observation of the
fluorescence upon addition of crown ether failed.
18 The fluorescence spectrum upon addition of TBAOH exhibits two
different superimposed maxima – one of them with a bathochromic
lmax which could stem from the non-polymerized hydroxide adduct.
19 As mentioned above fluorides provide a signal response in a time
frame of approx. 20 min; however, to allow for comparable results
with the less reactive other tested anions an incubation time of
t = 24 h was used.
c
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New J. Chem., 2011, 35, 994–997 997