Chemoselective and enantioselective fluorescent recognition of sugar alcohols by a bisboronic acid receptor

Article abstract of DOI:10.1039/b501249j

Bisboronic acid 3 binds strongly and enantioselectively with six-carbon sugar alcohols but does not bind strongly with five- or four-carbon sugar alcohols or monosaccharides. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b501249j

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Chemoselective and enantioselective fluorescent recognition of sugar
alcohols by a bisboronic acid receptor
Jianzhang Zhao and Tony D. James*
Received 25th January 2005, Accepted 5th April 2005
First published as an Advance Article on the web 28th April 2005
DOI: 10.1039/b501249j
Bisboronic acid 3 binds strongly and enantioselectively with six-carbon sugar alcohols but does
not bind strongly with five- or four-carbon sugar alcohols or monosaccharides.
on saccharide binding, thus the photoinduced electron
Introduction
transfer (PET) process from nitrogen to fluorophore is
Much recent attention has been paid to the development of
suppressed and the fluorescence of the fluorophore is switched
synthetic molecular receptors having the ability to selectively
on. The binding event is therefore converted into an optical
recognise small molecules involved in biological pathways.
output signal.
Fluorescent sensors are preferred because they are well suited
Many naturally occurring molecules, including saccharides,
to meet the needs of in vivo probes, such as mapping the spatial
involved in critical biological processes are chiral compounds.
and temporal distribution of the biological analytes.1,2 Boronic
Obviously, a chiral receptor will be inherently more selective
acid molecular receptors for saccharides have attracted
for chiral guests than achiral receptors since the resulting
considerable interest due to their ability to bind saccharides
complexes are diastereomeric. Chiral sensors can be enantio-
via covalent interactions in aqueous media.3–6 The most
selective but differences in the stability of diastereomeric
complexes can also result in enhanced chemoselectivity.2 Using
sensors 1 and 2 as the template,13,14 we decided to introduce a
common interaction of the boronic acids is with cis-1,2 or
1,3-diols of saccharides, to form five- or six-membered rings.
Over the last few years we have been interested in developing
chiral center in close proximity to the boronic acid binding site.
new saccharide selective sensors by employing a modular
approach.7–12 The basic idea was to break a sensor into
Therefore, we designed fluorescent chiral sensors 3 and 4. We
have previously reported the binding properties of 3 and 4 with
sugar acids (a-hydroxy acids).15 In the present work, we set out
three variable components: receptor units, linker units, and
‘read-out’ units. This approach can be illustrated with the
D-glucose selective fluorescence sensor 1 which contains
two boronic acids (receptor units) and an anthracene core (a
D-glucose selective linker and fluorophore read-out unit).13,14
The interaction of the boron atom (Lewis acid) and
neighbouring nitrogen atom (Lewis base) is strengthened
to probe the selectivity of our system for sugar alcohols
(Fig. 1). There are a few sensors specific to sugar alcohols and,
to the best of our knowledge, only one sensor for sugar
alcohols has been studied 5.16 No chiral sensors have been
reported. The design of selective sensors for sugar alcohols is
of significant interest because sugar alcohols are important
metabolic intermediates.17–19 For example, D-mannitol, a six-
hydroxyl sugar alcohol, is a naturally occurring polyol with
numerous applications. It can be used in diabetic food because
*T.D.James@bath.ac.uk
Fig. 1 Analytes used in the study.
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reducing the need for hemodialysis.20,21 Mannitol has found
applications in neurocritical care.22–24
Results and discussion
The synthesis of sensors 3 is shown in Scheme 1. As a control,
the monodentate sensor 4 was also synthesized in an analogous
fashion from 9-anthracenecarboxaldehyde. Because the syn-
thetic route is quite simple, a variety of chiral sensors will be
readily available using this strategy.
It is known that the binding behavior of the boronic acid
and its fluorescence response are pH-dependent,3,13 therefore,
the change in the fluorescence intensity of the sensor, with and
without analytes, versus pH was investigated to determine the
pH region with the optimal fluorescence response. The results
with D-glucose and D-mannitol are shown in Fig. 2. The pH
titration shows that the fluorescence enhancement of S,S-3 and
R,R-3 with D-glucose are similar. However, D-mannitol shows
Scheme 1 Reagents and conditions: (i) EtOH/THF (7 : 2), reflux, 6 h,
76.0%; (ii) THF, NaBH4, 78.9%; (iii) CH3CN, 2-(2-bromobenzyl)-1,3-
dioxaborinane, K2CO3, reflux, 10 h, 78.5%.
its uptake is insulin independent. It is an osmotic agent and
can be used in treatment of acute renal failure to flush out
cellular debris and prevent tubular cast formation, thus
Fig. 2 pH dependence of the fluorescence intensity of sensors 3 and 4 in the presence of D-mannitol or D-glucose. (a) Sensors R,R-3 and S,S-3
with and without D-mannitol (0.1 M); (b) sensors R,R-3 and S,S-3 with and without D-glucose (0.1 M). 3.0 6 1026 M sensor in 0.05 M NaCl
solution (52.1 wt% methanol). lex 5 365 nm, lem 5 429 nm. 22 uC. (c) Sensors R-4 and S-4 with and without D-mannitol (0.1 M); (d) sensor R-4
and S-4 with and without D-glucose (0.1 M). 3.0 6 1026 mol dm23 of sensor in 0.05 mol dm23 sodium chloride solution (52.1 wt% methanol),
lex 5 373 nm, lem 5 421 nm. 22 uC.
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a different fluorescence response with S,S-3 and R,R-3. These
titrations indicate significant enantioselectivity for D-mannitol
and minimal enantioselectivity for D-glucose. Moreover, the
pH titrations show that the optimal pH region for recognition
is between pH 6 and 10. More importantly, the fluorescence
is summarized in Table 1. For example, the results for
D-mannitol and D-glucose are shown in Fig. 4. The results
clearly demonstrate enantioslective recognition since only
S,S-3 shows fluorescence enhancement in the presence of
D-mannitol. The binding constant of D-mannitol with S,S-3 is
(2.13 ¡ 0.68) 6 103 M21 (r2 5 1.00). However, the binding
constant of the R,R-3 with D-mannitol is very small and could
not be determined. Therefore, the enantioselectivity in this case
is at the very least 2000 : 1. For the binding of D-glucose, no
enantioselectivity was observed. The chemoselectivity of S,S-3
for D-mannitol over D-glucose is 89 : 1. For D-sorbitol the
fluorescence enhancement with S,S-3 and R,R-3 is similar at a
saturated analyte concentration, but the binding constants are
significantly different (Table 1).
response at pH
7 is not sensitive to pH changes, a
characteristic which will be of great benefit if this system is
developed for analytical use.
The corresponding spectral changes of the sensors in
the presence of the D-mannitol are shown in Fig. 3. In
the case of bisboronic acid sensor 3, the recognition is
enantioselective for D-mannitol because the fluorescence
enhancement is drastically different for S,S-3 and R,R-3
(Fig. 3a and b) and only the fluorescence of S,S-3 is
significantly enhanced, whereas for the monoboronic acid
sensor 4 (Fig. 3c and d) no chiral discrimination was observed.
Significant enantioselectivity in the pH titration was also
found for the interaction of D-sorbitol with sensor 3. With
D-glucose, however, minimal enantioselectivity was observed
for both the bisboronic acid sensor 3 and the monoboronic
acid sensor 4.
Sensor 3 shows excellent selectivity for the six-hydroxyl
sugar alcohols over the five- or four-hydroxyl sugar alcohols
and monosaccharides, whereas sensor 4 displays much lower
chemoselectivity. The chemoselectivity of S,S-3 towards
six-hydroxyl sugar alcohols, five-hydroxyl sugar alcohols,
four-hydroxyl alcohols and monosaccharides is exemplified
by those for dulcitol, xylitol, L-threitol and D-glucose which
are 1267 : 8.5 : 1 : 2.6. As mentioned above, significant
enantioselectivity was found for the binding of the sensors
with D-mannitol and D-sorbitol. Our investigations also
indicated that the binding constants of the sugar alcohols
were independent of pH. For example, at pH 5.6, the binding
constants of D-sorbitol with sensor 3 are (73.7 ¡ 5.0) M21
Since the differences in fluorescence enhancement observed
with the pH titrations do not necessarily indicate differences
in binding constants and vice versa, the strength of binding
was determined by titration with the analytes at pH 8.3. The
binding of sensors 3 and 4 with six-hydroxyl, five-hydroxyl,
4-hydroxyl sugar alcohols and several monosaccharides
Fig. 3 Fluorescence spectra of sensors 3 and 4 in the presence of D-mannitol. (a) R,R-3 with D-mannitol (0.01 M); (b) S,S-3 with D-mannitol
(0.01 M). 3.0 6 1026 M sensor 3 in 0.05 M NaCl solution (52.1 wt% methanol), at pH 8.3, lex 5 365 nm, 22 uC. (c) R-4 with D-mannitol (0.01 M);
(d) S-4 with D-mannitol (0.01 M). 3.0 6 1026 mol dm23 sensor 4 in 0.05 mol dm23 sodium chloride solution (52.1 wt% methanol). lex 5 373 nm,
22 uC. pH 8.3.
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Table 1 Logarithm of observed stability constants, enantioselectivity (KR/KS) and fluorescence enhancement for the binding of sensor 3 and 4
with sugar alcohols and monosaccharides (0.05 M NaCl buffer, 52.1% CH3OH by weight, pH 8.3, 22 uC)
log K(r2)
Fluorescence Enhancement
Sensor
Analytes
R-sensor
S-sensor
KR/KS
R-sensor
S-sensor
3
—
3.33 ¡ 0.30 (1.00)
3.30 ¡ 0.14 (0.97)
.1 : 2000
1.0
6.71 ¡ 0.05
9.51 ¡ 0.28
D-Mannitol
1.93 ¡ 0.04 (1.00)
1 : 23.8
10.9 ¡ 0.13
D-Sorbitol
Dulcitol
4.04 ¡ 0.16 (0.98)
1.53 ¡ 0.10 (0.99)
4.07 ¡ 0.19 (0.98)
1.62 ¡ 0.05 (1.00)
1 : 1.1
1 : 1.3
4.75 ¡ 0.20
10.09 ¡ 0.38
4.88 ¡ 0.24
9.98 ¡ 0.17
D-Arabitol
Xylitol
Adonitol
1.86 ¡ 0.07 (0.99)
1.60 ¡0.14 (0.98)
—
1.90 ¡ 0.06 (1.00)
1.71 ¡ 0.15 (0.98)
0.97 ¡ 0.17 (0.99)
1 : 1.1
1 : 1.3
—
11.8 ¡ 0.26
5.31 ¡ 0.22
4.5
11.0 ¡ 0.19
5.03 ¡ 0.21
7.0
L-Threitol
1.45 ¡ 0.12 (0.99)
0.88 ¡ 0.19 (0.99)
1.52 ¡ 0.41 (0.99)
1.88 ¡ 0.12 (0.89)
0.60 ¡ 0.18 (1.00)
—
1.38 ¡ 0.08 (1.00)
0.51 ¡ 0.43 (0.98)
1.69 ¡ 0.30 (0.94)
1.45 ¡ 0.11 (1.00)
1.16 ¡ 0.22 (0.97)
1.10¡0.17 (0.99)
2.47 ¡ 0.02 (1.00)
3.36 ¡ 0.05 (1.00)
1.2 : 1
2.4 : 1
1 : 1.5
2.7 : 1
1 : 3.6
—
14.2 ¡ 0.76
12.8 ¡ 1.46
7.65 ¡ 0.25
9.86 ¡ 0.34
5.6
15.8 ¡ 0.58
13.5 ¡ 4.12
10.7 ¡ 0.30
9.23 ¡ 0.38
5.8
D-Glucose
D-Arabinose
D-Xylose
D-Fructose
D-Mannose
D-Galactose
D-Mannitol
4.0
6.5
4
2.60 ¡ 0.01 (1.00)
3.62 ¡ 0.03 (1.00)
1.4 : 1
1.8 : 1
5.75 ¡ 0.02
5.34 ¡ 0.02
5.62 ¡ 0.02
5.52 ¡ 0.05
D-Sorbitol
Dulcitol
2.83 ¡ 0.07 (1.00)
2.63 ¡ 0.02 (1.00)
2.86 ¡ 0.08 (1.00)
2.43 ¡ 0.03 (1.00)
1 : 1.0
1.6 : 1
5.77 ¡ 0.12
5.28 ¡ 0.02
5.53 ¡ 0.13
5.41 ¡ 0.03
D-Arabitol
Xylitol
Adonitol
L-Threitol
2.72 ¡ 0.02 (1.00)
1.72 ¡ 0.33 (1.00)
1.64 ¡ 0.03 (1.00)
2.71 ¡ 0.01 (1.00)
1.77 ¡ 0.03 (1.00)
1.91 ¡ 0.02 (1.00)
1 : 1
1 : 1.1
1 : 1.9
5.36 ¡ 0.02
5.25 ¡ 0.04
5.18 ¡ 0.05
5.41 ¡ 0.02
5.39 ¡ 0.04
5.03 ¡ 0.02
0.80 ¡ 0.12 (1.00)
1.81 ¡ 0.10 (0.99)
1.31 ¡ 0.13 (0.99)
2.48 ¡ 0.12 (0.98)
1.46 ¡ 0.09 (0.99)
1.37 ¡ 0.11 (0.99)
—
—
3.7
3.0
D-Glucose
D-Arabinose
D-Xylose
1.70 ¡ 0.10 (1.00)
1.29 ¡ 0.09 (1.00)
2.39 ¡ 0.08 (0.99)
1.54 ¡ 0.10 (0.99)
1.30 ¡ 0.16 (0.99)
1.3 : 1
1.0 : 1
1.2 : 1
1 : 1.2
1.2 : 1
5.99 ¡ 0.17
6.75 ¡ 0.36
5.94 ¡ 0.18
5.71 ¡ 0.18
6.66 ¡ 0.28
6.09 ¡ 0.19
6.11 ¡ 0.24
5.55 ¡ 0.11
6.76 ¡ 0.25
6.87 ¡ 0.47
D-Fructose
D-Mannose
D-Galactose
(r2 5 1.00) (R,R-3) and (2.02 ¡ 0.21) 6 103 M21 (r2 5 0.99)
(S,S-3), and for xylitol the binding constants are (1.03 ¡
0.03) 6 102 M21 (r2 5 1.00) (S-3) and (1.38 ¡ 0.09) 6
102 M21 (r2 5 1.00) (S,S-3), respectively, which are in good
agreement with the binding constants at pH 8.3.
alcohols (D-arabitol, xylitol and adonitol) and four-carbon
(four-hydroxy) sugar alcohol (L-threitol) is also weak. For
the six-hydroxyl sugar alcohols D-mannitol and D-sorbitol,
significant enantioselective recognition was also observed with
respect to both binding constant and fluorescence enhance-
ment and this will be beneficial in the development of an
analytical system. It should be pointed out that selectivity in
the sense of binding constants does not necessarily result in a
difference in fluorescence enhancement ability at saturated
analyte concentrations.
The data in Table 1 also indicate that the binding of sensor 3
with monosaccharides, such as D-glucose, D-fructose, etc., is
very weak compared to the binding with the six-carbon (six-
hydroxyl) sugar alcohols, such as D-mannitol, D-sorbitol and
dulcitol. The binding of the five-carbon (five-hydroxy) sugar
Fig. 4 Concentration titration curves of sensors 3 and 4 with D-mannitol. (a) Fluorescence intensity of R,R-3 and S,S-3 versus concentration of
D-mannitol. 3.0 6 1026 M of sensor in 0.05 M NaCl solution (52.1 wt% methanol), lex 5 365 nm, lem 5 429 nm. pH 8.3, 22 uC. (b) Fluorescence
intensity of R-4 and S-4 versus concentration of D-mannitol. 3.0 6 1026 mol dm23 of sensor 4 in 0.05 mol dm23 sodium chloride solution (52.1 wt%
methanol). lex 5 373 nm, lem 5 421 nm. 22 uC. pH 8.3.
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It was found that two binding sites are necessary for highly
chemoselective binding. The results with the monoboronic
sensor 4 show that with the monodentate sensor, the binding
constants for the six-hydroxyl sugar alcohols are usually lower
than the values obtained with the bidentate sensor 3. For
the five-hydroxyl sugar alcohols and the monosaccharides,
however, the binding constants increased compared to that of
the bisboronic acid 3. Therefore, the general chemoselectivity
observed with bisboronic acid 3 for the sugar alcohols
disappears in monoboronic acid 4. For example, with the
bisboronic acid sensor S,S-3, the chemoselectivity between
D-mannitol, D-arabitol, L-threitol and D-fructose is 228 : 4.5 :
1.0 : 3.0. For the monoboronic acid sensor S-4, however, the
values decrease to 3.6 : 3.4 : 1.0 : 3.1. It is interesting that for
the unfavorable analytes of sensor 3, the binding constants are
higher for the monoboronic acid than for the bisboronic acid
3. Usually higher binding constants are observed for the
bisboronic acid because of the collaborative effect of the 1 : 1
binding. We attribute this result to the rigid linker and the
sterically demanding binding pocket of 3. It should be pointed
out that the chiral binding pocket of sensor 3 is rather
crowded, therefore 1 : 1 complex formation with bulky
analytes, such as monosaccharides, is unfavorable and weaker
binding due to 2 : 1 complex formation is observed. Therefore,
monosaccharides and sugar alcohols having incorrect lengths
do not bind strongly with sensor 3 because 1 : 1 complex
formation is strongly disfavoured for these analytes. Also, due
to the steric requirements of sensor 3, 2 : 1 complex formation
is unfavorable. Thus, lower binding constants were observed
for sensor 3. For the favorable analytes, six-hydroxyl sugar
alcohols such as mannitol, the molecules have both the correct
length and geometry for the 1 : 1 complex formation, thus,
higher binding constants are observed for these sugar alcohols.
The crowded nature of the binding pockets of sensors 3 and
4 was confirmed by comparing the binding constants with
their achiral analogues, sensors 1 and 2.13 The binding of the
monosaccharides with the monodentate boronic sensor 4 is
lower than for sensor 2. For example, with D-glucose the
monodentate boronic acid R-4 gave a binding constant of
6.4 ¡ 0.9 M21 (for S-4, no reliable binding constant can be
enantioselectivity of sensors 3 and 4 with D-sorbitol decreased
from 23.8 : 1 to 1 : 1.8 even though the binding of D-sorbitol
with sensor 4 is stronger than with sensor 3. This clearly
indicates that the binding of D-mannitol and D-sorbitol with
sensor 3 is 1 : 1. These results demonstrate that two binding
sites are indispensable for strong binding of sugar alcohols and
significant enantioselectivity. More importantly, although the
binding constants were improved for the four- or five-hydroxyl
sugar alcohols relative to the six-hydroxyl sugar alcohols, the
enantioselectivity was reduced. This indicates that sensor 3
possesses a very specific chiral binding pocket. Indeed, in
sensor 3 the chiral centers are located in close proximity to the
binding sites and the linker separating the two binding sites
is a very rigid anthracene unit, thus sensor 3 shows high
chemoselectivity and enantioselectivity towards six-hydroxy
sugar alcohols over five- or four-hydroxy sugar alcohols and
monosaccharides.
As discussed previously, for chiral analytes a chiral sensor is
more favorable than an achiral sensor because with chiral
sensors, the chirality of the analytes can be utilized as an extra
differentiating factor to enhance discrimination. For example,
with R,R-3, the selectivity for D-sorbitol over D-glucose is 3 : 1,
but with S,S-3, the selectivity is improved to 83 : 1. For achiral
sensors, however, such optimization of selectivity is simply
impossible.
In order to unambiguously define the enantioselectivity of a
sensor system for chiral analytes, the best approach is to
recruit both enantiomers of the analytes in order to observe a
mirroring of the response with the two enantiomers of the
chiral sensor. However, this is not a simple task since the
enantiomers of the sugar alcohols are not readily available.
Fortunately, several sugar alcohols are achiral meso isomers
and with these isomers we should not observe enantioselec-
tivity with R,R-3 and S,S-3 i.e. for achiral sugar alcohols,
similar binding constants and fluorescence enhancement
should be observed with both R,R-3 and S,S-3. For meso-
dulcitol, no enantioselectivity was observed although its
binding with sensor 3 was the strongest for the anylates
studied, neither was enantioselectivity observed for meso-
xylitol and meso-adonitol.
determined), but the achiral sensor 2 gave a value of 63 M21
.
Sensor 3 also displays high enantioselectivity in the binding
with D- and L-tartaric acid. In this case, both the enantiomer
of tartaric acid and the sensor were used to prove that
enantioselective binding does occur.25 Similar results were
also observed for the monoboronic acid 4. It should be pointed
out that for the monoboronic acid sensor 4, small yet
significant enantioselectivity was also observed for the chiral
analytes, see (Table 1). We attribute this enantioselectivity to
the chiral center of sensor 4, which is in close proximity to the
binding site.
For the bisboronic acid sensor 3, the binding constant with
D-glucose is 28.2 ¡ 3.7 M21 (R,R-3) and 24.0 ¡ 2.0 M21 (S,S-
3), but for the achiral sensor 1, the value is 4.0 6 103 M21. The
lower binding constants of sensors 3 and 4 with these
saccharides indicate that the binding site is much more
sterically crowded than for the achiral analogues 1 and 2.
The data for other monosaccharides, such as D-fructose, also
support this analysis. Therefore, the binding pockets of chiral
sensors 3 and 4 are more crowded than their achiral analogues
and as a result, monosaccharides are not bound strongly.
Thus, the chemoselectivity of sugar alcohols over mono-
saccharides is improved for sensors 3 and 4 when compared to
sensors 1 and 2.
To the best of our knowledge, only one achiral sensor 5 for
sugar alcohols has so far been developed.16 Although the
small binding pocket of sensor 5 is selective for sugar alcohols
(such as D-mannitol, sorbitol, etc.) over monosaccharides, no
chemoselectivity was found for sugar alcohols with different
numbers of hydroxyl groups. For example, the selectivity of
sensor 5 for dulcitol over arabitol, and sorbitol over threitol
is 1.9 : 1 and 3.2 : 1, respectively. For the sensor S,S-3, the
selectivity is significantly improved to 282 : 1 and 216 : 1,
The results also indicate that two binding sites and a 1 : 1
complex formation are required for significant enantioselec-
tivity. For example, the enantioselectivity of sensors S,S-3 and
R,R-3 towards D-mannitol is greater than 2000 : 1. But this
value decreases to 1 : 1.4 for sensors S,S-4 and R,R-4. The
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respectively. We attribute this extraordinary chemoselectivity
to the rigidity of the linker between the two binding sites of
sensor 3. Thus, only sugar alcohols with the appropriate mole-
cular dimensions complement the binding pocket of sensor 3.
For sensors having a flexible linker, this selectivity will be
diminished due to self-adjustment of the binding complex.
hydroxide and hydrochloric acid solutions. For every con-
centration or pH data point the fluorescence was recorded
three times and the mean value was used for the titration
curves.
Concentration titration with analytes
The fluorescence spectra of the sensors in the presence of the
analytes were recorded as increasing amounts of the analyte
were added to the solution. For all titrations the final pH was
controlled to within less than ¡0.03 units of the desired pH.
Titration plots were generated using the Origin 5.0 (Microcal
software). The binding constants were calculated using
custom-written nonlinear least-square curve-fitting programs
implemented within SigmaPlot 2000 (SPSS Inc.).
Conclusions
In conclusion, the rigid linker, a chiral center close to the
binding site, and the crowded binding pocket of sensor 3 result
in significant enantioselectivity towards sugar alcohols, with
an enantioselectivity factor as high as 2000 : 1 for D-mannitol.
Sensor 3 also shows very high chemoselectivity towards six-
hydroxyl sugar alcohols over five- or four-hydroxyl sugar
alcohols and monosaccharides. The selectivity can be as high as
3600 : 1 (with S,S-3, dulcitol over D-arabinose). Furthermore,
the sensor is also highly sensitive for six-hydroxyl sugar
alcohols due to the tight binding, with a detection limit down
to the sub-millimolar concentration range. This offers the
opportunity to selectively monitor the concentration of
biologically important sugar alcohols in a variety of industrial
and medicinal applications. Moreover, the present design
strategy shows that by restricting the flexibility of the sensor,
e.g. by introducing a rigid linker and chiral center close to the
binding pocket, subtle structural differences between analytes
can be selectively amplified, resulting in a highly chemo- and
enantioselective sensor. We believe that the results obtained
with modular chiral sensors R,R-3 and S,S-3 provide a unique
insight into how to design other enantioselective sensors. We
are currently preparing sensors with different linkers and chiral
centers in a rigid arrangement in order to prepare sensors
selective for other analyates.
Jianzhang Zhao and Tony D. James*
Department of Chemistry, University of Bath, Bath, UK BA2 7AY.
E-mail: T.D.James@bath.ac.uk; Fax: +44 1225 386231;
Tel: +44 1225 383810
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Experimental
All of the solvents and chemicals were supplied by Frontier
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Fluorescence spectra
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365 nm and the emission intensity at 429 nm was used to
monitor the fluorescence intensity changes. A 0.05 M NaCl
(52.1% methanol, w/w) ionic buffer was used in the experi-
ment. The final concentration of the sensors was fixed at 5.0 6
1026 M (by dilution of the stock solution into the buffer by
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pH Titration
The fluorescence emission spectra of the sensors, with or
without the analytes, were recorded as the pH was changed
from pH 2 to 12 in approximate intervals of 0.5 pH units. The
pH was controlled using minimum volumes of sodium
25 J. Zhao, T. M. Fyles and T. D. James, Angew. Chem., Int. Ed.,
2004, 43, 3461.
This journal is ß The Royal Society of Chemistry 2005
J. Mater. Chem., 2005, 15, 2896–2901 | 2901