Full text of DOI:10.1016/j.tetlet.2009.04.108

Tetrahedron Letters 50 (2009) 4201–4204
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
A dual detecting polymeric sensor: chromogenic naked eye detection
of silver and ratiometric fluorescent detection of manganese
Narinder Singh, Navneet Kaur, Catriona Ni Choitir, John F. Callan *
School of Pharmacy and Life Sciences, The Robert Gordon University, Aberdeen, Scotland AB10 1FR, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Polymer 1, a poly(olefin) with pendant Schiff base units is prepared by reacting poly(allylamine) and 2-
Received 27 February 2009
Revised 9 April 2009
Accepted 28 April 2009
Available online 3 May 2009
2
hydroxynapthaldehyde in methanol. The optical properties of 1 are investigated in THF:H O (9:1) solvent
system buffered at pH 7.0 ± 0.1 using HEPES. When tested against a range of physiological and environ-
+
mentally relevant cations 1 displays selectivity for Ag from changes in the UV–vis spectra with linearity
+
established in the range of 10–60
l
M. The addition of Ag to a solution of 1 also causes a colour change
+
from colourless to dark yellow meaning Ag is also detectable by the naked eye. In contrast, selectivity is
observed for Mn2 from changes in the emission spectra again with good linearity observed in the range
+
+
2+
of 0–50 nM. This is the first reported polymeric sensor capable of detecting both Ag and Mn using two
different detection modes.
Ó 2009 Elsevier Ltd. All rights reserved.
The development of new synthetic sensing assemblies for the
recognition and detection of physiologically and environmentally
important analytes is of considerable interest. UV–vis and fluores-
Similarly, manganese is a toxic, yet essential trace element
meaning it is not only essential for human health but also can be
detrimental if present in larger quantities. Therefore sensors that
can monitor both silver and manganese concentrations have obvi-
ous benefits.
1
cence spectroscopy remains the most frequently used modes of
detection in these sensors due to their high sensitivity, adaptability
and relative inexpense. As selectivity is a major priority of any sen-
sor, most are designed to show high specificity for a single target,
and as such, are limited to reporting on the concentration/location
of one analyte per sensor. More recently however, there has been a
shift towards the development of sensors capable of simulta-
neously recognising more than one analyte.2 Molecular logic, a
concept pioneered by de Silva and co-workers is one such example,
where two (or more) analytes are required to bind with a probe at
+
We have developed a polymeric sensor capable of detecting Ag
2
+
and Mn by colorimetric and fluorescence detection, respectively.
The sensor consisted of a poly(olefin) backbone with pendant
naphthalene-containing Schiff base receptors. We have previously
demonstrated that the immobilisation of Schiff base receptors on a
Quantum Dot (QD) surface led to an enhancement in the selectivity
of the resulting QD-Schiff base conjugate relative to the receptor
9
alone. Similarly, we demonstrate here that immobilisation of this
3
predefined levels before a single fluorescence signal is observed.
class of receptors on a polymeric backbone results in selectivities
not observed for the receptor alone. To the best of our knowledge,
this is the first reported example of a polymeric sensor for the dual
Semi-selective receptors that enable the binding of two guests
4
with distinct spectral responses have also been reported. Such
+
2+
sensors have benefits over their single analyte analogues as many
physiologically deleterious conditions are indicated by the abnor-
detection of both Ag and Mn .
The sensor 1, was synthesised in one step from commercially
available precursors by reacting 15 kDa poly(allylamine) (PAA)
with an excess of 2-hydroxy-1-naphthaldehyde in dry methanol.
After stirring for 18 hours a product was observed to precipitate
from solution and was collected by filtration. It was further
purified by redissolving in chloroform and by precipitating with
methanol. Control compound 2 was prepared by reacting 2-hydro-
xy-1-naphthaldehyde with aniline in methanolic solvent as
5
mal concentration of more than one analyte. In addition, the abil-
ity to screen solutions for more than one target at a time leads to
faster analytical processing times and potential cost reduction.
Both silver and manganese can produce adverse health effects in
humans if present in sufficient quantities. For example, silver in
any form is a bioaccumulative toxic heavy metal and in extreme
cases, accumulation in humans can cause Argyria, a condition char-
6
10
acterised by a bluish-grey colouration of the skin. The increased
detailed by us in a previous communication.
7
use of silver nanoparticles as potential therapies and biochemical
The successful grafting of 2-hydroxy-1-naphthaldehyde onto
8
1
labels increases the likelihood of exposure to this element.
PAA was confirmed by H NMR spectroscopy, with Figure 1 show-
ing the stacked spectra of 2-hydroxy-1-naphthaldehyde (top) and
sensor 1 (bottom). The aldehyde proton, observed at 10.80 ppm
in 2-hydroxy-1-naphthaldehyde was absent in the spectrum of 1
*
Corresponding author. Tel.: +44 1224 262546; fax: +44 1224 262555.
E-mail address: j.callan@rgu.ac.uk (J.F. Callan).
0
040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.04.108

4
202
N. Singh et al. / Tetrahedron Letters 50 (2009) 4201–4204
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
(a)
Figure 1. 1H NMR spectra of (a) 2-hydroxy-1-naphthaldehyde and (b) Sensor 1,
recorded in CDCl
3
.
0
.7
.6
.5
0
0
while the hydroxy proton, present at 13.15 ppm in the starting
material was observed further downfield as a broad resonance cen-
tred at 14.75 ppm. The downfield shift of the hydroxy proton
accompanied by its significant broadening suggests a hydrogen
bonding interaction between neighbouring hydroxyls of adjacent
(b)
0.4
0.3
0
0
.2
.1
0
1
1
receptors. Formation of the Schiff base was confirmed by the
presence of the imine proton at 8.60 ppm which was not present
in the starting material. The aromatic region also experienced
slight changes in chemical shifts relative to the monomer and
the peaks were generally much broader, consistent with anchorage
on a polymer backbone.12 The backbone protons and first side-
chain methylene protons were observed in the upfield region as
a group of resonances centred at 1.35, 1.80 and 3.30 ppm. By
comparing the relative integration of the aromatic and backbone
regions the percentage incorporation was judged to be 100% from
which a molecular weight of ꢀ50 kDa was established (Scheme 1).
2
75 300
325 350 375
400 425 450
Wavelength (nm)
0
.6
(c)
0.5
0
0
.4
.3
The optical properties of 1 were investigated in a THF:H
2
O (9:1)
0
20
40
60
solvent system buffered at pH 7.0 (HEPES). The UV–vis spectrum
was characterised by two main bands centred at 308 nm and
[Ag(I)] micromolar
Figure 2. (a) Bar chart illustrating the % change in absorbance at 310 nm for 1 upon
addition of various metal ions (b) UV–vis spectra of 1 upon increasing concentration
4
18 nm. The effect of a range of environmentally and physiologi-
cally important metal ions on the UV–vis spectrum was investi-
gated by preparing solutions containing 1 (20 M) and the ion
100 M). A substantial enhancement of the 310 nm and 418 nm
+
(
0–125
l
M) of Ag (c) plot of absorbance intensity of
1
at 310 nm against
+
l
concentration for Ag . [1] = 20
l
M, solvent: THF:H O (9:1) HEPES buffer solution
2
(
l
at pH 7.0 ± 0.1.
+
bands was observed upon addition of Ag while the other ions pro-
duced minimal effects on the absorbance intensity at these wave-
lengths (see Supplementary data). When the changes at 310 nm
The fluorescence spectrum of 1 was characterised by three
weakly emissive bands centred at 357, 454 and 518 nm when
excited at 275 nm. As in the UV–vis study the effect of adding ions
on the emission profile of 1 was investigated (see Supplementary
data). Significant changes were observed upon addition of Mn
with substantial enhancements for the peaks at 357 and 454 nm,
the former being accompanied by a red shift (ꢀ19 nm) of the emis-
sion maximum. The weak fluorescence observed for 1 in the
absence of metal ions is most likely due to a rapid isomerisation
about the C=N bond leading to non-radiative decay of the excited
+
were monitored, good selectivity was observed for Ag with a
7
0% increase in absorbance when compared to the starting inten-
sity (Fig. 2a).
The titration of Ag+ ions with a solution of 1 caused a gradual
increase in intensity of both the 310 nm and the 418 nm bands,
the former accompanied by a slight blue shift and the latter accom-
panied by a slight red shift of the absorption maxima (Fig. 2b).
2+
,
+
When a plot of the absorbance intensity at 310 nm against Ag con-
centration was made, good linearity was observed in the range of
+
1
0–60
l
M (Fig. 2c). The effect caused by Ag was very significant
2+
state. Binding of Mn, however, locks one particular conformation
that it could be detected by the naked eye with a distinct change
in colour of the solution from colourless to dark yellow while the
colour of the solutions containing the other ions remained
relatively unchanged (Fig. 3).
(
i.e., cis or trans) in place resulting in an enhancement of fluores-
13
2+
2+
cence. Of the remaining ions only Co and Sr produced any
significant effect on the emission profile of 1 with enhancements
of 48% and 55% when compared to the starting intensity, and much
2
+
lower than the 195% enhancement observed for Mn (Fig. 4a).
The changes in the fluorescence spectra of 1 upon the continuous
2
+
addition of Mn ions are shown in Figure 4b. The results show a
gradual increase and red shifting of the 357 nm band, a gradual
increase of the 454 nm band and a small decrease in the 518 nm
band. The presence of an isoemission point at 492 nm was also
observed. These significant changes offer great flexibility in
selecting wavelengths for the ratiometric determination of Mn2+
by fluorescence. Ratiometric monitoring has benefits over single
wavelength monitoring in that it is free from the errors associated
with sensor concentration, photobleaching and local environmental
O
n
OH
Dry MeOH
N
N
n
+
OH
OH
NH2
PAA
1
2
Scheme 1.

N. Singh et al. / Tetrahedron Letters 50 (2009) 4201–4204
4203
1
Li+ Na+
K+ Mg2+ Ba2+ Sr2+ Ca2+ Mn2+ Fe3+ Co2+ Ni2+
Cu2+ Zn2+
Ag+ Cd2+
Figure 3. Photograph of 1 in the presence of various metal ions. [1] = 20 lM, [ions] = 100 lM in a THF:H O (9:1) HEPES buffer at pH 7.0 ± 0.1.
2
and excited states was not totally unexpected as it has previously
been shown by Bhardwaj et al. that tripodal Schiff base sensors
containing structural motifs similar to the repeat unit present in
2
1
1
00
50
00
(
a)
+
15
1
display chromogenic selectivity for Ag . In addition, we have
also shown that Schiff base receptors can undergo Excited State
Intramolecular Proton Transfer (ESIPT) to produce keto and enol
1
0
tautomers with different binding selectivities. Unfortunately, as
polymeric systems are characterised by non-exact molecular
weight distributions we were unable to determine the binding
5
0
0
+
2+
stoichiometry occurring between 1 and Ag /Mn
.
In summary, we have reported for the first time that, by
appending Schiff base receptors on a polymeric backbone, the
selectivity of the resulting macromolecule can be improved signif-
icantly. Moreover, we illustrate that different selectivities were
observed in the ground and excited states, with the former
2
00
50
00
(
b)
1
1
+
2+
enabling detection of Ag and the latter Mn . This is, to the best
of our knowledge, the first reported example of a dual detecting
+
2+
polymer capable of detecting both Ag and Mn . In addition, the
greater processing ability of polymers (i.e., into films, etc.) com-
pared to that of low molecular weight compounds is another
advantage of this system. We believe this approach could be
extended to many more receptors and may result in a new gener-
ation of optical sensors.
5
0
0
3
25
425
525
Wavelength (nm)
2
1
0
.5
2
(
c)
Acknowledgements
y =0.0242x + 0.745
2
R = 0.9957
The authors thank the E.P.S.R.C. and R.G.U. for financial support.
.5
1
Supplementary data
.5
0
Supplementary data (experimental procedures and spectral
data for various compounds) associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.2009.04.108.
0
10
20
30
40
50
[
Mn(II)] nanomolar
Figure 4. (a) Emission spectra of 1 in the presence of various metal ions, (b)
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