Hydrolytically stable Schiff base as highly sensitive aluminium sensor

Article abstract of DOI:10.1016/j.inoche.2012.05.032

A highly fluorescence sensitive sensor for selective detection of aluminium (III) and also to differentiate isomeric acids or two isomers of receptors are presented.

Full text of DOI:10.1016/j.inoche.2012.05.032

Inorganic Chemistry Communications 22 (2012) 98–100
Contents lists available at SciVerse ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Hydrolytically stable Schiff base as highly sensitive aluminium sensor
Soham Samanta, Bhaskar Nath, Jubaraj B. Baruah ⁎
Department of Chemistry, Indian Institute of Technology, Guwahati, 781 039, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A highly fluorescence sensitive sensor for selective detection of aluminium (III) and also to differentiate iso-
Received 11 April 2012
Accepted 15 May 2012
Available online 24 May 2012
meric acids or two isomers of receptors are presented.
©
2012 Elsevier B.V. All rights reserved.
Keywords:
Schiff base
Aluminium sensor, Fluorescence‐on
Hydrolysis
Aluminium is an indispensible element due to its abundance and
use in every sphere of life from utensils to medicine [1–3]. Detection
of aluminium in mixed metal environment is of general concern [4].
On the basis of their differences of response in fluorescence emission,
Schiff bases and related heterocyclic compounds are generally used
for detection of aluminium ions [5–9]. Schiff bases have tendency to
undergo hydrolysis, so, hydrolytic stability of such receptor is impor-
tant. Metal complexes having Schiff base ligands are often used in
various organic transformations, thus fluorophore containing Schiff
bases are of interest to monitor in situ catalytic reactions [10]. The
use of Schiff bases in homogeneous or heterogeneous conditions
may have long standing impact to design receptors for detection of al-
uminium ions. In the present study three imine containing ligands il-
lustrated in Fig. 1, each of them having a chelating site which is one of
the prerequisite structural features to have favourable interactions
with metal ions is chosen to study their selectivity in sensing different
ions. All these ligands 1–3 have two common features, namely a che-
lating site to metal ion and other is each of them possesses a naphtha-
lene ring as fluorophore.
and compared their spectroscopic properties with standard authentic
samples. The compound 1 has absorption maximum at 487 nm; upon
degradation the solution turns pink; new set of visible peak appears at
563 nm (Fig. 2). Generally, hydrolysis of imines is facilitated by acids,
but it is found that compound 1 is relatively more stable in protonated
state with respect to its neutral state, the later state undergoes facile hy-
drolysis. The compound 1–3 shows visible absorptions at 487, 389 and
445 nm respectively, hence the excitation in each case were done near
the absorption maximum and fluorescence emissions were studied
under different conditions. Hydrofluoric acid (5 μl) quenches fluores-
−
6
cence emission of 1 (5×10
M solution in methanol); other acids
such as HCl, HNO etc. also quench the fluorescence emission of 1 in
3
the similar concentration. However, the compounds 2 and 3 follow a re-
verse trend with respect to the compound 1 in emission upon addition
of these acids, and in the cases of 2 and 3 enhancement of emission in-
tensity with a shift in emission maximum occurred. The compounds 1
and 3 are positional isomers, by comparing the trends in fluorescence
emission of these two isomers 1 and 3 upon treatment with a mineral
acid the two isomers can be differentiated. The compound 1 is highly
solvatochromic and it shows a large shift in positions of absorption
maxima in different solvents (refer supplementary materials). Due to
hydrolytic instability, compound 1 is not useful for detection of metal
ions. Attempt to study detection of metal ions by this compound in
The imine group containing compound 1 is hydrolytically un-
stable; but it is stable in common organic solvents. Upon addition
of a drop of water to a DMSO or methanol solution of 1, it trans-
forms to corresponding 1,4-phenylenediamine and the 2-hydroxy-
2
+
2+
2+
2+
naphthaldehyde. The hydrolysis of 1 is studied in an NMR tube by re-
methanol, showed that the metal ions such as Co , Ni , Cu , Zn
1
3+
cording HNMR spectra of compound 1 by dissolving it in DMSO-d
6
and Al cause reduction of fluorescence emission, such reductions in
fluorescence intensity are so less that they are comparable with the di-
lution effect.
1
and followed by recording the HNMR of the same solution by adding
a drop of deuterium oxide. In situ generated degraded products by ad-
dition of D
O can be easily identified by interpreting the 1HNMR
On the other hand, compounds 2 and 3 have the ability to recog-
2
nize Al3 very selectively. They show strong green fluorescence emis-
+
(
supporting information). Further the degraded products are purified
3+
sion in the presence of traces of Al ions. The fluorescence emission
enhancement of 2 by Al3 occurs at 445 nm (λex =380 nm) whereas
it occurs at 489 nm (λex =440 nm) for compound 3. The emission
changes of a methanol solution of 2 on addition of Al3 are shown
+
⁎
Corresponding author. Tel.: +91 361 2582301; fax: +91 361 2690762.
+
E-mail address: juba@iitg.ernet.in (J.B. Baruah).
1387-7003/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2012.05.032

S. Samanta et al. / Inorganic Chemistry Communications 22 (2012) 98–100
99
HO
H
N
H
HO
HO
N
H
N
HO
N
N
2
H N
H
NO
2
OH
H
1
2
3
Fig. 1. Structure of the receptors.
in Fig. 3. The fluorescence enhancement of 3 in methanol by Al3+ ions
of identical concentration is about approximately 2.2 times higher
relative to fluorescence enhancement of 2 under similar conditions.
Fig. 3. Fluorescence (λex =380 nm) enhancement of 2 (2 ml, 10− M in methanol) on
4
3
+
−4
addition of aluminium chloride hexahydrate (3.8×10−3 M, 20 μL in methanol in each
Addition of Al solution in methanol (20 μl of 10
M) to a solution
−
5
aliquot).
of 2 (2 ml, 10
M in methanol) shows about 60 times fluorescence
intensity enhancement; whereas the same solution on dilution by equal
volume of methanol shows about 200 times higher fluorescence intensity
enhancement with respect to the fluorescence intensity of fluorescence
emission of a solution of compound 2 in a similar concentration. The com-
pounds 2 and 3 have characteristic fluorescence emission features; from
the changes in these features the sensing properties and selectivity to-
wards an ion can be made.
Cu2+, interfere in the fluorescence emission caused by Al3+ of a Schiff
base derived from salicylaldehyde [6]. Our compounds 2 and 3 are supe-
rior in this regard, for example, the compound 3 shows fluorescence en-
hancement when a mixture of Al3 and Fe (1:4 molar ratio) is added
to methanol solution of 3, in this case the emission intensity is changed in-
significantly with respect to fluorescence emission obtained by adding a
+
3+
3+
Similar to the compound 2, dilution affects the enhancement of the
fluorescence intensity while detecting aluminium. This indicates that dis-
crete units of compound 2 are associated with aluminium ions probably
through chelation and these units are responsible for increase in fluores-
pure solution of Al to a solution of 3 in methanol. A similar observation
was found in the case of Al3 and Cu in 1:4 ratio, which causes 40%
+
2+
3+
fluorescence quenching, and similarly in the case of a addition of Al
and Zn2 in 1:4 ratio causes about 80% fluorescence quenching with re-
+
cence intensity. The ions like Co2 , Ni , Cu and Zn , quench the fluo-
+
2+
2+
2+
spect to emission intensity shown by only Al ion containing methanol
3+
rescence emission of 2. Thus the receptor 2 provides fluorescence-on with
solution of 3 (refer to supplementary bar diagrams). The fluorescence
3+
2+
3+
3+
3+
2+
Al and fluorescence-off with Zn . The fluorescence-on by Al can be
emission spectra of 3 is insignificantly affected by In ; and In , Co
,
2+
2+
3+
made to off by Zn whereas the reverse is not observed. The visible spec-
Ni do not interfere in the emission changes of 3 caused by Al ions.
2+
3+
troscopic titration of 2 with Zn clearly shows isosbestic point showing
In this study we have used aluminium chloride as the source of Al for
2+
complexation of Zn to ligand 2 (Fig. S7). The compound 2 does not rec-
2
+
2+
ognize Hg , and Cd . The compound 3 shows fluorescence quenching
with Co2+, Ni , Cu , Zn and Fe . It was reported that ions of Fe
2+
2+
2+
3+
3+
,
Fig. 2. The changes in visible absorption of 1 (2 ml, 10− M solution in methanol) upon
addition of water (100 μL in each aliquot).
5
Fig. 4. Changes in fluorescence emission (λex =380 nm) of compound 2 (2 ml, 10
−5
M
in methanol) upon addition of maleic acid (10− M in methanol, 10 μL per aliquot).
2

100
S. Samanta et al. / Inorganic Chemistry Communications 22 (2012) 98–100
it's detection, Al3+ can also be easily produced from aluminium scrap by
treatment with hydrochloric acid, however, the acidic solutions are not
very sensitive for aluminium detection by 3, hence we have to carry out
such detection in neutral condition. The compound 2 has an extra ability
to distinguish maleic acid and fumeric acid. Independent addition of these
two isomeric acids to the solution of 2 in methanol show different fluores-
cence responses. When maleic acid solution is added to a solution of 2 in
methanol, a new emission peak at 438 nm is generated (λex=380 nm)
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Appendix A. Supplementary material
[
1
-phenyl-3-(quinolin-5-yl)urea, J. Mol. Struct. 969 (2010) 75–82.
Supplementary data to this article can be found online at http://
dx.doi.org/10.1016/j.inoche.2012.05.032.