A new fluorescent "off-on" sensor for Al3+ derived from L-alanine and salicylaldehyde

Article abstract of DOI:10.1007/s10895-015-1647-5

The condensation product of L-alanine and salicylaldehyde was synthesised and characterised which was found to be selective fluorescent "on" sensor for Al³⁺ ion with the detection limit 10^-6 M. The sensor is free of interferences from metal ions - Na⁺, K⁺, Ca²⁺, Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺, Pb²⁺, Cd²⁺, Hg²⁺ and Fe³⁺. The Fluorescence and the UV/visible spectral data reveals a 1:1 interaction between the sensor and Al³⁺ ion with binding constant 10^4.5. The DFT and TDDFT calculations confirm the structures of the sensor and the sensor-Al³⁺ complex.

Full text of DOI:10.1007/s10895-015-1647-5

J Fluoresc
DOI 10.1007/s10895-015-1647-5
ORIGINAL ARTICLE
3+
A New Fluorescent BOff-On^ Sensor for Al Derived
from L-alanine and Salicylaldehyde
1
2
1
Smita Sarma & Pradip Kr. Bhattacharyya & Diganta Kumar Das
Received: 20 May 2015 /Accepted: 18 August 2015
#
Springer Science+Business Media New York 2015
Abstract The condensation product of L-alanine and
salicylaldehyde was synthesised and characterised which
disease, Parkinsonism dementia, osteoporosis, colic, and rick-
3
+
ets. The Al ions also damages plant roots [4]. The World
3
+
3+
was found to be selective fluorescent Bon^ sensor for Al
Health Organization (WHO) listed Al as one of the prime
−6
−1
ion with the detection limit 10 M. The sensor is free of
food pollutants and limited its concentration to 200 μg L
+
+
2+
2+
2+
interferences from metal ions - Na , K , Ca , Mn , Co ,
(7.41 μM) in drinking water [5]. WHO recommended tolera-
2
+
2+
2+
2+
2+
3+
3+
−1
Ni , Cu , Pb , Cd , Hg and Fe . The Fluorescence and
the UV/visible spectral data reveals a 1:1 interaction between
the sensor and Al ion with binding constant 10 . The DFT
and TDDFT calculations confirm the structures of the sensor
ble weekly dietary human intake of Al is 7.0 mg kg body
3
+
weight [6].Therefore the detection of Al in water is of envi-
ronmental, biological and human health importance. At pres-
ent, different detection methods, like atomic absorption and
emission, spectrophotometry, electrochemiluminscence and
3
+
4.5
3
+
and the sensor-Al complex.
3
+
electrochemical methods are known for detection of Al ion
7–10]. But due to the expensive instrumentations, require-
[
.
.
.
.
Keywords Fluorescence Sensor Aluminium L-alanine
ment of highly-trained operators and complicated pre-
treatment makes these methods difficult for routine monitor-
ing and applications.
.
Salicylaldehyde TDDFT
Recently fluorescent sensors have gained considerable at-
tention due to their operational simplicity, low detection limit,
real-time detection and portability. A number of fluorescent
Introduction
3
+
Aluminium (Al) is the most abundant metal in the earth’s crust
which is approximately 8 % of the total mass of the earth [1].
Modern life is largely dependent on Al in various forms like -
light alloys, pharmaceuticals, water purification instruments,
house hold utensils etc. [2] Al as such is normally not harmful
but due to the environmental acidification the amount of
sensors for determination of Al have been reported in recent
3
+
past. Fluorescent turn Bon^ Al -sensor has been synthesized
from 8-hydroxyquinoline-7-carbaldehyde and 4-aminopyrine
[11]. Another turn Bon^ fluorescent sensor has been reported
based on quinoline-coumarine [12]. A perylene
tetracarboxylic bisimide based sensor reported to show green
fluorescence under UV radiation [13]. Blue fluorescence en-
3
+
harmful free Al ions in soil increases. [3] These ions may
reach human body through plants and may cause Alzheimer’s
3
+
hancement for Al was shown by Schiff base receptor,
-((E)-(1,3-dihydroxy-2-(hydroxymethyl)propan-2-
ylimino)methyl) naphthalene-2-ol [14]. A coumarin based
1
3
+
*
Diganta Kumar Das
digkdas@yahoo.com
Al ion sensor is reported to act as turn Bon^ fluorescent
3
+
Al sensor by emitting bright blue fluorescence under UV
radiation [15]. A dual fluorescent turn Boff^ and colorimetric
sensor which shows a large red shift in fluorescent emission
1
2
Department of Chemistry, Gauhati University, Guwahati,
Assam 781 014, India
3
+
upon interaction with Al is also reported [16]. They are
3
+
attractive, although most of these sensors for Al involve
Department of Chemistry, Arya Vidyapeeth College,
Guwahati, Assam 781 016, India
complicated synthetic procedure. Simple Schiff’s bases as

J Fluoresc
Scheme 1 Structure of L (left)
and DFT optimised structure of L
(right)
metal ion fluorescent sensors have attracted recent interest due
to their simple synthetic procedure [17–20]. Therefore
reporting of new fluorescent sensor which is of ‘easy to syn-
distilled in quartz distillation plant. The metal salts were re-
crystallised from water before use.
The Fluorescence spectra measurements were performed in
a Hitachi FL-2500 florescence spectrophotometer, using a
quartz cuvette (1 cm), and both the excitation and emission
3
+
thesize’ type, for Al is fascinating and significant.
In recent years the DFT (density functional theory)
methods have proven their wide range of applicability to as-
certain structures of molecular complexes while the TDDFT
1
band passes were set at 5.0 nm. H NMR spectra were record-
ed in a Bruker Ultrashield 300 MHz spectrometer. All NMR
(
time dependent DFT) methods are successful in explaining
spectra were obtained in CDCl at room temperature and the
3
the electronic properties [21–23].
chemical shifts were reported in δ values (ppm) relative to
TMS. The UV-visible spectra were recorded on a Shimadzu
UV-vis-1800 spectrophotometer. The metal salt solutions
(10 M) were prepared in phosphate buffer solution (PBS)
at pH 7.0.
In this paper we report a new Boff-on^ fluorescent sensor
3
+
for Al ion derived from L-alanine and salicylaldehyde
3
+
+
+
2+
2+
−3
which is selective for Al over - Na , K , Ca , Mn ,
2
+
2+
2+
2+
2+
2+
3+
Co , Ni , Cu , Pb , Cd , Hg and Fe . The DFT and
the TDDFT calculations for the sensor and its interaction with
3
+
Al ion have also been reported.
Synthesis of Sensor (L)
One m mol (0.112 g) of l-alanine and 1 m mol (0.90 mL) of
salicylaldehyde were taken in a 1:5 (v/v) mixture of water and
methanol (30 mL) in a round bottom flask. The mixture was
stirred for 24 h to get an oily product. The solvent was evap-
orated in rota evaporator to obtain a dark greenish coloured
product which was recrystallized from methanol
(mp = 110 °C, yield = 70 %) (Scheme 1).
Experimental
Chemicals and Experimental Techniques
All the chemicals (analytical grade) were from Loba Chemie
except Methanol (Merck). The Metal salts were sulphates ex-
cept Pb(NO ) , CaCl , and HgCl . The Water used was double
3
2
2
2
Fig. 2 Fluorescence spectra of L (5 × 10− M) in 1:1 (v/v) CH
6
OH:H
O
3
2
−6
3+
Fig. 1 Fluorescence spectral changes of L (5 × 10 M) at different
(pH = 7.0) in presence of one equivalent amount of Al ion and metal
3+
+
+
2+
2+
2+
2+
2+
2+
2+
2+.
3+
added concentration of Al ion in 1:1 (v/v) CH
3 2
OH:H O (pH = 7.0)
ions - Na , K , Ca , Mn , Co , Ni , Cu , Pb , Cd , Hg and Fe

J Fluoresc
Fig. 3 Bar diagram comparing
the I/I
in 1:1 (v/v) CH
pH = 7.0) upon interaction with
0
values of L (5 × 10− M)
6
3 2
OH:H O
(
3
+
+
+
Al and metal ions - Na , K ,
Ca , Mn , Co , Ni , Cu ,
Pb , Cd , Hg , and Fe
2
+
2+
2+
2+
2+
2
+
2+
2+
3+
FT-IR spectrum (KBr): 3400 cm due to ν ; 1635 cm−1
−
1
were recorded at different added concentration of Al . The
3+
O-H
−1
−1
due to ν ; 1645 cm due to νC=O; 1315 cm due to νC-N
;
fluorescence intensity was found to increase gradually with
Al accompanied by a blue shift in λmax to 460 nm (Fig. 1).
C=N
1
1
3+
7
61 cm− due to δ=C-H. HNMR (CDCl , δ in ppm, TMS):
3
1
1.03 (COOH); 8.32 (=N-CH); 7.45, 7.11, 6.86, 6.73 (phenyl
The enhancement in fluorescence intensity ceases when
3
+
ring H); 4.94 (−OH); 4.50 (N-CH); 1.70 (−CH3).
L:Al concentration ratio became 1:1. The final fluorescence
intensity was ca. 40 times to that of the initial one. Figure 1,
3
+
Computational Details
Inset shows the plot of I/I as a function of Al concentration
0
where I and I are the fluorescence intensities of L in absence
0
3
+
The ground state geometrical minima of the ligand was opti-
mized at B3LYP/6-311++G(d,p) [24–27] and was confirmed
by the absence of any imaginary frequency. Further, time de-
pendent DFT (TDDFT) calculations were performed on the
optimized structures of the L:Al complex to assign the ob-
served electronic transitions in the UV–Visible spectra
and presence of Al respectively.
Similar fluorescence titrations were performed for L
−6
+
(5 × 10 M) in 1:1 (v/v) CH OH:H O for metal ions - Na ,
3
2
+
2+
2+
2+
2+
2+
2+
2+
2+
3+
K , Ca , Mn , Co , Ni , Cu , Pb , Cd , Hg and Fe .
Figure 2 shows the effect of these metal ions and Al ion on
3
+
3+
fluorescence spectra of L. From the figure it can be seen that
3
+
[
28–30], which is implemented in Gaussian09 [31]. The
the effect of Al ion on the fluorescence intensity of L is quite
TDDFT calculations were carried out at the same level of
theory in gas phase using N states =40.
distinctive over the rest of metal ions. The effect of different
metal ions on I/I values of L has been compared by the bar
0
diagram shown in Fig. 3. Where, I and I are the fluorescence
0
intensities of L in presence and absence of a metal ion respec-
2
+
Results and Discussion
tively. Fluorescence quenching was observed for Co and
2
+
Cu while a moderate fluorescence enhancement of ca. 15
times was observed for Cd . No significant enhancement or
3
+
2+
Fluorescence Detection of Al by L
quenching in fluorescence intensity of L was observed for
L (5 × 10− M) in 1:1 (v/v) CH OH:H O showed moderate
6
interaction with the rest of the metal ions. Figures 2 and 3
3
2
3
+
fluorescence with intensity ca. 70 when excited with 380 nm
photon. The fluorescence emission was observed in 390 nm to
00 nm range with λmax at 495 nm. Fluorescence spectra of L
clearly prove that L acts as a fluorescent sensor for Al is
+
+
2+
2+
2+
2+
2+
over metal ions - Na , K , Ca , Mn , Co , Ni , Cu ,
2
+
2+
2+
3+
6
Pb , Cd , Hg and Fe .
Fig. 4 Bar diagram comparing
3+
0
the I/I values of L + Al (black
3+
n+
bars) and L+ Al +M (gray
bars) to show the selectivity of L
3+
towards Al over metal ions-
+
+
2+
2+
2+
2+
Na , K , Ca , Mn , Co , Ni ,
2+
2+
2+
2+
3+
Cu , Pb , Cd , Hg and Fe

J Fluoresc
3
+
In order to prove the selectivity of L towards Al in pres-
ence of other metal ions, fluorescence spectra were recorded
3
+
for L in presence of one equivalent each of Al and another
+
+
2+
2+
2+
2+
2+
2+
metal ion viz. Na , K , Ca , Mn , Co , Ni , Cu , Pb ,
2+
2+
3+
Cd , Hg and Fe . The I/I values have been compared
with I/I value of L in presence of one equivalent of Al
0
3
+
0
ion only (no other metal ion) through bar diagram (Fig. 4).
Here, I is the fluorescence intensity of L while I is the fluo-
0
3
+
rescence intensity of L in presence of one equivalent of Al
3
+
or Al and one equivalent of a metal ion. Comparable heights
3
+
3+
of the bars corresponding to L with Al and L with Al and
another metal ion were observed. This confirms that the inter-
3
+
action between L and Al is stronger than that between L and
+
other metal ions. However in presence of Na ion the selec-
3
+
tivity of L towards Al was found to be a little less.
The binding constant and stoichiometry of binding was
determined from fluorescence data as reported [17]. For this
−3
Fig. 6 Change in UV/Visible spectra of L (10 M) in 1:1 (v/v)
3
+
3+
purpose, log[(I -I)/(I-I )] was plotted against log[Al ],
0
max
CH
(A -A
s
3
OH:H
α
)] versus log[Al ]
2 0 s
O (pH 7.0) with the addition of Al . Inset Plot of log[(A -A )/
3+
where I is the fluorescence intensity of L, I is the fluorescence
0
3
+
intensity of L at a given added concentration of Al and Imax
is the fluorescence intensity of I at one equivalent concen-
tration of Al . The plot of log[(I -I)/(I-I )] versus log[Al ]
max
4
.2,
3
+
3+
slope of the plot, was 1:1 with binding constant 10 which is
given by the intercept at abscissa. These values are in good
agreement with those obtained from fluorescence data. The
detection limit of L towards Al was calculated from the plot
of normalised fluorescence intensity versus log[Al ], as re-
0
max
was linear (Fig. 5). The slope of the plot is 1.19 which implies
3
+
that the binding interaction between L and Al is 1:1. The
3
+
4
.5
binding constant was calculated as 10 .
3
+
The binding stoichiometry and the binding constant, ob-
−6
3
+
ported [32] and found to be ca. 10 M.
tained from fluorescence data, between L and Al were con-
firmed by UV/visible spectroscopy. UV/visible spectra were
recorded for L in 1:1 (v/v) CH OH:H O with peaks observed
The enhancement in fluorescence intensity of L on interac-
3
+
tion with Al could be explained on the basis of photoin-
duced electron transfer (PET) process. The electron density
on the lone pairs of N and the pair of O atoms in the sensor
gets transferred to the LUMO of the fluorophore part which is
comprised of the phenyl ring and the immine bond. This
3
2
3+
at λmax 260 nm, 340 nm and 420 nm. Effect of Al on the
UV/Visible spectra of L has been shown in Fig. 6. With the
3
+
addition of Al the peak at 420 nm gradually disappeared, the
peak at 340 nm was shifted to 350 nm with little increase in
absorbance and absorbance of the peak at 260 nm decreased a
3+
causes the low fluorescence of L in unbound to Al form.
3
+
When Al binds to L through the N and the O lone pairs, the
PET process stops and enhancement in fluorescence intensity
was observed.
little with no change in peak position. The plot of log[(A -A)/
0
3
+
(
A-Amax)] against log[Al ] was found to be linear (Fig. 6,
inset) where A is the absorbance of L, A is the absorbance
0
3
+
of L at a given added concentration of Al and A
absorbance at one equivalent concentration of Al . The bind-
ing stoichiometry between L and Al , which is given by the
is the
max
3
+
3
+
3
+
Fig. 5 Plot of log[(I
CH OH:H O (pH = 7.0). The solpe = 1.21 indicates 1:1 interaction
between L and Al ion, binding constant calculated as 10
0 s s α
-I )/(I -I )] versus log[Al ] for L in 1:1 (v/v)
3
+
3
2
Fig. 7 Optimized structure of the L:Al complex obtained at B3LYP/6–
311++G(d,p) level of theory
3+
4.5

J Fluoresc
3
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3
+
Fluorescence data have confirmed that L and Al interact in
3
+
1
:1 ratio. To ascertain the structure of L:Al complex, the
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3
+
5
5:389–394
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9
.
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3
+
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1
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1
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1
3. Malkondu S (2014) A highly selective and sensitive
3+
3+
the Al ion binds L, the electron density which was associated
with acetate O sites would no longer be available for electronic
transition (at 410 nm), and thus quenching the 410 nm peak.
perylenebisimide-based fluorescent PET sensor for Al determi-
nation in MeCN. Tetrahedron 70:5580–5584
4. Kima DH, Im YS, Kimb H, Kima C (2014) Solvent-dependent
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Conclusion
6. Patil R, Moirangthem A, Butcher R, Singh N, Basu A, Tayade K,
The condensation product of L-alanine and salicylaldehyde acts
3+
Fegade U, Hundiwale D, Kuwar A (2014) Al selective colorimet-
3+
as fluorescent sensor for Al ion by Boff-on^ mode (detection
ric and fluorescent red shifting chemosensor: application in living
cell imaging. Dalton Trans 43:2895–2899
limit, 10− M). The sensor was selective for Al ion over metal
6
3+
+
+
2+
2+
2+
2+
2+
2+
2+
2+
ions - Na , K , Ca , Mn , Co , Ni , Cu , Pb , Cd , Hg
17. Dutta K, Deka R C, Das D K (2014) A new fluorescent and elec-
2
+
3+
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trochemical Zn ion sensor based on Schiff base derived from
benzil and L-tryptophan. Spectrochim Acta A Mol Biomol
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and Fe . A 1:1 interaction between the sensor and Al ion
with binding constant 10 was proved from fluorescence as
well as UV/visible spectral data. DFT and TDDFT calculations
4.5
1
1
8. Dutta K, Deka RC, Das DK (2013) A new on-fluorescent probe for
manganese (II) ion. J Fluoresc 23:1173–1178
9. Kumar J, Bhattacharyya P, Das DK (2015) New duel fluo-
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copper(II) binds through N coordination and pi cation inter-
action to sensor. Spectrochim Acta A Mol Biomol Spectrosc
3+
confirmed the 1:1 interaction between Al and the sensor.
Acknowledgments DST, New Delhi and UGC, New Delhi are thanked
for financial support to the department through FIST-II and SAP
respectively.
1
38:99–104
2
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