A novel chemosensor based on rhodamine and azobenzene moieties for selective detection of Al3+ ions

Article abstract of DOI:10.1039/c8nj00436f

The high absorption coefficient, fluorescence quantum yield, photo stability and relatively long wavelengths of rhodamine dye have expanded its applicability as a promising fluorescent probe. In the present investigation, two dyes, namely rhodamine and azobenzene, have been conjugated for the detection of Al³⁺ in aqueous ethanol. The turn on fluorescence response of chemosensor L towards Al³⁺ is attributed to the inhibited PET (photo-induced electron transfer) and CHEF (chelation enhanced fluorescence) process along with the spirolactam (non-fluorescent) to ring-opened amide (fluorescent) process. The presence of the PET and CHEF process was corroborated by time-resolved photoluminescence study and the spirolactam ring opening was confirmed by ¹³C NMR and infrared spectroscopy. In the presence of Al³⁺, the opened spirolactam ring forms a 1:1 binding complex with the metal, which is supported by its high binding constant (K_a = 7.033 × 10³ M^-1). The limit of detection (LOD) and limit of quantification (LOQ) values are found to be 1.1 × 10^-7 M and 3.6 × 10^-7 M, respectively. The reversible recognition of Al³⁺ was also proved in the presence of Na₂EDTA by both UV-Vis and fluorometric titration. The sensing behaviour of the chemosensor towards Al³⁺ was supported by DFT/TDDFT calculations.

Full text of DOI:10.1039/c8nj00436f

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A novel chemosensor based on rhodamine and azobenzene moieties for selective
detection of Al3+ ion
S
ubhabrata Mabhai,a Malay Dolai,b Satyajit Dey,*,c Anamika Dhara, d Bhriguram Das,c
Atanu Jana,*,e
aDepartment of Chemistry, Mahishadal Raj College, East Midnapore, Mahishadal, West
Bengal, Pin No. 721628, India
bDepartment of Chemistry, Prabhat Kumar College, Contai, Purba Medinipur 721401, India
cDepartment of Chemistry, Tamralipta Mahavidyalaya, East Midnapore, West Bengal, Pin
No. 721636, India. Eꢀmail: satyajitdeyoc@gmail.com
dDepartment of Chemistry, Jadavpur University, Raja S. C. Mallick Road, Kolkata 700032,
India.
eDepartment of Chemistry, Indian Institute of Technology, Delhi, HauzKhas, New Delhiꢀ
110016, India. Eꢀmail: atanujanaic@gmail.com
Abstract
High absorption coefficient, fluorescence quantum yield, photo stability and relatively
long wavelengths of rhodamine dye expand its applicability as a promising fluorescent probe.
In the present investigation, two dyes namely rhodamine and azobenzene have been
conjugated for the detection of Al3+ in aqueous ethanol. The turn on fluorescence response of
chemosensor L towards Al3+ is attributed to the inhibited PET (photoꢀinduced electron
transfer) and CHEF (chelation enhanced fluorescence) process along with spirolactam (nonꢀ
fluorescent) to ringꢀopened amide (fluorescent) process. The presence of PET and CHEF
process was corroborated by timeꢀresolved photoluminescence study and the spirolactam ring
opening was confirmed by 13C NMR and infrared spectroscopy. In presence of Al3+, the
opened spirolactam ring forms a 1:1 binding complex with metal which is supported by its
high binding constant (Ka = 7.033×103 Mꢀ1). Limit of detection (LOD) and limit of
quantification (LOQ) value are found to be 1.1×10ꢀ7 M and 3.6×10ꢀ7 M, respectively. The
reversible recognition of Al3+ was also proved in presence of Na2EDTA by both UVꢀVis and
fluorometric titration. The sensing behaviour of chemosensor towards Al3+ was supported by
DFT/TDDFT calculations.
Keywords: Al sensor, CHEF, Chemosensor, Quantum yield.
Introduction
Malabsorption of Aluminium inhibits several biological activities such as enzyme
activity and protein synthesis, alterations in nucleic acid function, and changes in cell
membrane permeability. When taken in unregulated amount it causes to osteomalacia,
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Parkinson’s, Alzheimer’s and dialysis diseases.1–5 In order to monitor in vitro and in vivo
levels harmful effect in subꢀmilli and micro molar concentration in environmental, clinical
and biological purposes developing of effective Al3+ sensor is become crucial. Development
of appropriate fluorescent probe for detection of Al3+ compared to other transition metal ions
encounters some severe problems due to its poor coordination ability, strong hydration ability
and lack of spectroscopic characteristics.6–11 To overcome these problems, some serious
efforts has been made for the development of aluminium sensing fluorescent probes such as
BINOL,12 BODIPY,13 8ꢀhydroxyquinoline,14 anthraquinone,15 coumarin,9 Schiff base
molecules.16–19 Majority of these probes provide moderate success because of difficult
synthetic methodologies,13 limited solubility, poor selectivity and interfering sensitivity
towards other dꢀblock cations like Fe3+ and Zn2+. Hence, invention of new superior effective
probe for Al3+ is still demanding. Spirocyclic Rhodamine derivatives have wide applications
as chemodosimeter due to their unique photo physical properties.20–24 Rhodamine derivatives
exhibit some unique fluorescence “OFF–ON” behavior through change in architecture from
color less spirolactam ring to intense pink colored ring opened amide system in the presence
of analyte.
Herein, we report the design, synthesis and photophysical properties of a new 2ꢀ
Hydroxyꢀ5ꢀ(4ꢀnitrophenyl)diazenyl) benzaldehydeꢀappended rhodamine based scaffold (L)
as a chemosensor, which showed strong reversibility, sensitivity and selectivity towards Al3+
in a mixed solvent through binding induced changes in the electronic spectral pattern via the
chelation enhanced fluorescence (CHEF) process25–28 and spirolactam ring opening
phenomena. The sensing mechanism has also been corroborated by DFT calculations.
Experimental
Materials
Rhodamine B (99.0%), Salicylaldehyde (98%), pꢀnitro aniline (97%) and all other
analytical grade materials (metal salts and solvents) were purchased from SigmaꢀAldrich and
were used without further purification. Any spectral change of the chemosensor moiety was
recorded in a constant ligand concentration with the addition of metal perchlorate salts. Metal
perchlorate salts were prepared according to literature method.29 Stock solution for
chemosensor L was prepared in EtOHꢀH2O (4:1, v/v) pH = 7.2 using 20 µM HEPES buffer at
25 oC. Distilled water was used throughout the experiment.
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Physical measurements
The fluorescence spectra and relative fluorescence intensity were recorded using a
XENO Flash (PTI) fluorescence spectrophotometer with excitation wavelength at 500 nm for
both ligand and complex. An Evolutionꢀ201 spectrometer was used for absorption
measurements. IR spectra (KBr pellet, 400–4000 cmꢀ1) were recorded on a PerkinꢀElmer
infrared spectrophotometer (Model: 883) The 1H NMR spectra were performed on Bruckner
400 MHz instrument in CDCl3 using TMS as internal standard. Chemical shifts δ were
1
reported in ppm unit and Hꢀ1H coupling constants in Hz. Mass spectra were recorded on
Qtof Micro YA263 mass spectrometer. Elemental analysis (C, H, N) result was obtained from
Perkin Elmer 2400 CHN analyser. DFT calculations have been done using Gaussian
software. Fluorescence lifetimes were measured by the method of Time Correlated Singleꢀ
Photon Counting (TCSPC) using a HORIBA Jobin Yvon Fluorocubeꢀ01ꢀNL fluorescence
lifetime spectrometer.
Computational method
DFT is an important tool to investigate the geometry, electronic structure, and optical
properties. Ground state electronic structure calculations in gas phase of both the ligand and
complex have been performed using DFT30 method associated with the conductorꢀlike
31–33
polarizable continuum model (CPCM).
Becke’s hybrid function34 with the LeeꢀYangꢀ
Parr (LYP) correlation function30 was used through the study. The geometries of the ligand
and complex were fully optimized without any symmetry constraints. On the basis of the
optimized ground state geometry, the absorption spectral properties in methanol (CH3OH)
media were calculated by timeꢀdependent density functional theory (TDDFT)35–37 approach
associated with the conductorꢀlike polarizable continuum model (CPCM).31–33 We computed
the lowest 40 singlet – singlet transition and results of the TD calculations were qualitatively
very similar. The TDDFT approach had been demonstrated to be reliable for calculating
spectra properties of many transition metal complexes.38–41 Due to the presence of electronic
correlation in the TDDFT (B3LYP) method it can yield more accurate electronic excitation
energies. Hence TDDFT had been shown to provide a reasonable spectral feature for our
complex of investigation.
We have run the Gaussian for geometry optimization of both the ligand and the complex in
ground state with 6ꢀ31G under B3LYP. For Al atom, we used 6ꢀ31G**; for C, H, N and O
atoms we employed 6ꢀ31G** as basis set under B3LYP and compare the results, which didn’t
not show any significant change in geometrical parameters. All the calculations were done
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with the Gaussian 09W software package. GaussSum 2.1 program 42 was used to calculate the
molecular orbital contributions from groups or atoms.
Synthesis of 2ꢀHydroxyꢀ5ꢀ(4ꢀnitrophenyl) diazenyl) benzaldehyde:
The compound was prepared according to literature method (Fig. S1, ESI).43,44 The pꢀnitro
aniline (0.57 g, 4.12 mmol) was dissolved in 2 mL concentrated hydrochloric acid to obtain a
clear green solution. Temperature of the beaker was kept in between 0 oC – 5 oC by placing it
on an iceꢀsalt bath while the solution was stirred vigorously. A cold solution of NaNO2 was
prepared by dissolving (0.284 g, 4.12 mmol) sodium nitrite in 1 ml water which was added to
the green solution drop wise slowly with constant stirring. This time colour of the solution
o
changed from green to pale yellow. The pale yellow diazo solution was added at 5 C to a
solution of salicylaldehyde (0. 503 g, 4.12 mmol) in 20 ml H2O which contained NaOH
(0.165 g, 4.12 mmol) and Na2CO3 (0.266 g, 2.51 mmol ). The resulting solution was stirred
overnight at room temperature. The dark brown cake was then filtered by using whatmanꢀ41,
filtrate was discarded, and the darkꢀbrown residue was washed repeatedly with 10 % NaCl
solution. Solid residue was taken in a beaker and reslurried by adding water. Nearly 2ꢀ3 drops
of concentrated HCl was added to it to maintain the pH at 4ꢀ5. Finally the orange colour
product was kept in vacuum for drying and was chromatographed for confirmation. The yield
of the product was 80 %. M.P. 180 oC .
1H NMR (CDCl3, 400 MHz): δ 11.4 (s, 1H, ArꢀOH), 10.06 (s, 1H, ArꢀCHO), 8.41ꢀ8.39 (d,
2H, J = 8 Hz, ArꢀH), 8.31 (s, 1H, ArꢀH), 8.22 ꢀ 8.21 (d, 1H, J = 4 Hz, ArꢀH), 8.04ꢀ8.02 (d,
2H, J = 8 Hz, ArꢀH), 7.18ꢀ7.16 (d, 2H, J = 8 Hz, ArꢀH). (Fig. S1, ESI
)
Synthesis of Rhodamine B hydrazide:
Rhodamine B hydrazide was synthesized according to reported method.45
Synthesis of chemosensor L
The synthesis of chemosensor L was done by the condensation reaction of aldehyde
precursor with Rhodamine B hydrazide under refluxing condition [Scheme 1]. In this process
Rhodamine B hydrazide (0.536 g, 1.175 mmol) and 2ꢀHydroxyꢀ5ꢀ((4ꢀnitrophenyl) diazenyl)
benzaldehyde (0.317 g, 1.175 mmol) was taken in 20 ml ethanol and refluxed with CaCl2
guard tube at the top of the condenser. Progress of the reaction was monitored with TLC and
after nearly 4 hours of refluxing a maroon coloured solid was appeared which was cooled to
room temperature. The final volume of the suspended solution was reduced 1/3 of the total
volume by heating in water bath. The yellowish precipitate was obtained by normal
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gravimetric filtration and repeated washing with cold EtOHꢀEther (1:1 ratio). Yield ~ 65 %.
M.P. 260 oC.
1H NMR [400 MHz, CDCl3,δ (ppm)] : 11.65 (s, 1H, ArꢀOH (Hꢀbonded)), 9.21 (s, 1H, Arꢀ
CH=Nꢀ), 8.36ꢀ8.34 (d, 2H, , J = 9 Hz ArꢀH), 8.00ꢀ7.93 (m, 3H, ArꢀH), 7.87ꢀ7.80 (m, 2H, Arꢀ
H), 7.57ꢀ7.53 (m, 2H, ArꢀH),7.20ꢀ7.19 (d, 1H, J = 6.4 H, ArꢀH), 7.01ꢀ6.99 ((d, 1H, J = 8.4
Hz, ArꢀH), 6.52ꢀ6.50 (m, 4H, XantheneꢀH, J = 8 Hz), 6.29ꢀ6.27 (d, 2H, XantheneꢀH, J = 8
Hz), 3.34ꢀ3.32 (d, 8H, NꢀCH2ꢀCH3, J = 6.4 Hz), 1.16ꢀ1.19 (t, 12H, NꢀCH2ꢀCH3, J = 6.4 Hz),
(Fig. S2, ESI). 13C NMR [800 MHz CDCl3, δ (ppm)] : 12.62, 44.38, 66.45, 97.94, 105,
108.18, 118.23, 118.93, 123.12, 123.52, 124.24, 124.75, 125.49, 128.11, 128.49, 128.75,
129.47, 135.85, 145.55, 148.22, 149.18, 150.77, 150.9, 153.53, 156, 162.8, 164.51. (Fig. S3,
ESI). CHN Analysis for C41H39N7O5: calculated (%): C, 69.38; H, 5.54; N, 13.81; O, 11.27;
Found (%): C, 69.35; H, 5.56; N, 13.83; O, 11.25. ESIꢀMS: m/z calculated for C41H39N7O5
[M+H]+ : 710.30, found 710.30 (100 %). (Fig. S4, ESI)
Scheme 1. Synthesis of chemosensor L
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Synthesis of LꢀAl3+complex
Al(NO3)3.9H2O (0.19 g, 0.506 mmol) was added to a stirring solution of ligand (0.359
g, 0.506 mmol) in CH3CN and stirred overnight at room temperature. Solution was filtered
and filtrate kept one week for slow evaporation. The deep maroon solid appeared was
collected and vacuum dried after several time washing with methanol. ESIꢀMS: m/z
calculated for C41H39N7O5 [L+Al+H2O+H+]+ : 756.28, found 756.2885 (100 %) (Fig.S5,
ESI).
Association constant
The association constant for the formation of the complex, [LꢀAl3+] were determined
using the Benesi–Hildebrand (B–H) equation.46
1
1
1
=
+
(A− A0 ) [K(Amax − A0 )C] (Amax − A0 )
where A0 is the absorbance maxima of sensor is the observed absorbance at that
L, A
particular wavelength at different concentration of the metal ion (C), Amax is the maximum
absorbance value at λmax = 554 nm (for Mn+) during titration with varying [C], K is the
association constant and was determined from the ratio of slope and intercept of the linear
plot, and [C] is the concentration of the Mn+ ion added during titration studies. The goodness
of the linear fit of the B–H plot of 1/ (AꢀA0) vs. 1/ [Mn+] for 1:1 complex formation confirms
the binding stoichiometry between sensor and Mn+.
(b)The binding constant for sensor
L–Metal complex was also determined using the Benesi–
Hildebrand relation from spectroflurometric titration.
1
1
1
=
+
(I − I0 ) (I1 − I0 ) (I1 − I0 )k[M n+ ]m
where [Mn+] is the metal ion concentration, I0, I and I1 indicate emission intensities in the
absence of, at intermediate and at infinite concentrations of metal ions, respectively. For 1:1
complexation, m = 1 and for 1:2 complexation, m = 2.
Procedure for metal ion sensing
For both UVꢀVis and fluorescence spectral study titration was carried out by taking
2.5 mL solution of ligand (40 ꢁM) and (100 ꢁM) metal ions solution in a quartz cuvette (path
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length, 100 cm). Before recording the UVꢀVis spectra the resulting solution was incubated
for 30 min after shaking. The excitation wave length was 500 nm.
Fluorescence Quantum Yield
Fluorescence quantum yield of the sensor was determined in spectroscopic grade
methanol using Fluorescein as an optically matching reference with a known Φr (0.79)
according to the following equation.
A F ηs2
r
Φs = Φr (
s )(
)
A F ηr2
s
r
where As and Ar are the absorbance of the sample and reference solutions, respectively, at the
same excitation wavelength, Fs and Fr are the corresponding relative integrated fluorescence
intensities, and η is the refractive index of the solvents. We measured the quantum yield of
our sensor in presence and absence of Al3+, and our experimental results revealed that the
bare sensor has the quantum yield, Φ = 0.51 %, and that of LꢀAl3+ complex is 4.08 % which
is almost 8 times higher than bare sensor.
Result and Discussion:
UVꢀVisible spectroscopic studies:
The sensing behaviour of the sensor
and fluorescence spectra in the presence of different metal ions in EtOHꢀH2O (4:1, v/v) pH =
7.2 using 20 µM HEPES buffer at 25 °C. Initially the sensing ability of the sensor was
L was investigated by monitoring its UVꢀVisible
L
examined by recording the UVꢀVisible absorption spectra by adding 100 µM stock solutions
of several metal ions including Ba2+, Ca2+, Cd2+, Cr3+, Cu2+, Fe3+, Fe2+, Hg2+, K+ Mg2+ Mn2+
,
,
,
Na+ Ni2+ Pb2+ and Al3+ to 40 µM sensor solution taken in quartz tube containing 2 mL
,
,
,
EtOHꢀH2O (4:1, v/v) pH = 7.2 using 20 µM HEPES buffer at 25 °C. As shown in the Fig. 1
it is interesting to note that except Al3+ and Cu2+ ions no other metal cations altered the UVꢀ
Visible spectral nature of the sensor resulted in the
. Addition of Al3+ and Cu2+ to sensor
appearance of strong bands at λmax = 554 & 550 nm, respectively, in the visible region of the
absorption spectrum of associated with immediate changes of colourless sensor solutions to
,
L
L
L
deep pink colour which was clearly visible to naked eye for direct recognition [Fig. 2a].
Appearance of these new absorption spectral bands in the visible region of the UVꢀVisible
spectrum demonstrated that the conjugated xanthene framework was generated by opening of
the spirolactam ring due to the complex formation by metal ion chelation.
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2.5
2.0
1.5
1.0
0.5
0.0
L
Ba2+
Ca2+
Cd2+
Cr3+
Fe3+
Hg2+
K+
2+
Cu
Mg2+
Mn2+
Na+
Ni2+
Pb2+
Al3+
Cu2+
Fe2+
Al3+
300
400
500
600
Wavelength (nm)
Fig. 1: UVꢀVis spectral nature of L (40 µM) upon addition of various metal ions ( Ba2+,
Ca2+, Cd2+, Cr3+ Fe3+ Fe2+ Hg2+ K+ Mg2+ Mn2+ Na+ Ni2+ Pb2+ Al3+ and Cu2+ ) in a
,
,
,
,
,
,
,
,
,
,
,
HEPES buffer [50 µM, C2H5OHꢀH2O (4:1,v/v) pH = 7.2] at 25 oC.
Fig. 2: Visual display of
L plus metal ions: (a) naked eye (b) under UV cabinet.
UVꢀVisible titrations of the sensor
L L
with Al3+ and the resulting ꢀAl3+ complex with
Na2EDTA were carried out to check the reversible binding nature of the sensor. As depicted
in Fig. 3, it was clearly observed that gradual incremental Al3+ additions (0ꢀ110 ꢁM) to
sensor L (40 ꢁM) in EtOHꢀH2O (4:1, v/v) pH = 7.2 using 20 µM HEPES buffer at 25 °C
resulted in gradual enhancement of the absorption intensity of the absorption band at λmax
554 nm along with the development and deepening of the pink colour. However, the
incremental addition of a strong chelating agent, Na2EDTA (0ꢀ435 ꢁM), to the resulting
=
L
ꢀ
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Al3+ complex solution showed immediate decrease in intensity of the above absorption band
Fig. 4] with disappearance of the pink colour. This was due to the regeneration of free sensor
by demetallation of Al3+ from the ꢀAl3+ complex indicating the reversible binding nature of
the sensor. Similarly UVꢀVisible titrations of the sensor
with Cu2+ and the resulting Lꢀ
Cu2+ complex with Na2EDTA were carried out to check the binding nature and reversibility
of the sensor (Fig. S6, S7, ESI). Maximum absorbance of was observed upon incremental
addition of 20 µM Cu2+ due to strong (1:1) binding nature of with Cu2+ which was furthur
[
L
L
L
L
proved from Job’s plot experiment (Fig. S8 ESI).47 The binding constant for Cu2+ was
calculated from BꢀH equation to be 1.45 x 104 Mꢀ1 (Fig. S9 ESI).
0.9
L
0.5 eq Al 3+
1.0 eq Al 3+
0.8
1.5 eq Al 3+
2.0 eq Al 3+
110 ꢀM
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
2.5 eq Al 3+
3.0 eq Al 3+
3.5 eq Al 3+
4.0 eq Al 3+
4.5 eq Al 3+
5.0 eq Al 3+
5.5 eq Al 3+
0ꢀ M
250 300 350 400 450 500 550 600
Wavelength (nm)
Fig. 3: UVꢀVis spectral changes of
L
(40 µM) upon addition of Al3+ ions in a HEPES
buffer [50 µM, C2H5OHꢀH2O (4:1, v/v, pH= 7.2)] at 25 oC. [Al3+] = 0ꢀ110 µM.
1.0
0ꢀM EDTA
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
435 ꢀM EDTA
300 350 400 450 500 550 600 650
WaveLength (nm)
Fig. 4: UVꢀVis spectral changes of Al3+ complex of
HEPES buffer [50 µM, C2H5OHꢀH2O (4:1, v/v, pH= 7.2)] at 25 C. EDTA added = 0ꢀ435
µM
L (40 µM) upon addition of EDTA in a
o
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10000
8000
6000
4000
2000
0
Fig. 5: Fluorescence intensity of L (40 µM) in the presence of various metal cations in a
o
HEPES buffer [50 µM, C2H5OHꢀH2O (4:1, (v/v), pH 7.2] at 25 C. The blue bars represent
the fluorescence intensities of sensor L in the presence of metal cations of interest. The green
bars represent the change of fluorescence intensity that occurred upon subsequent addition of
Al3+ ion to the above mentioned solution. λex = 500 nm
In order to investigate the recognizing ability of L towards Al3+ in the presence of
other competing metal cations, we subsequently recorded UVꢀVisible spectra of the sensor L
in the presence of Al3+ (100 ꢁM) mixed with other competing metal cations (100 ꢁM) such as
Ba2+, Ca2+, Cd2+, Co2+, Cr3+, Cu2+, Fe3+ Fe2+ , Hg2+, K+ Mg2+ Mn2+ Na+ Ni2+ Pb2+, Zn2+ and
,
,
,
,
,
,
Al3+ as shown in Fig. 5, we found that the absorbance intensity and band position of the Lꢀ
Al3+ complex remained unaltered in the presence of different metal cations. This observation
strongly supports that the selectivity of this Al3+ sensor is not affected by the presence of
other metal ions.
Furthermore, in order to get deeper inside into the binding behaviour of the sensor L
with Al3+, the Job's plot using UVꢀVisible titrations of the sensor L and Al3+ with a total
concentration of 20 ꢁM was utilized. Analysis of the Job's plot showed that highest UVꢀ
Visible absorption occurred at 0.5 mole fraction of Al3+ implying the 1:1 stoichiometry of the
LꢀAl3+ complex (Fig. S13, ESI). Further evidence for 1:1 stoichiometric complexation
between the sensor L and Al3+ was obtained from ESIꢀMS spectrum of the complex (Fig. S5,
ESI) which exhibited a peak at m/z 755.286 for [L+Al3++H2O] +. Based on the 1:1 binding
stoichiometry, BenesiꢀHildebrand plot of the UVꢀVisible titration data provided a linear
curve and the association constant (ka) was calculated to be 7.033×103 Mꢀ1(Fig. S14, ESI).
The detection limit was also calculated by 3σ method48 and found to be 1.1x10ꢀ7 M. in
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EtOHꢀH2O(4:1, v/v) pH = 7.2 using 20 µM HEPES buffer at 25 °C (Fig. S15, ESI). The
potentiality for any chemosensor to be used successfully lies on its lower detection limit. The
limit of detection value of our sensor is much lower as compared to others (Table S1, ESI),
and hence it has a stronger potentiality to be used as environment friendly chemosensor.
Fluorescence spectroscopic studies
The sensing behaviour of the sensor L was also pursued by fluorescence spectroscopic
studies. The experimental results revealed that upon excitation at λex = 500 nm, the sensor L
itself and in the presence of different metal ions such as Ba2+, Ca2+, Cd2+, Co2+, Fe3+, Fe2+
,
Hg2+, K+ Mg2+ Mn2+ Na+ Ni2+ Pb2+, Zn2+ and Al3+ in EtOHꢀH2O (4:1, v/v), pH = 7.2 using
,
,
,
,
,
20 µM HEPES buffer at 25 °C displayed no noticeable emission band above λmax = 500 nm.
Addition of Cr3+ and Cu2+ to the sensor solution showed very weak emission band at λmax
=
578 & 580 nm, respectively. However, in the presence of Al3+, a remarkable enhancement of
fluorescence intensity was observed at λmax = 582 nm [Fig. 6a and 6b] due to the happening
of CHEF as well as inhibited PET process. The lone pair (lp) from ꢀNEt2 group can move to
azobenzene moiety causing L as nonꢀfluorescent molecule. But, this lp in L is unavailable
upon binding with Al3+ ion in Al3+ꢀcomplex making the L as fluorescent where PET process
is inhibited. This strong affinity towards Al3+ can be attributed by the fact that trivalent Al
have stronger binding energy (nearly double) to L than Cu2+, and hence it displaces Cu2+
almost completely making the L highly selective for Al3+.49 Al3+ possesses a high charge and
small ionic radius, and consequently, it bears a high ionic potential value making it a hard
acid. So the ligand (L) having binding site with N and O has a strong tendency for chelation
with Al3+ following the wellꢀknown SHAB (hard and soft acids and bases) principle and
hence sensor containing N and O as donor site is very common for Al3+.50–52 Ba2+, Ca2+ and
Cu2+ are hard acids and hence according to SHAB principle they can bind with the hard base
centres like N and O exhibiting color change under visible light. It is noteworthy that Cu2+, a
d9 system being paramagnetic showed strong fluorescence quenching phenomena upon
binding with L.53 Though Ca2+, Ba2+ and Al3+ are pꢀblock elements, the stabilisation energy
of LꢀAl3+ is much higher than the LꢀBa2+ and LꢀCa2+ complexes. Consequently, LꢀAl3+
complex exhibited prominent color under UV light.49
Furthermore, under UV lamp of long wave length (360 nm), addition of 2.5
equivalents of Al3+ to the sensor solution showed a prominent colour change from colourless
to an intense orange fluorescence, demonstrating Al3+ selective "turnꢀon" fluorescence
signalling behaviour of L [Fig. 2b]. The reversible behaviour of L was also examined by
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fluorimetric titration method. The experimental observation revealed that upon gradual
incremental addition of Al3+ to the sensor solution resulted in the gradual enhancement of
emission intensity of the emission band around λmax = 582 nm in the emission spectrum of the
sensor L, which was gradually decreased on successive excess of Na2EDTA addition [Fig. 7
and 8]. These observations once again confirmed that our sensor can definitely be utilized as
a selective fluorescent "onꢀoff" sensor for Al3+.
We have confirmed our proposed mechanism of LꢀAl3+ complex formation through
comparison of both IR and 13C NMR spectrum of free sensor (L) and LꢀAl3+ Complex. The
carbonyl stretching frequency of spirolactam ring in free sensor and the LꢀAl3+ complex are
1725 cmꢀ1 and 1647.5 cmꢀ1, respectively. This shifting (77.5 cmꢀ1) indicates the spirolactam
ring opening phenomena where carbonyl oxygen make a bond towards Al3+ ion as reported
earlier (Fig. S10 and S11, ESI).54,55 The spirolactam ring opening mechanism was also
established from the disappearance of the tertiary carbon signal appeared at δ = 66.45 ppm
and appearance of a new signal at δ = 134.28 ppm upon addition of Al3+ (Fig. S12, ESI).56,57
This is attributed to the conversion of sp3 spiro carbon of rhodamine to sp2 carbon upon
binding with Al3+ ion.
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200
175
150
125
100
75
L
Ba2+
Ca2+
Cd2+
Cr3+
Fe3+
Hg2+
K+
(a)
Al3+
Mg2+
Mn2+
Na+
Ni2+
Pb2+
Al3+
Cu2+
Fe2+
50
25
0
550
600
650
700
Wavelength(nm)
15000
10000
5000
0
Fig. 6: (a) Emission spectral nature of L (40 µM) upon addition of various metal ions (Ba2+,
Ca2+, Cd2+, Co2+, Cr3+, Cu2+, Fe3+ Fe2+ Hg2+, K+ Mg2+ Mn2+ Na+ Ni2+ Pb2+, Zn2+ Al3+) in a
,
,
,
,
,
,
,
,
HEPES buffer [50 µM, C2H5OHꢀH2O (4:1,v/v, pH= 7.2)] at 25 °C (b) Column plot for the
emission spectral intensity of L (50 µM) upon addition of different metal ions ( Ba2+, Ca2+,
Cd2+, Cr3+, Cu2+, Fe3+ Fe2+ Hg2+, K+ Mg2+ Mn2+ Na+ Ni2+ Pb2+, and Al3+ ) in a HEPES buffer
,
,
,
,
,
,
[50 µM, C2H5OHꢀH2O = (4:1,v/v, pH= 7.2)] at 25 oC.
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180
160
140
120
100
80
200 ꢀ
M Al3+
(a)
60
0 ꢀ
M Al3+
40
20
0
525 550 575 600 625 650 675 700 725
Wavelength(nm
)
Fig. 7: (a) Emission spectral changes of L (40 µM) upon addition of Al3+ ion in a HEPES
buffer [50 µM, C2H5OHꢀH2O (4:1, v/v, pH= 7.2)] at 25 oC. [Al3+] = 0ꢀ200 µM (b) only L and
L in presence of Al3+ ion under UV light.
200
180
0 ꢀM EDTA
160
140
120
100
80
60
600 ꢀM EDTA
40
20
0
550 575 600 625 650 675 700
Wavelength (nm)
Fig. 8: Emission spectral changes of Al3+ complex of L (40 µM) upon addition of EDTA in a
HEPES buffer [50 µM, C2H5OHꢀH2O (4:1,v/v, pH= 7.2)] at 25 oC. EDTA = 0ꢀ600 µM
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Scheme 2. Probable binding mode of chemosensor L with Al3+
Effect of pH
To explore our sensor L as an effective tool for detecting Al3+ ion in physiological
pH, we evaluated the sensitivity of L in the absence and presence of Al3+ ion in different
acidic and basic pH values. As shown in Fig. 9, in the absence of Al3+, no characteristic
change in the fluorescence spectra of the sensor was recorded between pH 4.0 and 8.0, which
indicates that L must remain in the spirocyclic from within this pH range. In presence of Al3+,
the spectral responses of the sensor remained almost steady without any interference by
proton between the pH 4.0 and 8.0. At pH lower than 4.0, the fluorescence intensity of the
sensor increased gradually in the absence and presence of analyte due to the generation of
ring opening xanthene moiety by protonation of the amide carbonyl.58,59 At basic pH i.e.; pH
> 8, the emission intensity of the LꢀAl3+ complex decreased slowly due to the generation of
free ligand by the removal of Al3+ ion as its oxide. As the spectral responses of the sensor as
well as sensor plus Al3+ remained almost unchanged within a broad pH range which covered
the physiological pH (7.2), so the sensor can be used for the detection of Al3+ ion in
physiological conditions.
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Fig. 9: Effect of pH in absence and in presence of Al3+ ion [50 µM C2H5OHꢀH2O (4:1,v/v, pH=
7.2)] at 25 oC and at excitation wave length = 500 nm and emission wavelength = 582 nm
Geometry optimization and electronic structure.
The optimized geometry of L and its Al (III) complex is shown in [Fig. 10]. The ground state
geometry optimization for the ligand (L) and complex 1 were performed in gas phase. Main
optimized geometrical parameters of the complex 1 are listed in [Table 1]. The geometry of
the pentaꢀcoordinated metal center can be measured by the Addison parameter (τ), which is
0.114 for 1 in this case [τ = (α ꢀ β)/60, where α and β are the two largest LigandꢀMetalꢀ
Ligand angles of the coordination sphere], suggesting a distorted square pyramidal geometry
of around the Al (III) center (τ = 0 for a perfect square pyramid and τ = 1 for a perfect
trigonal biꢀpyramidal). In 1, all calculated AlꢀN distances occur in the range 1.968 Å and Alꢀ
O distances are in the range 1.795ꢀ1.919 Å. On complexation, some CꢀN and CꢀO bond
lengths are changed with respect to that in free ligand and [Table 2] describes the change in
bond lengths in 1 compared to free ligand L.
In case of HL at ground state, the electron density at HOMO–2, , LUMO, LUMO+1 and
LUMO+2 orbitals are mainly reside on the benzene moiety while a considerable contribution
comes from pꢀnitro azoꢀbenzene moiety along with the contribution of rhodamine moiety in
HOMO–3, HOMO–1 and HOMO orbitals. The energy difference between HOMO and
LUMO is 2.08 eV of ligand (L). In case of 1, all the HOMO, HOMO–1 and HOMO–5
orbitals are mainly originating from ligand π and π* orbital contribution while the HOMO–2,
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LUMO and LUMO+1 orbitals arises from metal d orbital contribution along with ligand π
orbital contribution. The energy difference between HOMO and LUMO is 2.69 eV of 1.
These compositions are useful in understanding the nature of transition as well as the
absorption spectra of both the ligand and complex (vide infra). The ligand shows two
absorption band at 385 nm in methanol solution at room temperature. This assignment was
supported by TDDFT calculations. This absorption band can be assigned to the S0→S14
transition. The absorption energies along with their oscillator strengths, the main
configurations and their assignments calculated using TDDFT method using the ground state
geometry for HL is discussed here and the related data are given in [Table 3]. The complex
shows three absorption bands at 553, 514 and 400 nm in methanol solution at room
temperature. The TDDFT calculated absorption bands are located at 545, 513 and 399 nm for
1, which are in good agreement with experimental results of 553, 514 and 400 nm [Table 4].
These three absorption bands can be assigned to the S0→S2, S0→S3 and S0→S11 transitions,
respectively [Table 4].
L
Fig. 10: DFT/B3LYP optimized geometry of L as well as complex [Al(L)(H2O)(NO3)]+.
Table 1: Selected optimized geometrical parameters for [Al(L)(H2O)(NO3)]+ (1) in the
ground state calculated at B3LYP Levels.
Bond Lengths (Å)
AlꢀO1
AlꢀN2
AlꢀO2
1.872
1.968
1.795
AlꢀO3w
AlꢀO4
1.919
1.879
Bond Angles (o)
O1ꢀAlꢀN2
N2ꢀAlꢀO2
80.387 O1ꢀAlꢀO3w 84.931
90.992 O4ꢀAlꢀO3w 98.613
O2ꢀAlꢀO3w 92.704 O4ꢀAlꢀO1
N2ꢀAlꢀO3w 156.675 O4ꢀAlꢀO2
102.170
107.929
102.122
O2ꢀAlꢀO1
149.814 O4ꢀAlꢀN2
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Table 2: Change in bond lengths for 1 compared to free L in the ground state calculated at
B3LYP Levels
Bond Lengths (Å)
Free ligand (L)
1.418
[Al(L)(H2O)(NO3)]+
N1ꢀC1
O1 –C1
N1 –N2
N2 –C2
O2 –C4
1.323
1.348
1.414
1.313
1.350
1.238
1.335
1.288
1.392
L
Fig. 11: Frontier molecular orbital of complex as well as L optimized.
Table 3: Selected Parameters for the vertical excitation (UVꢀVis absorptions) of L; electronic
excitation energies (eV) and oscillator strengths (f), configurations of the lowꢀlying excited
states of L; calculation of the S0→S1 energy gaps based on optimized groundꢀstate
geometries (UVꢀVis absorption) (methanol used as solvent).
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Electronic
transition
Composition
Excitation
energy
Oscillator strength
CI
λexp
(nm)
385
(f)
S0 → S14 HOMO – 3→LUMO + 1
HOMO – 2→ LUMO + 1
3.1905 eV
(388nm)
0.1089
0.37097
0.11323
0.41736
HOMO – 1→ LUMO + 2
Fig. 12: Frontier molecular orbitals involved in the UVꢀVis absorption of L.
Table 4: Main calculated optical transition for the complex 1 with composition in terms of
molecular orbital contribution of the transition, vertical excitation energies, and oscillator
strength in methanol.
Electronic
transition
S0 → S2
Composition
Excitation
energy
Oscillator strength
CI
λexp
(nm)
553
(f)
HOMO – 2 →LUMO
HOMO →LUMO
2.4163 eV
(545 nm)
2.5413 eV
(513 nm)
3.1073 eV
(399 nm)
0.2106
0.15464
0.50524
0.17444
0.67454
0.67380
0.13700
S0 → S3
HOMO – 2 →LUMO
HOMO – 1 →LUMO
HOMO – 5→LUMO
HOMO – 2→LUMO + 1
0.6006
0.0505
514
400
S0 → S11
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Fig. 13: Frontier molecular orbitals involved in the UVꢀVis absorption of complex 1.
Fluorescence lifetime measurements
The sample was excited using a laser diode at 450 nm and the signals were collected
at the magic angle of 54.7° to eliminate any considerable contribution from fluorescence
anisotropy decay.52 The typical time resolution of our experimental setup is ~ 100 ps. The
decays were deconvoluted using DASꢀ6 decay analysis software. The acceptability of the fits
was judged by χ 2 criteria and visual inspection of the residuals of the fitted function to the
data. Mean (average) fluorescence lifetimes were calculated using the following equation.60,61
2
i
α τ
∑
i
τ av
=
α τ
∑
i
i
in which αi is the preꢀexponential factor corresponding to the ith decay time constant, τi.
To gain an insight into the sensing mechanism we performed TCSPC measurements
of L and LꢀAl3+ ensemble using a 450 nm nanoꢀLED as the excitation source. The decay
behaviour of the bare fluorophore, its metal complex is found to be complicated and is best
fitted to triexponential functions. Bare L showed three components having lifetimes 1.46,
5.21 and 0.12 ns respectively. The populations of the same were 0.08, 0.03 and 0.89 %
respectively. The average lifetime, calculated using standard methods was calculated to be
1.20 ns. Upon addition of Al3+, three components were obtained having lifetimes 0.95, 0.07
and 4.13 ns respectively, having populations of 0.18, 0.79 and 0.03 % respectively (Table 5).
The average lifetime was calculated to be 3.00 ns. Thus, LꢀAl3+ ensemble underwent an
increment in lifetime compared to free L. The results are indicative of inhibited PET
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mechanism and CHEF mechanism where freezing of nonꢀradiative pathways leads to an
increment in the excited state lifetime.
Table 5: Fluorescence lifetimes of L and LꢀAl3+ complex MeOH solvent
MeOH
τ1(ns) τ2(ns) τ3(ns)
α1
α2
α3
χ2
τav
1.46
0.95
5.21
0.07
0.12 0.08 0.03 0.89 1.12 1.20
4.13 0.18 0.79 0.03 1.08 3.00
L
LꢀAl3+
Fig. 14: TCSPC measurements of L and LꢀAl3+ (λex = 450 nm)
Conclusion
In summary, we developed a new sensor L, which displays a significant
change of both colorimetric and fluorometric response upon the Al3+ binding. Upon addition
of Al3+ chemosensor changes its colour from colour less to pink due to the both PET and
CHEF process which can be observed in naked eye. Fluorometric measurements of Al3+ with
receptor high binding constant, nonlinearꢀleast squares fitting of titration profiles and
nanomolar level detection limit all supports the 1:1 binding complex of metal with
chemosensor. We believe that excellent selectivity and reversibility of this chemosensor to
Al3+ helps us to consider it as a practical probe for detection of Al3+ in naked eye.
Conflicts of interest
There are no conflicts of interest to declare.
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Acknowledgements
S. Dey is grateful to the University Grants Commission, New Delhi, India, for financial
support by sanctioning a minor research project (Sanction letter No. PSWꢀ155/14ꢀ15(ERO)
ID No. WV6ꢀ 027, S. No. 223832 dated 03ꢀ02ꢀ2015). A. Dhara acknowledges the financial
support provided by University Grants Commission, India (Award letter No. F.4ꢀ
2/2006(BSR)/CH/14ꢀ15/0163; dated May, 2015) through the Dr. D. S. Kothari PostꢀDoctoral
Fellowship (DSKPDF). SM acknowledges the contribution of Dr. Milan Kr. Barman, NISER,
Bhubaneswar and Nirmalya Acharya, Mahishadal Raj College for their critical scientific
suggestions.
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DOI: 10.1039/C8NJ00436F
Graphical Abstract:
Both rhodamine and azobenzene moiety have been conjugated to prepare a novel
chemosensor for the detection of Al3+ through CHEFꢀPET and spirolactam ring opening
mechanism.
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