Dual channel selective fluorescence detection of Al(iii) and PPi in aqueous media with an 'off-on-off' switch which mimics molecular logic gates (INHIBIT and EXOR gates)

Article abstract of DOI:10.1039/c3dt50621e

In this study, we have synthesized a simple Schiff base type isophthaloyl salicylaldehyde hydrazone (ISH) moiety which selectively detects Al(iii) and PPi with a fluorescence enhancement at two different wavelengths in aqueous solution. The sensing phenomenon is also reversible and thus the sensor beautifully mimics logic gates (INHIBIT and EXOR gates).

Full text of DOI:10.1039/c3dt50621e

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Dual channel selective fluorescence detection of Al(III)
and PPi in aqueous media with an ‘off–on–off’ switch
which mimics molecular logic gates (INHIBIT and
EXOR gates)†
Cite this: Dalton Trans., 2013, 42, 8078
Shyamaprosad Goswami,* Abhishek Manna, Sima Paul, Krishnendu Aich,
Avijit K. Das and Shampa Chakraborty
Received 14th January 2013,
Accepted 9th March 2013
In this study, we have synthesized a simple Schiff base type isophthaloyl salicylaldehyde hydrazone (ISH)
moiety which selectively detects Al(III) and PPi with a fluorescence enhancement at two different wave-
lengths in aqueous solution. The sensing phenomenon is also reversible and thus the sensor beautifully
mimics logic gates (INHIBIT and EXOR gates).
DOI: 10.1039/c3dt50621e
www.rsc.org/dalton
So far, only a few fluorescent aluminum sensors10 and
water soluble PPi sensors11 have been reported, and there are
Introduction
The widespread use of aluminum in food additives, aluminum- many aspects left to be improved, including complicated syn-
based pharmaceuticals and storage/cooking utensils often thetic procedures, poor water solubilities, unsuitable pH
exposes people to aluminum ions. In addition, frequent use of ranges for physiological use and the lack of sensitivity. More
aluminum foil, vessels and trays for convenience results in a importantly, it is still a challenge to develop fluorescent
moderate increase in the Al3+ concentration in food. It has also sensors that can also be used as water-soluble devices. The
been widely used in many industrial fields, including the manu- sensitive recognition of Al3+ and PPi in water is a prerequisite
facturing of cars and computers.1 A tolerable weekly aluminum for understanding the underlying mechanism of how alumi-
intake in the human body is estimated to be 7 mg kg−1 body num ions cause aluminum-induced human diseases, and the
weight.2 Many symptoms of aluminum toxicity mimic those of role of PPi in metabolic processes. Thus, the detection of Al3+
Alzheimer’s disease and osteoporosis.3 Colic, rickets, gastroin- and PPi is very important from the human health perspective.
testinal problems, interference with the metabolism of
Exploring the possibilities of mimicking the function of
calcium, extreme nervousness, anemia, headaches, decreased silicon chips, and building electronic devices using organic or
liver and kidney function, memory loss, speech problems, soft- inorganic single molecules based on their spectral responses
ening of the bones, and aching muscles can be caused by toward an external stimulus,12 have recently gained much
aluminum toxicity.3 Environmental acidification due to the importance in sustaining the development of information
increased solubility of Al minerals at a lower pH increases the sciences and new technologies.13 Semiconductor logic gates
amount of available Al3+ which is deadly to growing plants.4
(or electronic logic gates), which perform binary arithmetic
In addition, pyrophosphate (PPi) is involved in a number of and logical operations, are the basis of current computers.14
important cellular metabolic processes such as DNA and RNA Logic gates are switches whose output state (0 or 1) depends
polymerization reactions and ATP hydrolysis.5 In addition, on the input conditions (0 or 1). Thus, intelligent molecules
researchers have recently reported a relationship between PPi which are capable of producing sequential logic operations are
concentration and cancer.6 In this regard, there is a growing extremely interesting. However there are only a few reports
interest in using different detection methods to monitor PPi in about “set–reset” logic devices,15–17 where the memorized unit
biological processes, with fluorescence techniques being the has been mimicked by the sequential integration of molecular
most popular.7–9
logic gates. A plentiful number of molecular-level machines,
elementary electronic devices, fluorescent logic gates, such as
the AND, OR, NOR, INHIBIT, XOR, YES, NOT, and XNOR logic
gates, and arranged logic circuits incorporating single logic gates
have been established recently.18–20 Most of the logic systems
rely on only one output mode. However, recent interest in the
development of integrated systems involving multiple fluorescent
Department of Chemistry, Bengal Engineering and Science University, Shibpur,
Howrah, India. E-mail: spgoswamical@yahoo.com; Fax: +91 33 2668 2916;
Tel: +91 33 2668 2961-3
†Electronic supplementary information (ESI) available: details of spectral data
and fluorescence titration graphs are available. See DOI: 10.1039/c3dt50621e
8078 | Dalton Trans., 2013, 42, 8078–8085
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output modes inspired us to fabricate a logic gate that can (2 ml). CH3OH : H2O (1 : 1) is used as the solvent at a pH of 7.5
utilize fluorescence behaviour in the presence of cations and which is obtained using HEPES buffer. The mole fraction (X)
anions. Such basic logic gates, programmed in a single mol- of Al3+ was varied from 0.1 to 1.0. The corrected fluorescence
ecular switch, have possible implications in the development at 455 nm was plotted against the molar fraction of the Al3+
of electronic and photonic devices.
solution. A series of solutions containing ISH (c = 2 × 10−5
Kim et al. recently reported the strong affinity of Al3+ ml−1) and Na4P2O7 (c = 2 × 10−5 ml−1) were prepared such that
towards the imine of salicylaldehyde21 and it is also well the sum of the total volume remained constant (2 ml).
known to all that amide protons have strong binding ten- CH3OH : H2O (1 : 1) was used as the solvent at a pH of 7.5
dencies towards anions. Yoon and Lee et al. in 2010 showed which is obtained using HEPES buffer. The mole fraction (X)
the strong attraction between Al3+ and PPi in aqueous media.22 of Na4P2O7 was varied from 0.1 to 1.0. The corrected fluore-
Thus combining all of the concepts and considering all of the scence at 506 nm was plotted against the molar fraction of the
above facts, we have synthesized an isophthalyl salicylaldehyde P2O74− solution.
hydrazone (ISH) type receptor for the selective detection of Al3+
and PPi with a dual mode channel enhancement of fluo-
Calculation of the detection limit
rescence. The receptor is based on inhibited CvN isomerisa-
tion23 and internal charge transfer (ICT) in aqueous media
The detection limit DL of ISH for Al(III) and PPi was deter-
which mimic the INHIBIT (integrated24 by combining a NOT,
mined using the following equation:
a YES, and an AND gate) (two outputs at different wave lengths
DL ¼ K ꢀ Sb1=S
i.e. output1 and output2) and EXOR gates i.e. the Exclusive-OR
where K = 2 or 3 (we take 2 in this case), Sb1 is the standard
deviation of the blank solution and S is the slope of the cali-
bration curve.
logic gate (any fluorescence signal as a output). From our
knowledge, this is the first report where the molecular logic
gates INHIBIT and EXOR switch on a single fluorescent sensor
using Al3+ and PPi ions as the input functions.
Synthetic procedure (Scheme 1)
Synthesis of isophthaloyl dihydrazide (A). A solution of
hydrazine hydrate (80% solution, 4 ml) in dry DCM was added
to a solution containing isophthaloyl chloride (1.035 g,
5 mmol) in dry DCM with 0.5 ml triethylamine. The mixture
was stirred for 1 h at room temperature under a nitrogen
atmosphere. The solvent was then evaporated using a pump.
The obtained white solid product was used for the next step
without any purification.
Synthesis of the receptor (ISH). A solution of isophthaloyl
dihydrazide (388 mg, 2 mmol) in absolute ethanol was added
to a solution containing salicylaldehyde (488 mg, 4 mmol) in
ethanol. The mixture was stirred for 8 h at room temperature
under a nitrogen atmosphere. The resulting precipitate was
then filtered and washed with absolute ethanol (3 × 10 ml).
The yellowish white colored product was then purified using
column chromatography using dichloromethane. The yield of
the receptor was 91%.
Experimental section
General
The chemicals and solvents were purchased from Sigma-Aldrich
Chemicals Private Limited and were used without further purifi-
cation. 1H-NMR and 13C NMR spectra were recorded on a
Brucker 300 MHz and 600 MHz respectively. For NMR spectra,
d6-DMSO was used as the solvent, with TMS as the internal
standard. Chemical shifts are expressed in δ units and H–1H
coupling constants are expressed in Hz. UV-vis titration experi-
1
ments were performed using
a JASCO UV-V530 spectro-
photometer and fluorescence experiments were performed
using a Perkin Elmer LS 55 fluorescence spectrophotometer
with a fluorescence cell path of 10 mm. IR spectra were recorded
using a JASCO FT/IR-460 plus spectrometer, using KBr discs.
A general method for the UV-vis and fluorescence titrations
For the UV-vis and fluorescence titrations, a stock solution of
the sensor was prepared (c = 1 × 10−5 ml−1) in MeOH : H2O
(1 : 1, v/v). The solution of the guest cation was prepared (2 ×
10−4 ml−1) in MeOH : H2O (1 : 1, v/v) at pH 7.5 by using 20 mM
HEPES buffer. The original volume of the receptor solution
was 2 ml. Solutions of the sensor at various concentrations
and solutions containing an increasing concentration of the
cation were prepared separately. The spectra of these solutions
were recorded by means of UV-vis and fluorescence methods.
A Job plot using the fluorescence method
A series of solutions containing ISH (c = 1 × 10−5 ml−1) and
Al(NO3)3·9H2O (c = 1 × 10−5 ml−1) were prepared such that the
sum of the total metal ion and ISH volume remained constant
Scheme 1
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1H NMR (d6-DMSO, 300 MHz) δ (ppm): 12.265 (s, 2H),
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11.208 (s, 2H), 8.675 (s, 2H), 8.508 (s, 2H), 8.299 (s, 1H), 8.146
(d, 2H, J = 10.4 Hz), 7.718 (s, 1H), 7.571 (d, 2H, J = 9.6 Hz),
7.312 (q, 2H, J = 5.46 Hz), 6.923 (t, 2H, J = 10 Hz).
13C NMR (d6-DMSO, 150 MHz): δ (ppm): 162.7, 158.3,
150.2, 133.3, 131.3, 128.8, 127.2, 119.3, 118.2, 116.8, 78.4, 39.9.
MS (ESI MS): (m/z, %): 425.03 [(M + Na)+, 100%].
FT-IR (KBr): 3450, 3250, 2917, 2346, 1646, 1538, 1356, 1270,
960, 750 cm−1
.
Synthesis of the Al3+-complex of the sensor. Al(NO3)3·9H2O
is added to a solution of ISH in methanol and stirred for
2 hours to obtain a precipitate which is filtered off and dried.
MS (ESI MS): (m/z, %): 529.01 [(ISH + 2Al3+ + 4H2O + H+)7+,
100%].
Fig. 1 The fluorescence titration spectra of ISH (c = 1.0 × 10−5 M) in the pres-
ence of Al3+ (c = 2.0 × 10−4 M) at pH 7.5 in MeOH : H2O = 1 : 1 (v/v) and the
visual color change of ISH with addition of 5 equivalents of Al(NO3)3·9H2O
under UV light (inset).
FT-IR (KBr): 3409, 2361, 1629, 1548, 1382, 1093, 829, 746,
614 cm−1
.
spectra, with the addition of Al3+, there is a remarkable
enhancement (250 fold increase of the fluorescence quantum
yield) (ESI†) in the fluorescence with an emission band shift
from 527 nm to 455 nm (72 nm blue shift, excitation at
330 nm), whereas upon the addition of other metal ions, the
change is negligible. The observed changes in the fluorescence
emission spectra with Al3+ are shown in Fig. 1.
It seems that the fluorescence enhancement is derived from
the more widespread formation of the π-conjugation system in
ISH, which occurs upon metal binding, and the fluorescence
is sensitive to specific metal ions. The selectivity for 2 equi-
valents of Al3+ with 10−5 M of ISH over other metal ions is
plotted as a bar graph in Fig. 2. In addition, the Job’s plot
(Fig. S1, ESI†), which is based on the changes in the emission
at 455 nm, confirmed the formation of 1 : 2 complex of ISH
with Al3+. ESI TOF mass spectral analysis also indicates the for-
mation of a binuclear complex of ISH with Al3+; the positive-
ion mass spectrum of ISH upon the addition of 2 equivalents
of Al3+ exhibited the most intense peak at m/z = 529.01, which
corresponds to the ion [ISH + 2Al3+ + 4H2O + H+]7+ (ESI†).
Ga3+ and In3+, which belong to the same group on the peri-
odic table, also generated a similar fluorescence enhancement
Results and discussion
The objective for designing our simple Schiff base type probe
is to achieve an efficient emission signal (output) and a
change in the photophysical properties upon interaction with
anions/metal ions that may mimic a molecular logic gate oper-
ation upon the chemical input of ions. The sensor in the
aprotic solvent acetonitrile exhibits weaker fluorescence than
MeOH and the mixed solvent H2O : MeOH (1 : 1), due to the
hydrogen bonding of the solvent to the N and O lone electron
pairs. This weakens the intramolecular radiation-less tran-
sition from the nπ* state and makes the emission maxima
(λem) undergo a gradual red shift with increasing protonation
by the solvent. Therefore, the receptor appears to be a promis-
ing candidate for enhancing the fluorescence emission upon
the binding of suitable metal ions if their radiation-less
channel could be well blocked. The detection of metal ions
using fluorescent sensors based on electron donors/acceptors
are usually affected by the presence of protons, and so it is
necessary to consider excluding the pH effect to find the
optimum sensing conditions. The observed emission peak of
ISH at ca. 452 nm under acidic conditions is probably due to
the protonation of the carbonyl oxygen, which is gradually
decreased with an increase in the pH and finally vanishes at a
neutral pH. A dramatic change is observed under alkaline con-
ditions. A red shift in the emission maxima of about 50 nm
occurs and at ca. 502 nm, a new peak is observed which is
probably due to the deprotonation of the OH of ISH (Fig. S2:
ESI†). Therefore, this experiment enables us to identify the
conditions, pH = 7.5 (near neutral pH), under which the
metal-sensing abilities of ISH could be probed.
The photophysical properties of ISH are investigated by
monitoring the fluorescence behaviour upon the addition of
several metal ions such as Na+, K+, Ba2+, Ga3+, Mg2+, Ca2+,
Mn2+, Fe2+, Ni2+, Co2+, Zn2+, Cu2+, Ag+, Cd2+, Hg2+, Pb2+, In3+,
Cr3+, Fe3+ and Al3+, and the anions investigated are AcO−, Cl−,
Fig. 2 The change in the emission intensity of ISH after the addition of 2.0
Br−, I−, F−, BzO−, SH−, H2PO4−, PO43−, S2−, N3−, P2O7 and
4−
equivalents of each of the investigated guest cations in MeOH : H2O (1 : 1, v/v,
ADP in MeOH : H2O (1 : 1, v/v) as their salts. In the emission at pH 7.5) at 455 nm and a plot of the fluoresence intensity vs. [Al3+] (inset).
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centered at 455 nm, but the intensity was significantly lower change in the fluorescence upon the addition of other anions.
compared to Al3+ under the same conditions. Only Al3+ The observed changes in the fluorescence emission spectra for
resulted in a pronounced fluorescence enhancement relative ISH with the addition of PPi are shown in Fig. 3 and the
to the control ion. The association constant (Ka) as determined changes caused by the addition of other ions are shown as a
using a florescence titration method for the ISH sensor with bar graph in Fig. 4. In addition, the Job’s plot (Fig. S2†), which
Al3+ is found to be 1.05 × 105 M−1 by using the Benesi–Hildeb- is based on the changes in the emission at 508 nm, confirmed
rand equation.25 However in this case, as two guest metal ions the formation of a 1 : 1 complex of PPi with ISH, and the detec-
form a complex with one host receptor, the association con- tion limit of 1.71 μM (ESI†) suggests that the receptor is very
stant is determined using the non-linear least square analysis sensitive towards PPi.
−
method.26 By this method we found that k11 is 5.36 × 104 M−1
However, when titrations of other anions such as H2PO4
,
and k12 is 1.76 × 105 M−1 for ISH during the complex for- PO43−, ADP, ATP, AcO−, Cl−, Br−, I−, F−, BzO−, SH−, S2− and
mation with Al3+ (ESI†) and the fluorescence intensity of ISH N3 were performed under similar experimental conditions,
−
maintained a good linear relationship with [Al3+] (Fig. 2). The no significant changes were observed in the emission spectra.
calculated detection limit of 0.77 μM (ESI†) suggests that the The selectivity for 1 equivalent of PPi with 10−5 M of ISH was
receptor is very sensitive towards the Al3+ ion.
plotted as a bar graph in Fig. 4. Interestingly, anions other
Pioneering work in 2007 by Wang et al. reported that this than PPi that contain the phosphate moiety do not interfere in
non-radiative process can be inhibited by the complexation of the sensing properties.
metal cations to the CvN group, thereby leading to a strong
Furthermore, to examine the selectivity of the sensor in a
fluorescence emission.23b It was found that CvN isomeriza- complex background of potentially competing species, the
tion is the predominant factor in the decay process of the fluorescence enhancement of ISH with Al3+ was investigated in
excited state in compounds with an unbridged CvN structure,
which usually results in the nonfluorescence of those com-
pounds. But when the restriction is introduced, the fluore-
scence of the compound is regained. In this case, most
probably the fluorescence of the conjugated sal moiety is off
due to the simultaneous change in the cis–trans isomerization
in ISH. The isomerization phenomenon is locked in the pres-
ence of the Al3+ ion and thus the original fluorescence of the
ISH is recovered. Interestingly the transition metal ions which
have a strong affinity towards CvN are inert in restricting the
cis–trans isomerization in ISH. The mechanism is illustrated in
Scheme 2. In addition, a complex with a coplanar structure is
probably formed, resulting in a large chelation induced
enhanced fluorescence (CHEF) effect which induces a large
enhancement in the fluorescence: the reaction of a metal ion
with a chelating agent induces rigidity in the resulting mole-
cule and tends to produce fluorescence.
With the addition of PPi, in the emission spectra, there is
an enhancement in the fluorescence with an emission band
Fig. 3 The fluorescence titration spectra of ISH (c = 1.0 × 10−5 M) in the pres-
4−
ence of P2O7 (c = 2.0 × 10−4 M) at pH 7.5 in MeOH : H2O = 1 : 1 (v/v) and the
visual color change of ISH with the addition of 5 equivalents of Na4P2O7 under
UV light (inset).
shift from 527 nm to 508 nm (19 nm blue shift, excitation at
330 nm; with an increase of about 125 fold in the fluorescence
quantum yield). With the exception of PPi, there is a negligible
Fig. 4 The change in the emission intensity of ISH after the addition of 2.0
equivalents of each of the investigated guest anions in MeOH : H2O (1 : 1, v/v, at
Scheme 2
pH 7.5) at 508 nm and a plot of the fluoresence intensity vs. [P2O74−] (inset).
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Fig. 6 The anion sensitivity profile for ISH: the change in the emission intensity
Fig. 5 The metal ion sensitivity profile for ISH: the change in the emission
of ISH + 8.0 equivalents of the investigated interfering An− + 2.0 equivalents of
PPi.
intensity of ISH + 8.0 equivalents of the investigated interfering Mn+ + 2.0
equivalents of Al3+
.
the presence of other metal ions and the fluorescence
enhancement of ISH with PPi was investigated in the presence
of other common anions. The experiment is performed by
adding the species under investigation i.e. Al3+ or PPi (2.0
equivalents) to the sensor in the presence of commonly
employed interfering species i.e. metal ions or anions, respecti-
vely (8.0 equivalents). With the exception of Fe3+, Cu2+, Ga3+
and In3+, a background of competing metal ions did not inter-
fere in the detection of Al3+ by ISH in methanol : H2O. On the
basis of the fluorescence intensities shown in Fig. 5, Fe3+ and
Cu2+ quenched about 50% of the fluorescence obtained with
Al3+ alone. Interestingly, Ga3+ and In3+ quenched just 20–30%
under the same conditions. On the other hand, the selectivity
profile diagram (Fig. 6) reveals that a PPi-induced fluorescence
enhancement remains unperturbed and does not get any inter-
ference from the co-existing metal anions. This indicates that
the sensor can be used potentially to quantitatively detect the
Al3+ and PPi concentration individually with a high selectivity.
Fig. 7 The fluorescence titration spectra of the ISH–Al complex with the
gradual addition of PPi.
reversible probe that can be used repeatedly for sensing pur-
poses and the ‘off–on–off’ behaviour of ISH towards Al3+ and
In 2006 Zhao et al. reported27 a similar type of compound PPi is proved. The mass spectra supports this fact with appear-
(involving a naphthyl moiety instead of benzene) that was
selective for F− with an exited state proton transfer (ESPT) sig-
ance of a new peck at m/z 425.01 (for ISH) when PPi added to
the Al-complex (Fig. S10, ESI†). The cyclic voltammogram of
naling transduction mechanism. However, they only used ISH with the addition of Al3+ and PPi also suggests the revers-
DMSO as a solvent in the spectral titration, whereas in this
study, we used an aqueous medium at a neutral pH (obtained
ible phenomenon (Fig. S19, ESI†).
These alternate additions (3.0 equivalents each) are
by using HEPES buffer) which is more favourable for further repeated. After 6 cycles, fluorescence ‘off–on–off’ behaviours
biological studies. Most probably, the use of water as a solvent
is the main reason for inertness of ISH towards fluoride.
The most interesting phenomenon is observed in the emis-
sion spectra when PPi is added to the solution containing a
ISH–Al(III) mixture (2 equivalents of Al3+). The emission band
are still clearly observed, even though the emission intensity
changes are smaller than those found initially (Fig. 8).
This type of reversible behaviour mimics the INHIBIT logic
gate (integrated by combining a NOT, a YES, and an AND gate)
at two different wave lengths i.e. at λmax = 455 nm (output1)
at 455 nm is abruptly quenched and finally diminished with and λmax = 508 nm (output2) i.e. it works as a dual channel
the addition of 2 equivalents of PPi (Fig. 7) and a new peak at
508 nm is observed with addition of another equivalent of PPi
output mode. The truth table and the circuit for the INHIBIT
logic gate are shown in Fig. 9. However, when we consider any
to the solution mixture. Again, when Al3+ (3 equivalents) is fluorescence signal from the receptor, i.e. ISH with Al3+ or PPi
added to the solution, the fluorescence is regenerated at
455 nm. Thus it is established that the receptor i.e. ISH is a
as an input factor (input1 and input2), output with ‘YES’ i.e. 1
or ‘NO’ i.e. 0 function interestingly behaves as an Exclusive-OR
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Fig. 8 The emission intensity change (λex = 330 nm) of the stepwise and alter-
nate 3.0 equivalent addition of Al3+ (1 + 2 equivalent addition in blue squares)
and PPi (2 + 1 equivalent addition in black circles) showing the reversibility of
the ISH complex (50 μM) in 10 mM HEPES buffer solution at pH 7.5 in MeOH :
H2O = 1 : 1 (v/v).
Fig. 10 (a) The truth table of the Exclusive-OR gate (EXOR gate) and (b) the
logic schemes of the following gate with a change in the emission spectra and
the output intensities (bar diagram) of ISH upon reversing the chemical inputs
of Al3+ and PPi.
Fig. 11 The partial 1H-NMR spectra of (a) ISH with the addition of (b) 1 equi-
valent Al3+ and (c) 2 equivalents Al3+ in d6-DMSO.
of ISH with Al3+, and probably because of this, the shift in the
amide protons to nearer the carbonyl ‘O’ occurs with the influ-
ence of the metal ions (Scheme 2). The protons of the aromatic
ring are shifted to a lower magnetic field due to the reduction
Fig. 9 (a) The truth table for the INHIBIT gate with respect to output1 or
output2 and (b) the logic schemes of the following gate.
gate (EXOR gate). The truth table and the circuit for the EXOR of electron density upon coordination to the metal ion and
gate are shown in Fig. 10.
broadened upon complexation. Thus the NMR data also
The binding mode of ISH with Al3+ and PPi was examined demonstrates the adduct formation between Al3+ and ISH
1
by H NMR in d6-DMSO. Spectra of ISH before and after treat- which results in these chemical shift changes.
ment with 1 and 2 equivalents of Al3+ are shown in Fig. 11. On
The spectra of ISH before and after treatment with 1 equi-
complexation of ISH with Al3+, the phenolic protons of the sal valent of PPi are shown in Fig. 12. An upfield shift occurs
group (Ha) are not much affected, the δ value is shifted to just (0.12 ppm) to the phenolic protons (Ha) of the sal group upon
0.022 ppm downfield with the addition of 2 equivalents of the association of ISH with PPi. With the addition of 1 equi-
metal ions. In contrast, the amide protons (Hb) are most valent of PPi the amide protons (Hb, δ = 11.208 ppm) are down-
affected in the complexation process of ISH with metal ions. field shifted (0.15 ppm), but the shifting is less compared to
Interestingly with the addition of 1 equivalent of metal ions the complexation of Al3+ with ISH. The imine proton of ISH is
the amide protons (δ = 11.208 ppm) almost disappear (but not downfield shifted with the addition of PPi. The other protons
completely) with initiation of a new peak (δ = 9.223 ppm). The of ISH are not much affected by the addition of PPi.
disappearance of the peak corresponding to the amide protons
The most interesting phenomenon is observed when we
and appearance of the new peak is completed with the added 2 equivalents of PPi to the solution mixture of Al(III) and
addition of 2 equivalents of metal ions. From the presence of ISH; we get back almost the same 1H-NMR spectra detected for
this type of change in the NMR spectra, we can conclude that a ISH (Fig. 13) which also suggests the reversible binding of
structural change must have occurred during the complexation Al(III) to ISH.
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Fig. 13 The partial 1H-NMR spectra of (a) ISH with addition of (b) 2 equi-
valents Al3+ and (c) 2 equivalents PPi in d6-DMSO.
Conclusions
In conclusion, we have developed a turn-on fluorescence
method to detect Al3+ and PPi using an isophthaloyl salicyl-
aldehyde hydrazone (ISH) type amide containing Schiff base
moiety with high selectivity in methanol : water. In our assay,
Al3+ and PPi could selectively bind to ISH, which resulted in a
fluorescence enhancement in two different wavelengths in a
reversible manner and thus it mimics the logic gates. The
reversible binding is also explained by NMR spectroscopy.
Acknowledgements
Authors thank the DST and CSIR (Govt. of India) for financial
support. A.M., K.A. and A.K.D. acknowledge the CSIR. S.P.
acknowledges the UGC and S.C. acknowledges the DST (SR/S1/
OC-58/2010) for providing fellowships. We acknowledge
Mr Prasenjit Hazra for the CV diagram.
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