Three novel input logic gates supported by fluorescence studies: Organic nanoparticles (ONPs) as chemo-sensor for detection of Zn2+ and Al3+ in aqueous medium

Article abstract of DOI:10.1016/j.saa.2015.03.082

Organic nanoparticles (ONPs) of N,N′-ethylenebis(salicylimine) (salen) were synthesized and applied for specific recognition of Zn²⁺ and Al³⁺ ions in an aqueous medium. The results show that fluorescence intensity rises with the incr

Full text of DOI:10.1016/j.saa.2015.03.082

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 146 (2015) 142–150
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Three novel input logic gates supported by fluorescence studies: Organic
2+
3+
nanoparticles (ONPs) as chemo-sensor for detection of Zn and Al in
aqueous medium
C.A. Huerta-Aguilar , T. Pandiyan , N. Singh , N. Jayanthi c
a
a,
⇑
b
a
Facultad de Química, Universidad Nacional Autónoma de México (UNAM), Ciudad Universitaria, Coyoacán 04510, México D.F., Mexico
Indian Institute of Technology (IIT), Ropar, Punjab, India
División de Nanotecnología, Universidad Politécnica del Valle de México (UPVM), Av. Mexiquense, C.P. 54910 Tultitlan, Estado de México, Mexico
b
c
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
ꢀ
ꢀ
Three novel input logic gates
supported by fluorescence studies.
Organic nanoparticles as chemo-
2
+
sensor for detection of Zn
.
pH and temperature influence on the
fluorescence in the detection of Zn2
+
.
a r t i c l e i n f o
a b s t r a c t
0
Article history:
Received 29 October 2014
Received in revised form 10 February 2015
Accepted 6 March 2015
Available online 12 March 2015
Organic nanoparticles (ONPs) of N,N -ethylenebis(salicylimine) (salen) were synthesized and applied for
2+
3+
specific recognition of Zn and Al ions in an aqueous medium. The results show that fluorescence
2+
intensity rises with the increasing concentration of Zn in salen solution, proving that salen-ONPs detect
Zn efficiently in the aqueous medium as chemo-sensor. Furthermore, the salen-ONPs/Zn system per-
forms as an ON–OFF switch between pH 6.0 and 4.0. Amusingly, although salen-ONPs/Al does not show
2+
2+
3
+
3+
any significant effect in the fluorescence spectra, highest fluorescence intensity was observed when Al
Keywords:
ion was added to salen-ONPs/Zn in a sequential order (addition of Zn to salen-ONPs, followed by Al3+).
2+
2+
Organic nanoparticles (ONPs)
Logic gates
This system can be applied as a novel three inputs logic gate supported by the fluorescence for the detec-
tion of Zn and Al in biological and environmental samples. It appears that photo induced electron
2
+
3+
Recognition of Zn2
+
2
+
transfer (PET) occurs in the salen-ONPs when the fluorophore is excited. For salen/Zn system, the
PET is being inhibited considerably by lowering the receptor HOMO energy due to the formation of a
bond between the metal ion and ligand, enhancing the fluorescence emission. This is consistent with
the theoretical study that the energy of HOMO of the ligand is lower than that of Zn(salen)2 complex.
Ó 2015 Elsevier B.V. All rights reserved.
Fluorescence studies
+
Introduction
molecular devices because they mimic or mirror the action of
macroscopic tools such as switches, motors and other machinery
New materials that exhibit electronic, magnetic and optical
properties are currently of great interest in the development of
[1–4]; moreover, the change of modes (OFF and ON) in those prop-
erties can be modulated by employing external inputs such as ions,
molecules, light etc [5–10]. Luminescence, one of most useful tech-
niques, is being considered to monitor the molecular level func-
tions; for example, in DNA-based logic gates, fluorescent signals
⇑
Corresponding author. Tel.: +52 55 56223499.
E-mail address: pandiyan@servidor.unam.mx (T. Pandiyan).
http://dx.doi.org/10.1016/j.saa.2015.03.082
386-1425/Ó 2015 Elsevier B.V. All rights reserved.
1

C.A. Huerta-Aguilar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 146 (2015) 142–150
143
are developed with respect to chemical reactions generated by
ions, light or molecules as inputs in the Boolean operations [11–
with Energy Dispersive and X-Ray Spectroscopy micro-analyzer
((EDS, Oxford ISIS) was used. UV–Vis spectrophotometer (Perkin-
Elmer Lambda 25) and fluorescence spectrophotometer (F-96
Pro) were employed to analyze electronic and fluorescence proper-
ties of salen-ONP as well as for the detection of metal ions.
1
5]. Thus molecular logic gates [16,9,17–22] were used for bio-
chemical optical sensors [23] or semiconductor QD [24–27]. In
the studies, mostly, metal based nanoparticles (NPs) have been
used as fluorescent biological labels [28–30], light-emitting diodes
(
LEDs) [31], and optical sensors [32,33].
Organic molecules hetero atoms or bio-molecules such DNA
0
Preparation of ligand N,N -ethylenebis(salicylimine)) (salen)
Salen was prepared as reported elsewhere [55]: To solution of
chains have been employed as surface capping agents for metal
NPs in order to prevent NPs aggregation [34–39] and also for
achieving bio-compatibility. Nevertheless, the studies of organic
molecules in the concept of NPs and also the development of
molecular logic gate based on Organic nanoparticles (ONP) are very
limited in the literature when compared to metal NPs [40–43]. The
electronic and optical properties of organic molecular crystals are
fundamentally different from those of inorganic metals or semi-
conductors due to the presence of weak intermolecular interaction
forces (van der Waals type) [44]. For example, in organic nanopar-
ticles studies, the size dependent fluorescent properties of pery-
lene, pyrazoline and phthalocyanine have been established [45–
2
-hydroxybenzaldehyde (2.0 mmol) dissolved in ethanol (30 mL),
ethylenediamine (1.0 mmol) was added, and then the resulting
solution was stirred for 30 min at 40 °C to yield a yellow product,
which was filtered and dried in vacuum (Scheme 1).
O
N
N
EtOH
0°C
H
2
N
NH
2
+
2
HO
OH
HO
4
Synthesis of Ligand 1 (salen)
4
7]. Thus the electronic/optical behaviors of ONPs are expected
to be interesting because of their variability and flexibility.
Numerous chemical and biochemical analytes can be detected
Yield, 88.95%. Elemental analysis for C16
1.62; H, 6.01; N, 10.44. Found, C, 71.94; H, 5.64; N, 10.85.
NMR (300 MHz, CD3OD) d (ppm): 3.94 (s, 2H, ethylenediamine),
.82 (t, 2H, phenyl ring), 6.84 (s, 2H, phenyl ring), 7.25 (d, 2H, phe-
16 2 2
H O N : Calcd.: C,
+
+
+
2+
2+
2+
1
by fluorescence methods: cations (H , Li , Na , Ca , Mg Zn
,
7
H
2
+
2+
2+
3+
Pb , Cd , Al , Cr , etc.), and anions (halides, citrates, carboxy-
lates, phosphates, etc.), neutral molecules (sugars e.g., glucose).
The development of these types of chemo-sensors is of major
6
nyl ring), 7.30 (m, 2H, phenyl ring), 8.44 (s, 2H, R–N@C–R).
importance for simultaneous detection of Zn2 and Al ions in
+
3+
environmental and biological systems at very low concentrations.
Preparation of organic nanoparticles
2
+
Generally, Zn is commonly present in environment and biological
systems, the detection of this ion becomes vital, and the presence
of aluminum ion can be used as indicator of some metabolic
malfunctions.
Conjugate-schiff base ligands are being considered for chemo-
sensors [48] because the C@N structure (p acceptors) is generally
non-fluorescent and becomes fluorescent when the schiff base is
adequately modified. For instance, the addition of aromatic struc-
ture to the schiff base is such that the isomerization of C@N struc-
ture is inhibited to change the fluorescence behavior [49]. Only a
few reports on the ONP of Schiff bases are present in the literature
The preparation of salen-ONPs is followed as reported pre-
viously [56]: salen ligand (14.5 mg, 0.05 mmol) is first dissolved
in tetrahydrofuran (1.0 ml), then it was injected slowly into deion-
ized water (100 ml) under sonication. The resulting mixture was
further sonicated for 15 min. to yield a colloidal suspension of
salen-ONP (0.5 mM) and they were characterized by TEM and
EDX analyses [57].
Results and discussion
The nature of salen-ONPs was analyzed by TEM and EDX, show-
ing that the sphere shaped small ONPs (2.0 nm) are uniformly
formed (Fig. 1). In the EDX analysis, signals corresponding to C
and O were observed, confirming the organic nature of the parti-
cles. Since the ONPs are generally volatile and are not stable under
typical conditions of TEM analyses, they differ in the elemental
composition of salen ONPs in the EDX.
[
50–52]; moreover, the application of these ONPs in developing a
logic gate system is unexplored. In the present work, the ONPs of
N,N -ethylenebis(salicylimine) (salen) is chosen to detect metal
ions through the function of logic gates. In particular, the specific
detection of Zn ions in aqueous medium is important as it plays
a key role in human metabolism and also in synaptic plasticity
0
2
+
2
+
[
53,54]. The present study shows that salen-ONPs detect Zn effi-
ciently in aqueous medium and acts as a chemo-sensor, perform-
ing a novel input logic gate supported by fluorescence, which
increases the intensity many fold in the presence of Al3 ions.
Metal binding studies with salen-ONPs
+
The selective recognition of different metal ions (Mg2+, Al3+
,
Cr3+, Mn , Fe , Ni , Cu , Zn , Sr , Sm , Ag , and Cd as nitrate
salts) with salen-ONPs was analyzed by UV–visible and fluores-
cence techniques. The nanoparticles (NPs) of salen are insoluble
2+
3+
2+
2+
2+
2+
3+
+
2+
Experimental section
Materials and methods
(
colloid nature) in water. The UV visible spectra of NPs of salen
in methanol and also in water are recorded, and there has been
no a change in the spectra in both solvents. This confirms that
hydrolysis does not take place in the ligand in the water medium.
Moreover, if the ligand undergoes hydrolysis, the fluorescence
intensity is expected to decrease or disappear due to the loss of
All chemicals for the preparation of salen ligand were used as
received from Sigma-Aldrich and THF (J.T. Baker) was used in the
synthesis of salen-ONPs.
Physical measurements
conjugation. In the experiment, each metal ion solution (50 lL,
Elemental analyses were carried out on a Fisons (Model EA 1108
CHNSO). NMR (300 MHz Varian) and Mass Spectrometry (LECO
Pegasus 4D) with a TOF analyzer were used to characterize the
ligand. To determine the size and composition of ONPs,
Transmission Electron Microscope (TEM, JEOL JEM 2010) equipped
0.1 mM) was added to salen-ONPs (3.0 mL of 0.15 mM) suspended
in water, then the full UV–Vis spectra (200–800 nm), and the fluo-
rescence emission (360–750 nm) were measured. In the UV–visible
⁄
spectra, two peaks (250 and 330 nm) corresponding to the –
p
⁄
and
p–p
transitions were observed. In the fluorescence studies,

1
44
C.A. Huerta-Aguilar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 146 (2015) 142–150
a
b
10 nm
Fig. 1. Microscopic analysis of salen-ONPs: (a) TEM at 40,000 magnification; (b) EDX spectra.
salen-ONPs were exited at 365 nm and the corresponding fluores-
cent emission signal was observed at 490 nm.
(recognition) and this causing the lowering of the receptor
HOMO energy, thus inhibiting the photo-induced electron transfer
from HOMO (donor) to fluorophore, and hence enhancing the fluo-
rescence emission.
In the fluorescence study, a high fluorescence response was
2
+
observed for Zn with respect to other cations, showing a specific
2
+
recognition of Zn ion when compare to other metal ions by salen-
ONPs (see Fig. 2). In addition, there is a change in the fluorescent
color from green to neon blue after addition of Zn2 ions to
salen-ONPs, shifting the wavelength from 490 to 450 nm.
2+
Stoichiometry determination of salen-ONPs against Zn
+
Job’s method was employed to determine the stoichiometry of
2
+
Furthermore, the salen-ONPs (0.15 mM) is able to detect Zn ion
even at low concentration (16.0 mM) in aqueous solution, and this
detection threshold level is efficient to measure the concentration
2+
the reaction between salen-ONPs and Zn ions (Fig. 4). In the
study, the fluorescence intensity increases proportionally up to
2+
ꢁ
2.0 equivalents during the addition of Zn ions to ONPs at a ratio
2
+
of Zn in biological or environmental samples.
[
Zn ion: salen-ONPs] of 2: 1; however, subsequently to this ratio,
the proportionality decreased slightly to 6.0 equivalents (ratio:
6:1 for Zn ion:selen-ONPs). The detection limit was 20 M for
Zn ion in aqueous medium, indicating that the binding propor-
tionality of ligand with metal ion was not seen after ꢁ2.0 equiva-
lents. This is probably due to the nature of ONPs, where some of
the molecules might be inside the particle, causing a low interac-
tion with the metal ion.
2
+
l
Titration studies of salen-ONPs against Zn ions
2
+
The titration analysis was performed in order to prove the bind-
ing selectivity of salen-ONPs with Zn2+ ions. The florescent emis-
sion intensity was measured for each subsequent addition of
2
+
Zn ions (20.0–500.0
lM) to salen-ONPs (15.0 mM), and it was
observed that the peak intensity increases with increasing concen-
2
+
tration of Zn in the ONPs (Fig. 3a). The intensity of fluorescence
Competitive binding analysis
2+
for salen-ONPs was also measured without Zn ion. After analyz-
ing the results, it was found that there is a good quasi-linear
relationship between the intensity of fluorescent emission and
the concentration of Zn ion in the salen-ONPs (Fig. 3b), suggest-
ing that salen-ONPs perform as chemo-sensors in recognizing
Zn in the aqueous solution. It appears that photo induced elec-
tron transfer (PET) occurs in the salen-ONPs when the fluorophore
is excited. i.e., an electron is transferred from the HOMO (donor) to
the low-lying HOMO of the acceptor (fluorophore), causing a low
fluorescence emission. However, when Zn2+ is added to ONPs,
metal ions involve in the binding with the salen moiety
The binding selectivity of salen-ONPs with Zn2+ was analyzed
through fluorescence study in order to understand a possible inter-
ference of other metal ions. To the salen-ONPs (0.15 mM)/Zn
(16.0 mM), different metal ions (100.0 mM) were added in such a
way that the final concentration is to be 16.0 mM. The resulting
solution mixture was stirred for 15 min and kept for 5 min. to
reach equilibrium, and then the fluorescence emission was mea-
sured in the visible region after excitation at 365 nm. The results
2
+
2+
2
+
3+
3+
2+
2+
show that there is a small interference from Cr , Fe , Co , Ni
2
+
2+
and Cu ions during the recognition of Zn through salen-ONPs
8
7
6
5
4
3
2
1
0
ONP
Al
Mn
Ni
Zn
Sm
Cd
Mg
Cr
Fe
Cu
Sr
14
12
a
b
1
0
8
6
4
2
0
Ag
375
425
475
525
575
625
Wavelength (nm)
Fig. 2. Metal binding test of salen-ONPs with different metal ions: (a) Fluorescence spectra of salen-ONPs with respect to different cations; (b) Normalized fluorescence
2
+
intensity for specific recognition of Zn ions (50 lmol) at 450 nm.

C.A. Huerta-Aguilar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 146 (2015) 142–150
145
1
1
1
1
6
4
2
0
8
6
4
2
0
25.0
0.0
15.0
0.0
a
b
2
1
5
0
.0
.0
0
1
2
3
4
5
6
3
75 400 425 450 475 500 525 550 575 600 625
[
Zn]/[Ligand 1]
Wavelength (nm)
Fig. 3. Titration studies of salen-ONPs against Zn2+ ions: (a) the change of fluorescence intensity with respect to subsequent addition of Zn ions (20–500
2+
lM) to salen-ONPs;
2
+
2+
(
b) quasi-linear relationship for (salen-ONPs vs. Zn ) and the fluorescence response with respect to the Zn content in the solution.
highly magnified (Fig. 6), and this means that salen-ONPs/Zn2+
which is an electronic conjugated fluorophore, has been further
activated in the presence of Al ion in the aqueous medium,
,
3
3
2
2
1
1
0
0
.5
.0
.5
.0
.5
.0
.5
.0
3
+
2
+
increasing the intensity of fluorescence for [salen-ONPs/Zn
/
3
+
Al ]. Furthermore, a quenching in the fluorescence intensity for
salen-ONPs/Zn /Al ] was observed for the addition of the metal
ions except for Sr , Ag , Cd and Mg , and in addition, the fluo-
rescence emission signal was slightly red shifted from 450 nm to
2
+
3+
[
2+
+
2+
2+
3
+
2+
5
00 nm during the Al addition to [salen-ONPs/Zn ].
3
+
To understand the binding of Al ions with the [salen-ONPs/
Zn2+] system, the titration study was carried out by adding aliquots
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
Salen/[Salen+Zn²⁺]
1
of Al(NO (100 mM) to the system (ratio: 2:1, salen-ONPs
3 3
)
2
+
(
15.0 mM))/Zn (7.5 mM). The results show that the intensity of
2
+
the characteristic peak (450 nm) of Zn /ONPs was dramatically
decreased, while growing a new peak around 500 nm during the
addition of Al (3.0–6.0
Fig. 4. Job’s plot: stoichometry determination of Zn2+ with salen-ONPs.
3
+
2+
lM) to the [salen-ONPs/Zn ] system.
The stoichiometry ratio of this system was determined by Job’s
(
Fig. 5). It means that salen ligand can also bind with other metal
2
+
3+
3
+
method and was 1:2 for [salen-ONPs/Zn ]: Al (Fig. 7).
For example, the fluorescence intensity (2.0 F.U.) measured for
salen-ONPs (15.0 mM) at 500 nm increased to 16.2 for [salen-
ions owing to their similar chemical nature (ionic radius: Cr
2
+
3+
2+
2+
(
0.73 Å), Mn (0.67 Å), Fe (0.61 Å), Co (0.65 Å), Ni (0.69 Å),
Cu (0.73 Å) and Zn2+ (0.74 Å)). Moreover, the metal ions which
2+
2
+
ONPs Zn ] (15.0 mM, 1:1 ratio) at 450 nm; interestingly, the
intensity is further increased to 90 F.U. at 500 nm after addition
have d orbitals can coordinate with salen in a similar manner
and they can interfere with the fluorescence signal originated from
Zn/ONPs. However, there is no considerable interference from Ag
(
3
+
2+
+
of Al solution to the system. For salen-ONPs-Zn (15.0 mM)/
3
+
2
+
Al (0.6 mM) (1:1 ratio), a considerable increase in the florescence
intensity was observed through Intramolecular photo-induced
electron transfer (IPETS) [58]. For salen/Zn system, the receptor
HOMO energy is lowered when compared to salen-ONPs, and it
inhibits PET from HOMO (donor) to fluorophore, enhancing the
1.29 Å) or Cd (1.09 Å) in the fluorescence because of their differ-
ent acid-base chemistry.
2
+
Interestingly, although salen-ONPs/Al3 does not show any sig-
+
nificant effect in the fluorescence spectrum, an enhancement of
fluorescence intensity was observed when Al ion was added to
salen-ONPs/Zn . This indicates that Al interacts efficiently with
salen-ONPs/Zn system in such a way that the fluorescence is
3+
2
+
3+
2
+
3+
fluorescence emission. For the [salen/Zn /Al
] system, the
2
+
HOMO energy of the receptor is further stabilized in the presence
Fig. 5. (a) Competitive binding analysis of Zn2+/salen-ONPs with different metal ions (Mg, Al3+, Cr3+, Mn2+, Fe3+, Co2+, Ni2+, Cu2+, Sr2+, Sm3+, Ag+ and Cd2+) by fluorescence
2
+
spectra. (b) Fluorescence intensity of competitive binding test for Zn (16 mM) in the presence of other metal ions (16 mM) for salen-ONPs in aqueous solution.

1
46
C.A. Huerta-Aguilar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 146 (2015) 142–150
00
1
80
90
80
70
60
50
40
30
20
10
0
a
b
7
6
5
0
0
0
40
30
20
10
0
3
75 400 425 450 475 500 525 550 575 600 625 650
0
100
200
300
400
500
600
Wavelength (nm)
Al(III) concentraꢀon [uM]
Fig. 6. Titration of [salen-ONPs/Zn ] against Al ions: (a) the increase fluorescence intensity with the subsequent addition of Al3+ (2.0–600.0
2
+
3+
l
M); b) fluorescence response
2
+
3+
for the binding of [salen-ONPs/Zn ] with Al ions.
(
see suppl. Mat.), developing the fluorescence (at 450 nm). The
intensity of the fluorescence has been further increased by comple-
mentary gate (other AND logic gate) in the presence of Al3+ ions.
This means that the fluorescence intensity resulted from salen-
THF/H O
2
2
+
ONPs/Zn couple is further increased by adding another input
3
+
Organic Nanoparꢀcles
ONPs)
(the presence of Al ions), indicating that salen-ONPs first recog-
2
+
3+
Ligand 1
nize Zn ion, and then recognize Al . The enhancement of fluores-
cence intensity occurs only when all inputs [A = 1, B = 1, C = 1] are
(
Scheme 1. Synthesis of salen-ONPs.
given and it forms
(
[
a
three inputs AND logic gate system
Scheme 3). However, for other combinations with negative inputs
A = 0, B = 0 or C = 0], no fluorescence was found. This observation
of Al3+; besides the planarity of the complex structure is also
is consistent with other previous report [59].
increased, leading a strong PET from the receptor, and this
enhances further the fluorescence emission (Scheme 2).
A= ONP
The interaction of Al3 ions with Zn /ligand is pH dependent. At
+
2+
Fluorescence
at 450 nm
(
a)
B= Zn++
high pH, high fluorescence intensity was observed, so there is a
high possibility of deprotonation. Furthermore, the pHs were mea-
sured during addition of Al3 ion to Zn /salen system (suppl. Mat.),
+
2+
showing that pH was decreased with increasing concentration of
Al ion in Zn /salen system.
A= ONP
(b) B= Zn++
3
+
2+
Enhanced
Fluorescence
at 500 nm
Fl@450 nm
C= Al+++
1
2
2
+
3+
Logic gate system salen-ONPs/Zn /Al
For salen-ONPs/Zn2+, there exists a logic gate system in the
3
+
presence of Al , indicating that high fluorescence intensity takes
place only when both Zn2 and Al are present in a sequential
order; however, the intensity was low for other combinations of
metal ions or in other sequential order. First, an AND logic gate per-
forms through the binding of Zn2 with salen-ONPs [A = 1, B = 1]
+
3+
Effect of pH on the fluorescence intensity
+
The effect of pH on the fluorescence intensity for salen-ONPs
was studied by adding HNO (10.0 mM) or NH OH (10.0 mM).
3
3
For each addition (acid or base), the intensity and the pH values
were measured at excitation of 365 nm (Fig. 8). Similarly, for
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
2
+
2+
3+
salen-ONPs/Zn
and also for salen-ONPs/Zn /Al , the same
method was adopted. The results show that pH strongly influences
the fluorescent emission. This observation agrees with other pub-
lished works [60,61]. In the acidic condition (pH = 1.0–5.0), the
fluorescence intensity was very low (OFF mode), and it becomes
ON mode (enhancement of the fluorescence) at basic condition
2
+
3+
(pH = 6–10). The study shows that [ONPs/Zn /Al ] exhibits a
maximum intensity at pH 8.0. In the acid condition, photo-induced
electron transfer in salen-receptor (fluorophore) is thermodynami-
cally disallowed (switched-OFF), resulting in a low fluorescence
owing to the protonation of imine amine nitrogens (C@N) in the
receptor. While in the basic condition, the deprotonation is favored
for the receptor, allowing the PET phenomenon, the system
becomes switched-ON, exhibiting the fluorescence intensity. This
shows that the salen ligand acts as ON–OFF sensor between pH
6.0 and pH 4.0.
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
Salen/[Salen+Al³⁺]
1
Fig. 7. Job’s plot: determination of salen ONPs/Zn2+/Al3+
.

C.A. Huerta-Aguilar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 146 (2015) 142–150
147
LUMO
LUMO
LUMO
HOMO
HOMO
HOMO
HOMO
HOMO
HOMO
Zn2+
Salen/Zn2+
excited
Al3+
Salen/Zn2+
excited
Salen
Salen
excited
Salen
Salen
2+
3+
2+
Zn /Al
Zn
Ground
state
Weak
fluorescence
emission
Light
Light
Enhancement of
fluorescence
emission
Light
Strong enhancement
of fluorescence
emission
Scheme 2. The color change of fluorescence emission: (a) salen-ONPs = weak green; (b) Zn2+/salen-ONPs (1:1 ratio) = neon blue, (c) Zn2+/salen-ONPs/Al3+ = strong green
3
+
2+
fluorescence emission after addition of Al ion (600 lM) to Zn /salen-ONPs (2:1).
Scheme 3. Three inputs Logic Gate System: salen-ONPs with selective recognition of Zn2+ and Al3+ in aqueous solution. Three inputs are: 1 = ONPs, 2 = Zn2+ ion and 3 = Al3+
ion.
Temperature effect on fluorescence intensity
Mat.), which is consistent with other studies [62–64]. For salen-
ONPs, the fluorescence intensity was 8.5 F.U. at 2.0 °C, and the
intensity was fluorescence quenched with increasing temperature.
The same behavior was seen for all the systems although the mag-
nitude of the emission differs for each system. This can be
explained in that the thermal energy of the solution increases
The effect of temperature (2.0–40 °C) on the fluorescence inten-
2
+
sity was studied for all systems (salen-ONPs, salen-ONPs/Zn and
2
+
3+
salen-ONPs/Zn /Al . The results show that a high fluorescence
emission was observed when the temperature is low (see Suppl.

1
48
C.A. Huerta-Aguilar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 146 (2015) 142–150
Fig. 8. pH effect (1.0–12) on the fluorescence intensity: (a) salen-ONPs, (b) salen-ONPs/Zn2+ and (c) salen-ONPs/Zn2+/Al3+
.
2
+
salen
Zn /salen
LUMO
LUMO
(
-1.36 eV)
(
-1.74 eV)
∆E=4.78 eV
∆E=3.89 eV
HOMO
HOMO
(
-5.63 eV)
(
-6.14 eV)
Fig. 9. Density of sates (electronic states) diagram: salen at B3LYP/6-311G(d,p) and [Zn-salen]2+ at B3LYP/LANL2DZ/6-311G(d,p).
the electron moments between the molecules and so decreasing
the intensity. In contrast, at low temperature, the intramolecular
transference of electrons increases and so increases the fluorescent
emission.
6-311+G(d,p) basis set [65], and Polarizable Continuum Mode
(IEF-PCM) was used to see the solvent effect (water as solvent).
The structural data obtained were used as input to calculate
Time-Dependent DFT (TD-DFT) [66,67]. To optimize the structure
of [Zn-salen] , B3LYP with LANLDZ basis set was employed, and
then UV–visible spectrum and Density of States (DOS) diagrams
were obtained through TD-DFT calculations (Fig. 9).
2
+
Computational procedure
For the salen-structure, the full optimization was performed by
Density Functional Theory (DFT) using Gaussian’09 at B3LYP with
In the structure, two nitrogen and two oxygen atoms are coor-
dinated with Zn ion due to their negative charge detected by the
2
+

C.A. Huerta-Aguilar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 146 (2015) 142–150
149
a
b
LUMO
ΔE=4.78 eV
ΔE=0.16 eV
ΔE=3.89 eV
ΔE=0.16 eV
HOMO
HOMO -1
Fig. 10. Frontie molecular orbital (FMO) diagram for (a) salen (b) salen/Zn2+
.
5
4
4
3
.0E+4
.5E+4
.0E+4
.5E+4
4
4
3
3
2
2
1
1
0
0
.5
a
b
.0
.5
.0
.5
.0
.5
.0
.5
.0
3.0E+4
2.5E+4
2.0E+4
1.5E+4
1.0E+4
5.0E+3
0.0E+0
300
320
340
360
380
400
Wavelength (nm)
230
280
330
380
Wavelength (nm)
430
480
2
Fig. 11. UV–Vis spectra: (a) experimental spectra of salen in solvent mixture (THF: H O, 1:99); (b) TD-DFT spectra of the ligand at B3LYP/6-311G(d,p) in THF as solvent.
electrostatic potential analysis (Suppl. Mat.). It shows that the
ligand (receptor) can coordinate efficiently with the metal ion
ligand (Fig. 10), and the values were changed to
g
= 1.945 and to
2
+
r
= 0.51 eV after its complex with Zn :
(
acceptor).
Hardnessð
g
Þ ¼ 1=2ðEHOMO ꢂ ELUMO
Þ
ð1Þ
ð2Þ
DOS diagram was derived for both the ligand and [Zn-salen]2+
complex through Energy DFT technique, and the results show that
for the ligand, the band gap between the energy levels are very
close at high density of states and all the individual electronic
states are merged to form a wide band gap above LUMO+3 levels.
However, for [Zn-salen]2 , since the orbitals are involved in bond-
ing with the metal ion, only few independent states are appeared
with small band but at high energy levels (E = 3.8 eV; LUMO
Softnessð
r
Þ ¼ 1=
g
The TD-DFT spectrum of the ligand was calculated by energy
DFT at B3LYP/6-311+G(d,p) basis set with using THF as a solvent
at PCM model. The calculated spectra is well comparable with
the experimental spectra (See Fig 11); two absorbance peaks
(kexp = 230 nm, 330 nm; kcalc = 320 nm, 335 nm) attributed to
+
⁄
⁄
+
15), reducing the unoccupied electronic states. This observation
n ? p and p ? p electronic transitions were observed [70].
is consistent with the fluorescence study that a high energy emis-
sion for the complex with respect to the ligand was observed.
However, there is a considerable red shift in the spectra at high
energy band, consisting with the experimental spectra, where the
solvent interacts significantly with the ligand, affecting the spec-
tral band position. However, in the experimental spectrum, a weak
With using the molecular orbitals (MOs), the hardness (
g=2.43
eV) and softness ( = 0.41 eV) were calculated [68,69] for the
r

1
50
C.A. Huerta-Aguilar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 146 (2015) 142–150
peak appeared around 390 nm was not seen in the calculated spec-
tra. After analyzing the previously study [47], it is seen that when
some organic compounds are confined to below 50 nm (e.g. ONPs),
the charge transfer phenomena becomes more dominant, thus a
new peak around 400 is generated (PCT) [71].
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Acknowledgements
The authors acknowledge the Dirección General de Asuntos del
Personal Académico (Project PAPIIT No IN217813) and Mexico-
India-Joint Research Project [CONACYT/MEX-DST/INT] for eco-
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