Fluorescent organic nanoparticles (FONs) for selective recognition of Al3+: Application to bio-imaging for bacterial sample

Article abstract of DOI:10.1039/c6ra01231k

The development of a novel chemo-sensor for the detection of Al³⁺ with high sensitivity in aqueous solution is widely considered an important research goal because of the importance of such probes in medicine, living systems and the environment. In this work, we describe a new fluorescent probe, a Schiff's base N,N′-propylenebis(salicylimine) (salpn) as fluorescent organic nanoparticles for Al³⁺. The study shows that salpn detects Al³⁺ with the detection limit as low as 1.24 × 10^-3 mM, indicating that the chemo-sensor has high sensitivity in aqueous medium, and the fluorescence intensity increases with the increasing Al³⁺ concentration in the presence of the salpn-ONPs which act as chemo-sensors. The interference of common coexistent metal ions such as Mn²⁺, Mg²⁺, Co²⁺, Fe³⁺, Ni²⁺, Zn²⁺, Sr²⁺, Ag⁺, Sm³⁺, Al³⁺, Cd²⁺, Ba²⁺, Na⁺ and K⁺ was tested, showing that salpn-ONPs efficiently detect Al³⁺ ions with small interference from Cu²⁺ and Cr³⁺. Finally, the efficiency of salpn to as a fluorescent probe for Al ion in living systems was evaluated in Gram-negative and Gram positive bacteria, and con-focal laser scanning microscopy confirms its utility that this chemo-sensor efficiently detects Al³⁺ ion in Staphylococcus aureus enclosed by a single membrane.

Full text of DOI:10.1039/c6ra01231k

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PAPER
Fluorescent organic nanoparticles (FONs) for
3
+
selective recognition of Al : application to bio-
Cite this: RSC Adv., 2016, 6, 37944
imaging for bacterial sample†
a
b
a
Carlos Alberto Huerta-Aguilar, Pushap Raj, Pandiyan Thangarasu*
and Narinder Singh
b
3+
The development of a novel chemo-sensor for the detection of Al with high sensitivity in aqueous
solution is widely considered an important research goal because of the importance of such probes in
medicine, living systems and the environment. In this work, we describe a new fluorescent probe,
0
3+
a Schiff's base N,N -propylenebis(salicylimine) (salpn) as fluorescent organic nanoparticles for Al . The
3+
ꢁ3
study shows that salpn detects Al with the detection limit as low as 1.24 ꢀ 10 mM, indicating that
the chemo-sensor has high sensitivity in aqueous medium, and the fluorescence intensity increases with
3+
the increasing Al concentration in the presence of the salpn-ONPs which act as chemo-sensors. The
2+
2+
2+
3+
2+
2+
2+
+
interference of common coexistent metal ions such as Mn , Mg , Co , Fe , Ni , Zn , Sr , Ag ,
3+
3+
2+
2+
+
+
3+
Sm , Al , Cd , Ba , Na and K was tested, showing that salpn-ONPs efficiently detect Al ions with
Received 14th January 2016
Accepted 7th April 2016
2+
3+
small interference from Cu and Cr . Finally, the efficiency of salpn to as a fluorescent probe for Al ion
in living systems was evaluated in Gram-negative and Gram positive bacteria, and con-focal laser
3+
DOI: 10.1039/c6ra01231k
scanning microscopy confirms its utility that this chemo-sensor efficiently detects Al
Staphylococcus aureus enclosed by a single membrane.
ion in
www.rsc.org/advances
presence in water is toxic to biological species. For example, the
presence of Al ion in drinking water can cause damage to
Introduction
3
+
The development of highly selective new uorescent sensors for certain human tissues and cells, and produce Alzheimer's and
5
7–61
the detection of various metal ions or anions is essential for Parkinson's diseases and amyotrophiclateral sclerosis.
3
+
1
–4
various biologically and environmentally important species.
Thus, the detection of Al in aqueous medium is important in
Although analytical techniques as atomic absorption spectros- order to minimize its effect on human health; yet the detection
3
+
copy, colorimetric assays, spectrophotometry and voltammetry of aqueous Al ion is generally difficult when compared with
can effectively detect and quantify metal ions, these methods those for other metal ions due to its poor coordination ability,
are either time consuming procedures or use sophisticated strong hydration ability, and the lack of suitable spectroscopic
instrumentation; however, spectro-uorescence has high transitions.
sensitivity, selectivity, rapidity and an easy operational proce-
The following chemo-sensors have recently been used for the
62–64
3
+
5–7
dure. Sensors based on changes in uorescence induced by detection of Al : 7-methoxychromone-3-carbaldehyde-((2-
chemical species are highly sensitive, rapid, simple and there is benzothiazolylthio)acetyl) hydrazone in ethanol, 1-(2-pyr-
7
2
4
0
real-time monitoring of the uorescence, and they are able to idylazo)-2-naphthol
in methanol, N -(4-diethylamino-2-
8
–13
64
detect numerous metal ions in biological samples
as follows: hydroxybenzylidene)-2-hydroxy-benzohydrazide
3
in CH CN/
14
15–17
18–20
21–23
24–27
28,29
65
Ca(II),
Mg(II),
Pb(II),
Co(II),
Cd(II),
Ni(II),
Al(III),
Cr(III),
Cu(II),
H
2
O, 1-(2-hydroxy-5-methylphenylimino)-naphthalen-2-ol in
solvent mixture (water : methanol 1 : 9), 3,5-bis((E)-(2-hydrox-
3
0–34
35
36,37
38,39
40–46
47–49
Hg(II),
Fe(II),
Zn(II),
5
0,51
52
53–56
66
Ag(I),
Mn(II); rare earth metal ions.
yphenylimino)methyl)-4-hydroxybenzoate
in
methanol,
0
Aluminum ion is generally employed to treat wastewater 8-formyl-7-hydroxyl-4-methyl coumarins-(2 -methylquinoline-4-
67
through co-precipitation or by coagulation process; however, its formyl) hydrazone
in ethanol, 2-((E)-(2-((E)-2-hydrox-
68
ybenzylidene-amino)phenylimino)methyl)phenol
in meth-
anol, 2-chloronicotin-aldehyde functionalized rhodamine B
a
69
0
Facultad de Qu ´ı mica, Universidad Nacional Aut ´o noma de M ´e xico (UNAM), Ciudad
derivative in CH
l,2-phenylenediamine
3
CN, N,N -bis(2-hydroxy-1-naphthaldehyde)-
Universitaria, Coyoac a´ n 04510, M ´e xico D. F., Mexico. E-mail: pandiyanyan@unam.
70
in DMF, 4-(8-hydroxyquinolin-7-yl)
mx
b
71
methylene-imino-1-phenyl-2,3-dimethyl-5-pyzole in mixture of
methanol : water. Thus most of the chemo-sensors are suitable
only in non-aqueous medium (methanol, ethanol, or DMF). The
Department of Chemistry, Indian Institute of Technology (IIT), Ropar, India
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c6ra01231k
1
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detection of Al ion in 100% aqueous in the literature is limited,
yet it is essential for real samples; moreover, although these
sensors efficiently detect the metal ion, their applicability to real
samples has not been studied, and there are only limited
3
+
reports on Schiff base-type Al sensors which are being used for
64,72–74
bioimaging rare.
The ability of bio-sensing molecules to
selectively monitor guest species in living systems is important
in biological applications. In the present work, salpn-ONPs have
3
+
been found to selectively detect Al ion at low detection limit
ppm level), and the receptor has been applied to real sample
like Staphylococcus aureus and Salmonella typhi for the
(
3
+
Fig. 2 Fluorescence and absorbance spectra of salpn in MeOH [0.1
mM] and ONPs suspension [0.1 mM].
measurements of Al ion.
Results and discussion
Metal recognition binding tests
The organic nanoparticles have been fabricated through a re- The selective recognition of different metal ions of salpn-ONPs
precipitation method. The organic ligand dissolved in THF was tested, and the different binding tests were performed for
was injected into doubly distilled water at room temperature. several anionic and cationic species. The binding test for anions
ꢁ
ꢁ
ꢁ
ꢁ
The resulting solution was sonicated for 30 min in order to form in the form of tetra-butyl ammonium salts (F , I , Cl , Br ,
ꢁ ꢁ
ꢁ
ꢁ
4
stable nanoparticles. The formation and stability of organic SO , ClO , COO and PO ) was analyzed, and there are no
4 4
nanoparticle was monitored by DLS and TEM techniques, and emission spectral responses, so salpn-ONPs are not suitable for
the results show that the particles have hydrodynamic radius of anion detection. Yet, for the metal binding test was carried out
1
00 ꢂ 2 nm (see Fig. 1); this is consistent with TEM studies that for sixteen different metallic ions in the form of nitrate salts
2
+
2+
2+
2+
3+
3+
2+
2+
2+
+
3+
organic nanoparticles are of spherical shape with size of 100 nm (Mn , Mg , Cu , Co , Cr , Fe , Ni , Zn , Sr , Ag , Sm ,
(
3
+
2+
2+
+
+
see Fig. S6†). Both DLS and TEM data match with each other, Al , Cd , Ba , Na and K ), and the ligand selectively detects
3
+
conrming their stability in aqueous medium.
Al ion. In the experiment, each solution of the metal ion
The absorption and emission spectra of salpn and salpn- (50 mL, 0.1 mM) was added to salpn-ONPs (3.0 mL, 0.15 mM)
ONPs were analyzed in order to see the spectral difference suspended in water, and then the uorescence emission
between ligand and its organic nanoparticles (Fig. 2). In the UV- (360–750 nm) was measured. Chemo-sensor salpn-ONPs were
Vis spectra, a peak around 400 nm was observed, corresponding exited at 365 nm and the corresponding uorescent emission
to the p / p* transitions originating from the aromatic rings signal was observed at 490 nm. There is a considerable increase
3
+
of the ligand, although there was a slight blue shi from 400 to in the uorescence intensity for Al compared to other cations,
3
+
3
90 nm for salpn ONPs.
In the uorescence emission spectra, a signicant spectral (see Fig. 3).
change was observed for salpn-ONPs compared to salpn when
Hydrated aluminium salts will be hydrolyzed to form
so salpn-ONPs are suitable for specic recognition of Al ion
they were excited from 350 to 405 nm with a maximum wave- hydroxylated aluminium compounds even with small amounts
ꢁ
length of 365 nm. The uorescence emission peak was observed of moisture, and the interaction of OH and H
2
O (hard base)
3
+
3+
around 460 nm for salpn, while the intensity of the peak was with hard-acid Al makes it easier to form the Al complex, so
enhanced to around 510 nm for the ONPs due to the formation the Al ion may be chelated by the counter anion or solvent in
of J-type aggregates, agreeing with the supra-molecular self- order to satisfy the need of six-coordination. The organic nano-
organization typically found in organic nanoparticles. This particles of salpn-ONPS are insoluble (colloid nature) in water,
type of interaction also leads to a redshi and broadening of the and their UV visible spectra in methanol show that there is no
emission peaks.
spectral change in both solvents. This conrms that hydrolysis
Fig. 3 Metal binding test for fluorescence of salpn-ONPs [0.1 mM] in
the presence of different metal ions in aqueous solution [10.0 mM]. (a)
Fig. 1 DLS analyses of salpn-ONPs (0.01 mM) nanomaterials in water Fluorescence scanning upon addition of metal salts; (b) comparison of
dispersion.
fluorescence intensity at 380 nm.
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of the ligand does not take place in the water medium. More-
over, if the ligand were to undergo hydrolysis, the uorescence
intensity would be expected to decrease or disappear due to the
loss of conjugation.
Competitive binding analysis
3
+
The binding selectivity of salpn-ONPs with Al was analyzed
through uorescence study in order to understand a possible
interference of other metal ions. To the solution of salpn-ONPs
3
+
(
(
0.1 mM, 30 mL)/Al (100.0 mM, 30 mL), different metal ions
30.0 mL, 100.0 mM) were added in such a way that the nal
3+
Fig. 5 Titration of Al (0.0 to 0.45 mM) in aqueous solution in pres-
ence of salpn-ONPs [0.1 mM]. (a) Changes in fluorescence upon
successive addition of Al ions; (b) titration profile of fluorescence.
concentration was 16.0 mM. The resulting solution mixture was
stirred for 15 min and kept for 5 min to reach equilibrium, and
then the uorescence emission was measured in the visible
region aer excitation at 365 nm. The results show that there is
3+
2
+
3+
a small interference from Cu and Cr ions during the
0.1 mM, the changes in uorescence aer each addition
3
+
recognition of Al through salpn-ONPs (Fig. 4) because the
decreased and aer 3.0 mM concentration they become negli-
salpn-ligand can also bind with other metal ions of similar
gible. The results suggest that salpn-ONPs perform as chemo-
3
+
2+
chemical nature (ionic radius: Cr (0.73 A) and Cu (0.73 A˚ ).
The metal ions with d orbitals can coordinate with salpn in
a similar manner and can interfere with the uorescence signal
˚
3+
sensors in recognizing Al in aqueous solution. It appears
that photo induced electron transfer (PET) occurs in the salpn-
ONPs when the uorophore is excited i.e., an electron is trans-
ferred from the HOMO (donor) to the low-lying HOMO of the
acceptor (uorophore), causing quenching in uorescence
3
+
originated from Al /ONPs. However, there is no signicant
interference from Ag (1.29 A) or Cd (1.09 A) in the uores-
cence because of their different acid–base chemistry.
+
2+
˚
˚
3+
emission. However, when Al is added to ONPs, metal ions
The possible interference of other metal ions in the recog-
bind with the salpn-ONPs moiety (recognition) and this causes
the lowering of the receptor HOMO energy, thus inhibiting the
photo-induced electron transfer from HOMO (donor) to uo-
3
+
nition of Al by salpn-ONPs was analyzed by performing a set of
interference experiments. In this case, salpn-ONPs suspension
3
+
(
30 mL, 0.1 mM) was mixed with 30.0 mL of Al (100 mM), and
rophore, and enhancing the uorescence emission.
the uorescence intensity was measured for the resulting
solution. With this spectral reference, a battery of tubes was
prepared, each one containing 30.0 mL of one of the metal ions
to be tested, and to these tubes 3.0 mL of salpn-ONPs (0.1 mM)
75,76
It appears that Photoinduced Electron Transfer (PET)
is
operating in the sensor salpn-ONPS i.e., electron transfer is
blocked when the receptor is coordinated to Al ion (the inhi-
bition of PET process from the receptor to the uorophore),
enhancing the uorescence intensity (Scheme 1). This means
that when irradiated by light, an electron of the donor highest
occupied molecular orbital (HOMO) is excited to the lowest
3
+
and 30.0 mL of Al were added.
3
+
Titration studies salpn-ONPs against Al ions

uorophore unoccupied molecular orbital (LUMO), so CH]N–
Titration analysis was performed in order to study the binding
transfers an electron of HOMO to HOMO of the uorophore,
and ]N– occupies the ground state of the uorophore. Then
the electron cannot transit back directly to the original ground-
state, and the emission of the uorophore is weakened. In the
3
+
selectivity of salpn-ONPs with Al ions. The uorescent emis-
sion intensity was measured for each subsequent addition of
3
+
Al ions (0.0 to 0.45 mM) to salpn-ONPs (5.0 mL, 0.1 mM), and
the peak intensity increased with increasing concentration of
3+
presence of Al , which binds with electron donating sites of
3
+
Al in the ONPs (Fig. 5). Hence it was concluded that the
uorescence emission around 490 nm increases linearly with an
ligand, the lone electron pair of the ]N– is lowered underneath
the HOMO of the uorophore. PET process inhibition then

3
+
increase of Al concentration from 0.0 mM up to 0.1 mM. Above
3+
Fig. 4 Interference binding test for recognition of Al [10.0 mm] Scheme 1 Illustration of chelation-enhanced fluorescence (CHEF) of
3+
through salpn-ONPs 99[0.1 mM].
the receptor after binding with Al .
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1
3+
occurs with subsequent uorescence enhancement and chelation
From H NMR analysis of the [salpn]Al complex (Fig. 7),
enhanced uorescence (CHEF). The high selectivity of receptor a certain amount of the complex was prepared in deuterated
3
+
towards Al can be explained by considering the high ionic methanol (CD OD) and its spectra were recorded. It was found
3
3
+
potential and the high charge density of the Al . The smaller that most of the signals previously identied were split as usual
radius of Al allows suitable coordination environment of the in organometallic complexes, which suggests that protons are
3
+
chemosensor and the larger charge density allows strong coor- now found in two different environments.
3
+
dination interaction between the receptor and Al . The observed
In case of aromatic protons, the splitting signals led to

uorescent intensity increment of salpn ONPs in the presence of multiplets which can be observed at d ¼ 6.76, 6.96, 7.18 and 7.38
3
+
Al may be attributed to chelation-enhanced uorescence ppm. Moreover, new peaks were found that conrm the
3
+
(
CHEF) of the receptor aer binding with Al imposed by the formation of a new complex. The doublet at d ¼ 3.98 ppm and
rigidity of the system. CHEF is one of the most widely applied the singlet at d ¼ 5.60 ppm suggest the presence of a methanol
3
+
mechanisms for uorescent cation sensing. The uorescence molecule attached to the Al ion and at the same time, the
intensity of the uorophore is signicantly reduced in the absence of the OH signal conrms the coordination of salpn
absence of Al ion, due to photoinduced electron transfer (PET), oxygen's with the metallic atom. The nal conrmation of the
98,99
but for its presence, PET
uorescence intensity due to rigid chelated complex formation. ppm that was attributed to HNO
There are only a few reports for the detection of these ions using interaction with salpn molecule according to Scheme 2.
is prevented, leading to an enhanced complex formation was the presence of a new peak at d ¼ 10.31

3
molecules formed aer
77–83
3+
organic nanoparticles
which bind by weak intermolecular
The geometry of the salpn molecule as well as the salpn–Al
forces (van der Waals type) in organic molecular crystals, thereby complex were optimized and frequency calculations were
84–87
affecting their electronic and optical properties.
executed, in both cases using 6-311G (d, p)/B3LYP basis set
pH effect. To check the effectiveness of the salpn-ONPs, the because of its ability to calculate both organic and organome-
emission spectra were recorded in the range of pH 3–10. The tallic complexes. The optimized structures of the ligand and the
3
+
3+

uorescence intensity increases with the decreasing pH (from Al complex are shown in Fig. 8. The structure of the salpn–Al
neutral to acidic, pH 7–3), probably due to the inhibition of PET complex corresponds to 1 : 1 stoichiometry based on the results
mechanism (see Fig. S7†). However, the uorescence intensity obtained from NMR and FTIR complexation analyses; more-
remains same in the range of pH 5–10, conrming that the over, this structure agrees with previous reports dealing with
9
1
sensor performs in the physiological range. The enhancement aluminum complexes.
3
+
of uorescent emission from [Al /salpn-ONPs] (due to inhibi-
Aer optimizing inputs, theoretical UV-Vis spectra of salpn
3
+
tion of PET mechanism) is approximately the same that and salpn–Al complex were calculated through time depen-
observed for salpns-ONPs in acidic pH, conrming the dent (TD-DFT) calculations (Fig. 9). In order to get proles
involvement of PET mechanism for the enhancement of closer to the experimental results, a polarizable continuum
8
8–90

uorescence.
Stoichiometry determination. Job's method was employed to
determine the stoichiometry of the reaction between salpn-ONPs
3
+
and Al ions (Fig. 6). In the study, the uorescence intensity
increases proportionally up to ꢃ1.0 equivalents during the
3
+
3+
addition of Al ions to ONPs at a ratio [Al : salpn-ONPs] of 1 : 1.
In order to conrm these results, complementary FTIR and NMR
analyses were performed by obtaining the spectra of the complex
3
+
at different salpn : Al ratios (1 : 2, 1 : 1 and 2 : 1 respectively).
These results agree with DFT calculations and show there is
formed a pseudo cavity between the two aromatic centers of the
3
+
salpn molecule that allows the interaction with Al ions.
1
3+
Fig. 7 Comparison of H NMR spectra of salpn (a) and [salpn]Al (b) in
CD
3
OD for determination of complex structure.
Fig. 6 Job's plot for the determination of stoichiometry between
salpn ONPs and Al ions in aqueous solution.
3+
3+
Scheme 2 Complex formation of [salpn]Al in methanol.
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The confocal micrographs show that only for Gram positive
Staphylococcus aureus, uorescence response was obtained
when they grew in the presence of both analytes (salpn-ONPs–
3
+
Al ) (Fig. 10) i.e., when salpn-ONPs/S. aureus cells were put in
3
+
contact with an aqueous solution containing Al [0.01 mM], it
was possible to observe the uorescent cells. However, for other
combinations of samples, negative uorescence response was
resulted. This indicates that the aluminum complex of salpn-
ONPs incorporates into the cytoplasm of the bacterium cell,
and readily reacts with the compound present in the media.
For Salmonella typhi, there is no change in uorescence
Fig. 8 DFT optimized geometry through 6-311+G(d,p) for (a) salpn
3+
and (b) salpn–Al
.
3
+
response upon treatment with salpn-ONPs and Al (Fig. S8 and
S9†). This behavior can be explained that Staphylococcus aureus
is a Gram positive bacteria with a thin cell wall which allows
nanoparticles into the bacterium, while for Gram negative
bacteria, the presence of thick cell wall hinders the introduction
of salpn-ONPs into the cell. This is consistent with the reported
observation that the interaction of Schiff bases is stronger for
94
Gram positive than for Gram negative bacteria.
In the SEM analysis, it has been found that salpn-ONPs were
Fig. 9 Theoretical and experimental UV-Vis absorption profiles of present within the bacterial cell (see Fig. S10†), not on the cell
3+
chemosensors and Al complex; (a) DFT theoretical profiles obtained
through 6-311+G(d,p) and using water as solvent and (b) experimental
profiles obtained for salpn-ONPs in water.
surface, conrming the selectivity of aluminium complex in
S. aureus through cellular uptake, and not by the cellular
adherence. This indicates that salpn-ONPs can easily enter into
cytoplasm of the cell and react with the compounds present in
the medium as observed previously in that the Schiff base metal
complexes interact strongly with Gram positive bacteria rather
model (IEF-PCM) was used by employing water as the
92,93
solvent.
3
+
Reorganization of Al in biological samples. The uorescent
probe of salpn-ONPs against Stapylococcus aureus and Salmo-
nella typhi were analyzed in order to demonstrate its potential
application in biological systems. The cells were incubated with
ꢄ
different substrates (see Table 1) at 36 C for 36 hours in Mueller
Hinton broth.
Aer the bacterial culture was completed, the cells were
centrifuged at 3000 rpm for three minutes, washed with 1%
Triton-X-100 solution, followed by distilled water twice. The cell
count was performed by using hemocytometer, and the results
show that the S. aureus grow about 0.9% faster than S. typhi. The
pellet in the bottom of the centrifuge tube was recovered and
analyzed through confocal microscopy with excitation at 405
nm and the emission was monitored between 420 and 540 nm.
Table 1 Treatments of Stapylococcus aureus and Salmonella typhi
3+
broths for recognition of Al in aqueous media
Sample
Bacteria
Fluorescence response
Control
S. aureus
—
—
+
—
—
—
—
—
—
—
Salpn-ONPs
Salpn-ONPs + Al
Salpn
Salpn + Al
Control
Salpn-ONPs
Salpn-ONPs + Al
Salpn
3
+
+
3
+
3+
S. typhi
Fig. 10 Recognition of Al
through bio-fluorescence Staphylo-
coccus aureus treated with different combinations: (a) S. aureus under
visible light; (b) control growth; (c) S. aureus treated with salpn-ONPs
3
3+
(0.1 mM); (d) S. aureus treated with salpn-ONPs (0.1 mM) and Al (0.01
mM); (e) S. aureus treated with salpn (0.1 mM); (f) S. aureus treated with
3+
Salpn + Al
3+
salpn (0.1 mM) and Al (0.01 mM).
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than Gram negative bacteria, incorporating the metal complex under sonication to form suspension. The resulting mixture
easily into the lipid membrane of the Gram positive was further sonicated for 15 min to yield a colloidal milky
95–99
bacteria.
yellowish suspension of salpn-ONP (0.5 mM) with a nal
concentration of 0.1 mM of salpn-ONPs.
Experimental
General remarks
Computational studies
For the salpn-structure, a full optimization was performed by
All chemical reagents were purchased from Sigma Aldrich and
used without further purication. The bacterial strains were
obtained from the Strain Bank (WFCC/WDCM-100), Faculty of
Chemistry, UNAM for cell culture, and maintained in Cysteine
Tryptic Agar (CTA). Cysteine Tryptic Agar medium (CTA, BIO-
XON) for cell culture maintenance, Mueller Hinton (DIBICO) for
inoculum dilution mixture.
Density Functional Theory (DFT) using Gaussian'09 (ref. 104) at
105
B3LYP with 6-311+G(d,p) basis set,
and Polarizable
Continuum Mode (IEF-PCM) was used to determine the effect of
water as solvent. The structural data obtained were used as
106,107
input to calculate Time-Dependent DFT (TD-DFT).
To
2+
optimize the structure of [Al–salpn] , B3LYP with LANLDZ
basis set was employed, and then UV-visible spectrum and
Density of States (DOS) diagrams were obtained through TD-
DFT calculations. In the structure, two nitrogen and two
Elemental analyses were carried out on a Fisons (Model EA
1108 CHNS). NMR (300 MHz Varian) and Mass Spectrometry
(LECO Pegasus 4D) with a TOF analyzer were used to charac-
3+
oxygen atoms are coordinated with the Al ion and their
terize the ligand. UV-Vis spectrophotometer (Perkin-Elmer
Lambda 25) and uorescence spectrophotometer (F-96 Pro)
were employed to analyze electronic and uorescence proper-
ties of salpn-ONP as well as for the detection of metal ions.
Confocal uorescence imaging was performed on a Leica TCS
SP5 laser scanning confocal microscope (Leica Microsystems,
negative charge was detected by electrostatic potential analysis.
The ligand (receptor) was found to coordinate efficiently with
the metal ion (acceptor).
Cell culture study
Wetzlar, Germany), with a blue diode laser (50 mW, lex ¼ 405 The ability of salpn-ONPs to recognize metallic ions was tested
nm) and acquisition as xyz at a speed of 400 Hz.
by inoculating the molecule into Staphylococcus aureus and also
Salmonella typhi bacterial cells suspended in Mueller Hinton
108
0
broth. Bacterial cells were obtained from a ceparium available
at our institution. For each test, bacteria were inoculated into
Synthesis of N,N -propylenebis(salicylimine) (salpn)
The ligand salpn was synthesized (Scheme 3) as reported else-
ꢄ
the broth and incubated at 36.0 C for 36 hours before confocal
100,101
where:
to 2-hydroxybenzaldehyde (2.0 mmol, 244.0 mg)
microscopy analyses. The culture medium was taken in culture
tubes protected with a plug. For inoculation, the microbes were
dissolved in water and one drop of this suspension was added to
each tube of a medium. The cultures were a clear solution (to
the naked eye) at the beginning and with the growth of
microbes, cloudiness appeared. Aer incubation, the bacterial
content of each tube was recovered by centrifugation at 3000
rpm and washed with 1% Triton-X-100 solution followed by two
times with doubly distilled water.
dissolved in methanol, 1,3-diaminopropane (1.0 mmol, 74.0
mg) was slowly added under vigorous stirring, and then the
resulting mixture was stirred for 2.0 hours. The color of the
solution changed from colorless to bright yellow, and yields
a yellow product which was ltered and washed three times with
ethanol. The compound obtained was crystallized from ethanol.
Yield: 79.69%; elemental analysis for salpn, calc.: C, 72.32; H,
1
6.43; N, 9.92. Found: C, 72.430; H, 6.300 N, 10.210; S, 0.075. H
NMR (400 MHz, THF-d
8
) d 13.147 (s), 8.449 (s), 7.297 (m), 7.278
In the cell counting process, to the centrifuge tube contain-
ing S. aureus cell suspension (100 mL), trypan blue (100 mL) was
added. The resulting mixture was rst agitated to homogenize
the suspension mixture, and then it was allowed to stand for 10
min. A small amount of trypan blue-cell suspension was care-
fully transferred to the chamber of hematocytometer and the
cells were viewed by microscope, and were counted in each
square of the grid. Similarly, the cell number of S. typhi was
1
3
(m), 6.874 (m), 6.826 (m), 3.726 (t), 2.090 (m); C NMR (101
MHz, THF-d ) d 165.99, 161.30, 131.74, 131.20, 119.00, 118.04,
8
+
1
(
16.45, 56.82, 31.93. GC-MS (m/z%): 282 [M , C H N O ]; 162
12.5%) [C H NO] ; 148 (100.0%) [C H NO] ; 134 (85.2%)
10 12 9 10
NO] ; 107 (49.6%) [C
Synthesized salpn was fully converted to organic nano-
particles (salpn-ONPs) through the re-precipitation method
1
7
18 2 2
+
+
+
+
+
[C
8
H
8
7 7 6 5
H O] ; 77 (66.7%) [C H ] .
102,103
published elsewhere.
Ligand salpn (0.01 mmol, 2.82 mg)
4
counted. The concentration of S. aureus was 18 ꢀ 10 cells per
was dissolved in 1.0 mL of THF, and then it was then slowly
injected through a micro syringe into 100 mL of deionized water
4
mL, and for S. typhi, it was 16 ꢀ 10 cells per mL. To maintain
equal cell count, and also to x the concentration of both
bacteria, 0.12 mL of nutrient medium was added to 0.888 mL of
S. aureus suspension.
In the uorescence microscopy, the respective bacterial cells
were inoculated with different combination of salpn, salpn-
3
+
ꢄ
ONPs and Al at 36 C at 36 hours (see Table S1†). The
growing bacteria were washed with 1% Triton-X-100 solution,
followed by distilled water twice. Finally, the samples were
analyzed by confocal microscopy with excitation at 405 nm, and
Scheme 3 Synthesis of salpn.
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the emission was observed around 420–540 nm. To understand
the selectivity of S. aureus with [salpn-ONPs + Al ] over S typhi,
14 J.-m. An, Z.-y. Yang, M.-h. Yan and T. Li, J. Lumin., 2013, 139,
79.
3
+
SEM analyses were performed. The bacteria were centrifuged,
15 P. S. Hariharan and S. P. Anthony, RSC Adv., 2014, 4, 41565.
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Commun., 2014, 48, 1.
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Acta, Part A, 2015, 140, 21.

xed with glutaraldehyde (2.5%) and then dehydrated with
ethanol. Samples were then placed in SEM with Au coating for
observation with 15.0 kV secondary electrons SEM (JEOL, JSM
5900-LV, Tokyo, Japan).
18 L. Guo, S. Hong, X. Lin, Z. Xie and G. Chen, Sens. Actuators,
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Conclusions
1
9 K. C. Tayade, A. S. Kuwar, U. A. Fegade, H. Sharma,
N. Singh, U. D. Patil and S. B. Attarde, J. Fluoresc., 2014,
The prepared organic nanoparticles are well characterized by
DLS and TEM, and the uorescence probe has been analyzed,
showing that salpn-ONPs explore as a uorescence sensor for
aluminum ions with little interference of copper and chro-
mium. Moreover, this sensor recognizes the aluminum ions in
biological sample such as Staphylococcus aureus bacterial cells.
24, 19.
2
0 J. Wang, S. Chu, F. Kong, L. Luo, Y. Wang and Z. Zou, Sens.
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848, 74.
3
+
22 X.-J. Jiang, M. Li, H.-L. Lu, L.-H. Xu, H. Xu, S.-Q. Zang,
The confocal microscopy conrmed the detection of Al ions in
Staphylococcus aureus over Salmonella typhi bacterial cells.
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2
2
2
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2
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Acknowledgements
The authors acknowledge the Direcci ´o n General de Asuntos de
Personal Acad ´e mico (Project PAPIIT No. IN209616) for
economic support, USAII, Faculty of Chemistry, UNAM for
instrumental, DGSCA, UNAM for computation facilities. Carlos
Alberto Huerta Aguilar thanks CONACyT for a scholarship.
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