Turn-off fluorescence chemosensor for iron with Bis(2-aminoethyl)-2-(9- fluorenyl)malonamide functionlized SBA-15

Article abstract of DOI:10.1007/s10895-013-1321-8

An bis(2-aminoethyl)-2-(9-fluorenyl)malonamide as fluorophore ligand was immobilized onto mesoporous silica type SBA-15 via post synthesis grafting. The obtained material was characterized by small and wide angle X-ray diffraction, N₂ adsorption-desorption, Fourier transform infrared spectroscopy, Raman spectroscopy and thermogravimetric analysis that indicate the successful immobilization of the ligand on the surface of mesoporous silica. The sensing ability of the obtained material was studied by addition of the cations Fe ³⁺, Mg²⁺, Cr³⁺, Co²⁺, Ni ²⁺, Cu²⁺, Hg²⁺ and Zn²⁺ to water suspensions of the assayed solid. Of all the cations tested addition of Fe ³⁺ ion to a suspension of this material resulted in the largest decrease in the fluorescence intensity. Turn-off photoluminescence of this material was remarkably observed for iron ions in comparing of the other cations. A good linearity between the fluorescence intensity of this material and the concentration of Fe³⁺ ion is constructed, which enables it as a fluorescence chemosensor for detecting the Fe³⁺ ion with a suitable detection limit of 1.35∈×∈10^-5. It can be introduced as a novel fluorescent sensor in aqueous solution for a lot of practical applications in chemical, environmental and biological systems.

Full text of DOI:10.1007/s10895-013-1321-8

J Fluoresc
DOI 10.1007/s10895-013-1321-8
ORIGINAL PAPER
Turn-off Fluorescence Chemosensor for Iron
with Bis(2-aminoethyl)-2-(9-fluorenyl)malonamide
Functionlized SBA-15
Marzieh Yadavi & Alireza Badiei
Received: 13 July 2013 /Accepted: 7 November 2013
#
Springer Science+Business Media New York 2013
Abstract An bis(2-aminoethyl)-2-(9-fluorenyl)malonamide
as fluorophore ligand was immobilized onto mesoporous
silica type SBA-15 via post synthesis grafting. The obtained
material was characterized by small and wide angle X-ray
Introduction
Synthesizing chemosensors based on ion-induced fluorescence
changes for specific metals detection are predominantly attrac-
tive because of their high selectivity, sensitivity, simplicity and
low detection limit of fluorescence recognition methods [1–3].
In the middle of it all, fluorescence ‘turn-off’ sensors for
detection of metal ions of biological importance still remain a
challenging aspect [4, 5]. Hence, much attention has been paid
to the design of ‘turn-off’ fluorescence sensors. Among the
metal ions, Fe is of particular interest owing to its established
role in biological and industrial areas that is indispensable for
most organisms, and both its deficiency and overload can
induce various disorders [6]. Furthermore improved method
diffraction, N adsorption–desorption, Fourier transform in-
2
frared spectroscopy, Raman spectroscopy and thermogravi-
metric analysis that indicate the successful immobilization of
the ligand on the surface of mesoporous silica. The sensing
ability of the obtained material was studied by addition of the
3
+
2+
3+
2+
2+
2+
2+
2+
cations Fe , Mg , Cr , Co , Ni , Cu , Hg and Zn to
water suspensions of the assayed solid. Of all the cations
3
+
3
+
tested addition of Fe ion to a suspension of this material
resulted in the largest decrease in the fluorescence intensity.
Turn-off photoluminescence of this material was remarkably
observed for iron ions in comparing of the other cations. A
good linearity between the fluorescence intensity of this ma-
3+
for the detection and sensing of Fe ions with high selectivity
is of current interest in the sensor research field. Only a few
examples of sensor showing fluorescence changes upon Fe
3
+
3+
terial and the concentration of Fe ion is constructed, which
enables it as a fluorescence chemosensor for detecting the
complexation have been reported [7, 8]. However, the majority
of these reports were just based on organic molecules as
fluorescent chemosensors [7–9]. For some practical applica-
tions the attachment of the fluorophore to a solid support has
advantages like the possibility of recovering the materials for
their repetitive use and operation at the nanoscale level [10, 11].
Organic-functionalized mesoporous silica type SBA-15
can provide synergistic properties of organic and inorganic
components, such as flexible surface chemistry to
functionalization, well-defined ordered structure, available
surface area, structural stability and the large pore diameter
allows grafting of bulky organic moieties have attracted much
attention in recent years [12–18]. Sensor selectivity towards
transition metal ions can be modified by changing the struc-
tural features of the mesoporous silica, we are currently de-
signing and testing new sensors consisted of fluorenyl and
other chelating units [19, 20]. These materials have selectivity
and sensitivity to detect metal ions. Some have been reported
3
+
−5
Fe ion with a suitable detection limit of 1.35×10 . It can
be introduced as a novel fluorescent sensor in aqueous solu-
tion for a lot of practical applications in chemical, environ-
mental and biological systems.
.
.
.
Keywords Mesoporous silica SBA-15 Fluorene
3
+
.
Fluorescence Fe ions chemosensor
:
M. Yadavi A. Badiei (*)
School of Chemistry, College of Science, University of Tehran,
Tehran, P. O. Box: 14155-6455, Iran
e-mail: abadiei@khayam.ut.ac.ir
M. Yadavi
e-mail: marziyadavi@khayam.ut.ac.ir
A. Badiei
Nanobiomedicine Centra of Excellence, Nanoscience and
Nanotechnology Research Centra, University of Tehran, Tehran, Iran
2
+
2+
2+
2+
to recognize Zn [21], Cu [22], Pb [23] and Hg [24] in

J Fluoresc
aqueous media. However, material has been no reported to
recognize iron ions that operate at the nanoscale level in
aqueous media [19, 20]. Herein we report the synthesis of a
functionlized mesoporous silica based material bis(2-
aminoethyl)-2-(9-fluorenyl)malonamide that functions as a
done in a 1 cm quartz cuvette containing a magnetic-stirred
−1
solution of compounds in aqueous solvent (0.1 g L ). Calci-
nations were carried out in a box furnace Gofa in the presence
of flowing air. Melting points were determined by the capillary
method on electro thermal 9100 apparatus.
3
+
Fe sensor at aqueous solution. In view of high chemical
stability and fluorescent emission of fluorenyl, first we linked
it as fluorophore to a dioxotetramine receptor then, to the inner
surface of cholor functionalized mesoporous support SBA-15
Synthesis SBA-15
SBA-15 material was prepared according to literature proce-
dures [19, 20, 26, 27].
3
+
(
SBA-Cl) and obtained a new fluorescent Fe sensor in
aqueous solution. To the best of our knowledge, this is the
first example of a fluorescent sensor based on bis(2-
aminoethyl)-2-(9-fluorenyl)malonamide functionlized SBA-
Synthesis Chloro-functionalized of SBA-15
3
+
1
5 that allows the selective detection of Fe in aqueous
0.85 g of 3-chlorpropyltriethoxysilane was added drop wise to
1.25 g of vacuum dried mesoporous silica SBA-15 dispersed
in 60 ml of dry toluene under an inert atmosphere (Ar) in a
flask fitted with a reflux condenser and a magnetic stirrer.
Then the mixture was refluxed at 95 °C for 24 h and the final
product was filtered, washed with toluene and dried at room
temperature.
3
+
media. Moreover, in the presence of Fe , this sensor shows
highly selective and sensitive recognition toward Fe in over
a wide range of tested cations including Mg , Cr , Co ,
Ni , Cu , Hg and Zn without fluorescence intensity
change. Compared with other reported fluorescent
3
+
2
+
3+
2+
2+
2+
2+
2+
3
+
chemosensors for Fe ions, this system showed the highest
3
+
selectivity toward Fe ions in aqueous solution.
Synthesis 9-bromofluorene
Experimental
9-bromofluorene was prepared by literature methods [19, 20].
Synthesis Diethyl 2 - (9 -fluorenyl)malonate (Flem)
Materials and Instruments
Poly (ethylene glycol)-block-poly (propylene glycol)-block-
poly (ethylene glycol) (EO PO EO ) (P123, MW=5800)
The diethyl 2 -(9 -fluorenyl)malonate material was synthe-
sized in accordance with the procedure reporte by Luo et al.
[28]. To a solution of diethyl malonate (4.28 g, 0.027 mol) in
anhydrous DMF (5 ml) were added Na CO (2.83 g,
20
70
20
was obtained from Aldrich. Fluorene, bromine, carbon tetra-
chloride, diethyl ether, tetraethylorthosilicate (TEOS), 3-
chlorpropyltriethoxysilane, sodium carbonate, diethyl
malonate, ethylendiamine and hydrochloric acid were obtained
from Merck. Iron (ΙΙΙ) nitrate, magnesium nitrate, cupper (ΙΙ)
nitrate, cobalt (ΙΙ) nitrate, nickel (ΙΙ) nitrate, chromium (ΙΙΙ)
nitrate, mercury (ΙΙ) nitrate and zinc nitrate were of analytical
grade. Toluene and DMF was dried according to the standard
purification methods [25]. Nitrogen physisorption isotherms
were obtained on a BELSORP mini-II at liquid nitrogen tem-
perature (77 K). Surface area was measured using the
Brunauer-Emmett-Teller (BET) method, pore size distributions
were calculated from the nitrogen isotherms by Barrett-Joyner-
Halenda (BJH) method. Small and wide angle x-ray scattering
2
3
0.027 mol) and a solution of 9-bromofluorene (6.56 g,
0.027 mol) in DMF (20 ml). The resulting mixture was stirred
at 70 °C for 24 h and poured into 250 ml ice water. The
suspension formed was extracted with three 25 ml portions
of diethyl ether. The organic layer was dried over Na SO and
2
4
the solvent was removed by rotary evaporation to give a
yellow solid. After recrystallization from ethanol, yellow nee-
−1
dle crystals were obtained (Fig. 1.); m.p. 69–72 °C, νmax/cm
2977 (νCH), 1727 (νCO), 1447 and 1367 (νCH), 735 (νC-C).
Synthesis Bis(2-aminoethyl)-2-(9-fluorenyl)malonamide
(Flen)
(
SAXS) patterns were recorded with a model Hecus S3-
MICROpix SAXS diffractometer with a one-dimensional
The bis(2-aminoethyl)-2-(9-fluorenyl)malonamide material
was synthesized in accordance with the procedure reported
by Luo et al. [28]. The mixture of the diethyl 2 -(9 -
fluorenyl)malonate (6.28 g, 0.001 mol) and freshly distilled
ethylenediamine (5 ml) was stirred at 35 °C for 72 h. Excess
ethylenediamine was distilled off under reduced pressure.
After cooling, the yellow residue was treated and washed with
PSD detector using Cu Kα radation (50KV, 1 mA) at wave
−1
length 1.542 Å. FT-IR spectra were recorded a 4000–600 cm
region on Equinox 55 spectrometer. Raman spectra were ob-
tained on SENTERRA spectrometer. Thermogravimetric anal-
ysis (TGA) was performed on a TA Q50 instrument. The scans
were performed between 20 °C and 800 °C at 10 °C/min.
Emission spectra were recorded on Perkin-Elmer LS50 model
luminescence spectrometer. Fluorescence measurements were
−1
diethyl ether (Fig. 1.). ν max/cm 3281 (νOH) 3057 and 2865
(νCH),1655 (νCO), 1538 (νNH), 744 (νC-C).

J Fluoresc
Fig. 1 Preparation of SBA-F
Synthesis Fluorene Functionalized SBA-15 (SBA-F)
typical SBA-15 materials, that is a generally recognized char-
acteristic of the long-range periodic [17]. For all the materials,
two additional peaks related to the higher ordering (110) and
(200) reflections can be also observed, which is associated
with a two-dimensional hexagonal (p6mm) structure [17].
However, in the case of functionalized SBA-15 materials,
the peak (100) intensity with a small peak shift decreases after
immobilizations due to the difference in the scattering contrast
of the pores and the walls, and to the irregular coverage of
organic groups on the nanochannels. This observation indi-
cates that the hexagonal structure of the SBA-15 is maintained
without any pore wall collapse during the modifying of the
surface. The larger wall thickness of the functionalized SBA-
0
2
.1 g of SBA-Cl was placed in a vacuum for 30 min, then
0 mL of dry toluene was added and stirred for 15 min; bis(2-
aminoethyl)-2-(9-fluorenyl)malonamide (0.1 g, 0.285 mmol)
was added to the resulting mixture under an inert atmosphere
and refluxed for 24 h. Next, it was allowed to cool at room
temperature and the toluene was removed by filtration under
vacuum and was dried at room temperature.
Results and Discussion
1
5 is consistent with grafting the organic groups onto the pore
Small and Wide Angle X-ray Diffraction
walls within the porous network (Table 1).
Figure 2 shows the small and wide angle X-ray diffraction
patterns of SBA-15 and the functionalized SBA-15 with
chloropropyl groups and Flen. All samples have a single
intensive reflection at 2θ angle at around 0.87° similar to the
N Adsorption-Desorption
2
The textural properties of the samples were evaluated by the
N adsorption-desorption isotherms (Fig. 3). All materials
2
exhibit a typical irreversible type IV nitrogen adsorption iso-
therm with an H1 hysteresis loop as defined by IUPAC [17].
There is a shift of the hysteresis position toward low relative
pressures and a slight decreasing trend in overall N adsorp-
2
tion volume as the loading of chloropropyl groups and FLen.
This indicates some modification of the tubular channels of
the parent SBA-15 silica material. The N adsorption at low
2
Table 1 Texture properties of prepared compounds
2
−1
3
−1
a
t
Sample
D
BJH
S
BET (m g )
V
total (cm g )
w
d 100(nm)
(nm)
(nm)
SBA-15 6.2
SBA-Cl 5.4
587
470
167
0.78
0.73
0.29
3.06 8.06
4.98 9.04
5.72 9.07
SBA-F
4.7
SBET is the BET surface area; V is the total pore volume; DBJH is the
average pore diameter calculated using BJH method
pffiffiffi
a
Fig. 2 Small angle XRD patterns a) SBA-15, b) SBA-Cl and c) SBA-F
Wall thickness ¼ 2= 3dð100Þ−pore diameter

J Fluoresc
Fig. 5 FT-IR spectra of (a) SBA-15, (b) SBA-Cl, (c) SBA-F
Fig. 3 N
c) SBA-F (Inset: BJH pore size distribution curves of SBA-15, SBA-Cl,
SBA-F)
2
adsorption-desorption isotherms of (a) SBA-15, (b) SBA-Cl,
(
Fig. 4 FT-IR spectra of (a) 9-bromofluorene, (b) Flem, (c) Flen
Fig. 6 Raman spectra of (a) SBA-15, (b) SBA-Cl, (c) SBA-F

J Fluoresc
Fig. 7 TGA analysis of (a) SBA-Cl (b) SBA-F
relative pressures is accounted for by monolayer adsorption of
The pore size distribution can be calculated from BJH method
N on the pore walls, and does not essentially involve the
2
based on the desorption branch of the N adsorption isotherm.
2
presence of micropores. The sharp inflection in the P/P range
As demonstrated in Fig. 3, a typical BJH plot from modified
SBA-15 with chloropropyl groups and FLen a narrow pore
size distribution are observed. The uniformity of the
mesopores in this modified SBA-15 is comparable to the
0
from 0.37 to 0.75 of the isotherm is characterized as capillary
condensation within uniform mesopores, the position of
which is clearly correlated to a diameter in the mesopore rang.
3
+
2+
Fig. 8 Bar graphs of the fluorescence emission intensity for Fe , Co ,
0.16 mM for each metal ion. I
metal ions
0
corresponds to emission of SBA-F without
2+
3+
2+
2+
2+
2+
Ni , Cr , Zn , Mg , Hg and Cu metal ions showing the metal ions
selectivity profile of SBA-F (200 μg/2.0 mL) with a concentration of

J Fluoresc
Fig. 9 Fluorescence emission
−1
spectra SBA-F (0.1 g.L ) in the
3+
presence of Fe ions from 0.16 to
.66 mM (λex=260 nm)
0
SBA-15, indicating that the integrity of the original inorganic
wall structure of the SBA-15, is retained. In comparison with
SBA-15 decreasment in the pore diameter value by about 0.8
and 1.5 nm are observed for SBA-Cl and SBA-F, respectively.
The textural parameters, specific surface areas (BET method),
pore diameters (BJH method) and total pore volumes are
given in Table 1, which shows a trend of decreasing in surface
area, pore volume and pore diameter due functionalization
inside the channels of SBA-15.
the typical stretching frequency of the carbonyl in amid
and the strong peak of 1450 cm was observed which
due to the C=C ring skeletal vibrations.
−
1
Raman Spectroscopy
Figure 6 shows the Raman spectra of SBA-15, SBA-Cl and
−1
SBA-F. The band at 2900 cm is attributed to C-H stretching
in the propyl chain in SBA-Cl and SBA-F. In the Raman
spectra of SBA-F clearly the strong bands at 1610 and
−
1
1
024 cm are related to the C-C and C=C ring skeletal
FT-IR Spectroscopy
−1
vibrations of FLen. The band at 1350 cm is attributed to
C-N vibrations that confirm FLen was attached on the surface
of SBA-15.
The IR spectra of precursor compounds Flem and Flen exhibit
−1
two bands at about 740 and 725 cm assigned to the skeleton
stretching vibration of the aromatic ring that observe in the 9-
−1
TGA Analysis
bromofluorene (Fig. 4). In Flem, the band at 1725 cm is the
typical stretching frequency of the carbonyl in the ester. In
−1
TGA analysis was carried out to verify the grafted organic
groups on the surface SBA-15. The TGA profiles samples are
illustrated in Fig. 7. The weights loss at temperature of ap-
proximately 100 °C corresponded to desorption of
physisorbed water, which is 0.7 % and 1.6 % for SBA-Cl
and SBA-F, respectively [30]. The weight loss 8.1 % at 180–
Flen, the band at 1672 cm is the typical stretching frequency
of the carbonyl in the amide.
Figure 5 shows FT-IR spectra of SBA-15, SBA-Cl and
SBA-F. In the FT-IR spectrum of the SBA-15 silica the band
at 3434 cm− can be assigned to the stretching vibration of OH
1
−1
groups. The bands at 800 and 1086 cm are attributed to
Si-O-Si and Si-O stretching vibrations, respectively [29].
There is an abundance of silanol groups present in the parent
SBA-15 silica in the surface of the mesoporous channels.
After grafting chloropropyl on the surface of SBA-15,
600 °C is mainly due to the decomposition of chloropropyl
groups in SBA-Cl. The weights loss (21.19 % and 10.18 %)
Table 2 The charge densities and radius of metal ions
−
1
the intensity of the band at 3434 cm is decreased. The
+2
2+
2+
2+
3+
2+
2+
3+
band at 2930 cm−1 is assigned to C-H stretching vibra-
Element Co
Zn
Mg
Ni
Cr
Cu
Hg
Fe
tions of the methylene groups. These results indicated
that the chloropropyl group was attached on the surface
of SBA-15. SBA-F showed the band at 1670 cm is
R
0.745 0.74 0.72
1.15 1.18 1.28
0.69 0.615 0.73 1.02 0.490
1.45 3.08 1.22 0.45 6.09
ρ
−
1

J Fluoresc
was quenched by transfer of electron from metal center to
3
+
photoexcited state of fluorenyl during coordination of Fe .
As shown in Fig. 9, a decrease of fluorescence intensity
3
+
could be remarkably observed with an increasing Fe
concentration.
This high selectivity can be interpreted in terms of differ-
ence in charge densities of metal ions. It is well known that the
charge density (ρ) of a metal ion is defined as the amount of
electric charge/unit volume. It is one of the most important
characterizations of the relative electrophilicity of a metal ion.
The charge densities of metal ions were calculated according
to the Eq. (1):
ꢀ
ꢁ
3
ρ ¼ q= 4=3πr
ð1Þ
Fig. 10 Variation of (I
0
- I)− of emission as a function of [Fe ] for
SBA-F
1
3+ −1
Where q is the formal charge and r denotes the Shannon
ionic radius (Table 2) [34]. The calculated charge densities of
2
+
2+
between 200 and 800 °C are due to the decomposition of
organic groups in SBA-F. A minor weights loss due to silanol
condensation at high temperature was observed [31].
the cations increase in the following order: Hg (0.45)<Co
2
+
2+
2+
2+
(
1.15)<Zn (1.18)<Cu (1.22)<Mg (1.28)<Ni (1.45)<
3
+
3+
Cr (3.08)<Fe (6.09).
3
+
The results indicate that the charge density of Fe ion is
the biggest among all of the competition ions. Accordingly,
the electrophilicity ability of Fe ion is the strongest among
these cations that can be used to interpret the capability of
forming a coordination complex with an electron-rich ligand.
Fluorescence Study
3
+
The fluorescence emission spectra of SBA-F (200 μg/2.0 mL)
in water exhibited one fluorescence emission at 310 nm and
3
+
the else at 410 nm (λ =260 nm) due to the fluorescent
Thus, the strong coordinate interaction between Fe ions with
the lone pair of the amine groups onto SBA-F significantly
quench the fluorescence of fluorene because of the strong
electrophilicity ability when it mixes with the other competi-
tion ions in the analytic solutions.
ex
molecule fluorene grafted to the mesoporous silica. The sec-
ond emission could be observed between 360 nm to 410 nm
for fluorene that depends on the media [32]. This emission is
attributed to a novel kind of twisted intermolecular charge
transfer (TICT) conformer within the life time of the lowest
−1
3+ −1
Figure 10 shows the plot of (I - I) vs. [Fe ] , where I
0
0
3
+
excited singlet state [33]. With the addition of Fe , the
emission peak intensity decreased significantly. No obvious
and I refer to the emission fluorescence intensity at 310 nm for
3
+
SBA-F in the absence and presence of Fe ions, respectively.
Generally, the mechanism of fluorescence quenching is
divided into three types: dynamic quenching; static
quenching; energy transfer [35]. Each of them is depicted
through its special equation. A good linearity should be de-
2
+
3+
responses could be observed upon the addition of Mg , Cr ,
2
+
2+
2+
2+
2+
Co , Ni , Cu , Hg and Zn (Fig. 8).
These observations indicated that SBA-F had a high sensi-
3
+
tivity and excellent selectivity for Fe in water. The results of
fluorimetric titration indicate that fluorescence of fluorenyl
−1
3+ −1
veloped in the plot of (I - I) vs. [Fe ] for the static
0
3+
Fig. 11 Fe binding
with SBA-F

J Fluoresc
quenching mechanism according to the following Line
weaver-Burk Eq. (2) [36]:
environment. A good linearity between the fluorescence in-
3
+
tensity of SBA-F and the concentration of Fe ion is con-
structed, which enables this martial as a fluorescence
−1
−1
−1
ðI − IÞ ¼ I0 þ K ðI ½QꢀÞ
ð2Þ
3+
0
D
0
chemosensor for detecting the Fe ion with a suitable detec-
−5
tion limit of 1.35×10 . We believe that the sensor can be
promoted for a lot of practical applications in chemical, envi-
ronmental and biological systems.
−
1
As seen in Fig. 10, a good linearity between (I - I) and
0
3
+
the reciprocal of the concentration of Fe from 0.16 to
.83 mM (linearly dependent coefficient R =0.98) is con-
2
0
structed. According to the above equation, the dissociation
Acknowledgments The authors are grateful for the financial support
from the University of Tehran.
constant K can be obtained from the slope and intercept of
D
the line and the corresponding association constant K (1/K )
D
4
is 1.4×10 . Also the detection limit of SBA-F for monitoring
3
+
of Fe are calculated according to Eq. (3), where DL is the
minimal analytical signal that can be detected, SD is the
standard deviation of blank and m is the plot slope of fluores-
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3
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