An Anthracene Excimer Fluorescence Probe on Mesoporous Silica for Dual Functions of Detection and Adsorption of Mercury (II) and Copper (II) with Biological In Vivo Applications

Article abstract of DOI:10.1002/smll.201804749

Dual functional activity by the same organic–inorganic hybrid material toward selective metal ion detection and its adsorption has drawn more attraction in the field of sensing. However, most of the hybrid materials in the literature are either for sensing studies or adsorption studies. In this manuscript, a fluorescent active hybrid material SiO₂@PBATPA is synthesized by covalent coupling of anthracene-based chelating ligand N,N′-(propane-1,3-diyl) bis(N-(anthracen-9-ylmethyl)-2-((3-(triethoxysilyl)propyl) amino) acetamide) (PBATPA) within the mesopores of newly synthesized cubic mesoporous silica. The synthetic strategy is designed to form an exclusively intramolecular excimer on a solid surface, which is then used as a sensory tool for selective detection of metal ions through fluorescence quenching by the destruction of excimer upon metal ion binding. The dual functions of sensing and adsorption studies show selectivity toward Hg²⁺ and Cu²⁺ among various metal ions with detection limits of 37 and 6 ppb, respectively, and adsorption capacities of 482 and 246 mg g^?1, respectively. This material can be used as a sensory cum adsorbent material in real food samples and living organisms such as the brine shrimp Artemia salina without any toxic effects from the material.

Full text of DOI:10.1002/smll.201804749

FULL PAPER
Excimer Probes
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An Anthracene Excimer Fluorescence Probe on Mesoporous
Silica for Dual Functions of Detection and Adsorption of
Mercury (II) and Copper (II) with Biological In Vivo Applications
Sobhan Chatterjee, Hardipsinh Gohil, Ishan Raval, Shruti Chatterjee,* and Alok Ranjan Paital*
microorganisms when overused. There-
Dual functional activity by the same organic–inorganic hybrid material
toward selective metal ion detection and its adsorption has drawn more
attraction in the field of sensing. However, most of the hybrid materials in
the literature are either for sensing studies or adsorption studies. In this
fore, it is also present in the U.S. Environ-
mental Protection Agency’s (EPA’s) list
of priority pollutants considering its haz-
[
2]
ardous nature. The recommended level
of intake of Copper is limited to 1.3 mg
per day by the World Health Organization
manuscript, a fluorescent active hybrid material SiO @PBATPA is syn-
2
thesized by covalent coupling of anthracene-based chelating ligand N,N′-
(
WHO). But Mercury and its compounds
(
propane-1,3-diyl) bis(N-(anthracen-9-ylmethyl)-2-((3-(triethoxysilyl)propyl)
are highly toxic, dangerous and can cause
[
3]
amino) acetamide) (PBATPA) within the mesopores of newly synthesized
cubic mesoporous silica. The synthetic strategy is designed to form an
exclusively intramolecular excimer on a solid surface, which is then used
as a sensory tool for selective detection of metal ions through fluorescence
quenching by the destruction of excimer upon metal ion binding. The dual
many serious diseases. The Minamata
disease resulted in a disastrous effect in
Minamata Bay of Japan due to accumu-
[
4]
lation methylmercury. Owing to the
adverse effects of Mercury, the threshold
limits in drinking water have limited
2
+
−1
functions of sensing and adsorption studies show selectivity toward Hg
to 2 µg L by WHO. Therefore, simul-
and Cu2 among various metal ions with detection limits of 37 and 6 ppb,
+
taneous detection and removal of toxic
metal ions from the aqueous system have
gained more attention due to their serious
−
1
respectively, and adsorption capacities of 482 and 246 mg g , respectively.
This material can be used as a sensory cum adsorbent material in real food
samples and living organisms such as the brine shrimp Artemia salina
without any toxic effects from the material.
[
5]
impact on environmental remediation.
In this regard, many organic–inorganic
hybrid materials got more importance
over organic probes only as they can be
easily recycled and devoid of any dissolu-
tion issues as compared to many organic fluorescence probes.
Also, the same organic molecule cannot be used for both pur-
poses as sensory probes need hydrophilic motif whereas, an
extractant needs hydrophobic motif. Because of the possibility
of these dual functional activities by the same functional-
ized material, they have attracted more attention over sen-
sory probes only. Therefore, many mesoporous solid supports
bearing fluorogenic organic motif with appropriate binding
sites are used to achieve both sensory and adsorption proper-
ties simultaneously. The various properties of Silica such as
easiness for chemical modification, good thermal and chem-
ical stability, large surface area, and tunable pore sizes makes
1
. Introduction
Metal ion toxicity is a well-known environmental issue and pos-
sesses a challenge for their early detection and removal. With
increasing metal based industrial activity, these metal ions
most often present in the industrial effluents and finally enters
into the food chain system, threatening human health and
the ecosystem.[ Though copper is considered as one of the
essential transition metal ion required for various metabolic
functions, but could be toxic to human health and
1]
[
6]
Dr. S. Chatterjee, H. Gohil, Dr. A. R. Paital
Salt and Marine Chemicals Division & Academy of Scientific
and Innovative Research (AcSIR)
CSIR-Central Salt & Marine Chemicals Research Institute
G.B. Marg, Bhavnagar 364002, Gujarat, India
E-mail: schatterjee@csmcri.res.in; arpaital@csmcri.res.in
it an attractive candidate for solid support. The fluorogenic
chelating motif is the recognition unit responsible for the
selective metal ion recognition and binding. Among various
polyaromatic fluorophores, anthracenes are one of the impor-
tant blue fluorescent materials that display well-defined mon-
[
7]
Dr. I. Raval, Dr. S. Chatterjee
omer and excimer emissions. Although traditional cognition
on excimer as fluorescence quencher is different from recent
views as efficient fluorescence materials observed in several
cases with red-shifted emission spectra and long lifetimes
Division of Biotechnology and Phycology
CSIR-Central Salt & Marine Chemicals Research Institute
G.B. Marg, Bhavnagar 364002, Gujarat, India
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/smll.201804749.
[
8]
compared to their monomers. Owing to the above advan-
tages, excimers has drawn attraction of scientific researchers
due to their significant roles in chemo- and biosensors and
DOI: 10.1002/smll.201804749
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white OLED.[9] The photophysical properties due to excimer
formation or destruction upon complexation is a detection
position of 1,3-diaminopropane) on the mesoporous silica sur-
face to get the final material (SiO @PBATPA). This fluorogenic
2
[
10]
tool most often used for sensory applications. As the con-
formational changes of the receptor have a profound effect on
intensity ratio of excimer to monomer emission (I /I ), they
material shows excimer based emissions used for dual func-
tions of selective detection and adsorption of Hg2 and Cu
+
2+
ions in the presence of various metal ions. This strategy can
be helpful in tuning the emission of the probe material as
observed here from blue (monomer) to green (excimer) for
need-based applications. As an application, this material can be
used as a fluorescence probe for detection of the above metal
ions in the living organism like Artemia salina as a biological
platform.
E
M
can be instrumental as sensory probe material for metal ion
detection and biological molecules.[ Though excimer forma-
11]
[
12]
tion by anthracene is a well-known process in solution but
[
13]
relatively rare on nanoporous solid surfaces
as compared
to pyrene and naphthalene.[ In the case of pyrene, it is also
documented that ground-state pairing leads to static excimer
emission when attached to silica surface even at low surface
coverages.[ It is also well known that intramolecular excimer
formation is desirable as compared to intermolecular excimer
14]
To best of our knowledge, this material represents a rare
example of anthracene derived heterogeneous excimer-based
15]
sensory probe for Hg2 and Cu which can be used as a
probe in living organisms without any toxicity. This mate-
rial also can detect and adsorb the above toxic metal ions in
real food samples. Therefore, the dual functional activity of
selective detection and removal by the same material with
recyclability of the material represent an ideal remediation
system.
+
2+
[
16]
formation for sensory applications.
It is also documented
that intramolecular excimer formation of diphenylalkanes (Ph-
(
CH )n-Ph) is possible if n = 3, though this may not be held so
2
[
17]
rigorously.
Considering the above facts about excimer formation, we
were interested in taking advantage of intramolecular excimer
formation on a solid surface as a sensory tool for simultaneous
applications of metal ion detection and removal as part of
our research interest. Therefore, we synthesized mesoporous
2
. Results and Discussion
cubic SiO as the solid support through a sol–gel technique
2
in the presence of tetraethyl orthosilicate (TEOS), NH , and
2.1. Synthesis and Characterization
3
hexadecyltrimethylammonium bromide (CTAB), following
a modified procedure.[ Then we have successfully fabri-
cated a new anthracene-based receptor N,N′-(propane-1,3-diyl)
bis(N-(anthracen-9-ylmethyl)-2-((3-(triethoxysilyl)propyl)
amino) acetamide) (PBATPA) (with two anthracene motifs at 1,3
18]
The synthetic procedure for the material SiO @PBATPA
2
is expressed in Scheme 1, and the details are explained
in the Experimental Section. The mesoporous silica was
synthesized by a modified method using CTAB as structure
Scheme 1. Synthetic scheme for mesoporous silica, ligand PBATPA, and final probe material SiO @PBATPA.
2
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Figure 1. A,B) FESEM images cubic SiO ; C,D) FESEM images of cubic SiO @PBATPA; E) low angle and broad angle PXRD of SiO , and SiO @
2
2
2
2
PBATPA; F) N adsorption/desorption isotherms of SiO , SiO @NH , and SiO @PBATPA.
2
2
2
2
2
directing template. The surface modification of above cubic
mesoporous silica was done by two synthetic approaches
surface. The broad angle PXRD patterns of cubic silica (SiO2)
and the functionalized material (Figure 1E) shows the similar
broadband at 2θ = 22° indicating the amorphous nature of both
[
cubic SiO → SiO @NH → SiO @PBATPA (steps A and B) or
2 2 2 2
[
19]
cubic SiO → SiO @PBATPA (step F)]. In the latter method, the
ligand ATPAA was synthesized first and then fabricated to con-
materials. Low angle PXRD patterns of both the materials
(Figure 1E) shows a strong diffraction peak at 2θ = 1.55°, and
three weak diffraction peaks at 2θ = 3.99°, 5.68°, and 6.65° with
the corresponding crystal plane d-spacing of 5.67, 2.21, 1.55,
and 1.32 nm, respectively. The low angle strong diffraction
peak indicates the porous nature of synthesized materials. To
investigate the BET surface area, pore size distribution (PSD),
and pore volume for the cubic SiO particle, SiO @NH , and
2
2
firm the formation of same product SiO @PBATPA through
2
various characterization data. In this work, the product from
the first approach was used for most of the studies. The reason
for synthesizing ligand PBATPA first in the second approach is
to understand the complexation behavior of the ligand toward
metal ions through mass spectral analysis discussed later. The
2
2
2
final material SiO @PBATPA was characterized through var-
SiO @PBATPA, N2 adsorption–desorption experiment car-
2
2
ious analytical techniques like scanning electron microscopy
ried out (Figure 1F; Table 1). It is observed that cubic SiO and
2
(
SEM), transmission electron microscopy (TEM), powder X-ray
SiO @NH exhibit type-IV isotherm which is the characteristic
of the mesoporous material. On the other hand, for material
2
2
[
20]
diffraction (PXRD), thermogravimetric analysis (TGA), solid
1
3
29
state C NMR, Si NMR, Fourier transform infrared spec-
troscopy (FT-IR), and Brunauer–Emmett–Teller (BET) surface
measurements.
SiO @PBATPA, the isotherm devoid of any capillary condensa-
2
tion, as observed for type-I isotherm, suggesting a decrease in
pore size after functionalization. The various physicochemical
parameters such as BET surface area, pore size distribution,
and pore volume (Figure S1, Supporting Information) gradually
2
−1
3
−1
2
.1.1. Structural Characterization
decreases from cubic SiO (927 m g , 0.8884 cm g , 3.8 nm)
2
Structural characterization of cubic mesoporous silica (SiO2)
Table 1. Surface areas, pore volumes, and pore diameters of the synthe-
and final SiO @PBATPA material carried out through low angle
sized materials.
2
PXRD, SEM, TEM, and N adsorption-desorption isotherms.
2
Material
Surface area [m g ] Pore volume [cm g−1] Pore diameter [nm]
2
−1
3
The SEM images of both silica (SiO ) (Figure 1A,B) and SiO @
2
2
PBATPA material (Figure 1C,D) shows cubic morphology with
uniform particle sizes in the range of 280–350 nm. In case
SiO2
927.48
385.59
4.20
0.8884
0.3933
0.0667
3.8
3.3
1.1
SiO2@NH2
SiO2@PBATPA
of the SiO @PBATPA material, surfaces are quite irregular
2
indicating that functionalization of the ligand on the SiO2
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Figure 2. A–C) TEM images of cubic SiO ; D–F) TEM images of cubic SiO @PBATPA material.
2
2
2
−1
3
−1
−1
to SiO @NH (385 m g , 0.3933 cm g , 3.3 nm) to SiO @
vibrations. The band at 1327 cm is for C–N vibration of alkyl
2
2
2
2
−1
3
−1
[24]
PBATPA (4.20 m g , 0.0667 cm g , 1.11 nm) respectively,
clearly suggest that functionalization happened inside the
mesopores.[
amine.
Comparison of FT-IR spectra of SiO @NH and
SiO @PBATPA material (Figure 3B) reveals some similar
2 2
2
21]
−1
bands at 1062, 793, 2930, and 2880 cm due to SiOSi,
The TEM images of SiO2 and SiO @PBATPA materials
SiOH, and CH vibration of (CH )  groups respectively.
2
2 3
−
1
(
Figure 2) are also in good agreement with similar morpho-
logical observations in SEM studies with particle sizes of
50–340 nm. As the pore size decreases from nanopores (3.8 nm)
in cubic SiO to micropores (1.11 nm) in SiO @PBATPA, the
The additional bands at 1665 and 1580 cm found in material
SiO @PBATPA due to the existence of “CO” stretching of
the amide group and NH deformation vibration respectively.
The above results provide strong evidence in favor of covalent
attachment of ligand PBATPA on the silica surface.
2
[
25]
2
2
2
pores are not visible in the TEM image of SiO @PBATPA and
2
are also in good compliance with pore size distribution studies
To further confirm the formation of SiO @PBATPA mate-
2
rial, both solid state 29Si CP (MAS) and C CP (MAS) NMR
13
from N adsorption–desorption experiment.
2
studies were done. Both the materials SiO @NH and SiO @
2
2
2
1
3
PBATPA shows three peaks at 12, 24, and 45 ppm in C CP
2
.1.2. Surface Characterization
(MAS) NMR spectrum (Figure 4A) which can be assigned to
[
26]
three methylene groups, (CH ) . Some additional peaks
2
3
Surface characterization of final SiO @PBATPA material car-
between 92–162 and 180 ppm in SiO @PBATPA are due to
2
2
ried out through FT-IR, solid-state 13C CP (MAS), and Si NMR
29
the presence of arene and carbonyl carbons, respectively.
[27]
spectrum, and TGA. FT-IR spectrum for SiO (Figure 3A) dis-
The 29Si CP (MAS) NMR (Figure 4B) shows Q type signals
2
n
plays bands at 3432, 1640, 1062, and 793 cm− while SiO @NH2
1
(−94, −102, −110 ppm) for silanol group in all the three com-
pounds and T type signals (−50 to −70 ppm) for SiO @NH
2
2
shows bands at 3438, 2930, 2880, 1631, 1560, 1327, 1062, 793,
n
2
−
1
−1
and 694 cm . The additional peaks at 694, 1631, and 1560 cm
and SiO @PBATPA due to silicon atoms of the organic linker
2
due to N–H stretching and bending vibrations[ are observed
22]
3-APTES.
[28]
for the SiO @NH , whereas the extra bands at 1489, 2930,
The loading of the organic motif on cubic silica surface
was studied by TGA measurements. The TGA of SiO @NH
2
2
−
1
and 2880 cm is from the (3-Aminopropyl) triethoxysilane
2
2
23]
(
APTES) group[ assigned for the C–H stretching and bending
and SiO @PBATPA (Figure 5) show three weight loss regions
2
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Figure 3. A) Comparison of FT-IR spectra of SiO and SiO @NH ; B) SiO @NH and SiO @PBATPA.
2
2
2
2
2
2
between 33–218, 218–440, and 440–800 °C. The foremost
pH range of 5–10 but in the lower pH range (2–5), a new band
regions show a weight loss of −17.29% and −11.92% in SiO @
at 300 nm appears due to protonation of NH groups at ligand
2
+
NH and SiO @PBATPA, respectively, owing to desorption of
sites (SiO @PBATPA-H ) (Figure 6A). As the materials sur-
2
2
2
physically adsorbed aqueous molecule on the surface of the
materials. The higher value with the SiO @NH is probably
face charges sensitive to the pH of the suspension which also
affects adsorption capacities, the zeta potential at different pH
ranges were evaluated (Figure 6B). It is observed that though
zeta potential does not change much from neutral to higher pH
range but a more positive value was observed at lower pH range
2
2
due to the existence of hydrophilic NH groups which leads to
2
more water adsorption.[ The weight loss of the second region
29]
showing −16.18% in SiO @PBATPA material compared to
2
[
30]
−
2.41% in SiO @NH due to more organic content in SiO @
PBATPA material. The last regions show similar weight losses
owing to protonation of ligand motif in the material. Though
the material is stable at higher pH, considering the formation
of metal hydroxides at higher pH range (>8), the neutral pH
(≈7; Na HPO : NaH PO ) was considered for all analytical and
2
2
2
for both the materials SiO @NH (−6.42%) and SiO @PBATPA
2
2
2
(−6.63%) due to the destruction of organosilicate frameworks.
2
4
2
4
adsorption studies.
To examine the sensing ability of material toward var-
ious metal ions both UV–Vis and fluorescence investiga-
tions were done in the aqueous buffer system at pH 7
(Na HPO :NaH PO ). The UV–Vis spectrum of SiO @PBATPA
2
.2. Sensing Studies
To determine the working pH condition with this mate-
rial for sensing and adsorption studies, the UV–Vis absorp-
tion studies were done in the pH range of 2–10 in aq. buffer
medium to know the stability of the material. It is observed
that this material shows its original absorption bands in the
2
4
2
4
2
[
31]
material shows three typical peaks of anthracene moiety at
354, 375, and 395 nm along with peaks at 254 and 268 nm.
In the presence of Cu2 and Hg metal ions, the anthracene
+
2+
peaks remain intact, but the peak position at lower wavelength
Figure 4. A) 13C solid CP NMR for SiO @NH and SiO @PBATPA; B) 29Si CP (MAS) NMR of SiO2, SiO @NH , and SiO @PBATPA.
2
2
2
2
2
2
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observed in these cases.[32] Though only a few reports on such
systems with excimer formation is known but never used as a
[
13]
sensory tool for detection of metal ions. In our case due to
the presence of two anthracene motifs at 1,3 position leads to a
ground-state pairing to exhibit excimer emission.
[
33]
To determine the Stern–Volmer quenching constant (K )
sv
2+
2+
of SiO @PBATPA material toward metal ions (Hg , Cu )
2
fluorescence titration was done with the incremental addi-
tion of these metal ions into the aqueous suspended solution
of SiO @PBATPA material with a saturation concentration of
2
−6
1
00 × 10 m (Figure 8A,D). The Stern–Volmer plots of both
2+
2+
metal ions (Hg , Cu ) shows a straight line with linear fitting
[5a,e]
(Figure 8B,E), which indicates a static mechanism
where the
quencher (metal ions) binds the fluorophore in the ground state
forming a nonfluorescent complex, also in good agreement
with UV–Vis spectral changes. More details about fluorescence
sensing mechanism of SiO @PBATPA material toward these
2
metal ions are discussed later in probable mechanism section.
Considering a linear fitting, a linear Stern–Volmer equation
Figure 5. TGA of SiO @NH and SiO @PBATPA material.
+
2
2
2
F0/F = 1 + Ksv [Mn ] was used. In this equation, F represents
0
the original fluorescence intensity of material SiO @PBATPA,
2
got shifted with increasing in absorbance (Figure 7A).
and F is the final fluorescence intensity of material after the
addition of increment amount of metal ions (Hg2 and Cu ),
+
2+
But no changes were observed with all other metal ions
+
+
+
2+
2+
2+
2+
3+
2+
2+
2+
(
Li , Na , K , Ca , Mg , Sr , Mn , Fe , Co , Ni , Zn , and
Ksv is the Stern–Volmer constant which is the slope of linear
2
+
4
4
−1
m
Cd ). Likewise, the fluorescence spectra of suspended SiO @
plot found to be 1.2563 × 10 and 1.5332 × 10
, respectively.
2
[5a,e]
for Hg2+ and Cu
2+
PBATPA (0.5 mg/2 mL) in aqueous buffer medium shows
To determine the limit of detection (LOD)
ions, incremental addition of low concentrations (1 × 10 –5 ×
10 m) of above metal ions to the aqueous suspension of
−6
characteristic monomeric peaks at 445, 417, and 395 nm (λem
)
−6
and a strong excimer emission band at 510 nm (λem) upon exci-
tation at 395 nm (λ ) (Figure 7B). The band at 510 nm probably
SiO @PBATPA material were made in deionized water at
ex
2
due to an intramolecular excimer formation (π–π stacking of
aromatics) whose intensity ratio (Iexcimer/Imonomer = 1.17–1.07)
does not change much with concentration variation as shown
pH ≈ 7 and the fluorescence quenching was recorded (Figure S2,
Supporting Information). The LOD calculations for the metal
ions (Hg2 and Cu ) found to be 37 and 5.44 ppb respectively
+
2+
[
10c,e]
2+
2+
in Figure 7C.
(
After addition of Cu and Hg ions
using the equation LOD = 3 S.D./S. Where S.D. is the standard
−
6
100 × 10 m) to a suspension of SiO @PBATPA, both the mon-
deviation, calculated from three blank reading of SiO @
2
2
omeric and excimer peak quenched dramatically with a more
pronounced effect on the latter whereas no drastic changes
were observed with other relevant metal ions (Figure 7B).
Though solid surface appended anthracene-based hybrid mate-
rials are known in the literature, but excimer formation is not
PBATPA material and S is the slope of linear plot between
fluorescence intensity and metal ions concentrations (Figure
S2, Supporting Information). The Stern–Volmer constant (K ),
sv
2
limit of detection and correlation factor (R ) for individual
Hg ions and Cu ions are listed in Table 2. A competitive
2+
2+
Figure 6. UV–Vis spectra of SiO @PBATPA at different pH range; B) change of zeta potential value of SiO @PBATPA surface with the change in pH
2
2
of the solution.
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Figure 7. A) UV–Vis spectra of SiO @PBATPA material with various metal ions in neutral buffer solution (Na HPO :NaH PO ). B) Fluorescence
2
2
4
2
4
spectra of SiO @PBATPA in the presence of various metal ions in the aqueous medium at neutral pH (7.0) with an excitation at 395 nm. C) Fluores-
2
cence spectra of SiO @PBATPA at different concentrations with excitation at 355 nm.
2
fluorescence study with five equivalent molar excess of other
metal ions shows similar quenching result that of the control
experiments with Hg2+ ions and Cu ions (Figure 8C,F).
quenching may be due to the photoelectron transfer (PET) from
[
10c]
anthracene to the amide carbonyl group.
But the quenching
2+
of excimer emission may be due to conformational change
upon metal ion complexation resulting in the destruction of
[
10c]
excimer.
As a result, in this conformation, parallel stalking
2
.2.1. Possible Sensing Mechanism
of two anthracene units is not possible as shown in Figure 9B.
To determine the binding constant and number of binding
As discussed above the fluorescence spectra of this material
0.5 mg/4 mL) shows three weak anthracene monomeric peak
at 395, 417, 445 nm, and a strong excimer peak at 510 nm
sites for such quenching interaction between SiO @PBATPA
2
material and Hg2 and Cu ion, we used Scatchard equation:
+
2+
(
[
34,5a]
Log{(F − F)/F} = Log K + n Log [Q].
Here, Q is the con-
0
b
2+
2+
due to intramolecular π–π stalking interactions at excitation at
centration of Hg and Cu ions, F and F represent the fluo-
0
+
2+
−6
3
55 nm. In the presence of excess Cu2 and Hg (300 × 10 m)
rescence intensity of the material before and after the addition
of Hg2 and Cu ions (values were taken from fluorescence
+
2+
the intensity of both monomeric and excimer peak decreases
simultaneously (Figure 9A). The monomer fluorescence
titration studies), Kb is the binding constant, and “n” is the
2
+
2+
Figure 8. A) Fluorescence titration with increasing concentration of Hg . B) The linear Stern–Volmer plot with Hg . C) Competitive fluorescence
2
+
2+
spectra in the presence of excess other metal ions with Hg . D) Fluorescence titration with increasing concentration of Cu . E) The linear Stern–
Volmer plot with the Cu ion. F) Competitive fluorescence spectra in the presence of excess other metal ions with Cu .
2
+
2+
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2+
Table 2. The detection parameters from fluorescence studies for Hg
and Cu ions.
adsorption experiments, 100 mg of SiO @PBATPA material
was treated with 20 ppm of each metal ion mixed solution
2
2
+
(
50 mL) for 4 h at room temperature. After 4 h, the mixture
Ksv [m−1]
R2
Analytes
LOD
was filtered, and the filtrate was analyzed by inductively cou-
pled plasma optical emission spectrometry (ICP-OES) analysis
to evaluate the final concentration of various metal ions. The
adsorption/extraction efficiency was calculated by employing the
equation % E = C − C /C × 100, where % E is the extraction
percentage, C and C are the initial and final concentration
Hg2
+
1.5332 × 104
37 ppb (184.4 × 10−9 m)
5.44 ppb (85 × 10−9 m)
0.9878
0.9940
Cu2+
1.2563 × 104
[33]
i
f
i
number of binding sites. The K and n can be calculated from
i f
the slope, and the intercept of the double logarithm regression
of metal ions in aqueous solution. It has been observed that
only Hg2 (98.20%) and Cu (95.35%) ions got substan-
tially extracted whereas for other relevant metal ions it was
negligible (Table S1, Supporting Information). To calcula-
+
2+
curve of Log {(F − F)/F} verses Log [Q] which shows a linear
0
fitting (Figure 10A,B). The calculated number of binding sites,
2
binding constant, and the correlation factor (R ) for Cu(II) and
Hg(II) ions are shown in Table 3.
tion of experimental equilibrium adsorption capacity (Q ) of
e
2+
2+
The mass spectral analysis of ligand and ligand–metal chlo-
ride adducts also supports the formation 1:1 complex adduct.
The electrospray ionization mass spectrometry (ESI-MS) of
SiO @PBATPA material toward Hg and Cu ions, batch
2
adsorption experiments were performed in aqueous buffer
medium as discussed in the Experimental Section. The max-
Ligand PBATPA-HgCl (DMF solution) shows peaks at 1249 for
imum equilibrium adsorption capacity (Q ) calculated from the
2
+
e
+
[
PBATPA-HgCl + H ] and 1379.95 for [PBATPA-HgCl -DMF-
2
plot between Q versus C , using the adsorption isotherm equa-
2
2
e
e
+
[5a]
H O + K ] (Figure S3, Supporting Information). Similarly,
PBATPA-CuCl adduct shows peaks at 1332.61 for [PBATPA-
CuCl -2DMF-3H O + Na ] and 1348.56 for [PBATPA-CuCl2-
2
tion Q = (C − C )V/W, where Q is the equilibrium adsorp-
e I e e
tion capacity, C and C are the initial and final concentration of
2
I
2+
e
+
+
2+
metal ions (Hg and Cu ) in aqueous solution, V is the volume
of aqueous solution (L), and W is the weight of the material (g).
From equilibrium adsorption isotherm (Figure 11B), the adsorp-
tion capacity for Hg (II) and Cu (II) ions by the material found
2
2
+
+
DMF-3H O+K ] (Figure S4, Supporting Information). Prob-
2
ably the complex undergo solvation due to presence of triethox-
ysilyl motif as observed earlier.[
5e]
−
1
to be 482 and 246 mg g , respectively at room temperature.
The above experimental data were evaluated by the Lang-
muir and Freundlich adsorption isotherm models, which
relates to heterogeneous surfaces. For a Langmuir adsorption
isotherm, the following equation C /q = 1/K ·q + C /q was
2
.3. Adsorption Studies
To evaluate various adsorption parameters and selectivity, the
competitive adsorption experiments with all metal ions and
batch adsorption experiments were carried out in aqueous
buffer medium (pH 7) at room temperature. To examine the
e
e
L
m
e
m
[
5a]
used. Where q and qm are the equilibrium adsorption and
e
maximum equilibrium adsorption capacity, C is the equilib-
e
−1
rium concentration of metal ions (after adsorption, mg L ),
KL (L mg ) is the Langmuir adsorption constant. Similarly,
−
1
adsorption equilibrium times (Q ) with one of the representa-
t
2+
[5a]
tive cases for Cu with fixed concentrations of metal ion and
adsorbent but variable treatment time shows an increase in
adsorption capacity with time which gets saturated after 2 h
the equation lnq = lnK + 1/n ln C
was used for Freundlich
e
f
e
−
1
adsorption isotherm model where KF (L g ) and n are the
Freundlich constants. The various Langmuir and Freundlich
adsorption parameter listed in Table 4 and the adsorption iso-
therms are illustrated in (Figure 12 and Figure S5, Supporting
(Figure 11A). So, in all experiments, the equilibrium adsorption
time was kept a little upper side for 3 h. For the competitive
Figure 9. A) The fluorescence spectra of SiO @PBATPA showing quenching of monomeric and excimer emission peak with Cu and Hg2+ ions.
2+
2
B) Schematic presentation of excimer on-off mechanism.
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2+
2+
Figure 10. A) The Scatchard plot showing one binding site for Cu . B) The Scatchard plot showing one binding site for Hg .
Information). The above results show a better agreement with
the Langmuir model which indicates monolayer adsorption
occurred on the material surface.
for the material SiO @PBATPA. Similar observations were
2
also made with Hg2 suggesting reversibility nature of metal
binding and stripping process (Figure S6, Supporting Informa-
tion). The reversibility nature of the material was also studied
through acid–base treatments by measuring the zeta potential
of isolated materials after treatments which shows similar zeta
potential for original and regenerated material (−21.75 and
+
To further support our claim and to track the adsorption pro-
cess, energy dispersive X-ray spectroscopy (EDXs) was used.
The EDX measurement of material obtained from competitive
adsorption experiment indicates the presence of signals for Cu,
Hg, and Cl elements along with C, O, Si, and N elements sig-
nify the adsorption of above metal chlorides (Figure 13). The
dual functional activities of this material are comparable with
most of the other materials known in the literature regarding
extraction capability (adsorption capacity), selectivity, extrac-
tion time, detection limit (LOD) and recyclability (Table S2,
Supporting Information).
−22.30 mV) due to the presence of electronegative donor atoms
+
(Figure 14B). The acid treated material SiO @PBATPA-H
2
shows a more positive value of zeta potential (+13.90 mV) due
to protonation of organic amine functionality in the material.
Based on these observations a probable mechanism is shown
in Figure 14C.
The recyclability studies were also investigated through four
cycles of adsorption/desorption of metal ions by acid–base
treatments (Figure 15A and Figure S7, Supporting Informa-
tion). For desorption process, 0.2 n HCl was used whereas
for regeneration process 0.2 n NaOH solution was used. After
every adsorption/desorption and regeneration process, the
material was washed with deionized water. Every adsorption
experiment was repeated thrice to minimize the experimental
error. For one of the representative cases with Cu2 adsorption,
it is noticed that the adsorption capacity falls by 1.62% after
1st cycle and 3.65% after 2nd cycle (Figure 15A) may be due
to little damage of binding sites but later on uniform adsorp-
tion capacity was observed which shows that the material is
stable enough for successive acidic stripping steps. The solid-
state 13C NMR spectra after the fourth cycle demonstrate the
organic motif is intact and comparable with the original spectra
(Figure 15B).
2
.4. Recyclability Studies
As reusability of material is one of the crucial factors from
the economic point of view, the reversibility nature of adsorp-
tion and desorption property was studied through absorbance
spectral studies, zeta potential measurements, and adsorp-
tion studies. As a representative case for Cu (II), it is observed
+
that the UV–Vis absorption band for SiO @PBATPA material
2
changes upon addition of metal ion due to complexation reac-
tion forming SiO @PBATPA@M (Figure 14A,C). Upon acidifi-
2
cation (0.2 n HCl), the bands at 254 and 268 nm vanishes and a
new peak at 300 nm appear due to decomplexation reaction and
+
protonation of secondary amine nitrogens (SiO @PBATPA-H ).
2
Upon addition of dilute NaOH (0.2 n NaOH), the band at
3
00 nm vanishes and shifted back to the original position
2
.5. Detection and Adsorption Studies in Real Samples
Table 3. The various binding parameters from the Scatchard equation.
K [m−1
R2
To evaluate the applicability of this probe material for detection
Metal ions
Hg2
]
Binding sites (n)
0.7930
b
of Cu2 and Hg in real samples, grape juice, and orange juice
were considered for copper ion detection and conger eel fish
was considered for mercury ion detection. Fruit juices were
+
2+
+
0.2149 × 104
0.8425 × 104
0.9876
0.9905
Cu2
+
0.9480
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2+
Figure 11. A) The Equilibrium adsorption time with one of the representative case with Cu . B) The Equilibrium adsorption capacity of SiO @PBATPA
material toward Cu and Hg .
2
2
+
2+
centrifuged and filtered before use while Conger eel fish tissue
was acid digested and filtered after neutralization as described
in experimental section. Considering the low concentration of
these metal ions in the real samples, spiked solutions were pre-
removal was observed, as all the mercury ion is expected to be
in free ion state in the acid-digested sample. These results dem-
onstrate the potential application of the material to detect and
adsorb these toxic metal ions in the real samples.
−
6
−6
−6
pared with 30 × 10 , 40 × 10 , and 50 × 10 m of metal ions
2
+
2+
(
Cu for fruit juice and Hg for fish). Initially, fluorescence
titrations of material SiO @PBATPA (2 mg/2 mL) were carried
2.6. Detection of Metal Ions in the Animal Model Brine Shrimp
2
out in water suspension in the presence of various concentra-
tions of individual metal ions to plot the calibration curves.
Artemia Nauplii
Then the emission intensities of the same material SiO @
To examine the application of the above material as a sensory
2
probe for detection of Cu2 and Hg in the living organism,
in vivo fluorescence imaging was carried out with whole brine
shrimp Artemia in the absence and presence of the above metal
ions (Figure 16). Considering a cost-effective animal model
for in vivo bioimaging studies Artemia was chosen as the bio-
+
2+
PBATPA (2 mg/2 mL) with various spiked solutions were
recorded, and the concentration of metal ions was estimated
from the calibration curves (Figures S8 and S9, Supporting
[
35]
Information). The results show that the obtained recoveries
are satisfactory in the range of 96–104% for copper ion in fruit
juices and 102–106% for mercury ion in fish as shown in
Table S3 (Supporting Information). For adsorption studies in
these real samples, all the samples were digested and analyzed
by ICP-OES analysis with and without spiking to know the
[
36]
logical platform. For this purpose, hatched Artemia nauplii
(50 organisms in 10 mL tube) taken in 10 mL seawater to which
100 µL (from 10 mg/10 mL stock solution) of the suspended
SiO @PBATPA aqueous solution was added and kept for 2 h.
2
initial total metal ion (Cu2 or Hg ) concentrations correctly.
For copper ion determination, raw fruit juices were treated
with the material (10 mg in 10 mL), filtered and finally acid
digested to know the final concentration after the adsorption
process. Similarly, for mercury ion adsorption, the neutralized
acid-digested fish liquor was treated with the material (10 mg in
+
2+
Fluorescence imaging shows that Artemia nauplii readily intake
the fluorescent materials from the surrounding water and can
accumulate it in the gastrointestinal (GI) tract supported by the
observation of bright green colored staining of GI tract under
the microscope (BX 53 OLYMPUS) (Figure 16C,D). The treated
Artemia nauplii were then incubated in a 10 mL brine solution
−
6
−6
2+
1
0 mL) and after filtration analyzed to know the final concentra-
containing 100 µL of 10 × 10 and 100 × 10 m of Hg and
2+
tion (Table S4, Supporting Information). It has been observed
that around 60.11% and 68.88% of copper ion adsorbed by the
material in grape and orange juices, respectively. These results
are impressive considering the complex system, where the
total copper ion may not be in free ion state to be adsorbed by
the material. In the case of fish sample, 96.3% of mercury ion
Cu ions and kept for 1 h. However, the Artemia treated with
10 × 10 m Hg and Cu displayed a weak green fluorescence
staining of GI tract whereas, with 100 × 10 m Hg and Cu
−
6
2+
2+
−
6
2+
2+
(Figure 16G,H) the green fluorescence staining of GI tract
almost vanishes. These observations further support the fluo-
rescence quenching of material in the presence of these metal
Table 4. The Langmuir and Freundlich adsorption parameter for analytes at 25 °C.
Analytes
Experimental
Langmuir linear isotherm
Freundlich linear isotherm
qmax [mg g−1]
505
KL [L g−1]
0.1548
R2
R2
Qe
KF
n
Hg2
Cu2
+
482
246
0.9995
0.9980
186.53
111.73
4.6360
5.8736
0.9781
0.9794
+
256
0.2019
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2
+
2+
Figure 12. A) Langmuir adsorption isotherm for Cu . B) Langmuir adsorption isotherm for Hg .
ions as discussed in fluorescence sensing studies. Therefore,
these results suggest that this material can be used to detect
the presence of these toxic metal ions in living organisms and
hence can be used as an in vivo sensory probe material.
numbers died after 24 h. Whereas, in the presence of mercury
ion, movements were restricted totally after 24 h and all Artemia
were found dead accounting for 100% mortality. Only Artemia
without any treatment was treated as the negative control. These
observations confirm the nontoxic nature of material but mor-
tality observed in the presence of these metal ions.
2
.7. In Vivo Toxicity Studies in Shrimp Artemia Nauplii
To examine the toxicity of the material to the brine shrimp
3
. Conclusion
Artemia nauplii in vivo toxicity studies[37] were carried out in the
2
+
2+
presence of the material and metal ions (Hg or Cu ). An ali-
In summary, a new functionalized mesoporous material was
synthesized by covalent coupling of anthracene-based chelator
within the mesopores of newly synthesized cubic mesoporous
silica. PXRD, SEM, and TEM data suggests retention of a highly
ordered cubic mesoporous structure after functionalization.
A steady decrease in surface area, pore volume, and pore dia-
meter from SiO to SiO @PBATPA from the N sorption studies
suggest grafting of chelating fluorophore inside the mesopores.
Taking advantage of intramolecular excimer formation by anthra-
cene motif on a solid surface as a sensory tool, selective detection
quot of material, Cu2 and Hg that was used for in vivo sensing
studies was used to check the lethality toward Artemia nauplii in
different time duration. In the presence of material only (100 µL
from 10 mg/10 mL stock solution added to 10 mL of seawater
with 50 ± 3 organisms), no significant mortality was observed
in Artemia nauplii after 8 and 24 h. Whereas, in the presence of
+
2+
2
2
2
2+
2+
−6
Cu or Hg (100 µL from 100 × 10 m) in 10 mL of seawater,
Artemia mortality was observed. In the case of copper ion only,
mortality percentage (≈15%) was similar after 8 and 24 h. How-
ever, in the case of mercury, after 24 h, all treated Artemia nau-
plii were died (Figure S10, Supporting Information) due to the
toxicity of metal ions. Similar results were also observed when
metal ions added to the solution with materials and Artemia.
When copper ion added to the solution with material and
Artemia, it was observed that Artemia were healthy except few
of toxic metal ions like Cu2 and Hg were done through the
fluorescence quenching mechanism. This material also shows
high adsorption capacity toward above metal ions by complex
formation supported by EDX analysis and mass spectral analysis
of ligand–metal ion adducts. Recyclability of material up to four
cycles shows the stability of the compound during adsorption,
desorption, and regeneration by acid–base treatments supported
by reversibility studies through UV–Vis absorption studies, zeta
potential measurements and solid-state 13C NMR studies. This
material also shows a promising result in real samples and in
vivo sensory probe studies for detection of above metal ions in a
living organism like A. salina as a biological platform. Therefore,
this material represents very rare example of excimer based het-
erogeneous sensory probe cum adsorbent material which can be
recycled easily with simple acid–base treatments.
+
2+
4
. Experimental Section
Figure 13. EDX spectrum of isolated material from competitive adsorp-
Materials: All reagents and chemicals used here were of analytical
tion experiments showing adsorption of metal chlorides (Cu and Hg).
grades. In all analytical experiments, deionized water was used.
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2+
Figure 14. A) Reversible absorption spectral studies of SiO @PBATPA material with one of the representative case (Cu ) with acid–base treatments.
2
B) Reversible zeta potential studies with acid–base treatments. C) Probable recycling mechanism of SiO @PBATPA material during metal ion uptake,
2
stripping and regeneration of material.
Solvents were dried in the laboratory as required. TEOS, APTES,
CTAB, 1,3-diaminopropane, 9-anthracenecarboxaldehyde, bromoacetyl
chloride, NaBH4 were purchased from TCI chemical Private Ltd. For
buffer solutions of pH 2–5, Na HPO /citric acid was used for pH
HCl (37%), dry N(CH CH ) were purchased from SD Fine Chemical
2 3 3
Private Ltd.
Instrumentation: Bruker Advance 500 MHz NMR was used to record
C and Si solid CP NMR spectra. UV–Vis absorption studies were
1
3
29
2
4
5
–7, NaH PO /Na HPO was used, for pH 8–10, KH PO /NaOH was
measured by Shimadzu UV 3101 PC spectrophotometer. Fluorescence
spectra were recorded by using Edinburgh Instruments model Xe-900.
FT-IR spectra were recorded using KBR disks on a Perkin-Elmer GX
spectrophotometer (USA). Mass spectra were collected by using
2
4
2
4
2
4
used which were purchased from Merck group private Ltd. All metal
chlorides were purchased from Spectrochem Private Ltd. All dry
solvents (toluene, methanol, acetonitrile), anhydrous K CO , NaOH,
2
3
Figure 15. A) The adsorption capacity of SiO @PBATPA material toward Cu2 ions up to four cycles. B) Comparison of 13C solid-state NMR of SiO @
+
2
2
PBATPA after 4th cycle.
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Figure 16. A) A brine Shrimp Artemia nauplii bright field image; B) Artemia nauplii under UV filter as negative control (blank); C,D) Artemia nauplii
−
6
2+
after intake of SiO @PBATPA material (under UV filter); E) Artemia nauplii in the presence of 10 × 10 m Hg ; F) Artemia nauplii in the presence of
2
−
6
2+
−6
2+
−6
2+
1
0 × 10 m Cu ; G) Artemia nauplii in the presence of 100 × 10 m Hg ; H) Artemia nauplii in the presence of 100 × 10 m Cu .
Q-TOF Micro TM LC–MS instrument. ICP-OES analyses were measured
through Perkin Elmer instrument (Optima 2000DV). Malvern instrument
BPD (2 g, 4.4 mmol) and triethylamine (1.12 g, 11 mmol) were taken
in 250 mL of dry chloroform and stirred under cold condition (Ice-NaCl
bath) with a temperature range of 0–5 °C for 30 min. After that bromo-
acetyl chloride (1.12 g, 11 mmol) was added dropwise to the above
mixture and stirred for 3 h under cold condition followed by stirring at
room temperature for 24 h. Formation of both sides substituted final
product was monitored through TLC spotting and mass spectral analysis
(Figures S11 and S12, Supporting Information). Then the organic part
was extracted with brine and normal deionized water 3–4 times. The
organic part was dried over anhydrous Na SO , and the solvent was
(Zetasizer, Nano series, Nano-ZS90) was used to measure the zeta
potentials. TGA was performed by using a Mettler-Toledo (TGA/SDTA
−
1
8
51e) instrument in the air with a heating rate of 10 °C min . PXRD
was measured by Miniflex-II (FD 41521) powder diffractometer (Rigaku,
Japan). SEM was performed by Leo series 1430 VP instrument equipped
with INCA. TEM was performed using a JEOL JEM 2100 microscope.
Synthesis—Synthesis of Cubic Mesoporous Silica: The synthesis of Cubic
SiO2 was performed with little modification of the reported method.[
Typically, 2 g of CTAB surfactant was dissolved in 300 mL of deionized
water and stirred for 1 h followed by addition of 10–12 mL of ammonia–
water (28%) solution, forming a clear solution which was stirred further
for another 1 h. Then a mixture of n-hexane (40 mL) and TEOS (10 mL) was
added drop-wise to the above mixture for 45 min under vigorous stirring at
a temperature of 45 °C. As the reaction proceeds a milky colloidal solution
gradually formed and stirring continued for 12 h. The white product
was collected by centrifugation and washed with deionized water and
methanol. To remove CTAB surfactant template, the solid white product
was dispersed into 200 mL methanol in the presence of 1 m HCl (8–10 mL)
under reflux condition for 12 h. The above solvent extraction process was
repeated 4–5 times, and finally, the sample was collected by centrifugation
and washing with hot methanol 3–4 times. The above product was oven
dried overnight at 80 °C and yielded 2.2 g of product (Scheme 1).
18]
2
4
removed through the rotary evaporator. The crude was purified by
recrystallization (2.20 g, 72%), which was used as it is in the second
step.
Synthesis—Direct Functionalization of Ligand PBAB onto SiO @NH2
2
to Form SiO @PBATPA: In this procedure, 3 g of SiO @NH refluxed
2
2
2
with PBAB ligand (1 g, 1.4 mmol) in the presence of anhydrous K CO3
2
(0.3982 g, 2.8 mmol,) in dry acetonitrile (200 mL) for 48 h at 80 °C.
The crude material was separated by filtration and repeatedly washed
with hot chloroform to removed excess unreactive PBAB ligand and
finally washed with deionized water to removed excess K CO . The final
2
3
compound was dried under a hot air oven at 80 °C for 2 h to yield 3.42 g
of the compound. This final material SiO @PBATPA was characterized
2
by various analytical techniques.
Synthesis—Synthesis of Ligand PBATPA and Functionalization on Cubic
SiO to Prepare SiO @PBATPA: For the synthesis of ligand PBATPA, both
Synthesis—Preparation of 3-APTES Functionalized Silica (SiO @NH ):
2
2
2
2
Functionalization of above synthesized cubic silica with 3-APTES
PBAB and 3-APTES were refluxed in dry MeCN in the presence of K CO3
2
done by following a reported procedure.[ For this, 1 g of cubic
38]
in an inert atmosphere. After 48 h, the solvent was removed by the rotary
evaporator, and crude solid was dissolved in CHCl3 and washed with
water several times. Then the solvent was removed, and the solid was
recrystallized from hot ethanol to give PBATPA ligand confirmed from the
mass spectral analysis (Figures S13 and S14, Supporting Information).
The above crude ligand (0.5 g) was refluxed with 1 g of cubic silica in
silica (SiO ) refluxed with 2.5 g of 3-APTES in toluene for 12 h under
2
an inert atmosphere. The white solid (SiO @NH ) was collected by
2
2
centrifugation and repeatedly washed with excess toluene, hot methanol,
and chloroform to removed excess unreactive 3-APTES and finally dried
in the oven at 80 °C for 12 h to give 1.026 g of product (Scheme 1D).
Synthesis—Synthesis of N1,N3-bis(anthracen-9-ylmethyl)propane-1,3-
diamine (BPD): This compound was synthesized following a literature
dry toluene for 12 h under argon. Finally, the SiO @PBATPA material
2
was filtered off and thoroughly washed with CHCl , deionized water
3
[
39]
procedure
,3-propanediamine, followed by the reduction reaction in the presence
of NaBH4.
Synthesis—Synthesis of N,N′-(propane-1,3-diyl)bis(N-(anthracen-9-
by a Schiff-base reaction between anthraldehyde and
and dried under vacuum for 48 h to give an off-white colored material
(1.18 g). The analytical data for both methods confirms the formation of
1
the same compound SiO @PBATPA.
2
Synthesis—UV–Vis and Fluorescence Studies: For both UV–Vis and
ylmethyl)-2-bromoacetamide) (PBAB): To a 500 mL round bottom flask,
fluorescence studies, 0.5 mg of SiO @PBATPA material was suspended
2
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in 2 mL buffer solution and shaken well on a vortex shaker for 1 min
to get a uniform suspension solution before any spectral studies. The
suspension was taken in a 1 cm quartz cuvette, and before and after
addition of various metal ions to the suspension the cuvette is shaken
well, and the process was repeated for every spectrophotometric/
fluorescence reading. UV–Vis spectra were recorded in the range
of 200–800 nm and fluorescence spectra were recorded in the range
of 420–700 nm with an excitation wavelength of 395 nm. For LOD
calculations, the fluorescent intensity was recorded by gradually
titrations with the material (2 mg/2 mL), and the concentration of copper
ion in these spiked solutions were estimated from the calibration curve
(intensity vs concentration). For adsorption experiment, 10 mL of each
juice after filtration were acid-digested and volume adjusted to 10 mL
before the analysis by ICP-OES to know the initial concentration of total
copper ions (1.434 ppm in grape juice, 0.587 ppm in orange juice). Each
juice (10 mL) were treated with 10 mg of materials for 3h under a vibrating
mixture, filtered and acid-digested and volume adjusted to 10 mL before
the analysis to know the final concentration (0.572 ppm in grape juice
and 0.183 ppm in orange juice). For mercury ion, Conger eel fish was
considered, and the samples were prepared by acid-digestion of 2 g of fish
tissue followed by neutralization with NaOH which was used for detection
adding 0–70 µm solution of analytes to the SiO @PBATPA suspension
2
(0.5 mg/2 mL). The slope (S) was obtained from the fluorescence
intensity versus concentration of analytes plot. Three blank fluorescence
measurements were done to calculate the standard deviation (SD).
The LOD was determined using the formula: LOD = (3SD/S).
−3
and adsorption studies. For detection studies, 1 × 10 m spiked fish
2+
solution with Hg was prepared. This mercury ion spiked fish solution
−6
−6
−6
Synthesis—Adsorption Experiments: To calculate various adsorption
parameters and adsorption isotherms, batch adsorption experiments
were carried out in the aqueous buffer solution at neutral pH following
(30 × 10 , 40 × 10 , 50 × 10 m) was used for fluorescence titrations
with the material (2 mg/2 mL), and the concentration of mercury ion in
these spiked solutions were estimated from the calibration curve. For
adsorption of mercury ion, initial concentration of mercury ion from acid-
digested solution found to be 0.326 ppm. After 10 mL of the neutralized
fish solution were treated with 10 mg of material and equilibrated
for 3 h, the final concentration found to be 0.012 ppm from ICP-OES
analysis. All the samples were spiked with a standard metal ion solution
[
5e]
a reported method. In a typical experiment, various concentrations
2
+
2+
(
20–200 ppm) of metal ions (Hg or Cu ) in 50 mL aqueous suspension
was treated with 5 mg SiO @PBATPA material at neutral buffer (pH 7)
2
for 3 h. Then the mixture was filtered through a polypropylene syringe
filter after equilibrating for 3 h, and the filtrate was analyzed to get the
(6.061 ppm for Cu2 and 16.240 for Hg ) to know the correctness of the
measurements. The differences of initial and final concentration were
considered for adsorption percentage calculations.
+
2+
equilibrium concentration (C ) of the metal ions by ICP-OES analysis.
e
−
1
The adsorption capacity (Q , mg g ) of the SiO @PBATPA material for
e
2
the analytes was found from the equation Q = (C − C )V/Wads. In this
e
I
e
equation, Q , C , and C represents the equilibrium adsorption capacity
Synthesis—In Vivo Metal Ion Detection Studies: A. salina cysts were
taken out from the freeze and were allowed to hatch overnight in a
vigorously aerated vessel under very high-intensity visible light. The next
day hatched cysts produced Artemia larvae (nauplii) those larvae were
then used for the experimental purpose. Using A. salina as animal model
sensing experiment was done as described earlier with slight modification.
Approximately 50 number of Artemia nauplii were taken in 10 mL of
seawater and to which 100 µL (from 10 mg/10 mL stock solution) of the
e
I
e
−
1
−1
(
mg g ), initial and final concentration of analytes (mg L ), respectively.
V and Wads represents the volume of analyte solution (L), and weight
of the adsorbent (g), respectively. All adsorption experiments were
repeated thrice to reduce the experimental error. To calculate adsorption
2
+
equilibrium time (Q ), the same amount of analytes (≈200 ppm Cu
t
ion solution was taken in five 50 mL glass vials) were treated with same
amount of material (5 mg of SiO @PBATPA) and subjected to different
2
treatment times. The solutions were filtered after the treatment times
of 30 min, 1, 2, 3, and 4 h and the filtrates were analyzed by ICP-OES
measurements to calculate the equilibrium adsorption time.
suspended SiO @PBATPA aqueous solution was added and kept for 2 h.
2
The treated Artemia nauplii were then incubated in a 10 mL brine solution
containing 100 µL of 10 µM and 100 µM of Hg2 and Cu ions. Some
organisms from the tube were transferred on a glass slide and viewed
under BX53 OLYMPUS microscope under bright and fluorescent filters.
Synthesis—In Vivo Toxicity Studies: The hatched mature Artemia
nauplii (50 ± 3 nos.) were randomly collected with a sterile glass
dropper and transferred into the test tubes with 10 mL of sterile
seawater. In one set of experiments, 100 µL of material suspension
(10 mg/10 mL) was added to the test tubes and kept under light at
room temperature, and numbers of live Artemia were counted after
8 and 24 h. Other sets of experiment were done similarly in the presence
+
2+
Synthesis—Striping and Reusability Studies: For reusability studies of
SiO @PBATPA material toward metal ion adsorption/desorption and
2
recyclability of the above material, simple acid–base treatments were
2
+
carried out. One of the representative cases with Cu ion is mentioned
here. Initially, various concentration cupric-ion aqueous solution ranging
from ≈20–200 ppm taken in 50 mL glass vials (10 nos.), to which 5 mg
SiO @PBATPA material was added individually in a neutral buffer
2
solution of pH 7 and stirred for 4 h. After that, these mixture solutions
were centrifuged and the solid adsorbent and aqueous part separated
by filtration through a syringe filter. The aqueous part was analyzed to
measure the final concentration of cupric ions by ICP-OES analysis.
This separated solid adsorbent was washed gently with deionized water
to removed nonbound cupric ions. Then the material was treated with
−
6
of 100 µL of metal ions only (100 × 10 m) in 10 mL of sterile seawater
and in the presence of both material and metal ions. All experiments
were performed in triplicate with 50 ± 3 nos. of Artemia under similar
experimental conditions, and mortality percentage was calculated
considering the standard deviations and using the formula: mortality
percentage = (mortality observed – mortality of control)/(total no of
artemia – mortality of control) × 100.
0
.2 n HCl (10 mL aqueous) for 5 min and Milli-Q water twice, and
the filtrate was analyzed to determine the stripped metal ions. In the
regeneration step, the above material treated with 0.2 n NaOH (10 mL
aqueous) for 5 min and washed with water and dried for reuse. Each
glass vials material undergoes similar acid–base treatments separately.
The adsorption, desorption, and regeneration process was studied up
to 4th cycles through calculation of adsorption capacity from each cycle. Supporting Information
Adsorption capacities were evaluated separately for each cycle from the
plot of adsorption isotherms.
Supporting Information is available from the Wiley Online Library or
from the author.
Synthesis—Detection and Adsorption Studies in Real Samples: For
detection and adsorption of copper ion in real samples both grape and
orange juices were considered. For this purpose, around 5 mL of each juice
−3
were centrifuged at 5000 rpm for 30 min before filtration. Then 1 × 10
m stock solutions of copper ion was prepared in these juices separately
Acknowledgements
−3
as spiked solutions. Similarly, 1 × 10 m stock solution of copper ion
Science and Engineering Research Board, Department of Science
and Technology (SERB-DST), India (EMR/2016/002135 and
YSS/2015/000009) is acknowledged for financial support. Analytical
Division and Centralized Instrumental Facility of CSIR-CSMCRI is
acknowledged for material characterizations. AcSIR is acknowledged
were prepared in deionized water for fluorescence titrations of material
SiO @PBATPA (2 mg/2 mL) in the presence of various concentration of
2
copper ion to plot the calibration curve. Then, the copper ion spiked juice
−6
−6
−6
solutions (30 × 10 , 40 × 10 , 50 × 10 m) were used for fluorescence
Small 2019, 1804749
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for providing the Ph.D. degree to S.C. and H.G. CSIR-CSMCRI
communication no. 188/2018.
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Conflict of Interest
The authors declare no conflict of interest.
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Keywords
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anthracene, cubic mesoporous silica, detection and adsorption, excimer
emission, mercury and copper ion
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