Selective removal and quantification of Cu(II) using fluorescent iminocoumarin-functionalized magnetic nanosilica

Article abstract of DOI:10.1039/c2cc18113d

A newly prepared iminocoumarin-functionalized magnetic nanosilica (Ni@SiO₂-1) was found to form a selective stable complex with Cu ²⁺ over other metal ions. Quantification of Cu²⁺ ions in aqueous solution using Ni@SiO

Full text of DOI:10.1039/c2cc18113d

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COMMUNICATION
Selective removal and quantification of Cu(II) using fluorescent
iminocoumarin-functionalized magnetic nanosilicaw
a
b
c
Hyo Sung Jung,z Minsung Park,z Ji Hye Han,z Jae Hong Lee,a Chulhun Kang,*c
Jong Hwa Jung*b and Jong Seung Kim*a
Received 28th December 2011, Accepted 27th January 2012
DOI: 10.1039/c2cc18113d
A newly prepared iminocoumarin-functionalized magnetic nanosilica
(Ni@SiO2-1) was found to form a selective stable complex with
Cu2+ over other metal ions. Quantification of Cu2+ ions in
aqueous solution using Ni@SiO2-1 is demonstrated through a
fluorescent demetallization ensemble process.
other metal ions and quantification of the entrapped Cu2+
ions by virtue of fluorescence changes.
The synthetic pathways to Ni@SiO2-1 and 2 are depicted in
Scheme 1. Compound 2 was prepared by adaptation of our
previously reported synthetic procedures.7 Nickel nano-
particles were prepared by a modified polyol process.8 The
particles were coated with silica shells through the Stober
¨
Copper is one of the abundant essential trace elements in the human
body and can be commonly found as Cu2+ in natural water.1
However, due to its potential to induce damages in proteins and
DNA structures via ROS formation in biological systems,2 excessive
uptake of Cu2+ even for a short period of time may cause
gastrointestinal disturbance and long-term exposure to it may result
in liver and kidney damages.3 Thus, efficient, economic and
environmentally viable scientific techniques for selective monitoring
and removal of Cu2+ from other metal mixtures or pollutants are
highly desirable. Moreover, these methods are very useful in
preventing copper poisoning in environmental and biological fields.4
Magnetic silica nanoparticles have been also of great interest in
biomedical and environmental applications such as bioseparation,
drug targeting, cell isolation etc.5 Development of hybrid nano-
materials based upon a combination of their magnetic property
with an additional function such as selective ligand binding
ability, for example, that would endow diverse and noble func-
tionalities to the nanomaterials is highly enviable. Indeed, studies
on removal techniques of Cu2+ by using fluorescent nanoparticles
have been introduced in biological and environmental systems.6
In this regard, herein we report the design and synthesis of
core/shell type magnetic nanosilica (Ni@SiO2-1) functionalized
with an iminocoumarin moiety 2 which can form a stable
complex with Cu2+, but undergo fluorogenic hydrolysis in its
absence in an aqueous environment. To prove the causative
factors for this fluorescent response, Ni@SiO2-1 was tested with
various metal ions in this study. Hence, we herein present the
feasibility of Ni@SiO2-1 for selective capture of Cu2+ over
method using tetraethyl orthosilicate (TEOS) as a silica source
in water to yield Ni@SiO2 core/shell particles9 as detailed in
ESI.w Finally, Ni@SiO2-1 was prepared by a coupling reaction
of 1 with Ni@SiO2 core/shell particles in toluene under reflux
conditions. Morphological change of the Ni@SiO2-1 was
observed by TEM. Detailed characterizations of the prepared
compounds are provided in ESIw as well.
In our earlier study, we found that a complex of 2–Cu2+
,
where the o-OH unit of 2 as an additional binding site is
essential to make the stable metal complex, is resistant to
hydrolysis in aqueous solution. The 1 : 1 complex of 2–Cu2+
was evidenced by the ESI-MS spectrum (Fig. S1, ESIw) and
X-ray crystal structure (Fig. S2, ESIw), respectively. However,
upon Cu2+ elimination, non-fluorescent 2 was rapidly hydrolyzed
to produce a strongly fluorescent 3 revealing an absorption band
a Department of Chemistry, Korea University, Seoul 136-701, Korea.
E-mail: jongskim@korea.ac.kr; Fax: +82-2-3290-3121
b Department of Chemistry and Research Institute of Natural Science,
Gyeongsang National University, Jinju, 660-701, Korea.
E-mail: jonghwa@gnu.ac.kr
c The School of East-West Medical Science, Kyung Hee University,
Yongin 446-701, Korea. E-mail: kangch@khu.ac.kr
w Electronic supplementary information (ESI) available. See DOI:
10.1039/c2cc18113d
z These authors equally contributed to this work.
Scheme 1 Synthetic routes to 1, 2 and Ni@SiO2-1.
c
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Fig. 1 (A) UV-Vis and (B) fluorescence spectra of 2 (10 mM) with
Fig. 2 (A) TEM image of Ni@SiO2-1. (B) Photograph of a magnet
attracting Ni@SiO2-1 in MOPS buffer solution (pH 7.0).
addition of Li+, Na+, K+, Rb+, Cs+, Ag+, Cd2+, Mg2+, Sr2+
Ba2+, Zn2+, Ca2+, Pb2+, Ni2+, Co2+, Cu2+, Hg2+, Fe2+, Al3+
,
,
Fe3+, and In3+ (5 equiv., respectively) in aqueous solution (10 mM
A transmission electron microscopic (TEM) image of
Ni@SiO2-1 revealed that it appears to have a spherical structure
with narrow size distribution (ca. 35 nm) with a 30 nm Ni core,
thereby it maintained a nanocrystalline appearance (Fig. 2). IR
and TOF-SIMS results are in good accordance with a bond
formation between Ni@SiO2 and 2. The IR spectrum showed
new strong bands at 3632, 3282, 2918, 2848, 2369, 2330, 1698,
1564, 1515 and 1094 cmꢀ1 which originated from 2 (Fig. S10,
ESIw). The TOF-SIMS spectrum of 1 residing on the Ni@SiO2
core/shell nanoparticles displayed the signals at m/z = 484, 543
attributable to 1 and its fragment, implicating that 1 is anchored
onto the surface of the Ni@SiO2 core/shell nanoparticles (Fig. S11,
ESIw). The amount of 2 on the Ni@SiO2 surface was estimated to
be 1.66 ꢁ 0.05 nmoles per mg (n = 4) of the nanomagnet, based
on the recovered fluorescence intensity from the complete hydro-
lysis of Ni@SiO2-1 compared to that of 2 (Fig. S12, ESIw).
Absorption and emission spectroscopic measurements of
Ni@SiO2-1 in the presence of biologically and environmentally
relevant metal ions were performed in 0.2 M MOPS, buffer pH
7. The absorption spectrum of free Ni@SiO2-1 in the absence of
metal ions showed a single absorption band at 465 nm and a
corresponding emission maximum at 515 nm (Fig. S13, ESIw).
In the aspect of fluorescence changes, the Ni@SiO2-1 emits a
strong fluorescence in the presence of metal ions except the
Cu2+ ion due to its rapid hydrolysis to generate 3 in aqueous
media as seen in Fig. 3 and 4. However, the Ni@SiO2-1–Cu2+
complex was not indeed hydrolyzed, but maintained its fluores-
cence intensity. In this regard, it should be also noteworthy that
PBS buffer, pH 7.4, containing 1.0% CH3CN) after 10 min.
at 465 nm and an emission band at 515 nm (Ff = 0.67) in
aqueous solution (10 mM PBS buffer, pH 7.4, containing 1.0%
CH3CN) (Fig. S3 and Table S2, ESIw). From this result, it can be
envisaged that 2 would play an important role in the selective
detection and removal of Cu2+ unless it is hydrolyzed by Cu2+
complexation in aqueous solution. Thereby, the hybrid nano-
magnet 1 would form a stable complex with Cu2+ followed by
separation of Cu2+ by its magnetic property. Then, the exact
amount of entrapped Cu2+ ions by Ni@SiO2-1 can be easily
estimated by fluorescence changes after Cu2+ ion stripping.
At first, to investigate the metal ion selectivity of 2, absorption
and emission spectra of 2 in aqueous solution (10 mM PBS buffer,
pH 7.4, containing 1.0% CH3CN) were examined by the addition
of various metal ions. As indicated in Fig. 1A, hydrolysis of 2 is
exclusively inhibited in the presence of Cu2+ whereas the hydro-
lysis was observed with other metal ions (Li+, Na+, K+, Rb+,
Cs+, Ag+, Cd2+, Mg2+, Sr2+, Ba2+, Zn2+, Ca2+, Pb2+, Ni2+
,
Co2+, Hg2+, Fe2+, Al3+, Fe3+, and In3+). Absorption and
emission bands of each product formed from the addition of
metal ions to a solution of 2 are the same as those of 3 (Fig. S4,
ESIw). In addition, the 1H NMR spectra of the hydrolysis
products are identical to that of 3 (Fig. S5, ESIw). The fluores-
cence quantum yields (Ff = 0.65) of a mixture of 2 with other
metal ions are also similar to that of 3 (Ff = 0.69) (Table S2,
ESIw). On the other hand, upon the addition of Cu2+, the
fluorescence intensity of 2 declined (Fig. 1B). These results
confirm again that 2 selectively encapsulates the Cu2+ ion which
is not hydrolyzed by aqueous media. Studies of fluorescence-
enhancement factor (FEF) of 2 (10 mM) with and without Cu2+
(5.0 equiv.) in the presence of other metals (Li+, Na+, K+, Rb+,
Cs+, Ag+, Cd2+, Mg2+, Sr2+, Ba2+, Zn2+, Ca2+, Pb2+, Co2+
,
Cu2+, Hg2+, Fe2+, Al3+, Fe3+, and In3+) in aqueous solution
(10 mM PBS buffer, pH 7.4, containing 1.0% CH3CN) are also
implemented and their results are shown in Fig. S7 (ESIw). Both
free 2 and metal complexes of 2 except Cu2+ ion were found to be
hydrolyzed to give strongly fluorescent 3. To the best of our
knowledge, this ensemble system based on the blocking hydrolysis
caused by Cu2+ ion complexation has been rarely exploited so far.
To prepare the hybrid nanomagnet (Ni@SiO2-1), the non-
fluorescent 1 was immobilized on the Ni@SiO2 core/shell
particles under reflux in toluene for 24 h. In this process, the
triethoxylsilyl group of 1 undergoes hydrolysis to covalently
attach to the surface of the Ni@SiO2 core/shell particles.
Cooling the reaction mixture to room temperature and filtering
the red precipitates provided Ni@SiO2-1 in quantitative yield.
Fig. 3 Fluorescence intensities of Ni@SiO2-1 (0.1 mM) (red bar) and
Ni@SiO2-1 with Cu2+ (10 equiv.) (black bar) upon addition of Li+
,
,
K+, Ag+, Cd2+, Mg2+, Zn2+, Ca2+, Pb2+, Ni2+, Co2+, Hg2+
Mn2+, Fe2+, and Fe3+ (10 equiv., respectively) in MOPS buffer
solution (pH 7.0) after 10 min.
c
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Fig. 6 Photogram of Ni@SiO2-1 (10 mg) (A) before and (B) after
immersion into Cu2+ (10 mM) by irradiation with a UV lamp.
based on the measurement of the fluorescence after treatment
with b-mercaptoethanol (see also Fig. 4).
In order to extend the above performance to a portable chemo-
sensor kit, a disk-type pellet has been prepared with Ni@SiO2-1
(Fig. 6). Upon dipping in Cu2+ (10 mM) solution the pale yellow
pellet of Ni@SiO2-1 changed into red with fluorescence quenched
due to its Cu2+ ion complexation (Fig. 6B). On the other hand, in
the parallel experiments with Co2+, Mn2+, Cd2+, Ag+ and Pb2+
solutions (10 mM each), pale yellow fluorescence was observed
caused by its hydrolysis. The result implies that the disk-type pellet
prepared from Ni@SiO2-1 is applicable as a portable chemosensor
for the detection of Cu2+ in the environmental field as well.
In conclusion, we synthesized a core/shell type magnetic nano-
silica (Ni@SiO2-1) functionalized with iminocoumarin moiety 2
which can form a stable complex with Cu2+ in an aqueous
environment. The stable complex is found to be not hydrolysed
even in an aqueous solution. We then found that the Ni@SiO2-1
is capable of selectively entrapping Cu2+ ions over other metals
and the captured Cu2+ ions can be quantified by means of
fluorescence spectroscopy. These are promising findings for the
development of a new category of biocompatible systems built by
immobilizing appropriate fluorogenic receptors on the surfaces of
novel magnetic nanomaterials for detection, recovery and separa-
tion of other heavy metal ions from environmental pollutants.
J.S.K acknowledges the CRI program (No. 2011-0000420)
from KRF in Korea. J.H.J also acknowledges World Class
University (WCU) Program (R32-2008-000-20003-0) supported
by Ministry of Education, Science and Technology, Korea.
Fig. 4 Schematic representation of formation of a stable complex of
Ni@SiO2-1 with Cu2+ ion followed by hydrolysis by a thiol to
generate fluorescence-on.
the Ni@SiO2-1 would be a great promise as a useful selective
chemosensor for detection and removal of Cu2+
.
We also examined the pH dependency of Ni@SiO2-1 on the
Cu2+ uptake. As seen in Fig. S14 (ESIw), the fluorescence band
was constantly observed in the pH range of 4–11, suggesting
that complexation of Ni@SiO2-1 with Cu2+ readily undergoes
in the pH range of 4–11, which is applicable for biological and
environmental fields. From the fluorescence changes of the
Ni@SiO2-1 upon binding with the Cu2+ ion, the detection
limit was calculated to be 0.15 ppb of [Cu2+] (Fig. S15, ESIw).
To get insight into the linear relationship of fluorescence
intensity vs. Cu2+ concentration, emission intensities of the
Ni@SiO2-1 were measured with various concentrations of Cu2+
ions (Fig. 5A). The fluorescence intensity of the Ni@SiO2-1
decreased at 515 nm as the [Cu2+] reached 1.0 equiv. (inset of
Fig. 5A). In order to quantify the entrapped Cu2+ ions in the
Ni@SiO2-1, the Cu2+ ion embedded Ni@SiO2-1 was separated
and treated with 1.0 mM of b-mercaptoethanol solution. We
then in turn observed a marked enhancement in fluorescence
from the solution, which is obviously attributed to the hydrolysis
of Ni@SiO2-1 to give compound 3. Within this implementation,
we noticed that variation of Cu2+ concentration provides linearity
as depicted in Fig. 5B, which should be one of the useful and
important standards to determine metal ions in aqueous
media. These results led us to conclude that Ni@SiO2-1 can
form a stable complex with Cu2+ ions in the aqueous solution,
and the amount of Cu2+ ions entrapped can be quantified
Notes and references
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Fig. 5 (A) Fluorescence titrations of Ni@SiO2-1 (0.1 mM) upon addition
of increasing Cu2+ concentrations (0, 0.1, 0.2, 0.4, 0.6, 0.8, 1, 2, 3, 5, 7, 10,
15, and 20 equiv.) in MOPS buffer solution (pH 7.0) after 10 min. The
inset: plot for fluorescence intensity of Ni@SiO2-1 as a function of Cu2+
concentration. (B) Fluorescence intensities of Ni@SiO2-1–Cu2+. The
fluorescence intensities and the image of 96 well plate were obtained by
fluorometry and a scanner.
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¨
c
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As featured in:
Showcasing research from Dr Jong Seung Kim’s
Laboratory, Korea University, Seoul, Republic of Korea
Selective removal and quantification of Cu(II) using fluorescent
iminocoumarin-functionalized magnetic nanosilica
A newly prepared iminocoumarin-functionalized magnetic
nanosilica (Ni@SiO2-1) forms a selective stable complex with
Cu2+ over other metal ions. Quantification of removed Cu2+ ions
from aqueous solution using Ni@SiO2-1 is demonstrated through
a fluorescent demetallization ensemble process.
See C. Kang, J. H. Jung, J. S. Kim, et al.,
Chem. Commun., 2012, 48, 5082.
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