Dansyl derivative functionalized Fe3O4@SiO 2 fluorescent probe for detection and removal of Hg2+ in aqueous solution

Article abstract of DOI:10.1039/c2ra21642f

In the present work, a novel dansyl group functionalized Fe ₃O₄@SiO₂ magnetic nanomaterial with a core-shell structure was developed, aiming to detect and remove Hg²⁺ from aqueous media. The multifunctional nanocomposite shows excellent fluorescence sensitivity and selectivity towards Hg²⁺ over other metal ions (Na⁺, K⁺, Mg²⁺, Al³⁺, Ca²⁺, Cd²⁺, Ni²⁺, Co²⁺, Fe ³⁺, Zn²⁺, Pb²⁺, Cu²⁺, Ag ⁺). Concentrations as low as 10^-8 M can be detected and the addition of other metal ions has a negligible influence on the fluorescence emission. Regarding the reversibility of the nanosized sensor, the fluorescence of the DAP-Fe₃O₄@SiO₂ nanocomposite in the presence of Hg²⁺ ions is found to be almost reversible when treated with iodide anion. Additionally, the nanocomposite can be easily separated from solutions by adding an external magnetic field. Moreover, the functionalized nanosphere shows a good adsorption capacity to Hg²⁺, which makes it repeatedly applicable for simultaneous Hg²⁺ sensing and removal from polluted water. These results indicate that the multifunctional nanocomposite may find potential applications for simple detection and easy removal of Hg ²⁺ in biological and environmental areas. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c2ra21642f

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Dansyl derivative functionalized Fe3O4@SiO2
fluorescent probe for detection and removal of Hg2+ in
Cite this: RSC Advances, 2013, 3,
1082
aqueous solution3
Zhengping Dong,* Xin Tian, Yuanzhe Chen, Yueping Guo and Jiantai Ma*
In the present work, a novel dansyl group functionalized Fe3O4@SiO2 magnetic nanomaterial with a core–
shell structure was developed, aiming to detect and remove Hg2+ from aqueous media. The
multifunctional nanocomposite shows excellent fluorescence sensitivity and selectivity towards Hg2+ over
other metal ions (Na+, K+, Mg2+, Al3+, Ca2+, Cd2+, Ni2+, Co2+, Fe3+, Zn2+, Pb2+, Cu2+, Ag+). Concentrations as
low as 1028 M can be detected and the addition of other metal ions has a negligible influence on the
fluorescence emission. Regarding the reversibility of the nanosized sensor, the fluorescence of the DAP-
Fe3O4@SiO2 nanocomposite in the presence of Hg2+ ions is found to be almost reversible when treated
with iodide anion. Additionally, the nanocomposite can be easily separated from solutions by adding an
external magnetic field. Moreover, the functionalized nanosphere shows a good adsorption capacity to
Hg2+, which makes it repeatedly applicable for simultaneous Hg2+ sensing and removal from polluted
water. These results indicate that the multifunctional nanocomposite may find potential applications for
simple detection and easy removal of Hg2+ in biological and environmental areas.
Received 1st August 2012,
Accepted 19th November 2012
DOI: 10.1039/c2ra21642f
www.rsc.org/advances
homogeneous phase is not suitable for the separation, removal
and enrichment of target species or in rapid screening
Introduction
applications.23,24
The development of new methods for the detection of heavy
transition metal ions has received considerable attention
because the metals have an extremely toxic impact on the
environment and the human body. Particularly, the detection
of Hg2+ has attracted continuous attention, as pollution by
Hg2+ has resulted in harmful effects on human health. Once
Hg2+ is introduced into the environment, bacteria convert
inorganic Hg2+ into methylmercury, which has been impli-
cated as a cause of mercury pollution related to serious
irreversible neurological damage to human beings.1,2 As
fluorescent probes benefit from both high sensitivity and
simple implementation, intense efforts have been made to
create small-molecule fluorescent organic probes that can be
used for sensing Hg2+ in recent years.3–10 Many organic
molecule sensors for Hg2+ based on rhodamine,11,12 pyr-
In order to achieve practical applications, the fluorescent
molecules have to be assembled on carriers with sufficiently
small dimensions. Various solid materials such as SBA-15,25,26
MCM-41,24,27 silicon nanowires,28 silicon nanoparticles29 and
optical fibers30 are used as carriers to improve the properties
of small molecule probes. The above mentioned silica
nanomaterials have abundant Si–OH groups on the surface.
Thus, it is easy to covalently graft fluorescent probes to their
surface.
Recently, magnetic Fe3O4@SiO2 silica nanospheres have
attracted much more interest because they can remove metal
ions from aqueous solution in an external magnetic field. On
the other hand, the nanoparticles are biocompatible, easily
renewable, and are stable against degradation.31,32 Meanwhile,
in recent years, the metal adsorption capacity of materials has
also attracted a great deal of attention and various materials
have been found to remove heavy metals from water. In
particular, materials that can simultaneously detect and
remove heavy metal ions in aqueous solution have been
extensively investigated.33–37
ene,13,14
naphthalimide,15
bispyrenyl,16
terphenyl,17,18
naphthylthiourea19 and dansyl20–22 have been reported. All
the small molecule ligands have been found to be good
fluorescent probes for quickly detecting Hg2+ in the field.
However, their use in related analytical techniques in the
State Key Laboratory of Applied Organic Chemistry, College of Chemistry and
Chemical Engineering, Lanzhou University, Lanzhou 730000, China.
E-mail: dongzhp@lzu.edu.cn; majiantai@lzu.edu.cn; Fax: +86-0931-8912582.;
Tel: +86-0931-8912577
Bearing this in mind, we synthesized a dansyl-based
receptor DAP and immobilized it on the surface of magnetic
Fe3O4@SiO2 silica nanoparticles as a new approach to the
development of a chemosensor for the detection and removal
of Hg2+ (Scheme 1). Moreover, we found that this kind of
Electronic supplementary information (ESI) available. See DOI: 10.1039/
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hydroxide (14 wt.%) was added rapidly to the solution, and it
immediately turned black. The reaction was kept at 90 uC for
2.5 h and then allowed to cool to room temperature. The black
precipitate was collected by and resuspended in chloroform
with an end concentration of 54.5 mg mL21 oleic acid-capped
Fe3O4. Two mg of oleic acid-capped Fe3O4 nanoparticles was
suspended in 10 mL of cyclohexane at room temperature.
Then 1.8 g of Triton X-100, 1.6 mL of hexanol, and 0.34 mL of
H2O were added with stirring to form a water-in-oil micro-
emulsion. After 15 min, 40 mL of TEOS was added to the
mixture. After 30 min, 0.1 mL of aqueous ammonia (28–30
wt%) was added to initiate the TEOS hydrolysis and
condensation. After 24 h, ethanol was added to destabilize
the microemulsion system and precipitate the Fe3O4@SiO2
nanoparticles. The Fe3O4@SiO2 nanoparticles were isolated via
centrifugation and washed with ethanol five times followed by
deionized water five times to remove any adherent surfactant
and unreacted chemicals.
Scheme 1 Synthesis of DAP and DAP-Fe3O4@SiO2.
material exhibited high performance not only for detection,
but also the removal of Hg2+ from aqueous solution.
Additionally, the materials could also be reused. The magnetic
silica nanoparticles were easily separated from solutions by
adding an external magnetic field, so the nanospheres with
Hg2+ were quickly removed. We hope that such inorganic–
organic hybrid fluorescent nanomaterials will find application
in the development of sensing and removing materials for
cations.
Preparation of DAP-Fe3O4@SiO2
Experimental
First, 100 mg of dried Fe3O4@SiO2 and 100 mg (0.22 mmol) of
DAP were suspended in 40 mL of anhydrous toluene in a 100
mL round bottomed flask under nitrogen. The mixture was
refluxed for 12 h. The functionalized material, DAP-
Fe3O4@SiO2, was collected by filtration and repeatedly washed
with anhydrous toluene, dichloromethane, and then ethanol
several times under ultrasonic conditions.
Reagents
Dansyl chloride, tetraethoxysilane (TEOS) and (3-aminopro-
pyl)triethoxysilane (APTES) were purchased from Aladdin
Chemistry Co. Ltd. Cationic compounds as their perchlorate
were purchased from Aldrich and used as received. All other
chemicals were of reagent-grade, obtained from Tianjing
Guangfu Chemical Company and used as supplied. All
solvents used for synthesis and measurements were redistilled
before use.
Characterization and testing of the materials
A Dupont-1090 thermal gravimetric analysis (TGA) instrument,
a high-resolution transmission electron microscope (HRTEM,
TecnaiTM G2 F30, FEI, USA), and vibrating sample magnet-
ometers (TOEI, VSM-5S-15) with a maximal applied field of 16
kOe were used to characterize the materials. IR spectra were
recorded on a Nicolet FT-170SX instrument using KBr discs in
the 400–4000 cm21 region. 1H NMR and 13C NMR spectra were
recorded using 400 and 100 MHz, respectively. Chemical shifts
were expressed in ppm and coupling constants (J) in Hz.
Absorption spectra were determined on a Varian UV-Cary100
spectrophotometer. Fluorescence spectra measurements were
performed on a Hitachi F-4500 spectrofluorimeter.
HEPES buffer solutions (20 mM, pH = 7.0) were prepared in
deionized water. All spectrophotometric titrations were per-
formed with a suspension of DAP-Fe3O4@SiO2 dispersed in
HEPES buffer (20 mM, pH = 7.0) containing 50% (v/v) H2O/
CH3CN. And the concentration of DAP-Fe3O4@SiO2 was 1 6
1025 M. Typical aliquots of freshly prepared metal perchlo-
rates (Hg2+, Na+, K+, Mg2+, Ca2+, Al3+, Cd2+, Zn2+, Ni2+, Co2+,
Pb2+, Fe3+ and Ag+) standard solutions (1 6 1022 to 1 6 1026
M solutions in CH3CN) were added and the fluorescence
spectra of samples were recorded.
Synthesis of 3-(dansylamino)propyltriethoxysilane (DAP)
DAP was synthesized following the reported procedure.38
A
solution of dansyl chloride (270 mg, 1 mmol) in anhydrous
CH3CN (10 mL) was added to a solution of APTES (221 mg, 1
mmol) and K2CO3 (165 mg, 1.2 mmol) in the same solvent (10
mL). The mixture was stirred at room temperature overnight.
Then the organic mixture was obtained by filtrating. The
solvent was evaporated under reduced pressure, and the crude
product was purified by flash chromatography (silica gel,
toluene/ethyl acetate = 1 : 1) to afford 386 mg (85%) of DAP as
a yellow-bright green oil. 1H NMR (CDCl3, 400 MHz): d 0.47 (t, J
= 8 Hz, 2H), 1.15 (t, J = 8 Hz, 9H), 1.52 (m, 2H), 2.90 (m, 8H),
3.71 (q, J = 8 Hz, 6H), 5.22 (s, 1H), 7.18 (d, J = 8.0 Hz, 1H), 7.51
(m, 2H), 8.23 (d, J = 8 Hz, 1H), 8.32 (d, J = 8 Hz, 1H), 8.52 (d, J =
8 Hz, 1H) ppm; 13C NMR (CDCl3, 100 MHz): d 7.39, 18.12,
22.97, 45.31, 45.50, 58.34, 115.05, 118.81, 123.08, 128.18,
129.39, 129.62, 129.86, 130.16, 135.07, 151.92 ppm; ESI: 455.5
[M+H]+.
Preparation of Fe3O4@SiO2
The Fe3O4-coated SiO2 nanoparticles were synthesized by a
similar procedure developed by Haynes et al.39 First, the
deionized water was purged with nitrogen gas for 10 min.
Then, 4.80 g of FeCl3?6H2O, 2.00 g FeCl2?4H2O, and 0.85 mL
oleic acid were added to 30 mL of deionized water under
nitrogen atmosphere with vigorous stirring. The mixture
solution was heated to 90 uC. Then, 20 mL of ammonium
Results and discussion
General Characterizations
Fig. 1 shows the XRD patterns of Fe3O4@SiO2 nanoparticles
and the functional nanocomposites. The broad peak at 20–30u
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Fig. 1 XRD patterns of (a) Fe3O4@SiO2 and (b) DAP-Fe3O4@SiO2.
Fig. 3 IR spectra of (a) DAP, (b) Fe3O4@SiO2 and (c) DAP-Fe3O4@SiO2.
of methyl and methylene groups. The weak band at 1519 cm21
(C–N stretching) confirms the incorporation of DAP onto the
surface of Fe3O4@SiO2.
in Fig. 1 corresponds to silica, which indicates that the silica is
successfully coated on the surface of the Fe3O4 nanoparticles.
The XRD patterns of Fe3O4@SiO2 nanoparticles and DAP-
Fe3O4@SiO2 nanocomposites show characteristic (311), (400),
(511) and (440) peaks. This is in good agreement with the data
for pure cubic Fe3O4, as reported in the JCPDS card (No. 88-
315, a = 8.375).
The HRTEM images of Fe3O4@SiO2 and DAP-Fe3O4@SiO2
are shown in Fig. 2. It can be observed that the nanoparticles
have clear core–shell architectures and most of them have
multiple magnetic cores. Fig. 2 also reveals that iron oxide
nanoparticles were trapped in the silica shell successfully, in
which an average particle size is about 60–70 nm and the
diameter of the Fe3O4 core is about 10 nm. Moreover, the
thickness of the silica shells is uniform and the shell surface
appears to be quite smooth.
TGA analyses are also used to determine the content of the
organic functional groups of the sample (Fig. 4). From the
weight loss curve of Fe3O4@SiO2 (Fig. 4a), the weight loss at
temperatures below 200 uC can be attributed to water
desorption from the surface of the silica layer, while weight
loss above 500 uC is associated with the release of water from
the structure. The TGA curve of DAP-Fe3O4@SiO2 (Fig. 4b) also
shows the two water loss events,41 but compared with
Fe3O4@SiO2, there is an extra weight loss of about 5%. This
is the decomposition of DAP groups grafted to the silica shell,
which is similar to the calculation result of the elemental
microanalysis (Table S1 ). On the basis of the results of TGA
3
and EA, about 0.12 mmol g21 of DAP is grafted onto the
Fe3O4@SiO2 support.
The successful conjugation of DAP onto the surface of the
Fe3O4@SiO2 nanoparticles can be confirmed by infrared (IR)
The magnetic hysteresis loop of the magnetic DAP-
Fe3O4@SiO2 measured at 300 K is shown in Fig. 5a. It
demonstrates that the nanoparticles exhibited superparamag-
netism at room temperature, since no hysteresis is observed
and both remanence and coercivity were approximately zero.42
The result suggests that this material is suitable for magnetic
separation. Complete magnetic separation of DAP-Fe3O4@SiO2
is achieved when a magnet is placed to the side of the vial
spectroscopy. Fig.
3 shows the FTIR spectra of DAP,
Fe3O4@SiO2 and DAP-Fe3O4@SiO2. From Fig. 3 (b) and (c), it
can be seen that the bands at 3400–3500 and 1000–1250 cm21
of the both samples are ascribed to the O–H and Si–O
stretching vibration. A strong peak around 1640 cm21 was
assigned to the O–H bonds in silanol groups and adsorbed
water molecules.40 Compared with the spectrum of
Fe3O4@SiO2, the peaks at 2905 cm21 found in the spectrum
of DAP-Fe3O4@SiO2 can be assigned to the stretching vibration
Fig. 4 TGA curves of (a) Fe3O4@SiO2 and (b) DAP-Fe3O4@SiO2.
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Fig. 2 HRTEM images of (a) Fe3O4@SiO2 and (b) DAP-Fe3O4@SiO2.
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Fluorescence spectral responses of of DAP-Fe3O4@SiO2
Fig. 7 shows the fluorescence excitation and emission spectra
of DAP-Fe3O4@SiO2. With an excitation wavelength of 341 nm,
the fluorescence emission peak occurs at 505 nm, located in
the visible region. This phenomenon makes the DAP-
Fe3O4@SiO2 ideal fluorescent monitors/sensors for in vivo or
on-line applications.
The titration experiments of DAP-Fe3O4@SiO2 for the
detection of Hg2+ with various concentrations in neutral
medium were studied. The results show that Hg2+ has a
particular effect on the fluorescence intensity of DAP-
Fe3O4@SiO2. From Fig. 8, it can be seen that the fluorescence
intensity of DAP-Fe3O4@SiO2 is progressively quenched as the
concentration of Hg2+ gradually increases. Upon the addition
of 50 eq. Hg2+, a strong decrease of the emission intensity can
be observed, due to the mercury complexation with the
compound DAP in these conditions, which can be tentatively
explained in terms of electron transfer from the excited dansyl
fluorophore to the complexed mercury cation.50–52 Under
current experimental conditions, the detection limit of DAP-
Fe3O4@SiO2 for Hg2+ is established at the 1028 M level, which
is about the same level as other fluorescent sensors reported
by Yang53 and Guo.54
Fig. 5 (a) Magnetization curves of DAP-Fe3O4@SiO2 and (b) photographs of an
aqueous suspension of DAP-Fe3O4@SiO2 (left) and after magnetic capture
within 5 min (right).
within
a few seconds, leaving the solution transparent
(Fig. 5b). Furthermore, the use of magnetic silica composites
as the sensor offers a promising approach for simple and
effcient separation and detection of toxic metal ions.23,34
Sensing properties of DAP-Fe3O4@SiO2
Mercury has been regarded as one of the most toxic metals and
it exists in a variety of different forms. Even at a low
concentration level its toxicity can be dangerous. Therefore,
developing a new method for tracing Hg2+ content in the
environment is very important.43,44 Dansyl and its derivatives
Tolerance of of DAP-Fe3O4@SiO2-Hg2+ over other metal ions
are well-known fluorescent probes for detecting Hg2+ 45,46
In
.
As the selectivity is also a very important factor for fluorescent
chemosensors, the fluorescence response of DAP-Fe3O4@SiO2
to various metal ions was investigated. Fig. 9 shows the
fluorescence response of DAP-Fe3O4@SiO2 to various metal
ions in HEPES buffer (20 mM, pH = 7.0) containing 50% (v/v)
H2O/CH3CN by excitation with 341 nm. The sensor shows
exclusive selectivity for Hg2+, and no obvious fluorescent
response to the other metal ions (Fig. 9, black bar). However,
the emission intensity is slightly decreased to some extent
upon the addition of Fe3+ and Ag+.
this paper, the dansyl derivative (DAP) can chelate Hg2+ via S
and N atoms. The fluorescence quenching may be attributed
to charge transfer within DAP and Hg2+ 47,48
The functionali-
.
zation of magnetic Fe3O4@SiO2 with DAP can make the sensor
easy to recycle.49
UV-vis spectral reponses of of DAP-Fe3O4@SiO2
The absorption spectra of DAP-Fe3O4@SiO2 with varying Hg2+
concentration were recorded, as shown in Fig. 6. Upon the
addition of Hg2+ to DAP-Fe3O4@SiO2, the band between 240
and 300 nm is significantly enhanced, suggesting that Hg2+
chelates with the DAP group.
The response of DAP-Fe3O4@SiO2 to Hg2+ in the presence of
other metal ions was measured to investigate the interference
effect of other metal ions on the selective ability of DAP-
Fe3O4@SiO2 to detect Hg2+ (Fig. 9, red bars). The fluorescence
changes of DAP-Fe3O4@SiO2 were measured by the treatment
of 50 equiv. Hg2+ ions in the presence of 50 equiv. of other
Fig. 6 UV-vis spectra of DAP-Fe3O4@SiO2 (1 6 1025 M) in HEPES buffer (20
mM, pH = 7.0) containing 50% (v/v) H2O/CH3CN in the presence of different
amounts of Hg2+ (0–1 equiv.).
Fig. 7 Excitation and emission spectra of DAP-Fe3O4@SiO2 (1 6 1025 M) in
HEPES buffer (20 mM, pH = 7.0) containing 50% (v/v) H2O/CH3CN.
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Fig. 10 Fluorescence spectra of DAP-Fe3O4@SiO2 (10 mM) in the presence of
Hg2+ (500 mM) and I2 (1 mM) in HEPES buffer (20 mM, pH = 7.0) containing
50% (v/v) H2O/CH3CN; lex = 500 nm.
Cu2+, Ag+) ions showed small or no interference with the
binding of DAP-Fe3O4@SiO2 with Hg2+. These facts suggested
that DAP-Fe3O4@SiO2 could recognize Hg2+ with high selectiv-
ity against other metal ions under physiological conditions.
Reproducibility and Absorptive capacities of DAP-Fe3O4@SiO2
Reversibility is an important aspect for a chemical sensor to be
widely employed in the detection of specific analyses, and so a
reversibility experiment of the sensor has been carried out. It
is well known that the iodide anion,55 TBAI18,56 and sulfidion57
have a strong ability to coordinate with Hg2+; therefore, the
iodide anion was chosen to test the reversibility of DAP-
Fe3O4@SiO2. The addition of Hg2+ to the dispersed solution of
DAP-Fe3O4@SiO2 caused fluorescence to be quenched. Upon
addition of an excess amount of I2 to the mixture of DAP-
Fe3O4@SiO2 and Hg2+, the fluorescent emission intensity of
the system was recovered to the original level, indicating that
I2 replaced the receptor R-2 to coordinate Hg2+ (Fig. 10). Thus,
the reversibility of the sensor was realizable by introduction of
iodide anion.
Fig. 8 Fluorescence emission spectra of DAP-Fe3O4@SiO2 (1 6 1025 M) in the
presence of increasing concentrations of Hg2+ (0–50 equiv.) in HEPES buffer (20
mM, pH = 7.0) containing 50% (v/v) H2O/CH3CN; lex = 341 nm.
interfering metal ions. It can be seen from Fig. 9 that the
various alkali, alkaline earth metal ions, and transition metal
(Na+, K+, Mg2+, Al3+, Ca2+, Cd2+, Ni2+, Co2+, Fe3+, Zn2+, Pb2+,
Finally, a preliminary investigation on the use of the DAP-
Fe3O4@SiO2 nanocomposite as a sorbent for the magnetically
guided removal of Hg2+ from waste water was carried out for
applicability of the materials. The DAP-Fe3O4@SiO2 (20 mg)
was added into 1 mL of H2O–CH3CN buffer solution contain-
ing Hg2+ (10 ppb). The Hg2+ loaded composite nanospheres
were then isolated from the solution by adding an external
magnetic field. Inductively coupled plasma mass spectrometry
(ICP-MS) was used to detect the amount of Hg2+ left in the
solution. Measurements suggested that less than 5% of the
original Hg2+ remained in the solution. The result indicates
that DAP-Fe3O4@SiO2 can be used as a potential sorbent in
water for Hg2+ removal and has potential environmental
applications.
Fig. 9 Fluorescence response of DAP-Fe3O4@SiO2 (1 6 1025 M) to various
cations and selectivity of DAP-Fe3O4@SiO2 for Hg2+ in the presence of other
metal ions in HEPES buffer (20 mM, pH = 7.0) containing 50% (v/v) H2O/CH3CN.
The dark bars represent the emission of DAP-Fe3O4@SiO2 in the presence of 50
equiv. of the cations. The red bars represent the change of the emission that
occurs upon the subsequent addition of 50 equiv. of Hg2+ to the solution
containing DAP-Fe3O4@SiO2 and 50 equiv. of the cation of interest; lex = 341
nm and lem = 505 nm.
Conclusion
In summary, a novel fluorescence sensor based on DAP-
Fe3O4@SiO2 has been successfully prepared. The sensor
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