A hybrid magnetic core-shell fibrous silica nanocomposite for a chemosensor-based highly effective fluorescent detection of Cu(II)

Article abstract of DOI:10.1039/c7ra08821c

Herein, a novel hybrid magnetic core-shell fibrous silica nanocomposite (RhB-Fe₃O₄/MnO₂/SiO₂/KCC-1) probe-based chemosensor was designed and its behaviour towards Cu(ii) metal ion was investigated using a fluorescence spectrometer. The organic receptor rhodamine B (RhB) fluorophore derivative was covalently grafted onto the surface of the magnetic core-shell fibrous silica nanocomposite. This sensing probe achieved the selectivity towards Cu(ii) in an aqueous solution, and other competing metal ions basically induced no spectral change. Thus, this sensing probe can work as a Cu(ii) selective fluorescent sensor. The synthesized material was characterized using transmission electron microscopy (TEM), scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), surface analysis (with BET), Fourier transform infrared spectroscopy (FT-IR), X-ray diffractometry, and thermogravimetric analysis (TGA). The rhodamine B derivative has a unique signalling probe that exhibits a turn-on fluorescence enhancement upon the recognition of Cu(ii) ion with an excitation at 569 nm. The linearity of the Stern-Volmer plot (R² = 0.9882) and the detection limit of 12.3 × 10^-8 M were achieved. Finally, the sensor was tested to detect Cu(ii) ion in different real water samples.

Full text of DOI:10.1039/c7ra08821c

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A hybrid magnetic core–shell fibrous silica
nanocomposite for a chemosensor-based highly
Cite this: RSC Adv., 2017, 7, 45824
effective fluorescent detection of Cu(II)
K. Radhakrishnan, P. Panneerselvam * and A. Ravikumar
3 4 2 2
Herein, a novel hybrid magnetic core–shell fibrous silica nanocomposite (RhB–Fe O /MnO /SiO /KCC-1)
probe-based chemosensor was designed and its behaviour towards Cu(II) metal ion was investigated using
a fluorescence spectrometer. The organic receptor rhodamine B (RhB) fluorophore derivative was
covalently grafted onto the surface of the magnetic core–shell fibrous silica nanocomposite. This
sensing probe achieved the selectivity towards Cu(II) in an aqueous solution, and other competing metal
ions basically induced no spectral change. Thus, this sensing probe can work as a Cu(II) selective
fluorescent sensor. The synthesized material was characterized using transmission electron microscopy
(TEM), scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), surface analysis (with
BET), Fourier transform infrared spectroscopy (FT-IR), X-ray diffractometry, and thermogravimetric
Received 9th August 2017
Accepted 6th September 2017
analysis (TGA). The rhodamine B derivative has a unique signalling probe that exhibits a turn-on
fluorescence enhancement upon the recognition of Cu(II) ion with an excitation at 569 nm. The linearity
of the Stern–Volmer plot (R2 ¼ 0.9882) and the detection limit of 12.3 ꢀ 10ꢁ8 M were achieved. Finally,
DOI: 10.1039/c7ra08821c
rsc.li/rsc-advances
the sensor was tested to detect Cu(II) ion in different real water samples.
signicance due to the wide distribution of copper in the
environment and its bioaccumulation through food chain.
1
. Introduction
Cu(II) is one of the most toxic heavy metals. Even at low
Therefore, the interest in developing rapid, specic, and cost-
concentrations, it is a threat to human health and environment effective tools to detect Cu(II) is steadily growing. The traditional
8
due to its high toxicity, mobility, and ability to accumulate quantitative methods to develop a Cu(II) sensor include induc-
1
through food chains or atmosphere in the ecological system.
tively coupled plasma mass spectroscopy (ICPMS), atomic
Copper(II), as the third most abundant essential trace metal ion absorption/emission spectroscopy (AAS/AES), electrochemical
9–13
in the human body, also plays an important role in various detection, and non-material-based probes.
However, these
environmental, chemical, and physiological systems. There are methods are time-consuming and require expensive instru-
many reactions in the biological process that involve the elec- mentation, complicated equipment operation etc. Thus, signif-
2
tron transfer that is catalyzed by copper-containing enzymes.
icant efforts have been dedicated towards the development of
Copper poisoning induces a wide variety of serious complica- selective and sensitive detection methods. Among them, uo-
tions, such as vomiting, lethargy, increased blood pressure, rescent and colorimetric sensors are the most convenient tech-
acute haemolytic anaemia, Meknes and Wilson diseases, niques to monitor the level of copper. The paramagnetic
9
neurotoxicity, neurodegenerative, and Alzheimer's and prion behaviour of Cu(II), with a 3d outer shell electron conguration,
3
–5
14
diseases, in humans. Additionally, recently, copper has been attributes to the quenching of the emission intensity. With
suspected to cause infant liver damage. Both Indian childhood regard to the detection of copper via a uorescent chemosensor,
cirrhosis (ICC) and non-Indian childhood cirrhosis (NICC) have many previous reports have been focused on the uorescence
been proven to be associated with an excessive intake of copper quenching process through energy or electron transfer
6
ions. The permissible limit of copper in drinking water set by processes. In term of the sensitivity, uorescence enhancement
the US Environmental Protection Agency (EPA) is 1.3 mg l (20 (turn-on) signals are much easier to detect than uorescence
mM) and that set by the World Health Organization (WHO) is quenching (turn-off) signals. The rhodamine receptor-based
ꢁ1
ꢁ
1
7
1
.0 mg l
.
Thus, detection of copper in water is of great uorescent chemosensor for the detection of metal cations has
received signicant attention in recent years by virtue of its
excellent spectroscopic properties and good water solubility. It is
an ideal moiety to construct turn-on uorescence chemosensor
in term of metal ion triggering the spirolactam ring opening
Department of Chemistry, SRM University, Kattankulathur-603 203, Chennai, India.
E-mail: panneerselvam.pe@ktr.srmuniv.ac.in; panneerchem82@gmail.com; Tel: +91
mechanism along with change in chromaticity and uorescence
9
6 88 53 88 42
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at the same time. Considering the high affinity of Cu(II) to N and toluene, dichloromethane, and ethyl acetate used in this study
O atoms, rhodamine hydrazone is an excellent platform to were obtained from Siscon research laboratories Pvt. Ltd (India)
design a novel uorescent chemosensor for specially sensing and puried by a standard procedure.
15–19
Cu(II) with high sensitivity in an aqueous solvent.
Transmission electron microscopy (TEM) analysis was per-
Mesoporous silica materials provide a good support for formed using JEOL/JEM 2100. For the sample preparation, the
uorescence probes due to their unique characteristics such as powders were dispersed in ethanol, and the suspension was

excellent solvent dispersibility, good optical transparency in the dropped onto a 200 mesh carbon-coated TEM grid. The
visible region, favourable biocompatibility, large surface area, morphologies of the as-prepared samples were investigated
open pore structure, and abundant functionalization sites in using the eld emission scanning electron microscope (FE-
20,21
the silica network.
Some silica-based mesoporous materials SEM) FEI Quanta FEG200. X-Ray diffraction (XRD) measure-
such as SBA-15, MCM-41, and MCM-48 have been reported as ments were carried out via a PAN analytical X'pert powder
2
2–25
˚
inorganic carriers for chemosensor.
However, most of these diffractometer using Cu-Ka radiation (l ¼ 0.15405 A). Fourier
optical nanoprobes have some limitations, which include the transform infrared (FT-IR) spectra were obtained using an Agi-
requirement of organic or aqueous organic solvent mixtures, lent Resolution Pro FT-IR spectrometer. The surface area was
poor performance in the removal of metal ions, and poor measured using a smart sorbs 92 surface area analyser, and N
2
26
accessibility of analytes to sensing sites inside the probe.
was used as an adsorbate. Thermogravimetric analysis (TGA,
Therefore, recently, the most promising brous silica spheres STA 2500 Regulus, NETZSCH) was performed at a heating rate
ꢂ
ꢁ1
ꢁ1
(
KCC-1) have been developed, and they exhibit excellent phys- of 10 C min , and the nitrogen ow was 10 ml min . The
ical properties such as brous surface morphology, thermal and magnetization property of the sample was measured using
mechanical stability, better sorption capacity, and ease of a vibrating sample magnetometer (Lakeshore VSM 7410).
functionalization of the organic molecule. This valuable mate- Fluorescence spectra of the sample were obtained using
rial is also used as a catalyst, drug delivery vehicle, chroma- a HORIBA JOBIN YVON Fluoromax-4 spectrouorometer with
27
tography support, and chemosensor.
a xenon lamp excitation source. The UV-vis absorption spectra
Herein, we explored a new hybrid nonmaterial that was were obtained using a Shimadzu UV-2600 spectrophotometer.
formed by graing the functionalized rhodamine B-based All pH values were obtained using a PHS-3C digital pH meter.
receptor onto the inner surface of magnetic core–shell brous
silica KCC-1. The new hybrid sensor displayed an extremely high
selectivity and sensitivity towards Cu(II) ion as compared to that
towards other metal ions. The prepared iron oxide nanoparticles
acted as a centre core, and they were then coated with manganese
2
.2. Preparation of the Fe
The Fe /MnO nanocomposite was prepared by a simple
hydrothermal process following the procedure reported in the
3 4 2 2
O /MnO /SiO nanocomposite
3
O
4
2
2
8
4
literature. In a typical preparation, 0.51 g of KMnO was dis-
solved in 35 ml of deionized water and 0.7 ml of HCl (37 wt%)
under stirring for 15 minutes. To this mixture, 0.3 g of Fe O was
added, and the mixture was continuously stirred for another
3
autoclave that was kept in an oven at 120 C for 6 h for hydro-
thermal treatment under a static condition. The required
product was obtained by applying an external magnet, washed
several times with ethanol and deionized water, and then dried
3 4
oxide over the surface of silica shell. The Fe O core was used to
saturate the magnetic behaviour, to protect the uorescence
quenching, and to avoid the aggregation of the nanocomposite.
This approach has improved the selectivity and enhancement of
3
4
0 min. Then, the mixture was transferred to a 50 ml Teon-lined
uorescence signal, as depicted in Scheme 1. Finally, the devel-
ꢂ
oped sensing system was successfully utilized for the detection of
Cu(II) ion in environmental water samples. The design strategy of
the hybrid sensor I is such that the recovery and reusability of the
sensing material can be achieved. To the best of our knowledge,
ꢂ
at 60 C. The abovementioned prepared Fe O /MnO nano-
3
4
2
this is the rst time that the detection of Cu(II) ion using an Fe
MnO magnetic nanocomposite on a brous silica core func-
tionalized with a rhodamine-based derivative has been reported.
3 4
O /
composite was dispersed in 30 ml ethanol and deionized water,
and the mixture was stirred with 0.7 ml of concentrated
ammonia solution using a magnetic stirrer. Aer this, 0.2 g of
TEOS was added dropwise, and the mixture was continuously
stirred at room temperature for 6 h. Finally, the resulting product
2
2. Experimental
Fe
3 4 2 2
O /MnO /SiO nanocomposite was separated via an external
2.1. Reagents and instruments
magnetic eld and washed several times with deionized water.
All chemicals were obtained from the commercial source and
used without further purication. Unless otherwise stated, all
chemicals were of analytical reagent grade. Rhodamine B, 5-
2
.3. Preparation of Fe
3
O
4
/MnO
2
/SiO
/MnO
2
was rst mixed with distilled water and
2
/KCC-1
chlorosalicylic aldehyde, ethylenediamine, cyclohexane, n-pen- For the facile synthesis of Fe
tanol, tetraethoxysilane (TEOS), cetylpyridinium bromide (CPB), Fe /MnO /SiO
3
O
4
2
/SiO /KCC-1, 0.3 g of
2
3
O
4
2
and 3-aminopropyltriethoxysilane (APTES) were purchased ultrasonicated for 30 min. Then, 0.3 g of urea dissolved in 20 ml
from Alfa-Aesar. Salts used in the stock solutions of metal ions of water was added dropwise to the solution and ultrasonicated
were KNO , AgNO , Ca(NO ) $4H O, Ni(NO )$6H O, Hg(NO ) - for 20 min. Subsequently, 0.5 g of CPB dissolved in 0.7 ml of n-
3
3
3 2
2
3
2
3 2
$
$
H O, Pb(NO ) , Zn(NO ) $6H O, Co(NO ) $6H O, Al(NO ) - pentanol and 30 ml of cyclohexane was added slowly under
2
3 2
3 2
2
3 2
2
3 2
9H
2
3 3 2
O, and Fe(NO ) $9H O. They were purchased from Sigma- stirring at room temperature. TEOS (1.25 g, 6.0 mmol) was
Aldrich (India). All other organic solvents such as ethanol, added dropwise to the mixture and it was stirred for 1 h. Then,
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Scheme 1 Schematic of the design and turn-on fluorescence mechanism for Cu(II) sensing.
ꢂ
the reaction mixture was kept in an autoclave at 120 C for 6 h. 100 ml of ethanol and reuxed for 8 h. Aer this, the reaction
The obtained nanocomposite was separated by a strong mixture was cooled down to room temperature, and then, the
magnet, washed sequentially with water and acetone, and then solvent was removed under vacuum. Finally, the resultant
ꢂ
dried overnight at 60 C. Finally, the prepared hybrid silica product was obtained and washed with CH
2
Cl. The organic
ꢂ
nanocomposite material was calcined at 550 C for 6 h.
layer was dried over anhydrous magnesium sulphate.
2
.4. Preparation of RhB–NH
2
2.5. Preparation of RhB-5-chlorosalicylaldehyde–APTES
The preparation of RhB–NH was carried out using the previ- Typically, 0.46 g of RhB–NH was dissolved in 30 ml of ethanol
2
2
29
ously reported method. First, rhodamine B (3.82 g, 8 mmol) and 4 mmol of 5-chlorosalicyladehyde was added dropwise to
and ethane-1,2-diamine (2.6 ml, 40 mmol) were dissolved in the solution. The mixture was reuxed for 8 h and allowed to
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cool down to room temperature. To the mixture, 10 ml of ethyl found that Fe
3
O
4
provides an effortless magnetic separation
acetate was added, and then, the mixture was kept in a refrig- and increases the durability of the material. The modied
erator overnight. The material was washed with distilled water deposited MnO nanoparticles have a high acid etching resis-
2
ꢂ
followed by ethanol and dried at 65 C. Aer the resulting tance, are stable in organic solvents such as methanol, and also
30,31
material RhB-5-chlorosalicylaldehyde (0.270 g, 1.0 mmol) was closely interact with Fe
obtained and reacted with (0.241 g, 0.1 mmol) APTES, K CO core–shell Fe /MnO in water, a multistep strategy is engaged
0.138 g, 1.0 mmol) in 30 ml acetonitrile was slowly added to the to deposit dense silica on the surface. SiO provides stability to
3
O
4
.
To avoid the solubility of the
2
3
3
O
4
2
(
2
solution mixture that was then reuxed for 8 h. Finally, the the inner core, and its uniform surface supports the growth of
product was puried by column chromatography with ethyl brous silica. Very importantly, the thickness of the fabricated
acetate : hexane solvent.
silica prevents the uorescence energy transfer process (FRET)
32
between the magnetic core–shell and the uorophore. Finally,
the nanocomposite (Fe /MnO /SiO ) is modied with brous
2.6. Synthesis of RhB–Fe
3
O
4
/MnO
2
/SiO
2
/KCC-1
3
O
4
2
2
silica on the outer surface. The brous mesoporous silica
surface could be easily functionalized with a silane reagent
In a typical synthesis, 200 mg of Fe O /MnO /SiO /KCC-1 was
3
4
2
2
dissolved in 40 ml of toluene, and 200 mg of RhB–Si was added.
The mixture was stirred for 12 h under a nitrogen atmosphere.
The resulting product was separated by a strong magnet and
washed several times with 30 ml of different solvents, including
toluene, dichloromethane, and ethanol, each time. The puried
(APTES), on the surface moieties, such that sensing molecules
of rhodamine B derivatives are uniformly covered on the silica
surface. The functionalized modied nanocomposite is a suit-
able solid base material with a better dispersibility. The
importance of this hybrid sensor I is that it has electron rich
species, such as carbonyl (O), imino (N), and phenol (O) atoms,
which effectively and selectively binding with Cu(II) over other
metal ions. The spirolactam ring-opening upon binding with
cation to enhance the uorescence is a special feature of the
hybrid sensor I to create a barrier for quenching and improving
the sensing performance of Cu(II).
ꢂ
product was dried at 60 C and then used for further analysis.
3. Results and discussion
3.1. Design and analysis of the hybrid sensor
The synthesis of rhodamine-based receptor and functionalized
mesoporous magnetic core–shell brous silica KCC-1 involves
several steps, as outlined in Scheme 1. The main aim of this
study was to identify the ability of the hybrid sensor I towards
the detection of Cu(II) ion over other metal ions and the recovery
3.2. Characterization
of this sensor. To achieve an effective sensing system, the The morphology of the hybrid magnetic core–shell brous silica
design of multicore–shell is architectured in a way that para- nanocomposite was observed by SEM and TEM, as shown in
magnetic microspheres of iron oxide core (Fe O ) are deposited Fig. 1a–f. The Fe O nanoparticles exhibited a spherical shape
3
4
3 4
with nanosized manganese oxide (MnO
and dipole interaction between magnetic nanoparticles. It is amorphous MnO
2
) to prevent aggregation with a diameter of about 480 nm (Fig. 1a). Aer coating by
, the average diameter of Fe /MnO
2
2
3
O
4
Fig. 1 SEM images of (a) Fe
3
O
3 4 2 3 4 2 2 3 4 2 2
4, (b) Fe O /MnO , and (c) Fe O /MnO /SiO /KCC-1. TEM images of (d) KCC-1 and (e) Fe O /MnO /SiO /KCC-1.
(
f) Elemental analysis of the Fe
3
O
4
/MnO
2
/SiO /KCC-1 nanocomposite.
2
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nanocomposite increased to 620 nm. This clearly indicates that
uniform and well-dispersed spherical structures were formed
without aggregation (Fig. 1b). Aer this, successful coating of
magnetic nanocomposite with brous silica was clearly
observed in Fig. 1c. Furthermore, in the TEM images (Fig. 1d
and e), the brous structure of the parent KCC-1 clearly shows
the unique brous surface with a spherical shape (Fig. 1d). As
shown in Fig. 1e, the inner surface clearly shows the core shell
3 4 2 2
modication (Fe O /MnO /SiO /KCC-1) compared to that of
parent KCC-1. In Fig. 1e, the black inner surface in the core–
shell is represented by Fe O /MnO and gray-coloured outer
3
4
2
surface is represented by brous silica that is covered all over
the surface. The EDS elemental analysis of brous silica Fe
MnO /SiO /KCC-1 exhibited the presence of Fe, Mn, Si, and O
Fig. 1f), further indicating the formation of the hybrid core–
3 4
O /
2
2
(
shell magnetic brous silica nanocomposite.
The X-ray diffraction (XRD) pattern was obtained to investi-
gate the formation of Fe O /MnO and Fe O /MnO /SiO /KCC-
3
4
2
3
4
2
2
Fig. 3 FT-IR spectra of (a) Fe
Fe /MnO /SiO /KCC-1.
3
O
4
/MnO
2
/SiO
2
/KCC-1 and (b) RhB–
1
. As shown in Fig. 2a, the characteristic diffraction peaks of
O
3 4
2
2
ꢂ
ꢂ
ꢂ
the magnetic nanocomposite located at 30.17 , 35.51 , 43.10 ,
ꢂ
ꢂ
ꢂ
5
(
3.75 , 57.32 , and 62.89 can be attributed to the (220), (311),
400), (422), (511), and (440) crystal planes, respectively, which
represent the inverse spinal structure of Fe nanoparticles
Moreover, the well-resolved
ꢁ1
2
962.5 cm can be assigned to the stretching vibration of
3 4
O
36,37
methylene and amide group,
B derivatives have been functionalized with hybrid core–shell
magnetic brous silica nanocomposite.
Nitrogen adsorption/desorption isotherm reveals the surface
modication for the hybrid sensor I. Upon investigation, it was
observed that the magnetic nanocomposite and rhodamine-
functionalized nanocomposite showed a modied textural
which conrm that rhodamine
33
(JCPDS card no. 19-0629).
ꢂ
ꢂ
diffraction peaks at around 28.6 and 37.3 indicated the pres-
ence of MnO2. As displayed in Fig. 2b, the broad diffraction
peak at around 22.48 indicated the growth of brous silica on
the surface of Fe O /MnO nanocomposite.
34
ꢂ
35
3
4
2
FT-IR analysis has been performed on Fe
3 4 2 2
O /MnO /SiO /
KCC-1 and the hybrid sensor I. As shown in Fig. 3a, the
property and surface area, as shown in Fig. 4. For the Fe O /
ꢁ
1
3 4
absorption band at 563.2 cm is assigned to the Fe–O–Fe
MnO /SiO /KCC-1 and RhB–Fe O /MnO /SiO /KCC-1 nano-
2
2
3
4
2
2
stretching and bending mode, and no characteristic peaks for
2
ꢁ1
composite, the BET surface area was 225.08 and 69.25 m g
,
the amorphous MnO
2
are observed. The vibration peaks of
ꢁ
1
ꢁ1
the pore diameter was 7.96 and 6.74 nm, and the pore volume

brous mesoporous silica at 467.4 cm , 809.3 cm , and
ꢁ1
ꢁ
1
was 0.56 and 0.13 cm g , respectively. These results describe
the effective functionalization of the organic probe on the
surface of the brous nanocomposite.
1092.5 cm are assigned to the symmetric and asymmetric
stretching of the Si–O–Si bonds. The strong absorption peak at
ꢁ
1
3
426.5 cm is assigned to the hydroxyl groups. As shown in
ꢁ
1
ꢁ1
Fig. 3b, new bands at 2856.4 cm , 2925.9 cm , and
Fig. 2 X-ray diffraction patterns of (a) Fe
MnO /SiO /KCC-1.
3
O
4
/MnO
2
and (b) Fe O
3 4
/
Fig. 4 The nitrogen adsorption–desorption isotherms of (a) Fe
MnO /SiO /KCC-1 and (b) RhB–Fe /MnO /SiO /KCC-1.
3 4
O /
2
2
2
2
3
O
4
2
2
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ꢂ
ꢁ1
a heating rate of 10 C min under a nitrogen atmosphere. As
shown in Fig. 6, the curve exhibits three-step weight loss over
ꢂ
ꢂ
the temperature ranges of 50–165 C, 166–367 C, and 368–
6
ꢂ
15 C. The rst weight loss of 8.17% was predominantly due to
the removal of physically absorbed water and organic solvent.
The second weight loss of 15.93% was attributed to the thermal
decomposition of functionalized organic compounds. The nal
weight loss of 20.23% was due to the complete decomposition
of the whole nanocomposite. Total weight loss was about
ꢂ
4
4.86% in the temperature range from 50 to 615 C. The TGA
results of the hybrid sensor I demonstrate that it has a superior
thermal stability at high temperatures.
3
.3. Sensing performance
.3.1. Absorption response for the detection of Cu(II) ion.
To optimize the sensing ability of the hybrid sensor I, CH CN/
3
Fig. 5 Magnetization curves of (a) Fe
3
O
4
, (b) Fe /MnO2, (c) Fe
3
O
4
3 4
O /
MnO
2
/SiO
2
/KCC-1, and (d) RhB–Fe
3
O
4
/MnO /SiO /KCC-1. The inset
2
2
3
image shows the magnetic separation behaviour of the hybrid sensor I. water solution (1 : 1, v/v) was preferred for UV-vis detection
process. Aer the addition of various metal ions (K(I), Ag(I),
Ca(II), Ni(II), Hg(II), Pb(II), Zn(II), Co(II), Ba(II), Fe(II), and Cd(II)) in
As shown in Fig. 5a–d, the magnetic measurements of pris- the form of their nitrates, only copper ion showed a higher
tine Fe
hybrid sensor I were 56.09, 32.6, 13.0, and 6.9 emu g
respectively. All samples exhibited super paramagnetic behav- in the solution in the range of 0–5 ꢀ 10 M, the hybrid sensor I
iour at room temperature. The magnetic saturation value turned from colourless to pink with a signicant enhancement
decreased from Fig. 5a to d due to the increase in the diameter of a new absorbance peak centred at 556 nm, and a smaller peak
3 4 3 4 2 3 4 2 2
O , Fe O /MnO , Fe O /MnO /SiO
/KCC-1, and the response than other metal ions such as Fe(II) and Co(II), as
ꢁ1
,
shown in Fig. 7c. Upon increasing the concentration of Cu(II) ion
ꢁ4
3 4
of the outer surface of Fe O
nanoparticles and the presence of at 522 nm was obtained. This proves the selectivity of Cu(II) ion
the organic molecule. As a result, the functionalized core–shell towards the binding site. The signicant appearance of colour
nanocomposite can rapidly redisperse and prevent the aggre- and the enhancement of absorbance peak indicate the ring-
38
gation of nanoparticles. It can be observed that the hybrid opened amide form of the probe. These results indicate that
sensor I can be homogeneously dispersed in different solvents the hybrid sensor I can be used for the rapid determination of
and rapidly separated within 1 min by an external magnet, as Cu(II) by the naked eye. The titration for Cu(II) concentration
2
illustrated in the inset of Fig. 5. In addition, the saturated variation showed a better linear relationship (R ¼ 0.9838), as
magnetic behaviour of the hybrid sensor I acted as an effective shown in Fig. 7b. The detection limit was found to be 26.40 nM.
barrier to the quenching uorophore.
The data obtained from the Stern–Volmer plot suggested that
The stability of the hybrid sensor I was determined by TGA the synthesized material had a good selectivity for Cu(II) ion.
ꢂ
measurements in the temperature range from 50 to 615 C at
3.3.2. Emission response for the detection of Cu(II). The

uorescence spectra of the hybrid sensor I have been used to
recognize the sensing behaviour and the molecular interaction
towards Cu(II). To identify the sensitivity of the process, inter-
related parameters in the sensing ability were analyzed. The
ꢁ1

uorescence titration of the hybrid sensor I (0.1 mg ml ) in
CH CN/water (1 : 1, v/v) system was carried out with Cu(II) in the
3
ꢁ
4
concentration range of 0–5 ꢀ 10 M. As shown in Fig. 8a, the
emission band of the hybrid sensor I (5 mM) exhibited a weak

uorescence at 569 nm (lex 520 nm), which was attributed to the
close ring spirolactam form of the probe with a photo induced
electron-transfer (PET) process that was induced by nitrogen
atoms bonded to a hydrogen atom. The uorescence emission
intensity was enhanced around 569 nm with the increase in
Cu(II) ion concentration. The rhodamine derivative coordinated
with Cu(II); this led to the spirolactam ring-untie mecha-
39,40
nism,
which was associated with a switch on UV-vis spectral
response at 556 nm and the emission response at 569 nm.
The Stern–Volmer plot, which expresses the linear relation-
0 0
ship between F /F values (where F and F indicate the presence
Fig. 6 TGA data of RhB–Fe
O
3 4
/MnO
2
/SiO
2
/KCC-1.
and absence of Cu(II) ion at 569 nm) and the Cu(II)
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ꢁ
1
Fig. 7 (a) Absorption spectra of RhB–Fe
5
3
O
4
/MnO
2
/SiO
2
/KCC-1 (0.4 mg ml ) in CH
3
CN/H
2
O (1 : 1, v/v) and different concentrations of Cu(II) (0–
ꢁ4
ꢀ 10 M). (b) The Stern–Volmer plot between various concentrations of Cu(II) ion and F
0 3 4 2 2
/F values. (c) Image of RhB–Fe O /MnO /SiO /KCC-1
with Cu(II) and different metal ions (K(I), Ag(I), Ca(II), Ni(II), Hg(II), Pb(II), Zn(II), Co(II), Al(II), and Fe(II)).
ꢁ1
Fig. 8 (a) Fluorescence spectra of RhB–Fe
10 mM, pH ¼ 7.20). (b) The Stern–Volmer plot between various concentrations of Cu(II) ion and F
deviations of the results from three independent experiments.
3
O
4
/MnO
2
/SiO
2
-KCC-1 (0.4 mg ml ) with different concentrations Cu(II) ion in the Tris–HCl solution
(
0
/F values. Error bars represents the standard
ꢁ4
concentration in the range of 0–5 ꢀ 10 M with the correlation metal ions including K(I), Ag(I), Ca(II), Ni(II), Hg(II), Pb(II), Zn(II),
2
coefficient R ¼ 0.9882 is shown in Fig. 8b. The detection limit Co(II), Ba(II), Fe(II), and Cd(II) via both UV-visible absorption and
was calculated by the 3s/k method (where s is the standard uorescence emission spectra under identical condition as
deviation of the blank measurement and k is the slope) to be used with Cu(II) (Fig. 9a). Compared to the remarkable
ꢁ8
about 12.3 ꢀ 10 M, which was comparable to that of other enhancement of absorption intensity in the uorescent spectra
methods (Table 1). Moreover, the linear relationship data tting of the hybrid sensor I induced by Cu(II), no signicant change
41
with Stern–Volmer plot expressed the 1 : 1 complex formation. was caused by other metal ions. Consistent with the absorp-
.3.3. Selectivity and coexisting ion effect on the hybrid tion enhancement, the hybrid sensor I system obviously
3
sensor I. The selectivity of the hybrid sensor I for Cu(II) ion was changed from colourless to pink aer the addition of Cu(II).
examined by testing the response of the system to various other This result clearly indicates that Cu(II) is one of the well-known
45830 | RSC Adv., 2017, 7, 45824–45833
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Table 1 Comparison between the proposed method and other methods for the detection of Cu(II)
Materials
Detection methods
Working range (M)
LOD
Ref. no.
ꢁ
ꢁ
6
ꢁ6
ꢁ7
ꢁ6
SBA-15
SBA-15
SBA-15
Colorimetric sensor
Electrochemical sensor
Fluorescence
0.01–1.0 ꢀ 10
8.0–100.0 ꢀ 10
0.015 ꢀ 10
2.0 ꢀ 10
0.1 ꢀ 10
43
44
45
7
ꢁ6
0–50 ꢀ 10
chemosensor
1.57 ꢀ 10 to 7.86 ꢀ 10ꢁ6
ꢁ8
3.1 ꢀ 10
2.13 ꢀ 10
3.2 ꢀ 10
ꢁ8
ꢁ6
46
47
48
HOM-11
Silica nanoparticles
SBA-15
Colorimetric sensor
Colorimetric sensor
Optical sensor
Fluorescence
chemosensor
ꢁ
4
0–6.0 ꢀ 10
ꢁ6
—
ꢁ
4
ꢁ8
KCC-1
0–5 ꢀ 10
12.3 ꢀ 10
Present wok
ꢁ
1
Fig. 9 Fluorescence spectra of (a) RhB–Fe
ions (5.0 ꢀ 10 M), (b) the fluorescence responses of RhB–Fe
3
O
4
/MnO
2
/SiO
2
/KCC-1 (0.4 mg ml ) in the Tris–HCl solution (10 mM, pH ¼ 7.2) with different metal
ꢁ4
ꢁ1
O
3 4
2
/MnO /SiO
2
/KCC-1 (0.4 mg ml ) various cations in water (pH ¼ 7.2). The blue
ꢁ4
bars represent the emission spectra of various interfering ions (5.0 ꢀ 10 M). The green bars represent the change in emission spectra of upon
ꢁ
4
the addition of Cu(II) (4.0 ꢀ 10 M). The intensities were recorded at lex ¼ 520 nm and lem ¼ 569 nm.
paramagnetic metal ions. This hybrid sensor I displayed the
chelation-enhanced uorescence (CEFE) that was attributed to
the spiro-opening of the bis rhodamine Schiff-base derivative
aer the complexation with Cu(II) ion, which was induced by the
intermolecular electron transfer from the rhodamine group to
42
cationic metal ion. To further explore the selectivity of the
hybrid sensor I towards Cu(II), a competition experiment was
conducted via the additional mixing of Cu(II) with a solution
containing other metal ions (Fig. 9b). The bar graph of the

uorescence emission intensity revealed that Cu(II) uorescence
enhancement of the hybrid sensor I almost remained unaf-
fected in the presence of other metal ions at higher concen-
trations. Hence, the abovementioned results of the optical
experiments indicated that the hybrid sensor I had a high
sensitivity and selectivity towards Cu(II) with no interference by
other metal ions even at higher concentrations. Thus, our result
reveals that the hybrid sensor I is a selective chemosensor for
the detection of Cu(II) ion.
Fig. 10 The fluorescence response of RhB–Fe
3 4 2 2
O /MnO /SiO /KCC-1
in CH CN/H O as a function of pH.
3
2
3
.3.4. Inuence of pH values. Usually, the pH values of the
intensity of the hybrid sensor I increases with the increasing pH
value because Cu(II) has a high affinity to chelate with uoro-
phoric carbonyl (O), imino (N), and phenol (O) atoms to form
a ring-opened amide form to enhance the uorescence inten-
sity. When the pH values changed from 5 to 8, the dissociation
reaction of amino groups played a key role on the surface of the
solution have a remarkable inuence on the uorescence probe
for the detection of target analytes. Thus, the Cu(II) sensing
ability of the hybrid sensor I at different pH values was inves-
tigated. The pH value was adjusted in the range from 2 to 10,
and the uorescence emission intensities were measured, as
shown in Fig. 10. We have observed that the uorescence
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hybrid sensor I. Thus, the strong uorescence response was obvious change in the uorescence emission. These results
observed when pH < 5 because the deprotonated imine, demonstrated that the hybrid sensor I had a superiority of
carbonyl, and phenol groups were protonated, and the resulting recycling performance for the detection of Cu(II) ion.
ligands were separated from the surface of the hybrid sensor I.
3.3.6. Analysis of the environmental water sample. To
The results indicate a weak emission response in an acidic identify the performance of the abovementioned chemosensor,
medium that can be attributed to the poor binding response of we attempted to detect the concentration of Cu(II) in the envi-
Cu(II) because the sensing probes tend to protonate. At a higher ronmental water samples. The water samples (including tap
pH in the range from 5 to 8, the amino groups tend to deprot- water, pond water, and industrial waste water) obtained near
onate; this create a feasible environment for the detection of the SRM University campus were ltered to remove the solid
Cu(II) ion. Taking this type of pH inuence behaviour into impurity, and pH was adjusted to 7.2 using the Tris–HCl buffer
account, the practical sensing in a CH CN/water solution (1 : 1, for effective analysis in a neutral medium. The desired amount
3
v/v) with a buffer solution is highly accurate and reliable.
.3.5. Recycling of the hybrid sensor I. The recycling followed by the addition of the hybrid sensor I to each sample.
performance of the hybrid sensor I aer the addition of Cu(II) The obtained results are summarized in Table 2. They prove
of Cu(II) ion was spiked (2, 4, 6, and 8 mM) into water samples
3
ꢁ
4
(
5.0 ꢀ 10 M) ions was investigated. As shown in Fig. 11A–D, that this sensing system is able to determine Cu(II) in environ-
the absence of Cu(II) ion in the hybrid sensor I caused no change mental water samples.
in the uorescence enhancement (bar graph A). Aer the
addition of Cu(II) ion to the abovementioned sensing system,
4
. Conclusion
the uorescence intensity dramatically increased (bar graph B)
ꢁ
4
and fully recovered upon the addition of EDTA (5.0 ꢀ 10 M)
We have successfully prepared a hybrid magnetic core–shell
brous silica nanocomposite and covalently functionalized it
(bar graph C). When Cu(II) was added in excess, the uorescence

emission intensity reached the saturation level again (bar graph
D). Further subsequent addition of Cu(II) ion showed no
with a rhodamine-based derivative. The rhodamine-based
derivative was modied with nitrogen and oxygen atoms,
which strongly bonded to Cu(II) ion, playing a critical role in the
selective detection of Cu(II) ion. This hybrid sensor exhibited
signicantly improved sensing ability towards Cu(II) ion with
high sensitivity and selective uorescence responses in an
aqueous solution. The detection limit was found to be 12.3 ꢀ
ꢁ8
10
M, which was superior to that of the previously reported
Cu(II) detection methods. In addition, this hybrid sensing
system recovered 95% original uorescence sensing signal for
the detection of Cu(II) ion. The abovementioned results
demonstrated the successful use of this nanocomposite for the
detection of Cu(II) in the environmental real samples. Thus, this
simple and reliable hybrid sensor can be used for Cu(II) ion
detection in aqueous solutions.
Conflicts of interest
There are no conicts to declare.
Fig. 11 Fluorescence spectra of RhB–Fe
3
O
4
/MnO
0.4 mg ml ) in the Tris–HCl solution (10 mM, pH ¼ 7.2). In bar graph
A) RhB–Fe /MnO /SiO /KCC-1, (B) RhB–Fe /MnO /SiO /KCC-1
Cu(II) (5.0 ꢀ 10 ), (C) RhB–Fe /MnO /SiO /KCC-1 + Cu(II) (5.0 ꢀ
) + EDTA (5.0 ꢀ 10 M), and (D) RhB–Fe /MnO /SiO /KCC-1 +
2 2
/SiO /KCC-1
ꢁ1
(
(
+
3
O
4
2
2
3
O
4
2
2
ꢁ4
Acknowledgements
3
O
4
2
2
ꢁ4
ꢁ4
1
0
3
O
4
2
2
ꢁ
4
This work was nancially supported by the SRM University,
Department of Chemistry, Tamil Nadu, India.
Cu(II) (10 ꢀ 10 ) + EDTA.
Table 2 Determination of Cu(II) ion in the environmental water
samples
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