Conjugation-induced fluorescence labelling of mesoporous silica nanoparticles for the sensitive and selective detection of copper ions in aqueous solution

Article abstract of DOI:10.1039/c4nj01428f

Although the development of sensors for the detection of Cu²⁺ ions has significant importance, sensitive and selective recognition systems are still limited. Herein, the non-fluorescent small molecule of 2-bromomaleimide (BM) was firstly converted to 2-aminomaleimide (NM) by a simple nucleophilic addition-elimination reaction with pre-grafted amino groups in the meso-space of mesoporous silica nanoparticles (MSNs), and then was designed as fluorescent on-off probes for Cu²⁺ ion recognition. The strong green fluorescence emitted from the pendent NM was induced by the interaction between the lone electron pairs of the N atoms in amino groups and the conjugated maleimide rings. The fluorescence of MSNs labelled NM (MSNs-NM) could be selectively quenched by Cu²⁺ ions in aqueous solution and consequently exhibited sensitive fluorescence on-off switching effects for Cu²⁺ ions. The detection limit was as low as 0.28 μM Cu²⁺, which was far below the U.S. EPA limit of 20 μM. This prefigured the potential applications of the current chemosensor for the detection of Cu²⁺ ions in aqueous solution and in living cells. The small-angle XRD, TEM, BET, FT-IR, UV-vis absorption measurements all agreed on the fact that the sensitive probes of BM had been successfully labelled at the pore surface of MSNs.

Full text of DOI:10.1039/c4nj01428f

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Conjugation-induced fluorescence labelling of
mesoporous silica nanoparticles for the sensitive
and selective detection of copper ions in aqueous
solution†
Cite this:DOI: 10.1039/c4nj01428f
Jingjing Cui, Shangfeng Wang, Kai Huang, Yongsheng Li, Wenru Zhao, Jianlin Shi
and Jinlou Gu*
2+
Although the development of sensors for the detection of Cu ions has significant importance, sensitive
and selective recognition systems are still limited. Herein, the non-fluorescent small molecule of
2-bromomaleimide (BM) was firstly converted to 2-aminomaleimide (NM) by a simple nucleophilic
addition–elimination reaction with pre-grafted amino groups in the meso-space of mesoporous silica
2+
nanoparticles (MSNs), and then was designed as fluorescent ‘‘on–off’’ probes for Cu ion recognition. The
strong green fluorescence emitted from the pendent NM was induced by the interaction between the lone
electron pairs of the N atoms in amino groups and the conjugated maleimide rings. The fluorescence of
2+
MSNs labelled NM (MSNs-NM) could be selectively quenched by Cu ions in aqueous solution and
Received (in Montpellier, France)
2+
2
4th August 2014,
consequently exhibited sensitive fluorescence ‘‘on–off’’ switching effects for Cu ions. The detection limit
2+
Accepted 16th September 2014
was as low as 0.28 mM Cu , which was far below the U.S. EPA limit of 20 mM. This prefigured the potential
2+
applications of the current chemosensor for the detection of Cu ions in aqueous solution and in living
cells. The small-angle XRD, TEM, BET, FT-IR, UV-vis absorption measurements all agreed on the fact that
the sensitive probes of BM had been successfully labelled at the pore surface of MSNs.
DOI: 10.1039/c4nj01428f
www.rsc.org/njc
their recycling. In recent years, many efforts have been devoted
to the design of organic–inorganic hybrid heterogeneous materials
The copper ion is a significant environmental pollutant and for ion recognition and sensing.
As one of the most attractive organic–inorganic hybrid
excess quantities of Cu can be toxic to biological systems, and nanomaterials, mesoporous silica nanoparticles (MSNs) appear
cause a series of severe diseases such as Alzheimer’s and to be good sensing materials in the heterogeneous solid–liquid
1
. Introduction
26–30
1
,2
also plays a critical role in various biological processes. The
2
+
3
,4
Wilson’s diseases.
for the detection of Cu ions is of great significance.
Therefore, the development of sensors phase due to their large surface area and homogeneous porosity,
2
+
5–9
31–35
easy-preparation and functionalization.
Encapsulation of
Recently, the fluorescence technique for detecting heavy metal organic fluorophores in MSNs as optical sensitive probes has
ions has attracted wide attention due to its exquisite sensitivity attracted a great deal of attention because it features with
3
6–46
and selectivity, cost-effectiveness, and superb spatial and selective, sensitive and rapid response properties.
temporal resolutions.
However,
1
0,11
So far, numerous fluorescent chemo- the fluorescent MSNs previously reported commonly have
sensors for heavy metal ions have been developed, which are several disadvantages such as complicated fluorophore synthesis
mainly based on small molecular fluorophores, polymers and procedures and harsh fluorophore grafting conditions.
1
2–25
inorganic nanoparticles.
Among them, most of the sensors
We serendipitously noticed that a simple small molecule of
are utilized in homogeneous detection, which critically affects 2-bromomaleimide (BM) had been previously reported as a novel
47,48
protein label.
Although the free BM molecule is non-fluorescent,
it was discovered by K. O’Reilly that the strong fluorescence of
dithiolmaleimide (DTM) could be resulted from the interaction
between the lone electron pairs of the S atoms in a protein molecule
Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials
Science and Engineering, East China University of Science and Technology,
Shanghai 200237, China. E-mail: jinlougu@ecust.edu.cn; Fax: +86-21-64250740;
Tel: +86-21-64252599
49
and the conjugated maleimide rings. This strongly inspired us to
†
Electronic supplementary information (ESI) available: NMR spectra of BM, TEM
explore whether this non-fluorescent small molecule would also
become fluorescent upon its introduction into the mesochannels of
MSNs if the nucleophilic groups were pre-grafted for its conjugation.
images, the particle size distribution histogram, UV-vis spectra, TGA curves,
fluorescence measurements, fluorescence response curves and time titration
profile. See DOI: 10.1039/c4nj01428f
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with cold chloroform for several times to afford crude 2,3-
dibromosuccinimide (5.0 g, 19 mmol). The crude product was
dissolved in THF (50 mL), and then 10 mL of THF with TEA
(
3.0 mL, 21 mmol) dissolved was added dropwise into the above
Scheme 1 Conversion of the small molecule of BM to NM by a nucleophilic
addition–elimination reaction with the grafted amino groups within the
mesochannels of MSNs under mild reaction conditions.
solution at 0 1C. The resultant mixture was stirred at room
temperature for 2 days. The solution was filtered, the obtained
filtrate was concentrated and further purified by silica gel
column chromatography using EtOAc/petroleum (1 : 5, v/v) as
the eluent. After drying in a vacuum oven at room temperature,
a pale yellow solid of BM was achieved (yield: 2.6 g, 76%).
1
H NMR (400 MHz, CDCl , Fig. S1, ESI†): d = 7.61 (br s, 1H, NH),
3
d = 6.88 (s, 1H, CQCH).
Mesoporous silica nanoparticles (MSNs). The MSNs were
5
1
synthesized based on our previous report. As a typical proce-
dure, a given amount of F127 and DMHA were mixed with the
structure directing agent of cetyltrimethylammonium bromide
(CTAB) in a beaker, and then distilled water was poured into
the mixture. Subsequently, sodium hydroxide (2 M, 3.5 mL)
solution was added into the above mixture under stirring. After
a clear and homogeneous solution was formed under constant
stirring at 80 1C, TEOS (5.0 mL, 22 mmol) was added dropwise
to the above reaction mixture. The temperature was maintained
at 80 1C for 2 h. Then the resulting white colloidal suspension
was cooled to room temperature spontaneously. The white
colloid was separated by centrifugation at 10 000 rpm for 15 min
and washed with ethanol. The molar composition of the reaction
mixture was 8.15 : 0.216 : 0.00289 : 1 : 2.55 : 9700 TEOS : DMHA :
Scheme 2 Schematic presentation of the proposed synthetic routes for
BM conjugation into the pore channels of MSNs, and the illustration of the
fluorescence emission and quenching mechanisms.
In the current work, the small molecule of BM was converted
to 2-aminomaleimide (NM) by a simple nucleophilic addition–
elimination reaction with pre-grafted amino groups under mild
4
7,50
reaction conditions (Scheme 1).
As expected, the strong
green fluorescence emitted from the pendent NM was induced
by this conjugation although free BM was non-fluorescent.
Since the metallic ions could coordinate with the imine groups
and consequently mediate their electron donor ability, the
fluorescence intensity of BM would be facilely tuned depending
on the different affinities between the metal ions and nucleo-
F127 : CTAB : NaOH : H O. The surfactant of CTAB was extracted
2
by suspending 1.0 g of the resultant MSNs in 100 mL of ethanol
with a little concentrated HCl (12 M, 1.0 mL) added, and the
mixture was refluxed for 24 h at 80 1C.
Amino group functionalized mesoporous silica nano-
2
particles (MSNs-NH ). The procedure for the modification of
1
9,24
philic groups (Scheme 2).
We found that the obtained
chemosensor exhibited sensitive fluorescence ‘‘on–off’’ switching
the MSNs with amino groups was the same as our reported
2+
effects for Cu via grafting BM onto ordered MSNs. The fluores-
52
work. Typically, the synthesized MSNs (1.0 g) were suspended
cence of MSNs labelled with BM (MSNs-NM) could be selectively
in dry toluene (100.0 mL), and aminopropyltriethoxysilane
2+
quenched by Cu ions in an aqueous solution at pH 7.4, and the
(
3-APTES, 1.2 mL) was subsequently administered. The mixture
was stirred at 80 1C for 12 h under a N atmosphere. The
resultant white solid of aminopropyl-functionalized MSNs
MSNs-NH ) was centrifuged, and washed repeatedly with
ethanol. The product was finally dried at 60 1C overnight.
-Aminomaleimide moiety functionalized mesoporous silica
nanoparticles (MSNs-NM). In a round-bottomed flask, MSNs-NH2
150.0 mg) was suspended in 20 mL of methanol. To this
detection limit was 0.28 mM, which prefigured their potential
2
2
+
application for the detection of Cu ions in aqueous solution
and in living cells.
(
2
2
2. Experimental
2.1 Materials
(
Maleimide, aminopropyltriethoxysilane (3-APTES) and N,N- suspension, BM (80.0 mg, 0.45 mmol) and TEA (75.0 mL,
dimethylhexadecylamine (DMHA) were purchased from Sigma- 0.54 mmol) were added. The mixture was stirred at room
Aldrich and used as received. All other reagents and chemicals temperature overnight. After centrifugation, the obtained
including metals salts were of analytical grade and used as yellow solid (MSNs-NM) was washed repeatedly with methanol
purchased from Sinopharm Chemical Reagent Co., Ltd without and H O. After drying at 60 1C overnight, the final yellow
2
further purification.
powder was obtained.
2.2 Synthesis
2.3 Fluorescence and UV-vis studies for metal ion detection
ꢀ
1
2
-Bromomaleimide (BM). To a solution of maleimide (2.0 g, The suspension of MSNs-NM (30.0 mg L ) was prepared in a
20 mmol) in CHCl
3
was added dropwise a solution of bromine 50 mM HEPES buffer solution at pH 7.4. The cation stock
over 1 h. The reaction mixture was solutions were prepared by dissolving the metal salts (NaCl,
refluxed for 4 h, and then filtered. The yellow solid was washed KCl, CaCl , MgSO , Cu(NO , Co(NO , Cd(NO , Mn(NO
(1.2 mL, 20 mmol) in CHCl
3
2
4
3
)
2
3
)
2
3
)
2
3
)
2
,
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3 2 3 3 2 3 3
Ni(NO ) , AgNO , Zn(NO ) , Fe(NO ) ) in deionized water at a
concentration of 0.1 M and were diluted as required before use.
Each time, 3.0 mL of dispersion solution of MSNs-NM
was firstly filled into a quartz cell of 1 cm optical path length,
and then an appropriate concentration and amount of metal
ions was added to the solution for fluorimetric and UV-vis
measurements. All fluorescence spectra were recorded at room
temperature with an excitation wavelength of 371 nm.
2.4 Characterization and measurements
X-ray diffraction (XRD) patterns were collected on a Bruker D8
ꢀ
1
using Cu Ka radiation (40 kV, 40 mA) at a rate of 0.61 min
over the range of 0.6–61 (2y). Transmission electron microscope and (c) MSNs-NM.
TEM) measurements were carried out on a JEM 2100F electron
microscope operated at 200 kV. N adsorption–desorption
Fig. 1 Small-angle XRD patterns of the synthesized (a) MSNs, (b) MSNs-NH
2
(
2
intensity change was tentatively ascribed to the decreased electron
contrast between the silica framework and pore channels with
organic groups therein in reasonable agreement with our previous
isotherms were obtained on a NOVA 4200E at 77 K under
continuous adsorption conditions. Prior to the measurements,
all samples were pre-treated for 12 h at 120 1C under nitrogen.
The Brunauer–Emmett–Teller (BET) method was utilized to
calculate the specific surface areas using adsorption data in a
relative pressure range from 0.1 to 0.3. By using the Barrett–
Joyner–Halenda (BJH) model, the pore volumes and pore size
distributions were derived from the adsorption branches of
isotherms, and the total pore volumes were estimated from the
53
observations, evidencing that the grafting was successfully done
and it took place inside the mesoporous channels.
TEM images were taken to directly observe the morphology
of the synthesized samples. The parent MSNs (Fig. 2a) exhibited
a very uniform spherical shape with a diameter of about 155 nm
(also see Fig. S2, ESI†). After modification, the morphology of
MSNs-NM was preserved. The parallel mesopore channels
penetrated throughout the whole nanoparticles. Meanwhile,
hexagonally arranged mesochannels were present as shown in
Fig. 2(b), corroborating that the prepared sample possessed a
well-ordered mesostructure and the BM grafting did not affect
the general morphology of MSNs. Thus, from the TEM image
analysis and the small-angle XRD patterns, it can be concluded
that the ordered mesoporous silica-based matrix remains intact
ꢀ1 throughout the modification steps.
0
adsorbed amount at a relative pressure P/P of 0.99. UV-visible
absorption spectra were recorded on a Shimadzu UV-vis 3101
spectroscope. FTIR spectra were recorded on a Nicolet 7000-C
ꢀ
1
spectroscope with a resolution of 4 cm using the KBr method.
The fluorescence intensity spectra were collected using a
1
Shimadzu RF5301pc fluoro-photometer. H NMR measurements
were carried out on a Bruker AV400 spectrophotometer. Thermo-
gravimetric analysis (TGA) was conducted on a Perkin-Elmer
ꢀ
1
thermogravimetric analyzer in air (50 mL min ) at 10 1C min
from 30 to 800 1C.
The changes in the textural parameters of the MSNs before
and after the BM grafting were further investigated by nitrogen
sorption techniques. The nitrogen adsorption curves (Fig. 3A)
revealed type-IV isotherms with sharp capillary condensation
steps for all the samples before and after BM grafting. This
evidences the presence of a mesoporous structure in these
3. Results and discussion
3.1 Preparation and characterization of the chemosensor
In this study, we designed a novel chemosensor based on MSNs materials and also indicates that the modification process did
2
+
containing NM for the fluorescence sensing of Cu ions. not destroy the structural integrity of the parent MSNs. The
MSNs-NM was prepared by a simple nucleophilic addition– sharp increase in the N
elimination reaction between the non-fluorescent small molecule
2
uptake of parent MSNs (Fig. 3a) is
of BM and pre-grafted amino groups in the meso-space of MSNs.
The performance of MSNs-NM was evaluated for the sensitivity
2+
and selectivity towards Cu ion detection in aqueous media using
fluorimetric techniques.
The small-angle XRD was applied to monitor the meso-
structure evolvement for the BM grafting procedures. As mani-
fested in Fig. 1, three distinct diffraction peaks indexed to (100),
(110) and (200) reflections, respectively, could be clearly identified,
characterizing their ordered 2D hexagonal (P6mm) symmetry. All
the diffraction peaks of MSNs-NH2 (Fig. 1b) and MSNs-NM
(
Fig. 1c) slightly shifted to a higher angle direction. Additionally,
the intensities of (110) and (200) reflections also slightly decreased
after grafting of amino groups and subsequently BM. This peak Fig. 2 TEM images of the synthesized (a) MSNs and (b) MSNs-NM.
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ꢀ1
absorption of C–H bonds at 2900–3000 cm after the extraction
using ethanol and concentrated HCl (Fig. 4a) indicated that the
surfactant had been effectively removed from the MSNs. An
apparent decrease in the intensity of the Si–OH vibration
ꢀ
1
bands at 960 cm can be observed while the new –CH –NH2
2
ꢀ1
mode at 1552 cm (labelled with arrow) and C–H bonds at 2900–
ꢀ1
3
000 cm
MSNs-NH
amino groups. The bands observed at 1708 and 1760 cm
labelled with stars) for MSNs-NM (Fig. 4c) could be assigned to
(Fig. 4b) for propyl groups in APTES emerge in
2
, clearly verifying the successful incorporation of the
ꢀ
1
(
54
the five membered rings of imide. These results indicated that
BM had been attached to the amino groups. We can also confirm
Fig. 3
N
2
adsorption–desorption isotherms (A) and their corresponding the successful linking of fluorescent probes from the UV-vis
BJH pore size distribution curves (B) of the samples for (a) MSNs, (b) MSNs-NH
2
measurements. The spectrum (Fig. S3, ESI†) of MNS-NMs exhibits
a new absorption band at 371 nm arising from BM while the
parent and amino groups modified MSNs do not show any
absorption at the same wavelength. TGA measurements illustrate
that the loading capacity of the fluorescent probe on the synthe-
sized chemosensor was determined to be 0.1 g per gram of MSNs
and (c) MSNs-NM.
Table 1 Structural parameters of the MSNs before and after BM grafting
2
ꢀ1
3
ꢀ1
Samples
S
BET (m g
)
D
BJH (nm)
V (cm g
)
(
Fig. S4, ESI†).
MSNs
MSNs-NH
MSNs-NM
1481
1099
871
3.10
2.75
2.46
1.55
0.83
0.70
2
2
+
3.2 Selectivity of the MSNs-NM towards Cu ions
To check whether the MSNs-NM could serve as a fluorogenic
sensor for metallic ions, we tested the fluorescence response of
observed at a relative pressure of 0.3–0.5, clearly indicating that
the mesopores are uniform in size. Meanwhile, the capillary
condensation steps shifted to a relatively lower pressure after
modification by amino groups and subsequently BM. This
illustrates the successful attachment of BM to the void space
of the MSNs, which causes the shrinkage of the pore volume as
well as the decrease of the specific surface area of the modified
materials (Table 1). The pore size distributions calculated by
the BJH method (Fig. 3B) clearly reflects a decrease in the pore
size. The gradual decrease in the pore width from 3.10 to 2.75
and to 2.46 nm also suggests that BM was successfully grafted
inside the MSNs.
2
+
3+
2+
MSNs-NM towards various metal ions such as Cu , Fe , Co ,
2
+
+
2+
2+
2+
2+
2+
+
+
Zn , Ag , Ni , Mn , Cd , Mg , Ca , K and Na . The
fluorescence response was recorded at 505 nm followed by
excitation at 371 nm in HEPES buffer solution (50 mM, pH = 7.4).
Fig. 5 illustrates the quenching ratio, ((F ꢀ F)/F ), of the fluores-
0
0
cence intensity upon addition of 150 mM of various metal ions to
ꢀ1
the MSNs-NM suspension (30 mg L ). The results show that no
obvious changes occurred in the presence of alkali and alkaline
+
+
2+
2+
2+
earth metal ions (Na , K , Mg , Ca ), or transition ions (Cd ,
2+
2+
2+
+
3+
Zn , Co , Ni , Ag ) except that Fe caused a slight fluorescence
decrease. However, it was found that the fluorescence intensity
decreased dramatically to 34% upon the addition of Cu to the
suspension. These results reveal that MSNs-NM exhibits high
2+
To further testify the successful labelling of the MSNs with
fluorescent probes, the FT-IR spectra were recorded to trace
the grafting procedures. As shown in Fig. 4, the reduced IR
2+
selectivity and sensibility towards Cu ions in aqueous solution
as we expected (see Fig. S5, ESI† for the corresponding
Fig. 5 Quenching ratio ((F
MSNs-NM suspension (30 mg L ) in HEPES solution (50 mM; pH = 7.4) in
0
ꢀ F)/F
0
) of the fluorescence response of the
ꢀ
1
Fig. 4 FT-IR spectra of the synthesized (a) MSNs, (b) MSNs-NH
c) MSNs-NM.
2
and
(
the presence of various metal ions (150 mM) (lex = 371 nm, lem = 505 nm).
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fluorescence spectra). Since the high specificity is crucial for most
probes to be applied in the real sample detection, we further
2+
added other metallic ions to the MSNs-NM–Cu system. No
significant fluorescence intensity changes were observed in the
experiments in which other metal ions were spontaneously added
2+
into the Cu -containing detecting systems (Fig. S6, ESI†). These
findings confirm that the currently obtained MSNs-NM can be
2+
useful for the selective detection of Cu in the presence of other
metal ions.
2
+
3
.3 Sensing response of MSNs-NM towards Cu ions
2+
To test the sensibility of MSNs-NM for Cu , fluorescence titration
2+
spectra with different Cu ion concentrations were conducted
ꢀ
1
followed by excitation at 371 nm at room temperature. Upon Fig. 7 Stern–Volmer plot of the MSNs-NM suspension (30 mg L ) in a
2+
gradual addition of Cu ions (from 0 to 150 mM) to the MSNs-NM HEPES solution (50 mM; pH = 7.4) in the presence of different amounts of
2
+
Cu ions (from 0 to 150 mM) (lex = 371 nm, lem = 505 nm).
suspension, it was found that the fluorescence intensity was
dramatically decreased (Fig. 6), confirming their potential for
2+
Cu detection. The response time of the fluorescent probe to
exceeds 90 mM, the Stern–Volmer plots deviate from linearity
toward the x-axis. This may be attributed to the partial inacces-
sibility of the fluorescent probes to the Cu ions in the
mesopore space of MSNs. This also rationalizes that the
fluorescence intensity of MSNs-NM is not completely quenched
2+
the Cu ions was also detected (Fig. S7, ESI†), and the calculated
value was as short as 75 seconds, which was much faster than that
2+
55,56
of the reported work.
Therefore, a reaction time of 90 seconds
was set to ensure the complete equilibration before measurement
for all the experiments.
2+
by the addition of Cu ions.
Quenching of fluorescence is described by the Stern–Volmer
58
The modified Stern–Volmer equation (eqn (2)) is applied
for quantitative evaluation of the accessible fraction of fluores-
cence molecules.
5
7
equation (eqn (1)):
F0
¼
1 þ Ksv½Qꢁ
(1)
F
F0
1
1
¼
þ
(2)
F0 ꢀ F
faKa½Qꢁ fa
In this equation F
MSNs-NM in the absence and presence of Cu , respectively;
Q is the concentration of Cu , and K is a Stern–Volmer
0
and F are the fluorescence intensities of
2
+
where f
the quencher and K
a
is the fraction of initial fluorescence that is accessible to
is a Stern–Volmer quencher constant of the
accessible fraction. This modified form of the Stern–Volmer
2
+
sv
a
quenching constant. Fig. 7 shows the plot of F /F versus
0
2
+
[
Cu ] with a good linear relationship between the relative
fluorescence intensity and the Cu concentration in the range
equation allows f and K to be determined graphically (Fig. 8).
a
a
2
+
ꢀ1
A plot of F
/(F
0 0
ꢀ F) versus 1/[Q] yields f
a
as the intercept and
is calculated to be 5.41 M and f
is obtained as 0.73, revealing that almost 73% of the MSNs-NM
are accessible to Cu ions. It can be taken into account as the
5
2+
from 0 to 90 mM, following the equation F
0
/F = 0.26 ꢂ 10 [Cu ] +
ꢀ1
ꢀ1
(
f
a
K
a
)
as the slope. Thus, K
a
a
2
1
.05 with a linearly dependent coefficient r = 0.997. The Ksv is
4
ꢀ1
2+
determined to be 2.6 ꢂ 10 M . When the Cu concentration
2+
rationalized reason for the remaining observed fluorescence.
ꢀ1
Fig. 6 Fluorescence spectra of the MSNs-NM suspension (30 mg L ) in a
ꢀ
1
HEPES solution (50 mM; pH = 7.4) in the presence of different amounts of Fig. 8 Modified Stern–Volmer plot of the MSNs-NM suspension (30 mg L
)
2+
Cu (from 0 to 150 mM). Inset: the fluorescence intensity at 505 nm as a
in a HEPES solution (50 mM; pH = 7.4) in the presence of different amounts of
2+
2+
function of Cu ion concentration. Excitation wavelength was set to 371 nm.
Cu ions (from 0 to 150 mM) (lex = 371 nm, lem = 505 nm).
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The detection limit (DL) was calculated using eqn (3):
NJC
3
s
DL ¼
(3)
m
where s is the standard deviation of the blank measurements,
2
+
and m is the slope of the F-[Cu ] plot with the concentrations
2
+
of Cu in the range from 0 to 30 mM. Herein, s and m are
6
calculated to be 2.84 and 9.97 ꢂ 10 , respectively. Then the
detection limit is calculated to be 0.28 mM, which is more than
2
+
sufficient to sense the Cu concentration in drinking water
5
9
with respect to the U.S. EPA limit (E20 mM).
3.4 Possible sensing mechanism
To explore the mechanism of the fluorescence quenching by
2
+
Fig. 10 Absorption ratios (A293 nm/A371 nm) of the MSNs-NM suspension
the Cu ions, we further tested the UV-vis absorption of the
current conjugated system. Fig. 9 shows the spectral variation
ꢀ1
2+
(
30 mg L ) in the presence of different amounts of Cu ions (from 0 to
50 mM) in a HEPES solution (50 mM; pH = 7.4).
1
ꢀ1
of the MSNs-NM suspension (30 mg L ) in a HEPES solution
2
+
upon the gradual addition of Cu ions. It is observed that free
MSNs-NM shows an absorbance maximum at 371 nm. When
2
+
Cu ions were introduced, a new absorption band peaked at
93 nm was observed while the intensity of the band at 371 nm
decreased. A clear isosbestic point at 356 nm was observed
2
2
+
simultaneously, indicating the conversion of NM to Cu –NM
complexes. This observation can be considered as a reason for
fluorescence quenching. It reveals that the low energy band still
exists but the intensity slightly decreases, and a new high
energy band forms.
Fig. 10 shows the UV-vis absorption intensity ratios, which
were measured at 293 and 371 nm, respectively. It shows a good
linear relationship between absorption intensity ratios and
2
+
ꢀ1
,
Cu ion concentrations. This indicates that MSNs-NM not only Fig. 11 Absorption spectra of the MSNs-NM suspension (30 mg L
2
+
2+
could serve as a fluorogenic sensor but also has the potential (a) red), MSNs-NM–Cu suspension (90 mM Cu , (b) green) and free
2
+
maleimide (50 mM, (c) blue) in a HEPES solution (50 mM; pH = 7.4).
for quantitative determination of Cu ions even using a UV-vis
3
analytical method.
To provide the direct proof of the coordination between
a new absorption band centred at 371 nm appears and it
becomes fluorescent when maleimide is conjugated to the
2
+
MSNs-NM and Cu , we measured the UV-vis spectra of free
maleimide and compared it with the spectra of MSNs-NM and
2
+
MSNs via amino groups. When Cu ions are added, the lone
2
+
MSNs-NM–Cu . As shown in Fig. 11, the spectrum of free
maleimide shows an obvious absorption band at 292 nm, while
2
+
electron pairs of N atoms are strictly restricted by Cu due to
their strong interaction, so the absorption band of maleimide
2
+
is partially recovered (Fig. 11b). The more Cu ions are
introduced, the more the intense absorption at 292 nm
assigned to free maleimide is recovered (Fig. 9) and thus causes
the fluorescence quenching and absorption changes in the
conjugated BM molecules.
4. Conclusions
In summary, a novel fluorescent ‘‘on–off’’ chemosensor for
2
+
Cu ions has been successfully developed via labelling meso-
porous silica nanoparticles (MSNs) with maleimide. Although
the free maleimide molecule is non-fluorescent, the introduction
of this small molecule into the mesochannels of MSNs makes
them fluorescent, thanks to the nucleophilic amino groups for
ꢀ
1
Fig. 9 Absorption spectra of the MSNs-NM suspension (30 mg L ) in the
2+
2
their conjugation. N sorption analysis, FT-IR and UV-vis spectro-
presence of different amounts of Cu ions (from 0 to 150 mM) in a HEPES
solution (50 mM; pH = 7.4).
scopy revealed that maleimide was successfully grafted onto the
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mesopores. Small-angle XRD and TEM data suggested that 17 Z. Yu, T. Kabashima, C. H. Tang, T. Shibata, K. Kitazato,
the highly ordered mesoporous structure was retained after all
processes for the sensor fabrication. The new chemosensor shows
N. Kobayashi, M. K. Lee and M. Kai, Anal. Biochem., 2010,
397, 197.
high selectivity toward Cu than other metal ions with detection 18 T. Kabashima, Z. Yu, C. H. Tang, Y. Nakagawa, K. Okumura,
limit as low as 0.28 mM. Additionally, the synthesized MSNs-NM
T. Shibata, J. Z. Lu and M. Kai, Peptides, 2008, 29, 356.
not only could serve as a fluorogenic sensor but also has the 19 M. Montalti, E. Rampazzo, N. Zaccheroni and L. Prodi,
2+
2+
potential for quantitative determination of Cu even using a
chromogenic method. A rationalized mechanism for fluorescence 20 J. T. Yeh, W. C. Chen, S. R. Liu and S. P. Wu, New J. Chem.,
quenching was proposed that the strong interaction between
2014, 38, 4434.
amino groups of 2-aminomaleimides and Cu ions restricted 21 G. J. He, X. W. Zhao, X. L. Zhang, H. J. Fan, S. Wu, H. Q. Li,
the lone electron pairs of N atoms to conjugate with the
C. He and C. Y. Duan, New J. Chem., 2010, 34, 1055.
maleimide ring. The above sensor system is expected to be 22 A. Ajayaghosh, P. Carol and S. Sreejith, J. Am. Chem. Soc.,
New J. Chem., 2013, 37, 28.
2+
2
+
used for the detection of Cu in intercellular environments
and water-quality monitoring.
2005, 127, 14962.
23 G. Zhang, D. Zhang, S. Yin, X. Yang, Z. Shuai and D. Zhu,
Chem. Commun., 2005, 2161.
24 X. Qi, E. J. Jun, L. Xu, S.-J. Kim, J. S. Joong Hong, Y. J. Yoon
Acknowledgements
and J. Yoon, J. Org. Chem., 2006, 71, 2881.
2
2
5 H. Son, G. Kang and J. H. Jung, Analyst, 2012, 137, 163.
6 S. J. Lee, S. S. Lee, M. S. Lah, J.-M. Hong and J. H. Jung,
Chem. Commun., 2006, 4539.
7 S. J. Lee, J. E. Lee, J. Seo, I. Y. Jeong, S. S. Lee and J. H. Jung,
Adv. Funct. Mater., 2007, 17, 3441.
This work was financially supported by the Natural Science
Foundation of China (51372084, 51072053), the Innovation
Program of Shanghai Municipal Education Commission
2
2
2
3
(
13zz040), the Nano-Special Foundation for Shanghai Committee
of Science and Technology (12nm0502600) and the 111 Project
B14018).
8 X. Qiu, S. Han and M. Gao, J. Mater. Chem. A, 2013, 1,
(
1319.
9 C. Wang, S. Tao, W. Wei, C. Meng, F. Liu and M. Han,
J. Mater. Chem., 2010, 20, 4635.
0 X.-C. Fu, J. Wu, C.-G. Xie, Y. Zhong and J.-H. Liu, Anal.
Methods, 2013, 5, 2615.
31 M. S. Moorthy, H.-J. Cho, E.-J. Yu, Y.-S. Jung and C.-S. Ha,
Chem. Commun., 2013, 49, 8758.
32 S. A. El-Safty, A. A. Ismail, H. Matsunaga and F. Mizukami,
Chem. – Eur. J., 2007, 13, 9245.
Notes and references
1
2
3
Y. W. Liu, J. L. Chir, S. T. Wang and A. T. Wu, Inorg. Chem.
Commun., 2014, 45, 112.
E. L. Que, D. W. Domaille and C. J. Chang, Chem. Rev., 2008,
108, 1517.
H. H. Wang, L. X. Zhang, J. F. Guo, P. Li and H. Jiang,
New J. Chem., 2010, 34, 1239.
33 R. Metivier, I. Leray, B. Lebeau and B. Valeur, J. Mater.
Chem., 2005, 15, 2965.
4
5
S. D. R. Brown and H. Kozlowski, Dalton Trans., 2004, 1907.
B. F. Gouanv ´e , T. Schuster, E. Allard, R. M ´e allet-Renault and 34 D. Lu, L. Yang, Z. Tian, L. Wang and J. Zhang, RSC Adv.,
C. Larpent, Adv. Funct. Mater., 2007, 17, 2746.
K. Viswanathan, Sens. Actuators, A, 2012, 175, 15.
P. Yang, Y. Zhao, Y. Lu, Q. Z. Xu, X. W. Xu, L. Dong and
S. H. Yu, ACS Nano, 2011, 5, 2147.
2012, 2, 2783.
6
7
35 X. Wan, S. Yao, H. Liu and Y. Yao, J. Mater. Chem. A, 2013,
1, 10505.
36 P. Pal, S. K. Rastogi, C. M. Gibson, D. E. Aston, A. L. Branen
and T. E. Bitterwolf, ACS Appl. Mater. Interfaces, 2011, 3, 279.
37 Y. Hu, L. Meng and Q. H. Lu, Langmuir, 2014, 30, 4458.
8
9
C. Yu, J. Zhang, R. Wang and L. X. Chen, Org. Biomol. Chem.,
2010, 8, 5277.
C. Yu, L. X. Chen, J. Zhang, J. H. Li, P. Liu, W. H. Wang and 38 Z. Jin, X. B. Zhang, D. X. Xie, Y. J. Gong, J. Zhang, X. Chen,
B. Yan, Talanta, 2011, 85, 1627.
G. L. Shen and R. Q. Yu, Anal. Chem., 2010, 82, 6343.
0 H. N. Kim, M. H. Lee, H. J. Kim, J. S. Kim and J. Yoon, Chem. 39 H. J. Kim, H. Lee, J. H. Lee, D. H. Choi, J. H. Jung and
Soc. Rev., 2008, 37, 1465.
J. S. Kim, Chem. Commun., 2011, 47, 10918.
1 T. Balaji, S. A. El-Safty, H. Matsunaga, T. Hanaoka and 40 H. J. Kim, J. H. Lee, H. Lee, J. H. Lee, J. H. Lee, J. H. Jung and
F. Mizukami, Angew. Chem., Int. Ed., 2006, 45, 7202.
J. S. Kim, Adv. Funct. Mater., 2011, 21, 4035.
2 K. Singh, D. Sareen, P. Kaur, H. Miyake and H. Tsukube, 41 Q. Zou, L. Zou and H. Tian, J. Mater. Chem., 2011, 21, 14441.
1
1
1
Chem. – Eur. J., 2013, 19, 6914.
3 L. Prodi, New J. Chem., 2005, 29, 20.
4 Y. Xiang, A. Tong, P. Jin and Y. Ju, Org. Lett., 2006, 8, 2863.
42 J. Liu, C. Li and F. Li, J. Mater. Chem., 2011, 21, 7175.
43 K. Sarkar, K. Dhara, M. Nandi, P. Roy, A. Bhaumik and
P. Banerjee, Adv. Funct. Mater., 2009, 19, 223.
1
1
1
5 C. Bhattacharya, Z. Yu, M. J. Rishel and S. M. Hecht, 44 J. Wang, W. Guo, J.-H. Bae, S.-H. Kim, J. Song and C.-S. Ha,
Biochemistry, 2014, 53, 3264.
J. Mater. Chem., 2012, 22, 24681.
6 Z. Yu, R. M. Schmaltz, T. C. Bozeman, R. Paul, M. J. Rishel, 45 H. J. Kim, S. J. Lee, S. Y. Park, J. H. Jung and J. S. Kim, Adv.
1
K. S. Tsosie and S. M. Hecht, J. Am. Chem. Soc., 2013,
35, 2883.
Mater., 2008, 20, 3229.
46 B. Leng, J. Jiang and H. Tian, AIChE J., 2010, 56, 2957.
1
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2014
New J. Chem.

View Article Online
Paper
NJC
4
4
4
7 M. E. B. Smith, F. F. Schumacher, C. P. Ryan, L. M. Tedaldi, 53 J. L. Gu, W. Fan, A. Shimojima and T. Okubo, Small, 2007,
D. Papaioannou, G. Waksman, S. Caddick and J. R. Baker,
J. Am. Chem. Soc., 2010, 132, 1960.
8 M. W. Jones, R. A. Strickland, F. F. Schumacher, S. Caddick,
J. R. Baker, M. I. Gibson and D. M. Haddleton, J. Am. Chem.
Soc., 2011, 134, 1847.
3, 1740.
54 O. E. Tall, Y. Hou, E. Abou-Hamad, I. U. Raja, M. N. Hedhili,
W. Peng, R. Mahfouz, O. M. Bakr and P. M. Beaujuge, Chem.
Mater., 2014, 26, 2766.
55 J. F. Zhang, M. S. Park, W. X. Ren, Y. Kim, S. J. Kim,
J. H. Jung and J. S. Kim, Chem. Commun., 2011, 47, 3568.
9 M. P. Robin, P. Wilson, A. B. Mabire, J. K. Kiviaho,
J. E. Raymond, D. M. Haddleton and R. K. O’Reilly, J. Am. 56 A. Lee, J. Chin, O. K. Park, H. Chung, J. W. Kim, S. Y. Yoon
Chem. Soc., 2013, 135, 2875. and K. Park, Chem. Commun., 2013, 49, 5969.
0 J. L. Gu, S. Su, Y. Li, Q. He and J. Shi, Chem. Commun., 2011, 57 P. Zarabadi-Poor, A. Badiei, A. A. Yousefi and J. Barroso-
7, 2101. Flores, J. Phys. Chem. C, 2013, 117, 9281.
1 J. L. Gu, K. Huang, X. Zhu, Y. Li, J. Wei, W. Zhao, C. Liu and 58 J. R. Lakowicz, Principles of Fluorescence Spectroscopy,
J. Shi, J. Colloid Interface Sci., 2013, 407, 236. Kluwer Acad./Plenum Publ., 1999, p. 288.
2 J. L. Gu, J. Shi, G. You, L. Xiong, S. Qian, Z. Hua and 59 S. Seo, H. Y. Lee, M. Park, J. M. Lim, D. Kang, J. Yoon and
5
5
5
4
H. Chen, Adv. Mater., 2005, 17, 557.
J. H. Jung, Eur. J. Inorg. Chem., 2010, 843.
New J. Chem.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2014