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PAPER www.rsc.org/nanoscale | Nanoscale
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Synthesis of highly luminescent cobalt(II)-bis(8-hydroxyquinoline) nanosheetsas isomeric aromatic amine probes†
Haibing Li* and Yuling Li
Received 14th April 2009, Accepted 12th July 2009
First published as an Advance Article on the web 28th August 2009
DOI: 10.1039/b9nr00019d
Highly luminescent and water-soluble cobalt(II)-bis(8-hydroxyquinoline) (CoQ2) nanosheets have been
successfully synthesized via a simple, rapid sonochemical method. The water-soluble CoQ2 nanosheets
were characterized by luminescence spectroscopy, UV–vis spectroscopy, FT-IR spectroscopy and
transmission electron microscopy (TEM). The CoQ2 nanosheets allow highly sensitive and selective
determination of p-nitroaniline via fluorescence quenching. Under optimal conditions, the relative
fluorescence intensities of nanosheets decreased linearly with increasing p-nitroaniline. However, the
sensitivity of CoQ2 nanosheets toward other aromatic amines including o-diaminobenzene,
m-diaminobenzene, p-diaminobenzene, p-toluidine, o-nitroaniline, m-nitroaniline, p-chloroaniline and
aniline is negligible. It is found that p-nitroaniline can quench the luminescence of CoQ2 nanosheets
in a concentration-dependent manner that is best described by a Stern–Volmer-type equation. The
possible underlying mechanism is discussed.
1. Introduction
Aromatic amines (AAMs) are widely used as raw materials or
intermediates in the manufacturing of dyes, pesticides, medicines
and pharmaceuticals.1 But, aromatic amines are highly toxic
materials that can easily permeate through soil and contaminate
groundwater and enter the body when people consume food or
water contaminated with them. Currently, the International
Agency for Research on Cancer (IARC) has classified six
aromatic amines as carcinogenic or probably carcinogenic to
humans, accordingly, the remnants of aromatic amines in envi-
ronment has raised a great concern.2,3 As a consequence,
aromatic amines are suspected to be harmful to humans and need
to be monitored regularly, and the determination method must
be simple, rapid and effective.
Several analytical methods have been reported for the determi-
nation of aromatic amines. Among them, the most commonly
employed techniques are gas chromatography-mass spectrometry
(GC-MS) and high-performance liquid chromatography
(HPLC).4–6 Nowadays, as a useful analytical technique, fluores-
cence (FL) detection has been extensively employed with high
sensitivity. Many organic dyes and special inorganic nanomaterials
have been reported as fluorescent probes.5–12 For instance, Dan-
ielsona and coworkers have developed an N-alkyl acridine orange
dye as a fluorescence probe for the determination of cardiolipin.13
Calixarene-modified CdTe, synthesized by our group, allowed
a highly sensitive determination of polycyclic aromatic hydrocar-
bons14 and pesticides.15 In comparison to traditional organic dyes,
Key Laboratory of Pesticide & Chemical Biology (CCNU), Ministry ofEducation, College of Chemistry, Central China Normal University,Wuhan 430079, PR China. E-mail: lhbing@mail.ccnu.edu.cn; Tel:+86-27-67866423
† Electronic supplementary information (ESI) available: XRD pattern ofthe CoQ2 nanosheets, the UV–vis absorption spectra and fluorescencespectra of ligand Q, CoQ2 and CoQ2 nanosheets. See DOI:10.1039/b9nr00019d
128 | Nanoscale, 2009, 1, 128–132
inorganic nanomaterials such as semiconductor quantum dots
(QDs) have attracted great interest in the past decade due to their
unique optical properties including a narrow, tunable, symmetric
emission and photochemical stability.14,15 However, in general,
QDs, as sensors, have to be surface modified before use.
Also, metal complexes have been widely used as optoelectronic
devices because of their unique electronic and optical proper-
ties.16 For the past few years, luminescent metal 8-hydroxy-
quinoline (MQn) chelates have attracted considerable interest
owing to their various applications in photoluminescence, elec-
troluminescence and field emission.17 Due to the unique opto-
electronic properties of nanomaterials, more attention is
attracted to nanostructured MQn chelates. AlQ3 and many other
MQn chelates have also been demonstrated to be useful emitter
materials, and some of them have been widely used in organic
light-emitting devices (OLEDs).18,19 For instance, AlQ3 nano-
structures, such as nanowires, nanorods, and nanometre-scale
crystalline films, exhibited field emission with a relatively low
turn-on voltage.20 In comparison with their extensive applica-
tions in OLEDs, only a few metal 8-hydroxyquinoline (MQn)
nanomaterials have been employed as fluorescent analytical
assays. Recently, fluorescent sensors based on MQn nano-
materials have attracted increasing attention, due to their high
quantum yields and their multifunctional groups provide affinity
sites for the binding of biomolecules.21 For example, Zhu and
coworkers have reported an optical strategy based on the ZnQ2
nanorods for protein sensing.22 Cobalt complexes, which are
important in vitamin-B12 model chemistry, are also known as
catalysts for the reduction of CO2.23 However, the potential
applications of cobalt 8-hydroxyquinoline complex-based
nanomaterials in environmental pollution analysis are still at an
early stage. To our knowledge, the use of cobalt complex-based
nanomaterials as selective probes for the fluorescent determina-
tion of aromatic amines is almost unexplored.
In this work, we synthesized water-soluble, stable and
highly fluorescent cobalt(II)-bis(8-hydroxyquinoline) complex
This journal is ª The Royal Society of Chemistry 2009
Fig. 1 IR spectrum of the CoQ2 nanosheets.
Fig. 2 TEM images of the CoQ2 nanosheets. Scale bars: 500 nm.
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nanosheets by a very simple sonochemical method and investi-
gated their potential application as a selective fluorescent probe
for p-nitroaniline.
2. Experimental section
2.1 Materials
All chemicals used were of analytical grade or of the highest
purity available. 8-Hydroxyquinoline was purchased from
Beijing Corp. (Beijing, China). Cobalt chloride and AAMs
(o-diaminobenzene, m-diaminobenzene, p-diaminobenzene,
p-toluidine, o-nitroaniline, m-nitroaniline, p-nitroaniline,
p-chloroaniline, aniline) were obtained from Beijing Chemical
Corp. (Beijing, China). All AAM standards were of 98–99%
purity and were dissolved in 50% (v/v) ethanol–water solution.
2.2 Preparation of cobalt(II)-bis(8-hydroxyquinoline) nano-
complexes
Cobalt(II)-bis(8-hydroxyquinoline) nano-complexes were
synthesized using a sonochemical method combined with
a microemulsion technique,22 although some slight modifications
were made here. Briefly, a water-in-oil (W/O) microemulsion was
prepared by mixing TX-100 (30 mL), cyclohexane (55 mL),
n-hexanol (15 mL), 0.2 M cobalt chloride aqueous solution
(2.5 mL), and ethanol (2.5 mL). 8-Hydroxyquinoline (1.5 mmol,
0.2175 g) was dissolved in an ethanol–water solution (50%, v/v,
5 mL) and then added into the microemulsion. The mixture
solution was exposed to ultrasound irradiation under ambient air
for 45 min. When the reaction was finished, a yellowish-green
precipitate was obtained. After cooling to room temperature, the
precipitate was separated by centrifuging at a rotation rate of
9000 rounds per min. It was purified further by repeated cycles of
centrifugation and dispersing in ethanol and then dried in air at
room temperature. The final products were redispersed in 50%
(v/v) ethanol–water solution for further usage.
2.3 Characterization
UV–vis absorption spectra were acquired on a TU-1901 UV–vis
spectrometer (Beijing Purkinje General Instrument Co. Ltd).
Fluorescence spectra were taken on a Fluoromax-P luminescence
spectrometer (HORIBA JOBIN YVON INC.). IR spectra were
measured with a NEXUS FT/IR spectrometer (Thermo Nicolet
Co.). Transmission electron microscopy (TEM) was recorded by
a JEOL-JEM 2010 electron microscope operating at 200 kV.
X-Ray diffraction (XRD) was carried out with a Shimadzu labx
XRD-6000. Elemental analysis (EA) was carried out using
a Heraeus CHN–O Rapid instrument.
3. Results and discussion
3.1 Spectra characterizations of cobalt(II)-bis(8-
hydroxyquinoline) nano-complexes
The EA of the sample shows that the content (%) of C, H and N
is 62.14, 3.51 and 8.10, respectively. The values are consistent
with the calculated values (C: 62.26%; H: 3.48%; N: 8.07%) and
the product can be confirmed to be cobalt(II)-bis(8-hydroxy-
quinoline) (CoQ2). The X-ray diffraction (XRD) pattern of the
This journal is ª The Royal Society of Chemistry 2009
as-prepared product is shown in Fig. S1 (ESI†). The diffraction
peaks can be indexed to be CoQ2.22,24 The component of the
nanostructures was further identified with an FT-IR spectrum.
As indicated in Fig. 1, the water of hydration in the samples was
readily identified by the presence of a broad infrared absorption
band in the region from 3000 to 3400 cm�1. The intensity ratio of
the 3431 cm�1 band to the 1126 cm�1 band is commonly used to
study the water molecule number in metal–quinoline chelates.
Similar to the congeneric compounds,22 the bands of 1600 cm�1
should correspond to a C]C stretching vibration in the quino-
line group. The bands at 1489 and 1470 cm�1 are assigned to
CC/CN stretching and CH bending vibration of the pyridyl and
phenyl groups in CoQ2.25 The bands observed in the spectrum
with peak positions at 1264, 1210, and 1033 cm�1 are attributed
to a CH/CCN bending and C–N/C–O stretching vibrations.
Peaks at 664, 603, and 559 cm�1 should correspond to Co–O
stretching vibrations, and the band at 502 cm�1 is attributed to
the Co–N stretching vibrations.26
The morphologies and sizes of the samples were characterized
by TEM, and the results are shown in Fig. 2. With the ultrasound
proceeding, the formed CoQ2 nanoparticles underwent fusion to
form small sheets composed of small particles. Further ultra-
sound irradiation led to the continuous growth of CoQ2 nano-
sheets. A possible formation mechanism is proposed in Fig. 3.
These nanosheets have regularly quadrate morphologies with
a width 100–150 nm and a length 2–5 mm. Microcrystals with
large dimensions could not be observed, which suggests that
microcrystals of CoQ2 are destroyed under ultrasound
Nanoscale, 2009, 1, 128–132 | 129
Fig. 3 Schematic diagram showing the formation of CoQ2 nanosheets.
Fig. 4 Effect of pH on luminescence response of the CoQ2 nanosheets.
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irradiation for long reaction times, and these microcrystals are
changed into nanosheets due to weak bonding interactions
between 2D coordination polymers.27
The photophysical properties of the CoQ2 nanosheets have
been investigated in an ethanol–water solution. Fig. S2 (ESI†)
shows the UV–vis absorption spectra of 8-hydroxyquinoline (Q),
CoQ2 and CoQ2 nanosheets. CoQ2 exhibits two resolved
absorption bands at 310 nm and 356 nm, which is attributed to
the transition of phenyl rings and the n–p transition, respec-
tively.28 The absorption of CoQ2 nanosheets is different from
that regular CoQ2. There is an approximate 30 nm red-shift,
which is due to the p–p stacking of the nanosheets (geometry
change). Photoluminescence is a very important characteristic
for the 8-hydroxyquinoline metal chelates. The difference
between the CoQ2 nanosheets and CoQ2 has also been monitored
by fluorescence spectra. It can be observed that the fluorescence
spectra of the CoQ2 nanosheets exhibits a band centered at
450 nm. There is distinct red-shift in the peak position compared
with that of the CoQ2, and the luminescence intensity increased
with the formation of CoQ2 nanosheets (Fig. S3, ESI†). Ligand
Q of CoQ2 exhibits free intramolecular rotation in the single
complex molecule, but the rotation is inhibited in the aggregated
nanosheet state. The inhibition of intramolecular rotation could
be an effective mechanism for fluorescence enhancement, there-
fore, the CoQ2 nanosheets show an aggregation-induced emis-
sion enhancement (AIEE) characteristic. The quantum yields
(QY) of CoQ2 and CoQ2 nanosheets in ethanol are measured in
comparison with the value of rhodamine B (QY ¼ 89%, EtOH)
at room temperature, and are about 3.5% and 6.4%, respectively.
Fig. 5 (a) Fluorescence spectra of CoQ2 nanosheets with the relevant
AAMs, (b) effect of 10�4 M relevant AAMs on the FL intensity of
CoQ2 nanosheets, (from 1 to 9: p-nitroaniline, o-diaminobenzene,
3.2 Effect of pH on the luminescence response
The effect of pH in the range from 1 to 13 was studied and is
shown in Fig. 4. The results show an obvious decrease in the
luminescence intensity of CoQ2 nanosheets in a pH medium
below 3 and beyond 8, meanwhile in the interval 4.0–6.0, it
increases and is considered stable. Clearly, at low pH the ligand is
dissolved and creates surface defects. At high pH the base can
nucleophilically attack the surface, displacing the ligand creating
surface defects. Finally, a pH of 6.0 is selected in the following
biology assays.
m-diaminobenzene, p-diaminobenzene, p-toluidine, o-nitroaniline,
m-nitroaniline, p-chloroaniline, aniline. Inset: fluorescence photographs
of (I) CoQ2 nanosheets and (II) CoQ2 nanosheets and p-nitroaniline
(under l ¼ 365 nm UV light irradiation).
3.3 Detection of aromatic amines
Fig. 5 shows the FL response of CoQ2 nanosheets to 10�4 M
aromatic amines including p-nitroaniline, o-diaminobenzene,
m-diaminobenzene, p-diaminobenzene, p-toluidine, o-nitroani-
line, m-nitroaniline, p-chloroaniline and aniline. It is shown that
the p-nitroaniline can quench the luminescence of CoQ2
130 | Nanoscale, 2009, 1, 128–132
nanosheets selectively, as indicated by the fluorescence photo-
graphs. From the fluorescence photographs, the fluorescence of
the CoQ2 nanosheets was quenched after p-nitroaniline was
This journal is ª The Royal Society of Chemistry 2009
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added. However, the sensitivity of the CoQ2 nanosheets towards
other aromatic amines including o-diaminobenzene, m-di-
aminobenzene, p-diaminobenzene, p-toluidine, o-nitroaniline,
m-nitroaniline, p-chloroaniline and aniline is negligible.
Fig. 6a show the effect of increasing concentrations of
p-nitroaniline on the fluorescence of the nanosheets. It is found
that p-nitroaniline quenches the fluorescence of CoQ2 nanosheets
in a concentration dependence that is best described by a Stern–
Volmer type equation:
Imax/I ¼ 1 + KSV [S]
I and Imax are the fluorescent intensities of CoQ2 nanosheets at
a given p-nitroaniline concentration and in p-nitroaniline free
solution, respectively. KSV is the Stern–Volmer quenching
constant, and [S] is the p-nitroaniline concentration. The
dependence of Imax/I as function of [S], is shown in Fig. 6b. KSV is
found to be 1.034 � 104 M�1. The detection limits (DLs),
calculated following the 3s IUPAC criteria, are a little down to
6.8 � 10�7 M (9.38 ng L�1), which achieved the level of the
current chromatographic technique detection. For example,
a GC-MS method was also applied for aromatic amine
compounds detected with DLs in the range 2–30 ng L�1.2
To explore this method further it was used for more complex
samples. Competition experiments were also performed for
Fig. 6 (a) Fluorescence spectra of the CoQ2 with increasing concen-
trations of p-nitroaniline. (b) Effect of p-nitroaniline concentration on the
fluorescence of CoQ2 nanosheets showing decreasing emission with
increasing p-nitroaniline concentration. Inset: Stern–Volmer plot of
p-nitroaniline concentration dependence on the FL intensity with
a 0.997 correlation coefficient.
This journal is ª The Royal Society of Chemistry 2009
CoQ2 nanosheets in a mixture of p-nitroaniline and background
ions such as Li+, Na+, K+, Mg2+, Ca2+, Ba2+, Cl�, NO3�, Ac�,
H2PO4�, HPO4
2�, C2O42� and SO3
2�. As shown in Fig. 7, other
ions resulted in nearly no disturbance to the selective sensing of
CoQ2 nanosheets toward p-nitroaniline.
Fluorescence quenching can be static (e.g., complex forma-
tion) or dynamic (e.g., collision quenching).29 The complex
formation between p-nitroaniline molecules and CoQ2 can be the
main reason for the red-shift and the fluorescence quenching.
From Fig. 6a, the wavelength of fluorescence red-shifts with
increasing concentrations of p-nitroaniline, due to p-nitroaniline
coordinating with CoQ2 to form a new product CoQ2Xn
(X ¼ p-nitroaniline) chelate as shown in Fig. 8. In addition, the
electron-deficient nitroaromatics are strong quenchers to the
electron-rich chromophores via an electron transfer mechanism
for various photoluminescence materials, which results in the FL
intensity of the new formation complex decreasing with
increasing concentrations of p-nitroaniline.29,30 The p-nitroani-
line isomer easily forms complexes with the Co ion due to a lack
of steric hindrance (linear configuration of p-nitroaniline), which
results in the quenching by p-nitroaniline being larger than for
m-nitroaniline and o-nitroaniline.
3.4 Analysis of water samples
Surface river water samples were collected from local rivers. The
samples were filtered through 0.45 mm Supor filters and stored in
Fig. 7 Fluorescence spectra of CoQ2 in the presence of the p-nitroaniline
and miscellaneous ions X including Li+, Na+, K+, Mg2+, Ca2+, Ba2+, Cl�,
NO3�, AcO�, H2PO4
�, HPO42�, C2O4
2� and SO32� (10 mM, excitation
wavelength 320 nm). All spectral data were recorded at 10 min after
p-nitroaniline addition.
Fig. 8 Schematic illustration of a possible formation mechanism of
CoQ2Xn (X ¼ p-nitroaniline).
Nanoscale, 2009, 1, 128–132 | 131
Table 1 Recovery of p-nitroaniline in water samples with p-nitroanilinein solution at different concentration levels
Spikedconcentration/mM
Foundconcentration/mM
Recovery(%)
p-Nitroaniline 0.1 0.105 105.0p-Nitroaniline 0.5 0.478 95.6p-Nitroaniline 0.75 0.742 98.9p-Nitroaniline 1 1.02 102.0p-Nitroaniline 2.5 2.47 98.8
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precleaned glass bottles. As no AAMs in the collected water
samples were detectable by the proposed method, a recovery
study was carried out on the samples spiked with 2.5–0.1 mM
AAMs to evaluate the developed method.
To further demonstrate the practicality of the proposed
method, the recovery test was studied by adding different
amounts of p-nitroaniline into the water samples. The results
were summarized in Table 1. The recoveries were from 95.6% to
105%. These results demonstrated that it was a promising
approach and highly accurate, precise and reproducible. It can be
used for the direct analysis of relevant samples.
Conclusions
Cobalt(II)-bis(8-hydroxyquinoline) nanosheets have been
successfully synthesized via a simple sonochemical method to
develop a novel and highly sensitive luminescence probe for the
optical recognition of AAMs. The synthesized CoQ2 nanosheets
allow the detection of p-nitroaniline as low as 9.38 ng L�1, thus
affording a very sensitive detection system for AAMs analysis.
Future studies will investigate obtaining more sensitive and
selective metal complex-based nanosensors for the determination
of AAMs in environmental samples.
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