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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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