Synthesis of highly luminescent cobalt(II)-bis(8-hydroxyquinoline)nanosheets as isomeric aromatic amine probes

Abstract

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 CoQ₂nanosheets were characterized by luminescence spectroscopy, UV–vis spectroscopy, FT-IR spectroscopy and transmission electron microscopy (TEM). The CoQ₂nanosheets allow highly sensitive and selective determination of p-nitroanilinevia fluorescence quenching. Under optimal conditions, the relative fluorescence intensities of nanosheets decreased linearly with increasing p-nitroaniline. However, the sensitivity of CoQ₂nanosheets 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 CoQ₂nanosheets in a concentration-dependent manner that is best described by a Stern–Volmer-type equation. The possible underlying mechanism is discussed.

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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 environment 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 determination 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, fluorescence (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, Danielsona and coworkers have developed an N-alkyl acridine orange dye as a fluorescence probe for the determination of cardiolipin.¹³Calixarene-modified CdTe, synthesized by our group, allowed a highly sensitive determination of polycyclic aromatic hydrocarbons¹⁴ and pesticides.¹⁵ In comparison to traditional organic dyes, 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 properties.¹⁶ For the past few years, luminescent metal 8-hydroxyquinoline (MQ_n) chelates have attracted considerable interest owing to their various applications in photoluminescence, electroluminescence and field emission.¹⁷ Due to the unique optoelectronic properties of nanomaterials, more attention is attracted to nanostructured MQ_n chelates. AlQ₃ and many other MQ_n 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 nanostructures, 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 applications in OLEDs, only a few metal 8-hydroxyquinoline (MQ_n) nanomaterials have been employed as fluorescent analytical assays . Recently, fluorescent sensors based on MQ_n nanomaterials 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 determination of aromatic amines is almost unexplored.

In this work, we synthesized water-soluble, stable and highly fluorescent cobalt(II)-bis(8-hydroxyquinoline) complex nanosheets by a very simple sonochemical method and investigated 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-hydroxyquinoline) (CoQ₂). The X-ray diffraction (XRD) pattern of the 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 quinoline 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

Fig. 1 IR spectrum of the CoQ₂nanosheets.

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 ultrasound irradiation led to the continuous growth of CoQ₂nanosheets. 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 µm. Microcrystals with large dimensions could not be observed, which suggests that microcrystals of CoQ₂ are destroyed under ultrasound irradiation for long reaction times, and these microcrystals are changed into nanosheets due to weak bonding interactions between 2D coordination polymers.²⁷

Fig. 2 TEM images of the CoQ₂nanosheets. Scale bars: 500 nm.

Fig. 3 Schematic diagram showing the formation of CoQ₂nanosheets.

The photophysical properties of the CoQ₂nanosheets have been investigated in an ethanol–water solution. Fig. S2 (ESI† ) shows the UV–vis absorption spectra of 8-hydroxyquinoline (Q), CoQ₂ and CoQ₂nanosheets. CoQ₂ exhibits two resolved absorption bands at 310 nm and 356 nm, which is attributed to the transition of phenyl rings and the n–π transition, respectively.²⁸ The absorption of CoQ₂nanosheets is different from that regular CoQ₂. There is an approximate 30 nm red-shift, which is due to the π–π stacking of the nanosheets (geometry change). Photoluminescence is a very important characteristic for the 8-hydroxyquinoline metal chelates. The difference between the CoQ₂nanosheets and CoQ₂ has also been monitored by fluorescence spectra. It can be observed that the fluorescence spectra of the CoQ₂nanosheets 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 CoQ₂nanosheets (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, therefore, the CoQ₂nanosheets show an aggregation-induced emission enhancement (AIEE) characteristic. The quantum yields (QY) of CoQ₂ and CoQ₂nanosheets in ethanol are measured in comparison with the value of rhodamine B (QY = 89%, EtOH) at room temperature, and are about 3.5% and 6.4%, respectively.

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 CoQ₂nanosheets 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 .

Fig. 4 Effect of pH on luminescence response of the CoQ₂nanosheets.

3.3 Detection of aromatic amines

Fig. 5 shows the FL response of CoQ₂nanosheets to 10⁻⁴ M aromatic amines including p-nitroaniline, o-diaminobenzene, m-diaminobenzene, p-diaminobenzene, p-toluidine, o-nitroaniline, m-nitroaniline, p-chloroaniline and aniline. It is shown that the p-nitroaniline can quench the luminescence of CoQ₂nanosheets selectively, as indicated by the fluorescence photographs. From the fluorescence photographs, the fluorescence of the CoQ₂nanosheets was quenched after p-nitroaniline was added. However, the sensitivity of the CoQ₂nanosheets towards other aromatic amines including o-diaminobenzene, m-diaminobenzene, p-diaminobenzene, p-toluidine, o-nitroaniline, m-nitroaniline, p-chloroaniline and aniline is negligible.

Fig. 5 (a) Fluorescence spectra of CoQ₂nanosheets with the relevant AAMs, (b) effect of 10⁻⁴ M relevant AAMs on the FL intensity of CoQ₂nanosheets, (from 1 to 9: p-nitroaniline, o-diaminobenzene, m-diaminobenzene, p-diaminobenzene, p-toluidine, o-nitroaniline, m-nitroaniline, p-chloroaniline, aniline. Inset: fluorescence photographs of (I) CoQ₂nanosheets and (II) CoQ₂nanosheets and p-nitroaniline (under λ = 365 nm UV light irradiation).

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 CoQ₂nanosheets in a concentration dependence that is best described by a Stern–Volmer type equation:

I_max/I = 1 + K_SV [S]

I and I_max are the fluorescent intensities of CoQ₂nanosheets at a given p-nitroaniline concentration and in p-nitroaniline free solution, respectively. K_SV is the Stern–Volmer quenching constant, and [S] is the p-nitroaniline concentration. The dependence of I_max/I as function of [S], is shown in Fig. 6b. K_SV is found to be 1.034 × 10⁴ M⁻¹. The detection limits (DLs), calculated following the 3σ 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

Fig. 6 (a) Fluorescence spectra of the CoQ₂ with increasing concentrations of p-nitroaniline. (b) Effect of p-nitroaniline concentration on the fluorescence of CoQ₂nanosheets 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.

To explore this method further it was used for more complex samples. Competition experiments were also performed for CoQ₂nanosheets in a mixture of p-nitroaniline and background ions such as Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Ba²⁺, Cl⁻, NO₃⁻, Ac⁻, H₂PO₄⁻, HPO₄²⁻, C₂O₄²⁻ and SO₃²⁻. As shown in Fig. 7, other ions resulted in nearly no disturbance to the selective sensing of CoQ₂nanosheets toward p-nitroaniline.

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⁻, C₂O₄²⁻ and SO₃²⁻ (10 µM, excitation wavelength 320 nm). All spectral data were recorded at 10 min after p-nitroaniline addition.

Fluorescence quenching can be static (e.g., complex formation) 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 CoQ₂X_n (X = 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-nitroaniline 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.

Fig. 8 Schematic illustration of a possible formation mechanism of CoQ₂X_n (X = p-nitroaniline).

3.4 Analysis of water samples

Surface river water samples were collected from local rivers. The samples were filtered through 0.45 µm Supor filters and stored in 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 µM 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.

Table 1 Recovery of p-nitroaniline in water samples with p-nitroaniline in solution at different concentration levels

	Spiked concentration/µM	Found concentration/µM	Recovery (%)
p-Nitroaniline	0.1	0.105	105.0
p-Nitroaniline	0.5	0.478	95.6
p-Nitroaniline	0.75	0.742	98.9
p-Nitroaniline	1	1.02	102.0
p-Nitroaniline	2.5	2.47	98.8

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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 CoQ₂nanosheets 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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Footnote

† Electronic supplementary information (ESI) available: XRD pattern of the CoQ₂nanosheets, the UV–vis absorption spectra and fluorescence spectra of ligand Q, CoQ₂ and CoQ₂ nanosheets. See DOI: 10.1039/b9nr00019d

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