Quinoline Group Modified Carbon Nanotubes for the Detection of Zinc Ions
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Carbon nanotubes (CNTs) were covalently modified by fluorescence ligand (glycine-N-8-quinolylamide) and formed a hybrid material which could be used as a selective probe for metal ions detection. The anchoring to the surface of the CNTs was carried out by the reaction between the precursor and the carboxyl groups available on the surface of the support. Fourier transform infrared spectroscopy (FTIR) and Thermogravimetric analysis (TGA) unambiguously proved the existence of covalent bonds between CNTs and functional ligands. Fluorescence characterization shows that the obtained organic–inorganic hybrid composite is highly selective and sensitive (0.2 μM) to Zn(II) detection.
KeywordsCarbon nanotubes Glycine-N-8-quinolylamide Zn(II) Fluorescent material Detection
There has been growing interest during the last decade in the development of fluorescent molecular sensors for cations and anions in solution [1–8]. Especially, fabricating fluorescent materials for the detection of Zinc cation has drawn much more attention [9–13], as Zinc not only plays important roles in human bodies [14, 15], but also closely relates to severe pathological diseases such as Alzheimer’s and Parkinson’s diseases . So far, much study has been done for the detection and real-time localization of Zn(II). Yasuhiro Shiraishi’s group has synthesized a quinoline–polyamine conjugate as a fluorescent chemosensor for quantitative detection of Zn(II) in water . Maarten Merkx et al. used chelating fluorescent protein chimeras for ratiometric detection of Zn(II) in living cells and the detection range was from 10 nM to 1 mM . Jinshi Ma and coworkers have synthesized several bis(pyrrol-2-yl-methyleneamine) ligands as fluorescent sensor for Zn(II) , and their results revealed that the ligands exhibit excellent fluorescent properties. However, much of the work was just based on organic molecules as fluorescent chemosensors. For a few practical applications the attachment of the fluorescent units to a solid support has advantages like the possibility of recovering the materials for their repetitive use. For this point, scientists chose silica nanoparticles , nanosized boehmite particles  and silicon nanowires  to support fluoresence ligands as fluorescence sensors. And these materials exhibit excellent selectivity and sensitivity to sense metal ions.
In this study, we chose multi-walled carbon nanotubes (MWNTs) as fluorescent support. Since its discovery, surface modification of MWNTs has received considerable attention [20–24]. The fluorophore in this study is glycine-N-8-quinolylamide (GNQ) molecule, in light of the fact that the 8-aminoquinoline derivatives could effectively coordinate with specific metal ions . The quinoline group has been covalently grafted to the surface of the MWNTs that can behave as recognition center for metal ions depending on its actual protonation state. We find that this new material (MWNTs-GNQ) has high selectivity and sensitivity to detect Zn(II), and the sensitivity is down to 0.2 μM., which is about the same as the silica nanoparticles-supported fluorescence sensors . For other sensing materials [26, 27], the fluorescence enhancement selectivity is not only for Zn(II) but also for Cd(II), which may reduce selectivity when they are used just for Zn(II) detection. On the other hand, the fluorescence enhancement of MWNTs-GNQ is only for Zn(II).
As the use of organic–inorganic hybrid materials for bio-application has become a hot subject in the research field currently [28–31], MWNTs-GNQ may be used to build nanosensor devices to sense directly in intracellular environment, because carbon nanotubes (CNTs) can penetrate into cells and almost have no toxicity to organism [32, 33].
The multi-walled carbon nanotubes (MWNTs, diameters: 20–40 nm, purity: 95–98%) prepared by the catalytic decomposition of CH4 were provided by Shengzhen Nanotech Port Ltd. Co (China). Methanol and Tetrahydrofuran were used after distillation. Other reagents were analytical and used without purification. Glycine-N-8-quinolylamide (GNQ) was synthesized according to the known method .
Purification of MWNTs
In a typical experiment, 300 mg pristine-MWNTs were added to a 180 mL 3:1 mixture of concentrated H2SO4and HNO3. The mixture was treated in an ultrasonic bath (40 kHz) for 20 min and stirred at 60 °C for 4 h under reflux. Then, the mixture was vacuum-filtered through a 0.22 μm Millipore polytetrafluoroethylene membrane and washed with distilled water until pH of the filtrate was 7. The filter cake was dried under vacuum at 40 °C for 24 h to obtain MWNTs-COOH.
Functionalization of MWNTs (Scheme 1)
MWNTs-COCl of 100 mg was dispersed in 10 mL anhydrous chloroform and the mixture was sonicated for 20 min to create a homogeneous suspension. The mixture was added with 50 mg GNQ under a nitrogen atmosphere, and then immersed in an oil bath at 70 °C accompained by mechanical stirring for 24 h. The resulting reaction medium was vacuum-filtered through a 0.22 μm polycarbonate membrane three times to yield MWNTs-GNQ.
All the reactions in the experimental procedure were carried out under a nitrogen atmosphere.
Characterization and Test of the Materials
Fourier transform infrared (FTIR) spectrometer (Bruker IFS66/S), Dupont-1090 Thermal gravimetric analysis (TGA) instrument and Gmbh Varioel Elementar Analysensyteme were used to characterize the materials. Perkin Elmer LS 55 spectrofluorimeter was used to obtain the fluorescence spectra of the fluorescence material.
Results and Discussion
Elemental microanalysis for MWNTs-GNQ
It was reported that most of the previous Zn(II) sensors do not exhibit good selectivity to these metal cations [26, 27] (for instance, the selectivity in many cases is close to 1:1 for Zn(II):Cd(II)). This may bring trouble to certain applications where Co(II), Ni(II), Cu(II), or Cd(II) may interfere (e.g., in environmental science). Herein, MWNTs-GNQ shows 2.8-fold fluorescence enhancement for Zn(II) versus just minimal fluorescence enhancement for Cd(II) and Ni(II). From these results, it is evident that MWNTs-GNQ have a high selectivity to Zn(II).
As for selectivity of MWNTs-GNQ to metal ions, when Zn(II) forms a complex with MWNTs-GNQ with a suitable radius and an electronic structure 3d104 s0, the electron-transfer process of MWNTs-GNQ is forbidden , and an extended π–electron conjugation system is formed synchronously. This conjugation system is involved in an internal charge transfer process from the ligand donor to the Zn(II) acceptor, and simultaneously inhibits the excited-state proton transfer and photo-induced electron transfer that strongly suppress the fluorescence of MWNTs-GNQ. Thus Zn(II) considerably enhances the fluorescence of MWNTs-GNQ.
We have prepared a new organic–inorganic hybrid sensing material based on CNTs as support and glycine-N-8-quinolylamide as fluorescent center. The results of the fluorescence characterization show that the composite has a highly selective and sensitive (0.2 μM) detection for Zn(II), and reveal that ratiometric Zn(II) sensing is possible with fluorophore chemically modified carbon nanotubes. This novel fluorescent material may be used as a fluorescent device in intracellular environment for the detection of Zn(II).