Luminophore-immobilized mesoporous silica for selective Hg2+ sensing
Graphical abstract
Introduction
Fluorescent molecules for transition and heavy metal cation sensors such as Hg2+, Cu2+, Pb2+, and Zn2+ ions have been increasingly important as tools for the quantitative and the qualitative monitoring of the target metal ions in many biological and environmental processes.1, 2, 3, 4, 5, 6, 7, 8 Then, based upon the concepts of the host–guest chemistry, cation sensing has recently risen to a dominant position in research devoted to the detection of designated species. An effective fluorescence chemosensor must convert the event of cation recognition by the ionophore into an easily monitored and highly sensitive light signal from the fluorophore.9, 10 As such a fluorogenic unit, rhodamine dyes have been used for conjugation with biomolecules for fluorescent probes11, 12, 13, 14, 15, 16 owing to their excellent fluorescence properties. A spirolactam form of rhodamine is generally non-fluorescent and colorless. By the addition of guest ions to a solution of the rhodamine, however, the rhodamine shows a strong fluorescence and a pink color.
Recently, the use of organic–inorganic materials has been interesting in the research for new methodologies for ion recognition and sensing. The receptor-immobilized inorganic materials such as SiO2, Al2O3, and TiO2 have some important advantages17, 18, 19, 20, 21, 22, 23, 24, 25 as a solid chemosensor in heterogeneous solid-liquid phase. First, immobilized receptor on the inorganic support can liberate the organic guest molecule (metal and anion ions) to the solution of the pollutant.
Second, the organic–inorganic hybrid nanomaterials can be recyclable by suitable chemical treatment. Lastly, functionalized nanomaterials combined with fluorophore or chromophore display high selective and sensitive fluorescence or absorption changes because of their large surface area and well-defined pores in comparison to spherical structures. So, taking advantage of the homogeneous porosity with the large surface area, the mesoporous silica as an inorganic support has been quite interesting.26, 27, 28, 29, 30, 31, 32, 33, 34
Most receptor-immobilized inorganic materials developed so far have been known to be related to their UV band changes upon metal ion introduction. For example, Nazeeruddin et al. reported colorimetric, fluorimetric, and electrochemical detection of mercury ions by functionalized ruthenium sensitizers in aqueous and non-aqueous solutions, on anchored TiO2 films, but the most concern would be the colorimetric Hg2+ sensing.6, 8
In this context, we have developed a mesoporous silica-immobilized rhodamine (MSIR) because of its significant fluorescence change as well as color variation in the event of the Hg2+ ion-induced spirolactam ring opening. Hence, we now report its adsorption ability along with fluorescence responses upon the addition of various metal cations.
Section snippets
Results and discussion
The MSIR was prepared by coupling reaction of 1 and mesoporous silica in toluene. Compound 1 was synthesized by condensation of rhodamine-tren 2 with 3-(triethoxysilyl)propylisocyanate in toluene as a solvent. The mesoporous silica was prepared by adaptation of synthetic methods published eariler.21, 22 For the comparison of the functional advantages of the MSIR, the commercial silica-bounded rhodamine was also prepared via similar synthetic route to MSIR (Scheme 1, Scheme 2).
The synthetic MSIR
Conclusions
A novel mesoporous silica-immobilized rhodamine (MSIR) targeting for selective response to Hg2+ ion over other metal ions is developed. The sensitivity of the MSIR for Hg2+ ion is greater than that of the SPIR. In adsorption ability for Hg2+ ion, MSIR (70%) is better than SPIR (40%). In addition, in the light of portable colorimetric and fluorimetric kit for detection of Hg2+ ion in the environmental field, the glass plate-coated MSIR is prepared and shows an excellent function in visual and
Preparation of MSIR and SPIR
Compound 1 (20 mg, 0.02 mmol) was dissolved in anhydrous toluene (5 mL) to which the mesoporous silica (100 mg) or commercial silica was also added and stirred in reflux condition under N2 for 24 h. The collected solid was washed several times with methylene chloride and acetone to rinse away any surplus 1. MSIR or SPIR was obtained as a solid (100 mg). IR (KBr pellet, cm−1): 3382, 2975, 1656, 1517/3326, 1634, 1554, 1063.
Preparation of 1
Compound 2 (160 mg, 0.28 mmol) and 3-(triethoxysilyl)propylisocyanate (0.15 g, 0.62
Acknowledgements
This work was supported by the SRC program (R11-2005-008-02001-0(2006)) and KOSEF (R01-2005-000-10229-0). In addition, this work was partially supported by Korea Ministry of Environments as ‘The Eco-technopia 21 Project’.
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