Abstract
Metal−organic frameworks (MOFs) are a class of porous coordination networks extraordinarily varied in physicochemical characteristics such as porosity, morphologies, and compositions. These peculiarities make MOFs widely exploited in a large array of applications, such as catalysis, chemicals and gas sensing, drug delivery, energy storage, and energy conversion. MOFs can also serve as nanostructured precursors of metal oxides with peculiar characteristics and controlled shapes. In this work, starting from MIL125-(Ti), a 1,4-benzenedicarboxylate (BDC)-based MOF with Ti as metallic center, mesoporous TiO2 powders containing both anatase and rutile crystalline phases were produced. A challenging utilization of these porous MOF-derived Ti-based oxides is the optically-based quantitative detection of molecular oxygen (O2) in gaseous and/or aqueous media. In this study, the photoluminescence (PL) intensity changes during O2 exposure of two MOF-derived mixed-phase TiO2 powders were probed by exploiting the opposite response of rutile and anatase in VIS-PL and NIR-PL wavelength intervals. This result highlights promising future possibilities for the realization of MOF-derived doubly-parametric TiO2-based optical sensors.
Highlights
Published: 29 June 2021The detection of gaseous oxygen (O2 ) in gas mixtures and/or aqueous environments is important for many of applications, such as air quality control, packaging, life sciences, and automotive and chemical industry [1,2,3,4].The most employed techniques for measuring the O2 concentration include Winkler titration, electroanalytic methods, pressure-based methods and optics-based methods [4].Optical sensors, or opt(r)odes, are chemical sensors in which the interaction of the electromagnetic radiation with the sample causes the change of a particular optical parameter, the resulting change being related to the concentration of the analyte [5]
From an application standpoint, it is worth underlining that this phenomenon allows using mixed-phase TiO2 for the preparation of inorganic ratiometric O2 optical sensors [29]. This finding highlights the potentialities of the photoluminescence-based sensing compared to conventional conductometric methods based on single-parameter responses. Starting from such a result, in this work we explored the possibility of producing mesoporous mixed-phase TiO2 from a metal organic framework (MOF) and testing it as a Catalysts 2021, 11, 795 dual-emitting O2 optical probe
A 2% weight loss was observable and ascribable to the removal of solvent molecules entrapped inside the MOF pores
Summary
The most employed techniques for measuring the O2 concentration include Winkler titration, electroanalytic methods, pressure-based methods and optics-based (optical) methods [4]. Opt(r)odes, are chemical sensors in which the interaction of the electromagnetic radiation with the sample causes the change of a particular optical parameter, the resulting change being related to the concentration of the analyte [5]. Optical sensors can be based on various optical principles (absorbance, reflectance, luminescence, fluorescence) and can cover different regions of the electromagnetic spectrum (UV, Visible, IR, NIR). Optical sensors can allow for the measurement of the light intensity, and of other related properties, such as lifetime, refractive index, scattering, diffraction, and polarization [5]. Optical oxygen sensors have become attractive in the last years thanks to: (a) The lack of oxygen consumption during measurements; (b) The
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