Unconventional ratiometric-enhanced optical sensing of oxygen by mixed-phase TiO2

Abstract

Abstract. We show that mixed-phase titanium dioxide (TiO2) can be effectively employed as an unconventional, inorganic, dual-emitting and ratiometric optical sensor of O2. Simultaneous availability of rutile and anatase TiO2 PL and their peculiar "anti-correlated" PL responses to O2 allow using their ratio as measurement parameter associated to O2 concentration, leading to an experimental responsivity being by construction larger than the one obtainable for single-phase PL detection. A proof of this concept in given, showing a two-fold enhancement of the optical responsivity provided by the ratiometric approach. Besides the peculiar ratiometric-enhanced responsivity, other characteristics of mixed phase TiO2 can be envisaged as favorable for O2 optical probing, namely: a) low production costs, b) absence of heterogeneous components, c) self-supporting properties. These characteristics encourage experimenting its use for applications requiring high indicator quantities at competitive price, possibly also tackling the need to develop supporting matrixes that carry the luminescent probes and avoiding issues related to the use of different components for ratiometric sensing.Determining the content of molecular oxygen (O2) in gas mixtures and/or aqueous environments is important in many different fields. For example, monitoring the O2 concentration in the exhaust stream of an internal combustion engine provides information on the air-fuel ratio of the combustion process, whose control is relevant for reducing harmful emission [1][2][3] . Knowing the concentration of O2 dissolved in water (also "dissolved oxygen", DO) is also crucial in many applications. DO level is intimately related to the state of health of marine habitats, to the concentration of organic pollutants in water 4 and is of great importance for biological studies. In fact, DO content in cell culture media affects cell growth rate, metabolism and protein synthesis, exerting a decisive influence on the cellular functions and behaviors. To mention a few examples, hypoxia/re-oxygenation induce stem cell like

Charge Carrier Processes and Optical Properties in TiO2 and TiO2-Based Heterojunction Photocatalysts: A Review

“…It is interesting to mention that a similar concept can be implemented for mixed phase TiO 2 due to the fact that the PL of the rutile and of the anatase forms react differently to O 2 exposure under UV illumination conditions. This peculiarity of TiO 2 has been indeed exploited in Reference [ 213 ], showing that by using a mixture of rutile and anatase nanoparticles the sensitivity of the O 2 -induced PL changes can be increased significantly—in comparison with the one obtainable by single-phase nanopowders—using the ratio (rutile PL/anatase PL) as the parameter that signal the presence of O 2 . That leads to a “ratiometric” responsivity: which is by construction larger than the one obtainable using single-phase titania (for the demonstration see Reference [ 213 ]).…”

“…This peculiarity of TiO 2 has been indeed exploited in Reference [ 213 ], showing that by using a mixture of rutile and anatase nanoparticles the sensitivity of the O 2 -induced PL changes can be increased significantly—in comparison with the one obtainable by single-phase nanopowders—using the ratio (rutile PL/anatase PL) as the parameter that signal the presence of O 2 . That leads to a “ratiometric” responsivity: which is by construction larger than the one obtainable using single-phase titania (for the demonstration see Reference [ 213 ]). In Equation (14) “NIR” and “VIS” refer to the near-infrared PL of rutile and to the visible PL of anatase (respectively), while and are the PL intensity in absence and in presence of O 2 , respectively.…”

“…( b ) Time-dependent optical responsivities R(t) at different O 2 concentrations for the mixed-phase sample (“mixed phase titania”, labelled as MPT) corresponding to the data shown in ( a ) and for two single-phase samples (anatase and rutile nanopowders). The responsivity for the mixed-phase sample is indicated in Equation (14), while the expression for the responsivities for anatase (showing a quenching response Rq) and rutile (showing an enhancement response RE) are reported in Reference [ 213 ]. The O 2 concentration values are reported in ( a ).…”

“…The O 2 concentration values are reported in ( a ). Reprinted from Reference [ 213 ], with the permission of AIP Publishing. Copyright 2016 AIP Publishing.…”

Ultrafast Hyperspectral Transient Absorption Spectroscopy: Application to Single Layer Graphene

“…We use Raman spectroscopy (not shown) to confirm the nature of our CVD grown SLG. It is a widely employed method to study the vibrational properties of several nanomaterials [47][48][49][50] and particularly useful for carbon-based nanostructures such as few-layer graphene [51,52]. Indeed it has been well established that Raman spectroscopy represents a robust metrics of the number of layers to be used in place of or in combination with AFM [53,54].…”