Emission Based Sensors

Luminescence of Lanthanide Lons in Coordination Compounds and Nanomaterials, First Edition. 

As described earlier, the lifetimes and quantum yields of emissive Ln complexes vary dramatically due to the extremely sensitive nature of the 4/-centred excited states to 0-H, N-H and C-H vibrational manifolds, which can provide efficient, non-radiative deactivation pathways the efficiency of energy transfer between the antenna and lanthanide ion also determines overall quantum yields. A classical approach to maximising the emissivity of Ln complexes is to therefore inhibit the approach of water solvent to the inner coordination sphere (and where q denotes the number of coordinated solvent molecules) high denticity, metal ion encapsulating ligands with hydrophobic peripheries can achieve this very effectively, reducing q to zero [4]. 

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The future targets of supramolecular photochemistry in CD chemistry will contain photoresponsive molecular machines, emission-based sensors, and energy transport systems. For construction of such systems, the design of three-dimen-sionally correct arrangement of component units will become important. The molecular modeling computation approach will be helpful for designing the systems and deeper understanding of structural features of chromophore-modified CDs and their complexes. 

Fluorescence resonance energy transfer (FRET) has also been used very often to design optical sensors. In this case, the sensitive layer contains the fluorophore and an analyte-sensitive dye, the absorption band of which overlaps significantly with the emission of the former. Reversible interaction of the absorber with the analyte species (e.g. the sample acidity, chloride, cations, anions,...) leads to a variation of the absorption band so that the efficiency of energy transfer from the fluorophore changes36 In this way, both emission intensity- and lifetime-based sensors may be fabricated. 

Even when the d-d state is at much higher energy than the emitting level, it can still be of paramount importance in the photophysics and photochemistry of the system. Indeed, a major contributor to the temperature-dependent loss of emission intensity in luminescent metal complex based sensor materials is nonradiative decay via high-energy d-d excited states.(15) The model for this is shown in Figure 4.4A. The excited state lifetime is given by... 

The present authors and coworkers observed intense CTL emission during the catalytic oxidation of ethanol or acetone vapor on a heated aluminum oxide powder [8]. This phenomenon was applied to the consumption-free CTL-based sensor for detecting combustible vapors. The CTL response was fast and reproducible for a change in concentration of a sample vapor in air. CTL emission has three distinct features ... 

As CTL emission is observed in the course of catalytic oxidation, we must consider the overall reaction process in order to describe the working mechanism of the CTL-based sensor. Figure 8 shows a schematic illustration of the catalyst layer to depict the simplified overall reaction processes involving CTL emission on the CTL-based gas sensor. 

Similar to the catalyst of the catalytic thermometry sensor, the catalytic activity of the CTL-based sensor depends not only on the kind of catalyst material and the surface-to-volume ratio of the powder but also on the preparation procedure of the powder. In considering these conditions, a detailed comparison of the CTL catalytic activity has not been reported so far. The present authors and coworkers observed the CTL emission by ethanol vapor on y-aluminum oxide, barium sulfate, calcium carbonate, and zirconium oxide at a few hundred degrees. On the other hand, CTL emission is not observed during the catalytic oxidation on metal and semiconductive materials, e.g., tin oxide, zinc oxide, and copper oxide. 

The phenomenon of CTL emission has a rather long history since it was first observed in 1976. Unfortunately, however, only a few researchers to date have studied this phenomenon and its application for gas sensors. Although various proposals for measuring techniques using CTL-based sensors have... 

Oxygen sensors are the most widely used solid electrolyte-based sensors [393-395], because the control of oxygen concentrations is critical to controlling the combustion process. For automotive applications, exhaust gas oxygen (EGO) sensors provide critical information for controlling the air-to-fuel ratio for internal combustion engines [396, 397]. Tlte use of an optimal air-to-fuel ratio leads to increased efficiency and reduced emissions. 

Castellano et al.221 reported the formation of a luminescence lifetime-based sensor for cyanide and other counterions using Ru11 diimines possessing MLCT excited states with the anion recognition capabilities of 2,3-di(l//-2-pyrrolyl)quinoxaline (DPQ). Using time-resolved photoluminescence decay, its viability as a lifetime-based sensor for anions has been tested. There were significant changes to the UV-vis and steady-state emission properties after the addition of several ions (e.g., fluoride, cyanide, and phosphate). 

A very important aspect of gas sensors in automotive exhaust-gas environments is aftertreatment of the electrodes to control a specific sensor behavior. For example, to measure nonequilibrium raw emissions, the sensor needs excellent catalytic ability. Various methods are known to improve electrodes in Zr02-based sensors. One well known method is to increase the active platinum surface area and the three-phase boundary area by partial reduction of zirconia close to the electrode. This occurs when the ceramic is exposed to a reducing atmosphere at high temperatures or when an electrical cathodic current is applied through the electrode and electrolyte. A similar effect can be achieved by chemical etching of the elec-... 

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