Abstract
Temperature is a key parameter in many fields and luminescence-based temperature sensing is a solution for those applications in which traditional (mechanical, electrical, or IR-based) thermometers struggle. Amongst the indicator dyes for luminescence thermometry, Ru(II) polyazaheteroaromatic complexes are an appealing option to profit from the widespread commercial technologies for oxygen optosensing based on them. Six ruthenium dyes have been studied, engineering their structure for both photostability and highest temperature sensitivity of their luminescence. The most apt Ru(II) complex turned out to be bis(1,10-phenanthroline)(4-chloro-1,10-phenanthroline)ruthenium(II), due to the combination of two strong-field chelating ligands (phen) and a substituent with electron withdrawing effect on a conjugated position of the third ligand (4-Clphen). In order to produce functional sensors, the dye has been best embedded into poly(ethyl cyanoacrylate), due to its low permeability to O2, high temperature sensitivity of the indicator dye incorporated into this polymer, ease of fabrication, and excellent optical quality. Thermosensitive elements have been fabricated thereof as optical fiber tips for macroscopic applications (water courses monitoring) and thin spots for microscopic uses (temperature measurements in cell culture-on-a-chip). With such dye/polymer combination, temperature sensing based on luminescence lifetime measurements allows 0.05 °C resolution with linear response in the range of interest (0–40 °C).
Highlights
Temperature is one of the most important parameters in industrial process control, environmental monitoring, medicine practice, and biology, to name a few areas [1]
In the case of Ru(II) polypyridyl complexes, it has been demonstrated [27,28,29] that thermal activation can promote the photoexcited electron of the metal dye, located in a luminescent metal-to-ligand charge transfer (3 MLCT) manifold of three closely spaced states (Figure 1), to a nearby 3 MLCT excited state, and often to a higher lying non-emissive metal-centered state (3 MC)
The non-radiative constant corresponding to the thermal activation process follows Arrhenius-type kinetics, so that the temperature dependence of the luminescence lifetime (τ, i.e., the inverse of the excited state deactivation rate constant, kd ) for a Ru(II) complex can be expressed according to Equation (1) [30], τ = 1/kd = [ A + Bexp(−∆E/kB T )]−1 (1)
Summary
Temperature is one of the most important parameters in industrial process control, environmental monitoring, medicine practice, and biology, to name a few areas [1]. It has the advantage of profiting from the vast amount of research and development put forward for fiber-optic O2 measurements with such phosphors Enough, it seems that the only temperature indicators of this class used so far have been those already employed for O2 sensing [2,7], namely the tris(2,20 -bipyridine)ruthenium(II) [16,17,18,19], tris(1,10-phenanthroline)ruthenium(II) [15,20,21,22,23], and tris(4,7-diphenyl-1,10-phenanthroline)ruthenium(II) complexes [15,24], with the exception of one particular low-temperature application of bis(terpyridine)ruthenium(II) [25]. The best novel Ru(II) complex has been used here in two different formats, both using a fiberoptic phase shift-based luminometer: (i) as mm-thick polymer monolith onto the optical fiber tips (e.g., for water monitoring applications), and (ii) as μm-thick films into organ-on-a-chip devices, monitored from outside the chip with the optical fibers placed onto the sensitive spots
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