Optical Gas Sensing of Ammonia and Amines Based on Protonated Porphyrin/TiO₂ Composite Thin Films.

Pedro Castillero,Juan Sánchez-Valencia,Agustín González-Elipe,Angel Barranco,José Pedrosa,Javier Roales,Tânia Lopes-Costa

doi:10.3390/s17010024

Abstract

Open porous and transparent microcolumnar structures of TiO2 prepared by physical vapour deposition in glancing angle configuration (GLAD-PVD) have been used as host matrices for two different fluorescent cationic porphyrins, 5-(N-methyl 4-pyridyl)-10,15,20-triphenyl porphine chloride (MMPyP) and meso-tetra (N-methyl 4-pyridyl) porphine tetrachloride (TMPyP). The porphyrins have been anchored by electrostatic interactions to the microcolumns by self-assembly through the dip-coating method. These porphyrin/TiO2 composites have been used as gas sensors for ammonia and amines through previous protonation of the porphyrin with HCl followed by subsequent exposure to the basic analyte. UV–vis absorption, emission, and time-resolved spectroscopies have been used to confirm the protonation–deprotonation of the two porphyrins and to follow their spectral changes in the presence of the analytes. The monocationic porphyrin has been found to be more sensible (up to 10 times) than its tetracationic counterpart. This result has been attributed to the different anchoring arrangements of the two porphyrins to the TiO2 surface and their different states of aggregation within the film. Finally, there was an observed decrease of the emission fluorescence intensity in consecutive cycles of exposure and recovery due to the formation of ammonium chloride inside the film.

Highlights

The detection and measurement of gas concentrations is important for understanding and monitoring a variety of phenomena, from industrial processes to environmental change
Optical Properties of MMPyP and TMPyP before and after Protonation state of these porphyrins, with their Soret bands centred at 417 nm and 425 nm, respectively
10 nm blue-shift with state.inThese results indicate different aggregation states nm, meaning a partial 10 nm blue-shift with respect to its monomeric state. These results indicate of the porphyrins into the TiO2 matrix

Summary

Introduction

The detection and measurement of gas concentrations is important for understanding and monitoring a variety of phenomena, from industrial processes to environmental change. There has been a growing demand for the development of sensitive, portable, and cost-effective sensors for gases such as ammonia and amines. Ammonia plays an important role in several environmental issues, such as eutrophication or acidification processes, human health, and atmospheric particle formation, contributing to climate change [1]. Its detection is of interest for medical applications [2,3]. Some amines appear as reaction intermediates in the production of chemicals, and their presence needs to be controlled because of their potential toxicity [4]. Other amines can be used as food freshness indicators, as they are present in rotting food as a result of bacterial decomposition [5,6,7,8]

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Results

Conclusion