Application of gold quenching of luminescence to improve oxygen sensing using a ruthenium (4,7-diphenyl-1,10-phenanthroline)3Cl2:TEOS thin film

A new method for enhancement of luminescence from sol–gel based sensors is demonstrated, utilising the quenching effect and filtering effect of a gold film. Ruthenium (4,7-diphenyl-1,10-phenanthroline)3Cl2 is encapsulated in a trimethoxysilane (TEOS) xerogel and spin-coated to an approximate thickness of less than 100 nm. Using off axis illumination of the xerogel film, luminescence was collected using a photodiode below the plane of the gold and doped thin film. The gold had the effect of improving all sensor figures of merit via two mechanisms. Firstly, the transmission spectrum of the gold film peaks at 520 nm but allows poor transmission of excitation light at 450 nm reducing stray light interference in addition to a 550 nm cut-on filter. Secondly, and more, importantly relatively insensitive luminophores within approximately 10 nm of the gold film are quenched resulting in a significant noise reduction by removal of insensitive residual metal ion–ligand complex luminescence. It is questioned whether an element of radiative-decay-engineering of luminescence is occurring with luminophore emission coupling to the gold film and initiating plasmonic emission from the underside of the film, but no evidence either in terms of solely p-polarisation or increased intensity of luminescence was evidenced. This confirms that the sensor enhancement mechanism is primarily the function of metal quenching as evidenced by reduced background intensity as noise and improved response time to oxygen quenching indicating that only the most sensitive luminophores are available to be dynamically quenched by formation of the ruthenium:oxygen charge transfer complex. This simple sensor geometry improves a basic oxygen sensor luminophore:matrix combination and stands comparison with more complex combinations such as those achieved with fluorinated sol–gel matrices.