Luminescent ratiometric method in the frequency domain with dual phase-shift measurements: Application to oxygen sensing

Abstract

A novel ratiometric method for luminescence measurements in the frequency domain is presented. The method is based on the large difference between the lifetimes of the phosphorescence and fluorescence emissions of a dual luminescent indicator. The intensity ratios long-lived/short-lived emission is calculated by measuring the phase shift between the excitation and emission signals at two different but close frequencies. Ratiometric measurements are preferred to alternative luminescent measurements due to their proved insensitivity to ambient or scattered light, instrumental fluctuations, etc. Thus, they can be applied to the development of robust luminescence optical sensors. Theoretical aspects of the new methodology proposed here, the design and construction of a fiber-optic measuring prototype system using low-cost optoelectronics, including a light emitting diode (LED) to excite the chemical sensor and a conventional photodiode to measure the luminescent emission, is presented. The drive of the LED to obtain a good ratiometric measurement is also discussed. The performance of the proposed measurement method and simple instrument has been assessed using an oxygen sensing phase prepared by immobilizing Al-Ferron in an inorganic sol–gel support. This oxygen sensitive indicator shows a strong oxygen-insensitive fluorescence emission overlapping significantly with a long-lifetime oxygen-dependent phosphorescence emission. As compared to other ratiometric approaches, the method here proposed requires only one optical path to measure the luminescent response and luminophors showing only emission intensity changes in response to a certain analyte can be used, since changes in lifetime are not required. Interestingly, the ratiometric method can even be used without losing accuracy even if a spectral overlap between the two measured emissions occurs.