A critical comparison between two different ratiometric techniques for optical luminescence sensing

Abstract

Luminescence has become an invaluable analytical tool for development of robust fiber optical sensors for many industrial, biomedical and environmental applications. Direct intensity measurements of the luminescent emission are, due to their simplicity, a very common measuring principle. Consequently, many optical fiber sensors are based on the intensity measurement of the luminescence emission. Unfortunately, direct luminescent intensity measurements suffer from a series of analyte-independent fluctuations, which make them inappropriate for the development of reliable instrumentation for chemical sensing applications. A possibility for solving this trouble is to measure the lifetime of the luminescence emission. The drawbacks of such approach are an increase in the complexity of the system measurement, particularly for measurement of low lifetimes (e.g. nanoseconds), but also the limited number of available chemical sensors that suffer from a change in the luminescence lifetimes after interaction with the analyte. The ratiometric measurement can solve many of the trouble previously exposed. As it is well know, ratiometric measurements are an alternative to classical intensity or lifetime measurements due to their proved insensitivity to background light and instrumental fluctuations. Thus, they can be applied to the development of robust luminescence optical sensors. In this paper two measurement systems based on ratiometric techniques are compared. One of them is a ratiometric method in the frequency domain and the other one is a wavelength ratiometric method. The analytical performance of the dynamic ratiometric method has been assessed using a phosphorescent oxygen sensor, which is a good example of a sensor that suffers a dynamic luminescence deactivation after interaction with the analyte. The potential of the proposed ratiometric systems to overcome the problems of accuracy in luminescent sensing is evaluated by constructing two respective optical fiber instruments. The interest in designing an instrumental prototype specially adapted to a particular chemical sensor is also analyzed.