| In biology and medicine, how we make measurements is extremely crucial, and this is especially true in terms of how we measure physiological exchange across cell membrane in single cells and/or tissues. There are many applications of electrochemical methods used for biological measurements. While these methods have significantly advanced our knowledge, they often suffer from experimental artifacts such as low sensitivity, drift, noise and fouling. In contrast optical methods for biological measurements have gained significant momentum due to their advantages. Optical dyes are typically more specific than redox chemistry and highly sensitive due to photodetectors such as photomultiplier tubes which are sensitive to a few photon levels. They are also immune to electromagnetic interference and do not require a reference electrode, which is a frequent source of noise and drift in electrochemical methods. We present the development and application of frequency-domain fluorescence lifetime based optical fiber sensors (optrodes) and in combination with self-referencing technology. In self-referencing modality the optrode is oscillated between two points a few microns apart in a concentration gradient. This converts the static concentration sensor to a dynamic flux sensor based on Fick's first law. Due to the inherent drift and noise filtering with self-referencing, it is possible to measure pico-molar flux levels using optrodes. We also present the application of frequency domain lifetime spectroscopy in micro-well plate format which can be used for high-throughput physiological sensing. |
