Fluorescent materials for chemical sensing in 3D hydrogel cell cultures

Abstract

Tracking concentrations of important metabolic substances such as glucose and carbon dioxide gives insights into cell metabolism and cell culture environment. Especially in the more complex 3D cell cultures it is essential to know how the substances flow and if they reach all the cells in the culture. Other environmental factors like temperature and pH should also be monitored since they have an effect on most of the cells’ metabolic reactions. This thesis investigates what kind of biosensors and measuring methods have been used so far in 3D hydrogel cell cultures to measure different conditions of the culture environment. The viewpoint is especially on the use of fluorescent materials in the sensing of these factors.

In this thesis the basics of the physical phenomena of fluorescence and phosphorescence are explained to gain sufficient information for the understanding of different detection methods based on these phenomena. Fluorescence and phosphorescence are based on the absorption and re-radiation of energy, usually light photons. The absorbed and emitted light have a different wavelength. Fluorescence Resonance Energy Transfer (FRET) is a fluorescence-based phenomenon where energy is transferred non-radiatively between two fluorescent probes. It is caused by dipole-dipole interactions between the probes and results is a different set of wavelengths than the probes usually have when they are apart. These phenomena can be utilized in biosensors by for example labelling detection molecules with fluorescent dyes. Then the change in the analyte level will cause a change in the fluorescence emission intensity or lifetime, which can be detected to calculate the analyte level.

Although measuring different analytes usually requires different approaches in the sensor design, FRET is a detection method that is widely used in various sensors. Other fluorescence-based detection methods are used too. It was found that a usual problem with the sensors is the lack of testing in 3D conditions and with various cell lines. There were multiple sensors with much potential though. Other disadvantages included small detection ranges and having the sensor genetically encoded in the cells. Various advantages were presented too, for example sensitivity, biocompatibility and tunability of the sensors. It was stated that more studies about different sensors for 3D cell cultures are still needed.