# Optical Properties of Buffers and Cell Culture Media for Optofluidic and Sensing Applications
## 1 Introduction
However, changes in the optical properties of the used buffers or CCM, apart from the color or turbidity changes, are typically not investigated, since they may not be crucial for achieving the research aim, which is usually cell growth with no microbiological and mycological infection. Several optical fiber sensors have been reported for microorganism monitoring [9]. These sensors mainly rely on a label-free sensing mechanism, where changes in optical properties or the thickness of the growing biological films are monitored in the vicinity of the sensor surface [10; 11; 12; 13]. For these devices, the influence of the optical properties of the applied buffers or CCM is important to identify sensitivity and the limit of detection, but it is usually neglected. For measurements and numerical analyses, the optical properties of these liquids are considered to be equivalent to those of water, which may have a negative impact on the performance of these devices and the interpretation of data obtained during measurements. There is also a number of fluorescence-based systems where CCM are applied and a change in their properties is crucial for the system's performance. The photonic explorer for bioanalysis with biologically localized embedding (PEBBLE) is considered to be one of the most developed methods of the real-time imaging of living cells [8; 14]. In this method, the response comes from the fluorescent dye incorporated into the analyzed cell. The fluorescence intensity is not only dependent on the fluorescence quantum yield of the dye and the excitation source but also on absorbance (i.e., optical path length, solute concentration, and molar extinction coefficient) [15]. The detected fluorescent signal originates from in-vitro-cultivated cells and can be partially absorbed or scattered by other cells. The optofluidic resonator rings laser, where riboflavin dissolves in water has been considered an active medium, should also be mentioned here [16]. Use of fluorescent proteins as a viable gain medium for optical amplification has been demonstrated [17; 18]. In this type of optofluidic application, proteins as well as bacteria and cells genetically modified with a fluorescent label need to stay in buffer or CCM. However, the putative impact of the optical properties of the medium is usually omitted for aqueous solutions. The fluorescent emission peak of the analyzed fluorescent probe can be strongly shifted due to the environment (i.e., polarity of the solute) [15]. For these purposes, the study of the optical properties of such media, which include scattering and material dispersion, are also highly demanded. In this study, we examined for the first time the optical properties of a set of solutions commonly used as buffers and CCM. We determined their optical transmission range, scattering properties, and refractive indices. As a reference, pure water was used for comparison with the obtained results. The influence of the long-term handling of these liquids on the investigated properties is also discussed based on the preliminary experimental results.