# Optical Properties of Buffers and Cell Culture Media for Optofluidic and Sensing Applications
## 1 Introduction  
Conducting research on microorganisms and cell cultures requires aqueous media that support their life functions. These media are often called buffers or cell culture media (CCM) when storage and growth are considered, respectively. While buffers are typically multicomponent electrolytes, CCM are additionally enriched with nutrients, growth factors, hormones, and other biologically active substances required for in vitro growth, storage, or propagation of living cells. The ingredients of CCM have to mimic the composition of the nutrients, vitamins, minerals, and growth stimuli present in the natural environment of the microorganisms and tissue or the extracellular fluid in the case of cell cultures [1; 2; 3]. Three phases in the cell culture growth process can be distinguished, namely: the lag phase, when little or no increase in cell number is observed; the logarithmic growth phase, when the cell number increases exponentially; and the stationary phase, also called the plateau phase, when the number of cells remains constant. Cells are able to grow and divide in the CCM when it has a suitable composition, stable pH and nutrients, as well as the appropriate gas atmosphere (CO\({}_{2}\), O\({}_{2}\)). Cell growth is limited mainly by the availability of nutrients and overcompaction of the culture, leading to a decrease in the number of living cells (death phase) [2; 3]. For optimizing the CCM properties corresponding to cell growth and survival, other factors, including temperature, humidity, stability of buffer components, and specific illumination, are also of high importance [1; 2]. Bacterial or fungal infection of CCM can lead to death of cultivated cells. Thus, to grow cells in CCM, it is necessary to respect the principles of sterile work--in laboratory practice, all manipulations are performed in laminar flow chamber in clean rooms. Moreover, to maintain the cell culture, regular check-ups and medium replacements are performed. Knowledge of interactions between light and various cultured organisms is essential for therapeutic [4] or detection [5] purposes. The most common optical methods are used to monitor cell growth condition and are based on densitometry evaluation [3]. Optical density measurements of a small volume of cultured cells suspended in culture medium, usually at \(\lambda\) = 600 nm (OD600), are made in order to determine the growth kinetics, especially the determination of entering the plateau phase. This method can also be used for establishing conditions when the cells reproduce the fastest or show the expected morphological changes [3; 6]. When the cultured cells change their morphological or cytological properties, or when they stop growing, the biochemical composition of the medium may also exhibit detectable differences. In most the cases, factors such as microbiological infection, use of inadequate media (lack of growth factors or other stimulants), presence of chemical contamination, or improper growth conditions (temperature, humidity, CO\({}_{2}\) atmosphere) are considered to be responsible for disturbing cell life functions. For this reason, some media contain a pH-dependent dye, changing their color when they become older, they accumulate cell metabolites, or there are products of nutrient degradation. This effect seems to be important mainly in colorimetric and spectrophotometric analyses, especially when the color of the used media disturbs the measurement [2; 3; 4]. For the majority of living cells, monitoring is performed in the visible and near-infrared spectral range (\(\lambda\) = 390-700 nm) to reduce the energy supplied to the cells and to maintain the properties of the dyes used in the analysis. Real-time analysis of tissues (millimeters to centimeters in depth) is carried on, in turn, in the near-infrared range (\(\lambda\) = 650-900 nm) to distinguish the absorption peaks corresponding to the absorption of blood or water [7; 8].