# Optical Properties of Buffers and Cell Culture Media for Optofluidic and Sensing Applications
## 4 Conclusions  
We have measured the optical properties of selected commercially available buffers, such as PBS and TBE, as well as selected cell culture media--EXPRESS 5 and DMEM. These liquids are widely used in biomedical research for the growth and storage of live cells. In particular, we verified to what extent the optical properties of the liquids correspond to those of water, which are often assumed to be equivalent in numerical modeling. The transmission properties of buffers are comparable to those of water, but cell culture media show a significant reduction of transmission in a spectral range below 550 nm. This property may strongly limit the application of these media in devices where  
Figure 6: Scattering properties of the investigated liquids. Legend: TBE—TRIS-Borate-EDTA Buffer; PBS—Dulbecco’s Phosphate Buffered Saline solution; EXPRESS 5—Cell culture medium Express FiveTM SFM; DMEM—Cell culture medium Dulbecco’s Modified Eagle’s Medium. Figure 7: Light scattering characteristics after 3- and 8-months-long storage time for (**a**) DMEM, (**b**) EXPRESS 5, (**c**) TBE, and (**d**) PBS. Legend: TBE—TRIS-Borate-EDTA Buffer; PBS—Dulbecco’s Phosphate Buffered Saline solution; EXPRESS 5—Cell culture medium Express FiveTM SFM; DMEM—Cell culture medium Dulbecco’s Modified Eagle’s Medium. florescence is measured (sensors) or used as a gain medium (optofluidic laser). Measurements of material dispersion, in turn, have shown that refractive indices of these liquids are significantly higher than that of water due to the presence of biochemical nutrients, as well as the weak acid and salt of this acid. The differences in the near-infrared range may reach up to 0.012 and 0.006 for culture media and buffers, respectively. They might be negligible for biological experiments, but when these liquids are used in label-free biosensors, the differences may be important and should be taken into account at the sensor-design stage. The measured properties are, in particular, important for the development of label-free optical biosensors as well as for optical active fiber systems, where fluorescent proteins or genetically modified bacteria are considered as a gain medium [16; 17; 18]. In the latter case, accurate knowledge of the optical properties of the media is required for the proper design of optical fibers with an active liquid core. In addition, we have also shown that scattering for the examined liquids in the visible spectral range is similar to that of water; therefore, only from this point of view water can be considered as equivalent in properties. In general, buffers and CCM have good optical properties and can be used as optical medium for various biophotonic systems, including novel ones that employ various genetically modified cells, where the media are required to sustain cell living functions and growth. However, the long-term stability of these media must be also taken into account. We observed their significant change in properties after 8 months of storage. The changes concern CCM and PBS, where they may be originated by infection or degradation and salt crystallization, respectively. The change was only limited for the TBE buffer, which can be explained by the slightly alkaline pH and the presence of EDTA (Mg\({}^{2+}\) ion chelator) inactivating bacterial enzymes and blocking the multiplication of bacteria. For the majority of the investigated media, scattering increased with time. The main reason for those phenomena in the case of CCM was the degradation of the biological components of the liquids and the putative development of bacteria and/or fungi. The components enable not only the growth of cells of interest but also the rapid growth of bacteria, which may strongly influence the optical properties of the liquid. The probable source of microorganism infection may be inappropriate CCM handling, as the spectrophotometric analyses were performed out of the laminar or clean room--the same conditions as in many photonic labs.