# Optical Properties of Buffers and Cell Culture Media for Optofluidic and Sensing Applications
## 3 Results and Discussion
Group and phase refractive indices of water and the other investigated liquids were calculated according to Equation (1) and using interferograms, as shown in Figure 5. The obtained material dispersion of water was similar to that previously reported by [PERSON] et al. [11], which confirms the correctness of our experimental results. The calculated Sellmeier coefficients are given in Table 2.  
Figure 3: The transmission of the investigated liquids measured in the 10-mm-long cuvette. Results for pure water are provided for reference. 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 4: The effect of long-term handling on transmission characteristics of (**a**) DMEM, (**b**) EXPRESS 5, (**c**) TBE, and (**d**) PBS. Legend: TBE—TRIS-Borate-EDTA Buffer; PBS—Dulbeccoo’s Phosphate Buffered Saline solution; EXPRESS 5—Cell culture medium Express FiveTM SFM; DMEM—Cell culture medium Dulbecco’s Modified Eagle’s Medium. Figure 5: (**a**) Group and (**b**) phase refractive indices measured for the investigated liquids. Legend: TBE—TRIS-Borate-EDTA Buffer; PBS—Dulbeccoo’s Phosphate Buffered Saline solution; EXPRESS 5—Cell culture medium Express FiveTM SFM; DMEM—Cell culture medium Dulbecco’s Modified Eagle’s Medium. Based on the obtained results, we can conclude that \(n\) in the near-infrared range of the CCM and buffers was higher than that of water by up to \(\Delta n\) = 0.012. The higher \(n\) was induced by added chemical and biochemical nutrients. The \(n\) of buffers was, on average, higher only by 0.08% than that of water. The \(n\) of CCM was, on average, higher by 0.2% than that of water, and the difference corresponds to the added biochemical nutrients which increase their \(n\). In the case of standard biological applications, these differences can be negligible. However, they should be taken into account when using those media for label-free biosensing devices. The influence of long-term handling on the \(n_{D}\) was investigated next and the obtained results are shown in Table 3. No significant changes for any of the considered media were observed. An increase of \(n_{D}\) for 8-months-stored PBS was probably a result of salt crystallization. This measurement supported data obtained in the case of transmission measurements, where the same sample behaved differently. Scattering losses in the investigated liquids were analyzed next. Normalized intensity \(\mathrm{P}\alpha/\mathrm{P}_{0}\), defined as the ratio between the intensity measured at angle \(\alpha\) and the intensity at angle \(\alpha=0\), was used to show the scattering of culture cell media and buffers (Figure 6). Also, the scattering properties of water are shown here for reference. We observed that normalized intensity decreased exponentially with the angle. For an angle higher than 45\({}^{\circ}\), scattering intensity dropped nearly to zero. The obtained characteristics of fresh samples were very similar to those of water. It seems that the biochemical nutrients present in the CCM do not induce any additional scattering with respect to that of water, mostly due to the fact that their size is much smaller than the wavelength of the laser source. Similar experiments were repeated after 3 and 8 months (Figure 7). For DMEM, EXPRESS 5, and PBS, we observed a significant increase of scattering. Only in the case of TBE did scattering remain unchanged. These results agree with the measurements of the transmission of the liquids, where DMEM, EXPRESS 5, and PBS changed in time, while TBE remained unchanged. This behavior is related to the presence of substances enabling microorganism growth (such as nutrients and simple salts) in DMEM and EXPRESS 5, or salt crystallization in the case of PBS. Moreover, in CCM, after longer storage, the nutrients decomposed due to their limited half-life, causing media degradation.