# Optical Properties of Buffers and Cell Culture Media for Optofluidic and Sensing Applications
## 2 Materials and Methods
### Optical Analysis  
The transmission properties of the liquids in the spectral range of 400-1600 nm were investigated using a standard high-quality fused silica cuvette with a square cross section (10 \(\times\) 10 mm and 100-\(\upmu\)m-thick walls). The influence of the cuvette on the obtained results in the specified range was negligible. As a light source, a fiber-coupled HL-2000 halogen lamp (Ocean Optics Inc., Largo, FL, USA) was used to ensure a stable and flat input spectrum. The light was collimated by an achromatic microscope objective L \(\times\) 20. Next, and the transmission was measured with two (visible and infrared) spectrometers (Red Tide, Ocean Optics Inc., Largo, FL, USA and AvaSpec-NIR256/512-1.7, Avantes BV, Apeldoorn, The Netherlands). The output beam after the cuvette was split into two parts by flip mirror \(M\) to ensure robust coupling conditions into both spectrometers. The experimental setup used for transmission measurements is shown in Figure 1a. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Stability** & **Storage** & **Composition** & **Producer, Formal** \\  & **(months)** & **Temperature** (\({}^{\circ}\)**C) & & **Characteristics** \\ \hline
**TBE** & & & 89 mM Tris, 89 mM boric & Buffer, \\ (TRIS-Borate-EDTA Buffer) & 24 & 15–30 & acid, 2 mM EDTA; pH & Merck-Millipore cat. \\  & & & \(\sim\)8.3 at 25 \({}^{\circ}\)C & No. 574795 \\ \hline
**PBS** & & & & 140 mM NaCl, 10 mM & Buffer, GE \\ (HyClone\({}^{\upmu\text{m}}\) Dulbecco’s Phosphate Buffered Saline solution) & 24 & 15–30 & phosphate buffer, 3 mM & Healthcare cat. No. \\  & & & KCl; pH 7.4 at 25 \({}^{\circ}\)C & SH30028.02 Hyclone \\ \hline
**EXPRESS 5** & & & sodium pyruvate & Used for protein \\ (Cell culture medium Express Five\({}^{\upmu\text{m}}\) SFM) & 18 & 2–8 & NaHCO\({}_{3}\), low glucose, low phenol red; serum-free medium & Wavelength & Wavelength \\ \hline
**DMEM** & & & sodium pyruvate, & Used for cell culture; \\ (Cell culture medium Dulbecco’s Modified Eagle’s Medium) & 6 & 2–8 & NaHCO\({}_{3}\), low glucose, phenol red. & SIGMA cat. No. \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The main properties of the investigated buffers and cell culture media (CCM). Figure 1: Scheme of the setup used for measuring (**a**) transmission and (**b**) material dispersion of the liquids. Group refractive index \(N\) was measured by the white light interferometric technique and phase refractive index \(n\) was measured with an Abbe refractometer (RL-3, PZO S.A. Warsaw, Poland) while \(n_{D}\) was defined at \(\lambda\) = 589 nm [11]. A Michelson interferometer setup was used for measurement \(N\) of the liquids (Figure 1b). In this setup, the cuvette was placed in the signal arm of the interferometer. The reference arm was equipped with a mirror \(M_{1}\) mounted on a translation stage, which enabled compensation and measurement of the optical path difference (OPD) introduced by the bio-liquid in the signal arm. The interference patterns were observed and analyzed with the spectrometers in the visible and near-infrared spectral range. Measurement of the OPD as a function of the equalization wavelength allowed for determination of the spectral dependence of \(N\). The obtained curve could be fitted by the function describing \(N(\lambda)\) by including Sellmeier coefficients, as in Equation (1), where the phase refractive index \(n(\lambda)\) is given by the Sellmeier formula, as in Equation (2), where, in turn, n is the refractive index, \(\lambda\) is the wavelength, and \(B_{1,2,3}\) and \(C_{1,2,3}\) are Sellmeier coefficients:  
\[N(\lambda)=n(\lambda)+\frac{\lambda^{2}}{n(\lambda)}\sum_{i=1}^{3}\frac{B_{i} C_{i}}{\left(\lambda^{2}-C_{i}^{2}\right)} \tag{1}\]  
\[n=\sqrt{1+\frac{B_{1}\lambda^{2}}{\lambda^{2}-C_{1}}+\frac{B_{2}\lambda^{2}}{ \lambda^{2}-C_{2}}+\frac{B_{3}\lambda^{2}}{\lambda^{2}-C_{3}}} \tag{2}\]  
To verify the homogeneity and scattering properties of the liquids, we measured their angular characteristics of scattered light.