The effects of magnetic fields exposure on relative permittivity of saline solutions measured by a high resolution SPR system

A measurement system for the relative permittivity of a physiological solution under 50 Hz magnetic fields (MF) is presented. It is based on a phase-sensitive surface plasmon resonance (SPR) system. Relative permittivity was analyzed for different solute concentrations of sodium chloride under various MF exposure parameters. We found that MF exposure at 0.2–4.0 mT step-wise decreased significantly the SPR phase signal of a 0.9% sodium chloride solution while 0.1 mT of MF exposure did not. The decreases in the SPR phase signal depended on the duration of MF exposure, and the signal reached a plateau after 15 min of exposure. Interestingly, the decreased SPR phase signal showed a gradual increase and approached the background level when the exposure was drawn off. In addition, we found that the response of the sodium chloride solution to MF also depended on its concentration. In brief, the relative permittivity of sodium chloride in solutions appears to be practically affected by 50 Hz MF exposure. Our data indicates that the relative permittivity of the saline solution influenced by MF exposure should be considered when investigating the biological effects of MF exposure on organisms in experimental study.


Detection Sensitivity and Resolution
Fig. 2(c) shows the SPR phase signals of glycerin solutions with different refractive index values. The SPR phase signals from 1.33300 to 1.338 were -0.882rad, -0.8819rad, -0.8874rad, -0.9019rad, -0.9151rad, -0.9531rad, -1.0172rad, -1.0781rad,-1.1346rad, -1.1896rad, -1.202rad, -1.214rad, -1.2777rad, -1.3039rad and -1.3208rad, respectively. The region when the glycerin refractive index value increased from 1.33309 to 1.33311 with the phase signal noise of 0.00065rad was selected to calculated the phase sensitivity and resolution. (S1) Where and are the change of the SPR phase signal and refractive index value, and are the phase sensitivity and resolution of the system, respectively.

Study on the Effects of ELF-MF on the Permittivity of Human Serum
Our results showed that human serums from five individual persons could be affected by the MF exposure (50 Hz, 4.0 mT, 15 min) which could cause the amplitude of SPR phase signal decreased ( Fig.  S1(a)). Then, we exposed the human serum from the second person to various exposure intensities of MF for 30min and measured the SPR phase signal ( Fig. S1(b, c)). The result showed that MF exposure could also affect the human serum in a dose dependent way, which was similar to the result of 0.9% sodium chloride solution.

Study on the Effects of ELF-MF on the Permittivity of 0.9%Sodium Chloride Solution
The SPR phase signals of 0.9% sodium chloride solution under (30 Hz, 2.0 mT, 15 min) MF exposure and (130 Hz, 2.0 mT, 15 min) MF exposure were decreased. Furthermore, MF exposure could affect the 0.9% sodium chloride solution in a frequency dependent way. The amplitude of SPR phase signal decreased with the increasing of MF exposure frequency (Fig. S2).

Measure the Refractive Index of 0.9%Sodium Chloride Solution Based on Abbe Refractometer
We also used an abbe refractometer (2WA-J) to detect the refractive index of 0.9% sodium chloride solution under ELF-MF exposure. The refractive index of ten groups of 0.9% sodium chloride solution after (50 Hz, 4.0 mT, 15 min) MF exposure and ten sham groups were detected. The result with statistical evaluation showed that MF exposure could cause the refractive index decrease (Fig. S3), which was agreement with the result that the refractive index decreased by 3.95×10 -4 RIU detected by the SPR sensing.

The Refractive Index Change Caused by MF Exposure
From Fig. 2(c), the curve between every two spots could be seen as a linear line. We could obtain the linear relationship as shown in Table 1. One spot (50 Hz, 4.0 mT, 15 min) of 0.9% sodium chloride solution was chosen to estimate the refractive index change caused by MF exposure. The SPR phase of 0.9% sodium chloride solution after MF exposure (50 Hz, 4.0 mT, 15min) was -1.2154rad. From Table  S1, the linear relationship could be chosen to calculate the corresponding refractive index of 0.9% sodium chloride solution. The calculated refractive index was 1.33427 RIU. Compared to the refractive index (1.334665 RIU) of 0.9% sodium chloride solution under no MF exposure, the refractive index changed by -3.95×10 -4 RIU.
Supplementary Table S1. The linear relationship between SPR phase signal and refractive index in every two spots in Fig. 2(c).

Data Processing
The interference intensity can be written as equation (6).
Three rhombic prisms are mounted on the motor-driven stage in different incident angles. As a result, three different modulated phases can be obtained, which represent the phase difference between ppolarized light and s-polarized light. A LabVIEW program is developed to manipulate the moving of the motor-driven and to save the acquisition data from the detector, which can move the first rhombic prism into the light path and then save the interference intensity. That means, as for one sample, one rhombic prism representing one phase modulation corresponds to one interference intensity . Three interference intensities can be obtained by, , , where , and obtained according to equation (3), are collected by the detector. As a result, can be calculated by the MATLAB processing. The phase of SPR can be extracted using equation (7). Ten groups of data were saved and processed to reduce the noise. In addition, statistical analysis was given to improve the accuracy of the experiment.
All the experiments were carried out in our clean room with a constant temperature at 25 °C , which could reduce the noise from the temperature fluctuations and ensure no temperature variation during the MF exposure. The sample was placed on the MF exposure system using a 5mL centrifuge tube and was