pI determination of biotinylated TMV particles by ZP measurements

The working principle of the TMV-modified EIS sensor is based on detecting the intrinsic charge of the virus particle. Therefore, the pI (defined as the pH value at which the electrical net charge of a particle is zero) is an important parameter for this detection method. The pI gives information about the sign of a particle charge depending on the pH value. By that, the expected direction of the signal change, induced by the adsorption of charged TMV particles to the gate surface, can be determined. The pI of native TMV (strain vulgare, U1) is described to be about pH 3.5: TMV particles are negatively charged at pH values above 3.5 and positively charged at pH values below 3.5 [50, 51]. To our best knowledge, there is no information in the literature about the pI of biotinylated TMV particles. Therefore, it was analyzed in the course of this study. As an established method for pI determination, the measurement of the ZP was selected [52]. The ZP represents the electric potential at the slipping plane between the immobile and mobile layers, which is defined by the electrical double layer theory [53]. In this work, ZP analysis was carried out with a Litesizer 500™ Particle Analyzer (Anton Paar GmbH, Graz, Austria) by dynamic light scattering method (DLS). The ZP was examined at a laser wavelength of 660 nm for different pH values between pH 3 and pH 7. For each pH value, three individual measurements were conducted at 25 °C.