Ellipsometric Detection of Bacteria using Dielectrophoresis

The aim of this research is to detect bacteria by ellipsometry and evaluate if dielectrophoresis (DEP) can be useful to improve the limit of detection of optical biosensors.<br/><br/>Spectroscopic ellipsometry allows the non-destructive and contactless analysis of thin films or biological samples in a time range of seconds. Ellipsometry is based on the investigation of the polarization state of reflected or transmitted light. The measured parameters are the amplitude ratio (Ψ) and the phase difference (Δ) of s- and p-polarized light. Our measurement set-up consists of a flowcell that guides bacteria in liquid environment to a sensing surface, which consists of a glass substrate coated with thin film gold electrodes. Measurements were performed in Kretschmann configuration utilizing a quartz prism and gold electrode substrates. The bacteria studied in our experiments is the well-known, Gram-negative <i>E. coli</i> K12 strain.<br/><br/>The irradiation of the electrodes under a certain angle of incidence at a certain wavelength of light allows the excitation of surface plasmons resonance (SPR). Under this condition p-polarized light is coupled to the mode of surface plasmons at the metal-liquid-interface. This phenomenon can be monitored in the spectra of the ellipsometric parameters Ψ and Δ in form of a sharp dip, that correlates to the intensity minimum of the reflected p-polarized light. The excitation of SPR is highly dependent on the refractive index at the metal-liquid-interface. The presence of bacteria in a liquid changes the refractive index near the gold surface and therefor also changes the excitation condition of the SPR. Due to the penetration depth (~300nm) of the evanescent wave associated with the excitation of SPR, SPR-based sensors can only provide information on the refractive index very close to the metal interface. Therefore, it is of high importance to increase the number of bacteria at the metal-liquid-interface. To reach our goal of a highly sensitive SPR sensor we utilize the effect of dielectrophoresis.<br/><br/>Dielectrophoresis (DEP) describes the movement of dielectric particles in suspension due to the presence of inhomogeneous electric fields. DEP is divided into positive and negative DEP, depending on the different movement of particles due to the outer electric field. For positive DEP (pDEP) particles, such as bacteria, are accelerated to regions of high field inhomogeneity, which occur at the electrode edges. Hence bacteria are collected directly on the electrodes, the change in the spectra of Ψ and Δ magnified.<br/><br/>To explore the efficiency of bacteria collection by pDEP and the behavior of different electrode geometries in ellipsometry, a variety of electrode structures were designed. The structures vary in complexity and symmetry. Furthermore, the dependence of dielectrophoretic bacteria collection and SPR excitation on the gold film thickness was investigated. The designed electrodes and measurement set-up were able to visualize the influence of DEP on an ellipsometry based biosensor.