We have demonstrated the feasibility of surface plasmon resonance (SPR) multisensing by monitoring four separate immunoreactions simultaneously in real time using a multichannel SPR instrument. A plasmon carrying gold layer, onto which a four-channel flow cell was pressed, was imaged at a fixed angle of incidence. First, the fourchannels were coated with antibodies and then the flow cell was turned by 90°such that the flow channels overlapped the areas coated in the first step. In that geometry, antigens were applied to the different antibodies on the surface. Thus, all antibody-antigen combinations can be measured in a two-dimensional array of sensor surfaces in real time. Our results do correlate with expected immunologic specificity. The emphasis will be on presenting this method to obtain data on immunosystems and not as much on the assessment of biological activity.

In this article we demonstrate how to obtain the ultimate lateral resolution in surface plasmon microscopy (SPM) (diffraction limited by the objective). Surface plasmon decay lengths are determined theoretically and experimentally, for wavelengths ranging from 531 to 676 nm, and are in good agreement. Using these values we can determine for each particular situation which wavelength should be used to obtain an optimal lateral resolution, i.e., where the plasmon decay length does not limit the resolution anymore. However, there is a trade-off between thickness resolution and lateral resolution in SPM. Because of the non-optimal thickness resolution, we use several techniques to enhance the image acquisition and processing. Without these techniques the use of short wavelengths results in images where the contrast has vanished almost completely. In an example given, a 2.5 nm SiO2 layer on a gold layer is imaged with a lateral resolution of 2 μm, and local reflectance curves are measured to determine the layer thickness. The SPM image is compared with an atomic force microscopy image of the same object. We obtain a 3 μm resolution when thickness differences within a lipid monolayer are imaged and measured.

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Dielectrophoresis (DEP) and electrorotation (ROT) are two electrokinetic phenomena exploiting nonuniform electric fields to exert a force or torque on biological particles suspended in liquid media. They are widely used in lab-on-chip devices for the manipulation, trapping, separation, and characterization of cells, microorganisms, and other particles. The DEP force and ROT torque depend on the respective polarizabilities of the particle and medium, which in turn depend on their dielectric properties and on the field frequency. In this work, we present a new software, MyDEP, which implements several particle models based on concentric shells with adjustable dielectric properties. This tool enables the study of the variation in DEP and ROT spectra according to different parameters, such as the field frequency and medium conductivity. Such predictions of particle behavior are very useful for choosing appropriate parameters in DEP experiments. The software also enables the study of the homogenized properties of spherical or ellipsoidal multishell particles and provides a database containing published cell properties. Equivalent electrical conductivity and relative permittivity of the cell alone and in suspension can be calculated. The software also offers the ability to create graphs of the evolution of the crossover frequencies with the electric field frequency. These graphs can be directly exported from the software.