Data evaluation for surface-sensitive label-free methods to obtain real-time kinetic and structural information of thin films: A practical review with related software packages

Abstract Interfacial layers are important in a wide range of applications in biomedicine, biosensing, analytical chemistry and the maritime industries. Along with the growing number of applications, the analysis of these layer properties and understanding their behavior is becoming crucial. Label-free surface sensitive methods are excellent tools to monitor the structure and formation kinetics of thin layers in real-time even at the nanoscale. In this study, we review the available label-free techniques and demonstrate how the experimentally obtained data can be utilized to extract kinetic and structural information about the formed layers. We outline the most traditional and some novel techniques using optical and mechanical transduction principles. With a special focus, we review the current possibilities o the combination of label-free methods which is a powerful tool to extend the range of detected parameters. We summarize the most important theoretical considerations for obtaining reliable information from measurements taking place in liquid environments and hence layers in a hydrated state. Detailing with both structural and kinetic analysis methods, a thorough overview of the various quantities obtained from the evaluation of the raw label-free data are also provided. Here, thickness, refractive index, optical anisotropy and the related chain orientation, hydration degree, viscoelasticity, footprint, association and dissociation rate constants are all mentioned in detail. To demonstrate the effect of variations in model conditions on the observed data, simulations of kinetic curves at various model settings are also included. Based on our own experience with optical waveguide lightmode spectroscopy (OWLS) and the quartz crystal microbalance (QCM), we have developed software packages for data analysis. Along with this paper, we make the developed software available for the wider scientific community.