Dual-Polarization Interferometry: A Novel Technique To Light up the Nanomolecular World

Abstract

[EN] The challenging lecture given in 1959 by physicist and Nobel Prize awarded R. P. Feynman: “There's plenty of room at the bottom” is considered to be the starting point for nanotechnology. With this peculiar title, Feynman encouraged researchers to explore beyond the atomic level and predicted exciting new phenomena that might revolutionize science and technology. Among these pioneering researchers are Eric Betzig, Stefan W. Hell and William E. Moerner, who have been awarded with the Nobel Prize in Chemistry 2014 for developing the super-resolved fluorescence microscopy. However, it is important to remark that the exploration of this amazing nanomolecular world began in the early 1980s with the invention of the scanning tunneling1 and the atomic force microscopes2 (Figure 1). Figure 1. Measurement scales. The study and manipulation of interactions of nanometric dimensions could begin as soon as measuring tools became more efficient. The last decades have witnessed the development of techniques able to obtain information at the sub-molecular level, and their applications especially on the biomedical field. As an example, the stimulating labor of studying the role of conformational dynamics in reaction mechanisms has resulted in Nanometers 10-1 10 102 103 1 104 105 106 107 108 DPI WORLD Atom IgG Erythrocyte Grain of salt Tennis ball Glucose Virus Amoeba Plea numerous advances in life sciences.3 In this regard, the precise knowledge about molecular interactions and their effects on protein function has greatly aided the discovery of new targets in medical chemistry. Figure 2. General scheme of different DPI applications. The role of the structure in the protein behavior is a fundamental step in the utilization, characterization and understanding of numerous biological processes. Consequently, highly advanced functional and structural measurement tools have been developed over the last decadess.4 Particularly, nuclear magnetic resonance (NMR), X-ray crystallography and neutron reflectivity (NR) provide structural measurements, whereas tools such as microcalorimetry or surface plasmon resonance (SPR) provide functional data. Furthermore, the more recent approach based on Dual Polarization Interferometry (DPI), is allowing the molecular interactions to be quantitatively measured at nanometric dimensions. DPI is currently one of the most powerful label-free biosensing techniques in heterogeneous format to record real-time data of conformational dynamics, which is efficiently employed in different applications, such as bionanotechnology, surface science, Biotechnology Drug Discovery Lipid Studies Crystallography Surface Science DPI and crystallography or drug discovery (Figure 2). Their measurements can provide information about the connection between the biomolecule function and its structural changes. This technique is, to our knowledge, the most well-built and well-thought through such waveguide sensor. It has its weaknesses, e.g. the necessity of using a relatively long sensor element, but the information it delivers is the interaction of two polarization modes of the propagating light with a molecular film at the top of the waveguide with which it interacts through the evanescent field. It is well known that the use of an interferometric readout and a long waveguide makes the measurements very stable and more accurate than those of the competitor techniques. Accordingly, DPI can be described as a molecular ruler whose quantitative values can be correlated directly with those from other usual techniques, such as NMR, X-ray crystallography and NR, providing higher sensitivity and accuracy than the classical acoustic and optical biosensors. In 1996, Dr Neville Freeman conceived of the idea behind DPI as a robust and reproducible biosensing technology and, together with Dr Graham Cross, developed the concept and filed the original patent.5,6 This novel technique has gained popularity among the scientific community in the last decade and the number of publications dealing with this technique has increased considerably since the initial report in 2003.7 In this review, DPI is compared with other techniques, and its theoretical basis and applications are outlined. The fundamentals are specified together with strategies for chip functionalization and applications of the aforementioned technology in a wide variety of research areas. All this gives a unique chance to learn from this sensing technique, which may be an essential reference to facilitate the work of future users.