The characterization and use of nanostructured films in sensing applications

Summary form only given. The control of molecular architectures in nanostructured films holds the promise to revolutionise sensing and biosensing, particularly in clinical diagnosis. Such organized films are suitable for immobilising biomolecules with preserved activity, and allows synergy to be sought among distinct types of materials. Indeed, a large number of organic, inorganic and hybrid materials are exploited in organised films produced with either the Langmuir-Blodgett (LB) or the electrostatic layer-by-layer (LbL) techniques. In biosensing, for instance, nanoparticles, nanotubes, polymers and biomacromolecules can be used, in conjunction with an equally wide variety of biorecognition elements, including enzymes, DNA, RNA, catalytic antibodies, antigens, peptides, aptamers, and labeled biomolecules. The layer-by-layer nature of the films is essential for combining different properties in the same sensing device, whose principle of detection may be based on optical, electrical and electrochemical methods. In this lecture, an overview will be presented of the use of nanomaterials for sensing, with emphasis on two major topics. The first topic is related to investigation of interface properties of functionalised surfaces, which is crucial for the successful design of sensing devices. Of particular importance is the combination of spectroscopic and microscopic methods, as they permit to determine the presence and orientation of functional groups on the nanostructured films, especially those taking part in the intermolecular interactions responsible for sensing. Surface-specific methods for this purpose are sum-frequency generation spectroscopy (SFG) and polarisation-modulated infrared reflection absorption spectroscopy (PM-IRRAS). A recent example was the use of SFG to show that interaction between the analyte lactose and the enzyme β-Galactosidase (β-Gal) immobilised in an LbL film induced the latter to lose order. The clear decrease in intensity of the amide bands assigned to (β-Gal) appears to be the first demonstration of structural effects induced by molecular recognition of lactose. The other method to probe intermolecular interactions in sensing is atomic force spectroscopy, whereby force curves are obtained upon approaching and retracting the atomic force microscope (AFM) tip onto the sample coated with a nanostructured film in a liquid cell. Experiments with an AFM tip functionalised with acetyl-CoA carboxylase enzyme (ACC enzyme) made it possible to determine the interaction force between said enzyme and its substrate, the herbicide diclofop. Steered molecular dynamics was used to model the force, which shows a possible way to design nanobiosensors and interpret experimental results. The second topic focuses the use of statistical and computational methods to treat sensing and biosensing data, particularly in cases where large amounts of data need to be generated, as in clinical diagnosis. Examples will be given of information visualisation and artificial intelligence methods applied to impedance spectroscopy data in electronic tongue systems, through which the results can be correlated with human perception of taste. Other applications include enhancing biosensing to detect tropical diseases and single molecule detection. Also relevant for clinical diagnosis is the variety of types of data, ranging from scientific data from sensors and biosensors, to images and written reports. An intelligent system is proposed for integrating information from such sources to be mined and generate the diagnostics.