ABSTRACT

Recent discovery of the point-contact gas-sensitive effect provided prerequisites for the development and the investigation of the new type nanosensors based on the principles of Yanson point-contact spectroscopy. Point-contact sensors demonstrate excellent and unprecedented characteristics and their performance exceeds essentially that of other analogs. These peculiarities are of great prospect for the further intensive research of the basic properties

of the point-contact gas-sensitive effect and the rapid progress of advanced applications of point-contact sensors. The review contains an analytical consideration of the state-of-the-art of the point-contact sensors, including description of the scientific and technical basis for their design and investigations. The principles of Yanson point-contact spectroscopy are discussed for understanding the point-contact gas-sensitive effect and for the point-contact sensor development. Possibilities are described of both the analysis of the composite gas media, using point-contact sensors and a new advanced approach to the sensor breath analysis based on the point-contact information-rich breath gas spectra providing data similar to exhaled gas profiling. The development of the new tools for non-invasive medical diagnosis using the point-contact sensory breath analysis is considered too. 11.1 Introduction

Scientific advances are impossible without high-technology approaches that control reliably and monitor all stages of the development process. The field of sensors has undergone rapid progress and is currently a subject to the worldwide research combining physics, chemistry, biology, and materials sciences [28, 38]. One of the prospective trends in this activity is the development of sensors based on different nano-objects [51]. Undoubtedly, the breakthrough toward nano-object technologies yields new unvalued opportunities to improve considerably the techniques of sensors. Therefore, efforts are being made to improve the crucial parameters of these devices by a serious know-how approach combining solid-state physics and materials science. Qualitatively, new sensor characteristics can be discovered by using advanced fundamental benchmarks that have not been applied up to now.Taking the above statements into account, a novel pointcontact (PC) approach in this field has been proposed and essential advantages of the new type of sensors have been demonstrated [42, 44]. The concept is based on the application of Yanson pointcontact spectroscopy [121] to sensor technologies and the unique nonlinear electric properties of point contacts, which provide the

rich physical information in many investigations [53, 88]. One can find broad possibilities of the PC approach among physical phenomena discovered by Yanson point-contact spectroscopy. Some of them are, for instance, • discovery and exploitation of the point-contact spectroscopy

method [53, 88, 121]; • discovery of the Kondo-effect in point contacts with magnetic impurities and the Kondo-size effect [75, 130]; • detection of the thermal effects in point contacts and establishing the modulation point-contact spectroscopy [117, 118]; • discovery of the high-frequency and laser point-contact spectroscopy [8, 116]; • establishing the point-contact noise spectroscopy of phonons in metals [1]; • revealing the sign reverse of the point-contact spectra in semimetals due to the strong localization [126, 129]; • invention of the method for investigation of the relaxation processes kinetics and the determination of lifetimes of nonequilibrium phonons with Debye energies in a variety of metals [9, 64, 125]; • discovery of the conductance quantization in 3D atomicsized metallic contacts and the shell-effect in the conductivity of alkali metal nanowires [59, 131]; • discovery of point-contact spectroscopy of electron-phonon interaction (EPI) in superconducting state and determination of spectra of electron-phonon interaction for some hightemperature superconductors [52, 124, 128]; • detection of anisotropy of the electron-phonon interaction in MgB2 [87]; • discovery of the spin-valve effect in point contacts [133]; • discovery of the point-contact gas-sensitive effect [42].