**Preface to "Fiber Optic Sensors in Chemical and Biological Applications"**

Research on optical fiber sensors for chemical and biological applications began soon after the production of optical fibers in 1970, and boomed at the end of the second millennium. At the beginning, optical fibers served to explore light transmission from a measured area to a detector, and research was focused on the fixation of known chemical indicators and/or sensitive biological parts to the surface of an optical fiber. Currently, such types of sensors may be used in harsh conditions, particularly in reactors. The miniaturization and availability of advanced electronic devices are the reasons why extrinsic optical fiber chemical/bio-sensors have often been superseded by nanosensors and optical sensors with wireless transmitters, above all, in medical and environmental applications. Thus, recent research pertaining to optical fiber sensors in chemical and biological applications has mostly been oriented around intrinsic optical fiber sensors, whereby the waveguide structure of the optical fibers plays an active role in sensing. Such label-free chemical/bio-sensors are highly sensitive; however, their low selectivities limit their practical use to very specific analyses, for the detection of impurities.

In this Special Issue, the subjects of the papers truly reflect the contemporary stage of research interests in the field of chemical and biological applications of optical fiber sensors.

> **Gabriela Kuncov´a** *Editor*
