Biosensors are analytical tools that convert a bio-signal into a measurable response. The applicability of these devices is rapidly expanding and their fabrication for continuous and real-time detection is gaining increased attention. Current research continues to further improve their features. Some of the major advantages of these devices are that they are portable and easy to operate, provide a rapid response, can be miniaturized, have a long-life cycle and a short response time and possess a high sensitivity, selectivity and specificity. Biosensors are more efficient and economical than conventional detection methods for a wide range of applications. This article is a tutorial review on the basic concepts of biosensors and their application in water quality monitoring, especially on-line detection systems. The following topics are discussed: (i) basic concepts of biosensors, (ii) biosensing elements and transducers, (iii) biosensor types and performance criteria and (iv) specific applications on water quality monitoring. In addition, this review highlights the various aspects of portable paper-based sensors for on-site detection of pesticides, heavy metals and bacteria in water. Online sensing technologies for water quality monitoring are preferable than offline/onsite detecting technologies, considering that online systems provide real-time data. © 2019, © 2019 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group on behalf of the University of Bahrain.

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