Electrochromic materials for new point-of-care and wearable biosensors: a review

by Virginia Greco

A scientific paper recently published in “Materials Today” highlights the potentialities offered by electrochromic materials for the development of portable and easy-to-use bio-sensing platforms. This work, coordinated by ICREA Prof. Arben Merkoçi, provides a comprehensive description and discussion about these materials, their properties, state-of-the art applications as biosensors and new opportunities and challenges in this field.

Electrochromism is the property that certain materials have of changing colour —as an effect of optical variations in absorbance, transmittance or reflectance— in response to an electrical stimulus, such as the application of a small electric field. These optical alterations are associated with an electrochemically induced oxidation-reduction reaction, which causes the material to pass from a transparent (bleached) state to a coloured one, or from one coloured state to another. Thanks to the reversibility of this change, electrochromic materials (ECMs) have already found application in smart windows for controlling temperature in buildings, car rear mirrors, self-dimming glasses and displays, etc. They have also been integrated in supercapacitors and batteries.

More recently, electrochromic materials are being explored for bio-sensing applications, since their ability to provide a naked-eye read-out makes them attractive for point-of-care sensing devices. Indeed, ECMs can work as electrochemical transducers, with the advantage that they give a direct indication of their status by changing colour hue or transparency. A review on emerging applications of electrochromism for biosensing technologies has recently been published in Materials Today. This work was coordinated by ICREA Prof. Arben Merkoçi, leader of the ICN2 Nanobioelectronics and Biosensors Group, and developed by members of his team in collaboration with researchers from the Sharif University of Technology of Tehran (Iran).

The aim of this review is to highlight the benefits of integrating electrochromic materials in bio-sensing platforms and to provide all the relevant information for the design, development and test of devices based on such materials. The currently known ECMs are introduced —classifying them as organic, inorganic, hybrids or nanostructured— and analysed with respect to the key properties to consider for this kind of applications —i.e. contrast, colouration efficiency, switching time, cycle stability and multicolour capability. Then, the paper describes the multi-layered structure of EC-based sensing devices and explains the functionalities of each component, focusing in particular on electrodes, electrolytes and power source.

An overview of different kinds of electrochromic sensors —categorised as resistive, self-powered or bipolar systems— is also provided, accompanied by examples of applications. Finally, the challenges and perspectives of future research in this field are discussed. The combination of great electrochemical sensing capacities and easy read-out, which electrochromic materials offer, paves the way for the development of portable, easy-to-use and accurate bio-sensing platforms to be integrated in point-of-care or even wearable devices. The detection precision can also be improved by connecting the device to a mobile phone and using image processing software, which would allow recording and analysing quantitatively the outcome of the test.

Reference article:

Mohammad Amin Farahmand Nejad, Saba Ranjbar, Claudio Parolo, Emily P. Nguyen, Ruslan Álvarez-Diduk, Mohammad Reza Hormozi-Nezhad, Arben Merkoçi, Electrochromism: An emerging and promising approach in (bio)sensing technology, Materials Today, 2021. DOI: 10.1016/j.mattod.2021.06.015