Materials Today
Volume 50, November 2021, Pages 476-498
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Electrochromism: An emerging and promising approach in (bio)sensing technology

https://doi.org/10.1016/j.mattod.2021.06.015Get rights and content

Abstract

Electrochromism (EC) is a unique property of certain materials that undergo an electrochemical-induced change in colouration. During the last decades, electrochromic materials (ECMs) have been applied in a variety of technologies ranging from smart windows to information displays and energy storage devices. More recently, ECMs have attracted the attention of the (bio)sensing community thanks to their ability to combine the sensitivity of electrochemical methods with the intuitive readout of optical sensors. Although still a nascent technology, EC-based sensors are on the rise with several targets (e.g. cancer biomarkers, bacteria, metabolites and pesticides), which have already been detected by (bio)sensors using ECMs as transducers. In this review, we provide the reader with all the information to understand EC and its use in the development of EC-based biosensors.

Introduction

In 1961, the term of electrochromism (EC) was introduced and discussed theoretically by J.R. Platt [1]. Since then, huge progresses have been made with the generation of more than a hundred papers and patents on electrochromic materials (ECMs), and the first EC-based device was reported by Deb [2] in 1969 using amorphous WO3 film. In general, EC refers to the reversible optical change (absorbance, transmittance, and reflectance) triggered by electron transfer processes of some materials while under the application of small electric fields (Fig. 1) [3]. The colour-tuning nature of ECMs makes them fascinating for fabricating a variety of devices [4], [5], such as smart windows and power buildings to control temperatures [6], [7], [8], self-dimming rear mirrors for use in automobiles [9], interactive display platforms [10], as well as supercapacitors [11], [12], [13], [14], [15], [16], and batteries [17], [18], [19], [20], [21] for energy harvesting and storage.

More recently, following the successful applications of ECMs in displays and energy storage devices [22], [23], [24], [25], [26], [27], [28], the biosensing community has been exploring their use as transducer components in several sensing techniques [28], [29]. The rationale behind this is the combination of the analytical performance of electrochemical sensors with the simple naked eye read out of optical sensors in a single sensing platform. This makes the resulting EC-based analytical devices ideal candidates for point-of-care (PoC) applications, providing speed, portability, low cost, and ease of use [30], [31], [32], [33].

This review aims to provide the reader with all the knowledge required to design, develop and analyse EC-based sensors. Specifically, we first define key properties (e.g., contrast, stability, speed) that ECMs require to work as sensing platforms. Second, a comprehensive overview of available ECMs dividing them into inorganic, organic, hybrids and nano materials is presented [34], [35]. Third, we discuss the main components (e.g., potentiostat, detectors) that EC-based sensors should have as well as their functionalities [36], [37]. Fourth, we present the state-of-the-art of EC-based sensors highlighting the most relevant works. Finally, we discuss the challenges and perspectives of future EC-based sensors. We hope that this review will inspire researchers to explore new ways to apply the unique features of ECMs to the development of innovative and much-needed PoC sensors.

Section snippets

Electrochromic characteristic factor

Before discussing the configuration and operation of EC sensing devices, we define below some key concepts essential to understand the EC-based sensors’ behaviour.

Candidate materials

The cornerstone of every ECD is the material displaying electrochromic behaviour, defined as electrochromophors or electrochroms. As we have already seen, ECMs are electroactive species, which change their optical behaviour (absorbance, transmittance) under the application of an electric field [76]. We can divide ECMs into two main sub-classes: mono-colour and multi-colour ECMs. The former has two redox states one of which is bleached and the other has a specific colour. The latter has multiple

EC sensing device structure

The general layout of EC-based sensors is based on the use of a potentiostat to apply the potential, a transparent and conductive electrode to support the ECM, suitable electrolyte solution to ionic transport, an ion storage layer, and a camera to record the optical signal. In addition to the hardware, the device also requires a suitable bioreceptors to interact with biomolecules [95], [158].

EC-based sensors

In the last five years, we have seen numerous applications of EC-based sensors to detect a wide range of analytes (from ions to bacteria) in environmental and clinical samples. In the following paragraphs, we present several examples of EC-based sensors dividing them into three main categories: resistive, self-powered, and bipolar systems.

Conclusion and future perspective

With simple manipulation, rapid recognition, affordable instrumentation, and easy transformational results, point-of-care (PoC) devices represent the answer for many analytical applications, ranging from environmental (e.g. pesticide detection) to healthcare (e.g., microbiome, cancer, infectious diseases) [196], [197], [198], [199]. However, conventional visual PoC devices have faced challenges of low sensitivities and long assay times [199], [200], [201]. Integrating electrochromism with PoC

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We acknowledge the MICROB-PREDICT project that has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 825694. Financial support from the EU Graphene Flagship Core 2 Project (No. 785219) is also acknowledged. This article reflects only the author’s view, and the European Commission is not responsible for any use that may be made of the information it contains. ICN2 is funded by the CERCA programme, Generalitat de Catalunya. The

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