Quantitative self-powered electrochromic biosensors

Abstract

Self-powered sensors are analytical devices able to generate their own energy, either from the sample itself or from their surroundings. The conventional approaches rely heavily on silicon-based electronics, which results in increased complexity and cost, and prevents the broader use of these smart systems. Here we show that electrochromic materials can overcome the existing limitations by simplifying device construction and avoiding the need for silicon-based electronics entirely. Electrochromic displays can be built into compact self-powered electrochemical sensors that give quantitative information readable by the naked eye, simply controlling the current path inside them through a combination of specially arranged materials. The concept is validated by a glucose biosensor coupled horizontally to a Prussian blue display designed as a distance-meter proportional to (glucose) concentration. This approach represents a breakthrough for self-powered sensors, and extends the application of electrochromic materials beyond smart windows and displays, into sensing and quantification.

Fig. 1. Electrochromic biosensor and its operating principle. (a and b) Photographs of the actual electrochromic biosensor device. (c) Schematic representation of the device side view, with the biosensor (anode) placed to the left of the cathode (electrochromic display). As the analyte concentration or reaction time increases, the charge consumed at the cathode increases, resulting in a gradual color change along the electrode length, following the path of least resistance.

Fig. 2. Behaviour of the horizontal electrochromic display. (a) Captures of the display at different times under a constant applied current of 40 μA. The points signal the position of the color front chosen arbitrarily by a user (circles) and determined by analysis of the corresponding color profiles (triangles). (b) Color profiles of the electrochromic display obtained with ImageJ software. (c) Captures of the display recorded 30 s after application of different current levels. (d) Color profiles of the electrochromic display obtained with ImageJ software. The triangles in (a and b) represent the calculated distance obtained with ImageJ software. Circles represent the position estimated subjectively by eye.

Fig. 3. Electrochemical characterization of the self-powered glucose sensor. (a) Biosensor current response to different glucose concentrations. Error bars represent the standard deviation of the measurements from four different devices. (b) Cyclic voltammograms showing the reduction and oxidation of Prussian blue on an ITO electrode (blue) and the response of a stencilled biosensor in contact with a saturating concentration of glucose (black). Measurements were carried out at 5 mV s^–1 against a stencilled Ag/AgCl pseudo-reference electrode. (c) Polarization curves taken from the OCP to 0 V at a scan rate of 1 mV s^–1. (d) Power curves obtained from the polarization curves.

Fig. 4. Self-powered electrochromic biosensor response. (a) Captures of the display 30 seconds after the addition of different glucose solutions. The points signal the position of the color front determined subjectively (circles) and from the analysis of color profiles (open triangles). The error bars correspond to the standard deviation of the measurements obtained from four different devices. (b) Color profiles of the electrochromic display obtained with ImageJ software. The triangles represent the position of the color front for each glucose concentration. (c) Plot of the normalized to the maximum biosensor current (blue triangles) and color front position (black circles) as a function of glucose concentration.