PRONTO project consisted of exploring the viability of detecting changes in ionic conductivity taking place during the amplification of a specific nucleic acid sequence of SARS-Covid and using a battery sensor to produce the readout signal. According to the literature, the consumption of primers and dNTPs, plus the formation of magnesium precipitates due to the interaction of magnesium ions and generated pyrophosphates, led to a detectable decrease of the overall ionic strength of the solution during amplification.
The project departed from an already stablished knowledge on self-powered conductimetric sensors and explored a chemistry, based on a Zn anode and Ag/AgCl cathode screen-printed electrodes. The battery was designed to operate with very small liquid volumes, compatible with the 25 µl required for NAA of SARS-Covid with LAMP chemistry.
EIS technique was used to determine the changes in ionic conductivity during the LAMP amplification process with a set of microfabricated electrodes. Results showed that conductivity values of LAMP chemistry were significantly higher than those reported in the literature and that there were no detectable changes in conductivity that could be attributed to nucleic acid amplification, even at extremely high viral loads. Therefore, this approach was considered not viable. Nevertheless, the battery developed in this project - which was fully printed – showed excellent performance both as power source and conductimetric sensor, which remains open to provide a low-cost and flexible self-powered device for other further applications.
In view of the technical negative results in conductimetry, we decided to explore another new alternative: the viability of developing a pH-sensitive battery, which consists of implementing a battery with a redox chemistry that involves either protons or hydroxide ions in its reactions. With this approach, the output power of the battery depended the pH of the solution used for its activation. This was used to detect the pH changes taking place upon nucleic acid amplification in the non-buffered LAMP amplification version of the SARS-Covid commercial kit. This approach, which required the substitution of the AgCl cathode by a pH-sensitive Pt thin layer, showed that it is possible to detect such pH changes with the novel device, although the detection process itself at neutral pH diminishes the proton concentration at the electrode surroundings.
As a final proof-of-concept, the battery was tested with negative and positive non-buffered colorimetric LAMP kits and the results were promising: the tested samples yielded a clear differentiation in response before and after the amplification protocol. Therefore, this project opens a new avenue for the development of pH-sensitive self-powered devices, with a wide range of redox reactions to be explored and application where pH changes indicate the occurrence of a relevant biological event.
