Development of a PNA-DiSc₂ based portable absorbance platform for the detection of pathogen nucleic acids

We report the development of a portable absorption (PortAbs)-based pathogen nucleic acid detection system using peptide nucleic acid (PNA) and a cyanine dye, DiSc₂(5). When the dye binds to the PNA-DNA hybrid, it results in a characteristic ∼110 nm shift in the dye absorbance, which we measure using PortAbs. The protocol involves amplification of the target DNA, PNA-DNA hybridization and dye complexing steps followed by absorption measurement. The system is built using a broad-spectrum photodiode whose output is amplified and then measured by a high resolution (24 or 32 bit) analog-to-digital converter. The excitation pulses of light are delivered by a color-changing LED. The sequence of excitation, measurement and display of results are all controlled by an embedded Raspberry-Pi board (or alternatively a laptop). At higher concentrations of the target amplicon (∼200 ng), the color change can be detected visually. At lower concentrations, PortAbs outperforms a plate reader and can detect target DNA as low as 30 ng or approximately 10 nM which is at least 10 fold better than previously reported studies. We validate the methodology using SARS-CoV-2 clinical samples containing about 1000 copies of the viral RNA and show that the entire workflow takes about 90 min. The cost of the complete standalone system is less than INR 40 000 (approx. 500 USD).