Semi-quantitative visual detection of loop mediated isothermal amplification (LAMP)-generated DNA by distance-based measurement on a paper device

A distance-based paper analytical device (dPAD) for loop mediated isothermal amplification (LAMP) detection based on distance measurement was proposed. This approach relied on visual detection by the length of colour developed on the dPAD with reference to semi-quantitative determination of the initial amount of genomic DNA. In this communication, E. coil DNA was chosen as a template DNA for LAMP reaction. In accordance with the principle, the dPAD was immobilized by polyethylenimine (PEI), which is a strong cationic polymer, in the hydrophilic channel of the paper device. Hydroxynaphthol blue (HNB), a colourimetric indicator for monitoring the change of magnesium ion concentration in the LAMP reaction, was used to react with the immobilized PEI. The positive charges of PEI react with the negative charges of free HNB in the LAMP reaction, producing a blue colour deposit on the paper device. Consequently, the apparently visual distance appeared within 5 min and length of distance correlated to the amount of DNA in the sample. The distance-based PAD for the visual detection of the LAMP reaction could quantify the initial concentration of genomic DNA as low as 4.14 x 10(3) copies mu L-1. This distance-based visual semi-quantitative platform is suitable for choice of LAMP detection method, particular in resource-limited settings because of the advantages of low cost, simple fabrication and operation, disposability and portable detection of the dPAD device.