Development of a disposable and highly sensitive paper-based immunosensor for early diagnosis of Asian soybean rust.

Soybean is one of the most important crops and plays a key role in the whole food chain production. Soybean crops are very susceptible to the fungus Phakopsora Pachyrhizi, the agent responsible by the Asian soybean rust. The spore of the fungus is easily disseminated by wind with adequate environment, leaf wetness, high humidity and temperatures, the crop can be totally lost within few days. A high sensitive, specific and easy test is the key for early diagnosing the soybean rust and therefore save the crop. Here we present a paper-based immunosensor for early stage diagnosis of soybean rust that can be performed by unskilled operators on-site. Nitrocellulose membrane was chosen as the substrate to stick the antigen due to its high binding properties. Polyclonal antibodies labeled with fluorescent nanoparticles were employed as the recognizers. An analytical curve with spiked samples shows a linear response range from 0.0032 to 3.2 μg/mL. This immunosensor presents a very low detection limit of 2.2 ng/mL, which corresponds approximately to 8-12 spores/mL. The paper-based sensor reachs the detection range of ELISA and PCR based test systems, and outranges the available commercial test kits by two order of magnitude. We believe this immunosensor has a great potential as a point-of-care device for the early diagnosis of Asian soybean rust.