Researchers have also demonstrated solid-phase isothermal recombinase polymerase amplification (RPA) for the detection of Citrus tristeza virus (CTV) for plant disease diagnostics. Gold NPs linked with DNA strands on top of an electrode were used to detect in situ amplified CTV (RPA assay for amplification of the P20 gene, 387 bp) using electrochemical impedance spectroscopy (EIS) in a Fe(CN6)4–/Fe(CN6)3– redox system, being able to detect 1000 fg μL–1 of nucleic acid quantitatively (Figure 4).145