Electrochemiluminescent detection of epilepsy biomarker miR-134 using a metal complex light switch

The detection of a key biomarker in epilepsy, miR-134, using an environmentally sensitive electrochemiluminescent luminophore, [Ru(DPPZ)₂ PIC]²⁺, is reported, DPPZ is dipyrido[3,2-a:2',3'-c]phenazine) and PIC is (2,2'-bipyridyl)-2(4-carboxy phenyl) imidazo [4,5][1,10] phenanthroline. A thiolated capture strand is first labelled with [Ru(DPPZ)₂ PIC]²⁺ and then adsorbed onto a gold electrode. No significant electrochemiluminescence, ECL, is observed for immobilised Ru-labelled capture strands which is consistent with the light-switch dye being exposed to the aqueous solution. In sharp contrast, binding of the target turns on ECL. The ECL intensity, I_ECL, depends on the number of adenine "spacer" bases between the end of the capture sequence and the dye. The ECL intensity for the optimised system increases linearly with increasing miR-134 concentration from 100 nM to approximately 20 μM. Single and double base mismatches produce I_ECL that are only approximately 30% and 8% respectively of that observed for the fully complementary target reflecting differences in their association constants. Significantly, the presence of BSA protein causes I_ECL to increase by less 5% in either the single or duplex circumstances. Finally, the ability of the sensor to quantify miR-134 in unprocessed plasma samples from healthy volunteers and people with epilepsy is reported.