**Two-Step Energy Transfer Dynamics in Conjugated Polymer and Dye-Labeled Aptamer-Based Potassium Ion Detection Assay**

**Inhong Kim 1,**†**, Ji-Eun Jung 2,**†**, Woojin Lee 3, Seongho Park 3, Heedae Kim 4, Young-Dahl Jho 1, Han Young Woo 2 and Kwangseuk Kyhm 3,\***


Received: 17 June 2019; Accepted: 18 July 2019; Published: 19 July 2019

**Abstract:** We recently implemented highly sensitive detection systems for photo-sensitizing potassium ions (K+) based on two-step Förster resonance energy transfer (FRET). As a successive study for quantitative understanding of energy transfer processes in terms of the exciton population, we investigated the fluorescence decay dynamics in conjugated polymers and an aptamer-based 6-carboxyfluorescein (6-FAM)/6-carboxytetramethylrhodamine (TAMRA) complex. In the presence of K<sup>+</sup> ions, the Guanine-rich aptamer enabled e fficient two-step resonance energy transfer from conjugated polymers to dyed pairs of 6-FAM and TAMRA through the G-quadruplex phase. Although the fluorescence decay time of TAMRA barely changed, the fluorescence intensity was significantly increased. We also found that 6-FAM showed a decreased exciton population due the compensation of energy transfer to TAMRA by FRET from conjugated polymers, but a fluorescence quenching also occurred concomitantly. Consequently, the fluorescence intensity of TAMRA showed a 4-fold enhancement, where the initial transfer e fficiency (~300%) rapidly saturated within ~0.5 ns and the plateau of transfer e fficiency (~230%) remained afterward.

**Keywords:** FRET; time-resolved photoluminescence; two-step FRET; potassium ion detection
