General Label-Free Fluorescent Aptamer Binding Assay Using Cationic Conjugated Polymers

With more and more new aptamers being reported, a general, cost-effective yet reliable aptamer binding assay is still needed. Herein, we studied cationic conjugated polymer (CCP)-based binding assays taking advantage of the conformational change of aptamer after binding with a target, which is reflected by the fluorescence change of the CCP. Poly(3-(3'-N,N,N-triethylamino-1'-propyloxy)-4-methyl-2,5-thiophene hydrochloride) (PMNT) was used as a model CCP in this study, and the optimal buffer was close to physiological conditions with 100 mM NaCl and 10 mM MgCl₂. We characterized four aptamers for K⁺, adenosine, cortisol, and caffeine. For cortisol and caffeine, the drop in the 580 nm peak intensity was used for quantification, whereas for K⁺ and adenosine, the fluorescence ratio at 580 over 530 nm was used. The longer stem of the stem-loop structured aptamer facilitated binding of the target and enlarged the detection signal. High specificity was achieved in differentiating targets with analogues. Compared with the SYBR Green I dye-based staining method, our method achieved equal or even higher sensitivity. Therefore, this assay is practicable as a general aptamer binding assay. The simple, label-free, quick response, and cost-effective features will make it a useful method to evaluate aptamer binding. At the same time, this system can also serve as label-free biosensors for target detection.