Near-Infrared Fluorescence Modulation of Refolded DNA Aptamer-Functionalized Single-Walled Carbon Nanotubes for Optical Sensing

Selective modulation of near-infrared (NIR) fluorescence of single-walled carbon nanotubes (SWNTs) is important for their applications as NIR optical sensors and devices. Here, we study the target-molecule-mediated NIR fluorescence modulation of refolded DNA aptamer-functionalized SWNTs, using platelet-derived growth factor (PDGF) and a PDGF-binding aptamer as a model system. The aptamer–SWNT complexes use SWNT as nanoscale NIR optical emitters and DNA aptamers as molecular recognition elements. The binding of target molecules, PDGFs in this study, to PDGF-binding aptamers on the surface of SWNTs induces a conformation change of the aptamers, which modulates the NIR fluorescence of SWNT emitters. This study suggests that PDGF-binding aptamers noncovalently assembled on the SWNT surface can undergo a temperature- and divalent-ion-induced conformational change into a folded structure through multiple stages, which renders aptamer-functionalized SWNTs optically responsive to target molecules. In addition, our experimental and theoretical results show that the aptamers have a nanotube-diameter-dependent affinity for SWNTs. We demonstrate that refolded aptamer-functionalized SWNTs reversibly modulate their NIR fluorescence in response to PDGF at the nanomolar range 0.1–10 nM with apparent dissociation constants of ∼0.71 nM (solution-phase complexes) and ∼3.1 nM (complexes in hydrogels). This study could open new opportunities to design label-free, reversible NIR optical sensors that can detect various target molecules upon availability or selection of their cognate aptamers.