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 similar to 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.