Towards Single-Molecule Analyte Sensing with Single-Walled Carbon Nanotubes

Abstract

Single-walled carbon nanotubes (SWCNTs) have been at the forefront of research for well over a decade due to their unique optical properties. Specifically, the inherent and photostable fluorescence of SWCNTs has been used to create sensitive near-infrared (NIR) sensors for a variety of target analytes, such as dopamine and cholesterol. However, most of the SWCNT sensors are employed in the solution-phase, severely limiting the efficacy due to ensemble measurements which can significantly confound any optical readout. One potential solution involves imaging individual SWCNTs deposited on a surface, unfortunately it is well known that SWCNT florescence is adversely affected by the inherently rough surfaces that are conventionally used for their observation, which emphasizes the need for a novel platform. We therefore devised a spin-coated hydrogel platform that contained SWCNTs in a single focal plane and enabled fluorescent imaging without significantly perturbing their optical properties when compared to the solution-phase. As SWCNT fluorescent microcopy is limited by diffraction, several strategies were employed to improve the sensitivity of analyte detection. When single SWCNTs longer than the diffraction limit were examined, spectral heterogeneity was observed along the length of each SWCNT, suggesting an application in single-molecule analyte detection. A common surfactant sodium deoxycholate (SDC) was investigated as an analyte as it exchanges with DNA oligonucleotides on the surface of DNA-SWCNTs, by which the kinetic response can be used to estimate DNA binding strength. We employed DNA-SWCNTs in the hydrogel platform and observed the kinetic response of individual SWCNTs in response to the addition of SDC. Importantly, the diffusion was intentionally slowed by modifying the top hydrogel layer to examine the kinetic response on a longer time scale, in contrast to the nearly instantaneous response in solution. We observed a decoupling of the spectral properties of each SWCNT, where the fluorescence intensity responded later than the emission wavelength. We then utilized a 1-dimensional diffusion model to estimate the local concentration of each SWCNT. We hypothesize that differences in the SDC functionalization morphology on SWCNTs have a significant impact on this observed effect.