Increasing the Sensitivity of Optical Nanobiosensors for Neurotransmitter Detection

Cells release biomolecules that provide information about cell physiology and diseases. Detection of these molecules is difficult due to their complex release patterns and chemical diversity. Optical sensors made of single-wall carbon nanotubes (SWCNTs) offer high precision in imaging these dynamics. Depending on their chirality, they fluoresce in the near-infrared (NIR, 850 – 1700 nm) biological transparency window and through a surface modification they detect target molecules by altering their fluorescence. Few scientists outside the ’SWCNT community’ have yet exploited the potential of SWCNT sensors. This is due to the requirement of expensive NIR cameras and the lack of integration in materials/devices. In this Ph.D. thesis, an efficient phase separation process for the isolation of monochiral (6,4)-SWCNTs (880 nm emission) from mixed SWCNT samples is developed. It enables the detection of SWCNTs using high-resolution silicon (Si) cameras, which are broadly accessible to biologists and neuroscientists. A simulation also confirms that (6,4)-SWCNTs are ideal for biological imaging with Si cameras. Functionalization of (6,4)- SWCNTs with (GT)10 single-stranded DNA (ssDNA) renders them sensitive to dopamine, an important neurotransmitter. These new SWCNT sensors exhibit 1.7x brighter fluorescence and 7.5x higher sensitivity compared to sensors based on mixed SWCNT samples. Fast (< 50 ms) and high-resolution imaging (> 50x more pixels) as well as imaging of cellular dopamine release with Si cameras are achieved. Furthermore, SWCNT coatings on glass are optimized. Homogeneous coatings are obtained by spin coating and varied in sensor density to maximize sensitivity. The resulting ’Smart Slides’ can be sterilized with UV light and remain functional for at least 6 weeks, both in dry conditions or buffer. Finally, measurements of altered dopaminergic cell responses in response to various psychotropic substances are demonstrated. In summary, this work demonstrates SWCNT-based sensing to monitor cellular release events. Advances in (6,4)-SWCNT separation, sensitivity improvement, and integration into common laboratory instruments provide an easy accessible tool with high-resolution imaging capability for optical monitoring of biochemical processes in cells.