| Near-infrared light (700--1000 nm) is important for in vivo sensing applications due to the decreased absorption, scattering and auto-fluorescent interference of tissue and whole blood at these wavelengths. However, the lack of photostable organic fluorophores that emit in this region has hampered the production of an implantable continuous analyte sensor. Single-walled carbon nanotubes (SWNT) have a tunable excitation and emission in the near-infrared (nIR) and do not photobleach. Additionally, their quasi one-dimensional nature renders them sensitive to molecular adsorption events. We explore methods to control the modulation of SWNT photoluminescence (PL) in response to glucose. We show that it is possible to assemble a glucose specific enzyme on the surface of nanotubes in solution while maintaining nanotube fluorescence and colloidal stability and preserving the activity of the enzyme. The enzyme coating exposes the nanotube surface area such that electroactive species can adsorb irreversibly to the nanotube surface. One such species, potassium ferricyanide, is shown to attenuate nanotube fluorescence emission through two distinct mechanisms; fluorescence quenching and electron withdrawal. By coupling the reaction of ferricyanide at the surface of the nanotube to the chemical action of the enzyme, nanotube fluorescence is modulated indirectly in response to glucose and has a sensitivity of 34.7 uM. It is also possible to coat the SWNT in dextran, a glucose-like polymer, for an affinity based sensor. Addition of concavalin A (ConA) causes the dextran coated nanotubes to aggregate resulting in SWNT PL intensity diminution, while subsequent additions of glucose cause the ConA-SWNT aggregates to dissolve and the PL to recover. Finally, we demonstrate hydrogel swelling as a mechanism to reversibly induce solvatochromic shifts in SWNT PL within a biocompatible hydrogel matrix. Individually dispersed nanotubes in a poly(vinyl alcohol) hydrogel matrix with varying cross-linking densities exhibit a shift in emission maxima as the cross-linking is increased, with shifts of up to -50 meV, -15 meV and -17 meV observed in swollen hydrogels versus solution-suspended nanotubes for the (6,5), (7,5) and (8,3) nanotubes, respectively. Additional shifts of up to -48 meV, -29 meV and 16 meV were observed for hydrogels that had been dried. Hydrogels, which are important materials for biomedical applications due to their biocompatibility and structural properties, develop an internal osmotic pressure in the presence of water. The electronic band gap of a single-walled carbon nanotube is known to increase or decrease with uniaxial strain or lattice deformation due to hydrostatic pressure. Although evidence of strain is present in our system, lattice deformation is insufficient to describe the observed photoluminescence shifts. Instead, we attribute the observed shifts in nanotube emission to changes in the local dielectric constant around the nanotube due to changes in the hydrogel internal pressure and cross-linking. |
