Fluorescent single-wall carbon nanotubes (SWCNTs) are used as nanoscale biosensors in diverse applications. Selectivity is built in by noncovalent functionalization with polymers such as DNA. Recently, covalent functionalization was demonstrated by conjugating guanine bases of adsorbed DNA to the SWCNT surface as guanine quantum defects (g-defects). Here, we create g-defects in (GT)10-coated SWCNTs (Gd-SWCNTs) and explore how this affects molecular sensing. We vary the defect densities, which shifts the E 11 fluorescence emission by 55 nm to a λmax of 1049 nm. Furthermore, the Stokes shift between absorption and emission maximum linearly increases with defect density by up to 27 nm. Gd-SWCNTs represent sensitive sensors and increase their fluorescence by >70% in response to the important neurotransmitter dopamine and decrease it by 93% in response to riboflavin. Additionally, the extent of cellular uptake of Gd-SWCNTs decreases. These results show how physiochemical properties change with g-defects and that Gd-SWCNTs constitute a versatile optical biosensor platform.
Fluorescent single wall carbon nanotubes (SWCNTs) are used as nanoscale biosensors in diverse applications. Selectivity is built in by non-covalent functionalization with polymers such as DNA. In general, fluorescence sensing with SWCNTs would benefit from covalent DNA-conjugation but it is not known how changes in conformational flexibility and photophysics affect the sensing mechanism. Recently, covalent functionalization was demonstrated by conjugating guanine bases of adsorbed DNA to the SWCNT surface as guanine quantum defects (g-defects). Here, we create guanine defects in (GT)10 coated SWCNTs (Gd-SWCNTs) and explore how this affects molecular sensing. We vary the defect densities, which shifts the E11 fluorescence emission by 55 nm to max = 1049 nm for the highest defect density. Furthermore, the difference between absorption maximum and emission maximum (Stokes shift) increases with increasing defect density by 0.87 nm per nm of absorption shift and up to 27 nm in total. Gd-SWCNTs represent sensitive sensors and increase their fluorescence >70 % in response to the important neurotransmitter dopamine and decrease 93 % in response to riboflavin. Additionally, cellular uptake of Gd-SWCNTs decreases. These results show how physiochemical properties alter with guanine defects and that Gd-SWCNTs constitute a versatile optical biosensor platform.
Small perturbations in the structure of materials significantly affect their properties. One example are single wall carbon nanotubes (SWCNTs), which exhibit chirality-dependent near infrared (NIR) fluorescence. They can be modified with quantum defects through the reaction with diazonium salts and the number or distribution of these defects determine their photophysics. However, the presence of multiple chiralities in typical SWCNT samples complicates the identifica-tion of defect-related emission features. Here, we show that quantum defects do not affect aqueous two-phase extraction (ATPE) of different SWCNT chiralities into different phases, which pinpoints to low numbers of defects. For bulk samples the bandgap emission (E11) of monochiral (6,5)-SWCNTs decreases and the defect related emission feature (E11*) increas-es with diazonium salt concentration and represents a proxy for the defect number. The high purity of monochiral sam-ples from ATPE allows us to image NIR fluorescence contributions (E11 = 986 nm, and E11* = 1140 nm) on the single SWCNT level. Interestingly, we observe a stochastic (Poisson) distribution of quantum defects. SWCNTs have most-likely 1 to 3 defects (for low to high (bulk) quantum defect densities). Additionally, we verify this number by following single reaction events that appear as discrete steps in the temporal fluorescence traces. We thereby count single reactions via NIR imaging and demonstrate that stochasticity plays a crucial role for the optical properties of SWCNTs. These results show that there can be a large discrepancy between ensemble and single particle experiments/properties of nanomateri-als.
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