Single-walled carbon nanotubes (SWCNTs) emit near-infrared (NIR) photoluminescence (PL) that spans the tissue transparency window, allowing them to optically penetrate biological tissue for deep-tissue optical sensing. Such optical SWCNT sensors are often functionalized with single stranded DNA (ssDNA) to promote biocompatibility, responsivity, and selectivity. However, the low brightness of these ssDNA-wrapped sensors restricts the depth at which such sensors can be implanted in biological tissue. In this work, we overcome this limitation by demonstrating the PL enhancement of ssDNA-wrapped SWCNTs through the incorporation of biocompatible graphene quantum dots (GQDs). We compare the enhancement effects of two preparations of GQDs, pristine (PGQDs) and nitrogen-doped (NGQDs) graphene quantum dots. Both preparations of GQDs were shown to significantly increase the fluorescence emissions of ssDNA-SWCNTs to a comparable extent after 3 h of incubation. We further observed a diameter dependence of this enhancement, with the larger (8,6) SWCNT chirality demonstrating a 30-fold enhancement as compared to the 2-fold enhancement observed for the (6,5) SWCNT chirality. This PL enhancement was shown to vary with the exposed surface area of the SWCNTs, whereby SWCNT chiralities with wrappings that yield larger exposed SWCNT surfaces resulted in increased PL enhancement on conjugation with the GQDs. Despite the similarities in the final fluorescence enhancements, the NGQDs and PGQDs showed kinetic differences in their brightening effects. The NGQDs were able to achieve stabilized SWCNT fluorescence brightening 2 min after their addition, whereas the SWCNTs showed continued PL increases even 1 h after the addition of the PGQDs. Electrochemical analysis of the NGQDs confirms their behavior as reducing agents, which have been previously shown to enhance the rate of SWCNT brightening through the passivation of oxygen defect sites. The brightening effects of the NGQDs were further demonstrated in confocal single-molecule measurements, where we observed a 3-fold increase in the detection of individual SWCNTs on conjugation to the NGQDs compared to the unconjugated ssDNA-SWCNTs. The photoluminescence enhancements enabled by the GQDs thus provides a promising tool for engineering the brightness of NIR sensors for biomedical applications and enabling improved detection in deep-tissue sensing and imaging applica

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