Photoluminescence brightening of single-walled carbon nanotubes through conjugation with graphene quantum dots

Spanning the tissue transparency window, the near-infrared (NIR) photoluminescence (PL) of single-walled carbon nanotubes (SWCNTs) can optically penetrate biological tissue for deep-tissue imaging and optical sensing. SWCNTs are often functionalized with single-stranded DNA (ssDNA) to yield biocompatible, responsive, and selective sensors. However, the low brightness of these ssDNA-wrapped SWCNTs sensors restricts the depth at which such sensors can be implanted in the tissue. This work demonstrates the PL enhancement of ssDNA-wrapped SWCNTs by incorporating biocompatible graphene quantum dots (GQDs). Two kinds of GQDs, pristine (PGQDs) and nitrogen-doped (NGQDs), were fabricated and characterized. Thermodynamically, both GQDs were shown to significantly increase the fluorescence efficiency of ssDNA-SWCNTs with the same degree of PL enhancement after 3 h. Furthermore, a correlation between the diameter of the SWCNTs and the PL enhancement factor was found; the larger the SWCNT diameter, the higher the PL increase upon adding GQDs. For instance, a 30-fold enhancement was achieved for (8,6) chirality while it was only 2-fold for the (6,5) chirality. Our experiments showed that adding GQDs increases the surface coverage of SWCNTs suspended by ssDNA, limiting water molecules access to the nanotube surface, thus increasing the fluorescence efficiency. Kinetically, NGQDs brightened SWCNTs much faster than PGQDs. The PL intensity reached a plateau in 2 min following the addition of NGQDs, while it was still increasing even after 1 h upon the addition of PGQDs. We show that NGQDs can act as reducing agents to decrease the amount of dissolved oxygen, which quenches the SWCNTs PL. This advancement provides a promising tool for engineering the brightness of NIR sensors for biomedical applications such as single-molecule imaging of individual SWCNTs using NIR confocal microscopy and deep tissue sensing.