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Not AvailableCarbon dots have received considerable attention due to their tunable emission. Single-particle techniques revealed that individual top-down and bottom-up green carbon dots can support several chromophores. In particular, several studies demonstrated that bottom-up synthesized carbon dots are typically made of amorphous carbon and are multichromophoric but may also just be chemically impure, with free dye in solution or polymerized in a carbon matrix. Carbon dots made by top-down precursors, however, are highly graphitic and more often single-chromophoric, begging the question if carbon dots made from bottom-up precursors could have similar optical properties compared to their top-down counterparts, if properly purified. Here, we compare green-emitting carbon dots made by two methods: top-down by chemical oxidation and bottom-up from small-molecule precursors in a solvothermal synthesis followed by rigorous purification. Such dots have cores of different crystallinity, but both types have oxidized surfaces. Just as ensemble absorption and emission spectra show only subtle differences, we find based on single-particle emission imaging that both types of carbon dots contain similar weights of carbon dots with single and multiple chromophores. Surprisingly, the carbon dots are optically similar, despite coming from opposing synthetic approaches. Although the majority of all carbon dots are single-chromophoric, top-down carbon dots are found to more likely have only one emitting chromophore, whereas bottom-up carbon dots are comparatively more multichromophoric. In the multichromophoric case, bottom-up carbon dots have on average a greater number of chromophores than top-down carbon dots. Our results showing that very differently made carbon dots with different structural properties exhibit strikingly similar emission properties reveal the important insight that out of structural heterogeneity emerges spectroscopic homogeneity. 

For carbon dots, careful purification and electronic structure calculations facilitate learning about the origin of optical properties. 

A major challenge in the “bottom-up” solvothermal synthesis of carbon dots (CDs) is the removal of small-molecule byproducts, noncarbonized polyamides, or other impurities that confound the optical properties. In previously reported benzene diamine-based CDs, the observed fluorescence signal already has been shown to arise from free small molecules, not from nanosized carbonized dots. Here we have unambiguously identified the small-molecule species in the synthesis of CDs starting with several isomers of benzene diamine by directly matching their NMR, mass spectrometry, and optical data with commercially available small organic molecules. By combining dialysis and chromatography, we have sufficiently purified the CD reaction mixtures to measure the CD size by TEM and STM, elemental composition, optical absorption and emission, and single-particle blinking dynamics. The results can be rationalized by electronic structure calculations on small model CDs. Our results conclusively show that the purified benzene diamine-based CDs do not emit red fluorescence, so the quest for full-spectrum fluorescence from isomers of a single precursor molecule remains open. 

Cancer affects one in three people worldwide. Surgery remains the primary curative option for localized cancers, but good prognoses require complete removal of primary tumors and timely recognition of metastases. To expand surgical capabilities and enhance patient outcomes, we developed a six-channel color/near-infrared image sensor inspired by the mantis shrimp visual system that enabled near-infrared fluorescence image guidance during surgery. The mantis shrimp’s unique eye, which maximizes the number of photons contributing to and the amount of information contained in each glimpse of its surroundings, is recapitulated in our single-chip imaging system that integrates arrays of vertically stacked silicon photodetectors and pixelated spectral filters. To provide information about tumor location unavailable from a single instrument, we tuned three color channels to permit an intuitive perspective of the surgical procedure and three near-infrared channels to permit multifunctional imaging of optical probes highlighting cancerous tissue. In nude athymic mice bearing human prostate tumors, our image sensor enabled simultaneous detection of two tumor-targeted fluorophores, distinguishing diseased from healthy tissue in an estimated 92% of cases. It also permitted extraction of near-infrared structured illumination enabling the mapping of the three-dimensional topography of tumors and surgical sites to within 1.2-mm error. In the operating room, during surgical resection in 18 patients with breast cancer, our image sensor further enabled sentinel lymph node mapping using clinically approved near-infrared fluorophores. The flexibility and performance afforded by this simple and compact architecture highlights the benefits of biologically inspired sensors in image-guided surgery.