Poster Z2 - Investigation and characterization of carbon dots and carbon nanoflakes synthesized from the hydrothermal synthesis of glucosamine hydrochloride

Introduction: Traditionally, carbon dots (CDS) are a class of carbon nanomaterials generally defined by quasi-spherical geometry; crystalline, semi-crystalline, or amorphous morphology; sizes less than or equal to 10 nm; stable and tunable photoluminescence across the visible and infrared spectra; and excellent biocompatibility. CDs have therefore found significant use in in vitro/ in vivo imaging, biosensing, and drug delivery. However, their broad definition reflects the heterogeneity of the products from their synthesis, non-rigorous purification steps, and their tendency for mischaracterization, especially for those synthesized via facile, bottom-up methods. These issues ultimately hinder us from understanding how and why these materials work and, consequently, undermine any positive potential these materials may have for biomedical applications. This research aims to investigate an underexplored carbon dot precursor, glucosamine hydrochloride, in the context of the growing theory that the fluorescence from some CDs synthesized using bottom-up methods primarily arises from molecular and polymeric fluorophores rather than classical CDs. Across various reaction conditions, we consistently found the presence of amorphous material mixed with CD-like crystalline material and crystalline carbon nanoflakes evidenced to be doped with nitrogen and chlorine. With this research we hope to inform the sensing and imaging communities of possible challenges with CD characterization while also highlighting other useful carbon nanomaterials that may generate from CD synthesis.

Materials and

Methods: All carbon nanomaterials discussed here were synthesized from the hydrothermal reaction of glucosamine hydrochloride. 5 g of glucosamine hydrochloride was dissolved in 20 mL of ultrapure water then placed in a Teflon-lined autoclave. All hydrothermal reactions were held at 200 degrees Celsius for varying reaction times before being slowly cooled to room temperature. After reaction, the thin film that formed at the air-water interface was removed and washed with ultrapure water. Product solutions were then diluted with water, probe sonicated, then centrifuged and washed to remove the char. The product solutions were then filtered with a 0.2 micron polyether sulfone filter to produce what we refer to as the crude product. Dialysis, using cellulose membranes with a molecular weight cut-off of 3.5 - 5 kDa, was performed against ultrapure water, over two days, with multiple water changes to remove lower molecular weight compounds, byproducts, and free ions. Chemical characterization of the particles was performed using fluorescence spectroscopy, absorbance spectroscopy, Raman spectroscopy, mass spectroscopy, energy dispersive x-ray spectroscopy (EDS), and fourier-transform infrared spectroscopy. The morphology of the particles was characterized using x-ray diffraction and transmission electron microscopy.

Results, Conclusions, and Discussions: TEM imaging of CD products produced after 1 hour of reaction (CD1) revealed the presence of high-contrast, nanosized carbon flakes surrounded by what appeared to be carbon quantum dots (Figure 1a, b). However, lattice spacings estimated from the fast-fourier transform do not match with those typical of graphene or graphene oxide (Figure 1d) Steady state fluorescence spectroscopy of the CD1 crude produced spectra typical of blue emitting carbon dots (Figure 2a). Increasing the reaction time resulted in a red shift of the fluorescent center (Figure 2a), which is typical of the incorporation of nitrogen at the surface or into the lattice of CDs. The flakes imaged in the CD1 case appear again in the TEM micrographs of the products of longer reaction times of 6 (Figure 2d, e) and 12 (Figure 2f, g) hours, however polymers become plainly visible. XRD of the dialyzed products indicated that the products, at all reaction times, predominantly contained amorphous rather than crystalline material (Figure 2h). Probe sonication of the film produced from the 12 hour reaction condition resulted in flakes and carbon quantum dots similar to those observed in the crude samples (Figure 3a-b). EDS analysis (Figure 3c) of an aggregate of flakes shows the presence of chlorine, nitrogen, and oxygen localized on the flakes, indicating either there incorporation on the surface or in the lattice of the flakes.

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