Spontaneously sp2‑Carbonized Fluorescent Polyamides as a Probe Material for Bioimaging

Considering the limits of current imaging contrast agents and the potential advantages of nanoparticles for early diagnosis and microstructure visualization, interest in nanotechnology for biomedical imaging is rapidly increasing. Nanoparticle fluorescence imaging has been used in gene detection, protein analysis, enzyme activity evaluation, element tracing, cell tracking, early stage disease diagnosis, tumor related research, and monitoring real time therapeutic effects. Carbon-based nanoparticles (CBNs) are gaining popularity as nanomaterials in the fields of science and medicine. Carbon exists in a variety of allotropes, including well-known allotropic phases like amorphous carbon, graphite, and diamonds as well as recently discovered allotropes like auspicious carbon nanotubes (CNTs), graphene oxide (GO), graphene quantum dots (GQDs), and fullerene. Because of their small size and unique quantum confinement characteristics, fluorescent carbon dots (CDs) are a novel class of fluorescent nanomaterials that have excellent fluorescence signal which can be utilized in photocatalysis, biochemical sensing and bioimaging.

The Knoevenagel reaction is an interaction between molecules containing an active methylene linkage between two strong electron-withdrawing groups and ketone compounds that form unsaturated products. In instance, malonyl chloride (or malonyl ester) spontaneously conducts the Knoevenagel-type self-condensation as a side reaction during the polycondensation reaction with diamines (or diols) under basic circumstances because it contains both carbonyl and activated methylene groups. As a result, these polyamides are partially carbonized and emit light (or polyesters). These polymers are thought to contain reactive residual groups like carboxylic acid and amino (or hydroxyl) groups as well as partially sp²-carbonized backbones; however, detailed examination of their structural characteristics and FL emission behaviors, as well as the possibility of using them as probe materials for bioimaging, have not yet been conducted. In a new study published in ACS Applied Bio Materials, researchers from Kyungpook National University (KNU): Dr. Young-Jae Jin, Hyojin Kim, Woo-Dong Jang, Sang-Joon Park, and Professor Giseop Kwak described the correlation between their chemical structures and FL emission based on spectroscopic analyses and microscopic observation, and they suggested using a probe material for bioimaging based on pH-dependent FL.

The research team created several aliphatic polyamides by reacting ethylene diamine with dicarbonyl chlorides like malonyl chloride, succinyl chloride, and glutaryl chloride at a low temperature of 5 °C in the presence of triethylamine in THF. They used malonyl moieties as side reaction sites to make the luminous polyamides PA1 and PA2 that are sp²-carbonized. As a byproduct of the polycondensation reaction of amidation, the two malonyl-based polymers PA1 and PA2 undergo Knoevenagel condensation, in which the activated acidic methylene is partially carbonized, and a conjugated structure is inserted into the main chain. The decreased activity of the beta- and gamma-carbons between the two carbonyl groups in PA3 and PA4 prevents them from going through the side reaction, which prevents the formation of a carbonized structure. The authors conducted various spectroscopic investigations in order to clarify their chemical composition and FL origin. These two polymers enabled the enol form of the π*-π transition through the radiative decay pathway based on the conjugated structure produced by the Knoevenagel condensation’s byproducts. These polymers were therefore very emissive in the visible range, and pH and excitation wavelength had a big impact on FL emission. Regarding PA2, it should be noted that it was easily transduced inside living cells and only marked the pH-lowest lysosomes. PA2 demonstrated outstanding biocompatibility and was not only rapid to endocytose but also low in cytotoxicity.

In a nutshell, Professor Giseop Kwak and his colleagues demonstrated that PA2, which is made up of the tertiary amide linkage, has rapid endocytosis, minimal cytotoxicity, great biocompatibility, and only labelled lysosomes with the lowest intracellular pH. The results of the research group will advance our understanding on the origin of the FL emission of carbonised nanomaterials and investigating more sophisticated functions in the realm of bioimaging. Moreover this research pave the way for nanoparticles promise revolutionary potential as new imaging agents for a variety of clinical applications.