Smart, Tunable CQDs with Antioxidant Properties for Biomedical Applications—Ecofriendly Synthesis and Characterization

Abstract

Carbon quantum dots (CQDs) are nanoobjects of a size below 10 nm. Due to their favorable features, such as tunable luminescence, unique optical properties, water solubility, and lack of cytotoxicity, they are willingly applied in biomedicine. They can be obtained via bottom-up and top-down methods. However, to increase their quantum yield they must undergo post-processing. The aim of the following research was to obtain a new type of CQDs modified with a rhodamine b derivative to enhance their fluorescence performance without biocompability deterioration. For their preparation glucose was used as a precursor and four different carbonizing agents which affected semi-and final products luminescence properties. The ready nanomaterials were investigated over their chemical structure by FTIR and NMR, whereas morphology was investigated by the TEM method. Their optical properties were determined by UV-VIS spectroscopy. Fluorescence behavior, photo-and pH-stability, as well as solvatochromism showed their applicability in various biomedical applications due to the controlled properties. The samples exhibited excellent antioxidant activity and lack of cytotoxicity on L929 mouse fibroblasts. The results showed that proposed strategy enables preparation of the superior nanomaterials with outstanding luminescence properties such as quantum yield up to 17% which can be successfully applied in cell labelling, bioimaging, and theranostics.Molecules 2020, 25, 736 2 of 18 can release cadmium ions leading to cell death. Importantly, it occurred that quantum dots may undergo certain modifications resulting in the change of their surface or chemical structure [27,28]. Finally, their preparation is expensive. Therefore, there was a need for a novel type of quantum dots which would be suitable for biomedical applications. In 2004, a new type of carbon-based materials was accidentally obtained during nanotube purification [29]. Until then, numerous types of carbon quantum dots (CQDs) have been obtained with various possible applications due to their very attractive properties, such as luminesce, good water solubility, high photostability, resistance to photobleaching or photoblinking, possibility, chemical inertness and, finally, lack of toxicity and biocompability [1][2][3]. Furthermore, CQDs' region of photoluminescence is very broad, starting in the ultraviolet and ending in the near-infrared. Although, until now, the luminescence behavior of carbon nanodots is still not fully clear, it is believed that crucial role plays two factors, namely, that the quantum size is below 10 nm and surface defects which may act as excitation energy traps. Interestingly, CQDs can successfully act as both electron donors and acceptors [2][3][4].CQDs can be prepared from various resources, such as peels (mango, orange, banana, pineapple, onion, seaweed etc.), meat, grains, nuts, or vegetable by-products, which makes them a low-cost material [30,31]. Currently, these nanomaterials are widely applied in drug delivery, gene delivery, biosensing, bioimag...

Facile Synthesis of Surface-Modified Carbon Quantum Dots (CQDs) for Biosensing and Bioimaging

“…What is interesting, the choice of the modifying agent affected the PL properties in various ways and there was no general tendency for all of the samples, similarly as in the case of the optical characteristics of the products. The fluorescence of the prepared samples was a result of the disruption of the crystalline structure along with the band gap transitions (conjugated π-type bonds), which is characteristic for carbon quantum dots [ 5 , 6 , 14 , 20 , 25 , 29 , 30 ]. Figure 7 presents the CQDs solutions irradiated with the commercially available diode, which exhibit blue fluorescence at λ = 365 nm.…”

“…They may have a crystalline or amorphous structure. Carbon quantum dots fluorescence can be explained by surface defects or conjugated π-bonds bandgap transitions [ 20 , 25 , 26 , 27 , 28 , 29 , 30 ]. The nanodots can be prepared via two different approaches, namely top down and bottom up.…”

“…They enable fast obtainment of the nano-objects with mostly a crystalline structure and uniform size. However, these methods that are commonly called top-down, require second step enabling semiproducts functionalization due to the unsatisfactory level of quantum yield [ 27 , 28 , 29 ]. Therefore, new pathways have been developed.…”

“…Aforementioned ways of nanodots preparation are considered as ecofriendly, since they enable the use of waste biomass of various origin as CQDs precursors such as coffee grounds, fruit and vegetable peels and others. Very often also polysaccharides, peptides, antibodies or amino acids (cysteine) are also applied due to the presence of heteroatoms such as oxygen, nitrogen or sulphur, which play a crucial role in the enhancement of the carbon dots fluorescence properties [ 29 , 30 , 31 , 32 ]. The CQD’s surface is rich in various functional groups such as amino, hydroxyl or carboxyl, which are responsible for their hydrophilicity, which is a significant advantage over graphene quantum dots (GQDs), which are known to be poorly soluble in water.…”

“…Carbon quantum dots have a great potential in the field of biology and medicine due to their numerous advantages. Nevertheless, there are still many issues to be solved to increase their applicability in biomedical applications such as biosensing and bioimaging [ 20 , 29 ].…”