Dispersion of optical and structural properties in gel column separated carbon nanoparticles
Graphical abstract
Introduction
Carbon nanodots (CNDs) are a new and unique class of luminescent nanoparticles. CNDs have many advantages such as their small size around units of nanometers [1], [2], [3], high surface area, special morphology, unique electronic, optical characteristics [4], biocompatibility and chemical inertness; they mainly consist of non-toxic C, O and N atoms. These important characteristics are potentially promising in biological labeling [5], biosensing [6], [7], photocatalysis [8], [9].
Another advantage of CNDs is a large variety of carbon sources to produce the desired product such as carbon materials with different structure (graphene, graphite rod etc.) [10], [11], carbohydrates (dextran, glucose, sucrose, starch, citric acid) [12], [13], natural products like coffee [14], banana juice [15], orange waste peels [16], and many other.
In addition, there is a possibility of synthesis with a range of chemical and physical methods such as hydrothermal synthesis [13], microwave synthesis [17], electrochemical synthesis [10], [11], laser ablation, metal-graphite intercalation [18], pyrolysis in concentrated acid and carbonization in a microreactors [19]9 etc. Many of these ways of CNDs synthesis allow creating photoluminescent (PL) nanoparticles with different structure, they are fast, cheap and usually without using strong reagents such as concentrated acids, organic solvents and so on.
Today the physical origin of PL and morphology of carbon nanoscale materials is still unclear, with numerous fundamental questions yet unresolved [20]. It is believed, that presence of surface defects in carbon systems is one of the main cause of CNDs PL properties [5], [20], [21], [22]. Unlike luminescent semiconductor quantum dots (such as CdSe) where PL is largely determined by carrier (electron and hole) confinement, carbon nanoparticles have a different luminescence mechanism with functional groups on CNDs surface playing the key role. The CNDs surface does not consist of isolated chemical groups but rather the hybridization of the carbon backbone and connected chemical groups [23]. Hybridized carbons (sp2 and sp3) and other functionalized surface defects, such as carbonyl-related localized electronic states are present in CNDs and contribute to the light emission. Thus, the bright surface defect-derived PL of CNDs is due to the recombination of electron–hole pairs in π and π* electronic levels of the sp2 sites. These sites lie between the bandgap of the σ and σ * states of the sp3 matrix, leading to strong visible emissions [24]. Furthermore, a higher degree of surface oxidation can result in more surface defects, resulting in the red-shifted emission [25]. Thus, the comprehensive and effective theoretical description developed for conventional quantum dots is not directly applicable to CNDs. Moreover, their PL response is not collective and represents a composition of individual emitters [20].
One of common features of CNDs is the dependence of the emission wavelength on the excitation wavelength. The main reason for that is the simultaneous presence luminescent centers with the different size and morphology [23], [26], [27]. The use of various synthetic methods allows obtaining the mixture of CNDs with different size and structure. There are various approaches to the separation and purification of CNDs such as centrifugation (after removal of soot) [28], [29] dialysis [10], [30], electrophoresis [1], chromatographic methods [6], [14], [31] including gel column chromatography [32]. Gel column chromatography allows purification of carbon nanoparticles from substrate soot, extraction of CNDs fractions of maximum quantum yield [33] and separation of CNDs fractions by size [32]. However, approaches described in literature so far do not yield CNDs separation that results in distinct PL spectral characteristic.
In this work, we report an effective size separation method to separate polydisperse CNDs solution into fractions by gel column chromatography for further detailed research. This method allows to quickly and easily isolate CNDs fractions with different size, morphology and to determine the effect of CNDs size on the position of maxima and luminescence intensity. The resulted structures divided on fractions are analyzed by luminescence, Raman spectroscopy and TEM. The obtainment of more size-homogenous fractions allows easy control of CNDs properties in many areas, including application in vivo.
Section snippets
CNDs preparation
Dextran sulfate sodium salt (DSS, Mw∼ 70 kDa) was purchased from BioChemika. CNDs were synthesized directly from DSS using hydrothermal synthesis in bidistilled water. The typical procedure includes preparation of DSS water solution (6 ml) 0.042 g (0.6 pmol). The solution was then transferred into a glass cup, placed into the teflon cup with tight-fitting cover, put into stainless steel autoclave and heated at 220 °C for 3 h. The resulting solution was cooled at room temperature and the uplayer
CNDs preparation
Carbon nanodots were synthesized by hydrothermal method (Fig. 1). This procedure includes preparation of DSS aqueous solution, placing the solution into a special teflon cup and putting it to a stainless-steel autoclave. An autoclave is heated at constant temperature for 3 h at 220 °C. After cooling we centrifuged the obtained product and collected the supernatant solution containing CNDs. CNDs were obtained by the hydrothermal method from natural polymer, under these condition one can expect
Conclusion
We have synthesized luminescent carbon nanoparticles in aqueous solution using natural polymer sodium dextran sulfate as template for hydrothermal carbonization. The emission of CNDs have a widely-reported excitation-dependent spectra. We found out that the PL intensity significantly depends on reabsorption effect due to the large number of different luminescent centers in water suspension of CNDs. In order to reveal the intrinsic PL there is a necessity to dilute CNDs water solutions to reduce
Acknowledgements
The work was supported by Russian Science foundation (project 16-13-10195).
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