Revealing the nature of optical activity in carbon dots produced from different chiral precursor molecules.

Carbon dots (CDs) are light-emitting nanoparticles that show great promise for applications in biology and medicine due to the ease of fabrication, biocompatibility, and attractive optical properties. Optical chirality, on the other hand, is an intrinsic feature inherent in many objects in nature, and it can play an important role in the formation of artificial complexes based on CDs that are implemented for enantiomer recognition, site-specific bonding, etc. We employed a one-step hydrothermal synthesis to produce chiral CDs from the commonly used precursors citric acid and ethylenediamine together with a set of different chiral precursors, namely, L-isomers of cysteine, glutathione, phenylglycine, and tryptophan. The resulting CDs consisted of O,N-doped (and also S-doped, in some cases) carbonized cores with surfaces rich in amide and hydroxyl groups; they exhibited high photoluminescence quantum yields reaching 57%, chiral optical signals in the UV and visible spectral regions, and two-photon absorption. Chiral signals of CDs were rather complex and originated from a combination of the chiral precursors attached to the CD surface, hybridization of lower-energy levels of chiral chromophores formed within CDs, and intrinsic chirality of the CD cores. Using DFT analysis, we showed how incorporation of the chiral precursors at the optical centers induced a strong response in their circular dichroism spectra. The optical characteristics of these CDs, which can easily be dispersed in solvents of different polarities, remained stable during pH changes in the environment and after UV exposure for more than 400 min, which opens a wide range of bio-applications.

Toward Bright Red-Emissive Carbon Dots through Controlling Interaction among Surface Emission Centers.

Relatively weak red photoluminescence of carbon dots (CDots) is a major challenge on the way to their successful implementation in biological and optoelectronic devices. We present a theoretical analysis of the interaction among the surface emission centers of CDots, showing that it may determine efficiency of the red photoluminescence of CDots. Based on the previous experimental studies, it is assumed that the optical response of the CDots is determined by the molecule-like subunits of polycyclic aromatic hydrocarbons (PAHs) attached to the CDots' surface. Three characteristic types of coupling of these PAH subunits are considered: non-interacting monomers, noncovalently bound dimers, and covalently bound dimers with two, three, or four carbon linkers. We demonstrate that the CDots' photoluminescence broadens, redshifts, and weakens by 2 orders of magnitude when the free monomers are substituted by the covalently bridged centers. These and other results of our study show that the realization of CDots with many weakly interacting surface emission centers may constitute an efficient way to achieve their efficient red photoluminescence, which is highly desirable for biological and optoelectronic applications.

Influence of the solvent environment on luminescent centers within carbon dots.

Carbon dots (CDs) are luminescent nanomaterials, with potential use in bioimaging and sensorics. Here, the influence of the surrounding solvent media on the optical properties of CDs synthesized from the most commonly employed precursors, namely citric acid and ethylenediamine, is investigated. The position of optical transitions of CDs can be tuned by the change of pH and solvent polarity. The most striking observation is related to the interaction of CDs with chlorine containing solvents, which results in resolving a set of narrow peaks within both the absorption and PL bands, similar to those observed for polycyclic aromatic hydrocarbons or organic dyes. We assume that the chlorine containing molecules penetrate the surface layers of CDs, which results in an increase of the distance between the luminescent centers; this correlates well with an enhanced D-band in their Raman spectra. A model of CDs composed of a matrix of hydrogenated amorphous carbon with the inclusions of sp2-domains formed by polycyclic aromatic hydrocarbons and their derivatives is suggested; the latter are stacked ensembles of the luminophores and are considered as the origin of the emission of CDs.

Amino Functionalization of Carbon Dots Leads to Red Emission Enhancement.

The availability of carbon dots (CDots) with bright red photoluminescence (PL) would significantly broaden the range of their biological and optoelectronic applications. We present a theoretical model that predicts that amino functionalization of CDots not only shifts their PL to longer wavelengths but also preserves large oscillator strengths of the fundamental radiative transitions of CDots. The model considers the optical response of amino-functionalized CDots determined by molecule-like subunits of polycyclic aromatic hydrocarbons with one, two, or three -NH2 groups at the CDots' surface; the excited state of those subunits is characterized by strong charge separation between the amino groups and CDots' carbon core. Such a separation determines the Stokes shift of the CDots' emission, which increases with the growing amount of the amino functional groups at the CDot surface. Our model explains the experimentally observed dependence of the PL spectra of CDots on the excitation wavelength, the phenomenon well documented in the literature for nitrogen-containing CDots.

sp2-sp3-Hybridized Atomic Domains Determine Optical Features of Carbon Dots.

Carbon dots (CDots) are a promising biocompatible nanoscale source of light, yet the origin of their emission remains under debate. Here, we show that all the distinctive optical properties of CDots, including the giant Stokes shift of photoluminescence and the strong dependence of emission color on excitation wavelength, can be explained by the linear optical response of the partially sp2-hybridized carbon domains located on the surface of the CDots' sp3-hybridized amorphous cores. Using a simple quantum chemical approach, we show that the domain hybridization factor determines the localization of electrons and the electronic bandgap inside the domains and analyze how the distribution of this factor affects the emission properties of CDots. Our calculation data fully agree with the experimental optical properties of CDots, confirming the overall theoretical picture underlying the model. It is also demonstrated that fabrication of CDots with large hybridization factors of carbon domains shifts their emission to the red side of the visible spectrum, without a need to modify the size or shape of the CDots. Our theoretical model provides a useful tool for experimentalists and may lead to extending the applications of CDots in biophysics, optoelectronics, and photovoltaics.

Circular dichroism of surface complexes based on quantum dots and azo dye.

Chiral properties of surface complexes based on CdSe/ZnS quantum dots (QDs) and 1-(2-pyridylazo)-2-naphthol (PAN) azo dye were investigated by circular dichroism spectroscopy. The use of L-, D-cysteine (Lcys, Dcys) capping ligands allowed us to obtain water-soluble chiral QD-PAN complexes. The characterization of the complexes was performed by UV-vis, FTIR, and CD spectroscopy. Quantum chemical TDDFT calculated CD spectra reproduced the experimentally observed sign patterns, which originate from binding Lcys or Dcys and PAN molecules to the same Zn atom on the QD surface. The resulting complex is characterized by a large circular dichroism in comparison with an ordinary QD chirality induced by cysteine molecules. The pattern of CD signal is the same for Lcys and Dcys ligands in chiral QD-PAN complex.