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

The development of nanomaterials for theranostics (combined diagnostics and therapy) is crucial. Carbon dots (CDs), luminescent carbon nanoparticles, are promising candidates due to their biocompatibility, bright emission, and ease of fabrication and functionalization. Significant progress has been made in synthesizing highly emissive CDs with photoluminescence (PL) quantum yields exceeding 50%, shifting their optical transitions to the deep-red and near-infrared regions for improved bioimaging resolution and sensitivity. Surface functionalization allows for targeted bonding to tissues, ion sensing, and enhanced photothermal conversion for cancer treatment. Optical chirality, an inherent property of many natural objects, is also relevant for biomedical applications like enantioselective recognition and chiral sensing. Chiral CDs can be created using chiral substrates or synthesized from chiral precursors. Post-synthetic treatment of achiral CDs with chiral molecules is another approach. Previous research compared these methods, revealing that different approaches (surface functionalization, one-pot synthesis) result in circular dichroism spectral transitions originating from inherited precursor chirality, energy level hybridization, and chiral core formation. However, the origin of chirality in CDs produced via one-pot syntheses and methods for controlling circular dichroism signals require further investigation. To improve bioimaging resolution and signal-to-noise ratio, minimizing autofluorescence by red-shifting optical transitions or employing multiphoton excitation is important. The development of synthetic methods for chiral CDs with high multiphoton absorption cross-sections is thus of significant interest, but challenging. This study aims to develop synthetic routes towards chiral CDs from different chiral precursors, aiming for superior optical properties (high PLQYs, chiral signals in the UV and visible regions, two-photon absorption) and stability for bioimaging, sensing, drug delivery, and theranostics applications.

Numerous studies have focused on synthesizing highly emissive CDs with PL QYs over 50%, manipulating their optical properties for deep-red and near-infrared bioimaging applications. Strategies include increasing the size of sp²-domains and N-doping. Surface functionalization techniques have been explored for attaching proteins and antibodies for targeted drug delivery and sensing. The incorporation of chirality into CDs has also received attention, with different approaches explored, including using chiral substrates like cellulose nanocrystals or chiral precursors like D-proline or L/D-glutamine during synthesis. Post-synthetic modification with chiral molecules such as proline, phenylalanine, histidine, tryptophan, alanine, and tyrosine have also been used. However, a comprehensive understanding of the origin of chirality and its control in one-pot synthesis remains an open area of research. The importance of multiphoton excitation for reduced autofluorescence, improved penetration depth, and less photodamage in bioimaging applications have also been highlighted in existing literature.

A set of four chiral CDs (CD-cys, CD-glu, CD-phe, and CD-try) were synthesized using a one-step hydrothermal method (190 °C, 8 h) from citric acid and ethylenediamine, along with different chiral precursors (L-cysteine, L-glutathione, L-phenylglycine, and L-tryptophan, respectively). An achiral CD sample (CD-eda) was synthesized under identical conditions without chiral precursors, serving as a control. The size and morphology of the CDs were characterized by transmission electron microscopy (TEM) and dynamic light scattering (DLS). High-resolution TEM (HRTEM) was used to analyze the presence of sp²-domains. Zeta potential measurements were performed to determine the surface charge. Fourier transform infrared (FTIR) spectroscopy was used for chemical composition analysis. UV-Vis absorption spectroscopy, photoluminescence (PL) spectroscopy, and photoluminescence excitation (PLE) spectroscopy were employed to determine the optical properties. PL quantum yields (PLQYs) and PL lifetimes were also measured. The effects of UV exposure (366 nm) on PLQY were also investigated.

TEM images confirmed the spherical morphology of all synthesized CDs. Average sizes varied from 4.0 nm (CD-cys) to 8.2 nm (CD-phe), with DLS measurements revealing larger hydrodynamic sizes, potentially due to surface hydration. HRTEM showed the presence of sp²-domains with interplanar spacing consistent with graphitic carbon. FTIR spectra revealed the presence of N-H, O-H, and C-H groups, with CD-phe and CD-try showing stronger C-H peaks from aromatic carbons. The presence of amide groups was also evident. UV-Vis absorption spectra showed peaks corresponding to π–π* and n–π* transitions, with variations in peak positions related to precursor composition. PL spectra showed emission peaks around 450 nm for excitation wavelengths below 400 nm. Red-shifting of PL peak positions with longer excitation wavelengths indicated the presence of lower energy states. PLE spectra generally mirrored the absorption spectra. PLQYs varied from 30% (CD-glu) to 57% (CD-try). CD-phe and CD-try, with benzene ring-containing precursors, showed higher PLQYs. PL lifetimes were also affected by the chemical composition of the precursors. Importantly, the optical properties showed minimal change after 400 min of UV exposure and with variations in pH, indicating high stability.

The findings demonstrate that the optical chirality of CDs synthesized using a one-pot hydrothermal method arises from a combination of factors: the presence of chiral precursors on the surface, the hybridization of energy levels of chiral chromophores within the CD core, and potentially the intrinsic chirality of the CD core itself. DFT calculations would be needed to support this hypothesis. The variations in PLQY and PL lifetime observed among different CD samples highlight the influence of the chemical structure of the chiral precursors on the CDs' optical properties. The high stability of the CDs' optical characteristics under varying conditions suggests promising prospects for their use in various bio-applications.

This study successfully synthesized chiral CDs from different chiral precursors, achieving high PLQYs and demonstrating stable optical properties under varying conditions. The chiral signals were shown to arise from a combination of surface, core, and chromophore effects. This work provides insights into the origin of optical activity in CDs and highlights their potential for various bio-applications. Future research should focus on further exploring the structure-property relationships using advanced characterization techniques, potentially including theoretical modeling, and investigating the CDs' performance in specific bio-applications.

The study primarily focused on the optical properties of the synthesized CDs. Further investigation into the CDs' biocompatibility, cytotoxicity, and in vivo behavior is warranted. Detailed mechanistic studies using advanced characterization and theoretical modeling techniques are needed to gain a deeper understanding of the origin and control of chirality in the CDs.