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

The study addresses how optical activity (chirality) arises in carbon dots synthesized from different chiral precursor molecules and how it can be controlled. CDs are promising for theranostics due to high photoluminescence, biocompatibility, facile synthesis, and surface functionalization. Achieving deep-red/NIR emission or multiphoton excitation improves bioimaging performance by reducing autofluorescence and photodamage. Chiral CDs are valuable for enantioselective recognition and chiral sensing. Previous approaches include growth on chiral substrates, one-pot synthesis with chiral precursors, and postsynthetic functionalization with chiral molecules. However, the origin and controllability of circular dichroism (CD) signals in one-pot synthesized CDs remain insufficiently understood. This work develops chiral CDs using various L-precursors and investigates their structure–optical property relationships, including chirality origins and multiphoton absorption, targeting robust bioimaging/sensing uses.

The paper reviews strategies to obtain chiral CDs: (1) assembly on chiral templates (e.g., cellulose nanocrystals); (2) one-pot syntheses using chiral precursors (e.g., citric acid with D-proline or L/D-glutamine); (3) postsynthetic functionalization of achiral CDs with chiral molecules (e.g., proline, phenylalanine, histidine, tryptophan, alanine, proline methyl ester; tyrosine, phenylalanine, tryptophan, serine, glutamic acid). Prior findings indicate that surface functionalization and one-pot synthesis can yield circular dichroism features of different origins: (i) inherited chirality from the precursor, (ii) hybridization of precursor and CD energy levels, and (iii) formation of a chiral CD core. Despite progress, the precise origins of chirality in one-pot synthesized CDs and means to control CD signals required further elucidation, motivating this study.

Synthesis: One-step hydrothermal synthesis in water at 190 °C for 8 h using citric acid and ethylenediamine combined with different L-chiral precursors: L-cysteine, L-glutathione, L-phenylglycine, and L-tryptophan, yielding CD-cys, CD-glu, CD-phe, and CD-try, respectively. A reference achiral sample (CD-eda) was synthesized from citric acid and ethylenediamine under the same conditions. Characterization: Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) for morphology and lattice fringes; dynamic light scattering (DLS) for hydrodynamic sizes; zeta potential measurements for surface charge; Fourier transform infrared spectroscopy (FTIR) for surface functional groups; UV–vis absorption spectroscopy; photoluminescence (PL) spectra including excitation-dependent PL; photoluminescence excitation (PLE) spectra; PL quantum yields (PLQYs) and time-resolved PL lifetimes; assessment of photostability under continuous UV irradiation at 366 nm up to ~420 min; evaluation of stability against pH changes; two-photon absorption–excited emission; density functional theory (DFT) analysis to model how incorporation of chiral precursors at optical centers affects circular dichroism responses. Analysis focused on correlating precursor chemistry with particle size, surface functionality, charge, absorption/PL features, PLQY/lifetimes, chiral optical signals, and multiphoton properties.

• Chiral CDs with O,N-doped cores (and S-doping for sulfur-containing precursors) and amide/hydroxyl-rich surfaces were obtained via one-pot hydrothermal synthesis. They disperse in solvents of different polarities and exhibit stable optical properties against pH changes and prolonged UV exposure (>400 min).
• Morphology and size: Spherical nanoparticles with sp²-domains showing 0.21 nm lattice spacing (graphitic (100) planes). TEM-average sizes: CD-cys 4.0 ± 0.4 nm; CD-glu 5.2 ± 0.4 nm; CD-phe 8.2 ± 0.8 nm; CD-try 5.2 ± 0.3 nm; CD-eda 6.3 ± 0.4 nm. DLS hydrodynamic diameters: CD-eda 18.2 ± 2.5 nm; CD-cys 6.5 ± 2.2 nm; CD-glu 11.7 ± 4.0 nm; CD-phe 8.7 ± 1.5 nm; CD-try 5.6 ± 1.9 nm. Zeta potentials (mV): CD-cys -8.0; CD-glu -5.2; CD-phe -11.0; CD-try -14.2; CD-eda -25.0.
• Surface chemistry (FTIR): Broad 3100–3500 cm⁻¹ band (N–H and OH); C–H stretching of aromatic (~3060 cm⁻¹) and aliphatic (2880–2940 cm⁻¹) carbons (more intense for CD-phe and CD-try); strong amide signals at 1645 cm⁻¹ (C=O) and 1535 cm⁻¹ (N–H bend); 1500–1600 cm⁻¹ bands (C=C in benzene rings); CD-phe and CD-try show a strong narrow band at 740 cm⁻¹.
• Optical absorption: High-energy bands at ~215–240 nm (π–π*); precursor-related bands shifted to ~220 and 280 nm; 300–400 nm region attributed to n–π* transitions with broad peaks at 340 nm (CD-eda, CD-cys, CD-glu) and red-shifted to 350 nm (CD-phe) and 365 nm (CD-try). CD-glu exhibited increased absorption >450 nm indicating surface molecular groups.
• Emission: PL peaks ~450 nm for excitation <400 nm; red-shifting of PL maxima with excitation >400 nm indicated lower-energy states. PLE monitored at 490 nm mirrored absorption: For CD-eda, CD-cys, CD-glu, peaks at 245 nm (shoulder 215) and 365 nm; for CD-phe at 230 and 370 nm; for CD-try at 250 and 370 nm (shoulder 350 nm). Emission primarily arises from similar n–π* transitions of molecular groups formed during synthesis, largely independent of precursor type.
• Efficiencies and dynamics: PLQYs in water: CD-eda 51%; CD-cys 41%; CD-glu 30%; CD-phe 55%; CD-try 57%. Average PL lifetimes reported alongside PLQYs (values not fully listed in excerpt) and remained stable after extended UV exposure.
• Chirality: CDs exhibit chiral optical signals in UV and visible regions. Signals originate from (i) chiral precursors attached to CD surfaces, (ii) hybridization of lower-energy levels of chiral chromophores formed within CDs, and (iii) intrinsic chirality of CD cores. DFT analysis showed that incorporation of chiral precursors at optical centers induces strong circular dichroism responses.
• Nonlinear optics: Emission can be excited via two-photon absorption, supporting suitability for multiphoton bioimaging.

The study demonstrates that one-pot hydrothermal synthesis with different L-chiral precursors yields CDs with robust photophysical properties and well-defined chiral optical responses. Despite similar emission origins (n–π* transitions from molecular groups), the precursor chemistry tunes absorption features, PLQY, particle hydrodynamics, and surface charge. Aromatic-containing precursors (L-phenylglycine, L-tryptophan) enhance PLQY and yield smaller hydrodynamic sizes with more negative surface charge, while aliphatic sulfur-containing precursors (L-cysteine, L-glutathione) yield smaller core sizes and reduced surface charge magnitude. The chiral signals stem from combined mechanisms: inherited precursor chirality, energy level hybridization between precursor-derived chromophores and CD states, and intrinsic core chirality. DFT supports the strong CD response upon precursor incorporation at optical centers. Stability against pH and prolonged UV irradiation, together with two-photon excitable emission, underlines the relevance of these CDs for bioimaging, chiral sensing, enantiomer recognition, and site-specific interactions, addressing the need for robust, bright, and chiral nanoprobes.

Hydrothermal one-pot synthesis from citric acid, ethylenediamine, and L-chiral precursors (cysteine, glutathione, phenylglycine, tryptophan) produces chiral CDs with high PLQYs (up to 57%), tunable absorption, strong circular dichroism signals in UV–vis, and two-photon-excited emission. Structural analyses reveal sp²-domains and amide/hydroxyl-rich surfaces; optical and surface properties depend on precursor chemistry. Chiral signals arise from a combination of surface-attached precursors, hybridized chiral chromophores, and intrinsic core chirality, as supported by DFT. The materials are photostable and pH-stable, making them promising for bioimaging, sensing, drug delivery, and theranostics. Future work may exploit precursor design to further control chirality strength and spectral position and to enhance multiphoton cross-sections for deep-tissue imaging.