Surface functionalization and doping strategies for carbon quantum dots are critical for tailoring their physicochemical properties, enhancing performance, and enabling compatibility with diverse systems. These modifications can be broadly categorized into covalent and non-covalent functionalization methods, as well as heteroatom doping techniques. Each approach influences the optical, electronic, and surface characteristics of CQDs, making them suitable for specialized uses.

Covalent functionalization involves the formation of chemical bonds between functional groups and the CQD surface. One common strategy is the introduction of amine groups through reactions with ammonia, ethylenediamine, or other nitrogen-containing compounds. Amine-functionalized CQDs exhibit improved water solubility due to the hydrophilic nature of the amino groups, while also enhancing photoluminescence by introducing new surface states. Carboxyl groups are another widely used modification, often achieved via oxidation with nitric acid or other strong oxidizers. Carboxylation increases the negative surface charge, improving colloidal stability and providing anchor points for further conjugation with biomolecules or polymers. For example, carboxylated CQDs show enhanced dispersion in polar solvents and higher reactivity in esterification or amidation reactions.

Polymer grafting is another covalent approach, where polymers such as polyethylene glycol (PEG) or polyethylenimine (PEI) are attached to the CQD surface. PEGylation improves biocompatibility and reduces nonspecific interactions, while PEI grafting introduces positive charges, facilitating binding with nucleic acids or other negatively charged species. Thiol-based functionalization is also employed, where molecules like mercaptoacetic acid are used to introduce -SH groups, enabling further conjugation with metals or biomolecules via thiol-chemistry.

Non-covalent functionalization relies on physical interactions such as electrostatic forces, π-π stacking, or hydrophobic effects. Surfactants like sodium dodecyl sulfate (SDS) or cetyltrimethylammonium bromide (CTAB) can adsorb onto CQD surfaces, altering their hydrophilicity and stability. Aromatic molecules with conjugated systems, such as pyrene derivatives, interact with CQDs via π-π stacking, modifying their electronic structure without disrupting the carbon core. This method preserves the intrinsic properties of CQDs while enabling surface modifications. Another non-covalent strategy involves wrapping CQDs with amphiphilic polymers or biomolecules like proteins or DNA, which adsorb onto the surface through multiple weak interactions, improving solubility and functionality.

Heteroatom doping is a powerful tool for tuning the electronic and optical properties of CQDs. Nitrogen doping is the most studied approach, where nitrogen atoms are incorporated into the carbon lattice during synthesis or post-treatment. N-doped CQDs often exhibit enhanced fluorescence quantum yields and red-shifted emission due to the introduction of new energy levels within the bandgap. For instance, N-doping can increase the quantum yield from 10% to over 40% by creating electron-rich surface states that facilitate radiative recombination. Sulfur doping introduces lone pairs of electrons, which modify the charge distribution and often result in longer-wavelength emissions. S-doped CQDs may also exhibit improved catalytic activity due to the presence of sulfur-containing functional groups like thiophene or sulfoxide.

Phosphorus and boron doping are less common but offer unique advantages. P-doping introduces electron-donating characteristics, while B-doping creates electron-deficient sites, both of which alter the electronic conductivity and optical properties. Co-doping, such as N-S or N-P dual doping, can synergistically enhance properties by combining the effects of individual dopants. For example, N,S-co-doped CQDs often show superior photoluminescence and electrochemical activity compared to singly doped counterparts.

The choice of functional groups and doping elements directly impacts CQD performance. Hydroxyl and carbonyl groups improve hydrophilicity and facilitate hydrogen bonding, while alkyl chains enhance solubility in nonpolar solvents. Aromatic groups can extend conjugation, leading to redshifted emission. The introduction of charged groups like sulfonate or quaternary ammonium compounds enhances electrostatic stabilization in colloidal systems. Doping with electron-withdrawing or electron-donating heteroatoms modifies the charge carrier density, influencing conductivity and catalytic behavior.

Surface modifications also affect stability. For instance, PEG-coated CQDs exhibit reduced aggregation in biological fluids, while carboxyl-rich surfaces prevent precipitation in aqueous media by electrostatic repulsion. The presence of dopants can increase resistance to photobleaching by altering the electronic structure and reducing non-radiative decay pathways.

Functionalization strategies must be carefully selected based on the intended use. Covalent methods provide permanent modifications but may require harsh conditions that damage the CQD core. Non-covalent approaches are milder but may lack long-term stability. Doping introduces atomic-level changes but requires precise control to avoid undesirable defects. The combination of these techniques enables the fine-tuning of CQD properties for optimized performance in various fields.

In summary, surface functionalization and doping are versatile tools for engineering carbon quantum dots with tailored properties. Covalent modifications like amine, carboxyl, or polymer grafting alter surface chemistry and solubility, while non-covalent methods provide reversible changes through physical interactions. Heteroatom doping adjusts electronic and optical characteristics by introducing new energy states. These strategies collectively enhance the stability, functionality, and compatibility of CQDs, making them adaptable to a wide range of specialized requirements.