Colloidal synthesis of zinc oxide quantum dots (ZnO QDs) offers precise control over particle size, morphology, and optical properties, making them highly suitable for applications in bioimaging and display technologies. The process typically involves the hydrolysis and condensation of zinc precursors in a solvent medium, often at moderate temperatures. A common approach utilizes zinc acetate dihydrate as the precursor, dissolved in a polar solvent like methanol or ethanol, with a stabilizing agent such as sodium hydroxide or triethanolamine to control nucleation and growth. The reaction proceeds under vigorous stirring and can be tuned to produce QDs with diameters ranging from 2 to 10 nm by adjusting parameters like temperature, precursor concentration, and reaction time.
The size-dependent photoluminescence (PL) of ZnO QDs arises from quantum confinement effects, which modify the electronic band structure as the particle dimensions approach the exciton Bohr radius of ZnO (approximately 2.34 nm). Smaller QDs exhibit a blue shift in their UV emission peak due to increased quantum confinement, while larger particles show a red shift. The near-band-edge emission typically occurs between 370 and 390 nm, while visible emission in the green-orange range (500–600 nm) is attributed to defect states such as oxygen vacancies or zinc interstitials. The PL quantum yield can exceed 50% for well-passivated QDs, making them attractive for optoelectronic applications.
Surface functionalization is critical for stabilizing ZnO QDs in colloidal suspensions and tailoring their properties for specific uses. Ligands like oleic acid, thiols, or amines are often employed to prevent aggregation and enhance dispersibility in organic or aqueous media. For bioimaging, biocompatible coatings such as polyethylene glycol (PEG) or silica shells are added to reduce cytotoxicity and improve cellular uptake. These modifications also minimize surface defect-related PL quenching, ensuring bright and stable emission.
In bioimaging, ZnO QDs serve as fluorescent probes due to their high photostability, low toxicity, and tunable emission. Their UV-excited fluorescence is useful for tracking cellular processes, while surface conjugation with biomolecules like antibodies or peptides enables targeted imaging. Studies have demonstrated their effectiveness in visualizing cancer cells, where their bright emission and biocompatibility outperform conventional organic dyes. Additionally, their ability to generate reactive oxygen species under UV illumination has been explored for photodynamic therapy, combining imaging and therapeutic functions.
For display technologies, ZnO QDs are integrated into quantum dot light-emitting diodes (QLEDs) and color conversion layers. Their narrow emission bandwidth and high color purity enhance the gamut of displays, particularly for blue and UV-emitting components. When combined with red and green-emitting QDs, ZnO QDs contribute to wide-color-gamut backlighting in liquid crystal displays (LCDs) or as active emitters in electroluminescent devices. Their stability under electrical excitation and resistance to photobleaching make them superior to organic emitters in long-term applications.
Challenges remain in achieving uniform size distribution and suppressing defect-related emission, which can degrade color purity. Advanced synthesis techniques, such as hot-injection methods or microwave-assisted reactions, have improved monodispersity and reduced surface defects. Post-synthetic treatments with passivating agents further enhance optical performance by eliminating non-radiative recombination sites.
Future developments may focus on scaling up production while maintaining quality, as well as exploring hybrid systems where ZnO QDs are combined with other nanomaterials to achieve multifunctional properties. Their compatibility with flexible substrates also opens avenues for wearable displays and foldable electronics.
In summary, colloidal ZnO QDs exhibit size-tunable photoluminescence with high quantum efficiency, enabling their use in bioimaging and advanced displays. Precise synthesis and surface engineering are key to optimizing their performance, while ongoing research addresses scalability and integration challenges for commercial applications.