Quantum Dots in Biomedicine: From Precision Imaging to Single-Virus Tracking and the Road to Clinical Translation
Key Insights
Quantum dots (search) (QDs) are semiconductor nanocrystals with size-tunable fluorescence, exceptional brightness, and photostability, enabling multiplexed bioimaging and long-term tracking applications.
Heavy-metal-free alternatives such as carbon quantum dots (search) (CQDs) and InP-based QDs address cadmium toxicity concerns while maintaining favorable optical properties for biomedical use.
QD-based single-virus tracking (QSVT) using CRISPR/Cas systems and biotin-streptavidin strategies has revealed detailed mechanisms of viral entry, transport, and genome release in live cells.
Quantum dots (search) (QDs), semiconductor nanocrystals typically ranging from 2 to 10 nanometers in diameter, have emerged as one of the most promising classes of fluorescent nanoprobes for biomedical applications. Their unique quantum confinement effects enable precise control of fluorescence emission through particle size engineering, allowing simultaneous generation of multiple emission wavelengths using a single excitation source. This tunability, combined with broad absorption spectra, narrow symmetric emission peaks, and remarkable resistance to photobleaching, has positioned QDs at the forefront of next-generation imaging technologies.
Optical Properties and Material Innovation
The optical performance of QDs is fundamentally governed by their size-dependent bandgap energy. As described by the Brus equation and subsequent modifications by Kayanuma, the emission wavelength increases as particle diameter increases—CdSe QDs, for instance, exhibit emission wavelengths ranging from approximately 500 nm (green) to 650 nm (red) as their diameter grows from 2 nm to 6 nm. Core/shell architectures further enhance performance: Kim et al. (2022) reported alloy-typed CdSeZnS/ZnS core/shell QDs with an initial quantum yield as high as 98.0%, which remained at 84.7% even after hydrophilic surface modification—a decrease of only approximately 13.6%, compared with conventional multilayer QDs that lost more than 50% of their quantum yield under the same treatment.
Early generations of QDs were primarily composed of cadmium-containing materials such as CdSe and CdTe, raising significant toxicity concerns. Cadmium ions released through surface degradation can bind to sulfhydryl groups in mitochondrial proteins, causing oxidative stress and functional impairment. Stan et al. (2014) reported that Si/SiO2 QDs increased reactive oxygen species (ROS) generation by 5.5 times at 200 μg/mL over 48 hours and 7.0 times at 200 μg/mL over 72 hours compared with controls. These concerns have driven substantial research into safer alternatives, including indium phosphide (search) (InP), carbon quantum dots (search) (CQDs), graphene quantum dots (search) (GQDs), and silicon quantum dots (SiQDs).
Carbon-based QDs have attracted particular attention due to their excellent biocompatibility and low toxicity. Abu et al. (2023) evaluated three types of CQDs derived from biomass waste—palm shells, oyster shells, and citric acid—across multiple biological models including HeLa cells, cardiomyocytes, induced pluripotent stem cells, and medaka fish embryos. In vitro assessments revealed no cytotoxicity up to 24 hours of incubation, and in vivo tests on Japanese medaka fish confirmed biocompatibility even after 17 days of exposure. Dhandapani et al. (2022) developed a simple, additive-free method to synthesize green fluorescent CQDs using fried food waste as a carbon source, while Saikia et al. (2023) fabricated biocompatible CQDs from petroleum coke and kitchen tea residue via an ultrasonic-assisted oxidation process, demonstrating high water solubility and non-genotoxic behavior.
Surface Functionalization and Targeting
Surface engineering critically determines the biological interactions of QDs. Three key parameters—surface functional groups, surface charge, and ligand exchange dynamics—must be optimized for biomedical applications. Liu et al. (2015) systematically compared three commercially available QDs with different surface coatings and found that charged QDs entered cancer cells and macrophages more efficiently than neutral QDs, with negatively charged QDs showing the highest internalization. Notably, positively charged QDs exhibited severe cytotoxicity attributed to disruption of cell membrane integrity and induction of ROS production, while negatively charged QDs showed reduced non-specific uptake and better biocompatibility.
Ligand exchange dynamics govern the substitution of native hydrophobic ligands with hydrophilic ones. Saad et al. (2026) demonstrated that dual-surface carboxylation of silicon quantum dots (search) serves as an optimal strategy for designing high-performance drug delivery systems, with dual -COOH functionalization significantly enhancing adsorption of the anticancer drug thioguanine, achieving the highest binding energy of −2.04 eV and the shortest interaction distance of 2.54 Å.
Targeted nanoprobes have been developed by conjugating QDs with antibodies, peptides, and small-molecule ligands. Sukhanova et al. (2021) reported single-domain antibody-conjugated QDs (sdAb-HER2-QDs and sdAb-CEA-QDs) that successfully detected micro-metastases and disseminated tumor cells in HER2- or CEA-expressing cancers using deep-tissue imaging in animal models. Zhao et al. (2025) developed a dual-modal PET/fluorescence imaging probe using radioisotope-chelated InP/ZnSe/ZnS QDs combined with copper-64, enabling successful positron emission tomography and fluorescence imaging in tumor-bearing mice.
Drug Delivery Applications
QDs have been engineered as traceable drug carriers with real-time imaging capability. Yezhelyev et al. (2008) reported a siRNA delivery system by coating QDs with a proton-sponge polymer, achieving a 10–20-fold improvement in gene-silencing efficiency compared with standard transfection agents. Kumar et al. (2012) developed DSPE-PEG micelles co-encapsulating CdSe QDs and doxorubicin, demonstrating approximately 50% cell death at a doxorubicin concentration of 5 μg/mL compared with more than 65% cell survival for free doxorubicin at the same concentration.
CQDs have enabled sophisticated stimuli-responsive delivery platforms. Feng et al. (2016a) developed a pH-responsive, charge-convertible CQDs nanocarrier for cisplatin(IV) delivery, termed CQDs-Pt(IV)@PEG-(PAH/DMMA), which exploits the pH difference between the tumor microenvironment and normal tissue to trigger drug release. Li et al. (2016b) designed a transferrin-conjugated CQDs-doxorubicin system (C-Dots-Trans-Dox) that achieved approximately 5-fold higher nuclear doxorubicin accumulation in pediatric brain tumor cells compared with doxorubicin alone, with the conjugate at 10 nM reducing viability by 14%–45% across multiple pediatric brain tumor cell lines.
Single-Virus Tracking Breakthroughs
QDs-based single-virus tracking (QSVT) has emerged as a transformative technique for studying viral infection mechanisms. The superior brightness and photostability of QDs enable extended tracking with low laser intensity, making them particularly favorable for acquiring time-series images and 3D reconstructions.
Hong et al. (2015) devised a hydrazine-aldehyde-based strategy to label influenza A virus (search) (IAV), achieving a labeling efficiency of 92%–93% without compromising infectivity. Subsequently, Hong et al. (2018) developed dual-color QD-labeled IAV by transferring QDs-labeled nucleoprotein antibodies into H1N1 to label vRNP complexes, maintaining 93% infectivity. Wen et al. (2014) reported a general method for labeling enveloped viruses using lipid-biotin conjugates and SA-QDs, achieving specificity up to 99% and labeling efficiency up to 98% within 2 hours for viruses including JEV, PRRSV, and IAV.
The integration of CRISPR/Cas systems with QDs has further advanced viral tracking capabilities. Yang et al. (2020a) first utilized a CRISPR imaging system with biotinylated dCas9 and gRNA against the US2 gene to label pseudorabies virus (search) (PRV) nucleic acids with QDs, enabling real-time monitoring of viral adsorption, cytoplasmic transport along microtubules, and nuclear entry. Ma et al. (2024) employed the CRISPR/Cas13 system combined with a bio-orthogonal metabolic method to label JEV genomes and envelopes with different-colored QDs in situ, visualizing uncoating and genome release in real-time near the endoplasmic reticulum.
Du et al. (2024) used QSVT to comprehensively dissect IAV entry, revealing three distinct pathways for the virus to reach dynein along microtubules: myosin VI-driven movement along actin filaments, actin tails assembled by an Arp2/3-dependent mechanism, and direct microtubule access without actin-related movement.
Biosafety and Clinical Translation Barriers
Despite promising preclinical advances, only 10 clinical trials involving QDs have been registered to date, with one completed and three subsequently withdrawn. The toxicity of QDs arises from three interconnected mechanisms: degradation-mediated release of heavy metal ions such as Cd²⁺, increased intracellular oxidative stress through ROS production, and direct interaction with cellular membranes causing structural damage.
Surface charge critically influences these interactions. Positively charged QDs tend to disrupt membrane integrity more severely than neutral or negatively charged ones. Arezki et al. (2022) performed the first quantitative proteomic analysis of the protein corona formed on carbon dots with different surface charges, finding that CDs with high ζ-potential (>+30 mV) and charge density (>2 μmol mg⁻¹) showed the highest internalization, strongly correlated with a corona enriched in vitronectin, fibulin, fetuin, adiponectin, and alpha-glycoprotein.
For efficient renal clearance, QDs require an optimal diameter below 5.5 nm, while larger particles (>10 nm) predominantly accumulate in the liver and spleen, with over 80% of the injected dose often localized in these organs within 24 hours. PEGylation can extend circulation half-life from 1 to 241 minutes, though multiple dosing may trigger anti-PEG antibodies leading to accelerated clearance.
Future Directions
The convergence of QDs with artificial intelligence presents an additional frontier. AI-assisted image analysis has the potential to extract subtle diagnostic information from QD-generated imaging datasets, enhancing sensitivity and reducing observer variability. Near-infrared (NIR) and second near-infrared window (NIR-II, 1000–1700 nm) QDs offer deeper tissue penetration, reduced autofluorescence, and improved signal-to-noise ratios for noninvasive imaging of tumors, vasculature, and deep tissue structures.
Future research should emphasize biodegradable and renal-clearable QDs that maintain excellent optical performance while minimizing biological risk. Advances in green synthesis methodologies, surface engineering, and scalable manufacturing—including PDMS-based devices and additive-manufactured microfluidics—will further facilitate translational progress. Continued interdisciplinary collaboration among materials scientists, chemists, engineers, biologists, clinicians, and regulatory agencies will be essential to unlock the full clinical potential of quantum dot technologies.