Intracellular Uptake of Quantum Dots by Microfluidic Stretching of Live Cells

The redox state of cells is fundamental in regulating many essential cellular functions. In cancer cells, redox behaviour is critical in understanding the progression of the disease and developing potential therapies. Redox sensing in live cells with nanoparticles (NPs) is an attractive prospect, particularly with quantum dots (QDs), due to their inherent stable photoluminescence and versatility in surface functionalisation. Typically, NPs are delivered into live cells through endocytic pathways and remain trapped in vesicles, limiting their ability to probe the cellular environment. Hence, high-throughput non-endocytic delivery of NPs into live cells is highly desirable for intracellular redox sensing.
In this study, non-endocytic uptake of QDs using hydrodynamic cell deformation via a cross-slot microfluidic device has been demonstrated. This shear-induced hydrodynamic stretching of the cells leads to transient membrane disruption, that allows endosome-free delivery of QDs into the cytoplasm. The method is vector-free, inexpensive, high-throughput and reproducible. Confocal fluorescence microscopy showed that increased hydrodynamic deformation leads to an increase in QD delivery. Scanning transmission electron microscopy confirmed that QDs were freely dispersed in the cytoplasm, free from endosomes. Quinone ligand-modified QDs showed reversible quenching depending on its oxidation-reduction state, which was suitable for redox sensing in live cells. The redox-sensitive QDs were introduced into the cells through cell deformation using the cross-slot microfluidic device in breast cancer and non-malignant cells. These redox-sensitive QDs in breast cancer cells having estrogen receptors (MCF7 and T47D) showed higher photoluminescence compared to the cancer cells without these receptors (MDA-MB-231) and the non-malignant cells (MCF10A). Hence, the QD-based redox-probes were capable of sensing the reducing environment in live cells. From the ultrafast fluorescence frequency multiplexer division microscopy, it was observed that there is an outflow of intracellular materials on cell deformation, suggesting that the transport of materials is bi-directional across the membrane pores.
With the potential biological applications, the photoluminescent quantum yield of CdSe quantum rods was enhanced significantly by epitaxial ZnS shell growth and chloride ion treatments. An effort was made to transfer the CdSe/ZnS core/shell quantum rods into the aqueous phase by amphiphilic polymer treatment.