Study on intracellular delivery of liposome encapsulated quantum dots using advanced fluorescence microscopy

Kristina Bruun,Carsten Hille

doi:10.1038/s41598-019-46732-5

Kristina Bruun, Carsten Hille

Open Access

https://doi.org/10.1038/s41598-019-46732-5

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Abstract

Quantum dots increasingly gain popularity for in vivo applications. However, their delivery and accumulation into cells can be challenging and there is still lack of detailed information. Thereby, the application of advanced fluorescence techniques can expand the portfolio of useful parameters for a more comprehensive evaluation. Here, we encapsulated hydrophilic quantum dots into liposomes for studying cellular uptake of these so-called lipodots into living cells. First, we investigated photophysical properties of free quantum dots and lipodots observing changes in the fluorescence decay time and translational diffusion behaviour. In comparison to empty liposomes, lipodots exhibited an altered zeta potential, whereas their hydrodynamic size did not change. Fluorescence lifetime imaging microscopy (FLIM) and fluorescence correlation spectroscopy (FCS), both combined with two-photon excitation (2P), were used to investigate the interaction behaviour of lipodots with an insect epithelial tissue. In contrast to the application of free quantum dots, their successful delivery into the cytosol of salivary gland duct cells could be observed when applying lipodots. Lipodots with different lipid compositions and surface charges did not result in considerable differences in the intracellular labelling pattern, luminescence decay time and diffusion behaviour. However, quantum dot degradation after intracellular accumulation could be assumed from reduced luminescence decay times and blue-shifted luminescence signals. In addition to single diffusing quantum dots, possible intracellular clustering of quantum dots could be assumed from increased diffusion times. Thus, by using a simple and manageable liposome carrier system, 2P-FLIM and 2P-FCS recording protocols could be tested, which are promising for investigating the fate of quantum dots during cellular interaction.

Highlights

During the last decades, the biomedical application of nanomaterials has rapidly increased, especially in the fields of targeted drug delivery, cancer treatment and therapeutics[1,2,3]
Confocal fluorescence microscopy has been mainly used to study the cellular uptake of quantum dots and their intracellular fate[19,30,31,32,33,34,35]
For empty DOPC:DOPE-liposomes, dynamic light scattering (DLS) measurements resulted in an average hydrodynamic size of 111 ± 1.42 nm (N = 3) with a polydispersity index of 0.08 ± 0.01

Summary

Introduction

The biomedical application of nanomaterials has rapidly increased, especially in the fields of targeted drug delivery, cancer treatment and therapeutics[1,2,3]. In cell culture media or physiological buffer solutions, quantum dots can exhibit low stability due to particle aggregation and surface degradation, which leads to lower cellular uptake efficiencies[16,17,18]. Only few FLIM studies are available showing a decrease in the quantum dot luminescence decay time after intracellular accumulation by applying cell culture experiments[42,43,44,45]. We encapsulated hydrophilic quantum dots into liposomes of different membrane compositions and surface charges, but without further surface modification

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Results

Conclusion