Model lipid bilayers mimic non-specific interactions of gold nanoparticles with macrophage plasma membranes

Absorption of Polypropylene in Dipalmitoylphosphatidylcholine Membranes: The Role of Molecular Weight and Initial Configuration of Polymer Chains

We study by molecular dynamics simulations the absorption of polypropylene (PP) chains within a dipalmitoylphosphatidylcholine (DPPC) lipid membrane in aqueous solvent. DPPC represents the most abundant phospholipid in biological membranes, while PP is one of the most common synthetic polymers diffused in the anthropic environment. By following in detail the absorption process, and the corresponding structural modification undergone by the membrane, we show how the initial configuration and the PP molecular weight determine the overall behavior of the system. Specifically, if PP chains initially lie on the DPPC surface, they are fully absorbed; likewise, polymers initially included within the membrane cannot escape from. On the other hand, if polymers are placed sufficiently apart from the membrane, they have time to join together and coalesce into a few nanoparticles. At contact, such nanoparticles may completely dissolve (for low molecular weight) and then be absorbed. For high molecular weight, not all of them dissolve, and therefore the system attains a condition in which some of the chains are absorbed, while others form a residual nanoparticle staying outside (but in contact with) the membrane. Such a state─albeit energetically unfavorable with respect to a condition in which all PP chains are absorbed─remains stable, at the least over a substantial simulation time, extending in our study up to 1.6 μs. The tendency for polymers to spontaneously form aggregates, which then prefer to stay in contact with the membrane, is further corroborated by calculation of the DPPC-nanoparticle potential of mean force.

Metabolomics reveals the phytotoxicity mechanisms of foliar spinach exposed to bulk and nano sizes of PbCO3

PbCO3 is an ancient raw material for Pb minerals and continues to pose potential risks to the environment and human health through mining and industrial processes. However, the specific effects of unintentional PbCO3 discharge on edible plants remain poorly understood. This study unravels how foliar application of PbCO3 induces phytotoxicity by potentially influencing leaf morphology, photosynthetic pigments, oxidative stress, and metabolic pathways related to energy regulation, cell damage, and antioxidant defense in Spinacia oleracea L. Additionally, it quantifies the resultant human health risks. Plants were foliarly exposed to PbCO3 nanoparticles (NPs) and bulk products (BPs), as well as Pb2+ at 0, 5, 10, 25, 50, and 100 mg·L-1 concentrations once a day for three weeks. The presence and localization of PbCO3 NPs inside the plant cells were confirmed by TEM-EDS analysis. The maximum accumulation of total Pb was recorded in the root (2947.77 mg·kg-1 DW for ion exposure), followed by the shoot (942.50 mg·kg-1 DW for NPs exposure). The results revealed that PbCO3 and Pb2+ exposure had size- and dose-dependent inhibitory effects on spinach length, biomass, and photosynthesis attributes, inducing impacts on the antioxidase activity of CAT, membrane permeability, and nutrient elements absorption and translocation. Pb2+ exhibited pronounced toxicity in morphology and chlorophyll; PbCO3 BP exposure accumulated the most lipid peroxidation products of MDA and H2O2; and PbCO3 NPs triggered the largest cell membrane damage. Furthermore, PbCO3 NPs at 10 and 100 mg·L-1 induced dose-dependent metabolic reprogramming in spinach leaves, disturbing the metabolic mechanisms related to amino acids, antioxidant defense, oxidative phosphorylation, fatty acid cycle, and the respiratory chain. The spinach showed a non-carcinogenic health risk hierarchy: Pb2+ > PbCO3 NPs > PbCO3 BPs, with children more vulnerable than adults. These findings enhance our understanding of PbCO3 particle effects on food security, emphasizing the need for further research to minimize their impact on human dietary health.

Novel Hybrid Silver-Silica Nanoparticles Synthesized by Modifications of the Sol-Gel Method and Their Theranostic Potential in Cancer

Cancer is one of the leading causes of death worldwide. Conventional therapies lack selectivity and suffer from toxicity and drug resistance, leading to metastasis. To overcome these limitations, a new category of nanomaterials exploiting the tumor characteristics has been developed in cancer nanotherapeutics. Among them, pH, metabolism, and the disrupted architecture of cells can be exploited for theranostic applications. Such nanomaterials can be inorganic nanoparticles with silver ones and gain high attention as diagnostic, therapeutic, and antibacterial compounds. Silver has been linked with triggering the death of cancer cells via DNA damage due to the production of reactive oxygen species (ROS) during photodynamic therapy. Thus, improvement of biocompatibility, modification with targeted agents, and drug conjugation promote the use of silver nanoparticles. In this work, we managed to synthesize hybrid Ag@SiO2 core-shell nanoparticles via a modified sol-gel method by tackling the known etching of silver caused by ammonia by employing different bases of the sol-gel reaction. The bases used in the synthetic route were diethylamine (DEA) and triethylamine (TEA) and were monitored with silver nanoparticles individually from the absorbance peak of silver in the UV-vis region, showing no etching of silver in contrast with ammonia, which is usually used in the sol-gel method. Furthermore, we synthesized biocompatible nanoparticles with anticancer and diagnostic properties toward breast cancer cells and glioblastoma cells. The nanoparticles were characterized both structurally and morphologically. Their biological evaluation suggests minor toxicity toward healthy cells and red blood cells (RBCs). Also, the diagnostic potential of the hybrid nanoparticles was exploited by optical fluorescence microscopy. Therefore, we strongly suggest the investigation of such nanostructures as a dual platform for the diagnosis and therapy of cancer.

The effect of biomolecular corona on adsorption onto and desorption from a model lipid membrane

The current lack of insight into nanoparticle-cell membrane interactions hampers smart design strategies and thereby the development of effective nanodrugs. Quantitative and methodical approaches utilizing cell membrane models offer an opportunity to unravel particle-membrane interactions in a detailed manner under well controlled conditions. Here we use total internal reflection microscopy for real-time studies of the non-specific interactions between nanoparticles and a model cell membrane at 50 ms temporal resolution over a time course of several minutes. Maintaining a simple lipid bilayer system across conditions, adsorption and desorption were quantified as a function of biomolecular corona, particle size and fluid flow. The presence of a biomolecular corona reduced both the particle adsorption rate onto the membrane and the duration of adhesion, compared to pristine particle conditions. Particle size, on the other hand, was only observed to affect the adsorption rate. The introduction of flow reduced the number of adsorption events, but increased the residence time. Lastly, altering the composition of the membrane itself resulted in a decreased number of adsorption events onto negatively charged bilayers compared to neutral bilayers. Overall, a model membrane system offers a facile platform for real-time imaging of individual adsorption-desorption processes, revealing complex adsorption kinetics, governed by particle surface energy, size dependent interaction forces, flow and membrane composition.

Interaction of nanoparticles with lipid films: the role of symmetry and shape anisotropy

The bioactivity, biological fate and cytotoxicity of nanomaterials when they come into contact with living organisms are determined by their interaction with biomacromolecules and biological barriers. In this context, the role of symmetry/shape anisotropy of both the nanomaterials and biological interfaces in their mutual interaction, is a relatively unaddressed issue. Here, we study the interaction of gold nanoparticles (NPs) of different shapes (nanospheres and nanorods) with biomimetic membranes of different morphology, i.e. flat membranes (2D symmetry, representative of the most common plasma membrane geometry), and cubic membranes (3D symmetry, representative of non-lamellar membranes, found in Nature under certain biological conditions). For this purpose we used an ensemble of complementary structural techniques, including Neutron Reflectometry, Grazing Incidence Small-Angle Neutron Scattering, on a nanometer lengthscale and Confocal Laser Scanning Microscopy on a micrometer length scale. We found that the structural stability of the membrane towards NPs is dependent on the topological characteristic of the lipid assembly and of the NPs, where a higher symmetry gave higher stability. In addition, Confocal Laser Scanning Microscopy analyses highlighted that NPs interact with cubic and lamellar phases according to two distinct mechanisms, related to the different structures of the lipid assemblies. This study for the first time systematically addresses the role of NPs shape in the interaction with lipid assemblies with different symmetry. The results will contribute to improve the fundamental knowledge on lipid interfaces and will provide new insights on the biological function of phase transitions as a response strategy to the exposure of NPs.

A plasmon-based nanoruler to probe the mechanical properties of synthetic and biogenic nanosized lipid vesicles

Nanosized lipid vesicles are ubiquitous in living systems (e.g. cellular compartments or extracellular vesicles, EVs) and in formulations for nanomedicine (e.g. liposomes for RNA vaccine formulations). The mechanical properties of such vesicles are crucial in several physicochemical and biological processes, ranging from cellular uptake to stability in aerosols. However, their accurate determination remains challenging and requires sophisticated instruments and data analysis. Here we report the first evidence that the surface plasmon resonance (SPR) of citrated gold nanoparticles (AuNPs) adsorbed on synthetic vesicles is finely sensitive to the vesicles' mechanical properties. We then leverage this finding to show that the SPR tracking provides quantitative access to the stiffness of vesicles of synthetic and natural origin, such as EVs. The demonstration of this plasmon-based "stiffness nanoruler" paves the way for developing a facile, cost-effective and high-throughput method to assay the mechanical properties of dispersions of vesicles of nanometric size and unknown composition at a collective level.

Advanced Static and Dynamic Fluorescence Microscopy Techniques to Investigate Drug Delivery Systems

In the past decade(s), fluorescence microscopy and laser scanning confocal microscopy (LSCM) have been widely employed to investigate biological and biomimetic systems for pharmaceutical applications, to determine the localization of drugs in tissues or entire organisms or the extent of their cellular uptake (in vitro). However, the diffraction limit of light, which limits the resolution to hundreds of nanometers, has for long time restricted the extent and quality of information and insight achievable through these techniques. The advent of super-resolution microscopic techniques, recognized with the 2014 Nobel prize in Chemistry, revolutionized the field thanks to the possibility to achieve nanometric resolution, i.e., the typical scale length of chemical and biological phenomena. Since then, fluorescence microscopy-related techniques have acquired renewed interest for the scientific community, both from the perspective of instrument/techniques development and from the perspective of the advanced scientific applications. In this contribution we will review the application of these techniques to the field of drug delivery, discussing how the latest advancements of static and dynamic methodologies have tremendously expanded the experimental opportunities for the characterization of drug delivery systems and for the understanding of their behaviour in biologically relevant environments.

Effect of Protein Corona on Nanoparticle-Lipid Membrane Binding: The Binding Strength and Dynamics

All-atom molecular dynamics simulations of the 10 nm-sized anionic polystyrene (PS) particle complexed with plasma proteins (human serum albumin, immunoglobulin gamma-1 chain-C, and apolipoprotein A-I) adsorbed onto lipid bilayers [asymmetrically composed of extracellular (zwitterionic) and cytosolic (anionic) leaflets] are performed. Free energies calculated from umbrella sampling simulations show that proteins on the particle more weakly bind to the zwitterionic leaflet than do bare particles, in agreement with experiments showing the suppression of the particle-bilayer binding by protein corona. Proteins on the particle interact more strongly with the anionic leaflet than with the zwitterionic leaflet because of charge interactions between cationic protein residues and anionic lipid headgroups, to an extent dependent on various plasma proteins. In particular, hydrogen bonds between proteins and zwitterionic leaflets restrict the motion of lipids and thus reduce the lateral dynamics of bilayers, while the tight binding between proteins and anionic leaflets disrupts the helical structure of proteins and disorders lipids, leading to an increase in the lateral dynamics of bilayers. These findings help explain the experimental observation regarding the fact that the bilayer dynamics decreases when interacting with protein corona and suggest that the effect of protein corona on the binding strength and bilayer dynamics depends on protein types and bilayer charges.

Interaction with Human Serum Proteins Reveals Biocompatibility of Phosphocholine-Functionalized SPIONs and Formation of Albumin-Decorated Nanoparticles

Nanoparticles (NPs) are increasingly exploited as diagnostic and therapeutic devices in medicine. Among them, superparamagnetic nanoparticles (SPIONs) represent very promising tools for magnetic resonance imaging, local heaters for hyperthermia, and nanoplatforms for multimodal imaging and theranostics. However, the use of NPs, including SPIONs, in medicine presents several issues: first, the encounter with the biological world and proteins in particular. Indeed, nanoparticles can suffer from protein adsorption, which can affect NP functionality and biocompatibility. In this respect, we have investigated the interaction of small SPIONs covered by an amphiphilic double layer of oleic acid/oleylamine and 1-octadecanoyl-sn-glycero-3-phosphocholine with two abundant human plasma proteins, human serum albumin (HSA) and human transferrin. By means of spectroscopic and scattering techniques, we analyzed the effect of SPIONs on protein structure and the binding affinities, and only found strong binding in the case of HSA. In no case did SPIONs alter the protein structure significantly. We structurally characterized HSA/SPIONs complexes by means of light and neutron scattering, highlighting the formation of a monolayer of protein molecules on the NP surface. Their interaction with lipid bilayers mimicking biological membranes was investigated by means of neutron reflectivity. We show that HSA/SPIONs do not affect lipid bilayer features and could be further exploited as a nanoplatform for future applications. Overall, our findings point toward a high biocompatibility of phosphocholine-decorated SPIONs and support their use in nanomedicine.

Multifunctional nanoassemblies target bacterial lipopolysaccharides for enhanced antimicrobial DNA delivery

The development of new therapeutic strategies against multidrug resistant Gram-negative bacteria is a major challenge for pharmaceutical research. In this respect, it is increasingly recognized that an efficient treatment for resistant bacterial infections should combine antimicrobial and anti-inflammatory effects. Here, we explore the multifunctional therapeutic potential of nanostructured self-assemblies from a cationic bolaamphiphile, which target bacterial lipopolysaccharides (LPSs) and associates with an anti-bacterial nucleic acid to form nanoplexes with therapeutic efficacy against Gram-negative bacteria. To understand the mechanistic details of these multifunctional antimicrobial-anti-inflammatory properties, we performed a fundamental study, comparing the interaction of these nanostructured therapeutics with synthetic biomimetic bacterial membranes and live bacterial cells. Combining a wide range of experimental techniques (Confocal Microscopy, Fluorescence Correlation Spectroscopy, Microfluidics, NMR, LPS binding assays), we demonstrate that the LPS targeting capacity of the bolaamphiphile self-assemblies, comparable to that exerted by Polymixin B, is a key feature of these nanoplexes and one that permits entry of therapeutic nucleic acids in Gram-negative bacteria. These findings enable a new approach to the design of efficient multifunctional therapeutics with combined antimicrobial and anti-inflammatory effects and have therefore the potential to broadly impact fundamental and applied research on self-assembled nano-sized antibacterials for antibiotic resistant infections.

Crystalline phase and surface coating of Al2O3 nanoparticles and their influence on the integrity and fluidity of model cell membranes

Aluminium oxide nanoparticles (Al₂O₃ NPs) potentially cause health hazards after their release into the environment. The crystalline phase of Al₂O₃ NPs determines their surface structure and the number of functional groups. The adsorption of natural organic matter (NOM) or biomolecules on the surface Al₂O₃ NPs also alters their surface properties and subsequent interactions with organisms. In this study, the roles of the Al₂O₃ crystalline phase and the surface coating of the nanoparticles on the membrane integrity and fluidity were investigated. Giant and small unilamellar vesicles (GUVs and SUVs) were prepared as model cell membranes to detect membrane disruption after exposure to Al₂O₃ NPs. Due to amorphous structure and high surface activity of γ-Al₂O₃ NPs, they had a stronger affinity with the membrane and caused more serious membrane rupture than that of α-Al₂O₃ NPs. The deposition of Al₂O₃ NPs on the membrane and the induced membrane disruption were monitored by a quartz crystal microbalance with dissipation (QCM-D) method. HA-coated Al₂O₃ NPs disrupted the SUV layer on the QCM-D sensor, while BSA-coated Al₂O₃ NPs only adhered to the membrane and induced unremarkable vesicle disruption. In addition, untreated γ-Al₂O₃ NPs induced remarkable gelation of a negatively charged membrane, but other types of Al₂O₃ NPs caused negligible membrane phase changes. The outcomes of this study demonstrate that the crystalline phase of the Al₂O₃ NPs affects the integrity and fluidity of cell membranes. The protein coatings on the NPs weaken the NP-membrane interaction, while HA coatings increase the damage of the NP-induced interaction.

Structural evolution of supported lipid bilayers intercalated with quantum dots

HYPOTHESIS

Supported lipid bilayers (SLBs) embedded with hydrophobic quantum dots (QDs) undergo temporal structural rearrangement.

EXPERIMENTS

Synchrotron X-ray reflectivity (XRR) was applied to monitor the temporal structural changes over a period of 24 h of mixed SLBs of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) / 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-ethanolamine (POPE) intercalated with 4.9 nm hydrophobic cadmium sulphide quantum dots (CdS QDs). The QD-embedded SLBs (QD-SLBs) were formed via rupture of the mixed liposomes on a positively charged polyethylene imine (PEI) monolayer. Atomic force microscopy (AFM) imaging provided complementary characterization of the bilayer morphology.

FINDINGS

Our results show time-dependent perturbations in the SLB structure due to the interaction upon QD incorporation. Compared to the SLB without QDs, at 3 h incubation time, there was a measurable decrease in the bilayer thickness and a concurrent increase in the scattering length density (SLD) of the QD-SLB. The QD-SLB then became progressively thicker with increasing incubation time, which - along with the fitted SLD profile - was attributed to the structural rearrangement due to the QDs being expelled from the inner leaflet to the outer leaflet of the bilayer. Our results give unprecedented mechanistic insights into the structural evolution of QD-SLBs on a polymer cushion, important to their potential biomedical and biosensing applications.

Interactions between PAMAM dendrimers and DOPC lipid multilayers: Membrane thinning and structural disorder

BACKGROUND

Understanding the structure of hybrid nanoparticle-lipid multilayers is of fundamental importance to their bioanalytical applications and nanotoxicity, where nanoparticle-membrane interactions play an important role. Poly(amidoamine) (PAMAM) dendrimers are branched polymeric nanoparticles with potential biomedical applications due to precise tunability of their physicochemical properties. Here, the effect of PAMAM dendrimers (2.9-4.5 nm) with either a hydrophilic amine (NH₂) or a hydrophobic C₁₂ chain surface termination on the 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) multilayers has been studied for the first time.

METHODS

DOPC multilayers were created by the liposome-rupture method via drop-casting dendrimer-liposome dispersions with the dendrimers added at different concentrations and at three different stages. The multilayer structure was evaluated via the analysis of the synchrotron X-ray reflectivity (XRR) curves, obtaining the bilayer d-spacing, the coherence length from the Scherrer (L_s) analysis of the Bragg peaks, and the paracrystalline disorder parameter (g).

RESULTS

Dendrimer addition led to lipid bilayer thinning and more disordered multilayer structures. Larger hydrophobic dendrimers caused greater structural disruption to the multilayers compared to the smaller dendrimers. The smallest, positively charged dendrimers at their highest concentration caused the most pronounced bilayer thinning. The dendrimer-liposome mixing method also affected the multilayer structure due to different dendrimer aggregation involved.

CONCLUSIONS

These results show the complexity of the effect of dendrimer physicochemical properties and the addition method of dendrimers on the structure of mixed dendrimer-DOPC multilayers.

GENERAL SIGNIFICANCE

These insights are useful for fundamental understanding of nanotoxicity and future biomedical application of nanocomposite multilayer materials in which nanoparticles are added for enhanced properties and functionality.

Nanoparticles and organized lipid assemblies: from interaction to design of hybrid soft devices

This contribution reviews the state of art on hybrid soft matter assemblies composed of inorganic nanoparticles (NP) and lamellar or non-lamellar lipid bilayers. After a short outline of the relevant energetic contributions, we address the interaction of NPs with synthetic lamellar bilayers, meant as cell membrane mimics. We then review the design of hybrid nanostructured materials composed of lipid bilayers and some classes of inorganic NPs, with particular emphasis on the effects on the amphiphilic phase diagram and on the additional properties contributed by the NPs. Then, we present the latest developments on the use of lipid bilayers as coating agents for inorganic NPs. Finally, we remark on the main achievements of the last years and our vision for the development of the field.

Nanoparticles at Biomimetic Interfaces: Combined Experimental and Simulation Study on Charged Gold Nanoparticles/Lipid Bilayer Interfaces

The poor understanding of the interaction of nanomaterials with biologically relevant interfaces is recognized as one of the major issues currently limiting the development of nanomedicine. The central purpose of this study is to compare experimental (confocal microscopy, fluorescence correlation spectroscopy, X-ray reflectivity) and computational (molecular dynamics simulations) results to thoroughly describe the interaction of cationic gold nanoparticles (AuNPs) with mixed zwitterionic/anionic lipid membranes. The adhesion of AuNPs to the lipid membrane is investigated on different length scales from a structural and dynamical point of view; with this approach, a series of complex phenomena, spanning from lipid extraction, localized membrane disruption, lateral phase separation, and slaved diffusion, are characterized and interpreted from a molecular level to macroscopic observations.

Polymer Coating and Lipid Phases Regulate Semiconductor Nanorods' Interaction with Neuronal Membranes: A Modeling Approach

The interplay between nanoparticles (NPs) and cell membranes is extremely important with regard to using NPs in biology applications. With the aim of unraveling the dominating factors on the molecular scale, we have studied the interaction between polymer-coated semiconductor nanorods (NRs) made of cadmium selenium/cadmium sulfur and model lipid membranes. The zeta potential (ζ) of the NRs was tuned from having a negative value (-24 mV) to having a positive one (+11 mV) by changing the amine content in the polymer coating. Supported lipid bilayers (SLBs) and lipid monolayers (LMs) were used as model membranes. Lipid mixtures containing anionic or cationic lipids were employed in order to change the membrane ζ from -77 to +49 mV; lipids with saturated hydrophobic chains were used to create phase-separated gel domains. NR adsorption to the SLBs was monitored by quartz crystal microbalance with dissipation monitoring; interactions with LMs with the same lipid composition were measured by surface pressure-area isotherms. The results showed that the NRs only interact with the model membrane if the mutual Δζ is higher than 70 mV; at the air-water interface, positively charged NRs remove lipids from the anionic lipid mixtures, and the negative ones penetrate the space between the polar heads in the cationic mixtures. However, the presence of gel domains in the membrane inhibits this interaction. The results of the Derjaguin-Landau-Verwey-Overbeek model frame indicate that the interaction occurs not only due to electrostatic and van der Waals forces, but also due to steric and/or hydration forces.

Lipid-Conjugated Rigidochromic Probe Discloses Membrane Alteration in Model Cells of Krabbe Disease

The plasma membrane of cells has a complex architecture based on the bidimensional liquid-crystalline bilayer arrangement of phospho- and sphingolipids, which in turn embeds several proteins and is connected to the cytoskeleton. Several studies highlight the spatial membrane organization into more ordered (L_o or lipid raft) and more disordered (L_d) domains. We here report on a fluorescent analog of the green fluorescent protein chromophore that, when conjugated to a phospholipid, enables the quantification of the L_o and L_d domains in living cells on account of its large fluorescence lifetime variation in the two phases. The domain composition is straightforwardly obtained by the phasor approach to confocal fluorescence lifetime imaging, a graphical method that does not require global fitting of the fluorescence decay in every spatial position of the sample. Our imaging strategy was applied to recover the domain composition in human oligodendrocytes at rest and under treatment with galactosylsphingosine (psychosine). Exogenous psychosine administration recapitulates many of the molecular fingerprints of a severe neurological disease, globoid cell leukodystrophy, better known as Krabbe disease. We found out that psychosine progressively destabilizes plasma membrane, as witnessed by a shrinking of the L_o fraction. The unchanged levels of galactosyl ceramidase, i.e., the enzyme lacking in Krabbe disease, upon psychosine treatment suggest that psychosine alters the plasma membrane structure by direct physical effect, as also recently demonstrated in model membranes.

Preparation of giant lipid vesicles with controllable sizes by a modified hydrophilic polydimethylsiloxane microarray chip

This paper presents an accessible method to prepare giant lipid vesicles (GLVs) with controllable sizes based on the quick formation of patterned lipid films. Lipid solutions naturally penetrate into arrayed micro-apertures on a modified hydrophilic Polydimethylsiloxane (PDMS) chip, and excess lipid films on the surface are removed by a glass slide. Three main factors, the depth and diameter of the micro-apertures and concentration of the lipid solution, were investigated to obtain an optimal preparation condition. Based on this condition, the formed GLVs have a controllable size and narrow size distribution (the standard deviation < 5 μm). By controlling the diameter of the micro-aperture and concentration of the lipid solution, GLVs with various sizes (23, 48, 66 and 82 μm) can be formed.

Supported lipid bilayers with encapsulated quantum dots (QDs) via liposome fusion: effect of QD size on bilayer formation and structure

Understanding interactions between functional nanoparticles and lipid bilayers is important to many emerging biomedical and bioanalytical applications. In this paper, we report incorporation of hydrophobic cadmium sulphide quantum dots (CdS QDs) into mixed 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)/1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) liposomes, and into their supported bilayers (SLBs). The QDs were found embedded in the hydrophobic regions of the liposomes and the supported bilayers, which retained the QD fluorescent properties. In particular, we studied the effect of the QD size (2.7-5.4 nm in diameter) on the formation kinetics and structure of the supported POPC/POPE bilayers, monitored in situ using quartz crystal microbalance with dissipation monitoring (QCM-D), as the liposomes ruptured onto the substrate. The morphology of the obtained QD-lipid hybrid bilayers was studied using atomic force microscopy (AFM), and their structure by synchrotron X-ray reflectivity (XRR). It was shown that the incorporation of hydrophobic QDs promoted bilayer formation on the PEI cushion, evident from the rupture and fusion of the QD-endowed liposomes at a lower surface coverage compared to the liposomes without QDs. Furthermore, the degree of disruption in the supported bilayer structure caused by the QDs was found to be correlated with the QD size. Our results provide mechanistic insights into the kinetics of the rupturing and formation process of QD-endowed supported lipid bilayers via liposome fusion on polymer cushions.