How nanomaterials interact with the environment after they have been disposed of has many implications for potential toxicity and health concerns. Whether nanomaterials are being incorporated into commercial goods for their anti-microbial properties such as in work-out clothes or used for targeted drug therapies their overall prevalence is increasing. Therefore, the likelihood of someone coming into contact with these materials is also increasing.
Quartz Crystal Microbalance with Dissipation (QCM-D) technology can be used as a step in better understanding the interactions of nanoparticles with cells by approximating the cell surface with a supported lipid bilayer (SLB) and then flowing nanoparticles across the model cell membrane to probe the nanoparticle-cell membrane interaction. Using this model cell membrane mimic allows the researcher to simplify the cell somewhat and to investigate specific aspects of the membrane by building up the complexity. A multitude of variables from either the standpoint of the model cell membrane or the nanoparticle itself can be investigated. These include membrane composition, presence of membrane proteins or other constituents such as cholesterol, nanoparticle type, nanoparticle size, nanoparticle coating, pH, ionic strength, temperature, flow rate, additives in nanoparticle solution, etc. In this post I will briefly give an introduction to this research area and highlight some key references and exciting recent work.
One of the great things about this kind of research is that the assay design details are already outlined for you. At least the background research is already performed. All the user needs to do is to follow previously published recipes to form a supported lipid bilayer onto the QCM-D sensor.
The first paper illustrating how QCM-D can uniquely probe lipid vesicle deposition and the mechanism by which the vesicles can rupture to form a SLB onto a solid substrate was written by “Surface specific kinetics of lipid vesicle adsorption measured with a quartz crystal microbalance” Keller C. A.; Kasemo B. Biophys J. 1998, 75(3), 1397-1402. In this reference three different surfaces were investigated 1) a hydrophobic surface 2) a silicon dioxide surface and 3) a gold surface. On the hydrophobic surface, a monolayer of lipid was found to deposit. On the gold surface, the vesicles adsorbed intact. Yet on the silicon dioxide surface, the vesicles initially adsorbed intact until a significant surface concentration was reached and then the vesicles ruptured to form a lipid bilayer.
Several years later a description of the step by step protocols for how to form SLBs onto different surfaces including silicon dioxide, titanium dioxide, and gold were detailed in “Quartz crystal microbalance with dissipation monitoring of supported lipid bilayers on various substrates”Cho, NJ; Frank, C. W.; Kasemo, B.; Höök, F. Nature Protocols 2010, 5, 1096–1106. This “Nature Protocols” reference is extremely useful for those new to lipid bilayers that are looking for detailed step by step instructions for how to prepare these kinds of surfaces. This kind of information is typically not included in journal articles nowadays so I strongly recommended you to check out this reference if you are interested in this kind of work.
Example QCM-D data demonstrating lipid bilayer formation is shown in Figure 2 along with a corresponding cartoon. The lipid in this example is 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and solutions of 0.25 mg/ml in PBS at 7.4 pH were prepared and flown across an SiO2 Q-Sensor at 25.00 oC with a flow rate of 0.1 ml/min. The 7th harmonic is shown for clarity with the arrows indicating vesicles being introduced into the system (1) and when the system was washed with buffer (2). The key feature of QCM-D data when making a SLB is the characteristic rupture step shown at (B). After a critical coverage of surface adsorbed vesicles is reached or when an external stimulus is applied such as a reactive peptide or a change in osmotic pressure the frequency increases as the mass of associated solvent inside the vesicle is lost as the vesicle ruptures and a lipid bilayer forms. This is also why the dissipation decreases, because the adsorbed layer is going from a soft viscoelastic vesicle film with quite a bit of associated solvent to a rigid lipid bilayer with little to no associated solvent. This vesicle rupture to lipid bilayer “fingerprint” is unique to the QCM-D technology and allows probing the mechanism for bilayer formation. Surface support composition, ionic strength, pH, vesicle complexity, and constituents such as cholesterol or membrane proteins can be altered and their effects probed on how the resulting lipid bilayer forms.
While not shown in this specific example, it should be noted that many studies also take advantage of the fact that a layer of intact vesicles can be prepared and used to probe further interactions. The vesicle – surface interaction can be tailored to keep the vesicles intact.
After the lipid bilayer (or intact vesicle layer) is prepared then usually a particle solution is flown across the top and the resulting signal changes correspond to how the particle interacts with the membrane surface. The effects of particle type, size, concentration, outer coating chemistry, solution ionic strength, pH, and temperature can all be measured. A number of different groups are working in this area including:
Open Access “Formation of supported lipid bilayers containing phase-segregated domains and their interaction with gold nanoparticles” Eric S. Melby, Arielle C. Mensch, Samuel E. Lohse, Dehong Hu, Galya Orr, Catherine J. Murphy, Robert J. Hamers and Joel A. Pedersen Environ. Sci.: Nano2016, 3, 45-55. Published by The Royal Society of Chemistry.
“TiO2 nanoparticle interactions with supported lipid membranes – an example of removal of membrane patches” Fang Zhao, Jenny Perez Holmberg, Zareen Abbas, Rickard Frost, Tora Sirkka, Bengt Kasemo, Martin Hassellöv and Sofia Svedhem RSC Adv., 2016, 6, 91102-91110.
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