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.
QCM-D (Quartz crystal microbalance with dissipation monitoring) 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 cell membrane mimic allows the researcher to simplify the cell, 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:
presence of membrane proteins or other constituents such as cholesterol
additives in nanoparticle solution
In this post I will briefly give an introduction to this research area and highlight some key references and exciting recent work.
Supported lipid bilayer formation via QCM-D - Key references
On the (1) hydrophobic surface, a monolayer of lipid was found to deposit. On the (3) gold surface, the vesicles adsorbed intact. Yet on the (2) 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.
The mechanism of supported lipid bilayer formation characterized by QCM-D
Example QCM-D data, demonstrating lipid bilayer formation, along with a corresponding illustration, is shown in the figure to the right.
The lipid in this example is 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC)
Solution of 0.25 mg/ml DOPC in PBS at 7.4 pH was prepared and flown across an SiO2 QSensor at 25°C 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 (step 1) and when the system was washed with buffer (step 2).
"The vesicle rupture to lipid bilayer “fingerprint” is unique to the QCM-D technology"
The key feature of QCM-D data when making a SLB is the characteristic rupture step shown at (B) in the figure. 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.
Investigating the effects of nanoparticles on the SLB model cell membrane - five examples of publications
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. Below I list five different studies:
Terri Camesano’s group at Worcester Polytechnic Institute recently published how both the size of a nanoparticle and the presence of natural organic matter (NOM) affects how the particle interacts with a lipid membrane. They found that the lack of NOM caused very little nanoparticle - surface interactions but that the presence of this NOM caused significant interactions leading in some cases to significant membrane removal with the largest nanoparticles tested. “Size dependence of gold nanoparticle interactions with a supported lipid bilayer: A QCM-D study” Christina M. Bailey, Elaheh Kamaloo, Kellie L. Waterman, Kathleen F. Wang, Ramanathan Nagarajan, Terri A. Camesano Biophysical Chemistry2015, 203–204, 51–61. .
Joel Pedersen’s group from the University of Wisconsin recently published work detailing how ordered membrane domains (or lipid rafts with phase segregated domains) affect charged nanoparticle interactions. They found that the presence of liquid ordered domains increased the attachment between positively charged particles and the underlying membrane. They even have their own blog post about this work that can be found here: http://sustainable-nano.com/2015/11/19/whats-in-a-name/
Kai Loon Chen’s group from Johns Hopkins University recently illustrated how the outer coating of a nanoparticle can affect the way it interacts with a SLB. Specifically, they were interested in how an outer protein layer that could encapsulate a silver nanoparticle in the human body would affect the particle’s interaction with cells. They found the protein layer caused disruption of the electrostatic interactions between the particle and the bilayer and therefore caused less overall interaction.
Sofia Svedhem’s group from Chalmers Institute of Technology recently published an example of how certain nanoparticles (in this case TiO2) can actually tear holes in SLBs in the presence of calcium ions. A mechanism of the TiO2 particle – calcium ion - lipid bilayer interaction was proposed with the particles interacting with the membrane near where calcium had loosened the ionic interaction between the lipid and the surface. The particle could then remove a portion of the bilayer due to this weakened attraction between the bilayer and surface.
Nathalie Tufenkji’s group from McGill University recently published work showing how decreasing the interaction of a SLB with a substrate by tailoring the solution chemistry affected how nanoparticles interacted with said bilayer. The idea being that these free floating bilayers could potentially be more representative of an actual cell membrane and thus this method may be a better way to make and study the effects of nanoparticles with cell membranes overall.
Model membrane platforms, such as vesicles, monolayers, and bilayers are used in various research areas. To characterize these model systems, it is often necessary to use more than one analysis method. So which analysis methods should you use?