Abstract
Nanoplasmonic sensing (NPS), based on localized surface plasmon resonance, with sensors composed of glass covered with golden nanodisks and overlaid with a SiO2 coating was applied in this study. Egg phosphatidylcholine (eggPC), being an easily accessible membrane-forming lipid, was used for preparation of biomimicking membranes. Small unilamellar vesicles with an approximate hydrodynamic diameter of 30 nm, formed by sonication in 4-(2-hydroxyethyl)piperazine-1-ethanesulfonic acid buffer, were adsorbed within 10 min on the sensor surface either as intact vesicles or as a planar bilayer. The adsorbed biomembrane systems were further utilized for interaction studies with four different well-known surfactants (negatively and positively charged, zwitterionic, and nonionic) and each surfactant was tested at concentrations below and above the critical micelle concentration (CMC). Our results allowed the evaluation of different NPS patterns for every particular supported membrane system, surfactant, and its concentration. The most significant effect on the membrane was achieved upon the introduction of zwitterionic surfactant micelles, which in fact completely solubilized and removed the lipid membranes from the sensor surface. Other surfactant micelles interacted with the membranes and formed mixed structures remaining on the sensor surface. The studies performed at the concentrations below the CMCs of the surfactants showed that different mixed systems were formed. Depending on the supported membrane system and the type of surfactant, the mixed systems indicated different formation kinetics. Additionally, the final water rinse revealed the stability of the formed systems. To investigate the effect of the studied surfactants on the overall surface charge of the biomembrane, capillary electrophoresis (CE) experiments were carried out in parallel with the NPS analysis. The electroosmotic flow mobility of an eggPC-coated fused silica capillary was used to measure the total surface charge of the biomembrane after its treatment with the surfactants. Our results indicated in general good correlation between CE and NPS data. However, some discrepancies were seen while applying either zwitterionic or positively charged surfactants. This confirmed that CE analysis was able to provide additional data about the investigated systems. Taken together, the combination of NPS and CE proved to be an efficient way to describe the nature of interactions between biomimicking membranes and amphiphilic molecules.
Highlights
The cellular membrane is one of the key cellular organelles, which separates the inner environment of a cell from the surrounding medium and plays a crucial role in many biochemical processes
A model of vesicle adsorption, which was described by Jackman et al, showed that the time needed for vesicles to reach low and moderate surface coverages scaled with r−5/3 (r: vesicle hydrodynamic radius).[10]
The coating was done in 5 and 10 min for supported lipid bilayers (SLBs) and supported vesicle layers (SVLs), respectively, using 30 nm sonicated Egg L-α-phosphatidylcholine (eggPC) vesicles. This approach saves a considerable amount of phospholipids, which is of great importance for systems with limited amounts of lipids and increases the throughput of the utilized nanoplasmonic sensing (NPS) method
Summary
The cellular membrane is one of the key cellular organelles, which separates the inner environment of a cell from the surrounding medium and plays a crucial role in many biochemical processes. A method that could determine the dynamics of membrane interaction would enable fast prescreening of newly synthesized amphiphilic substances. Egg L-α-phosphatidylcholine (eggPC) is a highly available phospholipid, which can provide biomembrane-mimicking platforms for studying interactions with analytes.[1] Depending on the surface investigated, eggPC can remain as intact adsorbed vesicles or supported lipid bilayers (SLBs) on surfaces.[2,3] A newly emerging technique for studying surface interactions is nanoplasmonic sensing (NPS), which is based on localized surface plasmon resonance (LSPR).[4−7] It can provide insight into a number of different changes happening within the sensing region.[5,8] The sensitivity to changes of refractive index (RI) is confined to the region localized near to the metal nanostructures on the sensor’s surface, with a decay depth of a few tens of nanometers from the nanostructures.[9] This is a particular benefit of NPS compared with competing techniques where bulk liquid is sensed at sensing depths of hundreds of Received: April 3, 2018 Revised: April 30, 2018 Published: May 1, 2018
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