Edward T. Castellana

Solid supported lipid bilayers: From biophysical studies to sensor design

Abstract The lipid bilayer is one of the most eloquent and important self-assembled structures in nature. It not only provides a protective container for cells and sub-cellular compartments, but also hosts much of the machinery for cellular communication and transport across the cell membrane. Solid supported lipid bilayers provide an excellent model system for studying the surface chemistry of the cell. Moreover, they are accessible to a wide variety of surface-specific analytical techniques. This makes it possible to investigate processes such as cell signaling, ligand–receptor interactions, enzymatic reactions occurring at the cell surface, as well as pathogen attack. In this review, the following membrane systems are discussed: black lipid membranes, solid supported lipid bilayers, hybrid lipid bilayers, and polymer cushioned lipid bilayers. Examples of how supported lipid membrane technology is interfaced with array based systems by photolithographic patterning, spatial addressing, microcontact printing, and microfluidic patterning are explored. Also, the use of supported lipid bilayers in microfluidic devices for the development of lab-on-a-chip based platforms is examined. Finally, the utility of lipid bilayers in nanotechnology and future directions in this area are discussed.

Label-Free Biosensing with Lipid-Functionalized Gold Nanorods

The fabrication of a label-free mass spectrometry and optical detection-based biosensor platform for the detection of low-abundance lipophilic analytes in complex mixtures is described. The biosensor consists of a lipid layer partially tethered to the surface of a gold nanorod. The effectiveness of the biosensor is demonstrated for the label-free detection of a lipophilic drug in aqueous solution and a lipopeptide in serum.

Sol-gel-derived silver-nanoparticle-embedded thin film for mass spectrometry-based biosensing.

A porous silver-nanoparticle (AgNP)-embedded thin film biosensor was produced by the sol-gel method. The thin films were used as matrix-free laser desorption ionization mass spectrometry (LDI-MS) biosensors applicable to several chemical classes. In these experiments, UV laser irradiation (337 nm) of the AgNP facilitates desorption and ionization of a number of peptides, triglycerides, and phospholipids. Preferential ionization of sterols from vesicles composed of olefinic phosphosphatidylcholines is also demonstrated, offering the possibility for a simplified approach for sterol analysis from complex mixtures. The composition of the nanoparticles was confirmed by X-ray photoelectron spectroscopy (XPS) and UV-vis spectroscopy. XPS data revealed a binding energy of 368.2 eV, consistent with the previous assignment of the binding energy for the Ag 3d(5/2) peak from Ag(0) at 368.1 ± 0.1 eV. The surface morphology of the thin films was studied by field-emission scanning electron microscopy (FE-SEM) and revealed the presence of nanoparticles and the porous nature of the biosensor.

Benchtop chemistry for the rapid prototyping of label-free biosensors: Transmission localized surface plasmon resonance platforms

Herein, a simple label-free biosensor fabrication method is demonstrated based on transmission localized surface plasmon resonance (T-LSPR). The platform, which consists of a silver nanoparticle array, can be prepared in just a few minutes using benchtop chemistry. The array was made by a templating technique in conjunction with the photoreduction of Ag ions from solution. This metal surface was functionalized with biotin-linked thiol ligands for binding streptavidin molecules from solution. For an array of 19 nm diameter silver nanoparticles, a redshift in the T-LSPR spectrum of 24 nm was observed upon protein-ligand binding at saturation. The binding constant was found to be 2 × 1012 M−1. Platforms were also fabricated with silver nanoparticles of 34, 55, and 72 nm diameters. The maximum LSPR wavelength shift was nanoparticle size dependent and the maximum sensitivity was obtained with the smaller nanoparticles.

Nanoparticles for Selective Laser Desorption/Ionization in Mass Spectrometry

The selective desorption/ionization of analytes using nanomaterials is investigated using metallic nanoparticles. By replacing the sodium citrate capping of gold nanoparticles with self-assembled monolayers, we are able to both enhance analyte ionization and selectively capture analytes. Capping gold nanoparticles with a monolayer of 4-mercaptobenzoic acid enhances analyte ionization while greatly decreasing chemical noise resulting from alkali adducted species. Selective capture and sequential desorption/ionization of the peptide bradykinin (1–7) from a two peptide mixture is achieved using β-cyclodextrin capped gold nanoparticles. Finally, by switching from gold to silver nanoparticles, we are able to ionize both folic acid and amphotericin B. These results demonstrate that through careful control of nanoparticle surface chemistry and composition one can achieve selective analyte ionization for MS applications.

Imaging large arrays of supported lipid bilayers with a macroscope

Herein, the authors present fluorescence resonance energy transfer (FRET) and two-dimensional protein saturation data acquired from spatially addressed arrays of solid supported lipid bilayers (SLBs). The SLB arrays were imaged with an epifluorescence/total internal reflection macroscope. The macroscope allowed 1× imaging and had a relatively high numerical aperture (0.4). Such powerful light gathering and large field of view capabilities make it possible to simultaneously image dozens of addressed SLBs. Three experiments have been performed. First, a 9×7 array of supported lipid bilayer was fabricated and imaged in which each bilayer element was individually addressed. Second, a FRET assay was developed between Texas Red-DHPE (1,2-dihexadecanoyl-sn-glycero-3-phosphoethanolamine) and NBD-PE (1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine-n-(7-nitro-2-1,3-benzoxadiazol-4-yl)). The concentration of dye could be varied at each address and the value of the Förster radius (7.3±0.6 nm) was easily abstracted. Third, a ligand/receptor recognition assay was designed to show the two-dimensional number density of proteins which can be bound at saturation. It was found for the streptavidin/biotin pair that the protein saturated at the interface above 3 mol % biotin concentration. This corresponded to a two-dimensional footprint of 40 nm2 for the streptavidin molecule. These results clearly open the door to using individually addressed bilayers for obtaining large amounts of biophysical data at the supported bilayer/aqueous interface. Such abilities will be crucial to obtaining sufficient data for determining the interfacial mechanisms for a variety of membrane/ protein interactions.