Fabrication of elliptical nanorings with highly tunable and multiple plasmonic resonances

Herein, a new and facile patterning method is demonstrated for the scalable fabrication of gold elliptical rings (ERs) in a controlled manner over large areas. In this method, well-ordered hexagonally arrayed polystyrene (PS) rings, fabricated by colloidal lithography, were used as masters to generate poly(dimethylsiloxane) (PDMS) stamps with circular apertures. The stamps were then stretched and utilized as molds for creating elliptical PS rings by a capillary filling process. Through subsequent reactive ion etching and chemical wet-etching, the elliptical PS rings could be readily transferred into an underlying gold film, leading to the formation of gold ERs. Since the aspect ratio (AR) of the elliptical PS rings could be controlled by varying the applied strain during the capillary filling process, gold ERs with different ARs could be fabricated in a scalable manner. The optical properties of the gold ERs were characterized by UV-vis/NIR and IR extinction measurements. The ERs exhibited only odd modes of polarization-dependent plasmonic resonances at normal incidence. The experiments and corresponding theoretical studies illustrated that all resonant modes could be tuned across a broad spectral range from the visible to the mid infrared (550-4700 nm) by simply varying the AR of the ERs. Moreover, the experimental data were confirmed by COMSOL simulations.

Phosphatidylserine reversibly binds Cu2+ with extremely high affinity

Phosphatidylserine (PS) embedded within supported lipid bilayers was found to bind Cu(2+) from solution with extraordinarily high affinity. In fact, the equilibrium dissociation constant was in the femtomolar range. The resulting complex formed in a 1:2 Cu(2+)-to-PS ratio and quenches a broad spectrum of lipid-bound fluorophores in a reversible and pH-dependent fashion. At acidic pH values, the fluorophores were almost completely unquenched, while at basic pH values significant quenching (85-90%) was observed. The pH at which the transition occurred was dependent on the PS concentration and ranged from approximately pH 5 to 8. The quenching kinetics was slow at low Cu(2+) concentrations and basic pH values (up to several hours), while the unquenching reaction was orders of magnitude more rapid upon lowering the pH. This was consistent with diffusion-limited complex formation at basic pH but rapid dissociation under acidic conditions. The tight binding of Cu(2+) to PS may have physiological consequences under certain circumstances.

Deflected capillary force lithography

Herein we introduce a novel strategy based on capillary force lithography (CFL) to fabricate asymmetric polymeric ring structures by applying both shear and nomal forces to a poly(dimethylsiloxane) stamp. The mechanism for the formation of asymmetric rings is caused by the deflection of cylindrical PDMS pillars due to the shear load, which is therefore termed deflected CFL (dCFL). The asymmetric polymeric rings could be readily transferred to an underlying gold layer to generate split ring structures with tunable opening angles. Asymmetric structures based upon trigular and square-shaped pillars were also fabricated. These elements were formed into periodic arrays over surface areas as large as 1 cm(2) and may have optical and electromagnetic applications.

Methyl groups of trimethylamine N-oxide orient away from hydrophobic interfaces

The molecular orientation of trimethylamine N-oxide (TMAO), a powerful protein stabilizer, was explored at aqueous/hydrophobic interfaces using vibrational sum frequency spectroscopy (VSFS). The systems studied included the octadecyltrichlorosilane (OTS)/water interface, which represents an aqueous solution in direct contact with a hydrophobic medium. Surprisingly, the measurements revealed that the methyl groups of TMAO pointed into the aqueous phase and away from the OTS. This orientation may arise from the more hydrophilic nature of methyl groups attached to a strongly electron-withdrawing atom such as a quaternary nitrogen. Additional studies were performed at the air/water interface. This interface showed a high degree of TMAO alignment, but the dangling OH from water was present even at 5 M TAMO. Moreover, the addition of this osmolyte modestly increased the surface tension of the interface. This meant that this species was somewhat depleted at the interface compared to the bulk solution. These findings may have implications for the stabilizing effect of TMAO on proteins. Specifically, the strong hydration required for the methyl groups as well as the oxide moiety should be responsible for the osmolyte's depletion from hydrophobic/aqueous interfaces. Such depletion effects should help stabilize proteins in their folded and native conformations on entropic grounds, although orientational effects may play an additional role.

Protein separation by electrophoretic-electroosmotic focusing on supported lipid bilayers

An electrophoretic-electroosmotic focusing (EEF) method was developed and used to separate membrane-bound proteins and charged lipids based on their charge-to-size ratio from an initially homogeneous mixture. EEF uses opposing electrophoretic and electroosmotic forces to focus and separate proteins and lipids into narrow bands on supported lipid bilayers (SLBs). Membrane-associated species were focused into specific positions within the SLB in a highly repeatable fashion. The steady-state focusing positions of the proteins could be predicted and controlled by tuning experimental conditions, such as buffer pH, ionic strength, electric field, and temperature. Careful tuning of the variables should enable one to separate mixtures of membrane proteins with only subtle differences. The EEF technique was found to be an effective way to separate protein mixtures with low initial concentrations, and it overcame diffusive peak broadening to allow four bands to be separated simultaneously within a 380 μm wide isolated supported membrane patch.

Supported bilayer electrophoresis under controlled buffer conditions

A pH controlled flow cell device was constructed to allow electrophoretic movement of charged lipids and membrane associated proteins in supported phospholipid bilayers. The device isolated electrolysis products near the electrodes from the electrophoresis process within the bilayer. This allowed the pH over the bilayer region to remain within ±0.2 pH units or better over many hours at salt concentrations up to 10 mM. Using this setup, it was found that the electrophoretic mobility of a dye conjugated lipid (Texas Red 1,2-dihexadecanoyl-sn-glycero-3-phosphoethanolamine (TR-DHPE)) was essentially constant between pH 3.3 and 9.3. In contrast, streptavidin, which was bound to biotinylated lipids, shifted from migrating cathodically at acidic pH values to migrating anodically under basic conditions. This shift was due to the modulation of the net charge on the protein, which changed the electrophoretic forces experienced by the macromolecule. The addition of a polyethylene glycol (PEG) cushion beneath the bilayer or the increase in the ionic strength of the buffer solution resulted in a decrease of the electroosmotic force experienced by the streptavidin with little effect on the Texas Red-DHPE. As such, it was possible in part to control the electrophoretic and electroosmotic contributions to streptavidin independently of one another.

Specific anion effects on water structure adjacent to protein monolayers

Vibrational sum frequency spectroscopy (VSFS) was used to explore specific ion effects on interfacial water structure adjacent to a bovine serum albumin (BSA) monolayer adsorbed at the air/water interface. The subphase conditions were varied by the use of six different sodium salts and four different pH values. At pH 2 and 3, the protein layer was positively charged and it was found that the most chaotropic anions caused the greatest attenuation of water structure. The order of the salts followed an inverse Hofmeister series. On the other hand, when the protein layer was near its isoelectric point (pH 5), the most chaotropic anions caused the greatest increase in water structure, although the effect was weak. In this case, a direct Hofmeister series was obeyed. Finally, virtually no effect was observed when the protein layer was negatively charged with a subphase pH of 9. For comparison, similar experiments were run with positively charged, negatively charged, and zwitterionic surfactant monolayers. These experiments gave rise to nearly the identical results as the protein monolayer which suggested that specific anion effects are dominated by the charge state of the interfacial layer rather than its detailed chemical structure. In a final set of experiments, salt effects were examined with a monolayer made from an elastin-like polypeptide (ELP). The peptide consisted of 120 pentameric repeats of the sequence Val-Pro-Gly-Val-Gly. Data from this net neutral biopolymer followed a very weak, but direct Hofmeister series. This suggested that direct anion binding to the amide groups in the backbone of a polypeptide is quite weak in agreement with the BSA data. The results from the variously charged protein, surfactant, and polymer monolayers were compared with a modified Gouy-Chapman-Stern model. The agreement with this simple model was quite good.

Chemistry of Hofmeister anions and osmolytes

The study of the interactions of salts and osmolytes with macromolecules in aqueous solution originated with experiments concerning protein precipitation more than 100 years ago. Today, these solutes are known to display recurring behavior for myriad biological and chemical processes. Such behavior depends both on the nature and concentration of the species in solution. Despite the generality of these effects, our understanding of the molecular-level details of ion and osmolyte specificity is still quite limited. Here, we review recent studies of the interactions between anions and urea with model macromolecular systems. A mechanism for specific ion effects is elucidated for aqueous systems containing charged and uncharged polymers, polypeptides, and proteins. The results clearly show that the effects of the anions are local and involve direct interactions with macromolecules and their first hydration shell. Also, a hydrogen-bonding mechanism is tested for the urea denaturation of proteins with some of these same systems. In that case, direct hydrogen bonding can be largely discounted as the key mechanism for urea stabilization of uncollapsed and/or unfolded structures.

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 2x10(12) 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.

The inverse and direct Hofmeister series for lysozyme

Anion effects on the cloud-point temperature for the liquid-liquid phase transition of lysozyme were investigated by temperature gradient microfluidics under a dark field microscope. It was found that protein aggregation in salt solutions followed 2 distinct Hofmeister series depending on salt concentration. Namely, under low salt conditions the association of anions with the positively charged lysozyme surface dominated the process and the phase transition temperature followed an inverse Hofmeister series. This inverse series could be directly correlated to the size and hydration thermodynamics of the anions. At higher salt concentrations, the liquid-liquid phase transition displayed a direct Hofmeister series that correlated with the polarizability of the anions. A simple model was derived to take both charge screening and surface tension effects into account at the protein/water interface. Fitting the thermodynamic data to this model equation demonstrated its validity in both the high and low salt regimes. These results suggest that in general positively charged macromolecular systems should show inverse Hofmeister behavior only at relatively low salt concentrations, but revert to a direct Hofmeister series as the salt concentration is increased.

Multivalent ligand-receptor binding on supported lipid bilayers

Fluid supported lipid bilayers provide an excellent platform for studying multivalent protein-ligand interactions because the two-dimensional fluidity of the membrane allows for lateral rearrangement of ligands in order to optimize binding. Our laboratory has combined supported lipid bilayer-coated microfluidic platforms with total internal reflection fluorescence microscopy (TIRFM) to obtain equilibrium dissociation constant (K(D)) data for these systems. This high throughput, on-chip approach provides highly accurate thermodynamic information about multivalent binding events while requiring only very small sample volumes. Herein, we review some of the most salient findings from these studies. In particular, increasing ligand density on the membrane surface can provide a modest enhancement or attenuation of ligand-receptor binding depending upon whether the surface ligands interact strongly with each other. Such effects, however, lead to little more than one order of magnitude change in the apparent K(D) values. On the other hand, the lipophilicity and presentation of lipid bilayer-conjugated ligands can have a much greater impact. Indeed, changing the way a particular ligand is conjugated to the membrane can alter the apparent K(D) value by at least three orders of magnitude. Such a result speaks strongly to the role of ligand availability for multivalent ligand-receptor binding.

Detecting protein-ligand binding on supported bilayers by local pH modulation

Herein, we describe a highly sensitive technique for detecting protein-ligand binding at the liquid/solid interface. The method is based upon modulation of the interfacial pH when the protein binds. This change is detected by ortho-Texas Red DHPE, which is doped into supported phospholipid bilayers and used as a pH-sensitive dye. The dye molecule fluoresces strongly at acidic pH values but not basic ones and has an apparent pK(A) of 7.8 in 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine membranes containing 0.5 mol % biotin-cap-PE. This method was used to detect antibiotin/biotin binding interactions as well as the binding of cholera toxin B subunits to GM(1). Since these proteins are negatively charged under the conditions of the experiment the interface became slightly more acidic upon binding. In each case, the equilibrium dissociation constant was determined by following the rise in fluorescence as protein was introduced. This change is essentially linear with protein coverage under the conditions employed. For the biotin/antibiotin system it was determined that K(D) = 24 +/- 5 nM, which is in excellent agreement with classical measurements made by total internal reflection fluorescence microscopy involving fluorophore-conjugated antibody molecules. Moreover, the limit of detection was approximately 350 fM at the 99% confidence level. This corresponds to 1 part in 69,000 of the K(D) value. Such a finding compares favorably with surface plasmon resonance studies of similar systems and conditions. The assay could be run in imaging mode to obtain multiple simultaneous measurements using a CCD camera.

Evaporation-induced assembly of quantum dots into nanorings

Herein, we demonstrate the controlled formation of two-dimensional periodic arrays of ring-shaped nanostructures assembled from CdSe semiconductor quantum dots (QDs). The patterns were fabricated by using an evaporative templating method. This involves the introduction of an aqueous solution containing both quantum dots and polystyrene microspheres onto the surface of a planar hydrophilic glass substrate. The quantum dots became confined to the meniscus of the microspheres during evaporation, which drove ring assembly via capillary forces at the polystyrene sphere/glass substrate interface. The geometric parameters for nanoring formation could be controlled by tuning the size of the microspheres and the concentration of the QDs employed. This allowed hexagonal arrays of nanorings to be formed with thicknesses ranging from single dot necklaces to thick multilayer structures over surface areas of many square millimeters. Moreover, the diameter of the ring structures could be simultaneously controlled. A simple model was employed to explain the forces involved in the formation of nanoparticle nanorings.

Effects of Hofmeister anions on the phase transition temperature of elastin-like polypeptides

The modulation of the lower critical solution temperature (LCST) of two elastin-like polypeptides (ELPs) was investigated in the presence of 11 sodium salts that span the Hofmeister series for anions. It was found that the hydrophobic collapse/aggregation of these ELPs generally followed the series. Specifically, kosmotropic anions decreased the LCST by polarizing interfacial water molecules involved in hydrating amide groups on the ELPs. On the other hand, chaotropic anions lowered the LCST through a surface tension effect. Additionally, chaotropic anions showed salting-in properties at low salt concentrations that were related to the saturation binding of anions with the biopolymers. These overall mechanistic effects were similar to those previously found for the hydrophobic collapse and aggregation of poly(N-isopropylacrylamide), PNIPAM. There is, however, a crucial difference between PNIPAM and ELPs. Namely, PNIPAM undergoes a two-step collapse process as a function of temperature in the presence of sufficient concentrations of kosmotropic salts. By contrast, ELPs undergo collapse in a single step in all cases studied herein. This suggests that the removal of water molecules from around the amide moieties triggers the removal of hydrophobic hydration waters in a highly coupled process. There are also some key differences between the LCST behavior of the two ELPs. Specifically, the more hydrophilic ELP V5A2G(3)-120 construct displays collapse/aggregation behavior that is consistent with a higher concentration of anions partitioning to polymer/aqueous interface as compared to the more hydrophobic ELP V(5)-120. It was also found that larger anions could bind with ELP V5A2G(3)-120 more readily in comparison with ELP V(5)-120. These latter results were interpreted in terms of relative binding site accessibility of the anion for the ELP.

Chain conformation of poly(dimethyl siloxane) at the air/water interface by sum frequency generation

This is to report a study of chain conformation of poly(dimethylsiloxane) (PDMS) in spread monolayers at the air/water interface (A/W) with the aid of vibrational sum frequency spectroscopy (VSFS). We find that methyl groups of PDMS chains at the interface are completely disordered in the dilute regime of the surface density. At higher surface densities, however, the two methyl groups on the repeating unit point into the air asymmetrically; one points more normal to the interface, whereas the other lies more parallel to the interface. In the first collapsed regime, where the surface pressure of the PDMS monolayer reaches a plateau value of 8.7 mN/m, the signal intensity at 2915 cm (-1), assigned to the symmetric vibrational frequency of the methyl groups, is found independent of the surface density. On the basis of this finding, we propose that PDMS chains, in the first collapse regime at the A/W, form asymmetric layers. Thus, our proposal lends support to earlier works by Langevin's group to refute a widely speculated helix model that was based on energy minimization in the crystalline state of PDMS. In short, the energy consideration in the bulk crystalline state does not provide meaningful guidance as to the chain conformation of the monolayer at the A/W.

Multiplexing ligand-receptor binding measurements by chemically patterning microfluidic channels

A method has been designed for patterning supported phospholipid bilayers (SLBs) on planar substrates and inside microfluidic channels. To do this, bovine serum albumin (BSA) monolayers were formed via adsorption at the liquid/solid interface. Next, this interfacial protein film was selectively patterned by using deep UV lithography. Subsequently, SLBs could be deposited in the patterned locations by vesicle fusion. By cycling through this process several times, spatially addressed bilayer arrays could be formed with intervening protein molecules serving as two-dimensional corrals. By employing this method, phospholipid bilayers containing various concentrations of ganglioside GM1 were addressed along the length of individual microfluidic channels. Therefore, the binding of GM1 with pentameric cholera toxin B (CTB) subunits could be probed. A seven-channel microfluidic device was fabricated for this purpose. Each channel was simultaneously patterned with four chemically distinct SLBs containing 0, 0.2, 0.5, and 2.0 mol % GM1, respectively. Varying concentrations of CTB were then introduced into each of the channels. With the use of total internal reflection fluorescence microscopy, it was possible to simultaneously abstract multiple equilibrium dissociation constants as a function of ligand density for the CTB-GM1 system in a single shot.

Double cushions preserve transmembrane protein mobility in supported bilayer systems

Supported lipid bilayers (SLBs) have been widely used as model systems to study cell membrane processes because they preserve the same 2D membrane fluidity found in living cells. One of the most significant limitations of this platform, however, is its inability to incorporate mobile transmembrane species. It is often postulated that transmembrane proteins reconstituted in SLBs lose their mobility because of direct interactions between the protein and the underlying substrate. Herein, we demonstrate a highly mobile fraction for a transmembrane protein, annexin V. Our strategy involves supporting the lipid bilayer on a double cushion, where we not only create a large space to accommodate the transmembrane portion of the macromolecule but also passivate the underlying substrate to reduce nonspecific protein-substrate interactions. The thickness of the confined water layer can be tuned by fusing vesicles containing polyethyleneglycol (PEG)-conjugated lipids of various molecular weights to a glass substrate that has first been passivated with a sacrificial layer of bovine serum albumin (BSA). The 2D fluidity of these systems was characterized by fluorescence recovery after photobleaching (FRAP) measurements. Uniform, mobile phospholipid bilayers with lipid diffusion coefficients of around 3 x 10(-8) cm2/s and percent mobile fractions of over 95% were obtained. Moreover, we obtained annexin V diffusion coefficients that were also around 3 x 10(-8) cm2/s with mobile fractions of up to 75%. This represents a significant improvement over bilayer platforms fabricated directly on glass or using single cushion strategies.

Sub-100 nm patterning of supported bilayers by nanoshaving lithography

Sub-100 nm wide supported phospholipid bilayers (SLBs) were patterned on a planar borosilicate substrate by AFM-based nanoshaving lithography. First, a bovine serum albumin monolayer was coated on the glass and then selectively removed in long strips by an AFM tip. The width of vacant strips could be controlled down to 15 nm. Bilayer lines could be formed within the vacant strips by vesicle fusion. It was found that stable bilayers formed by this method had a lower size limit of approximately 55 nm in width. This size limit stems from a balance between a favorable bilayer adhesion energy and an unfavorable bilayer edge energy.