The evolution of solution aggregates of the anionic form of the native monorhamnolipid (mRL) mixture produced by Pseudomonas aeruginosa ATCC 9027 is explored at pH 8.0 using both experimental and computational approaches. Experiments utilizing surface tension measurements, dynamic light scattering, and both steady-state and time-resolved fluorescence spectroscopy reveal solution aggregation properties. All-atom molecular dynamics simulations on self-assemblies of the most abundant monorhamnolipid molecule, L-rhamnosyl-β-hydroxydecanoyl-β-hydroxydecanoate (Rha-C10-C10), in its anionic state explore the formation of aggregates and the role of hydrogen bonding, substantiating the experimental results. At pH 8.0, at concentrations above the critical aggregation concentration of 201 μM but below ~7.5 mM, small premicelles exist in solution; above ~7.5 mM, micelles with hydrodynamic radii of ~2.5 nm dominate, although two discrete populations of larger lamellar aggregates (hydrodynamic radii of ~10 and 90 nm) are also present in solution in much smaller number densities. The critical aggregation number for the micelles is determined to be ~26 monomers/micelle using fluorescence quenching measurements, with micelles gradually increasing in size with monorhamnolipid concentration. Molecular dynamics simulations on systems with between 10 and 100 molecules of Rha-C10-C10 indicate the presence of stable premicelles of seven monomers with the most prevalent micelle being ~25 monomers and relatively spherical. A range of slightly larger micelles of comparable stability can also exist that become increasing elliptical with increasing monomer number. Intermolecular hydrogen bonding is shown to play a significant role in stabilization of these aggregates. In total, the computational results are in excellent agreement with the experimental results.

The potential of glycolipid surfactants, composed of a sugar headgroup and lipid tail, as highly biodegradable and less toxic alternatives to commonly used surfactants motivates the systematic study of structure-function relationships of various glycolipid surfactants. Advances in the efficient synthesis of high purity amphipathic glycolipid surfactants enable the analysis of a diverse set of glycolipids. The solution phase properties of two suites of glycolipid surfactants, nalkyl-O-melibiosides and n-alkyl-O-cellobiosides, with varying straight-chain alkyl tails of 8, 10 and 12 carbons are investigated in this work. This study substantiates their efficient surfactant performance and reveals their aggregate structure and microenvironment as it relates to molecular structure. Surface tensiometry demonstrates critical micelle concentrations of 0.2-40 mM with minimum surface tension values of 36-40 mN/m. Time resolved fluorescence quenching(TRFQ) spectroscopy is used to calculate micelle aggregation number (12-70 molecules/micelle) and assess relative micropolarity and microfluidity within the aggregates. The average aggregation numbers determined by TRFQ and apparent hydrodynamic radii determined by dynamic light scattering vary with alkyl chain length and headgroup size, but these properties are relatively insensitive to changes in concentration. The aggregates form nearly spherical, tightly-packed micelles with a small population of vesicles and polydispersity present at low concentrations or with short alkyl chain length.

The penetration depth of surface plasmons is commonly determined empirically from the observed response for adsorbate loading on gold surface plasmon resonance (SPR) substrates. However, changes in the SPR spectrum may originate from both changes in the effective refractive index near the metal surface and changes in the metal permittivity following covalent binding of the adsorbate layer. Herein, the significance of incorporating an additional adsorbate-metal bonding effect in the calculation is demonstrated in theory and in practice. The bonding effect is determined from the nonzero intercept of a SPR shift versus adsorbate thickness calibration and incorporated into the calculation of penetration depth at various excitation wavelengths. Determinations of plasmon penetration depth with and without the bonding response for alkanethiolate-gold are compared and are shown to be significantly different for a thiol monolayer adsorbate system. Additionally, plasmon penetration depth evaluated with bonding effect compensation shows greater consistency over different adsorbate thicknesses and better agreement with theory derived from Maxwell's equation, particularly for adsorbate thicknesses that are much smaller (<5%) than the plasmon penetration depth. The method is also extended to a more practically applicable polyelectrolyte multilayer adsorbate system.

Nanohole arrays exhibit unique surface plasmon resonance (SPR) characteristics according to hole periodicity, diameter, and excitation wavelength (λ(SPR)). This contribution investigates the SPR characteristics and surface sensitivity of various nanohole arrays with the aim of tuning the parameters for optimal sensing capability. Both the Bragg surface plasmons (SPs) arising from diffraction by the periodic holes and the traditional propagating SPs are characterized with emphasis on sensing capability of the propagating SPs. Several trends in bulk sensitivity and penetration depth were established, and the surface sensitivity was calculated from bulk sensitivity and penetration depth of the SPs for different analyte thicknesses. Increased accuracy and precision in penetration depth values were achieved by incorporating adsorbate effects on substrate permittivity. The optimal nanohole array conditions for highest surface sensitivity were determined (820 nm periodicity, 0.27 diameter/periodicity, and λ(SPR) = 1550 nm), which demonstrated an increase in surface sensitivity for the 10 nm analyte over continuous gold films at their optimal λ(SPR) (1300 nm) and conventional visible λ(SPR) (700 nm).