Inter-Leaflet Phospholipid Exchange Impacts the Ligand Density Available for Protein Binding at Supported Lipid Bilayers

Surface-Area Enhanced Raman Spectroscopy of DNA in Porous Silica: A Quantitative and Reproducible Alternative to Plasmonic-Based SERS

Despite the success of surface-enhanced Raman spectroscopy (SERS) for detecting DNA immobilized on plasmonic metal surfaces, its quantitative response is limited by the rapid falloff of enhancement with distance from the metal surface and variations in sensitivity that depend on orientation and proximity to plasmonic "hot spots". In this work, we assess an alternative approach for enhancing detection by immobilizing DNA on the interior surfaces of porous silica particles. These substrates provide over a 1000-fold greater surface area for detection compared to a planar support. The porous silica substrate is a purely dielectric material with randomly oriented internal surfaces, where scattering is independent of proximity and orientation of oligonucleotides relative to the silica surface. We characterize the quantitative response of Raman scattering from DNA in porous silica particles with sequences used in previous SERS investigations of DNA for comparison. The results show that Raman scattering of DNA in porous silica is independent of distance of nucleotides from the silica surface, allowing detection of longer DNA strands with constant sensitivity. The surface area enhancement within particles is reproducible (<4% particle-to-particle variation) owing to the uniform internal pore structure and surface chemistry of the silica support. DNA immobilization with a bis-thiosuccinimide linker provides a Raman-active internal standard for quantitative interpretation of Raman scattering results. Despite the high (30 mM) concentrations of immobilized DNA within porous silica particles, they can be used to measure nanomolar binding affinities of target molecules to DNA by equilibrating a very small number of particles with a sufficiently large volume of low-concentration solution of target molecules.

Rapid Generation of Therapeutic Nanoparticles Using Cell-Free Expression Systems

The surface modification of membrane-based nanoparticles, such as liposomes, polymersomes, and lipid nanoparticles, with targeting molecules, such as binding proteins, is an important step in the design of therapeutic materials. However, this modification can be costly and time-consuming, requiring cellular hosts for protein expression and lengthy purification and conjugation steps to attach proteins to the surface of nanocarriers, which ultimately limits the development of effective protein-conjugated nanocarriers. Here, the use of cell-free protein synthesis systems to rapidly create protein-conjugated membrane-based nanocarriers is demonstrated. Using this approach, multiple types of functional binding proteins, including affibodies, computationally designed proteins, and scFvs, can be cell-free expressed and conjugated to liposomes in one-pot. The technique can be expanded further to other nanoparticles, including polymersomes and lipid nanoparticles, and is amenable to multiple conjugation strategies, including surface attachment to and integration into nanoparticle membranes. Leveraging these methods, rapid design of bispecific artificial antigen presenting cells and enhanced delivery of lipid nanoparticle cargo in vitro is demonstrated. It is envisioned that this workflow will enable the rapid generation of membrane-based delivery systems and bolster our ability to create cell-mimetic therapeutics.

In Situ Confocal Raman Microscopy of Redox Polymer Films on Bulk Electrode Supports

A spectroelectrochemical cell is described that enables confocal Raman microscopy studies of electrode-supported films. The confocal probe volume (∼1 μm³) was treated as a fixed-volume reservoir for the observation of potential-induced changes in chemical composition at microscopic locations within an ∼20 μm thickness layer of a redox polymer cast onto a 3 mm diameter carbon disk electrode. Using a Raman system with high collection efficiency and wavelength reproducibility, spectral subtraction achieved excellent rejection of background interferences, opening opportunities for measuring within micrometer-scale thickness redox films on widely available, low-cost, and conventional carbon disk electrodes. The cell performance and spectral difference technique are demonstrated in experiments that detect transformations of redox-active molecules exchanged into electrode-supported ionomer membranes. The in situ measurements were sensitive to changes in the film oxidation state and swelling/deswelling of the polymer framework in response to the uptake and discharge of charge-compensating electrolyte ions. The studies lay a foundation for confocal Raman microscopy as a quantitative in situ probe of processes within electrode-immobilized redox polymers under development for a range of applications, including electrosynthesis, energy conversion, and chemical sensing.