Visualization of Surfactant Dynamics to and along Oil-Water Interfaces Using Solvatochromic Fluorescent Surfactants.

Although many theories exist to understand the transport of surfactants to and along interfaces under flow, few techniques are available to directly monitor surfactant dynamics at interfaces. Due to the broad use of surfactants, from oil spill remediation to healthcare and household products, a better means of assessing surfactant dynamics would be widely applicable. Here, we describe and demonstrate the use of fluorescent surfactant-dyes as a means of dynamic visualization of surfactant transport and have synthesized Nile Red/polyethylene glycol (NR-PEG) derivatives for this purpose. These NR-PEG surfactant-dyes fluoresce differently at oil-water interfaces than in bulk aqueous solution, allowing for the selective monitoring of bound and free surfactant using confocal microscopy. We have visualized the transport of surfactants to and along interfaces of droplets in a pipe flow and at the interfaces of liquid-infused surfaces undergoing shear-driven drainage. Imaging surfactant adsorption to interfaces at these time scales will allow for the development of improved dispersant systems for applications such as oil spill remediation and can also be used in understanding other fundamental transport phenomena involving dispersants.

Investigation of the Local Environment of Hydrophobic End Groups on Polyethylene Glycol (PEG) Brushes Using Fluorometry: Relationship to Click Chemistry Conjugation Reactions on PEG-Protected Nanoparticles.

Targeted nanoparticles often require conjugating targeting ligands to polyethylene glycol (PEG) chains of a nanoparticle's dense protecting corona. "Click" chemistries are commonly employed for their bioorthogonality, with strain-promoted azide-alkyne cycloadditions (SPAAC) increasingly chosen to avoid cytotoxic copper catalysts. However, conjugation becomes compromised if reactive PEG chain ends cannot encounter their reaction counterparts. We use fluorescence to probe the location of Nile Red, methylpyrene, and butylpyrene, dyes with comparable hydrophobicities to SPAAC alkynes (logP = 3.2-5.7), tethered to PEG chains on 100 nm NPs. Using fluorescence peak shifts, we find that Nile Red resides 43% of the time in the 5k PEG corona and 57% at the more hydrophobic nanoparticle core. Increasing the PEG MW to 67k doubles the corona dye fraction to 86% (14% core). More hydrophobic methylpyrene and butylpyrene, monitored with I1/I3 ratios, reside 1% in the corona (99% core). These results explain difficulties with using SPAAC reactions for conjugating large ligands to nanoparticles with PEG coronae.

Highly sensitive flexible pressure sensors with microstructured rubber dielectric layers.

The development of an electronic skin is critical to the realization of artificial intelligence that comes into direct contact with humans, and to biomedical applications such as prosthetic skin. To mimic the tactile sensing properties of natural skin, large arrays of pixel pressure sensors on a flexible and stretchable substrate are required. We demonstrate flexible, capacitive pressure sensors with unprecedented sensitivity and very short response times that can be inexpensively fabricated over large areas by microstructuring of thin films of the biocompatible elastomer polydimethylsiloxane. The pressure sensitivity of the microstructured films far surpassed that exhibited by unstructured elastomeric films of similar thickness, and is tunable by using different microstructures. The microstructured films were integrated into organic field-effect transistors as the dielectric layer, forming a new type of active sensor device with similarly excellent sensitivity and response times.