RESUMEN
Chemically modified elastomer surfaces are important to many applications, including microfluidics and soft sensors. Sensitive characterization of the interfacial chemistry of soft materials has been a persistent challenge, given their structural and chemical complexity. This article reports a method to probe local chemical states of elastomer surfaces that leverages the interference effects observed in micro-Raman spectroscopy. Unexpectedly, systematic variations of Raman scattering intensity were observed across a chemical wettability gradient grafted to the surface of a poly(dimethylsiloxane) (PDMS) film. Specifically, hydrophobic surface regions with a high graft density of long-chain hydrocarbon molecules showed suppressed Raman intensity. An optical interference model that accounts for molecular filling and swelling of an interfacial glassy layer during chemical modifications of the PDMS surface quantitatively reproduces experimental observations. This work establishes the spectroscopic signatures of interfacial chemical modifications on elastomer surfaces and enables a noncontact optical probe of local chemical states at the micro- and nanoscale compatible with the complex interfaces of soft materials.
RESUMEN
Domain features and domain walls in lead halide perovskites (LHPs) have attracted broad interest due to their potential impact on optoelectronic properties of this unique class of solution-processable semiconductors. Using nonpolarized light and simple imaging configurations, ferroelastic twin domains and their switchings through multiple consecutive phase transitions are directly visualized. This direct optical contrast originates from finite optical reflections at the wall interface between two compositionally identical, orientationally different, optically anisotropic domains inside the material bulk. The findings show these domain walls serve as internal reflectors and steer energy transport inside halide perovskites optically. First-principles calculations show universal low domain-wall energies and modest energy barriers of domain switching, confirming their prevalent appearance, stable presence, and facile moving observed in the experiments. The generality of ferroelasticity in halide perovskites stems from their soft bonding characteristics. This work shows the feasibility of using LHP twin domain walls as optical guides of internal photoexcitations, capable of nonvolatile on-off switching and tunable positioning endowed by their universal ferroelasticity.
RESUMEN
Development of controlled release biomolecules by surface modification of hydroxyapatite nanoparticles has recently gained popularity in the areas of bionanotechnology and nanomedicine. However, optimization of these biomolecules for applications such as drug delivery, nutrient delivery requires a systematic understanding of binding mechanisms and interfacial kinetics at the molecular level between the nanomatrix and the active compound. In this research, urea is used as a model molecule to investigate its interactions with two morphologically different thin films of hydroxyapatite nanoparticles. These thin films were fabricated on quartz crystal piezoelectric sensors to selectively expose Ca2+ and PO43- sites of hydroxyapatite. Respective urea adsorption and desorption on both of these sites were monitored in situ and in real time in the phosphate buffer solution that mimics body fluids. The measured kinetic parameters, which corroborate structural predisposition for controlled release, show desorption rates that are one-tenth of the adsorption rates on both surfaces. Furthermore, the rate of desorption from the PO43- site is one-half the rate of desorption from the Ca2+ site. The Hill kinetic model was found to satisfactorily fit data, which explains cooperative binding between the hydroxyapatite nanoparticle thin film and urea. Fourier transform infrared spectra and X-ray photoemission spectra of the urea adsorbed on the above surfaces confirm the cooperative binding. It also elucidates the different binding mechanisms between urea and hydroxyapatite that contribute to the changes in the interfacial kinetics. These findings provide valuable information for structurally optimizing hydroxyapatite nanoparticle surfaces to control interfacial kinetics for applications in bionanotechnology and nanomedicine.