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 Ca 2+ and PO 4 3− 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 PO 4 3− site is one-half the rate of desorption from the Ca 2+ 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.

An asymmetrical spectral continuum is observed in low-frequency surface-enhanced Raman spectroscopy (LF-SERS) in the terahertz (THz) region. This background continuum appears as a flat baseline in the Stokes side and a slope in the anti-Stokes side. Analysis shows this asymmetry originates from the different scaling of anti-Stokes and Stokes scattering with respect to the Bose–Einstein distribution. Such asymmetry is readily visible, under room temperature, in the low-frequency THz spectral range. Accounting for this spectral intensity asymmetry reveals the intrinsic continuum background of electronic origin, differing from local normal modes and collective relaxational motions. We also describe a numerical method, independent of line shape models, to determine sample temperatures while simultaneously accounting for this asymmetrical background continuum. Finally, we show that such asymmetrical spectral continuum is generally observable in low-frequency Raman scattering.

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.