Respiratory monitoring is a fundamental method to understand the physiological and psychological relationships between respiration and the human body. In this review, we overview recent developments on ultrafast humidity sensors with functional nanomaterials for monitoring human respiration. Key advances in design and materials have resulted in humidity sensors with response and recovery times reaching 8 ms. In addition, these sensors are particularly beneficial for respiratory monitoring by being portable and noninvasive. We systematically classify the reported sensors according to four types of output signals: impedance, light, frequency, and voltage. Design strategies for preparing ultrafast humidity sensors using nanomaterials are discussed with regard to physical parameters such as the nanomaterial film thickness, porosity, and hydrophilicity. We also summarize other applications that require ultrafast humidity sensors for physiological studies. This review provides key guidelines and directions for preparing and applying such sensors in practical applications.

Two kinds of organosilica nanoparticles (NPs) which were fabricated from thiol containing precursors, (3-mercaptopropyl)trimethoxysilane (MPMS) and (3-mercaptopropyl)methyldimethoxysilane (MPDMS), are potential delivery vehicles of anticancer drugs. MPMS can form three siloxane bonds, but MPDMS forms two siloxane bonds at a maximum limit. Hence, disulfide bonds can be involved in the three-dimensional (3D) morphology of MPDMS NPs. In addition, NPs containing disulfide bonds are potentially degraded by reduced form of glutathione (GSH). To examine reactions between the organosilica NPs and GSH, the NPs were incubated in 10 mM GSH aqueous solution at 37 °C for 7 d and the products were analyzed using a field-emission scanning electron microscopy (FE-SEM), Raman spectroscopy and soft X-ray photoelectron spectroscopy (XPS). The Raman spectra showed the presence of disulfide bonds in the MPDMS NPs and the absence of disulfide bonds in MPMS NPs. The results of XPS measurements suggested that the disulfide bonds in the outer layer of MPDMS-NP were reduced to thiol groups. FE-SEM observations of MPDMS NPs detected changes in NP morphology after the GSH incubation. These results support the idea that MPDMS NPs contain disulfide bonds and are degradable by GSH. Therefore, MPDMS NPs possess biodegradable feature that is advantageous for clinical translation, i.e., nanomedicine.

Reports of marine organisms ingesting microplastics—formed from large plastic litter drifting in the marine environment by mechanical forces such as waves and photochemical processes initiated by sunlight, particularly ultraviolet rays—are increasing. However, the degradation process from microplastics to nanoplastics that are easily consumed by plankton located in the lower part of the food chain is not clear. Therefore, 200 nm diameter polystyrene (PS) latex particles—nanoparticle tracking analysis (NTA) calibration particles—dispersed in ultrapure deionized water were subjected to three types of agitation: rotation mixing, shaking, and flowing in addition to standing without agitation, and the physical degradation caused by agitation for 1 week at room temperature (23 °C) was evaluated. The degradation of the particles into nanosized particles was assessed by particle size distribution measurement using NTA and shape observation using field emission scanning electron microscopy (FE-SEM). In addition, the ratio of particle aggregation during incubation was calculated from the number of particles present in the region exceeding the particle size distribution range of the monodisperse particles before agitation with respect to the total number of particles. A shape change to a particle size of 100 nm or less was observed for the rotating and flowing of samples, and the influence of aggregation was suppressed to within 21% of the total particles calculated by NTA at the maximum. These results show that chemically stable PS can be degraded from micro- to nanosize with simple agitation in ultrapure deionized water.