Respiratory Monitoring by Ultrafast Humidity Sensors with Nanomaterials: A Review.

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.

Liquid-dependent impedance induced by vapor condensation and percolation in nanoparticle film.

A liquid-dependent impedance is observed by vapor condensation and percolation in the void space between nanoparticles. Under the Laplace pressure, vapor is effectively condensed into liquid to fill the nanoscale voids in an as-deposited nanoparticle film. Specifically, the transient impedance of the nanoparticle film in organic vapor is dependent on the vapor pressure and the conductivity of the condensed liquid. The response follows a power law that can be explained by the classical percolation theory. The condensed vapor gradually percolates into the void space among nanoparticles. A schematic is proposed to describe the vapor condensation and percolation dynamics among the nanoparticles. These findings offer insights into the behavior of vapor adsorbates in nanomaterial assemblies that contain void space.

Rapid Prototyping of a Nanoparticle Concentrator Using a Hydrogel Molding Method.

Nanoparticle (NP) concentration is crucial for liquid biopsies and analysis, and various NP concentrators (NPCs) have been developed. Methods using ion concentration polarization (ICP), an electrochemical phenomenon based on NPCs consisting of microchannels, have attracted attention because samples can be non-invasively concentrated using devices with simple structures. The fabrication of such NPCs is limited by the need for lithography, requiring special equipment and time. To overcome this, we reported a rapid prototyping method for NPCs by extending the previously developed hydrogel molding method, a microchannel fabrication method using hydrogel as a mold. With this, we fabricated NPCs with both straight and branched channels, typical NPC configurations. The generation of ICP was verified, and an NP concentration test was performed using dispersions of negatively and positively charged NPs. In the straight-channel NPC, negatively and positively charged NPs were concentrated >50-fold and >25-fold the original concentration, respectively. To our knowledge, this is the first report of NP concentration via ICP in a straight-channel NPC. Using a branched-channel NPC, maximum concentration rates of 2.0-fold and 1.7-fold were obtained with negatively and positively charged NPs, respectively, similar to those obtained with NPCs fabricated through conventional lithography. This rapid prototyping method is expected to promote the development of NPCs for liquid biopsy and analysis.

Characterization of Nanoparticle Adsorption on Polydimethylsiloxane-Based Microchannels.

Nanoparticles (NPs) are used in various medicinal applications. Exosomes, bio-derived NPs, are promising biomarkers obtained through separation and concentration from body fluids. Polydimethylsiloxane (PDMS)-based microchannels are well-suited for precise handling of NPs, offering benefits such as high gas permeability and low cytotoxicity. However, the large specific surface area of NPs may result in nonspecific adsorption on the device substrate and thus cause sample loss. Therefore, an understanding of NP adsorption on microchannels is important for the operation of microfluidic devices used for NP handling. Herein, we characterized NP adsorption on PDMS-based substrates and microchannels by atomic force microscopy to correlate NP adsorptivity with the electrostatic interactions associated with NP and dispersion medium properties. When polystyrene NP dispersions were introduced into PDMS-based microchannels at a constant flow rate, the number of adsorbed NPs decreased with decreasing NP and microchannel zeta potentials (i.e., with increasing pH), which suggested that the electrostatic interaction between the microchannel and NPs enhanced their repulsion. When exosome dispersions were introduced into PDMS-based microchannels with different wettabilities at constant flow rates, exosome adsorption was dominated by electrostatic interactions. The findings obtained should facilitate the preconcentration, separation, and sensing of NPs by PDMS-based microfluidic devices.

Capillary-condensed water in nonporous nanoparticle films evaluated by impedance analysis for nanoparticle devices.

We study capillary-condensed water in nonporous nanoparticle films and evaluate its effect on impedance analysis for practical nanoparticle devices. Nanoparticle-based electronic/optoelectronic devices have been intensively studied for applications in ambient air. Non-sintered nanoparticle films have porous structures and a vapor phase of water molecules condenses in nanopores between nanoparticles at a lower vapor pressure. This condensed water hinders intrinsic electrical properties of nanoparticle films. To clarify the effects of capillary-condensed water between nanoparticles on impedance, we electrically investigate 50 nm and 10 nm nonporous silica nanoparticle films. In a 50 nm nanoparticle film, an impedance hysteresis is observed at higher than 80% of relative humidity (RH). On the contrary, a larger impedance hysteresis appears in a 10 nm nanoparticle film at higher than 50% RH. We evaluate critical pore sizes in 50 nm and 10 nm nanoparticle films as 5-10 and 1.6 nm, respectively. These values correspond to the critical size of nanopores where adjacent menisci between nanoparticles coincide as following the Kelvin equation. This condensation of gas/vapor molecules will be an important issue for developments of practical devices using nanoparticle films.

Nonporous Inorganic Nanoparticle-Based Humidity Sensor: Evaluation of Humidity Hysteresis and Response Time.

Fast-response humidity sensors using nanomaterials are attractive and have been intensively studied. Among the various nanomaterials, nonporous inorganic nanoparticles are suitable for use in humidity sensitive films for sensors. Here, we focus on a nonporous inorganic nanoparticle film and investigate a humidity sensor using the film. Hysteresis error and a dynamic response to a change of humidity are fundamental specifications of humidity sensors. A humidity sensor using a 50 nm silica nanoparticle film shows a hysteresis error of 2% at 85% RH and a response/recovery time of 2.8/2.3 s in 30% RH to 70% RH. We also summarize response/recovery times and hysteresis errors of state-of-the-art humidity sensors. As compared to those of commercial sensors and porous nanoparticle-based sensors evaluated using saturated salt solutions, the fabricated sensor shows a comparative hysteresis error and shorter response time.

Effect of Agitation Method on the Nanosized Degradation of Polystyrene Microplastics Dispersed in Water.

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.

Development of mesoporous silica-based nanoparticles with controlled release capability for cancer therapy.

Nanoparticles that respond to internal and external stimuli to carry out controlled release of anticancer drugs have been developed. In this review, we focus on the development of mesoporous silica based nanoparticles, as this type of materials provides a relatively stable material that is amenable to various chemical modifications. We first provide an overview of various designs employed to construct MSN-based controlled release systems. These systems respond to internal stimuli such as pH, redox state and the presence of biomolecules as well as to external stimuli such as light and magnetic field. They are at a different stage of development; depending on the system, their operation has been demonstrated in aqueous solution, in cancer cells or in animal models. Efforts to develop MSNs with multi-functionality will be discussed. Safety and biodegradation of MSNs, issues that need to be overcome for clinical development of MSNs, will be discussed. Advances in the synthesis of mechanized theranostic nanoparticles open up the possibility to start envisioning future needs for medical equipment.