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.

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.

Digital image analysis for measuring nanogap distance produced by adhesion lithography.

A simple digital image analysis for measuring nanogap distance produced by adhesion lithography is proposed. Adhesion lithography produces metal electrodes with sub-15 nm undulated space and µm to mm scale width without using electron beam lithography. Although the process has been rapidly improved in recent years, there has been no generalized procedure to evaluate the nanogap distance. In this study, we propose a procedure to evaluate a nanogap electrode with large width/gap distance ratios (>1000). The procedure is to determine the average distance of nanogap space from the area and the perimeter of the space by the analysis of the grayscale image. This procedure excludes any arbitrariness of the estimation and gives quantitative comparison of nanogap electrodes produced by different processes.

Respiratory rate on exercise measured by nanoparticle-based humidity sensor.

In this study, we show the measurement of respiratory rate on exercise using a nanoparticle-based humidity sensor. A portable respiratory rate sensor is comprised of a colloidal silica nanoparticle-based humidity sensor chip. The impedance of the silica nanoparticle film is dependent on humidity and it is used for the detection of humid exhaled air. The respiratory rate sensor can be attached on an oxygen mask and the sensor signal is remotely monitored via Bluetooth. We show that the sensor follows a respiratory rate up to 60 bpm. We compare the sensor signal with that of a conventional respiratory measurement unit, which monitors a respiratory volume. The nanoparticle-based sensor can monitor a respiratory rate of an exercising person on a treadmill. The sensor operates stably for almost one year.

Solution Processing of Hydrogen-Terminated Silicon Nanocrystal for Flexible Electronic Device.

We demonstrate solution processing of hydrogen-terminated silicon nanocrystals (H-Si NCs) for flexible electronic devices. To obtain high and uniform conductivity of a solution-processed Si NC film, we adopt a perfectly dispersed colloidal H-Si NC solution. We show a high conductivity (2 × 10-5 S/cm) of a solution-processed H-Si NC film which is spin-coated in air. The NC film (area: 100 mm2) has uniform conductivity and responds to laser irradiation with 6.8 and 24.1 µs of rise and fall time. By using time-of-flight measurements, we propose a charge transport model in the H-Si NC film. For the proof-of-concept of this study, a flexible photodetector on a polyethylene terephthalate substrate is demonstrated by spin-coating colloidal H-Si NC solution in air. The photodetector can be bent in 5.9 mm bending radius at smallest, and the device properly works after being bent in 2500 cycles.

Silicon quantum dots with heavily boron and phosphorus codoped shell.

Heavily boron and phosphorus codoped silicon quantum dots (QDs) are dispersible in water without organic ligands and exhibit near infrared luminescence. We summarize the fundamental properties and demonstrate the formation of a variety of nanocomposites.

Size-dependent donor and acceptor states in codoped Si nanocrystals studied by scanning tunneling spectroscopy.

The electrical and optical properties of semiconductor nanocrystals (NCs) can be controlled, in addition to size and shape, by doping. However, such a process is not trivial in NCs due to the high formation energy of dopants there. Nevertheless, it has been shown theoretically that in the case of B and P (acceptor/donor) codoped Si-NCs the formation energy is reduced relative to that of single type doping. Previous comprehensive measurements on ensembles of such codoped Si-NCs have pointed to the presence of donor and acceptor states within the energy gap. However, such a conjecture has not been directly verified previously. Following that, we investigate here the electronic properties of B and P codoped Si-NCs via Scanning Tunneling Spectroscopy. We monitored the quantum confinement effect in this system, for which the energy gap changed from ∼1.4 eV to ∼1.8 eV with the decrease of NC diameter from 8.5 to 3.5 nm. Importantly, all spectra showed two in-gap band-states, one close to the conduction band edge and the other to the valence band edge, which we attribute to the P and B dopant levels, respectively. The energy separation between these dopants states decrease monotonically with increasing NC diameter, in parallel to the decrease of the conduction-to-valence bands separation. A fundamental quantity that is derived directly for these Si-NCs is the intrinsic like position of the Fermi energy, a non-trivial result that is very relevant for understanding the system. Following the above results we suggest an explanation for the character and the origin of the dopants bands.

Fast-Response and Flexible Nanocrystal-Based Humidity Sensor for Monitoring Human Respiration and Water Evaporation on Skin.

We develop a fast-response and flexible nanocrystal-based humidity sensor for real-time monitoring of human activity: respiration and water evaporation on skin. A silicon-nanocrystal film is formed on a polyimide film by spin-coating the colloidal solution and is used as a flexible and humidity-sensitive material in a humidity sensor. The flexible nanocrystal-based humidity sensor shows a high sensitivity; current through the nanocrystal film changes by 5 orders of magnitude in the relative humidity range of 8-83%. The response/recovery time of the sensor is 40 ms. Thanks to the fast response and recovery time, the sensor can monitor human respiration and water evaporation on skin in real time. Due to the flexibility and the fast response/recovery time, the sensor is promising for application in personal health monitoring as well as environmental monitoring.

Three-input gate logic circuits on chemically assembled single-electron transistors with organic and inorganic hybrid passivation layers.

Single-electron transistors (SETs) are sub-10-nm scale electronic devices based on conductive Coulomb islands sandwiched between double-barrier tunneling barriers. Chemically assembled SETs with alkanethiol-protected Au nanoparticles show highly stable Coulomb diamonds and two-input logic operations. The combination of bottom-up and top-down processes used to form the passivation layer is vital for realizing multi-gate chemically assembled SET circuits, as this combination enables us to connect conventional complementary metal oxide semiconductor (CMOS) technologies via planar processes. Here, three-input gate exclusive-OR (XOR) logic operations are demonstrated in passivated chemically assembled SETs. The passivation layer is a hybrid bilayer of self-assembled monolayers (SAMs) and pulsed laser deposited (PLD) aluminum oxide (AlO[Formula: see text]), and top-gate electrodes were prepared on the hybrid passivation layers. Top and two-side-gated SETs showed clear Coulomb oscillation and diamonds for each of the three available gates, and three-input gate XOR logic operation was clearly demonstrated. These results show the potential of chemically assembled SETs to work as logic devices with multi-gate inputs using organic and inorganic hybrid passivation layers.

Conversion efficiency of an energy harvester based on resonant tunneling through quantum dots with heat leakage.

We study the conversion efficiency of an energy harvester based on resonant tunneling through quantum dots with heat leakage. Heat leakage current from a hot electrode to a cold electrode is taken into account in the analysis of the harvester operation. Modeling of electrical output indicates that a maximum heat leakage current is not negligible because it is larger than that of the heat current harvested into electrical power. A reduction of heat leakage is required in this energy harvester in order to obtain efficient heat-to-electrical conversion. Multiple energy levels of a quantum dot can increase the output power of the harvester. Heavily doped colloidal semiconductor quantum dots are a possible candidate for a quantum-dot monolayer in the energy harvester to reduce heat leakage, scaling down device size, and increasing electrical output via multiple discrete energy levels.

Size-Dependence of Acceptor and Donor Levels of Boron and Phosphorus Codoped Colloidal Silicon Nanocrystals.

Size dependence of the boron (B) acceptor and phosphorus (P) donor levels of silicon (Si) nanocrystals (NCs) measured from the vacuum level was obtained in a very wide size range from 1 to 9 nm in diameter by photoemission yield spectroscopy and photoluminescence spectroscopy for B and P codoped Si-NCs. In relatively large Si-NCs, both levels are within the bulk Si band gap. The levels exhibited much smaller size dependence compared to the valence band and conduction band edges. The Fermi level of B and P codoped Si-NCs was also studied. It was found that the Fermi level of relatively large codoped Si-NCs is close to the valence band and it approaches the middle of the band gap with decreasing the size. The results suggest that below a certain size perfectly compensated Si-NCs, that is, Si-NCs with exactly the same number of active B and P, are preferentially grown, irrespective of average B and P concentrations in samples.

Radio-frequency capacitance spectroscopy of metallic nanoparticles.

Recent years have seen great progress in our understanding of the electronic properties of nanomaterials in which at least one dimension measures less than 100 nm. However, contacting true nanometer scale materials such as individual molecules or nanoparticles remains a challenge as even state-of-the-art nanofabrication techniques such as electron-beam lithography have a resolution of a few nm at best. Here we present a fabrication and measurement technique that allows high sensitivity and high bandwidth readout of discrete quantum states of metallic nanoparticles which does not require nm resolution or precision. This is achieved by coupling the nanoparticles to resonant electrical circuits and measurement of the phase of a reflected radio-frequency signal. This requires only a single tunnel contact to the nanoparticles thus simplifying device fabrication and improving yield and reliability. The technique is demonstrated by measurements on 2.7 nm thiol coated gold nanoparticles which are shown to be in excellent quantitative agreement with theory.

Control of charging energy in chemically assembled nanoparticle single-electron transistors.

We show the control of a charging energy in chemically assembled nanoparticle single-electron transistors (SETs) by altering the core diameter of Au nanoparticles. The charging energy is a fundamental parameter that decides the operating temperature of SETs. Practical application of SETs requires us to regulate the value of the charging energy by tuning the diameter of quantum dots. In this study, we used 3.0, 5.0 and 6.2 nm diameter synthesized Au nanoparticles as a quantum dot in the SETs. The total capacitances and charging energy of the SETs were evaluated from the rhombic Coulomb diamonds attributed to a single Coulomb island. The capacitance and charging energy matched with a concentric sphere model much better than with a simple sphere model. The operating temperatures of the SETs suggested that a charging energy 2.2 times greater than the thermal energy was required for stable operation, in theory. These results will help us to select an appropriate core diameter for the Au nanoparticles in practical SETs.

Nanoparticle characterization based on STM and STS.

In this review, we describe recent progress made in the study of nanoparticles characterized by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). Basic principles of STM measurements and single-electron tunneling phenomena through a single NP are summarized. We highlight the results of electrical and photonic properties on NPs studied by STM and STS. Because nanoparticles are single-digit nanometre in diameter, a single-electron transport on individual nanoparticles such as Coulomb blockade and resonant tunneling through discrete energy levels are investigated. Photon-emission from NPs is also introduced based on STM measurements. Novel single-nanoparticle functions such as stochastic blinking and one-write erasing behaviours are presented. This review provides an overview of nanoparticle characterization methods based on STM and STS that include the detailed understanding of the electrical and photonics properties of nanoparticles.

Granulomatous cheilitis with intralymphatic histiocytosis possibly associated with calcium deposition caused by chronic inflammation owing to dental metals and periodontitis.

Granulomatous cheilitis (GC) is an uncommon disease and the pathogenesis of GC has not been clear. Here, we present two cases of GC associated with calcium deposition caused by chronic inflammation owing to dental metal allergy and periodontitis. Histopathologically, intralymphatic histiocytosis (IH) in addition to non-caseating granulomas was seen in both cases. The results of a patch test were positive for some dental metals. Qualitative analyses by energy-dispersive X-ray spectroscopy detected calcium from biopsy specimens. We considered that partial destruction of teeth by chronic inflammation induced reactive proliferation of histiocytes, resulting in IH. Our study suggests that GC and IH is a consequence of chronic inflammatory diseases caused by dental metals and periodontitis. Dental care is necessary for treatment of GC.

Ideal discrete energy levels in synthesized Au nanoparticles for chemically assembled single-electron transistors.

Ideal discrete energy levels in synthesized Au nanoparticles (6.2 ± 0.8 nm) for a chemically assembled single-electron transistor (SET) are demonstrated at 300 mK. The spatial structure of the double-gate SET is determined by two gate and drain voltages dependence of the stability diagram, and electron transport to the Coulomb box of a single, nearby Coulomb island of Au nanoparticles is detected by the SET. The SET exhibits discrete energy levels, and the excited energy level spacing of the Coulomb island is evaluated as 0.73 meV, which well corresponds to the expected theoretical value. The discrete energy levels show magnetic field evolution with the Zeeman effect and dependence on the odd-even electron number of a single Au nanoparticle.

Coulomb blockade behaviors in individual Au nanoparticles as observed through noncontact atomic force spectroscopy at room temperature.

Coulomb blockade behaviors in individual Au nanoparticles of 2 nm core diameter in double-barrier structures have been studied by means of noncontact atomic force spectroscopy (NC-AFS) at room temperature. The Au nanoparticles with a 1-decanethiol ligand were chemisorbed by 1,10-decanedithiol molecules of a mixed 1-octanethiol/1,10-decanedithiol self-assembled monolayer coated on a Au(111) surface; these particles were observed through NC-AFS. NC-AFS measurements of the cantilever frequency shift-sample voltage (Δf-V(S)) curves were sequentially conducted on three Au nanoparticles under the same experimental conditions; the Δf-V(S) curves were found to deviate from the parabolic (Δf(N)) curve in the cases where no extra charge existed on the Au core. The experimental Δf(CB)(=Δf-Δf(N)) and Δf(CB)/V curves agree well with the theoretical curves obtained using a golden-rule calculation and the same parabolic parameters. All the results, through NC-AFS, suggest Coulomb blockade behaviors in the Au nanoparticles at room temperature.