Silica Nanoparticle Films Research Articles

Water Vapor Condensation in Nanoparticle Films: Physicochemical Analysis and Application to Rapid Vapor Sensing

Nanomaterial-based humidity sensors hold great promise for water vapor detection because of their high sensitivity and fast response/recovery. However, the condensation of water in nanomaterial films remains unclear from a physicochemical perspective. Herein, the condensation of water vapor in silica nanoparticle films was physicochemically analyzed to bridge the abovementioned gap. The morphology of surface-adsorbed water molecules was characterized using infrared absorption spectroscopy and soft X-ray absorption spectroscopy, and the effect of RH on the amount of adsorbed water was observed using a quartz crystal microbalance. The adsorbed water was found to exist in liquid- and ice-like states, which contributed to high and low conductivity, respectively. The large change in film impedance above 80% RH was ascribed to the condensation of water between the nanoparticles. Moreover, RH alteration resulted in a colorimetric change in the film’s interference fringe. The obtained insights were used to construct a portable device with response and recovery times suitable for the real-time monitoring of water vapor. Thus, this study clarifies the structure of water adsorbed on nanomaterial surfaces and, hence, the action mechanism of the corresponding nanoparticle-based sensors, inspiring further research on the application of various nanomaterials to vapor sensing.

Use of Silica Nanoparticle Langmuir Films to Determine the Effect of Surface Roughness on the Change in the Forces between Two Silica Surfaces by a Liquid Flow.

We aimed to determine how surface roughness changes the effect of a liquid flow on the forces between two charged surfaces. This is because many applications require liquid to flow through charged confined areas and because all surfaces show a degree of roughness. We prepared films of different roughness by making mixed Langmuir films of silica nanoparticles (NPs) of two different diameters at air-100 mM NaCl aqueous interfaces and then by transferring and sintering these films to silicon wafers. Atomic force microscope (AFM) imaging showed that the film roughness decreased (1) with an NP diameter decrease and (2) an increased ratio of small NPs in a mixed film of small and larger NPs. This decrease was explained by a decreased NP aggregation in the film, due to the increased Brownian velocity that accompanies an NP diameter decrease. Force-separation curves were next measured with an AFM between a microsized silica particle (probe) and a smooth substrate (silicon wafer) or the rough NP films in 1 mM NaCl. In the absence of a liquid flow, the repulsive forces decreased with an increased substrate roughness. This reduction was explained by an increased difference between the real zero separation distance and apparent zero separation distance (the distance between the first point of mechanical contact between the probe and substrate) with an increased surface roughness. In the presence of a liquid flow, the repulsive forces decreased in the case of a smooth substrate. However, the repulsive forces were reduced less by a liquid flow for rough substrates. This result was explained by the difference in the effect of liquid flow on the diffuse layer for the smooth and rough surfaces. Surface roughness is postulated to modify the liquid flow trajectory near the surfaces and to cause ion concentration gradients near the surface. These factors are proposed to lessen the change in the diffuse layer brought about by a liquid flow. This would then reduce the change in the forces.

Proton transport over nanoparticle surface in insulating nanoparticle film-based humidity sensor

We study proton transport on the surface of insulating nanoparticles for humidity sensors. We use this approach to reveal proton transfer mechanisms in humidity-sensitive materials. Hydrophilic and hydrophobic ligand-terminated silica nanoparticle films are adopted for evaluating the temperature dependence of the ion conductivity. According to the activation energy of the conductivity, we explain that Grotthuss (H+ transfer) and vehicular (H3O+ transfer) mechanisms are mainly dominant on hydrophilic (−OH terminated) and hydrophobic (acrylate terminated) surfaces of the nanoparticles, respectively. This investigation gives us a clue to understanding the proton transfer mechanism in solution-processed flexible humidity-sensitive nanomaterial films.

Ultralow friction of graphene-coated silica nanoparticle film

Reductions in friction and wear are widely sought to promote energy-saving and reliability in ubiquitous moving machine components. In this work, we demonstrate ultralow friction of graphene-coated silica nanoparticle films on diamond (111) surfaces by performing molecular dynamics simulations. Coated nanoparticles are prepared by enclosing 2.7 nm sized silica nanoparticles in C720 fullerene cages, which are used to construct a monolayer film with the face-centered-cubic (111) structure. Results indicate an ultralow friction coefficient, μ ∼ 0.07, for loads up to 18 GPa. The ultralow friction and high compressive strength are attributed to the coated structure that synergizes the outstanding low friction coefficient of C720 fullerenes with the nanobearing capability of silica nanoparticles. The realization of coated nanoparticles film-based ultralow friction has the potential to enhance the reliability and extend the working life of machines and devices while reducing their operating energy requirements.

FDTD simulation on transmittance of silica microsphere thin films with varying embedding in an optical adhesive

Moth-eye inspired nanostructure is widely used in optical applications. In this numerical study, a new type of anti-reflection film is designed using finite difference time domain method (FDTD). A three-dimensional model is established, simulating a monolayer of close-packed silica nanoparticle film embedded in a layer of polymeric optical adhesive. FDTD simulation is carried out in the visible band of wavelength, in order to investigate the transmittance of light propagation through the silica nanoparticle and adhesive layer. The effect of size of nanostructure as well as the embedding degree of optical adhesive are compared on different substrates, including visible transparent substrates with lower refractive index, i.e., fused silica glass, PMMA, PS, and visible opaque silicon substrate, with high refractive index. The numerical results reveal that we can obtain the best optical performance with nanoparticle size of 200 nm in diameter, while the immersion degree has different effects on different substrates.

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.

AC-STEM and HRSEM Investigation of Silica Nanoparticle Film Structure

AC-STEM and HRSEM Investigation of Silica Nanoparticle Film Structure

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.

Adsorption of a Polyethoxylated Surfactant from Aqueous Solution to Silica Nanoparticle Films Studied with In Situ Attenuated Total Reflection Infrared Spectroscopy and Colloid Probe Atomic Force Microscopy.

Polyethoxylated (PEO) surfactant adsorption to silica under aqueous conditions is an important physical process in a multitude of industries. Consequently, a considerable number of spectroscopic and other studies have been carried out to ascertain the molecular/structural details of the adsorbed surfactant and the kinetics of PEO surfactant adsorption. However, the use of infrared spectroscopy to probe surfactant adsorption at the silica/aqueous solution interface has been limited because of the instability of silica particle films under aqueous conditions and the opacity of silicon prisms below 1300 cm-1 typically employed for these studies. The work presented here provides infrared spectroscopic measurements of silica particle films formed from differing suspension pH on a diamond internal reflection prism to probe silica particle film stability as a function of pH. The films formed from a suspension pH of 2.5 were found to be the most stable owing to a sol-gel transition of the colloidal suspension upon drying and the reduction in electrostatic repulsion between silica nanoparticles, creating a tightly packed nanoparticle film. Colloid probe atomic force microscopy (CP-AFM) was used to confirm the alteration of surface forces between silica nanoparticles as a function of pH. Particle films from silica suspensions of pH 2.5 were formed in situ on an attenuated total reflection infrared diamond prism and used to probe Triton X-100 adsorption from an aqueous solution. The obtained infrared spectra revealed a critical surface aggregation concentration at a solution concentration of 0.14 mmol L-1, Triton X-100 forms discrete micelles at the silica surface, and the PEO head group preferentially adopts a helical conformation. Most intriguingly, a breakup of the silica particle film was observed at the critical micelle concentration of the surfactant. This is due to the repulsive steric forces arising from the interactions between the PEO corona of the surfactant micelles formed at the silica surface, as confirmed by the CP-AFM measurements.

Layer by Layer Mesoporous Silica-Hyaluronic Acid-Cyclodextrin Bifunctional "Lamination": Study of the Application of Fluorescent Probe and Host⁻Guest Interactions in the Drug Delivery Field.

The layer-by-layer technique was exploited to adjust the magnitude of the host–guest interactions between adamantane and cyclodextrin. The effect depends on numerous complex and changeable growth profiles of the films and the number of bilayers. These composite films of mesoporous silica nanoparticles and hyaluronic acid–cyclodextrin(HA-CD) were constructed to load the fluorescent dyes and peptides. The release rates of these molecules would decrease with an increase in the number of layers. A laser scanning confocal microscope was utilized to obtain the diffusion coefficient of fluorescein isothiocyanate. Hybrid films could be applied to increase the loading of different kinds of molecules and could also be integrated into the lamination to delay the rate of release.

Ultraviolet laser-induced damage of freestanding silica nanoparticle films

Silica nanoparticle antireflection films are essential for the development of high-power laser systems because of their high laser damage threshold in ultraviolet (e.g., 351 nm or 355 nm). However, most laser damage reports focus on the films standing on substrates, and the intrinsic laser damage behaviors of the silica nanoparticle films are poorly understood due to the influence of the substrates. Here, we investigate the ultraviolet laser damage behaviors of the freestanding silica nanoparticle films. We find that silica nanoparitcle films demonstrate higher laser damage threshold than the fused silica substrates under the irradiation of 355-nm laser. For the films on the fused silica substrates, the laser damage is caused by the substrates. In the case of freestanding films, laser-damage resistance is strongly dependent on the film thickness and the amount of organic groups in the films. The laser damage threshold increases from 103 to 159 J/cm2 @ 355 nm with decreasing the film thickness from 322 to 31 nm. In addition, removal of organic groups from the films results in higher laser damage threshold. Our findings provide insight into the laser damage behavior of the silica nanoparticle films and foreshow a possible way to improve the laser-damage resistance.

Interfacing superhydrophobic silica nanoparticle films with graphene and thermoplastic polyurethane for wear/abrasion resistance

HypothesisNanoparticle films are one of the most suitable platforms for obtaining sub-micrometer and nanometer dual-scale surface texture required for liquid repellency. The assembly of superhydrophobic nanoparticles into conformal and strongly adherent films having abrasion-induced wear resistance still poses a significant challenge. Various techniques have been developed over the years to render nanoparticle films with good liquid repellent properties and transparency. However, forming abrasion resistant superhydrophobic nanoparticle films on hard surfaces is challenging. One possibility is to partially embed or weld nanoparticles in thin thermoplastic primers applied over metals. ExperimentsHexamethyldisilazane-functionalized fumed silica nanoparticle films spray deposited on aluminum surfaces were rendered abrasion resistant by thermally welding them into thermoplastic polyurethane (TPU) primer applied a priori over aluminum. Different solvents, nanoparticle concentrations and annealing temperatures were studied to optimize nanoparticle film morphology and hydrophobicity. FindingsThermal annealing at 150 °C enhanced stability and wear resistance of nanoparticle films. A thin thermal interface layer of graphene nanoplatelets (GnPs) between the primer and the nanoparticle film significantly improved superhydrophobic wear resistance after annealing. As such, superhydrophobic nanocomposite films with the GnPs thermal interface layer displayed superior abrasion-induced wear resistance under 20 kPa compared to films having no GnPs-based thermal interface.

Silica Nanoparticles Functionalized with Zwitterionic Sulfobetaine Siloxane for Application as a Versatile Antifouling Coating System.

The growing need to develop surfaces able to effectively resist biological fouling has resulted in the widespread investigation of nanomaterials with potential antifouling properties. However, the preparation of effective antifouling coatings is limited by the availability of reactive surface functional groups and our ability to carefully control and organize chemistries at a materials' interface. Here, we present two methods of preparing hydrophilic low-fouling surface coatings through reaction of silica-nanoparticle suspensions and predeposited silica-nanoparticle films with zwitterionic sulfobetaine (SB). Silica-nanoparticle suspensions were functionalized with SB across three pH conditions and deposited as thin films via a simple spin-coating process to generate hydrophilic antifouling coatings. In addition, coatings of predeposited silica nanoparticles were surface functionalized via exposure to zwitterionic solutions. Quartz crystal microgravimetry with dissipation monitoring was employed as a high throughput technique for monitoring and optimizing reaction to the silica-nanoparticle surfaces. Functionalization of nanoparticle films was rapid and could be achieved over a wide pH range and at low zwitterion concentrations. All functionalized particle surfaces presented a high degree of wettability and resulted in large reductions in adsorption of bovine serum albumin protein. Particle coatings also showed a reduction in adhesion of fungal spores (Epicoccum nigrum) and bacteria (Escherichia coli) by up to 87 and 96%, respectively. These results indicate the potential for functionalized nanosilicas to be further developed as versatile fouling-resistant coatings for widespread coating applications.

The topographical properties of silica nanoparticle film preserve the osteoblast-like cell characteristics in vitro

The Transplantation of osteoblasts, along with an artificial implant, is experimentally considered as a therapeutics for degenerative bone diseases. However, osteoblasts have several limitations for application of transplantation in therapeutics, including a low-efficiency for bone mineralization and easy loss of characteristics in in vitro culture condition. In this study, we fabricated silica nano-particle (SNP) films using particles of different sizes to culture osteoblast-like cells for analysis the effect of topography on cellular behavior and characteristics. The physical parameters of films, such as intervals, height and roughness, were proportionally increased depending on the SNP diameter. When osteoblast-like cells were cultured on the various SNP films, the cell attachment rate on SNP-300 and SNP-700 was significantly decreased when it compared to tissue culture polystyrene (TCPS) group. In addition, the genes responsible for cell adhesion showed differential expression profiles in SNP films. The expression and activity of alkaline phosphatase were elevated in SNP-300 and SNP-700, and the extra-cellular matrix and osteoblast marker showed increased gene expression in these SNP films when compared to TCPS group. In the present study, we demonstrate that the topographical property of a nano-scale structure preserves the characteristics of osteoblast-like cells, and regulates the cellular behavior.

Mesoporous silica nanoparticle film as sorbent for in situ and real-time monitoring of volatile BTX (benzene, toluene and xylenes)

A strategy is described for the real time and selective monitoring of benzene, toluene and xylenes (BTX) in air. Rapid preconcentration and recovery have been achieved with films made of mesoporous silica nanoparticles assembled on a quartz plate. The resulting nanoparticle films displayed high transmittance and allowed the detection of toluene up to 1ppm within a minute and a regeneration of the sensor in less than 30s. The sensor response increased while increasing the concentration of benzene (detection at 252nm), toluene (detection at 267nm) or p-xylene (detection at 274nm) in the polluted air from 1 to 100ppm. The order of sensitivity, i.e. p-xylene>toluene>benzene, was correlated with their respective molar extinction coefficients. The BTX adsorption on mesoporous silica nanoparticle films has been analyzed with the Freundlich adsorption model. The repeatability of the detection was good in intraday (2%) and extra-day (4%) experiments with a given sensor. The ability to detect toluene in the presence of volatile compounds often encountered in working environments (i.e., methyl ethyl ketone, ethanol, acetone and cyclohexane) has been shown. The introduction of a gas dryer before the measurement cell allowed application of the sensor in humid atmosphere. This sensor could be used to alert if permissible values of exposure to toluene and xylenes have been reached during work.

Polyvinylpyrrolidone adhesion layer for increased uniformity and optical transmittance of silica nanoparticle antireflective coatings

The uniformity of silica nanoparticle antireflective coatings deposited from aqueous solutions on glass substrates is limited by the high surface tension and low evaporation rate of water. In this work, thin films of polyvinylpyrrolidone (PVP) were utilized as an adhesion layer to increase the uniformity and optical transmittance of silica nanoparticle coatings. The increase in adhesive force caused by the presence of the PVP layer was measured using atomic force microscopy (AFM). The micro- and nanoscale uniformities of silica nanoparticle films with and without PVP adhesion layers were characterized using scanning electron microscopy and AFM. It was found that a thin PVP adhesion layer provides the adhesion required to form uniform films of silica nanoparticles. Solar weighted transmittance of 97.6% over a wavelength range of 330–1000 nm was achieved with soda-lime glass substrates coated on both sides.

Controlling surface energy of glass substrates to prepare superhydrophobic and transparent films from silica nanoparticle suspensions

We fabricated superhydrophobic and transparent silica nanoparticle (SNP) films on glass plates via spray-coating technique. When suspensions containing 1-propanol and hydrophobic SNPs were sprayed over glass plates that were modified with dodecyl groups, superhydrophobic and transparent SNP films were formed on the substrates. Surface energy of the glass plates had a significant role to obtain superhydrophobic and transparent SNP films. SNP films did not show superhydrophobicity when bare glass plates were used as substrates, because water droplets tend to adhere the exposed part of the hydrophilic glass plate. Glass plates having extreme low surface energy were not also suitable because suspension solution was repelled from the substrates, which resulted in forming non-uniform SNP films.

Mesoporous silica nanoparticle-based substrates for cell directed delivery of Notch signalling modulators to control myoblast differentiation

Biochemical cues are critical to control stem cell function and can be utilized to develop smart biomaterials for stem cell engineering. The challenge is to deliver these cues in a restricted manner with spatial and temporal control. Here we have developed bilayer films of mesoporous silica nanoparticles for delayed cellular delivery of Notch modulators to promote muscle stem cell differentiation. We demonstrate that drug-loaded particles are internalized from the particle-covered surface, which allows for direct delivery of the drug into the cell and a delayed and confined drug release. Substrates of particles loaded with γ-secretase-inhibitors, which block the Notch signalling pathway, promoted efficient differentiation of myoblasts. The particle substrates were fully biocompatible and did not interfere with the inherent differentiation process. We further demonstrate that impregnating commercially available, biocompatible polymer scaffolds with MSNs allows for a free standing substrate for cell directed drug delivery.

Luminescence features of nanocomposites of silicon-organic polymer/porous SiO2 and TiO2 films

Photoluminescence properties of poly(di-n-hexylsilane) (PDHS) films as well as of PDHS adsorbed in nanoporous silica and titania films, and dispersed in films of silica nanoparticles have been investigated in a wide temperature range of 15–300K. Photoluminescence spectra of all compounds were found to be dominated by the aggregate band. PDHS photoluminescence has been quenched by about 5 times in nanoporous titania film in comparison with the silica film. Weak temperature dependences of PDHS photoluminescence intensities, lifetimes, band positions, and bandwidths have been observed in nanoporous silica and titania films in comparison with neat PDHS film and PDHS dispersed in films of silica nanoparticles. It has been interpreted as a consequence of slow energy migration and weak photoluminescence quenching in nanoporous films.

Transparent self-cleaning and antifogging silica nanoparticle films

Self-cleaning and antifogging coatings are of great interest for application in outdoor solar cell installations to mitigate the performance loss and associated maintenance costs due to environmental contamination. These coatings, applied to the cover glass of solar panels, can remove contaminants when wetted with rain water and transmit more sunlight in foggy weather. Herein, we report a superhydrophilic silica nanoparticle film that exhibits the self-cleaning effect without relying on photocatalytic materials. When wetted with simulated light raining conditions, the coated glass removed 90% of surface contaminants, compared to only 48% removed from the bare glass. The deposited film created an antifogging surface in addition to increasing the solar transmittance (AM 1.5) of glass substrates by 4.3% over bare glass in the wavelength range of 350–1100nm.