Flexible nanohybrid substrates utilizing gold nanocubes/nano mica platelets with 3D lightning-rod effect for highly efficient bacterial biosensors based on surface-enhanced Raman scattering.

In this study, gold nanocubes (AuNCs) were quickly synthesized using the seed-mediated growth method and reduced onto the surface of two-dimensional (2D) delaminated nano mica platelets (NMPs), enabling the development of AuNCs/NMPs nanohybrids with a 3D lightning-rod effect. First, the growth-solution amount can be changed to easily adjust the AuNCs average-particle size within a range of 30-70 nm. The use of the cationic surfactant cetyltrimethylammonium chloride as a protective agent allowed the surface of AuNCs and nanohybrids to be positively charged. Positively charged nanohybrid surfaces presented a good adsorption effect for detecting molecules with negative charges on the surface. Additionally, the NMP surfaces were rich in ionic charges and provided a large specific surface area for stabilizing the growth of AuNCs. Delaminated AuNCs/NMPs nanohybrids can generate a 3D hotspot effect through self-assembly to enhance the Raman signal. Surface-enhanced Raman scattering (SERS) is highly sensitive in detecting adenine biomolecules. Its limit of detection (LOD) and Raman enhancement factor reached 10-9 M and 3.6 × 108, respectively. Excellent reproducibility was obtained owing to the relatively regular arrangement of AuNC particles, and the relative standard deviation (RSD) was 10.7%. Finally, the surface of NMPs was modified by adding the hydrophilic poly(oxyethylene)-diamine (POE2000) and amphiphilic PIB-POE-PIB copolymer at different weight ratios. The adjustment of the surface hydrophilicity and hydrophobicity of AuNCs/NMPs nanohybrids led to better adsorption and selectivity for bacteria. AuNCs/POE/NMPs and AuNCs/PIB-POE-PIB/NMPs were further applied to the SERS detection of hydrophilic Staphylococcus aureus and hydrophobic Escherichia coli, respectively. The SERS-detection results suggest that the LOD of hydrophilic Staphylococcus aureus and hydrophobic Escherichia coli reached 92 CFU mL-1 and 1.6 × 102 CFU mL-1, respectively. The AuNCs/POE/NMPs and AuNCs/PIB-POE-PIB/NMPs nanohybrids had different hydrophilic-hydrophobic affinities, which greatly improved the selectivity and sensitivity for detecting bacteria with different hydrophilicity and hydrophobicity. Therefore, fast, highly selective, and highly sensitive SERS biological-detection results were obtained.

Application of Nanohybrid Substrates with Layer-by-Layer Self-Assembling Properties to High-Sensitivity Surface-Enhanced Raman Scattering Detection.

The present study was conducted to prepare and investigate large-area, high-sensitivity surface-enhanced Raman scattering (SERS) substrates. Organic/inorganic nanohybrid dispersants consisting of an amphiphilic triblock copolymer (hereafter referred to simply as "copolymer") and graphene oxide (GO) were used to stabilize the growth and size of gold nanoparticles (AuNPs). Ion-dipole forces were present between the AuNPs and copolymer dispersants, while the hydrogen bonds between GO and the copolymer prevented the aggregation of GO, thereby stabilizing the AuNP/GO nanohybrids. Transmission electron microscopy (TEM) revealed that the AuNPs had particle sizes of 25-35 nm and a relatively uniform size distribution. The AuNP/GO nanohybrids were deposited onto the glass substrate by using the solution drop-casting method and employed for SERS detection. The self-assembling properties of two-dimensional sheet-like GO led to a regular lamellar arrangement of AuNP/GO nanohybrids, which could be used for the preparation of large-area SERS substrates. Following removal of the copolymer by annealing at 300 °C for 2 h, measurements were obtained under scanning electron microscopy. The results confirmed that 2D GO nanosheets were capable of stabilizing AuNPs, with the final size reaching approximately 40 nm. These AuNPs were adsorbed on both sides of the GO nanosheets. Because the GO nanosheets were merely 5 nm-thick, a good three-dimensional hot-junction effect was generated along the z-axis of the AuNPs. Lastly, the prepared material was used for the SERS detection of rhodamine 6G (R6G), a commonly used highly fluorescent dye. An enhancement factor (EF) of up to 3.5 × 106 was achieved, and the limit of detection was approximately 10-10 M. Detection limits of 10-10 M and < 10-10 M were also observed with the detection of Direct Blue 200 and the biological molecule adenine. It is therefore evident that AuNP/copolymer/GO nanohybrids are large-area flexible SERS substrates that hold great potential in environmental monitoring and biological system detection applications.

PSPS: A Step toward Tamper Resistance against Physical Computer Intrusion.

Cyberattacks are increasing in both number and severity for private, corporate, and governmental bodies. To prevent these attacks, many intrusion detection systems and intrusion prevention systems provide computer security by monitoring network packets and auditing system records. However, most of these systems only monitor network packets rather than the computer itself, so physical intrusion is also an important security issue. Furthermore, with the rapid progress of the Internet of Things (IoT) technology, security problems of IoT devices are also increasing. Many IoT devices can be disassembled for decompilation, resulting in the theft of sensitive data. To prevent this, physical intrusion detection systems of the IoT should be considered. We here propose a physical security system that can protect data from unauthorized access when the computer chassis is opened or tampered with. Sensor switches monitor the chassis status at all times and upload event logs to a cloud server for remote monitoring. If the system finds that the computer has an abnormal condition, it takes protective measures and notifies the administrator. This system can be extended to IoT devices to protect their data from theft.

Reducing Contact Time of Droplets Impacting Superheated Hydrophobic Surfaces.

Reducing the contact time (tc ) of a droplet impacting a solid surface is crucial in various fields. Superhydrophobic (SHB) surfaces are used to reduce tc at room temperature. However, at high temperatures, SHB surfaces cannot achieve tc reduction because of the failure of the coating materials or the Leidenfrost (LF) effect. Therefore, a surface that can suppress the LF effect and reduce tc at high temperatures is required. To create such a surface, a double-reentrant groove (DRG) array surface with an overhanging structure on top of the microgrooves is developed. The overhanging structure renders the surface hydrophobic (HB). Despite its HB nature, the DRG surface's LF point (LFP) is observed at ≈530 °C, which is higher than the LFP on other HB surfaces. Moreover, a tc smaller than the inertia-capillary limit on the DRG surface is observed at between 400 and 500 °C. Accordingly, the DRG surface is currently the only HB surface for tc reduction at high temperatures. The DRG surface avoids the limitation of low LFPs observed on HB surfaces. Due to its HB properties, the DRG surface is determined to exhibit self-cleaning characteristics and can be used in various applications at high temperatures.

Thermal conductivity and electrical resistivity of single copper nanowires.

Copper nano-interconnects are ubiquitous in semiconductor devices. The electrical and thermal properties of copper nanowires (CuNWs) profoundly affect the performance of electronics. In contrast to the intensively studied electrical properties of CuNWs, the thermal conductivities of CuNWs have seldom been examined. In this study, the electrical resistivity and thermal conductivity of single CuNWs were investigated. The Bloch-Grüneisen formula was introduced to determine the mechanisms responsible for the obtained electrical resistivity of the CuNWs. High residual resistivity was found, which indicated strong structural scattering on the electron transport resulting from defect scattering and boundary scatterings at the copper-copper oxide interface and grain boundaries. The mean structural scattering distance was employed to appreciate the degree of structural scattering in the CuNWs. The residual resistivity and electron-phonon coupling parameter were found to increase with the degree of structural scattering. Moreover, the unified thermal resistivity was introduced to illustrate the mechanisms responsible for the CuNWs' thermal conductivities. Similarly, large values of residual unified thermal resistivity and electron-phonon-induced unified thermal resistivity were found. The obtained unified thermal resistivities of the CuNWs could also be qualitatively explained by the degree of structural scattering in the CuNWs. The results suggested that structural scattering was predominant in the electrical current transport and heat transfer in the nanowires. This study revealed the mechanisms of electrical resistivity and thermal conductivity of CuNWs, and the insights could assist in improving the design of semiconductor architectures.

Heat Transfer of Semicrystalline Nylon Nanofibers.

Polymers are generally regarded as thermal insulators. The efficient heat transfer observed in the low-dimensional polymers in the literature mainly result from the larger crystallinity or improved polymer chain orientation in the low-dimensional polymers. However, the role of the amorphous domain on heat transfer in polymers remains unexplored. In this work, we report that the semicrystalline nylon polymer nanofibers can exhibit a very large thermal conductivity of 59.1 ± 3.1 W m-1 K-1 and the heat transfer in the semicrystalline polymer nanofibers was time-dependent. The thermal conductivity of the nanofibers could be modulated to span 3 orders of magnitude from being nearly insulated (∼0.27 ± 0.02 W m-1 K-1) to being highly thermal conductive after annealing (∼59.1 ± 3.1 W m-1 K-1). The time-dependent thermal conductivity was observed at a temperature lower than the gamma transition temperature of the polymer and was a result of the physical aging of the semicrystalline polymer. A phenomenological model was adopted to explain the time-dependent heat transfer of the semicrystalline nanofibers. The physical aging reduced the configuration disorder in the polymer and caused the heat transfer of the semicrystalline polymer to increase during the annealing process.

Ultrafast Diameter-Dependent Water Evaporation from Nanopores.

Evaporation from nanopores plays an important role in various natural and industrial processes that require efficient heat and mass transfer. The ultimate performance of nanopore-evaporation-based processes is dictated by evaporation kinetics at the liquid-vapor interface, which has yet to be experimentally studied down to the single nanopore level. Here we report unambiguous measurements of kinetically limited intense evaporation from individual hydrophilic nanopores with both hydrophilic and hydrophobic top outer surfaces at 22 °C using nanochannel-connected nanopore devices. Our results show that the evaporation fluxes of nanopores with hydrophilic outer surfaces show a strong diameter dependence with an exponent of nearly -1.5, reaching up to 11-fold of the maximum theoretical predication provided by the classical Hertz-Knudsen relation at a pore diameter of 27 nm. Differently, the evaporation fluxes of nanopores with hydrophobic outer surfaces show a different diameter dependence with an exponent of -0.66, achieving 66% of the maximum theoretical predication at a pore diameter of 28 nm. We discover that the ultrafast diameter-dependent evaporation from nanopores with hydrophilic outer surfaces mainly stems from evaporating water thin films outside of the nanopores. In contrast, the diameter-dependent evaporation from nanopores with hydrophobic outer surfaces is governed by evaporation kinetics inside the nanopores, which indicates that the evaporation coefficient varies in different nanoscale confinements, possibly due to surface-charge-induced concentration changes of hydronium ions. This study enhances our understanding of evaporation at the nanoscale and demonstrates great potential of evaporation from nanopores.

Elongated Bouncing and Reduced Contact Time of a Drop in the Janus State.

Fast drop bouncing is desired in numerous applications. However, it has never been realized on a superheated surface with concurrent contact boiling and the Leidenfrost effect (the so-called Janus thermal state). This is presumably because of the increased drop adhesion as a result of bubbling on a Janus surface. Nevertheless, on a hydrophilic SiN x surface with v-shaped Si microgrooves in this thermal state, an elongated bouncing for a water drop was observed and a small drop contact time of 10.36 ms was obtained. We propose that the elongated bouncing was originated from an asymmetric momentum force created by the vapor bubbles at the rim of the drop. Analytical expressions for the contact time and the asymmetric momentum force were established and the predictions qualitatively agreed with the experimental results. The obtained contact time was the lowest value that has ever been reported in the Janus thermal state. The proposed approach has promising applications in state-of-the-art technologies requiring high liquid mobility.

Control of Ice Formation.

Ice formation is a catastrophic problem affecting our daily life in a number of ways. At present, deicing methods are costly, inefficient, and environmentally unfriendly. Recently, the use of superhydrophobic surfaces has been suggested as a potential passive anti-icing method. However, no surface is able to repel frost formation at a very cold temperature. In this work, we demonstrated the abilities of spatial control of ice formation and confinement of the ice-stacking direction. The control and confinement were achieved by manipulating the local free energy barrier for frosting. The V-shaped microgroove patterned surface, which possessed these abilities, exhibited the best anti-icing and deicing performances among the studied surfaces. The insight of this study can be applied to alleviate the impact of icing on our daily life and in many industrial systems.

Silicon Nanowires for Solar Thermal Energy Harvesting: an Experimental Evaluation on the Trade-off Effects of the Spectral Optical Properties.

Silicon nanowire possesses great potential as the material for renewable energy harvesting and conversion. The significantly reduced spectral reflectivity of silicon nanowire to visible light makes it even more attractive in solar energy applications. However, the benefit of its use for solar thermal energy harvesting remains to be investigated and has so far not been clearly reported. The purpose of this study is to provide practical information and insight into the performance of silicon nanowires in solar thermal energy conversion systems. Spectral hemispherical reflectivity and transmissivity of the black silicon nanowire array on silicon wafer substrate were measured. It was observed that the reflectivity is lower in the visible range but higher in the infrared range compared to the plain silicon wafer. A drying experiment and a theoretical calculation were carried out to directly evaluate the effects of the trade-off between scattering properties at different wavelengths. It is clearly seen that silicon nanowires can improve the solar thermal energy harnessing. The results showed that a 17.8 % increase in the harvest and utilization of solar thermal energy could be achieved using a silicon nanowire array on silicon substrate as compared to that obtained with a plain silicon wafer.

Scale effect on dropwise condensation on superhydrophobic surfaces.

Micro/nano (two-tier) structures are often employed to achieve superhydrophobicity. In condensation, utilizing such a surface is not necessarily advantageous because the macroscopically observed Cassie droplets are usually in fact partial Wenzel in condensation. The increase in contact angle through introducing microstructures on such two-tier roughened surfaces may result in an increase in droplet departure diameter and consequently deteriorate the performance. In the meantime, nanostructure roughened surfaces could potentially yield efficient shedding of liquid droplets, whereas microstructures roughened surfaces often lead to highly pinned Wenzel droplets. To attain efficient shedding of liquid droplets in condensation on a superhydrophobic surface, a Bond number (a dimensionless number for appraising dropwise condensation) and a solid-liquid fraction smaller than 0.1 and 0.3, respectively, are suggested.

A solar-thermal energy harvesting scheme: enhanced heat capacity of molten HITEC salt mixed with Sn/SiO(x) core-shell nanoparticles.

We demonstrated enhanced solar-thermal storage by releasing the latent heat of Sn/SiO(x) core-shell nanoparticles (NPs) embedded in a eutectic salt. The microstructures and chemical compositions of Sn/SiO(x) core-shell NPs were characterized. In situ heating XRD provides dynamic crystalline information about the Sn/SiO(x) core-shell NPs during cyclic heating processes. The latent heat of ∼29 J g(-1) for Sn/SiO(x) core-shell NPs was measured, and 30% enhanced heat capacity was achieved from 1.57 to 2.03 J g(-1) K(-1) for the HITEC solar salt without and with, respectively, a mixture of 5% Sn/SiO(x) core-shell NPs. In addition, an endurance cycle test was performed to prove a stable operation in practical applications. The approach provides a method to enhance energy storage in solar-thermal power plants.

Specific heat capacity of molten salt-based alumina nanofluid.

There is no consensus on the effect of nanoparticle (NP) addition on the specific heat capacity (SHC) of fluids. In addition, the predictions from the existing model have a large discrepancy from the measured SHCs in nanofluids. We show that the SHC of the molten salt-based alumina nanofluid decreases with reducing particle size and increasing particle concentration. The NP size-dependent SHC is resulted from an augmentation of the nanolayer effect as particle size reduces. A model considering the nanolayer effect which supports the experimental results was proposed.

Thermodynamic investigations using molecular dynamics simulations with potential of mean force calculations for cardiotoxin protein adsorption on mixed self-assembled monolayers.

Understanding protein adsorption onto solid surfaces is of critical importance in the field of bioengineering, especially for applications such as medical implants, diagnostic biosensors, drug delivery systems, and tissue engineering. This study proposed the use of molecular dynamics simulations with potential of mean force (PMF) calculations to identify and characterize the mechanisms of adsorption of a protein molecule on a designed surface. A set of model systems consisting of a cardiotoxin (CTX) protein and mixed self-assembled monolayer (SAM) surfaces were used as examples. The set of mixed SAM surfaces with varying topographies were created by mixing alkanethiol chains of different lengths. The results revealed that CTX proteins underwent similar conformal changes upon adsorption onto the various mixed SAMs but showed distinctive characteristics in free energy profiles. Enhancement of the adsorption affinity, i.e., the change in free energy of adsorption, for mixed SAMs was demonstrated by using atomic force microscopic measurements. A component analysis conducted to quantify the physical mechanisms that promoted CTX adsorption revealed contributions from both SAMs and the solvent. Further component analyses of thermodynamic properties, such as the free energy, enthalpy, and entropy, indicated that the contribution from SAMs was driven by enthalpy, and the contribution from the solvent was driven by entropy. The results indicated that CTX adsorption was an entropy-driven process, and the entropic component from the solvent, i.e., the hydrophobic interaction, was the major driving force for CTX adsorption onto SAMs. The study also concluded that the surfaces composed of mixtures of SAMs with different chain lengths promoted the adsorption of CTX protein.

Evaporation-induced cavitation in nanofluidic channels.

Cavitation, known as the formation of vapor bubbles when liquids are under tension, is of great interest both in condensed matter science as well as in diverse applications such as botany, hydraulic engineering, and medicine. Although widely studied in bulk and microscale-confined liquids, cavitation in the nanoscale is generally believed to be energetically unfavorable and has never been experimentally demonstrated. Here we report evaporation-induced cavitation in water-filled hydrophilic nanochannels under enormous negative pressures up to -7 MPa. As opposed to receding menisci observed in microchannel evaporation, the menisci in nanochannels are pinned at the entrance while vapor bubbles form and expand inside. Evaporation in the channels is found to be aided by advective liquid transport, which leads to an evaporation rate that is an order of magnitude higher than that governed by Fickian vapor diffusion in macro- and microscale evaporation. The vapor bubbles also exhibit unusual motion as well as translational stability and symmetry, which occur because of a balance between two competing mass fluxes driven by thermocapillarity and evaporation. Our studies expand our understanding of cavitation and provide new insights for phase-change phenomena at the nanoscale.

Nanowires for enhanced boiling heat transfer.

Boiling is a common mechanism for liquid-vapor phase transition and is widely exploited in power generation and refrigeration devices and systems. The efficacy of boiling heat transfer is characterized by two parameters: (a) heat transfer coefficient (HTC) or the thermal conductance; (b) the critical heat flux (CHF) limit that demarcates the transition from high HTC to very low HTC. While increasing the CHF and the HTC has significant impact on system-level energy efficiency, safety, and cost, their values for water and other heat transfer fluids have essentially remained unchanged for many decades. Here we report that the high surface tension forces offered by liquids in nanowire arrays made of Si and Cu can be exploited to increase both the CHF and the HTC by more than 100%.

Enhanced thermal conductivity of nanofluids diagnosis by molecular dynamics simulations.

Molecular Dynamics simulations are performed to calculate the thermal conductivity of nanofluids, and to understand the fundamental physics of the enhancement of thermal conductivity observed in experiments. Based on the analysis, intermolecular interactions between copper-copper atoms, layer structure surrounding nanoparticles, convection effect induced by the Brownian motion of copper atoms, as well as particle-particle interactions are identified and confirmed on the enhancement using Green-Kubo method in thermal conductivity.

Design of passive mixers utilizing microfluidic self-circulation in the mixing chamber.

This paper proposes the design of a passive micromixer that utilizes the self-circulation of the fluid in the mixing chamber for applications in the Micro Total Analysis Systems (microTAS). The micromixer with a total volume of about 20 microL and consisting of an inlet port, a circular mixing chamber and an outlet port was designed. The device was actuated by a pneumatic pump to induce self-circulation of the fluid. The self-circulation phenomenon in the micromixer was predicted by the computational simulation of the microfluidic dynamics. Flow visualization with fluorescence tracer was used to verify the numerical simulations and indicated that the simulated and the experimental results were in good agreement. Besides, an index for quantifying the mixing performance was employed to compare different situations and to demonstrate the advantages of the self-circulation mixer. The mixing efficiencies in the mixer under different Reynolds numbers (Re) were evaluated numerically. The numerical results revealed that the mixing efficiency of the mixer with self-circulation was 1.7 to 2 times higher than that of the straight channel without a mixing chamber at Re= 150. When Re was as low as 50, the mixing efficiency of the mixer with self-circulation in the mixing chamber was improved approximately 30% higher than that in the straight channel. The results indicated that the self-circulation in the mixer could enhance the mixing even at low Re. The features of simple mixing method and fabrication process make this micromixer ideally suitable for microTAS applications.