Design of non-autonomous pH oscillators and the existence of chemical beat phenomenon in a neutralization reaction.

The beat in physical systems is a transparent and well-understood phenomenon. It may occur in forced oscillatory systems and as a result of the interference of two waves of slightly different frequencies. However, in chemical systems, the realization of the latter type of the beat phenomenon has been lacking. Here we show that a periodic titration of acid and alkaline solutions with each other using programmable syringe pumps in a continuous stirred-tank reactor exhibits the beat phenomenon in the temporal pH oscillation pattern if the time periods of sinusoidal inflow rates of the reagents are slightly different. Interestingly, the frequency of the chemical beat pattern follows the well-known relationship from physics, namely the frequency of the beat is equal to the absolute value of the difference of the two wave frequencies. Based on our strategy, we can design and engineer non-autonomous pH oscillatory systems, in which the characteristics of the temporal oscillations (amplitude, time period) can easily and precisely be controlled by the experimental conditions such as the inflow rates and feed concentrations. The demonstrated phenomena can be exploited in practical applications, we use the non-autonomous pH oscillators to drive the reversible assembly and disassembly of pH-sensitive building blocks (oleic acid and gold nanoparticles), both highly relevant in nanotechnology and biomedical applications.

Nanocrystals Assembled by the Chemical Reaction of the Dispersion Solvent.

The development of methods to pattern nanocrystals with different sizes and shapes remains a challenge. In this study, we demonstrate a unique class of bottom-up approaches to assemble nanocrystals into patterns. Our approach for patterning nanocrystals focuses on the utilization and control of the chemical reaction of solvents surrounding nanocrystals. The photopolymerization of solvent molecules through a photomask creates time-dependent concentration gradients of the solvents. Dispersed nanocrystals such as silver nanowires (AgNWs) migrate and are gradually organized and integrated into the polymerizing films based on the concentration gradients. The AgNW-embedded film properties are determined by the organized AgNW structures and include light transmission and electrical conductivity. Overall, the demonstrated method is very simple, widely applicable to various nanocrystals and solvents, and can thus contribute to the development of a new class of nanocrystal patterning methods.

Eco-Friendly, Direct Deposition of Metal Nanoparticles on Graphite for Electrochemical Energy Conversion and Storage.

Simple, green, and energy-efficient methods for preparing electroactive materials used to generate and store renewable energy are important for a sustainable future. In this study, we showed that noble and certain non-noble metal nanoparticles can be deposited on graphite without the aid of any reducing agent. This method of reducing metal ions to metal nanoparticles by graphite involves only one step (i.e., immersion into a solution) and one chemical (i.e., a metal salt). Hence, the method is exceedingly simple, green, and does not require any energy input. Large amounts of metal nanoparticles are generated both on the surface and deep into the bulk of graphite (∼100 µm). Despite the simplicity of this method, the metal deposited on graphite showed good electrocatalytic performance for ethanol oxidation and oxygen evolution reactions and also functioned as electrodes for supercapacitors. This method is thus ideal for preparing electrocatalytic materials and electrochemical energy storage devices due to its simplicity and environmental sustainability. The simplicity of the method is due to the inherent reducing potential of graphite (i.e., a material that is generally perceived as inert). Results from analyses showed that functionalization of the reactive edges in the regions of defects allowed the graphite to serve as a reducing agent. Increasing the amount of defects (e.g., via chemical or simple mechanical treatments) is shown to be the fundamental principle for increasing the reactivity of graphite.

Effects of Nanowire Length on Charge Transport in Vertically Aligned Gold Nanowire Array Electrodes.

In this study, we demonstrate that vertically aligned gold nanowire array electrodes provide rapid ion and electron transport to the electrode-electrolyte interface. The charge-transport properties of the nanowire electrodes were investigated through cyclic voltammetry, galvanostatic charge/discharge measurements, and electrochemical impedance spectroscopy under a constant-volume device configuration. The total charge stored in the corresponding devices increases monotonically with the length of the nanowires owing to the concomitant increase in the electroactive real surface area of the electrode. A remarkable feature of the electrodes is that the internal resistance associated with charge transport decreases with increasing nanowire length. The electric double-layer capacitance per unit electroactive surface area remains constant up to high charge/discharge rates. Our results demonstrate that charge migration occurs rapidly on the surfaces of the nanowires regardless of their length and the charge/discharge rate used. Thus, vertically aligned nanowire array electrodes show promise as current collectors for next-generation electrochemical energy-storage devices.

Interaction of Positively Charged Gold Nanoparticles with Cancer Cells Monitored by an in Situ Label-Free Optical Biosensor and Transmission Electron Microscopy.

Functionalized nanoparticles (NPs) can penetrate into living cells and vesicles, opening up an extensive range of novel directions. For example, NPs are intensively employed in targeted drug delivery and biomedical imaging. However, the real-time kinetics and dynamics of NP-living cell interactions remained uncovered. In this study, we in situ monitored the cellular uptake of gold NPs-functionalized with positively charged alkaline thiol-into surface-adhered cancer cells, by using a high-throughput label-free optical biosensor employing resonant waveguide gratings. The characteristic kinetic curves upon NP exposure of cell-coated biosensor surfaces were recorded and compared to the kinetics of NP adsorption onto bare sensor surfaces. We demonstrated that from the above kinetic information, one can conclude about the interactions between the living cells and the NPs. Real-time biosensor data suggested the cellular uptake of the functionalized NPs by an active process. It was found that positively charged particles penetrate into the cells more effectively than negatively charged control particles, and the optimal size for the cellular uptake of the positively charged particles is around 5 nm. These conclusions were obtained in a cost-effective, fast, and high-throughput manner. The fate of the NPs was further revealed by electron microscopy on NP-exposed and subsequently fixed cells, well confirming the results obtained by the biosensor. Moreover, an ultrastructural study demonstrated the involvement of the endosomal-lysosomal system in the uptake of functionalized NPs and suggested the type of the internalization pathway.

Metal Nanowire-Based Hybrid Electrodes Exhibiting High Charge/Discharge Rates and Long-Lived Electrocatalysis.

Nanostructured electrodes are at the forefront of advanced materials research, and have been studied extensively in the context of their potential applications in energy storage and conversion. Here, we report on the properties of core-shell (gold-polypyrrole) hybrid nanowires and their suitability as electrodes in electrochemical capacitors and as electrocatalysts. In general, the specific capacitance of electrochemical capacitors can be increased by faradaic reactions, but their charge transfer resistance impedes charge transport, decreasing the capacitance with increasing charge/discharge rate. The specific capacitance of the hybrid electrodes is enhanced due to the pseudocapacitance of the polypyrrole shells; moreover, the electrodes operate as an ideal capacitive element and maintain their specific capacitance even at fast charge/discharge rates of 4690 mA/cm3 and 10 V/s. These rates far exceed those of other types of pseudocapacitors, and are even superior to electric double layer-based supercapacitors. The mechanisms behind these fast charge/discharge rates are elucidated by electrochemical impedance spectroscopy, and are ascribed to the reduced internal resistance associated with the fast charge transport ability of the gold nanowire cores, low ionic resistance of the polypyrrole shells, and enhanced electron transport across the nanowire's junctions. Furthermore, the hybrid electrodes show great catalytic activity for ethanol electro-oxidation, comparable to bare gold nanowires, and the surface activity of gold cores is not affected by the polypyrrole coating. The electrodes exhibit improved stability for electrocatalysis during potential cycling. This study demonstrates that the gold-polypyrrole hybrid electrodes can store and deliver charge at fast rates, and that the polypyrrole shells of the nanowires extend the catalytic lifetime of the gold cores.