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Interfacial chemical dynamics on a droplet generate various self-propelled motions. For example, ballistic and random motions arise depending on the physicochemical conditions inside the droplet and its environment. In this study, we focus on the relationship between oxidant concentrations in an aqueous droplet and its mode of self-propelled motion in an oil phase including surfactant. We demonstrated that the chemical conditions inside self-propelled aqueous droplets were changed systematically, indicating that random motion appeared at higher concentrations of oxidants, which were H2SO4 and BrO3-, and ballistic motion at lower concentrations. In addition, spontaneous mode switching from ballistic to random motion was successfully demonstrated by adding malonic acid, wherein the initially observed reduced state of the aqueous solution suddenly changed to the oxidized state. Although we only observed one-time transition and have not yet succeeded to realize alternation between ballistic (reduced state) and random motion (oxidized state), such spontaneous transitions are fundamental steps in realizing artificial cells and understanding the fundamental mechanisms of life-like behavior, such as bacterial chemotaxis originating from periodical run-and-tumble motion.

A five day activity that involves the construction and characterization of a small car whose motion is controlled by the vitamin C clock reaction is presented. The purpose of the activity is to engage first-year Science, Technology, Engineering, and Mathematics (STEM) students not majoring in the chemical sciences. Throughout the activity students learn and put into practice basic concepts of electronics and chemical kinetics, while testing four different types of electric power devices: galvanic cells, thermoelectrical cells, fuel cells, and solar panels. They also learn to analyze the vitamin C clock reaction with a simple kinetic model to calculate the time elapsed for a color transition from transparent to dark. A target-distance competition takes place on the last day of the activity.

Chemical gardens and clock reactions are two of the best-known demonstration reactions in chemistry. Until now these have been separate categories. We have discovered that a chemical garden confined to two dimensions is a clock reaction involving a phase change, so that after a reproducible and controllable induction period it explodes., J.H.E.C. and C.I.S.D. acknowledge the financial support of the Spanish MINCINN projects FIS2016-77692-C2-2-P and PCIN-2017-098. The authors acknowledge the contribution of the COST Action CA17120.

Quantitative demonstration of chemical kinetics often requires sophisticated equipment and materials. By using inexpensive and commonly available chemicals, we propose here two clock reactions, oxygen–safranin–benzoin clock and cysteine–iodine–hydrogen peroxide clock. Students simply measure and add stock solutions to start clock reactions. Sudden discoloration or coloration of the reaction mixture marks the end of the reaction, and this can be conveniently timed. Students then complete calculations and graphing in a spreadsheet file by following common protocols. Rates of reaction are calculated from the ratio of [limiting reactant]0 and clock time. Orders of reactions and rate constants are determined by the method of initial rate. Additional discussions on topics such as catalysis, reaction mechanism, redox reaction, and pseudo-zeroth-order reaction are also possible.

The thiourea dioxide-iodate reaction has been reinvestigated spectrophotometrically under acidic conditions using phosphoric acid-dihydrogen phosphate buffer within the pH range of 1.1-1.8 at 1.0 M ionic strength adjusted by sodium perchlorate and at 25 °C. The system was found to exhibit clock behavior, having a well-defined and reproducible time lag called Landolt time, though elementary iodine may even be detected in substrate excess; hence, under these conditions, the reaction can be classified as an autocatalysis-driven clock reaction. It is clearly demonstrated that the previously proposed kinetic model suffers from serious drawbacks from both theoretical and experimental points of view. The reaction may be characterized by either sigmoidal-shaped or rise-and-fall kinetic traces, depending on the initial concentration ratio of the reactants. Iodide significantly accelerates the appearance of the clock species iodine acting therefore as an autocatalyst. The age of stock TDO solution also has a great, so far completely overlooked impact on the Landolt time. On the basis of evaluating simultaneously the kinetic curves, a 16 step kinetic model including 5 well-known rapidly established equilibria is proposed with 7 fitted rate coefficients in which the rate coefficients of both forms of TDO were determined.

We report here elaboration of a cascade strategy for naphthyne formation using Kobayashi benzynes as diynophiles in a subsequent in situ hexadehydro-Diels-Alder reaction. Density functional theory computations suggest that the strained benzynes act as "super-diynophiles" in this transformation. The reaction requires only mildly basic and ambient temperature conditions and allows for rapid construction of various naphthalenic products. The trapping efficiencies of several arynophiles were determined using the benzyne to naphthyne cyclization as an internal clock reaction.

Programming supramolecular assembly in the time domain is a fundamental aspect of the design of biomimetic materials. We achieved the time-controlled sol–gel transition of a poly(vinyl alcohol)–iodine supramolecular complex by generating iodine in situ with a clock reaction. We demonstrate that both the gelation time and the mechanical properties of the resulting hydrogel can be tuned by properly selecting the clock parameters or through competitive iodine complexation., Soft Matter, 17 (5), ISSN:1744-683X, ISSN:1744-6848

The urea-urease clock reaction is a pH switch from acid to basic that can turn into a pH oscillator if it occurs inside a suitable open reactor. We study the confinement of the reaction to lipid vesicles, which permit the exchange with an external reservoir by differential transport, enabling the recovery of the pH level and yielding a constant supply of urea molecules.For microscopically small vesicles, the discreteness of the number of molecules requires a stochastic treatment of the reaction dynamics. Our analysis shows that intrinsic noise induces a significant statistical variation of the oscillation period, which increases as the vesicles become smaller. The mean period, however, is found to be remarkably robust for vesicle sizes down to approximately 200 nm. The observed oscillations are explained as a canard-like limit cycle that differs from the wide class of conventional feedback oscillators., Comment: 11 pages, 4 figures with 2 pages of Supporting info; published in the Journal of Physical Chemistry Letters

The bell-shaped reactivity-pH curve is the fundamental reason that the temporal programmable kinetic switch in clock reactions can be obtained in bio-competitive enzymatic reactions. In this work, urease was loaded on small resin particles through ionic binding. Experimental results reveal that the immobilization not only increased the stability of the enzyme and the reproducibility of the clock reaction, but also shifted the bell-shaped activity curve to lower pHs. The latter change enables the clock reaction to occur from an initial pH of 2.3, where the free enzyme had already lost its activity. Two mechanisms explain the influence of the immobilization on the clock reaction. Immobilization modified the pH sensitive functional groups on the enzyme, shifting the activity curve to a more acidic region, and reduced diffusion alters the enzyme dynamics.

Herein we present a modified iodine clock experiment which replaces starch with cellulose paper. This provides the reaction with a white solid surface in which color change can be clearly observed and reduces reagent amounts required to 540 μL per group. After data acquisition, students are required to calculate reaction orders and the reaction constant k, culminating in their ability to discern the rate equation for this reaction. This experiment is ideal for teaching kinetics in high school and undergraduate settings, particularly if liquid waste disposal is not readily available or if high importance is placed upon minimizing their environmental impact.

We report a visual chronometric method for the detection of copper(II) ions (Cu2+) using the clock reaction between methylene blue (MB) and hydrazine (N2H4). The addition of Cu2+ greatly accelerates the clock reaction due to the formation of the Cu cluster between Cu2+ and N2H4. By recording the change of the reaction time and solution color, different concentrations of Cu2+ in buffer and environmental water have been detected. As low as 300 nM Cu2+ can be visually identified by the naked eye without the assistance of any advanced instruments. This visual chronometric assay shows a dynamic range from 0.2 to 16 μM with a limit of detection of 20 nM. Additionally, it has good selectivity toward Cu2+ and against other common metal cations.

Designing a simple, inexpensive, and sensitive colorimetric sensor for the detection of toxic species in drinking water is of great importance. In this paper, we have demonstrated a simple and sensitive colorimetric assay for naked-eye detection of mercury ions (Hg2+) in water based on gold nanoparticles-catalyzed clock reaction. Citrate-capped Au nanoparticles (AuNPs) were used to catalyze the reduction of methylene blue (MB) to leucomethylene blue (LMB) by hydrazine. However, Hg2+ ions can reduce the catalytic activity of AuNPs in this clock reaction, resulting in the longer reaction time for color transformation of MB from blue to colorless LMB solution. Based on this principle, we have developed an ultrasensitive colorimetric assay that can selectively detect Hg2+ in the concentration range of 5 to 1000 nM with a limit of detection (LOD) of 4.3 nM, which is well below the maximum allowable concentration for Hg2+ in drinking water (10 nM) as defined by the United States Environmental Protection Agency. Owing to the rapidity and simplicity of the naked eye detection process, we expect that this AuNPs-catalyzed clock reaction could put forward an insight for the detection of toxic Hg2+ ions in real water samples, opening up new avenues for developing cost-effective and portable nanosensors for environmental applications.

Chemical clocks usually achieve well-defined temporal delays through concentration thresholding coupled to the production, degradation, activation, or inhibition of downstream effectors. In this way, the stochastic dynamics of many individual molecules yield essentially deterministic bulk behavior through ensemble averaging. As a result, their temporal evolution is governed by ensemble dynamics rather than by the behavior of an individual molecule or complex. Here, we present a general approach for the design of single-molecule clocks that permits quasi-deterministic control over the lifetime of single molecular interactions without any external synchronization. By coupling the dissociation of a bimolecular complex to a series of irreversible chemical steps, we interpose a well-defined time delay between binding and dissociation. The number and speed of irreversible steps can be varied to systematically tune both the lifetimes of complexes and the precision of the time delay, raising the prospect of localized timekeeping in nanoscale systems and devices.

Feedback driven by enzyme catalyzed reactions occurs widely in biology and has been well characterized in single celled organisms such as yeast. There are still few examples of robust enzyme oscillators in vitro that might be used to study nonlinear dynamical behavior. One of the simplest is the urea–urease reaction that displays autocatalysis driven by the increase in pH accompanying the production of ammonia. A clock reaction was obtained from low to high pH in batch reactor and bistability and oscillations were reported in a continuous flow rector. However, the oscillations were found to be irreproducible and one contributing factor may be the lack of stability of the enzyme in solution at room temperature. Here, we investigated the effect of immobilizing urease in thiol-poly(ethylene glycol) acrylate (PEGDA) hydrogel beads, prepared using emulsion polymerization, on the urea–urease reaction. The resultant mm-sized beads were found to reproduce the pH clock and, under the conditions employed here, the stability of the enzyme was increased from hours to days.

A visual chronometric assay for glutathione (GSH) and cysteine (Cy) is reported based on a gold nanoparticle (AuNP)-catalyzed clock reaction of methylene blue and hydrazine. The blue MB is reduced ...

We report a visual chronometric assay for Cr3+ based on the Cu2O nanocube-mediated clock reaction of methylene blue (MB) and N2H4. MB with a blue color can be easily reduced by excess N2H4 to form colorless leuco-MB in the presence of Cu2O nanocubes as a catalyst in a short time. However, the addition of Cr3+ decreases the catalytic activity of Cu2O nanocubes owing to the coordination interactions between them, leading to an increase in the reaction time. The reaction time change was employed for selective detection of Cr3+. This assay has a wide dynamic range from 0.03 to 600 μM with a limit of detection of 2.7 nM. Remarkably, different concentrations of Cr3+ can be directly detected with the naked eye by observing the color-fading speed. Additionally, this assay was successfully used to determine Cr3+ in real water samples.

The thiourea dioxide-periodate reaction has been investigated under acidic conditions using phosphate buffer within the pH range of 1.1-2.0 at 1.0 M ionic strength adjusted by sodium perchlorate. Absorbance-time series are monitored as a function of time at 468 nm, the isosbestic point of the I2-I3- system. The profile of these kinetic runs follows either sigmoidal-shaped or rise-and-fall traces depending on the initial concentration ratio of the reactants. The clock species iodine appears after a well-defined but reproducible time lag even in substrate excess, meaning that the system may be classified as an autocatalysis-driven clock reaction. It is also demonstrated that the age of the thiourea dioxide solution markedly shortens the Landolt time, suggesting that the original form of thiourea dioxide (TDO) rearranges into a more reactive form and reacts faster than the original one. The behavior found is consistent with that recently observed in other oxidation reactions of TDO. To characterize the system quantitatively, a 22-step kinetic model is constructed from adapting the kinetic model of the TDO-iodate reaction published recently by supplementing it with six different reactions of periodate. By the help of seven fitted rate coefficients a sound agreement between the measured and calculated absorbance-time traces is obtained.

Acid generators, such as cyclic esters, have recently emerged as powerful modulators of clock reactions. Here, we show that a transient pH burst emerges when δ-gluconolactone is coupled to the chlorite-iodide reaction. The timing and strength of the bursts depend on the concentration of the cyclic ester and can be further modulated by sequestration of iodine by tetrabutylammonium cations or α-cyclodextrin. These findings could be useful for the design of new, transient pH- and complexation-driven self-assembly systems, as well as for analytical purposes.

We demonstrate an analytical approach to monitor the effect of pH on protein tertiary structure. An autocatalytic enzyme reaction is used to stimulate refolding of proteins during real-time analysis. The method takes advantage of a nonlinear pH ramp generated by the urea-urease clock reaction. In this study, alterations to the structures of model proteins were monitored by mass spectrometry (charge pattern shift) and fluorometry (tryptophan fluorescence quenching). The pH measurements were conducted at different points of the sample flow line by different methods to minimize artifacts. Interestingly, different protein ions (corresponding to native and unfolded proteins) show distinct temporal mass spectral profiles, which reveal gradual refolding and concomitant deprotonation of higher charge state ions in the course of the clock reaction. Every multiply charged ion of a protein is characterized with its own "clock time". This approach does not require major modification of standard instrumentation. It enables determination of "high sensitivity" pH intervals for small and large molecules within a single experiment. Thus, it can be useful for characterizing the protein folding in response to pH change.

Uniquely, the circadian clock of cyanobacteria can be reconstructed outside the complex milieu of live cells, greatly simplifying the investigation of a functioning biological chronometer. The core oscillator component is composed of only three proteins, KaiA, KaiB, and KaiC, and together with ATP they undergo waves of assembly and disassembly that drive phosphorylation rhythms in KaiC. Typically, the time points of these reactions are analyzed ex post facto by denaturing polyacrylamide gel electrophoresis, because this technique resolves the different states of phosphorylation of KaiC. Here, we describe a more sensitive method that allows real-time monitoring of the clock reaction. By labeling one of the clock proteins with a fluorophore, in this case KaiB, the in vitro clock reaction can be monitored by fluorescence anisotropy on the minutes time scale for weeks.

The thiourea dioxide (TDO)-bromate reaction has been reinvestigated spectrophotometrically under acidic conditions using phosphoric acid-dihydrogen-phosphate buffer within the pH range of 1.1-1.8 at 1.0 M ionic strength adjusted by sodium perchlorate and at 25 °C. The title system shows a remarkable resemblance to the classical Landolt reaction, namely, the clock species (bromine) may only appear after the substrate TDO is completely consumed. Thus, the title system can be classified as substrate-depletive clock reaction. Despite the well-known slow rearrangement characteristic of TDO in acidic solution, it is surprisingly found that the Landolt time of the title reaction does not depend at all on the age of TDO solution applied. It is, however, shown experimentally that the inverse of Landolt time linearly depends on the initial bromate concentration as well as on the square of the hydrogen ion concentration. In addition to this, it is also noticed that dihydrogen phosphate markedly affects the Landolt time as well, and this feature may easily be taken into consideration by the H2PO4- dependence of the rate of bromate-bromide reaction quantitatively. Based on the experiments, a simple three-step kinetic model is proposed from which a complex formula is derived to indicate the exact concentration dependence of the Landolt time.

A pH-changing clock chemical system, also known to induce changes in viscosity, is shown experimentally to induce a viscous fingering instability during the displacement of reactive solutions in a Hele-Shaw cell. Specifically, a low-viscosity solution of formaldehyde is displaced by a more viscous solution of sulfite and of a pH-sensitive poly(acrylic acid) polymer. The pH change triggered by the formaldehyde-sulfite clock reaction in the reactive contact zone between the two solutions affects the polymer and induces a local increase of the viscosity that destabilizes the displacement via a viscous fingering instability. The influence of changes in the chemical parameters on this fingering instability is analyzed using different techniques and the results are compared with numerical simulations., SCOPUS: ar.j, info:eu-repo/semantics/published

It is reported that iodine volatilization can occur in any elemental analysis of total iodine by ICP-MS. This problem affects the accuracy of the results, and it has been neither rationalized nor solved up to now. In this work, the formation of volatile iodine compounds in concentrated acid solutions (nitric acid or aqua regia) under microwave heating was studied by UV–Vis spectrophotometry, linear sweep voltammetry, and cyclic voltammetry. It was evidenced that molecular iodine (I2) can unexpectedly form in concentrated hot HNO3 solutions, irrespective of the starting iodine compound (iodide, iodate, periodate, 3-iodo- l -tyrosine, 3,5-diiodo- l -tyrosine dehydrate). I2 is produced by the nitrogen oxides existing in these conditions. The formation of volatile iodine is minimized in aqua regia, as chloride is able to keep iodine in solution due to the formation of charged chloro-iodo complexes (e.g. I2Cl−). The dilution of the concentrated acid solution, required prior to the ICP-MS analysis, causes the disruption of I2Cl− so that I2 can again volatilize. To avoid this, 0.1 M thiocyanate can be added, as it forms a strong I2SCN− complex which keeps I2 in solution as ion. Also other iodine species possibly occurring in the explored conditions, iodide and iodate, were demonstrated to be converted to I2SCN− in diluted HNO3 solutions and in the presence of 0.1 M thiocyanate. Recovery tests demonstrated that iodine volatilization is minimized if samples containing iodine are treated in aqua regia and, after dilution, they are added with 0.1 M thiocyanate.

Dynamic self-assembly is of great interest in the fields of chemistry, physics and materials science and provides a flexible bottom-up approach to build assemblies at multiscale levels. We propose a method to control the time domain of self-assembling systems in a closed system, from molecular to material level using a driving chemical system: methylene glycol-sulfite pH clock reaction coupled to lactone hydrolysis. The time domain of the transient pH state (alkaline) and the time lag between the initialization of the reaction and the pH change can be efficiently fine-tuned by the initial concentration of the reagents and by the chemical composition of the lactone. The self-assembly of pH-responsive building blocks can be dynamically driven by this kinetic system, in which the time course of the pH change is coded in the system. This approach provides a flexible and autonomous way to control the self-assembly of pH responsive building blocks in closed chemical systems far from their thermodynamic equilibrium.

Mixed-valent iron–iron oxide nanoparticles were synthesized by the reduction of Fe2+ to Fe0 followed by aerial oxidation, and were fully characterized using powder XRD, FE-SEM, EDX, and XPS. A clock reaction of organic dyes (methylene blue, nile blue and crystal violet) was performed with iron–iron oxide nanoparticles, where periodic colour changes were observed. The decolourization of the dyes was very fast (within 7–12 min), whereas colour regeneration took a relatively longer time (up to 90 min). Furthermore, this clock reaction was found to be dependent on light and atmospheric oxygen. Light speeds up the generation of electron–hole pairs that are responsible for the reduction of the dyes, whereas oxygen is required for the colour regeneration.

Clock reactions allow precise control of chemical composition in the time domain. Such nonlinear chemical systems have recently been introduced to mimic the self-assembly pathways common in living organisms. Here, we demonstrate the use of a clock reaction to trigger the formation of polymeric nanoparticles. By adjusting the delay of a formaldehyde clock reaction, we controlled the precipitation of chitosan to form particles with sizes tunable in a wide range (from about 200 to 600 nm diameter). The chemical structure of chitosan was not significantly perturbed by the clock reagents.

In order to practice green chemistry, a simple and safe spectrophotometric method for iodide determination has been successfully developed based on the formation of a blue starch-iodine complex . Iodide was oxidized to form iodine prior to the addition of a starch solution, and the blue s tarch-iodine complex was directly detected spectrophoto-metrically at a wavelength of 615 nm. The chemical parameters, such as type, reaction time, as well as concentration of oxidizing agents and solution pH were optimized with respect to sensitivity and analysis time. The method showed optimum results under iodate oxidant with a mol ratio of IO 3 - :I - =1:3, reaction time of 5 minutes, and solution pH of 5. Under the se optimum conditions, the method showed linierity measurements from 5-4 0 mg/L iodide with a correlation ( R 2 ) of 0.9 889 . This technique offers a simple, safe, accurate, and relatively fast method for iodide determination, which is prospective for monitoring iodide samples.

Controlling the composition and crystal phase is an important issue to tune material physical/chemical properties. Herein, it was found that triphenyl phosphine (TPP) can be used as a phase transfer agent to transform CuS, Cu39S28 phases into pure low-sulfur Cu1.8S phase. When mixed phase copper sulfides were reacted with triphenyl phosphine under suitable temperature, sulfur was extracted to produce the low-sulfur Cu1.8S phase. It was also demonstrated that the Cu–S product can effectively catalyze a clock reaction between methylene blue and hydrazine in aqueous medium. In addition, the photothermal conversion properties of the Cu–S based products were studied. The results show that the purified Cu1.8S materials show enhanced or similar properties than the original mixed-phase Cu–S products.

Synergetic chemomechanical oscillators represent a fundamentally new class of oscillators, where a clock reaction, owning no oscillatory chemical kinetics, generates shrinking-swelling cycles in a chemoresponsive gel under appropriate fixed nonequilibrium boundary conditions. Sufficiently large size-changes are a condition for continually switching between a reacted and an unreacted chemical state in the gel through sufficiently large differences in the diffusion time between the environment and the core of the gel. Two former experimental demonstrations with acid autocatalytic reactions were frustrated either by complex behaviors (chlorite-tetrathionate system) or by side reactions with the gel matrix (bromate-sulfite system). With the Landolt (iodate-sulfite) reaction, regular large-amplitude chemomechanical oscillations can be sustained for more than a week. This enabled a fine study of the temperature and stoichiometry range of operation. I have identified several key steps that are experimentally essential to the systematic design of further synergetic oscillators. The robust realization of this type of self-organization in artificial systems is currently unique.

Using cyanine dye-coupling 2-pyridylmethyl chelating unit as fluorescent probe, we successfully developed an indicator of copper clock reaction to monitor the process of cysteine (Cys) oxidation. During the monitoring assay, the fluorescent changes reflected three sequential reaction and binding behaviors including oxidation of Cys, oxidation of Cu(I) and binding between Cu(II) and the oxidation product cystine (Cys-Cys). Because the different kinetic trajectories were obtained when used Cys, GSH and Hcy as the substrates, respectively, a new strategy for distinguishing Cys from GSH and Hcy was further developed using the cyanine dye-coupling Cu(II) complex.

Copper nanowires (CuNWs) with a high aspect ratio of ~2600 have been successfully synthesized by using a facile hydrothermal method. The reductions of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) and methylene blue (MB) to leucomethylene blue (LMB) by using sodium borohydride (NaBH4) were used as models to test the catalytic activity of CuNWs. We showed that by increasing the CuNWs content, the rate of reduction increased as well. The CuNWs showed an excellent catalytic performance where 99% reduction of 4-NP to 4-AP occurred in just 60 s by using only 0.1 pg of CuNWs after treatment with glacial acetic acid (GAA). The rate constant (kapp) and activity factor (K) of this study is 18 and ~1010 fold in comparison to previous study done with no GAA treatment applied, respectively. The CuNWs showed an outstanding catalytic activity for at least ten consecutive reusability tests with a consistent result in 4-NP reduction. In clock reaction of MB, approximately 99% of reduction of MB into LMB was achieved in ~5 s by using 2 &mu, g CuNWs. Moreover, the addition of NaOH can improve the rate and degree of recolorization of LMB to MB.

An iodine clock is the basis for studying the kinetics of the fast reaction between iodine and ascorbic acid in aqueous solution. UV–visible absorbance, where the molar absorptivity of the triiodide ion is high, is equated with the total concentration of iodine, through the equilibrium, I2 + I– ⇌ I3–, established when the iodine clock is concluding its steady-state titration of ascorbic acid. Persulfate ion is the primary oxidant catalyzed by the iodide ion, which produces the iodine titrant and is recycled. A method testing the second-order rate equation, R1 = k[iodine][ascorbic acid], uses the coefficient of variation, CV. This locates equivalence, enabling [ascorbic acid] to be measured and contributes to a procedure where rate of reaction, R1, at different concentrations of iodine and ascorbic acid determines an average value of the overall rate constant, k. In combination with results from another source, the rate constants of the two individual forms of iodine reacting with ascorbic acid also are de...

Electrochemical processing has already manifested its prominence for obtaining well structured composite materials. The method is highly precise and is reduction potential driven. This methodology has resulted in a hierarchical Au–CuO nanocomposite from a chosen redox transformation reaction between the newly synthesized spherical and roughened Cu2O nanoparticles and HAuCl4. Thus the attractive shape as well as reducing capability of Cu2O made it promising as a starting material. The proposed redox transformation reaction does not need any additional reducing or stabilizing agents. The reduction potential value of CuO/Cu2O (+0.66 V vs. SHE) supports the quantitative reduction of Au(III) ions because of its higher reduction potential (+1.69 V vs. SHE) i.e., for the AuCl4−/Au half cell. The prescribed conditions, spontaneous redox reaction and the morphology of Cu2O nanoparticles help to produce the unique Au–CuO nanoflowers. The formation of Au–CuO nanocomposites from Cu2O nanospheres is characterised by several physical techniques such as XRD, XPS and FTIR. Finally the flower like Au–CuO nanocomposite shows higher SERS activity with 4-aminothiophenol (4-ATP) as a probe molecule than what is evident from the individual components (Au or Cu2O or CuO). Additionally the derived Au–CuO nanocomposite has also been found to be an effective catalyst for the clock reaction employing methylene blue and ascorbic acid in solution.

Ultrathin two-dimensional (2D) nanosheets are a conceptually new category of nanoscale materials. Integration/assembly of individual 2D nanosheets into 3D hierarchical structures is an enormous challenge and an essential requirement for their application. Here we first report the direct synthesis of Cu7S4 hierarchical hollow cubic cages assembled by ultrathin nanosheets based on the Kirkendall effect. Slowly released Cu+ from Cu2O cubic template-crystals and S2− from decomposed thioacetamide (TAA) can react with each other and form a diffusion pair, which provides a thermodynamic and kinetic equilibrium to be responsible for the formation of ultrathin Cu7S4 nanosheets and the Cu7S4 hierarchical hollow cubic cages. Using this unique hollow structure and the outstanding catalytic property of the Cu7S4 nanosheets, as an example, we successfully demonstrate that Cu7S4 nanocages can effectively catalyse the “clock reaction”, which is a periodic cycle redox oscillation reaction between methylene blue (MB) and colorless leucomethylene blue (LMB). The unique hierarchical structure has been found to enhance the rate of this redox reaction via the ultrathin nanocatalyst. This work develops a facile strategy for synthesizing 3D hierarchical structures constructed by ultrathin nanosheets and demonstrates their superior ability to optimize the nanosheet-catalyzed clock reaction.

Clock reactions are rare kinetic phenomena, so far limited mostly to systems with ionic oxoacids and oxoanions in water. We report a new clock reaction in cyclohexanol that forms molybdenum blue from a noncharged, yellow molybdenum complex as precursor, in the presence of hydrogen peroxide. Interestingly, the concomitant color change is reversible, enabling multiple clock cycles to be executed consecutively. The kinetics of the clock reaction were experimentally characterized, and by adding insights from quantum chemical calculations, a plausible reaction mechanism was postulated. Key elementary reaction steps comprise sigmatropic rearrangements with five-membered or bicyclo[3.1.0] transition states. Importantly, numerical kinetic modeling demonstrated the mechanism's ability to reproduce the experimental findings. It also revealed that clock behavior is intimately connected to the sudden exhaustion of hydrogen peroxide. Due to the stoichiometric coproduction of ketone, the reaction bears potential for application in alcohol oxidation catalysis.

The use of the 2-O-(2-trimethylsilylmethallyl) group as intramolecular nucleophile and cation clock reaction in the glucopyranose series depends on the nature of the glycosyl donor. As previously reported, with trichloroacetimidates the anticipated intramolecular Sakurai reaction proceeds efficiently and is an effective clock, whereas with sulfoxides complications arise. The source of these complications is now shown to be an intramolecular sulfenyl transfer reaction between the tethered allylsilane and the activated sulfoxide. These results illustrate how a different unimolecular clock reaction may be required for a given cation when it is generated from different donors in order to avoid side reactions. The synthesis and cyclization of a 2-O-(3-hydroxypropyl) glucopyranosyl sulfoxide leading on activation to the formation of a trans-fused acetal is also described. The formation of this crystallographically-established trans-fused acetal is discussed in terms of the high effective concentration of the intramolecular nucleophile which leads to a high degree of a SN2 character in the displacement of the α-glucosyl triflate or at the level of the corresponding α-CIP. The possible use of such intramolecular alcohols as clock reactions and their limitations is discussed.

In this paper, we show experimentally that inside a microfluidic device, where the reactants are segregated, the reaction rate of an autocatalytic clock reaction is accelerated in comparison to the case where all the reactants are well mixed. We also find that, when mixing is enhanced inside the microfluidic device by introducing obstacles into the flow, the clock reaction becomes slower in comparison to the device where mixing is less efficient. Based on numerical simulations, we show that this effect can be explained by the interplay of nonlinear reaction kinetics (cubic autocatalysis) and differential diffusion, where the autocatalytic species diffuses slower than the substrate.

Truncated Cu2O cubes with well-defined morphology have been successfully synthesized using a stable Cu(II)–EDTA precursor and glucose as the reducing agent in alkaline conditions under 5 min of microwave irradiation. The truncated cubic Cu2O particles as a solid powder were characterized by different physical methods. Curiously enough, upon the addition of Cu2O particles, the blue-colored methylene blue (MB) in aqueous acidic (pH ≈ 1.0) solution successively bleaches to colorless leuco-MB (LMB). The redox reaction generates a colorless solution which easily reverts back to blue in air. The reaction has been quantitatively monitored by UV–vis spectrophotometry. The reversible color change, i.e., oscillation between a blue MB solution and colorless LMB solution happens to be a periodic phenomenon for more than 50 cycles and is reproducibly demonstrated as a simple “clock reaction”. Acidic (pH ≈ 3.0) conditions favor zeroth-order kinetics for the underlying redox phenomena. Dilute H2SO4 has been proven to be...

Two types of gold ores were subjected to iodide/iodine leaching at room temperature. The effects of three factors, including ore type (a carbonaceous and an oxide gold ores), iodide/iodine concentration, and the presence of oxygen in solution on gold leaching performance were investigated. The carbonaceous ore showed only 20% gold extraction, since gold–iodide complexes readily adsorb on organic matter. In contrast, gold extraction from the oxide ore in a solution containing 20 g/L iodide and 4 g/L iodine reached 77% in 6 h and 89% in 24 h. With iodate as the oxidant, no gold could be leached in 48 h. When the iodate was partially converted to iodine by adding HCl to the solution, gold leaching was then started. Iodine reactions with the sulfide and ferrous minerals of the oxide ore slowly consumed iodine within 48 h. Based on the obtained experimental data, a power law rate equation was applied to model the gold extraction kinetics in iodide/iodine solutions. The reaction was found to be first order with respect to tri-iodide concentration.

The thiosulfate-periodate reaction has been studied spectrophotometrically in a slightly acidic medium at 25.0 ± 0.1 °C in an acetate/acetic acid buffer by monitoring the absorbance in the 250-600 nm wavelength range at a constant ionic strength adjusted by the buffer component sodium acetate. In agreement with a previous study, we found that the reaction cannot be described by a single stoichiometric equation, tetrathionate and sulfate are simultaneously formed, and its ratio strongly depends on the pH. As expected at certain initial concentration ratios of the reactants, the reaction behaves as a clock reaction, but after its appearance, iodine is slowly consumed mainly because of the moderate tetrathionate-iodine reaction. It is also enlightened that the initial rate of the reaction is completely independent of the pH, which apparently contradicts a previous study, which postulates a "supercatalytic" behavior of the hydrogen ion on the title reaction. Significant buffer assistance that may change the absorbance-time profiles was also observed. On the basis of the kinetic data, a robust 28-step kinetic model with 22 fitted parameters is proposed and discussed to explain adequately all of the important characteristics of the kinetic curves.

A microscale laboratory for teaching chemical kinetics utilizing the iodine clock reaction is described. Plastic pipets, 3 mL volume, are used to store and deliver precise drops of reagents and the reaction is run in a 24 well plastic tray using a total 60 drops of reagents. With this procedure, students determine the rate of reaction and the order of reaction as a function of iodide ion concentration or hydrogen peroxide concentration. Six reaction concentrations are run concurrently in about an hour. The students also calculate the value of the rate constant and determine the concentration of an unknown iodide ion sample. The procedure has been adopted to run at a lower temperature to determine the energy of activation. The experiment teaches the principles of chemical kinetics to first-year college students and can also be used in second-year or advanced-placement high school chemistry classes.

Alternate solutions to third order rate law expressions which are first order in one reactant and second order in another for the generalized reaction \({{aA}+{bB}\mathop \rightarrow \limits^{k_A }{P}}\) with a limiting reactant and for simple cubic autocatalysis \({{A}+{ 2B}\mathop \rightarrow \limits^{k_A }{3B}}\) are presented and discussed. These solutions are expressed in terms of the Lambert function W[x] with argument x(t) = α exp(α − βt), where α and β are empirical parameters. These expressions permit the concentrations of the reactant species to be explicitly represented as dependent functions of time. For the generalized reaction, the solution involves considering exactly which species is limiting, so that the correct branch of the Lambert function can be determined. For simple cubic autocatalysis, the solution is shown to be consistent with the characteristics associated with clock reactions. An exact expression for the “induction time” associated with a clock reaction described by this mechanism is also derived.

The oxidation reaction of triiodide, I3−, by chlorate is investigated in a slightly acidic and neutral media. The reaction was verified and monitored both potentiometrically and spectrophotometrically. Generally, a slow linear decay preceded by an induction period was observed for the triiodide concentration following the addition of chlorate. The induction period is likely to be related to the time required for the generation of suitable concentrations of plausible intermediates (HIO and HIO2), which are assumed to auto-catalyse the reaction. We examined the effect of acidity and concentrations of both chlorate and triiodide on the induction time for this reaction. The acidity of the medium influenced the induction period, while the oxidation of iodide by chlorate competed with that of iodine as the medium acidity increased, making the reaction more complicated. Therefore, a suitable pH is highly recommended for studying the chlorate–triiodide reaction. A plausible mechanism involving the HIO, HIO2, and I2O species is proposed.

A time-lapse polymerization exhibits a programable timebetween the mixing of the reagents and the onset of poly-merization. In fact, a free-radical polymerization systemcontaining an inhibitor can be thought of this way. Anotherapproach was invented by Norling that consisted of aperoxide or persulfate initiating system using iron or copperwith a special chelating agent and an organic reducingagent.