Stochastic Current Fluctuations Research Articles

Machine Learning Guided Synthesis of Flash Graphene.

Advances in nanoscience have enabled the synthesis of nanomaterials, such as graphene, from low-value or waste materials through flash Joule heating. Though this capability is promising, the complex and entangled variables that govern nanocrystal formation in the Joule heating process remain poorly understood. In this work, machine learning (ML) models are constructed to explore the factors that drive the transformation of amorphous carbon into graphene nanocrystals during flash Joule heating. An XGBoost regression model of crystallinity achieves an r2 score of 0.8051 ± 0.054. Feature importance assays and decision trees extracted from these models reveal key considerations in the selection of starting materials and the role of stochastic current fluctuations in flash Joule heating synthesis. Furthermore, partial dependence analyses demonstrate the importance of charge and current density as predictors of crystallinity, implying a progression from reaction-limited to diffusion-limited kinetics as flash Joule heating parameters change. Finally, a practical application of the ML models is shown by using Bayesian meta-learning algorithms to automatically improve bulk crystallinity over many Joule heating reactions. These results illustrate the power of ML as a tool to analyze complex nanomanufacturing processes and enable the synthesis of 2D crystals with desirable properties by flash Joule heating.

Correlated trapping of sugar molecules by the trimeric protein channel chitoporin

The protein channel chitoporin (ChiP), which is used by marine bacteria to translocate selected sugar molecules through the outer cell membrane, is studied via single channel current measurements in water and heavy water sugar solutions. The dynamic trapping and escape probabilities of sugar molecules from different monomers in the trimeric channel are characterized, including their dependence on channel orientation and sensitivity to a deuterium isotope effect. A detailed analysis of stochastic current fluctuations reveals that the trapping properties of chitoporin exhibit memory effects: the rate of trapping transitions depends on the previous sequence of transitions; and intermonomer correlations: the average trapping rate of an unblocked monomer is larger when its neighboring monomers are blocked. The latter, likely resulting from rapid re-trapping of recently escaped sugar molecules, is considered as a possible design strategy to enhance sugar transport.

Investigation of Filamentary Current Fluctuations Features in the High-Resistance State of Ni/HfO2-Based RRAM

In this paper, the presence of filamentary current instabilities in the high resistance state of Ni/HfO2-based RRAM devices and their associated current fluctuations mechanisms are explored. After systematic measurements, the advanced weighted time-lag plot method is employed to accurately identify the contribution of multiple electrically active defects and to minimize the negative effect of the background noise. Special attention is given to the physical analysis of multilevel random telegraph noise (RTN) signals caused by both independent and interactive defects located in the vicinity of the conductive filament (CF), which may alter the tunneling path and induce large stochastic current fluctuations. In addition, irreversible current changes, owing to trap density variations inside or near the CF, are also identified. Finally, the voltage dependence of the RTN phenomena and the occurrence of stress-induced complex fluctuations is evaluated.

Stochastic Pore Blocking and Gating in PDMS–Glass Nanopores from Vapor–Liquid Phase Transitions

Polydimethylsiloxane (PDMS) is commonly used in research for microfluidic devices and for making elastomeric stamps for soft lithography. Its biocompatibility and nontoxicitiy also allow it to be used in personal care, food, and medical products. Herein we report a phenomenon observed when patch clamp, a technique normally used to study biological ion channels, is performed on both grooved and planar PDMS surfaces, resulting in stochastic current fluctuations that are due to a nanopore being formed at the interface of the PDMS and glass surfaces and being randomly blocked. Deformable pores between 1.9 ± 0.7 and 7.4 ± 2.1 nm in diameter, depending on the calculation method, form upon patching to the surface. Coulter blocking and nanoprecipitation are ruled out, and we instead propose a mechanism of stochastic current fluctuations arising from transitions between vapor and liquid phases, consistent with similar observations and theory from statistical mechanics literature. Interestingly, we find that [Ru(bp...

Charge transport in nanoparticle chains influenced by stabilizer molecules

AbstractWe have bridged a pair of gold electrodes through chains and arrays of gold nanoparticles (NPs), which were stabilized by a coating of citrate molecules. We performed a systematic and comparative analysis of current–voltage (I–V) characteristics for chains of NPs having variable lengths and configurations. Besides stochastic current fluctuations at a constant bias voltage and quasi‐periodic fluctuations of the differential conductance arising from conformational changes of citrate molecules, we also observed that the arrangement and distribution of NPs can be changed by the applied electric field, contributing to conductance fluctuations and leading to irreversible changes and finally rupture of the conducting bridge. Although in all cases gold is bridged by the same citrate molecules, a significantly higher resistance between gold electrodes and the citrate coated gold NPs was found as compared to the resistance between identical NPs. This difference is attributed to the fact that citrate molecules are chemically attached to the NPs, but are only physically interacting with the electrodes. Thus, the resistance of the bridge is not only a function of the number of molecular contacts but also depends on the strength of the individual interactions between metal conductor and molecules.

Dorsal–ventral organization of theta‐like activity intrinsic to entorhinal stellate neurons is mediated by differences in stochastic current fluctuations

The membrane potential dynamics of stellate neurons in layer II of the medial entorhinal cortex are important for neural encoding of location. Previous studies suggest that these neurons generate intrinsic theta-frequency membrane potential oscillations, with a period that depends on neuronal location on the dorsal–ventral axis of themedial entorhinal cortex, and which in behaving animals could support generation of grid-like spatial firing fields. To address the nature and organization of this theta-like activity, we adopt the Lombmethod of least-squares spectral analysis. We demonstrate that peaks in frequency spectra that differ significantly from Gaussian noise do not necessarily imply the existence of a periodic oscillator, but can instead arise from filtered stochastic noise or a stochastic random walk. We show that theta-like membrane potential activity recorded fromstellate neurons in mature brain slices is consistentwith stochastic mechanisms, but not with generation by a periodic oscillator. The dorsal–ventral organization of intrinsic theta-likemembrane potential activity, and themodification of this activity during block of HCN channels, both reflect altered frequency distributions of stochastic spectral peaks, rather than tuning of a periodic oscillator. Our results demonstrate the importance of distinguishing periodic oscillations from stochastic processes.We suggest that dorsal–ventral tuning of theta-like membrane potential activity is due to differences in stochastic current fluctuations resulting from organization of ion channels that also control synaptic integration.

Effects of stochastic channel gating and distribution on the cardiac action potential

Ion channels exhibit stochastic conformational changes determining their gating behavior. In addition, the process of protein turnover leads to a natural variability of the number of membrane and gap junctional channels. Nevertheless, in computational models, these two aspects are scarcely considered and their impacts are largely unknown. We investigated the effects of stochastic current fluctuations and channel distributions on action potential duration (APD), intercellular conduction delays (ICDs) and conduction blocks using a modified ventricular cell model (Rudy et al.) with Markovian formulations of the principal ion currents (to simulate their stochastic open–close gating behavior) and with channel counts drawn from Poisson distributions (to simulate their natural variability). In single cells, APD variability (coefficient of variation: 1.6% at BCL=1000 ms) was essentially caused by stochastic channel gating of I Ks, persistent I Na and I Ca,L. In cell strands, ICD variability induced by stochastic channel gating and Poissonian channel distributions was low under normal conditions. Nonetheless, at low intercellular coupling levels, Poissonian gap junctional channel distribution resulted in a large ICD variability (coefficient of variation >20%), highly heterogeneous conduction patterns and conduction blocks. Therefore, the stochastic behavior of current fluctuations and channel distributions can contribute to the heterogeneity of conduction patterns and to conduction block, as observed previously in experiments in cardiac tissue with altered intercellular coupling.

Effects of Stochastic Channel Gating and Stochastic Channel Distribution on the Cardiac Action Potential

Cardiac ion channels exhibit stochastic conformational changes that determine their open-close gating behavior, leading ultimately to the action potential (AP). However, in computational models of conduction, ion currents are usually represented deterministically. Moreover, the natural intercellular variability of the number of membrane and gap junctional channels is never considered. Our aim was to quantify the effects of stochastic current fluctuations and channel distributions on AP duration (APD) and intercellular conduction delays (ICDs) using a ventricular cell model (Rudy et al.) with Markovian formulations of the principal ion currents (INa, ICa,L, IKr, IKs and IK1). Stochastic channel transitions were simulated explicitly and channel counts were drawn randomly from Poisson distributions. In single cells paced at 1 Hz, stochastic channel gating generated APD variability (APD = 143±1.8 ms) with a coefficient of variation (CVar) of 1.3%. APD variability decreased at higher pacing frequencies. IKs fluctuations contributed most (85%) to APD variance, followed by those of ICa,L and IKr (12% and 2%, respectively). Poissonian channel distribution induced APD variability with a CVar of 0.65%. In cell strands, the CVar of APD was strongly decreased by intercellular coupling. During conduction, stochastic channel gating generated ICD variability with a CVar of 0.25%. Reduction of INa or gap junctional coupling slowed conduction, but did not increase the CVar of ICDs above 1%. Poisson distribution of membrane channels exerted a similar small effect. However, during strong gap junctional uncoupling (60-200 channels/junction, conduction velocity <1 cm/s), Poisson distribution of gap junction channels resulted in a large ICD variability (>20%), highly heterogeneous conduction patterns and conduction blocks. Therefore, the variability of the number of channels in different gap junctions contributes to the heterogeneity of conduction patterns observed previously in experiments in cardiac tissue with altered intercellular coupling.

MAGNETIC FIELDS PRODUCED BY THE COMBUSTION OF METALS IN OXYGEN

Weak magnetic fields produced by the combustion of pure metal and metal/metal-oxide powders in Rowing oxygen were measured by High-Tc Superconducting Quantum Interference Devices (SQUIDs). Two qualitatively different types of magnetic signals were observed for various reactions. The combustion of Nb, Cr, and Co generates a slowly oscillating field, while the combustion of Zr, Hf, Mn, Ti, Mg, and Fe generates a slowly oscillating field on which rapid oscillations are superimposed. The magnetic power spectra for these reactions scales as a power-law up to 100 Hz with an exponent between 0·64 and 0·96. This suggests that the magnetic field oscillations are generated by stochastic current fluctuations over a wide range of time scales. This may result from fluctuations in either the instantaneous shape and/or the velocity of the reaction zone. A simple electromagnetic model is used to predict the qualitative features observed in our experiments.

Features of electrochemical noise generated during pitting of inhibited A516-70 carbon steel in chloride solutions

The features of electrochemical noise generated during pitting of A516-70 carbon steel in chloride solutions containing Na2CrO4 were investigated. Polarization curves showed that the threshold Cl− concentration in 0.01m Na2CrO4 solution to initiate pitting and the threshold CrO42− concentration in 0.1m NaCl solution to maintain passivation for A516-70 carbon steel were approximately 0.1m and 0.01m, respectively. Noise measurements indicated that above the critical Cl− concentration, the stochastic current fluctuations appeared after 5h of immersion, which was an indication of pit initiation. Increasing CrO42− concentration to 0.05m led to the production of symmetrical current spikes at the start of immersion. The noise pattern changed to frequent fluctuations with small amplitude during immersion. Noise analysis in the frequency domain showed that pitting initiation was characterized by the slope of the PSD plot approaching -2. Extending the immersion time decreased the pitting initiation rate, as indicated by the decreased PSD slope and increased PSD frequency.

Control of rat GluR6 glutamate receptor open probability by protein kinase A and calcineurin.

1. We have used non-stationary variance analysis to examine the single channel conductance and the probability of channel opening at the peak of the homomeric GluR6 response (Po,peak) to 100-200 ms application (10-90% exchange time, 0.3 ms) of glutamate onto excised membrane patches from transiently transfected human embryonic kidney cells (HEK 293). 2. Our determinations of both Po,peak and single channel conductance of simulated current responses are insensitive to system filtering, response rise time, desensitization rate and measured variation in our drug perfusion speed. Isolation of stochastic current fluctuations using the local mean response waveform minimizes problems associated with modest rundown of response amplitude during the experiment. 3. The slope conductance calculated from the weighted mean unitary currents for the channels activated in response to glutamate application is 16 pS. Chord conductance between-40 and -80 mV is independent of agonist concentration. Conversion of the codon for glutamine621 to arginine (Q621R) by RNA editing reduces conductance by more than 35-fold to less than 0.4 pS without changing response time course, desensitization, or Po,peak. 4. Po,peak is high at saturating glutamate concentrations (0.65 +/- 0.23; mean +/- S.D.) and varies with agonist concentrations. The half-maximally effective glutamate concentration (EC50) determined for Po,peak (0.2 mM; Hill slope = 0.6) is similar to that determined for the macroscopic peak current amplitude (0.5 mM; Hill slope = 1.0) in response to rapid agonist application. 5. Inclusion of the purified catalytic subunit of cAMP-dependent protein kinase A (PKA) in the patch pipette increases Po,peak to 0.85 +/- 0.12 and co-transfection of cells with a cDNA encoding the catalytic subunit of PKA (C alpha-PKA) increases Po,peak to 0.94 +/- 0.09. 6. Inclusion of purified calcineurin plus its coactivators 200 nM Ca2+ and calmodulin in the patch pipette decreases Po,peak to 0.48 +/- 0.10. The calcineurin-stimulated decrease of Po,peak in cells co-transfected with C alpha-PKA is blocked by 800 nM deltamethrin, a calcineurin inhibitor. Calmodulin, 200 nM Ca2+ and deltamethrin have no effect on Po,peak in the absence of calcineurin. As predicted from its effects on Po,peak, inclusion of calcineurin in the patch pipette accelerates the run-down of whole cell GluR6 responses in cells co-transfected with C alpha-PKA. 7. The effects of both calcineurin and PKA on Po,peak for GluR6 receptors in excised patches occur without any detectable changes to response time course, desensitization, or chord conductance. 8. We conclude that the binding of glutamate to homomeric GluR6 receptors is associated with a high probability of channel opening, which is under the control of two signalling systems that are known to be co-localized at the neuronal membrane: PKA (Po,peak near 1.0) and calcineurin (Po,peak near 0.5).