Tunable Intervalence Charge Transfer in Ruthenium Prussian Blue Analog Enables Stable and Efficient Biocompatible Artificial Synapses.

Emerging concepts for neuromorphic computing, bioelectronics, and brain-computer interfacing inspire new research avenues aimed at understanding the relationship between oxidation state and conductivity in unexplored materials. This report expands the materials playground for neuromorphic devices to include a mixed valence inorganic 3D coordination framework, a ruthenium Prussian blue analog (RuPBA), for flexible and biocompatible artificial synapses that reversibly switch conductance by more than four orders of magnitude based on electrochemically tunable oxidation state. The electrochemically tunable degree of mixed valency and electronic coupling between N-coordinated Ru sites controls the carrier concentration and mobility, as supported by density functional theory computations and application of electron transfer theory to in situ spectroscopy of intervalence charge transfer. Retention of programmed states is improved by nearly two orders of magnitude compared to extensively studied organic polymers, thus reducing the frequency, complexity, and energy costs associated with error correction schemes. This report demonstrates dopamine-mediated plasticity of RuPBA synapses and biocompatibility of RuPBA with neuronal cells, evoking prospective application for brain-computer interfacing.

ECRAM Materials, Devices, Circuits and Architectures: A Perspective.

Non-von-Neumann computing using neuromorphic systems based on two-terminal resistive nonvolatile memory elements has emerged as a promising approach, but its full potential has not been realized due to the lack of materials and devices with the appropriate attributes. Unlike memristors, which require large write currents to drive phase transformations or filament growth, electrochemical random access memory (ECRAM) decouples the "write" and "read" operations using a "gate" electrode to tune the conductance state through charge-transfer reactions, and every electron transferred through the external circuit in ECRAM corresponds to the migration of ≈1 ion used to store analogue information. Like static dopants in traditional semiconductors, electrochemically inserted ions modulate the conductivity by locally perturbing a host's electronic structure; however, ECRAM does so in a dynamic and reversible manner. The resulting change in conductance can span orders of magnitude, from gradual increments needed for analog elements, to large, abrupt changes for dynamically reconfigurable adaptive architectures. In this in-depth perspective, the history of ECRAM, the recent progress in devices spanning organic, inorganic, and 2D materials, circuits, architectures, the rich portfolio of challenging, fundamental questions, and how ECRAM can be harnessed to realize a new paradigm for low-power neuromorphic computing are discussed.

Electrochemical Synthesis of Individual Core@Shell and Hollow Ag/Ag2S Nanoparticles.

This letter presents an electrochemical methodology for structure-tunable synthesis, characterization, and kinetic monitoring of metal-semiconductor phase transformations at individual Ag nanoparticles. In the presence of HS- in aqueous solution, the stochastic collision and adsorption of Ag nanoparticles at a Au microelectrode initiates the partial anodic transformation of Ag to Ag2S at each particle. A single continuous current transient is observed for each Ag nanoparticle reacted. The characteristic shapes of the transients are distinct from previously reported amperometric recordings of electrochemical reactions involving single nanoparticles and are highly uniform at a constant applied potential. The average maximum current increases while the event duration decreases as a function of increasing potential. Independent of applied potential, the electrochemical transformation event abruptly stops after converting ∼80% of the Ag in the nanoparticle to Ag2S, a self-terminating process that does not occur for bulk Ag electrodes under similar conditions. The resulting products are a mixture of core@shell Ag@Ag2S nanoparticles with and without voids in the core, as characterized by transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDX). Both the frequency and size of voids increase at more positive potentials. The average size of the core@shell nanoparticles determined by coulometric analysis of the current transients agrees well with TEM measurements.

Single Ag nanoparticle collisions within a dual-electrode micro-gap cell.

An adjustable width (between 600 nm and 20 µm) gap between two Au microelectrodes is used to probe the electrodissolution dynamics of single Ag nanoparticles. One Au microelectrode is used to drive the oxidation and subsequent dissolution of a single Ag nanoparticle, which displays a multi-peak oxidation behavior, while a second Au microelectrode is used to collect the Ag+ that is produced. Careful analysis of the high temporal resolution current-time traces reveals capacitive coupling between electrodes due to the sudden injection of Ag+ ions into the gap between the electrodes. The current-time traces allow measurement of the effect of citrate concentration on the electrodissolution dynamics of a single Ag nanoparticle, which reveals that the presence of 2 mM citrate significantly slows down the release of Ag+. Intriguingly, these experiments also reveal that only a portion (ca. 50%) of the oxidized Ag nanoparticle is released as free Ag+ regardless of citrate concentration.

Collision Dynamics during the Electrooxidation of Individual Silver Nanoparticles.

Recent high-bandwidth recordings of the oxidation and dissolution of 35 nm radius Ag nanoparticles at a Au microelectrode show that these nanoparticles undergo multiple collisions with the electrode, generating multiple electrochemical current peaks. In the time interval between observed current peaks, the nanoparticles diffuse in the solution near the electrolyte/electrode interface. Here, we demonstrate that simulations of random nanoparticle motion, coupled with electrochemical kinetic parameters, quantitatively reproduce the experimentally observed multicurrent peak behavior. Simulations of particle diffusion are based on the nanoparticle-mass-based thermal nanoparticle velocity and the Einstein diffusion relations, while the electron-transfer rate is informed by the literature exchange current density for the Ag/Ag+ redox system. Simulations indicate that tens to thousands of particle-electrode collisions, each lasting ∼6 ns or less (currently unobservable on accessible experimental time scales), contribute to each experimentally observed current peak. The simulation provides a means to estimate the instantaneous current density during a collision (∼500-1000 A/cm2), from which we estimate a rate constant between ∼5 and 10 cm/s for the electron transfer between Ag nanoparticles and the Au electrode. This extracted rate constant is approximately equal to the thermal collisional velocity of the Ag nanoparticle (4.6 cm/s), the latter defining the theoretical upper limit of the electron-transfer rate constant. Our results suggest that only ∼1% of the surface atoms on the Ag nanoparticles are oxidized per instantaneous collision. The combined simulated and experimental results underscore the roles of Brownian motion and collision frequency in the interpretation of heterogeneous electron-transfer reactions involving nanoparticles.

Observation of Multipeak Collision Behavior during the Electro-Oxidation of Single Ag Nanoparticles.

The dynamic collision behavior of the electro-oxidation of single Ag nanoparticles is observed at Au microelectrodes using stochastic single-nanoparticle collision amperometry. Results show that an Ag nanoparticle collision/oxidation event typically consists of a series of 1 to ∼10 discrete "sub-events" over an ∼20 ms interval. Results also show that the Ag nanoparticles typically undergo only partial oxidation prior to diffusing away from the Au electrode into the bulk solution. Both behaviors are characterized and shown to exist under a variety of experimental conditions. These previously unreported behaviors suggest that nanoparticle collision and electro-dissolution is a highly dynamic process driven by fast particle-electrode interactions and nanoparticle diffusion.

Addressing Colloidal Stability for Unambiguous Electroanalysis of Single Nanoparticle Impacts.

Herein the problem of colloidal instability on electrochemically detected nanoparticle (NP) collisions with a Hg ultramicroelectrode (UME) by electrocatalytic amplification is addressed. NP tracking analysis (NTA) shows that rapid aggregation occurs in solution after diluting citrate-stabilized Pt NPs with hydrazine/phosphate buffers of net ionic strength greater than 70 mM. Colloidal stability improves by lowering the ionic strength, indicating that aggregation processes were strongly affected by charge screening of the NP double layer interactions at high cation concentrations. For the system of lowest ionic strength, the overwhelming majority of observed electrocatalytic current signals represent single NP/electrode impacts, as confirmed by NTA kinetic monitoring. NP diffusion coefficients determined by NTA and NP impact electroanalysis are in excellent agreement for the stable colloids, which signifies that the sticking probability of Pt NPs interacting with Hg is unity and that the observed NP impact rate agrees with the expected steady-state diffusive flux expression for the spherical cap Hg UME.

Electrocatalytic amplification of DNA-modified nanoparticle collisions via enzymatic digestion.

We report a new and general approach that will be useful for adapting the method of electrocatalytic amplification (ECA) to biosensing applications. In ECA, individual collisions of catalytic nanoparticles with a noncatalytic electrode surface lead to bursts of current. In the work described here, the current arises from catalytic electrooxidation of N2H4 at the surface of platinum nanoparticles (PtNPs). The problem with using ECA for biosensing applications heretofore, is that it is necessary to immobilize a receptor, such as DNA (as in the case here) or an antibody on the PtNP surface. This inactivates the colliding NP, however, and leads to very small collision signatures. In the present article, we show that single-stranded DNA (ssDNA) present on the PtNP surface can be detected by selectively removing a fraction of the ssDNA using the enzyme Exonuclease I (Exo I). About half of the current associated with collisions of naked PtNPs can be recovered from fully passivated PtNPs after exposure to Exo I. Experiments carried out using both Au and Hg ultramicroelectrodes reveal some mechanistic aspects of the collision process before and after treatment of the ssDNA-modified PtNPs with Exo I.

Electronic coupling between ligand and core energy states in dithiolate-monothiolate stabilized Au clusters.

Electron transfer activities of metal clusters are fundamentally significant and have promising potential in catalysis, charge or energy storage, sensing, biomedicine and other applications. Strong resonance coupling between the metal core energy states and the ligand molecular orbitals has not been established experimentally, albeit exciting progress has been achieved in the composition and structure determination of these types of nanomaterials recently. In this report, the coupling between core and ligand energy states is demonstrated by the rich electron transfer activities of Au130 clusters. Quantized electron transfers to the core and multi-electron transfers involving the durene-dithiolate ligands were observed at lower and higher potentials, respectively, in voltammetric studies. After a facile multi-electron oxidation from +1.34 to +1.40 V, several reversal reduction processes at more negative potentials, i.e. +0.91 V, +0.18 V and -0.34 V, were observed in an electrochemically irreversible fashion or with sluggish kinetics. The number of electrons and the shifts of the respective reduction potentials in the reversal process were attributed to the electronic coupling or energy relaxation processes. The electron transfer activities and subsequent relaxation processes are drastically reduced at lower temperatures. The time- and temperature-dependent relaxation, involving multiple energy states in the reversal reduction processes upon the oxidation of ligands, reveals the coupling between core and ligand energy states.

Increasing the Collision Rate of Particle Impact Electroanalysis with Magnetically Guided Pt-Decorated Iron Oxide Nanoparticles.

An integrated microfluidic/magnetophoretic methodology was developed for improving signal response time and detection limits for the chronoamperometric observation of discrete nanoparticle/electrode interactions by electrocatalytic amplification. The strategy relied on Pt-decorated iron oxide nanoparticles which exhibit both superparamagnetism and electrocatalytic activity for the oxidation of hydrazine. A wet chemical synthetic approach succeeded in the controlled growth of Pt on the surface of FeO/Fe3O4 core/shell nanocubes, resulting in highly uniform Pt-decorated iron oxide hybrid nanoparticles with good dispersibility in water. The unique mechanism of hybrid nanoparticle formation was investigated by electron microscopy and spectroscopic analysis of isolated nanoparticle intermediates and final products. Discrete hybrid nanoparticle collision events were detected in the presence of hydrazine, an electrochemical indicator probe, using a gold microband electrode integrated into a microfluidic channel. In contrast with related systems, the experimental nanoparticle/electrode collision rate correlates more closely with simple theoretical approximations, primarily due to the accuracy of the nanoparticle tracking analysis method used to quantify nanoparticle concentrations and diffusion coefficients. Further modification of the microfluidic device was made by applying a tightly focused magnetic field to the detection volume to attract the magnetic nanoprobes to the microband working electrode, thereby resulting in a 6-fold increase to the relative frequency of chronoamperometric signals corresponding to discrete nanoparticle impact events.

Electrochemical monitoring of single nanoparticle collisions at mercury-modified platinum ultramicroelectrodes.

Here, we report a potentiometric method for detecting single platinum nanoparticles (Pt NPs) by measuring a change in open-circuit potential (OCP) instead of the current during single Pt NP collisions with the mercury-modified Pt ultramicroelectrode (Hg/Pt UME). Similar to the current-time (i-t) response reported previously at Hg/Pt UMEs, the OCP-time (v-t) response consists of repeated potential transient signals that return to the background level. This is because Hg poisons the Pt NP after collision with the Hg/Pt UME due to amalgamation and results in deactivation of the redox reaction. For individual Pt NP collisions the amplitude of the OCP signal reaches a maximum and decays to the background level at a slower rate compared to the comparable i-t response. Due to this, OCP events are broader and more symmetrical in shape compared to i-t "spikes." The collision frequency of Pt NPs derived from v-t plots (0.007 to 0.020 pM(-1) s(-1)) is in good agreement with the value derived from i-t plots recorded at Hg/Pt UMEs (0.016 to 0.024 pM(-1) s(-1)) under similar conditions and was found to scale linearly with Pt NP concentration. Similar to the current response, the amplitude of the OCP response increased with the NP's size. However, unlike the change in current in a i-t response, the change in OCP in a v-t response observed during single Pt NP collisions with Hg/Pt UME is larger than the estimated change in OCP based on the theory. Therefore, the Pt NP sizes derived from the v-t response did not correlate with the TEM-derived Pt NP sizes. In spite of these results the potentiometric method has great value for electroanalysis because of its significant advantages over the amperometric method such as a simpler apparatus and higher sensitivity.

Influence of the redox indicator reaction on single-nanoparticle collisions at mercury- and bismuth-modified Pt ultramicroelectrodes.

Single-Pt nanoparticles (NPs) can be detected electrochemically by measuring the current-time (i-t) response associated with both hydrazine oxidation and proton reduction during individual Pt NP collisions with noncatalytic Hg- and Bi-modified Pt ultramicroelectrodes (Hg/Pt and Bi/Pt UMEs, respectively). At Hg/Pt UMEs, the i-t response for both hydrazine oxidation and proton reduction consists of repeated current "spikes" that return to the background level as Hg poisons the Pt NP after collision with the Hg/Pt UME due to amalgamation and deactivation of the redox reaction. Furthermore, at a Hg/Pt UME, the applied potential directly influences the interfacial surface tension (electrocapillarity) that also impacts the observed i-t response for single-Pt NP collisions for proton reduction that exhibits a faster decay of current (0.7-4 ms) to background levels than hydrazine oxidation (2-5 s). Because the surface tension of Hg is lower (-0.9 V), Pt NPs possibly react faster with Hg (amalgamate at a faster rate), resulting in sharp current spikes for proton reduction compared to hydrazine oxidation. In contrast, a stepwise "staircase" i-t response is observed for proton reduction for single-Pt NP collisions at a Bi/Pt UME. This different response suggests that electrostatic forces of negatively charged citrate-capped Pt NPs also influence the i-t response at more negative applied potentials, but the Pt NPs do not poison the electrochemical activity at Bi/Pt UMEs.

Ultrasensitive electroanalytical tool for detecting, sizing, and evaluating the catalytic activity of platinum nanoparticles.

Here we describe a very simple, reliable, low-cost electrochemical approach to detect single nanoparticles (NPs) and evaluate NP size distributions and catalytic activity in a fast and reproducible manner. Single NPs are detected through an increase in current caused by electrocatalytic oxidation of N(2)H(4) at the surface of the NP when it contacts a Hg-modified Pt ultramicroelectrode (Hg/Pt UME). Once the NP contacts the Hg/Pt UME, Hg poisons the Pt NP, deactivating the N(2)H(4) oxidation reaction. Hence, the current response is a "spike" that decays to the background current level rather than a stepwise "staircase" response as previously described for a Au UME. The use of Hg as an electrode material has several quantitative advantages including suppression of the background current by 2 orders of magnitude over a Au UME, increased signal-to-noise ratio for detection of individual collisions, precise integration of current transients to determine charge passed and NP size, reduction of surface-induced NP aggregation and electrode fouling processes, and reproducible and renewable electrodes for routine detection of catalytic NPs. The NP collision frequency was found to scale linearly with the NP concentration (0.016 to 0.024 pM(-1)s(-1)). NP size distributions of 4-24 nm as determined from the current-time transients correlated well with theory and TEM-derived size distributions.

Mixed dithiolate durene-DT and monothiolate phenylethanethiolate protected Au130 nanoparticles with discrete core and core-ligand energy states.

A new type of gold nanoparticle with interesting energetics has been created by employing a mixture of dithiol durene (Durene-DT) and monothiol phenylethanethiol (PhC2S) in the synthesis. The average composition of these mixed thiolate clusters is characterized to be Au(130)(Durene-DT)(29)(PhC2S)(22). Continuous quantized core charging behaviors were observed at lower potentials in voltammetric measurements, while ligand reaction and core-ligand interactions were observed at higher potentials. The absorbance spectrum displays discrete absorption bands at ca. 355, 490, 584, and 718 nm. The electrochemical and absorbance features are correlated through the determined energy states and charging energy. Broad near-IR luminescence was observed, associated with significant relaxation of excitation energy. Such interesting optical and electrochemical properties are attributed to the nanoparticle core size, ligand composition, and core-ligand charge delocalization determined by the dithiolate molecular structure.

Monolayer reactions of protected Au nanoclusters with monothiol tiopronin and 2,3-dithiol dimercaptopropanesulfonate.

The novel thiol bridging "staple motif RS-Au-SR" discovered at the Au-thiolate interface has tremendously advanced the structural understanding of monolayer protected Au clusters (AuMPCs). In this paper, multidentate dithiol ligands are introduced into the monolayer of the Au clusters. The impacts of dithiols on the Au-monothiolate interfacial bonding and related physical properties are explored. A correlation is established of the near-IR luminescence with Au-tiopronin monothiol interactions that are constrained by the dithiol molecule structures. Two types of monolayer reaction are studied: (1) monothiol tiopronin AuMPCs with dithiol molecule 2,3-dimercaptopropanesulfonate (DMPS) and (2) DMPS Au dithiol clusters (AuDTCs) with tiopronin monothiol ligands. Upon the addition of excess DMPS molecules into tiopronin MPC solution, tiopronin molecules are efficiently liberated from the original AuMPCs monitored by proton NMR. The process is accompanied by the decrease of near-infrared luminescence of the tiopronin AuMPCs. A slower enhancement of the 282 nm absorption band is observed, a signature of DMPS Au4DTCs characterized by mass spectrometry. The analysis of the reaction kinetics reveals a two-step mechanism: a facile ligand replacement followed by a sluggish core etching process. The reverse approach, tiopronin molecules reacting with DMPS DTCs, results in the addition of tiopronin into DMPS monolayer instead of ligand exchange. Near-IR luminescence intensifies with the monolayer addition of tiopronin.