Piece by Piece—Electrochemical Synthesis of Individual Nanoparticles and their Performance in ORR Electrocatalysis

Impact electrochemistry for biosensing: advances and future directions

Impact electrochemistry allows for the investigation of the properties of single entities, ranging from nanoparticles (NPs) to soft bio-particles. It has introduced a novel dimension in the field of biological analysis, enhancing researchers' ability to comprehend biological heterogeneity and offering a new avenue for developing novel diagnostic devices for quantifying biological analytes. This review aims to summarize the recent advancements in impact electrochemistry-based biosensing over the past two to three years and provide insights into the future directions of this field.

Label-Free DNA Biosensor Based on Reduced Graphene Oxide and Gold Nanoparticles

Currently available DNA detection techniques frequently require compromises between simplicity, speed, accuracy, and cost. Here, we propose a simple, label-free, and cost-effective DNA detection platform developed at screen-printed carbon electrodes (SPCEs) modified with reduced graphene oxide (RGO) and gold nanoparticles (AuNPs). The preparation of the detection platform involved a two-step electrochemical procedure based on GO reduction onto SPCEs followed by the electrochemical reduction of HAuCl4 to facilitate the post-grafting reaction with AuNPs. The final sensor was fabricated by the simple physical adsorption of a single-stranded DNA (ssDNA) probe onto a AuNPs-RGO/SPCE electrode. Each preparation step was confirmed by morphological and structural characterization using scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy, respectively. Furthermore, the electrochemical properties of the modified electrodes have been investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The results demonstrated that the introduction of AuNPs onto RGO/SPCEs led to an enhancement in surface conductivity, a characteristic that favored an increased sensitivity in detection. The detection process relied on the change in the electrochemical signal induced by the binding of target DNA to the bioreceptor and was particularly monitored by the change in the charge transfer resistance of a [Fe(CN)6]4-/3- redox couple added in the test solution.

Real-Time Plasmonic Imaging of the Compositional Evolution of Single Nanoparticles in Electrochemical Reactions

Real-time probing of the compositional evolution of single nanoparticles during an electrochemical reaction is crucial for understanding the structure-performance relationship and rationally designing nanomaterials for desirable applications; however, it is consistently challenging to achieve high-throughput real-time tracking. Here, we present an optical imaging method, termed plasmonic scattering interferometry microscopy (PSIM), which is capable of imaging the compositional evolution of single nanoparticles during an aqueous electrochemical reaction in real time. By quantifying the plasmonic scattering interferometric pattern of nanoparticles, we establish the relationship between the pattern and composition of single nanoparticles. Using PSIM, we have successfully probed the compositional transformation dynamics of multiple individual nanoparticles during electrochemical reactions. PSIM could be used as a universal platform for exploring the compositional evolution of nanomaterials at the single-nanoparticle level and offers great potentials for addressing the extensive fundamental questions in nanoscience and nanotechnology.

Sonoelectrosynthesis of monodisperse metal nanoparticles

Traditional colloidal syntheses of metal nanoparticles (NPs) are highly sensitive to the selection of and quality of chemical reducing agents and metal precursors. To address these challenges, we demonstrate the complete sonoelectrochemical synthesis of monodisperse metal NPs starting from bulk metal, using Cu as a model system. Electrochemical syntheses of NPs are of great interest as the oxidation and reduction processes that account for product formation can occur directly at the anode and cathode, respectively. This ability has the potential to improve reproducibility by simplifying the chemical pathway to NPs, with electrosyntheses often also providing unique kinetic pathways toward green product formation. Herein, ultrasound is coupled with electrosynthesis to clean the electrode surface, dispersing the NPs produced at the electrode into solution. We were able to shift the size distribution to form monodispersed metal NPs through control of applied potential (V_applied) and ultrasonic pulses. The synthesis begins with electrooxidation of bulk Cu metal to directly dissolve metal ions into a microemulsion system. This step is followed by sonoelectroreduction of the ions, which facilitates the formation of dispersible, monodisperse Cu NPs with diameters <10 nm. The size distribution can be controlled by adjusting the V_applied, pulse intensity, and pulse sequence implemented during sonoelectroreduction. We view this technique as a scalable method to synthesize metal NPs from bulk metal without chemical reducing agents.

Mechanochemical Synthesis of Fluorine-Containing Co-Doped Zeolitic Imidazolate Frameworks for Producing Electrocatalysts

Catalysts derived from pyrolysis of metal organic frameworks (MOFs) are promising candidates to replace expensive and scarce platinum-based electrocatalysts commonly used in polymer electrolyte membrane fuel cells. MOFs contain ordered connections between metal centers and organic ligands. They can be pyrolyzed into metal- and nitrogen-doped carbons, which show electrocatalytic activity toward the oxygen reduction reaction (ORR). Furthermore, metal-free heteroatom-doped carbons, such as N-F-Cs, are known for being active as well. Thus, a carbon material with Co-N-F doping could possibly be even more promising as ORR electrocatalyst. Herein, we report the mechanochemical synthesis of two polymorphs of a zeolitic imidazole framework, Co-doped zinc 2-trifluoromethyl-1H-imidazolate (Zn_0.9Co_0.1(CF₃-Im)₂). Time-resolved in situ X-ray diffraction studies of the mechanochemical formation revealed a direct conversion of starting materials to the products. Both polymorphs of Zn_0.9Co_0.1(CF₃-Im)₂ were pyrolyzed, yielding Co-N-F containing carbons, which are active toward electrochemical ORR.

Electrochemistry under confinement

Although the term 'confinement' regularly appears in electrochemical literature, elevated by continuous progression in the research of nanomaterials and nanostructures, up until today the various aspects of confinement considered in electrochemistry are rather scattered individual contributions outside the established disciplines in this field. Thanks to a number of highly original publications and the growing appreciation of confinement as an overarching link between different exciting new research strategies, 'electrochemistry under confinement' is the process of forming a research discipline of its own. To aid the development a coherent terminology and joint basic concepts, as crucial factors for this transformation, this review provides an overview on the different effects on electrochemical processes known to date that can be caused by confinement. It also suggests where boundaries to other effects, such as nano-effects could be drawn. To conceptualize the vast amount of research activities revolving around the main concepts of confinement, we define six types of confinement and select two of them to discuss the state of the art and anticipated future developments in more detail. The first type concerns nanochannel environments and their applications for electrodeposition and for electrochemical sensing. The second type covers the rather newly emerging field of colloidal single entity confinement in electrochemistry. In these contexts, we will for instance address the influence of confinement on the mass transport and electric field distributions and will link the associated changes in local species concentration or in the local driving force to altered reaction kinetics and product selectivity. Highlighting pioneering works and exciting recent developments, this educational review does not only aim at surveying and categorizing the state-of-the-art, but seeks to specifically point out future perspectives in the field of confinement-controlled electrochemistry.

Enabling practical nanoparticle electrodeposition from aqueous nanodroplets

The rapid rise of technology in the modern world has led to an increased demand for energy. Consequently, it is essential to increase the efficiency of current energy-producing systems due to the poor activity of their catalysts. Nanoparticles play a significant role in energy storage and conversion; however, electrodeposition of nanoparticles is difficult to achieve due to surface heterogeneities, nanoparticle diffusion layer overlap, and the inability to electrodeposit multi-metallic nanoparticles with stoichiometric control. These problems can be solved through nanodroplet-mediated electrodeposition, a technique where water nanodroplets are filled with metal salt precursors that form stable nanoparticles when they collide with a negatively-biased electrode. Further, this method has demonstrated control over nanoparticle size and morphology, displaying a wide variety of applications for the generation of materials with excellent catalytic properties. Historically, the cost of nanodroplet-mediated electrodeposition experimentation is prohibitive because practitioners use 0.1 M to 0.5 M tetrabutylammonium perchlorate (TBAP) dissolved in the oil phase (∼10 mL). Such high concentrations of electrolytes have been used to lower ohmic drop and provide ions to maintain charge balance during electrodeposition. Here, we show that supporting electrolyte is not necessary for the oil phase. In fact, one can use a suitable salt (such as lithium perchlorate) in the aqueous phase to achieve nanoparticle electrodeposition. This simple change, grounded in an understanding of ion transfer, drives down the cost per experiment by nearly three orders of magnitude, representing a necessary step forward in enabling practical nanoparticle electrodeposition from water nanodroplets. This approach is a promising procedure for future cost-effective energy conversion systems relying on electrocatalytic nanoparticles.

Unravelling the last milliseconds of an individual graphene nanoplatelet before impact with a Pt surface by bipolar electrochemistry

Contactless interactions of micro/nano-particles near electrochemically or chemically active interfaces are ubiquitous in chemistry and biochemistry. Forces arising from a convective field, an electric field or chemical gradients act on different scales ranging from few microns down to few nanometers making their study difficult. Here, we correlated optical microscopy and electrochemical measurements to track at the millisecond timescale the dynamics of individual two-dimensional particles, graphene nanoplatelets (GNPs), when approaching an electrified Pt micro-interface. Our original approach takes advantage of the bipolar feedback current recorded when a conducting particle approaches an electrified surface without electrical contact and numerical simulations to access the velocity of individual GNPs. We evidenced a strong deceleration of GNPs from few tens of μm s-1 down to few μm s-1 within the last μm above the surface. This observation reveals the existence of strongly non-uniform forces between tens of and a thousand nanometers from the surface.

Acid-base chemistry at the single ion limit

We present the results of acid–base experiments performed at the single ion (H⁺ or OH⁻) limit in ∼6 aL volume nanopores incorporating electrochemical zero-mode waveguides (E-ZMWs). At pH 3 each E-ZMW nanopore contains ca. 3600H⁺ ions, and application of a negative electrochemical potential to the gold working electrode/optical cladding layer reduces H⁺ to H₂, thereby depleting H⁺ and increasing the local pH within the nanopore. The change in pH was quantified by tracking the intensity of fluorescein, a pH-responsive fluorophore whose intensity increases with pH. This behavior was translated to the single ion limit by changing the initial pH of the electrolyte solution to pH 6, at which the average pore occupancy 〈n〉_pore ∼3.6H⁺/nanopore. Application of an electrochemical potential sufficiently negative to change the local pH to pH 7 reduces the proton nanopore occupancy to 〈n〉_pore ∼0.36H⁺/nanopore, demonstrating that the approach is sensitive to single H⁺ manipulations, as evidenced by clear potential-dependent changes in fluorescein emission intensity. In addition, at high overpotential, the observed fluorescence intensity exceeded the value predicted from the fluorescence intensity-pH calibration, an observation attributed to the nucleation of H₂ nanobubbles as confirmed both by calculations and the behavior of non-pH responsive Alexa 488 fluorophore. Apart from enhancing fundamental understanding, the approach described here opens the door to applications requiring ultrasensitive ion sensing, based on the optical detection of H⁺ population at the single ion limit.

Visualizing dynamic change in the number of protons during electroreduction of protons in attoliter volume zero-mode waveguides.

Voltammetric Analysis of Redox Reactions and Ion Transfer in Water Microdroplets

We report a set of voltammetric experiments for studying redox reactions and ion transfer in water microdroplets emulsified in 1,2-dichloroethane (DCE). The electrochemistry of microdroplets (r_drop ∼ 700 nm) loaded with either ferrocyanide ([Fe(CN)₆]^4-) or ferricyanide ([Fe(CN)₆]^3-), chosen due to their hydrophilic nature, was tracked using cyclic voltammetry. These heterogeneous reactions necessitated ion transfer at the droplet interface to maintain charge balance in the two liquid phases during oxidation or reduction, which was facilitated by the tetrabutylammonium perchlorate ([TBA][ClO₄]) salt in the DCE phase. Experiments were performed with (1) a single macrodroplet (10^-7 L) on a macroelectrode (r ∼ 1.5 mm), (2) millions of microdroplets (10^-15 L) adsorbed on to a macroelectrode (r ∼ 1.5 mm), and (3) at the single microdroplet level via observing individual microdroplet collisions at an ultramicroelectrode (r ∼ 5 μm). We demonstrate that when millions of microdroplets are adsorbed onto a macroelectrode, there are two surprising observations: (1) the half-wave potential (E_1/2) for the [Fe(CN)₆]^3-/4- redox couple shifts by +100 mV, which is shown to depend on the number of droplets on the electrode surface. (2) The reduction of [Fe(CN)₆]^3-, which is assisted by the transfer of TBA⁺ into the water droplet, displays two waves in the voltammogram. This dual-wave behavior can be explained by the formation of TBA_xK_3-xFe(CN)₆, which is soluble in DCE. Additionally, we demonstrate that the adsorption of microdroplets onto an electrode surface offers significant amplification (×10³) of the water/oil/electrode three-phase boundary when compared to the adsorption of larger macrodroplets, permitting a rigorous evaluation of heterogeneous chemistry at this distinct interface. In combination, these experiments provide new energetic and mechanistic insights for droplet systems, as well as reactivity differences between microscale and bulk multiphase systems.

Visualizing Phase Boundaries with Electrogenerated Chemiluminescence

Reactivity at phase boundaries is central to many areas of chemistry, from synthesis to heterogeneous catalysis. New tools are necessary to gain a more detailed understanding of processes occurring at these boundaries. We describe a series of experiments to visualize phase boundaries using electrogenerated chemiluminescence (ECL) on glassy carbon electrodes. By taking advantage of the solubilities of the ECL luminophore and the coreactant in different liquid phases, we demonstrate that the interface of various phases (i.e., the boundaries formed between a water microdroplet, 1,2-dichloroethane, and a glassy carbon electrode and the boundaries formed between an oxygen bubble, water, and a glassy carbon electrode) can be evaluated. We measured microdroplet contact radii, the three-phase boundary thickness, and growth dynamics of electrogenerated O₂ bubbles. These experimental tools and the fundamental knowledge they yield will find applications in biology, nanoscience, synthesis, and energy storage and conversion, where understanding phase boundary chemistry is essential.

Operando Studies of the Electrochemical Dissolution of Silver Nanoparticles in Nitrate Solutions Observed With Hyperspectral Dark-Field Microscopy

Since nanoparticles are frequently used in commercial applications, there is a huge demand to obtain deeper insights into processes at the nanoscale. Especially, catalysis, chemical and electrochemical reaction dynamics are still poorly understood. Thus, simultaneous and coupled opto-and spectro-electrochemical dark-field microscopy is used to study in situ and operando the electrochemically driven dissolution mechanism of single silver nanoparticles in the presence of nitrate ions as non-complexing counter-ions, herein. Hyperspectral imaging is used to probe the intrinsic localized surface plasmon resonance of individual silver nanospheres before, during and after their electrochemical oxidation on a transparent indium tin oxide (ITO) electrode. Furthermore, optical video imaging was performed for additional information. Based on the complete loss of spectral information and intensity, a dissolution of the particles during the reaction was concluded. This way it is revealed that the dissolution of individual particles proceeds over several seconds, indicating a hindrance by the nitrate ions. Only electrochemical analysis does not provide this insight as the measured current does not allow distinguishing between successive fast dissolution of one particle after another or slow dissolution of several particles in a concerted manner. For comparison, experiments were performed in the presence of chloride ions. It was observed that the silver chloride formation is an instantaneous process. Thus, it is possible to study and define the reaction dynamics on the single nanoparticle level in various electrochemical systems and electrolyte solutions. Accordingly, operando opto- and spectro-electrochemical studies allow us to conclude, that the oxidation of silver to solvated silver cations is a kinetically slow process, while the oxidation to silver chloride is fast. We propose this approach as a new method to study electrocatalyst materials, their transformation and degradation under operando conditions.

Advances and challenges for experiment and theory for multi-electron multi-proton transfer at electrified solid–liquid interfaces

Understanding microscopic mechanism of multi-electron multi-proton transfer reactions at complexed systems is important for advancing electrochemistry-oriented science in the 21st century.