Using Ambient Ion Beams to Write Nanostructured Patterns for Surface Enhanced Raman Spectroscopy

Enhancing Energy Storage Capacity of 3D Carbon Electrodes Using Soft Landing of Molecular Redox Mediators

The incorporation of redox-active species into the electric double layer is a powerful strategy for enhancing the energy density of supercapacitors. Polyoxometalates (POM) are a class of stable, redox-active species with multielectron activity, which is often used to tailor the properties of electrochemical interfaces. Traditional synthetic methods often result in interfaces containing a mixture of POM anions, unreactive counter ions, and neutral species. This leads to degradation in electrochemical performance due to aggregation and increased interfacial resistance. Another significant challenge is achieving the uniform and stable anchoring of POM anions on substrates to ensure the long-term stability of the electrochemical interface. These challenges are addressed by developing a mass spectrometry-based subambient deposition strategy for the selective deposition of POM anions onto engineered 3D porous carbon electrodes. Furthermore, positively charged functional groups are introduced on the electrode surface for efficient trapping of POM anions. This approach enables the deposition of purified POM anions uniformly through the pores of the 3D porous carbon electrode, resulting in unprecedented increase in the energy storage capacity of the electrodes. The study highlights the critical role of well-defined electrochemical interfaces in energy storage applications and offers a powerful method to achieve this through selective ion deposition.

Water Charge Transfer Accelerates Criegee Intermediate Reaction with H2O- Radical Anion at the Aqueous Interface

Criegee intermediates (CIs) are important carbonyl oxides that may react with atmospheric trace chemicals and impact the global climate. The CI reaction with water has been widely studied and is a main channel for trapping CIs in the troposphere. Previous experimental and computational reports have largely focused on reaction kinetic processes in various CI-water reactions. The molecular-level origin of CI's interfacial reactivity at the water microdroplet surface (e.g., as found in aerosols and clouds) is unclear. In this study, by employing the quantum mechanical/molecular mechanical (QM/MM) Born-Oppenheimer molecular dynamics with the local second-order Møller-Plesset perturbation theory, our computational results reveal a substantial water charge transfer up to ∼20% per water, which creates the surface H₂O⁺/H₂O^- radical pairs to enhance the CH₂OO and anti-CH₃CHOO reactivity with water: the resulting strong CI-H₂O^- electrostatic attraction at the microdroplet surface facilitates the nucleophilic attack to the CI carbonyl by water, which may counteract the apolar hindrance of the substituent to accelerate the CI-water reaction. Our statistical analysis of the molecular dynamics trajectories further resolves a relatively long-lived bound CI(H₂O^-) intermediate state at the air/water interface, which has not been observed in gaseous CI reactions. This work provides insights into what may alter the oxidizing power of the troposphere by the next larger CIs than simple CH₂OO and implicates a new perspective on the role of interfacial water charge transfer in accelerating molecular reactions at aqueous interfaces.

Catalyst-Free Accelerated Three-Component Synthesis of Betti Bases in Microdroplets

Due to their important roles in medicine and asymmetric metal catalysis, the formation of Betti bases has attracted wide interest in organic chemical community. Traditional multicomponent reaction methods for synthesizing Betti bases normally require long reaction times under harsh conditions (high temperature, microwave or ultrasonic irradiation, etc.) in the presence of various catalysts. In this study, we developed a mild, highly efficient and environmentally friendly method to synthesize Betti bases without the use of any catalysts in microdroplets. The Betti reaction was accelerated by 6.53×10³ in microdroplets by comparing the measured rate constant in bulk. Fifteen Betti bases were synthesized by the microdroplet method using a variety of aldehydes, naphthols and amines with 68-98 % yields at a scaled-up amount of 1.9 g h^-1 . Overall it is an attractive alternative to classic organic synthesis for the construction of Betti bases and derivatives.

Surface-Enhanced Raman Spectroscopic Probing in Digital Microfluidics through a Microspray Hole

We report a novel approach for surface-enhanced Raman spectroscopy (SERS) detection in digital microfluidics (DMF). This is made possible by a microspray hole (μSH) that uses an electrostatic spray (ESTAS) for sample transfer from inside the chip to an external SERS substrate. To realize this, a new ESTAS-compatible stationary SERS substrate was developed and characterized for sensitive and reproducible SERS measurements. In a proof-of-concept study, we successfully applied the approach to detect various analyte molecules using the DMF chip and achieved micro-molar detection limits. Moreover, this technique was exemplarily employed to study an organic reaction occurring in the DMF device, providing vibrational spectroscopic data.

Hydrophilic-Hydrophobic Nanohybrids of AuNP-Immobilized Two-Dimensional Nanomica Platelets as Flexible Substrates for High-Efficiency and High-Selectivity Surface-Enhanced Raman Scattering Microbe Detection

A flexible hybrid substrate was developed by affixing gold nanoparticles (AuNPs) onto the surface of two-dimensional nanomica platelets (NMPs). The substrate was successfully used in biosensors with high efficiency and high selectivity through surface-enhanced Raman scattering (SERS). By controlling the amphiphilicity of the hybrid substrate, the flexible substrate was made highly selective toward biomolecules. Four different SERS substrate systems were constructed, including intercalated mica, exfoliated NMPs, hydrophilic exfoliated NMPs, and hydrophobic exfoliated NMPs. NMPs were only 1 nm thick. AuNPs adsorbed on both sides of NMPs and thus created excellent three-dimensional hot junction effects in the z-axis direction. For the detection of adenine in DNA, a satisfactory Raman enhancement factor (EF) of up to 8.9 × 10⁶ was achieved with the detection limit as low as 10^-8 M. Subsequently, the AuNP/NMP hybrids were adopted to rapidly detect hydrophilic Staphylococcus hominis and hydrophobic Escherichia coli. The AuNP/PIB-POE-PIB/NMP nanohybrid was concurrently hydrophilic and hydrophobic. This amphiphilic property greatly enhanced the detection selectivity and signal intensity for hydrophilic or hydrophobic bacteria. Overall, AuNPs/PIB-POE-PIB/NMPs developed as SERS substrates enable rapid, sensitive biodetection.

Fabrication of a New, Low-Cost, and Environment-Friendly Laccase-Based Biosensor by Electrospray Immobilization with Unprecedented Reuse and Storage Performances

The fabrication of enzyme-based biosensors has received much attention for their selectivity and sensitivity. In particular, laccase-based biosensors have attracted a lot of interest for their capacity to detect highly toxic molecules in the environment, becoming essential tools in the fields of white biotechnology and green chemistry. The manufacturing of a new, metal-free, laccase-based biosensor with unprecedented reuse and storage capabilities has been achieved in this work through the application of the electrospray deposition (ESD) methodology as the enzyme immobilization technique. Electrospray ionization (ESI) has been used for ambient soft-landing of laccase enzymes on a carbon substrate, employing sustainable chemistry. This study shows how the ESD technique can be successfully exploited for the fabrication of a new promising environment-friendly electrochemical amperometric laccase-based biosensor, with storage capability up to two months without any particular care and reuse performance up to 63 measurements on the same electrode just prepared and 20 measurements on the one-year-old electrode subjected to redeposition. The laccase-based biosensor has been tested for catechol detection in the linear range 2-100 μM, with a limit of detection of 1.7 μM, without interference from chrome, cadmium, arsenic, and zinc and without any memory effects.

Picomole-Scale Transition Metal Electrocatalysis Screening Platform for Discovery of Mild C-C Coupling and C-H Arylation through in Situ Anodically Generated Cationic Pd

Development of new transition-metal-catalyzed electrochemistry promises to improve overall synthetic efficiency. Here, we describe the first integrated platform for online screening of electrochemical transition-metal catalysis. It utilizes the intrinsic electrochemical capabilities of nanoelectrospray ionization mass spectrometry (nano-ESI-MS) and picomole-scale anodic corrosion of a Pd electrode to generate and evaluate highly efficient cationic catalysts for mild electrocatalysis. We demonstrate the power of the novel electrocatalysis platform by (1) identifying electrolytic Pd-catalyzed Suzuki coupling at room temperature, (2) discovering Pd-catalyzed electrochemical C-H arylation in the absence of external oxidant or additive, (3) developing electrolyzed Suzuki coupling/C-H arylation cascades, and (4) achieving late-stage functionalization of two drug molecules by the newly developed mild electrocatalytic C-H arylation. More importantly, the scale-up reactions confirm that new electrochemical pathways discovered by nano-ESI can be implemented under the conventional electrolytic reaction conditions. This approach enables in situ mechanistic studies by capturing various intermediates including transient transition metal species by MS, and thus uncovering the critical role of anodically generated cationic Pd catalyst in promoting otherwise sluggish transmetalation in C-H arylation. The anodically generated cationic Pd with superior catalytic efficiency and novel online electrochemical screening platform hold great potential for discovering mild transition-metal-catalyzed reactions.

Ambient microdroplet annealing of nanoparticles

Conversion of polydisperse nanoparticles to their monodisperse analogues and formation of organized superstructures using them involve post synthetic modifications, and the process is generally slow. We show that ambient electrospray of preformed polydisperse nanoparticles makes them monodisperse and the product nanoparticles self-assemble spontaneously to form organized films, all within seconds. This phenomenon has been demonstrated with thiol-protected polydisperse silver nanoparticles of 15 ± 10 nm diameter. Uniform silver nanoparticles of 4.0 ± 0.5 nm diameter were formed after microdroplet spray, and this occurred without added chemicals, templates, and temperature, and within the time needed for electrospray, which was of the order of seconds. Well organized nanoparticle assemblies were obtained from such uniform particles. A home-made and simple nanoelectrospray set-up produced charged microdroplets for the generation of such nanostructures, forming cm² areas of uniform nanoparticles. A free-standing thin film of monodisperse silver nanoparticles was also made on a liquid surface by controlling the electrospray conditions. This unique method may be extended for the creation of advanced materials of many kinds.

Polydisperse silver nanoparticles were converted to a highly ordered assembly of nanoparticles by microdroplet-induced chemistry, under ambient conditions, within seconds.

Synthetic strategies of supported atomic clusters for heterogeneous catalysis

Supported atomic clusters with uniform metal sites and definite low-nuclearity are intermediate states between single-atom catalysts (SACs) and nanoparticles in size. Benefiting from the presence of metal–metal bonds, supported atomic clusters can trigger synergistic effects among every metal atom, which contributes to achieving unique catalytic properties different from SACs and nanoparticles. However, the scalable and precise synthesis and atomic-level insights into the structure–properties relationship of supported atomic clusters is a great challenge. This perspective presents the latest progress of the synthesis of supported atomic clusters, highlights how the structure affects catalytic properties, and discusses the limitations as well as prospects.

Supported atomic clusters with precise nuclearity are intermediate states between single-atom catalysts and nanoparticles in size. Here the authors summarize and discuss synthetic strategies of supported atomic clusters with unique catalytic properties for heterogeneous reactions.

Structural insights into metal-metalloid glasses from mass spectrometry

Despite being studied for nearly 50 years, smallest chemically stable moieties in the metallic glass (MG) could not be found experimentally. Herein, we demonstrate a novel experimental approach based on electrochemical etching of amorphous alloys in inert solvent (acetonitrile) in the presence of a high voltage (1 kV) followed by detection of the ions using electrolytic spray ionization mass spectrometry (ESI MS). The experiment shows stable signals corresponding to Pd, PdSi and PdSi₂ ions, which emerges due to the electrochemical etching of the Pd₈₀Si₂₀ metallic glass electrode. These fragments are observed from the controlled dissolution of the Pd₈₀Si₂₀ melt-spun ribbon (MSR) electrode. Annealed electrode releases different fragments in the same experimental condition. These specific species are expected to be the smallest and most stable chemical units from the metallic glass which survived the chemical dissolution and complexation (with acetonitrile) process. Theoretically, these units can be produced from the cluster based models for the MG. Similar treatment on Pd₄₀Ni₄₀P₂₀ MSR resulted several complex peaks consisting of Pd, Ni and P in various combinations suggesting this can be adopted for any metal-metalloid glass.

Reaction of chloroauric acid with histidine in microdroplets yields a catalytic Au-(His)2 complex

An aqueous solution containing histidine (His, 100 μM) and chloroauric acid (HAuCl4, 10 μM) is electrosprayed (-4.5 kV) from a capillary (50 μm in diameter) with N2 nebulizing gas (120 psi). The resulting microdroplets entered a mass spectrometer with a 2 cm flight path. The mass spectrum recorded in negative ion mode showed several peaks including the Au5 nanocluster with the major one being [Au + 2His-2H]-, which is a catalytically active species. The reaction time was less 1 ms, and the yield of [Au + 2His-2H]- was 76%. In contrast, the bulk reaction for the same concentration run at room temperature for 2 h did not produce this species but instead formed Au10 nanocluster. When a solution of water and acetonitrile (1 : 1) containing indoline (100 mM) and the phenylacetylene (200 mM) as well as histidine and chloroauric acid at the same concentrations as above was electrosprayed, the mass spectrum showed the formation of the intermediate [Au + 2His + phenylacetylene + H]+. Upon collecting the microdroplets, the 4-methyl-4,6-diphenyl-1,2-dihydro-4H-pyrrolo[3,2,1-ij] quinolone product was observed by 1H nuclear magnetic resonance and liquid chromatography with a yield of 44%. The microdroplet synthesis using the Au-(His)2 complex as a catalyst was scaled up using room-temperature ultrasonic nebulization to produce the product at the rate of 35 mg min-1, which is semi-preparative and demonstrates the promise of using microdroplet reactions for chemical synthesis.

High-yield gram-scale organic synthesis using accelerated microdroplet/thin film reactions with solvent recycling

A closed system has been designed to perform microdroplet/thin film reactions with solvent recycling capabilities for gram-scale chemical synthesis. Claisen-Schmidt, Schiff base, Katritzky and Suzuki coupling reactions show acceleration factors relative to bulk of 15 to 7700 times in this droplet spray system. These values are much larger than those reported previously for the same reactions in microdroplet/thin film reaction systems. The solvent recycling mode of the new system significantly improves the reaction yield, especially for reactions with smaller reaction acceleration factors. The microdroplet/thin film reaction yield improved on recycling from 33% to 86% and from 32% to 72% for the Katritzky and Suzuki coupling reactions, respectively. The Claisen-Schmidt reaction was chosen to test the capability of this system in gram scale syntheses and rates of 3.18 g per h and an isolated yield of 87% were achieved.

Well-Defined Materials for Heterogeneous Catalysis: From Nanoparticles to Isolated Single-Atom Sites

The use of well-defined materials in heterogeneous catalysis will open up numerous new opportunities for the development of advanced catalysts to address the global challenges in energy and the environment. This review surveys the roles of nanoparticles and isolated single atom sites in catalytic reactions. In the second section, the effects of size, shape, and metal-support interactions are discussed for nanostructured catalysts. Case studies are summarized to illustrate the dynamics of structure evolution of well-defined nanoparticles under certain reaction conditions. In the third section, we review the syntheses and catalytic applications of isolated single atomic sites anchored on different types of supports. In the final part, we conclude by highlighting the challenges and opportunities of well-defined materials for catalyst development and gaining a fundamental understanding of their active sites.

Multi-metal electrohydrodynamic redox 3D printing at the submicron scale

An extensive range of metals can be dissolved and re-deposited in liquid solvents using electrochemistry. We harness this concept for additive manufacturing, demonstrating the focused electrohydrodynamic ejection of metal ions dissolved from sacrificial anodes and their subsequent reduction to elemental metals on the substrate. This technique, termed electrohydrodynamic redox printing (EHD-RP), enables the direct, ink-free fabrication of polycrystalline multi-metal 3D structures without the need for post-print processing. On-the-fly switching and mixing of two metals printed from a single multichannel nozzle facilitates a chemical feature size of <400 nm with a spatial resolution of 250 nm at printing speeds of up to 10 voxels per second. As shown, the additive control of the chemical architecture of materials provided by EHD-RP unlocks the synthesis of 3D bi-metal structures with programmed local properties and opens new avenues for the direct fabrication of chemically architected materials and devices.

DRILL Interface Makes Ion Soft Landing Broadly Accessible for Energy Science and Applications

Polyoxometalates (POM) have been deposited onto carbon nanotube (CNT) electrodes using benchtop ion soft landing (SL) enabled by a vortex-confined electrohydrodynamic desolvation process. The device is based on the dry ion localization and locomotion (DRILL) mass spectrometry interface of Fedorov and co-workers. By adding electrospray emitters, heating the desolvation gas, and operating at high gas flow rates, it is possible to obtain stable ion currents up to -15 nA that are ideal for deposition. Coupled with ambient ion optics, this interface enables desolvated ions to be delivered to surfaces while excluding solvent and counterions. Electron microscopy of surfaces prepared using the device reveal discrete POM and no aggregation that degrades electrode performance. Characterization of POM-coated CNT electrodes in a supercapacitor showed an energy storage capacity similar to that achieved with SL in vacuum. For solutions that produce primarily a single ion by electrospray ionization, benchtop SL offers a simpler and less costly approach for surface modification with applications in catalysis, energy storage, and beyond.

Soft- and reactive landing of ions onto surfaces: Concepts and applications

Soft- and reactive landing of mass-selected ions is gaining attention as a promising approach for the precisely-controlled preparation of materials on surfaces that are not amenable to deposition using conventional methods. A broad range of ionization sources and mass filters are available that make ion soft-landing a versatile tool for surface modification using beams of hyperthermal (<100 eV) ions. The ability to select the mass-to-charge ratio of the ion, its kinetic energy and charge state, along with precise control of the size, shape, and position of the ion beam on the deposition target distinguishes ion soft landing from other surface modification techniques. Soft- and reactive landing have been used to prepare interfaces for practical applications as well as precisely-defined model surfaces for fundamental investigations in chemistry, physics, and materials science. For instance, soft- and reactive landing have been applied to study the surface chemistry of ions isolated in the gas-phase, prepare arrays of proteins for high-throughput biological screening, produce novel carbon-based and polymer materials, enrich the secondary structure of peptides and the chirality of organic molecules, immobilize electrochemically-active proteins and organometallics on electrodes, create thin films of complex molecules, and immobilize catalytically active organometallics as well as ligated metal clusters. In addition, soft landing has enabled investigation of the size-dependent behavior of bare metal clusters in the critical subnanometer size regime where chemical and physical properties do not scale predictably with size. The morphology, aggregation, and immobilization of larger bare metal nanoparticles, which are directly relevant to the design of catalysts as well as improved memory and electronic devices, have also been studied using ion soft landing. This review article begins in section 1 with a brief introduction to the existing applications of ion soft- and reactive landing. Section 2 provides an overview of the ionization sources and mass filters that have been used to date for soft landing of mass-selected ions. A discussion of the competing processes that occur during ion deposition as well as the types of ions and surfaces that have been investigated follows in section 3. Section 4 discusses the physical phenomena that occur during and after ion soft landing, including retention and reduction of ionic charge along with factors that impact the efficiency of ion deposition. The influence of soft landing on the secondary structure and biological activity of complex ions is addressed in section 5. Lastly, an overview of the structure and mobility as well as the catalytic, optical, magnetic, and redox properties of bare ionic clusters and nanoparticles deposited onto surfaces is presented in section 6.

Rational design of efficient electrode-electrolyte interfaces for solid-state energy storage using ion soft landing

The rational design of improved electrode-electrolyte interfaces (EEI) for energy storage is critically dependent on a molecular-level understanding of ionic interactions and nanoscale phenomena. The presence of non-redox active species at EEI has been shown to strongly influence Faradaic efficiency and long-term operational stability during energy storage processes. Herein, we achieve substantially higher performance and long-term stability of EEI prepared with highly dispersed discrete redox-active cluster anions (50 ng of pure ∼0.75 nm size molybdenum polyoxometalate (POM) anions on 25 μg (∼0.2 wt%) carbon nanotube (CNT) electrodes) by complete elimination of strongly coordinating non-redox species through ion soft landing (SL). Electron microscopy provides atomically resolved images of a uniform distribution of individual POM species soft landed directly on complex technologically relevant CNT electrodes. In this context, SL is established as a versatile approach for the controlled design of novel surfaces for both fundamental and applied research in energy storage.