Electrochemical Characterization and Catalytic Application of Gold-Supported Ferrocene-Containing Diblock Copolymer Thin Films in Ethanol Solution

This paper reports the electrochemical behavior and catalytic property of electrode-supported thin films of polystyrene-<i>block</i>-poly­(2-(acryloyloxy)­ethyl ferrocenecarboxylate) (PS-<i>b</i>-PAEFc) in an ethanol (EtOH) solution. The electrochemical properties of PS-<i>b</i>-PAEFc films with different PAEFc volume fractions (<i>f</i>_PAEFc = 0.47, 0.30, and 0.17) in 0.1 M ethanolic sodium hexafluorophosphate (NaPF₆) were compared with those in an acetonitrile (MeCN) solution of 0.1 M tetrabutylammonium hexafluorophosphate. Pristine PS-<i>b</i>-PAEFc films did not afford significant faradaic currents in the EtOH solution because EtOH is a nonsolvent for both PS and PAEFc. However, the films could be rendered redox-active in the EtOH solution by applying potentials in the MeCN solution to induce the redox-associated incorporation of the supporting electrolytes into the films. Atomic force microscopy images verified the stability of PAEFc microdomains upon electrochemical measurements in these solutions. Cyclic voltammograms measured in the EtOH solution for PS-<i>b</i>-PAEFc with the larger <i>f</i>_PAEFc were diffusion-controlled regardless of ellipsometric film thickness (23–152 nm) at relatively slow scan rates, in contrast to those in the MeCN solution that were controlled by surface-confined redox species. The electron propagation efficiency in the EtOH solution was significantly lower than that in the MeCN solution because of the poorer swelling of the films, which limited the migration of counterions and the collisional motions of the ferrocene moieties. PS-<i>b</i>-PAEFc films were applied as electrochemically responsive heterogeneous catalysts based on the ferrocenium moieties for Michael addition reaction between methyl vinyl ketone and ethyl 2-oxocyclopentanecarboxylate (E2OC) in 0.1 M NaPF₆/EtOH. The catalytic activities of thin films were similar regardless of <i>f</i>_PAEFc, suggesting that the catalytic reaction took place for the reactants that could penetrate through the film and reach PAEFc microdomains communicable with the underlying electrode. Interestingly, the permeability of PS-<i>b</i>-PAEFc films provided a means to control the reaction selectivity, as suggested by negligible reaction of E2OC with <i>trans</i>-4-phenyl-3-buten-2-one

Electron Propagation within Redox-Active Microdomains in Thin Films of Ferrocene-Containing Diblock Copolymers

This paper reports the electrochemical behavior of redox-active microdomains in thin films of ferrocene-containing diblock copolymers, polystyrene-<i>block</i>-poly­(2-(acryl­oyloxy)­ethyl ferrocene­carboxylate) (PS-<i>b</i>-PAEFc). PS-<i>b</i>-PAEFc with different PAEFc volume fractions (PS₁₅₄-<i>b</i>-PAEFc₅₁, PS₁₅₄-<i>b</i>-PAEFc₂₆, and PS₁₅₄-<i>b</i>-PAEFc₁₂, where the subscripts represent the polymerization degree of each block; <i>f</i>_PAEFc = 0.47, 0.30, and 0.17, respectively) was synthesized by sequential atom transfer radical polymerization. PS-<i>b</i>-PAEFc films of controlled thicknesses (20–160 nm) were prepared on gold substrates via spin-coating and characterized by ellipsometry. Microdomains were observed via atomic force microscopy on the surfaces of PS₁₅₄-<i>b</i>-PAEFc₅₁ and PS₁₅₄-<i>b</i>-PAEFc₂₆ thin films but not on the surfaces of PS₁₅₄-<i>b</i>-PAEFc₁₂ thin films. Electrochemical behavior of films was assessed by cyclic voltammetry and chronocoulometry in acetonitrile solution. The redox potential of ferrocene moieties was similar (ca. + 0.29 V vs Fc⁺/Fc) regardless of <i>f</i>_PAEFc and film thickness. For PS₁₅₄-<i>b</i>-PAEFc₅₁ and PS₁₅₄-<i>b</i>-PAEFc₂₆, thicker films afforded larger faradaic peak currents and exhibited diffusion-controlled voltammograms at faster sweep rates. PS₁₅₄-<i>b</i>-PAEFc₂₆ produced voltammograms less influenced by solvent-induced swelling than PS₁₅₄-<i>b</i>-PAEFc₅₁, reflecting the improved morphological stability of PAEFc microdomains by redox-inert PS frameworks. In contrast, PS₁₅₄-<i>b</i>-PAEFc₁₂ films yielded similar faradaic peak currents regardless of film thickness and exhibited voltammograms indicative of surface-confined species. These observations suggest that PS₁₅₄-<i>b</i>-PAEFc₅₁ and PS₁₅₄-<i>b</i>-PAEFc₂₆ films contain continuous PAEFc microdomains extending from the electrode to the surface, in contrast to the PS₁₅₄-<i>b</i>-PAEFc₁₂ films which contain isolated PAEFc microdomains buried within the PS matrix. Electron propagation took place only through PAEFc microdomains that could electrically communicate with the underlying electrode. Apparent diffusion coefficients within PAEFc microdomains were similar (≈ 2 × 10^–11 cm²/s) for PS₁₅₄-<i>b</i>-PAEFc₅₁ and PS₁₅₄-<i>b</i>-PAEFc₂₆. The relatively low efficiency in electron propagation was attributable to ineffective electron self-exchange reaction within the PAEFc microdomains and/or limited counterion migration through the acetonitrile-swollen microdomains. These results provide guidance in design of redox-active metalloblock copolymers for various applications, which include electrocatalysis, electrochemical mediation in enzyme sensors, and redox-controlled molecular deposition

Nanoconfinement and Crowding Enhanced Single-Molecule Detection of Small Molecules with Nanopipettes

Glass nanopipettes have gained widespread use as a versatile single-entity detector in chemical and biological sensing, analysis, and imaging. Its advantages include low cost, easy accessibility, simplicity of use, and high versatility. However, conventional nanopipettes based on the volume exclusion mechanism have limitations in detecting small biomolecules due to their small volume and high mobility in aqueous solution. To overcome this challenge, we have employed a novel approach by capitalizing on the strong nanoconfinement effect of nanopipettes. This is achieved by utilizing both the hard confinement provided by the long taper nanopipette tip at the cis side and the soft confinement offered by the hydrogel at the trans side. Through this approach, we have effectively slowed down the exit motion of small molecules, allowing us to enrich and jam them at the nanopipette tip. Consequently, we have achieved high throughput detection of small biomolecules with sizes as small as 1 nm, including nucleoside triphosphates, short peptides, and small proteins with excellent signal-to-noise ratios. Furthermore, molecular complex formation through specific intermolecular interactions, such as hydrogen bonding between closely spaced nucleotides in the jam-packed nanopipette tip, has been detected based on the unique ionic current changes

Probing the Intermediates of Catalyzed Dehydration Reactions of Primary Amide to Nitrile in Plasmonic Junctions

Visible light can effectively drive chemical reactions in plasmonic molecular junctions owing to the high reactivity of adatoms at the surface of plasmonic metal nanostructures and the localized surface plasmon resonance (LSPR)-induced energetic charge carriers (electrons and holes) and heat. Here, we investigated the dehydration reaction of primary amides, which is important to generate valuable nitrile molecules, in the visible-light-irradiated self-assembled gold nanoparticle–aromatic primary amide–gold nanoelectrode junctions in aqueous solution under ambient conditions. At present, the research on the dehydration reaction of the primary amide group is only at the macroscopic level, limiting the mechanistic study of reaction dynamics and intermediates. Using time-resolved surface enhanced Raman spectroscopy (SERS) with tens of millisecond time resolution, we successfully followed the evolution of the SERS spectra along with various transient spectral changes during the rise of the nitrile vibration peak. Combined with density functional theory and a picocavity model, we revealed that most pronounced transient spectral changes were from the gold surface adatom-coupled reaction intermediates. The adatoms produced picocavities with a strong atomic size local field, which strongly enhanced the SERS signals of the intermediates down to sub-single-molecule resolution. The active adatoms played critical roles in producing, interacting, and stabilizing the intermediates. We have determined the complex reaction pathway involving multiple proton transfer steps and intermediates with signature carbon–nitrogen double and triple bonds

Diffusion Behavior of Differently Charged Molecules in Self-Assembled Organic Nanotubes Studied Using Imaging Fluorescence Correlation Spectroscopy

The diffusion behavior of fluorescent molecules within bolaamphiphile-based organic nanotubes (ONTs) was systematically investigated using imaging fluorescence correlation spectroscopy (imaging FCS). Anionic sulforhodamine B, zwitterionic/cationic rhodamine B, or cationic rhodamine 123 was loaded into ONTs having cylindrical hollow structures (ca. 10 nm in inner diameter) with amine and glucose groups on the inner and outer surfaces, respectively. Wide-field fluorescence video microscopy was used to acquire imaging FCS data for dye-doped ONTs in aqueous solutions of different ionic strengths (1–500 mM) at different pH (3.4–8.4). The diffusion behavior of these dyes was discussed on the basis of their apparent diffusion coefficients (D) that were determined by autocorrelating the time transient of fluorescence intensity at each pixel on an ONT. Molecular diffusion in the ONTs was significantly slowed by the molecule–nanotube interactions, as shown by the very small D (10–1 to 10–2 μm2/s). The pH dependence of D revealed that dye diffusion was basically controlled by electrostatic interactions associated with the protonation of the amine groups on the ONT inner surface. The pH-dependent change in D was observed over a wide pH range, possibly because of electrostatically induced variations in the pKa of the densely packed ammonium ions on the ONT inner surface. On the other hand, the influence of ionic strength on D was relatively unclear, suggesting the involvement of non-Coulombic interactions with the ONTs in molecular diffusion. Importantly, individual ONTs of different lengths (1–5 μm) afforded similar diffusion coefficients for each type of dye at each solution condition, implying that the properties of the ONTs were uniform in terms of solute loading and release. These results highlight the characteristics of the molecular diffusion behavior within the ONTs and will help in the design of ONTs better suited for use as drug vehicles and contaminant adsorbents

Diffusion Behavior of Differently Charged Molecules in Self-Assembled Organic Nanotubes Studied Using Imaging Fluorescence Correlation Spectroscopy

The diffusion behavior of fluorescent molecules within bolaamphiphile-based organic nanotubes (ONTs) was systematically investigated using imaging fluorescence correlation spectroscopy (imaging FCS). Anionic sulforhodamine B, zwitterionic/cationic rhodamine B, or cationic rhodamine 123 was loaded into ONTs having cylindrical hollow structures (ca. 10 nm in inner diameter) with amine and glucose groups on the inner and outer surfaces, respectively. Wide-field fluorescence video microscopy was used to acquire imaging FCS data for dye-doped ONTs in aqueous solutions of different ionic strengths (1–500 mM) at different pH (3.4–8.4). The diffusion behavior of these dyes was discussed on the basis of their apparent diffusion coefficients (D) that were determined by autocorrelating the time transient of fluorescence intensity at each pixel on an ONT. Molecular diffusion in the ONTs was significantly slowed by the molecule–nanotube interactions, as shown by the very small D (10–1 to 10–2 μm2/s). The pH dependence of D revealed that dye diffusion was basically controlled by electrostatic interactions associated with the protonation of the amine groups on the ONT inner surface. The pH-dependent change in D was observed over a wide pH range, possibly because of electrostatically induced variations in the pKa of the densely packed ammonium ions on the ONT inner surface. On the other hand, the influence of ionic strength on D was relatively unclear, suggesting the involvement of non-Coulombic interactions with the ONTs in molecular diffusion. Importantly, individual ONTs of different lengths (1–5 μm) afforded similar diffusion coefficients for each type of dye at each solution condition, implying that the properties of the ONTs were uniform in terms of solute loading and release. These results highlight the characteristics of the molecular diffusion behavior within the ONTs and will help in the design of ONTs better suited for use as drug vehicles and contaminant adsorbents

Diffusion Behavior of Differently Charged Molecules in Self-Assembled Organic Nanotubes Studied Using Imaging Fluorescence Correlation Spectroscopy

The diffusion behavior of fluorescent molecules within bolaamphiphile-based organic nanotubes (ONTs) was systematically investigated using imaging fluorescence correlation spectroscopy (imaging FCS). Anionic sulforhodamine B, zwitterionic/cationic rhodamine B, or cationic rhodamine 123 was loaded into ONTs having cylindrical hollow structures (ca. 10 nm in inner diameter) with amine and glucose groups on the inner and outer surfaces, respectively. Wide-field fluorescence video microscopy was used to acquire imaging FCS data for dye-doped ONTs in aqueous solutions of different ionic strengths (1–500 mM) at different pH (3.4–8.4). The diffusion behavior of these dyes was discussed on the basis of their apparent diffusion coefficients (D) that were determined by autocorrelating the time transient of fluorescence intensity at each pixel on an ONT. Molecular diffusion in the ONTs was significantly slowed by the molecule–nanotube interactions, as shown by the very small D (10–1 to 10–2 μm2/s). The pH dependence of D revealed that dye diffusion was basically controlled by electrostatic interactions associated with the protonation of the amine groups on the ONT inner surface. The pH-dependent change in D was observed over a wide pH range, possibly because of electrostatically induced variations in the pKa of the densely packed ammonium ions on the ONT inner surface. On the other hand, the influence of ionic strength on D was relatively unclear, suggesting the involvement of non-Coulombic interactions with the ONTs in molecular diffusion. Importantly, individual ONTs of different lengths (1–5 μm) afforded similar diffusion coefficients for each type of dye at each solution condition, implying that the properties of the ONTs were uniform in terms of solute loading and release. These results highlight the characteristics of the molecular diffusion behavior within the ONTs and will help in the design of ONTs better suited for use as drug vehicles and contaminant adsorbents