Nanointerfaces: Concepts and Strategies for Optical and X-ray Spectroscopic Characterization

Interfaces at the nanoscale, also called nanointerfaces, play a fundamental role in physics and chemistry. Probing the chemical and electronic environment at nanointerfaces is essential in order to elucidate chemical processes relevant for applications in a variety of fields. Many spectroscopic techniques have been applied for this purpose, although some approaches are more appropriate than others depending on the type of the nanointerface and the physical properties of the different phases. In this Perspective, we introduce the major concepts to be considered when characterizing nanointerfaces. In particular, the interplay between the characteristic length of the nanointerfaces, and the probing and information depths of different spectroscopy techniques is discussed. Differences between nano- and bulk interfaces are explained and illustrated with chosen examples from optical and X-ray spectroscopies, focusing on solid-liquid nanointerfaces. We hope that this Perspective will help to prepare spectroscopic characterization of nanointerfaces and stimulate interest in the development of new spectroscopic techniques adapted to the nanointerfaces.

Room-Temperature One-Pot Synthesis of pH-Responsive Pyridine-Functionalized Carbon Surfaces

Carbon surfaces (glassy carbon, graphite, and boron-doped diamond) were functionalized with layers composed of linked pyridinium and pyridine moieties using simple electrochemical reduction of trifluoroacetylpyridinium. The pyridinium species was generated in situ in solution by the reaction of trifluoroacetic anhydride and pyridine precursors and underwent electrochemical reduction at -1.97 V vs Fc/Fc⁺, as determined by cyclic voltammetry. The pyridine/pyridinium films were electrodeposited at room temperature, on a timescale of minutes, and were characterized using X-ray photoelectron spectroscopy. The as-prepared films have a net positive charge in aqueous solution at pH 9 and below due to the pyridinium content, confirmed by the electrochemical response of differently charged redox molecules at the functionalized surfaces. The positive charge can be enhanced further through protonation of the neutral pyridine component by controlling the solution pH. Moreover, the nitrogen-acetyl bond can be cleaved through base treatment to purposefully increase the neutral pyridine proportion of the film. This results in a surface that can be "switched" from functionally near neutral to a positive charge by treatment in basic and acidic solutions, respectively, through manipulation of the protonation state of the pyridine. The functionalization process demonstrated here is readily achievable at a fast timescale at room temperature and hence can allow for rapid screening of surface properties. Such functionalized surfaces present a means to test in isolation the specific catalytic performance of pyridinic groups toward key processes such as oxygen and CO₂ reduction.

The stability mechanism of Pickering emulsions fabricated by multi-functional amylose-based nanoparticles in a delivery system

In this work, multi-functional amylose-based nanoparticles (OSA-AM-9/VE NPs) were fabricated via simple and sustainable esterification, encapsulation, and co-precipitation processes of amylose (AM), octenyl succinic anhydride (OSA), and vitamin E (VE). These nanoparticles showed a nanometer size of 243.2 nm and a regular spherical shape which contributed to their excellent physical and oxidative stability and the outstanding pH-responsive performance of a Pickering emulsion. Compared with OSA-AM-9 and OSA-AM-9 NPs, the Pickering emulsion stabilized by OSA-AM-9/VE NPs presented higher stability and stronger antioxidant capacity. The delivery system of the OSA-AM-9/VE NP stabilized emulsion could protect fish oil from gastric juice and then was digested to facilitate the absorption of ω-3 polyunsaturated fatty acids in the intestine due to the pH-induced protonation/deprotonation of carboxyl groups in OSA-AM-9/VE NPs.

Fabrication and characterization of octenyl succinate anhydride modified amylose with pH-responsive Pickering emulsion behavior

To make stable Pickering emulsion in stomach acid condition which are suitable for small intestine targeted delivery, the emulsifying ability and pH responsiveness mechanisms of octenyl succinate anhydride modified amylose (OSA-AM) with the formless state and nanoparticles (NP) form in the Pickering emulsion were compared. OSA-AMs and OSA-AM NPs were obtained by successively modification of OSA esterification reaction with amylose and nanoprecipitation process, respectively. OSA-AM NPs showed higher contact angle and lower interfacial tension than OSA-AMs, which suggested OSA-AM NPs have the stronger ability to form stable Pickering emulsion. In addition, to compare the stability of Pickering emulsion in different environment, the emulsion index (EI), photographs and microscopy images during storage time of 180 days in pH 2.0, pH 4.0 and pH 7.0 were monitored. Pickering emulsion formed by OSA-AM NPs exhibited stronger stability in acid environment (pH 2.0) than pH 4.0 and pH 7.0. However, Pickering emulsion stabilized by OSA-AMs presented the opposite pH-responsive behaviors with OSA-AM NPs. To further study the pH responsiveness mechanisms of Pickering emulsion, the morphology, contact angle, particle size and surface charge of OSA-AMs and OSA-AM NPs with pH changing were measured. These results suggested that the protonation/deprotonation process of carboxyl groups in difference pH condition revealed the pH-responsible behaviors of Pickering emulsion.

In Situ Determination of pH at Nanostructured Carbon Electrodes Using IR Spectroscopy

Changes in pH at electrode surfaces can occur when redox reactions involving the production or consumption of protons take place. Many redox reactions of biological or analytical importance are proton-coupled, resulting in localized interfacial pH changes as the reaction proceeds. Other important electrochemical reactions, such as hydrogen and oxygen evolution reactions, can likewise result in pH changes near the electrode. However, it is very difficult to measure pH changes located within around 100 µm of the electrode surface. This paper describes the use of in situ attenuated total reflectance (ATR) infrared (IR) spectroscopy to determine the pH of different solutions directly at the electrode interface, while a potential is applied. Changes in the distinctive IR bands of solution phosphate species are used as an indicator of pH change, given that the protonation state of the phosphate ions is pH-dependent. We found that the pH at the surface of an electrode modified with carbon nanotubes can increase from 4.5 to 11 during the hydrogen evolution reaction, even in buffered solutions. The local pH change accompanying the hydroquinone-quinone redox reaction is also determined.

In-Situ Infrared Spectroscopy Applied to the Study of the Electrocatalytic Reduction of CO2 : Theory, Practice and Challenges

The field of electrochemical CO₂ conversion is undergoing significant growth in terms of the number of publications and worldwide research groups involved. Despite improvements of the catalytic performance, the complex reaction mechanisms and solution chemistry of CO₂ have resulted in a considerable amount of discrepancies between theoretical and experimental studies. A clear identification of the reaction mechanism and the catalytic sites are of key importance in order to allow for a qualitative breakthrough and, from an experimental perspective, calls for the use of in-situ or operando spectroscopic techniques. In-situ infrared spectroscopy can provide information on the nature of intermediate species and products in real time and, in some cases, with relatively high time resolution. In this contribution, we review key theoretical aspects of infrared reflection spectroscopy, followed by considerations of practical implementation. Finally, recent applications to the electrocatalytic reduction of CO₂ are reviewed, including challenges associated with the detection of reaction intermediates.

Photochemistry of Organic Retinal Prostheses

Organic devices are attracting considerable attention as prostheses for the recovery of retinal light sensitivity lost to retinal degenerative disease. The biotic/abiotic interface created when light-sensitive polymers and living tissues are placed in contact allows excitation of a response in blind laboratory rats exposed to visual stimuli. Although polymer retinal prostheses have proved to be efficient, their working mechanism is far from being fully understood. In this review article, we discuss the results of the studies conducted on these kinds of polymer devices and compare them with the data found in the literature for inorganic retinal prostheses, where the working mechanisms are better comprehended. This comparison, which tries to set some reference values and figures of merit, is intended for use as a starting point to determine the direction for further investigation.

Insight into the Nature of Iron Sulfide Surfaces During the Electrochemical Hydrogen Evolution and CO2 Reduction Reactions

Greigite and other iron sulfides are potential, cheap, earth-abundant electrocatalysts for the hydrogen evolution reaction (HER), yet little is known about the underlying surface chemistry. Structural and chemical changes to a greigite (Fe₃S₄)-modified electrode were determined at -0.6 V versus standard hydrogen electrode (SHE) at pH 7, under conditions of the HER. In situ X-ray absorption spectroscopy was employed at the Fe K-edge to show that iron-sulfur linkages were replaced by iron-oxygen units under these conditions. The resulting material was determined as 60% greigite and 40% iron hydroxide (goethite) with a proposed core-shell structure. A large increase in pH at the electrode surface (to pH 12) is caused by the generation of OH^- as a product of the HER. Under these conditions, iron sulfide materials are thermodynamically unstable with respect to the hydroxide. In situ infrared spectroscopy of the solution near the electrode interface confirmed changes in the phosphate ion speciation consistent with a change in pH from 7 to 12 when -0.6 V versus SHE is applied. Saturation of the solution with CO₂ resulted in the inhibition of the hydroxide formation, potentially due to surface adsorption of HCO₃^-. This study shows that the true nature of the greigite electrode under conditions of the HER is a core-shell greigite-hydroxide material and emphasizes the importance of in situ investigation of the catalyst under operation to develop true and accurate mechanistic models.

Calcium carbonate crystallisation at charged graphite surfaces

For identical solution conditions, the crystallisation of calcium carbonate (polymorph and crystal orientation) at Highly Oriented Pyrolytic Graphite substrates is highly dependent on substrate surface charge.