Five years of scanning electrochemical cell microscopy (SECCM): new insights and innovations.

Scanning electrochemical cell microscopy (SECCM) is a nanopipette-based technique which enables measurement of localised electrochemistry. SECCM has found use in a wide range of electrochemical applications, and due to the wider uptake of this technique in recent years, new applications and techniques have been developed. This minireview has collected all SECCM research articles published in the last 5 years, to demonstrate and celebrate the recent advances, and to make it easier for SECCM researchers to remain well-informed. The wide range of SECCM applications is demonstrated, which are categorised here into electrocatalysis, electroanalysis, photoelectrochemistry, biological materials, energy storage materials, corrosion, electrosynthesis, and instrumental development. In the collection of this library of SECCM studies, a few key trends emerge. (1) The range of materials and processes explored with SECCM has grown, with new applications emerging constantly. (2) The instrumental capabilities of SECCM have grown, with creative techniques being developed from research groups worldwide. (3) The SECCM research community has grown significantly, with adoption of the SECCM technique becoming more prominent.

Drop-cast gold nanoparticles are not always electrocatalytically active for the borohydride oxidation reaction.

The next-generation of energy devices rely on advanced catalytic materials, especially electrocatalytic nanoparticles (NPs), to achieve the performance and cost required to reshape the energy landscape towards a more sustainable and cleaner future. It has become imperative to maximize the performance of the catalyst, both through improvement of the intrinsic activity of the NP, and by ensuring all particles are performing at the level of their capability. This requires not just a structure-function understanding of the catalytic material, but also an understanding of how the catalyst performance is impacted by its environment (substrate, ligand, etc.). The intrinsic activity and environment of catalytic particles on a support may differ wildly by particle, thus it is essential to build this understanding from a single-entity perspective. To achieve this herein, scanning electrochemical cell microscopy (SECCM) has been used, which is a droplet-based scanning probe technique which can encapsulate single NPs, and apply a voltage to the nanoparticle whilst measuring its resulting current. Using SECCM, single AuNPs have been encapsulated, and their activity for the borohydride oxidation reaction (BOR) is measured. A total of 268 BOR-active locations were probed (178 single particles) and a series of statistical analyses were performed in order to make the following discoveries: (1) a certain percentage of AuNPs display no BOR activity in the SECCM experiment (67.4% of single NPs), (2) visibly-similar particles display wildly varied BOR activities which cannot be explained by particle size, (3) the impact of cluster size (#NP at a single location) on a selection of diagnostic electrochemical parameters can be easily probed with SECCM, (4) exploratory statistical correlation between these parameters can be meaningfully performed with SECCM, and (5) outlying "abnormal" NP responses can be probed on a particle-by-particle basis. Each one of these findings is its own worthwhile study, yet this has been achieved with a single SECCM scan. It is hoped that this research will spur electrochemists and materials scientists to delve deeper into their substantial datasets in order to enhance the structure-function understanding, to bring about the next generation of high-performance electrocatalysts.

Structure-dependent CO2 reduction on molybdenite (MoS2) electrocatalysts.

Scanning electrochemical cell microscopy (SECCM) is employed to directly identify the structure-dependent electrochemical CO2 reduction reaction (eCO2RR) activity of molybdenite (MoS2) electrocatalysts in an aqueous imidazolium-based aprotic ionic liquid electrolyte. Nanoscale defects, where the edge plane (EP) is exposed, are directly targeted, revealing heightened overall activity (eCO2RR + the competing hydrogen evolution reaction, HER) over the relatively inactive basal plane (BP). In addition, certain types of defects (e.g., step edges) only exhibit heightened activity under a CO2 atmosphere (i.e., compared to N2), indirectly confirming higher selectivity at these surface sites. Overall, this work will guide the bottom-up design of earth-abundant electrocatalysts for use in large-scale CO2 electrolysis.

Direct electrochemical identification of rare microscopic catalytic active sites.

Local voltammetric analysis with a scanning electrochemical droplet cell technique, in combination with a new data processing protocol (termed data binning and trinisation), is used to directly identify previously unseen regions of elevated electrocatalytic activity on the basal plane (BP) of molybdenum disulfide (2H-MoS2). This includes BP-like structures with hydrogen evolution reaction activities approaching that of the edge plane and rare nanoscale electrocatalytic "hot-spots" present at an areal density of approximately 0.2-1 µm-2. Understanding the nature of (sub)microscopic catalytic active sites, such as those identified herein, is crucial to guide the rational design of next-generation earth-abundant materials for renewable fuels production.

Anticancer Activity and Cisplatin Binding Ability of Bis-Quinoline and Bis-Isoquinoline Derived [Pd2L4]4+ Metallosupramolecular Cages.

New bis-quinoline (L q) and bis-isoquinoline-based (L iq) ligands have been synthesized, along with their respective homoleptic [Pd2(L q or L iq)4]4+ cages (C q and C iq). The ligands and cages were characterized by 1H, 13C and diffusion ordered (DOSY) NMR spectroscopies, high resolution electrospray ionization mass spectrometry (HR-ESIMS) and in the case of the bis-quinoline cage, X-ray crystallography. The crystal structure of the C q architecture showed that the [Pd2(L q)4]4+ cage formed a twisted meso isomer where the [Pd(quinoline)4]2+ units at either end of the cage architecture adopt the opposite twists (left and right handed). Conversely, Density Functional Theory (DFT) calculations on the C iq cage architecture indicated that a lantern shaped conformation, similar to what has been observed before for related [Pd2(L tripy)4]4+ systems (where L tripy = 2,6-bis(pyridin-3-ylethynyl)pyridine), was generated. The different cage conformations manifest different properties for the isomeric cages. The C iq cage is able to bind, weakly in acetonitrile, the anticancer drug cisplatin whereas the C q architecture shows no interaction with the guest under the same conditions. The kinetic robustness of the two cages in the presence of Cl- nucleophiles was also different. The C iq cage was completely decomposed into free L iq and [Pd(Cl)4]2- within 1 h. However, the C q cage was more long lived and was only fully decomposed after 7 h. The new ligands (L iq and L q) and the Pd(II) cage architectures (C iq and C q) were assessed for their cytotoxic properties against two cancerous cell lines (A549 lung cancer and MDA-MB-231 breast cancer) and one non-cancerous cell line (HDFa skin cells). It was found that L q and C q were both reasonably cytotoxic (IC50S ≈ 0.5 µM) against A549, while C iq was slightly less active (IC50 = 7.4 µM). L iq was not soluble enough to allow the IC50 to be determined against either of the two cancerous cell lines. However, none of the molecules showed any selectivity for the cancer cells, as they were all found to have similar cytotoxicities against HDFa skin cells (IC50 values ranged from 2.6 to 3.0 µM).