Surface Charge Measurements with Scanning Ion Conductance Microscopy Provide Insights into Nitrous Acid Speciation at the Kaolin Mineral-Air Interface

Unveiling the underestimated direct emissions of nitrous acid (HONO)

Gaseous nitrous acid (HONO) is a critical source of hydroxyl radicals (OH) in the troposphere. While both direct and secondary sources contribute to atmospheric HONO, direct emissions have traditionally been considered minor contributors. In this study, we developed δ15N and δ18O isotopic fingerprints to identify six direct HONO emission sources and conducted a 1-y case study on the isotopic composition of atmospheric HONO at rural and urban sites. Interestingly, we identified that livestock farming is a previously overlooked direct source of HONO and determined its HONO to ammonia (NH3) emission ratio. Additionally, our results revealed that spatial and temporal variations in atmospheric HONO isotopic composition can be partially attributed to direct emissions. Through a detailed HONO budget analysis incorporating agricultural sources, we found that direct HONO emissions accounted for 39~45% of HONO production in rural areas across different seasons. The findings were further confirmed by chemistry transport model simulations, highlighting the significance of direct HONO emissions and their impact on air quality in the North China Plain. These findings provide compelling evidence that direct HONO emissions play a more substantial role in contributing to atmospheric HONO than previously believed. Moreover, the δ15N and δ18O isotopic fingerprints developed in this study may serve as a valuable tool for further research on the atmospheric chemistry of reactive nitrogen gases.

Charge Mapping of Pseudomonas aeruginosa Using a Hopping Mode Scanning Ion Conductance Microscopy Technique

Scanning ion conductance microscopy (SICM) is a topographic imaging technique capable of probing biological samples in electrolyte conditions. SICM enhancements have enabled surface charge detection based on voltage-dependent signals. Here, we show how the hopping mode SICM method (HP-SICM) can be used for rapid and minimally invasive surface charge mapping. We validate our method usingPseudomonas aeruginosaPA14 (PA) cells and observe a surface charge density of σ_PA = -2.0 ± 0.45 mC/m² that is homogeneous within the ∼80 nm lateral scan resolution. This biological surface charge is detected from at least 1.7 μm above the membrane (395× the Debye length), and the long-range charge detection is attributed to electroosmotic amplification. We show that imaging with a nanobubble-plugged probe reduces perturbation of the underlying sample. We extend the technique to PA biofilms and observe a charge density exceeding -20 mC/m². We use a solid-state calibration to quantify surface charge density and show that HP-SICM cannot be quantitatively described by a steady-state finite element model. This work contributes to the body of scanning probe methods that can uniquely contribute to microbiology and cellular biology.

A comparative study on adsorption and catalytic degradation of tetracycline by five magnetic mineral functional materials prepared from steel pickling waste liquor

Palygorskite (Pal), bentonite (Bent), sepiolite (Sep), zeolite (Zeol), and kaolin (Kaol) were used with steel pickling waste liquor to synthesize magnetic palygorskite (Pal@Fe₃O₄), magnetic bentonite (Bent@Fe₃O₄), magnetic sepiolite (Sep@Fe₃O₄), magnetic zeolite (Zeol@Fe₃O₄), and magnetic kaolin (Kaol@Fe₃O₄), for adsorption and catalytic degradation of tetracycline (TC), respectively. Through the study of adsorption kinetics and adsorption isotherms, the maximum adsorption capacity of Pal@Fe₃O₄ to TC was 149.439 mg/g, which was 1.239 times, 2.260 times, 3.161 times, and 3.448 times of Bent@Fe₃O₄, Zeol@Fe₃O₄, Kaol@Fe₃O₄, and Sep@Fe₃O₄, respectively. The kinetic study of tetracycline degradation demonstrated that the maximum reaction rate constant of Bent@Fe₃O₄/H₂O₂ system was K_(obs) = 2.12 × 10^-2 min^-1, which was close to that of Pal@Fe₃O₄/H₂O₂, Kaol@Fe₃O₄/H₂O₂ system, and was 2.000 times, 2.356 times, 2.650 times, and 4.711 times of Fe₃O₄/H₂O₂, Zeol@Fe₃O₄/H₂O₂, Sep@Fe₃O₄/H₂O₂, and H₂O₂ system, respectively. The results showed that Pal@Fe₃O₄ and Bent@Fe₃O₄ were more advantageous in the treatment of wastewater containing tetracycline, and efficient reuse of exhausted magnetic minerals and deep mineralization of organic pollutants were achieved by constructing an advanced oxidation system. The BET, VSM, SEM, XPS, XRD, and FTIR were used to characterize the five clay minerals before and after magnetic modification. It was speculated that the surface structure - OH groups of clay minerals might be significant factors influencing the adsorption performance of magnetic minerals on TC, and reduction ability of clay minerals to Fe³⁺ importantly affected the catalytic performance of magnetic minerals. The specific surface area and morphological structure of clay minerals both affected the adsorption and catalytic degradation of TC by the five magnetic minerals.

Probing and Visualizing Interfacial Charge at Surfaces in Aqueous Solution

Surface charge density and distribution play an important role in almost all interfacial processes, influencing, for example, adsorption, colloidal stability, functional material activity, electrochemical processes, corrosion, nanoparticle toxicity, and cellular processes such as signaling, absorption, and adhesion. Understanding the heterogeneity in, and distribution of, surface and interfacial charge is key to elucidating the mechanisms underlying reactivity, the stability of materials, and biophysical processes. Atomic force microscopy (AFM) and scanning ion conductance microscopy (SICM) are highly suitable for probing the material/electrolyte interface at the nanoscale through recent advances in probe design, significant instrumental (hardware and software) developments, and the evolution of multifunctional imaging protocols. Here, we assess the capability of AFM and SICM for surface charge mapping, covering the basic underpinning principles alongside experimental considerations. We illustrate and compare the use of AFM and SICM for visualizing surface and interfacial charge with examples from materials science, geochemistry, and the life sciences.

Gauging surface charge distribution of live cell membrane by ionic current change using scanning ion conductance microscopy

The distribution of surface charge and potential of cell membrane plays an indispensable role in cellular activities. However, probing surface charge of live cells under physiological conditions, until recently, remains an arduous challenge owing to the lack of effective methods. Scanning ion conductance microscopy (SICM) is an emerging imaging technique for imaging a live cell membrane in its native state. Here, we introduce a simple SICM based imaging technique to effectively map the surface charge contrast distribution of soft substrates including cell membranes by utilizing the higher surface charge sensitivity of the ionic current when the nanopipette tip is close to the substrate with a relatively high current change. This technique was assessed on charged model substrates made of polydimethylsiloxane, and the surface charge sensitivity of ionic current change was supported by finite element method simulations. With this method, we can distinguish the surface charge difference between the cell membrane and the supporting collagen matrix. We also observed the surface charge change induced by the small membrane damage after 1% dimethyl sulfoxide (DMSO) treatment. This new SICM technique provides opportunities to study interfacial and cell membrane processes with high spatial resolution.