Combined Ion Conductance and Atomic Force Microscope for Fast Simultaneous Topographical and Surface Charge Imaging

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.

Concluding remarks: next generation nanoelectrochemistry - next generation nanoelectrochemists

The aim of this paper is to describe the scientific journey taken to arrive at present-day nanoelectrochemistry and consider how the area might develop in the future, particularly in light of papers presented at this Faraday Discussion. By adopting a generational approach, this brief contribution traces the story of the nanoelectrochemistry family within the broader electrochemistry field, with a focus on scientific capability and themes that were important to each generation. I shall consider research questions and the impact of technology that was developed or available in each period. Nanoelectrochemistry is still somewhat niche, but is attracting increasing numbers of researchers. It is set to become a major part of electrochemistry and interfacial science. It is studied by people with a fairly unique skillset, and I shall speculate on the skills and expertise that will be needed by nanoelectrochemists to address the challenges and opportunities that lie ahead. I conclude by asking: who will be the nanoelectrochemists of the future and what will they do?

Scanning Ion Conductance Microscopy

Scanning ion conductance microscopy (SICM) has emerged as a versatile tool for studies of interfaces in biology and materials science with notable utility in biophysical and electrochemical measurements. The heart of the SICM is a nanometer-scale electrolyte filled glass pipette that serves as a scanning probe. In the initial conception, manipulations of ion currents through the tip of the pipette and appropriate positioning hardware provided a route to recording micro- and nanoscopic mapping of the topography of surfaces. Subsequent advances in instrumentation, probe design, and methods significantly increased opportunities for SICM beyond recording topography. Hybridization of SICM with coincident characterization techniques such as optical microscopy and faradaic electrodes have brought SICM to the forefront as a tool for nanoscale chemical measurement for a wide range of applications. Modern approaches to SICM realize an important tool in analytical, bioanalytical, biophysical, and materials measurements, where significant opportunities remain for further exploration. In this review, we chronicle the development of SICM from the perspective of both the development of instrumentation and methods and the breadth of measurements performed.

Scanning ion-conductance microscopy with a double-barreled nanopipette for topographic imaging of charged chromosomes

Scanning ion conductance microscopy (SICM) is useful for imaging soft and fragile biological samples in liquids because it probes the samples' surface topography by detecting ion currents under non-contact and force-free conditions. SICM acquires the surface topographical height by detecting the ion current reduction that occurs when an electrolyte-filled glass nanopipette approaches the sample surface. However, most biological materials have electrically charged surfaces in liquid environments, which sometimes affect the behavior of the ion currents detected by SICM and, especially, make topography measurements difficult. For measuring such charged samples, we propose a novel imaging method that uses a double-barrel nanopipette as an SICM probe. The ion current between the two apertures of the nanopipette desensitizes the surface charge effect on imaging. In this study, metaphase chromosomes of Indian muntjac were imaged by this technique because, owing to their strongly negatively charged surfaces in phosphate-buffered saline, it is difficult to obtain the topography of the chromosomes by the conventional SICM with a single-aperture nanopipette. Using the proposed method with a double-barrel nanopipette, the surfaces of the chromosomes were successfully measured, without any surface charge confounder. Since the detailed imaging of sample topography can be performed in physiological liquid conditions regardless of the sample charge, it is expected to be used for analyzing the high-order structure of chromosomes in relation to their dynamic changes in the cell division.

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

Unique surface properties of aluminosilicate clay minerals arise from anisotropic distribution of surface charge across their layered structures. Yet, a molecular-level understanding of clay mineral surfaces has been hampered by the lack of analytical techniques capable of measuring surface charges at the nanoscale. This is important for understanding the reactivity, colloidal stability, and ion-exchange capacity properties of clay minerals, which constitute a major fraction of global soils. In this work, scanning ion conductance microscopy (SICM) is used for the first time to visualize the surface charge and topography of dickite, a well-ordered member of the kaolin subgroup of clay minerals. Dickite displayed a pH-independent negative charge on basal surfaces whereas the positive charge on edges increased from pH 6 to 3. Surface charges responded to malonate addition, which promoted dissolution/precipitation reactions. Results from SICM were used to interpret heterogeneous reactivity studies showing that gas-phase nitrous acid (HONO) is released from the protonation of nitrite at Al-OH₂⁺ groups on dickite edges at pH well above the aqueous pK_a of HONO. This study provides nanoscale insights into mineral surface processes that affect environmental processes on the local and global scale.

Force-Controlled Formation of Dynamic Nanopores for Single-Biomolecule Sensing and Single-Cell Secretomics

Nanopore sensing of single nucleotides has emerged as a promising single-molecule technology for DNA sequencing and proteomics. Despite the conceptual simplicity of nanopores, adoption of this technology for practical applications has been limited by a lack of pore size adjustability and an inability to perform long-term recordings in complex solutions. Here we introduce a method for fast and precise on-demand formation of a nanopore with controllable size between 2 and 20 nm through force-controlled adjustment of the nanospace formed between the opening of a microfluidic device (made of silicon nitride) and a soft polymeric substrate. The introduced nanopore system enables stable measurements at arbitrary locations. By accurately positioning the nanopore in the proximity of single neurons and continuously recording single-molecule translations over several hours, we have demonstrated this is a powerful approach for single-cell proteomics and secretomics.

Single-Nanoparticle Thermometry with a Nanopipette

Thermal measurements at the nanoscale are key for designing technologies in many areas, including drug delivery systems, photothermal therapies, and nanoscale motion devices. Herein, we present a nanothermometry technique that operates in electrolyte solutions and, therefore, is applicable for many in vitro measurements, capable of measuring and mapping temperature with nanoscale spatial resolution and sensitive to detect temperature changes down to 30 mK with 43 μs temporal resolution. The methodology is based on local measurements of ionic conductivity confined at the tip of a pulled glass capillary, a nanopipettete, with opening diameters as small as 6 nm. When scanned above a specimen, the measured ion flux is converted into temperature using an extensive theoretical support given by numerical and analytical modeling. This allows quantitative thermal measurements with a variety of capillary dimensions and is applicable to a range of substrates. We demonstrate the capabilities of this nanothermometry technique by simultaneous mapping of temperature and topography on sub-micrometer-sized aggregates of thermoplasmonic nanoparticles heated by a laser and observe the formation of micro- and nanobubbles upon plasmonic heating. Furthermore, we perform quantitative thermometry on a single-nanoparticle level, demonstrating that the temperature at an individual nanoheater of 25 nm in diameter can reach an increase of about 3 K.

Electrokinetics in Micro-channeled Cantilevers: Extending the Toolbox for Reversible Colloidal Probes and AFM-Based Nanofluidics

The combination of atomic force microscopy (AFM) with nanofluidics, also referred to as FluidFM, has facilitated new applications in scanning ion conductance microscopy, direct force measurements, lithography, or controlled nanoparticle deposition. An essential element of this new type of AFMs is its cantilever, which bears an internal micro-channel with a defined aperture at the end. Here, we present a new approach for in-situ characterization of the internal micro-channels, which is non-destructive and based on electrochemical methods. It allows for probing the internal environment of a micro-channeled cantilever and the corresponding aperture, respectively. Acquiring the streaming current in the micro-channel allows to determine not only the state of the aperture over a wide range of ionic strengths but also the surface chemistry of the cantilever’s internal channel. The high practical applicability of this method is demonstrated by detecting the aspiration of polymeric, inorganic and hydrogel particles with diameters ranging from several µm down to 300 nm. By verifying in-situ the state of the aperture, i.e. open versus closed, electrophysiological or nano-deposition experiments will be significantly facilitated. Moreover, our approach is of high significance for direct force measurements by the FluidFM-technique and sub-micron colloidal probes.

Localized detection of ions and biomolecules with a force-controlled scanning nanopore microscope

Proteins, nucleic acids and ions secreted from single cells are the key signalling factors that determine the interaction of cells with their environment and the neighbouring cells. It is possible to study individual ion channels by pipette clamping, but it is difficult to dynamically monitor the activity of ion channels and transporters across the cellular membrane. Here we show that a solid-state nanopore integrated in an atomic force microscope can be used for the stochastic sensing of secreted molecules and the activity of ion channels in arbitrary locations both inside and outside a cell. The translocation of biomolecules and ions through the nanopore is observed in real time in live cells. The versatile nature of this approach allows us to detect specific biomolecules under controlled mechanical confinement and to monitor the ion-channel activities of single cells. Moreover, the nanopore microscope was used to image the surface of the nuclear membrane via high-resolution scanning ion conductance measurements.