High-Content Label-Free Single-Cell Analysis with a Microfluidic Device Using Programmable Scanning Electrochemical Microscopy

Label-Free Imaging of Mesenchymal Stem Cell Spheroid Differentiation with Flexible-Probe SECM and a Microfluidic Device

Mesenchymal stem cells (MSCs) have emerged as an indispensable source for stem cell research and preclinical studies due to their capacity for in vitro proliferation and their potential to differentiate into mesodermal lineages, particularly into osteoblasts. This capability has propelled their application in the fields of bone regeneration and osteochondral repair. Traditional methodologies for assessing the differentiation status of MSCs necessitate invasive procedures such as cell lysis or fixation. In this study, we introduce a nondestructive technique that utilizes an integrated label-free approach to evaluate the osteogenic maturation of MSC spheroid aggregates. This method employs scanning electrochemical microscopy (SECM) with a flexible probe in conjunction with a top-removable microfluidic device designed for easy SECM access. By tracking the production rate of p-aminophenol (PAP) in the generation/collection mode and assessing morphological changes via the negative feedback mode using [Ru(NH3)6]Cl3 (Ruhex), we can discern variations in the alkaline phosphatase (ALP) activity indicative of osteogenic differentiation. This innovative strategy enables the direct evaluation of osteogenic differentiation in MSC spheroids cultured within microwell arrays without necessitating any labeling procedures. The utilization of a flexible microelectrode as the probe that scans in contact mode (with probe-substrate distances potentially as minimal as 0 μm) affords enhanced resolution compared to the traditional stiff-probe technique. Furthermore, this method is compatible with subsequent molecular biology assays, including gene expression analysis and immunofluorescence, thereby confirming the electrochemical findings and establishing the validity of this integrative approach.

Exploring the Living Cell: Applications and Advances of Scanning Electrochemical Microscopy

A living cell is a complex network of molecular, biochemical and physiological processes. Cellular activities, such as ion transport, metabolic processes, and cell-cell interactions can be determined electrochemically by detecting the electrons or ions exchanged in these processes. Electrochemical methods often are noninvasive, and they can enable the real-time monitoring of cellular processes. Scanning electrochemical microscopy (SECM) is an advanced scanning probe electroanalysis technique that can map the surface topography and local reactivity of a substrate with high precision at the micro- or nanoscale. By measuring electrochemical signals, such as redox reactions, ion fluxes, and pH changes, SECM can provide valuable insights into cellular activity. As a result of its compatibility with liquid medium measurements and its nondestructive nature, SECM has gained popularity in living cell research. This review aims to furnish an overview of SECM, elucidating its principles, applications, and its potential to contribute significantly to advancements in cell biology, electroporation, and biosensors. As a multidisciplinary tool, SECM is distinguished by its ability to unravel the intricacies of living cells and offers promising avenues for breakthroughs in our understanding of cellular complexity.

Operando Scanning Electrochemical Probe Microscopy during Electrocatalysis

Scanning electrochemical probe microscopy (SEPM) techniques can disclose the local electrochemical reactivity of interfaces in single-entity and sub-entity studies. Operando SEPM measurements consist of using a SEPM tip to investigate the performance of electrocatalysts, while the reactivity of the interface is simultaneously modulated. This powerful combination can correlate electrochemical activity with changes in surface properties, e.g., topography and structure, as well as provide insight into reaction mechanisms. The focus of this review is to reveal the recent progress in local SEPM measurements of the catalytic activity of a surface toward the reduction and evolution of O₂ and H₂ and electrochemical conversion of CO₂. The capabilities of SEPMs are showcased, and the possibility of coupling other techniques to SEPMs is presented. Emphasis is given to scanning electrochemical microscopy (SECM), scanning ion conductance microscopy (SICM), electrochemical scanning tunneling microscopy (EC-STM), and scanning electrochemical cell microscopy (SECCM).

Flexible Disk Ultramicroelectrode for High-Resolution and Substrate-Tolerable Scanning Electrochemical Microscopy Imaging

A simple and universal strategy for fabricating flexible 25 μm platinum (Pt) disk ultramicroelectrodes (UMEs) was proposed, where a pulled borosilicate glass micropipette acted as a mold for shaping the flexible tip with flexible epoxy resin. The whole preparation procedure was highly efficient, enabling 10 or more probes to be manually fabricated within 10 h. Intriguingly, this technique permits an adjustable RG ratio, tip length, and stiffness, which could be tuned according to varying experimental demands. Besides, the electroactive area of the probe could be exposed and made renewable with a thin blade, allowing its reuse in multiple experiments. The flexibility characterization was then employed to optimize the resin/hardener mass ratio of epoxy resin and the tip position during HF etching in the fabrication process, suggesting that more hardener, a larger RG value, or a longer tip length obtained stronger deformation resistance. Subsequently, the as-prepared probe was examined by optical microscopy, cyclic voltammetry, and SECM approach curves. The results demonstrated the probe possessed good geometry with a small RG ratio of less than 3 and exceptional electrochemical properties, and its insulating sheath remained undeformed after blade cutting. Owing to the tip's flexibility, it could be operated in contactless mode with an extremely low working distance and even in contact mode scanning to achieve high spatial resolution and high sensitivity while guaranteeing that the tip and samples would suffer minimal damage if the tip crashed. Finally, the flexible probe was successfully employed in three scanning scenarios where tilted and 3D structured PDMS microchips, a latent fingerprint deposited on the stiff copper sheet, and soft egg white were included. In all, the flexible probe encompasses the advantages of traditional disk UMEs and circumvents their principal drawbacks of tip crash and causing sample scratches, which is thus more compatible with large specimens of 3D structured, stiff, or even soft topography.