Smart Materials and Devices has published a new Mini Review, "Bridging the scales: Advanced scanning probe microscopy across molecular redox regimes," which examines how advanced scanning probe microscopy and nanospectroscopy can help connect molecular redox chemistry across bulk, nanoscale, and single-molecule environments.
The Mini Review was led by Dr. Noah Al-Shamery of the Leibniz Institute of Surface Engineering (IOM) in Leipzig, Germany, in collaboration with Dr. Rafael A. Vicente, Christian Zocher, Prof. Berthold Kersting, and Dr. Kirill Monakhov. Dr. Al-Shamery conceptualized and drafted the review together with Dr. Monakhov, bringing together expertise in advanced scanning probe microscopy, nanospectroscopy, and redox-active molecular materials. The work highlights the importance of interdisciplinary collaboration in connecting fundamental molecular electrochemistry with emerging applications in smart materials and molecular-scale devices.
Redox-active molecules and materials are important building blocks for smart materials, molecular electronics, energy storage, sensing, switching, and neuromorphic-device concepts. Their functional behavior, however, is strongly shaped by the environment in which they are measured and used. A molecule that displays well-defined electrochemical behavior in bulk solution may exhibit substantially different charge-transfer, structural, electronic, or spin-related properties once it is confined at an interface, incorporated into a thin film, or integrated into a molecular junction.
Using polyoxometalates (POMs) as a representative molecular platform, this Mini Review provides a cross-scale perspective on how redox behavior evolves across three experimental regimes:
- Bulk ensembles, where cyclic voltammetry and spectroelectrochemistry establish reference redox windows, reversibility, charge-transfer kinetics, and chemical stability;
- Confined nanoscale ensembles, where techniques such as scanning electrochemical cell microscopy (SECCM), scanning electrochemical microscopy (SECM), and scanning ion conductance microscopy (SICM) enable spatially localized electrochemical measurements under liquid confinement;
- Single-molecule and few-molecule junctions, where electrochemical scanning tunneling microscopy (EC-STM), scanning tunneling spectroscopy (STS), and electron-spin-resonance STM (ESR-STM) can probe local structure, electronic states, tunneling behavior, and spin-related phenomena.
The review emphasizes that measurements obtained in these different regimes should not be treated as directly interchangeable. Faradaic current, tunneling current, local density of states, vibrational fingerprints, and spin-dependent signals capture different physical consequences of a redox process. Their relationship must therefore be established through carefully designed, complementary experiments rather than inferred from apparently similar potential- or bias-dependent features.
A central focus of the article is the potential of advanced scanning probe techniques to bridge the conceptual gap between classical electrochemistry and molecular electronics. Bulk electrochemistry provides ensemble-averaged information, while confined electrochemical methods introduce local addressability and controlled liquid environments. EC-STM and STS offer nanoscale access to surface-confined molecular structure and electronic states, whereas ESR-STM extends this perspective toward spin-sensitive measurements. Optical and synchrotron-coupled nanospectroscopic approaches, including scattering-type near-field optical microscopy and nano-FTIR, add chemical and vibrational information beyond electrical signals alone.
The article further outlines a practical regime-bridging workflow: first establish a molecular redox map in bulk solution; then assess local electrochemical programmability under confined conditions; subsequently correlate redox states with surface electronic structure through tunneling-based methods; and finally verify local chemical identity using optical or synchrotron-enabled nanospectroscopy.
This framework is relevant not only to polyoxometalates, but also to coordination complexes, molecular electrolytes, ionic liquids, redox polymers, molecular perovskites, and hybrid organic–inorganic frameworks. It offers a useful basis for researchers seeking to translate molecular redox chemistry into programmable functions for sensing, memory, switching, catalysis, energy storage, and molecular-scale electronic devices.
Importantly, the review identifies a remaining metrological challenge: a fully matched experiment that quantitatively follows the same POM-based molecular system from bulk electrochemistry through confined electrochemistry to a single-molecule junction has not yet been established. Addressing this gap could help clarify which molecular redox functions remain robust across environments and which depend critically on confinement, hydration, electrode coupling, or local interfacial structure.
