Beyond ensemble averages: single-entity approaches for complex systems

Analytical chemistry is undergoing a paradigm shift from measuring ensemble-averaged properties to directly probing the individuality, heterogeneity, and fluctuations of molecules and particles. Single-Entity analytical chemistry addresses this need by resolving structure, dynamics, and reactions at the level of single molecules or single particles using techniques such as nanopores, nanogap electrodes, surface-enhanced Raman scattering, and nanofluidic devices. In this Mini-Review, we highlight recent developments in Single-Entity measurements with a particular focus on studies reported in Analytical Sciences during 2024-2025, while also placing them in the context of global advances. We first discuss the conceptual significance of Single-Entity approaches as a framework for accessing "chemistry of individuality." We then summarize emerging detection technologies and nanodevice platforms that provide nanoscale spatiotemporal resolution, and outline how machine learning (ML) and artificial intelligence (AI) enable high-dimensional signal interpretation and robust extraction of rare events. Finally, we discuss future directions, including multimodal Single-Entity platforms and statistical schemes that connect event-level fluctuations to bulk properties, positioning Single-Entity analytical chemistry as a next-generation foundation for complex systems analysis.