Calibration-free electrochemical sensing is increasingly promoted as a route to simpler, more deployable analytical devices. However, the term is used inconsistently, ranging from genuinely absolute measurement to factory-calibrated, ratiometric, self-referenced, drift-corrected or model-assisted operation. This review critically examines what calibration-free sensing can and cannot mean in electrochemical analysis. We argue that a strict claim requires that the reported measurand be obtained from an internally constrained physical, chemical or stoichiometric relationship, with the required parameters known, controlled or independently measured within an uncertainty framework. Potentiometric, amperometric, coulometric, impedimetric, biosensing and affinity-based approaches are compared to show where empirical calibration is removed and where it is shifted to fabrication, internal correction, model fitting, matrix correction or context-specific validation. Particular attention is given to coulometric and thin-layer systems, geometry-constrained devices, electrochemical aptamer-based sensors and self-diagnostic platforms. We propose a classification scheme, a decision tree and a minimum assumption map linking measurand definition, electrochemical signals, signal-to-result relationships, parameter sources, uncertainty, matrix transfer, reproducibility and independent-method agreement. The review provides a practical framework for distinguishing genuinely calibration-free measurements from calibration-reducing, conditionally calibration-free and drift-corrected strategies.
Angel A. J. Torriero (Fri,) studied this question.