Optical Molecular Sensing Strategies for Fuel Quality Monitoring: From Polarity-Sensitive Probes to Emerging Portable Technologies for Adulterant Detection.

Fuel adulteration remains a persistent global challenge with significant economic, environmental, and regulatory implications. Conventional methods, such as chromatography and vibrational spectroscopy, provide high sensitivity but require laboratory infrastructure and trained personnel, limiting their use for decentralized monitoring. This review examines solvatochromic and fluorescent molecular probes as alternatives for fuel quality assessment. The discussion focuses on how changes in optical signals, driven by variations in polarity, hydrogen bonding, viscosity, aggregation, and fluorescence quenching, can be used to detect common fuel adulterants, such as ethanol, methanol, water, and kerosene in different fuel matrices. Analytical performance parameters, including limits of detection, response time, ratiometric capability, and matrix robustness, are compared across representative probe architectures reported between 2006 and 2025. Emphasis is placed on field-deployable formats, such as paper-based test strips, smartphone-assisted fluorescence readout systems, and low-cost portable spectrophotometers. Finally, current technical limitations, calibration challenges, and regulatory barriers are discussed, outlining future directions for integrating optical molecular sensing into practical fuel quality control strategies.