Chitosan-Based Materials for the Fluorescence and Colorimetric Detection of Pathogenic Bacteria: A Comprehensive Review (2011–2025)

The increasing threat of pathogenic bacteria to public health has necessitated the development of rapid, sensitive, and cost-effective detection methods. Chitosan (CS), a naturally occurring biopolymer with excellent biocompatibility, biodegradability, and antimicrobial properties, has emerged as a versatile material in the construction of fluorescence and colorimetric biosensors. Recently substantial progress has been made in the design and application of CS-based sensing platforms for bacterial detection. Moreover, the integration of CS with quantum dots, metallic nanoparticles (such as AgNPs and AuNPs), organic dyes, and stimuli-responsive polymers has significantly enhanced the optical performance and specificity of these sensors. Reported limits of detection (LOD) for CS-based fluorescence and colorimetric biosensors range from as low as 6.8 CFU/mL to 900 CFU/mL, highlighting their remarkable sensitivity compared to conventional methods. This review comprehensively explores the advancements in CS-based fluorescence and colorimetric sensors for detecting a wide range of pathogenic bacteria. Emphasis is placed on the synthesis strategies, functionalization techniques, detection mechanisms, and sensitivity improvements achieved through nanomaterial hybridization. By critically analyzing the strengths and emerging trends, the review provides a roadmap for future research and development of CS-based optical biosensors for bacterial diagnostics in clinical, environmental, and food safety applications.