Advances in metal complex-based colorimetric sensors

Metal complexes and their composites have recently emerged as the pivotal elements of colorimetric analysis, driving substantial progress in sensor technology. Owing to their advantageous optical properties, metal complex-based colorimetric sensors are well suited for detecting various analytes. The sensing properties of these composites can be tailored to specific analytes by varying the metal center (transition metal, lanthanide, or main-group metal ion). Compared with metal complexes, metal complex-based composites achieve markedly higher sensitivities and stabilities in colorimetric sensing, primarily because of the synergistic integration of metal complexes with diverse nanomaterials, including metals, metal oxides, and carbon-based materials. Despite the considerable progress in this field, the structural design of metal complexes and the preparation and performance regulation of their composites remain challenging. Herein, we review the metal complex-based colorimetric sensors developed in the past decade from the perspectives of metal centers, composite nanomaterials, colorimetric mechanisms, and applications, highlighting the need for further research to foster the advancement and utilization of colorimetric sensing based on metal complexes and their composites.