The chapter introduces the concept of functional nucleic acid sensors, which are devices that respond to specific chemical or biological stimuli and produce detectable signals. These sensors are crucial for various applications, including environmental monitoring, industrial quality control, and medical diagnostics. The text highlights the advantages of nucleic acids, such as DNA and RNA, over antibodies in terms of target recognition, signal transduction, cost-effectiveness, stability, and reusability. It also discusses the development of nucleic acid enzymes (NAEs) and aptamers, which are highly specific and sensitive for detecting various analytes, including metals. The chapter details the combinatorial selection methods used to evolve NAEs and aptamers, and it covers various signaling methods for constructing FNA sensors, including fluorescent, colorimetric, and electrochemical detection. The text emphasizes the potential of these sensors in practical applications, such as on-site and real-time monitoring, and highlights the advantages of NAE-based sensors over traditional methods.The chapter introduces the concept of functional nucleic acid sensors, which are devices that respond to specific chemical or biological stimuli and produce detectable signals. These sensors are crucial for various applications, including environmental monitoring, industrial quality control, and medical diagnostics. The text highlights the advantages of nucleic acids, such as DNA and RNA, over antibodies in terms of target recognition, signal transduction, cost-effectiveness, stability, and reusability. It also discusses the development of nucleic acid enzymes (NAEs) and aptamers, which are highly specific and sensitive for detecting various analytes, including metals. The chapter details the combinatorial selection methods used to evolve NAEs and aptamers, and it covers various signaling methods for constructing FNA sensors, including fluorescent, colorimetric, and electrochemical detection. The text emphasizes the potential of these sensors in practical applications, such as on-site and real-time monitoring, and highlights the advantages of NAE-based sensors over traditional methods.