Optical Visualization of Cadmium Ion Using a Label-Free Nucleic Acid Aptamer for Recognition

Aptamer-based biosensors have been demonstrated to provide real-time physical and chemical signals for biomedicine, food safety, drug identification, and metal ion determination. In this study, cadmium ion (Cd2+) was determined in drinking water by color signaling using the specific binding of the aptamer and target by its regulation of the catalytic capacity of gold nanoparticles (AuNPs). When sodium borohydride (NaBH4) and a probe with a catalytic function were present, the solution containing 4-nitrophenol changed from yellow to colorless. Under optimal conditions, the naked eye was sufficient to determine Cd2+ with a limit of detection (LOD) of 5 μM. The discoloration time showed a positive correlation with Cd2+ concentration up to 30 μM. This method was label-free, efficient, rapid, and selective for Cd2+ in water without expensive instruments, suggesting practical applications.