Microscale thermophoresis sensor for Cd2+using aptamer with tetramethylrhodamine label at an internal site

Rapid sensitive detection of toxic cadmium ion (Cd2+) is demanded in environmental analysis and food safety. Aptamer against Cd2+ becomes attractive recognition element for Cd2+ sensors. The emerging microscale thermophoresis (MST) technique brings new power for optical biosensors construction by employing fluorescently labeled ligand, which measures fluorescence response of samples inside capillaries to localized infrared (IR) laser heating. Herein, we reported a MST aptasensor for Cd2+ by using the aptamer with tetramethylrhodamine (TMR) label at an internal site, and demonstrated that TMR position had large influence on MST responses to Cd2+. The 15-mer aptamer with a TMR conjugated on the internal 12th thymine (T) generated a much larger MST signal change upon Cd2+ binding than the aptamer with TMR labeled at terminals. By utilizing the optimal aptamer probe with TMR labeled at the internal T site, we achieved detection of Cd2+ with a detection limit about 0.5 nM, and the MST aptasensor allowed to detect Cd2+ in real water samples. This work is helpful for developing sensitive aptamer MST sensors for wide applications.