The present disclosure relates to particular nucleotide sequences comprising of at least one stem and/or loop structure and which optionally bind fluorescent labels, for example DFHBI and a process for obtaining said sequences. The present disclosure also provides nucleic acid sensor for cAMP comprising reporter domain, communication module and target recognition domain. Further, the present disclosure provides for ratiometric sensors 10 for quantifying cAMP using the nucleic sensors of the instant invention. The instant disclosure further provides method for obtaining the sensors, method for detecting and measuring small molecules, such as cAMP using the sensors of the instant disclosure and kits thereof.