Introducing Nanoscale Electrochemistry in Small-Molecule Detection for Tackling Existing Limitations of Affinity-Based Label-Free Biosensing Applications

Electrochemical sensing techniques for small moleculeshave progressedin many applications, including disease diagnosis and prevention aswell as monitoring of health conditions. However, affinity-based detectionfor low-abundance small molecules is still challenging due to theimbalance in target-to-receptor size ratio as well as the lack ofa highly sensitive signal transducing method. Herein, we introducednanoscale electrochemistry in affinity-based small molecule detectionby measuring the change of quantum electrochemical properties witha nanoscale artificial receptor upon binding. We prepared a nanoscalemolecularly imprinted composite polymer (MICP) for cortisol by electrochemicallycopolymerizing & beta;-cyclodextrin and redox-active methylene blueto offer a high target-to-receptor size ratio, thus realizing "bind-and-read"detection of cortisol as a representative target small molecule, alongwith extremely high sensitivity. Using the quantum conductance measurement,the present MICP-based sensor can detect cortisol from 1.00 x10(-12) to 1.00 x 10(-6) M witha detection limit of 3.93 x 10(-13) M (S/N =3), which is much lower than those obtained with other electrochemicalmethods. Moreover, the present MICP-based cortisol sensor exhibitedreversible cortisol sensing capability through a simple electrochemicalregeneration process without cumbersome steps of washing and solutionchange, which enables "continuous detection". In situdetection of cortisol in human saliva following circadian rhythm wascarried out with the present MICP-based cortisol sensor, and the resultswere validated with the LC-MS/MS method. Consequently, thispresent cortisol sensor based on nanoscale MICP and quantum electrochemistryovercomes the limitations of affinity-based biosensors, opening upnew possibilities for sensor applications in point-of-care and wearablehealthcare devices.