Biocompatible folic acid-chitosan-coated gold nanorods as a dual-mode aptasensing platform for thyroxine detection

Thyroxine is the primary thyroid hormone (TH) regulated by thyrotropin-releasing hormone (TRH). This study demonstrates the detection of thyroxine (T4) using an aptamer-based dual-modality platform constructed with folic acid-chitosan-conjugated gold nanorods (FACS-AuNR). FACS-AuNR functions as a reversible and highly specific optical probe, translating aptamer-analyte recognition into quantifiable fluorescence changes via an ON-OFF-ON signal in the presence of aptamer and thyroxine. The probe demonstrated a limit of detection of 1.89 ± 0.41 nM for T4. The electrochemical behaviour of the probe-aptamer on glassy carbon electrode (GCE) (Apt@FCCS-AuNR@GCE) was evaluated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in the presence of the external redox probe [Fe(CN)₆]^4-. Under optimal conditions at pH 4.0, the limit of detection for T4 was determined to be 6.47 ± 0.13 pM within the 10-100 pM range by differential pulse voltammetry (DPV). Furthermore, the practical applicability of Apt@FCCS-AuNR@GCE was demonstrated for the detection of commercially available thyroxine tablets in serum samples.