Plasmonic Biosensor of Cruzipain-Antibody Complexes Based on Gold-Nanogratings for Chagas Disease Screening

This work describes the development of a label-free optical immunosensor for the diagnosis of Chagas disease (CD). The proposed plasmonic sensor is based on nanograting gold-coated substrates of 725-nm period, which are functionalized with a highly immunogenic Trypanosoma cruzi protein, cruzipain, and subsequently passivated with bovine serum albumin (BSA). Verification of surface modifications and immunocomplex formation resulting from the antigen-antibody interaction was successfully conducted using atomic force microscopy (AFM). The biosensor signal was retrieved by measuring the reflected spectra of the gold-coated surface, being based on grating-coupled surface plasmon resonance (GCSPR) in wavelength modulation mode using incident light at 60°. The binding of molecules to the sensor surface led to an increase in surface refractive index, causing blueshifts in the resonance wavelength. The immunosensor yielded a refractometric sensitivity of 1240 nm/RIU with a limit of detection (LOD) of $60~\mu $ g/mL, which is almost three orders of magnitude smaller than the antibody levels commonly found in patients with chronic CD. The sensors functionalized with cruzipain were successfully tested with clinical serum samples. Significant shifts were observed in the resonance wavelength that was $2\times $ higher in positive samples compared to negative samples. Looking forward, the development of a unique cruzipain-based sensor surface for the label-free detection of CD biomarkers is anticipated by integrating GCSPR and electrochemical impedance spectroscopy (EIS). This integration holds promise for enhancing detection sensitivity and specificity, paving the way for advanced diagnosis of infectious diseases.