Desenvolvimento de imunossensores baseados em redes de Bragg

Abstract

The present work explores sensors based on unsaturated and saturated Fiber Bragg Gratings (FBG), according to their sensitivity to changes in the refractive index of the surrounding medium. It also investigates the application of saturated FBG sensors coated with gold nanoparticles for functionalizing and detecting Green Fluorescent Protein (GFP). The FBGs were inscribed using the direct phase mask exposure method, with variations in the exposure time and the aperture of the optical recording system. After the inscription, the gratings underwent a chemical etching process. When saturated, the results show that these etched fiber Bragg gratings (EFBG) have refractive index sensitivities like those of unsaturated EFBGs. However, after chemical etching, the saturated EFBGs exhibit spectra with coupling to higher-order modes, showing greater sensitivity to variations in the refractive index of the surrounding medium compared to the fundamental mode. Moreover, the refractive index range in which signal readings are possible is broader for the etched saturated gratings, spanning from 1 to 1.456 RIU, compared to the unsaturated ones, which range from 1.33 to 1.445 RIU. The enhancement of EFBG sensor sensitivities was also analyzed using the deposition of nanoparticle coatings. In this work, gold nanoparticles (AuNPs) functionalized with sodium citrate were deposited on the sensing region using the drop-casting technique on three sensors: FBG-1 (unsaturated), SFBG-2, and SFBG-3 (saturated). The depositions limited the refractive index range that could be measured, 1.333 to 1.406 RIU for FBG-1 (unsaturated) and from 1 to 1.418 RIU for SFBG-2, and in the range of 1 to 1.4459 RIU for SFBG-3. The deposition, which proved efficient and resistant to immersion in aqueous solutions, led to an increase in sensitivity to external refractive index variations, although this increase was not constant across the analyzed refractive index ranges. For immunosensing applications, saturated etched fiber Bragg gratings coated with AuNPs synthesized with sodium citrate were used, providing carboxyl groups for binding antibodies to the nanoparticle surface. Nanoparticle depositions were conducted, followed by the deposition of EDC/NHS to activate the carboxyl groups and, subsequently, the deposition of anti-GFP (antibody) or GFP (antigen), with a final BSA deposition to prevent nonspecific binding during target material detection. SFBG sensors provedeffective in detecting biological material in antigen and antibody measurements, demonstrating potential as diagnostic test tools for diseases. It is important to note that the sensitivity of the anti-GFP functionalized sensor for antigen detection was higher, with a more significant wavelength shift after blocking nonspecific bindings and washing, with absolute values of 0.568 nm and 0.092 nm for the GFP (antigen) detection sensor and the anti-GFP (antibody) detection sensor, respectively.