Simultaneous fluorometric and chirality based aptasensing of sulfamethazine by using upconversion nanoparticles and Au@Ag@Au core-shell nanoparticles.

A dual-mode aptasensor was employed for ultrasensitive determination of sulfamethazine (SMZ). The assay is based on simultaneous quantification by using fluorometry and chirality. The aptamer against SMZ was immobilized on upconversion nanoparticles (UCNPs) while the complementary DNA of SMZ aptamer was immobilized on Au@Ag@Au core-shell nanoparticles (Au@Ag@AuNPs). Based on complementary base-pairing reactions, the aptamer of SMZ and its complementary DNA sequence (cDNA) were hybridized to form duplex structure. Thus, Au@Ag@AuNPs and UCNPs were in close proximity. Efficient inner filter effect (IFE) from UCNPs (energy donor) to Au@Ag@AuNPs (energy acceptor) occurred under the excitation of 980 nm laser. In the presence of targets (SMZ), as the aptamer of SMZ coupled with SMZ to form stable complex structure. As a result, the hybridization of aptamer and its cDNA deceased and the fluorescence signal recovered. Furthermore, as the degree of the assembly decreased, the circular dichroism (CD) signal also decreased. Fluorescence was measured at excitation/emission wavelengths of 980/655 nm. The linear ranges for detection of SMZ are between 0.10-100 ng·mL and 1.00-100 ng·mL for the fluorescence and circular dichroism modes, respectively. The method was applied to the determination of SMZ in spiked milk with high recoveries. Conceivably, it can be extended to the analysis of numerous other targets for which adequate antibodies or aptamers are available. Graphical abstract Schematic representation of a chirality assay and a fluorometric assay based on inner filter effect (IFE) between upconversion nanoparticles (UCNPs) and Au@Ag@Au core-shell nanoparticles (Au@Ag@AuNPs) for the determination of sulfamethazine (SMZ).