Plasmonic response of aptamer-functionalized gold nanoparticles via target-induced aggregation for oxytetracycline detection.

The widespread use of oxytetracycline (OTC) in sericulture to control bacterial infections and improve silkworm productivity has raised significant concerns regarding the emergence of antimicrobial resistance and antibiotic residue accumulation in seri-products, highlighting the need for reliable monitoring strategies. In this work, we report a plasmonic, interface-engineered colorimetric aptasensor based on aptamer-functionalized gold nanoparticles for the rapid, sensitive, and selective detection of OTC. The sensing strategy exploits the target-induced dissociation of an aptamer-cDNA duplex immobilized on the gold nanoparticle (AuNPs) surface. Recognition of OTC by the aptamer triggers its release from the nanoparticle interface, reducing colloidal stability and promoting salt-induced AuNPs aggregation. This event produces a distinct red-to-blue color transition accompanied by a localized surface plasmon resonance (LSPR) shift, enabling rapid, sensitive, and label-free colorimetric detection of OTC. Uniform citrate-stabilized gold nanoparticles with an average diameter of 8.6 ± 2.5 nm were synthesized and employed as a stable and biocompatible plasmonic platform for aptamer immobilization. Under the optimized assay conditions, the developed aptasensor exhibited a linear analytical response to OTC, with a limit of detection (LOD) of 0.55 µM and a limit of quantification (LOQ) of 1.67 µM. The sensor demonstrated excellent selectivity against structurally related antibiotics and enabled rapid visual detection within 6-8 min, highlighting its potential for simple, sensitive, and on-site monitoring of OTC residues. Its analytical performance was successfully demonstrated in sericulture-relevant biological matrices, including silkworm hemolymph and excretory samples. Furthermore, an aptamer-based dot blot assay was also developed to enable rapid visual detection of OTC through aggregation-induced colorimetric changes. The successful validation of both detection formats in complex biological matrices underscores the robustness, versatility and practical applicability of the proposed detection platform for real-world antibiotic residue analysis. With its portability, low cost, rapid response, and instrument-free visual readout, this plasmonic aptasensing strategy represents a promising tool for on-site monitoring of OTC residues in sericulture and other complex biological samples.