Seeing Molecular Recognition in Action: A Visual, Amplification-Free Sandwich Assay for DNA Detection Using Noble Metal Nanoparticles

Abstract

This laboratory experiment introduces upper-division undergraduate and early stage graduate students to the design and implementation of a nanoparticle-based DNA sandwich assay for colorimetric DNA detection. The assay leverages sequence-specific hybridization between DNA-functionalized magnetic microparticles (MMPs) and noble metal nanoparticles (Au and Ag NPs), forming a “sandwich” complex in the presence of a complementary target DNA. Students synthesized Au and Ag NPs (20–80 nm) and functionalized them with thiolated DNA. In parallel, streptavidin-coated MMPs were conjugated with biotinylated DNA probes. Upon hybridization, the MMP–DNA–NP complexes were magnetically separated and subsequently dissociated to release signal Au and Ag NPs into solution. The color and absorbance intensity of the released particles, quantified via UV–vis spectroscopy, correlated with target DNA concentration, enabling both qualitative and quantitative analysis. Varying NP size, material, and mixing conditions (vortex vs rotation) revealed that larger NPs and vigorous agitation enhanced assay sensitivity via higher extinction coefficients and increased hybrid formation. The experiment reinforces concepts including plasmonic signal transduction, hybridization thermodynamics, and colloidal stability, while providing hands-on experience with synthesis, conjugation, and spectroscopy. Students demonstrated strong engagement and the ability to relate NP properties to analytical performance, interpreting assay results in the context of molecular recognition and surface chemistry. Overall, this modular, inquiry-driven experiment effectively bridges nanotechnology and bioanalysis in the undergraduate and graduate curriculum, offering a versatile platform for teaching interdisciplinary chemical principles in a visually intuitive and experimentally accessible format.