Fluorescence-enhanced bio-detection platforms obtained through controlled "step-by-step" clustering of silver nanoparticles.

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Metal nanoparticle coatings are widely employed as fluorescence-enhanced platforms for high-throughput biological detection; however, complex manufacturing technologies and stringent fabrication procedures hinder their development for use in bioassays. Here, we present the preparation of fluorescence-based bioassay platforms using spray-assisted step-by-step assembly of silver nanoparticles (Ag NPs) and poly(diallyldimethylammonium chloride) (PDDA). This approach allowed us to control the density and the degree of aggregation of Ag NPs on large surfaces which are prerequisites for the development of bioassay platforms with a substantial fluorescence enhancement. After one assembly cycle (1-Ag platform) the adsorbed particles are not forming aggregates or ones composed of very few particles which, as expected, led to poor fluorescence enhancement (1.1) for cyanine 5. Further assembly steps induce the clustering of Ag NPs by multiple electrostatic interactions between PDDA and Ag NPs and thus increase the number of nanoparticles per aggregate in a controlled way. We observed that the nanoparticle island growth takes place first mainly in the plane (2D) and then in the plane and in the third dimension and that the aggregate morphology (2D versus 3D) strongly affects the plasmonic fluorescence enhancement of the fluorescent dye. A substantial fluorescence enhancement (12.3) was measured for a Ag NP platform obtained after twelve assembly cycles. This result is within the ballpark of values reported in the literature for bioassay platforms using metal nanoparticles and opens the route towards the preparation of fluorescence-based bioassay platforms on the large scale.