Hydrogen-Bonding Assembly of Monolayer Metallic Silver Nanocluster Superlattices Improving Guest Fluorescence

Constructing ordered plasmonic superstructures with sub-2 nm interparticle gaps via spontaneous self-assembly remains a formidable challenge; however, it is the key to generating strong electromagnetic fields for molecular optical amplification. Herein, we report the synthesis and structure determination of a metallic all-alkynyl-protected silver nanocluster, (Et4N)3[Ag215(ArC≡C)96] (Ag215, ArC≡C = 4-ethynyl-α,α,α-trifluorotoluene), featuring a shell-by-shell Ag13@Ag42@Ag86@Ag74 metal kernel. Remarkably, directional C-H···F hydrogen bonds between neighboring nanoclusters drive the spontaneous self-assembly of Ag215 into a micrometer-scale monolayer hexagonal close-packed (hcp) superlattice with exceptionally uniform interparticle nanogaps of ∼1.32 nm-an ideal regime for maximizing near-field plasmonic coupling. This well-defined superlattice boosts the fluorescence of various guest dyes (eosin Y, rhodamine 6G, rhodamine B, and methylene blue) by 7.4- to 12.8-fold. In addition, owing to its high molar extinction coefficient and metallic nature, Ag215 exhibited exceptional photothermal performance with a temperature rise of ∼35.6 °C and a conversion efficiency of 67.7% under 660 nm irradiation (0.5 W, 15.0 μM). To the best of our knowledge, this is the first example of an atomically precise two-dimensional monolayer superlattice self-assembled from metallic nanoclusters. The exceptional structural uniformity and functional reproducibility render these nanogap-enhanced optical probes promising platforms for advanced sensing, imaging, and photothermal therapeutics.