Chemically Programmable Fano Resonances via Colloidal Nanocrystal‐Ligand Chemistry for Ultra‐Sensitive Ion Detection

Abstract Thin‐film‐based Fano resonators (TFFRs) offer a promising route to scalable, high‐sensitivity refractive index (RI) sensing without complex nanostructure fabrication. However, their vacuum‐deposited, porous, and lossy dielectrics are limited compositionally to perform ion detection in biochemical applications. This study introduces an ion‐responsive TFFR platform by integrating two resonators, namely a colloidal metal nanocrystal (NC) film and a metal–insulator–metal (MIM) cavity. Upon precise ligand ion treatment, the solution‐processed NC film can provide widely tunable RI for coupling with the MIM cavity, enabling continuous spectral tuning of the TFFR between Fano and Lorentzian line shapes in calculations. It showcases this design using Au and Ag NCs and SCN ⁻ and Cl ⁻ ligand ions, respectively. Chemical, structural, and optical analyses track the RI evolution within the NC film as the ligand ion concentration changes stepwise from 1 × 10 ⁻⁶ to 10 mg mL ⁻¹ , revealing reproducibility and sample‐to‐sample variation <3.5%. The measured distinctive spectral signatures corroborate simulations to enable efficient machine learning algorithms that predict SCN ⁻ concentrations below 1 × 10 ⁻³ mg mL ⁻¹ with 97.4% accuracy. The NC/MIM TFFR achieves electrode‐free detection of SCN ⁻ and Cl ⁻ ions with exceptional detection limits of 245.6 and 3.83 nmol L ⁻¹ , respectively, in a dynamic range exceeding 10⁷.