Tailoring Chiral Gold Nanorods via Single‐Step Seed‐to‐Au(III) Ratio and Unlocking their Potential in Refractive Index Sensing

Colloidal chiral plasmonic nanoparticles are garnering growing interest due to their interaction with circularly polarized light, offering advanced optical applications. Their circular dichroism (CD) spectra are significantly narrower and more defined than extinction spectra, making them ideal for refractive index‐based sensing. Despite progress in colloidal synthesis, this field remains relatively underexplored. In this work, a one‐step, seed‐mediated route to chiral Au nanorods is introduced in which the molar ratio Au 3 ⁺/Au⁰ enables continuous control of the CD response (intensity, sign, and position), using L‐ and D‐cysteine as chiral inducers. CD‐based refractive index sensitivity (RIS) measurements reveal a figure of merit (FoM = RIS/linewidth) exceeding 1000 RIU −1 , outperforming the conventional extinction‐based approaches. Thin films of C‐AuNRs fabricated via a layer‐by‐layer assembly retain the bisignate CD response and show RIS values comparable to colloidal samples. These films demonstrate excellent stability, reusability, and resilience in highly absorbing media. All the experimental data are supported by advanced calculations performed using full‐wave M3 Maxwell's solver and using electron tomography reconstructions as direct input. Finally, their applicability in RI‐based quantitative detection of bovine serum albumin (BSA) is demonstrated, highlighting their potential for biomolecular sensing.