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Spurred by outstanding optical properties, chemical stability, and facile bioconjugation, plasmonic metals have become the first-choice materials for optical signal transducers in biosensing. While the design rules for surface-based plasmonic sensors are well-established and commercialized, there is limited knowledge of the design of sensors based on nanoparticle aggregation. The reason is the lack of control over the interparticle distances, number of nanoparticles per cluster, or multiple mutual orientations during aggregation events, blurring the threshold between positive and negative readout. Here we identify the geometrical parameters (size, shape, and interparticle distance) that allow for maximizing the color difference upon nanoparticle clustering. Finding the optimal structural parameters will provide a fast and reliable means of readout, including unaided eye inspection or computer vision. J.L.M.-P., N.Z., and J.A. acknowledge financial support from Spanish MICIN/AE/DOI 10.13039/501100004837, Reg. No. PID2019-107432GB-I00 and from the Department of Education of the Basque Government under Project IT1526-22. M.G. acknowledges Euskampus Foundation for financial support (Resilience COVID19). This work received computational support from DIPC's HPC cluster ATLAS, operated by DIPC Supercomputing Center.

We show that the magnetic-modulation of the optical response induced by an external magnetic field in giant-magneto-resistance rod-slit arrays is more sensitive to the presence of a molecular vibration than the direct optical response.