Noble metal nanoparticles (NPs) possess size- and shape-dependent optical properties, suggesting the possibility of tuning desired optical properties of ensemble NPs at single NP resolution and underscoring the importance of probing the sizes and shapes of single NPs in situ and in real-time. In this study, we synthesized twelve colloids of Ag NPs. Each colloid contains various sizes and shapes of single NPs, showing rainbow colors with peak-wavelength of absorption spectra from 393 to 738 nm. We correlated the sizes and shapes of single NPs determined by high-resolution transmission electron microscopy (HRTEM) with scattering localized surface plasmon resonance (LSPR) spectra of single NPs characterized by dark-field optical microcopy and spectroscopy (DFOMS). Single spherical (2–39 nm in diameter), rod (2–47 nm in length with aspect ratios of 1.3–1.6), and triangular (4–84 nm in length with thickness of 2–27 nm) NPs show LSPR spectra (λmax) at 476 ± 5 or 533 ± 12, 611 ± 23, and 711 ± 40 nm, respectively. Notably, we observed new cookie-shaped NPs, which exhibit LSPR spectra (λmax) at 725 ± 10 nm with a shoulder peak at 604 ± 5 nm. Linear correlations of sizes of any given shape of single NPs with their LSPR spectra (λmax) enable the creation of nano optical rulers (calibration curves) for identification of the sizes and shapes of single NPs in solution in real time using DFOMS, offering the feasibility of using single NPs as multicolored optical probes for study of dynamics events of interest in solutions and living organisms at nm scale in real time.
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