Enhanced optical absorption of molecules in the vicinity of metallic nanostructures is key to a number of surface-enhanced spectroscopies and of great general interest to the fields of plasmonics and nano-optics. Yet, experimental access to this absorbance has long proven elusive. We here present direct measurements of the intrinsic absorbance of dye-molecules adsorbed onto silver nanospheres, and crucially, at sub-monolayer concentrations where dye-dye interactions become negligible. With a large detuning from the plasmon resonance, distinct shifts and broadening of the molecular resonances reveal the intrinsic properties of the dye in contact with the metal colloid, in contrast to the often studied strong-coupling regime where the optical properties of the dye-molecules cannot be isolated. The observation of these shifts together with the ability to routinely measure them has broad implications in the interpretation of experiments involving resonant molecules on metallic surfaces, such as surface-enhanced spectroscopies and many aspects of molecular plasmonics.Over the last two decades, the optical properties of metallic nanoparticles (NPs) of various sizes and shapes [1] [18][19][20]. Many of these existing and emerging applications are underpinned by the fact that the optical (electronic) absorption of molecules on the surface of metallic NPs is enhanced. But spectral changes induced by molecular adsorption are often ignored because of the experimental challenge of measuring surface absorbance spectra on nanoparticles, despite early attempts more than 30 years ago [21].This question is not directly addressed in the great number of recent studies devoted to the topic of strongcoupling between plasmons and molecules [22][23][24][25][26][27][28][29][30][31][32][33][34][35][36]; in this regime, the plasmon-molecule interaction is evidenced by a typical anti-crossing of the two resonances as a function of detuning [25,33], but in such a strongly interacting system the molecular response cannot be isolated. Moreover, in such studies the dye concentration is often large (typically monolayer coverage and above) to maximize dye/plasmon interactions. Dye-dye interactions cannot therefore be neglected and are expected to induce resonance shifts of the dye layer independently of any plas- * Eric.LeRu@vuw.ac.nz monic effects; in fact many studies specifically work with J-aggregates rather than isolated dyes [22, 25-27, 31, 33]. As a result, the intrinsic effect (chemical and/or electromagnetic) of the NP on an isolated adsorbed molecule cannot be elucidated. To this aim, we will show that it is necessary to measure the absorbance of the dye when the dye and plasmon resonances are detuned (to avoid dye-plasmon interaction effects) and at low surface coverage (to avoid dye-dye interaction effects). This low surface coverage poses a significant experimental challenge because the dye absorbance is then very small and obscured by the large optical response (absorption and scattering) of the NPs.We here propose to measure NP/d...

This study highlights a crucial but often overlooked consideration during sample preparation involving surface-adsorbing species: the competition between analyte adsorption and analyte diffusion/mixing strongly affects the distribution of analytes throughout the sample. In cases of fast analyte adsorption, we argue that the use of large-dilution factors, a common approach for sample preparation in surface-enhanced Raman spectroscopy (SERS), may result in an extreme nonuniformity of the surface coverage. This has a direct effect on the aggregation state of the colloidal solution and therefore on the overall SERS signal. Explicitly, we show that the average SERS signal obtained from typical dyes in colloidal solutions can be drastically different for two seemingly equivalent samples, differing only in the method by which the dye molecules were diluted. We, in addition, discuss the implications of such nonuniformity on the statistics of SERS intensities in the context of single-molecule detection. These results vividly highlight the importance of the dilution step in any experiments involving surface-adsorbing species and position SERS as an ideal tool to evidence such effects. In such cases, a simple half-half dilution procedure should be adopted as the standard method to mitigate these effects.

The wavelength-dependent complex linear polarizability of a dye is a crucial input for the modeling of the optical properties of dye-containing systems. We here propose and discuss methods to obtain an accurate polarizability model by combining absorption spectrum measurements, Kramers–Kronig (KK) tranformations, and density functional theory (DFT) calculations. We focus, in particular, on the real part of the polarizability and its link with static polarizability. In addition, we introduce simple KK-consistent analytic functions based on the theory of critical points as a much more accurate approach to model dye polarizabilities compared with existing models based on Lorentz oscillators. Accurate polarizability models based on critical points and DFT calculations of the static polarizability are derived for five commonly used dyes: Rhodamine 6G, Rhodamine 700, Crystal Violet, Nile Blue A, and Methylene Blue. Finally, we demonstrate explicitly, using examples of Mie Theory calculations of nanoparticle–dye interactions, how an inaccurate polarizability model can result in fundamentally different predictions, further emphasizing the importance of accurate models, such as the one proposed here.