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Quantifying the Sensitivity of Multipolar (Dipolar, Quadrupolar and Octapolar) Surface Plasmon Resonances in Silver Nanoparticles: The Effect of Size, Composition and Surface Coating. Neus G Bastús, Jordi Piella, and Víctor F. Puntes Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b03859 • Publication Date (Web): 09 Dec 2015 Downloaded from http://pubs.acs.org on December 15, 2015
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Quantifying the Sensitivity of Multipolar (Dipolar, Quadrupolar and Octapolar) Surface Plasmon Resonances in Silver Nanoparticles: The Effect of Size, Composition and Surface Coating. Neus G. Bastús1,*, Jordi Piella1,2 and Víctor Puntes1,3,* 1-Institut Català de Nanociència i Nanotecnologia (ICN2), Campus UAB, 08193 Bellaterra, Barcelona, Spain. 2- Universitat Autònoma de Barcelona (UAB), Campus UAB, 08193 Bellaterra, Barcelona, Spain. 3- Institut Català de Recerca i Estudis Avançats (ICREA), 08010 Barcelona, Spain. *
[email protected], *
[email protected] ABSTRACT The effect of composition, size and surface coating on the sensitivity of localized multipolar surface plasmon resonances has been spectroscopically investigated in high-quality silver colloidal solutions with precisely controlled sizes from 10 to 220 nm and well-defined surface chemistry. Surface plasmon resonance modes have been intensively characterized, identifying the size-dependence of dipolar, quadrupolar and octapolar modes. Modifications of NP’s surface chemistry revealed the higher sensitivity of larger sizes, dipolar than higher-order modes, thiol than amine groups, and long than short molecules. We also extend this study to gold nanoparticles, aiming to compare the sensitivity of both materials, quantifying the higher sensitivity of silver.
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INTRODUCTION Metal nanoparticles (NPs), particularly those of the noble metals, exhibit strong surface plasmon resonances (SPRs) with intense and broad optical absorption bands that arise from the coherent oscillations of conduction electrons near the NP’s surfaces and that result in strong localized electric fields at its vicinity. The ability to control those SPRs is critical for achieving advances in many areas, including chemical and biological sensing, imaging, optoelectronics, energy harvesting and conversion, and medicine.1, 2 Interestingly, the intrinsic properties of NPs can be tailored by controlling their morphology3 (e.g. size and shape), composition4 (e.g. monocomponent vs. alloys), structure5 (e.g. solid vs. hollow), surface chemistry6 and structure,7 and the refractive index of the local environment.8 Among the three metals that display plasmon resonances in the visible spectrum (Ag, Au, Cu), Ag exhibits the highest efficiency of plasmon excitation and it is the only material whose plasmon resonance can be tuned to any wavelength in the visible and near infrared (NIR) range by modifying Ag NP’s morphology.9 Owing to these unique optical properties, Ag is probably the most important material for the next-generation plasmonic technologies.2 A well-known example is the plasmonic enhancement of Raman scattering, known as surface-enhanced Raman scattering (SERS), where the strong near-fields near NP’s surface allow enhancing spectroscopic signals from molecules, providing ultrasensitive and ultraselective information about its structure, and offering molecular-specific signatures for chemical imaging.10 Other examples are the plasmonic field-enhanced emission from fluorophores,11 the relatively unexplored near-field enhancing processes such as natural and magnetic circular dichroism,12 the local photolithography,13 the photochemistry and photocatalysis,14 and the fast dissipation of energy upon plasmon excitation for localized heating.15 In this context, one of the key priorities for expanding the applicability of plasmonic NPs is to produce advanced NPs with engineered SPRs modes (frequency and intensity) and fields (nearfield enhancements, far-field signatures and spatial profiles). Up to date, most investigations have been focused on the dipolar and quadrupolar resonances in the small particle size regime (2-100 nm),16,
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while high-order multipole modes in larger or anisotropic NPs have only
recently begun to attract attention.18 The identification of multipolar resonances have been traditionally difficult to achieve due to the relatively wide distributions of Ag NP’s sizes and morphologies involved, and/or the restricted size ranges studied, which is translated into the overlapping of the resonance frequencies of the various multipolar plasmons in a broad spectrum19 (for instance, the calculated separation between the two peaks of dipolar and quadrupolar absorption in Ag NPs is less than 75 nm20). In this regard, previous attempts have 2
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been restricted to the detection of multipolar modes in non-spherical geometries21
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shells.23, 24 Several studies also report the size-dependent optical properties of spherical Au
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and Ag 27-32 NPs observing, in addition to the dipolar resonances, the quadrupolar mode. Highorder modes, in particular the octapolar and hexadecapolar modes, were theoretically predicted33 and recently observed for spherical Ag NPs18 and metallic nanoshells.23, 34 Beyond the size-control, an important requisite is the control of the interaction between the NPs and the surrounding environment, which is usually achieved by its functionalization with the appropriate molecules. In the case of plasmonic NPs, these molecules act as transducers that, after binding to the NPs, convert small increases in the local refractive index (most of these molecules have a higher refractive index than the hosting solution) into spectral red-shifts. The magnitude of this red-shift usually depends on the size, morphology and composition of the metal particle,35 the nature of the molecule, in detail, its length (the thickness of the dielectric media surrounding the metal core)36 and chemical bonds formed with surface atoms (the degree of confinement of surface electrons),16 and its degree of packaging at the surface of the NPs.37 This molecular binding, which is routinely observed after NP’s functionalization,38-41 has been monitored in real time with high sensitivity42-44 but not systematically studied, due in part to the complexity of physical and chemical processes involved (oxidation and dissolution) and uncertainties in experimental samples (lack of size control and aggregation). Pioneering studies were reported by Mulvaney and Henglein in Ag NPs.45, 46 However, since then, only few reports address the study of how optical properties are affected by modifications of the nature of the NP-molecule chemical bonding16 and/or the thickness of the inorganic47 or organic coating.48 In this work, we present a comprehensible study on how the optical properties of noble metal NPs are affected by systematic variations of NP’s size, composition and surface coating. First of all, we exhaustively analyze the size-dependent optical properties of colloidal solutions of Ag NPs from 10 to 220 nm, with very narrow size distributions (
