Morphology- and Composition-Modulated Sensing
Nanoparticles with various sizes and shapes produce unique localized surface plasmon resonance bands and exhibit different physical and chemical properties. For instance, catalytic ability, sensitivity to changes in the surrounding medium, and biocompatib
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Morphology- and Composition-Modulated Sensing
Abstract Nanoparticles with various sizes and shapes produce unique localized surface plasmon resonance bands and exhibit different physical and chemical properties. For instance, catalytic ability, sensitivity to changes in the surrounding medium, and biocompatibility are all dependent on the morphology of nanoparticles. In recent decades, various types of nanostructures have been fabricated to tune plasmon resonance bands, enhance the electromagnetic field around metal nanoparticles, and determine the relationship between the size and shape of nanoparticles and their LSPR band. In this chapter, we discuss the effect of morphology on plasmonic properties and the related applications. Keywords Size of nanoparticles • Shape of nanoparticles • Composition of nanoparticles • Core–shell nanoparticles • Polarisation
4.1 Nanorods LSPR property is dependent on the shape of nanoparticles that it is able to fabricate nanoplasmonics with different resonance band and sensing functions [1–3]. Particularly, nanorods have been widely applied in catalysis, biosensing, and photothermal therapy [4–7]. Nanorods with various aspect ratios have been fabricated, and their LSPR spectra can be predicted using the Gans theory [4]. Gans predicted that the surface plasmon mode can be divided into two parts for nanorods (also referred to as ellipsoids) when the dipole is constant. The Gans formula has been developed over the course of 40 years, and the polarisability of nanoparticles is now described by:
χx,y =
4πab2 (εAu − εm ) 3εm + 3Lx,y (εAu − εm )
Y.-T. Long and C. Jing, Localized Surface Plasmon Resonance Based Nanobiosensors, SpringerBriefs in Molecular Science, DOI: 10.1007/978-3-642-54795-9_4, © The Author(s) 2014
(4.1)
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4 Morphology- and Composition-Modulated Sensing
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Fig. 4.1 a Surface plasmon absorption spectra of gold nanorods of different aspect ratios, showing the sensitivity of the strong longitudinal band to the aspect ratios of the nanorods. b TEM image of nanorods of aspect ratio of 3.9, the absorption spectrum of which is shown as the orange curve in panel (a). Reprinted with permission from Ref. [8]. Copyright (2006), American Chemical Society
where a and b denote the lengths of the nanoparticle along the x- and y-axes (a > b), εAu is the dielectric constant of Au, and εm is the dielectric constant of the medium. We can get Lx, y as:
Lx =
1+e 1 1 − e2 )) ln( (−1 + e2 2e 1−e
(4.2)
1 − Lx 2
(4.3)
Ly =
where Lx, y represents the depolarisation, e is the ellipticity, and the polarisability is easily related to Cabs and Csca in Eq. (2.51). Based on this theory, we conclude that, as the aspect ratio of nanorods increases, the LSPR band will shift to longer wavelengths, as shown in Fig. 4.1 [8]. By tuning the aspect ratio of nanorods, it is easy to obtain resonance band of nanoplamsonics at near-infrared region from 600 nm to more than 1,000 nm, providing multiple materials for diagnosis, theranostics, and mapping in vitro and in vivo.
4.2 Pla
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