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Signal Generation with Gold Nanoparticles: Photophysical Properties for Sensor and Imaging Applications

Signal Generation with Gold Nanoparticles: Photophysical Properties for Sensor and Imaging Applications

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320



Chapter 10 Signal Generation with Gold Nanoparticles: Photophysical Properties

10.8 NONLINEAR OPTICAL PROPERTIES

10.8.1 MULTIPHOTON LUMINESCENCE

10.8.2 NONLINEAR SCATTERING AND SECOND

HARMONIC GENERATION



338

339



10.9 CONCLUDING REMARKS



343



REFERENCES



343



341



10.1 INTRODUCTION

Nanostructured materials are well suited to support the diverse activities currently

being explored in contemporary supramolecular science. Nanomaterials and supramolecular systems operate on similar length scales, and in recent years, chemists

have been successful in controlling the number and types of molecules presented

on the surfaces of nanoparticles, ranging from single-molecule functionalization to

multilamellar structures. Like molecules, nanoparticles can be prepared in a wide

variety of shapes and sizes, and in many cases exhibit tunable physical properties

that can be exploited for applications in analytical chemistry and information technology. Nanoparticles are also building blocks in their own right, and can be directed

by various modes of self-assembly to create unique architectures with novel materials

properties.

Here, we focus on Au nanoparticles as complementary functional materials for

supramolecular systems, and discuss recent demonstrations of hybrid nanosystems

in chemical and biomolecular sensing, biomedical imaging, photothermal actuation,

or a combination of these. Nearly all of the properties to be discussed are derived

from surface plasmons, an electrodynamic phenomenon involving the collective excitation of conduction electrons at specific wavelengths of light. Plasmon-resonant

nanomaterials can amplify optical signals by many orders of magnitude, and the

resonance conditions are highly sensitive to both structural and surface effects. This

tunability creates a wealth of opportunities for coupling the optical properties of

Au nanoparticles to supramolecular function.

In this chapter, we provide a brief discussion on surface plasmons, intended for

the nonspecialist. This is followed by a closer examination of some photophysical

properties intrinsic to Au nanostructures, as well as examples of their application to

supramolecular or bioanalytical systems. Readers interested in a deeper understanding

of the underlying physics of surface plasmons are directed to the monographs or

reviews in References 1 to 13. With respect to supramolecular chemistry, it must be

emphasized that a successful fusion with metal nanoparticles requires a good working

knowledge of surface chemistry and the various chemisorptive strategies for introducing ligands onto metal surfaces. A comprehensive treatise on this subject can be

found in Chapter 4. Lastly, due to space limitations and the enormous number of publications involving the application of Au nanoparticles in chemical systems, we regret

that only a selected number of examples can be presented, to illustrate some of the

many possible applications supported by these fascinating materials. Indeed, we



10.2 Surface Plasmons



321



have intentionally left out some of the more obvious applications, such as the changes

in optical absorbance caused by the aggregation of gold nanoparticles, in order to

focus on some more recent (and possibly underexploited) photophysical effects.



10.2 SURFACE PLASMONS

Coinage metals such as Au and Ag are well known for their ability to support surface

plasmons. For bulk metal with smooth surfaces, the conduction electrons can be

excited as a plasmon “wave” by incident photons at the surface (metal –dielectric

interface), and can propagate for several microns before the energy is reemitted (or

scattered) as light. This effect, which is commonly referred to as surface plasmon

resonance (SPR), also involves a strong attenuation in reflectance at a specific angle

of incidence. The resonance condition is highly sensitive to the local environment

and is useful for detecting changes in molecular surface adsorption, but its quantitative

analysis requires precise alignment of the collection optics (Fig. 10.1).

Metal nanoparticles with dimensions below the wavelength of light are also

highly responsive to light; however, the conduction electrons are trapped within the

particle and thus forced to oscillate as induced dipoles in a localized fashion. Here,

the resonance condition is defined by the intrinsic material properties (dielectric function) of the metal as well as the surrounding medium, and also by nanoparticle size and

shape. In the case of spherical Au nanoparticles in the size range of 5 to 40 nm, the

localized surface plasmon resonance (LSPR) is strongest at green wavelengths



Figure 10.1



(a) Surface plasmon wave propagating (in the x direction) along a metal–dielectric

interface; (b) localized surface plasmon resonance (oscillating induced dipole) produced by Au

nanospheres.7 (Reprinted with permission from Y. Xia and N. J. Halas, MRS Bull. 2005, 30, 338 –343.)



322



Chapter 10 Signal Generation with Gold Nanoparticles: Photophysical Properties



(lSPR ¼ 520 to 540 nm), with colloidal dispersions producing a characteristic winered color. If the Au nanoparticles are smaller than 5 nm, the LSPR peak is broadened

due to surface scattering.14 If the nanospheres are much greater than 40 nm (the

approximate mean free path of an electron in Au), the LSPR shifts to longer wavelengths due to retardation effects, and secondary peaks may appear due to multipolar

plasmon modes.

The intense optical properties of colloidal gold have been appreciated for a very

long time, with famous examples dating back to the days of the Roman empire, but the

physical origin of “red gold” was first established by Faraday in the nineteenth

century.15 The electrodynamic nature of surface plasmons was first described by

Mie for spherical Au particles at the beginning of the twentieth century,16 then

extended by Gans toward ellipsoidal particles.17 In the simple case of small metal

nanoparticles (r , 20 nm) the LSPR can be attributed solely to the dipolar oscillation

of conduction electrons, whereas higher-order plasmon modes increase in significance

at the expense of dipolar modes for large or anisotropic particles.18

The extinction cross-section Cext for spherical particles can be approximated by

using Mie theory:









24p2 r3 13=2

100

m



(10:1)

Cext ẳ



2

2

0

00

l

(1 ỵ 21m ) ỵ (1 )



Figure 10.2 Simulated and normalized experimental LSPR spectra from different Au nanostructures.

(a, d) Au nanospheres with varying diameters; (b, e) Au nanorods with varying aspect ratio; (c, f )

nanospheres loaded onto thin glass films with varying surface coverages.8 Horizontal and vertical arrows

correspond to the progressions in lLSPR with increasing size or aspect ratio. (Reprinted with permission

from L. M. Liz-Marza´n, Langmuir 2006, 22, 32– 41. Copyright 2006 American Chemical Society.)



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