Novel Optical Properties and Emerging Applications of Metal

 Copyright 2008 by the American Chemical Society

VOLUME 112, NUMBER 28, JULY 17, 2008

CENTENNIAL FEATURE ARTICLE Novel Optical Properties and Emerging Applications of Metal Nanostructures† Adam M. Schwartzberg‡ and Jin Z. Zhang*,§ Department of Chemistry, UniVersity of California at Berkeley, Berkeley, California 94720, and Department of Chemistry and Biochemistry, UniVersity of California at Santa Cruz, Santa Cruz, California 95064 ReceiVed: February 29, 2008; ReVised Manuscript ReceiVed: May 1, 2008

This paper provides a brief overview of recent research activities concerning metal nanomaterials, including their synthesis, structure, surface plasmon absorption, surface enhanced Raman scattering (SERS), electron dynamics, emerging applications, and the historical context by which to view these subjects. We emphasize coinage metals, particularly silver and gold. Silver and gold nanostructures exhibit fascinating optical properties due to their strong optical absorption in the visible as a result of the collective oscillation of conduction band electrons, known as the surface plasmon. This is the origin of many interesting physical phenomena and related applications such as surface plasmon resonance (SPR) and SERS useful in chemical and biomedical detection and analysis. SERS offers high sensitivity and molecular specificity that are attractive for sensing and imaging applications. Electron dynamics in metal nanostructures have been studied using ultrafast laser techniques to gain fundamental insight into electron-phonon interaction as well as coherent lattice oscillation in different metal nanostructures. Relevant theoretical work and models are also discussed in conjunction with the related experimental work. Synthesis and structural characterization are also discussed to make this paper self-contained and easier to follow. The paper ends with some emerging applications of optical properties of metal nanomaterials including photothermal therapy for cancer. I. Introduction The particles are easily rendered eVident, by gathering the rays of the sun (or a lamp) into a cone by a lens, and sending the part of the cone near the focus into the fluid; the cone becomes Visible, and though the illuminated particles cannot be distinguished because of their minuteness, yet the light they reflect is golden in character, and seen to be abundant in proportion to the quantity of gold present. sMichael Faraday † This year marks the Centennial of the American Chemical Society’s Division of Physical Chemistry. To celebrate and to highlight the field of physical chemistry from both historical and future perspectives, The Journal of Physical Chemistry is publishing a special series of Centennial Feature Articles. These articles are invited contributions from current and former officers and members of the Physical Chemistry Division Executive Committee and from J. Phys. Chem. Senior Editors. * To whom correspondence should be addressed. ‡ University of California, Berkeley. § University of California, Santa Cruz.

Adam Schwartzberg received his B.S. and Ph.D. degrees from University of California, Santa Cruz in 2000 and 2006, respectively. He worked extensively with surface enhanced Raman scattering and metal nanoparticle synthesis for detection and sensing at both UC Santa Cruz and Lawrence Livermore National Labs as a SEGRF fellow. He is currently a postdoctoral researcher at University of California, Berkeley working in broadband coherent anti-Stokes Raman scattering and X-ray imaging of silsesquioxane based intermetal dielectrics.

The insights of Michael Faraday into the nature of colloidal solutions ring as true today as they did in 1857.1 In his seminal article on finely particulated gold, he described many unusual and wonderful properties of these solutions, but the most important revelation lies in his supposition that, rather than a solution of dissolved gold atoms or inorganic compounds, the gold was present in the form of “pure gold in a divided state”. This observation is fundamental and at the root of all nanoscience. Nanoparticles had been used for generations before Faraday’s work, e.g., in stained glasses and the like.2 However, until his

10.1021/jp801770w CCC: $40.75  2008 American Chemical Society Published on Web 06/06/2008

10324 J. Phys. Chem. C, Vol. 112, No. 28, 2008 Jin Zhong Zhang received his B.Sc. degree in Chemistry from Fudan University, Shanghai, China, in 1983 and his Ph.D. in Physical Chemistry from University of Washington, Seattle, USA in 1989. His Ph.D. work focused on experimental and theoretical studies of molecular reaction dynamics in the gas phase. He was a postdoctoral research fellow at University of California, Berkeley from 1989 to 1992, where he studied reaction dynamics in solutions using ultrafast laser and computer simulation techniques. In 1992, he joined the faculty at University of California, Santa Cruz, where he is currently full professor of Chemistry and Biochemistry. Prof. Zhang’s recent research interests focus on design, synthesis, characterization, and exploration of applications of advanced materials including semiconductor, metal oxide, and metal nanomaterials, with emphasis on optical and dynamic properties and applications in areas such as solar energy conversion, chemical sensing and detection, and biomedical detection and therapy.

work little was known as to the true origin of the colors observed upon doping glasses with gold and silver. One of the reasons for the great deal of attention paid to metal nanomaterials, both then and now, originates from the strong absorption they possess in the visible region of the electromagnetic (EM) spectrum. So much so, they actually absorb more light than would be indicated by their geometrical cross section.3 In fact, it can be shown that silver particles are capable of interacting with ten times more light than would be expected by this measure.4 These particles act as “photon catchers,” concentrating a significant amount of electromagnetic energy into a very small volume. They can improve any application that can benefit from this kind of light harnessing or field enhancement. The most prevalent application to exploit these properties has been surface enhanced Raman scattering (SERS), which uses the evanescent field at the surface of nanostructured materials to greatly amplify the weak, but molecule specific Raman signal.5,6 Amplification by many orders of magnitude is possible and has been suggested to be capable of detecting the Raman spectra of single molecules.7–15 SERS has been used for a wide range of applications in physics, chemistry, and, more recently, biology as well as biochemistry.16–33 However, SERS is not the only application that has found traction among those taken with metal nanomaterials. Surface enhanced fluorescence,34–47 surface plasmon resonance imaging,48–55 surface plasmon resonance spectroscopy,56–61 and surface enhanced second harmonic generation62–65 are other examples that have demonstrated the advantages and promises of metal nanostructure for different applications. Another area of significant interest is the electron dynamics of nanomaterials.66–72 For small particles of several thousand atoms or so, a high percentage of these atoms are on the surface making the electronic properties of the system much more sensitive to surface changes such as capping materials and solvent. This, coupled with the strong optical absorption, has made metal nanoparticles a fundamentally interesting system for understanding the solid-liquid interface and the effect of size, shape, and surface on electron dynamics including electron-phonon interaction. Techniques such as ultrafast transient absorption and second harmonic generation have proven to be powerful tools for probing electron as well as lattice dynamics in metal nanomaterials. Throughout this paper, we will attempt to give an overview of the field of metal nanomaterials, focusing on the coinage metals due to their unique and interesting optical properties. We will briefly introduce synthesis and characterization techniques that are currently used and try to provide some perspective as to the direction of the field. Next we will discuss the nature of the plasmon absorption and how shape, size, and material play important roles in the nature of their resulting photophysical properties. We will also introduce SERS and

Schwartzberg and Zhang highlight some current theoretical and experimental results that are important to the field. We then discuss electron dynamics of metal nanoparticles and how important results here can lead to a better understanding of their fundamental properties. Finally, some examples of emerging applications of metal nanostructures, e.g., in biomedical imaging and therapy, are highlighted to illustrate the usefulness of metal nanomaterials. II. Synthesis and Structural Characterization of Metal Nanomaterials II.1. Synthesis of Metal Nanomaterials. Metal nanomaterials are generally synthesized by reduction of metal ions or salts with appropriate reducing reagents. Gold nanoparticles were discovered early in the evolution of nanomaterials largely because of the ease with which they can be produced. A gold salt solution mixed with an appropriate reducing agent or heated in a glass matrix will rapidly turn from yellow to bright red or burgundy, indicating the transformation from a dissolved salt (Au3+ ions) to a collection of miniscule gold metal particles. While synthetic control has improved over the decades, the basic mechanisms involved are unchanged. Here we will examine several basic processes for generation and control over shape and size of metal nanostructures. The most popular and, in many ways, least complex means for producing gold nanoparticles is the Turkevich method.73 This was later improved upon by Frens and is continually refined.74–76 The synthesis involves simply boiling a solution of gold chloride in the presence of a weak reducing agent, sodium citrate. The equivalent of this reaction for silver nanoparticles is the method introduced by Lee and Meisel,77 which also utilizes sodium citrate as the reducing agent. Although the Turkevich method is normally used to produce relatively small, homogeneously distributed particles, the silver synthesis is used to produce large, inhomogeneous, spherical particles. An inhomogeneous sample size can be limiting for some applications; however, these particles in particular have become the “gold” standard for surface enhanced Raman scattering (SERS) due to their strong SERS activities. The mechanism of particle formation for both the Turkevich and Lee-Meisel methods are essentially identical and the general principal can be applied to many colloidal syntheses. The most fundamental parts of any colloidal synthesis are the reducing and capping agents. In this case, sodium citrate acts as both a capping material and as the reducing reagent. At room temperature, in fact, the combination of gold chloride or silver nitrate with sodium citrate does not result in any reaction at all. However, by boiling the solution, it is possible to impart enough energy into the system to push a few ions over the reductive potential energy. Once a “seed” atom has formed the potential energy at its surface is significantly lower than that of the free solution.78 It is at these “seed” sites that particle growth will begin. At this point, the capping material becomes critical. If the material is too weakly associated with the metal, there will be little or no protection and the formed nanocrystals will begin to aggregate. However, if the capping material is too strongly bound to the surface, it can limit or stop growth (this becomes especially important for shape control, which we will discuss later). We have found that, in the case of some capping materials, the interaction can be too strong, paradoxically inducing interparticle aggregation, then stabilizing the aggregates once equilibrium is reached.79–82 In the case of citrate, it is only weakly bound to the surface, mostly electrostatically. However, there is enough association and ionic repulsion to halt aggregation. In many cases, the capping material will also limit the size of the particle. This is most obvious in micelle or reverse micelle

Centennial Feature Article

J. Phys. Chem. C, Vol. 112, No. 28, 2008 10325

Figure 1. Representative TEM micrographs of Au spheres obtained after subsequent growth steps. Average diameters are 66 (a), 100 (b), 139 (c), 157 (d), and 181 nm (e). (Figure taken from ref 89, copyright American Chemical Society, 2006.)

directed reactions where the physical limitations of the micelle determine size.83,84 While initially it was believed that citrate as a reducing agent was responsible for particle size, it has been shown to be a more complicated problem.76 Simply speaking, the sodium citrate acts most strongly as a capping material as, in all cases, there is excessive citrate to perform the reduction of the gold salt. At high citrate concentration, many small particles can be stabilized, however, at low concentration, fewer large particles will be supported. In general, as particle size increases, the sample dispersity becomes large, which is undesirable for some applications. Because seed formation and particle growth can overlap in time, seed particles continue to form while the initial particles are already growing. As a solution to this problem, the technique of seed mediated growth was developed.85–88 Small seed particles are produced by the Turkevich method, and then they are diluted and mixed with additional gold ions. By adding a weak reducing agent (normally hydrazine or ascorbic acid) the gold will only grow at the surface of the particles and few or no additional particles will form. By separating the seed formation stage and the particle growth stage in time, it is possible to produce relatively large particles (>100 nm) with high homogeneity. Some TEM images of different sized Au nanoparticles formed by this method are shown in Figure 1.89 This method has also been found to be useful as a starting point for structural control. Depending on surfactant (capping material), it is possible to produce shapes other than spheres during seed mediated growth. The most notable example of this is the generation of nanorods via seed mediated growth in the presence of cetyltrimethylammonium bromide (CTAB). There are several theories as to the nature of nanorod formation. They normally center on the preferential adsorption of the surfactant to certain crystal facets on seed particles. In other words, in the case of nanorods, CTAB binds to the surface radially but not axially.90,91 This blocks crystal growth on these surfaces and the particle can only grow in the axial direction. What is perhaps even more interesting is initially the ratio of rods to spheres was rather poor, but over time it was found that by adding silver ions to the growth medium it was possible to force nearly all the seed particles to grow into rods.92–96 It is clear that there are several complex processes responsible for the resulting shape in metal nanoparticle synthesis. More recently Murphy et al. and others have shown great ability to control particle shape and size merely by altering CTAB and ionic concentrations in solution. An example of this is shown in Figure 2 where a variety of shapes is achieved by altering ascorbic acid (AA) and seed particle concentrations. In addition to producing particles with nonspherical dimensions, it is also possible to alter the internal structure of a particle, which has drastic effects on their optical properties. Metal/dielectric core/shell and hollow metal structures are excellent examples. In order to produce hollow structures we and others have used a technique called galvanic replacement. In essence, the

Figure 2. Top: TEM (inset SEM) images of Au nanoparticles synthesized under different conditions. [AA] increases from A to C, and seed concentration increases from C to D. Scale bar ) 100 nm.225 Bottom: TEM images of gold nanorods with plasmon band energies at (a) 700, (b) 760, (c) 790, (d) 880, (e) 1130, and (f) 1250 nm. The scale bar is 50 nm.96

reduction potential difference between a metal nanoparticle and a metal salt in solution is used to turn the particles into essentially nanobatteries. As the salt interacts with the surface the potential difference strips electrons off the metal particle, reducing the salt to its zero state, i.e. metal. In the case of the work of Xia et al., silver particles formed by the high temperature polyol synthesis are used as the “template” and gold ions are added to the solution.97–99 Because the standard reduction potential of the AuCl4-/Au pair (0.99 V, vs SHE) is higher than that of the Ag+/Ag pair (0.80 V vs SHE) gold ions are reduced at the surface of Ag nanoparticles, and silver metal will be oxidized into Ag+ ions. This can be done with almost

10326 J. Phys. Chem. C, Vol. 112, No. 28, 2008

Schwartzberg and Zhang

Figure 3. Transmission electron micrographs of the HGNs (top). (A) High resolution TEM of a single, 30 nm HGN. The wall thickness is approximately 4 nm and large areas of crystallinity are clearly visible. (B-E) Low resolution TEM images of particles of 71 ( 17 nm (B), 50 ( 5 nm (C), 40 ( 3.5 nm (D), and 28 ( 2.3 nm (E).101 High resolution TEM of a representative HGN.101

Figure 4. Schematic illustration of surface plasmon resonance of a nanoparticle in an electrical field (e.g., light) with wavelength much longer than the dimension of the particle.

any shape; nanostructures such as hollow rods, spheres, rattles, cubes, and wires have been demonstrated by this technique.97,100 Recently we have used a similar approach with cobalt nanoparticles and gold ions as the redox pair to produce highly uniform hollow gold nanospheres (HGNs) with tunable optical properties.101,102 We found that it was possible to control the size and shell thickness to tune the plasmon absorption in the entire visible and near IR region of the spectrum. Some

representative TEM and HRTEM images of HGNs are shown in Figure 3. It is interesting to note that these hollow spheres of gold do not grow as single crystals as is often thought. It is clear that there are large single crystal domains; however, there is significant twinning when the entire shell is examined closely (Figure 3A and bottom). The shell is apparently polycrystalline. It is extremely challenging, if possible at all, to produce a shell that is entirely single crystalline. Structural characterization at the atomic scale is essential for nanomaterials. Lack of detailed structural study can often be the cause of controversies in the literature.82 II.2. Structural Characterization of Metal Nanomaterials. Structural determination is essential for nanomaterials research. Since the nanostructures are usually too small to be visualized with a conventional optical microscope, it is important to use appropriate tools to adequately characterize their structure and surface in detail at the molecular or atomic level. This is important not only for understanding their fundamental properties but also for exploring their functional and technical performance in technological applications. There are several experimental techniques that can be used to characterize



structural and surface properties of nanomaterials either directly or indirectly, e.g. XRD (X-ray diffraction), STM (scanning tunneling microscopy), AFM (atomic force microscopy), SEM (scanning electron microscopy), TEM (transmission electron microscopy), XAS (X-ray absorption spectroscopy), EXAFS (extended X-ray absorption fine structure) and XANES (X-ray absorption near edge structure), EDX (energy dispersive X-ray), XPS (X-ray photoelectron spectroscopy), IR (infrared), Raman, and DLS (dynamic light scattering).103–107 Some of these techniques are more surface sensitive than others. Some of the techniques are directly element-specific, whereas others are not. The choice of technique depends strongly on the information being sought about the material. X-ray diffraction (XRD) is a popular and powerful technique for determining crystal structure of crystalline materials.108–110 By examining the diffraction pattern, one can identify the crystalline phase of the material. Small angle scattering is useful for evaluating the average interparticle distance while wideangle diffraction is useful for refining the atomic structure of nanoclusters.111 The widths of the diffraction lines are closely related to strain and defect size and distribution in nanocrystals. As the size of the nanocrystals decrease, the line width is broadened due to loss of long-range order relative to the bulk. This XRD line width can be used to estimate the size of the particle by using the Debye-Scherrer formula.

D)

0.9λ β cos θ

(1)

where D is the nanocrystal diameter, λ is the wavelength of light, β is the full width half-max (fwhm) of the peak in radians, and θ is the Bragg angle. Scanning probe microscopy (SPM) represents a group of techniques, including scanning tunneling microscopy (STM), atomic force microscopy (AFM), and chemical force microscopy (CFM), that have been extensively applied to characterize nanostructures.103,112 A common characteristic of these techniques is that an atom sharp tip scans across the specimen surface and the images are formed by either measuring the current flowing through the tip or the force acting on the tip. SPM can be operated in a variety of environmental conditions, in a variety of different liquids or gases, allowing direct imaging of inorganic surfaces and organic molecules. It allows viewing and manipulation of objects on the nanoscale and its invention is a major milestone in nanotechnology. For nonconductive nanomaterials, atomic force microscopy (AFM) is a better choice.112,113 AFM operates in an analogous mechanism except the signal is the force between the tip and the solid surface. The interaction between two atoms is repulsive at short-range and attractive at long-range. The force acting on the tip reflects the distance from the tip atom(s) to the surface atom, thus images can be formed by detecting the force while the tip is scanned across the specimen. A more generalized application of AFM is scanning force microscopy, which can measure magnetic, electrostatic, frictional, or molecular interaction forces allowing for nanomechanical measurements. Scanning electron microscopy (SEM) is a powerful and popular technique for imaging the surfaces of almost any material with a resolution down to about 1 nm.104,105 The image resolution offered by SEM depends not only on the property of the electron probe but also on the interaction of the electron probe with the specimen. Interaction of an incident electron beam with the specimen produces secondary electrons, with energies typically smaller than 50 eV, the emission efficiency of which sensitively depends on surface geometry, surface

chemical characteristics and bulk chemical composition.114 Transmission electron microscopy (TEM) is a high spatial resolution structural and chemical characterization tool.115 A modern TEM has the capability to directly image atoms in crystalline specimens at resolutions close to 0.1 nm, smaller than interatomic distance. An electron beam can also be focused to a diameter smaller than ∼0.3 nm, allowing quantitative chemical analysis from a single nanocrystal. This type of analysis is extremely important for characterizing materials at a length scale from atoms to hundreds of nanometers. TEM can be used to characterize nanomaterials to gain information about particle size, shape, crystallinity, and interparticle interaction.104,116 X-ray based spectroscopies are useful in determining the chemical composition of materials. These techniques include X-ray absorption spectroscopy (XAS) such as extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES), X-ray fluorescence spectroscopy (XRF), energy dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS).117,118 They are mostly based on detecting and analyzing radiation absorbed or emitted from a sample after excitation with X-rays, with the exception that electrons are analyzed in XPS. The spectroscopic features are characteristic of specific elements and thereby can be used for sample elemental analysis. For complex nanostructures, a combination of techniques is often needed to unambiguously determine the structure. One example is the distinction between so-called “core/shell” structures versus aggregates in the case of gold nanostructures from the reaction of HAuCl4 and Na2S.119 Although the initial and several follow-up reports claimed Au2S/Au core/shell structure,120,121 more detailed studies have shown that the reaction product is mostly aggregates of gold nanoparticles.82,122–124 This example shows that care must be taken in structural determination. III. Optical Properties: Surface Plasmon Absorption The optical and other physical properties of coinage metals are intimately related to their delocalized conduction band electrons that only weakly interact with the lattice of cations that make up their crystal structure.125 The delocalized electrons account for the apparently high electrical and heat conductivities of the metals. Less obvious though is the role played by these conduction band electrons in the optical properties of nanostructures of these metals. When light is incident on a metal particle that has a diameter much less than the wavelength of light (d , λ), the electromagnetic (EM) field across the entire particle is essentially uniform, as shown in Figure 4. As the EM field oscillates back and forth (in the direction of E field polarization), the weakly bound electrons in the metal will respond collectively. When the incident light frequency matches the intrinsic electron oscillation frequency, light will be absorbed, resulting the well-known surface plasmon absorption. Coulombic attraction between the electrons and the metal cations or lattice serves as the restoring force for the electrons when they are off their equilibrium positions. There is transient net charge displacement (appearance of surface charges: negative for the electrons and positive for the metal core, as shown in Figure 4) and therefore the equivalent of a transiently induced dipole moment during oscillation. The less tightly bound the electrons are, the lower the resonance frequency or longer the wavelength of the plasmon absorption is. Because gold and silver particles have essentially “free” or weakly bound conduction electrons, this oscillation lies low enough in energy to be



Figure 6. UV-visible absorption spectra (top) and photo of different colors (bottom) of several HGN samples with varying shell diameters and wall thicknesses.101

Figure 5. Schematic illustration of plasmon resonance in solid metal, e.g., gold, nanoparticle (A), nanorod (B), HGN (C), and aggregate (D).

in the visible spectral region, typically around 390 nm for Ag and 520 nm for Au nearly spherical nanoparticles. The spectral shape of the plasmon band for a given particle was determined over a century ago by Gustav Mie and can be represented in this way

[

24π2Na3εex3/2 εi(λ) E(λ) ) λ ln(10) (εr(λ) + χεex)2 + εi(λ)2

]

(2)

where εr and εi are respectively, the real and imaginary components of the dielectric function of the metal, εex is the external environment dielectric function, r is the radius of the particle, χ is factor related to the eccentricity of the particle, and N is the number of atoms present in the particle. From this equation one can reasonably predict the position and shape of plasmon absorption for spherical and spheroidal metal particles.126 For particles with highly nonspherical geometries, some approximations must be made and the methods of discrete dipole approximation (DDA) and finite-difference time-domain (FDTD) have been used to make these calculations.127–129 Because spherical particles are in principle completely symmetrical, there is only one plasmon resonance, or rather, all possible modes are degenerate (Figure 5A). By extending this particle in one

dimension, however, it is possible to form a second, lower energy resonance band in the longitudinal direction while the original transverse mode persists, as shown schematically in Figure 5B. As particles become more complex, many more nondegenerate modes arise and their optical absorption becomes more complex with many bands over a broad spectral region. Examples include nanorods, nanoprims, nanocages, aggregates, and core/shells.82,90,96,130–142 One interesting example is the HGNs that have essentially a single, narrow resonance observed due to their highly uniform shell structure, despite its seemingly complex structure (Figure 5C).101 To understand the origin of this single resonance mode, it is helpful to take a closer look at the shell structure and its relation to plasmon resonance. While one may consider the plasmon resonance as due to electrons moving back and forth, from one to the other side of a particle, this is not a correct interpretation of the phenomenon. The resonance is due to collective motion of the electrons where any individual electron does not move significantly from its origin. The effect of size and shape is to alter the energy of this collective mode, not necessarily the path length under which a given electron may travel. While a gold rod will experience resonant energies along the two different axes (Figure 5B), a hollow sphere has no such degeneracy, which is similar to solid spherical particles. The electron cloud will “feel” a single energy resonance in whichever direction the incoming electric field is applied. The total energy of this resonance is tunable based on the size of the particle and the thickness of the shell. At a constant particle diameter, as the shell thickens, the energy of the resonance will blue shift continuously until the particle becomes solid with the corresponding resonance. We have been able to show wide tunability of the plasmon absorption of these particles by changing particle size and shell thickness as shown in Figure 6.101 The results are in good agreement with theoretical calculations.127 This is an excellent example of plasmon engineering, but only one of many examples.93,132,143,144 An example of a nanostructure with multiple modes or plasmon resonances is aggregates of nano-

Centennial Feature Article particles, as illustrated in Figure 5D.80,82 Another parameter which may play a large role in plasmon physics is the polycrystalline nature of the particles. As was pointed out for the HGNs in figure 3, there is significant crystalline twinning in the crystal. This type of structure is seen in many nanostructures of gold and silver and is not unique to hollow particles. The effect of grain boundaries on electron oscillation is not known, however, it is likely that the type and concentration of these defects will have a great effect on optical properties. In general, the more complex and lower symmetry the nanostructure, the more different modes it possesses and the broader the overall plasmon absorption spectrum is. One can in principle manipulate the structure to control and obtain narrow and tunable plasmon absorption in a very broad spectral range, covering easily the entire visible to near IR region, which is often desired for different technological applications, including SERS, as discussed later. Shape is not the only parameter that affects the plasmon resonance of the particle. From eq 2 it is clear that both imbedding medium and metal dielectric (and indexes of refraction) play a key role in determining the position and quality of the plasmon band. By changing the particle material from silver to gold, spherical particle resonance shifts from ∼400 nm to ∼520 nm. Similarly, by changing the dielectric of the imbedding medium, it is possible to shift the plasmon resonance of a particle significantly.145–147 In addition, interaction between nanoparticles, especially when strong, also significantly affect the plasmon absorption.148–150 Although a weak or moderate interaction results in a small shift of the plasmon band, a strong interaction results in new absorption bands due to electronic coupling between particles, such as in aggregates.80,82,151 IV. Optical Properties: Surface Enhanced Raman Scattering Surface enhanced Raman scattering (SERS) is based on the enhancement of Raman scattering of an analyte molecule near or on a roughened metal substrate surface. It has become the ubiquitous application of metal nanoparticles. The origin of the Raman enhancement is largely due to an enhanced EM field at the metal substrate surface due to increased absorption of the incident light. This is related to several other surface enhancement phenomena, including surface plasmon resonance and surface-enhanced fluorescence.40,42,43,45,47,54,61,136,152–154 Since its initial discovery in 1928, Raman scattering has become one of the most widely used and powerful spectroscopic techniques for chemical sensing, detection, and imaging. Its key advantage is molecular specificity that allows for unique identification of molecules. The weakness of Raman is the very low yield of scattering (approximately 1 in 107 photons).155 Thus normal Raman spectroscopy requires high incident laser intensity, long data acquisition time, and high analyte concentration. The discovery of SERS over 30 years ago has made Raman more popular and practical for real world applications. In 1977 surface enhanced Raman scattering was discovered almost simultaneously by Van Duyne et al. and Creighton et al.24,25 In their experiments, they observed an unusually intense Raman signal from pyridine adsorbed onto an electrochemically roughened silver electrode. Previous studies had observed an enhanced signal; however, the effect was assumed to be due to the increase in electrode surface area.23 Van Duyne and Creighton were the first to realize that some other effect was taking place. Nevertheless, it was Moskovitz and co-workers who made the first correct interpretation of the data.156 They surmised that the roughened electrode was essentially a two-

J. Phys. Chem. C, Vol. 112, No. 28, 2008 10329 dimensional array of silver nanoparticles, and that these nanoparticles possessed what was called at the time an “optical conduction resonance” or surface plasmon as it is known today. By boosting the number of Raman scattered photons by 5-10 orders of magnitude, SERS brought Raman to a point where not only could data be collected with reasonable analyte concentrations, laser intensity, and collection times, but also detection of a single molecule using SERS has been demonstrated.12–14,157–161 With the power of SERS realized, it was not long before a significant number of other applications of surface enhancement using metal nanostructures began to appear. There are two theories as to the origin of SERS. The main source of enhancement is believed to be due to the amplified electromagnetic field at the surface of the metal particle.5–8,162 The second, and less prominent, mechanism is the so-called chemical enhancement mechanism in which metal-molecule charge transfer complexes are formed, which improve resonance with the Raman excitation laser. The chemical enhancement is sensitive to the surface properties of the SERS substrate and the nature of the analyte molecules. We will only discuss the EM mechanism here as the chemical enhancement is thought to be responsible for at most 2 orders of magnitude of the total enhancement which can be greater than 1013. Also, the majority of other surface enhanced processes are due solely to the EM mechanism. For a complete discussion of the chemical mechanism, please see the excellent review by Otto.163 It is easiest to understand the EM enhancement by examining a relatively simple model, the quasistatic treatment of an isolated sphere. This model makes the assumption that the metal particle, e.g. gold, is essentially a scaffold of positive charges with a cloud of electrons which can move, more or less, freely. When the system is polarized, e.g. by an EM field, the electrons will respond but the positively charged nuclei will remain static. Assuming that light is incident on a spherical metal particle, imbedded in some medium of dielectric constant ε0, and has the electromagnetic field vector E0 pointing along the z axis and independent of coordinates for distances on the order of the size of the sphere, one can determine the EM field inside and outside of the sphere by the quasi-static approximation of Maxwell’s equations.7,125 The field outside the sphere is written as:

[

Eout ) E0z - RE0

]

z 3z - (zz + xx + yy) r3 r5

(3)

R is the metal polarizability, x, y, and z are the Cartesian coordinates, r is the radial distance, and x, y, and z are the unit vectors. E0 is the magnitude of E0. The first term here is the applied field of the light and the second is the induced dipole of the polarized sphere. Now, assuming that the sphere has a dielectric constant of i and a radius of a, one can write the polarizability as:

R ) gR3

(4)

with

g)

εi - εo εi + 2εo

(5)

It is at this point that we can begin to realize how the plasmon dipole mode affects the external field. When the real part of εi is -2εo and the imaginary part is small, R becomes large, making the induced field large. This implies that the dielectric of the particle must be large and negative for SERS to take place. This is the case for metals with relatively “free” electrons,



e.g., gold and silver at long wavelengths of light. When the resonance condition is met, i.e. the light frequency matches the intrinsic collective electron oscillation frequency, absorption of light will take place, resulting in what is termed the surface plasmon absorption. This resonance is essential for effective SERS enhancement. It is also interesting to note that the plasmon resonance is not only determined by εi, but also by εo. If the dielectric function of the imbedding medium increases by ∆εo, the resonance condition will take place at a position on the dielectric function where εI is proportional to εi - ∆εo. Since the dielectric of the metal generally becomes more negative with increasing wavelength, a decrease in imbedding dielectric will shift the plasmon resonance to the red. This has been observed experimentally and can be useful for various applications.61,136,145–147,152,164 Next we can look at what kind of Raman enhancements are expected with this simple model. The Raman intensity depends on the absolute square of Eout

Eout2 ) E02[|1 - g|2 + 3 cos2 θ(2Re(g) + |g|2)]

(6)

θ is the angle between the excitation field vector and the analyte molecule in question at the surface of the particle. When g is large, as is true when the resonance condition has been met, and this equation reduces to

E2 ) E2|g|2(1 + 3cos2 θ)

(7)

It is clear that the Raman intensity will be greatest on axis with the excitation laser, at 0 and 180°. There is one more factor to take into account, however. Not only can this surface field increase the probability of the vibrationally exited molecule emitting a Raman photon, but there is also a possibility of enhanced emission from the particle’s dipole. Raman shifted photons can be reabsorbed by the particle, enhanced, and reemitted. This implies that the Raman intensity can be enhanced by two separate processes. The overall enhancement from both the incident and scattered fields is

Er )

Eout2E′out2 4

E

16|g|2|g′|2

(8)

Here, the primed variables represent the reradiated fields. This is based on the assumption that the Raman photons are close in frequency to the excitation light, i.e., Raman photons with small Raman shifts. With larger shifts, resonance with the plasmon band becomes less and the reradiation effect becomes less pronounced. This should give a rough idea as to the nature of the EM field enhancement and how the plasmon oscillation plays a part. There are several excellent reviews devoted to this should a more analytical treatment be desired.7,8 For SERS in particular, besides detection of a variety of organic and biological molecules, one of the new directions in which the field has been moving is intracellular detection and imaging.27,165 Nanoparticles are generally small enough to fit into cells without impeding their normal function. There is a size limit, too large or too small, where the particles will not be stable inside the cell or may perturb the cell significantly. If too small (below 20 nm), the particles will be able to slip out of the cell. If too large (above 80 nm), they begin to damage or impede the function of the cell.166 There is a fine line where implanting SERS active particles into living cells is practical. Aside from size considerations, surface properties also play a strong role in cell compatibility and interactions. This is clearly a complex problem and currently an active area of research.

The first example of performing intracellular SERS was by Kneipp et al. who were able to observe the SERS spectrum at several positions within the cell with greater than 1 µm resolution.167 What they observed was a wide variety of different Raman signals, implying a very complex chemical environment, as one might expect within a cell. Later, it was discovered that the SERS active particles are largely particles that have aggregated within the cell and were immobile.168,169 The strong signal from these aggregates allowed them to observe a transient signal as the cell performed its normal functions. Other groups have extended this work to specific analyte sensing using particles with an active surface coating. For instance, mercaptobenzoic acid functionalized silver aggregates have been investigated for SERS sensing applications.170 By monitoring the ratio of non-pH sensitive to pH sensitive Raman bands it is possible to sense, at the nanolevel, the pH of the local environment. This type of pH sensing is now an important area of research and has been expanded by others.171–174 The general field of using SERS sensitive particles for intracellular sensing seems to be moving in the direction of surface functionalization and using reactive species to detect all manner of analytes.175 Many of these SERS sensors utilize large particles or aggregates and it is not clear how these large particles may affect cell function. In order to improve intracellular mobility and minimize negative interaction, we have recently introduced a small SERS probe using HGNs.176 Our first study mimicked earlier ones in which mercaptobenzoic acid was used as a pH sensitive probe. However, rather than attaining large enhancement factors with aggregated particles, we produced HGNs that were demonstrated to be more SERS active than solid gold spheres.127 The unique optical and structural properties of hollow spheres afford several advantages over solid nanoparticles. First, the plasmon resonance is broadly tunable. By controlling size and wall thickness we have been able to produce solutions with absorption maxima ranging from 520 to 800 nm.101 This is particularly useful for biological applications since tissues have an absorption minimum in the near-infrared and near IR light is thus preferred for deep tissue penetration. Nanostructures with strong near-IR absorption are thus highly sought after for biological detection, imaging, and therapy. Second, HGNs possess narrow, tunable absorption in a broad spectral range that solid spherical particles cannot, even though they both have simply spherical structures. Third, a single 30-60 nm HGN provides SERS sensitivity that is comparable to that of an aggregate. However, compared to aggregates or other complex nanostructures that have tunable and broad plamson absorption, the hollow spheres have much simpler structure and narrower plamson absorption, which are both ideal for many applications including SERS. Furthermore, while sensitive for SERS, aggregates tend to have a broad distribution of shapes and sizes that have different resonance absorptions, resulting in substantial inhomogeneous spectral broadening. This is problematic for SERS because this means most of the aggregates are not likely to be on resonance with the incident light frequency since a single or very narrow frequency light source is required for Raman or SERS. This is perhaps partly responsible for some aggregates or particles being “hot” (on resonance) while others are not (off resonance) for SERS. In an ensemble-averaged case, where there is significant inhomogeneity in structure and plasmon resonance, any spectral fluctuations in SERS will average out, leading to a relatively normal spectrum and information on inhomogeneity is lost. When looking at SERS on individual particles, however, where



Figure 8. UV-vis spectra of HGNs and hollow Au-Ag double nanospheres with incremental addition of silver ions during silver shell growth. There are 10-20 µL increments of 1 mM AgNO3 between each consecutive spectrum.177 Figure 7. The ensemble average solution absorption spectrum of an as prepared solution (black trace, top), and the Rayleigh scattering spectrum of a single HGN (red trace, middle) immobilized on a glass coverslip in air. Plotted against the right axis is a histogram of the peak wavelength in the scattering spectra (λmax) of 100 particles (average 621 ( 10.6 nm) (blue bars, bottom). The absorption spectrum is shifted in intensity for clarity. Inset: Rayleigh scattering spectra of two different silver aggregates. 176

peak intensity ratios are important, as for pH sensing, it is important that the enhancement response is consistent from one nanostructure to another. This requires high homogeneity in structure and plasmon absorption for all of the nanostructures. Without this, the peak ratios cannot be used as a valid indicator of the information to be obtained, such as pH.13,14,170 With the normal Ag nanoparticles, the Raman peak intensity ratio changes drastically from particle to particle (Figure 7, inset). In contrast, with HGNs, the SERS peak ratio is highly consistent, attributed to the uniform structure and narrow plamson dispersity. The plasmon line width of the ensemble solution is only slightly broader than the single particle scattering spectrum as shown in Figure 7. The single particle SERS results from these samples showed very narrow calculated pH variations from particle to particle which is reflected in a very narrow scattering maxima distribution of less than 2% (Figure 7, inset). Although the HGNs are indeed very uniform and show highly consistent SERS, they provide relatively weak SERS signal compared to Ag nanoparticles. There are several possible reasons for this. First, because they are hollow, their absorption cross sections are relatively small in comparison to solid particles. Second, because the plasmon resonance of a HGN is relatively narrow, there is little enhancement at Raman bands shifted significantly from the Rayleigh line. By 1000 cm-1 there will be reduced resonance leading to smaller enhancement when using a HeNe excitation source at 632.8 nm. Third, while gold is an excellent, nonreactive material to make sensors out of, it does not possess the same enhancement ability as silver due to their fundamentally different electronic properties. Ideally, one would produce hollow silver, instead of gold, HGNs to improve their SERS sensitivity. However, using the galvanic replacement procedure with cobalt, it is impossible to produce homogeneous particles. Because two silver ions are reduced per cobalt atom the shell grows too quickly leading to a combination of very small particles which have been ejected from the surface of the cobalt and very large, lumpy, inhomogeneous clumps of silver. In order to get around this we have

capped very thin HGNs with silver using the seed mediate growth method.177 By using the HGNs as the seeds we have been able to controllably grow a layer of silver with tunable thickness. The first thing to note of these new structures is the significantly shifted and broadened plasmon band which can be tuned to some extent by controlling silver thickness (Figure 8). The plasmon absorption changes from predominantly that of gold to that of silver with increasing the silver coating as the outer shell. Unlike the HGNs, with excitation at 400 nm the silver coated HGNs (i.e., hollow Ag-Au double nanospheres) will have full resonance for over 6000 cm-1 covering more than enough spectral area for just about any application. Also, there is a greatly increased Raman enhancement over HGNs (top panel of Figure 9) due likely to a combination of the increased volume of metal and the improved enhancement factor of silver. The bottom panel of Figure 9 shows a representative TEM image of a hollow Au-Ag double nanosphere. As mentioned before, metal nanoparticle absorption is dependent on several factors including imbedding material dielectric. This is a key component in a new type of sensing platform called surface plasmon resonance spectroscopy (SPRS).61,178 Using the metal film over nanosphere lithography developed by Van Duyne et al., it has been shown that surface the surface plasmon of these nanoparticulate films is highly sensitive to adsorbates.60,133,179 The relation of adsorbate dielectric to plasmon shift is given as:56,180

∆λmax ) εi∆εads[1 - e-2d⁄ld]

(9)

where εi and εads are the dielectric constant of the particle and the change in dielectric induced by the adsorbate respectively, d is the effective adsorbate layer thickness, and ld is the EM field decay length. Even small changes in adsorbate dielectric can have an observable effect on resonance wavelength. This can be used in a general dielectric (or refractive index) sensing mechanism; however, it is far more powerful in functionalized sensing. A monolayer of receptors sensitive to the desired analyte is applied to the surface of the film. When even a relatively small number of analyte molecules bind to these surfaces, the change in dielectric is enough that the plasmon resonance will detectibly change. This has been shown for the first time in a clinical diagnostic procedure and has the potential to be a powerful and highly sensitive technique for detection.57



Figure 9. (Top) Comparison of averaged single particle SERS for (A) HGNs with and without silver shell, (B) 48 nm Au nanoparticles with and without silver shell, (C) 35 nm Au nanoparticles with and without silver shell, and (D) 48 nm Ag nanoparticles. The sharp lines are artifacts caused by some laser lines not fully filtered.177 (Bottom) Representative TEM image of hollow Au-Ag double nanospheres.

V. Electron and Lattice Dynamics in Metal Nanostructures Study of charge carrier dynamics of metal NPs has received considerable attention recently since it helps to gain insight into the fundamental electron-phonon interaction. Hot electron relaxation in metals occurs on ultrafast timescales because of strong electron-phonon interaction. One of the important issues in the dynamics of metal NPs is whether the relaxation is dependent on particle size or not. Theoretically, electron and phonon spectral overlap is expected to decrease with decreasing

size, leading to weaker electron-phonon interaction. Early electronic relaxation measurements by Zhang et al. suggested a possible size dependence of the electronic relaxation in the 1-40 nm range.181 However, later on Harland et al. and ElSayed et al. found no size dependence on the relaxation down to the size of 2.5 nm.182–184 The relaxation time in Au NPs was reported to be same as that of bulk gold (∼1 ps). Excitation intensity dependence of the relaxation time has been found and could be the reason for the discrepancy. It is likely that the excitation intensities used in earlier work are much higher than

Centennial Feature Article those used in later measurements. Another possible explanation for the difference is surface and/or solvent environment, which was found to affect electronic relaxation of Au NPs.185–187 It seems convincing that the electron-phonon coupling constant is the same for Au NPs as for bulk, at least for particles down to 2 nm in diameter.69 When the particle size is smaller than 1 nm, the electronic relaxation time seems to become significantly longer than that of bulk. An earlier study of Au13 and Au55 found that the electron lifetime becomes significantly longer for Au13 nanoclusters than for Au55 or larger nanoparticles, indicating bulk to molecule transitions in the size regimes of 55 and 13 Au atoms.181 A study of Au28 clusters found biexponential decays with a subpicosecond and a ns component.188 The fast component was attributed to relaxation from a higher lying excited stated to lower electronic states, and the longer nanosecond component was assigned to radiative lifetime of the Au28 clusters.188 More recently, studies of electronic relaxation in Au11 clusters revealed a similar long-lived component (∼1 ns).189 The longer lifetime again suggests that for very small metal clusters, particles are becoming molecule-like in nature and the electron-phonon interaction becomes weaker, similar to that found for Au13.181 Another interesting issue concerning metal NPs is the lattice vibrational oscillations observed in the electronic relaxation dynamics of Au and bimetallic core/shell particles.69,190–193 Femtosecond transient absorption data for Au particles probed at 550 nm following 400 nm pulse excitation show clear modulations with a period of about 16 ps. The frequency of the oscillation was found to increase linearly with decreasing particle size. The oscillations have been attributed to a coherent excitation of the radial breathing vibrational modes of the particles. Photoexcited electrons can transfer their energy into the lattice, heating up the particle and causing a rapid expansion. The expansion and contraction of particle volume over time cause the electron density of particle to change and periodic shift of the surface plasmon absorption band, manifesting itself as a modulation in the transient absorption signal. Similar oscillation has been observed more recently in silver ellipsoids,194 Au nanorods,195 strongly coupled Au aggregates,196 and Au and Ag nanocages.70,197 Time-resolved laser spectroscopy has been used to investigate Au nanorods with aspect ratio from 2 to 5.195 Coherent excitation of the acoustic vibrational modes in Au nanorods results in the oscillation of transient absorption signals. The period of the oscillation has been found to be 2L/ct, where L is the length of the rod and ct is the transverse speed of sound in bulk gold. This is different from the results of silver ellipsoids, where the period is determined to be 2d/cl, where d is the length or width of the ellipsoid and cl is the longitudinal speed of sound in silver.194 The discrepancy has been explained by the different natures of the vibrational motion and elastic properties.195 Aggregates of metal NPs are fundamentally interesting in that they can be used to study interparticle interactions. The interaction between particles can be roughly divided into three regimes: weak, moderate, and strong. In weakly or moderately interacting systems such as DNA-linked Au NPs198,199 and superlattice structures of Au NPs,200 the transverse plasmon band shift noticeably to the red. In strongly interacting systems such as Au nanoparticle aggregates, a whole new absorption band, termed extended plasmon band (EPB), appears near 700-950 nm in addition to the transverse surface plasmon band (∼520 nm).80,201 The EPB is similar to the longitudinal plasmon absorption in nanorods and its appearance is a signature of strong

J. Phys. Chem. C, Vol. 112, No. 28, 2008 10333 interaction between nanoparticles (illustrated in Figure 5D). In a recent dynamic study of strongly interacting Au aggregates, the electronic relaxation time appeared to be similar to those of isolated particles and bulk.196 Surprisingly, periodic oscillations and probe wavelength dependence of the oscillation period were observed in the dynamic profiles of Au nanoparticle aggregates. The oscillations have been attributed to the coherent excitation of vibration of the aggregates. The dependence of oscillation period on probe wavelength indicates that the broad EPB in the static absorption spectrum is inhomogeneously broadened due to different aggregate sizes, supported by a spectral hole burning experiment.196 Samples with such inhomogeneously broadened spectrum is undesirable for SERS since only a subset, often small percentage, of the aggregates will have plasmon absorption resonant with the incident light and thereby will not be effective for SERS. One interesting observation is that the oscillation period is longer than that predicted based on an elastic sphere model. A possible explanation is that the vibrational motion in aggregates is “softer” than that of isolated hard spherical particles. This study shows that transient absorption measurements can provide information on size distribution and vibrational frequencies of metal nanoparticle aggregates or similar systems. VI. Emerging Applications of Metal Nanostructures Metal NPs have long been of interest because of not only their intriguing fundamental properties but also their applications in areas such as optical waveguides,144 catalysis,202 sensors,203 and in surface-enhanced Raman scattering (SERS).157,204 Recently this interest has extended to photovoltaic devices,205–208 biological imaging,209,210 and detection of DNA, RNA, and proteins.211–213 Treatment of diseases such as cancer based on metal nanomaterials are also emerging as good possibilities.214–227 One new and particularly promising application of metal nanostructures is in photothermal imaging and therapy of cancer. Photothermal imaging is based on detection and imaging of thermal energy converted from light, while photothermal therapy is based on destruction or killing of cancer cells by thermal energy converted from light. Metal nanostructures are excellent thermal conductors and photothermal converters in the sense that they strongly absorb light at a particular wavelength and can convert light energy into thermal energy very quickly (1-2 ps) and very efficiently. Therefore, they have good potential for photothermal imaging and therapy of cancer. Indeed, several preliminary studies have successfully demonstrated the use of metal nanostructures in the detection, imaging, and therapeutic treatment of cancer tissues.210,215,217,220,224 When on resonance with an excitation source, the temperature change of a gold or silver nanoparticle can be quite drastic.66,68,70,195,214–217 Initially, exposure to or injection into tumors was used to deliver nanoparticles.218–222 This can be effective but lacks specificity. Wherever the particles end up (and the excitation light can reach) cells will be destroyed. For more effective particle delivery, one approach is to functionalize the particles with cancer specific agents of several different types.210,223–226 One of the attractive features of metal nanostructures for this purpose is their tunable optical absorption that allows good match to specific laser wavelength to improve or enhance the photothermal conversion process. For instance, HGNs discussed earlier have tunable absorption covering the entire visible and near IR region and the wavelength can be simply controlled by controlling the thickness and diameter of the shell.228 In comparison to solid nanoparticles, aggregates, nanorods, or other more complex nanostructures, the HGNs are unique in their


Schwartzberg and Zhang achieve better control and understanding of the effects of size, surface, shape, internanostructure interaction, embedding media on the optical and other properties and functionalities of metal nanostructures. Nanocomposites involving metal nanostructures with other nanomaterials such as organic, biological, inorganic semiconductor or insulator present many other opportunities for exploration of new materials properties and potential applications. Surface and interfaces of such nanocomposite materials need to be better understood theoretically and experimentally at the atomic scale. This would require development of new theoretical and experimental tools that can provide the atomic precision as well as nanoscale probe capability. Acknowledgment. We are grateful to financial support from the National Science Foundation, NASA-UARC, and the U.S. Army. References and Notes

Figure 10. Schematic illustration of the idea of photothermal therapy using HGNs based on antibody (C225)-antigen (EGFR) interaction that targets A431 carcinoma cancer cells.224

structure and optical property combination in that it is the only system that has a small (

Novel Optical Properties and Emerging Applications of Metal

Recommend Documents