Plasmons: Why Should We Care?

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edited by Advanced Chemistry Classroom and Laboratory Joseph J. BelBruno Dartmouth College Hanover, NH 03755 Plasmons: Why Should We Care?

Dean J. Campbell* Department of Chemistry and Biochemistry, Bradley University, Peoria, IL 61625; *[email protected]

Younan Xia Department of Chemistry, University of Washington, Seattle, WA 98195

The physical phenomenon of plasmons has helped to at least three types of SPs: enhance methods of chemical analysis in recent decades, even •Propagating SPs, which occur on extended metal sur- to the point where it is possible to detect a single molecule. faces Though many of these methods have not yet found their way •Localized SPs, which occur in small volumes such as into traditional chemistry classes and labs, it is likely that metal particles many of today’s chemistry and biochemistry majors will en- counter plasmon-based techniques in their future. Some tech- •Acoustic SPs, which are predicted to exist for some niques, such as surface plasmon resonance, staining for structures and metal surfaces and are the subject of microscopy, and colorimetry have already been commercial- continued study (3). These plasmons have not yet seen ized. Other applications for plasmons are still being explored, extensive use in chemistry and will not be discussed for example, plasmon-based alternatives to traditional elec- further in this article. tronic circuits. This review will provide a brief overview of The combination of photons and SPs produce electro- plasmons and the techniques that build upon them. magnetic excitations on extended surfaces known as surface Strictly speaking, plasmons are quantized waves in a col- plasmon polaritons (SPPs) or propagating surface plasmons lection of mobile electrons that are produced when large num- (PSPs) (1, 5). PSPs have lower frequencies and energies than bers of these electrons are disturbed from their equilibrium bulk plasmons; metal surfaces in contact with a vacuum have ͞ positions (1, 2). Electrons present in classic gaseous plasmas PSPs with a theoretical frequency of ωp √2 (1, 3). Figure can support plasmons, hence the name of these waves. The 1A shows a representation of PSPs. As the waves of electron collection of mobile electrons in metals is referred to as a density travel along the surface, alternating regions of posi- quantum plasma (2). This is composed of delocalized electrons that bathe the metal nuclei and their localized core electrons, as described by the free-electron theory of metals (1). This description of metallic bonding has been used to explain many metallic properties. Metals that best exhibit this free-electron plasma behavior include the alkali metals, Mg, Al, and noble metals such as Cu, Ag, and Au (1). Plasmons can exist within the bulk of metals, and their existence was used to explain energy losses associated with electrons beamed into bulk metals (3). For a bulk metal of infinite size, the frequency of oscil- lation, ωp, of the plasmons can be described by the follow- ing equation (1):

2͞ 1͞2 ωp = (Ne ε0me) (1) where N is the number density of mobile electrons, ε0 is the dielectric constant of a vacuum, e is the charge of an electron, and me is the effective mass of an electron. Surface plasmons (SPs) are a type of plasmon associated with the surfaces of metals. They are significantly lower in frequency (and energy) than bulk plasmons and can interact, under certain conditions, with visible light in a phenomenon called surface plasmon resonance (SPR). SPs have been Figure 1. (A) Depiction of propagating surface plasmons (1). (B) the most useful to chemists. For example, the electric fields Depiction of localized surface plasmons (1). Regions of greater elec- of SPs amplify optical phenomena such as Raman scattering tron density are represented by lighter shading. Electric fields are (1, 4), which will be discussed later in the article. There are represented by arrows.

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propagate PSPs along a thin layer of metal placed at the interface between the media with differing refractive indices (5, 6, 9). This SP resonance angle is susceptible to the refractive index of the media near the metal film, making it sensitive to chemical changes that involve changes in refractive index (or its square, the dielectric constant). Other methods to enable light to access a metal surface and produce PSPs involve roughening the surface, thereby changing its momentum. If light shines on a metal diffraction grating with parallel, linear features, it can be diffracted by that grating. At the same time, however, PSPs will be propagated along the surface of the grating in the direction perpendicular to the linear features (5, 10, 11). SPs can also be enhanced by randomly roughened metal surfaces. The rough surfaces can be considered as a superposition of many gratings (11). Localized surface plasmons (LSPs), are the collective elec- Figure 2. Extinction spectra of 10 nm, 50 nm, and 100 nm Au tron oscillations in small volumes, Figure 1B. LSPs also have particles. The spectra are scaled to the same maximum absorbance. lower frequencies and energies than bulk plasmons; metal Note the red shift of the plasmons as particle size increases. The particles in contact with a vacuum have LSPs with a theo- colloids were obtained from British Biocell International, Cardiff, ͞ retical frequency of ωp √3 (1, 3). LSP resonances are pro- Britain. duced in a somewhat different fashion from PSPs. An example of this interaction between light and the electrons of a metal tive and negative charges are produced. The electric fields pro- particle is illustrated in Figure 1B (1, 12). For this phenom- duced by these regions of differing charge decay exponen- enon to occur, the particle must be much smaller than the tially away from the metal surface (5). A macroscopic physical wavelength of incident light. The electric field of the inci- analogy that has been used to describe surface plasmons is dent light can induce an electric dipole in the metal particle that of seaweed floating in water near a shoreline. The waves by displacing many of the delocalized electrons in one direc- of water lapping along the shoreline represent the electric tion away from the rest of the metal particle and thus pro- fields and the seaweed sloshed back and forth by these waves ducing a net negative charge on one side. Since the rest of represent electrons (6). the metal particle is effectively a cationic lattice of nuclei and Though PSPs can be produced with light, simply shin- localized core electrons, the side opposite the negative charge ing light on a smooth metal surface in air is not sufficient has a net positive charge. LSPs have also been referred to as because the momentum of the light does not match that of dipole plasmons, but the oscillating field of the incident light the SPs (3). Neither will the smooth metal surface emit light can induce quadrupole as well as dipole resonances, especially from plasmons.1 Various methods are employed to enable for particles greater than 30 nm in diameter (12, 13). If a light to produce and to be produced by PSPs. Some meth- particle with a dipole can be considered to have a positively ods involve passing the light through a medium with a higher charged pole and a negatively charged pole, then a particle refractive index than air, such as glass, before it comes near with a quadrupole can be considered to have two positively to the metal surface (3, 5). To better explain the role of re- charged poles and two negatively charged poles. fractive index, consider what happens when a beam of light The energy of light required to produce LSP resonance passes from a medium with a higher index into a medium depends on a number of factors, including the size, shape, with a lower index. In many cases, part of the incident beam and composition of the particles, as well as the composition is reflected from the interface and part of the incident beam of the surrounding media. The interactions of light with passes into the medium with the lower refractive index (and spherical metal particles can be described by Mie theory, is refracted in the process). However, when the angle of the which is based upon an exact solution to Maxwell’s equa- incident light exceeds a critical angle, no portion of the light tions (1). This theory also predicts what fraction of light im- leaves the medium with higher refractive index and is com- pinging upon colloidal metal particles will be absorbed and pletely reflected. This is the condition of total internal re- what fraction will be scattered. The sum of absorption and flectance (7). Many fiber optic cables utilize the same scattering is the extinction of light due to the particles. This principle to transmit light down curved strands: the refrac- is actually what is measured when one places a colloidal metal tive index of the core of the fiber optic is higher than that of suspension into a UV–vis spectrometer. Figure 2 shows the its cladding layer, and the condition of total internal reflec- extinction spectra for spherical Au colloids with differing di- tance keeps the light in the fiber (8). Under the conditions ameters. Generally speaking, as the particle size increases, the of total internal reflectance, light waves that strike the inter- plasmon resonant frequency decreases (shifts to longer wave- face between two materials with differing refractive indices lengths) (1). Small Au particles (with diameters less than 20 produce new light waves that propagate from the interface a nm) exhibit extinction that is primarily due to absorption; very short distance into the medium with the lower refrac- larger particles tend to exhibit much stronger scattering (1). tive index. These evanescent waves decay exponentially as the Though solid spherical Au and Ag particles are easy to distance from the interface increases. At a particular angle of synthesize, their plasmon energies do not cover much of the incident light beyond the critical angle, evanescent waves can electromagnetic spectrum. Particles with other shapes are ca-

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adsorbates surface + − − + plasmons + − + − + − + − metal

+ − θ + − + − + − + − + − + − plasmon + − incident reflected + + − − of sphere + − light light + − plasmon hybridized of cavity plasmons glass prism

Figure 3. A hybridization model for the plasmon behavior of Au Figure 4. Schematic of the surface plasmon resonance technique (4, nanoshells (15). The plasmons of a spherical shell can be consid- 9). Light passing through a prism onto a metal film at the proper inci- ered a hybribization of the plasmon from a solid sphere and the dent angle θ will amplify propagating surface plasmons in the metal plasmon of a spherical void. film. The nature of the adsorbates helps determine the actual SPR angle. pable of covering much wider energy ranges (14). Moving the spherical cavity (that is, as the thickness of the metal shell beyond spherical particles requires more advanced method- decreases) their plasmons interact more strongly and increase ology, both synthetically and theoretically. Many of these plas- the energy separation of the hybridized plasmons and ulti- mon resonant structures feature dimensions between 1 and mately decrease the energy of the lower-energy hybridized 100 nm, making them “nanostructures” (1). Technology that plasmon. The net result is that the shells with thinner metal deals with such nanostructures is called “nanotechnology”. layers have plasmons that are shifted to lower frequencies (18). Non-spherically symmetric particles can be synthesized by Aggregation of colloidal metal particles can also lower their directionally dependent growth rate control with polymers plasmon frequencies (1, 19). Finally, the refractive index of or surfactants (15, 16). Another method for controlling par- the medium surrounding the metal particles can influence ticle shapes uses other particles or surfactant assemblies as the frequency of their LSPs (1, 18, 20). Typically, a higher templates (15, 16). Other template-directed patterning meth- refractive index (or dielectric constant) of the medium pro- ods have been used to produce structures on surfaces that duces a lower plasmon frequency. A macroscale physical anal- exhibit LSPs (17). ogy one might make to this phenomenon is using an Plasmons and their associated electric fields can be cal- oscillating weighted spring to represent the electric field of culated for a variety of different particle shapes. The the LSP. A high dielectric medium can be represented by a plasmonic behavior of these particles has been successfully viscous, higher-friction medium like oil. A spring in a vacuum described by the discrete dipole approximation (DDA) will oscillate with a higher frequency than the spring in the method, which subdivides a particle of interest into an array oil. of polarizable cubes (1, 17). It seems that the most intense plasmon electric fields tend to be located near areas of the Applications Using PSPs particles with the sharpest curvature (15). Particles of different shapes have different plasmon One of the major applications of the phenomenon of properties. For example, rod-shaped Ag or Au particles ex- PSP is surface plasmon resonance analysis. The specific ge- hibit two plasmons: a longitudinal plasmon, corresponding ometry of this technique varies. The Kretchmann–Raether to the long axis of the rod, and a transverse plasmon, corre- arrangement is described in Figure 4 (10). This involves shin- sponding to the short axis of the rod. Keeping the diameters ing a monochromatic light source through a prism onto a of the rods constant while increasing their lengths result in thin metal film deposited along one of its flat faces. A detec- the transverse plasmon remaining essentially constant in fre- tor senses the light reflected from the film. As the angle be- quency (or energy) while the longitudinal plasmon decreases tween the incident light and the normal to the film is in frequency (1, 15). Hollow metal particles tend to have increased through the critical angle, the intensity of light lower plasmon resonant frequencies than solid metal particles reaching the detector increases. However, when the angle is (18). The plasmon behavior of small hollow spherical metal increased even further through the SPR angle, the intensity particles, or shells, has recently been described by using a hy- of light reaching the detector decreases to a minimum. The bridization model that resembles, at first glance, the hybrid- specific SPR angle of the system varies as the refractive index ization of molecular orbitals, Figure 3. In this model, the of the medium adjacent to the metal film changes. Molecules plasmons associated with a shell can be thought of as a com- can be attached to the metal film, and the changes in inter- bination of the plasmon from a solid sphere of the same di- actions of these molecules under different conditions can be ameter as the exterior of the shell and a plasmon from a monitored as changes to their collective refractive index. spherical cavity in extended metal with the same diameter as These SPR systems are commercially available and have been the interior of the shell. The plasmons hybridize to produce used to study the binding interactions of biomolecules (6, a new higher energy plasmon and a new lower energy plas- 9). These systems can be integrated with other analytical sys- mon (the lower energy plasmon is more likely to interact with tems, for example, mass spectrometry may be performed on light). As the diameter of the solid sphere approaches that of the species adsorbed to the metal film (9).

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Another application of PSPs is based on a phenomenon selective binding to tissue surfaces (16). Visibility of the called extraordinary transmission (21). Ordinarily one would bound Au colloids can be enhanced by depositing Ag onto expect a screen placed in front of a light beam to block a the metal particles (36). Most colloidal particles of Au can- significant fraction of the light being transmitted through the not be easily made to produce light like many colloidal semi- screen. However, if a metal film containing a two dimensional conductor particles. Very small (< 5 nm) Au clusters can array of holes of the proper size and periodicity is placed in produce light by photoluminescence. Though the photolu- the light beam, more light is transmitted through the screen minescence phenomenon itself is attributed to electronic tran- at certain wavelengths than is expected. It is postulated that sitions within the Au clusters, it is believed that plasmons the light resonates with PSPs at the incident side of the metal associated with these very small particles amplify the inten- film. The PSPs propagate to the other side of the film, where sity of the incoming light and therefore enhance their pho- they produce light that transmits away from the other side. toluminescence (37). Recently Au colloids have also been This phenomenon is being explored as a method for study- induced to produce light by a phenomenon called third har- ing species adsorbed to the metal films, for example, the in- monic generation (38). This may enable these Au particles frared absorbance of some alkanethiols on Cu are enhanced to be used as light-producing labels in the future. Fluores- by a factor of 102 (22). cence of chemical species near a metal surface can also be The technique of scanning near-field optical microscopy enhanced by plasmons, but the challenge in using this method (SNOM) can access PSPs on a metal surface. In this tech- of enhancement is that if the species get too close (less than nique, light is shone down a glass fiber that has been reduced about 5 nm) to the metal the fluorescence is actually dimin- to a diameter less than the wavelength of light. When the ished (quenched) (11, 15). tip of this fiber is brought very close to the metal surface, One application based on the intense light absorbance circular PSPs will radiate from the tip to the metal (5). This of Au colloids is photothermal therapy. Au colloids them- process, which has been explained by both evanescent waves selves are fairly chemically and biologically inert, but Au par- from the high refractive index glass fiber and diffraction from ticles bound to living tissue can absorb light energy and the fiber tip itself (5), enables PSPs to be initiated at specific convert it to thermal energy, destroying the tissue. Hollow locations on a surface with great precision. Au particles can now be produced with plasmon resonances Finally, PSPs are being explored as an alternative means in the near infrared region of the electromagnetic spectrum. of transmitting information (23, 24). Plasmons travel through Since water and most biomolecules do not absorb in this re- metals with optical frequencies, about 105 times faster than gion of the electromagnetic spectrum, it is possible to beam the frequency of today’s microprocessors. At the same time, this energy some distance into tissue to heat the Au particles there is not as much need for light transmitting materials as (15, 18). One way to illustrate this principle to a class would would be necessary for entirely light-based (photonic) com- be to place Au colloid (e.g., by evaporating a drop of sus- puters; many plasmonic components would use the same pension) onto a polystyrene Petri dish and to place the dish materials as electronic components. One challenge to over- near the Fresnel lens focal point of a high intensity overhead come is that the distance plasmons can travel is currently less projector. We used a 7800 lumen projector with the mirror than that the dimensions of a computer chip (24). and the focusing lens removed. A ring stand and clamp were used to hold the assemblage about 40 cm above the glass top Applications Using LSPs surface of the projector. (CAUTION: The focused light from an overhead projector can be dazzlingly bright and can heat One of the most recognizable results of LSP resonance some objects dramatically.) The transparent Petri dishes them- is the dramatic absorption of light by some small metal par- selves are not melted by the focused light, but light-absorb- ticles. Solid Au and Ag particles can have extinction coeffi- ing materials such as Au colloids, carbon nanotubes, and even cients 106 times greater than molecular chromophores (1). dark permanent marker ink can absorb sufficient energy to The deep red and purple colors associated with colloidal Au damage the polystyrene dish at the location of these light ab- have been known since antiquity, for example, in tinted glass sorbing materials (39). The surrounding transparent polysty- such as the Lycurgus cup from the 4th century C.E. (25). rene is not affected. Some hollow colloidal metal cages “Purple of Cassius” was used as a coloring agent for glass and synthesized by the Xia group absorb light energy so well that enamel and is thought to be a colloidal mixture of Au and the light of a camera flash can melt the cages (40). These hydrated SnO2 (26). Michael Faraday first reported the pro- flash-induced shape changes have been verified by transmis- duction of aqueous colloidal Au suspensions (using phospho- sion electron microscopy (Figure 5) and scanning electron rus as a reducing agent) in 1857. Some of these colorful microscopy. These colloidal cages can also melt transparent suspensions are still preserved today at the Faraday Museum polystyrene when irradiated by intense projector light. in London (1, 27). Au and Ag colloids can be produced and Au colloids and their LSPs can also be used as colori- used conveniently in the student laboratory, and some of these metric chemical sensors. The pink test stripe of a positive educational preparations and applications have been pub- pregnancy test can be composed of Au colloids coated with lished in this Journal and elsewhere (28–34). antibodies that have bound to hormone human chorionic The high light-absorbing properties of Au colloids en- gonadotropin (hCG) in urine. These complexes are then cap- able them to serve as structural and chemical labels for light tured by more antibodies in the test band region (41). The microscopy (35). For example, Au colloids can be placed into pink control stripe can also be composed of colloidal Au. Au biological tissue for use as optical contrast agents to enhance colloids coated with other biologically interacting molecules imaging of these tissues. The colloid particle surfaces can be have shown aggregation-induced changes to their plasmon derivatized with molecules such as antibodies to enhance their frequencies (1, 13, 35). For example, the surfaces of some

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Au particles have been derivatized with strands of DNA. When these fixed strands match complementary strands of free-floating DNA, the strands all link together by hydrogen bonding and the colloidal particles aggregate together. The Au colloid plasmons shift in frequency during aggregation, and the suspension changes color, indicating a positive test for that particular sequence of free-floating DNA (42). An- other biochemical detection method that is somewhat remi- niscent of the SPR analysis method mentioned previously uses LSP-active features patterned onto an Au surface. Binding of molecules of interest to other molecules already attached to these Au features produces a change in the refractive index near these features, resulting in a change in their plasmon energy. This energy change can be monitored by UV–vis spectroscopy. This method has recently been shown to be ca- pable of detecting amyloid-β derived diffusible ligands (ADDLs), which are believed to play a role in Alzheimer’s disease (43). Resonance of LSPs amplifies electric fields, E, near the particle surfaces. The |E|2 of the plasmon electric field can be 102 to 104 of the |E|2 of the incident light (17). The electric fields of plasmons can amplify the Raman signals of chemical species near the metal surface, thus enabling the technique of surface-enhanced Raman spectroscopy (SERS).2 Weaver and Norrod have already described SERS in this Jour- nal and experiments suitable for the undergraduate laboratory are available (28, 33, 44). Briefly, Raman spectroscopy is a type of vibrational spectroscopy that is sensitive to polarizable bonds within molecules (typically different than Figure 5. Flash melting of Au cages (35). (A) Transmission electron microscope (TEM) image of unmelted Au cages formed by reduc- bonds that are already polarized—the domain of infrared tion around polyhedral Ag templates (the Ag is oxidized away in spectroscopy) (45). The electric field interacts with polariz- the process). (B) TEM image of Au droplets formed by melting Au able molecules to produce dipoles, described by the equa- cages with a camera flash. tion (28): p = αE where α is polarizability of the molecule, E is the applied elec- ticles that are very close (on the order of one nanometer) to tric field, and p is the electric dipole induced in the molecule one another have enabled the detection of SERS signals from As the equation shows, p can be made larger by increas- single molecules located between these particles (48). ing either α or E, but plasmons tend to enhance E. As the intensity of the induced dipoles increases, the intensity of the Conclusion Raman signals from those molecules increase. This surface enhancement of Raman signals was first observed from pyri- This brief survey has merely scratched the surface, so to dine adsorbed onto a roughened Ag substrate (46). Repro- speak, of many of the chemical applications of plasmons. Plas- ducible control of the degree of enhancement has been mon-enhanced applications have begun to make inroads into problematic. It appears that many of the roughened substrates chemistry education and are well-suited for introduction in have “hot spots” with exceptionally good signal enhancement physical chemistry and instrumental analysis classes. Some (18). Precise control of the nature of the surface roughness methods of fabrication and analysis of plasmon-producing (e.g., by patterning techniques) allows for better control of structures are sufficiently simple for use in labs in general, the degree of Raman enhancement and of the wavelength physical, and inorganic chemistry. Readers who are interested required to access the surface plasmons (17). Recently, a pat- in knowing more about research and educational aspects of terned Ag substrate was used to detect calcium dipicinolate, plasmons and plasmon-producing structures are encouraged a compound present in anthrax spores, by SERS (47). It is to peruse the references provided, especially the May, 2005 estimated that this method can detect as little as 2600 spores, issue of the MRS Bulletin (1,15–18, 23, 42). which is less than the infectious dose of approximately 10,000 spores. The Raman signal of the adsorbed species can be dra- Acknowledgments matically enhanced with colloids as well. Enhancement of the electric fields of both the incident and Raman scattered DJC would like to acknowledge Bradley University for light by 104 produces an overall theoretical enhancement of sabbatical support. DJC would like to thank Benjamin Wiley 108 (17). The regions where colloidal particles come into close and Jingyi Chen for helpful discussions, supplying the Au proximity appear to be the location of intense plasmon elec- cages, and assistance with transmission electron microscopy. tric fields (15). The fields that can arise between two par- YX has been supported in part by a grant from the NSF

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