BIG Physics for small Science

The Science of Surface Plasmon Resonance

Metallic particles can be synthesized which are adequately small to be suspended in a liquid phase, where the particles exist on a size scale such that buoyancy in the medium and forces of gravity are balanced, and the particle solution, known as a colloid, is stable. In this state, these nanoparticles, named after their dimensions on the nanometer scale (1/billionth of a meter), will stay dispersed instead of precipitating out of solution. It might help to think of the similar way in which large sugar cubes might fall to the bottom of your iced tea, but smaller sugar crystals will disperse and dissolve if you warm up and stir that same drink of tea. However, there is more to very small metallic particles, or nanoparticles, than their small size and defiance of our macroscopic ideas of gravity. Indeed, there is more here than meets the eye. Ironically, it is due to interactions with the very medium by which we have vision at all – electromagnetic energy (light) – that nanoparticles derive some of their most unique and interesting properties. Tweet

Colloidal solutions of metallic nanoparticles, of gold and silver especially, display optical properties, or interactions with light in the visible to near-infrared (NIR) regions (Link 2000), which make these particles very attractive for imaging and sensing applications. By attractive, I am not referring to how ‘pretty’ the colorful colloids look arranged in delicate glass bottles for a nanochemistry display. Metallic nanoparticles have applications far and wide for optical imaging, photo-acoustic (light combined with ultrasound) imaging, nuclear medicine imaging, assays based on color changes, i.e. colorimetric assays, and many other medical and sensing techniques. One that may be familiar is the dip-stick assay, or lateral-flow immunoassay, commonly used for home pregnancy tests. In this case, gold nanoparticles dotted with antibodies (bio-markers) are used to create the characteristic red bands for yes/no readouts.

Although the colorimetric properties of metallic nanoparticles are used in many other bio-sensing applications, the greater interest in their usage arises due to the fact that nanostructures of various sizes and shapes can strongly enhance the local electromagnetic field at distances very near their surfaces. These enhancements result from the phenomenon of Surface Plasmon Resonance (SPR), the optical effects of which were first accurately described mathematically as solutions to Maxwell’s equations for spherical nano-objects by Gustav Mie (Mie 1908). The phenomenon is known as or Localized Surface Plasmon Resonance (LSPR) when the effect is localized to the surfaces of nano-scale particles. Plasmon resonance produces the brilliant colors observed when light passes through metallic colloidal solutions. For example, light passing through a typical 30nm spherical silver (Ag) colloid appears yellow-green due to the fact that silver particles of this size absorb light in the violet-blue region. Spherical gold (Au) nanoparticle colloids of similar sizes appear red, absorbing light maximally in the green region (Stockman Physics Today 2011). These optical properties were the reason behind gold-silver alloy colloids being use as coloring agents for stained glass windows such as those displayed in the Sainte Chapelle in Paris, decades before the physics behind their optical properties were first described.

The term ‘surface’ in LSPR arises due to the fact that the special interactions between metallic nanoparticles and light incident upon them, these interactions generally being the absorption of a photon (Klabunde Nanoscale Materials in Chem), produce charge density oscillations which resonate at optical frequencies only for particle boundaries. Charge density oscillations are periodic fluctuations of the particle’s electron gas cloud, where an electron can be represented as a single negative charge. In simpler terms, we can consider a plasma, which is a conducting material like a metal which consists of negatively charged electrons plus a heavier core of positively charged ions, the latter containing the ‘heavy’ atomic nuclei. Upon excitation by an electromagnetic wave, the electrons are collectively displaced from their ion cores. However opposites attract, so the electrons may not stray too far from their positive counterparts, creating a restoring force much like a spring that has been stretched. This restoring force at the curved particle surface is what creates the oscillation of the electron cloud, much as a stretched spring once released will bounce back and forth, being repeatedly stretched, contracted, and stretched again. On the nanoscale, instead of a physical spring, will are dealing with a quantized electron oscillator which resonates at frequencies determined by the restoring force and the ’effective mass of the electron". (Stockman Physics Today 2011). This resonance state, similar to a resonant state achieved on a plucked violin string (harmonic oscillator), varies with nanoparticle composition, morphology, and chemical environment.

As the external light induces collective oscillations of free / conduction electrons at a rough or curved-particle surface (where metal meets air, water, or any other dielectric medium), we can begin to observe a strong absorption and scattering of light. This absorption and scattering, together known as extinction, occurs to a maximum at a specific resonance frequency dependent on the oscillation frequency for a particular electron cloud and its partner ionic lattice. This is where the ‘resonance’ term comes into LSPR. As wavelength is inversely proportional to frequency, the resonance condition will occur at different wavelengths (different colors of light) for different nanoparticle types. The resonance frequency, i.e. the wavelength where light is most strongly absorbed and/or scattered, will depend on a nanostructure’s material composition (ex. gold vs. silver), size, shape, and environment. The difference between absorption and scattering may be explained as follows: absorption is the process of conversion of incident resonant photons (particles of light) into phonons, which are vibrations of metal lattice (Klabunde); the process of scattering occurs due to re-emission of incident resonant photons in all directions (Klabunde). By resonant photon, we mean a photon that has a wave propagation pattern with a frequency equal to that of the maximum plasmon frequency of the nanoparticle in question. Non-resonant photons, with frequencies that do not overlap the oscillation frequency, will not have such strong interactions with and resultant excitation of the nanoparticle into a state of plasmonic resonance. This has implications in choosing light sources to match the specific nanomaterials we may use in imaging or biosensing studies, and vice versa choosing a particular nanoparticle to match a desired wavelength (light color) for detection. What’s the Big Deal about Small Science?

“The ability to quickly and robustly detect minute amounds of chemical substances and biological pathogens makes nanoplasmonic sensing important not only in science, engineering, and biomedicine but also in environmental monitoring, homeland security, and national defense.” – Stockman Physics Today 2011

It has been established that surface plasmons are “collective oscillations of free electron gas density relative to the positive ion lattice in a metal”, and that these oscillations occur at visible frequencies for nanoscale materials, and can thus be detected via methods such as absorption spectroscopy (Klabunde Nanoscale Materials in Chem). Surface plasmons are essentially electromagnetic waves that can propagate for great distances along a metal-dielectric interface (see FDTD simulation video above). The surface confinement of such oscillations in the case of nanoscale metallic particles allows adsorbed molecules to experience localized enhancements of the incident electromagnetic field. These enhancements can be harnessed to increase fluorescence or scattering signals from adsorbed dyes or other molecules. This is the principle behind surface-enhanced, or field-enhanced, spectroscopy techniques.

A fair question that may be asked is how nanoparticles, on the size scale of 10-100nm, can enhance electromagnetic energy in the visible spectrum (i.e. visible light) which is characterized by wavelengths on the order of 400-800nm? The answer is the now familiar confinement of the excited oscillatory plasmon to the interface between nanoparticle surface and surrounding dielectric medium, which could be for example water or air. This confinement ‘squeezes’ the original electromagnetic energy down to sub-wavelength dimensions. This ‘squeezing’ is more accurately put as a concentration of the electromagnetic field, or near-field, in the vicinity of the nanoparticle surface. This localized enhancement of the electric field is especially strong at sharp (nanoscale) tips (lightening rod effect – Stockman 1996), and in the gaps between closely spaced particles (‘Hot-spots’), for example nanoparticle dimers. Photons of light incident upon or near a plasmonic nanoparticle surface are converted to surface plasmons which exist at sub-wavelength dimensions, i.e. several orders of magnitude smaller than the wave-property dimensions of the original photon. Near-field strengths can easily be 10x stronger than the surrounding electric field. Once a photon has been converted at the metallic surface, the surface plasmon will exist but begin to decay or relax over time. This decay may be radiative (re-emission of a photon, i.e. light), or non-radiative (not producing light). Non-radiative pathways to relaxation of the surface plasmon include collisions between the oscillator electrons and themselves or other electrons, and conversion of the surface plasmon to vibrations inside the ionic lattice, resulting in heat. We can make an analogy of a stretched spring which has just been released. Although now it vibrates or resonates by a series of contractions and elongations, eventually the spring will come to rest due to losses associated with friction forces and conversions to heat.

So how can the special size and optical related properties of metallic nanostructures be harnessed for imaging, bio-sensing, and other biomedical applications? As we have already seen, the presence of strong absorption and scattering in the visible spectrum for plasmon resonant nanoparticles make them useful as organic dye replacements in colorimentric assays. For example, the red stipe(s) that appear on a home pregnancy test are the result of gold nanoparticles (forms a reddish colloidal solution at high concentrations) attaching through antibody-antigen interactions to specific the regions on the absorbent test strip. Indeed the extinction coefficient, which is a measure of how well a dye or nanoparticle both absorb and scatters light, for most metallic nanoparticles are several (3-4) orders of magnitude larger than typical organic dyes (Link 2000). The magnitude of extinction coefficient, which can also be expressed as absorption and/or scattering cross-section, for a plasmonic nanoparticle is essentially derived from the sheer numbers of electrons that ‘coherently contribute to the surface plasmon’ (Stockman 2011), the sum of which equals the oscillator strength. In other words, the large number of electrons oscillating in unison contributes to the high efficiency of a metallic nanostructure in absorbing or scattering incident light. The oscillator strength of a single dye molecule is near 1 (Stockman 2011) causing the organic dye to be much less efficient in light conversion than a nanoplasmonic particle. Nanostructures also have use in sensing applications due to the dependence of their special optical properties on the surrounding chemical and physical environment. Sensitivity of the surface plasmon band to the nanoparticle environment allows detection of changes to the local environment, for example adsorption of proteins onto the particle surface, or antibody binding to ligands attached to the surface during particle synthesis. These changes in environment cause shifts in the LSPR extinction peak, changing the way the particle absorbs and scatters light. Because the optical properties of the particle are different before and after a change in the local environment, we can use the nanostructures as sensors to detect if target molecules are present in a solution. Binding events, such as the hydridization of two complementary nucleic acids, can also be detected if they cause two particles to come into direct or near contact, i.e. cause aggregation. Because the surface plasmons on one nanostructure can couple to those on an adjacent nanostructure, aggregation and thus the binding event can be detected as red-shifting of the resonance wavelength (the particles will now absorb and scatter light at a longer, i.e redder, wavelength than previously) and general broadening of the plasmon bandwidth. The plasmon bandwidth represents the range of wavelengths over which light is absorbed and scattered by the nanostructure.

The physics between red-shifting and band broadening caused by chemical changes at the nanostructure surface can be described in terms of electron transfers. Excited electrons in the surface plasmon may couple with the lowest unoccupied molecular orbitals (LUMOs) of molecule attached or adsorbed on the metallic surface (Link 2000, Persson 1993 Surf Sci 281, 153). Now the conduction band of the metallic nanoparticle can serve as an electron donor for surface molecules with matching acceptor electron energy levels. Exicted electrons participating in the oscillations of the surface plasma will be transferred back and from from the nanoparticle surface to the LUMOs of nearby molecules. During these transfers, the plasma electrons will lose their coherence with the other electrons in the cloud, causing a dephasing and damping of plasmon resonance and thus broadening of the absorption and scattering bandwidths. (Link 2000, Persson 1993 Surf Sci 281, 153). Because different molecules have different electronic structures (different energy levels of the LUMOs), the various degrees of resultant change in the plasmon peak and bandwidth make for the additional suitability of nanoparticles as chemical sensors.

The last decade has seen the applications and influences of metallic, plasmonic nanostructures spread into such diverse fields including nanophotonics, optical imaging, drug delivery and cancer therapy. Metallic nanostructures are excellent labels and contrast agents for optical imaging, due to their strong propensity to absorb light in the visible spectrum. As described above, nanostructures are particularly suited for use as components of chemical and biological sensors, due to the sensitivity of their surface plasmon resonance peak and bandwidth to the chemical state at their surfaces. However, one of the most interesting and unique applications of plasmonic nanostructures exists in the realm of surface-enhanced spectroscopy. Related to this field of study, plasmon resonance has helped to increase the sensitivity of Raman spectroscopy for surface-adsorbed molecules due to the large enhancements of local electromagnetic fields which are observed near a nanoparticle surface (Link 2000).

Join me back next week, when I will discuss the science behind surface-enhanced spectroscopy!

References:

1) Mie, G., Ann.Physik, ⁴ 25, 377(1908).

2) Link and El-Sayed, Int. Reviews in Physical Chemistry, 2000, Vol. 19, No. 3, 409±453

3) Stockman, Physics Today, Feb 2011 39-44

4) String Vibrations – Photographs by Andrew Davidhazy

5) Colored Biosensor Image – Flickr by Matthew Everitt

All images compiled and property of Paige Brown

Hutter, E., & Fendler, J. (2004). Exploitation of Localized Surface Plasmon Resonance Advanced Materials, 16 (19), 1685-1706 DOI: 10.1002/adma.200400271