Nanotechnology is becoming a quickly advancing field. For instance, extraordinary advances are being made in nanometer-scale electronics and mechanics, including handling and assembly of nanometer-scale components. For instance, suggestions are beginning to be made as to designs for molecular-scale or atomic-scale devices. Someday devices may be assembled on the molecular or even atomic scales, e.g., providing precision in such devices at the molecular or atomic levels. As design and/or analysis continues to progress on such a small scale, suitable analytical techniques for properly identifying and/or analyzing such small-scale building blocks as molecules or atoms, as well as their interaction, become increasingly important.
Various techniques have been developed in the prior art to aid in recognizing/analyzing molecular arrangements. For instance, spectroscopy is a well-known analytical technique concerned with the measurement of the interaction of radiant energy with matter and with the interpretation of such interaction. Interpretation of the spectra produced by various spectroscopic instrumentation has been used to provide fundamental information on atomic and molecular energy levels, the distribution of species within those levels, the nature of processes involving change from one level to another, molecular geometries, chemical bonding, and interaction of molecules in solution, as examples.
One type of spectroscopy is known as vibrational spectroscopy. Vibrational spectroscopy provides a relatively useful technique for characterizing molecules and for determining their chemical structure. The vibrational spectrum of a molecule, based on the molecular structure of that molecule, is a series of distinguishable lines which constitutes a unique fingerprint of that specific molecular structure. Optical fibers may be used to measure the vibrational spectrum by an optical absorption process, wherein optical energy from a source is delivered to a sample via one fiber, and after passage through the sample, an optical signal generated by the exciting optical energy is collected by the same or, more preferably, another fiber. The collected light may be directed to a monochrometer/or a photodetector for analyzing its wavelength and/or intensity.
For many years, it has been known that when certain molecules are illuminated by a beam of light, for example ultraviolet, visible, or near infrared, a small fraction of the incident photons are retained momentarily by some of the molecules, causing a transition of the electrons within the energy levels of some of those molecules to higher vibrational levels of the ground electronic state. Often, these are elastic collisions, and the molecules return to their original vibrational level by releasing photons. Photons are emitted in all directions at the same wavelength as the incident beam (i.e., they are scattered). This is commonly known as “Rayleigh scattering.”
Another important type of spectroscopy is known as “Raman spectroscopy,” which is a process that makes use of “Raman scattering” to investigate molecular vibrations and rotations. “Raman scattering” is generally defined as the scattering of light by molecules in which there is a change of frequency due to the molecules gaining or losing energy as a result of transitions between vibrational or rotational energy levels. The phenomenon was discovered in 1928 by C. V. Raman. More specifically, Raman discovered that when certain molecules are illuminated, a small percentage of the molecules which have retained a photon, drop to a different vibrational level of the ground electronic state. The radiation emitted from these molecules will therefore be at a different energy and hence a different wavelength. If the molecule drops to a higher vibrational level of the ground electronic state, the photon emitted is at a lower energy or longer wavelength than that absorbed. This is commonly referred to as “Stokes-shifted Raman scattering.” If a molecule is already at a higher vibrational state before it absorbs a photon, it can impart this extra energy to the remitted photon thereby returning to the ground state. In this case, the radiation emitted is of higher energy (and shorter wavelength) and is commonly referred to as “anti-Stokes-shifted Raman scattering.” In a set of molecules under normal conditions, the number of molecules at ground state is generally much greater than those at an excited state. Therefore, the odds of an incident photon interacting with an excited molecule and being scattered with more energy than it carried upon collision is typically very small. Thus, when a set of molecules are under study, photon scattering at frequencies higher than that of the incident photons (anti-Stokes frequencies) is typically minor relative to that at frequencies lower than that of the incident photons (Stokes frequencies). Consequently, the Stokes frequencies are usually analyzed in Raman spectroscopy processes of the prior art.
The amount of energy lost to, or gained from, a molecule in this way is quantized, resulting in the scattered photons having discrete wavelength shifts. These wavelength shifts can be measured by a spectrometer. Raman scattering was initially considered to have the potential to be useful as an analytical tool to identify certain molecules, and as a means of studying molecular structure. However, interest in Raman scattering faded somewhat as other methods, such as infrared spectroscopy, gained popularity.
Interest in Raman spectroscopy was renewed with the advent of the laser as a light source. Its intense coherent light overcame some of the sensitivity drawbacks initially encountered in Raman spectroscopy. Moreover, it was discovered that when the wavelength of the incident light is at or near the maximum absorption frequency of the molecule, and hence can cause electronic as well as vibrational transitions in the molecules, resonance Raman scattering is observed. With resonance Raman scattering, the re-emitted photons still show the differences in vibrational energy associated with Raman scattering. However, with resonance Raman scattering, the electronic vibrational absorption is approximately 1000 times more efficient. Even with the increased signal from resonance Raman scattering, its usefulness as an analytic tool was limited due to its still comparatively weak signal.
Interest in Raman spectroscopy further increased when, in 1974, M. Fleischmann et al. discovered surface-enhanced Raman spectroscopy (SERS), though did not recognize it as such. See M. Fleischmann, P. J. Hendra, and A. J. McQuillan, Chem. Phys. Lett., 1974, 26, 163. Specifically, Fleischmann et al. observed intense Raman scattering from pyridine adsorbed onto a roughened silver electrode surface from aqueous solution. Fleischmann's approach was to roughen the electrode to increase its surface area and, hence, the number of adsorbed molecules available for study. Dr. Richard P. Van Duyne et al. later recognized that the large intensities observed could not be accounted for simply by the increase in the number of scatterers present and proposed that an enhancement of the scattered intensity occurred in the adsorbed state. See D. L. Jeanmaire and R. P. Van Duyne, J. Electroanal Chem., 1977, 84, 1. Also in 1977 Creighton et al. recognized that the increased Raman signal was not possible by more scatterers alone and proposed an enhancement mechanism. See M. G. Albrecht and J. A. Creighton, J. Am. Chem. Soc. 99, 5215 (1977). Thus, it was recognized that Raman scattering efficiency can be enhanced when a compound is adsorbed on or near special metal surfaces. That is, a significant increase in the intensity of Raman light scattering can be observed when molecules are brought into close proximity to (but not necessarily in contact with) certain metal surfaces that are atomically “roughened.” Metal colloids also demonstrate such signal enhancement effect. Such phenomenon is referred to as “surface-enhanced Raman scattering” (SERS), and use of such surface-enhancement has enabled enhancements in Raman scattering efficiency by factors of 106 to be observed.
The cause of the SERS effect is not completely understood. However, at least two separate factors contributing to SERS have been advanced in the prior art. First, the metal surface contains minute irregularities. These irregularities may be thought of as spheres (in a colloid, they are spheroidal or nearly so). Those particles with diameters of approximately {fraction (1/10)}th the wavelength of the incident light have been thought to contribute most to the effect. The incident photons induce a field across the particles which, being metal, have very mobile electrons.
In certain configurations of metal surfaces or particles, groups of surface electrons can be made to oscillate in a collective fashion in response to an applied oscillating electromagnetic field. Such a group of collectively oscillating electrons is called a “plasmon.” The incident photons supply this oscillating electromagnetic field. The induction of an oscillating dipole moment in a molecule by incident light is the source of the Raman scattering. The effect of the resonant oscillation of the surface plasmons is to cause a large increase in the electromagnetic field strength in the vicinity of the metal surface. This results in an enhancement of the oscillating dipole induced in the scattering molecule and hence increases the intensity of the Raman scattered light. The effect is to increase the apparent intensity of the incident light in the vicinity of the particles.
A second factor considered to contribute to the SERS effect is molecular imaging. A molecule with a dipole moment, which is in close proximity to a metallic surface, will induce an image of itself on that surface of opposite polarity (i.e., a “shadow” dipole on the plasmon). The proximity of that image is thought to enhance the power of the molecules to scatter light. Put another way, this coupling of a molecule having an induced or distorted dipole moment to the surface plasmons greatly enhances the excitation probability. The result is a very large increase in the efficiency of Raman light scattered by the surface-absorbed molecules.
The SERS effect can be enhanced through combination with the resonance Raman effect. When an excitation light source (e.g., laser) used to excite SERS is in resonance with an electronic transition of the substance, such condition is referred to as surface-enhanced resonance Raman scattering (or “SERRS” or “resonant SERS”). As described above, an enhancement in the efficiency of Raman scattering on the order of 106 fold has been observed with SERS. An additional 103 fold enhancement in the efficiency of Raman scattering has been observed with SERRS.
Accordingly surface-enhanced Raman spectroscopy (both SERS and SERRS) are capable of providing great information for use in identifying and analyzing molecules. Accordingly surface-enhanced Raman spectroscopy is being used in a variety of applications, including detection of molecules and analysis of molecular structure, as examples.
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