Surface plasmon resonance (SPR) spectroscopy, as well as its counterpart, localized surface plasmon resonance (LSPR) spectroscopy, has been recognized as an invaluable tool for label-free chemical and biological sensing; as well as nanostructure characterization. The most common application of SPR spectroscopy is in biosensing, particularly the study of binding affinities, such as antibody-antigen interactions.
LSPR spectroscopy, on the other hand, is predominantly used as a signal enhancement technique for trace-molecule detection. In these cases, LSPR is the underlying physical phenomenon behind methods such as surface-enhanced Raman, infrared, absorption, and fluorescence spectroscopies.
In this application note, we will discuss the underlying physics behind surface plasmons, including their wavelength-dependent interactions. Next, we will discuss the differences between SPR and LSPR, examining the mechanisms behind signal enhancement. Lastly, we will provide two brief case studies demonstrating applications of both SPR and LSPR spectroscopy.
Surface Plasmon Physics
Surface plasmons are surface waves generated from the coherent oscillation of conduction band electrons along with the interface between a metal and a dielectric. Like any other electromagnetic wave, surface plasmons have an associated wavevector whose magnitude is dependent on the relative permittivity (also known as the dielectric constant) of the media it is propagating in. In non-magnetic materials, the relative permittivity is equal to the square root of the index of refraction; therefore, the relative permittivity has a similar wavelength dependence as the index of refraction. This dependency means that if the parallel component of the incident light’s wavevector is in resonance with the wavevector of the surface plasmon, it will induce wavelength-dependent constructive or destructive interference of the two waves. Therefore, under certain conditions, SPR and LSPR can result in either a wavelength increase or a decrease in the reflected or transmitted spectrum.
Traditional SPR sensors are made up of a thin metal film (typically gold or silver) deposited on a dielectric material (usually glass), and broadband light is directed at the interface through total internal reflection (TIR). Traditional SPR sensors ensure TIR through the use of prisms, similar to an ATR (attenuated total reflection) tip in FT-IR. As we will discuss in more detail later, some more novel approaches make use of TIR within coated optical fibres to induce SPR. In SPR spectroscopy, the sensor is then exposed to an analyte, which can bond to the sensor, causing a slight change to the relative permittivity, which in turn changes the resistance frequency of the surface plasmons. In SPR, the redetect spectra will cause a dip in the spectrum corresponding to destructive interference when the incident light is in residence with the surface plasmons. Therefore, the local minima of the transmission spectrum will shift as the sensor collects more and more analyte, allowing for extremely sensitive quantitation.
According to Gauss’s Law, the surface charge density of a conductor is inversely proportional to its radius, so, in LSPR, nanofabrication techniques are employed to take advantage of this localization effect. While a detailed description of the underlying physics is beyond the scope of this article, it is essential to understand that the resultant extremely high surface charge density can lead to considerable signal enhancement. Furthermore, this enhancement only occurs when the real part of the substrate’s relative permittivity is a negative multiple of that of the environment, greatly enhancing the signal strength. This relationship is why gold, which has a relative permittivity of -22.855 + 1.4245i at 785 nm, and silver, with a relative permittivity of -11.755 + 0.37038i, are commonly used in both SPR and LSPR, which is why in surface-enhanced Raman spectroscopy (SERS) gold is typically used for 785nm excitation and silver for 532 nm excitation. Additionally, since the magnitude of the scaling factor mentioned earlier is dependent on the geometry of the substrate (this typically varies between two and twenty), the ‘enhancement’ wavelength can be tuned depending on the shape of the nanostructure. As we will show, this geometric dependency is also rather useful in characterizing nanomaterials.
Fibre-Optic SPR Probes
The integration of SPR sensors into fibre-optic probes is now allowing for targeted sensors to be deployed in dangerous environments. These probes are typically produced by stripping the cladding off of a section of fibre- optic cable and subsequently coating that section with a metallic layer and finally a dialectic layer. One group at the Indian Institute of Technology Delhi recently published results using just his type of fibre-optic cable, where the fibre core was coated with a layer of silver and then with a layer of zinc oxide, to detect chlorine gas . In their experiment, broadband light was coupled into the probe from an Avantes AvaLight-HAL, tungsten halogen lamp. As the chlorine gas molecules interacted with the zinc oxide, it caused the generation of zinc chloride, changing the relative permittivity and therefore changing the residence wavelength of the surface plasmons. Since a small portion of the light travels within the cladding of the fibre, this resulted in a change in the transmission spectrum, which was measured using an Avantes AvaSpec-ULS4096CL-EVO fibre-coupled spectrometer. Figure 1 shows measured spectra as a function of chlorine concentration, demonstrating a detection range from 10ppm to 100ppm.
Nano-Characterization by LSPR
LSPR spectroscopy is a powerful characterization tool, since it is highly dependent on the nanostructure of a particle or substrate. Recently, researchers in Hungary took advantage of the fact that line width of the LSPR absorbance spectrum is highly dependent on the uniformity of nanoparticles . In their experiment, they sputtered four different thicknesses of gold nanoparticles with an estimated layer thickness of 7.5 nm, 12.5 nm, 15 nm, and 30 nm. They then utilized an Avantes AvaSpec-ULS2048CL-EVO four-channel high-resolution spectrometer and an Avantes AvaLight-DH-S-BAL halogen light source to measure the absorption spectra of the four samples, in air (n = 1), water (n=1.33), and oil (n=1.616).
From this data, shown in Figure 2, they were able to show a clear dependence on the relationship between the layer thickness and the FWHM of the surface plasmon resonance, indicating a higher degree of variability in nanoparticle size as the layer thickness was increased.
The above examples are just two of many applications where SPR and LSPR spectroscopy are being used for applications ranging from biological and chemical sensors to materials characterization. Furthermore, the availability of high-resolution, low-noise modular fibre-coupled spectrometers can help facilitate new SPR and LSPR sensors transition from the laboratory to the field. Avantes’ AvaSpec instruments are ideally suited for integration into OEM systems, particularly those requiring high-speed, continuous measurements such as biological and chemical hazard detection. All of the spectrometers discussed above are also available as OEM modules and can be integrated into turnkey laboratory sensing devices, in addition to working as an add-on to existing laboratory equipment. These units can communicate via USB, Ethernet, and the native digital & analogue input/output capabilities of the Avantes AS-7010 EVO electronics board, which provides for a superior interface with other devices. Additionally, the Avantes AvaSpec DLL software development application, with sample programs in Delphi, Visual Basic, C#, C++, LabView, MatLab, and other programming environments, enables users to develop code for their own applications.
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 Usha, S.P., Mishra, S.K. and Gupta, B.D., 2015. Fabrication and characterization of a SPR-based fibre-optic sensor for the detection of chlorine gas using silver and zinc oxide. Materials, 8(5), pp.2204-2216.
 Bonyár, A., Wimmer, B. and Csarnovics, I., 2014, May. Development of a localised surface plasmon resonance sensor based on gold nanoparticles. In Proceedings of the 2014 37th International Spring Seminar on Electronics Technology (pp. 369-374). IEEE.