Label Free Chemical and Biological Sensing Using SPR Spectroscopy

Surface plasmon resonance (SPR) spectroscopy, along with its counterpart, localized surface plasmon resonance (LSPR) spectroscopy, is known as an indispensable technique for label-free chemical and biological sensing, along with nanostructure characterization.  

SPR spectroscopy is most commonly applied in the discipline of biosensing, especially in the study of binding affinities, for example, for antibody-antigen interactions.

Conversely, LSPR spectroscopy is mainly utilized as a method for signal enhancement for the detection of trace molecules.

In these applications, LSPR is the primary physical process behind techniques like surface-enhanced Raman, absorption, fluorescence, and infrared spectroscopies.

This article will outline the fundamental physics of surface plasmons, together with their wavelength-dependent interactions.

The variations between LSPR and SPR will then be examined by illustrating the processes that achieve signal enhancement. Two short case studies will then be presented that show the applications of both LSPR and SPR spectroscopy.

Surface Plasmon Physics

The consistent oscillation of conduction band electrons and the interface between a dielectric and metal produce surface waves that are known as surface plasmons.

As with any alternative electromagnetic wave, surface plasmons have a related wavevector. The magnitude of the wavevector is determined by the relative permittivity (also called the dielectric constant) of the media in which it is cultivated.

The relative permittivity matches the square root of the index of refraction in materials that are non-magnetic. As a result of this, the relative permittivity and the index of refraction have an equivalent wavelength dependence.

If the parallel component of the wavevector of the incident light is in resonance with the surface plasmon’s wavevector, this dependency will produce wavelength-dependent destructive or constructive interference of the two waves.

In some circumstances, LSPR and SPR can cause either a decrease or increase in wavelength in the transmitted or reflected spectrum.

Conventional SPR sensors comprise a thin metal film (usually silver or gold) positioned on a dielectric material (typically glass). Broadband light is focused on the interface via total internal reflection (TIR).

Standard SPR sensors achieve TIR utilizing prisms, like an ATR (attenuated total reflection) tip in FT-IR. Some of the methods, which will be described in further detail later, utilize TIR inside of coated optical fibers to generate SPR.

In SPR spectroscopy, the sensor is exposed to an analyte, which can bond to the sensor, resulting in a small difference to the relative permittivity. This, in turn, modifies the resistance frequency of the surface plasmons.

The detected spectra in SPR will have a decline in the spectrum related to destructive interference once the incident light is in residence with the surface plasmons.

The local minima of the transmission spectrum will change as the sensor gathers an increasing amount of analyte, which enables highly sensitive measurements.  

A conductor’s surface charge density is inversely proportional to its radius, as stated by Gauss’s law. Nanofabrication methods are used in LSPR to gain the benefits of this localization effect.

A complete description of the physics behind this effect will not be covered in this article. Still, it is crucial to understand that the exceptionally high surface charge density can produce significant signal enhancement.

This enhancement is only attainable when the real part of the substrate’s relative permittivity is a negative multiple to that of the environment, significantly increasing the strength of the signal.  

This correlation is the reason why silver and gold are frequently used in both SPR and LSPR. Silver has a relative permittivity of -11.755 + 0.37038i at 532 nm, and gold has a relative permittivity of -22.855 + 1.4245i at 785 nm. This is why silver is normally used for 532 nm excitation and gold for 785 nm excitation in surface-enhanced Raman spectroscopy (SERS).

As the magnitude of the scaling factor cited earlier is reliant on the substrate’s geometry (normally differs between two and twenty), the “enhancement” wavelength can be tuned according to the shape of the nanostructure.

This geometric dependency is also useful in the characterization of nanomaterials.

Case Studies

Fiber Optic SPR Probes

Targeted sensors can now be utilized in hazardous environments due to the integration of SPR sensors into fiber-optic probes.

These probes are normally created by removing the cladding from a segment of fiber optic cable and then coating that area with a metallic layer and then a dialectic layer.

Recent results published by a group of researchers at the Indian Institute of Technology in Delhi evaluated this method. A layer of silver and a layer of zinc oxide were used to coat the fiber core to identify chlorine gas¹.

Using an Avantes AvaLight-HAL tungsten halogen lamp, broadband light was coupled into the probe in this experiment.

The production of zinc chloride was caused by the interaction of chlorine gas molecules with zinc oxide. This altered the relative permittivity and subsequently modified the residence wavelength of the surface plasmons.

As a fraction of the light reaches inside of the fiber’s cladding, a change in the transmission spectrum was detected. This was quantified utilizing an Avantes AvaSpec-ULS3648-USB2 fiber-coupled spectrometer.  

Figure 1 demonstrates the quantified spectra as a function of chlorine concentration, showing a detection range between 10 ppm to 100 ppm.

Figure 1. Transmission spectra for various concentrations of chlorine gas (left) and the peak wavelength shift as a function of concentration (right)¹.

Nano-Characterization by LSPR

As it is highly reliant on the nanostructure of a substrate or a particle, LSPR spectroscopy is an effective characterization method.

Researchers in Hungary recently utilized the fact that the line width of the LSPR absorbance spectrum is extremely reliant on nanoparticle uniformity².

Four different thicknesses of gold nanoparticles were sputtered in this experiment. Each had an estimated layer thickness of 30 nm, 15 nm, 12.5 nm, and 7.5 nm.

An Avantes Avalight DHS halogen light source and an Avaspec-ULS2048-4DT-USB2 (four-channel high-resolution spectrometer) were employed to quantify the absorption spectra of the four samples, in air (n = 1), water (n=1.33), and oil (n=1.616).

Figure 2 outlines the data, which shows a strong dependence on the relationship between the FWHM of the surface plasmon resonance and the layer thickness. This demonstrates that as the layer thickness was increased, there was a larger amount of variability in nanoparticle size.

Figure 2. Full width half maximum (FWHM) peak width of four sputtered gold nanoparticles of different thicknesses and a function of the environmental index of refraction². 

Final Thoughts

Both of the case studies are just two of the range of applications where LSPR and SPR spectroscopy are being utilized for fields ranging from chemical and biological sensors to the characterization of materials.

The capabilities of low noise, high-resolution modular fiber-coupled spectrometers can assist in facilitating the latest LSPR and SPR sensors to be adapted to the field from the laboratory.

The Avaspec instruments from Avantes are optimized to be integrated within OEM systems, especially those that demand continuous, high-speed measurements, for example, in the detection of chemical and biological hazards.

The spectrometers described above can all be integrated into turnkey laboratory sensing devices and are available as OEM modules. They can also operate as an add-on to current laboratory equipment.

The units can communicate through Ethernet, USB, and the native digital and analog input/output function of the Avantes AS7010 EVO electronics board, which offers an exceptional interface with different devices.

The Avantes AvaSpec DLL software development package, with sample programs in Visual Basic, Delphi, MatLab, LabView, C#, C++, and further programming platforms, allows users to produce code for the applications they are working on.

References and Further Reading

1. Usha, S.P., Mishra, S.K. and Gupta, B.D., 2015. Fabrication and characterization of a SPR based fiber optic sensor for the detection of chlorine gas using silver and zinc oxide. Materials, 8(5), pp.2204-2216.
2. 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.

This information has been sourced, reviewed and adapted from materials provided by Avantes BV.

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