With many biosensing technologies available, is conventional SPR still the right choice? A clear-eyed look at where SPR outperforms alternatives and where it doesn't.
Introduction
When it comes to real-time, label-free optical techniques for biosensing, environmental monitoring, clinical diagnostics, or elucidating protein-protein interactions, surface plasmon resonance (SPR) often comes to mind for most researchers. In general, SPR signals arise from a change in local refractive index due to the presence of a biomolecular interaction on a thin metallic surface, thus eliminating the need for labels, and the data are collected in real-time. Conventional SPR has been established for decades. Localized SPR (LSPR), on the other hand, may be less familiar to some people.
Strengths of Conventional SPR
Affinité's portable SPR instruments' core technology uses the Kretschmann configuration in which the incident light interacts with surface plasmons on a thin gold film that is coupled to a glass prism. Upon generation of surface plasmons, evanescent waves propagate perpendicular to the metal surface into the surrounding sample media. These evanescent waves are sensitive to any refractive index changes within 200–300 nm from the thin metallic film surface — the key to the surface sensitivity of SPR.
The gold film on the prism is ~50 nm thick, which has been shown to provide the most sensitive measurement. Affinité's SPR instruments use wavelength mode, not angle. Thus, the optical components allow our platforms to stay compact and robust and less subjected to noise generated by physical vibration or perturbations. Most importantly, due to the refractometric nature of this technique, complex matrices such as serum and plasma can be used as the light does not go through the sample.
What About LSPR?
Instead of a thin metallic film, LSPR uses round nanoparticles as well as other shapes such as nanodisks, nanorods, and nanotriangles that are usually less than 100 nm in diameter, which are used either in solution or immobilized on a clear substrate. LSPR uses transmission mode, in which a peak absorption wavelength shift corresponds to a change in the local refractive index on the nanoparticle surfaces.
Although LSPR seems like a straightforward process, there are some caveats in terms of signal reproducibility, sensitivity, and signal-to-noise. Sensor performance issues such as low result repeatability may be encountered due to SPR plasmon peak broadening, which may occur due to a number of physical nanoparticle processes and a combination of inhomogeneity in size, shape, surface roughness, and material used for these nanoparticles.
Although the number of publications on LSPR has begun to increase since the early 2000s, there is still a gap in the optimization of surface chemistry of nanoparticles. According to one author, there are certainly features that are unique to LSPR, but it is not yet practical for routine biosensing applications.
Conclusions
Conventional SPR and LSPR provide signals through different pathways. However, the surface chemistry on planar thin films is more established and the SPR signal does not rely on the quality and/or behaviour of nanoparticles as in LSPR. The surface chemistry and synthesis of nanoparticles must be well controlled to maximize performance. Conventional SPR instruments such as those by Affinité Instruments remain the more reliable choice for biosensing, assay development, clinical and environmental monitoring applications, and protein interaction studies.