A direct comparison of SPR and bio-layer interferometry (BLI) for prostate-specific antigen detection in human serum — with implications for which technology best suits clinical biosensing.
Introduction
One of the most challenging goals in biosensor development for clinical applications is to create surfaces that can detect low concentrations of biomarkers in complex media such as human serum. These surfaces should also ideally possess low biofouling properties, high binding capacity, and high sensitivity. One possible way to obtain these characteristics is to build 3D polyelectrolyte structures.
Surface plasmon resonance (SPR) is a label-free technique which is based on total internal reflection of light, and it is sensitive to any changes in refractive index at the sensing surface. Therefore, it can be used for opaque and complex media such as human serum. This blog highlights the capability of SPR (P4SPR™, Affinité Instruments) to detect prostate-specific antigen (PSA) in human serum samples by using various types of 3D polyelectrolyte structures compared to their 2D counterpart. On the other hand, biolayer interferometry (BLI) failed to achieve high sensitivity in detecting PSA in undiluted human serum samples.
3D Polyelectrolyte Structures — A Case Study
A research article by Pan et al. described the synthesis of 3D polyelectrolyte structure as a novel biosensing surface to detect clinical biomarkers such as prostate-specific antigen (PSA). The cationic polyelectrolyte backbone is based on poly-L-lysine (PLL) and was electrostatically adsorbed onto gold or glass surfaces. The 3D-PETx structures at various grafting ratios of biotinylated-oligoethylene glycol (OEG-biotin) sidechains, as well as their 2D-PET counterpart, were compared for their binding capacity and extent of antifouling.
Detection of PSA in Serum with SPR
The researchers switched from BLI to Affinité's P4SPR™ because a stable signal could not be achieved with BLI unless the serum was diluted to 10%. It was deduced that the light was being scattered in an opaque and complex medium. On the contrary, SPR can handle such complex biological samples.
SPR chips were coated with anti-PSA-modified 3D-PETx or 2D-PET, and they were then introduced to PSA in undiluted human serum. The P4SPR generated clear sensorgrams, and the height of the sensorgrams correlated well with PSA concentrations. Furthermore, 3D-PETx surface outperformed the 2D-PET ones, as they were more sensitive especially in the lower PSA concentration range (<5 ng/mL). This result is especially promising as the total PSA concentration in serum in this range is often used to diagnose prostate cancer.
SPR vs. BLI for Complex Samples
BLI is an optical-based technique similar to SPR in that the signal collected is also reflected light. However, instead of a wavelength shift due to a change in refractive index, the amount of specific binding at the end of a glass fiber tip causes a proportionate change in interference pattern. The critical difference is that BLI cannot provide a stable signal in undiluted complex biological matrices such as serum — while SPR can.
Conclusions
BLI could not provide a stable signal for polyelectrolyte surfaces exposed to serum samples unless the serum was diluted to 10%. Instead, Affinité's P4SPR was able to measure the sensitivity of the two polyelectrolyte surfaces in undiluted human serum, concluding that the 3D-PETx had better sensitivity than the 2D-PET surfaces, especially in PSA concentrations lower than 5 ng/mL, which is of clinical importance. The ability of Affinité's P4SPR in conjunction with these 3D-PETx surfaces to detect low concentrations of biomarkers such as PSA in human serum is extremely significant in the development of diagnostic tools for clinical applications.