A novel SPR sensor surface based on ionic liquids enables detection of a breast cancer biomarker directly in cell lysates — demonstrating the power of surface chemistry innovation.
Introduction
The detection of biomarkers in real biological samples poses a real challenge in medical diagnostics. Samples may be blood, serum, plasma, cell lysate, urine, and saliva; these biofluids contain numerous cellular components that are amenable to high biofouling on any sensor surfaces. Cell lysates, in particular, have a higher lipid content than serum. Thus, any biosensing surfaces that were previously thought to be suitable to detect biomarkers in serum while minimizing non-specific adsorption — such as hydrophilic PEG-based surfaces — may not work as well for cell lysates.
An alternative to hydrophilic surfaces is to use ionic liquids. Surface chemistries that are hydrophobic and positively charged have been observed to reduce non-specific adsorption from cell lysate samples. The research group led by Prof. Masson at Université de Montréal screened a number of ionic liquids and other surface chemistries for their level of non-specific adsorption in the presence of cell lysates. This blog briefly summarizes their findings of how an ionic liquid was selected to build an SPR platform to detect a breast cancer biomarker, HER2, in cell lysates.
Evaluation of Ionic Liquid and Other Surfaces for Non-Specific Adsorption
The surface chemistry used for biosensing must enable immobilization of the desired capture ligand and minimize biofouling. Some of the surfaces studied were very efficient at reducing the non-specific adsorption; however, they cannot be used to immobilize capture ligands due to the lack of chemically reactive groups. Bare Au performed the worst in terms of non-specific adsorption level.
The first part of the study concluded that all the screened ionic liquids performed well and similarly in terms of reducing the level of biofouling. After a thorough study of alkyl chain length, counterions, and immobilization efficiency, the optimal ionic liquid monolayer was chosen for the SPR surface to detect HER2 in cell lysate. The SH- group allows for self-assembly onto the gold surface while the -COOH group allows for covalent linkage to antibodies through activation by EDC/NHS chemistry.
Detection of HER2 Biomarker in Cell Lysate
To overcome the increase in biofouling on antibody-modified ionic liquid surfaces and the low concentration of HER2 in cell lysate, the researchers implemented a sandwich assay in which a different polyclonal antibody was introduced to bind to the captured biomarker. The control sample used was MCF-7 cells, which contain low levels of HER2. Another MCF-7 sample was spiked with recombinant HER2 as a positive control. The third sample was the SK-BR-3 cells that should contain the HER2 biomarker.
The SPR shifts were between 7–10 times higher for the HER2-spiked MCF-7 sample and the SK-BR-3 cells compared to the MCF-7 cells only sample (control). This demonstrated that the sandwich detection using two polyclonal antibodies by SPR was able to efficiently detect HER2 biomarkers in cell lysate.
Conclusions
Through optimization and design, the HER2 biomarker from a crude breast cancer cell lysate was detected on a novel ionic liquid-based biosensing surface by SPR. A secondary detection method involving a different antibody proved to be essential in observing the signal of HER2 biomarker in cell lysate. These results show promise in using ionic liquids to develop biosensing surfaces for SPR analysis for a variety of crude cell lysate samples.
Reference
[1] Alexandra Aubé, Shirley Campbell, Andreea R. Schmitzer, Audrey Claing, and Jean-François Masson. Analyst, 2017, 142, 2343-2353.