Evaluation of back scatter interferometry, a method for detecting protein binding in solutionElectronic supplementary information (ESI) available. See DOI: 10.1039/c4an01129e

Back Scatter Interferometry (BSI) has been proposed to be a highly sensitive and versatile refractive index sensor usable for analytical detection of biomarker and protein interactions in solution. However the existing literature on BSI lacks a physical explanation of why protein interactions in general should contribute to the BSI signal. We have established a BSI system to investigate this subject in further detail. We contribute with a thorough analysis of the robustness of the sensor including unwanted contributions to the interferometric signal caused by temperature variation and dissolved gasses. We report a limit of the effective minimum detectability of refractive index at the 10 −7 level. Long term stability was examined by simultaneously monitoring the temperature inside the capillary revealing an average drift of 2.0 × 10 −7 per hour. Finally we show that measurements on protein A incubated with immunoglobulin G do not result in a signal that can be attributed to binding affinities as otherwise claimed in literature. We evaluate the sensitivity and stability of back scatter interferometry, a method proposed for detecting protein binding in solution by changes in refractive index.