Draft:Backscatter Interferometry (BSI)

Backscatter Interferometry (BSI) is an analytical technique used for the detection and analysis of fluid bulk properties at micro and nanoliter volumes. This method leverages the principles of interferometry to measure changes in the refractive index of fluids, making it highly sensitive to minute volume changes. BSI is particularly useful in various scientific fields, including microfluidics, capillary electrophoresis, and clinical diagnostics.

Core Principles
BSI operates by directing an unfocused laser beam at a cylindrical tube of capillary dimensions, which produces a backscattered interference pattern. This pattern contains information about the refractive index (RI) of the fluid within the tube. Positional changes in the interference fringes correspond to changes in the fluid's RI, enabling highly sensitive measurements. BSI can detect changes in the refractive index at the level of 10-7, making it suitable for probing extremely small volumes, down to 350 picoliters.

Micro-Interferometric Backscatter Detection (MIBD)
Developed by Bornhop in 1995, MIBD measures relative refractive index changes in small volumes without needing special optical alignment or beam-conditioning optics. It is applicable to capillary tubes ranging from 75 µm to 1.0 mm in diameter.

Noninvasive Thermometry
Demonstrated by Swinney and Bornhop in 2001, this application uses an on-chip interferometric backscatter detector (OCIBD) to facilitate sensitive, small volume temperature measurements, achieving a resolution of 9.9 × 10-4 °C.

Flow Measurements in Microfluidic Channels
Explored by Markov et al. in 2004, BSI was used for noninvasive fluid flow measurements in microfluidic channels, capable of accurately measuring fluid velocities with detection limits as low as 0.127 nL/s.

Clinical Diagnostics
Kussrow et al. in 2010 highlighted BSI's potential as an in vitro clinical diagnostic tool for detecting antibody-antigen interactions in human serum, significant for serological diagnosis of infectious diseases.

Heat Index Flow Monitoring
StClaire and Hayes (2000) demonstrated the application of BSI for real-time flow monitoring in capillaries using heat indexing, enabling accurate flow measurements in the range of 500 nL/s to 7 mL/s.

Molecular Interaction Studies
Weinberger et al. (2012) applied BSI to measure the dissociation constants (Kd) of protein complexes, demonstrating its high sensitivity and quantitative capabilities.

Aptamer-Protein Interactions
Olmsted et al. (2011) used BSI to measure aptamer-protein interactions, revealing allosteric effects that influence binding affinities.

Hydrogen Bonding in Organic Solvents
Pesciotta et al. (2011) extended BSI's application to study hydrogen bonding interactions in organic solvents, demonstrating its high sensitivity in non-aqueous environments.

Membrane Protein Interactions
Gerhart et al. (2015) explored the application of BSI in studying the interactions of membrane proteins within lipid membranes. This study highlighted BSI's ability to quantify association constants and stability free energy of membrane proteins under various conditions, illustrating its non-perturbing and physiologically relevant measurement capabilities.

Verification of BSI's Accuracy
Baksh and Finn (2017) validated BSI's accuracy by determining association constants for well-known biomolecular interactions, reinforcing BSI's reliability and precision.

Label-Free Quantification of Protein-Protein Interactions
Abbas and Koch (2021) demonstrated the label-free quantification of protein-protein interactions using BSI, providing accurate equilibrium dissociation constants without labeling or immobilization.

High-Speed Capillary Electrophoresis
Dunn (2020) implemented high-speed capillary electrophoresis (HSCE) combined with BSI, reducing analysis time and improving temperature stabilization.

Electric Field-Enhanced Detection
De Silva and Dunn (2024) introduced an electric field-enhanced BSI for capillary electrophoresis, enhancing the BSI signal with high field strengths.

Dual Detection in HSCE
Opallage, De Silva, and Dunn (2022) developed a high-speed capillary electrophoresis platform capable of simultaneous serum protein electrophoresis (SPE) and immunoassay measurements. Using a single laser excitation source, the platform measures both RI and fluorescence signals. This dual detection method enables the analysis of serum proteins and immunocomplexes in the same run, enhancing diagnostic capabilities. Optimized buffer systems like 20 mM CHES at pH 10 provided suitable conditions for both SPE and immunoassays, yielding a limit of detection (LOD) of 23 nM and a limit of quantification (LOQ) of 70 nM for fluorescein detection.