Measuring the transverse concentration gradient between adjacent laminar flows in a microfluidic device by a...

Area of Science:

• Microfluidics and Analytical Chemistry
• Optical Detection Methods
• Biochemical Analysis

Background:

• Molecular transport in microfluidic laminar flows primarily occurs via diffusion.
• Accurate quantification of analytes in microfluidic systems requires sensitive detection methods.
• Refractive index gradients (RIGs) can be generated at the interface of laminar flows with differing concentrations.

Purpose of the Study:

• To develop and evaluate a novel detection scheme for microfluidic devices based on probing refractive index gradients.
• To assess the sensitivity and dynamic range of this detection method for various analytes.
• To demonstrate the universal detection capability of the scheme for polymer, pharmaceutical, and life science applications.

Main Methods:

• A diode laser probes the transverse concentration gradient, creating a refractive index gradient (RIG).
• The deflection of the laser beam by the RIG is monitored using a position sensitive detector (PSD).
• The RIG is probed at a position minimizing molecular diffusion to optimize detection sensitivity.

Main Results:

• Quantitative detection of sucrose achieved with a limit of detection (LOD) of 96 ppm (280 µM) and a dynamic range of 96 ppm to 50% sucrose.
• Lower LODs were obtained for larger molecules with smaller diffusion coefficients, such as poly(ethylene glycol) at 56 ppm (4.7 µM).
• Favorable LODs were achieved for proteins: ribonuclease A (RNAse) at 46 ppm (3.4 µM) and bovine serum albumin (BSA) at 54 ppm (780 nM).

Conclusions:

• The developed laser-based RIG probing method provides sensitive and universal detection in microfluidic systems.
• Detection sensitivity is influenced by analyte diffusion coefficients, enabling tailored detection for different molecules.
• This technique is highly applicable to chemical monitoring in polymer, pharmaceutical, and life sciences.