Capacitively coupled contactless conductivity detection for microfluidic capillary isoelectric focusing

Area of Science:

• Analytical Chemistry
• Biophysics
• Microfluidics

Background:

• Microfluidic capillary isoelectric focusing (μCIEF) is a powerful protein separation technique.
• Contactless conductivity detection (C4D) offers a label-free approach for analyzing biomolecules.
• Integrating C4D with μCIEF presents opportunities for enhanced protein analysis.

Purpose of the Study:

• To develop and optimize capacitively coupled contactless conductivity detection (C4D) for proteins separated by microfluidic capillary isoelectric focusing (μCIEF).
• To investigate the characteristics of conductivity peaks during isoelectric focusing (IEF) and establish conditions for sensitive detection.
• To demonstrate the utility of the developed technique for protein quantification and mixture separation.

Main Methods:

• Numerical simulations were performed using green fluorescence protein (GFP) and carrier ampholytes (CAs).
• Assay conditions for C4D in μCIEF were optimized using a pressure-mobilization approach.
• Co-detection of conductivity and fluorescence signals was employed to validate the C4D signal attribution.

Main Results:

• Successful application of C4D to μCIEF with optimized conditions.
• Confirmation that negative C4D peaks correspond to the actual protein, not background conductivity.
• Quantification of GFP from 10 nM to 30 μM with a detection limit of 10 nM.
• Separation and conductivity measurement of three fluorescent proteins (R-phycoerythrin, GFP-F64L, RK-GFP) with an average resolution of 2.06.

Conclusions:

• The developed label-free μCIEF-C4D technique enables sensitive and quantitative protein analysis.
• The method demonstrates high resolving power for separating complex protein mixtures.
• This technique is expected to become a widely adopted portable, electronics-only protein analysis tool.