Molecular Weight Determination by Counting Molecules.

Area of Science:

• Analytical Chemistry
• Materials Science
• Spectroscopy

Background:

• Molecular weight (MW) is a critical macromolecular property.
• Conventional methods like gel permeation chromatography (GPC) can yield inaccurate MW measurements due to aggregation and conformation.
• Accurate MW determination is vital for understanding material properties and device performance.

Purpose of the Study:

• To develop and validate a novel method for precise average molecular weight (MW) measurement.
• To address the limitations of conventional MW determination techniques, particularly for challenging samples.
• To demonstrate the applicability of the proposed method across diverse fluorescent molecules and materials.

Main Methods:

• Utilized single-molecule spectroscopy for MW determination.
• Quantified average MW by counting individual fluorescent molecules within a matrix film of known mass concentration.
• Validated the method using dye molecules, a labeled protein, and the conjugated polymer MEH-PPV.

Main Results:

• Single-molecule spectroscopy accurately measured the average MW of tested samples.
• Gel permeation chromatography (GPC) with polystyrene calibration significantly overestimated the MW of MEH-PPV by 40-fold due to aggregation and stiffness.
• The proposed method proved effective for fluorescent molecules, biological objects, and nanoparticles at low concentrations.

Conclusions:

• Single-molecule spectroscopy provides a robust, conformation-independent, and reference-free method for accurate MW determination across a wide range.
• This technique overcomes limitations of traditional methods like GPC, especially for aggregated or conformationally complex macromolecules.
• The findings are crucial for correlating polymer length with photophysics and device performance in conjugated polymers.