Wanted: scalable tracers for diffusion measurements.

Area of Science:

• Physical Chemistry
• Materials Science
• Biophysics

Background:

• Diffusion is crucial in biological and chemical systems, but accurately measuring diffusion coefficients in complex environments is challenging.
• Existing methods often confound size-dependent diffusion with variations in tracer properties like shape or surface chemistry.
• Scalable tracers offer a solution by maintaining consistent properties across different sizes.

Purpose of the Study:

• To introduce and define the concept of scalable tracers for studying diffusion mechanisms.
• To explore the application of scalable tracers in both two-dimensional (2D) and three-dimensional (3D) diffusion systems.
• To propose novel scalable tracer candidates for enhanced diffusion studies.

Main Methods:

• Defining scalable tracers as a series of molecules with identical shape, structure, surface chemistry, and deformability, differing only in size.
• Ensuring chemical homology and constant dynamics (no size-dependent branching or diffusion mechanism transitions).
• Reviewing existing correlations for predicting diffusion coefficients and suggesting new tracer types.

Main Results:

• Measurements with scalable tracers directly yield the mean diffusion coefficient as a function of size.
• Non-scalable tracers introduce confounding variables, leading to inaccurate diffusion coefficient estimations.
• Candidate scalable tracers were identified for 2D (membranes) and 3D (aqueous solutions) diffusion.

Conclusions:

• Scalable tracers are a powerful tool for dissecting diffusion mechanisms and predicting diffusion coefficients, especially in hindered diffusion scenarios.
• Synthetic dendrimers, hyperbranched polymers, core-shell quantum dots, and reinforced polymers (e.g., "reinforced Ficoll") are promising candidates for scalable tracers.
• Further development of scalable tracers will significantly advance our understanding of molecular transport in complex systems.