Binary colloidal nanoparticle concentration gradients in a centrifugal field at high concentration


With the development of more effective characterization and data acquisition tools, the behaviors of various crystalline phases of the binary colloidal nanoparticles have been observed. Unfortunately, measuring and simulating binary hard-sphere colloidal nanoparticles has remained difficult more so at higher particle concentration gradient.

Recently studies have shown that accessing radial concentration gradient in analytical ultra-centrifugal experiments have been hindered by strong turbidity due to high refractive index difference and difficulty in distinguishing the sized nanoparticles. Consequently, the aforementioned reasons have prevented the determination of the correlation between the nanoparticle’s concentration gradients and the phase behavior of the binary colloidal particles. Therefore, researchers have been looking for alternative solution methods and have identified the use of ultra-centrifugal theory as a promising solution.

To this note, Xufing Xu (PhD candidate) and Professor Dr. Nico Sommerdijk at the Eindhoven University of Technology in collaboration with Dr. Tina Franke, Dr. Kristian Schilling at Nanolytics GmbH and Professor Helmut Cölfen at the University of Konstanz explored binary colloidal nanoparticle concentration gradients in analytical ultracentrifugation. Consequently, they measured and simulated the nanoparticle concentration gradient at high concentration of 30% and higher. In particular, they examined the possibility of obtaining a continuous phase diagram from a given preparative ultracentrifuges experiment by determining the variation of the concentration ratio along the centrifugal field. Their work is currently published in the research journal, Nano Letters.

Briefly, the research team synthesized fluorescence-labeled stabilized silica nanoparticles. Next, two distinct silica particles were dispersed in refractive index solvent and a multiwavelength analytical ultracentrifuge detector used for measuring the concentration gradient for the individual nanoparticles. To address the initially experienced challenges, appropriate turbidity correction procedures were taken into consideration. Eventually, sedimentation diffusion equilibrium model was constructed to determine the necessary parameters: diffusion, sedimentation and nonideality coefficients for prediction and simulation of the binary nanoparticle concentration gradients.

The authors effectively measured and simulated binary colloidal nanoparticle concentration gradients, achieving the highest particle concentration of 30 vol %. This was attributed to the ability to measure individual concentration gradients as well as effective turbidity correction procedures. In addition, the fitted parameters were used to simulate the nanoparticle concentration gradient to obtain similar results as those in the existing literature.

In summary, that was the first report to successfully conduct such experiments at high concentration levels. In general, they conducted a single experiment to determine the continuous phase diagram by examining the structure evolution along the centrifugal field. Altogether, the study paves the way for conducting experimental simulations at very high particle concentration. This will allow exploration of more potential applications of binary colloidal nanoparticles.

Binary colloidal nanoparticle concentration gradients in a centrifugal field at high concentration - Advances in Engineering

About the author

Xufeng Xu is currently a PhD candidate under EU H2020-MSCA-ITN-2015 project “MULTIMAT”. He works on the project “Ultracentrifugation techniques for the fractionation and controlled assembly of nanoparticle” under the supervision by Prof.Sommerdijk (TU/e) and Prof.Coelfen (UoK) since 2016. His current research interests include the analytical/preparative ultracentrifugation, synthesis and assembly of colloids.


About the author

HELMUT CÖLFEN is full professor for physical chemistry at the university of Konstanz. His research interests are in the area of nucleation, classical and non-classical crystallization, Biomineralization, synthesis of functional polymers, directed self assembly of nanoparticles and fractionating methods of polymer and nanoparticle analysis.

His group has made contributions in high resolution particle size analysis with Angström resolution in solution, Mesocrystals, Nonclassical Nucleation and Crystallization, CaCO3 crystallization, bio-inspired mineralization, synthesis of double hydrophilic block copolymers and additive controlled crystallization.

He has published more than 360 papers and was listed among the top 100 chemists 2000 – 2010 by Thomson Reuters.


Xu, X., Franke, T., Schilling, K., Sommerdijk, N., & Cölfen, H. (2019). Binary Colloidal Nanoparticle Concentration Gradients in a Centrifugal Field at High ConcentrationNano Letters19(2), 1136-1142.

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