Purification of Colloidal Nanocrystals

Introduction

During the preparation of colloidal nanocrystals, in order to maintain the morphology and colloidal stability of crystal particles during the growth process, excessive surfactants and other reagents are often used. The decomposition products of these excess ligands and reagents remain in the crude product solution and need to be separated from the solution phase mixture. Due to the dynamic exchange of colloidal nanocrystals in free solution, the choice of purification method directly affects the surface chemistry of colloidal nanocrystals, so it is extremely important to use appropriate purification methods.

Fig.1 Conceptual diagram of purification of colloidal nanocrystals^[1]

Our Technologies

Alfa Chemistry achieves purification by manipulating polarity (relative solubility), electrophoretic mobility and particle size property differences between three impurities and nanoparticles. Our methods include precipitation, extraction, electrophoretic methods and size-based methods (ultracentrifugation, ultrafiltration, diafiltration and size exclusion chromatography).

• Polarity-Based Purification Technology
  One approach is to achieve flocculation of colloidal nanocrystals by adding an antisolvent that increases the polarity of the solvent mixture. After decanting the supernatant containing impurities and excess ligand, the colloidal precipitate was redissolved in clean solvent. Repeat several times until the desired purity is achieved. In addition, Alfa Chemistry can also use the liquid-liquid extraction method to take advantage of the difference in solubility between colloids and impurities to distribute impurities and colloids into different phases.

Fig. 2 Schematic diagram of purification schemes.^[2]

• Electrophoresis-Based Techniques
  In addition to using electrophoresis as a technique of choice to separate metal nanoparticles of different shapes and sizes, we were able to use it to purify nanocrystals containing synthetic by-products and excess surfactant impurities.

Fig. 3 Quantum dot electrophoretic purification setup.^[3]

• Size-Based Separation
  Alfa Chemistry is not only able to use ultracentrifugation to prepare monodisperse nanoparticle samples, but also to purify nanoparticles after surface modification. Dialysis, filtration, ultrafiltration and diafiltration are also commonly used size-based membrane separation techniques. Finally, Alfa Chemistry is also able to efficiently purify nanoparticles using size exclusion chromatography.

Our Advantages

• Comprehensive purification methods, selected according to customer needs
• Ultra-high purification efficiency to ensure the removal of impurities
• Standardized project management process to achieve zero-distance communication with customers
• Strong service quality system to ensure perfect service delivery

References

1. Gee, et al. Purification technologies for colloidal nanocrystals. Chemical communications, 2017.
2. Yang Y, et al. An efficient and surface-benign purification scheme for colloidal nanocrystals based on quantitative assessment. Nano Research, 2015, 8(10):3353-3364.
3. D. Kim, et al. Continuous flow purification of nanocrystal quantum dots. Nanoscale, 2014, 6(23):14467-14472.

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