A new study shows that polyethylene glycol coatings keep nanoparticles stable during freeze drying, while the protein albumin helps protect other surfaces and improves their safety in use.
(Nanowerk Spotlight) Turning fragile nanoparticle suspensions into dry, reconstitutable solids that can travel through supply chains without losing performance is a difficult manufacturing step across many fields, from pharmaceuticals to catalysis and materials science. Nanoparticles carry drugs, imaging agents, or catalysts and are engineered to remain uniform and active. That behavior depends on keeping them well dispersed. Suspensions can drift over time as particles clump, shed components, or react with the surrounding liquid.
Lyophilization, conventionally known as freeze drying, offers a way to extend shelf life and simplify transport. The process freezes the suspension and removes ice as vapor under vacuum so that a dry solid remains and can be rehydrated before use. It avoids heat damage but introduces new stresses.
As ice crystals form, salts and buffers concentrate, acidity shifts, and water that once separated particles disappears. Surfaces come into direct contact and particles often stick together. Formulators use sugars, proteins, and polymers to cushion these effects, yet success depends strongly on the chemistry of the particle surface.
Advances in coating design have introduced polymers such as polyethylene glycol, which resists protein buildup, and zwitterionic groups that contain both positive and negative charges to reduce unwanted interactions in liquids. Whether these coatings can also protect particles during freezing and drying has remained uncertain.
A study in ACS Applied Nano Materials (“Role of Surface Coatings in Preventing Nanoparticle Aggregation Induced by Freeze-Drying”) addresses this problem directly. The researchers selected silica nanoparticles as a model because the material is chemically stable, easy to modify, and widely used in industrial and biomedical contexts. They compared several surface chemistries and two common protectants to determine which combinations preserve dispersion after a freeze-drying cycle.
(A) Chemical sketch of the surface moieties on the different silica nanoparticles (SNPs) used throughout this work. (B) Representative scanning electron microscopy (SEM) image of bare SNPs. (C) Box and whisker plot showing the size distribution statistics for each SNP type, derived from SEM image analysis (n = 250 particles per sample). The lower and upper limits of the colored boxes represent the 25th and 75th percentiles, respectively. The line across each box indicates the median. The lower and upper whiskers correspond to the 5th and 95th percentiles, respectively. (Image: Reprinted from DOI:10.1021/acsanm.5c02655, CC BY) (click on image to enlarge)
The team synthesized spherical silica nanoparticles about sixty to 65 nanometers in diameter. They coated one set with polyethylene glycol, a flexible polymer that forms a soft, hydrated layer around a surface. Two sets carried zwitterionic groups that include both positive and negative charges within the same molecule, creating a neutral, water-binding surface. Another two sets carried amine groups, which are positively charged at common pH, or phosphonate groups, which are negatively charged. A final set combined amine and phosphonate groups, producing a mixed surface that behaves like a weak zwitterion depending on acidity.
Before testing, the team confirmed size and surface properties through electron microscopy, dynamic light scattering, and zeta-potential measurements. They then froze each suspension in liquid nitrogen, dried it under vacuum for twenty-four hours, and rehydrated the resulting solid with water or buffer. After mixing and brief sonication, they measured the average particle size and compared it with the pre-drying value. A size ratio near one indicated stable dispersion; higher values pointed to aggregation.
Only the polyethylene glycol coated nanoparticles maintained their original size without any additives. Every other surface coating produced visible aggregation after drying. Small-angle X-ray scattering confirmed these findings. For polyethylene glycol coated samples, scattering curves before and after drying overlapped closely, showing that individual particles stayed isolated. For zwitterionic surfaces, the post-drying curve rose sharply at low scattering vector and showed a broad feature characteristic of dense clusters that persisted after rehydration.
These results illustrate how physical forces differ during freeze drying. Zwitterionic coatings perform well in liquids because their balanced charges attract a thin, stable layer of water that prevents molecular attachment. When the suspension freezes, that hydration layer collapses, and opposite charges on neighboring particles can form ion pairs across the gap. Once those pairs form and the sample dries, they are difficult to reverse.
Polyethylene glycol behaves differently. Its flexible chains create a hydrated brush that resists compression. Pushing two brushes together costs entropy and produces a repulsive force that keeps particles apart throughout freezing and drying.
The team next added free polyethylene glycol to uncoated silica suspensions to test whether the polymer could act as a substitute for the grafted layer. They tested concentrations from 0.1 to 3 percent by weight and found noticeable protection only above about 0.5 percent, with the best results between 1.5 and 3 percent.
In comparison, the grafted particles carried roughly 2 percent polymer by weight, corresponding to a much lower concentration in the suspension. Anchoring the polymer directly to the surface proved far more efficient than adding it freely in solution. Fixed chains remain at the interface where they are needed, whereas free chains must diffuse into thin liquid films during freezing to be effective.
The researchers also evaluated two familiar protectants: glucose and human serum albumin. Glucose, a small sugar, forms a glassy matrix as water leaves and can replace hydrogen bonds normally provided by water. Human serum albumin, a major blood protein, adsorbs onto surfaces to create a soft, protective layer.
Albumin worked reliably across all tested surfaces. At 1.5 percent by weight, it preserved the original size, and at 0.3 percent it still limited aggregation. Glucose showed mixed results. It stabilized uncoated silica, polyethylene glycol coated particles that were already stable, and phosphonate or mixed amine phosphonate surfaces. It failed for zwitterionic coatings and performed poorly for amine surfaces. In one case, glucose triggered aggregation before drying, suggesting an unfavorable surface interaction.
Small-angle X-ray scattering supported these trends. For phosphonate surfaces, both glucose and albumin produced patterns consistent with individual nanoparticles, and albumin contributed an additional signal from the protein itself. For zwitterionic surfaces, glucose reduced but did not remove the features associated with clusters, while albumin restored patterns typical of isolated particles.
These data indicate that albumin not only cushions nanoparticles mechanically during freezing but also forms a temporary coating that prevents them from touching.
To separate the influence of freezing from that of drying, the team carried out freeze and thaw cycles without vacuum drying. Samples that later aggregated strongly after full freeze drying already showed moderate size increases after a single freeze and thaw cycle. Polyethylene glycol coated particles remained stable in both situations. The finding shows that freezing alone can initiate particle contact and that drying amplifies the effect.
The researchers then examined blood compatibility using hemolysis assays, which measure the rupture of red blood cells. Before processing, uncoated silica, amine, and phosphonate surfaces caused moderate hemolysis. Polyethylene glycol and the zwitterionic coatings caused almost none. When the team added albumin as a protectant, hemolysis disappeared across all cases. Glucose reduced hemolysis for uncoated silica and prevented it for phosphonate surfaces but produced little change where values were already low.
Finally, the group stored selected powders containing glucose or albumin for up to sixty days and then tested how well they redispersed. Most formulations retained stability. One case, the amine surface protected by albumin, showed increased aggregation after one week, highlighting a condition that warrants further attention.
Overall, the results confirmed that suitable protectants can maintain performance during storage.
The study offers clear practical guidance. Silica nanoparticles coated with polyethylene glycol withstand freeze drying without additional stabilizers and maintain their dispersion after rehydration. For other surface chemistries, human serum albumin acts as a broadly effective protectant that also improves compatibility with blood. Glucose can assist in specific cases but is unreliable for the zwitterionic surfaces examined here. The findings caution that coatings effective in liquid environments may not behave the same under freezing or drying, where physical forces and available water differ sharply.
Within its defined scope—silica nanoparticles near 60 nanometers and the tested processing conditions—the work provides a useful map of how coatings and additives interact during freeze drying. It links formulation stability with biological safety and supplies data that can shorten development cycles for those creating powdered nanoparticle systems. By identifying where anchored polymers and protein additives succeed or fail, the study supports the broader effort to move delicate suspensions toward reliable dry storage across many scientific and industrial domains.
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