Highlights
- Gold nanoparticle surface charge and hydrodynamic size were systematically varied and tested for aggregation inside agarose hydrogel.
- Negatively charged nanoparticles aggregated in neutral gel, with charge magnitude controlling the degree of aggregation.
- Positively charged nanoparticles showed almost no aggregation under the same conditions.
- An extended DLVO model combining electrostatic repulsion and steric hindrance explained the observed trends.
Abstract
This study looks at how a gold nanoparticle's surface charge (zeta-potential) and hydrodynamic size affect whether it aggregates once trapped inside a hydrogel, rather than free in solution. Gold nanoparticles were coated with polymers of different charge and tested in agarose gel with varying salt concentration, with aggregation triggered by a chemical cross-linking reaction. Negatively charged particles aggregated in neutral gel, with the degree of aggregation depending on how strongly negative the surface charge was, while positively charged particles showed almost no aggregation. Hydrodynamic size also mattered, but its effect depended on how much polymer coated the particle surface. A theoretical model combining electrostatic repulsion and steric hindrance explained the trends, and a follow-up test with DNA-functionalized particles showed that surface charge similarly controlled their aggregation behavior in gel.
Research summary
Nanoparticles embedded in hydrogels are used in biosensing and diagnostic applications, where whether the particles aggregate together or stay dispersed can determine how well a device works. Many of the forces known to control nanoparticle behavior in free solution, such as electrostatic repulsion and van der Waals attraction, may act differently once particles are confined inside a gel network. This study systematically varies two properties of gold nanoparticles, their surface charge (zeta-potential) and their hydrodynamic size, to see how each affects aggregation inside agarose hydrogel.
What the study examined
- Gold nanoparticles capped with polymers carrying different surface charges, from strongly negative to positive
- Aggregation behavior of these particles inside agarose gel at different salt (NaCl) concentrations, triggered using a chemical cross-linking reaction
- The separate contribution of hydrodynamic size, by comparing particles with different amounts of surface-bound polymer
- A theoretical model, extended DLVO theory, combining electrostatic repulsion and steric hindrance to explain the trends
- A follow-up test using DNA-functionalized gold nanoparticles to see whether the same charge-driven behavior applied
Main findings
Negatively charged gold nanoparticles could aggregate even in neutral agarose gel, and the degree of aggregation scaled with how strongly negative the surface charge was. Positively charged nanoparticles, by contrast, showed almost no aggregation under the same gel conditions. Confirmed with cryo-electron microscopy, this charge-dependent pattern held up as a reliable predictor of aggregation behavior.
Hydrodynamic size also influenced aggregation, with smaller particles generally aggregating more than larger ones, but this trend was not independent of surface charge: the amount of polymer coating a particle, which itself sets the hydrodynamic size, had to be accounted for separately. An extended DLVO model that combined electrostatic repulsion with steric hindrance from the polymer coating was able to explain these combined effects. When the authors extended the approach to DNA-functionalized nanoparticles, adjusting particle charge to near-neutral let target DNA trigger visible aggregation in gel, while charge that was too negative suppressed the desired response.
Why it matters
By separating out the roles of surface charge and hydrodynamic size, and showing they must be considered together rather than independently, the study gives a more complete design framework for engineers building nanoparticle-based sensors and diagnostic tools that operate inside gel matrices. The DNA-aggregation demonstration also points toward practical assay designs where controlled nanoparticle aggregation, visible as a color change, signals the presence of a target molecule.
Citation
S. H. Kim, B. Lee, J. H. Heo, K. E. Lee, Prabakaran Shankar, K.-H. Han, J. H. Lee. The Effect of ζ-Potential and Hydrodynamic Size on Nanoparticle Interactions in Hydrogels. Particle & Particle Systems Characterization 36 (2019) 1800292.
