Aminated Silica Nanoparticle Surface Chemistry

Amine-functionalized silica provides a reactive, positively charged surface for nanoparticle conjugation and binding applications. Primary amines at the silica surface can react with carboxyl-containing molecules through EDC/NHS coupling and with other amine-reactive chemistries, including isothiocyanates and activated esters.

The protonation state of the surface amines also provides pH-dependent control of particle charge. Under acidic conditions, protonated amines produce a strongly positive zeta potential, while the surface becomes less positively charged as the pH approaches its isoelectric region.

Need a standard silanol surface rather than terminal amines? See Silica Surface Chemistry.

Aminated Silica Surface Properties

• Surface functionality: Primary amines available for binding and chemical modification
• Surface charge: Positive when surface amines are protonated
• Isoelectric behavior: Dependent on particle and surface formulation; the silica-shelled gold dataset below exhibits a high isoelectric region
• Conjugation: Supports coupling to carboxyl-containing molecules using EDC/NHS chemistry and reaction with other amine-reactive groups
• Structure: Functionalized silica layer rather than a readily displaced molecular capping ligand
• Solvent compatibility: Dispersible in selected polar solvents depending on particle formulation

Representative silica precursor: Tetraethyl orthosilicate (TEOS; Sigma-Aldrich, 333859)

Why Use an Aminated Silica Surface?

Aminated silica combines the structural versatility of a silica coating with chemically accessible primary amines. This provides a convenient surface for covalent attachment of molecules containing carboxyl groups or other amine-reactive functionality.

Common coupling approaches include:

• Carboxyl-containing molecules: EDC/NHS chemistry can activate a carboxyl group for reaction with surface amines to form an amide bond
• Isothiocyanates: ITC-functionalized dyes and other molecules can react directly with primary amines
• Amine-reactive esters: Activated ester chemistries can be used to attach appropriately functionalized molecules to the surface

The silica layer also provides a platform for controlling shell thickness, surface chemistry, particle spacing, and additional functionality. For metal nanoparticle cores, the silica shell can physically separate neighboring cores and reduce direct plasmonic interactions when particles are incorporated into coatings or composites.

Applications

• Covalent conjugation and binding studies
• Attachment of dyes, proteins, and other functional molecules
• Diagnostic and biosensing research
• Drug-delivery and nanomedicine research
• Photothermal and optical applications
• Controlled particle assembly and aggregation
• Functionalized solid and mesoporous silica nanoparticles

For broader information about silica nanoparticle design and applications, see Introduction to Silica Nanoparticles and Mesoporous Silica Nanoparticles.

Aminated Silica Stability

Silica is not completely inert in aqueous environments. Depending on pH, shell composition, degree of condensation, particle structure, and storage conditions, silica can gradually dissolve to soluble silicic acid species.

This behavior can be particularly important for functionalized silica because loss of the outer silica layer may also remove surface groups that are important to particle performance. For applications requiring greater resistance to aqueous dissolution, nanoComposix can prepare aluminosilicate shells using a solution-based process. Existing silica shells can also be converted to aluminosilicate using nanoComposix technology described in U.S. Patent 9,675,953.

Aminated Silica Surface Charge

The figure above shows representative zeta potential versus pH data for amine-functionalized silica-shelled 40 nm gold nanoparticles. These data were generated by manual titration with HCl and NaOH followed by zeta potential measurement.

Aminated silica exhibits substantially different surface-charge behavior from an unmodified silanol surface. At acidic pH, protonation of the surface amines produces a positive zeta potential. As pH increases and the amines become less protonated, the magnitude of the positive charge decreases and the particle approaches its isoelectric region.

In this silica-shelled gold dataset, the apparent isoelectric region occurs at relatively high pH, approximately pH 9. The exact crossover is not an intrinsic constant of all aminated silica surfaces and can vary with particle architecture, amine density, surface preparation, and measurement conditions.

For example, measurements on solid amine-terminated silica nanoparticles can produce somewhat different isoelectric behavior. The relevant surface-charge profile should therefore be considered in the context of the specific particle formulation.

Learn more about how pH, ionic strength, and particle surface chemistry affect these measurements in Zeta Potential Measurements.

Salt Stability of Aminated Silica Nanoparticles

The salt stability of aminated silica is particularly dependent on pH because protonation of the surface amines determines the magnitude of the positive zeta potential.

The figure above shows UV-Visible spectra of amine-functionalized silica-shelled 40 nm gold nanoparticles under different sodium chloride (NaCl) conditions. The particles were diluted into DI water at approximately pH 7, spiked with the indicated NaCl concentrations, and incubated for 10 minutes before UV-Vis measurement.

Nonaggregated particles exhibit a strong optical feature near 525 nm. Under these unbuffered conditions, colloidal destabilization is evident at the lowest salt concentration tested, 5 mM NaCl. At approximately pH 7, the magnitude of the particle's positive zeta potential is relatively low, reducing electrostatic repulsion between particles.

Lowering and controlling the pH increases protonation of the amine groups and produces a larger positive zeta potential. nanoComposix has routinely evaluated aminated silica particles after dilution into 2 mM acetate buffer at pH 5. Under appropriately buffered acidic conditions, the particles exhibit substantially greater salt stability and have demonstrated stability in comparable acetate buffers at concentrations up to 100 mM.

These results are specific to the aminated silica-shelled gold formulation and experimental conditions shown here. Particle size, silica architecture, amine density, concentration, buffer composition, and ionic strength can all influence colloidal stability.

See Salt Stability of Nanoparticles and Silica Surface Chemistry for additional context.

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Related silica and surface chemistry resources

• Silica Surface Chemistry
• Silica Nanoparticle Physical Properties
• Silica Nanoparticle FAQs
• Mesoporous Silica Nanoparticles
• Zeta Potential Measurements
• Custom Nanoparticle Development