Surface charge plays a critical role in determining the stability, dispersion, and interaction of nanoparticles in colloidal systems. Zeta potential, the electrokinetic potential at the slipping plane of the particle-liquid interface, serves as a key indicator of colloidal stability. Modifying the surface of nanoparticles to control zeta potential is essential for applications ranging from drug delivery to wastewater treatment. Three primary approaches—chemical functionalization, polymer coatings, and surfactant adsorption—are widely employed to tailor surface charge characteristics. Each method introduces distinct changes to the nanoparticle surface, influencing electrostatic repulsion and colloidal behavior.

Chemical functionalization involves the covalent attachment of specific functional groups to the nanoparticle surface, directly altering its charge properties. Carboxylation introduces carboxylic acid (-COOH) groups, which deprotonate in aqueous solutions above pH 4–5, generating a negative surface charge. For example, carboxylated polystyrene nanoparticles exhibit zeta potentials ranging from -30 mV to -60 mV at neutral pH due to the dissociation of surface carboxyl groups. Amination, in contrast, grafts amine (-NH₂) groups that protonate in acidic conditions (pH < 7), creating a positive surface charge. Silica nanoparticles functionalized with (3-aminopropyl)triethoxysilane (APTES) display zeta potentials shifting from negative to positive (+20 mV to +40 mV) at pH 3–5. Sulfonation incorporates sulfonic acid (-SO₃H) groups, which are strongly acidic and remain deprotonated across a wide pH range, imparting a highly negative zeta potential (up to -70 mV). These modifications not only adjust the magnitude of zeta potential but also influence its pH dependence, enabling precise control over colloidal stability under varying environmental conditions.

Polymer coatings provide a versatile means of modulating zeta potential through the adsorption or grafting of charged macromolecules onto nanoparticle surfaces. Polyelectrolytes such as poly(acrylic acid) (PAA) or poly(styrene sulfonate) (PSS) confer negative charges due to their ionizable carboxylate or sulfonate groups, respectively. Coating iron oxide nanoparticles with PAA can shift their zeta potential from near-neutral to -50 mV at pH 7. Conversely, cationic polymers like poly(ethylene imine) (PEI) or poly(allylamine hydrochloride) (PAH) introduce positive charges, with zeta potentials reaching +30 mV to +60 mV depending on polymer molecular weight and coating density. The thickness and conformation of the polymer layer further influence charge distribution, as extended chains enhance charge accessibility while dense brushes may partially shield ionizable groups. Stimuli-responsive polymers, such as poly(N-isopropylacrylamide) (PNIPAM), enable dynamic zeta potential tuning via temperature or pH changes, though their primary effect stems from co-grafted charged monomers rather than the neutral polymer backbone.

Surfactant adsorption relies on the non-covalent attachment of amphiphilic molecules to nanoparticle surfaces, where the charged headgroup dictates the resultant zeta potential. Anionic surfactants like sodium dodecyl sulfate (SDS) impart negative charges through sulfate groups, with adsorption densities of 1–3 molecules per nm² typically yielding zeta potentials of -40 mV to -70 mV on metal oxide nanoparticles. Cationic surfactants such as cetyltrimethylammonium bromide (CTAB) introduce positive charges via quaternary ammonium moieties, often producing zeta potentials between +30 mV and +60 mV. The hydrophobic tail length and surfactant concentration critically affect monolayer formation and charge saturation, with excess surfactant leading to bilayer formation and potential charge reversal. Nonionic surfactants (e.g., Tween 80) minimally impact zeta potential but are sometimes used in combination with charged surfactants to fine-tune interfacial properties. Mixed surfactant systems can produce intermediate zeta potentials by varying the ratio of anionic to cationic components, though competitive adsorption kinetics must be considered.

The choice of surface modification technique depends on the desired zeta potential range, environmental stability, and application-specific requirements. Chemical functionalization offers permanent charge modification but requires compatible surface chemistry and may alter core material properties. Polymer coatings provide thicker, tunable layers with potential for multifunctionality but can increase hydrodynamic diameter and may desorb under extreme conditions. Surfactant adsorption is simple and reversible but sensitive to concentration and ionic strength changes. In all cases, the ionic strength and pH of the dispersion medium critically influence the measured zeta potential by compressing the electrical double layer or protonating/deprotonating surface groups.

Practical considerations for zeta potential control include colloidal stability thresholds (typically >|30 mV| for electrostatic stabilization), biocompatibility requirements for biomedical applications, and environmental factors such as salinity for water treatment uses. Combining multiple modification strategies, such as carboxylation followed by polyelectrolyte coating, can further refine zeta potential profiles while adding functionality. Careful characterization via titration curves, isoelectric point determination, and stability assays ensures predictable performance across application conditions.

Advances in surface modification continue to expand the precision of zeta potential control, with emerging techniques including molecular layer deposition for nanoscale coatings and zwitterionic modifications for salt-tolerant charge stabilization. The relationship between surface chemistry, zeta potential, and colloidal behavior remains foundational to nanomaterial design across diverse fields, driving ongoing innovation in charge-directed assembly, targeted delivery, and interfacial engineering. Understanding these principles enables rational selection and optimization of surface modification strategies to meet specific zeta potential requirements.