{"id":80,"date":"2025-07-02T18:09:09","date_gmt":"2025-07-02T18:09:09","guid":{"rendered":"https:\/\/libraryresources.nse.org.ng\/nanoengineering\/chapter\/laboratory-3\/"},"modified":"2026-03-16T14:21:01","modified_gmt":"2026-03-16T14:21:01","slug":"laboratory-3","status":"publish","type":"chapter","link":"https:\/\/libraryresources.nse.org.ng\/nanoengineering\/chapter\/laboratory-3\/","title":{"raw":"Laboratory 3 - Electrophoretic Light Scattering","rendered":"Laboratory 3 – Electrophoretic Light Scattering"},"content":{"raw":"
\n\nNote to readers:\u00a0<\/strong>Relevant theory for this laboratory on electrophoretic lights scattering is typically covered in two lectures. Here, we present only a short introduction to theory, a standards-reading exercise, and the procedures, data analysis, and discussion questions corresponding to a laboratory exercise. Theory sections may be expanded into additional chapters at a later time.\n\n<\/div>\n

1. Introduction<\/strong><\/h1>\nThe surface charge or potential of nanoparticles or colloids is an important factor contributing to their colloidal stability in suspension. Surface charge arises from charged moieties or absorbates on the particle surface and leads to a surface potential. This potential is screened by counterions in solution, leading to a decline in the magnitude of the potential away from the surface. Counterions at higher concentrations or higher valences provide more effective charge screening, i.e., a more compressed electric double layer with a shorter Debye length (\u03ba<\/em>\u20131<\/sup>). Charge screening results in diminished electrostatic repulsion forces and poorer colloidal stability for like-charged particles.\n\nElectrophoretic light scattering (ELS)<\/em><\/strong> measurements are often used to evaluate the zeta potential (\u03b6<\/em><\/strong>)<\/em><\/strong> of the particles, i.e., the potential at the hydrodynamic shear plane. These measurements are based on the principle of the motion of charged particles in an electric field as evaluated through a force balance \u2013 particles with a higher magnitude of charge or potential will experience a stronger force in the electric field relative to the opposing drag or friction force and attain a higher steady-state velocity. For particles in motion, the frequency of their scattered light will change following the Doppler effect. ELS uses laser Doppler velocimetry measurements to determine the particle speed by evaluating the frequency shift between the scattered light and a reference beam of the incident light. The instrument utilized in this laboratory (Malvern Zetasizer Nano ZS) further implements mixed mode measurement \u2013 phase analysis light scattering (M3-PALS). PALS yields improved measurements of particles with lower velocities (or particles under a lower applied electric field), and the M3 (fast field reversal and slow field reversal) measurements yield higher quality measurements of zeta potential distributions by correcting for electro-osmotic effects.[1]<\/a>\n\nThe ELS measurements of particle velocity are normalized by the applied electric field strength to determine the electrophoretic mobility<\/em><\/strong> (u<\/em>e<\/sub>) of the particles. Various model equations have been derived to convert u<\/em>e<\/sub> to \u03b6<\/em>. The H\u00fcckel approximation, u<\/em>e<\/sub> = 2\/3(\u03b5\u03b6<\/em>\/\u03b7<\/em>), applies for spherical particles with a very small radius (R<\/em>s<\/sub>) relative to the Debye length, i.e., R<\/em>s<\/sub> < 0.1\u03ba<\/em>\u20131<\/sup>, whereas the Helmholtz-Smoluchowski approximation, u<\/em>e<\/sub> = \u03b5<\/em>\u03b6<\/em>\/\u03b7<\/em>, applies for particles with a very large radius relative to the Debye length, i.e., R<\/em>s<\/sub> > 100\u03ba<\/em>\u20131<\/sup>. For nanoparticles having diameters between 1 nm and 100 nm in monovalent electrolyte from 1 mM to 100 mM ionic strength, neither regime applies and the Henry equation should be used, in which the multiplicative factor is a function of k<\/em>R<\/em>s. Simplified expressions for f(\u03ba<\/em>R<\/em>s<\/sub>) have been derived in various references, e.g., by Ohshima (1994),[2]<\/a> where f(\u03ba<\/em>R<\/em>s<\/sub>)\u00a0ranges from 2\/3 to 1 for u<\/em>e<\/sub> = f(\u03ba<\/em>R<\/em>s<\/sub>)(\u03b5<\/em>\u03b6<\/em>\/\u03b7<\/em>). (Note that many other references, including the Malvern Zetasizer software, write the Henry equation as ue<\/sub><\/em> = f(\u03ba<\/em>R<\/em>s<\/sub>)(2\/3)<\/strong>(\u03b5<\/em>\u03b6<\/em>\/\u03b7<\/em>), such that f(\u03baRs<\/sub>)<\/em>\u00a0ranges from 1 to 1.5.)<\/em>\n\nThis laboratory will apply ELS measurements to determine zeta potentials for gold (Au) nanoparticles (nominally 60 nm diameter) in background electrolytes of varying concentration (1 mM, 20 mM, 50 mM, and 100 mM NaCl), valence (1 mM CaCl2<\/sub>), and with a polymeric surface coating (polyvinylpyrrolidone coating, in 1 mM NaCl). The learning objectives of the laboratory include: (1) evaluating and explaining trends in the measured zeta potential; (2) identifying uncertainties in the measurement and data analysis; and (3) implementing best practices for ELS measurements.\n\nThe laboratory is paired with NCL Method PCC-2 \u2014 Measuring Zeta Potential of Nanoparticles<\/strong> to support the learning objectives.\n

2. Relevant Documentary Standards and Reading Exercises<\/strong><\/h1>\n
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Exercise L3.1<\/strong><\/p>\n\n<\/header>\n

\n\nReview the following method published by the Nanotechnology Characterization Laboratory (NCL) of the National Cancer Institute:\n