Electroacoustic Quantification of Surface Bound Ligands in Functionalized Silica and Iron Oxide Nanoparticles

Abstract

The electrokinetic sonic amplitude (ESA) is one of the most versatile techniques for deriving the zeta potential of nanoparticles before, during and after surface functionalization with different ligands, thus overcoming current limitations of well‐established electrophoretic measurement principles. We present a novel method for direct quantification of accessible hydroxide groups available on the surface of silica nanoparticles by the ESA effect, supported by simultaneous electrical conductivity measurements. Moreover, the in‐operando determination of varying zeta potential during addition of carboxylic acids demonstrate a profound way to quantify surface‐bound ligands, which still poses a challenge in following the surface modification of nanoparticles. In this study, phase pure cuboidal hematite particles were chosen as a model system with an initial zeta potential between +50 and +55 mV in ethanol. Using both citric acid and 10‐undecynoic acid as surface‐modifiers, the correlation between the degree of surface functionalization and zeta potential was investigated. The decrease of zeta potential of hematite particles during the titration of 10‐undecynoic acid or citric acid could be directly correlated to a successful surface functionalization in contrast to a surface protonation, which would be expected in case of both carboxylic acids. Furthermore, the addition of 10‐undecynoic acid led to a stabilization effect on the zeta potential of cuboidal hematite particles. These results highlight the hitherto unexplored potential of ESA techniques as a quantification method in nanoparticle surface modification.

Structuration and deformation of colloidal hydrogels

“…Our experiments were performed with stable aqueous dispersions of Ludox TM50 (solid volume fraction φ of 0.30, particle diameter 2a=22±2 nm, polydispersity index 0.2 from Sigma-Aldrich). The pH is about 9.0 which ensures that the particle surface bears a high negative charge density with a zeta potential of about -38 mV 15 . In the absence of evaporation, the electrostatic repulsion between the particles ensures the stability of the colloidal dispersion at those volume fractions.…”

“…The pH is about 9.0 which ensures that the particle surface bears a high negative charge density with a zeta potential of about −38 mV. 15 In the absence of evaporation, the electrostatic repulsion between the particles ensures the stability of the colloidal dispersion at those volume fractions.…”

Analyzing the surface of functional nanomaterials—how to quantify the total and derivatizable number of functional groups and ligands

“…Adapted with author permission from ref. [88] (CC BY-NC 4.0) are more acidic than the respective base, and the ratios of the amounts of the different oxygen-containing FG can then be calculated directly from the base consumption. Boehm titration has been utilized to characterize the FG on different kinds of CNT [106][107][108] as well as on other carbon-based materials like ozone-treated nanodiamonds, carbon NP derived from organic resin, graphite-decorated MnFe 2 O 4 nanocomposites, and natural char nano-and microparticles [109][110][111][112].…”

“…By addition of Fe(III) ions before the titration and comparison of the results with measurements done without metal ions, the authors could also derive information on metal ion-CD interactions. Renner et al compared potentiometric pH titrations with Zeta potential and conductivity measurements to quantify the number of hydroxy groups on silica and iron oxide NP [ 88 ]. The results obtained for silica particles, shown in Fig.…”

“…Important parameters such as the necessity of a label, sample requirements, and the need for method calibration are also summarized Analytical method Description/reporter (Examples) Functional groups/Ligands Requirements Ref. Target Type Label Sample Calibration Electrochemical titration Potentiometric titration Electrochemical potential as function of added H + /OH − All (de)protonable FG (e.g., -COOH, -NH 2 , -SH) Total FG Label-free Aqueous dispersion, free of (de)protonable contaminants Calibration with titrants of known concentration [ 75 – 88 ] Conductometric titration Conductivity as function of added H + /OH − All (de)protonable FG (e.g., -COOH, -NH 2 , -SO 4 ) [ 63 , 64 , 89 – 100 ] Boehm titration Potent. titration with bases of different p K a , back titration of residual titrant Oxygen containing, acidic FG [ 101 – 113 ] Optical spectroscopy Labeling with conventional dyes FITC, Dansyl chloride Corresponds to reactive group of the label (e.g., -NH 2 , -COOH, -N 3 ) Derivatizable FG Dye labeling Aqueous dispersion, free of contaminants bearing the same FG Calibration required: Knowledge of the reporter’s optical properties used for quantification (ε λ and/or Φ PL ) needed [ 63 , 64 , 114 , 117 – 119 ] Activatable or chromogenic dyes Fluram, Ninhydrin, Ellman’s reagent Corresponds to reactive group of the dye (e.g., -NH 2 , -SH) Dye formation upon reaction [ ...…”