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

Abstract

Functional nanomaterials (NM) of different size, shape, chemical composition, and surface chemistry are of increasing relevance for many key technologies of the twenty-first century. This includes polymer and silica or silica-coated nanoparticles (NP) with covalently bound surface groups, semiconductor quantum dots (QD), metal and metal oxide NP, and lanthanide-based NP with coordinatively or electrostatically bound ligands, as well as surface-coated nanostructures like micellar encapsulated NP. The surface chemistry can significantly affect the physicochemical properties of NM, their charge, their processability and performance, as well as their impact on human health and the environment. Thus, analytical methods for the characterization of NM surface chemistry regarding chemical identification, quantification, and accessibility of functional groups (FG) and surface ligands bearing such FG are of increasing importance for quality control of NM synthesis up to nanosafety. Here, we provide an overview of analytical methods for FG analysis and quantification with special emphasis on bioanalytically relevant FG broadly utilized for the covalent attachment of biomolecules like proteins, peptides, and oligonucleotides and address method- and material-related challenges and limitations. Analytical techniques reviewed include electrochemical titration methods, optical assays, nuclear magnetic resonance and vibrational spectroscopy, as well as X-ray based and thermal analysis methods, covering the last 5–10 years. Criteria for method classification and evaluation include the need for a signal-generating label, provision of either the total or derivatizable number of FG, need for expensive instrumentation, and suitability for process and production control during NM synthesis and functionalization. Graphical abstract

Physical Characterization and Cellular Toxicity Studies of Commercial NiO Nanoparticles

“…The selected materials represent three different types of surface coatings: Ni-05 with a physically adsorbed polymer (PVP) ( Figure 2 ), Ni-06 with stearic acid attached as a carboxylate, and Ni-07 covalently modified with a commonly used aminopropylsilane. Although a number of approaches for the quantification of coatings on nanomaterials has been reported, these vary in their range of applicability and sensitivity and validated standard procedures are still not available [ 27 ]. We removed the coating from the particles for quantification by quantitative NMR after removal of the particles by centrifugation.…”

Revisiting carboxylic group functionalization of silica sol–gel materials

“…As Geißler et al recently pointed out, reliable determinations of the chemical moieties of nanomaterials aimed for health/clinical applications will demand protocols for surface analysis and the need of nanoscale reference materials for method calibration and/ or validation. 122 There exists an ample variety of materials modified with this procedure: silica-coated Eu-encoded polystyrene beads, polystyrene core/SiO 2 shell particles, SiO 2 -coated Fe 3 O 4 particles and SiO 2 NPs doped with Ru(bpy) 3 Cl 2 , among others. [123][124][125][126][127][128][129][130][131][132][133][134][135][136] Alternatively, it is possible to synthesize anhydrides from polycarboxylic acids such as EDTA as shown in Scheme 15.…”

“…As Geißler et al recently pointed out, reliable determinations of the chemical moieties of nanomaterials aimed for health/clinical applications will demand protocols for surface analysis and the need of nanoscale reference materials for method calibration and/or validation. 122…”

Atomic spectrometry update: review of advances in the analysis of metals, chemicals and materials

“…A theory section was included and the modelling of the XPS background for the investigation of the coating is discussed along with the use of hard-energy X-ray sources and a discussion about the sources of uncertainty for the determination of the thickness of the particle coatings. An overview of the analytical methods used for analysis and quantification of surface bound functional groups (FGs) on NPS, with emphasis on bioanalytically relevant FGs such as proteins, peptides, and oligonucleotides has been authored by Geissler et al 247 The analytical techniques reviewed (with 259 references) included electrochemical titration methods, optical assays, NMR and vibrational spectroscopies, X-ray based and thermal analysis methods. The final review reported on here concerned the use of laser and plasma spectroscopy during NP production.…”