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
Highlights
Need for and importance of functional group quantificationFunctionalized nanomaterials (NM) are of increasing industrial and economic importance in the life sciences and the health sector as well as for applications in nano(bio)technology, optical and sensor technologies, solid state lighting and photovoltaics, as well as opto-electronic and electronic devices and security applications
Potentiometric titrations have been used for characterizing the surface chemistry of different types of organic and inorganic NM with various functional groups (FG), and have been applied to determine the number and nature of acidic sites on carbonbased materials like carbon dots (CD), nanocellulose/ nanobentonite composites, biochar particles, multi-walled carbon nanotubes, or cellulose nanocrystals [76–80], or to quantify hydroxy and thiol groups on hybrid silica particles [81]
Boehm titration is presently the only electrochemical titration method considered by international standardization organizations like IEC TC 113: Nanotechnology for Electrotechnical Products and Systems for surface FG analysis and quantification in the currently evaluated standardization document 62607-6-13: Nanomanufacturing – Key control characteristics – Part 6-13: Determination of Oxygen Functional Groups Content of Graphene Materials with
Summary
Need for and importance of functional group quantificationFunctionalized nanomaterials (NM) are of increasing industrial and economic importance in the life sciences and the health sector as well as for applications in nano(bio)technology, optical and sensor technologies, solid state lighting and photovoltaics, as well as opto-electronic and electronic devices and security applications. Potentiometric titrations have been used for characterizing the surface chemistry of different types of organic and inorganic NM with various FG, and have been applied to determine the number and nature of acidic sites (carboxy, lactone, phenol, and ester groups) on carbonbased materials like carbon dots (CD), nanocellulose/ nanobentonite composites, biochar particles, multi-walled carbon nanotubes, or cellulose nanocrystals [76–80], or to quantify hydroxy (silanol) and thiol groups on hybrid silica particles [81].
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