Ligand-free metal nanoparticle colloids: benefits in standardized toxicity assays and functional bioconjugates

The interactions between metal nanoparticles and biological systems are known to be highly complex as effects occurring at the nano-bio interface are ruled by multiple, often interdependent factors like particle size, particle... [ view full abstract ]

The interactions between metal nanoparticles and biological systems are known to be highly complex as effects occurring at the nano-bio interface are ruled by multiple, often interdependent factors like particle size, particle dose, surface charge, surface functionalization¹ as well as the protein corona formed upon contact with body fluids². This complexity makes systematic studies on the biocompatibility as well as in vitro and in vivo functionality of nanoparticles and their bioconjugates highly challenging. One drawback, in this context, is that nanoparticles from chemical synthesis always contain large quantities of artificial ligands, which cannot be quantitatively removed or exchanged³ and may interfere with biomedical applications.

An alternative approach is the use of ultrapure totally ligand-free metal nanoparticles available via a modern physical synthesis route called pulsed laser ablation in liquids (PLAL)⁴. These nanoparticles can be easily generated on a gram/hour scale⁵ and exhibit completely “naked” partially oxidized surfaces which allow their good stability in diluted aqueous media⁶. These ligand-free ultrapure nanoparticles constitute an ideal platform for systematic studies in biology and medicine. One example is the risk assessment of colloidal nanoparticles, which still suffers from a lack of standardization. In this context, ligand-free nanoparticles could serve as ideal reference materials in order to differentiate toxic effects originating from the nanoparticle core and the ligand shell⁷. Extensive studies on the cytotoxic effects of ligand-free laser-generated gold, silver and gold-silver alloy nanoparticles on spermatozoa, oocytes and embryo development verify the suitability of these materials for systematic toxicological trials⁸. Another example is the use of laser-generated nanoparticles in functional bioconjugates. In this case the ligand-free surfaces of laser-generated metal nanoparticles allows their facile and defined functionalization with biomolecules like peptides⁹ and oligonucleotides¹⁰ via thiol-based chemistry. PLAL-fabricated bioconjugates were successfully utilized for cellular imaging¹¹ as well as transfection experiments e.g. with regulatory T-cells¹². In a recent series it was furthermore demonstrated that multivalent peptide conjugates based on laser-generated gold nanoparticles can be used to prevent pathological protein aggregation and were even more efficient than the free functional ligand due to avidity effects¹³.

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