Detection of gold cysteine thiolate complexes on gold nanoparticles with time-of-flight secondary ion mass spectrometry.

Heng-Yong Nie,Valentin Romanovski,Jonas Hedberg,Yolanda S Hedberg,Elena Romanovskaia

doi:10.1116/6.0000910

Abstract

Gold (Au) nanoparticles (NPs) are widely used in nanomedical applications as a carrier for molecules designed for different functionalities. Previous findings suggested that biological molecules, including amino acids, could contribute to the dissolution of Au NPs in physiological environments and that this phenomenon was size-dependent. We, therefore, investigated the interactions of L-cysteine with 5-nm Au NPs by means of time-of-flight secondary ion mass spectrometry (ToF-SIMS). This was achieved by loading Au NPs on a clean aluminum (Al) foil and immersing it in an aqueous solution containing L-cysteine. Upon rinsing off the excessive cysteine molecules, ToF-SIMS confirmed the formation of gold cysteine thiolate via the detection of not only the Au-S bond but also the hydrogenated gold cysteine thiolate molecular ion. The presence of NaCl or a 2-(N-morpholino)ethanesulfonic acid buffer disabled the detection of Au NPs on the Al foil. The detection of larger (50-nm) Au NPs was possible but resulted in weaker cysteine and gold signals, and no detected gold cysteine thiolate signals. Nano-gold specific adsorption of L-cysteine was also demonstrated by cyclic voltammetry using paraffine-impregnated graphite electrodes with deposited Au NPs. We demonstrate that the superior chemical selectivity and surface sensitivity of ToF-SIMS, via detection of elemental and molecular species, provide a unique ability to identify the adsorption of cysteine and formation of gold-cysteine bonds on Au NPs.

Highlights

Gold nanoparticles (Au NPs) are widely considered for different nanomedical applications, including tumor targeting, drug delivery, imaging, and molecular sensing.1–6 Their fate in the human body, including changing surface chemistries, degradation kinetics, toxicity, and their clearance from the body, is complex and remains a hindering factor for successful clinical use.2,7 All considered Au NPs are functionalized with a avs.scitation.org/journal/bip capping agent to hinder their aggregation and enable their dispersion in aqueous solutions
Nano-gold specific adsorption of L-cysteine was demonstrated by cyclic voltammetry using paraffine-impregnated graphite electrodes with deposited Au NPs
The experimental design of the ToF-SIMS and electrochemical studies involved the deposition of the gold suspension on a substrate (PIGE or Al foil) and drying in air at room temperature, prior to exposure to L-cysteine-containing solutions

Summary

Introduction

Gold nanoparticles (Au NPs) are widely considered for different nanomedical applications, including tumor targeting, drug delivery, imaging, and molecular sensing. Their fate in the human body, including changing surface chemistries, degradation kinetics, toxicity, and their clearance from the body, is complex and remains a hindering factor for successful clinical use. All considered Au NPs are functionalized (surface-coated) with a avs.scitation.org/journal/bip capping agent to hinder their aggregation and enable their dispersion in aqueous solutions. We have earlier shown that citrate-coated Au NPs dissolve up to 14% after one week of incubation in cell medium at physiological pH and 37 °C, when triggered by stimulated macrophages This effect is size-dependent, that is, largest for 5 nm Au NPs (14% dissolution for 5 nm and 0.6% for 50 nm Au NPs under these conditions).. The smaller size can influence dissolution and surface reactions in several ways: by changing the diffusion layer thickness, by a larger surface to bulk atom ratio of smaller-sized nanoparticles, and by changing lattice parameters and surface energies affecting dissolution equilibrium constants.11 Most of these nano-specific effects dominate only well below 10 nm.. Biomolecules have been seen to play an important role for the nano-specific dissolution of Au NPs. We studied L-cysteine, which is an important amino acid with a thiol side chain known to interact with gold and to displace other ligands from Au NPs.

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