Bifunctional Calix[4]arene-Coated Gold Nanoparticles for Orthogonal Conjugation

Ligation of Gold Nanoparticles with Self-Assembling, Coiled-Coil Peptides

The surface of gold nanoparticles (AuNPs) can be conjugated with a wide range of highly functional biomolecules. A common pitfall when utilizing AuNPs is their tendency to aggregate, especially when their surface is functionalized with ligands of low molecular weight (no steric repulsion) or ligands of neutral charge (no electrostatic repulsion). For biomedical applications, AuNPs that are colloidally stable are desirable because they have a high surface area and thus reactivity, resist sedimentation, and exhibit uniform optical properties. Here, we engineer the surface of AuNPs so that they remain stable when decorated with coiled-coil (CC) peptides while preserving the native polypeptide properties. We achieve this by using a neutral, mixed ligand layer composed of lipoic acid poly(ethylene glycol) and lipoic acid poly(ethylene glycol) maleimide to attach the CCs. Tuning the surface fraction of each component within the mixed ligand layer also allowed us to control the degree of AuNP labeling with CCs. We demonstrate the dynamic surface properties of these CC-AuNPs by performing a place-exchange reaction and their utility by designing an energy-transfer-based caspase-3 sensor. Overall, this study optimizes the surface chemistry of AuNPs to quantitatively present functional biomolecules while maintaining colloid stability.

A fluoride-induced aggregation test to quickly assess the efficiency of ligand exchange procedures from citrate capped AuNPs

Hypothesis

Citrate capped gold nanoparticles (AuNPs-citrate) are the starting material for most of the academic and industrial applications using gold nanoparticles. AuNPs-citrate must usually be functionalized with organic (bio)molecules, through a ligand exchange process, to become suitable for the envisaged application. The evaluation of the efficiency of the ligand-exchange process with a simple and convenient procedure is challenging.

Experiments

Fluoride was used to evaluate the efficiency of a ligand exchange process from AuNPs-citrate with five standard types of ligands. The relationship between the aggregation level of the AuNPs exposed to fluoride and the amount of residual citrate ligands at the surface of the AuNPs was studied. The fluoride-induced aggregation process was characterized with various techniques such as TEM, UV-Vis, ATR-FTIR or MANTA and then used to quickly identify the optimal conditions for the functionalization of AuNPs-citrate with a new ligand, i.e. a PEGylated calixarene-tetradiazonium salt (X4-(PEG)4).

Findings

It was observed that the fluoride-induced aggregation of AuNPs is proportional to the efficiency of the ligands exchange. We believe that these results could benefit to everyone engineering AuNPs for advanced applications, as the fluoride-aggregation of AuNPs can be used as a general and versatile quality test to verify the coating density of organic (bio)molecules on AuNPs.

Di-Arginine Additives for Dissociation of Gold Nanoparticle Aggregates: A Matrix-Insensitive Approach with Applications in Protease Detection

We report the reversible aggregation of gold nanoparticles (AuNPs) assemblies via a di-arginine peptide additive and thiolated PEGs (HS-PEGs). The AuNPs were first aggregated by attractive forces between the citrate-capped surface and the arginine side chains. We found that the HS-PEG thiol group has a higher affinity for the AuNP surface, thus leading to redispersion and colloidal stability. In turn, there was a robust and obvious color change due to on/off plasmonic coupling. The assemblies' dissociation was directly related to the HS-PEG structural properties such as their size or charge. As an example, HS-PEGs with a molecular weight below 1 kDa could dissociate 100% of the assemblies and restore the exact optical properties of the initial AuNP suspension (prior to the assembly). Surprisingly, the dissociation capacity of HS-PEGs was not affected by the composition of the operating medium and could be performed in complex matrices such as plasma, saliva, bile, urine, cell lysates, or even seawater. The high affinity of thiols for the gold surface encompasses by far the one of endogenous molecules and is thus favored. Moreover, starting with AuNPs already aggregated ensured the absence of a background signal as the dissociation of the assemblies was far from spontaneous. Remarkably, it was possible to dry the AuNP assemblies and solubilize them back with HS-PEGs, improving the colorimetric signal generation. We used this system for protease sensing in biological fluids. Trypsin was chosen as the model enzyme, and highly positively charged peptides were conjugated to HS-PEG molecules as cleavage substrates. The increase of positive charge of the HS-PEG-peptide conjugate quenched the dissociation capacity of the HS-PEG molecules, which could only be restored by the proteolytic cleavage. Picomolar limit of detection was obtained as well as the detection in saliva or urine.