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.

Ultrastable Silver Nanoparticles for Rapid Serology Detection of Anti-SARS-CoV-2 Immunoglobulins G.

Dipstick assays using silver nanoparticles (AgNPs) stabilized by a thin calix[4]arene-based coating were developed and used for the detection of Anti-SARS-CoV-2 IgG in clinical samples. The calixarene-based coating enabled the covalent bioconjugation of the SARS-CoV-2 Spike Protein via the classical EDC/sulfo-NHS procedure. It further conferred remarkable stability to the resulting bioconjugated AgNPs, as no degradation was observed over several months. In comparison with lateral-flow immunoassays (LFIAs) based on classical gold nanoparticles, our AgNP-based system constitutes a clear step forward, as the limit of detection for Anti-SARS-CoV-2 IgG was reduced by 1 order of magnitude and similar signals were observed with 10 times fewer particles. In real clinical samples, the AgNP-based dipstick assays showed impressive results: 100% specificity was observed for negative samples, while a sensitivity of 73% was determined for positive samples. These values match the typical sensitivities obtained for reported LFIAs based on gold nanoparticles. These results (i) represent one of the first examples of the use of AgNP-based dipstick assays in the case of real clinical samples, (ii) demonstrate that ultrastable calixarene-coated AgNPs could advantageously replace AuNPs in LFIAs, and thus (iii) open new perspectives in the field of rapid diagnostic tests.

Ultra-stable silver nanoplates: efficient and versatile colorimetric reporters for dipstick assays.

Noble metal anisotropic nanostructures, such as silver nanoplates (AgNPls), are interesting because they possess enhanced plasmonic properties compared to their spherical counterparts: increased extinction coefficient and tunable maximum of absorption wavelength. However, their use for biosensing application is limited as these structures are intrinsically unstable and, to maintain the anisotropic structure, a coating protecting the metallic surface is required. In this work, we report on the capacity of a thin but robust coating based on calixarene-diazonium salts to maintain the structure anisotropy of silver nanoplates in conditions in which traditionally used coatings fail. We synthesized AgNPls of various sizes and coated them with two different calixarenes, differing by the functional groups attached to their small rim. After characterization of the efficiency of the ligand exchange process between the initial citrate anions and the calixarenes, the chemical and colloidal stabilities of the resulting calixarene-coated AgNPls were compared to citrate-capped AgNPls. A radical improvement of the lifetime of the material from 1 day for AgNPls coated with citrate to more than 900 days for calixarene-coated AgNPls, as well as the stability in acidic conditions, phosphate saline buffer (PBS) or biofluid, were observed. Benefiting from this exceptional robustness, calixarene-coated AgNPls were exploited to design dipstick assays. Rabbit immunoglobulin G (IgG) detection was developed first as proof-of-concept. The optimal system was then used for the detection of Anti-SARS-CoV-2 IgG. In both cases, a picomolar limit of detection (LOD) was achieved as well as the detection in 100% of pooled human plasma. This sensitivity competes with that of ELISA and is better than the one previously obtained with gold or even silver nanospheres for the same target and in similar conditions. Finally, the wide range of colors provided by the AgNPls allowed the design of a multicolor multiplex assay for the simultaneous detection of multiple analytes.

Empirical Optimization of Peptide Sequence and Nanoparticle Colloidal Stability: The Impact of Surface Ligands and Implications for Colorimetric Sensing.

Surface ligands play a critical role in controlling and defining the properties of colloidal nanocrystals. These aspects have been exploited to design nanoparticle aggregation-based colorimetric sensors. Here, we coated 13-nm gold nanoparticles (AuNPs) with a large library of ligands (e.g., from labile monodentate monomers to multicoordinating macromolecules) and evaluated their aggregation propensity in the presence of three peptides containing charged, thiolate, or aromatic amino acids. Our results show that AuNPs coated with the polyphenols and sulfonated phosphine ligands were good choices for electrostatic-based aggregation. AuNPs capped with citrate and labile-binding polymers worked well for dithiol-bridging and π-π stacking-induced aggregation. In the example of electrostatic-based assays, we stress that good sensing performance requires aggregating peptides of low charge valence paired with charged NPs with weak stability and vice versa. We then present a modular peptide containing versatile aggregating residues to agglomerate a variety of ligated AuNPs for colorimetric detection of the coronavirus main protease. Enzymatic cleavage liberates the peptide segment, which in turn triggers NP agglomeration and thus rapid color changes in <10 min. The protease detection limit is 2.5 nM.

Peptide-Conjugated Silver Nanoparticles for the Colorimetric Detection of the Oncoprotein Mdm2 in Human Serum.

The development of efficient, reliable, and easy-to-use biosensors allowing early cancer diagnosis is of paramount importance for patients. Herein, we report a biosensor based on silver nanoparticles functionalized by peptide aptamers for the detection of a cancer biomarker, i. e. the Mdm2 protein. Silver nanoparticles (AgNPs) were produced and stabilized with a thin PEGylated-calix[4]arene layer that allows (i) the steric stabilization of the AgNPs and (ii) the covalent conjugation of the peptide aptamers via the formation of an amide bond. These peptide-conjugated AgNPs were then used to detect Mdm2 via a dual trapping strategy that was previously reported with gold nanoparticles (AuNPs). Our results showed that replacing AuNPs by AgNPs improves the detection limit by nearly one order of magnitude, down to 5 nM, while the high selectivity of the system and the stability of the particles provided by the calixarene coating allow the detection of Mdm2 in human serum.

Peptide-Conjugated Silver Nanoparticles for the Colorimetric Detection of the Oncoprotein Mdm2 in Human Serum.

Invited for this month's cover are the collaborating groups of Prof. Gilles Bruylants and Prof. Ivan Jabin, Université libre de Bruxelles, Belgium. The cover picture shows the principle of a colorimetric sensor, based on peptide-conjugated silver nanoparticles, for the detection of the cancer biomarker Mdm2. The particles were functionalized via a recently developed strategy based on the use of calixarene diazonium salts. The calixarene-based coating provides an unprecedented stability to the silver nanoparticles, enabling their use as colorimetric reporters for in vitro diagnostics. The cover was designed by I. Jabin. More information can be found in the Research Article by I. Jabin, G. Bruylants, and co-workers.

Highly stable silver nanohybrid electrocatalysts for the oxygen reduction reaction.

Silver nanoparticles (AgNPs) were deliberately functionalized via aryl diazonium chemistry with a monolayer of calix[4]arenes. The resulting nanohybrids show high efficiency and high selectivity toward the ORR in alkaline media along with an exceptional durability and a high methanol tolerance.