Tailored Ultrastable Core–Shell Au@Ag Nanoparticles for Enhanced Colorimetric Detection in Lateral Flow Assays

In the quest for more effective colorimetric reporters compared with traditional gold nanoparticles (AuNPs), a family of Au@Ag core–shell nanoparticles was designed and synthesized using a seed growth-mediated approach starting from commercial 37 nm AuNPs. This method enabled precise control over the thickness of the silver shell by employing hydroquinone for the reduction of silver and citrate for stabilization of the resulting core–shell particles. Core–shell NPs with an Ag shell of 7 nm (Au@Ag5NPs) and 18 nm (Au@Ag10NPs) were synthesized, resulting in orange and milky yellow suspension, respectively. Additionally, the impact of an external gold layer on Au@Ag10NPs (Au@Ag10@AuNPs), which significantly altered their optical properties from milky yellow to gray, was investigated. The core–shell Au@AgNPs exhibited substantially higher molar extinction coefficients than their parent AuNPs: from 3.5-fold for Au@Ag5NPs and 9-fold for Au@Ag10NPs and Au@Ag10@AuNPs. Subsequently, all core–shell NPs were functionalized with a calix[4]arene layer, imparting superior stability against external stresses, such as dispersion in PBS, when compared to NPs functionalized with traditional ligands. This calixarene coating enabled the covalent bioconjugation of antibodies on all NP types without inducing noticeable aggregation. Their performance as colorimetric reporters was evaluated in a lateral flow assay for troponin I detection, demonstrating positive signals down to 1 ng/mL, surpassing the detection limit of the parent gold NPs (2.5 ng/mL). Notably, the gray color of the core–shell Au@Ag10@AuNPs provided strong contrast against the white NC membrane, facilitating T-line visualization even at low signal intensity. Despite the lack of optimization of our LFA, it competes with the limit of quantification of commercial LFAs for troponin I detection, offering the potential for the development of a highly sensitive assay. The diverse core–shell NPs employed in this study form a library of colorimetric reporters with distinct optical properties, paving the way for multiplexed detection systems targeting multiple proteins simultaneously and enhancing diagnostic reliability. Furthermore, the choice of colorimetric reporters allows tailoring the detection range based on the pertinent limit of quantification desired for the analyte, dictated by the reporter’s light extinction properties

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

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