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

Rapid and Selective Detection of Proteins by Dual Trapping Using Gold Nanoparticles Functionalized with Peptide Aptamers

A colorimetric platform for the fast, simple, and selective detection of proteins of medical interest is presented. Detection is based on the aggregation of two batches of peptide functionalized gold nanoparticles via the dual-trapping of the protein of interest. As proof of concept, we applied our platform to the detection of the oncoprotein Mdm2. The peptide aptamers used for the functionalization are based on the reported binding sequences of proteins p53 and p14 for the oncoprotein. Rapid aggregation, and a color change from red to purple, was observed upon addition of Mdm2 with concentrations as low as 20 nM. The selectivity of the system was demonstrated by the lack of response upon addition of bovine serum albumin (in large excess) or of a truncated version of Mdm2, which lacks one of the peptide binding sites. A linear response was observed between 30 and 50 nM of Mdm2. The platform reported here is flexible and can be adapted for the detection of other proteins when two binding peptide aptamers can be identified. Unlike current immunoassay methods, it is a one-step and rapid method with an easy readout signal and low production costs

Versatile Self-Adapting Boronic Acids for H‑Bond Recognition: From Discrete to Polymeric Supramolecules

Because of the peculiar dynamic covalent reactivity of boronic acids to form tetraboronate derivatives, interest in using their aryl derivatives in materials science and supramolecular chemistry has risen. Nevertheless, their ability to form H-bonded complexes has been only marginally touched. Herein we report the first solution and solid-state binding studies of the first double-H-bonded DD·AA-type complexes of a series of aromatic boronic acids that adopt a <i>syn</i>–<i>syn</i> conformation with suitable complementary H-bonding acceptor partners. The first determination of the association constant (<i>K</i>_a) of <i>ortho</i>-substituted boronic acids in solution showed that <i>K</i>_a for 1:1 association is in the range between 300 and 6900 M^–1. Crystallization of dimeric 1:1 and trimeric 1:2 and 2:1 complexes enabled an in-depth examination of these complexes in the solid state, proving the selection of the −B­(OH)₂ <i>syn</i>–<i>syn</i> conformer through a pair of frontal H-bonds with the relevant AA partner. Non-<i>ortho</i>-substituted boronic acids result in “flat” complexes. On the other hand, sterically demanding analogues bearing <i>ortho</i> substituents strive to retain their recognition properties by rotation of the ArB­(OH)₂ moiety, forming “T-shaped” complexes. Solid-state studies of a diboronic acid and a tetraazanaphthacene provided for the first time the formation of a supramolecular H-bonded polymeric ribbon. On the basis of the conformational dynamicity of the −B­(OH)₂ functional group, it is expected that these findings will also open new possibilities in metal-free catalysis or organic crystal engineering, where double-H-bonding donor boronic acids could act as suitable organocatalysts or templates for the development of functional materials with tailored organizational properties