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.

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.

Atomic Force Manipulation of Single Magnetic Nanoparticles for Spin-Based Electronics.

Magnetic nanoparticles (MNPs) are instrumental for fabrication of tailored nanomagnetic structures, especially where top-down lithographic patterning is not feasible. Here, we demonstrate precise and controllable manipulation of individual magnetite MNPs using the tip of an atomic force microscope. We verify our approach by placing a single MNP with a diameter of 50 nm on top of a 100 nm Hall bar fabricated in a quasi-two-dimensional electron gas (q2DEG) at the oxide interface between LaAlO3 and SrTiO3 (LAO/STO). A hysteresis loop due to the magnetic hysteresis properties of the magnetite MNPs was observed in the Hall resistance. Further, the effective coercivity of the Hall resistance hysteresis loop could be changed upon field cooling at different angles of the cooling field with respect to the measuring field. The effect is associated with the alignment of the MNP magnetic moment along the easy axis closest to the external field direction across the Verwey transition in magnetite. Our results can facilitate experimental realization of magnetic proximity devices using single MNPs and two-dimensional materials for spin-based nanoelectronics.

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

Gold nanoparticles (AuNPs) are currently intensively exploited in the biomedical field as they possess interesting chemical and optical properties. Although their synthesis is well-known, their controlled surface modification with defined densities of ligands such as peptides, DNA, or antibodies remains challenging and has generally to be optimized case by case. This is particularly true for applications like in vivo drug delivery that require AuNPs with multiple ligands, for example a targeting ligand and a drug in well-defined proportions. In this context, we aimed to develop a calixarene-modification strategy that would allow the controlled orthogonal conjugation of AuNPs, respectively, via amide bond formation and copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC). To do this, we synthesized a calix[4]arene-tetradiazonium salt bearing four PEG chains ended by an alkyne group (C1) and, after optimization of its grafting on 20 nm AuNPs, we demonstrated that CuAAC can be used to conjugate an azide containing dye (N3-cya7.5). It was observed that AuNPs coated with C1 (AuNPs-C1) can be conjugated to approximately 600 N3-cya7.5 that is much higher than the value obtained for AuNPs decorated with traditional thiolated PEG ligands terminated by an alkyne group. The control over the number of molecules conjugated via CuAAC was even possible by incorporating a non-functional calixarene (C2) into the coating layer. We then combined C1 with a calix[4]arene-tetradiazonium salt bearing four carboxyl groups (C3) that allows conjugation of an amine (NH2-cya7.5) containing dye. The conjugation potential of these bifunctional AuNPs-C1/C3 was quantified by UV-vis spectroscopy: AuNPs decorated with equal amount of C1 and C3 could be conjugated to approximately 350 NH2-dyes and 300 N3-dyes using successively amide bond formation and CuAAC, demonstrating the control over the orthogonal conjugation. Such nanoconstructs could benefit to anyone in the need of a controlled modification of AuNPs with two different molecules via two different chemistries.

Magnetic Self-Healing Composites: Synthesis and Applications.

Magnetic composites and self-healing materials have been drawing much attention in their respective fields of application. Magnetic fillers enable changes in the material properties of objects, in the shapes and structures of objects, and ultimately in the motion and actuation of objects in response to the application of an external field. Self-healing materials possess the ability to repair incurred damage and consequently recover the functional properties during healing. The combination of these two unique features results in important advances in both fields. First, the self-healing ability enables the recovery of the magnetic properties of magnetic composites and structures to extend their service lifetimes in applications such as robotics and biomedicine. Second, magnetic (nano)particles offer many opportunities to improve the healing performance of the resulting self-healing magnetic composites. Magnetic fillers are used for the remote activation of thermal healing through inductive heating and for the closure of large damage by applying an alternating or constant external magnetic field, respectively. Furthermore, hard magnetic particles can be used to permanently magnetize self-healing composites to autonomously re-join severed parts. This paper reviews the synthesis, processing and manufacturing of magnetic self-healing composites for applications in health, robotic actuation, flexible electronics, and many more.

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.

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.

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.

Synthesis of Ultrastable and Bioconjugable Ag, Au, and Bimetallic Ag_Au Nanoparticles Coated with Calix[4]arenes.

Compared to gold nanoparticles, silver nanoparticles are largely underexploited for the development of plasmonic nanosensors. This is mainly due to their easy chemical degradation through oxidation, poor colloidal stability, and usually broad size distribution after synthesis, which leads to broad localized surface plasmon resonance bands. Coatings based on polymers such as poly(ethylene glycol) (PEG) or poly(vinylpyrrolidone) (PVP) and plant extracts have been used for the stabilization of AgNPs; however, these thick coatings are not suitable for sensing applications as they isolate the metallic core. The examples of stable AgNPs coated with a thin organic layer remain scarce in comparison to their gold counterparts. In this work, we present a convenient one-step synthesis strategy that allows to obtain unique gold, silver, and bimetallic NPs that combine all of the properties required for biosensing applications. The NPs are stabilized by a tunable calix[4]arene-based monolayer obtained through the reduction of calix[4]arene-tetradiazonium salts. These multidentate ligands are of particular interest as (i) they provide excellent colloidal and chemical stabilities to the particles thanks to their anchoring to the surface via multiple chemical bonds, (ii) they allow the subsequent (bio)conjugation of (bio)molecules under mild conditions, and (iii) they allow a control over the composition of mixed coating layers. Ag and Ag_Au nanoparticles of a high stability are obtained, opening perspectives for development of numerous biosensing applications.

Ultrastable PEGylated Calixarene-Coated Gold Nanoparticles with a Tunable Bioconjugation Density for Biosensing Applications.

Many in vivo and in vitro applications using gold nanoparticles (AuNPs) require (i) their PEGylation, as it increases their stability and prevents nonspecific protein adsorption, and (ii) their conjugation to biomolecules, that provides them with specific recognition properties. Currently, the functionalization of AuNPs is based on thiol chemistry that suffers from two major drawbacks: (i) the Au-S bond is labile and confers limited chemical robustness to the organic layer, and (ii) control over the bioconjugation density is highly challenging. We report here a novel functionalization strategy based on calix[4]arene-tetradiazonium platforms for the coating of AuNPs with a robust PEG layer and their controlled bioconjugation. AuNPs were first modified with a functional calix[4]arene-diazonium salt bearing three PEG chains ended by a methoxy group and one by a carboxyl group. The resulting particles showed excellent chemical and colloidal stabilities, compared to similar systems obtained via a classical thiol chemistry, and could even be dispersed in human serum without degrading or aggregating. In addition to that, the carboxyl groups protruding from the PEG layer allowed their conjugation via amide bond formation with amine-containing biomolecules such as peptides. The control of the bioconjugation was obtained by grafting mixed layers of functional and nonfunctional PEGylated calix[4]arenes, that allowed varying the number of functional groups carried by the AuNPs and subsequently their bioconjugation capacity while preserving their dense protective PEG shell. Finally, we used these nanomaterials, modified with peptide aptamers, for the in vitro biosensing of a cancer biomarker, Mdm2.

Adaptable DNA interactions regulate surface triggered self assembly.

DNA-mediated multivalent interactions between colloidal particles have been extensively applied for their ability to program bulk phase behaviour and dynamic processes. Exploiting the competition between different types of DNA-DNA bonds, here we experimentally demonstrate the selective triggering of colloidal self-assembly in the presence of a functionalised surface, which induces changes in particle-particle interactions. Besides its relevance to the manufacturing of layered materials with controlled thickness, the intrinsic signal-amplification features of the proposed interaction scheme make it valuable for biosensing applications.

Limits of thiol chemistry revealed by quantitative analysis of mixed layers of thiolated-PEG ligands grafted onto gold nanoparticles.

HYPOTHESIS: The functionalization of gold nanoparticles is commonly based on the use of thiol groups for the anchoring of organic ligands. To functionalize gold nanoparticles with mixed layers in defined proportions, different thiolated ligands are often used and assumed to graft equally on the surface. This assumption is however generally not verified and a quantitative investigation of the grafting density of mixed organic layers of thiolated ligands is therefore required. EXPERIMENTS: Gold nanoparticles were exposed to solutions containing various proportions of two PEG ligands containing a thiol group at one extremity and a methoxy, carboxylate, or alkyne group at the other. A systematic study was performed on the resulting particles in order to quantify the composition of the PEG layer by quantitative 1H NMR spectroscopy. FINDINGS: Our results showed that the grafting of the PEG ligands with either a carboxylate or an alkyne group is strongly hindered in the presence of the methylated PEG ligands, despite the use of identical thiol anchoring groups. This is the first report on the quantification of mixed layers of PEGylated ligands on gold nanoparticles that demonstrates the severe limits of thiol chemistry for the functionalization of gold nanoparticles with mixed monolayers.

Kinetics of Nanoparticle-Membrane Adhesion Mediated by Multivalent Interactions.

Multivalent adhesive interactions mediated by a large number of ligands and receptors underpin many biological processes, including cell adhesion and the uptake of particles, viruses, parasites, and nanomedical vectors. In materials science, multivalent interactions between colloidal particles have enabled unprecedented control over the phase behavior of self-assembled materials. Theoretical and experimental studies have pinpointed the relationship between equilibrium states and microscopic system parameters such as the ligand-receptor binding strength and their density. In regimes of strong interactions, however, kinetic factors are expected to slow down equilibration and lead to the emergence of long-lived out-of-equilibrium states that may significantly influence the outcome of self-assembly experiments and the adhesion of particles to biological membranes. Here we experimentally investigate the kinetics of adhesion of nanoparticles to biomimetic lipid membranes. Multivalent interactions are reproduced by strongly interacting DNA constructs, playing the role of both ligands and receptors. The rate of nanoparticle adhesion is investigated as a function of the surface density of membrane-anchored receptors and the bulk concentration of nanoparticles and is observed to decrease substantially in regimes where the number of available receptors is limited compared to the overall number of ligands. We attribute such peculiar behavior to the rapid sequestration of available receptors after initial nanoparticle adsorption. The experimental trends and the proposed interpretation are supported by numerical simulations.

Submerging a Biomimetic Metallo-Receptor in Water for Molecular Recognition: Micellar Incorporation or Water Solubilization? A Case Study.

Molecular recognition in water is an important topic, but a challenging task due to the very competitive nature of the medium. The focus of this study is the comparison of two different strategies for the water solubilization of a biomimetic metallo-receptor based on a poly(imidazole) resorcinarene core. The first relies on a new synthetic path for the introduction of hydrophilic substituents on the receptor, at a remote distance from the coordination site. The second involves the incorporation of the organosoluble metallo-receptor into dodecylphosphocholine (DPC) micelles, which mimic the proteic surrounding of the active site of metallo-enzymes. The resorcinarene ligand can be transferred into water through both strategies, in which it binds ZnII over a wide pH window. Quite surprisingly, very similar metal ion affinities, pH responses, and recognition properties were observed with both strategies. The systems behave as remarkable receptors for small organic anions in water at near-physiological pH. These results show that, provided the biomimetic site is well structured and presents a recognition pocket, the micellar environment has very little impact on either metal ion binding or guest hosting. Hence, micellar incorporation represents an easy alternative to difficult synthetic work, even for the binding of charged species (metal cations or anions), which opens new perspectives for molecular recognition in water, whether for sensing, transport, or catalysis.

Controlled Functionalization of Gold Nanoparticles with Mixtures of Calix[4]arenes Revealed by Infrared Spectroscopy.

Labile ligands such as thiols and carboxylates are commonly used to functionalize AuNPs, though little control over the composition is possible when mixtures of ligands are used. It was shown recently that robustly functionalized AuNPs can be obtained through the reductive grafting of calix[4]arenes bearing diazonium groups on the large rim. Here, we report a calix[4]arene-tetradiazonium decorated by four oligo(ethylene glycol) chains on the small rim, which upon grafting gave AuNPs with excellent stability thanks to the C-Au bonds. Mixtures of this calixarene and one with four carboxylate groups were grafted on AuNPs. The resulting particles were analyzed by infrared spectroscopy, which revealed that the composition of the ligand shell clearly reflected the ratio of calixarenes that was present in solution. This strategy opens the way to robustly protected AuNPs with well-defined numbers of functional or postfunctionalizable groups.

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 syn-syn conformation with suitable complementary H-bonding acceptor partners. The first determination of the association constant (Ka) of ortho-substituted boronic acids in solution showed that Ka 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 syn-syn conformer through a pair of frontal H-bonds with the relevant AA partner. Non-ortho-substituted boronic acids result in "flat" complexes. On the other hand, sterically demanding analogues bearing ortho 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.

Temperature Dependence of Triplet-Triplet Annihilation Upconversion in Phospholipid Membranes.

Understanding the temperature dependency of triplet-triplet annihilation upconversion (TTA-UC) is important for optimizing biological applications of upconversion. Here the temperature dependency of red-to-blue TTA-UC is reported in a variety of neutral PEGylated phospholipid liposomes. In these systems a delicate balance between lateral diffusion rate of the dyes, annihilator aggregation, and sensitizer self-quenching leads to a volcano plot, with the maximum upconversion intensity occurring near the main order-disorder transition temperature of the lipid membrane.

Extremely robust and post-functionalizable gold nanoparticles coated with calix[4]arenes via metal-carbon bonds.

Gold nanoparticles stabilized with a thin layer of post-functionalizable calix[4]arenes were prepared through the reductive grafting of a calix[4]arene-tetra-diazonium salt. These particles show exceptional stability towards extreme pH, F(-), NaCl, and upon drying. Post-functionalization of the calix-layer was demonstrated, opening the way to a wide range of applications.