Synthesis and characterization of a calix[4]arene amphiphilie bearing cysteine and uniform Au nanoparticle formation templated by its four cysteine moieties

Advances in the design of amino acid and peptide synthesized gold nanoparticles for their applications

The field of therapeutics and diagnostics is advanced by nanotechnology-based approaches including the spatial-temporal release of drugs, targeted delivery, enhanced accumulation of drugs, immunomodulation, antimicrobial action, and high-resolution bioimaging, sensors and detection. Various compositions of nanoparticles (NPs) have been developed for biomedical applications; however, gold NPs (Au NPs) have attracted tremendous attention due to their biocompatibility, easy surface functionalization and quantification. Amino acids and peptides have natural biological activities as such, their activities enhance several folds in combination with NPs. Although peptides are extensively used to produce various functionalities of Au NPs, amino acids have also gained similar interests in producing amino acid-capped Au NPs due to the availability of amine, carboxyl and thiol functional groups. Henceforth, a comprehensive review is needed to timely bridge the synthesis and the applications of amino acid and peptide-capped Au NPs. This review aims to describe the synthesis mechanism of Au NPs using amino acids and peptides along with their applications in antimicrobial, bio/chemo-sensors, bioimaging, cancer therapy, catalysis, and skin regeneration. Moreover, the mechanisms of various activities of amino acid and peptide capped-Au NPs are presented. We believe this review will motivate researchers to better understand the interactions and long-term activities of amino acid and peptide-capped Au NPs for their success in various applications.

Triazolated calix[4]arenes from 2-azidoethylated precursors: is there a difference in the way the triazoles are attached to narrow rims?

A large series of narrow-rim 4-R-1-triazolated calix[4]arenes was prepared, and these compounds were compared in terms of their cation-binding ability with the ‘inverted’ 1-R-4-triazolated calix[4]arenes.

Rediscovering the Monodispersity of Sulfonatocalix[4]arene-Based Micelles

When the micellar aggregation number ( N_agg) is small enough (<30), the N_agg matches the value of vertexes of a regular polyhedron: Platonic solids, and demonstrates perfect monodispersity. These micelles are named Platonic micelles and are particularly found in the system of calix[4]arene-based micelles due to the rigid structure of the backbone molecule. Although sulfonatocalix[4]arene-based micelles are among the most studied host molecules in supramolecular chemistry, their micellar properties as Platonic micelles have thus far been overlooked. In this study, we prepared various sulfonatocalix[4]arene-based amphiphiles bearing alkyl chains with different lengths and investigated their aggregation behavior. When the amphiphiles formed spherical micelles, they demonstrated monodispersity in terms of N_agg, whose value changed from 4 to 17, and then to 24, upon increasing the carbon number in each alkyl chain from C5 to C6, and then to C7, respectively. Although the numbers 17 and 24 do not match the vertices of regular polyhedra, these values can be reasonably explained by the Thomson problem, which considers the Coulomb potential for calculating the best packing on a sphere with multiple identical spherical caps. This study describes rediscovery of the monodispersity of sulfonatocalix[4]arene-based micelles, which is consistent with the idea of Platonic micelles.

Self-Assembly of Calix[4]arene-Based Amphiphiles Bearing Polyethylene Glycols: Another Example of "Platonic Micelles"

The aggregation number of classical micelles exhibits a certain distribution, which is a recognizable feature of conventional micelles. However, we recently identified perfectly monodisperse calix[4]arene-based micelles whose aggregation numbers agree with the vertex numbers of regular polyhedra, that is, Platonic solids, and thus they are named "Platonic micelles". Regarding our hypothesis of the formation mechanism of Platonic micelles, both repulsive interactions including steric hindrance and electrostatic repulsions among the headgroups are important for determining their aggregation number; however, neither of these is necessarily needed to consider. In this study, we employed polyethylene glycols (PEGs) as the nonionic headgroup of calix[4]arene-based amphiphiles to study the effects of only repulsive interactions caused by steric hindrance on the formation of Platonic micelles. The amphiphiles containing relatively low-molecular-weight PEGs (550 or 1000 g mol^-1) form dodecamer or octamer micelles, respectively, with no variation in the aggregation number. However, relatively high-molecular-weight PEGs (2000 g mol^-1) produce polydispersed micelles with a range of aggregation number. PEG 2000 exhibits a greater affinity for water than PEG 550 and 1000, resulting in fewer hydrophobic interactions in micelle formation, as indicated by the drastic increase of the critical micelle concentration (CMC) value in the PEG 2000 system. The instability of the structure of PEG_2kCaL5 micelles might contribute to the higher mobility of PEG in the micellar shell, resulting in a non-Platonic aggregation number with polydispersity.

Glutamic Acids Bearing Calix[4]arene Micelles: pH-Controllable Aggregation Number Corresponding to Regular Polyhedra

We have prepared a new calix[4]arene-based lipid containing glutamic acid as the hydrophilic group. The α-amine and the γ-carboxylic acid groups of the glutamic acid moiety allowed a continuous change in the state of the headgroup from cationic to zwitterionic and then to anionic with increasing pH. Accompanying this headgroup change, micelles of the lipid underwent a morphological transformation from spherical to cylindrical and again to spherical. The morphological transition was ascribed to the change in the lipid conformation corresponding to the pH conditions. Interestingly, at acidic and basic pH, the spherical micelles demonstrated monodispersity in terms of the aggregation number, which agreed with the vertex numbers of Platonic solids, indicating the formation of Platonic micelles. At acidic and basic pH, the lipid conformations were almost identical, but there was a slight difference in the hydrophilic volume, which might affect the packing behavior of the lipid in micelles and account for the difference in the aggregation number. This study clearly demonstrates the precise pH-controllable aggregation number of micelles, which belong to the Platonic micelle systems.

Platonic Micelles: Monodisperse Micelles with Discrete Aggregation Numbers Corresponding to Regular Polyhedra

The concept of micelles was first proposed in 1913 by McBain and has rationalized numerous experimental results of the self-aggregation of surfactants. It is generally agreed that the aggregation number (Nagg) for spherical micelles has no exact value and a certain distribution. However, our studies of calix[4]arene surfactants showed that they were monodisperse with a defined Nagg whose values are chosen from 6, 8, 12, 20, and 32. Interestingly, some of these numbers coincide with the face numbers of Platonic solids, thus we named them "Platonic micelles". The preferred Nagg values were explained in relation to the mathematical Tammes problem: how to obtain the best coverage of a sphere surface with multiple identical circles. The coverage ratio D(N) can be calculated and produces maxima at N = 6, 12, 20, and 32, coinciding with the observed Nagg values. We presume that this "Platonic nature" may hold for any spherical micelles when Nagg is sufficiently small.

Supramolecular Chirality: Vesicle-to-Chiral Helix Transition of the Micelles Consisting of a Sugar-Bearing Calix[4]arene Surfactant

Supramolecular self-assembly and the resulting chiral transfer from the molecular level to the nanoscale is a major topic in modern supramolecular chemistry. We synthesized a galactose-bearing calix[4]arene surfactant (chiral) and mixed it with a primary amine-bearing analogue (achiral). The mixture showed strong induced circular dichroism (ICD) at an almost 3:2 molar ratio of the two surfactants, and exothermic heat was observed upon mixing. The magnitude of ΔH was comparable to that of van der Waals interactions. This phenomenon indicated that the ICD can be ascribed to the formation of a new supramolecular assembly in which the stoichiometric interaction between the two molecules leads to complexation and the resultant complex has chiral morphology. Transmission electron microscopy and small-angle X-ray scattering showed that the galactose-bearing surfactant forms vesicles, and the mixing induces a transition from the vesicles to threadlike cylinders with a diameter of ∼3.0 nm. We presume that these cylinder are twisted because of chiral transfer from the chiral galactose moiety and show ICD.

Calix-Based Nanoparticles: A Review

Calixarenes are considered as third generation supramolecules with hollow cavity-like architecture whereas nanoparticles are small entities with dimensions in the nanoscale. Many exciting achievements are seen when the calix system merges with nanoparticles which produces many fascinating facets in all fields of contemporary chemistry. The properties of nanoparticles which are tuned by calixarenes find applications in sensing, catalysis, molecular recognition, etc. Here, we have reviewed the chemistry of calix-based nanoparticles, and emphasis is laid on the modified, reducing, templated and stabilizing roles of calixarenes. This review covers the research being carried out in the domain of calix protected metal nanoparticles during last 18 years under the canopy of important 109 references. This article contains 58 figures which include 81 easy to understand structures. Calix-protected nanoparticles have enthralled researchers in the field of nanoscience with a tremendous growth in its applications, which heralds much promise to become in future a separate area of research.

Facile Decoration of Multiwalled Carbon Nanotubes with Hetero-oligophenylene Stabilized-Gold Nanoparticles: Visible Light Photocatalytic Degradation of Rhodamine B Dye

A hetero-oligophenylene derivative 3 has been designed and synthesized which forms fluorescent spherical aggregates in mixed aqueous media due to its aggregation-induced emission enhancement characteristics. These fluorescent aggregates act as a ratiometric probe for the detection of gold ions in aqueous media and serve as reactors and stabilizers for the preparation of gold nanoparticles. The in situ generated gold nanoparticles have been decorated on multiwalled carbon nanotubes to form AuNPs@MWCNTs nanohybrid materials, which serve as recyclable photocatalysts for carrying out degradation of rhodamine dye in aqueous media.

Nitrogen and sulfur codoped graphene quantum dots as a new fluorescent probe for Au3+ ions in aqueous media

The N and S codoped GQDs can be used as a promising label free fluorescent probe for the sensitive and selective detection of Au³⁺ in aqueous media, which provides a new application of the functionalized GQDs to the detection of metal ions.

Preparation and application of tubular assemblies based on amphiphilic tetramethoxyresorcinarenes

A silver nanoparticle/organic hybrid micro-tubular material was prepared by fabricating tetramethoxyresorcinarene tetraaminoamide-functionalized silver nanoparticles on templates of microtubular assemblies.

Micelles consisting of choline phosphate-bearing calix[4]arene lipids

We synthesized new calix[4]arene-based lipids, denoted by CPCaLn, bearing the choline phosphate (CP) group which is an inverse phosphoryl choline (PC) structure. Small-angle X-ray scattering and multi-angle light scattering coupled with field flow fractionation showed that these lipids form monodisperse micelles with a fixed aggregation number and diameters of 1.9 and 2.6 nm for lipids bearing C3 and C6 alkyl tails, respectively. Furthermore, when CPCaLn was mixed with the fluorescein isothiocyanate (FITC)-bearing lipids and added to cells, strong fluorescence was observed at 37 °C, but not at 4 °C, indicating that the micelles were taken up by the cells through endocytosis. Recent studies have shown that replacement of polymer-attached PC groups with CP groups markedly promotes cellular uptake, even though the surface charge is neutral. On the basis of the idea, CPCaLn micelles interacted with cells in the same way, suggesting that the micelles bearing CP groups are expected to use as carriers in the drug delivery system.