Metal-Enhanced Hg2+-Responsive Fluorescent Nanoprobes: From Morphological Design to Application to Natural Waters.

Metal-enhanced fluorescence (MEF) is a powerful tool in the design of sensitive chemical sensors by improving brightness and photostability of target-responsive fluorophores. Compounding these advantages with the modest hardware requirements of fluorescence sensing compared to that of centralized elemental analysis instruments, thus expanding the use of MEF to the detection of low-level inorganic pollutants, is a compelling aspiration. Among the latter, monitoring mercury in the environment, where some of its species disseminate through the food chain and, in time, to humans, has elicited a broad research effort toward the development of Hg2+-responsive fluorescent sensors. Herein, a Hg2+-sensitive MEF-enabled probe was conceived by grafting a Hg2+-responsive fluorescein derivative to concentric Ag@SiO2 NPs, where the metallic core enhances fluorescence emission of molecular probes embedded in a surrounding silica shell. Time-resolved fluorescence measurements showed that the fluorophore's excited-state lifetime decreases from 3.9 ns in a solid, coreless silica sphere to 0.4 ns in the core-shell nanoprobe, granting the dye a better resistance to photobleaching. The Ag-core system showed a sizable improvement in the limit of detection at 2 nM (0.4 ppb) compared to 50 nM (10 ppb) in silica-only colloids, and its effectiveness for natural water analysis was demonstrated. Overall, the reported nanoarchitecture hints at the potential of MEF for heavy metal detection by fluorescence detection.

Nucleation Points: The Forgotten Parameter in the Synthesis of Hydrogel-Coated Gold Nanoparticles.

The implementation of gold-hydrogel core-shell nanomaterials in novel light-driven technologies requires the development of well-controlled and scalable synthesis protocols with precisely tunable properties. Herein, new insights are presented concerning the importance of using the concentration of gold cores as a control parameter in the seeded precipitation polymerization process to modulate-regardless of core size-relevant fabrication parameters such as encapsulation yield, particle size and shrinkage capacity. Controlling the number of nucleation points results in the facile tuning of the encapsulation process, with yields reaching 99% of gold cores even when using different core sizes at a given particle concentration. This demonstration is extended to the encapsulation of bimodal gold core mixtures with equally precise control on the encapsulation yield, suggesting that this principle could be extended to encapsulating cores composed of other materials. These findings could have a significant impact on the development of stimuli-responsive smart materials.

Thinking outside the shell: novel sensors designed from plasmon-enhanced fluorescent concentric nanoparticles.

The alteration of photophysical properties of fluorophores in the vicinity of a metallic nanostructure, a phenomenon termed plasmon- or metal-enhanced fluorescence (MEF), has been investigated extensively and used in a variety of proof-of-concept demonstrations over the years. A particularly active area of development in this regard has been the design of nanostructures where fluorophore and metallic core are held in a stable geometry that imparts improved luminosity and photostability to a plethora of organic fluorophores. This minireview presents an overview of MEF-based concentric core-shell sensors developed in the past few years. These architectures expand the range of applications of nanoparticles (NPs) beyond the uses possible with fluorescent molecules. Design aspects that are being described include the influence of the nanocomposite structure on MEF, notably the dependence of fluorescence intensity and lifetime on the distance to the plasmonic core. The chemical composition of nanocomposites as a design feature is also discussed, taking as an example the use of non-noble plasmonic metals such as indium as core materials to enhance multiple fluorophores throughout the UV-Vis range and tune the sensitivity of halide-sensing fluorophores operating on the principle of collisional quenching. Finally, the paper describes how various solid substrates can be functionalized with MEF-based nanosensors to bestow them with intense and photostable pH-sensitive properties for use in fields such as medical therapy and diagnostics, dentistry, biochemistry and microfluidics.

Revealing the Hydrolysis Mechanism of a Hg2+-Reactive Fluorescein Probe: Novel Insights on Thionocarbonated Dyes.

As one of the most toxic metal pollutants, mercury is the subject of extensive research to improve current detection strategies, notably to develop sensitive, selective, fast, and affordable Hg2+-responsive fluorescent probes. Comprehending the sensing mechanism of these molecules is a crucial step in their design and optimization of their performance. Herein, a new fluorescein-based thionocarbonate-appended Hg2+-sensitive probe was synthesized to study the hydrolysis reactions involved in the sensing process. Autohydrolysis was revealed as a significant component of the signal generation mechanism, occurring concurrently with Hg2+-catalyzed hydrolysis. This knowledge was used to investigate the effects of key experimental conditions (pH, temperature, chloride ions) on sensing efficiency. Overall, the chemical and physical properties of this new thionocarbonated dye and the insights into its sensing mechanism will be instrumental in designing reliable and effective portable sensing strategies for mercury and other heavy metals.

Low-Temperature Synthesis of Carbon-Rich Nanoparticles with a Clickable Surface for Functionalization.

Carbon nanoparticles (CNPs) are promising materials for optoelectronic and biomedical applications thanks to their optical properties, low production cost, and superior biocompatibility compared to traditional semiconductor quantum dots. The countless synthetic methods reported allow a library of diverse CNP structures and optical properties, guiding their subsequent applications. However, the current drawbacks lie mainly within these synthetic processes, as many of them require harsh conditions preventing control over morphology and often generating chemically inert nanoparticles. Thus, more advances on low temperature and controllable synthetic processes are desirable. In this study, we suggest a new strategy to synthesize CNPs with tunable size, while avoiding the use of harsh conditions and allowing easy surface functionalization. The metastable state of polyyne-containing materials appoints them as ideal precursors for low-temperature preparation of carbon-rich structures. Our approach is to synthesize octatetrayne-containing particles prompt to spontaneous reaction, including topochemical polymerization, followed by aromatization, to avoid harsh carbonization steps. For the particle synthesis, the well-known dispersion polymerization process has been adapted for homocoupling of terminal butadiynes, generating the octatetrayne-containing particles. The method was proven reproducible, scalable, and versatile, as the particles' size can be modulated between 50 and 170 nm. Surface functionalization via thiol-yne click chemistry was completed with a pyrene-modified thiol ligand to provide the CNPs with photoactive properties in the visible range. The functionalized particles exhibit fluorescence at 470 nm arising from excimer formation.

Discriminating Lipid- from Protein-Calcium Binding To Understand the Interaction between Recoverin and Phosphatidylglycerol Model Membranes.

Recoverin is a protein involved in the phototransduction cascade by regulating the activity of rhodopsin kinase through a calcium-dependent binding process at the surface of rod outer segment disk membranes. Understanding how calcium modulates these interactions and how it interacts with anionic lipid membranes is necessary to gain insights into the function of recoverin. In this work, infrared spectroscopy allowed us to show that the availability of calcium to recoverin is modulated by the presence of complexes involving phosphatidylglycerol (PG), which in turn regulates its interactions with this negatively charged lipid. Calcium can indeed be sequestered into strongly bound complexes with PG and is thus sparingly available to recoverin. The thermal stability of recoverin then decreases, which results in weakened interactions with PG. By contrast, when calcium is fully available to recoverin, the protein is thermally stable, indicating that it binds two calcium ions, which results in favorable interactions with negatively charged lipids. Consequently, the protein induces an increase in the chain-melting phase transition temperature of PG, which is indicative of an enhanced lipid chain packing resulting from the peripheral location of the protein. The secondary structure of recoverin is not affected by its interactions with anionic membrane lipids. Similar results have been obtained with saturated and unsaturated anionic lipids. This work shows that the recruitment of recoverin at the surface of anionic lipid membranes is dependent on the availability of calcium.

Regioselective Synthesis of Nanographenes by Photochemical Cyclodehydrochlorination.

Novel nanographenes were prepared by a photochemical cyclodehydrochlorination (CDHC) reaction. Chlorinated precursors were irradiated in acetone in the presence of a base or in pure benzene and underwent multiple (up to four) regioselective cyclization reactions to provide rigid π-conjugated molecules. Pure compounds were recovered in good yields by simple filtration at the end of the reaction. The CDHC reaction showed compatibility with both electron-poor and electron-rich substrates, thus allowing the synthesis of pyridine- and thiophene-fused nanographenes. It also enabled the synthesis of sterically hindered contorted π-conjugated molecules without causing full aromatization. A kinetic study showed that the CDHC reaction under the conditions used is a very fast process, and some reactions are completed within minutes. The CDHC reaction thus shows great potential as an alternative to other reactions involving harsher conditions for the preparation of nanographenes.

The thermal stability of recoverin depends on calcium binding and its myristoyl moiety as revealed by infrared spectroscopy.

To evaluate the structural stability of recoverin, a member of the neuronal calcium sensor family, the effect of temperature, myristoylation, and calcium:protein molar ratio on its secondary structure has been studied by transmission infrared spectroscopy. On the basis of the data, the protein predominantly adopts α-helical structures (∼50-55%) with turns, unordered structures, and ß-sheets at 25 °C. The data show no significant impact of the presence of calcium and myristoylation on secondary structure. It is found that, in the absence of calcium, recoverin denatures and self-aggregates while being heated, with the formation of intermolecular antiparallel ß-sheets. The nonmyristoylated protein (Rec-nMyr) exhibits a lower temperature threshold of aggregation and a higher intermolecular ß-sheet content at 65 °C than the myristoylated protein (Rec-Myr). The former thus appears to be less thermally stable than the latter. In the presence of excess calcium ions (calcium:protein ratio of 10), the protein is thermally stable up to 65 °C with no significant conformational change, the presence of the myristoyl chain having no effect on the thermal stability of recoverin under these conditions. A decrease in the thermal stability of recoverin is observed as the calcium:protein molar ratio decreases, with Rec-nMyr being less stable than Rec-Myr. The data overall suggest that a minimal number of coordinated calcium ions is necessary to fully stabilize the structure of recoverin and that, when bound to the membrane, i.e., when the myristoyl chain protrudes from the interior pocket, recoverin should be more stable than in a Ca-free solution, i.e., when the myristoyl chain is sequestered in the interior.