Pyrene-Functionalized Nanoparticles: Two Independent Sensors, the Excimer and the Monomer

The high surface-to-volume ratio of nanoparticles has been used to obtain a high local concentration of pyrene units on their periphery, making the formation of both pyrene emissive species possible using amazingly small pyrene concentrations. The sensing properties of model pyrene-functionalized nanoparticles was investigated by using different nitroaromatic compounds [<i>m</i>-nitroaniline and <i>p</i>-nitroaniline] and nitrobenzenes [nitrobenzene, <i>p</i>-nitrotoluene, 2,4-dinitrotoluene, and 2,6-dinitrotoluene]. The hybrid system acts as a dual-fluorescence sensor, in which the decrease of the pyrene emission, induced by the quencher, is hardly reflected in the pyrene excimer emission. The encapsulation capacity of the NPs also plays a key role in their sensitivity to the analyte

Texture and Phase Recognition Analysis of β‑NaYF₄ Nanocrystals

Texture and phase recognition analysis (TPRA) based on electron nanodiffraction technique is used to characterize the geometry of up-conversion nanocrystals (UCNCs) synthesized by the common thermal-decomposition protocol in the presence of a stoichiometric amount of NH₄F. Here, we confirmed experimentally that despite the apparently different shapes of samples (hexagons, rods, and cubes), all the nanocrystals are actually β-phase hexagonal prisms. This is of relevance since many biological features of nanostructures, such as cellular internalization and cytotoxicity, are governed by their geometry. In addition, reproducibility in biological experiments is paramount

Sensitive and Selective Plasmonic Assay for Spermine as Biomarker in Human Urine

A simple, fast, and highly selective and sensitive colorimetric assay to detect nanomolar levels of spermine in human urine (healthy donors, cancer patients) is reported. This assay is based on the absence of a competitive organic capping on the gold nanoparticles together with the high affinity of the amine groups of the analyte for the nanoparticle surface

Unconventional Fluorescence Quenching in Naphthalimide-Capped CdSe/ZnS Nanoparticles

Core–shell (CS) CdSe/ZnS quantum dots (QD) capped with ligands that possess a mercapto or an amino group and a naphthalimide (NI) as chromophore unit, linked by a short ethylene chain (CS@S–NI and CS@H₂N–NI, respectively), have been synthesized and fully characterized by infrared and nuclear magnetic resonance spectroscopies, high-resolution transmission electron microscopy, and voltammetry as well as by steady-state absorption and emission spectroscopies. The organic ligands HS–NI and H₂N–NI act as bidentate ligands, thereby causing a drastic decrease in the QD emission. This was particularly evident in the case of CS@S–NI. This behavior has been compared with that of commercially available QDs with octadecylamine as the surface ligand and a QD capped with decanethiol ligands (CS@S–D). The interaction between the anchor groups and the QD surface brings about different consequences for the radiative and nonradiative kinetics, depending on the nature of the anchor group. Our results suggest that the naphthalimide group “stabilizes” empty deep trap states due to the carbonyl group capacity to act as both a σ-donor and a π-acceptor toward cations. In addition, the thiolate group can induce the location of electron density at shallow trap states close to the conduction band edge due to the alteration of the QD surface provoked by the thiolate binding

Thin Amphiphilic Polymer-Capped Upconversion Nanoparticles: Enhanced Emission and Thermoresponsive Properties

Upconversion nanoparticles (β-NaYF₄:Er³⁺, Yb³⁺, UCNPs) were capped with a thin polymer shell by replacing the oleate ligand of hydrophobic UCNPs by multidentate thiolate-grafting of P­(MEO₂MA-<i>co</i>-SEMA) copolymers. The presence of the 2-(2-methoxyethoxy)­ethyl side chains of MEO₂MA extending out of the nanohybrid made them water-dispersible. The UCNP@P­(MEO₂MA-<i>co</i>-SEMA) nanohybrids exhibited an enhanced emission by up to a factor of 10, as compared with that of their hydrophobic precursor in dichloromethane and even in water (a factor of 2). Moreover, their thermoresponsiveness was modulated by the pH; this is consistent with the presence of some thiol groups at the nanohybrid periphery. Remarkably, the nanohybrid emission, as well as its stability, was almost independent of the aggregation state (in the basic-acid and temperature range studied here). The formation of stable water-dispersible UCNPs with enhanced emission, together with their amphiphilic and temperature-responsive polymer coating, is promising for building multifunctional nanostructures for intracellular imaging, therapy, and drug delivery

Colloids of Naked CH₃NH₃PbBr₃ Perovskite Nanoparticles: Synthesis, Stability, and Thin Solid Film Deposition

A novel preparation of lead halide, CH₃NH₃PbBr₃, perovskite nanoparticle solid films from colloidal “naked” nanoparticles, that is, dispersible nanoparticles without any surfactant, is reported. The colloids are obtained by simply adding potassium ions, whose counterions are both more lipophilic and less coordinating than bromide ions, to the perovskite precursor solutions (CH₃NH₃Br/PbBr₂ in dimethylformamide) following the reprecipitation strategy. The naked nanoparticles exhibit a low tendency to aggregate in solution, and they effectively self-assembled on a substrate by centrifugation of the colloid, leading to homogeneous nanoparticle solid films with arbitrary thickness. These results are expected to spur further the interest in lead halide perovskites due to the new opportunities offered by these films

In Situ Colorimetric Quantification of Silver Cations in the Presence of Silver Nanoparticles

Silver ions (Ag⁺) can be quantified in situ in the presence of AgNPs by using a colorimetric sensing probe (3,3′,5,5′-tetramethylbenzidine). Interestingly, it also enables detection of the Ag⁺ adsorbed on the AgNP surface. This is relevant to design new methods to make AgNPs while ensuring the total reduction of Ag⁺

Delayed Luminescence in Lead Halide Perovskite Nanocrystals

The mechanism responsible for the extremely long photoluminescence (PL) lifetimes observed in many lead halide perovskites is still under debate. While the presence of trap states is widely accepted, the process of electron detrapping back to the emissive state has been mostly ignored, especially from deep traps as these are typically associated with nonradiative recombination. Here, we study the photophysics of methylammonium lead bromide perovskite nanocrystals (PNCs) with a photoluminescence quantum yield close to unity. We show that the lifetime of the spontaneous radiative recombination in PNCs is as short as 2 ns, which is expected considering the direct bandgap character of perovskites. All longer (up to microseconds) PL decay components result from the rapid reversible processes of multiple trapping and detrapping of carriers with a slow release of the excitation energy through the spontaneous emission channel. As our modeling shows, the trap (dark) and excitonic states are coupled by the trapping–detrapping processes so that they follow the same population decay kinetics, while a majority of excited carriers are in the dark state. The lifetime of the PNCs delayed luminescence is found to be determined by the depth of the trap states, lying from a few tens to hundreds meV below the emitting excitonic state. The delayed luminescence model proposed in this work can serve as a basis for the interpretation of other photoinduced transient phenomena observed in lead halide perovskites

Nontemplate Synthesis of CH₃NH₃PbBr₃ Perovskite Nanoparticles

To date, there is no example in the literature of free, nanometer-sized, organolead halide CH₃NH₃PbBr₃ perovskites. We report here the preparation of 6 nm-sized nanoparticles of this type by a simple and fast method based on the use of an ammonium bromide with a medium-sized chain that keeps the nanoparticles dispersed in a wide range of organic solvents. These nanoparticles can be maintained stable in the solid state as well as in concentrated solutions for more than three months, without requiring a mesoporous material. This makes it possible to prepare homogeneous thin films of these nanoparticles by spin-coating on a quartz substrate. Both the colloidal solution and the thin film emit light within a narrow bandwidth of the visible spectrum and with a high quantum yield (ca. 20%); this could be advantageous in the design of optoelectronic devices

Photoluminescence Enhancement of CdSe Quantum Dots: A Case of Organogel–Nanoparticle Symbiosis

Highly fluorescent organogels (QD–organogel), prepared by combining a pseudopeptidic macrocycle and different types of CdSe quantum dots (QDs), have been characterized using a battery of optical and microscopic techniques. The results indicate that the presence of the QDs not only does not disrupt the supramolecular organization of the internal fibrillar network of the organogel to a significant extent, but it also decreases the critical concentration of gelator needed to form stable and thermoreversible organogels. Regarding the photophysical properties of the QDs, different trends were observed depending on the presence of a ZnS inorganic shell around the CdSe core. Thus, while the core–shell QDs preserve their photophysical properties in the organogel medium, a high to moderate increase of the fluorescence intensity (up to 528%) and the average lifetime (up to 1.7), respectively, was observed for the core QDs embedded in the organogel. The results are relevant for the development of luminescent organogels based on quantum dots, which have potential applications as advanced hybrid materials in different fields