Light Management in Upconverting Nanoparticles: Ultrasmall Core/Shell Architectures to Tune the Emission Color

Ultrasmall NaGdF₄ nanoparticles with core/shell and core/shell/shell architectures have been synthesized following a microwave-based thermolysis procedure, allowing us to rapidly obtain homogeneous nanoparticles compared to conventional heating. To analyze the possibilities of the proposed structure in terms of tuning the emission color, core and shells have been doped with different lanthanide ion pairs (either Er³⁺/Yb³⁺ and/or Tm³⁺/Yb³⁺), keeping them therefore spatially separated inside the different layers of the nanoparticles. Here, we demonstrate that the position of the dopants inside the nanoparticles affects the intensity of the different emission bands of the luminescing Tm³⁺ and Er³⁺ ions and show how it has a relevant effect on the overall emission color of the luminescence obtained after 975 nm excitation

Effect of Surface Oxidation on the Interaction of 1-Methylaminopyrene with Gold Nanoparticles

The effect of the surface chemistry of gold nanoparticles (GNPs) on the GNP–amine (−NH₂) interaction was investigated via conjugating an amine probe1-methylaminopyrene (MAP) chromophorewith three Au colloidal samples of the same particle size yet different surface chemistry. The surface of laser-irradiated and ligand-exchanged-irradiated GNPs is covered with acetonedicarboxylic ligands (due to laser-introduced citrate oxidization) and citrate ligands, respectively, and both surfaces contain oxidized Au species which are essentially lacking for the citrate-capped GNPs prepared by the pure chemical approach. Both laser-irradiated samples show inferior adsorption capacity of MAP as compared with the purely chemically prepared GNPs. Detailed investigations indicate that MAP molecules mainly complex directly with Au atoms via forming Au-NH₂R bonds, and the oxidization of the GNP surface strongly influences the ratio of this direct bonding to the indirect bonding originating from the electrostatic interaction between protonated amine (−NH₃⁺) and negatively charged surface ligands. The impact of the oxidized GNP surface associated with the laser treatment is further confirmed by aging experiment on GNP–MAP conjugation systems, which straightforwardly verifies that the surface oxidation leads to the decrease in the MAP adsorption on GNPs

Yolk–Shell Nanoparticles with CO₂‑Responsive Outer Shells for Gas-Controlled Catalysis

Yolk–shell nanoparticles (YSNPs) consisting of a gold nanoparticle core, a CO2-responsive crosslinked poly­(N,N-diethylaminoethyl methacrylate) shell, and a void in between, denoted as Au@void@PDEAEMA, were synthesized, characterized, and utilized as a catalyst for the gas-controllable reduction of 4-nitrophenol (4-NP) and nitrobenzene (NB). We show that the rate of the catalytic reaction can be regulated by controlling the diffusion of reactant molecules through the polymer shell by switching the latter between a hydrophilic state upon CO2 bubbling and a hydrophobic state by bubbling N2 to remove CO2. While for the reduction of 4-NP, it is possible to turn on and turn off the reaction through alternating bubbling of the two gases, with a mixture of 4-NP and NB, the respective reactions of either reactant can be selectively favored using the gases (CO2 for the relatively faster reaction of 4-NP and N2 for the faster reaction of NB). Moreover, the gas-controlled reversible dispersion and the agglomeration state of Au@void@PDEAEMA were explored for in situ recovery of the nanoparticle catalyst and their reuse for catalytic reactions without using ultracentrifugation. Finally, the effect of structural parameters of such YSNPs on the catalytic activity was investigated by either varying the thickness of the polymer shell at the same void space or changing the void while keeping the same polymer shell thickness

High-Efficiency Broadband C₃N₄ Photocatalysts: Synergistic Effects from Upconversion and Plasmons

A plasmon and upconversion enhanced broadband photocatalyst based on Au nanoparticle (NP) and NaYF₄:Yb³⁺, Er³⁺, Tm³⁺ (NYF) microsphere loaded graphitic C₃N₄ (g-C₃N₄) nanosheets (Au-NYF/g-C₃N₄) was subtly designed and synthesized. The simple one-step synthesis of NYF in the presence of g-C₃N₄, which has not been reported in the literature either, leads to both high NYF yield and high coupling efficiency between NYF and g-C₃N₄. The Au-NYF/g-C₃N₄ structure exhibits high stability, wide photoresponse from the ultraviolet (UV), to visible and near-infrared regions, and prominently enhanced photocatalytic activities compared with the plain g-C₃N₄ sample in the degradation of methyl orange (MO). In particular, with the optimization of Au loading, the rate constant normalized with the catalysts mass of the best-performing catalyst 1 wt % Au-NYF/g-C₃N₄ (0.032 h^–1 mg^–1) far surpasses that of NYF/g-C₃N₄ and g-C₃N₄ (0.009 h^–1 mg^–1) by 3.6 times under λ > 420 nm light irradiation. The high performance of the Au-NYF/g-C₃N₄ nanocomposite under different light irradiations was ascribed to the distinctively promoted charge separation and suppressed recombination, and the efficient transfer of charge carriers and energy among these components. The promoted charge separation and transfer were further confirmed by photoelectrochemical measurements. The 1 wt % Au-NYF/g-C₃N₄ exhibits enhanced photocurrent density (∼6.36 μA cm^–2) by a factor of ∼5.5 with respect to that of NYF/g-C₃N₄ sample (∼1.15 μA cm^–2). Different mechanisms of the photodegradation under separate UV, visible, and NIR illuminations are unveiled and discussed in detail. Under simulated solar light illumination, the involved reactive species were identified by performing trapping experiments. This work highlights the great potential of developing highly efficient g-C₃N₄-based broadband photocatalysts for full solar spectrum utilization by integrating plasmonic nanostructures and upconverting materials

Tuning the Charge-Transfer Property of PbS-Quantum Dot/TiO₂-Nanobelt Nanohybrids via Quantum Confinement

A newly designed photoactive nanohybrid structure based on the combination of near-infrared PbS quantum dots (QDs) as light harvester and one-dimensional TiO₂ nanobelts (NBs) to guide the flow of photogenerated charge carriers is reported. Efficient electron transfer from photoexcited PbS QDs to TiO₂ NBs has been demonstrated to occur in the developed PbS-QD/TiO₂-NB nanohybrids, and the charge-transfer property can be tuned through the size quantization effect of PbS QDs. Moreover, the use of TiO₂ NBs instead of TiO₂ NPs permits a larger critical size of PbS QDs capable of injecting electrons into TiO₂ NBs, which, in turn, markedly extends the “effective” absorption of the PbS-QD/TiO₂-NB nanohybrids to a longer wavelength region up to 1400 nm. Such an extension of the “effective” absorption is a major asset for improving the overall photoconversion efficiency of PbS-QD/TiO₂-NB nanohybrids-based photovoltaic devices

Size Dependence of Temperature-Related Optical Properties of PbS and PbS/CdS Core/Shell Quantum Dots

The effect of PbS core size on the temperature-dependent photoluminescence (PL) of PbS/CdS quantum dots (QDs) in the temperature range of 100–300 K was thoroughly investigated and compared with shell-free PbS QDs. The core/shell QDs show significantly smaller PL intensity variation with temperature at a smaller PbS size, while a larger activation energy when the PbS domain size is relatively large, suggesting both different density and different distribution of defects/traps in the PbS and PbS/CdS QDs. The most remarkable difference consists in the PbS size dependence of the energy gap temperature coefficient (d<i>E</i>/d<i>T</i>). The PbS/CdS QDs show unusual non-monotonic d<i>E</i>/d<i>T</i> variation, resulting in the reversal of the d<i>E</i>/d<i>T</i> difference between the PbS and PbS/CdS QDs at a larger PbS size. In combination with theoretical calculations, we find that, although lattice dilation and carrier-phonon coupling are generally considered as dominant terms, the unique negative contribution to d<i>E</i>/d<i>T</i> from the core/shell interfacial strain becomes most important in the relatively larger-core PbS@CdS QDs

Detection of Adenosine Triphosphate with an Aptamer Biosensor Based on Surface-Enhanced Raman Scattering

A simple, ultrasensitive, highly selective, and reagent-free aptamer-based biosensor has been developed for quantitative detection of adenosine triphosphate (ATP) using surface-enhanced Raman scattering (SERS). The sensor contains a SERS probe made of gold nanostar@Raman label@SiO₂ core–shell nanoparticles in which the Raman label (malachite green isothiocyanate, MGITC) molecules are sandwiched between a gold nanostar core and a thin silica shell. Such a SERS probe brings enhanced signal and low background fluorescence, shows good water-solubility and stability, and exhibits no sign of photobleaching. The aptamer labeled with the SERS probe is designed to hybridize with the cDNA on a gold film to form a rigid duplex DNA. In the presence of ATP, the interaction between ATP and the aptamer results in the dissociation of the duplex DNA structure and thereby removal of the SERS probe from the gold film, reducing the Raman signal. The response of the SERS biosensor varies linearly with the logarithmic ATP concentration up to 2.0 nM with a limit of detection of 12.4 pM. Our work has provided an effective method for detection of small molecules with SERS

Plasmonic Nanorice Antenna on Triangle Nanoarray for Surface-Enhanced Raman Scattering Detection of Hepatitis B Virus DNA

The sensitivity and the limit of detection of Raman sensors are limited by the extremely small scattering cross section of Raman labels. Silver nanorice antennae are coupled with a patterned gold triangle nanoarray chip to create spatially broadened plasmonic “hot spots”, which enables a large density of Raman labels to experience strong local electromagnetic field. Finite difference time domain simulations have confirmed that the quasi-periodic structure increases the intensity and the area of the surface plasmon resonance (SPR), which enhances the surface-enhanced Raman scattering (SERS) signal significantly. The SERS signal of the nanorice/DNA/nanoarray chip is compared with that of the nanorice/DNA/film chip. The SERS signal is greatly enhanced when the Ag nanorices are coupled to the periodic Au nanoarray instead of the planar film chip. The resulting spatially broadened SPR field enables the SERS biosensor with a limit of detection of 50 aM toward hepatitis B virus DNA with the capability of discriminating a single-base mutant of DNA. This sensing platform can be extended to detect other chemical species and biomolecules such as proteins and small molecules

Silver Nanorice Structures: Oriented Attachment-Dominated Growth, High Environmental Sensitivity, and Real-Space Visualization of Multipolar Resonances

We have synthesized and investigated the anisotropic growth of interesting silver nanorice. Its growth is kinetically controlled at 100 °C, and both oriented attachment and Ostwald ripening are involved, with the former growth mode dominating the anisotropic growth of the nanorice along the ⟨111⟩ direction. This one-directional growth is initiated by an indispensable seed-selection process, in which oxygen plays a critical role in oxidatively etching twinned silver crystals. The inhibition of this process by removing oxygen essentially blocks the nanorice growth. Although increasing reaction temperature to 120 °C accelerates the one-dimensional growth along the ⟨111⟩ direction, further temperature increase to 160 °C makes the oriented attachment dominated one-directional growth disappear; instead, the diffusion-controlled two-dimensional growth leads to the emergence of highly faceted truncated triangular and hexagonal plates mainly bound by low energy faces of {111}. Interestingly, we also found that the longitudinal surface plasmon resonance of the nanorice structures is highly sensitive to the refractive index of surrounding dielectric media, which predicts their promising applications as chemical or biological sensors. Moreover, the multipolar plasmonic resonances in these individual nanorice structures are visualized in real space, using high-resolution electron energy-loss spectroscopy

Facile and Mild Strategy to Construct Mesoporous CeO₂–CuO Nanorods with Enhanced Catalytic Activity toward CO Oxidation

CeO₂–CuO nanorods with mesoporous structure were synthesized by a facile and mild strategy, which involves an interfacial reaction between Ce₂(SO₄)₃ precursor and NaOH ethanol solution at room temperature to obtain mesoporous CeO₂ nanorods, followed by a solvothermal treatment of as-prepared CeO₂ and Cu­(CH₃COO)₂. Upon solvothermal treatment, CuO species is highly dispersed onto the CeO₂ nanorod surface to form CeO₂–CuO composites, which still maintain the mesoporous feature. A preliminary CO catalytic oxidation study demonstrated that the CeO₂–CuO samples exhibited strikingly high catalytic activity, and a high CO conversion rate was observed without obvious loss in activity even after thermal treatment at a high temperature of 500 °C. Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and hydrogen temperature-programmed reduction (H₂-TPR) analysis revealed that there is a strong interaction between CeO₂ and CuO. Moreover, it was found that the introduction of CuO species into CeO₂ generates oxygen vacancies, which is highly likely to be responsible for high catalytic activity toward CO oxidation of the mesoporous CeO₂–CuO nanorods