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A fluorescent biosensor for real-time monitoring the release of Zn 2+ in plants was constructed through immobilization of DNAzyme-containing hairpin DNA on nanofertilizer ZnO@Au nanoparticles (ZnO@Au NPs). A specially designed hairpin DNA containing both DNAzyme and its substrate sequence, which was also labeled with 5'-FAM and 3'-SH groups, was modified on ZnO@Au NPs through the Au-S bond. The fluorescent signal of FAM was initially quenched by AuNPs. When Zn 2+ was released from ZnO@Au NPs, DNAzyme was activated and the substrate sequence in hairpin DNA was cleaved. The restored fluorescent signal in Tris-HCl buffer (pH 6.5) was correlated with the concentration of the released Zn 2+ . The performance of the biosensor was first demonstrated in the solution. The linear detection range was from 50 nM to 1.5 μM, with a detection limit of 30 nM. The biosensor system can penetrate into maize leaves with ZnO@Au NPs. With the release of Zn 2+ in leaves, the restored fluorescence can be imaged by a confocal laser scanning microscope and used for monitoring the release and distribution of Zn 2+ . This work may provide a novel strategy for tracing and understanding the mechanism of nanofertilizers in organisms.

The insufficient utilization of sunlight of ZnO, due to its broad band gap, results in low efficiency for photocatalytic hydrogen production. In this work, plasmonic noble metal nanoparticles (NPs) with different shapes (spheres and rods) were combined with mesoporous ZnO forming core-shell nanostructure to enhance the photocatalytic efficiency of ZnO in visible-light region. The photoelectrochemical water splitting activities of the metal@ZnO core-shell nanocomposites (NCs) were investigated. The photocurrent response of metal@ZnO NCs was found higher than pure ZnO or the mixture of metal NPs and ZnO ascribed to the effective charge transfer mechanism. It was also found that the photocurrent of metal@ZnO NCs was related to the thickness of ZnO and there was optimized shell for each kind of metal cores. Moreover, the introduction of Ag shell can get a higher photoelectrocatalytic efficiency compared to pure Au NPs core due to lower Schottky barrier between Ag and ZnO and wider extinction range in the visible light of Au@Ag NPs., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

By solving the continuity equation of excitons under steady excitation, a theoretical model for the excitonic luminescence of semiconductors was developed taking into account the exciton diffusion and surface recombination. The theoretical model was used to analyze the photoluminescence (PL) spectra of ZnO obtained from the bulk single-crystal samples with and without surface passivation, showing that the nonradiative recombination on the surface is an important channel of losing excitons, thus substantially reducing the PL quantum efficiency of excitons at room temperature. In addition, the surface recombination was found to have impacts on the fine structure of excitonic luminescence at low temperature. Using the theoretical model, the diffusion length of excitons at room temperature was estimated and found to be different from sample to sample, strongly depending on the sample processing. The theoretical model was demonstrated to be capable of accurately fitting the temperature-dependent PL intensity of passivated samples and showed that the exciton diffusion has significant impacts on the dynamics of excitonic luminescence at high temperature. [ABSTRACT FROM AUTHOR]

The effects of surface conditions on photo-induced-carrier (PIC) diffusion are studied by solving one-dimensional continuity equation. Using various forms of ZnO samples with different surface conditions, the PICs lost at sample surface are substantiated to have important contributions to the visible luminescence. For unpassivated ZnO nanorods and single-crystal samples that undergo a high-temperature annealing in air, the visible luminescence originating from the PICs diffused to the surface is estimated more than 99.8 % and 70 % at room temperature, respectively, and more than 99.99 % and 97 % at low temperature. The structureless luminescence caused by PICs, such as the green, yellow, and orange luminescence bands are well fitted under the framework of tail-state luminescence, while the structured green band is suggested to be in association with a specific type of surface states formed during high-temperature annealing in oxidizing atmosphere and can be fitted with Huang-Rhys model by taking into account tail-state luminescence. [ABSTRACT FROM AUTHOR]

Co-doped ZnO was studied using first-principles methods with comparison to experimental results taken from epitaxial Zn 1− x Co x O ( x ~0.05) films. Density of Co 2+ ions was determined using absorption spectra for the first time, and then a definite correlation between metallic Co clusters and the magnetism of the ZnCoO films was proved and the average number of Co atoms in the metallic Co clusters was estimated to be less than 200 using a superparamagnetic model. First-principles calculations of ZnCoO alloys and the relevant problems were discussed by comparing the electronic structures with absorption spectra and the results calculated by Tanabe-Sugano theory. U correction was proved to be necessary for calculating the band-gap energy of ZnCoO alloys, but other optical properties related to Co 2+ ions are incorrect and the conclusion for magnetic properties is ambiguous due to uncertainty of the calculated highly localized states, which are in pressing for solution in study of material properties relevant to electronic structure. [ABSTRACT FROM AUTHOR]

The flower-like Ag-ZnO nanorod exhibited a high SERS activity and a long-term optical stability with a shelf life longer than two years. • The flower-like Ag-ZnO NRs exhibited high SERS activity and enable 10−14 M R6G to be detected. • The SERS substrates have a shelf life longer than two years prior to use. • Electron transfer between Ag and ZnO was proved to be important for the SERS stability. The optical stability of silver-coated flower-like ZnO nanorod (NR) arrays were studied in terms of localized surface plasmon resonance and surface-enhanced Raman spectroscopy (SERS). As a highly sensitive SERS substrate, the silver-coated ZnO NR arrays enabled as low as 10−14 M Rhodamine-6G (R6G) molecules to be detected and exhibited a long-term optical stability with a shelf life longer than two years. Using specially prepared samples, the Ag-ZnO nanostructures were studied as a metal–semiconductor junction and the importance of electron transfer between the silver and ZnO NRs for optical stability of silver in the SERS substrates was demonstrated. By addressing the growth behavior of silver in different samples, the good crystallinity of silver was suggested to be another crucial factor of maintaining the SERS activity long-term stable. [ABSTRACT FROM AUTHOR]

ZnO nanorod (NR) arrays in-situ grown on indium tin oxide (ITO) substrates by a solution method are investigated by optimizing the solution concentration and the thickness of ZnO seed layer. Using 5–1000 ppm ethanol as the target gas, the gas sensor prepared with as-grown ZnO NR arrays in 25 mM solution with ∼ 10-nm thick ZnO seed layer is found to have the best gas response at 250 °C and exhibit a pretty good stability. The morphology of ZnO NR arrays is found important for optimizing the gas sensing property and is predominated by coalescence, which varies with solution concentration and the thickness of ZnO seed layer, thus leading to a variation in the defect density and surface state of ZnO NRs and enabling the gas sensing property not to be determined only by the specific surface area of ZnO NRs. Different thick ZnO seed layers will convert into the low-resistance states within a few seconds to minutes after applying an electric voltage and thus the role of ZnO seed layers in determining the gas sensing performance is mainly to control the diameter distribution of ZnO NRs, in which the arrays with a relatively large difference in the diameter of ZnO NRs would exhibit good gas response. [Display omitted] • Gas sensing property of ZnO NR sensors is determined by the morphology. • Coalescence dominates the morphology of ZnO NRs and varies with solution. • A large difference in the diameters of ZnO NRs leads to a good gas response. • Thickness of ZnO seed layer is important for optimizing ZnO NR sensors. • ZnO seed layer on ITO converts into low-resistance state within a short time. [ABSTRACT FROM AUTHOR]

Due to the high sensitivity and lithographic compatibility, gas sensors prepared with ZnO nanorod (NR) arrays in-situ grown on substrates receive much attention. In this work, ZnO NR arrays are grown on ITO glass by a solution method and the thermal stability is studied. It is found that the instability of as-grown sample can be associated with the metastable defects formed during the growth of ZnO NR arrays and the variation of surface states during the measurement. After high-temperature annealing, the thermal stability of ZnO NR arrays are improved, while the best working temperature is shifted to ∼350 °C. By studying the effects of annealing temperature on the thermal stability, the lowest annealing temperature is suggested to be 400 °C for stabilizing ZnO NR arrays. After annealing at 400 °C in air, the ZnO NR arrays exhibit sufficiently good reproducibility with the gas response of ∼45. Based on the dependences of electric currents on working temperature and O 2 partial pressure, chemisorbed O− is suggested to play a predominant role in detecting reducing gases for the ZnO NR arrays and the best working temperature is probably in association with the conversion from irreversible to reversible chemisorption of O−. • Instability of as-grown ZnO NRs is related to instable defects and surface states. • The best working temperature of annealed ZnO NR arrays is shifted to ∼350 °C. • 400 °C is the lowest annealing temperature for stabilizing ZnO NR arrays. • Chemisorbed O− plays a predominant role in detecting reducing gases. • Irreversible to reversible chemisorption decides the best gas response. [ABSTRACT FROM AUTHOR]

Chloride solutions are able to modify the morphology of Ag-coated ZnO nanorod arrays and enhanced the Raman signals of R6G molecules by enhancing adsorption. [Display omitted] • Cl− solution is able to modify the Ag-decorated ZnO nanorod arrays. • Modified SERS substrates depress the molecule thermalization during detection. • Cl− ions is able to enhance the adsorption of R6G on Ag surface. • Chemical enhancement is due to the enhanced R6G adsorption. Chloride solutions are used to modify flower-like Ag-coated ZnO nanorod (Ag-ZnO-NR) arrays, leading to the formation of Ag-nanosphere decorated ZnO-NR arrays, which have a morphology with each sphere-like Ag nanoparticle independently lying at the top of ZnO-NR and thus can be used as a new kind of substrates for the study of surface-enhanced Raman spectroscopy (SERS). The chloride modification is dominated by Ag abnormal migration on ZnO NRs in the environment of chloride solution. The concentration of chloride ions for effective modification is higher than ∼1 × 10−4 M and the final morphology is nearly steady as the soaking time is longer than 10 min. On the modified SERS substrates, the thermalization of probing molecules during Raman measurement is substantially depressed. Using the SERS samples prepared by loading the probing molecules in the presence of chloride ions, the Raman signals are obviously enhanced with an extremely high signal-to-noise ratio. The Raman enhancement is evidenced to be due to the enhanced adsorption of probing molecules on Ag surface in the presence of chloride ions. [ABSTRACT FROM AUTHOR]