Growth of Au nanostructures by annealing electron beam evaporated thin films

Aptamer-functionalized AuNPs for the high-sensitivity colorimetric detection of melamine in milk samples

Although aptamer-functionalized AuNPs technology exhibits great potential in analytical and biological chemistry, direct analysis of molecules at a low concentration using colorimetric assay remains challenging. The development of intuitive methods has attracted interest for promising detection of melamine in milk samples due to a demand for stable and understandable process. In this study, we propose a rapid and facile colorimetric measurement method of melamine combined aptamer-functionalized AuNPs in contaminated milk samples. To realize the high stability and the lower limit of detection, the aptamer-functionalized surface of AuNPs via a coordinated bond was used in combination with ultra-sonication. The kinetics of this colorimetric assay based on aptamer-functionalized AuNPs was analyzed to illustrate that the higher the concentration of melamine, the faster the aggregation of AuNPs induced. The sensitivity, selectivity, limit of detection and recovery rate were sufficiently validated to understand the measurement principle of melamine using aptamer-functionalized AuNPs. The calibration curve established by the absorption peak ratio (A640 /A520) was linear in the concentration range of 0~1μM of melamine in aqueous solutions with the correlation coefficient (R²) of 0.986 and the limit of detection (LOD) of 22 nM, whereas, the correlation coefficient (R²) of 0.998 and the LOD of 14.9 nM were achieved at the concentration of melamine below 0.5 μM in milk samples. The optimized performance of this colorimetric assay of melamine using aptamer-functionalized AuNPs in milk samples was obtained with 100 μL of 13 nm AuNPs solution, 40 μL of 1 μM (100 dilutions) aptamers and the pre-reaction time of 30 min. This simple colorimetric measurement of melamine using aptamer-functionalized AuNPs provides a promising target for various applications of the sample source with complex sample matrices.

Investigation on the morphological and optical evolution of bimetallic Pd-Ag nanoparticles on sapphire (0001) by the systematic control of composition, annealing temperature and time

Multi-metallic alloy nanoparticles (NPs) can offer additional opportunities for modifying the electronic, optical and catalytic properties by the control of composition, configuration and size of individual nanostructures that are consisted of more than single element. In this paper, the fabrication of bimetallic Pd-Ag NPs is systematically demonstrated via the solid state dewetting of bilayer thin films on c-plane sapphire by governing the temperature, time as well as composition. The composition of Pd-Ag bilayer remarkably affects the morphology of alloy nanostructures, in which the higher Ag composition, i.e. Pd_0.25Ag_0.75, leads to the enhanced dewetting of bilayers whereas the higher Pd composition (Pd_0.75Ag_0.25) hinders the dewetting. Depending on the annealing temperature, Pd-Ag alloy nanostructures evolve with a series of configurations, i.e. nucleation of voids, porous network, elongated nanoclusters and round alloy NPs. In addition, with the annealing time set, the gradual configuration transformation from the elongated to round alloy NPs as well as size reduction is demonstrated due to the enhanced diffusion and sublimation of Ag atoms. The evolution of various morphology of Pd-Ag nanostructures is described based on the surface diffusion and inter-diffusion of Pd and Ag adatoms along with the Ag sublimation, Rayleigh instability and energy minimization mechanism. The reflectance spectra of bimetallic Pd-Ag nanostructures exhibit various quadrupolar and dipolar resonance peaks, peak shifts and absorption dips owing to the surface plasmon resonance of nanostructures depending on the surface morphology. The intensity of reflectance spectra is gradually decreased along with the surface coverage and NP size evolution. The absorption dips are red-shifted towards the longer wavelength for the larger alloy NPs and vice-versa.

Thickness Dependent Nanostructural, Morphological, Optical and Impedometric Analyses of Zinc Oxide-Gold Hybrids: Nanoparticle to Thin Film

The creation of an appropriate thin film is important for the development of novel sensing surfaces, which will ultimately enhance the properties and output of high-performance sensors. In this study, we have fabricated and characterized zinc oxide (ZnO) thin films on silicon substrates, which were hybridized with gold nanoparticles (AuNPs) to obtain ZnO-Au_x (x = 10, 20, 30, 40 and 50 nm) hybrid structures with different thicknesses. Nanoscale imaging by field emission scanning electron microscopy revealed increasing film uniformity and coverage with the Au deposition thickness. Transmission electron microscopy analysis indicated that the AuNPs exhibit an increasing average diameter (5–10 nm). The face center cubic Au were found to co-exist with wurtzite ZnO nanostructure. Atomic force microscopy observations revealed that as the Au content increased, the overall crystallite size increased, which was supported by X-ray diffraction measurements. The structural characterizations indicated that the Au on the ZnO crystal lattice exists without any impurities in a preferred orientation (002). When the ZnO thickness increased from 10 to 40 nm, transmittance and an optical bandgap value decreased. Interestingly, with 50 nm thickness, the band gap value was increased, which might be due to the Burstein-Moss effect. Photoluminescence studies revealed that the overall structural defect (green emission) improved significantly as the Au deposition increased. The impedance measurements shows a decreasing value of impedance arc with increasing Au thicknesses (0 to 40 nm). In contrast, the 50 nm AuNP impedance arc shows an increased value compared to lower sputtering thicknesses, which indicated the presence of larger sized AuNPs that form a continuous film, and its ohmic characteristics changed to rectifying characteristics. This improved hybrid thin film (ZnO/Au) is suitable for a wide range of sensing applications.