Alexander Yu. Polyakov

Non-classical growth of water-redispersible spheroidal gold nanoparticles assisted by leonardite humate

The growth of gold nanoparticles (AuNPs) assisted by humate – a natural hyperbranched polyelectrolyte – was studied using in situ and ex situ techniques. The conditions for formation of almost monodisperse gold nanospheres of 13 ± 3 nm diameter with pronounced plasmonic properties were defined. A striking similarity was found between humate- and citrate-mediated growth of AuNPs: the formation of gold nanospheres involved rapid nucleation of gold seeds, slow growth within the intermediate agglomerates (visible as nanoworms after drying on a TEM grid), and rapid peptization into the final gold nanospheres. Both humate and citrate syntheses produced ultrastable gold sols with pronounced plasmonic properties. The substantial difference was slower kinetics of humate synthesis, 240 min versus 15 min for citrate, and the water redispersible properties of the humate-capped gold nanospheres after freeze drying, which was not seen with the citrate AuNPs. Theoretical calculations revealed a leading role of steric factors in the formation of intermediate aggregates of capped AuNPs at the stage of their slow growth in the case of both citrate and humate. We suggested that it was the polyelectrolyte nature of humate which enabled the water-redispersibility of humate- versus citrate-capped gold nanoparticles.

Silver Eco-Solvent Ink for Reactive Printing of Polychromatic SERS and SPR Substrates

A new reactive ink based on a silver citrate complex is proposed for a photochemical route to surface-enhanced Raman spectroscopy active substrates with controllable extinction spectra. The drop-cast test of the ink reveals homogeneous nucleation of silver and colloid particle growth originating directly from photochemical in situ reduction in droplets, while the following evaporation of the deposited ink produces small nano- and micron-size particles. The prepared nanostructures and substrates were accurately characterized by electron microscopy methods and optical extinction spectroscopy. Varying the duration of UV irradiation allows tuning the morphology of individual silver nanoparticles forming hierarchical ring structures with numerous “hot spots” for most efficient Raman enhancement. Raman measurements of probe molecules of rhodamine 6G and methylene blue reached the largest signal enhancement of 106 by the resonance effects.

Gold Decoration and Photoresistive Response to Nitrogen Dioxide of WS2 Nanotubes

Composites of WS2 nanotubes (NT-WS2 ) and gold nanoparticles (AuNPs) were prepared using aqueous HAuCl4 solutions and subjected to surface analysis. The obtained materials were jointly characterized by X-ray photoelectron (XPS), Raman scattering (RSS), and ultraviolet photoelectron (UPS) spectroscopies. Optical extinction spectroscopy and electron energy loss spectroscopy in the scanning transmission electron microscopy regime (STEM-EELS) were also employed to study plasmon features of the nanocomposite. It was found that AuNPs deposition is accompanied by a partial oxidative dissolution of WS2 , whereas Au-S interfacial species could be responsible for the tight contact of metal nanoparticles and the disulfide. A remarkable sensitivity of n-type resistance of NT-WS2 and Au-NT-WS2 to the adsorption of NO2 gas was also demonstrated at room temperature using periodical illumination by a 530 nm light-emitting diode. Au-NT-WS2 nanocomposites are found to possess a higher photoresponse and enhanced sensitivity in the 0.25-2.0 ppm range of NO2 concentration, as compared to the pristine NT-WS2 . This behaviour is discussed within the physisorption-charge transfer model to explore sensing properties of the nanocomposites.

Facile chemical routes to mesoporous silver substrates for SERS analysis

Mesoporous silver nanoparticles were easily synthesized through the bulk reduction of crystalline silver(I) oxide and used for the preparation of highly porous surface-enhanced Raman scattering (SERS)-active substrates. An analogous procedure was successfully performed for the production of mesoporous silver films by chemical reduction of oxidized silver films. The sponge-like silver blocks with high surface area and the in-situ-prepared mesoporous silver films are efficient as both analyte adsorbents and Raman signal enhancement mediators. The efficiency of silver reduction was characterized by X-ray diffraction and X-ray photoelectron spectroscopy. The developed substrates were applied for SERS detection of rhodamine 6G (enhancement factor of about 1–5 × 105) and an anti-ischemic mildronate drug (meldonium; enhancement factor of ≈102) that is known for its ability to increase the endurance performance of athletes.