A photoswitchable double-shell structure on Au nanoparticles, consisting of photochromic spiropyran as the first shell, which regulates the assembly and release of an outer shell of amino acid derivatives upon irradiation, is being reported for the first time. The light-regulated changes in the topographic properties of spiropyran-capped Au nanoparticles (i.e., interconversion between the zwitterionic and neutral forms) are exploited for the assembly and release of amino acid-based therapeutic agents such as l-DOPA.

Since many metallic nanostructures with different shapes exhibit unique chemical and physical properties, a systematic attempt to synthesize these shape-controlled structures for property-shape correlation remains an important challenge in contemporary materials chemistry. Main difficulties like poor shape selectivity, low yield, presence of impurity phases, difficulty of separation, etc., are exacerbated since metallic structures have high surface energies which favour lower surface areas, and consequently many synthesis strategies, including the use of hard and soft templates and external nucleating agents, are being employed along with theoretical guidelines from density functional calculations on simpler systems. One of the important application areas where these structures have gained profound attention is in electrocatalysis, where the kinetics of many structure-sensitive reactions of technological relevance have been experimentally observed to show drastic changes with shape especially in the nanosize domain, at least in one dimension. Considering their scientific and technological importance, this feature article provides an overview of the recent progress on the shapecontrolled synthesis of metallic nanostructures with special emphasis on platinum, and their crucial role in the electrocatalysis of anodic reactions for polymer electrolyte fuel cells.

Single-step preparation of smaller sized (ca. 3 nm, approximate composition Au 923 ATP 241 ) gold nanoparticles (AuNPs) followed by their self-assembly is demonstrated using 4-aminothiophenol (ATP) as a reducing agent in water/N,N-dimethylformamide (DMF). Water and DMF play a crucial role during the reduction process, since nanoparticles are formed neither in water nor in DMF alone at room temperature. Moreover, the morphology of the particles is found to be strongly dependent on the pH of the medium. The instantaneous UV-visible absorption spectrum shows a relatively sharp peak at 550 nm, which becomes a broad band after 1 h of mixing, due to the formation of aggregates. The size of the gold nanoparticles is controlled in the stipulated range by maintaining a critical AuCl 4 -/ATP ratio. Transmission electron microscopic images reveal close-packed assembly of gold nanoparticles induced by the bifunctionality of ATP. Powder X-ray diffraction patterns confirm the metallic face-centered cubic (fcc) lattice structure with ( 111), ( 200), ( 220), and (311) crystal planes. Thermogravimetric analysis shows 22% organic molecules on the surface of AuNPs. The molecular level analysis of the as prepared gold nanoparticles by Fourier transform infrared spectrum shows the presence of -SO stretching. X-ray photoelectron spectroscopic results also confirm the oxidation of -SH during the reduction of AuCl 4ions. The cyclic voltammograms of the monolayer-protected Au nanoparticles show quasi-reversible redox behavior, though the electrochemical features are different from those of the self-assembled monolayer (SAM) of ATP on a gold electrode.

We here demonstrate a remarkable potential-dependent morphological evolution of platinum mesostructures in the form of multipods, discs, and hexagons using a porous anodic alumina membrane (PAAM). These structures prepared potentiostatically at -0.7, -0.5 and -0.3 V, respectively, reveal unique shape-dependent electrocatalytic activity toward both formic acid and ethanol oxidation reactions. A comparison of the electrooxidation kinetics of these structures illustrates that hexagons show better performance toward formic acid oxidation whereas, for ethanol oxidation, multipods show significantly enhanced activity. Interestingly, the enhancement factor (R) for these mesostructures with respect to that of commercial platinized carbon toward formic acid oxidation ranges up to 2000% for hexagons whereas for multipods and disc they are about 700% and 300%, respectively. Similarly, for ethanol oxidation, the calculated value of R varies up to 600% for multipods while for disc and hexagons these values are 500% and 200%, respectively. These shape-dependent electrocatalytic activity of Pt mesostructures have been further correlated with XRD results. Thus, the present results demonstrate the importance of precise control of morphology by an electric field and their potential benefits especially for fuel cell applications since designing a better electrocatalyst for many fuel cell reactions continues to be an important challenge.

We here demonstrate the formation of bundles of RuO2 nanoneedles (ca. 100 nm diameter) by a template-assisted electrodeposition from aqueous RuCl3 solution under potentiostatic conditions at room temperature. Cyclic voltammetric measurements in 0.5 M H2SO4 show significantly higher redox-related charging behavior for the RuO2 nanoneedles compared to that of the commercial sample, which is also supported by the electrochemical impedance data. A comparison of the specific capacitance reveals a higher value for nanoneedles (3 F/g instead of 0.4 F/g for the bulk), which has been explained on the basis of enhanced reactivity. More interestingly, electrical transport measurements reveal a transition from metallic to semiconducting behavior especially at low-temperature caused by an impurity scattering mechanism. We anticipate that the present simple route for the fabrication of RuO2 nanostructures will be useful to exploit their potentials in various fields such as electrocatalysis, nanoelectronics, and more importantly for designing supercapacitors.

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Monolayer-protected quantum dots (Q-dots) show multivalent redox property, popularly known as the quantized double-layer (QDL) charging phenomena. In this report, we demonstrate the QDL behavior of the larger-sized Au Q-dots (ca.3.72nm) protected with dodecanethiol using differential pulse voltammetry (DPV) and cyclic voltammetry (CV). The voltammetric results show that the QDL property is evident even for these larger-sized Q-dots as reflected by a large population of well-resolved charging events in a narrow potential range with an almost equidistant voltage (ΔV) spacing. The theoretical calculation of the variation of charging energy with size using the well-known concentric sphere capacitance model facilitates the understanding of electrochemical behavior of these sidelined larger-sized Au Q-dots. The calculated capacitance value is in well agreement with the experimentally obtained value of 1.6aF.

Coral-like branched architectures comprised of single-crystal copper nanocrystals were synthesized at room temperature through a galvanic displacement reaction between aqueous CuCl2 and Al foil in the presence of a cationic double-chain surfactant diocta-decyl-dimethyl-ammonium bromide. The corals are monolithic single crystals consisting of nanorod stems with an axis along ⟨001⟩ and orthogonal branches along ⟨110⟩. The branch diameters fall in a narrow range between 80 and 100 nm, and the branch lengths vary between 200 and 800 nm. The branch density is controllable by adjusting the surfactant/metal-ion ratio in solution and reaction time. These branched structures could serve as attractive building blocks for creating interconnected nanorod networks or porous materials for diverse applications, including catalysts, sensors, solid-state refrigerators, and nanodevices.