A Novel approach for phase-tranfer of metal ions and its applications in nanoparticle synthesis

One of the crucial step preceding the synthesis of nanoparticles is the phase transfer of metal salts from water to an organic medium. Metal ions could not be transferred to the organic phase by direct mixing of an aqueous metal salt solution with an organic solvent. The most well-known approach is the use of long chain ammonium salt like tetraoctylammonium bromide (TOAB) to tranfer metal ions from aqueous phase to organic phase. The phase tranfer approach suggested in a recent research article in Nature Materials, involves mixing the aqueous solution of metal ions with an ethanolic solution of dodecylamine (DDA), and extracting the coordinating compounds formed between the metal ions and DDA into toluene. It has several advantages:

(a) good ion uptake by the complexing agent, enabling fast binding with the metal ion, (b) high stability against hydrolysis, (c) selective ion complexation of heavy metals, along with no affinity for alkali or alkaline earth ions that are usually present in high concentrations in water and soil, (d) sufficiently high binding strength for the metal ions to be extracted and (e) preference of the metal complex derived for the organic phase over the aqueous phase, which would be of interest for applications in environmental remediation, such as the extraction of heavy metals from water and soil.

Figure on the right :TEM images of metal nanoparticles. (1) Ag derived with HDD, (2) Au, (3) worm-like Pd and (4) Pt from Pt(IV), derived with TBAB. Alloy nanoparticles of (5) Ag–Au, (6) Pd–Pt, (7) Pt–Rh and (8) Pt–Ru, synthesized by co-reduction of the metal precursors with TBAB. Core–shell nanoparticles of (9) 7.4nm Au@Ag, (10) 12.7nm Au@Ag, (11) 3.9nm Pt@Ag and (12) 9.2nm Pt@Ag, synthesized by seed-mediated growth. Core–shell nanoparticles of (13) Ag@Au and (14) Ag@Pt, synthesized by the replacement reaction. (15) Pt hollow spheres synthesized by BSPP treatment of Ag@Pt nanoparticles. (16) Ag–Pd alloy synthesized by the replacement reaction. Semiconductor nanocrystals of (17) Ag2S, (18) CdS, (19) HgS and (20) PbS. Hybrid nanoparticles of (21) Ag2S–Au, (22) CdS–Au, (23) CuS–Au, (24) PbS–Au, (25) Ag2S–Ag, (26) CdS–Ag, (27) CuS–Ag and (28) PbS–Ag. Core–shell nanoparticles of Au@Ag2S synthesized with Au/Ag2S precursor molar ratios of (29) 1:1 and (30) 1:3.

Ref: Nature Materials 8, 683 - 689 (2009)

Authors: Jun Yang, Edward Sargent, Shana Kelley& Jackie Y. Ying

Improved sensing and catalytic properties of Tungsten oxide nanorods using Gold nanoparticles

In present world, with industrial and scientific developments, the effective detection of toxic and hazardous gases, as well as the degradation of organic pollutants has become imperative. Among other materials, tungsten oxide has attracted greater attentions for its distinctive photocatalytic and electrochromic properties. Many attempts have been made to enhance the gas sensitivity of semiconductor gas sensors, one of which involved the doping of dopants in the films.Also, the photocatalytic properties are usually known to improve by functionalizing the material with catalytical active metals (Pt, Pd and Au). Present protocols for synthesizing such materials often require high temperature and induce impurities in the final products when catalysts and templates are introduced into the reaction system, further limiting their application field.

In a current study carried out in China, a self-assembly approach for building nanoarchitectures with WO₃ nanorods (WO₃ NRs) and Au nanoparticles (Au NPs) as building blocks and further fabricating chemical sensors with highly enhanced performances has been reported. It is generally considered that monodispersed Au NPs act as a catalyst not only in the gas response but also in the photocatalytic activity.

Compared with pure WO₃NRs, Au NP@WO₃ NRs exhibit not only highly improved response and selectivity for H₂ gas detection but also high photocatalytic activity for the degradation of rhodamine B(RhB) under irradiation of simulated sunlight.

For further details, please read : J. Phys. Chem. C, Article ASAP

Authors: Q. Xiang, G. F. Meng, H. B. Zhao, Y. Zhang, H. Li, W. J. Ma and J. Q. Xu

Multifunctional thin film assemblies of molecularly-linked metal nanoparticles

Ability to tune the size, shape, and composition at the nano-particles has led to the growth of extensive research in the area of novel multifunctional materials. The ability to engineer the interparticle properties for fabricating nanoparticle assemblies in macroscopic scales is also very important from the perspective of exploiting the collective properties of the nanopoarticle assemblies as multifunctional materials for practical applications that involve surface or interfacial processes.

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The exploration of nanoparticle-structured molecular recognition is an important front in many emerging chemical and biological sensor technologies. The viability of introducing noncovalent character (e.g., hydrogen bonding) through shell molecules provides tunable molecular interactions for enhancing selectivity, which parallels synthetic or biological receptors.Through investigation of these nano-particle assemblies implies that both non-covalent and covalent interactions are important for manipulating the interparticle properties in the multifunctional nanostructures. The noncovalent interparticle interactions are useful for regulating molecular recognition and sensing properties, whereas the covalent interactions facilitate the assembly of stable interfacial nanostructures.

The viability of the one-step molecularly mediated assemblies also open technological prospects to combine the nanostructure-tuning capabilities with the electrical, optical, magnetic, and spectroscopic properties for designing chemical sensors, biosensors, and medical probes. Implications of insights to expanding the exploration of nanoparticle thin film assemblies for a wide range of technological applications has been discussed in details in a recent article in Langmuir.

Ref: Langmuir, 2010, 26 (2), pp 618–632

Authors: Lingyan Wang, Jin Luo, Mark J. Schadt and Chuan-Jian Zhong

Early diagnosis of lung cancer by breath analysis using gold nanoparticle sensors

Gold nanoparticles based sensors array in combination with pattern recognition method provide an effective diagnostic tool for lung cancer. This system can distinguish between the order prints of non-small cell lung cancer (NSCLC) and negative control with 100% accuracy. Among other advantages over conventional diagonistic method like GC-MS based systems, this is non-invasive, inexpensive scheme which do not need any preconcentrator and high expertise to use.

Scientists have designed an array of 18 chemiresistors based on functionalized Gold NPs for detecting headspace of NSCLC and the control medium. Each sensor is widely responsive to variety of odorants. Hence, each analyte yields a distinct signature from the array of broadly cross-reactive sensors.Sensors of Gold NPs mostly coated with hydrophobic functionalities that are almost insensitive to water and hence perticularly suitable for breath testing, since exhaled breath contains ~ 80% relative humidity. This journal article further establish that AuNPs sensors can also be used as highly sensitive, simple-to-use tool for understanding the biochemical background of endogenous compound appearing in exhaled breath.

For details, please refer to Small, 2009, 5, No. 22, 2618-2624

Palladium Nanorods with Magnetic Properties

Palladium as free atom has all filled atomic orbitals, hence diamagnetic behavior is observed based on Hund's rule. However, in confined nanoscale systems, more localized electronic states as well as narrower bands are usually supposed to increase the densities of states and lead to the exotic magnetic behavior.

These magnetic behavior can be controlled by changing the shape and size of nano-particles. Studies based on the feasibility of tuning the magnetic property of Pd at the nanoscale is facilitating, both the understanding of fundamental magnetism and the future application of spintronic technology.

Despite all the efforts put by scientific community in this field, it remains a great challenge to achieve shape-controlled synthesis of single crystalline palladium nanostructures in a large scale, particularly the 1D structure.

In a recently published scientific article, authors have successfully demonstrated the synthetic approach toward single crystalline Pd nanorods with a controlled aspect ratio in large quantities via a simple one-pot solution method.They have observed the ferromagnetic properties of Pd nanorods at room temperature, and proved that the onset of ferromagnetic behavior to be highly related to the unique 1D growth behavior.

In a simple solution phase chemistry, through tuning the molar ratio of two surfactants, cetyltrimethylammonium bromide (CTAB) and poly(vinyl pyrrolidone) (PVP), single crystalline palladium nanorods with different aspect ratios have been synthesized in large quantities.

The introduction of cosurfactant CTAB is critical to the growth of palladium nanorods. Its headgroup CTA+ and counterion Br- anion act as stabilizing species and etchant, respectively.

Compared with PVP-capped Pd nanoparticles, the Pd nanorods show a ferromagnetism behavior at 5 and 300 K. Pd nanorods at different temperatures were measured by SQUID. It has been concluded that a permanent magnetic moment is an intrinsic property of a Pd nanorod based on EELS study and M-H curve of PVP.

Although the detailed mechanism is still elusive, the morphology- related magnetism is believed to be meaningful in the exploration of novel magnetism in other nonmagnetic metals at nanoscale.

Ref: J. Phys. Chem. C, 2009, 113 (31), 13466–13469