Green synthesis of chitosan/glutamic acid/agarose/Ag nanocomposite hydrogel as a new platform for colorimetric detection of Cu ions and reduction of 4-nitrophenol

In this study, the green synthesis of chitosan/glutamic acid/agarose/Ag (Chi/GA/Aga/Ag) nanocomposite hydrogel was obtained via in situ reduction of Ag ions during the crosslinking process of chitosan-agarose double network hydrogels. The rich hydroxyl, carboxyl and amino groups in both agarose, chitosan, and glutamic acid can effectively control the growth, dispersion and immobilization of nearly spherical Ag nanoparticles (70 nm) in the Chi/GA/Aga/Ag composite hydrogel. Glutamic acids can act as the structure-directing agents to induce the formation of chitosan/glutamic acid hydrogel. The mechanical strength of the Chi/GA/Aga/Ag composite hydrogel can be enhanced by the introduction of chitosan-agarose double network hydrogels, which guarantees that it can be directly used as a visual test strip of the Cu ions with a lower detection limit of 1 μM and an active catalyst for the reduction of 4-nitrophenol within 18 min. The quantitative and semi-quantitative measurement of Cu ions can be carried out by UV-visible absorption spectroscopy and visual measurement, which provided a convenient, portable, and "naked-eye" solid-state detection methodology.

Functionalized agarose hydrogel with in situ Ag nanoparticles as highly recyclable heterogeneous catalyst for aromatic organic pollutants

In the present research work, a highly recyclable catalyst of Ag-based agarose (HRC-Ag/Agar) hydrogel was successfully fabricated through a simple and efficient in situ reduction method without the aid of additional surface active agent. The interaction between the rich hydroxyl functional (-OH) groups in agarose and Ag can effectively control the growth and dispersion of Ag nanoparticles (NPs) in the HRC-Ag/Agar hydrogel and keep Ag NPs free from chemical contamination, which also guarantees the reusability of HRC-Ag/Agar hydrogel as catalysts. HRC-Ag/Agar hydrogel without freeze drying and calcination was investigated for their potential applications as highly active/recyclable catalysts in reducing aromatic organic pollutants (p-nitrophenol (4-NP), methylene blue (MB) and rhodamine B (RhB)) by KBH₄. The optimal HRC-Ag/Agar-1.9 hydrogel can complete the catalytic reduction of 4-NP within 11 min. Moreover, HRC-Ag/Agar-1.9 hydrogel achieves the high conversion rate (> 99%) through ten catalytic runs. Similarly, HRC-Ag/Agar-1.9 hydrogel was able to achieve a reduction efficiency of RhB at 98% within 17 min and that of MB at 95% within 40 min. The advantages of simple synthetic procedure, no secondary pollution, strong stability and easily separated make the HRC-Ag/Agar hydrogel have great potential prospect for environmental applications.

Large-Area Monolayer Films of Hexagonal Close-Packed Au@Ag Nanoparticles as Substrates for SERS-Based Quantitative Determination

In this work, quasi-spherical, small-sized, citrate-stabilized, core-shell (CS)-structured Au_5.5@Ag_m nanoparticles (NPs) with Ag shells of controlled thicknesses (m = 0, 1.25, 3.25, and 5.25) were successfully synthesized by using Au NPs with sizes of 5.5 nm as seeds. The as-prepared Au@Ag NPs after the phase transfer process were further used for the fabrication of high-quality large-area monolayer films of hexagonal close-packed Au@Ag nanoparticles (LAMF-HCP-Au@Ag NPs) by our improved self-assembly at the interface of toluene-DEG containing a proper amount of water (10% v/v). Moreover, after transferring the as-prepared LAMF-HCP-Au@Ag NPs onto polydimethylsiloxane (PDMS) substrates (LAMF-HCP-Au@Ag NP@PDMS substrates), the resulting LAMF-HCP-Au@Ag NP@PDMS substrates can exhibit uniformity in the intensity of the surface-enhanced Raman scattering signals. Furthermore, taking LAMF-HCP-Au_5.5@Ag_5.25 NP@PDMS substrates as an example, they can achieve quantitative detection with high sensitivity for crystal violet (CV) and 4-aminothiophenol (4-ATP) in the range from 10^-12 to 10^-7 M and from 10^-13 to 10^-7 M, respectively. Also, their limit of detection (LOD) for CV and 4-ATP are 10^-12 and 10^-13 M, respectively. Especially, the LOD for CV can also be as low as 10^-13 M by extending the immersing time.

Fe(II)-Assisted one-pot synthesis of ultra-small core-shell Au-Pt nanoparticles as superior catalysts towards the HER and ORR

In this work, uniform ultra-small core-shell Au-Pt nanoparticles (denoted as USCS Au-Pt NPs) with Au-decorated Pt surfaces are successfully prepared by Fe(ii)-assisted one-pot co-reduction of Au(iii) ions and Pt(ii) ions in a citrate solution. The as-prepared USCS Au38.4@Au9.3Pt52.3 NPs have an average diameter of 2.3 ± 0.5 nm. It is found that the morphology, composition and size of Au-Pt NPs are highly dependent on the reaction conditions including the addition sequence of the precursors, and the concentrations of Fe(ii) ions, Au(iii) ions and Pt(ii) ions. In addition, USCS Au38.4@Au9.3Pt52.3-NP/C catalysts (USCS Au38.4@Au9.3Pt52.3 NPs loaded on the Vulcan XC-72R carbon black) exhibit excellent electrocatalytic performance towards the hydrogen evolution reaction (HER) and the oxygen reduction reaction (ORR) in acidic media due to the higher electrochemically active surface area (ECSA) and electronic effect between Pt and Au. For instance, USCS Au38.4@Au9.3Pt52.3-NP/C catalysts exhibited greatly enhanced HER activity in terms of overpotential (16 mV at a current density of -10 mA cm-2) and are better than commercial Pt/C catalysts (31 mV at a current density of -10 mA cm-2) reported in the literature thus far, to the best of our knowledge. Strikingly, their mass activity is about 13.1-fold higher than that of commercial Pt/C catalysts. Moreover, they also show an improved ORR activity, Eonset = 1.015 V and E1/2 = 0.896 V, which are positively shifted by nearly 28 mV and 21 mV than those of commercial Pt/C catalysts (0.987 V and 0.875 V), respectively. In addition, they also showed a higher kinetic current density (12.85 mA cm-2 at 0.85 V) and a better long-term durability. Our synthetic strategy presented here may be extended to the preparation of ultra-small Au-based bimetallic or multi-metallic NPs.

Size Control Synthesis of Monodisperse, Quasi-Spherical Silver Nanoparticles To Realize Surface-Enhanced Raman Scattering Uniformity and Reproducibility

In this work, we reported the synthesis of monodisperse, quasispherical Ag nanoparticles (NPs) with sizes of 40-300 nm by using ascorbic acid reduction of a silver-ammonia complex onto preformed, 23 nm Ag-NP seeds in the aqueous solution with an optimal pH of about 9.6 at room temperature. The as-prepared Ag NPs with such a large size span (from 40 to 300 nm) and high quality by one-pot seeded growth method are reported for the first time to the best of our knowledge. It is found that the key in the present seed-mediated growth method is to introduce a proper amount of ammonia water for the formation of a stable complex with a silver precursor (silver-ammonia complex) while maintaining the pH value of the growth solution simultaneously. By using rhodamine 6G molecules as probes, the surface-enhanced Raman scattering (SERS) activities of the as-prepared Ag NPs in ethanol solution are highly dependent on the sizes of Ag NPs at the fixed 633 nm laser and at the fixed particle number, which show a volcano-like curve. Moreover, 125 nm Ag NPs bear the largest SERS activity among them. Furthermore, Ag NPs with narrow distributions in shape and size (say, less than 10%) can achieve the uniformity and reproducibility of their SERS signals in solution; their relative standard deviations can be as low as 5% in space and temporal scale.

Fabrication of Au aerogels with {110}-rich facets by size-dependent surface reconstruction for enzyme-free glucose detection

Au^6–50 aerogels with {110}-rich facets (35.5%) are fabricated for glucose detection by size-dependent surface reconstruction between two differently-sized gold nanoparticles.

Regulating Surface Facets of Metallic Aerogel Electrocatalysts by Size-Dependent Localized Ostwald Ripening

It is well known that the activity and stability of electrocatalysts are largely dependent on their surface facets. In this work, we have successfully regulated surface facets of three-dimensional (3D) metallic Au ^{m- n} aerogels by salt-induced assembly of citrate-stabilized gold nanoparticles (Au NPs) of two different sizes and further size-dependent localized Ostwald ripening at controlled particle number ratios, where m and n represent the size of Au NPs. In addition, 3D Au ^{m- n}-Pd aerogels were further synthesized on the basis of Au ^{m- n} aerogels and also bear controlled surface facets because of the formation of ultrathin Pd layers on Au ^{m- n} aerogels. Taking the electrooxidation of small organic molecules (such as methanol and ethanol) by the resulting Au ^{m- n} and Au ^{m- n}-Pd aerogels as examples, it is found that surface facets of metallic aerogels with excellent performance can be regulated to realize preferential surface facets for methanol oxidation and ethanol oxidation, respectively. Moreover, they also indeed simultaneously bear high activity and excellent stability. Furthermore, their activities and stability are also highly dependent on the area ratio of active facets and inactive facets on their surfaces, respectively, and these ratios are varied via the mismatch of sizes of adjacent NPs. Thus, this work not only demonstrates the realization of the regulation of the surface facets of metallic aerogels by size-dependent localized Ostwald ripening but also will open up a new way to improve electrocatalytic performance of 3D metallic aerogels by surface regulation.

Simple Synthesis of Au-Pd Alloy Nanowire Networks as Macroscopic, Flexible Electrocatalysts with Excellent Performance

The present work introduces a new way to prepare Au-Pd alloy nanowire networks (NWNs) via deposition of Pd atoms onto Au nanowires in reaction media at room temperature without the aid of additional reducing agents. Thanks to their excellent colloidal stability in water as well as in ethanol, the resulting NWNs can be utilized to produce composite thin films with Nafion (perfluorinated sulfonic acid) with dimensions above dozens of square centimeters by means of solution casting on the glass substrate. Most importantly, these films can be easily transferred onto different solid substrates by lift-off technology. Moreover, the resulting Au-Pd alloy NWNs can also be easily and thoroughly loaded into macroscopic carbon fiber cloth (CFC). Both the Au-Pd alloy NWN/Nafion composite film and the Au-Pd alloy NWN-loaded CFC can be used as flexible electrodes for electrocatalysis of ethanol oxidation, with electrocatalytic performance at different distorted states superior by 2 orders of magnitude to those reported in the literature (e.g., commercial Pd/C catalysts and Pd-based nanostructured catalysts). This work opens new possibilities for the large-scale manufacturing of electrodes for fuel cells.

Revitalizing the Frens Method To Synthesize Uniform, Quasi-Spherical Gold Nanoparticles with Deliberately Regulated Sizes from 2 to 330 nm

In this work, we have successfully developed a new and consistent model to describe the growth of gold nanoparticles (Au NPs) via citrate reduction of auric acid (HAuCl4) by carefully assessing the temporal evolution of the NP sizes and surface charges by means of dynamic light scattering (DLS) and zeta-potential measurements. The new model demonstrates that the nucleation and growth of the Au NPs occur exclusively in the particles of the complexes of Au(+) ions and sodium acetone dicarboxylate (SAD) derived from the citrate/HAuCl4 redox reaction, which proceeds as described by the classic LaMer model. Concomitant with the Au NP growing therein, the Au(+)/SAD complex particles undergo reversible agglomeration with the reaction time, which may result in an abnormal color change of the reaction media but have little impact on the Au NP growth. Built on the new model, we have successfully produced monodisperse quasi-spherical Au NPs with sizes precisely regulated from 2 to 330 nm via simple citrate reduction in a one-pot manner. To date, highly uniform Au NPs with sizes spanning such a large size range could not be formed otherwise even via deliberately controlled seeded growth methods.