Chuchaat Thammacharoen

Influence of the metal film thickness on the sensitivity of surface plasmon resonance biosensors

The influence of the metal film thickness (i.e., the chromium adhesion promoting film and the gold film) on the sensitivity of surface plasmon resonance (SPR) signals (i.e., resonance angle shift and reflectance change) towards the thickness variation of the nonabsorbing dielectric film is investigated. The sensitivity of reflectance change decreases when a thick chromium film or a thin gold film is employed. Its linear range becomes narrower as the thickness of the metal films increases. The sensitivity and linear range of the resonance angle shift are not affected by the thickness variation of the metal films. The phenomena were theoretically explained based on the attenuated total reflection (ATR) generated evanescent field at the prism/metal interface and the SPR-generated evanescent field at the metal/dielectric interface.

H2O2-triggered shape transformation of silver nanospheres to nanoprisms with controllable longitudinal LSPR wavelengths

A novel approach for the synthesis of colloidal silver nanoprisms (AgNPrs) with controllable localized surface plasmon resonance (LSPR) via a chemical shape transformation of silver nanospheres (AgNSs) is presented. The shape conversion is carried out by feeding hydrogen peroxide (H2O2) solution into a starch-stabilized AgNS colloid under ambient conditions. Oxidative dissolution and the mild reducing action of H2O2 under alkaline conditions serve as the principal reactions for the shape transformation process. After addition of H2O2, the instantaneous shape transformation events can be visualized by the naked eye through the color change of the colloid. Initial concentration of AgNSs, molar ratio of H2O2:AgNSs, H2O2 injection rate, and mixing efficiency are the key parameters for controlling the LSPR wavelengths of AgNPrs as the in-plane dipole plasmon resonance can be selectively tuned across visible and near infrared regions (i.e., 460–850 nm). The obtained AgNPrs exhibited mixed geometries e.g. hexagonal, truncated triangular, rounded-tip triangular prisms, and circular disks with average bisector lengths of 30 to 120 nm and the thickness of 10 to 20 nm. A colloid of highly concentrated AgNPrs having a final concentration up to 11 mM can be produced within 10 min.

Colorimetric determination of hydrogen peroxide by morphological decomposition of silver nanoprisms coupled with chromaticity analysis

A novel colorimetric method with image colour analysis for highly sensitive and accurate detection of hydrogen peroxide using starch-stabilized silver nanoprisms (AgNPrs) is proposed. AgNPrs were morphologically decomposed by a low concentration of hydrogen peroxide revealed by UV-visible absorption spectroscopy and transmission electron microscopy (TEM). The morphological changes of AgNPrs led to an appreciable colour change in the AgNPr solution from red to orange, and finally yellow. A good linear relationship between the wavelength shift of AgNPrs and the H2O2 concentration can be obtained. The solution phase detection of H2O2 by the direct morphological change can be accomplished without any surface modification of AgNPrs. In addition to the conventional determination of the H2O2 concentration utilizing spectroscopic data, a new and simple colorimetric strategy based on the chromaticity analysis of AgNPr solution was demonstrated. The strategy can be employed not only for visual detection of H2O2 by the naked eye but also for reliable and convenient methods for quantification of H2O2. The hydrogen peroxide concentration at 1.57 μM can be recognized by naked-eye observation with good accuracy, stability and reproducibility. Furthermore, the proposed protocol can be applied to determine the glucose concentration through the glucose-oxidase system. A good linearity between the red chromaticity of the solution colour and the glucose concentrations was observed. The new colorimetric determination of hydrogen peroxide utilizing digital image analysis on colour changes from AgNPr shape decomposition will open up an alternative method for simple, rapid and reliable detection of hydrogen peroxide and can realize its future applications in biochemical analysis or clinical diagnosis.

Enhancement of the reduction efficiency of soluble starch for platinum nanoparticles synthesis

In this work, the efficiency of soluble starch as a reducing and a stabilizing agent in the synthesis of platinum nanoparticles under acidic-alkaline treatment is systematically studied. The degraded intermediates with reducing potential (i.e., small molecules containing aldehyde and α-hydroxy ketone moieties) are concomitantly generated when the alkaline concentration is greater than 0.025 M. The in situ generated species could completely reduce platinum ions (20 mM) and sufficiently stabilize the obtained platinum nanoparticles (5 mM) of uniform particle size (2-4 nm). The reduction is efficient and rapid as a complete conversion is achieved within 5 min. In a stronger alkaline condition, the platinum nanoparticles tend to aggregate and form a bigger domain because extensive degradation generates small starch fragments with less stabilization efficiency. This observation suggests that starch is a promising green material which could be chemically treated and transformed to a powerful reducing agent and stabilizer for the synthesis of metal nanoparticles.

Formation of large H2O2-reduced gold nanosheets via starch-induced two-dimensional oriented attachment

We report here a simple yet efficient approach for fabricating large gold nanosheets using H2O2 as the reducing agent with starch as the stabilizer and shape-controlling agent. The weak reducing capability of H2O2 enables a kinetically controlled growth of nanosheets out of flower-like nanostructures formed at the early stage. Small nanosheets with starch-bound {111} facet undergo oriented attachment and become large nanosheets having lateral size as large as 50 μm with 20–50 nm thickness. Without starch, gold quasi-microspheres with diameters of 5–10 μm become the dominant product as they also grow out of the flower-like nanostructures by filling gold atoms in the gaps between nano-petals. This starch-enable selective formation of large nanosheets is rapid and efficient as a 100% conversion could be attained even at a high concentration of gold ions (25.4 mM). A large-scale production of 2.5 g glittering gold nanosheets with a 5-L batch 10 h synthesis was demonstrated. The synthetic protocol was not only achieving atomic economy using an environmental friendly reducing reagent and renewable stabilizer, the mass-scale production prototype was accomplished at room temperature without a need for extensive waste treatment. The slightly acidic starch solution obtained after the reaction was recycled for the next synthesis or neutralized before discharging.

Green synthesis of size controllable and uniform gold nanospheres using alkaline degradation intermediates of soluble starch as reducing agent and stabilizer

AbstractA green synthesis of size-controllable, uniform gold nanospheres is reported. The size of the gold particles can be selectively tuned from nanometer to submicrometer regimes with narrow size distribution through pH adjustment of the solution. Based on the employed green chemical reduction method, soluble starch is used as both reducing agent and stabilizer. Soluble starch is generally a very weak reducing agent. However, under an alkaline condition, its reducing efficiency is enhanced by the concomitant generation of reducing species as the starch molecules are alkaline degraded. The in situ generated reducing species nucleate and grow gold nanoparticles. The growth mechanism of gold particles is systematically investigated and proposed. The synthesized gold colloid is very stable and can be kept over 4 months without precipitation, aggregation, or any significant changes. Moreover, all processes of this method are simple and environmentally friendly, and no complex instrument is needed.

3D AgCl microstructures selectively fabricated via Cl−-induced precipitation from [Ag(NH3)2]+

Herein various AgCl microstructures including octapods, octapods with fishbone pods, hexapods, hexapods with 4-blade arrowhead pods, concave octahedra, and octahedra are selectively precipitated from a [Ag(NH3)2]+ solution by addition of Cl−. The microstructures were rapidly formed and precipitated within 5 min. In a Cl−-rich environment, the octapod grew from cubic seeds as the growth along the 〈111〉 directions was favorable. The hexapod grew from octahedral seeds in an NH4OH-rich environment as the growth along the 〈100〉 directions dominated. By manipulating the seed morphology and the growth environment, complex AgCl microstructures of thermodynamically unfavorable structures (hexapods and octahedra) can be selectively fabricated. A potential application of AgCl microstructures as photocatalysts was explored by partial photo-reduction of the surface AgCl into isolated AgNPs. The fabricated AgCl microstructures were turned into efficient Ag@AgCl visible-light photocatalysts as complete decomposition of methyl orange was achieved within 20 min. This work not only explores a method to control the preferential growth of AgCl microstructures along the 〈100〉 and 〈111〉 directions but also provides an in-depth understanding on the crystal growth mechanisms and how to manipulate them efficiently.

Nanoporous silver microstructure for single particle surface-enhanced Raman scattering spectroscopy

The potential of a nanoporous Ag microstructure (np-AgMs) for use as a single particle for surface-enhanced Raman scattering spectroscopy (SERS), with the added advantages of being easy to manipulate and reusable, was successfully demonstrated. The np-AgMs with interconnected pore and controllable pore size were fabricated from symmetric hexapod AgCl via a galvanic replacement reaction in NaCl solution with zinc (Zn) as the sacrificed metal. The clean surface of np-AgMs enables rapid surface functionalization with easy handling and sample preparation as no particle aggregation occurs. The SERS acquisition spots on the np-AgMs can be visually selected using a normal Raman microscope. SERS spectra of p-aminothiophenol (PATP) with a concentration range of 10−8–10−3 M can be achieved. The position-dependent enhancement of np-AgMs was expendably evaluated. The signal-position correlation was confirmed by electric filed enhancement obtained from Finite-difference time-domain (FDTD) calculation. In addition, the highly stable substrate showed insignificant loss of the enhanced Raman signal after several cycles of chemical re-generation. Finally, the potential application of np-AgMs in label-free detection of biomolecules including hemoprotein, protein without chromophore and DNA strains at low concentration of 500 μg mL−1 was demonstrated.

Rapid fabrication of silver microplates under an oxidative etching environment consisting of O2/Cl−, NH4OH/H2O2, and H2O2

Morphologically controlled micro-/nanostructures have gained considerable interest as they offer unique properties associated with size, shape, and crystallographic facets. In this paper, we report a simple yet rapid protocol for large scale synthesis of silver microplates (AgMPls) from a silver ammine complex ([Ag(NH3)2]+) under an etching environment containing O2/Cl−, NH4OH/H2O2, and H2O2 capable of dissolving silver crystals except plate structures. H2O2 is employed as the sole reducing agent. Chloride ions are essential for creating an etching environment capable of selective dissolution of singly and multiply twinned crystals, while leaving plate structures unaffected. Without chloride ions, H2O2 reduces the [Ag(NH3)2]+ complex to silver microparticles containing truncated cubes, icosahedra, pentagonal rods, and plate microstructures with icosahedra as the major product. The developed protocol enables environmentally friendly fabrication of highly pure AgMPls and AgMPs directly from the AgCl precipitate.

Palladium nanoparticles synthesized by reducing species generated during a successive acidic/alkaline treatment of sucrose.

Uniform spherical palladium nanoparticles with an average particle size of 4.3±0.5 nm were successfully synthesized by reducing H2PdCl4 with intermediates in situ generated during a successive acidic/alkaline treatment of sucrose. A successive acidic/alkaline treatment plays an important role on converting the non-reducing sucrose into efficient reducing species containing aldehyde functionality. The Benedicts test corroborates the development and vanishing of the in situ generated reducing species upon prolonged degradation. An increase in alkalinity drastically improves the reduction efficiency. ATR FT-IR spectroscopy indicated spontaneous development of carboxylate after the alkaline treatment. Under the employed condition, small organic species with carbonyl groups (aldehyde, acid, and acid salt) were generated through the sucrose degradation before being oxidized to carbonate after an hour of the treatment. Sucrose was completely decomposed into carbonate after a 24-h successive acidic/alkaline treatment. The synthesized palladium nanoparticles express a good catalytic activity in the decolorization process of Congo red by sodium borohydride.