Synthesis of Organic Nanofibers with Embedded Palladium Nanoparticles for Decolourization of Organic Dyes

Review Article

Austin J Chem Eng. 2014;1(1): 1005.

Synthesis of Organic Nanofibers with Embedded Palladium Nanoparticles for Decolourization of Organic Dyes

Bera T and Fang JY*

Department of Materials Science and Engineering, University of Central Florida, USA

*Corresponding author: Fang JY, Department of Materials Science and Engineering, University of Central Florida, Orlando, Florida 32816, USA

Received: May 16, 2014; Accepted: July 09, 2014; Published: July 14, 2014


We report the synthesis of organic nanofibers with embedded palladium nanoparticle (PdNP) by the self-assembly of Pd2+-coordinated lithocholic acid (LCA)in the presence of sodium borohydride(NaBH4) or ascorbic acid (AA) reducing agents. The in-situ reduction of Pd2++ ions by NaBH4or AA leads to the formation of PdNPs in the interior of self-assembled LCA nanofibers. The catalytic activity of LCA nanofibers with embedded PdNPs in the decolourization of 4-phenyl azo benzoic acid (4-PABA) is investigated.

Keywords: Lithocholic acid; Self-assembly; Palladium nanoparticles; Nanofibers; Recyclable nanocatalysts


LCA: Lithocholic Acid; PdNP: Palladium Nanoparticle; NaBH4: Sodium Borohydride; AA: Ascorbic Acid; 4-PABA: 4-phenyl Azo Benzoic acid


There has been great interest in metal nanoparticles due to their applications as efficient nanocatalysts for chemical productions, energy processes, and pollution controls [1-3]. It is known that small metal nanoparticles are thermodynamically unstable. The aggregation of metal nanoparticles can lead to the reduction of their catalytic activities. Thus, surfactants are often used as a stabilizing agent for synthesizing metal nanoparticles [4-13]. However, the recyclability of surfactant-stabilized small metal nanoparticles still remains to be a challenge because they are difficult to be separated and recovered from reaction solution. Recently, efforts have been made for synthesizing one-dimensional metal nanostructures [14-17] or constructing metal nanoparticles on polymer nanofibers [18-21] in order to achieve the good recyclability.

Contrary to the commonly used surfactants in stabilizing metal nanoparticles, a number of lipids and bile acids can self-assemble into one-dimensional (1D) nanostructures with well-defined sizes and morphologies including fibers, ribbons and tubes [22-24]. These 1D nanostructures are promise as organized templates for growing and patterning inorganic nanoparticles [25-33]. The self-assembly of metal ion-coordinated lipids has been proven to an efficient method in the synthesis of metal ion-coordinated nanotubes [34-36]. The Cu ion-and Ni ion-coordinated lipid nanotubes have shown good catalytic activities for the effective oxidation of organic compounds [35-36].

Lithocholic acid (LCA) is a secondary bile acid. It has a rigid, nearly planar steroid nucleus and a short alkyl chain with a carboxyl terminal group. It has been shown that LCA can self-assemble in aqueous solution into helical ribbons and tubes [37-43], depending on the experimental conditions under which self-assembly occurs. In a previous publication [43], we reported the formation of PdNP-LCA hybrid nanofibers by the self-assembly of Pd2+-coordinated LCA molecules, followed by an ex-situ reduction with sodium borohydride (NaBH4) or ascorbic acid (AA), in which PdNPs grow on the surface of the self-assembled LCA nanofibers [44]. In this paper, we report the synthesis of hybrid nanofibers by the self-assembly of Pd2+- coordinated LCA molecules in the presence of NaBH4 or AA. The in-situ reduction by NaBH4 or AA can lead to the formation of LCA nanofibers with embedded PdNPs. Azo dyes are widely used in the textile, pharmaceutical, food and cosmetics industries. They are one of primary sources to cause water pollution. The decolourization of organic dyes in water is critical in improving the quality of water. As a proof-of-concept, 4-phenyl azo benzoic acid (4-PABA) is chosen as a model dye. We demonstrate that LCA nanofibers with embedded PdNPs can be used as a recyclable nanocatalyst for the decolourization of 4-PABA in water.

Experimental section

Lithocholic acid (LCA), 4-phenyl azo benzoic acid (4-PABA), ammonium hydroxide (NH4OH), palladium chloride (PdCl2), ascorbic acid (AA), and sodium borohydride (NaBH4) were purchased from Sigma-Aldrich. Pd2++(NH4OH)6Cl2 used as a precursor for the synthesis of PdNPs was prepared by dissolving PdCl2 in NH4OH aqueous solution. NaBH4 and AA were used as reducing agents. Water used in our experiments was purified with Easy pure II system (18MΩ cm, pH 5.7). Carbon-coated copper grids were purchased from Electron Microscopy Science.

Transmission electron microscopy (TEM) was performed with a JEOL1011-EM microscope operating at an acceleration voltage of 100 kV. Ultraviolet-visible (UV-vis) spectra were recorded using a Cary300 spectrophotometer. X-ray diffraction (XRD) measurements were carried out with a Rigaku D/max diffractometer with CuKa radiation (λ = 1.542 Å) operated at 40 kV and 30 mA.

Results and Discussion

The chemical structures of LCA and 4-PABA are shown in Figure 1. In our experiments, 30 mg of LCA was dissolved in 4mL of 0.01 M NH4OH aqueous solution at pH 11.0. The pre-prepared Pd2+(NH4OH)6Cl2 solution was then added into 20mM LCA solution. The concentration of Pd2++ ions in the mixed solution varied from 0.1mM to 1mM. The mixed solution was stirred, followed by adding 400μL of 100mM of NaBH4 solution. Finally, the solution was vigorously stirred for 60 min and then sealed in a glass vial at room temperature. Figure 2a shows a low-resolution TEM image of nanofibers formed at the Pd2+ ion concentration of 1mM. The nanofibers are over 100 μm long with diameters in the range from 70 nm to 180 nm. As can be seen from the high-resolution TEM image (Figure 2b), PdNPs (dark dots) with a diameter of 30-40 nm are embedded in the LCA nanofibers. In the synthesis process, the deprotonated LCA molecules provide negatively charged COO-1 groups to coordinate with positively charged Pd2+ ions. The Pd2+- coordinated LCA molecules then self-assemble into nanofibers in the presence of NaBH4reducing agent. The formation of PdNPs inside the LCA nanofibers suggests that the coordinated Pd2+ ions are in situ reduced by NaBH4 to form Pd nuclei, which then grow into PdNPs inside the LCA nanofibers. X-ray diffraction patterns of LCA nanofibers with embedded PdNPs show five characteristic peaks (Figure 3), corresponding to the scattering from the (111), (200), (220), (311), and (222) planes of the face centered cubic PdNPs.