金屬奈米粒子修飾溶菌酶類澱粉纖維在亞甲基藍催化反應上之應用

Abstract

金屬奈米粒子具有高的表面能量及原子比例，擁有良好的催化活性，常用於催化反應中，但高的表面能量同時也導致金屬奈米粒子易產生聚集現象，為了提升金屬奈米粒子的穩定性及分散性，將金奈米粒子和鈀奈米粒子固定於纖維狀的溶菌酶類澱粉纖維上，製備異相催化材料。 以溶於Glycine緩衝溶液(pH 2)的雞蛋白溶菌酶(1 mg/mL)，在55 ℃及470 rpm攪拌下，培養一天生成溶菌酶類澱粉纖維(AF)，利用靜電吸引力將金屬鹽類吸附於溶菌酶類澱粉纖維後，加入硼氫化鈉作為還原劑，還原金屬鹽類生成金屬奈米粒子，合成AuNP@AF或PdNP@AF複合催化材料；而未經溶菌酶類澱粉纖維固定的金屬奈米粒子，直接由硼氫化鈉還原金屬鹽類生成。在本研究中選用兩種金屬鹽類濃度進行複合材料備製，以10.4 mM金屬鹽類濃合成之奈米粒子標示為NPH，而以5.2 mM金屬鹽類濃合成之奈米粒子標示為NPL。 由TEM分析AuNPL@AF時，圓球狀的AuNPL分散於束狀的溶菌酶類澱粉纖維上，平均粒徑為5.16 (±1.29) nm；經由EDX分析AuNPH@AF時，可得到來自AuNPH的Au特徵峰；經由TGA分析，AuNPL@AF中AuNPL的重量約為18.51 %。由TEM分析PdNPL@AF時，圓球狀的PdNPL亦分散於溶菌酶類澱粉纖維上，平均粒徑為10.87(±2.54) nm，未經固定化的PdNPL雖呈圓球狀，但出現明顯聚集；經由EDX分析PdNPH@AF時，可得來自PdNPH的Pd特徵峰；經由TGA分析，PdNPL@AF中PdNPL的重量約為18.01 %。 常溫下，以1 mL的亞甲基藍為反應物(0.03 mM)、0.4 mL的硼氫化鈉(15 mM)為還原劑，進行亞甲基藍還原反應。以0.4 mL金屬奈米濃度約為0.0041 mM的AuNPL@AF或AuNPL為催化劑時，反應時間30秒，亞甲基藍還原率分別約為77.24和99.57 %，反應速率常數分別為0.3102(±0.0048)和0.6373(±0.0117) s-1，未經固定的AuNP具較佳的催化活性；以0.4 mL金屬奈米濃度約為0.0040 mM的PdNPL@AF或PdNPL為催化劑時，在反應時間60秒，亞甲基藍還原率分別約為95.33 %和23.56 %，PdNPL@AF的反應速率常數為0.1092(±0.0049) s-1、活化能為44.49(±6.09)，經固定的PdNP催化活性大幅提升，且放置時間15週時，亞甲基藍的還原率仍可達94 %，說明經過溶菌酶類澱粉纖維固定化後，PdNP的穩定性增加，催化活性大幅提升。

Metal nanoparticles have been widely used as the catalysts because of their high surface energy and catalytic efficiency. However, the high surface energy of metal nanoparticles also lend to aggregation. In order to improve the stability and dispensability of the catalytic metal nanoparticles, we deposited gold nanoparticles (AuNP) and palladium nanoparticles (PdNP) on egg white lysozyme amyloid fibrils (AF) to produce heterogeneous catalytic materials. Moreover, we compared the catalytic activity of AuNP, AuNP@AF, PdNP and PdNP@AF on the catalytic reduction of methylene blue (MB) by reducing agent NaBH4. Lysozymes (1 mg/mL) dissolved in glycine buffer (pH 2) and incubated at 55oC with stirring at 470 rpm for 24 hr to produce AF. Under acidic conditions, due to electrostatic interactions, AuCl4- and PdCl2- were spparately adsorbed on positively charged AF. NaBH4 was used as the reducing agent to reduce metal ions into AuNP and PdNP. In addition, AuNP and PdNP were produced by adding NaBH4 into metal ions solution. TEM analysis revealed the presence of spherical AuNPL (5.16 ± 1.29 nm) on AF without aggregation. The EDX spectrum of AuNPH@AF contained intense peaks of C, O, and Au. The C and O signals were from AF. TGA analysis revealed that AuNPL on the AuNP L@AF was about 18.51 wt%. For PdNPL@AF, TEM analysis revealed the presence of spherical PdNPL (10.87 ± 2.54 nm) on AF without aggregation. Without AF supports, spherical PdNPL were greatly aggregated. The EDX spectrum of PdNPH@AF contained intense peaks of C, O, and Pd. TGA analysis revealed that PdNPL on the PdNPL@AF was about 18.01 wt%. At room temperature, the reduction of 1 mL MB (0.03 mM) by 0.4 mL BH4− (15 mM) was monitored. At a reaction time of 30 s, the degree of reduction of MB catalyzed by 0.4 mL AuNPL@AF or AuNPL (0.0041 mM) was about 77.24 and 99.57 %, respectively. The reaction followed pseudo-first order kinetics with reaction rate constant of 0.3102(±0.0048) and 0.6373(±0.0117) s-1, respectively. The degree of reduction of MB was better which suggests that AuNP itself has high stability. For PdNPL@AF (0.0040 mM), at a reaction time of 60 s, the degree of reduction of MB was about 95.33 % with a reaction rate constant of 0.1092 ±0.0049 s-1. Moreover, the degree of reduction of MB could still reach 94% at 15 weeks. However, the degree of reduction of MB catalyzed by 0.4 mL PdNPL (0.0040 mM) was only about 23.56 %. In summary, depositing PdNP on AF, not only increased the dispersion of PdNP, but also effectively increased the catalytic activity of PdNP.