Study Of Silver Nanoprticles Loaded Temperature Responsive Hybrid Nanofibrous Membrane

Silver nanoparticles(AgNPs) of less than 10 nm particle diameter hold strong catalytic activity as well as surface enhanced Raman scattering(SERS) efficiency, so they have attractive application prospect in catalyst and SERS substrate. However, the AgNPs of small size are prone to self-aggregation and easily oxidized due to their very high surface energy. An effective approach to solve the problem is to load them into a specific carrier. If the AgNPs are loaded into intelligent hydrogel carrier, their self-aggregation can not only be avoided, but their properties can be regulated by external stimulus. The previously reported intellligent hydrogel carriers for AgNPs are stimulus responsive bulky hydrogels or microgels. The former has much slower stimulus response, and the later usually exists as colloidal particles in aqueous medium, suffering from poor colloidal stability and recyclability. In this thesis, fast temperature-responsive hybrid nanofibrous membrane with macroscopic mechanical strength was used as the carrier of AgNPs. The temperature-responsive hybrid nanofibous membrane fabricated by electrospinning technique followed by heat treatment process can be used as an intelligent recyclable catalyst and free-standing intelligent SERS substrate. The membrane has the ability of real-time SERS monitoring intermediate product of its catalyzed reaction for speculating the reaction mechanism.The obtained research results of this thesis are described as the following three aspects:(1) The temperature-responsive polymer poly(N-isopropylacrylamide-co-Nhydroxymethylacrylamide)(PNN) was firstly synthesized via free radical copolymerization of N-isopropylacrylamide(NIPAM) and N-hydroxymethylacrylamide(NMA). The spinning solution containing silver nanoparticles(AgNPs) was prepared by reducing Ag+ in PNN aqueous solution via ultraviolet irradiation of 365 nm with no using small molecule compounds like reductant,stabilizer and organic solvent. Then AgNPs loaded temperature-responsive hybrid nanofibrous membrane(AgNPs/PNN membrane) was fabricated by electrospinning technique, followed by heat treatment process. The results obtained by scanning electron microscopy and transmission electron microscopy show that AgNPs/PNN membrane consists of randomly aligned nanofibers with homogeneous diameters, their average diameter is decreased from 525 nm to 421 nm with increasing AgNO3 amount used for preparing the spinning solution. The AgNPs of ca. 4 nm average particle size, independent of the used AgNO3 amount, are distributed evenly inside its constituent nanofibers. The good stability of the hybrid nanofibrous membrane in aqueous medium was identified by oscillating it at 400 rpm for 2 h with less than 2% of mass loss. When the temperature of the aqueous medium was raised from 25 ℃ to 50 ℃, the area of AgNPs/PNN membrane at 50 ℃ was about half of the one at 25 ℃,with the phase transition temperature around 38 ℃. It is able to reach swelling and deswelling equilibrium state within 15 s with the aqueous medium temperature jumping or plunging between 25 ℃ and 50 ℃. These results indicates that AgNPs/PNN hybrid nanofibrous membrane possesses remarkable temperature-responsiveness as well as highly fast response rate.(2) AgNPs/PNN hybrid nanofibrous membrane hold catalytic activity for the reduction of4-nitrothiophenol(4-NTP) into 4-aminothiophenol(4-ATP) by NaBH4, and it is enhanced by the AgNPs content. The catalytic activity is obviously dependent on reaction temperature, which declines during temperature inducing phase transition of the membrane, showing that it is able to be tuned by temperature. In addition, AgNPs/PNN hybrid nanofibrous membrane as a catalyst has well stability and recyclability.(3) AgNPs/PNN hybrid nanofibrous membrane can be used as a free-standing SERS substrate and its SERS efficiency is enhanced by its AgNPs content. When it was used as the SERS substrate for detecting trace amount of 4-NTP in aqueous solution, its detection limit is less than10-12 M, with well reproducibility of the detection results. The SERS activity of AgNPs/PNN membrane is notably temperature-dependent, which is elevated with temperature within the range from 35 ℃ to 40 ℃, indicating that the detection sensitivity based on the membrane as SERS substrate can be improved by elevating detection temperature. Moreover, the intermediate 4,4’-dimercaptoazobenzene(4, 4’-DMAB) of the reduction of 4-NTP to 4-ATP by Na BH4 can be detected, which is helpful to identifying its mechanism, and the intermediate production is a ratedetermining step of the whole reaction.