I. INTRODUCTION
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ChooseTop of pageABSTRACTI. INTRODUCTION <<II. EXPERIMENTIII. RESULTS AND DISCUSSI...IV. SUMMARY AND CONCLUSIO...REFERENCESIn this work, AgNPs were composited onto nanofibers by thermal bonding. Two polymers with different melting points were selected to construct nanofibers with shell-core structures by the coaxial electrospinning technique. Polyglycolic acid (PGA) with high melting point (Tm) was used as the core layer to maintain the fiber structure and polycaprolactone (PCL) with low Tm was used as the shell layer to thermally bond the particles. AgNPs were electrosprayed on the surface of the nanofibers and were thermally bonded onto the shell layer by heat treatment. In this way, AgNPs were robustly anchored onto the nanofibers with a more significant surface effect, while the membranes were also strengthened by fusion and cross-linking of nanofibers. Then, the structure of shell-core nanofibers and the heat treatment condition were optimized, and the binding stability of AgNPs and the antibacterial effect of the composite nanofibers were investigated.
II. EXPERIMENT
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ChooseTop of pageABSTRACTI. INTRODUCTIONII. EXPERIMENT <<III. RESULTS AND DISCUSSI...IV. SUMMARY AND CONCLUSIO...REFERENCESA. Materials
Silver nitrate (AgNO3) was obtained from Shanghai Lingfeng Chemical Reagent Co., Ltd. Polyvinylpyrrolidone (PVP, K-30), 3-aminopropyltriethoxysilane (KH-550), and PCL (Mw = 80 000) were purchased from Sigma-Aldrich (Shanghai) Trading Co., Ltd. PGA was purchased from Shandong gain Technology Co., Ltd. 2,2,2-trifluoroethylamine (TFEA) was purchased from Aladdin reagent (Shanghai) Co., Ltd. Hexafluoroisopropanol (HFIP) was purchased from Shanghai Macklin Biochemical Co., Ltd. Staphylococcus aureus (S. aureus, ATCC6538) was obtained from Shanghai Luwei Technology Co., Ltd. All the chemicals were used as purchased without further purification.
B. Preparation of coaxial electrospinning nanofibers
PCL-PGA nanofiber membranes were prepared by the coaxial electrospinning technique [Fig. 1(a)]. They were obtained by coaxial electrospinning utilizing PCL spinning solution as the shell layer and PGA spinning solution as the core layer. First, PCL was dissolved in TFEA (10% w/v), and PGA/HFIP (6% w/v) was heated until PGA was totally dissolved. Electrospinning was carried out with two syringes (5 ml) connected to a stainless steel coaxial needle (the inner diameter was 0.40 mm and the outer diameter was 1.00 mm). The thickness of the core and shell layer was controlled by tuning the flow rates. The nanofibers obtained at different shell-core flow rates (0.0015:0.0015 mm/s, 0.002:0.001 mm/s, 0.0024:0.0008 mm/s, 0.0024:0.0006 mm/s) were termed PCL1-PGA1, PCL2-PGA1, PCL3-PGA1, and PCL4-PGA1. The receiving distance was set to 15 cm and the receiving drum speed was 200 rpm, while the voltage was adjusted in the range of 10–13 kV.C. Synthesis of AgNPs
AgNPs were synthesized via a solvothermal method. AgNO3 (0.085 g) was added to 50 ml of anhydrous ethanol, stirred to dissolve completely, followed by being added drop by drop to 100 ml PVP/ethanol solution (5 mg/ml). Then, the above-mentioned mixture was transferred to a 200 ml polytetrafluoroethylene-lined autoclave and placed in an oven (DHG-9015A, Shanghai Yiheng Instrument Co., Ltd.) at 180 °C for 18 h. The obtained product was washed by centrifugation with purified water and anhydrous ethanol three times, and then freeze-dried to get a brownish-yellow powder with a metallic luster, i.e., AgNPs.
D. Preparation of Ag/PCL-PGA composite nanofiber membranes
AgNPs (0.5 mg/ml) were dispersed in anhydrous ethanol with 1% (w/v) of KH-550 followed by being sonicated for 20 min. Then, the AgNPs suspension was electrosprayed onto the surface of the PCL-PGA coaxial nanofibrous membrane. The receiving distance was set to 10 cm, the receiving drum speed was 200 r/min, the flow rate was 0.003 mm/s, and the voltage was 20 kV.
Then, the prepared AgNP-composited nanofibers (termed Ag/PCL3-PGA1 nanofibers) were heated in an oven for 5 min at temperatures of 60, 80, and 100 °C, respectively. Another part of nanofibers was thermally treated for 2.5, 5, 7.5, and 10 min, respectively, with the temperature being set to 80 °C. Throughout the study, the nonthermally bonded Ag/PCL3-PGA1 was named Ag/PCL3-PGA1 (NTB), and the thermally bonded Ag/PCL3-PGA1 was named Ag/PCL3-PGA1 (TB).
E. Characterization
The morphological structures of AgNPs and nanofibers were observed by scanning electron microscopy (SEM, vltra55, Carl Zeiss SMT Pte Ltd., Germany) and transmission electron microscopy (TEM, JEM-2100, Electronics Co., Ltd., Japan). The chemical components and elemental distribution of Ag/PCL3-PGA1 composite nanofiber were analyzed by an energy dispersive spectrometer (EDS). The optical property and crystal structure of AgNPs were characterized by a UV-vis spectrophotometer (Lambda 35, Perkin Elmer, USA) and x-ray diffraction instrument (XRD, A8 Advance, Bruker AXS).
The mechanical tensile properties of the composite membrane were tested using a universal material testing machine (Instron 3367 ITW, Instron Corporation). Detailly, the specimens were cut to a size of 1 × 6 cm2, the clamping distance was set to 3 cm, and the stretching speed was 5 mm/min. The water contact angle (WCA) of the specimen was measured using a video contact angle meter (FM40MK2, Krus's company, Germany).
F. Binding stability of AgNPs on Ag/PCL3-PGA1 composite nanofiber membranes
Ag/PCL3-PGA1 (NTB) and Ag/PCL3-PGA1 (TB) were cut to the same size (1 × 1 cm2) and placed into centrifuge tubes containing 5 ml of purified water, respectively, and sonicated in an ultrasonic cleaner at 100 W for 20 min, room temperature. Subsequently, the intensity of the characteristic absorption peaks of AgNPs in solution was measured using a UV-vis spectrophotometer. Also, the amount of AgNPs in the solution was detected by an inductively coupled plasma mass spectrometer (ICP-MS, Optima 8300DV/CPMS, NexION 300X, PerkinElmer, USA). The bonding stability of AgNPs on PCL3-PGA1 nanofibers after thermal bonding was evaluated by calculation.
G. Determination of Ag+ content released from Ag/PCL3-PGA1 (TB) composite nanofiber membrane
Ag/PCL3-PGA1 (TB) nanofiber membranes of the same size (1 × 1 cm2) were immersed in 5 ml of purified water and shaken in an incubator (TQZ-312, Shanghai Jinghong Experimental Equipment Co., Ltd) at 37 °C, 150 rpm for different times. Afterward, the membranes were removed and the Ag+ content in the aqueous solution was tested by ICP-MS.
H. Antibacterial properties of Ag/PCL3-PGA1(TB) composite nanofiber membrane
The antibacterial activity of the composite nanofiber membranes was tested against Gram-positive S. aureus. Ag/PCL3-PGA1 (TB) and PCL3-PGA1 were cut into the same size (1 × 1 cm2) and UV sterilized for 2 h, followed by being immersed into 5 ml of bacterial suspension (107 CFU/ml), respectively. Then, they were incubated in a shaking incubator at 37 °C, 150 rpm. After 8 h, the bacterial suspension was diluted and spread on nutrient agar plates with a glass applicator, followed by incubation in an incubator (GHP-9050, Shanghai Yiheng Scientific Instruments Co., Ltd.) at 37 °C for 24 h. Finally, the plates were photographed and the colonies were counted.
IV. SUMMARY AND CONCLUSIONS
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ChooseTop of pageABSTRACTI. INTRODUCTIONII. EXPERIMENTIII. RESULTS AND DISCUSSI...IV. SUMMARY AND CONCLUSIO... <<REFERENCESIn summary, we established a facile way to construct a robust AgNP-composited nanofiber through thermal bonding. Two polymers with different Tm were used to prepare coaxial nanofibers, with PGA as the core layer and PCL as the shell layer. By regulating the shell-core flow ratio, the thickness of the PCL layer was regulated, and AgNPs were anchored to the fibers through thermal bonding. This method is simple and easy to operate and the binding stability between the particles and the fiber surfaces was effectively improved. The optimal condition for the heat treatment was found and the antibacterial efficiency against S. aureus reached over 97%. It provides a universal method for the stable combination of particles and fiber surfaces.
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