Study On Preparation Of Nanoarrays By Block Copolymer Micelle Method And Their Fluorescence Enhancement Effect

Fluorescence-based devices and techniques have been widely used in the field of chemistry, biology, medicine, photonics, and materials science. Enhancing the fluorescence intensity of fluorophore would greatly improve the performance of optoelectronic devices as well as other fluorescence-based techniques. One of important strategies to achieve fluorescence enhancement is utilizing noble metal nanostructures due to the generation of surface plasmon resonances upon resonant light excitation. At present, a wide range of metal nanostructures such as nanowires, nanorods, nanoholes, nanotriangles, have been reported to enhance fluorescence of various fluorophores, especially small dye molecules. However, fluorescence enhancement based on highly ordered metal nanoarray substrates was rarely reported. To fabricate highly ordered nanoarrays as metal-enhanced fluorescence (MEF) substrates, a number of methods have been developed, including anodic alumimum oxide (AAO) template deposition, electron-beam lithography, polystyrene microspheres template deposition and so on.Here we report a simple and economical approach for fabricating large-area and well-ordered nanoparticle arrays on silicon substrate by block copolymer micelle self-assembly. The morphologies and compositions were characterized using scanning electron microscope (SEM), transmission electron microscopy (TEM), and energy dispersive spectrometer (EDS). Moreover, fluorescence enhancement effect of poly(3-hexylthiophene) (P3HT) on different kinds of nanoparticle arrays were systematically investigated by ultraviolet and visible spectrometer, fluorescence spectrometer, Raman spectroscopy, and FDTD simulation.Well-ordered Au nanoparticle arrays with different particle sizes were fabricated by block copolymer self-assembly and adjusting the molar ratios of HAuCl4 precursor to vinyl pyridine units. Fluorescence spectrum analysis showed that the emission intensities of the conjugated polymer P3HT on ordered Au nanoparticle arrays were much stronger than those on the bare silicon substrate due to the electromagnetic field enhancement based on the localized surface plasmon resonances and coupling. Furthermore, the fluorescence intensities of P3HT on Au nanoparticle arrays were significantly enhanced with the Au nanoparticle sizes increasing. The reason is that the fluorescence intensities of P3HT could be stronger with the higher overlap between extinction spectra of the P3HT film and the Au nanoparticle arrays. Moreover, the FDTD simulation results showed that larger Au nanoparticles can effectively enhance the electromagnetic fields in larger region, enabling stronger coupling. And fluorescence lifetime testing showed that the decay times of P3HT has no significant change.The particle size is a major factor that determines the surface plasmon resonance frequency and the fluorescence enhancement factor. In this paper, a metal overgrowth process was utilized to tuned the particle sizes. As a seed layers, the Au nanoparticle arrays fabricated by block copolymer technology was soaked in an aqueous solution of HAuCl4, silver nitrate, ascorbic acid and hexadecyltrimethylammonium bromide (Au growth solution). SEM and EDS results showed that the nanoparticles successfully growed after 10min. The extinction spectrum testing showed that the localized surface plasmon resonance peak of Au nanoarrys after growth red-shift and overlap better with the absorption peak of the P3HT. Furthermore, FDTD simulation results showed that the electromagnetic fields intensity was effectively enhanced due to the stronger coupling between adjacent particles. As a result, the fluorescence intensity of P3HT films was enhanced greatly on the Au nanoparticle array after growth.Au/ZnO binary nanoparticle arrays were fabricated by two simple processes. First, the Au nanoparticle arrays were prepared by block copolymer micelle method. Then using Zn(CH3COO)2 as precursor, ZnO nanoparticle arrays were fabricated on the prepared Au nanoparticle arrays, making up Au/ZnO binary nanoparticle arrays. The results showed that the addition of ZnO shows no effect to the plasmon resonance properties of the Au nanoparticle arrays. However, compared to those on the Au nanoparticle arrays, the fluorescence intensities of P3HT were greatly enhanced on the binary nanoparticle arrays.