Metal Nanoparticles Based Plasmon Resonance Enhanced In-Plane Structures And Their Optical/Thermal Applications

Metal nanoparticles such as gold and silver nanoparticles with unique surface plasmon resonance（SPR）effect,have been widely applied in many fields such as energy,catalysis,biology,environment,sensing and medicine.The SPR effect of metal nanoparticles can enhance the local electromagnetic field around them and produce strong selective spectral absorption in the UV-Vis-NIR region,which is closely related to the parameters such as the composition,size,shape,spacing and arrangement structure of these nanoparticles.Thus,the researchers focused on designing many kinds of metal nano-structures to enhance their SPR effect,so as to improve the optical and thermal properties and extend their applications in surface enhanced spectroscopy,photothermal conversion,etc.In recent years,with the rapid development of nanotechnology and the improved requirements of practical application,people have put forward higher requirements for the portability,lightness and miniaturization of functional nanodevices.Therefore,the preparation and regulation of in-plane structures of metal nanoparticles has gradually become a hot research field.However,conventional methods such as etching and vapor deposition for preparing metal nanostructures are not favorable for large-scale production due to their low production efficiency,high cost,complex operation and low structure resolution,which limits their subsequent planar applications.In this paper,a simple and versatile self-assembly method is developed to realize the fabrication of in-plane metal nanostructures.We detailedly analyzed and discussed about the design idea and mechanism of action of enhanced SPR effect,and the regulation of optical/thermal performance and application.The research contents are as following:（1）The two-dimensional Au/Ag nanoparticle-island structure with abundant nanogaps was prepared on the planar substrate through the hierarchical assembly of different-sized metal nanoparticles.Moreover,the fluorescence enhanced detection platform was constructed by combining with near-infrared fluorescent dyes in order to achieve improved fluorescence enhancement performance.According to the random sequential adsorption（RSA）model,the small sized gold nanoparticle owned the reduced repulsive energy barrier,thus they can insert into the vacant spaces between large sized silver nanoparticles on the substrate,and then form a large number of nanogaps to enhance its electromagnetic field around the hot spots.The two-dimensional Au/Ag nanoparticle-island structure on the planar substrate produced plasmon coupling peak toward the near-infrared（NIR）region,and amplify the fluorescence emission intensity of NIR fluorescent dyes.This fluorescence detection platform based on in-plane Au/Ag nanoparticles-island structure can achieve a maximum fluorescence enhancement factor of 3.6 times.Compared with the pre-assembled Ag nanoparticle platform,the introduction of small sized Au nanoparticle enhanced the fluorescence emission intensity of the fluorescent dye Cy5,especially the fluorescence enhancement efficiency of the two-dimensional arranged Au/Ag nanoparticle-island structure reached 161.6%.In addition,the finite difference time domain（FDTD）method was used to illustrate the electromagnetic field enhancement mechanism and fluorescence enhancement mechanism of Au/Ag nanoparticle-island hierarchical structure.This study demonstrates the advantage of hierarchical assembly structure of Au/Ag nanoparticles in two-dimensional platform for fluorescence enhancement detection.（2）To further enhance the surface plasmon resonance effect of on-plane metal nanostructures and broaden their near-infrared absorption spectra,the hierarchical electrostatic field was constructed by using the gold and silver nanoparticles with different sizes and surface charges,which drove the nanoparticles to achieve linear assembly on the planar substrate.The one-dimensional nanochain structures of metal nanoparticles in plane were prepared and used as the solar heating surface to achieve improved photothermal conversion performance.Through adding the small sized gold nanoparticles to the silver nanoparticles with large size and surface charge,the electrostatic field was established according to the Derjaguin-Landau-Verwey-Overbeek（DLVO）theory,the hierarchical electrostatic fields combined with the effects of attractive van der Waals forces and dipolar interactions,contributed to the random generation of NP oligomers and the growth of nanochains on the substrate,and thus realized the in-plane linear assembly of metal nanoparticles.The metal nanochains structures enhanced the longitudinal plasmon coupling effect,and the resulted chain-like distribution of hot spots broadened the spectral absorption in the near infrared range,thus improving the photothermal conversion effect.The surface temperature of metal nanochains on the planar substrate was increased by about 32°C after being exposed to 3.2 k W·m^-2 of simulated sunlight for 10 min.In addition,the solar heating surface has high visible light transmittance（65%）,meanwhile the window surface with metal nanochains increased 9.8°C than the bare glass after 1 h of natural sun exposure outdoors.Thus,it can be used for energy saving in buildings as transparent solar radiator in the cold winters,in order to maintain comfortable indoor environment while achieve energy saving effect.（3）To further optimize the in-plane nanochain structures and improve the photothermal conversion performance,the linear assembly of metal nanoparticles with selective adsorption behaviors were achieved by adjusting electrostatic effect between the nanoparticles in the hierarchical electrostatic fields,and thus preparing optimized metal nanochain structures with adjustable chain length and morphology on the planar substrate.Then the in-plane optimized metal nanochain structures were combined with thermal responsive hydrogel to construct smart windows,which achieved improved adaptive solar modulation performance.According to DLVO theory,the competitive effect of electrostatic repulsive force and van der Waals attraction in the linear assembly process promotes the selective adsorption of subsequent nanoparticles on the ends or sides of the short nanochains,which leads to the generation of optimized structures such as long chains,bended chains and folded chains on planar substrates.The strong longitudinal plasmon coupling effect of these optimized structures causes the redshift of their plasmon absorption peak,which further broadens and enhances the spectral absorption in the near infrared region.The optimized structure can further improve the photothermal conversion performance compared with the short nanochain structure.The surface temperature of in-plane optimized structure assembled for 2 h reached 53.7°C under the irradiztion of 2.8 k W·m^-2 of simulated sunlight for 5 min.The obtained solar heating surface can act as light-stimulated nanoheaters to drive the phase transition of hydrogel and thus change the light transmittance.The sandwich-structured smart windows integrated with in-plane nanochains and hydrogel showed favorable initial visible light transmittance（71.2%）at room temperature,which satisfied the requirement of the transparency of the window in buildings.In addition,the smart window under strong light has fast response speed and higher near-infrared light shielding ability,which can block 94.1%of solar radiation from 300 to 2500 nm.Furthermore,the smart window based on gold nanochains showed good indoor cooling performance in the test of outdoor model houses.After 1 h of exposure to natural sunlight,the smart window could achieve a cooling performance of 7.8°C compared with the ordinary window,indicating its great potential in the practical application of energy-saving buildings.（4）To expand the photothermal application of in-plane assembled structures of metal nanopaeticles,the composite coated fabric with densely deposited silver and gold nanoparticles was prepared by spraying assembly method on the surface of flexible fabric.After wearing on the knuckles of human,the composite coated fabric achieved continuous and steady photothermal conversion performance and thermotherapy effect for human body.Field emission scanning electron microscopy（SEM）was used to observe the assembly morphology of nanoparticles on the fabric surface.It was found that the gold and silver nanoparticles were randomly distributed on the surface of the fabric,and the small sized Au NPs were densely deposited in the gap of the large size Ag NPs.Compared with the blank fabric without coating,the absorption spectra of the coated fabric showed the characteristic absorption peaks of Ag and Au NPs respectively and enhanced the absorption spectra in the near infrared direction.Moreover,the gold/silver nanoparticles composite coated fabric showed good photothermal conversion performance under simulated sunlight of 1k W·m^-2,and the surface temperature of the coated fabric reached 48.8°C after 5 min of solar irradiation,meanwhile achieved about 20.9°C of temperature increment compared with the bare fabric.Besides,the surface temperatures of the coated fabric satisfy the temperature requirements of keeping warmth and thermotherapy for human body.This gold/silver nanoparticles coated on the fabric surface through electrostatic interactions also has good washing stability and photothermal stability,meanwhile keeping the air permeability of the bare fabric,which illustrated that the gold/silver nanoparticles composite coated fabric is very suitable for flexible,breathable and wearable textiles for thermothertapy.