| Local surface plasmon resonance (LSPR) sensing technology attracts much attention for biochemical monitoring due to its advantages of high sensitivity, fast measurement speed, real-time monitoring and unnecessariness of marking the samples. Biosensor is based on LSPR measures resonant wavelength shift with changing of refractive index in the surrounding environment. The relationship between the shape and size of the gold nanoparticles and LSPR, and the way to use the shape and size to adjust wavelength and optimize sensing properties for LSPR, were the points which researchers focus on. These also are key technical issues in practical applications. To solve these problems, the relationship between LSPR properties and shape and size of metal nanoparticles is numerically studied by Finite Difference Time Domain (FDTD), and the functional relationship was obtained in this paper. The relationship between extinction efficiency and FWHM and the shapes and sizes in three directions were studied. The LSPR sensing properties were optimized under a specific wavelength for nanoparticles of four kinds of shapes, and then the structures with high quality factors were obtained.The main contents and results of this paper are as follows:1) The effects of the shape and size in different directions on LSPR properties were different. The shape and size in the direction parallelling to polarization direction of incident light play a main role for inspiring LSPR. Increasing this size and the shape becoming sharper in the direction, resonance wavelength redshifted, while blueshifted. The shape and size in the direction being perpendicular to polarization direction of incident light plays a secondary role. Increment of the size of the direction results in the resonant wavelength blueshifted.2) Shape parameter L that can confirm LSPR wavelength in the air has a good linear relationship withζ:1/L=aζ+b, wherein the coefficient a embodys the sharpness in light polarization direction.3) The relationship between LSPR wavelength and the sizes of ellipse, rectangle, rhombus and triangle nanoparticle in each direction was also studied and their functional relationship were obtained. Resonant wavelength change linearly with the size in the direction parallelling to polarization direction of incident light, which can be fitted with the function of λ(x)=k1x+b1. The reciprocal of the size in the direction perpendicular to polarization direction of incident light was a linear relationship with LSPR wavelength, it can be fitted by λ(y)=k2/y+b2function. The reciprocal of the size in the direction parellelling to the incident is also linear. The relationship between the reciprocal of the size and LSPR wavelength can be expressed by a linear function of λ(z)=k3/z+b1. Where the values of coefficients k1, b1, k2, b2, k3, b3are greater than zero.4) All shapes and sizes in each direction exhibit saturation effect. There are smallest value as start-up resonant and the bigest value as saturation. When the size is smaller than start-up resonant value extinction efficiency is low and FWHM is very wide. FWHM is wider when the size is bigger than saturation value. And FWHM is the best between start-up resonant and saturation value.5) LSPR sensing properties can be adjusted and optimized under a specific wavelength by controlling the sizes of the different shapes nanoparticles in different directions. For ellipse, rectangle, rhombus and triangle gold nanoparticles, when the resonant wavelength is of700nm the ellipse nanoparticle has the highest quality factor, to2.10, and the triangle nanoparticle has the worst. The quality factor of the rhombus nanoparticle is the best for900nm and1100nm, respectively to3.92and5.50. That of ellipse nanoparticle is the lower, only3.63. Quality factors optimized by the shape and size improved by more than30%than unoptimized.The results of this paper provide theoretical references for controlling a wavelength and the optimization of sensing properties in the applications of LSPR sensor. |
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