Gold and silver nanoparticles in sensing and imaging: Sensitivity of plasmon response to size, shape, and metal composition
Gold and silver nanoparticles in sensing and imaging: Sensitivity of plasmon response to size, shape, and metal composition
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DOI：
10.1021/jp062536y
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发表时间：
2006-10-05
影响因子：
3.3
通讯作者：
El-Sayed, Mostafa A.
中科院分区：
文献类型：
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作者：
Lee, Kyeong-Seok;El-Sayed, Mostafa A.
由于等离子体金属纳米粒子对纳米粒子表面局部环境的敏感光谱响应，以及由于其强烈的散射或吸收而易于监测光信号，因此在化学和生物传感器应用方面具有巨大的潜力。在这项工作中，我们研究了表面等离子体共振响应(频率和带宽)对周围环境变化的敏感性以及光散射对总消光的相对贡献与纳米棒的尺寸和形状以及金属的类型，即Au和Ag的关系。对表面等离子体共振条件的理论分析表明，光谱灵敏度有两个控制因素：一是体等离子体波长，这是一个与金属类型有关的性质；二是纳米棒的长径比，这是一个几何参数。结果表明，灵敏度与这两个因素均呈线性关系。为了定量考察光谱灵敏度与纳米棒金属成分和长径比的关系，采用离散偶极子近似方法计算了银-金合金纳米棒的光谱随银浓度的变化关系。我们观察到，灵敏度并不取决于金属的类型，而是很大程度上取决于纳米棒的长径比。纳米棒尺寸越大，灵敏度与长径比的直接依赖关系越显著。然而，使用较大的纳米颗粒可能会由于多极激发的贡献增加而导致共振谱的过度加宽。这限制了传感分辨率。等离子体激元响应对金属成分的不敏感性是由于金属的体等离子体频率对贵金属具有类似的值，该频率决定了金属的真实介电函数的光谱色散和表面等离子激元共振条件。另一方面，银浓度越高的纳米棒，等离子体共振带的大小和锐度都有很大的提高，即使等离子体响应相似，也能提供更好的传感分辨率。此外，与相同尺寸的金纳米棒相比，银纳米棒作为更好的散射体还有一个额外的优势。
Plasmonic metal nanoparticles have great potential for chemical and biological sensor applications, due to their sensitive spectral response to the local environment of the nanoparticle surface and ease of monitoring the light signal due to their strong scattering or absorption. In this work, we investigated the dependence of the sensitivity of the surface plasmon resonance (frequency and bandwidth) response to changes in their surrounding environment and the relative contribution of optical scattering to the total extinction, on the size and shape of nanorods and the type of metal, that is, Au vs Ag. Theoretical consideration on the surface plasmon resonance condition revealed that the spectral sensitivity, defined as the relative shift in resonance wavelength with respect to the refractive index change of surrounding materials, has two controlling factors: first the bulk plasma wavelength, a property dependent on the metal type, and second on the aspect ratio of the nanorods which is a geometrical parameter. It is found that the sensitivity is linearly proportional to both these factors. To quantitatively examine the dependence of the spectral sensitivity on the nanorod metal composition and the aspect ratio, the discrete dipole approximation method was used for the calculation of optical spectra of Ag-Au alloy metal nanorods as a function of Ag concentration. It is observed that the sensitivity does not depend on the type of the metal but depends largely on the aspect ratio of nanorods. The direct dependence of the sensitivity on the aspect ratio becomes more prominent as the size of nanorods becomes larger. However, the use of larger nanoparticles may induce an excessive broadening of the resonance spectrum due to an increase in the contribution of multipolar excitations. This restricts the sensing resolution. The insensitivity of the plasmon response to the metal composition is attributable to the fact that the bulk plasma frequency of the metal, which determines the spectral dispersion of the real dielectric function of metals and the surface plasmon resonance condition, has a similar value for the noble metals. On the other hand, nanorods with higher Ag concentration show a great enhancement in magnitude and sharpness of the plasmon resonance band, which gives better sensing resolution despite similar plasmon response. Furthermore, Ag nanorods have an additional advantage as better scatterers compared with Au nanorods of the same size.