Huygens-Kamerlingh Onnes Laboratory, Leiden University, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands.
J Chem Phys. 2022 Mar 7;156(9):094201. doi: 10.1063/5.0073208.
Plasmonic metallic nanoparticles are commonly used in (bio-)sensing applications because their localized surface plasmon resonance is highly sensitive to changes in the environment. Although optical detection of scattered light from single particles provides a straightforward means of detection, the two-photon luminescence (TPL) of single gold nanorods (GNRs) has the potential to increase the sensitivity due to the large anti-Stokes shift and the non-linear excitation mechanism. However, two-photon microscopy and spectroscopy are restricted in bandwidth and have been limited by the thermal stability of GNRs. Here, we used a scanning multi-focal microscope to simultaneously measure the two-photon excitation spectra of hundreds of individual GNRs with sub-nanometer accuracy. By keeping the excitation power under the melting threshold, we show that GNRs were stable in intensity and spectrum for more than 30 min, demonstrating the absence of thermal reshaping. Spectra featured a signal-to-noise ratio of >10 and a plasmon peak width of typically 30 nm. Changes in the refractive index of the medium of less than 0.04, corresponding to a change in surface plasmon resonance of 8 nm, could be readily measured and over longer periods. We used this enhanced spectral sensitivity to measure the presence of neutravidin, exploring the potential of TPL spectroscopy of single GNRs for enhanced plasmonic sensing.
等离子体金属纳米粒子通常用于（生物）传感应用，因为它们的局域表面等离子体共振对环境变化非常敏感。虽然从单个粒子散射光的光学检测提供了一种直接的检测手段，但由于大的反斯托克斯位移和非线性激发机制，单个金纳米棒（GNRs）的双光子荧光（TPL）有可能提高灵敏度。然而，双光子显微镜和光谱学受到带宽的限制，并且受到 GNRs 热稳定性的限制。在这里，我们使用扫描多焦点显微镜以亚纳米精度同时测量数百个单个 GNR 的双光子激发光谱。通过将激发功率保持在熔化阈值以下，我们证明 GNR 在强度和光谱上稳定超过 30 分钟，表明没有热重塑。光谱的信噪比大于 10，等离子体峰值宽度通常为 30nm。可以很容易地测量到小于 0.04 的介质折射率变化，对应于 8nm 的表面等离子体共振变化，并且可以在更长的时间内进行测量。我们利用这种增强的光谱灵敏度来测量中性亲和素的存在，探索单根 GNR 的 TPL 光谱在增强等离子体传感方面的潜力。
