Engineering of single molecule fluorescence with uneven nano antennas

PICTURE: (a) Scheme of a double rod nano-antenna coated with AIEE1000 molecules (black double arrows) in PMMA (light blue) on a glass substrate (light gray). The inset shows the chemical structure of AIEE1000. (bg) … view More

Photo credit: by Wenqi Zhao, Xiaochaoran Tian, ​​Zhening Fang, Shiyi Xiao, Meng Qiu, Qiong He, Wei Feng, Fuyou Li, Yuanbo Zhang, Lei Zhou, and Yan-Wen Tan

Single-molecule fluorescence detection (SMFD) can dynamically examine processes that are of crucial importance for understanding the functional mechanisms in biosystems. Near infrared (NIR) fluorescence provides an improved signal-to-noise ratio (SNR) by reducing the scattering, absorption and autofluorescence of biological cell or tissue samples and therefore offers high image resolution with increased tissue penetration depth, which is important for biomedical applications. Most NIR emitters, however, suffer from a low quantum yield, and the weak NIR fluorescence signal makes detection extremely difficult.

Plasmonic nanostructures can convert localized electromagnetic energy into free radiation and vice versa. This ability makes them efficient nano-antennas for modulating molecular fluorescence. The plasmonic nano-antenna generally enhances the fluorescence of a nearby molecule by increasing the excitation rate and the quantum yield of the molecule. In order to optimally intensify the fluorescence, the plasmonic mode of the nano-antenna must 1) be strongly coupled to the molecule and 2) radiate strongly into free space. The simultaneous fulfillment of the two requirements represents a challenge that cannot be overcome with conventional symmetrical plasmonic nanostructures.

In a new article published in Light Science & Application, scientists from the State Key Laboratory for Surface Physics, Physics Department at Fudan University, China, establish a novel, universal approach to improving single-molecule fluorescence in the NIR regime without increasing the photostability of the molecule affect.

They construct asymmetrical nano-antennas that consist of two bars of unequal length (Fig. 1) and deliver several plasmonic modes with adjustable resonance frequencies that correspond to both the excitation and emission frequencies of the fluorophore. The added tuning parameter, ie the ratio of the rod lengths, in such asymmetrical structures offers new possibilities to modulate the near-field and far-field properties of the plasmonic modes, whereby both excitation and emission processes are further improved. As a result, they experimentally obtain a single-molecule fluorescence amplification factor of up to 405 (Fig. 2), and the corresponding theoretical calculations show that the quantum yield can be up to 80%. Since the quantum yield plays an important role in this setup, this improvement is achieved without impairing the survival time of the molecules under laser irradiation.

In addition, compared to reference groups of molecules located on a glass substrate, the authors observed a significantly increased photobleaching time in molecules located around asymmetrical double-bar nano-antennas (Fig. 3), which indicates a much higher number of fluorescence photons, which are emitted by these molecules. The nano-antennas are therefore able to drastically suppress the photo-bleaching. Since the local field enhancement does not improve the photostability, the suppression is mainly based on the increased quantum yield due to the competition between the photobleaching rate and the energy transfer rate to the antenna.

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