Photoluminescence Mechanism of DNA-Templated Silver Nanoclusters: Coupling between Surface Plasmon and Emitter and Sensing of Lysozyme

DNA-templated silver nanoclusters (DNA-AgNCs) have now been thrust into the limelight with their superior optical properties and potential biological applications. However, the origin of photoluminescence from DNA-AgNCs still remains unclear. In this work, DNA-AgNCs were synthesized and the photoluminescence properties as well as the biosensing applications of the designed DNA-AgNCs were investigated. The photoluminescence properties of the DNA-AgNCs were studied under three regions of excitation wavelength based on the UV-visible absorption spectra. It was deemed that the photoluminescence originated from coupling between the surface plasmon and the emitter in AgNCs when they were excited by visible light above 500 nm, and thus the emission wavelength varied with changing the excitation wavelength. The photoluminescence of the red-emitting-only AgNCs was the intrinsic fluorescence when excited from 200 to 400 nm, which was only related to the emitter; but for two components of blue- and red-emitting AgNCs, the emission wavelength varied with the excitation wavelength ranging from 300 to 360 nm, and the photoluminescence was a coupling between the surface plasmon and the emitter. The photoluminescence was only related to the surface plasmon when the AgNCs were excited from 400 to 500 nm. Four DNA probes were designed and each contained two parts: one part was the template used to synthesize AgNCs and it was same to all, and the other part was the lysozyme binding DNA (LBD) used to bind lysozyme and two kinds of LBD were studied. It was deemed that the difference in DNA bases, sequence, and secondary structure caused the synthesized DNA-AgNCs to be different in photoluminescence properties and sensing ability to lysozyme, and the sensing mechanism based on photoluminescence enhancement was also presented. This work explored the origin of photoluminescence and the sensing ability of DNA-AgNCs, and is hoped to make a better understanding of this kind of photoluminescence probe.