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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.

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Core-shell structured quantum dot (QD)-silica fluorescent nanoparticles have attracted a great deal of attention due to the excellent optical properties of QDs and the stability of silica. In this study, core-shell structured CdTe/CdS@SiO @CdTe@SiO fluorescent nanospheres were synthesized based on the Stöber method using multistep silica encapsulation. The second silica layer on the CdTe QDs maintained the optical stability of nanospheres and decreased adverse influences on the probe during subsequent processing. Red-emissive CdTe/CdS QDs (630 nm) were used as a built-in reference signal and green-emissive CdTe QDs (550 nm) were used as a responding probe. The fluorescence of CdTe QDs was greatly quenched by added S , owing to a S -induced change in the CdTe QDs surface state in the shell. Upon addition of Cd to the S -quenched CdTe/CdS@SiO @CdTe@SiO system, the responding signal at 550 nm was dramatically restored, whereas the emission at 630 nm remained almost unchanged; this response could be used as a ratiometric 'off-on' fluorescent probe for the detection of Cd . The sensing mechanism was suggested to be: the newly formed CdS-like cluster with a higher band gap facilitated exciton/hole recombination and effectively enhanced the fluorescence of the CdTe QDs. The proposed probe shows a highly sensitive and selective response to Cd and has potential application in the detection of Cd in environmental or biological samples.

In this study, three kinds of red-emitting Ag nanoclusters capped with denatured lysozyme (dLys-AgNCs) were synthesized from different reactant molar ratio of AgNO 3 and lysozyme, called dLys-AgNCs 1, dLys-AgNCs 2 and dLys-AgNCs 3, respectively.The thus synthesized dLys-AgNCs 1 contained only Ag(0), while dLys-AgNCs 2 and dLys-AgNCs 3 contained both Ag(0) and Ag(I). The maximum emission wavelengths of the three dLys-AgNCs were all located near 640 nm when excited at 490 nm, but with different photoluminescence intensity. The photoluminescence properties of these dLys-AgNCs in the presence of common inorganic anions were studied. The three dLys-AgNCs had distinct photoluminescence responding mode to S 2-. The photoluminescence of dLys-AgNCs 1 could be quenched by S 2-, as S 2-destroyed the nanostructure of dLys-AgNCs 1 by forming Ag 2 S. While, the photoluminescence of dLys-AgNCs 3 could be enhanced by S 2-, as S 2-would combine with Ag(I) contained in dLys-AgNCs 3 and altered the ligand-to-metal-metal charge transfer process. For dLys-AgNCs 2, the photoluminescence was enhanced firstly with small amount of S 2-and then quenched with more S 2-added, and this could be due to that S 2-combined with Ag(I) first at low concentration and then with Ag(0) in high concentration. Other anions had no obvious effects except that I -could quench the photoluminescence of these three dLys-AgNCs. The responding mechanisms were further studied by XPS, MS, zeta potential, DLS, and lifetime measurements. Moreover, the as-prepared dLys-AgNCs 1 and dLys-AgNCs 3 were used as fluorescent probe for S 2-