Mn-Doped AgZnInS/ZnS Nanocrystals (NCs): Effects of Zn Etching on the NC Optical Properties.

Mn-doped I(II)-III-VI NCs (e.g., Mn-doped AgZnInS/ZnS NCs) possessing low-energy excitation, high brightness and long fluorescence lifetimes have been desired for time-gated fluorescence biosensing/imaging. In this type of NCs, their optical properties are significantly affected by the microscopic interactions between Mn and Mn and between Mn and host NC, the compositions of NCs, and the defects in NCs. On the other hand, it is known that Zn etching to core I(II)-III-VI NCs in NC synthesis can significantly enhance the NC brightness because Zn can exchange surface atoms (e.g., Ag and In) in NCs to minimize NC surface-defects. But for Mn-doped I(II)-III-VI NCs, Zn etching could etch out not only surface-atoms of host NCs (e.g., Ag and In) but also Mn in NCs. As a result, it could significantly affect the NC compositions and the microscopic interactions between Mn and Mn as well as between Mn and host NC, and thus the optical properties of NCs (like lifetime and absorption/emission spectra). Therefore, it is needed to investigate how Zn etching would affect the optical properties of such Mn-doped NCs. In this study, a series of Mn-doped AgZnInS NCs with different Mn doping levels were prepared through nucleation doping, and then Zn etching was applied to etch these core NCs. To identify the effects of Zn etching on NC optical properties, ZnS coating (a different ZnS shelling approach by injecting Zn precursor and S precursor alternately in synthesis) was performed on the same Mn:AgZnInS NCs, and the optical properties of NCs with these two different ZnS shelling approaches were compared. Experimental results showed that under appropriate Mn doping levels in synthesis, Zn etching instead of ZnS coating can produce low-energy excitable NCs with higher QYs and longer lifetimes, which would further facilitate the use of such NCs in time-gated fluorescence measurement. To understand the reasons for the different optical properties under different ZnS shelling approaches, the material characteristics of the prepared NCs were further measured/analyzed and the possible fluorescence mechanisms were discussed.

Optical Properties of Mn-Doped CuGa(In)S-ZnS Nanocrystals (NCs): Effects of Host NC and Mn Concentration.

Time-gated fluorescence measurement (TGFM) using long-life fluorescence probes is a highly sensitive fluorescence-measurement technology due to the inherently high signal-to-background ratio. Although many probes for TGFM such as luminescent-metal-complex probes and lanthanide-doped nanoparticles are in development, they generally need sophisticated/expensive instruments for biosensing/imaging applications. Probes possessing high brightness, low-energy (visible light) excitation, and long lifetimes up to milliseconds of luminescence, are highly desired in order to simplify the optical and electronic design of time-gated instruments (e.g., adopting non-UV-grade optics or low-speed electronics), lower the instrument complexity and cost, and facilitate broader applications of TGFM. In this work, we developed Mn-doped CuGa(In)S-ZnS nanocrystals (NCs) using simple and standard synthetic steps to achieve all the desired optical features in order to investigate how the optical properties (fluorescence/absorption spectra, brightness, and lifetimes) of the Mn-doped NCs are affected by different host NCs and Mn concentrations in host NCs. With optimal synthetic conditions, a library of Mn-doped NCs was achieved that possessed high brightness (up to 47% quantum yield), low-energy excitation (by 405 nm visible light), and long lifetimes (up to 3.67 ms). Additionally, the time-domain fluorescence characteristics of optimal Mn-doped NCs were measured under pulsed 405 nm laser excitation and bandpass-filter-based emission collection. The measurement results indicate the feasibility of these optimal Mn-doped NCs in TGFM-based biosensing/imaging.

A Compact and Sensitive Time-Resolved-Optical-Reader for Bioassay Using Low-Energy Excitable and Long-Lived-Fluorescence Nanolabels.

Time-resolved fluorescence measurement, a technique utilized to improve sensitivity for fluorescence detection, is becoming more accessible due to the recent development of bright nanolabels with long fluorescence lifetimes and low-energy excitation. Currently, instruments taking advantage of this technique for broader applications (in-field or point-of-care testing) are lagging behind in their development. In this work, a low-cost, compact, and sensitive time-resolved-optical-reader was developed aiming to advance such a technique for wider applications.