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Abstract
Ag2S semiconductor nanoparticles (NPs) are near-infrared luminescent probes with outstanding properties (good biocompatibility, optimum spectral operation range, and easy biofunctionalization) that make them ideal probes for in vivo imaging. Ag2S NPs have, indeed, made possible amazing challenges including in vivo brain imaging and advanced diagnosis of the cardiovascular system. Despite the continuous redesign of synthesis routes, the emission quantum yield (QY) of Ag2S NPs is typically below 0.2%. This leads to a low luminescent brightness that avoids their translation into the clinics. In this work, an innovative synthetic methodology that permits a 10-fold increment in the absolute QY from 0.2 up to 2.3% is presented. Such an increment in the QY is accompanied by an enlargement of photoluminescence lifetimes from 184 to 1200 ns. The optimized synthetic route presented here is based on a fine control over both the Ag core and the Ag/S ratio within the NPs. Such control reduces the density of structural defects and decreases the nonradiative pathways. In addition, we demonstrate that the superior performance of the Ag2S NPs allows for high-contrast in vivo bioimaging.
Highlights
Biomedical imaging techniques are used routinely in clinical practice as they provide physiological information of organs/tissues of living beings that is, otherwise, unreachable.[1,2] These noninvasive techniques allow the diagnose and/or prognosis of different diseases and have gained increasing interest in clinical and medical research
Among them, computed tomography (CT), positron emission tomography (PET) or single-photon emission tomography (SPECT), magnetic resonance imaging (MRI), ultrasonography (Echo), and optical coherent tomography are by far the most used techniques. Some drawbacks such as the use of ionizing radiation (CT, PET, and SPECT), limited spatial resolution (MRI, Echo, and PET), poor temporal resolution (CT, MRI, PET, and SPECT), and complex and expensive instrumentation restrict their access to third-level hospitals.[3,4]
The energy-dispersive spectrometry (EDS) elemental mappings showed in Figure 1 reveal the anisotropic spatial distribution of Ag and S within the NPs
Summary
Tissues of living beings that is, otherwise, unreachable.[1,2] These noninvasive techniques allow the diagnose and/or prognosis of different diseases and have gained increasing interest in clinical and medical research. A systematic study of the steady-state luminescence, emission lifetime, and absolute QYs of NPs with different metallic silver contents demonstrates that the presence of a specific amount of metallic silver within the NPs increases the QY of the NPs by more than 1 order of magnitude This increment is concomitant with an enlargement of the PL lifetime which scales from 180 to 1220 ns as well as an emission red shift of the produced Ag2S/Ag NPs. The potential use of the improved Ag2S NPs for high-contrast in vivo imaging in the second biological window has been evaluated. Luminescence decay curves were obtained by exciting the colloidal suspensions of NPs by an OPO oscillator (Lotis) tuned to nm, which provides 8 ns pulses at a repetition rate of 10 Hz. Fluorescence intensity was detected with a Peltier cooled photomultiplier tube with enhanced sensitivity in the NIR-II
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