sı Supporting Information 8 ABSTRACT: Ag 2 S semiconductor nanoparticles (NPs) are near-infrared luminescent 9 probes with outstanding properties (good biocompatibility, optimum spectral operation 10 range, and easy biofunctionalization) that make them ideal probes for in vivo imaging. 11 Ag 2 S NPs have, indeed, made possible amazing challenges including in vivo brain imaging 12 and advanced diagnosis of the cardiovascular system. Despite the continuous redesign of 13 synthesis routes, the

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... sion quantum yield (QY) of Ag 2 S NPs is typically below 0.2%. 14 This leads to a low luminescent brightness that avoids their translation into the clinics. In 15 this work, an innovative synthetic methodology that permits a 10-fold increment in the 16 absolute QY from 0.2 up to 2.3% is presented. Such an increment in the QY is 17 accompanied by an enlargement of photoluminescence lifetimes from 184 to 1200 ns. The 18 optimized synthetic route presented here is based on a fine control over both the Ag core 19 and the Ag/S ratio within the NPs. Such control reduces the density of structural defects 20 and decreases the nonradiative pathways. In addition, we demonstrate that the superior 21 performance of the Ag 2 S NPs allows for high-contrast in vivo bioimaging. 22 * Unknown * | ACSJCA | JCA11.2.5208/W Library-x64 | manuscript.3f (R5.0.i3:5004 | 2.1) 2020/02/05 13:43:00 | PROD-WS-116 | rq_4928177 | 3/02/2020 06:32:53 | 10 | JCA-DEFAULT 61 NIR-II region with a QY of about 40%; 13,14 however, these 62 classes of NPs have been discarded because of their known 63 acute and chronic toxicity. 15,16 64 Silver chalcogenides can be considered as promising 65 alternative candidates for NIR-II fluorescent imaging because 66 of their negligible toxicity and high photostability. 17−19 67 However, their low luminescence brightness is still a big 68 obstacle that limits their application for high-contrast, deep 69 tissue in vivo imaging. Many works dealing with Ag 2 S NP 70 obtain the QY as high as 10%; 20,21 however, most of these 71 results are obtained by comparison with NIR-II fluorescent 72 dyes that do not match with the absolute QYs measured with 73 an integrating sphere and which are typically below 1%. 22 74 These low QYs agree with short photoluminescence (PL) 75 lifetimes observed in these NP, ranging between 50 and 200 ns, 76 which reveal the dominant role of nonradiative pathways 77 within the emission process. 23,24 These nonradiative processes 78 are associated with the presence of certain regions on the 79 surface of the NP that act as charge acceptors, dissociating 80 excitons and therefore reducing the PL efficiency. 25 Some of 81 these traps are related to variations in solvent or surface-bound 82 coordinating ligands (dangling bonds) that influence the 83 steady-state, time-resolved PL and QY. 26 In addition, structural 84 defects in the crystalline structure can also strongly affect the 85 PL efficiency of the NPs because they induce the formation of 86 midgap states that favor non-PL pathways and reduce the PL 87 efficiency of the NPs. 27 Furthermore, in the case of Ag 2 S NPs, 88 the high redox potential of silver ions and the high temperature 89 used during their synthesis can lead to the formation of a 90 metallic silver core when the reaction is carried out in the 91 presence of amines, forming Ag + −amine complexes that are 92 highly reactive and can form Ag 2 S as well as Ag NPs. 28,29 To 93 minimize the formation of the metallic silver core, different 94 authors have proposed to remove, totally or partially, the 95 presence of amine-bearing molecules and use dodecanethiol 96 instead, thereby reducing the reactivity and passivating the 97 surface of the NPs. 30 However, the resulting NPs still 98 presented low QY 31 as well as short PL lifetimes. 32,33 99 In this work, we have developed a new synthesis route 100 toward the improvement of the fluorescence properties of Ag 2 S 101 NPs. A systematic study of the steady-state luminescence, 102 emission lifetime, and absolute QYs of NPs with different 103 metallic silver contents demonstrates that the presence of a 104 specific amount of metallic silver within the NPs increases the 105 QY of the NPs by more than 1 order of magnitude. This 106 increment is concomitant with an enlargement of the PL 107 lifetime which scales from 180 to 1220 ns as well as an 108 emission red shift of the produced Ag 2 S/Ag NPs. The potential 109 use of the improved Ag 2 S NPs for high-contrast in vivo 110 imaging in the second biological window has also been 111 evaluated. EXPERIMENTAL SECTION 112 2.1. Chemicals. Silver nitrate (99%), sodium diethyldithiocarba-113 mate (NaDDTC) (ACS reagent grade), oleylamine (70%) (OLA), 1-114 dodecanethiol (≤98%) (DDT), 11-mercaptoundecanoic acid (95%) 115 (MUA), CHCl 3 (99.6%), ethanol (99.9%), N-(3-(dimethylamino)-116 propyl)-N′-ethylcarbodiimide hydrochloride (EDC) (99%), N-117 hydroxysulfosuccinimide sodium salt (sulfo-NHS) (98%), O-(2-118 aminoethyl)-O′-methyl-polyethylene glycol (PEG-NH 2 , M p = 5000 119 g/mol), and phosphate-buffered saline (PBS) tablets were purchased 120 from Sigma-Aldrich and used as received. Commercial Ag 2 S-PEG NPs 121 were bought from NIR Optics Technology. 122 2.2. Characterization. Transmission electron microscopy (TEM) 123 studies were carried out using a TEM Talos F200X operated at 80 kV. 124 Zeta potential experiments were carried out using a Malvern Nano-125 157 555