Boosting the Near-Infrared Emission of Ag2S Nanoparticles by a Controllable Surface Treatment for Bioimaging Applications

Ag2S nanoparticles are the staple for high-resolution preclinical imaging and sensing owing to their photochemical stability, low toxicity, and photoluminescence (PL) in the second near-infrared biological window. Unfortunately, Ag2S nanoparticles exhibit a low PL efficiency attributed to their defective surface chemistry, which curbs their translation into the clinics. To address this shortcoming, we present a simple methodology that allows to improve the PL quantum yield from 2 to 10%, which is accompanied by a PL lifetime lengthening from 0.7 to 3.8 μs. Elemental mapping and X-ray photoelectron spectroscopy indicate that the PL enhancement is related to the partial removal of sulfur atoms from the nanoparticle’s surface, reducing surface traps responsible for nonradiative de-excitation processes. This interpretation is further backed by theoretical modeling. The acquired knowledge about the nanoparticles’ surface chemistry is used to optimize the procedure to transfer the nanoparticles into aqueous media, obtaining water-dispersible Ag2S nanoparticles that maintain excellent PL properties. Finally, we compare the performance of these nanoparticles with other near-infrared luminescent probes in a set of in vitro and in vivo experiments, which demonstrates not only their cytocompatibility but also their superb optical properties when they are used in vivo, affording higher resolution images.


S12.
Lifetime values reported for Ag 2 S NPs in aqueous media S13. PLQY measurements and size characterization of the commercial Ag 2 S NPs S14. Scheme of surface modification and reduction of ligand density upon sonication. S15. Supplementary references S1. Band-gap energy of Ag 2 S NPs analized by Tauc´s method S-3 Figure S1. Band-gap energy of Ag 2 S NPs analized by Tauc´s method The Tauc's plot obtained from the absorption spectrum in the range between 1000 and 1400 nm is depicted in the inset of Figure 2A and in Figure S1. This analysis permits inferring the band-gap energy of the synthesized NPs. The Tauc's method is based on the assumption that the energy-dependent absorption coefficient α can be expressed by the following equation (S1): where h is the Planck constant, ν is the photon's frequency, E g is the band-gap energy and B is a constant. The n factor depends on the nature of the electron transition and is equal to ½ or 2 for the direct and indirect transition band-gaps, respectively. 1 From this analysis, we conclude a direct band-gap transition of 1.04 eV, which agrees with the value obtained for bulk Ag 2 S. 2 S2. PL emission of the NPs upon excitation at 978 nm Figure S2. Normalized PL spectrum of Ag 2 S through excitation at 800 nm (black) and S-4 S3. PL properties of the Ag 2 S NPs during the sonication treatment. TEM images at 40 min of sonication treatment.
In Figure S3A, we can observe the evolution of the PL intensity and the average lifetime along the sonication process showing an enhancement of the PL properties up to 15 min of sonication. At longer sonication treatments de PL properties progressively decrease.
The TEM images ( Figure S3B) reveal that at longer sonication durations the Ag 2 S lose their original shape and stability. After 40 min of sonication most of the Ag 2 S NPs are completely disintegrated.    Blue lines are trapping and detrapping rates.
We theoretically described the PL emission of the Ag 2 S NPs following previous models based on trapping and detrapping of the excited charges. 3,4 We considered a three-level model with a ground state G, an intrinsic exciton state X, and an in-gap state S related to NP defects. The 800 nm excitation laser populates the intrinsic exciton state. The exciton decays (radiatively and mostly non-radiatively) to the ground state but it can also be trapped in an in-gap state. This state S can relax to the ground state (radiatively or mostly non-radiatively) and it can also be detrapped to the intrinsic exciton state. The kinetic equations governing the population of the three states are: where N D , N C , and N E are the populations of the ground state G, the intrinsic exciton state X, and the in-gap state S, respectively, with N D + N C + N E = 1. γ C = γ C 5F/ + γ C #5F/ (γ E = γ E 5F/ + γ E #5F/ ) is the radiative decay rate of X (S), where the fast non-radiative contribution dominates being responsible for the emission quenching. The probability that an exciton S-9 is trapped in an in-gap state is given by k CE , whereas k EC is the detrapping rate. R is the rate of exciton generation by the excitation laser. The values of the parameters were chosen taking into account the experimental values of the QY and the long decay time (in the microseconds range). We took slow radiative decay rates γ C 5F/ = 5 × 10 G s -1 and γ E 5F/ = 1.7 × 10 G s -1 , and faster trapping and detrapping rates k CE = 2 × 10 H s -1 and k EC = 6.6 × 10 A s -1 (by Boltzmann distribution). The rate of laser induced exciton generation was considered much lower than the radiative decay rate R = 1.6 × 10 G s -1 to ensure the operation within the excitation linear regime. Finally, to simulate the sonication treatment, we varied the nonradiative decay rates of both excited states to follow the elimination of surface quenchers. We changed γ C #5F/ = γ E #5F/ = γ #5F/ from the nanoseconds to the microsecond range.
We calculated the steady-state populations from Equations (S5)-(S6) which give us information about the PL emission intensity and the contribution of the intrinsic exciton emission and the in-gap emission.
We plotted in Figure S9A the number of emitted photons per second for both excited states (intrinsic exciton state γ C 5F/ N C and in-gap state γ E 5F/ N E ) as the non-radiative decay rate γ #5F/ is decreased. Initially, i.e., large γ #5F/ , the emission is strongly quenched for both states. As the sonication time increases, the reduction of non-radiative deexcitation pathways simultaneously increases both emissions. Furthermore, the overall enhancement in PL can be analyzed in terms of the PLQY by computing the total number of emitted photons per second and the number of absorbed laser photons per second: The simulated PLQY given by Equation (S9) shows a good agreement with the measured values (see Figure S9B). That is, an increase from 2% to 10% is obtained. = γ 2 + γ 23 + γ 32 + γ 3 2 ∓ 1 2 )(γ 2 + γ 23 + γ 32 + γ 3 ) 4 − 4(γ 2 γ 32 + γ 2 γ 3 + γ 3 γ 23 ) (S10) Figure S10 shows the two decay times (Equation (S10)) as a function of γ #5F/ for the same parameters than in Figure S9. In agreement with the experiments (symbols in Figure   S10), the long decay time (in the microseconds range) strongly increases as the nonradiative decay rate decreases whereas the short decay time is less affected by the reduction of nonradiative pathways. A close inspection of Equation (S10) reveals that the long decay time behaves as the excited decay rate τ I&#$ ≃ γ E :! (see dashed line in Figure   S-11 S10), which increases as γ #5F/ decreases. On the other hand, the short decay time nearly behaves as the trapping rate, i.e., τ 21&5-≃ (γ CE + γ EC ) :! .  S-13 S11. Hydrodynamic sizes of the NPs transferred to water.

Figure S12. Hydrodynamic diameter distributions of the NPs functionalized with three
PEGs of different molecular weights (750, 2000 and 5000 g·mol -1 ). Ag 2 S@Thyoglicolic acid 0.003 10 S-14 S13. PLQY measurements and size characterization of the commercial Ag 2 S NPs.

Core@Coating Lifetime (µs) Reference
Commercial Ag 2 S NPs in water were purchased from Sinano corp.