Near-infrared Ag₂Se quantum dots with distinct absorption features and high fluorescence quantum yields

Abstract

Near-infrared Ag₂Se quantum dots (QDs) with distinct absorption features ranging between 830–954 nm and fluorescence quantum yields up to 23.4% were controllably synthesized using a phosphine-free approach. Based on the distinct absorption features, the molar extinction coefficients of the Ag₂Se QDs have been determined.

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Silver chalcogenide (Ag₂X, X = S, Se and Te) quantum dots (QDs) are ideal materials for bioimaging because of their small size, low cytotoxicity and tunable fluorescence emission in the near-infrared (NIR) region.^1–9 With a narrow bulk bandgap of 0.15 eV, Ag₂Se QDs are much more promising because of their potential emission in the NIR IIa window with high quantum yield.^10–12 Moreover, NIR Ag₂Se QDs with NIR absorption features are more desirable because NIR fluorescence imaging is performed under NIR excitation.^12,13 So far, Ag₂Se QDs are generally prepared using hot-injection,^11,14 solvothermal¹⁰ and aqueous synthesis processes.¹⁵ The QDs prepared using an aqueous synthesis process usually exhibit some unfavorable properties, such as featureless absorption, relatively low fluorescence quantum yield (FL QY) and a broad size distribution,^2,15–17 which limit their biomedical application. A hot-injection approach, which is able to produce nanocrystals with a narrow size distribution,^18,19 is desirable for synthesizing high-quality Ag₂Se QDs with distinct absorption features and high quantum yields. During the hot-injection synthesis of Ag₂Se QDs, hazardous TOPSe (Se powder directly dissolved in TOP) was used as the Se precursor, and the prepared Ag₂Se QDs exhibited indistinct NIR absorption features and low FL QYs,^11,14,20 which was induced by the low reactivity of TOPSe.²¹ Therefore, it is still a great challenge to synthesize NIR Ag₂Se QDs with distinct absorption features and high FL QYs.

In this work, we use an alternative Se precursor ODE–Se (Se powder directly dissolved in 1-octadecene (ODE)), which has been proven to be more reactive than TOPSe for the synthesis of high-quality CdSe QDs.²² Though ODE–Se has been employed to synthesize Ag₂Se seeds (7–12 nm) for the preparation of Ag₂Se–ZnS nanorods and nanowires,²³ it has not been used to synthesize NIR Ag₂Se QDs. With ODE–Se as the phosphine-free selenium precursor, the synthesized Ag₂Se QDs show distinct absorption features and high FL QYs. Thereby, the extinction coefficient (ε) values of the Ag₂Se QDs were determined.

To synthesise the Ag₂Se QDs, ODE and Se powder were mixed, heated and cooled to produce a ODE–Se stock solution. Then, the ODE–Se stock solution was swiftly injected into a heated mixture of AgAc, ODE and 1-octanethiol (OT). Subsequently, the temperature was reduced to allow slow growth of the nanocrystals. The prepared nanocrystals were precipitated using ethanol and redispersed in non-polar solvents for further characterization. To compare the reactivity of ODE–Se and TOPSe, both of them were used as Se precursors to synthesize Ag₂Se nanocrystals in identical conditions. The growth kinetics were monitored by measuring the absorption spectra of the products at different reaction times. As shown, the absorption spectra of the products were featureless with TOPSe as the Se precursor (Fig. 1A), whereas, when using ODE–Se as the Se precursor, the products displayed distinct absorption features (Fig. 1B) similar to those of CdSe²⁴ and PbSe²⁵ QDs. These distinct absorption features indicated that small-sized nanocrystals with narrow size distributions were obtained.¹⁸ The TEM image showed that the products were spherical particles with sizes of 2.8 ± 0.35 nm (Fig. 2A). The high-resolution TEM (HRTEM) image showed an interplanar spacing of 0.241 nm (Fig. 2A, inset) which could be indexed as the (013) crystallographic facet of orthorhombic Ag₂Se. The powder X-ray diffraction (XRD) pattern (Fig. 2B(a)) of the obtained products did not show distinguishable diffraction peaks due to the ultra-small size and the organic ligands on the surface of the nanocrystals.² After annealing at 180 °C for 30 min under Ar, all of the diffraction peaks in the XRD spectra became distinguishable (Fig. 2B(b)), matching well with those of orthorhombic Ag₂Se (JCPDS card no. 24-1041). These results confirmed that the products were Ag₂Se nanocrystals. The 2.8 nm Ag₂Se nanocrystals exhibited a narrow fluorescence spectrum centered at 1010 nm (Fig. 2C(b)), with a FL QY up to 23.4% (Fig. S1, ESI†) referenced to the organic dye ICG (QY ≈ 13%), which is superior to that of previously reported results (Table S1, ESI†). Interestingly, the Stokes shift of 66 nm was much smaller than the previously reported values of 178–362 nm.^11,26 The small Stokes shift might be attributed to the predominant direct excitonic recombination rather than the nonradiative recombination which usually results in a large Stokes shift.²⁵ Moreover, the small Stokes shift is favorable for exciting NIR emission from the QDs using NIR light to reduce the absorption and scattering by the body during in vivo imaging.¹² After hydrophobic encapsulation with amphiphilic polymers,²⁷ the prepared Ag₂Se nanocrystals were successfully transferred to the aqueous phase. The water-soluble Ag₂Se nanocrystals retained excellent optical properties (Fig. S2, ESI†), such as the distinct NIR absorption feature and a high FL QY of 17.7%. Dynamic laser scattering measurements showed that the Ag₂Se nanocrystals had good solubility and stability (Fig. S3, ESI†). The energy-dispersive X-ray (EDX) results (Fig. 2D) of the 2.8 nm Ag₂Se nanocrystals showed a Ag/Se atomic ratio of 2.55/1, which agreed well with the inductively coupled plasma atomic emission spectroscopy (ICP-AES) result of 2.70/1 (Table S2, c, ESI†), indicating that the prepared Ag₂Se nanocrystals were Ag-rich nanoparticles. To further confirm the surface composition of the Ag₂Se nanocrystals, X-ray photoelectron spectroscopy (XPS) (Fig. 3) analysis was employed. As shown in Fig. 3, the binding energies of Ag 3d_5/2 and Ag 3d_3/2 appeared at 368 eV and 374 eV, respectively.²⁸ The peak at 54.2 eV could correspond to Se 3d.²⁸ The high resolution XPS spectrum of S 2p (Fig. 3D) showed peaks at 160.7 eV, 161.8 eV, and 166.0 eV, which could be assigned to Ag–S,²⁹ thiolate,³⁰ and disulfides,³¹ respectively. The appearance of Ag–S bonds suggested that the Ag₂Se cores were coated with a thin Ag surface shell passivated with the –SH group of the OT.²⁵ The Fourier transform infrared (FT-IR) spectrum (Fig. S4, ESI†) indicated that the nanocrystals were capped with OT alone. Thus, it could be concluded that the prepared Ag₂Se nanocrystals were Ag-rich and well passivated by OT. With strong Ag–S bonds between Ag(I) and OT on the surface of the Ag₂Se nanocrystals, trap states could be reduced greatly, increasing the probability of direct excitonic recombination. Consequently, the obtained Ag₂Se nanocrystals exhibited a high FL QY and a small Stokes shift, just as mentioned above.

Fig. 1 Temporal evolution of the absorption spectra of the Ag₂Se nanocrystals synthesized with different Se precursors: (A) TOPSe; (B) ODE–Se.

Fig. 2 (A) TEM and HRTEM (inset) images of the as-prepared Ag₂Se nanocrystals; (B) XRD patterns of the as-prepared Ag₂Se nanocrystals before (a) and after (b) annealing at 180 °C for 30 min under an Ar flow; (C) UV-vis-NIR absorption spectrum (a) and fluorescence emission spectrum (b) of the as-prepared Ag₂Se nanocrystals; (D) EDX spectrum of the as-prepared Ag₂Se nanocrystals.

Fig. 3 XPS survey spectrum (A), and the corresponding high-resolution XPS spectra of Ag 3d (B), Se 3d (C) and S 2p (D) of the as-prepared Ag₂Se nanocrystals.

The injection temperature is a key factor in determining the amounts of monomer and crystal nucleus upon mixing the cation and anion precursors, so the temporal evolutions of the absorption spectra of Ag₂Se nanocrystals obtained at different injection temperatures were studied. The results showed that the distinct absorption features remained with a growth time of less than 30 min (Fig. 4A–C). In addition, the absorption peaks displayed a red-shift with increasing injection temperature and growth time (Table S3, ESI†), indicating that a rapid nucleation and slow growth occurred.³²

Fig. 4 Temporal evolutions of the absorption spectra of Ag₂Se nanocrystals synthesized at different injection temperatures: (A) 125 °C, (B) 145 °C, and (C) 165 °C; absorption (D) and FL emission spectra (E) of the as-prepared Ag₂Se nanocrystals with different sizes: (a) 1.9 nm, (b) 2.6 nm, (c) 2.8 nm, and (d) 3.1 nm; plots of absorbance versus concentrations (F) of the Ag₂Se QDs at (a) 830 nm, (b) 895 nm, (c) 944 nm, and (d) 954 nm.

The initial ratio of Ag to Se is another key factor in determining the amount of monomer for nucleation and growth of the Ag₂Se nanocrystals. When the injection/growth temperatures were set at 165 °C/120 °C and the growth time was fixed at 15 min, the initial ratio of Ag to Se was varied from 6 : 1 to 1 : 1 to obtain different sized Ag₂Se nanocrystals with identical concentrations of Ag precursor. TEM images (Fig. S5, ESI†) showed that the size of the Ag₂Se nanocrystals increased from 1.9 to 3.1 nm with increasing amounts of ODE–Se. It was because the greater amount of ODE–Se was used (no more than the amount of Ag), the more monomer was formed for nucleation and growth at a certain temperature and thus the sizes of the formed Ag₂Se nanocrystals increased. Notably, with identical concentrations of Ag precursor, the initial ratios of Ag to Se of 2 : 1 and 1 : 1 should produce identical amounts of monomer and identical sizes of nanocrystal according to the stoichiometric ratio of Ag₂Se. However, TEM images (Fig. S5C and D, ESI†) showed that different-sized nanocrystals were formed. This may be ascribed to the incomplete conversion of Se precursor, as was further confirmed from the ICP-AES measurement. The ICP-AES results (Table S2, a–d, ESI†) suggested that the atomic ratios of Ag/Se in the different-sized samples were not stoichiometric but Ag-rich, and the Ag/Se ratios decreased as the size of the Ag₂Se nanocrystals increased, which could be attributed to the surface effect of the nanoparticles, similar to CdSe³³ and PbSe²⁵ nanocrystals. With identical concentrations of the Ag precursor mentioned above, we also tested an initial ratio of Ag to Se of 1 : 2, and as shown in Fig. S6, ESI,† the product with emission at 1125 nm exhibited no distinct absorption features. The ICP-AES results (Table S2, e, ESI†) suggested that the product had a Ag/Se ratio of 1.89/1. Taken together, it could be inferred that the Ag-rich structure contributed to the distinct absorption features.³⁴ As the sizes of the nanocrystals increased from 1.9 to 3.1 nm, the exciton absorption peaks shifted from 830 to 954 nm (Fig. 4D), and the fluorescence emission varied from 958 to 1020 nm (Fig. 4E), indicating obviously size-dependent optical properties. Thus, the prepared Ag₂Se nanocrystals were indeed Ag₂Se QDs. Taking advantage of their distinct absorption features, the ε for the absorption peaks of the Ag₂Se QDs were measured using the absorption-based method (Fig. 4F), which has been employed extensively for II–VI and IV–VI QDs.^24,25 The ε values (Table S4, ESI†) were 0.17–1.15 × 10⁵ cm⁻¹ M⁻¹, comparable to those of Ag₂Se and Ag₂Te QDs with similar sizes.^20,34 It was notable that the ε values increased with increasing sizes of the Ag₂Se QDs, indicating that the ε of the QDs was also size-dependent, just as in previous reports.^20,24,25

In conclusion, high-quality NIR Ag₂Se QDs with distinct absorption features and high FL QYs have been successfully prepared using ODE–Se as the Se precursor. In addition, the as-prepared Ag₂Se QDs have uniform size, a passivated surface and a small Stokes shift. Moreover, the ε values of the Ag₂Se QDs were determined owing to the distinct absorption peaks. By optimizing this phosphine-free approach, the quality of Ag₂Se QDs was further improved and may provide a reference for the synthesis of Ag₂S and Ag₂Te QDs, which would be beneficial to nanobiomedicine.

Acknowledgements

This work was supported by the National Basic Research Program of China (973 Program, 2011CB933600), the 863 Program (2013AA032204), the National Natural Science Foundation of China (21375100) and the Natural Science Foundation of Hubei Province (2014CFA003).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra04987g

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