Synthesis and characterization of chiral Ag₂S and Ag₂S–Zn nanocrystals

Abstract

This study focused on the synthesis of novel chiral Ag₂S and Ag₂S–Zn nanocrystals (NCs) with chiral Pen as a capping reagent in an aqueous solution. Luminescence studies indicated that all the prepared Ag₂S and Ag₂S–Zn NCs exhibited size-tunable photoluminescence (PL) emission at 500–700 nm. Compared with Ag₂S, the PL emission of the Ag₂S–Zn NCs could improve by around 2.4-fold. XRD peaks of the as-prepared Ag₂S NCs were weak, whereas the XRD peaks of the Ag₂S–Zn NCs had the characteristics of a monoclinic crystal structure. The circular dichroism (CD) test showed that the prepared NCs revealed a clear mirror-image relationship in their CD signals at 300–700 nm, and Zn²⁺ played a key role in the Cotton effect of the NCs. The chiral and fluorescent properties of these NCs are likely to find widespread applications in bioimaging, chemical and biosensing, and possibly in asymmetry catalysis.

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Introduction

In the last two decades, the development of semiconductor nanocrystals (NCs) with multi-functionality, such as using as fluorescence probes in cell imaging and biological labeling, as well as chiral detection, has attracted considerable attention.^1–3 Compared with organic dyes, semiconductor NCs exhibit a series of excellent optical properties, including size-dependent tunable photoluminescence, broad excitation spectrum, narrow emission bandwidth and high photochemical stability.^4–6 Unfortunately, most highly luminescent NCs are cytotoxic because they contain toxic heavy metals elements (Cd, Hg, Pb, etc.).^7–9 Ag₂S is an I₂-VI semiconductor with a direct band gap of 1.1 eV, and does not contain highly toxic heavy metals.^10–12 This material might be a promising candidate for fluorescence probes. Although various routes for the synthesis of Ag₂S NCs have been reported, they involved preparations at high temperatures and dispersions in organic solvents, both of which make the nanocrystals inapplicable to biological systems.^13–16 Generally, with water-soluble thiols as transfer reagents, hydrophobic quantum dots can be transferred to hydrophilic semiconductor nanoparticles.¹⁷ Hong et al. reported that the Ag₂S NCs were first synthesized in an organic phase, and the hydrophobic Ag₂S NCs were coated with a surfactant dihydrolipoic acid (DHLA) and reacted with amine-functionalized PEG using ethyl(dimethylaminopropyl) carbodiimide/N-hydroxysuccinimide (EDC/NHS) to afford highly water soluble Ag₂S NCs.¹⁸ Despite this, there are few reports on the aqueous synthesis of Ag₂S NCs.¹⁹ This can be partly attributed to the fact that water-soluble Ag₂S clusters have a strong tendency to aggregate into a bulk material, which complicates their synthesis considerably. Compared to organic synthesis,^20–22 aqueous synthesis of NCs is more reproducible, cheaper and less toxic. The hydrophilicity of the products holds great promise in biological applications. Recently, aqueous synthesis allows the formation of near-IR emitting Ag₂S NCs using 2-mercaptopropionic acid as a coating.²³ Apart from near-IR luminescence, however, other optical properties, such as vis-luminescence and chirality, have not been reported. Therefore, it is a great challenge to synthesize water-soluble Ag₂S NCs with multi-functionality.

Recently, there has been considerable interest in elucidating chirality in NCs because of the widespread use of chirally modified nanoclusters surfaces for enantioselective catalysis and chiral analysis applications.^24–27 Surface in situ functionalization by chiral molecules is an effective strategy for chirality on nanoclusters. According to this strategy, we reported the synthesis and chiroptical properties of optically active silver nanoclusters protected by a pair of enantiomers of chiral penicillamine.²⁸ A search of the literature revealed few reports of chiral NCs. One example is D- or L-penicillamine-capped NCs²⁹ but the origin of chirality or optical activity in NCs is not fully understood. Here, with D-penicillamine (D-Pen) and L-penicillamine (L-Pen) as the capping reagent, we present a novel method for the synthesis of chiral Ag₂S and Ag₂S–Zn NCs in aqueous solutions. In these experiments, Zn²⁺ ions played a key role in the chirality of the NCs.

Experimental section

Chemicals

D-Penicillamine (99%, D-Pen), L-penicillamine (99%, L-Pen) and NaBH₄ (96%) were obtained from Sigma, Inc. Thioacetamide (TAA) and AgNO₃ were purchased from Shanghai Chemical Reagents Company. High purity deionized water (>18.3 MΩ cm⁻¹) was produced by Millipore A10 Milli-Q.

Synthesis of Pen capped-Ag₂S NCs

All the solutions were freshly prepared with deionized water prior to synthesis. The Ag₂S NCs were prepared by a simple one-pot process using silver nitrate as a precursor, NaOH as a basic catalyst and TAA as a sulfide source at 60 °C. In a typical synthesis, 74.6 mg (0.5 mmol) of Pen was dissolved in a solution containing 95 mL of water and 0.9 mL of 0.5 M NaOH. After stirring at 60 °C for 30 min, 2.5 mL of a 0.1 M silver nitrate aqueous solution and 1.2 mL of a 0.1 M TAA solution were added. The resulting mixture solution of Ag⁺ and TAA was heated to 60 °C under open-air conditions and refluxed for different times to control the size of the NCs. Aliquots of the reaction solution were taken at regular intervals for the UV absorption and PL experiments.

Synthesis of Pen capped-Ag₂S–Zn NCs

A typical synthesis of Ag₂S–Zn NCs is described hereafter. 100 mL of the as-prepared Pen-capped Ag₂S solution was heated to 60 °C under open-air conditions, and then 1.2 mL of 0.1 M ZnAc₂ and 0.6 mL of 0.1 M TAA were added. The solution was heated to 60 °C for different times to control the sizes of the NCs. Aliquots of the reaction solution were removed at regular intervals for the UV absorption and PL experiments.

Characterization

UV-vis absorption and photoluminescence (PL) spectra were measured at room temperature using a Shimadzu UV-3100 spectrophotometer and a Hitachi 7000 fluorescence spectrometer, respectively. The PL spectra were obtained at the excitation wavelength λ_ex = 400 nm. Time-resolved luminescence measurements were carried out on a Fluorolog-3 spectrofluorimeter with a LED lamp as the light source. Ludox was applied for the PL lifetime measurement to eliminate the influence of light scattering (i.e., excitation and emission). The circular dichroism (CD) spectra were measured using an Applied Photophysics Ltd. Chirasan spectrophotometer. The concentrations of the Ag₂S and Ag₂S–Zn NCs were the same and diluted to the original solution (10%), and the NCs were purified with a 10 kDa dialysis membrane for CD characterization. For all structural characterization, the Ag₂S (A) and Ag₂S–Zn (B) samples were obtained after heating for 31 and 14 min, respectively. Powder XRD was conducted on a Philips X'Pert PRO X-ray diffractometer. High-resolution transmission electron microscopy (HRTEM) was performed on a Philips FEI Tecnai G² 20 S-TWIN.

Results and discussion

XRD, TEM and XPS characterization

Powder X-ray diffraction (XRD) was carried out on the as-formed products. XRD of the Ag₂S NCs powder sample (Fig. 1A) revealed a broad and intense (121) peak of silver at 2θ = ∼35°. For Ag₂S–Zn NCs, all the peaks in the XRD patterns (Fig. 1B) matched those of monoclinic α-Ag₂S (JCPDS 14-0072). The characteristic monoclinic planes of −111, 111, 112, −112, 121, 220 and −213 locating at 25.79°, 29.18°, 31.66°, 34.94°, 36.78°, 43.47°, and 53.29° have been observed. Compared to the XRD patterns of the Ag₂S and Ag₂S–Zn samples, the XRD patterns of the as-prepared Ag₂S NCs were indistinguishable, whereas XRD of the Ag₂S–Zn NCs revealed a monoclinic crystal structure. The results showed that the prepared Ag₂S–Zn NCs had possibly better PL emission than that of Ag₂S NCs. The XRD intensity of the Ag₂S–Zn NCs was similar to the Ag₂S QDs, which might be due to the excess ligands attached to the surfaces of the Ag₂S–Zn NCs.

Fig. 1 XRD patterns of Ag₂S and Ag₂S–Zn NCs.

Generally, the size and size distribution are critically important for luminescent NCs. The TEM images in Fig. 2A and B show that the Ag₂S and Ag₂S–Zn NCs are well-dispersed and have a near-spherical shape. The average diameters of the Ag₂S and Ag₂S–Zn NCs were 4.5 + 0.1 and 5.0 + 0.2 nm, respectively. Energy dispersive X-ray spectroscopy (EDX) measurements of the Pen-capped Ag₂S (A) and Ag₂S–Zn (B) NCs revealed the existence of Ag and S in the Ag₂S (A) and Ag₂S–Zn (B) samples (Fig. 3). Compared to Ag₂S, the Zn peak at 1.01 keV in the new sample was clear, indicating the formation of Ag₂S–Zn nanoparticles. The N peaks at 0.27 keV were assigned to the stabilizer Pen.

Fig. 2 TEM images of the Ag₂S (A) and Ag₂S–Zn (B) NCs. Inset: HR-TEM image of the Ag₂S–Zn NCs.

Fig. 3 Typical EDX spectra of Pen-capped Ag₂S (A) and Ag₂S–Zn (B) NCs.

To confirm the formation of Ag₂S–Zn NCs in aqueous solution, X-ray photoelectron spectroscopy (XPS) was conducted. A full survey scan and the Ag, Zn, and S photoelectron spectra of the Ag₂S–Zn are displayed in Fig. 4. In addition to the Ag levels, the spectra were dominated by the C1s and O1s signals stemming from the capping agent. The XPS spectra in Fig. 4 revealed Ag3d, Zn2p and S2p. The two peaks in Fig. 4B at 1021.9 eV and 1045.1 eV were assigned to the binding energy of Zn 2p3/2 and Zn 2p1/2, respectively, which corresponds to Zn²⁺ according to previous results.^30–32 The peaks with binding energies of 367.8 eV and 373.8 eV were assigned to Ag 3d5/2 and Ag 3d3/2, respectively, which is characteristic of Ag⁺ in the Ag₂S product in Fig. 4C.³³ The peak at 161.3 eV belongs to S2p1/2 in a zinc–sulfur bond and silver–sulfur bond in composites.³³ Overall, XPS provided direct evidence of the formation of Ag₂S–Zn NCs.

Fig. 4 The XPS spectra recorded from (A) Ag₂S–Zn NCs: binding energy spectra of (B) Zn2p, (C) Ag3d, and (D) S2p.

The optical properties of Ag₂S NCs

Aqueous Ag₂S NCs could be synthesized easily using Pen as a stabilizer at 60 °C. Fig. 5 presents the typical evolutions of both the absorption and PL spectra of Pen-stabilized Ag₂S NCs prepared in an aqueous solution. Before heating, there was a UV-vis absorption peak at 501 nm due to un-passivated Ag₂S NCs because the extremely low solubility of Ag₂S (K_SP = 6.3 × 10⁻⁵⁰) results in rapid crystal growth, even in the presence of Pen. On the other hand, no fluorescence was observed with this crude solution. Under heating at 60 °C, these colloid clusters began to crystallize and the fluorescence of the solution appeared. As shown in Fig. 5A, during heating from 0 to 15 min, the absorptivity increased significantly and the absorption band-edge red-shifted, which indicated that the sizes of Ag₂S NCs increased gradually. At longer reaction times from 15 to 79 min, the absorption onset showed no apparent change and a significant increase in absorptivity was observed. As shown in Fig. 5B, after heating for 15 min, the as-synthesized Ag₂S NCs exhibited orange luminescence with an emission peak around 577 nm. For a heating time of 18, 26, 31, and 79 min, Ag₂S NCs were obtained with maximum PL emission at 584, 588, 596, and 610 nm, respectively (Fig. 5B). After heating for 31 min, the PL intensity reached a plateau and started to decrease with further heating. The red-shift of the PL spectrum is in accord with the absorption band-edge, possibly due to the increase in NCs.³⁴

Fig. 5 Typical temporal evolution of the absorption (A) and corresponding emission (B) spectra of Ag₂S NCs. Curves a–f represent the absorption (A) and corresponding emission (B) spectra of Ag₂S NCs obtained for heating 0, 15, 18, 26, 31, and 79 min, respectively. The excitation wavelength was 400 nm.

The optical properties of Ag₂S–Zn NCs

Fig. 6 presents typical evolutions of both the absorption and PL spectra of Pen-stabilized Ag₂S and Ag₂S–Zn NCs prepared in an aqueous solution. The absorption spectra of the Ag₂S–Zn NC fractions taken after 15, 18, 26, 31 and 79 min of heating at 60 °C were compared with the spectra of Ag₂S NCs. Compared to the Ag₂S NCs, the absorption maximum of the first electronic transition of the as-prepared Ag₂S–Zn NCs evidently showed a blue shift in wavelength, which was attributed to the addition of a ZnS shell confining the Ag₂S NCs wavefunction. The absorption in Ag₂S–Zn NCs shifted to longer wavelengths as the particles grew during the course of heating, which is a typical characteristic of core–shell NCs.^35–37 The emission spectra of Ag₂S–Zn NCs heated for different times are shown in Fig. 6B with a comparison to the Ag₂S reference sample. As shown in Fig. 6B, after heating for 3 min, the PL emission of the as-synthesized Ag₂S–Zn NCs also showed a blue-shift with an emission peak at around 575 nm, which is similar to the absorption spectrum. Further heating for 6, 14, 24, 39, 54, and 84 min resulted in Ag₂S–Zn NCs with maximum PL emission at 577, 578, 579, 584, 592, and 603 nm, respectively. After heating for 14 min, the PL intensity reached a plateau and the maximum emission of the NCs increased around 2.4-fold. The observed PL enhancement was caused by passivation of the surface trap states due to the formation of core–shell NCs and the PL decreased as a function of the heating time possibly due to the formation of new surface trap along with a thicker ZnS shell.³⁸

Fig. 6 Typical temporal evolution of the absorption (A) and corresponding emission (B) spectra of Ag₂S and Ag₂S–Zn NCs. Curve a represents the absorption (A) and corresponding emission (B) spectra of Ag₂S NCs. Curves b–h represent the absorption (A) and corresponding emission (B) spectra of Ag₂S–Zn NCs obtained for heating 3, 6, 14, 24, 39, 54 and 84 min, respectively. The excitation wavelength was 400 nm.

According to relevant documents,^39,40 red NCs with <700 nm PL emission are generally very small (<2.6 nm in diameter). For example, Pang's group successfully synthesized octylamine-capped Ag₂S NCs (690 nm) with sizes of 1.5 nm.³⁹ Gui et al. reported that using multidentate polymers (poly(acrylic acid)-graft-cysteamine-graftethylenediamine) as a stabilizer, aqueous Ag₂S NCs with 2.6 nm sizes were prepared, displaying 687 nm PL.⁴⁰ More studies of Ag₂S NCs have shown that QD diameters of 4–5 nm generally display a NIR PL spectrum (>700 nm).^39–41 In this paper, we report that using small molecules thiol (Pen) as a stabilizer, Ag₂S and Ag₂S–Zn NCs with sizes ranging from 4 to 5 nm display visible PL emission. The reason may be because in the presence of a Pen stabilizer, extremely small Ag₂S NCs (such as 1–2 nm) aggregate in aqueous solution and the observed visible PL is actually from the extremely small NCs.

To reveal the intrinsic reason for the unique evolution of PL spectrum, PL relaxations of the NCs were characterized. The obtained PL decay curves of the Ag₂S and Ag₂S–Zn NCs are shown in Fig. 7. In general, these PL decay curves at a peak wavelength of 600 nm (λ_ex = 390 nm) can be well fitted using a biexponential equation, I(t) = A₁ exp(−t/τ₁) + A₂ exp(−t/τ₂).^42–44 The constants obtained by the fitting are tabulated in Table 1. The observed lifetimes were 3.14 ± 0.12 ns (74.87%) and 27.10 ± 0.32 ns (25.13%) for Ag₂S NCs, and 3.47 ± 0.11 ns and 33.72 ± 0.35 ns for Ag₂S–Zn NCs. The average lifetime of the Ag₂S and Ag₂S–Zn NCs were 9.16 ns and 10.92 ns, respectively. Obviously, the average emission lifetime of the latter was longer than that of the former. Surface defect states are formed during the surface modification of Ag₂S NCs with Pen, and they lead to the nonradiative decay of excitons. For the Ag₂S–Zn NCs, the excitons generated by excitation with a wavelength of 390 nm were created in the cores of Ag₂S and confined to the cores energetically lower than ZnS shells, which effectively protected them from nonradiative decay. The reduction of the non-radiative decay rate was accompanying by an increase in the radiative lifetime, resulting in an improvement of the luminescence properties of the NCs. Therefore, compared with that of Ag₂S NCs, the longer average lifetime of Ag₂S–Zn NCs was attributed to the decrease in surface defect states and the increase in radiative lifetime.

Fig. 7 PL time evolution spectra of the Ag₂S (A) and Ag₂S–Zn (B) samples. Ag₂S (A) and Ag₂S–Zn (B) NCs were obtained after heating 31 and 14 min, respectively. The excitation and emission wavelengths were 390 and 600 nm, respectively.

Table 1 PL decay constants obtained from I(t) = A₁ exp(−t/τ₁) + A₂ exp(−t/τ₂)

Sample	τ1/ns	A1/%	τ2/ns	A2/%
Ag2S	3.14 ± 0.12	74.87	27.10 ± 0.32	25.13
Ag2S–Zn	3.47 ± 0.11	74.76	33.72 ± 0.35	25.24

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The chiroptical properties of the Ag₂S and Ag₂S–Zn NCs

Fig. 8 shows the absorption spectra of the Ag₂S and Ag₂S–Zn samples and the CD spectra of Pen, the complex of Pen and Ag⁺, Ag₂S and Ag₂S–Zn NCs. The absorption spectra and CD spectra were measured at the same time using an Applied Photophysics Ltd. Chirasan spectrophotometer. From Fig. 8A and B, it is evident that the absorption peak of the D-Pen capped Ag₂S NCs coincide with that of the L-Pen capped Ag₂S NCs, as expected, and the absorptivity of chiral Pen capped Ag₂S and Ag₂S–Zn NCs were almost the same at 300 to 700 nm. The Pen provides opposite CD responses at 224 nm. With respect to pure Pen, the additional chiral features appear at 249, 278 and 322 nm for a mixture of Ag cations and chiral Pen (Fig. 8D), which can be ascribed to the formation of an Ag-Pen complex. Compared to Pen (Fig. 8C) and a mixture of (D, L)-Pen and Ag⁺, which present prominent peaks at 224, 249, 278, and 322 nm, Ag₂S and Ag₂S–Zn NCs stabilized by chiral Pen showed additional broad CD features between 322 and 700 nm (Fig. 8E and F). Therefore, Pen molecules adsorbed on NC surfaces not only preserve their own chirality but also induce chirality on the Ag₂S and Ag₂S–Zn core. Compared to Ag₂S, it is evident that the Ag₂S–Zn NCs exhibit strong chiral peaks at 371, 492 and 594 nm, which shows that Zn²⁺ can greatly improve the CD features of the NCs. The origin of the difference of chirality in the NCs between Ag₂S and Ag₂S–Zn is unclear, but some possibilities can be suggested. (i) The interaction of the chiral ligand and NCs. The Ag₂S NCs tends to aggregate, whereas Ag₂S–Zn NCs not only have good stability, but also exhibit strong PL and CD features. Compared with the Ag₂S NCs, the strong interaction between chiral-Pen ligand and Ag₂S–Zn NCs possibly improves the stability of the NCs, resulting in strong CD signals. (ii) The difference in the electronic structure between Ag₂S and Ag₂S–Zn might influence their deformability. Therefore, the passivation of NCs with chiral molecules results in unique electronic and chiroptical responses that are unlike those of the component parts. These optically active NCs exhibited Cotton effects or a clear mirror-image relationship in their CD signals with the anisotropy factors increasing with the addition of Zn²⁺ to the NCs.

Fig. 8 Absorption spectra of D-Pen capped (A) and L-Pen capped (B) Ag₂S and Ag₂S–Zn samples. Circular dichroism spectra of Pen (C), the complex of Pen and Ag⁺ (D), chiral Ag₂S (E) and Ag₂S–Zn NCs (F).

The stability of the Ag₂S and Ag₂S–Zn NCs

The as-prepared Ag₂S NCs exhibited photostability for several days, whereas Ag₂S–Zn NCs showed excellent colloidal and photostability over three months. To further study the stability of the NCs, we investigated them by the Zeta potential (ζ). The Zeta potential was used to determine the surface charge densities of the Ag₂S and Ag₂S–Zn NCs. Fig. 9 shows that the ζ of the Ag₂S and Ag₂S–Zn solution reach 20.8 mV and 34.4 mV, respectively, indicating that the surface charge was dominated by the bound Pen. The ζ of the Ag₂S–Zn solution increased to 13.6 mV, resulting in an increase in the repulsion of the different NCs, which agrees with the stability of the NCs. The changes in ζ may be due to the different electronic interaction of NCs (positive charges) and Pen (negative charges). Therefore, using chiral Pen as stabilizer, we synthesized not only highly luminescent NCs with strong chiral signals, but also more stable NCs in aqueous solution.

Fig. 9 Zeta-potential measurements of Pen-capped Ag₂S and ZnS–Ag₂S NCs in aqueous solution.

Summary

In summary, to the best of our knowledge, this is the first report of the synthesis of chiral Ag₂S and Ag₂S–Zn NCs. The prepared Pen-stabilized NCs exhibited size-tunable PL emission at 500 to 700 nm. Compared to Ag₂S, the PL emission of the Ag₂S–Zn NCs could improve by around 2.4-fold. The prepared NCs exhibited a clear mirror-image relationship in their circular dichroism (CD) signals at 300 to 700 nm. Zn²⁺ played a key role in the Cotton effect of the NCs. The chiral and fluorescent properties of these NCs are likely to find widespread applications in bioimaging, chemical and biosensing, and possibly in asymmetry catalysis.

Acknowledgements

This work was financially supported by the Natural Science Foundation of China (nos. 21171150 and 21271159) and Henan Province Science and Technology Programs (no. 112102210002).

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