Synthesis of near-infrared fluorescent, elongated ring-like Ag₂Se colloidal nanoassemblies

Abstract

A low-temperature, air-exposed, hot-injection process has been developed for the synthesis of water-dispersible Ag₂Se quantum dots. The 3-mercaptopropionic acid capped quantum dots were shown to be elongated ring-like self-assembled nanostructures with prominent near-infrared fluorescent absorptions and emissions.

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Near-infrared fluorescent quantum dots (NIRQDs) have attracted much research interest in many areas, especially in biomedical applications and in efficient solar energy conversion, due to their stable and tunable absorptions and emissions in the NIR band.^1,2 The expansion of photovoltaic solar cells to the NIR band is highly desirable in order to capture a broader band of the solar spectrum and this can be well achieved by NIRQD-sensitized solar cells or polymer-NIRQD hybrid solar cells.^2–4 In addition, as alternative candidates for biolabelling materials, NIRQDs offer enormous advantages over both conventional QDs and organic dyes, which generally absorb and emit in the visible band. As some biological constituents such as water and haemoglobin may absorb and scatter visible light, whereas others such as collagen have significant autofluorescence in the visible band, conventional QDs and organic dyes suffer from fluorescence quenching as well as interference.^5,6 It is thought that NIRQDs have great potential in these applications.

Silver chalcogenide (specifically Ag₂Se and Ag₂S), a versatile semiconductor material with narrow band gaps⁷ and no significant toxicity,^8,9 unlike traditional Cd- and Pb-containing NIRQDs, has been rigorously investigated to develop novel NIRQDs for biomedical applications. Notably, several groups have carried out much pioneer work in this area. Sahu and co-workers^10,11 developed a facile hot-injection process for the synthesis of a series of silver chalcogenides and systematically studied the quantum confinement effects in Ag₂Se nanocrystals. Wang and co-workers,^12-17 co-operating with Dai and co-workers,^13,14 optimized the synthetic process and developed Ag₂S and Ag₂Se QD-based fluorescent biolabels in the second NIR window (1.0–1.4 μm) as well as dual-colour emitting Ag₂S–ZnS heterostructures.^16,17 Gu et al.¹⁸ exploited various reaction conditions and introduced a quasi-biosystem. They obtained emission-tunable Ag₂Se and Ag₂S QDs for in vivo imaging and subsequently utilized the NIR electrogenerated chemiluminescences of ultra-small Ag₂Se QDs for detecting dopamine.^19–22

In the work reported here, we developed a low-temperature hot-injection process for the synthesis of water-dispersible, structurally well defined, elongated ring-like nanoassemblies (ER-like NAs) of pristine Ag₂Se QDs. An Na₂SeSO₃ (ref. 23 and 24) aqueous solution was selected as the Se source for the reaction as it was more active and less toxic than Se powder, SeO₂, Na₂Se and other classical Se sources; 3-mercaptopropionic acid (3-MPA) was used as the capping agent. It was relatively a facile route because the reaction was directly conducted at ambient atmospheric conditions and the reaction temperature was much lower than in common hot-injection processes. These ER-like NAs were shown to have optical absorption and emission at 819 nm and 938 nm, respectively, which gives this novel-structured nanomaterial great potential in biomedical and energy conversion applications.

The X-ray diffraction (XRD) pattern and Raman spectrum were recorded for the identification of the phase structure and purity of the ER-like NAs. The XRD pattern, shown in Fig. 1(a), demonstrates the pure and single phase of orthorhombic β-Ag₂Se, with all the characteristic peaks distinctly corresponding to JCPDS file no. 24-1041. The Raman spectrum in Fig. 1(b), with the peak position located at 135 cm⁻¹, characteristic of the Ag–Se bond,^25,26 may also suggest the formation of Ag₂Se.

Fig. 1 Representative (a) X-ray diffraction pattern and (b) Raman spectrum of the synthesized elongated ring-like Ag₂Se nanoassemblies.

Fourier transform infrared (FT-IR) spectra of the Ag₂Se ER-like NAs and the as-purchased 3-MPA (Fig. 2) were recorded to confirm the surface binding of the capping molecules. The bands at 2924, 1709, 1412 and 663 cm⁻¹ can be ascribed to the symmetric stretching vibrations of the C–H bonds of alkyl chains, the C=O and C–O bonds of carboxylic acid groups and the C–S bonds of 3-MPA molecules, respectively.^21,27 The absence of the S–H stretching mode at 2663 and 2564 cm⁻¹, which clearly appears in the spectrum of a liquid film of pure 3-MPA, indicates that the 3-MPA molecules were bound to the surface of the Ag₂Se nanocrystals via Ag–thiol bonds and that the surfaces of the Ag₂Se clusters were hence terminated by carboxylic acid groups. In addition, the peaks at 1625 and 1565 cm⁻¹, which do not appear in the spectrum of pure 3-MPA, can be assigned to the vibration of the carboxylate anions of 3-MPA molecules and this phenomenon, namely the existence of carboxylate anion peaks, has been observed and well discussed in previous work^28,29 on Cd chalcogenide systems capped with thioalkyl acid.

Fig. 2 Fourier transform infrared spectra of the 3-mercaptopropionic acid (3-MPA) capped Ag₂Se nanocrystals and a liquid film of pure 3-MPA.

Fig. 3 displays the transmission electron microscopy (TEM) images of the ER-like NAs of Ag₂Se QDs at a reaction time of 15 min. As can be seen in Fig. 3(a), superior monodispersity of the NAs was achieved overall, and, in addition, the hollow ER-like structures are quite discernible. The high-resolution TEM (HRTEM) image shown in Fig. 3(d) reveals that these ER-like structures were actually side-by-side assemblies of pristine Ag₂Se QDs. These Ag₂Se QDs possess a uniform size distribution within about 4–5 nm, as marked by the white circles in Fig. 3(d). The (121) and (032) crystal planes also demonstrate the formation of the single orthorhombic β-Ag₂Se phase, although the crystallinity of the ER-like Ag₂Se NAs does not appear to be so good based on either the XRD results or the low contrast of the HRTEM image. This may be due to the short reaction time and relatively low reaction temperature (90 °C).

Fig. 3 Representative (a–c) transmission electron microscopy (TEM) and (d) high-resolution TEM images of the elongated ring-like Ag₂Se nanoassemblies.

To investigate the effects of reaction time and the capping agent on the morphology of Ag₂Se nanocrystals, different reaction times (typically, 5 min, 1 h and 5 h) and capping agents [2-MPA and thioglycolic acid (TGA)] were used in experiments to provide comparisons. The TEM images in Fig. 4(a) and (b) show that the very small Ag₂Se QDs have assembled into linear NAs like chains of beads at a reaction time of 5 min. In these NAs, Ag₂Se QDs are arranged almost one-by-one along linear tracks, as shown in Fig. 4(b). Also, the corresponding size distribution profile [the top panel of Fig. 4(g)], which gives us a small average diameter of about 5.69 nm and a large length of about 98.10 nm, also indicates these linear NAs of Ag₂Se QDs. When these linear NAs further transformed into ER-like NAs, as shown in the green panel of Fig. 4(g), the average diameter of NAs increased to 20.66 nm while the length decreased to 75.70 nm. The fact that the average diameter of ER-like NAs was more than twice that of linear NAs while the length was less than that of linear NAs is rational, because the formation of a ring-like structure would naturally need a decrease in length; also, the existence of a hollow section within the ring will increase the radial dimension. Fig. 4(c–f) show the Ag₂Se nanocrystals obtained with prolonged reaction times of 1 and 5 h. At a reaction time of 1 h, the previous ER-like NAs aggregated and therefore lost their hollow ring-like structures and consequently became aggregated rod-like NAs (AR-like NAs). The size distribution results in the yellow part of Fig. 4(g) show that the average length of these AR-like NAs (75.88 nm) and that of ER-like NAs are approximately equal, whereas the average diameter (15.19 nm) has decreased, which indicates the loss of the hollow parts of the ER-like NAs. At a reaction time of 5 h, the TEM images in Fig. 4(e) and (f) and the size distribution profile in the blue part of Fig. 4(g) reveal that the close-packing degree of these AR-like NAs is significantly enhanced compared with a reaction time of 1 h. It can also be inferred that these AR-like NAs have grown larger at this point in time as both the average diameter (38.04 nm) and length (129.40 nm) have increased distinctly. Nevertheless, in spite of the morphological transformations, the pure phase of orthorhombic β-Ag₂Se could be demonstrated in all cases, as all the existing crystal planes in HRTEM images, i.e. the (121), (102) and (211) planes, match well. It seems that the crystallinity of pristine Ag₂Se nanocrystals was gradually improved as the reaction time was increased because the contrast in the corresponding TEM and HRTEM images is slightly enhanced.

Fig. 4 (a–c and e) Transmission electron microscopy (TEM) and (d and f) high-resolution (HR) TEM images of the Ag₂Se nanoassemblies obtained with reaction times of (a and b) 5 min, (c and d) 1 h and (e and f) 5 h. The HRTEM image in (d) corresponds to the marked area in (c); (f) corresponds to the marked area in (e). (g) Size distribution profiles of the Ag₂Se nanoassemblies obtained with different reaction times.

A probable mechanism for the formation and morphological evolution of these Ag₂Se NAs is given in Fig. 5. Once the Se source, an aqueous solution of Na₂SeSO₃, was injected, large quantities of Ag₂Se clusters were generated. Previous FT-IR characterization has revealed that these 3-MPA-capped Ag₂Se clusters are terminated by carboxylic acid groups. It has been intensively reported and discussed that, in the case of 3-MPA-capped Au nanoparticles, hydrogen bonding can serve as a kind of molecular link to achieve the linear assembly of Au nanorods or nanoparticles.^30–32 In addition, the successful synthesis of rod-like Ag³³ and CdS³⁴ nanostructures in 3-MPA aqueous solutions further demonstrated the ability of 3-MPA to bring about nanostructures of noble metals or semiconductors with high aspect ratios. Therefore this step can be described here as the hydrogen bond (attraction 1 in Fig. 5) mediated linear assembly of Ag₂Se QDs. It was because these linear NAs were moving freely in an aqueous environment that they had the potential to link to each other if any two approached closely enough. The fact that the linking behavior was finally initiated at both ends of the NAs and consequently resulted in the formation of ER-like NAs might be attributed to the availability of more free carboxylic acid groups to form hydrogen bonds. Inside these ER-like NAs, two kinds of attractions, referred to as strong attraction 1 and weak attraction 2 in Fig. 5, can be assumed to exist. At both ends of the ER-like NAs, hydrogen bonds formed directly among 3-MPA molecules due to the relatively shorter distance between Ag₂Se clusters and thus the 3-MPA molecules (that is, the strong attraction 1 between two specific Ag₂Se QDs), whereas in the middle parts of the ER-like NAs hydrogen bonds were assumed to form among the oligomers of 3-MPA molecules. Considering the longer distance between the two specific Ag₂Se clusters that the 3-MPA oligomers were bound to in this case, we regard it as a weaker attraction than the other. These two kinds of attraction together led to the further aggregating assembly of the ER-like NAs. Although the cluster attachment happened at the same time, the aggregating process was dominant in this period and hence the average diameter decreased from 20.66 nm of the ER-like NAs at 15 min to 15.19 nm of the AR-like NAs at a reaction time of 1 h. The cluster attachment process then continued to occur on these AR-like NAs, accompanied by the Ostwald ripening process, and these more closely packed AR-like NAs of Ag₂Se finally grew larger and the average diameter and length increased to 38.04 nm and 129.40 nm, respectively.

Fig. 5 Schematic diagram of the mechanism for the formation and morphological evolution of the various nanoassemblies of Ag₂Se nanocrystals capped by 3-mercaptopropionic acid.

The TEM images in Fig. S1 and S2 of the ESI† show the appearance of Ag₂Se QDs obtained using 2-MPA and TGA as the capping agent. In both cases it can be seen clearly that highly monodispersed QDs (MDQDs) were obtained. No obvious self-assembled units can be observed, although some irregular QD aggregates appeared in the 5 h products of Fig. S1(c) and S2(c).† The chemical structures of 2-MPA, TGA and 3-MPA are given in Fig. S3 of the ESI† in order to understand the reasons why no NAs formed in 2-MPA- or TGA-capped Ag₂Se nanocrystals. From Fig. 5 we know that the initial key step in assembling the nanocrystals was the hydrogen bond mediated linear assembly. According to the various published reports, 3-MPA, 11-mercaptoundecanoic acid and cysteine are probably the most commonly used hydrogen-bonding-type molecular linkers.^30–32,35 It can be summarized from the structures of these molecules that an alkyl main chain containing at least three carbon atoms and no pendant groups, except groups containing N or S atoms, are required for molecular linkers in the linear assembly of nanocrystals. It is obvious that neither the 2-MPA or TGA molecules can meet these requirements. Both the short alkyl main chains of 2-MPA and TGA and the pendant methyl group of 2-MPA would bring about large steric hindrance and thus have an adverse effect on the linear assembly. Therefore no distinct indication of linear assembly could be observed in these two examples.

The TEM and HRTEM images of the ER-like Ag₂Se NAs obtained without ethanol washing are given in Fig. S4 of the ESI.† It can be inferred that no morphological change or structure collapse occurred after washing with ethanol because similar ER-like nanostructures can be observed. The solubility in water of the ER-like NAs of Ag₂Se is shown in Fig. S5 of the ESI.† It can be inferred that superior solubility in water of these NAs has been achieved as uniform dispersions are still maintained and no apparent precipitate was seen when they were kept at room temperature for 2 h after preparation, although the concentration reached as high as 5 mg ml⁻¹.

As a typical, narrow-band gap semiconductor material, Ag₂Se shows a fundamental absorption edge at 0.15 eV,⁷ which gives it great potential for optical absorption and photoluminescent (PL) emission in the NIR band. The corresponding NIR absorbance and PL spectra of the Ag₂Se linear NAs, ER-like NAs and AR-like NAs capped by 3-MPA, as well as MDQDs capped by 2-MPA, are shown in Fig. 6. It is obvious that, for the ER-like Ag₂Se NAs, a discernible absorption peak was detected in the NIR window near 819 nm and the corresponding PL spectrum shows a symmetrical emission peak centred on 938 nm. Also, the MDQDs possess distinct absorption and PL peaks at 797 and 893 nm. In all these NAs, the positions of the absorption peaks red shifted from 809, 819 to 820 nm and then returned back to 812 nm, while the corresponding PL peaks red shifted from 925, 938, 943 to 950 nm, respectively, as the reaction time gradually increased. Based on previously published reports, the red-shifted absorption and PL peaks of the various NAs may be attributed to two main factors. First, the particle and grain sizes of Ag₂Se clusters should have increased gradually as the reaction time was extended and this would lead to the red-shift phenomenon due to the quantum confinement effect.^36–38 In addition, it can be seen clearly in the TEM images in Fig. 3 and 4 and the schematic mechanism in Fig. 5 that the degree of close packing of the pristine Ag₂Se QDs was promoted during the morphological transformations from linear NAs to ER-like NAs and, eventually, to the AR-like NAs, which resulted in stronger interdot coupling interactions³⁹ and, consequently, more non-radiative energy loss for these NAs. The gradually decreased PL intensities could also be evidence of the increasing non-radiative energy loss in the excitation state. The reason why the absorption peak of the AR-like NAs obtained at a reaction time of 5 h finally blue-shifted to 812 nm from the 820 nm of the AR-like NAs obtained at 1 h could be ascribed to the correspondingly decreased aspect ratio from 4.88 to 3.40 as it has been demonstrated previously that a blue shift of the optical absorption peaks would take place if the aspect ratio of the nanorods decreased.^40–42 Finally, it is notable that the calculated full width at half-maximum (FWHM) of all emission peaks did not exceed 25 nm; this could be attributed to the narrow size distribution of the synthesized Ag₂Se NAs and pristine QDs. The fluorescent stability against photobleaching of the synthesized Ag₂Se ER-like NAs is also shown in Fig. S6 of the ESI.† It is possible that both the narrow FWHM and the fluorescent stability would help long-term signal analysis if the QDs are used as fluorescent labels in biomedical and other fields.

Fig. 6 Corresponding optical absorbance and PL spectra of the 3-MPA capped linear NAs (5 min), ER-like NAs (15 min) and AR-like NAs (1 h and 5 h) of Ag₂Se nanocrystals, and the 2-MPA capped MDQDs of Ag₂Se. All measurements were conducted using deionized water as solvent and the concentration of nanocrystals was kept at 10 mg l⁻¹.

In conclusion, we have demonstrated for the first time a facile air-exposed and low-temperature hot-injection process for the synthesis of water-dispersible, structurally well defined, ER-like NAs of pristine Ag₂Se colloidal QDs. The influences of the reaction time as well as the capping agent on the assembling behavior and morphological evolution were investigated meticulously. It seems that during the initial stage of the reaction, the capping molecule, 3-MPA, has served as a kind of molecular linker to bring about the hydrogen bond mediated linear assembly of pristine Ag₂Se clusters and, as the reaction time increased, these linear NAs gradually transformed into the distinctive ER-like NAs and the more closely packed AR-like NAs of Ag₂Se colloidal nanocrystals. These ER-like NAs of Ag₂Se nanocrystals had appreciable water solubility. Notably, the absorption and emission peaks of the ER-like NAs were centred near 819 and 938 nm, respectively, and the NIR emission peak showed a very small FWHM of less than 25 nm. Considering their unique structure, facile synthesis, broadband absorption and narrowband emission and inherent low toxicity, it is possible that the novel ER-like Ag₂Se NAs described here have great potential in biomedical applications such as bioimaging, diagnostics and photodynamic treatment or energy-related applications such as light-emitting diodes and QD solar cells.

Experimental section

Chemicals

3-MPA (99%), 2-MPA (97%) and TGA (97+%) were purchased from Alfa-Aesar and the Se powder (99.99%) from Sigma-Aldrich. AgNO₃ (≥99.8%), Na₂SO₃ (≥97%) and absolute ethanol (≥99.8%) were purchased from Sinopharm Chemical Reagent Company. All chemicals were used as received without further purification. For all experiments, extremely pure water (Millipore) with a resistivity greater than 18.0 MΩ cm was used.

Synthetic procedures

Na₂SeSO₃ solution (0.1 M) was prepared using a published method.²³ In a typical process, AgNO₃ (1 mmol), 3-MPA (or 2-MPA, TGA; 5 mmol) and deionized water (10 ml) were mixed and placed in a 25 ml three-necked flask. The ivory, cloudy mixture was continuously stirred and heated to 90 °C for 15 min in air. Then 5 ml of the Na₂SeSO₃ solution was quickly injected into the flask and kept for another 15 min (or 5 min, 1 h, 5 h). After cooling down naturally, the resulting product was washed and collected by adding ethanol accompanied by centrifugation several times.

Characterization

TEM and HRTEM images were obtained with a JOEL-JEM2100F transmission electron microscope at 200 kV acceleration voltage. The powder XRD pattern was obtained using a Bruker D8-Advance X-ray diffractometer with Cu Kα (1.5406 angstrom) radiation. Raman and FT-IR spectra were recorded using a Renishaw inVia reflex Raman spectrometer and a Shimadzu IRAffinity-1 spectrophotometer, respectively. The size distribution profiles were obtained using a Malvern Zeta-sizer ZEN3600 dynamic light scattering instrument which was excited by a 632.8 nm He–Ne laser. Finally, the absorbance spectra were obtained with a Shimadzu UV-3600 UV-visible-NIR spectrophotometer and the PL spectra were recorded using a Prinston NIR-CCD as the detector and an 808 nm laser as the excitation source.

Acknowledgements

This work was financially supported by the National Natural Foundation of China (no. 11274066, 51172047, 50872145 and 51102050) and the Ministry of Science and Technology of China (973 Project no. 2013CB932901 and 2009CB930803). The authors are also grateful to the “Shu Guang” project supported by the Shanghai Municipal Education Commission and the Shanghai Education Development Foundation (09SG01).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Additional TEM images, solubility and photostability test results are included. See DOI: 10.1039/c4ra00613e

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