Reduced carbon dots employed for synthesizing metal nanoclusters and nanoparticles

Abstract

Carbon dots (CDs) have been considered as ideal and promising fluorescent materials owing to their excellent optical, electronic and biocompatible properties including photoluminescence, photostability, electron transfer behavior and biocompatibility. Interestingly, reduced state carbon dots (r-CDs) have emerged as a new type of CDs. Herein, we successfully prepared CDs based on cysteine serving as the carbon source, and further creatively synthesized r-CDs via simply introducing NaBH₄ into the solution of CDs. Significantly, this type of r-CDs innovatively played the role of a reducer for directly synthesizing AuNCs and AuNPs (AgNPs), the reductive groups on their surfaces providing r-CDs with excellent electron-donating capability, thereby facilitating the fast reduction of metal ions to corresponding metal nanoclusters or nanoparticles. Besides, this exploration may open a new scope of CD applications in synthesis of metal nanomaterials.

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Introduction

Carbon dots (CDs), known as small oxygenous carbon nanoparticles (<10 nm), have been emerging as superior fluorescent nanomaterials owing to their excellent properties including low cytotoxicity,¹ biocompatibility,² photoluminescence,³ photostability and electron transfer behavior.⁴ Thus, CDs have received significant attention as a promising alternative to traditional semiconducting quantum dots. Interestingly, enhanced electron transfer is related to the surface passivation of CDs, which provides new opportunities for energy conversion and other applications.^5–9 For instance, Yu et al. designed a method of employing CDs as the energy donor for detecting H₂S¹⁰ and CDs also have been used as visible-light photocatalysts to reduce CO₂ more efficiently.¹¹

As regards their structures, most CDs, rather than nanodiamonds, are composed of sp² carbon atoms hybridized with abundant oxygenous residues, and it is proved that there exist oxygen-containing functional groups (hydroxyl, carbonyl, carboxyl and epoxy groups) on their surfaces.¹² Consequently, CDs certainly play roles as both electron acceptors and electron donors,¹³ indicating their potential as oxidizing or reducing agents. The electron-donating capability of CDs has been successfully verified by reducing metal salts to their corresponding metals.^14,15 For example, Shi et al. reported synthesis of Au@CDs by utilizing surface-enhanced Raman scattering;¹⁶ Lv et al. obtained an Ag@CDs composite according to the reduction of Ag(NH₃)₂OH by CDs themselves.¹⁷ However, these methods suffered from disadvantages like high synthesis temperature and introduction of additional chemicals.¹⁸ To date, however, studies on synthesizing noble metal nanomaterials by using CDs as reducing agents have not been reported.

Basically, preparations of CDs are mainly achieved by treating various carbon sources such as graphite,^19–21 soot from a burning candle^22,23 and natural gas,²⁴ saccharide²⁵ and graphene oxide^1,26 through oxidation processes. As a result, most CDs appear in high oxidation forms and this oxidation does not facilitate the fluorescence emission of CDs; however, reduced CDs (r-CDs) display enhanced fluorescence.^27,28 Thus, developing r-CDs will broaden their meaningful applications.

Noble metal nanoclusters (NCs), typically consisting of several to tens of atoms, have attracted the attention of numerous researchers during recent years due to their unique physical, electrical, and optical properties.²⁹ Among metal NCs, AuNCs have been extensively studied because of their intrinsic characteristics like facile synthesis, chemical stability, and high quantum yield.^30–32 So far, a few reducing agents mainly including sodium borohydride (NaBH₄),^33,34 sodium citrate^35,36 and ascorbic acid (Vc)³⁷ have been introduced to synthesize AuNCs.

For the conventional methods of synthesizing Au or Ag nanoparticles (AuNPs or AgNPs), sodium citrate or NaBH₄ usually serves as reducing agent.³² In addition, alcohols sometimes act as reducing agent for the synthesis of metal nanoparticles.^38,39 As previously reported, the oxyethylene groups on the surface of poly(ethylene glycol)s showed an ability for reducing Au³⁺ or Ag⁺ to form gold or silver nanoparticles.^40,41 Also, poly(vinyl pyrrolidone) terminating with hydroxyl groups (–OH) was employed to prepare nanoparticles, proving the reducing function of –OH.^42,43

Herein, we first creatively prepared CDs based on cysteine serving as the carbon source, and further successfully synthesized r-CDs via simply adding NaBH₄ into the solution of CDs. Importantly, this type of r-CDs were first successfully employed for preparing Au nanoclusters and nanoparticles. This exploration may open a new field of applications of CDs, especially for synthesis of metal nanomaterials.

Experimental

Apparatus

All fluorescence measurements were performed using a Hitachi F-7000 fluorescence spectrophotometer (Tokyo, Japan) with excitation slit set at 5 nm band pass and emission at 5 nm band pass in a 1 cm × 1 cm quartz cell. Meanwhile, UV/visible absorption spectra were recorded by a Shimadzu UV-2450 spectrophotometer (Tokyo, Japan). High-resolution transmission electron microscopy (HR-TEM) images were obtained by using a Tecnai G² F20 microscope (FEI, USA) at 200 kV. Samples for HR-TEM analysis were prepared by evaporating a drop of aqueous product on a lacey carbon copper TEM grid. Fourier transform infrared (FTIR) spectra were recorded using a Thermo Nicolet-380 IR spectrophotometer (Tokyo, Japan) with the KBr pellet technique ranging from 400 to 4000 cm⁻¹. X-ray photoelectron spectroscopy (XPS) analysis was performed with a PHI 5000 Versaprobe system (Shimadzu, Japan), using monochromatic Al Kα radiation (1486.6 eV) operating at 25 W. The quantum yields were obtained by an Absolute PL quantum yield spectrometer (C11347, Hamamatsu, Japan). Powders of CDs, r-CDs and r-CDs–AuNCs were obtained by lyophilization in PiloFD8-4.3V (Charlotte, USA). Mass spectra were obtained by UltrafleXtreme matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI TOF/TOF MS, USA). A Fangzhong pHS-3C digital pH meter (Chengdu, China) was employed to measure the pH values of various aqueous solutions and a vortex mixer (QL-901, Haimen, China) was used for blending the solutions. Zeta potential was measured after suitably diluting CDs and r-CDs solutions at 25.0 ± 0.5 °C, using a Nano Zetasizer (Malvern, England). The thermostatic water bath (DF-101s) was purchased from Gongyi Instrument Co. Ltd (Gongyi, China).

Chemicals and materials

Chloroauric acid (HAuCl₄, M_w = 411.85, AR), albumin from bovine serum (BSA), silver nitrate (AgNO₃, M_w = 169.87, AR) and cysteine were obtained from Shanghai Sangon Biotechnology Co. Ltd (Shanghai, China). Sodium hydroxide (NaOH, M_w = 40.00, AR), sodium citrate, ascorbic acid (Vc, M_w = 176.12, AR), sodium borohydride (NaBH₄, M_w = 37.83, AR) and hydrazine hydrate (80%) were purchased from Dingguo Changsheng Biotechnology Co. Ltd (Beijing, China). Ultrapure water, 18.25 MΩ cm, produced with an Aquapro AWL-0502-P ultrapure water system (Chongqing, China) was employed for all the experiments.

Synthesis of CDs and r-CDs

Cysteine (1.5 g) was initially added to ultrapure water (2 mL) and mixed with NaOH (2 M, 1 mL). Then, this mixture was heated in a domestic 750 W microwave oven for 3 min. During this time, the solution changed from a colorless liquid to a yellowish and finally clustered solid. After cooling to room temperature, this solid was dissolved with 10 mL ultrapure water. This aqueous solution of the CDs was purified by centrifuging (3000 rpm, 15 min) to remove large or agglomerated particles. Finally, the fluorescent CDs were collected by dialysis against deionized water via a dialysis membrane (300 MWCO) for 24 h. The powder of CDs was obtained by lyophilization, and further dissolved in ultrapure water with a final concentration of 1 mg mL⁻¹. The purified CDs thus prepared were kept at 4 °C prior to use.

Next, sodium borohydride (0.05 g) was introduced into the CDs solution (1 mg mL⁻¹, 20 mL) obtained as described above and then stirred gently overnight at room temperature. Excess reducing agent was removed by dialysis as mentioned above.

Synthesis of r-CDs–AuNCs

In a typical procedure, an aqueous HAuCl₄ solution (300 μL, 10 mM) was added into the r-CDs solution (200 μL, 1 mg mL⁻¹) under vigorous stirring at 37 °C. Subsequently, BSA (300 μL, 50 mg L⁻¹) was introduced at 37 °C, and this mixture was incubated at 37 °C for 12 h.

Preparation of r-CDs–AgNPs

The r-CDs–AgNPs were prepared by treating AgNO₃ aqueous solution in the presence of r-CDs. As a typical run, 200 μL of r-CDs (1 mg mL⁻¹) and 150 μL of AgNO₃ (10 mM) were mixed and incubated at 50 °C in a water bath for 10 min, and the color of this solution varied from light yellow to dark brown, suggesting that the reduction of Ag ions to Ag nanoparticles had finished. The products were washed three times with ultrapure water by centrifugation at 8000 rpm for 10 min.

Preparation of r-CDs–AuNPs

Similarly, 1 mL r-CDs (1 mg mL⁻¹) solution was mixed with 200 μL (10 mM) of HAuCl₄. Then this mixture was incubated at 60 °C in a water bath for 30 min, and the color of this solution varied from light yellow to wine-red, indicating that the reduction of Au ions to Au nanoparticles had finished. The products were washed three times with ultrapure water by centrifugation at 8000 rpm for 10 min.

Results and discussion

To explore the properties of CDs and r-CDs initially, a series of characterizations were performed. As shown in Fig. 1A, the original CDs exhibited maximum excitation and emission wavelengths at 410 nm and 475 nm, respectively. However, the r-CDs were excited at 370 nm, the surface structure facilitating this blue shift of the emission peak for r-CDs. Simultaneously, brighter blue fluorescence of r-CDs appeared compared with that of CDs, and the fluorescence of their related lyophilized powder (Fig. S1, ESI†) showed an agreement with that of Fig. 1A. Besides, the CDs displayed more obvious excitation-dependent behavior than r-CDs, and similar infrared upconversion property for the two types of CDs was observed (Fig. S2, ESI†). The average quantum yields of the r-CDs were obtained around 31.4%, while those of CDs were about 4.7% (Table S1, ESI†). These data demonstrated that r-CDs showed fluorescent properties superior to those of CDs.

Fig. 1 (A) The maximum excitation and emission spectra of CDs and r-CDs (inset: photographs under visible light (I and III) and UV light (II and IV) respectively). (B) FTIR spectra of CDs and r-CDs. HR-TEM images of CDs (C) and r-CDs (D).

Next, FTIR spectroscopy and XPS were applied to explore the surface groups, structure and components of CDs and r-CDs in detail. As revealed in Fig. 1B, there were O–H, N–H, C=O, S–H, C–N, and C–O groups on the surface of both CDs and r-CDs. Specifically, the absorption bands of O–H and N–H stretching vibrations appeared at 3510 cm⁻¹ and 3496 cm⁻¹. However, the vibrational absorption band of C=O at 1639 cm⁻¹ shifted to 1651 cm⁻¹, indicating that the surface carbonyl groups of the CDs have been reduced.³⁸ Furthermore, XPS analysis also showed that the carbonyl peak at 287.9 eV obviously decreased after reduction, whereas sp² C=C peak at 284.5 eV retained its original intensity (Fig. S3, ESI†). The S_2p XPS spectrum demonstrated a peak at 163.9 eV corresponding to the binding energy for the C–S bond, owing to spin–orbit couplings. And the S–H peak at 164.6 eV revealed that there existed S–H in the two types of CDs. The latter peak can be fit with oxidized sulfur species at 167.6 eV originating from a C–SOx (x = 2, 3, 4) species such as sulfate or sulfonate.^45,46 (Fig. S4, ESI†) Moreover, these oxygen-containing surface groups (hydroxyl, carbonyl and carboxyl) resulted in CDs being more hydrophilic and negatively charged. The r-CDs' zeta potential value was about −20 mV (pH = 7.0) and the CDs' zeta potential value was about −11.1 mV (pH = 7.0) (Table S1, ESI†). This evidence shows that the CDs were reduced by NaBH₄ and that NaBH₄ selectively reduced the carbonyl groups to hydroxyl groups and without reducing other species (C=C and COOH). HR-TEM images were employed to directly observe the morphology and particle size distributions. As shown in Fig. 1C and D, no aggregation of CDs occurred, depicting their satisfactory dispersion. In particular, the size distribution of CDs was 1–3 nm with an average diameter of 2.0 nm. Meanwhile, r-CDs remained well dispersed after reduction with their related average size of about 2.3 nm (Fig. S5, ESI†).

To elucidate the formation mechanism of r-CDs, we have built up a model for this reduction process (Fig. 2). To be specific, NaBH₄ selectively reduced carbonyl and epoxy rather than other species (C=C and COOH), inducing an increased amount of hydroxyl groups on the r-CDs. Meaningfully, the increased amount of hydroxyl groups endowed reducibility to the r-CDs.⁴⁴

Fig. 2 Schematic illustration of synthesis of reduced-state carbon dots.

Considering the advantage of their reducibility, we investigated whether r-CDs can be utilized to prepare nanoclusters. Using r-CDs, HAuCl₄ and BSA, r-CDs–AuNCs were successfully synthesized on the basis of electron transfer from r-CDs (Fig. 3A). To describe these AuNCs, the brown solution of r-CDs–AuNCs exhibited excitation and emission peaks at 375 and 630 nm respectively with related visualization (photographs I and II). At the same time, the powder of r-CDs–AuNCs obtained via lyophilization exhibited striking red fluorescence under UV light (photograph III), whereas a yellowish color was observed under daylight. To investigate the nanostructures of r-CDs–AuNCs, HR-TEM was employed to directly observe the morphology and particle size distributions. As shown in Fig. 3C, r-CDs–AuNCs obtained here existed with a major population within the size range of 2–3 nm and no aggregation occurred. Next, FTIR spectroscopy revealed the groups of these AuNCs. In particular, there are fractions of –NH₂, –OH and C=O upon formation of the r-CDs–AuNCs (Fig. S6, ESI†). Furthermore, matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry was applied to characterize the current AuNCs. As shown in Fig. S7 (ESI†), the as-prepared r-CDs–AuNCs exhibited a peak at 70 kDa, and BSA molecular weight of 66 kDa was well known. Thereby, the r-CDs–AuNCs showed a peak shift of ∼4 kDa, suggesting that there were 21 gold atoms in r-CDs–AuNCs. In accordance with the FTIR data, MS results provided further evidence of r-CDs–AuNCs' formation.

Fig. 3 (A) Schematic illustration of preparation of r-CDs–AuNCs. (B) Fluorescence excitation and emission spectra of r-CDs–AuNCs. Inset: photographs of r-CDs–AuNCs solution (I and II) and powder (III). (C) HR-TEM image of r-CDs–AuNCs (inset: the crystalline structure of an individual nanocluster).

Likewise, the r-CDs–AuNCs displayed excitation-independent behavior (Fig. S8, ESI†), indicating their stable fluorescent property. Additionally, the r-CDs only reduced by NaBH₄ showed efficiency for synthesizing r-CDs–AuNCs with distinct fluorescence, indicating that NaBH₄ rather than other reducers can be used for reducing CDs for the synthesis of AuNCs (Fig. S9, ESI†). To identify the optimal conditions for synthesizing r-CDs–AuNCs, a series of experiments were further performed. As revealed in Fig. S10 (ESI†), the fluorescence intensity of r-CDs–AuNCs exhibited variations along with varying concentrations of r-CDs and Au³⁺, demonstrating that synthesis of r-CDs–AuNCs was dependent on these proposed factors. Thus, 2.0 g L⁻¹ of r-CDs and 3.0 mM of Au³⁺ served as the optimal conditions.

Since the r-CDs described here behaved as an effective reducing agent, we further investigated whether they can be employed for synthesizing AuNPs or AgNPs. Importantly, we have successfully prepared AgNPs and AuNPs by using r-CDs without any additional reducing agent or external photoirradiation under a mild water-bath condition (Fig. 4). For the mechanism, r-CDs acted as an electron donor for ensuring the reduction of metal ions to atoms.⁴⁷ To be specific, Ag⁺ (Au³⁺) ions were firstly attracted to the surface of r-CDs via electrostatic interactions with hydroxyl, amine, and other functional groups. Then, Ag⁺ (Au³⁺) ions were reduced by r-CDs via electron transfer from the r-CDs to the metal ions.⁴⁸ Eventually, Ag or Au atoms functioned as the nucleation centers, followed by the growth of metallic nanocrystals, leading to formation of AgNPs or AuNPs. During this process, r-CDs also played the role of a stabilizer, preventing AgNPs and AuNPs from aggreation.¹⁵

Fig. 4 Schematic illustration of preparation of r-CDs–metal nanoparticles.

Next, various characterizations of the AgNPs obtained by using r-CDs were performed. Compared with the original r-CDs, the mixture including r-CDs and AgNPs apparently exhibited a lower fluorescence intensity and a red-shift of about 30 nm as well as the solution color varying from light yellow to brown (Fig. S11, ESI†). As illustrated in the UV-visible spectrum (Fig. 5A), the adsorption band of r-CDs–AgNPs at 480 nm was attributed to the characteristic surface plasmon absorption of r-CDs–AgNPs, and their related resonance light scattering (RLS) was at 480 nm (Fig. 5B). Taken together, the data above demonstrated that r-CDs–AgNPs indeed formed by virtue of r-CDs. As regards the nanostructure of r-CDs–AgNPs, HR-TEM was employed to directly observe the morphology and particle size distributions. As shown in Fig. 5C and D, r-CDs–AgNPs obtained here existed with a major population at a size of 8 nm and no aggregation occurred. For the size distribution analysis, the diameter of r-CDs–AgNPs was in the range of 6–10 nm determined by dynamic light scattering (DLS).

Fig. 5 (A) UV-visible spectrum of r-CDs–AgNPs. (B) RLS spectrum of r-CDs–AgNPs. (C and D) HR-TEM images of r-CDs–AgNPs. Inset: DLS data of r-CDs–AgNPs.

Again, different kinds of r-CDs reduced by NaBH₄, ascorbic acid, sodium citrate, and hydrazine hydrate separately were introduced to synthesize AgNPs. As shown in Fig. S12 (ESI†), only the solution in the presence of r-CDs derived from NaBH₄ showed an absorption band, indicating that other reducing agents, except NaBH₄, cannot be applied to synthesize r-CDs for further reducing Ag⁺ to AgNPs. Furthermore, to identify the appropriate conditions for synthesizing r-CDs–AgNPs, various experiments were performed. As revealed in Fig. S13 (ESI†), the absorbance intensities of r-CDs–AgNPs implied variations along with varying concentrations of r-CDs and Ag⁺, reaction time and temperature, demonstrating that synthesis of r-CDs–AgNPs was dependent on these selected conditions. Thereby, 2.0 g L⁻¹ of r-CDs, 2.0 mM of Ag⁺, 50 °C and 10 min finally served as the optimal conditions.

Similarly, we also obtained AuNPs using r-CDs. In UV-visible spectra, the absorption band at 520 nm was attributed to the characteristic surface plasmon absorption of r-CDs–AuNPs, and the obvious RLS was at 570 nm (Fig. 6A and B), indicating the formation of AuNPs. TEM images and DLS in Fig. 6C and D showed that r-CDs–AuNPs are uniformly distributed, and their size was 13.4 ± 7.3 nm. Besides, these data proved that AuNPs formed only in the presence of r-CDs reduced by NaBH₄ (Fig. S14, ESI†), in agreement with the data for the preparation of AgNPs. Likewise, we further identified the optimal conditions for synthesizing AuNPs as 1.0 g L⁻¹ of r-CDs, 1.5 mM of Au³⁺, 60 °C and 30 min (Fig. S15, ESI†).

Fig. 6 (A) UV-visible spectrum of r-CDs–AuNPs. (B) RLS spectrum of r-CDs–AuNPs. (C and D) HR-TEM images of r-CDs–AuNPs. Inset: DLS data of r-CDs–AuNPs.

Conclusions

In conclusion, we have successfully synthesized r-CDs by reducing conventional CDs, with cysteine serving as the carbon source. Meaningfully, metal nanoclusters and nanoparticles were creatively prepared by using the r-CDs as a reducing agent on the basis of electron transfer. To be specific, the hydroxyl groups on their surfaces provided r-CDs with excellent electron-donating capability, thereby facilitating the fast reduction of metal ions to corresponding metal nanoclusters or nanoparticles. Significantly, this electron transfer mechanism of r-CDs will broaden their potential applications in the fields of energy-conversion systems or catalysis, such as solar cells, electro-catalysis and photo-catalysis.

Acknowledgements

We gratefully acknowledge financial support by National Natural Science Foundation of China (31100981), Research Fund for the Doctoral Program of Higher Education of China (20110182120014), Natural Science Foundation Project of CQ CSTC (cstc2013jcyjA10117), Fundamental Research Funds for the Central Universities (XDJK2015A005, 2362014xk07), and Innovative Research Project for Postgraduate Students of Chongqing (CYS14049).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra02598b

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