Growth and Stabilization of Silver Nanoparticles on Carbon Dots and

Article pubs.acs.org/Langmuir

Growth and Stabilization of Silver Nanoparticles on Carbon Dots and Sensing Application Liming Shen,† Meiling Chen,† Linlin Hu,† Xuwei Chen,*,† and Jianhua Wang*,†,‡ †

Research Center for Analytical Sciences, College of Sciences, Northeastern University, Box 332, Shenyang 110819, China Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, China

‡

S Supporting Information *

ABSTRACT: Carbon dots (C-dots) have been proven to show the capability for direct reduction of Ag+ to elemental silver (Ag0) without additional reducing agent or external photoirradiation by incubating Ag+ with C-dots for 5 min in a water bath at 50 °C. Silver nanoparticles (Ag-NPs) are simultaneously formed with an average size of 3.1 ± 1.5 nm and grew on carbon dots. This process involves the oxidation of amine or phenol hydroxyl groups on the aromatic ring of Cdots. Meanwhile C-dots protect and stabilize the Ag-NPs from aggregation in aqueous medium; that is, the Ag-NPs are stable at least for 45 days in aqueous medium. The formed Ag-NPs cause significant resonance light scattering (RLS), which correlates closely with the concentration of silver cation, and this facilitates quantitative detection of silver in aqueous medium.

1. INTRODUCTION Carbon dots (C-dots) have recently attracted extensive interest for their unique properties of tunable photoluminescence, low toxicity, and favorable biocompatibility.1−3 C-dots, as photoactive nanomaterials, are physicochemically and photochemically stable and nonblinking.4,5 These features offer them great popularity in the field of one- or two-photon and multicolor in vitro and in vivo bioimaging.6−8 On the other hand, ascribing to the abundant surface oxygen-containing functional groups and favorable hydrophilicity, C-dots have been recognized as a promising support for construction of optical sensors for quantitative assay of biomolecules and environmental pollutants.9−12 Similar to the luminescent emission of traditional semiconductor quantum dots, the emission of C-dots might also be attributed to the radiative recombination of surfacetrapped electrons and holes.13 It has been demonstrated that photoexcited C-dots can serve as both electron donors and electron acceptors. Its photoluminescence could be quenched efficiently by either electron acceptor or donor molecules.14 The electron-donating capabilities of C-dots have found successful applications in photoreduction metal salts into the corresponding metal coating on the surface of C-dots with no metal nanoparticles under certain photoirradiation, where photoexcited C-dots act as the electron donors to harvest photons for subsequent efficient photocatalytic process.15 It is reported to be feasible to grow metal nanoparticles on C-dots surface. However, the presence of an additional reducing reagent, that is, ascorbic acid, is required. The C-dots serve as a structural scaffold whose peripheral charges help to stabilize the metal particles in aqueous solution.16 Hitherto, no attempt has been reported for the growth of metal nanoparticles on the © 2013 American Chemical Society

surface of C-dots in the absence of additional reducing reagent or photoirradiation. Silver-containing compounds are widely used in electrical, photographical, pharmaceutical industries, and disinfection of drinking water.17,18 Every year, a huge amount of industrial wastes containing silver is released into surface water, which makes silver the second maximum contaminant.19 The binding of silver into active sites of enzymes leads to their distinctive inhibition; thus excessive Ag+ ions are greatly toxic to water organisms and have an extremely harmful side-effect on human health.20 It is crucial to monitor the concentration level of silver in environmental waters and offer timely warning of its potential toxic effect. Numerous spectroscopic approaches are available for silver detection, for example, atomic spectrometry,21 UV−vis spectroscopy,22 and fluorescence spectrometry.23 Generally, conventional methods require either sophisticated equipment or chromogenic agents. In this respect, it is highly necessary to develop simple, economical, and sensitive sensing systems for silver detection. Herein, we report a one-step approach for the growth of silver nanoparticles (Ag-NPs) on C-dots surface without any additional reducing agent or external photoirradiation under a mild water-bathing condition. C-dots act as an electron donor ensuring the reduction of Ag+ to Ag0 within a very short time, that is, 5 min. Meanwhile, C-dots serve as a capping agent to maintain the Ag-NPs stable for at least 45 days in aqueous medium. The Ag-NPs cause obvious resonance light scattering Received: November 6, 2013 Revised: December 5, 2013 Published: December 6, 2013 16135

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according to a reported protocol.24 The average size of the Cdots is derived to be 29.4 ± 6.9 nm in diameter (Figure 1A).

(RLS), which correlates closely with the concentration of silver cation, and this facilitates quantitative detection of silver in aqueous medium.

2. EXPERIMENTAL SECTION Materials and Reagents. All of the chemicals used in this study including chitosan, acetic acid, AgNO3, hydrogen tetrachloroaurate hydrate (HAuCl4), cetyltrimethylammonium chloride (CTAC), H2N(CH2)10COOH, citric acid, activated carbon, and NaOH are obtained from commercial sources (Sinopharm Chemical Reagent Co.) and are used without further purification. Deionized water of 18 MΩ cm is used throughout. Instrumentation. UV−vis absorption spectra are recorded with a T6 UV/vis spectrophotometer (Beijing Purkinje General Instrument, China) with a 1.0-cm quartz cell. Transmission electron microscopy (TEM) images are recorded on a field-emission transmission electron microscope at an accelerating voltage of 200 kV (JEM-2100F, JEOL, Ltd., Japan). The photoluminescence and resonance light scattering (RLS) studies are carried out on an F-7000 fluorescence spectrophotometer (Hitachi High Technologies, Japan) equipped with a 0.2 cm quartz cuvette. The excitation and emission slits are both set at 5.0 nm, with a scan speed of 1200 nm min−1. The size of composites is investigated with a Nano Zetasizer (Malvern, England). Fourier-transform infrared (FT-IR) spectra are recorded on a Vertex 70 FT-IR spectrophotometer (Bruker, Germany) from 400 to 4000 cm−1. An ESCALAB 250 X-ray photoelectron spectrometer (Thermo, America) is used to investigate the surface groups of C-dots and composites. Preparation of the Fluorescent C-Dots and Silver/Gold Nanoparticles. C-Dots derived from chitosan are prepared by heating 2 g of chitosan in 18 mL of 2% acetic acid in a Teflon equipped stainless steel autoclave at 180 °C for 12 h. After being cooled to room temperature, the obtained dark brown product is centrifuged at a high speed (12 000 rpm) for 25 min to remove the less-fluorescent deposit. The upper brown solution is then filtered with a 0.22 μm filter membrane.24 H2N(CH2)10COOH (2 g in 25 mL of H2O) was first neutralized by NaOH (0.45 g) to obtain sodium 11-amino-undecanoate [H2N− (CH2)10COONa], and then the amine groups were protonated with citric acid (2 g) to form the corresponding ammonium carboxylate salt, that is, the molecular precursor. The precursor was directly oxidized in air at 300 °C for 2 h at a heating rate of 10 °C min−1 in a muffle oven. The crude product was extracted with 25 mL of hot water with sonication and then centrifuged to remove the insoluble particles. The supernatant was collected and adjusted to pH 2 with HCl to produce a bulk precipitate, which was dissolved by 0.2 g of NaOH after washing with water. The final product is a deep brown colloidal dispersion.25 One gram of activated carbon powder was put into 100 mL of HNO3 (4 mol L−1) and refluxed for 24 h. After being naturally cooled to room temperature, the suspension was neutralized with NaOH and subsequently centrifuged at 10 000 rpm for 15 min to remove the nonfluorescent deposit. The supernatant was further dialyzed against double distilled water through a dialysis membrane (MWCO of 1 kDa) to remove inorganic salt.26 For the growth of Ag-NPs, an aqueous solution consisting of C-dots (33 mg L−1), AgNO3 (0.06 mmol L−1), and NaOH (0.01 mol L−1) was incubated at 50 °C in a water bath for 5 min. Similarly, the growth of Au-NPs was conducted in an aqueous solution consisting of C-dots (0.17 g L−1), HAuCl4 (0.13 mmol L−1), and CTAC (3 mmol L−1) at 70 °C in a water bath for 10 min. The acidity of the reaction media for the growth of Au-NPs was maintained at pH 4.4.

Figure 1. (A) HRTEM images of the derived C-dots; and (B) HRTEM images of the Ag-NPs/C-dots composites made from AgNO3 (0.06 mM) and C-dots (33 mg L−1) in 0.01 M NaOH incubating at 50 °C for 5 min. Inset: HRTEM images of Ag-NPs with fringe spacing.

The as-prepared C-dots are then mixed with an AgNO3 solution and incubated in a water bath of 50 °C for 5 min. The reaction mixture turned yellow, indicating the formation of Ag-NPs. As illustrated in the UV−vis spectra of Ag-NPs (Figure 2A), the adsorption band at 408 nm is attributed to the characteristic surface plasmon absorption of Ag-NPs, while no absorption is observed for the control experiment where no AgNPs are formed in the absence of C-dots. To exclude the possibility that the growth of Ag-NPs is induced by chitosan itself or by circumambient photoirradiation, control experiments have been performed by incubating AgNO3 with chitosan only or incubating it with C-dots in dark. The results show that no Ag-NPs are formed when chitosan is adopted. It is notable that similar UV−vis spectra are observed in the presence of C-dots, no matter whether there is circumambient photoirradiation (Figure 2A). This observation indicated that the obtained C-dots are able to accomplish the catalytic reaction for the conversion of small sized Ag-NPs, which is quite different from most reported protocols where external photoirradiation is a prerequisite for the reduction of Ag+ to elemental silver.14,15,27 It is also noteworthy that no color change and aggregation are observed for the colloidal Ag-NPs after storage for a long time, for example, 45 days (Figure 2B). This indicates that the C-dots can serve as a stabilizing agent for the metal nanoparticles to prevent their aggregation, revealing significant affinity between the Ag-NPs and C-dots. TEM images confirm the presence of well-dispersed Ag-NPs in the AgNO3−C-dots mixture after heating as shown in Figure 1B. It is noted that the C-dots are hardly seen in the image because of their low contrast on the substrate used, and this phenomenon has also been encountered in the Pd particles− graphene QDs composites.28 The diameters of the Ag-NPs are centered at 2−4 nm, with an average of 3.1 ± 1.5 nm (based on statistical analyses of more than 300 nanoparticles), which are much smaller than the previously reported Ag-NPs obtained from synergistic function of C-dots and additional reducing reagent ascorbic acid.16 HRTEM image further shows high crystallinity of the Ag-NPs with a well-defined crystal lattice of 0.244 nm (inset in Figure 1B) attributed to Ag(111). The distribution of Ag-NPs on the surface of C-dots is further demonstrated by elemental mapping as illustrated in Supporting Information Figure S1. We have thoroughly investigated the factors affecting the growth of Ag-NPs on the surface of C-dots. Figure 3A shows UV−vis spectra of the Ag-NPs achieved by using different concentrations of C-dots. With the increase of C-dots concentration, a bathochromic shift of absorption maxima is

3. RESULTS AND DISCUSSION Catalytic Production of Ag-NPs with C-Dots as Reducing Agent. C-Dots with abundant surface functional groups are first prepared by one-step hydrothermal carbonization of chitosan followed by purification with centrifugation 16136

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Figure 2. (A) UV−vis spectra of the reaction mixtures. AgNO3 (0.06 mM) incubating with chitosan (33 mg L−1) under natural light (a); AgNO3 (0.06 mM) incubating with C-dots (33 mg L−1) under natural light (b); and AgNO3 (0.06 mM) incubating with C-dots (33 mg L−1) in the dark (c). Inset: Photographs of sample. (B) UV−vis spectra of the Ag-NPs/C-dots composites. Fresh Ag-NPs/C-dots composites (a); and Ag-NPs/C-dots composites after storage for 45 days (b). Inset: Photographs of sample.

dynamic light scattering (DLS), which increased from 35.5 ± 2.8 to 111.0 ± 4.2 nm, with the original C-dots concentration increasing from 55 to 110 mg L−1 (as shown in Figure 4). As the size of C-dots in solution stays the same, we speculate the size increase of the nanocomposites was induced by Ag-NPs, and the growth of multi big size Ag-NPs on C-dots leads to a dramatic increase of the size of nanocomposites. Meanwhile, the mono distribution of DLS plot indicates Ag-NPs are attached on the surface of C-dots, instead of isolated from each other. It is also noted that a weak basic solution is preferential for the growth of Ag-NPs (Figure 3C). Surface charge analysis indicated that the C-dots are negatively charged under basic conditions; therefore, the cationic Ag+ is prone to coordinate with the surface functional groups of C-dots, for example, −OH, and in situ reduced to Ag0 therein. It is widely believed that oxygen-containing functionalities, such as carboxylic, carbonyl, and phenolic groups, are necessary for anchoring of metal nanoparticles on the carbon supports.29,30 When Ag-NPs are formed on the surface of Cdots, the peripheral charges of surrounding C-dots help to stabilize the metal nanoparticles in aqueous solution and prevent their aggregation. Further studies on the reaction temperature and incubation time indicate that the catalysis proceeds rapidly at 50 °C and the absorbance of the Ag-NPs reaches maximum and remains constant after incubation for 5 min (Figure 3D). Mechanisms for the Formation of Nanoparticles. Further attempts have been made to elucidate the mechanisms

Figure 3. (A) UV−vis spectra of the nanocomposites after incubating AgNO3 (0.06 mM) with C-dots (11−110 mg L−1) in 0.01 M NaOH at 50 °C for 5 min. (B) Photographs of sample solutions with 11−110 mg L−1 C-dots. (C) NaOH concentration-dependent absorbance at 408 nm for the derived Ag-NPs. (D) The incubation temperature- and time-dependent absorbance at 408 nm for the derived Ag-NPs.

observed along with a gradual decrease in the absorption intensity. It is known that the bathochromic shift of absorption maximum corresponds to the size increment of Ag-NPs. We investigate hydrodynamic diameter of the composites by

Figure 4. Size distribution of the Ag-NPs/C-dots composites measured by DLS technique: (A) C-dots (55 mg L−1) and AgNO3 (0.06 mM) in 0.01 M NaOH incubated at 50 °C for 5 min; and (B) C-dots (110 mg L−1) and AgNO3 (0.06 mM) in 0.01 M NaOH incubated at 50 °C for 5 min. 16137

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metal nanoparticles resulting in a quinone-like structure.32−34 The peaks of binding energies centered at 531.7 eV in the O1 s spectra of C-dots (Figure 5B) indicate the existence of −OH of phenol hydroxyl groups in the aromatic structure of C-dots. While a new binding energy peak centered at 532 eV appears after reduction (Figure 5C), this can be ascribed to CO in quinone structure, indicating the structure change of C-dots during the reduction process. FT-IR spectra of C-dots (Figure 6A) showed that before reduction the broad O−H stretching vibration is clearly observed at 3390 cm−1 along with absorption at around 1000 cm−1, indicating the presence of C−OH in C-dots. The absorptions at 3390 and 1660 cm−1 are due to asymmetric stretching and deformational vibration of −NH2 respectively, suggesting the presence of abundant −NH2 group on the surface of C-dots. The typical bands at 1564, 1498, 1440, and 1369 cm−1 are ascribed to the CC stretching vibrations of aromatic ring, and those at 865 and 686 cm−1 are due to the out-of-plane bending vibration of CH group of aromatic ring. The obtained C-dots also exhibit characteristic absorptions of CCH wagging vibration at 898 cm−1. Moreover, the sharp bands at 1751, 1297, and 1245 cm−1 further indicate the presence of CO and C−N bonds. These results suggest that the derived C-dots are of aromatic structure with abundant amino and phenol hydroxyl groups on the surface, serving as the electron donor for the reduction of Ag+ to elemental silver.35 The decrease of FT-IR intensity at 3390 cm−1 after the reduction should be induced by the consumption of −NH2 and C−OH groups during the reduction process. The amount of −NH2 groups is determined using 2,4,6-trinitrobenzenesulphonic acid as reported previously.36 The results reveal that there is 0.29 and 0.09 mol g−1 −NH2 groups for C-dots before and after reduction, respectively, providing direct evidence for the consumption of NH2 groups. For amine group, azo bonds are the final oxidation products.37 The obtained composites exhibit apparently lower photoluminescence intensity and a blue-shift of 8/19 nm for excitation/emission maximum in comparison to those of the original C-dots (Figure 7). As the photoluminescence is attributed to radiative combination of electrons and holes confined on the C-dots surface, the above observation implies that the surface states of C-dots are affected by in situ growth of the Ag-NPs on the surface.38 Figure 6B illustrated that C-dots derived from different carbon sources; for example, activated carbon and 11-amino-undecanoate

of nanoparticles formation by investigating the changes of surface configuration of C-dots before and after the reduction process. The X-ray photoelectron spectroscopy (XPS) results (Figure 5A) exhibit the main peaks of C atoms in the C-dots at

Figure 5. XPS spectra of the corresponding materials including C-dots and Ag-NPs/C-dots composites. (A) C 1s spectra of the C-dots; (B) O 1s spectra of the C-dots; (C) O 1s spectra of the Ag-NPs/C-dots composites; (D) N 1s spectra of the C-dots; and (E) Ag 3d spectra of the Ag-NPs/C-dots nanocomposites.

288.6 eV (CO), 286 eV (C−N), 285.4 eV (C−OH), and 284 eV (CC). The binding energy at 284.7 eV belongs to aromatic ring sp2 C−C of C-dots. The peaks at 399 eV in the N spectra of C-dots suggest the existence of −NH2 (Figure 5D). After the reduction process, the binding energies of 368 and 374 eV in the Ag 3d spectra of composites are attributed to Ag 3d5/2 and Ag 3d3/2, which well indicate the formation of metallic Ag (Figure 5E).31 The phenol hydroxyl groups on the aromatic ring are reductive and are able to induce the growth of

Figure 6. (A) FT-IR spectra of C-dots and the Ag-NPs/C-dots composites. (B) UV−vis spectra of C-dots derived from activated carbon (a) and 11amino-undecanoate (b), along with the corresponding Ag-NPs/C-dots composites after incubating with AgNO3 at 50 °C for 5 min (c, C-dots from activated carbon) and at 80 °C for 20 min (d, C-dots from 11-amino-undecanoate). 16138

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filtered through a 0.22 μm filter membrane. The method is further validated by spiking recoveries of silver in the two river water samples. The analytical results are illustrated in Table 1. Table 1. Silver Detection in Surface Watersa samples

foundb (μM)

spiked (μM)

recoveredb (μM)

recov. (%)

Songhua River Zhu River

5.2 ± 0.1 4.8 ± 0.3

2 2

7.2 ± 0.2 6.9 ± 0.2

103 106

River waters, n = 3, 95% confidence level. bn = 3, means ± t·s/√n, f = 95%, t = 3.18.

a

Figure 7. Photoluminescent excitation and emission of C-dots and the Ag-NPs/C-dots nanocomposite.

It is obvious that satisfactory recoveries are achieved for the spiked silver, that is, 103% and 106% for the Songhua River and Zhu River water samples, respectively. These results demonstrated that the present method has potentials for quantitative analysis of silver in water samples.

whose cores consist of aromatic regions and surface abundant with oxygen-containing functional groups can also effectively reduce Ag+ to Ag0 and produce Ag-NPs/C-dots composites with similar UV−vis absorption spectra as compared to that achieved by using chitosan as carbon source. Detection of Ag+ with C-Dots as Reducing Agent. Herein, we construct an optical sensor for Ag+ detection with C-dots as reducing agent, based on the observations as described in the previous sections. Shortly, the catalytic reducing effect of C-dots produces Ag-NPs, which result in resonance light scattering (RLS) at a wavelength of 340 nm (as shown in Supporting Information Figure S3). There is a linear relationship between the ΔRLS intensity and Ag+ concentration within a range of 0.5−6 μM (as illustrated in Figure 8). A

4. CONCLUSIONS C-Dots are proven to be an excellent catalyst for the reduction of Ag+ to elemental silver under a very mild condition, and AgNPs are simultaneously formed. The nucleated Ag-NPs are efficiently stabilized by the negatively charged C-dots against flocculation in aqueous medium. The formed Ag-NPs cause significant resonance light scattering, which facilitates quantitative detection of silver in water samples. On the other hand, C-dots can also be utilized for the catalytic growth of Au-NPs. This study exhibits potential applications of C-dots in the synthesis of metal nanomaterials.

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ASSOCIATED CONTENT

S Supporting Information *

Elemental mapping, UV−vis absorption spectra, and resonance light scattering spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the Natural Science Foundation of China (21275027, 21235001, and 21075013), the Program of New Century Excellent Talents in University (NCET-11-0071), and Fundamental Research Funds for the Central Universities (N110705002, N110405005, and N110805001).

Figure 8. The linear calibration graph between RLS intensity and the Ag+ concentration. C-dots 33 mg L−1, NaOH 0.01 M, 50 °C for 5 min.

regression equation of ΔRLS = 280.32 × CAg+ − 45.665 is achieved along with a detection limit of 0.13 μM (3σ/s, n = 11) and a precision of 2.8% RSD at 3 μM. The U.S. Environmental Protection Agency (EPA) has launched a restriction of 0.9 μM silver in drinking water.39 In this respect, the sensitivity of the present sensing system is sufficient for the detection of silver on the requirement of EPA, requiring no expensive biological reagent and sophisticated instruments. We have investigated the practical applicability of the proposed resonance light scattering assay system for the detection of silver contents in environmental water samples. Surface water samples are collected from Songhua River (Harbin, China) and Zhu River (Guangzhou, China), which are

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REFERENCES

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dx.doi.org/10.1021/la404270w | Langmuir 2013, 29, 16135−16140