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Silver Nanoparticles/N-doped Carbon-Dots Nanocomposites Derived from Siraitia Grosvenorii and Its Logic Gate and Surface-Enhanced Raman Scattering Characteristics Yubin Su, Bingfang Shi, Suqi Liao, Jingjing Zhao, Lini Chen, and Shulin Zhao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b01698 • Publication Date (Web): 22 Jan 2016 Downloaded from http://pubs.acs.org on January 23, 2016

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ACS Sustainable Chemistry & Engineering

Silver Nanoparticles/N-doped Carbon-Dots Nanocomposites Derived from Siraitia Grosvenorii and Its Logic Gate and Surface-Enhanced Raman Scattering Characteristics Yubin Su,† Bingfang Shi,†,‡ Suqi Liao,† Jingjing Zhao,*,† Lini Chen† and Shulin Zhao*,†

†

Key Laboratory for the Chemistry and Molecular Engineering of Medicinal Resources (Ministry of Education), Guangxi Normal University, 15 Yucai Road, Guilin 541004, China

‡

Department of Chemistry and Life Sciences, Baise University, 21 Zhongshan Road, Baise 533000, China

ABSTRACT: Silver/carbon dots (CDs) nanocomposites receive significant attention for diverse applications owing to their unique physical and chemical properties. Herein, a green method is proposed for synthesizing silver nanoparticle/N-doped CDs (AgNPs/N-CDs) nanocomposites, wherein the AgNPs are grown on the surface of reduced N-CDs derived from Siraitia grosvenorii. The N-CDs were used as a reducing agent and stabilizer, no additional reducing agent and stabilizer were necessary. The as-synthesized AgNPs/N-CDs nanocomposites were characterized using ultraviolet-visible spectroscopy, transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). An AND logic system based on the obtained N-CDs was proposed, which avoids complicated modifications and chemical labeling. The surface-enhanced Raman scattering (SERS) properties of the prepared 1

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AgNPs/N-CDs nanocomposites were also investigated, indicating the potential application for SERS detection.

KEYWORDS: AgNPs/N-doped carbon dots nanocomposites, siraitia grosvenorii logic gate, surface-enhanced Raman scattering

INTRODUCTION Silver nanoparticles (AgNPs) have received significant attention for diverse applications owing to their unique physical and chemical properties, such as sensing,1,2 bioimaging,3 surface-enhanced Raman scattering (SERS),4,5 and catalysis.6 Recently, because of the large surface area and interaction with the target molecules, carbon materials are becoming increasingly important for the gathering of target molecules.7 Silver–carbon nanocomposites, including AgNPs–graphene oxide nanocomposites,8

silver/carbon

nanotube

networks,9

and mesoporous-AgNPs

nanohybrids,10 have been extensively investigated. For example, the AgNPs–graphene oxide

hybrids

could

act

as

effective

substrates

for

SERS.11,12

Using

AgNP-DNA@graphene quantum dots (GQDs) as a sensing platform, sensitive detection of H2O2 and glucose was achieved.13 Considerable effort has been directed toward the synthesis of silver–carbon nanomaterials, and the typical synthesis process for AgNPs/carbon material nanocomposites requires extra additives such as ascorbic acid14 and NaBH4.15 However, such methods have drawbacks: they are time-consuming, inducing toxicity, and are complicated, which limits their wide 2


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application. Thus, it is a challenge to develop facile, nontoxic, and eco-friendly methods for the synthesis of AgNPs/carbon material nanocomposites.

Fluorescent carbon materials, such as GQDs and carbon dots (CDs), have attracted great attention because of their fascinating properties, including low toxicology, photostability,

and

good

biocompatibility

for

bioimaging

and

biomedical

applications.16-20 In addition to their excellent photoluminescence and biological properties, they can serve as an excellent electron acceptor and electron donor owing to the rich oxygen-containing functional groups (hydroxyl, carbonyl, carboxyl, and epoxy groups) on their surfaces, indicating the potential as an oxidizing or reducing agent.21-23 In the past few years, the preparation of noble-metal NPs and noble-metal NP-carbon hybrids using reduced carbon materials have attracted extensive interest.24,25 For example, Zhang’s group used hydroxyl-rich CDs as both the reducing agent and stabilizer for the preparation of noble-metal NPs.26 Wang et al. reported a one-step approach for the growth of AgNPs under a mild water-bathing condition without any reducing agents or external photoirradiation.27 More recently, they successfully fabricated stable AgNPs with CDs acting as a catalytic reductant and capping agent.28 In the same year, they proposed an approach for the preparation of a GQDs/AgNPs hybrid by the in situ growth of AgNPs on the surface of GQDs.29 However, these reported methods have some disadvantages regarding the preparation of the reduced CDs or GQDs. Most of them require a high temperature, as well as complicated and time-consuming procedures. In this regard, the facile, green synthesis of a new AgNPs/carbon material nanocomposite at relatively low temperatures 3



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without any extra additives is of great importance.

Recently, lots of interest has been directed toward the development of green methods for preparing CDs using inexpensive and renewable materials such as natural products or food.30,31 Herein, we report a facile, cost-effective, green synthetic method to fabricate a novel AgNPs/N-CDs nanocomposite, using CDs as a reducing agent and stabilizer which were derived from Siraitia grosvenorii. The preparation procedures for the N-CDs are shown in Scheme 1. The synthesized N-CDs do not require further processing or surface modification and have an excellent reducibility. Impressively, in this work, the prepared N-CDs can be acted as a reductant for the synthesis of AgNPs/N-CDs nanocomposites within 120 min by simply mixing AgNO3 and NaOH simultaneously. To our knowledge, the discovery of the CDs in natural products and its function as a reducing agent to prepare the AgNPs/carbon materials nanocomposites is hardly reported. This method can potentially be applied to establish a simple AND logic system, without further modification and labeling. In addition, the SERS properties of as-synthesized AgNPs/N-CDs nanocomposites were investigated.

EXPERIMENTAL SECTION Reagents and Materials. Dry Siraitia grosvenorii was purchased from a local market (Guilin, China). The other chemicals used in the tests were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All the reagents were used as received, without further purification. Ultrapure water obtained from a Millipore water purification system (≥18 MΩ, Milli-Q, Millipore) was used in all of the runs. 4


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Synthesis of the N-CDs. First, the Siraitia grosvenorii sample was prepared by powdering dry Siraitia grosvenorii. Then, 3.0 g of dry Siraitia grosvenorii powder was added to 20 mL of ultrapure water at 90 oC, and the mixture solution was vigorously stirred for 30 min. The resulting supernatant was filtered through a 0.22 µm membrane to remove large or agglomerated particles, and the aqueous solution was centrifuged at 12,000 rpm for 15 min to dislodge the deposit, and purified over 24 h through a dialysis membrane (1,000 MWCO). The upper N-CDs aqueous solution was dried by evaporation, and the product was stored at 4 oC for later characterization and use.

Synthesis of the AgNPs/N-CDs Nanocomposites. In a typical preparation process, 0.483 mg of N-CDs was diluted to a 0.483 mg/·mL aqueous solution. Then, 100 µL of the diluted N-CDs aqueous solution was mixed with NaOH (200 µL, 70 mM, final concentration of 35.0 mM) and AgNO3 (100 µL, 140 mM, final concentration

of

35.0

mM)

in

sequence.

Subsequently,

yellowish-brown

AgNPs/N-CDs nanocomposites were produced at room temperature within 120 min. The nanocomposites were then washed repeatedly with deionized water in three times, and dried by freeze. The mass yield of the nanocomposites is about 83.3%. Finally, the product was stored at 4 ℃ for later characterization and use.

Characterization. Absorption measurement was performed using a Cary 60 ultraviolet-visible (UV-vis) spectrometer (Agilent Technologies, USA). Fluorescence spectra were recorded on a Cary Eclipse fluorescence spectrometer (Agilent

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Technologies, USA). Fourier transform infrared spectroscopy (FTIR) was conducted using KBr pellets and a Perkin-Elmer FTIR spectrophotometer (Perkin-Elmer, USA). Transmission electron microscopy (TEM) images were taken using a Tecnai G2 F20 transmission electron microscope (FEI, USA) operating at 200 kV. X-ray photoelectron spectroscopy (XPS) was performed using a Thermo ESCALAB 250Xi Multitechnique Surface Analysis spectrometer (Thermo, USA). Atomic force microscopy (AFM) images were captured using a MultiMode Nanoscope IIIa controller atomic force microscope (Veeco, USA). SERS spectra were recorded using a confocal microscope Raman spectrometer (Renishaw, UK). Scanning electron microscope (SEM) images were obtained on environmental scanning electron microscope FEI Quanta 200 FEG (Philips, Netherlands). The overall quantum yield was measured on FLS980 Edinburgh Fluorescence Spectrometer (Edinburgh, UK).

Establishment of the Logic Gates. For the AND logic gate, 100 µL of the diluted N-CDs aqueous solution was mixed with 300 µL of water [input = (0,0)]; 100 µL of 140 mM AgNO3 (final concentration is 35.0 mM) and 200 µL of water [input = (1, 0)]; 200 µL of 70 mM NaOH (final concentration is 35.0 mM) and 100 µL of water [input = (0,1)]; or 100 µL of 140 mM AgNO3 and 200 µL of 70 mM NaOH [input = (1,1)]. The solution was allowed to react at room temperature for 120 min, followed by the UV-vis spectrometer measurements.

SERS Measurement. In a typical process, 4 µL of the AgNPs/N-CDs nanocomposite suspension was pipetted onto the glass slide and dried under N2 gas,

6


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and then 2 µL of Rhodamine B solutions with different concentrations (1.0 µM, 5.0 µM, 10 µM, 50 µM) were dropped on the dried AgNPs/N-CDs nanocomposites for the SERS analysis. The Raman spectra were recorded immediately after the sample preparation.

RESULTS AND DISCUSSION Characterization of the N-CDs and AgNPs/N-CDs Nanocomposites. Figure 1A shows the TEM image of the prepared N-CDs and their size distribution. The average size of the N-CDs is ~9.0 nm in diameter. The high-resolution TEM (HRTEM) image clearly shows the lattice spacing of 0.21 nm (inset in Figure 1A, Figure S1A, Fourier-transform diffraction was shown in Figure S1C), which is similar to the (110) facet of graphitic carbon.32 Figure 1B is the TEM image of the AgNPs/N-CDs nanocomposites. Although the size distribution is slightly wide, more than 83 % AgNPs/N-CDs display similar diameters concentrated in 11±2 nm. The inset in Figure 1B and Figure S1B show HRTEM images (Fourier-transform diffraction was shown in Figure S1D), clearly indicating a highly crystalline structure with lattice spacing of 0.21 nm which agrees with the (110) lattice spacing of sp2 graphitic carbon, and a well-defined crystal lattice 0.24 nm in size which was attributed to the Ag(111).29,33 The AFM images indicates that the height of the N-CDs was ~1.0 nm (Figure 1C and 1D), whereas the height of the AgNPs/N-CDs nanocomposites was ~4.5 nm (Figure 1E and 1F). These results demonstrate the growth of the AgNPs on the surface of the N-CDs. The formation of AgNPs/N-CDs nanocomposite was further confirmed by

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energy dispersive spectroscopy, as illustrated in Figure S2 in the Supporting Information.

XPS was further performed to investigate the surface configuration of N-CD and AgNPs/N-CDs nanocomposites. The survey XPS spectrum clearly indicates the existence of C, N, and O in N-CDs (Figure 2A), and C, N, O, and Ag in the AgNPs/N-CDs nanocomposites (Figure 2B). The high resolution N 1s spectrum of N-CDs (Figure 3A) can be divided into three component peaks with binding energies at 399.5, 400.1, and 401.4 eV, which were attributed to C−N, C−N−C, and −NH2, respectively.34,35 The C 1s high resolution XPS spectrum of N-CDs (Figure 3C) exhibits four main peaks at 284.6, 285.1, 286.3 and 288.0 eV, which were assigned to C−C/C=C, C−N, C-O and C=O, respectively.35 After the reduction process, two peaks at 368.8 and 374.8 eV which were attributed to the binding energies of the Ag 3d5/2 and Ag 3d3/2 were observed (Figure 3E), clearly indicating the formation of metallic Ag on the surface of N-CDs.36,37 In addition, the intensity of C-O bond was decreased (Figure 3D), indicating that the C-O groups on the surface of N-CDs were oxidized during the formation of AgNPs/N-CDs nanocomposites. And the intensity of O-C=O bond was increased, which suggested that some oxygen-contained groups were oxidized and changed to O-C=O groups.38 The intensity of N 1s XPS signal (Figure 3B) at 399.5 disappeared and the intensity at 400.2 eV decreased after the reduction process, which is similar to the previous reported AgNPs/N-CDs nanocomposites38 FTIR measurements were also used to investigate the changes of surface groups before and after the reduction process. As shown in Figure 3F, for N-CDs (curve a), a 8


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broad O−H or N−H stretching vibration was obviously observed at 3,341 cm-1, and the strong peak at 1,631 cm-1 was attributed to the C=O vibrations. The C-O bond was observed at 1,384 cm-1.39 These results reveal that the N-CDs derived from Siraitia grosvenorii was rich in oxygen (hydroxyl, carbonyl, or carboxylic acid groups) and nitrogen-containing functional groups, which could serve as the electron donor for the reduction of Ag+ to element silver. 27,40 For the AgNPs/N-CDs nanocomposites (curve b), the intensity at 1,384 cm-1 was decreased, and the relative intensity at 1,630 cm-1 was increased, indicating the surface structure change of N-CDs after the reduction, which agrees with the XPS results.

As illustrated in Figure 4A, the N-CDs exhibited an absorption band around 280 nm, which is similar to that of a previously reported CDs because of the n−π* transition of the C=O domains (curve a).18 The adsorption band at 405 nm is attributed to the characteristic surface plasmon absorption of the AgNPs (curve b), whereas it could not be observed if no AgNPs were formed on the N-CDs. In addition, the photoluminescence (PL) spectra (Figure 4B) were investigated to evaluate the optical properties of the N-CDs (curve a) and AgNPs/N-CDs nanocomposites (curve b). First, different excitation wavelengths were tested to investigate the fluorescent properties of the prepared N-CDs. As shown in Figure S3, the as-synthesized N-CDs exhibited excitation-dependent fluorescence behavior, similar to N-CDs fabricated using other different approaches.18,30,31 The strongest emission was achieved around 470 nm when the prepared N-CDs were excited at 340 nm (Figure 4B and Figure S3). The corresponding solution exhibited a bright blue color under 365 nm UV lamp (inset of 9



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Figure 4B). The overall quantum yield is 0.05%, which is detected by the absolute method according to a previous report (Figure S4).41 The obtained AgNPs/N-CDs nanocomposites exhibited an apparently lower PL intensity. This might be attributed to the change of the surface states of the N-CDs which was induced by the in situ reduction of Ag+ to form AgNPs /N-CDs nanocomposites.27,42

The colloidal stability of AgNPs/N-CDs nanocomposites. As previously stated, the as-prepared N-CDs were rich in oxygen and nitrogen-containing functional groups on their surface, which could served as electron donors for the reduction of Ag+ to element silver, and they were necessary for anchoring AgNPs on the N-CDs surface.27 It has been reported that the peripheral charges of N-CDs surface could help the stabilization of immobilized AgNPs in aqueous solutions and prevent the aggregation.27 In this work, the zeta potential of AgNPs/N-CDs nanocomposites was analyzed to be -36.7 mV which revealed a negative charged surface (Figure S5). Furthermore, no significant changes in absorbance intensity were observed when deionized water-dispersed AgNPs/N-CDs nanocomposites were stored for 20 days under 4℃ and ambient conditions (Fig. S6). The results above imply that the as-prepared AgNPs/N-CDs nanocomposites have a good stability.

Establishment of the Logic System. Before performing the logic system, to obtain the ideal operation conditions, the concentrations of AgNO3 and NaOH were optimized. As shown in Figure 5A, the absorption intensity at 405 nm of AgNPs/N-CDs nanocomposites increased gradually as the AgNO3 concentration

10


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increased from 0.5 to 45.0 mM. Figure 5B reveals the relationship between the absorbance increase at the value of A405-A320 and the AgNO3 concentration. A linear correlation (R2 = 0.9850) was observed between A405-A320 and the AgNO3 concentration in the range of 0.5 to 25.0 mM. Similarly, with increasing NaOH concentration from 1.0 to 45.0 mM, the absorption intensity at 405 nm of AgNP/N-CDs nanocomposites also enhanced gradually (Figure 5C). And a linear correlation (R2 = 0.9930) was obtained between the value of A405-A320 and the NaOH concentration in the range of 1.0 to 30.0 mM (Figure 5D). These results indicated that the presence of AgNO3 and NaOH together was essential for the effective formation of AgNPs/N-CDs nanocomposites. An AND logic gate can then be constructed by using AgNO3 and NaOH as the inputs, and the absorption intensity at 405 nm as the outputs. Thus, to obtain the effective legible output signal, the concentrations of AgNO3 and NaOH were both used 35.0 mM. Finally, the effect of the reaction time was also investigated (Figure S7), and 120 min of preparation time was used in the following study.

The above results demonstrate the potential AND logic response using AgNO3 (35.0 mM) and NaOH (35.0 mM) as the inputs and the absorption intensity at 405 nm of the obtained AgNPs/N-CDs nanocomposites as the outputs. The true table and corresponding symbol were illustrated in Figure 6A and 6B, respectively. In the logic system, the presence of inputs (35.0 mM AgNO3 or NaOH) was defined as 1, and the absence of inputs was defined 0. When no inputs were added (0/0), no obvious absorption was observed at 405 nm (Figure 6C). The absorbance was low and the 11



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corresponding solution was colorless (Figure 6D). In addition, the sole addition of AgNO3 or NaOH (1/0 or 0/1) was also did not induce a color change or absorbance increase (Figure 6C and 6D). As expected, when the AgNO3 and NaOH were added together (1/1), the absorbance at 405 nm enhanced remarkably, and the solution color changed to be brown. Thus, the present preparation system can perform the AND logic operation.

SERS Measurement. The AgNPs/N-CDs nanocomposites were dropped on the glass slide and the dried to obtain particle aggregates. The aggregates may provide hot spots for SERS. SEM images of dried aggregates were illustrated in Figure S8. The Rhodamine B solutions were then dropped on the aggregates followed by the SERS measurements. Figure 7 shows the typical SERS spectra of Rhodamine B with different concentrations in the range of 1.0 µM to 50.0 µM. A good linear correlation (R2=0.98) is observed over the concentration range of 1.0 to 50.0 µM. The results imply that the synthesized AgNPs/N-CDs nanocomposites may be used as the substrates for the other potential SERS applications.

CONCLUSION A green synthetic method was developed to prepare novel reduced N-CDs derived from Siraitia grosvenorii, which is widely available at a low cost. Stable AgNPs/N-CDs nanocomposites were successfully synthesized by using Ag+ as the precursor and the obtained N-CDs as a reducing agent and stabilizer. Compared with the previously reported methods, the proposed method has several advantageous 12


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features. First, only AgNO3, NaOH, and Siraitia grosvenorii were necessary in the synthesis process of the AgNPs/N-CDs nanocomposites. No additional reducing agents or stabilizers were needed, making the synthesis simple and cost efficient. Second, an AND logic system based on the novel reduced CDs was established, which avoids complicated modifications and chemical labeling. Third, the SERS properties of the AgNPs/N-CDs nanocomposites indicated that the as-synthesized AgNPs/N-CDs nanocomposites may hold potential applications in SERS.

ASSOCIATED CONTENT

Supporting Information

Figures S1−S8, including the HRTEM image of N-CD and AgNPs/N-CD nanocomposites, and corresponding Fourier-transform diffraction patterns of N-CD and AgNPs/N-CD nanocomposites; EDS of AgNPs/N-CD nanocomposites; excitation wavelength dependent fluorescence spectra of N-CD; the absolute quantum yield of N-CDs; Zeta potential of AgNPs/N-CDs nanocomposites; UV-vis spectra of AgNPs/N-CDs nanocomposites in different days; the effect of reaction time; SEM images of each dried nanocomposite sample.

AUTHOR INFORMATION

Corresponding Author

Shulin Zhao, E-mail: [email protected].

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Page 14 of 32

Jingjin Zhao, E-mail: [email protected].

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundations of China (No. 21305020),

the

Natural

Science

Foundations

of

Guangxi

Province

(No.

2014GXNSFBA118041) and Guangxi Normal University (2013ZD002), as well as BAGUI Scholar Program.

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Figure Captions Scheme 1. Schematic illustrations for the synthesis process of N-CDs (A) and AgNPs/N-CDs nanocomposites (B). Figure 1. (A) TEM image of N-CDs. Insets are HRTEM images and particle size distribution histogram. (B) TEM image of AgNPs/N-CDs. Insets are HRTEM images and particle size distribution histogram. (C) AFM image of N-CDs. (D) The height profile of N-CDs. (E) AFM image of AgNPs/N-CDs nanocomposites. (F) The height profile of AgNPs/N-CDs nanocomposites. Figure 2. The XPS spectra of N-CDs (A) and AgNPs/N-CDs nanocomposites (B). Figure 3. N 1s XPS spectra of N-CDs (A) and AgNPs/N-CDs nanocomposites (B); C 1s XPS spectra of N-CDs (C) and AgNPs/N-CDs nanocomposites (D); Ag 3d XPS spectra of N-CDs (E-a) and AgNPs/N-CDs nanocomposites (E-b); FIRT spectra of N-CDs (F-a) and AgNPs/N-CDs nanocomposites (F-b). Figure 4. (A) UV-vis spectra of 7.82 µg/mL AgNPs/N-CDs nanocomposites (curve a) and N-CDs (curve b); (B) Photoluminescence spectra of N-CDs (curve a) and AgNPs/N-CDs nanocomposites (curve b). Insets depict the photographs of the N-CD (a) and AgNPs/N-CDs nanocomposites (b) with a UV lamp (365 nm). Figure 5. (A) UV-vis spectra of the N-CDs (0.121mg/mL) reduction systems containing different concentrations of AgNO3 from 0.5 mM to 45.0 mM. 22


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The concentration of NaOH was 35.0 mM. (B) Plot of the A405-A320 value as a function of the AgNO3 concentration. Inset is the linear calibration graph between A405-A320 value and the concentrations of AgNO3. (C) UV-vis spectra of the N-CD (0.121mg/mL) reduction systems containing different concentrations of NaOH from 1.0 mM to 45.0 mM. The concentration of AgNO3 was 35.0 mM. (D) Plot of the A405-A320 value as a function of the NaOH concentration. Inset is the linear calibration graph between A405-A320 value and the concentrations of NaOH. Figure 6. (A) Truth table of the AND logic. (B) Symbol of the AND logic. (C) UV-vis spectra in the presence or absence of each input. (D) The absorbance at 405 nm under different inputs. Inset is the optical photographs of the corresponding samples. Figure 7. SERS spectra of Rhodamine B with different concentration on AgNPs/N-CDs nanocomposites substrate. Inset is the relationship between the SERS intensity at ~ 1645 cm-1 and the concentrations of Rhodamine B from 1.0 to 50 µM. The error bars represent the standard deviation of three measurements.

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Scheme 1.

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Figure 1.

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Figure 2.

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Figure 3.

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Figure 4.

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Figure 5.

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Figure 6.

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Figure 7.

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For Table of Contents Use Only

Silver Nanoparticles/N-doped Carbon-Dots Nanocomposites Derived from Siraitia Grosvenorii and Its Logic Gate and Surface-Enhanced Raman Scattering Characteristics Yubin Su,† Bingfang Shi,†,‡ Suqi Liao,† Jingjing Zhao,*,† Lini Chen† and Shulin Zhao*,†

†

Key Laboratory for the Chemistry and Molecular Engineering of Medicinal Resources (Ministry of Education), Guangxi Normal University, 15 Yucai Road, Guilin 541004, China. E-mail: [email protected];[email protected].

‡

Department of Chemistry and Life Sciences, Baise University, 21 Zhongshan Road, Baise 533000, China

A novel AgNPs/N-CDs nanocomposite, which were derived from Siraitia grosvenorii, was prepared and applied as the substrates for SERS.

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N-Doped Carbon-Dots ... - ACS Publications

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