CNT-Intercalated GO Membranes for

Subscriber access provided by CORNELL UNIVERSITY LIBRARY

Article

Highly Reproducible Ag NPs/CNT-Intercalated GO Membranes for Enrichment and SERS Detection of Antibiotics Lu-Lu Qu, Ying-Ya Liu, Mingkai Liu, Guo-Hai Yang, Da-Wei Li, and Haitao Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b08790 • Publication Date (Web): 27 Sep 2016 Downloaded from http://pubs.acs.org on October 2, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Highly Reproducible Ag NPs/CNT-Intercalated GO Membranes for Enrichment and SERS Detection of Antibiotics Lu-Lu Qu,a Ying-Ya Liu,b Ming-Kai Liu,a Guo-Hai Yang,a Da-Wei Li,b* Hai-Tao Lia* a

School of Chemistry and Chemical Engineering, Jiangsu Normal University, Xuzhou, Jiangsu

221116, P. R. China b

Shanghai Key Laboratory of Functional Materials Chemistry & College of Chemistry and

Molecular Engineering, East China University of Science and Technology, Shanghai, 200237, P. R. China

ABSTRACT The increasing pollution of aquatic environments by antibiotics makes it necessary to develop efficient enrichment and sensitive detection methods for environmental antibiotics monitoring. In this work, silver nanoparticles and carbon nanotube-intercalated graphene oxide laminar membranes (Ag NPs/CNT-GO membranes) were successfully prepared for enrichment and surface-enhanced Raman scattering (SERS) detection of antibiotics. The prepared Ag NPs/CNTGO membranes exhibited a high enrichment ability because of the π-π stacking and electrostatic interactions of GO toward antibiotic molecules, which enhanced the sensitivity of SERS

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 25

measurements and enabled the antibiotics to be determined at sub-nM concentrations. In addition, the nanochannels created by the intercalation of CNTs into GO layers resulted in an 8-fold enhancement in the water permeance of Ag NPs/CNT-GO membranes compared to that of pure GO membranes. More importantly, the Ag NPs/CNT-GO membranes exhibited high reproducibility and long-term stability. The spot-to-spot variation in SERS intensity was less than 15%, and the SERS performance was maintained for at least 70 days. The Ag NPs/CNT-GO membranes were also used for SERS detection of antibiotics in real samples; the results showed that the characteristic peaks of antibiotics were obviously recognizable. Thus, the sensitive SERS detection of antibiotics based on Ag NPs/CNT-GO offers great potential for practical applications in environmental analysis.

KEYWORDS: GO membranes, SERS, enrichment, excellent uniformity, high sensitivity, antibiotic detection

INTRODUCTION Antibiotics, as an important group of pharmaceuticals, are frequently used in the treatment of humans and in livestock farming to promote growth.1 However, extensive evidence has demonstrated that antibiotic residues in aquatic environments such as wastewaters, groundwater, surface water, and even drinking water, pose a risk of undesirable health effects.2, 3 Therefore, there is increasing demand for the development of efficient analytical methods for rapid and sensitive identification and quantification of antibiotics in water. The routine detection strategies for antibiotics are chromatography-based methods4,

5

and

enzyme-linked immunosorbent assays (ELISA).6, 7 However, chromatographic methods require sophisticated and time-consuming pretreatment procedures. The ELISA method is not readily


2

Page 3 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


adapted to the detection of different antibiotics in real samples and generally requires additional antibodies.8 To overcome these limits, researchers have developed nanomaterial-based sensors using quantum dots (QDs),9 magnetic nanoparticles,10 carbon nanotubes (CNTs),11 and molecularly imprinted materials12 to detect antibiotics in environmental samples. Compared to routine analytical techniques, these emerging analytical methods are sensitive, rapid, and costeffective. However, the analysis of antibiotics in the environment remains a difficult task because of both the high complexity of the analyzed matrices and the commonly low concentrations of target analytes in environmental samples. Surface-enhanced Raman scattering (SERS) has been demonstrated to be a promising method for environmental analysis due to its excellent molecular specificity and high sensitivity.13-15 To improve SERS detection sensitivity, extensive studies have recently focused on optimizing the structures of SERS substrates;16, 17 however, an important barrier limits the practical applications of SERS. Specifically, SERS detection depends critically on high statistical binding of target analytes to active sites or “hot spots” of noble metal nanostructures, which makes the detection of target analytes in highly diluted solutions challenging.18-20 A number of approaches have been developed for improving the sensitivity of SERS detection. For example, a pinning-free platform has been constructed, which allows for the delivery and preconcentration of analytes into SERSactive regions, and brings the highly sensitive R6G detection to fM levels.21 A disposable Aggraphene sensor has been explored to response polar molecules in water samples rapidly and sensitively using electrostatic pre-enrichment and SERS.22 Additionally, solid-phase extraction has been combined with SERS to achieve rapid collection of analytes and in situ detection.23, 24 The ability to detect analytes at low concentration levels using SERS can bring out many potential real-world applications.


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 25

Membrane enrichment technology plays significant roles in water treatment, and in the modern pharmaceutical and chemical industries.25, 26 Of particular note is graphene, which has been proved to be a highly effective platform for exploring molecular-enrichment membranes attributed to its atomic thickness, chemical inertness, and high mechanical strength.27,

28

Graphene consists of a single layer of carbon atoms arranged in an sp2-bonded aromatic structure. The delocalized electron clouds of π-orbitals occupy the voids of aromatic rings in a graphene sheet, thereby effectively enriching the molecules via π-π stacking interactions.29 On the basis of this effect, graphene has been extensively used as an enrichment material for various molecules. However, the enrichment capacity is generally estimated by collecting the UV-vis or florescence spectra of the residual solution, which is tedious and can result in a large relative error. Thus, the combination of membrane enrichment technology with other analytical methods for simultaneous enrichment and determination is important. Herein, silver nanoparticles (Ag NPs) and carbon nanotube (CNT)-intercalated graphene oxide (GO) laminar membranes (Ag NPs/CNT-GO membranes) were successfully prepared and subsequently used for the enrichment and direct SERS detection of antibiotics. The prepared Ag NPs/CNT-GO membranes exhibit a strong ability to enrich the antibiotic molecules on graphene oxide via π-π stacking interactions and electrostatic interactions, resulting in strong SERS activity from “hot spots” formed between intercalated Ag NPs. In addition, nanochannels created by the intercalation of CNTs into GO layers greatly improve water permeation. More importantly, the Ag NPs/CNT-GO membranes exhibit high reproducibility and a long shelf-life. Thus, the Ag NPs/CNT-GO membranes fabricated using this simple and low-cost method have potential applications in monitoring environmental pollutants.

EXPERIMENTAL SECTION


4

Page 5 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Materials. All reagents were of analytical-reagent grade and were used without further purification. Oxytetracycline hydrochloride (OHC), ampicillin trihydrate (AT), tetracycline hydrochloride

(TH),

rhodamine

6G

(R6G),

silver

nitrate,

sodium

citrate,

and

cetyltrimethylammonium bromide (CTAB) were purchased from Aladdin Chemical Company (Shanghai, China). Poly(diallyldimethylammonium chloride) (PDDA) (40,000 ≤ MW ≤ 50,000, 20 wt% in H2O) was obtained from Sigma-Aldrich (St. Louis, MO). Carbon nanotubes (CNTs) were supplied by Shenzhen Nanotech Port Co. Ltd. (Shenzhen, China). Deionized water (18 MΩ·cm-1) used throughout the experiments was produced by a water puriﬁcation system (Billerica, MA, USA). All glassware were immersed into freshly prepared 3:1 HCl/HNO3 (aqua regia) overnight and rinsed thoroughly in deionized water prior to use. Apparatus. The scanning electron microscopic (SEM) images of the prepared SERS membrane were acquired by a field-emission scanning electron microscopy (FESEM, Ultra 55, Carl Zeiss Ltd., Germany). The graphene oxide, functionalized Ag NPs and CNT were dropped onto carbon-coated copper grids and dried under ambient conditions. Then, the transmission electron microscopy (TEM) was performed using a HITACHI H-800 electron microscope with an accelerating voltage of 200 kV. Raman spectra were detected by a small portable Raman spectrometer (BWS465, B&W Tek Inc., USA) with an excitation wavelength of 532 nm, a laser beam diameter of 10 µm, and a resolution of 3 cm-1. Preparation of CTAB-Functionalized Ag NPs. Positive Ag NPs were fabricated by a seeded-growth procedure.30 Under vigorous stirring, silver seeds were synthesized by dropwise addition of freshly prepared NaBH4 solution (5.3 mM, 5 mL) to mixed solution of AgNO3 (0.1 mM, 400 mL) and sodium citrate (0.1 mM). Then, the reaction mixture was stirred vigorously for another 1 h and aged for complete degradation of NaBH4 under ambient conditions for 7


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 25

days. The Ag NPs growth was achieved by adding L-ascorbic acid (0.1 M, 6 mL) into AgNO3 solution (1.8 mM, 170 mL) in the presence of CTAB (19.9 mM), followed by the addition of 30 mL the synthesized silver seeds. The pH of the system was adjusted with NaOH (1.0 mL, pH 13.0) and the mixture was stirred for 30 min. The resulting silver colloids were centrifuged at 6000 rpm for 15 min to remove excess CTAB and finally resuspended in 40 mL of deionized water. Synthesis of PDDA-Functionalized CNTs. PDDA-functionalized CNTs were prepared as follows.31 Briefly, 100 mg of CNTs were initially suspended in 400 mL of deionized water by ultrasonication in the presence of PDDA (5 wt%), yielding a stable dispersion of CNTs. After filtering, the precipitate was washed several times with deionized water and then dried in a vacuum oven at 70 °C for 24 h. Synthesis of Graphene Oxide (GO) Dispersion. The GO sheets were synthesized using natural graphite powder via a modified Hummer’s method.32,

33

Graphite powder (3 g) was

gradually added to a mixture of P2O5 (2.5 g), K2S2O8 (2.5 g) and concentrated H2SO4 (12 mL). The mixture was maintained at 80 °C for 4.5 h under stirring and cooled down to room temperature. Subsequently, the reaction product was collected by vacuum filtration, washed with deionized water, and then dried under ambient conditions for 12 h. Pre-oxidized graphite was added to cold (0 °C), concentrated H2SO4 (120 mL) under continuous stirring, followed by a slow and careful addition of KMnO4 (15 g). After the mixture was reacted at 35 °C for 2 h, it was diluted with deionized water (250 mL), and continued to react for another 2 h. Then, deionized water (700 mL) and H2O2 (20 mL, 30%) was added successively. The mixture color changed immediately to a brilliant yellow, accompanied by bubbling. The solution was washed with HCl solution (10%) for several times to remove metal ions, followed by washing with deionized


6

Page 7 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


water to completely remove residual acid. A stable, brownish, and transparent GO suspension was obtained by sonicating GO in deionized water for 2 h. Fabrication of Ag NPs/CNT-intercalated GO (Ag NPs/CNT-GO) Membrane. CNTs (2.5 mg) were added to GO aqueous solution (5 mL, 2 mg·mL-1) under stirring. The resulting solution was then mixed with Ag NPs colloidal solution (4 mL). The mixture was stirred for 10 min and sonicated for 30 min. It was subsequently filtered on a polycarbonate (PC, Whatman) membrane with a pore size of 0.22 µm, an effective filtration area of 12.56 cm2, and a porosity of 10%. After the product was dried in a vacuum oven at 40 °C for 5 min, the Ag NPs/CNT-intercalated GO membrane could be easily peeled from the PC membrane. Batch Sorption Experiments and SERS Detection. Batch experiments were conducted to evaluate the adsorption of antibiotics onto the Ag NPs/CNT-GO membranes. A 1 µM stock solution was prepared by dissolving antibiotics in deionized water (50 mL). Working solutions of the required concentrations were prepared by further diluting the stock solution with deionized water. Next, 2 mL of the antibiotic solution was filtered through Ag NPs/CNT-GO membranes on a PC membrane at room temperature. Once the solution passed through the Ag NPs/CNT-GO membranes, the concentration of the antibiotics in the retentate was detected using a portable Raman spectrometer.

RESULTS AND DISCUSSION Fabrication and Characterization of Ag NPs/CNT-GO Membranes. To construct the Ag NPs/CNT-intercalated GO membrane, we prepared a homogeneous dispersion containing negatively charged GO sheets and positively charged CNTs and Ag NPs, as illustrated in Figure 1A. The Ag NPs/CNT-GO membranes were then fabricated by vacuum filtration of the mixed aqueous dispersion on a porous support. The positively charged Ag NPs (an average size of 30 ±


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 25

5 nm in diameter) and CNTs with lengths up to tens of micrometers and diameters of about 11 nm assembled tightly with the negatively charged GO sheets via electrostatic interactions (Figure S1). Specifically, GO sheets interlocked with each other in a horizontal manner to form lamellar structures, whereas Ag NPs and CNTs intercalated into the stacked GO layers to enhance Raman signals and establish nanochannels for fast water delivery, respectively (Figure 1B). When the solution of antibiotics was filtered through Ag NPs/CNT-GO membranes, the antibiotic molecules were enriched on the GO membranes via electrostatic and π-π stacking interactions. This process may cause an increase in the concentration of antibiotic molecules at the zone of the Ag NP electromagnetic field or interparticle junctions (SERS “hot spots”), thus resulting in large intensification of the Raman signals of the target molecules.


8

Page 9 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1 (A) A schematic of the fabrication process for Ag NPs/CNT-GO membranes. (B) The process of enrichment and SERS detection of antibiotics by Ag NPs/CNT-GO membranes. Figures 2A and 2B show optical images of the Ag NPs/CNT-GO membrane. The membrane was uniform and exhibited excellent flexibility (Figure 2C). In addition, no damage was observed even when the curled membrane was immersed into water. This result illustrates that the Ag NPs/CNT-GO membrane is highly flexible and robust, and may provide a promising source for in situ detection. Figure 2D shows the top-view SEM image of the Ag NPs/CNT-GO membrane. Corrugation and nanochannels constructed by CNTs are evident. Additionally, Ag NPs are uniformly decorated over a large area of the GO surface, indicating a uniform distribution of Ag NPs in the stacked GO layers. Figure 2E displays a cross-sectional SEM image of the Ag NPs/CNT-GO membrane. It shows that the membrane thickness was approximately 6.4 µm and the Ag NPs and CNTs intercalated into the stacked GO layers. Figure S2 displays the Raman spectrum of the Ag NPs/CNT-GO membrane. The characteristic bands at 1326 cm-1 (D band) and 1597 cm-1 (G band) result from the sp3 carbon atoms of defects and the disorder and vibrations of sp2 carbon atoms in the graphitic 2D hexagonal lattice, respectively.34 To confirm that the prepared Ag NPs/CNT-GO membrane was SERS-active, we immersed it into a 1.0 × 10-7 M solution of R6G (a conventional probe molecule) for 10 min and recorded the corresponding SERS spectrum, as presented in Figure S2b. The SERS signal of R6G was enhanced dramatically, demonstrating that the Ag NPs/CNTGO membrane can be used as a SERS substrate.


9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 25

Figure 2 Photographs of the Ag NPs/CNT-GO membrane (A) top-view and (B) side-view. (C) Photograph showing the flexibility of the Ag NPs/CNT-GO membrane, which can be folded or bent. The prepared Ag NPs/CNT-GO membrane is 4.0 cm in diameter. (D) SEM image showing the surface of the Ag NPs/CNT-GO membrane. The inset shows a higher-magnification SEM image of Ag NPs. (E) Cross-sectional view of the Ag NPs/CNT-GO membrane (thickness: 6.4 µm). The inset shows a higher-magnification image of the Ag NPs/CNT-GO membrane. Optimized Fabrication of Ag NPs/CNT-GO Membranes. The SERS signal is well known to mainly result from interparticle plasmon coupling, which largely depends on the density of Ag NPs, in turn, is related to the prepared concentration.34 Thus, to obtain the best signal enhancement, we optimized the fabrication conditions by varying the Ag NP concentration. The variation of SERS signals with the amount of Ag NPs is displayed in Figure S3. A small SERS signal of R6G was initially observed from a membrane with only 0.4 mM Ag NPs. As the amount of Ag NPs was gradually increased from 0.4 mM to 4.0 mM, many more Ag NPs were illuminated in the laser spot, resulting in markedly enhanced Raman signals. With a further


10

Page 11 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


increase in the gross amount of Ag NPs, the planar density of Ag aggregates per unit area became saturated. As a result, the intensity of the Raman peak increased only slightly and eventually tended toward a constant value. Therefore, the large enhancement of Raman signals is attributed primarily to coupling between the Ag NPs. In addition, the number of nanochannels in the Ag NPs/CNT-GO membranes is expected to increase with increasing mass faction of CNTs, thus improving the water permeation flux of the membrane.26, 28 However, these nanochannels decrease the robustness of the Ag NPs/CNT-GO membrane because the lamellar structures of the membrane are destroyed by the CNTs. To attain maximum permeation flux with an acceptable robustness, we fabricated Ag NPs/CNT-GO membranes with various CNT-to-GO mass ratios and measured the water permeation flux; the results are shown in Figure S4. The water permeation flux increased to 0.077 mL·min-1 when the mass ratio of CNT-to-GO was increased to 1:4, which represents an 8-fold enhancement compared to that of pure GO membranes. When the mass ratio was further increased to 1:2, the permeation flux increased to 0.133 mL·min-1; however, the resulting membranes were more easily destroyed. Thus, the Ag NPs/CNT-GO membranes fabricated with a CNT-to-GO mass ratio of 1:4 were used in subsequent experiments. SERS Activity of Ag NPs/CNT-GO Membranes. To confirm the sensitivity of the Ag NPs/CNT-GO membranes, we recorded SERS signals for different concentrations of R6G by filtering 2 mL of R6G solution through Ag NPs/CNT-GO membranes. Figure 3 clearly shows that the Raman intensity decreased with decreasing concentration of R6G (from curve g to a). Numerous characteristic bands are distinctly visible even for an R6G concentration as low as 1.0 × 10-13 M, revealing the high sensitivity of the Ag NPs/CNT-GO membranes. The inset of Figure 3 presents a plot of the logarithmic concentration versus the SERS intensity for the bands at 614


11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 25

and 1364 cm-1. A linear dependence is observed between the SERS response and the logarithmic R6G concentration (see the inset) in the range from 1.0 × 10-13 M to 1.0 × 10-9 M. The limit of detection (LOD) was approximately 8.6 × 10-14 M at a signal-to-noise ratio of 3σ, which is superior to those obtained from silver dendrite and Ag NPs/CNTs.35,

36

In addition, the

enhancement factor (EF) of the Ag NPs/CNT-GO membranes was estimated to be 7.2×106 (Figure S5, Text S1), implying that the Ag NPs/CNT-GO membranes could be effective for highly sensitive SERS detection.

Figure 3 SERS spectra for different concentrations of R6G adsorbed onto the Ag NPs/CNT-GO membrane: (a) 1.0 × 10-13 M, (b) 1.0 × 10-12 M, (c) 1.0 × 10-11 M, (d) 1.0 × 10-10 M, (e) 1.0 × 10-9 M, (f) 1.0 × 10-8 M, and (g) 1.0 × 10-7 M. Inset: Plot demonstrating the changes in the Raman intensities of the 614 cm-1 and 1364 cm-1 bands upon the addition of different concentrations of R6G. Uniformity and Stability of Ag NPs/CNT-GO Membranes. An excellent SERS substrate is expected to exhibit not only superior enhancement performance but also high uniformity. Thus, to evaluate the uniformity of the Ag NPs/CNT-GO membrane, we collected 20 SERS signals of 1.0 × 10-7 M R6G from a randomly selected area, and showed the corresponding results in Figure


12

Page 13 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4A. Figure 4B displays the corresponding contour plots of the Raman bands at 614 cm-1 and 1364 cm-1. These results illustrate that the intensities of the 20 points were highly reproducible. According to the statistics of predominant band intensities, the relative standard deviations (RSDs) for the 614 cm-1 and 1364 cm-1 band vibrations of R6G were 11.2% and 12.8%, respectively. The good reproducibility mainly attributes to the uniform Ag NP distribution in covered area of laser spot, which causes the resulting SERS signal is an average response from covered nanoparticles.37 Figures 4C and 4D show the Raman waterfall plots and corresponding contour plots of 20 different Ag NPs/CNT-GO membranes. The RSDs for the 614 cm-1 and 1364 cm-1 band vibrations of R6G were 12.6% and 13.2%, respectively. These RSDs clearly demonstrate that the enhancement achieved with the Ag NPs/CNT-GO membranes is highly reproducible. These results further demonstrate that our Ag NPs/CNT-GO membranes are uniform over a large area and from batch to batch, and are capable of generating SERS signals with high reproducibility. The temporal stability of the Ag NPs/CNT-GO membranes is also an essential parameter for SERS detection. To test the temporal stability of the Ag NPs/CNT-GO membranes, we recorded SERS spectra of 1.0 × 10-7 M R6G from Ag NPs/CNT-GO membranes at different storage times (up to 70 days), as shown in Figure 5. We observed no obvious change in the overall shape and only a slight change in the intensity of the SERS bands after storage for 60 days. In addition, the Ag NPs/CNT-GO membrane exhibited good stability for detection of solution sample with high ionic strength (Figure S6). The long and outstanding SERS lifetimes of the Ag NPs/CNT-GO membranes may be a consequence of most of the Ag NPs having intercalated into the stacked GO layers, which provide protection from the surrounding environment. Our results demonstrate


13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 25

that the Ag NPs/CNT-GO membranes possess good temporal stability and can meet the requirements for routine SERS detection.

Figure 4 Reproducibility of the SERS signals on Ag NPs/CNT-GO membranes. (A) Successively measured SERS spectra of 1.0 × 10-7 M R6G on 20 different positions of an Ag NPs/CNT-GO membrane. (B) The SERS intensity distribution of the 614 cm-1 and 1364 cm-1 bands. The average intensity of 20 spectra are indicated by the red line; intensity variations of ±10% and ±10-15% are indicated by the yellow and orange zones, respectively. (C) SERS spectra of 1.0 × 10-7 M R6G recorded from 20 different Ag NPs/CNT-GO membranes. (D) SERS intensity distributions of the 614 cm-1 and 1364 cm-1 bands. The average intensity of 20 spectra are indicated by the red line; intensity variations of ±10% and ±10-15% are indicated by the yellow and orange zones, respectively.


14

Page 15 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5 (A) Stability of Ag NPs/CNT-GO membranes. SERS spectra of 1.0 × 10-7 M R6G, recorded from Ag NPs/CNT-GO membranes stored over a number of days under ambient condition. (B) Intensity versus storage time based on the measurement at 614 cm-1 up to 70 days. SERS Detection of Antibiotics Using the Ag NPs/CNT-GO Membranes. The previously presented results demonstrate that the Ag NPs/CNT-GO membranes exhibit excellent SERS activity, high reproducibility, and long-term stability. We next used the Ag NPs/CNT-GO membranes to analyze antibiotic molecules. The enrichment process was carried out on a vacuum filtration apparatus with an Ag NPs/CNT-GO membrane. Two milliliters of antibiotic solutions of various concentration were poured onto the membranes at a rate of approximately 0.077 mL·min-1. After a certain amount of time, SERS detection was performed using a portable


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 25

Raman spectrometer. Distinct SERS spectra of TH, AT, and OHC were acquired, as shown in Figure S7, and the typical bands were assigned as shown in Table S1. Even at the nanomolar level, the characteristic Raman peaks at 1275 and 1601 cm-1 for TH, 1115 and 1600 cm-1 for AT, and 1278 and 1533 cm-1 for OHC were easily identified (Figure 6). To evaluate the SERS-active membranes for quantitative analysis, we plotted the Raman intensity for each antibiotic against its logarithmic concentration, as shown in Figure 6A2-C2. The Raman intensity increased with increasing antibiotic concentration and became saturated at 6.0 × 10-7 M for TH, 4.0 × 10-6 M for AT, and 1.0 × 10-6 M for OHC. A linear dependence between the SERS intensities and logarithmic concentrations was observed within the concentration ranges from 2.0 × 10-9 to 6.0 × 10-7 M, from 4.0 × 10-9 to 1.0 × 10-6 M, and from 1.0 × 10-9 to 1.0 × 10-6 M for TH, AT, and OHC, respectively. The LODs were estimated to be 1.5 × 10-9 M (1275 cm-1), 3.2 × 10-9 M (1600 cm-1), and 7.6 × 10-10 M (1533 cm-1) for TH, AT, and OHC at a signal-to-noise ratio of 3σ, respectively (Table S2). The enrichment of the antibiotic by the Ag NPs/CNT-GO membrane enhanced the sensitivity of the SERS measurements by 1-2 orders of magnitude (Figure S8). Table S3 summarizes recent developments in the detection of antibiotics by SERS and a wide array of other methods. The LOD of our method is 1-2 orders of magnitude lower than those of the commonly reported methods. This result further confirms that antibiotics can be effectively adsorbed onto the high enhancement region of Ag NPs/CNT-GO membranes by π-π stacking and electrostatic interactions, even when their concentration in solution is below the nM level.


16

Page 17 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6 Raman spectra of TH (A1), AT (B1) and OHC (C1) at different concentration absorbed onto the Ag NPs/CNT-GO membranes by a flow-through method. (A2) SERS intensities at 1275 cm-1 and 1601 cm-1 as a function of logarithmic TH concentration. (B2) SERS intensities at 1115 cm-1 and 1600 cm-1 as a function of logarithmic AT concentration. (C2) SERS intensities at 1278 cm-1 and 1533 cm-1 as a function of logarithmic OHC concentration. Environmental Sample Detection. On the basis of the previously discussed promising results, we used Ag NPs/CNT-GO membranes for SERS-sensing of antibiotics in a real-life sensing scenario. Wastewater was collected from the Xuzhou Industry District and was used to represent a typical set of background conditions and contaminants. The results demonstrate that the concentrations of antibiotics were below the detection limits and SERS detection would not be interfered by environmental and man-made species (Figure S9). Recovery experiments were performed using wastewater spiked with TH and AT. The final concentration of spiked TH and AT was 5.0 × 10-8 M. The characteristic bands of the individual antibiotics were distinguished obviously from SERS spectrum of the complex mixture. As displayed in Table S1, the typical


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 25

bands at 1275 and 1346 cm-1 indicate the presence of TH. The bands at 1004, 1115, and 1504 cm1

indicate the presence of AT. Matching the detection results with the calibration curves in Figure

6, we determined the corresponding concentrations of TH and AT to be 3.8 × 10-8 M and 4.2 × 10-8 M, respectively. The recoveries of spiked TH and AT were 76.0 ± 5.7% and 84.0 ± 6.8%, respectively, for industrial wastewater. The sample was also analyzed using HPLC-MS, demonstrating that AT and TH were detected at concentrations of 4.0 × 10-8 M and 4.2 × 10-8 M, respectively. A comparison of the results for the two methods demonstrates that the detection performance of the Ag NPs/CNT-GO membranes for antibiotics is similar to that of the more sophisticated HPLC-MS technique, and the deviation in the detected concentrations was less than 15% between the two techniques. These results further indicate that the Ag NPs/CNT-GO membrane is reliable in determining trace amounts of antibiotics in environmental water samples.

CONCLUSION Herein, Ag NPs/CNT-GO membranes were prepared as SERS substrates for the enrichment and detection of antibiotics in water. GO and Ag NPs, on which reproducible and ultrasensitive SERS signals of antibiotics were obtained, were used to enrich and enhance the Raman signals of the antibiotic molecules, respectively. CNTs were used to create nanochannels for improving the filtration ability. The results demonstrated that the minimum detectable concentration of the antibiotic was at the sub-nM level. The capability of Ag NPs/CNT-GO membranes for SERS detection of antibiotics in real wastewater was also investigated, and the characteristic bands of the antibiotics were still recognizable. We expect that the reported ultrasensitive SERS detection of antibiotics can improve practical applications of Ag NPs/CNT-GO in environmental analysis.


18

Page 19 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. TEM images of Ag NPs, GO and CNTs; SERS performance of Ag NPs/CNT-GO membrane; SERS spectra denpending on Ag NP solution concentrations; permeation flux depending on CNT-to-GO mass ratios; estimation of Raman enhancement factor; chemical structures, SERS spectra and Raman bands assignments of TH, AT, and OHC; SERS detection of antibiotics in wastewater. AUTHOR INFORMATION Corresponding Author E-mail: [email protected] (Da-Wei Li); [email protected] (Hai-Tao Li) Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundations of China (21505057, 21575041, 21375051), the Natural Science Foundation of Jiangsu Province (BK20150227), the Natural Science Foundation of Jiangsu Normal University (14XLR011), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the Brand Major of Universities in Jiangsu Province, the Top-notch Academic Programs Project of Jiangsu Higher Education Institution (TAPP), and the Shanghai Municipal Natural Science Foundation (14ZR1410800).


19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 25

REFERENCES 1. Rosi-Marshall, E. J., Kelly, J. J. Antibiotic Stewardship Should Consider Environmental Fate of Antibiotics. Environ. Sci. Technol., 2015, 49(9), 5257-5258. 2. Kümmerer, K. Antibiotics in the Aquatic Environment-A Review-Part I. Chemosphere, 2009, 75(4), 417-434. 3. Su, S., Wu, W. H., Gao, J. M., Lu, J. X., Fan, C. H. Nanomaterials-Based Sensors for Applications in Environmental Monitoring. J. Mater. Chem., 2012, 22(35), 18101-18110. 4. Liang, N., Huang, P., Hou, X., Li, Z., Tao, L., Zhao, L. Solid-Phase Extraction in Combination with

Dispersive

Liquid-Liquid

Microextraction

and

Ultra-High

Performance

Liquid

Chromatography-Tandem Mass Spectrometry Analysis: The Ultra-Trace Determination of 10 Antibiotics in Water Samples. Anal. Bioanal. Chem., 2016, 408(6), 1701-1713. 5. Ye, Z., Weinberg, H. S., Meyer, M. T. Trace Analysis of Trimethoprim and Sulfonamide, Macrolide, Quinolone, and Tetracycline Antibiotics in Chlorinated Drinking Water Using Liquid Chromatography Electrospray Tandem Mass Spectrometry. Anal. Chem., 2007, 79(3), 11351144. 6. Kumar, K., Thompson, A., Singh, A. K., Chander, Y., Gupta, S. C. Enzyme-Linked Immunosorbent Assay for Ultratrace Determination of Antibiotics in Aqueous Samples. J. Environ. Qual., 2004, 33(1), 250-256. 7. Huet, A. C., Charlier, C., Tittlemier, S. A., Singh, G., Benrejeb, S., Delahaut, P. Simultaneous Determination of (Fluoro)quinolone Antibiotics in Kidney, Marine Products, Eggs, and Muscle by Enzyme-Linked Immunosorbent Assay (ELISA). J. Agric. Food Chem., 2006, 54(8), 28222827.


20

Page 21 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


8. Cháfer-Pericás, C., Maquieira, Á., Puchades, R. Fast Screening Methods to Detect Antibiotic Residues in Food Samples. TrAC, Trends Anal. Chem., 2010, 29(9), 1038-1049. 9. He, Y., Wang, H. F., Yan, X. P. Exploring Mn-Doped ZnS Quantum Dots for the RoomTemperature Phosphorescence Detection of Enoxacin in Biological Fluids. Anal. Chem., 2008, 80(10), 3832-3837. 10. Font, H., Adrian, J., Galve, R., Estévez, M. C., Castellari, M., Gratacós-Cubarsí, M., Sánchez-Baeza, F., Marco, M. P. Immunochemical Assays for Direct Sulfonamide Antibiotic Detection in Milk and Hair Samples Using Antibody Derivatized Magnetic Nanoparticles. J. Agric. Food Chem., 2008, 56(3), 736-743. 11. Ji, L., Shao, Y., Xu, Z., Zheng, S., Zhu, D. Adsorption of Monoaromatic Compounds and Pharmaceutical Antibiotics on Carbon Nanotubes Activated by KOH Etching. Environ. Sci. Technol., 2010, 44(16), 6429-6436. 12. Benito-Peña, E., Martins, S., Orellana, G., Moreno-Bondi, M. C. Water-Compatible Molecularly Imprinted Polymer for the Selective Recognition of Fluoroquinolone Antibiotics in Biological Samples. Anal. Bioanal.Chem. 2009, 393(1), 235-245. 13. Yang, L. B., Li, P., Liu, H. L, Tang, X. H., Liu, J. H. A Dynamic Surface Enhanced Raman Spectroscopy Method for Ultra-Sensitive Detection: from the Wet State to the Dry State. Chem. Soc. Rev., 2015, 44(10), 2837-2848. 14. Nie, S., Emory, S. R. Probing Single Molecules and Single Nanoparticles by SurfaceEnhanced Raman Scattering. Science, 1997, 275(5303), 1102-1106. 15. Li, J. F., Huang, Y. F., Ding, Y., Yang, Z. L., Li, S. B., Zhou, X. S., Fan, F. R., Zhang W., Zhou, Z. Y., Wu, D. Y., Ren, B., Wang, Z. L., Tian Z. Q. Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy. Nature, 2010, 464(7287), 392-395.


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 25

16. Fan, W., Lee, Y. H., Pedireddy, S., Zhang, Q., Liu, T. X., Ling, X. Y. Graphene Oxide and Shape-Controlled Silver Nanoparticle Hybrids for Ultrasensitive Single-Particle SurfaceEnhanced Raman Scattering (SERS) Sensing. Nanoscale, 2014, 6(9), 4843-4851. 17. Severyukhina, A. N., Parakhonskiy, B. V., Prikhozhdenko, E. S., Gorin, D. A., Sukhorukov, G. B., Möhwald, H., Yashchenok, A. M. Nanoplasmonic Chitosan Nanofibers as Effective SERS Substrate for Detection of Small Molecules. ACS Appl. Mater. Interfaces, 2015, 7(28), 1546615473. 18. Dieringer, J. A., Lettan, R. B., Scheidt. K. A., Van Duyne, R. P. A Frequency Domain Existence Proof of Single-Molecule Surface-Enhanced Raman Spectroscopy. J. Am. Chem. Soc., 2007, 129(51), 16249-16256. 19. Liu, H. L., Sun, Y. D., Jin, Z., Yang, L. B., Liu, J. H. Capillarity-Constructed Reversible Hot Spots for Molecular Trapping inside Silver Nanorod Arrays Light up Ultrahigh SERS Enhancement. Chem. Sci., 2013, 4(9), 3490-3496. 20. Huang, Y. P., Miao, Y. E., Ji, S. S., Tjiu, W. W., Liu, T. X. Electrospun Carbon Nanofibers Decorated with Ag-Pt Bimetallic Nanoparticles for Selective Detection of Dopamine. ACS Appl. Mater. Interfaces, 2014, 6(15), 12449-12456. 21. Yang, S., Dai, X., Stogin, B. B., Wong, T. S. Ultrasensitive Surface-Enhanced Raman Scattering Detection in Common Fluids. Proc. Natl. Acad. Sci. U. S. A., 2016, 113(2), 268-273. 22. Li, Y. T., Qu, L. L., Li, D. W., Song, Q. X., Fathi, F., Long, Y. T. Rapid and Sensitive in-situ Detection of Polar Antibiotics in Water using a Disposable Ag-Graphene Sensor based on Electrophoretic Preconcentration and Surface-Enhanced Raman Spectroscopy. Biosens. Bioelectron., 2013, 43(10), 94-100.


22

Page 23 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


23. Meng, Y. J., Lai, Y. C., Jiang, X. H., Zhao, Q. Q, Zhan, J. H. Silver Nanoparticles Decorated Filter Paper via Self-Sacrificing Reduction for Membrane Extraction Surface-Enhanced Raman Spectroscopy Detection. Analyst, 2013, 138(7), 2090-2095. 24. Shi, Y. E., Li, L., Yang, M., Jiang, X., Zhao, Q., Zhan, J. A Disordered Silver Nanowires Membrane for Extraction and Surface-Enhanced Raman Spectroscopy Detection. Analyst, 2014, 139(10), 2525-2530. 25. Wei, J., Zhu, H., Li, Y., Chen, B., Jia, Y., Wang, K., Wang Z., Liu W., Luo, J., Zheng, M., Wu, D., Zhu, Y., Wei, B. Ultrathin Single-Layered Membranes from Double-Walled Carbon Nanotubes. Adv. Mater., 2006, 18(13), 1695-1700. 26. Gao, S. J., Qin, H., Liu, P., Jin, J. SWCNT-Intercalated GO Ultrathin Films for Ultrafast Separation of Molecules. J. Mater. Chem. A., 2015, 3(12), 6649-6654. 27. Fan, W., Miao, Y. E., Ling, X. Y., Liu, T. X. Free-Standing Silver Nanocube/Graphene Oxide Hybrid Paper for Surface-Enhanced Raman Scattering. Chin. J. Chem., 2016, 34(1), 7381. 28. Huang, H., Song, Z., Wei, N., Shi, L., Mao, Y., Ying, Y., Sun, L., Xu, Z., Peng, X. Ultrafast Viscous Water Flow through Nanostrand-Channelled Graphene Oxide Membranes. Nat. Commun., 2013, 4, 2979. 29. Li, W., Gao, S., Wu, L., Qiu, S., Guo, Y., Geng, X., Chen, M., Liao, S., Zhu, Y., Gong, Y., Long, M., Xu, J., Wei, X., Sun, M., Liu, L. High-Density Three-Dimension Graphene Macroscopic Objects for High-Capacity Removal of Heavy Metal Ions. Sci. Rep., 2013, 3, 2125. 30. Wu, W., Zhou, T., Berliner, A., Banerjee, P., Zhou, S. Smart Core-Shell Hybrid Nanogels with Ag Nanoparticle Core for Cancer Cell Imaging and Gel Shell for pH-Regulated Drug Delivery. Chem. Mater., 2010, 22(6): 1966-1976.


23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 25

31. Wang, S. Y., Yu, D. S., Dai, L. M. Polyelectrolyte Functionalized Carbon Nanotubes as Efficient Metal-Free Electrocatalysts for Oxygen Reduction. J. Am. Chem. Soc., 2011, 133(14), 5182-5185. 32. Stankovich, S., Dikin, D. A., Piner, R. D., Kohlhaas, K. A., Kleinhammes, A., Jia, Y., Wu, Y., Nguyenb, S. T., Ruoff, R. S. Synthesis of Graphene-Based Nanosheets via Chemical Reduction of Exfoliated Graphite Oxide. Carbon, 2007, 45(7), 1558-1565. 33. Guo, H. L., Wang, X. F., Qian, Q. Y., Wang, F. B., Xia, X. H. A Green Approach to the Synthesis of Graphene Nanosheets. ACS Nano, 2009, 3(9), 2653-2659. 34. Zhang, Y., Liu, S., Wang, L., Qin, X., Tian, J., Lu, W., Chang, G., Sun, X. One-Pot Green Synthesis of Ag Nanoparticles-Graphene Nanocomposites and their Applications in SERS, H2O2, and Glucose Sensing. RSC Adv., 2012, 2(2), 538-545. 35. Gu, H. X., Xue, L., Zhang, Y. F., Li, D. W., Long, Y. T. Facile Fabrication of a Silver Dendrite-Integrated Chip for Surface-Enhanced Raman Scattering. ACS Appl. Mater. Interfaces, 2015, 7(4), 2931-2936. 36. Peng, Y., Qiu, L., Pan, C., Wang, C., Shang, S., Yan, F. Facile Preparation of Water Dispersible Polypyrrole Nanotube-Supported Silver Nanoparticles for Hydrogen Peroxide Reduction and Surface-Enhanced Raman Scattering. Electrochim. Acta, 2012, 75, 399-405. 37. Qu, L. L., Li, D. W., Xue, J. Q., Zhai, W. L., Fossey, J. S., Long, Y. T. Batch Fabrication of Disposable Screen Printed SERS Arrays. Lab Chip, 2012, 12(5), 876-881.


24

Page 25 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table of Contents


25

CNT-Intercalated GO Membranes for

Recommend Documents