Ti mesh loaded with Ag “nanobosk”: A highly sensitive Raman sensing platform for trace norfloxacin in water

Accepted Manuscript Title: Ti mesh loaded with Ag “nanobosk”: A highly sensitive Raman sensing platform for trace norfloxacin in water Authors: Xiaomeng Dou, Weiming Hao, Xiangqing Li, Lixia Qin, Shi-Zhao Kang PII: DOI: Reference:

S0925-4005(18)31909-9 https://doi.org/10.1016/j.snb.2018.10.132 SNB 25566

To appear in:

Sensors and Actuators B

Received date: Revised date: Accepted date:

13-2-2018 25-10-2018 25-10-2018

Please cite this article as: Dou X, Hao W, Li X, Qin L, Kang S-Zhao, Ti mesh loaded with Ag “nanobosk”: A highly sensitive Raman sensing platform for trace norfloxacin in water, Sensors and amp; Actuators: B. Chemical (2018), https://doi.org/10.1016/j.snb.2018.10.132 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Ti mesh loaded with Ag “nanobosk”: A highly sensitive

Raman

sensing

platform

for

trace

norfloxacin in water

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Xiaomeng Dou, Weiming Hao, Xiangqing Li, Lixia Qin, Shi-Zhao Kang* School of Chemical and Environmental Engineering, Shanghai Institute of Technology,

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Corresponding author: Shi-Zhao Kang, Tel./fax: +86 21 60873061.

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*

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100 Haiquan Road, Shanghai 201418, China

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E−mail address: [email protected] (S.-Z. Kang)

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Highlights

► Preparation of Ti mesh loaded with Ag “nanobosk”

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► Ultrasensitive SERS substrate for trace norfloxacin in water (1  10-10 mol L-1) ► Good reproducibility and satisfactory rapid detection capability

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► SERS substrate used in direct detection from solution

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For Table of Contents Use Only Ti mesh loaded with Ag “nanobosk”: A highly sensitive Raman sensing platform for trace norfloxacin in water Xiaomeng Dou, Weiming Hao, Xiangqing Li, Lixia Qin and

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Shi-Zhao Kang

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Synopsis

Preparation of Ti mesh loaded with Ag “nanobosk” and the

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SERS behavior of norfloxacin on Ti mesh loaded with Ag

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“nanobosk” are reported.

ABSTRACT In the present work, Ti mesh loaded with Ag “nanobosk” was prepared through electro-deposition and characterized with X-ray diffraction, scanning electron 2

microscopy and X-ray photoelectron spectroscopy. Then, the surface enhanced Raman scattering of norfloxacin on the Ti mesh loaded with Ag “nanobosk” was directly explored in norfloxacin aqueous solution. In addition, for practical purposes, the surface enhanced Raman scattering of norfloxacin was studied as a function of

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content of Ag, pH of solution, ions in water, and salinity, respectively. At last, the surface enhanced mechanisms were discussed preliminarily. The results indicate that

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the Ti mesh loaded with Ag “nanobosk” is an ultrasensitive surface enhanced Raman scattering substrate for trace norfloxacin in water. When the concentration of

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norfloxacin is 1  10-5 mol L-1, the enhancement factor is approximately 5.1  103 at

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1389 cm-1. And the detection limit is up to 1  10-10 mol L-1 for norfloxacin under

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optimal conditions. Our work provides a new strategy for designing a sensing

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platform which is sensitive to trace norfloxacin in water and can be applied in the

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routine detection.

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norfloxacin

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Keywords: surface enhanced Raman scattering; Ti mesh; nanostructured Ag;

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1. Introduction Norfloxacin (NFX) is one of the commonest antibiotics used for both humans and animals, and has been widely applied in the treatment of respiratory diseases, enteric bacterial infections and urinary tract infections [1]. At present, the overuse and

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misuse of NFX are very serious, which leads to the extensive presence of NFX residues in natural water. The NFX residues in natural water would trigger a series of

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public health problems, such as formation of antimicrobial resistance, allergic reaction

and toxic reaction [2]. Therefore, it is necessary to fabricate a sensing platform for

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routine monitoring NFX in natural water. However, most of the studies reported were

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focused on the detection of NFX with fluorescence quenching immune

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chromatographic strip [2], liquid chromatography [3], liquid chromatography-tandem

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mass spectrometry [4], high-performance liquid chromatography coupled with mass

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spectrometry [5] and capillary electrophoresis [6]. Although these methods exhibit high sensitivity and good selectivity to NFX, they are unfit for rapid screening of

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large numbers of samples and field assays due to their requirements of complex

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pretreating procedures, expensive instruments and professional operators. As a result, it is still desirable to develop a sensing platform which is sensitive to trace NFX in

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water and can be applied in the rapid field assays of large numbers of samples. Recently, increasing attention has been paid in the field of the rapid assays of

antibiotic residues in water and food samples. A series of sensitive analytic methods have been reported, such as electrochemical detection [7], surface enhanced Raman scattering (SERS) analysis [8], colorimetric aptasensor [9], fluorimetric method [10] 4

and biosensor [11]. Among these methods, SERS analysis is regarded as a promising candidate

due

to

ultra-sensitivity,

outstanding

selectivity,

non-destructive

characterization, rapid analysis ability without complicated sample pretreating and low cost. Hence, a lot of efforts have been devoted to the SERS detection of antibiotic

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residues. And great progress has been successfully made in the SERS assays of various antibiotic residues in water and food samples, including oxytetracycline

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hydrochloride [12], tetracycline hydrochloride [12], ampicillin [13], ciprofloxacin [14], levofloxacin [15] and so on. However, to our best knowledge, there are few

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papers on the detection of trace NFX in water based on the SERS effect so far. Here,

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one of important barriers which limit the application of SERS in the detection of trace

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NFX is the lack of sensitive, stable, cost-effective and user-friendly SERS substrates.

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Therefore, it is meaningful to fabricate a new SERS substrate which is stable,

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cost-effective, user-friendly and sensitive to NFX in water. Nowadays, most of the SERS substrates reported are colloid-based substrates [15]

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or the solid substrates loaded with functional nanomaterials, such as Si substrate

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covered with nitrogen-enriched carbon nanosheets [16], glass slide loaded with Ag-coated C60 nanoclusters [17], Ag clusters on Cu substrate [18]. Although theses SERS substrates exhibit high sensitivity and dramatic rapid detection capability, they

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have to face some difficulties when they are applied in monitoring of antibiotic residues in natural water. In the case of colloid-based substrates, their performance is inconsistent [19]. And the preservation and the operation of the colloid-based substrates are annoying due to their poor stability. For the solid SERS substrates, the 5

functional nanomaterials immobilized on solid substrates often demonstrate a small surface area, and the amount of nanomaterials is not enough, which leads to insufficient display of their properties. In order to overcome the mentioned problems, numerous newfashioned SERS substrates with high sensitivity were fabricated, such

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as Ag nanoplate hierarchical turreted array on Si nanocone template [20], Ag nanoparticles/carbon nanotube-intercalated graphene oxide laminar membrane [12],

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vertically perforated three dimensional Au nanoparticles stack on Si wafer [21], Au nanoparticles modified Si nanorod array [22], self-assembly of large Au nanoparticles

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on Si wafer [23] etc.. However, it is still highly worthy of fabricating a newfashioned

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SERS substrate with high sensitivity and rapid detection capability to solve the

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problems above better. On the other hand, most of SERS detections are carried out in

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a dried environment in order to cause efficient attachment of target molecules to

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SERS substrates. However, there exists “coffee ring effect” in the drying process, which results in inconsistence of the Raman signal measured. Theoretically, this

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problem could be solved when the Raman signals of target molecules are measured

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directly from solutions [24]. Unfortunately, although the direct detection from solution begins to attract attention [24,25], there are still limited papers on the SERS substrate which can be applied in direct detection from solution. For these reasons, it

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is very necessary to fabricate a SERS substrate which can be applied in direct detection from solution, and possesses high sensitivity and rapid detection capability. In the present work, Ag “nanobosk” was loaded on Ti mesh through electro-deposition. As a SERS substrate, the Ti mesh loaded with Ag “nanobosk” 6

possesses some promising advantages as follows. (1) The highly branched structure of Ag “nanobosk” would provide large surface area and larger effective hot spot area so as to adsorb target molecules effectively and further enhance Raman signal intensity. In addition, the tips of the Ag branches can also generate efficient plasmonic hot spots.

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(2) Compared with common solid substrates such as Si wafer and glass slide, Ti mesh is favor to increasing the amount of Ag “nanobosk” and enhancing the effective

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contact between analytes and the Ag “nanobosk”. (3) The conductivity of Ti mesh might enhance the chemical enhancement originated from the charge transfer. (4)

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When the Ti mesh loaded with Ag “nanobosk” is applied in the direct detection from

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solution, there may exist coupling effect between Ag “nanobosks” loaded on adjacent

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Ti wires, which can further enhance the Raman response. (5) It will become easy to

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focus target molecules on the SERS substrate if the Ti mesh loaded with Ag

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“nanobosk” is used in the direct detection from solution. (6) Ti possesses excellent chemical and physical stability, which is attractive when a SERS substrate is used

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under acidic or alkaline conditions. Moreover, it can be also expect that the

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inconsistence of the Raman signal due to “coffee ring effect” in the drying process would be avoided through direct detection from NFX aqueous solution. Based on these viewpoints above, the surface enhanced Raman scattering of NFX on the

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as-prepared Ti mesh was directly explored in NFX aqueous solution. In addition, for practical purposes, the SERS effect of the aforementioned Ti mesh was studied as a function of content of Ag, pH, salinity, type of salt and mesh number, respectively. At last, the surface enhanced mechanisms were discussed preliminarily. 7

2. Experimental 2.1 Materials NFX (99%), AgNO3 (A.R.), ethanol (A.R.), acetone (A.R.) and chloroform (A.R.) purchased

from

Shanghai

Titan

Scientific

Co.

Ltd.

(China).

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were

2,4-Dichlorophenoxyacetic acid (97%) and clofibric acid (97%) were purchased from

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ANPEL Laboratory Technologies Inc. (China). 4-Chlorophenoxyacetic acid (98%) was purchased from Alfa Aesar Chemicals Co. Ltd. (China). Glyphosate (96%) was

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purchased from J&K Scientific Ltd. (China). Teracycline hydrochloride (USP) was

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purchased from Shanghai Macklin Biochemical Co. Ltd. (China). Chloroamphenicol

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(98%) was purchased from Sinopharm Chemical Reagent Co. Ltd. (China).

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Ampicillin (96%) was purchased from Aladdin Industrial Corporation (China). All of

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reagents were used without further purification. Double distilled water was used as solvent.

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2.2 Fabrication of Ti mesh loaded with Ag “nanobosk” (Ti mesh-Ag)

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Through varying AgNO3 concentrations, a series of Ti mesh-Ag were prepared using Ti meshes with various mesh numbers by an electrolytic deposition process (Scheme 1). In this electrolytic deposition process, AgNO3 serves as a precursor. The

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main electrochemical reaction occurred on the cathode is the reduction of Ag+ [26]. Typically, a piece of Ti mesh (2.3 cm × 2.5 cm, 100 mesh, wire diameter 0.1 mm) was rinsed ultrasonically with deionized water, ethanol, acetone, chloroform and ethanol, respectively. After washed with deionized water, the Ti mesh was dried at room 8

temperature in air. Then the edges of the Ti mesh were packaged using epoxy resin to prepare a work electrode. The bare area of the Ti mesh was 2 cm  2 cm. The electrolytic deposition was performed with a DC power source (Agilent E3643A, Agilent Technologies Inc., USA) in a homemade two-electrode cell containing

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aqueous AgNO3 solution (5.9 mmol L-1, 25 mL). The packaged Ti mesh and the platinum electrode were used as cathode and anode, respectively. The deposition was

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carried out under the constant voltage mode, and the bias voltage was 5 V. After

electrolytic depositing for 20 min, the mesh was taken out from the electrolyte,

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washed with deionized water several times and dried at room temperature. The control

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experimental results indicate that AgNO3 in the electrolyte was completely converted

equation 1.

(1)

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Content of Ag = MAg  C  V / S

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into Ag. Therefore, the amount of Ag deposited can be estimated according to

Where MAg is the mol mass of Ag, C is the concentration of the AgNO3 solution, V is

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the volume of the AgNO3 solution, S is the area of Ti mesh when the mesh is assumed

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to be a solid sheet.

For convenience, the product is marked as Ti mesh (y)-Ag/x, in which x is the

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content of Ag (mg cm-2), y is the mesh number.

/Scheme 1/

2.3 SERS measurement 9

The Raman spectra of NFX were measured on a BWS465-532S Raman spectrometer (B&WTek Inc., USA) with an excitation wavelength of 532 nm. The incident laser power was 40 mW, and the spot was 0.1 mm. Twenty seconds acquisition time was employed. The accumulation parameter was 1 time. The

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baselines of the spectra were automatically corrected using the software (BWSpec 4.04_00) which was provided by B&WTek Inc.. The calibration parameter was

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1000000. Typically, Ti mesh (100)-Ag/4 was cut into a rectangle piece with lateral dimensions of 0.5 cm  0.5 cm. Then, Ti mesh (100)-Ag/4 was put in a homemade

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sealed measuring cell (Scheme 1) containing NFX aqueous solution (1  10-5 mol L-1,

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2 mL). After 10 min, the Raman spectra of NFX were collected at five random

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locations of Ti mesh (100)-Ag/4. Besides, it is well known that there exist some salts

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in fresh water. In order to further evaluate the practical application of Ti mesh-Ag, the

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SERS effect of Ti mesh-Ag was explored when some common salts, including NaCl, KCl, MgCl2, CaCl2, FeCl3, NaNO3, Na2SO4, Na2SO3 and Na2CO3, are present in the

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NFX solutions.

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2.4 Characterization

The scanning electron microscope (SEM) images were taken with an S-3400 N

Hitachi scanning electronic microscopy (Japan). The X-ray photoelectron

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spectroscopy (XPS) analysis was carried out using a Thermo Fisher ESCALAB 250Xi X-ray photoelectron spectrometer equipped with a monochromatic Al (Kα) source (USA). The X-ray diffraction (XRD) pattern was measured with a Bruker D8-Advance X-ray diffractometer (Germany). 10

3. Results and discussion Fig. 1A shows the XRD pattern of Ti mesh (100)-Ag/4. As can be seen from Fig. 1A, some diffraction peaks appear at 35.1°, 38.1°, 40.2°, 44.3°, 53.0°, 62.9°, 64.4°,

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70.6°, 76.2°, 77.4° and 81.5°, respectively. Therein, the diffraction peaks at 38.1°, 44.3°, 64.4°, 77.4° and 81.5° should correspond to the (111), (200), (220), (311) and

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(222) planes of face-centered cubic Ag (JCPDS No. 04-0783) [27]. The peaks at 35.1°, 40.2°, 53.0°, 62.9°, 70.6° and 76.2° could be assigned to diffractions corresponding to

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(100), (101), (102), (110), (103) and (112) planes of metallic titanium (JCPDS No.

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44-1294) [28]. This result indicates that face-centered cubic Ag is deposited on the Ti

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mesh in the electrolytic deposition process. Moreover, no diffraction peaks can be

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observed except the aforementioned peaks. Therefore, it can be deduced that the

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as-prepared sample is the Ti mesh loaded with pure Ag. The composition of the sample was further clarified with XPS analysis. The XPS

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survey spectrum (supplementary Fig. S1) displays that there exist some XPS peaks

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ascribed to Ti, Ag, O and C elements. One possible explanation is that the as-prepared sample is composed of the elements Ti and Ag. The signals of C and O may be ascribed to the partial oxidation of Ti and Ag as well as the adsorption of organic

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molecules during the ambient isolation and drying processes. Fig. 1B shows the high-resolution XPS spectrum of Ag 3d. From Fig. 1B, it can be observed that there exist two XPS peaks at 374.1 eV and 368.1 eV, respectively. These peaks should correspond to the Ag 3d 3/2 peak and Ag 3d 5/2 peak of Ag0 [29], implying that 11

metallic silver is loaded on the Ti mesh through electrolytic deposition process. In addition, we cannot find the XPS peaks ascribed to Ag2O from Fig. 1B. This result indicates that the aforementioned peaks of O may be ascribed to TiO2 rather than Ag2O. Therefore, it can be concluded that Ag is successfully introduced on the Ti

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mesh, and its purity is high. Moreover, it is interested that the Ag 3d peaks of Ag loaded on Ti mesh shift to

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higher binding energy in comparison with those of pure Ag (374.02 eV and 368.02 eV) [30]. One possible explanation is that the work function of Ti is 4.33 eV [31] while

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the work function of Ag is 4.22 eV [32]. There exists a slight transfer of electron from

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Ag to Ti, and some low electron density spots would form along the interface between

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Ag and Ti. Thus, the Ag 3d peaks of Ag loaded on Ti mesh shift to higher binding

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energy. This phenomenon implies that there is a strong electron interaction between

/Figure 1/

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Ag and Ti, which is favor to fabrication of the SERS substrate with high sensitivity.

Fig. 2 shows the SEM images of Ti mesh (100)-Ag/4. For comparison, the SEM

image of bare Ti mesh was also shown in Fig. 2. From Fig. 2, it can be found that the

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surface of the bare Ti wires is smooth. After electrolytic deposition, the surface of the Ti wires becomes rougher, and some hierarchical structures appear on the Ti mesh. The high-magnification SEM images (Fig. 2C and D) indicate that these structures adopt a bosk-like morphology which is composed of randomly oriented and 12

cross-linked dendritic nanorods. These dendritic nanorods possess a 3.5-8.5 m trunk (approximate 400 nm in diameter) and 140-500 nm sider branches (approximate 150 nm in diameter). Moreover, we can also observe that Ti mesh (100)-Ag/4 possesses fairly good uniformity in a large area, which is beneficial for the SERS effect.

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Combined with the results of XRD and XPS, it is confirmed that the Ag “nanobosks”

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are successfully loaded on the Ti mesh according to the procedure described above.

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/Figure 2/

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The Raman spectra of NFX solutions (1  10-5 mol L-1) on Ti mesh (100)-Ag/4

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and the bare Ti mesh are shown in Fig. 3. For comparison, the Raman spectrum of the

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NFX solution (1  10-5 mol L-1) was also measured without any SERS substrate. As

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can be seen from Fig. 3, when the concentration of NFX is 1  10-5 mol L-1, the Raman peaks of NFX can hardly be found in the absence of Ti mesh-Ag. In contrast,

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when Ti mesh-Ag is present, we can observe very strong Raman signals, suggesting

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that Ti mesh-Ag possesses significant SERS effect for NFX. These Raman peaks from NFX mainly appear at 1039 cm-1, 1083 cm-1, 1258 cm-1, 1332 cm-1, 1389 cm-1, 1445 cm-1, 1548 cm-1 and 1610 cm-1, respectively. Here, the peaks at 1039 cm-1 and 1083

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cm-1 may be ascribed to the pyrazine ring breathing vibration. The peak at 1258 cm-1 ought to be assigned to the C-F bond stretching vibration. The peak at 1332 cm-1 may be ascribed to the mixed vibration. The peaks at 1389 cm-1 and 1445 cm-1 may respond to the O-C-O (carboxyl group) symmetric and asymmetric stretching 13

vibrations, respectively. The peak at 1548 cm-1 may be assigned to the stretching vibration of the quinolone ring. The Raman band at 1610 cm-1 ought to be assigned to the C=C bond asymmetrical stretching vibration of aromatic rings [33,34]. And they are in good agreement with those of pure NFX (powder). Therefore, it can be

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concluded that these Raman signals would originate from the NFX molecules on Ti mesh-Ag, and Ti mesh-Ag possesses significant SERS effect for NFX in water.

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Of cause, the Raman spectrum of NFX on Ti mesh (100)-Ag/4 slightly differs

from that of pure NFX powder. At first, the peaks ascribed to the O-C-O stretching

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vibrations (1389 cm-1 and 1445 cm-1) red-shift about 5 cm-1 and 9 cm-1 when the

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Raman spectrum is measured on Ti mesh (100)-Ag/4. The peak at 1548 cm-1 blue

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shifts about 10 cm-1. In contrast, the shift of the peaks ascribed to the pyrazine ring

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can be hardly observed. These phenomena imply that the SERS effect may originate

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from the interaction between the quinolone moiety of NFX and Ti mesh-Ag. From Fig. 3, we can also find that the bare Ti mesh does not possess SERS effect

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for NFX in water. When the bare Ti mesh is put in the NFX solution, the Raman

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signals of NFX molecules cannot be observed. This phenomenon indicates that the SERS effect of Ti mesh-Ag originates from the Ag “nanobosks” loaded on Ti mesh. Because the Raman peaks of NFX (1  10-5 mol L-1) cannot be observed in the

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absence of Ti mesh-Ag, the corresponding Raman signal of the NFX solution with higher concentration (1  10-3 mol L-1) is measured under the same conditions and used as a benchmark in the calculation of the enhancement factor (EF). EF of Ti mesh (100)-Ag/4 is calculated according to the equations below [35]. 14

EF = (ISERS / NNFX1) / (Isolution /NNFX2)

(2)

NNFX1 = C1  V1

(3)

NNFX2 = C2  V2

(4)

Where ISERS is the Raman peak intensity of NFX (1  10-5 mol L-1) measured in the

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presence of Ti mesh-Ag, Isolution is the Raman signal intensity of NFX solution (1  10-3 mol L-1) measured in the absence of Ti mesh-Ag. NNFX1 and NNFX2 are the

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numbers of NFX molecules in the solutions measured. C1 and C2 are the

concentrations of the NFX solutions used. V1 and V2 are the corresponding solution

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volumes. Thus, EF of Ti mesh (100)-Ag/4 is up to 5122 at 1389 cm-1, which is about

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30 percent more than that of the common Ag colloid-based substrate. Moreover, the

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control experimental results indicate that the uniformity of the SERS signal is

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satisfactory. When the SERS spectra are measured at ten random locations on Ti mesh

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(100)-Ag/4 (0.5 cm  0.5 cm), the Raman spectra are highly reproducible and stable, and the relative standard deviation of the peak intensities (1389 cm-1) is about 2.5%

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(Fig. S2). Moreover, the control experiment results display that the batch-to-batch

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variation of the SERS signals is also unobvious on Ti mesh-Ag. When the concentration of NFX is 1  10-5 mol L-1, the relative standard deviation of the mean Raman signal intensities (1389 cm-1) which are measured over various substrates is

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about 1% (Fig. S2). The results above suggest that Ti mesh-Ag may be a promising SERS substrate for the Raman detection of trace NFX in water.

/Figure 3/ 15

Fig. 4 shows the SERS spectra of NFX on Ti mesh (100)-Ag/4 measured in NFX aqueous solutions with various concentrations. From Fig. 4A, it can be found that the Raman signal of NFX is gradually weakened with the NFX concentration decreasing

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(from curve 1 to curve 9). In addition, the result shows that Ti mesh-Ag is a highly sensitive SERS substrate for NFX in water. When the concentration of NFX is 1 

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10-10 mol L-1, the Raman band from 1000 to 1800 cm-1 is still obviously observed (Fig. 4A curve 8). And the most strong, stable and obvious Raman peak appears at 1389

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cm-1. This result implies that the trace NFX in water can be detected by monitoring

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the Raman signal at 1389 cm-1. The dependence of the SERS intensity at 1389 cm-1 on

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concentration is plotted in an I-log(C) style (Fig. 4B). Fig. 4B exhibits good linearity

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(R2 = 0.998) over the range of 1  10-9 to 1  10-5 mol L-1. The linear dependence can

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be described as I = 68100 + 7255 lg(C) (mol·L-1). This indicates that it is possible to use Ti mesh-Ag for the quantitative detection of NFX. In addition, the control

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experimental result shows that the enhancement of Raman signal is very fast on Ti

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mesh-Ag (Fig. S3). When the concentration of NFX is 1 × 10-5 mol·L-1, the Raman peak will reach its maximum intensity after Ti mesh-Ag is put in the NFX aqueous solution for 20 minutes. Even if the Raman spectrum is measured as soon as Ti

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mesh-Ag is put in the NFX aqueous solution, the Raman peak can reach seventy five percent of its maximum intensity. Therefore, it can be indicated that Ti mesh-Ag possesses satisfactory rapid detection capability, and would be a promising sensing platform for the qualitative as well as quantitative detection of trace NFX in water. 16

/Figure 4/

Fig. 5 shows the SERS spectra of NFX on the Ti meshes loaded with various

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contents of Ag. As can be seen from Fig. 5, the SERS effect of Ti mesh-Ag is strengthened with the content of Ag increasing from 1 mg cm-2 to 4 mg cm-2.

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Afterwards, the SERS signals of NFX are weakened on Ti mesh-Ag when the content

of Ag further increases. This phenomenon may be ascribed to two causes. One is that

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the hot spot area would increase with the content of Ag increasing. Another is that the

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morphology of Ag would change due to increasing the content of Ag. The SEM

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images of the Ti meshes loaded with various contents of Ag (Fig. S4) confirm our

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assumption. As can be seen from Fig. S4, when the content of Ag is 1 mg cm-2, some

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hierarchical structures composed of Ag nanoplates can be found on the Ti mesh besides the dendritic nanorods. In the case of Ti mesh (100)-Ag/10, the Ag loaded on

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Ti mesh adopts a straw heap-like morphology which is composed of nanorods.

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Although the surface rough increases with the content of Ag increasing, there exists some conglutination between the dendritic nanorods, which leads to decreasing of the gaps between dendritic nanorods and the number of the tips of the Ag branches.

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Meanwhile, the Ag branches become blunter with increasing the content of Ag. As a result, compared with Ag “nanobosk”, either the hierarchical structure composed of Ag nanoplates or Ag “straw heap” possesses less plasmonic hot spots originated from the tips of the Ag branches. Moreover, because Ag “straw heap” is more compact than 17

Ag “nanobosk”, the hot spots originated from the gaps between dendritic nanorods would also decrease. Therefore, at the beginning, the number of hot spots increases with the content of Ag increasing. However, if the content of Ag is more than the optimal content, the number of hot spots would decrease with the content of Ag

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increasing. By this taken, the morphology of Ag plays an important role in the

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enhancement of Raman scatting.

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/Figure 5/

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On the basis of the experimental results above, we infer that this remarkable

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SERS effect on Ti mesh-Ag may be related to the several factors as follows: (1) the

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electromagnetic enhancement from the Ag “nanobosk”, (2) the bosk-like morphology

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which is composed of cross-linked dendritic nanorods, (3) more Ag “nanobosk” loaded and more effective contact between NFX and the Ag “nanobosk” ascribed to

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the usage of Ti mesh, (4) the coupling effect between Ag “nanobosks” loaded on

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adjacent Ti wires, (5) direct detection from NFX solution. Here, the electromagnetic enhancement from the Ag “nanobosk” is the key contribution. The bosk-like morphology and the coupling effect between Ag “nanobosks” loaded on adjacent Ti

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wires play important roles in the enhancement of Raman scatting. In order to confirm our assumptions above, some control experiments were carried out. The results indicate that when the same amount of Ag is deposited on Ti foil under the same conditions, the mechanical strength of the Ag loaded on Ti foil is much weaker than 18

that loaded on Ti mesh. The Ag loaded on Ti foil is apt to fall off very much, indicating that the load capacity of Ti foil is poor in comparison with that of Ti mesh. The SERS effect of Ti foil loaded with Ag is also much weaker than that of Ti mesh-Ag. Its EF is only 2000 at 1389 cm-1. This result confirms that the contact

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between NFX and the Ag loaded Ti foil might be inefficient. In addition, the Raman spectra of NFX were also measured on Ti mesh-Ag with various mesh numbers (Fig.

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S5). The result shows that the SERS effect of Ti mesh-Ag is strengthened

monotonously with the mesh number increasing, which preliminarily confirms that

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there exists some coupling effect between Ag “nanobosks” loaded on adjacent Ti

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wires.

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Fig. 6A shows the effect of pH on the SERS behavior of NFX on Ti mesh

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(100)-Ag/4. The curves (A1 - A5) shown in Fig. 6A were measured in turn at pH 4, 5,

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6, 7 and 8. From Fig. 6A, it can be observed that the SERS effect of Ti mesh-Ag is strengthened obviously when the pH value is adjusted from 4 to 5. However, with

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further increasing the pH value, the SERS effect of Ti mesh-Ag is weakened. One

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possible explanation is that NFX is regarded as an amphoteric compound. The pKa values are approximately 6.2 and 8.5 [36]. Thus, there are four existing forms in aqueous solution, i.e. the neutral form, tautomeric form, anionic form and cationic

A

form [36]. And the existing state of NFX in aqueous solution will change with the pH value. When pH increases from 4 to 8, the cationic form of NFX would transform into the neutral form of NFX. The results reported previously indicate that the interaction between carboxyl groups and Ag would decrease with pH decreasing [37]. Meanwhile 19

the interaction between piperazine groups and Ag would also decrease when pH decreases. Thus, the neutral form of NFX is more favorable to its attachment to Ag “nanobosk” in comparison with the cationic form. On the other hand, the Ag branches cross-link more closely in acidic condition than those in neutral or basic condition

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[38], which would result in stronger SERS signals. Therefore, the maximum SERS enhancement of NFX can be observed when the pH value of the NFX aqueous

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solution is 5.

According to the results reported previously, there exist some salts, such as NaCl,

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KCl, MgCl2, CaCl2, FeCl3, NaNO3, Na2SO4, Na2SO3 and Na2CO3, in fresh water. The

N

TDS (total dissolved solids) is usually < 1000 mg L-1 [39]. Therefore, in order to

A

further evaluate the practical application of Ti mesh-Ag, the SERS effect of Ti

M

mesh-Ag was explored when some common salts (concentration 1  10-3 mol L-1 –

ED

0.1 mol L-1) are present in the NFX solutions. Fig. 6B shows the spectra of NFX on Ti mesh (100)-Ag/4 in the presence of NaCl. These spectra (curve B1 - curve B6) were

PT

measured when the concentrations of NaCl were 0 mol L-1, 0.001 mol L-1, 0.005 mol

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L-1, 0.01 mol L-1, 0.05 mol L-1 and 0.1 mol L-1, respectively. As can be seen from Fig. 6B, the SERS effect of Ti mesh-Ag is obviously influenced by NaCl. And the SERS effect of Ti mesh-Ag is weakened monotonously with the concentration of NaCl

A

increasing. Fig. 6C shows the spectra of NFX on Ti mesh (100)-Ag/4 which were measured in the presence of NaCl (curve C1), KCl (curve C2), CaCl2 (curve C3), MgCl2 (curve C4) and FeCl3 (curve C5). From Fig. 6C, it can be found that all salts can cause weakening of the SERS effect of Ti mesh-Ag. Therein, the SERS effect of 20

Ti mesh-Ag in the presence of NaCl is almost same as that in the presence of KCl. Compared with K+ and Na+, Mg2+ and Ca2+ influence the SERS effect of Ti mesh-Ag more obviously. The SERS effect of Ti mesh-Ag can hardly be observed when FeCl3 is present. One possible explanation is that the existing form of NFX is mainly the

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cationic form at pH 5. Because of adsorption of cations on Ti mesh-Ag, the surface of Ti mesh-Ag might be positively charged in the present of salts. As a result, the

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adsorption of NFX on Ti mesh-Ag would be blocked, which leads to the weakening of

the SERS effect of Ti mesh-Ag. Besides, because an excess amount of metal ion was

U

used in the experiments, most of NFX molecules would form complexes with Mg2+,

N

Ca2+ and Fe3+. And the metal ions coordinate with the carboxyl groups of quinolone

A

molecules [40,41]. Thus, the Raman signals of NFX are further weakened on Ti

M

mesh-Ag. Among Mg2+, Ca2+ and Fe3+, the complexes of Mg2+ and Ca2+ are colorless

ED

while the complex of Fe3+ is deep yellow. Their UV-vis spectra (Fig. S6) also show that the complexes of Mg2+ and Ca2+ cannot absorb visible light while the complex of

PT

Fe3+ possesses a fairly strong absorption band in the range of 230 nm - 600 nm. Hence,

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the laser used (532 nm) cannot effectively reach Ti mesh-Ag across the NFX solution in the presence of Fe3+. For these reasons above, Fe3+ ions most influence the SERS

A

effect of Ti mesh-Ag among K+, Na+, Mg2+, Ca2+ and Fe3+. Fig. 6D shows the spectra of NFX on Ti mesh (100)-Ag/4 which were measured

in the presence of NaCl (curve D1), Na2SO4 (curve D2), NaNO3 (curve D3), Na2SO3 (curve D4) and Na2CO3 (curve D5). As can be seen from Fig. 6D, the effects of NO3-, SO42- and Cl- are weaker than those of SO32- and CO32-. This phenomenon may be 21

ascribed to the difference in the pH values of the solutions. Na2SO3 and Na2CO3 are strong alkali and weak acid salts. When they are introduced into the NFX aqueous solutions (pH = 5), the pH values would increase slightly, which lead to greater weakening of the SERS signals.

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However, the SERS effect of Ti mesh-Ag is still satisfactory in the presence of salts. When the concentration of NaCl is 0.1 mol L-1, the SERS peak intensity at 1389

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cm-1 can be up to 24126, which is comparable with that of the Ag colloid-based substrate in the absence of NaCl. Even if Mg2+ ions (0.01 mol L-1), Ca2+ ions (0.01

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mol L-1), SO32- ions (0.01 mol L-1) or CO32- ions (0.01 mol L-1) is present, the SERS

N

peak intensities at 1389 cm-1 are still 17000, 22000, 22500 and 21000, respectively.

A

These results indicate that it is possible to use Ti mesh-Ag as a sensing platform for

M

the detection of trace NFX in natural water. Fe3+ ions should be removed from water

/Figure 6/

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PT

ED

prior to measuring the Raman spectrum of NFX on Ti mesh-Ag.

In order to verify the application of Ti mesh-Ag in real life, some common

chemicals, including teracycline hydrochloride, chloroamphenicol, ampicillin,

A

2,4-dichlorophenoxyacetic acid, 4-chlorophenoxyacetic acid, clofibric acid and glyphosate, were selected as interfering substances, and their SERS responses were measured on Ti mesh (100)-Ag/4. The results (Fig. 7) show that NFX can exhibit much higher SERS signal at 1389 cm-1 on Ti mesh-Ag in comparison with any of the 22

aforementioned interfering substances, although the concentrations of the interfering substances (1  10-3 mol·L-1) are 100 times the concentration of NFX (1  10-5 mol·L-1). It is also worth pointing out that both chloroamphenicol and ampicillin possess obvious Raman peak around 1389 cm-1. However, even if the solution of NFX

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is very dilute (1  10-7 mol L-1), the Raman signal of NFX is still 2 and 3 times stronger than those of chloroamphenicol and ampicillin (1  10-3 mol·L-1),

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respectively. These phenomena imply that the interference from the aforementioned interfering substances can be ignored when Ti mesh-Ag is applied in the detection of

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trace NFX in natural water. Therefore, it can be concluded that Ti mesh-Ag possesses

A

N

remarkable selectivity for NFX in water.

4. Conclusion

ED

M

/Figure 7/

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In summary, Ti mesh loaded with Ag “nanobosk” can be prepared through

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electro-deposition. And the Ti mesh loaded with Ag “nanobosk” is an ultrasensitive SERS substrate with outstanding selectivity, good reproducibility and satisfactory rapid detection capability for trace norfloxacin in water. It is possible to use Ti

A

mesh-Ag as a sensing platform for the direct detection of trace NFX in natural water. Here, this remarkable SERS effect on Ti mesh-Ag may be mainly ascribed to the electromagnetic enhancement from the Ag “nanobosk”. Meanwhile, the bosk-like morphology and the coupling effect between Ag “nanobosks” loaded on adjacent Ti 23

wires also play important roles in the enhancement of Raman scatting. Our work provides a new strategy for designing a sensing platform which is sensitive to trace norfloxacin in water, and can be applied in the routine detection.

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Acknowledgments

This work was financially supported by Shanghai Municipal Education

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Commission (Plateau Discipline Construction Program) and the National Natural

A

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M

A

N

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Science Foundation of China (No. 21771125).

24

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Figure captions

Fig. 1. (A) XRD pattern and (B) high-resolution XPS spectrum of Ag 3d of Ti mesh

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(100)-Ag/4.

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Fig. 2. SEM images of (A) bare Ti mesh and (B, C, D) Ti mesh (100)-Ag/4.

Fig. 3. SERS spectra of NFX solutions in the presence of (a) Ti mesh (100)-Ag/4, (b)

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bare Ti mesh with 100 mesh, and (c) without any SERS substrate (NFX concentration

A

N

1  10-5 mol L-1); (d) SERS spectrum of pure NFX (powder).

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Fig. 4. (A) SERS spectra of NFX on Ti mesh (100)-Ag/4 measured in NFX aqueous

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solutions with various concentrations: (1) 1  10-5 mol L-1, (2) 5  10-6 mol L-1, (3) 1  10-7 mol L-1, (4) 1  10-8 mol L-1, (5) 5  10-9 mol L-1, (6) 1  10-9 mol L-1, (7) 5 

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10-10 mol L-1, (8) 1  10-10 mol L-1 and (9) 0 mol L-1; (B) SERS intensities of NFX at

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1389 cm-1 as a function of logarithmic NFX concentrations.

Fig. 5. SERS spectra of NFX solution (1  10-5 mol L-1) on the Ti meshes loaded with

A

various contents of Ag: (a) 1 mg cm-2, (b) 2 mg cm-2 (c) 4 mg cm-2 (d) 6 mg cm-2 and (e) 10 mg cm-2.

Fig. 6. (A) Raman spectra of NFX on Ti mesh (100)-Ag/4 measured at pH (A1) 4, 31

(A2) 5, (A3) 6, (A4) 7 and (A5) 8; (B) Raman spectra of NFX on Ti mesh (100)-Ag/4 in the presence of NaCl: (B1) 0, (B2) 0.001 mol L-1, (B3) 0.005 mol L-1, (B4) 0.01 mol L-1, (B5) 0.05 mol L-1 and (B6) 0.1 mol L-1; (C) Raman spectra of NFX on Ti mesh (100)-Ag/4 in the presence of (C1) NaCl (0.01 mol L-1), (C2) KCl (0.01 mol

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L-1), (C3) CaCl2 (0.01 mol L-1), (C4) MgCl2 (0.01 mol L-1) and (C5) FeCl3 (0.01 mol L-1); (D) Raman spectra of NFX on Ti mesh (100)-Ag/4 in the presence of (D1) NaCl

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(0.01 mol L-1), (D2) Na2SO4 (0.01 mol L-1), (D3) NaNO3 (0.01 mol L-1), (D4) Na2SO3

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(0.01 mol L-1) and (D5) Na2CO3 (0.01 mol L-1) (the NFX solution 1  10-5 mol L-1).

N

Fig. 7. SERS signal intensities at 1389 cm-1 of teracycline hydrochloride (1  10-3 mol

A

L-1), chloroamphenicol (1  10-3 mol L-1), ampicillin (1  10-3 mol L-1),

M

2,4-dichlorophenoxyacetic acid (1  10-3 mol L-1), 4-chlorophenoxyacetic acid (1 

A

CC E

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(1  10-5 mol L-1).

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10-3 mol L-1), clofibric acid (1  10-3 mol L-1), glyphosate (1  10-3 mol L-1) and NFX

32

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Figure 1

A

40

60

80

M

20

A

N

U

Intensity (a. u.)

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Ag Ti

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2 Theta (degree)

CC E A

380

378

Ag 3d 5/2 368.1 eV

Ag 3d 3/2 374.1 eV

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Intensity (a. u.)

B

376

374

372

370

368

Binding energy (eV)

33

366

364

362

A ED

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A

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Figure 2

34

a

b

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c

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Intensity (a. u.)

5000

ED

M

A

N

5000

900

1200

d

1500

1800 -1

A

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Raman shift (cm )

35

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Figure 3

35000

A 1

20000

9

15000

5000

15000 10000

1200

M

5000

1600

ED

0 800

20000

A

10000

25000

U

25000

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30000

Intensity (a. u.)

Intensity (a. u.)

30000

B

N

35000

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Figure 4

2000 -1

0

-10 -9 -8 -7 -6 -5 -1

Raman shift (cm )

A

CC E

PT

lg CNFX (mol L )

36

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Figure 5

5000

U

e

M

A

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Intensity (a. u.)

d

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PT

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b

a

1200

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1800 -1

Raman shift (cm )

A

900

c

37

Figure 6

5000

A

A4

SC R

A3

A2

A1

1200

1600

2000

U

800

-1

M

5000 B6

ED

B5

B4

B3

B2 B1

800

1200

1600 -1

Raman shift (cm )

A

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PT

Intensity (a. u.)

A

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Raman shift (cm )

B

38

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Intensity (a. u.)

A5

2000

5000 C

Intensity (a. u.)

C5

C4

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C3

C2

800

1200

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2000

-1

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Raman shift (cm )

5000

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D5

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D4

D3

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D

D2

D1

1200

1600 -1

Raman shift (cm )

A

CC E

800

SC R

C1

39

2000

Figure 7

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Norfloxacin Teracycline hydrochloride Chloroamphenicol

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Ampicillin Glyphosate Clofibric acid 4-Chlorophenoxyacetic acid

5000 10000 15000 20000 25000 30000 35000 40000

N

0

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2,4-Dichlorophenoxyacetic acid

A

CC E

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M

A

Intensity (a. u.)

40

Scheme captions

Scheme 1 Schematic diagram of the preparation of Ti mesh-Ag, schematic diagram of the SERS detection of NFX on Ti mesh-Ag and the molecular structure of NFX.

A

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M

A

N

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Scheme 1

41