Multifunctional Fe3O4@mTiO2@noble metal composite NPs as ultrasensitive SERS substrates for trace detection

Hongyan Guo; Aiwu Zhao; Qinye He; Ping Chen; Yuanyuan Wei; Xiangyu Chen; Haiying Hu; Min Wang; He Huang; Rujing Wang

doi:10.1016/j.arabjc.2019.01.007

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Original article

12 (

); 2017-2027

doi:

10.1016/j.arabjc.2019.01.007

Multifunctional Fe₃O₄@mTiO₂@noble metal composite NPs as ultrasensitive SERS substrates for trace detection

Hongyan Guo^{^a,^b,^c}, Aiwu Zhao^{^a,^b,^c,⁎}, Qinye He^{^a,^b,^c}, Ping Chen^{^a,^b,^c}, Yuanyuan Wei^{^a}, Xiangyu Chen^{^a}, Haiying Hu^{^a}, Min Wang^{^a}, He Huang^{^a}, Rujing Wang^{^a,⁎}

a Institute of Intelligent Machines, Chinese Academy of Sciences, Hefei 230031, People’s Republic of China

b Department of Chemistry, University of Science and Technology of China, Hefei 230026, Anhui, People’s Republic of China

c State Key Laboratory of Transducer Technology, Chinese Academy of Sciences, Hefei 230031, People’s Republic of China

⁎Corresponding author. awzhao@iim.ac.cn (Aiwu Zhao)

Received: 2018-11-24, Accepted: 2019-1-27, Published: 2019-12-01

Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

Multifunctional materials have become the development trend of current material preparation. We reported a typical layer-by-layer method for the fabrication of multifunctional Fe₃O₄@mTiO₂@noble metal triplex core-shell composite nanoparticle (NP), which is composed of a magnetic Fe₃O₄ particle as the core, a mesoporous TiO₂ interlayer and a layer of Ag nanoparticles or Au nanorods as the shell. The obtained Fe₃O₄@mTiO₂@noble metal composite NPs have shown excellent surface enhanced Raman scattering (SERS) sensitivity. Raman results present that the limit of detection (LOD) for crystal violet (CV), p-aminothiophnol (p-ATP) and p-mercaptobenzoic acid (p-MBA) of the Fe₃O₄@mTiO₂@noble metal composite NPs substrates are as low as 1.0 × 10⁻⁹ M, 1.0 × 10⁻¹² M and 1.0 × 10⁻⁹ M, respectively. In addition, the composite NPs also show high reproducibility and stability across the entire area with relative standard deviations (RSD) less than 15.00%. These highly sensitivity with good reproducibility can be attributed to the presence of plentiful “hot spots” produced by magnetic aggregation and target molecules enrichment by mesoporous TiO₂ adsorption for practical application. Fe₃O₄@mTiO₂@Ag composite NPs were used for thiram detection and the detection limit can reach to 5.0 × 10⁻⁸ M (about 0.012 ppm), which is lower than the maximal residue limit of 7 ppm in fruit prescribed by the U.S. Environmental Protection Agency. These multifunctional composite NPs provide easy separation, enrichment and trace detection of the analyte, exhibiting a great prospect as a potential SERS sensor in complex environments.

Keywords

Surface enhanced Raman scattering

Fe3O4@mTiO2@Ag

Fe3O4@mTiO2@Au NR

Thiram

Trace detection

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1 Introduction

Surface-enhanced Raman Scattering (SERS) spectroscopy has been well known as a powerful and versatile analytical tool for application in the field of biology, medicine, food security, environmental monitoring and national security due to its high sensitivity, specificity, and fingerprint effect in trace detection (Xia et al., 2011; Liu et al., 2014a; 2014b; Zong et al., 2012; Zong et al., 2012; Lane et al., 2015; Liao and Lu, 2016; Wang et al., 2014; Xu et al., 2015; Xu et al., 2017 ). Up to now, multiple nanostructures of traditional noble metals (Au, Ag and Cu), including nanoparticles (NPs) (Hu et al., 2005), nanorods (NRs) (Guo et al., 2009), nanocubes (Huang et al., 2017), core/shell structure (Bao et al., 2008; Li et al., 2010; Tao et al., 2014 ) have been successively fabricated as substrates, showing excellent SERS properties. Although these great progress have been made, most of SERS substrates are single functional. For the application viewpoint, it is of great prospect to develop multifunctional SERS substrates not only providing high SERS sensitivity but also showing other functions, for example, enrichment and recycling.

In recent years, hybrid nanostructures consisting of magnetic materials and noble metals have been of great interest, mainly because of the combination of magnetic and plasmonic properties. These multifunctional materials have wide application potentials in the field of recycled catalysis (Gan et al., 2013), SERS (Hu et al., 2010), bio imaging (Qu et al., 2013) and photothermal therapy (Wang et al., 2013; Ding et al., 2012). Thus, various binary or triplex micro-/nano- particles have been prepared by one-step or multi-step processes (Zhang et al., 2012; Zhou et al., 2010; Tang et al., 2015; Lv et al., 2010; Chen et al., 2013; Niu et al., 2016; Sun et al., 2016 ). For example, Jiang’s group have reported a facile one-step approach to synthesize rattle-type noble metal@Fe₃O₄ nanocomposites (Jiang et al., 2012). Zhai et al prepared Fe₃O₄ core/Au shell submicrometer spheres through the layer-by-layer technique which had very rough surfaces on the nanoscale for efficient surface-enhanced Raman scattering (Zhai et al., 2009). Ye’s group synthesized sea-urchin-like Fe₃O₄@C@Ag particles with efficient SERS property for organic pollutants detection (Ye et al., 2013). It has remained challenging to prepare triplex magnetic complex with excellent SERS activity.

It is well known that mesoporous materials with large surface area and excellent adsorption are usually used as enrichment elements of hybrid SERS substrates. Especially, TiO₂, a typical wide-band gap semiconductor nano-material, has become a potential candidate for SERS substrate, because it can enhance the Raman scattering via charge-transfer mechanism from the TiO₂ to the adsorbed mercapto group probe molecules. Furthermore, TiO₂ has also been recognized as the best photocatalysts, due to its high catalytic activity, low cost, long-term stability, non-toxicity and versatile degradation capability of organic pollutants (Thompson and Yates, 2006). Therefore, the introduction of mesoporous TiO₂ in the magnetic-based noble metal composites would promote multifunction of SERS substrates. However, the combination of Fe₃O₄, mTiO₂ and noble metals in a composite material system is still rarely reported (Zhang et al., 2014; Deng et al., 2008; Yan et al., 2016 ).

Herein, in this paper, we synthesized novel Fe₃O₄@mTiO₂@noble metal composite nanoparticles including Fe₃O₄@mTiO₂@Ag and Fe₃O₄@mTiO₂@Au NR which can be used as mutifuntional SERS substrates (as shown in scheme 1). Fe₃O₄ was prepared by a conventional solvothermal method, which acts as a magnetic core for enrichment and recycling. These composite NPs have a mesoporous TiO₂ interlayer prepared by sol–gel and hydrothermal process which can play a role of enrichment. Thus, we respectively prepared Fe₃O₄@mTiO₂@Ag by the in-situ reduction process and Fe₃O₄@mTiO₂@Au NR by electrostatic interaction with polyelectrolyte. The introduction of Ag and Au gives the composite NPs excellent SERS properties. Therefore, the multifunctional Fe₃O₄@mTiO₂@noble metal triplex core-shell composite NPs have great potential applications in rapid on-site detection of chemical, biological, and environment pollutants.

Schematic illustration of procedure for the fabrication of: Fe3O4@mTiO2@Ag and Fe3O4@mTiO2@Au NR composite NPs.

2 Experimental section

2.1

2.1 Chemicals and materials

Ferric chloride hexahydrate (FeCl₃·6H₂O), trisodium citrate, anhydrous sodium acetate, ethylene glycol (EG), acetonitrile, ammonia aqueous solution (28%), tetrahydrate chlorhexionic acid (HAuCl₄·4H₂O), silver nitrate (AgNO₃), Tetrabutyl titanate (TBOT), butylamine, sodium borohydride (NaBH₄), cetyltrimethylammonium bromide (CTAB), ascorbic acid (AA), thiram, Poly(diallyldimethylammonium chloride) (PDDA, Mw = 200000–350000, 20% in H₂O) and poly(styrenesulfonate) (PSS, Mw = ～70000, 30% in H₂O) were purchased from Shanghai Reagents Co (China). p-aminothiophnol (p-ATP), crystal violet (CV) and p-mercaptobenzoic acid (p-MBA) was supplied by Alfa-Aesar. All of the chemical reagents were of analytical grade and used as received without further purification. All glass wares were cleaned with aqua regia, thoroughly rinsed with deionized water, and dried prior to use.

2.2

2.2 Synthesis of Fe₃O₄ NPs

Fe₃O₄ NPs were synthesized by a modified solvothermal reaction according to previous report (Deng et al., 2008). Briefly, FeCl₃·6H₂O (1.08 g), trisodium citrate (0.2 g) and anhydrous trisodium acetate (1.2 g) were dissolved in ethylene glycol (20 mL) with magneitic stirring at room temperature for 0.5 h until all the reactants were fully dissolved. Then, the obtained yellow solution was transferred into a Teflon-lined stainless-steel autoclave and heated at 200℃ for 12 h. When the temperature drops to room temperature, the obtained Fe₃O₄ NPs were collected, then washed with deionized water and ethanol for several times. Finally, the obtained Fe₃O₄ NPs were dried under vacuum at 40℃ for 6 h and stored for future use.

2.3

2.3 Synthesis and pretreatment of core-shell Fe₃O₄@mTiO₂ NPs

The Fe₃O₄@TiO₂ core-shell NPs were synthesized as follows. The as-prepared Fe₃O₄ NPs (30 mg) were redispersed into a mixture of ethanol (90 mL), acetonitrile (30 mL) and ammonium hydroxide (0.5 mL, 28 wt%) under ultrasound for 15 min. Then 0.5 mL TBOT was dropwise added into the suspension under stirring for 1.5 h. The resultant products Fe₃O₄@TiO₂ were magnetically collected and washed with ethanol for at least 6 times. The Fe₃O₄@mTiO₂ NPs with porous TiO₂ shell were prepared through a hydrothermal method. Fe₃O₄@TiO₂ NPs were dispersed into a mixture of ethanol (20 mL), water (10 mL,) and ammonium hydroxide (2 mL). They were transferred into a Teflonlined stainless-steel autoclave at 160℃ for 20 h. At last, the obtained Fe₃O₄@mTiO₂ magnetic products were collected, washed with ethanol and dried under vacuum at 40℃ for 6 h for further use.

2.4

2.4 Preparation of Fe₃O₄@mTiO₂@Ag NPs

Typically, the freshly prepared 10 mg Fe₃O₄@mTiO₂ were dispersed into 15 mL 20 mM AgNO₃ solution under sonication for 30 min. Then 20 μL butylamine were added into the suspension and the reaction mixture was incubated for 50 min at 50 °C with vigorously mechanical stirring. Finally, the resulting Fe₃O₄@mTiO₂@Ag NPs were separated with an external magnet and rinsed with ethanol for several times, then were redispersed in deionized water for future use.

2.5

2.5 Preparation of Fe₃O₄@mTiO₂@Au NR

Fe₃O₄@mTiO₂@Au NR were prepared via a layer-by layer assembly approach (Ye et al., 2013; Ge et al., 2013). The suspension of Fe₃O₄@mTiO₂ (100 mg in 1 mL deionized water) was added to 99 mL of 2% PDDA solution containing 2 × 10⁻² M trisodium citrate and 2 × 10⁻² M NaCl under mechanical stirring at 30 ℃ for 1 h. The excess PDDA on Fe₃O₄@mTiO₂ core-shell NPs were washed with water at least six times. To modify the surface charge of Fe₃O₄@mTiO₂-PDDA (+) from positive to negative, the microspheres were further decorated with anionic PSS (Hu et al., 2005). The Fe₃O₄@mTiO₂-PDDA (+) magnetic NPs (40 mg) were dispersed in 40 mL PSS solution (3% in 2 × 10⁻² M NaCl) under mechanical stirring at 30 ℃ for 30 min. The residual PSS was removed with a magnet and the Fe₃O₄@mTiO₂-PSS (-) magnetic NPs were rinsed with water at least six times. Au NRs were synthesized in advance by a typical seed-growth method according to the literature as shown in S1 (ESI†) (Tang et al., 2015; Perez-Juste et al., 2004; Xu et al., 2012; Ye et al., 2012 ). The excess gold NRs were added to the above Fe₃O₄@mTiO₂-PSS (-) magnetic NPs in three-necked flask, mechanically stirred for 30 min. Then the product Fe₃O₄@mTiO₂@Au NR NPs were collected, washed, and finally dispersed in deionized water for future use.

2.6

2.6 Preparation of samples for SERS detection

First, a series of standard concentrations of CV (p-ATP, p-MBA and thiram) were prepared before use. The composite NPs suspension was added to the probe molecule solutions. After shaking for 1 h, take a drop of the mixture directly onto the silicon wafer for the collection of spectra in dispersed state. Then, insert another silicon wafer into the remaining mixture and the composite NPs were enriched onto the surface with the magnet. The photograph of Fe₃O₄@mTiO₂@Ag NPs magnetically enriched on the silicon wafer is shown in S2 (ESI†). The laser was applied to the enriched area for collecting the spectra in enriched state.

2.7

2.7 Characterization

The morphologies of the samples were observed with field emission scanning electron microscope images (FESEM) on a JEOL JSM-6300F SEM with a primary electron energy of 10 kV. Transmission electron microscopy (TEM) studies were performed with a JEOL-2010 microscope operated at an accelerating voltage of 200 kV with a tungsten filament. The phase and composition of the products were determined by a Rigaku D/Max-γA rotating-anode X-ray diffractometer equipped with monochromatic high-intensity Cu-Kα radiation (λ = 1.54187 Å). The Raman scattering spectra were recorded on a portable-Raman spectrometer (B&W TEK, i-Raman) equipped with a diode laser emitting at 785 nm, the laser power was 30 mW and the integration time was 5 s.

3 Results and discussion

3.1

3.1 The characterization of the as-prepared composite NPs

The strategy for preparing multifunctional Fe₃O₄@mTiO₂@noble metal composite NPs is shown in Scheme 1, involving three main steps: fabrication of magnetic Fe₃O₄ NPs as the core, coating amorphous TiO₂ onto the Fe₃O₄ NPs, and Ag nanoparticles or Au nanorods-assembling around the Fe₃O₄@mTiO₂ NPs. Fig. 1(a) shows the typical SEM image of uniform and monodispersed Fe₃O₄ NPs with a diameter of approximately 500 nm. Fig. 1(b) presents the SEM image of Fe₃O₄@TiO₂ NPs with a core-shell structure. The uniform gray amorphous TiO₂ shell with a thickness of about 30 nm can be clearly observed. Moreover, the thickness of amorphous TiO₂ shells can be well controlled by varying the amount of TOBT. After hydrothermal, a mesoporous TiO₂ shell on Fe₃O₄ NPs were obtained as shown in Fig. 1c. It can be seen obviously that the smoothly TiO₂ shell (as shown in Fig. 1b) was changed to mesoporous structure. In this work, the mesoporous TiO₂ shell on the Fe₃O₄ NPs not only improves the dispersibility of the magnetic NPs but also play a role of adsorption which is advantage for molecule and noble metal ions loading.

SEM images of the (a) Fe3O4 NPs, (b) Fe3O4@TiO2 core-shell NPs, (c) Fe3O4@mTiO2 core-shell NPs. The scale bar is 1 μm.

After loading noble metal, Fe₃O₄@mTiO₂@Ag composite NPs or Fe₃O₄@mTiO₂@Au NR composite NPs were obtained. Fig. 2a and b are the typical SEM and TEM images of the Fe₃O₄@mTiO₂@Ag composite NPs. They clearly illustrate that the silver nanoparticles are strongly anchored onto the surface of Fe₃O₄@mTiO₂ NPs, forming uniform three-ply micro-/nano- particles. In the fabrication process, the surfaces of Fe₃O₄@mTiO₂ NPs were amino-functionalized after hydrothermal, thus the assembly of the silver NPs on the surface of Fe₃O₄@mTiO₂ NPs can be obtained facilely by the reduction of AgNO₃ with butylamine under the action of the amino groups. In order to investigate the elemental distribution of the composite, the elemental mapping of Ti, Fe, Ag and O were performed by EDS area scans as shown in Fig. 2d–g. And EDS spectrum analysis can be exhibited in Fig. 2c, which indicates that as-prepared product consists of Fe, O, Ti and Ag. As a note, the Cu element was detected in the sample originating from copper grids. From Fig. 2f, the profile of Ag La1 is similar with but slightly larger than those of Ti Ka1 (d), Fe Ka1 (e) and O Kal (g), indicating that the Ag element distributes homogeneously around the mTiO₂ interlayer.

Typical SEM image of Fe3O4@mTiO2@Ag composite NPs (a). Typical TEM image of Fe3O4@mTiO2@Ag composite NPs (b). EDS spectrum of Fe3O4@mTiO2@Ag composite NPs (c). Elemental mappings of Ti Ka1 (d), Fe Ka1 (e), Ag La1 (f) and O Ka1 (g).

Meanwhile, the morphology and composition of the as-prepared Fe₃O₄@mTiO₂@Au NR composite NPs were also characterized by SEM, TEM and EDS in Fig. 3. As can be seen from Fig. 3a and b, Au NRs were assembled on the surface of Fe₃O₄@mTiO₂ NPs with large quantity and good uniformity. Fig. 3c is the EDS spectrum analysis and Fig. 3d–g are the elemental mapping of O, Ti, Fe and Au performed by EDS area scans. They demonstrated that the as-prepared product consists of Fe, O, Ti and Au, plus Au element distributes homogeneously around the mTiO₂ interlayer. In this work, Au NRs used were protected with CTAB and the surface was positively charged, while the surface of Fe₃O₄@mTiO₂ NPs were negatively charged due to modification with PDDA and PSS in turn, thus making the assembly of Au NRs on the surfaces of Fe₃O₄@mTiO₂ NPs more easily based on the electrostatic interaction.

Typical SEM image of Fe3O4@mTiO2@Au NRs composite NPs (a). Typical TEM image of Fe3O4@mTiO2@Au NR composite NPs (b). EDS spectrum of Fe3O4@mTiO2@Au NR composite NPs (c). Elemental mappings of O Ka1 (d), Ti Ka1 (e), Fe Ka1 (f) and Au La1 (g).

Furthermore, the crystal structure and phase purity of the as-prepared products were demonstrated by X-ray diffraction (XRD). Fig. 4 shows the XRD patterns of Fe₃O₄ NPs, Fe₃O₄@mTiO₂ NPs, Fe₃O₄@mTiO₂@Ag composite NPs, and Fe₃O₄@mTiO₂@Au NR composite NPs. The specific XRD of Fe₃O₄ (Fig. 4a) is characterized by six peaks at the 2θ degree of 30.0°, 35.3°, 42.9°, 53.5°, 56.9 and 62.4°, which correspond to the (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1), and (4 4 0) lattice planes of the cubic phase of Fe₃O₄ (JCPDS card no. 01-075-0449), respectively. Compared with Fig. 4a, it can be clearly seen from Fig. 4b that there are four additional peaks located at 25.3°, 37.9°, 48.0° and 53.9° which match with the (1 0 1), (0 0 4), (2 0 0) and (1 0 5) planes of the anatase phase (JCPDS card no. 01-075-2545). After loading the Ag NPs or Au NRs on the surface of the Fe₃O₄@mTiO₂ NPs, there were several additional diffraction peaks appeared in Fig. 4c and d, corresponding to the cubic phase of Ag phase (JCPDS card no. 00-004-0783) and cubic phase of Au (JCPDS card no. 00-004-0784) respectively. Meanwhile, the XRD peaks of the Fe₃O₄@mTiO₂ NPs became weak and nearly disappeared, which clearly reflects the formation of Ag NPs or Au NRs and large coverage on the surface of Fe₃O₄@mTiO₂ NPs.

XRD patterns of the (a) Fe3O4 NPs, (b) Fe3O4@mTiO2 NPs, (c) Fe3O4@mTiO2@Ag composite NPs and (d) Fe3O4@mTiO2@Au NR composite NPs.

3.2

3.2 SERS performances of Fe₃O₄@mTiO₂@Ag composite NPs

Fe₃O₄@mTiO₂@Ag composite NPs are expected to exhibit excellent SERS enhancements. Therefore, to evaluated their potential application as SERS substrate, CV and p-ATP were chosen as the probe molecules. The process of SERS detection in dispersion and enriched state is shown in Fig. 5A. The two SERS spectra comparison chart is shown in Fig. 5B. It is clear that the Raman intensity of enriched state is obviously stronger than the dispersion state. That’s because the composite NPs were tightly gathered together which greatly increased the number of plasmonic hot spots. In the paper, we all used the assembled particles for further SERS investigation.

(A) Schematic diagram of SERS acquisition process in the dispersion and enrichment state; (B) SERS spectra of 1.0 × 10−6 M CV adsorbed on the Fe3O4@mTiO2@Ag NPs in dispersion state (a) and magnetic enrichment state (b).

The performance of Fe₃O₄@mTiO₂@noble metal composite NPs as SERS-active substrates was initially investigated by using a portable Raman spectrometer. CV, p-ATP and p-MBA were chose as probe molecules. Fig. 6A shows the SERS spectra of CV with different concentrations adsorbed on the Fe₃O₄@mTiO₂@Ag composite NP substrate. The bands at 912, 1173 and 1388 cm⁻¹ are assigned to the ring skeletal vibrations, C—H in-plane bending vibrations and N-phenyl stretching of CV respectively (Osawa et al., 1994). The bands at 1535, 1588 and 1617 cm⁻¹ are attributed to ring C—C stretching (Orendorff et al., 2006). The spectral intensities and resolutions are decreased by diluting the concentrations of the probe molecules. it is found that the CV peaks still appeared at a low concentration of 1.0 × 10⁻⁹ M, indicating that this SERS substrate is highly sensitive and promising for detection of other probe molecules. Therefore, we also examined the SERS sensitivity of the Fe₃O₄@mTiO₂@Ag composite NPs for detection of p-ATP molecules. Fig. 6B shows the SERS spectra of p-ATP with different concentrations adsorbed on the Fe₃O₄@mTiO₂@Ag composite NP substrate. All the peaks are attributed to p-ATP molecules (Osawa et al., 1994). The two strong peaks appearing at 1073 and 1580 cm⁻¹ which are originated from the a₁ vibrational modes can be assigned to C—S stretching vibrations and parallel C—C stretching vibrations. The other three peaks at 1142, 1389 and 1438 cm⁻¹ can be attributed to the b₂ vibrational modes (Orendorff et al., 2006). Clearly, the Fe₃O₄@mTiO₂@Ag composite NPs substrate can detect the p-ATP as low as 1.0 × 10⁻¹² M. these results indicated that the Fe₃O₄@mTiO₂@Ag composite NPs substrate can achieve the trace detection of analytes. The main reason for such high SERS sensitivity due to the presence of plentiful “hot spots” produced by magnetic aggregation and target molecules enrichment by mesoporous TiO₂ adsorption.

SERS spectra of CV (A) and p-ATP (B) with different concentrations adsorbed on Fe3O4@mTiO2@Ag composite NPs. The spectra were collected by i-Raman portable spectrometer with 785 nm laser excitation.

3.3

3.3 SERS performances of Fe₃O₄@mTiO₂@Au NR composite NPs

The SERS activities of Fe₃O₄@mTiO₂@Au NR composite NPs were also studied using p-MBA and p-ATP as probe molecules. Fig. 7 shows the SERS spectra of p-MBA and p-ATP with different concentrations adsorbed on the Fe₃O₄@mTiO₂@Au NR composite NP substrate. In Fig. 7A, the two strong SERS peaks appearing at 1074 and 1589 cm⁻¹ are assigned to ν_8a and ν₁₂ aromatic ring vibrations, respectively (Guo et al., 2009). Their SERS intensities gradually increase with the increase of concentration of p-MBA. The characteristic bands of p-MBA can be identified clearly at the concentration as low as 1.0 × 10⁻⁹ M. We examined the SERS activity of Fe₃O₄@mTiO₂@Au NR composite NPs for the detection of p-ATP molecules. The characteristic bands of p-ATP appeared in Fig. 7B and the concentration can be detected down to 1.0 × 10⁻¹⁰ M. The results both indicate that Fe₃O₄@mTiO₂@Au NR composite NPs show high SERS activity and detection sensitivity. In previous reports, Orendorff et al. proved that enhancement factors are 10–100 greater for the gold NRs that have surface plasmon resonance (SPR) band overlap with the excitation line than for the gold NRs whose SPR bands do not (Orendorff et al., 2006). The gold NRs were used in the paper with longitudinal SPR peak at 787 nm which was resonance with the 785 nm excitation line, leading to the greater SERS enhancement. While for p-ATP detection, the enhancement of Fe₃O₄@mTiO₂@Au NR was weaker than that of Fe₃O₄@mTiO₂@Ag composite NPs. This may be because the initial SERS enhancement of silver is stronger than gold. Nevertheless, due to the high stability, low toxicity, bio-affinity and excellent photothermal conversion of gold NRs, the Fe₃O₄@mTiO₂@Au NR composite NPs can be widely used in the field of life sciences.

SERS spectra of p-MBA (A) and p-ATP (B) with different concentrations adsorbed on Fe3O4@mTiO2@Au NR composite NPs. The spectra were collected by i-Raman portable spectrometer with 785 nm laser excitation.

3.4

3.4 Reproducible Fe₃O₄@mTiO₂@noble metal composite SERS substrates for ultrasensitive detection

The reproducibility and stability of the as-prepared Fe₃O₄@mTiO₂@noble metal composite SERS substrates were also demonstrated by collecting twenty points of SERS spectra on the randomly selected area (50 μm × 50 μm) of the substrates. The concentration of CV, p-ATP and p-MBA used in the experiment is 1.0 × 10⁻⁷ M. Fig. 8A showed a series of the SERS spectra of p-ATP collected on randomly selected 20 points of the Fe₃O₄@mTiO₂@Ag substrate. The major characteristic peaks intensity distributions at 1073, 1438 and 1580 cm⁻¹ are shown in Fig. 8B–D. The corresponding RSD values for these peaks were calculated as 3.80%, 2.71% and 5.68% respectively. A series of the SERS spectra of CV were also collected on randomly selected 20 points of the substrate and the RSD values for the main peaks at 912, 1173 and 1588 cm⁻¹ were calculated as 8.65%, 11.52% and 13.34% in S4 (ESI†). Fig. 9. and S4 (ESI†) showed separately of p-ATP and p-MBA collected on randomly selected 20 points of the Fe₃O₄@mTiO₂@Au NR substrate. The RSD values for main Raman vibrations of p-ATP at 1073, 1438 and 1580 cm⁻¹ are calculated as 12.98%, 14.80% and 13.48% respectively, while the RSD values for main Raman vibrations of p-MBA at 1073, and 1589 cm⁻¹ are calculated as 11.98% and 13.51% respectively. All the RSD values were consistently less than 15.00% which further indicated the as-prepared Fe₃O₄@mTiO₂@noble metal composite SERS substrates were uniform and reproducible across the entire area (Zhang et al., 2018). However, the uniformity and reproducibility of Fe₃O₄@mTiO₂@Au NR was inferior to that of Fe₃O₄@mTiO₂@Ag. This was possibly because the spherical Ag was more easily and densely coated on the surface of the TiO₂ shell than the rod-shaped Au.

(A) A series of the SERS spectra of 1.0 × 10−7 M p-ATP collected on randomly selected 20 points of the Fe3O4@mTiO2@Ag substrate; (B-D) The intensity distribution histogram respectively of the 1073, 1438 and 1580 cm−1 peak of p-ATP.

(A) A series of the SERS spectra of 1.0 × 10−7 M p-ATP collected on randomly selected 20 points of the Fe3O4@mTiO2@Au NR substrate; (B-D) The intensity distribution histogram respectively of the 1073, 1438 and 1580 cm−1 peak of p-ATP.

3.5

3.5 SERS detection of pesticides with Fe₃O₄@mTiO₂@Ag composite NPs

Based on the excellent detection sensitivity and reproducibility, in order to investigate the feasibility of practical application of the Fe₃O₄@mTiO₂@Ag composite NPs substrates, thiram was chose as a target analyst for rapid detection. Thiram, as a particular dithiocarbamate fungicide with dithiocarbamate group, are widely used on food crops. Excessive use can cause environmental pollution and pesticide residue problems. Thiram molecule has an S—S bond which can be spontaneously broken when encounters silver NPs and binds to the surface of silver NPs by forming Ag—S bond. This provide the possibility for its identification and detection by SERS techniques. Fig. 10 illustrates series of SERS spectra of thiram at different concentrations. There were four main characteristic peaks of thiram respectively at 560, 1145, 1383, and 1516 cm⁻¹ (Guo et al., 2015). The strongest peak at 1383 cm⁻¹ is attributed to the CN stretching mode and symmetric CH₃ deformation mode. The peak at 560 cm⁻¹ is attributed to S—S stretching vibration modes, and those at 1145 and 1516 cm⁻¹ to the CN stretching vibrations and the rocking CH₃ mode, respectively. The concentration of thiram can be detected as low as 5.0 × 10⁻⁸ M (about 0.012 ppm), which is lower than the maximal residue limit of 7 ppm in fruit prescribed by the U.S. Environmental Protection Agency. In addition, the Fe₃O₄@mTiO₂@Ag composite NPs are magnetic, easy to separate and enrich from the sample, which has potential application in the field of pesticide on-site detection.

A series of the SERS spectra of thiram with different concentrations collected on Fe3O4@mTiO2@Ag composite NPs. The spectra were collected by i-Raman portable spectrometer with 785 nm laser excitation.

4 Conclusions

In conclusion, we have successfully synthesized the multifunctional Fe₃O₄@mTiO₂@Ag and Fe₃O₄@mTiO₂@Au NR composite triplex core-shell NPs. The as-prepared composite NPs comprised of Fe₃O₄ core providing good magnetism, mesoporous TiO₂ interlayer for adsorption of the probe molecules, and a dense Ag or Au NRs shell providing sufficient plasmonic hot spots and further capture of the probe molecules. Raman experiments indicated that the as-prepared composite NPs showed high SERS sensitivity and good reproducibility to CV, p-ATP and p-MBA molecules. Finally, the Fe₃O₄@mTiO₂@Ag composite NPs were applied in pesticide testing and the detection limit is 5.0 × 10⁻⁸ M. We believe that these composite NPs can be potentially applied in rapid trace detection of chemical, biological and hazardous molecules.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (No. 61378038 and 61875255), the Major/Innovative Program of Development Foundation of Hefei Center for Physical Science and Technology (No. 2018ZYFX005), the Dean Foundation of Hefei Institute of Physical Science, Chinese Academy of Sciences (No. YZJJ201514), the “Thirteenth Five-Year“ National Key Research and Development Program of China (No. 2017YFD0700501).

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Appendix A

Supplementary material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.arabjc.2019.01.007.

Appendix A

Supplementary material

The following are the Supplementary data to this article:

Supplementary Data 1

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Show Sections

[1] Bao F., Li J.F., Ren B., Yao J.L., Gu R.A., Tian Z.Q., . Synthesis and characterization of Au@Co and Au@Ni core-shell nanoparticles and their applications in surface-enhanced Raman spectroscopy. J. Phys. Chem. C. 2008;112(2):345-350.
[Google Scholar]

[2] Chen H., Wang Z.Y., Ma X.Q., Zong S.F., Cui Y.P., . Magnetically controllable dual-mode nanoprobes for cell imaging with an onion-liked structure. Talanta. 2013;116:978-984.
[Google Scholar]

[3] Deng Y.H., Qi D.W., Deng C.H., Zhang X.M., Zhao D.Y., . Superparamagnetic high-magnetization microspheres with an Fe₃O₄@SiO₂ core and perpendicularly aligned mesoporous SiO₂ shell for removal of microcystins. J. Am. Chem. Soc.. 2008;130(1):28-29.
[Google Scholar]

[4] Ding Q.Q., Liu H.L., Yang L.B., Liu J.H., . Speedy and surfactant-free in situ synthesis of nickel/Ag nanocomposites for reproducible SERS substrates. J. Mater. Chem.. 2012;22(37):19932-19939.
[Google Scholar]

[5] Gan Z.B., Zhao A.W., Zhang M.F., Tao W.Y., Guo H.Y., Gao Q., Mao R.R., Liu E.H., . Controlled synthesis of Au-loaded Fe₃O₄@C composite microspheres with superior SERS detection and catalytic degradation abilities for organic dyes. Dalton T.. 2013;42:8597-8605.
[Google Scholar]

[6] Ge X.X., Tao Y., Zhang A.D., Lin Y.H., Du D., . Electrochemical detection of dual exposure biomarkers of organophosphorus agents based on reactivation of inhibited cholinesterase. Anal. Chem.. 2013;85(20):9686-9691.
[Google Scholar]

[7] Guo H.Y., Ruan F.X., Lu L.H., Hu J.W., Pan J.G., Yang Z.L., Ren B., . Correlating the shape, surface plasmon resonance, and surface-enhanced Raman scattering of gold nanorods. J. Phys. Chem. C. 2009;113(24):10459-10464.
[Google Scholar]

[8] Guo H.Y., Zhao A.W., Wang R.J., Wang D.P., Wang L.S., Gao Q., Sun H.H., Li L., He Q.Y., . J. Nanopart. Res.. 2015;17(12):494.

[9] Hu H.B., Wang Z.H., Pan L., Zhao S.P., Zhu S.Y., . Ag-coated Fe₃O₄@SiO₂ three-ply composite microspheres: synthesis, characterization, and application in detecting melamine with their surface-enhanced Raman scattering. J. Phys. Chem. C. 2010;114(17):7738-7742.
[Google Scholar]

[10] Hu X., Cheng W., Wang T., Wang Y., Wang E., Dong S.J., . Fabrication, characterization, and application in SERS of self-assembled polyelectrolyte-gold nanorod multilayered film. J. Phys. Chem. B. 2005;109:19385-19389.
[Google Scholar]

[11] Huang Z.L., Meng G.W., Huang Q., Chen B., Lu Y.L., Wang Z.M., Zhu X.G., Sun K.X., . Surface-enhanced Raman scattering from plasmonic Ag-nanocube@Au-nanospheres core@satellites. J. Raman Spectrosc.. 2017;48(2):217-223.
[Google Scholar]

[12] Jiang W.Q., Zhou Y.F., Zhang Y.L., Xuan S.H., Gong X.L., . Superparamagnetic Ag@Fe₃O₄ core-shell nanospheres: fabrication, characterization and application as reusable nanocatalysts. Dalton Trans.. 2012;41(15):4594-4601.
[Google Scholar]

[13] Lane L.A., Qian X.M., Nie S.M., . SERS nanoparticles in medicine: from label-free detection to spectroscopic tagging. Chem. Rev.. 2015;115:10489-10529.
[Google Scholar]

[14] Li J.F., Zhang W., Zhou Z.Y., Wu D.Y., Ren B., Wang Z.L., Tian Z.Q., . Shell-isolated nanoparticle-enhanced Raman spectroscopy. Nature. 2010;464 392–395. [12]
[Google Scholar]

[15] Liao W., Lu X.N., . Determination of chemical hazards in foods using surface-enhanced Raman spectroscopy coupled with advanced separation techniques. Trends Food Sci. Technol.. 2016;54:103-113.
[Google Scholar]

[16] Liu M., Wang Z.Y., Zong S.F., Che H., Zhu D., Wu L., Hu G.H., Cui Y.P., . SERS detection and removal of mercury(Ⅱ)/silver(Ⅰ) using oligonucleotide-functionalized core/shell magnetic silica sphere@Au nanoparticles. ACS Appl. Mater. Inter.. 2014;6(10):7371-7379.
[Google Scholar]

[17] Liu Z., Cheng L., Zhang L., Jing C., Shi X., Yang Z.B., Long Y.T., Fang J.X., . Large-area fabrication of highly reproducible surface enhanced Raman substrate via a facile double sided tape-assisted transfer approach using hollow Au-Ag alloy nanourchins. Nanoscale. 2014;6:2567-2572.
[Google Scholar]

[18] Lv B.L., Xu Y., Tian H., Wu D., Sun Y.H., . Synthesis of Fe₃O₄\SiO₂\Ag nanoparticles and its application in surface-enhanced Raman scattering. J. Solid State Chem.. 2010;183(12):2968-2973.
[Google Scholar]

[19] Niu C.Y., Zou B.F., Wang Y.Q., Cheng L., Zheng H.H., Zhou S.M., . Highly sensitive and reproducible SERS performance from uniform film assembled by magnetic noble metal composite microspheres. Langmuir. 2016;32:858-863.
[Google Scholar]

[20] Orendorff C.J., Gearheart L., Jana N.R., Murphy C.J., . Aspect ratio dependence on surface enhanced Raman scattering using silver and gold nanorod substrates. J. Phys. Chem. Chem. Phys.. 2006;8:165-170.
[Google Scholar]

[21] Osawa M., Matsuda N., Yoshii K., Uchida I., . Charge transfer resonance Raman process in surface-enhanced Raman scattering from p-aminothiophenol adsorbed on silver: Herzberg-Teller contribution. J. Phys. Chem.. 1994;98(48):12702-12707.
[Google Scholar]

[22] Perez-Juste J., Liz-Marzan L.M., Carnie S., Chan D.Y.C., Mulvaney P., . Electric-field-directed growth of gold nanorods in aqueous surfactant solutions. Adv. Funct. Mater.. 2004;14:571-579.
[Google Scholar]

[23] Qu H., Lai Y., Niu D., Sun S., . Surface-enhanced Raman scattering from magneto-metal nanoparticle assemblies. Anal. Chim. Acta. 2013;763:38-42.
[Google Scholar]

[24] Sun Z.L., Du J.J., Yan L., Chen S., Yang Z.L., Jing C.Y., . Multifunctional Fe₃O₄@SiO₂-Au satellite structured SERS probe for charge selective detection of food dyes. ACS Appl. Mater. Interf.. 2016;8:3056-3062.
[Google Scholar]

[25] Tang X.H., Dong R.L., Yang L.B., Liu J.H., . Fabrication of Au nanorod-coated Fe₃O₄ microspheres as SERS substrate for pesticide analysis by near-infrared excitation. J. Raman Spectrosc.. 2015;46(5):470-475.
[Google Scholar]

[26] Tao W.Y., Zhao A.W., Sun H.H., Gan Z.B., Zhang M.F., Li D., Guo H.Y., . Periodic silver nanodishes as sensitive and reproducible surface-enhance Raman scattering substrates. RSC Adv.. 2014;4(7):3487-3493.
[Google Scholar]

[27] Thompson T.L., Yates J.T., . Surface science studies of the photoactivation of TiO₂-new photochemical processes. Chem. Rev.. 2006;106(10):4428-4453.
[Google Scholar]

[28] Wang J.P., Yang L., Liu B.H., Jiang H.J., Liu R.Y., Yang J.W., Han G.M., Mei Q.S., Zhang Z.P., . Inkjet-printed silver nanoparticle paper detects airborne species from crystalline explosives and their ultratrace residues in open environment. Anal. Chem.. 2014;86(7):3338-3345.
[Google Scholar]

[29] Wang Y., Wang K., Zou B., Gao T., Zhang X., Du Z., Zhou S., . Magnetic-based silver composite microspheres with nanosheet-assembled shell for effective SERS substrate. J. Mater. Chem. C. 2013;1(13):2441-2447.
[Google Scholar]

[30] Xia X.H., Zeng J., McDearmon B., Zheng Y.Q., Li Q.G., Xia Y.N., . Silver nanocrystals with concave surfaces and their optical and surface-enhanced Raman scattering properties. Angew. Chem. Int. Ed.. 2011;50:12542-12546.
[Google Scholar]

[31] Xu L., Kuang H., Xu C., Ma W., Wang L., Kotov N.A., . Regiospecific plasmonic assemblies for in situ Raman spectroscopy in live cells. J. Am. Chen. Soc.. 2012;134(3):1699-1709.
[Google Scholar]

[32] Xu S.C., Man B.Y., Jiang S.Z., Wang J.H., Wei J., Xu S.D., Liu H.P., Gao S.B., Liu H.L., Li Z.H., Li H.S., Qiu H.W., . Graphene/Cu nanoparticle hybrids fabricated by chemical vapor deposition as surface-enhanced Raman scattering substrate for label-free detection of adenosine. ACS Appl. Mater. Interf.. 2015;7:10977-10987.
[Google Scholar]

[33] Xu S.C., Zhan J., Man B.Y., Jiang S.Z., Yue W.W., Gao S.B., Guo C.G., Liu H.P., Li Z.H., Wang J.H., Zhou Y.Q., . Real-time reliable determination of binding kinetics of DNA hybridization using a multi-channel grapheme biosensor. Nat. Commun.. 2017;8:14902.
[Google Scholar]

[34] Yan L., Wang Y.F., Li J., Shen H.D., Wang C., Yang S.B., . A facile approach for the synthesis of sea urchin-like Fe₃O₄@TiO₂@Ag nanocomposites as highly efficient and recyclable photocatalysts. J. Mater. Sci-Mater. El.. 2016;27:10616-10621.
[Google Scholar]

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[Google Scholar]

[36] Ye Y.J., Chen J., Ding Q.Q., Lin D.Y., Dong R.L., Yang L.B., Liu J.H., . Sea-urchin-like Fe₃O₄@C@Ag particles: an efficient SERS substrate for detection of organic pollutants. Nanoscale. 2013;5(13):5887-5895.
[Google Scholar]

[37] Zhai Y.M., Zhai J.F., Wang Y.L., Guo S.J., Ren W., Dong S.J., . Fabrication of iron oxide core/gold shell submircrometer spheres with nanoscale surface roughness for efficient surface-enhanced Raman scatrering. J. Phys. Chem. C. 2009;113(17):7009-7014.
[Google Scholar]

[38] Zhang M.F., Meng J.T., Wang D.P., Tang Q., Chen T., Rong S.Z., Liu J.Q., Wu Y.C., . Biomimetic synthesis of hierarchical 3D Ag butterfly wing scale arrays/grapheme composites as ultrasensitive SERS substrates for efficient trace chemical detection. J. Mater. Chem. C. 2018;6:1933-1943.
[Google Scholar]

[39] Zhang X.Q., Zhu Y.H., Yang X.L., Zhou Y., Yao Y.F., Li C.Z., . Multifunctional Fe₃O₄@TiO₂@Au magnetic microspheres as recyclable substrates for surface-enhance Raman scattering. Nanoscale. 2014;6:5971-5979.
[Google Scholar]

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[Google Scholar]

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[Google Scholar]

[42] Zong S.F., Wang Z.Y., Yang J., Wang C.L., Xu S.H., Cui Y.P., . A SERS and fluorescence dual mode cancer cell targeting probe based on silica coated Au@Ag core-shell nanorods. Talanta. 2012;97:368-375.
[Google Scholar]

Multifunctional Fe₃O₄@mTiO₂@noble metal composite NPs as ultrasensitive SERS substrates for trace detection