Facile Synthesis of Au-Coated Magnetic Nanoparticles and Their

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Facile synthesis of Au-coated magnetic nanoparticles and their application in bacteria detection via a SERS method Junfeng Wang, Xuezhong Wu, Chongwen Wang, Zhen Rong, Hongmei Ding, Hui Li, Shaohua Li, Ningsheng Shao, Peitao Dong, Rui Xiao, and Sheng-Qi Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07528 • Publication Date (Web): 15 Jul 2016 Downloaded from http://pubs.acs.org on July 16, 2016

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ACS Applied Materials & Interfaces

Facile synthesis of Au-coated magnetic nanoparticles and their application in bacteria detection via a SERS method Junfeng Wang,†, § Xuezhong Wu,†, ‡ Chongwen Wang,§, ┴ Zhen Rong,§ Hongmei Ding,ǁ Hui Li,ǁ Shaohua Li,ǁ Ningsheng Shaoǁ, Peitao Dong,†, * Rui Xiao,§, * Shengqi Wang§, * † College of Mechatronics and Automation and ‡ Laboratory of Science and Technology on Integrated Logistics Support, National University of Defense Technology, Changsha 410073, Hunan, P. R. China. § Beijing Institute of Radiation Medicine, Beijing 100850, P. R. China. ǁ Beijing Institute of Basic Medical Sciences, Beijing 100850, P. R. China. ┴ College of Life Sciences & Bio–Engineering, Beijing University of Technology, Beijing 100124, P. R. China.

Corresponding Author * Author to whom correspondence should be addressed. (P.T. D.) TEL.: +86–731–84574958; E–mail: [email protected] (R. X.) TEL.: +86–10–66931422–5; E–mail: [email protected] (S.Q. W.) TEL: +86–10–66932211; E–mail:[email protected]

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ABSTRACT This study proposes a facile method for synthesis of Au-coated magnetic nanoparticles (AuMNPs) core/shell nanocomposites with nanoscale rough surfaces. MnFe2O4 nanoparticles (NPs) were first modified with a uniform polyethyleneimine layer (2 nm) through selfassembly under sonication. The negatively charged Au seeds were then adsorbed on the surface of the MnFe2O4 NPs through electrostatic interaction for Au shell formation. Our newly developed sonochemically assisted hydroxylamine seeding growth method was used to grow the adsorbed gold seeds into large Au nanoparticles (AuNPs) to form a nanoscale rough Au shell. Au-coated magnetic nanoparticles (AuMNPs) were obtained from the intermediate product (Au seeds decorated magnetic core) under sonication within 5 min. The AuMNPs were highly uniform in size and shape and exhibited satisfactory surface-enhanced Raman scattering (SERS) activity and strong magnetic responsivity. PATP was used as a probe molecule to evaluate the SERS performance of the synthesized AuMNPs with a detection limit of 10−9 M. The synthesized AuMNPs were conjugated with Staphylococcus aureus (S. aureus) antibody for bacteria capture and separation. The synthesized plasmonic AuNR– DTNB NPs, whose LSPR wavelength was adjusted to the given laser excitation wavelength (785 nm), were conjugated with S. aureus antibody to form a SERS tag for specific recognition and report of the target bacteria. S. aureus was indirectly detected through SERS based on sandwich-structured immunoassay, with a detection limit of 10 cells/mL. Moreover, the SERS intensity at Raman peak of 1331 cm−1 exhibited a linear relationship to the logarithm of bacteria concentrations ranging from 101 cells/mL to 105 cells/mL.

KEYWORDS Au-coated magnetic nanoparticles, AuNRs, surface-enhanced Raman spectroscopy, antibody, Staphylococcus aureus


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Introduction Both magnetic and noble-metal nanoparticles (NPs) have been extensively explored due to their wide range of potential applications.1-9 Superparamagnetic NPs have gained increased interest because of their special magnetic properties, which enable the particles to be manipulated conveniently by an external magnet and rapidly redispersed in the absence of magnetic field. Thus, superparamagnetic NPs can be used to catch and concentrate targets with the assistance of an external magnetic field, instead of centrifugation, to avoid sample aggregation. Noble metal NPs, especially Au nanoparticles (AuNPs), have also received increased attention because of their biocompatibility and facile biochemical modification and conjugation. The nanoscale rough surface of Au can amplify the Raman signal of the adsorbed molecules. The combination of a magnetic core and metallic shell in a nanocomposite could form a unique multifunctional material, such as Fe3O4@Au, Fe3O4@Ag, and [email protected] In particular, Au-coated magnetic nanoparticles (AuMNPs) with good magnetic responsivity, enhanced stabilization and biofunctionalization have extended the application of such nanocomposites. Therefore, various synthetic routes have been explored to prepare Au-coated magnetic nanoparticles (AuMNPs) in the past decades. Previous studies have been reported to synthesize small AuMNPs (smaller than 20 nm).14-17 For example, Zhong’s group used thermally activated processing protocol to synthesize AuMNPs with a mean diameter of 6.3 nm.16-17 The thermal processing treatment of AuMNPs involved molecular desorption, nanocrystal core coalescence, and molecular reencapsulation in the evolution of nanoparticle precursors at high temperature (149 °C). The entire experiment was conducted in toluene at a very high temperature for 1 h and could be risky for the conductor. The weak magnetic responsivity of small AuMNPs causes slow and low magnetic enrichment. As such, small AuMNPs are unsuitable for capture and fast


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separation of biological samples, especially bacteria (size of several hundred nanometers). In addition, the SERS-based sandwich immunoassay for the detection of bacteria is based on the formation of a sandwich structure of SERS tags/bacteria/AuMNPs (Scheme 1). AuMNPs are used to capture the target bacteria, and the SERS tags are used to report the presence of the target bacteria. Small AuMNPs are densely attached on the bacterial surface for fast and effective separation of bacteria, thereby affecting subsequent binding to SERS tags. Small AuMNPs are unsuitable for capture and fast separation of bacteria because of their weak magnetic responsivity and steric hindrance. Various synthetic methods have been explored to prepare large AuMNPs (larger than 100 nm). Bao et al. synthesized bifunctional Fe3O4-Au nanoparticles by linking two separately prepared Fe3O4 and AuNPs through chemical bonds (Au-S).18 Qiu et al. prepared Fe3O4-Au NPs through electrostatic interaction between the positively charged Fe3O4 NPs and negatively charged Au NPs.19 AuMNPs, which were prepared by directly connecting the magnetic core and the gold nanoparticles, did not possess a continuous shell and had partially exposed magnetic cores, causing a relatively poor SERS effect. Ji et al. coated the Fe3O4 core by a silica shell which can be functionalized with amino groups to facilitate the deposition of gold seeds and further covered with an outer gold shell by reduction of K-gold solution (HAuCl4 0.015% wt%, K2CO3 0.025% wt%) with formaldehyde.20 This method, with the assistance of a silicon shell interlayer, causes relatively weak magnetic responsivity. Li et al. developed a facile one-pot hydrothermal approach to synthesize Fe3O4@Au NPs.21 However, this process exhibits poor controllability and uniform particle size of the synthesized AuMNPs. Iterative hydroxylamine seeding is another method used to synthesize AuMNPs. Thermodynamically, hydroxylamine or hydroxylamine hydrochloride can reduce gold chloride acid to elemental gold (Au0); gold particle surfaces can accelerate the occurrence of this reaction. Thus, newly reduced Au0 can be adsorbed on the surfaces of existing Au seeds


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and grows into large NPs, rather than new nucleation particles. Hydroxylamine seeding growth method was first used to synthesize colloidal Au particles of mean diameters between 20 and 100 nm based on 12 nm Au seeds by Natan’s group in 2000.22 AuNPs produced by this method exhibit uniform size and shape because of separation of nucleation and growth stages. In 2004, Lyon et al. synthesized AuMNPs for the first time by modifying the hydroxylamine seeding procedure of Natan; the modified approach is named as iterative hydroxylamine seeding, which includes five repeat hydroxylamine hydrochloride and chloroauric acid addition process (with at least 10 min between additions).23 Although the entire experimental process is cumbersome and time-consuming, this iterative hydroxylamine seeding growth method has been an important synthetic route for the synthesis of gold shellcoated magnetic nanoparticles. For example, Gu et al. synthesized Fe2O3/Au core/shell nanoparticles by deposition of Au on the synthesized Fe2O3 nanoparticles via Lyon’s method.24 The five-addition process of Lyon’s iterative hydroxylamine seeding procedure was shortened into a three-addition one. However, at least 50 min stirring is required after each addition. Zhou et al. synthesized cluster/shell Fe3O4/Au nanoparticles based on Lyon’s iterative hydroxylamine seeding growth with some modifications; the synthesized cluster/shell Fe3O4/Au nanoparticles were applied to free-PSA detection through a SERS method.25 However, the application of iterative hydroxylamine seeding is restricted by its complex procedure and time-consuming preparation process. Therefore, a facile synthesis of magnetic NPs with a continuous gold shell that exhibits good dispersity and strong magnetic responsivity remains a challenge. Surface-enhanced Raman scattering (SERS) spectroscopy is an important analytical technique for biosensing and trace detection because of its enormous Raman enhancement.2633

Because of its high sensitivity and finger-printing capability, SERS has been applied in


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various fields such as the detection of pesticide residue, DNA and bacteria34-41 The key of SERS application lies in the preparation of high-performance SERS substrates. Staphylococcus aureus (S. aureus) is an important human pathogen that can cause various diseases such as abscesses, pneumonia, meningitis, endocarditis, and septicemia. S. aureus can also produce different enterotoxins that can cause food poisoning and is ranked as one of the top five pathogens that cause food-borne illnesses worldwide. Therefore, a rapid, sensitive, and specific detection method for S. aureus must be developed. The time consuming and laborious conventional detection methods for S. aureus involve repeated enrichment, colony isolation, and various biochemical and serological identification tests. This process usually takes days to identify the pathogenic bacteria, and such a delay is unacceptable when facing emergencies, such as life-threatening diseases. Previous studies focused on developing new detection methods for S. aureus; such methods include enzymelinked immunosorbent assay, polymerase chain reaction amplification, ligase chain reaction, electrochemical, fluorescence, colorimetric, light-scattering, two-photon Rayleigh scattering, and SERS methods.42-52 This work has great originality and innovation, mainly in the following two aspects. First, we propose a sonochemically assisted hydroxylamine seeding growth method for the facile synthesis of AuMNPs for the first time. The entire reaction process for the synthesis of AuMNPs from the intermediate product (Au seeds decorated magnetic core) was completed within 5 min and the resulted AuMNPs were highly uniform in size and shape with good magnetic responsivity, stability and high SERS activity. Au shell of different thicknesses can be easily formed outside magnetic cores of different sizes depending on the bacteria detection demands. To the best of our knowledge, the proposed technique is the most convenient synthesis route for preparation of high-quality gold shell-coated magnetic nanoparticles to date. Furthermore, a high-sensitive sandwich-structured SERS detection platform for S.


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aureus was constructed. The synthesized AuMNPs were used for bioseparation and biodetection of bacteria. S. aureus was detected through SERS method with a plasmonic SERS tag, and the detection limit of the process was 10 cells/mL.

Scheme 1. (a) Synthetic route for gold shell-coated magnetic nanoparticles. (b) Schematic illustration of the operating procedures for bacteria detection via a SERS method.

Results and discussion Detection principle of SERS biosensor for S. aureus through immunomagnetic separation.


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As illustrated in Scheme 1b, the SERS-based sandwich immunoassay for the detection of bacteria is based on the formation of a sandwich structure of SERS tags/target/AuMNPs, creating many “hotspots” in the adjacent slits between AuMNPs and the SERS tags. Antibody-conjugated AuMNPs were used to catch and concentrate the target bacteria through immunomagnetic separation with the assistance of an external magnet instead of centrifugation to avoid sample aggregation, and meanwhile the synthesized AuNR–DTNB plasmonic NPs, whose LSPR wavelength was adjusted to 785 nm, were conjugated with antibody to form a SERS tag that can specifically recognize and report the existence of the target bacteria. In other words, this kind of detection is an indirect detection, and the signal is reported by the SERS tag, namely Antibody-conjugated AuNR-DTNB. The weak magnetic responsivity of small AuMNPs usually causes the speed and loss issues of magnetic enrichment. Besides, small AuMNPs usually cause the steric hindrance issue, leaving no enough space for the attachment of SERS tags. Thus, AuMNPs of proper size must be well designed and synthesized to achieve the capture and fast separation of biological samples especially bacteria (size of several hundred nanometers). Synthesis and structure characterization of Au-coated magnetic NPs. AuMNPs were synthesized by our newly developed sonochemically assisted hydroxylamine seeding growth method. This method is a general method, which can be used to coat gold shell of different thicknesses on the surface of magnetic core with different sizes to form the core/shell magnetic nanoparticles. Scheme 1a shows the synthetic route of gold shell–coated magnetic nanoparticles. With the increase of the amount of HAuCl4, the Au seeds absorbed outside the MNPs gradually grew bigger and bigger and finally they intersected with each other and formed a continuous gold shell. The continuous Au shell just means the adjacent AuNP grew bigger enough to intersect with each other and entirely wrapped the magnetic core, not meaning a smooth shell. In fact, this gold shell is a nanoscale rough gold surface (as


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shown in Fig. 1), which can amplify the Raman signal of the adsorbed molecules. If the AuNP grew not bigger enough to intersect with each other, then the synthesized AuMNPs did not possess a continuous shell and the magnetic cores are partially exposed, while the partially exposed magnetic core did not possess any SERS effect and may even weaken the SERS effect, so it will cause a relatively poor SERS effect. Therefore, the completed gold shell is not a must but better for the SERS effect. Superparamagnetic ferrite NPs (MnFe2O4, ~190 and 300 nm), which were used as the magnetic core for the synthesis of AuMNPs, were synthesized by a binary solvent solvothermal reaction following Leung’s method with some modifications.53 FeCl3 and MnCl2 were used as precursors, and a mixture of EG/DEG acted as both the solvent and reductant. NaOAc was selected for electrostatic stabilization to prevent particle agglomeration. NaOAc can also increase the alkalinity of reaction system after hydrolysis, which promotes the hydrolysis of FeCl3 and MnCl2 to form Fe(OH)3 and Mn(OH)2, respectively. Finally, MnFe2O4 NPs were formed after dehydration. The size of the synthesized magnetic nanoparticles can be precisely controlled by simply varying the volume ratio of EG/DEG while all other parameters were kept constant. When the volume ratio of EG/DEG varied from 10/10 to 20/0, the diameters of the resulting MNPs were roughly 190 and 300 nm. The synthetic route for AuMNPs was illustrated in Scheme 1a. First, the magnetic core (MnFe2O4) was sonochemically modified with a uniform PEI layer of 2 nm for 20 min, as shown in Figure S1. The intermediate product MnFe2O4@PEI-AuNPs was obtained via the electrostatic interaction between the primary amine groups of PEI wrapped outside the MnFe2O4 NPs and the 4 nm negatively charged Au seeds. The zeta potentials of MnFe2O4@PEI and the 4 nm Au seeds are +41.0 and −33.9 mV respectively, as shown in Figure S2. The adsorbed Au seeds provided many randomly oriented crystalline domains for


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the following seed-mediated growth of the Au shell. Under the stabilization of PVP, hydroxylamine hydrochloride was used to reduce HAuCl4 within 5 min to form a continuous Au shell outside the MNPs with the Au seeds as nuclei under sonication. After the reduction, the synthesized products were separated by an external magnet and the supernatant was essentially colorless, indicating that basically no new nucleation of gold happening in solution existed. TEM, HRTEM, and SEM were employed to examine the morphology of the as-obtained samples during different stages. Figures 1a and 1e showed the TEM images of the synthesized MnFe2O4 NPs with different volume ratios of EG/DEG (10/10 and 20/0), respectively. The prepared monodisperse spherical MnFe2O4 NPs had a diameter of approximately 190 and 300 nm, respectively. After coating the MnFe2O4 NPs with Au seeds, the TEM images (Figures 1b and 1f) and the HRTEM images (the inset of Figures 1b and 1f) showed that many small Au seeds were homogeneously adhered to the surface of the PEIcoated MnFe2O4 NPs. These Au seeds provide an abundance of small, randomly oriented crystalline domains for the subsequent seed-mediated growth of the Au shell. After further reduction of HAuCl4 by hydroxylamine hydrochloride, a rough Au shell was formed outside the magnetic cores, as shown in Figures 1c and 1g. The SEM images (Figures 1d and 1h) of the prepared AuMNPs showed the rough Au shell was consisted of many large adjacent AuNPs. The Au shell formation was investigated by controlling the amount of HAuCl4, as shown in Figure S3. As described in the experimental section of Supporting information, the amount of PVP and hydroxylamine hydrochloride was maintained as constant, whereas that of HAuCl4 varied from 0 to 500 µL. As shown in Figure S3 and Table S1, the Au shell thickness was positively correlated with the amount of HAuCl4. In summary, our proposed method to


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synthesize AuMNPs is a general method which can be used to coat gold shell of different thicknesses on the surface of magnetic core with different sizes to form the core/shell MNPs.

Figure 1. TEM and SEM images of the synthesized magnetic nanoparticles. TEM images of MnFe2O4 (a, e), MnFe2O4@PEI–AuNPs (b, f), MnFe2O4@Au (c, g), and the corresponding SEM images of MnFe2O4@Au (d, h). The as-prepared samples were also examined by X-ray diffraction (XRD), as shown in Figure 2. In curve (a) for MnFe2O4, all the diffraction peaks (square) can be indexed to the Powder Diffraction File (PDF) database (PDF 74-2403, International Center for Diffraction Data (ICDD), 2004). The peaks of MnFe2O4 (curve a of Figure 2) can be clearly observed at 2θ values of 29.8, 35.3, 42.8, 52.9, 56.7, and 61.9, which corresponded to the reflections of the (220), (311), (400), (422), (511), and (440) crystalline planes of the synthesized MnFe2O4, respectively. After the absorption of Au seeds on the MnFe2O4 NPs, a new XRD peak (circle, curve b of Figure 2) was observed at a 2θ value of 38.3, which corresponded to the (111) crystalline plane of Au (PDF 04-0784, ICDD, 2004). After the continuous Au shell was formed outside the MnFe2O4 NPs, five new peaks of Au (triangle, curve c of Figure 2) were clearly observed at 2θ values of 38.3, 44.3, 64.5, 77.4, and 81.6, which corresponded to the


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reflections of the (111), (200), (220), (311), and (222) crystalline planes of the synthesized Au shell (PDF 04-0784, ICDD, 2004), respectively.

Figure 2. XRD patterns of (a) MnFe2O4, (b) MnFe2O4@PEI–AuNPs, and (c) MnFe2O4@Au. The synthesized products were also examined by energy-dispersive spectroscopy (EDS), as shown in Figure S4. The adsorption of Au seeds and the Au shell formation can be clearly seen from Figure S4. Optical and Magnetic Properties of AuMNPs UV–vis spectra were obtained to investigate the synthesis process and the optical properties of the as-prepared MnFe2O4, MnFe2O4@PEI–AuNPs, and MnFe2O4@Au nanoparticles. As shown in Figure 3a, no new obvious absorption peak was observed after the absorption of Au seeds on the MnFe2O4 NPs. After the continuous Au shell was formed outside the MnFe2O4 NPs, even the Au shell is relatively thin (curve c of Figure 3a, 50 µL HAuCl4 used (Table S1)), an absorption peak at 525 nm showed up. Upon depositing more Au on the magnetic core, an increasing red shift of the absorption peak was observed. The absorption peak red shifted to 586 nm when 200 µL HAuCl4 was used (Table S1). The UV–vis spectra of the synthesized MNPs based on 300 nm magnetic core exhibited a similar phenomenon and are


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shown in Figure S5. Such a red shift phenomenon coming along with increasing Au shell layer thickness can be understood in terms of plasmon hybridization with the Au shell layer growth.54 Besides, the Au shell formed outside the magnetic core was a rough surface after a 5 min reaction (Figures 1d and 1h), thus strong interaction and coupling of the surface plasmon existed between neighboring gold nanoparticles, which may also contribute to the plasmon resonance red shift.55 Furthermore, the surface plasmon coupling resulting from such a highly rough surface of AuMNPs was also efficient for SERS detection.

Figure 3. UV-vis spectra (a) and magnetic hysteresis curves (b) of the synthesized MnFe2O4, MnFe2O4@PEI–AuNPs, and MnFe2O4@Au nanoparticles based on 190 nm magnetic core.


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The magnetic properties of the synthesized MnFe2O4, MnFe2O4@PEI–AuNPs, and MnFe2O4@Au nanoparticles were investigated with a SQUID magnetometer (Figure 3b). No hysteresis loops were observes for all the synthesized NPs, indicating that all of the NPs exhibited superparamagnetic properties.56 The superparamagnetic property can prevent aggregation and enable the magnetic nanoparticles to be rapidly redispersed in the absence of magnetic field, thus, paving the way for their application in the bioseparation and biodetection fields. As shown in Figure 3b, the magnetic saturation (MS) value of MnFe2O4 NPs was approximately 74 emu/g. After the absorption of Au seeds, the MS value decreased to 55 emu/g. Further coating with the Au shell resulted in a further decrease of the MS value to 36 emu/g. The decrease in the overall MS values indicated that the MnFe2O4 surface was covered with nonmagnetic materials, such as PEI and AuNPs, as confirmed by the TEM and SEM images. Besides, as shown in the inset of Figure 3b, the synthesized MNPs quickly concentrated on the side of the vials within 30 s upon placement of an external magnet beside the vials, leaving the solution basically transparent. Such excellent magnetic properties implied that the synthesized MnFe2O4@Au NPs had strong magnetic responsivity and can be easily separated from the solution with the help of an external magnetic force. SERS activity of AuMNPs. PATP was used as a probe molecule to evaluate the SERS performance of the synthesized AuMNPs because of there being a thiol group that can bind to gold surface through chemical bond (Au-S) and its well-characterized Raman bands.11 When evaluating the SERS performance using PATP, 1 mL of PATP alcoholic solution (from 10−5 M to10−10 M) and 5 µL of AuMNPs were subsequently added to a 1.5 mL centrifuge tube. After 1 h sonication, the AuMNPs were magnetically separated and rinsed three times with ethanol to remove the excess free PATP. After the washing procedure, the resulted AuMNPs were redispersed in 5


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µL of ethanol and dropped on a silicon substrate. After drying in air, SERS spectra of the sample were conducted and recorded as shown in Figure 4a. The two main Raman peaks of PATP (1077 and 1586 cm-1) can be clearly seen down to 10-9 M. We conducted five tests for each of the samples and made a statistical analysis of the results to obtain the intensity−concentration calibration curve (polynomial fitting), which plotted the intensity of the SERS peak at 1077 cm−1 versus the logarithmic concentration of the sample (Figure 4b).

Figure 4. (a) SERS spectra taken from the synthesized AuMNPs with various concentrations of PATP (from 10−5 M to10−10 M), (b) Calibration curve for PATP at a concentration range of 10−9 to 10−5 M obtained by using SERS intensity at 1077 cm−1. Synthesis and characterization of AuNR–DTNB plasmonic NPs AuNRs typically exhibiting an LSPR of 840 nm, which corresponds to an aspect ratio of approximately 4, were obtained using the seed-mediated method according to our previous


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publication.12 On the basis of previous experiments, AuNRs with three different aspect ratios were successfully synthesized only by changing the amount of AgNO3 (Figures 5a-c). It is a very mature approach to synthesize AuNRs with three different aspect ratios by changing the amount of AgNO3, as first proposed in 2003 by El-Sayed.57

Figure 5. TEM images of the synthesized AuNRs at different conditions (a) 0.2 mL AgNO3, (b) 0.28 mL AgNO3, and (c) 0.3 mL AgNO3. The insets are the corresponding optical images. (d) UV–visible spectra of the synthesized nanoparticles. (e) Raman intensity of DTNB adsorbed on the three different kinds of AuNRs under the same conditions. Figure 5d shows the UV–vis spectra of the synthesized AuNRs. The LSPR wavelength of the synthesized AuNRs (Figures 5a-c) was 710, 785, and 840 nm respectively, consistent with the simulation results (Figure S6). These AuNRs were modified with 20 µM Raman reporter molecule DTNB under sonication for 1 h. After removing the excess DTNB molecules by centrifugation, the precipitate was redispersed and 5 µL of AuNR−DTNB solution was dropped on a silicon substrate. After drying in air, SERS spectra of the sample


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were conducted and recorded as shown in Figure 5e. The AuNRs, whose LSPR was adjusted to match the given laser excitation wavelength (785 nm), had the strongest Raman signal (about 2 times stronger than the other two kinds of AuNRs). Detection of S. aureus by SERS To prove the practicality and great potential in bioapplication, the synthesized AuMNPs were applied in bacteria detection. The 785 nm incident laser was the most commonly used for the detection of biological samples (such as bacteria, tumor cells, and biomarks) due to the sample damage issue in published literatures.58-60 Therefore, 785 nm incident laser was chosen for the SERS detection of S. aureus. AuMNPs were conjugated with antibody to achieve the bacteria capture and separation. Meanwhile, the synthesized AuNR–DTNB plasmonic NPs, whose LSPR wavelength was adjusted to 785 nm, were conjugated with antibody to form a SERS tag that can specifically recognize and report the existence of the target bacteria. The detailed process was described in the experimental section of Supporting information.

Figure 6. XPS spectra of (a) the synthesized AuMNPs, (b) MU/MUA and EDC/NHS successively modified AuMNPs, and (c) antibody-conjugated AuMNPs.


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X-ray photoelectron spectroscopy (XPS) was used to prove that AuMNPs were conjugated with antibody. The XPS spectrum of AuMNPs (Figure 6a) indicated the presence of Au 4f (~83.7 eV), Au 4d5 (~334.0), Au 4d3 (~353.0 eV), Au 4p3 (~546.3 eV), Au 4p1 (~642.4 eV), and Au 4s (~761.2 eV) peaks. Note that the appearance of C 1s (~285.0 eV), N 1s (~398.4 eV) and O 1s (~531.8 eV) in Figure 6a was attributed to PVP which was used as a surfactant for the synthesis of AuMNPs. The absence of Fe or Mn peaks indicated that a continuous gold shell was formed outside the magnetic core. AuMNPs were then carboxyl group–functionalized through the modification of MU/MUA. EDC/NHS was employed to activate the carboxyl groups and form an active ester that can rapidly couple with amines on target antibodies. After the successive modification of MU/MUA and EDC/NHS outside the surface of AuMNPs, the S 2p (~164.0 eV) peak, which was attributed to mercapto groups (– SH) of MU/MUA, appeared (Figure 6b). The ratio of C 1s, N1s, and O 1s peaks to Au peaks was significantly increased upon conjugating with the antibody (Figure 6c), which is caused by a large quantity of C, N, and O elements in the antibody molecules. With the decrease of the relative content of Au elements, the Au 4p1 and Au 4s disappeared. These results confirmed that AuMNPs were successfully conjugated with antibody. As illustrated in Scheme 1b, the SERS biosensor based on antibody recognition included the following processes performed in 1.5 mL microtubes. First, the antibody-conjugated AuMNPs were mixed and incubated with S. aureus under shaking condition, during the incubation, S. aureus bound to the antibody-conjugated AuMNPs. Upon external magnetic separation, the resulting bacteria–AuMNPs complexes (Figures 7a and 7b) were magnetically separated and rinsed with PBST to remove the excessive free bacteria. Afterwards, the SERS tags (AuNR–DTNB–antibody) were added and incubated for binding together with the bacteria, leading to the formation of the SERS tags/S. aureus/AuMNPs sandwich architecture (Figures 7 and Figure S8), creating many “hotspots” in the adjacent slits between AuMNPs


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and the SERS tags. After washing with PBST under magnetic confinement to remove the free SERS tags, the resultant SERS tags/S. aureus/AuMNPs were redispersed in ethanol and dropped on a silicon substrate. After drying in air, SERS spectroscopy was conducted.

Figure 7. (a) TEM image, (b) SEM image of S. aureus binding to antibody-conjugated AuMNPs, (c) TEM image, and (d) SEM image of the SERS tags/S. aureus/AuMNPs sandwich architecture. Figure 8a shows the SERS spectra that corresponded to different concentrations of S. aureus. Several strong Raman bands of the reporter molecule (DTNB) introduced by the plasmonic SERS tag were observed. The detection is based on the main Raman peak (1331 cm–1) of the SERS tag and then quantified by its intensity. The main Raman peak of DTNB can be clearly seen down to 10 cells/mL. As shown in Figure 8b, the calibration curve between the SERS intensity at Raman peak 1331 cm–1 and the logarithm of S. aureus bacteria concentrations (101–105 cells/mL) was plotted and a good linear relationship was observed from 101 to105 cells/mL S. aureus concentration (correlation coefficient is calculated to be


19


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0.9789). Control experiments using culturing and plate counting method were also conducted to confirm our SERS results, as shown in Figure S7. In summary, the synthesized AuMNPs can be used as multifunctional SERS substrate. The inner magnetic core MnFe2O4 enables the composites with good magnetic responsivity and the outer rough gold shell provides good stability and high SERS activity. With the aid of a plasmonic SERS tag, the bioseparation and biodetection of S. aureus were achieved with the detection limit down to 10 cells/mL.

Figure 8. (a) SERS spectra taken from the SERS tag/bacteria/AuMNPs sensing platform with various concentrations of S. aureus (105, 104, 103, 102, 101, and 100 cells/mL) and blank control; (b) Calibration curve for S. aureus at a concentration range of 101–105 cells/mL obtained by using SERS intensity at 1331 cm–1.


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Table 1. The characteristics of the SERS biosensor developed in this work compared to other recently reported methods for bacteria detection Dynamic range (cfu/mL) 3.5×102-3.5 ×107 10-106 1.5×103-1.5 ×105

Detection limit (cfu/mL)

Ref.

35

Tamer et al. 201261

9

Yuan et al. 201445

1.5×103

Sung et al. 201346

Two-Photon Rayleigh Scattering

50-2100

50

Singh et al. 200949

S. aureus

Amperometric

4.4×105-1.8 ×107

1.7×105

S. aureus

Amperometric

10-108

10

S. aureus

Potentiometric

103-108

8×102

S. aureus

SERS

10-105

10

Escamilla-Gómez et al. 200862 Majumdar et al. 201363 Zelada-Guillén et al. 201264 This work

Target

Detection method

E. coli

SERS

S. aureus

Colorimetric

S. aureus

Colorimetric

E. coli

Table 1 shows the characteristics of our proposed SERS biosensor using immunomagnetic separation compared with other recently reported methods for bacteria detection. As can be seen from the table, our results are superior or equivalent than the results of other researchers. Conclusions A sonochemically assisted hydroxylamine seeding growth method for the facile synthesis of AuMNPs is proposed for the first time. AuMNPs are obtained within 5 min from the intermediate product (Au seeds decorated magnetic core) under sonication. To the best of our knowledge, this is the most convenient route for the synthesis of high-quality gold shellcoated magnetic nanoparticles to date. We successfully achieve the perfect growing of gold seeds, which were electrostatically adsorbed on the surfaces of magnetic cores, into gold shells using our sonochemically assisted hydroxylamine seeding growth method under the stabilization of PVP. The synthesized AuMNPs were highly uniform in size and shape with


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good

SERS

activity,

strong

magnetic

responsivity,

enhanced

Page 22 of 28

stabilization,

and

biofunctionalization. Au shell of different thicknesses can be easily formed outside magnetic cores of different sizes as needed. Furthermore, the bioseparation and biodetection of S. aureus were achieved via a SERS method with the aid of a plasmonic SERS tag. The coupling enhancement between AuMNPs and the detection probes produces multiple “hotspots”, and consequently SERS signals can be enhanced. These “hotspots” inside a single sandwich architecture, together with the interparticle “hotspots” produced under magnetinduced aggregation, further amplify the Raman signal of detection probes to achieve highsensitive detection. We expect that our proposed synthesis method for AuMNPs can provide some ideas for the preparation of core/shell nanocomposites for biomedical application in the future. We expect this SERS bioassay can be extended to the detection of a wide variety of bacterial pathogens or even cells by using the corresponding antibodies. We expect that our proposed bioassay to be applied for pathogenic bacteria detection in clinical diagnosis and biomedical research in the future. ASSOCIATED CONTENT Supporting Information Available: Figure S1–S8, Table S1, and Experimental section. This material is available free of charge via the Internet at http://pubs.acs.org. ACKNOWLEDGMENT This work was funded by National Natural Science Foundation of China (No. 51475468) and the 863 Program of China (No. 2012AA022501). Junfeng Wang, Xuezhong Wu, and Chongwen Wang contributed equally.


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