Au Nanoparticles Deposited on Magnetic Carbon Nanofibers as the

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Au nanoparticles deposited on magnetic carbon nanofibers as the ultrahigh sensitive substrate for surface-enhanced Raman scattering Hung-Chi Wu, Tse-Ching Chen, Hsing-Jui Tsai, and Ching-Shiun Chen Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02488 • Publication Date (Web): 01 Nov 2018 Downloaded from http://pubs.acs.org on November 3, 2018

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Au nanoparticles deposited on magnetic carbon nanofibers as the ultrahigh sensitive substrate for surface-enhanced Raman scattering: detections of Rhodamine 6G and aromatic amino acids

Hung-Chi Wua, Tse-Ching Chenb, Hsing-Jui Tsaia and Ching-Shiun Chena,b*

aCenter

for General Education, Chang Gung University, 259, Wen-Hua 1st Rd., Guishan Dist., Taoyuan City 33302, Taiwan, Republic of China

bDepartment

of Pathology, Chang Gung Memorial Hospital Linkou, 5, Fusing St., Guishan Dist., Taoyuan City 33302, Taiwan, Republic of China

KEYWORDS

surface-enhanced Raman scattering, Rhodamine 6G, gold, magnetic carbon

nanofibers, phenylalanine and tyrosine amino acids

ACS Paragon Plus Environment

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ABSTRACT

Surface-enhanced Raman scattering (SERS) is a unique spectroscopy that can offers high-sensitive detection for many molecules. Herein, the Au particles deposited on carbon nanofiberencapsulated magnetic Ni nanoparticles (Ni@CNFs@Au) have been successfully synthesized for SERS measurements. The Ni@CNFs@Au substrates have the advantages of a high SERS sensitivity and good magnetic response. The Ni@CNFs could be directly obtained from CO2 hydrogenation on a Ni catalyst, which has been characterized as having rich carboxylic acid groups, graphitic structures and a high surface area. The Ni@CNFs surface could effectively increase the density of hot-spots during Au NP aggregation and influence the morphology of the Au nanostructures. The spherical shape, oval shape and coral-like Au nanostructures were prepared on Ni@CNFs with various Au concentrations. BET, zeta potential, HRTEM, XRD and XPS measurements were used to characterize the Ni@CNFs@Au samples. The Au NPs deposited on the Ni@CNFs presented a suitable oval shape, and an average size of ~30-40 nm. The size allowed surprisingly ultrasensitive SERS detection of Rhodamine 6G (R6G) with a resolution of approximately a single molecule under an excitation wavelength of 532 nm. Using 785 nm excitation, a low R6G concentration of ~1×10-14 M was detected . Moreover, the Ni@CNFs@Au substrates could be rapidly magnetically separated after adsorption. Phenylalanine and tyrosine amino acids, which are associated with liver disease, were examined using SERS with the Ni@CNFs@Au substrate. Ultralow concentrations of ~1×10-11 M for phenylalanine and ~1×10-13 M for tyrosine were clearly measured. The Ni@CNFs@Au substrates exhibited applicability as excellent SERS detection platforms that combine a high-sensitivity and rapid magnetic separation for various adsorption molecules.


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1. Introduction Over the past few years, surface-enhanced Raman scattering (SERS) spectroscopy has attracted attention due to its applications for the identification and detection of trace chemical, medical and biological molecules.1-15 To date, Au and Ag colloidal nanoparticles (NPs) have been widely used as SERS platforms, and a high SERS sensitivity can be obtained due to the generation of “hotspots” caused by colloidal aggregation. The localized surface plasmon resonance (LSPR) of noble metal NPs, namely, the electromagnetic enhancement, is proposed to be associated with the SERS sensitivity. Recently, a variety of substrates, including 1-D nanowires, 2-D silicon wafers, polymer films and carbon nanotubes with high surface area, have been developed to support Au and/or Ag NPs to increase the hot-spot density in order to reduce the SERS detection limit.16-20 On the other hand, the chemical enhancement from the template, such as graphene-based materials, has also been investigated to determine its influence in SERS applications. Graphene oxide and semiconductor particles may provide a strong chemical enhancement to the electron transfer between the graphite structure and adsorbed molecules.21-29 Traditionally, Au or Ag NPs must be mixed with probe molecules in a solution and dried in air before SERS measurements. In other words, noble metal NPs with and without a template are not easy to rapidly separate from solution, and thus to resolve this drawback, magnetic materials combined with Au or Ag NPs have been further developed to rapidly pretreat SERS substrates.3034

Fe3O4@Au NP clusters as SERS substrates can preconcentrate analytic molecules from diluted

solutions, allowing a detection limit for Rhodamine 6G (R6G) molecules of ~1 × 10-7 M.31 Fe3O4@TiO2@Au triplex core-shell magnetic microspheres were synthesized with multiplefunctions to eliminate the ‘single-use’ problem of traditional SERS substrates, and they exhibited a detection limit of 1×10-7 M for R6G.32 A TiO2 composite can play an auxiliary role in the removal


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of adsorbed molecules through a photocatalytic process.32 Wang and his co-workers synthesized Ag NP-coated spherical Fe3O4@carbon core-shell microspheres (Fe3O4@C@Ag) for SERS detection of aromatic molecules.33 The carbon material in Fe3O4@C@Ag can effectively adsorb aromatic molecules and improve the detection limit because the analyzed molecules adsorbed on the carbon layer may be located near the electromagnetic field with the excited Ag NPs.33 Nevertheless, spherical Fe3O4 NPs usually possess a low surface area, which may continue to be a challenge of these newly developed magnetic SERS substrates, leading to poor SERS sensitivity due to a low density of hot-spots on the surface. Several magnetic transition metals, such as Ni or Fe, have been employed to be efficient catalysts for the production of carbon nanotubes (CNTs) from the decomposition of carbon monoxide process.35-39 However, it has been assumed that the combination of hydrophilic CNTs with magnetic NPs is inherently difficult. The raw CNTs materials usually provided extremely hydrophobic and poor dispersion in an aqueous solution. Thus, the carbon surface of CNTs must be treated with concentrated acids (HNO3 or H2SO4) for generating hydrophilic groups, such as carboxylic acid and carboxylate groups. However, the treatment of CNTs with concentrated acids may lead to the removal of metal NPs from the CNTs surface. We have developed a novel CO2 utilization process to produce carboxylic-functionalized carbon nanofiber-encapsulated magnetic Ni nanoparticles (Ni@CNFs) that are directly synthesized from the catalytic hydrogenation of CO2 on a Ni catalyst.40 The Ni@CNFs material uniquely combines strong magnetization and a high surface area (>100 m2/g) and shows improvements in the drawbacks of previous materials, as described above.41,42 The Ni@CNF material has the potential to increase the hot-spot density , while providing a good magnetic response. In this work, we report the design of Ni@CNFs with Au NPs as a SERS substrate (Ni@CNFs@Au) for high sensitivity SERS detection, and the SERS


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activities of the Ni@CNF@Au substrates were estimated using R6G and aromatic amino acids, such as phenylalanine and tyrosine, as probe molecules. These hybrid nanomaterials provide the advantages of a low SERS detection limit and rapid separation from solution. 2. Experimental Preparation of the Ni@CNFs Commercial 12 wt% Ni/Al2O3 (Süd-Chemie Catalysts, Inc., catalyst # FCR-42) was used as the catalyst to synthesize the carbon nanofiber-encapsulated Ni magnetic nanoparticles (Ni@CNFs) via CO2 hydrogenation. Initially, an aqueous NaNO3 solution was impregnated onto the 12 wt% Ni/Al2O3 sample containing 3 wt% Na, and then, the as-prepared sample was subsequently airdried at 80°C for 10 h. The NiNa/Al2O3 catalyst was further calcined in air and reduced in H2 at 500°C for 5 h. The reduced NiNa/Al2O3 catalyst was fed by a stream of H2/CO2 (1:1) at 100 mL/min and 500°C for 15 h to synthesize the Ni@CNF material. 2.2 Characterization of the Ni@CNFs The chemical composite and oxidation state of the elements in the NiNa/Al2O3 catalyst with and without CNFs deposition were probed by X-ray photoelectron spectroscopy (XPS). All XPS data were obtained using a Thermo VG-Scientific Sigma Probe spectrometer at the Precision Instrument Center of the College of Engineering at the National Central University, Taiwan. The spectrometer was equipped with an Al K X-ray source with 1486.6 eV operated at 108 W with a pass energy of 50 eV through a hemispherical analyzer. The analysis chamber pressure of the XPS instrument was held at approximately 1×10–9 Torr. The N2 physisorption isotherms were measured at 77 K using a Quantachrome Autosorb-1-MP instrument. Brunauer-Emmett-Teller (BET) surface areas were calculated from the adsorption branches based on a relative pressure in the range of 0.05-0.30. The isotherm was analyzed by the


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non-local density functional theory (NLDFT) method that can evaluate the pore size of a sample using the kernel of the NLDFT equilibrium capillary condensation nitrogen isotherms at 77 K on silica (adsorption branch, assuming cylindrical pore geometry), and the total pore volumes were evaluated at a relative pressure of 0.95. The zeta potential of the Ni@CNFs dispersed in deionized water was determined using a zeta potential analyzer (Nano ZS90, Malvern Instruments, Ltd., UK) with a combination of laser Doppler velocimetry (LDV) and phase analysis light scattering techniques. The cuvette cells were used and filled with deionized water after dispersing the Ni@CNFs samples. The optical response was measured by changing the reflectance (resulting in color differences) after applying a reverse bias voltage of ±5 V using an LCD electro-optical measurement system (LCD 5200, Otsuka Electronics Co., Ltd.) with a BaSO4 standard in diffuse reflectance mode. 2.3 Synthesis of Au NPs on Ni@CNFs and Au NPs A 10 mg of Ni@CNFs adsorption was dispersed in 25 mL of deionized water and placed in a 50-mL three-necked round-bottomed flask at room temperature. Then, 20 L, 40 L, or 50 L of a 0.5 M HAuCl4 aqueous solution was added into the Ni@CNF colloidal solution to deposit different Au concentrations on the Ni@CNFs material. The HAuCl4 and Ni@CNFs mixed solutions were heated by a heating mantle to the boiling point under vigorous stirring, and immediately, 5 mL of a 0.2 M sodium citrate solution was injected into the reactor over 15 min to generate Ni@CNFs@Au colloids. The 20 L, 40 L and 50 L additions of 0.5 M HAuCl4 reduced on the Ni@CNFs were denoted Ni@CNFs@Au(20), Ni@CNFs@Au(40) and Ni@CNFs@Au(50), respectively. Using the same synthesis process, Au NPs without Ni@CNFs were also prepared with various HAuCl4 concentrations using 20 L, 40 L, and 50 L of 0.5 M HAuCl4 for the Au(20), Au(40) and Au(50) samples, respectively.


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2.4 SERS spectral measurements A Raman spectrometer (UniRaman, Protrustech Co.) equipped with 532 nm and 785 nm wavelength laser sources and a CCD detector was used to conduct the SERS measurements. The laser power is 1 mW. The acquisition time was 10 s. The scattering signals were collected using a long working distance 100× objective. The SERS measurements were performed from 10 different position over the samples. All SERS spectra revealed in this study were recorded by taking the average of 10 measurements. R6G and the aromatic amino acids, phenylalanine and tyrosine, were used as probe molecules to test the SERS activity of the Ni@CNFs@Au substrates and Au NPs. All probe molecules were prepared in aqueous solutions with different concentrations. First, 10 L of a solution for probe molecule was added to 10 L of Au(x) or Ni@CNFs@Au(x) colloidal solutions for 30 min and mixed with vigorous shaking. The 20 L droplet of mixing the probe molecule and pure Au was deposited on the measurement substrate (glass plate covered by aluminium foil) and dried at room temperature. The samples for probe molecules adsorbed onto Ni@CNFs@Au were separated by a magnet from the aqueous solution, and then were transformed to the measurement substrate. Figure S1 shows the images of Au(x) and Ni@CNFs@Au(x) deposited onto measurement substrate. 3. Results and Discussion 3.1 Characterization of the Ni@CNFs material Figure S2 shows the N2 adsorption-desorption isotherms and pore size distribution curves of the Ni@CNFs. Figure S2A shows that an inflection point occurred near the completion of the first adsorbed monolayer, and thus, the curve could be classified as a type II isotherm. The Ni@CNFs material was estimated to have a specific surface area of 122 m2/g. The pore size distribution curve is provided in Figure S2B, and no apparent distribution peak can be observed in this figure. Thus,


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the Ni@CNFs are typical nonporous materials with diameters exceeding micropore sizes. The Xray diffraction (XRD) spectra of the NiNa/Al2O3 material before and after CNFs deposition are compared in Figure S3. The diffraction peaks associated with the metallic Ni0, NiAl2O4, and Al2O3 species were observed in spectrum (a) for the NiNa/Al2O3 sample. A new diffraction peak at 26.7° belonging to the typical (002) graphitic basal plane can be observed in spectrum (b) as a result of the carbon deposition that occurred during the formation of the Ni@CNFs material, This peak implied that a graphite structure might be generated during the carbon deposition. Figure 1 shows the C 1s and Ni 2p2/3 XPS spectra of NiNa/Al2O3 and the Ni@CNFs material, which can be used to identify carbon functional groups and Ni species. For the NiNa/Al2O3 sample without a CNF coating, the major C 1s XPS peak at 284.7 eV can be attributed to unavoidable residual carbon with an sp3 C-C structure on NiNa/Al2O3. Moreover, the Ni 2p2/3 XPS band was fitted to two peaks positioned at 854.8 and 852.5 eV, which were assigned to Ni2+ in the NiAl2O4 spinel structure and the reduced Ni0 species, respectively.40 The C 1s XPS band of the Ni@CNFs shows that multi-carbon species form via CO2 hydrogenation to form CNFs on NiNa/Al2O3 (Ni@CNFs). The C 1s XPS band was fitted with five peaks at 289.1 eV for the -* satellite, 287.9 eV for -COOH, 285.9 eV for -C=C-O (keto-enol equilibria structure), 284.7 eV for sp3 C-C and 284.4 eV for sp2 C=C.43 These results indicate that the presence of oxygen-containing functional groups (-COOH and -C=C-O) might generate the hydrophilic properties of the Ni@CNFs material. Moreover, the peak at 284.4 eV likely correlates to the characteristic graphitic structure (sp2 C=C). The Ni 2p2/3 XPS signal of the Ni@CNFs was too weak to distinguish from that of the background, which suggested that the surface Ni species were completely encapsulated by the CNFs.


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Figure 1 (A) C 1s and (B) Ni 2p2/3 XPS spectra of NiNa/Al2O3 and the Ni@CNFs samples. The zeta potentials of all the Ni@CNFs samples in the PBS-buffered aqueous solutions at various pH values are shown in Figure S4. The acidity or alkalinity of each surface was determined in addition to its isoelectric point (IEP), which indicates the pH at which the zeta potential is zero. For the Ni@CNFs samples, the zeta potential was initially positive under acidic conditions, and the potential gradually became negative as the pH increased. The IEP value (pHIEP) was 6.5 for the Ni@CNFs material. In our previous paper, we reported that a Ni@CNFs sample dispersed in pure water (pH 7.0) can generate a weakly basic solution with a pH value of 7.7. The formation of a weak base likely depends on the presence of R-COO－ groups on the CNFs.41 3.2 Characterization of the Ni@CNFs@Au(x) materials Figure S5 shows the XRD spectra of different concentrations of Au particles on Ni@CNFs@Au(x) with x=20, 40 and 50. The diffraction patterns of crystalline Au at 38.2°for Au


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(111) and 44.5° for the Au(200) facets were observed. Based on the full-width at half-maximum (FWHM) value of the Au(111) peak, the average size of the Au particles for all the Ni@CNF@Au(x) samples was estimated from the Scherrer equation, which provided average Au sizes of 22 nm for Ni@CNFs@Au(20), 42 nm for Ni@CNFs@Au(40) and 69 nm for Ni@CNFs@Au(50). The pure Au solid is usually difficult to separate from a colloidal solution. Thus, the size of the pure Au NPs was evaluated using the HRTEM image results, as described later. Figures 2 reveals the HRTEM images and size distribution of the Au(x) (x=20, 40 and 50) samples. The literature reports that Au3+ is often reduced by trisodium citrate to form a spherical shape.44-46 As expected, many sphere-like Au NPs were observed in the Au(x) (x=20, 40 and 50) samples in this case, as shown in Figure 2, and the average size of the Au NPs increased from ~11.4 to ~36.8 nm with the increasing Au concentration based on the HRTEM results. The NPs of the Au(50) sample appeared to be slightly larger than those formed at low Au concentrations due to the likely formation of elongated particles.


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Figure 2 HRTEM and particle distribution of Au(x) samples with x=20, 40 and 50. The SEM image and the energy-dispersive spectroscopy (EDS) of the Ni@CNFs are shown in Figures S6A and S6B, revealing that the Ni particle could remain at the tip of the carbon structure. The analysis of EDS spectrum could give low intensity for Ni element. The TEM images for the large quantity of CNFs and isolated CNFs were observed in Figures S6C and S6D. It can see that the carbon film encapsulated Ni might be formed during CNFs growth. The HRTEM images of the Au NPs deposited on the Ni@CNFs are shown in Figure 3. Notably, a significant change in the shape of the Au NPs on the Ni@CNFs was found with the increasing Au concentration. For the Ni@CNF@Au(20) sample, the sphere-like Au NPs maintained an average size of ~8.9±1.6 nm, which was similar to the that of the Au (20) sample. As the Au concentration increased (Ni@CNF@Au(40)), several oval-shaped Au NPs were


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observed on the Ni@CNFs, and the average size increased to ~ 30.8±5.8 nm, as estimated from the statistical measurements of the particles in Figure 3. Ni@CNF@Au(50) had additional inhomogeneous Au NPs containing coral-like and long-rod morphologies. Excluding the corallike structures in the analysis, the elongated Au particles had an average particle size of ~ 80±25.1 nm. Based on the results described above, the Ni@CNF material may influence the shape of the Au nanocrystals. The colloidal Au(x) with a sphere-like morphology may closely resemble icosahedral or decahedral structures.46 By controlling the kinetics, the low Au concentration of Ni@CNFs@Au(20) could be rapidly reduced, and thus, spherical Au NPs were still synthesized on the [email protected],47 For the Ni@CNF@Au(40) sample, forming a stable, spherical structure on the CNF surface might be difficult because the direction of the crystal growth at high Au concentrations might be influenced by the carbon surface and abundant oxygen-containing functional groups, resulting in the formation of oval Au NPs. As more Au3+ ions are reduced on the Ni@CNFs, as seen for the Ni@CNF@Au(50) sample, the subsequent growth of inhomogeneous crystal structures could generate coral-like and long-rod morphologies. Our previous study has reported that the –COO- group can be formed on the Ni@CNFs material surface during carbon deposition.32 Thus, it was reasonably proposed that the –COO- group with negatively charge might have the ability for binding the Au3+ ions on the carbon surface. The dense Au3+ adsorbed on Ni@CNFs through electrostatic interaction may the important factor for generating the oval, long-rod or coral-like morphologies rather than spherical shape. The SEM and EDS results of Ni@CNF@Au(40) sample are shown in Figure S7. It was found that large amount of Au particles deposited on CNFs could be observed. On the other hand, the clear Ni and Au signals for the EDS measurement could be found. These results represented that the Ni@CNFs


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could have the positive effect for Au aggregation on CNFs surface, and thus led to generate different crystal shape.

Figure 3 HRTEM and particle distribution of Ni@CNF@Au(x) samples with x=20, 40 and 50. The optical properties of colloidal Au NPs and Ni@CNF@Au solutions were measured by UVVis spectroscopy and are shown in Figure 4. The intensity and position of the plasmon resonance peak are usually sensitive to the size and shape of pure Au NPs. The results in Figure 4 reveal that the LSPR band has a significant redshift from 518 to 538 nm with an increase in the particle size,


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according to the report in the literature. The peak maxima appeared at 518 nm for Au(20), 530 nm for Au(40) and 538 nm for Au(50). Figure 4 shows a comparison of the UV-Vis spectra of the Ni@CNF@Au(x) samples. Compared to the UV-Vis measurement of the Au(20) samples, Ni@CNF@Au(20) exhibited the same plasmon resonance position at 518 nm because a similar spherical Au shape and diameter size still formed on the Ni@CNFs. However, the large and ovalshaped Au NPs that formed on Ni@CNFs@Au(40) may have caused the shift in the peak position to 552 nm and the increase in the plasmon intensity. Literature reports show that spherical particles elongated into an oval shape can cause the plasmon resonance to shift to a longer wavelength and enhance the peak intensity.48,49 For Ni@CNFs@Au(50), the formation of a large, coral-like Au nanostructure was responsible for the bandwidth broadening and the significant redshift to 580 nm observed in the resonant Rayleigh scattering spectrum, which indicated that the thin layers of pure Au induced a larger redshift in the LSPR spectra, band broadening and a decay in the intensity.50,51 Furthermore, a large Au structure with low surface area might result in poor intensity during resonant Rayleigh scattering.

Figure 4 UV-Vis spectra of Au(x) and Ni@CNF@Au(x) with x=20, 40 and 50.


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Figure 5 Magnetic hysteresis loops of Ni@CNFs and Ni@CNF@Au(x) samples. Figure 5 shows the magnetization hysteresis of Ni@CNFs with and without Au NPs as a function of the magnetic field at 25°C on a SQUID magnetometer. The magnetic saturation value decreased from 9.9 to 3.5 as the Au concentration deposited on the Ni@CNFs increased. The Ni@CNF@Au(50) sample was dispersed in water, resulting in a dark-colored suspension. However, fast aggregation could be observed from the suspension solution, when an external magnet was used (inset images in Figure 5). The results demonstrated that the Ni@CNF@Au(x) substrates possessed a good magnetic response in the presence of an external magnetic field. 3.3 Measurements of R6G SERS spectra on Au(x) and Ni@CNF@Au(x) The SERS functions of the Au(x) and Ni@CNF@Au(x) (x=20, 40 and 50) surfaces with different Au loadings and particle sizes were compared using R6G as a target molecule. Figure 6


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shows the SERS spectra of the R6G molecules adsorbed on the Au(x) substrates using an excitation wavelength of 532 nm. The SERS spectra of R6G on pure Au nanocrystals revealed an effective detection concentration of ~1×10-7 M. The SERS sensitivity on the Au(x) substrates was slightly enhanced as the Au particle size increased. However, poor SERS activity was demonstrated for R6G adsorbed on the Au(x) substrates because an R6G concentration of less than 1×10-7 M was not effectively detected.

Figure 6 SERS spectra of R6G molecules on Au(x) with x=20, 40 and 50. The excitation wavelength was 532 nm. When the Au NPs were deposited on the Ni@CNFs to form various Ni@CNF@Au(x) substrates, their SERS detection sensitivity was higher than that observed in the same experiments performed on Au(x), as shown in Figure 7. The Ni@CNF@Au(40) substrate can induce a stronger spectral sensitivity than the Ni@CNF@Au(x) samples with x=20 and 50, resulting in the following order: Ni@CNF@Au(40) > Ni@CNF@Au(20) > Ni@CNF@Au(50). The Ni@CNF@Au(x)


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samples without R6G adsorption are also compared in this figure and show the typical D- and Gbands for CNF materials at 1336 and 1568 cm-1, respectively. No Raman signals of R6G could be observed on the Ni@CNF@Au(x) substrates, even if the Au NPs formed on the R6G pre-covered Ni@CNFs. Notably, the Ni@CNF@Au(40) substrate provided surprisingly ultrasensitive SERS detection of R6G molecules. In Figure 7, an extremely low concentration of R6G of 1×10-18 M (10 L R6G solution contains ~6 molecules) adsorbed on the Ni@CNF@Au(40) substrate was clearly detected, demonstrating the ability to detect a single R6G molecule. Figure S8 shows the SERS spectra of R6G on the Ni@CNF@Au(40) substrate from different location. The characteristic peaks of R6G molecules were highly reproducible.

Figure 7 SERS spectra of R6G molecules on Ni@CNF@Au(x) with x=20, 40 and 50. The excitation wavelength was 532 nm. Undoubtedly, the high sensitivity for R6G on Ni@CNF@Au(x) correlates to the growth and aggregation of Au NPs on the Ni@CNF surface. Literature has reported that the formation of first


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and second generation hot-spots can influence SERS sensitivity.52 The first generation hot-spot can associate with the single nanostructure, such as nanocubes, nanospheres and nanorods etc., which exhibits moderate SERS activity.52 On the other hand, the junction between neighboring Au NPs during particle aggregation might lead to the formation of second generation hot-spots, which can greatly amplify the electromagnetic field and enhance the SERS signals of molecules adsorbed on the Au surface. In general, reducing space between neighboring Au NPs can significantly increase the sensitivity in SERS spectra. The TEM images of Au(x) in Figure 2 show that a large quantity of spherical Au NPs can be observed for the Au(20) and Au(40) samples; however, the small spherical particle aggregates exhibited large spaces among the small spherical particles. The synthesis of the slightly elongated Au NPs in the Au(50) sample might reduce the space between neighboring particles, which may account for the higher SERS activity of Au(50) for R6G detection than that of Au(20) and Au(40). Compared to the Au(x) samples, the use of Ni@CNFs undoubtedly induced the formation of densely aggregated Au NPs on the CNF surface, as revealed in Figure 3. Thus, the high SERS sensitivity for R6G detection might be due to the presence of high-density hot-spots during Au NP aggregation. Additionally, the SERS activity on various Ni@CNF@Au(x) substrates should depend on the size and shape introduced during the growth of the Au nanostructures. The size dependence of the resonant Rayleigh light scattering spectra has been demonstrated, and small Au NPs may have a small scattering cross-section, resulting in weak scattered light. In general, the LSPR of Au NPs strongly affects the electromagnetic enhancement, which is closely related to the size, shape and aggregation of the Au NPs. The relative intensity of the UV-Vis spectra for the three Ni@CNF@Au(x) samples directly reflects the SERS sensitivity. The smaller spherical Au NPs of the Ni@CNF@Au(20) substrate caused a low scattering cross-section and a weak electromagnetic


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enhancement for SERS. Studies in the literature reported that 30-40 nm Au NPs may provide better SERS detection efficiencies.48 Moreover, elongated nanoparticles can generate a strong electromagnetic field associated with electron oscillation along the longitudinal direct.39,40 Thus, Au nanorods provide a higher SERS efficiency than a spherical morphology. The formation of oval-shaped Au nanostructures in Ni@CNF@Au(40) may cause the enhanced SERS sensitivity in this work. The high Au3+ concentration of Au(50) on the Ni@CNFs tended to form a coral-like nanostructure, but the formation of highly inhomogeneous and large Au nanostructures may result in a low Au surface area, causing a weaker SERS signal. Figure 6 shows that in the SERS spectra of R6G on the Au(x) samples a strong fluorescence appeared in the background when using an excitation wavelength of 532 nm. In general, R6G is considered a fluorescent molecule with a strong fluorescence emission under 532 nm excitation, leading to difficulties in detecting R6G using Raman spectroscopy. The strong fluorescence background might be ascribed to electrons in the R6G molecules being excited by the incident light (532 nm) and decaying to the ground state. In this case, a R6G concentration of ~1×10-7 M could be effectively detected on pure Au NPs due to the LSPR effect, but the intense fluorescence background restricted the SERS measurement. Nevertheless, the strong fluorescence background of Ni@CNF@Au(x) did not appear during the SERS detection. The graphitic structure of the Ni@CNFs may play an important role in reducing the fluorescence background of R6G. The electrons of the R6G molecule in the excited state may relax to the ground state or react with the ambient environment. Several studies have indicated that the electron transfer between R6G in the excited state and a graphitic structure might occur when graphene or carbon materials containing sp2 structures are used as the substrate, which implies that substrates containing graphitic structures might reduce the likelihood of excited molecules relaxing to the ground state through a


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spontaneous emission.21-26 In this work, the XPS, Raman and XRD spectra clearly indicated that the Ni@CNFs contain an abundant graphitic structure. Thus, the laser-excited electrons in R6G may be favorably transferred to the sp2 C=C of the graphitic structure, leading to a low fluorescence background and enhanced resonant Raman signals. The SERS activity of R6G adsorbed on the Ni@CNF@Au(40) substrate was further examined using an excitation wavelength of 785 nm, as shown in Figure S9. Compared to the results using an excitation wavelength of 532 nm, the results using a 785 nm excitation light resulted in poor sensitivity for the detection of R6G because of the long wavelength. In general, the peak maximum of optical absorption for R6G is usually positioned at ~528 nm. Thus, the excited wavelength of 785 nm might lead to lower absorbance than that of 532 nm. In other words, the low optical absorption for R6G at 785 nm should directly reflect on the weaker SERS signals in this study. However, the Ni@CNF@Au(40) substrate still exhibited a high-sensitivity toward R6G, and the SERS signal was clearly observed at a low concentration of ~1×10-14 M in Figure S9. The enhancement factor (EF) of the SERS spectra

with

Ni@CNF@Au(40)

and

Au(50)

was

calculated

using

the

formula:

EF=ISERSN0/I0NSERS25, where ISERS and I0 are the peak intensities in the SERS spectrum with 1×10-7 M R6G and normal Raman spectrum with a 4×10-2 M R6G solution, respectively. N0 and NSERS are the number of R6G molecules in the Raman and SERS measurements, respectively. Figure 8A compares the Raman spectrum of the 4×10-2 M R6G solution and the SERS spectra of 1×10-7 M R6G adsorbed on Ni@CNF@Au(40) and Au(50) when a 785 nm wavelength laser was used. The Raman spectrum of Ni@CNF@Au(40) exhibited weak peaks associated with the D- and G- bands of the CNFs. Thus, the peaks at 760, 1173 and 1289 cm-1 were selected to calculate the EF values and are compared in Figure 8B. As expected, the EF of Ni@CNF@Au(40) was higher than that of Au(50).


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Figure 8 (A) Comparison of Raman spectra for (a) a 0.04 M R6G solution, (b) 1×10-7 M adsorbed on Au(50), (c) 1×10-7 M adsorbed on Ni@CNF@Au(40) and Ni@CNF@Au(40) without R6G; (B) comparison of the enhancement factor for Ni@CNF@Au(40) and Au(50). The excitation wavelength was 785 nm. 3.4 SERS spectra of the aromatic amino acids, phenylalanine and tyrosine, on Au(x) and Ni@CNFs@Au(x) As described above, the Ni@CNFs@Au(40) substrate exhibited SERS activity for R6G detection. Additionally, Ni@CNFs@Au(40) and Au(50) were further employed to compare their SERS activities for the aromatic amino acids, phenylalanine and tyrosine, which are important biomarkers metabolized in a diseased liver.53 Figure 9 shows the concentration-dependent SERS spectra for a phenylalanine molecule on both substrates. The excited wavelength with 785 nm was also used, because the weak D- and G- bands of the CNFs could give a neglected background in the course of SERS experiments. In Figure 9A, the normal Raman spectrum of the phenylalanine


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powder exhibited peaks at 1001 cm-1, 1173 cm-1, 1311 cm-1, 1532 cm-1 and 1588 cm-1.54,55 The phenylalanine molecules adsorbed onto Au(50) demonstrated a poor detection limit of ~1×10-5 M. The characteristic peaks of phenylalanine were difficult to clearly differentiate from the background as the concentration decreased to 1 × 10-6 M. However, the Ni@CNF@Au(40) substrate generated a high SERS activity for phenylalanine detection, which led to an effective measurement of 1×10-11 M , as shown in Figure 9B. Interestingly, the strong band at 1001 cm-1 belonging to the ring-breathing mode almost vanished on the Ni@CNF@Au(40) substrate, implying that the aromatic ring of phenylalanine likely strongly adsorbed to the Au surface. The strong interaction between the aromatic ring of phenylalanine and the Au surface could restrict the stretching mode associated with ring breathing. Nevertheless, molecular phenylalanine could be clearly differentiated from other characteristic peaks at extremely low concentrations.

Figure 9 SERS spectra of phenylalanine molecules on (A) Au(50) and (B) a Ni@CNF@Au(40) substrate. The excitation wavelength was 785 nm.


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Figure 10 compares the SERS spectra of tyrosine on Au(x) and Ni@CNF@Au(x) at different concentrations. The typical Raman spectrum of tyrosine powder was not observed in the experiment. The Ni@CNF@Au(40) substrate had a better detection activity for tyrosine molecules than the Au(50) substrate. The detection limit for the target molecule reached 1×10-13 M.

Figure 10 SERS spectra of tyrosine molecules on (A) Au(50) and (B) a Ni@CNF@Au(40) substrate. The excitation wavelength was 785 nm. 4. Conclusion In summary, we successfully developed Ni@CNF@Au substrates with an excellent magnetic response and SERS activity. The Ni@CNF material was characterized as being rich in carboxylic acid groups and having a high surface area. The morphology of the Au NPs during crystal growth could be influenced by the Ni@CNF surface, and spherical, oval and coral-like nanostructures formed with the increasing Au concentration. The surface of the Ni@CNFs effectively improved the density of hot-spots during Au NP aggregation. The Au NPs on the Ni@CNF@Au(40) sample exhibited a suitable oval shape, and an average size of ~30-40 nm, which allowed surprisingly


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ultrasensitive SERS detection of R6G. The SERS measurements generate an approached a single molecule detection limit when an excitation wavelength of 532 nm was used. The graphitic structure of the Ni@CNFs can significantly minimize the fluorescence emission from electrons in the R6G molecule, which are excited by the incident light (532 nm) and decay to the ground state. Thus, a weak SERS signal is observed for low R6G concentrations. Using a 785 nm excitation wavelength, the Ni@CNF@Au(40) substrate provides a high sensitivity and can detect ~1×10-14 M R6G. Moreover, the Ni@CNF@Au substrates can be rapidly magnetically separated after the adsorption of molecules. The Ni@CNF@Au(40) substrate was further employed to detect phenylalanine and tyrosine, which are amino acids associated with a diseased liver. Ultralow concentrations of ~1×10-11 M for phenylalanine and ~1×10-13 M for tyrosine were clearly measured. The Ni@CNF@Au substrates exhibited great potential for high-sensitivity and quick SERS measurements under a magnetic environment. AUTHOR INFORMATION Corresponding Author *E-mail:

[email protected] (Ching-Shiun Chen)

ACKNOWLEDGMENT Financial support from the Ministry of Science and Technology (MOST106-2113-M-182-002) and Chang-Gung Memorial Hospital (CMRPD5F0022) is gratefully acknowledged.

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Table of Contents The Ni@CNFs@Au substrates can provide surprisingly ultrasensitive SERS detection of Rhodamine 6G and aromatic amino acids.


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Au Nanoparticles Deposited on Magnetic Carbon Nanofibers as the

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