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A Highly Sensitive, Selective and Reproducible SERS Sensor for Detection of Trace Metalloids in the Environment Shuyu Xu, Wenqiong Tang, D. Bruce Chase, Donald L. Sparks, and John F. Rabolt ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00301 • Publication Date (Web): 09 Feb 2018 Downloaded from http://pubs.acs.org on February 14, 2018

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ACS Applied Nano Materials

A Highly Sensitive, Selective and Reproducible SERS Sensor for Detection of Trace Metalloids in the Environment Shuyu Xu1, Wenqiong Tang2, D. Bruce Chase1, Donald L. Sparks3 and John F. Rabolt1* 1

Department of Materials Science and Engineering, University of Delaware, Newark, Delaware 19716, United States EAG Laboratories, 810 Kifer Road, Sunnyvale, California 94086, United States 3 Department of Plant and Soil Sciences, University of Delaware, Newark, Delaware 19716, United States Silver coated gold nanorods, fiber electrospinning, polymer/nanoparticle composite SERS sensor, reproducibility, arsenic detection 2

ABSTRACT: We have fabricated 3D surface enhanced Raman scattering (SERS) substrates composed of silver coated gold nanorods (Ag/AuNRs) supported on polycaprolactone (PCL) electrospun fibers for use as sensors of environmental contaminants. The successful fabrication and immobilization of Ag/AuNRs onto PCL fibers further demonstrates the universal nature of our fabrication strategy, benefiting from immobilization based on electrostatic attraction, a non-specific interaction. For the first time, Ag/AuNRs with different Ag coating thicknesses have been synthesized, characterized by UV-Vis spectroscopy and electron microscopy, and assembled onto PCL fibers. The variation of the Ag coating thickness allows the tuning of the surface plasmon resonance (SPR) so as to be in close proximity to the laser wavelength of the probing laser. Using 4-Mercaptopyridine (4-Mpy) as a probe molecule, the SERS performance of the Ag/AuNR based SERS substrate has been investigated. This substrate allowed detection of 4-Mpy at a concentration as low as 10 nanomolar with excellent reproducibility. SERS substrates fabricated with Ag/AuNRs with different Ag coating thicknesses have been compared with substrates using AuNRs. It was found that when fabricated with a Ag coating, the substrates exhibited larger SERS enhancement than that found for substrates fabricated with AuNRs alone. This could be attributed to electron transfer between the gold core and silver shell in a bimetallic rod-shape nanostructure. In addition, considering the surface chemistry provided by silver coating, we have demonstrated that this new SERS substrate can be used as an environmental sensor for arsenic detection.

INTRODUCTION Surface Enhanced Raman Spectroscopy (SERS) is a surfacesensitive resonance scattering technique that amplifies the normal Raman scattering signal by many orders of magnitude. Since its discovery on a roughened silver electrode in the 1970s,1 a large number of fundamental and applied SERS studies have appeared in the literature. The dramatic growth in this technique demonstrates the significant interest in its potential use as a powerful analytical tool in chemical,2 biomedical,3 and environmental sensing,4 combining the fingerprint specificity of Raman spectroscopy with the high sensitivity found with surface enhancement. A major topic in the field of SERS research is the fabrication of SERS substrates, which usually incorporate noble metal nanoparticles (NPs) in order to take advantage of their surface plasmonic properties.5-6 Since the properties of NPs are significantly size- and shape- dependent,7-10 efforts have been made to control the synthesis of metallic NPs to produce particles with various sizes and geometries to serve as SERS substrates. An effective SERS substrate requires both high sensitivity and reproducibility.10-12 An essential factor for achieving high sensitivity is the generation of “hot spots” among closely packed metallic nanostructures, where theoretical calculations suggest that the highest SERS enhancement occurs because of plasmon coupling.13-16 In solution phase, “hot spots” are usually generated by the formation of clusters of SERS-active NPs. Unfortunately, once the NPs aggregate, the solutions become

unstable and cannot sustain the required structures for high levels of enhancement. This makes solid SERS substrates more favorable for practical applications.17 Solid SERS substrates have been produced by utilizing various techniques, including electrochemical deposition,18 Langmuir−Blodgett assembly,19 and electron beam lithography.20 However, these techniques mostly yield two dimensional (2D) SERS substrates. Recently, substrates composed of NPs supported on electrospun polymer fiber mats have been shown to be effective as SERS substrates.21-22 This approach provides opportunities to readily fabricate SERS substrates with more complicated and controlled 3D architectures, providing much higher surface area available for loading SERS-active NPs, which can lead to better sensitivity. Our previous studies23 have proposed the fabrication strategy of an effective SERS substrate composed of highly dense AuNRs assembled on a PCL fiber surface. Thanks to the nonspecific electrostatic attraction which is the driving force for the immobilization of AuNRs onto PCL fibers, this substrate has demonstrated a high reproducibility as well as versatility for detection of various types of analytes by utilizing both 4Mpy (chemically bound via Au-S bonds) and Rhodamine 6G (physically bound via electrostatic absorption) as probe molecules. This 3D AuNR SERS substrate has been compared to a similar 2D planar AuNR/PCL film, showing a 6-fold increase in Raman intensity. To demonstrate that the nonspecific nature of the immobilization process featured in this fabrication protocol can be readily extended to various metallic NPs, we fab-

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ricated SERS substrates based on Ag/AuNRs with various Ag coating thicknesses. In addition, a fibrous mat electrospun from poly(3-hydroxybutyrate-co-3-hydroxyhexanoate), was used as another example to further illustrate the fabrication protocol on other polymers. Ag/AuNRs were chosen for our study because silver is more SERS-active than gold in the visible due to dielectric differences.24 However, significant effort is usually required to obtain anisotropic silver NPs, whereas many methods and techniques have been developed to synthesize highly monodisperse AuNRs with various aspect ratios.25-27 Coating a silver layer onto the AuNRs offers an easy way to obtain bimetallic NPs carrying the desired SERS activity of silver. Studies also suggest Ag-Au bimetallic NPs with Ag as the coating layer show superior SERS sensitivity over their monometallic NPs and Au-Ag bimetallic NPs with Au as the outer shell.28-29 This is thought to originate from a charge transfer between the two metallic components.28 In addition, the silver surface can exhibit chemical properties not found for gold, which could allow the substrate to have multiple functions including antibacterial properties30 and catalytic activity31 simultaneously with SERS detection. As an application of this substrate as environmental sensor for arsenic detection, the silver surface of the Ag/AuNRs can anchor various arsenic species directly via Ag-O-As bonds for SERS analysis. This cannot be done using the previous AuNR substrate. To carry out this SERS fabrication protocol more systematically in the present work, we further compared the SERS performance of 3D Ag/AuNR substrates compared to the previously studied 3D AuNR substrates and the Ag/AuNRs showed an overall improvement in Raman intensities using 4-Mpy as the probe molecule.

EXPERIMENTAL SECTION Poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHx, with 3.9 mol % Hx content Mw = 843 000 g/mol, PDI = 2.2) was supplied by the Procter & Gamble Company. Cetyltrimethylammonium bromide (CTAB) and cetyltrimethylammonium chloride (CTAC) were purchased from TCI. All the other chemicals were purchased from Sigma-Aldrich. All chemicals were used as received. Preparation of SERS-active Metallic Nanorods. AuNRs were first synthesized by following the classical protocol, seed-mediated wet chemical method, developed and refined by Murphy25 and El-Sayed.26 As for Ag/AuNRs synthesis, a silver inorganic salt was mildly reduced and deposited onto AuNRs using L-ascorbic acid under basic conditions as reported in previous studies.32 To introduce charges onto the Ag/AuNR surface for immobilization, Ag/AuNRs were further coated with a layer of poly (sodium 4-styrenesulfonate) (PSS). (PSS/Ag/AuNRs, See Supporting Information for details.) SERS-active metallic NPs were characterized using transmission electron microscopy (TEM, JEOL JEM-3010, 300 kV accelerating voltage) and UV-Vis spectroscopy (Agilent Cary 60 UV-Vis Spectrometer). Preparation and Surface Modification of PCL Fibrous Mat. PCL fibrous mats were produced by electrospinning. To establish positive charges on the surface of the PCL fibrous mat, it was subjected to polyelectrolyte Layer-by-Layer (LbL) deposition with positively charged PDADMAC as the outermost layer in preparation for immobilization of the NPs. (See Supporting Information for details.) A morphology change

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with increasing deposition of layers was observed using scanning electron microscopy (SEM, Auriga 60 CrossBeam). Immobilization of PSS/Ag/AuNRs on PCL Fibrous Mats. The modified PCL mat was cut into small pieces (typically 1.0 cm x 4.0 cm), and immersed into 6 mL of the PSS/Ag/AuNRs solution for the desired period of time. Afterwards, the mat was washed with DI water to remove loosely bound PSS/Ag/AuNRs and then left to dry under ambient conditions. Thus, a SERS substrate with PCL fibrous mat decorated with PSS/Ag/AuNRs was formed. Evaluation of SERS Sensitivity and Reproducibility. To evaluate the sensitivity of the substrate, 4-Mpy was used as a probe molecule. A SERS substrate was cut into identical small pieces and immersed into 9 mL of 4-Mpy/ethanol solutions with a series of concentrations ranging from 10-4 M to 10-8 M for 2 hours. Afterwards, the SERS substrates are taken out of the solution, washed with ethanol, and dried prior to Raman measurement. To evaluate the reproducibility of the substrate, a SERS substrate immersed into 10-5 M 4-Mpy/ethanol solution was used. SERS measurements were taken on 9 randomly chosen spots on the substrate. The SERS measurements are performed using a Kaiser Holospec1.8 Raman Spectrograph equipped with a TE cooled Andor CCD and an ONDAX 785 laser and probe head. The Raman instrument uses an 800 lines/mm transmission holographic grating and the numerical aperture is 0.9 with a magnification of 65x. Spectra were recorded using 785 nm excitation (~1mW at sample surface) using a 30 s exposure.

RESULTS AND DISCUSSION Characterization of SERS-active Metallic Nanorods. The UV-Vis absorption spectra of AuNRs, Ag/AuNRs and PSS/Ag/AuNRs aqueous solutions are shown in Figure 1A. The longitudinal surface plasmon resonance (SPR) peak of AuNRs typically is centered at approximately 765 nm, and blue shifts to 705 nm after Ag coating. Adding a PSS coating does not change the position of the longitudinal SPR peak, which indicated that no aggregation occurred during the coating process. The transverse SPR peak of AuNRs is found near 515 nm, and only exhibits a very slight blue shift after Ag coating. The TEM images of AuNRs and Ag/AuNRs are shown in Figure 1B and 1C, respectively. The dimensions of the as-synthesized AuNRs are estimated to be 60 nm in length and 16 nm in width with an average aspect ratio of 3.75. The centers of longitudinal SPR peak of as-synthesized AuNRs typically ranges from 800 nm to 760 nm due to slight variations in the synthetic protocol (i.e. temperature). After Ag coating, a thin layer (approximately 3 nm) of Ag follows the contour of the AuNRs as shown in Figure 1C. Zeta potentials for AuNRs, Ag/AuNRs and PSS/Ag/Au were measured as +31.1 mV, +49.6 mV and -39.8 mV respectively (data not shown). The inversion of the zeta potential after PSS coating onto Ag/AuNRs demonstrated that a successful PSS coating was formed. AgNO3 was the source of Ag (I) ions for coating Ag onto the AuNRs in this study. With an increasing amount of AgNO3 added, the color of the Ag/AuNR solution turns from light brown to bright green (digital photo inserted in Figure 2A), the longitudinal SPR peak of Ag/AuNRs gradually blue shifts to approximately 650 nm as the UV-Vis spectrum shows in Figure 2A, and a thicker Ag coating layer is deposited onto the surface of the AuNRs as the TEM images show Figure 2B-D.


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The thickness of the Ag coating layer increases from about 3 nm to over 7 nm when the amount of AgNO3 is increased. In Figure 2D, the AuNR core is observed to distort its cylindrical shape, becoming thicker at both ends, and thinner in the middle. In addition, the silver coating seems to thicken more than lengthen. However, since the same recipe is used in the synthesis of silver coatings, the crystalline properties of the silver layers should be identical. Thus, variations in shapes of the rods could be ascribed to experimental variations from batch to batch during nanoparticle synthesis. Overall, the molar ratio of silver and gold in the Ag/AuNRs calculated from the estimated dimensions in the TEM results corresponds to the amounts of reagents, particularly silver nitrite and chloroauric acid used in the synthesis.

lets on the pores of the meshes with thinner fibers prevents penetration into the decreasing pore sizes. The reduction in penetration of polyelectrolytes could result in a less uniform surface modification of the fibrous mesh. In addition, it takes a longer time to wash the NaCl off the mesh with smaller fiber diameters. Cleaning the salts off the mesh is significant for the following immobilization since the salt can induce the aggregation and precipitation of the NPs, resulting in a non-uniform coverage of NPs on the fiber surface. Considering time efficiency and uniformity of the substrate fabrication, a PCL fibrous mesh with fiber diameters of approximately 3 µm was typically used in our study, and the optimum number of LbL deposition layers has been determined to be 7 (designated as (PDADMAC/PSS)3.5-PCL denoting three and a half cycles of PDADMAC/PSS deposition onto PCL fibers with the 7th PDADMAC layer as the outermost layer).

Figure 1. (A) UV-Vis spectra of AuNRs, Ag/AuNRs and PSS/Ag/AuNRs. (B-C) TEM images of AuNRs and Ag/AuNRs. The scale bar is 20 nm.

Figure 2. (A) UV-Vis spectra, digital photo (the insert), (B-D) TEM images of Ag/AuNRs, with molar ratio of Au to Ag: 4.25, 2.13, and 1.42 respectively. The scale bar is 20, 10, and 5 nm, respectively.

Polyelectrolyte LbL Deposition on PCL Fibers. Pristine PCL fiber surfaces can be modified by polyelectrolyte LbL deposition. The process has been well studied in previous investigations where changes of surface chemistry, morphology and water contact angle have been elucidated in detail.23 By changing the CHCl3/DMF ratio of the solvent for electrospinning, PCL fibrous meshes with various fiber diameters, ranging from hundreds of nanometers to several micrometers, can be obtained (Figure S2), and LbL deposition can be readily carried out on these meshes. For instance, as the LbL deposition cycles increase, the surface morphology of PCL fibers (approximately 1 µm in diameter) changes from rough to smooth, indicating an increasing extent of polyelectrolyte deposition, which can easily be observed in the SEM images (Figure S3). It is worth mentioning that we have observed the restriction of polyelectrolyte solution penetration into the mesh with thinner, submicron fibers. Presumably this occurs because the surface tension of the LbL polymer solution drop-

Immobilization of PSS/Ag/AuNRs. The dark green color of the substrate (digital photo inserted in Figure 3A) and the lack of UV-Vis absorbance (Figure 3A) of the PSS/Ag/AuNRs solution after immersing a piece of (PDADMAC/PSS)3.5-PCL substrate into the solution for 24 hours suggest a highly effective immobilization process has occurred with a complete transfer of PSS/Ag/AuNRs from the aqueous solution to the solid substrate. Under moderate shaking (reciprocating shaker, CMS EQUATHERM), which facilitates the penetration and accessibility of the PSS/Ag/AuNRs onto inner fiber surfaces, the immobilization process can be completed within hours, increasing the fabrication efficiency significantly. A densely packed monolayer assembly of PSS/Ag/AuNRs immobilized on the PCL fiber surface is illustrated in the SEM images in Figure 3B-C. A direct immobilization of Ag/AuNRs without a PSS coating was first attempted, since the as-synthesized Ag/AuNRs are



stabilized with surfactants (CTAB and CTAC) and the zeta potential was measured above +40 mV, which theoretically suggests a stable colloidal solution ready for immobilization onto a negatively charged surface.33 Uniform decoration of Ag/AuNRs onto PCL fibrous mesh, however, was not obtained consistently, and Ag/AuNRs were found to be partially aggregated and precipitated during the immobilization. Since, surfactant molecules absorbed on NPs form bilayers34 and undergo a dynamic absorption/desorption equilibrium, we hypothesize that a direct immobilization of NPs is not robust when the surfactant bilayers on NPs interacting with the negatively charged fiber surface could experience a displacement off the NPs that cause the resultant desorption of surfactants and aggregation of the NPs. Therefore, a PSS coating onto NPs is preferred for further protection and stabilization during immobilization.

Figure 3. (A) UV-Vis spectra of PSS/Ag/AuNRs before and after a 24h immobilization. The insert is the digital photo of the substrates after a 24h immobilization. (B-C) SEM images of PSS/Ag/AuNR decorated PCL fibers. The scale bars are 5 µm and 1 µm, repectively.

Immobilization occurring on the fiber surface in solution is a subtle process. Parameters including centrifugation cycles, speed, and time need to be engineered and optimized in order to obtain densely packed coverage of metallic NPs on the fiber surface. Multiple competing processes exist during immobilization. One is the competition between the free surfactant molecules left in solution after centrifugation and surfactants deposited on the NPs. Once the free surfactants, which have better mobility than those on the NPs, adsorb onto the fiber surface, the surface area of fibers occupied by free surfactants is no longer available for immobilization due to the electrostatic repulsion. Consequently, this results in more sparse coverage of NPs. Another competition is between the affinity of NPs among themselves and their affinity with the substrate. It

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should be noted that there exists a range of zeta potentials for colloids that make them associate with each other via hydrophobic interactions rather than with the substrate, but neither necessarily leads to a considerable precipitation.35 The aggregation of colloids with themselves rather than with substrate can result in a very low density of NPs on the fiber surface. Ionic strength of the PSS solution is also critical for effective PSS coating. A high ionic strength can directly induce aggregation of NPs. The negatively charged side groups are repulsive and force the PSS molecular chain to extend itself, while rod-shape NPs are highly curved. Thus, for a specific molecular weight of PSS (Mw = 70 000 g/mol in this study), a certain ionic strength is needed to partially screen the electrostatic charges on the chain and help to relax the chain conformation needed for a complete wrap around the contour of the particle. Usually, the higher molecular weight of a polyelectrolyte, the higher ionic strength is needed for deposition onto curved NPs.36 SERS Evaluation. The SERS performance of the Ag/AuNR based substrate was investigated by using 4-Mpy, which binds onto the Ag/AuNR surface chemically via Ag-S bond formation. The SERS spectra of 4-Mpy at varying concentrations is shown Figure 4A. The spectra demonstrate the sensitivity of this substrate by the detection of 4-Mpy at a concentration as low as 10-8 M (10 nM). When the concentration of 4-Mpy is lowered, the background of the bare SERS substrate appeared, arising primarily from the PSS coating around the Ag/AuNRs (see Raman spectrum of PSS powder inserted in Figure 4A). The background, however, does not interfere with the spectra of 4-Mpy at low concentrations where peaks at 1098 cm-1 and 1008 cm-1 are still detectable. Actually, since we have excellent spectral reproducibility, specific peaks from the background can be selected as a reference for background subtraction yielding neater spectra and better quantification for trace level detection. The reproducibility of the substrate was also examined by randomly selecting spots on the substrate. The SERS spectra measured are shown in Figure 4B. The average and variations of peak intensities of 4-Mpy were measured (Figure S4) and are summarized in Table 1 along with the peak assignments.3738 The relative standard deviation (RSD) for major peaks of 4Mpy are all less than 7%, which demonstrates excellent reproducibility across a specific preparation of substrate material. This results from uniform monolayer coverage through Ag/AuNR immobilization, and, in turn, reinforces the fact that uniformity can be achieved by this immobilization strategy. Beyond sensitivity and reproducibility, the shelf-life of Ag/AuNR-based substrates is of concern, since it is wellknown that silver is easily oxidized. Oxidation of the silver surface can severely impact the SERS performance of the substrate. We tested SERS performance of a Ag/AuNR-based substrate from the same batch after aging under ambient conditions for 3 months. As shown in Figure 4C, the SERS signals obtained on a 3-month-aged substrate are comparable in sensitivity to measurements on a fresh substrate. The long shelf-life of the Ag/AuNR- based SERS substrate could be attributed to the protection provided by the surfactants and PSS coating, which hinder the transport of oxygen to the surface of Ag/AuNRs thus inhibiting oxidation.


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Figure 5. (A) SEM images of PSS/Ag/AuNR decorated PHBHx electrospun fibrous mat. The scale bar is 5 µm (1 µm for the insert). (B) SERS spectrum of 4-Mpy at a concentration of 10-5 M on PSS/Ag/AuNRs decoreated PHBHx substrate. The insert is the molecular structure of PHBHx polymer.

Figure 4. (A) SERS spectra of 4-Mpy at concentrations from 10-4 M to 10-8 M, each spectrum is an average of three measurements on the substrate. Insert: Magnified spectra of 4-Mpy at 10-7 M and 10-8 M from 1500 cm-1 to 500 cm-1, background spectrum of bare SERS substrate and Raman spectrum of PSS powder. (B) SERS spectra of 10-5 M 4-Mpy at 9 randomly selected spots on the Ag/AuNR based SERS substrate. (C) SERS spectra of 4-Mpy at a concentration of 10-5 M on the same batch of Ag/AuNR based substrate when freshly fabricated and aged for 3 months.

To simply reinforce the universal nature of the non-specific immobilization, the fabrication can be readily implemented on electrospun PHBHx fibers (shown in Figure 5A). PHBHx is a novel type of biodegradable and biocompatible polyester.

Comparison between AuNR- and Ag/AuNR- Based SERS Substrates. The SERS performance of substrates fabricated with AuNRs and Ag/AuNRs having different Ag coating thicknesses have been compared. The spectra are labeled with the type of the SERS-active NPs used to fabricate the substrate followed by the longitudinal SPR peak of these NPs measured in the solution phase. Ag/AuNR substrate_673nm, for instance, denotes a SERS substrate fabricated with Ag/AuNRs having a longitudinal SPR peak at 673 nm in solution. The SERS spectra of 10-5 M 4-Mpy on these substrates were measured randomly on three different spots, averaged and are shown in Figure 6A. The peak intensities of 4-Mpy on these substrates are shown in Figure 6B. It is known that spacing between nanoparticles is critical for the generation of “hot spots” and increased SERS sensitivity. To assure that the performance difference is not governed by a difference in the density of NPs immobilized, SEM images of these substrates have been taken to confirm that there is little, if any, difference in the densities of NPs for different substrates (Figure S5). Although the spatial resolution of the SEM images is insufficient to provide quantitative information on spacings between the rodshape nanoparticles on the 3D substrates, the size of the PCL mat and the amount of nanoparticles for fabrication are all



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Table 1. Peak assignments, averaged intensities, intensity variations of 4-Mpy major peaks Peak (cm-1)

Symmetry Assignment37-38

Average Peak Intensity

RSD (%)

1576

8B2

ν(CC) with deprotonated nitrogen

404±27

6.6

1217

12A1

β(CH)/δ(NH)

173±8

4.6

1098

12A1

ν (ring breathing)/ν(CS)

871±40

4.6

1061

18A1

β(CH)

292±11

3.8

1008

1A1

ring breathing

618±39

6.4

709

6A1

β(CC)/ν(CS)

95±5

5.6

432

7A1

δ(CS)/γ(CCC)

217±6

2.7

ν, stretching; β, bending; δ, in-plane deformation; γ, out-of-plane deformation.

well controlled. The surface area of PCL fibrous mesh was examined using BET surface area measurements (0.90 ± 0.10 m2/g). From the UV-Vis spectra before and after immobilization, the number of nanoparticles adsorbed onto substrates can also be well estimated. The SERS intensity comparisons in Figure 6B show that not all the peak intensities obtained from Ag/AuNR-based substrate are larger than those obtained from the AuNR-based one. The fact that the peak intensities at 432 cm-1 and 709 cm-1 are comparable among all the substrates also indicates that the different performance of various SERS substrates is not because of a nanoparticle density difference on the substrates. Figure 6B also shows that the Ag/AuNRbased SERS substrates had better performance than the AuNR-based substrates particularly for peaks 1008 cm-1, 1098 cm-1 and 1576 cm-1, which are assigned to ring breathing, ν (ring breathing)/ν(CS) and δ(CS)/γ(CCC) respectively.37-38 The 4-Mpy peaks are enhanced more on Ag/AuNRs compared to the AuNRs even though 785 nm excitation is usually more favorable for gold in SERS because of its dielectric properties.24 This could be due to the properties of the bimetallic junction where charge transfer occurs between the bimetallic nanostructures. When silver is coated onto the AuNRs, the surface is covered gradually by metallic silver, so the surface of the nanorods becomes gradually silver-like, allowing charge transfer from the gold core to silver shell. Given the Fermi levels of Au (-5.0 eV) and Ag (-4.6 eV),28 there is likely an increase in the localized electromagnetic fields and the SERS enhancement of analytes around the NPs. The 4-Mpy molecule is known to exhibit C2v symmetry with two in-plane vibrational modes A1, B2, and two out-of-plane vibrational modes A2 and B1.37, 39 As SERS selection rules suggest,40 in-plane modes of 4-Mpy are much more sensitive to the localized electromagnetic field than those out-of-plane modes. Thus, the intense peaks found in the SERS spectrum of 4-Mpy, 1008 cm-1 [ring breathing mode, A1], 1098 cm-1 [ν (ring breathing)/ν(CS),12A1] and 1576 cm-1 [ δ(CS)/γ(CCC), 8B2], experienced more enhancement than other peaks when the substrate is Ag/AuNR-based instead of AuNR-based. In this study, the “Ag/AuNR substrate_673nm” showed the highest enhancement, while the “Ag/AuNR substrate_664nm” showed decreasing SERS intensities compared to “Ag/AuNR substrate_673nm” for 10-5 M 4-Mpy solutions. This could be

Figure 6. (A) The SERS spectra of 10-5 M 4-Mpy measured on different substrates. (B) A comparison of peak intensities between different SERS substrates.

attributed to the gradual offset of longitudinal SPR peak for the Ag/AuNRs relative to the laser wavelength, namely 785nm, which could decrease the coupling effects between the Ag/AuNRs and laser light, decreasing the SERS intensities. In addition, the SERS enhancement factor (EF) of the “Ag/AuNR substrate_703nm” is calculated by following the method used in our previous study (Figure S6).23 The EF of the peak origi-


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nating from the ring breathing mode (1116 cm-1 for normal Raman, 1008 cm-1 for SERS) is estimated to be 3.4 x 104 and the EF of the peak originating from ν(ring breathing)/ν(CS) (1001 cm-1 for normal Raman, 1098 cm-1 for SERS) is estimated to be 1.6 x105. Compared with AuNR-based substrates investigated in our previous study, which showed EFs of 1.5x104 and 1.3x105 respectively, the increase of the SERS signals showed in Figure 6B is consistent. That is, “Ag/AuNR substrate_703nm” has a 2-fold increase for the ring breathing mode and a 1.2-fold increase for ν(ring breathing)/ν(CS) mode in peak intensity compared to “AuNR substrate_778nm”. Potential application as environmental sensors. Arsenic(As) is a highly toxic element that naturally exists in drinking water in various forms. Considering the dentate complexation of arsenic compounds to a silver surface through As-O,41-42 the Ag/AuNR based SERS substrate can be used as an environmental sensor for arsenic detection. As shown in Figure 7, both inorganic arsenic, arsenate and organic arsenic, parsanilic acid and roxarsone can be detected on this substrate at trace concentration. The latter two compounds are widely used poultry antimicrobials. A more expanded investigation of chemisorption of arsenic species onto this substrate are currently being studied.

Supporting Information. Additional information is noted in this section. This material is available free of charge via the Internet at http://pubs.acs.org. Experimental details of the AuNRs synthesis, Ag/AuNRs synthesis, PSS coating onto Ag/AuNRs, electrospinning of the polymer fibrous mat, surface modification of polymer fibrous mat; TEM images of AuNRs and Ag/AuNRs; SEM images of PCL fibers electrospun from different solvents; SEM images of PDADMAC/PSS layer-by-layer deposition on PCL fibers; peak intensities and variations of 10-5 M 4-Mpy on Ag/AuNR SERS substrate; SEM images of AuNR- and Ag/AuNR- based SERS substrates; Normal Raman and SERS of 4-Mpy for EF calculation.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. Fax: 302-831-4545. Tel.: 302-8314476

Funding Sources The authors would like to acknowledge the support from Delaware NSF EPSCoR Grant # 1301765 and the NSF DMR Polymers Program Grant DMR-1407255.

Notes The authors declare no competing financial interest.

REFERENCES

Figure 7. SERS detection of different arsenic species onto a Ag/AuNR based SERS substrate.

CONCLUSION The SERS performance of a substrate is affected by many factors, including the properties of metallic NPs, the type of analyte molecules, the absorption efficiency and the laser excitation used. In this study, we demonstrate that, by incorporating Ag/AuNRs, the fabricated 3D SERS substrate performs significantly better than AuNR-based substrates under the same conditions. This superiority stems from the charge transfer between the gold core and silver layer in the bimetallic structure. It is our conjecture that this charge transfer increases the localized electromagnetic field generated around the NPs and enhances the SERS signal. In addition, Ag surfaces can expand the surface chemistries available for molecular interaction. Arsenic compounds, for instance, can be chemisorbed on silver based SERS substrates directly via Ag-O bonding.42-43 This environmental sensing application to arsenic species detection is currently being pursued by utilizing Ag/AuNR-based SERS substrates in our studies.

ASSOCIATED CONTENT

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Figure 1. (A) UV-Vis spectra of AuNRs, Ag/AuNRs and PSS/Ag/AuNRs. (B-C) TEM images of AuNRs and Ag/AuNRs. The scale bar is 20 nm. 134x159mm (96 x 96 DPI)


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Figure 2. (A) UV-Vis spectra, digital photo (the insert), (B-D) TEM images of Ag/AuNRs, with molar ratio of Au to Ag: 4.25, 2.13, and 1.42 respectively. The scale bar is 20, 10, and 5 nm, respectively. 136x144mm (96 x 96 DPI)



Figure 3. (A) UV-Vis spectra of PSS/Ag/AuNRs before and after a 24h immobilization. The insert is the digital photo of the substrates after a 24h immobilization. (B-C) SEM images of PSS/Ag/AuNR decorated PCL fibers. The scale bars are 5 µm and 1 µm, repectively. 129x147mm (96 x 96 DPI)


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Figure 4. (A) SERS spectra of 4-Mpy at concentrations from 10-4 M to 10-8 M, each spectrum is an average of three measurements on the substrate. Insert: Magnified spectra of 4-Mpy at 10-7 M and 10-8 M from 1500 cm-1 to 500 cm-1, background spectrum of bare SERS substrate and Raman spectrum of PSS powder. (B) SERS spectra of 10-5 M 4-Mpy at 9 randomly selected spots on the Ag/AuNR based SERS substrate. (C) SERS spectra of 4-Mpy at a concentration of 10-5 M on the same batch of Ag/AuNR based substrate when freshly fabricated and aged for 3 months. 137x289mm (96 x 96 DPI)



Figure 5. (A) SEM images of PSS/Ag/AuNR decorated PHBHx electrospun fibrous mat. The scale bar is 5 µm (1 µm for the insert). (B) SERS spectrum of 4-Mpy at a concentration of 10-5 M on PSS/Ag/AuNRs decoreated PHBHx substrate. The insert is the molecular structure of PHBHx polymer. 139x215mm (96 x 96 DPI)


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Figure 6. (A) The SERS spectra of 10-5 M 4-Mpy measured on different substrates. (B) A comparison of peak intensities between different SERS substrates. 137x230mm (96 x 96 DPI)



Figure 7. SERS detection of different arsenic species onto a Ag/AuNR based SERS substrate. 225x185mm (300 x 300 DPI)


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