Immobilization of Gold Nanorods onto Electrospun Polycaprolactone

Oct 18, 2013 - Wenqiong Tang, D. Bruce Chase, and John F. Rabolt*. Department of Materials Science and Engineering, University of Delaware, Newark, ...
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Immobilization of Gold Nanorods onto Electrospun Polycaprolactone Fibers Via Polyelectrolyte DecorationA 3D SERS Substrate Wenqiong Tang, D. Bruce Chase, and John F. Rabolt* Department of Materials Science and Engineering, University of Delaware, Newark, Delaware 19716, United States S Supporting Information *

ABSTRACT: We report the fabrication of a homogeneous and highly dense gold nanorod (AuNR) assembly on electrospun polycaprolactone (PCL) fibers using electrostatic interaction as the driving force. Specifically, decoration of a poly(sodium 4styrenesulfonate) (PSS) layer onto the AuNRs imposed negative charges on the nanorod surface, and the interactions between PSS and the AuNRs were investigated using attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). Positive charges on the PCL fibrous substrate were established via polyelectrolyte layer-by-layer deposition, which was investigated using multiple characterization techniques. Driven by the attractive electrostatic interaction, immobilization of AuNRs on the PCL fibers was initiated upon substrate immersion, and the kinetics of the immobilization process were studied using UV−vis spectroscopy. Electron microscopy characterization of the AuNR/PCL nanocomposite fibers reveals a uniform AuNR coating on the fiber surface with the immobilized AuNR density being high enough to provide full surface coverage. By using both 4-mercaptopyridine and Rhodamine 6G as probe molecules, the performance of the AuNR/PCL fibrous mesh as a three-dimensional (3D) surface-enhanced Raman scattering (SERS) substrate was investigated. The nanocomposite fibers allowed detection at concentrations as low as 10−7 M of the probe molecule in solution and exhibited excellent reproducibility in the SERS measurements. In addition, a comparison between the 3D AuNR/ PCL fibrous mesh and a 2D AuNR/PCL film reveals that the enhanced surface area in the 3D substrate effectively improved the SERS performance with a 6-fold increase in the Raman intensity.

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Langmuir−Blodgett assembly,13,16,17 assembly driven by the interaction between the capping molecules on nanorod surfaces,18,19 assembly at the liquid−liquid interface,20 and so forth. Unfortunately, the applicability of the aforementioned approaches is generally limited to two-dimensional (2D) planar substrates. When it comes to sensing applications, threedimensional (3D) substrates with hierarchical structures (e.g., fibrous meshes, micro- or nanosphere aggregates, etc.) are preferred due to the enhanced surface area, which leads to better detection efficiency.21−23 To produce MNP assemblies on 3D structures, substrates carrying metal-affinitive functional groups (e.g., thiol groups, pyridyl groups, etc.) are generally chosen.21,24,25 The exclusive chemistry which occurs between the MNPs and the substrate is then utilized as the driving force for MNP immobilization. Since the MNP immobilization relies on the formation of specific chemical bonds, the applications of those proposed methods are confined to a limited range of substrate materials. Unlike the MNP immobilization driven by the chemical interaction, electrostatic attraction is a nonspecific interaction and thus can be utilized to develop a universal MNP immobilization strategy. Up until now, quite a few studies on

etallic nanoparticles (MNPs) have generated extensive research interest during the past few decades due to their fascinating optical, electronic, and catalytic properties.1 Since the properties of the MNPs are size- and shapedependent,2−5 significant research efforts have been devoted to the controlled synthesis of MNPs with anisotropic geometries, such as gold nanorods (AuNRs).6−8 Due to the geometrical change, AuNRs exhibit substantially different properties compared to spherical gold nanoparticles,9−11 one of which is their plasmonic properties. Unlike spherical gold nanoparticles which give rise to one single surface plasmon resonance (SPR) band in the extinction spectrum, the SPR for AuNRs splits into two modes. The electron oscillation along the short and long axis of the nanorods results in the appearance of a transverse SPR band and a much stronger longitudinal SPR band, respectively.1,11 Besides the strong light adsorption, the longitudinal SPR band of the AuNRs can be effectively tuned from the visible to the near-infrared region,3,11 facilitating easy coupling to commercial laser sources. Thus, AuNRs have become very promising building blocks for surface-enhanced Raman scattering (SERS) substrates. One critical feature of an effective SERS substrate is the presence of “hot spots”, which arise from the plasmon coupling between the closely packed metallic nanostructures.12,13 Various techniques have been utilized to produce such highly dense nanorod assemblies, including solvent evaporation,14,15 © 2013 American Chemical Society

Received: February 6, 2013 Accepted: October 18, 2013 Published: October 18, 2013 10702

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ATR accessory. All the ATR-FTIR spectra were recorded with 4 cm−1 resolution. Preparation of Positively Charged PCL Fibrous Substrates. The PCL fibrous substrates were first fabricated using the electrospinning technique (see Supporting Information for details). To develop the positive charges on the substrate, the polyelectrolyte LBL deposition technique was utilized. Briefly, polyelectrolyte multilayers were deposited onto PCL mesh by the alternate immersion of PCL mesh into poly(diallyldimethylammonium chloride) (PDADMAC) solution (3 mg/mL) and PSS solution (3 mg/mL) with washing steps between. The ionic strength of the polyelectrolyte solutions was adjusted to 1 M by the addition of NaCl, and the deposition cycle was repeated several times until the desired number of polyelectrolyte layers had been deposited. ATR-FTIR measurements were carried out to study the polyelectrolyte multilayer deposition process. The ATR-FTIR spectra of PSS, pristine PCL mesh, and PCL meshes coated with various numbers of polyelectrolyte layers were recorded with 4 cm−1 resolution. Immobilization of Gold Nanorods. The polyelectrolyte multilayer decorated PCL mesh (PEM-PCL) was cut into small pieces, each with a size of approximately 1 cm2. The 1 cm2 mesh was subsequently immersed into 9 mL of the PSSdecorated AuNR (PSS-AuNR) solution for 24 h. Afterward, the PEM-PCL mesh was taken out from the PSS-AuNR colloidal solution and washed with DI water to remove the loosely bound AuNRs. The AuNR/PCL nanocomposite mesh obtained was left to dry under ambient condition before further use. To elucidate the role of electrostatic attractive forces in the immobilization process, the AuNR immobilization kinetics were studied using UV−vis spectroscopy. Basically, aliquots of PSS-AuNR solution were transferred into the UV cuvettes at fixed immersion time intervals, and the corresponding UV−vis spectra were recorded. Immediately afterward, the solution in the cuvette was transferred back into its vial to maintain a constant volume of stock solution during the AuNR immobilization process. At the same time, a control experiment was conducted by immersing a piece of pristine PCL mesh (instead of the PEM-PCL mesh) into the AuNR solution. Visualization of the AuNR assembly on the PCL fiber surface was achieved using scanning electron microscopy (SEM, JSM7400F). The SEM images were recorded under either LEI or SEI mode at various magnifications. SERS Evaluation. SERS measurements on the AuNR/PCL nanocomposite fibers were investigated using both 4mercaptopyridine (4-MPy) and Rhodamine 6G (Rh6G) as probe molecules. To prepare the samples of 4-MPy, five pieces of AuNR/PCL nanocomposite meshes were soaked in 7 mL of 4-MPy/ethanol solutions with varying concentrations (from 10−3 to 10−7 M) for 2 h. Afterward, the meshes were washed first with ethanol followed by washing with DI water to remove the loosely bound 4-MPy molecules. The substrates were left to dry under reduced pressure before SERS measurements. In addition, for a negative control, a sample was fabricated by soaking a piece of PEM-PCL mesh in 7 mL of 5 × 10−3 M 4MPy/ethanol solution for 2 h. Similar procedures were carried out in the preparation of Rh6G samples. The SERS measurements were performed on a Raman instrument, which was composed of an Invictus diode laser with 785 nm excitation and a Kaiser Optical Systems, Inc. (Ann Arbor, Michigan) Holospec VPT System. The power of the

the electrostatic-attraction-assisted AuNR immobilization have been reported. For example, Murphy et al.26,27 fabricated the AuNR monolayer assemblies on polyelectrolyte pretreated glass slides. In addition, by replacing one of the polyelectrolytes with the charged AuNRs in the traditional polyelectrolyte layer-bylayer (LBL) deposition process, Hu et al. created the AuNR multilayer assemblies on an ITO substrate after multiple deposition cycles.28 What is worth mentioning is that in both of these two cases, AuNR assemblies were formed on 2D planar substrates. The electrostatic-attraction-assisted AuNR immobilization on 3D substrates, especially polymer fibrous meshes, is scarcely seen in the reported literature.29−31 In this article, we demonstrate the effective fabrication of AuNR assemblies on a 3D electrospun polycaprolactone (PCL) mesh using electrostatic attraction as the driving force. Opposite charges were first established on the AuNRs and the PCL fibers via polyelectrolyte LBL deposition. Immobilization of AuNRs was initiated upon immersing the positively charged PCL fibrous mesh into the negatively charged AuNR colloidal solution. With the proper immersion time, a homogeneous and highly dense AuNR assembly was produced on the PCL fiber surface. The obtained 3D AuNR/PCL fibrous mesh was highly SERS active and exhibited a 6-fold higher enhancement in the Raman signal of 4-mercaptopyridine (with excellent reproducibility) compared to the 2D AuNR/PCL film counterpart. Since the polyelectrolyte LBL deposition technique is independent of both the material composition and the morphology of the substrate,32−34 the fabrication approach proposed in this article can be readily extended to the production of nanocomposite materials with various compositions and morphologies.

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EXPERIMENTAL SECTION All the chemicals were purchased from Sigma-Aldrich and used as received unless specified otherwise. A schematic illustration showing the fabrication of AuNR/PCL nanocomposite fibers can be found in Figure S1 in the Supporting Information. Preparation of Negatively Charged Gold Nanorods. The AuNRs were first synthesized using the well-established seed-mediated growth protocol6 (see Supporting Information for details) and were characterized using transmission electron microscopy (TEM, JEOL JEM-2000FX, 200 kV accelerating voltage) and UV−vis spectroscopy (Shimadzu UV-3600 UV− vis-NIR spectrophotometer) before further use. Negative charges were then established on AuNRs through the deposition of a poly(sodium 4-styrenesulfonate) (PSS) layer. In a typical procedure, aliquots of the as-synthesized AuNR solution were transferred into 2 mL microcentrifuge tubes and centrifuged at 13 000 rpm for 12 min. The precipitates from each tube were then redispersed in 0.75 mL of a 2 mg/mL PSS solution. The ionic strength of the PSS solution was previously adjusted to 1 mM by the addition of NaCl (99%+, ACROS). The redispersed AuNR solutions were left undisturbed for 1 h to allow for the PSS adsorption. Thereafter, the solutions were centrifuged twice at 13 000 rpm for 12 min to wash off the excess PSS. The final precipitates were redispersed in DI water (18.2 MΩ·cm, Millipore Co.) with a AuNR concentration that was 2/3 of its original concentration. To demonstrate the decoration of PSS onto the AuNRs, attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) measurements were carried out on a Nexus 670 FTIR instrument (Thermo Nicolet) with a PIKE MIRacle 10703

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laser was 2 mW at the sample surface, and all the SERS spectra were collected using a 30s exposure time.

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RESULTS AND DISCUSSION PSS Decoration on Gold Nanorods. The as-synthesized AuNRs were estimated to be 37.3 ± 7.6 nm in length and 12.1 ± 1.9 nm in width (average aspect ratio ∼3), exhibiting the transverse SPR absorbance at 521 nm and the longitudinal SPR absorbance at 786 nm, respectively. A representative TEM image and the UV−vis absorption spectrum of the AuNRs are provided in Figure S2 in the Supporting Information. Theoretically, the CTAB-stabilized gold nanorods which carry net positive charges in solution can be readily immobilized onto a negatively charged surface.35 However, one critical feature of an effective SERS substrate is the presence of hot spots, which usually exist at the junction of densely packed nanorod assemblies.12,13 Deposition of an additional PSS layer onto the as-synthesized CTAB stabilized AuNRs reverses their surface charges and increases their zeta potential, 26 which further guarantees a more effective immobilization process and favors the formation of a denser nanorod assembly on the substrate. To validate the successful deposition of the PSS, ATR-FTIR spectra of pure PSS, CTAB-stabilized AuNRs (CTAB-AuNRs), and PSS-decorated AuNRs (PSS-AuNRs) were collected and are shown in Figure 1A, B, and C, respectively. A comparison between the spectra of CTAB-AuNRs and PSS-AuNRs indicates that the two most intense peaks at around 964 and 908 cm−1 (labeled by the green asterisks) in the spectrum of CTAB-AuNRs, which arise from the vibration of the C−N+ groups in the CTAB bilayer on the AuNR surface,35 show up at the same position in the spectrum of PSS-AuNRs. In addition, the peaks at around 1124, 1034, and 1009 cm−1 (labeled by the black arrows) in the spectrum of PSS-AuNRs also match nicely with those specific peaks from PSS. Besides the perfectly matched peaks, two distinct spectral features in the spectrum of PSS-AuNRs have been noticed: First, a new peak shows up at 1215 cm−1 (as labeled by the red dashed line) in the spectrum of the PSS-AuNRs, which cannot be simply attributed to either PSS or CTAB-AuNRs. Second, the peak at around 1178 cm−1 in pure PSS has shifted to 1188 cm−1 in the spectrum of PSSAuNRs, as has been indicated by the blue dashed line. Elucidation of the origin of those spectral features is of critical importance in the interpretation of the interaction between PSS and the CTAB-AuNRs. With the deposition of a PSS layer onto the CTAB-AuNRs, a likely interaction that would occur is the electrostatic attraction between the sulfonate anions in PSS and the quaternary ammonium cations in CTAB. Therefore, the ATR-FTIR spectrum of the CTAB + PSS complex was recorded and is shown in Figure 1D. The CTAB + PSS complex was produced by mixing the CTAB solution with the PSS solution at a molar ratio of 1:1. The precipitates were collected, and their spectra were recorded. By comparison, the matching peak for the new peak at 1215 cm−1 is found in the spectrum of the CTAB + PSS complex, which suggests that this new peak most likely arises due to electrostatic interactions. To further validate our assumption, the spectrum of the CTAB/PSS mixture with the presence of an extensive amount of salt (CTAB + PSS + NaCl) was acquired and is shown in Figure 1E. The salt was added to screen the electrostatic interaction between the two oppositely charged species, and its effectiveness was shown by subtracting the spectrum of CTAB + NaCl (Figure S3-A) from the

Figure 1. ATR-FTIR spectra of (A) pure PSS, (B) the as-synthesized CTAB-stabilized AuNRs (CTAB-AuNRs), (C) the PSS-decorated AuNRs (PSS-AuNRs), (D) the CTAB + PSS complex, and (E) the CTAB/PSS mixture with the presence of salt (CTAB + PSS + NaCl).

spectrum of CTAB + PSS + NaCl. The subtraction result (Figure S3-C) resembles the spectrum of PSS + NaCl (Figure S3-B), which suggests that there is no electrostatic interaction between CTAB and PSS in the presence of salt. From the three spectra shown in Figure 1C, D and E, the new peak at 1215 cm−1 appears with the presence of electrostatic interaction and disappears with its absence. Thus, we conclude that this peak originates from the electrostatic interaction between the PSS and the CTAB bilayer on the AuNR surface. In addition, a comparison among the three spectra shown in Figure 1C, D, and E indicates that there is no shift in the peak at 1178 cm−1 upon CTAB + PSS complexation. Since the peak at 1178 cm−1 arises from the antisymmetric vibration of the sulfonate groups in PSS,36 we believe that the shift observed in the spectrum of PSS-AuNRs most likely originates from the interaction between the sulfonate groups with the AuNRs. The interaction between oxyanions and gold nanoparticles has been previously observed by other researchers as well.37,38 The presence of all the abovementioned features in the spectrum of PSS-AuNRs provides strong evidence of the successful deposition of PSS onto the CTAB-AuNRs with electrostatic interactions between PSS and both the CTAB bilayer and the AuNR itself. Polyelectrolyte LBL Deposition on PCL Fibers. To immobilize the negatively charged PSS-AuNRs onto the 10704

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polymer fibrous mesh, positive surface charges need to be established on the substrate. Introduction of proper surface charges can be realized through different approaches, such as self-assembly of a naturally charged chemical onto the substrate via specific chemical bond formation27 and introduction of charged functional groups onto the substrate by chemical treatment.32 Among those methods, polyelectrolyte LBL deposition is the most simple and versatile due to its ease of operation and its high tolerance for variable material composition and morphology of the substrate.32−34 In our experiment, poly(sodium 4-styrenesulfonate) (PSS) and poly(diallyldimethylammonium chloride) (PDADMAC) have been chosen as the polyanion and polycation, respectively, as both of them are strong polyelectrolytes which could remain fully charged under a wide range of pH conditions.39,40 To monitor the deposition of PDADMAC/PSS layers onto the PCL fibrous substrate, ATR-FTIR characterization has been carried out on a series of PCL meshes coated with a variable number of polyelectrolyte layers. Referring to the ATR-FTIR spectra of pristine PCL and PSS in Figure 2A,B, the peaks in

confirms the successful deposition of the polyelectrolyte multilayer. Determination of the Minimal Number of Polyelectrolyte Deposition Cycles. Complete polyelectrolyte coverage on the surface of PCL fibers is a prerequisite for the establishment of a uniform charge density, which is crucial to the formation of a homogeneous AuNR assembly on the PCL fibers. Previous studies40 suggest that for a neutral substrate like PCL fibers in our experiment, only partial coverage can be achieved with the first few cycles of polyelectrolyte deposition due to the lack of anchoring sites for the initial polyelectrolyte layer. To determine the starting point for the formation of complete polyelectrolyte coverage, water contact angle (WCA) measurements were carried out. The change of WCA with the number of deposited polyelectrolyte layers is shown in Figure 3.

Figure 3. Change of water contact angle values with the LBL deposition of polyelectrolyte layers.

Layer 0 corresponds to the pristine PCL fibrous substrate without polyelectrolyte coating, and the measured WCA value is approximately 133 degrees. The odd numbered layers correspond to the PDADMAC-terminated layers, and the even numbered layers correspond to the PSS-terminated layers. The WCA begins to decrease upon deposition of the polyelectrolyte layers due to the more hydrophilic nature of the polyelectrolytes. In addition, the PDADMAC-terminated layers always exhibit higher WCA values than the PSS-terminated layers, indicating a higher hydrophobicity of PDADMAC.41,42 From Figure 3, the starting point for a well-defined oscillation of the WCA with the deposition of polyelectrolytes was determined to be layer 4, which suggests the formation of complete surface coverage. However, since the PSS-AuNRs carry net negative charges, immobilization driven by electrostatic attraction requires the substrate to be positively charged. Thus, the minimal number of polyelectrolyte layers to be deposited is five. Furthermore, the zigzag trend of the measured WCAs can be divided into two stages, as has been labeled in Figure 3. In stage I, a decreasing trend of WCAs measured on both the odd- and even-numbered layers has been observed. When entering stage II, the WCAs begin to stabilize at two distinct levels, around 105 degrees for the PDADMAC-terminated layers and 85 degrees for the PSS-terminated layers. As is known, the WCA values can be affected by both the chemical composition and

Figure 2. ATR-FTIR spectra of (A) pure PSS, (B) pristine PCL mesh and PCL meshes decorated with various numbers of polyelectrolyte multilayers: (C) PCL-(PDADMAC/PSS)1, (D) PCL-(PDADMAC/ PSS)2.5, (E) PCL-(PDADMAC/PSS)3.5, and (F) PCL-(PDADMAC/ PSS)5.5. Note: the number N in the nomenclature PCL-(PDADMAC/ PSS)N denotes the number of PDADMAC/PSS bilayers that has been deposited on the PCL mesh.

the spectrum of PSS at 1009 and 673 cm−1 are chosen as the indicators to study the deposition of polyelectrolyte layers because those two peaks arise from the benzene ring structure exclusively existing in PSS, and they show up at positions where there is no overlap with the signals from PCL, making the spectra easier to analyze. As is shown in Figure 2C−F, the peak at 1009 cm−1 begins to appear after two layers of PSS have been deposited, and it becomes more evident as more PSS layers are added. Similarly, the peak at 673 cm−1 first shows up as a shoulder and gradually develops into an easily distinguishable peak. These observations lend strong support to the claim of successful deposition of the polyelectrolyte multilayer. More direct evidence for the presence of a polyelectrolyte multilayer on the PCL fiber surface was provided by the XPS survey spectrum recorded on the PCL-(PADAMAC/PSS)5.5 mesh (shown in Figure S4). Detection of S and N, which exist exclusively in PSS and PDADMAC, respectively, further 10705

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physical morphology of the substrate.41,43,44 Interpenetration of adjacent polyelectrolyte layers has been observed in the polyelectrolyte multilayer coating process that leads to the change in the chemical composition of the substrate and thus the change in the WCA values. However, typical evidence of interpenetration is the observation of a converging trend of the WCA,41 which contradicts the results in our experiment. The WCAs measured on the PDADMAC-terminated layers are always well-separated from those measured on the following PSS-terminated layers by approximately 20 degrees in both stage I and II. As for the effect of the physical morphology of the substrate, it was investigated using SEM characterization. The SEM images of the PCL fibrous meshes coated with various numbers of polyelectrolyte layers are shown in Figure S5. The pristine PCL fibers exhibit small pores on the surface,45 which contribute to a high surface roughness. As more polyelectrolyte layers are deposited, the fiber surface becomes smoother for both the odd- and even-numbered layers. It has been previously reported that a higher fiber surface roughness leads to an increase in hydrophobicity.43,44 The SEM characterization reveals that deposition of polyelectrolytes gradually fills in the pores, and a smooth fiber surface was produced with the deposition of the 11th layer (stage I). Further deposition to the 19th layer (stage II) did not cause any obvious change to the surface morphology. This is consistent with the WCA results. As the deposition goes into stage II, the WCA values stabilize due to the total elimination of the surface pores. Therefore, the decreasing trend observed in stage I is attributed to the loss of surface roughness. Again, the results from both the WCA measurements and the SEM characterization suggest the successful deposition of the polyelectrolyte multilayers. Immobilization of Gold Nanorods. With the proper surface charges being developed on both the AuNRs and the PCL fibrous mesh, the immobilization process was initiated by immersing the PEM-PCL substrate into the AuNR colloidal solution. To elucidate the role of electrostatic interactions in the AuNR immobilization process, the UV−vis spectra of the PSS-AuNR solution were recorded at fixed immersion time intervals and are shown in Figure 4A. From the first seven UV− vis spectra recorded hour by hour, a steady decrease in the absorbance of both the transverse and longitudinal SPR bands has been observed, indicating the gradual consumption of AuNRs in solution upon substrate immersion. Extending the immersion time to 24 h led to a further decrease in the UV absorbance, which suggests that more AuNRs have been effectively transferred from the solution onto the fibrous substrate. The change of UV absorbance at the longitudinal SPR band with immersion time is plotted and shown in Figure 4B. Instead of a steady decreasing trend exhibited by the red dots, the absorbance fluctuated around the starting value in our control experiment (black squares) where the PEM-PCL mesh was replaced by a pristine PCL mesh. The fluctuation resulted from the physisorption and desorption of the AuNRs. Through this comparison, we conclude that the electrostatic attraction provided a stronger binding, which irreversibly immobilized the AuNRs onto the fibrous substrate and thus guarantees the stability of the substrate. By setting the immersion time to 24 h, a sample was fabricated, and visualization of the AuNR assembly formed on the PCL fiber surface was realized using SEM. From the digital photograph shown in Figure 5C, the color of the fibrous substrate changed (from white for a PCL mesh) to dark purple

Figure 4. (A) UV−vis absorption spectra of the AuNR solution after PEM-PCL mesh being immersed for varied time intervals. (B) Change of absorbance at 786 nm with immersion time.

after AuNR immobilization, resembling the color of the AuNR colloidal solution. The uniform coloration of the substrate indicates the formation of a uniform AuNR assembly throughout the whole mesh. In addition, on each single PCL fiber, a homogeneous AuNR coating was produced with no severe aggregation being observed, as is shown in Figure 5A,B. Moreover, the immobilized AuNR density is high enough to provide a full surface coverage. SERS Evaluation. The performance of the AuNR/PCL nanocomposite fibers as a SERS substrate was first investigated using 4-mercaptopyridine (4-MPy) as the probe molecule. The choice of the probe molecule was made on the basis of its distinctive Raman spectral features and the effective binding to AuNR surface via the thiol groups.46−48 The SERS spectra shown in Figure 6A were recorded at varied probe molecule concentrations, ranging from 10−3 to 10−7 M. In the case of covalent binding to AuNRs, the pyridyl ring of the 4-MPy molecule adopts a perpendicular orientation with respect to the AuNR surface, favoring the formation of a densely packed 4MPy monolayer.49 In our SERS measurements, the remarkable enhancement in the X-sensitive band at 1097 cm−147,48,50 supports the adsorption of 4-MPy via the thiol group. According to the literature,49 for a monolayer of densely packed 4-MPy molecules exhibiting a perpendicular orientation to the metal surface, the molecular packing density is estimated to be around 5 × 10−10 mol/cm2. Using this number together with the AuNR adsorption information from the UV−vis spectra (see Supporting Information), the theoretical 4-MPy 10706

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Figure 5. SEM images of AuNR/PCL nanocommposite fibers fabricated with immersion time of 24 h (A) Recorded at a magnification of 35K. (B) Enlarged image of the rectangular area in (A). (C) Recorded at a magnification of 10K. Inset: digital photograph of the AuNR-PCL nanocomposite mesh.

The enhancement factor (EF) of 4-MPy on the AuNR/PCL nanocomposite fibrous substrate is calculated using eq 1 shown below: EF = (ISERS/NSERS)/(INR /NNR )

(1)

In eq 1, ISERS and INR represent the intensity of a vibrational mode in the SERS spectrum and the intensity of the same vibrational mode in the normal Raman spectrum, respectively. NSERS and NNR are the number of probe molecules being sampled in the SERS and the normal Raman measurements, respectively. EF of the ring breathing mode, which gives rise to the peak at 1008 cm−1 in the SERS spectrum, was estimated to be ∼104. A ∼105 EF was obtained for the peak at 1097 cm−1, which arises due to the ring breathing/CS stretching mode (see Supporting Information for detailed calculation). The reproducibility of the SERS measurements was also investigated by collecting a series of spectra from seven randomly selected spots on our nanocomposite fibers, and the spectra are shown in Figure 6B. The average intensities of the two peaks at 1097 and 1008 cm−1 were calculated to be 1576 ± 72 (relative standard deviation, RSD ∼4.57%) and 749 ± 37 (RSD ∼4.94%), respectively. The intensity variations of both peaks are less than 5%, indicating an excellent reproducibility of the SERS results. The high reproducibility further confirms the production of a homogeneous AuNR assembly on the PCL fibers. Since the SERS substrate fabricated in our work is constructed by immobilizing the AuNRs onto a 3D fibrous mesh, it has the advantage of enhanced surface area compared to the traditional 2D planar SERS substrates. The effect of enhanced surface area on the SERS performance was investigated by comparing the SERS response of the 3D AuNR/PCL fibrous mesh to its 2D counterparta AuNR/ PCL film. The AuNR/PCL film was fabricated using the same procedures as the AuNR/PCL fibers except that the initial substrate was a smooth PCL film instead of a PCL fibrous mesh. The AuNR/PCL film was characterized by SEM before further use, and the photos are shown in Figure 7A−D. As can be seen, the density of AuNRs on the 2D film is similar to the 3D mesh. The concentration of 4-MPy is chosen at 1 mM, which is well-above the saturation concentration. Under those conditions, the SERS signal recorded on the 3D mesh shows a 6-fold increase in intensity relative to that of the 2D film, as is shown in Figure 7E. Our result is comparable to what has been reported by Lee et al.21 In addition, our fabrication protocol has the advantage of universality over the protocol proposed in their work. Besides the covalently bound 4-MPy molecules, SERS detection of another common Raman probe molecule Rhodamine 6G (Rh6G)was also conducted. Rh6G is a

Figure 6. (A) SERS spectra of 4-MPy recorded on the AuNR/PCL nanocomposite fibers at various probe molecule concentrations: (a) 10−3 M, (b) 10−4 M, (c) 10−5 M, (d) 10−6 M, (e) 10−7 M, and (f) control experiment5 × 10−3 M 4-MPy on PEM-PCL mesh. (B) SERS spectra of 10−6 M 4-MPy recorded on seven randomly selected spots on the AuNR/PCL nanocomposite mesh.

saturation concentration on our AuNR/PCL nanocomposite fibers was calculated to be approximately 3 × 10−6 M, which is consistent with the experimental results. As shown in Figure 6A, the SERS intensity remains almost unchanged as the probe molecule concentration decreases from 10−3 to 10−5 M and begins to level off as the concentration further decreases, which indicates the saturation concentration is between 10−6 and 10−5 M. Furthermore, the as-fabricated AuNR/PCL nanocomposite fibers can detect as low as 10−7 M 4-MPy in solution. 10707

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Figure 7. (A,B) SEM images of AuNR/PCL film taken under magnifications of 40 and 10K; (C,D) SEM images of AuNR/PCL fibers taken under magnifications of 40 and 10K; (E) SERS spectra of 1 mM 4-MPy recorded on the 2D and 3D SERS substrates with the acquisition time of 30s.

reproducibility. Due to the enhanced surface area in the 3D fibrous substrate, the AuNR/PCL mesh provided a 6-fold increase in the SERS intensity compared to its 2D AuNR/PCL film counterpart. Owing to the nonspecific nature of the electrostatic interaction, the fabrication protocol proposed in this work can be readily extended to production of nanocomposite materials with various compositions and morphologies.

cationic dye which carries positive charge upon dissociation. Therefore, the Rh6G molecules can electrostatically bind to the negatively charged PSS-AuNRs on the PCL fiber surface. The SERS spectra of Rh6G recorded at various probe molecule concentrations are shown in Figure 8. The effective binding facilitated by electrostatic attraction allows the detection of as low as 10−7 M Rh6G under the nonresonant conditions.

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ASSOCIATED CONTENT

S Supporting Information *

Additional information as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].. Fax: 302-831-4545. Tel.: 302-8314476. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Chaoying Ni and Mr. Frank Kriss for their help with the acquisition of the electron microscopy images. Also, we gratefully acknowledge Mr. Nopporn Rujisamphan’s assistance with conducting the XPS measurements. This work is supported by the National Science Foundation (NSF) under the Polymer program (Award Number DMR-0704970), the Biomaterials program (Award Number DMR-1206310), the Delaware EPSCoR program (Award Number DMR-0643226), the State of Delaware, and partially by the Department of Materials Science and Engineering at the University of Delaware.

Figure 8. SERS spectra of Rh6G recorded on AuNR/PCL nanocomposite fibrous mesh at various probe molecule concentrations: (A) 10−4 M, (B) 10−5 M, (C) 10−6 M, (D) 10−7 M, and (E) control experiment10−3 M Rh6G on PEM-PCL fibrous mesh.

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CONCLUSIONS In this article, we have demonstrated the effective fabrication of a 3D SERS substrate composed of a highly dense AuNR assembly on the surface of an electrospun PCL fibrous mesh. The nonspecific electrostatic attraction was utilized as the driving force to irreversibly bind the AuNRs onto the PCL fibers. The AuNR/PCL nanocomposite fibers serve as an effective SERS substrate, which detects both the chemisorbed 4-MPy and electrostatically bound Rh6G with excellent

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