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Small Molecule Detection in Thiol-Yne Nanocomposites via Surface-Enhanced Raman Spectroscopy Darryl A. Boyd, Francisco J Bezares, Dennis B. Pacardo, Maraizu Ukaegbu, Charles Hosten, and Frances S Ligler Anal. Chem., Just Accepted Manuscript • Publication Date (Web): 10 Nov 2014 Downloaded from http://pubs.acs.org on November 10, 2014

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Analytical Chemistry

Small Molecule Detection in Thiol-Yne Nanocomposites via Surface-Enhanced Raman Spectroscopy Darryl A. Boyd*1, Francisco J. Bezares2, Dennis B. Pacardo3, Maraizu Ukaegbu4, Charles Hosten4, Frances S. Ligler3,5 1

Optical Sciences Division, Naval Research Laboratory, 4555 Overlook Ave SW, Washington, DC 20375, USA ICFO – The Institute of Photonic Sciences, Av. Carl Friedrich Gauss, 3, 08860 Castelldefels (Barcelona), Spain 3 Department of Biomedical Engineering, University of North Carolina and North Carolina State University, EB3, Mail Stop 7115, Raleigh, NC 27695-7115, USA 4 Department of Chemistry, Howard University, 525 College Street, NW, Washington, DC 20059, USA 5 Former NRL Employee 2

*[email protected] - 202-404-6140

ABSTRACT Surface enhanced Raman spectroscopy (SERS) is generally performed on planar surfaces, which can be difficult to prepare and may limit the interaction of the sensing surface with targets in large volume samples. We propose that nanocomposite materials can be configured that both include SERS probes and provide a high surface area-to-volume format, i.e. fibers. Thiol-yne nanocomposite films and fibers were fabricated using exposure to long-wave ultraviolet light after the inclusion of gold nanoparticles (AuNPs) functionalized with thiophenol. A SERS response was observed that was proportional to the aggregation of the AuNPs within the polymers and the amount of thiophenol present. Overall, this proof-of-concept fabrication of SERS active polymers indicated that thiol-yne nanocomposites may be useful as durable film or fiber SERS probes. Properties of the nanocomposites were evaluated using various techniques including UV-vis spectroscopy, µ-Raman spectroscopy, dynamic mechanical analysis, differential scanning calorimetry, thermogravimetric analysis and transmission electron microscopy.

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INTRODUCTION Surface-enhanced Raman scattering (SERS) is a powerful analytical tool that may be useful in numerous sensor applications ranging from chemical warfare detection to disease identification.1-7 In general the goal is to use SERS in applications for detection of specific molecules, relying on the chemical affinity of the analyte to the metal surface, and relying on the analyte’s ability to access the metal surface. In recent years various methods and materials have been developed to take advantage of SERS, including self-assembled noble-metal thin films,8 hyperbranched polymer nanoparticle hybrids,9-11 molecular sieve composites,12 nanopillar arrays,13 polymer composite particles fabricated via microfluidics,14 and fibers with gold nanorods attached to the surface.15 The emergence of thiol-yne chemistry,16,17 and its use in nanocomposite fabrication,18,19 gives rise to new opportunities for implementing SERS.

Recently, we reported the fabrication of polymer nanocomposites by the incorporation of gold and silver nanospheres into the matrices of thiol-ene and thiol-yne polymer films and fibers.18 That report determined that the optical and mechanical properties of the materials could be altered by functionalizing the nanoparticles with various ligands prior to polymerization, as well as by adjusting the nanoparticle concentration within the polymers.18 During the course of studying those thiol-ene and thiol-yne nanocomposites, SERS behavior was also observed within the material when the incorporated AuNPs were surface-functionalized with various ligands. Though interesting, the ligands in that study were too uncommon to truly evaluate the worth of the nanocomposites as SERS substrates. Further investigation was deemed worthwhile, however, because there is tremendous practical value in developing durable materials with high sensitivity that can detect harmful toxins, particularly in field environments. Specifically, polymer nanocomposites have recently been shown to be valuable conduits for small molecule detection via SERS behavior.20,21 Thus the current work explores the possibility of using the thiol-yne polymer platform to develop stronger, more durable and more easily fabricated


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nanocomposite materials that also have the ability to sense small molecules. This potential is explored by functionalizing AuNPs with the well-studied SERS reporter molecule thiophenol (TP), embedding the functionalized AuNPs within thiol-yne polymers, and measuring the SERS signal in a proof-of-concept demonstration. This approach allowed us to determine if detection within the material was at all possible and feasible, and eliminated the variable of the efficiency with the target analyte to diffuse through the polymer matrix to reach the AuNPs. The tweaking of the polymer chemistry to optimize diffusion or partition of the target into the polymer to reach the AuNP is an issue to be addressed in future work.

Advantages to using thiol-yne polymers as SERS probe materials include their transparency, their strength and durability,22-24 their amenability for use as coatings and resins,25,26 and the possibility that porous thiol-yne nanocomposite membranes could be used as detectors by allowing air or fluids containing a desired target analyte to pass through them.27,28 Finally, recent reports have shown that AuNPs devoid of surface molecules can be embedded into polymers via the related thiol-ene chemistry,29 and that macroporous hybrid monoliths fabricated via thiol-ene chemistry can be used in capillary columns,30 highlighting the possibility that combining these two concepts may lead to the development of materials capable of detecting or separating small molecules. Herein we demonstrate the potential for thiol-yne nanocomposites to be used for small molecule detection via SERS analysis.

EXPERIMENTAL SECTION Materials.

Pentaerythritol tetrakis (3-mercaptopropionate) (PETMP); 1,7-octadiyne (ODY); 2,2-

dimethoxy-2-phenylacetophenone (DMPA); thiophenol (TP); tetrahydrofuran (THF); chloroform;



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polyethylene glycol (PEG) were purchased from Sigma Aldrich and used as received. 10 nm gold nanospheres (AuNPs) were purchased from SPI Supplies.

Nanocomposite Fabrication.

Thiol-yne nanocomposites were fabricated as previously described.18

First, 10 nm AuNPs were functionalized with TP, using the method of Fontana, et.al.31

These

functionalized AuNPs were then added to a thiol-yne prepolymer that was composed of ODY and PETMP monomer components; the monomers were combined in a 1:1 ratio, with 2 mol % DMPA photoinitiator. The AuNP concentrations (given in vol %) for the different thiol-yne nanocomposites were 3.0 X 10-6, 3.0 X 10-5, 7.5 X 10-5, 1.5 X 10-4 and 3.0 X 10-4. Thiol-yne films were fabricated in a custom-built mold as previously described.18

AuNP thiol-yne fibers were fabricated via microfluidics as previously

described.18,32

Characterization. Images of the thiol-yne nanocomposites were taken using a digital camera (Nikon Coolpix S5200). SEM images were obtained using a LEO Supra 55 scanning electron microscope (Carl Zeiss SMT Inc., Peabody, MA). TEM images were obtained using a JEOL 2000FX S/TEM with LaB6 electron source, operated at 200kV with magnification of 25k and 60k, as previously described.18 Glass transition temperatures were determined by differential scanning calorimetry (DSC), as previously described.18

The thermal stability of the nanocomposites was determined by thermogravimetric

analysis (TGA) as the change in percent weight versus temperature using a TA instruments Hi-Res TGA 2950 Analyzer instrument. Samples were analyzed in platinum pans while being heated at a rate of 10 °C/min for the temperature range of 25−800 °C. The degradation onset temperature was determined at 3% mass loss.

Young’s moduli of thiol-yne nanocomposite fibers were determined by dynamic

mechanical analysis (DMA) in triplicate as previously described.18,32 UV-vis spectra were recorded using a Varian Cary 5000 UV-Vis-NIR Spectrophotometer (Agilent Technologies).


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The SERS measurements were performed using two different experimental setups; 1) a custom-built Mitutoyo microscope with a 514 nm incident Coherent Inova 90 Ar-ion laser. A 200 µm optical fiber collected the back-reflected light into an Ocean Optics QE65000 Vis-HIR spectrometer. The power of the source was 4.8 mW at the sample surface, focused into a ~3 µm spot via a 50X, 0.75 NA microscope objective. 2) A commercially-available DeltaNuExaminR micro-Raman system, in back-reflection mode, using 532 nm, 633 nm and 785 nm incident laser modules. The power of this system at the sample was also 4.8 mW with a spot size of approximately 3 µm via a 50X, 0.75 NA microscope objective. Detection in both cases was achieved using a thermoelectrically-cooled CCD array, with acquisition times of 10 seconds.

RESULTS AND DISCUSSION Thiol-yne Nanocomposite Fabrication and Mechanical Properties. In preparation for inclusion into the thiol-yne polymer matrix, preformed gold nanospheres were functionalized with TP.8 The functionalized AuNPs were added to a thiol-yne combination consisting of the monomers pentaerythritol tetrakis (3mercaptopropionate) (PETMP) and 1,7-octadiyne (ODY), as previously described.18 Bulk films were produced by photopolymerization of the prepolymer solution in custom-made molds. Microfibers were fabricated via hydrodynamic focusing in a microfluidic channel as previously described, and had a ribbon-shaped cross sectional profile (Figure 1).18,32-34 The five discrete AuNP concentrations for which thiol-yne nanocomposite films were produced were 3.0 X 10-6, 3.0 X 10-5, 7.5 X 10-5, 1.5 X 10-4 and 3.0 X 10-4 (vol %). Thiol-yne nanocomposite fibers were made with a AuNP concentration of 3.0 X 10-4 AuNP vol %. The properties of these nanocomposites were compared to the properties of the native thiol-yne polymer made of the same monomer components (PETMP and ODY), but devoid of AuNPs.32



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Figure 1. (a) Thiol-yne nanocomposite films containing (i) 7.5 X 10-5, (ii) 1.5 X 10-4 and (iii) 3.0 X 10-4 AuNP by vol %; scale bar represents 10 mm. (b) Representative TEM image of 10 nm AuNPs within a thiol-yne nanocomposite (3.0 X 10-4AuNP vol %); scale bar represents 20 nm. (c) Thiol-yne nanocomposite fibers aligned on a glass slide. (d) SEM cross-section image of a thiol-yne nanocomposite fiber; scale bar represents 50 µm. The bulk nanocomposite films had a uniform thickness of 0.25 mm and had a blue hue, yet were translucent (Figure 1). Increasing the concentration of AuNPs led to nanocomposites that were bluer in appearance and less transparent.18 However, even at the greatest concentration of AuNPs, the thiol-yne nanocomposite films remained translucent (Figure 1). The fabricated AuNP thiol-yne fibers appeared bluer than their film counterparts (Figure 1); they were, however, also translucent when imaged with an optical microscope. Fabricating the fibers such that they had ribbon-shaped cross sectional profiles allowed for greater surface area (when compared to typical round cross section fibers), which increased the overall small molecule detection area (Figure 1).32


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Table 1. Thiol-yne Nanocomposite Thermal Properties

3.0 X 10

-6

47.6 + 0.2

thermal degradation onset (°C) 346.0 341.7

3.0 X 10

-5

46.9 + 0.6

344.4

7.5 X 10

-5

38.1 + 1.6

343.3

1.5 X 10-4

36.8 + 2.1

344.8

3.0 X 10-4

44.4 + 0.3

343.9

vol % AuNP

Tg (°C)

0.0

46.2 + 0.5

Extensive analysis of the mechanical properties of these nanocomposites, and comparison to similar nanocomposite materials from our previous report,18 is beyond the scope of this present work, and will be addressed in future work. However, to demonstrate the viability of the nanocomposite materials presented in this report for SERS applications, the following brief analysis of their mechanical properties is offered.

Although the trend in degradation temperature was not linear with respect to increasing AuNP concentration, all of the nanocomposites were thermally stable at temperatures below 340 °C (Table 1). Though the variation in degradation temperature of just a few degrees is not trivial, the range in degradation temperature within the set of data was less than 5 °C, indicating that the thiol-yne nanocomposites were nearly as thermally stable as their native thiol-yne polymer counterparts. With respect to the glass transition temperatures (Tg), the two nanocomposite samples that contained the least AuNPs had Tg values nearly identical to that of the native polymer; otherwise the addition of AuNPs to the polymer matrix of thiol-yne resulted in glass transition temperatures that were less than that of the native material (Table 1). Our previous study indicated that ligands on the AuNP surface that covalently linked to the polymer matrix could limit aggregation. Consistent with this observation, functionalizing the AuNPs with TP, a ligand that cannot crosslink with the polymer monomer



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components, led to the formation of dense aggregates of AuNPs within the thiol-yne nanocomposites, and these dense aggregates were situated in isolated pockets throughout the material (Figure 1).

The lack of a linear trend in degradation temperature was likely due to variance in the AuNP distribution within each nanocomposite; an issue accounted for and addressed in the SERS calculations later in this report. The non-linear nature of the degradation temperature, as well as that of the Tg values, might also be attributed to the fact that significant branching can occur in thiol-yne polymerizations, leading to greater variance in AuNP distribution within the thiol-yne nanocomposites.

Unlike thiol-ene

nanocomposites, for which the Tg tends to increase as the AuNP concentration increases,18,29 a definitive Tg trend as the AuNP concentration increases has not yet been shown for thiol-yne nanocomposites.18 The difference in thiol-ene and thiol-yne nanocomposite Tg behavior may further be explained by the differences in the kinetics of thiol-ene and thiol-yne chemistries.35,36

Thiol-yne nanocomposite fibers with 3.0 X 10-4 vol % AuNPs were fabricated via hydrodynamic focusing in a microfluidic channel (Figure 1).18,32-34,37,38 The Young’s (E) moduli values for the fibers were determined by dynamic mechanical analysis (DMA),18,32 and calculated to be 5.0 + 0.7 MPa. By comparison, this E-modulus value was significantly less than that of native thiol-yne fibers made of the same monomer components (15.0 + 2.0 MPa),32 as well as similarly fabricated AuNP thiol-yne nanocomposites.18 However, it is important to note that an E-modulus value of 5.0 MPa is still much greater than E-moduli given for many commonly used polymer materials.

For example,

polydimethylsiloxane, a polymer material that is ubiquitous and has many practical applications (e.g. soft contact lenses, accelerometer springs),39-41 can have an E-modulus of only 750 kPa.41,42 Thus, thiolyne nanocomposite fibers, in addition to thiol-yne nanocomposite films, offer significant potential for


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use as SERS substrates (in applications such as filtration) as the amenability of these materials for SERS applications will depend in part on their strength and durability.

Figure 2. UV-vis analysis of thiol-yne nanocomposites. Dotted line represents the typical λMAX for 10 nm AuNP in water; dashed line represents 600 nm. UV-Vis and SERS Analysis of Thiol-yne Nanocomposites. UV-vis analysis of each of the thiol-yne nanocomposite films revealed red-shifted λMAX (or peak) values (Figure 2). The 3.0 X 10-5, 7.5 X 10-5, 1.5 X 10-4 AuNP vol % nanocomposites each had λMAX values at ~600 nm. As seen in Figure 2, there was a much more significant red-shift for the 3.0 X 10-4 AuNP vol % nanocomposite material, as well as more broadening of its peak. Based on the typical resonance wavelength for a single 10 nm gold nanosphere in water (~510-525 nm),43 this red-shift suggested that the AuNPs were aggregated.44 In addition, the broadness of the peaks also suggested nanoparticle aggregation45,46 and that the size of the aggregates within the nanocomposites was large.45 This was confirmed by the large AuNP aggregates imaged by TEM (Figure 1).



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Figure 3. SERS spectra of a thiol-yne nanocomposite sample with fixed AuNP concentration measured at different incident wavelengths. The inset represents the SERS spectrum observed on the thiol-yne nanocomposites before background subtraction. It is expected that the resonant nature of the absorption peak observed in Figure 2 will result in a maximal enhancement of the SERS signal at a given wavelength, determined by the peak position of the resonance.13 This can be more clearly seen in Figure 3, where the SERS spectra of a sample with a fixed AuNP concentration of 3.0 X 10-4, measured at incident wavelengths of 514 (black), 532 (blue), 633 (red) and 785 nm (green), is shown. All spectra in the figure have been uniformly background-subtracted and the intensities offset along the y-axis for clarity. Here, background spectra were defined by the broad spectral features within the detection window, underlying all Raman mode features. These broad spectral features were due to hybridized opto-electronic states of the S–Au bonds between the TP molecule and the nanosphere surface.47 A spectrum before background subtraction of the sample, taken with a 633 nm incident, is presented in the Figure 3 inset. The spectral features of the surfaceenhanced Raman modes of the TP molecule on Au nanostructures13 at approximately 998, 1021, 1071 and 1569 cm-1 were significantly more prominent on the spectrum taken at 633 nm incident. Although some of these features were present at other incident wavelengths, their intensities were weak. With the exception of the 998 cm-1 Raman mode, which was minimal, no other clear TP spectral features can


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be identified on the spectrum obtained for the 785 nm incident wavelength. This is expected as this wavelength lies far away from the plasmonic resonance maxima, and thus no large SERS enhancement occurs within this spectral region.

There was no appreciable SERS intensity detected for nanocomposites with AuNP concentrations of 3.0 X 10-6 or 3.0 X 10-5 at any of the incident wavelengths used for excitation (Figure 4). However, increasing the AuNP concentration from 3.0 X 10-5 to 1.5 X 10-4 significantly increased the SERS intensity, as clearly seen in Figure 4a, where the average SERS intensity of the C–H wagging mode of the TP molecule at 998 cm-1 (see inset, Figure 3) is plotted as a function of vol % of AuNP concentration. Each average value and the shown error bars correspond to 10 individual measurements. The intensity of this mode was specifically monitored and plotted in Figure 4a as this mode is furthest removed from the S–Au bond between the TP molecule and the metal surface, thus avoiding variation on the SERS intensities due to interactions with hybridized states within the molecular HOMO-LUMO gap introduced by the S–Au bond.47 Figure 4b compares the spectra obtained on samples with AuNP variation ranging from concentrations of 0 to 3.0 X 10-4. This spectral graph shows that the SERS features of the TP molecule gradually increase in intensity. That is, the SERS modes at approximately 998, 1021, 1071 and 1569 cm-1, which correspond to different vibrational modes of the TP molecule,48-52 were not observed in the absence of AuNP and their intensities grow as the AuNP concentration increases. At the same time, the most prominent intensities of the spectral features of the samples without AuNPs (i.e. Raman modes corresponding to the native polymer material) decreased, possibly due to the efficiency of the SERS phenomena in enhancing the electromagnetic fields locally, very close to the AuNP surfaces, as well as due to the decrease of polymer material within the probed volume, as it was displaced by AuNP while their concentration increased. Greater AuNP concentrations increased the number of TP molecules interacting with light near the plasmonic surface of the particles, thus enhancing the detected SERS



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signal. The rate of increase of the average SERS intensities for the samples with AuNP concentrations of 1.5 X 10-4 and 3.0 X 10-4 seemed to be decreasing towards a threshold value suggesting saturation, with the greatest SERS intensity values measured at these concentrations (Figure 4a). The data for thiol-yne nanocomposites fabricated with the 3.0 X 10-4 concentration will be the focus of the SERS analysis from here on.

Figure 4. (a) SERS intensities of thiol-yne nanocomposites, after background subtraction, with various AuNP concentrations under 633 nm incident. (b) Raman signals for thiol-yne nanocomposites with varying AuNP concentrations; the arrows point to the SERS peaks corresponding to the TP molecules. Determination of SERS Enhancement Factor. Generally, calculations of the SERS enhancement factor (EF) provide a quantitative description of the SERS properties of a particular material. However, calculating the SERS EF of the nanocomposite system described here was more challenging compared to other previously studied SERS-active systems. For example, in large-area periodic arrays of metal NPs in which the nanoparticle distribution is uniform, the nanoparticle surface area exposed to the laser beam is approximately constant at different areas within the samples. This differs from the present work as suggested by the TEM and UV-Vis spectroscopy analysis discussed above. Nevertheless, in order to provide a more quantitative analysis of the polymer nanocomposite samples’ SERS capabilities, an approximate SERS EF was calculated by making the following assumptions:


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1) Multiple scattering events have a minimal impact on the enhancement of the detected SERS signal. The probability for detection of multiply scattered signals was typically negligible under the experimental conditions used here. 2) The EF is largely determined by the interaction of light with the TP molecules near the surface of AuNPs that are themselves closer to the surface of the nanocomposites. Particles within the bulk (beneath the surface) are effectively screened by particles near the surface and only interact with light through (negligible) multiple scattering events. 3) The surface area of the agglomeration of nanoparticles was calculated by assuming clusters with uniform particle distribution and a spherical cross section. This assumption contrasts with the TEM images in which non-uniform particle distribution was observed (Figure 1). Note that, although the assumption of uniform distribution leads to an under-estimation of the EF, it provides a basis for the approximate calculation of this value. 4) To obtain the SERS intensity per molecule, monolayer coverage of the TP molecule on the surfaces of the AuNPs was also assumed, as previously established in the literature.48-52

The (average ) was calculated using the formula

= (ISERSNRaman)/(IRamanNSERS)

where ISERS = I0SERS[(P)(t)]-1, I0SERS is the observed Raman signal, P is the power of the laser probe and t is the acquisition time. For this work, an EF per molecule value of 1.4 + 0.4 x 103 for the 998 cm-1 Raman mode of TP was calculated for the 3.0 X 10-4 vol % AuNP thiol-yne nanocomposites. An additional set of data was taken on a 3.0 X 10-4 vol % AuNP thiol-yne nanocomposite sample in order to determine the



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repeatability of the EF. The calculations that resulted from the additional data set returned an EF per molecule value of 1.2 + 0.4 x 103 for the 998 cm-1 Raman mode of TP.

Despite the modest EF values determined, and the ~33% standard deviations, it is important to consider that these results were obtained from nanocomposites in which the AuNPs were non-uniformly distributed, yet reproducibility in the EF values was still obtained. It should be noted that these values are consistent with a recent report in which a SERS EF of 103 was observed in a monolayer of silver nanowires.53 That report focused on layering SERS active materials to improve SERS EF.53 Furthermore, the calculations obtained in this present report largely underestimate the EF of the EM field near the surface of the AuNP, as they neglect the absorption of light by the polymer thin film as well as scattering effects. Finally, further optimization of the nanocomposite system to achieve higher AuNP uniformity and optimization of the average particle-to-particle distance can significantly increase the EF of the detected SERS signals. For instance, although the largest SERS EFs in the literature necessary for single molecule detection are observed at “hot spots” between NP gaps on the order of a few nm, the “hot spot” density is typically low in systems of NPs of non-uniform distribution. Thus, the probability of analyte detection is significantly increased when the density of “hot spots” is increased by increasing large-area uniformity.13,52 At the same time, closely-arranged NPs throughout large-areas will increase the signal EF as the intensity of the SERS enhancement has been shown to increase exponentially with interparticle distance.54-56

CONCLUSIONS In conclusion, this proof-of-concept report suggests that the fabrication of nanocomposites via thiol-yne chemistry can lead to the development of strong, durable films and fibers with molecule detecting capabilities at the surfaces of incorporated AuNPs via SERS analysis. Using thiophenol (TP), which has


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well-studied SERS signatures, we calculated a SERS EF on the order of 103 for the nanocomposites with the greatest concentration of AuNPs tested. Various methods could be employed to improve both the mechanical and SERS properties of these nanocomposites to achieve optimal durability and sensor capabilities. Methods we are currently exploring include using specific thiol-yne polymer combinations targeted to have certain kinetics and mechanical properties,35,36,57,58 engineering the fibers to have nonround cross-sections or porous morphologies to increase accessibility to the AuNPs,27,28,32-34,37,59,60 and using anisotropic nanoparticles of various sizes in the nanocomposites to generate greater SERS EF values.61-63 Modifying the ligands on the surface of the NPs to include ligands important for sensing, as well as ligands that interact with the prepolymer, can lead to increased control of aggregate size. Finally, tailoring nanocomposites to have specific properties (e.g. by using thiol-ene chemistry instead of thiol-yne chemistry or by embedding AgNPs or Fe3O4NPs in place of AuNPs) may lead to nanocomposites with varied mechanical properties and unique small molecule sensing capabilities.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; Phone: 202-404-6140 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS The work was supported by ONR/NRL Work Unit 9899 and the Jerome & Isabella Karle Fellowship. The views are those of the authors and do not represent the opinion or policy of the US Navy or Department of Defense. The authors acknowledge the use of experimental facilities in the Hosten lab at Howard University. The authors also acknowledge the use of the Analytical Instrumentation Facility (AIF) at the



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North Carolina State University, which is supported by the State of North Carolina and the National Science Foundation.

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