Surface Enhanced Raman Spectroscopy at Electrochemically

20 Nov 2015 - Ward , D. R.; Corley , D. A.; Tour , J. M.; Natelson , D. Nat. Nanotechnol. 2011, 6, 33– 38 DOI: 10.1038/nnano.2010.240. [Crossref], [...
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Surface Enhanced Raman Spectroscopy at Electrochemically Fabricated Silver Nanowire Junctions Radhika Dasari† and Francis P. Zamborini* Department of Chemistry, University of Louisville, Louisville, Kentucky 40292, United States S Supporting Information *

ABSTRACT: Here we describe enhanced Raman scattering at Au electrode 1 (E1)/Ag nanowire (NW)/4-aminothiophenol (4ATP)/Au electrode 2 (E2) nanojunctions fabricated by combining self-assembly and metal electrodeposition at microgap electrodes (E1 and E2). In this method we assemble the 4-ATP on electrode E2 and electrodeposit Ag on the opposite electrode E1 of an Au interdigitated array (IDA) electrode device. The electrodeposited Ag grows in the form of NWs on E1 and makes nanoscale contact to E2 to form the junctions. The presence of the Ag NW leads to strong Raman scattering of the 4-ATP molecules within the nanojunction leading to estimated enhancement factors ranging from 103 to 106. Scanning electron microscopy (SEM) images provide insight into the morphology of the junctions. The magnitude of the Raman enhancement depends on the extent of contact between the Ag NW and the 4-ATP self-assembled monolayer (SAM). With this approach we could detect 4-ATP molecules diluted by a factor of 1000 with hexanethiol molecules within the junctions. Our approach is simple and fast with the potential to correlate electronic measurements of molecules with Raman spectroscopy data of the same molecules in a nanoscale junction for molecular electronics or chemiresistive sensing applications.

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several experimental studies of SERS at nanogap electrodes to better understand fundamental issues. For example, Tian and co-workers studied the effect of gap width on the SERS intensity of 1,4-benzenedithiol molecules assembled at nanogap electrodes fabricated using the mechanically controlled break junction method.33 Moskovits and co-workers also studied the effect of gap size and incident light polarization in the SERS of rhodamine 6G molecules at Ag gaps formed by electromigration.36 In addition, several theoretical studies exist, discussing the importance of gap distance, polarization, electrode geometry, and metal−molecule interaction on the electric field enhancement and SERS response at nanogaps.41−43 The main motivation behind most SERS studies at nanogap molecular junctions is to correlate the molecular structure with its electronic properties to improve fundamental understanding of molecular electronics.44−47 Experimentally, Mirkin and coworkers used on-wire lithography to fabricate heterometallic nanogaps and assembled oligophenylene ethylene (OPE) molecules in the nanogap to fabricate molecular transport junctions, using SERS to characterize the molecules in the junction.32 Allara and co-workers measured the electronic properties of oligophenylene ethylene (OPE) molecules along with SERS in a crossed Au nanowire molecular junction.40 Ward et al. fabricated Au nanogap electrodes by electromigration for correlating the conductance of p-mercaptoaniline

trongly enhanced Raman scattering signals appear when a Raman active molecule is in close proximity to a nanostructured Ag or Au surface due to the well-known electric field enhancement mechanism, which is the result of exciting localized surface plasmons within the Ag or Au with the visible Raman laser, and the charge transfer mechanism that occurs due to chemisorption of the molecule on the metal. This is collectively known as surface enhanced raman spectroscopy (SERS),1,2 with the electric field mechanism being dominant in enhancing the signals. SERS was first discovered on roughened Ag surfaces3−5 and has since then been widely employed on a variety of better defined, lithographically patterned surfaces,6−8 individually synthesized nanostructures,9−12 and homo- or heterocoupled nanostructures,13 such as fabricated nanoparticle dimers,10,14−16 aggregates,17,18 and nanowire bundles.19,20 It is now well-accepted that the region between coupled plasmonic Ag and Au nanostructures serves as “hot spots” for maximizing the SERS signals.21,22 These types of structures have been able to achieve sensitivity down to the single molecule level11,23,24 with analytical applications in environmental sensing,25 the monitoring of heterogeneous catalytic reactions,26 chemical warfare stimulant detection,27,28 pH sensing,29 and biological applications, such as in vivo glucose sensing6 and the identification of DNA and proteins.30,31 Another common geometry for SERS that has emerged over the years is that of two metal Au or Ag electrodes separated by a nanoscale gap.32−37 These types of “nanogap” electrodes have been previously fabricated by lithography,32,38 electromigration of nanowires,34,36,39 crossed and parallel configurations of synthesized nanowires,37,40 and electrodeposition combined with the mechanical break junction technique.33 There are © XXXX American Chemical Society

Received: June 22, 2015 Accepted: November 20, 2015

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and fluorinated OPE molecules in the gap with the timedependent SERS spectra.34,39 They observed Raman blinking and spectral diffusion with correlations and anticorrelations between the conductance and SERS spectrum over different time ranges. They concluded that such fluctuations would only be observed in single molecule measurements, which they attributed to molecular movement and different bonding geometries of the molecule in the junction over time. Konishi et al. studied molecular junctions of Au nanogaps formed by the mechanical break junction method containing 4,4′-bipyridine in solution.48 They also observed fluctuations in the Raman intensity and energy of the various modes that correlated with the conductance, which they attributed to different molecular orientations. Matsuhita et al. studied benzenedithiol (BDT) in the junction of Au nanogaps, concluding that the BDT made direct contact with the Au and there was photon driven charge transfer from the metal to the molecule based on the combined conductivity and SERS measurement.49 Li et al. reported voltage-induced tuning of the SERS spectrum of C60 molecules in an Au nanogap that was larger than that expected for a pure Stark effect, showing that Raman modes can shift as a result of electronic charge on the molecule from the bias.50 Several groups have described theoretically the effects of electricity passing through molecules on the SERS spectrum,46,51−53 including the consideration of bias voltage,54 charge transfer,55 and joule heating.56 In this work we describe the electrochemical fabrication of Au(E1)/Ag NW/4-aminothiophenol (4-ATP)/Au(E2) junctions, which exhibit strong Raman enhancement of the 4-ATP that is sandwiched between the Ag NW and Au(E2) electrode. This approach is fundamentally different from almost all of those described in the literature in that the junction is formed by bringing the Ag NW to the molecule of interest by electrodeposition instead of forming the metallic nanogap junction first and then bringing the molecules to the junction. The Ag NW in our work is somewhat similar to the tip in tip enhanced Raman spectroscopy (TERS), which involves bringing a sharp Ag or Au tip toward a planar surface containing the Raman active molecule of interest.57,58 TERS has previously provided sufficient enhancements for single molecule detection as well as electronic measurements on 4,4′bipyridine molecules on Au, for example.59 In our strategy, the Ag NW is brought to the molecules being detected by electrodeposition in a planar configuration, where the Ag NW is ultimately stationary as opposed to being movable (scanned) in the case of TERS. Campbell and co-workers similarly electrodeposited Pt at a nanogap junction for making molecular electronics measurements, but did not explore the technique for SERS.60,61 The benefits of our approach are (1) processes such as “nanolithography”, electromigration, or mechanical break junctions, which can be challenging and tedious and have low success rates, are not required; (2) any molecule of interest can be studied in the junction as long as it can be assembled as a thin film on the Au electrode; (3) the Ag NW/molecule/ electrode “hot spot” can be easily identified under an optical microscope for fast SERS measurements; and (4) the SERS and conductivity of the junction can be measured simultaneously without any worry about the Ag NW contact resistance since it is formed electrochemically. Our approach can be useful for molecular electronics applications but also for dual SERS and conductivity based sensing strategies.

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EXPERIMENTAL SECTION Electrode Wiring and Cleaning Procedure. Au interdigitated array (IDA) electrodes (10 or 14 fingers, 5 μm gap) used for this study were fabricated in a clean room by photolithography on a Si/SiOx substrate. Wire leads were attached to the electrode contact pads with Ag epoxy (Stan Rubinstein Associates Inc., Foxboro, MA, USA) and placed in an oven for ∼12 h at 80 °C in order to cure the epoxy. The wires were further insulated with an overlayer of torr-seal epoxy and cured ∼12 h at 80 °C. The electrode was then cleaned by rinsing in acetone, ethanol, 2-propanol, nanopure water, and drying under N2. The electrode was placed in a UV ozone cleaner (Jelight Co. Inc., Irvine, CA, USA; Model 4C442) for 15 min in order to remove organic impurities. Electrodes were also cleaned electrochemically by cycling several times from 0 to 1.2 V at 100 mV/s in 0.1 M H2SO4 until we observed a wellpronounced oxidation and reduction peak for Au. Fabrication of Electrode/AgNW/4-ATP(Monolayer and Sub-monolayer)/Electrode Junctions. Junctions were fabricated using our previously reported procedure.62,63 Briefly, the method involves the formation of a monolayer of 4-ATP or mixed monolayer of 4-ATP/hexanethiol on both electrodes of the Au IDA device (E1 and E2). Mixed monolayers of 4-ATP/ hexanethiol were formed by placing the electrode overnight in ethanolic 4-ATP solutions containing hexanethiol at concentrations 9, 99, and 999 times larger than 4-ATP. The hexanethiol/4-ATP solutions were prepared by adding 1, 0.1, and 0.01 mL of 2 mM 4-ATP solution to 9, 9.9, and 9.99 mL of 2 mM hexanethiol solution. The 4-ATP: hexanethiol ratio in these solutions was 1:9, 1:99, and 1:999, respectively, providing dilutions of 10, 100, and 1000. Later, the mixed self-assembled monolayer (SAM) was desorbed from electrode E1 electrochemically.62 Finally, 6.0 × 10−5 C of Ag was electrodeposited onto electrode E1 from a solution containing 0.005 M AgNO3 in 0.1 M H2SO4 at −0.3 V (∼12 s) versus an Ag/AgCl reference with a Pt counter electrode. Raman Measurements. Raman scattering measurements were acquired with an in-Via Renishaw micro-Raman system in the backscattering configuration. All Raman measurements were obtained at room temperature. A He:Ne laser source with 632.8 nm radiation was used as an excitation source. All Raman measurements were obtained using 0.17 mW laser power and an acquisition time of 10 s. The laser beam was focused with a 50× objective lens which resulted in a 1.3 μm spot diameter.

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RESULTS AND DISCUSSION Scheme 1 shows the steps involved in the fabrication of Au(E1)/Ag NW/4-ATP/Au(E2) junctions. In step 1, a SAM of 4-ATP or mixed SAM of 4-ATP/hexanethiol was formed on both Au electrodes (E1 and E2). In step 2, the monolayer or mixed monolayer was electrochemically desorbed from E1. In step 3, Ag was electrodeposited onto E1, where it crossed over to E2 to form the junction. Figure 1A shows a SEM image of an entire Au IDA device where we chemisorbed 4-ATP on E2 and electrodeposited Ag on E1. Ag NWs deposited on the edges of E1 and made several connections between the two electrodes leading to the formation of Au(E1)/Ag NW/4-ATP/Au(E2) junctions as previously described.62 Numbers 1−8 in Figure 1A highlight the number of E1/Ag NW/4-ATP/E2 junctions in the device. Figure 1B shows a zoomed in SEM image of the circled portion in Figure 1A (junction no. 3). It shows one of the Ag NWs connecting the two electrodes of the device shown B

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Analytical Chemistry Scheme 1. Schematic Representation of the Method for Fabricating E1/Ag NW/4-ATP (Monolayer or Submonolayer)/E2 Junctions

Figure 2. SEM image of one E1/Ag NW/4-ATP/E2 junction (top left). Red open circles represent the regions from which the Raman spectra were obtained, and frames A−E represent the optical images of the same E1/Ag NW/4-ATP/E2 junction as in frame A with the crosshairs showing where the Raman spectra were collected.

the Raman scattering there relative to the other areas. This is due to the electric field enhancement mechanism made possible by the plasmon properties of the Ag NW tip coupled to the Au electrode. The Ag NW acts in a fashion similar to that of a tip in TERS, but in a planar format. The Raman modes observed in Figure 3A are consistent with the photooxidation of 4-ATP to 4,4′-dimercaptoazobenzene (DMAB) under normal laser illumination conditions.64 Therefore, although we will discuss the signal as being 4-ATP, it really is DMAB. We tested the reproducibility of the Raman enhancement from the same Ag NW junction and from different Ag NW junctions from the same and different IDAs. We first tested the reproducibility of the same junction. Figure 4A shows the SERS spectra obtained three times from the same region of the same E1/Ag NW/4-ATP/E2 junction identified as no. 3 in Figure 1A. The relative standard deviation in the signal of the Raman band at 1576 cm−1 was 6% for the same junction. We also tested the reproducibility of different E1/Ag NW/4-ATP/E2 junctions within the same device. Supporting Information Figure S1 shows optical images of different E1/Ag NW/4-

in Figure 1A. The circled portion in Figure 1B shows the E1/ Ag NW/4-ATP/E2 junction region. The SEM image in the top left frame of Figure 2 shows the same Ag NW in Figure 1B, and the five circles (A−E) show the locations from which Raman spectra were obtained. Frames A− E in Figure 2 show the optical images from the Raman spectrometer of the same junction and locations as shown and labeled A−E in the SEM image. Figure 3A shows the SERS spectra of the 4-ATP molecules obtained from regions A to E shown in Figure 2. Figure 3B shows the zoomed in SERS spectra obtained from regions B−E. The SERS signal of the 4ATP molecules from the edges and the surface of the Au electrodes and directly on the Ag NW (regions B−E) are extremely weak or nonexistent. Interestingly, there is a barely noticeable signal for 4-ATP on the electrode edge that is slightly larger than the signal from the middle of the Au electrode. In contrast, the signal from the nanojunction (region A) is strong, showing that the Ag NW significantly enhanced

Figure 1. (A) SEM image of the entire 14 finger Au IDA device with (E1)Au/Ag NW/4-ATP/Au(E2) junctions. The red circle indicates the area that is shown as an expanded SEM image in frame B. These devices contain multiple Ag NWs crossing the electrodes. The average number of E1/Ag NW/4-ATP/E2 junctions on devices prepared this way, based on top-view SEM images, is 6 ± 2. (B) Expanded SEM image of the circled region in frame A. The red circle indicates the Ag NW/4-ATP/Au(E2) junction. This is one of 8 junctions on this device, labeled as no. 3 in frame A. C

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since some show almost no enhancement at all (wires 1 and 5). Supporting Information Figure S2 shows a side-view SEM image of a sample with Ag NW junctions, revealing that Ag NWs grow horizontally and vertically from the Au. Some of the Ag NWs that appear to be in contact with E2 from the top-view SEM image in Figure 1 and the optical images in Supporting Information Figure S1 are likely above the E2 electrode and not in close contact. This explains the low or zero Raman enhancement at some of the Ag NWs. The Raman signal is therefore indicative of the Ag NW/4-ATP/E2 contact. Differences in the Ag NW morphology can also lead to differences in the Raman enhancement. Supporting Information Table S1 shows the maximum intensity of the Raman signal for different E1/Ag NW/4-ATP/E2 junctions within the same device and in different devices along with the current measured through the device. The device with the largest current also showed the largest SERS signal, indicating the extent of contact is important for both. It is important to estimate the Raman enhancement factor for these Ag NW junctions to determine their potential use for analytical applications. The enhancement factor (EF) is calculated according to EF = (ISERSNbulk )/(IbulkNSERS) Figure 3. (A) SERS spectra (632.8 nm laser) of 4-ATP molecules obtained at different regions on the electrode device represented by the red open circles in Figure 2 and (B) zoomed in spectra collected from B to E regions. The spectrum in A is consistent with photoxidation of 4-ATP to 4,4′-dimercaptoazobenzene (DMAB).

(1)

where ISERS is the intensity of the Raman band (we use the 1576 cm−1 band) obtained from the junction and Ibulk is the intensity of the Raman signal obtained away from the junction, but at the edge of or on the electrode where 4-ATP is chemisorbed but the Ag NW is absent. Nbulk is the number of 4ATP molecules giving rise to the Raman signal in the absence of the Ag NW, and NSERS is the number of 4-ATP molecules giving rise to the signal in the presence of the Ag NW. It has been reported that each 4-ATP molecule occupies an area of ∼0.2 nm2.65 Since the laser illumination area has a diameter of ∼1.3 μm, or area of 1.33 × 106 nm2, this means that there would be approximately 6.6 × 106 4-ATP molecules in the illumination area. We used half that value (3.3 × 106 molecules) for Nbulk since the laser spot is on the edge of the Au electrode. We estimate Ibulk to be 10 counts based on spectra B and E in Figure 3 obtained at the electrode edge. The highest ISERS from Supporting Information Table S1 is 57,820 counts. If we consider that Ibulk is 10 counts for a 1.3 μm diameter spot size in the absence of the Ag NW at the electrode edge, then the intensity due to the Ag NW site itself is approximately 57,820 − 10, or 57,810. To get NSERS, we must estimate the number of 4ATP molecules that are enhanced and giving rise to the SERS signal. This has the most uncertainty. To do this, we estimate the Ag NW contact area from the average resistance of Ag NWs grown across the same IDAs without a SAM in the junction, which we previously found was about 1024 nm2, based on a perfect cylindrical geometry for the Ag NW.62 This is actually a high estimate since the Ag NW is more of a cone shape. The end is responsible for Raman enhancement, but it is smaller than the estimated geometric area based on the resistance, which is determined by the base and cone angle of the Ag NW as well. The estimated value is closer to the average crosssection of the wire along the total length, not just the end. Using this estimate, though, the number of 4-ATP molecules in a 1024 nm2 area is 5120. Using this calculation for the number of molecules, NSERS, gives a maximum enhancement factor of

Figure 4. SERS spectra of 4-ATP molecules obtained from (A) the same spot of the E1/Ag NW/4-ATP/E2 junction three times and (B) different E1/Ag NW/4-ATP/E2 junctions numbered 1−8 in Figure 1A.

ATP/E2 junctions numbered as 1−8 in the SEM image shown in Figure 1A. Figure 4B shows the SERS spectra corresponding to the junctions numbered as 1−8 in Figure 1A. Clearly, there is significant deviation from junction to junction, especially

EF = (57,810 × 3.3 × 106)/(10 × 5120) = 3.7 × 106 D

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Analytical Chemistry Supporting Information Table S1 shows the signals and estimated enhancement factors of all of the junctions at three different devices. The EF values range from 103 to 106. Inaccuracies in the Ag NW contact area could affect the EF estimates, but we feel we have been conservatively high in our estimated contact area, making it likely that the EF values are actually higher. Recently, it was reported that SERS enhancements of 10 8 −10 10 are sufficient for single molecule detection.66 Others have reported EF values of 106−108 for chemically synthesized Ag NRs and Ag NWs, consistent with the highest value for our electrochemically fabricated Ag NWs.67−69 The intensity of the Raman band at 1576 cm−1 varies from 2000 to 60,000 for the different junctions. This large deviation is partly due to differences in the Ag morphology, but mostly due to the different level of contact made between the Ag NWs and 4-ATP/Au(E2). In some cases, the Ag NW may not be in close electrical contact with the 4ATP at all. Supporting Information Table S1 shows the device current in parentheses along with the Raman enhancement of different devices. The device with the highest current displayed the largest SERS signal, suggesting that the Ag NW/4-ATP contact is responsible for the deviation in Raman enhancement from junction to junction. We reported an estimated range of EF values in Supporting Information Table S1 instead of an average and standard deviation of the measurements due to the large wire-to-wire variability. In an attempt to determine the minimum number of molecules we can detect at these junctions, we diluted the 4ATP molecules in the SAM with non-Raman active hexanethiol (C6S) molecules. Figure 5 shows the SERS spectra of 4-ATP molecules obtained from E1/Ag NW/4-ATP:C6S/E2 junctions with different ratios of 4-ATP:C6S of 1:0, 1:9, 1:99, and 1:999. Supporting Information Table S2 shows the intensities of the signal for the Raman band at 1576 cm−1 obtained from the different junctions of three different devices with the same 4ATP:C6S ratios. The intensities ranged from 200 to 2000, from 200 to 2500, and from 50 to 200 for the 1:9, 1:99, and 1:999 mixed 4-ATP:hexanethiol SAMs, respectively. The median values were 1575, 830, and 115 for the 1:9, 1:99, and 1:999 ratios, respectively, which is consistent with the 4-ATP dilution. No signal above the noise level was observed for samples with a 1:9999 4-ATP:hexanethiol ratio. Although the Raman intensities vary considerably for the different junctions, there is good general correlation between median Raman intensity and the concentration of 4-ATP in the SAM. Based on the estimated 1024 nm2 contact area holding about 5000 thiol molecules, the 1:999 4-ATP:C6S SAM would contain about five 4-ATP molecules in the junction, assuming that there is no phase segregation under the Ag NW and the ratio of 4ATP:C6S on the surface reflects the ratio it was in solution. This represents a very small number of molecules in the junction region.

Figure 5. SERS spectra of ATP molecules obtained from E1/Ag NW/ 4-ATP:C6S/E2 junctions with different ratios of 4-ATP:C6S (1:0, 1:9, 1:99, and 1:999).

us to determine the extent of contact between the Ag NW and 4-ATP SAM as evidenced by some correlation with the device resistance. Sub-monolayer coverages of 4-ATP, specifically 1:999 4-ATP:C6S SAMs, could be detected within the junctions by SERS. Experiments to improve the reproducibility of the junctions and prepare single Ag NW devices are currently underway. In order to correlate the electronic properties of molecules in the junction with structural changes in the future, we will use these devices as a platform for measuring both conductivity and SERS spectra simultaneously for a small number of molecules. This could be a powerful approach for better understanding various molecular electronics systems and serve as a dual probe for chemiresistive and optical based sensing.

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CONCLUSIONS In summary, we described a simple approach to fabricate E1/ Ag NW/4-ATP/E2 junctions by combining thiol self-assembly and metal electrodeposition at microgap electrodes. The electrodeposited Ag grew in the form of NWs and made nanoscale contact to form E1/Ag NW/4-ATP/E2 junctions. The presence of the Ag NW at the junction led to strong Raman scattering of the molecules within the nanojunction, leading to enhancement factors estimated as high as 106. The SERS enhancement of several junctions within a device allowed

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.5b02343. Optical and SEM images of different E1/Ag NW/4ATP/E2 junctions and Raman intensities of 4-ATP molecules from different E1/Ag NW/4-ATP/E2 junctions containing monolayer and submonolayer amounts of 4-ATP (PDF) E

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: 502-852-8149. Present Address †

Department of Chemistry, Eastern Kentucky University, 521 Lancaster Ave., NSB 4126, Richmond, KY 40475, United States. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the National Science Foundation (Grant CHE-1308763) for full financial support of this research. R.D. acknowledges Chandrashekar Pendyala for assistance with Raman measurements.

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DOI: 10.1021/acs.analchem.5b02343 Anal. Chem. XXXX, XXX, XXX−XXX