Probing Local Potentials inside Metallic ... - Wiley Online Library

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Yi Li,* Chang Chen,* Kherim Willems, Sarp Kerman, Liesbet Lagae, Guido Groeseneken, Tim Stakenborg, and Pol Van Dorpe

considerable attention. In addition to being used as electrodes, optical nanostructures for extraordinary light transmission,[8] fluorescence,[9] and surface-enhanced Raman spectroscopy (SERS)[10] have demonstrated additional detection and/or control capabilities. For these optical techniques, metal layers are usually unbiased, so-called floating electrode mode, whereas applying a bias voltage may induce a potential drop over the floating metallic nanopore because of its high resistance. In the field of macroscale fluidics, a sufficient DC voltage difference between the floating metal layer and the solution enables the activation of oxidation and reduction reactions.[11,12] However, on the nanoscale, the study on the local potential and electrochemical effects on floating metallic nanopores remains challenging. Local potential and related electrochemistry on floating metal electrodes has been termed “bipolar electrochemistry”[12,13] In an unbiased state, bipolar electrochemistry requires either highly confined electric fields or extremely high concentration-gradients adjacent to bipolar electrodes[14–16] to drive reactions and investigate optical properties. Such investigation techniques include electrochemiluminescence,[17] electrochemical corrosion microscopy,[18] epifluoresence,[19] and surface plasmon resonance spectroscopy,[20] which are ultimately restricted by the diffraction of light. Here, working toward subwavelength detection, we propose the use of SERS to study the local potentials.[21–25] To simultaneously align localized SERS hot spots with fluidic focus for high spatial resolution and high specificity,[26] our technique probes the local potential and characterizes nanoscale bipolar electrochemical effects on metallic nanopores for the first time. Figure 1a depicts a schematic of the experiments on the metallic nanopores. The nanopore chip was mounted into a custom-made flow cell, separating the electrolyte into two reservoirs. Next, a 785 nm laser was tightly focused at the nanopore cavity. The electrical potential was applied to the top side of the membrane as shown in Figure 1a, and the other side was connected to ground. This defines the applied bias direction for positive and negative values in the experiments. A doublesided gold (MM) nanopore is used for the demonstration of bipolar electrochemical SERS. To maximize the plasmonic enhancement effect and subsequent SERS signals at 785 nm,

It is essential to understand the local potential distribution of solid-state nanopores in nanofluidic systems. However, applying gate voltage or adding external electrical probes tends to disturb the electric field and/or flow patterns. To solve this problem, an approach is described to monitor the local potential using electrochemical surface enhanced Raman spectroscopy (EC-SERS) in two types of nanocavity pores: doubled-sided gold nanopores (MM nanopores) and single-sided gold nanopores with a dielectric passivation layer on the backside (MD nanopores). Numerical simulations predict an electrical polarization reversal in the two nanopore geometries. Consequently, the redox SERS changes of Nile Blue A on the two gold nanopores are found to be reversed, which is consistent with the variation of polarizations. The driving voltage of metallic nanopore devices is about an order of magnitude lower than that of microfluidic bipolar devices. Our method will not only prove valuable for the design of metallic nanopores, but also will find applications in the measurement of contactless metallized nanofluidic devices.

Solid-state nanopores integrated with metallic elements have been used extensively to investigate localized physical or chemical processes.[1,2] Applications such as tunneling current through electrode pairs[3,4] or biasing a gate electrode[5–7] for molecular sensing and manipulation, for instance, have drawn Dr. Y. Li,[+] Dr. C. Chen, K. Willems, S. Kerman, Prof. L. Lagae, Prof. G. Groeseneken, Dr. T. Stakenborg, Prof. P. Van Dorpe imec Kapeldreef 75, 3001 Leuven, Belgium E-mail: [email protected]; [email protected] Dr. Y. Li, Prof. G. Groeseneken Department of Electrical Engineering KU Leuven Kasteelpark Arenberg 10, 3001 Leuven, Belgium Dr. C. Chen, S. Kerman, Prof. L. Lagae, Prof. P. Van Dorpe Department of Physics and Astronomy KU Leuven Celestijnenlaan 200D, 3001 Leuven, Belgium K. Willems Department of Chemistry KU Leuven Celestijnenlaan 200F, 3001 Leuven, Belgium [+]

Present address: Experimental Solid State Group, Department of Physics, Imperial College London, London SW7 2AZ, UK

DOI: 10.1002/adom.201600907 Adv. Optical Mater. 2017, 5, 1600907

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Probing Local Potentials inside Metallic Nanopores with SERS and Bipolar Electrochemistry


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Figure 1. Bipolar electrochemical surface-enhanced Raman spectroscopy (EC-SERS) through single gold nanopores. a) Schematic drawing of the experiments and typical Raman spectrum of NBA molecules. 1 × 10−5 m Nile Blue A (NBA) molecules (in blue) are dispersed in the electrolyte, acting as electrochemical active Raman probes. The SERS hot spots are illustrated in red. b) The SERS spectra are at +1 V (blue), 0 V (green), −1 V (red) and the control group without the NBA probes (black). The fingerprint peak at 594.8 cm−1 of NBA is marked with an asterisk.

nanopores with a gap size of 10 nm and the length of ≈100 nm were fabricated. The plasmonic resonance of devices in water was found to be ≈785 nm,[27,28] with the SERS enhancement factor of 108 for the width of 10 nm in our numerical calculation.[27] It is worth noting that the SERS hot spots with enormous enhancement has been shown from the bottom of the nanopores.[26] The processing details are explained in the Supporting Information. Nile Blue A (NBA) was used as the electrochemical active Raman probe. The voltage was applied across the nanopore device in the conductive electrolyte, providing potential drop concentrated inside the nano pores and overlapping with the SERS hot spots. Typical SERS spectra of NBA are shown in Figure 1b. The fingerprint peak at 594.8 cm−1 appears at 0 V and +1 V but not at −1 V. The spectrum at −1 V also differs from the control spectrum without NBA in the solution, featuring a higher low-wavenumber background. This indicates that the NBA molecules become SERSinactive at −1 V. Our observation is in line with the reports for bianalyte single-molecule NBA measurements,[21,29,30] showing that this vibration mode is electrochemically active in our system.

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To determine the local potential through SERS, we investigate two types of metalized devices (shown in Figure 2): doubled-sided gold nanopores (MM nanopores) and single-sided gold nanopores with a backside dielectric (e.g., silicon nitride, SiN) passivation layer (MD nanopores). The MM nanopores as fully covered with gold, have been realized as an opposite polarization in bipolar chemistry, while the MD nanopores have not been fully explored. We performed simulations using COMSOL Multiphysics v4.3b, with details in the Supporting Information. A distinct polarization dependent properties are shown in Figure 2a,b. An opposite induced polarization of free charges can be found for MM nanopores. However, for MD nanopores, the continuity of induced free charge redistribution ends at the interface of the backside dielectric layer, resulting in an incomplete polarization. To demonstrate that the applied electric field along the gold/electrolyte interfaces drives the bipolar effects vertically, we plot the simulated profiles of DC electric field component in the z direction Ez in Figure 2c,d. The highest DC electric field located along the center of the MD nanopore channels, whereas the field intensity becomes much weaker for the MM nanopores. This can be attributed to an incomplete polarization of the metal layer. In addition to floating metals, bulk dielectrics can become polarized by applying an electric field with high intensity.[31–33] For the MM nanopores, the sign of Ez at +1 V (applied voltage) is negative, indicating the gold holds a lower potential than the electrolyte near the hot spots, while the positive sign of Ez at −1 V results in a higher potential on the gold than on the electrolyte. The converse is true for MD nanopores. The simulation model predicts the reversal of DC polarizations around the SERS hot spot regions for the two kinds of nanopores. To experimentally support the reversal of the electric field distribution, we utilize electrochemical SERS for the two kinds of nanopore devices. These devices feature strong electromagnetic field enhancements (SERS hot spots) properly aligned to the electrochemical reaction site at the fluid channel.[26] The DC potential difference applied in the electrolyte across the nano pores device induces charge redistribution, polarizes the unbiased gold layer and therefore triggers bipolar electrochemical reactions. Figure 3 displays the integrated SERS intensities at the 594.8 cm−1 band for NBA (illustrated in Figure 1b) at different bipolar driving voltages. First, the SERS intensity from the MM nanopore increases with an increasing potential. This voltagedependent SERS behavior confirms the functioning of the bipolar electrochemistry in our metallic nanopores. Conversely, the intensity response from the MD nanopore decreases with the increasing potential. The different SERS responses between the two devices are consistent with the numerical simulations as described in Figure 2. For the MM nanopores, the increase of SERS intensity can be attributed to the bipolar electrochemical induced oxidation of the SERS probe (NBA), while the intensity decreasing on the MD nanopore implies the genera tion of a reduced form. Notably, the polarization of MD nanopore takes place both on gold and dielectrics—SiN due to high electric fields, which was previously demonstrated by Bouchet and co-workers.[31–33] Additionally, the threshold of driving voltage is found to be ≈0.7–0.8 V, higher than the threshold for redox potential of 0.48 V (see Figure S1, Supporting Information). This suggests, in our nanopore, 1.46-fold of the redox


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SERS intensities, the oxidized form of NBA gives rise to a higher intensity of 300 a.u. at 594.8 cm−1, locating in a spot with full width at half maximum (FWHM) of 2–3 µm. The size of this spot is mainly due to the convolution of the laser beam size and the pore geometry. The completely diminished SERS maps indicate that not only does the reduced form yield little to no SERS signal inside the nanopore, but also oxidized forms move out of the detection region (on the top of the cavity). Therefore, the spatial and spectroscopic integration SERS signal can only report the reaction process that occurs at the hot spots. Benefiting from the understanding of the local potential inside our gold nanopores, we turn to study the manipulation of different small molecules based on both bipolar electrochemistry and nanofluidics for sensing. Two kinds of small molecules are used: 2′-deoxyadenosine 5′-monophosphate (dAMP) for nanofluidic control due to its Figure 2. Simulated Ez distribution of the DC electric field along the x-axis for a) double-sided charges and concentrations, while 4-amingold (MM) nanopores and b) top-sided gold nanopores (MD) with a passivation dielectric obenzenethiol (4-ATP) further enables the (SiN) layer on the backside at −1 V. The charge distributions are labeled for eye guide. The forming of a monolayer via AuS chemical zoomed-in Ez distribution in x–z plane for c) MM nanopores and d) MD nanopores. The dashed bond onto the gold surface. We will compare lines indicate the cutline for the main graph. the electrochemical displacement of 4-ATP with nonchemical process with dAMP. Figure 4 shows a three-step electrochemical process inside potential is required to drive the bipolar effects for these a gold nanopore channel. An MM nanopore was shown in devices. Compared to the observed driving voltage in microfluFigure 4a, which was initially coated with a self-assembled idic channels (usually >10-folds) with bipolar electrodes,[18,34] monolayer of 4-ATP and was then immersed with 1 × 10−3 m the voltage threshold of our gold nanopores is around seven times lower. dAMP dissolved in 0.1 m KNO3. The molecular structures of The spatial mappings of SERS are illustrated as the insets dAMP and 4-ATP are displayed in Figure 4b. The transmemin Figure 3. First, we can observe clear ON/OFF intensity brane bipolar voltages used in the experiments are plotted in changes of the nanopores at ±1 V. In line with the average Figure 4c. We switched the transmembrane voltage from +1 V to −1 V and later back to +1 V, dividing the entire process into the three stages. Figure 4d illustrates the schemes of molecules surrounding the SERS hot spots of our nanopores for all the stages of I, II, and III, which we will discuss into details later. Figure 4e shows the typical SERS spectra of 4-ATP and dAMP at these three stages: the nonresonant dAMP contributes to a weak SERS band at 731 cm−1 in the brown region, while SERS peaks from 1000 to 1700 cm−1 are mainly attributed to 4-ATP and its chemical transformation to 4,4-dimercaptoazo-benzene (DMAB, purple window). Among them, the 1139, 1387, and 1425 cm−1 have been named as “b2 modes”—a chemical transformation from two 4-ATP to DMAB mediated by electrons transfer. The temporal traces of integrated SERS band are depicted in Figure 4f, which are synchronized with the applied voltage of Figure 4c. For the initial condition of bias voltage at +1 V, the SERS spectrum recorded at 15 s shows the peaks Figure 3. Integrated Raman band intensities at 594.8 cm−1 as a function both for dAMP and 4-ATP, including the hot electron induced of applied voltage. Yellow dots illustrate the SERS intensities of doubleSERS peaks. At this stage (Scheme I in Figure 4d), both 4-ATP sided gold nanopores while black squares represent the results of gold and dAMP are in close proximity to the gold surface or even nanopores with backside SiN. The mean value and the error bar are adsorbed onto it. derived from 50 spectra at each potential level. The insets in the blue and When the transmembrane voltage switches to −1 V at stage II, red boxes are the x–y plane SERS maps at +1 V and −1 V, respectively. The we observe a steep rise in temporal response of 600 cm−1 band, white scale bar is 1 µm.


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Figure 4. Bipolar electrochemistry of complex chemical replacements inside a single MM nanopore. a) MM Nanopore configuration. b) Molecular structures of 2′-deoxyadenosine 5′-monophosphate (dAMP) and 4-aminobenzenethiol (4-ATP). The arrows indicate the corresponding SERS bands in panel (d). c) Transmembrane bipolar voltage as a function of time. The three dots on the voltage levels (I, II, and III) correspond to the time stamps in panel (e) (15, 291, and 500 s). d) Schematics of the interfacial activities for the three stages (I, II, and III) corresponding to panel (c). Stage I represents that 4-ATP is coated on the gold surface and dAMP gets close to the interface. Stage II for the generation of AuO stretch and the repelling of dAMP away from the interface. Stage III for the termination of AuO stretch and dAMP can reach the interface again. e) Measured SERS spectra at 15, 291, and 500 s. The yellow window shows the spectra of AuO stretch, the brown one for dAMP and the purple one for 4-ATP. f) The temporal integrated SERS band intensities. Yellow, brown, and purple traces are the integrated band intensity of AuO, dAMP, and 4-ATP.

which is in a line with AuO vibration mode in the literatures, and the broad width indicates the amorphous structure of the oxide.[35,36] In Figure S2 (Supporting Information), we confirm the voltage threshold of AuO formation at −0.2 V. Then we turn to the changes of SERS peaks of dAMP (731 cm−1) and 4-ATP (the region from 1000 to 1700 cm−1). Both SERS peaks of 4-ATP and dAMP steadily decrease until all peaks are gone after around 5 min. We attribute the disappearance of SERS peaks to the electrochemical removal of the molecules. It is known for thiol-functionalized molecules that the electrochemical desorption occurs at around −0.7 V,[20,31] which fits with our equivalent local potential generated by bipolar electrochemistry. Second, negatively charged dAMP can be repelled by a negative bias from the surface, resulting in an undetectable level of dAMP in the observed SERS spectra. The SERS spectra response rapidly when the transmembrane potential turns back to +1 V. At stage III, the intensity of AuO band immediately drops back to its original resting state. The 731 cm−1 mode for dAMP appears again, indicating that dAMP can freely reach the gold-electrolyte interfaces in this condition. However, no signal of the released 4-ATP mole cules can be observed in the spectrum. This becomes reasonable as the molecules diffuse away from the pore, resulting in an extremely low concentration of 4-ATP. In summary, a bipolar electrochemical modulation of SERS signals has been investigated on single gold nanopores without disturbing the DC electric field distribution. We demonstrate that the local potentials of metallic nanopores can be influenced by a significant bipolar electrochemical displacement. We also show the opposite voltage dependency of the electric field distribution for both MM and MD nanopores, which is supported by numerical simulations. Benefiting from the strong

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confinement within the surrounding metal inside the nanopores, our devices outperform microfluidic bipolar devices with a driving voltage almost an order of magnitude lower. Our findings can offer better designs of nanofluidic chips and will be able to contribute to the understanding of plasmonic induced hot electron SERS effects[37,38] and the applications of (bio) chemical sensing,[39] concentration and separation,[40,41] as well as contactless sidewall functionalization.[33]

Experimental Section Chemicals: Sodium phosphate dibasic (99.95% trace metals basis), potassium nitrate (ACS reagent, ≥99.0%), dAMP (Sigma Grade, 98%–100%), and 4-ATP (97%) were purchased from Sigma-Aldrich, Germany. Device Fabrication: The fabrication procedure of 200 mm wafer scale silicon nanocavity arrays was described previously.[42] Briefly, the nanocavity structures were defined on 8 in. wafer by deep ultraviolet (DUV) lithography and anisotropic etched by tetramethylammonium hydroxide (TMAH). Then the wafer was bonded to a carrier wafer and thinned down to 200 µm. A vertical fluidic channel of 70 µm was open by deep reactive ion etching. The wafer was diced into 20 mm × 20 mm pieces which contains single cavity arrays and then remove the bonding polymer HT10.10 (Brewer Science Inc.) is removed by RCA-1 cleaning. Vapor hydrogen fluoride (VHF) etching (Gemetec Vapor Phase Decomposition) was performed for 35 min to remove the buried oxide layer. For double sided gold nanopore chips, a 10 nm Ti adhesion layer was sputtered on the backside (Pffeifer Spider 630), followed by sputtering a 10 nm Ti and 200 nm Au on the top of the cavity. Finally, a layer of 20 nm gold was sputtered on the backside. For top-side gold sample, 10 nm Ti and 200 nm Au were sputtered on the top of the cavity. For gold nanopores with backside SiN, a 30 nm SiN layer was deposited on the backside by plasma-enhanced chemical vapor deposition (PECVD) at 250 °C (Plasmalab System 100, Oxford





Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements Y.L. and C.C. contributed equally to this work. C.C. acknowledges the financial support from the FWO (Flanders). The authors appreciate Xubin Chen for his help with cyclic voltammetry measurements, and Dr. Jiaqi Li and Brock Doiron for critical reading of the manuscript. Received: October 31, 2016 Revised: January 15, 2017 Published online: February 24, 2017

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Instruments plc), followed by 10 nm Ti and 200 nm Au sputtered on the top of the cavity. Experimental Setup: For the experiments in Figure 3, the chip was diced into 3 × 3 mm2 and rinsed by acetone and Isoproponyl alcohol (IPA). After drying with N2 gas, the chips were glued with Kwik-Cast (World Precision Instrument) on polyacrylamide substrates, containing a 2 mm diameter hole. To clean the chip, 1 min O2 plasma treatment was performed on each side to clean the chip. The chip was amounted in a custom-made optical flow cell (made by polydimethylsiloxane) and filled with 10−5 m Nile Blue A, 1 m KNO3, 10 × 10−3 m Tris-acetate, and 1 × 10−3 m EDTA buffer (pH = 8.0). A pair of Ag/AgCl electrodes were inserted into both of the reservoirs and connected with the headstage of the Axopatch 200B patch clamp amplifier (Molecular Devices Co.). The amplifier was connected to a 1440A digitizer (Molecular Devices Co.) and the voltage was applied by using this instrument. For optical measurements, a 60× water immersion lens (Olympus LUMPlanFl/IR 60×/0.90 W) was dipped into the top reservoir. An incident 785 nm laser (Toptica Photonics AG, Germany), with a 100 µm IRVIS fiber (QMMJ3A3A-IRVIS-100/140-3AS-3) was focused on the nanocavity pore with its polarization initially perpendicular to the longitudinal axis of the structure. The diameter of the laser spot was estimated to be ≈1 µm and the laser intensity was 8 mW. The Raman spectra were taken from a WITec α300 Raman setup (Wissenschaftliche Instrumente und Technologie GmbH, Germany), with the integration time of 0.1 ms. Measurement in Figure S2 (Supporting Information) was performed with the pretreated chips. The MM nanopore chip was clean by O2 plasma for 1 min. Then, the chip was immersed into ethanol solution with 1 × 10−3 m 4-ATP overnight. Before mounting into the optical flowcell, the chip was rinsed with excess ethanol and dried with N2 gas. 0.1 m KNO3 and 1 × 10−3 m dAMP was used as the bulk electrolyte. Numerical Calculation: The detailed configuration of the simulation has been described previously.[43] We calculated the steady state of electrical field distribution using three modules in the COMSOL v4.3b environment: electrostatics (AC/DC module), transport of diluted species (chemical species transport module) for the calculation of K+ ions and NO3− ions, and laminar flow (fluidic flow module). The computational domain was filled with 1 m KNO3 as buffer medium. Corresponding to the reported data of SiN at pH 8.0,[44] the surface charge density of the SiN wall was configured to be −49 mC/m2.

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