Plasmonic Vertically Coupled Complementary Antennas for Dual

Plasmonic Vertically Coupled Complementary Antennas for Dual-Mode Infrared Molecule Sensing Xiahui Chen,†,‡ Chu Wang,†,‡ Yu Yao,*,†,‡ and Chao Wang*,†,‡,§ †

School of Electrical, Computer and Energy Engineering, ‡The Center for Photonics Innovation, and §Biodesign Center for Molecular Design & Biomimetics, Arizona State University, Tempe, Arizona 85287, United States S Supporting Information *

ABSTRACT: Here we report an infrared plasmonic nanosensor for label-free, sensitive, specific, and quantitative identification of nanometer-sized molecules. The device design is based on vertically coupled complementary antennas (VCCAs) with densely patterned hot-spots. The elevated metallic nanobars and complementary nanoslits in the substrate strongly couple at vertical nanogaps between them, resulting in dual-mode sensing dependent on the light polarization parallel or perpendicular to the nanobars. We demonstrate experimentally that a monolayer of octadecanethiol (ODT) molecules (thickness 2.5 nm) leads to significant antenna resonance wavelength shift over 136 nm in the parallel mode, corresponding to 7.5 nm for each carbon atom in the molecular chain or 54 nm for each nanometer in analyte thickness. Additionally, all four characteristic vibrational fingerprint signals, including the weak CH3 modes, are clearly delineated experimentally in both sensing modes. Such a dual-mode sensing with a broad wavelength design range (2.5 to 4.5 μm) is potentially useful for multianalyte detection. Additionally, we create a mathematical algorithm to design gold nanoparticles on VCCA sensors in simulation with their morphologies statistically identical to those in experiments and systematically investigate the impact of the nanoparticle morphology on the nanosensor performance. The nanoparticles form dense hotspots, promote molecular adsorption, enhance near-field intensity 103 to 104 times, and improve ODT refractometric and fingerprint sensitivities. Our VCCA sensor structure offers a great design flexibility, dual-mode operation, and high detection sensitivity, making it feasible for broad applications from biomarker detection to environment monitoring and energy harvesting. KEYWORDS: plasmonic nanoantennas, refractometric sensing, molecular fingerprint, surface-enhanced infrared absorption, vibrational spectroscopy, self-assembled gold nanoparticles, near-field enhancement red,21−23 and color-based molecular detection.24,25 Most plasmonic nanosensors are sensitive to the physical properties of the analyte such as its thickness and refractive index but insensitive to the biochemical properties; therefore, they require labor-intensive sample labeling to achieve a high specificity. Surface-enhanced Raman scattering (SERS) can

H

igh-sensitivity and high-specificity detection of biological and chemical molecules is crucial for accurate determination of threat- and disease-relevant species and biomarkers, which have great implications in forensic and homeland security, environment monitoring, early stage disease diagnosis, etc.1−7 Plasmonic nanosensors7−11 are advantageous in their reliable molecular binding,12,13 strong enhancement of signal intensity, flexible working wavelength design for detection of a variety of molecules,14−16 and feasibility of different sensing mechanisms, including Raman,17−20 infra© 2017 American Chemical Society

Received: April 18, 2017 Accepted: July 10, 2017 Published: July 10, 2017 8034

DOI: 10.1021/acsnano.7b02687 ACS Nano 2017, 11, 8034−8046

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Figure 1. Vertically coupled complementary antenna (VCCA). (a) Three-dimensional schematic of the VCCA design. (b, c) Charge distribution and schematic electric field distribution of the device working at (b) parallel mode and (c) perpendicular mode. The cross section in (b) and (c) is cut along the axis of structure symmetry in x−z and y−z planes in (a), respectively. (d, e) Simulated near-field intensity corresponding to the (d) parallel mode and (e) perpendicular mode. The nanobars in parts d and e are chosen as both 125 nm wide but 720 and 830 nm long, respectively, so that their resonance wavelengths match the ODT CH2− asymmetric vibrational signals in simulation. (f−h) Scanning electron microscope images of the fabricated VCCA nanosensors: (f) top view and (g, h) side view. The nanobars in parts f−h are 620 nm long, 125 nm wide, and 1 μm in period, and the vertical gap size is approximately 40 nm.

distinguish biochemical species,26 but it remains a challenge for precise quantification,27,28 because of uncertainties in determining the amount of scattering molecules, which tend to be unstable over time due to morphology change in the SERS substrate by laser radiation, photobleaching of molecules, photochemical effects, interference from fluorescence signals, etc. In comparison, surface-enhanced infrared absorption (SEIRA) is a label-free, nondestructive, and specific technique that provides quantitative information about molecule fingerprints at infrared spectral range that can faithfully reflect the molecule structures. In the mid-infrared (mid-IR) range, vibrational signals of crucial functional groups (4000 to 1300 cm−1, or 2.50 to 7.69 μm) and single-bond and skeletal vibrations of molecules (from 1300 to 650 cm−1, or 7.69 to 15.38 μm) are both critical to identification of biochemical molecules.29−31 One challenge in SEIRA is the accurate detection of small molecules due to their small IR absorption. To boost the sensitivity, a variety of plasmonic nanoantenna designs, such as nanoparticles, 32 nanorods, 23,33,34 nanocrosses, 21,35 split rings,36,37 and fan-shaped antennas,22 have been used to amplify the molecular absorption signals by enhancing the near-field intensity. Typically, a very small nanoantenna gap (as small as sub-10 nm)23 or a very sharp antenna tip (radius of curvature as small as 5 nm)22 is necessary to create the “hotspots” where the near-field is significantly enhanced (up to 4−5 orders of magnitude). In these designs, the hot-spots are essentially an isolated nanofocus that greatly confines the local electromagnetic field.38 Despite a high enhancement factor, however, such sensors suffer a large signal fluctuation at lowabundance analyte detection due to the random molecule binding nature to the low-density hot-spots. In addition, the

stringent fabrication requirement of such small nanostructures22,23 makes it very challenging to scale the production of the plasmonic sensors to an application-relevant level. Here we report an infrared plasmonic nanosensor that has densely packed hot-spots without requiring complex nanofabrication. In our vertically coupled complementary antenna (VCCA) scheme, vertical nanogaps form between elevated anisotropic metal nanobars and complementary nanoslits on the substrate. Densely packed metal nanoparticles are assembled within the nanogaps to form hot-spots with significantly enhanced near-field coupling (by 3−4 orders of magnitude). In this prototyped demonstration, we chose monolayer octadecanethiol (ODT) as the test molecule because the assembly procedure of ODT on gold plasmonic sensors and its sensitivity are well-documented,21,22,32,33,36,39−44 allowing us to better evaluate our VCCA sensor performance. Our sensor can detect a 7.5 nm resonance shift per carbon atom in the alkanethiol chain or a 54 nm shift per 1 nm molecule thickness. Moreover, our sensor scheme can unambiguously identify the ODT molecules by clearly delineating all four symmetric and asymmetric CH3− and CH2− vibrational modes. Further, the identification mechanism of CH2− and CH3− vibrational modes can be readily used for other stretching modes, such as N−H and O−H modes, in the same wavelength range and further extended to shorter or longer wavelength range for identification of molecular overtones, unsaturated molecular bonds, and skeletal vibrations. Furthermore, our sensor can detect molecules with dual sensing modes depending on the polarization alignment to the nanobars, demonstrating its potential for multiband biochemical sensing. 8035


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RESULTS AND DISCUSSION Design and Fabrication of the VCCA Sensor. The VCCA consists of a top metallic nanoantenna, a SiO2 pillar, a nanoslit in the metal thin film, and self-assembled nanoparticles between the metal nanobar and nanoslit (Figure 1a). Such a design is advantageous for IR molecular sensing due to the following reasons. First, in conventional designs the antenna sensors are patterned directly on a planar substrate, and accordingly the enhanced near field is largely confined within the substrate film but not accessible to the analyte.33−37 Recent research demonstrates that elevating gold nanoantennas from a dielectric substrate can boost the molecular signal by 1 order of magnitude.35 Here, our design elevates the top metal nanobars above coupled metallic nanoslits, thus creating hot-spots at the vertical nanogaps between the bars and slits that are accessible to molecular bonding and sensing. Second, unlike many vertically coupled nanoantennas that are round and isotropic,17,45 the bar-shaped VCCA design breaks the structural symmetry and yields two distinct operation modes at different resonance wavelengths, namely, “parallel mode”, when light polarization is parallel to the top nanobar (Figure 1b), and “perpendicular mode”, when light polarization is perpendicular to the nanobar (Figure 1c). In parallel mode, the polarized light parallel to the nanobar antenna excites collective electrons, which accumulate at the ends of top nanobars and effectively form an electric dipole. As a result, charges are induced in the substrate nanoslit metal film, strongly interacting with the charges in the nanobars, resulting in strong vertical coupling and electrical field intensity enhancement in the vertical gap between the nanobars and nanoslits. Therefore, the highest detection sensitivity can be achieved within the vertical gaps, i.e., the hot-spots. In perpendicular mode, incident light polarized perpendicular to the nanobar excites magnetic dipoles instead and charges distribute along the edge of the length of the aperture.46 This can be understood by Babinet’s principle, in which the inverse structure to the nanobar features similar plasmonic resonance behavior and electric and magnetic field distribution interchange in complementary systems.46−48 The dual-mode design potentially allows our sensors to detect molecular fingerprint signals over a broad wavelength range. Third, gold nanoparticles nucleate within the vertical nanogap during metal deposition and create a large amount of fieldenhancing hot-spots with a spacing as small as 100 for the parallel and perpendicular modes for small d, respectively, and then slowly drops as d increases due to the exponential decay of the plasmonic near-field intensity. Additionally, the gold nanoparticles can promote the molecular bonding surface areas and at the same time enhance |E|2 by ∼20% and ∼50% in the parallel and transverse mode (Supplementary Figure S11b), explaining the enhanced ODT sensitivity in our VCCA structures. Additionally, the parallel mode is ∼3 times better than the perpendicular mode in |E|2 , and thus it has a higher ODT sensitivity despite a smaller volume where the field is strongly enhanced (Figure 6). ODT Identification by Molecular Fingerprints. Besides detecting the spectral difference induced by the ODT refractive index, our VCCA sensor is also capable of identifying the unique ODT IR fingerprints, hence feasible for label-free and specific detection (Figure 7). We extracted the ODT signals from the IR reflectance signals by a baseline correction algorithm adapted from asymmetric least-squares smoothing (AsLSS, Supplementary Section 4.1). This algorithm removes the superimposed nanoantenna signal and generates only the ODT vibrational fingerprint signals; hence it has been used in 8040


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Figure 6. FDTD-simulated field intensity of the bare VCCA nanosensors. (a) Schematic of parallel mode operation. (b, c) FDTD-simulated near-field distribution at resonance wavelength of (b) our VCCA nanosensor with nanoparticles and (c) a hypothetical sensor with the same geometry but no nanoparticles. The field distribution is log-plotted in the x−z plane 5 nm below the metal nanobar. (d) The field intensity is monitored along the nanobar short-side edge in the y-direction and 3 nm away from the nanobar edge (red, with gold nanoparticles; blue, without gold particles). (e) Schematic of perpendicular mode operation. (f, g) FDTD-simulated near-field distribution at resonance wavelength (f) with nanoparticles and (g) without nanoparticles. The field distribution is log-plotted in the x−y plane (5 nm above the nanoslit film). (h) The field intensity is monitored along the nanobar long-side edge in the x-direction and 3 nm way from the nanobar edge (red, with gold nanoparticles; blue, without gold particles). In parts a−h, the nanosensors working in parallel mode and perpendicular mode both have a 40 nm gap and 125 nm bar width. The bar lengths are selected as 720 and 830 nm, respectively, so that the sensors’ resonance wavelengths match the ODT CH2− asymmetric vibrational signals in simulation upon ODT coating.

Figure 7. ODT fingerprint signal detection. (a) Fingerprint signal extraction by the AsLSS algorithm. (b, c) Comparisons of ODT fingerprint intensity at different gap sizes (40 nm, blue line; 50 nm, black line; and 65 nm, red line) in (b) parallel mode and (c) perpendicular mode. The nanosensors are chosen such that their plasmonic resonance best matches with ODT vibrational modes.

mentary Table S2).21,22,33,36,41,42 The demonstrated strong ODT fingerprint signals and experimental identification of the weak CH3− symmetric vibrational signals by our VCCA sensor indicate that our sensor is capable of retrieving the critical chemical and structural information on small molecules, which cannot be achieved by solely refractometric sensing in conventional surface plasmon resonance sensors. The high ODT fingerprint sensitivity is due to the strong coupling scheme in our VCCA design. On one hand, the vertical coupling between the bar and the slit has a strong impact on the ODT fingerprint sensitivity (Figure 7). Apparently, the ODT signal intensity increases as the gap size shrinks, which is consistent with the previous experimental

and simulation observations (Figures 4, 5, and 6) that a smaller gap enhances the vertical coupling and hence the refractometric molecule detection sensitivity. On the other hand, the selfassembled nanoparticles further enhance the near field and thus strengthen the excited ODT signals, as well as increase the binding sites of ODT molecules in the hot-spots. Unlike other localized surface plasmon sensors that use very narrow gaps or sharp tips to define a very localized hot-spot,22,23 our VCCA sensors have densely packed nanoparticles as the hot-spots (3000 to 5000 per μm2 depending on the nanogap size, Supplementary Figure S2) and therefore provide a large amount of bonding sites to each sensor, which is desired to 8041


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Figure 8. ODT fingerprint signal intensity dependence on the VCCA nanosensor resonance. (a, b) AsLSS-extracted ODT signals for nanosensors with different bar lengths in (a) parallel and (b) perpendicular mode. The vertical gap size is 40 nm, and nanobars are from 520 nm at the bottom to 925 nm at the top. (c, d) Impact of the detuning effect on normalized ODT CH2− asymmetry fingerprint signal intensity in (c) parallel mode and (d) perpendicular mode. The experimental data (black dots) are fitted by a Lorentzian function (red curve).

Table 2. Comparison of ODT Fingerprint Sensitivity in Parallel Mode on Nanosensors with Different Gap Sizes sensor

nanogap (nm)

bar length (nm)

bar width (nm)

λres,ODT (nm)

CH2− symmetric (2850 cm−1) intensity


CH2− asymmetric (2918 cm−1) intensity


1 2 3

40 50 65

655 675 730

125 125 125

3449 3412 3477

1.7% 1.5% 1.4%

0.7% 0.6% 0.4%

4.1% 3.7% 3.3%

1.1% 1.0% 0.6%

Table 3. Comparison of ODT Fingerprint Sensitivity in Perpendicular Mode on Nanosensors with Different Gap Sizes sensor

nanogap (nm)

bar length (nm)

bar width (nm)

λres,ODT (nm)





1 2 3

40 50 65

770 790 810

125 125 125

3412 3439 3447

1.2% 0.8% 0.7%

0.5% 0.3% 0.2%

2.5% 1.9% 1.5%

0.7% 0.5% 0.2%

suppress noise from molecular bonding fluctuation and improve the signal-to-noise ratio. We also investigated the dependence of the ODT fingerprint signal intensity on the VCCA nanobar design (Figure 8). By changing the bar length from 520 nm to 925 nm in parallel and perpendicular operational modes (Figure 8a and b), we noticed a Fano-type resonance in the reflection signals attributed to the interference nature of plasmonic nanoatenna resonance and molecular vibration.65−67 Here we use a resonance detuning factor (defined as ωres/ωODT) to characterize how well the

resonance of the VCCA antenna (ωres) matches that of the ODT fingerprints (ωODT) (Figure 8c and d). Clearly, when the VCCA antenna resonance well matches the ODT IR fingerprint range (2850−2965 cm−1, or 3370 to 3510 nm), the ODT molecules are best excited to yield the highest ODT signal intensity (IODT) and result in the most pronounced Fano resonance effect. The ODT CH2− asymmetric vibrational signal intensities clearly depend on the VCCA sensor resonance frequency, and this can be understood by a model of strongly coupled harmonic oscillators.68−70 Intuitively, the near-field 8042


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ACS Nano intensities of the VCCA sensors roughly follow a Lorentzian distribution and reach the maximum values at resonance; therefore the ODT molecules are best excited when their fingerprint frequencies overlap with the VCCA resonance, following a Lorentzian distribution similar to the near field of the antenna structures. For the VCCA sensors, its resonance in the perpendicular mode is much broader than in the parallel mode, and therefore the molecular signals in a relatively broader wavelength range are all enhanced, feasible for multiplexed sensing. On the other hand, the near-field enhancement in the parallel mode is a few times stronger than that in the perpendicular mode, making it feasible for highsensitivity detection. ODT Signal Sensitivity and Detection Limit. Accurate and high-sensitivity molecular detection depends on both the sensor design and the experiment conditions. We experimentally evaluated the impact of nanopatterning uniformity from four different areas of one sample (Supplementary Figure S13). Our results found the sensor resonance at 2934.2 ± 1.35 cm−1 and a small ODT fingerprint signal fluctuation of

Plasmonic Vertically Coupled Complementary Antennas for Dual

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