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Plasmonics – Nanofluidics Hydrid Metamaterial: An Ultra-Sensitive Platform for Infrared Absorption Spectroscopy and Quantitative Measurement of Molecules Thu H. H. H. H. Le, and Takuo Tanaka ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b02743 • Publication Date (Web): 25 Sep 2017 Downloaded from http://pubs.acs.org on September 26, 2017
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Plasmonics – Nanofluidics Hydrid Metamaterial: An Ultra-Sensitive Platform for Infrared Absorption Spectroscopy and Quantitative Measurement of Molecules AUTHOR NAMES Thu H. H. Le1, Takuo Tanaka1†, 2, 3 AUTHOR ADDRESS †1. Innovative Photon Manipulation Research Team, RIKEN Center for Advanced Photonics, Wako, Saitama 351-0198, Japan. 2. Metamaterials Laboratory, RIKEN, Wako, Saitama 351-0198, Japan. 3. Department of Chemical Science and Engineering, Tokyo Institute of Technology, Yokohama, Kanagawa 226-8503, Japan. KEYWORDS infrared absorption spectroscopy, metamaterials, plasmons, hot-spot, nanofluidics, SEIRA, quantitative measurement.
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ABSTRACT
One of the most attractive potentials of plasmonic metamaterials is the amplification of intrinsically weak signals such as molecular infrared (IR) absorption or Raman scattering for detection applications. This effect, however, is only effectual when target molecules are located at the enhanced electromagnetic field of the plasmonic structures (i.e. hot-spots). It is thus of significance to control the spatial overlapping of molecules and hot-spots, yet it is a longstanding challenge, since it involves the handling of molecules in nanoscale spaces. Here metamaterial consisting of a nanofluidic channel with depth of several tens of nanometers sandwiched between plasmonic resonators and a metal film enables the controllable delivery of small molecules into the most enhanced field arising from the quadrupole mode of the structures, forming a plasmon-molecular coupled system. It offers an ultra-sensitive platform for detection of IR absorption and molecular sensing. Notably, the precise handling of molecules in a fixed and ultra-small (10 – 100 nm) gap also addressed some critical issues in IR spectroscopy such as quantitative measurement, and measurement in aqueous solution. Moreover, a drastic change in the reflectance characteristic resulting from the strong coupling between molecules and plasmonic structures indicates that molecules can also be utilized as trigger for actively switching the optical property of metamaterials.
Vibrational spectroscopy, including IR absorption and Raman spectroscopy is one of the most powerful biochemical analysis tools as they extract essential information of chemical bonds and molecular structures in a label-free fashion. They are especially useful in tracing subtle changes of the conformational structures in response to surrounding environment or probing the kinetics of chemical events.1-3 Although vibrational spectroscopies can offer a large information content,
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the low sensitivity originated from the intrinsic low absorption/scattering cross-sections severely limits their practical applications. Surface-enhanced and the later developed plasmonic metamaterials-based IR absorption and Raman spectroscopies have emerged as promising approaches that improve the sensitivity by several orders of magnitude, owning to the localized enhanced electromagnetic field (hot-spots) in plasmonic materials.4-6 Vibrational modes of molecules, which do not interact strongly with the incident light, nevertheless, can strongly couple with plasmonic resonators. The coupling results in a large change in the far-field spectral response of the system, in which signals of molecules are apparently amplified. This is the mechanism for the enhancement of sensitivity.7-10 This effect, however, is only effectual when molecules are located in the vicinity of hot-spots, thus it is a common sense that positioning target molecules exactly at the hot-spots is crucial to effectively utilize the plasmonic-molecular coupling. 11-13 Despite recent efforts in this field, most of the studies have focused on either engineering or tailoring the plasmonic structures, while molecules are randomly adsorbed on sensing surfaces by chemisorption or physisorption.14-16 In such cases, the small spatial overlapping of hot-spots and molecules restricts the sensitivity. This problem gets much more difficult in complex 3D or multi-layer metamaterials, in which hot-spots are embedded inside the structures. In addition, the inhomogeneity and the uncertainty in number of adsorbed molecules are serious obstacles for the reproducibility and determination of molecular quantity. Those issues can be addressed, in part, by using hollow-core waveguides,17 resonant cavities18 or microfluidics19 for continuously introducing molecules onto the plasmonic surface, however the improvement was still limited because the sample volume in such cases is much larger than the hot-spot sizes. The challenge lies behind the precise handling of molecules at the nanoscale hotspots. On the other hand, nanofluidics, which is the science and technology of fluids confined
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within nanoscale structures, has recently attracted much attention in the field of single molecule and single cell analysis, as it enables the precise and automatic handling of molecules in ultrasmall spaces at our will.20-22 Here we proposed and demonstrated a metamaterial that leverages the resonant coupling between molecules and plasmonic structures to its fullest, by combining molecules and plasmonic resonators in a heterogeneous configuration. It composed of plasmonic resonators and a metal film sandwiched by a nanofluidic channel. This structure enables a controllable and effective delivery of molecules into the most enhanced electromagnetic field of the resonance arising from the interference between top resonators and bottom metal mirror, which is known as “MIM absorber” structure. This configuration induces a strong coupling between molecules and plasmonic structure, thus it is expected to significantly enhance the IR absorption signal of molecules with respect to conventional approaches. Notably, the manipulation of molecules by fluidic operation in 10 – 100 nm spaces also allow us to quantify the number of molecules with a good reproducibility, as well as to measure biomolecules in aqueous solution, since herein the disturbance from large absorption of water is effectively suppressed.
RESULT AND DISCUSSION Concept and device fabrication The concept of the device is depicted in Figure 1(a). A nanofluidic channel is embedded with gold (Au) nano square-disk arrays on the top wall and an Au mirror on the bottom one. The nanogap g between two structures is controlled at several tens of nanometers. The numerical calculation of the corresponding system carried out by finite-element-method (FEM) reveals that there is a strong resonant mode, at which the electric dipole on the Au nano square-disk induces
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an opposite dipole on the mirror, and the two dipoles form a quadrupole mode (i.e. magnetic dipole mode). Figure 1(b) illustrates the conceptual diagram of a unit cell, in which the width w of square-disk, periodicity p and the lattice structure are controlled to tune the resonant condition. To standardize the structures of different lattices, we define the filling factor f as the coverage ratio of nano square-disk in a unit cell in term of surface area. Figure 1(c) shows the electric E and magnetic H fields distribution under x-polarized irradiation, indicating that at resonance, E field and H field are accumulated inside the gap (i.e. inside the channel). This distribution suggests that when molecules are introduced into the channel, their large spatial overlapping with the hot-spots can induce a profound rise to the plasmonic-molecular coupling. This resonant mode is easily affected by the spacer layer thickness, therefore it is essential to control the nanogap thickness g (or the channel depth) at a fixed value to realize the structure. The fabrication of micro/nanofluidic device, so far, relies on soft substrate materials such as dimethylpolysiloxane (PDMS), yet controlling nanogap in such devices is quite challenging since they easily deform. Although hard wafers such as SiO2 or Si can solve this problem, they have low compatibility with plasmonic metamaterials, especially due to the bonding process that is the most critical step in fabrication of fluidic chips. The bonding between the SiO2 substrates is usually performed at extremely high temperatures, for example, around 1000°C for fused-silica glass, after cleaning in piranha solution.23-25 Unfortunately, most of plasmonic materials cannot tolerate that high temperatures nor harsh chemical condition. It so far has been a technical bottleneck for integrating metallic objects into the nanofluidic chip. Here we developed a room temperature (RT) bonding technique for SiO2 fluidic chips to fabricate the proposed device. We also confirmed its validity for CaF2 substrate, which is transparent to IR light, but weak to thermal and chemical treatments.
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The fabrication process is described in Figure 2(a). On a SiO2 or CaF2 substrate coated with resist, nano square-disk arrays with different patterns (different width w, periodicity p, and lattice geometry) were written by electron beam (EB) lithography. It was then deposited with a 5 nmCr/65 nm-Au film and subsequently lifted-off to obtain Au nano square-disk arrays. On another SiO2 substrate, the microchannels used for guiding fluidics into the nanochannels were fabricated by UV photolithography following by reactive ion etching (RIE). Nanochannels (designed width: 200 µm; depth: 180 nm; and length: 400 µm) were fabricated by UV-photolithography and wetetching in buffered hydrofluoric acid. After that, an Au film thicker than 90 nm (5 nm-Cr/ >85 nm-Au) was deposited inside nanochannels by using UV photolithography and lift-off technique. A careful position adjustment was necessary in all steps to ensure the matching between upper and lower patterns. The two substrates then underwent an UV/O3 treatment to activate the surface –OH groups, before they were aligned and bonded together at RT. To reinforce the bonding strength, in case of CaF2 chip, the chip was applied with a pressure of 250 Ncm-2 in one hour right after the bonding. Details of bonding process and possible bonding mechanism are described in the supporting information SI-2. Figure 2(b) shows the fabricated device, the microscopic image of nanochannel, and the representative scanning electron microscope (SEM) images of Au nano square-disk arrays. The bright part in the image represents the bottom Au mirror, while the eight darker patterns indicated by arrow are the square-disk arrays. In one device, several arrays of various patterns can be simultaneously integrated to finely tune the resonant frequency. In this experiment, the nanogap between two structures was designed at 15 nm (experimental value: 14.3 ± 0.9 nm). The nanogaps were well controlled from batches to batches, while they varied from experiments to experiments at several nanometers (see supporting information SI-5). The bonding strength was tested by introducing water under
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pressure-driven flow. The device was verified to tolerate an applied pressure up to ~360 kPa, which corresponds to a flow velocity of ~26 µm/s through the nanogap. Characterization of optical properties The device embedded with different patterns was measured by FT-IR spectrometer under nonpolarized irradiation. As shown in the reflectance spectra in Figure 3(a), each pattern exhibited several resonant dips. Among them, the strongest one can be attributed to the quadrupole resonance, at which the reflected light is significantly eliminated. The origin of this mode will be discussed further below. Since the bottom Au film is thick enough to block the transmitted light, the combined zero transmission and cancellation of reflection implies a nearly perfect (~90 %) absorption of light. The result also confirms the possibility of finely tuning the resonance in midIR regime. Since larger field enhancement effect can be expected in structures with large absorbance (i.e. low reflectance), several parameters were taken into consideration to optimize the absorbance, in conjunction with tailoring the resonant frequency to the mode of interest. According to the spectra of devices with different widths w, and filling factors f (Figure 3(a)), at a certain gap thickness, even with the same width w, the resonant frequency and the absorbance slightly varied with filling factor. Under fixed gap thickness of 15 nm, by controlling the width w and filling factor f, the fine tuning of resonant frequencies with optimum reflectance dip was demonstrated as shown in Figure 3(a). Next, we verified the interaction of plasmonic structure and molecular vibrational modes by introducing water into the nanochannel. The structure was designed to exhibit the resonance at 3200 – 3500 cm-1, which is overlapped with the O-H stretching band of H2O molecules (detailed design in supporting information SI-4). Heavy water D2O was chosen as reference sample, since it shares the same refractive index with H2O (real part of complex refractive index), while its
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absorption band (imaginary part of complex refractive index) is shifted to the much lower energy region (2400 – 2600 cm-1). The IR reflectance spectra of device filled with D2O and H2O are shown in Figure 3(b). A strong resonance dip exhibiting a symmetric Lorentzian line in 3200 – 3600 cm-1 was clearly observed in the spectrum of “square-disk - D2O - Au film” system. This dip can be attributed to the light absorption by the plasmonic resonance of the structure itself, since D2O and structure are in off-resonance. When filling fluidic was changed from D2O to H2O, spectrum transformed to a Fano-like resonance line shape, revealing the typical characteristic of coupled plasmonic systems.26, 27 The drastic change in reflectance spectra in 3000 – 3600 cm-1 range is a solid proof of a strong coupling between H2O molecules and plasmonic structure. A numerical calculation was carried out to study the mechanism of the above systems (supporting information SI-6). The calculated electric field and electric current profiles in Figure 3(c) have elucidated the origin of the observed spectra. Under x-polarized light illumination, at resonant frequency (indicated by the broken line in Figure 3(b)) an electric dipole is resonantly excited in the upper Au square-disk with dipole oriented along the electric incident field. This localized plasmon mode induces an opposite dipole, which is its own mirror image, in the Au film. They appear as antiparallel currents running in the top Au square-disk and the bottom Au mirror in Jx distribution map displayed in Figure 3(c - d). The two dipole modes were interfered and form a quadrupole mode (i.e. magnetic dipole mode), which is the non-radiative mode, leading to the anti-reflection.28, 29 The opposite dipoles are clearly observed in the electric field Ez profile. When H2O is introduced into the gap, their vibrational modes couple with this quadrupole mode and break the anti-reflection condition. This is observed as the disappearance of both quadrupole mode and antiparallel currents Jx in Figure 3(e - f). As a consequence, the
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light is reflected back at the frequency range corresponding to the vibrational absorption bands of H2O, and the obtained spectrum exhibits a Fano-like line shape. Detection and tracing the quantity of molecules Exploiting this mechanism, even weak molecular absorber whose coupling coefficient with incident light is low can be effectively detected. To demonstrate the performance of our device in detecting and tracing the quantities of molecules, we used octadecane C18H38 dissolved in carbon tetrachloride CCl4 solvent as target molecules. CCl4 solvent was chosen here because it is free of vibrational absorption in the frequency range of interest. The device was purposely designed to exhibit the resonance at 2800 – 3000 cm-1 that covered the C-H vibrational bands of C18H38 molecules. Figure 4(a) shows the reflectance spectra of channel filled with CCl4 solvent and C18H38/CCl4 solution, respectively. It unveiled three C-H vibrational modes including the asymmetric, symmetric stretching of –CH2 and asymmetric stretching of –CH3 as distinct peaks in the broad reflectance dip of plasmonic structure. Although the shape of those peaks were different from the case of water (Figure 3(b)) due to the difference in Q-factor of the intrinsic vibrational modes of molecules, this result agreed well with the mechanism discussed above. In order to quantitatively analyze the coupling between molecules and plasmonic structure, we proposed the following theoretical model. The observed spectrum is a result of the coupling between three resonators: (a) the plasmonic structure whose resonance was measured as channel was entirely filled with CCl4, (b) the asymmetric stretching of –CH2 at ~2930 cm-1, and (c) the symmetric stretching of –CH2 at ~2850 cm-1. The asymmetric stretching of –CH3 was omitted in this model as its peak intensity was negligibly small. The three-resonator coupled system was fitted by employing the temporal coupled theory model (TCTM), which was generally used in coupled plasmonic systems.19, 30, 31 This model allows us to numerically analyze the underlying
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coupling coefficients between resonators, which can be used to extract the molecular IR absorbance. We assume that resonator (a) couples with resonator (b) and (c) independently, while (b) and (c) do not interfere each other, the coupled mode equations for the configuration become: !" !" !" !" !" !"
= 𝑗𝜈! 𝑎 − 𝛾!" + 𝛾!! 𝑎 + 𝑗𝜇!" 𝑏 + 𝑗𝜇!" 𝑐 + 𝜅𝑆!!
(1)
= 𝑗𝜈! − 𝛾!! 𝑏 + 𝑗𝜇!" 𝑎
(2)
= 𝑗𝜈! − 𝛾!! 𝑐 + 𝑗𝜇!" 𝑎
(3)
In these equations, a, b, c represent the mode amplitudes of resonator (a), (b), and (c) respectively; 𝜈! , 𝜈! , 𝜈! are the intrinsic center frequency of resonators; 𝛾!" is the radiative loss rate of plasmonic resonator (a), while 𝛾!! , 𝛾!!, 𝛾!! are the non-radiative damping rate of resonators (a), (b) and (c), respectively. µab and µac represent the coupling coefficient of (a) - (b) and (a) – (c), while 𝑆!! is the incoming wave. Solving those equations to extract the reflectance and fitting it with the experimental data (see supporting information SI-1), we obtained the coupling coefficients µab, µac for each spectrum. The fitted spectrum matched perfectly with the experimental one as shown in Figure 4(a), strongly supporting the validity of our analysis model. Furthermore, assuming that vibrational mode of every single molecule is independent resonator that shares the same coupling coefficients µ with the plasmonic structure. According to this assumption, for example, µab also presents the number of ~2930 cm-1 vibrational modes involving in the coupling, or in other words it corresponds to the number of molecules contributed to the coupled system. To fairly evaluate the sensitivity of our device, we employed the molecular density that is defined by the number of sensing molecules per surface area of nanostructure. Most of studies on
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sensing the self-assembled monolayer (SAM) of the same analyte species on engineered plasmonic structures reported the detection limit (LOD) at a molecular density of ~10-2 moleculesÅ-2 (e.g. ~4.6 × 10-2 moleculesÅ-2).9, 16, 32 We defined the molecular density in our device as the total number of molecules inside the nanogap (i.e. concentration × gap volume) divided by the gap surface area. For example, the reflectance spectrum when device was filled with 7.8 mM C18H38 in Figure 4(b) confirmed detectable peaks with sufficient signal-to-noise ratio (SNR). This concentration in 15 nm gap corresponds to a molecular density of 7.1 × 10-4 moleculesÅ-2. It should be noted that this value is two orders lower than previous reports, indicating the enhancement of sensitivity up to two orders of magnitude. The achieved sensitivity of our device firmly verified our proposal that exactly positioning molecules at the hot-spots has contributed to the enhancement of plasmonic-molecular coupling, and thus improved the sensitivity. Taking the advantage of precisely controlling the number of molecules inside nanogap, our device can simultaneously detect and trace the quantity of molecules. Solutions with different concentrations were introduced into the device, and the corresponding spectra were recorded and fitted using the fitting procedure mentioned above. We experimentally confirmed a linear relationship between coupling coefficients and molecular density in a narrow concentration range. The calibration curve in Figure 4(c) in which the summation of coupling coefficients µab and µac was plotted against the molecular density (moleculesÅ-2) revealed a good linearity at molecular density lower than 3.0 × 10-2 moleculesÅ-2. Note that it is essential to compare the summation of the two peak strengths since the tail of the first peak may affect height of the second one and vice versa.32 The relationship between coupling coefficients and number of molecules in a large range of concentration was discussed more details in supporting information
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SI-1. The small deviation of signal intensities standing for a prominent reproducibility strongly supports that our device performs as an excellent platform for quantifying the number of molecules. The detection limit is defined as the analyte density corresponding to the signal of sample blank value adding three times of the standard deviation. According to this definition, we estimated the LOD to be 4.4 × 10-4 moleculesÅ-2, showing the improvement by 105 folds compared to previous studies. 9, 16, 32-34 It is also worth mentioning that the minimum detectable volume is estimated to be ~0.5 fL, which is the smallest one to the best of our knowledge. Our device, not only does it offer a pronounced sensitivity, but it also allows us to address some critical issues in conventional and surface enhanced IR absorption (SEIRA) spectroscopies. The first demonstration is the quantitative measurement of molecules that has been verified above. The later one is the measurement of biomolecules in aqueous solution. Measurement of protein in aqueous solution In bioanalysis, measurement in water solution is an inevitable process, but the comparatively large absorption of water becomes a crucial obstacle in IR spectroscopy in aqueous solution. Specifically, the O-H bending band of water overwhelms the amide I and II bands which carry essential information of protein secondary structures. Various efforts have been made so far to limit the optical path length (
