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Metamaterials-Enhanced Infrared Spectroscopic Study of Nanoconfined Molecules by Plasmonics-Nanofluidics Hydrid Device Thu H. H. Le, Akihiro Morita, Kazuma Mawatari, Takehiko Kitamori, and Takuo Tanaka ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00398 • Publication Date (Web): 04 Jun 2018 Downloaded from http://pubs.acs.org on June 5, 2018
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ACS Photonics
Metamaterials-Enhanced Infrared Spectroscopic Study of Nanoconfined Molecules by Plasmonics-Nanofluidics Hydrid Device Thu H. H. Le,1 Akihiro Morita,2, 3 Kazuma Mawatari, 4 Takehiko Kitamori, 4 Takuo Tanaka1, 5, 6*
1. Innovation Photon Manipulation Research Team, RIKEN Center for Advanced Photonics, Wako, Saitama 351-0198, Japan. 2. Department of Chemistry, Graduate School of Science, Tohoku University, Sendai, Miyagi 980-8578, Japan. 3. Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University, Saikyo, Kyoto 615-8245, Japan. 4. Department of Applied Chemistry, Graduate School of Engineering, University of Tokyo, Bunkyo, Tokyo 113-8654, Japan. 5. Metamaterials Laboratory, RIKEN, Wako, Saitama 351-0198, Japan. 6. Department of Chemical Science and Engineering, Tokyo Institute of Technology, Yokohama, Kanagawa 226-8503, Japan.
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KEYWORDS nanoconfinement, local mode surface plasmons, plasmon-enhanced IR spectroscopy, SEIRA, plasmonics-nanofluidics hydrid device, nanoconfined water.
ABSTRACT
The behavior of molecules under nanoconfinement is crucial for understanding the chemical processes in biological and nanomaterial systems. We demonstrated here an infrared spectroscopic method to characterize the molecular structures of molecules confined in several tens of nanometer cavities by employing the plasmonics-nanofluidics hybrid device. This device consists of an array of metal nanostructures and a metal mirror separated by a nanofluidic cavity. Its configuration enables the confinement of both molecules and light energy as localized surface plasmons inside the physico-chemically well-defined nanocavity. Exploiting the plasmonsmolecular coupling, the vibrational modes of the nanoconfined molecules are selectively detected with a prominent sensitivity. Applying water as a proof-of-concept sample, we have successfully measured the infrared absorption characteristic, and elucidate the molecular structures of water confined in 10 nm cavity. It unveiled the presence of a strong H-bond network with respect to bulk water. Our method was also able to distinguish the subtle differences in the molecular structures, revealing the scaling behavior of confined water in several tens of nanometer size regime. This effect is also found not being driven by the interaction with the interfaces, yet the constrained geometry itself promotes the intermolecular interactions of water and results in the modification of the H-bond network. This study has offered an ultrasensitive platform for in-situ probing the nanoconfined molecules and chemical
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reactions in their intact condition, and thus gives us a fundamental insight into the nanoconfinement effects.
Chemical reactions confined in a nanoscale geometry (molecular capsule, nanotube, bioreactor, etc.) are unique and remarkably different from those of macroscopic systems.1, 2 Even the origin of this phenomena is still a subject of controversy, it has been confirmed that the nanoconfinement might affect the chemical structures and physical properties of molecules,3 consequently influence on the mechanisms and thermodynamics of the chemical reactions.4 In particular, the behavior of water under nanoconfinement has become a very active and important research topic as it is directly concerned with the reactions in biological and nanomaterial systems.5-7 The unique structure of confined water is of significance to understanding the physiological processes that take place in the living organisms.8,
9
However, most of the
theoretical and experimental studies so far have focused on the water in sub-10 nm graphitic structures such as carbon nanotubes (CNTs) or graphene cavities.10-13 The confinement in larger geometries, especially in several tens of nanometer spaces (i.e., ~101 nm) still remains poorly understood, despite its scientific interests as well as practical relevance to various applications such as catalysis,14,
15
sensing,16 single molecule detection,17,
18
molecular separation,19 or
functional materials,20 etc. Furthermore, this scale is equivalent to the interaction length of biomolecules, it is thus also crucial to in-vivo chemical reactions.21-23 Elucidating the molecular structures is a key to clarify the anomalous properties of the nanoconfined water. In this research field, the challenge lies behind the fabrication and characterization within a nanoscale geometry. Micro- and nanochemistry on a chip, which is facilitated by the top-down nanofabrication and micro/nanofluidic operation is a promising technology that allows us to construct a physico-
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chemically well-defined structure. For example, exploiting this technique one can manipulate the geometries and functionalities, or even a complex chemical reaction within a nanoscale structure.24-26 However, the limited number of structures that can be integrated in one device extremely restricts the total sample volume (e.g., atto- to femto- liter). As a consequence, the characterization becomes a serious issue. Especially for the detection by optical means, a threedimensional dense integration is required to gain sufficient spatial interaction with light, which is still technically challenging. 27, 28 The
surface-enhanced
and
plasmonic
metamaterials-based
infrared
(IR)
absorption/Raman spectroscopies have emerged as promising detection and identification methods for a small number of molecules. They exploit the localized and enhanced electromagnetic field (i.e., hot-spots) in plasmonic structures to amplify the signals of molecules.29-33 The vibrational modes of molecules, which do not interact strongly with the incident light, nevertheless, can couple with plasmonic resonators, resulting in a significant change in the far-field spectral response of the system. Recent efforts in this research fields, have focused on hot-spot engineering to improve the field enhancement and its spectral mode overlapping with molecular vibrations to reach atto/zeptomole sensitivity. 29-33 On the other hand, the enhancement effect and the spatial localization characteristic of plasmon field have suggested a promising approach for detection in nanoscale geometry. Here we proposed an idea of utilizing metamaterials-based IR spectroscopy for the study of nanoconfined molecules. To realize it, however, a precise manipulation of the plasmonic field distribution and the control of its spatial overlapping with molecules are required. In this study, we developed the so-called plasmonicsnanofluidics hydrid device that allows simultaneously the confinement of plasmonic energy and
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molecules in a physico-chemically well-defined ~101 nm cavity, and demonstrated the IR spectroscopic study of molecules confined within the nanocavities. EXPERIMENTAL AND RESULTS Concept and Configuration of Device The device comprises of a metal mirror and a batch of periodic nano square-disks separated by a nanofluidic channel of several tens of nanometers in depth, as described in Figure 1(a).34 The structure forms a plasmonic resonant mode that exhibits the intrinsic property of trapping the plasmonic energy inside the gap between two metal layers. This feature is exploited to probe the IR absorption characteristic and elucidate the molecular structures of the molecules confined within the nanogap. The device configuration thus can be considered as integrating a plasmonic ultrasensitive IR detector in every cavity. Herein, we chose water a proof-of-concept sample to verify the methodology, as well as to clarify the molecular structures and the scaling behavior of water confined in several tens of nanometer cavities. On a SiO2 substrate, a nanofluidic channel was fabricated by UV photolithography, followed by wet etching in buffered hydrofluoric acid solution. After that, a thick gold (Au) film, which plays the part of the mirror layer, was deposited inside the nanochannel. On another SiO2 substrate, Au square-disk nanostructures were fabricated by employing the electron beam (EB) lithography and lift-off technique. The two substrates then underwent the UV/O3 treatment before bonded together at room temperature to form a sealed fluidic device. Liquid sample is introduced into the nanochannel by a pressure-driven flow; comprising nanocavities bounded by two Au interfaces, as shown in Figure 1(a). This structure forms the “metal-insulator-metal” perfect absorber configuration in the mid IR regime, which is well known for the localization of
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electromagnetic field in between two metal layers.34-37 As depicted in Figure 1(b), the channel depth d at the mirror part and the height h of the square-disk were carefully controlled to tune the final nanogap g to the value of interest. The depth d and the height h were measured by surface profiling methods right prior to bonding, and the final gap g was determined by subtracting the channel depth d to the height h of nanostructures. In fact, this gap g can be precisely controlled at an accuracy of several nanometers from experiment to experiment. It is worth noting that the confinement characteristic of the electric field assures the detection of water confined in the gap, even the device configuration generates two confinement geometries of different sizes: the nanogap g bounded by two Au layers and the larger one outside of the square-disks. Figure 1(c) shows the typical SEM image of the fabricated nano square-disk array. The width w, and the density of nano square-disks were taken into consideration to finely tune the plasmonic resonant condition. We evaluate the density by the filling factor parameter f defined as the coverage ratio of nano square-disks in term of surface area. The plasmonic structure was purposedly designed to spectrally overlap with the stretching band O-H of H2O at 3000-3600 cm-1. In one device, various liquid samples can be replaced repeatedly without causing any ill effects to the optical responses. Here, we used heavy water (D2O) and ultrapure water (H2O) with a conductivity of 18.2 MΩ from a Millipore Alpha-Q apparatus as samples. Since D2O shares a similar refractive index with H2O (real part of the complex refractive index) but it is free of vibrational modes in this frequency regime, D2O and H2O can be considered as interacting with the same plasmonic resonator but in off- and on-resonance, respectively. In other words, the “D2O-device” exhibits the property of the original plasmonic resonator, while the “H2O-device” represents the coupled system. Optical Property: Numerical Calculation and Experimental Data
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To clarify the optical property and the field distribution of the corresponding devices, a set of numerical calculations was carried out to study the plasmonic resonances. To better understand the nature of the original plasmonic resonant mode (without coupling with H2O), the system is calculated as the nanocavity is filled with D2O. Under x-polarized irradiation, there is a strong resonant mode originating from the antiparallel currents excited in the Au square-disk and the Au mirror layer, forming a quadrupole mode (i.e., the first mode or the magnetic dipole mode).34 At resonance, the enhanced electric field is localized in between two layers of Au, as shown in the electric field distribution in Figure 2(a). It indicates that the photon energy is efficiently trapped inside the nanocavity and consequently no light is reflected back. This gives rise to a pronounced reflectance dip with nearly zero reflection in the corresponding calculated reflectance spectrum in Figure 2(b). Since the mirror layer blocks all the transmitted light, the combination of zero reflectance (R) and transmittance (T) results in a perfect absorption of light. Moreover, the physico-chemical properties of the nanocavities can be well controlled by topdown fabrication. It implies that our device allows the manipulation of the plasmonic confinement characteristic. The plasmon enhanced electromagnetic field leverages the coupling between plasmonic resonance and the vibrational modes of molecules. Therefore, the aforementioned spatial distribution of the enhanced field indicates that only the water molecules presents inside the nano-cavity are selectively coupled with the plasmon mode. It is worth noting that the spatial distribution of the electric field maintains its confinement characteristic (i.e., trapped inside the nanocavity) throughout the response spectrum of the quadrupole mode, for example at both on- and off-resonant wavenumber indicating by the broken lines in Figure 2(b). The only difference between them is the field intensity, which is quantitatively expressed as the volumetric integration of |E|2 within the nanocavity. Quantitatively, the strength of this coupling
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depends upon the field intensity |E|2 at which molecules are present. The field intensity is one of the important parameters determining the coupling between plasmonic resonance and molecular vibrational modes that will be discussed later. The optical responses of devices were measured at normal incident angle by using an FTIR spectrometer equipped with a microscope under nonpolarized light. An objective lens with NA of 0.57 was used, corresponding to the field angle of 34.8 deg. As explained above, here we exploited the first mode of the MIM structure for the detection. This mode can be excited by any incident angle, and its incident angle dependency is negligibly small. Importantly, once the mode is fixed, the spatial distribution of the electric field inside the nanogap does not change. Therefore, the measured spectra themselves are independent of the incident angle or angular variation of the light illumination. It should be noticed that the vibrational modes of water are detected via their interactions with the enhanced electric field produced by the plasmonic structures and this localized field has all three components Ex, Ey, and Ez. Our method thus can be considered to measure both in-plane and out-of-plane absorption, regardless of the angle of incidence. Several devices of different nanogaps g ranging from 10 nm to 50 nm were fabricated and their optical responses were investigated. Figures 2(c-e) show the reflectance spectra of devices with different gaps as they were filled with H2O and D2O, respectively. The spectra of “D2O-devices” show strong reflectance dips corresponding to the plasmonic quadrupole modes. In case of “H2O-devices”, the spectra exhibit the asymmetric Fano line-shapes, which are very typical for the coupled systems. Surprisingly, the drastic change of reflectance from less than 10% to more than 70% in the presence of water indicates the detection of water with a pronounced sensitivity. The recovery of reflection can be explained as the breaking of the
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antireflection quadrupole mode when it couples with vibrational modes of water molecules. It should further be noted that, for example the water confined in a 10 nm cavity is equivalent to merely 30 layers of water molecules. Even the optical path length of device is limited; the accumulating of light energy by the plasmonic mode enhances the interaction of light and molecules, resulting in a significant improvement of sensitivity. However, here the IR absorption of water appears in the spectrum of a coupled system, which is determined by both the original plasmonic resonance and the intrinsic vibrational absorption of water. Therefore, the spectra in Figure 2(c-e) themselves do not directly give us information about the IR absorption characteristic of the confined water. Data Analysis Procedure for Retrieving the IR Absorption Spectra of Nanoconfined Water In the plasmon-molecular coupled system, molecular vibration can be expressed as a single harmonic oscillator,34, 36, 38, 39 and its coupling with the plasmonic resonance is usually numerically modeled based on the temporal coupled theory model (TCTM). 34, 38-41 This model allows us to extract the underlying coupling coefficients (i.e., coupling strength) between the plasmonic resonator and the vibrational modes of molecules from the experimental spectral data. The reliability of this theoretical model and the fitting procedure in retrieving the exact spectral information of a single molecular vibrational mode has been confirmed in previous reports.34, 3839
In a plasmon coupled system, the coupling behavior varies depending on the plasmon decay
channels including the scattering and absorption.38, 39 In this study, the stretching band O-H of water comprises of multiple vibrational modes and every mode represents one characteristic bonding of water molecules. It is relatively broader than molecular vibrational modes in prior literature; the coupling thus undergoes multiple coupling channels, resulting in a complicated spectral shape as observed. As assuming that the O-H stretching band of water is comprised of N
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harmonic oscillators (bi) corresponding to N different vibrational modes and the plasmonic resonator (a) (i.e., quadrupole mode) couples with those oscillators (bi) independently; while the molecular oscillators themselves do not interfere with each other. The mode equations become: 34, 38-41
= − + + ∑, + = − +
(1) (2)
=
(3)
a, bi represent the mode amplitudes of the resonators (a), (bi), respectively; a, are the
intrinsic center frequencies of the corresponding resonators; is the radiative loss rate of the
plasmonic resonator (a), and , are the nonradiative damping rate of resonators (a), (bi).
stands for the coupling coefficients of (a) with the ith molecular vibrational mode (bi), while
is the coupling constant of the overall system to the incident light. S1+ and S1- denote the
amplitudes of the incident and reflected light fields, respectively. In our system, are set to zero, based on the fact that the direct couplings of molecular vibrational modes with incident light are negligibly small. "#$ &
=!
"#%
The reflectance can be extracted as
!
(4)
Solving (1), (2), (3) and substituting to (4) yields the reflectance of the coupled system: =
'(
2.3 4 )*+, – ,. / 0.1 0. ∑:#,; ) 567 – 73 8%93
4
(5)
The equation (5) was used to fit the experimental reflectance spectra using a minimum number of N. Here a linear correction term was added to correct the baseline; in consideration
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there was an overall slight baseline drift in the FT-IR measurements. The coupling coefficients were extracted from the fitting, and they relate to the original molecular vibrational
intensities CD can be expressed as the volumetric integration of |E|2 within the nanocavity. Therefore, it is possible to retrieve the vibrational intensity or the vibrational & strength of each mode
