Surface-enhanced Infrared Absorption Signals Research Articles

Over-coupled resonator for broadband surface enhanced infrared absorption (SEIRA)

Detection of molecules is a key issue for many applications. Surface enhanced infrared absorption (SEIRA) uses arrays of resonant nanoantennas with good quality factors which can be used to locally enhance the illumination of molecules. The technique has proved to be an effective tool to detect small amount of material. However, nanoresonators can detect molecules on a narrow bandwidth so that a set of resonators is necessary to identify a molecule fingerprint. Here, we introduce an alternative paradigm and use low quality factor resonators with large radiative losses (over-coupled resonators). The bandwidth enables to detect all absorption lines between 5 and 10 μm, reproducing the molecular absorption spectrum. Counterintuitively, despite a lower quality factor, the system sensitivity is improved and we report a reflectivity variation as large as one percent per nanometer of molecular layer of PMMA. This paves the way to specific identification of molecules. We illustrate the potential of the technique with the detection of the explosive precursor 2,4-dinitrotoluene (DNT). There is a fair agreement with electromagnetic simulations and we also introduce an analytic model of the SEIRA signal obtained in the over-coupling regime.

Lanthanide and Actinide Ion Complexes Containing Organic Ligands Investigated by Surface-Enhanced Infrared Absorption Spectroscopy.

A new technique, surface-enhanced infrared absorption (SEIRA) spectroscopy, was used for the structural investigation of lanthanide (Ln) and actinide (An) complexes containing organic ligands. We synthesized thiol derivatives of organic ligands with coordination sites similar to those of 2-[N-methyl-N-hexanethiol-amino]-2-oxoethoxy-[N',N'-diethyl]-acetamide [diglycolamide (DGA)], Cyanex-272, and N,N,N',N'-tetrakis(2-pyridinylmethyl)-1,2-ethanediamine (TPEN), which have been used for separating Ln and An through solvent extraction. These ligands were attached on a gold surface deposited on an Si prism through S-Au covalent bonds; the gold surface enhanced the IR absorption intensity of the ligands. Aqueous solutions of Ln (Eu3+, Gd3+, and Tb3+) and An (Am3+) ions were loaded onto the gold surface to form ion complexes. The IR spectra of the ion complexes were obtained using Fourier transform infrared spectroscopy in the attenuated total reflection mode. In this study, we developed a new sample preparation method for SEIRA spectroscopy that enabled us to obtain the IR spectra of the complexes with a small amount of ion solution (5 μL). This is a significant advantage for the IR measurement of radiotoxic Am3+ complexes. In the IR spectra of DGA, the band attributed to C═O stretching vibrations at ∼1630 cm-1 shifted to a lower wavenumber by ∼20 cm-1 upon complexation with Ln and An ions. Moreover, the amount of the red shift was inversely proportional to the extraction equilibrium constant reported in previous studies on solvent extraction. The coordination ability of DGA toward Ln and An ions could be assessed using the band position of the C═O band. The Cyanex-272- and TPEN-like ligands synthesized in this report also showed noticeable SEIRA signals for Ln and An complexes. This study indicates that SEIRA spectroscopy can be used for the structural investigation of ion complexes and provides a microscopic understanding of selective extraction of Ln and An.

Elevating Surface-Enhanced Infrared Absorption with Quantum Mechanical Effects of Plasmonic Nanocavities

Plasmonic nanocavities, with the ability to localize and concentrate light into nanometer-scale dimensions, have been widely used for ultrasensitive spectroscopy, biosensing, and photodetection. However, as the nanocavity gap approaches the subnanometer length scale, plasmonic enhancement, together with plasmonic enhanced optical processes, turns to quenching because of quantum mechanical effects. Here, instead of quenching, we show that quantum mechanical effects of plasmonic nanocavities can elevate surface-enhanced infrared absorption (SEIRA) of molecular moieties. The plasmonic nanocavities, nanojunctions of gold and cadmium oxide nanoparticles, support prominent mid-infrared plasmonic resonances and enable SEIRA of an alkanethiol monolayer (CH3(CH2)n-1SH, n = 3-16). With a subnanometer cavity gap (n < 6), plasmonic resonances turn to blue shift and the SEIRA signal starts a pronounced increase, benefiting from the quantum tunneling effect across the plasmonic nanocavities. Our findings demonstrate the new possibility of optimizing the field enhancement and SEIRA sensitivity of mid-infrared plasmonic nanocavities.

Classical Model of Surface Enhanced Infrared Absorption (SEIRA) Spectroscopy.

The molecule-plasmon interaction is the key to the mechanisms of surface enhanced infrared absorption (SEIRA) and surface enhanced Raman scattering (SERS). Since plasmons are well described by Maxwell's equations, one fundamental treatment involves the classical interpretation of infrared absorption and resonance Raman spectroscopies. We can understand the molecule-plasmon interaction using electromagnetic theory if the classical field effect on a transition dipole moment or transition polarizability is properly described. In previous work, we derived the Raman excitation profile of a model molecule using a classical driven spring attached to a charged mass with a perturbative force constant due to vibrational oscillations. In this study we generalize the interactions of plasmons with molecules by considering the N2O asymmetric stretch SEIRA signal on a Dy doped CdO (CdO:Dy) film. This semiconductor has tunable plasmon dispersion curves throughout the near-and mid-infrared that can interact directly with vibrational absorption transitions. We have demonstrated this using the Kretschmann configuration with a CaF2 prism and a MgO substrate. The model predicts the phase behavior of SEIRA. The calculated enhancement factor relative to an Au control is 6.2, in good agreement with the value of 6.8 ± 0.5 measured under the same conditions.

Reshaping plasmonic resonances using epsilon-near-zero materials for enhanced infrared vibrational spectroscopy

Surface-enhanced infrared absorption (SEIRA) spectroscopy is a powerful technique for a label-free identification of molecular species. The low infrared absorption cross sections of molecules are made up for the huge electromagnetic field enhancement provided by the resonant excitation of collective electron oscillations in metallic nanoantennas. Since these surface plasmons are localized at the nanometer scale, a minute amount of materials is detected leading to a weak SEIRA signal. The design of actual plasmonics detectors is a trade-off between the detection of very small volumes of molecules and the signal to noise ratio level. We demonstrate that an epsilon-near-zero (ENZ) material combined with nano-slits lifts this constraint and provides both extreme enhancement factor up to 107 and highly contrasted SEIRA signal for an extremely low amount of material of interest. These results are explained by the modification of the electromagnetic field of the gap plasmon mode sustained by the slits in the presence of the ENZ material. We propose to implement this concept with a semiconductor whose doping level engineering provides a versatile way to scan the whole molecules' fingerprint frequency range.

Ultrasensitive Molecule Detection Based on Infrared Metamaterial Absorber with Vertical Nanogap.

Surface-enhanced infrared absorption (SEIRA) spectroscopy is a powerful methodology for sensing and identifying small quantities of analyte molecules via coupling between molecular vibrations and an enhanced near-field induced in engineered structures. A metamaterial absorber (MA) is proposed as an efficient SEIRA platform; however, its efficiency is limited because it requires the appropriate insulator thickness and has a limited accessible area for sensing. SEIRA spectroscopy is proposed using an MA with a 10nm thick vertical nanogap, and a record-high reflection difference SEIRA signal of 36% is experimentally achieved using a 1-octadecanethiol monolayer target molecule. Theoretical and experimental comparative studies are conducted using MAs with three different vertical nanogaps. The MAs with a vertical nanogap are processed using nanoimprint lithography and isotropic dry etching, which allow cost-effective large-area patterning and mass production. The proposed structure may provide promising routes for ultrasensitive sensing and detection applications.

Waveguide-integrated mid-infrared plasmonics with high-efficiency coupling for ultracompact surface-enhanced infrared absorption spectroscopy.

Waveguide-integrated plasmonics is a growing field with many innovative concepts and demonstrated devices in the visible and near-infrared. Here, we extend this body of work to the mid-infrared for the application of surface-enhanced infrared absorption (SEIRA), a spectroscopic method to probe molecular vibrations in small volumes and thin films. Built atop a silicon-on-insulator (SOI) waveguide platform, two key plasmonic structures useful for SEIRA are examined using computational modeling: gold nanorods and coaxial nanoapertures. We find resonance dips of 90% in near diffraction-limited areas due to arrays of our structures and up to 50% from a single resonator. Each of the structures is evaluated using the simulated SEIRA signal from poly(methyl methacrylate) and an octadecanethiol self-assembled monolayer. The platforms we present allow for a compact, on-chip SEIRA sensing system with highly efficient waveguide coupling in the mid-IR.

Importance of Plasmonic Scattering for an Optimal Enhancement of Vibrational Absorption in SEIRA with Linear Metallic Antennas

Surface-enhanced infrared absorption (SEIRA) and surface-enhanced Raman scattering (SERS) represent very effective techniques to detect molecular vibrational fingerprints. These techniques can be improved thanks to the use of plasmonic antennas that produce strong resonant near-fields in their vicinity, enhancing the signal of vibrational samples. Here we study the role of plasmonic absorption and scattering of the hosting antennas in the resulting SEIRA signal. Using numerical simulations of the antenna–sample infrared response, we show that the optimal SEIRA signal measured in transmittance (as extinction) is achieved when the spectral maxima of absorption and scattering of the antennas are of similar magnitude. Paradoxically, when the optimal condition for SEIRA is fulfilled, the decomposition of the signal into the contribution from scattering and from absorption show that the vibrational fingerprint is exclusively a result of the scattering, with no contribution from absorption. Using a simple analyt...

Plasmonic Enhancement of Infrared Vibrational Signals: Nanoslits versus Nanorods

We report on systematic investigations of plasmonically active nanoslits as a beneficial substrate for surface-enhanced infrared absorption (SEIRA). Arranged in arrays, nanoslits with the proper geometry feature strong nanorod-like resonances in the infrared spectral range, as predicted by Babinet’s principle for the same geometrical dimensions. SEIRA enhancement as studied with a thin self-assembled molecular layer of octadecanethiol reaches the values obtained with nanorods if the slit geometry is optimized. We show by systematically examining the important parameters that the slit width has an especially strong influence on the near-field intensity and therefore on the SEIRA signal. Furthermore, the transversal and longitudinal couplings of nanoslits are studied. Compared to nanorod arrays, a stronger influence of the array periodicities on the plasmonic excitations is observed, which indicates coupling via surface plasmon polaritons. So the array periodicity could be further optimized toward higher SE...

Fan-shaped gold nanoantennas above reflective substrates for surface-enhanced infrared absorption (SEIRA).

Here, we report a new nanoantenna for surface-enhanced infrared absorption (SEIRA) detection, consisting of a fan-shaped Au structure positioned at a well-specified distance above a reflective plane with an intervening silica spacer layer. We examine how to optimize both the antenna dimensions and the spacer layer for optimal SEIRA enhancement of the C-H stretching mode. This tunable 3D geometry yields a theoretical SEIRA enhancement factor of 10(5), corresponding to the experimental detection of 20-200 zeptomoles of octadecanethiol, using a standard commercial FTIR spectrometer. Experimental studies illustrate the sensitivity of the observed SEIRA signal to the gap dimensions. The optimized antenna structure exhibits an order of magnitude greater SEIRA sensitivity than previous record-setting designs.

Electroless reduction of silver chloride precipitates for the preparation of highly sensitive substrates for surface-enhanced infrared absorption (SEIRA) measurements.

To prepare silver nanoparticles (AgNPs) on infrared-transmitting crystal for surface-enhanced infrared absorption (SEIRA) measurements, a new strategy is proposed and demonstrated using electroless reduction of preformed silver chloride (AgCl) particles. Silver chloride precipitates were formed using an additive of polyvinyl pyrrolidone (PVP) to vary the size and shape of the precipitates. After settling on germanium substrates, the preformed particles of AgCl were reduced electrolessly and spontaneously coagulated to AgNPs. The resulting AgNPs showed a multilayer structure, but the AgNPs were isolated, as shown by the lack of absorption-band distortion in the SEIRA measurements. Hence, the sensitivity and analyte-loading capacity for SEIRA measurements are improved significantly. To optimize the chemical deposition and electroless reduction method, we examined several parameters, including the concentrations of reagents during AgCl precipitation and the reaction time required in the deposition-reduction steps. We used para-nitrobenzoic acid (pNBA) to probe the intensity of the SEIRA effect for the prepared substrates. To better correlate the SEIRA performances with each variable, we examined the prepared substrates using a scanning electron microscope and SEIRA. The results indicate that two major morphologies of AgNPs are observed: nanoparticles and nanorods. The distributions of nanorods we observed were related to the procedures used to prepare the substrates. Based on SEIRA signals, we observed enhancement factors approaching three orders of magnitude compared to conventional transmission measurement. Also, based on the morphologies, the large signals were mainly caused by the formation of multilayers of non-percolated AgNPs.

Preparation of high-capacity substrates from polycrystalline silver chloride for the selective detection of tyrosine by surface-enhanced infrared absorption (SEIRA) measurements

In this work, stacked but isolated silver nanoparticles (AgNPs) on silver chloride were prepared in order to create sensitive substrates that could be used to determine analytes in aqueous solution by performing surface-enhanced infrared absorption (SEIRA) measurements. These substrates have a high loading capacity, which improves sensitivity, thus allowing the detection of biospecies such as weak infrared absorbers (including amino acids) in aqueous solution. AgNPs were obtained by converting the surface layers of the silver chloride crystals into stacked but isolated AgNPs using reducing agents. To optimize the conditions used to prepare the SEIRA substrates, the roles of four common reducing agents-hydrazine, sodium borohydride, glucose, and formaldehyde-were explored by probing the finished substrates with para-nitrobenzoic acid (PNBA). Factors that influence the morphologies of the AgNPs were examined systemically. These factors include the pH of the reducing solution, the reaction time, and the concentration of the reducing agent. Results indicate that the concentration of the reducing agent and the pH of the solution strongly influence the AgNP morphology and hence the SEIRA signals. Under optimized conditions, the resulting substrates showed intense SEIRA spectra from PNBA, with enhancement factors of around two orders of magnitude compared to conventional transmission methods. The new substrates offer a high loading capacity and good heat tolerance, allowing the direct infrared detection of tyrosine in aqueous solution.

Chemical Reduction Method for Preparation of Silver Nanoparticles on a Silver Chloride Substrate for Application in Surface-Enhanced Infrared Optical Sensors

A new method for the preparation of silver nanoparticles (AgNPs) on silver chloride discs was developed to integrate the unique properties of plasmonic nanoparticles into infrared optical sensing technologies. AgNP layers exhibiting strong infrared surface enhancement were prepared by reacting silver chloride discs in a solution containing hydrazine, which acts as a reducing agent. The silver ions in the outer layer of the disc could be reduced under proper conditions and the reduced silver coagulated to form suitable AgNPs for surface-enhanced infrared absorption (SEIRA) measurements. To examine the influences of the reaction solution composition and also to optimize preparation of SEIRA substrates, factors such as pH value, reaction time, and concentration of reducing agent were examined. Results indicated that both the concentrations of hydrazine and hydroxide strongly influenced the SEIRA signals. The strongest signals were observed when AgNPs on a AgCl substrate were prepared by using a reducing solution of 20 mM NaOH with 0.75 mM hydrazine. Using the optimized substrates, intense SEIRA spectra were observed with an enhancement factor around two orders of magnitude compared to measurements made using conventional sampling techniques.

Surface-Controlled Electroless Deposition Method in the Preparation of Stacked Silver Nanoparticles on Germanium for Surface-Enhanced Infrared Absorption Measurements

A new method to prepare highly sensitive sensing elements for surface-enhanced infrared absorption (SEIRA) measurements was investigated. A surface-controlled procedure was employed to grow round and stacked silver nanoparticles (Ag-NPs) on germanium substrates. In this method, an initial layer of Ag-NPs was prepared using a common method of electroless deposition. After subsequently placing a controlled layer of p-aminothiophenol (pATP) on the surface of the initial layer of Ag-NPs, the substrates were placed in a silver nitrate solution to grow a second layer of Ag-NPs. By repeating these growing procedures, multi-layers of stacked Ag-NPs can be obtained. To examine the influence of morphology of the formed Ag-NPs on the resulting SEIRA signals, the factors affecting the reactions were systematically examined. These factors included the concentrations of silver nitrates, the reaction times to prepare both the initial layer and the second layer of Ag-NPs, and the coverage of pATP. Results indicate that the Ag-NPs making up the second layer were round in shape and much more densely distributed than those in the initial layer. The observed SEIRA spectra did not show derivative-shaped absorption bands for pATP on the Ag-NPs after re-growth, indicating that pATP was sandwiched between the two layers of Ag-NPs, preventing the nanoparticles from coming into direct contact with one another. Also, the SEIRA signals of the controlled molecules between the particles were found to be two to five times more intense than the signals before growing another layer of Ag-NPs. The reaction conditions can be adjusted to vary the morphology and thickness of the Ag-NP layers, and, by repeating the growing procedures, a thick layer of stacked Ag-NPs with suitable size for SEIRA measurements can be obtained that is highly suited to chemical sensing applications.

Characterization of Thio Compounds for a Surface-Controlled Electroless Deposition Method in the Preparation of Silver Nanoparticles on Germanium for Surface-Enhanced Infrared Absorption Measurements

An electroless re-growing method has been developed to prepare silver nanoparticles (Ag-NPs) on substrates for surface-enhanced infrared absorption (SEIRA) measurements. To understand the roles of specific molecules in controlling the properties of Ag-NPs on germanium substrates, thio compounds bearing different functional groups were examined systematically. Four classes of thio compounds were examined in this work, including aromatic thio compounds with second functional group, alkyl thio compounds with second functional group, structural isomers of aromatic thio compounds, and alkyl thio compounds with different alkyl chain lengths. By correlating SEIRA signals with the observed morphologies of the Ag-NPs, it was found that thio compounds bearing amine polar functional groups produced Ag-NPs that were the most regular in shape and size. Also, aromatic thio compounds provided better performance than did alkyl thio compounds in terms of producing suitable shapes and sizes for SEIRA. The Ag-NPs prepared in this work were generally around 50 nm in diameter and round in shape. To examine the prepared substrates for chemical sensing purposes, the enhancement factors were determined by deposition of para-nitrothiophenol (pNTP) on the Ag-NP substrates produced by this surface-controlled electroless deposition method. Results indicated that the surface of the newly formed Ag-NPs actively interacted with pNTP, and the enhancement factors obtained ranged from 50 to 150.

Silver Nanocrescents with Infrared Plasmonic Properties As Tunable Substrates for Surface Enhanced Infrared Absorption Spectroscopy

We exploit the unique infrared plasmonic properties of silver nanocrescents (AgNCs) in preparing tunable substrates for surface enhanced infrared absorption (SEIRA) spectroscopy. Fabrication provides good control over the crescents' structural properties which enables tuning of the localized surface plasmon resonances (LSPRs) from the visible through the infrared (IR) regions of the spectrum. Using AgNCs as uniquely tunable IR LSPR substrates, we demonstrate the impact of spectral tuning on maximizing SEIRA signal enhancements measured for adsorbed alkylthiolates. The AgNCs demonstrate the largest reported area-normalized SEIRA signal enhancements which increase from 7,700 to 46,000 depending on the relative positions of the AgNC's LSPR frequency and the molecular vibration frequency. The SEIRA enhancement increases and the absorption band line shape becomes more asymmetric as the AgNCs' LSPR frequency overlaps more extensively with the frequency of the probed molecular vibration. The tunability of the LSPR properties will enable fundamental SEIRA studies and the development of optimized SEIRA substrates for detection and identification of molecular adsorbates.

Preparation and characterization by surface-enhanced infrared absorption spectroscopy of silver nanoparticles formed on germanium substrates by electroless displacement

In this paper, the feasibility of applying electroless displacement to prepare silver nanoparticles (AgNPs) on the surface of germanium (Ge) substrate is demonstrated, and the performances of surfaces prepared in this manner for surface-enhanced infrared absorption (SEIRA) spectroscopy are reported. The process used to produce suitable AgNPs for SEIRA by electroless deposition is simple and effective, requiring only pretreatment of the germanium surface with hot air, immersion of the substrate in a dilute solution of silver nitrate, and washing of the resulting plate. To quantify the behavior of AgNPs on a Ge substrate and to optimize the conditions for the preparation of AgNPs on Ge substrates, a monolayer of p-nitrothiophenol (PNTP) was bonded to the surface of the AgNPs by immersion of the plate in a dilute solution of PNTP and measurement of the transmission spectrum. The factors that influenced the formation of AgNPs, and hence the SEIRA signals, included the concentration of AgNO3, the reaction time and the temperature. Results indicated that stronger absorption bands in the SEIRA spectrum of a monolayer of PNTP were obtained if the reaction rate for the displacement of silver ions by Ge was slow. This condition was achieved by keeping the concentration of AgNO3 and the reaction temperature low. Under the optimal conditions found in this work, an enhancement factor of approximately 100 was achieved for commonly used probe molecules in SEIRA measurements.