Surface-enhanced Infrared Absorption Research Articles

Spectroelectrochemical studies of structural changes during reduction of oxygen catalyzed by laccase adsorbed on modified carbon nanotubes

Reduction of oxygen catalyzed by laccase was studied by surface-enhanced infrared absorption spectroscopy (SEIRAS) combined with electrochemical techniques. Laccase molecules were adsorbed on modified carbon nanotubes (CNTs). SEIRAS provided spectra of Amide I band of adsorbed laccase. Fourier self-deconvolution (FSD) and two-dimensional correlation spectroscopy (2D COS) techniques were employed to reveal the sub-band structure of the Amide I band. The analysis demonstrated that laccase adsorbed on CNTs remains its native state both in the oxidized and reduced states. The detailed analysis showed that change of the oxidation state leads to a small change of the secondary structure of the protein. This change is comparable to the change observed for similar blue-copper oxidases in solution during redox titration. The results of this study demonstrated that CNT's immobilized laccase is an excellent catalyst of oxygen reduction. This study illustrates power of quantitative analysis of IR data to provide information about three-dimensional structure of surface immobilized proteins. • Laccase adsorbed at naphthalene modified SWCNT has superb electrocatalytic activity • SEIRAS spectra demonstrated that it preserves its native state • Potential induced changes in the secondary structure of laccase are small

Modulating plasmonic signatures of gold nanoparticles by fine tuning of surface roughness

Localized surface plasmon resonance peak of a plasmonic nanoparticle dictates its unique optical properties which are responsible for various applications like surface enhanced Raman scattering (SERS), surface enhanced Infra-red absorption (SEIRA), surface enhanced fluorescence (SEF) etc. In this report we have fine-tuned the surface morphology/roughness of pseudo spherical gold nanoparticles by reducing different metal precursors onto their surface using mild reducing ability of versatile polymer polyvinyl pyrrolidone (PVP) in alcoholic medium containing. The significant morphology/roughness dependent optical plasmonic signature defines optical parameters of plasmonic resonators like plasmon’s dephasing time, its quality factor as well as its bandwidth. Moreover, variation of these parameters with additive metal salts and its concentration is studied and a superlative conclusion was drawn between surface roughness and these optical parameters which were further confirmed by TEM data.

Gold nanonails for surface-enhanced infrared absorption.

Surface-enhanced infrared absorption (SEIRA) can dramatically enhance the vibrational signals of analyte molecules owing to the interaction between plasmons and molecular vibrations. It has huge potential for applications in various detection and diagnostic fields. High-aspect-ratio rod-like metal nanostructures have been the most widely studied nanomaterials for SEIRA. However, nearly all of the rod-like nanostructures reported previously are fabricated using physical methods. They suffer from damping and low areal number densities. In this work, high-aspect-ratio Au nanorods are synthesized, and Au nanonails are prepared through Au overgrowth on the as-prepared Au nanorods. The aspect ratios of the Au nanorods and nanonails can be varied in the range of ∼10 to ∼60, and their longitudinal dipolar plasmon resonance wavelengths can be correspondingly tailored from ∼1.6 to ∼8.3 μm. The Au nanonails exhibit superior SEIRA performance with 4-aminothiophenol used as the probe molecules. They are further used to detect the common biomolecule l-cysteine. Numerical simulations are further performed to understand the experimental results. They match well with the experimental observations, revealing the mechanism of the SEIRA enhancement. Our study demonstrates that colloidal high-aspect-ratio Au nanonails and nanorods can function as SEIRA nanoantennas for highly sensitive molecular detection in various situations.

Nanostructured copper sulfide thin film via a spatial successive ionic layer adsorption and reaction process showing significant surface-enhanced infrared absorption of CO2

A highly sensitive CO2 sensor, comprising nanostructured copper sulfide that exhibits desired localized surface plasmon resonance, demonstrates a significant surface-enhanced infrared absorption for 100 ppm CO2 with an enhancement factor of 104.

Nanoantenna Structure with Mid-Infrared Plasmonic Niobium-Doped Titanium Oxide.

Among conductive oxide materials, niobium doped titanium dioxide has recently emerged as a stimulating and promising contestant for numerous applications. With carrier concentration tunability, high thermal stability, mechanical and environmental robustness, this is a material-of-choice for infrared plasmonics, which can substitute indium tin oxide (ITO). In this report, to illustrate great advantages of this material, we describe successful fabrication and characterization of niobium doped titanium oxide nanoantenna arrays aiming at surface-enhanced infrared absorption spectroscopy. The niobium doped titanium oxide film was deposited with co-sputtering method. Then the nanopatterned arrays were prepared by electron beam lithography combined with plasma etching and oxygen plasma ashing processes. The relative transmittance of the nanostrip and nanodisk antenna arrays was evaluated with Fourier transform infrared spectroscopy. Polarization dependence of surface plasmon resonances on incident light was examined confirming good agreements with calculations. Simulated spectra also present red-shift as length, width or diameter of the nanostructures increase, as predicted by classical antenna theory.

Surface-Enhanced Absorption Spectroscopy for Optical Fiber Sensing.

Visible and near-infrared spectroscopy are widely used for sensing applications but suffer from poor signal-to-noise ratios for the detection of compounds with low concentrations. Enhancement by surface plasmon resonance is a popular technique that can be utilized to increase the signal of absorption spectroscopy due to the increased near-field created close to the plasmons. Despite interest in surface-enhanced infrared absorption spectroscopy (SEIRAS), the method is usually applied in lab setups rather than real-life sensing situations. This study aimed to achieve enhanced absorption from plasmons on a fiber-optic probe and thus move closer to applications of SEIRAS. A tapered coreless fiber coated with a 100 nm Au film supported signal enhancement at visible wavelengths. An increase in absorption was shown for two dyes spanning concentrations from 5 × 10−8 mol/L to 8 × 10−4 mol/L: Rhodamine 6G and Crystal Violet. In the presence of the Au film, the absorbance signal was 2–3 times higher than from an identically tapered uncoated fiber. The results confirm that the concept of SEIRAS can be implemented on an optical fiber probe, enabling enhanced signal detection in remote sensing applications.

Surface-Enhanced IR-Absorption Microscopy of Staphylococcus aureus Bacteria on Bactericidal Nanostructured Si Surfaces.

Surface-enhanced IR absorption (SEIRA) microscopy was used to reveal main chemical and physical interactions between Staphylococcus aureus bacteria and different laser-nanostructured bactericidal Si surfaces via simultaneous chemical enhancement of the corresponding IR-absorption in the intact functional chemical groups. A cleaner, less passivated surface of Si nanoripples, laser-patterned in water, exhibits much stronger enhancement of SEIRA signals compared to the bare Si wafer, the surface coating of oxidized Si nanoparticles and oxidized/carbonized Si (nano) ripples, laser-patterned in air and water. Additional very strong bands emerge in the SEIRA spectra on the clean Si nanoripples, indicating the potential chemical modifications in the bacterial membrane and nucleic acids during the bactericidal effect.

Host-Guest Chemistry Meets Electrocatalysis: Cucurbit[6]uril on a Au Surface as a Hybrid System in CO2 Reduction.

The rational control of forming and stabilizing reaction intermediates to guide specific reaction pathways remains to be a major challenge in electrocatalysis. In this work, we report a surface active-site engineering approach for modulating electrocatalytic CO2 reduction using the macrocycle cucurbit[6]uril (CB[6]). A pristine gold surface functionalized with CB[6] nanocavities was studied as a hybrid organic–inorganic model system that utilizes host–guest chemistry to influence the heterogeneous electrocatalytic reaction. The combination of surface-enhanced infrared absorption (SEIRA) spectroscopy and electrocatalytic experiments in conjunction with theoretical calculations supports capture and reduction of CO2 inside the hydrophobic cavity of CB[6] on the gold surface in aqueous KHCO3 at negative potentials. SEIRA spectroscopic experiments show that the decoration of gold with the supramolecular host CB[6] leads to an increased local CO2 concentration close to the metal interface. Electrocatalytic CO2 reduction on a CB[6]-coated gold electrode indicates differences in the specific interactions between CO2 reduction intermediates within and outside the CB[6] molecular cavity, illustrated by a decrease in current density from CO generation, but almost invariant H2 production compared to unfunctionalized gold. The presented methodology and mechanistic insight can guide future design of molecularly engineered catalytic environments through interfacial host–guest chemistry.

Optical Properties of Plasmonic Nanoantennas Based on Arrays of Interacting V-Shaped Structures in the Mid-IR Range

Polarization-selected IR transmission, IR reflection, and IR absorption spectra of arrays of V-shaped gold structures are experimentally studied for various structure periods. To form a local region of the enhanced electromagnetic field in subwavelength nanogaps, arrays with unit cells consisting of two V-shaped structures, facing each other by acute angles with gaps of 0, 50, 100 nm were also used. The results of comparison of spectral data with measurements of dimensional parameters using atomic-force microscopy and scanning electron microscopy are presented. The possibility of using the fabricated structures to achieve the effect of surface-enhanced IR absorption by quantum dots is shown.

Using a Graphene-Polyelectrolyte Complex Reducing Agent To Promote Cracking in Single-Crystalline Gold Nanoplates.

It is a challenge to produce single-crystalline gold nanoparticles having regular size definition designed for controlled light absorbance and internal structural inhomogeneities to enhance electro-magnetic fields. Here, we report a synthetic strategy to generate large single-crystalline triangular or hexagonal gold nanoplates with multiple cracks within the plates using a graphene-polyelectrolyte complex as both a surface adsorbent and bulk reducing agent. Large-scale gold nanoplates can be synthesized within 48 h. First-principles calculations indicate that the nanoplates have a kinetically limited morphology resulting from prior growth of {111} facets confined by the graphene-polyelectrolyte multilayer. The nanocracks result from the inability of the bulk reducing agent to enter narrow defect spaces during growth that remained permanently. The nanoplates had extraordinary physical-chemical detection sensitivity when used for surface-enhanced Raman scattering (SERS) and surface-enhanced infrared absorption (SEIRA). The limit of rhodamine 6G (Rh6G) SERS detection is as low as 5 × 10-13 M. The gold nanoplates also showed a remarkable light-to-heat conversion efficiency (68.5%). The approach described may be applicable to other metals so that tunable nanostructures can be generated by the graphene-polyelectrolyte multilayer strategy.

Understanding the Synergic Mechanism of Weak Interactions between Graphene Oxide and Lipid Membrane Leading to the Extraction of Lipids.

Revealing how weak forces interact synergistically to induce differences in nanobio effects is critical to understanding the nature of the nanobio interface. Herein, graphene oxide (GO) and a lipid membrane are selected as a nanobio model, and interaction forces at the GO-biomembrane interface are modulated by varying the amounts and species of oxygenated functional groups on the surface of GO. A synergic mechanism of interfacial interaction forces is investigated by a combination of surface-enhanced infrared absorption (SEIRA) spectroscopy, confocal laser scanning microscopy (CLSM), and electrochemical impedance spectroscopy (EIS). The results reveal that after balancing with electrostatic repulsion, the moderate attraction between GO and lipid headgroups (such as electrostatic and/or hydrophobic interactions) is most favorable for lipid extraction, whereas lipid extraction is inhibited under an attraction that is too strong or too weak. Under moderate attraction between GO and the headgroups of lipids, the appropriate degree of rotation freedom is maintained for GO, which is beneficial to the hydrogen-bonding interaction between the C═O group in the phosphatide hydrophobic region and GO, thus triggering the insertion of GO into the lipid alkyl chain region, resulting in the rapid and significant extraction of lipids. Our results have important guiding significance for how to reveal the synergistic mechanism of weak interactions at the nanobio interface.

Visualizing the movement of the amphipathic helix in the respiratory complex I using a nitrile infrared probe and SEIRAS

Conformational movements play an important role in enzyme catalysis. Respiratory complex I, an L-shaped enzyme, connects electron transfer from NADH to ubiquinone in its peripheral arm with proton translocation through its membrane arm by a coupling mechanism still under debate. The amphipathic helix across the membrane arm represents a unique structural feature. Here, we demonstrate a new way to study conformational changes by introducing a small and highly flexible nitrile infrared (IR) label to this helix to visualize movement with surface-enhanced IR absorption spectroscopy. We find that labeled residues K551CL and Y590CL move to a more hydrophobic environment upon NADH reduction of the enzyme, likely as a response to the reorganization of the antiporter-like subunits in the membrane arm.

Point mutation in the TGFBI gene: surface-enhanced infrared absorption spectroscopy (SEIRAS) as an analytical method

A novel method to detect DNA mutations based on gold nanoparticles is described. This bioconjugate was prepared by conjugation of the TGFBI gene on the surface of gold nanoparticles. The surface plasmon resonance band observed in the ultraviolet–visible spectrum of the gold nanoparticles showed a shift to a lower energy after the surface was covered with the TGFBI gene. Surface-enhanced infrared spectroscopy (SEIRAS) showed the absorption bands of the bioconjugate due to their proximity to the nanoparticles. The SEIRAS effect was able to detect small and specific changes in the sequence of the TGFBI gene using interactions of the bioconjugates. The study was performed on patients clinically diagnosed with lattice corneal dystrophy (LCD). To our knowledge, this is the first report of FTIR spectroscopy employed to analyse this gene. We used genotyping results previously obtained by next-generation DNA sequencing technology to ensure that the analysed samples had the mutation. Our results show highly reproducible signals, which could be used for analytical studies of mutations in the genome.

Selective Chemical Enhancement via Graphene Oxide in Infrared Attenuated Total Reflection Spectroscopy

Graphene oxide (GO) as a readily prepared graphene derivative comprises negatively charged oxygenated functional groups, and a mixture of sp2 and sp3 carbon domains. In comparison with hydrophobic graphene, GO is characterized by superior water dispersibility, and may noncovalently interact with a significantly wider range of molecules. Taking advantage of GO and infrared attenuated total reflection spectroscopy (IR-ATR), an efficient strategy enabling unprecedented absorption signal amplification via chemical enhancement of various molecular species in aqueous environment was demonstrated in this study. The evolution of selectively enhanced molecular fingerprints across the entire mid-infrared spectral range was in situ monitored in real time, while retaining the characteristic molecular IR absorptions, and confirms the utility of GO as a suitable and versatile material in surface-enhanced infrared absorption spectroscopy (SEIRA) scenarios.

Physico-chemical analysis of molecular binding to the colloidal metal nanostructure: Multiple micro- and nanospectroscopy study

The work presents a detailed analysis of adsorption geometry of nocodazole, an antineoplastic agent, deposited onto gold nanoparticle surface. This type of study is fundamental because of improving drugs delivery specifically to cancer cells. Simultaneously, it will revolutionize oncology, dramatically improving the effect of drugs while reducing or even eliminating adverse side effects for the patient. The analytical means are now mature to study very precisely the solid-liquid interactions that will foreshadow the development of new anti-cancer therapies.This research demonstrates the feasibility of surface-enhanced infrared absorption spectroscopy (SEIRA), surface-enhanced Raman spectroscopy (SERS) and polarization-modulated surface-enhanced infrared absorption nanospectroscopy (PM-SEIRA) to study adsorption phenomenon of a molecule at the solid/liquid interface. A novel approach combining the classical infrared spectroscopy (IR) with atomic force microscopy (AFM) is employed to more comprehensively describe the behavior of the drug adsorbed on metal surface at the nanoscale level.The performed analysis enabled to track subtle changes in nocodazole bonds arrangement upon adsorption. The observed changes provide evidence for strong interaction between the electron-rich thiophene ring and metal surface. It can also be inferred that charge-transfer mechanism contributes considerably to the SERS signal amplification.

Silver nanoparticles on copper foam as substrate for full range mid-infrared surface enhanced infrared absorption spectroscopy in transmission configuration

The availability of sensitive, reproducible, and broad spectral window range surface enhanced infrared absorption spectroscopy (SEIRAS) substrate is essential to the wide application of SEIRAS technique. Here, a facile method was proposed for the preparation of full mid-infrared range transmission surface-enhanced infrared absorption spectroscopy (T-SEIRAS) substrate, where silver nanoparticles (Ag NPs) was synthesized in situ by immersing copper foam into the silver nitrate solution with the help of hydroxylamine hydrochloride. The T-SEIRAS substrate was characterized with Fourier Transform Infrared Spectroscopy (FT-IR), Scanning Electron Microscope (SEM) and X-ray Photoelectron Spectroscopy (XPS). It was found that the spectral window of the substrate was from 600 to 4000 cm−1. The apparent signal enhancement factor of the T-SEIRAS substrate was found to be about 35 times with the utilization of 11-mercaptoundecanoic acid (MUA) as probe molecule. Finally, the T-SEIRAS spectrum of E. coli was successfully obtained. It is expected that this type of T-SEIRAS substrate could be used in inexpensive IR instruments for field sensing applications.

Multiple-resonant pad-rod nanoantennas for surface-enhanced infrared absorption spectroscopy

Due to the ability to tightly confine electromagnetic energy, plasmonic nanoantennas have been widely studied for surface-enhanced infrared absorption (SEIRA) spectroscopy, surface-enhanced Raman spectroscopy as well as refractive index sensing. However, most of the nanoantennas are limited by narrow resonant band and it is rather challenging to detect multiple molecular fingerprints. In this work, we report dual and triple- resonant pad-rod plasmonic nanoantennas which are nanorods with large pads at their ends placed above gold (Au) mirror separated by a spacer layer. By adjusting the geometries, the nanoantennas have demonstrated dual and triple resonant bands enabling detection of molecular fingerprints at different wavelength. The calculated maximum SEIRA enhancement factor is around 1.8 × 106, which is among the highest reported so far. The pad-rod plasmonic nanoantennas have been used for the detection of molecules of polymethyl methacrylate (PMMA) by SEIRA and fingerprints of C=O and C–H bands are clearly identified. This work has shown that the multiple-resonant pad-rod plasmonic nanoantennas are promising for chemical and biomolecular sensing by the detection of vibrational fingerprints with SEIRA.

Optimization of a Commercial Variable Angle Accessory for Entry Level Users of Electrochemical Attenuated Total Reflection Surface-Enhanced Infrared Absorption Spectroscopy (ATR-SEIRAS).

An evaluation of several experimental aspects that can optimize electrochemical attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) performance using a commercially available, specular reflection accessory is provided. A comparison of different silicon single-bounce internal reflection elements (IREs) is made with emphasis on different face-angled crystal (FAC) options. Selection of optimal angle of incidence for maximizing signal and minimizing noise is shown to require consideration of the optical throughput of the accessory, reflection losses at the crystal surfaces, and polarization effects. The benefits of wire-grid polarizers and antireflective (AR) coatings on the IREs is discussed. High signal-to-noise ratios can be achieved by omitting polarizers, using an AR-coated FAC with a larger face angle, and working at angles of incidence close to the maximum throughput angle of the accessory.

Graphene-Enhanced Spectro-Electrochemistry on Boron-Doped Diamond Waveguides

With an increasing usage of engineered nanoparticles, a considerable amount of nanoparticles is inevitably released into the environment. This circumstance raises increasing environmental pollution and human health concerns. To mitigate their negative impacts, it is essential to understand the transport, transformation, and ultimate fates of these species within the environment. Among the most important parameters is their interaction with dissolved organic matters (DOM), which may significantly affect the surface properties and aggregation behavior of engineered nanoparticles. Consequently, this affects not only their geochemical cycling, but also any potential environmental remediation strategy. Understanding the heterogeneity of nanoparticle-DOM interactions, and unravelling the mechanisms in molecular details, as well as the factors regulating such interactions calls for effective analytical tools. Here, we present the development of advanced infrared attenuated total reflection (IR-ATR) sensing technologies for environmental analysis with specific focus on elucidating DOM-nanoparticle interactions in aquatic environments. The interaction dynamics of DOM with nanoparticles will be studied via thin-film diamond ATR waveguides combined with tunable quantum cascade laser and signal-enhancing graphene and graphene oxide surface structures (a.k.a. surface enhanced infrared absorption (SEIRA) spectroscopy) enabling unprecedently sensitive detection of DOM-nanoparticle interactions via molecular spectroscopy. Doping the diamond waveguide with boron (BDD) establishes an IR-transparent electrode material facilitating simultaneous electrochemical studies. Based on these orthogonal IR-spectroscopic and electrochemcial data sets derived from the same sample it is anticipated that DOM-nanoparticle interactions may be elucidated in yet unprecedented detail.

Surface Modifications of Pt Cathode Catalysts in Polymer Electrolyte Fuel Cells with Site-Selective or Nonselective Overlayers

For the wider commercialization of polymer electrolyte fuel cells (PEFCs), it is essential to improve the long-term durability of cathode catalysts, which are exposed to harsh conditions of potential cycles at low pH and high temperature (> 60°C) during the operation. Improving the durability without sacrificing the i-V performance is, however, a big challenge because those two properties have often a trade-off relation. Surface modifications of Pt catalysts by foreign materials are a promising method for improving both the activity and durability, and as the materials, ionic liquids [1-3] and a carbon thin layer formed by the heat-treatment (700°C) of dopamine [4] have been reported. The roles of the modifiers in the improvements, however, have not been clarified, and are studied in the present study. The role of ionic liquid (IL) was examined by using a stepped Pt single crystal. Figure 1 shows the cyclic voltammograms (CV) of Pt (443) in an aqueous electrolyte of 0.1 M HClO4 with and without the modification by the IL of [C4C1im][NTf2]. Only the Pt(110) step peaks for underpotential hydrogen deposition (Hupd) disappeared by the modification while the Pt(111)-terrace plateaus remained, and thus the IL molecules are judged to be selectively adsorbed on the step sites. This behavior is reminiscent of the results with the modification by a solution of Au complex, which exhibited selective depositions of Au atoms on step sites and improvements in both the activity and stability of Pt surface [5]. Although the improvements in the activity and stability by the IL has not been confirmed for the Pt single crystal electrode, where the IL molecules face the bulk electrolyte and can easily dissolve away, the CV results can explain the reported beneficial effects of ILs on high-surface-area Pt catalysts [1-3], where IL molecules can remain in micro pores of the catalysts for a long term. Analyses with surface-enhanced infrared absorption spectroscopy (SEIRAS) are underway to clarify the adsorption state of the IL molecules on Pt surface. The effect of the dopamine modification followed by the heat-treatment (DM-HT) was examined using a carbon-supported single-metal Pt nanoparticles (Pt/C) catalyst in the present study whereas a Pt-alloy catalyst (PtFe/C) was used in the previous study [4]. The analysis with the single-metal Pt/C enables us to evaluate the effect of the carbon thin layer separately from that of the formation of ordered intermetallic PtFe phase through the heat-treatment. Figure 2a shows the effect of the DM-HT on the activity of Pt/C. The results show the increase in the ORR activity accompanied by the suppression of sulfonate adsorptions on the Pt surfaces with the increase in the amount of dopamine added to the cathode catalyst, indicating the blockages of detrimental sulfonate adsorptions from ionomer by the carbon layers. The DM-HT Pt/C also exhibits a larger electrochemical surface area (ECSA) throughout potential cycles and thus a higher durability than the pristine Pt/C and a heat-treated Pt/C without the dopamine modification (HT-Pt/C) as shown in Fig.2b. The effect of the carbon layer on the O2 transport property near the catalyst surface (power density of the cell) will also be discussed in the meeting.