Redox-linked protein dynamics of cytochrome c probed by time-resolved surface enhanced infrared absorption spectroscopy

Time-resolved surface enhanced infrared absorption (SEIRA) spectroscopy is employed to analyse the dynamics of the protein structural changes coupled to the electron transfer process of immobilised cytochrome c (Cyt-c). Upon electrostatic binding of Cyt-c to Au electrodes coated with self-assembled monolayers (SAMs) of carboxyl-terminated thiols, cyclic voltammetric measurements demonstrate a reversible redox process with a redox potential that is similar to that of Cyt-c in solution, and a non-exponential distance-dependence of the electron transfer rate as observed previously (D. H. Murgida and P. Hildebrandt, Chem. Soc. Rev. 2008, 37, 937). On the basis of characteristic redox-state-sensitive amide I bands, the protein structural changes triggered by the electron transfer are monitored by rapid scan and step scan SEIRA spectroscopy in combination with the potential jump technique. Whereas the temporal evolution of the conjugate bands at 1693 and 1673 cm(-1) displays the same rate constants as electron transfer, the time-dependent changes of the 1660-cm(-1) band are slower by about a factor of 2. The study demonstrates that time-resolved SEIRA spectroscopy provides further information about the dynamics and mechanism of interfacial processes of redox proteins, thereby complementing the results obtained from other surface-sensitive techniques. In comparison with previous surface enhanced resonance Raman spectroscopic findings, the present results are discussed in terms of the local electric field strengths at the Au/SAM/Cyt-c interface.     

Attenuated total reflection Fourier transform infrared spectra of faceted diamonds

In this work, the contributions of cationic and elemental gold on roughened gold substrates to surface-enhanced Raman scattering (SERS) of polypyrrole (PPy) films were first investigated. First, a gold substrate was roughened by a triangular wave oxidation-reduction cycle (ORC) in an aqueous solution containing 0.1 M KCl. Then, the roughened gold substrate was further reduced by applying a cathodic potential for a fixed time to control the quantity of unreduced cationic Au on the roughened Au substrate. The result indicates that the content of cationic Au is responsible for the improved SERS of PPy electrodeposited on this roughened Au substrate. This phenomenon can be attributed to the interfacial charge transfer from PPy to the roughened Au substrate by the aid of cationic Au.     

ATR FT-IR absorption enhancement of a thin film under the photon-tunneling condition.

A simple, yet very powerful technique for the spectral acquisition of an extremely thin film with enhanced absorption was explored. An infrared absorption of an extremely thin film confined between media of high refractive indices was greater than that of its bulk when the spectrum was acquired under the attenuated total reflection (ATR) condition with parallel (p) polarized radiation. The absorption enhancement was not observed under perpendicular (s) polarized radiation. Theoretical investigations indicated that the absorption enhancement was proportional to the integration of the mean square evanescent field within the film. The field integration under p-polarized radiation increased, while that under s-polarized radiation decreased as the thickness of the confined film became thinner. The maximum enhancement was observed when the film was sufficiently thinner than the penetration depth. The phenomena were experimentally investigated, and the results agreed very well with theoretical predictions.     

3D SERS Imaging Using Chemically Synthesized Highly Symmetric Nanoporous Silver Microparticles

3D surface‐enhanced Raman scattering (SERS) imaging with highly symmetric 3D silver microparticles as a SERS substrate was developed. Although the synthesis method is purely chemical and does not involve lithography, the synthesized nanoporous silver microparticles possess a regular hexapod shape and octahedral symmetry. By using p‐aminothiophenol (PATP) as a probe molecule, the 3D enhancement patterns of the particles were shown to be very regular and predictable, resembling the particle shape and exhibiting symmetry. An application to the detection of 3D inhomogeneity in a polymer blend, which relies on the predictable enhancement pattern of the substrate, is presented. 3D SERS imaging using the substrate also provides an improvement in spatial resolution along the Z axis, which is a challenge for Raman measurement in polymers, especially layered polymeric systems.     

Flexible superhydrophobic gold film for magnetical manipulation of droplets

Herein, we present a simple, efficient, and economical approach for the preparation of superhydrophobic gold film embedded on polydimethylsiloxane (PDMS) sheets without the requirement of surface pretreatment. The reduction reaction between chloroauric acid (HAuCl₄) and sodium formate (HCOONa) at room temperature was performed to generate the aggregated gold microstructures on a PDMS sheet without chemical residuals. Superhydrophobic property was achieved when deposition time was reached to 2 h with water contact angle >160° and low contact angle hysteresis (H = 1.93°). Systematic investigations of the size, morphology, and mechanism of the generated gold films are presented. The generated gold film contains two different layers involving uniform spherical gold particles attached to the PDMS surface with the complex hierarchical structures on top. The complex structures play an important role in the superhydrophobic property, as they strongly promote the roughness to the PDMS surface. The durability of the fabricated gold film was elucidated by dropping ~7,200 waterdrops and external physical forces (e.g. stretch, bend, and twist). The main structures and their superhydrophobic properties have not disoriented after the tests. Moreover, the surface of the gold film demonstrated the potential applications as magnetical manipulation of droplets and a robust Surface enhanced Raman spectroscopy (SERS substrate).     

Micrometer-sized gold nanoplates: starch-mediated photochemical reduction synthesis and possibility of application to tip-enhanced Raman scattering (TERS)

In this report, we propose a novel starch-mediated photochemical reduction method for synthesizing micrometer-sized gold nanoplates and the possibility of using them as a tip-enhanced Raman scattering (TERS) substrate. To reduce gold ions, a starch chain firstly forms a complex with AuCl(4)(-), and the gold ion is subsequently reduced by receiving an electron from a chloride ion and generating a chloride radical when the [AuCl(4)(-)-starch] complex is irradiated by sunlight. Due to the slow reaction rate and the capability of starch as a template, gold structure can thermodynamically grow along the (111) facet which is the lowest energy facet of the gold face-centered cubic (fcc) crystal. This method can provide various shapes of gold plates such as triangle, truncated triangle, hexagon, polygon, etc. The plate size can be controlled in the range from a few micrometers to more than one hundred micrometers by increasing the acidity of solution while the plate thickness is less than 100 nm. Potential application of the gold plates as TERS substrates is demonstrated by collecting Raman signals while approaching a silver-coated tungsten tip to the surface of the micrometer-sized gold nanoplate covered by crystal violet (CV) molecules. The results show that less than one hundred CV molecules can be detected in our study.     

An ionic surfactant-mediated Langmuir-Blodgett method to construct gold nanoparticle films for surface-enhanced Raman scattering

A gold nanoparticle film for surface-enhanced Raman scattering (SERS) was successfully constructed by an ionic surfactant-mediated Langmuir-Blodgett (LB) method. The gold film was formed by adding ethanol to a gold colloid/hexane mixture in the presence of dodecyltrimethylammonium bromide (DTAB). Consequently, gold nanoparticles (AuNPs) assembled at the water/hexane interface due to the decrease in surface charge density of AuNPs. Since DTAB binds the gold surface by a coulombic force, rather than a chemical bonding, it is easily replaced by target molecules for SERS purposes. The SERS enhancement factor of the 80 nm gold nanoparticle film was approximately 1.2 × 10(6) using crystal violet (CV) as a Raman dye. The SERS signal from the proposed DTAB-mediated film was approximately 10 times higher than that from the octanethiol-modified gold film, while the reproducibility and stability of this film compared to an octanethiol-modified film were similar. This method can also be applied to other metal nanostructures to fabricate metal films for use as a sensitive SERS substrate with a higher enhancement factor.     

Distinguishing Enantiomers by Tip‐Enhanced Raman Scattering: Chemically Modified Silver Tip with an Asymmetric Atomic Arrangement

Discrimination between enantiomers is achieved by tip‐enhanced Raman scattering (TERS) using a silver tip that is chemically modified by an achiral para‐mercaptopyridine (pMPY) probe molecule. Differences in the relative intensities of the pMPY spectra were monitored for three pairs of enantiomers containing hydroxy (?OH) and/or amino (?NH₂) groups. The N: or N⁺?H functionality of the pMPY‐modified tip participates in hydrogen‐bond interactions with a particular molecular orientation of each chiral isomer. The asymmetric arrangement of silver atoms at the apex of the tip induces an asymmetric electric field, which causes the tip to become a chiral center. Differences in the charge‐transfer (CT) states of the metal‐achiral probe system in conjunction with the asymmetric electric field produce different enhancements in the Raman signals of the two enantiomers. The near‐field effect of the asymmetric electric field, which depends on the number of analyte functional groups capable of hydrogen‐bond formation, improves the degree of discrimination.     

Investigation of Gold Quantum Dot Enhanced Organic Thin Film Solar Cells

Gold quantum dots (AuQDs) are employed as photosensitizers in organic thin‐film solar cells (OSCs) to improve their photoelectric conversion properties. Three types of AuQDs with different fluorescence emission wavelengths are used: blue (B‐AuQDs), green (G‐AuQDs), and red (R‐AuQDs). AuQDs are loaded into the poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonate) thin‐film layer of OSCs. UV–vis spectra, atomic force microscope images, current density–voltage characteristics, and impedance spectra of the fabricated devices are measured for the three aforementioned types of AuQDs. All types of AuQDs improve the photoelectric conversion properties, and the G‐AuQD‐loaded OSCs exhibit the best improvement, exhibiting an efficiency increase of 10% compared with OSCs without the AuQDs. The fluorescence/photosensitization of the AuQDs plays an important role in the enhancement of the OSCs. Finite‐difference time‐domain simulations indicate increased electric field intensity due to a small degree of AuQDs aggregation.