Nanorods and nanofilms of ZnO generated at the liquid-liquid interface

Reaction between zinc cupferron and alkali at the organic-aqueous interface has been investigated in detail. In the presence of an organic amine, this reaction yields ZnO nanorods, the morphology depending on the concentration of reactants. In the absence of the amine, ultrathin films of ZnO are produced. Amazingly, both the nanorods and thin films are single crystalline in nature, even though the synthesis is carried out at room-temperature. Single-crystalline, substrate-free ultrathin films of ZnO, is indeed an extraordinary feature of synthesis at the liquid-liquid interface. Photoluminescence spectra show defect-related bands besides that due to band edge emission. The nanostructures exhibit ferromagnetism due to surface defects.     

Conventional and microwave-assisted synthesis of ZnO nanorods and effects of PEG400 as a surfactant on the morphology

Conventional and microwave-assisted synthesis of ZnO nanorods have been performed with and without using PEG400. ZnO nanorods were synthesized with 50-250 nm of diameter which depends on the used surfactant and methods. Surfactant effects of PEG400 on the size and morphology of ZnO nanorods were investigated. The microwave method was compared to the conventional heating method. Morphologies were investigated by using scanning electron microscopy (SEM).     

A sensitive diagnostic assay of rheumatoid arthritis using three-dimensional ZnO nanorod structure

We synthesized a three-dimensional nanorod structure of zinc oxide (ZnO) using a simple sol-gel process and systematically investigated properties of the ZnO nanorods regarding protein adsorption and effect on fluorescence emission. As compared to conventional polystyrene plate that has been widely used for strong protein adsorption, the ZnO nanorods had a superior protein adsorption capacity and significantly amplified fluorescence emission, suggesting the ZnO nanorods are attractive for fluorescence-based biomolecular detection assays. When applied to diagnostic assay of rheumatoid arthritis (RA) using cyclic citrullinated peptide (CCP) probe with a RCGRS motif that reportedly has a strong affinity for ZnO, the ZnO nanorods gave apparently high positive signals for all the RA-positive standards and patient sera, whereas upon the detection using conventional polystyrene plate, all the detection signals were relatively negligible. Moreover, the streptavidin-mediated immobilization of well oriented CCP further enhanced sensitivity, even for a 5000-times diluted patient serum. A highly sensitive detection of a very small amount of RA autoantibodies is important because individuals at high risk of developing RA can be identified several years before the clinical onset. Consequently, the fluorescence-based sensitive assay of RA was successfully performed using the three-dimensional ZnO nanorods, owing to the fluorescence amplification and protein/peptide adsorption properties and dimensionality of ZnO nanorods that in turn increases probe accessibility to anti-CCP RA autoantibodies. Although RA was assayed here for proof-of-concept, the ZnO nanorods-based assay can be applied in general to sensitive detection of a wide variety of antibody or protein targets.     

Interfacial Engineering of P3HT/ZnO Hybrid Solar Cells Using Phthalocyanines: A Joint Theoretical and Experimental Investigation

Atomistic simulations and experimental investigations are combined to study heterojunction interfaces of hybrid polymer solar cells, with the aim to better understand and precisely predict their photovoltaic properties. The focus is on a hybrid ternary model system based on a poly(3‐hexylthiophene) (P3HT)/zinc phthalocyanine (ZnPc)/ZnO interface, in which a ZnPc interlayer is applied to improve the performance of the hybrid interface. Theoretical predictions of the ternary system are validated against the properties of a concrete P3HT/ZnPc/ZnO planar heterojunction device. The theoretical predictions closely agree with the photovoltaic properties obtained in P3HT/ZnPc/ZnO solar cells, indicating the strength of the method for modeling hybrid heterojunction interfaces. The theoretical and experimental results reveal that: i) ZnPc molecules in direct contact with a ZnO surface insert new energy levels due to a strong ZnPc/ZnO coupling, ii) electron injection from these new energy levels of ZnPc into ZnO is highly efficient, iii) the ZnPc/ZnO coupling strongly influences the energy levels of the ZnO and P3HT leading to a reduction of the open circuit voltage, and iv) charge carrier recombination at the P3HT/ZnO interface is reduced by the ZnPc interlayer. The intercalation of ZnPc leads to an increase in photocurrent as well as to an overall increase in power conversion.     

Plasmonic Control of Near-Interface Exciton Dynamics in Defect-Rich ZnO Thin Films

Zinc oxide (ZnO) is an attractive material for many electro-optical applications, but the control of impurities remains an issue in device fabrication. For this paper, the dynamics of defect states produced by annealing ZnO thin films at temperatures of 400–800 °C were probed by band-edge pump-probe spectroscopy in differential reflection and transmission. The distinction between the differential reflection and transmission spectra allowed for the analysis of ultrafast near-interface dynamics, which cannot be separated from the bulk thin-film dynamics by traditional ultrafast spectroscopies. In particular, simultaneous differential reflection and transmission spectroscopy provided clear evidence that the band-edge recombination dynamics in samples annealed at 400 °C were absent near the ZnO/substrate interface. However, the Purcell enhancement observed in Ag/ZnO heterostructures resulted in the dramatic emergence of the band-edge recombination signal nearly two orders of magnitude greater in intensity than the defect differential reflectivity. This indicates that the spatial range of the Purcell effect is at least twice as large as inferred from previous photoluminescence studies.     

Large‐Area,Ultrathin Inorganic Network Coverages–Graphene Hierarchical Electrodes for Flexible,Heat‐Resistant Energy Storage Application

Graphene and quasi‐2D graphene‐like materials with an ultrathin thickness have been investigated as a new class of nanoscale materials due to their distinctive properties. A novel “molecular tools‐assistances” strategy is developed to fabricate two kinds of graphene‐based electrodes, ultrathin Fe‐doped MnO₂ network coverage–graphene composites (G‐MFO) and ultrathin MoS₂ network coverage–graphene composites (G‐MoS₂) with special hierarchical structures. Such structures enable a large contact interface between the active materials and graphene and thus fully exploit the synergistic effect from both the high specific capacitance of MFO or MoS₂ and the superb conductivity of graphene. Benefiting from their unique structural features, G‐MFO and G‐MoS₂ films directly use as free‐standing electrodes for flexible asymmetric supercapacitors with a nonaqueous gel electrolyte. The device achieves a high energy/power density, superior flexibility, good rate capability as well as outstanding performance stability even at a high temperature. This work represents a promising prototype to design new generation of hybrid supercapacitors for future energy storage devices.     

New nanocomposite films for biosensors: layer-by-layer adsorbed films of polyelectrolytes, proteins or DNA

This report describes the construction of ultrathin multicomponent films with an internal structure on the nanometre scale. In the simplest case, the films are built-up by the subsequent adsorption of polyanions and polycations. They can be fabricated on inorganic substrates such as glass, quartz or silicon wafers, or on various organic materials. The polymeric interlayers can incorporate materials with desired electrical or optical properties. The average thickness of the layers can be fine-tuned with Angstrom precision by the addition of suitable salts. They are temperature stable up to at least 200°C and can be laterally structured using conventional photolithographic techniques. The films provide for a well-defined substrate-independent interface for the immobilization of biological macromolecules, such as proteins or DNA, in their active state. The immobilization of streptavidin enables the controlled attachment of any biotinylated molecule with no resulting loss in its biological activity. Area-selective immobilization provides the possibility of built-in quality control for the fabrication of biosensors with separated reference and sample areas on the same substrate.     

Studies of Electronic Excitations of Rectangular ZnO Nanorods by Electron Energy-Loss Spectroscopy

Electronic excitations of single ZnO rectangular nanorod have been investigated by electron energy-loss spectroscopy in conjunction with scanning transmission electron microscopy (STEM-EELS). We focus primarily on the surface excitations greatly enhanced at the grazing incidence parallel to the surfaces of ZnO nanorods. An uncommon kind of surface excitation known as surface exciton polaritons occurring near interband transitions is found to dominate in the spectral range between the band gap at 3.4 eV and the surface plasmon peak at 15.8 eV. In addition, the dielectric function of ZnO up to 25 eV has also been derived from the bulk excitation spectra using the Kramers–Kronig analysis on a single nanorod. Theoretical EELS simulations are also compared with the experimental results and good agreements are obtained.     

Interface Design to Improve the Performance and Stability of Solution‐Processed Small‐Molecule Conventional Solar Cells

A systematic study on the effect of various cathode buffer layers on the performance and stability of solution‐processed small‐molecule organic solar cells (SMOSCs) based on tris{4‐[5‐(1,1‐dicyanobut‐1‐en‐2‐yl)‐2,2‐bithiophen‐5‐yl]phenyl}amine (N(Ph‐2T‐DCN‐Et)₃):6,6‐phenyl‐C71‐butyric acid methyl ester (N(Ph‐2T‐DCN‐Et)₃:PC₇₀BM) is presented. The power conversion efficiency (PCE) in these systems can be significantly improved from approximately 4% to 5.16% by inserting a metal oxide (ZnO) layer between the active layer and the Al cathode instead of an air‐sensitive Ba or Ca layer. However, the low work‐function Al cathode is susceptible to chemical oxidation in the atmosphere. Here, an amine group functionalized fullerene complex (DMAPA‐C₆₀) is inserted as a cathode buffer layer to successfully modify the interface towards ZnO/Ag and active layer/Ag functionality. For devices with ZnO/DMAPA‐C₆₀/Ag and DMAPA‐C₆₀/Ag cathodes the PCEs are improved from 2.75% to 4.31% and to 5.40%, respectively, compared to a ZnO/Ag device. Recombination mechanisms and stability aspects of devices with various cathodes are also investigated. The significant improvement in device performance and stability and the simplicity of fabrication by solution processing suggest this DMAPA‐C₆₀‐based interface as a promising and practical pathway for developing efficient, stable, and roll‐to‐roll processable SMOSCs.     

DNA complexes, formed on aqueous phase surfaces: new planar polymeric and composite nanostructures

The formation of DNA complexes with Langmuir monolayers of the cationic lipid octadecylamine (ODA) and the new amphiphilic polycation poly-4-vinylpyridine with 16% of cetylpyridinium groups (PVP-16) on the surface of an aqueous solution of native DNA of low ionic strength was studied. Topographic images of Langmuir-Blodgett films of DNA/ODA and DNA/PVP-16 complexes applied to micaceous substrates were investigated by the method of atomic force microscopy. It was found that films of the amphiphilic polycation have an ordered planar polycrystalline structure. The morphology of planar DNA complexes with the amphiphilic cation substantially depended on the incubation time and the phase state of the monolayer on the surface of the aqueous DNA solution. Complex structures and individual DNA molecules were observed on the surface of the amphiphilic monolayer. Along with quasi-linear individual bound DNA molecules, characteristic extended net-like structures and quasi-circular toroidal condensed conformations of planar DNA complexes were detected. Mono- and multilayer films of DNA/PVP-16 complexes were used as templates and nanoreactors for the synthesis of inorganic nanostructures via the binding of metal cations from the solution and subsequent generation of the inorganic phase. As a result, ultrathin polymeric composite films with integrated DNA building blocks and quasi-linear arrays of inorganic semiconductor (CdS) and iron oxide nanoparticles and nanowires were obtained. The nanostructures obtained were characterized by scanning probe microscopy and transmission electron microscopy techniques. The methods developed are promising for investigating the mechanisms of structural organization and transformation in DNA and polyelectrolyte complexes at the gas-liquid interface and for the design of new extremely thin highly ordered planar polymeric and composite materials, films, and coatings with controlled ultrastructure for applications in nanoelectronics and nanobiotechnology.     

Characterization of zinc oxide nanocrystals with different morphology for application in ultraviolet‐light photocatalytic performances on rhodamine B

ZnO nanostructures of different morphology (nanorods, nano‐leaf, nanotubes) were favourably grown using a chemical precipitation process. The prepared ZnO nanostructures were characterized systematically using absorption spectroscopy, emission spectroscopy, X‐ray diffraction (XRD), scanning electron microscopy (SEM) and Fourier transform infrared studies. XRD results showed the hexagonal wurtzite phase of the synthesized ZnO nanostructures. Structural properties such as average crystallite size, lattice constants, volume of the unit cell, atomic fraction, and structural bonds were also studied. The optical band gap of the synthesized ZnO nanocrystals varied from 3.52 eV to 3.69 eV with high quantum yield of the blue emission (~420 nm). Urbach energy for ZnO nanocrystals was calculated to be 0.702 eV, 0.901 eV, and 0.993 eV for nanorods, nano‐leaf, and tube like ZnO crystals, respectively. Morphology of the fabricated nanostructures was investigated using SEM. Photocatalytic degradation of rhodamine B (Rh B) in solution under UV irradiation was explored with different ZnO morphology. Photocatalytic experiments showed that ZnO nano‐leaf had a higher degradation rate of photocatalytic activity of photodegrading Rh B compared with the other tube shape and rods shape nanostructures. The Rh B dye degraded considerably by ～79.05%, 74.41%, and 69.8% within 120 min in the presence of the as‐fabricated fern nano‐leaf, nanotubes, and nanorods of the ZnO nanocrystals at room temperature.     

Nanostructural zinc oxide hollow spheres: A facile synthesis and catalytic properties

The development of reproducible procedures for the synthesis and organization of nanostructured metal oxides is important in order to exploit the unique properties of these materials for practical applications. The present work describes the transformation of Zn(NH₃)₄]²⁺ into hollow structured ZnO materials through solvothermal decomposition. An increase in ammonia concentration in the reaction medium, significantly changes the morphology of ZnO from spheres made of nanoparticles (20-30 nm) to hollow spheres composed of nanorods (200-350 nm) or to free microrods as evidenced from scanning and transmission electron micrographs (SEM/TEM). The powder X-ray diffraction (XRD) pattern of ZnO confirms formation of the wurtzite structure. Raman and Energy-dispersive spectroscopic (EDS) studies indicate the presence of oxygen deficiency in ZnO. The investigation on the catalytic behavior of ZnO in the synthesis of (4-methoxyphenyl)(phenyl) methanone (MPPM) by Friedel-Crafts acylation of anisole with benzoyl chloride has also been carried out. The results reveal that the prepared ZnO could produce ∼98% of yield compared to 41% produced by commercial ZnO.     

Layer-by-layer Synthesis and Transfer of Freestanding Conjugated Microporous Polymer Nanomembranes

CMP as large surface area materials have attracted growing interest recently, due to their high variability in the incorporation of functional groups in combination with their outstanding thermal and chemical stability, and low densities. However, their insoluble nature causes problems in their processing since usually applied techniques such as spin coating are not available. Especially for membrane applications, where the processing of CMP as thin films is desirable, the processing problems have hindered their commercial application.Here we describe the interfacial synthesis of CMP thin films on functionalized substrates via molecular layer-by-layer (l-b-l) synthesis. This process allows the preparation of films with desired thickness and composition and even desired composition gradients.The use of sacrificial supports allows the preparation of freestanding membranes by dissolution of the support after the synthesis. To handle such ultra-thin freestanding membranes the protection with sacrificial coatings showed great promise, to avoid rupture of the nanomembranes. To transfer the nanomembranes to the desired substrate, the coated membranes are upfloated at the air-liquid interface and then transferred via dip coating.     

The Interaction between Steroid Hormones and Lipid Monolayers on Water

The interaction of progesterone, testosterone, androsterone, and etiocholanolone with insoluble lipid films (cholesterol and saturated hydrocarbons containing either alcohol, ester, acetamide, phosphate, amine, or carboxyl groups) was studied. In addition to surface pressure and surface potential measurements of the surface films, radioactive tracers were used to measure the concentration of adsorbed steroid in the lipid films. In general, steroids form mixed films with the insoluble lipid films. Compression of the insoluble lipid films to their most condensed state leads to complete ejection of adsorbed steroid from the surface in all cases except with the amine, for which a small amount of steroid is still retained in the surface. Interactions between the steroids and insoluble lipids are primarily due to van der Waals or dispersion forces; there were no significant contributions from dipole-dipole interactions (except possibly with the amine). Specific interactions between cholesterol and the soluble steroids were not observed. Evidence suggests that low steroid concentrations influence structure of lipid films by altering the hydration layer in the surface film. In contrast to a specific site of action, it is proposed that steroid hormones initiate structural changes in a variety of biological sites; this model of steroid action is consistent with the ubiquity of many steroid hormones.     

A review on biological synthesis of ZnO nanostructures and its application in photocatalysis mediated dye degradation: An overview

Zinc oxide (ZnO) has several industrial applications due to its versatile properties, which lead to its continuously increasing demand in different industrial sectors. Additionally, ZnO nanostructures possess unique photocatalytic activity, and because of this, they are being applied to degrade organic dyes through photocatalysis for wastewater treatment. Nevertheless, chemical synthesis methods to develop ZnO nanostructures have raised concerns related to environmental issues, furthermore, these methods are found to be costly and tedious. As a result, the synthesis of ZnO nanostructures using green methods is gaining popularity due to its low cost and eco-friendly mode, while avoiding the use of toxic chemicals. Green synthesis of ZnO nanostructures using different biological approaches involving plants, algae, and different microorganism-derived bioactive compounds has been well reported for diverse applications. Among different applications, ZnO nanostructures that enable photocatalysis to degrade dye have been found to be imperative for wastewater treatments. Therefore, the current review explores recent studies on green synthesis approaches to prepare ZnO nanostructures via adopting different biological methods that rely on plants, algae, and bacterial microorganisms. The properties of ZnO nanostructures, along with their green synthesis routes and feasible mechanisms, have also been discussed in this review. This review focuses on the use and efficiency of green route synthesized ZnO nanostructures as nanophotocatalysts for the degradation of organic dyes in wastewater treatment. Additionally, existing challenges in green synthesis methods and the efficiency of ZnO nanostructures to degrade organic dyes following photocatalysis has been discussed.     

Ethanedithiol Treatment of Solution‐Processed ZnO Thin Films: Controlling the Intragap States of Electron Transporting Interlayers for Efficient and Stable Inverted Organic Photovoltaics

The surface defects of solution‐processed ZnO films lead to various intragap states. When the solution‐processed ZnO films are used as electron transport interlayers (ETLs) in inverted organic solar cells, the intragap states act as interfacial recombination centers for photogenerated charges and thereby degrade the device performance. Here, a simple passivation method based on ethanedithiol (EDT) treatment is demonstrated, which effectively removes the surface defects of the ZnO nanocrystal films by forming zinc ethanedithiolates. The passivation by EDT treatment modulates the intragap states of the ZnO films and introduces a new intragap band. When the EDT‐treated ZnO nanocrystal films are used as ETLs in inverted organic solar cells, both the power conversion efficiency and stability of the devices are improved. The control studies show that the solar cells with EDT‐treated ZnO films exhibit reduced charge recombination rates and enhanced charge extraction properties. These features are consistent with the fact that the modulation of the intragap states results in reduction of interfacial recombination as well as the improved charge selectivity and electron transport properties of the ETLs. It is further demonstrated that the EDT treatment‐based passivation method can be extended to ZnO films deposited from sol–gel precursors.     

Structure, composition, enzymatic activities of human erythrocyte and sarcoplasmic reticulum membrane films

Air/water interface films were obtained from human erythrocytes and rabbit sarcoplasmic reticulum membranes at 'zero surface pressure. according to Verger, R and Pattus, F. (Chem. Phys. Lipids (1976) 16, 285-291). The lipid and protein distribution of these membrane films suggest that the film composition is determined by the composition of the membrane and the mode of integration of its components. When kept at low surface pressure, slow film expansion occurred due to unfolding of proteins at the interface. This process can be stopped by compressing the films at a higher surface pressure than 15 dyn/cm. Acetylcholinesterase activity from human erythrocyte films is highly dependent on the condensation state of the film. Ca2+-ATPase from sarcoplasmic reticulum films was still activable by Ca2+. Freeze-fracture studies on erythrocyte membrane films suggest the such films are monolayers in which proteins are randomly distributed.     

Ultrathin antibiotic walled microcapsules

Ultrathin microcapsules comprised of anionic polyelectrolytes (PE) and a polycationic aminoglycoside (AmG) antibiotic drug were prepared by depositing PE/AmG multilayers on zinc oxide (ZnO) colloid particles using the layer-by-layer self-assembly technique and subsequently dissolving the ZnO templated cores. The polyelectrolytes, dextran sulfate sodium (DxS) and poly(styrenesulfonate) (PSS), were selected owing to their different backbone structure. An aminoglycoside, tobramycin sulfate (TbS), was used for studying DxS/TbS or PSS/TbS multilayer films. The multilayer growth on ZnO cores was characterized by alternating zeta potential values that were different for the DxS/TbS and PSS/TbS multilayers due to the PE chemistry and its interaction with Zn(2+) ions. Transmission and scanning electron microscopy provide evidence of PE/TbS multilayer coating on ZnO core particles. The slow acid-decomposition of the ZnO cores using weak organic acids and the presence of sufficient quantity of Zn(2+) in the dispersion were required to produce antibiotic multilayer capsules. There was no difference in the morphological characteristics of the two types of capsules; although, the yield for [PSS/TbS](5) capsules was significantly higher than for [DxS/TbS](5) capsules which was related to the physicochemical properties of DxS/TbS/Zn(2+) and PSS/TbS/Zn(2+) complexes forming the capsule wall. The TbS quantity in the multilayer films was determined using a quartz crystal microbalance and high performance liquid chromatography techniques which showed less TbS loading in both, capsules and multilayers on planar gold substrate, than the theoretical DxS:TbS or PSS:TbS stoichiometric ratio. The decomposition of the [PE/TbS](6) multilayers was fastest in physiological buffer followed by mannitol and water. The decomposition rate of the [PSS/TbS](6) multilayers was slower than [DxS/TbS](6) monolayers. The incomplete decomposition of DxS/TbS under saline conditions suggests the major role of hydrogen bonding for stability of DxS/TbS multilayers. A combination of hydrogen bonding and hydrophobic interaction between phenyl rings in PSS was responsible for PSS/TbS multilayer stability. In vivo studies in rabbits highlight the safety and sustained drug delivery potential of the PE/AmG microcapsules. The antibiotic walled ultrathin capsules presented here are suitable for sustained ophthalmic antibiotic delivery.     

Boronization‐Induced Ultrathin 2D Nanosheets with Abundant Crystalline–Amorphous Phase Boundary Supported on Nickel Foam toward Efficient Water Splitting

The conversion of crystalline metal–organic frameworks (MOFs) into metal compounds/carbon hybrid nanocomposites via pyrolysis provides a promising solution to design electrocatalysts for electrochemical water splitting. However, pyrolyzing MOFs generally involves a complex high‐temperature treatment, which can destroy the coordinated surroundings within MOFs, and as a result not taking their full advantage of their electrolysis properties. Herein, a simple and room‐temperature boronization strategy is developed to convert nickel zeolite imidazolate framework (Ni‐ZIF) nanorods into ultrathin Ni‐ZIF/Ni? B nanosheets with abundant crystalline–amorphous phase boundaries. The combined experiment, and theoretical calculation results disclose that the ultrathin thickness allows fast electron transfer and ensures increased exposure of surface coordinatively unsaturated active sites while the crystalline–amorphous interface elaborately changes the potential‐determining step to energetically favorable intermediates. As a result, Ni‐ZIF/Ni? B nanosheets supported on nickel foam (NF) require overpotentials of 67 mV for the hydrogen evolution reaction and 234 mV for the oxygen evolution reaction to achieve a current density of 10 mA cm^?2. Remarkably, Ni‐ZIF/Ni? B@NF as a bifunctional electrocatalyst for overall water splitting enables an alkaline electrolyzer with 10 mA cm^?2 at an ultralow cell voltage of 1.54 V. The present work may open a new avenue to the design of MOF‐derived composites for electrocatalysis.     

Orientation and lateral mobility of cytochrome c on the surface of ultrathin lipid multilayer films.

We have previously shown that cytochrome c can be electrostatically bound to an ultrathin multilayer film having a negatively charged hydrophilic surface; furthermore, x-ray diffraction and absorption spectroscopy techniques indicated that the cytochrome c was bound to the surface of these ultrathin multilayer films as a molecular monolayer. The ultrathin fatty acid multilayers were formed on alkylated glass, using the Langmuir-Blodgett method. In this study, optical linear dichroism was used to determine the average orientation of the heme group within cytochrome c relative to the multilayer surface plane. The cytochrome c was either electrostatically or covalently bound to the surface of an ultrathin multilayer film. Horse heart cytochrome c was electrostatically bound to the hydrophilic surface of fatty acid multilayer films having an odd number of monolayers. Ultrathin multilayer films having an even number of monolayers would not bind cytochrome c, as expected for such hydrophobic surfaces. Yeast cytochrome c was covalently bound to the surface of a multilayer film having an even number of fatty acid monolayers plus a surface monolayer of thioethyl stearate. After washing extensively with buffer, the multilayer films with either electrostatically or covalently bound cytochrome c were analyzed for bound protein by optical absorption spectroscopy; the orientation of the cytochrome c heme was then investigated via optical linear dichroism. Polarized optical absorption spectra were measured from 450 to 600 nm at angles of 0 degrees, 30 degrees, and 45 degrees between the incident light beam and the normal to the surface plane of the multilayer. The dichroic ratio for the heme alpha-band at 550 nm as a function of incidence angle indicated that the heme of the electrostatically-bound monolayer of cytochrome c lies, on average, nearly parallel to the surface plane of the ultrathin multilayer. Similar results were obtained for the covalently-bound yeast cytochrome c. Furthermore, fluorescence recovery after photobleaching (FRAP) was used to characterize the lateral mobility of the electrostatically bound cytochrome c over the monolayer plane. The optical linear dichroism and these initial FRAP studies have indicated that cytochrome c electrostatically bound to a lipid surface maintains a well-defined orientation relative to the membrane surface while exhibiting measurable, but highly restricted, lateral motion in the plane of the surface.