Advances in Colloid and Interface Science (v.116, #1-3)

by Dieter Vollhardt (viii-x).

Sensors based on double-decker rare earth phthalocyanines by J.A. de Saja; M.L. Rodríguez-Méndez (1-11).
Phthalocyanines are interesting materials for sensing applications because their physicochemical properties are susceptible to be modified by the presence of certain molecules. Among the large family of the phthalocyanines, the sensing properties of double-decker phthalocyanines (which are sandwich-type complexes where a lanthanide metal is co-ordinated with two phthalocyanine rings) are of special interest due to their unique physicochemical properties. Their high intrinsic semiconductivity and their rich electrochemical and electrochromic behaviour, facilitate enormously the measurement of the changes in the physicochemical properties caused by the environment.In spite of their remarkable sensing behaviour, bisphthalocyanines have not been so extensively studied as sensitive materials as the parent monophthalocyanine compounds. This is due to the difficulty of the synthesis and purification processes, and to the fact that these compounds are not commercially available. In this paper, the sensing devices constructed using rare earth bisphthalocyanines are revised (including sensors foe electronic noses and electronic tongues) and their advantages discussed.
Keywords: Phthalocyanine; Bisphthalocyanine; Sensor; Electronic nose; Electronic tongue;

Phthalocyanines are macrocycles with an appealing diversification of characteristics of fundamental interest in contemporary advanced technologies. On the other side, the Langmuir–Blodgett method permits to deposit films with a substantial control over thickness and molecular organisation. As a natural consequence, this review deals with an actual subject of attention in up-to-date research, the employment of Langmuir–Blodgett films of phthalocyanines in chemical sensors for the revelation of analytes in both gas and liquid phases. Experimental data on the structure, morphology and surface properties of the multilayers are connected with the most relevant characteristics of sensors, in order to discover the intimate relationships between the sensor performances and the peculiarities and molecular organisation induced by the deposition technique. The integration of the unconventional electrical and optical properties of phthalocyanines with the potentialities of the Langmuir–Blodgett thin films has generated not only promising expectations, but has given also certainties about the realisation of functional sensing devices.
Keywords: Langmuir–Blodgett films; Phthalocyanines; Chemical sensors; Spectroscopic characterisation; Electrical characterisation;

Surface-enhanced Raman scattering on colloidal nanostructures by R.F. Aroca; R.A. Alvarez-Puebla; N. Pieczonka; S. Sanchez-Cortez; J.V. Garcia-Ramos (45-61).
Surface-enhanced Raman scattering combines extremely high sensitivity, due to enhanced Raman cross-sections comparable or even better than fluorescence, with the observation of vibrational spectra of adsorbed species, providing one of the most incisive analytical methods for chemical and biochemical detection and analysis. SERS spectra are observed from a molecule-nanostructure enhancing system. This symbiosis molecule-nanostructure is a fertile ground for theoretical developments and a realm of applications from single molecule detection to biomedical diagnostic and techniques for nanostructure characterization.
Keywords: SERS; Colloids; Nanostructure; Raman; Vibrational spectroscopy; Single molecule detection;

Recent progress in studies of the main characteristics of supramolecular assemblies formed by interfacial molecular recognition between an amphiphilic monolayer and a non-surface-active species, which is dissolved in the aqueous subphase, by complementary hydrogen bonding and/or electrostatic interaction at the air–water interface is reviewed.Systems consisting of an amphiphilic melamine-type monolayer and an pyrimidine derivative dissolved in the aqueous subphase are representative model systems for molecular recognition on the basis of complementary hydrogen bonding. Most of the studies have been performed with 2,4-di(n-undecylamino)-6-amino-1,3,5-triazine (2C11H23-melamine) monolayers as host component and thymine, uracil or barbituric acid as dissolved non-surface-active pyrimidine derivatives.The combination of surface pressure studies with Brewster angle microscopy (BAM) imaging and Grazing incidence X-ray diffraction (GIXD) measurements is optimal for the characterization of the change in structure and phase behavior at the interfacial recognition process. The molecular recognition of all pyrimidine derivatives dissolved in the aqueous subphase changes drastically and in a specific way the characteristic features (πA isotherms, morphology of the condensed phase domains) of the 2C11H23-melamine monolayer. The small condensed phase domains of the pure 2C11H23-melamine monolayer are compact without an inner texture. The monolayers of the supramolecular 2C11H23-melamine entities with thymine or uracil form specifically well-shaped condensed phase domains with an inner alkyl chain texture essentially oriented parallel to the periphery. The completely different morphology of the 2C11H23-melamine-barbituric acid monolayers is characterized by the formation of large homogeneous areas of condensed phase that transfer at smaller areas per molecule to a homogeneous condensed monolayer.The striking differences in the main characteristics between the supramolecular entities are related to their different chemical structures: complementary hydrogen bonding of two thymine or uracil molecules by one 2C11H23-melamine molecule and a linearly extended hydrogen bonding network between 2C11H23-melamine and barbituric acid. The high values of hydrogen bonding energy obtained by quantum chemical calculations on the basis of the semi-empirical PM3 method state the high stability of the supramolecular entities.The GIXD results reveal that the formation of hydrogen-bond based superstructures between the polar head groups of the amphiphilic 2C11H23-melamine monolayer and the non-surface-active pyrimidine derivatives gives rise only to quantitative changes in the two-dimensional lattice structure of the alkyl chains.The alternative possibility to construct interfacial molecular recognition systems on the basis of acid–base interaction is demonstrated by the experimental results obtained by molecular recognition of the heptadecyl-benzamidinium chloride monolayers with dissolved non-surface-active phenylacetate ions. The formation of supramolecular assemblies causes also drastical changes of the surface features in these systems. Here, the development of a substructure in the condensed phase domains consisting of long filigree strings and the favoured formation of bilayers overgrowing the strings indicates a linearly extended amidinium-carboxylate interfacial structure of the base and acid component in alternating sequence.
Keywords: Interfacial molecular recognition; Monolayers; Supramolecular chemistry; Brewster angle microscopy; Grazing incidence X-ray diffraction; Quantum chemical calculations;

Development and optimization of porphyrin gas sensing LB films by T.H. Richardson; C.M. Dooling; L.T. Jones; R.A. Brook (81-96).
A series of meso-substituted metal-free porphyrins has been developed which show high sensitivity to NO2 gas in the sub-5 ppm concentration range. By selecting different substituents, it has been possible to improve in a systematic manner the response time and sensitivity of the porphyrin LB film to NO2. Initially, a sulphonamino substituent yielded a fairly long response time of around 450 s but this was shortened considerably when this substituent was changed for a stearamido group. Further modifications resulted in achieving a porphyrin LB film which exhibited a t 50 response time of only 11 s. By using an optically inert calixarene host material in which the porphyrin guest was incorporated, it was possible to obtain t 50 values as low as 5 s.

Kinetics of photochromic reactions in condensed phases by J. Sworakowski; K. Janus; S. Nešpůrek (97-110).
A review of kinetic data reported for a few organic photochromic systems is given. The kinetics of processes taking place in solid matrices and in crystals was briefly discussed. The effect of solid matrices manifests itself in the kinetics being controlled by distributions rather than by discrete rate constants. The photochromic reactions often require a substantial free volume to occur, hence they seldom take place in crystals without a destruction of the crystal lattice. The activation energies of thermally driven reactions are in this case related rather to crystal parameters than to the reactions themselves.
Keywords: Photochromism; Kinetics; Rate constant; Polymer matrix; Crystal;

Thirty years of haemoglobin electrochemistry by Frieder W. Scheller; Nikitas Bistolas; Songqin Liu; Michael Jänchen; Martin Katterle; Ulla Wollenberger (111-120).
Electrochemical investigations of the blood oxygen carrier protein include both mediated and direct electron transfer. The reaction of haemoglobin (Hb) with typical mediators, e.g., ferricyanide, can be quantified by measuring the produced ferrocyanide which is equivalent to the Hb concentration. Immobilization of the mediator within the electrode body allows reagentless electrochemical measuring of Hb. On the other hand, entrapment of the protein within layers of polyelectrolytes, lipids, nanoparticles of clay or gold leads to a fast heterogeneous electron exchange of the partially denatured Hb.
Keywords: Redox reactivity; Amperometric Hb determination; Acetylated or glycated haemoglobin; Modified electrodes;

Our recent studies in the field of ultrathin membranes prepared upon layer-by-layer assembly of various polyionic compounds such as polyelectrolytes, calixarenes and polyelectrolytes, and metal hexacyanoferrate salts such as Prussian Blue are reviewed. It is demonstrated that polyelectrolyte multilayers can be used (a) as nanofiltration and reverse osmosis membranes suitable for water softening and seawater desalination and (b) as molecular sieves and ion sieves for size-selective separation of neutral and charged aromatic compounds. Furthermore, hybrid membranes of p-sulfonato-calixarenes and cationic polyelectrolytes showing specific host–guest interactions with permeating ions are described. The membranes exhibit high selectivities for distinct metal ions. Finally, it is demonstrated that purely inorganic membranes of Prussian Blue (PB) and analogues can be prepared upon multiple sequential adsorption of transition metal cations and hexacyanoferrate anions. Due to the porous lattice of PB, the membranes are useful as ion filters able to separate cesium from sodium ions, for example.
Keywords: Polyelectrolyte; Multilayer; Membrane; Layer-by-layer; Nanofiltration; Reverse osmosis; Calixarene; Prussian Blue; Ion separation;

Photoinduced phenomena in corona poled polar organic films by I. Muzikante; E. Fonavs; L. Brehmer; B. Stiller (133-141).
Organic materials have received considerable attention because of their large dipole moments and optical nonlinearities. The optically induced switching of material properties is important for studying the optoelectronic effects including second harmonic generation. Organic materials for photonic applications contain chromophore dipole which consist of acceptor and donor groups bridged by a delocalized π-electron system. Both theoretical and experimental data show a reversible highly dipolar photoinduced intra molecular charge transfer in betaine type molecules accompanied by change of the sign and the value of the dipole moment.The arrangement of polar molecules in films is studied both by atom force microscopy and surface potential measurements. To understand the photo response of these materials, their spectroscopic and electrical properties are studied. The morphology and photoinduced surface potential switching of the self-assembled monolayers and polymer films are investigated.
Keywords: Polar betaine-type molecules; Photoinduced intramolecular charge transfer; Host–guest polymers; Chemically attached polymers; Surface potential; Optical switching; AFM;

Self-organization of substituted 1,3,4-oxadiazoles in the solid state and at surfaces by Burkhard Schulz; Ingo Orgzall; Anke Freydank; Chenggang Xü (143-164).
Different aspects of the structure formation for a class of molecules containing the diphenyl-1,3,4-oxadiazole fragment are discussed. Starting from the bulk state with the ideal crystal lattice and the derivation of some common packing motifs the formation of liquid-crystalline states are described. This leads to the consideration of structures found in Langmuir–Blodgett films and those obtained by organic molecular beam deposition. These structures may again be compared to those for the bulk crystalline state. Common features as well as characteristic differences due to peculiarities of the individual molecular structures are discussed.
Keywords: Oxadiazoles; Molecular crystals; Liquid crystals; Vacuum deposition; Self-organization;

Polymer based interfaces as bioinspired ‘smart skins’ by Danilo De Rossi; Federico Carpi; Enzo Pasquale Scilingo (165-178).
This work reports on already achieved results and ongoing research on the development of complex interfaces between humans and external environment, based on organic synthetic materials and used as smart ‘artificial skins’. They are conceived as wearable and flexible systems with multifunctional characteristics. Their features are designed to mimic or augment a broad-spectrum of properties shown by biological skins of humans and/or animals. The discussion is here limited to those properties whose mimicry/augmentation is achievable with currently available technologies based on polymers and oligomers. Such properties include tactile sensing, thermal sensing/regulation, environmental energy harvesting, chromatic mimetism, ultra-violet protection, adhesion and surface mediation of mobility. Accordingly, bioinspired devices and structures, proposed as suitable functional analogous of natural architectures, are analysed. They consist of organic piezoelectric sensors, thermoelectric and pyroelectric sensors and generators, photoelectric generators, thermal and ultra-violet protection systems, electro-, photo- and thermo-chromic devices, as well as structures for improved adhesion and reduced fluid-dynamic friction.
Keywords: Bioinspired; Electroactive polymers; Interface; Multifunctional; Smart skin; Wearable;

Optical storage and surface-relief gratings in azobenzene-containing nanostructured films by Osvaldo N. Oliveira; David S. dos Santos; Débora T. Balogh; Valtencir Zucolotto; Cleber R. Mendonça (179-192).
This paper brings an overview of photoisomerization-derived properties in azobenzene-containing nanostructured films produced with the Langmuir–Blodgett (LB) and layer-by-layer (LbL) methods. Emphasis was placed on the optical storage and formation of surface-relief gratings (SRGs), where the distinctive properties of the nanostructured films were highlighted. For optical storage, in particular, a discussion was made of the higher birefringence induced in LB films from azopolymers due to their organized nature, and of the strong effects from ionic interactions on the photoisomerization of azochromophores in LbL films. The molecular-level control of film properties in the LbL films is described as being responsible for the varied behavior observed in the photoinscription of SRGs, which may involve considerable level of photodegradation depending on the film fabrication conditions and materials used.
Keywords: Azobenzene; Photoisomerization; Optical storage; Surface-relief gratings; Langmuir–Blodgett; Layer-by-layer;

Langmuir–Blodgett films based on inorganic molecular complexes with magnetic or optical properties by M. Clemente-León; E. Coronado; A. Soriano-Portillo; C. Mingotaud; J.M. Dominguez-Vera (193-203).
Keywords: Langmuir–Blodgett films; Magnetic properties; Mn12; Polyoxometalates; Ferritin; Single molecule nanomagnets; Superparamagnetism; Electrochromism;

Enzyme association with lipidic Langmuir–Blodgett films: Interests and applications in nanobioscience by Agnès P. Girard-Egrot; Stéphanie Godoy; Loïc J. Blum (205-225).
This review presents the recent advances in the achievement of organized proteo-lipidic nanostructures based on Langmuir–Blodgett technology and their potential applications in the nanobioscience area. By using the self-assembled properties of amphiphilic biomolecules at the air–water interface, the Langmuir–Blodgett (LB) technique offers the possibility to prepare ultrathin layers suitable for biomolecule immobilization at the molecular level. This review will provide a general overview of the enzyme association with preformed Langmuir–Blodgett films in connection with their potential applications in biosensing device developments, and then introduce the design of a new functionalised biomimetic nanostructure with oriented recognition site. The potential applications of such an organized proteo-lipidic nanostructure for biocatalysis investigations of an immobilised enzyme in a biomimetic situation and for the development of bioelectronic devices are finally discussed.
Keywords: Langmuir–Blodgett films; Biomimetic nanostructure; Protein orientation; Biosensing devices; Enzyme kinetic behaviour;

The role of interface between molecular material and electrode on currents and photocurrents is considered. Mechanisms of charge carrier injection, electrode recombination and transport are discussed. Particularly thermal, excitonic, photo and tunneling injection of charge carriers, diffusion in presence of image force, interface barrier between electrode and organic materials and two organic materials, non-uniformity of electrodes and other phenomena on charge carrier injection are considered. The data presented in the review which complete theoretical considerations have been taken from previous as well as current literature. The considered phenomena are very important for the analysis of many practical problems for molecular electronic devices such as rectification of current, organic transistors, electroluminescence, photovoltaic effects and some similar problems.
Keywords: Organic; Interface; Injection; Photoinjection; Excitons;

Keywords: Nanostructures; Nanomaterials; Gas sensors; Sensitivity;

Differential approach to the study of integral characteristics in polymer films by P. Smertenko; L. Fenenko; L. Brehmer; S. Schrader (255-261).
The differential approach is based on the determination of dimensionless differential slope, for instance, of current–voltage characteristics (IVC), I  =  f(V). This slope (α) is given by formula α  = d(lgI)/d(lgV). With such definition the ranges of constancy of the α(V) dependency correspond to the part of IVC characterized by the power behaviour (I  ∼  V α ). The differential slope of α(V) dependency γ  = d(lgα)/dlgV determines the exponent behaviour of curve (I  ∼ exp{eV γ /kT}). Processing by the differential approach of the investigated theoretical or experimental characteristics permits us to determine the peculiarity of charge flow mechanisms, temperature behaviour of conductivity, etc. The theoretical base and some applications of differential approach to the investigation of the current–voltage, temperature and degradation characteristics of the polyaniline and poly(p-phenilenevinilene) based structures have been shown.
Keywords: Differential approach; Dimensionless differential slope; Charge flow mechanism; Current–voltage characteristic;

This paper reviews the fabrication of organic and metal nanoclusters in polymer matrices by three co-deposition techniques. In particular, the structure and properties of polytetrafluoroethylene (PTFE), polychlortrifluoroethylene (PCTFE), polyparaphenylene sulphide (PPS), polystyrene (PS) and polyparaxylylene (PPX) films, containing gold (Au) and dye clusters are discussed. For the first time, dye-filled polymers and multi-component films, consisting of both Au nanoparticles and dye molecules, dispersed in the PTFE matrix were studied. A low temperature plasma was used for film structure modification.Cluster formation process was studied using optical spectroscopy in situ. Transmission electron microscopy (TEM), atomic force microscopy (AFM) and ellipsometry were used for characterisation of the grown films. During Au–PTFE film growth plasmon band shifted from 460–480 nm to 560 nm. Au cluster diameter was in the 3–7 nm range. Plasma treatment of the vapours led to formation of smaller, but more aggregated clusters. During Au–PPS film deposition a two-step growth mechanism was discovered. At the beginning of film growth the plasmon band at 540 nm appeared, but as thickness increased, the band at 430 nm dominated. Without plasma treatment a disordered mixture was deposited, while with plasma treatment large Au aggregates confined with PPS matrix having plasmon band at 620 nm were formed.Dye cluster formation depends on the dye ability to aggregate, its concentration and the properties of the polymer matrix. But cluster formation can also be tuned by varying the deposition conditions. Laser beam evaporation promoted cluster formation, while plasma treatment and dilution in a polymer matrix prevented cluster formation. In all cases both equilibrium and non-equilibrium film structure can be formed using kinetic factor. Asymmetric molecules with bulky substituents were oriented in polymer matrices by applying an electric field in situ or by corona poling. These molecules did not aggregate even at high dye load. The films exhibited second harmonic generation, which demonstrated chromophore orientation in the polymer matrices.
Keywords: Nanocluster; Dye; Polymer; Co-deposition; Thin film;

Silicon-containing polyoxadiazoles—synthesis and perspectives by Maria Bruma; Thomas Köpnick (277-290).
A review is presented on those polymers which contain diphenylsilylene units and 1,3,4-oxadiazole rings either in the main chain or in the pendent groups. The synthesis of silicon-containing monomers as well as of the polyoxadiazoles based on them is described. The properties of these polymers, such as solubility, film forming ability, thermal stability, electrochemical behavior and photoluminescence properties, and their potential applications are discussed.
Keywords: Polyoxadizoles; Polycondensation; Polyheterocycles;