New hybrid films based on cellulose and hydroxygallium phthalocyanine. Synergetic effects in the structure and properties

Ammonia and Humidity Sensing by Phthalocyanine-Corrole Complex Heterostructure Devices

The versatility of metal complexes of corroles has raised interest in the use of these molecules as elements of chemical sensors. The tuning of the macrocycle properties via synthetic modification of the different components of the corrole ring, such as functional groups, the molecular skeleton, and coordinated metal, allows for the creation of a vast library of corrole-based sensors. However, the scarce conductivity of most of the aggregates of corroles limits the development of simple conductometric sensors and requires the use of optical or mass transducers that are rather more cumbersome and less prone to be integrated into microelectronics systems. To compensate for the scarce conductivity, corroles are often used to functionalize the surface of conductive materials such as graphene oxide, carbon nanotubes, or conductive polymers. Alternatively, they can be incorporated into heterojunction devices where they are interfaced with a conductive material such as a phthalocyanine. Herewith, we introduce two heterostructure sensors combining lutetium bisphthalocyanine (LuPc2) with either 5,10,15-tris(pentafluorophenyl) corrolato Cu (1) or 5,10,15-tris(4-methoxyphenyl)corrolato Cu (2). The optical spectra show that after deposition, corroles maintain their original structure. The conductivity of the devices reveals an energy barrier for interfacial charge transport for 1/LuPc2, which is a heterojunction device. On the contrary, only ohmic contacts are observed in the 2/LuPc2 device. These different electrical properties, which result from the different electron-withdrawing or -donating substituents on corrole rings, are also manifested by the opposite response with respect to ammonia (NH3), with 1/LuPc2 behaving as an n-type conductor and 2/LuPC2 behaving as a p-type conductor. Both devices are capable of detecting NH3 down to 10 ppm at room temperature. Furthermore, the sensors show high sensitivity with respect to relative humidity (RH) but with a reversible and fast response in the range of 30-60% RH.

Ultrathin Films of Cellulose: A Materials Perspective

A literature review on ultrathin films of cellulose is presented. The review focuses on different deposition methods of the films-all the way from simple monocomponent films to more elaborate multicomponent structures-and the use of the film structures in the vast realm of materials science. The common approach of utilizing cellulose thin films as experimental models is therefore omitted. The reader will find that modern usage of cellulose thin films constitutes an exciting emerging area within materials science and it goes far beyond the traditional usage of the films as model systems.

A ZnO/porous GaN heterojunction and its application as a humidity sensor

A heterojunction of ZnO/porous GaN (ZnO/PGAN) was fabricated and directly applied to a diode-type humidity sensor. ZnO disks were loaded onto PGAN using a spraying process. The structure and surface morphology of the ZnO/PGAN were characterized using X-ray diffraction and scanning electron microscopy. The heterojunction displayed an excellent diode nature, which was investigated using photoluminescence spectra and I-V characteristics. The excellent transport capability of ZnO/PGAN contributes to enhanced electron transfer, and hence results in high sensitivity and quick response/recovery properties under different relative humidity (RH) levels. In the range of 12-96% RH, a fast sensing response time as low as 7 s and a recovery time of 13 s can be achieved. The simple design of a ZnO/PGAN based humidity sensor highlights its potential in various applications.

Cellulose films: designing template-free nanoporous cellulose films on semiconducting surfaces

In this work, we report the preparation of ultrathin submicro- and nanoporous cellulose films onto Si (100). The effect of different experimental conditions of preparation on the film surface morphology was studied, namely the role of the film casting method (spin- versus dip-coating), solvent (toluene or tetrahydrofuran), substrate pretreatment (hydrophilicity degree), and regeneration procedure with HCl vapors (two consecutive dips followed by regeneration or regeneration after each dip). The surface morphological structures presented in this work were never obtained before without the use of templates. A rather regular two-dimensional pore network was obtained onto the less hydrophilic Si substrate (contact angle≅68°), after two consecutive dips (with an intercalary rotation of 180º) in trimethylsilyl cellulose diluted in toluene and regeneration at the end. All the surfaces were characterized by atomic force microscopy.

From the solution processing of hydrophilic molecules to polymer-phthalocyanine hybrid materials for ammonia sensing in high humidity atmospheres

We have prepared different hybrid polymer-phthalocyanine materials by solution processing, starting from two sulfonated phthalocyanines, s-CoPc and CuTsPc, and polyvinylpyrrolidone (PVP), polyethylene glycol (PEG), poly(acrylic acid-co-acrylamide) (PAA-AM), poly(diallyldimethylammonium chloride) (PDDA) and polyaniline (PANI) as polymers. We also studied the response to ammonia (NH₃) of resistors prepared from these sensing materials. The solvent casted films, prepared from s-CoPc and PVP, PEG and PAA-AM, were highly insulating and very sensitive to the relative humidity (RH) variation. The incorporation of s-CoPc in PDDA by means of layer-by-layer (LBL) technique allowed to stabilize the film, but was too insulating to be interesting. We also prepared PANI-CuTsPc hybrid films by LBL technique. It allowed a regular deposition as evidenced by the linear increase of the absorbance at 688 nm as a function of the number of bilayers. The sensitivity to ammonia (NH₃) of PANi-CuTsPc resistors was very high compared to that of individual materials, giving up to 80% of current decrease when exposed to 30 ppm NH₃. Contrarily to what happens with neutral polymers, in PANI, CuTsPc was stabilized by strong electrostatic interactions, leading to a stable response to NH₃, whatever the relative humidity in the range 10%–70%. Thus, the synergy of PANI with ionic macrocycles used as counteranions combined with their simple aqueous solution processing opens the way to the development of new gas sensors capable of operating in real world conditions.

Phthalocyanine-based hybrid materials for chemosensing

In the present review, we show how the chemical variability of phthalocyanines allowed to synthesize a broad range of hybrid materials. The combination of phthalocyanines or related derivatives with polymers or carbonaceous materials led to efficient chemical sensors. It is shown how the incorporation of macrocyclic molecules in hybrid materials highly modifies the structural and morphological characteristics of the materials. Rugosity, specific surface and porosity being key parameters in the analyte-sensing material interactions, these modifications highly improve the performance of chemical sensors. This is the reason why they are particularly promising materials for the development of new chemical sensors, associated with electrochemical, conductometric or optical transducers.

Humidity effect on ammonia sensing properties of substituted and unsubstituted cobalt phthalocyanines

In this paper, we studied the effect of humidity on the response of cobalt phthalocyanine-containing resistors to ammonia, in the ppm range. We pointed out the fact that, when alternating exposure periods with recovery periods, the humidity effect had to be carefully studied, in correlation with the flow variation. Thus, for a sulfonated cobalt phthalocyanine, the effect of NH(3) was totally screened as soon as the relative humidity (RH) was above 10%. On the contrary, when using unsubstituted cobalt phthalocyanine (CoPc) as sensing material, the sensors' response to NH(3) appears to be quite stable in a wide RH range, allowing a discrimination between 12, 25 and 50 ppm of NH(3) over the 10-70% RH range. Finally, CoPc offers a promising perspective as sensing material for air quality control applications, even at relatively high humidity levels.

Nanostructured thin films based on phthalocyanines: electrochromic displays and sensors

The group of the University of Valladolid is a multidisciplinary team formed by chemists, physicists and engineers. The activities of the group are focused to the study of the physicochemical properties of nanostructured Langmuir-Blodgett thin films based on phthalocyanines and their applications. Films of a variety of phthalocyanine molecules including several metallophthalocyanines, lanthanide double decker phthalocyanines and heteroleptic derivatives have been prepared. Their spectroelectrochemical properties have been described in detail and compared with those observed in disordered casted films or microcrystalline evaporated films. The group has dedicated special attention to films based on rare earth double decker compounds due to their unique semiconducting, optical and electrochemical properties. A rich electrochromism has been demonstrated in thin films of this family of compounds. The reversibility is improved in nanostructured Langmuir-Blodgett films. This has permitted development of an electrochromic display that can change its color from blue to green and finally to red. At the present moment, our main objective is the design of sensors able to detect gases and liquids. It has been demonstrated that thin film assemblies based on rare earth bisphthalocyanines modify their conductivity and their optical properties in the presence of electron donor or electron acceptors gases. Changes are also observed when the devices are exposed to Volatile Organic Compounds such as esters, alcohols or aldehydes which are responsible of odors in foods and beverages. Liquid sensors have also been developed. Their working principle is based in the fact that the rich electrochemical properties of phthalocyanine thin films are extremely sensitive to the nature of the electrolytic solution. Arrays of phthalocyanines have been used to construct an electronic nose able to discriminate odors from a variety of foods and beverages. Similarly, phthalocyanines have also been used to construct an electronic tongue based on voltammetric sensors. This is one of the main contributions of the group to the field of sensors.

Hybrid systems of silver nanoparticles generated on cellulose surfaces

A method to produce hybrid systems of cellulose ultrathin films containing immobilized silver nanoparticles (Ag NPs) generated and grown at the surface is presented. Ag NPs were produced via a mild wet chemistry technique on cellulose ultrathin films spin-coated on GaAs substrates and on modified films after grafting of diaminoalkanes activated by N,N'-carbonyldiimidazole. Appended amine groups operate as anchoring centers of the silver NPs enabling selective generation and immobilization of Ag NPs. The different phases of the modification process were followed by Fourier transform infrared spectroscopy (FTIRS) in attenuated total reflection in multiple internal reflections (ATR/MIR), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). The generation of NPs is observed even on untreated cellulose surfaces with sizes ranging from 7 to 30 nm but not specifically at the surface. For modified surfaces with diaminoalkanes, higher NP density regions including extensive plates are obtained, which are specifically located at the film extreme surface. The highest NP density is achieved when the NP generation is performed on these modified surfaces in the presence of a carboxylic salt.

Grafting of porphyrins on cellulose nanometric films

Ultrathin films of cellulose were functionalized with iron protoporphyrin IX (FePP). Spin-coating allows the production of silylated cellulose films in a controlled way. Cellulose regeneration is achieved through the hydrolyzation of the silane groups, exposing the film to acidic vapors. To enhance the reactivity of the cellulose surface to the protoporphyrin, carbonyldiimidazole (CDI) was used as an activator. The effect of different spacers on the porphyrin grafting such as 1,8-diaminooctane and 1,4-phenylenediamine was studied. The highest level of cellulose functionalization with FePP was achieved when both the cellulose film and FePP were activated by CDI and a diaminoalkane was used as a spacer between the surface and the FePP. ATR/MIR (attenuated total reflection in multiple internal reflections) was performed in situ to follow the kinetics of the different chemical reactions with the cellulose surface. ATR/MIR proved again to be a powerful tool for probing the surface reaction. X-ray photoelectron spectroscopy permitted the elemental analysis of the cellulose surface after the chemical modification.