José M. Pedrosa

The optical gas-sensing properties of an asymmetrically substituted porphyrin

In this paper we have investigated the NO2 gas-sensing properties of LB film assemblies of 5,15-bis(4-aminophenyl)-10,20-bis[3,4-bis(2-ethylhexyloxy)phenyl]-21H,23H-porphine (CAH4). The optical absorbance spectrum of these films is dramatically affected when exposed to low concentrations of NO2 gas. LB films of CAH4 were prepared by using ultra-fast deposition and characterized by imaging ellipsometry. The high deposition rates employed (500 mm min−1) led to an inhomogeneous structure with high porosity. The LB film exposed to 4.6 ppm NO2 showed a sensitivity of 60% relative absorbance change at 439 nm. The response was found to be faster than that measured in similar systems. The fast response can be explained in terms of the molecular structure of the porphyrin as well as the enhanced surface area of the porous film. The optical response of the CAH4 film gradually decreases as its temperature is increased, a result of a shift in the adsorption–desorption equilibrium towards desorption. An activation energy of 0.48 eV is obtained. Full recovery of the original spectrum after exposure to NO2 is obtained and can be dramatically accelerated with gentle heating (353 K). The concentration dependence of the optical response over the range 0.46–4.6 ppm NO2 obeyed a Langmuir adsorption model. Ageing experiments have shown that the basic response of the CAH4 assemblies is not affected over a time period of at least 1 year.

Electrochromic behavior of WxSiyOz thin films prepared by reactive magnetron sputtering at normal and glancing angles

This work reports the synthesis at room temperature of transparent and colored W(x)Si(y)O(z) thin films by magnetron sputtering (MS) from a single cathode. The films were characterized by a large set of techniques including X-ray photoelectron spectroscopy (XPS), Rutherford backscattering spectrometry (RBS), Fourier transform infrared (FT-IR), and Raman spectroscopies. Their optical properties were determined by the analysis of the transmission and reflection spectra. It was found that both the relative amount of tungsten in the W-Si MS target and the ratio O(2)/Ar in the plasma gas were critical parameters to control the blue coloration of the films. The long-term stability of the color, attributed to the formation of a high concentration of W(5+) and W(4+) species, has been related with the formation of W-O-Si bond linkages in an amorphous network. At normal geometry (i.e., substrate surface parallel to the target) the films were rather compact, whereas they were very porous and had less tungsten content when deposited in a glancing angle configuration. In this case, they presented outstanding electrochromic properties characterized by a fast response, a high coloration, a complete reversibility after more than one thousand cycles and a relatively very low refractive index in the bleached state.

Selective Detection of Volatile Organic Compounds by Spectral Imaging of Porphyrin Derivatives Bound to TiO2 Porous Films

In this work, the carboxylic acid derivatives of a free-base porphyrin, 5,10,15,20-tetrakis(4-carboxyphenyl)-21H,23H-porphyrin, and 10 of its metal derivatives (TCPPs) have been used for optical gas sensing. For this purpose, microstructured columnar TiO(2) thin films prepared by GAPVD (glancing angle physical vapor deposition) have been used as host materials for the porphyrins as they are non-dispersive and porous, allowing their use for UV-visible spectroscopy and gas sensing. The chemical binding between the dye molecules and the TiO(2) has been studied through infrared spectroscopy, and the obtained spectral changes have been found to be compatible with chelating and/or bidentate binding modes of the carboxylate groups on the TiO(2) surface. When hosted in the film, the UV-visible spectra of the porphyrins featured a blue shift and broadening of the Soret band with respect to the solution, which has been attributed to the formation of π-π aggregates between porphyrin molecules. The composite porphyrin/TiO(2) films obtained from each of the 11 porphyrins have been exposed to 12 different volatile organic compounds (VOCs), and their respective gas-sensitive properties have been analyzed as a function of the spectral changes in their Soret band region in the presence of the analytes. The set of composite films has shown high selectivity to the analyzed volatile compounds. For each analyte, an innovative way of showing the different responses has been developed. By means of this procedure, an imagelike recognition pattern has been obtained, which allows an easy identification of every compound. The kinetics of the exposure to several analytes showed a fast, reversible and reproducible response, with response times of a few seconds, which has been attributed to both the sensitivity of the porphyrins and the high porosity of the TiO(2) films. Also, increasing concentrations of the analytes resulted in an increase in the magnitude of the response, indicating that the sensor behavior is also concentration-dependent.

Aggregate formation in mixed monolayers at the air–water interface of metal-complex tetracationic water-soluble porphyrins attached to a phospholipid matrix

The conformational behaviour of mixtures of the metallated Ni-TMPyP/DMPA and Co-TMPyP/DMPA, with molar ratio 1∶4, has been investigated at the air–water interface. In order to characterize the interactions of the metallated TMPyP derivatives with DMPA, different surface methods have been used. Thus, the surface pressure–surface area, π–A, and surface potential–surface area, ΔV–A, isotherms were measured and valuable information about the presence of the metallated porphyrin molecules underneath the lipid matrix of DMPA, as well as the stability and the hysteresis phenomena of these monolayers was obtained. To reveal the molecular organization of the metallated porphyrins in the mixed monolayers, the reflection spectra of the mixed monolayers at the air–water interface were measured. Additional information on the organization of Ni-TMPyP and Co-TMPyP was obtained by using Brewster angle microscopy (BAM). An analysis of the molecular organization of Ni-TMPyP and Co-TMPyP in a mixed monolayer containing DMPA as lipid anchor, by using the oscillator strength and in accord with the extended dipole approximation, leads us to propose different organization models of these metal-complex porphyrins at the air–water interface depending upon the coordination number of the metal ion.

Preparation of Luminescent Metal-Organic Framework Films by Soft-Imprinting for 2,4-Dinitrotoluene Sensing

A novel technique for the creation of metal-organic framework (MOF) films based on soft-imprinting and their use as gas sensors was developed. The microporous MOF material [Zn2(bpdc)2(bpee)] (bpdc = 4,4′-biphenyldicarboxylate; bpee = 1,2-bipyridylethene) was synthesized solvothermally and activated by removing the occluded solvent molecules from its inner channels. MOF particles were characterized by powder X-ray diffraction and fluorescence spectroscopy, showing high crystallinity and intense photoluminescence. Scanning electron microscope images revealed that MOF crystals were mainly in the form of microneedles with a high surface-to-volume ratio, which together with the high porosity of the material enhances its interaction with gas molecules. MOF crystals were soft-imprinted into cellulose acetate (CA) films on quartz at different pressures. Atomic force microscope images of soft-imprinted films showed that MOF crystals were partially embedded into the CA. With this procedure, mechanically stable films were created, with crystals protruding from the CA surface and therefore available for incoming gas molecules. The sensing properties of the films were assessed by exposing them to saturated atmospheres of 2,4-dinitrotoluene, which resulted in a substantial quenching of the fluorescence after few seconds. The soft-imprinted MOF films on CA/quartz exhibit good sensing capabilities for the detection of nitroaromatics, which was attributed to the MOF sensitivity and to the novel and more efficient film processing method based on soft-imprinting.

Optical Gas Sensing of Ammonia and Amines Based on Protonated Porphyrin/TiO2 Composite Thin Films

Open porous and transparent microcolumnar structures of TiO2 prepared by physical vapour deposition in glancing angle configuration (GLAD-PVD) have been used as host matrices for two different fluorescent cationic porphyrins, 5-(N-methyl 4-pyridyl)-10,15,20-triphenyl porphine chloride (MMPyP) and meso-tetra (N-methyl 4-pyridyl) porphine tetrachloride (TMPyP). The porphyrins have been anchored by electrostatic interactions to the microcolumns by self-assembly through the dip-coating method. These porphyrin/TiO2 composites have been used as gas sensors for ammonia and amines through previous protonation of the porphyrin with HCl followed by subsequent exposure to the basic analyte. UV–vis absorption, emission, and time-resolved spectroscopies have been used to confirm the protonation–deprotonation of the two porphyrins and to follow their spectral changes in the presence of the analytes. The monocationic porphyrin has been found to be more sensible (up to 10 times) than its tetracationic counterpart. This result has been attributed to the different anchoring arrangements of the two porphyrins to the TiO2 surface and their different states of aggregation within the film. Finally, there was an observed decrease of the emission fluorescence intensity in consecutive cycles of exposure and recovery due to the formation of ammonium chloride inside the film.

Determination of porphyrin dimer in a mixed monolayer of porphyrin/ phospholipid transferred on ITO electrodes

Abstract The Langmuir–Blodgett method has been used to transfer mixed monolayers of a porphyrin (TMPyP) and a phospholipid (DMPA) from the air|water interface onto optically transparent indium–tin oxide (ITO) electrodes. The surface concentration of porphyrin, Γ , transferred on the ITO surface, has been obtained by integration of the reduction current from the cyclic voltammograms. The experimental Γ values ranged from Γ m =5.19×10 −11 mol cm −2 , and corresponding to a compact monolayer of porphyrin monomers in a plane orientation with respect to the surface, to Γ d =8.65×10 −11 mol cm −2 , and equivalent to the total amount of the porphyrin molecules at the air|water interface under a compression of 35 mN m −1 . Prior to the electrochemical experiments, the transmission spectrum was recorded. The surface concentration obtained of the porphyrin is not directly proportional to the transmission of the film, Δ T . This phenomenon is assigned to the dimer formation and, depending on the surface activity of the ITO electrodes, toward porphyrin adsorption. The dimer structure of TMPyP on an intact ITO electrode is altered with respect to that found at the air|water interface. A simple model has been developed to evaluate the contribution of monomer and dimer phases of the porphyrin in the mixed monolayer. Furthermore, spectroscopic measurements with linearly polarized light under oblique incidence have been performed in order to infer the plane orientation of the TMPyP molecules with respect to the ITO surfaces.

Highly pH-responsive sensor based on amplified spontaneous emission coupled to colorimetry

We demonstrated a simple, directly-readable approach for high resolution pH sensing. The method was based on sharp changes in Amplified Spontaneous Emission (ASE) of a Stilbene 420 (ST) laser dye triggered by the pH-dependent absorption of Bromocresol Green (BG). The ASE threshold of BG:ST solution mixtures exhibited a strong dependence on BG absorption, which was drastically changed by the variations of the pH of BG solution. As a result, ASE on-off or off-on was observed with different pH levels achieved by ammonia doping. By changing the concentration of the BG solution and the BG:ST blend ratio, this approach allowed to detect pH changes with a sensitivity down to 0.05 in the 10–11 pH range.

Molecular organization and electrochemical reduction of a Ni(II)Porphyrin complex in LB films

Abstract In this work, mixed films of a tetra-cationic porphyrin, Ni(II)TMPyP, and an anionic phospholipid, DMPA, in molar ratio of 1:4, were formed at the air–water interface and transferred onto glass and optically transparent indium tin oxide (ITO) electrodes. Transmission spectroscopy (on glass and ITO) and cyclic voltammetry (on ITO) were used to infer the molecular organization and the electrochemical reduction of these LB films. Likewise, we compare the electrochemical reduction of the Ni(II)TMPyP in water solution with that in LB films. The porphyrin molecules in water solution show three two-electron reduction waves, which are related to the two-electron reduction of the central ring of the porphyrin and to the one-electron reductions of the four methyl–pyridyl groups of the molecule, respectively, while only two reversible one-electron reduction waves are observed in LB films corresponding to the reduction of the central ring of the porphyrin and to the Ni(II) to Ni(I) reduction, respectively.