Chemical Sensors for Hostile Environments


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However, the codification is not trivial, and the correlation between the wavelength and the responsiveness of the silicon photodetectors has to be considered. The optimal measure of spectral changes in the blue region of chemical indicators requires a proper design of silicon detectors. The first demonstration of the analytical properties of consumer electronic devices was obtained with a flatbed scanner, a system devised by K.

Suslick at the University of Illinois. The scanner was used to image an array of a set of metalloporphyrins immobilized onto reverse silica thin-layer-chromatography plates. An interesting method to extend the set of sensed molecules including less reactive volatile compounds consists in the preoxidation of volatile compounds in a tube filled with chromic acid on silica Figure This approach was also found appropriate to detect volatile compounds relevant to diseases such as lung cancer in vivo and to identify pathogenic microorganisms.

The preparation of the sensitive spots was optimized along the years from the original porphyrins spotted on a silica gel layer to a mix with plasticizers and finally to nanoporous pigments created from the immobilization of dyes in ormosils. In general, plasticizer formulations were preferred for polypropylene while ormosil is the optimal choice for polyvinylidene fluoride. The two indicators were used to characterize artificial saliva modified with the addition of alleged markers of stomach cancer such as NH 3 and CO 2.

In the past few years, the method devised by Suslick has attracted the interest of several researchers, resulting in a number of applications of similar colorimetric arrays in different studies. Starting from ammonia detection and quantification, the majority of these articles are devoted to the quality assessment of food matrices, such as the determination of the freshness of meat, , fish, and beverages such as tea, , liquor, rice wine, and vinegar.

All these papers were supported by a generic illustration of the interaction mechanisms between the VOCs and the various indicators. An attempt to study the interactions between VOCs and metallo tetraphenylporphyrin was performed with density functional theory. Results are in fair agreement with the experiments, although they are still hardly exploitable for a practical sensor array design. This captivating representation is inherent in the RGB codification of the absorbance spectrum of the dye.

The color indeed is a synthesis of the whole spectrum, typically from to nm integrated over three filtering curves approximating the spectral response of three color receptors in humans. Suslick introduced a simple method to display the spectral changes calculating the absolute difference between the RGB colors of the dye during the exposure to the gaseous sample and before being exposed to a reference air. However, the representation as a color is slightly deceiving because colors are positively defined while the spectral changes can be either positive or negative.

In particular, in the case of a spectral shift, if the Soret band in the blue region moves toward larger wavelengths then the intensity of blue channels decreases and the intensity of green channels increases. The color representation, being based on absolute values, cannot discriminate the situation depicted above from the opposite case when the shift occurs toward smaller wavelengths.


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The approach introduced by Suslick was extended, complementing the image sensor device with a computer controlled light source. The most obvious source is the computer screen, which can be programmed to deliver light controlling both position and color. This approach is called computer screen-photoassisted technique CSPT , and the first demonstration of the concept, applied to bio chemically sensitive optical reporters, was given in Since the emission spectra of displays and the spectral response of digital cameras are known, the CSPT fingerprint can be theoretically calculated from the absorbance spectra and the overall method can be properly calibrated.

CSPT captures specifically composed subjects optically responsive chemical sensing devices under conditioned illumination in order to enhance the analytical information. Just like in photography, every indicator is a unique subject, and the technique needs to be adapted to capture its character. The separation between illumination and detection enables the measurement of both absorbance and luminescence.

However, fluorophores on transparent surfaces emit most fluorescence within the substrate and above the critical angle, confining most of the light within the substrate. To this end, arrays of transparent SU8 micromachined pillar structures, with a height of a few tens of micrometers, were demonstrated to provide an efficient fluorescence collection. CSPT is frequently used to measure the changes of optical properties of porphyrins. The first paper documenting its use for a sensor array based on porphyrins appeared in A rainbow of 50 colors from purple to red monitors the interaction between gas molecules ammonia, amines, ethanol, CO, and NO and receptors.

For each illuminating color, the camera captured an image composed of red, green, and blue channels. Each measurement results in a fingeprinting vector of elements, 50 illumination colors for each of the 3 camera channels. CSPT was used to study the optical properties of porphyrin nanotubes achieved by the ionic self-assembly of two oppositely charged porphyrins.

However, the analysis of the CSPT fingerprint allowed the identification of other compounds such as acetic acid, ethanol, and NOx. HDR is aimed at extending the dynamic range of a picture allowing for a correct exposure of luminous and dark portions of the image.

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HDR is typically performed with digital cameras processing images taken at different exposure setups, and in CSPT it was obtained changing the intensity of the light of the display. As demonstrated by Suslick, a practical use of colorimetric arrays cannot be exclusively based on porphyrins because only a restricted kind of interactions can modify porphyrin optical properties. However, interaction among porphyrins may promote additional sensing effects. Interactions among porphyrins in solid-state lead to a noteworthy broadening of the visible spectrum features.

The absorption of weakly interacting molecules, driven by, for example, van der Waals forces, could then modulate the mutual interaction between contiguous porphyrins, eliciting a detectable change of the absorbance spectrum. Results show the correct identification of 13 VOCs belonging to different families including alkanes.

The use of common equipment such as webcams and scanners raises the question about the performance of such systems. Suslick demonstrated that many toxic compounds can be detected well below their allowed limits. The change of mass is a straightforward consequence of absorption. From a relative point of view, the change of mass due to adsorption is large. However, the detection limit of the mass detector fixes the minimum number of detectable molecules. It is important to remark that mass transducers are not the primary choice of transducers.

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Indeed, contrary to electric conductivity and optical properties where the alteration requires some sort of charge transfer, and in the case of mass, the transducer detects, in principle, any form of bindings of the volatile compound onto the sensing layer, even the weak and ubiquitous van der Waals interactions. For this reason, mass transducers are seldom used as a stand-alone sensor but rather as elements of sensor arrays.

Piezoelectricity offers the grounds to the fabrication of mass detectors. Devices such as QMB and surface acoustic waves SAW are endowed with sufficient resolution for chemical sensing purposes. These sensors offer a surface that can be properly coated with the sensitive material. Then, the absorption of molecules into the sorbent layer produces a change of mass whose evaluation allows estimating the amount of adsorbed molecules. In literature, quartz microbalances have been extensively indicated using the QCM acronym; however, since quartz is a crystal itself, we consider this denomination a tad redundant, and in this paper we refer to this device as QMB.

QMBs are thin quartz plates cut along a particular crystalline direction that confers the device the thickness shear resonance mode. They thus belong to the more general class of thickness shear mode resonators. The range of the fundamental resonant frequency depends on the angle of the cut. Crystal cut may be either AT or BT. The AT cut is the most commonly used. Due to the piezoelectricity of quartz, the mechanical resonance of the crystal is coupled with its electric resonance. Since the mechanical resonance of the crystal is characterized by very low energy dissipation, the electric resonance exhibits a very large quality factor.

This property is largely exploited in electronics to build stable oscillators such as clock references. The Sauerbrey equation is strictly valid for rigid coatings that do not store elastic energy. In accordance with the Sauerbrey equation, the mass sensitivity of a QMB depends on the resonance frequency of the crystal and the resonant frequency depends on the thickness of the crystal. The thinner the quartz plate, the higher the resonant frequency and the mass sensitivity.

However, in order to ensure mechanical rigidity, thickness is never too small. The diameter of commercially available AT-cut quartzes is between 1 and 3 cm, and the resonance frequency is between 5 and 50 MHz. Quartz crystals are typically connected to oscillator circuits so the change of mass on the quartz surface results in a shift of the output signal of the oscillator.

The theoretical sensitivity eq 1 of a QMB with a fundamental frequency of 20 MHz and a diameter of 1 cm is 1. The wider use of QMBs is in thin film technology where they complement growing machines, such as sputtering and evaporator, providing a direct measurement of the amount of material deposited onto the substrate.

The use of QMBs as chemical sensors began at the beginning of the sixties when the gas sensitivity of quartz crystals coated with gas chromatographic solid phase was demonstrated. The first use of porphyrins as coating of quartz microbalances dates back to half way through the nineties, where an array of four QMB functionalized with H 2 TPPs with different metals rhodium, ruthenium, cobalt, and manganese was investigated. Results show that sensors were cross selective to alcohols and amines but each with a distinct profile of sensitivities.

These findings paved the way to the development of porphyrin-coated QMB arrays and to their applications in various fields. Among the applications worthy of mention are those aimed at identifying diseases by measuring the released volatile compounds. In , a porphyrin-based quartz microbalance provided the first evidence that lung cancer can be diagnosed measuring breath with an array of solid-state gas sensors. The role of porphyrins as sensitive materials for cancer detection is intriguing considering that oxidative stress is one of the major causes of production of cancer-related VOCs.

Further studies indicated that arrays of porphyrin-coated QMBs could also distinguish among different stages of lung cancer. The sensing properties of porphyrin solid films are strongly related to their tridimensional arrangement. For this reason, a number of efforts were made to study the sensing properties of solid films with different compositions. An example is offered by the combination of metal oxide semiconductors and porphyrins. In the previous sections, we have seen the progress made in optical and electric transduction.

To this regard, microporous organic networks have shown unique physical properties such as a high surface area and a pore size smaller than 2 nm. For instance, a microporous metal-free porphyrin network MP was formed on the surface of ZIF-8 via the Sonogashira coupling of tetrakis 4-ethynylphenyl porphyrin with 1,4-diiodobenzene.

The sensor was tested with several gases. The sensor is cross selective, and the smallest detection limit was found for 3-methylbutanol in the order of 0. QMB is also particularly suitable for humidity sensing using different nano arrangements of porphyrins in order to enhance interaction with water vapor.

A layer-by-layer growth structure of poly diallyldimethylammonium chloride and poly sodium 4-styrenesulfonate provided the basis for the deposition of Mn TPPS Cl. The spontaneous deposition of each layer of the previous element gave rise to a multistack film that revealed a constant sensitivity in the whole relative humidity range. A similar approach was shown with a polylysinated substrate onto which porphyrins are spontaneously deposited. In this case too, an excellent and constant sensitivity in the whole relative humidity range was found with H 2 TPPS.

An interesting variation of the QMB device working in solution is the electrochemical quartz crystal microbalance EQMB , where mass measurement and electrochemistry can be performed simultaneously on the same film. The technique has been applied to measure the sensitive properties of two electropolymerizable zinc II porphyrins bearing the 2-phenoxyacetamide binding group Scheme 6. Scheme 6.

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The exploitation of Mn corrole LB films as sensing layers of QMB sensors was probably one of the first applications of these macrocycles, since the peculiar chemistry of such a porphyrinoid immediately kindled interest for analyte detection. The possibility of detecting CO by corrole layers has also been reported for tris-3,5 dihydroxyphenylcorrole-functionalized QMB, although in this case, the binding mechanism cannot be simply ascribed to metal coordination. The possible exploitation of the different binding behaviors of porphyrins and corroles as sensing layers was recently investigated to prepare cross sensitive sensor arrays, by measuring the response of porphyrinoids-functionalized QMB toward model VOCs.

In particular, free base corroles showed higher sensitivities than corresponding porphyrins. These results revealed the possibility of tuning sensing layer properties by structural modifications, and that porphyrin and corrole derivatives can positively cooperate to enhance the performance of sensor arrays. Piezoelectric materials and micromachined silicon structures offer the possibility of fabricating more sophisticated mass transducers.

The principle is the same as with QMBs, namely, the link between mass and frequency in mechanical resonators. With respect to a QMB, an SAW is a planar device where an acoustic wave, traveling at the surface of a crystal, is launched and collected by pairs of interdigitated electrodes. The path between launcher and receiver is the place where a sensitive film can be applied.

As a first approximation, a SAW is sensitive to the mass of the film, just as the QMB, even if, in the case of a SAW, all mechanical properties, including rigidity and viscosity, are sensed. SAWs are operated at frequencies in the range of — MHz. They are in fact supposed to be more sensitive than QMBs whose use is devised for applications requiring low detection limits. On the other hand, it is important to remember that the resolution of a sensor, namely the minimum measurable change of analytes, is the ratio between the noise of the sensor signal and its sensitivity.

The highest frequencies, therefore the greatest sensitivities, can be achieved with resonating structures made with materials with superior piezoelectric properties such as AlN. A typical configuration is the so-called film bulk acoustic resonator FBAR where an AlN layer is sandwiched between two metal electrodes. The resonance frequency of such a structure can be on the order of GHz.

Thanks to their superior sensitivity they have mainly been used as biosensors. This is chiefly because the complexity of their fabrication and the sophisticated high-frequency electronics needed hinder their competitiveness with low-frequency devices such as QMBs and SAWs. H 2 OEP was used as a coating of a 4. It is important to consider that in FBAR devices as sensitivity to mass increases greatly, the overall dimension of the detector is smaller with respect to a less sensitive QMB. Eventually, the greatest sensitivity to mass is not paralleled by a similar improvement of the sensitivity with respect to the concentration of the analyte.

Cantilevers are another important category of mass transducers. These devices stem from the development of atomic force microscopy tips. They can be adequately coated with a sensitive film in order to measure the adsorption of volatile compounds. The variation of mass can be measured either as a change of resonance frequency or as a change of the cantilever strain. An example of porphyrin-coated cantilever was based on a SU8 micromachined cantilever functionalized 5,10,15,tetrakis 4,5-dimethoxyphenyl porphyrin iron III chloride and aimed at the detection of carbon monoxide.

The adsorption of mass was evaluated measuring the strain of the cantilever. For the scope, the nonconductive SU8 was turned into a conductor by a dispersion of carbon black nanoparticles in SU8. The porphyrin-coated cantilever was sensitive to few parts per million of CO with a good rejection of interferents such as CO 2 , O 2 , and N 2 O. Although Zn complex and free-base porphyrin were reported not to be responding, since they have no coordinating site for CO, it should be noted that also Fe III porphyrin complexes do not bind CO, so the sensing mechanism should be more complex than the metal coordination.

Electrochemical methods have often been used for sensor analysis of liquid samples, relying on the direct transformation of an electrical signal into the target analyte concentration, based on known theoretical principles of electrode processes. Unlike other transduction mechanisms, which usually consider the homogeneous solution, the electrochemical processes occur at the electrode—solution surface.

Two main groups of electrochemical processes can be used for sensor development: potentiometric and volt-amperometric methods. In the first case, no current flows in the electrochemical cell and the electrode potential is measured. In the second case, the interesting phenomenon is the current flowing in the cell due to the oxidation and reduction processes at the electrodes. Both methods require dedicated electronic setups, making use of high input impedance amplifiers.

In accordance with this schematic separation, we illustrate the different devices in the following sections. Porphyrins and their metal complexes have been amply exploited as ionophores in potentiometric ion selective electrodes ISEs. The working mechanism of these devices has been previously described in literature.

The versatile character of porphyrins as ionophores is due to their coordination chemistry, since they can act as ligand for cations as free bases, or ion carriers for anion binding, when used as metal complexes. It should be noted that porphyrin-based ISEs have also been used to detect organic species in aqueous solutions. The four nitrogen atoms present in the porphyrin macrocyclic core are probably one of the most versatile chelating systems known, and the vast majority of the elements of the periodic table have been coordinated to a porphyrin.

For this reason, porphyrins are ideal ionophores to detect cations, where their selectivities can be finely tuned by changing the peripheral substituents. Since chemical sensors require a reversible interaction, in the case of porphyrins the coordination of the metal ions is supposed to occur through the so-called sitting-atop complex, where the ion is not completely inserted in the porphyrin plane Scheme 7.

Scheme 7.


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  8. The developed ISE demonstrated a good stability and reproducibility, and its performances were tested for the detection of Hg II ion in the environmental wastewater of the Xiang River. The influence of pH in the ISE response was also studied, showing the possible use in the pH range of 2. The ISE was stable during a working period of 6 weeks, and it was tested for the detection of Fe III ion both in tap water and in synthetic solutions mimicking those of spent lithium ion batteries, presenting good results.

    Scheme 8. The same group studied the performances of a tetrakis 4-allylphenyl porphyrin-based ISE in a similar application, to detect and remove Cu II ion in synthetic solutions, similar to those of spent lithium ion batteries. The ISE can be used in the pH range of 2—8, with a near-Nernstian response, with a six-week lifetime here too. The ionophore was also tested in removing Cu II ions from solutions, showing that this porphyrin can be used as an efficient adsorbent for the target ion. The detection limit of lead determination was 0.

    Metalloporphyrins have been exploited on a large scale for the development of anion selective electrodes, since they can bind different anions thanks to their axial coordination chemistry. In this case, the selectivity of the resulting ISE can be modulated by changing the porphyrin coordinated metal ion. For this reason, the selectivity pattern of metalloporphyrin-based ISEs differs from the Hofmeister series.

    Selectivity might also be influenced by the peripheral substitution of the macrocycle or by the addition of ionic additives to the polymeric membrane. These different ways of modulating the properties of the corresponding ISE make metalloporphyrins very promising materials; for this reason they have been extensively studied for anion detection.

    One of the major drawbacks of metalloporphyrin-based ISEs is represented by the frequent super-Nernstian behavior exhibited by these electrodes, that is, when the slopes of the sensor responses to ion exposure are much higher than those expected by the theoretical Nernstian law. For this reason the hydroxy induced metalloporphyrin dimerization in the membrane could be avoided, together with the pH cross influence.

    Pt porphyrins have been used extensively in polymeric membranes to detect molecular oxygen, but they have not been investigated as ionophores, probably due to the reduced axial chemistry of such square planar complexes. Three Pt complexes were studied Scheme 9 , with PtTPPCl 2 as the first, since we hypothesized that the Pt IV oxidation state could allow an active axial chemistry useful for the target sensing mechanism.

    Scheme 9. Molecular Structures of Pt Porphyrins. DFT studies indicated a potential selectivity toward the iodide ion, also confirming a reduced affinity in terms of binding energies toward oxoanions. This behavior was made explicit by controlling the optical feature of the membrane, showing a variation of the UV—vis spectrum over time Figure The presence of both complexes induced the mixed mode response of the resulting membrane. Both complexes functioned as neutral carriers and needed cationic additives to demonstrate a good selectivity of iodides, with PtOEP displaying a better performance.

    The Nernstian slope of the ISE responses suggested the possibility of preventing ionophore dimerization by using Pt porphyrins. This feature was also confirmed by the absence of variations in the UV—vis spectra of the macrocycles in the PVC membranes. Mn tetraarylporphyrins have been used as ionophores to develop ISEs devoted to the detection of organic anions of pharmaceutical interest. Its universal diffusion makes it one of the emerging pollutants of wastewaters. Monitoring its concentration is becoming urgent. The influences of the different components of the PVC membrane, such as plasticizers and cationic and anionic additives, on sensor responses were investigated.

    The sensor also showed good diclofenac detection, with SCN — being the strongest interferent. The developed sensor can work in the 5. The addition of cationic additives increased the response slope to a value closer to the Nernstian behavior, while the linear range was not changed. ZnTPP was recently exploited as an ionophore to develop a cyanide selective electrode. Metalloporphyrins are ideal binding agents for cyanide ions, since they are a good ligand for metal ions. The developed ISE showed a Nernstian linear response in the 0.

    The most interfering anions were thiocyanate and iodide, but only the first can be present in applications of interest to the mining industry. The lifetime of the developed ISE can be improved by using freshly distilled THF for the membrane preparation and by storing the membrane in the dark.

    All these features indicated an oxidative decomposition of the ionophore due to the formation of peroxide species. This mechanism is difficult to operate with colored matrices, such as wines. Potentiometry can exploit the same chemistry without this drawback. The influence of the DEA concentration in the conditioning solution and the addition of ionic additives to the membrane on sensing performances were evaluated, to optimize SO 2 detection. The pH was fixed at 1. When the pH value was increased to 5, no response was observed, since all the SO 2 was converted to a bisulfite anion, not useful for the membrane sensing mechanism.

    The selectivity of the developed ISE was tested toward the ions potentially present in wine samples and toward the compounds present in wines that bear amine groups, such amino acids or biogenic amines. For all these species, no interferences were observed at the contents expected in wine samples. The developed ISE was tested for SO 2 analysis in wines, and the results obtained compared with those of a standard method for SO 2 quantification.

    The results were comparable with those of the reference method. The developed ISEs showed a good selectivity to a series of anions, and the PVC membrane electrode was tested to detect the selenite ion in selected applications, delivering satisfactory performances. The exploitation of metalloporphyrins grafted on graphene oxide GO as ionophores was proposed almost contemporaneously by the same group.

    The hybrid material was dissolved in a PVC membrane and used to develop a coated wire ISE, tested for fluoride detection. The sensor showed a Nernstian linear response, with a good selectivity in the 10 —7 to 0. The possibility of tuning the binding properties of the macrocyclic ionophore by skeletal modification led to studying the exploitation of porphyrinoids as binding units of ISEs. This opportunity has been one of the seminal applications of corrole in the chemical sensor field, first using free base corroles as ionophores.

    In an initial work, 5,10,tris pentafluorophenyl corrole was used as an ionophore for Ag I -selective PVC polymeric membrane electrode, showing a close to Nernstian potentiometric response slope in the 5. At almost the same time, Dehaen and co-workers reported the exploitation of free base corroles in PVC membranes for the potentiometric detection of salicylic acid. The most positive responses were obtained at pH 2, where neutral salicylic acid is present.

    The response improved with the presence of a cationic exchanger. High super Nernstian response slopes were observed in these conditions, indicating a complex mechanism for the potentiometric response. At a higher pH, the response slope decreased, while the ISE was not sensitive to the salicylate ion at a basic pH.

    Scheme The strong pH influence on the corrole-based ISE was later confirmed, studying the behavior of triphenylcorrole-based PVC membrane electrodes toward different anions. Creating an ISE is quite problematic, due to the strong influence of the background pH of the analyzed solution. The exploitation of metal complexes of corrole is also interesting for anion detection, since the peculiar coordination chemistry of this macrocycle offers the opportunity to tune the selectivity pattern of the developed ISE.

    The same article investigated the use of Cu, Mn, and Fe complexes of corroles as ionophores. The addition of cationic additives improved the response behavior of the corresponding electrodes, which showed an enhanced selectivity toward carbonate and monohydrogen phosphate anion, with a response slope close to the Nernstian value. UV—vis measurements demonstrated no dimerization of the Cu corrole when the membrane was immersed into the NaH 2 PO 4 solution.

    The observed selectivity toward hydrophilic anions is quite interesting, since it is difficult to obtain with analogous porphyrin complexes. The developed ISE showed a low pH response and a good selectivity toward common anions, such as nitrate, nitrite, bromide, and perchlorate ions. Furthermore, optical investigations revealed that Mn TPC Cl did not suffer from dimerization in the membrane, a drawback that usually affects metalloporphyrins.

    The influence of the peripheral substituents on the sensing behavior was investigated for iron complexes of corrole. The response slope for this latter anion was close to the theoretical Nernstian behavior, while for carbonate a supernernstian slope probably indicated a partial corrole dimerization.

    A modest increase of selectivity toward nitrate ion was also obtained by using tributylphosphate as plasticizer. Among other porphyrinoids, only sparse examples of their exploitation as ionophores have been reported in the past decade. The electrode performances were not affected by pH in the 4. The electrode that was developed was tested to determine Mg content in real matrices, such as baby foods and soft drinks, with satisfactory results that make this Mg selective electrode a good alternative to the existing sensors for Mg quantification.

    Octamethylcalix[4]pyrrole has been tested as ionophore in ISEs designed for the selective detection of Ti III ions in industrial wastewaters. The selectivity of the resulting membrane was excellent toward a series of mono-, di-, and trivalent cations tested. The electrode lifetime was satisfying i.

    The developed ISE was successfully exploited for the quantitative detection of Ti III ions in industrial wastewater samples and in tap water. Different techniques have been developed based on the observation of the current flowing in the cell, but all mainly deal with metallic electrodes and observe the situation when the current flows and the analyte concentration changes as a result of an electron-transfer red-ox reaction on the electrode surface.

    The corresponding sensors can be named amperometric or voltammetric, depending on the form of the applied potential amperometry uses a constant applied potential, while voltammetry varies the applied potential. Porphyrinoids are applied to functionalize the working electrodes. The following sections describe recent examples in literature, according to the different techniques exploited and the target analytes. Porphyrin-based electrodes to detect nitric oxide produced by cancer cells are rarely reported.

    The modification of the electrode surface with metalloporphyrins, carbon based materials, or permselective membranes is crucial to increase the analytical performances in terms of selectivity and sensitivity. The NO sensor registered an amperometric current sensitivity of 0. Another example of microelectrochemical sensor for NO detection based on 3-aminophenylboronic acid APBA and metalloporphyrin cofunctionalized reduced graphene oxide rGO was reported by Huang and co-workers. The further covalent functionalization of these nanostructures with a small cell-adhesive molecule APBA also provides the sensor with excellent cytocompatibility and practicable reusability.

    The authors also fabricated a patterned ITO microelectrode for the sensitive and selective real-time monitoring of NO molecules released from attached human umbilical vein endothelial cells cultured directly on the sensor surface. The developed device featured fast responses about ms and a calculated LOD of about 55 pM in PBS and 90 pM in cell medium, which are lower than those previously reported.

    Monitoring amounts of H 2 O 2 is of great importance in various fields, ranging from food control, pharmaceutical, clinical, and environmental protection. The amperometric detection of this analyte is usually accomplished based on its reduction rather than oxidation, since the latter is more affected by many other electroactive interferents in the biological fluids. Although the use of biosensors is rather common, the development of nonenzymatic H 2 O 2 sensors is particularly appealing, being economic and more stable in comparison.

    An amperometric sensor for H 2 O 2 detection in beverages that uses a nanocomposite porphyrin material was reported by Chen and co-workers. The functionalization of MWNTs by an iron porphyrin complex led to enhanced electron transfer by synergic effect that produces a highly sensitive amperometric sensor for the reduction of hydrogen peroxide with a low overpotential.

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    The last feature enables the practical use of the developed amperometric sensor to detect H 2 O 2 in different commercial beverages without any sample pretreatment. The analytical results obtained with this system were in satisfactory agreement with those determined by the classical titration method with KMnO 4.

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