Overview of Gas Sensors Focusing on Chemoresistive Ones for Cancer Detection

The necessity of detecting and recognizing gases is crucial in many research and application fields, boosting, in the last years, their continuously evolving technology. The basic detection principle of gas sensors relies on the conversion of gas concentration changes into a readable signal that can be analyzed to calibrate sensors to detect specific gases or mixtures. The large variety of gas sensor types is here examined in detail, along with an accurate description of their fundamental characteristics and functioning principles, classified based on their working mechanisms (electrochemical, resonant, optical, chemoresistive, capacitive, and catalytic). This review is particularly focused on chemoresistive sensors, whose electrical resistance changes because of chemical reactions between the gas and the sensor surface, and, in particular, we focus on the ones developed by us and their applications in the medical field as an example of the technological transfer of this technology to medicine. Nowadays, chemoresistive sensors are, in fact, strong candidates for the implementation of devices for the screening and monitoring of tumors (the second worldwide cause of death, with ~9 million deaths) and other pathologies, with promising future perspectives that are briefly discussed as well

Fabrication of a Highly NO2-Sensitive Gas Sensor Based on a Defective ZnO Nanofilm and Using Electron Beam Lithography

Hazardous substances produced by anthropic activities threaten human health and the green environment. Gas sensors, especially those based on metal oxides, are widely used to monitor toxic gases with low cost and efficient performance. In this study, electron beam lithography with two-step exposure was used to minimize the geometries of the gas sensor hotplate to a submicron size in order to reduce the power consumption, reaching 100 °C with 0.09 W. The sensing capabilities of the ZnO nanofilm against NO2 were optimized by introducing an enrichment of oxygen vacancies through N2 calcination at 650 °C. The presence of oxygen vacancies was proven using EDX and XPS. It was found that oxygen vacancies did not significantly change the crystallographic structure of ZnO, but they significantly improved the electrical conductivity and sensing behaviors of ZnO film toward 5 ppm of dry air

Chemoresistive Nanosensors Employed to Detect Blood Tumor Markers in Patients Affected by Colorectal Cancer in a One-Year Follow Up

Simple Summary Since colorectal cancer represents one of the most diffused pathologies worldwide, usually lacking specific symptoms, it is crucial to develop and validate innovative low-invasive techniques to detect it. Here, a device based on an array of nanostructured gas sensors has been employed to analyze and discriminate the exhalations of blood samples collected from colorectal cancer-affected patients at different stages of their pre- and post-surgery therapeutic path. The device was clearly able to distinguish between the pre-surgery samples, where the tumor was present, and the one-year post-surgery ones, following the tumor removal. These results raise high hopes for the device's clinical validation and its future use in clinical follow-up protocols, patient health status monitoring, and to detect possible post-treatment relapses. Colorectal cancer (CRC) represents 10% of the annual tumor diagnosis and deaths occurring worldwide. Given the lack of specific symptoms, which could determine a late diagnosis, the research for specific CRC biomarkers and for innovative low-invasive methods to detect them is crucial. Therefore, on the basis of previously published results, some volatile organic compounds (VOCs), detectable through gas sensors, resulted in particularly promising CRC biomarkers, making these sensors suitable candidates to be employed in CRC screening devices. A new device was employed here to analyze the exhalations of blood samples collected from CRC-affected patients at different stages of their pre- and post-surgery therapeutic path, in order to assess the sensor's capability for discriminating among these samples. The stages considered were: the same day of the surgical treatment (T1); before the hospital discharge (T2); after one month and after 10-12 months from surgery (T3 and T4, respectively). This device, equipped with four different sensors based on different metal-oxide mixtures, enabled a distinction between T1 and T4 with a sensitivity and specificity of 93% and 82%, respectively, making it suitable for clinical follow-up protocols, patient health status monitoring and to detect possible post-treatment relapses

Performance optimization of metal oxides for gas sensing: the case of WO3 and SnO2

Electrical gas sensors based on semiconducting metal oxides are now used in a wide range of applications and provided by many companies. They attracted the attention of many users and scientists due to the low cost, flexibility of production, ease of use, long-term stability and large number of detectable gases. The rising demand for gas sensors for a wide range of applications has highlighted not only the capabilities of these devices, but also their limits. Although common metal oxides, such as SnO2, TiO2, WO3 and ZnO, are catalytically active, different strategies are required to improve their selectivity and sensitivity. The quest for highly selective and high-performance gas sensors encouraged research into new sensing materials. Two approaches were used in this thesis to enhance the sensing capabilities of metal oxide-based thick films for detection of ethanol and hydrogen, i.e., two analytes of widespread interest for several applications. The first strategy aimed to control the size and shape of WO3 nanostructured powders used to produce thick films. WO3 nanoflakes have been synthetized through a simple and time effective solvothermal method. Two-dimensional (2D) WO3 was evaluated as most promising for the optimization of active surface area and film porosity for ethanol sensing. The second approach tuned the chemical composition and structure of SnO2 through substitution of Sn sites with Ti and Nb in different contents. Ti, Nb and Sn have similar ionic radii and bimetallic oxide solid solution of (Sn,Ti)xO2 and (Ti,Nb)xO2 have been claimed to enhance the sensing properties of single oxides, although some limitations remain. Nevertheless, the large number of compositional and structural combinations that these materials offer, makes it possible still unexplored possibilities. Indeed, what emerged from this work was that the incorporation of Nb in (Sn,Ti)xO2 offer a number of advantages, including increased film conductance and structural stability, as well as improved sensitivity to some gases, i.e. ethanol and hydrogen. Moreover, humidity (a common interferent) had a negligible influence on the baseline conductance of the (Sn,Ti,Nb)xO2 solid solution. While the reactions between the target gas and the surface of WO3 are well documented in the literature, those that occur over (Sn,Ti,Nb)xO2 are unknown due to the new chemical nature of the material. Therefore, operando Diffuse Reflectance Infrared Fourier Transform (DRIFT)-spectroscopy was employed to explore the interactions between ethanol, hydrogen and water vapour with the surface of the most promising (Sn,Ti,Nb)xO2 sensors while they were in operation.I sensori elettrici a base di ossidi metallici semiconduttori sono ad oggi ampiamente usati in numerose applicazioni e venduti da diverse compagnie. Hanno attratto l’attenzione di molti utilizzatori e scienziati grazie al loro basso costo, adattabilità di produzione, facilità di utilizzo, stabilità a lungo termine e largo numero di gas rilevabili. La crescente domanda per sensori di gas in applicazioni diversificate ha portato alla luce non solo le capacità di questi sensori, ma anche i loro limiti. Nonostante i comuni ossidi metallici, come SnO2, TiO2, WO3 e ZnO, siano cataliticamente attivi, diverse strategie devono essere adottate per migliorare la loro selettività e sensibilità. La richiesta di sensori di gas altamente performanti e selettivi ha incoraggiato la ricerca verso nuovi materiali sensibili. In questa tesi sono stati usati due approcci per ottimizzare film spessi a base di ossidi metallici per il rilevamento di etanolo ed idrogeno, ovvero due analiti di interesse diffuso per molte applicazioni. La prima strategia aveva l’obiettivo di controllare dimensione e forma di polveri nanostrutturate a base di WO3 usate per produrre film spessi. Nano lamine di WO3 sono state sintetizzate con un metodo solvotermale semplice e veloce. Il WO3 bidimensionale (2D) è stato considerato come il più promettente per ottimizzare la superficie attiva e la porosità del film verso il rilevamento di etanolo. Il secondo approccio ha modificato la struttura e composizione chimica dell’SnO2 tramite sostituzione di siti Sn con Ti e Nb, in diverse concentrazioni. Ti, Nb e Sn hanno raggi ionici simili e soluzioni solide di ossidi bimetallici come (Sn,Ti)xO2 and (Ti,Nb)xO2 hanno dimostrato di migliorare le proprietà di rilevamento dei singoli ossidi metallici, anche se rimangono alcune limitazioni. Ciononostante, l’ampio numero di combinazioni composizionali e strutturali che questi materiali offrono consentono ancora possibilità inesplorate. Ad esempio, dal lavoro di tesi è emerso che l’aggiunta di Nb in (Sn,Ti)xO2 offre diversi vantaggi, tra cui una maggiore conduttanza del film e stabilità strutturale, nonché una migliore sensibilità verso alcuni gas, come etanolo ed idrogeno. Inoltre, l’umidità (un comune interferente) ha un’influenza trascurabile sulla conduttanza della soluzione solida di (Sn,Ti,Nb)xO2. Mentre le reazioni tra i gas target e la superficie del WO3 sono documentate nella letterature, quelle che avvengono su (Sn,Ti,Nb)xO2 sono sconosciute a causa della nuova composizione chimica del materiale. Quindi, è stata impiegata la spettroscopia DRIFT (Diffuse Reflectance Infrared Fourier Transform) operando per investigare le interazioni tra etanolo, idrogeno e vapore acqueo con la superficie dei sensori (Sn,Ti,Nb)xO2 più promettenti mentre erano in funzione

DNA-templated nanowires for sensing volatile organic compounds

PhD ThesisThe fabrication of gas sensors with semi-conducting nanowires has attracted considerable interest in recent times because of their potential of selective and fast detection of low quantities of gaseous analyte when incorporated into miniature and low-power consumer electronics. DNA templating is a relatively new process for fabrication of nanowires at room temperature without the requirement for vacuum technology. This thesis describes the synthesis, characterization and gas sensing application of DNA templated metal sulfides and carbon nanotube nanowires. DNA templated CdS, CdZnS2 and ZnS were synthesized in solution to form smooth and continuous nanowires.The reaction involves initial coordination of the metal ion(s) with DNA and subsequent co-precipitation with sulfide ions upon addition of Na2S.The nanowires were deposited on the substrate via molecular combing to form a wellaligned network for electrical characterisation and gas sensing experiments. The structure, chemical composition and morphology of the nanowires were characterised by atomic force microscopy (AFM), powder X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, UV-vis spectroscopy, photoluminescence (PL), fluorescence microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy and Raman spectroscopy. These techniques showed that the metal sulfides interact with the DNA template to form microcrystalline nanowires of typical diameter < 10 nm and controllable Cd:Zn ratio. The current-voltage (I-V) properties as a function of temperature were measured using micro-band electrodes on a probe station, Impedance spectroscopy and current transients were used to estimate contact resistances. The nanowires showed weak conductivity with I-V curves typical of metalsemiconductor-metal systems and described by the space charge limited conduction model. The temperature dependent properties of the nanowires showed simple Arrhenius behaviour. The room temperature sensing properties of the nanowires to volatile organic compounds (VOCs) such as ethanol, acetone, chloroform and hexane were also determined. They demonstrated good and reversible sensing response to the VOCs but with a higher sensitivity towards ethanol. The result also suggests that the room temperature sensing mechanism of the VOCs molecules on CdS/DNA, ZnS/DNA and CdZnS2/DNA nanowire sensor is essentially driven by their direct adsorption on the surface and interaction with charges in the nanowires

Synthesis and Gas Sensing Properties of Transition Metal Dichalcogenides materials (TMDs)

En el procés de monitorització industrial, el control d'emissions dels cotxes, la seguretat de la qualitat de l'aire interior i exterior i la protecció del medi ambient, la detecció contínua i fiable de diversos gasos és fonamental. Els òxids metàl·lics semiconductors, els materials més utilitzats en aplicacions de detecció de gasos, tenen limitacions substancials com ara un alt consum d'energia, una mala estabilitat a llarg termini, una selectivitat limitada i, sobretot, una alta sensibilitat creuada a la humitat. Els materials nous que permeten un funcionament a baixa temperatura poden resoldre problemes relacionats amb l'energia, donant lloc a xarxes de sensors millors i més fiables. Com a resultat, materials 2D com els dicalcogenurs de metalls de transició (TMD) han sorgit com a opcions viables per a la detecció de gasos. Aquests materials de nova generació tenen el potencial de millorar les propietats de detecció dels materials sensibles als gasos, com ara la sensibilitat, la selectivitat, l'estabilitat i la velocitat (temps de resposta-recuperació). Això es deu a les seves propietats úniques inherents, que inclouen el gruix a nanoescala, una gran superfície específica, abundants llocs de vora actiu i una alta sensibilitat a les molècules de gas a temperatures més baixes i fins i tot a temperatura ambient. La tesi actual intenta augmentar la fabricació d'aquests materials en capes 2D de nova generació i utilitzar-los per a aplicacions de detecció de gasos en aquest camp d'estudi. A més, els materials de detecció de gasos investigats en aquesta tesi tenen el potencial d'abordar l'esmentat anteriorment en la seva forma prístina o després d'alguna funcionalització. En aquest sentit, aquesta tesi proposa sensors de gas quimioresistius basats en diversos materials TMD.En el proceso de monitoreo industrial, el control de emisiones de automóviles, la seguridad de la calidad del aire interior y exterior y la protección del medio ambiente, la detección continua y confiable de varios gases es fundamental. Los óxidos de metales semiconductores, los materiales más utilizados en aplicaciones de detección de gases, tienen limitaciones sustanciales, como un alto consumo de energía, poca estabilidad a largo plazo, selectividad limitada y, sobre todo, alta sensibilidad cruzada a la humedad. Los nuevos materiales que permiten el funcionamiento a baja temperatura podrían resolver los problemas relacionados con la energía, lo que daría como resultado redes de sensores mejores y más fiables. Como resultado, los materiales 2D como los dicalcogenuros de metales de transición (TMD) han surgido como opciones viables para la detección de gases. Estos materiales de próxima generación tienen el potencial de mejorar las propiedades de detección de los materiales sensibles al gas, como la sensibilidad, la selectividad, la estabilidad y la velocidad (tiempo de respuesta-recuperación). Esto se debe a sus propiedades únicas inherentes, que incluyen espesor a nanoescala, gran área de superficie específica, abundantes sitios de borde activos y alta sensibilidad a las moléculas de gas a temperaturas más bajas e incluso a temperatura ambiente. La tesis actual intenta ampliar la fabricación de estos materiales en capas 2D de próxima generación y utilizarlos para aplicaciones de detección de gases en este campo de estudio. Además, los materiales de detección de gases investigados en esta tesis tienen el potencial de abordar lo mencionado anteriormente, ya sea en su forma original o después de alguna funcionalización. En este sentido, esta tesis propone sensores de gas quimiorresistivos basados en varios materiales TMDs.In the industrial monitoring process, car emission control, indoor and outdoor air quality safety, and environmental protection, continuous and reliable detection of various gases is critical. Semiconducting metal oxides, the most extensively used materials in gas sensing applications, have substantial limitations such as high power consumption, poor long-term stability, limited selectivity, and, most notably, high humidity cross-sensitivity. Novel materials that allow for low-temperature operation might solve power-related issues, resulting in better and more reliable sensor networks. As a result, 2D materials like transition-metal dichalcogenides (TMDs) have emerged as viable options for gas sensing. These next-generation materials have the potential to improve gas-sensitive materials' sensing properties such as sensitivity, selectivity, stability, and speed (response-recovery time).This is owing to their inherent unique properties, which include nanoscale thickness, large specific surface area, abundant active edge sites, and high sensitivity to gas molecules at lower temperatures and even at room temperature. The current thesis attempts to scale up the fabrication of these next-generation 2D layered materials and utilise them for gas sensing applications in this field of study. Furthermore, the gas sensing materials investigated in this thesis have the potential to address the aforementioned either in their pristine form or after some functionalization. In this regard, this thesis proposes chemoresistive gas sensors based on several TMDs materials

Organic receptors for chemical sensors realized on flexible substrates

The aim of this research was to carry out synthesis and characterization of series of dithienyl pyrrole (SNS) based conducting polymers and their applications as chemical gas sensors in the perspective of development of flexible multisensing radio frequency identification (RFID) system for perishable goods monitoring, the aim of the EU project ‘FlexSmell’. In this context, number of dithienyl pyrrole derivatives were synthesized and polymerized by both chemical and electrochemical methods. The synthesis of SNS based polymers with different functionalities on their backbone was undertaken in order to study the effect of electron donating/withdrawing substituents on the properties of the polymers. The SNS polymers with halogen atoms (F, Cl, Br and I) were also prepared and studied for their effects on the properties of the polymers. Flexible chemoresistive sensors were fabricated by electrochemical deposition of the SNS polymers onto interdigitated electrodes (IDE) substrates. The sensors were characterized against the analytes responsible for decay of perishable goods, such as humidity, ammonia, ethanol etc. The optical absorption spectra of the SNS conducting polymers showed well defined absorption bands due to π-π* transition or to the transitions among polaron, bipolaron and band states. These features correlate with the good conductivity shown by the investigated compounds when regarded in the frame of the conduction models for organic materials owning delocalised π bonds. The influence of the substituents on the electrical conductivities of the polymers was analysed. The polymers have their electrical conductivity linked to the electron donating character and electronegativity (for the polymers with halogen atoms) of the substituents. The polymers are also studied for their thermal stability, morphology etc. The SNS polymers characterized for their sensing performances against humidity, ammonia and ethanol showed linear increase in their resistances with the relative humidity and a power function one in respect with the concentrations of the other analytes. Attempts have also been made towards the synthesis of dithienyl pyrrole-dialkylbithiazoles copolymers for the synthesis of easily soluble and environmentally stable polymeric materials intended to be used for chemical sensing. The main goal of the FlexSmell project, development of flexible multisensing RFID system was achieved by working in collaboration with Ecole Polytechnique Federale de Lausanne (EPFL), Switzerland, The University of Manchester (UK) and Holst Centre (The Netherlands). The multisensor platform was developed at EPFL whereas RFID tag at Holst Centre. Multisensor platforms with sensors of different transduction principles were fabricated by ink-jet printing of Ag-nanoparticle ink on flexible polyethylene terephthalate foils. The platforms have two IDE capacitors for humidity sensing, one resistive temperature detector for temperature measurement and two IDE resistive devices for ammonia and VOCs detection. The capacitive devices were functionalised with cellulose acetate butyrate or polyether urethane layers at University of Tübingen whereas resistive ones with polyaniline and polypyrrole layers at The University of Manchester. The RFID tag was integrated with the multisensor platform through a hybrid approach. In comparison with the currently available RFID sensing systems based on silicon technology, our prototype of low cost flexible multisensing platform with wireless communication capabilities represents a very promising approach for the next generations of smart RFID tags. Another part of the work explored the possibility to incorporate porcine odorant binding proteins in the structure of field effect gas biosensors through chemical and physical immobilization of the biological material on gold coated substrates. The concept has been tested by differential Kelvin probe measurements. This investigation was also performed in the frame of FlexSmell project for future developments of biosensor based RFID systems. Keywords: Conducting polymers, Dithienyl pyrrole, Chemical gas sensors, Smart multisensing RFID, Biosensors

Gas Sensors Based on Conducting Polymers

Since the discovery of conducting polymers (CPs), their unique properties and tailor-made structures on-demand have shown in the last decade a renaissance and have been widely used in fields of chemistry and materials science. The chemical and thermal stability of CPs under ambient conditions greatly enhances their utilizations as active sensitive layers deposited either by in situ chemical or by electrochemical methodologies over electrodes and electrode arrays for fabricating gas sensor devices, to respond and/or detect particular toxic gases, volatile organic compounds (VOCs), and ions trapping at ambient temperature for environmental remediation and industrial quality control of production. Due to the extent of the literature on CPs, this chapter, after a concise introduction about the development of methods and techniques in fabricating CP nanomaterials, is focused exclusively on the recent advancements in gas sensor devices employing CPs and their nanocomposites. The key issues on nanostructured CPs in the development of state-of-the-art miniaturized sensor devices are carefully discussed. A perspective on next-generation sensor technology from a material point of view is demonstrated, as well. This chapter is expected to be comprehensive and useful to the chemical community interested in CPs-based gas sensor applications

Gas Sensing at Low Temperatures with Different Semiconducting Materials

Since the late 1960s, chemo-resistive gas sensors have been getting a lot of attention. The materials used for this type of gas sensors have some inherent advantages like their low cost, as well as the ease of manufacture and miniaturization. These sensors, however, also have some inherent drawbacks. These include a lack of selectivity and high power consumption since they are constantly heated to temperatures between 200 and 500 °C. In this Thesis, two different classes of materials were investigated for the use in chemoresistive gas sensors operated at temperatures below 100 °C. First, with the help of supercritical fluid reactive deposition, WO3 nanoparticles were surface loaded with metallic Pt clusters. Most publications of Pt or Pd-loaded semiconducting metal oxide materials reported the presence of oxidized noble metal clusters and found a Fermi level pinning mechanism. In this Thesis, experimental proof was found that SFRD results in metallic Pt clusters and a spillover sensing mechanism. The second investigated class of materials for their gas sensing properties below 100 °C were semiconducting metal sulfides. The amount of research on this class of materials for gas sensors is relatively low and when this work was started, there were no publications with experimental proof for sensing mechanisms to any gas. In this Thesis, PbS colloidal quantum dots and Bi2S3 nanorods were investigated for their gas sensing properties. The sensing mechanisms to NO2 for both materials were revealed. For Bi2S3, the sensing mechanism to O3 was investigated as well, in addition to the interference of O3 with the sensing mechanism to NO2. It was found that the investigated metal sulfides in this Thesis react selectively with oxidizing gases. For PbS, it was additionally found that the presence of organic ligands used to stabilize the colloidal quantum dots have a significant effect on the stability of the sensor as well as the sensing mechanism to NO2. It was revealed that NO2 reacted with the residual organic ligand which resulted in the formation of an insulating organic shell around the CQDs as well as an initial boost of the sensing response due to a byproduct formed during the decomposition reaction of the organic ligand. For Bi2S3, it was found that the sensing mechanism with NO2 changed in dependence on the present concentration. While low NO2 concentrations resulted in healing of sulfur vacancies, higher concentrations resulted in the formation of nitrates.Seit den späten 1960ern bekommen chemo-resistive Gassensoren viel Aufmerksam. Die Materialien, die für diese Art von Sensoren verwendet werden, haben einige inhärente Vorteile wie einen geringen Preis, sowie die einfache Verarbeitung und Miniaturisierung. Allerdings haben sie auch inhärente Nachteile, wie zum Beispiel die Selektivität und relativ hohe Stromverbräuche, da die Sensoren konstant auf Temperaturen zwischen 200 und 500 °C beheizt werden. In dieser Thesis wurden zwei verschiedene Materialklassen für chemo-resistive Gassensoren, betrieben bei unter 100 °C, untersucht. Zunächst wurden mit Hilfe der Überkritischen Reaktivabscheidung WO3 Nanopartikel mit metallischen Pt- Clustern beladen. Die meisten Publikationen, die Pt- oder Pd-beladenes WO3 behandeln, berichten von oxidierten Edelmetall-Clustern und dem Fermi-level pinning Mechanismus. Im Rahmen dieser Thesis wurde experimentell bewiesen, dass metallische Pt-Cluster in einem Spillover-Mechanismus resultieren. Die zweite Materialklasse, die auf ihre Gassensoreigenschaften bei unter 100 °C untersucht wurden, waren halbleitende Metallsulfide. Die verfügbare Menge an Literatur für diese Materialklasse ist vergleichsweise gering und zu Beginn der hier präsentierten Thesis gab es keine Publikationen mit experimentellen Beweisen für Detektionsmechanismen. Im Rahmen dieser Thesis wurden kolloidale PbS Quantenpunkte und Bi2S3 Nanostäbchen auf ihre Gassensoreigenschaften untersucht. Der NO2-Detektionsmechanismus wurde für beide Materialien aufgedeckt. Für Bi2S3 wurde zudem der O3-Detektionsmechanismus und die daraus resultierende Interferenz mit der NO2-Detektion untersucht. Beide Metallsulfide reagierten selektiv auf oxidierende Gase. Mit PbS wurde zudem entdeckt, dass der organische Ligand, der zur Stabilisierung der Quantenpunkte verwendet wurde, erheblichen Einfluss auf sowohl die Sensorstabilität als auch den NO2 Detektionsmechanismus hat. NO2 reagierte mit dem organischen Liganden, was sowohl in der Bildung einer isolierenden Schicht um die halbleitenden Quantenpunkte resultierte als auch in einer Verstärkung des Sensorsignals aufgrund eines gebildeten Nebenprodukts der Reaktion. Bei Bi2S3 wurde entdeckt, dass der NO2-Detektionsmechanismus sich mit zunehmender NO2-Konzentration verändert. Während bei niedrigen Konzentration Schwefel-Leerstellen geheilt werden, bilden sich bei höheren Konzentrationen Nitrate