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

Nanocomposite Films for Gas Sensing

Nanocomposite films are thin films formed by mixing two or more dissimilar materials having nano-dimensional phase(s) in order to control and develop new and improved structures and properties. The properties of nanocomposite films depend not only on the individual components used but also on the morphology and the interfacial characteristics. Nanocomposite films that combine materials with synergetic or complementary behaviours possess unique physical, chemical, optical, mechanical, magnetic and electrical properties unavailable from that of the component materials and have attracted much attention for a wide range of device applications such as gas sensors.NRC publication: Ye

First Fifty Years of Chemoresistive Gas Sensors

The first fifty years of chemoresistive sensors for gas detection are here reviewed, focusing on the main scientific and technological innovations that have occurred in the field over the course of these years. A look at advances made in fundamental and applied research and leading to the development of actual high performance chemoresistive devices is presented. The approaches devoted to the synthesis of novel semiconducting materials with unprecedented nanostructure and gas-sensing properties have been also presented. Perspectives on new technologies and future applications of chemoresistive gas sensors have also been highlighted

Breakthroughs in the Design of Novel Carbon-Based Metal Oxides Nanocomposites for VOCs Gas Sensing

Nowadays, the detection of volatile organic compounds (VOCs) at trace levels (down to ppb) is feasible by exploiting ultra-sensitive and highly selective chemoresistors, especially in the field of medical diagnosis. By coupling metal oxide semiconductors (MOS e.g., SnO2, ZnO, WO3, CuO, TiO2 and Fe2O3) with innovative carbon-based materials (graphene, graphene oxide, reduced graphene oxide, single-wall and multi-wall carbon nanotubes), outstanding performances in terms of sensitivity, selectivity, limits of detection, response and recovery times towards specific gaseous targets (such as ethanol, acetone, formaldehyde and aromatic compounds) can be easily achieved. Notably, carbonaceous species, highly interconnected to MOS nanoparticles, enhance the sensor responses by (i) increasing the surface area and the pore content, (ii) favoring the electron migration, the transfer efficiency (spillover effect) and gas diffusion rate, (iii) promoting the active sites concomitantly limiting the nanopowders agglomeration; and (iv) forming nano-heterojunctions. Herein, the aim of the present review is to highlight the above-mentioned hybrid features in order to engineer novel flexible, miniaturized and low working temperature sensors, able to detect specific VOC biomarkers of a human's disease

Sol-Gel Production of Semiconductor Metal Oxides for Gas Sensor Applications

As they are widely utilized in industries including the food packaging industry, indoor air quality testing, and real-time monitoring of man-made harmful gas emissions to successfully combat global warming, reliable and affordable gas sensors represent enormous market potential. For environmental monitoring, chemical safety regulation, and many industrial applications, the detection of carbon monoxide (CO), carbon dioxide (CO2), nitrogen dioxide (NO2), and methane (CH4) gases is essential. To reliably and quantitatively detect these gases, much-improved materials and methods that are adaptable to various environmental factors are needed using low-cost fabrication techniques such as sol–gel. The advantages of employing metal oxide nanomaterials-based chemoresistive for creating high-performance gas sensors are shown by key metrics such as selectivity, sensitivity, reaction time, and detection. The primary sensing methods are also grouped and thoroughly covered. In light of the current constraints, anticipated future developments in the field of sol–gel nanomaterial-based chemoresistive gas sensors are also highlighted

Development of New Generation Chemoresistive VOC/Gas Sensors for Real-Field Applications

Volatile organic compound (VOC) and gas sensors based on semiconducting metal oxides and metal oxide composites have paid much attention since 1960s due to their sensing capability. Compared to conventional methods (e.g., micro-extraction (SPME) and gas chromatography mass spectrometry (GC–MS)) for detecting VOCs and toxic gases, these fabricated devices are anticipated to play a vital role in personal healthcare, environmental protection, securities and industries. Consequently, in recent years, intensive research has been directed to advance their sensing deeds, predominantly to augment the sensitivity and limit of detection for such sensing devices. Besides, sensing technologies become the heart of the future intelligent system due to the prompt enhancement of the Internet of Things (IoT)-enabled real-field applications and associated automation. Sensor devices that can detect trace amounts of VOC/gas analytes presents the favourable ubiquitous nodes of these sensor platforms. For VOC/gas sensing application case selective, highly sensitive, and portable devises are required by employing the various gas sensing materials including metals, carbon nanotubes, conducting polymers, and two-dimensional (2D) materials for meeting the criteria of real-field application. This thesis aims to develop chemoresistive sensors to address the challenges associated with current conventional sensors for VOC/gas detection by employing different fabrication methods. The specific objectives of this thesis are organized into seven chapters that will be presented in the form of a collection of the published papers which are the outcomes of the research. In addition, a literature review has been provided to establish the background of this research, which is also published in a review article. Overall, the main contributions of this thesis to the VOC/gas sensors field are designing and fabricating new sensing devices, and are summarized in following chapters: Chapter 3: Graphene inks for 3D extrusion micro printing of chemo-resistive sensing devices for VOCs detection (paper 1). Chapter 4: Fractal Design for Advancing the Performance of Chemoresistive Sensors (paper 2). Chapter 5: Extrusion printed CNT-graphene sensor array with embedded MXene/PEDOT: PSS heater for enhanced NO2 sensing at low temperature (paper 3). Chapter 6: Fast response hydrogen gas sensor based on Pd/Cr nanogaps fabricated by a single-step bending deformation (paper 4).Thesis (Ph.D.) -- University of Adelaide, School of Chemical Engineering and Advanced Materials, 202

Clinical validation results of an innovative non-invasive device for colorectal cancer preventive screening through fecal exhalation analysis

Screening is recommended to reduce both incidence and mortality of colorectal cancer. Currently, many countries employ fecal occult blood test (FOBT). In Emilia-Romagna (Italy), since 2005, FOBT immunochemical version (FIT) is performed every two years on people aged between 50 and 69 years. A colonoscopy is then carried out on those who are FIT positive. However, FIT shows approximately 65% false positives (non-tumoral bleedings), leading to many negative colonoscopies. The use of an economic and easy-to-use method to check FOBT-positives will improve screening effectiveness, reducing costs to the national health service. This work illustrates the results of a three-year clinical validation protocol (started in 2016) of a patented device composed of a core of nanostructured gas sensors. This device was designed to identify CRC presence by fecal volatile compounds, with a non-invasive, in vitro and low-cost analysis. Feces are, in fact, affected by tumor-volatile biomarkers, produced by cellular peroxidation and metabolic alterations. The protocol consisted in the analysis of fecal samples of FIT-positive subjects, using colonoscopy as a gold standard. A total of 398 samples were analyzed with machine learning techniques, leading to a sensitivity and specificity of 84.1% and 82.4%, respectively, and a positive predictive value of 72% (25–35% for FIT)

Detection of acetone using nanostructured WO3 for diabetes mellitus monitoring applications

Diabetes mellitus which is characterized by a high levels of blood glucose is a major source of mortality, morbidity and health costs worldwide. Major gaps exist in efforts to comprehend the burden nationally and globally, especially in developing nations, due to a lack of accurate, cheap and non-invasive data and devices for monitoring and surveillance. In Africa, type 2 diabetes mellitus represents 90% of diabetes cases. The disease mainly relies on management and monitoring. Although reliable blood glucose monitoring techniques and devices exist worldwide, the challenge is with the cost, invasiveness, and long sample preparation. Herein this study, the challenge was addressed by synthesizing WO3 materials for the detection of acetone in a simulated human breath. Acetone has been reliably confirmed to be the biomarker of diabetes mellitus. The Gas Chromatography-Mass Spectrometry (GC-MS) was employed to quantify acetone in type 2 diabetes mellitus. A statistically significant correlation (R=0.756) between blood glucose and breath acetone was observed, between blood acetoacetate and breath acetone (R=0,897), and between beta-hydroxybutyrate and breath acetone (R=0,821). Furthermore, we used semiconducting metal oxide (WO3) to investigate its selectivity, sensitivity, and response towards acetone. Semiconducting metal oxides sensor has the potential to detect volatile organic compound (VOCs) at low concentrations as low as 0.1 ppb. Other advantages of semiconducting metal oxides sensors include, facile and cheap device fabrication, portability, real-time analysis, and facile operating principle. We used two synthesis methods for fabrication of acetone sensors namely solvothermal method whereby solvent ratios were varied, and the sol-gel method where carbon nanospheres were used as a template and cobalt as a dopant. The sensor fabricated with 51:49 water: ethanol is found to demonstrate high response and good selectivity to 2 ppm level of acetone when compared with the one fabricated with pure ethanol, 18:92 (ethanol: water) and 92% water. Furthermore, the sensor could respond to low concentrations of acetone ranging from 0.5 to 4.5 ppm of acetone at 100 °C. For the sol-gel method, the 0.6 % Co-doped WO3 showed higher response and selectivity towards acetone gas from as low as 0.5 ppm at a very low operating temperature of 50 °C. Contrary, there was a very low response from other gases including toluene, NO2, NH3, CH4 and H2S operating at a similar temperature. This highlights the acetone selectivity of our 0.6 % Co-doped WO3 sample. Based on the two methods used for the synthesis of the acetone sensor, we can conclude that the Co-doped sensor shows better performance as compared to the as-prepared WO3. This is from the findings that the Co-doped WO3 can respond and select acetone concentration at 50 ◦C, which is a very low temperature in comparison to other platforms described in literature. An envisioned portable point of care diabetic device could therefore be operated at 50 ◦C in any point of care setting.Thesis (PhD (Biochemistry))--University of Pretoria, 2020.Council for Scientific and Industrial ResearchBiochemistryPhD (Biochemistry)Unrestricte

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