Developing new chemical-based sensors for the detection of volatile compounds

This research project aims to enhance the performance of a series of chemical-based sensor devices. In particular, metal oxide gas sensors. These materials are well known and primarily employed in several applications, thanks to their characteristic of interacting with volatile compounds, giving a response in terms of reversible change in the electrical resistance. Another exciting feature of metal oxide semiconductors, in particular rare earth-dope ones, is represented by their cataluminescence (a type of light emission when they interact with certain volatile compounds). We attempt to register simultaneously and combine the two different sensor responses (electrical resistance and light emission) to enhance sensor sensitivity and selectivity. This mode of operation can be defined as "dual-modality", and it represents a novel approach to sensor technology.For this purpose, the first part of the research project involved the synthesis and the characterisation of metal oxide nanoparticles to be employed in the sensing experiments. Afterwards, the materials obtained were used as sensing elements in in-house made equipment, recording the electrical resistance and the cataluminescence simultaneously. We tested each material under investigation with the following volatile compounds: acetone, ethanol, hydrogen peroxide, nitroglycerine, ethylene glycol dinitrate, 2,3-dimethyl-2,3-dinitrobutane, 2,4-dinitrotoluene and triacetone triperoxide. The experiments were repeated at different sensor temperatures in the range of 150-400°C. These ensured a full screening of the operating conditions and, consequently, the possibility of identifying the best parameters to ensure optimal sensor performance.The results obtained are encouraging in terms of sensor performances. In particular, the europium-doping of the zirconium oxide gas sensor allowed achieving a higher cataluminescence production, especially in the temperature range 250-275°C, and consequently a better sensor sensitivity. Recording the light emission simultaneously with the resistance response was shown to be very promising in terms of selectivity. If two given compounds showed a similar response in terms of resistance, discrimination is still possible thanks to their different cataluminescence response profiles.This thesis work showed for the first time and promisingly the possibility of combining more than one response from a single sensor to enhance its performance. We found out that the dual-modality can increase the sensor's possibility to distinguish among different volatiles. Especially the doping with rare earth metals, such as europium, increased the general response and consequently, they represent a promising material for the employment of dual-modality sensing

Preparation of single wall carbon nanotube-pyrene 3D hybrid nanomaterial and its sensor response to ammonia

In this work, the structural features and sensor response toward ammonia of a three dimensional (3D) SWCNTs material covalently functionalised with 1,6-diethynylpyrene were studied. The target SWCNTs hybrid material was prepared by the reaction of azido substituted SWCNTs with the 1,6-diethynylpyrene containing double terminal alkyne groups via the azide-alkyne Huisgen cycloaddition (Click) reaction. The structure of the 1,6-diethynylpyrene compound was determined by different spectroscopic methods such as FT-IR, 1H-NMR, MALDI-TOF mass, fluorescence and UV–Visible, while its SWCNT-Pyrene 3D hybrid material was characterized by FT-IR, Raman, UV-Visible spectroscopies and thermogravimetric analysis. The morphology of the hybrid films was investigated by scanning electron microscope (SEM). The sensing performance of the SWCNT-Pyrene 3D hybrid material was studied against low-concentrations of NH3 in the range of 0.1-40 ppm by measuring changes in the films' conductivity at different levels of relative humidity. The reversible electrical sensor response toward ammonia was observed both in the case of SWCNT and SWCNT-Pyrene 3D hybrid films however the response values of SWCNT-Pyrene 3D hybrid film were higher than those of SWCNT

State of the Art in Alcohol Sensing with 2D Materials.

Since the discovery of graphene, the star among new materials, there has been a surge of attention focused on the monatomic and monomolecular sheets which can be obtained by exfoliation of layered compounds. Such materials are known as two-dimensional (2D) materials and offer enormous versatility and potential. The ultimate single atom, or molecule, thickness of the 2D materials sheets provides the highest surface to weight ratio of all the nanomaterials, which opens the door to the design of more sensitive and reliable chemical sensors. The variety of properties and the possibility of tuning the chemical and surface properties of the 2D materials increase their potential as selective sensors, targeting chemical species that were previously difficult to detect. The planar structure and the mechanical flexibility of the sheets allow new sensor designs and put 2D materials at the forefront of all the candidates for wearable applications. When developing sensors for alcohol, the response time is an essential factor for many industrial and forensic applications, particularly when it comes to hand-held devices. Here, we review recent developments in the applications of 2D materials in sensing alcohols along with a study on parameters that affect the sensing capabilities. The review also discusses the strategies used to develop the sensor along with their mechanisms of sensing and provides a critique of the current limitations of 2D materials-based alcohol sensors and an outlook for the future research required to overcome the challenges

Conducting polymer nanowires for multi-analyte chemiresistive sensing

A conducting polymer nanowire-based chemiresistive sensor array was developed for the liquid-phase multi-analyte detection. The ability to distinguish and quantify multiple chemical species with a single sensory device can be useful in many areas including food industry, pollution control, biosensors, and explosives detection. A polyaniline nanowire is a good candidate for use as a chemiresistive sensing material due to its large resistivity change and ease of synthesis. However the two most important issues in chemiresistive sensors are the reproducibility in sensing and the selectivity in chemical species. For improving the reproducibility in polyaniline-based chemiresistive sensing, a self-calibration mechanism was proposed. This method utilizes two unique properties of polyaniline: one is the rate of the conductivity decay upon repeated cycling of the electrochemical potential, and the other is the position of the second redox potential, both of which are pH-dependent. These two properties were minimally affected by the polyaniline’s inherent limitations, i.e. hysteresis and degradation, and therefore were effective in obtaining repeatable measurements. In order to enhance the selectivity, a catalyst-based selective detection was proposed. This method is based on the concept that the catalytic reaction between the species and the catalysts causes a local pH change near the polyaniline nanowire network which changes the resistance of the polymer. Finally, a sensor array consisting of polyaniline nanowire-based chemiresistors with each sensing element modified with a unique catalyst was implemented for multi-analyte sensing of ascorbic acid, dopamine, and hydrogen peroxide. Principal component algorithm was applied for the classification and semi-quantification of the chemical species

The application of nanomaterials in electrochemical sensors and biosensors

Nanotechnology has recently become one of the most exciting forefront fields in analytical chemistry. A wide variety of nanoscale materials of different sizes, shapes and composltlons at the 1-100 nm scale, are now available. Metal and polymeric nanoparticles were applied in this work for desigining novel sensing systems, enhancing the perfomance of bioanalytical assays and improving the visualisation of biointeractions ocurring on sensing surfaces. A novel nanoparticulate formulation of the condructing polymer polyaniline (PANI) was applied foi the development of a chemlcal sensor device capable of detecting both ascorbic acid and hydrogen peroxide. The "nanoPANI"-modified electrode showed enhanced electrocatalysls over tradltlonal bulk PAN1 films for hydrogen peroxide. lnkjet printing deposition of this highly processable nanomatcrial onto screen-printed electrodes was also demonstrated for simple and rapld sensor device production. An enzyme-channelling system for the detection of glucose was optmused with HRP and GOX enzymes and applied to an mununosensor platform to report the immunological interaction between biotin and avidin. After the evaluation of the efficrency of this system, a slgrd enhancement approach was then attempted by means of AuNPs as multi-enzyme carriers. Charactensahon of the enzyme-NP conjugates was also performed by spectrophotometzlc and electrochemical analyses. AuNPs were also used to develop a multi-detection lmmunoassay system A sandwch-type platform was prepared using stueptavidln-modified paramagnetic beads as supporting material, blotinylated anti-human IgG as primary antibodys pecific to human IgG and Au-labelled anti-human-HRP as secondary antibody Using AuNPs as labels offered the posslbllity of the spectrophotometnc analysis based on elther AuNP absorphon or HRP enzymatic activity aid also electrochemcal analysls based on the drrect detection of AuNPs Both the opt~cal and the electrochemical analysis of a human IgCr model proteln resulted in enhanced senwfivity when compared to the classical ELISA tests where HRP-labelled antibodies are used. Silver-enhanced AuNPs were finally used to visualize an immunomteraction occurring at an electrode surface by means of SEM A AuNP-labelled anti-goat antibody was used as the target proteln to interact with two imunosensor platforms prepared immobilising anti-atrazine (single chain) and mtr-biotin antlbohes onto PAM-modified electrodes Comparing the images of the immunosensor surfaces with those of different control surfaces, it was possible to gain an appreciation of the extent and distribuhon of the munological interaction and the level of non-specific binding occurring at the electrode. In general, the application of these nanoparticles resulted in many advantages for the senslng systems investigated m this work These Include the observabon of enhanced electrocatalytic pheilomena wth benefits in chemical and biosensing, in improved analytical performance of classical senslng platforms where metal NPs were used as electrical tracers, as well as the application of metal NPs to assist in the detailed physical charactensation of immunosensing systems

Novel Graphene Oxide Based Nanocomposites: Synthesis and Application Towards Adsorptive Removal of Toxic Inorganic/Organic Pollutants from Aqueous Media

In this doctoral work, we have synthesized a series of GO based composite nanoadsorbents such as MgO-MgFe2O4 decorated GO (MgO-MgFe2O4/GO), amine functionalized GO mounted with ZnO-ZnFe2O4 (NH2-GO/ZnO-ZnFe2O4), AlOOH-FeOOH nanorods functionalized GO (GO/AlOOH-FeOOH) and GO/g-C3N4 decorated with Fe3O4 (GO/g-C3N4-Fe3O4) nanomaterials by using hydrothermal method. Then the prepared GO based metal oxide nanocomposites were used as novel adsorbent for adsorption study of inorganic pollutants such as fluoride ions (F-), hexavalent chromium (Cr(VI)), arsenate (As(V)) and organic pollutants like methylene blue (MB) dye, tetracycline (TC) antibiotic from water. The formation, composition, bonding, crystalline phase, surface morphology, size, and surface area of these prepared nanocomposites were analyzed by XRD, FTIR, Raman, XPS, FESEM, HRTEM, and BET analytical techniques. Batch adsorption experiments were carried out under various conditions including pH, time, concentration, adsorbent dose and temperature. The synthesized MgO-MgFe2O4/GO magnetic nanocomposite was used as adsorbent for removal of F- ions from water. The maximum adsorption capacity for F- ions removal is found to be 34 mg/g, which is higher as comparable to MgO-Fe2O3 nanocomposite. The amine functionalized GO decorated with ZnO-ZnFe2O4 (NH2-GO/ZnO-ZnFe2O4) nanocomposite material was used for remediation of Cr (VI) from water. It was observed that introduction of NH2 groups to GO/ZnO-ZnFe2O4 nanocomposite play a very important role for remediation of hexavalent chromium with a maximum uptake capacity of 109.89 mg/g. Apart from this we have also prepared GO/AlOOH-FeOOH composite nanomaterials by one step hydrothermal method and have used for decontamination of arsenate (As(V)) ions from water. Experimental finding reveals that the prepared GO based nanocomposite material is highly efficient for remediation of As(V) ions from water. Furthermore, we have also synthesized GO/g-C3N4 (graphitic carbon nitride) 2D layered composite materials decorated with Fe3O4 nanoparticles and have used for removal of methylene blue (MB) dye and tetracycline (TC) antibiotic from aqueous media. It was found that the adsorption of TC and MB was pH dependent and maximum adsorption capacities of 120 and 220 mg/g were achieved for TC and MB respectively. All the prepared GO based nanoadsorbents were regenerated and reused up to 5 successive cycles without major loss in their sorption capacity. From the obtained experimental results, plausible adsorption mechanism has been proposed for all adsorption process

MOF-derived synthesis of Co<inf>3</inf>O<inf>4</inf> nanospheres with rich oxygen vacancies for long-term stable and highly selective n-butanol sensing performance

Long-term stability and selectivity are crucial for the practical application of gas sensors, which are closely related to the microstructure and composition of sensor materials. In this work, Co-based metal organic framework (MOF) is used as a precursor and prepared by a simple hydrothermal method. After calcination, a series of Co3O4 nanospheres with various microstructures are derived. When the calcination temperature increases from 300 °C to 500 °C, the microstructure of Co3O4 nanospheres changed from rough solid to porous, and then transformed into porous core-shell. When assembled into the gas sensors, the Co3O4 nanospheres with porous structure calcined at 400 °C (Co3O4-400) show the highly selective response of 53.78 for 100 ppm n-butanol at the operating temperature of 140 °C. Moreover, the theoretical limit of detection was calculated to be 150 ppb. The reproducibility, selectivity and stability of the gas sensor were further verified to be excellent. After 45 days, the response value of Co3O4-400 is at 86.74%, even after 75 days, the response value remains at 74.93%. The main reason can be attributed to the large specific surface area, abundant pore structure and a large number of oxygen vacancies on its surface. These findings provide reference for the development of p-type metal oxide semiconductor (MOS) sensors with long-term stability and high performance

Greenhouse Gas Sensors Fabricated with New Materials for Climatic Usage: A Review

With the increasing utilization of fossil fuels in today’s technological world, the atmosphere’s concentration of greenhouse gases is increasing and needs to be controlled. In order to achieve this goal, it is imperative to have sensors that can provide data on the greenhouse gases in the environment. The recent literature contains a few publications that detail the use of new methods and materials for sensing these gases. The first part of this review is focused on the possible effects of greenhouse gases in the atmosphere, and the second part surveys the developments of sensors for greenhouse gases with coverage on carbon nano-materials and composites directed towards sensing gases like CO2, CH4, and NOx. With carbon dioxide measurements, due consideration is given to the dissolved carbon dioxide gas in water (moisture). The density functional calculations project that Pd-doped single-walled carbon nanotubes are ideal for the development of NOx sensors. The current trend is to make sensors using 3D printing or inkjet printing in order to allow for the achievement of ppb levels of sensitivity that have not been realized before. This review is to elaborate on the need for the development of greenhouse gas sensors for climatic usage by using selected examples