Cecilia Jiménez

Application of ion sensitive field effect transistor based sensors to soil analysis

Abstract Standard methods to measure nutrient levels in soil are complex and time consuming due to the extraction and pre-treatment processes involved. Besides, the instrumentation used for these measurements is also expensive. Therefore, the use of chemical sensors warrants investigation since they can be placed directly in the soil and results can be provided in real or quasi-real time at a moderate cost. The control of nutrients with sensors will permit an optimisation of irrigation and fertilisation management systems and thus will be useful for reducing the environmental impact caused by the runoff of nutrients into surface and ground waters. In this work, the use of chemical sensors based on ion sensitive field effect transistors (ISFETs) for soil analysis is proposed. These devices are fabricated with microelectronic technology – providing some important advantages such as robustness, small size, low output impedance and mass production. Fabrication of pH, Ca2+, K+ and NO3− ISFETs with photocurable polymeric membranes and their evaluation in aqueous solutions is reported. Studies of their response in horticulture soils and comparison with standard methods have been performed. The results confirm the feasibility of ISFET based sensors for in-soil monitoring and the promising future applications they have.

Photosensitive polyurethanes applied to the development of CHEMFET and ENFET devices for biomedical sensing

Chemical microsensors based on ion-selective field effect transistor (ISFET) transducers with ion-selective and enzymatic membranes have been fabricated. In this case, photolithographically patterned membranes based on acrylated urethanes have been developed and applied onto the gate area of ISFET chips. Aliphatic urethane diacrylate has been used for K+ and NH+4 membranes, while a photocurable hydrogel formulation based on other type of acrylated urethane has been optimized for urea-FET sensors. Resulting potassium and ammonium sensors show similar performances to those found when PVC membranes are employed. An integrated packaging process for ISFET-based sensors has been developed giving the possibility of carrying out most of the encapsulation on wafer level. For this purpose, a photocurable polyurethane encapsulant formulation has been optimized to be microstructured by photolithography. Finally, a preliminary study of biocompatibility of photosensitive formulations containing urethane oligomers has been performed in order to examine future applications in biomedical and clinical analysis.

Continuous-Flow System for On-Line Water Monitoring Using Back-Side Contact ISFET-Based Sensors.

A multiparametric continuous-flow system for on-line monitoring of water based on ISFET sensors is described. The ISFETs used have silicon nitride as gate material, and the electrical contacts are placed on the back side of the chip. This is a technological improvement that allows for a more compact ISFET packaging and greatly increases the lifetime of the sensor compared with planar type ISFETs, since the electrical parts are separated from the chemical environment. A special probe has been designed in order to encapsulate and apply these ISFETs into the flow system. Further, a reference electrode based on standard Ag/AgCl technology has been constructed according to the ISFET probe design in order to integrate both sensors in the same flow-through cell. These probes can be easily replaced in the flow system and are made of cheap and easily mechanized materials. Using these flow-through sensors, a continuous-flow system for the determination of pH, NH(4)(+), Ca(2+), and NO(3)(-) in waters has been designed. The system configuration is based on a modular design (one setup for each parameter and a common sampling channel), which allows simple manipulation and maintenance as well as a good flexibility for different analytical requirements. A study of the system characteristics was performed by following the specifications for water monitoring. Under the conditions established for the flow system, a sampling rate of 20 h(-)(1) was obtained for each parameter, and long-term stabilities of at least 3 weeks of daily work for ISFET sensors and 5 months for the reference electrode have been achieved. The response performances obtained show the feasibility of the BSC ISFET probe use in continuous-flow monitoring.

Development of a screen-printed thick-film nitrate sensor based on a graphite-epoxy composite for agricultural applications

Abstract The present report describes the development of a thick-film sensor with intended applications for soil analysis. A probe incorporates three sensors, each at a different depth and featuring a copper plate as the reference electrode. In addition, the probe also contains all necessary instrumentation required for processing the sensor signal and to transmit it via radio. The developed sensors were calibrated in situ using two different soil compositions by adding standard solutions. Results show a quasi-Nernstian response in soil, in response to the addition of fertilisers. Extracts of soil samples analysed by the Kjeldahl method were compared with results from the probe. This comparison confirms that the sensors are capable or measuring chemical parameters in situ in an automated fashion.

Development of Durable Nitrate-Selective Membranes for All-Solid State ISE and ISFET Sensors Based on Photocurable Compositions

Novel nitrate-sensitive membranes have been developed using an aliphatic urethane diacrylate photocurable polymer. This polymer has been chosen due to its easy and fast deposition procedure compared with PVC membranes and its compatibility with microelectronic techniques. These membranes were obtained after UV irradiation during no more than 60 seconds. To optimize their composition and evaluate the chemical response ion selective electrodes (ISEs) with solid inner reference were used. Studies of different plasticizers for nitrate PVC membranes and compositions increasing the ratio between oligomer and diluting agent were performed. ISEs developed presented a sensitivity of −56 mV/dec and a lifetime up to one year. Membrane compositions containing dibutyl phthalate and trioctyl phosphate as plasticizers were deposited onto ISFETs. Studies to elongate ISFETs lifetime were carried out modifying the encapsulation methodology, by using the same polymer family. ISFETs showed a sensitivity between 65–69 mV/decade, depending on the plasticizer, and a linear range of 2.4×10−5−0.06 M. The long-term stability of ISFET sensors with trioctyl phosphate plasticizer was up to 10 months, being superior to that described in the literature for PVC membranes and even for other kind of polymers.

Ecological aspects of the land crab Gecarcinus planatus (Decapoda: gecarcinidae) in Socorro Island, Mexico

Abstract The results of a study are presented on the Socorro Island crab Gecarcinus planatus , a species inhabiting this oceanic island in a wide range of communities and altitudes. In February 1990, two transects in the forest community at Socorro Island were sampled and all crabs captured were recorded. Through mark-recapture methods the population density was estimated, and the individual home ranges were determined by means of Component Bivariate Model. The structure and phenology of the vegetation was recorded in the same two transects. It seems that crab density is related to cover of trees, particularl to Ficus cotinifolia, Guettarda insularis , and Bumelia socorrensis . In addition, the phenology of G. insularis also seems to be important to crab distribution. Home range of G. planatus is inversely associated to crab population density. Vegetation communities in Socorro Island are endangered by sheep overgrazing, so densities and home ranges of the crab in this forest may be affected in the future. To determine the real impact of exotic species on the structure and function of the communities present in Socorro Island (such as feral sheep) it will be necessary to study the relationships between vegetation and the different animal populations in the areas not strongly disturbed.

Portable system based on microsensors for environmental monitoring applications

A versatile and portable system based on microsensors for measuring simultaneously pH, oxidation–reduction potential (ORP), conductivity and temperature in samples of environmental interest has been assembled and evaluated. An ion selective field effect transistor (ISFET) for pH measurements and platinum microelectrodes for ORP and conductivity measurements are used within the system. The multi-parametric system includes a commercial temperature sensor and the circuitry for each one of the mentioned sensors which can be exchanged or replaced. A commercial module with a USB interface is used for the acquisition and control of data from a laptop PC. Validation of the whole system has been carried out with standard solutions, providing high device-to-device reproducibility and stable response of all sensors during at least 24 h measured in a standard solution. Analysis of waste water samples with the portable system has been carried out to test its feasibility in environmental applications. Data have been compared with commercial electrodes.

Potentiometric pH sensors based on urethane–acrylate photocurable polymer membranes

Abstract A pH sensitive membrane based on a photocurable urethane–acrylate polymer with dioctylsebacate as plasticiser and tetradodecylamine as ionophore is described. This kind of polymer has been chosen as the membrane matrix because it is prepared and deposited more easily than PVC membranes. The deposition procedure, compatible with IC technology, permits the production of 100 μm thick membranes after few s of UV light exposure. The composition of the membrane (monomer, cross-linker, sensitive ion-pair and plasticiser) has been studied to achieve good mechanical properties and optimal sensor response. This pH membrane has been tested using a potentiometric transducer based on an all-solid-state design. A sensitivity of 55 mV pH −1 and a linear range from pH 2 to 10 was obtained with this sensor. The long-term stability was up to 9 months, which is comparable with PVC membranes. The results obtained with this polymer demonstrate the feasibility of using this material for sensor construction and its promising future application to urea determination by deposition of an enzymatic membrane based on the same polymer on the pH membrane developed.

Nanostructures for chemical recognition using ISFET sensors

Abstract The application of Langmuir–Blodgett (LB)-fabricated nanostructures on Ion selective Field Effect Transistors (ISFET) is of great interest since it permits the combination of a micro-scale device with a nanoscale material. In this article, the optimization of a LB-monolayer for calcium and sodium detection is carried out by characterization with XPS and AFM techniques, followed by electrochemical characterisation onto ISFET sensors. Physico-chemical modifications of the surface and good cation complexation of the ionophores fixed in the monolayers were observed with XPS studies. AFM images showed the presence of a unhomogeneous layer all over the ISFET surface. The electrochemical response of those monolayers onto ISFETs demonstrated the feasibility of this technology for sensor membrane fabrication and it opens further studies for other kind of devices such as optochemical sensors.

Analysis and identification of several apple varieties using ISFETs sensors

There are a number of variables that are useful to determine the optimal moment for the fruit collection such as starch content, sugar content, acidity and firmness. Other variables, including calcium and potassium concentrations, may establish better the state of ripeness of fruit and can help to optimise the collection process as well as augmenting the nutritional value of fruits. At present, these novel parameters cannot be used for the control of fruit collection due to the slow standard methods required. The need for in situ and in real-time ion measurements calls for fast response sensors and simpler and portable instrumentation. Solid-state sensors respond to these requirements. This work describes the application of ISFET sensors to analyse calcium, potassium and nitrates in several apple varieties, both in juice and in situ fruit. Results show that the analysis of potassium, calcium and nitrate permits to distinguish among apple varieties and can also be used to determine correctly the concentrations of these ions.