Antoni Baldi

ISFET Based Microsensors for Environmental Monitoring

The use of microsensors for in-field monitoring of environmental parameters is gaining interest due to their advantages over conventional sensors. Among them microsensors based on semiconductor technology offer additional advantages such as small size, robustness, low output impedance and rapid response. Besides, the technology used allows integration of circuitry and multiple sensors in the same substrate and accordingly they can be implemented in compact probes for particular applications e.g., in situ monitoring and/or on-line measurements. In the field of microsensors for environmental applications, Ion Selective Field Effect Transistors (ISFETs) have a special interest. They are particularly helpful for measuring pH and other ions in small volumes and they can be integrated in compact flow cells for continuous measurements. In this paper the technologies used to fabricate ISFETs and a review of the role of ISFETs in the environmental field are presented.

Fabrication of biofunctionalized microfluidic structures by low-temperature wax bonding.

In this work, a new fabrication technology for microfluidics based on the use of wax is described. Microfluidic structures are assembled using wax as both a thermoplastic adhesive layer between two glass substrates and a spacer layer defining the microchannels. Wax patterns with dimensions down to 25 μm are easily produced on glass substrates using specially developed decal-transfer microlithography. A complete microfluidic system is created by bonding the wax patterned layer with an additional glass substrate. On the basis of the special melting behavior of waxes, an effective glass-wax bonding is achieved at 40 °C by applying a soft pressure and without the requirement of any glass pretreatment. Wax bonding provides an effective sealing of the fluidic networks even on nonflat glass substrates (i.e., containing metal electrodes). The mild conditions required for the bonding process enables the fabrication of lab-on-a-chip devices incorporating biomolecules, as is demonstrated with the implementation of a simple heterogeneous immunoassay in a microfluidic device with amperometric detection.

Electrochemical Nanocomposite-Derived Sensor for the Analysis of Chemical Oxygen Demand in Urban Wastewaters

This work reports on the fabrication and comparative analytical assessment of electrochemical sensors applied to the rapid analysis of chemical oxygen demand (COD) in urban waste waters. These devices incorporate a carbon nanotube-polystyrene composite, containing different inorganic electrocatalysts, namely, Ni, NiCu alloy, CoO, and CuO/AgO nanoparticles. The sensor responses were initially evaluated using glucose as standard analyte and then by analyzing a set of real samples from urban wastewater treatment plants. The estimated COD values in the samples were compared with those provided by an accredited laboratory using the standard dichromate method. The sensor prepared with the CuO/AgO-based nanocomposite showed the best analytical performance. The recorded COD values of both the sensor and the standard method were overlapped, considering the 95% confidence intervals. In order to show the feasible application of this approach for the detection of COD online and in continuous mode, the CuO/AgO-based nanocomposite sensor was integrated in a compact flow system and applied to the detection of wastewater samples, showing again a good agreement with the values provided by the dichromate method.

Integrated Multisensor for FIA-Based Electronic Tongue Applications

The design, fabrication, and application of a monolithically integrated array of chemical sensors is presented. The multisensor chip includes six independent ion selective field effect transistors (ISFETs), a pair of interdigitated platinum electrodes, and a diode temperature sensor. Simultaneous polarization of multiple ISFETs is enabled by electrical isolation of the devices using two different approaches: by trenches and by p-n junction. The degradation of ISFET parameters by the fabrication steps of the interdigitated electrodes has been also studied and solutions consisting of additional annealing steps proposed. The multisensor chip has been assembled within a flow cell and applied as an electronic tongue to the measurement of bottled drinking water. Sensitivity to different ions has been achieved by deposition of organic membranes on top of the ISFET devices. Discrimination of different commercial waters by means of principal component analysis (PCA) of the data is demonstrated.

A microfluidic device for the automated electrical readout of low-density glass-slide microarrays.

Microarrays are a powerful platform for rapid and multiplexed analysis in a wide range of research fields. Electrical readout systems have emerged as an alternative to conventional optical methods for microarray analysis thanks to its potential advantages like low-cost, low-power and easy miniaturization of the required instrumentation. In this work an automated electrical readout system for low-cost glass-slide microarrays is described. The system enables the simultaneous conductimetric detection of up to 36 biorecognition events by incorporating an array of interdigitated electrode transducers. A polydimethylsiloxane microfluidic structure has been designed that creates microwells over the transducers and incorporates the microfluidic channels required for filling and draining them with readout and cleaning solutions, thus making the readout process fully automated. Since the capture biomolecules are not immobilized on the transducer surface this readout system is reusable, in contrast to previously reported electrochemical microarrays. A low-density microarray based on a competitive enzymatic immunoassay for atrazine detection was used to test the performance of the readout system. The electrical assay shows a detection limit of 0.22±0.03 μg L(-1) similar to that obtained with fluorescent detection and allows the direct determination of the pesticide in polluted water samples. These results proved that an electrical readout system such as the one presented in this work is a reliable and cost-effective alternative to fluorescence scanners for the analysis of low-density microarrays.

Reusable conductimetric array of interdigitated microelectrodes for the readout of low-density microarrays

Low-density protein microarrays are emerging tools in diagnostics whose deployment could be primarily limited by the cost of fluorescence detection schemes. This paper describes an electrical readout system of microarrays comprising an array of gold interdigitated microelectrodes and an array of polydimethylsiloxane microwells, which enabled multiplexed detection of up to thirty six biological events on the same substrate. Similarly to fluorescent readout counterparts, the microarray can be developed on disposable glass slide substrates. However, unlike them, the presented approach is compact and requires a simple and inexpensive instrumentation. The system makes use of urease labeled affinity reagents for developing the microarrays and is based on detection of conductivity changes taking place when ionic species are generated in solution due to the catalytic hydrolysis of urea. The use of a polydimethylsiloxane microwell array facilitates the positioning of the measurement solution on every spot of the microarray. Also, it ensures the liquid tightness and isolation from the surrounding ones during the microarray readout process, thereby avoiding evaporation and chemical cross-talk effects that were shown to affect the sensitivity and reliability of the system. The performance of the system is demonstrated by carrying out the readout of a microarray for boldenone anabolic androgenic steroid hormone. Analytical results are comparable to those obtained by fluorescent scanner detection approaches. The estimated detection limit is 4.0 ng mL(-1), this being below the threshold value set by the World Anti-Doping Agency and the European Community.

Stripping voltammetric detection of trace heavy metals using gold ultramicroelectrode arrays

The sensitive and simultaneous detection of zinc (Zn), cadmium (Cd) and lead (Pb) at gold ultramicroelectrode arrays (UMEAs) is reported. The device performance was based on the square-wave anodic stripping voltammetric technique. This consisted of the co-deposition of a layer of bismuth (Bi) together with the heavy metal target analytes onto the UMEA surface by applying a constant negative potential of -1.3 V for 360 s, followed by the stripping of the heavy metal alloy deposited by an anodic potential scan in a suitable potential window. Peaks at potentials ca. -1.0, -0.75 and -0.5 V were recorded and could be ascribed to the Zn, Cd and Pb stripping processes, respectively. Measurements were carried out in standard 0.1 M acetate buffer solutions pH 4.5, containing increasing concentrations of the three analytes. By using the peak areas, the corresponding calibration curves were constructed. The results showed a good sensitivity with a linear range from 10 to 70 ppb and a limit of detection of 7.5 ppb Pb, 5.3 ppb Cd and 9.2 ppb Zn, respectively.