Multisensing Devices

In the recent past, multianalyte determination has found increased applications, i.e. specific and multiple reactions favor a system that allows the specific determination of each reaction, using the same principal measurement methods, detectors and conditions. In keeping with this idea, a flow injection thermometric method based on an enzyme reaction and an integrated sensor device was proposed for the determination of multiple analytes. In principle the technique relies on the specificity of enzyme catalysis and the universality of 

Inlet Thermistor Geld tubing Outlet Fig. 7. Schematic illustration of the thermal micro-biosensor fabricated onto a silicon chip 

The integrated system, including transducer and enzyme reactor, provides improved reliability and stability in multianalyte determinations, as compared with discrete thermal sensor systems. In addition, application of micromachining and IC technologies is of benefit for the manufacture of uniform, cheap thermal transducers with flexible shape, size, and resistance, as well as delicate microstructure on the chips. The good thermal insulation of the transducers from the flow stream eliminates interference from the reactants on the transducers, and the intrinsic stability of the transducers obviates the need for frequent recalibration of the sensors. 

---

Challenges remain in the development of lab-on-a-chip sensing systems. The overall lifetime of a sensor chip is always determined by the sensor with the shortest lifetime, which in most cases is the depletion of reference electrolytes. Measures to minimize cross-talking among sensors, especially when biosensors are integrated in the system, also should be implemented [122], The development of compatible deposition methods of various polymeric membranes on the same chip is another key step in the realization of multisensing devices. 

Based on the plethora of applications of thermistor/thermopile based devices, it can be concluded that the field of thermometric sensing offers several avenues of progress in materials science, process monitoring, process control, molecular level detection, characterization of biocatalysts, hybrid sensing and multisensing devices, as well as in telemedicine and other areas of biomedical analysis. 

It is important that all indicated devices can function at room temperatures. This means that polymer-based sensors have low power consumption (of the order of microwatts) because no heater element is required for their operation. Properties of polymers that influence the operating parameters of sensors can be physicochemical, chemical, optical (photo- and electroluminescence, optoelectronic), redox, hydrophobic/ hydrophilic, piezoelectric/pyroelectric, and electrical (conductivity, resistivity). Moreover, the polymer itself can be modified to bind biomolecules to a biosensor (Mulchandani and Wang 1996). It is mentioned above that polymers have considerable potential for fabrication of multisensing arrays required for e-nose fabrication (Janata and Huber 1985). 

---