OPTICAL AND FIBER OPTIC CHEMICAL SENSORS 423 reagents. Furthermore, since the chemiluminescence reagent needs to be mixed with the separated ana- lytes before detection, a more complex microchip layout is required. Regarding a more advanced approach to designing microfluidic platforms, which can be classified as "the on-chip approach," research in this direction has produced very promising results (Ng et al. 2002). According to the general conception, the integration of optical components or functions in a microfluidic platform that should be able to perform all chemical functions and detection in a single device requires increased integration of not only fluidic elements, but also electrical or other types of elements. Research has shown that such microelectromechanical systems (MEMS) can be realized. For example, Malic and Kirk (2007), using integrated optical waveguide technology, developed a miniatur- ized optical detection system for sensing in microchannels. The design allowed the implementation of both absorption and fluorescence measurement methods. An array of optical waveguides was fabricated using spin-on polymer technology on a silicon substrate and monolithically integrated with a micro- fluidic channel and V-groove fiber alignment. 16. OPTICAL MULTIPLE-CHEMICAL SENSING The optical approach is one of the approaches which is capable of multianalyte monitoring, meaning that multiple parameters can be determined simultaneously (Nagl and Wolfbeis 2007). Such sensors are called multisensors. A subgroup of these are the so-called compensating sensors, meaning that, apart from the analyte itself, the sample is also evaluated for a second species which interferes with the result. For example, all known fluorescence-based oxygen sensors suffer from interference by temperature. A second (temperature) sensor is needed to compensate for its effects (Coyle and Gouterman 1999). Dual sensors compensating for oxygen include those for glucose, in which an oxygen sensor is used to compensate for the variable supply to glucose oxidase (Li and Walt 1995; Wolfbeis et al. 2000), and for halothane, in which the indicator is quenched by both halothane and oxygen (Wolfbeis 1985). Several strategies can be employed to achieve optical multisensing. The most straightforward and easily implemented approach is based on so-called multispot sensors, which are also called sensor arrays if there are more than a few individual sensors. In these, individual sensors are placed in a way that the sam- ple is contacted simultaneously or successively with all of the sensors. The analytical signal is then read out and processed (Figures 6.76a and 6.76b). Multispot sensors have been described in many varieties. For example, multihalide sensors have been made (Zhu et al. 1990; Huber 2003). Other multisensors (a) (b) (c) (d) Figure 6.76. Types of dual sensors: (a) two-spot sensors; (b) sensor arrays; (c) dual-layer sensors; (d) single-layer sensors. (Reprinted with permission from Nagl and Wolfbeis 2007. Copyright 2007 Royal Society of Chemistry.)