An optochemical sensor is a device that detects and/or quanti-šes the presence of specišc target chemicals by optical methods. A good sensor should be sensitive to the target analyte but not to the other analytes, and it should not in°uence the quantity of the target analyte. ™e performance of a sensor is determined by various factors listed as in Table 33.1. Compared to other traditional sensors such as electrochemical sensors, optical sensors have signišcant advantages such as high sensitivity, wide dynamic range, and freedom from electromagnetic interference. Sensors made of optical šbers have been a typical form of optochemical sensors, with advantages such as multiplexing ability and applicability for remote sensing, which were not possible with traditional electrode sensors. ™ese optical šber sensors, however, are not suitable for intracellular applications because of their relatively large size, typically hundreds of micrometers in diameter, in comparison to cells that have a typical dimension of tens of micrometers. ™ey have been miniaturized to the nanoscale, down to 20 nm (Cullum and Vo-Dinh 2000), by limiting the sensor area to the tip of a šnely pulled šber, especially for application to intracellular measurements in single mammalian cells (Barker and Kopelman 1998; Barker et al. 1998; Tan et al. 1992, 1999). Still, their use has been limited by their not insignificant physical invasiveness. While the šber tip is very small compared to traditional sensors, the pulled šber sha¥ is not small enough to prevent physical perturbation to samples of micrometer dimensions, such as live cells or their subcompartments. For example, the inserted volume of a šber sensor with a 200 nm tip was found to be nearly 4% of the volume of a mouse oocyte cell,
which is larger than most mammalian cells (Buck et al. 2004). On the other hand, like larger chemical sensors, these sensors contain a chemically inert matrix (like silica or hydrogel) that minimizes the chemical perturbations of the sample.