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4 Smell and Taste

different solutions, the sensor can determine different concentrations of bitter and as-

tringent tastes in solution but is less effective when additional tastes are added to the

mixture [27].

There is a large variety of different sensing molecules for different applications.

A common method is a calorimetric sensor array using metalloporphyrins, solva-

tochromic dyes, or pH-sensitive dyes, which have been used for artificial noses [24,

28–30] and artificial tongues [31, 32]. Other optical sensors are based on fluorescence

[33, 38] or luminescence [33]. Light sources, when needed, can be LEDs of different col-

ors. Detectors are often photomultiplier tubes or CCD cameras [24, 32, 33]. An example

for such a sensor is a calorimetric sensor developed for identifying different types of

green tea [30] (Figure 4.9). The 36 sensor elements were different porphyrins that each

reacted with a slightly different color to the different tea samples. A CCD camera took

a picture before and after the sample and the difference between the two signals was

identified in a difference map for all sensing elements, which was used to identify the

tea sample.

Figure 4.9: Calorimetric sensor with 36 sensor elements. Compounds are identified via pattern recognition

of difference maps [30].

A similar sensor is used to identify gut bacteria (Figure 4.10) [34]. Twelve different fluo-

rescence monomers in a PEG-block-PLL polymer are used as the sensing molecules. The

fluorescence spectra are measured, and the difference pattern is analyzed by artificial

intelligence for identifying different amounts and types of gut bacteria. The same sensor

is used to identify differentiated stem cells [35].

An optical sensor is also used to measure glyphosphate concentration in solutions

[36]. The sensing molecules are immobilized enzymes that have glyphosphate as their

substrate. These enzymes are combined with competing glyphosate-functionalized

poly(ethylene glycol) colloidal probes. Reflection interference contrast microscopy was

used to measure interactions with a sensitivity down to 100 pM. This is a relatively low-