## 3 Biotechnological Approaches for Sensing and Actuation
### Biosensors for Environmental Monitoring  
\"Biosensors\" usually refer to sensors with biological sensing parts, operating in cells and organisms to measure biochemical substances. In environmental monitoring, biosensors have become a subject of interest for many researchers due to the following advantages:  
* Frequent low power consumption [52], including self-powered sensors [58; 59];
* The ability to measure parameters that are difficult to measure in place and on-line and that are usually measured by laboratory methods [60];
* The possibility of multiparametric measurements using one cell-based biosensor [61; 62]. All of the discussed biosensors can be divided into the three big groups:
* Bioelectrochemical sensors with enzymes or cells as the sensing part that generate an analog electrical signal;
* Biomolecular (DNA, antibody, and enzyme) sensors that require an optical detector;
* Cell-based sensors with genetically engineered cells that are able to provide multiparametric measurements and/or some intracellular processing [63]. Bioelectrochemical sensors are the most widely used because they have an analog electrical signal (current or voltage) that is easy to handle and process, in almost the same way as in the case of classical electrochemical sensors. The sensing part is the cell or enzyme and provides a bioelectrochemical reaction that in its turn generates output. There are a number of approaches to using such biosensors for measuring toxic pollutants, heavy metals, ammonium, and bioorganic compounds [64; 65; 30; 66]. In many cases, such sensors are combined with BFCs because the measuring process involves the generation of electrical currents as a result of a bioelectrochemical reaction catalyzed by an enzyme or cell [64]. This approach leads to the possible application of such self-powered biosensors in rain gardens; they are able to start their operation with the start of measurements, so they can be self-activated during water flow appearance. Summarizing, it is necessary to say that bioelectrochemical sensors are an extremely promising field of research to enhance the sensing capability of a city's IoT. Biomolecular (DNA, antibody, and enzyme) sensors are mostly used for laboratory measurements [67]. The fast development of optical biosensing [68] and related technology might give rise to opportunities for future applications of such biosensors outside the laboratory. The fast development of synthetic biology in recent decades has yielded advancements that can lead to IoT applications. In the following, approaches that are able to expand sensing possibilities will be discussed. The development of multiparametric cell-based biosensors is a synthetic biology achievement. One interesting approach to develop it is based on the integration of simple Boolean logic gates that synthetic biology can provide for IoT applications [22; 69]. The logic process, in this case, provided through genetic logic circuits, is based on the variation of synthesis of active elements such as repressors and actuators that influence the biosynthesis processes from another part of the DNA [63]. It has been shown that tenths of logic gates can be applied in the cell [62; 70]. Providing such computations in the cell can provide the following advantages for whole-cell biosensors:  
* One cell can be made a multiparametric biosensor with an indication via fluorescent protein biosynthesis [63; 71];
* Output can vary depending on the input parameters;
* A system can be developed with cells with different functions that can contact each other via chemical signals [22; 70] and generate more readable and informative output [72]. Thus, it is possible to develop a multisensory array from several groups of cells that can provide not only sensing but also preliminary data analysis. The main advantage of such an approach is that microorganisms can support their operation via their metabolism, and the energy consumption ofwhole sensor systems begins from detection signals from cells. Figure 3 schematically illustrates such a sensor system.