**6. Conclusions**

**Figure 13** illustrates the layers that comprise the paper-based biosensors and compares their sensibility for two different colorimetric indicators: AB (4-aminoantipyrine + 3,5-dichloro-

The 3D device shows important advantages compared to the 2D lateral flow sensor, given its higher coloration homogeneity. The 2D lateral flow sensor, on the other hand, is more subject to some carryover of the colored products that hinder the coloration intensity. This test runs in less than 5 min and a calibration color gradient scale can be printed directly on the paper device in order to compare with test results, allowing the patient to infer an approximate concentration of glucose in the sample. Eventually, a more complex device able to perform a wide range of analysis could take advantage of a solar cell to power sensors, a CMOS to perform

Electrochemically active bacteria (EAB) have the ability to extracellularly transfer their electrons produced during microbial respiration [81, 122, 123]. This ability sparked interest given

**Figure 13.** (a) Schematic representation of the 2D lateral flow device—enzyme and colorimetric indicators are deposited together. (b) Schematic representation of the 3D device. (c) Comparison between the 2D and 3D glucose sensors, when tested with increasing glucose concentrations (0.01–100 mM), with the graphical representations of the color intensity of detection zones containing the AB indicator (top) and the KI indicator (bottom). Adapted from Costa et al. [104], with

2-hydroxy-benzenesulfonic acid) and KI (potassium iodide).

54 Nanostructured Solar Cells

logic calculations and a display to show information.

*5.2.2. Electrochemically active bacteria detection*

permission from IOP Publishing.

Cellulose as a substrate for electronics is a highly promising booming field, not only for its low cost and recyclability, but also because of its compatibility with fast and inexpensive manufacturing printing processes. However, despite the fact that paper properties can be tuned to the intended purpose, there are still numerous challenges to address (e.g., water absorption, need for encapsulation, porosity) usually related with the internal structure, morphology, and chemistry of the fiber surface.

Unlike inorganic platforms (e.g., crystalline silicon, glass), the physical properties of paperbased materials have a considerable degree of polidispersion which can greatly affect the reproducibility of devices employing such materials. Therefore, extensive characterization protocols have to accompany the manufacturing processes, mainly targeted to provide a full definition of the morphology and composition of the paper as they widely vary and affect immensely the properties of the electrical devices. The analysis of the different devices here described (solar cells, FETs, electrochromic displays, and lab-on-paper) highlight how certain paper substrates are appropriate in some cases, but detrimental to other devices. Nonetheless, in all cases cellulose substrates allow for working devices with similar performance to those produced on plastic or glass. These are exceptional reasons to make electronic device applications on paper a reality and encourage the progress on printed solar cells to power more complex systems such as intelligent packages or diagnostic platforms.
