the transducer surface which in turn gives an electrical response. Figure 22.4a is

the schematic representation of this reprinted from [11]. Mostly enzyme-based

biosensors are in this category, where an enzyme is immobilized directly over a

transducer, and signals of the reaction product are measured.

2. Second generation: These are mediator-dependent sensors. Herein, redox med­

iators like ferricyanide, ferrocene, Prussian blue, thionin, azure, methyl violet, etc.

are used to immobilize the bioreceptors or carry out the shuttling of the electron.

Redox mediators can be added to electrolytes or can be immobilized on the

surface of the electrode depending on the application. Figure 22.4b is the sche­

matic representation of this [11].

3. Third generation: Herein, wiring of a bioreceptor on the electrode surface is direct

via co-immobilizing the bioreceptor and conductive polymer. This electro­

deposition takes place layer by layer. The monomer is mixed with a bioreceptor

and electropolymerized. Figure 22.4c is the schematic representation of this

reprinted from [11].

22.1.2 Characterization Techniques

Printable, as well as flexible biosensors, have several layers of matrices. Starting with the base

electrode usually, carbon or solid metal or flexible polymer, followed by carbon or metallic or

polymer matrix. Over this, a bioreceptor and a transducer further interact with the analyte.

A synergetic mechanism must exist between these layers. This mechanism plays a key role in

electron transfer activity, electro-oxidation/reduction, hence, is the major driving force of

sense. The association of each layer can be manipulated based on desired features. Hence,

morphological nature has to be examined. To study the morphological and chemical char­

acteristics and features, several techniques mentioned below are employed [12]:

1. Microscopic: To study the surface morphology, scanning electron microscopy

(SEM) and transmission electron microscopy (TEM) are usually adopted. This

shows the basic shape, size of the various matrices used for immobilization.

FIGURE 22.4

Schematic representation of (a) 1st-generation, (b) 2nd-generation, (c) 3rd-generation biosensor types. Adapted

with permission [ 11]. Copyright The Authors, some rights reserved; exclusive licensee [MDPI]. Distributed

under a Creative Commons Attribution License 4.0@2016.

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