*5.4. Conductometric Biosensors*

Conductometric biosensors measure the conducting current between the electrodes and reference electrodes where the analyte or the medium plays a vital role. Usually, a differential measurement is performed between the working electrode with an enzyme and an identical reference electrode without an enzyme in a biosensor. The sensitivity of the sample amount is hampered by the parallel conductance of the target solution. The technique is significantly like conventional conductometers. An alternating current with the operating frequency is applied to the active electrodes to measure the potential. Conductance is measured by using both the current and voltage. Glucose, urea, creatinine acetaminophen, and phosphate are reported as different analytes to be determined using conductometric biosensors [117].

G. A. Zhalyak et al. [118] reported an alkaline phosphate-based conductometric biosensor for assessing the heavy metal ions in water. Gold-based electrode and residual enzyme activity measured in tris-nitrate buffer without metal preincubation. Various metal toxicity can be measured in the range as follows: Cd2+ > Co2+ > Zn2+ > Ni2+ > Pb2+. A similar method (Figure 3) is reported [119] to identify the heavy metals in water. The alkaline phosphate activity (APA) was collected from cyanobacterium to immobilize directly on the substrate by physical absorption. The response time was 12 s. Other works [120–122] are also reported for heavy metal detection in water.

**Figure 3.** (**A**) Schematic of the microelectrodes with the gold electrode, the working electrode is immobilized with Spirulina cells, the reference electrode is immobilized with inhibited APA, which also includes Spirulina cells, and S.E.M. image of spirula cells with the gold electrodes of interdigitated transducers. (**B**) The real-time response of the conductometric transducer. (**C**) Standard calibration curve for the detection of the alkaline phosphatase activity (reproduced with the permission from [119]). (**D**) Fabrication process. (**E**) the response time of the sample solution. (**F**) The averaged calibration curve (reproduced with the permission from [123]). (**G**) Schematics of the measurement setup of the FET sensor. (**H**) Differential threshold voltage (ΔVth) measurement of the gold-coated NWs vs concentration of the electrolyte and pH. (**I**) Response of the ionic strength of the gold-coated NW fitted with a blended site-binding model for deprotonation, protonation, and Cl– adsorption (reproduced with the permission from [124]).

In this work [125], the proposed biosensor was developed to determine the organic matter in water by immobilizing the enzyme bilayer with bovine serum albumin in glutaraldehyde vapour. It can detect the protein as a biomarker in water to identify urban pollution. The proposed method shows excellent sensitivity, reproducibility, and a long lifetime. C. Chouteau et al. [126] reported the whole cell *Chlorella vulgaris* microalgae as a bioreceptor on the interdigitated conductometric electrodes for detecting the toxic compounds in aquatic habitats. N. Kolahchi et al., proposed a fast, sensitive miniaturized conductometric biosensor for determining the phenol in water. *Pseudomonas* sp. (GSN23) bacteria were immobilized on the gold interdigitated microelectrodes to create the sensor assembly to determine phenol in river water.
