biomarkers or therapeutic targets for disease progressive stage monitoring. Over- and

under-regulation of miRNA leads to either inhibition of tumor suppressor genes or acti­

vation of oncogenes in cancer cells. miRNAs are more stable than mRNA and act as in­

vasive and robust biomarkers. The quantification of miRNA is made either as label-free or

label-based electrochemical assays. Oligonucleotide miRNA microarray analysis is used as

a high throughput technique for analyzing cancer-specific expression levels of hundreds of

miRNAs in cancer samples. In a study, an osmium-based nitrogenous ligand was syn­

thesized onto which hybridization occurs between DNA probe and miRNA. Through this

hybridization, a sequence of specific miRNA is detected in femtomolar levels from the

mixture of other non-complementary miRNAs. This osmium-based electroanalytical sensor

can be used in cancer stage diagnosis and monitoring the therapeutic systems [31].

DNA methylation is an epigenetic event of DNA repair and coordination of gene ex­

pression in higher eukaryotes. Changes in DNA methylation lead to changes in normal

cellular functions and generate biomarkers of cancer and other diseases. In normal cells,

DNA methylation occurs in the presence of DNA methyltransferase (MTase) and ab­

normality in MTase activity leads to acute monocytic leukemia. Therefore, DNA MTase

activity analysis is critical in understanding the regulation of genetic information, early-

stage cancer diagnosis, and quantification of disease pathogenesis. Electrochemical assays

detect the DNA MTase activity. These assays are low-cost, sensitive, selective, and offer

ease of miniaturization. In a study, gold nanoparticles (Au NPs) were used for the detection

of DNA MTase. Au NPs were modified with single-strand DNA hybrid and a com­

plementary sequence of DNA hybrid was assembled with modified Au NPs at a specific

recognition sequence. The modified electrochemical assay for MTase activity exhibited the

limit of detection of about 0.12 U/mL with a linear range of 0.2 U/mL to 10 U/mL. The

mechanism of electrode fabrication for detecting DNA MTase is explained in Figure 8.4 [32].

Escherichia coli (E. coli) is the versatile bacteria found in the digestive tract as beneficial

probiotics and act as a poisonous pathogen in food and environment and cause food

poisoning, urinary tract infections, and meningitis. In a study, a microfluidic electro­

chemical sensor was used for the detection and quantification of E. coli. The as-fabricated

microfluidic sensor detected and quantified the bacteria as 24 CFU/mL and 8.6 fg/µL

DNA in 60 minutes. The fabricated microfluidic sensor can be used as a point of care

diagnostic device in clinical and hospital applications [33].

Obesity is a physiological state characterized by high amounts of fats in the body. Obesity

is linked to diabetes, hypertension, respiratory system disease, and cerebrovascular dis­

eases. Adiponectin is an anti-inflammatory adipokine and has a role in lipids and glucose

metabolism. It is the biomarker of metabolic syndrome. In obesity, adiponectin is dysre­

gulated. Adiponectin has a role in vascular healing and angiogenesis and is the clinical

biomarker of cardiovascular disease. It is observed that the adiponectin levels are lower in

the plasma of obese people than the non-obese ones. In the early diagnosis of obesity,

biosensing systems were employed in the form of electrochemical impedance spectroscopy.

In a study, a graphite paper (GP) electrode was used as a working electrode for the de­

tection of adiponectin. GP electrode has electrical conductivity with usability in a wide

linear range and stable physical and electrochemical properties. GP electrode was used to

analyze the interaction between anti-adiponectin and adiponectin through impedimetric

and cyclic voltammetric studies. The as-fabricated sensor detected adiponectin with a limit

of detection of 0.003 pg/mL in a linear range of 0.05–25 pg/mL. The as-fabricated im­

munosensor was also tested for the detection of other proteins (leptin, creatine kinase,

parathyroid hormone, etc.) in the presence of adiponectin. The sensor exhibited long

storage life with usability in detecting proteins from real samples [34].

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