**2.2 Oligo-based sensing methods**

Besides the carrier of genetic information, breakthroughs in DNA-based biotechniques, CRISPR-based gene editing and other tools revolutionized the nucleotide-based sensing platforms. Among these sensing techniques, PCR, RT-PCR, antisense technology, single nucleotide polymorphisms in cancer and other deadly genetic disorders played an important role and often been converted into convenient biosensors. In the current COVID pandemic era, DNA-based POC diagnostics played a critical role in early and rapid detection of viral infections and followed to better patient management [5]. Even, to combat the urgent medical emergencies against SARS-CoV2, synthetic DNA-based vaccine was a lifesaver in the process of vaccine development. These nucleotide-based vaccines are amenable to scale-up, cheaper, cold-chain free stable, which are of critical attributes for delivery to resource-limited spaces.

Oligonucleotides having specific target binding or catalytic functions are termed as "Functional DNA". Such an example of functional DNA is "Aptamers", also called "Magic bullets" are considered to be chemical equivalent of monoclonal antibodies for their specificity and sensitivity to their cognate targets [6]. The target repertoire is diverse ranging from small molecules, metabolites, to proteins, cell surface receptors, cells and tissues and even whole organisms. Another major variant of functional DNA is ribozymes or deoxy ribozymes which are analogues of enzymes having specific recognition sites and cleavage activity. They also catalyze certain biochemical reactions. Due to stability, specificity, and variability for the base-pairing hybridization for detection and diagnosis, oligonucleotide-based biosensors are the key shareholder in clinical diagnosis, genome mutation detection and environmental pollutant detection [7, 8].

Recent developments in DNA nanotechnology include advent of various modified functional DNAs (XNAs), DNA computing, logic gates, DNA-based signal amplification strategy which further extended the applications of DNA-based sensors in POC detection, live cell imaging and monitoring, disease prognosis and overall, in health management systems [9].

This book presents a collection of new developments in modified oligonucleotides, cutting edge theranostic applications of oligonucleotides which provide insights and comprehensive overview of this exciting topic to the scientific community.
