*2.3.1.5 Antibiotics and antibiotic residues*

Antimicrobial resistance has emerged as one of the most essential problems in public health for the 21st century. This phenomenon is a threat to the effective disease prevention and treatment as increasing number of pathogens gain resistance to common medicines that used to treat them. In recent years, steady increase and intensification of animal production due to the increasing demand for animal protein has also lead to the increase in the use of antimicrobials as growth promoters in addition the specific use of antibiotics to treat specific diseases and to prevent the spread of particular diseases. This practice has been an essential contributor to the development and spread of resistance. On the other side of the coin, the livestock industry cannot support the growing demand for animal protein to the growing population without this modern miracle – antibiotics.

Research groups around the world has developed aptamer nanoparticle-based detection of antibiotics and its derivatives. Point-of-care detection of antibiotics is important in One Health approach as tainted products with antibiotics and its derivatives can be intercepted before penetrating the market and table of consumers.

Oxytetracycline (OTC) is one of the most commonly used tetracyclines (TCs) in veterinary medicine. TCs are extensively used as growth promoters that can lead to bioaccumulation in livestock products and by-products. This bioaccumulation of antibiotics may lead to serious human health issues ranging from allergies to incurable disease such as aplastic anemia, however, still the greatest threat is antimicrobial resistance. Thus, point-of-care detection for antimicrobials and its derivative is important to prevent the development and spread of antimicrobial resistance. Kim *et al.* [62] developed a colorimetric aptasensor using gold nanoparticle for the detection of OTC. Using a highly specific ssDNA aptamer to bind to OTC that can discriminate to doxycycline (DOX) and tetracycline (TET), aggregation of AuNPs was specifically induced via desorption of OTC binding aptamers (OBAs) from the surface of AuNPs as a result of aptamer-target interaction, thus a colorimetric change from red to purple. Kim *et al.* [62] aptasensor can detect up to 25 nM of OTC which is 20-fold lower than the limit of USA-EPA regulations. Thus, this colorimetric aptasensor is advantageous over traditional methods with simple signal generation and detection with naked eye specially during on-site detection of antimicrobial agents.

Kanamycin is one the frequently used aminoglycoside antibiotics produced by *Streptomyces kanamyceticus* [63, 64]. The increasing use of kanamycin is a threat to human health due to is ototoxicity, nephrotoxicity and neurotoxicity due to its residue in animal-derived products [65, 66]. The European Commission has set the maximum residue limits (MRLs) or kanamycin in milk at 150 μg/kg [64, 67, 68]. Thus, a convenient, fast, economical, accurate and sensitive point-of-care detection test is vital to promote healthy and safe animal derived products [69, 70]. Ou *et al.* [64] developed an aptamer-based strip biosensor for visual detection of kanamycin. The strip design uses the easy separation of magnetic microspheres (MMS) with target-mediated chain displacement of ssDNA and capture of the visible DNA-functionalized AuNP probe. The presence of kanamycin will competitively bind to the aptamer and release the cDNA to the supernatant. The free cDNA concentration is directly proportional to the concentration of kanamycin. The capture of DNA functionalized AuNPs on the test zone is through cDNA-induced hybridization that provide visual detection signal or the presence of line in the test zone. The limit of detection of the aptamer test strip is 50 nM and 4.96 nM for visual detection limit by naked eye and quantitative determination, respectively. This lateral flow strip biosensor can detect presence of kanamycin in different food samples and has a potential in medicine and for everyday use. Furthermore, this is vital as kanamycin side effects from animal derived foods has become a serious public health issue on a global scale [68].

### *2.3.2 Point-of-care animal disease diagnosis using silver nanoparticles (AgNPs)*
