# Recent Advances in Optical Detection of Aminoglycosodies
## 3 Colorimetric Assays
### Based on Target-Induced Aggregation of Metal Nanoparticles  
Metal NPs have specific colors. For example, well-dispersed spherical Au NPs (15 nm) have a wine-red color and a characteristic absorption band located around 520 nm. In terms of Ag NPs, the color is light yellow and the characteristic absorption band is around 420 nm. The aggregation of metal NPs leads to a hypochromic shift in the maximum absorption band. Based on the color change of metal NPs, various colorimetric sensors have been developed. Cirrate-capped metal NPs carry a negative charge, while the aminoglycoses have a positive charge under certain conditions. Thus, colorimetric methods for the determination of kanamycin were developed based on the electrostatic interaction between the negatively charged metal NPs and cationic AGs. For example, gentamicin could induce the aggregation of citrate-capped Ag NPs directly. The absorption ratio of 510 nm and 394 nm was employed for the quantitative detection of gentamicin  
Figure 5: Typical colorimetric assays (reprinted with permission from [51] Copyright (2018) Royal Society of Chemistry and [53] Copyright (2020) Royal Society of Chemistry). with a LOD of 720 nM [54]. This direct method is simple; however, it suffers from low sensitivity and poor specificity. In order to improve the specificity, kanamycin-aptamer has been employed. The aptamer, as single-strand DNA, can be adsorbed on the surface of Au NPs and prevent the slat-induced aggregation of Au NPs. The binding of antibiotics to the aptamer causes the dissociation of the aptamer from the Au NPs surface and subsequent aggregation in the salt solution. Thus, the addition of target antibiotics leads to the color of Au NPs changing from red to bluish violet. The employment of aptamers not only improves the detection specificity but also improves the detection sensitivity. The aptamer-based colorimetric sensing strategy has been developed for the detection of kanamycincin, tobramycin, and neomycin with LODs of 8.1 nM, 23.1 nM, and 27 nM, respectively, in which the ratio A620/A520 was employed as the parameter for quantitative detection [55; 56; 57]. In order to improve the performance of the Au NP-based colorimetric aptasensor for the detection of antibiotics, [PERSON]'s group employed a polyA-tailed aptamer and poly(diallyl dimethyl ammonium chloride) (PDDA) to replace the original aptamer and salt, respectively, and constructed a colorimetric assay for the detection of kanamycin (Figure 5B). The polyA at the end of the aptamer as an anchor improved the affinity of the aptamer to the Au NPs surface and prevented the Au NPs from the PDDA-induced aggregation. The binding of kanamycin to the aptamer liberated the polyA-tailed aptamer from the Au NPs surface, causing the aggregation of Au NPs by the PDDA. The colorimetric assay based on PDDA-induced aggregation lowered the LOD to 0.05 nM [53]. AGs have hydroxyl groups and aliphatic amine groups in their structure. Cysteamine-modified Au NPs were employed instead of citrate-capped Au NPs for the development of the colorimetric determination of gentamicin based on the strong affinity of gentamicin to cysteamine through the hydrogen bonding interactions. As a result, gentamicin could directly cause the aggregation of cysteamine-modified Au NPs. The cysteamine-modified Au NPs showed a capability for the detection of gentamicin concentrations as low as 12.45 nM in water [58]. In recent years, various Ag NPs with different capping agents on the surface have been synthesized and employed for the determination of aminoglycos. Typically, [PERSON]'s group developed a pH-triggered green synthesis method for the preparation of Ag NPs by using epigallocatechin gallate (EGCG) as both a reducing agent and a capping agent. The EGCG was oxidized by Ag\({}^{+}\) and formed quinone moieties on the surface of Ag NPs.