# Recent Advances in Optical Detection of Aminoglycosodies
## 2 Fluorescent Assays
### Turn-on Fluorescent Analysis Strategies  
The turn-on FL analysis strategy means the FL signal is enhanced in the presence of a target analyte. The FL signals are proportional to the concentration of the target analyte. Compared with turn-off FL analysis methods, the turn-on FL methods have a lower background. Mesoporous silica NPs loaded with organic dye were prepared and applied to the turn-on FL sensing of kanamycin. The NH\({}_{2}\)-modified complementary DNA was conjugated on the surface of Rhodamine B, filling mesoporous silica nanoparticles. The hybridization of the kanamycin aptamer with the complementary DNA increased the steric hindrance and inhibited the release of Rhodamine B from the mesoporous silica nanoparticles. The binding of kanamycin to the aptamer dissociated the aptamer from the complementary DNA and triggered the release of Rhodamine B to the solution, causing the increase in FL intensity. The LOD for kanamycin was 7.5 nM [34]. In addition, the kanamycin-aptamer was employed as a template for the preparation of Ag NCs, which had weak FL intensity initially. In the presence of kanamycin, the Ag NCs exhibited an enhanced FL intensity due to the formation of a hairpin structure and following the closeness of G and C bases contained in the aptamer template to the silver nanoclusters. The enhanced FL ratio was employed for the detection of kanamycin, which showed a linear range of 0.075-8.0 \(\upmu\)M with a LOD of 36 nM [35]. Despite the fact that these are strategies for turn-on FL sensing, the background of these methods is also very high because of the native FL emission of dye-loaded silica NPs and Ag NCs. Fluorescence resonance energy transfer (FRET) has been employed to establish an FL sensing platform for AGs. In a FRET process, the energy of the donor molecules is transferred to the acceptor molecules. When FRET occurs, the FL intensity of acceptor molecules enhances, and at the same time, the FL intensity of the donor molecules decreases. The distance and the spectral overlap between the donor and the acceptor are two main factors affecting the FRET efficiency. Dabcyl and Black Hole Quencher (BHQ) are the most commonly used quenchers. The approach of quencher to fluorophore can cause the FRET between the fluorophore and the quencher, resulting in significant FL quenching. [PERSON] and [PERSON]'s group designed a simple FL biosensor for kanamycin based on the signal transduction principle of FRET between the fluorophore carboxyfluorescein (FAM) and quencher Dabcyl. The kanamycin-aptamer was labeled with FAM and its complementary DNA was labeled with Dabcyl. The hybridization of complementary DNA with the aptamer brought the proximity of FAM and Dabcyl, leading to FL quenching, while the binding of kanamycin to the aptamer dissociated the complementary DNA from the aptamer, causing the departure of FAM from Dabcyl and subsequent FL recovery. They found that the complementary DNA, with a length of seven bases, showed the best performance in the sensing of kanamycin. The FRET-based sensing method showed a linear range from 100 nM to 600 nM for kanamycin in milk samples, and the LOD was 13.52 nM [36]. Nanomaterials such as Au NCs, Ag NCs, CDs, QDs, etc., have unique FL properties. These nanomaterials emit intense FL. Compared with traditional FL molecules, these nanomaterials have advantages such as size-dependent FL, good photostability, long FL lifetime, etc. FL-emitting nanomaterials have been employed in FRET assays to replace organic FL molecules. For example, [PERSON]'s group synthesized graphene oxide quantum dots (GQDs) with a bottom-up method using citrate as the starting material. The GQDs with carboxyl groups on the surface were conjugated with single-stranded DNA-binding protein (SSB) through an EDC/NHS reaction. An FL sensing platform composed of SSB-modified GQDs and BHQ-labelled aptamers was constructed (Figure 4A). The binding of the BHQ-aptamer to the SSB-modified GQDs enabled the proximity of the quencher BHQ to the FL-emitter GQDs causing the FL quenching.