Handbook of Aggregation-Induced Emission, Volume 2. Группа авторов
micellar concentration. Moreover, the nanoparticles also have excellent biocompatibility, which makes them have potential applications in cell imaging.
2.2.4 Fluorescent Probes for Chemical and Biological Sensing
Fluorescence analysis technology has attracted much attention in the fields of chemistry, biology, and environmental science due to low background noise, high sensitivity, excellent selectivity, and easy operation. However, the fluorescence of materials will be quenched in high concentration or aggregate state, which greatly reduces the detection signal. The practical application of the fluorescence sensing system is severely limited. Introducing a fluorescent probe with AIE characteristics into the sensing platform, on the one hand, can effectively avoid the fluorescence quenching effect; on the other hand, the sensitivity of the sensing system can be improved by the “off”‐to‐“on” process of the fluorescent signal. Figure 2.11 shows some molecules based on DSA applied in the fluorescent probes for chemical and biological sensing.
2.2.4.1 Fluorescent Probes for Chemical Sensing
Heavy metal pollutions such as lead ions, mercury ions, and silver ions pose great threats to human life and health. Therefore, the development of heavy metal ion detection technology with high sensitivity is greatly significant for the environmental protection. The probe molecule 4‐1 exhibits a weak fluorescence in the solution. As Hg2+ is added, the fluorescence of the solution gradually increases [87] due to the specific recognition group toward Hg2+ in compound 4‐1. When thymine meets Hg2+, the imino group in thymine forms a coordinate bond with Hg2+ to limit the vibrational motion of the probe molecule, which makes it to emit strong fluorescence. Coordination bonds cannot be formed when other heavy metal ions are added, and there is no obvious change for the fluorescence intensity.
Figure 2.11 Molecules based on DSA for chemical and biological sensing.
Hydrosoluble AIE probe molecule 3‐5 (i.e. the molecule 9,10‐distyrylanthracene with two ammonium groups [DSAI] in Figure 2.12) in combination with the nucleic acid aptamer oligo‐C and nuclease S1 was used to achieve highly sensitive detection of Ag+ [88]. Figure 2.12a shows the design strategy for Ag+ sensing. Compound 3‐5 (DSAI) cannot emit fluorescence when completely dissolved in the solution. However, after oligo‐C was added, the positively charged probe molecule combined with the negatively charged oligo‐C, and the solution emitted weak fluorescence. When Ag+ was added, the cytosine base of oligo‐C and Ag+ forms a coordinate bond, inducing oligo‐C changing from a random structure to a stable U‐shaped structure, and prevented from being hydrolyzed by nuclease S1. Then, the AIE probe molecule 3‐5 (DSAI) can aggregate on the surface of the U‐shaped structure, and the inner rotation of the probe is restricted, which makes the fluorescence of the solution enhanced (see Figure 2.12b). On the contrary, when other heavy metal ions are added, the conformation of oligo‐C does not change, and oligo‐C can be hydrolyzed into fragments by the nuclease S1. Then, the probe molecule 3‐5 cannot aggregate, and the solution has no fluorescence (see Figure 2.12c). This method achieves a nonlabeled, highly sensitive detection of Ag+ with a detection limit of 155 nM. In addition, guanine base‐rich single‐stranded DNA (ssDNA) can form a stable G‐quadruplex (G4) complex in the presence of cations. Therefore, a method for detecting lead ions (Pb2+) based on the probe molecule 4‐5 and thrombin aptamers with high selectivity and high sensitivity can be established [21].
Figure 2.12 (a) Design strategy for Ag+ sensing; (b) fluorescence spectra of 3‐5 in the presence of different concentrations of Ag+; (c) fluorescence intensity (I − I0) of 3‐5 at 516 nm in the presence of different ions [88].
Source: Reproduced from Ref. [88] with permission from the Springer.
Compound 3‐5 was also used to develop a label‐free fluorescent aptasensor for detecting ochratoxin A (OTA) with specificity and super sensitivity. In this aptasensor, specific aptamer of OTA (OSA) serves as a recognition element, AIE molecule 3‐5 as a fluorescent probe, and graphene oxide (GO) as a quencher. In the absence of OTA, the AIE probe 3‐5 and OSA can form a complex of 3‐5/OSA and bright fluorescence was produced. After the addition of GO, the fluorescence of the complex 3‐5/OSA was quenched due to the adsorption of the complex on the GO surface, which results from the FRET from 3‐5 to GO. After OTA was added, a more stable complex of OSA‐OTA was formed and released from GO. At the same time, the AIE probe 3‐5 and OSA‐OTA can form a new complex of 3‐5/OSA‐OTA, and the fluorescence of AIE probe 3‐5 recovers gradually. Therefore, the ultrasensitive detection of OTA can be easily realized by monitoring the fluorescence change of compound 3‐5 before and after the addition of OTA. This aptasensor exhibits a high sensitivity with a detection limit of 0.324 nmol/l, and the linear detection range is 10–200 nmol/l. In addition, the aptasensor has a high selectivity to OTA against other analogues. Moreover, it also exhibits a good detection result when applied to the detection of OTA in red wine [90].
Yang's research group synthesized dinuclear zinc compounds 4‐2 and 4‐3 based on DSA to achieve the detection of citrate [91]. The fluorescence of compounds 4‐2 and 4‐3 can be quenched significantly through energy transfer mechanism upon the binding of a common indicator – phenol red. When the indicator phenol red is added, phenol red forms a new complex with 4‐2 or 4‐3, resulting in energy transfer from 4‐2 or 4‐3 to phenol red, and the fluorescence of the solution is quenched. With the addition of the target anionic citrate, the phenol red is released, and the combination of compound 4‐2 or 4‐3 with citrate will limit the intramolecular rotation of DSA, and the fluorescence will gradually increase to realize the quantitative analysis of citrate.
DSA derivatives with amino and hydroxyl groups are very sensitive to the pH of the solution [92]. Compound 3‐1 has almost no fluorescence in the solution of pH > 10. However, when pH < 10, the fluorescence of the solution gradually increases. At pH = 6, the fluorescence intensity of the solution reaches a maximum, which is 140 times that at pH = 10.3. It is because under basic conditions, the hydroxyl group of the molecule is converted into a sodium alkoxide to dissolve itself in the solution. As the pH of the solution decreases, the number of the sodium alkoxide structure gradually decreases, leading to the molecules starting to aggregate and the fluorescence of the solution increasing. Compound 4‐4 can also be used to detect pH based on the same mechanism, and the fluorescence is turned on when pH is high [92].
Compound 4‐5 was used to develop a facile, sensitive, and label‐free aptamer‐based fluorescent biosensor for chloramphenicol (CAP) detection [93]. In this aptasensor, C‐Apt,