The key substances inside the human body, including metal ions [1], biological small molecules ^[2][3][2,3], reactive oxygen species ^[4][5][6][4,5,6] and reactive nitrogen ^[7][8][7,8], are closely related to biological events, regulating physiological functions and metabolism. For instance, metal ions (e.g., Fe³⁺, Na⁺, Al³⁺ and Mg²⁺) play important regulatory roles in metabolism and osmolality ^[9][10][11][9,10,11]. Biological small molecules, such as homocysteine (Hcy), cysteine (Cys) and glutathione (GSH) participate in metabolism and redox reactions ^[12][13][14][12,13,14]. The content of reactive oxygen species and reactive nitrogen reflects many pathological states, especially the development of cancer ^{[15][16][17][18]}[15,16,17,18]. Therefore, monitoring the levels of these substances is helpful for understanding human health status.

With the development of in vivo imaging technology, more and more tracer techniques have been applied in biomedical research. Traditional tracing methods are usually required to remove tissues from the body, which cannot realize the real-time dynamic monitoring of the target substances. Currently, the commonly used in vivo tracer techniques include radionuclide imaging, magnetic resonance imaging (MRI), optical imaging and so on [19]. Radionuclide imaging can achieve quantitative and localization analysis of cells, but it has the shortcomings of low spatial resolution and the need of expensive equipment ^[20][21][20,21]. MRI with high spatial and temporal resolution enables monitoring of changes in cellular function, but this technique faces the problem of long imaging time [22]. In vivo luminescence imaging is a non-invasive technology, including bioluminescence imaging (BLI) and fluorescence imaging, mainly used to study gene expression and cell activity. Compared with BLI, the signal of fluorescence imaging is stronger and the detection accuracy is higher ^[23][24][25][23,24,25]. At present, in vivo fluorescence imaging has become a hotspot in biomedical research due to its advantages of low toxicity, high spatiotemporal resolution and utilization of an inexpensive instrument ^[26][27][28][26,27,28]. For example, fluorescent dye indocyanine green (ICG) imaging has been used in a variety of abdominal surgery applications, such as lymph node localization, ureteral detection and tumor identification [29]. In recent years, some fluorescent probes with high specificity have also been developed and used for in vivo imaging ^[30][31][32][30,31,32].

To date, many types of fluorescent probes, such as fluorescent organics ^{[33][34][35][36]}[33,34,35,36], fluorescent proteins [37], inorganic nanoparticles ^[38][39][38,39] and semiconductor polymer nanoparticles ^[40][41][40,41] have been developed and widely used in bioimaging. Fluorescent proteins can be generated by cells themselves through genetic engineering, which is convenient for in vivo imaging. However, they are hard to metabolize in the body. Inorganic nanoparticles have good spectral properties and light stability, but the biological toxicity limits their application in bioimaging. Semiconductor polymer nanoparticles have high fluorescence brightness, but slow metabolism causes them to accumulate in the liver. Comparatively, fluorescent organic small molecule probes (FOSMPs) attracted more attention because of their controllable synthesis, stable luminescence, good biocompatibility, sensitive response and high signal-to-noise ratio ^[42][43][42,43]. When applied to imaging, the photochemical properties of FOSMPs are generally more stable than other types of probes. Additionally, small molecules also help to achieve higher fluorophore density and spatial resolution. Therefore, a large number of FOSMPs have been designed and applied to detect substances in various matrix ^{[44][45][46][47][48][49][50][51]}[44,45,46,47,48,49,50,51]. Fluorescent organic nanoparticles (FONPs), which were designed with FOSMPs as fluorophores, show the ability to build a multifunctional biosensing platform through surface functionalization and drug encapsulation [52]. A summary of the progress on the applications of FOSMPs in bioimaging and detection is of significance for the development of new diagnostic tools. TAs far as we know, there are no special reviews focusing on this field.

2. FOSMPs for Bioimaging and Detection

Fluorescence is the phenomenon that electrons return from the first singlet state to the ground state with concomitant energy release in the form of light. The characteristics of the fluorescent probes such as excitation and emission wavelength, intensity, lifetime and polarization are easily influenced by the environment conditions, thereby providing sensitive signals for the tracking and monitoring of analytes ^{[53][54][55][56][57][58]}[53,54,55,56,57,58]. The structure of FOSMPs is mainly composed of three parts: the recognition group, the fluorophore, and the linking group (linker) (Figure 1). The recognition group endows the fluorescent probe with detection selectivity. The fluorophore provides the signal response when the probe recognizes the target. The linker connects the recognition group and the fluorophore and is not necessary for the fluorescent probe. As shown in Figure 2, according to the interaction manner between FOSMPs and targets, the recognition mechanisms can be divided into five types: photo-induced electron transfer (PET), intramolecular charge transfer (ICT), fluorescence resonance energy transfer (FRET), excited state intramolecular proton transfer (ESIPT) and aggregation-induced emission (AIE). Based on these recognition mechanisms, various FOSMPs for bioimaging and detection have been developed, as listed in Table 1. In the following, the applications of FOSMPs in bioimaging and analysis based on these recognition mechanisms are reviewed.