Recent Progress in Fluorescent Probes for Diabetes Visualization and Drug Therapy

: Diabetes has become one of the most prevalent endocrine and metabolic diseases that threaten human health, and it is accompanied by serious complications. Therefore, it is vital and pressing to develop novel strategies or tools for prewarning and therapy of diabetes and its complications. Fluorescent probes have been widely applied in the detection of diabetes due to the fact of their attractive advantages. In this report, we comprehensively summarize the recent progress and development of ﬂuorescent probes in detecting the changes in the various biomolecules in diabetes and its complications. We also discuss the design of ﬂuorescent probes for monitoring diabetes in detail. We expect this review will provide new ideas for the development of ﬂuorescent probes suitable for the prewarning and therapy of diabetes in future clinical transformation and application.


Introduction
Diabetes, as a metabolic disease with hyperglycemia, is caused by defective insulin secretion or impaired biological action, or both [1,2]. Significantly, some diseases caused by diabetes, including neurodegenerative diseases, liver diseases, and kidney diseases, are essential and prevalent problems in patients with diabetes [3][4][5]. Therefore, it is one of the frontiers and difficulties to illuminate the precise pathogenesis of diabetes in biological and medical research. As we know, there are two types of diabetes. One is type I diabetes, which is mainly found in kids and teenagers. It is mainly caused by abnormal immune systems and hereditary factors [6]. The other is type II diabetes, which mostly occurs in middle-aged and older people who are fatigued and obese [7,8]. Inhibition of the levels or concentrations of blood glucose is fundamental to diabetes management. As far as diabetes treatment is concerned, it is relatively effective to treat type I diabetes by injecting insulin, reducing the amount of sugar in the daily diet, or increasing exercise [9]. Noninsulin drugs, including sulfonylureas and biguanide hypoglycemic drugs, may be used to treat type II diabetes [10,11]. At present, the diagnosis of diabetes is mainly based on the blood glucose levels of patients and analysis of typical symptoms of diabetes, including polyphagia, obesity, and fatigue [12,13]. However, these phenomena are all diagnosed after diabetes occurs; therefore, the opportunity for early treatment may be missed. Thus, it is vital and pressing to diagnose diabetes with precise and early warning and to cure diabetes and its complications.
Compared with classical imaging techniques, such as magnetic resonance imaging (MRI), computed tomography (CT), and positron emission tomography, fluorescence imaging has been widely used in various fields owing to its attractive advantages, including outstanding sensitivity, excellent selectivity, simple operation, membrane permeability, real-time visualization, and inexpensiveness [32][33][34]. In addition, it has become a potent instrument for in situ and real-time imaging of biomolecules in organisms [35,36].
In this review, we provide an overview of the recent progress in the design of fluorescent probes, fluorescence imaging mechanisms, and their application for detecting various biomarkers of diabetes including polarity, viscosity, ROS, RSS, MGO in active carbonyl compounds, enzymes, and pH values ( Figure 1). In addition, we also pay attention to a variety of vital biomarkers and their biological applications and significance in the occurrence and development of diabetes. Finally, we discuss the limitations of fluorescent probes currently used for diabetes diagnosis and imaging and put forward ideas on how to overcome these shortcomings. Therefore, this review will provide new ideas for the development of fluorescent probes suitable for the prewarning and cure of diabetes in future clinical transformation and application.

Design of Fluorescent Probes for Diabetes
Organic fluorescent probes designed for prewarning and cure of diabetes should meet the following demands: (1) It has a certain water solubility, and the amount of organic solvent added should be as low as possible, because the high proportions of organic solvents usually destroy the normal functions of biomolecules. (2) The recognition system cannot react with the detected bioactive species. For example, dimethyl sulfoxide (DMSO) is not suitable for examining hypochlorous acid (HClO), because it is easily oxidized by

Design of Fluorescent Probes for Diabetes
Organic fluorescent probes designed for prewarning and cure of diabetes should meet the following demands: (1) It has a certain water solubility, and the amount of organic solvent added should be as low as possible, because the high proportions of organic solvents usually destroy the normal functions of biomolecules. (2) The recognition system cannot react with the detected bioactive species. For example, dimethyl sulfoxide (DMSO) is not suitable for examining hypochlorous acid (HClO), because it is easily oxidized by HClO [37]. Similarly, HEPES (4-(2-hydroxyethyl)piperazine-1-ethanesulfonic acid) buffer solution is also easily oxidized by HClO; thus, HEPES buffer solution cannot be used as a system for detecting HClO [38]. (3) Fluorophores have high fluorescence quantum yield and excellent biocompatibility. (4) There are obvious fluorescence changes before and after The NIR-II region (1000-1700 nm) has drawn increasing attention owing to its noninvasiveness and superior temporal resolution and spatial resolution [55][56][57][58]. In 2020, Liu et al. reported four viscosity-activated NIR-II emissive probes 2-4 [59], which consisted of boron dipyrromethenes (BODIPY) dye as a reporting unit and 1-ethyl-2-methyl-benz [c,d]  The NIR-II region (1000-1700 nm) has drawn increasing attention owing to its non-invasiveness and superior temporal resolution and spatial resolution [55][56][57][58]. In 2020, Liu et al. reported four viscosity-activated NIR-II emissive probes 2-4 [59], which consisted of boron dipyrromethenes (BODIPY) dye as a reporting unit and 1-ethyl-2-methyl-benz [c,d] iodolium salt as a molecular rotor and electron-withdrawing group ( Figure 3A). Probe 3, compared to the other three molecules, exhibited the best advantages in viscosity sensitivity, environmental insensitivity, and quantum yield. In addition, probe 3 displayed the ability to monitor the viscosity behaviors in organisms and image the viscosity variation in liver injury caused by streptozotocin (STZ, an antitumor antibiotic)-induced diabetes. When treated with resveratrol (RVT, a drug for liver injury treatment) to prevent liver damage or lower blood sugar levels, the viscosity of diabetic mice decreased ( Figure 3B). right 2019, the Royal Society of Chemistry.
The NIR-II region (1000-1700 nm) has drawn increasing attention invasiveness and superior temporal resolution and spatial resolution [5 et al. reported four viscosity-activated NIR-II emissive probes 2-4 [59], boron dipyrromethenes (BODIPY) dye as a reporting unit and 1-ethyl-2 iodolium salt as a molecular rotor and electron-withdrawing group (Fi compared to the other three molecules, exhibited the best advantages tivity, environmental insensitivity, and quantum yield. In addition, pro ability to monitor the viscosity behaviors in organisms and image the in liver injury caused by streptozotocin (STZ, an antitumor antibiotic) When treated with resveratrol (RVT, a drug for liver injury treatmen damage or lower blood sugar levels, the viscosity of diabetic mice decr  In 2019, a red-emitting probe 6 for detecting viscosity, which consists of dicyanoisophorone as acceptor and modified julolidine as donor, was obtained by Zhu and coworkers ( Figure 4A) [60]. With the environmental viscosity aggrandized, a prominent fluorescence enhancement (400-fold) of probe 6 at approximately 675 nm was observed. Probe 6 possessed the advantages of high sensitivity, good water solubility, and low cytotoxicity; thus, it is befitting for monitoring the behavior of viscosity in vitro and in vivo. Probe 6 is regarded as the first probe to estimate the viscosity behavior between blood samples of diabetic and hypertensive patients ( Figure 4B). phorone as acceptor and modified julolidine as donor, was obtained by Zhu and cowork-ers ( Figure 4A) [60]. With the environmental viscosity aggrandized, a prominent fluorescence enhancement (400-fold) of probe 6 at approximately 675 nm was observed. Probe 6 possessed the advantages of high sensitivity, good water solubility, and low cytotoxicity; thus, it is befitting for monitoring the behavior of viscosity in vitro and in vivo. Probe 6 is regarded as the first probe to estimate the viscosity behavior between blood samples of diabetic and hypertensive patients ( Figure 4B). Zhou's group created a mitochondria-targeted NIR viscosity fluorescent probe (7) [61], consisting of 2,3,3-trimethyl-3H-indolenine and 4-(dimethylamino) cinnamaldehyde ( Figure 5A). Probe 7 had a double twistable ethylene structure with twisted intramolecular charge transfer (TICT) capacity. In probe 7, the cationic indoline moiety, as an acceptor, improved the aqueous solubility, the aniline group was essential for electron emission, and the conjugated diethylene bond structure would rotate sensitively with the difference in environmental viscosity. Probe 7 had some merits of NIR emission at 700 nm and good selectivity, sensitivity, and biocompatibility. Figure 5B shows that the viscosity of the insulin group was lower than that of the diabetic group, which indicates that the probe is suitable for monitoring viscosity behavior and imaging pancreatic tissue in diabetic mice. (B) the fluorescence spectra of probe 6 in the fresh blood of normal and diabetic mice, respectively, λ ex = 510 nm. Adapted from Reference [60]. Copyright 2020, the Royal Society of Chemistry.
Zhou's group created a mitochondria-targeted NIR viscosity fluorescent probe (7) [61], consisting of 2,3,3-trimethyl-3H-indolenine and 4-(dimethylamino) cinnamaldehyde ( Figure 5A). Probe 7 had a double twistable ethylene structure with twisted intramolecular charge transfer (TICT) capacity. In probe 7, the cationic indoline moiety, as an acceptor, improved the aqueous solubility, the aniline group was essential for electron emission, and the conjugated diethylene bond structure would rotate sensitively with the difference in environmental viscosity. Probe 7 had some merits of NIR emission at 700 nm and good selectivity, sensitivity, and biocompatibility. Figure 5B shows that the viscosity of the insulin group was lower than that of the diabetic group, which indicates that the probe is suitable for monitoring viscosity behavior and imaging pancreatic tissue in diabetic mice. Lin's group fabricated a NIR viscosity fluorescent probe 8 [62]. Probe 8 possessed a D-π-A configuration, which consisted of phenolic dihydroxanthene, with methoxy as the donor and quinolone as the acceptor through a flexible conjugated linker ( Figure 6A). The quaternary ammonium salt group was able to ensure the solubility of probe 8 in water (B) distribution of organ fluorescence in three groups, λ ex / em = 570/700 nm, * p < 0.05, # p < 0.05. Adapted from Reference [61]. Copyright 2021, the Royal Society of Chemistry.
Lin's group fabricated a NIR viscosity fluorescent probe 8 [62]. Probe 8 possessed a D-π-A configuration, which consisted of phenolic dihydroxanthene, with methoxy as the donor and quinolone as the acceptor through a flexible conjugated linker ( Figure 6A). The quaternary ammonium salt group was able to ensure the solubility of probe 8 in water and target mitochondria. A large Stokes shift was observed in probe 8, which effectively avoided the interference from overlapping excitation and emission. As the environmental viscosity rises, the fluorescence signal of the probe at 730 nm displayed 13-fold enhancement. In addition, probe 8 had the ability to visualize the viscosity behavior in mitochondria of living cells and zebrafish. Importantly, visualizing the differences in cell viscosity in the kidneys of diabetic and normal mice was achieved by probe 8 for the first time ( Figure 6B).
A NIR fluorescent probe (9) for examining the viscosity behavior of LDs was ob by Lin and colleagues [63]. Probe 9 was composed of triphenylamine as a fluores group and benzothiazole as viscosity-sensitive group through a benzene linker (F 7A). Probe 9 showed a larger stokes shift of approximately 195 nm, good photosta and high sensitivity of viscosity. With the viscosity boost, the fluorescence signal nm displayed remarkable enhancement (95-fold). The blood viscosity in normal mic lower than that of STZ-induced diabetic mice, and the cell localization experimen played that probe 9 accumulated obviously in LDs. In addition, probe 9 had the abi track metformin (Met, an antidiabetic drug)-treated diabetes by monitoring viscos havior ( Figure 7B). (B) HE staining, photos, and fluorescence imaging of the kidney in a normal mouse and a diabetic mouse treated with probe 8, respectively; HE staining of (Ba1) normal mouse kidney and (Bb1) diabetic mouse kidney; photos of (Ba2) normal mouse kidney and (Bb2) diabetic mouse kidney; fluorescence imaging of (Ba3) normal mouse kidney and (Bb3) diabetic mouse kidney, λ ex / em = 560/740 nm. Adapted from Reference [62]. Copyright 2021, Elsevier B.V.
A NIR fluorescent probe (9) for examining the viscosity behavior of LDs was obtained by Lin and colleagues [63]. Probe 9 was composed of triphenylamine as a fluorescence group and benzothiazole as viscosity-sensitive group through a benzene linker ( Figure 7A). Probe 9 showed a larger stokes shift of approximately 195 nm, good photostability, and high sensitivity of viscosity. With the viscosity boost, the fluorescence signal at 650 nm displayed remarkable enhancement (95-fold). The blood viscosity in normal mice was lower than that of STZ-induced diabetic mice, and the cell localization experiment displayed that probe 9 accumulated obviously in LDs. In addition, probe 9 had the ability to track metformin (Met, an antidiabetic drug)-treated diabetes by monitoring viscosity behavior ( Figure 7B).

Probes Targeting Polarity
Changes and developments in polarity may be caused by many cellular processes linked to spatial arrangement and protein composition such as cell differentiation, activation of the immune response, and localized membrane growth [64][65][66]. Abnormal polarity behavior may result in impaired function of subcellular organelles as well as the onset and development of various physiological or pathological activities [67]. For example, excessive accumulation of partially unfolded or glycosylated proteins in the ER stress environment can induce a polarity change [68]. The degree of liver injury caused by diabetes can be evaluated by differences or changes in ER polarity [69]. During autophagy, the fusion between the lysosomal membrane and the autophagosome membrane leads to an increase in polarity [70]. Therefore, tracking intracellular polarity behavior in different organelles is of great significance for illuminating cell state variation.
The fluorescent probe 10 with NIR emission, ratiometric photoacoustic (PA) imaging dual-mode, and ER-targeting was constructed by Tang and coworkers [71], which was used to image the behavior of polarity in the liver of normal and diabetic mice. Probe 10 was composed of merocyanine, with a tertiary amine as the donor and difluoroboronate as the acceptor ( Figure 8A). The conjugated system was able to offer NIR absorption and a strong PA signal. The fluorescence signal of probe 10 at approximately 800 nm exhibited dramatic enhancement by reducing the polarity in the environment. Probe 10 was able to examine the polarity changes with sensitivity and selectivity and had excellent biocompatibility and ER-targeting ability. Higher polarity behavior in the liver of diabetic mice than in normal mice was observed by ratiometric PA imaging, and when metformin was

Probes Targeting Polarity
Changes and developments in polarity may be caused by many cellular processes linked to spatial arrangement and protein composition such as cell differentiation, activation of the immune response, and localized membrane growth [64][65][66]. Abnormal polarity behavior may result in impaired function of subcellular organelles as well as the onset and development of various physiological or pathological activities [67]. For example, excessive accumulation of partially unfolded or glycosylated proteins in the ER stress environment can induce a polarity change [68]. The degree of liver injury caused by diabetes can be evaluated by differences or changes in ER polarity [69]. During autophagy, the fusion between the lysosomal membrane and the autophagosome membrane leads to an increase in polarity [70]. Therefore, tracking intracellular polarity behavior in different organelles is of great significance for illuminating cell state variation.
The fluorescent probe 10 with NIR emission, ratiometric photoacoustic (PA) imaging dual-mode, and ER-targeting was constructed by Tang and coworkers [71], which was used to image the behavior of polarity in the liver of normal and diabetic mice. Probe 10 was composed of merocyanine, with a tertiary amine as the donor and difluoroboronate as the acceptor ( Figure 8A). The conjugated system was able to offer NIR absorption and a strong PA signal. The fluorescence signal of probe 10 at approximately 800 nm exhibited dramatic enhancement by reducing the polarity in the environment. Probe 10 was able to examine the polarity changes with sensitivity and selectivity and had excellent biocompatibility and ER-targeting ability. Higher polarity behavior in the liver of diabetic mice than in normal mice was observed by ratiometric PA imaging, and when metformin was used, the polarity behavior in the liver tissue of diabetic mice showed an obvious decline ( Figure 8B). used, the polarity behavior in the liver tissue of diabetic mice showed an obvious decline ( Figure 8B). Probe 11 is an electro-negative, water-soluble Ir (III) complex for detecting polarity behavior that was reported by Zhang and coworkers [72]. With the increase or decrease in polarity in the environment, probe 11 exhibited excellent sensitivity ( Figure 9A). When the solvent polarity was reduced, the phosphorescence peak of probe 11 changed from 632 to 585 nm. Probe 11 with ER-targeting was able to trace the polarity behavior in situ during ER stress in vitro. In addition, the polarity difference between the fresh blood of diabetic and normal mice was examined by probe 11 ( Figure 9B). Qiang and colleagues fabricated a two-photon probe (12) with the features of lysosome targeting and polarity-sensitive properties ( Figure 10A) [73]. The probe featured several advantages of a large two-photon action cross-section, good water solubility, and excellent biocompatibility. When autophagy occurred, the fluorescence intensity in the green channel showed a dramatic decline. Probe 12 successfully visualized the polarity behavior of lysosomes in BV-2 cells. Furthermore, probe 12 was used to visualize the development of inflammation caused by skin ulcers in diabetic mice ( Figure 10B). Probe 11 is an electro-negative, water-soluble Ir (III) complex for detecting polarity behavior that was reported by Zhang and coworkers [72]. With the increase or decrease in polarity in the environment, probe 11 exhibited excellent sensitivity ( Figure 9A). When the solvent polarity was reduced, the phosphorescence peak of probe 11 changed from 632 to 585 nm. Probe 11 with ER-targeting was able to trace the polarity behavior in situ during ER stress in vitro. In addition, the polarity difference between the fresh blood of diabetic and normal mice was examined by probe 11 ( Figure 9B). used, the polarity behavior in the liver tissue of diabetic mice showed an obvious decline ( Figure 8B). Probe 11 is an electro-negative, water-soluble Ir (III) complex for detecting polarity behavior that was reported by Zhang and coworkers [72]. With the increase or decrease in polarity in the environment, probe 11 exhibited excellent sensitivity ( Figure 9A). When the solvent polarity was reduced, the phosphorescence peak of probe 11 changed from 632 to 585 nm. Probe 11 with ER-targeting was able to trace the polarity behavior in situ during ER stress in vitro. In addition, the polarity difference between the fresh blood of diabetic and normal mice was examined by probe 11 ( Figure 9B). Qiang and colleagues fabricated a two-photon probe (12) with the features of lysosome targeting and polarity-sensitive properties ( Figure 10A) [73]. The probe featured several advantages of a large two-photon action cross-section, good water solubility, and excellent biocompatibility. When autophagy occurred, the fluorescence intensity in the green channel showed a dramatic decline. Probe 12 successfully visualized the polarity behavior of lysosomes in BV-2 cells. Furthermore, probe 12 was used to visualize the development of inflammation caused by skin ulcers in diabetic mice ( Figure 10B). Qiang and colleagues fabricated a two-photon probe (12) with the features of lysosome targeting and polarity-sensitive properties ( Figure 10A) [73]. The probe featured several advantages of a large two-photon action cross-section, good water solubility, and excellent biocompatibility. When autophagy occurred, the fluorescence intensity in the green channel showed a dramatic decline. Probe 12 successfully visualized the polarity behavior of lysosomes in BV-2 cells. Furthermore, probe 12 was used to visualize the development of inflammation caused by skin ulcers in diabetic mice ( Figure 10B).

Probes Targeting ROS
Diabetes is a disease characterized by hyperglycemia, which threatens human health [70]. The excessive production of ROS is due to the oxidative phosphorylation of glucose, which results in the activation of non-enzyme glycosylation and the reduction of the antioxidant capacity of superoxide dismutase [74][75][76][77][78]. In addition, the burst of ROS may be linked to the onset and evolution of diabetes and related complications, including liver diseases, neurodegenerative diseases, and kidney diseases [79,80]. Therefore, revealing the detailed mechanism of the occurrence and evolution of diabetes has attracted great attention in the field of biology and medical research.

Probes Targeting ROS
Diabetes is a disease characterized by hyperglycemia, which threatens human health [70]. The excessive production of ROS is due to the oxidative phosphorylation of glucose, which results in the activation of non-enzyme glycosylation and the reduction of the antioxidant capacity of superoxide dismutase [74][75][76][77][78]. In addition, the burst of ROS may be linked to the onset and evolution of diabetes and related complications, including liver diseases, neurodegenerative diseases, and kidney diseases [79,80]. Therefore, revealing the detailed mechanism of the occurrence and evolution of diabetes has attracted great attention in the field of biology and medical research.
Superoxide anion (O 2 •− ), as a kind of cell signaling mediator of ROS, is related to the physiological and pathological process of diabetes [81][82][83]. In 2017, Tang and coworkers reported the ER-targeted two-photon fluorescent probe 13 [84], which was composed of 1,8-naphthalimide, benzothiazoline, and methyl sulphonamide ( Figure 11A). When O 2 •− was introduced, probe 13 exhibited a remarkable fluorescence enhancement (11-fold) around 450 nm, and the limit of detection was 60 nM. In addition, probe 13 featured the advantages of excellent sensitivity, selectivity, and stability and low toxicity. Fluorescence imaging experiments suggested that O 2 •− levels in ER were increased under different conditions. The fluorescence imaging of two-photon showed that endogenous O 2 •− in the liver and abdomen of normal mice was less than that of diabetic mice. Moreover, the diabetic mice were treated with an antidiabetic drug of metformin, and the levels of O 2 •− were reduced ( Figure 11B).
Chemosensors 2022, 10, x FOR PEER REVIEW 11 of 32 was introduced, probe 13 exhibited a remarkable fluorescence enhancement (11-fold) around 450 nm, and the limit of detection was 60 nM. In addition, probe 13 featured the advantages of excellent sensitivity, selectivity, and stability and low toxicity. Fluorescence imaging experiments suggested that O2 •− levels in ER were increased under different conditions. The fluorescence imaging of two-photon showed that endogenous O2 •− in the liver and abdomen of normal mice was less than that of diabetic mice. Moreover, the diabetic mice were treated with an antidiabetic drug of metformin, and the levels of O2 •− were reduced ( Figure 11B). In 2021, Lin and coworkers developed a two-photon fluorescent probe that could detect aberrant levels of O 2 •− in diabetes [85]. The probe adopted Rho as the red luminescent group and used diphenylphosphine as the quenching group and O 2 •− response site ( Figure 12A). After reacting with O 2 •− , probe 14 emitted red fluorescence at 638 nm with the detection limit of 2.09 µM and reached a response plateau after 150 s. Probe 14 had high sensitivity, excellent selectivity, and low toxicity. In addition, the concentrations of intracellular O 2 •− under different stimuli were observed by two-photon fluorescence imaging experiments. Probe 14 achieved the changes of O 2 •− in diabetic mice and mice after treatment ( Figure 12B).
In 2021, Lin and coworkers developed a two-photon fluorescent pr tect aberrant levels of O2 •− in diabetes [85]. The probe adopted Rho as th group and used diphenylphosphine as the quenching group and O2 •− re 12A). After reacting with O2 •− , probe 14 emitted red fluorescence at 63 tection limit of 2.09 μΜ and reached a response plateau after 150 s. P sensitivity, excellent selectivity, and low toxicity. In addition, the conce cellular O2 •− under different stimuli were observed by two-photon flu experiments. Probe 14 achieved the changes of O2 •− in diabetic mice an ment ( Figure 12B).  Peroxynitrite (ONOO − ), as a crucial oxidant in organisms, participates in the reaction of many biomolecules, such as proteins, lipids, and nucleic acids, thus leading to some diseases including diabetes, cancer, and autoimmune [86][87][88][89][90]. In 2016, an ONOO − fluorescent probe 15 was successfully designed and synthesized by Guo and coworkers [91], and it featured the advantages of fast response, a low detection limit (<2 nM), and excellent selectivity. When treated with ONOO − , the photoinduced electron transfer (PET) quenching process was inhibited, and probe 15 displayed remarkable green fluorescence enhancement (560-fold) around 517 nm ( Figure 13A). The colocalization results demonstrated that probes 16 and 17 were able to image ONOO − in the corresponding subcellular organelles, respectively. The visualization of endogenous ONOO − in RAW264.7 cells, EA.hy926 cells after oxygen-glucose deprivation (OGD), and kidneys of diabetic rats was realized by probe 15 ( Figure 13B). selectivity. When treated with ONOO − , the photoinduced electron transfer (PET) quenching process was inhibited, and probe 15 displayed remarkable green fluorescence enhancement (560-fold) around 517 nm ( Figure 13A). The colocalization results demonstrated that probes 16 and 17 were able to image ONOO − in the corresponding subcellular organelles, respectively. The visualization of endogenous ONOO − in RAW264.7 cells, EA.hy926 cells after oxygen-glucose deprivation (OGD), and kidneys of diabetic rats was realized by probe 15 ( Figure 13B). Probe 15 exhibited a fast response and good selectivity for ONOO − [91]. However, it could be found that probe 15 possessed excitation and emission wavelengths of 400-650 nm (visible region), which may be a defect for in vivo imaging applications. To overcome the above limitation, Guo's group successfully obtained the NIR fluorescent probe 18 for examining and visualizing endogenous ONOO − [92]. Probe 18 was composed of a functional group with aromatic tertiary amine and a Si-rhodamine fluorophore ( Figure 14A). When ONOO − was introduced to probe 18, a fast and specific reaction occurred along with an arresting fluorescence off-on response around 680 nm. Probe 18 also displayed outstanding sensitivity, with a detection limit of 3.0 nM and had the potential for visualizing and examining exogenous and endogenous ONOO − in cells. Probe 18 had the ability to analyze the pathogenesis in activated pancreatic β-cells and diabetic rats. In addition, visualization of ONOO − in diabetic rats during the development of diabetic nephropathy was also carried out ( Figure 14B). Probe 15 exhibited a fast response and good selectivity for ONOO − [91]. However, it could be found that probe 15 possessed excitation and emission wavelengths of 400-650 nm (visible region), which may be a defect for in vivo imaging applications. To overcome the above limitation, Guo's group successfully obtained the NIR fluorescent probe 18 for examining and visualizing endogenous ONOO − [92]. Probe 18 was composed of a functional group with aromatic tertiary amine and a Si-rhodamine fluorophore ( Figure 14A). When ONOO − was introduced to probe 18, a fast and specific reaction occurred along with an arresting fluorescence off-on response around 680 nm. Probe 18 also displayed outstanding sensitivity, with a detection limit of 3.0 nM and had the potential for visualizing and examining exogenous and endogenous ONOO − in cells. Probe 18 had the ability to analyze the pathogenesis in activated pancreatic β-cells and diabetic rats. In addition, visualization of ONOO − in diabetic rats during the development of diabetic nephropathy was also carried out ( Figure 14B).
Diabetes is closely related to excessive HClO induced by ER stress [93,94]; thus, it is vital to test HClO behavior in the ER to study the relationships between HClO and diabetes. In 2021, Lin et al. constructed a two-photon fluorescent probe (19) for examining HClO behavior in diabetes [95]. Probe 19 consisted of rhodol fluorophore, p-methylbenzene sulfonamide serving as an ER-specific part, and 4-Bromo-2-hydroxybenzaldehyde for improving its lipid solubility ( Figure 15A). When probe 19 was treated with various concentrations of HClO, the fluorescence signal (556 nm) enhanced depending on the concentration. In addition, probe 19 exhibited high selectivity and sensitivity with a detection limit of 0.785 µM for detecting HClO. Moreover, probe 19 was applied in the monitoring of HClO levels in vivo ( Figure 15B). Diabetes is closely related to excessive HClO induced by ER stress [93,94]; thus, it is vital to test HClO behavior in the ER to study the relationships between HClO and diabetes. In 2021, Lin et al. constructed a two-photon fluorescent probe (19) for examining HClO behavior in diabetes [95]. Probe 19 consisted of rhodol fluorophore, p-methylbenzene sulfonamide serving as an ER-specific part, and 4-Bromo-2-hydroxybenzaldehyde for improving its lipid solubility ( Figure 15A). When probe 19 was treated with various concentrations of HClO, the fluorescence signal (556 nm) enhanced depending on the concentration. In addition, probe 19 exhibited high selectivity and sensitivity with a detection limit of 0.785 μM for detecting HClO. Moreover, probe 19 was applied in the monitoring of HClO levels in vivo ( Figure 15B).  (20) to observe HClO production associated with diabetic cataracts [96]. The benzophenothiazine moiety of the probe not only functions as a fluorophore but also acts as a specific recognition group for HClO, and pyridinium salt was introduced to increase the water solubility of the probe ( Figure 16A). Probe 20 showed a significant emission enhancement of HClO at 562 nm, with high selectivity, excellent sensitivity, a low detection limit (LOD = 12.6 nM), and fast  (20) to observe HClO production associated with diabetic cataracts [96]. The benzophenothiazine moiety of the probe not only functions as a fluorophore but also acts as a specific recognition group for HClO, and pyridinium salt was introduced to increase the water solubility of the probe ( Figure 16A). Probe 20 showed a significant emission enhancement of HClO at 562 nm, with high selectivity, excellent sensitivity, a low detection limit (LOD = 12.6 nM), and fast response (within seconds). Probe 20 has been successfully used to detect exogenous and endogenous HClO in living cells. In addition, probe 20 was successfully used to monitor HClO levels in diabetic HLE cells, as well as downregulation of HClO during antioxidant (NAC or AA) treatment ( Figure 16B). Adopting quinolinium-xanthene dye as a fluorophore and borate est sponse group of H2O2, the NIR fluorescent probe 21 for examining H2O2 was achieved by Li's group ( Figure 17A) [97]. After H2O2 was introduced into pro Adopting quinolinium-xanthene dye as a fluorophore and borate ester as the response group of H 2 O 2 , the NIR fluorescent probe 21 for examining H 2 O 2 was successfully achieved by Li's group ( Figure 17A) [97]. After H 2 O 2 was introduced into probe 21, a new remarkable NIR fluorescence peak around 772 nm was found, with a detection limit of 0.17 µM. Probe 21 featured outstanding sensitivity and selectivity for testing H 2 O 2 . Moreover, visualizing exogenous and endogenous H 2 O 2 in different cells and zebrafish was realized by probe 21. Figure 17B shows that the fluorescence intensity of the control mice was weaker than that of the diabetic mice, and the signal in the diabetic mice was reduced obviously with treatment with metformin. In 2021, a fluorophore-dapagliflozin dyad (22), as a theranostic system, w and synthesized by Wu's group [98]. Probe 22 consisted of NIR fluorescent d the SGLT2 inhibitor, dapagliflozin (DA), and they were connected by a bora which acted as a linker between the fluorescence quencher and the responsi ure 18A). Overexpressed H2O2 triggered the theranostic system of the probe and DA were released. Meanwhile, the absorption band from 590 to 685 n obvious changes, and the fluorescence intensity around 705 nm also exhibite enhancement. Probe 22 had excellent sensitivity, good selectivity, low toxici biosafety. Furthermore, the probe not only offered visualization of liver/kidn duced by diabetes but also improved the therapeutic effect of type II diabetes In 2021, a fluorophore-dapagliflozin dyad (22), as a theranostic system, was designed and synthesized by Wu's group [98]. Probe 22 consisted of NIR fluorescent dye (DX) and the SGLT2 inhibitor, dapagliflozin (DA), and they were connected by a borate ester bond which acted as a linker between the fluorescence quencher and the responsive ROS ( Figure 18A). Overexpressed H 2 O 2 triggered the theranostic system of the probe such that DX and DA were released. Meanwhile, the absorption band from 590 to 685 nm displayed obvious changes, and the fluorescence intensity around 705 nm also exhibited significant enhancement. Probe 22 had excellent sensitivity, good selectivity, low toxicity, and good biosafety. Furthermore, the probe not only offered visualization of liver/kidney injury induced by diabetes but also improved the therapeutic effect of type II diabetes ( Figure 18B).

Probes Targeting H2S and Cys
H2S, a vital endogenous signaling molecule with reductive properties, is thought to be associated with diabetes [24,99,100]. H2S not only adjusts the insulin sensitivity in insulin-targeted organs but also preserves intracellular redox homeostasis [101]. Cys serves as a vital scavenger of ROS under mitochondrial oxidative stress, which is very vital in the biosystem [102]. Therefore, it is of great significance to research the relations and connections between mitochondrial oxidative stress and diabetes by monitoring the levels or behaviors of Cys in mitochondria.
In 2022, Wei et al. obtained the dual-locked fluorescent probe 23 with NIR emission for testing the H2S behavior in organisms [103]. In probe 23, rhodamine B was used as a fluorophore with NIR emission, and aromatic azide and NBD-piperazine acted as the reactive site of H2S ( Figure 19A). The fluorescence signal at 663 nm was locked and quenched by the processes of intramolecular charge transfer (ICT) and PET, and the detection limit was calculated as 33 nM. Probe 23 showed excellent selectivity and good sensitivity for visualizing the changes of H2S. Importantly, probe 23 had the ability to image the behavior of endogenous H2S in diabetic mice ( Figure 19B).

Probes Targeting H 2 S and Cys
H 2 S, a vital endogenous signaling molecule with reductive properties, is thought to be associated with diabetes [24,99,100]. H 2 S not only adjusts the insulin sensitivity in insulin-targeted organs but also preserves intracellular redox homeostasis [101]. Cys serves as a vital scavenger of ROS under mitochondrial oxidative stress, which is very vital in the biosystem [102]. Therefore, it is of great significance to research the relations and connections between mitochondrial oxidative stress and diabetes by monitoring the levels or behaviors of Cys in mitochondria.
In 2022, Wei et al. obtained the dual-locked fluorescent probe 23 with NIR emission for testing the H 2 S behavior in organisms [103]. In probe 23, rhodamine B was used as a fluorophore with NIR emission, and aromatic azide and NBD-piperazine acted as the reactive site of H 2 S ( Figure 19A). The fluorescence signal at 663 nm was locked and quenched by the processes of intramolecular charge transfer (ICT) and PET, and the detection limit was calculated as 33 nM. Probe 23 showed excellent selectivity and good sensitivity for visualizing the changes of H 2 S. Importantly, probe 23 had the ability to image the behavior of endogenous H 2 S in diabetic mice ( Figure 19B). In 2022, Wei and coworkers reported a mitochondrial-targeted Cys NIR fluorescent probe 24 [104]. The cyanine in the probe acted as a NIR fluorescent dye and its positive charge could be used as a mitochondrial targeting group, and thiobenzoate was used as a Cys-specific recognition group ( Figure 20A). The fluorescence signal at 628 nm gradually increased depending on Cys concentration, and the detection limit was 36.8 nM. The levels of Cys in mitochondrial were remarkably reduced in diabetic cells and mice. Figure  20B showed that airborne particulate matter (APM) promoted the development of diabetes, and the main damaged organs of diabetic mice were liver and kidney. In 2022, Wei and coworkers reported a mitochondrial-targeted Cys NIR fluorescent probe 24 [104]. The cyanine in the probe acted as a NIR fluorescent dye and its positive charge could be used as a mitochondrial targeting group, and thiobenzoate was used as a Cys-specific recognition group ( Figure 20A). The fluorescence signal at 628 nm gradually increased depending on Cys concentration, and the detection limit was 36.8 nM. The levels of Cys in mitochondrial were remarkably reduced in diabetic cells and mice. Figure 20B showed that airborne particulate matter (APM) promoted the development of diabetes, and the main damaged organs of diabetic mice were liver and kidney. In 2022, Wei and coworkers reported a mitochondrial-targeted Cys NIR fluorescent probe 24 [104]. The cyanine in the probe acted as a NIR fluorescent dye and its positive charge could be used as a mitochondrial targeting group, and thiobenzoate was used as a Cys-specific recognition group ( Figure 20A). The fluorescence signal at 628 nm gradually increased depending on Cys concentration, and the detection limit was 36.8 nM. The levels of Cys in mitochondrial were remarkably reduced in diabetic cells and mice. Figure  20B showed that airborne particulate matter (APM) promoted the development of diabetes, and the main damaged organs of diabetic mice were liver and kidney.

Probes Targeting Methylglyoxal (MGO)
Methylglyoxal (MGO), as a small molecule by-product, is produced by a variety of metabolic pathways, such as glycolysis, polyol pathway, and threonine [105][106][107]. In addition, MGO also acts as an effective saccharifying agent, which has the ability to help the formation of terminal glycation products and has a remarkable reactivity of 20,000 folds [108]. Currently, studies have shown that MGO concentrations often rise in conditions of hyperglycemia, including diabetes [109]. In particular, for type II diabetes, MGO plays a potentially critical role in visualizing disease development and therapeutic effects [110,111].
In 2018, an ER-targeted two-photon fluorescent probe 25 for detecting MGO levels was achieved by Fan and coworkers [112]. After o-phenylenediamine moiety of the probe reacted with MGO, the PET process was repressed, and an obvious fluorescence signal appeared at 460 nm ( Figure 21A). The probe could be used for examining MGO with excellent selectivity, excellent sensitivity, low toxicity, and a lower detection limit (56 nM). Monitoring MGO in living cells and diabetic mice was realized by probe 25. When metformin was introduced to diabetic mice, the level of MGO in the liver and kidney displayed an obvious reduction ( Figure 21B).

Probes Targeting Methylglyoxal (MGO)
Methylglyoxal (MGO), as a small molecule by-product, is produced by a variety of metabolic pathways, such as glycolysis, polyol pathway, and threonine [105][106][107]. In addition, MGO also acts as an effective saccharifying agent, which has the ability to help the formation of terminal glycation products and has a remarkable reactivity of 20000 folds [108]. Currently, studies have shown that MGO concentrations often rise in conditions of hyperglycemia, including diabetes [109]. In particular, for type II diabetes, MGO plays a potentially critical role in visualizing disease development and therapeutic effects [110,111].
In 2018, an ER-targeted two-photon fluorescent probe 25 for detecting MGO levels was achieved by Fan and coworkers [112]. After o-phenylenediamine moiety of the probe reacted with MGO, the PET process was repressed, and an obvious fluorescence signal appeared at 460 nm ( Figure 21A). The probe could be used for examining MGO with excellent selectivity, excellent sensitivity, low toxicity, and a lower detection limit (56 nM). Monitoring MGO in living cells and diabetic mice was realized by probe 25. When metformin was introduced to diabetic mice, the level of MGO in the liver and kidney displayed an obvious reduction ( Figure 21B).  [113]. The probe used 2-aminoacetamide as the reaction site and exhibited outstanding fluorescence intensity around 525 nm during the process of detecting MGO, which was free from the interference of NO and FA ( Figure  22A). Probe 26 featured some merits, with excellent sensitivity and selectivity and a low detection limit (0.24 μM). In addition, probe 26 was applied to track the behavior of  [113]. The probe used 2-aminoacetamide as the reaction site and exhibited outstanding fluorescence intensity around 525 nm during the process of detecting MGO, which was free from the interference of NO and FA ( Figure 22A). Probe 26 featured some merits, with excellent sensitivity and selectivity and a low detection limit (0.24 µM). In addition, probe 26 was applied to track the behavior of intracellular MGO and to assess the changes of MGO in clinical blood samples from patients with diabetes and normal people ( Figure 22B).
Chemosensors 2022, 10, x FOR PEER REVIEW 2 intracellular MGO and to assess the changes of MGO in clinical blood samples from tients with diabetes and normal people ( Figure 22B).  [114]. When the probes reacted with MGO (GOS), deprotonation-cyclization of the guanidium ion process occurred, and an exoc five-membered dihydroxyimi-dazolidines was produced ( Figure 23A). Upon GOS b introduced into probe 27, a new fluorescence signal peak appeared around 564 nm. W GOS coexisted with aminoguanidine or N-acetylcysteine (quenchers of GOS), pro displayed reversible fluorescence turn-on responses. Probe 27 displayed excellent fea of quick, reversible, and good selectivity for testing GOS. The spectral response beh of ER-targeting probe 28 for detecting GOS was similar to probe 27, and it had the a to investigate the GOS changes of ER under various stress environments (Figure 23B , type II diabetic foot; (Be,Bk) sample 5 (person 5), type I diabetics; (Bf,Bl) sample 6 (person 6), type II diabetic retinopathy; (C) integrated emission intensity ratios (I blue /I green ) of images. Conditions: λ ex = 405 nm; for blue channel, λ em = 420-480 nm; for green channel, λ em = 500-650 nm. Scale bar: 20 µm. Adapted from Reference [113]. Copyright 2019, American Chemical Society.
Two fluorescent probes (27 and 28) based on 4-(2-guanidino)-1,8-naphthalimide were reported by Lou and coworkers in 2020 [114]. When the probes reacted with MGO/GO (GOS), deprotonation-cyclization of the guanidium ion process occurred, and an exocyclic five-membered dihydroxyimi-dazolidines was produced ( Figure 23A). Upon GOS being introduced into probe 27, a new fluorescence signal peak appeared around 564 nm. When GOS coexisted with aminoguanidine or N-acetylcysteine (quenchers of GOS), probe 27 displayed reversible fluorescence turn-on responses. Probe 27 displayed excellent features of quick, reversible, and good selectivity for testing GOS. The spectral response behavior of ER-targeting probe 28 for detecting GOS was similar to probe 27, and it had the ability to investigate the GOS changes of ER under various stress environments ( Figure 23B).

Probes Targeting Enzymes
Enzymes, as one potential biomarker, have aroused wide concern in recent yea ing to their intriguing and vital roles in various processes including physiology, p ogy, and pharmacology [32,115,116]. Studies have shown that caspases (Cas), dipe peptidase-IV (DPP-IV), nitroreductase, and leucine aminopeptidase (LAP) are po indicators of specific diseases [117,118]. For example, DPP-IV can be used as a tar the diagnosis and cure of diabetes, and it plays a vital role in adjusting blood g levels in the biosystem [28]. Hence, precisely detecting the activity of various enzy complex biosystems, particularly the activity of diabetes-related enzymes in plasm be helpful for early diagnosis of disease, drug discovery, and clinical application.
In 2020, Li et al. fabricated a fluorescent probe (29) with NIR emission and ex water solubility for testing the activity of alkaline phosphatase (ALP) [119]. The f cence of the free probe 29 was interdicted by the phosphate group due to the supp ICT effect ( Figure 24A). When ALP was added to probe 29, a remarkable fluorescen sion signal appeared at 770 nm. Probe 29 exhibited excellent sensitivity and selectiv detecting ALP, and the detection limit was calculated as 0.017 U mL −1 . Moreover 29 successfully testified that the concentration of ALP in diabetic mice was highe that in normal mice. When diabetic mice were treated with a hypoglycemic drug (m min), a marked decrease in ALP occurred ( Figure 24B). for (Ba), λ ex = 425 nm, λ em = 564 nm; for (Bc), λ ex = 440 nm, λ em = 540 ± 25 nm. *** p < 0.0001. Adapted from Reference [114]. Copyright 2020, American Chemical Society.

Probes Targeting Enzymes
Enzymes, as one potential biomarker, have aroused wide concern in recent years owing to their intriguing and vital roles in various processes including physiology, pathology, and pharmacology [32,115,116]. Studies have shown that caspases (Cas), dipeptidyl peptidase-IV (DPP-IV), nitroreductase, and leucine aminopeptidase (LAP) are potential indicators of specific diseases [117,118]. For example, DPP-IV can be used as a target for the diagnosis and cure of diabetes, and it plays a vital role in adjusting blood glucose levels in the biosystem [28]. Hence, precisely detecting the activity of various enzymes in complex biosystems, particularly the activity of diabetes-related enzymes in plasma, will be helpful for early diagnosis of disease, drug discovery, and clinical application.
In 2020, Li et al. fabricated a fluorescent probe (29) with NIR emission and excellent water solubility for testing the activity of alkaline phosphatase (ALP) [119]. The fluorescence of the free probe 29 was interdicted by the phosphate group due to the suppressed ICT effect ( Figure 24A). When ALP was added to probe 29, a remarkable fluorescent emission signal appeared at 770 nm. Probe 29 exhibited excellent sensitivity and selectivity for detecting ALP, and the detection limit was calculated as 0.017 U mL −1 . Moreover, probe 29 successfully testified that the concentration of ALP in diabetic mice was higher than that in normal mice. When diabetic mice were treated with a hypoglycemic drug (metformin), a marked decrease in ALP occurred ( Figure 24B). A fluorescent probe (30) with features of good water solubility and NIR emission, was developed in 2020 by Gao and coworkers [120]. Probe 30 was composed of a cyanine skeleton containing a sulfonic acid group (as a fluorophore) and glycylproline dipeptidyl aminopeptidase (GPDA, as a specific recognition site) ( Figure 25A). When DPP-IV was introduced to the probe solution, probe 30 exhibited a remarkable NIR fluorescence emission. Probe 30 exhibited the excellent features of selectivity, high sensitivity, and a low detection limit of 0.19 ng mL −1 . In addition, probe 30 had the ability to monitor DPP-IV changes in serum samples from normal and diabetic people and visualize DPP-IV in vivo ( Figure 25B).  A fluorescent probe (30) with features of good water solubility and NIR emission, was developed in 2020 by Gao and coworkers [120]. Probe 30 was composed of a cyanine skeleton containing a sulfonic acid group (as a fluorophore) and glycylproline dipeptidyl aminopeptidase (GPDA, as a specific recognition site) ( Figure 25A). When DPP-IV was introduced to the probe solution, probe 30 exhibited a remarkable NIR fluorescence emission. Probe 30 exhibited the excellent features of selectivity, high sensitivity, and a low detection limit of 0.19 ng mL −1 . In addition, probe 30 had the ability to monitor DPP-IV changes in serum samples from normal and diabetic people and visualize DPP-IV in vivo ( Figure 25B). A fluorescent probe (30) with features of good water solubility and NIR emission, was developed in 2020 by Gao and coworkers [120]. Probe 30 was composed of a cyanine skeleton containing a sulfonic acid group (as a fluorophore) and glycylproline dipeptidyl aminopeptidase (GPDA, as a specific recognition site) ( Figure 25A). When DPP-IV was introduced to the probe solution, probe 30 exhibited a remarkable NIR fluorescence emission. Probe 30 exhibited the excellent features of selectivity, high sensitivity, and a low detection limit of 0.19 ng mL −1 . In addition, probe 30 had the ability to monitor DPP-IV changes in serum samples from normal and diabetic people and visualize DPP-IV in vivo ( Figure 25B).   Figure 26A) [121]. A large Stokes shift of 135 nm was found in probe 31. Upon DPP-IV being reacted with probe 31, the fluorescence emission behavior in the mixture solution was changed from yellow (564.8 nm) to red (616.8 nm) with the detection limit of 5.8 ng/mL. Comparing the fluorescence signals of diabetic mice with those of normal mice and tumor-bearing mice, the diabetic mice displayed outstanding fluorescence signals. Organ experiments suggested that DPP-IV was relatively gathered in diabetic mice livers. Moreover, probe 31 possessed the ability to monitor the behavior of DPP-IV and diagnose diabetic mice ( Figure 26B). In 2021, Liu et al. constructed a fluorescent probe (31) with ratiometric emission and long-wavelength emission for examining DPP-IV ( Figure 26A) [121]. A large Stokes shift of 135 nm was found in probe 31. Upon DPP-IV being reacted with probe 31, the fluorescence emission behavior in the mixture solution was changed from yellow (564.8 nm) to red (616.8 nm) with the detection limit of 5.8 ng/mL. Comparing the fluorescence signals of diabetic mice with those of normal mice and tumor-bearing mice, the diabetic mice displayed outstanding fluorescence signals. Organ experiments suggested that DPP-IV was relatively gathered in diabetic mice livers. Moreover, probe 31 possessed the ability to monitor the behavior of DPP-IV and diagnose diabetic mice ( Figure 26B).

Probes Targeting pH Values
The skin surface of healthy people was found to be subacid (4.2-5.9); however, obvious alkaline (7.14-8.9) was found at the chronic wound stage when delayed healing occurred in certain pathological environments [122][123][124]. Chronic wounds, including diabetic foot ulcers, venous leg ulcers, and bedsores, decrease patients' quality of life and increase their financial burden [125][126][127]. A serious type of body condition in patients with type I diabetes is diabetic ketoacidosis (DKA) [128]. It is a phenomenon of hyperglycemia

Probes Targeting pH Values
The skin surface of healthy people was found to be subacid (4.2-5.9); however, obvious alkaline (7.14-8.9) was found at the chronic wound stage when delayed healing occurred in certain pathological environments [122][123][124]. Chronic wounds, including diabetic foot ulcers, venous leg ulcers, and bedsores, decrease patients' quality of life and increase their financial burden [125][126][127]. A serious type of body condition in patients with type I diabetes is diabetic ketoacidosis (DKA) [128]. It is a phenomenon of hyperglycemia and metabolic acidosis caused by the deposition of ketone bodies in the blood and urine, and the accumulation of these ketones and the resulting ketoacidosis can reduce the pH of blood and urine [129]. There are currently known methods for detecting pH values, including glass electrodes, luminescent probes, and electrochemical methods of detection of chronic wounds, which have the disadvantages of only single-spot value detecting or tracking in real time [127]. Thus, developing novel and convenient methods with high spatiotemporal resolutions to monitor pH behavior is crucial.
In 2019, Wu and coworkers reported a NIR fluorescent probe (32) for sensitivity examining pH ( Figure 27A) [130]. The pH experiments revealed that the fluorescence emission of probe 32 decreased drastically in the region of pH from 7.0 to 10.5. Probe 32 exhibited good selectivity and excellent photostability in acidic and alkaline environments. Moreover, the pH behavior in the course of chronic wound development in diabetic mice was visualized by probe 32 for the first time ( Figure 27B). and metabolic acidosis caused by the deposition of ketone bodies in the blood and urine, and the accumulation of these ketones and the resulting ketoacidosis can reduce the pH of blood and urine [129]. There are currently known methods for detecting pH values, including glass electrodes, luminescent probes, and electrochemical methods of detection of chronic wounds, which have the disadvantages of only single-spot value detecting or tracking in real time [127]. Thus, developing novel and convenient methods with high spatiotemporal resolutions to monitor pH behavior is crucial. In 2019, Wu and coworkers reported a NIR fluorescent probe (32) for sensitivity examining pH ( Figure 27A) [130]. The pH experiments revealed that the fluorescence emission of probe 32 decreased drastically in the region of pH from 7.0 to 10.5. Probe 32 exhibited good selectivity and excellent photostability in acidic and alkaline environments. Moreover, the pH behavior in the course of chronic wound development in diabetic mice was visualized by probe 32 for the first time ( Figure 27B). In 2021, Radhakrishnan's group fabricated a ratiometric fluorescent probe for monitoring pH changes in patients with type I diabetes ( Figure 28A) [131]. With the augment of basicity, the fluorescence emission wavelength of probe 33 gradually red-shifted from 414 to 447 nm. In addition, probe 33 had the excellent advantages of good water solubility, photostability, reversibility, and selectivity. The probe could directly detect the DKA status of patients with type I diabetes ( Figure 28B).  (20 μM) in PBS were measured for three different ratios of acetoacetate and 3-hydroxy butyric acid (i.e., 1:1, 1:3, and 1:6); (C) the fluorescence response of probe 33 in PBS was measured for three different ratios of acetoacetate and 3-hydroxy butyric acid (i.e., 1:1, 1:3, and 1:6); (D) expanded graph of (C). λex = 350 nm. Adapted from Reference [131]. Copyright 2021, Elsevier B.V. In 2021, Radhakrishnan's group fabricated a ratiometric fluorescent probe for monitoring pH changes in patients with type I diabetes ( Figure 28A) [131]. With the augment of basicity, the fluorescence emission wavelength of probe 33 gradually red-shifted from 414 to 447 nm. In addition, probe 33 had the excellent advantages of good water solubility, photostability, reversibility, and selectivity. The probe could directly detect the DKA status of patients with type I diabetes ( Figure 28B). and metabolic acidosis caused by the deposition of ketone bodies in the blood and urine, and the accumulation of these ketones and the resulting ketoacidosis can reduce the pH of blood and urine [129]. There are currently known methods for detecting pH values, including glass electrodes, luminescent probes, and electrochemical methods of detection of chronic wounds, which have the disadvantages of only single-spot value detecting or tracking in real time [127]. Thus, developing novel and convenient methods with high spatiotemporal resolutions to monitor pH behavior is crucial. In 2019, Wu and coworkers reported a NIR fluorescent probe (32) for sensitivity examining pH ( Figure 27A) [130]. The pH experiments revealed that the fluorescence emission of probe 32 decreased drastically in the region of pH from 7.0 to 10.5. Probe 32 exhibited good selectivity and excellent photostability in acidic and alkaline environments. Moreover, the pH behavior in the course of chronic wound development in diabetic mice was visualized by probe 32 for the first time ( Figure 27B). In 2021, Radhakrishnan's group fabricated a ratiometric fluorescent probe for monitoring pH changes in patients with type I diabetes ( Figure 28A) [131]. With the augment of basicity, the fluorescence emission wavelength of probe 33 gradually red-shifted from 414 to 447 nm. In addition, probe 33 had the excellent advantages of good water solubility, photostability, reversibility, and selectivity. The probe could directly detect the DKA status of patients with type I diabetes ( Figure 28B).  (20 μM) in PBS were measured for three different ratios of acetoacetate and 3-hydroxy butyric acid (i.e., 1:1, 1:3, and 1:6); (C) the fluorescence response of probe 33 in PBS was measured for three different ratios of acetoacetate and 3-hydroxy butyric acid (i.e., 1:1, 1:3, and 1:6); (D) expanded graph of (C). λex = 350 nm. Adapted from Reference [131]. Copyright 2021, Elsevier B.V.  (20 µM) in PBS were measured for three different ratios of acetoacetate and 3-hydroxy butyric acid (i.e., 1:1, 1:3, and 1:6); (C) the fluorescence response of probe 33 in PBS was measured for three different ratios of acetoacetate and 3-hydroxy butyric acid (i.e., 1:1, 1:3, and 1:6); (D) expanded graph of (C). λ ex = 350 nm. Adapted from Reference [131]. Copyright 2021, Elsevier B.V.

Probes Targeting O 2 •− and Polarity
Diabetic cardiomyopathy (DCM) is a diabetic complication that threatens human health, and the occurrence of DCM is closely related to abnormal fluctuations of ROS [75,[132][133][134][135]. DCM also may be able to change the polarity in myocardial cells [136]. In 2018, the ER-targeting fluorescent probe 34, including 1,8-naphthalimide as a potent fluorophore, caffeic acid group as O 2 •− the recognition unit, and methylsulphonamide as an ER-targeting moiety, was successfully obtained by Tang et al. (Figure 29A) [137]. Upon the introduction of O 2 •− , the fluorescence signal at 545 nm displayed obvious enhancement owing to caffeic acid being instantaneously oxidized to benzoquinone and releasing strong a fluorescence of 1,8-naphthalimide. Probes 34 and 10 exhibited different fluorescence spectra (405/545 nm for probe 34 and 633/800 nm for probe 10), and endogenous O 2 •− level and polarity differences in normal and diabetic myocardium were determined using this dual-color imaging method ( Figure 29B).

Probes Targeting O2 •-and Polarity
Diabetic cardiomyopathy (DCM) is a diabetic complication that threatens human health, and the occurrence of DCM is closely related to abnormal fluctuations of ROS [75,[132][133][134][135]. DCM also may be able to change the polarity in myocardial cells [136]. In 2018, the ER-targeting fluorescent probe 34, including 1,8-naphthalimide as a potent fluorophore, caffeic acid group as O2 •− the recognition unit, and methylsulphonamide as an ER-targeting moiety, was successfully obtained by Tang et al. (Figure 29A) [137]. Upon the introduction of O2 •− , the fluorescence signal at 545 nm displayed obvious enhancement owing to caffeic acid being instantaneously oxidized to benzoquinone and releasing strong a fluorescence of 1,8-naphthalimide. Probes 34 and 10 exhibited different fluorescence spectra (405/545 nm for probe 34 and 633/800 nm for probe 10), and endogenous O2 •− level and polarity differences in normal and diabetic myocardium were determined using this dual-color imaging method ( Figure 29B).

Conclusions
Diabetes is associated with complications in various organs, especially the liver, kidneys, heart, and blood vessels. Effective and accurate diagnosis or tracking of early diabetes and its complications is of importance for early diagnosis and treatment of diabetes. To date, great progresses have been achieved in the development of organic fluorescent probes for detecting and treating diabetes. In this review, we mainly summarized the organic fluorescent probes used in the study of diabetes and its complications, and analyzed their design concept, potential detection mechanism, advantages, and disadvantages.

Conclusions
Diabetes is associated with complications in various organs, especially the liver, kidneys, heart, and blood vessels. Effective and accurate diagnosis or tracking of early diabetes and its complications is of importance for early diagnosis and treatment of diabetes. To date, great progresses have been achieved in the development of organic fluorescent probes for detecting and treating diabetes. In this review, we mainly summarized the organic fluorescent probes used in the study of diabetes and its complications, and analyzed their design concept, potential detection mechanism, advantages, and disadvantages. Currently, most fluorescent probes are designed and synthesized based on abnormally expressed biomarkers during the development of diabetes or its complications such as polarity, viscosity, ROS, RSS, pH values, MGO, and enzymes. Among them, some probes can achieve subcellular localization, which provides evidence for studying the relationship between diabetes and subcellular organelles such as lysosomes, mitochondria, and endoplasmic reticulum. In addition, several probes with two-photon or NIR/NIR-II regions, especially for photoacoustic imaging applications, have been developed. They are helpful for further imaging in tissue or in vivo, which will promote the application of probes in clinical diagnostics.
Although considerable progresses have been made, there are still some challenges to overcome. Firstly, for the diagnosis and treatment of diabetes, most fluorescent probes are basically monochromatic at present, which are easily interfered with by external factors. In addition, most organic small molecules always face some disadvantages, such as short fluorescence lifetime, small Stokes shift, and susceptibility to spontaneous fluorescence background in cell imaging applications. In order to solve these problems, it is vital to fabricate novel probes with some excellent advantages of two-photon fluorophores, NIR luminescence, and even NIR-II luminescence for early diagnosis and cure of diabetes and complications efficiently and accurately. Secondly, some probes possess poor water solubility and generally require organic solvents such as DMSO and acetonitrile as auxiliary solvents. Therefore, designing and constructing novel fluorescent probes with excellent water solubility and biocompatibility are necessary and important for further biological applications. Finally, dual-recognition fluorescent probes that can detect two or more biomarkers of diabetes and complications remain rare. In biological systems, changes in one active substance may lead to changes in another related active substance. Thus, developing bifunctional or multifunctional fluorescent probes is beneficial for the simultaneous recognition of two or more indicators in the diagnosis and cure of diabetes and complications.
In summary, fluorescent probes possess excellent advantages in the diagnosis and cure of diabetes and complications, and we strongly believe that fluorescent probes for monitoring diabetes and complications will become increasingly important tools in future work.

Conflicts of Interest:
The authors declare no conflict of interest.