The interrelationships and synergistic regulations of bioactive molecules play pivotal roles in physiological and pathological processes involved in the initiation and development of some diseases, such as cancer and neurodegenerative and cardiovascular diseases. Therefore, the simultaneous, accurate and timely detection of two bioactive molecules is crucial to explore their roles and pathological mechanisms in related diseases. Fluorescence imaging associated with small molecular probes has been widely used in the imaging of bioactive molecules in living cells and in vivo due to its excellent performances, including high sensitivity and selectivity, noninvasive properties, real-time and high spatial temporal resolution. Single organic molecule fluorescent probes have been successively developed to simultaneously monitor two biomolecules to uncover their synergistic relationships in living systems. Hence, in this review, we focus on summarizing the design strategies, classifications, and bioimaging applications of dual-response fluorescent probes over the past decade. Furthermore, future research directions in this field are proposed.

H₂S is well-known as a colorless, acidic gas, with a notoriously rotten-egg smell. It was recently revealed that H₂S is also an endogenous signaling molecule that has important biological functions, however, most of its physiology and pathology remains elusive. Therefore, the enthusiasm for H₂S research remains. Fluorescence imaging technology is an important tool for H₂S biology research. The development of fluorescence imaging technology has realized the study of H₂S in subcellular organelles, facilitated by the development of fluorescent probes. The probes reviewed in this paper were categorized according to their chemical mechanism of sensing and were divided into three groups: H₂S reducibility-based probes, H₂S nucleophilicity-based probes, and metal sulfide precipitation-based probes. The structure of the probes, their sensing mechanism, and imaging results have been discussed in detail. Moreover, we also introduced some probes for hydrogen polysulfides.

Heterocyclic compound quinoline and its derivatives exist in natural compounds and have a broad spectrum of biological activity. They play an important role in the design of new structural entities for medical applications. Similarly, indoles and their derivatives are found widely in nature. Amino acids, alkaloids and auxin are all derivatives of indoles, as are dyes, and their condensation with aldehydes makes it easy to construct reaction sites for nucleophilic addition agents. In this work, we combine these two groups organically to construct a rapid response site (within 30 s) for H₂S, and at the same time, a ratiometric fluorescence response is presented throughout the process of H₂S detection. As such, the lower detection limit can reach 55.7 nmol/L for H₂S. In addition, cell imaging shows that this probe can be used for the mitochondrial targeted detection of endogenous and exogenous H₂S. Finally, this probe application was verified by imaging H₂S in nude mice.

Chemosensor arrays have a great potential for on-site applications in real-world scenarios. However, to fabricate on chemosensor array a number of chemosensors are required to obtain various optical patterns for multi-analyte detection. Herein, we propose a minimized chemosensor array composed of only two types of carboxylate-functionalized polythiophene derivatives for the detection of eight types of metal ions. Upon recognition of the metal ions, the polythiophenes exhibited changes in their fluorescence intensity and various spectral shifts. Although both chemosensors have the same polymer backbone and recognition moiety, only the difference in the number of methylene groups contributed to the difference in the fluorescence response patterns. Consequently, the metal ions in aqueous media were successfully discriminated qualitatively and quantitatively by the chemosensor microarray on the glass chip. This study offers an approach for achieving a minimized chemosensor array just by changing the alkyl chain lengths without the necessity for many receptors and reporters.

Na⁺, Cl^‒ and K⁺ are the most abundant electrolytes present in biological fluids that are essential to the regulation of pH homeostasis, membrane potential and cell volume in living organisms. Herein, we report synthetic crown ether-thiourea conjugates as a cation/anion symporter, which can transport both Na⁺ and Cl⁻ across lipid bilayers with relatively high transport activity. Surprisingly, the ion transport activities were diminished when high concentrations of K⁺ ions were present outside the vesicles. This unusual behavior resulted from the strong affinity of the transporters for K⁺ ions, which led to predominant partitioning of the transporters as the K⁺ complexes in the aqueous phase preventing the transporter incorporation into the membrane. Synthetic membrane transporters with Na⁺, Cl^‒ and K⁺ transport capabilities may have potential biological and medicinal applications.

Mitochondrial DNA has a special structure that is prone to damage resulting in many serious diseases, such as genetic diseases and cancers. Therefore, the rapid and specific monitoring of mitochondrial DNA damage is urgently needed for biological recognition. Herein, we constructed an in situ hydrophobic environment-triggering reactive fluorescence probe named MBI-CN. The fluorophore was 2-styrene-1H-benzo[d]imidazole, and malononitrile was introduced as a core into a molecule to initiate the hydrolysis reaction in the specific environment containing damaged mitochondrial DNA. In this design, MBI-CN conjugates to mitochondrial DNA without causing additional damages. Thus, MBI-CN can be hydrolyzed to generate MBI-CHO in an in situ hydrophobic environment with mitochondrial DNA damage. Meanwhile, MBI-CHO immediately emitted a significative fluorescence signal changes at 437 and 553 nm within 25 s for the damaged mitochondria DNA. Give that the specific and rapid response of MBI-CN does not cause additional damages to mitochondrial DNA, it is a potentially effective detection tool for the real-time monitoring of mitochondrial DNA damage during cell apoptosis and initial assessment of cell apoptosis.

Uridine diphosphate (UDP)-glucuronosyltransferases (UGTs) are enzymes involved in the biotransformation of important endogenous compounds such as steroids, bile acids, and hormones as well as exogenous substances including drugs, environmental toxicants, and carcinogens. Here, a novel fluorescent probe BDMP was developed based on boron-dipyrromethene (BODIPY) with high sensitivity for the detection of UGT1A8. The glucuronidation of BDMP not only exhibited a red-emission wavelength (λ_ex/λ_em = 500/580 nm), but also displayed an excellent UGT1A8-dependent fluorescence signal with a good linear relationship with UGT1A8 concentration. Based on this perfect biocompatibility and cell permeability, BDMP was successfully used to image endogenous UGT1A8 in human cancer cell lines (LoVo and HCT15) in real time. In addition, BDMP could also be used to visualize UGT1A8 in tumor tissues. These results suggested that BDMP is a promising molecular tool for the investigation of UGT1A8-mediated physiological function in humans.

The design of three novel fatty nitrogen mustard-based anticancer agents with fluorophores incorporated into the alkene structure (CXL 118, CXL121, and CXL122) is described in this report. The results indicated that these compounds are selectively located in lysosomes and exhibit effective antitumour activity. Notably, these compounds can directly serve as both reporting and imaging agents in vitro and in vivo without the need to add other fluorescent tagging agents.

Lysine lipoylation plays vital roles in cell metabolism and redox processes. For example, removal of lipoylation will decrease pyruvate dehydrogenase activity and affect the citric acid cycle. Despite the important functions of lysine lipoylation, the mechanisms for the addition and removal of this modification remain largely unexplored. Very few useful chemical tools are available to study the interactions of lysine lipoylation with its regulatory delipoylation proteins. For example, immunoaffinity purification-mass spectrometry is one of such tools, which highly relies on antibody efficiency and purification techniques. Single-step activity based fluorogenic probes developed by our groups and others is also an efficient method to study the deacylation activity. Affinity-based labeling probe using photo-cross-linker is a powerful platform to study the transient and dynamic interactions of peptide ligands with the interacting proteins. Herein, we have designed and synthesized a dual-function probe KTLlip for studying enzymatic delipoylation (eraser) activity and interaction of lysine lipoylation with the eraser at the same time. We show that KTLlip can be used as a useful tool to detect delipoylation as demonstrated by its ability to fluorescently label the eraser activity of recombinant Sirt2. We envision that the probe will help delineate the roles of delipoylation enzyme in biology.

The development of fluorophores emitting in the near-infrared spectral window has gained increased attention given their suitable features for biological imaging. In this work, we have optimised a general and straightforward synthetic approach to prepare a small library of near-infrared-emitting C-bridged nitrobenzodiazoles using commercial precursors. C-bridged benzodiazoles have low molecular weight and neutral character as important features that are not common in most near-infrared dyes. We have investigated their fluorescence response in the presence of a wide array of 60 different biomolecules and identified compound 3i as a potential chemosensor to discriminate between Fe²⁺ and Fe³⁺ ions in aqueous media.