As such the selective labeling and tracking of endogenous VDPs may provide a better understanding of the biological and physiological function of vicinal dithiols in cells or organelles of interest. Since 1947, arsenic agents have been used to study vicinal dithiols, such as arsenic affinity chromatography, direct and indirect chemical tests . These methods have several disadvantages including the requirement of cell lysis, and indirect detection signal readout, which causes problems in reproducibility and operation. In addition, some vicinal dithiols are oxidized to disulfides quickly and lose their function during sample preparation procedures. This necessitates the development of a non-destructive tracking and analysis technique for live cells. Fluorescent probes have become the most popular tool for this purpose, owing to their convenience and visual signal readout. A significant amount of effort in this field has been expended in attempts to determine, label, and assess endogenous thiol groups. However, amongst the fluorescent probes developed to detect biothiols, only a few could be used for the in-situ tracking of endogenous vicinal dithiols on VDPs in live cells. In 2011, we discovered a fluorophore with arsenite derivatives to serve as a fluorescent probe (NPE) for VDPs. 30 NPE used 1,2-ethanedithiol (EDT) to protect trivalent arsenic and form a stable ring structure, which can improve the membrane permeability by increasing the lipophilicity and enhance the stability of the trivalent arsenic from oxidation and polymerization. Through exchange between EDT on the arsenic group and the vicinal dithiols of the VDPs, the NPE probe can capture endogenous VDPs in live Chang liver cells. Subsequent research in this area has been extended and some additional probes for VDPs have been developed. In particular, in 2014, we reported a ratiometric fluorescent probe for the quantification of endogenous vicinal dithiols of VDPs in living cells, using trivalent arsenic as a specific receptor that connected the fluorescence donor (coumarin) and fluorescence receptor (naphthalimide). Despite such ground-breaking research, the detailed behavior of the endogenous vicinal dithiols of VDPs in different types of live cell lines are still unknown. For instance, in addition to the fundamental role in redox signalling, it has also been suggested that cell surface protein vicinal dithiols may participate in cellular delivery and virus entry into cells (such as the HIV and COVID-19 entry into target cells). And some new drug delivery vectors have been developed for the efficient transport of cell-impermeable proteins, nucleic acids using endogenous cell surface thiols. However, the question of the role of cell surface protein vicinal dithiols in cellular uptake and virus entry into cells remains uncertain. The effective monitoring of such processes requires an effect tool to simultaneous monitoring the behavior of cellular vicinal dithiols on VDPs over an extended period of time. Additionally, there are still no visual methods that can reveal the presence of cell surface protein vicinal dithiols through direct observation in live cells despite published research that has confirmed the key roles of cell surface protein vicinal dithiols through proteomic analysis. Herein we developed a small rhodamine-azo based fluorescent probe (RhQ) for the long-term tracing of VDPs with simultaneous fluorescence imaging in live cells. The design rationale is depicted in Scheme 1 and illustrated as follows. The trivalent arsenic unit acts as the specific reagent for vicinal dithiols on VDPs. In addition a naphthalimide azo based group was introduced to obtain an "off-on" response as a fluorescence quencher for the rhodamine dye through Förster resonance energy transfer (FRET). The fluorescence signal of RhQ is initially silent due to the quenching effect of the azo group then when the trivalent arsenic interacts with the VDPs FRET is interrupted. Upon interaction, the fluorophore is covalently tagged to the VDPs through the interaction between the vicinal dithiols on the VDPs and the arsenic group of RhQ, thus realizing the selective imaging and dynamic visual tracking of VDPs in living cells through the monitoring of the recovered fluorescence from the rhodamine group. RhQ exhibits a unique "off-on" fluorescence response enabling the time-lapse, specific, and direct imaging of