Sulfite (SO32−), which exists as a form of the sulfur dioxide (SO2) in aqueous, is a highly reactive molecule. It is generated from sulfur-containing amino acids (cysteine) through catalyzing by cytosolic isoform glutamate oxaloacetate transaminase 1 (GOT1) in living systems [1], [2], [3]. Normally, sulfite oxidase can rapidly convert excess sulfite to sulfate, which is excreted from the body subsequently [4], [5], [6]. However, abnormal sulfite accumulation in plasma will be found after inhaling high concentrations of SO2, ingesting high volumes of sodium sulfites. Also, some pathological mechanisms such as acute pneumonia, chronic renal failure, and rare congenital sulfite oxidase deficiency can result in accumulation of sulfite in organisms [7], [8], [9], [10]. Under the condition of oxidative stress or inflammatory response, additional sulfite could be generated from H2S [11], [12].

As one of the reactive sulfur species (RSS) [13], [14], excessive sulfite accumulation can enhance the generation of interleukin (IL)-12, IL-8 [15], [16] and oxygen radicals [17], [18], [19] by activating human neutrophils. These inflammatory cytokines and living radical can destroy biological molecules, following cause oxidative damage and signal disturbances [20], [21]. Therefore, sulfite has been recognized as an endogenous mediator in the process of inflammation [22], [23]. Substantial evidence has suggested that the abnormal concentration of sulfite is associated with many health issues, including diarrhea, hypotension, myocardial ischemia, ischemic heart diseases, migraine headaches, allergic reactions, strokes, acute asthma, lung cancer, and etc [24], [25], [26], [27], [28], [29], [30], [31]. Accordingly, a comprehensive understanding of the fluctuation of sulfite will help us monitor the initiation and progression of diseases, hence facilitate their early diagnosis and the drug development.

Even though several methods like titration [32], electrochemistry [33], chromatography [34], and capillary electrophoresis [35] have been exploited to detect sulfite, there are unmet needs in this this field due to the methods above couldn’t exert the real-time detection of sulfite in cellular and in organisms. By contrast, fluorescence imaging technology has many advantages, such as non-invasive, high sensitivity and selectivity, good biocompatibility, and economical, making it receive great attention for monitoring of numerous biomarkers and the diagnosis of several diseases in recent years [36], [37], [38]. It should be noted that fluorescent probes for the detecting sulfite have gotten considerable progress [39], [40]. The were widely used for sulfite/bisulfite detection in food and biological samples [41], [42].

In this work, we developed a novel fluorescent probe (QX-OA) for monitoring the fluctuation of sulfite in the inflammation model. For the probe, we chose the quinolinium-xanthene group as the fluorophore, and levulinate was used as the recognition group due to its steady characteristics (Fig. 1). From the imaging results, QX-OA displayed a negligible fluorescence signal. According to the reported response mechanism to SO32− [43], [44], we considered that the prohibited ICT (intramolecular charge transfer) effect by levulinate group in QX-OA is responsible for this result. Upon adding sulfite, the ICT effect formed along with the leaving of levulinate group. Meanwhile, a strong green fluorescence at 550 nm was observed. QX-OA displayed excellent sensitivity and selectivity for sulfite in vitro and thus was used for imaging sulfite in the mice model with inflammation.