Introduction Cerium is the most abundant rare-earth element and is widely used in industry as catalyst, magnet, phosphor, polishing powder and ceramic colorant. Cerium occurs naturally in the form of Ce3+. Ce4+ is easily extracted and separated from cerium fluocarbonate. Therefore, Ce3+ is often converted to Ce4+ during the reprocessing stage in mineral extraction and metallurgical engineering. Ce4+ is likely to accumulate in the environment, food chain and organisms due to its extensive development and application, causing serious damage to the environment and biological systems. It is thus necessary to detect Ce4+. AA can be also called vitamin C as one of the most important vitamins to maintain the normal activities of the human body. Previous studies indicate that the level of AA in the human body is closely related to many diseases, lack of AA may suffer from the following diseases like mouth ulcer, hair loss, Alzheimer's disease, scurvy and so on. A simple and healthy way to consume AA is to eat more fruits and vegetables that contain AA. AA concentration is an important index to determine fruit freshness and nutritional quality. It is thus of great significance to check the AA of fruits and vegetables correctly and reliably to prevent diseases and evaluate the freshness of vegetables. The development of a simple, reliable and highly selective AA assay is crucial for analytical applications and clinical disease diagnosis, becoming a key topic in current chemistry research. In the past few decades, a series of methods for the detection of Ce4+ or AA are reported, and a fluorescence method has attracted recent attention due to its advantages such as simple instrument, low cost and low sample consumption. However, such fluorescence analysis methods still have some technical challenges and inevitable shortcomings. For instance, the detection results with single emission fluorescence method are often interfered by some factors unrelated to the detection environment, such as background signal, fluorescence substance concentration, and surrounding environment. To solve this problem, a fluorescence analysis strategy, namely ratio fluorescence analysis, was proposed. The ratio fluorescence analysis method realized the self-calibration of the analysis signal through the double fluorescence signal, easily obtaining the more accurate detection results rather than the fluorescence method with single emission signal. It provides an effective way for the analysis and detection of target objects.Methods According to the method described in the literature, 0.25 g thiourea and 0.25 g citric acid were dissolved in 20 mL ultra-pure water, treated with ultrasound for 30 min, then placed at 80 ℃ for 8 h, centrifuged to collect the supernatant, and dialyzed twice with 1 000 Da dialysis bag. The solution obtained was N, S-CQDs solution.BSA-AuNCs were prepared according to the method reported in the literature. After 50 mg/mL BSA solution and 10 mmol/L HAuCl4 solution were mixed under stirring for 2 min, 1 mL NaOH (1 mol/L) was added drop by drop and stirred at 100 ℃ for 7 min. After dialysis in ultra-pure water with 1 000 Da bag for 12 h, BSA-AUNCS solution was obtained.BSA-AuNCs and N, S-CQDs were mixed at a volume ratio of 1:2 and stored at 4 ℃ for the coming use. 5 mL N, S-CQDs@BSA-AuNCs as a working solution was mixed with Ce4+ solution at different concentrations. Deionized water was added into the mixture for a fixed volume to 10 mL, and stabilized for a period of time. Afterwards, the fluorescence test was performed at the excitation wavelength of 345 nm. 5 mL N, S-CQDs@BSA-AuNCs solution was firstly mixed with 100 μL 1 mmol/L Ce4+, and then mixed with AA solution at different concentrations at a constant volume of 10 mL under stirring for 2 min. The fluorescence test was performed at the excitation wavelength of 345 nm after stability for a period of time.Results and discussion In this work, nitrogen and sulfur doped carbon quantum dots (N, S-CQDs) and gold nanoclusters (BSA-AuNCs) are combined to form a N, S-CQDs@BSA-AuNCs fluorescence sensor for the continuous determination of Ce4+ ion and AA. Bsa-auncs were synthesized with bovine serum protein (BSA) as a stabilizer. The BSA-AuNCs have superior fluorescence properties, good biocompatibility, high quantum yield, excellent colloidal stability and ultra-small size. It is indicated that BSA-AuNCs selectively recognizes Ce4+ ions based on a fluorescence enhancement mechanism, and an "open" NIR fluorescence sensor platform for detecting Ce4+ is established. The Ce4+ mediated BSA-AuNCs "off" NIR fluorescence sensing strategy is developed to detect AA via adding AA to the NIR fluorescence sensor platform of N, S-CQDs@BSA-AuNCs system.Conclusions The N, S-CQDs@BSA-AuNCs ratio fluorescence sensor was constructed by N, S-CQDs and BSA-AuNCs for the continuous determination of Ce4+ and AA. When Ce4+ was added to the system, BSA-AuNCs were oxidized by Ce4+ and aggregated into coarser particles with an enhanced fluorescence. Therefore, an "open" NIR fluorescence sensing platform for Ce4+ detection was constructed. The fluorescence intensity ratio of IF429/IF692 showed a linear detection relationship with the concentration of Ce4+, and the detection limit was 0.12 μmol/L. Subsequently, a Ce4+ mediated NIR fluorescence sensing strategy based on N, S-CQDs@BSA-AuNCs "off" was developed to detect AA, with a detection limit of 0.19 μmol/L via adding the NIR fluorescence intensity of AA quenching N and S-CQDs@BSA-AuNCs/Ce4+ system. The ratio fluorescence sensor was applied to the detection of Ce4+ concentration in actual tap water samples, AA concentration in commercial peach juice and vitamin C tablets.