Dual-mechanism detecting fluoride and tetracycline in food matrices using red-emitting carbon dots.

Rapid industrialization and excessive consumption have led to the accumulation of fluoride (F^–) and tetracycline (TC) in the environment, ultimately contaminating water and food sources [1,2]. Excessive intake of these substances poses serious risks to both ecosystems and human health, including skeletal damage and dental disorders [3,4]. Consequently, the accurate and efficient detection of F^– and TC is critical for food safety, water quality assessment, and environmental monitoring [5,6]. Fluorescence-based detection has gained significant attention due to its simplicity, rapid response, and high selectivity [7,8]. Various fluorescent materials, including organic molecules, metal-organic frameworks, and semiconductor quantum dots, have been explored for F^– and TC sensing [[9], [10], [11], [12], [13], [14]]. However, many of these materials suffer from drawbacks such as complex synthesis, low solubility, high toxicity, and poor optical stability [15,16], limiting their practical applicability. Therefore, developing alternative materials that overcome these limitations is essential. In this regard, carbon dots (CDs) have attracted significant attention.

As a rapidly emerging class of zero-dimensional fluorescent carbon nanomaterials, CDs have been widely applied in sensing and imaging, particularly in medical diagnostics, due to their simple synthesis, high solubility, excellent biocompatibility, and tunable optical properties [17]. Extensive research has explored CDs-analyte interactions, establishing key sensing mechanisms such as the inner filter effect (IFE), Förster resonance energy transfer (FRET), and photo-induced electron transfer (PET) [18]. These mechanisms enable fluorescence quenching or enhancement, providing valuable tools for analyte detection. Additionally, CDs with high fluorescence quantum yield, long-wavelength emission, and large Stokes shift help minimize interference from biological autofluorescence, improving sensitivity in analytical applications [19,20]. Long-wavelength emissive CDs, in particular, have emerged as promising candidates for food safety and environmental monitoring. Several long-wavelength emissive CDs have been developed for F^– detection [[21], [22], [23], [24]]. However, most suffer from a small Stokes shift, which limits their sensitivity. Moreover, while numerous fluorescence sensors have been designed for detecting F^– or TC, few studies have explored the use of a single type of CDs for the independent detection of both through multiple sensing mechanisms.

In this study, we introduce red-emissive carbon dots (R-CDs) capable of detecting fluoride (F^–) and tetracycline (TC) via two distinct mechanisms (see Scheme 1). For F^– detection, F^– displace Fe³⁺ from the surface of R-CDs, restoring the fluorescence previously quenched by Fe³⁺. This competitive binding process forms the basis of a fluorescence-enhanced sensor with high selectivity and sensitivity. For TC detection, the significant spectral overlap between TC’s absorption and the excitation spectrum of R-CDs leads to fluorescence quenching via a combination of static quenching and IFE. This dual-sensing approach enables the independent quantification of F^– and TC in food matrices and water samples with satisfactory results. Our findings highlight the potential of R-CDs for practical applications in food safety and environmental monitoring. This study also presents a novel strategy for multi-analyte detection, demonstrating the versatility of carbon dots in advanced sensing technologies.