Aloe vera derived carbon dots as multifunctional fluorescent probe for temperature, pH, and ferric ion sensing

Carbon dots (CDs) synthesized from natural precursors with excellent fluorescent properties are drawing a lot of attention for sensing and biosensing applications. We offer a sustainable carbon precursor derived from Aloe vera gel, utilizing a green one-step hydrothermal synthesis of carbon dots. A systematic optimization of the synthesis parameters was conducted by varying the reaction temperature, ranging from 180 °C to 240 °C in a 20 °C interval, while adjusting the reaction duration (4, 8, 12, and 16 h). We studied the absorption and emission of the prepared CDs using optical characterizations, such as UV-Vis and photoluminescence spectroscopy. X-ray diffraction (XRD), transmission electron microscopy (TEM), and Fourier transform infrared (FTIR) spectroscopy were used to study the structure and shape of the dots. These tests confirmed the creation of uniformly dispersed, amorphous, and functionalized carbon dots. The quantum yield and time-resolved PL decay were used in the determination of the most optimal synthesis conditions, yielding the highest photoluminescence quantum yield (PLQY) at 240 °C and 12 h. Results show that the synthesis parameters have a clear impact on the quantum yield, absorption, photoluminescence (PL) spectra, and morphology. CDs synthesized at 240 °C for 12 h with a carbon-to-oxygen ratio of Rc/o ≈ 2.37 showed the highest PLQY and were selected as sensitive fluorescence probes. As a first CDs powder-state temperature probe, the PL-intensity decrease from 298 to 393 K evidence strong thermal responsivity over a wide range. The fluorescence works as a pH sensor by changing the emission-intensity ratio across pH 3-12 with two calibration lines. For metal-ion sensing, the CDs exhibit outstanding selectivity toward Fe³⁺ with near-complete PL quenching and a linear range 0-500 nM. Notably, the limit of detection (LOD) is 16.15 nM, the lowest among hydrothermally synthesized carbon dots (CDs) from natural precursors.