This study incorporates the formation of carbon quantum dots (CQDs) via a hydrothermal approach, recording the first-time use of castor leaves as a natural precursor. The used precursor offers various benefits including novelty, abundance, elemental composition, and biocompatibility. CQDs were further characterized with multiple techniques including high-resolution transmission electron microscope (HR-TEM), X-ray photoelectron microscopy (XPS), X-ray diffraction (XRD), Fouriertransform infrared spectroscopy (FTIR), Raman spectroscopy, UV–visible spectroscopy, Zeta analysis, and optical spectroscopy. They are fundamentally composed of carbon (71.37%), nitrogen (3.91%), and oxygen (24.73%) and are nearly spherical, and uniformly distributed with an average diameter of 2.7 nm. They possess numerous interesting characteristics like broad excitation/emission bands, excitation-sensitive emission, marvelous photostability, reactivity, thermo-sensitivity, etc. A temperature sensor (thermal sensitivity of 0.58% C− 1) with repeatability and reversibility of results is also demonstrated. Additionally, they were found selective and sensitive to ions in aqueous solutions. So, they are also utilized as a fluorescent probe for metal ion ( Fe3+) sensing. The lowest limit of detection (LOD) value for the current metal ion sensor is 19.1 μM/L.

A dual-mode fluorescent probe for temperature-sensing and metal ions detection based on castor leaves-derived carbon quantum dots
    Abstract
    1 Introduction
    2 Experimental section
        2.1 Materials
        2.2 Characterization
        2.3 CQD synthesis
        2.4 PL measurements in various solvents and pH levels
        2.5 Photostability measurements
        2.6 Fe3+ ion sensing process
        2.7 Detection limit (LOD) calculations
        2.8 Thermal sensing experiment
    3 Results and discussion
        3.1 Structural and elemental analysis
        3.2 Optical analysis
            3.2.1 Photostability
    4 Applications
        4.1 CQDs as temperature sensors
        4.2 Metal ion detection
            4.2.1 Fe3+ detection in real water samples
            4.2.2 Proposed mechanism
    5 Conclusions
    References

• Avinash Kumar(Department of Physics and Photonics Science, National Institute of Technology Hamirpur, Hamirpur, Himachal Pradesh 177005, India)
• Sandeep Kumar(Department of Physics and Photonics Science, National Institute of Technology Hamirpur, Hamirpur, Himachal Pradesh 177005, India)
• Anchal Sharma(Department of Physics and Photonics Science, National Institute of Technology Hamirpur, Hamirpur, Himachal Pradesh 177005, India)
• Arvind K. Gathania(Department of Physics and Photonics Science, National Institute of Technology Hamirpur, Hamirpur, Himachal Pradesh 177005, India) Corresponding author
• Ishant Kumar(Department of Physics and Photonics Science, National Institute of Technology Hamirpur, Hamirpur, Himachal Pradesh 177005, India, Rabindranath Tagore Government College Sarkaghat, Mandi, Himachal Pradesh 175024, India)
• Amit Sharma(Bharati Vidyapeeth’s College of Engineering, New Delhi 110063, India)