Senna Leaves Turned Into Glowing Nanosensors That Light Up for Mercury

Mercury is the only metal that is liquid and volatile at room temperature, and it is also one of the most toxic and widely distributed heavy metal pollutants in the world. It persists in the environment, migrates easily between air, soil and water, and accumulates continuously in living organisms. Inorganic mercury discharged into water can be converted by microorganisms into methylmercury, its most notorious organic form, which then climbs the food chain and concentrates in fish and other animals consumed by humans. Mercury is a recognized neurotoxin that inflicts particularly severe damage on the early neurodevelopment of children, and long-term exposure has been linked to cardiovascular diseases such as hypertension, coronary heart disease and myocardial infarction. Against this backdrop, a research team in China has now reported a surprisingly elegant solution to the problem of detecting mercuric ions in water: fluorescent carbon dots made from ordinary senna leaves, which glow brighter rather than dimmer when they encounter mercury.

The study, published in Frontiers in Analytical Science by Yishan Ding of Kuitun Traditional Chinese Medicine Hospital and Chenxi Guo, Zhihua Xu, Ting Zhou, Xinyu Cheng, Alang Zhang, Kedong Guo and Feng Shi of Shihezi University, describes a one-pot hydrothermal synthesis of nitrogen-doped fluorescent carbon dots, abbreviated FL-CDs. The team used senna leaf powder as a green carbon source and m-phenylenediamine as both the nitrogen dopant and a functional monomer. This precursor combination was chosen deliberately. Senna leaves are rich in anthraquinone structures, which can enhance the binding ability with metal ions, while m-phenylenediamine introduces abundant amino groups onto the particle surface. Together, these components build a recognition interface with strong affinity for Hg2+, the inorganic form of mercury that dominates contamination in aquatic systems.

The synthesis itself is strikingly simple. One gram of senna leaf powder was mixed with m-phenylenediamine at various mass ratios, suspended in ten milliliters of deionized water, sealed in a PTFE-lined stainless steel autoclave and heated in an oven. The researchers systematically varied reaction time, temperature and precursor ratio, testing durations from two to twelve hours and temperatures from 120 to 220 degrees Celsius. Fluorescence intensity climbed as carbonization proceeded, reaching a maximum at 200 degrees Celsius for six hours with a senna-to-m-phenylenediamine mass ratio of 5:1. Beyond these conditions, excessive heat or overly long reaction times caused particle agglomeration and loss of surface functional groups, while too much nitrogen dopant created structural defects and non-radiative recombination centers that weakened the glow. After centrifugation and 24 hours of dialysis, the purified aqueous dispersion of carbon dots was stored at 4 degrees Celsius for further testing.

Transmission electron microscopy revealed regularly spherical particles with good dispersibility, clear edges and no obvious agglomeration. The size distribution ranged from 0.36 to 2.62 nanometers, with an average diameter of about 2.19 nanometers, placing these dots firmly in the ultra-small category of carbon nanomaterials. High-resolution TEM showed clear, continuous lattice fringes with a spacing of about 0.206 nanometers, corresponding to the (100) crystal plane characteristic of graphitic carbon, and a core composed of graphitic and amorphous carbon structures. Fourier transform infrared spectroscopy identified a broad O-H and N-H stretching band between 3150 and 3450 inverse centimeters, along with C-H, C=O, C=C and C-N peaks, confirming the presence of amino groups on the surface. X-ray photoelectron spectroscopy independently verified carbon, nitrogen and oxygen as the dominant elements, with high-resolution peak fitting revealing amino-type nitrogen, pyrrolic nitrogen, carbonyl groups and ether or hydroxyl linkages. The two techniques cross-validated each other, establishing the amino groups as the key active sites for mercury recognition.

Optically, the dots are impressive. Excited at 469 nanometers, they emit bright green fluorescence at 518 nanometers, visible to the naked eye under blue light illumination. Their relative fluorescence quantum yield was calculated at 23.54 percent, a high figure that guarantees strong signal output during sensing. The ultraviolet-visible absorption spectrum showed a characteristic peak at 383 nanometers attributed to n-pi* electron transitions of surface carbonyl and amino groups. Stability testing was equally thorough. Under continuous 365-nanometer ultraviolet irradiation for five hours, fluorescence intensity dropped only marginally, from roughly 4.8 to 4.6 arbitrary units, demonstrating strong resistance to photobleaching. The dots tolerated pH values from 2 to 13, peaked in brightness near pH 10, remained stable between 15 and 30 degrees Celsius, kept detectable signals even in sodium chloride concentrations up to 500 millimolar, and retained essentially stable fluorescence after two months of storage at room temperature, with no obvious attenuation beyond 30 days.

The most striking result came from the metal ion selectivity tests. When the dots were exposed to a battery of common ions including lithium, magnesium, potassium, sodium, calcium, iron in both oxidation states, aluminum, chromium, cobalt, manganese, zinc and copper, the fluorescence signal barely moved. Only Hg2+ triggered a significant enhancement, a turn-on response that contrasts sharply with the quenching behavior of most heavy metal fluorescent probes. Turn-off sensors, which rely on fluorescence quenching, often suffer from low sensitivity and susceptibility to interference, because many factors can dim a signal, whereas a signal that brightens specifically in the presence of one target is inherently more robust against false positives. The pH dependence of the response added a further clue: the enhancement was strong in acidic to neutral conditions from pH 2 to 7 and weakened markedly in alkaline environments.

Quantitatively, the fluorescence increase followed a Stern-Volmer type relationship, with the relative change in intensity F/F0 minus 1 showing a good linear correlation with Hg2+ concentration across the range of 20 to 100 micromoles per liter, with a correlation coefficient of 0.9566. Using the IUPAC-recommended formula of three times the standard deviation of the blank signal divided by the calibration slope, the team calculated a limit of detection of 10.65 micromoles per liter. While this detection limit is higher than some previously reported mercury sensors, the authors argue that their single-component nitrogen-doped dots avoid the tedious blending of two different carbon dot types required by ratiometric designs, and the broader linear window allows direct quantification of moderately or heavily polluted wastewater without repeated dilution, a distinct advantage for practical field analysis. All measurements were performed in triplicate, with relative standard deviations below 5 percent, confirming reproducibility.

Real-world validation followed in tap water and lake water. The researchers spiked samples at three concentrations, 12, 50 and 90 micromolar, chosen to probe trace-level performance near the detection limit and quantitative accuracy within the linear range. Recoveries ranged from 99.96 to 100.83 percent, with relative standard deviations all below 2.2 percent, demonstrating that the sensor resists interference from complex natural water matrices and delivers accurate, precise results in genuine environmental samples rather than only in purified laboratory solutions.

The mechanism behind the turn-on behavior emerged from a combination of spectroscopic evidence. After mercury addition, the ultraviolet absorption band between 300 and 450 nanometers decreased in intensity without any shift in peak position or profile, indicating that the graphitic carbon core and its quantum confinement effect remained untouched. Time-resolved fluorescence measurements showed nearly identical average lifetimes before and after mercury binding, 8.62 versus 9.08 nanoseconds, ruling out changes in the radiative recombination rate of the excited state. X-ray diffraction, however, told a different story about the surface: the broad amorphous peak at 2-theta of about 20.23 degrees shifted slightly to larger angles with reduced intensity after mercury treatment, consistent with Hg2+ coordinating to surface hydroxyl and carboxyl groups, compressing the interlayer spacing and increasing structural disorder. The authors attribute the fluorescence enhancement primarily to inhibition of photoinduced electron transfer, or PET. In the pristine dots, lone pair electrons on surface nitrogen atoms transfer to empty orbitals of the excited carbon core, opening a non-radiative decay pathway that suppresses emission. When Hg2+ coordinates with those nitrogen atoms, their electron-donating ability is weakened, the PET process is blocked, and radiative recombination becomes dominant, so the dots glow brighter. The pH dependence fits this picture perfectly: in acidic to neutral solution, amino groups are protonated and can form strong complexes with mercury, whereas in alkaline conditions they are deprotonated and lose that binding strength.

Beyond the immediate sensing application, the work carries broader significance for materials chemistry. It introduces senna leaves as a new biomass precursor for carbon dot fabrication, expanding the palette of sustainable raw materials available for nanomaterial synthesis, and it demonstrates a function-oriented design strategy in which the natural anthraquinone content of the plant and controlled nitrogen doping are combined to build a targeted recognition interface. The authors suggest the platform offers a promising alternative for environmental monitoring and food safety analysis, and contributes to the rational design of advanced functional materials derived from natural resources. As mercury contamination continues to threaten ecosystems and human health worldwide, a cheap, green, stable and selective fluorescent probe that lights up in the presence of the very toxin it hunts may prove to be exactly the kind of tool that on-site water testing has been waiting for.

Subject of Research: Nitrogen-doped carbon dots synthesized from senna leaves for fluorescent detection of mercuric ions in water

Article Title: Senna leaf derived nitrogen-doped fluorescent carbon dots for Hg2+ detection

Article References: Ding, Y., Guo, C., Xu, Z., Zhou, T., Cheng, X., Zhang, A., Guo, K., & Shi, F. (2026). Senna leaf derived nitrogen-doped fluorescent carbon dots for Hg2+ detection. Frontiers in Analytical Science, 6, Article 1837916. https://doi.org/10.3389/frans.2026.1837916

Image Credits: AI Generated

DOI: 10.3389/frans.2026.1837916

Keywords: carbon dots, mercury detection, fluorescent sensor, senna leaf, nitrogen doping, heavy metal pollution, turn-on fluorescence, hydrothermal synthesis, photoinduced electron transfer, water quality monitoring, green nanomaterials, quantum yield