Green N, S-codoped carbon dots from fennel and garlic enabling selective Fe³⁺ sensing and robust colorimetric thermometry

Abstract

Dual-mode optical probes that are low-cost, sustainable, and stable over long periods are attractive for fieldable chemical and temperature readouts. We report a one-pot hydrothermal synthesis of nitrogen- and sulfur-codoped carbon dots (N, S-CDs) from fennel and garlic. The N, S-CDs exhibit green emission (λex/λem ≈ 459/525 nm) with a quantum yield of 9.98%. As a Fe³⁺ fluorescent probe, the material shows a linear response over 20–100 μM with a limit of detection of 0.37 μM (3σ/k). As a thermometric probe, the fluorescence intensity decreases linearly from 0–100 °C with a relative sensitivity of 0.58% °C⁻¹, accompanied by a progressive blue shift that enables intuitive colorimetric readout. After one year of ambient storage, the thermometric performance remains essentially unchanged (e.g., intensity variation at 0 °C ≤ ~4%), underscoring exceptional long-term stability. The combination of green synthesis, dual-function sensing (Fe³⁺ and temperature), and year-long durability positions these N, S-CDs as a practical platform for environmental monitoring and low-power visual thermometry.