Foods · 7 authors, 2 centres
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This study developed a fluorescent and colorimetric dual-mode detection method for mercury ions (Hg²⁺) in seafood using a rhodamine 6G hydrazide (R6GH) probe. The probe showed a low detection limit of 2.5 × 10⁻² μM and good linearity (R² = 0.9888) in acetonitrile, with high selectivity over other metal ions. A paper-based sensor combined with a portable color reader enabled rapid, on-site, and semiquantitative visual detection of Hg²⁺ in oysters, yellow croaker, and prawn samples.
**Background:** Heavy metal pollution, particularly mercury (Hg), poses serious risks to food safety and human health due to its nondegradability and bioaccumulation in the food chain. Conventional detection methods such as ICP-MS, AAS, and AFS are accurate but expensive, bulky, and require complex sample pretreatment, limiting their use for on-site and large-scale screening. Rhodamine-based fluorescent probes, especially rhodamine 6G hydrazide (R6GH), offer an alternative due to their 'On–Off' fluorescence switching upon metal ion binding. This study aimed to develop a dual-mode (fluorescent and colorimetric) detection strategy for Hg²⁺ in seafood using an R6GH probe.
**Methods:** The R6GH probe was synthesized by reacting rhodamine 6G (0.5 g) with hydrazine hydrate (85%, 2.0 mL) in ethanol, stirred for 10 h at room temperature. The probe was tested in three solvent systems: THF/H₂O (v/v, 1:1), acetonitrile (ACN), and MeOH/H₂O (v/v, 3:1). For fluorescence detection, Hg²⁺ at concentrations of 0–10 μM was added to the R6GH probe (20 μM) in ACN, and fluorescence intensity was measured. Selectivity was assessed against nine common metal ions (Cu²⁺, V²⁺, Cd²⁺, Mn²⁺, Zn²⁺, Cr³⁺, Co²⁺, Ag⁺, K⁺) at 10 μM. Reversibility was tested using EDTA over five cycles. For paper-based sensors, filter paper strips (1 cm × 1 cm) were infiltrated with R6GH probe solution (20 μM) in THF/H₂O, dried, then exposed to Hg²⁺ standards (0–100 μM). Colorimetric LAB values were measured using a portable colorimeter. Seafood samples (oysters, yellow croaker, prawn) were spiked with Hg²⁺ at 0.5, 2.0, and 4.0 μM, pretreated with HNO₃, and analyzed by both the R6GH probe and ICP-MS.
**Key Results:** The R6GH probe in ACN showed the highest UV absorbance and fluorescence intensity. Fluorescence intensity increased linearly with Hg²⁺ concentration from 0–5 μM (R² = 0.9888), with a limit of detection of 2.5 × 10⁻² μM (S/N = 3). Selectivity tests showed that Hg²⁺ (10 μM) induced a fluorescence intensity of 9435.1, while other metal ions produced much weaker signals. Interference factor K values were all < 1.0: Cd²⁺ (0.47), Mn²⁺ (0.29), V²⁺ (0.27), Cu²⁺ (0.24), Ag⁺ (0.002), Zn²⁺ (0.0014), K⁺ (0.0012), Co²⁺ (0.00094), Cr³⁺ (0.0008). EDTA reversibility experiments showed the probe could be cycled five times without significant fluorescence loss. In spiked seafood samples, recoveries ranged from 88.0–108.3% with RSDs < 5% (n = 3). Correlation with ICP-MS was excellent (r² > 0.99). The paper-based sensor showed a linear relationship between chromatic aberration parameter ΔE and Hg²⁺ concentration from 2.5–50 μM (y = 0.439x + 6.472, R² = 0.9875).
**Clinical Implications:** This dual-mode detection strategy provides a rapid, sensitive, portable, and low-cost method for on-site screening of Hg²⁺ in seafood, addressing the limitations of conventional instrument-based methods. The paper-based sensor combined with a portable color reader enables semiquantitative visual detection without specialized equipment. The R6GH probe platform could potentially be adapted for detecting other heavy metal targets in food and environmental samples, supporting food safety monitoring and public health protection.