Hierarchically porous RhB-encapsulated ZIF-7 as a dual-emission fluorescence probe for ultrasensitive detection of melamine in infant formulations

Abstract

Melamine is an unauthorized food additive and a highly concerning adulterant in foods that can occur either accidently or intentionally in dairy products, with potential health risks upon exposure to higher concentrations. An ultrasensitive fluorescent probe based on dual emissive RhB_x@ZIF-7 was developed to detect melamine. In this study, a fluorescent dye, Rhodamine B (RhB), was successfully encapsulated into the metal–organic framework (MOF) pores of ZIF-7 to form a fluorescent probe (RhB₃₀@ZIF-7), with dual emission properties to enable the detection of melamine at low concentrations. RhB₃₀@ZIF-7 was optimized by varying experimental parameters, including temperature (25 °C), pH (7.0), incubation time (10 min), and probe concentration (1 mg mL⁻¹), to enhance its sensitivity and selectivity. The observed fluorescent quenching towards melamine was primarily attributed to the mechanisms of the internal filtering effect (IFE), due to absorption of the excitation wavelength by melamine, causing a turn-off response in the system. The limit of detection (LOD) and limit of quantification (LOQ) were found to be 0.47 μM and 1.4 μM, respectively, with an R² of 0.99. This study reveals the previously unexplored enhanced fluorescence of RhB₃₀@ZIF-7 and elucidates the contribution of the intermolecular interaction between RhB and ZIF-7 to fluorescence sensing, paving the way for food safety monitoring.