A Simple Method for Synthesizing Nitrogen-Doped Carbon Quantum Dots for Fluorescent "Turn off" Mercury (II) Ion Sensing

Here, straightforward and environmentally friendly fluorescent nitrogen doped carbon quantum dots (N-CQDs) with a high blue fluorescence emission at 455 nm are used for ultrasensitive Hg2+ ion detection. Folic acid and urea are used as carbon sources in the carbonization process. Two broad absorption bands at around 280 and 370 nm from UV-Vis spectrum and characteristic absorption peaks from infrared spectrum confirms the successful synthesis of the N-CQDs. Energy dispersive X-Ray analysis confirmed the elemental composition of the N-CQDs. Transmission electron microscopy showed the homogeneous globular morphology of the N-CQDs with an average particle size of 65 nm. Zeta potential measurement established the stability and surface charge of N-CQDs. Dynamic light scattering measurement showed the average size of N-CQDs. With the addition of Hg2+ ion to N-CQDs, the blue fluorescence emission is quenched. Moreover, the N-CQDs can be applied to real water sample such as pond water, river water, and tap water. The detection limit is approximately calculated to be 12 nM and linear range is 0-30 parts per billion.

A magnetic relaxation switching and visual dual-mode sensor for selective detection of Hg2+ based on aptamers modified Au@Fe3O4 nanoparticles

The solvated mercuric ion (Hg²⁺) from industrial pollutants are highly toxic to the ecological environment and human health. Driven by urgent need for the selective and sensitive detection of Hg²⁺, a magnetic relaxation switching (MRS) based on Fe₃O₄ nanoparticles (NPs) was designed. Practically, the concentrations of Hg²⁺ in industrial pollutant is usually much higher than the detection range. Thus, gold nanoparticles (AuNPs) were synthesized on the surface of Fe₃O₄ NPs to enable the visual detection of Au@Fe₃O₄ NPs. The presence of Hg²⁺ in sample can specifically cause the aggregation of Au@Fe₃O₄-aptamers NPs through T-Hg²⁺-T base pairs, leading to the change in transverse relaxation time T₂ value of detection solution. The MRS sensor showed excellent response for Hg²⁺ ions in the range of 10 nM-100 nM and 100 nM to 5 μM. A highly sensitive and selective measurement of Hg²⁺ was obtained with a limit of detection of 2.7 nM. Noticeably, the visual detection can qualitatively analyze the Hg²⁺ beyond 5 μM by naked eye without advanced instrumentation and skilled operators.

Water-soluble mercury ion sensing based on the thymine-Hg2+-thymine base pair using retroreflective Janus particle as an optical signaling probe

Herein, we report an optical sensing platform for mercury ions (Hg²⁺) in water based on the integration of Hg²⁺-mediated thymine-thymine (T-T) stabilization, a biotinylated stem-loop DNA probe, and a streptavidin-modified retroreflective Janus particle (SA-RJP). Two oligonucleotide probes, including a stem-loop DNA probe and an assistant DNA probe, were utilized. In the absence of Hg²⁺, the assistant DNA probe does not hybridize with the stem-loop probe due to their T-T mismatch, so the surface-immobilized stem-loop DNA probe remains a closed hairpin structure. In the presence of Hg²⁺, the DNA forms a double-stranded structure with the loop region via Hg²⁺-mediated T-T stabilization. This DNA hybridization induces stretching of the stem-loop DNA probe, exposing biotin. To translate these Hg²⁺-mediated structural changes in DNA probe into measurable signal, SA-RJP, an optical signaling label, is applied to recognize the exposed biotin. The number of biospecifically bound SA-RJPs is proportional to the concentration of Hg²⁺, so that the concentration of Hg²⁺ can be quantitatively analyzed by counting the number of RJPs. Using the system, a highly selective and sensitive measurement of Hg²⁺ was accomplished with a limit of detection of 0.027nM. Considering the simplified optical instrumentation required for retroreflection-based RJP counting, RJP-assisted Hg²⁺ measurement can be accomplished in a much easier and inexpensive manner. Moreover, the detection of Hg²⁺ in real drinking water samples including tap and commercial bottled water was successfully carried out.