Fig. 1. Illustration of the synthesis process of chiral Cys-MoO2 NPs and their application for Hg2+ sensing.

Fig. 2. (a) TEM image of D-Cys-MoO2 NPs. The corresponding scale bar is 100 nm. (b) Histogram distribution of the diameter of NPs. Measurements of (c) circular dichroism spectrum and (d) absorption spectrum of MoO3 (black line), L-Cys-MoO2 (red line), and D-Cys-MoO2 NPs (blue line). XPS spectra of (e) MoO3 NPs and (f) D-Cys-MoO2 NPs with deconvoluted molybdenum 3d peaks. The blue and orange peak areas are corresponding to the different valence states of Mo(VI) and Mo(IV), respectively.

Fig. 3. Chiroptical sensing of Hg2+ using Cys-MoO2 NPs. (a) CD and (b) absorption measurements for Hg2+ mixing with aqueous D-Cys-MoO2 and L-Cys-MoO2 NPs solution. The concentration of Hg2+ in the mixture varied from 0.1 nM to 30 nM. (c) Calculated g-factor curves of specimens in (a). (d) Differences of g-factor [values at 384 nm shown in (c)] versus Hg2+ concentration and corresponding fitting curve. The inset image is the calibration plot.

Fig. 4. (a) TGA curves of L-Cys-MoO2 NPs with different amounts of mercury after dialysis. (b) Ligand density varies with mercury ion concentrations. (c) CD and (d) absorption spectra of pure L-Cys-MoO2 NPs as well as L-Cys-MoO2 mixing with Hg2+ (10 nM in the mixture) under different reaction times.

Fig. 5. TD-DFT simulation for different amounts of D-Cys capped Mo4O8 nanoclusters. Calculated frontier molecular orbital of (a), (c) HOMO and (b), (d) LUMO for one Cys molecule capped and six Cys molecule capped Mo4O8 nanoclusters. Calculated (e) CD spectra and (f) absorption spectra.

Fig. 6. Selectivity of D-Cys-MoO2-based Hg2+ sensor. (a) CD spectra of D-Cys-MoO2 solution mixed with different heavy metal ions: Zn2+, Cd2+, Pb2+, Ag+, Cu2+, and Hg2+. (b) CD signal at 384 nm of mixtures measured in (a). The concentrations of all metal ions are settled at 10 nM.

Table 1. Comparison of the Proposed Probe with Previously Reported Hg2+ Sensors Based on Different Methods^a

Table 2. Summary of Calculated Mass Loss and Ligand Density for L-Cys-MoO2 NPs with Different Hg2+ Additions