Results and Discussion .1 Sensing mechanism

Scheme 3.1 shows the detection of Hg²⁺, mainly through the cation exchange reaction between the Fe²⁺ ions in FeTe NRs and Hg²⁺ ions as shown in Equation (1), leading to the formation of HgTe nanostructures and consequently the release of Fe²⁺

ions.

FeTe + Hg²⁺ → HgTe + Fe2+ (1)

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We note that the FeTe NRs were stable at 4 °C for at least 30 days. Although the solubility product of FeTe is unavailable from literature, the solubility product of FeTe is not much smaller than that of FeS (10^-19),^{22, 23} which is much larger than that (10^-70) of HgTe.The released Fe²⁺ ions present in the supernatant catalyze the reaction of ABTS with H2O2 as shown in Equation (2).

H2O2 + ABTS 𝐅𝐞²⁺

→ 2H2O + Oxidized ABTS (2)

Scheme 3.1 Colorimetric detection of Hg²⁺ ions using FeTe NRs. Fe²⁺ ions released from the FeTe NRs mainly through the cation exchange reaction with Hg²⁺ ions catalyze the reaction of ABTS (60 mM) with H2O2 (0.1 µM).

The amount of ABTS product (formed is proportional to the concentration of Fe²⁺ ions and thus to that of the Hg²⁺ ions. We note that the oxidized product of ABTS has an absorption wavelength maximum at 418 nm. Thus, by using a calibration curve of the absorbance values at λmax = 418 nm against the concentration of the Fe²⁺ ions in the standard solutions, the concentration of Fe²⁺ displaced and thus that of Hg²⁺ ions in the sample solution can be determined.

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Fig. 3.1 Characterization of the synthesized FeTe and HgTe NRs. (A) and (B) TEM image of FeTe and HgTe NRs, respectively. (C) Raman and (D) EDAX spectra of the as-formed HgTe. The concentrations of FeTe and Hg²⁺ ions are 5 and 0.5 mM, respectively.

To support the formation of HgTe nanostructures from the reaction of FeTe NRs and Hg²⁺ ions, we conducted TEM, EDAX, and Raman measurements. The TEM image of the as-prepared FeTe NRs (Fig. 3.1A) is shown, with average length and width (the widest part) of (105  21) and (19 ± 2) nm (100 counts), respectively. On the other hand, the TEM image of HgTe NRs depicted in Fig. 3.1B displays their average length and width were (112  26) and (23 ± 8) nm, respectively. The HgTe NRs has a marginally larger lattice constant (6.458 Å) in comparison to that (6.26 Å) of the FeTe NRs.^{24, 25} Moreover, the HgTe NRs relative to the FeTe NRs had a comparatively smoother surface. The Raman scattering spectrum of the HgTe NRs (Fig. 3.2C) reveals characteristic transverse optical (TO) and longitudinal optical (LO) peaks at 119 and 139 cm^-1, which are close to those (118 and 140 cm^-1, respectively)

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for the bulk HgTe. In comparison, the Raman peaks of the Te NWs are at 46, 62 and 182 cm^1,²⁶ while those for the FeTe NRs are at 148 (A1g) and 168 cm^-1 (B1g).²¹ The EDAX pattern clearly show the Hg and Te signals (Fig. 3.1D), further confirming the

complete replacement of Fe²⁺ ions by Hg²⁺ and the formation of HgTe NRs.

Fig. 3.2 EDAX spectrum of HgTe NRs displaying the displacement of Fe²⁺ ions by Hg²⁺ ions during the course (30 min) of the reaction

Fig. 3.3 Fluorescence spectra of FITC (1 µM) in the mixtures of FeTe NRs (1.25 µg mL^-1) and Hg²⁺ ions (a: 500, b: 250, c: 100, d: 50, e: 25, f: 5 nM). Mixtures were prepared in phosphate buffer (10 mM, pH 6.4) containing iodine (0.3 µM).

The Fe peak was still apparent in the FeTe NRs in intermediate steps (e.g.

after the reaction for 30 min as depicted in Fig. 3.2). We also detected the

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concentration of Fe²⁺ ions released by using FITC as a fluorophore. The fluorescence of FITC increased upon increasing the concentration of Fe²⁺ ions (Fig. 3.3), supporting the formation of Fe²⁺ ions between the reaction of FeTe NRs and Hg²⁺

ions. In addition, the Fe²⁺ ions released were stable in the acidic solution during the analysis.

3.3.2 Effect of reaction time, temperature, and pH

It is known that Fe²⁺ ions have the highest catalytic activity toward the reaction of ABTS with H2O2 at pH 4.0 (0.2 M acetate buffer).²¹

Fig. 3.4 Absorbance of a mixture of FeTe NRs (1.25 µg mL^-1) and Hg²⁺ ions (500 nM) against reaction time.

Figure 3.4 shows response curve against reaction time, showing that 90 min was optimal. The absorbance decreased slightly upon further increase in reaction time, mainly because of the decomposition of ABTS product. When the reaction was conducted at pH 4.0 and at the temperature range over 30–75 ºC for 90 min, the optimal temperature was 60 ºC (Fig. 3.5A).

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Fig. 3.5 (A) Temperature dependent response curves for the reaction between FeTe NRs and Hg²⁺ ions. Inset to (A) ΔA = A418 (in the presence of Hg²⁺ ions) – A418 (in the absence of Hg²⁺ ions) against temperature. (B) pH dependent response curves (■ for FeTe NRs and ▲ for the control).

The kinetic energy increased upon increasing reaction temperature, accelerating the reaction. At a temperature higher than 60 ºC, the oxidation of Fe²⁺ by oxygen and decomposition of ABTS took place more rapidly. The solvent effect is negligible since our sensing mechanism is mainly through the reaction between FeTe

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and Hg²⁺ ions, with support of no effect from methanol, ethanol, hexane and chloroform. We further investigated the pH effect on the reaction of the FeTe NRs with Hg²⁺ ions over pH values 2.012.0, as it might also play a critical role in the reaction of the FeTe NRs with Hg²⁺ ions. Taking the account of pH having a significant effect on the formation of iron oxide (at high pH values) and the dissolution of FeTe NRs (at low pH values).²⁷

Figure 3.5B shows prominent differences in the absorbance values occurred at pH 3.0 and 4.0. At the pH values > 4.0, the formation of iron oxide and mercury hydroxide is a concern. The solubility product values of iron oxide and mercury hydroxide are 2.6  10^39 and 3.6  10^26, respectively.²⁸ At the pH values < 3.0, dissolution of FeTe and HgTe NRs occurred.²⁹

3.3.3 Detection of Hg²⁺ ions

Figure 3.6 shows that the absorbance of ABTS product increases upon increasing the Hg²⁺ concentration, displaying the dose response curve for the detection of Hg²⁺ ions, with a linear relationship between the absorbance and the HgCl2 concentration ranging from 5 to 500 nM (R² = 0.99) and a linear range from 5 to 100 nM (Fig. 3.6A inset). This approach provided a limit of detection at a signal-to-noise ratio of 3 of 1.31 nM for Hg²⁺ ions.

We also conducted ICP-MS measurements to determine the amounts of the Fe and Hg in the as-formed HgTe NRs (100 nM). The amount of Fe²⁺ ions decreased upon increasing the amount of Hg²⁺ ion detected in the as-formed HgTe NRs (Fig.

3.7), further supporting the displacement of Fe²⁺ ions in the FeTe NRs by Hg²⁺ ions.

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Fig. 3.6 (A) Absorption spectra of FeTe NRs solutions containing ABTS (60 mM) with H2O2 (0.1 µM) in the presence of Hg²⁺ ions at the concentrations of 5 to 500 nM (a: 5, b: 25, c: 50, d: 100, e: 250, f: 500 nM). Inset to (A): linearity of absorbance against Hg²⁺ concentration over a range of 5100 nM. (B) Selectivity of the assay using FeTe NRs for the detection of Hg²⁺ ions. Concentrations of metal ions are 50 nM for Hg(II) and 1 µM for the other metal ions.

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Fig. 3.7 The concentrations of Fe²⁺ ions remaining in the FeTe NRs against those for Hg²⁺ ions added. The concentrations of the two metal ions were determined by ICP-MS measurements. The reaction conditions are the same as in Fig. 2A. Hg²⁺ ions at the concentrations 10–100 nM were added to the FeTe NRs solution (0.32 µg mL^-1). After the reaction, the FeTe NRs were subjected to three cycles of centrifugation/wash and the pellets were dissolved in HNO3 solution (5 mL, 2%).

3.3.4 Sensitivity and selectivity of FeTe NRs towards Hg²⁺ ions

Control experiments were carried out to test the specificity of the developed system for Hg²⁺ ions (50 nM) under optimal conditions in the presence of various metal ions and anions such as acetate and nitrate ions (each at a concentration of 1 µM). The results displayed in Fig. 3.6B reveal that the sensing system is specific to Hg²⁺ ions. Relative to the solubility product of HgTe (10^-70), those for the other metal complexes with Te^2- are much higher. In other words, the other metal ions relative to Hg²⁺ ions displaced Fe²⁺ ions from the FeTe NRs with much lower degree.^{29, 30}

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