Characterization of FeTe NRs

Fig. 2.1 Characterization of the as-synthesized Te NWs and FeTe NRs. (A) and (B) TEM image of Te NWs and FeTe NRs, respectively. (C) Raman and (D) EDAX spectra of FeTe NRs.

The lengths of the as-prepared Te NWs and FeTe NRs were estimated from

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their corresponding TEM images (each 100 counts) to be (785  170) and (45  11) nm, respectively (Fig. 2.1A). The width of FeTe NRs (Fig. 2.1B) in the middle section is close to that of Te NWs (13.3 ± 1.8) nm vs. (16.2 ± 3.1) nm). The Raman scattering spectrum of the as-prepared FeTe NRs (Fig. 2.1C) reveals characteristic FeTe Raman active modes of A1g (Te) and B1g (Fe) at 148 and 168 cm^1, respectively.²⁰ In comparison, the Raman peaks of pure Te NWs are known to be at 46, 62 and 182 cm^1.²¹ The detection of Te and Fe in the EDAX spectrum (Fig. 2.1D) further confirms the formation of FeTe NRs. FeTe NRs were formed through a galvanic reaction between Te and Fe³⁺ ions by (equation 1)

7Te + 18OH¯ +4Fe³⁺

→ 3TeO32¯ + 4FeTe +9H2O (1)

Similar to that in the preparation of Au NRs,^{14, 15} CTAB prefers binding to the {110}

facet, leading to the anisotropic growth of FeTe seeds to form FeTe NRs. In addition, the CTAB bilayers on the surfaces of the FeTe NRs further stabilize them. The formation of FeTe NRs became apparent after a reaction time period of 20 min at 60

°C.To evaluate the practical potential of as-prepared FeTe NRs, we determined the peroxidase-like activity of the as-prepared FeTe NRs; FeTe NRs catalyze the reaction of H2O2 with peroxidase substrate ABTS (equation 2). Because the amount of the

The as-prepared FeTe NRs (Fig. 2.2) provided about 9-fold absorbance value at 418 nm of that in the absence of the FeTe NRs.

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Fig. 2.2 Absorption spectra of solutions containing ABTS and H2O2 in the presence of (A) FeTe NRs (B) Fe3O4 NPs and (C) ABTS solution as a control. Inset: Photograph (from left to right: FeTe NRs, Fe3O4 NPs, and control). Concentrations: ABTS (60 mM), H2O2 (100 mM), Fe3O4 NPs (2.17 mg/ml), and FeTe NRs (2.17 mg/ml). Acetate buffer: (0.2 M, pH 4.0).

Under these studied conditions, the catalytic activity of the FeTe NRs was higher than that of the Fe3O4 NPs.⁸ The catalytic activity of Fe3O4 NPs is dependent on the solution pH and reaction temperature.^{7, 8} Thus, the pH- and temperature-dependent activities of the FeTe NRs and Fe3O4 NPs were investigated in the current study. Fig. 2.3 shows the response curves in acetate buffer solutions (0.2 M) containing 100 M H2O2 over a pH range (2.012.0) at 30 C. In the absence of any NMs, the absorbance decreased gradually from pH values 2.0 to 5.0 and almost no reaction was observed at pH values higher than 5.0. Decomposition (hydrolysis) of H2O2 tends to occur rapidly at a pH higher than 5.0, leading to loss of its oxidation

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activity toward ABTS.^22-24 Similar trends occurred in the presence of the FeTe NRs and Fe3O4 NPs.

Fig. 2.3 Fluorescence spectra of FITC (0.1 mM) in the presence of (A) various concentrations of Fe²⁺ ions and (B) FeTe NRs and Fe3O4 NPs at 30 ºC. (A) FITC concentrations range from 0.5 to 100 µM. Inset is the linear range from 1 to 100 µM. Concentrations: Iodine solution (0.3 µM), Phosphate buffer: (10 mM, pH 6.4).

In addition to instability of H2O2 at high pH values, no sufficient amount of

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Fe²⁺ ions available in the systems also accounts for their low activity.^{25, 26} We note that the solubility product of Fe(OH)3 is (2.6 ± 0.2)  10^39.²⁷ Maximum difference in the absorbance values in the presence and absence of the FeTe NRs and Fe3O4 NPs occurred at pH 4.0 and 3.0, respectively. Both in the absence and presence of the FeTe NRs and Fe3O4 NPs, the absorbance increased when the reaction temperatures were raised from 20 to 60 ºC. We plotted the differential absorbance (ΔA) values against reaction temperature, in which ΔA = A418 nm (FeTe NRs or Fe3O4 NPs) – A418 nm (no NMs) (inset). 30 ºC and 45 ºC, respectively. The maximum values for the FeTe NRs and Fe3O4 NPs occurred at 30 ºC and 45 ºC, respectively. This is because the released Fe²⁺ ions from FeTe NRs reached a maximum value at 30 ºC (Fig. 2.3B).

2.3.2 Detection of H2O2.

The catalytic activity of FeTe NRs toward ABTS oxidation is proportional to the amount of released Fe²⁺ ions. However, in the case of Fe3O4 NPs, the temperature for the generation of a maximum amount of Fe²⁺ ions occurred at 45 ºC. The difference in temperature reaching maximum amounts of Fe²⁺ ions released is mainly because the bond strength of Fe with O is stronger in Fe3O4 than that with Te in FeTe.²⁸

It has been shown that the Fe²⁺ ions released from Fe3O4 NPs play a dominant role in the catalytic ABTS oxidation with H2O2.^{7, 8} To prove it, we determined the concentration of Fe²⁺ ions released from the FeTe NRs and Fe3O4 NPs using FITC as a fluorophore (Fig. 2.3).²⁹ The amount of Fe²⁺ ions released from the FeTe NRs was found to be one order in magnitude higher than that released from the Fe3O4 NPs. This leads to a plausible explanation of the greater enhancement in the absorbance observed when using the FeTe NRs than using the Fe3O4 NPs.

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Fig. 2.4 Effects of pH and temperature on the catalytic activity of the Fe3O4 NPs and FeTe NRs for H2O2-mediated ABTS reaction. (A) pH and (B) temperature dependent response curves: ⋆ for FeTe NRs, ■ for Fe3O4 NPs, and ● for the control. Inset to (B) ΔA against temperature, where ΔA = A418 nm (FeTe NRs or Fe3O4 NPs) – A418 nm (Blank).

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Fig. 2.5 A dose-response curve for H2O2 detection when using FeTe NRs.

Inset: linearity of absorbance against H2O2 concentration ranging over 0.15 µM.

The optimum pH and temperature are 4.0 (0.2 M acetate buffer) and 30 ºC, respectively (Fig. 2.3). Under these conditions, we determined the effect of H2O2 on the absorbance of the solutions containing the FeTe NRs and ABTS. The Fig. 2.5 displays the increase in absorbance upon increasing the H2O2 concentration. A linear relationship (inset to Fig. 2.5) between the absorbance and the H2O2 concentration ranging from 0.1 to 5 µM (R² = 0.99) was obtained, with a limit of detection at a signal-to-noise ratio of 3 of 55 nM. The LOD provided by using the FeTe NRs is two orders in magnitude lower than that provided by the Fe3O4 NPs.⁸ Relative to HRP,⁷ the FeTe NRs provided at least 150 fold higher sensitivity.