Mixtures of L-amino acids as reaction medium for formation of iron nanoparticles: the order of addition into a ferrous salt solution matters

Abstract

Owing to Mössbauer spectroscopy, an advanced characterization technique for iron-containing materials, the present study reveals previously unknown possibilities using l-amino acids for the generation of magnetic particles. Based on our results, a simple choice of the order of l-amino acids addition into a reaction mixture containing ferrous ions leads to either superparamagnetic ferric oxide/oxyhydroxide particles, or magnetically strong Fe0-Fe2O3/FeOOH core-shell particles after chemical reduction. Conversely, when ferric salts are employed with the addition of selected l-amino acids, only Fe0-Fe2O3/FeOOH core-shell particles are observed, regardless of the addition order. We explain this phenomenon by a specific transient/intermediate complex formation between Fe2+ and l-glutamic acid. This type of complexation prevents ferrous ions from spontaneous oxidation in solutions with full air access. Moreover, due to surface-enhanced Raman scattering spectroscopy we show that the functional groups of l-amino acids are not destroyed during the borohydride-induced reduction. These functionalities can be further exploited for (i) attachment of l-amino acids to the as-prepared magnetic particles, and (ii) for targeted bio- and/or environmental applications where the surface chemistry needs to be tailored and directed toward biocompatible species.

Figure 1

(a) Mössbauer spectrum of Fe2ArgBH recorded at room temperature; (b) Mössbauer spectrum of Fe2ArgBH recorded at 5 K; (c) Mössbauer spectrum of Fe2GluBH recorded at room temperature.

Figure 1

(a) Mössbauer spectrum of Fe2ArgBH recorded at room temperature; (b) Mössbauer spectrum of Fe2ArgBH recorded at 5 K; (c) Mössbauer spectrum of Fe2GluBH recorded at room temperature.

Figure 2

Mössbauer spectrum of Fe2pH10BH recorded at room temperature.

Figure 3

(a) Mössbauer spectrum of Fe3ArgBH recorded at room temperature; (b) X-ray powder diffraction (XRD) patterns of Fe3ArgBH; (c) Mössbauer spectrum of Fe3GluBH recorded at room temperature; (d) XRD patterns of Fe3GluBH.

Figure 4

(a) Mössbauer spectrum of Fe2Arg recorded at room temperature; (b) Mössbauer spectrum of Fe2Glu recorded at room temperature.

Figure 5

(a) Surface plasmon extinction (SPE) spectra of Ag colloid without and with Arg and/or ArgBH; (b) SPE spectra of Ag colloid without and with Glu and/or GluBH; (c) SERS spectra of Ag colloid with Arg and/or ArgBH; (d) SERS spectra of Ag colloid with Glu and/or GluBH.

Figure 6

Mössbauer spectra of (a) Fe2ArgGluBH freshly prepared, recorded at room temperature; (b) Fe2GluArgBH freshly prepared, recorded at room temperature; (c) Fe2ArgGluBH two-years aged, recorded at room temperature; (d) Fe2GluArgBH two-years aged, recorded at room temperature; (e) Fe3ArgGluBH, recorded at room temperature; (f) Fe3GluArgBH, recorded at room temperature.

Figure 6

Mössbauer spectra of (a) Fe2ArgGluBH freshly prepared, recorded at room temperature; (b) Fe2GluArgBH freshly prepared, recorded at room temperature; (c) Fe2ArgGluBH two-years aged, recorded at room temperature; (d) Fe2GluArgBH two-years aged, recorded at room temperature; (e) Fe3ArgGluBH, recorded at room temperature; (f) Fe3GluArgBH, recorded at room temperature.

Figure 7

TEM images of (a,b) Fe2GluArgBH and (c,d) Fe2ArgGluBH.

Figure 8

(a) 5 K and room-temperature hysteresis loops of Fe2GluArgBH; (b) ZFC (zero-field-cooled) and FC (field-cooled) magnetization curves of Fe2GluArgBH; (c) 5 K and room-temperature hysteresis loops of Fe2ArgGluBH; (d) ZFC and FC magnetization curves of Fe2ArgGluBH.

Scheme 1

Depiction of intermediate complexes formed by Glu (a) and/or Arg (b) with ferrous salt dissolved in solution and reduced by NaBH₄ in the next step. Hypothetical structures of intermediate complexes (at pH 3 for Glu-FeSO₄ and at pH 10 for Arg-FeSO₄) are suggested. Resulting types of iron nanoparticles are also schematically depicted.