Iron Oxide Nanoparticles: An Inorganic Phosphatase

Xiao-Lan Huang

#### Abstract

Phosphorus is one of the most important macronutrients for the primary production. The transformation of dissolved organic phosphorus in the environment and its contribution to biological production in the different ecosystems is still a mystery. Recently, it was demonstrated that phosphate ester can be rapidly hydrolyzed in solutions containing iron oxide nanoparticles with enzyme kinetics. The catalyst is sensitive to temperature and pH changes and inhibited by tetrahedral oxyanions with an order of PO4 < MoO4 < WO4. The oxo-Fe structure in the iron oxide nanoparticles, like the metal center of natural phosphatase (e.g., purple acid phosphatase, PAP), might contribute to the observed catalytic activity. Iron oxide nanoparticles are very common and widely exist in the current earth environment, and phosphate esters are the main component of dissolved organic phosphorus in soil and waters. It is expected that iron oxide nanoparticles in aqueous environments, as an inorganic phosphatase, play a critical role for the phosphorus transformation from the view of the phosphorus cycle.

Keywords: enzyme, hydrolysis, iron oxide, nanoparticles, phosphate ester, phosphorus cycle

#### 1. Introduction

Phosphorus is one of the most important macronutrients for the primary production, which is primarily taken up by plants in the form of phosphate ions (HPO4 <sup>2</sup> and H2PO4 ). Most of the knowledge of phosphorus in the environment, including the phosphorus geochemistry cycle, comes from inorganic phosphates [1–4]. The dissolved organic phosphorus transformation and its contribution to the biological production in the different ecosystems, e.g., soil, lake, estuary and ocean, is still a mystery [5–7]. Recently some limited works have indicated that different phosphate esters, especially monoesters are the main components in the dissolved organic phosphorus in soils [8–11] and waters [12–17], which might be an important source of P phytoavailability and a potential source of water eutrophication. The phosphomonoesters in supra-/macro-molecular structures were found to account for the majority (61–73%) of soil organic P in diverse agricultural soils across the world and the monoester P pool was estimated to account for 33% of the total phosphorus (587 32 kg ha<sup>1</sup> ) by a recently review [18].

In general, the phosphate ester hydrolysis is catalyzed by various enzymes, including purple acid phosphatases (PAPs), which have been identified and characterized from plant, animal and bacterial organisms [19]. On the other hand, several studies have already demonstrated that the phosphate ester can be hydrolyzed with the interaction of minerals in the aqueous environments [5, 20–25]. Here, the results of laboratory study on the hydrolysis of phosphorus esters, promoted by the iron oxide nanoparticles in water, including the aged nanomolar inorganic iron ion solutions [26–28], were summarized. Additionally, the potential role of inorganic iron oxide nanoparticle for the phosphorus cycles due to the intrinsic phosphoesterase activity is postulated.

The corresponding reaction rate constant (k) was 3.02 <sup>10</sup><sup>6</sup> <sup>s</sup>

Iron Oxide Nanoparticles: An Inorganic Phosphatase DOI: http://dx.doi.org/10.5772/intechopen.82650

half-life (t1/2) was 6.38 h. Similar to the initial 20 μM of G6P, the Pi concentration of a initial 100 μM of G6P in the aged inorganic iron solution at 1, 3 and 6.7 h was 4.95, 10.74 and 20.62 <sup>μ</sup>M, respectively. The corresponding <sup>k</sup> was 8.83 <sup>10</sup><sup>6</sup> <sup>s</sup>

fresh unaged nanomolar inorganic iron [26] and millimole metals [30–32] solutions. Like aged inorganic iron solution, the concentration of phosphate esters and condensed inorganic phosphate decreased, and inorganic orthophosphate (Pi) increased in a solution bearing iron oxide (IO) nanoparticles, which consists of a dialysis membrane tube (DMT, e.g., Spectra/Por 1 membranes, molecular weight cut-off (MWCO) 6000–8000 Da) filled with iron oxide (DMT-IO). The iron oxide (D) was synthesized by Fe(NO3)3 following the basic protocol of Atkinson [37] and aged at 80°C [28]. The k for 100 μM G6P, Glycerol-2-phosphate

and the t1/2 was 21.8 h. It is highlighted that these k in the aged iron solution were much higher than the previously reported rates in the presence of the

(3-carbon, G2P), and three energy metabolism compounds, i.e., adenosine monophosphate (AMP), adenosine diphosphate (ADP), adenosine triphosphate (ATP), as well as two inorganic condensed phosphates, i.e., polyphosphate (poly-Pi), and pyrophosphate (PPi) at room temperature (22°C) was

esters and inorganic condensed phosphates was 7.1, 11.5, 12.5, 33.6, 33.4, 50.7,

aged inorganic iron solutions or the solutions bearing inorganic iron oxide nanoparticles (DMT-IO), are listed in Table 1. The half-life for aged 4-month Fe (NO3)3 (16.5 nM), FeCl3 (10 nM) and Fe(ClO4)3(10 nM) was 37.8, 58.6 and 78.4 h, respectively, whereas the half-life of IO from the same source (Fe(NO3)3, JT Baker), aged at 5–80°C was 11, 2.7, 3.2 and 2.8 h, though they were in the same order of magnitude. The same patterns were observed for the ATP as well [28]. These results further indicate, as expected, that the behavior of catalysis depends on the sources of iron oxides nanoparticles in solutions—whether FeCl3, or Fe(NO3)3, and even on the different manufacturers, as well as with the different aging temperatures for IO (5–80°C) [37, 38]. No clear relationships between ferric ion (III) sources, age processing, and catalytic activity, with the hydrolysis rate constant,

These inorganic iron solutions also have the same promotion effects on hydrolysis of different sugar phosphates, including G2P, ribose-5-phosphate (5-carbon, R5P), and fructose 1-phosphate (6-carbon, F1P) (Table 2). As expected, the promotion effect was also found on the hydrolysis of AMP, ADP and ATP, and

inorganic condense phosphates (poly-Pi and PPi) as well as the RNA model

As expected, the catalytic activity is related to the soaked time of DIW with DMT-IO and the nature of IO, which can be described by the hydrolysis reaction rate constant. The kinetics of k of 100 μM G6P and ATP in three different IOs is presented in Figure 2. These results can be explained by the changes of the nanoparticles concentration in the water. It was expected that the concentration of the IO nanoparticles in these solutions would initially increase up to 10 days and then reach equilibrium. However, the total dissolved iron concentrations in these solutions were still beneath the detection limits of

2-aminoethylphosphonic acid, phosphono-formic acid) and inositol

hexakisphosphate (IP6) (data not shown).

compound (4-nitrophenyl phosphate ester, pNPP). However, no promotion effects were observed for the hydrolysis of phosphonates (C-P bonded compounds, e.g.,

Measured k of the initial 20 μM G6P with different sources of iron, either the

, 5.73 <sup>10</sup><sup>6</sup>

, respectively. The corresponding t1/2 of these phosphorus

, 5.76 <sup>10</sup><sup>6</sup>

, 1.54 <sup>10</sup><sup>5</sup>

2.69 <sup>10</sup><sup>5</sup>

and 5.09 <sup>10</sup><sup>6</sup> <sup>s</sup>

were observed.

iron (0.1 nM) [39].

99

and 37.8 h, respectively.

, 1.68 <sup>10</sup><sup>5</sup>

1

1

, 3.8 <sup>10</sup><sup>6</sup>

,

, and the

1 ,
