Stability of Superparamagnetic Iron Oxide Nanoparticles at Different pH Values: Experimental and Theoretical Analysis

Abstract

The detection of superparamagnetic nanoparticles using NMR logging has the potential to provide enhanced contrast in oil reservoir rock formations. The stability of the nanoparticles is critical because the NMR relaxivity (R(2) ≡ 1/T(2)) is dependent on the particle size. Here we use a molecular theory to predict and validate experimentally the stability of citric acid-coated/PEGylated iron oxide nanoparticles under different pH conditions (pH 5, 7, 9, 11). The predicted value for the critical surface coverage required to produce a steric barrier of 5k(B)T for PEGylated nanoparticles (MW 2000) was 0.078 nm(-2), which is less than the experimental value of 0.143 nm(-2), implying that the nanoparticles should be stable at all pH values. Dynamic light scattering (DLS) measurements showed that the effective diameter did not increase at pH 7 or 9 after 30 days but increased at pH 11. The shifts in NMR relaxivity (from R(2) data) at 2 MHz agreed well with the changes in hydrodynamic diameter obtained from DLS data, indicating that the aggregation behavior of the nanoparticles can be easily and quantitatively detected by NMR. The unexpected aggregation at pH 11 is due to the desorption of the surface coating (citric acid or PEG) from the nanoparticle surface not accounted for in the theory. This study shows that the stability of the nanoparticles can be predicted by the theory and detected by NMR quantitatively, which suggests the nanoparticles to be a possible oil-field nanosensor.

Effect of PEG molecular weight on stability, T2 contrast, cytotoxicity, and cellular uptake of superparamagnetic iron oxide nanoparticles (SPIONs)

“…[12] Briefly, iron(III) tri(acetylacetonate) [Fe(acac) 3 ] (2 mmol), benzyl ether (40 mL), 1,2-tetradecanediol (10 mmol), oleic acid (6 mmol), and oleylamine (6 mmol) were mixed and stirred magnetically under a flow of nitrogen. The mixture was heated at 2 degrees per min to 100 °C and kept for 45 min, followed by heating to 200 °C at 2 degrees per min and kept for 2 h. Subsequently the reaction mixture was heated to reflux (∼ 300 °C) and held for another 30 min to 1 h. The oleic acid – coated SPIONs were stored after washing steps with an excess of ethanol and after a drying step in a vacuum oven.…”

“…The polyethylene glycol (PEG) coating of SPIONs provides stability between the particles via steric repulsion, [12] and reduces undesired interactions with plasma proteins and their subsequent opsonization. [13] PEG also allows targeting ligands to be conjugated onto the SPIONs.…”

Control of magnetite primary particle size in aqueous dispersions of nanoclusters for high magnetic susceptibilities

“…The biggest challenges for using IO NPs for oil reservoir applications lie in (i) synthesizing a IO NP core with high initial magnetic susceptibility which make it possible to generate enough signal for mapping under even low external magnetic field, and (ii) developing a surface coating procedure which enable the IO NPs to be stable in the subsurface oil reservoir environment, namely high temperature, high salinity aqueous solutions. Recently, long term colloidal stability of IO and other NPs was reported even over a wide pH range in high salinity, high temperature brines typically encountered in oil reservoirs [10][11][12][13][14][15][16] . For example polymers containing 2-acrylamido-2-methylpropane sulfonic acid (AMPS), which bind weakly to Ca 2+ , remain extended after coated on ~100 nm IO nanoclusters in American Petroleum Institute brine (API brine, 8% w/w NaCl + 2% w/w CaCl 2 , anhydrous basis) and provide efficient steric and electrostatic stabilization [16][17][18][19][20] .…”

Antimicrobial and Controlled Release Studies of a Novel Nystatin Conjugated Iron Oxide Nanocomposite

“…Different researchers studied the diameter stability of magnetic nanoparticles and degradation stability by using dynamic light scattering (DLS) [27] and inductively coupled plasma (ICP) [28]. Yoonjee Park and his research group showed that the effective diameter for citric acid-coated/PEGylated iron oxide nanoparticles did not increase at pH 7 or 9 after 30 days but increased at pH 11 [27].…”

“…Yoonjee Park and his research group showed that the effective diameter for citric acid-coated/PEGylated iron oxide nanoparticles did not increase at pH 7 or 9 after 30 days but increased at pH 11 [27]. Similarly Hoskins showed that magnetic nanoparticles coated with poly ethylenimine and poly ethylene glycol were stable at neutral pH and at an acidic environment at a pH of 4.6 in the presence of sodium citrate [28].…”