Synthesis and Reaction Chemistry of Nanosize Monosodium Titanate

Summary

The surfactant mediated sol-gel synthesis of nanosized monosodium titanate is described, along with preparation of the corresponding peroxide modified material. An ion-exchange reaction with Au(III) is also presented.

Abstract

This paper describes the synthesis and peroxide-modification of nanosize monosodium titanate (nMST), along with an ion-exchange reaction to load the material with Au(III) ions. The synthesis method was derived from a sol-gel process used to produce micron-sized monosodium titanate (MST), with several key modifications, including altering reagent concentrations, omitting a particle seed step, and introducing a non-ionic surfactant to facilitate control of particle formation and growth. The resultant nMST material exhibits spherical-shaped particle morphology with a monodisperse distribution of particle diameters in the range from 100 to 150 nm. The nMST material was found to have a Brunauer-Emmett-Teller (BET) surface area of 285 m²g^-1, which is more than an order of magnitude higher than the micron-sized MST. The isoelectric point of the nMST measured 3.34 pH units, which is a pH unit lower than that measured for the micron-size MST. The nMST material was found to serve as an effective ion exchanger under weakly acidic conditions for the preparation of an Au(III)-exchange nanotitanate. In addition, the formation of the corresponding peroxotitanate was demonstrated by reaction of the nMST with hydrogen peroxide.

Introduction

Titanium dioxide and alkali metal titanates are widely used in a variety of applications such as pigments in paint and skin care products and as photocatalysts in energy conversion and utilization.^1-3 Sodium titanates have been shown to be effective materials to remove a range of cations over a wide range of pH conditions through cation exchange reactions.^4-7

In addition to the applications just described, micron-sized sodium titanates and sodium peroxotitanates have recently been shown to also serve as a therapeutic metal delivery platform. In this application, therapeutic metal ions such as Au(III), Au(I), and Pt(II) are exchanged for the sodium ions of monosodium titanate (MST). In vitro tests with the noble metal-exchanged titanates indicate suppression of the growth of cancer and bacterial cells by an unknown mechanism.^8,9

Historically, sodium titanates have been produced using both sol-gel and hydrothermal synthetic techniques resulting in fine powders with particle sizes ranging from a few to several hundred microns.^4,5,10,11 More recently, synthetic methods have been reported that produced nanosize titanium dioxide, metal-doped titanium oxides, and a variety of other metal titanates. Examples include sodium titanium oxide nanotubes (NaTONT) or nanowires by reaction of titanium dioxide in excess sodium hydroxide at elevated temperature and pressure,^12-14 sodium titanate nanofibers by reaction of peroxotitanic acid with excess sodium hydroxide at elevated temperature and pressure,¹⁵ and sodium and cesium titanate nanofibers by delamination of acid-exchanged micron-sized titanates.¹⁶

The synthesis of nanosize sodium titanates and sodium peroxotitanates is of interest to enhance ion exchange kinetics, which are typically controlled by film diffusion or intraparticle diffusion. These mechanisms are largely controlled by the particle size of the ion exchanger. In addition, as a therapeutic metal delivery platform, the particle size of the titanate material would be expected to significantly affect the nature of the interaction between the metal-exchanged titanate and the cancer and bacterial cells. For example, bacterial cells, which are typically on the order of 0.5 – 2 µm, would likely have different interactions with micron size particles versus nanosized particles. In addition, non-phagocytic eukaryotic cells have been shown to only internalize particles with a size of less than 1 micron.¹⁷ Thus, the synthesis of nanosize sodium titanates is also of interest to facilitate metal delivery and cellular uptake from the titanate delivery platform. Reducing the size of sodium titanates and peroxotitanates will also increase the effective capacity in metal ion separations and enhance photochemical properties of the material.^16,18 This paper describes a protocol developed to synthesize nanosize monosodium titanate (nMST) under mild sol-gel conditions.¹⁹ The preparation of the corresponding peroxide modified nMST; along with an ion-exchange reaction to load the nMST with Au(III) are also described.

Protocol

1. Synthesis of Nano-monosodium Titanate (nMST)

1. Prepare 10 ml of solution #1 by adding 0.58 ml of 25 wt % sodium methoxide solution to 7.62 ml of isopropanol followed by 1.8 ml of titanium isopropoxide.
2. Prepare 10 ml of solution #2 by adding 0.24 ml of ultrapure water to 9.76 ml of isopropanol.
3. Add 280 ml of isopropanol to a 3-neck 500-ml round bottom flask, followed by 0.44 ml of Triton X-100 (average MW: 625 g/mol). Stir the solution well with a magnetic stir bar.
4. Prepare a dual channel syringe pump to deliver solutions #1 and #2 at a rate of 0.333 ml/min.
5. Load solutions #1 and #2 into two separate 10-ml syringes fitted with a length of tubing that will allow delivery of the solution from the syringe pump to below the solution level in the 500-ml round bottom flask.
6. While stirring, add all of solutions #1 and #2 (10 ml each) to the reaction using the syringe pump programmed in step 1.4.
7. After the addition is complete, cap the flask and continue to stir for 24 hr at RT.
8. Uncap the flask and heat the reaction mixture to ~82 °C (refluxing isopropanol) for 45-90 min, followed by purging with nitrogen while maintaining heating. As isopropanol is evaporated, add ultrapure water intermittently to replace the evaporated isopropanol.
9. After most of the isopropanol has evaporated and the volume of water added is approximately 50 ml, remove the heat and allow the reaction mixture to cool.
10. Collect the product by filtering through a 0.1-µm nylon filter paper, and wash several times with water to remove the surfactant and any residual isopropanol. Do not filter to dryness. After washing is complete, transfer the slurry from the filter into a pre-weighed bottle or vial, and store as an aqueous slurry.
11. Determine the yield by determining the weight percent solids of the slurry. This can be done by measuring the weight of an aliquot of the slurry before and after drying.

2. Au(III) Ion Exchange

1. Transfer 6.50 g of 4.23 wt % nMST slurry to a 50-ml centrifuge tube. This amount may vary based on the actual weight percent of the nMST slurry produced in step 1.10 above, and determined in step 1.11.
2. Weigh out 0.0659 g of HAuCl₄·3H₂O into a 1-dram glass vial. The target Ti:Au mass ratio is 4:1.
3. Dissolve the HAuCl₄·3H₂O in ~ 1 ml of water, then transfer to the centrifuge tube containing the nMST. Rinse the vial several times with additional water to ensure all of the HAuCl₄·3H₂O is transferred to the centrifuge tube containing the nMST.
4. Dilute the suspension with additional water, as necessary, to bring the total volume to 11 ml. Target a final Au(III) concentration of approximately 15 mM.
5. Wrap the centrifuge tube in foil to keep the suspension in the dark, and then tumble the suspension on a shaker rotisserie for a minimum of 4 days.
6. Collect the product by centrifuging at 3,000 x g for 15 min to isolate the solids. Wash the solids three times with distilled water by redispersing in water, and reisolate by centrifuging at 3,000 x g for 15 min to remove any unexchanged Au(III).
7. Store the final product either as an aqueous suspension by redispersing in water, or as a moist solid by decanting off the final wash water and capping the tube. Store the product in the dark.

3. Preparation of the Peroxotitanate

1. Transfer 5 g of a 9.8 wt % slurry of nMST to a small flask.
2. Weigh out 0.154 g of 30 wt % H₂O₂ solution. The target H₂O₂:Ti molar ratio is 0.25:1.
3. While stirring the nMST suspension well add the 0.154 g of H₂O₂ solution drop-wise. Upon H₂O₂ addition, the suspension of white solids immediately turns yellow.
4. After the addition is complete stir the reaction at ambient temperature for 24 hr.
5. Collect the product by filtering through a 0.1-µm nylon filter, and wash several times with water to remove any unreacted H₂O₂. Do not filter to dryness. After washing is complete, transfer the slurry from the filter into a pre-weighed bottle or vial, and store as an aqueous slurry.

Results

MST is synthesized using a sol-gel method in which tetraisopropoxytitanium(IV) (TIPT), sodium methoxide, and water are combined and reacted in isopropanol to form seed particles of MST.⁴ Micron-sized particles are then grown by controlled addition of additional quantities of the reagents. The resultant particles feature an amorphous core and an outer fibrous region having dimensions of about 10 nm wide by 50 nm in length.²⁰

Figure 1A shows a typical parti...

Discussion

The presence of extraneous water, for example from impure reagents, can alter the outcome of the reaction, leading to larger or more polydisperse particles. Therefore, care should be taken to ensure dry reactants are used. The titanium isopropoxide and sodium methoxide should be stored in a desiccator when not in use. High purity isopropanol should also be used for the synthesis.

Temperature was found to play a key role in the conversion of the product from a gel to particulate form. TEM image...

Materials

Name	Company	Catalog Number	Comments
Titanium(IV) isopropoxide	Sigma Aldrich	377996	99.999% trace metals basis
Isopropyl alcholol, 99.9%	Sigma Aldrich	650447	HPLC grade (Chomasolv)
Sodium methoxide in methanol	Sigma Aldrich	156256	25 wt%
Triton X-100	Sigma Aldrich	T9284	BioXtra
hydrogen tetrachloroaurate(III) trihydrate	Sigma Aldrich	G4022	ACS reagent grade
hydrogen peroxide (30 wt%)	Fisher	H325	Certified ACS
10-ml syringes	Fisher	14-823-16E
Dual channel syringe pump	Cole Parmer	EW-74900-10	Or equivalent programmable dual channel syringe pump
Tygon tubing 1/8 inch ID, 1/4 inch OD	Cole Parmer	EW-0640776
Tygon tubing 1/16 inch ID, 1/8 inch OD	Cole Parmer	EW-0740771
0.1-µm Nylon filter	Fisher	R01SP04700
Labquake shaker rotisserie	Thermo Scientific	4002110Q