Enhanced antibactericidal function of W⁴⁺-doped titania-coated nickel ferrite composite nanoparticles: A biomaterial system

Abstract

The study demonstrates a distinct enhancement of antimicrobial activity of W⁴⁺-doped titania that is coated on nickel ferrite nanoparticles in comparison to undoped titania. The composite nanoparticles were synthesized by uniquely combining reverse micelle and chemical hydrolysis synthesis methods [Rana S, Rawat J, Misra RDK, Acta Biomater 2005;1:691]. The superior antimicrobial activity of W⁴⁺-doped titania is related to the inhibition of electron–hole recombination and decrease in the band gap energy of titania. The function of the ferrite is to facilitate the removal of nanoparticles from the sprayed surface using a small magnetic field. The coating of ferrite nanoparticles with titania retains superparamagnetic character and magnetic strength of composite nanoparticles signifying non-deterioration of magnetic properties and promoting their use as removable antimicrobial photocatalyst nanoparticles.

Introduction

Photocatalytic semiconductor oxides are being increasingly synthesized as nanoparticles or as nanostructured materials for degradation of organic and inorganic microorganisms because of their large surface area [1], [2], [3], [4], [5], [6], [7], [8], [9]. In this regard, titania is considered a preferred and potential semiconductor photocatalytic material for applications requiring antimicrobial and sterilizing characteristics [10], [11], [12], [13], [14], [15]. Unfortunately, pure titania as a semiconductor photocatalyst is characterized by low quantum efficiency because of high band gap energy (∼3.2 eV) and high rate of e⁻ and h⁺ recombination. Increasing the quantum efficiency requires a lower e⁻/h⁺ recombination rate and the minimization of the band gap energy. Besides the large band gap characteristic of titania, the absorption spectrum is in the ultraviolet light range such that it corresponds to about 5–8% of solar light. The utilization of the photocatalytic character of titania for antimicrobial activity involving visible light requires that the absorption spectrum is extended to the visible light range. An effective method to enhance the photocatalytic activity of titania is to narrow the band gap by doping titania with a lanthanide metal cation.

The experience of doping with metal cations to promote the photocatalytic activity with dopant is varied because of their different roles in trapping electrons and/or holes on the surface. Experiments with ferric ions (Fe³⁺) indicated an increase in the photoreduction efficiency of nitrogen [13] and methyl viologen [14] and a decrease in electron/hole pair recombination in titania. However, photoreactivity was insignificant with regard to photodegradation of phenol and 4-nitrophenol when titania was doped with Fe³⁺ (ferric ions) [15]. Enhanced photoreactivity for water cleavage [16] and nitrogen reduction [15] with Cr³⁺-doped titania was observed, while some researchers concluded that Cr³⁺ was detrimental to photocatalytic activity [17] Titania doped with vanadium exhibited significantly reduced photoreactivity [18], but Gratzel and Howe [19] predicted the inhibition of electron/hole pair recombination based on electron paramagnetic resonance (EPR) data. The lanthanide ion-doped titania exhibited a stronger photoresponse and superior reactivity for the photocatalytic degradation of rhodamine B in comparison with undoped titania nanoparticles [20]. Thus, the results of the doping effects of metal ions on the reactivity of titania have been generally inconsistent primarily because of the significant variation in the fabrication methods used for the synthesis of the undoped and doped titania and the approach adopted to make a relative comparison between the photoreactivity of the undoped and doped photocatalyst. The previous fabrication methods involved sol–gel or solid state reaction, which led to the formation of amorphous or mixed crystalline–amorphous titania.

Another important disadvantage of pure titania from the viewpoint of application is that titania is an electrical insulator and is therefore not practically extractable from the sprayed surface or wound after treatment. However, a practical solution to this limitation is to consider synthesizing composite particles characterized by magnetic nanoparticles that are subsequently coated with titania. The magnetic part of the composite particles facilitates their removal by using a small magnetic field and also enables the controlled and targeted delivery of particles [21], [22]. The present work is aimed at illustrating the superior antimicrobial activity of tungsten (W⁴⁺)-doped titania composite nanoparticles by making a relative comparison between undoped and tungsten-doped titania.