TiO₂ photocatalytic degradation of cholesterol: SIMS study

Abstract

TiO₂ is an efficient photocatalyst for chemical degradation. Nanocrystalline TiO₂ is used in the form of film supported on glass substrate. Since the photocatalytic process is occurring on the surface, a surface-sensitive technique as a direct probe is desirable. In order to study TiO₂ photoactivity and photodegradation of cholesterol, as a model biomolecule, secondary ion mass spectrometry is utilized. Cholesterol (C₂₇H₄₆O), stated as M, with mass of 386.65 u, adsorbed on TiO₂ surface, activated with water, and irradiated with different ultraviolet (UV) exposure times is characterized under impact of 25-keV Bi⁺ primary ions. The molecular positive ions in the forms of M–OH and M–H with masses of 369.3 u and 385.3 u, respectively, are of main interest. Due to the specific group of OH, the M-OH molecular ion might be considered as the primary identification species of the intact cholesterol molecule. The intensities of cholesterol fragment ions M–C₅H₁₀, M–C₇H₁₄, M–C₇H₁₄OH, M + C₄H₇, M + C₄H₉O, M + C₄H₉O₂, M + C₈H₁₅, and 2 M–2(C₈H₁₇), 2 M–2(C₇H₁₅) decrease significantly after 7 h of UV exposure. Intensities of M–OH and M–H peaks decrease after 24 h by 88% and 62%, respectively. These measurements show the potential to degrade cholesterol, as a main bacteria membrane component and in such a way to terminate bacteria effectively. Copyright © 2012 John Wiley & Sons, Ltd.

Introduction

The photocatalytic properties of TiO₂ have been extensively studied[1, 2], and TiO₂ is well known to degrade organic substances. TiO₂ is considered as a safe, efficient, and economical photocatalyst for chemical degradation. TiO₂ in the anatase form appears to be most photoactive and useful for the widespread environmental applications, such as degradation of organic or biological pollutants in water or in atmosphere. There are many studies of photocatalytic degradation processes running on the surface of TiO₂ induced by ultraviolet (UV) radiation.[3-7] In recent years, nanocrystalline TiO₂ (nc - TiO₂) has attracted great interest due to its potential in environmental applications. The basic mechanism of photoinduced TiO₂ reactions can be described in several steps,[8] where TiO₂ is a semiconductor with a band gap of E_g ~ 3.2 eV and UV photon with energy greater than E_g then generates an electron–hole pair in TiO₂ structure. If these charge carriers do not recombine, they induce reduction/oxidation processes of adsorbed molecules, leading eventually to the catalytic degradation of any pollutant species present on the surface. Redox potential of generated holes is +2.53 eV against Standard Hydrogen Electrode. Reactions with adsorbed water or adsorbed hydroxide ions can produce hydroxide radicals. Redox potential of electrons is −0.52 eV what can reduce adsorbed oxygen to superoxide. Both pathways are described in Eqns (1)-(6)

• (1)

• (2)

• (3)

• (4)

• (5)

• (6)

Depending on the conditions, the presented reactants and products in Eqns (1)-(6) can play an important role in photocatalytic reaction mechanism. They can decompose many hydrocarbons or terminate bacteria, where the ideal degradation of hydrocarbons is described in Eqn (7).

• (7)

The goal of this work was to use secondary ion mass spectrometry (SIMS)[9] to study the degradation process of cholesterol on nc - TiO₂ surface depending on the duration of UV irradiation. Such reactions have been mostly studied in suspensions containing nc - TiO₂ particles. In our experiment, nc - TiO₂ is used in the form of film supported on the glass substrate. Hence, a direct probe of photocatalytic process occurring on the nc - TiO₂ surface is readily suitable by utilizing advantages of SIMS determining the surface species. Cholesterol due to its structure and chemical composition is almost completely insoluble in water, providing an essential part of bacteria cell membrane. Hydrophobicity of cholesterol protects cells from environmental effects. Investigation of cholesterol degradation is used as a model for the potential termination of bacteria. In the present work, the samples of cholesterol on the nc - TiO₂ film are analyzed before and after UV irradiation. The SIMS intensities of selected species are determined as a function of UV irradiation time exposure.

Experimental

Nanocrystalline anatase TiO₂ particles with a nominal size of 25 nm in powder form (Sigma-Aldrich) were used in the present work. Suspension was prepared from the TiO₂ particles and organic solvent (mixture of ethanol: water = 3: 2). Nanocrystalline thin film was deposited by sedimentation of the nc - TiO₂ colloidal suspension on glass substrate in Petri dish. The film was then dried in desiccator at room temperature for 24 h and subsequently annealed at 300 °C in air for 1 h. Such a prepared film was ready for the cholesterol deposition. Cholesterol was dissolved in chloroform (0.30 M), and 10 µl of this solution was applied on the nc - TiO₂ film. A spin-coating was used in order to maintain the preparation of uniform cholesterol layer.

The UV exposures were performed in air by using Hg lamp, and the irradiation wavelength of 365 nm was selected by using appropriate filter. The intensity of incident light was approximately 300 μW/cm². Note that the absorption spectral shoulder of cholesterol starts approximately below 300 nm, with no absorption at 365 nm.[10]

The SIMS measurements were carried out in a time-of-flight secondary ion mass spectrometer which is equipped with a Bi liquid-metal ion source. For the analyses, 25-keV Bi⁺ ion gun was employed with a raster scan over the surface area of 100 × 100 µm², while maintaining PIDD of 10¹³ ions/cm².

Results and discussion

The SIMS spectra of cholesterol on nc - TiO₂ with different exposure times of UV irradiation were measured. Measurements provide results for non-irradiated sample and for irradiated samples for 3, 7, 16, and 24 h. The focus of the SIMS measurements was on the cholesterol molecular positive ions in the forms of M–OH and M–H with masses of 369.3 u and 385.3 u, respectively. M states for the molecule of cholesterol C₂₇H₄₆O with mass of 386.65 u. The M–OH and M–H ions are then only slightly different compared to the intact cholesterol molecule; therefore. they best represent the concentration of cholesterol on the surface. However, to provide a complete picture, three regions of SIMS spectra were analyzed. The first region is spanning from 1 to 200 u, the second from 200 to 400 u, and the third from 400 to 600 u.

The first region is shown in Fig. 1, where Fig 1a, provides spectra without UV exposure as an initial concentration of cholesterol in the cholesterol/nc - TiO₂ system. The UV exposure times, which are discussed, were 7 and 24 h, and the results are shown in Fig. 1b and Fig. 1c, respectively. The peaks in the first region were assigned as C_xH_y, where x spans from 2 to 7. The result of the cholesterol degradation after 24 h shows the higher population of smaller C_xH_y fragments between masses 40 u and 60 u as a result of the cholesterol fragmentation. The peak of Ti is progressively better observed as the degradation of cholesterol proceeds. This result might be attributed to the cleaning of the surface due to the lower cholesterol concentration or eventually to a change of the matrix due to the different composition of hydrocarbons on the nc - TiO₂ surface.

Figure 1. SIMS spectra of positive secondary ions from cholesterol/nc - TiO₂ system in the 1–200 u region. (a) no UV exposure, (b) 7 h, and (c) 24 h of exposure.

The peaks in the second region of 200–400 u were assigned as positive ions M + CH₃, M–C₅H₁₀, M–C₇H₁₄, and M–C₇H₁₄OH with masses of 401.5 u, 316.5 u, 288.5 u, and 271.5 u, respectively. In this case, the molecule of cholesterol has cleaved off the –C_xH_y groups or attracted + CH₃ group, and the spectra are shown in Fig. 2. The proposed structures of such M–C₅H₁₀ and M–C₇H₁₄ fragments are shown as an inset in Fig. 2a, suggesting the initial phase of degradation mechanism. The cholesterol molecular fragments M–C₅H₁₀, M–C₇H₁₄, and M–C₇H₁₄OH are shown in Fig. 2a, together with molecular ions of M–OH and M–H as an initial state before the UV exposure. The cholesterol molecular fragments M–C₅H₁₀, M–C₇H₁₄, and M–C₇H₁₄OH disappeared after 7-h exposure as shown in Fig. 2b. Interestingly, the positive ion M + CH₃ intensity increased after 7 h, probably due to the higher concentration of CH₃ fragment, originating from the degraded cholesterol molecule. However, after 24 h, the intensity of positive ion M + CH₃ decreased to initial level.

Figure 2. SIMS spectra of positive secondary ions from cholesterol/nc - TiO₂ system in the 200–400 u region. (a) no UV exposure, (b) 7 h, and (c) 24 h of exposure.

The most important changes were attributed to the changes in the intensities of molecular ions M–OH and M–H. The initial intensity of M–OH ion is higher than the intensity of M–H ion, presumably due to the lower and higher mobility of OH and H, respectively, in the ions formation SIMS process. Due to the more specific group of OH, the M–OH molecular ion might be considered as the primary identification of the intact cholesterol molecule. The intensity of M–OH peak decreases after 24 h of irradiation by 88% compared to the intensity of the non-irradiated cholesterol/nc - TiO₂ system. The decrease of the M–H ion intensity is by 62% after 24 h of irradiation. The difference is due to the different mechanisms of M–OH and M–H ion formation and eventually the observation can shed some light also on differences in the mechanism of photoinduced degradation.

The results of degradation reactions of cholesterol with radicals arising from the nc - TiO₂ surface after exposure of UV light are summarized in Fig. 3 for the molecular ions of M–OH and M–H. The times of irradiation were selected to cover the whole day period within the initial concentration, 3, 7, 16, and 24 h. We discussed only 7 and 24 h. The reason to show the irradiation time of 7 h was not arbitrary, but it was approximately half time of decay, where the faster degrading molecular ion of M–OH decreased to 50% of the initial intensity within 7 h. The irradiation time of 24 h was used a complete day period. The intensities of the M–OH molecular ion decreased to 74, 54, 29, and 12% within the time periods of 3, 7, 16, and 24 h, respectively. The intensities of the M–H molecular ion decreased to 92, 83, 61, and 38% within the time periods of 3, 7, 16, and 24 h, respectively.

Figure 3. SIMS intensities of M-OH and M-H molecular ions as a function of UV irradiation times.

The peaks in the third region are shown in Fig. 4 and were assigned as M + C₂H₅, M + C₄H₇, M + C₄H₉O, M + C₄H₉O₂, and M + C₈H₁₅, where + C_xH_y are added groups to the molecule of cholesterol. Peaks, which were assigned as 2 M–2(C₈H₁₇) and 2 M–2(C₇H₁₅), correspond to a dimer of cholesterol with cleaved groups of −2(C_xH_y). The intensity of M + C₂H₅, similar to M + CH₃ ion, increased after 7 h, probably due to the higher concentration of C₂H₅ fragment, originating from the degraded cholesterol molecule, and then slightly decreased after 24 h. The intensities of M + C₄H₇, M + C₄H₉O, M + C₄H₉O₂, and M + C₈H₁₅ ions decreased significantly after 7 h exposure and gradually decreased after 24 h, as shown in Fig. 4b and Fig. 4c, respectively. The intensities of 2 M–2(C₈H₁₇) and 2 M–2(C₇H₁₅) dimer ions decreased significantly after 7 h exposure and disappeared after 24 h, as shown in Fig. 4b and Fig. 4c, respectively.

Figure 4. SIMS spectra of positive secondary ions from cholesterol/nc - TiO₂ system in the 400–600 u region. (a) no UV exposure, (b) 7, and (c) 24 h of exposure.

Although the bacterial cell wall is not formed only with pure cholesterol, it is likely that the process of complete surface cleaning and terminating bacteria might take no longer than 24 h.

Conclusions

The cholesterol degradation on the nc - TiO₂ surface with different UV light exposure times was investigated by using SIMS technique. SIMS measurements characterizing the samples as non-irradiated and irradiated for 3, 7, 16, and 24 h were determined. Three regions of SIMS spectra were analyzed. The cholesterol degradation after 24 h showed the higher concentration of smaller C_xH_y fragments between masses 40 u and 60 u as a result of the cholesterol fragmentation. The peak of Ti was progressively better observed as the degradation of cholesterol proceeds and the surface is cleaned. The cholesterol fragments M–C₅H₁₀, M–C₇H₁₄, and M–C₇H₁₄OH disappeared after 7-h UV exposure. The intensity of M–OH peak decreased after 24 h of irradiation by 88% compared to the intensity of the non-irradiated cholesterol/nc - TiO₂ system. The decrease of the M–H ion intensity was by 62% after 24 h of irradiation. Due to the specific group of OH, the M–OH molecular ion was considered as the primary identification species of the intact cholesterol molecule. The intensities of M + C₄H₇, M + C₄H₉O, M + C₄H₉O₂, and M + C₈H₁₅ ions decreased significantly after 7-h exposure, and the intensities of 2 M–2(C₈H₁₇) and 2 M–2(C₇H₁₅) dimer ions disappeared after 24 h. These measurements showed the potential to degrade cholesterol, as a main bacteria membrane component and in such a way to effectively terminate bacteria.

Acknowledgements

This research was supported by ERDF OP R&D, Project ‘meta-QUTE- Centrum excelentnosti kvantových technológií’ and APVV-0491-07.