pH Switchable Water Dispersed Photocatalytic Nanoparticles

Abstract

Photogeneration of Reactive Oxygen Species (ROS) finds applications in fields as different as nanomedicine, art preservation, air and water depollution and surface decontamination. Here we present photocatalytic nanoparticles (NP) that are active only at acidic pH while they do not show relevant ROS photo-generation at neutral pH. This dual responsivity (to light and pH) is achieved by stabilizing the surface of TiO₂ NP with a specific organic shell during the synthesis and it is peculiar of the achieved core shell-structure, as demonstrated by comparison with commercial photocatalytic TiO₂ NP. For the investigation of the photocatalytic activity, we developed two methods that allow real time detection of the process preventing any kind of artifact arising from post-treatments and delayed analysis. The reversibility of the pH response was also demonstrated as well as the selective photo-killing of cancer cells at acidic pH.

Keywords: fluorescence; nanoparticles; photocatalysis; reactive oxygen species (ROS); stimuli responsive materials.

Scheme 1

Templated synthesis of DSR NP by hydrolysis and condensation of the molecular precursor TIP in the PPG core of the Pluronic F127 micelles.

Figure 1

a) HR TEM images of the DSR NP. In the inset: top, size distribution histogram of the DSR NP; bottom, Fast Fourier Transform of the HR‐TEM micrograph. b) Absorption spectra of the DSR NP (0.2 mg/mL) at different pH. In the inset: absorption spectra of RhB (0.5 μM) in the presence (red line) and in the absence (black line) of DSR NP.

Figure 2

a) Scheme of the method for investigating the kinetic of photodegradation. The same beam (340 nm) is used for excitation of the photocatalyst (DSR NP) and of the target (RhB). Photogenerated ROS degrade RhB causing a decrease of the fluorescence. Fluorescence at 590 nm is followed to detect the concentration of RhB. b) Fluorescence at 590 nm as a function of time during irradiation at different pH (1.0–6.0).

Figure 3

a) Cycles of photo‐catalytic degradation of RhB by DSR NP. Red curves show fluorescence at 590 nm at pH=2.0 and blue curves at pH=5.0. b) Pseudo‐zero order rate constant calculated for each cycle.

Figure 4

a) Schematic representation of the mechanism responsible for the decreased photocatalytic activity of DSR NP at pH 7.0 with respect to acidic pH. The killing of cancer cell line by DSR NP upon photo and chemical stimulation was evaluated on HeLA cell treated with DSR NP diluted from 1 : 10 (d10) to 1 : 100 000 (d100 000) in either b) PBS pH 7 or c) PBS pH 5. After treatment cells were irradiated with white lamp (white bars) or kept under dark conditions (dark grey bars). Cell viability is expressed in percentage with respect to the untreated sample (dashed line). *= p‐value <0.05, ***= p‐value <0.01, ***= p‐value <0.001.