a. H2O2 sensing
130. The experimental conditions including anodizing parameters (time and voltage) and hydrogen peroxide concentration had a direct impact on the LSPR of MoO3 hence colorimetric performance.
131. To explore the capability of MoO3-x nanosheets for sensing hydrogen peroxide, samples S10V-15, S20V-15 and S30V-15, prepared at different potentials but similar anodizing times were selected.
132. These samples were selected for investigation of hydrogen peroxide sensing because they have similar LSPR wavelengths for 15 min anodizing time.
133. Sample S30V-25 was also included for the study of the effect of anodizing time in order to obtain a wider detection range.
134. Also, for correct comparison of spectral changes, corresponding blank samples were prepared by adding the same volume of water used in all hydrogen peroxide dilution processes.
135. Fig.4(a) represents the optical absorption spectra of the MoO3-x samples mixed with variable concentrations of H2O2 in the range of 0-100 mM.
136. The photographic images of sample S30V-25 in the presence of different concentrations of hydrogen peroxide are shown in Fig.4(b), which indicates a pronounced visual change in the appearance of the sample, evidencing oxidation of the MoO3-x particles to MoO3 by hydrogen peroxide.
137. All the LSPR bands with peaks at around 730 nm begin to drop with increasing concentration of hydrogen peroxide and diminish at above certain ranges of concentration.
138. The normalized absorptions at LSPR peaks of different samples in terms of concentration of hydrogen peroxide (calibration curves) are plotted in Fig.5.
139. The linear range and limit of detection (LOD) are presented in Table.2.
140. The LOD is calculated using the calibration curves according to the following relation:
141. LOD=3.3(σ/S)
142. Where σ and S are the standard deviation and the slope of calibration curves.
143. As shown in Fig.7a, a linear response was observed in the concentration ranges depending on the sample fabrication conditions.
144. The linear detection range increases from 0.001-0.1 to 0.01-0. 6 mM as the anodizing voltage increase from 10 to 30 V at constant 15 min anodizing time but it increases to 0.04-1 mM when anodizing time increase to 25 min at a constant potential of 30 V (see vertical arrows in Fig.5).
145. The wider detection range of S30V-25 is attributed to the higher MoO3-x to hydrogen peroxide ratio because anodizing at 30 V for a period of 25 min produces relatively many MoO3-x nanosheets (see also Fig.3).
146. However, the LOD is lower at lower anodizing potentials
147.
148. Fig.4 (a) LSPR absorption spectra of the MoO3-x colloidal solutions mixed with variable concentrations of H2O2 in the range of 0-100 mM. (b) Photographs of sample S30V-25 after adding varying concentrations of hydrogen peroxide.
149.
150. Fig.5 Relative absorption curves at LSPR wavelength (~740 nm) for different samples at mM concentration range of hydrogen peroxide.
Sample name Linear range (mM) LOD (mM)
S10V-15 0.001-0.08 0.022
S20V-15 0.01-0.1 0.025
S30V-15 0.01-0.6 0.044
S30V-25 0.04-1 0.283
151. Fig.7 LSPR wavelength as a function of hydrogen peroxide concentration for sample S30V-25.
152. The absorption wavelength in terms of hydrogen peroxide concentration is plotted for sample S30V-25 in Fig.7, which increases from about 743 to 749 nm with hydrogen peroxide concentration.
153. As a strong oxidizer, H2O2 fills the oxygen vacancies created in the anodizing process, causing the free charge carriers to reduce, and the LSPR band to diminish and the deep blue color to transform to pale blue. As the number of charge carriers decreases due to the oxidation of MoO3-x, ꞷLSPR decreases and as a result the LSPR red-shifts.
154.