Determination of Tio2 Nanoparticles in Sunscreen Using N-Doped Graphene Quantum Dots As

ELECTRONIC SUPPORTING INFORMATION

Determination of TiO2 nanoparticles in sunscreen using N-doped graphene quantum dots as a fluorescent probe

Sandra Benítez-Martínez, Ángela Inmaculada López-Lorente, Miguel Valcárcel*

Department of Analytical Chemistry, University of Córdoba, E-14071 Córdoba, SpainPhone/Fax +34 957 218616; E-mail:

Study of the conditions for quenching measurements

Firstly, the influence of the pH of the N-GQDs solution over the quenching response of the method was investigated. The pH obtained directly from the synthesis of N-GQDs, which is basic, was selected since higher decrease in fluorescence was observed when mixing the nanomaterial with a solution of TiO2 nanoparticles, thus, enabling better sensitivity of the methodology.

The solvent employed for the reconstitution of the extract and the subsequent interaction with the N-GQDs also play a significant role in the performance of the method. Different solvents were investigated, namely water, methanol (MeOH), dimethylsulfoxide (DMSO), acetonitrile (ACN) and dimethylformamide (DMF). As can be seen in Figure S1a with water better response in terms of quenching signal was obtained. However, the reaction is slower in this medium and the signal need a long time to reach a stable value. A similar effect is observed in the case of acetonitrile, needing also a long time to obtain a stable measurement. When the TiO2 NPs sample is reconstituted and mixed with the N-GQDs in methanol medium the signal become stable in a short time. Thus, a compromise between sensibility and reaction time was acquired, finally selecting methanol as the medium for further measurements.

The concentration of the N-GQD solution was also investigated. The analytical response when using different dilutions of the synthesized N-GQDs was studied. Higher fluorescence value of the N-GQDs solution was observed when the sample was diluted 4-fold. Moreover, the ratio of volume between the N-GQD and sample solution containing the TiO2 NPs was also investigated. Different volume ratio of N-GQDs and TiO2 NPs-extract were tested (1:1, 1:2, 2:3, 3:2 and 2:1), observing that at a 1:2 ratio the higher values in terms of sensitivity were observed (Figure S1b), finally selecting 100 μL of the N-GQDs solution (4-fold diluted) and 200 μL of the extract in methanol as the final conditions for further measurements.

Figure S1.(a) Study of the response of the N-GQDs probe to the presence of 200 μg·g-1 ofTiO2 nanoparticles in different medium: dimethylformamide (DMSO), acetonitrile (ACN), dimethylformamide (DMF), methanol (MeOH) and water. (b) Influence of the volume ratio of N-GQDs and sample extract in the quenching response of 200 μg·g-1 of TiO2 nanoparticles.

Stern-Volmer plot

The data were analysed using the Stern-Volmer equation (I0/I=1+KSV[analyte]), where I0 and I are the fluorescence intensities in absence and presence of TiO2 NPs, for evaluating fluorescence quenching of N-GQDs. The straight-line calibration data set adjusted accordingly to the Stern-Volmer model yielded the equation I0/I = 1.065 + 0.0018[TiO2 NPs], with the concentration expressed in μg·g−1. The response was linear in the range of concentrations tested (R2 = 0.9895).

Figure S2.Calibration graph according to the Stern-Volmer equation of TiO2 standards aqueous solutions under the optimal conditions: 100 µL of a 4-fold diluted N-GQD solution mixedwith 200 µL of themethanol solution containing TiO2 NPs.