A green-PAD array combined with chemometrics for pH measurements

This work presents the development of a green paper-based analytical device (Green-PAD) array for pH
detection. The array was obtained with natural dyes extracted from red cabbage (Brassica oleracea) and
butterfly pea flower (Clitoria ternatea); a filter paper was used as a substrate. The RGB indexes of the PADs’
colors were extracted from the pictures taken using a smartphone or using a specifically developed RGB
detector (Arduino-based) to obtain RGB indexes not affected by the light and the photocamera sensitivity.
Multi-technique chemometric models were developed for calculating the pH value, starting from the RGB
triplet of each sensing PAD. A preliminary and explorative chemometric analysis with PCA (partial
component analysis) and TWPCA (3-way PCA) was carried out. Partial least square regression, PLS, was then
applied to correlate the color of the PAD’s picture with the solutions’ pH. Different solutions at various pHs,
ranging from 1 to 13, were obtained by titrating orthophosphoric acid with standardized NaOH, and they
were used to create PLS models. Some real samples were examined as a test set, and the results were
validated with pH-meter measurements. The ability of the PLS to model the experimental data was
satisfactory since a good agreement between the experimental and fitted pH values was obtained. The
proposed PADs were prepared with natural dyes and filter papers, so they are completely biodegradable and
eco-friendly. Their fabrication does not require toxic or expensive reagents and sophisticated equipment.
Also, the developed RGB detector, built-up with low-cost components and recycled batteries, adds value
making the measurement cheap, easy and feasible also by non-expert people.