A new, smartphone-based colourimetric method for the assay of d-penicillamine formulations relying on the Ellman's reaction was developed by performing the colourimetric reaction in a microplate. Subsequently, the plate was positioned on a white illuminating screen of an iPad placed in a dark box in order to capture a top-view image using an iPhone 5s back camera. The intensity of yellow colour was converted to Red-Green-Blue pixels using a free mobile application. Under the optimal conditions for the reaction and photography, the intensity of blue colour, which was logarithmically transformed, showed an excellent linearity over the drug concentration range of 5-40 µg/mL. The assay was validated and successfully applied to the assay of drug content and the determination of drug amount released in the dissolution test in the capsule dosage forms. Apart from that, a smartphone was employed for the colour measurement as an alternative to a spectrophotometer in the currently used method for the quantitation of free sulfhydryl groups in polymers. Using cysteine-conjugated chitosan as a sample and l-cysteine as a standard, the smartphone method gave the results in agreement with those obtained from the absorbance measurement on a microplate reader. In conclusion, smartphone-based colourimetry has been proved to be a reliable, fast, simple and affordable alternative means for the analysis of d-penicillamine and cysteine-conjugated polymer and can be potentially applied to other thiol-containing drugs and excipients.
Keywords: 5,5′-Dithiobis (2-nitrobenzoic acid) (PubChem CID: 6254); Colourimetric assay; Ellman; Penicillamine; RGB; Smartphone; Thiolated polymer; d-Penicillamine (PubChem CID: 5852); l-Cysteine hydrochloride monohydrate (PubChem CID: 5862).
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