A job for quantum dots: use of a smartphone and 3D-printed accessory for all-in-one excitation and imaging of photoluminescence

Point-of-care (POC) diagnostic technologies are needed to improve global health and smartphones are a prospective platform for these technologies. While many fluorescence or photoluminescence-based smartphone assays have been reported in the literature, common shortcomings are the requirement of an excitation light source external to the smartphone and complicated integration of that excitation source with the smartphone. Here, we show that the photographic flash associated with the smartphone camera can be utilized to enable all-in-one excitation and imaging of photoluminescence (PL), thus eliminating the need for an excitation light source external to the smartphone. A simple and low-cost 3D-printed accessory was designed to create a dark environment and direct excitation light from the smartphone flash onto a sample. Multiple colors and compositions of semiconductor quantum dot (QD) were evaluated as photoluminescent materials for all-in-one smartphone excitation and imaging of PL, and these were compared with fluorescein and R-phycoerythrin (R-PE), which are widely utilized molecular and protein materials for fluorescence-based bioanalysis. The QDs were found to exhibit much better brightness and have the best potential for two-color detection. A model protein binding assay with a sub-microgram per milliliter detection limit and a Förster resonance energy transfer (FRET) assay for proteolytic activity were demonstrated, including imaging with serum as a sample matrix. In addition, FRET within tandem conjugates of a QD donor and fluorescent dye acceptor enabled smartphone detection of dye fluorescence that was otherwise unobservable without the QD to enhance its brightness. The ideal properties of photoluminescent materials for all-in-one smartphone excitation and imaging are discussed in the context of several different materials, where QDs appear to be the best overall material for this application.