Background-Free Referenced Luminescence Sensing and Imaging of pH Using Upconverting Phosphors and Color Camera Read-out

Abstract

Fluorescence background and problems with proper signal referencing severely disrupt the read-out of luminescence sensors and images. We present a pH sensor film in combination with a simple read-out technique that overcomes issues of background signals and autofluorescence. It consists of micrometer-sized upconversion phosphors (UCPs) and a pH indicator (Neutral Red) that absorbs their green emission. Both are embedded in a proton permeable hydrogel matrix. The UCPs generate green and red luminescence upon excitation with IR light of 980 nm wavelength. Solely the green light of the UCPs is affected by the pH indicator, while the red emission acts as inert reference signal for ratiometric measurements. The emission peaks of the UCPs match the red and green color channels of standard digital cameras. Thereby, the devised sensor film can be used for referenced ratiometric sensing and 2D imaging of pH using a color camera read-out. The sensor setup using common and hand-held devices is cheap and straightforward and allows for point-of-care measurements. Finally, pH measurements in human serum samples show the potential of this sensor for imaging free of interfering background signals.