Spatial, spectral, radiometric, and temporal analysis of polymer-modified paper substrates using fluorescence microscopy

Optical microscopy has long been recognized as a method to characterize the heterogeneous and complex structure of paper. With fluorescence detection, the functionality has even been extended to provide chemical selectivity, e.g. to determine the distribution of secondary modifications like coatings and fillers throughout a sheet of paper. The full spectrum of capabilities offered by fluorescence microscopy, which is able to deliver information with high spatial, spectral, radiometric, and temporal resolution simultaneously and non-destructively, however has yet to be exploited. With paper more and more coming into focus as a versatile platform for the development of functional devices, static structural and compositional information is no longer sufficient to describe the properties of these systems. Rather, a likewise versatile method is required that delivers spatially resolved, quantitative, sensitive, and, most importantly, also dynamic measurements. Here we show that quantitative widefield and confocal fluorescence microscopy are able to meet this set of demands. In a proof of concept analysis on PMMA-modified microfluidic paper substrates, we exploit all four types of resolution provided by fluorescence microscopy, by analyzing the distribution of rhodamine labeled polymer in relation to calcofluor white labeled cellulose fibres with high selectivity and spatial resolution and by imaging the static and dynamic distribution of a FITC labeled dextran solution inside the polymer coated fibre network.