Diagnostic tests in resource-limited settings require technologies that are affordable and easy to use with minimal infrastructure. Colorimetric detection methods that produce results that are readable by eye, without reliance on specialized and expensive equipment, have great utility in these settings. We report a colorimetric method that integrates a paper-based immunoassay with a rapid, visible-light-induced polymerization to provide high visual contrast between a positive and a negative result. Using Plasmodium falciparum histidine-rich protein 2 as an example, we demonstrate that this method allows visual detection of proteins in complex matrices such as human serum and provides quantitative information regarding analyte levels when combined with cellphone-based imaging. It also allows the user to decouple the capture of analyte from signal amplification and visualization steps.

Although polymerization-based amplification (PBA) has demonstrated promise as an inexpensive technique for use in molecular diagnostics, oxygen inhibition of radical photopolymerization has hindered its implementation in point-of-care devices. The addition of 0.3-0.7 μM eosin to an aqueous acrylate monomer solution containing a tertiary amine allows an interfacial polymerization reaction to proceed in air only near regions of a test surface where additional eosin initiators coupled to proteins have been localized as a function of molecular recognition events. The dose of light required for the reaction is inversely related to eosin concentration. This system achieves sensitivities comparable to those reported for inert gas-purged systems and requires significantly shorter reaction times. We provide several comparisons of this system with other implementations of polymerization-based amplification.

Despite the numerous applications of eosin Y as an organic photoredox catalyst, substantial mechanistic aspects of the photoredox process have remained elusive. Through deductive, steady-state kinetic studies, we first propose a mechanism for alkaline, aqueous photoredox catalysis using eosin Y, triethanolamine, and oxygen, integrating photo- and nonphotochemical steps. The photoredox cycle begins with a single-electron transfer (SET) induced when eosin Y absorbs green light. This photoinduced SET leads to the formation of a metastable radical trianion that can be fully reduced to inactivated leuco eosin Y via H+/e–/H+ transfer or regenerated to eosin Y via ground-state SET to oxygen. Since the radical trianion absorbs violet light, we tested the effect of radical trianion photoexcitation on catalyst regeneration. We found that excitation of the metastable radical trianion in the presence of a threshold concentration of oxygen enabled ∼100% regeneration of eosin Y. The response to violet light supports the important role of the metastable radical trianion and indicates that the photoredox cycle can be closed via a secondary photoinduced SET event. The idea of photoredox cycles with two consecutive photoinduced electron transfer (PET) steps is not intuitive and is introduced as a tool to increase photocatalyst turnover by selectively favoring regeneration over “death”. This alludes to the Z-scheme in biological photosynthesis, where multiple PET reactions, often triggered by different frequencies, promote highly selective biochemical transformations by precluding unproductive SET events in plants and bacteria. We expect that the simple Z-scheme model introduced here will enable more efficient use of organic photoredox catalysts in organic and materials chemistry.

Widely used medical diagnostic devices and assays that sense the presence of a particular molecule in a bodily fluid often rely on either a nanoparticle label or an enzymatic reaction to generate a signal that is easily detectable. In many cases, it is desirable if the magnitude of the signal correlates with the concentration of the molecule of interest. Photo-initiated polymerization reactions are an alternative means of generating amplified signals that can be used to quantify biological molecules in complex fluids. In this case, the formation of a polymer, typically a cross-linked hydrogel, signifies the presence of the molecule of interest. This tutorial review explains how photo-initiated polymerization reactions have been used in a conditional manner to detect and quantify molecular recognition events. We weigh the advantages and disadvantages of using photo-initiated reactions in comparison with other approaches and highlight exciting directions and opportunities in this area.