Exponential molecular amplification such as the polymerase chain reaction is a powerful tool that allows ultrasensitive biodetection. Here we report a new exponential amplification strategy based on photoredox autocatalysis, where eosin Y, a photocatalyst, amplifies itself by activating a non-fluorescent eosin Y derivative (EYH2) under green light. The deactivated photocatalyst is stable and rapidly activated under low intensity light, making the eosin Y amplification suitable for resource-limited settings. Through steady-state kinetic studies and reaction modeling, we found that EYH2 is either oxidized to eosin Y via one-electron oxidation by triplet eosin Y and subsequent 1e─/H+ transfer, or activated by singlet oxygen with the risk of degradation. By reducing the rate of the EYH2 degradation, we successfully improved EYH2- to-eosin Y recovery, achieving efficient autocatalytic eosin Y amplification. Additionally, to demonstrate its flexibility in output signals, we coupled the eosin Y amplification with photo-induced chromogenic polymerization, enabling sensitive visual detection of analytes. Finally, we applied the exponential amplification methods in developing bioassays for detection of biomarkers including SARS-CoV-2 nucleocapsid protein, an antigen used in the diagnosis of COVID-19<br>

Exponential molecular amplification such as the polymerase chain reaction is a powerful tool that allows ultrasensitive biodetection. Here we report a new exponential amplification strategy based on photoredox autocatalysis, where eosin Y, a photocatalyst, amplifies itself by activating a non-fluorescent eosin Y derivative (EYH2) under green light. The deactivated photocatalyst is stable and rapidly activated under low intensity light, making the eosin Y amplification suitable for resource-limited settings. Through steady-state kinetic studies and reaction modeling, we found that EYH2 is either oxidized to eosin Y via one-electron oxidation by triplet eosin Y and subsequent 1e─/H+ transfer, or activated by singlet oxygen with the risk of degradation. By reducing the rate of the EYH2 degradation, we successfully improved EYH2- to-eosin Y recovery, achieving efficient autocatalytic eosin Y amplification. Additionally, to demonstrate its flexibility in output signals, we coupled the eosin Y amplification with photo-induced chromogenic polymerization, enabling sensitive visual detection of analytes. Finally, we applied the exponential amplification methods in developing bioassays for detection of biomarkers including SARS-CoV-2 nucleocapsid protein, an antigen used in the diagnosis of COVID-19<br>