The chloroplast contains densely stacked arrays of light-harvesting proteins that harness solar energy with theor. max. glucose conversion efficiencies approaching 12%. Few studies have explored isolated chloroplasts as a renewable, abundant, and low cost source for solar energy harvesting. One impediment is that photoactive proteins within the chloroplast become photodamaged due to reactive oxygen species (ROS) generation. In vivo, chloroplasts reduce photodegrdn. by applying a self-repair cycle that dynamically replaces photodamaged components; outside the cell, ROS-induced photodegrdn. contributes to limited chloroplast stability. The incorporation of chloroplasts into synthetic, light-harvesting devices will require regenerative ROS scavenging mechanisms to prolong photoactivity. Herein, we study ROS generation within isolated chloroplasts extd. from Spinacia oleracea directly interfaced with nanoparticle antioxidants, including dextran-wrapped nanoceria (dNC) previously demonstrated as a potent ROS scavenger. We quant. examine the effect of dNC, along with cerium ions, fullerenol, and DNA-wrapped single-walled carbon nanotubes (SWCNTs), on the ROS generation of isolated chloroplasts using the oxidative dyes, 2',7'- dichlorodihydrofluorescein diacetate (H2DCF-DA) and 2,3-bis(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide sodium salt (XTT). Electrochem. measurements confirm that chloroplasts processed from free soln. can generate power under illumination. We find dNC to be the most effective of these agents for decreasing oxidizing species and superoxide concns. while preserving chloroplast photoactivity at concns. below 5 μM, offering a promising mechanism for maintaining regenerative chloroplast photoactivity for light-harvesting applications. [on SciFinder(R)]