An　ultrasensitive　photoelectrochemical　(PEC)　immunoassay　protocol　for　quantitative　detection　of　low-abundant　proteins　at　a　low　potential　was　designed　by　utilizing　porphyrin-sensitized　titanium　dioxide　(TiO2)　nanostructures.　Experimental　results　demonstrated　that　the　water-soluble　5,10,15,20-tetra(4-sulfophenyl)-21H,23H-porphyrin　(TSPP)　could　be　bound　onto　titanium　dioxide　via　the　sulfonic　group.　TSPP-sensitized　TiO2　nanostructures　exhibited　better　photoelectrochemical　responses　and　stability　in　comparison　with　TiO2　nanoparticles　alone　under　continuous　illumination.　Using　carcinoembryonic　antigen　(CEA)　as　a　model　analyte,　a　typical　PEC　immunosensor　by　using　TSPP-TiO2　as　the　affinity　support　of　anti-CEA　capture　antibody　(Ab1)　to　facilitate　the　improvement　of　photocurrent　response　was　developed.　Bioconjugates　of　secondary　antibody　and　glucose　oxidase　with　gold　nanoparticles　(Ab2/GOx-AuNPs)　was　introduced　by　an　antigen-antibody　immunoreaction.　AuNP　acted　as　a　powerful　scaffold　to　bind　with　bioactive　molecules,　while　GOx　catalyzed　glucose　to　in　situ　generate　hydrogen　peroxide　(H2O2).　The　generated　H2O2　as　a　sacrificial　electron　donor　could　be　oxidized　by　the　photogenerated　holes　to　assist　the　signal　amplification　at　a　low　potential　under　light　excitation,　thus　eliminating　interference　from　other　species　coexisting　in　the　samples.　Under　optimal　conditions,　the　PEC　immunosensor　showed　a　good　linear　relationship　ranging　from　0.02　to　40　ng　mL-1　with　a　low　detection　limit　of　6　pg　mL-1　CEA.　The　precision,　reproducibility,　and　specificity　were　acceptable.　In　addition,　the　method　accuracy　was　also　evaluated　for　quantitatively　monitoring　human　serum　samples,　giving　results　matching　with　the　referenced　CEA　ELISA　kit.　©　2015　American　Chemical　Society.