A　heterojunction　with　excellent　photocatalytic　performance　based　on　graphene-like　carbon　nitride　(g-C3N4)　nanosheets　and　dendritic　zinc(ii)　phthalocyanine　was　proposed.　Herein,　the　g-C3N4　with　excellent　photo-activity　and　high　nitrogen　content　was　readily　available　as　a　functional　material.　The　g-C3N4　acted　as　an　electron　pair　donor　for　dendritic　zinc(ii)　phthalocyanine　through　axial　coordination,　forming　the　p-n　heterojunction.　Then　by　taking　advantage　of　the　distortion　of　dendritic　zinc(ii)　phthalocyanine,　the　spatial　charge　separation　of　photo-generated　charge　carriers　in　this　metal　macrocycle　achieved　high　efficiency,　resulting　in　the　enhanced　photo-to-electric　conversion　efficiency.　Therefore,　the　optoelectronic　sensing　device　based　on　the　heterojunction　led　to　an　enhanced　photocurrent,　and　made　it　a　promising　candidate　for　establishing　photoelectrochemical　biosensors.　Moreover,　the　p-n　heterojunction　was　successfully　applied　to　the　detection　of　choline　with　a　wide　linear　range　from　10　nM　to　5　μM,　which　could　be　oxidized　by　the　photo-generated　holes.　Along　with　these　attractive　features,　the　as-proposed　biosensor　also　displayed　a　remarkable　specificity　against　other　interferents　and　could　be　successfully　used　for　detecting　choline　in　real　samples.　The　heterojunction　with　enhanced　photoelectronic　properties　provides　a　promising　format　for　the　future　development　of　photoelectrochemical　biosensors.　©　The　Royal　Society　of　Chemistry　2014.