Carbon　based　dots　(CDs)　have　attracted　broad　attention　exhibit　due　to　the　unique　optical　properties.　However,　the　exact　origins　of　their　optical　properties　are　still　controversial.　Citric　acid　(CA)　coupled　with　some　amino　group-containing　small　molecules　are　believed　to　be　ideal　precursors　for　the　synthesis　of　high　luminescent　CDs　through　various　thermal　treatment　processes.　Herein,　CA　coupled　with　four　amino　group-containing　small　molecules　are　chosen　as　models　to　synthesize　CDs　for　a　systematical　study　on　the　photoluminesce　(PL)　properties.　It　is　found　that　the　PL　properties　of　CDs　are　resulted　from　the　synergistic　effect　of　the　contained　luminescent　pyridine-derivatives　and　the　defect　states.　A　reasonable　mechanism　of　PL　emission　from　the　CDs　has　been　proposed.　The　results　presented　here　must　be　critical　for　understanding　the　origins　of　PL,　and　also　for　preparing　CDs　with　strong　and　wavelength　tunable　PL　emission.　(C)　2017　Elsevier　Ltd.　All　rights　reserved.