Reactive boronic acid-functionalized carbon dots (rBA-CDs) for dopamine (DA) detection were prepared by solvothermal synthesis of CDs using lactose and diacrylate polyethylene glycol (DAPEG) and subsequent 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide coupling with 3-aminophenylboronic acid. The solid-state thin film of the DAPEG/rBA-CD composite was prepared on an acrylate-functionalized glass substrate by ultraviolet curing. The rBA-CD in the solid-state PEG thin film on the glass substrate showed excellent DA detection performance in the presence of small amounts of DA by fluorescence color change from blue to green when excited at 360 nm. The fluorescence intensity increased as the DA concentration increased, with a linear detection range of 0–40 μM and a limit of detection of 0.2 μM. The developed solid-state fluorescent sensor offered high selectivity for DA over other components (e.g., glucose, lactose, ascorbic acid, urea, and uric acid) in blood. Furthermore, real samples of human serum spiked with dopamine showed excellent recovery. Thus, the designed reactive rBA-CDs can be utilized as a DA biosensor on a solid substrate after their immobilization. Owing to their good water dispersity, fluorescence color change, and reactive nature, rBA-CDs may provide a new strategy for the naked eye detection of dopamine.