## 3 Flow-based optical detection
### Fluorescence  
Fluorescence is a luminescence that is more sensitive and selective than UV-vis spectrophotometry. Fluorescence detectors can be used as sensors for flow systems. For example, fluorescence determination of the pesticide asulam was carried out by flow injection analysis [109]. This study focused on the influence of surfactants on the native fluorescence of asulam, as an effect of ultraviolet irradiation on the fluorescence of asulam by forming photo-fragments with high intensities. Moreover, [PERSON] et. al. [110] has determined tannin in green tea infusions using the quenching effect of 3-aminophtalate without interference from the ascorbate.  
For immunoassay applications, [PERSON] et. al. [111] developed time-resolved fluoroimmunoassays coupled with flow injection analysis for the determination of carcinoembryonic antigens. In addition, [PERSON] and [PERSON] [112] presented a method using co-immobilized gluconate kinase (GK) and 6-phosphogluconate dehydrogenase (PGDH) reactors for the determination of total D-gluconate.  
Figure 4: Manifold of the SIA system in the proposed method. HC\({}_{1}\), holding coil\({}_{1}\) (0.5 mm i.d., 60 cm); HC\({}_{2}\), holding coil2 (0.5 mm i.d., 55 cm); HC\({}_{3}\), holding coil\({}_{3}\) (1 mm i.d., 50 cm); TC, temperature controller (45.C); CS, 5\(\times\)10\({}^{\text{-}5}\) mol l\({}^{\text{-}1}\) 5-Br-PSAA in 0.01 mol l\({}^{\text{-}1}\) HCl; R\({}_{\text{Cu}}\), 4\(\times\)10-4 mol l\({}^{\text{-}1}\) 5- Br-PSAA; R\({}_{\text{Fe}}\), 7\(\times\)10-4 mol l\({}^{\text{-}1}\) 5-Br-PSAA; S, sample containing copper and/or iron; AS, 5\(\times\)10-4 mol l\({}^{\text{-}1}\) ascorbic acid; BS, 0.2 mol l\({}^{\text{-}1}\) acetate buffer at pH 4.6.  
Bead injection can provide the advantage of a renewable surface. This system coupled with fluorescence detection was applied to determine vanadium(V) [113]. V(V) was reacted with a fluorogenic agent 1,2-dihydroxyanthraquinone-3-sulfuric acid (Alizarin Red S) immobilized on the bead to form the fluorescent chelate. Then, the bead was subsequently discarded from the flow cell by reversing the flow. To increase the selective adsorption, flow injection renewable drop spectrofluorimetry [114] with Sephadex CMC-25 was used in the sequential determination of vitamins B\({}_{1}\), B\({}_{2}\), and B\({}_{6}\). Also simple fluorescence coupled to sequential injection analysis can be used for determination of drugs in pharmaceutical preparations [115],[116],[117]. To obtain rapid analysis and reduce the reagent, innovative stirring actions of magnetic beads in the sequential injection system was proposed [118]. The magnetic beads were immobilized with antibiotics and injected in the microreactor where they were vigorously agitated by a magnetic field of several hundred gauss. Furthermore to enhance of sample/reagent mixing, a controlled pulsed flow was applied to implement SIA methodology to determine indomethacin [119]. In order to accomplish the separation and preconcentration/detection, a multicommented fluorescence-flow based sensing system was employed for the determination of vitamins B\({}_{2}\) and B\({}_{6}\)[120]. This method achieved a linear range of 0.01-0.4 ug mL-1 for B\({}_{2}\) and 0.15-3 ug mL-1 for B\({}_{6}\) with detection limit of 0.003 for B\({}_{2}\) and 0.045 ug mL-1 for B\({}_{6}\).  
To reach the lower detection limit, micro-sequential injection or lab-on-value was developed [121]. The low sample consumption and miniaturized system were demonstrated. The system was employed for the quantification of small amounts of nucleic acids, particularly dsDNA (double stranded DNA) with laser-induced fluorescence detection. A detection limit of 0.009 ug mL-1 was achieved.