NOVELTY - The continuous-flow injection reactor comprises a mixing zone unit (110), a first fluid source, a second fluid source, a residence time unit (120), a first heater, a second heater, a removable fitting (118), and a heater coupled to the mixing zone unit. The mixing zone unit comprises an outer housing comprising top inlets, a bottom outlet and a side inlet (113) positioned between the top inlets and the bottom inlet, injection tubes inserted into the top inlets and positioned within the outer housing, and a mixing zone present between lower ends of the injection tubes.    USE - The continuous-flow injection reactor is useful for synthesizing nanoparticles (all claimed), which are useful for producing solar cells.    ADVANTAGE - The continuous-flow injection reactor is capable of rapidly, efficiently, simply and economically synthesizing nanoparticles with high purity and accuracy and controlled shape and size.    DETAILED DESCRIPTION - The continuous-flow injection reactor comprises a mixing zone unit (110), a first fluid source, a second fluid source, a residence time unit (120), a first heater, a second heater, a removable fitting (118), and a heater coupled to the mixing zone unit. The mixing zone unit comprises an outer housing comprising top inlets, a bottom outlet and a side inlet (113) positioned between the top inlets and the bottom inlet, injection tubes inserted into the top inlets and positioned within the outer housing, and a mixing zone present between lower ends of the injection tubes and the bottom outlet of the mixing zone unit. Each of the injection tubes is of sufficient length to extend past the side inlet while terminating above the bottom outlet. The first fluid source is operably coupled to the side inlet. The second fluid source is operably coupled to each of the injection tubes inserted into the top inlets. The residence time unit: has a diameter cooperatively dimensioned such that it is removably coupled to the bottom outlet of the mixing zone unit; and comprises a conduit of defined length, an outer housing having a length (l), where the outer housing has a shape and diameter cooperatively dimensioned such that it is removably coupled to the mixing zone unit, and inner tubes positioned within the outer housing. The residence time unit outer housing comprises an inlet and an outlet. The side inlet and the injection tube are positioned perpendicular to one another. The first heater is disposed between the first fluid source and the side inlet such that a fluid flows from the first fluid source through the first heater before flowing into the side inlet. The second heater is disposed between the second fluid source and the injection tube inserted into the top inlets such that the fluid flows from the second fluid source through the second heater before flowing into the injection tube. The removable fitting is configured: to removably couple the residence time unit to the mixing zone unit; and such that a heat transfer fluid flows into the removable fitting through the inlet of the outer housing and flows out of the removable fitting through the outlet of the outer housing without directly contacting the mixing zone unit or the residence time unit. A portion of the conduit is in a temperature-controlled environment. The heater is positioned to provide heat to the mixing zone. The injection tubes are positioned parallel to one another and perpendicular to the side inlet. The inner tubes are positioned parallel to one another and extending to the length (l). A heat-transfer fluid source is operably coupled to the residence time unit outer housing inlet such that a heat transfer fluid (119) flows into the residence time unit through the outer housing inlet and flows out of the residence time unit through the outer housing outlet, where the heat-transfer fluid partially fills the outer housing and surrounds the inner tubes as it flows through the residence time unit. The heat-transfer fluid has a temperature T1 when it flows into the residence time unit and a temperature T2 when it flows out of the residence time unit, and the heat-transfer fluid that flows out through the residence time unit outer housing outlet is adjusted to temperature T1 and recycled to the outer housing inlet. An INDEPENDENT CLAIM is included for a method of synthesizing nanoparticles.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of an assembled continuous-flow injection reactor.    Mixing zone unit (110)    Side inlet (113)    Removable fitting (118)    Heat transfer fluid (119)    Residence time unit. (120)