All 3D-printed parts should be printed using black filament (polylactic acid [PLA] is suitable), using a layer thickness of 0.2 mm or less, and an infill of at least 10 %. Assembly of the instrument is straightforward, and is illustrated in Fig. for the flow cell. The LED and OPT101 are glued into pre-formed recesses within their respective mounting plates using a small amount of hot glue. Next, the OPT101 is plugged into its socket on the PCB. The instrument is assembled by stacking the PCB and individual layers of the optical stack as shown in Fig. , and slotting them -as a single unit -into the base; the optical stack is held together and retained in the base-plate by a friction fit. The flow-cell is formed using a ~10-cm length of 2-mm-OD, 1-mm-ID semi-transparent polytetrafluoroethylene (PTFE) tubing (228-4118, VWR) secured within the two 3D-printed halves using either hot glue or epoxy resin (see Fig. ). If required, the PTFE tubing could be replaced with more transparent fluorinated ethylene-propylene (FEP) tubing of the same dimensions, but this comes at a small cost premium. Fluidic connections to the internal tubing are made using nut-and-ferrule style Luer adaptors. The construction steps are almost identical for the cuvette holder, except the retaining clip is omitted to compensate for the greater depth of the cuvette holder. Side-by-side images of the two configurations are shown in Fig. . ), mounting plate for LED and short-pass filter (3), short-pass filter (4), flow-cell assembly (5), long-pass filter (6), mounting plate for long-pass filter , mounting plate for amplified photodiode , amplified photodiode (9) and PCB . (The PCB is shown unpopulated for clarity). The unit is assembled by stacking together parts 1 to 10 and slotting into the base-plate . The ring-mounted short-pass filter (4) presses into its mounting plate (3) with a friction fit, whereas the long-pass filter is held in place via two protrusions that extend from the flow-cell assembly. The LED and photodiode package are glued into their respective mounting plates. (b) Exploded view of flow cell. The flow-cell is constructed from a short (~10 cm), doubled-over length of 2-mm-OD, 1-mm-ID PTFE tubing glued between two 3D-printed retaining halves.