One step multi-material 3D printing for the fabrication of a photometric detector flow cell.

Optical detection is the most common detection mode for many analytical assays. Photometric detection systems and their integration with analytical systems usually require several assembly parts and manual alignment of the capillary/tubing which affects sensitivity and repeatability. 3D printing is an innovative technology for the fabrication of integrated complex detection systems. One step multi-material 3D printing has been explored to fabricate a photometric detector flow cell from optically transparent and opaque materials using a dual-head FDM 3D printer. Integration of the microchannel, the detection window and the slit in a single device eliminates the need for manual alignment of fluidic and optical components, and hence improves sensitivity and repeatability. 3D printing allowed for rapid design optimisation by varying the slit dimension and optical pathlength. The optimised design was evaluated by determining stray light, effective path length and the signal to noise ratio using orange G. The optimised flow cell with extended path length of 10 mm and 500 µm slit yielded 0.02% stray light, 89% effective path length and detection limit of 2 nM. The sensitivity was also improved by 80% in the process of optimisation, using a blue 470 nm LED as a light source.

3D printed LED based on-capillary detector housing with integrated slit.

A 3D printed photometric detector body with integrated slit was fabricated to position a LED and photodiode either side of capillary tubing using a fused deposition modelling (FDM) printer. To make this approach suitable for capillaries down to 50 µm i.d. the dimension of the in-built slit is the critical element of the printed housing. The spatial orientation of the model for printing was found to significantly impact on the resolution of the structures and voids that can be printed. By designing a housing with a slit positioned in the XY plane in parallel with the print direction, the narrowest void (slit) that could be printed was 70 µm. The potential use of the 3D printed slit for photometric detection was characterised using tubing and capillary from 500 down to 50 µm i.d, demonstrating a linear response from 632 to 40 mAU. The effective pathlength and stray light varied from 383 to 22 µm and 3.8% - 50% for 500- 50 µm i.d tubing and capillary. The use of a V-shaped alignment feature allowed for easy and reliable positioning of the tubing inside the detector, as demonstrated by a RSD of 1.9% (n = 10) in peak height when repositioning the tubing between measurements using flow injection analysis (FIA). The performance of the 3D printed housing and 70 µm slit was benchmarked against a commercially available interface using the CE separation of Zn2+ and Cu2+ complexes with PAR. The limit of detection with the 3D printed slit was 6.8 and 4.5 µM and is 2.8 and 1.6 µM with the commercial interface.