A novel 3D printed negative pressure small sampling system for bubble-free liquid core waveguide enhanced Raman spectroscopy

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Liquid core waveguide (LCW) is well-known as an effective fiber enhanced approach for Raman spectroscopy with features of long optical path and small sampling size. However, inevitable air bubbles introduced in the LCW tube possibly caused light scattering, refraction and reflection so as to further hamper the quantitative analysis. In this work, to eliminate air bubbles, a novel negative pressure system combined with 3D printing was first utilized for the enhanced Raman spectroscopy on the principle of the gas permeability of LCW tube. After optimization, the LCW tube made of Teflon-AF was inserted into a D-shaped support with an internal channel manufactured by 3D printing to create a sealed space; then, air pressure outside the LCW tube was reduced to create a negative pressure via diaphragm pump and magnetic valve controlled by computer. Under adjustable negative pressure, not only can liquid sample be introduced into the LCW tube automatically, but air bubbles can also be removed through the tube wall simply and completely. For real samples, the assembled apparatus was employed as the small sampling system of Raman spectrometer to measure rhodamine B and ethanol in solutions, with the highest 82-fold (ethanol) enhancement of analytical sensitivity vs. the traditional colorimetric ware. The limit of detections (LODs) were 0.7 μg/mL rhodamine B and 0.03% (v:v) ethanol with only 250 μL sample consumption; their linear correlation coefficients (r) were 0.998 and 0.999 in the range from 2 μg/mL to 25 μg/mL (rhodamine B) and 0.1%–5% (ethanol), respectively. It is worth mentioning that the intraday stability and 7-days reproducibility can be both controlled within 7%, which is extremely superior to the previous enhanced Raman spectroscopy. For another, 3D printing enables the LCW detection system more integrated and easier to assemble. So, the proposed method proves many advantages, such as stability, sensitivity, and quickness, in addition of effective physical enhancement, low sample consumption, and long light path. Considering the flexibility of LCW tube, as a versatile module, the negative pressure LCW system should be further suitable to ultraviolet, fluorescence and other detectors, which reveals a favorable application prospect for the fast testing instruments.