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A NIR-spectroscopy-based approach for detection of fluids in rectangular glass micro-capillaries
Valentina Bello * , Elisabetta Bodo
1  Department of Electrical, Computer and Biomedical Engineering, University of Pavia


Rectangular glass micro-capillaries are very interesting devices that can be inserted in a micro-fluidic path and exploited for label-free optical sensing of ultra-low volumes of fluids. In the past, such devices have been used for the detection of the refractive index of fluids. In this work, we developed a smart micro-opto-fluidic platform that can distinguish water and alcohol samples flowing in the micro-channel thanks to the profile of their absorption spectrum in the near infrared (NIR) region from 1.15 to 1.65 µm. The readout technique is non-contact, remote and non-invasive. The micro-capillary, with wall thickness of 280 µm and channel depth equal to 400 µm, is laid flat onto an Aluminum bulk mirror and the light from a Tungsten lamp is shone on its upper flat side with an angle of incidence of 14°. The readout beam crosses the glass walls and the channel depth twice, since it is reflected by the mirror, and it is then coupled to the monochromator input of an optical spectrum analyzer. The theoretical transmission spectra T(λ) of the capillary filled just with air as well as with distilled water, isopropanol, ethylene glycol, and 95% ethanol (with 5% water content) are obtained using analytical equations including the wavelength-dependent attenuation due to fluid absorption. Then experimental measurements are carried out and the experimental spectral response, defined as SR(λ) = Tsample(λ)/Tair(λ), is compared with the theoretical one, revealing a very good level of agreement.

Keywords: rectangular glass micro-capillaries; optical sensing; optical absorption; near infrared; spectroscopy; micro-opto-fluidic
Comments on this paper
Marco César Prado Soares
Small question
Carissima Valentina,

Come sa, eravamo nella stessa sessione, in chi Lei ha fatto una apresentazione veramente interessante.

Solo chi non ho capito bene è se Lei ha considerato l'effetti della velocità del fluido, o se i fluidi erano statitche. D'altra parte, volevo chiederLe se il routine computazionale era semplicemente una sequenza multipplicativa, o se doveva fare altri processamenti dei sinali, usare programme come Comsol, etc.

Nuovamente, le congratulo per il bravissimo lavoro. Non c'é nessun problema se preferisce rispondermi in italiano o in inglese, fa già due anni che ho iniziato a impare il Suo idioma.

Un caro saluto,
Valentina Bello
Dear Marco,
first of all, complimenti, il tuo italiano è molto buono.
I will reply in English just in case other people want to read our comments.

All the measurements that I have reported were carried out with the fluids in static conditions inside the channel, that is with a velocity of 0 mm/s.
As a matter of fact, we have also done some in-flow-measurements (not shown in my presentation) with water flowing in the channel at different velocities from 15 to 90 mm/s and we could conclude that the flow rate does not any significant effect on the measurements.

About the model routine, as you wrote, we just multiplied all the Fresnel contributions and the abosprtion contribution (given by the Beer-Lambert law) to obtain the "spectral response".

Thank you for your questions, I hope that everything is clearer now.

Warm wishes,

Marco César Prado Soares
Dear Valentina,

Thank you very much for your response and for the compliment!

It is very interesting to know that depending on the situation the velocity will not substantially affect the power transmitted (in fact, it is something that I was wondering regarding a project that we are starting). Last year, when we were working with particulate suspensions to analyze the scattering there was something a little similar: depending on the particulate, the thermal variation caused little variation of the signal, despite the increase that we expect on the diffusivity. Our hypothesis was that if the particular was bigger and heavier (e.g. yeast, with about 1 micrometer of diameter) the effect on the diffusivity was substantially lower than in nanoparticles until ~200 nm of diameter. So, the second case allowed to verify thermal differences that were not possible in the case of yeast. This was not a problem, though: since the fermentation is complex, not to have to account for thermal variations in a range of +- 5 ºC facilitated the correlation.

Again, congratulations on your work!
Best regards
Valentina Bello
Dear Marco,
it sounds like a very interesting project: nanoparticles are such an hot topics during these times!

Best of luck with it,