Endoscopic Fourier-transform infrared spectroscopy through a fiber microprobe

TL;DR

This paper introduces an endoscopic fiber microprobe for FTIR spectroscopy that enables chemical analysis inside liquids, overcoming traditional limitations caused by IR-absorbing media, with promising applications in biological and chemical systems.

Contribution

Development of a novel IR fiber microprobe for FTIR that can be inserted into liquids for in situ chemical analysis, demonstrating its effectiveness in water.

Findings

Microprobe accurately measures IR transmittance in water

Spectra are consistent with standard IR properties of water

Enables endoscopic chemical profiling in liquids

Abstract

Fourier-transform infrared spectroscopy (FTIR) is a powerful analytical method for not only the chemical identification of solid, liquid, and gas species, but also the quantification of their concentration. However, the chemical quantification capability of FTIR is significantly hindered when the analyte is surrounded by a strong IR absorbing medium, such as liquid solutions. To overcome this limit, here we develop an IR fiber microprobe that can be inserted into liquid medium, and obtain full FTIR spectra at points of interest. To benchmark this endoscopic FTIR method, we insert the microprobe into bulk water covering a ZnSe substrate and measure the IR transmittance of water as a function of the probe-substrate distance. The obtained vibrational modes, overall transmittance vs z profiles, quantitative absorption coefficients, and micro z-section IR transmittance spectra are all consistent with the standard IR absorption properties of water. The results pave the way for endoscopic chemical profiling inside bulk liquid solutions, promising for applications in many biological, chemical, and electrochemical systems.