In a study published in the journal Advanced Photonics, Chinese researchers presented a new miniaturized all-fiber photoacoustic spectrometer (FPAS) that can analyze nanoliter-sized samples with millisecond response times and detect trace gases at the ppb level, making it ideal for continuous intravascular gas analysis.
Miniature spectroscopic instruments capable of detecting trace concentrations at the parts per billion (ppb) level are essential for applications such as industrial process control, environmental monitoring and biomedical diagnostics. However, traditional benchtop spectrometers are too large, complex and impractical for use in limited spaces.
Traditional laser spectroscopy also has limitations for less invasive applications such as intravascular diagnostics, as these systems rely on bulky components such as light sources, mirrors, detectors and gas cells to measure light absorption or scattering, making them unsuitable for these situations.
We have endeavored to address the key challenge of miniaturizing current photoacoustic spectrometers down to the microscale while maintaining high sensing performance, especially for intravascular diagnostics and minimally invasive lithium battery condition monitoring.
Bai Ou Guang, corresponding author of the study and professor at Jinan University
The proposed FPAS overcomes these challenges by using photoacoustic spectroscopy (PAS) to detect sound waves generated by gas molecules excited by modulated light. Unlike conventional PAS systems that rely on bulky microphones and resonant air cells for acoustic sensitivity, FPAS integrates key components into one compact design.
FPAS has a laser-patterned elastomeric membrane integrated with a silica capillary section at the tip of a single optical fiber to form a microscale Fabry-Perot (FP) cavity. The silica capillary acts as an acoustically stiff boundary, effectively focusing and amplifying the acoustic waves towards the flexible membrane. This design compensates for the loss of sensitivity due to miniaturization and produces a size-independent photoacoustic response.
Both excitation and detection of the photoacoustic signal are achieved through the same optical fiber, eliminating the need for bulky free-space optics. The FPAS system is extremely compact, with an FP cavity length of just 60 micrometers and a diameter of 125 micrometers. Despite its small size, the FPAS achieves a low detection limit of 9 ppb for acetylene gas, comparable to large laboratory spectrometers. The short cavity length enables an extremely fast response time of 18 ms, which is 2-3 orders of magnitude faster than conventional PAS systems.
FPAS has demonstrated its versatility in a variety of applications: Researchers have used it to monitor dissolved CO2 concentrations in vivo in the blood vessels of mice via tail vein injection , and to detect fermentation of yeast solutions in samples as small as 100 nanoliters, and to monitor CO2 . 2. Displays the flow of air in real time .
The spectrometer efficiently measures CO2 concentrations under hypoxic (low oxygen concentrations) and hypercapnic (high CO2 concentrations) conditions , demonstrating its potential for real-time monitoring of vascular blood gases without the need to take blood samples.
Ma Jun, Associate Professor of Jinan University
Optical fibers can be easily coupled to low-cost distributed feedback laser sources and integrated into existing fiber optic networks, providing a compact, flexible and cost-effective optical solution.
This miniature spectrometer combines the requirements of small size, high sensitivity and low sample volume to deliver laboratory-level accuracy in a microprobe format. Potential applications include continuous monitoring of blood gases in blood vessels, minimally invasive assessment of lithium-ion battery status and remote detection of explosive gas leaks in confined spaces.
Journal References:
Ma, J., et al. (2024) In situ and real-time trace gas sensing by microfiber photoacoustic spectroscopy. Advanced Photonics . Translation: doi.org/10.1117/1.AP.6.6.066008
