Miniaturized all-fiber photoacoustic spectrometer for intravascular gas detection rivals lab-based systems

Miniaturized spectroscopy systems that can detect trace concentrations at the parts-per-billion (ppb) level are of the utmost importance in applications ranging from environmental monitoring and industrial process control to biomedical diagnostics. However, the existing bench-top spectroscopy systems are too large, complex, and impractical for narrow-space use. Furthermore, the traditional laser spectroscopy techniques use bulky components like light sources, mirrors, detectors, and gas cells to detect light absorbed or scattered by a sample, which makes them unsuitable for scenarios calling for minimal invasiveness such as intravascular diagnosis.

In a study published in the journal Advanced Photonics, researchers from China have introduced an innovative miniaturized all-fiber photoacoustic spectrometer (FPAS), designed to detect trace gases at the ppb level and analyze nanoliter-sized samples with millisecond response times, making it ideal for continuous intravascular gas analysis.

“We attempted to address the significant challenge of shrinking the current photoacoustic spectrometer into a microscale size while preserving its high sensing performance, particularly for intravascular diagnosis and lithium battery health monitoring that require minimal invasiveness,” explains Professor Bai-Ou Guan from Jinan University, the corresponding author of the article.

While the current laser spectroscopy systems, mostly in open-path configurations, have an inherent scaling-down sensitivity with the device footprint, the proposed FPAS operates using photoacoustic spectroscopy (PAS), which detects sound waves generated by gas molecules when excited by modulated light.

Instead of using bulky resonant gas cells for acoustic amplification or large microphones for higher acoustic sensitivity like traditional PAS systems, the all-fiber photoacoustic spectrometer integrates a laser-patterned elastic membrane into a single optical fiber tip with a section of silica capillary to construct a microscale Fabry–Perot (F–P) cavity.

The silica cavity acts as a sound-hard boundary, effectively confining and accumulating the acoustic waves generated by the gas molecules toward the flexible membrane. This local acoustic amplification compensates for the sensitivity loss caused by the reduction in the membrane diameter and results in a size-independent photoacoustic response.

In addition, both the pump and probe light beams are directly delivered through the same fiber for the excitation and detection of the photoacoustic signal, avoiding bulky free-space optics for the light delivery.

With the F–P cavity measuring only 60 micrometers (1 µm = 10^-6 m) in length and 125 µm in diameter, the system is remarkably compact. Despite its small size, it achieves a detection limit for acetylene gas as low as 9 ppb, which is almost as sensitive as larger, traditional laboratory spectrometers. The short cavity length also enables ultrafast measurements, with response times as quick as 18 milliseconds, which is 2 to 3 orders of magnitude faster than conventional photoacoustic spectroscopy systems.

Researchers successfully monitored real-time carbon dioxide (CO₂) concentrations in flowing gas, detected fermentation in yeast solutions with sample volumes as small as 100 nanoliters, and tracked dissolved CO₂ levels in rat blood vessels in vivo by inserting the FPAS into the tail vein via a syringe.

“The spectrometer effectively measured CO₂ levels under hypoxic (low oxygen) and hypercapnic (high CO₂) conditions, highlighting its potential for real-time intravascular blood gas monitoring without the need for blood sample collection,” explains Associate Professor Jun Ma from Jinan University.

Moreover, the optical fiber can be easily connected to a low-cost distributed-feedback laser source and integrated with existing fiber-optic networks, making the system a cost-effective, compact, and flexible solution for spectroscopy.

With its small size, high sensitivity, and low sample volume requirement, the proposed miniaturized spectrometer delivers laboratory-level precision in a microscale probe format, with potential for such applications as continuous intravascular blood gas monitoring, minimally invasive health assessment of lithium-ion batteries, and remote detection of explosive gas leakage in extremely narrow spaces.