Real-time in Vivo monitoring of photosynthesis in individual leaves by frequency-locked quartz-enhanced photoacoustic spectroscopy

This paper presents a method for real-time measurement of in vivo leaves photosynthetic rates using quartz-enhanced photoacoustic spectroscopy (QEPAS) with first-harmonic frequency- locking (1f-locking). A compact gas sensor structure was constructed by integrating an in-plane acoustic micro-resonator (AmR) with a commercial 30.720 kHz quartz tuning fork (QTF). The DFB laser’s output wavelength was locked to the CO 2 absorption line at 4991.26 cm −1 via 1f-locking, enabling real-time monitoring of CO 2 uptake during leaf photosynthesis. Compared to the scanning mode, the standard deviation (STD) in 1f-locking mode is significantly reduced, with detection sensitivity increased by nearly threefold. The system achieved a 1 s measurement cycle, with detection linearity R 2 = 0.999. When the integration time is 127 s, the minimum detection limit (MDL) is 2.44 ppmv. The normalized noise equivalent absorption coefficient (NNEA) is 4.78 × 10 −9 cm −1 ·W·Hz −1/2 . Results obtained align with reported photosynthetic rate ranges, validating the system’s feasibility. This system provides a portable, highly sensitive, rapid, and reliable method for leaves photosynthetic rate determination.