Broadband surface plasmon resonance sensor for fast spectroscopic photoacoustic microscopy.

High-speed optical-resolution photoacoustic microscopy (OR-PAM), integrating the merits of high spatial resolution and fast imaging acquisition, can observe dynamic processes of the optical absorption-based molecular specificities. However, it remains challenging for the evaluation to morphological and physiological parameters that are closely associated with photoacoustic spectrum due to the inadequate ultrasonic frequency response of the routinely-employed piezoelectric transducer. By utilizing the galvanometer for fast optical scanning and our previously-developed surface plasmon resonance sensor as an unfocused broadband ultrasonic detector, high-speed spectroscopic photoacoustic imaging was accessed in the OR-PAM system, achieving an acoustic bandwidth of ∼125 MHz and B-scan rate at ∼200 Hz over a scanning range of ∼0.5 mm. Our system demonstrated the dynamic imaging of the moving phantoms' structures and the simultaneous characterization of their photoacoustic spectra over time. Further, fast volumetric imaging and spectroscopic analysis of microanatomic features of a zebrafish eye ex vivo was obtained label-freely.

In Vivo Reflection-Mode Photoacoustic Microscopy Enhanced by Plasmonic Sensing with an Acoustic Cavity.

Relying on high-sensitivity refractive index sensing and a highly constrained evanescent field of surface plasmon resonance (SPR), broadband photoacoustic (PA) pressure transients were measured using an SPR sensor instead of routinely used piezoelectric ultrasonic transducers. An acoustic cavity made from stainless steel and having a designed ellipsoidal inner surface redirected laser-induced PA waves from the PA excitation spot to the SPR sensor. By incorporating the SPR sensor with the acoustic cavity, we developed optical-resolution photoacoustic microscopy (OR-PAM) with multiple advantages, including reflection-mode signal capture, improved PA detection sensitivity, increased PA spectral bandwidth as broad as ∼98 MHz, and micrometer-scale lateral resolution. This allowed label-free volumetric PA imaging of vasculature in not only the thin ear but also the thick forelimb of living mice. With these combined advantages, our OR-PAM system potentially offers more opportunities for biomedical investigation, for example, when studying microcirculations in the eye and cortex.

Isometrically Resolved Photoacoustic Microscopy Based on Broadband Surface Plasmon Resonance Ultrasound Sensing.

Photoacoustic microscopy (PAM) can measure optical absorption-based molecular specificities within tissues. Despite the diffraction-limited lateral resolution in optical-resolution photoacoustic microscopy (OR-PAM), the ongoing challenge is poor axial resolution because of an insufficient ultrasound detection bandwidth, which hampers PAM volumetric imaging. We propose polarization-differential surface plasmon resonance (SPR) sensing for broadband and high-sensitivity photoacoustic (PA) detection, allowing OR-PAM with comparable resolution along lateral and axial directions. This sensor possesses an estimated noise-equivalent-pressure sensitivity of ∼477 Pa over an approximately linear pressure response up to 107 kPa. Moreover, an improved PA detection bandwidth of ∼173 MHz permits an axial resolution (∼7.6 µm) that approaches the lateral resolution (∼4.5 µm) of our OR-PAM system. The capability in spatially isometric micrometer-scale resolution enables in vivo volumetric label-free imaging of the microvasculature of a mouse ear. The SPR sensing technology promises broader applications of PAM in biomedical studies such as microcirculation.