Design and Synthesis of a Ratiometric Photoacoustic Probe for In Situ Imaging of Zinc Ions in Deep Tissue In Vivo

Abstract

As a noninvasive deep-tissue imaging technique, photoacoustic (PA) imaging has great application potential in biomedicine and molecular diagnosis. The zinc ion (Zn2+), which is a necessary metal ion in the human body, plays a very important role in the regulation of gene transcription and metalloenzyme function. The imbalance of Zn2+ homeostasis is also associated with a variety of neurological diseases. Therefore, it is critically important to accurately image the steady-state changes of Zn2+ in vivo. However, no PA imaging method is currently available for Zn2+. To this end, we designed and synthesized the first PA probe of Zn2+, namely, CR-1 for in situ ratiometric imaging of Zn2+ in deep tissue in vivo. The CR-1molecule, combined with Zn2+, weakened the conjugation system of the π-electron in the CR-1 molecule, which resulted in the blue shift of its absorption peak from 710 nm to 532 nm. The PA signal intensity decreased at 710 nm and increased at 532 nm, and the ratiometric PA signal at these two wavelengths (PA532/PA710) showed a good linear relationship with the concentration of Zn2+ in the range of 0–50 μM, with a detection limit as low as 170 nM. Furthermore, this probe exhibits extremely fast responsiveness, is highly selective, and has excellent biocompatibility. We have used the developed PA probe for the ratiometric PA imaging of Zn2+ in the thigh tissue of mice, and we still can accurately image Zn2+ after covering chicken breast tissue on the surface of mice thigh. In light of these outstanding features, the developed PA probe has high potential for imaging Zn2+ in deep tissues; thus, it will open up new avenues for the study of the complex biochemical processes involving Zn2+ in vivo.

Design Strategies of Photoacoustic Molecular Probes

“…Due to the variation of molecular ICT after interaction with the chelation of Pd 2+ , the probe displayed an absorption shift to imaging Pd 2+ with a ratiometric PA response. Zhao's group utilized a similar strategy to achieve a Zn 2+ PA probe by linking tris(2‐pyridylmethyl) amine to ICG framework (CR‐1, Figure 4i) [50] . Electrophilic Zn 2+ will weaken the molecular conjugation system of the π‐electron that led a remarkable absorption blue shift from 710 to 532 nm after chelation with CR‐1, which resulted ratiometric PA signals at these two wavelengths (PA 532 /PA 710 ).…”

“…Zhao's group utilized a similar strategy to achieve a Zn 2 + PA probe by linking tris(2-pyridylmethyl) amine to ICG framework (CR-1, Figure 4i). [50] Electrophilic Zn 2 + will weaken the molecular conjugation system of the π-electron that led a remarkable absorption blue shift from 710 to 532 nm after chelation with CR-1, which resulted ratiometric PA signals at these two wavelengths (PA 532 / PA 710 ). Likewise, Lin's group developed a turn-on PA probe (LET-2) base on heptamethine cyanine analogue and a dipicolylethylenediamine moiety for detecting Cu 2 + in living plant and mouse (Figure 4k).…”

“…j) The application of ratiometric PA imaging of Zn 2 + in vivo. Reproduced with permission from ref [50]…”

A General Approach to Convert Hemicyanine Dyes into Highly Optimized Photoacoustic Scaffolds for Analyte Sensing

“…21 Due to these limitations, our design has not yet been widely adopted by other probe developers. Beyond aza-BODIPYs, other NIR fluorescent platforms such as cyanines [22][23][24][25][26][27][28][29][30] and hemicyanine dyes (HD) [31][32][33][34][35][36][37][38][39] have also been repurposed extensively (Scheme 1a).…”

High-performance Photoacoustic Probe for Biopsy-free Assessment of Copper Status in Murine Models of Wilson’s Disease and Liver Metastasis

“…4,5 Notable examples include those that can visualize dysregulated enzymatic activities, [6][7][8][9] properties of the disease tissue microenvironment, [10][11][12] as well as small molecule-and metal ion-based disease biomarkers. [13][14][15][16][17][18] However, replacing an invasive medical procedure such as a liver biopsy with an acoustogenic probe is an immense challenge since the in vivo performance and safety profile of such a chemical tool must be exceptional. Thus, in spite of undesirable shortcomings such as the potential to develop severe infections, false negatives due to collection of nondiseased tissue, and the inability to directly monitor disease progression in real-time, 19 liver biopsies are still commonly employed to assess biomarkers in conditions such as Wilson's disease (WD) 20 and cancer.…”