The presence of room temperature phosphorescence emission in metalloporphyrin, via the transition from the excited triplet state (T1) to the ground state (S0), relies on the chelated heavy metal ions, which is known as the heavy atom effect (HAE). Despite the HAE being a reliable method to tune the phosphorescence process widely, the realization of the HAE nature is a tough task as the induced phosphorescence process is sensitive to not only the specie of bonded heavy atoms but also chemical environments such as the oxygen quenching and solvent effect. In this study, we have aimed at a quantitative determination of the intrinsic phosphorescent transition rate (kP) in metalloporphyrin gadolinium-labeled hematoporphyrin monomethyl ether (Gd-HMME). After the theoretical analysis based on the rate equation model to remove the nonintrinsic contribution and the experimental results of phosphorescence, the kP is calculated to be ∼2.4 × 10–4 μs–1. This study enables us to approach the intrinsic energy characteristic of metalloporphyrins; moreover, our work provides an effective pathway for the further optimization of the varied functional metalloporphyrin.
Environmental heavy atoms can further enhance the room temperature phosphorescence (RTP) emissions of gadolinium-labeled hematoporphyrin monomethyl ether (Gd-HMME) by way of the external heavy atom effect (HAE). However, the macroscopic phosphorescence intensity covered the intrinsic effect of the environmental heavy atoms. In this study, a method of separating the external HAE from the total is performed, and a quantity to describe the intrinsic nature of external HAE is defined. The environmental Gd3+ concentration evolution of the phosphorescent transition rate (kP) is obtained by correlated absorption, emission, and time-resolved spectroscopy. The kP increases linearly with environmental Gd3+ concentration, while the intercept kP0 coincides with that of the internal HAE. The slope κ could be calculated, and it is a quantity free of the Gd3+ concentration and only relies on the type of environmental heavy atoms. In addition, the environmental Lu3+ exhibits similar functionality to Gd3+ in external HAE. Environmental Pd2+ quenches the phosphorescence intensity macroscopically, while it enhances the HAE intrinsically. Our method provides an alternative insight into the intrinsic nature of environmental heavy atoms.
Magnetic resonance contrast agent employs the use of gadolinium chelates, which has many limitations for clinical use including a low contrast effect, a short diagnostic window, and a brief blood circulation time. On this basis, we designed a gadolinium-labeled dendrimer nanocluster (GdDN) loaded graphene oxide nanosheet (GO-GdDN) to boost T1 contrast ability for imaging in vivo and improve blood circulation time. GO-GdDN presented an ultrahigh r1 relaxivity up to 19.07 mM–1 s–1 in a 9.4 T MR scanner, and a bright contrast image in vitro experiment. In addition, GO-GdDN could be internalized by HepG2 cells and presented strong cell contrast enhancement and reduction of the T1 value in HepG2 cells. In vivo, the retention time of GO-GdDN was significantly improved, so that the accumulation of GO-GdDN in liver was enhanced. Moreover, compared to Gd-DTPA, systemic delivery of GO-GdDN dramatically enhanced the signal-to-noise ratio of liver images, which was helpful for accurate imaging of liver and detection of liver lesions in vivo. Thus, this study demonstrates the utility of a powerful diagnosis tool for liver tumor or lesions.
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