image: Figure 1 | Schematic of micro-scale rigid frame engineering. Due to the weak spin-orbit coupling (SOC) and the molecular motion in the surrounding environment, as well as inhibitors like oxygen and water, triplet excitons are easily dissipated (Figure 1 left). Previously, researchers have employed various methods to protect triplet excitons, including multiple interactions such as hydrogen bonds and covalent bonds to limit molecular motion and reduce non-radiative transitions. Based on this, the authors' team proposed a rigid framework strategy to suppress the non-radiative transition from the excited triplet state to the ground state of CNDs through restricted interactions, thereby achieving efficient phosphorescent emission (Figure 1 right).
Credit: Yachuan Liang et al.
Triplet excitons with long-lived excited states can emit photons for an extended period, thereby enhancing the signal-to-noise ratio and tissue penetration, which is crucial for non-invasive and deep tissue imaging. Traditional stimuli-responsive fluorescence materials play an important role in optical sensing, used for detecting chemical and environmental changes, non-invasive diagnostics, real-time tracking, and data encryption through dynamic emission characteristics. In recent years, stimuli-responsive room temperature phosphorescent (RTP) materials have garnered significant attention in these fields due to their unique excited state properties. Their long-lived luminescent characteristics can effectively eliminate background fluorescence interference, providing a new solution for practical applications.
In a new paper published in Light: Science & Applications, a team led by Liang Yachuan from Zhengzhou University of Light Industry, in collaboration with Liu Kaikai's team from Zhengzhou University, proposed a micro-scale rigid framework engineering strategy. This strategy utilizes the self-assembly characteristics of cyclodextrin in aqueous solution, promoting the self-assembly of cyclodextrin through ultrasound to enhance the rigidity around carbon dots, thereby activating triplet excitons. They constructed ultrasound-responsive phosphorescent carbon dots with a lifespan of 1.25 seconds in aqueous solution. These carbon dots exhibit high sensitivity to the surrounding ultrasound environment and show a linear response to ultrasound stimulation under specific conditions. Such ultrasound-responsive phosphorescent carbon dots demonstrate great potential applications in ultrasound radar detection and in vivo afterglow imaging.
Background fluorescence typically exhibits nanosecond-level emission lifetimes, while the long-lived excited state characteristics of RTP materials allow for time-gated recording, thereby avoiding interference from background fluorescence. This feature is particularly important in optical detection and bioimaging technologies, as it enables high-contrast signal detection in complex environments. However, the development of stimuli-responsive RTP materials is still in its infancy, facing issues that need to be addressed: first, the long-lived triplet excitons dissipate rapidly through non-radiative relaxation pathways and are quenched by oxygen. Secondly, it is extremely challenging to simultaneously regulate both the triplet excitons and the stimuli-responsive sites. Therefore, achieving tunable stimuli-responsive RTP materials remains a daunting challenge.
Here, the author team reports a novel method for regulating phosphorescent carbon dots (CNDs): constructing micro-scale rigid frameworks by modulating the self-assembly of cyclodextrin in aqueous solution through ultrasonic means, which enables RTP CNDs with ultrasonic responsive characteristics. Initially, the phosphorescence of CNDs could not be detected in the absence of an ultrasonic environment due to non-radiative transitions of triplet excitons in the aqueous solution. However, after a period of ultrasonic treatment, the assembly of cyclodextrin gradually completed, and CNDs were confined within the cyclodextrin framework through hydrogen bonding interactions, thus enhancing the RTP performance in the aqueous solution, while the RTP lifetime increased to 1.25 seconds.
The study found that the ultrasonic responsiveness of CNDs is positively correlated with the crystallinity of the cyclodextrin framework. Additionally, by utilizing Förster resonance energy transfer theory, multi-color afterglow responsive to ultrasound can also be achieved in aqueous solution. Based on this characteristic, the author team demonstrated some preliminary applications of ultrasound-responsive RTP CNDs in ultrasonic radar detection and in vivo afterglow imaging.
Journal
Light Science & Applications
Article Title
Ultrasound-responsive phosphorescence in aqueous solution enabled by microscale rigid framework engineering of carbon nanodots