Two-dimensional transition metal carbides and nitrides known as MXenes, with a general formula of Mn+1Xn (n = 1-3), integrate the advantages of metallic conductive transition metals with large groups of carbides, nitrides, or carbonitrides. They have led to a burgeoning research interest in biomedical applications due to their ultrathin structure and fascinating physiochemical (electronic, optical, magnetic, etc.) properties. In this review, we summarize recent advances in biomedical applications for MXenes. We first introduce the preparation methods and surface modifications with respect to MXenes. Their unique properties are then elaborated. Thirdly, we highlight their various biomedical applications, such as with biosensors, antibacterial materials, bioimaging probes, therapeutics, and theranostics. In the end, the current challenges and new opportunities for MXenes in regard to their biomedical applications are also discussed.

Because of the limited penetration depth of visible light that generally excites most of the available photosensitizers (PSs), conventional photodynamic therapy (PDT) is limited to the treatment of superficial and flat lesions. Recently, the application of deep penetrating near-infrared (NIR) light excitable upconversion nanoparticles (UCNs) in conjunction with PDT has shown to have clear potential in the treatment of solid tumors due to its ability to penetrate thick tissue. However, various constructs developed so far have certain limitations such as poor or unstable PS loading, reducing their therapeutic efficacy and limiting their application to solution or cell-based studies. In this work, we present a method to fabricate uniform core-shell structured nanoconstruct with a thin layer of photocatalyst or PS-titanium dioxide (TiO2) stably coated on individual UCN core. Our design allows controllable and highly reproducible PS loading, preventing any leakage of PS compared to previously developed nanoconstructs, thus ensuring repeatable PDT results. Further surface modification of the developed nanoconstructs with polyethylene glycol (PEG) rendered them biocompatible, demonstrating good therapeutic efficacy both in vitro and in vivo.

Organic triplet–triplet annihilation upconversion (TTA-UC) materials have considerable promise in areas as broad as biology, solar energy harvesting, and photocatalysis. However, the development of highly efficient near-infrared (NIR) light activatable TTA-UC systems remains extremely challenging. In this work, we report on a method of systematically tailoring an annihilator to attain such outstanding systems. By chemical modifications of a commonly used perylene annihilator, we constructed a family of perylene derivatives that have simultaneously tailored triplet excited state energy (T1) and singlet excited state energy (S1), two key annihilator factors to determine TTA-UC performance. Via this method, we were able to tune the TTA-UC system from an endothermic type to an exothermic one, thus significantly elevating the upconversion performance of NIR light activatable TTA upconversion systems. In conjunction with the photosensitizer PdTNP (10 μM), the upconversion efficiency using the optimal annihilator (100 μM) identified in this study was measured to be 14.1% under the low-power density of NIR light (100 mW/cm2, 720 nm). Furthermore, using such a low concentration of perylene derivative, we demonstrated that the optimal TTA-UC pair developed in our study can act as a highly effective light wavelength up-shifter to enable NIR light to drive a photoredox catalysis that otherwise requires visible light. We found that such an NIR driven method is highly effective and can even surpass directly visible light driven photoredox catalysis. This method is important for photoredox catalysis as NIR light can penetrate much deeper in colored photoredox catalysis reaction solutions, especially when done in a large-scale manner. Furthermore, this TTA-UC mediated photoredox catalysis reaction is found to be outdoor sunlight operable. Thus, our study provides a solution to enhance NIR activatable organic upconversion and set the stage for a wide array of applications that have previously been limited by the suboptimal efficiency of the existing TTA upconversion materials.

A strategy to expand anti-Stokes shifting from the far-red to deep-blue region in metal-free triplet–triplet annihilation upconversion (TTA-UC) is presented and its utility in the in vivo titration of the photorelease of an anticancer prodrug is demonstrated. This new TTA system has robust brightness and the longest anti-Stokes shift of any reported TTA system. TTA core-shell-structured prodrug delivery capsules that benefit from these properties are developed; they can operate with low-power-density far-red light-emitting diode (LED) light. These capsules contain mesoporous silica nanoparticles preloaded with TTA molecules as the core and amphiphilic polymers encapsulating anticancer prodrug molecules as the shell. When stimulated by far-red light, the intense TTA upconversion blue emssison in the system activates the anticancer prodrug molecules and shows effective tumor growth inhibition in vivo. This work paves the way for the design of new organic TTA upconversion with regard to in vivo photocontrollable drug release and other biophotonic applications.