Real-Time Monitoring of ATP-Responsive Drug Release Using Mesoporous-Silica-Coated Multicolor Upconversion Nanoparticles

Stimuli-responsive drug delivery vehicles have garnered immense interest in recent years due to unparalleled progress made in material science and nanomedicine. However, the development of stimuli-responsive devices with integrated real-time monitoring capabilities is still in its nascent stage because of the limitations of imaging modalities. In this paper, we describe the development of a polypeptide-wrapped mesoporous-silica-coated multicolor upconversion nanoparticle (UCNP@MSN) as an adenosine triphosphate (ATP)-responsive drug delivery system (DDS) for long-term tracking and real-time monitoring of drug release. Our UCNP@MSN with multiple emission peaks in UV-NIR wavelength range was functionalized with zinc-dipicolylamine analogue (TDPA-Zn2+) on its exterior surface and loaded with small-molecule drugs like chemotherapeutics in interior mesopores. The drugs remained entrapped within the UCNP-MSNs when the nanoparticles were wrapped with a compact branched polypeptide, poly(Asp-Lys)-b-Asp, because of multivalent interactions between Asp moieties present in the polypeptide and the TDPA-Zn2+ complex present on the surface of UCNP-MSNs. This led to luminescence resonance energy transfer (LRET) from the UCNPs to the entrapped drugs, which typically have absorption in UV visible range, ultimately resulting in quenching of UCNP emission in UV visible range while retaining their strong NIR emission. Addition of ATP led to a competitive displacement of the surface bound polypeptide by ATP due to its higher affinity to TDPA-Zn2+, which led to the release of the entrapped drugs and subsequent elimination of LRET. Monitoring of such ATP-triggered ratiometric changes in LRET allowed us to monitor the release of the entrapped drugs in real-time. Given these results, we envision that our proposed UCNP@MSN-polypeptide hybrid nanoparticle has great potential for stimuli-responsive drug delivery as well as for monitoring biochemical changes taking place in live cancer and stem cells.