Upconversion nanoparticle incorporated oleogel as probable skin tissue imaging agent

Although upconversion nanoparticle (UCNP) has shown promising role as a near infra-red (NIR) excitation triggered theranostic agent, its application in dermatology is limited due to skin permeation issues. Herein, we report the delivery of UCNP to the inner skin structure in transdermal fashion using oleogel based topical formulation. For this purpose, we have developed a series of oleylamine coated UCNP (OA-UCNP) incorporated oleogel through a simple two-step heat-cool procedure by varying the weight % of OA-UCNP and keeping the weight ratio of soybean oil:stearic acid constant. The UCNP-oleogels were characterized by various microscopic techniques as well as XRD and FTIR in order to understand the changes occurring in the microstructures of the oleogels with the incorporation of OA-UCNP in a concentration-dependent manner. The photoluminescence (PL) spectroscopy studies revealed that the UCNP-oleogel with the highest concentration of OA-UCNP (UG4) showed maximum luminescence among all the other compositions. The skin tissue penetration ability of UCNP was validated using UG4 as a topical agent through in vitro PL experiments involving agarose gel and animal skin models. The permeation of UCNP across the full thickness of intact pig ear skin was evident when UCNP was incorporated in oleogel, whereas UCNP itself in the form of OA-UCNP did not show any detectable skin permeation. The inner layer of animal skins were imaged by capturing the PL of the UCNP under NIR (980 nm) laser excitation.