Fabrication of Chitosan-Alginate Core-Shell Mircogels Incorporated with luminescent Cabron Dots for Biomedical Applications

Abstract

Biopolymer microgels present many opportunities in biomedicine and tissue engineering. Among diverse types of microgels, core-shell microgels are of special significance since they may be designed to have a solid core surrounded by liquid-like shell or vice-versa and can be made responsive to external stimuli. Under suitable stimulus (e.g. pH or temperature of a solution containing the microgels) the outer shell may be diluted, thereby releasing the core’s material. Such strategies provide promising possibilities for controlled drug delivery and other biological applications. Additionally, nanoparticles having different functionalities can be embedded in these microgels to enhance or tune their overall properties, thereby making them amenable for variety of applications. In this investigation we develop a novel method to produce chitosan-sodium alginate (CS-SA) core-shell microgels in a single step process using a specially designed high throughput centrifugal microfluidic device (HTCMD). We subsequently incorporated nitrogen functionalized graphene quantum dots (NGQDs) in the core-shell microgels which render them luminescent under UV excitation and are expected to enhance the physical and biological characteristics of the hydrogel microspheres. Initial part of this study was focused on designing and fabricating a microfluidic device that could generate core-shell microgels with controllable geometry and sizes. After several attempts with planar structures we converged on a 3D printed multichannel cylindrical HTCMD that could produce core-shell structures with desired control over size and shape. An analysis of the of microgels generated using the specially designed HTCMD was carried out in order to develop an understanding of the ways in which characteristics of the device such as the diameter of the nozzle and the rotating velocity influence the size, shape, and homogeneity of the generated microparticles. Using a nozzle diameter of 310 µm we could obtain core-shell microspheres having an average diameter of 444 µm at 2500 rpm. On the other hand, a variation of angular velocity between 900 to 2500 rpm allowed us to generate microspheres with average diameters varying between 1500 to 400 µm depending on the nozzle diameter. Following successful fabrication of the HTCMD and controlled generation of spherical core-shell microgels, we investigated the structural, compositional and optical properties of the microgels using a variety of techniques. Fourier transform infrared spectroscopy (FTIR) spectra of the as-prepared core-shell microgels for different concentrations of CS and SA and for the NGQD incorporated microgels revealed that the overall bonding architecture is strongly dependent on the concentrations of CS and SA and is marginally affected by the presence of NGQDs. X-ray diffraction (XRD) of NGQD, CS-SA core-shell microgels and the NGQD incorporated CS-SA particles reveal signatures of crystallinity in all the three samples although sharp crystalline features are not present in any of the samples. In case of NGQD this is attributed to the nanometric size of the crystalline domains while in CS and SA samples the presence of amorphous constituents dominate. Scanning electron microscopy (SEM) alongside brightfield microscopy showed the formation of distinct core-shell interface between CS and SA core-shell structure. UV-vis absorption spectra of all the samples exhibit standard absorption characteristics suggesting the formation of structures with desired electronic transitions. The NGQDs demonstrate excellent room temperature photoluminescence (PL) emission with a PL peak at 444 nm for an excitation of 350 nm. The CS SA core-shell particles exhibit a very weak room temperature PL but following NGQD incorporation their emission is completely defined by the characteristics of the NGQDs. The size and shape controlled, luminescent hydrogel core-shell microspheres have immense potential applications in the fields of drug delivery and tissue engineering. This work proposes a simplified method for the synthesis of microgels by utilizing the pH-dependent sol-gel transition qualities of chitosan, the ionic crosslinking capabilities of alginate. The simplicity of the centrifugal microfluidic platform utilized in the research make it possible to exert exact control over the architecture of the microgel, which in turn promotes a synthesis process that is both easy and extremely effective.