Fabrication of Chitosan Nanoparticles Containing Samarium Ion Potentially Applicable for Fluorescence Detection and Energy Transfer

Chitosan is a natural polysaccharide that has ideal properties as a polymer nanoparticle for drug delivery applications because it is easy to synthesize, inexpensive, biocompatible, biodegradable, non-immunogenic, and non-toxic. In this study, chitosan nanoparticles were fabricated in an acidic solution in the presence of potassium persulfate using a microwave technique. The effects of the reaction time, temperature, and weight ratio of potassium persulfate/chitosan on the yield and particle size were evaluated. It was found that the yield increased non-linearly, whereas the size of chitosan nanoparticles was 3 nm in the absence of potassium persulfate, which tended to increase with an increase in the potassium persulfate concentration. The chitosan nanoparticles were also treated with samarium for fluorescence detection. The fluorescence intensity at 590 nm of samarium-treated chitosan nanoparticles increased by a factor of more than 20 when compared with the samarium ion itself and was significantly higher than that of the untreated chitosan nanoparticles. It is indicated that chitosan nanoparticles are not only useful for drug carriers, where the drug delivery can be traced by monitoring fluorescence emission, but with the photoemissive properties of chitosan nanoparticles treated with samarium, they could also be applicable as environmentally friendly photocatalysts for the photodegradation of discharged pollutants as well as efficient photosensitizers that participate in energy transfer.

Chitosan; Microwave technique; Nanoparticles; Potassium persufate; Samarium

Nanotechnology is a promising field because the materials in nanometer scales (Moura et al., 2008) can be manipulated due to their large surface area, large surface reactivity, and unique physicochemical properties for novel applications (Sonia & Sharma, 2011). The reactivity of the nanometer-sized materials is determined by the atoms on their surface (Abdullah et al., 2008). Though most efforts have been devoted to fabrications and applications of semiconductor nanoparticles, those for biomaterials have also attracted attention due to their biocompatible and biodegradable properties (Moura et al., 2008). For instance, natural polysaccharides, such as chitosan, are ideal biomaterials and have become important due to their availability in large quantities, their low cost (Lee et al., 2009), and their non-immunogenic and non-toxic properties (Tiyaboonchai, 2003), which allow them to be applied as drug carriers (Kocak et al., 2011; Kusrini et al., 2014). For this drug carrier application, chitosan is strongly considered due to its antimicrobial activity (Dutta et al., 2009). The use of composites of chitosan with clay, which can be applied for packaging materials, has also been widely studied (Haerudin et al., 2010). In particular, the chitosan particles and composites can be fabricated to be within a few hundreds of nanometers. 

Chitosan nanoparticles with sizes between 100–600 nm, for instance, have been fabricated by various methods, including coacervation from suspension (Aloys et al., 2016), ionotropic gelation (Calvo et al., 1997), emulsion cross-linking (Riegger et al., 2018), emulsion-droplet coalescence (Balcerzak et al., 2013), reverse micellar process (Zhao, 2011), and sieving (Agnihotri, 2004). All these methods involve surfactants and cross-linking agents, such as glutaraldehyde, making the fabrication of chitosan nanoparticles complicated and non-environmentally friendly. To overcome this issue, a simple method using potassium persulfate in formic acid without involving any organic solvents, surfactants, or precipitation agents to produce 50–110 nm-sized chitosan nanoparticles has been proposed (Kusrini et al., 2015). Persulfate ions are able to cut the biopolymeric chains of chitosan (Hsu et al., 2002). The use of an acid as a solvent has been exhibited to have strong effects on the size and morphology of the resulting chitosan nanoparticles (Soltani et al., 2012). The effect of potassium persulfate in an acidic solvent, including acrylic acid, methyl methacrylic acid, and methacrylic acid, has been explored (Hsu et al., 2002; Moura et al., 2008). The size of the chitosan nanoparticles is within the range of 60–300 nn. Compared with conventional heating, the chemical process using a microwave has also received considerable attention due to the higher conversion and shorter reaction times required to form chitosan nanoparticles by the ionic gelation method with acetic acid as the solvent (Kocak et al., 2011).

In this study, chitosan nanoparticles were fabricated from chitosan using potassium persulfate in formic acid. The microwave technique was used to accelerate the bond breaking reaction of chitosan. Using this technique, chitosan nanoparticles with different sizes and morphologies were fabricated. The size of the chitosan nanoparticles was as small as 3 nm. The chitosan nanoparticles were then treated with samarium to enhance their fluorescence intensity so that they could potentially be used for light-electricity conversion as well as for drug carrier applications. For the former, the chitosan nanoparticles containing samarium or dyes might be capable of electron and energy transfers, while the latter is promising for tracing drug delivery by monitoring the emission of chitosan nanoparticles.

Chitosan nanoparticles have been fabricated through a depolymerization process of chitosan utilizing potassium persulfate and formic acid and a microwave technique. The potassium persulfate is a depolymerization agent, and formic acid acts as a proton donor. The yield of chitosan nanoparticles increased non-linearly with the presence of potassium persulfate, and similarly, the size increased with increasing potassium persulfate concentrations. The chitosan nanoparticles fabricated at a specific ratio of potassium persulfate/chitosan were found to be monodispersed. The infrared vibrational spectrum of chitosan nanoparticles is similar to that of chitosan, indicating that the chitosan nanoparticles still contained O–H and N–H groups. With these functional groups, the chitosan nanoparticles are potentially capable of acting as matrices to encapsulate electron-accepting or -donating dyes for bulk heterojunction solar cells, optimizing the photoactive layer. The chitosan nanoparticles were then treated with samarium for fluorescence detection. It was found that samarium was chelated by chitosan oligomers via the hydroxyl and amine groups of chitosan, and the composition of samarium in the complex was 1.10%. The fluorescence intensity at 590 nm of chitosan nanoparticles containing samarium was 585 a.u., which increased by a factor of more than 20 when compared with samarium ion itself (38.75 a.u.) and was significantly higher than that of the untreated chitosan nanoparticles (6.96 a.u.). These results suggest that when chitosan nanoparticles are applied to drug carriers, drug delivery can be traced by monitoring the fluorescence emission. With their photoemissive properties, chitosan nanoparticles treated with samarium developed herein could also be applicable as environmentally friendly photocatalysts for the photodegradation of discharged pollutants. The chitosan nanoparticles treated with samarium can also be efficient photosensitizers and can participate in energy transfers. 

This research was financially supported by Universitas Indonesia through the grant PITTA No. 2430/UN2.R3.1/HKP.05.00/2018.

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