ZnS:Mn2+ Phosphors Capped with Chitosan

Abstract

Luminescent II-VI semiconductor like ZnS, CdS, CdSe nanocrystals have attracted a lot of attention in the past two decades due to their unique properties and potential applications in electronics, lighting industry, military and medical fields. Controlling crystallite size and/or doping with luminescent ionic centers can tailor the optical properties of the nanocrystals. Cadmium Selenide (CdSe) quantum particles are today the standard dots for most biolabeling applications involving luminescent tagging. Unfortunately, these quantum dots are not very efficient for electromagnetic or electrical energy conversion into photons of visible light due to the 'unsaturated bonds' on their surfaces providing trapping centers for nonradiative recombination processes to take place. To avoid this drawback for converting the quantum dot into an efficient light emitter, the energy traps on its surface is generally eliminated by growing a thin shell of zinc sulphide (ZnS) around the core quantum dots of CdSe. Not only does this increases the quantum efficiency but also avoids the direct exposure to cadmium or selenium atoms, that are toxic.In the present work, we introduced a novel biocompatible passivation layer 'Chitosan' on the ZnS nanophosphors, which showed an interesting enhancement in the luminescence efficiency compared to other passivative agents that were generally used in this literature. The nanophosphors were doped with Mn 2+ ions in order to exhibit orange color when exited with UV light. The synthesis of particles in a solution occurs by chemical reactions resulting in the formation of nuclei and subsequent particle growth. The inorganic wet chemical synthesis used to prepare ZnS:Mn 2+ nanocrystals is similar to that described in the literature [1] except that synthesis was carried out in water because of its inherent advantages of being simple and environment friendly. All steps of the synthesis were performed at room temperature and under ambient conditions. Scanning electron microscopy (SEM) images reveal that the particles have smooth surfaces due to the surface passivation by chitosan roughly around 30 nm in size (figure 1). High Resolution Transmission Electron Microscopy (HRTEM) analysis reveals that the 30nm agglomerates are polycrystalline with distinct grain boundaries (figure 2) with average crystallite sizes of 2.5 nm. This is consistent with the estimation of the crystallite sizes from X-ray diffraction (XRD) analysis that leads to an average crystallite size of 2.2nm. FTIR spectra of chitosan-passivated nanoparticles show strong absorption at 1547 cm -1 and weak one at 642 cm -1 . This confirms that amide group is attached to the particles and functionalisation of the particle surfaces with the amide groups will allow surface modifications of the nanoparticles using simple chemical routes. It was found that the chitosan-capped nanoparticles consisted of higher density of crystallites than 1912

Novel Hybrid Materials And Their Applications

“…Chitosan 144 Precipitation Cetyltriammonium bromide 145 Stable at low concentrations of ZnS:Mn 2+ Cysteine [146][147][148][149] Precipitation Diethylene glycol 150,151 Stable for 2-3 days Hexametaphosphate 152 Precipitation Hydroxyethyl cellulose 153 Stable for 2-3 days, poor luminescence Mercaptoacetic acid 147 Stable for 2-3 days Mercaptopropionic acid 154 Stable for 2-3 days, corrosive Mercaptosuccinic acid 155 Stable for months, excellent luminescence Methacrylic acid 134 Stable for 1-2 days, corrosive NP-5/NP-9…”