Quantum dot/glycol chitosan fluorescent nanoconjugates

Mansur and Mansur Nanoscale Research Letters Quantum dot/glycol chitosan fluorescent nanoconjugates Alexandra AP Mansur 0 Herman S Mansur 0 0 Center of Nanoscience , Nanotechnology and Innovation - CeNano2I , Department of Metallurgical and Materials Engineering, Federal University of Minas Gerais , Av. Antonio Carlos, 6627 - Escola de Engenharia, Bloco 2 - Sala 2233, Belo Horizonte, MG 31.270-901 , Brazil In this study, novel carbohydrate-based nanoconjugates combining chemically modified chitosan with semiconductor quantum dots (QDs) were designed and synthesised via single-step aqueous route at room temperature. Glycol chitosan (G-CHI) was used as the capping ligand aiming to improve the water solubility of the nanoconjugates to produce stable and biocompatible colloidal systems. UV-visible (UV-vis) spectroscopy, photoluminescence (PL) spectroscopy, and Fourier transform infrared (FTIR) spectroscopy were used to characterise the synthesis and the relative stability of biopolymer-capped semiconductor nanocrystals. The results clearly demonstrated that the glycol chitosan derivative was remarkably effective at nucleating and stabilising semiconductor CdS quantum dots in aqueous suspensions under acidic, neutral, and alkaline media with an average size of approximately 2.5 nm and a fluorescent activity in the visible range of the spectra. Glycol chitosan; Nanoparticle; Quantum dot; Colloid; Biopolymer; Bioconjugates; Nanomaterials - Background Approximately 3 decades ago, quantum dots (QDs) emerged as a notable class of nanomaterials because of their unique set of optical, electronic, magnetic, and chemical properties [1-3]. Essentially, QDs are ultra-small semiconductor crystalline nanoparticles with size-dependent properties, which possess higher luminescence, narrower emission band, broader excitation wavelength range, and greater photostablility compared with fluorescent organic dyes [2]. Due to their small dimensions with extremely high surface area, these fluorescent nanocrystals must be stabilised by capping agents during their synthesis to restrict the growth of the nucleated nanoparticles [4]. Thus, QDs have been produced using a myriad of processes, such as entrapment in molecular films [5,6] and glasses [7,8], as well as encapsulation in polymer nanoparticles [9], organic solvents [10], and colloidal dispersions [11]. Since the seminal work of Murray et al. [12], the majority of QDs have been developed using organometallic routes at high temperature because they commonly result on monodisperse nanoparticles with high luminescent behaviour. However, water-soluble QDs have increasingly attracted the attention of the research community based on their potential use in biomedical and environmentally friendly applications [1,13,14]. Therefore, water-soluble polymers are a promising platform to develop innovative QD nanohybrids because they offer an attractive set of physicochemical properties associated with broad availability, large variety of chemical structures at relative low cost. In addition, polymers can be chemically functionalised and conjugated with other molecules for designed and specific applications [15-17]. Among the numerous alternative polymers for biomedical applications, chitosan (CHI) and its derivatives have often been selected due to their multidimensional properties [18,19]. However, chitosan is reasonably water-soluble only under acidic conditions, and it is practically insoluble at neutral and alkaline pH (at pH higher than its pKa 6.5), which significantly restricts its applications in medicine and biology at physiological pH (approximately 7.4). Hence, the chemical modifications of chitosan for producing watersoluble derivatives in a broader pH range, mainly under physiological conditions, are highly attractive for the preparation of nanohybrids and nanoconjugates for nanomedicine [9,20-25]. Surprisingly, only few reports have been published in the literature using chitosan and its derivatives as direct capping ligands for the synthesis of QDs in aqueous media [21-24]. Glycol chitosan (G-CHI) is a commercially available derivate of chitosan with improved hydrophilicity and biocompatibility and is frequently used in various biomedical applications such as drug delivery, siRNA carrier, cancer imaging, and therapy [26]. Interesting reports using G-CHI combined with nanomaterials (e.g. gold nanoparticles) have been published by Kim and collaborators [26,27], as well as studies on PEG-conjugated chitosan derivatives for the preparation of QDs [28]. Nevertheless, no study was found in the consulted literature addressing the direct synthesis of QDs using glycol chitosan as capping ligands by aqueous colloidal chemistry. Thus, in this study, novel carbohydrate-based nanoconjugates combining glycol chitosan with CdS semiconductor QDs were designed and synthesised via a singlestep aqueous process at room temperature. G-CHI was used as the capping ligand to produce water-soluble colloidal bioconjugates. The results demonstrated that the glycol chitosan derivative was effective at nucleating and stabilising luminescent CdS QDs in aqueous colloidal dispersions under acidic, physiological, and alkaline media, indicating considerable potential for biomedical and pharmaceutical applications in nanomedicine. Methods Materials All of the reagents and precursors, including cadmium perchlorate hydrate (Sigma-Aldrich, St. Louis, MO, USA, Cd(ClO4)2 6H2O), sodium sulphide (Synth, Diadema, Brazil, >98%, Na2S 9H2O), and hydrochloric acid (SigmaAldrich, St. Louis, MO, USA, 36.5% to 38%, HCl) were used as received. Glycol chitosan (G-CHI; Sigma-Aldrich, St. Louis, MO, USA, PN# G7753; degree of polymerization 400, lot supplied = 2,000 (Mw ~ 410 kDa); degree of deacetylation DD 60%, lot supplied = 76.2%) was used as the ligand. Chitosan (Aldrich Chemical, St. Louis, MO, USA, catalogue#419419; high molecular weight, Mw = 310 to >395 kDa; degree of deacetylation DD 75.0%; viscosity 800 to 2,000 cPoise, 1 wt.% in 1% acetic acid) was used as the reference polysaccharide ligand. Unless otherwise indicated, deionised water (DI water; Millipore Simplicity, Millipore, Billerica, MA, USA) with a resistivity of 18 M cm was used to prepare the solutions, and the procedures were conducted at room temperature (RT; 23C 2C). Synthesis of CdS quantum dots A chitosan solution (1%, w/v) was prepared by dispersing CHI powder in an aqueous solution (2%, v/v) of acetic acid. The mixture was placed under constant stirring overnight at room temperature, until complete solubilisation had occurred (pH ~ 3.6). Glycol chitosan solution (1.0%, w/v) was prepared by dissolving G-CHI powder in DI water under moderate magnetic stirring for 2 h until complete solubilisation had occurred (pH ~ 8.4). Before synthesising the CdS QDs, chitosan and glycol chitosan solutions were diluted with DI water to a concentration of 0.4 mg.mL1 and the pH was adjusted with NaOH or HCl solutions (0.1 (...truncated)