Chitosan-Capped Au Nanoparticles for Laser Photothermal Ablation Therapy: UV-Vis Characterization and Optothermal Performances

Hindawi Journal of Spectroscopy Volume 2018, Article ID 8271254, 11 pages https://doi.org/10.1155/2018/8271254 Research Article Chitosan-Capped Au Nanoparticles for Laser Photothermal Ablation Therapy: UV-Vis Characterization and Optothermal Performances Giancarlo Margheri ,1 Silvana Trigari,1 Mariabeatrice Berti,2 Maurizio Muniz Miranda,1 and Rita Traversi 2 1 2 Institute for Complex Systems, National Research Council, Via Madonna del Piano 10, 50019 Sesto Fiorentino, Firenze, Italy Department of Chemistry “Ugo Schiff”, University of Florence, Via della Lastruccia 13, 50019 Sesto Fiorentino, Firenze, Italy Correspondence should be addressed to Giancarlo Margheri; Received 31 January 2018; Accepted 12 April 2018; Published 10 July 2018 Academic Editor: Artem E. Masunov Copyright © 2018 Giancarlo Margheri et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. We have reported on the synthesis and characterization of near-infrared- (NIR-) absorbing colloidal nanoparticles prepared by exploiting the one-step reaction of HAuCl4 and Na2S2O3, followed by their stabilization with chitosan. This reaction also produces a big amount of unwanted nanoparticles detuned with respect to the NIR spectral region. For this reason, it is usually assumed that the product has to be filtered and enriched to enhance its NIR absorption, and the possible exploitation of the simpler raw product has never been worthy to be considered. Aiming to investigate this missing aspect, we chose to avoid the purification steps and rather focused on the preparation of the unrefined colloid, identifying the synthesis conditions that maximize its NIR absorbance and, subsequently, testing it as an optothermal transducer by measuring its molar heating rate (MHR). As expected, we found that the performances of the raw colloid are indeed lower than those of its refined version, but only to a limited extent. Moreover, MHR is unexpectedly higher than that deducible for other classical NIR-absorbing nanoparticles, like Au nanorods or Au nanostars. Thus, the product of the simpler preparation protocol appears as a competitive trade-off solution between easy manufacturing and optothermal performances. 1. Introduction Laser photothermal ablation assisted by plasmonic nanoparticles has emerged as one of the most promising approaches to defeat cancer diseases. A large amount of work has been produced in this field, considering the various crucial aspects for the assessment of a protocol ensuring the effectiveness of therapy, safety, and optimal tumor targeting at the same time. While the specificity of tumor targeting is generally enhanced by proper particle decoration [1–3] or by the manufacturing of specific biological vectors doped with Au nanoparticles (AuNPs) [4–7], biocompatibility is ensured by decoration with nontoxic moieties [6, 7]. Effectiveness of light/heat conversion is mainly related to the shape and size of nanoparticles or their agglomerates. In fact, such features manage the absorption efficiency of laser radiation and of heat delivery to the surrounding media with benefits exploited for instance in bio-oriented and sensing applications. In order to obtain maximum penetration in biological tissues, infrared radiation has to be used to stimulate AuNPs and this restricts the choice to particles which underwent uneven growth. As of today, the current literature is rich in examples: nanorods, nanostars, nanocages, nanohexapods [8], nanoprisms [9], or even bio-induced AuNP clusters [10], that absorb fairly well in the near-infrared (NIR) region of the electromagnetic spectrum. Nonspherical growth is usually obtained via chemical routes using a highly toxic surfactant, namely, cetyltrimethylammonium bromide (CTAB), which must be mandatorily removed after the synthesis. Moreover, in order to keep colloidal nanoparticles stable, a new nontoxic surfactant must be added to the nanoparticle surface, for instance, polyethylene glycol (PEG), one of the most commonly used 2 compound. The process thus requires multiple steps, and synthesis yield is an important concern. To overcome these problems, nanoparticles synthesized without CTAB and based on the reaction between HAuCl4 and sodium thiosulfate (Na2S2O3) [11–13] using chitosan as stabilizer [14] have been taken into consideration. Chitosan is a biobased polysaccharide, derived from shrimps and crab shells. It is nontoxic, biodegradable, biocompatible, and easily processable [15, 16]. Margheri et al. just demonstrated the efficiency of this reaction’s product [7] in tumor treatment and cell therapy. However, no systematic study was performed to assess the characteristic features of the colloid, for instance, the molar ratio of the two reagents needed to obtain the highest NIR absorption. This reaction, derived from that proposed for the first time by Zhou et al. [17], between tetrachloroauric acid and sodium sulfite is, by itself, effective to generate NIRabsorbing AuNPs. Unfortunately, it can also follow another pathway, whose result is a large amount of small spheroidal nanoparticles that cannot be used to absorb NIR radiation. Moreover, in clinically oriented applications, they would only increase off-target accumulation in the reticuloendothelial system (RES) organs, whose cells are furthermore expected to strongly interact with the positively charged chitosan-capped nanoparticles, unless they are safely carried by proper shielding cell vectors [7]. The natural way to overcome the presence of non-NIR nanoparticles and increase NIR efficiency is to filter them out and obtain a refined colloidal solution of concentrated NIRabsorbing nanoparticles. Following this path, efficient colloids capable of reaching hyperthermic regimes in few minutes at low laser intensities have been obtained. However, the refinement of the raw colloidal solution is not trivial as it suffers from typical issues induced by standard manufacturing steps (centrifugation and filtration) necessary to obtain NIR colloid with an enhanced content of NIRresonating AuNPs. The question arises whether the raw colloidal suspension shows sufficiently high thermal performances to counterbalance the drawbacks of further refinement stages without any further handling. In this paper, we report on our recent spectroscopic and thermal characterization of chitosan-stabilized colloids, based on HAuCl4/Na2S2O3 reduction synthesis, that did not undergo further treatments. First, we identified the necessary molar ratio of HAuCl4 and Na2S2O3 to produce the highest absorbance around 810 nm wavelength. Using this ratio, we checked the optothermal performances of the product and found that, as expected, they are lower than the ones of the refined product, but only up to a moderate extent. Thus, our product seems to be highly competitive when pointing at applications specificall (...truncated)