J. Chil. Chem. Soc., 61, Nº 4 (2016) 3228 A CHEMOMETRIC APPROACH TO THE INFLUENCE OF THE SYNTHESIS PARAMETERS ON THE OPTICAL RESPONSE OF GOLD NANOPARTICLES AND STUDY OF THEIR ELECTROPHORETIC DEPOSITION ON SILICON RENZO MILESI L. 1 , EMILIO NAVARRETE S. 1 , JAVIER ROMÁN S. 1 , VÍCTOR ROJAS C. 1 , RODRIGO HENRÍQUEZ N. 1 , RICARDO SCHREBLER G. 1 , RICARDO CÓRDOVA, EDUARDO C. MUÑOZ 1 1 Pontifcia Universidad Católica de Valparaíso, Instituto de Química, Facultad de Ciencias, Avenida Universidad 330, Valparaíso, Chile. ABSTRACT This study describes gold nanoparticle synthesis in an aqueous solution by reducing tetrachloroauric acid (HAuCl 4 ), using sodium citrate as a reducing agent. Synthesis was optimized through an experimental design, and particle characterization was obtained through UV-Vis spectroscopy. In a frst stage, a factorial design was conducted to establish the signifcance of the variables used in the synthesis, i.e. reducer concentration, temperature, stirring rate. In the second stage, the obtained nanoparticles were positioned on a silicon p-type substrate through electrophoretic deposition. The modifed substrates were characterized by means of refectance measurements and their morphology using transmission electron microscopy (TEM) and atomic force microscopy (AFM). Finally, a brief discussion was carried out concerning the sizes of the determined nanoparticles based on a model taken from the literature that relates maximum absorption with the nanoparticle diameter, comparing these with the sizes obtained through AFM and TEM observation. Keywords: Gold nanoparticles, AFM, TEM, experimental design, electrophoretic deposition. 1. INTRODUCTION Nanotechnology has grown as an area of study and development over the last 50 years. This is due in part to the fact that controlling the physical- chemical properties of nanoparticles allows them to be used in diverse felds, such as medicine, optics, physics, biology and chemistry 1–3 . Furthermore, these properties can be modulated to change the size of the nanoparticles without needing to change their material composition. Currently, important advances have been made in synthesizing high-quality nanostructures 4 , particularly metallic nanoparticles 5, 6 and semiconductor nanocrystals 7 . Within the range of metallic nanomaterials, gold nanoparticles (AuNPs) are of great interest, both for their properties as well as for their applications, which include i) chemical properties, e.g. molecular recognition 8, 9 , reactivity 10, 11 ; ii) biological properties, e.g. assembly of DNA-AuNPs, conjugation of AuNPs with cancerous cells for detection 12, 13 ; iii) physical properties e.g. optical applications 14 and iv) electrochemical properties 15 . One of the current applications of interest for AuNPs is the feld of biosensors for the detection of distinct biological species, such as glucose 16, 17 , enzymes 18 , peroxidase 19 , DNA 20 , among others. However, before application can take place, the nanoparticles must be functionally integrated in the device, which requires controlling their interactions with other materials and their spatial organization. In this context, one alternative is to deposit the AuNPs on a conductor or semiconductor substrate, a process that must meet certain criteria, such as irreversibility of the deposition process 21 , stability, and a high nuclear density. The deposition of AuNPs can be carried out using different methods. For example, the electrophoretic deposition method (EPD) 22, 23 , uses an external uniform electric feld that conducts the suspended particles from the dissolution toward substrate surface. Traditionally, AuNPs can be obtained both from organic and aqueous media. In the former case, this can be carried out using gold halides, e.g. AuCl and AuBr, in the presence of alkylamines 24 . On the other hand, aqueous phase synthesis is achieved mainly through the reduction of tetrachloroauric acid (HAuCl 4 ) with sodium citrate (Na 3 C 6 H 5 O 7 ) 25–27 . The sodium citrate allows for the control of nanoparticle size, acting as a stabilizing agent. This generates the dissolution of colloidal AuNPs as a result of the surface charges adopted by both the citrate particles and the AuNPs. The surface charge can be obtained by (1) ion adsorption according to the Paneth-Fajans rule 28–30 , or (2) through a surface dissociation process 31 . The reduction of ionic force leads to an increase in the Debye length of the double layer around the particles, which then stabilizes the colloids according to DLVO theory 32 . By modulating the size of the AuNPs, it is possible to control the synthesis conditions, i.e. the use of stabilizing agents, temperature, pH, ionic strength, reaction time, molar ratio of the reaction precursors, which creates a signifcant number of experiments. For this reason, experimental chemometric design strategies are an important statistical tool that allows researchers to know the infuence and interaction of experimental variables in relation to an interesting response, allowing them to optimize the process with minimal experiments 33–35 . In view of the above, the e-mail: eduardo.munoz.c@pucv.cl objective of this work is to study the infuence of the AuNPs synthesis variables through the application of experimental design in order to later deposit them on silicon substrates and characterize them according their refectance and morphology. 2. EXPERIMENTAL SECTION 2.1 Experimental Design Construction In this stage, an experimental matrix was created using a 2 3 factorial design, using the following parameters as experimental variables: i) concentration of reducing agent; ii) temperature; iii) stirring rate as shown in Table I. The concentration of HAuCl 4 used in each experiment was kept at a constant value of 5.0 mM. Table I: AuNPs Synthesis Variables Experiment Sodium Citrate Concentration (mM) Temperature (°C) Stirring Rate (rpm) Maximum value 15.40 110 1000 Central Value 8.40 100 950 Minimum Value 1.40 90 900 Depending on the values of the experiment matrix, synthesis of the AuNPs was carried out in two phases: i) the dissolution of HAuCl 4 was maintained with the corresponding stirring rate and temperature and ii) adding the reducing agent to the experiment at the corresponding temperature. All experiments were carried out for a constant time of 20 minutes following the process of precursor mixing. After this time, the AuNPs were stored at 4°C for their subsequent characterization. 2.2 Optical and size distribution characterization of AuNPs suspension The spectral absorbance and refectance of the AuNPs were obtained using a Shimadzu UV-2600 spectrophotometer between 450 and 600 nm. In the case of the optical refectance measurements, an integrating sphere was coupled. AuNPs size was determined by a Transmission Electron Microscope, TEM, model JEOL JEM-1010 operated to 100 kV. Samples were dispersed in water and subjected to ultrasound. Subsequently, a drop of each treated sample was positioned on a copper grid with a carbon flm, cleaned, and dried under a plasma fow. 2.3 AuNPs deposited on Silicon The following procedure was employed when positioning the AuNPs on silicon substrates: The electrodes used for positioning the AuNPs were p-Si