Slight pH Fluctuations in the Gold Nanoparticle Synthesis Process Influence the Performance of the Citrate Reduction Method.

Braulio Contreras-Trigo,Sebastián Godoy,Ignacio Sanhueza,Víctor Díaz-García,Patricio Oyarzún,Sergio Torres,Pablo Coelho,Enrique Guzmán-Gutierrez

doi:10.3390/s18072246

Braulio Contreras-Trigo, Sebastián Godoy + Show 6 more

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https://doi.org/10.3390/s18072246

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Abstract

Gold nanoparticles (AuNPs) are currently under intense investigation for biomedical and biotechnology applications, thanks to their ease in preparation, stability, biocompatibility, multiple surface functionalities, and size-dependent optical properties. The most commonly used method for AuNP synthesis in aqueous solution is the reduction of tetrachloroauric acid (HAuCl4) with trisodium citrate. We have observed variations in the pH and in the concentration of the gold colloidal suspension synthesized under standard conditions, verifying a reduction in the reaction yield by around 46% from pH 5.3 (2.4 nM) to pH 4.7 (1.29 nM). Citrate-capped AuNPs were characterized by UV-visible spectroscopy, TEM, EDS, and zeta-potential measurements, revealing a linear correlation between pH and the concentration of the generated AuNPs. This result can be attributed to the adverse effect of protons both on citrate oxidation and on citrate adsorption onto the gold surface, which is required to form the stabilization layer. Overall, this study provides insight into the effect of the pH over the synthesis performance of the method, which would be of particular interest from the point of view of large-scale manufacturing processes.

Highlights

Gold nanoparticles (AuNPs) have emerged as a promising platform for a growing number of biomedical and biotechnology applications in the fields of sensing [1], molecular diagnostic [2], therapeutic [3], and imaging [4], owing to their stability, biocompatibility, remarkable physicochemical properties, and easy surface functionalization with a wide range of ligands [5,6]
Citrate-capped AuNPs were synthesized by the citrate reduction method and immediately characterized in terms of the pH of the solutions, as well as by UV-visible spectroscopy, size, morphology, elemental composition, and surface charge properties
(4.7, 5.0, and 5.3), showing the typical curve with a characteristic maximum at 520 nm associated to the surface plasmon resonance (SPR) band

Summary

Introduction

Gold nanoparticles (AuNPs) have emerged as a promising platform for a growing number of biomedical and biotechnology applications in the fields of sensing [1], molecular diagnostic [2], therapeutic [3], and imaging [4], owing to their stability, biocompatibility, remarkable physicochemical properties, and easy surface functionalization with a wide range of ligands [5,6]. A prominent optic feature of AuNPs arises from the collective oscillation of the conduction electrons in the presence of an incident light, the so-called surface plasmon resonance (SPR) [7]. This phenomenon causes a sharp and intense absorption band in the visible range, which can be readily tuned by varying the particle size, shape, and the surrounding physicochemical environment [8]. As concerns the preparation of the colloidal gold nanoparticles, various chemical routes, including the use of chemical reductants [13] and several photochemical methods based on UV irradiation [14], γ-irradiation [15], of and laser irradiation have been widely for different. Label-free colorimetric sensors based on AuNPs (nanobiosensors) have been widely proposed as a promising analytical for selective binding and detection of chemical and biological targets, including metal ions [9], antibiotics [10], mycotoxins [11], as well as a large number of microorganisms [12].

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