Size-controlled Gold Nanoparticles Research Articles

Gold nanoparticles with natural polymer synthesized by plasma–liquid interactions: Size-control, characterization and colorimetric detection of melamine based on the size effect of gold nanoparticles

This work reports the synthesis of size-controlled gold nanoparticles as AuNP colloids in aqueous solutions with capping natural polymer matrices – the sodium salt of carboxymethyl cellulose, using a liquid-phase plasma process. The effects of the solution plasma process discharge time, concentrations of gold(III) chloride trihydrate precursor (0.06–2.5 mmol/L) and correlations of the precursor/CMC ratios on the characteristics on the synthesized Au NPs, including size distribution and optical absorption properties, were investigated. The average size (20–90 nm) and form (spherical, hexagonal and triangular) of the AuNPs can be change depending on the initial concentration of Au3+ and the ratios of precursor/CMC on the process synthesis. Gold nanoparticles were used to study a straightforward colorimetric detection of melamine. Melamine caused the AuNPs to agglomerate in aqueous solution, changing their visible color from red to blue. The prepared AuNPs can be used for the detection of melamine with a low LOD and an extremely wide linear range (0.005–0.21 μM.).

In situ tuning and investigating the growth process of size controllable gold nanoparticles and statistical size prediction analysis

A critical understanding of the formation and growth process of gold nanoparticles (GNPs) is crucial, in order to synthesize monodisperse gold nanocrystals of controllable size and shape in a predictable way. GNPs of different shapes and sizes can be produced by altering different physicochemical reaction conditions, such as pH, temperature, and the ratio of reactants. The significant variations in the morphological features of GNPs can be monitored in situ in a time-dependent manner by using UV–visible spectroscopy. In this study, we have synthesized seedless GNPs by reduction of HAuCl4 by using various molar ratios of HEPES and Na2HPO4. The shape and the geometry of the gold nanostructures were optimized by varying the pH (5, 7, and 9) of the HEPES, molar concentrations of disodium phosphate to HAuCl4, and reaction temperatures (20 °C, 40 °C, and 60 °C). The changes in the color of the reaction mixtures over time were recorded in situ in terms of the absorbance of the UV–visible light to tune and investigate the growth process of gold nanostructures. Transmission electron microscopic (TEM) images indicated that the gold nanostructures are anisotropic, stable, and exist in the size range of < 1–100 nm. The present study also confirms that the change in the color of nanostructure reaction mixtures is a function of the surface plasmon resonance bands under the influence of physicochemical reaction conditions and corroborates with the size and shape of nano-gold. Physicochemical parameters such as temperature, pH, and the molar concentration of the reactants act synergistically to influence the growth, molecular mechanics, and reaction thermodynamics that aid to affect the particle size, shape, and surface corona of the GNPs. By tracking the growth process, we can confirm that the nucleation, growth process, size, and shape of the gold nanostructures depend on the HEPES free radicals and phosphate ions which cluster to form polymeric chains on the gold nanocore. The present study's findings explain the growth process of the GNPs of various colors that can be observed by the naked eye and have promising applications, for example in the development of the biosensing rapid lateral flow immunoassays (LFIA) for the detection of mycotoxins in food samples.

The effect of size control of Gold Nanoparticles Stabilized with α-cyclodextrin and β-cyclodextrin and their antibacterial activities

This research aims to control gold nanoparticles (AuNPs) size by monitoring the relationship between pH stabilizer and Au (III) concentration on the resulting AuNPs. α-cyclodextrin (α-CDs) and β-cyclodextrin (β-CDs) were used as stabilizers for AuNPs that had been reduced by ortho-hydroxybenzoic acid through the one-pot synthesis method. Based on the optimization of CDs pH and Au3+ concentration, the optimal condition for AuNPs with α-cyclodextrin (AuNPs-αCDs) were at pH 11 and Au3+ concentration of 180 ppm, while AuNPs with β-cyclodextrin (AuNPs-βCDs) were at pH 6 and Au3+ concentration of 180 ppm. The results of the dynamic light scattering (DLS) analysis showed that AuNPs-αCDs and AuNPs-βCDs had smaller sizes (12.89 and 13.10 nm, respectively) compared to those without CDs (AuNPs-ortho), which was 29.57 nm. The results of X-ray Diffraction (XRD), Transmission Electron Microscope (TEM), Dynamic Light Scattering (DLS), zeta potential, and stability test results show that AuNPs-αCDs had a smaller size, higher crystallinity, and better stability than AuNPs-βCDs. AuNPs-αCDs performed better as gram-negative antibacterial (Escherichia coli and Pseudomonas aeruginosa) than AuNPs-β-CDs.

Synthesis of amino talc-like clay-supported Au nanoparticles with outstanding catalytic activity for 4-nitrophenol reduction

Studies on synthesizing gold nanoparticle-based catalysts to reduce the amount of gold and improve the catalytic activity at the same time are still crucial. The reduction reaction of 4-nitrophenol (a toxic compound) to 4-aminophenol (a non-toxic compound and essential material for the pharmaceutical industries) is a standard reaction for analyzing the catalytic activity of AuNPs. A capping agent is indispensable for producing stable and size-controlled gold nanoparticles (AuNPs) and in maintaining the catalytic activity of the AuNPs. However, some previous studies reported that capping agents reduced the catalytic activity of AuNPs through a surface-blocking phenomenon. Moreover, introducing a framework for capped AuNPs could be another consideration to increase the dispersion of AuNPs in an aqueous medium, eventually improving the catalytic activity of the AuNPs. Amino talc-like clay, a relatively non-toxic and well-dispersed layered material in aqueous solution, has been used as a framework for AuNPs. The present study reports the AuNP synthesis using delaminated amino talc-like clay (DAC) as a framework that brings about gold nanoparticles with superior catalytic activity to other supports. Moreover, the effect of the reducing agent concentration, Mn addition and pH conditions are discussed in determining this excellent property. DAC shows a promising role as a AuNP framework which increases the catalytic activity by improving the dispersion and stability of the material in the colloidal system. The pH-dependent stacking structure arrangement of the DAC support was a determining factor of Kapp for the 4-nitrophenol reduction reaction by DAC stabilized AuNPs. Under acidic conditions, the stacking structure of DAC was destroyed, providing more open space for the AuNPs attached to the clay surface, which increased the Kapp value. On the contrary, under basic conditions, the stacking structure of DAC was re-arranged, closing the surface space of the AuNPs attached to the interlayer clay, which decreased the Kapp value. Moreover, the DAC stabilizer provided a partially capped AuNP structure in neutral and acidic aqueous solutions, which prevented the catalytically active surface blocking phenomenon occurring in the ordinary capping type. Mn addition during the synthesis process controlled the production of small-size gold nanoparticles and increased the catalytic activity of the material.

A cellulose nanocrystal templating approach to synthesize size-controlled gold nanoparticles with high catalytic activity

Advanced templating methods have shown precise regulation of the micro/nanostructures of inorganic catalysts. Here, on the basis of controlled self-assembly and micro-structures of cellulose nanocrystals (CNCs), a new bio-mass-mediated templating approach is proposed to control the growth of gold nanoparticles (Au NPs). The catalytic performance of the as-prepared Au NPs was evaluated using p-nitrophenol as a model pollutant. TEM, POM, zeta-potential, and rheological measurements were conducted to investigate the structure and catalytic activity of the nano-materials. By regulating the chiral nematic liquid crystal texture formed by the self-assembly of CNCs, the size of Au NPs could be adjusted at the nanoscale dimension, from 1.38 ± 0.38 nm to 4.25 ± 1.24 nm. Depending on the Au size, a high catalytic effect, namely, 98.0% conversion rate, was obtained within 30 min. The conversion rate was maintained at 97.0% even after 3-run cyclic application. Such findings demonstrate the potential of using CNCs as a bio-template to control the growth of nanomaterials.

Carbon dots as a ‘green’ reagent to produce shape and size controlled gold nanoparticles for application in pollutant degradation

Gold nanoparticles (Au NPs) have attracted wide attention as catalysts for in situ degradation of organic pollutants. Current synthetic methods that enable precise control of Au NPs size and shape cause adverse effects on the environment. In this work, a sustainable, one-step method was used to synthesize Au NPs by reducing Au3+ with norepinephrine and ethylamine-based carbon dots (NE/EA CDs). CDs not only act as reducing agent but also stabilize the resulting Au NPs. The mechanism of Au3+ reduction was investigated by studying the effects of functional groups present in the CDs, which were confirmed from UV–Vis and XPS spectral analysis. Three different morphologies of Au NPs were observed after reduction: nanotriangles, nanourchins and nanospheres. Fabricated Au NPs were used to study a model catalytic reaction, reduction of 4-nitrophenol, indicating that CDs are an attractive option for fabrication of Au NPs for water treatment applications.

An effective method for size-controlled gold nanoparticles synthesis with nonthermal microplasma

A simple, effective and interesting method for gold nanoparticles (AuNPs) synthesis with nonthermal microplasma is developed in this study. The device of dielectric barrier discharge (DBD) microplasma generator with a spray portion is designed and fabricated for uniform AuNPs synthesis. The AuNPs can be synthesized effectively in situ by the DBD microplasma generated on the nozzle of the pneumatic micro-nebulizer. The mechanism of the AuNPs formation under microplasma, the effect of nebulization for uniform AuNPs synthesis and other significant parameters are investigated in the experiment. The morphology and optical properties of the synthesized gold nanoparticles are also characterized. The minimum particle size in average obtained by the proposed method is 4.9 ± 1.1 nm. The particle size of AuNPs can be controlled in the range of 4.9–16.8 nm by the various aqueous solution conditions.

Surface plasmon resonance-induced visible light photocatalytic TiO2 modified with AuNPs for the quantification of hydroquinone

The impregnation of size-controlled gold nanoparticles (AuNPs) on an anatase TiO2 structure (AuNPs@TiO2) was studied for the photoelectrochemical detection of hydroquinone (HQ) under visible light illumination integrated into a flow injection analysis (FIA) setup. The crystalline form of TiO2 was preserved during synthesis and the homogeneous distribution of AuNPs over the TiO2 structure was confirmed. Its photoelectrocatalytic activity was improved due to the presence of AuNPs, preventing charge recombination in TiO2 and improving its light absorption ability by the surface plasmon resonance effect (SPR). The FIA system was used in order to significantly reduce the electrode fouling during electroanalysis through periodic washing steps of the electrode surface. During the amperometric detection process, reactive oxygen species (ROS), generated by visible light illumination of AuNPs@TiO2, participate in the oxidation process of HQ. The reduction of the oxidized form of HQ, i.e. benzoquinone (BQ) occurs by applying a negative potential and the measurable amperometric response will be proportional to the initial HQ concentration. The influencing parameters on the response of the amperometric photocurrent such as applied potential, flow rate and pH were investigated. The linear correlation between the amperometric response and the concentration of HQ was recorded (range 0.0125 – 1.0 µM) with a limit of detection (LOD) of 33.8 nM and sensitivity of 0.22 A M−1 cm−2. In this study, we illustrated for the first time that the impregnation of AuNPs in TiO2 allows the sensitive detection of phenolic substances under green laser illumination by using a photoelectrochemical flow system.

Size-controlled synthesis of gold nanoparticles by ultrafine bubbles and pulsed ultrasound

Size-controlled gold nanoparticles (AuNPs) were synthesized by ultrasonic irradiation of HAuCl4 aqueous solutions with the aid of ultrafine bubbles (UFBs) in the absence of any capping and reducing agents. Upon addition of air-UFBs, the mean diameter of the spherical AuNPs decreased. This result was attributed to the sonochemical reduction of gold ions in aqueous solution being accelerated by the UFBs. Moreover, the AuNPs were stable in a solution containing UFBs because AuNPs electrostatically adsorbed onto UFBs, whose lifetime in water was very long. Compared with the mean diameter of AuNPs synthesized with argon-, nitrogen- and oxygen-UFBs, that of AuNPs synthesized with air-UFBs was smaller. Pulsed ultrasound delivered with the same time-averaged power as continuous-wave irradiation further decreased the mean diameter of the AuNPs. Size-controlled synthesis of AuNPs without the use of a capping or reducing agent was successful through optimization of the number air-UFBs and the pulsed ultrasound conditions.

Nano Gold Hybrid Polyvinyl Alcohol Films for Sensing of Cu2+ ions

AbstractAn efficient and eco‐friendly approach is adopted for the synthesis of highly stable L‐alanine assisted nano gold based polyvinyl alcohol nanocomposite hybrid films (Au/PVA).The –COOH groups of L‐alanine and oxygeneous groups of polyvinyl alcohol are utilized for the synthesis of size controlled gold nanoparticles. The as‐synthesized nanocomposites show rapid and selective detection of Cu2+ ion in aqueous medium. With addition Cu2+ ion to as‐synthesized nanocomposites, the SPR peaks of Au NPs (538 nm) decrease with occurrence of a red shift as well as a new peak is observed at higher wavelength which is due to the aggregation of gold nanoparticles in the nanocomposites hybrid films. Scanning electron microscope (SEM) micrograph and energy dispersive spectroscopy (EDS) reveal the aggregation of gold nanoparticles in the nanocomposite hybrid films. The limit of detection and limit of quantification for Cu2+ ion are 40 nM and 133 nM respectively. This technique for detection of Cu2+ ion in waste water samples is inexpensive and eco‐friendly since no any toxic chemicals are used. In the presence of Cu2+ ion, the colour of the hybrid film is changed hence, this method also involves visual detection of Cu2+ ion without use of electrolyte or transducer. Further, many reported methods represent the in situ monitoring of Cu2+ ion, but the present hybrid film can be used for in situ as well as ex situ monitoring of Cu2+ ion.

Biosynthesis of size-controlled gold nanoparticles using M. lucida leaf extract and their penetration studies on human skin for plastic surgery applications

Recently, majority of the studies were focusing on the nanoparticles (NPs) and their abilities of penetrating Stratum Corneum (SC), as they can be prominently utilized in the plastic surgeries. In the current work, we demonstrated the penetrating abilities of gold NPs (AuNPs) through anthropological skin with diameters of 10 and 15 nm, varying in sizes, with the help of Multiphoton Microscopy. In addition, we also demonstrated a rapid facile environment friendly process of synthesizing AuNPs of adjustable sizes with the help of aqueous M. lucida leaf extract. Surface plasmon resonance was performed to confirm the synthesis of AuNPs at 530 nm with the help of UV–vis spectrophotometer. By differentiating the quantities of M. lucida leaf aqueous extracts, we studied the reduction time, morphological differences and size of the AuNPs. By performing Fourier Transformation Infrared Spectroscopy (FTIR), UV–vis spectroscopy, Transmission Electron Microscopy (TEM), Powder X-ray Diffraction (XRD), Energy Dispersive X-ray Spectroscopy (EDAX) and Selected Area Electron Diffraction (SAED), we characterized the fabricated AuNPs. The further aggregation and growth of AuNPs was protected by the polyphenols in the oxidised form by having a coordination with the surface of AuNPs. Moreover, the experiments of skin penetration showed an effort to deeply examine the factors leading to the penetration of particles into the human skin. These responses indicate that NPs at the determined size ranges penetrate the SC in the same pattern of the drug molecules, mostly by the intercellular paths. These responses attained were essential for developing a unique transdermal transporter as well as for understanding the basic interaction of skin–NPs for the application of plastic surgeries.

A novel, rapid, seedless, in situ synthesis method of shape and size controllable gold nanoparticles using phosphates

We hereby report a novel synthesis method of size and shape controllable gold nanoparticles that is rapid, in situ and seedless. Unlike most currently employed size and shape controllable synthesis methods, it takes place in a single step under room temperature within ~15 minutes. While mixtures of gold nanospheres around 70 nm and gold nanoplates with width ranging from 100 nm to 1000 nm can be synthesized in about 15 minutes by standard synthesis method using N-2-hydroxyethylpiperazine-N-2-ethanesulphonic acid (HEPES) to reduce Au(III), gold nanoflowers or mixtures of smaller gold nanospheres and nanoplates can be synthesized with the addition of disodium phosphate (Na2HPO4) or monosodium phosphate (NaH2PO4), respectively. Increasing the concentration of phosphate added significantly reduces the formation time of gold nanoparticles to seconds. By increasing the molar ratio of Na2HPO4: HEPES and NaH2PO4: HEPES, the size of gold nanoflowers and gold nanoparticle mixtures can be tuned from ~60 nm down to 1 nm and from ~70 nm to ~2.5 nm, respectively. The systematic structural changes are accompanied by similarly systematic colour changes associated with shifting of the surface plasmon resonance. The proposed mechanism of the synthesis process is also presented.

Tuning Optical and Granulometric Properties of Gold Nanostructures Synthesized with the Aid of Different Types of Honeys for Microwave-Induced Hyperthermia.

Size-controlled gold nanoparticles (AuNPs) were synthesised with solutions of three types of Polish honeys (lime, multiflower, honeydew) and used in microwave-induced hyperthermia cancer treatment. Optical and structural properties of nanostructures were optimized in reference to measurements made by using UV/Vis absorption spectrophotometry (UV/Vis), transmission electron microscopy (TEM) supported by energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), and attenuated total reflectance Fourier transformation infrared spectroscopy (ATR FT-IR). In addition, concentrations of reducing sugars and polyphenols of honeys applied were determined to reveal the role of these chemical compounds in green synthesis of AuNPs. It was found that the smallest AuNPs (20.6 ± 23.3 nm) were produced using a 20% (w/v) multiflower aqueous honey solution and 25 mg·L−1 of Au(III) ions. These AuNPs were then employed in microwave-induced hyperthermia in a system simulating metastatic tissues. This research illustrated that AuNPs, as produced with the aid of a multiflower honey solution, could be suitably used for microwave-induced heating of cancer. A fluid containing resultant Au nanostructures, as compared to water, revealed facilitated heating and the ability to maintain a temperature of 45 °C required for hyperthermia treatment.

Synthesis of Size Controlled Gold Nanoparticles and Surface Enhanced Raman Spectroscopy (SERS) Effect

Synthesis of Size Controlled Gold Nanoparticles and Surface Enhanced Raman Spectroscopy (SERS) Effect

Construction of size-controllable gold nanoparticles immobilized on polysaccharide nanotubes by in situ one-pot synthesis

A green method for in situ one-pot synthesis of size-controlled gold nanoparticles (AuNPs) was provided in this work. In our findings, a β-1,3-D-glucan, AF1, extracted from black fungus was used as a support of AuNPs. The AuNPs were dispersed in the AF1 aqueous solution without additional reducing and stabilizing or dispersing agents. The infrared spectrum (FT-IR), thermogravimetry (TG), X-ray photoelectron spectra (XPS) and atomic force microscope (AFM) results indicated that the AuNPs synthesized by AF1 were immobilized on the AF1 nanotubes with controllable shape and size of 5–45nm. The hydroxyl groups on AF1 nanotubes not only acted as reducing agents but also were able to stabilize AuNPs by the strong interaction between the surface Au atoms of AuNPs and oxygen atoms of AF1 nanotubes. The AuNPs exhibited a good catalytic activity for the rapid reduction from 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) by excess borohydride, further confirming the dispersion of AuNPs in water.

Monodispersed Size-Controlled Gold Nanoparticles from Electrodeposited Aminoferrocenyl Dendrimer-Templates and Their Application as Efficient Hydrogen Peroxide Electrocatalyst

The main target of this work is the development of efficient hydrogen peroxide non-enzymatic sensors based on size-controlled gold nanoparticles from electrodeposited aminoferrocenyl dendrimer-templates. In this work we study the kinetics and electrochemistry of the modified electrodes as well as the kinetics and the analytical responses of them to both the oxidation and reduction of hydrogen peroxide via different processes related with the absence or presence of gold nanoparticles and their size. The best responses as non-enzymatic sensors were observed at 0.5 V with the largest nanoparticles, obtained from the first generation dendrimer-template, with two linear ranges (0.001–1.2 and 1.2–4.0 mM), sensitivities 249.5 and 185.6 μA/mM cm2 and limit of detection of 0.07 μM; and at −0.2 V with the smallest ones, obtained from the fifth generation, with three linear ranges (0.001–2.0, 2.0–4.0 and 4.0–15.0 mM), sensitivities 172.1, 888.4 and 437.0 μA/mM cm2 respectively, and limit of detection of 0.06 μM. The non-enzymatic peroxide sensors showed an interference-free behavior at −0.2 V.

BSA-caged metal clusters to exfoliate MoS2 nanosheets towards their hybridized functionalization

Herein, we develop a facile exfoliation and in situ functionalization strategy to produce hybridized Au/MoS2 nanostructures comprised of size-controlled gold nanoparticles (Au NPs) and ultrathin MoS2 nanosheets by using bovine serum albumin (BSA)-caged Au25 clusters as both exfoliating and functionalizing agents. As revealed, BSA molecules are strongly adsorbed on MoS2via their hydrophobic interaction, and this drives the expansion of the BSA molecules that initially protect Au25 cores at pH 4, leading to the effective exfoliation of MoS2 nanosheets together with the epitaxial growth of Au25 cores into 5 nm-sized Au NPs on MoS2 nanosheets due to their reduced surface protection. Upon the addition of H2O2, the resulting Au NPs can further grow to achieve a controlled size from 5 to 30 nm with an increase of the reaction time. It is demonstrated that the hybridized Au/MoS2 nanosheets exhibit a better performance in the photocatalytic degradation of substrates compared to the individual components or their mixture. Moreover, the hybridized Ag/MoS2, Au/WO3 and Au/graphene nanosheets are further produced by the usage of BSA-caged Ag and Au clusters, respectively. Overall, this work reports the first utilization of protein-caged metal clusters for the exfoliation and hybridized functionalization of 2D materials, and this brings more opportunities to exploit unusual properties of hybridized 2D materials for novel applications.

Size-controllable gold nanoparticles prepared from immobilized gold-containing ionic liquids on SBA-15

A series of gold nanoparticles (AuNPs) with controllable size were prepared from immobilized gold-containing ionic liquid on SBA-15 (AuCl_me-Im@SBA-15) at room temperature. AuNPs size and shape could be controlled inside porous SBA-15 in the presence of immobilized 1-methyl-3-(3-trimethoxysilyl- propyl)-imidazolium chloride by controlling a concentration and a feed rate of a reducing agent NaBH4. The smallest AuNPs with the average size of 1.6nm was obtained at a fixed feed rate of 0.2ml/min with 4mM NaBH4. The transformation of AuNPs shape from spherical to capsule-like NPs was observed at low feed rate of 0.04ml/min. The prepared AuNPs catalysts were characterized by TEM, XPS, FTIR, and XRD. The catalyst with the smallest NPs, AuNP(1.6nm)_me-Im@SBA-15, exhibited the highest catalytic performance in p-nitrophenol hydrogenation reaction at room temperature. Moreover this catalyst was reusable up to four recycle processes.

Size-controlled gold nanoparticles obtained from electrodeposited amidoferrocenylpoly(propyleneimine) dendrimer-templates for the electrochemical sensing of dopamine

Nanometer-scale gold particles exhibit size-dependent electronic properties with important sensing and biosensing applications. In the same way, a lot of analytes show some type of surface-sensitive reaction and the electrode material has a strong influence on the catalytic activity. In this work we study the kinetics and electrochemistry of electrodes with size controlled gold nanoparticles, obtained by electrodeposited amidoferrocenylpoly(propyleneimine) dendrimers of two generations as templates, and the kinetics and the analytical response to the oxidation of dopamine. We demonstrate that the four-types of modified electrodes show good catalytic responses toward the oxidation of dopamine via different processes in relation with the absence or presence of gold nanoparticles and their size. The best response was obtained with the largest nanoparticles, obtained with the first generation dendrimer-template at 0.3V vs. SCE, with three linear ranges (0–70, 70–600 and 600–1000μM), with sensitivities 585.7; 466.0 and 314.3μA/mMcm2, and limit of detection of 0.01μM. The effect of interfering substances has been studied by differential pulse voltammetry and the developed sensor has been successfully used for the determination of dopamine in a commercial dopamine hydrochloride injection and in spiked Human urine.

Synthesis and optical properties of size-controlled gold nanoparticles

Au nanoparticles (NPs) of various sizes were synthesized by controlling the NP growth process. When the ratio of the HAuCl4 and NaBH4 solution concentrations was controlled, the experimental results showed that the average Au NP size decreased as the HAuCl4 concentration of the seed solution increased, owing to the increase in the number of Au seeds. The activity of the seed solution decreased as the additive volume of the HAuCl4 solution increased, resulting in the formation of large Au seeds. At low seed solution additive volumes, the peak absorbance wavelength and average size of the Au NPs decreased rapidly as the seed solution additive volume increased. The final size of the Au NPs with negatively charged Au seeds was relatively large since the size of the negatively charged Au seeds was relatively large. In addition, it was easier for the negatively charged seed solution to absorb Au atoms in the positively charged growth solution. A high ascorbic acid concentration was beneficial for improving the production of Au NPs. The average size of the Au seeds added to the growth solution increased gradually with the number of NPs grown, which resulted in an increase in the average Au NP size.