Effects of Caramelization Products on Synthesis of Silver Nanoparticles by Using Silver-Containing Glass Powder and Various Reducing Sugars in an Aqueous System

Abstract

Silver-containing glass powder can be used as a sustainable source of silver ions in inhomogeneous synthetic systems of strictly size-controlled silver nanoparticles using an aqueous solution of reducing sugars. The caramelization products derived from sugars in this synthetic system contained promoters of silver ion release from silver-containing glass powder and suppressers of nanoparticle growth. In this study, the effects of caramelization products on the size control of nanoparticles were investigated using four types of reducing sugars: glucose, fructose, maltose, and lactose. It was found that acidic components of caramelization products were corresponding to promoters of silver ion release. The acidity of the product suspensions increased with increasing initial concentrations of each sugar. For monosaccharides, glucose and fructose, nanoparticle sizes significantly increased (3.5–50 nm) with the acidity of the product suspensions. In contrast, for disaccharides, maltose and lactose, the nanoparticle sizes slightly increased (4.1–7.1 nm) with acidity. This difference arose from a balance of the effects of both silver ion release promoters and nanoparticle growth suppressers: the monosaccharide systems were dominated by the effect of silver ion release promoters to promote particle growth, whereas the disaccharide systems were regulated by the relationship between the effects of both components.

1. Introduction

Controlling the size and shape of metal nanoparticles is essential for the optimization of their characteristic properties for applications in areas such as optics,^1,2) chemical catalysis,³⁾ biosensing,⁴⁾ and antimicrobial activities.^5–7) For instance, in antiviral applications, the diameter of spherical silver nanoparticles (Ag NPs) affects their antiviral effects against HIV-1,⁸⁾ H1N1 influenza virus,⁹⁾ SARS-CoV-2,¹⁰⁾ and other viruses.⁵⁾ More precisely, Ag NPs with diameters in the range of 2–15 nm are effective against extracellular SARS-CoV-2.¹⁰⁾ Particularly for biomedical and hygiene applications, Ag NPs synthesized by environmentally friendly processes, in which harmful materials are not used and produced, are preferable.^11,12) However, strict size control and environmentally friendly processes are usually in a trade-off relationship with the synthesis of Ag NPs.^12,13)

In our previous study, a simple and environmentally friendly synthesis method for size-controlled spherical Ag NPs was developed by the reaction of silver-containing glass powder (Ag-GP) as a source of Ag⁺ and glucose as a reducing agent in aqueous media at 121°C.¹⁴⁾ To avoid excessive growth and aggregation by decreasing their surface energy, it was not necessary to add a stabilizing agent because caramel was generated from glucose during the synthetic process, which acts as the stabilizing agent. We found a simple rule between the fed glucose concentration and the particle size of the generated Ag NPs, whose diameters proportionally increased with the square root of the fed glucose concentration in the range of 2.5–80 g/L (corresponding to an average diameter of 3.5 to 20.0 nm). This synthetic system was considered to be inhomogeneous; the particle size of Ag NPs was regulated by the release, reduction, and diffusion rate of Ag⁺ near the surfaces of the Ag-GP. However, the precise mechanism of generation and growth of Ag NPs in this synthetic system has not been well investigated, although this system has great potential for synthesizing size-controlled metal nanoparticles, not only silver, but also other elements.

In this study, four types of reducing sugars, namely, glucose, fructose, maltose, and lactose, were used in the Ag NPs synthetic system with Ag-GP to investigate the effects of caramelization products (CPs) on the inhomogeneous reaction system. Fructose is a ketose monosaccharide, unlike glucose, which is an aldose monosaccharide, and it has been reported less reduction activity for Ag⁺ than for glucose in a homogeneous system.¹⁵⁾ Both maltose and lactose are disaccharides composed of reducing glucose units. The caramelization rate of these disaccharides, evaluated by spectrophotometry, has been reported to be similar to that of glucose in an aqueous solution (pH 6.0 aqueous solution),¹⁶⁾ whereas their molecular weight was almost two times greater than that of glucose. Compared with glucose, maltose and lactose were expected to generate more CPs per reduction group. Nevertheless, the effects of the generated CPs on the synthesis of Ag NPs should be investigated carefully because caramelization is a complicated oxidative reaction of sugars to produce miscellaneous compounds by various reaction routes, including degradation and polymerization.^17,18)

In this work, the synthesis of Ag NPs was conducted using a method similar to that utilized in the previous work for glucose,¹⁴⁾ that is, autoclaving a mixture of Ag-GP and an aqueous solution of reducing sugar at 121°C. The product suspensions were characterized by UV-visible spectroscopy, pH measurements, and transmission electron microscopy (TEM).

2. Experimental Procedure

2.1 Materials

Ag-GP (BSP21, silver content: 1 mass% (93 µmol/g), average grain size: 10 µm) was obtained from Kankyo Science Co., Ltd. (Kyoto, Japan). D(+)-Glucose (>98.0%), D(−)-fructose (>99.0%), maltose monohydrate (Wako 1st grade), lactose monohydrate (Wako special grade), and 2N sulfuric acid aqueous solution (volumetric analysis grade) were purchased from Wako Pure Chemical Industries, Ltd. (Osaka, Japan). Distilled water was used as the medium in all the experiments in this study. All materials were used as received without further purification.

2.2 Synthesis and characterization of Ag NPs

Ag NPs were synthesized using a method similar to that described previously.¹⁴⁾ For example, 0.50 g of Ag-GP was dispersed in 50 mL of an aqueous solution of reducing sugar (10 g/L) in a 100 mL glass vial. The mass concentration of Ag-GP was fixed at 10 g/L. The range of mass concentrations of reducing sugar was 2.5–80 g/L, which contained an excess amount of reducing groups relative to the silver content of the glass powder; the molar ratio was not less than 10. The mixture was heated in an autoclave at 121°C and 200 kPa for 5 or 20 min to obtain a suspension of Ag NPs. After the reaction, the mixture was cooled gradually to room temperature. To remove the insoluble residue, the mixture was centrifuged at 3,000 rpm for 10 min. The experimental procedure to remove Ag NPs from the suspensions by using chitosan was described in another literature.⁹⁾

The UV-visible spectra of the Ag NP suspensions were measured using a JASCO V-660 spectrometer with a polystyrene cuvette (cell length = 1 cm) at room temperature. All suspensions were diluted 10-fold with distilled water before measuring their UV-visible spectra. The pH of the suspensions was measured at room temperature using a HORIBA LAQUAtwin B-712 compact pH meter.

2.3 Transmission electron microscopy observation and particle size analysis

TEM specimens of Ag NPs were prepared by casting a drop of a 5 µL Ag NP suspension onto a carbon-coated Cu microgrid. The excess suspension was removed using a filter paper, and the specimens were then gradually dried at room temperature. TEM observations were performed using a JEOL JEM-1010 microscope at 80 kV.

The particle sizes of the Ag NPs were analyzed from the TEM images using the image-processing software ImageJ (version 1.49v). The average particle size and standard deviation of the specimens were calculated for more than 100 particles. The relationship between the particle size of Ag NPs and the concentration of glucose was obtained from a previous report.¹⁴⁾

2.4 An acid–base titration for the Ag-GP

An aqueous suspension of Ag-GP (1.0 g/L) was titrated with 0.10 N sulfuric acid aqueous solution at 20°C. The pH of the suspensions was monitored using a HORIBA LAQUA F-74 pH meter with a 9615S-10D electrode.

3. Results and Discussions

3.1 Synthesis of Ag NPs using Ag-GP and various reducing sugars

3.1.1 Glucose

The Ag NP synthetic system using Ag-GP and glucose has been investigated previously.^9,14) The peak absorbance at approximately 400 nm, corresponding to the surface plasmon absorbance of spherical Ag NPs, increased with the concentration of glucose. In a previous study, it was found that Ag NPs in suspension could be removed using chitosan.⁹⁾ After the removal of Ag NPs from the suspension, the absorbance derived from CPs was observed (Fig. 1). The UV-visible spectra of CPs were monotonically decreased with wavelengths in the range of 300–600 nm. By comparing the UV-visible spectra before and after the removal of Ag NPs, it was found that caramel generation in the suspension could be roughly estimated from the absorbance at 300 nm, which was a measurement limit of a polystyrene cuvette, even before the removal of Ag NPs. Regarding the absorbance at 300 nm, caramel generation appeared to be enhanced with increasing concentrations of glucose. However, it should be noted that a quantitative evaluation of CPs from UV-visible spectra is difficult because CPs consist of miscellaneous compounds, including both colored and non-colored, and vary according to the synthetic conditions such as pH, water activity, temperature, and types of original sugars.^16,19–21)

Fig. 1

UV-visible spectra of caramel solutions derived from Ag NPs suspensions synthesized with 2.5 (gray) and 10 g/L (black) of glucose at 121°C for 20 min. Solid line: caramel solutions, dotted line: original Ag NPs suspensions.

3.1.2 Maltose and lactose

Before autoclaving, the mixtures of Ag-GP and solutions of maltose or lactose were turbid suspensions. After autoclaving for 20 min, the color of the mixtures turned dark yellow or brown, and the strength of the color corresponded to the sugar concentration. All autoclaved mixtures had a caramel-like sweet smell.

Figure 2 shows the UV-visible spectra of Ag NP suspensions synthesized with various concentrations of maltose (a) or lactose (b). The peaks in the range of 400–410 nm correspond to the surface plasmon absorbance of the spherical Ag NPs. Considering the absorbance at 300 nm, the generation of colored CPs also increases with the concentration of sugars, as in the glucose system.⁷⁾ Thus, the stabilizing agents for Ag NPs were considered to be some CPs, as in previous works.^9,14)

Fig. 2

UV-visible spectra of Ag NP suspensions synthesized with various concentrations (g/L) of maltose (a) or lactose (b) at 121°C for 20 min.

3.1.3 Fructose

As with synthetic systems using maltose or lactose, the mixtures of fructose systems before autoclaving were turbid suspensions. The reaction time for the fructose system was either 5 or 20 min. Regarding the autoclaved mixture after both reaction times, the caramel-like sweet smell was also observed to be similar to that of the other reducing sugar systems. The effect of fructose concentration on the color strength of suspensions after autoclaving had a reverse trend compared to that of the other reducing sugar systems; the generation of both Ag NPs and colored CPs decreased with the concentration of fructose. Additionally, gray precipitation, which was not observed in the other reducing sugar systems, was generated with an increase in the fructose concentration. The gray precipitate corresponded to bulk silver and could be removed by centrifugation.

Figure 3 shows the UV-visible spectra of the Ag NP suspensions synthesized with various concentrations of fructose for 5 min. A decrease in the peak strength around 410 nm with fructose concentration indicated a decrease in nanosized silver, corresponding to the generation of bulk silver. A peak shift toward longer wavelengths with increasing fructose concentration suggests size enlargement of the generated nanosized silver.²⁾ These results imply a lack of stabilizing agents for the generation of Ag NPs in the higher fructose concentration systems. Figure 4 shows a comparison between the reaction times of 5 and 20 min for 20 (a) and 50 (b) g/L fructose. At 20 g/L fructose, a larger amount of colored CPs was observed in the spectrum at 20 min than in that at 5 min. The peak intensity of Ag NPs from the baseline at 20 min was similar to that at 5 min. The comparison of these spectra suggested that the generation of Ag NPs was almost complete at 5 min, although the generation of colored CP continued during a period of 5 to 20 min. This trend was observed in all the suspensions that reacted with fructose in the concentration range of 5–50 g/L. At 50 g/L fructose, the generation of colored CPs after the completion of the bulk silver generation was observed more clearly.

Fig. 3

UV-visible spectra of Ag NPs suspensions synthesized with various concentrations (g/L) of fructose at 121°C for 5 min.

Fig. 4

Time evolution of UV-visible spectra of Ag NP suspensions synthesized with 20 (a) and 50 (b) g/L of fructose at 121°C.

3.2 Transmission electron microscopy observation and particle size analysis

Figure 5 shows typical TEM micrographs of Ag NPs synthesized with maltose, lactose, and fructose. For both the maltose and lactose systems at 20 min, spherical Ag NPs were observed in a range of 5–60 g/L of disaccharides. Unlike the glucose system, in which an increase in particle sizes of Ag NPs with increasing concentrations of glucose was observed from TEM micrographs,¹⁴⁾ the particle sizes of Ag NPs synthesized with maltose or lactose seemed almost visually unaffected by the concentration of reducing sugars. Conversely, for the fructose system at 5 min, the effect of fructose concentration on particle sizes of the generated Ag NPs was clearly observed.

Fig. 5

TEM micrographs of Ag NPs synthesized in various conditions at 121°C: (a) 10 g/L of maltose for 20 min, (b) 60 g/L of maltose for 20 min, (c) 10 g/L of lactose for 20 min, (d) 60 g/L of lactose for 20 min, (e) 10 g/L of fructose for 5 min, (f) 30 g/L of fructose for 5 min.

To consider the time dependence of Ag NPs synthesized with fructose, particle size histograms of Ag NPs generated with 20 g/L fructose at 5 and 20 min are displayed in Fig. 6. The samples were independently synthesized at each time point. At 5 min, the size distribution of the Ag NPs was unimodal, with a peak in the range of 15–20 nm. At 20 min, a new smaller component of less than 10 nm was generated, whereas the unique peak at 5 min (in the range of 15–20 nm) remained. Regarding the slight increase in the surface plasmon absorption of Ag NPs between 5 and 20 min (Fig. 4(a)), it was suggested that the available silver content in the Ag-GP was almost exhausted during the period of 5–20 min.

Fig. 6

Time dependence of size distributions of Ag NPs synthesized with 20 g/L of fructose at 121°C.

In a previous study on glucose, it was found that the particle sizes of generated Ag NPs were unimodal and independent of the reaction time while stable sustained-release of Ag⁺ from Ag-GP was provided.¹⁴⁾ It should be noted that Ag NPs are not gradually grown during a reaction time in the Ag-GP system. In other words, Ag NPs were continuously generated and grown near the surface of Ag-GP and then dispersed to an area with a negligible concentration of Ag⁺, where the growth of Ag NPs no longer occurred. Thus, a steady-state approximation between stable Ag⁺ supply and generation of well-size-controlled Ag NPs can be hypothesized for the glucose system. To investigate Ag NPs generation near the surface of Ag-GP experimentally, centrifuged precipitation of Ag-GP after the reaction was observed with TEM although real-time microscopic observation of reacting Ag-GP was difficult. Figure 7 shows TEM micrographs of centrifuged precipitation of Ag-GP reacted with 40 g/L of glucose for 20 min at 121°C and a size distribution of Ag NPs observed near the surface of Ag NPs. A gray mottled pattern, which would be containing CPs and dissolved Ag-GPs, was observed at a Ag NPs generating area near the surface of Ag-GP. The average size of Ag NPs in the area was 3.2 ± 1.5 nm, which was much smaller than that in a supernatant (12.9 ± 2.5 nm). As a stable generation of Ag NPs with 40 g/L of glucose was continued for 20 min, Ag NPs near the surface of Ag-GP were growing ones. When Ag⁺ supply was nearly exhausted, smaller particles were dispersed into a supernatant, and size control of Ag NPs was no longer enable.¹⁴⁾

Fig. 7

(a) A TEM micrograph of centrifuged precipitation of Ag-GP after a reaction with 40 g/L of glucose for 20 min at 121°C, (b) a magnified image of the Ag NPs generating area, and (c) a size distribution of Ag NPs at the generating area.

According to the fructose system in this study, stable generation of Ag NPs was continued for at least 5 min. The size distributions of Ag NPs synthesized with fructose, maltose, or lactose were also displayed in Fig. 8. The size distributions of Ag NPs synthesized with maltose or lactose in the range of 5–60 g/L for 20 min were unimodal, indicating a stable generation of Ag NPs. According to the fructose system for 5 min, a bimodal trend was still observed at 30 g/L or more although unimodal size distributions were observed at 20 g/L or less. Consequently, a steady-state approximation between stable Ag⁺ supply and generation of well-size-controlled Ag NPs can be also hypothesized for the fructose, maltose, and lactose systems (except for 30 and 40 g/L of fructose), as well as the glucose system.

Fig. 8

Size distributions of Ag NPs synthesized at 121°C. Reaction time: Fructose (5 min), maltose and lactose (20 min). Fructose (20 g/L) was displayed again.

In addition, the generation rate of the fructose system was higher than that of the glucose system because exhaustion was not observed in the 20 g/L glucose system at 20 min.¹⁴⁾ These results appear to contradict the lower reduction activity of fructose for Ag⁺ than that of glucose in a homogeneous system.¹⁵⁾

Figure 9 shows relationships between sugar concentrations and average particle sizes analyzed from TEM images. As the particle size of Ag NPs generated in a period of exhaustion of silver content in the glass powder was not focused in this work, the particle size analysis of Ag NPs generated from the fructose reaction systems was carried out with the products at 5 min. A numerical list of average particle sizes and standard deviation is summarized in Table 1. The data of the glucose series were obtained from another report.¹⁴⁾ As in the previously studied glucose series,¹⁴⁾ the average particle sizes of Ag NPs generated with the other reducing sugars investigated in this work also increased with sugar concentrations. For both the maltose and lactose series, the increases in average particle size with increasing concentrations of reducing sugar were smaller than those with the glucose series. Conversely, for the fructose series, a drastic increase in the average particle size of Ag NPs was observed in the range of 5–30 g/L. Although the average particle size at 40 g/L decreased compared with that at 30 g/L, both an increase in bulk silver generation and a decrease in the surface plasmon absorption of the centrifuged supernatants (shown in Fig. 3) suggested that the increasing trend in the size of generated particles with increasing fructose concentration still continued after 30 g/L.

Fig. 9

Relationships between the average particle sizes of Ag NPs and concentration of sugar used for synthesis. Data of the glucose series were obtained from a previous study.¹⁴⁾ Reaction time: 5 min (fructose), 10 min (marked by *), and 20 min (the others).

Table 1 Average particle sizes and standard deviations of the Ag NPs.

3.3 Relationship between pH of the product suspensions and average particle sizes of Ag NPs

To investigate the effect of the pH of the reaction systems on the particle size of the generated Ag NPs, the pH of the product suspensions after autoclaving was measured. Although in-situ measurement of pH during autoclaving is difficult, pH values of the product suspensions are important to investigate the mechanism of Ag NP generation using Ag-GP and reducing sugars. The relationship between the pH of the product suspensions and the concentration of each reducing sugar is shown in Fig. 10. At the same mass concentration of sugars, the acidity of the suspensions increased in the order of fructose > glucose > maltose > lactose. At 20 g/L or more, the pH values of the product suspensions of the fructose series decreased more than those of the other reducing sugars; the pH values of the fructose series were less than 5.0, whereas those of the other reducing sugar series were saturated at approximately 6.0.

Fig. 10

Relationships between pH of the product suspensions and concentration of sugar used for synthesis. Reaction time: 5 min (fructose), 10 min (marked by *), and 20 min (the others).

The Ag-GP used in this work is made from SiO₂, B₂O₃, Na₂O, and Ag₂O and can gradually release Ag⁺ into aqueous surroundings by dissolving gradually.^22,23) The values of the base equivalent per unit weight (n_b), pH at the neutralization point, and solubility product constant (K_sp) of Ag-GP as a poorly soluble basic compound were evaluated by acid–base titration (Table 2). For acid–base considerations, Ag-GP was simplistically assumed as a monovalent compound because the titration curve was single stepping in a pH range of 8.80–2.71, which covered the Ag NP synthetic systems in this work. Thus, the chemical equation for the dissolution of Ag-GP and the solubility product constant can be described as follows:   

\begin{equation} \text{BOH} \rightleftarrows \text{B$^{+}$} + \text{OH$^{-}$} \end{equation} (1)

  

\begin{equation} K_{sp} = [\text{B$^{+}$}] [\text{OH$^{-}$}] \end{equation} (2)

where BOH and B⁺ are model compounds of the basic components of Ag-GP and its cations, including AgOH and Ag⁺, respectively.

Table 2 Acid–base characteristics of the Ag-GP.

According to these equations, H⁺ promotes the dissolution of Ag-GP and the release of Ag⁺. In the synthesis of Ag NPs with Ag-GP and reducing sugars, oxidation of reducing sugars by Ag⁺ produces a stoichiometric amount of sugar acids (eq. (3)).   

\begin{align} &\text{R} - \text{CHO} + \text{2Ag$^{+}$} + \text{OH$^{-}$}\\ &\quad \to \text{R} - \text{COO$^{-}$} + \text{2H$^{+}$} + \text{2Ag} \end{align} (3)

However, the produced amount of H⁺ by the reduction of Ag⁺ was insufficient for the sustainable release of Ag⁺ because the molar ratio of Ag⁺ to basic cations in Ag-GP was 0.020, which was calculated from n_b and the Ag content of Ag-GP (93 µmol/g). The production of acid during the reaction was considered to be mainly due to caramelization of the sugars. The higher acidity of the fructose systems was consistent with that in a previous research by de Wilt and Kuster, who reported that the acidity of the products generated by the oxidation of fructose was higher than that of glucose.²⁴⁾ It should be noted that the caramelization of sugars did not proceed without Ag-GP in aqueous media at 121°C; therefore, the basicity of Ag-GP was necessary to promote the caramelization.^20,21)

Figure 11 shows the relationship between the pH of the product suspensions derived from each reducing sugar and the average particle size of the Ag NPs in the suspensions. There are two trends in the particle sizes: one is an exponential-like increase with the acidity of the suspensions (monosaccharides: glucose and fructose), and the other is almost constant (disaccharides: maltose and lactose).

Fig. 11

Relationships between average particle sizes of Ag NPs and pH of the product suspensions. Dashed line: fitting curve for the monosaccharide series, dotted line: fitting curve for the disaccharide series. Reaction time: 5 min (fructose), 10 min (marked by *), and 20 min (the others).

From experimental results of size control of Ag NPs in both the previous study on the glucose system¹⁴⁾ and this study on the other sugars, a steady-state approximation between stable Ag⁺ supply and generation of well-size-controlled Ag NPs can be hypothesized near the surface of Ag-GP. Although La Mer’s theory applies to a homogeneous system,^25,26) an extended theory in a steady-state condition was also proposed by Vreeland et al.²⁷⁾ According to these theories, the particle size is regulated by the relationship between the initial number of particle nuclei and the subsequent monomer supply to growing particles; a greater initial number of nuclei produces smaller particles, and a greater monomer supply to growing particles produces larger particles. Regarding the basic characteristics of Ag-GP, the release of Ag⁺ is accelerated by an increase in acidity. It was also suggested that the acidic components of CPs rarely acted as stabilizing agents for Ag NPs, according to fructose systems. For monosaccharide systems, the monomer supply to the growing particles was considered to be superior to the initial nuclei generation because the particle size increased with acidity. The earlier exhaustion of silver content in the fructose systems compared to that in the other sugars was also consistent with the above discussion. The acidity-regulating consumption of Ag⁺ indicates that the rate of Ag⁺ reduction was faster than the release rate of Ag⁺ from Ag-GP. Consequently, the rate-determining step of the entire reaction in this inhomogeneous system was found to be the dissolution of Ag-GP or the generation of acidic CPs.

3.4 Discussion of the effects of CPs on both initial nuclei generation and monomer supply in disaccharide systems

For the disaccharide systems, the variation in average particle size with acidity was less than that of the monosaccharide series in the pH range of 6.1–8.3. The release rate of Ag⁺ from Ag-GP was considered to be similar between the disaccharides and monosaccharides at a similar pH. Thus, the differences in the CPs derived from each sugar should be focused on to discuss the effects on both initial nuclei generation and monomer supply to nuclei; higher nuclei generation and/or lower monomer supply than those of the monosaccharide systems would suppress particle growth.

According to the classical theory of nucleation, the critical radius r_c, the smallest radius to be crystallized, can be written as follows:²⁸⁾   

\begin{equation} r_{c} = \frac{2\gamma V_{m}}{RT\ln S} \end{equation} (4)

where γ, V_m, R, T, and S are the surface free energy per unit area, molar volume of the monomer, gas constant, temperature, and degree of monomer supersaturation, respectively. The values of V_m and T were the same for each disaccharide and monosaccharide system, respectively. The value of S, which corresponds to the release rate of Ag⁺ from Ag-GP, was considered to be similar between the disaccharides and monosaccharides at similar pH. Unlike the above parameters, the value of γ at nuclei generation surfaces varied with both types and amounts of CPs, which acted as stabilizing agents for Ag NPs.

There are two limiting factors for the growth of nanoparticles: the diffusion of monomers to the surface of the particle, and the reaction rate of the monomers on the surface. The diffusion-limiting model was considered reasonable for this steady-state system because the size distributions of Ag NPs were unimodal and rather narrow for all the synthetic systems in this work, except for higher fructose concentration systems, in which bulk silver was partially generated.²⁹⁾ The growth rates of the particles for the diffusion-limiting model according to the classical theory are as follows:²⁹⁾   

\begin{equation} \frac{dr}{dt} = \frac{DV_{m}}{r}([M]_{b} - [M]_{r}) \end{equation} (5)

where r, D, [M]_b, and [M]_r are the particle size, diffusion constant, monomer concentration in the bulk solution, and monomer solubility of the particle, respectively. The value of [M]_b was considered to be similar between the disaccharides and monosaccharides at similar pH values, as well as the value of S. The value of [M]_r was also considered similar because there was no factor affecting the solubility of the silver atoms. Meanwhile, the value of D would vary with both the types and amounts of CPs, corresponding to the viscosity of the media surrounding the growing Ag NPs.

Based on the above considerations, two causes were expected for the difference in particle sizes between monosaccharides and disaccharides: (1) the surfactant activity of the CPs derived from the disaccharides for the initially generated nuclei was higher than that derived from the monosaccharides, and (2) the viscosity of the CPs derived from the disaccharides was higher than that derived from the monosaccharides. Wang and Hartel previously reported the contact angles with water and viscosity of caramels derived from corn syrups with various dextrose equivalents (DEs), which indicated an inversely proportional relationship with the degree of polymerization.³⁰⁾ The literature showed that the viscosity of caramel significantly decreased with the DEs, whereas contact angles with water were not necessarily affected by the DE. Thus, polymeric CPs with higher viscosities were estimated from disaccharides rather than from monosaccharides. Nevertheless, it was difficult to confirm the viscosity and/or the surfactant activity near the surface of Ag-GP, where nuclei generation and growth were proceeded, from the experimental results in this study. Microscopic investigations of CPs near the surface of Ag-GP were required to understand their effects on the generation of Ag NPs in the future. The entire reaction process of the Ag NP synthetic system with Ag-GP and reducing sugars is schematically summarized in Fig. 12.

Fig. 12

An entire reaction scheme of the Ag NP synthetic system with Ag-GP and reducing sugars.

4. Conclusions

In this study, the effects of CPs on the size control of Ag NPs synthesized in an inhomogeneous aqueous reaction system using Ag-GP and four reducing sugars (glucose, fructose, maltose, and lactose) were investigated. CPs derived from the sugars played two essential roles in the Ag NP synthetic systems: (1) an Ag⁺ release promoter from the Ag-GP by acidic components, and (2) a nuclei growth suppressor (also acting as a stabilizing agent for nuclei and Ag NPs). The effect of the concentration of fructose on the particle sizes of the generated Ag NPs was stronger than that of glucose. The generation of bulk silver was also observed by increasing the fructose concentration, unlike in the glucose system. The acidity of the product suspensions of the fructose system was higher than that of other reducing sugars. The higher acidity of the fructose systems promoted an excess release of Ag⁺ from Ag-GP to generate larger Ag NPs and bulk Ag. The particle size of the Ag NPs increased with the acidity of the product suspensions of both the glucose and fructose systems. Regarding monosaccharide systems, the monomer supply to promote particle growth was superior to the initial nuclei generation to suppress particle growth. The disaccharides, maltose and lactose, showed similar results throughout the study in a range of 5–60 g/L of disaccharides. The effect of the disaccharide concentration on the particle size of the generated Ag NPs was weaker than that of the monosaccharides, although the pH values of the product suspensions at the same mass concentration were similar. The effect of the acidity of the product suspensions of the disaccharide systems on the particle sizes of the Ag NPs was less than that of the monosaccharide systems. Comparing the glucose and disaccharide systems, it was considered that suppression of particle growth was promoted by some components of CPs. Consequently, the acidic components of the CPs acted to increase the particle sizes of the generated Ag NPs, and some components acted adversely in the Ag NPs synthesis system using Ag-GP and reducing sugars.

Acknowledgments

I gratefully acknowledge Mr. Toyohiko Kuno, president of Kankyo Science Co., Ltd., for providing the Ag-GP. This study was supported by JSPS KAKENHI Grant Number 15K17892.

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