RESULTS AND DISCUSSION
The shape and size of silver nanoparticles are possible to control in synthesised solutions by varying the temperature, the molar ratio of used reagents, and other experimental conditions. Ethylene glycol as both solvent and reducer helps to produce an angular particle shape. In the typical polyol synthesis, it is the most popular agent with a good solubility in water. Polyvinylpyrrolidone as a stabilizer helps to avoid the aggregation of nanoparticles and control the shape. It also serves as an additional reductant. Several other critical ingredients in the synthesis are chlorides and oxygen. These components initiate and affect oxidative etching, which results in sharp edges of nanostructures [20, 21].
Our experience in the microwave-assisted synthesis of silver nanoparticles [22] allowed using a simple precursor and reducer – silver nitrate and sodium citrate. The optimal molar ratio of the reagents (10 : 10 mM) was chosen by Faxian’s protocol [23]. Because both salts are soluble in water, a mixture of ethylene glycol and polyvinylpyrrolidone in an aqueous medium was taken. After studying the literature, a strategy way to obtain as angular particles as possible was chosen (Fig. 1b). The presence of O2 from air and Cl– ions in polyol synthesis causes the formation of etched spheres, sharp or truncated cubes, and nanowires [20]. Therefore, avoiding long uniform nanowires, we chose HCl as an etching agent and did not use an environment of nitrogen or other inert gas.
Fig. 1. The strategy of Ag NPs synthesis: resulting mixture of nanoproducts in the presence of air (a), in the presence of air and Cl– ions (b), and in the presence of inert N2 gas (c)
It is also well-known that the morphology and dimensions of the nanoproducts strongly depend on temperature and growth time. The polyol process reduction is carried out at the temperature higher than 150°C for 30 min or more [24]. In our case, the aqueous medium limited the choice of temperature regime. Consequently, the Ag NPs synthesis time was achieved experimentally and provided under microwave irradiation in a boiling aqueous solution (Table 1). To maintain similar refluxing conditions, the temperature regime was broken down into stages, raising the temperature consistently. The length of each step was chosen according to the technical capabilities of the microwave reactor to raise the temperature evenly with a power of 100 W. At higher power, particles were more likely to aggregate, and degradation occurred during synthesis. The 16–20°C primary temperatures were suitable, and using higher primary temperatures, the nanoparticles degraded during the synthesis. We also aimed to shorten the total synthesis time to get as stable products as it was possible.
| Stage | Temperature, °C | Time, min | Power of microwave reactor, W |
|---|---|---|---|
| 1 | 20–60 | 3 | 100 |
| 2 | 60–95 | 6 | 100 |
The growth of nanoparticles was monitored by UV/Vis technique. The UV/Vis absorption peak at 438 nm (Fig. 2a) and the visual exchange in colour from colourless to yellow-green suggest the formation of Ag NPs. After three days keeping at 8–10°C, the exchange in colour from yellow-green to greyish was noticed. The beginning of Ag NPs aggregation was also improved by a specific UV/Vis spectrum (Fig. 2c).
To increase stability, Ag NPs were covered with silica shell (Table 2) by the reported earlier procedure [22]. Ag NPs were not separate from the reaction mixture and were used for the silica coating procedure without any purification. Silica shell was formed over the nanoparticles in the presence of sodium citrate, ethylene glycol, polyvinylpyrrolidone, and HCl residues. HR-TEM images improved (Fig. 3), that 70 ± 5 Ag nanospheres were decorated with 2–10 nm bubble silica shell. The green-brown suspension of synthesised Ag@SiO2 NPs was stable keeping at 8–10°C for three weeks (Table 3). No changes in colour and UV/Vis spectrum were observed (Fig. 2b). After three weeks, a grey precipitate and changes in the UV/Vis spectrum occur.
Fig. 2. UV/Vis spectra of Ag NPs (a), Ag@SiO2 NPs (b), and aggregated nanoparticles (c)
Fig. 3. HR-TEM images of Ag NPs (a) and Ag@SiO2 NPs (b)
| Stage | Temperature, °C | Time, min | Power of microwave reactor, W |
|---|---|---|---|
| 1 | 25–60 | 1.5 | 100 |
| 2 | 60–95 | 6 | 100 |
| Obtained property | Ag NPs | Ag@SiO2 NPs |
|---|---|---|
| Total time of formation under MW | 9 min | 9 min (Ag NPs) + 7.5 min (SiO2) |
| Colour | yellow-green | green-brown |
| Form | nanospheres | specific bubble form |
| Diameter | 70 ± 5 nm | 80 ± 5 nm |
| Thickness of silica shell | – | 2–10 nm |
| UV/Vis absorption peak | 438 nm | 445 nm |
| Stability keeping at 8–10°C | 3 days | 3 weeks |
It is well-known that larger than 50 nm nanospheres with a thin 2–4 nm inert shell allow obtaining more intensive Raman spectra of analytes [25]. Synthesised Ag@SiO2 NPs suited for SHINERS were 80 ± 5 nm in diameter and protected by a 2–10 nm layer of silica.
For further experiments, a self-assembled monolayer of 4-MBA onto smooth gold was formed. The non-informative Raman spectrum of 4-MBA onto gold without nanoparticles was recorded (Fig. 4a). For comparison, the spectrum of stable Ag@SiO2 NPs onto the steel substrate was recorded (Fig. 4b). The nanoparticles amplified contaminants from a nearby environment, but the intensity of the spectrum was very low. An enhanced SHINERS spectrum of 4-MBA was obtained with Ag@SiO2 NPs (Fig. 4c). All assignments of the spectrum were reported earlier [22] and are represented in Table 4.
Fig. 4. Raman spectrum of 4-MBA adsorbed on a smooth Au surface without nanoparticles (a); Raman spectrum of Ag@SiO2 NPs onto steel substrate (b); SHINERS spectrum of 4-MBA on a smooth Au surface with Ag@SiO2 NPs (c)
| Assignments | Δν, cm–1 |
|---|---|
| Ring out-of-plane bending | 525 |
| CCC out-of-plane bending | 714 |
| COO– bending | 845 |
| Ring breathing | 1012 |
| Ring breathing, C-S stretching, C-H symmetric in-plane bending | 1077 |
| C-H bending | 1141 |
| C-H bending | 1185 |
| COO– symmetric stretching | 1384 |
| Ring bending | 1480 |
| Ring C-C stretching, C-H asymmetric in-plane bending | 1587 |
We succeeded in producing silver core-silica decorated nanoparticles (80 ± 5 nm) by rapid (16.5 min) microwave irradiation at atypical low temperatures (95 ± 5°C) for the polyol process. We failed to get angled nanostructures, but synthesised Ag@SiO2 NPs suitable for SHINERS experiments. In addition, stable particles were successfully isolated from the reaction mixture by centrifugation and had low traces of impurities bands in the Raman spectrum. This opens up new possibilities for applying more widely the polyol process to synthesise stable core-shell particles of different shapes and forms.