Since at least one dimension of silver nanocrystals decreases to the range of 1–100 nm, the optical, electronic, catalytic and chemical properties of silver nanoparticles are quite different from bulk metal [1
]. Silver nanoparticles have received considerable attention during the past decades due to their unique properties. For instance, with optical properties of localized surface plasmon resonance (LSPR), silver nanostructures were used as substrates for surface-enhanced Raman scattering (SERS), contrast agents for bioimaging, and biosensors [2
]. The intrinsic properties of a silver nanoparticle are mainly depended on particles size, shape, composition, crystallinity, and structure, among which size and shape are more significant [3
]. Theoretically, the properties of silver nanoparticle can be regulated by controlling one of these parameters [4
]. For example, the high enhancement effect is achieved with hundreds of nanometer-sized particles in SERS [5
]; while, the optimal size of silver nanoparticles is about 50 nm for nonresonant SERS [6
]. Therefore, achieving well-controlled size and shape is a fundamental requirement for the synthesis of silver nanoparticles with desirable properties.
In spite of the importance of controlled synthesis, the synthesis of silver nanoparticles with controllable size is a challenge. Varieties of methods have been reported to prepare size-controlled silver nanostructures. The classical methods can be classified into two categories: seed-mediated synthesis and photochemical synthesis. Wan et al. synthesized quasi-spherical silver nanoparticles with tunable size by following a controlled seed-mediated growth approach via thermal reduction of silver nitrate with citrate [7
]. Moreover, Bastús et al. synthesized silver nanoparticles by using a combination of two chemical reducing agents, sodium citrate and tannic acid [8
]. The size of silver nanoparticles was controlled via adjusting reaction temperature, pH, and the concentration of reducing agents and seed. As a result, citrate-coated spherical silver nanoparticles with controllable size from 10 to 200 nm were obtained. However, it is complicated to achieve large-sized silver nanoparticles in most similar methods, because stepwise seeding growth is needed. The size of silver nanocrystals prepared via photochemical pathway was mainly affected by the irradiating time, intensity and wavelength [9
]. However, the wavelength of light source available in the laboratory is limited, limiting the synthesis of silver nanocrystals with controlled size in a wide range, and silver nanostructure shape can change during the irradiation process [11
]. In addition, template-based method is also commonly used to synthesize size-controlled silver nanowires or nanorods by the steric and structure-directing effect. The templates are mesoporous materials, carbon nanotubes, polymer materials, and micelles [14
]. However, the structures of templates, especially for polymer and biological templates, are highly sensitive to the surroundings, and impurities may be introduced during the process of removing templates.
Another rapidly developing research filed in recent years is droplet microreactors based on microfluidic technology. Droplets can precisely control entire nanoparticles preparation processes. Moreover, the micro-scale size of droplets facilitates the achievement of rapid heat and mass-transfer rates. Droplet microreactors have been proved a powerful tool for biological and chemical reactions [15
]. For example, colloidal nanoparticles were synthesized via the droplet microreactors [17
]. Compared with the conventional synthesis in large reaction vessel, droplet microreactors possess the following advantages [18
]. First, the consumption of reagents is small during the process of optimizing reduction condition, especially, for expensive or toxic reagents. Second, the enhanced mass and heat transfers due to the large specific surface area, contribute a better control of reaction concentration and temperature, which govern particle size and size distribution. Third, scaling up the production of nanoparticles can be achieved just by extending the duration of the synthesis, without adjusting other experimental parameters, which not scale linearly with the volume of the reaction solution [20
]. Moreover, droplet microreactors can eliminate concentration dispersion in single phase microreactors and avoid the fouling by reagents or nanocrystal contacting with the inner of channel. These features make droplet microreactors uniquely suitable for synthesizing size-controlled nanomaterials.
There have been many reports about the synthesis of metal nanoparticles in droplet-based microreactors. Additionally, Pa, Pd, Au, and Ag nanocrystals with controlled sizes and shapes were synthesized in droplet-based microreactor by the group of Xia [21
]. Some researchers investigated the controlled synthesis of silver nanoparticles in microdroplet reactors. Xia et al. synthesized silver nanocubes with controlled sizes in droplet microreactors for the first time [24
]. The edge lengths of silver nanocubes can be tuned in the range of 30 to 100 nm by changing the reaction time, the amount of silver nitrate, and the amount of silver seeds in the droplets. To prepare silver nanoparticles with controlled size, we constructed microdroplet reactors. Herein, a method of seed-mediated growth was developed, and silver nitrate was reduced by sodium citrate on the surface of silver seeds. The size of obtained silver nanoparticles and the color of the sol could be easily changed by varying the reaction time, heating temperature, and concentration of silver seeds and silver precursors. For instance, nanoparticles size increased and LSPR wavelength bathochromic shifted when the temperature increased (40–80 °C). The LSPR wavelength bathochromic shifted with the increase of droplets heating time or concentration of silver nitrate. This work provided a novel way for synthesizing nanomaterials with high quality.
2. Experimental Section
All the used chemicals were analytical grade and used as received from their corresponding suppliers, unless otherwise noted. Silver nitrate (AgNO3, ≥99.8%) and sodium borohydride (NaBH4, 96.0%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Trisodium citrate dihydrate (Na3C6H5O7·2H2O, 99.0%) and liquid paraffin were obtained from Tianjin Fengchuan Chemical Reagent Co., Ltd. (Tianjin, China). Sorbitanmonooleate (Span 80) was provided by Tianjin Kemiou Chemical Reagent Co., Ltd. (Tianjin, China). Sylgard 184 silicone elastomer kits including polydimethylsiloxane (PDMS) and a crosslinking agent were purchased from Dow Corning Co. (San Diego, CA, USA). Ultrapure deionized water with resistance of 18 MΩ·cm (filtration system: KL-MINI4-T, Kertone Water Treatment Co., Ltd., Kertone, UK) was used for all experiments.
2.2. Silver Seed Synthesis
The silver seed were prepared by using the method reported by Jia et al. In brief, nearly spherical silver nanoparticle seeds (~4 nm on average) were synthesized by drop-wise adding 0.5 mL freshly prepared sodium borohydride solution (50 mM) to a mixture of aqueous silver nitrate (50 mL of 0.1 mM) and TSC (0.5 mL of 30 mM) solution under vigorous stirring.
2.3. Microfluidic Device Fabrication
Photolithographic technique was used to fabricate a SU-8 photoresist mold on a silicon wafer (Delta 80, Suss MicroTec, Germany). A mixture of PDMS oligomer and crosslinking agent (10:1, w/w) was cast onto the mold to form a replication, then it was degassed in a vacuum chamber for 30 min and heated in an oven for 2 h at 60 °C for the polymer to cure. After curing, a PDMS replication was removed from the mold and was drilled at the channel inlets. The patterned surface of PDMS replication was treated with corona discharge for 1 min, and then directly bonded to a 2-mm-thick blank PDMS plate. The bonded device was then baked at 60 °C for at least 2 h in an oven.
2.4. Seed-Mediated Growth of AgNPs in Microdroplet Reactors
A flow focusing microchip (Figure 1
A) was used to produce uniform microdroplets containing silver seeds and growth solution at first. The width and height of the microchannel were 100 μm and 100 μm, respectively. Oil phase (continuous phase) and aqueous phase (dispersed phase) were delivered by two syringe pumps (Harvard, PHD 2000) into the microchip to produce microdroplets. In a standard synthesis, a mixture solution of AgNO3
(5.0 mM, 3.6 mL), TSC (30.0 mM, 6.0 mL), silver seeds (1.0 mL) and pure water (7.4 mL) serving as aqueous phase was introduced into I
1 at a flow velocity of 14 μL/min, while liquid paraffin with 1.5% (w/w) Span 80 used as oil phase was introduced into I
2 at a flow velocity of 80 μL/min. Polytetrafluoroethylene (PTFE) tubes with an inner diameter of 0.5 mm were used for connecting the syringes to the microchip inlets, and for transferring the product microdroplets from the microchip outlet to a 50 mL glass vial to collect microdroplets.
The collected microdroplets containing reactants were then heated at 60 °C for a period of time to ensure silver seed grow up within microdroplets. During the heating process, aliquots of microdroplets were taken out from glass vial and were centrifuged at 8000 r/min for 10 min to separate aqueous phase from microdroplets, and then at 15,000 r/min for 10 min to precipitate Ag particles from aqueous phase. The as-obtained Ag particles were washed with ethanol once and twice with deionized water before dispersed in deionized water by ultrasonication.
The optical properties of the obtained silver colloidal solutions were characterized by using UV-Vis spectrometer (UV-2450, SHIMADZU) with a resolution of 0.5 nm.