Seed-mediated Plasmon-driven Regrowth of Silver Nanodecahedrons (NDs)
We report the synthesis of silver nanodecahedrons (NDs) for extending the localized surface plasmon resonance (LSPR) of silver nanostructures from blue to green-orange (~590 nm), which will enable much wider application opportunities using common laser light sources. In our photo-assisted method, we use a light-emitting-diode (LED) to control regrowth of silver ND from precursor seeds. Highly uniform silver NDs are synthesized when the LED emission peak coincides with the LSPR peak of the seeds. A two-step process involving precursor self-transformation into silver nanoprisms and nanoplates, and subsequent photo-activated regrowth of silver NDs has been proposed. Surface-enhanced Raman scattering of silver NDs in different sizes has been studied, and the average enhancement factor for each size is estimated to be in the order of ~106.
KeywordsLocalized surface plasmon resonance (LSPR) Silver nanodecahedron (AgND) Photochemical synthesis Surface-enhanced Raman scattering (SERS)
Noble-metal nanocrystals have received considerable attention in recent years for their size- and shape-dependent plasmonic properties, and they have been used in various application areas including photonics , electronics , catalysis , sensing , and biomedicine . Highly uniform silver (Ag) nanostructures of different shapes (spheres , rods , bars , belts , wires , prisms , disks ) have been widely reported in the literatures. It is interesting to note that plasmonic applications associated with the longer wavelength region (e.g., 500 nm and above) of the visible spectrum is largely covered by gold (Au) nanoparticles (NP)—instead of silver nanoparticles—despite that gold offers relatively lower field enhancement factors because of higher losses. Until now, reported cases of AgNPs primarily cover the UV-blue region. This impedes widespread applications of AgNPs. With the aim of extending the wavelength of localized surface plasmon resonance (LSPR) in AgNPs, Xia et al. successfully synthesized silver nanocubes that have LSPR peaks in blue-green region (420–500 nm) [13, 14]. On the other hand, silver nanodecahedrons (AgNDs) should hold some promise on expanding LSPR wavelength further in to the red region. Because of their sharp corners/edges, AgND solutions demonstrate remarkable optical properties including narrow scattering peaks and highly reproducible transmission spectra [15, 16]. Despite such special optical properties, literature reporting the growth of AgNDs is quite limited. Earlier works on the synthesis of AgNDs using photochemical [15, 17] or N, N-dimethylformamide (DMF) reduction  methods have been reported by several groups. Kitaev et al. reported that growth of nanodecahedrons in a mixture of NP seeds and precursor solution can be activated by white light illumination . While our experimental results indicate that a broadband light source will lead to the formation of poly-dispersive Ag nanoparticles (see Supplementary Information), it is the aim of this work to investigate the possibility of controlling the growth uniformity by using varying the wavelength of the light source. Our experiments reveal that when wavelength of the LED illumination matches with the LSPR wavelength of the AgND seeds, a seed-mediated plasmon-driven regrowth mechanism will occur, and the resultant product is highly mono-dispersive AgNDs with LSPR anywhere in the range of 489–590 nm.
Silver nitrate (ACS reagent, ≥99.0%), Poly(vinyl pyrrolidone) (PVP, Mw = 40,000 g/mol), l-Arginine (BioUltra, ≥99.5%), and 4-Methylbenzenethiol (98%) were purchased from Sigma-Aldrich. Sodium citrate dehydrate and sodium borohydride (99%) were purchased from Fisher Scientific. All chemicals were used with no further purification. Water used in the synthesis experiment was purified by Spectra-Teknik ultrapure water purification system. High-brightness light-emitting diodes (LEDs) with emission peaks at 465, 500, 520, and 578 nm were purchased from RS Components.
Synthesis of Ag Nanodecahedron Seeds
Silver nanodecahedron (ND) seeds were prepared using the method reported in ref. . A 0.5 mL trisodium citrate (0.05 M), 0.015 mL poly(vinyl pyrrolidone) (PVP, Mw = 40,000 g/mol, 0.05 M), 0.05 mL l-arginine (0.01 M), and 0.15 mL silver nitrate (0.005 M) were mixed with 7 mL deionized water in a clear 10 ml beaker. An 80-μL ice-cold sodium borohydride (0.1 M) was rapidly injected into the mixed solution with continuous stirring. The solution was incubated for 2 h in a dark environment at room temperature. The final bright yellow solution, which was also used for precursor in the following regrowth steps, was exposed to 465 nm radiation from a blue LED for 6 h.
Seed-mediated Regrowth of Ag Nanodecahedrons
In a typical synthesis experiment, 2 mL AgNDs seed solution was mixed with 2–4 mL precursor solution. The final mixture was then exposed to LED illumination for 6–9 h depending on the light intensity. The spectral peak of the LED was chosen such that it was close to the LSPR peak of the ND seeds. All experiments were conducted under room temperature.
SERS Assessment of Ag Nanodecahedrons
In a standard procedure, 100 μL AgNDs colloid after two times centrifugation wash was mixed with 900 μL 0.1 mM 4-methylbenzenethiol (4-MBT) solution in ethanol for 1 h. Then, 10 μL of the mixture was transferred to the surface of a silicon substrate and left to dry. The substrate was then rinsed by ethanol thoroughly to remove any unbounded 4-MBT. SERS measurement of single ND was conducted immediately after sample preparation. Normal Raman spectroscopy was measured with solution of 14 mM 4-MBT in NaOH (~6 M) for the calculation of average enhancement factor (EF) of the single AgND.
Field-emission scanning electron microscopy (FESEM) was performed using a Carl Zeiss Ultra Plus FESEM and the operating voltage was 4.96 kV. Transmission electron microscopy (TEM) images were acquired using a FEI CM120 microscope at 120 kV. The average size was determined from SEM and TEM images by averaging the edge length measurements obtained from at least 100 AgNDs. Extinction spectra were measured using a Shimadzu UV-3101 UV/Vis/NIR scanning spectrophotometer. SERS spectra were recorded using a Renishaw inVia confocal Raman spectrometer coupled to a Leica microscope. Backscattered photons were collected using a × 100 objective (NA = 0.85). The 514-nm excitation source was obtained from an argon laser. The scattering spectra were recorded in the range of 550–2,000 cm−1. Except for the normal Raman spectrum of 4-MBT, which required 20 mW and 30 s, typical accumulation time was 10 s per data point, and laser power was 1 mW.
Results and Discussions
Variation of LSPR Peak with Size of Silver Nanodecahedrons
Summary of final products obtained from exposing seed and precursor solution to LED illumination at various wavelengths
LED illumination peak (nm)
Seed solution volume (mL)
Precursor solution volume (mL)
Extinction peak of final silver NDs (nm)
2 mL sample 1
2 mL sample 1
2 mL sample 1
2 mL sample 2
2 mL sample 3
2 mL sample 4
2 mL sample 5
2 mL sample 6
2 mL sample 7
LED Irradiation at Wavelength away from LSPR Peak of Silver ND Seed
SERS Characterization of Silver Nanodecahedrons
Summary of calculated average EFs of different sizes silver NDs at 1,077 cm−1 and 1,581 cm−1 band
Extinction peak (nm)
Average EF (1,077 cm−1)
Average EF (1,581 cm−1)
7.66 × 105
1.64 × 106
1.34 × 106
3.87 × 106
5.77 × 105
1.67 × 106
1.77 × 105
3.79 × 105
By controlling and tuning the mixing ratio of seed and precursor concentrations, uniform silver nanodecahedrons (NDs) with major localized surface plasmon resonance (LSPR) ranging from 489 to 590 nm have been synthesized by a seed-mediated plasmon-driven method. It is found that two processes are happening in the synthesis. The choice of LED illumination wavelength that matches with the LSP peak of the silver ND seeds can drastically suppress the formation of nanoprims and nanoplates, thus improving the uniformity of the desired nanostructures. Due to their sharp tips and edges, silver NDs provide up to 106 electric field enhancement, with the possibility of further increase if one optimize the matching between the LSP peak and the irradiation laser wavelength. The reported silver NDs should find applications in biosensing and bioimaging.
The authors thank Drs. Isakov Dmitry and Ning Ke for conducting FESEM and TEM characterizations of the samples, respectively. The project is supported by SIMTech collaborative research grant SIM/09-220001. HFL's research studentship and a Group Research Grant 3110048 from The Chinese University of Hong Kong are gratefully acknowledged.