Autocatalytic Oxidization of Nanosilver and Its Application to Spectral Analysis

The stable yellow nanosilver (AgNP) and blue nanosilver (AgNPB) sols were prepared by the NaBH4 procedure. The new nanocatalytic reaction of AgNP-NaCl-H2O2 was investigated by surface plasmon resonance (SPR) absorption, resonance Rayleigh scattering (RRS), surface-enhanced Raman scattering (SERS) and scanning electron microscope (SEM) techniques. The autocatalytic oxidization of Ag on AgNP surface by H2O2 was observed firstly and the AgNP/AgCl nanoparticles were characterized. The [Ag+] from AgNP is different to the Ag+ from AgNO3 that adsorb on the AgNP surface. An autocatalytic oxidization mechanism was proposed to explain experimental phenomena. The relationship between the SPR absorption peaks and the RRS peaks of AgNPB was studied, and three characteristic RRS peaks called as out-of-plane quadrupole, out-of-plane dipole and in-plane dipole RRS peaks were observed firstly. Using AgNP as nanoprobe, a simple, sensitive and selective RRS method was developed for assay of H2O2 in the range of 2.0 × 10−8-8.0 × 10−5 mol/L.

system, there is an autocatalytic oxidation reaction on the surface of AgNP to generate large Ag n /AgCl particles with an average size of 60 6 15 nm (Fig. 1b). In AgNP-H 2 SO 4 -NaCl-FeSO 4 -H 2 O 2 system, on one hand the autocatalytic oxidation reaction of AgNP generate Ag 1 on the surface, on the other hand surface atoms of AgNP also can generate Ag 1 by the Fenton oxidation reaction, so the large Ag n /AgCl particles with an average size of 75 6 16 nm was formed (Fig. 1c). SEM of AgNPB system shows that they are nearly spherical, with particle size between 6-100 nm and an average size of 40 6 8 nm (Fig. 1d). The shape of AgNPB can not be observed satisfactorily by SEM, and the TEM of AgNPB system was done. Figure 2e indicated that there triangle nanosilver particles in the system, with the side length between 30-90 nm and an average side length of 45 6 10 nm, in addition to the nearly spherical particles.  With addition of different concentration of AgNO 3 to the system of 2.0 3 10 23 mol/L NaCl-0.035% sodium citrate, AgCl particles were generated and exhibited strong scattering signal at 335 nm (Fig. 2S). The increased intensity DI 335 nm was linear to AgNO 3 concentration in the range of 12. 116. This suggests that RRS signal's enhancement of AgNP-NaCl-H 2 O 2 system is the result of the formation of AgCl particles. When adding different concentration of Ag 1 to the AgNP-NaCl-sodium citrate system, the RRS spectrum ( Fig. 4S) is different with that of AgNP-NaCl-H 2 O 2 and the former is weaker. It also suggests that [Ag 1 ] which produced by AgNP surface oxidation is different with that adsorption on the surface of the AgNP by adding AgNO 3 . Compare to RRS spectra of NaCl-sodium citrate-AgNO 3 system, the RRS intensity of AgNP-NaCl-sodium citrate system is greatly reduced and has a valley at 395 nm, as the result of the strongest absorption of AgNPs at 395 nm.
SPR absorption spectra. Mie theory 49 pointed out that, spherical nanoparticles have only one SPR absorption peak. Spherical AgNP with diameter of 20-30 nm has the strongest SPR peak near 400 nm 50 , which is out-of-plane dipole SPR absorption peak 51 . In the systems of NaCl and NaCl-H 2 SO 4 -FeSO 4 , both have an AgNP SPR absorption peak at 395 nm (Fig. 2B, Fig. 5S). The absorbance at 395 nm of the two systems decreased linearly with the H 2 O 2 concentration increased and can be chosen to determine H 2 O 2 . The AgNO 3 -NaCl and AgNO 3 -NaCl-H 2 SO 4 -FeSO 4 systems were examined by spectrophotometry. With addition of different AgNO 3 concentration to the two systems of 2.0 3 10 23 mol/L NaCl-0.035% sodium citrate and 2.0 3 10 23 mol/L NaCl-0.035% sodium citrate 22.0 3 10 23 mol/L H 2 SO 4 23.75 3 10 25 mol/L FeSO 4 , the produced AgCl particles exhibited weak SPR peak at   285 nm (Fig. 6S,7S). The absorbance increased slowly with the AgNO 3 concentration increased in the range of 12.5-100 3 10 26 mol/L because AgCl particles have weak absorption. In the medium of 5.0 3 10 24 mol/L NaCl, AgNPB has two SPR absorption peaks at 330 nm and 530 nm (Fig. 8S) (Fig. 9S). This demonstrated that there are AgNP/AgCl aggregates in the system.

Discussion
Mechanism of autocatalytic oxidization of AgNP. Stabile AgNP in size of 10 nm was prepared conveniently by NaBH 4 reduction of Ag 1 . When NaCl was added, Cl 2 can be adsorbed on the surface of AgNP, and the signals of SPR absorption and RRS are still very weak that indicated no aggregation in the system.  (Fig. 3), and made AgNP become smaller and its SPR absorption weaker. When H 2 O 2 increased, the RRS intensity increased linearly due to more big AgNP/AgCl aggregates forming, and the SPR absorption decreased linearly due to much less small nanosilver forming. Thus, two new SPR absorption and RRS methods were established to determine H 2 O 2 . According to the generation mechanism of HO? and the autocatalytic oxidation mechanism of AgNP 53,54 (AgNP 5 Ag n 5 Ag m 1 2k ), the main reactions of AgNP-NaCl-H 2 O 2 system are as follows, Ag n HOOH~Ag=HOO : ð2Þ 2Ag n HOO :~2 Ag n z2HO : zO 2 ð3Þ Ag n{1 Ag z zCl {~A g n{1 AgCl ð5Þ In the presence of NaCl, the reducing ability of Ag was enhanced and made the reaction of H 2 O 2 oxidize AgNP to form Ag m-2k /[AgCl] 2k complex particles become easily, as the result of the formation of AgCl particle with low solubility. The total reaction is as follow, Relationship between the SPR absorption peaks and the RRS peaks of AgNPB. AgNPBs exhibited special optical property that have one sharp out-of-plane quadrupole SPR absorption peak at 330 nm (Fig. 4A), one out-of-plane dipole peak at 390 nm, and one broad in-plane dipole peak at 580 nm 54,55 . The absorbance of the three peaks was linearly increased with AgNPB concentration increased. The study of RRS spectra of nanoparticles in liquid phase shown that, their RRS peaks are closely related with the emission intensity of light source and the SPR absorption peaks 56,57 . The light source of model F-7000 Hitachi fluorescence spectrometer has the strongest emission wavelength at 280 nm that cause a scattering peak at 280 nm, and the emission intensity weaken as the increase of the wavelength. AgNPB has a sharp scattering peak at 330 nm which is corresponding to the out-of-plane quadrupole SPR absorption peak (Fig. 4B) that was called as out-of-plane quadrupole RRS peak. AgNPB has a strong RRS peak at 390 nm that was ascribed to outof-plane dipole SPR absorption, was called as out-of-plane dipole RRS peak. Besides, the in-plane dipole peak at 530 nm is violetshift 50 nm compare to its SPR absorption peak because the light source has strong emission at 530 nm. Though the RRS intensity of the three peaks increased with AgNPB concentration increased, they had no linear relationship since the sols exist in multiple scattering. .8% respectively, this showed that the RRS method has good accuracy. The linear range of AgNP-NaCl-H 2 SO 4 -FeSO 4 system was 1.0 3 10 27 -2.5 3 10 25 mol/L, with a detection limit of 2 3 10 28 mol/L. In the RRS analytical system, the AgNP-NaCl is most sensitive, simple, stabile and low blank (Table 1S), and it was chosen to detect H 2 O 2 concentration. The SPR methods of the two systems also can be used to determine H 2 O 2 with low-cost, though they were not as sensitive as RRS methods. According to the procedure, a standard solution containing 20 3 10 26 mol/L H 2 O 2 and various coexistent compounds were examined, with a relative error of less than 6 10%. A 100 times of ClO 4 2 and SO 4 22 , 70 times of Ca(II) and Mg(II), 50 times of Cu 21 , Mo 61 , I 2 , triethanolamine, Co 21 , NO 2 2 , 10 times of Mn 21 , Br 2 , citric acid, and 2.5 mg/L HSA did not interfere with the determination. This indicated that the method has good selectivity.

Methods
Apparatus and reagents. A Model F-7000 fluorescence spectrophotometer (Hitachi Company, Japan) was used to record the RRS spectra by means of synchronous scanning excitation wavelength l ex and emission wavelength l em (l ex -l em 5 Dl 5 0) and the RRS intensity. A Model TU-1901 double beams spectrophotometer (Puxi Tongyong Apparatus Limited Company, Beijing) was used to record the SPR spectra and the SPR intensity. A model JSM-6380LV scanning electron microscope (Electronic Stock Limited Company, Japan), a model of JEM-2100F field emission transmission electron microscope (Electronic Stock Limited Company, Japan), a model DXR smart Raman spectrometer (Thermo Fisher Scientific Co., Ltd., USA), a moldel SK8200LH ultrasonic reactor (Kedao Company, Shanghai, China), and a model magnetic stirrer (Zhongda Instrumental Plant, Jiangsu, China) were used. A 1.0 3 10 23 mol/L AgNO 3 solution, 1% (W/V) sodium citrate solution, 0.05 mol/ L NaCl solution, and 1.0 3 10 23 mol/L FeSO 4 solution were used. A 0.05% (W/V) NaBH 4 was prepared freshly. A 0.100 mol/L H 2 O 2 standard solution was prepared as follows: 1.02 mL H 2 O 2 (30%) was diluted to 100 mL with water, it was standardized by KMnO4 procedure, and was diluted to 5.00 3 10 24 mol/L before use. A 1.85 3 10 24 mol/L AgNP was prepared as follows: 9.25 mL 1.0 3 10 23 mol/L AgNO 3 and 3.5 mL 1% trisodium citrate were added into a conical flask with stirring and diluted to 40 mL with water, then 4 mL 0.05% NaBH 4 was added slowly with about 15 min, the mixture was diluted to 50 mL, and it can be used after 24 h to make the NaBH 4 decomposing completely. The preparation of AgNP sols was repeated five times, the SPR peak is at 395 nm with an average absorption value of 2.50 6 0.10, and the sols were stabile within 20 days (Fig.20S). A 1.0 3 10 24 mol/L AgNPB was prepared as follows: 40 mL of water, 500 mL 1.0 3 10 22 mol/L AgNO 3 , 1.5 mL 6.0 3 10 22 mol/L sodium citrate, 120 mL 30% H 2 O 2 , and 200 mL 0.1 mol/L NaBH 4 were added into a triangle flask in turn with constantly stirring for 15 min. Then heat to ð6Þ www.nature.com/scientificreports SCIENTIFIC REPORTS | 4 : 3990 | DOI: 10.1038/srep03990 boil for 5 min to get rid of the excess H 2 O 2 and the solution was diluted to 50 mL. All reagents were of analytical grade and the water was highly pure sub-boiling water.
Procedure. A 1.0 mL 1.85 3 10 24 mol/L AgNP solution, 80 mL 0.05 mol/L NaCl (or adding 80 mL 5.0 3 10 22 mol/L H 2 SO 4 , 75 mL 1.0 3 10 23 mol/L FeSO 4 ), and a certain amount of H 2 O 2 solution were added into a 5 mL calibrated tube in turn, then diluted to 2 mL and mixed well. The RRS intensity at 330 nm (I) was recorded by a fluorescence spectrophotometer with synchronous scanning (l ex -l em 5 Dl 5 0). A blank (I 0 ) without H 2 O 2 was recorded and the value of DI 5 I-I 0 was calculated.