Utilization of triangle nanosilver to prepare spherical nanosilver and quantitatively detect trace titanium by SERS
- Qingye Liu†1,
- Guiqing Wen†1,
- Xinghui Zhang1,
- Aihui Liang1Email author and
- Zhiliang Jiang1Email author
© Liu et al.; licensee Springer. 2014
Received: 4 October 2014
Accepted: 2 December 2014
Published: 10 December 2014
The blue triangle nanosilver (BAgNP) sol was prepared by the two reducers of NaBH4 and H2O2. Using BAgNP as the precursor, a small spherical nanosilver (AgNP) sol in yellow was synthesized by addition of suitable amounts of X− (X = Cl, Br, and I). The oxidization process of BAgNP to AgNP was studied in detail by resonance Rayleigh scattering (RRS), surface-enhanced Raman scattering (SERS), laser scattering, surface plasmon resonance (SPR) absorption, and microscope techniques. It has been observed that NaCl accelerated the oxidizing BAgNP to form AgNP, and an oxidizing mechanism and quasi-nanograting Raman-scattering enhanced mechanism were developed to explain the phenomena. Using the BAgNP sol as substrate and based on the catalysis of Ti(IV) on the BrO3− oxidizing safranine T (ST) molecular probe with a strong SERS peak at 1,535 cm−1, a new catalytic SERS quantitative method was developed for the determination of 1.0 to 100 ng/mL Ti, with a detection limit of 0.4 ng/mL.
KeywordsBlue triangle nanosilver Spherical nanosilver Safranine T SERS quantitative Ti
Metal nanoparticles, especially nanogold and nanosilver, are of novel physical and chemical properties that become one of the hot spots in physics, chemistry, materials, and sensors. Nanogold sol has good biocompatibility and stability and excellent optical and catalytic properties and has been used widely in several fields. Comparing to nanogold, the cost of nanosilver is lower, the mol absorption coefficient is higher, and the optical property is more excellent such as a very high surface-enhanced Raman scattering (SERS) effect and very low mole absorption coefficient of its aggregations. In addition, its studies and applications are less than the nanogold. These are interesting to people to study the nanosilver preparation, property, and applications [1–3].
It is significant to prepare nanosilver sol because it exhibits novel optical properties such as different color and high SERS activity and can be used as a biochemical label and nanocatalyst in liquid phase synthesis. In general, nanosilver sols were prepared by citrate heating and NaBH4 procedures. Lee et al.  used NaBH4 as the reducer to prepare a brown nanosilver sol with a surface plasmon resonance (SPR) peak at 400 nm. Using citrate as the reducer, an unstable kelly nanosilver sol was obtained with a SPR peak at 420 nm. Munro et al  improved the citrate procedure to prepare stable nanosilver sol by addition of a stable reagent. However, the addition of the stable reagent made the procedure complicated, and a serious problem may be caused in that it interfered with the subsequent study. For example, the stable reagent may affect the nanosilver functionalizing and restrain the optical property. A stable blue triangle nanosilver sol was prepared by NaBH4 and H2O2 reducers [6, 7], using polyvinylpyrrolidone (PVP) as the stable reagent. However, the PVP strongly restrains the SERS effect that decreased SERS sensitivity. Thus, it is important to prepare stable, simple, highly SERS-active nanosilver sol without a restraining stabilizer. To our best knowledge, there are no reports that used big triangle nanosilver to prepare small spherical nanosilver and to determine trace Ti by SERS technique. In this article, the blue triangle nanosilver (BAgNP)-NaCl system was studied firstly by resonance Rayleigh scattering (RRS) [8–10] and SERS [11, 12] spectral techniques. A simple and rapid preparation procedure for yellow nanosilvers (AgNPs) was developed using BAgNPs as the precursor. In addition, titanium is a necessary trace element for organisms that enhanced the immune function and stimulated plant growth. Therefore, it is important to develop a simple, rapid, sensitive, and selective method for the determination of trace Ti in plant and foods. At present, several methods including atomic, molecular, and mass spectrometry have been reported for the analysis of Ti [13, 14]. However, there are no SERS methods with rapidity, high sensitivity, and selectivity for quantitative analysis of Ti in foods such as tea. Thus, a new catalytic SERS method was developed for the quantitative analysis of trace Ti, based on its catalysis of BrO3− oxidization of safranine T (ST) that can be utilized to amplify the analytical signal, and using ST as the SERS molecular probe in the BAgNP sol substrate that formed highly SERS-active AgNP/AgCl composite aggregations in the presence of NaCl.
Stock standard solutions of 1.0 × 10−3 mol/L AgNO3, 1.0% (W/V) trisodium citrate, 0.05 mol/L NaCl, 30% H2O2, freshly prepared 0.05% NaBH4, 10 mmol/L KBrO3, 1.0 × 10−5 mol/L ST, 1.0 mol/L H3PO4, and 1.00 mmol/L Ti(IV) were prepared. A 1.0 × 10−4 mol/L BAgNP sol was prepared as follows: into a triangle flask containing about 40 mL water, 500 μL 1.0 × 10−2 mol/L AgNO3, 1.5 mL 6.0 × 10−2 mol/L trisodium citrate, 200 μL 0.1 mol/L NaBH4, and 120 μL 30% H2O2 were added in turn with constant stirring for 15 min and diluted to 50 mL to obtain the BAgNP sol. To obtain BAgNP sol without H2O2 (hBAgNP), the BAgNP sol could be heated at 60°C for 15 min to get rid of the excess H2O2, and the solution was also in blue. The stable AgNP sol in yellow was prepared by mixing 10 mL 1.0 × 10−4 mol/L BAgNP with 30 μL 0.50 mol/L NaCl or 10 μL 0.005 mol/L NaBr solutions. All reagents were of analytical grade and the water was highly pure sub-boiling water.
Apparatus and measurements
A model F-7000 fluorescence spectrophotometer (Hitachi Company, Chiyoda-ku, Japan) was used to record the RRS intensity, and the RRS spectra were recorded by means of synchronous scanning excited wavelength λex and emission wavelength λem (λex − λem = Δλ = 0). A model DXR smart Raman spectrometer (Thermo Fisher Scientific Co., Ltd., Waltham, MA, USA) was used to record the SERS spectra and the intensity using a laser wavelength of 633 nm, power of 2.0 mW, and collection time of 2.0 s. A model of TU-1901 double beam UV-vis spectrophotometer (Beijing Purkinje General Instrument Co. Ltd., Beijing, China), a model of JEM-2100 F field emission transmission electron microscope (Electronic Stock Limited Company, Tokyo, Japan), and a model of nanoparticle and Zeta potential analyzer (Malvern Instruments Ltd., Malvern, England) were used.
Procedure for preparation and spectral characterization of AgNP
Into a test tube, 1.0 mL 1.85 × 10−4 mol/L BAgNP solution and certain amounts of X− were added, diluted to 2 mL with water, and mixed well to obtain the AgNP sol. The RRS spectra and the intensity (I) were recorded by a fluorescence spectrophotometer with the synchronous scanning technique (λex − λem = Δλ = 0). A blank (I0) without X− was recorded, and the value of ΔI = I − I0 was calculated. Meanwhile, the SPR absorption spectra were also recorded by spectrophotometer. If the ST molecular probes were added after the addition of X−, the SERS spectra and the intensity were recorded by the laser Raman spectrometer.
Procedure for SERS detection of Ti
A solution of 100 μL 1.0 × 10−5 mol/L ST, 250 μL 10 mmol/L KBrO3, and 100 μL 1 mol/L H2SO4 and a certain amount of Ti(IV) solution were added into a 5-mL marked test tube, diluted to 1.0 mL and mixed well. The mixture was placed in 60°C for 10 min and cooled with tap water, and 120 μL 1.0 mol/L NaCl and 500 μL 100 μmol/L BAgNP solutions were added, diluted to 2.0 mL, and mixed well. Then, the mixture was transferred into a 1-cm quartz cell. The SERS spectrum and the SERS intensity at 1,535 cm−1 (I1,535cm-1) were recorded. Meanwhile, a reagent blank (I1,535cm-1)0 without Ti(IV) was recorded, and a value of ΔI = (I1,535cm-1)0 − I1,535cm-1 was calculated.
Results and discussion
Principle for SERS detection of Ti
Preparation of nanosilver sols
The conditions for preparing BAgNP sol, including the precursor AgNO3, stabilizer sodium citrate, reducer of NaBH4 and H2O2, and reaction temperature and time, were considered. Those conditions as in the ‘Methods’ section were selected to prepare stable BAgNP and hBAgNP sols. Using the BAgNP or hBAgNP sol as the precursor, the preparing conditions of yellow nanosilver such as NaX and H2O2 were also examined. Without addition of H2O2, there are still micro-amounts of H2O2 in the BAgNP sol that come from the preparation process. Upon the addition of NaCl in the range of 5 × 10−4 to 100 × 10−4 mol/L, the color changed from blue to yellow, the characteristic SPR absorption peak shifts from 550 to 395 nm, and the RRS intensities enhanced due to the formation of rigid AgNP and loose AgNP/AgCl particles. The NaCl concentration increased continuously, and the color is gray due to the aggregation of AgNPs. In the absence of NaCl, the residual H2O2 in the BAgNP sol cannot oxidize the BAgNP. When the H2O2-added concentration is higher than 0.003%, the blue color comes out immediately. In the presence of NaCl, as low as 0.0001% H2O2 also oxidizes BAgNP to form Ag+. In short, even with no addition of H2O2, the stable yellow nanosilver sol can be obtained by mixing BAgNP sol and a suitable X−.
Characterization of nanosilver
SPR absorption spectra
SERS quantitative analysis of Ti
Effect of coexistent ions on the SERS quantitative analysis of 40 ng/mL Ti
Relative error (%)
Relative error (%)
Analytical results for Ti in tea samples using the catalytic SERS and AAS methods
Single value (μg/g Ti)
Average (μg/g Ti)
Added Ti (μg/g Ti)
Found Ti (μg/g Ti)
AAS (μg/g Ti)
9.50, 9.85, 10.3, 9.40, 10.5
11.1, 11.0, 12.5, 11.8, 11.9
13.5, 14.3, 13.2, 14.2, 14.5
In summary, a SERS-active and stable BAgNP sol was prepared by the NaBH4-H2O2 procedure without PVP surfactant. Using the BAgNPs as the precursor, a simple and fast procedure was developed for preparation of stable yellow nanosilver sol by mixing it with NaCl that can be used as a SERS sol substrate with strong SERS activity. The nanosilver sols and their oxidization were studied in detail, and the oxidization and quasi-nanograting enhanced mechanisms were proposed to explain the phenomena. Using BAgNP-NaCl as the SERS substrate and ST as the probe, a new catalytic SERS method was developed for determination of trace Ti in tea samples, with high sensitivity and selectivity.
This work supported by the National Natural Science Foundation of China (No. 21165005, 21267004, 21367005, 21447006, 21467001, 21465006, 21477025) and the Natural Science Foundation of Guangxi (No. 2013GXNSFFA019003, 2014GXNSFAA118050, 2014GXNSFAA118059).
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