ARTICLE pubs.acs.org/ac
Highly Sensitive and Selective Determination of Iodide and Thiocyanate Concentrations Using Surface-Enhanced Raman Scattering of Starch-Reduced Gold Nanoparticles Prompong Pienpinijtham,†,‡ Xiao Xia Han,‡ Sanong Ekgasit,*,† and Yukihiro Ozaki*,‡ † ‡
Sensor Research Unit, Department of Chemistry, Faculty of Science, Chulalongkorn University, Bangkok 10240, Thailand Department of Chemistry, School of Science and Technology, Kwansei Gakuin University, Sanda, Hyogo 669-1337, Japan
bS Supporting Information ABSTRACT: In this report, we propose a novel technique for the determination of the concentrations of iodide and thiocyanate by surface-enhanced Raman scattering (SERS) of starch-reduced gold nanoparticles. Starch-reduced gold nanoparticles show an intrinsic Raman peak at 2125 cm1 due to the CtC stretching mode of a synthesized byproduct. Because of the high adsorptivity of iodide on a gold surface, the intensity of the SERS peak at 2125 cm1 decreases with an increase in the iodide concentration. Thiocyanate also strongly adsorbs on a gold surface, and a new peak appears at around 2100 cm1, attributed to the CtN stretching vibration in a SERS spectrum of starch-reduced gold nanoparticles. These two peaks were successfully used to determine the iodide and thiocyanate concentrations separately, even in their mixture system. The detection limit of this technique for iodide is 0.01 μM with a measurement range of 0.012.0 μM, while the detection limit of this technique for thiocyanate is 0.05 μM with a measurement range of 0.0550 μM. This technique is highly selective for iodide and thiocyanate ions without interference from other coexisting anions such as other halides, carbonate, and sulfate.
oth iodide (I) and thiocyanate (SCN) are very crucial anions related to the human body and health science. In the thyroid gland, I is used to produce thyroid hormones, triiodothyronine (T3) and thyroxine (T4), which control many metabolic activities in the human body.1 Many diseases originate from the deficiency and excess of iodide, such as goiter, hypothyroidism, and hyperthyroidism (Graves disease).1,2 Moreover, I is also employed in organic syntheses, medicine, analytical chemistry, and other applications. A small amount of SCN, which is produced by the digestion of some vegetables or by the intake of thiocyanate-containing foods (e.g., milk and cheese), can be found in human body fluids (e.g., serum, saliva, and urine).3,4 When one is exposed to cyanide or inhales tobacco smoke, a higher SCN level is usually found due to a metabolite of cyanide. The presence of SCN in body fluids, especially saliva, is very useful in indicating cyanide exposure and is considered to be a biomarker for the identification of nonsmokers and smokers.35 Currently, there are several methods for determining the concentrations of I and SCN in aqueous solutions. Ion chromatography, high-performance liquid chromatography (HPLC), combined solid-phase extraction/diffuse reflectance spectroscopy, gel electrophoresis coupled to inductively coupled plasma-mass spectrometry (ICP-MS), atomic emission spectrometry (AES), inductively coupled plasma atomic emission spectrometry (ICP-AES), colorimetric and fluorometric
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r 2011 American Chemical Society
chemosensors, capillary electrophoresis, ion-selective electrode polarography, voltammetry, and neutron activation analysis are techniques employed for the determination of the concentration of I.1,614 Ion pair chromatography, gas chromatography mass spectrometry, colorimetric techniques, capillary electrophoresis, and electrochemical methods are the techniques employed for determining the concentration of SCN.3,5,1518 As mentioned above, most of these techniques need separation processes before the measurement and are often time-consuming and involve complicated sample preparation. Moreover, the coexistence of other anions easily interferes with the performance of some techniques, whereas some techniques require complex and expensive instruments. In this study, we have developed a novel technique based on surface-enhanced Raman scattering (SERS) using starchreduced gold nanoparticles for the determination of the concentration of iodide and thiocyanate. SERS is a Raman scattering technique in which the Raman signal of molecules adsorbed on a surface of a noble metal, transition metal, or semiconductor substrate is dramatically enhanced (1031014) via two possible mechanisms: electromagnetic and chemical mechanisms.1921 One of the most popular substrates for SERS is gold Received: November 4, 2010 Accepted: April 12, 2011 Published: April 12, 2011 3655
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Figure 1. A scheme of the proposed method for I and SCN detection.
nanoparticles; these nanoparticles are very stable and are responsible for high enhancement in many SERS systems.22,23 In our previous work, gold nanoparticles were synthesized via an environmentally friendly chemical reduction method, and soluble starch was employed as both the reducing agent and the stabilizer.24 Starch was degraded by alkali in order to enhance its reducing power. As a result, starch-reduced gold nanoparticles were obtained. The SERS spectrum showed a sharp peak at 2125 cm1 due to the CtC stretching mode of a synthesized byproduct that is originated from the alkaline degradation of starch, and stabilizes the gold nanoparticles.25 This peak is very useful because its position is far from the fingerprint region and the positions of other Raman bands. Hence, we can exploit the peak as a probe for I detection. I adsorbs very strongly on a gold surface, and the SERS intensity at 2125 cm1 decreases upon I adsorption due to the replacement of CtC species by I on a gold surface (see Figure 1). SCN also strongly adsorbs on a gold surface, resulting in the appearance of a strong peak at around 2100 cm1 due to the CtN stretching vibration.25 This peak can be used to determine the SCN concentration, as shown in Figure 1. Even in an I and SCN mixture, the concentration of these two individual ions can be determined simultaneously. In addition, this technique is highly selective for I and SCN. The coexisting anions, like other halides, carbonate, and sulfate, do not interfere with the measurement. Using this method, the measurement can be performed directly without separating the anions and without chromatographic process; only a Raman microspectrometer is required. The SERS substrate for the measurement is very easy to fabricate and can be utilized immediately after synthesis without surface modification. Thus, the measurement process is easy, quick, and inexpensive.
’ EXPERIMENTAL METHODS Chemicals. Tetrachloroauric acid (HAuCl4) was prepared from 99.99% gold metal dissolved by aqua regia (1:3 ratio of concd nitric acid and concd hydrochloric acid by volume). All the acids used were purchased from Merck. Sodium hydroxide (NaOH), potassium iodide (KI), potassium thiocyanate (KSCN), and soluble starch were purchased from Wako Co., Ltd. Human serum was purchased from Sigma Aldrich. Synthesis of Starch-Reduced 80-nm Gold Nanoparticles. One thousand ppm gold nanoparticles were synthesized by our previously proposed synthesis method.24 Briefly, 5 mL of 4000 ppm HAuCl4 (pH 7) was mixed with 5 mL of 0.04 M NaOH and allowed to stand for 15 min. Five milliliters of 2% starch solution was incubated with 5 mL of 0.02 M NaOH at 80 °C for 15 min. After incubation, the starchNaOH solution was added to the HAuCl4NaOH solution at 80 °C and stirred vigorously
Figure 2. (a) Plasmon extinction spectra (black line represents gold colloid; red and green lines represent dried gold film before and after soaking in 10 mM KI solution, respectively; dashed line is located at 785 nm which is an excitation laser wavelength), (b) AFM, (c) TEM, and (d) SEM images of starch-reduced gold nanoparticles.
for 15 min. Finally, the gold colloid was cooled to room temperature, and the volume of the colloid was adjusted to 20 mL to compensate for evaporation. Using this procedure, 80-nm starchreduced gold nanoparticles were obtained with a plasmon extinction maximum at 548 nm. Before iodide and thiocyanate measurements, the gold colloid was centrifuged and redispersed multiple times for removing synthesis residues and for preconcentration from 1000 to 10 000 ppm. The morphology of the synthesized gold nanoparticles was characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), and atomic force microscopy (AFM) techniques. Determination of Iodide and Thiocyanate Concentrations. One microliter of 10 000 ppm gold nanoparticle colloid was placed on a silicon (Si) wafer and dried at room temperature for 12 h. Dried gold nanoparticles on the Si wafer were soaked in a solution of KI, KSCN, or a mixture of the two at various concentrations for 1 h and then rinsed with triply distilled water multiple times. The drop-dried gold film remains intact after dipping into the aqueous samples (see Supporting Information). The SERS spectra of dried starch-reduced gold nanoparticles were collected by a HoloSpec f/1.8i spectrograph (Kaiser Optical Systems Inc.) using the 785-nm line of an NIR diode laser (Invictus) as the excitation source. Laser power at the sample position was set at ∼15 mW. The exposure time for each SERS measurement in this study was 20 s. All SERS spectra were analyzed without any spectral treatments except for baseline correction and normalization. All the presented data were averaged from eight repeated measurements (four samples two measurements). In the case of SCN concentration determination, there were two overlapping peaks around 2100 cm1 from CtC and CtN stretching vibrations. There are two major mechanisms that lead to broadening of a spectral line shape of these two peaks, i.e., “natural line shape” (pressure broadening) described by a Lorentzian profile, and “Doppler line shape” (thermal motion broadening) described by a Gaussian profile.26 To determine the overlapping-area intensity of these 3656
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Table 1. Raman Band Assignment of Soluble Starch27 Raman shift (cm1) 441, 479,
Raman band assignment skeletal modes of pyranose ring
577, and 718 764
CC stretching
862
C(l)-H, CH2 deformation
939
skeletal mode vibrations of a-1,4 glycosidic
1047
OH deformation
1082 1124
COH bending CO stretching, COH bending
linkage (COC)
Figure 3. (a) Raman and SERS spectra of soluble starch powder in the 35002000 cm1 and 1600280 cm1 regions, and starch-reduced (b) 20-nm and (c) 80-nm gold nanoparticle colloids.
two peaks separately, curve fitting was performed via the Voigt function, which is a combination of Gaussian and Lorentzian functions.
’ RESULTS AND DISCUSSION Starch-reduced gold nanoparticles can be obtained by a environmentally friendly chemical reduction method.24 Figure 2 shows their (a) plasmon extinction spectrum, (b) AFM image, (c) TEM image, and (d) SEM image. The plasmon extinction spectrum of the gold colloid shows an extinction maximum at 548 nm corresponding to a particle diameter of ∼80 nm, which was verified by AFM, TEM, and SEM techniques. The images show the spherical shape and narrow size distribution of the gold nanoparticles. The size and size distribution of these gold nanoparticles are ideal for use as a SERS substrate because smaller particles (
