Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 151 (2015) 308–314
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Resonance Rayleigh scattering method for determination of 2-mercaptobenzothiazole using gold nanoparticles probe Hooshang Parham ⇑, Nahid Pourreza, Farzaneh Marahel Chemistry Department, Faculty of Sciences, Shahid Chamran University, 6135714168 Ahvaz, Iran
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A novel resonance Rayleigh scattering
Schematics of the reaction between AuNPs and 2MBT fungicide which produces AuNP–2MBT complex at pH 6.
method was developed for trace 2MBT detection. AuNPs can be applied as sensor for 2MBT determination in real water samples. Resonance Rayleigh scattering of AuNPs was used for 2MBT detection in water samples. The method is fast and shows good accuracy and sensitivity for 2MBT detection.
a r t i c l e
i n f o
Article history: Received 18 December 2014 Received in revised form 5 June 2015 Accepted 28 June 2015 Available online 29 June 2015 Keywords: Resonance Rayleigh scattering 2-Mercaptobenzothiazole Gold nanoparticles Determination
a b s t r a c t A sensitive, simple and novel method was developed to determine 2-mercaptobenzothiazole (2MBT) in water samples. This method was based on the interaction between gold nanoparticles (AuNPs) and 2MBT followed by increasing of the resonance Rayleigh scattering (RRS) intensity of nanoparticles. The change in RRS intensity (DIRRS) was linearly correlated to the concentration of 2MBT over the ranges of 5.0–100.0 and 100.0–300.0 lg L 1. 2MBT can be measured in a short time (5 min) without any complicated or time-consuming sample pretreatment process. Parameters that affect the RRS intensities such as pH, concentration of AuNPs, standing time, electrolyte concentration, and coexisting substances were systematically investigated and optimized. Interference tests showed that the developed method has a very good selectivity and could be used conveniently for determination of 2MBT. The limit of detection (LOD) and limit of quantification (LOQ) were 1.0 and 3.0 lg L 1, respectively. Relative standard deviations (RSD) for 20.0 and 80.0 lg L 1 of 2MBT were 1.1 and 2.3, respectively. Possible mechanisms for the RRS changes of AuNPs in the presence of 2MBT were discussed and the method was successfully applied for the analysis of real water samples. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction The quality of drinking water, pollution of surface water and water used in agricultural plants and removal of toxic substances ⇑ Corresponding author. Tel.: +98 61 33331042; fax: +98 61 33331042. E-mail address: [email protected] (H. Parham). http://dx.doi.org/10.1016/j.saa.2015.06.108 1386-1425/Ó 2015 Elsevier B.V. All rights reserved.
from water samples are some critical environmental problems occurring in recent years. Therefore, this study aims to face the challenge of protecting clean water from pollution by these toxic substances and to suggest a removal mechanism for elimination of hazardous materials such as fungicides from water samples. Mercaptans are a class of high production chemicals employed in various industrial processes. 2-Mercaptobenzothiazole (2MBT)
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(Fig. 1) is a member of mercaptans and is the most important members of the benzothiazole group, which are heterocyclic aromatic compounds. These chemicals are toxic and poorly biodegradable pollutants [1]. The production of 2MBT and its related salts was reportedly in excess of about 46 million pounds annually in the USA [2]. Estimates from the US EPA have reported that 1 million pounds of 2MBT are released annually into the environment. Its toxicity and allergenicity make its presence in the environment a challenge and a cause for concern [2–6]. The pollution effects of 2MBT on the environment are a major problem for environmental protection agencies (EPA) worldwide. The US EPA estimated that over 450 tons of mercaptobenzothiazole is deposited into the environment every year [7,8]. 2MBT is used as corrosion inhibitors to protect copper [9], it also has other applications such as in medicine as an antifungal drug [10]; a coating agent for metallic surfaces [11] and predominately, as a vulcanization accelerator in the rubber industry [12–14]. It is frequently found in effluent of wastewater treatment plants, in surface water and they are volatile organic compounds contributing to odor problems in wastewater treatment plants [15]. Environmental pollution by sulfur compounds is always of great concern for aquatic organisms and humans. Several methods have been reported for detection and determination of 2MBT and thiols in environmental samples. Spectrophotometric and electrochemical methods have been mainly used for analysis of mercaptans [1,16–18]. Other methods for determination of aromatic thiols and aliphatic thiols are based on different chromatographic techniques [19–25]. However, to the best of our knowledge there has not been any report based on RRS for determination of trace amounts of 2MBT in water solution samples. Resonance Rayleigh scattering (RRS) has drawn much more attention in recent years and made important contributions in many scientific areas. When a particle is exposed to an electromagnetic radiation, the electrons in the particle oscillate at the same frequency as the incident wave. Resonance Rayleigh scattering takes place when the wavelength of Rayleigh scattering is located at or close to the molecular absorption band. The properties of scattered light depend on the size, composition, shape, homogeneity of the nanoparticles, and refractive index of the medium [26,27]. Light scattering methods such as resonance Rayleigh scattering (RRS) or resonance light-scattering (RLS) has been widely applied to the determination of different analytes [28–30]. This method showed its high potential for the determination of metal ions [31], non-metallic inorganic substances [32,33], surfactants [34], biomacromolecules [35], and pharmaceuticals [36]. The method is characterized by high sensitivity, convenience in performance and simplicity in apparatus (usually common spectrofluorophotometer). Gold nanoparticles (AuNPs) and silver nanoparticles (AgNPs) possess novel physical and chemical properties, especially the surface Plasmon resonance (SPR) or resonance Rayleigh scattering (RRS), which resulted they are widely used in the fields of analytical chemistry and biomedical science as probes and sensors in recent years [37–39], exhibit their signals in the visible spectral region under appropriate conditions and give corresponding localized surface plasmon resonance light scattering (LSPR-LS) band of the NPs [40–42].
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Scheme 1. Schematic of reaction of AuNPs with 2MBT which produces AuNP–2MBT complex at pH 6.
These noble metal nanoparticles show special optical properties such as strong resonance light scattering in the orders of magnitude higher than light emission from strongly fluorescent dye molecules [43]. Such a character makes them ideal optical probes for chemical, biological and clinical applications [44–46]. Gold nanoparticles (AuNPs) exhibit certain advantages such as higher extinction coefficients, sharper extinction bands and higher ratio of scattering to extinction. More recently, AuNPs are rapidly gaining popularity as a consequence, and some research groups have been developing several strategies for optical sensors and imaging techniques using AuNPs as building blocks and labeling probes [47–49]. Herein, a simple and sensitive method for determination of 2MBT using AuNPs as probes was established on the basis of the formation of 2MBT-AuNPs aggregates and intensifying the RRS intensity of aggregated particles. Scheme 1 shows the aggregation of nanoparticles occurs when AuNPs react with thiol groups of 2MBT. The reaction leads to an increase in the size of nanoparticles and also enhancement of the RRS intensity [28–32]. The detection sensitivity can be significantly improved to lg L 1 level by monitoring of signal changes of high sensitivity RRS by AuNPs. 2. Experimental 2.1. Materials and reagents All chemicals used in the experiments were of analytical grade or higher without further purification. 2MBT was purchased from Sigma–Aldrich (America) and a working solution of 10.0 mg L 1 was prepared for use in the experiment. Sodium citrate and sodium borhydrate were purchased from Merck (Darmstadt, Germany). Buffer solutions were prepared by adjusting the pH of 0.1 mol L 1 citric acid and phosphoric acid solutions to 6 using NaOH solution (0.1 mol L 1). All solutions were prepared in high-purity water. 2.2. Apparatus A Shimadzu RF-5301PC spectrofluorophotometer (Japan) was used for recording and measuring the RRS spectra. A pH-meter (827 pH lab, Metrohm1, Herisau, Switzerland) was used for pH adjustment. Transmission electron microscopy (906E, LEO, Germany) was used to study the morphology of AuNPs and 2MBT-AuNPs. 2.3. Preparation of AuNPs
Fig. 1. Chemical structure of 2MBT fungicide.
Stock solution of Au (III) (1000 lg mL 1) was prepared by dissolving 0.100 g of pure gold (24-carat) metal in concentrated HCl:HNO3 (3:1) solution. Working gold solution was prepared by appropriate diluting of stock solution. AuNPs were synthesized by slow addition (drop wise) of 0.25 mL of citrate–borohydride
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mixture solution (10:0.75 w/v) into a beaker containing 10 mL Au (III) (10 lg mL 1) and 10 mL of sodium citrate (1% w/v) solution at room temperature. The color of this solution changed gradually from colorless to purple. The final concentration of AuNPs was 5 lg mL 1 (2.53 10 5 mol L 1). Above solution was stored at 4 °C. 2.4. Measurement of the RRS intensity of AuNPs–2MBT system Appropriate amounts of the AuNPs solution (250 lL of 5 lg mL 1), 1 mL of citrate buffer (pH 6), 1.0 mL of the 0.001 mol L 1 of KCl electrolyte, and certain volumes of 2MBT standard solutions were added into a 10.0-mL flask. The resulting solution was diluted to 10 mL and was vortex-shaken to mix thoroughly and stand for 5 min. The RRS spectra of the solutions were recorded with synchronous scanning at kex = ksc = 331 nm (i.e., Dk = 0 nm), slit widths were kept at 1.5 nm, and RRS intensity of AuNP solutions in the absence (I0) and the presence of 2MBT (IRRS) was recorded. Fig. 2 shows the recorded RRS spectra of the blank solution (red) and the test solution (gray) and the difference in RRS intensity values (DIRRS = I0 IRRS) in the wavelength range of 230–430 nm. As it is seen in Fig. 2, at kex = ksc = 331 nm a large increase in RRS intensity occurs after the addition of 2MBT to AuNPs solution. So, 331 nm was selected as optimum RRS wavelength for further works. Fig. 3 shows the absorption spectrum (red, wavelength range 200–700 nm) and RRS spectrum (green, kex = ksc = 331 nm, and 2kex = ksc = 662 nm scattering wavelength range 200–700 nm) of AuNPs solution (blank). It must be mentioned that AgNPs were also examined for detection of 2MBT and the results were compared with those obtained by AuNPs. The results showed that AuNPs can detect the target analyte (2MBT) in a shorter response time (5 min in comparison to 7 min for AgNPs) and better sensitivity (bigger DIRRS) with respect to AgNPs at the same conditions.
Fig. 3. Absorption (black) and RRS spectra of AuNPs (red) and AuNPs + 2MBT (blue). Conditions: [AuNPs] = 0.3 lg lmL 1; kex = 331 nm; scattering spectrum was scanned in the wavelength range of 200–700 nm and maximum RRS occurred at ksc = 331 nm; slit of excitation, 1.5 nm; slit of emission, 1.5 nm. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
3. Results and discussion 3.1. Shape and size of AuNPs and AuNPs–2MBT The structural characteristics such as shape and size of AuNPs and AuNPs–2MBT were investigated by TEM (Fig. 4). It can be seen from these TEM images that the average diameters of the as synthesized AuNPs were about 25 nm and also particles were comparatively homogenous, well dispersed, and almost spherically shaped without any obvious aggregation (Fig. 4A and C). It is well known that thiols will self-assemble into strictly arranged monolayers (SAMs) onto the surface of the metals, especially gold, silver, and copper [50]. These SAMs have been intensely studied and are of great interest due to unique properties of the resulting surfaces
Fig. 4. TEM and images of AuNPs before and after addition of 2MBT: (A) TEM of AuNPs, (B) TEM of AuNPs–2MBT, (C) close up TEM image of AuNPs, (D) close up TEM image of AuNPs–2MBT.
Fig. 2. RRS spectra of AuNPs in the absence (red) and presence of 2MBT (gray). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
[50]. These properties include stabilization and passivation to other reactions. Gold metal has high affinity for reaction with thio groups and a new product is formed through the combination with 2MBT (containing 2 sulfur atoms). The shape of the new particles is different from that of the AuNPs and they are aggregated together and form an inhomogeneous cluster which is obviously bigger than the original AuNPs (Fig. 4B and D). The semitransparent layer of MBT is clearly seen around and between AuNPs in Fig. 4B and C. Overall, the combination of 2MBT with the AuNPs not only increased the size of the nanoparticles to about 60 nm, but also changed their apparent shape. The AuNPs–2MBT complex has a bigger size and contains an inhomogeneous aggregate in which the AuNPs were complexed by 2MBT molecules which led to an increase of RRS intensity of gold nanoparticles. It must be mentioned that prepared AuNPs are stable for 2 month (stored at
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refrigerator, 4 °C) and the recorded RRS intensity of nanoparticles was changed from 990 (first day) to 930 after 50 days (changing about 6%). Dynamic light scattering (DLS) method is used for particle size distribution of nanoparticles before and after the addition of MBT to AuNPs solution. Fig. 5 shows the particle size distribution of nano particles before and after the analyze addition. 3.2. Spectral characteristics As well known, RRS is a process produced by the resonance of scattering and absorption of light when the wavelength of RRS is located at or close to its molecular absorption band. In such a case, the frequency of the exciting electromagnetic wave is equal to that of scattering light. AuNPs showed intense scattering of light in the wavelength region of 300–360 nm and in presence of 2MBT fungicide, the RRS intensity is increased significantly. The RRS spectra of AuNPs, AuNPs–2MBT and 2MBT solutions were examined at excitation wavelengths of 300, 310, 320, 330, 340, 350 and 360 nm and the maximum DI is located at 331 nm for kex = ksc = 331 nm. The RRS spectra of the AuNPs, AuNPs–2MBT complex and 2MBT were overlaid (seven spectra for each solution) and the results showed (Fig. 6) that: (1) the RRS intensity of the 2MBT solution is very weak and nearly zero; (2) the AuNPs shows somewhat strong RRS intensity, and the maximum RRS wavelength is located at around 330 nm; (3) the RRS intensity of the AuNPs is greatly increases after the addition of 2MBT due to aggregation of nanoparticles [51–54] and the maximum difference between scattering signals from AuNPs and AuNPs–2MBT (DIRRS) occurs at 331 nm. This observation led to the development of a sensitive methodology using gold nanoparticles as optical sensor for 2MBT determination. 3.3. Optimization of the experimental conditions 3.3.1. Effects of pH, type and volume of buffer solution on RRS intensities It is well known that pH value can readily affect the interaction between AuNPs and 2MBT molecules and influence the RRS signals of detection system. Therefore, the influence of pH on the RRS intensity of the system was studied over a pH range of 4–7, and the results are shown in Fig. 7. The pH of the working solutions was adjusted by dilute (0.01 mol L 1) HCl and NaOH solutions. As can be seen from the figure, the DIRRS of the system depends greatly on the pH value. It increased sharply when the pH
Fig. 6. Overlaid RRS spectra of the AuNPs, AuNPs–2MBT complex and 2MBT at wavelength range of 300–360 nm.
increased from 4 to 6, whereas it decreased greatly with the continuous increase of pH above 6. At low pHs, thio groups of 2MBT interact with H3O+ ions (protonation of thio groups of 2MBT) which can compete with AuNPs and DIRRS decreases. However, at pH values higher than 6, hydroxide ions seems to compete with 2MBT in terms of adsorption onto the surface of gold nanoparticles leading to inhibition of interaction between AuNPs and 2MBT and also increasing the I0 and consequently decreasing the DIRRS (DIRRS = I0 IRRS). Two buffer solutions (pH 6) were examined and citrate (0.1 mol L 1) produced better results with respect to phosphate (0.1 mol L 1). Into a 10 mL volumetric flask were added 250 lL of 5.0 lg mL 1 of AuNPs solution, 1 mL of buffer (pH 6) and appropriate amounts of 2MBT solution, respectively. The solution was finally diluted to 10 mL, mixed thoroughly and stands for 4 min. The effect of citrate buffer volume was also studied and the results showed that 1.0 mL can provide maximum DIRRS. Therefore, 1.0 mL of the citrate buffer with pH 6 was selected as the optimum experimental medium. 3.3.2. Effect of AuNPs concentration By increasing the amount of AuNPs in the experimental solution, the intensity of RRS of this system increased. Due to the limitations of spectrofluorophotometer which could not read intensities higher than 1000, a concentration of 0.125 lg mL 1 of AuNPs in the final solution was used as optimum. Considering the high RRS intensity of the system regent blank,
Fig. 5. Particle size distribution of (A) AuNPs and (B) AuNPs–MBT.
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aggregate size. Therefore, it is possible that KCl or NaCl could change the surface-state of AuNP in addition to driving the aggregation of AuNPs [56]. Therefore, 1 mL of the 0.001 mol L 1 of NaCl was used as the optimum electrolyte amount in the test solution (10 mL) medium.
Fig. 7. Influence of pH on DIRRS of AuNPs in presence of 0.1 lg mL pH of test solution was adjusted with dilute NaOH or HCl.
1
of 2MBT; the
0.125 lg mL 1 AuNPs, and low RRS intensity of AuNPs–2MBT mixture, high DIRRS was obtained and used for determination of trace concentrations of 2MBT in water samples. Fig. 8 shows the obtained results. 3.3.3. Effect of surfactants The effects of different surfactants on the AuNPs–2MBT system were studied. The presence of surfactants controls the size growth and causes more dispersion and also prevents the aggregation of nanoparticles which leads to decreasing the RRS intensity of particles [55]. The results showed that the presence of cationic surfactants, such as cetyltrimethylammonium bromide (CTAB), can decrease the RRS intensity of both AuNPs and AuNPs–2MBT solutions. The RRS intensity of AuNPs–2MBT solution (IRRS) decreases more than the RRS intensity of AuNPs (I0) and so DIRRS decreases in presence of such cationic surfactant. Sodium dodecyl sulfate (SDS) as an anionic surfactant was also tested on the system and RRS intensity of both solutions decreased dramatically. Based on the above results, it could be seen that the biggest DIRRS and higher sensitivity was obtained in the absence of common surfactants. 3.3.4. Effect of ionic strength The effect of ionic strength on RRS processes was examined using different salts such as KCl, KNO3, NaCl and Na2SO4 as electrolyte and NaCl gave better results. Results show that DIRRS is increased by increasing the salt concentration up to 0.001 mol L 1 of NaCl. But, I0 of the blank is decreased in more concentrated (up to 0.01 mol L 1) solutions due to the aggregation of nanoparticles and DIRRS decreases as the result. The decrease in RRS intensity of AuNPs (I0) in the presence of KCl or NaCl should be related to the surface-state of nanoparticles rather than the size of nanoparticle aggregates. The RRS intensity should be sensitively dependent on the surface-state of nanoparticles as well as the
3.3.5. The order of addition and standing time Effect of the adding order of different reagents was investigated. The results indicated that the order of AuNPs – buffer–electrolyte– 2MBT is the best. Under the optimum condition, the effect of standing time on the stability of RRS intensity was studied. The results showed that the DIRRS reached a maximum at 5 min after all reagents were added, and it remained stable for over 1.0 h without any significant change. Therefore, this system exhibits good stability and a standing time of 5 min was selected for further works. 3.3.6. Effect of interfering substances Under the optimal conditions, various coexisting substances such as Mg2+, Co2+, Cu2+, Na+, K+, NH+4, Ca2+, Ac , NO3 , Fe3+, SO24 , 2-mercaptobenzimidazole (2MBI) and 2-mercaptobenzole (2MBO) were examined for the interference effect on determination of 100 lg L 1 of 2MBT. The permitted relative deviation from the DIRRS value was ±5%. The results indicated that most of the interfering substances tested could be tolerated at relatively higher levels (20,000 lg L 1). 2MBI and 2MBO do interfere at 500 lg L 1. 4. Analytical applications 4.1. Calibration graphs and detection limits Calibration curve for the determination of 2MBT by RRS was obtained under the optimal experimental conditions. By increasing the concentration of 2MBT to the test solution RRS intensity by AuNPs increases and the results show two good linear relationships over the ranges 5–100 and 100–300 lg L 1. The overlaid RRS spectra and calibration graph are presented in Fig. 9. The linear regression equations were DIRRS = 3.033 C (lg L 1) + 83.84 (r = 0.994) and DIRRS = 0.910 C (lg L 1) + 290.3 (r = 0.994) for 5– 100 and 100–300 lg L 1, respectively. The limit of detection (LOD) and quantification (LOQ) were calculated in accordance to the official compendia methods by k(Sb)/m, where k = 3 for LOD and k = 10 for LOQ, Sb is the standard deviation from 8 replicate blank measurements (Sb = 1.02) and m (m = 3.033) is the slope of the calibration curve. The LOD and LOQ estimated were 1.0 and 3.0 lg L 1, respectively. Intra- and inter-day precision and accuracy data (showing reproducibility and repeatability terms) for RRS detection of 2MBT in quality controlled (QC) water samples are summarized in Table 1. The precision and accuracy of the present method conform to the criteria for the analysis of water samples according to the guidance of US-FDA where the RSD determined at each concentration level is required not to exceed 15% (20% for LOQ) and R.E. within ±15% (±20% for LOQ) of the actual value [57]. The recovery values from QC water sample solutions containing low, middle and high concentrations of 2MBT (20, 100 and 200 lg L 1) were between 98.5% and 113%. 4.2. Determination of 2MBT in water samples
Fig. 8. Effect of AuNPs concentration on (DIRRS = I0 IRRS). Instrument conditions: kex = ksc = 331 nm; slit of excitation, 1.5 nm; slit of emission, 1.5 nm.
In order to test the validity of the method, the developed procedure was applied for determination of 2MBT in real water samples. Recovery tests were used to examine the reliability and accuracy of the method, different standard amounts of 2MBT were spiked into the Ramin power plant cooling water and drinking water samples
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Table 3 The comparison of the figures of merit of proposed RRS method with some of other reported methods for 2MBT determination. Method b
SWV Uv-HPLCc HPLC–ECDd SBSE–GC SPE–LC–MSe SPE–HPLC RRS a b c d e
LDRa (lg L
1
)
7000–40,000 10,000–120,000 17–167,000 – 0.05–0.20 10–10,000 5–100 and 100–300
LOD (lg L 140 2800 4.8 0.25 0.05 2.7 1.0
1
)
RSD
Refs.
1.2–2.3 3.5 2 9.8 2.5 3.0 1.1–2.3
[1] [17] [18] [21] [22] [25] Present work
Linear dynamic range. Square wave voltammetry. Uv-high performance liquid chromatography. High performance liquid chromatography–electron capture detector. Solid phase extraction–liquid chromatography–mass spectrometry.
5. Conclusions
Fig. 9. Overlaid RRS spectra of AuNPs–2MBT (1–18) and calibration graphs for determination of 2MBT. [2MBT]: 0, 1, 2, 3, 5, 8, 10, 15, 20, 25, 30, 50, 60, 80, 90, 120, 140, and 200 lg mL 1. Instrument conditions: kex = ksc = 331 nm; slit of excitation, 1.5 nm; slit of emission, 1.5 nm.
Table 1 Precision and accuracy data for detection of 2MBT in water sample using RRS of AuNPs (intra-day: n = 7; inter-day: n = 7 runs per day, 5 days). 2MBT conc. lg L
1
RSD (%)
Relative error (%)
Added
Found (mean ± S.D.)
Intra-day
Inter-day
20 100 200
18 ± 0.009 108 ± 0.021 193 ± 0.043
4.0 3.3 2.6
4.3 4.4 3.7
10.0 +8.0 3.5
Table 2 Analytical results of the determination of 2MBT content and recovery test of 2MBT in water samples with the proposed method (n = 3) [conditions: 100 mL of water sample; 250 lL of 5.0 lg mL 1 of AuNPs solution; 1 mL of citrate buffer pH 6; standing time: 5 min; excitation wavelength: 331 nm; scattering wavelength: 331 nm; slit width: 1.5 nm]. 2MBT founda (lg L 1)
Sample
2MBT added (lg L 1)
Ahwaz drinking waterb
– 10 20
5.6 ± 0.07 15.9 ± 0.07 25.3 ± 0.08
– 103 98.5
Ramin power plant cooling water
– 10 20
85.2 ± 0.09 96.5 ± 0.05 104.4 ± 0.07
– 113 99
a b
CO23
%Recovery
Mean ± standard deviation (n = 3). Ahwaz drinking water main components: Ca2+ = 82; Mg2+ = 49; Na+ = 68; = 71; Cl = 44; SO24 = 35; NO3 = 9 lg mL 1; pH 7.1; TDS = 35; EC = 546.
(100 mL each) and 2MBT content of each sample was determined at optimum conditions. The 2MBT content of different water samples and recoveries of added analyte were evaluated and the results showed that it is possible to determine the 2MBT concentration in real sample solutions using the proposed method outlined in this investigation (Table 2). Water samples were taken from the Ramin power plant and Ahwaz drinking water. After standing for 24 h in refrigerator, the samples were filtered by a piece of filter paper. Known amounts of 2MBT were added (10 and 20 lg L 1) and were determined by the aforementioned procedure.
A novel and innovative methodology was developed for detection and quantification of ultra-traces of 2MBT and successfully applied for its determination in water solutions. The developed methodology in this study is simple, fast, sensitive and cheap, especially when more sophisticated techniques such as chromatography are not available. In this paper, a new resonance Rayleigh scattering method was used based on an increase of scattered light from the gold nanoparticles (AuNPs) after the addition of 2MBT. TEM images showed the formation of aggregated particles of AuNPs–2MBT and formation of a complex. The increasing of RRS intensity of AuNPs in the presence of trace amounts of 2MBT was studied and aggregation mechanism was proposed for such RRS phenomenon. The proposed method is simple, fast, sensitive and needs a simple spectrofluorophotometer. The main advantage of the proposed method is the possibility of direct 2MBT determination with very good accuracy, sensitivity and tolerance. The method needs no time consuming pretreatment processes such as solid phase extraction and micro-extractions for samples used. The LDR, LOD and RSD of the method are good and better than most of the reported methods (Table 3). The method needs no pretreatment processes like clean up of the sample and pre-concentration step which are mostly used in chromatographic procedures. The RSD of the method is better than most of the reported methods. The obtained results showed that the AuNPs can be applied as sensor for 2MBT determination in real water samples. Acknowledgements The authors wish to thank Shahid Chamran University Research Council and Environment Protection Agency (EPA) of Khosestan Province, Iran, for the financial support of this work (Grant 1393). References [1] H. Parham, B. Aibaghi, J. Ghasemi, J. Hazard. Mater. 151 (2008) 636–641. [2] Anon, 2-mercaptobenzothiazole; proposed test rule, Fed. Reg. 50 (1985) 46121–46133. [3] M.A. Gaja, J.S. Knapp, Water Res. 32 (1998) 3786–3789. [4] S.F. Vogt, 2-Mercaptobenzothiazole; Final Test Rule, vol. 53, Office of the Federal Register (United States), 1998, pp. 34514–34521. [5] A. Allaouia, M.A. Maloukia, P. Wong-Wah-Chung, J. Photochem. Photobiol. A: Chem. 212 (2010) 153–160. [6] H. Yang, S. Yji, W. Song, X. Zhu, Y. Yao, Z. Zhang, Corros. Sci. 50 (2008) 3160– 3167. [7] S.F. Vogt, 2-Mercaptobenzothiazole: final test rule, Fed. Regist. 53 (1988) 34514–34521. [8] C.M. Reddy, J.G. Quinn, Environ. Sci. Technol. 31 (1997) 2847–2853. [9] C.W. Yan, H.C. Lin, C.N. Cao, Electrochim. Acta 45 (2000) 2815–2821. [10] H. Bujdakova, T. Kuchta, E. Sidoova, A. Gvozdjakova, Microbiol. Lett. 112 (1993) 329–334.
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Resonance Rayleigh scattering method for determination of 2-mercaptobenzothiazole using gold nanoparticles probe Hooshang Parham ⇑, Nahid Pourreza, Farzaneh Marahel Chemistry Department, Faculty of Sciences, Shahid Chamran University, 6135714168 Ahvaz, Iran
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A novel resonance Rayleigh scattering
Schematics of the reaction between AuNPs and 2MBT fungicide which produces AuNP–2MBT complex at pH 6.
method was developed for trace 2MBT detection. AuNPs can be applied as sensor for 2MBT determination in real water samples. Resonance Rayleigh scattering of AuNPs was used for 2MBT detection in water samples. The method is fast and shows good accuracy and sensitivity for 2MBT detection.
a r t i c l e
i n f o
Article history: Received 18 December 2014 Received in revised form 5 June 2015 Accepted 28 June 2015 Available online 29 June 2015 Keywords: Resonance Rayleigh scattering 2-Mercaptobenzothiazole Gold nanoparticles Determination
a b s t r a c t A sensitive, simple and novel method was developed to determine 2-mercaptobenzothiazole (2MBT) in water samples. This method was based on the interaction between gold nanoparticles (AuNPs) and 2MBT followed by increasing of the resonance Rayleigh scattering (RRS) intensity of nanoparticles. The change in RRS intensity (DIRRS) was linearly correlated to the concentration of 2MBT over the ranges of 5.0–100.0 and 100.0–300.0 lg L 1. 2MBT can be measured in a short time (5 min) without any complicated or time-consuming sample pretreatment process. Parameters that affect the RRS intensities such as pH, concentration of AuNPs, standing time, electrolyte concentration, and coexisting substances were systematically investigated and optimized. Interference tests showed that the developed method has a very good selectivity and could be used conveniently for determination of 2MBT. The limit of detection (LOD) and limit of quantification (LOQ) were 1.0 and 3.0 lg L 1, respectively. Relative standard deviations (RSD) for 20.0 and 80.0 lg L 1 of 2MBT were 1.1 and 2.3, respectively. Possible mechanisms for the RRS changes of AuNPs in the presence of 2MBT were discussed and the method was successfully applied for the analysis of real water samples. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction The quality of drinking water, pollution of surface water and water used in agricultural plants and removal of toxic substances ⇑ Corresponding author. Tel.: +98 61 33331042; fax: +98 61 33331042. E-mail address: [email protected] (H. Parham). http://dx.doi.org/10.1016/j.saa.2015.06.108 1386-1425/Ó 2015 Elsevier B.V. All rights reserved.
from water samples are some critical environmental problems occurring in recent years. Therefore, this study aims to face the challenge of protecting clean water from pollution by these toxic substances and to suggest a removal mechanism for elimination of hazardous materials such as fungicides from water samples. Mercaptans are a class of high production chemicals employed in various industrial processes. 2-Mercaptobenzothiazole (2MBT)
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(Fig. 1) is a member of mercaptans and is the most important members of the benzothiazole group, which are heterocyclic aromatic compounds. These chemicals are toxic and poorly biodegradable pollutants [1]. The production of 2MBT and its related salts was reportedly in excess of about 46 million pounds annually in the USA [2]. Estimates from the US EPA have reported that 1 million pounds of 2MBT are released annually into the environment. Its toxicity and allergenicity make its presence in the environment a challenge and a cause for concern [2–6]. The pollution effects of 2MBT on the environment are a major problem for environmental protection agencies (EPA) worldwide. The US EPA estimated that over 450 tons of mercaptobenzothiazole is deposited into the environment every year [7,8]. 2MBT is used as corrosion inhibitors to protect copper [9], it also has other applications such as in medicine as an antifungal drug [10]; a coating agent for metallic surfaces [11] and predominately, as a vulcanization accelerator in the rubber industry [12–14]. It is frequently found in effluent of wastewater treatment plants, in surface water and they are volatile organic compounds contributing to odor problems in wastewater treatment plants [15]. Environmental pollution by sulfur compounds is always of great concern for aquatic organisms and humans. Several methods have been reported for detection and determination of 2MBT and thiols in environmental samples. Spectrophotometric and electrochemical methods have been mainly used for analysis of mercaptans [1,16–18]. Other methods for determination of aromatic thiols and aliphatic thiols are based on different chromatographic techniques [19–25]. However, to the best of our knowledge there has not been any report based on RRS for determination of trace amounts of 2MBT in water solution samples. Resonance Rayleigh scattering (RRS) has drawn much more attention in recent years and made important contributions in many scientific areas. When a particle is exposed to an electromagnetic radiation, the electrons in the particle oscillate at the same frequency as the incident wave. Resonance Rayleigh scattering takes place when the wavelength of Rayleigh scattering is located at or close to the molecular absorption band. The properties of scattered light depend on the size, composition, shape, homogeneity of the nanoparticles, and refractive index of the medium [26,27]. Light scattering methods such as resonance Rayleigh scattering (RRS) or resonance light-scattering (RLS) has been widely applied to the determination of different analytes [28–30]. This method showed its high potential for the determination of metal ions [31], non-metallic inorganic substances [32,33], surfactants [34], biomacromolecules [35], and pharmaceuticals [36]. The method is characterized by high sensitivity, convenience in performance and simplicity in apparatus (usually common spectrofluorophotometer). Gold nanoparticles (AuNPs) and silver nanoparticles (AgNPs) possess novel physical and chemical properties, especially the surface Plasmon resonance (SPR) or resonance Rayleigh scattering (RRS), which resulted they are widely used in the fields of analytical chemistry and biomedical science as probes and sensors in recent years [37–39], exhibit their signals in the visible spectral region under appropriate conditions and give corresponding localized surface plasmon resonance light scattering (LSPR-LS) band of the NPs [40–42].
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Scheme 1. Schematic of reaction of AuNPs with 2MBT which produces AuNP–2MBT complex at pH 6.
These noble metal nanoparticles show special optical properties such as strong resonance light scattering in the orders of magnitude higher than light emission from strongly fluorescent dye molecules [43]. Such a character makes them ideal optical probes for chemical, biological and clinical applications [44–46]. Gold nanoparticles (AuNPs) exhibit certain advantages such as higher extinction coefficients, sharper extinction bands and higher ratio of scattering to extinction. More recently, AuNPs are rapidly gaining popularity as a consequence, and some research groups have been developing several strategies for optical sensors and imaging techniques using AuNPs as building blocks and labeling probes [47–49]. Herein, a simple and sensitive method for determination of 2MBT using AuNPs as probes was established on the basis of the formation of 2MBT-AuNPs aggregates and intensifying the RRS intensity of aggregated particles. Scheme 1 shows the aggregation of nanoparticles occurs when AuNPs react with thiol groups of 2MBT. The reaction leads to an increase in the size of nanoparticles and also enhancement of the RRS intensity [28–32]. The detection sensitivity can be significantly improved to lg L 1 level by monitoring of signal changes of high sensitivity RRS by AuNPs. 2. Experimental 2.1. Materials and reagents All chemicals used in the experiments were of analytical grade or higher without further purification. 2MBT was purchased from Sigma–Aldrich (America) and a working solution of 10.0 mg L 1 was prepared for use in the experiment. Sodium citrate and sodium borhydrate were purchased from Merck (Darmstadt, Germany). Buffer solutions were prepared by adjusting the pH of 0.1 mol L 1 citric acid and phosphoric acid solutions to 6 using NaOH solution (0.1 mol L 1). All solutions were prepared in high-purity water. 2.2. Apparatus A Shimadzu RF-5301PC spectrofluorophotometer (Japan) was used for recording and measuring the RRS spectra. A pH-meter (827 pH lab, Metrohm1, Herisau, Switzerland) was used for pH adjustment. Transmission electron microscopy (906E, LEO, Germany) was used to study the morphology of AuNPs and 2MBT-AuNPs. 2.3. Preparation of AuNPs
Fig. 1. Chemical structure of 2MBT fungicide.
Stock solution of Au (III) (1000 lg mL 1) was prepared by dissolving 0.100 g of pure gold (24-carat) metal in concentrated HCl:HNO3 (3:1) solution. Working gold solution was prepared by appropriate diluting of stock solution. AuNPs were synthesized by slow addition (drop wise) of 0.25 mL of citrate–borohydride
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mixture solution (10:0.75 w/v) into a beaker containing 10 mL Au (III) (10 lg mL 1) and 10 mL of sodium citrate (1% w/v) solution at room temperature. The color of this solution changed gradually from colorless to purple. The final concentration of AuNPs was 5 lg mL 1 (2.53 10 5 mol L 1). Above solution was stored at 4 °C. 2.4. Measurement of the RRS intensity of AuNPs–2MBT system Appropriate amounts of the AuNPs solution (250 lL of 5 lg mL 1), 1 mL of citrate buffer (pH 6), 1.0 mL of the 0.001 mol L 1 of KCl electrolyte, and certain volumes of 2MBT standard solutions were added into a 10.0-mL flask. The resulting solution was diluted to 10 mL and was vortex-shaken to mix thoroughly and stand for 5 min. The RRS spectra of the solutions were recorded with synchronous scanning at kex = ksc = 331 nm (i.e., Dk = 0 nm), slit widths were kept at 1.5 nm, and RRS intensity of AuNP solutions in the absence (I0) and the presence of 2MBT (IRRS) was recorded. Fig. 2 shows the recorded RRS spectra of the blank solution (red) and the test solution (gray) and the difference in RRS intensity values (DIRRS = I0 IRRS) in the wavelength range of 230–430 nm. As it is seen in Fig. 2, at kex = ksc = 331 nm a large increase in RRS intensity occurs after the addition of 2MBT to AuNPs solution. So, 331 nm was selected as optimum RRS wavelength for further works. Fig. 3 shows the absorption spectrum (red, wavelength range 200–700 nm) and RRS spectrum (green, kex = ksc = 331 nm, and 2kex = ksc = 662 nm scattering wavelength range 200–700 nm) of AuNPs solution (blank). It must be mentioned that AgNPs were also examined for detection of 2MBT and the results were compared with those obtained by AuNPs. The results showed that AuNPs can detect the target analyte (2MBT) in a shorter response time (5 min in comparison to 7 min for AgNPs) and better sensitivity (bigger DIRRS) with respect to AgNPs at the same conditions.
Fig. 3. Absorption (black) and RRS spectra of AuNPs (red) and AuNPs + 2MBT (blue). Conditions: [AuNPs] = 0.3 lg lmL 1; kex = 331 nm; scattering spectrum was scanned in the wavelength range of 200–700 nm and maximum RRS occurred at ksc = 331 nm; slit of excitation, 1.5 nm; slit of emission, 1.5 nm. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
3. Results and discussion 3.1. Shape and size of AuNPs and AuNPs–2MBT The structural characteristics such as shape and size of AuNPs and AuNPs–2MBT were investigated by TEM (Fig. 4). It can be seen from these TEM images that the average diameters of the as synthesized AuNPs were about 25 nm and also particles were comparatively homogenous, well dispersed, and almost spherically shaped without any obvious aggregation (Fig. 4A and C). It is well known that thiols will self-assemble into strictly arranged monolayers (SAMs) onto the surface of the metals, especially gold, silver, and copper [50]. These SAMs have been intensely studied and are of great interest due to unique properties of the resulting surfaces
Fig. 4. TEM and images of AuNPs before and after addition of 2MBT: (A) TEM of AuNPs, (B) TEM of AuNPs–2MBT, (C) close up TEM image of AuNPs, (D) close up TEM image of AuNPs–2MBT.
Fig. 2. RRS spectra of AuNPs in the absence (red) and presence of 2MBT (gray). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
[50]. These properties include stabilization and passivation to other reactions. Gold metal has high affinity for reaction with thio groups and a new product is formed through the combination with 2MBT (containing 2 sulfur atoms). The shape of the new particles is different from that of the AuNPs and they are aggregated together and form an inhomogeneous cluster which is obviously bigger than the original AuNPs (Fig. 4B and D). The semitransparent layer of MBT is clearly seen around and between AuNPs in Fig. 4B and C. Overall, the combination of 2MBT with the AuNPs not only increased the size of the nanoparticles to about 60 nm, but also changed their apparent shape. The AuNPs–2MBT complex has a bigger size and contains an inhomogeneous aggregate in which the AuNPs were complexed by 2MBT molecules which led to an increase of RRS intensity of gold nanoparticles. It must be mentioned that prepared AuNPs are stable for 2 month (stored at
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refrigerator, 4 °C) and the recorded RRS intensity of nanoparticles was changed from 990 (first day) to 930 after 50 days (changing about 6%). Dynamic light scattering (DLS) method is used for particle size distribution of nanoparticles before and after the addition of MBT to AuNPs solution. Fig. 5 shows the particle size distribution of nano particles before and after the analyze addition. 3.2. Spectral characteristics As well known, RRS is a process produced by the resonance of scattering and absorption of light when the wavelength of RRS is located at or close to its molecular absorption band. In such a case, the frequency of the exciting electromagnetic wave is equal to that of scattering light. AuNPs showed intense scattering of light in the wavelength region of 300–360 nm and in presence of 2MBT fungicide, the RRS intensity is increased significantly. The RRS spectra of AuNPs, AuNPs–2MBT and 2MBT solutions were examined at excitation wavelengths of 300, 310, 320, 330, 340, 350 and 360 nm and the maximum DI is located at 331 nm for kex = ksc = 331 nm. The RRS spectra of the AuNPs, AuNPs–2MBT complex and 2MBT were overlaid (seven spectra for each solution) and the results showed (Fig. 6) that: (1) the RRS intensity of the 2MBT solution is very weak and nearly zero; (2) the AuNPs shows somewhat strong RRS intensity, and the maximum RRS wavelength is located at around 330 nm; (3) the RRS intensity of the AuNPs is greatly increases after the addition of 2MBT due to aggregation of nanoparticles [51–54] and the maximum difference between scattering signals from AuNPs and AuNPs–2MBT (DIRRS) occurs at 331 nm. This observation led to the development of a sensitive methodology using gold nanoparticles as optical sensor for 2MBT determination. 3.3. Optimization of the experimental conditions 3.3.1. Effects of pH, type and volume of buffer solution on RRS intensities It is well known that pH value can readily affect the interaction between AuNPs and 2MBT molecules and influence the RRS signals of detection system. Therefore, the influence of pH on the RRS intensity of the system was studied over a pH range of 4–7, and the results are shown in Fig. 7. The pH of the working solutions was adjusted by dilute (0.01 mol L 1) HCl and NaOH solutions. As can be seen from the figure, the DIRRS of the system depends greatly on the pH value. It increased sharply when the pH
Fig. 6. Overlaid RRS spectra of the AuNPs, AuNPs–2MBT complex and 2MBT at wavelength range of 300–360 nm.
increased from 4 to 6, whereas it decreased greatly with the continuous increase of pH above 6. At low pHs, thio groups of 2MBT interact with H3O+ ions (protonation of thio groups of 2MBT) which can compete with AuNPs and DIRRS decreases. However, at pH values higher than 6, hydroxide ions seems to compete with 2MBT in terms of adsorption onto the surface of gold nanoparticles leading to inhibition of interaction between AuNPs and 2MBT and also increasing the I0 and consequently decreasing the DIRRS (DIRRS = I0 IRRS). Two buffer solutions (pH 6) were examined and citrate (0.1 mol L 1) produced better results with respect to phosphate (0.1 mol L 1). Into a 10 mL volumetric flask were added 250 lL of 5.0 lg mL 1 of AuNPs solution, 1 mL of buffer (pH 6) and appropriate amounts of 2MBT solution, respectively. The solution was finally diluted to 10 mL, mixed thoroughly and stands for 4 min. The effect of citrate buffer volume was also studied and the results showed that 1.0 mL can provide maximum DIRRS. Therefore, 1.0 mL of the citrate buffer with pH 6 was selected as the optimum experimental medium. 3.3.2. Effect of AuNPs concentration By increasing the amount of AuNPs in the experimental solution, the intensity of RRS of this system increased. Due to the limitations of spectrofluorophotometer which could not read intensities higher than 1000, a concentration of 0.125 lg mL 1 of AuNPs in the final solution was used as optimum. Considering the high RRS intensity of the system regent blank,
Fig. 5. Particle size distribution of (A) AuNPs and (B) AuNPs–MBT.
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aggregate size. Therefore, it is possible that KCl or NaCl could change the surface-state of AuNP in addition to driving the aggregation of AuNPs [56]. Therefore, 1 mL of the 0.001 mol L 1 of NaCl was used as the optimum electrolyte amount in the test solution (10 mL) medium.
Fig. 7. Influence of pH on DIRRS of AuNPs in presence of 0.1 lg mL pH of test solution was adjusted with dilute NaOH or HCl.
1
of 2MBT; the
0.125 lg mL 1 AuNPs, and low RRS intensity of AuNPs–2MBT mixture, high DIRRS was obtained and used for determination of trace concentrations of 2MBT in water samples. Fig. 8 shows the obtained results. 3.3.3. Effect of surfactants The effects of different surfactants on the AuNPs–2MBT system were studied. The presence of surfactants controls the size growth and causes more dispersion and also prevents the aggregation of nanoparticles which leads to decreasing the RRS intensity of particles [55]. The results showed that the presence of cationic surfactants, such as cetyltrimethylammonium bromide (CTAB), can decrease the RRS intensity of both AuNPs and AuNPs–2MBT solutions. The RRS intensity of AuNPs–2MBT solution (IRRS) decreases more than the RRS intensity of AuNPs (I0) and so DIRRS decreases in presence of such cationic surfactant. Sodium dodecyl sulfate (SDS) as an anionic surfactant was also tested on the system and RRS intensity of both solutions decreased dramatically. Based on the above results, it could be seen that the biggest DIRRS and higher sensitivity was obtained in the absence of common surfactants. 3.3.4. Effect of ionic strength The effect of ionic strength on RRS processes was examined using different salts such as KCl, KNO3, NaCl and Na2SO4 as electrolyte and NaCl gave better results. Results show that DIRRS is increased by increasing the salt concentration up to 0.001 mol L 1 of NaCl. But, I0 of the blank is decreased in more concentrated (up to 0.01 mol L 1) solutions due to the aggregation of nanoparticles and DIRRS decreases as the result. The decrease in RRS intensity of AuNPs (I0) in the presence of KCl or NaCl should be related to the surface-state of nanoparticles rather than the size of nanoparticle aggregates. The RRS intensity should be sensitively dependent on the surface-state of nanoparticles as well as the
3.3.5. The order of addition and standing time Effect of the adding order of different reagents was investigated. The results indicated that the order of AuNPs – buffer–electrolyte– 2MBT is the best. Under the optimum condition, the effect of standing time on the stability of RRS intensity was studied. The results showed that the DIRRS reached a maximum at 5 min after all reagents were added, and it remained stable for over 1.0 h without any significant change. Therefore, this system exhibits good stability and a standing time of 5 min was selected for further works. 3.3.6. Effect of interfering substances Under the optimal conditions, various coexisting substances such as Mg2+, Co2+, Cu2+, Na+, K+, NH+4, Ca2+, Ac , NO3 , Fe3+, SO24 , 2-mercaptobenzimidazole (2MBI) and 2-mercaptobenzole (2MBO) were examined for the interference effect on determination of 100 lg L 1 of 2MBT. The permitted relative deviation from the DIRRS value was ±5%. The results indicated that most of the interfering substances tested could be tolerated at relatively higher levels (20,000 lg L 1). 2MBI and 2MBO do interfere at 500 lg L 1. 4. Analytical applications 4.1. Calibration graphs and detection limits Calibration curve for the determination of 2MBT by RRS was obtained under the optimal experimental conditions. By increasing the concentration of 2MBT to the test solution RRS intensity by AuNPs increases and the results show two good linear relationships over the ranges 5–100 and 100–300 lg L 1. The overlaid RRS spectra and calibration graph are presented in Fig. 9. The linear regression equations were DIRRS = 3.033 C (lg L 1) + 83.84 (r = 0.994) and DIRRS = 0.910 C (lg L 1) + 290.3 (r = 0.994) for 5– 100 and 100–300 lg L 1, respectively. The limit of detection (LOD) and quantification (LOQ) were calculated in accordance to the official compendia methods by k(Sb)/m, where k = 3 for LOD and k = 10 for LOQ, Sb is the standard deviation from 8 replicate blank measurements (Sb = 1.02) and m (m = 3.033) is the slope of the calibration curve. The LOD and LOQ estimated were 1.0 and 3.0 lg L 1, respectively. Intra- and inter-day precision and accuracy data (showing reproducibility and repeatability terms) for RRS detection of 2MBT in quality controlled (QC) water samples are summarized in Table 1. The precision and accuracy of the present method conform to the criteria for the analysis of water samples according to the guidance of US-FDA where the RSD determined at each concentration level is required not to exceed 15% (20% for LOQ) and R.E. within ±15% (±20% for LOQ) of the actual value [57]. The recovery values from QC water sample solutions containing low, middle and high concentrations of 2MBT (20, 100 and 200 lg L 1) were between 98.5% and 113%. 4.2. Determination of 2MBT in water samples
Fig. 8. Effect of AuNPs concentration on (DIRRS = I0 IRRS). Instrument conditions: kex = ksc = 331 nm; slit of excitation, 1.5 nm; slit of emission, 1.5 nm.
In order to test the validity of the method, the developed procedure was applied for determination of 2MBT in real water samples. Recovery tests were used to examine the reliability and accuracy of the method, different standard amounts of 2MBT were spiked into the Ramin power plant cooling water and drinking water samples
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Table 3 The comparison of the figures of merit of proposed RRS method with some of other reported methods for 2MBT determination. Method b
SWV Uv-HPLCc HPLC–ECDd SBSE–GC SPE–LC–MSe SPE–HPLC RRS a b c d e
LDRa (lg L
1
)
7000–40,000 10,000–120,000 17–167,000 – 0.05–0.20 10–10,000 5–100 and 100–300
LOD (lg L 140 2800 4.8 0.25 0.05 2.7 1.0
1
)
RSD
Refs.
1.2–2.3 3.5 2 9.8 2.5 3.0 1.1–2.3
[1] [17] [18] [21] [22] [25] Present work
Linear dynamic range. Square wave voltammetry. Uv-high performance liquid chromatography. High performance liquid chromatography–electron capture detector. Solid phase extraction–liquid chromatography–mass spectrometry.
5. Conclusions
Fig. 9. Overlaid RRS spectra of AuNPs–2MBT (1–18) and calibration graphs for determination of 2MBT. [2MBT]: 0, 1, 2, 3, 5, 8, 10, 15, 20, 25, 30, 50, 60, 80, 90, 120, 140, and 200 lg mL 1. Instrument conditions: kex = ksc = 331 nm; slit of excitation, 1.5 nm; slit of emission, 1.5 nm.
Table 1 Precision and accuracy data for detection of 2MBT in water sample using RRS of AuNPs (intra-day: n = 7; inter-day: n = 7 runs per day, 5 days). 2MBT conc. lg L
1
RSD (%)
Relative error (%)
Added
Found (mean ± S.D.)
Intra-day
Inter-day
20 100 200
18 ± 0.009 108 ± 0.021 193 ± 0.043
4.0 3.3 2.6
4.3 4.4 3.7
10.0 +8.0 3.5
Table 2 Analytical results of the determination of 2MBT content and recovery test of 2MBT in water samples with the proposed method (n = 3) [conditions: 100 mL of water sample; 250 lL of 5.0 lg mL 1 of AuNPs solution; 1 mL of citrate buffer pH 6; standing time: 5 min; excitation wavelength: 331 nm; scattering wavelength: 331 nm; slit width: 1.5 nm]. 2MBT founda (lg L 1)
Sample
2MBT added (lg L 1)
Ahwaz drinking waterb
– 10 20
5.6 ± 0.07 15.9 ± 0.07 25.3 ± 0.08
– 103 98.5
Ramin power plant cooling water
– 10 20
85.2 ± 0.09 96.5 ± 0.05 104.4 ± 0.07
– 113 99
a b
CO23
%Recovery
Mean ± standard deviation (n = 3). Ahwaz drinking water main components: Ca2+ = 82; Mg2+ = 49; Na+ = 68; = 71; Cl = 44; SO24 = 35; NO3 = 9 lg mL 1; pH 7.1; TDS = 35; EC = 546.
(100 mL each) and 2MBT content of each sample was determined at optimum conditions. The 2MBT content of different water samples and recoveries of added analyte were evaluated and the results showed that it is possible to determine the 2MBT concentration in real sample solutions using the proposed method outlined in this investigation (Table 2). Water samples were taken from the Ramin power plant and Ahwaz drinking water. After standing for 24 h in refrigerator, the samples were filtered by a piece of filter paper. Known amounts of 2MBT were added (10 and 20 lg L 1) and were determined by the aforementioned procedure.
A novel and innovative methodology was developed for detection and quantification of ultra-traces of 2MBT and successfully applied for its determination in water solutions. The developed methodology in this study is simple, fast, sensitive and cheap, especially when more sophisticated techniques such as chromatography are not available. In this paper, a new resonance Rayleigh scattering method was used based on an increase of scattered light from the gold nanoparticles (AuNPs) after the addition of 2MBT. TEM images showed the formation of aggregated particles of AuNPs–2MBT and formation of a complex. The increasing of RRS intensity of AuNPs in the presence of trace amounts of 2MBT was studied and aggregation mechanism was proposed for such RRS phenomenon. The proposed method is simple, fast, sensitive and needs a simple spectrofluorophotometer. The main advantage of the proposed method is the possibility of direct 2MBT determination with very good accuracy, sensitivity and tolerance. The method needs no time consuming pretreatment processes such as solid phase extraction and micro-extractions for samples used. The LDR, LOD and RSD of the method are good and better than most of the reported methods (Table 3). The method needs no pretreatment processes like clean up of the sample and pre-concentration step which are mostly used in chromatographic procedures. The RSD of the method is better than most of the reported methods. The obtained results showed that the AuNPs can be applied as sensor for 2MBT determination in real water samples. Acknowledgements The authors wish to thank Shahid Chamran University Research Council and Environment Protection Agency (EPA) of Khosestan Province, Iran, for the financial support of this work (Grant 1393). References [1] H. Parham, B. Aibaghi, J. Ghasemi, J. Hazard. Mater. 151 (2008) 636–641. [2] Anon, 2-mercaptobenzothiazole; proposed test rule, Fed. Reg. 50 (1985) 46121–46133. [3] M.A. Gaja, J.S. Knapp, Water Res. 32 (1998) 3786–3789. [4] S.F. Vogt, 2-Mercaptobenzothiazole; Final Test Rule, vol. 53, Office of the Federal Register (United States), 1998, pp. 34514–34521. [5] A. Allaouia, M.A. Maloukia, P. Wong-Wah-Chung, J. Photochem. Photobiol. A: Chem. 212 (2010) 153–160. [6] H. Yang, S. Yji, W. Song, X. Zhu, Y. Yao, Z. Zhang, Corros. Sci. 50 (2008) 3160– 3167. [7] S.F. Vogt, 2-Mercaptobenzothiazole: final test rule, Fed. Regist. 53 (1988) 34514–34521. [8] C.M. Reddy, J.G. Quinn, Environ. Sci. Technol. 31 (1997) 2847–2853. [9] C.W. Yan, H.C. Lin, C.N. Cao, Electrochim. Acta 45 (2000) 2815–2821. [10] H. Bujdakova, T. Kuchta, E. Sidoova, A. Gvozdjakova, Microbiol. Lett. 112 (1993) 329–334.
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