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Colorimetric determination of nabumetone based on localized surface plasmon resonance of functionalized gold nanoparticles as a chemical sensor
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Colorimetric determination of nabumetone based on localized surface plasmon resonance of functionalized gold nanoparticles as a chemical sensor Javad Khodaveisi a , Shayessteh Dadfarnia a,∗ , Ali Mohammad Haji Shabani a , Dariush Saberi b a b
Department of Chemistry, Faculty of Science, Yazd University, 89195-741, Yazd, Iran Fisheries and Aquaculture Department, Faculty of Agriculture and Natural Resources, Persian Gulf University, Boushehr, 75169, Iran
a r t i c l e
i n f o
Article history: Received 9 July 2016 Received in revised form 18 September 2016 Accepted 19 September 2016 Available online xxx Keywords: Localized surface plasmon resonance Nabumetone Thiolated -cyclodextrin Gold nanoparticle Colorimetric chemical sensor
a b s t r a c t A highly selective and sensitive colorimetric sensor for the determination of nabumetone (NAB) based on the aggregation of the thiolated -cyclodextrin (T-CD) functionalized gold nanoparticles (Au-NPs) with NAB in the presence of polyvinylpyrrolidone (PVP) was developed. Thiolated -cyclodextrin bonds to the surface of Au-NPs and forms a complex with NAB. The presence of PVP changed the NAB:T-CD complex to the NAB:(T-CD)2 :PVP ternary complex which resulted in the aggregation of the Au-NPs. As a result of this aggregation, the localized surface plasmon resonance (LSPR) band of Au-NPs around 520 nm decreased and a new red shifted band at 650 nm appeared which gradually increased with an increase in the NAB concentration. Under the optimized conditions, the calibration curve derived from the ratio of absorbance intensity at 650 nm to the original wavelength of 520 nm against NAB concentration was linear in the concentration range of 1–120 g L−1 . The limits of detection (LOD) and quantification (LOQ) were 0.2 and 0.7 g L−1 , respectively. The relative standard deviation at 20 g L−1 of NAB was found to be 3.1%. The selectivity of the method was demonstrated through the analysis of the synthetic samples containing the major interference compound reported in the literature. Finally, the method was successfully applied to the determination of NAB in pharmaceutical, urine and wastewater samples. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Nabumetone (NAB), (4-(6-methoxynaphthalen-2-yl)-butan-2one), is a non-steroidal anti-inflammatory drug (NSAID) belonging to the 2, 6-disubstituted naphthyl-alkanones class (Fig. 1) [1]. NAB is a less aggressive drug with some side effects metabolizing to the active 6-methoxy-2-naphthylacetic acid (6-MNA) [2,3]. NAB is efficient as an anti-inflammatory, analgesic and antipyretic pre-drug due to the inhibition of the cyclooxygenase enzymes responsible for the synthesis of prostaglandins [4]. Various techniques were used for the determination of NAB including high performance liquid chromatography (HPLC) [5,6] micellar liquid chromatography (MLC) [7] differential pulse polarography (DPP), osteryoung square wave (OSW) [8],
∗ Corresponding author. E-mail addresses: [email protected] (J. Khodaveisi), [email protected] (S. Dadfarnia), [email protected] (A.M.H. Shabani), [email protected] (D. Saberi).
ultraviolet-visible (UV–vis) spectrophotometry [9–13], flow injection analysis (FIA) with UV detector, time-resolved fluorescence (TRF) [15] and micelle-stabilized room temperature phosphorescence (MS-RTP) [16]. Among these methods, spectrophotometry has attracted more attention due to its low cost, availability in most laboratories as well as the ease of operation. Cyclodextrins are cyclic oligosaccharides constituted by 5 or more d-glucopyranoside units containing almost a conical hydrophobic cavity which is able to form an inclusion complex with a large variety of molecules [17]. They are often used to increase the aqueous solubility and the chemical stability of some molecules through the formation of an inclusion complex which modifies the properties of the guest molecule [18]. -Cyclodextrin (-CD) with 7-membered sugar ring molecules usually possesses better complexation efficiency with drugs than other cyclodextrins [19–21]. In recent years, the noble metal nanoparticles based colorimetric sensors have received great interest for the potential analytes determination due to the localized surface plasmon resonance (LSPR) phenomenon, which is responsible for their optical dependent properties [22–25]. Noble metal nanoparticles are emerging
http://dx.doi.org/10.1016/j.snb.2016.09.110 0925-4005/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: J. Khodaveisi, et al., Colorimetric determination of nabumetone based on localized surface plasmon resonance of functionalized gold nanoparticles as a chemical sensor, Sens. Actuators B: Chem. (2016), http://dx.doi.org/10.1016/j.snb.2016.09.110
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had a maximum absorption at 520 nm with the extinction coefficient () of ∼6 × 108 mol−1 L cm−1 for the 13 nm Au-NPs and based on the Beer’s law the particle concentration in the solution was estimated to be 15 nmol L−1 [31,32]. 2.3. Synthesis of thiolated ˇ-cyclodextrin (Tˇ-CD)
Fig. 1. Structural formula of NAB.
as a promising tool for the spectrophotometric quantification of a wide variety of analytes due to their very high extinction coefficients as well as the dependence of LSPR wavelength and intensity on the shape, size and dielectric constant of the medium [26–28]. The objective of this work is to combine the phenomenon of formation of ternary complex of NAB with -CD in the presence of PVP and the localized surface plasmon resonance (LSPR) of Au-NPs to develop a colorimetric sensor for the determination of NAB. Thiolated -cyclodextrin is able to bond to the surface of Au-NPs and form a complex with NAB. However, the presence of PVP changes the NAB:T-CD complex to the NAB:(T-CD)2 :PVP ternary complex which results in the aggregation of the Au-NPs and causes the LSPR band of Au-NPs around 520 nm to decrease while a new red shifted band at 650 nm appears. Furthermore, the ratio of absorbance intensity at 650 nm to the original wavelength of 520 nm was found to be directly proportional to NAB concentration. Thus, based on this observation, a highly selective and sensitive spectrophotometric method for the determination of NAB was designed and the factors affecting the aggregation of Au-NPs such as the ionic strength, pH and concentration of PVP and Au-NPs were studied and optimized using the univariable technique. Finally, the applicability of the method for the determination of NAB in real samples was investigated. 2. Experimental 2.1. Reagents and apparatus Tetrachloroauric acid (HAuCl4 ·4H2 O, 99.99%) and NAB were purchased from Sigma-Aldrich (St. Louis, MO, USA) and cyclodextrin (-CD, 99%) was supplied by Acros (Geel, Belgium). All the other reagents which were analytical grade, were purchased from Merck (Darmstadt, Germany). Deionized water was used for the preparation of the solutions. All the experiments were performed at the ambient temperature of 25 ± 2 ◦ C. The absorbance spectra were recorded with a UV–vis spectrophotometer (HACH DR 3900, Loveland, Colorado, USA) using a 1.0 cm glass cell. The pH was measured with an EcoMet Model P25 pH meter (Guro-gu, Seoul, Korea) equipped with a combined glasscalomel electrode. The size of the Au-NPs was characterized by transmission electron microscopy (TEM) using a Zeiss transmission electron microscope (Jena, Germany) operating at an accelerating voltage of 80 kV. 2.2. Procedure for the preparation of Au-NPs Citrate-capped Au-NPs with an average diameter of about 13.1 ± 1.3 nm were synthesized according to the method reported by Turkevich et al. [29,30], i.e. 50 mL of 1 mmol L−1 solution of HAuCl4 was prepared and heated under reflux. At the boiling point, 5 mL of 38.8 mmol L−1 trisodium citrate was added under vigorous stirring and the mixture was heated under reflux for 30 min. In this stage, the color changed to deep red which indicates the formation of Au-NPs. Then, the solution was cooled down to room temperature and was stored at 4 ◦ C for further use. The resulted solution
T-CD was prepared from per-6-iodo--cyclodextrin (CD-I). CD-I was synthesised according to previous reported method by Stoddart and Kaifer et al. [33,34]; briefly, 13.3 g (48 mmol) of Ph3 P was dissolved in 30 mL of anhydrous DMF under stirring, 13.5 g (53 mmol) of I2 was also dissolved in 30 mL of anhydrous DMF and was added drop wise to the Ph3 P solution under nitrogen protection. When the addition was completed, the solution was heated to 80 ◦ C and then anhydrous -CD (4.0 g, 3.3 mmol) was added to this dark brown solution and was stirred at 80 ◦ C for 15 h. Then, the solvent was evaporated to half of its original volume under vacuum and the pH of the resulting solution was adjusted to 9–10 upon the addition of sodium methoxide in an ice-water bath. The reaction mixture was stirred at ambient temperature for 30 min and it was then added to 500 mL of methanol to form precipitate. The precipitate was separated via filtration and then it was washed with methanol. The product was purified through Soxhlet extract with methanol for 1 day and after drying under vacuum, CD-I was recovered as a white powder (yield: 69%). Then, the T-CD was prepared from the synthesized CD-I according to the previously reported method [35]. Thus, 3.86 g of CD-I and 1.21 g of thiourea were dissolved in 40 mL of DMF and the mixture was heated at 70 ◦ C under nitrogen atmosphere for 19 h. Next, the DMF was removed under reduced pressure and the obtained yellow oil was dissolved in 200 mL of water. In the next step, 1.04 g of sodium hydroxide was added and the reaction mixture was heated to a gentle reflux under nitrogen atmosphere. After 1 h, the resulting suspension was acidified with aqueous KHSO4 . So, a white precipitate of T-CD was obtained and it was filtered, washed thoroughly with water and dried under vacuum (yield: 81%). 2.4. Procedure for the determination of NAB For the determination of NAB, 2 mL of the prepared Au-NPs, 1 mL of 50 mol L−1 T-CD and 5 mL of deionized water were transferred to a 10 mL volumetric flask and the mixture was vigorously stirred for 6 min. Then, respectively, 100 L of NaCl (1.0 mol L−1 ), 0.5 mL of PVP (0.4 g L−1 ) and 1.2 mL of NAB standard or sample solution containing not more than 1.2 g of NAB were added to the flask and the volume was adjusted upon the addition of deionized water. In this stage the color of solution was changed from red to blue which could be detected by naked eyes. The absorbance spectrum was recorded after 6 min. The ratio of intensity of absorbance at 650 nm to 520 nm was directly proportional to the NAB concentration and was used as the analytical signal. It is to be reminded that the order of the addition of the reagents is supreme importance and the alteration of the mentioned order causes inaccurate results with a low level of reproducibility. 2.5. Preparation of real samples 2.5.1. Pharmaceutical tablets The 500 mg NAB tablets (Meda Pharma S.A.U., San Fernando de Henares, Madrid), were purchased from a drugstore. Ten tablets were powdered and mixed thoroughly and an amount equivalent to one-tenth of the weight of one tablet (∼0.102 g) was dissolved in deionized water. It was then stirred for 30 min, filtered, and diluted to a volume of 1 L with deionized water. After that, the solution was
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Fig. 2. Schematic diagram for aggregation of functionalized Au-NPs by NAB in the presence of T-CD and PVP.
properly diluted in order to get into the linear range of the developed method and was treated according to the given procedure. 2.5.2. Urine sample The urine sample was offered by a healthy volunteer in our laboratory who was not taking any drugs during the last four months. The sample was frozen after sampling, it was then thawed at room temperature just before analysis, centrifuged for 15 min at 5000 rpm and filtered through a 0.45 m Millipore filter. Afterwards, 1 mL of the sample was diluted to 100 mL and was treated according to the given procedure. 2.5.3. Wastewater sample The wastewater sample was collected from the Boushehr wastewater treatment plant (Boushehr, Iran) in glass bottles and was transferred to the laboratory. The samples were filtered through a 0.45 m Millipore filter, diluted properly with deionized water and were treated according to the given procedure. 3. Results and discussion Valero et al. [36] reported that the presence of PVP elicits a strong increase in the affinity of -CD for NAB, interacts with the free NAB and the NAB:-CD forming an inclusion complex. In the ternary complex, NAB is wrapped at both ends by -CD and the PVP binds them together via a bridge. Furthermore, it was reported that the thiolated molecules have high affinity toward the surface of Au-NPs due to the hard-soft acid-base interaction which displaces the citrate groups [37]. Based on these reports, the -CD was thiolated and immobilized on the surface of the synthesized Au-NPs. Then, in the presence of PVP, the immobilized T-CD interacted with NAB and formed the ternary complex of NAB:(-CD)2 -PVP. This resulted in the aggregation of NPs (Fig. 2) and caused the LSPR band of Au-NPs around 520 nm to decrease while a new red shifted band appeared at 650 nm due to the near-field coupling in the resonant wavelength peak of the interacting particles [38]. The typical TEM images and corresponding UV–vis absorption spectra of Au-
NPs, before and after of aggregation, are displayed in Fig. 3. The ratio of intensity of band at 650 nm to band at 520 nm was also found to be directly proportional to the NAB concentration. Thus, based on this observation, a spectrophotometric method for the determination of NAB was designed. In order to establish the best analytical conditions for the detection of NAB, the effect of the critical parameters including ionic strength, pH, PVP and Au-NPs concentration was optimized. 3.1. Effect of ionic strength and time Ionic strength has an important role in the aggregation of nanoparticles due to the ability of strong electrolytes to reduce the volume of electrical double-layer aroused from the capping agent. Therefore, some experiments were conducted by varying the concentration of NaCl between 0 and 14 mol L−1 . It illustrated that in the absence of strong electrolytes the Au-NPs did not undergo the aggregation even at high concentration of NAB, nevertheless, an increase in the salt concentration up to 10 mol L−1 caused an increase in the aggregation along with the augmentation of the A650 /A520 signal which leveled off up to 16 mol L−1 of salt (Fig. 1S). However, when the concentration of NaCl was more than 16 mol L−1 , the nanoparticles aggregated even in the absence of the analyte. Thus, a concentration of 10 mol L−1 of NaCl with no aggregation of Au-NPs in the absence of analyte, was selected as the optimum concentration of NaCl. Furthermore, the change in the absorbance spectrum as the function of time after the addition of NAB, was studied (Fig. 4). It showed that after 6 min, there was no change in the absorbance signals indicating that the aggregation was complete. So, the spectrophotometric measurement was done after 6 min. 3.2. Effect of pH The pH of the solution is an important factor that may influence the form of the analyte in the solution as well as the surface charge and aggregation of NPs. The Au-NPs are stable in the pH range of
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Fig. 3. Typical TEM images (A) and UV–vis absorption spectra (B) of functionalized Au-NPs; I) before and II) after the addition of 120 g L−1 of NAB. Conditions: Au-NPs 3.0 nmol L−1 , NaCl 10 mol L−1 , NAB 60 g L−1 , pH 7.0 PVP 0.02 g L−1 .
Fig. 4. Variation in the surface plasmon spectrum of Au-NPs at times interval of 60 s. Conditions: Au-NPs 3.0 nmol L−1 , PVP 0.02 g L−1 , NaCl 10 mol L−1 , pH 7.0, NAB 120 g L−1 .
4.5–10 [39] and the internal cavity of the -CD is non-polar. Thus, the pH of the solution should properly be adjusted so that the AuNPs is stable and the neutral form of analyte with high affinity for interaction with -CD dominates. The pH of solution was varied in the range of 2.0–11.0 and as expected, it was found that (Fig. 5) in the pH range of 5.0–8.0 the signal was maximized. The decrease in signal at lower pH can be due to the instability of the Au-NPs, whereas, the decrease at pH greater than 8.0, can be because of the formation of ionic form of the NAB. In further experiments, a pH of approximately 7.0 was selected as the optimum.
Fig. 5. Effect of pH on the aggregation of Au-NPs. Conditions: Au-NPs 3.0 nmol L−1 , PVP 0.02 g L−1 , NaCl 10 mol L−1 , NAB 60 g L−1 .
3.3. Effect of concentration of Au-NPs In order to increase the sensitivity of the technique, the effect of the concentration of Au-NPs on its aggregation in the presence of 50 g L−1 NAB and 10 M NaCl at the pH 7.0 was investigated. The results of this study (Fig. 6) revealed that an increase in the concentration of Au-NPs could cause an increase in analytical signal which leveled off at 3 nmol L−1 of Au-NPs. It should be reminded that the decrease in signal at lower concentration of NPs can be related to the kinetic of the aggregation of NPs, as the speed of aggregation improves with an increase in the Au-NPs concentration. Therefore,
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Table 2 Results of the analysis of NAB in pharmaceutical real sample. Sample
Added (g L−1 )
Founda (g L−1 )
Recovery (%)
Pharmaceutical tablet
0.0 10.0 30.0 60.0
50.2 ± 1.2 59.8 ± 1.4 79.8 ± 1.8 108.8 ± 2.2
– 96.0 98.6 97.6
Urine sample
0.0 20.0 60.0 100.0
ND 19.2 ± 0.3 60.8 ± 1.0 102.6 ± 2.3
– 96.0 101.3 102.6
Wastewater sample
0.0 20.0 60.0 100.0
ND 20.2 ± 0.3 59.2 ± 0.8 104.2 ± 1.8
– 101.0 98.6 104.2
a The results are mean of three measurements ± standard deviation; ND = Not detected.
Fig. 6. Effect of Au-NPs concentration on the surface plasmon band intensity. Conditions: PVP 0.02 g L−1 , NaCl 10 mol L−1 , NAB 60 g L−1 , pH 7.0.
Table 1 Tolerance limits of interferences species in the determination NAB (50 g L−1 ) at the optimum conditions. Species
Mole ratio of interfering species to analyte
Salicylic acid, mefenamic acid, paracetamol 6-Methoxy-2-naphthylacetic acid (6-MNA), Naproxen Sucrose, glucose, lactose, sodium benzoate, Glycerin
2000 500 1000 20000 30000
with regards to this experiment, a concentration of 3 nmol L−1 of the Au-NPs was selected for further experiments.
3.4. Effect of PVP concentration The effect of PVP concentration in the range of 0.01-0.05 g L−1 on encapsulation of NAB through the formation of ternary complex of NAB:(T-CD)2 :PVP was surveyed (Fig. 2S). The results showed that the analytical signal would increase with an increase in the PVP concentration up to 0.02 g L−1 , remain constant up to 0.03 g L−1 and then it would decrease with further increase in PVP concentration. The decrease in signal at concentration greater than 0.03 g L−1 of PVP can be due to the inhibition effect of PVP as a capping agent in aggregation of Au-NPs [27]. Hence, a PVP concentration of 0.02 g L−1 was selected for further studies.
3.5. Interference study To assess the selectivity of the designed method for the determination of nabumetone, several experiments were performed in the presence of the compounds reported in the literature to interfere with the determination of NAB including naproxen, 6-methoxy2-naphthylacetic acid (6-MNA), salicylic acid, mefenamic acid and paracetamol [5,9,10,15] as well as in the presence of some common excipients such as sucrose, glucose, lactose, sodium benzoate and glycerin [16]. The tolerance limit was defined as the maximum concentration of the interference species that produce a relative error of less than ±5%. The results presented in Table 1 revealed that at the given mole ratio, no major interference in the determination of NAB was observed. Thus, the procedure is very selective for the determination of NAB.
Fig. 7. (A): A typical absorption spectra of Au-NPs in presence of different concentration of NAP. (B): Calibration curve of NAP under optimum conditions.
3.6. Figures of merit The linear range of the determination of NAB was evaluated under the optimized conditions. A linear relation was observed between the ratio of A650 /A520 and the concentration of NAB in the range of 1–120 g L−1 with R2 = 0.9954 (Fig. 7). The limit of detection (LOD) and quantification (LOQ) (defined as 3Sb /m and 10Sb /m) were found to be 0.2 and 0.7 g L−1 , respectively. The study of precision was made through five independent experiments and a relative standard deviation (RSD%) of 3.1% was determined for 20 g L−1 of NAB. Furthermore, some of the figure of merit of developed method was compared with the literature values. It was found
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Table 3 Comparison of Figures of merit of the developed method with other reported methods for determination of NAB. Method
LODa (g L−1 )
LDRb (g L−1 )
Ref.
Reverse phase HPLC with UV detection HPLC with diode array detector Micellar liquid chromatography Differential pulse polarography Osteryoung square wave voltammetry First-order derivative spectrophotometric method Ultraviolet-visible method-area under curve method Spectrophotometric methods-Statistical Ultraviolet spectrophotometric method Dual wavelength spectrophotometric method Flow injection analysis with Ultraviolet detector Time-resolved fluorescence Micelle-stabilized room temperature phosphorescence Localized surface plasmon resonance
1.5 50 12 17.5 8.2 560 260 230 210 300 100 0.96 18.2 0.2
24–288 100–4560 500–25000 230–18300 230–18300 3000–18000 5000–25000 10000–30000 5000–45000 2000–20000 320–6300 up to 3000 up to 1000 1–120
[6] [8] [9] [10] [10] [11] [12] [13] [14] [15] [16] [17] [18] This method
that the detection limit of the developed method was lower than the other reported method and its linear range was comparable to others (Table 3). 3.7. Real sample analysis The applicability of the method was tested by determining the amount of NAB in commercial pharmaceutical tablet, wastewater and urine samples. The accuracy of the method was confirmed through the recovery experiments by spiking the samples with three concentration levels as well as by comparing the results of tablet analysis with that of manufacturer’s specification. The results of these analyses are summarized in Table 2 and as indicated, the recoveries of the spiked samples were satisfactory (96.0–104.3%) and at 95% confidence limit, there was a good agreement with the result of the tablet analysis (502 ± 13) and the manufacturer’s specification (500 mg). Thus, the method is suitable for the determination of NAB in the sample types examined. 4. Conclusions In this study, the phenomenon of the formation of ternary complex of NAB with -CD in the presence of PVP and the localized surface plasmon resonance (LSPR) of Au-NPs were combined and a colorimetric chemical sensor for the determination of NAB based on the aggregation of the thiolated -cyclodextrin (T-CD) functionalized gold nanoparticles (Au-NPs) with NAB in the presence of polyvinylpyrrolidone (PVP) was reported. Furthermore, it was demonstrated that the presence of PVP could change the NAB:TCD complex to the NAB:(T-CD)2 :PVP ternary complex resulting in the aggregation of the Au-NPs and decrease in the LSPR band of AuNPs around 520 nm as well as formation of a new red shifted band at 650 nm. Developed method has good selectivity, wide linear range, low detection limit, acceptable accuracy as well as good reproducibility. Furthermore, the developed method does not require any expensive equipment and can be used conveniently for the routine analysis and quality control of NAB in tablet formulation.
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12] [13]
[14]
[15]
[16]
Appendix A. Supplementary data
[17]
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2016.09.110.
[18]
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Colorimetric determination of nabumetone based on localized surface plasmon resonance of functionalized gold nanoparticles as a chemical sensor Javad Khodaveisi a , Shayessteh Dadfarnia a,∗ , Ali Mohammad Haji Shabani a , Dariush Saberi b a b
Department of Chemistry, Faculty of Science, Yazd University, 89195-741, Yazd, Iran Fisheries and Aquaculture Department, Faculty of Agriculture and Natural Resources, Persian Gulf University, Boushehr, 75169, Iran
a r t i c l e
i n f o
Article history: Received 9 July 2016 Received in revised form 18 September 2016 Accepted 19 September 2016 Available online xxx Keywords: Localized surface plasmon resonance Nabumetone Thiolated -cyclodextrin Gold nanoparticle Colorimetric chemical sensor
a b s t r a c t A highly selective and sensitive colorimetric sensor for the determination of nabumetone (NAB) based on the aggregation of the thiolated -cyclodextrin (T-CD) functionalized gold nanoparticles (Au-NPs) with NAB in the presence of polyvinylpyrrolidone (PVP) was developed. Thiolated -cyclodextrin bonds to the surface of Au-NPs and forms a complex with NAB. The presence of PVP changed the NAB:T-CD complex to the NAB:(T-CD)2 :PVP ternary complex which resulted in the aggregation of the Au-NPs. As a result of this aggregation, the localized surface plasmon resonance (LSPR) band of Au-NPs around 520 nm decreased and a new red shifted band at 650 nm appeared which gradually increased with an increase in the NAB concentration. Under the optimized conditions, the calibration curve derived from the ratio of absorbance intensity at 650 nm to the original wavelength of 520 nm against NAB concentration was linear in the concentration range of 1–120 g L−1 . The limits of detection (LOD) and quantification (LOQ) were 0.2 and 0.7 g L−1 , respectively. The relative standard deviation at 20 g L−1 of NAB was found to be 3.1%. The selectivity of the method was demonstrated through the analysis of the synthetic samples containing the major interference compound reported in the literature. Finally, the method was successfully applied to the determination of NAB in pharmaceutical, urine and wastewater samples. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Nabumetone (NAB), (4-(6-methoxynaphthalen-2-yl)-butan-2one), is a non-steroidal anti-inflammatory drug (NSAID) belonging to the 2, 6-disubstituted naphthyl-alkanones class (Fig. 1) [1]. NAB is a less aggressive drug with some side effects metabolizing to the active 6-methoxy-2-naphthylacetic acid (6-MNA) [2,3]. NAB is efficient as an anti-inflammatory, analgesic and antipyretic pre-drug due to the inhibition of the cyclooxygenase enzymes responsible for the synthesis of prostaglandins [4]. Various techniques were used for the determination of NAB including high performance liquid chromatography (HPLC) [5,6] micellar liquid chromatography (MLC) [7] differential pulse polarography (DPP), osteryoung square wave (OSW) [8],
∗ Corresponding author. E-mail addresses: [email protected] (J. Khodaveisi), [email protected] (S. Dadfarnia), [email protected] (A.M.H. Shabani), [email protected] (D. Saberi).
ultraviolet-visible (UV–vis) spectrophotometry [9–13], flow injection analysis (FIA) with UV detector, time-resolved fluorescence (TRF) [15] and micelle-stabilized room temperature phosphorescence (MS-RTP) [16]. Among these methods, spectrophotometry has attracted more attention due to its low cost, availability in most laboratories as well as the ease of operation. Cyclodextrins are cyclic oligosaccharides constituted by 5 or more d-glucopyranoside units containing almost a conical hydrophobic cavity which is able to form an inclusion complex with a large variety of molecules [17]. They are often used to increase the aqueous solubility and the chemical stability of some molecules through the formation of an inclusion complex which modifies the properties of the guest molecule [18]. -Cyclodextrin (-CD) with 7-membered sugar ring molecules usually possesses better complexation efficiency with drugs than other cyclodextrins [19–21]. In recent years, the noble metal nanoparticles based colorimetric sensors have received great interest for the potential analytes determination due to the localized surface plasmon resonance (LSPR) phenomenon, which is responsible for their optical dependent properties [22–25]. Noble metal nanoparticles are emerging
http://dx.doi.org/10.1016/j.snb.2016.09.110 0925-4005/© 2016 Elsevier B.V. All rights reserved.
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had a maximum absorption at 520 nm with the extinction coefficient () of ∼6 × 108 mol−1 L cm−1 for the 13 nm Au-NPs and based on the Beer’s law the particle concentration in the solution was estimated to be 15 nmol L−1 [31,32]. 2.3. Synthesis of thiolated ˇ-cyclodextrin (Tˇ-CD)
Fig. 1. Structural formula of NAB.
as a promising tool for the spectrophotometric quantification of a wide variety of analytes due to their very high extinction coefficients as well as the dependence of LSPR wavelength and intensity on the shape, size and dielectric constant of the medium [26–28]. The objective of this work is to combine the phenomenon of formation of ternary complex of NAB with -CD in the presence of PVP and the localized surface plasmon resonance (LSPR) of Au-NPs to develop a colorimetric sensor for the determination of NAB. Thiolated -cyclodextrin is able to bond to the surface of Au-NPs and form a complex with NAB. However, the presence of PVP changes the NAB:T-CD complex to the NAB:(T-CD)2 :PVP ternary complex which results in the aggregation of the Au-NPs and causes the LSPR band of Au-NPs around 520 nm to decrease while a new red shifted band at 650 nm appears. Furthermore, the ratio of absorbance intensity at 650 nm to the original wavelength of 520 nm was found to be directly proportional to NAB concentration. Thus, based on this observation, a highly selective and sensitive spectrophotometric method for the determination of NAB was designed and the factors affecting the aggregation of Au-NPs such as the ionic strength, pH and concentration of PVP and Au-NPs were studied and optimized using the univariable technique. Finally, the applicability of the method for the determination of NAB in real samples was investigated. 2. Experimental 2.1. Reagents and apparatus Tetrachloroauric acid (HAuCl4 ·4H2 O, 99.99%) and NAB were purchased from Sigma-Aldrich (St. Louis, MO, USA) and cyclodextrin (-CD, 99%) was supplied by Acros (Geel, Belgium). All the other reagents which were analytical grade, were purchased from Merck (Darmstadt, Germany). Deionized water was used for the preparation of the solutions. All the experiments were performed at the ambient temperature of 25 ± 2 ◦ C. The absorbance spectra were recorded with a UV–vis spectrophotometer (HACH DR 3900, Loveland, Colorado, USA) using a 1.0 cm glass cell. The pH was measured with an EcoMet Model P25 pH meter (Guro-gu, Seoul, Korea) equipped with a combined glasscalomel electrode. The size of the Au-NPs was characterized by transmission electron microscopy (TEM) using a Zeiss transmission electron microscope (Jena, Germany) operating at an accelerating voltage of 80 kV. 2.2. Procedure for the preparation of Au-NPs Citrate-capped Au-NPs with an average diameter of about 13.1 ± 1.3 nm were synthesized according to the method reported by Turkevich et al. [29,30], i.e. 50 mL of 1 mmol L−1 solution of HAuCl4 was prepared and heated under reflux. At the boiling point, 5 mL of 38.8 mmol L−1 trisodium citrate was added under vigorous stirring and the mixture was heated under reflux for 30 min. In this stage, the color changed to deep red which indicates the formation of Au-NPs. Then, the solution was cooled down to room temperature and was stored at 4 ◦ C for further use. The resulted solution
T-CD was prepared from per-6-iodo--cyclodextrin (CD-I). CD-I was synthesised according to previous reported method by Stoddart and Kaifer et al. [33,34]; briefly, 13.3 g (48 mmol) of Ph3 P was dissolved in 30 mL of anhydrous DMF under stirring, 13.5 g (53 mmol) of I2 was also dissolved in 30 mL of anhydrous DMF and was added drop wise to the Ph3 P solution under nitrogen protection. When the addition was completed, the solution was heated to 80 ◦ C and then anhydrous -CD (4.0 g, 3.3 mmol) was added to this dark brown solution and was stirred at 80 ◦ C for 15 h. Then, the solvent was evaporated to half of its original volume under vacuum and the pH of the resulting solution was adjusted to 9–10 upon the addition of sodium methoxide in an ice-water bath. The reaction mixture was stirred at ambient temperature for 30 min and it was then added to 500 mL of methanol to form precipitate. The precipitate was separated via filtration and then it was washed with methanol. The product was purified through Soxhlet extract with methanol for 1 day and after drying under vacuum, CD-I was recovered as a white powder (yield: 69%). Then, the T-CD was prepared from the synthesized CD-I according to the previously reported method [35]. Thus, 3.86 g of CD-I and 1.21 g of thiourea were dissolved in 40 mL of DMF and the mixture was heated at 70 ◦ C under nitrogen atmosphere for 19 h. Next, the DMF was removed under reduced pressure and the obtained yellow oil was dissolved in 200 mL of water. In the next step, 1.04 g of sodium hydroxide was added and the reaction mixture was heated to a gentle reflux under nitrogen atmosphere. After 1 h, the resulting suspension was acidified with aqueous KHSO4 . So, a white precipitate of T-CD was obtained and it was filtered, washed thoroughly with water and dried under vacuum (yield: 81%). 2.4. Procedure for the determination of NAB For the determination of NAB, 2 mL of the prepared Au-NPs, 1 mL of 50 mol L−1 T-CD and 5 mL of deionized water were transferred to a 10 mL volumetric flask and the mixture was vigorously stirred for 6 min. Then, respectively, 100 L of NaCl (1.0 mol L−1 ), 0.5 mL of PVP (0.4 g L−1 ) and 1.2 mL of NAB standard or sample solution containing not more than 1.2 g of NAB were added to the flask and the volume was adjusted upon the addition of deionized water. In this stage the color of solution was changed from red to blue which could be detected by naked eyes. The absorbance spectrum was recorded after 6 min. The ratio of intensity of absorbance at 650 nm to 520 nm was directly proportional to the NAB concentration and was used as the analytical signal. It is to be reminded that the order of the addition of the reagents is supreme importance and the alteration of the mentioned order causes inaccurate results with a low level of reproducibility. 2.5. Preparation of real samples 2.5.1. Pharmaceutical tablets The 500 mg NAB tablets (Meda Pharma S.A.U., San Fernando de Henares, Madrid), were purchased from a drugstore. Ten tablets were powdered and mixed thoroughly and an amount equivalent to one-tenth of the weight of one tablet (∼0.102 g) was dissolved in deionized water. It was then stirred for 30 min, filtered, and diluted to a volume of 1 L with deionized water. After that, the solution was
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Fig. 2. Schematic diagram for aggregation of functionalized Au-NPs by NAB in the presence of T-CD and PVP.
properly diluted in order to get into the linear range of the developed method and was treated according to the given procedure. 2.5.2. Urine sample The urine sample was offered by a healthy volunteer in our laboratory who was not taking any drugs during the last four months. The sample was frozen after sampling, it was then thawed at room temperature just before analysis, centrifuged for 15 min at 5000 rpm and filtered through a 0.45 m Millipore filter. Afterwards, 1 mL of the sample was diluted to 100 mL and was treated according to the given procedure. 2.5.3. Wastewater sample The wastewater sample was collected from the Boushehr wastewater treatment plant (Boushehr, Iran) in glass bottles and was transferred to the laboratory. The samples were filtered through a 0.45 m Millipore filter, diluted properly with deionized water and were treated according to the given procedure. 3. Results and discussion Valero et al. [36] reported that the presence of PVP elicits a strong increase in the affinity of -CD for NAB, interacts with the free NAB and the NAB:-CD forming an inclusion complex. In the ternary complex, NAB is wrapped at both ends by -CD and the PVP binds them together via a bridge. Furthermore, it was reported that the thiolated molecules have high affinity toward the surface of Au-NPs due to the hard-soft acid-base interaction which displaces the citrate groups [37]. Based on these reports, the -CD was thiolated and immobilized on the surface of the synthesized Au-NPs. Then, in the presence of PVP, the immobilized T-CD interacted with NAB and formed the ternary complex of NAB:(-CD)2 -PVP. This resulted in the aggregation of NPs (Fig. 2) and caused the LSPR band of Au-NPs around 520 nm to decrease while a new red shifted band appeared at 650 nm due to the near-field coupling in the resonant wavelength peak of the interacting particles [38]. The typical TEM images and corresponding UV–vis absorption spectra of Au-
NPs, before and after of aggregation, are displayed in Fig. 3. The ratio of intensity of band at 650 nm to band at 520 nm was also found to be directly proportional to the NAB concentration. Thus, based on this observation, a spectrophotometric method for the determination of NAB was designed. In order to establish the best analytical conditions for the detection of NAB, the effect of the critical parameters including ionic strength, pH, PVP and Au-NPs concentration was optimized. 3.1. Effect of ionic strength and time Ionic strength has an important role in the aggregation of nanoparticles due to the ability of strong electrolytes to reduce the volume of electrical double-layer aroused from the capping agent. Therefore, some experiments were conducted by varying the concentration of NaCl between 0 and 14 mol L−1 . It illustrated that in the absence of strong electrolytes the Au-NPs did not undergo the aggregation even at high concentration of NAB, nevertheless, an increase in the salt concentration up to 10 mol L−1 caused an increase in the aggregation along with the augmentation of the A650 /A520 signal which leveled off up to 16 mol L−1 of salt (Fig. 1S). However, when the concentration of NaCl was more than 16 mol L−1 , the nanoparticles aggregated even in the absence of the analyte. Thus, a concentration of 10 mol L−1 of NaCl with no aggregation of Au-NPs in the absence of analyte, was selected as the optimum concentration of NaCl. Furthermore, the change in the absorbance spectrum as the function of time after the addition of NAB, was studied (Fig. 4). It showed that after 6 min, there was no change in the absorbance signals indicating that the aggregation was complete. So, the spectrophotometric measurement was done after 6 min. 3.2. Effect of pH The pH of the solution is an important factor that may influence the form of the analyte in the solution as well as the surface charge and aggregation of NPs. The Au-NPs are stable in the pH range of
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Fig. 3. Typical TEM images (A) and UV–vis absorption spectra (B) of functionalized Au-NPs; I) before and II) after the addition of 120 g L−1 of NAB. Conditions: Au-NPs 3.0 nmol L−1 , NaCl 10 mol L−1 , NAB 60 g L−1 , pH 7.0 PVP 0.02 g L−1 .
Fig. 4. Variation in the surface plasmon spectrum of Au-NPs at times interval of 60 s. Conditions: Au-NPs 3.0 nmol L−1 , PVP 0.02 g L−1 , NaCl 10 mol L−1 , pH 7.0, NAB 120 g L−1 .
4.5–10 [39] and the internal cavity of the -CD is non-polar. Thus, the pH of the solution should properly be adjusted so that the AuNPs is stable and the neutral form of analyte with high affinity for interaction with -CD dominates. The pH of solution was varied in the range of 2.0–11.0 and as expected, it was found that (Fig. 5) in the pH range of 5.0–8.0 the signal was maximized. The decrease in signal at lower pH can be due to the instability of the Au-NPs, whereas, the decrease at pH greater than 8.0, can be because of the formation of ionic form of the NAB. In further experiments, a pH of approximately 7.0 was selected as the optimum.
Fig. 5. Effect of pH on the aggregation of Au-NPs. Conditions: Au-NPs 3.0 nmol L−1 , PVP 0.02 g L−1 , NaCl 10 mol L−1 , NAB 60 g L−1 .
3.3. Effect of concentration of Au-NPs In order to increase the sensitivity of the technique, the effect of the concentration of Au-NPs on its aggregation in the presence of 50 g L−1 NAB and 10 M NaCl at the pH 7.0 was investigated. The results of this study (Fig. 6) revealed that an increase in the concentration of Au-NPs could cause an increase in analytical signal which leveled off at 3 nmol L−1 of Au-NPs. It should be reminded that the decrease in signal at lower concentration of NPs can be related to the kinetic of the aggregation of NPs, as the speed of aggregation improves with an increase in the Au-NPs concentration. Therefore,
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Table 2 Results of the analysis of NAB in pharmaceutical real sample. Sample
Added (g L−1 )
Founda (g L−1 )
Recovery (%)
Pharmaceutical tablet
0.0 10.0 30.0 60.0
50.2 ± 1.2 59.8 ± 1.4 79.8 ± 1.8 108.8 ± 2.2
– 96.0 98.6 97.6
Urine sample
0.0 20.0 60.0 100.0
ND 19.2 ± 0.3 60.8 ± 1.0 102.6 ± 2.3
– 96.0 101.3 102.6
Wastewater sample
0.0 20.0 60.0 100.0
ND 20.2 ± 0.3 59.2 ± 0.8 104.2 ± 1.8
– 101.0 98.6 104.2
a The results are mean of three measurements ± standard deviation; ND = Not detected.
Fig. 6. Effect of Au-NPs concentration on the surface plasmon band intensity. Conditions: PVP 0.02 g L−1 , NaCl 10 mol L−1 , NAB 60 g L−1 , pH 7.0.
Table 1 Tolerance limits of interferences species in the determination NAB (50 g L−1 ) at the optimum conditions. Species
Mole ratio of interfering species to analyte
Salicylic acid, mefenamic acid, paracetamol 6-Methoxy-2-naphthylacetic acid (6-MNA), Naproxen Sucrose, glucose, lactose, sodium benzoate, Glycerin
2000 500 1000 20000 30000
with regards to this experiment, a concentration of 3 nmol L−1 of the Au-NPs was selected for further experiments.
3.4. Effect of PVP concentration The effect of PVP concentration in the range of 0.01-0.05 g L−1 on encapsulation of NAB through the formation of ternary complex of NAB:(T-CD)2 :PVP was surveyed (Fig. 2S). The results showed that the analytical signal would increase with an increase in the PVP concentration up to 0.02 g L−1 , remain constant up to 0.03 g L−1 and then it would decrease with further increase in PVP concentration. The decrease in signal at concentration greater than 0.03 g L−1 of PVP can be due to the inhibition effect of PVP as a capping agent in aggregation of Au-NPs [27]. Hence, a PVP concentration of 0.02 g L−1 was selected for further studies.
3.5. Interference study To assess the selectivity of the designed method for the determination of nabumetone, several experiments were performed in the presence of the compounds reported in the literature to interfere with the determination of NAB including naproxen, 6-methoxy2-naphthylacetic acid (6-MNA), salicylic acid, mefenamic acid and paracetamol [5,9,10,15] as well as in the presence of some common excipients such as sucrose, glucose, lactose, sodium benzoate and glycerin [16]. The tolerance limit was defined as the maximum concentration of the interference species that produce a relative error of less than ±5%. The results presented in Table 1 revealed that at the given mole ratio, no major interference in the determination of NAB was observed. Thus, the procedure is very selective for the determination of NAB.
Fig. 7. (A): A typical absorption spectra of Au-NPs in presence of different concentration of NAP. (B): Calibration curve of NAP under optimum conditions.
3.6. Figures of merit The linear range of the determination of NAB was evaluated under the optimized conditions. A linear relation was observed between the ratio of A650 /A520 and the concentration of NAB in the range of 1–120 g L−1 with R2 = 0.9954 (Fig. 7). The limit of detection (LOD) and quantification (LOQ) (defined as 3Sb /m and 10Sb /m) were found to be 0.2 and 0.7 g L−1 , respectively. The study of precision was made through five independent experiments and a relative standard deviation (RSD%) of 3.1% was determined for 20 g L−1 of NAB. Furthermore, some of the figure of merit of developed method was compared with the literature values. It was found
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Table 3 Comparison of Figures of merit of the developed method with other reported methods for determination of NAB. Method
LODa (g L−1 )
LDRb (g L−1 )
Ref.
Reverse phase HPLC with UV detection HPLC with diode array detector Micellar liquid chromatography Differential pulse polarography Osteryoung square wave voltammetry First-order derivative spectrophotometric method Ultraviolet-visible method-area under curve method Spectrophotometric methods-Statistical Ultraviolet spectrophotometric method Dual wavelength spectrophotometric method Flow injection analysis with Ultraviolet detector Time-resolved fluorescence Micelle-stabilized room temperature phosphorescence Localized surface plasmon resonance
1.5 50 12 17.5 8.2 560 260 230 210 300 100 0.96 18.2 0.2
24–288 100–4560 500–25000 230–18300 230–18300 3000–18000 5000–25000 10000–30000 5000–45000 2000–20000 320–6300 up to 3000 up to 1000 1–120
[6] [8] [9] [10] [10] [11] [12] [13] [14] [15] [16] [17] [18] This method
that the detection limit of the developed method was lower than the other reported method and its linear range was comparable to others (Table 3). 3.7. Real sample analysis The applicability of the method was tested by determining the amount of NAB in commercial pharmaceutical tablet, wastewater and urine samples. The accuracy of the method was confirmed through the recovery experiments by spiking the samples with three concentration levels as well as by comparing the results of tablet analysis with that of manufacturer’s specification. The results of these analyses are summarized in Table 2 and as indicated, the recoveries of the spiked samples were satisfactory (96.0–104.3%) and at 95% confidence limit, there was a good agreement with the result of the tablet analysis (502 ± 13) and the manufacturer’s specification (500 mg). Thus, the method is suitable for the determination of NAB in the sample types examined. 4. Conclusions In this study, the phenomenon of the formation of ternary complex of NAB with -CD in the presence of PVP and the localized surface plasmon resonance (LSPR) of Au-NPs were combined and a colorimetric chemical sensor for the determination of NAB based on the aggregation of the thiolated -cyclodextrin (T-CD) functionalized gold nanoparticles (Au-NPs) with NAB in the presence of polyvinylpyrrolidone (PVP) was reported. Furthermore, it was demonstrated that the presence of PVP could change the NAB:TCD complex to the NAB:(T-CD)2 :PVP ternary complex resulting in the aggregation of the Au-NPs and decrease in the LSPR band of AuNPs around 520 nm as well as formation of a new red shifted band at 650 nm. Developed method has good selectivity, wide linear range, low detection limit, acceptable accuracy as well as good reproducibility. Furthermore, the developed method does not require any expensive equipment and can be used conveniently for the routine analysis and quality control of NAB in tablet formulation.
[3]
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[12] [13]
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Appendix A. Supplementary data
[17]
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2016.09.110.
[18]
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Please cite this article in press as: J. Khodaveisi, et al., Colorimetric determination of nabumetone based on localized surface plasmon resonance of functionalized gold nanoparticles as a chemical sensor, Sens. Actuators B: Chem. (2016), http://dx.doi.org/10.1016/j.snb.2016.09.110