Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 122 (2014) 737–743
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CuO nanosheets-enhanced flow-injection chemiluminescence system for determination of vancomycin in water, pharmaceutical and human serum A.R. Khataee a,⇑, A. Hasanzadeh a, M. Iranifam b, M. Fathinia a, Y. Hanifehpour c, S.W. Joo c,⇑ a b c
Research Laboratory of Advanced Water and Wastewater Treatment Processes, Department of Applied Chemistry, Faculty of Chemistry, University of Tabriz, Tabriz, Iran Department of Chemistry, Faculty of Science, University of Maragheh, Maragheh, East Azerbaijan, Iran School of Mechanical Engineering, Yeungnam University, Gyeongsan 712–749, South Korea
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
Preparation of CuO nanosheets (NSs)
by a simple one-step sonochemical method. Enhancement of luminolH2O2 CL system by prepared CuO NSs. Determination of vancomycin using a luminolH2O2CuO flow-injection CL system. Appropriate sensitivity and selectivity of the proposed method.
a r t i c l e
i n f o
Article history: Received 19 October 2013 Received in revised form 29 November 2013 Accepted 8 December 2013 Available online 12 December 2013 Keywords: Flow-chemiluminescence Vancomycin CuO nanosheets Sonochemical Luminol
a b s t r a c t A novel, rapid and sensitive CuO nanosheets (NSs) amplified flow-injection chemiluminescence (CL) system, luminol–H2O2–CuO nanosheets, was developed for determination of the vancomycin hydrochloride for the first time. It was found that vancomycin could efficiently inhibit the CL intensity of luminol–H2O2–CuO nanosheets system in alkaline medium. Under the optimum conditions, the inhibited CL intensity was linearly proportional to the concentration of vancomycin over the ranges of 0.5–18.0 and 18.0–40.0 mg L1, with a detection limit (3r) of 0.1 mg L1. The precision was calculated by analyzing samples containing 5.0 mg L1 vancomycin (n = 11) and the relative standard deviation (RSD) was 2.8%. Also, a high injection throughput of 120 sample h1 was obtained. The CuO nanosheets were synthesized by a sonochemical method. Also, X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses were employed to characterize the CuO nanosheets. The method was successfully employed to determine vancomycin hydrochloride in environmental water samples, pharmaceutical formulation and spiked human serum. Ó 2013 Elsevier B.V. All rights reserved.
Introduction Vancomycin hydrochloride is a glycopeptide antibiotic that is widely employed in the treatment of serious infections by Gram⇑ Corresponding authors. Tel.: +98 411 3393165; fax: +98 411 3340191 (A.R. Khataee). Tel.: +82 53 810 1456 (S.W. Joo). E-mail addresses: [email protected], [email protected] (A.R. Khataee), [email protected] (S.W. Joo). 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.12.014
positive bacteria [1,2]. It was chosen for therapy of infections in patients allergic to b-lactam antibiotics. This antibiotic acts by preventing the peptidoglycan synthesis of bacterial cell wall [3]. Monitoring vancomycin in the biological fluids and in pharmaceutical products is of importance to prevent side effects in patients under treatment and to achieve optimum therapeutic concentrations [4]. Moreover, antibiotics including vancomycin have been found in the aquatic environment such as waste and polluted
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water resources. This compound can play a role in the maintenance or extension of antibiotic resistance bacteria, finally resulting in hazards to human health [5,6]. Therefore, screening vancomycin both in biological and environmental studies seems to be very important. The common analytical methods for determination of vancomycin in different real samples are voltammetry [7], spectrophotometry [8–12], immunoassay [13–15], high-performance liquid chromatography (HPLC) with ultraviolet detection [16–24], electrochemical detection [25], mass spectroscopy detection [4,5,26–28] fluorescence detection [29] and capillary electrophoresis (CE) [30–32]. Nevertheless, to the best of authors knowledge, there have been no reports regarding the use of CL methods for the determination of vancomycin in real samples. The above-mentioned methods for determination of vancomycin have their own advantages and disadvantages. However, high cost instrumentation, time-consuming procedure or unsuitability for automatic analysis and the need for large amounts of expensive and toxic solvents are some of the reported drawbacks of these methods [33]. Hence, the development of simple and sensitive techniques for the determination of vancomycin in real samples is of great importance. Moreover, in recent decades, CL methods have received extensive attention due to their high sensitivity, simplicity, inexpensive equipment, reproducibility and rapidity [34,35]. In addition, the coupling of a flow-injection analysis technique with CL detection presents high analytical throughput [36]. Owing to these advantages, flow-CL methods have been developed as useful and powerful tools in many different fields, such as clinical assay, pharmaceutical, environmental and food analysis [37]. It should be noted that in some of the common CL systems, the produced CL emission during oxidation of an analyte is of relatively low intensity due to the low quantum yield in some oxidation reaction. In this context, metal and semiconductor nanomaterials with unique physical and chemical properties have been widely used in various CL reactions as signal amplifier catalysts [38,39]. As an example, nanomaterials of gold [40], silver [41], platinum [42], CdTe [43], TiO2 [44], Se [39] and CuO nanoparticles [45] have been used as signal enhancers. Therefore, the integration of nanomaterials with CL systems could provide new approaches to enhance the sensitivity of the used method, which is an essential factor for analytical applications [38]. The catalytic performance of CuO nanostructures is highly dependent on the morphology and size. CuO nanosheets exhibit several advantages including non-toxicity, high density, uniformity and good crystalline array structure [46]. In the past, extensive efforts have been focused on controlling the size and shape of CuO nanomaterials. Recently, CuO nanomaterials have been synthesized via various techniques such as the solution-based, vapor phase growth and sonochemical technique [47]. Herein, we have applied the sonochemical-based method for the preparation of high uniform CuO nanosheets due to its several attractive features such as high yields of the prepared nanomaterials and the low cost. To the best of our knowledge, there is no previous report concerning the use of CuO nanosheets as a signal amplifier in CL systems. The reaction between luminol and H2O2 is a popular CL reaction that has been extensively applied for the determination of various substances [24]. The absence of a simple and fast CL method for the determination of vancomycin caught our attention in this context. Here in, vancomycin was subjected to some introductory tests and it was found that vancomycin was able to inhibit the CL emission of luminol–H2O2–CuO nanosheets. The resultant decrease in CL intensity was proportional to vancomycin concentration. In the present work, we have proposed a novel and sensitive flow-injection CL method for the determination of the vancomycin hydrochloride in real samples. Within this context, the effect of
CuO nanosheets on the luminol–H2O2 CL system was investigated. In addition, it was found that CuO nanosheets could act as a nanocatalyst on the luminol–H2O2 CL reaction under alkaline conditions, capable of generating a significant increase in CL emission. Therefore, under the optimized conditions, the novel CuO nanosheets-enhanced CL system could be used as an alternative method for the analysis of vancomycin in environmental water samples, pharmaceutical formulation and spiked human serum. Experimental details Materials All the chemicals and reagents used in this work were of analytical grade and used without further purification and purchased from Merck Co. (Germany), except for vancomycin hydrochloride, which was obtained from Jaber Ebne Hayyan pharmaceutical Co. Tehran, Iran. Doubly distilled water was used in all experiments. A stock standard solution of 2 102 mol L1 luminol was prepared by dissolving 0.354 g luminol in 100 mL of 0.1 mol L1 sodium hydroxide in a brown volumetric flask. A stock solution of 1 mol L1 sodium hydroxide was prepared by dissolving 4 g sodium hydroxide in 100 mL double distilled water. The working solutions of hydrogen peroxide were freshly prepared daily by diluting appropriate amounts of 30% (w/v) H2O2 with doubly distilled water. A 0.02 mol L1 copper (II) acetate monohydrate solution was prepared by dissolving 0.998 g in 250 mL distilled water. A 100 mg L1 stock standard solution of vancomycin was prepared by dissolving 25 mg vancomycin in 500 mL doubly distilled water and stored at 4 °C in refrigerator. All working solutions were prepared by diluting their related stock solutions. Vancomycin hydrochloride capsules were purchased from Flynn pharmaceutical Co. Ireland. Drug-free human serum used in this study was taken from healthy volunteers and stored in freezer until analysis. Apparatus The schematic of the flow-injection CL is illustrated in Fig. 1. Polytetrafluoroethylene (PTFE) tubing with an inner diameter (i.d.) of 1.0 mm was utilized as a connection material in the flow system. A peristaltic pump was used to deliver all respective solutions through the flow system at a flow rate of 2.0 mL min1 for each channel. A six-port valve equipped with a 100 lL sample loop was applied for injection. The light generation from the CL reaction in the flow cell was monitored by a FB12 luminometer (Berthold Detection Systems, Germany). Ultraviolet–visible (UV–Vis) spectra were recorded by UV–Vis spectrophotometer (WPA lightwave S2000, England) in the range of 200–800 nm. A bath type sonicator (Sonica, 2200 EP S3, Italy) with 50–60 Hz frequency and heating arrangement was used for sonosynthesis of CuO nanosized samples. To characterize the crystal structure, mean crystal size and phase purity of as-prepared
Fig. 1. Schematic diagram of flow-injection CL system; (a) luminol in NaOH solution; (b) sample or blank solution; (c) H2O as the carrier; (d) H2O2 solution; P: peristaltic pump; M: mixing tube; V: injection valve; F: flow cell; W: waste; D: detector (luminometer); R: recorder (personal computer).
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samples, XRD measurements were performed at room temperature by using Siemens X-ray diffraction D5000 (California, USA), with Cu Ka radiation. The accelerating voltage of 40 kV and emission current of 30 mA were utilized. The Debye–Scherrer formula was employed to calculate the average crystalline size of the catalysts from XRD peak [48]. SEM (S-4200, Hitachi, Japan) was used to observe the surface state and the morphology of the prepared nanosheets using an electron microscope. General procedure for flow-injection CL system As depicted in Fig. 1, the alkaline solution of luminol (a), sample or standard solution of mixture of vancomycin and CuO nanosheets (b), H2O as the carrier (c) and H2O2 solution (d) were pumped by a peristaltic pump. Solutions of a and b were mixed via a mixing tube (silicon tubing, 1.0 mm i.d.); then 100 lL of mixing solution was injected into the carrier stream and premixed with H2O2 stream through a Y-piece; after that, the mixture was delivered into the flow cell. The peak height of the CL signals was continuously monitored on a computer connected to the luminometer. Determination of vancomycin was carried out based on the decrease of CL intensity as DI = I0–Is, where I0 and Is denote CL intensity in the absence and presence of vancomycin, respectively. Pretreatment of real samples solution before CL assay Tap water was analyzed without any pretreatment. Ground and river water samples were freshly collected and filtered with polyamide membrane filters of 0.45 lm to eliminate the suspended solid matter and stored in dark at 4 °C in the refrigerator [39]. They were used within 1 week. Prior to analysis, 30 mL water samples were spiked with 0.5, 1, 1.5, 2, 2.5 and 3 mL vancomycin standard solutions (100 mg L1) and diluted to 50 mL with doubly distilled water to prepare solutions of 1, 2, 3, 4, 5 and 6 mg L1, respectively. For commercial capsules samples containing vancomycin hydrochloride, 5 capsules were powdered and mixed in mortar. Then, 100 mg of vancomycin, was weighted and dissolved in distilled water, and the obtained solution was filtered via an ordinary filter paper and diluted to 100 mL with doubly distilled water to prepare the solution of 1000 mg L1. For human serum samples, only a deproteinization pretreatment step utilizing trichloroacetic acid (CCl3COOH) was performed; an extraction process was not needed [49]. To prepare the spiked samples, known amounts of vancomycin were spiked into 1.0 mL of serum and then, for each sample, 5.0 mL of 10% (w/v) trichloroacetic acid was added. These mixtures were centrifuged at 3000 rpm for 15 min. 2.5 mL of the protein-free supernatant was diluted to 50 mL with doubly distilled water. The amount of vancomycin in the spiked samples was determined by means of the general procedure using standard addition method. Also, the blank value was determined by vancomycin-free samples in the same procedure. Synthesis of CuO nanosheets CuO nanosized crystals were synthesized by sonochemical method as follows: 200 mL of 0.02 mol L1 copper(II) acetate aqueous solution was added to 1 mL of glacial acetic acid. Then, the solution was put into a round-bottomed flask equipped with a refluxing device. Subsequently, the flask was transferred to the sonicator, heated to boiling under ultrasonic irradiation; then about 20 mL of 1 mol L1 NaOH solution was added gradually into the above solution in ambient air. The obtained black precipitate was centrifuged and washed thoroughly for several times with
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ethanol and distilled water to remove residual impurities. The resultant product was dried in air at room temperature. Results and discussion Characterization of CuO nanosheets The XRD patterns of the synthesized CuO samples are depicted in Fig. 2a. The XRD diffraction peaks at 2h of 32.4°, 35.5°, 38.7°, 48.7°, 53.2°, 57.9°, 61.3°, 66.1°, 68.0°, 71.9° and 74.8° can be classified into (1 1 0), (0 0 2), (1 1 1), (2 0 2), (0 2 0), (2 0 2), (1 1 3), (0 2 2), (2 0 0), (3 1 1) and (2 2 2) plane reflections, which are associated with monoclinic crystal structure CuO according to the standard powder diffraction data (JCPDS 05-0661) [50]. No peak for impurities was detected, confirming that the applied sonochemical method in this study was successful in synthesizing the nanosized CuO samples. Moreover, the sharp diffraction peaks in the XRD spectrum of the synthesized sample indicated that the synthesized product was highly crystalline. By calculating the Debye–Scherrer formula [48], the average size of the crystals was found to be about 5 nm. In order to further clarify the size and morphology of the synthesized product, SEM images were taken at different magnifications. Fig. 2b shows the SEM microphotographs of the nanosized samples at different magnifications. The low magnification SEM image in the left side of Fig. 2b obviously showed that the obtained samples possessed sheet-like morphology. Moreover, the high magnification SEM image in the right side of the Fig. 2b confirmed that the grown nanosheets were randomly grown. They exhibited clean and flat surfaces with the surface and wall thickness being in the range of 120 and 50 nm, respectively. CuO nanosheets-enhanced CL system The reaction between luminol and H2O2 is a popular CL reaction that has been extensively applied for the detection of diverse substances [36,38]. The CL intensity–time response curves are shown in Fig. 3. As presented in Fig. 3, the oxidation of luminol by H2O2 generated a relatively weak CL emission in alkaline media. However, we found that in the presence of CuO nanosheets, the CL intensity was significantly enhanced by about 9 times. In order to confirm the enhancing effect of CuO nanosheets, blank experiments were carried out using copper(II) acetate and glacial acetic acid aqueous solution in concentrations used for the preparation of CuO nanosheets. No enhancing effect was observed in the presence of glacial acetic acid, but an enhancing effect in the presence of copper(II) acetate was observed, not much remarkable in comparison with CuO nanosheets amplified CL system. These results confirmed that the catalytic effect was due to the CuO nanosheets. So, CuO nanosheets-enhanced CL reaction was used for the determination of vancomycin. The results indicated that the CL intensity of the CuO nanosheets catalyzed luminol–H2O2 system was significantly decreased upon the addition of 5 mg L1 vancomycin; however, in the absence of the CuO nanosheets, the inhibition of the luminol–H2O2 was negligible in the presence of 5 mg L1 vancomycin. Optimization of experimental conditions A series of experiments were run to optimize the CL reaction conditions, including the chemical concentration, flow rate, and the length of mixing tube. The CL intensity was dependent on the concentration of CuO nanosheets. As indicated in Fig. 4a, the inhibition of CL intensity was increased with the increase in the CuO nanosheet
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Fig. 2. (a) XRD pattern of the synthesized CuO NSs and (b) SEM images of synthesized CuO NSs.
increasing H2O2 concentration from 0.001 to 0.015 mol L1 and slightly decreased when H2O2 concentration was higher than 0.015 mol L1. So, 0.015 mol L1 H2O2 was chosen as the most suitable concentration. Moreover, the influence of the flow rate of pump on the CL reaction was assayed over the range of 1.0–3.5 mL min1. The inhibition of CL intensity was increased with the flow rate of 1.0–2.0 mL min1. When the flow rate was higher than 2.0 mL min1, the CL response fairly remained constant. Therefore, 2.0 mL min1 of flow rate was applied in our measurements in which high sample through put could be achieved. The effect of the mixing tubing length on the inhibition of CL intensity was examined in the ranges of 5–20 cm. The best results with good reproducibility were obtained with 10.0 cm mixing tube. Fig. 3. Kinetic curve for (a) luminol–H2O2 CL system (b) luminol–H2O2–vancomycin; (c) luminol–H2O2–Cu(II) (d) luminol–H2O2–CuO NSs and (e) luminol–H2O2– CuO NSs in the presence of vancomycin. The concentrations of luminol, H2O2, NaOH, CuO NSs, Cu(II) and vancomycin were 100 lmol L1, 0.015 mol L1, 0.01 mol L1, 1 mg L1, 0.02 mol L1and 5 mg L1, respectively.
concentration up to 1 mg L1 and then decreased with the further increase in the catalyst concentration. So, it is clear that the optimum concentration of CuO nanosheets was 1 mg L1, with the highest inhibition of CL intensity. Luminol CL intensity is dependent on the concentration of NaOH. Therefore, the effect of different concentrations of NaOH on the CL system was examined in the ranges of 0.0001– 0.04 mol L1. The results revealed that the highest inhibition of CL signals was obtained in the NaOH concentration of 0.01 mol L1. Thus, 0.01 mol L1 NaOH was chosen for the subsequent experiment (see Fig. 4b). The effect of luminol concentration on the inhibition of CL intensity was studied over the range of 0.5–300 lmol L1. According to the results (Fig. 4c), inhibition of CL intensity was increased with the increase of luminol concentration from 0.5 to 100 lmol L1;for amounts beyond the concentration of 100 lmol L1, the inhibition of CL intensity was declined. So, 100 lmol L1 luminol was taken as an optimum concentration. As shown in Fig. 4d, the effect of H2O2 concentration was investigated in the range of 0.001–0.03 mol L1. The results indicated that the inhibition of CL intensity was increased with
Analytical features Under the optimum conditions, the inhibition of CL intensity of luminolH2O2CuO nanosheets by vancomycin was linear over the concentration ranges of 0.5–18.0 and 18.0–40.0 mg L1. The linear regression equations were y = 769706x + 106 (r2 = 0.996) and y = 78525x + 107 (r2 = 0.994), respectively, where y was the DI and x was the vancomycin concentration in mg L1. The detection limit (3r) of the method was calculated to be 0.1 mg L1. The relative standard deviation for the determination of 5 mg L1 vancomycin was 2.8% (n = 11). The results showed that the proposed CL system had a good linearity, a relatively high sensitivity and adequate precision. Moreover, the experimental results obtained from our proposed method were compared with those of other previously reported works for the determination of vancomycin (see Table 1). It is noteworthy to mention that in comparison with most methods used for the determination of vancomycin, the proposed simple and rapid CL system in the present work featured acceptable analytical figures of merit with a relatively cheap apparatus. Moreover, the reagents were stable and inexpensive. Interference study In order to validate the possible analytical application of the proposed method for real samples, the interference effects of some
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CL signal for vancomycin. The results listed in Table 2 indicated that substances possibly found in real sample including environmental water samples, pharmaceutical formulation and human serum did not interfere with the determination except Cu2+ ion, which was found to be the most serious interferent. However, the interference from this metal ion could be removed with the addition of ethylene diamine tetra acetic acid (EDTA). EDTA with concentration 15 times greater than vancomycin had an insignificant effect on the CL signal. It can be concluded that the proposed CL method presented high selectivity for the determination of vancomycin in environmental water samples, pharmaceutical formulation and human serum. Analytical application In order to evaluate the applicability of the proposed method to real samples, the method was applied to determine the vancomycin in environmental water samples, pharmaceutical formulation and spiked human serum. The samples were prepared and analyzed according to the general procedure described in the experimental section. The obtained results for environmental water samples, pharmaceutical formulation and spiked human serum are summarized in Tables 3–5, respectively. The results for analysis of pharmaceutical formulation were also compared with those obtained by an official method (antibiotics microbial assays) [51]. Statistical analysis of these results using Student t-test showed that there is no significant difference between the results of two methods (see Table 4). The accuracy of the method was evaluated by performing recovery experiments for samples solution. The obtained results confirmed the applicability and accuracy of the method. Possible CL mechanism
Fig. 4. Effects of chemical concentration on the inhibition of CL intensity (a) effect of CuO NSs concentration. Conditions: the concentrations of vancomycin, luminol, NaOH and H2O2 were 5 mg L1, 80 lmol L1, 0.005 mol L1 and 0.01 mol L1, respectively; (b) effect of NaOH concentration. Conditions: CuO NSs concentration was 1 mg L1, other conditions were as in (a); (c): effect of luminol concentration. Conditions: NaOH concentration was 0.01 mol L1, other conditions were as in (b and d): effect of H2O2 concentration. Conditions: luminol concentration was 100 lmol L1, other conditions were as in (c).
species often found in water samples, pharmaceutical formulation and human serum were investigated. Analyses were performed for a fixed concentration of vancomycin (5 mg L1) with increasing interference concentrations. The tolerable limit of each foreign species was taken if it caused a relative error less than 5% in the
The mechanism of luminolH2O2 system under alkaline conditions in aqueous solution has been widely studied [40,43,52]. Also, it has been well confirmed that reactive oxygen species (ROS) such as hydrogen peroxide (H2O2), hydroxyl radicals (OH) and superoxide radical anion O 2 are the most important oxidizing species. Moreover, oxidation of luminol by the resulted ROS generates luminol radical and diazaquine, which rapidly react with O 2 or OH, giving rise to excited state 3-aminophthalate anion (3-AMP ). CL emission emanates from relaxation of 3-AMP to the ground state at 425 nm [38]. In this context, some authors reported that [41] the reaction of luminolH2O2 in alkaline medium in the absence of a catalyst was relatively slow, so the produced CL emission was weak. Many attempts have been made to elucidate the mechanism of enhanced CL system in the presence of nanomaterials [40,53,54]. According to the literature review [43,54,55], the enhancement of CL emission is due to the interaction of nanomaterials surface with the reactants or the intermediates of the CL reactions which facilitate ROS generation and electron-transfer processes occurring on the surface of the nanoparticles. In the present work, in order to elucidate the possible CL mechanism, UV–Vis absorption spectra (Fig. 5), in addition to CL intensity–time curve (Fig. 3), were recorded for the CL system in the presence and absence of CuO nanosheets. As shown in Fig. 5, vancomycin had a maximum absorption peak at about 281 nm (curve a) and the CuO nanosheets showed a linear absorption peak (curve b). The luminol–H2O2 system had two absorption peaks at 296 nm and 346 nm (curve c), the absorption peak of a mixed system of CuO nanosheets and luminol–H2O2 system (curve e) was almost equal to the sum of the absorption peak of the two individual systems, indicating that there was no chemical reaction between CuO nanosheets and luminol–H2O2 system during the CL reaction. Comparing our re-
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Table 1 Analytical performance data of previously reported works for the determination of vancomycin. Method
Matrix
Linear range (mg L1)
Detection limit (mg L1)
Reference
Spectrophotometry Voltammetry HPLC/UVa
Pharmaceutical formulation Pharmaceutical formulation Human plasma
HPLC/ECb LC/MSc LC/MS/MSd
Human plasma Human serum Water samples Rat plasma Human plasma Pharmaceutical formulation Human serum, pharmaceutical formulation and water samples
2.0–18.0 4.0–32.0 0.4–100.0 1.0–80.0 0.5–50.0 0.05–10.0 0.016–2.0 0.01–20.0 0.005–1.0 0.25–5.0 0.5–40.0
0.02 2.7 0.2 0.5 0.25 0.001 0.01 0.007 0.002 0.1 0.1
[9] [7] [17] [18] [25] [4] [5]
HPLC/FLe CE/UVf CL a b c d e f
High performance liquid chromatography (HPLC) with UV detection. HPLC with electrochemical detection. Liquid chromatography (LC) with mass spectrometry. LC with tandem mass spectrometry. LC with fluorescence detection. Capillary electrophoresis with UV detection.
Table 2 Tolerable concentration ratios with respect to 5 mg L1 of vancomycin. Species
Table 5 Results for the determination of vancomycin in spiked human serum samples.
Tolerable concentration ratio
Sample
Added (mg L1)
Found
½C interferent
Human serum sample No. 1
0 3 7 4 8 10 14
Not detected 2.84 ± 0.25 7.13 ± 0.12 3.86 ± 0.37 7.90 ± 0.42 10.44 ± 0.35 14.46 ± 0.26
ðmg L1 Þ=C vancomycin ðmg L1 Þ
1000
Na , Cl , K , SO2 4 , CH3COO 3 2+ 2+ , PO , Mg , Ca , Br, CO2 3 4 +
+
500
Tartaric acid, Lactose, Valine Alanine, Sucrose, Glucose, Starch,
No. 2
400
2 Threonine, NO 3 , SO3 Methyl parabene, Propyl parabene, Stearic acid Ethylenediaminetetraacetic acid (EDTA) Fe2+, Citrate Ascorbic acid Cu2+
No. 3
100
a
10 2 1
a
Sample
Added (mg L1)
Found
Water Tap water
0 1 3 2 5 4 6
Not detected 0.94 ± 0.22 3.12 ± 0.12 1.93 ± 0.35 5.08 ± 0.48 4.25 ± 0.15 5.84 ± 0.36
Ground water River water
a
(mg L1)
Recovery (%) – 94.67 101.86 96.50 98.75 104.40 103.28
Mean of six determinations ± standard deviation.
15
Table 3 Results for the determination of vancomycin in environmental water samples.
a
[29] [30] This work
(mg L1)
Recovery (%) – 94 104 96.5 101.6 106.25 94.33
Mean of six determinations ± standard deviation.
sults with those of previous reported papers, we can conclude that the enhanced CL signals (Fig. 3) were ascribed to the generation of more ROS due to the fast transfer of electron on the surface of CuO nanosheets and the catalytic effects of CuO
nanosheets. This phenomena accelerated 3-AMP and 3-AMP generation, leading to the higher CL intensity. On the other hand, we have investigated the inhibiting effect of vancomycin in the proposed CL system. Curve f in Fig. 5 shows that the absorbance of the luminol–H2O2–CuO nanosheets system in the presence of vancomycin had a slight shift to red area at 296 nm and the absorption intensity in 281 nm disappeared (curve f). Also, it can be deduced from this curve that the light absorption of the mixed system was not equal to the sum of the light absorption of the two individual systems (curves a and e). This implied that vancomycin might be involved and oxidized in the luminol–H2O2–CuO nanosheets reaction system, thereby creating new productions. Moreover, the inhibiting role of vancomycin could be related to the presence of the reducing groups such as OH, NH and NH2. It was reported that [38,56–58] the functional groups of vancomycin could react and scavenge the produced ROS, leading to the reduced CL emission intensity. In addition, vancomycin molecules could cover the surface of CuO nanosheets and prevent them from acting as a catalyst in CL system.
Table 4 Results for the determination of vancomycin in pharmaceutical formulation. Sample Pharmaceutical formulation
a b c
Sample solution number a
1 2 3 4
Sample taken (5 mg L1). Mean of six determinations ± standard deviation. t-Critical = 4.30 for n = 6 and P = 0.05.
b
Added (mg L1)
Found
(mg L1)
0 10 20 30
5.39 ± 0.12 15.72 ± 0.15 26.48 ± 0.16 34.85 ± 0.21
Found (official method) (mg L1)
t-Statistic
5.03 ± 0.52 – – –
1.79 2.08 1.90 2.38
c
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Fig. 5. UV–Vis absorption spectra of CL system. Conditions: the concentrations of luminol, H2O2, NaOH, CuO NSs and vancomycin were 100 lmol L1, 0.015 mol L1, 0.01 mol L1, 1 mg L1and 5 mg L1, respectively.
Conclusion Based on the inhibition effect of vancomycin on the CL reaction of luminol–H2O2–CuO nanosheets in alkaline medium, a novel, sensitive and selective flow-injection CL method was reported for the determination of vancomycin. The proposed method was rapid with a linear dynamic range of 0.5–40.0 mg L1. Moreover, it was successfully applied for the determination of vancomycin in environmental water samples, pharmaceutical formulation and spiked human serum. Acknowledgments The authors thank the University of Tabriz and Maragheh, Iran for all of the support provided. We also thank Jaber Ebne Hayyan pharmaceutical Co. Tehran, Iran for providing the pharmaceutical. This work was funded by the Grant 2011-0014246 of the National Research Foundation of South Korea. References [1] K.C. Nicolaou, C.N.C. Boddy, S. Bräse, N. Winssinger, Angew. Chem. Int. Ed. 38 (1999) 2096–2152. [2] M.D. Nailor, J.D. Sobel, Med. Clin. North. Am. 95 (2011) 723–742. [3] N.E. Allen, T.I. Nicas, FEMS Microbiol. Rev. 26 (2003) 511–532. [4] T. Zhang, D.G. Watson, C. Azike, J.N.A. Tettey, A.T. Stearns, A.R. Binning, C.J. Payne, J. Chromatogr. B 857 (2007) 352–356. [5] B. Li, T. Zhang, Z. Xu, H.H.P. Fang, Anal. Chim. Acta 645 (2009) 64–72. [6] F. Tamtam, F. Mercier, B. Le Bot, J. Eurin, Q. Tuc Dinh, M. Clément, M. Chevreuil, Sci. Total. Environ. 393 (2008) 84–95. [7] F. Belal, S.M. El-Ashry, M.M. El-Kerdawy, D.R. El-Wasseef, Arzneimittel-Forsch 51 (2001) 763–768. [8] A.R. Júnior, M.M.D.C. Vila, M. Tubino, Anal. Lett. 41 (2008) 822–836. [9] S.M. El-Ashry, F. Belal, M.M. El-Kerdawy, D.R. El Wasseef, Microchim. Acta 135 (2000) 191–196. [10] M.M.D.C. Vila, A.A. Salomão, M. Tubino, Eclet. Quim. 33 (2008) 67–72. [11] A.M. El-Didamony, A.S. Amin, A.K. Ghoneim, A.M. Telebany, Cent. Eur. J. chem. 4 (2006) 708–722. [12] C.S.P. Sastry, T.S. Rao, P.S.N.H.R. Rao, U.V. Prasad, Microchim. Acta 140 (2002) 109–118. [13] M.A. Pfaller, D.J. Krogstad, G.G. Granich, P.R. Murray, J. Clin. Microbiol. 20 (1984) 311–316. [14] M.T. Lam, X.C. Le, Analyst 127 (2002) 1633–1637. [15] S.Y. Tetin, K.M. Swift, E.D. Matayoshi, Anal. Biochem. 307 (2002) 84–91.
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
CuO nanosheets-enhanced flow-injection chemiluminescence system for determination of vancomycin in water, pharmaceutical and human serum A.R. Khataee a,⇑, A. Hasanzadeh a, M. Iranifam b, M. Fathinia a, Y. Hanifehpour c, S.W. Joo c,⇑ a b c
Research Laboratory of Advanced Water and Wastewater Treatment Processes, Department of Applied Chemistry, Faculty of Chemistry, University of Tabriz, Tabriz, Iran Department of Chemistry, Faculty of Science, University of Maragheh, Maragheh, East Azerbaijan, Iran School of Mechanical Engineering, Yeungnam University, Gyeongsan 712–749, South Korea
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
Preparation of CuO nanosheets (NSs)
by a simple one-step sonochemical method. Enhancement of luminolH2O2 CL system by prepared CuO NSs. Determination of vancomycin using a luminolH2O2CuO flow-injection CL system. Appropriate sensitivity and selectivity of the proposed method.
a r t i c l e
i n f o
Article history: Received 19 October 2013 Received in revised form 29 November 2013 Accepted 8 December 2013 Available online 12 December 2013 Keywords: Flow-chemiluminescence Vancomycin CuO nanosheets Sonochemical Luminol
a b s t r a c t A novel, rapid and sensitive CuO nanosheets (NSs) amplified flow-injection chemiluminescence (CL) system, luminol–H2O2–CuO nanosheets, was developed for determination of the vancomycin hydrochloride for the first time. It was found that vancomycin could efficiently inhibit the CL intensity of luminol–H2O2–CuO nanosheets system in alkaline medium. Under the optimum conditions, the inhibited CL intensity was linearly proportional to the concentration of vancomycin over the ranges of 0.5–18.0 and 18.0–40.0 mg L1, with a detection limit (3r) of 0.1 mg L1. The precision was calculated by analyzing samples containing 5.0 mg L1 vancomycin (n = 11) and the relative standard deviation (RSD) was 2.8%. Also, a high injection throughput of 120 sample h1 was obtained. The CuO nanosheets were synthesized by a sonochemical method. Also, X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses were employed to characterize the CuO nanosheets. The method was successfully employed to determine vancomycin hydrochloride in environmental water samples, pharmaceutical formulation and spiked human serum. Ó 2013 Elsevier B.V. All rights reserved.
Introduction Vancomycin hydrochloride is a glycopeptide antibiotic that is widely employed in the treatment of serious infections by Gram⇑ Corresponding authors. Tel.: +98 411 3393165; fax: +98 411 3340191 (A.R. Khataee). Tel.: +82 53 810 1456 (S.W. Joo). E-mail addresses: [email protected], [email protected] (A.R. Khataee), [email protected] (S.W. Joo). 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.12.014
positive bacteria [1,2]. It was chosen for therapy of infections in patients allergic to b-lactam antibiotics. This antibiotic acts by preventing the peptidoglycan synthesis of bacterial cell wall [3]. Monitoring vancomycin in the biological fluids and in pharmaceutical products is of importance to prevent side effects in patients under treatment and to achieve optimum therapeutic concentrations [4]. Moreover, antibiotics including vancomycin have been found in the aquatic environment such as waste and polluted
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water resources. This compound can play a role in the maintenance or extension of antibiotic resistance bacteria, finally resulting in hazards to human health [5,6]. Therefore, screening vancomycin both in biological and environmental studies seems to be very important. The common analytical methods for determination of vancomycin in different real samples are voltammetry [7], spectrophotometry [8–12], immunoassay [13–15], high-performance liquid chromatography (HPLC) with ultraviolet detection [16–24], electrochemical detection [25], mass spectroscopy detection [4,5,26–28] fluorescence detection [29] and capillary electrophoresis (CE) [30–32]. Nevertheless, to the best of authors knowledge, there have been no reports regarding the use of CL methods for the determination of vancomycin in real samples. The above-mentioned methods for determination of vancomycin have their own advantages and disadvantages. However, high cost instrumentation, time-consuming procedure or unsuitability for automatic analysis and the need for large amounts of expensive and toxic solvents are some of the reported drawbacks of these methods [33]. Hence, the development of simple and sensitive techniques for the determination of vancomycin in real samples is of great importance. Moreover, in recent decades, CL methods have received extensive attention due to their high sensitivity, simplicity, inexpensive equipment, reproducibility and rapidity [34,35]. In addition, the coupling of a flow-injection analysis technique with CL detection presents high analytical throughput [36]. Owing to these advantages, flow-CL methods have been developed as useful and powerful tools in many different fields, such as clinical assay, pharmaceutical, environmental and food analysis [37]. It should be noted that in some of the common CL systems, the produced CL emission during oxidation of an analyte is of relatively low intensity due to the low quantum yield in some oxidation reaction. In this context, metal and semiconductor nanomaterials with unique physical and chemical properties have been widely used in various CL reactions as signal amplifier catalysts [38,39]. As an example, nanomaterials of gold [40], silver [41], platinum [42], CdTe [43], TiO2 [44], Se [39] and CuO nanoparticles [45] have been used as signal enhancers. Therefore, the integration of nanomaterials with CL systems could provide new approaches to enhance the sensitivity of the used method, which is an essential factor for analytical applications [38]. The catalytic performance of CuO nanostructures is highly dependent on the morphology and size. CuO nanosheets exhibit several advantages including non-toxicity, high density, uniformity and good crystalline array structure [46]. In the past, extensive efforts have been focused on controlling the size and shape of CuO nanomaterials. Recently, CuO nanomaterials have been synthesized via various techniques such as the solution-based, vapor phase growth and sonochemical technique [47]. Herein, we have applied the sonochemical-based method for the preparation of high uniform CuO nanosheets due to its several attractive features such as high yields of the prepared nanomaterials and the low cost. To the best of our knowledge, there is no previous report concerning the use of CuO nanosheets as a signal amplifier in CL systems. The reaction between luminol and H2O2 is a popular CL reaction that has been extensively applied for the determination of various substances [24]. The absence of a simple and fast CL method for the determination of vancomycin caught our attention in this context. Here in, vancomycin was subjected to some introductory tests and it was found that vancomycin was able to inhibit the CL emission of luminol–H2O2–CuO nanosheets. The resultant decrease in CL intensity was proportional to vancomycin concentration. In the present work, we have proposed a novel and sensitive flow-injection CL method for the determination of the vancomycin hydrochloride in real samples. Within this context, the effect of
CuO nanosheets on the luminol–H2O2 CL system was investigated. In addition, it was found that CuO nanosheets could act as a nanocatalyst on the luminol–H2O2 CL reaction under alkaline conditions, capable of generating a significant increase in CL emission. Therefore, under the optimized conditions, the novel CuO nanosheets-enhanced CL system could be used as an alternative method for the analysis of vancomycin in environmental water samples, pharmaceutical formulation and spiked human serum. Experimental details Materials All the chemicals and reagents used in this work were of analytical grade and used without further purification and purchased from Merck Co. (Germany), except for vancomycin hydrochloride, which was obtained from Jaber Ebne Hayyan pharmaceutical Co. Tehran, Iran. Doubly distilled water was used in all experiments. A stock standard solution of 2 102 mol L1 luminol was prepared by dissolving 0.354 g luminol in 100 mL of 0.1 mol L1 sodium hydroxide in a brown volumetric flask. A stock solution of 1 mol L1 sodium hydroxide was prepared by dissolving 4 g sodium hydroxide in 100 mL double distilled water. The working solutions of hydrogen peroxide were freshly prepared daily by diluting appropriate amounts of 30% (w/v) H2O2 with doubly distilled water. A 0.02 mol L1 copper (II) acetate monohydrate solution was prepared by dissolving 0.998 g in 250 mL distilled water. A 100 mg L1 stock standard solution of vancomycin was prepared by dissolving 25 mg vancomycin in 500 mL doubly distilled water and stored at 4 °C in refrigerator. All working solutions were prepared by diluting their related stock solutions. Vancomycin hydrochloride capsules were purchased from Flynn pharmaceutical Co. Ireland. Drug-free human serum used in this study was taken from healthy volunteers and stored in freezer until analysis. Apparatus The schematic of the flow-injection CL is illustrated in Fig. 1. Polytetrafluoroethylene (PTFE) tubing with an inner diameter (i.d.) of 1.0 mm was utilized as a connection material in the flow system. A peristaltic pump was used to deliver all respective solutions through the flow system at a flow rate of 2.0 mL min1 for each channel. A six-port valve equipped with a 100 lL sample loop was applied for injection. The light generation from the CL reaction in the flow cell was monitored by a FB12 luminometer (Berthold Detection Systems, Germany). Ultraviolet–visible (UV–Vis) spectra were recorded by UV–Vis spectrophotometer (WPA lightwave S2000, England) in the range of 200–800 nm. A bath type sonicator (Sonica, 2200 EP S3, Italy) with 50–60 Hz frequency and heating arrangement was used for sonosynthesis of CuO nanosized samples. To characterize the crystal structure, mean crystal size and phase purity of as-prepared
Fig. 1. Schematic diagram of flow-injection CL system; (a) luminol in NaOH solution; (b) sample or blank solution; (c) H2O as the carrier; (d) H2O2 solution; P: peristaltic pump; M: mixing tube; V: injection valve; F: flow cell; W: waste; D: detector (luminometer); R: recorder (personal computer).
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samples, XRD measurements were performed at room temperature by using Siemens X-ray diffraction D5000 (California, USA), with Cu Ka radiation. The accelerating voltage of 40 kV and emission current of 30 mA were utilized. The Debye–Scherrer formula was employed to calculate the average crystalline size of the catalysts from XRD peak [48]. SEM (S-4200, Hitachi, Japan) was used to observe the surface state and the morphology of the prepared nanosheets using an electron microscope. General procedure for flow-injection CL system As depicted in Fig. 1, the alkaline solution of luminol (a), sample or standard solution of mixture of vancomycin and CuO nanosheets (b), H2O as the carrier (c) and H2O2 solution (d) were pumped by a peristaltic pump. Solutions of a and b were mixed via a mixing tube (silicon tubing, 1.0 mm i.d.); then 100 lL of mixing solution was injected into the carrier stream and premixed with H2O2 stream through a Y-piece; after that, the mixture was delivered into the flow cell. The peak height of the CL signals was continuously monitored on a computer connected to the luminometer. Determination of vancomycin was carried out based on the decrease of CL intensity as DI = I0–Is, where I0 and Is denote CL intensity in the absence and presence of vancomycin, respectively. Pretreatment of real samples solution before CL assay Tap water was analyzed without any pretreatment. Ground and river water samples were freshly collected and filtered with polyamide membrane filters of 0.45 lm to eliminate the suspended solid matter and stored in dark at 4 °C in the refrigerator [39]. They were used within 1 week. Prior to analysis, 30 mL water samples were spiked with 0.5, 1, 1.5, 2, 2.5 and 3 mL vancomycin standard solutions (100 mg L1) and diluted to 50 mL with doubly distilled water to prepare solutions of 1, 2, 3, 4, 5 and 6 mg L1, respectively. For commercial capsules samples containing vancomycin hydrochloride, 5 capsules were powdered and mixed in mortar. Then, 100 mg of vancomycin, was weighted and dissolved in distilled water, and the obtained solution was filtered via an ordinary filter paper and diluted to 100 mL with doubly distilled water to prepare the solution of 1000 mg L1. For human serum samples, only a deproteinization pretreatment step utilizing trichloroacetic acid (CCl3COOH) was performed; an extraction process was not needed [49]. To prepare the spiked samples, known amounts of vancomycin were spiked into 1.0 mL of serum and then, for each sample, 5.0 mL of 10% (w/v) trichloroacetic acid was added. These mixtures were centrifuged at 3000 rpm for 15 min. 2.5 mL of the protein-free supernatant was diluted to 50 mL with doubly distilled water. The amount of vancomycin in the spiked samples was determined by means of the general procedure using standard addition method. Also, the blank value was determined by vancomycin-free samples in the same procedure. Synthesis of CuO nanosheets CuO nanosized crystals were synthesized by sonochemical method as follows: 200 mL of 0.02 mol L1 copper(II) acetate aqueous solution was added to 1 mL of glacial acetic acid. Then, the solution was put into a round-bottomed flask equipped with a refluxing device. Subsequently, the flask was transferred to the sonicator, heated to boiling under ultrasonic irradiation; then about 20 mL of 1 mol L1 NaOH solution was added gradually into the above solution in ambient air. The obtained black precipitate was centrifuged and washed thoroughly for several times with
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ethanol and distilled water to remove residual impurities. The resultant product was dried in air at room temperature. Results and discussion Characterization of CuO nanosheets The XRD patterns of the synthesized CuO samples are depicted in Fig. 2a. The XRD diffraction peaks at 2h of 32.4°, 35.5°, 38.7°, 48.7°, 53.2°, 57.9°, 61.3°, 66.1°, 68.0°, 71.9° and 74.8° can be classified into (1 1 0), (0 0 2), (1 1 1), (2 0 2), (0 2 0), (2 0 2), (1 1 3), (0 2 2), (2 0 0), (3 1 1) and (2 2 2) plane reflections, which are associated with monoclinic crystal structure CuO according to the standard powder diffraction data (JCPDS 05-0661) [50]. No peak for impurities was detected, confirming that the applied sonochemical method in this study was successful in synthesizing the nanosized CuO samples. Moreover, the sharp diffraction peaks in the XRD spectrum of the synthesized sample indicated that the synthesized product was highly crystalline. By calculating the Debye–Scherrer formula [48], the average size of the crystals was found to be about 5 nm. In order to further clarify the size and morphology of the synthesized product, SEM images were taken at different magnifications. Fig. 2b shows the SEM microphotographs of the nanosized samples at different magnifications. The low magnification SEM image in the left side of Fig. 2b obviously showed that the obtained samples possessed sheet-like morphology. Moreover, the high magnification SEM image in the right side of the Fig. 2b confirmed that the grown nanosheets were randomly grown. They exhibited clean and flat surfaces with the surface and wall thickness being in the range of 120 and 50 nm, respectively. CuO nanosheets-enhanced CL system The reaction between luminol and H2O2 is a popular CL reaction that has been extensively applied for the detection of diverse substances [36,38]. The CL intensity–time response curves are shown in Fig. 3. As presented in Fig. 3, the oxidation of luminol by H2O2 generated a relatively weak CL emission in alkaline media. However, we found that in the presence of CuO nanosheets, the CL intensity was significantly enhanced by about 9 times. In order to confirm the enhancing effect of CuO nanosheets, blank experiments were carried out using copper(II) acetate and glacial acetic acid aqueous solution in concentrations used for the preparation of CuO nanosheets. No enhancing effect was observed in the presence of glacial acetic acid, but an enhancing effect in the presence of copper(II) acetate was observed, not much remarkable in comparison with CuO nanosheets amplified CL system. These results confirmed that the catalytic effect was due to the CuO nanosheets. So, CuO nanosheets-enhanced CL reaction was used for the determination of vancomycin. The results indicated that the CL intensity of the CuO nanosheets catalyzed luminol–H2O2 system was significantly decreased upon the addition of 5 mg L1 vancomycin; however, in the absence of the CuO nanosheets, the inhibition of the luminol–H2O2 was negligible in the presence of 5 mg L1 vancomycin. Optimization of experimental conditions A series of experiments were run to optimize the CL reaction conditions, including the chemical concentration, flow rate, and the length of mixing tube. The CL intensity was dependent on the concentration of CuO nanosheets. As indicated in Fig. 4a, the inhibition of CL intensity was increased with the increase in the CuO nanosheet
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Fig. 2. (a) XRD pattern of the synthesized CuO NSs and (b) SEM images of synthesized CuO NSs.
increasing H2O2 concentration from 0.001 to 0.015 mol L1 and slightly decreased when H2O2 concentration was higher than 0.015 mol L1. So, 0.015 mol L1 H2O2 was chosen as the most suitable concentration. Moreover, the influence of the flow rate of pump on the CL reaction was assayed over the range of 1.0–3.5 mL min1. The inhibition of CL intensity was increased with the flow rate of 1.0–2.0 mL min1. When the flow rate was higher than 2.0 mL min1, the CL response fairly remained constant. Therefore, 2.0 mL min1 of flow rate was applied in our measurements in which high sample through put could be achieved. The effect of the mixing tubing length on the inhibition of CL intensity was examined in the ranges of 5–20 cm. The best results with good reproducibility were obtained with 10.0 cm mixing tube. Fig. 3. Kinetic curve for (a) luminol–H2O2 CL system (b) luminol–H2O2–vancomycin; (c) luminol–H2O2–Cu(II) (d) luminol–H2O2–CuO NSs and (e) luminol–H2O2– CuO NSs in the presence of vancomycin. The concentrations of luminol, H2O2, NaOH, CuO NSs, Cu(II) and vancomycin were 100 lmol L1, 0.015 mol L1, 0.01 mol L1, 1 mg L1, 0.02 mol L1and 5 mg L1, respectively.
concentration up to 1 mg L1 and then decreased with the further increase in the catalyst concentration. So, it is clear that the optimum concentration of CuO nanosheets was 1 mg L1, with the highest inhibition of CL intensity. Luminol CL intensity is dependent on the concentration of NaOH. Therefore, the effect of different concentrations of NaOH on the CL system was examined in the ranges of 0.0001– 0.04 mol L1. The results revealed that the highest inhibition of CL signals was obtained in the NaOH concentration of 0.01 mol L1. Thus, 0.01 mol L1 NaOH was chosen for the subsequent experiment (see Fig. 4b). The effect of luminol concentration on the inhibition of CL intensity was studied over the range of 0.5–300 lmol L1. According to the results (Fig. 4c), inhibition of CL intensity was increased with the increase of luminol concentration from 0.5 to 100 lmol L1;for amounts beyond the concentration of 100 lmol L1, the inhibition of CL intensity was declined. So, 100 lmol L1 luminol was taken as an optimum concentration. As shown in Fig. 4d, the effect of H2O2 concentration was investigated in the range of 0.001–0.03 mol L1. The results indicated that the inhibition of CL intensity was increased with
Analytical features Under the optimum conditions, the inhibition of CL intensity of luminolH2O2CuO nanosheets by vancomycin was linear over the concentration ranges of 0.5–18.0 and 18.0–40.0 mg L1. The linear regression equations were y = 769706x + 106 (r2 = 0.996) and y = 78525x + 107 (r2 = 0.994), respectively, where y was the DI and x was the vancomycin concentration in mg L1. The detection limit (3r) of the method was calculated to be 0.1 mg L1. The relative standard deviation for the determination of 5 mg L1 vancomycin was 2.8% (n = 11). The results showed that the proposed CL system had a good linearity, a relatively high sensitivity and adequate precision. Moreover, the experimental results obtained from our proposed method were compared with those of other previously reported works for the determination of vancomycin (see Table 1). It is noteworthy to mention that in comparison with most methods used for the determination of vancomycin, the proposed simple and rapid CL system in the present work featured acceptable analytical figures of merit with a relatively cheap apparatus. Moreover, the reagents were stable and inexpensive. Interference study In order to validate the possible analytical application of the proposed method for real samples, the interference effects of some
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CL signal for vancomycin. The results listed in Table 2 indicated that substances possibly found in real sample including environmental water samples, pharmaceutical formulation and human serum did not interfere with the determination except Cu2+ ion, which was found to be the most serious interferent. However, the interference from this metal ion could be removed with the addition of ethylene diamine tetra acetic acid (EDTA). EDTA with concentration 15 times greater than vancomycin had an insignificant effect on the CL signal. It can be concluded that the proposed CL method presented high selectivity for the determination of vancomycin in environmental water samples, pharmaceutical formulation and human serum. Analytical application In order to evaluate the applicability of the proposed method to real samples, the method was applied to determine the vancomycin in environmental water samples, pharmaceutical formulation and spiked human serum. The samples were prepared and analyzed according to the general procedure described in the experimental section. The obtained results for environmental water samples, pharmaceutical formulation and spiked human serum are summarized in Tables 3–5, respectively. The results for analysis of pharmaceutical formulation were also compared with those obtained by an official method (antibiotics microbial assays) [51]. Statistical analysis of these results using Student t-test showed that there is no significant difference between the results of two methods (see Table 4). The accuracy of the method was evaluated by performing recovery experiments for samples solution. The obtained results confirmed the applicability and accuracy of the method. Possible CL mechanism
Fig. 4. Effects of chemical concentration on the inhibition of CL intensity (a) effect of CuO NSs concentration. Conditions: the concentrations of vancomycin, luminol, NaOH and H2O2 were 5 mg L1, 80 lmol L1, 0.005 mol L1 and 0.01 mol L1, respectively; (b) effect of NaOH concentration. Conditions: CuO NSs concentration was 1 mg L1, other conditions were as in (a); (c): effect of luminol concentration. Conditions: NaOH concentration was 0.01 mol L1, other conditions were as in (b and d): effect of H2O2 concentration. Conditions: luminol concentration was 100 lmol L1, other conditions were as in (c).
species often found in water samples, pharmaceutical formulation and human serum were investigated. Analyses were performed for a fixed concentration of vancomycin (5 mg L1) with increasing interference concentrations. The tolerable limit of each foreign species was taken if it caused a relative error less than 5% in the
The mechanism of luminolH2O2 system under alkaline conditions in aqueous solution has been widely studied [40,43,52]. Also, it has been well confirmed that reactive oxygen species (ROS) such as hydrogen peroxide (H2O2), hydroxyl radicals (OH) and superoxide radical anion O 2 are the most important oxidizing species. Moreover, oxidation of luminol by the resulted ROS generates luminol radical and diazaquine, which rapidly react with O 2 or OH, giving rise to excited state 3-aminophthalate anion (3-AMP ). CL emission emanates from relaxation of 3-AMP to the ground state at 425 nm [38]. In this context, some authors reported that [41] the reaction of luminolH2O2 in alkaline medium in the absence of a catalyst was relatively slow, so the produced CL emission was weak. Many attempts have been made to elucidate the mechanism of enhanced CL system in the presence of nanomaterials [40,53,54]. According to the literature review [43,54,55], the enhancement of CL emission is due to the interaction of nanomaterials surface with the reactants or the intermediates of the CL reactions which facilitate ROS generation and electron-transfer processes occurring on the surface of the nanoparticles. In the present work, in order to elucidate the possible CL mechanism, UV–Vis absorption spectra (Fig. 5), in addition to CL intensity–time curve (Fig. 3), were recorded for the CL system in the presence and absence of CuO nanosheets. As shown in Fig. 5, vancomycin had a maximum absorption peak at about 281 nm (curve a) and the CuO nanosheets showed a linear absorption peak (curve b). The luminol–H2O2 system had two absorption peaks at 296 nm and 346 nm (curve c), the absorption peak of a mixed system of CuO nanosheets and luminol–H2O2 system (curve e) was almost equal to the sum of the absorption peak of the two individual systems, indicating that there was no chemical reaction between CuO nanosheets and luminol–H2O2 system during the CL reaction. Comparing our re-
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Table 1 Analytical performance data of previously reported works for the determination of vancomycin. Method
Matrix
Linear range (mg L1)
Detection limit (mg L1)
Reference
Spectrophotometry Voltammetry HPLC/UVa
Pharmaceutical formulation Pharmaceutical formulation Human plasma
HPLC/ECb LC/MSc LC/MS/MSd
Human plasma Human serum Water samples Rat plasma Human plasma Pharmaceutical formulation Human serum, pharmaceutical formulation and water samples
2.0–18.0 4.0–32.0 0.4–100.0 1.0–80.0 0.5–50.0 0.05–10.0 0.016–2.0 0.01–20.0 0.005–1.0 0.25–5.0 0.5–40.0
0.02 2.7 0.2 0.5 0.25 0.001 0.01 0.007 0.002 0.1 0.1
[9] [7] [17] [18] [25] [4] [5]
HPLC/FLe CE/UVf CL a b c d e f
High performance liquid chromatography (HPLC) with UV detection. HPLC with electrochemical detection. Liquid chromatography (LC) with mass spectrometry. LC with tandem mass spectrometry. LC with fluorescence detection. Capillary electrophoresis with UV detection.
Table 2 Tolerable concentration ratios with respect to 5 mg L1 of vancomycin. Species
Table 5 Results for the determination of vancomycin in spiked human serum samples.
Tolerable concentration ratio
Sample
Added (mg L1)
Found
½C interferent
Human serum sample No. 1
0 3 7 4 8 10 14
Not detected 2.84 ± 0.25 7.13 ± 0.12 3.86 ± 0.37 7.90 ± 0.42 10.44 ± 0.35 14.46 ± 0.26
ðmg L1 Þ=C vancomycin ðmg L1 Þ
1000
Na , Cl , K , SO2 4 , CH3COO 3 2+ 2+ , PO , Mg , Ca , Br, CO2 3 4 +
+
500
Tartaric acid, Lactose, Valine Alanine, Sucrose, Glucose, Starch,
No. 2
400
2 Threonine, NO 3 , SO3 Methyl parabene, Propyl parabene, Stearic acid Ethylenediaminetetraacetic acid (EDTA) Fe2+, Citrate Ascorbic acid Cu2+
No. 3
100
a
10 2 1
a
Sample
Added (mg L1)
Found
Water Tap water
0 1 3 2 5 4 6
Not detected 0.94 ± 0.22 3.12 ± 0.12 1.93 ± 0.35 5.08 ± 0.48 4.25 ± 0.15 5.84 ± 0.36
Ground water River water
a
(mg L1)
Recovery (%) – 94.67 101.86 96.50 98.75 104.40 103.28
Mean of six determinations ± standard deviation.
15
Table 3 Results for the determination of vancomycin in environmental water samples.
a
[29] [30] This work
(mg L1)
Recovery (%) – 94 104 96.5 101.6 106.25 94.33
Mean of six determinations ± standard deviation.
sults with those of previous reported papers, we can conclude that the enhanced CL signals (Fig. 3) were ascribed to the generation of more ROS due to the fast transfer of electron on the surface of CuO nanosheets and the catalytic effects of CuO
nanosheets. This phenomena accelerated 3-AMP and 3-AMP generation, leading to the higher CL intensity. On the other hand, we have investigated the inhibiting effect of vancomycin in the proposed CL system. Curve f in Fig. 5 shows that the absorbance of the luminol–H2O2–CuO nanosheets system in the presence of vancomycin had a slight shift to red area at 296 nm and the absorption intensity in 281 nm disappeared (curve f). Also, it can be deduced from this curve that the light absorption of the mixed system was not equal to the sum of the light absorption of the two individual systems (curves a and e). This implied that vancomycin might be involved and oxidized in the luminol–H2O2–CuO nanosheets reaction system, thereby creating new productions. Moreover, the inhibiting role of vancomycin could be related to the presence of the reducing groups such as OH, NH and NH2. It was reported that [38,56–58] the functional groups of vancomycin could react and scavenge the produced ROS, leading to the reduced CL emission intensity. In addition, vancomycin molecules could cover the surface of CuO nanosheets and prevent them from acting as a catalyst in CL system.
Table 4 Results for the determination of vancomycin in pharmaceutical formulation. Sample Pharmaceutical formulation
a b c
Sample solution number a
1 2 3 4
Sample taken (5 mg L1). Mean of six determinations ± standard deviation. t-Critical = 4.30 for n = 6 and P = 0.05.
b
Added (mg L1)
Found
(mg L1)
0 10 20 30
5.39 ± 0.12 15.72 ± 0.15 26.48 ± 0.16 34.85 ± 0.21
Found (official method) (mg L1)
t-Statistic
5.03 ± 0.52 – – –
1.79 2.08 1.90 2.38
c
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Fig. 5. UV–Vis absorption spectra of CL system. Conditions: the concentrations of luminol, H2O2, NaOH, CuO NSs and vancomycin were 100 lmol L1, 0.015 mol L1, 0.01 mol L1, 1 mg L1and 5 mg L1, respectively.
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