Highly sensitive detection of chromium (III) ions by resonance Rayleigh scattering enhanced by gold nanoparticles

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 776–781

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Highly sensitive detection of chromium (III) ions by resonance Rayleigh scattering enhanced by gold nanoparticles Min Chen a,b, Huai-Hong Cai c,⇑, Fen Yang c, Dewen Lin c, Pei-Hui Yang c, Jiye Cai c a

Department of Pulmonary Medicine, The First Affiliated Hospital of Jinan University, Guangzhou 510630, China Institute of Respiratory Diseases, Guangdong Medical College, Zhanjiang 524001, China c Department of Chemistry, Jinan University, Guangzhou 510632, China b

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

3+

 Cr –citrate chelation was used to

selectively detect Cr3+ ions.  The assay was based on the enhancement of resonance Rayleigh scattering by gold nanoparticles. 3+  A very low limit of detection for Cr has been achieved at room temperature within only 20 min.  This assay was suitable for highthroughput routine applications in environment and food samples.

a r t i c l e

i n f o

Article history: Received 21 May 2013 Received in revised form 3 September 2013 Accepted 16 September 2013 Available online 24 September 2013 Keywords: Cr3+ ion Gold nanoparticles Sensors Resonance Rayleigh scattering Nanoparticle aggregates

a b s t r a c t Simple and sensitive determination of chromium (III) ions (Cr3+) has potential applications for detecting trace contamination in environment. Here, the assay is based on the enhancement of resonance Rayleigh scattering (RRS) by Cr3+-induced aggregation of citrate-capped gold nanoparticles (AuNPs). Transmission electron microscopy (TEM) and UV–vis absorption spectroscopy were employed to characterize the nanostructures and spectroscopic properties of the Cr3+–AuNP system. The experiment conditions, such as reaction time, pH value, salt concentration and interfering ions, were investigated. The combination of signal amplification of Cr3+–citrate chelation with high sensitivity of RRS technique allow a selective assay of Cr3+ ions with a detection limit of up to 1.0 pM. The overall assay can be carried out at room temperature within only twenty minutes, making it suitable for high-throughput routine applications in environment and food samples. Ó 2013 Elsevier B.V. All rights reserved.

Introduction Contamination of the environment with heavy metal ions has been an important worldwide concern for decades. Chromium pollution originates mainly from coal-burning power plants, gold mining, volcanic emissions, and waste combustion. Chromium (Cr), which can accumulate in vital organs and tissues, such as the liver, brain and heart muscle, is toxic and ⇑ Corresponding author. Tel./fax: +86 20 85223569. E-mail address: [email protected] (H.-H. Cai). 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.09.058

can have lethal effects on living systems [1,2]. Chromium   oxidation commonly exists in chromium III (Cr3+) and VI Cr2 O2 7 states. An essential trace element of Cr (III) in human nutrition has great impacts on the metabolism of carbohydrates, fats, proteins and nucleic acids by activating certain enzymes [3,4]. However, high levels of Cr (III) can bind to DNA, negatively affecting cellular structures and damaging the cellular components, although its toxicity observed in vivo is less serious than that of Cr (VI) [5,6]. The two oxidation states of Cr (III) and Cr (VI) can be interchangeable [7,8]. Therefore, it is highly desirable to develop a sensitive and selective Cr (III) detection method

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that can provide simple, practical, and high-throughput routine determination of levels of Cr3+ ions for both environmental and food samples. According to US Environmental Protection Agency (EPA) standard, World Health Organization (WHO) and European Community (EC), the MAL (maximum allowable level) of Cr3+ ions in drinking water are 100 lg/L, 50 lg/L and 50 lg/L, respectively. Studies showed that long-term accumulation of low Cr3+ concentration could induce diagnose chromium poisoning. Serum chromium levels normally range from less than 50 up to 500 ng/mL. Thus, the development of a highly sensitive, facile, and practical assay for Cr3+ ions remains a challenge. A number of sensitive techniques have been employed in determining trace amount of Cr3+, such as atomic absorption spectrophotometry [9,10] and plasmas mass spectrometry [11,12], but each of these processes is time consuming and necessitates expensive equipments. Much effort has been devoted towards the design of sensing systems for Cr3+ ions, including sensors based on organic chromophores [13,14], enzymes [15], and specific proteins [16]. However, most of these methods have some limitations such as expensive reagents with conjugation of specific biomolecules, complex fabrication process, insufficient sensitivity, and in certain cases are bioactivity in biosensors. Another emerging approach for the detection of Cr3+ ions involves the use of Cr3+ ions can specifically induce nanoparticle aggregates. Cr3+ ion detection assays based on this property of Cr3+-nanoparticle coordination chemistry have been developed in recent years [17,18]. Wu and coworkers have described a colorimetric assay based on Cr3+-induced silver nanoparticle aggregates, which yields color change corresponding to enhanced localized surface Plasmon resonance (LSPR) signals, and then allows the detection of Cr3+ ions with high selectivity and sensitivity [19]. Ouyang and coworkers fabricated a flower-like self-assembly of gold nanoparticles (AuNPs) on a glassy carbon electrode (GCE) as a highly sensitive platform for Cr ions detection with the detection limit of up to 2.9 ng/L [20]. Hughes and coworkers reported the hyper Rayleigh scattering detection of Cr ions using 5,50 -dithiobis-(2-nitrobenzoic acid)-modified gold nanoparticles with specifically designed Cr3+-nanoparticle conjugates with a sensitivity of up to 25 ppb [21]. Resonance Rayleigh scattering (RRS), a special re-scattering, takes place when the wavelength of Rayleigh scattering is located at or close to the molecular absorption band [22]. Because RRS detection limit is lowered by several orders of magnitude when it compared with other spectroscopic techniques, RRS technique was therefore used to sensitively determine molecules [23–25]. Importantly, the magnitude of light scattering by gold nanoparticles (AuNPs) can be orders of magnitude higher than light emission from fluorescent dyes [23]. Considering signal amplification of re-scattering signals for AuNPs, it is natural to hypothesize that nanoparticle-involved RRS method can be a sensitive technique for quantitative detection of metal ions at trace concentration [26,27]. AuNPs-based enhancement functions have been employed to substantially improve the performance and sensitivity of various biosensing systems [28]. The enhanced effects were implemented by the use of AuNPs as labels for amplified quartz crystal microbalance detection [29] and electrochemical detection [30], by the application of AuNPs as electron relays for the facilitation of interfacial electron transfer on electrodes [31,32]. Recently, AuNPs-enhanced effects in resonance Rayleigh scattering (RRS) method [33] were also reported. To our knowledge, no reports have shown quantitative detection for Cr3+ using AuNPs enhancement of RRS signals. Herein, we present a novel highly sensitive RRS method for the detection of Cr3+ ions on the basis of the formation of Cr3+-nanoparticle aggregates (Scheme 1). The detection sensitivity can be

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significantly improved to picomolar level by combinating of signal enhancement of nanoparticles with high sensitivity of RRS. Materials and methods Reagents and instruments All chemicals used in this work were analytical grade and directly used without additional purification. Hydrogen tetrachloroaurate (III) hydrate (HAlCl4) was purchased from Shanghai Chemical Reagent Co. (Shanghai, China). The other chemicals were purchased from Sigma–Aldrich (St. Louis, MO, USA). The stock solution of Cr3+ (1.0 mM) was prepared by dissolving Cr(NO3)3 in high purity water (Milli-Q, Millpore). Other metal ion stock solution (1.0 mM) was also prepared in high purity water. RRS measurements were performed using a 970CRT fluorescence spectrophotometer (Shanghai, China) with a 1.0 cm quartz cell. The magnitude of RRS intensity was obtained by the synchronous scanning at kex = kem (Dk = 0 nm). The relative RRS intensity (DIRRS) was obtained by the difference between the assay system (IRRS) and the regent blank (I0), namely, DIRRS = IRRSI0. Transmission electron microscope (TEM) images of nanoparticles were characterized by a Philips TECNAI-10 TEM. Sample was prepared by dropping onto copper grid. UV–vis absorption spectra were measured using a UV-1901 UV–Vis spectrophotometer (Beijing, China) by quartz cell with the path length of 1.0 cm. Preparation of citrate-capped AuNPs and analytical procedure AuNPs of 20 nm were prepared by the citrate reduction of HAuCl4 according to a reported literature method [34]. 50 ml of HAuCl4 solution (1.0  103 M) was added 10 ml of 1% trisodium citrate solution and was rapidly stirred and kept boiling for 20 min. Then nanoparticle solution was cooled to room temperature. Excess reagents were then removed by centrifugation at 12,000 rpm at 4 °C for 20 min. Finally, citrate-capped AuNPs were re-dispersed in 1 ml high purity water (pH 7). The concentration of AuNPs was quantified based on the absorption of A520 by UV–vis absorption spectroscopy. Analytical procedure for Cr3+ detection AuNPs coupling with RRS method was applied for sensitive detection of the Cr3+. Firstly, solution of AuNPs (1.0 lL, 25 nM particle concentration) was added to a 1.5 mL eppendorf tube containing 1 mL water (pH 7). Subsequently, 1 lL of Cr(NO3)2 solution (at different concentrations) or solutions of other metal ions were added, and the solution was incubated for an additional 20 min at room temperature before RRS measurements. All experiments were repeated two times. Each sample was measured four times. Interfering measurement To check the selectivity of proposed sensor for Cr3+ ion, the solutions of different interference ions, such as Na+, Ba2+, Ca2+, 3+ Mg2+, K+, Cl, and PO3 4 , were added to Cr –AuNPs solution, and then were investigated by RRS measurements. Results and discussion Characterizations of Cr3+-induced AuNP aggregates The AuNPs-enhanced RRS method is outlined in Scheme 1. The important step for developing useful metal ion assay is to choose appropriate nanoparticles that can sensitively recognize Cr3+ ions.

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Scheme 1. Schematic illustration of the strategy of Cr3+ ion detection using RRS enhancement by AuNPs.

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Under TEM, individual AuNPs exhibited spherical and well-shared shape (Fig. 1A). In the presence of Cr3+, the network-liked aggregations of nanoparticles were observed, visually demonstrating that Cr3+ could induce large-scaled nanoparticle aggregation through chelating reaction [19] (Fig. 1B). Another evidence of aggregation of AuNPs can be provided by UV–vis absorption spectroscopy. AuNPs exhibited the absorption band at 520 nm (Fig. 2, curve a), a typical characteristic of the surface Plasmon band. In the presence of Cr3+, nanoparticles solution exhibited a new band at 650 nm (Fig. 2, curve b), showing that Cr3+ could induce aggregations of nanoparticles. The decrease in the absorbance of the band at 520 nm was observed, further demonstrating the successful conjugation of Cr3+ with AuNPs.

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Furthermore, the red shift of the band at 650 nm suggests that the average size of the Cr3+–AuNPs system was increased, further supporting the results obtained from TEM. These results provide functional evidence that Cr3+ can induce aggregations of citratecapped AuNPs.

AuNPs-based RRS enhancement by Cr3+ ions

Fig. 1. TEM images of AuNPs (A) and Cr3+-induced aggregations of AuNPs (B).

To demonstrate the feasibility and sensitivity of RRS technique for Cr3+ detection, we seek to determine RRS characteristics of AuNPs and their sensitivity for capturing specific ions. As shown in Fig. 3A (curve a), AuNPs exhibited a sharp peak at 470 nm [33]. Introduction of Cr3+ ions onto citrate-capped AuNPs solution induced the significant increase of RRS intensity, demonstrating that carboxyl groups of citrate layer on AuNP surface were chelated with Cr3+ ions and thus led to the increase in nanoparticle size, and hence the enhancement of RRS intensity. It is important to note the RRS intensity of the Cr3+-AuNPs system is greater than that of AuNPs only, indicating that AuNPs are able to specifically capture Cr3+ ions. Therefore, the relative RRS intensity (DIRRS) at 470 nm can be measured as the readout to examine and quantitate targeted heavy metal ions. The determination of Cr3+ was then conducted in solution using citrate-capped AuNPs coupled with RRS measurement. Fig. 3B is the RRS spectra of Au NPs in the presence of different concentrations of Cr3+. DIRRS intensity enhanced with the increasing concentration of Cr3+, indicating that the linear detection for Cr3+ was established. In contrast, control experiment is carried out using citrate-capped AuNPs with water. The slight response for water with AuNPs was attributed to combination of unspecific surface adsorption because of larger surface area of nanoparticles, and electrostatic interactions of other heavy metal ions with the negatively

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Wavelength (nm) Fig. 3. RRS enhancement of AuNPs (1.5 nM) upon addition of different concentrations of Cr3+ ions. (A) Curves from (a) to (b) correspond to 0, 1  105 M Cr3+. (B) Curves from (a) to (d) correspond to 0, 1.0  1012, 5.0  1012 and 1.0  1011 M Cr3+.

charged citrate-capped AuNPs. Therefore, control experiment further validated high affinity of citrate-capped AuNPs with Cr3+, and low interference by other ions in linear detection of Cr3+. Optimal experiment conditions for Cr3+ detection The influence of reaction time on the chelating reaction between Cr3+ and citrate-capped AuNPs was investigated by monitoring the change of DIRRS magnitude at 470 nm. DIRRS value increased initially and then tended to be stable at the reaction time more than 20 min after Cr3+ was added to the solution containing AuNPs (Fig. 4A). These changes demonstrated that chelation-based nanoparticle aggregates was stable after 20 min interaction. Therefore, 20-min time course was introduced to record chelating reaction in this detection system. It is well known that pH value can readily affect the recognition ability of AuNPs and targeted molecules, correspondingly, influence the DIRRS signal of detection system. DIRRS magnitude initially tended to be stable at the pH range of 6–7 (Fig. 4B), while decreased with the increase of pH value. It is well-known that the hydrolysis constants of citric acid for pKa1, pKa2 and pKa3 are 3.1, 4.8 and 6.4, respectively. When pH value is greater than 6.4, citric acid group is existed in trivalent anions, which has the highest ability to chelate Cr3+ ions. This indicated that an increase in pH value in buffer benefited the existence of trivalent anions for citrate and resulted in the increasing of affinity of chelating reaction, therefore correspondingly generated a greater magnitude of DIRRS. However, when pH value was higher than an optimal pH range, an obvious

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decrease in the magnitude of DIRRS was observed in the Cr3+ determination. This showed that alkaline solution condition (pH > 8) induced hydrolysis of metal ions, destroyed chelating reaction of citrate-capped AuNPs with Cr3+. In acid solution, when pH value was lower than 4, the decrease in DIRRS was also observed, which was ascribed to the highest ability of citric acid group with hydrogen ion. This change in pH value resulted in the reduction in the magnitude of DIRRS. Therefore, the optimal pH range for this detection system was 6–7. The influence of salt concentration was investigated. Salt concentration has some effect on the DIRRS value of the assay. The optimal salt concentration for the detection is 1.0  104 M. In this salt concentration, the maximum DIRRS magnitude for detecting Cr3+ was obtained. Analytical performance Under the optimal experiment conditions, the proportional correlation of DIRRS intensity with Cr3+ concentration was investigated. The relative RRS intensity (DIRRS) value increased as the concentration of Cr3+ ions increased (Fig. 5). The present limit of detection for this method was 1.0 pM Cr3+, which, to the best of our knowledge, was the lowest ever reported for Cr3+sensing without signal and PCR amplification using enzymes. The result also illustrated that this method has a good detection range of Cr3+ ions from 1.0 pM to 10.0 pM (DIRRS = 47.85 + 12.11 [Cr3+]; correlation coefficient, r = 0.9965), and the relative standard derivation (R.S.D) was 2.24%. This demonstrated that nanoparti-

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than that of the other metal ions when the reaction time reached 20 min. It showed that selective detection was probably because the aggregation rates of other metal ions were relatively slow as compared to Cr3+ ions. As a transition metal ion with electronic configuration of 3d54s04p0 and high value of z2/r, Cr3+ ions are then easily chelated by citrate ions, which resulted in the formation of stable nanoparticle aggregates [35].

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cle-based RRS enhancement has picomolar sensitivity for determining heavy metal ions. The picomolar detection limit was attributed to signal amplification of nanoparticles and specific chelation reaction of citrate-capped AuNPs with Cr3+ ions. Interference analysis

Acknowledgements 3+

A major challenge for Cr detection in real samples is the elimination of interferences. The selectivity of the method has also been investigated by testing the response of this assay to other metal ions, including Mn2+, Pb2+, Mg2+, Cu2+, K+, Zn2+, and Fe3+ at a concentration of 10.0 lM—1000 times greater than that of the Cr3+. As shown in Fig. 6, Cr3+ ions showed a significant DIRRS magnitude, while there was very little increase of DIRRS value in the presence of other metal ions. The results demonstrated excellent selectivity over alkali, alkaline earth, and heavy transition-metal ions. The highly selective aggregation of AuNPs induced by Cr3+, we suppose, is ascribed to the chelation of Cr3+ with the citrate ions capped on AuNPs. To further evaluate the selectivity of Cr3+-induced AuNP aggregates, we monitored the aggregation kinetics of citrate-capped AuNPs after addition of 10.0 lM of above metal ions. Changes in the time-dependent DIRRS magnitude over 50 min of AuNPs with interference metal ions were observed. All DIRRS intensity at 470 nm were increased with time of ions reacting with AuNPs, however in all cases, Cr3+ ions exhibited a stronger DIRRS signal 120

This work was supported by National Natural Science Foundation China (21071064 and 21375048), National 973 Project (2010CB833603), Specialized Research Fund for the Doctoral Program of Higher Education (20104401120004), the Fundamental Research Funds for the Central Universities (21610427 and 21612402), and the PhD Start-up Fund of Natural Science Foundation of Guangdong Province (S2012040006713). References [1] [2] [3] [4] [5] [6] [7] [8]

[9] [10]

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In summary, we developed a highly sensitive and selective Cr3+ ion determination method at room temperature using Cr3+–citrate chelation reaction and RRS enhancement by gold nanoparticles. This method demonstrated several analytical advantages. Firstly, it has high sensitivity with a detection limit of 1.0 pM, which can be two to three orders of magnitude more sensitive than many other techniques. Secondly, it is selective, which allows detection of Cr3+ ions in the presence of an excess (1000-fold) of other metal ions in samples. Thirdly, it takes only approximately 20 min to determine the concentration of Cr3+ in aqueous media. Finally, the assay can be carried out in 96- or 384-well plates, rendering it suitable for routine high-throughout applications. The method has enormous potential for the application of Cr3+ ion monitoring in environment, water, and food samples.

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Fig. 6. The selectivity of Cr detection in the presence of different interference metal ions. The concentration of AuNPs was 3.0 nM, the concentration of Cr3+ ions was 10.0 nM, the other metal ions were tested at 10.0 lM.

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