Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 76–81
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Effect of aggregated silver nanoparticles on luminol chemiluminescence system and its analytical application Yingying Qi a,b, Baoxin Li b,⇑, Furong Xiu a a b
Department of Environment and Equipment Engineering, Fujian University of Technology, Fuzhou 350108, PR China Key Laboratory of Analytical Chemistry for Life Science of Shaanxi Province, School of Chemistry & Chemical Engineering, Shaanxi Normal University, Xi’an 710062, PR China
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
For AgNPs’s catalysis on luminol CL 5000
15 nm dispersed 7 nm aggregated 15 nm aggregated 7 nm dispersed 55 nm dispersed 55 nm aggregated blank
4000
CL intensity
system, aggregation was an important factor. The aggregated AgNPs’s effect characteristic was closely related to AgNPs’s sizes. Aggregation led to CL increase for 7 nm, decline for 15 nm and no change for 55 nm. Aggregated AgNPs’s effect is due to electron density’s change in conduction band.
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Article history: Received 29 September 2013 Received in revised form 15 February 2014 Accepted 21 February 2014 Available online 12 March 2014 Keywords: Aggregated silver nanoparticles Catalysis Chemiluminescence Luminol
a b s t r a c t We found that after silver nanoparticles (AgNPs) aggregated, its catalytic activity on luminol CL reaction obviously changed, and the change characteristic was closely related to the sizes of AgNPs. UV–visible spectra, X-ray photoelectron spectra, zeta potential and transmission electron microscopy studies were carried out to investigate the CL effect mechanism. The different CL responses of aggregated AgNPs with different size were suggested to be due to the two effects of quantum size and electron density in nanoparticle’s conduction bands, and which one played a major role. The poisonous organic contaminants such as anilines, could induce the aggregation of AgNPs, were observed to affect effectively the luminol–H2O2–7 nm and 15 nm AgNPs CL systems and were detectable by use of a flow injection method with the enhanced or inhibited CL detection. Ó 2014 Elsevier B.V. All rights reserved.
Introduction Metal nanomaterials have received significant attention for their special characteristics such as quantum size effect, surface effect, macro-quantum tunnel effect [1,2] and their potential applications in microelectronics, optics, electronics, magnetic devices, and catalysis [3–5]. Silver nanoparticles (AgNPs), mostly hydrosols, are perhaps most widely studied because of their important ⇑ Corresponding author. Tel.: +86 29 85308184; fax: +86 29 85307774. E-mail addresses: [email protected], [email protected] (B. Li). http://dx.doi.org/10.1016/j.saa.2014.02.139 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.
applications in catalysis [6–8], photographic processes, intrinsic antimicrobial properties, and surface-enhanced Raman spectroscopy (SERS) [9,10]. Ag+ is one rather strong oxidant. With the lower redox potential (E°Ag+/Ag = +0.799 V), silver has the better chemical activity than gold and platinum. Chemiluminescence (CL) is known as a powerful and important analytical technique, because of its extremely high sensitivity along with its other advantages, such as simple instrumentation, wide calibration ranges, and suitability for miniaturization in analytical chemistry [11,12]. The catalysis of nanoparticles for CL reactions becomes an expanding area in recent years
Y. Qi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 76–81
[6–8,13–19]. As silver has the better chemical activity than gold and platinum, many studies on AgNPs catalytic CL system have been reported [8,20–25]. In 2007, AgNPs were firstly applied into CL system and it was found that AgNPs could catalyze the luminol–H2O2 CL reaction [8]. Afterward, AgNPs were used as catalysts in other CL reactions [20–25]. In 2010, Haghighi’ group [20] investigated the effect of AgNPs on the luminol–isoniazid system. Cui et al. [21] found that AgNPs in the presence of nucleophiles and Cu2+ could induce luminol to produce CL. A flow injection CL determination of norfloxacin (NFLX) was developed based on the Ce(IV)–Na2SO3 redox system in the presence of AgNPs [22]. In 2011, Liu and Li [23] developed AgNPs–luminol–AgNO3 CL system and used the CL system in immunoassay for IgG. Chen et al. [24] reported that bisphenol A (BPA) could inhibit AgNPs-enhanced luminol–KMnO4 CL system and realized CL detection of BPA. Very recently, Liu et al. [25] developed a CL method for the determination of nitrofurans (NFs) based on the luminol–H2O2–AgNPs CL system. Indeed, AgNPs exhibited a better CL catalysis ability than gold and platinum nanoparticles [15,23]. In general, the catalysis of nanoparticles for CL system was found to be related to their size [8,13], surface state [26], and morphology [27–29]. Certainly, different state of aggregation of nanoparticles may also influence their catalytic activity for CL system. Actually, the aggregation of AgNPs is a frequent phenomenon and can occur in the presence of salt or organics [30–33]. It was reported that aniline and their derivatives could induce the aggregation of AgNPs [30]. Hence, the catalytic activity of AgNPs on CL system may be changed when they are transformed from dispersion to aggregation. The applicability of aggregated AgNPs used in the direct CL assay determination of the poisonous organic contaminants such as aniline and their derivatives depends on whether or not the aggregated AgNPs induced by the poisonous organic contaminants show different catalytic activity for the CL reaction. In this work, we chose luminol–H2O2 CL reaction as model system and the systematic study regarding the difference of the catalysis behavior for CL reaction between dispersed and aggregated AgNPs was conducted. The catalytic mechanism of aggregated AgNPs on luminol–H2O2 CL reaction was investigated. The aniline and their derivatives, which could induce the aggregation of AgNPs, were detectable by luminol–H2O2–7 nm and 15 nm AgNPs CL system combined with a flow injection method with the enhanced and inhibited CL detection, respectively.
Experimental Chemicals and solutions Luminol stock solution (2.5 10 2 M) was prepared by dissolving 4.43 g luminol (obtained from Shaanxi Normal University, Xi’an, China) in 20 mL of 0.10 M NaOH and then diluting to 1 L with water. The luminol solution was stored in dark for one week prior to use ensured that the reagent property was stabilized. Working solutions of luminol were prepared by diluting the stock solution. Working solutions of H2O2 were prepared fresh daily from 30% (w/w) H2O2 reagent solution (Shanghai Chemical Plant, Shanghai, China). AgNO3, trimethyl ammonium bromide (CTAB), hydrazine hydrate, ethylenediamine and NaBH4 were obtained from Sinopharm Chemical Reagent Co., Ltd., (Shanghai, China). Sodium citrate, aniline, o-phenylenediamine, p-phenylenediamine and m-phenylenediamine were purchased from Tianjin Chemical Reagent Company (Tianjin, China). Other reagents and chemicals were of analytical grade and used without further purification. Doubly distilled and deionized water was used throughout.
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Apparatus The CL intensity was measured and recorded with a model IFFL-D chemiluminescence Analyzer (Xi’an Ruimai Electronic Sci. Tech. Co., Ltd., Xi’an, China). Absorption spectra were recorded on a TU-1901 UV–visible spectroscopy (Beijing Purkinje General Instrument Co., Ltd., Beijing, China). The transmission electron microscopy (TEM) images of AgNPs were taken using a JEM-2100 TEM (Japan Electronics Co., Ltd.). The CL spectra of this system were recorded with a Hitachi FL-4600 spectrofluorometer (Tokyo, Japan) combined with a flow-injection system, with its excitation source turned off. The X-ray photoelectron spectra (XPS) analysis of AgNPs before and after the CL reaction was performed with an ESCALab 220I-XL spectrometer. The determination of zeta potential was carried out by Malvern Zetasizer 2000. Preparation and characterization of AgNPs All glassware used in these preparations was thoroughly cleaned in aqua regia (1:3 HNO3–HCl), rinsed in doubly distilled water, and oven-dried prior to use. 7 nm AgNPs were prepared according to the literature [34]. Firstly, after the pH value of 0.10 mol/L AgNO3 aqueous solutions was adjusted to 10 with ethylenediamine, 10 mL of 1 10 3 mol/ L cetyltrimethyl ammonium bromide (CTAB) was added to be mixed completely. Then 5 mL of the hydrazine hydrate (N2H4H2O) solution was added to the above mixture drop-wise under vigorous stirring. Thereafter, the above mixture solution was kept to react under room temperature for 5 min, resulting in the formation of AgNPs, which was stored in 4 °C refrigerator before use. 15 nm AgNPs were prepared by the chemical reduction of silver nitrate in NaBH4 according to the reported method [35]. Briefly, 25 mL 1 10 3 mol/L silver nitrate was added into 75 mL 2 10 3 mol/L fresh NaBH4 solution drop-wise under vigorous stirring. After reacted for 10 min, 5 mL 1% (w/w) sodium citrate as stabilizer was added and continued to stir for an additional 30 min, and was left overnight before use. 55 nm AgNPs were prepared by the chemical reduction method according to the literature [36]. First, 100 mL of a solution containing AgNO3 (0.4 mol/L), NaBH4 (0.2 mol/L) and citric acid (0.4 mol/L) was prepared under vigorous stirring. The mixed solution was stirred for an additional 30 min to obtain dark brown AgNPs colloid and was left overnight before use. The size and shape of the synthesized AgNPs were characterized by TEM. Statistical analysis of TEM data revealed that their average diameter (nm) was about 7 ± 2.5, 15 ± 2.1 and 55 ± 4.1, respectively, and their diameters were uniform and their dispersion was very good. In addition, UV–visible absorption spectra of the prepared AgNPs of various particle sizes were measured as shown in Fig. 1. The AgNPs of different particle sizes have different absorption spectra, which was consistent with the reports [34–36]. Procedures for CL detection A schematic diagram of the flow-injection CL system is shown in Fig. S1 luminol and H2O2 were first mixed. Double distilled water was used as a carrier to carry the AgNPs solution to mix with luminol and H2O2. The solutions of luminol, H2O2, and double distilled water were pumped into the flow cell by the peristaltic pump at the rate of 1.6 mL/min, respectively. The AgNPs or the mixture solution of AgNPs and salt was injected by a valve injector with a 120 lL sample loop. When the CL system was used for investigating the effects of target analytes on the CL system, the target analytes and AgNPs were first mixed and then injected by a valve injector. The CL signals were monitored by the PMT adjacent to the flow CL cell, and imported to the computer for data acquisition.
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Change of 7 nm and 15 nm AgNPs induced by salt 15nm
Change of luminol CL after the aggregation of AgNPs
Optimization of the experimental conditions
The dispersed AgNPs could catalyze the luminol–H2O2 CL reaction [8]. The effects of dispersed and aggregated AgNPs on the luminol–H2O2 CL system were investigated. As shown in Fig. 2, for the dispersed AgNPs, 15 nm AgNPs showed the strongest catalysis and the catalytic effect of 7 nm AgNPs was the second, and 55 nm AgNPs showed negligible catalysis. It was reported that nanoparticles’s catalysis for luminol CL system might be influenced by the electron density in the conduction bands of nanoparticles, which was indicated by SPR absorption intensity from UV–visible absorption spectra [13,37]. So, 15 nm AgNPs showed the strongest catalysis probably because of high electron density in the conduction bands, which was manifested by UV–visible absorption spectra (Fig. 1). Moreover, the effect of AgNPs’ aggregation on the catalysis for luminol–H2O2 CL system also can be found from Fig. 2. In comparison with dispersed AgNPs, the catalytic activity increased significantly after its aggregation for the 7 nm AgNPs, while decreased for the 15 nm AgNPs, and no obvious change could be found for the 55 nm AgNPs. The results indicated that AgNPs’s aggregation was an important influencing factor for luminol–H2O2 CL system.
The experimental conditions were optimized for the luminol– H2O2–7 nm AgNPs and 15 nm AgNPs CL system. The concentration of AgNPs, a vital parameter for our research, was explored. The CL intensity increased steadily with increasing the concentration of AgNPs. Considering the CL intensity and the consumption of the reagents, we used 1.6 10 11 mol/L of 7 nm AgNPs and 6.2 10 10 mol/L of 15 nm AgNPs for all experiments. In order to obtain higher sensitivity, 0.02 mol/L NaCl was used to induce the aggregation of AgNPs when AgNPs’s concentration was fixed in the actual experiment. The important experimental parameters influencing the CL system of luminol–H2O2–7 nm AgNPs and 15 nm AgNPs, including the concentrations of luminol and H2O2, and media pH, were optimized. It was found that both signal and background increased as the luminol concentration was raised, and the signal/noise ratio was the highest at 5.0 10 5 mol/L luminol. The effect of the H2O2 concentration on the CL was studied in the range 0.01–20 mM, and the maximum CL emission was observed at 5.0 10 3 mol/L H2O2 solutions. The experimental results showed that the optimal pH for this CL system was 12.0, which is in agreement with the results of the previous studies [26,28]. The effect of flow rate on the CL intensity was studied in the range of 0.5–2.2 mL/min (each channel). It was observed that the CL intensity increased with the increase of flow rate from 0.5 mL/ min to 1.6 mL/min. When the flow rate was above 1.6 mL/min, the CL intensity decreased probably because that the maximum CL emission in the CL flow cell was absent. So, the flow rate of 1.6 mL/min was employed. The volume of injection solution (AgNPs, AgNPs/salt and AgNPs/ target analytes) was investigated. When the injection volume was smaller than 120 lL, the signal change before and after AgNPs’s aggregation was not very distinct. The high sensitivity was obtained when the injection volume was up to 120 lL. Therefore, the injection volume of AgNPs solution was all the same (120 lL) in the experiment.
Abs
Results and discussion
UV–visible absorption spectra and TEM analysis were carried out to investigate the change of AgNPs induced by salt. As shown in Fig. S2, before the addition of salt, the prepared AgNPs were stable due to the electrostatic repulsion against van der Waals attraction between AgNPs [32]. The AgNPs were highly dispersed and had a specific absorption spectrum (Fig. 3). The addition of salt screened the repulsion between the AgNPs to lead to the aggregation of AgNPs, and the nanoparticles with big size appeared simultaneously. Remarkably, after the addition of salt, UV–visible absorption peak position of 7 nm AgNPs shifted slightly and absorption intensity increased obviously (Fig. 3A). Analogously, absorption peak position of 15 nm AgNPs also changed very little after the addition of salt. However, absorption intensity of 15 nm AgNPs was reduced significantly (Fig. 3B), which was in contrast to the 7 nm AgNPs. The change of UV–visible absorption intensity after AgNPs’s aggregation was probably ascribed to different electric charge accumulation and oscillation effect occurred in nanoparticles’ specific sites of the dispersed and aggregated AgNPs [38,39].
55nm
7nm
Wavelength/nm Fig. 1. UV–visible absorption spectra of AgNPs with various particle sizes.
5000 15 nm dispersed 7 nm aggregated 15 nm aggregated 7 nm dispersed 55 nm dispersed 55 nm aggregated blank
CL intensity
4000 3000 2000 1000 0
Mechanism discussion 0
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Time/s Fig. 2. CL profiles of luminol–H2O2 mixed with AgNPs. Conditions: luminol: 5 10 5 M; H2O2: 5 10 3 M. Blank: H2O.
In order to explore the luminophor of luminol–H2O2–7 nm AgNPs and 15 nm AgNPs CL reaction, the CL spectra for 7 nm, 15 nm, and 55 nm AgNPs mixed with luminol–H2O2 were acquired as shown in Fig. 4, and it was clearly indicated that the maximum
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A
B
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before
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Wavelength/nm
Fig. 3. UV–visible absorption spectra of AgNPs before and after the addition of salt: (A) 7 nm AgNPs, and (B) 15 nm AgNPs.
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15 nm dispersed 7 nm aggregated 15 nm aggregated 7 nm dispersed 55 dispersed 55 aggregated blank
CL intensity
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15 nm Ag after reaction 7 nm Ag before reaction 7 nm Ag after reaction
Relative Intensity/a.u.
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15 nm Ag before reaction
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Binding Energy/eV Fig. 4. CL spectra for the luminol–H2O2–AgNPs system. Conditions: luminol: 5 10 5 M; H2O2: 5 10 3 M. Blank: H2O.
emission for all the cases was about 425 nm, revealing that the luminophor for the CL system was still the excited-state 3-aminophthalate anions [40]. Therefore, the aggregation of AgNPs did not lead to the generation of a new luminophor for this CL system. The changes of CL signals were thus ascribed to the possible catalysis from aggregated AgNPs itself. The active species for catalysts may also be changed for the aggregated AgNPs before and after the CL reaction. For 7 nm and 15 nm AgNPs, XPS studies were carried out to investigate the oxidation state of Ag before and after the CL reaction. The results are shown in Fig. 5. It could be found that the binding energy of silver for all the cases had no change before and after the CL reaction, indicating that the oxidation state of Ag was not involved in catalysis process of the CL reaction. It was further inferred that the active species of silver catalysts were only metallic AgNPs before and after the CL reaction. Therefore, the change of catalytic effect may have originated from the aggregated AgNPs itself. The previous researches [26,28] have indicated that the surface charge properties of metal nanoparticle are an important effect factor for the catalytic activity on luminol CL reaction. Zeta potential is considered to be an indicator of the charge density on the surface of nanoparticles. To investigate the change of different size AgNPs’ surface charge property before and after salt-induced aggregation, the zeta potentials of 7 nm and 15 nm AgNPs were detected. It was found that the zeta potentials of dispersed 7 nm and 15 nm AgNPs before the addition of salt and salt-induced aggregated 7 nm and 15 nm AgNPs were 26 mv, 42 mv, 15 mv, and 29 mv, respectively (Table 1). As previously reported [29,32], the addition of salt would screen the surface negative charge on the surface of
Fig. 5. XPS of AgNPs before and after the luminol–H2O2 CL reaction.
Table 1 Zeta potential of 7 nm and 15 nm AgNPs before and after salt induced aggregation. Particle size (nm)
Zeta potential before (mV)
Zeta potential after (mV)
7 15
26 42
15 29
nanoparticles and easily induce the aggregation of nanoparticles, and the aggregated nanoparticles have low negative charge density, so it can more easily interact with the anionic HO2 (or anionic luminol), resulting in a higher catalytic effect on the luminol–H2O2 reaction. However, it was interested that an opposite result was observed in the AgNPs–luminol CL system. It could be found from Table 1 that after salt induced aggregation, the zeta potential of 7 nm AgNPs decreased from 26 mv to 15 mv, while its catalytic activity for luminol–H2O2 CL system was inversely enhanced (Fig. 2). The interested result was also found in the 15 nm AgNPs system. The zeta potential of 15 nm AgNPs increased from 42 mv to 29 mv while its catalytic activity was inversely reduced (Fig. 2). The results indicated that some other factors played a role in influencing the CL intensity of luminol–H2O2–AgNPs system except the effect factor of surface charge properties of nanoparticle, and such factors might hold dominant position among all effect factors, which was further confirmed by the fact that catalytic effect of 7 nm dispersed AgNPs for luminol CL system was even lower than that of 15 nm dispersed AgNPs (Fig. 2), although 7 nm AgNPs was positively charged (zeta potential = 26 mv), whose
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negative charge density was much lower than that of 15 nm AgNPs (zeta potential = 42 mv). In addition to the surface charge properties, the specific surface areas and electron density in the conduction bands of metal nanoparticle could also influence the catalytic activity for luminol CL system [13]. The catalytic activity of AgNPs was probably a comprehensive result from more than one effect factors. In general, AgNPs with smaller particle size have the larger ratio of CL intensity to superficial area and the catalytic activity is higher [37]. For the effect of electron density in the conduction bands, the higher electron density in the conduction bands of AgNPs can result in the stronger particle-mediated electron transfer ability [37] and the better catalytic activity for luminol CL system [13]. SPR absorption intensity from UV–visible absorption spectra is considered to be able to indicate the electron density in the conduction bands of nanoparticles [38,39]. As shown in Fig. 1, SPR absorption intensity from UV–visible absorption spectra of 15 nm AgNPs is the strongest among the studied AgNPs with three sizes, and its particle size is between the other two. So, dispersed 15 nm AgNPs showed the strongest catalytic effect (Fig. 2) because of proper surface area and high electron density in the conduction bands (Fig. 1), which was the similar to the strongest catalysis of 38 nm gold nanoparticles (AuNPs) for luminol CL reaction in Cui’s study [13]. After salt-induced aggregation of 15 nm AgNPs, the specific surface area decreased (Fig. S2B) and the SPR absorption intensity decreased significantly (Fig. 3B). Both the two effects of specific surface area and electron density in nanoparticle’s conduction bands resulted in the decrease of catalytic activity of 15 nm AgNPs after salt-induced aggregation. On the other hand, after salt-induced aggregation of 7 nm AgNPs, the increase of particle size could lead to the decrease of specific surface area, nevertheless, the electron density in the conduction bands increased significantly after aggregation for the 7 nm AgNPs, which could be found from the distinct enhancement of SPR absorption intensity in Fig. 3A. Therefore, we inferred that the higher catalytic activity of 7 nm aggregated AgNPs on the luminol–H2O2 CL reaction compared to the dispersed one (Fig. 2) could be attributed to the effect of electron density in nanoparticle’s conduction bands, which had surpassed the effects of specific surface area and surface charge properties of AgNPs. In another word, the effect of electron density in the conduction bands plays the most important role among all of the influence factors. Furthermore, for 55 nm AgNPs, extremely weak CL intensity was observed either before or after aggregation (Fig. 2), probably because that quantum size effects began to function with an increase in band gap energy, leading to a higher activation energy that was needed for electron transfer.
Comparison with the catalysis of aggregated AuNPs for luminol CL system Herein, we reasoned that in luminol–H2O2 CL system involved aggregated precious metal nanoparticles (AuNPs or AgNPs), the change of catalytic activity of nanoparticles after aggregation on luminol–H2O2 CL system might be caused by three factors: (1) the variation of negative charge density on the surface of nanoparticles, (2) the variation of nanoparticles diameter, and (3) the variation of electron density in nanoparticle’s conduction bands. The variation characteristic of nanoparticle’s catalytic activity depends on which factor above mentioned holds the dominant position during the catalytic process. The previous research [29] has shown that organic compounds containing OH, NH2, or SH groups were capable to enhance the CL intensity of luminol–H2O2–AuNPs, and the primary reason was that they could induce the aggregation of AuNPs and decrease the surface negative charge density of AuNPs. Based on this find, the determination of this kind of organic compounds was realized. However, the regularity of surface negative charge density for AuNPs did not work in the luminol– H2O2–AgNPs CL system according to our experimental results in this study. It seems that in AgNPs (7 nm and 15 nm) CL system, the variation of electron density in conduction bands after AgNPs’s aggregation plays a leading role in influencing the catalytic activity of AgNPs. Application Anilines, as an important organic chemical raw materials and fine chemical intermediates, have been widely used in pharmaceutical and dyeing and textile industries. Owing to its high toxicity and carcinogenicity, it is vital to detect these residues in environmental samples. It has been reported that the poisonous organic compounds like phenylamine, p-phenylenediamine, o-phenylenediamine and m-phenylenediamine can induce the aggregation of AgNPs [30]. According to the mechanism discussed above, it can be expected that the interaction between these organic compounds and AgNPs (7 nm and 15 nm) can have an effect on the CL signal of luminol–H2O2–AgNPs systems. Herein, the effects of such organics on the luminol–H2O2–7 nm and 15 nm AgNPs CL systems were investigated. As expected, all the tested compounds were capable to enhance luminol–H2O2–7 nm AgNPs CL signal, and quench luminol–H2O2–15 nm AgNPs CL signal in that they could induce the aggregation of AgNPs and further alter (increase or decrease) the electron density in the conduction bands of AgNPs (7 nm or 15 nm).
Table 2 Analytical performance of the proposed luminol–H2O2–7 nm AgNPs CL system. Sample
Linear range (g/mL)
Phenylamine p-Phenylenediamine o-Phenylenediamine m-Phenylenediamine
2 10 2 10 3 10 3 10
9
–2 10 9 –6 10 9 –1 10 9 –8 10
6 6 6 7
Equation of linear regression
Related coefficient
Detection limit (3r, n = 5) (g/mL)
DI = 2.115lnC + 21.49 DI = 7.537lnC + 35.82 DI = 6.745lnC + 3.207 DI = 1.575lnC + 35.57
0.9942 0.9914 0.9926 0.9863
6.2 10 2.6 10 1.2 10 1.6 10
10 10 9 9
Table 3 Analytical performance of the proposed luminol–H2O2–15 nm AgNPs CL system. Sample
Linear range (g/mL)
Phenylamine p-Phenylenediamine o-Phenylenediamine m-Phenylenediamine
2 10 2 10 3 10 3 10
9
–6 10–6 9 –2 10–6 9 –1 10–6 9 –8 10 6
Equation of linear regression
Related coefficient
Detection limit (3r, n = 5) (g/mL)
DI = DI = DI = DI =
0.9999 0.9892 0.9916 0.9926
3.6 10 1.1 10 8.6 10 6.3 10
1.274lnC + 57.46 10.23lnC 4.003 10.54lnC + 0.051 31.57lnC + 310.5
10 9 10 10
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The analytical potential for these compounds which could induce the aggregation of AgNPs on the proposed luminol–H2O2– 7 nm AgNPs CL system and luminol–H2O2–15 nm AgNPs CL system was explored by using a flow injection procedure. Under the optimized experimental conditions, the CL intensity linearly responded to the concentration of target compounds within a wide range, and the results are presented in Tables 2 and 3. It can be seen that the linear range for all these compounds could reach three or more orders of magnitude. Combined with separation techniques such as HPLC or high-performance capillary electrophoresis, this CL system has a wide application for the simultaneous determination of such poisonous organic compounds. Conclusions In this work, aggregation of AgNPs was found to be an important effect factor for the luminol–H2O2 CL system. Aggregated AgNPs could effectively affect the CL signal compared with the dispersed one, and the effect characteristic was closely related to the sizes of AgNPs. In comparison with dispersed AgNPs, the catalytic activity increased significantly after its aggregation for the 7 nm AgNPs, while decreased for the 15 nm AgNPs. No obvious change could be found for the 55 nm AgNPs. The CL responses of aggregated AgNPs were suggested to be due to the two effects of quantum size and electron density in nanoparticle’s conduction bands. It was found that the enhanced effect of aggregated AgNPs of 7 nm was mainly due to the increase of electron density in AgNPs’s conduction bands, while both the two effects of quantum size and electron density in AgNPs’s conduction bands resulted in the decrease of catalytic activity of 15 nm AgNPs after aggregation. Toxic organic compounds such as aniline, phenylenediamine and their derivatives, could induce AgNPs’s aggregation and further enhance (for 7 nm AgNPs) or quench (for 15 nm AgNPs) luminol– H2O2–AgNPs CL signal, were detectable within a wide range by using a flow injection method with the CL detection. This work is important to broaden the investigation and application of catalysis of AgNPs for CL reactions and has some practical applied value for environment monitoring. Acknowledgements This work was supported financially by the Natural Science Foundation of Fujian Province of China (2011J05016), the Foundation of Fujian Educational Committee (JA11177 and JA12237), the Special Science Funding of Provincial Universities of Fujian (JK2012032), and the Scientific Research Foundation of Fujian University of Technology (GY-Z10055).
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Effect of aggregated silver nanoparticles on luminol chemiluminescence system and its analytical application Yingying Qi a,b, Baoxin Li b,⇑, Furong Xiu a a b
Department of Environment and Equipment Engineering, Fujian University of Technology, Fuzhou 350108, PR China Key Laboratory of Analytical Chemistry for Life Science of Shaanxi Province, School of Chemistry & Chemical Engineering, Shaanxi Normal University, Xi’an 710062, PR China
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
For AgNPs’s catalysis on luminol CL 5000
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system, aggregation was an important factor. The aggregated AgNPs’s effect characteristic was closely related to AgNPs’s sizes. Aggregation led to CL increase for 7 nm, decline for 15 nm and no change for 55 nm. Aggregated AgNPs’s effect is due to electron density’s change in conduction band.
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Article history: Received 29 September 2013 Received in revised form 15 February 2014 Accepted 21 February 2014 Available online 12 March 2014 Keywords: Aggregated silver nanoparticles Catalysis Chemiluminescence Luminol
a b s t r a c t We found that after silver nanoparticles (AgNPs) aggregated, its catalytic activity on luminol CL reaction obviously changed, and the change characteristic was closely related to the sizes of AgNPs. UV–visible spectra, X-ray photoelectron spectra, zeta potential and transmission electron microscopy studies were carried out to investigate the CL effect mechanism. The different CL responses of aggregated AgNPs with different size were suggested to be due to the two effects of quantum size and electron density in nanoparticle’s conduction bands, and which one played a major role. The poisonous organic contaminants such as anilines, could induce the aggregation of AgNPs, were observed to affect effectively the luminol–H2O2–7 nm and 15 nm AgNPs CL systems and were detectable by use of a flow injection method with the enhanced or inhibited CL detection. Ó 2014 Elsevier B.V. All rights reserved.
Introduction Metal nanomaterials have received significant attention for their special characteristics such as quantum size effect, surface effect, macro-quantum tunnel effect [1,2] and their potential applications in microelectronics, optics, electronics, magnetic devices, and catalysis [3–5]. Silver nanoparticles (AgNPs), mostly hydrosols, are perhaps most widely studied because of their important ⇑ Corresponding author. Tel.: +86 29 85308184; fax: +86 29 85307774. E-mail addresses: [email protected], [email protected] (B. Li). http://dx.doi.org/10.1016/j.saa.2014.02.139 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.
applications in catalysis [6–8], photographic processes, intrinsic antimicrobial properties, and surface-enhanced Raman spectroscopy (SERS) [9,10]. Ag+ is one rather strong oxidant. With the lower redox potential (E°Ag+/Ag = +0.799 V), silver has the better chemical activity than gold and platinum. Chemiluminescence (CL) is known as a powerful and important analytical technique, because of its extremely high sensitivity along with its other advantages, such as simple instrumentation, wide calibration ranges, and suitability for miniaturization in analytical chemistry [11,12]. The catalysis of nanoparticles for CL reactions becomes an expanding area in recent years
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[6–8,13–19]. As silver has the better chemical activity than gold and platinum, many studies on AgNPs catalytic CL system have been reported [8,20–25]. In 2007, AgNPs were firstly applied into CL system and it was found that AgNPs could catalyze the luminol–H2O2 CL reaction [8]. Afterward, AgNPs were used as catalysts in other CL reactions [20–25]. In 2010, Haghighi’ group [20] investigated the effect of AgNPs on the luminol–isoniazid system. Cui et al. [21] found that AgNPs in the presence of nucleophiles and Cu2+ could induce luminol to produce CL. A flow injection CL determination of norfloxacin (NFLX) was developed based on the Ce(IV)–Na2SO3 redox system in the presence of AgNPs [22]. In 2011, Liu and Li [23] developed AgNPs–luminol–AgNO3 CL system and used the CL system in immunoassay for IgG. Chen et al. [24] reported that bisphenol A (BPA) could inhibit AgNPs-enhanced luminol–KMnO4 CL system and realized CL detection of BPA. Very recently, Liu et al. [25] developed a CL method for the determination of nitrofurans (NFs) based on the luminol–H2O2–AgNPs CL system. Indeed, AgNPs exhibited a better CL catalysis ability than gold and platinum nanoparticles [15,23]. In general, the catalysis of nanoparticles for CL system was found to be related to their size [8,13], surface state [26], and morphology [27–29]. Certainly, different state of aggregation of nanoparticles may also influence their catalytic activity for CL system. Actually, the aggregation of AgNPs is a frequent phenomenon and can occur in the presence of salt or organics [30–33]. It was reported that aniline and their derivatives could induce the aggregation of AgNPs [30]. Hence, the catalytic activity of AgNPs on CL system may be changed when they are transformed from dispersion to aggregation. The applicability of aggregated AgNPs used in the direct CL assay determination of the poisonous organic contaminants such as aniline and their derivatives depends on whether or not the aggregated AgNPs induced by the poisonous organic contaminants show different catalytic activity for the CL reaction. In this work, we chose luminol–H2O2 CL reaction as model system and the systematic study regarding the difference of the catalysis behavior for CL reaction between dispersed and aggregated AgNPs was conducted. The catalytic mechanism of aggregated AgNPs on luminol–H2O2 CL reaction was investigated. The aniline and their derivatives, which could induce the aggregation of AgNPs, were detectable by luminol–H2O2–7 nm and 15 nm AgNPs CL system combined with a flow injection method with the enhanced and inhibited CL detection, respectively.
Experimental Chemicals and solutions Luminol stock solution (2.5 10 2 M) was prepared by dissolving 4.43 g luminol (obtained from Shaanxi Normal University, Xi’an, China) in 20 mL of 0.10 M NaOH and then diluting to 1 L with water. The luminol solution was stored in dark for one week prior to use ensured that the reagent property was stabilized. Working solutions of luminol were prepared by diluting the stock solution. Working solutions of H2O2 were prepared fresh daily from 30% (w/w) H2O2 reagent solution (Shanghai Chemical Plant, Shanghai, China). AgNO3, trimethyl ammonium bromide (CTAB), hydrazine hydrate, ethylenediamine and NaBH4 were obtained from Sinopharm Chemical Reagent Co., Ltd., (Shanghai, China). Sodium citrate, aniline, o-phenylenediamine, p-phenylenediamine and m-phenylenediamine were purchased from Tianjin Chemical Reagent Company (Tianjin, China). Other reagents and chemicals were of analytical grade and used without further purification. Doubly distilled and deionized water was used throughout.
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Apparatus The CL intensity was measured and recorded with a model IFFL-D chemiluminescence Analyzer (Xi’an Ruimai Electronic Sci. Tech. Co., Ltd., Xi’an, China). Absorption spectra were recorded on a TU-1901 UV–visible spectroscopy (Beijing Purkinje General Instrument Co., Ltd., Beijing, China). The transmission electron microscopy (TEM) images of AgNPs were taken using a JEM-2100 TEM (Japan Electronics Co., Ltd.). The CL spectra of this system were recorded with a Hitachi FL-4600 spectrofluorometer (Tokyo, Japan) combined with a flow-injection system, with its excitation source turned off. The X-ray photoelectron spectra (XPS) analysis of AgNPs before and after the CL reaction was performed with an ESCALab 220I-XL spectrometer. The determination of zeta potential was carried out by Malvern Zetasizer 2000. Preparation and characterization of AgNPs All glassware used in these preparations was thoroughly cleaned in aqua regia (1:3 HNO3–HCl), rinsed in doubly distilled water, and oven-dried prior to use. 7 nm AgNPs were prepared according to the literature [34]. Firstly, after the pH value of 0.10 mol/L AgNO3 aqueous solutions was adjusted to 10 with ethylenediamine, 10 mL of 1 10 3 mol/ L cetyltrimethyl ammonium bromide (CTAB) was added to be mixed completely. Then 5 mL of the hydrazine hydrate (N2H4H2O) solution was added to the above mixture drop-wise under vigorous stirring. Thereafter, the above mixture solution was kept to react under room temperature for 5 min, resulting in the formation of AgNPs, which was stored in 4 °C refrigerator before use. 15 nm AgNPs were prepared by the chemical reduction of silver nitrate in NaBH4 according to the reported method [35]. Briefly, 25 mL 1 10 3 mol/L silver nitrate was added into 75 mL 2 10 3 mol/L fresh NaBH4 solution drop-wise under vigorous stirring. After reacted for 10 min, 5 mL 1% (w/w) sodium citrate as stabilizer was added and continued to stir for an additional 30 min, and was left overnight before use. 55 nm AgNPs were prepared by the chemical reduction method according to the literature [36]. First, 100 mL of a solution containing AgNO3 (0.4 mol/L), NaBH4 (0.2 mol/L) and citric acid (0.4 mol/L) was prepared under vigorous stirring. The mixed solution was stirred for an additional 30 min to obtain dark brown AgNPs colloid and was left overnight before use. The size and shape of the synthesized AgNPs were characterized by TEM. Statistical analysis of TEM data revealed that their average diameter (nm) was about 7 ± 2.5, 15 ± 2.1 and 55 ± 4.1, respectively, and their diameters were uniform and their dispersion was very good. In addition, UV–visible absorption spectra of the prepared AgNPs of various particle sizes were measured as shown in Fig. 1. The AgNPs of different particle sizes have different absorption spectra, which was consistent with the reports [34–36]. Procedures for CL detection A schematic diagram of the flow-injection CL system is shown in Fig. S1 luminol and H2O2 were first mixed. Double distilled water was used as a carrier to carry the AgNPs solution to mix with luminol and H2O2. The solutions of luminol, H2O2, and double distilled water were pumped into the flow cell by the peristaltic pump at the rate of 1.6 mL/min, respectively. The AgNPs or the mixture solution of AgNPs and salt was injected by a valve injector with a 120 lL sample loop. When the CL system was used for investigating the effects of target analytes on the CL system, the target analytes and AgNPs were first mixed and then injected by a valve injector. The CL signals were monitored by the PMT adjacent to the flow CL cell, and imported to the computer for data acquisition.
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Change of 7 nm and 15 nm AgNPs induced by salt 15nm
Change of luminol CL after the aggregation of AgNPs
Optimization of the experimental conditions
The dispersed AgNPs could catalyze the luminol–H2O2 CL reaction [8]. The effects of dispersed and aggregated AgNPs on the luminol–H2O2 CL system were investigated. As shown in Fig. 2, for the dispersed AgNPs, 15 nm AgNPs showed the strongest catalysis and the catalytic effect of 7 nm AgNPs was the second, and 55 nm AgNPs showed negligible catalysis. It was reported that nanoparticles’s catalysis for luminol CL system might be influenced by the electron density in the conduction bands of nanoparticles, which was indicated by SPR absorption intensity from UV–visible absorption spectra [13,37]. So, 15 nm AgNPs showed the strongest catalysis probably because of high electron density in the conduction bands, which was manifested by UV–visible absorption spectra (Fig. 1). Moreover, the effect of AgNPs’ aggregation on the catalysis for luminol–H2O2 CL system also can be found from Fig. 2. In comparison with dispersed AgNPs, the catalytic activity increased significantly after its aggregation for the 7 nm AgNPs, while decreased for the 15 nm AgNPs, and no obvious change could be found for the 55 nm AgNPs. The results indicated that AgNPs’s aggregation was an important influencing factor for luminol–H2O2 CL system.
The experimental conditions were optimized for the luminol– H2O2–7 nm AgNPs and 15 nm AgNPs CL system. The concentration of AgNPs, a vital parameter for our research, was explored. The CL intensity increased steadily with increasing the concentration of AgNPs. Considering the CL intensity and the consumption of the reagents, we used 1.6 10 11 mol/L of 7 nm AgNPs and 6.2 10 10 mol/L of 15 nm AgNPs for all experiments. In order to obtain higher sensitivity, 0.02 mol/L NaCl was used to induce the aggregation of AgNPs when AgNPs’s concentration was fixed in the actual experiment. The important experimental parameters influencing the CL system of luminol–H2O2–7 nm AgNPs and 15 nm AgNPs, including the concentrations of luminol and H2O2, and media pH, were optimized. It was found that both signal and background increased as the luminol concentration was raised, and the signal/noise ratio was the highest at 5.0 10 5 mol/L luminol. The effect of the H2O2 concentration on the CL was studied in the range 0.01–20 mM, and the maximum CL emission was observed at 5.0 10 3 mol/L H2O2 solutions. The experimental results showed that the optimal pH for this CL system was 12.0, which is in agreement with the results of the previous studies [26,28]. The effect of flow rate on the CL intensity was studied in the range of 0.5–2.2 mL/min (each channel). It was observed that the CL intensity increased with the increase of flow rate from 0.5 mL/ min to 1.6 mL/min. When the flow rate was above 1.6 mL/min, the CL intensity decreased probably because that the maximum CL emission in the CL flow cell was absent. So, the flow rate of 1.6 mL/min was employed. The volume of injection solution (AgNPs, AgNPs/salt and AgNPs/ target analytes) was investigated. When the injection volume was smaller than 120 lL, the signal change before and after AgNPs’s aggregation was not very distinct. The high sensitivity was obtained when the injection volume was up to 120 lL. Therefore, the injection volume of AgNPs solution was all the same (120 lL) in the experiment.
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Results and discussion
UV–visible absorption spectra and TEM analysis were carried out to investigate the change of AgNPs induced by salt. As shown in Fig. S2, before the addition of salt, the prepared AgNPs were stable due to the electrostatic repulsion against van der Waals attraction between AgNPs [32]. The AgNPs were highly dispersed and had a specific absorption spectrum (Fig. 3). The addition of salt screened the repulsion between the AgNPs to lead to the aggregation of AgNPs, and the nanoparticles with big size appeared simultaneously. Remarkably, after the addition of salt, UV–visible absorption peak position of 7 nm AgNPs shifted slightly and absorption intensity increased obviously (Fig. 3A). Analogously, absorption peak position of 15 nm AgNPs also changed very little after the addition of salt. However, absorption intensity of 15 nm AgNPs was reduced significantly (Fig. 3B), which was in contrast to the 7 nm AgNPs. The change of UV–visible absorption intensity after AgNPs’s aggregation was probably ascribed to different electric charge accumulation and oscillation effect occurred in nanoparticles’ specific sites of the dispersed and aggregated AgNPs [38,39].
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In order to explore the luminophor of luminol–H2O2–7 nm AgNPs and 15 nm AgNPs CL reaction, the CL spectra for 7 nm, 15 nm, and 55 nm AgNPs mixed with luminol–H2O2 were acquired as shown in Fig. 4, and it was clearly indicated that the maximum
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emission for all the cases was about 425 nm, revealing that the luminophor for the CL system was still the excited-state 3-aminophthalate anions [40]. Therefore, the aggregation of AgNPs did not lead to the generation of a new luminophor for this CL system. The changes of CL signals were thus ascribed to the possible catalysis from aggregated AgNPs itself. The active species for catalysts may also be changed for the aggregated AgNPs before and after the CL reaction. For 7 nm and 15 nm AgNPs, XPS studies were carried out to investigate the oxidation state of Ag before and after the CL reaction. The results are shown in Fig. 5. It could be found that the binding energy of silver for all the cases had no change before and after the CL reaction, indicating that the oxidation state of Ag was not involved in catalysis process of the CL reaction. It was further inferred that the active species of silver catalysts were only metallic AgNPs before and after the CL reaction. Therefore, the change of catalytic effect may have originated from the aggregated AgNPs itself. The previous researches [26,28] have indicated that the surface charge properties of metal nanoparticle are an important effect factor for the catalytic activity on luminol CL reaction. Zeta potential is considered to be an indicator of the charge density on the surface of nanoparticles. To investigate the change of different size AgNPs’ surface charge property before and after salt-induced aggregation, the zeta potentials of 7 nm and 15 nm AgNPs were detected. It was found that the zeta potentials of dispersed 7 nm and 15 nm AgNPs before the addition of salt and salt-induced aggregated 7 nm and 15 nm AgNPs were 26 mv, 42 mv, 15 mv, and 29 mv, respectively (Table 1). As previously reported [29,32], the addition of salt would screen the surface negative charge on the surface of
Fig. 5. XPS of AgNPs before and after the luminol–H2O2 CL reaction.
Table 1 Zeta potential of 7 nm and 15 nm AgNPs before and after salt induced aggregation. Particle size (nm)
Zeta potential before (mV)
Zeta potential after (mV)
7 15
26 42
15 29
nanoparticles and easily induce the aggregation of nanoparticles, and the aggregated nanoparticles have low negative charge density, so it can more easily interact with the anionic HO2 (or anionic luminol), resulting in a higher catalytic effect on the luminol–H2O2 reaction. However, it was interested that an opposite result was observed in the AgNPs–luminol CL system. It could be found from Table 1 that after salt induced aggregation, the zeta potential of 7 nm AgNPs decreased from 26 mv to 15 mv, while its catalytic activity for luminol–H2O2 CL system was inversely enhanced (Fig. 2). The interested result was also found in the 15 nm AgNPs system. The zeta potential of 15 nm AgNPs increased from 42 mv to 29 mv while its catalytic activity was inversely reduced (Fig. 2). The results indicated that some other factors played a role in influencing the CL intensity of luminol–H2O2–AgNPs system except the effect factor of surface charge properties of nanoparticle, and such factors might hold dominant position among all effect factors, which was further confirmed by the fact that catalytic effect of 7 nm dispersed AgNPs for luminol CL system was even lower than that of 15 nm dispersed AgNPs (Fig. 2), although 7 nm AgNPs was positively charged (zeta potential = 26 mv), whose
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negative charge density was much lower than that of 15 nm AgNPs (zeta potential = 42 mv). In addition to the surface charge properties, the specific surface areas and electron density in the conduction bands of metal nanoparticle could also influence the catalytic activity for luminol CL system [13]. The catalytic activity of AgNPs was probably a comprehensive result from more than one effect factors. In general, AgNPs with smaller particle size have the larger ratio of CL intensity to superficial area and the catalytic activity is higher [37]. For the effect of electron density in the conduction bands, the higher electron density in the conduction bands of AgNPs can result in the stronger particle-mediated electron transfer ability [37] and the better catalytic activity for luminol CL system [13]. SPR absorption intensity from UV–visible absorption spectra is considered to be able to indicate the electron density in the conduction bands of nanoparticles [38,39]. As shown in Fig. 1, SPR absorption intensity from UV–visible absorption spectra of 15 nm AgNPs is the strongest among the studied AgNPs with three sizes, and its particle size is between the other two. So, dispersed 15 nm AgNPs showed the strongest catalytic effect (Fig. 2) because of proper surface area and high electron density in the conduction bands (Fig. 1), which was the similar to the strongest catalysis of 38 nm gold nanoparticles (AuNPs) for luminol CL reaction in Cui’s study [13]. After salt-induced aggregation of 15 nm AgNPs, the specific surface area decreased (Fig. S2B) and the SPR absorption intensity decreased significantly (Fig. 3B). Both the two effects of specific surface area and electron density in nanoparticle’s conduction bands resulted in the decrease of catalytic activity of 15 nm AgNPs after salt-induced aggregation. On the other hand, after salt-induced aggregation of 7 nm AgNPs, the increase of particle size could lead to the decrease of specific surface area, nevertheless, the electron density in the conduction bands increased significantly after aggregation for the 7 nm AgNPs, which could be found from the distinct enhancement of SPR absorption intensity in Fig. 3A. Therefore, we inferred that the higher catalytic activity of 7 nm aggregated AgNPs on the luminol–H2O2 CL reaction compared to the dispersed one (Fig. 2) could be attributed to the effect of electron density in nanoparticle’s conduction bands, which had surpassed the effects of specific surface area and surface charge properties of AgNPs. In another word, the effect of electron density in the conduction bands plays the most important role among all of the influence factors. Furthermore, for 55 nm AgNPs, extremely weak CL intensity was observed either before or after aggregation (Fig. 2), probably because that quantum size effects began to function with an increase in band gap energy, leading to a higher activation energy that was needed for electron transfer.
Comparison with the catalysis of aggregated AuNPs for luminol CL system Herein, we reasoned that in luminol–H2O2 CL system involved aggregated precious metal nanoparticles (AuNPs or AgNPs), the change of catalytic activity of nanoparticles after aggregation on luminol–H2O2 CL system might be caused by three factors: (1) the variation of negative charge density on the surface of nanoparticles, (2) the variation of nanoparticles diameter, and (3) the variation of electron density in nanoparticle’s conduction bands. The variation characteristic of nanoparticle’s catalytic activity depends on which factor above mentioned holds the dominant position during the catalytic process. The previous research [29] has shown that organic compounds containing OH, NH2, or SH groups were capable to enhance the CL intensity of luminol–H2O2–AuNPs, and the primary reason was that they could induce the aggregation of AuNPs and decrease the surface negative charge density of AuNPs. Based on this find, the determination of this kind of organic compounds was realized. However, the regularity of surface negative charge density for AuNPs did not work in the luminol– H2O2–AgNPs CL system according to our experimental results in this study. It seems that in AgNPs (7 nm and 15 nm) CL system, the variation of electron density in conduction bands after AgNPs’s aggregation plays a leading role in influencing the catalytic activity of AgNPs. Application Anilines, as an important organic chemical raw materials and fine chemical intermediates, have been widely used in pharmaceutical and dyeing and textile industries. Owing to its high toxicity and carcinogenicity, it is vital to detect these residues in environmental samples. It has been reported that the poisonous organic compounds like phenylamine, p-phenylenediamine, o-phenylenediamine and m-phenylenediamine can induce the aggregation of AgNPs [30]. According to the mechanism discussed above, it can be expected that the interaction between these organic compounds and AgNPs (7 nm and 15 nm) can have an effect on the CL signal of luminol–H2O2–AgNPs systems. Herein, the effects of such organics on the luminol–H2O2–7 nm and 15 nm AgNPs CL systems were investigated. As expected, all the tested compounds were capable to enhance luminol–H2O2–7 nm AgNPs CL signal, and quench luminol–H2O2–15 nm AgNPs CL signal in that they could induce the aggregation of AgNPs and further alter (increase or decrease) the electron density in the conduction bands of AgNPs (7 nm or 15 nm).
Table 2 Analytical performance of the proposed luminol–H2O2–7 nm AgNPs CL system. Sample
Linear range (g/mL)
Phenylamine p-Phenylenediamine o-Phenylenediamine m-Phenylenediamine
2 10 2 10 3 10 3 10
9
–2 10 9 –6 10 9 –1 10 9 –8 10
6 6 6 7
Equation of linear regression
Related coefficient
Detection limit (3r, n = 5) (g/mL)
DI = 2.115lnC + 21.49 DI = 7.537lnC + 35.82 DI = 6.745lnC + 3.207 DI = 1.575lnC + 35.57
0.9942 0.9914 0.9926 0.9863
6.2 10 2.6 10 1.2 10 1.6 10
10 10 9 9
Table 3 Analytical performance of the proposed luminol–H2O2–15 nm AgNPs CL system. Sample
Linear range (g/mL)
Phenylamine p-Phenylenediamine o-Phenylenediamine m-Phenylenediamine
2 10 2 10 3 10 3 10
9
–6 10–6 9 –2 10–6 9 –1 10–6 9 –8 10 6
Equation of linear regression
Related coefficient
Detection limit (3r, n = 5) (g/mL)
DI = DI = DI = DI =
0.9999 0.9892 0.9916 0.9926
3.6 10 1.1 10 8.6 10 6.3 10
1.274lnC + 57.46 10.23lnC 4.003 10.54lnC + 0.051 31.57lnC + 310.5
10 9 10 10
Y. Qi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 76–81
The analytical potential for these compounds which could induce the aggregation of AgNPs on the proposed luminol–H2O2– 7 nm AgNPs CL system and luminol–H2O2–15 nm AgNPs CL system was explored by using a flow injection procedure. Under the optimized experimental conditions, the CL intensity linearly responded to the concentration of target compounds within a wide range, and the results are presented in Tables 2 and 3. It can be seen that the linear range for all these compounds could reach three or more orders of magnitude. Combined with separation techniques such as HPLC or high-performance capillary electrophoresis, this CL system has a wide application for the simultaneous determination of such poisonous organic compounds. Conclusions In this work, aggregation of AgNPs was found to be an important effect factor for the luminol–H2O2 CL system. Aggregated AgNPs could effectively affect the CL signal compared with the dispersed one, and the effect characteristic was closely related to the sizes of AgNPs. In comparison with dispersed AgNPs, the catalytic activity increased significantly after its aggregation for the 7 nm AgNPs, while decreased for the 15 nm AgNPs. No obvious change could be found for the 55 nm AgNPs. The CL responses of aggregated AgNPs were suggested to be due to the two effects of quantum size and electron density in nanoparticle’s conduction bands. It was found that the enhanced effect of aggregated AgNPs of 7 nm was mainly due to the increase of electron density in AgNPs’s conduction bands, while both the two effects of quantum size and electron density in AgNPs’s conduction bands resulted in the decrease of catalytic activity of 15 nm AgNPs after aggregation. Toxic organic compounds such as aniline, phenylenediamine and their derivatives, could induce AgNPs’s aggregation and further enhance (for 7 nm AgNPs) or quench (for 15 nm AgNPs) luminol– H2O2–AgNPs CL signal, were detectable within a wide range by using a flow injection method with the CL detection. This work is important to broaden the investigation and application of catalysis of AgNPs for CL reactions and has some practical applied value for environment monitoring. Acknowledgements This work was supported financially by the Natural Science Foundation of Fujian Province of China (2011J05016), the Foundation of Fujian Educational Committee (JA11177 and JA12237), the Special Science Funding of Provincial Universities of Fujian (JK2012032), and the Scientific Research Foundation of Fujian University of Technology (GY-Z10055).
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