Anal Bioanal Chem DOI 10.1007/s00216-016-9393-3
RESEARCH PAPER
Improved sensitivity via layered-double-hydroxideuniformity-dependent chemiluminescence Zenghe Li 1 & Dan Wang 1 & Zhiqin Yuan 1 & Chao Lu 1
Received: 7 December 2015 / Revised: 22 January 2016 / Accepted: 2 February 2016 # Springer-Verlag Berlin Heidelberg 2016
Abstract In the last two decades nanoparticles have been widely applied to enhance chemiluminescence (CL). The morphology of nanoparticles has an important influence on nanoparticle-amplified CL. However, studies of nanoparticle-amplified CL focus mainly on the size and shape effects, and no attempt has been made to explore the influence of uniformity in nanoparticle-amplified CL processes. In this study we have investigated nanoparticle uniformity in the luminol–H2O2 CL system using layered double hydroxides (LDHs) as a model material. The results demonstrated that the uniformity of LDHs played a key role in CL amplification. A possible mechanism is that LDHs with high uniformity possess abundant catalytic active sites, which results in high CL intensity. Meanwhile, the sensitivity for H2O2 detection was increased by one order of magnitude (1.0 nM). Moreover, the uniform-LDH-amplified luminol CL could be applied to selective detection of glucose in human plasma samples. Furthermore, such a uniformity-dependent CL enhancement effect could adapted to other redox CL systems—for example, the peroxynitrous acid (ONOOH) CL system.
Published in the topical collection Highlights of Analytical Chemical Luminescence with guest editors Aldo Roda, Hua Cui, and Chao Lu. Electronic supplementary material The online version of this article (doi:10.1007/s00216-016-9393-3) contains supplementary material, which is available to authorized users. * Chao Lu
[email protected]
1
State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing `100029, China
Keywords Nanoparticle . Uniformity . Chemiluminescence . Layered double hydroxides
Introduction The chemiluminescence (CL) intensity is very important in applications in analytical-related fields [1–4]. For example, through enhancement of the CL intensity of the luminol– H2O2 CL system, Wang et al. [5] improved the sensitivity for H2O2 detection to 0.02 μM, which is about 30-fold higher than in a previous report [6]. So far, much effort has been dedicated to CL enhancement by the choosing of an appropriate solvent, catalyst, surfactant, etc. [7, 8]. Particularly, nanoparticles as catalysts have been widely used to amplify CL intensities [9–12]. Several reports have proved that the morphology of nanoparticles largely influences their catalytic performance toward CL systems [13, 14], and it is generally accepted that small nanoparticles possess high catalytic ability [15–17]. However, recent reports indicate the smallest nanoparticle does not show the highest catalytic capability in gold-nanoparticleamplified CL systems [18, 19]. For example, Zhang et al. [18] studied the size effect on the luminol CL system using five types of gold nanoparticles with different sizes (16, 25, 38, 68, and 99 nm), and they concluded that the 38-nm gold nanoparticles rather than the 16-nm gold nanoparticles possessed the highest catalytic activity. Similarly, Duan et al. [19] showed that 25-nm gold nanoparticles exhibited optimal catalytic activity over gold nanoparticles of four other sizes (11, 16, 39, and 71 nm). These results are inconsistent with previous reports [15–17], which suggests the existence of other factors that influence CL intensity besides the nanoparticle size. It has been reported that a uniform Ni/Al2O3 nanocatalyst exhibits the highest coking resistance and stability during partial oxidation of methane because of the high nanoparticle
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uniformity [20]. In addition, Li et al. [21] demonstrate that tin oxide nanoparticles with high uniformity display higher photoactivity in rhodamine B degradation compared with commercial P25-type tin oxide nanoparticles. These findings provide supporting evidence that uniformity of nanoparticles may play an important role in nanoparticle-amplified CL. However, there has been no report on the effect of uniformity on CL intensity. In view of its versatility in the study of crystal tectonics, the reverse microemulsion method has been widely used to synthesize nanocrystals with a uniform size distribution (e.g., silicon oxide nanoparticles, metal oxide nanoparticles, a n d c o p o l y m e r n a no p a r t i c l e s ) [ 2 2 – 29 ] . R e ve r s e microemulsion techniques produce monodisperse and uniform colloids via the restraining of water pools within surfactantconfined micelles by precise control of the water–surfactant molar ratio (ω = [H2O]/[surfactant]) [30–32]. The nanoparticles thus prepared usually possess high catalytic capability due to the excellent uniformity [20, 21]. Layered double hydroxides (LDHs) are hydrotalcite-like compounds, which are widely used as adsorbents and catalysts [33–36]. In particular, magnesium aluminum carbonate LDHs synthesized by the coprecipitation method could serve as a catalyst and buffer solution for the luminol–H2O2 CL reaction, allowing sensitive detection of H2O2 and glucose [37, 38]. However, magnesium aluminum carbonate LDHs produced by the coprecipitation method at low supersaturation always have a nonuniform size distribution [39], which encouraged us to investigate the effect of LDH uniformity on CL intensity using uniform LDHs produced by the reverse microemulsion method. We synthesized uniform LDHs by the reverse microemulsion method and investigated their catalytic performance on the luminol–H2O2 CL system. The results showed that the uniformity of LDHs had an obvious enhancement effect on CL intensity. Such a uniformity-dependent CL character was further demonstrated by an aging temperaturedependent experiment. The mechanism of this phenomenon was that LDHs with a uniform size distribution possessed sufficient catalytic active sites, leading to CL enhancement. With improved CL intensity, the sensitivity for H2O2 of the luminol– H2O2 CL system was improved by an order of magnitude and the CL system could be used as a biosensor for glucose detection in human plasma samples with good reliability. Furthermore, this uniformity-dependent CL property is also adaptable to other redox CL systems—for example, the peroxynitrous acid (ONOOH) CL system.
Materials and methods Chemicals and solutions Magnesium nitrate hexahydrate (Mg(NO3)2.6H2O), aluminum nitrate nonahydrate (Al(NO3)3.9H2O), sodium hydroxide
(NaOH), sodium carbonate (Na2CO3), concentrated hydrochloric acid (HCl; 38 wt %), n-butanol, isooctane, ammonium hydroxide (NH4OH; 25 wt %), hydrogen peroxide (H2O2; 30 vol %), acetonitrile, phenol, and cyclohexane were purchased from Beijing Chemical Reagent Company (Beijing, China). Sodium nitrite (NaNO2), potassium sodium tartrate, and sodium metasulfite were purchased from Tianjin Chemical Reagent Company (Tianjin, China). Luminol (3aminophthalhydrazide) was purchased from Arcos (USA). Hexadecyltrimethylammonium bromide (CTAB) was purchased from J&K Scientific (Shanghai, China). Glucose oxidase (GOD) was obtained from Aspergillus niger (at least 264 U/mg). 3,5-Dinitrosalicylic acid (DNS) was purchased from HWRK Chem (Beijing, China). We prepared a 0.01 M stock solution of luminol by dissolving luminol in 0.1 M NaOH solution without purification and we used it 2 weeks later. A working solution of 1 × 10-4 M H2O2 was prepared by dilution of H2O2 (10 M) with deionized water. We prepared a 0.1 M nitrite stock solution by dissolving 0.345 g NaNO2 in 50 mL deionized water. H2O2 solution (0.05 M) was prepared by dilution of H2O2 with deionized water. HCl solution (0.07 M) was prepared by dilution of concentrated HCl with deionized water. We prepared the phenol solution by dissolving phenol in cyclohexane. We prepared the 10 mM glucose stock solution by dissolving 36 mg glucose in 20 mL deionized water. We prepared the GOD solution by dissolving 20 mg GOD in 10 mL phosphate buffer (pH 7.4). The DNS solution was prepared and stored in the dark for 7 days before use. All the experimental chemicals were of analytical grade and were used without further purification. Ultrapure water (18.2 MΩ cm) was obtained from a Millipore system. Synthesis of LDHs by the reverse microemulsion method LDHs with a Mg/Al molar ratio of 3 were synthesized by use of a CTAB–isooctane–water–n-butanol quaternary reverse microemulsion system [40]. Briefly, microemulsion A contained 1.36 mmol Mg(NO3)2 and 0.425 mmol Al(NO3)3, microemulsion B contained 17 mmol ammonium hydroxide and 0.21 mmol Na2CO3, and the two microemulsions were mixed dropwise in equal volumes under stirring. This mixture was stirred until the solution became translucent. After the mixture had been stirred for another 30 min at room temperature, the flask was tightly sealed and heated at 60 °C for 24 h with refluxing and stirring. The preparation of LDHs in the reverse microemulsion system with different ω values is summarized in Table S1. A gel-like substance was separated by centrifugation at 10,000 rpm for 10 min. The resulting colloids were first washed with ethanol three times and then washed with deionized water three times to remove all the organic solvent, nitrate salts, and the surfactant CTAB. Finally, the colloids were redispersed with 20 mL deionized water and stored at 4 °C for further use.
Improved sensitivity via layered-double-hydroxide-uniformity
Apparatus
Determination of LDH nanoparticle uniformity
Transmission electron microscope (TEM) images of LDHs were obtained with a Tecnai G220 high-resolution TEM (FEI, USA) at an accelerating voltage of 200 kV. X-ray diffraction (XRD) patterns were collected with a D8 ADVANCE X-ray diffractometer (Bruker, Germany) equipped with graphite-monochromatized Cu Kα radiation (λ =1.54178). The scan rate was 10°/min, and the 2θ angle ranged from 5° to 70°. Infrared spectra were obtained with a Nicolet 6700 Fourier transform infrared spectrometer (Thermo, USA) with KBr disk technology. CL signals were collected by a CL analyzer (BPCL, Institute of Biophysics, Chinese Academy of Science, Beijing, China). The hydrodynamic diameter distribution of LDHs was measured by a Zetasizer 3000HS nanogranularity analyzer (Malvern Instruments, UK). Ultraviolet–visible spectra were obtained with a Shimadzu UV-2401 PC spectrophotometer (Tokyo, Japan).
The uniformity of the LDH nanoparticles was characterized by the relative standard deviation (RSD), defined as vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 uX n u d −d t i 1 i¼1 RSD ¼ 100%; ð1Þ n−1 d
CL measurements A schematic of the static CL detection for the luminol–H2O2 system is shown in Fig. S1. Typically, we mixed 100 μL LDH colloidal solution with 100 μL of 5 × 10-5 M luminol solution in a CL quartz vial above the photomultiplier tube. Then we injected 100 μL of 10-4 M H2O2 into the mixture, and the photons generated from the CL reaction were recorded by the BPCL analyzer via the single photon counting technique with a data integration time of 0.1 s per spectrum at a working voltage of -1000 V. The protocol for ONOOH system CL measurements was the same as that for the luminol–H2O2 system. Briefly, we injected 200 μL LDHs in a CL quartz vial above the photomultiplier tube, and then injected 100 μL of 0.05 M H2O2–0.07 M HCl solution and 100 μL of 10 μM NaNO2 solution simultaneously into the quartz vial. The photons generated from the CL reaction were recorded by the BPCL analyzer via the single photon counting technique with a data integration time of 0 .1 s per spectrum at a working voltage of -1000 V. The CL signal was imported to the computer for data acquisition. All CL responses were recorded at least three times. Measurement of catalytic active sites The concentration of base active sites on the surface of LDHs was obtained by our measuring the adsorption isotherms of phenol dissolved in cyclohexane at 25 °C [41, 42]. The adsorbed amount of phenol on the LDH solid was measured with an ultraviolet–visible spectrophotometer (λmax = 271.6 nm). The amount of phenol adsorbed on the LDHs conformed to the Langmuir-type equation and represented the concentration of base sites. The ratio of cyclohexane solution to LDHs was 50 mL/0.1 g.
where d is the mean diameter. Sample pretreatment Human plasma samples were obtained from the China-Japan Friendship Hospital (Beijing, China). The blood samples were stored at -20 °C and thawed at room temperature before use. After 50 μL blood samples had been mixed with 200 μL acetonitrile under stirring and the solution had been centrifuged, the supernatant was recovered. The test samples were obtained by dilution of the supernatant 100 times with deionized water.
Results and discussion Controllable synthesis of LDHs As a starting point for our study, we synthesized LDHs with different ω values using the reverse microemulsion method by changing the ω value from 11 to 40. With increasing ω values, all synthesized LDHs showed symmetric and sharp reflections for the (003), (006), (110), and (113) planes, and asymmetric and broad reflections for the (012), (015), and (018) planes, which are reflections characteristic of a hydrotalcite-like structure (Fig. S2) [43]. Similar Fourier transform infrared spectra (Fig. S3) also demonstrated the quasi structure and surface environment of the prepared LDHs. As shown in Fig. 1, the TEM images and hydrodynamic diameter distribution data revealed that the LDH nanoparticle size increased with increasing ω values. Figure 2 shows the CL intensity of luminol–H2O2 in the absence and presence of LDHs, and reveals that the CL intensity was dramatically amplified by the LDHs. In addition, the CL intensity was enhanced obviously with increasing ω values, implying that the CL intensity was proportional to LDH nanoparticle size. The results seemed to be contrary to conventional results indicating that catalytic activity decreases with increasing nanoparticle size [15–17]. Uniformity-dependent CL We further clarified the interesting phenomenon of the enhanced CL intensity with increase of the nanoparticle size.
Z. Li et al. Fig. 1 Transmission electron microscope (TEM) images and hydrodynamic diameter distributions of layered double hydroxides (LDHs) synthesized with different ω values
We synthesized LDHs with uniformity comparable with that of LDHs with ω = 11 (the RSD is shown in Table S2) but their nanoparticle size was larger (denoted as n-LDHs). The XRD pattern of n-LDHs exhibited reflections characteristic of a hydrotalcite-like structure (Fig. 3a). The TEM image of nLDHs showed a circular nanoplate morphology, similar to that of the LDHs (Fig. 3b). Next, the CL catalytic activity of nLDHs on the luminol–H2O2 CL system was compared with that of LDHs (ω = 11). Figure 4 shows that the CL intensity of
the luminol–H2O2 system in the presence of the LDHs (ω = 11) was much higher than that obtained by addition of n-LDHs. Despite the similar crystalline structure and uniformity, the LDHs with smaller nanoparticle size led to higher catalytic activity, which was in accordance with the literature findings [15–17]. We thus speculated that the reason for CL enhancement was mainly ascribed to the increase of nanoparticle uniformity. In addition, the CL intensity of the luminol–H2O2 system with the addition of LDHs with ω = 40 was much higher than that in the presence of n-LDHs. In comparison with LDHs with ω = 40, the synthesized n-LDHs possessed a similar average diameter but poor uniformity. Therefore, we speculated the reason that the CL intensity increased with increasing ω values was because of an increase in the LDH nanoparticle uniformity. In conclusion, the LDH nanoparticle uniformity had a larger effect on the luminol–H2O2 CL intensity than did LDH nanoparticle size. Validation of uniformity-dependent CL
Fig. 2 Chemiluminescence (CL) intensity of the luminol–H2O2 CL system in the absence and presence of LDHs synthesized with ω values from 11 to 40. Luminol concentration 5 × 10-5 M, H2O2 concentration 1 × 10-4 M
To further validate the uniformity-dependent CL, we prepared LDHs with various uniformities by controlling the aging temperature [44]. We investigated the synthesis of LDHs (ω = 22) at three different aging temperatures (40, 60, and 80 °C). The reverse microemulsion can be affected by temperature and
Improved sensitivity via layered-double-hydroxide-uniformity Fig. 3 X-ray diffraction pattern (a) and TEM image (b) of nLDHs
become unstable at higher temperature (above 90 °C), but higher aging temperature was not investigated here. As shown in Fig. S4, the XRD spectra revealed that all the resulting LDHs exhibit reflections characteristic of hydrotalcite-like structure. In addition, the CL intensity for LDH particles aged at different temperatures (Fig. S5) and the RSDs (Table S2) calculated from the hydrodynamic diameter distributions (Fig. S6) showed that the LDHs prepared at 60 °C generated the strongest CL intensity as a result of their having the highest uniformity. These experimental results further demonstrated that the uniformity of the LDHs played a vital role in the CL enhancement.
Discussion of the mechanism It has been reported that the physical and chemical properties of nanoparticles partially depend on their uniformities, including optical and catalytic properties [45–48]. In the present system, LDHs served as a catalyst [38], and we thus speculated that the number of catalytic active sites of LDHs has an important role in CL enhancement. To verify our hypothesis, we determined the number of catalytic active sites of the
synthesized LDHs by measuring the adsorption isotherms of phenol in cyclohexane at 25 °C. As shown in Fig. S7, the amount of phenol absorbed on the surface of LDHs increased with the increase of LDH uniformity, which meant the number of catalytic active sites on the surface of LDHs increased with the increase of LDH uniformity. For LDHs, the active sites were mainly located on orderly arranged hydroxyl sites on the surface of the LDH particle rather than on the edges [41]. The number of basic sites was decreased with increasing numbers of hydroxyl defects, which is proportional to the fraction of the surface defects [49]. On the other hand, an increased number of surface defects would lead to an irregular surface charge distribution and cause the aggregation of LDHs, which buries the basic sites and diminishes the catalytic activity [50]. Moreover, it was reported that the number of point defects on the LDH surface is inversely proportional to the crystallinity of LDH. In other words, the number of surface defects is decreased with the increase of crystallinity. In our work, the XRD data revealed that LDHs with higher uniformity possesses better crystallinity (Fig. S2). Therefore LDHs with high uniformity show fewer surface defects and more basic sites. It can thus be concluded that LDH-uniformity-dependent catalytic activity is due to the increase in number of catalytic active sites, which stimulates the generation of excited-stated intermediates and results in the CL enhancement [37, 38]. Sensitivity improvement
Fig. 4 CL intensity of the luminol–H2O2 system in the presence of nLDHs and LDHs (ω = 11) respectively. Luminol concentration 5 × 10-5 M, H2O2 concentration 1 × 10-4 M
Generally, the enhancement of CL intensity will result in improvement of sensitivity. Since the increased numbers of uniform LDHs could assist the generation of high CL intensity in the luminol–H2O2 CL system, we speculated that the sensitivity for H2O2 detection by the luminol–H2O2 CL system should be improved with increasing LDH uniformity. This assumption was supported by H2O2 detection results in the presence of LDHs (ω = 40) and n-LDHs. As shown in Fig. 5, the CL intensity increased with the increase of H2O2 concentration in the presence of both LDHs (ω = 40) and n-LDHs. However, the CL intensity in the presence of LDHs (ω = 40) is much higher than that resulting from addition of n-LDHs. For
Z. Li et al.
Fig. 5 CL intensity of the luminol–H2O2 system in the presence of n-LDHs (a) and LDHs (ω = 40) (b) on addition of different concentrations of H2O2. Insets the calibration curves for H2O2
example, on addition of 10 μM H2O2, the CL intensity in the presence of LDHs (ω = 40) was about 30-fold higher than that in the presence of n-LDHs (data not shown). Meanwhile, both the linear range and the detection limit showed an obvious difference. For the n-LDH-nanoparticle-catalyzed luminol– H2O2 CL system, the CL intensity showed good linearity in the H2O2 concentration range from 0.1 to 100 μM, and the limit of blank (LOB), limit of detection (LOD; signal-to-noise ratio of 3), and limit of quantification (LOQ; signal-to-noise ratio of 10) for H2O2 were 3, 10, and 100 nΜ respectively. For the LDH (ω = 40)-nanoparticle-amplified luminol–H2O2 CL system, the linear range was 0.01–10 μM. The LOB, LOD, and LOQ for H2O2 were 0.2, 1.0, and 10 nM respectively. These results indicate that the sensitivity for H2O2 detection indeed increased with the increase of LDH uniformity. The intraassay and interassay imprecision were tested (Table S3), and the RSDs were below 5 % and 15 % respectively, indicating good precision of the system [51]. Meanwhile, the stability of the system was investigated by 20 repeated injection of 10 μΜ Η2Ο2; the RSD was 2.06 % (Fig. S8). Because of the improved sensitivity, detection of H2O2 in real samples would be facilitated. Performances for biological sensing To verify the feasibility of the sensor, the LDH (ω = 40)amplified luminol CL was used to detect glucose in human plasma samples. Because GOD can catalyze the oxidation of glucose to generate H2O2, we can indirectly measure glucose by detection of the H2O2 content generated. Therefore we mixed 700 μL glucose solution of different concentrations with Table 1 Determination of glucose in human plasma samples
300 μL GOD solution, and incubated the mixture at 37 °C for 0.5 h before the CL experiment. As shown in Fig. S9, the CL signals were linear from 1.2 to 150 μΜ, with the regression line y = 0.66637x + 3.3990 (R2 = 0.9980). The LOB, LOD, and LOQ for glucose were 0.1, 0.3, and 1.2 μΜ respectively. In addition, the kinetic CL intensity–time profile is shown in Fig. S10; the decay time was within 100 s. The standard addition method was used to measure the glucose concentration in human plasma samples. The test results were in accordance with the results obtained by the DNS method (Table 1) [52]. To investigate the accuracy of the method, known amounts of glucose were added to the samples and the recovery was measured. The recovery values ranged from 98 % to 105 %, representing good accuracy of our method.
Universality of uniformity-dependent CL The remarkable catalytic activity of the higher-uniformity LDHs on the luminol–H2O2 CL system was exploited in the ONOOH CL system [53]. We investigated the CL of the ONOOH CL system in the presence of n-LDHs and LDHs (ω = 40). As shown in Fig. S11, the CL intensity of the ONOOH CL system in the presence of the LDHs (ω = 40, high uniformity) was much higher than that obtained by addition of n-LDHs (low uniformity). These results were in agreement with those for the luminol–H2O2 system. The enhancement of CL intensity might also result in sensitivity improvement in further analytical applications. We propose nanoparticleuniformity-dependent CL is universal.
Sample
Present method (mM)
Standard method (mM)
Added (mM)
Found (mM)
Recovery (%)
Sample 1 Sample 2
7.3 ± 0.3 5.0 ± 0.2
7.1 ± 0.2 4.9 ± 0.1
2.0 1.0
2.1 ± 0.03 0.98 ± 0.01
105 ± 1.5 98 ± 1.0
Improved sensitivity via layered-double-hydroxide-uniformity
Conclusions
7.
In summary, we investigated the effect of nanoparticle uniformity on the luminol–H2O2 CL system using various uniformities of LDHs obtained by the reverse microemulsion method. The results demonstrated that the uniformity played a very important role in the LDH nanoparticle-amplified CL because LDHs with high uniformity possess plentiful catalytic activity sites, leading to dramatic CL enhancement. Moreover, the LOD for H2O2 sensing was decreased from 0.01 to 0.001 μM. The CL system could be applied to detect glucose in human plasma with good accuracy. Furthermore, the uniformity-dependent CL enhancement was extended to another CL system (i.e., ONOOH system). This work provides a novel possibility to increase CL intensity by the preparation of uniform nanoparticles rather than only by the tuning of the size of nanoparticles.
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Acknowledgments This work was supported by the National Basic Research Program of China (973 Program, 2014CB932103), the National Natural Science Foundation of China (21375006 and 21575010), and the Innovation and Promotion Project of Beijing University of Chemical Technology.
15.
16. Compliance with ethical standards Conflict of interest The authors declare that they have no competing interest.
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18. Ethics approval The use of the blood samples for research was approved by the Ethics Committee of the China-Japan Friendship Hospital. 19.
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