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Lucigenin-tris(2-carboxyethyl)phosphine chemiluminescence for selective and sensitive detection of TCEP, superoxide dismutase, mercury (II), and dopamine Muhammad Saqib, Shahida Bashir, Haijuan Li, Shan-shan Wang, and Yongdong Jin Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b05486 • Publication Date (Web): 28 Jan 2019 Downloaded from http://pubs.acs.org on January 29, 2019

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Analytical Chemistry

Lucigenin-Tris(2-carboxyethyl)phosphine Chemiluminescence for Selective and Sensitive Detection of TCEP, Superoxide Dismutase, Mercury (II), and Dopamine Muhammad Saqib,† Shahida Bashir,‡ Haijuan Li,† ShanShan Wang,† Yongdong Jin*,†,§ †State

Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun, Jilin 130022, P. R. China ‡Faculty of Science, Department of Mathematics, University of Gujrat, Gujrat 50700, Pakistan §

University of Science and Technology of China, Hefei 230026, P. R. China

ABSTRACT: Development of simple and effective chemiluminescence (CL) systems for multiple sensing applications is significantly important but still a challenge. Until now, majority of CL systems primarily utilized hydrogen peroxide (H2O2) as coreactant, which is limited in its stability and selectivity due to the easy decomposition of H2O2 in the presence of several ions. In this study, we develop a new and intense CL system by combined use of tris(2-carboxyethyl)phosphine (TCEP), a highly solution stable and pH-tolerant tertiary phosphine, with lucigenin for the first time. The effective pairing leads to a significant ~ 23 times CL enhancement over classic lucigenin-H2O2 system without employing additional catalysts. By virtue of this fascinating platform, a sensitive CL method has been developed for the multiple detection of TCEP (LOD = 70 nM), lucigenin (LOD = 4.0 nM), superoxide dismutase (LOD = 0.8 ng/ml), Hg2+ (LOD = 0.3 nM), and dopamine (LOD = 3.0 nM), with a linear range of 0.1-320 µM, 0.01-55 µM, 0.005-0.5 µg/mL, 1.0-600 nM and 0.01-0.8 µM, respectively. Remarkably, this CL method exhibited superior selectivity over several potential interferents. Moreover, the proposed method achieved excellent recoveries in the range of 94.0–102.3% for both Hg2+ detection in lake water and dopamine detection in human serum real samples. We envision that broad applications of TCEP may lead to construct new CL systems, pushing forward for efficient detection of various analytes.

Developing high performance detection methods for multiple sensing applications has attracted great interest in recent years. Chemiluminescence (CL) has been witnessed as one of the most sensitive analytical technique to accomplish higher signal to noise ratio optical signal readout in many chemical and biological sensing protocols.1 CL based sensing platforms are extensively documented due to quick response, wide linear dynamic range, higher sensitivity, low background noise, and easy availability of instrumentation.2 Lucigenin (N,N-dimethyl9,9'-biacridinium dinitrate) is one of the most efficient CL luminophore with broad range of analytical applications. Since 1935, the classic lucigenin CL systems are paired with H2O2.3 However, the traditional lucigenin-H2O2 CL system usually suffer from poor stability and selectivity for practical applications in the presence of common interferents including metal ions and their complexes.4 Recently, several catalysts and metal nanoparticles reductants have been incorporated into CL systems to improve and widen the scope of CL applications.5 Although numerous efforts have been made, it is still highly desired to develop new lucigenin CL systems to overcome the limitations and to further improve the CL performance without applying sophisticated catalysts. Tris(2-carboxyethyl)phosphine (TCEP), a solution stable and pH-tolerant tertiary phosphine, has been reported as a strong reducing agent and an effective maintainer for free sulphydryl groups and cleaver for disulfide bonds.6-7 Due to these

favourable features, it has been extensively exploited for versatile uses in pathological and biochemical analysis in vitro and in vivo,8-9 drug delivery and drug synthesis,10-11 and physiological macromolecule modification.12 Recently, its potential in medicinal practice has also been explored. For example, the use of TCEP can remarkably boost the cisplatin reactivity towards zinc finger proteins, whose bonding has been reported as a new tactic for anticancer developments.13 In addition, TCEP offers a dual role in retinal therapy; on one side it serves as a neuro-protective agent for retinal ganglion cells,14 on the other side it offers a novel strategy for the treatment of oxidative damage to photoreceptors and photic injury.15 Because of its unique reductivity and coordination properties, TCEP may react with lucigenin to generate CL. Developing efficient methods for TCEP detection could help for its monitoring in biochemical, pathological analysis, and its therapeutic effects and related reaction processes. In the past, TCEP detection has been achieved by using chromatographic and UV-vis spectrometry methods. As TCEP shows negligible native absorbance, the UV-vis method usually needs optical labelling reagents (e.g. 5, 5-dithiobis) for its indirect sensing. However, this strategy was limited in the presence of several thiolate compounds which exhibit similar reduction behaviour.16 Although high-performance liquid chromatography coupled with evaporative light scattering detector (HPLC-ELSD) had also been utilized for the detection of TCEP,17 the technique needs pre-oxidation of TCEP to

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TCPEO with the help of H2O2 to achieve better separation by ion exchange, which gives sum of the concentrations of both and makes the detection complicated. Additionally, the technique needs expensive ELSD equipment, which limits its usage in broad range of applications. Recently, a ratiometric fluorescent sensor has also been reported for the detection of TCEP based on the dual emissive gold nanoclusters and bovine serum albumin.18 Although it exhibited good performance than previously reported methods, it requires complicated and costly multistep preparations of the fluorescent materials. Oxygen-derived free radicals (e.g. O2•−) are harmful for cells and can cause serious oxidative damages to proteins, nucleic acids, lipids, and cell structures, and ultimately results in cancer, apoptosis, heart disease, aging, and neurological disorders.19-20 Therefore, sensitive detection of superoxide radicals (O2•−) is very crucial for the understanding of its pathogenic mechanisms, and disease diagnosis, treatment and health screening. Notably, superoxide dismutase (SOD) which is broadly considered as a vital antioxidant metalloenzyme has a pivotal role for biological defense to detoxify O2•−.19 SOD catalyze the dismutation of O2•− to H2O2 and molecular oxygen (O2) and ultimately scavenge the activity of O2•−.19 To date, a variety of methods have been reported for the detection of SOD, however, majority of the methods required sophisticated materials, relatively high cost, and complicated steps (Table S1). Nowdays, metal ions accumulation is already a severe threat to the environment, human health, and ecological systems. Mercury ion (Hg2+) as one of the highly toxic metal ions is therefore attracted immense focus due to its bioaccumulation and severe toxicity by food intake and water drinking. The allowable limit of Hg in drinking water is as low as 10 nM approved by the U.S. Environmental Protection Agency (EPA). Several studies have proved that Hg2+ poses serious damage to the immune system, brain, central nervous system, kidney, and endocrine system.21 Hence, it is highly desired to develop a simple and rapid method for selective and sensitive detection of Hg2+ for the environmental analysis.22 Until now, numerous sensing methods including UV–vis absorption spectrometry,23 fluorescence,24 inductively coupled plasma mass spectrometry (ICPMS),25 colorimetric analysis,26 CL,27 and electrochemical28 and electrochemiluminescence (ECL)29 methods have been reported for the detection of Hg2+. However, chromatographic methods involved exhaustive steps, expert personnel, and expensive instrumentation. Although fluorescence methods have made more progress for the detection of Hg2+, however, many of the methods limit practical use due to cross-sensitivity toward other metal ions, poor solubility of some fluorophores in aqueous medium, and multistep synthesis of probe materials.30 In addition, dopamine (DA) detection is also highly desired and undoubtedly of clinical significance since DA is a vital neurotransmitter within the central nervous system and brain, which plays significant roles in various physiological processes. The disruption of its normal functioning could leads to several diseases, including depression,31 schizophrenia,32 neurological disorders, and Parkinson’s disease.33 DA also plays important functions in the immune system,34 the cardiovascular system,35 and the renal system.36 Up to now, a number of analytical methods have been established for the detection of DA, including chromatography coupled with spectroscopy (e.g., HPLC-MS),37 spectrophotometry,38 electrochemistry,39 38 40 spectrophotometry, CL, fluorescence,41 and ECL.42

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However, chromatographic methods involve time-consuming procedures and expensive instrumentation. Similarly, colorimetric and fluorescent probes synthesis also need multistep and complicated procedures. While electrochemical methods usually show poor detection selectivity with the interference of ascorbic acid (AA) and uric acid (UA). In the present work, we exploit the use of TCEP for the first time as an efficient coreactant, instead of classic H2O2, for lucigenin CL and successfully apply this new lucigenin-TCEP CL system for multiple detection of TCEP, Lucigenin, SOD, Hg2+ and DA with high sensitivity and selectivity. The lucigenin-TCEP CL system displays an enhanced CL emission. In the presence of SOD, Hg2+, and DA, the CL signals of the system were strongly decreased, thus allowed to rapid, selective, and sensitive detection of these potential targets with a simple CL system. Meanwhile, this CL method is sensitive to the concentration of TCEP. Notably, this system demonstrated decent selectivity in the presence of several interferences. To the best of our knowledge, this is also the first report on using lucigenin for CL detection of TCEP and Hg2+. Finally, the practical applicability of this CL platform is further evaluated in the measurement of spiked Hg2+ in lake water and DA in human serum samples. EXPERIMENTAL SECTION Chemicals and apparatus. TCEP and DA were purchased from Aladdin (Shanghai, China). AA, UA, and glucose (Glu) were purchased from Sigma-Aldrich (USA). Glutathione (GSH) was bought from Beijing Dingguo Changsheng Biotechnology Co Ltd. L-Cysteine (Cys), L-tryptophan (Tryp), L-lysine (Lys), thymine (Thy), and cytosine (Cyto) were supplied by Sinopharm Chemical Reagent Co. Ltd. (Beijing, China). dLalanine (Ala), L-proline (Pro), L-glutamic acid (Glut), Larginine (Arg), and L-methionine (Meth) were supplied by Shanghai Yuanju Biotechnology Co. Ltd. (Shanghai, China). SOD and sodium azide (NaN3) was purchased from Aladdin (Shanghai, China) and Tianjin Fuchen Chemical Reagents Factory (China), respectively. Ethylenediaminetetraacetic acid disodium salt (EDTA) was bought from VWR Life Science (USA). 5,5-dimethyl-1-pyrroline N-oxide (DMPO) and creatinine (Creat) were purchased from Aladdin (Shanghai, China). Lucigenin was bought from TCI (Shanghai, China). H2O2 was purchased from Xinke Reagent Factory (Bengbu, China). Metal salts, thiourea, thiourea dioxide (TD), hydroxylamine (HA), and mannitol (Mann) were purchased from Beijing Chemical Reagent Company (Beijing, China). All the stock solutions were prepared by doubly distilled water. Different pH carbonate buffers were prepared using sodium carbonate, sodium bicarbonate, and small amount of sodium hydroxide to adjust the pH. All the reagents used in the experiments were of analytical grade and water was purified with a Millipore system. The CL experiments were carried out in an eppendorf tube cell with transparent bottom and CL signals were recorded on a model MPI A capillary electrophoresis ECL system (Xi’an Remex Electronics Co. Ltd., Xi’ an, China) at room temperature by batch CL method. The intensities were captured by putting cell in dark black (light-tight) box of the luminescent analyser by applying different photomultiplier tube (PMT) voltages to get optimum intensity. A range of band-pass filters were applied to record the wavelength emission spectrum from 400640 nm. Electron paramagnetic resonance (EPR) spectra were


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measured on a Model JES-FA200 spectrometer operating at room temperature. Procedure of lucigenin detection. For a typical CL measurement, first, TCEP (10 μL, 40 mM) was mixed with 380 μL of 0.1 M carbonate buffer (pH=11.4) in an eppendorf tube cell, and then different concentrations of lucigenin solution (10 μL) were injected with the help of a microliter syringe in above solution, thoroughly mixed and immediately used for CL detection. Procedure of TCEP detection. First, a volume of 10 μL of different concentrations of TCEP were mixed with 380 μL of 0.1 M carbonate buffer (pH=11.4) in an eppendorf tube cell, and then lucigenin solution (10 μL, 4 mM) was orderly added by a microliter syringe in above solution, well mixed and directly applied for capturing CL intensity of the resulting solutions. Procedure of SOD detection. First, TCEP (10 μL, 40 mM) was mixed with 375 μL of 0.1 M carbonate buffer (pH=11.4) in an eppendorf tube cell, and then different concentrations of SOD solution (5 μL) were orderly added by a microliter syringe in above solutions and mixed again. Finally, lucigenin solution (10 μL, 4 mM) was added to the above mixed solution, thoroughly mixed and immediately used for recording CL intensity of the resulting solution. Procedure of Hg2+ detection. First, TCEP (10 μL, 40 mM) was mixed with 370 μL of 0.1 M carbonate buffer (pH=11.4) in an eppendorf tube cell, and then EDTA (5 μL, 20 mM) was pipetted into above solution and thoroughly mixed, then different concentrations of Hg2+ solution (5 μL) were orderly injected by a microliter syringe in above solutions. Finally, lucigenin solution (10 μL, 4 mM) was added to the above solution, well mixed and immediately used for CL detection. Procedure of DA detection. First, TCEP (10 μL, 40 mM) was mixed with 375 μL of 0.1 M carbonate buffer (pH=11.4) in an eppendorf tube cell, and then different concentrations of DA solution (5 μL) were injected by a microliter syringe in above solutions. Finally, lucigenin solution (10 μL, 4 mM) was added to the above solution, well mixed and readily applied for CL detection. Procedure of real sample preparation. The present method was further applied to detect Hg2+ and DA in real samples. For Hg2+ real sample application, water samples were analyzed from the Changchun South Lake, China. For DA real sample application, human serum samples were collected from healthy individuals. The serum samples were diluted 100 times to avoid other potentially interfering matrix components in serum samples. The percentage recoveries of the method were evaluated by spiking Hg2+ and DA at three different stated concentrations, 5, 10, 50 nM and 0.05, 0.10 and 1.0 µM, respectively, using standard addition method. All measurements were recorded three times. RESULTS AND DISCUSSION CL of lucigenin-TCEP system. Figure 1 illustrates the CL intensity-time curves for TCEP, lucigenin, lucigenin-H2O2, and lucigenin-TCEP, respectively. It is observed that TCEP and lucigenin alone has nearly no CL emission, while lucigeninH2O2 exhibited weak CL emission. However, upon combined use of TCEP with lucigenin, an intense CL signal was observed. The lucigenin-TCEP pairing leads to a significant ~ 23 times enhancement in CL peak intensity over classic lucigenin-H2O2 system without employing additional catalysts. It proves that the introduced TCEP can efficiently react with lucigenin and is

therefore a potential coreactant for lucigenin CL. As shown in Figure 2, a range of band-pass filters were first applied to record the wavelength-dependent emission spectra in the range from 400 to 640 nm. The maximum CL intensity for lucigeninTCEP system was recorded at ~ 490 nm, which is consistent with the typical spectrum of lucigenin reported in the literature.

Figure 1. Comparison of CL intensity–time curves of TCEP (black line), lucigenin (red line), lucigenin–H2O2 (blue line), and lucigenin–TCEP (green line). The inset shows the enlarged version of TCEP, lucigenin, and lucigenin–H2O2 curves. c(lucigenin), 0.1 mM; c(TCEP), 1 mM; c(H2O2), 1 mM; pH,11.4; PMT, 600 V. We then investigated the effects of the known radical scavengers on the CL intensities to disclose the underlying mechanism. Figure S1 shows the dependence of CL intensities on the concentrations of thiourea, NaN3, and SOD, which are effective radical scavengers of hydroxyl radical (HO•), singlet oxygen (1O2), and O2•–, respectively.43 Interestingly, even at higher concentrations (1-2 mM) of thiourea and NaN3 there was no obvious decrease in CL intensities. In contrast, the addition of SOD even with very low concentrations decreased significantly the CL intensity. This suggests that O2•– plays a critical role in present CL system. It has been reported that TCEP is a very stable, faster and stronger reducing agent,7 which reduces dissolved oxygen in alkaline solution via auto-oxidation reaction to produce ROS, such as O2•–, HO•, and H2O2.44-45 And usually the predominantly generated species is O2•– in such reactions.44, 46 Therefore, we tested the effect of dissolved oxygen on CL of the lucigeninTCEP system. The reacting solution was purged with nitrogen gas for 30 minutes to bubble out the dissolved oxygen. After deaeration of the reacting solution, the CL intensity decreased about 40% (Figure S2). The result showed that the dissolved oxygen was participating in the generation of CL from lucigenin-TCEP system. Therefore, it can be concluded that TCEP can react with dissolved oxygen to generate O2•–. This was confirmed by using SOD as a specific O2•– scavenger during the CL measurements. Upon addition of SOD (0.5 μg/ml), the CL emission completely disappeared (cf. Figure S1). In addition, EPR spectroscopy was performed to further confirm the generation of O2•– in this CL reaction, by using DMPO which is frequently applied as a spin trap to identify O2•–. Although DMPO can react with both HO• and O2•–, it has been reported that above pH 7.7, the reaction between DMPO and O2•– was predominated.47 As seen from Figure S3, the characteristic g factor values of EPR peaks from TCEP, lucigenin and lucigenin-TCEP-DMPO suggested the generation of O2•– in this CL system.48 Scheme 1 displays the proposed CL mechanism of the reaction system. It is possible that two different reaction



mechanisms, in the presence and absence of oxygen, are working simultaneously. In the reaction system, TCEP can not only react with dissolved oxygen to generate O2•–,44 but also can reduce lucigenin (Luc++) through one electron transfer to lucigenin cation radical (Luc•+) which can react with the dissolved oxygen to generate O2•–.49 Then, Luc•+ react with O2•– to generate highly unstable dioxetane intermediate (LucO2) and the primary emitter, singlet N-methylacridone (NMA), and subsequently resulting in intense CL emission (Scheme 1A).50 On the other hand, TCEP is a good nucleophile behaving as an electron donor toward good electron acceptors.51 It has been reported that the oxidation of tertiary phosphines (R3P, TCEP) generates radical species and R3P oxides (R3P=O) as the stable product in the presence of an oxygen-centered nucleophile.52-53 In the presence of lucigenin (Luc++), R3P undergoes rapid oneelectron transfer to give the corresponding Luc•+ and R3P radical cations R3P•+, respectively (eq 1 in Scheme 1B).53 Lucigenin (Luc++) can also convert to Luc•+ by reaction with OH- in alkaline medium (eq 1 in Scheme 1B).49 Due to the presence of an unpaired electron and a positive charge, R3P•+ displays dual reactivities as a free radical and an electrophile which reacts with various nucleophiles such as water, hydroxide anions and so on.53 The nucleophilic attack of H2O or OH- toward the phosphorus atom of R3P•+ generates phosphoranyl radical species (R3P•-OH), which further undergo several radical chain processes (including O2•– and H2O2) and subsequently gives the R3P=O as the final stable product (eq 2 in Scheme 1B).52 Previous reports of unusual reducing properties of R3P by utilizing H2O or OH- as a source of oxygen rather than O2 54 further supports this mechanism. Luc•+ then react with R3P•-OH radicals to generate the excited N-methylacridone (emitter) and R3P=O. Finally, the excited state N-methylacridone relaxes to its ground state, resulting in CL emission (eq 3 in Scheme 1B). In fact, lucigenin has been reported to react with nucleophiles (such as TCEP in this study) in the absence of molecular oxygen or oxidizing agents to generate CL.55 The proposed CL mechanisms were further confirmed by the control experiment shown in Figure S2, where the CL intensity decreased only ~ 40% even after deaeration of the CL solution. Moreover, the effect of several reductants including AA, UA, TD, Creat, Glu, Mann, HA, and GSH was also investigated on the CL generation with lucigenin.45 As shown in Figure S4, TCEP shows much higher CL emission with lucigenin as compared to aforementioned reducing agents under the same experimental conditions. Effect of pH on CL. By studying the lucigenin oxidation mechanism, we known that highly basic medium gives more efficient CL emission. Hence, the effect of pH from ~ 9.2 to 11.9 on lucigenin-TCEP CL system was tested. As displayed in Figure S5, the CL response intensifies sharply with the increment of pH from 9.2 to 11.4, and afterwards it shows no sharp enhancement as pH goes above 11.4. This phenomenon concludes that the sharp rise in CL intensities with the increase of pH up to 11.4 is primarily because of the faster generation of O2•– from TCEP and subsequently highly unstable dioxetane intermediate formation from lucigenin. Therefore, pH value of 11.4 was selected for the subsequent CL experiments due to fast reaction speed and high CL intensity.

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Scheme 1. Schematic drawing of the reaction mechanisms of the lucigenin-TCEP CL system in the presence (A) and absence (B) of oxygen.

Figure 2. (A) Repeated CL intensity–time curves recorded with band pass filters in the wavelength range from 400 to 640 nm, (B) CL intensity versus wavelength spectrum plotted from 400-640 nm. c(lucigenin), 0.1 mM; c(TCEP), 1 mM; pH, 11.4; PMT, 500 V. CL Detection of lucigenin. The lucigenin–TCEP CL system was then applied for sensitive detection of lucigenin. Figure S6 shows the linear relationship between CL intensities and lucigenin concentrations from 0.01 to 55 µM with linear equation of I = -74.65 + 307.44 c (c symbolizes the concentrations in µM) (R2 =0.998). The limit of detection (LOD) is calculated as 4.0 nM at a signal-to-noise ratio of 3 (S/N=3), which is lower than the previous reports.56 The relative standard deviation (RSD) of nine replicate measurements (n=9) of 0.1


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µM lucigenin was 2.43%, indicating the good reproducibility of the CL system. CL Detection of TCEP. By virtue of lucigenin-TCEP sensing platform, a simple, rapid, and sensitive CL method for TCEP detection was developed. Under the optimized conditions, as shown in Figure 3, the CL intensity exhibited the concentration dependent performance for TCEP. The CL intensity increases linearly with the increase of TCEP concentration from 0.1 to 320 µM with linear equation of I = 76.55 + 54.41 c (R2 = 0.999). A low LOD (S/N=3) of 70 nM was achieved for TCEP detection. In comparison with those aforementioned HPLC/ELSD method (20−2000 μM),21 DNTBreduction-based strategy (0.1−10 mM),19 and fluorescent detection method (500-50000 nM),22 our method is simple, rapid, and sensitive. To our knowledge, this is the first lucigenin CL approach for the detection of TCEP. RSD (n=9) of 1 µM TCEP was 2.59%, indicating the good reproducibility of the method.

Figure 3. (A) Repeated CL emission–time curves recorded with varied concentration of TCEP from 0.1 to 320 μM and (B) linear relationship of CL enhancement versus concentration of TCEP from 0.1 to 320 μM. c(lucigenin), 0.1 mM; pH, 11.4; PMT, 700 V. CL Detection of SOD. Based on the lucigenin-TCEP CL, a highly sensitive method was developed for the detection of SOD. Figure 4 shows that the CL intensity of the system strongly quenched with the increase of SOD concentrations because of the strong scavenging effect of SOD on O2•–. The CL intensity decreased linearly with the increase of SOD concentration from 0.005 to 0.5 µg/mL following a typical stern-volmer equation: I0/I=1+KSV[SOD] Where I0 and I symbolizes the CL intensity of the lucigeninTCEP system in the absence (before) and after the SOD addition, respectively. KSV represents the stern-volmer quenching constant, while [SOD] refers to the SOD concentration. The linearity equation is I0/I = 1.01 + 20.24 c

(R2=0.998), with KSV value of 20.24. The LOD (S/N = 3) was calculated as 0.8 ng/mL. In comparison with previous reports for the detection of SOD, our method is simple and sensitive without adding catalysts or luminescent materials (Table S1). Moreover, our method shows good selectivity in the presence of several potential interferences such as Na+, K+, Cl-, SO4-, Glu, UA, AA, H2O2, thiourea, ClO-, NO3-, and Cys (Figure S7). Incidentally, since H2O2 can induce oxidation of TCEP to its oxide TCEPO, a stable and nonreactive product,17 the presence of H2O2 at much higher concentrations may decrease CL of the system. RSD (n=9) of 0.05 µg/mL SOD concentration was 2.20% (Figure S8), indicating the good reproducibility of the CL system.

Figure 4. (A) CL kinetic profiles with the addition of different concentrations of SOD from 0.005 to 0.5 μg/mL; (B) linear calibration curve for SOD. c(lucigenin), 0.1 mM; c(TCEP), 1 mM; pH,11.4; PMT, 500 V. CL Detection of Hg2+. The developed CL system was also applied for selective and sensitive detection of Hg2+. Figure 5A displays that the CL intensities decreased proportionally to the increasing concentrations of Hg2+ from 1.0 to 600 nM. The decrease in CL intensity vs. concentration of Hg2+ follows a linear trend according to a typical stern-volmer equation expression: I0/I=1+KSV[Hg2+] The linear equation was calculated as I0/I = 0.99 + 0.005 c (R2=0.998), with KSV value of 0.005 (Figure 5B). The LOD (S/N = 3) was estimated to be 0.3 nM, which is comparable with several previously reported methods (Table S2) and also satisfies the sensitivity requirement of U.S. EPA. In addition, this method is simple, selective, and sensitive than several other techniques (Table S2). RSD (n=9) of 10 nM Hg2+ was 1.64% (Figure S9), indicating the good reproducibility of the method. Selectivity of the method for Hg2+ detection. The effect of 12 potential interfering typical metal ions including Fe3+, Cu2+,



Co2+, Mn2+, Ca2+, Zn2+, Cd2+, Ni2+, Sn2+, Li+, Na+, K+, and Hg2+ on the CL method was further investigated, respectively (Figure 5C). It has been reported that tertiary phosphines can behave as outstanding ligands towards several transition metals and metal ions.57 The quenching effect may primarily ascribe to the redox reaction of Hg2+ with the TCEP. TCEP has been reported as a weak chelating agent for metal ions such as Cd2+, Zn2+, Pb2+ and Ni2+,57 and it is stable in acidic solutions (i.e. phosphonium salt) towards weak oxidants including Hg2+. In fact, TCEP undergoes a redox reaction with Hg2+ at pH higher than 6.5 to form Hg and TCEPO (P-oxide, a stable and nonreactive product),58 leading to the quenching of CL response. In addition, due to the lower electronegativity of phosphorus, the stability constants corresponding to the bond with a free electron pair are lower than of nitrogen ligands (such as EDTA).58 Therefore, EDTA plays a supportive role in this study and can easily remove the interference from other weakly complexed metal ions. It is important to mention that Hg2+ quenches the CL intensity of this system in the presence or absence of EDTA, which also supports the aforementioned mechanism.

Figure 5. (A) Repeated CL emission-time curves recorded in different concentrations of Hg2+, (B) linear relationship of CL quenching versus concentration of Hg2+ from 1 to 600 nM, (C) selectivity for the detection of Hg2+. The concentration of Hg2+ was 25 µM and all the interfering metal ions were 50 µM for

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selectivity test. c(lucigenin), 0.1 mM; c(TCEP), 1 mM; c(EDTA), 0.25 mM; pH,11.4; PMT, 500 V. Remarkably, no significant decrease in CL intensities was observed upon addition of several metal ions even at higher concentrations (50 µM) than Hg2+ (25 µM). These results proved that the CL method has exquisite selectivity for Hg2+ over several potential interferents. It was found that EDTA could enhance the CL intensity to some extent (Figure S10). It has been reported that the addition of metal chelators (e.g. EGTA) significantly decreased the TCEP stability and catalyzed its oxidation,7 leading to the faster generation of ROS including O2•–, thus enhancing the CL emission. In addition, EDTA can increase the life of radicals/ROS generated during the CL reaction, further improving the CL reproducibility, and CL intensity.59 To the best of our knowledge, this is also the first lucigenin CL method for the detection of Hg2+. CL Detection of DA. By virtue of the efficient sensing platform, a simple and selective method was further developed for sensitive detection of DA. The strong CL quenching of the lucigenin-TCEP system by DA was observed. As shown in Figure 6A, with the increase of DA concentration, the CL intensity was monotonically decreased from 0.01 to 0.8 µM. The linear decrease in CL intensity is well described by sternvolmer equation: I0/I=1+KSV[DA] The linear equation is I0/I = 0.99 + 2.04 c (R2=0.999), with KSV value of 2.04. The LOD (S/N = 3) was calculated as 3.0 nM (Figure 6 B). As shown in Table S3, our method is simple, selective and sensitive, and avoiding troublesome preparations of specific catalysts and luminescent materials. RSD (n=9) of 0.05 μM DA was 2.58% (Figure S11), indicating the good reproducibility of the method. Selectivity of the method for DA detection. In the experiment, the effect of 14 common interfering biological compounds including AA, UA, Cys, Glu, Ala, Tryp, GSH, Meth, Pro, Arg, Thy, Lys, Glut, and Cyto on the detection of DA was investigated. As shown in Figure 6C, none of these showed any notable quenching effect even at very high concentrations of 0.1 mM (concentration of DA was 50 µM), indicating excellent selectivity of the method for DA detection. The good selectivity may stem from the complex roles of DA in the CL system. Firstly, in an alkaline solution DA can be oxidized by ambient O2 to generate DA-quinone species, which act as electron accepters and react with TCEP, making it lose the reduction ability and leading to the CL quenching. A good selectivity can be obtained since TCEP can reduce such quinone species at much faster rate than other antioxidant species.60 Secondly, DA acting as a strong reducing agent will compete with TCEP for radicals generation by donating H+ and e−, thus recycles/inactivates it back to TCEP and thereby prevents the light-yielding reaction.61-62 Thirdly, the reaction of phenolic hydroxyl groups of DA with O2•− produces aryloxy radicals which can be stabilized by the formation of intramolecular hydrogen bonds with ortho-phenolic hydroxyl groups (Figure S12) (resonates to o-benzoquinone structure). Therefore, O2•− were consumed competitively by DA, leading to the decreased CL (Figure S12).63


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Analytical Chemistry analytes may not be seriously interfered with each other due to the different sample media. CONCLUSIONS In this work, we exploited the use of TCEP as an efficient coreactant for lucigenin CL, without the addition of specific catalysts or luminescent materials. The new lucigenin CL system enables sensitive detections of TCEP, lucigenin, SOD, Hg2+ and DA with good reproducibility. Notably, it achieves excellent selectivity in the presence of several biomolecules and common metal ions. In addition, it was successfully applied for Hg2+ and DA real sample detections with excellent recoveries. Since TCEP is an important tertiary phosphine for broad applications in medicinal, biological, and pathological analysis, the developed lucigenin-TCEP CL system may find broad applications in biological and chemical sensing, and clinical analysis.

ASSOCIATED CONTENT Supporting Information Effect of different radical scavengers, effect of oxygen, EPR spectra, effect of different reducing agents, effect of pH, CL effect and detection of lucigenin, selectivity check for SOD, reproducibility check for SOD, Hg2+, and DA, effect of EDTA, proposed reaction mechanism of DA with O2•−, comparison of different methods for the detection of SOD, Hg2+, and DA, respectively, analytical results for the detection of Hg2+ and DA in real samples. The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author * Tel: +86-431-85262661. Fax: +86-431-85262661. E-mail: [email protected].

Author Contributions Figure 6. (A) Repeated CL emission-time curves recorded in different concentrations of DA from 0.01 to 1 µM, (B) linear relationship of CL quenching versus concentration of DA from 0.01 to 1 µM, (C) selectivity for the detection of DA. The concentration of DA was 50 µM and all the other biological interferents were 100 µM for selectivity check. c(lucigenin), 0.1 mM; c(TCEP), 1 mM; pH,11.4; PMT, 500 V. Determination of Hg2+ and DA in real samples. We also applied the developed method for CL detection of Hg2+ and DA in real samples. As shown in Table S4, recovery experiments were carried out to determine the Hg2+ in lake water and DA in human serum samples. Three stated concentrations of 5, 10, 15 nM Hg2+ were added in lake water samples. The human serum samples were diluted hundred times and spiked with three standard DA concentrations of 0.05, 0.10 and 1.0 µM. Finally, the spiked samples were tested and the percentage recoveries were calculated in the range of 94.01–102.3% (Table S4), which proves its applicability for real sample applications. Selectivity note for Hg2+, DA, and SOD detections. The detections of Hg2+, DA, and SOD are based on quenching effects of CL but these substances almost don’t coexists in most different samples to be tested. Hg2+ detection is usually urgent for environmental samples such as water, which is less affected by DA and SOD. Whereas DA exists mainly in biological samples, of which Hg2+ is not a main interference with the detection of DA. Therefore, the detection of all these target

All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This project was kindly supported by the National Key Research and Development Program of China (No. 2016YFA0201300), the CAS President’s International Fellowship Initiative (PIFI) project.

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