In Situ Formation of Metal Coordination Polymer: A Strategy for

Article pubs.acs.org/ac

In Situ Formation of Metal Coordination Polymer: A Strategy for Fluorescence Turn-On Assay of Acetylcholinesterase Activity and Inhibitor Screening Dongli Liao, Jian Chen, Huipeng Zhou, Yan Wang, Yongxin Li, and Cong Yu* State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China Graduate School of the Chinese Academy of Sciences, Beijing 100039, P. R. China S Supporting Information *

ABSTRACT: A novel method for the sensing of acetylcholinesterase (AChE) activity and inhibitor screening based on the formation of metal coordination polymer has been developed. Acetylthiocholine (ATCh) was selected as the substrate. In the presence of AChE, ATCh was hydrolyzed to thiocholine and acetate. Thiocholine interacted with Ag(I) to form a metal coordination polymer. A positively charged perylene probe (probe 1) was employed. The fluorescence of probe 1 was very efficiently quenched by a polyanion [PVS, poly(vinyl sulfonate)]. In the presence of acetylcholinesterase, the positively charged metal coordination polymer newly formed in situ would interact with PVS, probe 1 monomer molecules were released, and a turn on fluorescence signal was detected. The assay is highly sensitive, a limit of detection of 0.04 mU/mL AChE was obtained. The assay is also highly selective, a number of potential interference proteins (enzymes) were tested, and none of them show noticeable interference. Sensing of AChE inhibitor was also demonstrated. Our assay is fairly simple and inexpensive. We envision that it could be used for the sensitive detection of other hydrolytic enzyme activities with properly selected substrates and for the screening of potential inhibitor drugs.

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novel AChE assay strategies. However, certain drawbacks still exist, such as low detection sensitivity, the use of other enzymes for signal amplification and readout, complicated and timeconsuming assay procedures, and expensive materials or instruments. Therefore, the development of a new, highly sensitive, selective, and simple method for the sensing of AChE activity and screening for its potential inhibitors is highly desirable. Herein we report a novel fluorescence turn on method for the ultrasensitive detection of AChE activity and for the sensing of its potential inhibitors. Acetylthiocholine (ATCh) was used as the substrate. AChE hydrolyzed ATCh to give thiocholine. Thiocholine interacted with Ag(I) to form a positively charged metal coordination polymer. A polyanion could induce aggregation and fluorescence quenching of a perylene probe (probe 1). However, in the presence of the positively charged metal coordination polymer, the polycation interacted with the polyanion and caused the release of the free probe 1 monomer molecules, and a fluorescence turn on signal was therefore detected. Our assay is simple, highly sensitive, and selective. Its

cetylcholinesterase (AChE) is a key enzyme in the central and peripheral nervous system. Its primary biological function is the termination of neurotransmission at the cholinergic synapse, by the rapid hydrolysis of acetylcholine (a neurotransmitter) into choline and acetate. The enzyme possesses extraordinarily high catalytic activity, with a rate approaching that of a diffusion controlled reaction.1−6 AChE inhibitors are currently used for the treatment of a number of neuromuscular disorders.7,8 In addition, literature research works have shown that a low level of acetylcholine is closely related to the formation of amyloid fibrils, a possible cause of Alzheimer’s disease (AD), the most common form of dementia among older people worldwide.9−14 Inhibitors of AChE have been suggested to increase the level of acetylcholine and are widely used as drugs to treat AD. Sensing of AChE activity and screening for its potential inhibitors are therefore of great importance. Many efforts have been made to sense the enzymatic activity of AChE and to screen for its potential inhibitors. The widely used traditional UV−vis spectroscopic method offers low detection sensitivity.15 In recent years, a number of fluorescent, colorimetric, and electrochemical AChE assays have been developed.16−28 In addition, advanced materials such as fluorescent conjugated polymers,16 quantum dots,19 and gold nanoparticles24 have also been used for the construction of © 2013 American Chemical Society

Received: October 12, 2012 Accepted: February 4, 2013 Published: February 4, 2013 2667

dx.doi.org/10.1021/ac302971x | Anal. Chem. 2013, 85, 2667−2672

Analytical Chemistry

Article

use for the sensing of AChE inhibitors has also been demonstrated.

Among them, perylene derivatives are of particular interest because of their high fluorescence quantum yield and photostability.29 The perylene probe (probe 1) employed in the current investigation contains four positive charges. It is very soluble in water, shows very little background selfaggregation, and emits very strong green fluorescence at 488 nm. The synthetic polymer used in our study was poly(vinyl sulfonate) (PVS). It is a polyanion and very soluble in an aqueous solution. It is well-known that thiol containing organic compounds could form metal coordination polymers in the presence of Ag(I), Au(I), or Cu(I) ions (Scheme S1, Supporting Information).39−41 Their use for medical applications,39 for synthesis of supramolecular hydrogels,42 organometallic clusters, and 43 nanoparticles,44 and for sensing applications40,41 has been reported. The overall AChE sensing strategy is schematically illustrated in Scheme 1. (1) Acetylthiocholine (ATCh) was selected as the

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EXPERIMENTAL SECTION Materials. Acetylcholinesterase (AChE), poly(vinyl sulfonate) (PVS, sodium salt, MW 4000−6000), acetylthiocholine (ATCh) chloride, butylcholinesterase, and lipase were obtained from Sigma-Aldrich (St Louis, MO). Silver nitrate was purchased from Shanghai Chemical Reagent Co., Ltd. (Shanghai, China). Cupric chloride was from Beijing Chemical Works (Beijing, China). Alkaline phosphatase (ALP) and exonuclease I (Exo I) were purchased from Takara (Dalian, China). The AChE stock solution (2.5 U/μL) was prepared by dissolving AChE with 50% glycerol aqueous solution and stored at −20 °C before use. PVS concentration used in the current investigation was the concentration of the repeating unit. Also it should be noted that the chloride salt (not the iodide salt) of ATCh should be used since iodide ion may have a quenching effect. Other reagents were all of analytical grade and used freshly without further purification. Water was doubly distilled and purified by a Milli-Q system (Millipore, Billerica, MA). The perylene probes (probes 1 and 2) were synthesized according to literature procedures.29 Instrumentation. UV−vis absorption spectra were obtained with a Cary 50 Bio spectrophotometer (Varian Inc., CA). Fluorescence measurements were carried out on a Fluoromax-4 spectrofluorometer (Horiba Jobin Yvon Inc.). Sample solutions were excited at 442 nm, and the emission spectra were recorded with slits for excitation and emission both of 1.5 nm. All spectra were collected at an ambient temperature of 22 °C. Assay Procedures. A volume of 27.5 μL of the substrate (acetylthiocholine, 1 mM), 27.5 μL of Tris-HAc (100 mM, pH 7.4) buffer, and water were mixed with certain amounts of acetylcholinesterase (final sample volume, 400 μL). The sample mixture was incubated at 37 °C for a certain period of time. In total, 7 μL of water, 55 μL of Ag(I) (1 mM), 33 μL of PVS (1 mM), and 55 μL of probe 1 (100 μM) were then added, and the emission spectra were immediately recorded (final sample volume 550 μL). Sensing AChE Activity in Lake Water Samples. Water samples were collected from the South Lake of Changchun (Jilin province, China). The samples were filtered through a 0.22 μm membrane filter. In total, 27.5 μL of the substrate (acetylthiocholine, 1 mM), 27.5 μL of Tris-HAc (100 mM, pH 7.4) buffer, lake water (330 μL), different concentrations of the inhibitor, and 0.1 U/mL AChE were mixed together (final sample volume, 400 μL). The sample mixture was incubated at 37 °C for a certain period of time. A volume of 7 μL of water, 55 μL of Ag(I) (1 mM), 33 μL of PVS (1 mM), and 55 μL of probe 1 (100 μM) were then added, and the emission spectra were immediately recorded (final sample volume 550 μL).

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Scheme 1. Schematic Illustration of the Assay Strategy for the Detection of Acetylcholinesterase Activity

substrate. AChE catalyzed the hydrolysis of ATCh to thiocholine and acetate. (2) The positively changed perylene probe (probe 1) is highly fluorescent in an aqueous buffer solution. The probe 1 aggregates are not fluorescent. A polyanion (PVS) could induce aggregation of probe 1 and result in efficient quenching of the perylene probe monomer fluorescence. (3) When mixed with Ag(I), thiocholine interacted with Ag(I) and formed a metal coordination polymer through S−Ag bonding interactions. Since thiocholine is positively charged, the coordination polymer is a polycation. (4) The positively charged coordination polymer interacted very strongly through electrostatic and hydrophobic interactions with the negatively charged polyanion. The perylene probe aggregates were replaced, probe monomer molecules were released, and a turn on fluorescence was detected. The degree of the restoration of the probe 1 monomer fluorescence was directly related to the amount of AChE added to the assay solution. (5) In the presence of an AChE inhibitor, the enzyme activity was inhibited. Also a lesser degree of probe 1

RESULTS AND DISCUSSION

The acetylcholinesterase (AChE) used in the current investigation was from Electrophorus electricus (electric eel). It is a globular enzyme and composed of four equal subunits (a tetramer). Each subunit contains an active site and has a molecular weight of 70 kDa. Small molecular probes have drawn considerable attentions in recent years. Several novel strategies based on these probes have been developed for various sensing applications.17,18,29−38 2668



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fluorescence recovery was observed, which provides a facile way for the screening of potential AChE inhibitors. The PVS induced perylene probe aggregation was studied. When probe 1 was mixed with PVS, strong electrostatic, hydrophobic, and π−π stacking interactions induced aggregation and strong fluorescence quenching of probe 1. Figure 1

Figure 2. (Upper) Structure of the negatively charged perylene probe (probe 2). (Lower) Positively charged coordination polymer induced probe 2 aggregation and its fluorescence quenching. Conditions: 10 μM probe 2, 50 μM ATCh, 100 μM Ag(I), AChE (0, 20, 40, 60, 80 mU/mL). Buffer: 5 mM Tris-HAc, pH 7.4. Reaction time: 1 h.

coordination polymer as a result of the hydrolysis of ATCh by AChE. UV−vis absorption spectra of the assay mixture also clearly show changes (Figure 3). Without the addition of AChE, probe

Figure 1. (A) Changes in emission spectrum of probe 1 upon the addition of increasing concentrations of poly(vinyl sulfonate). (B) Quenching efficiency at 488 nm versus PVS concentration. Conditions: 1 μM probe 1; buffer, 5 mM Tris-HAc, pH 7.4.

shows that with the increase of the solution PVS concentration, a gradual decrease of the probe 1 monomer fluorescence was observed, which indicates a gradually increased degree of induced probe 1 aggregation. When 6 μM PVS was used, complete quenching of the probe 1 monomer emission (quenching efficiency ∼99.9%) was obtained. One probe 1 molecule contains four positive charges. The results indicate that for efficient quenching, the charge ratio of the perylene probe to PVS needs to be 1:1−1.5 [(1 μM probe 1 × 4): 4−6 μM PVS]. The metal coordination polymer induced perylene probe aggregation was studied. When ATCh was hydrolyzed by AChE, thiocholine was released and it would bond to Ag(I) ions and form a positively charged coordination polymer. Scheme S2 (Supporting Information) and Figure 2 show that the positively charged coordination polymer induced aggregation of the negatively charged perylene probe (probe 2), and strong quenching of probe 2 monomer fluorescence was observed. The degree of fluorescence quenching was very much dependent on the amount of AChE added to the assay solution. In addition, when a positively charged perylene probe (probe 1) was used instead, little induced fluorescence quenching was observed (Figure S1, Supporting Information). The results strongly suggest the in situ formation of the positively charged

Figure 3. UV−vis absorption spectra of the assay mixture with (1) or without (2) the addition of AChE. Conditions: 10 μM probe 1, 60 μM PVS, 50 μM ATCh, 100 μM Ag(I), 1.0 U/mL AChE. Buffer: 5 mM Tris-HAc, pH 7.4. Reaction time: 1 h.

1 molecules were mostly in the aggregated forms, and in the presence of AChE, probe 1 molecules were mostly in the free monomeric form.29 The results are in consistent with the design strategy, that is the recovery of the probe 1 monomer emission was a result of the transition of the aggregated forms of probe 1 to the free monomeric molecules. The assay conditions were optimized to get the best sensing performance. The optimized conditions are as follows: 10 μM probe 1, 60 μM PVS, 50 μM ATCh, 100 μM Ag(I), and 1 h enzymatic reaction time (Figures S2−S4, Supporting Information). Figure 4 shows that with the addition of increasing concentrations of AChE, the emission intensity of probe 1 gradually increased. Also the changes in emission could be clearly seen by the naked eye (inset of Figure 4). The detection limit of our assay was estimated to be 0.04 mU/mL, which is 2669



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when AChE was added to the assay solution of ATCh and Ag(I), almost complete recovery of probe 1 monomer emission was observed (over 800-fold probe 1 emission intensity increase) (Figure 5I). The results further confirm that the observed probe 1 emission recovery was a result of AChE catalyzed hydrolysis of ATCh. Selectivity of the assay was studied (Figure 6). A number of proteins/enzymes were tested. They were alkaline phosphatase

Figure 4. (A) Changes in emission spectrum of probe 1 as a function of AChE concentration. Inset: probe 1 emission intensity changes at 488 nm with AChE concentration. Photographs: (1) no AChE, (2) with 0.1 U/mL AChE added. Conditions: 10 μM probe 1, 60 μM PVS, 50 μM ATCh, 100 μM Ag(I). Buffer: 5 mM Tris-HAc, pH 7.4. Reaction time: 1 h.

among the most sensitive AChE assay methods reported to date (Figure S5, Supporting Information). Also it is about 100− 1000 times more sensitive than a number of novel fluorometric methods, such as the fluorescence conjugated polymer, quantum dot, and aggregation induced emission based methods developed in recent years.16−28 The total time needed for the assay was estimated to be about 65−70 min including the sample preparation, the incubation time, and the detection process. Kinetic analysis for the ATCh hydrolysis catalyzed by AChE was performed (Figure S7, Supporting Information). A KM value of 34.9 μM and a Vmax value of 3.12 μM min−1 were obtained. These values are comparable to those reported in the literature.15 Figure 5 shows that when probe 1 was mixed with either AChE, Ag(I), ATCh, or ATCh + Ag(I), hardly any probe 1

Figure 6. Selectivity of the assay: (A−H) AChE, ALP, Exo I, Hinc II, collagenase, lysozyme, lipase, and blank, respectively. The experimental conditions were the same as those described in Figure 4. Enzyme concentration: 1.0 U/mL each. Reaction time: 1 h.

(ALP), exonuclease I (Exo I), restriction endonuclease Hinc II, collagenase, lysozyme, and lipase. The results show that none of these proteins gave any noticeable probe 1 fluorescence recovery, which clearly suggests that these proteins did not interfere with the assay, and the assay is highly specific for AChE. Acetylcholine and acetylthiocholine are known substrates of butylcholinesterase (BChE).45,46 It is therefore understandable that BChE could also recover the probe 1 monomer fluorescence (Figure S9, Supporting Information). The results suggest that our assay could also be used for the sensing of BChE activity. The inhibition effect of two AChE inhibitors (donepezil and 3-hydroxycarbofuran) was tested.16,25 With the addition of an AChE inhibitor to the assay solution, the AChE activity would be reduced (inhibited), and the probe 1 monomer fluorescence recovery would be reduced accordingly depending on the amount and type of the inhibitor added. Figure 7 shows that the F/F0 value decreased with the addition of increasing concentrations of the inhibitor.47 The results clearly show that our assay could be used for the sensing of AChE inhibitors. The detection limit was estimated to be 100 pM for donepezil and 250 pM for 3-hydroxycarbofuran. The total time needed for the inhibition study was estimated to be about 20−25 min including the sample preparation, the incubation time, and the detection process. In addition, known quantities of the AChE inhibitors were added to the lake water samples, and their inhibition effects were monitored. Figure 7 shows that the changes in F/F0 value are similar to those obtained with samples prepared in deionized water. The results clearly suggest that our assay could be applied to the detection of AChE inhibitor in a real sample.

Figure 5. Changes in emission intensity of probe 1 at 488 nm under different assay conditions: (A) probe 1 only; (B−E) probe 1 + AChE, Ag(I), ATCh, or ATCh + Ag(I), respectively; (F) probe 1 + PVS; (G) probe 1 + PVS + ATCh + Ag(I); (H) probe 1 + PVS + ATCh + AChE; (I) probe 1 + PVS + ATCh + Ag(I) + AChE. Conditions: 10 μM probe 1, 60 μM PVS, 50 μM ATCh, 100 μM Ag(I), 1.0 U/mL AChE. Buffer: 5 mM Tris-HAc, pH 7.4. Reaction time: 1 h.

emission intensity changes were observed. The results clearly suggest that these compounds did not interfere with the assay. However, when probe 1 was mixed with PVS, complete quenching of probe 1 monomer emission was observed. Addition of ATCh and Ag(I) did not make a noticeable probe 1 emission recovery. In addition, the addition of free thiocholine (ATCh + AChE) without Ag(I) also did not make noticeable probe 1 emission recovery (Figure 5H). However,

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CONCLUSIONS In conclusion, an ultrasensitive and selective assay method for AChE activity and inhibitor screening has been developed. AChE catalyzed the hydrolysis of ATCh, and thiocholine was 2670



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ASSOCIATED CONTENT

S Supporting Information *

Additional information as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: (+86)-431-85262710. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the “100 Talents” program of the Chinese Academy of Sciences, the National Natural Science Foundation of China (Grants 21075119, 91027036, and 21275139), the National Basic Research Program of China (973 Program, Grant No. 2011CB911002), and the Pillar Program of Changchun Municipal Bureau of Science and Technology (Grant No. 2011225).

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REFERENCES

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Figure 7. Changes in F/F0 value as a function of the inhibitor concentration: (A) donepezil and (B) 3-hydroxycarbofuran. F0 was the recovered probe 1 emission intensity at 488 nm without the addition of an inhibitor. Conditions: 10 μM probe 1, 60 μM PVS, 50 μM ATCh, 100 μM Ag(I), 0.1 U/mL AChE. Buffer: 5 mM Tris-HAc, pH 7.4. Reaction time: 15 min. Black line, samples prepared in deionized water, red line, samples prepared in lake water.

released. Thiocholine interacted with Ag(I) and produced a positively charged coordination polymer in situ. PVS could induce aggregation and complete quenching of probe 1 monomer fluorescence. The newly formed coordination polymer interacted with PVS and resulted in the release of the probe 1 monomer. A turn on fluorescence signal was therefore detected. The intensity increase of probe 1 was directly related to the amount of AChE added to the assay solution. The assay is highly sensitive, and a limit of detection of 0.04 mU/mL AChE was obtained. The assay is also highly selective. A number of unrelated proteins/enzymes were tested, and none of them gave noticeable interference. Two AChE inhibitors were also tested, and clear inhibition effects were observed. In addition, our assay offers a “mix and detect” protocol, which is quite simple and convenient. It is based on a fluorescence “turn-on” mode, which could considerably reduce the likelihood of false-positive signals associated with other methods. The perylene probe could be easily prepared, and all materials used are fairly inexpensive. Thus, the assay is fairly cost-effective. We envision that our assay method could be used for the detection of AChE activity in related biological/ biochemical research areas and for the high-throughput screening of AChE inhibitors as potential drugs for the effective treatment of Alzheimer’s disease. 2671



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In Situ Formation of Metal Coordination Polymer: A Strategy for

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