A Luminescence Turn-On Assay for Acetylcholinesterase Activity and


Jan 29, 2019 - A Luminescence Turn-On Assay for Acetylcholinesterase Activity and Inhibitor Screening Based on Supramolecular Self-Assembly of ...
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A Luminescence Turn-On Assay for Acetylcholinesterase Activity and Inhibitor Screening Based on Supramolecular Self-Assembly of Alkynylplatinum(II) Complexes on Coordination Polymer Angela Sin-Yee Law, Margaret Ching-Lam Yeung, and Vivian Wing-Wah Yam* Institute of Molecular Functional Materials (Areas of Excellence Scheme, University Grants Committee (Hong Kong)) and Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, People’s Republic of China

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S Supporting Information *

ABSTRACT: A new approach toward acetylcholinesterase (AChE) detection has been demonstrated based on the electrostatic interactions between anionic alkynylplatinum(II) complex molecules and cationic coordination polymer, together with the spectroscopic and emission characteristics of alkynylplatinum(II) complexes upon supramolecular selfassembly. This process involves strengthening of distinct noncovalent Pt(II)···Pt(II) and π−π stacking interactions, which is evidenced by UV−vis absorption, emission, and resonance light scattering results. Such a method has been applied to AChE inhibitor screening, which is important as the demand for AChE inhibitor assays arises along with the drug development for Alzheimer’s disease. It affords an emission turn-on response and operates in a continuous and label-free fashion. The low-energy red emission and large Stokes shift of alkynylplatinum(II) complexes are advantageous to biological applications. KEYWORDS: acetylcholinesterase assay, aggregation, biosensors, platinum, supramolecular chemistry

1. INTRODUCTION Abnormal enzymatic activities are closely associated with the growth of diseases, and accordingly, a variety of probes have been developed for sensitive detection of disease-relevant enzymes.1−3 Alzheimer’s disease, one of the most commonly encountered dementia, is defined as the chronic neurodegenerative disease characterized by symptoms including short-term memory loss, primary progressive aphasia, and mental disorientation.4 It affects a great proportion of aged people and costs hundreds of billions a year in different areas such as medical diagnosis and hospitalization.5 Acetylcholinesterase (AChE) is an enzyme that is believed to accelerate the abnormal aggregation of amyloid β peptides into Alzheimer’s amyloid fibrils in the central nervous system for inducing Alzheimer’s disease.6,7 At present, a number of AChE inhibitors are employed in medication treatments of patients with Alzheimer’s disease, such as donepezil,8 galantamine,8 rivastigmine,8 and tacrine.9 As a result, significant attention has been drawn to the development of assays for AChE activity and screening of inhibitors. A number of methods have been developed. These include Ellman’s test,10 fluorometric approaches,11−15 electrochemical assays,16,17 and so on. Although Ellman’s test remains widely used, increasing analytical studies suggest that there are drawbacks and limitations because of false-positive effects and poor precision.18,19 Also, due to the lower sensitivities of traditional © XXXX American Chemical Society

UV−vis assays in comparison to luminescence assays, one of the most attractive strategies to determine AChE activity is to develop a convenient, sensitive, and simple analytical approach based on luminescence sensing. Luminescent d8 platinum(II) complexes have drawn a lot of attention and are one of the most important classes of complexes in supramolecular chemistry.20 This is attributed to the square-planar molecular geometry and the ability of platinum(II) complexes to form noncovalent Pt(II)···Pt(II) and π−π stacking interactions.21−47 One example is the solvent-induced aggregation of alkynylplatinum(II) terpyridine complexes, which was first reported in 2002.48 Later, various studies have been performed on the supramolecular selfassembly of alkynylplatinum(II) terpyridine complexes upon variations in counteranions,49 temperature,50 pH,51 addition of anionic polymers,52 and so on. Apart from alkynylplatinum(II) terpyridine complexes, platinum(II) 2,6-bis(benzimidazol-2′yl)pyridine (bzimpy) complexes have also been designed and synthesized, which can display interesting solvent-dependent spectroscopic and morphological properties.53 At the same time, alkynylplatinum(II) complex-based assays for biological analytes including single-stranded nucleic acid,54 G-quadruReceived: October 25, 2018 Accepted: January 14, 2019

A

DOI: 10.1021/acsami.8b18739 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces plex,55,56 enzymes,55,57−60 glucose,58 human serum albumin,61 heparin,62 and phosphate derivatives59 have been achieved. When compared to traditional organic probes, platinum(II) complexes exhibit low-energy red to near-infrared luminescence properties and do not easily suffer from autofluorescence. Also, owing to the large Stokes shift of platinum(II) complexes, the signal-to-noise ratio and the sensitivity of assays are distinctly improved due to the absence of self-absorption and the reduction in scattering interference, which is beneficial to biological assays and diagnostic applications.63 Thanks to the rich spectroscopic and emission characteristics of platinum(II) complexes that arise from noncovalent Pt(II)···Pt(II) and π−π stacking interactions, one can envisage that alkynylplatinum(II) complexes possess potential ability in the continuous monitoring of biological processes. Also, to the best of our knowledge, sensing approach based on the supramolecular self-assembly of alkynylplatinum(II) complexes on organometallic coordination polymers has not been previously reported. This has provided us the inspiration to investigate the capability of this approach to detect AChE activity with higher sensitivity and to attempt earlier detection of Alzheimer’s disease. In this work, an anionic alkynylplatinum(II) complex 1 has been employed in the luminescence turn-on assay for AChE activity and inhibitor screening (Figure 1). In the presence of AChE, acetylthiocho-

2. RESULTS AND DISCUSSION The complexation ability of thiocholine toward Ag(I) ions has been examined. According to the UV−vis absorption spectra, upon introduction of both thiocholine and Ag(I) ions at pH 8.0, the low-energy absorption tails at around 380 nm are found to increase in absorbance (Figure S1 in the Supporting Information), which are absent without the addition of Ag(I) ions (Figure S2). Based on previous studies on Ag(I)−thiolate coordination polymers such as Ag(I)−thioglycolic acid,64 Ag(I)−cysteine, 65 and Ag(I)−4-mercaptophenylboronic acid,66 such absorption tails can be attributed to ligand-tometal−metal charge transfer (LMMCT) transition, which is associated with the formation of Ag(I)···Ag(I) or argentophilic interactions.67,68 To further verify the formation of Ag(I)− thiocholine coordination polymer, the size distribution profile of particles in a solution containing thiocholine and Ag(I) ions has been analyzed using dynamic light scattering (DLS) technique. The DLS data show that the average hydrodynamic diameter is around 200 nm, which can be ascribed to the presence of large aggregates due to coordination polymerization (Figure S3). The effect of pH on the formation of Ag(I)−thiolate coordination polymers has also been studied. At pH 5.0, it is found that the LMMCT absorption tails characteristic of the Ag(I)−thiolate coordination polymers at around 380 nm are enhanced gradually upon introduction of both thiocholine and Ag(I) ions, which is similar to that observed at pH 8.0 (Figure S4). This implies that lowering the pH value does not affect the electrostatic interactions among adjacent ligands as well as the argentophilic interactions along the Ag(I)−thiocholine polymeric backbone. On the other hand, at pH 10.0, the LMMCT absorption tails increase to a smaller extent, and accordingly, it is anticipated that the argentophilic interactions are weakened at high pH (Figure S5). Cationic Ag(I)−thiocholine coordination polymer-induced supramolecular self-assembly of 1 has been examined. At pH 8.0, in the presence of Ag(I) ions, the absorbance of absorption tails at around 550 nm is found to increase when the amount of thiocholine increases in Tris−HAc buffer solution (Figure S6). By analogy to the previously reported spectral assignment on platinum(II) bzimpy complexes,53,69−71 the low-energy absorption tails are typical of metal−metal-to-ligand charge transfer (MMLCT) transitions. At the same time, the lowenergy luminescence at 650 nm displays an intensity enhancement, which is assigned to the triplet MMLCT (3MMLCT) excited state, indicative of noncovalent Pt(II)··· Pt(II) and π−π stacking interactions associated with aggregate formation (Figure 2). It is worth mentioning that the addition of Ag(I) ions does not affect the luminescence behavior of 1 (Figure S7). The excitation spectra of a solution containing 1 and Ag(I) ions in the absence and presence of thiocholine are compared. It is shown that the excitation spectrum exhibits a growth in intensity at around 550 nm upon introduction of thiocholine, which corresponds to the region of MMLCT

Figure 1. Chemical structure of complex 1.

line (ATCh) can be hydrolyzed to produce thiocholine (Scheme 1). It has been found that upon the addition of Ag(I) ions, thiocholine can form a cationic Ag(I)−thiocholine coordination polymer in situ in aqueous solution (Scheme 2). The supramolecular self-assembly properties of the anionic complex 1 upon introduction of this in situ formed oppositely charged coordination polymer have been analyzed, and the capability of the complex to monitor AChE activity has been investigated. Since AChE inhibitors are used in the central nervous system nowadays to alleviate Alzheimer’s disease, the feasibility of this assay to screen inhibitors has also been examined.

Scheme 1. Reaction Scheme for the Hydrolysis of Acetylthiocholine Catalyzed by AChE

B

DOI: 10.1021/acsami.8b18739 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Scheme 2. Reaction Scheme for the Formation of Ag(I)−Thiocholine Coordination Polymer between Thiocholine and Ag(I) Ions

Figure 2. (a) Corrected emission spectra of 1 (50 μM) in Tris−HAc buffer (10.0 mM, pH = 8.0) containing AgNO3 (100 μM) and various concentrations of thiocholine (0−50 μM). (b) Plot of relative emission intensity at 650 nm against thiocholine concentration.

emission intensity of the ensemble does not show any apparent decay, indicating a satisfactory photostability of the system (Figure S12). The supramolecular self-assembly of 1 onto the cationic Ag(I)−thiocholine coordination polymer has been found to be pH-sensitive. At pH 5.0, addition of thiocholine to 1 in the presence of Ag(I) ions results in an emergence of the MMLCT absorption tails at 550 nm (Figure S13), as well as an enhancement of the 3MMLCT emission bands at 650 nm that are typical of aggregates of 1 (Figure S14). It is believed that the electrostatic interactions between 1 and the coordination polymer allow the platinum(II) complex molecules to assemble together into close proximity to form aggregates. On the other hand, at pH 10.0, insignificant changes in the MMLCT absorption tails are observed upon increasing the concentration of thiocholine (Figure S15). In addition, the 3 MMLCT emission bands show minor changes in intensity (Figure S16). It is unlikely that 1 has a tendency to aggregate onto the coordination polymer, probably as a result of the weakening of argentophilic interactions at high pH, which sets the cationic sites on the polymer further apart. Apart from Ag(I)−thiocholine coordination polymer, Au(I)−thiocholine coordination polymer has been explored for inducing the supramolecular self-assembly of 1. According to the literature,72−74 Au(I)−thiolate coordination polymers can be prepared by the addition of thiolates to Au(III) ions. After Au(III) ions are reduced to Au(I) ions by thiolates, Au(I)−thiolate coordination polymers are formed.72−74 In the presence of Au(III) ions and thiocholine, the supramolecular self-assembly of 1 has been examined by a number of

absorption of platinum(II) complexes (Figure S8). To affirm that the spectral changes are a result of aggregation, resonance light scattering (RLS) study has been carried out. It is noted that the intensity of the band at around 550 nm, which originated from the highly ordered supramolecular structures formed by platinum(II) complexes,56,71 increases gradually, further supporting the aggregation of the platinum(II) complex molecules induced by the in situ formation of cationic coordination polymer via electrostatic interactions (Figure S9). In addition, the particle size distribution of 1 in Tris−HAc buffer solution has been analyzed by DLS technique. As a control, DLS measurement has been performed on 1 in the presence of Ag(I) ions in the Tris−HAc buffer solution, confirming that the complexes are relatively dispersed and the average hydrodynamic diameter is around 10 nm (Figure S10). In the presence of thiocholine, the average hydrodynamic diameter increases to around 260 nm. This obvious shift in particle size further supports the appearance of aggregates between the platinum(II) complex molecules and coordination polymer. To further affirm the formation of supramolecular self-assembly of 1, transmission electron microscopy (TEM) has been conducted to investigate the morphological features of the sample. As shown in the TEM image, an inherent sheetlike structure is observed (Figure S11). The sheets are probably made up of multilayer structures. From the TEM study, it is suggested that platinum(II) complexes undergo selfassembly to form supramolecular structures, implying the possible aggregation of the platinum(II) complex molecules onto the coordination polymer. More importantly, when the 1−thiocholine ensemble is continuously scanned by the spectrofluorometer in the presence of Ag(I) ions, the relative C

DOI: 10.1021/acsami.8b18739 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Scheme 3. Schematic Illustration of the Design Rationale for the Luminescence Turn-On Assay for AChE Using Anionic Alkynylplatinum(II) Bzimpy Complex, Acetylthiocholine, and Ag(I) Ions

Figure 3. (a) Corrected emission spectra of 1 (50 μM) in Tris−HAc buffer (10.0 mM, pH = 8.0) containing acetylthiocholine (50 μM), AgNO3 (100 μM), and AChE (0.08 U mL−1) after incubation at 37 °C for different times. (b) Plot of relative emission intensity at 650 nm against incubation time.

hydrolysis of acetylthiocholine to thiocholine, followed by the in situ formation of cationic Ag(I)−thiocholine coordination polymer (Scheme 3). At the same time, an increase in emission intensity is observed for the 3MMLCT emission band at 650 nm (Figure 3). The longer the incubation time, the higher is the emission intensity. Also, RLS study shows a gradual increase in the intensity of the band at around 550 nm upon introduction of AChE, further supporting the fact that the electrostatic interactions between the positively charged coordination polymer and the negatively charged platinum(II) complex molecules promote the strengthening of noncovalent Pt(II)···Pt(II) and π−π stacking interactions (Figure S21). At the same time, a control experiment has been carried out. It demonstrates that the addition of 1 to a solution containing AChE-hydrolyzed acetylthiocholine and Ag(I) ions has a similar effect on the supramolecular self-assembly and emission properties of 1 as compared to that obtained from the abovementioned sensing strategy, which indicates that the spectral changes are due to the aggregation of platinum(II) complex molecules by the AChE-catalyzed hydrolysis of acetylthiocholine (Figure S22).

spectroscopic techniques. Accompanying the growth of the MMLCT absorption tails at 550 nm (Figure S17) is the emergence of the 3MMLCT emission bands at 650 nm typical of aggregates of 1 (Figure S18). At the same time, it is noted that the RLS intensity at around 550 nm is enhanced progressively (Figure S19). All these spectroscopic changes have been attributed to aggregation formation of 1 via noncovalent Pt(II)···Pt(II) and π−π stacking interactions. It has been proposed that the supramolecular self-assembly of 1 is induced by the electrostatic interactions between the negatively charged platinum(II) complex molecules and the oppositely charged Au(I)−thiocholine coordination polymer formed. However, the photostability of Au(I)−thiocholine coordination polymer is not satisfactory, making it not applicable to detect AChE in a continuous fashion. Attempts have been made to understand the effects of AChE on the supramolecular self-assembly of 1 by conducting a number of spectroscopic studies. As shown in the UV−vis absorption spectra of 1−acetylthiocholine ensemble in the presence of Ag(I) ions, addition of AChE results in an obvious enhancement in the MMLCT absorption tail (Figure S20). This is ascribed to the aggregation of 1 by the AChE-catalyzed D

DOI: 10.1021/acsami.8b18739 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces After the introduction of various concentrations of AChE, the time-dependent emission changes of 1−acetylthiocholine ensemble at 650 nm are compared (Figure 4). It is apparent

Figure 5. Plots of relative emission intensity of 1 (50 μM) at 650 nm in Tris−HAc buffer (10.0 mM, pH = 8.0) against incubation time containing AgNO3 (100 μM), AChE (0.08 U mL−1), and various concentrations of acetylthiocholine (0, 10.0, 20.0, 30.0, 45.0, and 50.0 μM) after incubation at 37 °C for different times.

Figure 4. Plots of relative emission intensity of 1 (50 μM) at 650 nm in Tris−HAc buffer (10.0 mM, pH = 8.0) against incubation time containing acetylthiocholine (50 μM), AgNO3 (100 μM), and various concentrations of AChE (0, 0.01, 0.03, 0.06, 0.07, and 0.08 U mL−1) after incubation at 37 °C for different times.

μM min−1, respectively. The value of Km is in good agreement with those previously reported in the literature.10,78,79 This result shows that the design rationale of this assay and the addition of 1 will not affect the AChE activity, establishing the suitability of this assay for biological applications. Screening for AChE inhibitors has attracted increasing attention over the last decade, as it can be employed in drug discovery to develop new pharmaceutical drugs for Alzheimer’s disease. One can anticipate that the AChE-catalyzed hydrolysis of acetylthiocholine will slow down after the introduction of AChE inhibitors. For the purpose of examining the feasibility of screening AChE inhibitors using this assay, neostigmine, which is one of the representative inhibitors for AChE, has been selected for the study. Variation of the relative emission intensity at 650 nm of 1 has been plotted against the hydrolysis time with different concentrations of neostigmine added (Figure 6). The addition of neostigmine leads to a smaller emission growth, and the initial reaction rate drops when the neostigmine concentration increases. In accordance with the relationship between the relative emission intensity and the concentration of thiocholine in the presence of Ag(I) ions (Figure 2b), the concentration of thiocholine at a certain time

that the emission change is dependent on AChE concentration. In the absence of AChE, insignificant changes in luminescence intensity are observed. By increasing the concentration of AChE, a higher initial hydrolysis rate can be achieved and a larger degree of emission turn-on can be observed. Owing to the strong correlation between the relative emission intensity and the concentration of thiocholine in the presence of Ag(I) ions (Figure 2b), the concentration of thiocholine at a certain time can be plotted against the incubation time upon addition of various concentrations of AChE (Figure S23a). The limit of detection of this assay is estimated to be 18.5 mU mL−1,75 which is more sensitive than that of some reported methods,12,14,76 indicating that this luminescence turn-on assay can be employed to detect AChE with satisfactory sensitivity (Figure S23b). To study the enzyme kinetics, the initial rate of AChEcatalyzed hydrolysis reaction has been measured by varying the substrate concentration. The relative emission intensity at 650 nm has been plotted against the reaction time with different concentrations of acetylthiocholine added (Figure 5). It has been found that the initial hydrolysis rate depends crucially on the initial acetylthiocholine concentration. Without the addition of acetylthiocholine, the emission band at 650 nm showed minor changes in intensity even after the addition of AChE. In the presence of acetylthiocholine, it is shown that the extent of emission enhancement of 1 increases with initial substrate concentration. According to the relationship between the relative emission intensity and the concentration of thiocholine in the presence of Ag(I) ions (Figure 2b), the concentration of thiocholine at a certain time can be plotted against the incubation time with various concentrations of acetylthiocholine added (Figure S24a). The Lineweaver−Burk plot for the determination of Km and Vmax of AChE-catalyzed hydrolysis of acetylthiocholine is acquired for this luminescence assay, in which the inverse of the initial reaction rate, 1/Vo, is plotted against the inverse of the substrate concentration, 1/[S] (Figure S24b). The y-intercept of the resulting straight line is 1/Vmax, whereas its slope is Km/Vmax.77 Km and Vmax values are determined and are 149 μM and 7.22

Figure 6. Plots of relative emission intensity of 1 (50 μM) at 650 nm in Tris−HAc buffer (10.0 mM, pH = 8.0) against incubation time containing acetylthiocholine (50 μM), AgNO3 (100 μM), AChE (0.08 U mL−1), and various neostigmine concentrations (0, 2.0, 5.0, 20.0, 50.0, and 100.0 nM) after incubation at 37 °C for different times. E

DOI: 10.1021/acsami.8b18739 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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revealing that the present assay is a highly selective and specific sensing platform. As AChE is present in both bovine serum and human serum, determination of AChE activity in serum solutions is important for a number of applications such as medical diagnosis. To achieve this aim, AChE activity has also been determined by this assay in commercially available adult bovine serum as well as human serum. Under the same assay conditions, the emission intensity of 1 is measured in diluted serum solutions with acetylthiocholine, Ag(I) ions, and AChE added. The result obtained is comparable to that performed in Tris−HAc buffer solution, hence the present sensing approach can be applied in biological samples (Figure 7).

can be plotted against the incubation time upon introduction of various concentrations of neostigmine (Figure S25a). To measure the effectiveness of neostigmine in inhibiting AChE activity, the half maximal inhibitory concentration (IC50), which is the amount of an inhibitor needed to inhibit the activity of enzyme by half, is evaluated. By constructing a plot of inhibition efficiency against the concentration of inhibitor, the IC50 value is found to be 37.3 nM, which is comparable to that reported by previous literature with similar assay conditions (Figure S25b).80 This result indicates that the present methodology can provide a sensitive strategy for screening AChE inhibitors. Meanwhile, control experiments have been carried out. When 1 is mixed with acetylthiocholine, Ag(I) ions, and AChE, a drastic emission enhancement is noticed. On the contrary, after it is mixed with either acetylthiocholine, Ag(I) ions, or AChE alone, no emission intensity change is observed, implying that these components do not cause any interference to the assay (Figure S26). Addition of acetylthiocholine and Ag(I) ions, acetylthiocholine and AChE, or Ag(I) ions and AChE, respectively, to 1 does not trigger the emission turn-on as well. The Ag(I)−thiocholine coordination polymer is also nonemissive. These findings further verify that the emission turn-on observed is a consequence of the aggregation of 1 due to AChE-catalyzed hydrolysis. As there is a strong complexation ability of thiolates toward Ag(I) ions,64−66 it is essential that biologically related sulfurcontaining compounds other than thiocholine will not bring about any emission change. Here, when thiols including thiocholine, coenzyme A, L-cysteine, D-cysteine, DL-cysteine, and L-glutathione are added to 1 in the presence of Ag(I) ions, respectively, only thiocholine gives a positive response in emission intensity (Figure S27). L-Cystine and L-methionine, which are sulfur-containing amino acids, are found not to cause any emission turn-on, meaning that this sensing technique is very selective to thiocholine and will not be affected by other sulfur-containing compounds. As this assay allows AChE detection upon addition of Ag(I) ions, it is necessary to evaluate the selectivity toward Ag(I) ions. The emission intensity is measured after adding various interfering metal ions respectively instead of Ag(I) ions to 1− acetylthiocholine ensemble under the same assay conditions. The result indicates that the addition of Au(I) ions can lead to similar emission change as compared to that of Ag(I) ions. As it is well-known that thiolates can also form coordination polymers with Au(I) ions,72−74 it is believed that such emission change is ascribed to the formation of cationic Au(I)−thiocholine coordination polymer and subsequently the aggregation of 1 (Figure S28). In contrast, the emission change is negligible after the introduction of other metal ions. In competition experiments, the simultaneous addition of Ag(I) ions and the respective interfering metal ions do not exert much influence on the emission turn-on, indicating the excellent resistance of this assay against metal-ion interference. For the purpose of evaluating the selectivity and specificity of the present assay, several proteins have been selected for control experiments. These proteins are examined under the above-mentioned assay conditions. It is found that the emission intensity is enhanced only upon addition of AChE, with no emission turn-on upon introduction of other proteins (Figure S29). Simultaneous addition of a mixture of AChE and the respective interfering proteins give emission intensity changes that are similar to that of adding AChE alone,

Figure 7. Analysis for the practicality of the AChE assay. Relative emission intensity of 1 (50 μM) at 650 nm containing acetylthiocholine (ATCh) (50 μM), AgNO3 (100 μM), and AChE (0.08 U mL−1) after incubation at 37 °C for 30 min in (A) Tris−HAc buffer solution (10.0 mM, pH = 8.0), (B) diluted adult bovine serum, and (C) diluted human serum. The emission intensity was relative to that of 1 (50 μM) containing acetylthiocholine (ATCh) (50 μM) and AgNO3 (100 μM) in the corresponding media.

3. CONCLUSIONS We have successfully developed a luminescence turn-on assay for monitoring AChE activity in a continuous and label-free fashion based on the supramolecular self-assembly of alkynylplatinum(II) complexes. The success of this convenient and sensitive analytical approach is built upon the interesting spectroscopic and supramolecular self-assembly behaviors of platinum(II) complexes through noncovalent Pt(II)···Pt(II) and π−π stacking interactions. Upon addition of AChE, acetylthiocholine undergoes hydrolysis to thiocholine, leading to the in situ formation of a cationic Ag(I)−thiocholine coordination polymer. The electrostatic interactions between the anionic platinum(II) complex molecules and the cationic coordination polymer are responsible for facilitating aggregation and a low-energy red emission turn-on. The present work can detect AChE at a concentration as low as 18.5 mU mL−1. Other applications such as inhibitor screening have been introduced, which is important for drug development for Alzheimer’s disease. Also, this assay allows monitoring of AChE activity in serum solutions and is inherently suitable for exploring the biological activity of enzymes in biological specimens. 4. EXPERIMENTAL SECTION 4.1. Materials and Reagents. Silver nitrate (Alfa Aesar, >99%), potassium tetrachloroaurate(III) (Strem Chemicals, >99%), acetylthF

DOI: 10.1021/acsami.8b18739 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces iocholine iodide (Sigma-Aldrich, ≥98%), acetylcholinesterase from Electrophorus electricus (electric eel) (Sigma-Aldrich), neostigmine bromide (Alfa Aesar), coenzyme A sodium salt hydrate (SigmaAldrich), L-cysteine (Alfa Aesar, ≥98%), D-cysteine (Alfa Aesar, 98%), DL-cysteine (TCI Chemicals, >98%), L-glutathione (reduced) (Alfa Aesar, ≥98%), L-cystine (Alfa Aesar, 99%), L-methionine (Alfa Aesar, >98%), α-amylase from human saliva (Sigma-Aldrich), albumin from bovine serum (Sigma-Aldrich), albumin from human serum (SigmaAldrich), alkaline phosphatase from bovine kidney (Sigma-Aldrich), insulin from bovine pancreas (Sigma-Aldrich), lysozyme from chicken egg white (Sigma-Aldrich), trypsin from bovine pancreas (SigmaAldrich), adult bovine serum (Sigma-Aldrich), and human serum (Sigma-Aldrich) were purchased from the corresponding chemical company. [Pt{bzimpy(PrSO3)2}{CC−C6H3−(CH2OH)2-3,5}]K60 and thiocholine iodide13 were prepared according to literature procedure respectively. All other reagents and solvents were of analytical grade and used without further purification. Deionized water used was purified with Elga Purelab UHQ system. 4.2. Physical Measurements and Instrumentation. UV−vis absorption spectra were collected on an Agilent Cary 60 UV−vis Spectrophotometer. Emission spectra, excitation spectra, and RLS spectra were obtained from a Horiba Scientific Spex Fluorolog-3 Model FL3-211 Spectrofluorometer equipped with a Hamamatsu R2658P Photomultiplier Tube. The DLS data were collected on a Malvern Zetasizer Nano ZS90 equipped with an internal HeNe laser (λo = 633.0 nm). TEM experiment was performed on a Philips CM100 Transmission Electron Microscope with an accelerating voltage of 100 kV. The microscope was equipped with a TENGRA 2.3k × 2.3k camera for digital imaging. 4.3. Formation of Ag(I)−Thiocholine Coordination Polymer. Different amounts of thiocholine (0−50 μM) were added to a Tris− HAc buffer (10.0 mM, pH = 5.0, 8.0, or 10.0) in the presence and absence of AgNO3 (100 μM) respectively. The UV−vis absorption spectra were recorded at 37 °C with various amounts of thiocholine. The DLS data were recorded at 37 °C with thiocholine (50 μM) and AgNO3 (100 μM). 4.4. Aggregation of 1 upon Addition of Different Amounts of Thiocholine. Different amounts of thiocholine (0−50 μM) were added to a solution of 1 (50 μM) in a Tris−HAc buffer (10.0 mM, pH = 5.0, 8.0, or 10.0) in the presence of AgNO3 (100 μM). UV−vis absorption spectra, emission spectra, excitation spectra, RLS spectra, and DLS data were recorded at 37 °C with various amounts of thiocholine. The emission spectra were recorded at an excitation wavelength of 350 nm. The excitation spectra of 1 in the absence and presence of thiocholine were monitored at 673 and 650 nm, respectively. TEM sample was prepared by adding a few drops of a solution containing 1 (50 μM), Ag(I) ions (100 μM), and thiocholine (50 μM) in a Tris−HAc buffer (10.0 mM, pH = 8.0) onto a carboncoated copper grid, followed by drying in air overnight. Similarly, spectroscopic measurements were carried out in a Tris−HAc buffer (10.0 mM, pH = 8.0) using KAuCl4 (100 μM) in place of AgNO3. 4.5. Aggregation of 1 upon Addition of AChE. AChE (0.08 U mL−1) was added to a solution of 1−acetylthiocholine ensemble ([1] = 50 μM and [acetylthiocholine] = 50 μM) in a Tris−HAc buffer (10.0 mM, pH = 8.0) in the presence of AgNO3 (100 μM). UV−vis absorption spectra, emission spectra, and RLS spectra were recorded at 37 °C at a desired time interval in real-time. The emission spectra were recorded at an excitation wavelength of 350 nm. 4.6. Control Experiment upon Addition of AChE-Hydrolyzed Acetylthiocholine and Ag(I) Ions. AChE (0.08 U mL−1) was added to a solution of acetylthiocholine (50 μM) in a Tris−HAc buffer (10.0 mM, pH = 8.0) in the presence of AgNO3 (100 μM). After incubation at 37 °C for 30 min, 1 (50 μM) was added to this mixture. The emission spectrum was recorded at an excitation wavelength of 350 nm. 4.7. Hydrolysis of Acetylthiocholine upon Addition of Different Amounts of AChE. Different amounts of AChE (0, 0.01, 0.03, 0.06, 0.07, and 0.08 U mL−1) were added to a solution of 1−acetylthiocholine ensemble ([1] = 50 μM and [acetylthiocholine] = 50 μM) in a Tris−HAc buffer (10.0 mM, pH = 8.0) in the presence

of AgNO3 (100 μM). The emission spectra were recorded at 37 °C at a desired time interval in real-time at an excitation wavelength of 350 nm. 4.8. Lineweaver−Burk Plot for AChE Activity. Different amounts of acetylthiocholine (0, 10.0, 20.0, 30.0, 45.0, and 50.0 μM) were added to a solution of 1 (50 μM) in a Tris−HAc buffer (10.0 mM, pH = 8.0) in the presence of AgNO3 (100 μM). To each mixture was added AChE (0.08 U mL−1). The emission spectra were recorded at 37 °C at a desired time interval in real-time at an excitation wavelength of 350 nm. Km and Vmax for AChE were determined using the Lineweaver−Burk Plot, which was based on the derivation of the Michaelis−Menten equation shown below77 K 1 1 1 = m + Vo Vmax [S] Vmax where Vo is the initial reaction rate, Km is the Michaelis−Menten constant, which is the substrate concentration at which the initial reaction rate is at half maximum, Vmax is the maximum reaction rate when all the enzyme exists in the form of enzyme−substrate complexes, and [S] is the substrate concentration. 4.9. Inhibition of AChE Activity upon Addition of Different Amounts of Neostigmine. Different amounts of AChE inhibitor, neostigmine (0, 2.0, 5.0, 20.0, 50.0, and 100.0 nM), were added to a solution of 1−acetylthiocholine ensemble ([1] = 50 μM and [acetylthiocholine] = 50 μM) in a Tris−HAc buffer (10.0 mM, pH = 8.0) in the presence of AgNO3 (100 μM). To each mixture was added AChE (0.08 U mL−1). The emission spectra were recorded at 37 °C at a desired time interval in real-time at an excitation wavelength of 350 nm. The inhibition efficiency of neostigmine toward AChE was calculated using the equation shown below ij V yz inhibition efficiency (%) = jjj1 − zzz × 100% j Vo z{ k

where Vo and V are the initial reaction rates before and after the addition of inhibitor, respectively. 4.10. Control Experiments upon Addition of Different Components. One or more components (1 (50 μM), acetylthiocholine (50 μM), AgNO3 (100 μM), and AChE (0.08 U mL−1) and/or thiocholine (50 μM)) were added to a Tris−HAc buffer (10.0 mM, pH = 8.0). The emission spectra were recorded after incubation at 37 °C for a desired time interval at an excitation wavelength of 350 nm. 4.11. Control Experiments upon Addition of Different Sulfur-Containing Compounds. Thiocholine (50 μM) and/or other sulfur-containing compounds (50 μM), including coenzyme A, L-cysteine, D-cysteine, DL-cysteine, L-glutathione, L-cystine, and Lmethionine, were added to a solution of 1 (50 μM) in a Tris−HAc buffer (10.0 mM, pH = 8.0) in the presence of AgNO3 (100 μM). The emission spectra were recorded at 37 °C at an excitation wavelength of 350 nm. 4.12. Control Experiments upon Addition of Different Metal Ions. AgNO3 (100 μM) and/or other metal ions (100 μM), including Au+, Na+, Mg2+, K+, Ca2+, Mn2+, Fe2+, and Fe3+, were added to a solution of 1−acetylthiocholine ensemble ([1] = 50 μM and [acetylthiocholine] = 50 μM) in a Tris−HAc buffer (10.0 mM, pH = 8.0). To each mixture was added AChE (0.08 U mL−1). The emission spectra were recorded after incubation at 37 °C for 30 min at an excitation wavelength of 350 nm. 4.13. Analysis for Selectivity and Specificity of AChE Assay. AChE (0.08 U mL−1) and/or other proteins (1.0 μM), including αamylase, albumin from bovine serum, albumin from human serum, alkaline phosphatase, insulin, lysozyme, and trypsin, were added to a solution of 1−acetylthiocholine ensemble ([1] = 50 μM and [acetylthiocholine] = 50 μM) in a Tris−HAc buffer (10.0 mM, pH = 8.0) in the presence of AgNO3 (100 μM). The emission spectra were recorded after incubation at 37 °C for 30 min at an excitation wavelength of 350 nm. 4.14. Determination of AChE Activity in Serum Solutions. Dilution is a commonly adopted pretreatment procedure for protein detection in samples of high complexity. Here, both adult bovine G

DOI: 10.1021/acsami.8b18739 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

(11) Maeda, H.; Matsuno, H.; Ushida, M.; Katayama, K.; Saeki, K.; Itoh, N. 2,4-Dinitrobenzenesulfonyl Fluoresceins as Fluorescent Alternatives to Ellman’s Reagent in Thiol-Quantification Enzyme Assays. Angew. Chem., Int. Ed. 2005, 44, 2922−2925. (12) Feng, F.; Tang, Y.; Wang, S.; Li, Y.; Zhu, D. Continuous Fluorometric Assays for Acetylcholinesterase Activity and Inhibition with Conjugated Polyelectrolytes. Angew. Chem., Int. Ed. 2007, 46, 7882−7886. (13) Peng, L.; Zhang, G.; Zhang, D.; Xiang, J.; Zhao, R.; Wang, Y.; Zhu, D. A Fluorescence “Turn-On” Ensemble for Acetylcholinesterase Activity Assay and Inhibitor Screening. Org. Lett. 2009, 11, 4014− 4017. (14) Wang, M.; Gu, X.; Zhang, G.; Zhang, D.; Zhu, D. Convenient and Continuous Fluorometric Assay Method for Acetylcholinesterase and Inhibitor Screening Based on the Aggregation-Induced Emission. Anal. Chem. 2009, 81, 4444−4449. (15) Zhou, G.; Wang, F.; Wang, H.; Kambam, S.; Chen, X.; Yoon, J. Colorimetric and Fluorometric Assays Based on Conjugated Polydiacetylene Supramolecules for Screening Acetylcholinesterase and Its Inhibitors. ACS Appl. Mater. Interfaces 2013, 5, 3275−3280. (16) Gill, R.; Bahshi, L.; Freeman, R.; Willner, I. Optical Detection of Glucose and Acetylcholine Esterase Inhibitors by H2O2-Sensitive CdSe/ZnS Quantum Dots. Angew. Chem., Int. Ed. 2008, 47, 1676− 1679. (17) Miao, Y.; He, N.; Zhu, J.-J. History and New Developments of Assays for Cholinesterase Activity and Inhibition. Chem. Rev. 2010, 110, 5216−5234. (18) Rhee, I. K.; van Rijn, R. M.; Verpoorte, R. Qualitative Determination of False-Positive Effects in the Acetylcholinesterase Assay Using Thin Layer Chromatography. Phytochem. Anal. 2003, 14, 127−131. (19) George, P. M.; Abernethy, M. H. Improved Ellman Procedure for Erythrocyte Cholinesterase. Clin. Chem. 1983, 29, 365−368. (20) Lehn, J.-M. Supramolecular Chemistry: Concepts and Perspectives; Wiley-VCH: Weinheim, FRG, 1995. (21) Jennette, K. W.; Lippard, S. J.; Vassiliades, G. A.; Bauer, W. R. Metallointercalation Reagents. 2-Hydroxyethanethiolato(2,2′,2″-Terpyridine)-Platinum(II) Monocation Binds Strongly to DNA by Intercalation. Proc. Natl. Acad. Sci. U.S.A. 1974, 71, 3839−3843. (22) Jennette, K. W.; Gill, J. T.; Sadownick, J. A.; Lippard, S. J. Metallointercalation Reagents. Synthesis, Characterization, and Structural Properties of Thiolato(2,2′,2″-Terpyridine)platinum(II) Complexes. J. Am. Chem. Soc. 1976, 98, 6159−6168. (23) Miskowski, V. M.; Houlding, V. H. Electronic Spectra and Photophysics of Platinum(II) Complexes with α-Diimine Ligands. Solid-State Effects. 1. Monomers and Ligand π Dimers. Inorg. Chem. 1989, 28, 1529−1533. (24) Biedermann, J.; Gliemann, G.; Klement, U.; Range, K. J.; Zabel, M. Spectroscopic Studies of Cyclometalated Platinum(II) Complexes: Superposition of Two Different Spectroscopic Species in the Electronic Spectra of a Single Crystal of [Pt(bpm)(CN)2] (bpm = 2,2′-Bipyrimidine). Inorg. Chem. 1990, 29, 1884−1888. (25) Houlding, V. H.; Miskowski, V. M. The Effect of Linear Chain Structure on the Electronic Structure of Pt(II) Diimine Complexes. Coord. Chem. Rev. 1991, 111, 145−152. (26) Miskowski, V. M.; Houlding, V. H. Electronic Spectra and Photophysics of Platinum(II) Complexes with α-Diimine Ligands. Solid-State Effects. 2. Metal−Metal Interaction in Double Salts and Linear Chains. Inorg. Chem. 1991, 30, 4446−4452. (27) Yip, H.-K.; Cheng, L.-K.; Cheung, K.-K.; Che, C.-M. Luminescent Platinum(II) Complexes. Electronic Spectroscopy of Platinum(II) Complexes of 2,2′:6′,2″-Terpyridine (Terpy) and pSubstituted Phenylterpyridines and Crystal Structure of [Pt(terpy)Cl][CF3SO3]. J. Chem. Soc., Dalton Trans. 1993, 2933−2938. (28) Aldridge, T. K.; Stacy, E. M.; McMillin, D. R. Studies of the Room-Temperature Absorption and Emission Spectra of [Pt(trpy)X]+ Systems. Inorg. Chem. 1994, 33, 722−727. (29) Herber, R. H.; Croft, M.; Coyer, M. J.; Bilash, B.; Sahiner, A. Origin of Polychromism of Cis Square-Planar Platinum(II)

serum and human serum were diluted with the Tris−HAc buffer solution (10.0 mM, pH = 8.0) before detection. All the emission spectra were recorded at 37 °C at a desired time interval at an excitation wavelength of 350 nm.



ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b18739. UV−vis absorption spectra, DLS data, emission spectra, excitation spectra, RLS spectra, TEM image, Lineweaver−Burk plot (PDF)



AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] ORCID

Vivian Wing-Wah Yam: 0000-0001-8349-4429 Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS V.W.-W.Y. gratefully thanks the University Grants Committee Areas of Excellence Scheme (AoE/P-03/08) and the Research Grants Council of Hong Kong Special Administrative Region, People’s Republic of China (HKU 17334216) for financial support on this work. A.S.-Y.L. acknowledges the receipt of a Postgraduate Scholarship and a University Postgraduate Fellowship by The University of Hong Kong. Dr A. K.-W. Chan is thankfully acknowledged for his fruitful discussion on this project.



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DOI: 10.1021/acsami.8b18739 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acsami.8b18739 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX