J. Phys. Chem. C 2009, 113, 2541–2546
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An Anthracene-Based Chemosensor for Multiple Logic Operations at the Molecular Level Guoqiang Zong†,‡ and Gongxuan Lu*,† State Key Laboratory for Oxo Synthesis and SelectiVe Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, China, Graduate UniVersity of Chinese Academy of Sciences, Beijing, 100049, China ReceiVed: October 18, 2008; ReVised Manuscript ReceiVed: December 8, 2008
A chemosensor consisting of an L-histidine covalently linked to an anthracene unit, L, has been synthesized, and its sensing behavior toward pH values and metal ions has been investigated by fluorescence and absorption spectroscopy. In aqueous solution, compound L may serve as an “off-on-off” fluorescence switch over 4.0 pH units. At neutral pH, the fluorescence intensities of L are enhanced by addition of Zn2+ and Cd2+ and quenched by metal ions such as Cu2+, Ni2+, Co2+, Hg2+, and Pb2+. Furthermore, the binding constants for L with Zn2+, Cu2+, Ni2+, Co2+, and Hg2+, respectively, are also determined by fluorescence titrations. On the other hand, compound L displays a drastic decrease in absorbance selectively with Hg2+ over other metal ions. On the basis of the above results, several logic gates (XNOR, OR, NOR, INHIBIT) and a half-subtractor have been achieved at the molecular level by changing the initial states of system L and chemical inputs. 1. Introduction
CHART 1: Structure of Compound L
There is a great deal of current interest in molecular systems capable of performing ion-induced logic operations, due to their potential applications in the creation of nanometer-scale molecular devices.1 In particular, a system consisting of chemically encoded information as input and fluorescennt signal as output has attracted great attention. In this regard, the three basic logic operations (AND,2 NOT,3 and OR4) and more complex logic functions, such as INH,5 NOR,6 XNOR,7 and XOR,8 have been reproduced at the molecular level. On the basis of functional integration in a single molecule, half adder8a,9 and halfsubtractor10 having superior processing capabilities have been constructed. By harnessing the principles of molecular Boolean logic, several molecular systems to ensure information security against illegal invasion have also been developed in the past 2 years.11 Simultaneously, the original demonstration of a few basic logic operations has now been extended to cover many of the one- and two-input varieties and even some of the threeinput types.6a,7b,12 In addition, construction of molecules whose logic functions can be reconfigured multiply by varying the chemical inputs has now been a focus. In general, the construction of reconfigurable logic gates requires that the optical sensor exhibit multiple output results in response to different combinations of input signals. Following these principles, a variety of molecule-based systems behaving as logic gates with multiply configurable multiple outputs have been proposed;7b,13 however, such systems capable of operating in an aqueous solution of physiological pH are less common.13a,e,i A fluorescent sensor is essentially a two-component compound in which a fluorophore is covalently linked to a receptor specific for a particular ion. When the receptor is combined with the various analytes, the molecule can display fluorescence enhancement or quenching of the fluorophore, which can be used for building logic gates.1a,f Due to its strong luminescence and chemical stability, the anthracene unit has been widely used * Corresponding author. Phone and Fax: +86-931-4968178. E-mail:
[email protected]. † Lanzhou Institute of Chemical Physics. ‡ Graduate University of Chinese Academy of Sciences.
in the development of fluorescent sensors.14 In recent years, our groups have been involved in the synthesis and photochemical characterization of chemosensors containing an anthracene fluorophore and an amino acid receptor unit, which could carry out various logic operations.15 Among the amino acids, Lhistidine is one of the strongest metal-coordinating ligands and plays an important role in the binding of metal ions by proteins. L-Histidine has amino, imidazole, and carboxylate groups as three potential metal binding sites. At physiological pH, the tridentate chelation of L-histidine ligand has been established for bis-metal complexes of Co(II), Ni(II), Zn(II), and Cd(II).16 Earlier, we reported an L-histidine derivative bearing an anthracene unit (L) (Chart 1), which functions as a two-input NOR fluorescence logic gate.15d In the present work, we have extended our investigation to mimic the logic operations with multiply configurable multiple outputs. Depending on the effect of pH values and metal coordination on the spectral properties of system L, various logic operations, including XNOR, OR, threeinput NOR, INHIBIT, and a half-subtractor are demonstrated at the molecular level. Since all those logic operations mentioned above can be operated by two or three inputs but within a single sensor molecule, the system of L reported here shows a high versatility. 2. Experimental Section Materials and Methods. 9-Anthraldehyde was analytically pure and recrystalized from ethanol before use. All of the other chemicals used here were analytical reagents. 1H NMR and 13C NMR data were recorded on an INOVA-400 spectrometer (Varian INOVA Ltd. Co., USA) with chemical shifts reported
10.1021/jp8092379 CCC: $40.75 2009 American Chemical Society Published on Web 01/20/2009
2542 J. Phys. Chem. C, Vol. 113, No. 6, 2009
Zong and Lu
Figure 1. pH-dependent changes in maximum emission at 418 nm of L (5 µM) (λ ) 368 nm) measured in aqueous solution (CH3OH/H2O ) 1:99, v/v) with 1 equiv of respective metal cations.
Figure 2. Fluorescence spectra changes of L (5 µM) upon addition of different metal cations (5 equiv) in CH3OH/H2O solution (1:99, v/v, 10 mM HEPES buffer, pH 7.0).
as parts per million (in CD3OD, TMS as an internal standard). ESI measurement was obtained with a Waters ZQ-4000 instrument (Manchester, UK). Elemental analyses were performed on a Vario EL elemental analysis instrument (Elementar Co, Germany). UV-vis absorption spectra were measured with a HP8453 spectrophotometer (Hewlett-Packard Co. Ltd., USA), and fluorescence spectra were recorded on a F-4500 spectrometer (Hitachi, Japan). Both excitation and emission bands were set at 2.5 nm. The pH values were adjusted by the addition of 0.1 mol/L HC1 or 0.1 mol/L NaOH and determined on a Markson 6200 pH meter (Markson Science, Phoenix, AZ). Synthesis. L-Histidine (0.78 g, 5 mmol) and KOH (0.28 g, 5 mmol) were dissolved in hot anhydrous ethanol (50 mL), and 9-anthraldehyde (1.0 g, 5 mmol) was added portion-wise under stirring. The mixture was then stirred at room temperature for 36 h to produce a yellow precipitate. NaBH4 (1.0 g, 25 mmol) was added portion-wise, and the resulting solution was stirred at room temperature for 24 h. After distilling off the solvent under reduced pressure, the residue was treated with 50 mL of water and acidified with 37% HCl to pH 6.0-7.0 under stirring. The resultant solid was then filtered and recrystalized twice from 95% ethanol (addition of HCl to strong acidity) and dried in vacuum at 333 K to afford a yellow needle crystal L · 2HCl (0.94 g, yield: 45%). mp 228-229 °C. 1H NMR (CD3OD, 400 MHz) δ: 8.82-7.49 (m, ArH, imidazole-CH, 11H), 5.43-5.34 (m, ArCH2, 2H), 4.65-4.61 (m, CHCOOH, 1H,), 3.51-3.30 (m,CH2, 2H). 13C NMR (CD3OD, 400 MHz) δ: 170.3, 135.5, 132.9, 132.5, 132.1, 130.1, 129.0, 128.8, 126.7, 124.6, 122.3, 119.4, 60.7, 44.8, 26.4. MS (ESI): 346.4 [M + H]+. Anal. Calcd. for C21H19N3O2 · 2HCl: C, 60.30; H, 5.06; N, 10.04. Found: C, 60.66; H, 5.12; N, 10.13.
intensity is observed under strongly acidic conditions in the range of pH 4.0-1.0. This is probably due to the protonation of the imidazole nitrogen of L under strong acid conditions, which may cause an electron-transfer process from the excited anthracene unit to the electron-deficient imidazole part, inducing a decrease in emission intensity.10c,18 On the other hand, unlike the significant effects of pH on the emission of L, acid and base appear to have little affect on the absorption spectra (Figure S1 of the Supporting Information), which supports the proposition of a PET mechanism for the fluorescence response of L to pH values.5d,19 3.2. Effect of Metal Ions on the Spectra. The fluorescence spectra changes of L (5 µM) upon addition of various metal ions (5 equiv) in CH3OH/H2O solution (1:99, v/v, 10 mM HEPES buffer, pH 7.0) are illustrated in Figure 2. A selective chelation-enhanced fluorescence (CHEF) effect is observed upon addition of Zn2+, although a relatively small CHEF effect is observed with Cd2+. Nevertheless, there is a significant chelation-enhanced fluorescence quenching (CHEQ) effect upon addition of Cu2+, Ni2+, Hg2+, and Co2+ and a relatively small CHEQ effect for Pb2+. There are almost no fluorescence changes when other metal ions are added, such as Mg2+, Ca2+, Ba2+, Al3+, Fe3+, Mn2+, Cr3+, and La3+. The CHEF effect for Zn2+ and Cd2+ can be explained by the blocking of the PET process from the unprotonated amine nitrogen to the excited anthracene moiety because of the coordination action of these ions with L at physiological pH. The CHEQ effects for Cu2+, Ni2+, and Co2+ occur because these coordinated ions lead to an energy transfer from the excited anthracene unit to the low-lying empty d orbital of the ions, although the CHEQ effects for Hg2+ and Pb2+ are probably due to the acceleration of intersystem crossing from the singlet excited-state anthracene to its triplet excited-state via the heavy atom effect.13a,b,i,20 The complex formation between L and these ions is analyzed by 1HNMR and electronic absorption spectroscopy. The addition of 1 equiv of metal ions to L in DMSO-d6/D2O (8:1 v/v) results in an upper-field shift of the anthracene moiety resonances, and the NMR resonance signals on the imidazole unit of L also shift and broaden after addition of Zn2+ and Hg2+ (these signals disappear upon addition of paramagnetic Cu2+, Ni2+ and Co2+) (Figure S2 of the Supporting Information). In UV-vis spectra, L (5 × 10-5 M) shows an increase or red shift of absorption corresponding to the anthracene moiety upon addition of Co2+, Ni2+, Cu2+, and Hg2+ (Figures S3-S6 of the Supporting Information). These results support the formation of a stable complex between L and the respective metal ions. In addition, it should be mentioned that the fluorescence intensities of the complex systems are
3. Results and Discussion 3.1. Effect of pH on the Spectra. Compound L in aqueous solution displays the typical emission spectrum of anthracene, the maximum emission of which appears at 418 nm (λex ) 368 nm). Figure 1 shows pH-dependent intensity changes (λem ) 418 nm) of L (5 µM) with 1 equiv of respective metal ions in aqueous solution (CH3OH/H2O ) 1:99, v/v). In strong basic conditions (pH >10.0), the fluorescence of L is very weak and then starts to increase considerably with a pH decrease over the pH range 10.0-4.0. This fluorescence enhancement behavior in acidic solution is due to the inhibition of the photoinduced electron transfer (PET) processes from the amine bridge nitrogen atom to the photoexcited anthryl group by the protonation of the amine group of L, which is responsible for the fluorescence quenching.14,17 However, a gradual decrease in fluorescence
Anthracene-Based Chemosensor
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TABLE 1: Binding Constant and Stoichiometry Determined of Cations (Chloride Salt) with L in Aqueous Solution (CH3OH/H2O ) 1:99, v/v, pH 7.0) at 25°C cation 2+
Zn Cu2+ Ni2+ Hg2+ Co2+
stoichiometry cation/L 1:1 1:2 1:2 1:2 1:1
binding constant 3.32 ×104 2.66 ×106 1.02 ×106 4.68 ×105 2.40 ×104
consistent with that of system L under strong acidic or strong basic conditions due to the disassociation of metal complexes to liberate the receptor under these conditions (Figure 1). Table 1 summarizes the binding constants (kb) and stoichiometries of different metal ions (Zn2+, Cu2+, Ni2+, Hg2+, and Co2+) with L in CH3OH/H2O solution (1:99, v/v, 10 mM HEPES buffer, pH 7.0). The kb values of other metal ions cannot be estimated due to the small changes in fluorescence intensities. Figure 3A demonstrates the fluorescence changes of L (5 µM) upon addition of Zn2+. The intensities of the emission peaks progressively increase in the presence of varying concentrations of Zn2+. From the fluorescence titrations, the kb value is 3.32 × 104 M-1 using the Benesi-Hildebrand method.21 The 1:1 binding stoichiometry is also supported by this method (Figure 3B). In contrast, the fluorescence intensity of L (5 µM) exhibits a gradual decrease with increasing Cu2+ concentration (Figure 4A). A Job’s plot for the binding between L and Cu2+ shows a 2:1 stoichiometry (Figure 4B). From the fluorescence titration data, the association constant for Cu2+ is 2.66 × 106 M-2.22 According to the methods mentioned above, the binding constants and stoichiometries for Ni2+, Co2+, and Hg2+ with L are also determined (Figures S7-S9 of the Supporting Information). On the other hand, compound L (5 µM) displays a drastic decrease in maximum absorption at 255 nm with Hg2+ (5 equiv) in CH3OH/H2O solution (Figure 5). However, only a minor decrease in absorbance at 255 nm is observed upon addition of Cu2+, and the absorption peaks exhibit negligible absorbance changes when 5 equiv of the other 13 metal ions are added. This effect is probably due to the difference in binding features between mercury and some other metals. As is well-known, the Hg2+ ion typically binds two ligands very strongly in a linear configuration and shows only a small tendency to form a third and a fourth coordinate bond, whereas the other metals, such as Co2+, Ni2+, and Cu2+, generally have coordination numbers
Figure 3. (A) Changes in fluorescence spectra of L (5 µM) in CH3OH/ H2O solution (1:99, v/v, 10 mM HEPES buffer, pH 7.0) after addition of 0, 1.0, 1.5, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 7.5, 10.0, 15.0, or 20.0 equiv of Zn2+. (B) The Benesi-Hildebrand plot analysis for Zn2+ binding with L.
Figure 4. (A) Changes in fluorescence spectra of L (5 µM) in CH3OH/ H2O solution (1:99, v/v, 10 mM HEPES buffer, pH 7.0) after addition of 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0, or 5.0 equiv of Cu2+. (B) Job’s plot for Cu--L system ([L] + [Cu2+] ) 5 µmol L-1).
Figure 5. UV/vis spectral changes of L (5 µM) upon addition of different metal cations (5 equiv) in CH3OH/H2O solution (1:99, v/v, 10 mM HEPES buffer, pH 7.0).
of 4 or 6. These spectral changes as well as a marked red shift of absorption corresponding to the anthracene moiety (Figure S6 of the Supporting Information) indicate that L and Hg2+ interact significantly in the ground state. The absorption spectra of L observed by titration with Hg2+ are shown in Figure S10 (Supporting Information). The maximum absorption band at 255 nm gradually decreases in intensity with an increase in the Hg2+ concentration. Meanwhile, the three weaker absorption bands at 330-400 nm also gradually red-shift and decrease. 3.3. XNOR Logic Gate Based on System L at pH 4.0. XNOR can be operated by the situation only when both or neither of two inputs is computed. As shown in Figure 1, a unique feature of system L (5 µM) is its fluorescence intensity vs pH profile which shows an “off-on-off” fluorescence switch over 4.0 pH units. Then, starting with system L at pH 4.0, an XNOR logic gate can be operated when H+ (0.1 M HCl) and OH- (0.1 M NaOH) are used as the inputs. With no addition of inputs, the fluorescence output exceeds the threshold value (>375, output: 1). When a strong acid or base is added solely, the output falls below the threshold value (200, output: 1) when no ions are added. However, when one, or both, or all of the three inputs are operated, the fluorescence emission of L is quenched (375, output: 1). In the presence of 5 equiv of Cu2+ or Ni2+ or both of them, the resultant outputs are all 0 (
