Selective Anion Sensing through a Self-Assembled Monolayer of Thiol

Jul 1, 2009 - Beer , P. D.; Davis , J. J.; Drillsma-Milgrom , D. A.; Szemes , F. Chem. Commun. ...... Paul A. Bertin , Michael J. Ahrens , Kinjal Bhav...
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Selective Anion Sensing through a Self-Assembled Monolayer of Thiol-End-Functionalized Porphyrin Fupeng Zhi, Xiaoquan Lu,* Jiandong Yang, Xiaoyan Wang, Hui Shang, Shaohua Zhang, and Zhonghua Xue College of Chemistry & Chemical Engineering, Northwest Normal UniVersity, Lanzhou 730070, P. R. China ReceiVed: January 13, 2009; ReVised Manuscript ReceiVed: May 31, 2009

Self-assembled monolayers (SAMs) of thiol-derivatized tetraphenylporphyrin on gold are able to bind anions reversibly from aqueous solutions. Electrochemical impedance spectroscopy (EIS) and scanning electrochemical microscopy (SECM) are employed to investigate the binding of anions by porphyrin SAMs and proved to be efficient and convenient techniques for detecting anions in aqueous solutions using the redox couple Fe(CN)63-/4- as reporter ion. SAMs of porphyrins are shown to selectively bind dihydrogenphosphate over various other anions, including Cl-, Br-, NO2-, or NO3-. Gold electrodes modified with porphyrins can detect H2PO4- even in the presence of a 10-fold excess of other anions studied. 1. Introduction Self-assembled monolayers (SAMs) of thiol-derivatized molecules on gold substrates have recently received considerable attention in connection with their potential applications for elaborate designs of molecular-based electronics, chemical sensors, and nanopatterning.1-4 The use of SAMs has been extensively employed for this development because of the ease of their preparation, stability, and the densely packed structures and because the possibility to introduce different functional groups provides the facile means to prepare surfaces with tailormade properties.5-9 The interest in using these modified surfaces in sensing devices has led to the development of resorcin[4]arene SAMs which are able to detect organic compounds from gas phase.10 SAMs of synthetic receptors have also been used to detect the binding of guests from aqueous solutions. The cation recognition properties of some SAMs have been widely reported by Reinhoudt, Echegoyen, and others using a range of receptors including SAMs of oligoethylene glycol derivatives,11 12-crown4, 15-crown-5 adsorbates,12 and the calix[4]crown-6 derivative.13,14 Of course, the recognition of anions has indeed been the subject of special scrutiny, given their role in biology.15 In recent years, some of the few examples that have been extensively investigated include a dihydrogenphosphate anion sensor derived from a SAM-modified gold electrode by an amidoferrocene derivative,16 the anion recognition properties of ferrocene SAMs on gold surfaces,17 and metalloporphyrin-functionalized gold nanoparticles.18 Echegoyen and co-workers reported the acetate ion recognition properties by SAMs modified with a rigid and electrochemically inactive cyclotriveratrylene (CTV) derivative.19 Echegoyen and co-workers also reported the fluoride ion recognition properties of SAMs based on the calix[6]crown-4 derivative.20 Ohsaka et al. investigated anion recognition by SAMs modified with a nickel azamacrocycle complex.21 In all of these cases, electrochemically active groups were included to monitor the anion recognition processes. However, specific porphyrin receptors for inorganic anions that exist in natural aqueous environments are greatly needed because porphyrins * To whom correspondence should be addressed. Tel: +86-931-7971276. Fax: +86-931-7971323. E-mail: [email protected].

exist widely in nature and play a significant role in the metabolism of living organisms. Porphyrin is one of the most attractive molecules for its wellknown functions in biological and biomimetic systems, which are focused on their excellent stability and unique optical and electronic properties.22,23 In particular, researching on the selfassembled structures of porphyrins is attractive for the precise surface chemical modification that allows for the study of surface-dependent phenomena, such as molecular and electronic devices, sensors, energy harvesting, storage, molecular recognition, and catalysis.24-30 In past decades, considerable efforts have also been devoted to developing free base porphyrin derivatives as anion receptors. Sessler and co-workers have done excellent work in this field.31,32 The anion recognition properties have also been extensively explored by Lindsey and Bocian using a range of porphyrin SAMs.33-36 Porphyrin compounds have attracted considerable interest as building blocks for the construction of receptors for the dihydrogenphosphate anion recognition because of advantages such as its high efficiency, good regioselectivity, and compatibility with the reaction environment, and the unique properties of the meso-tetraphenylporphyrin core in terms of its ability to participate in hydrogen bonds with the dihydrogenphosphate anion in aqueous medium. Especially, it is of particular significance to detect the dihydrogenphosphate anion in aqueous solution because of its biological importance. For example, in the human body, phosphate plays an important role not only in the control of pH in blood or lymph fluid but also in the energy and nitrogen metabolism in cells. Moreover, phosphate analysis in soil and wastewater are indexes of nutrition, environmental monitoring, and biomedical research. The analysis of phosphate ion is thus very important in clinical chemistry, environmental chemistry, biochemistry, pharmacology, and others.37,38 Therefore, our ongoing interest in developing a new type of porphyrin-based receptor for anion recognition and the ability of the meso-tetraphenylporphyrin core to act as a hydrogen bond donor and dihydrogen-phosphate as a hydrogen bond acceptor makes it an amide/dihydrogenphosphate bond mimic. More importantly, it also establishes the potential of the porphyrin system as an anion recognition moiety and the importance of recognition and the recognition of anions.

10.1021/jp9003278 CCC: $40.75  2009 American Chemical Society Published on Web 07/01/2009

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SCHEME 1: Molecular Structure of Porphyrins SH-TPP (a) and TPPS4 (b)

2. Experimental Section Synthesis and characterization of porphyrins (Scheme 1) are described in the Supporting Information. SH-TPP was dissolved in chloroform and assembled on a precleaned gold disk electrode. The disk electrodes were polished carefully and cleaned with pure water four times. The preparation of SHCoTPP and TCA-ATP SAMs are also described in the Supporting Information. SAMs were identified by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) (Princeton Applied Researcher, USA), scanning electrochemical microscopy (SECM) (CHI 900, CH Instruments Co. Ltd., Austin, USA), and a Fourier transform infra red (IR) spectrometer (Bruker, Germany). UV-vis absorption spectra were taken by absorption mode with an Agilent 8453 UV-vis spectrophotometer. All anions studied here were used as their corresponding sodium salts. 3. Results and Discussion 3.1. Monolayer Characterization. Infrared reflectionabsorption spectroscopy was used as a convenient method to identify the functional groups of the adsorbates which had selfassembled onto gold surfaces, usually in the reflection-absorption mode, using thick Au film to obtain effective reflection. The SAMs of SH-TPP have characteristic antisymmetric and symmetric modes of methylene C-H stretching vibrations at 2915 cm-1 and 2847 cm-1, respectively, almost coinciding with the bulk spectra, which are indicative of adsorbed SH-TPP since these two bands are often observed in the SAMs of alkanethiol and aromatic thiols derivatives.39,40 The fact that two methylene stretching vibrations are observed can be explained by the presence of chemically different methylene groups. The oxygenbound methylene groups have absorptions at 2847 cm-1, as was found for a solid sample of the SH-TPP. The band at 3011 cm-1 is ascribed to the C-H stretching vibration of ring hydrogens. As the literature reported previously, the bands at 3064 and 3031 cm-1 are assigned to the CH stretching modes of aromatics (pyrrole and phenyl).41 Besides the high-wavenumber band, the bands at 1261, 1174, 900, and 721 cm-1 were due to CH inplane and out-of-plane of benzene ring and porphyrin, respectively.42 The self-assembly of SH-TPP on a gold electrode was monitored by CV. Fe(CN)63-/4- couple peaks were used to identify the electron transfer efficiency through the SAMs. With SH-TPP molecules bound to the gold surface step by step, electron transfer was blocked gradually.43 It appeared on CVs

as Fe(CN)63-/4- peak currents decreasing and becoming more and more irreversible (as shown in Figure 1). The Nyquist plots of the ac impedance displayed the increasing resistance of the SAM with SH-TPP adsorption (EIS, Figure 2). After 36 h, the peak current of the CV was suppressed to its lowest value, and then small changes in current and resistance were observed even though the assembly time was prolonged, which indicated that the SAM reached saturation. The saturation of the SAM was further demonstrated by SECM results. The inset of Figure 3 proves the good reversibility of the probe. The feedback current increased on the bare gold electrode (positive feedback), whereas it decreased on the insulating surface when the SH-TPP was self-assembled over a period of 36 h (negative feedback) in Figure 3. The change indicated that the Au electrode was fully coated. The surface coverage of saturated SAMs was also estimated using a method reported by Weisshaar et al.44 The charge under a desorption wave was used to provide a measure of the surface coverage of thiol-porphyrin. The surface coverage Γ (mol/cm2) was calculated, on the basis of the relationship Γ ) Q/nFA (where Q is the total charge (C), A is the electrode surface area (cm2), and n and F have their usual electrochemical meanings). By integrating the charges (Q) passing on the cathodic wave, the estimated surface coverage for the SH-TPP SAMs was 1.99 × 10-10 mol/cm2. The surface coverage Γ value was consistent with those of similar porphyrin SAMs reported previously.45-47

Figure 1. CVs of SH-TPP SAM on gold electrode in 1 mM Fe(CN)63-/ 4- with 0.1 M KPF6 as supporting electrolyte at different deposition times, with a scan rate of 50 mV/s.

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Figure 2. Impedance spectra corresponding to a bare gold electrode modified with SH-TPP at different times. (a) Bare Au, (b) 2 h, (c) 24 h, and (d) 36 h in 1 mM Fe(CN)63-/4- with 0.1 M KPF6 as supporting electrolyte. Inset: the equivalent circuit model used to obtain equations for Zre and Zim. W.E is the working electrode, R.E is the reference electrode, and C.E is the counter electrode.

Figure 3. Probe approach curves of the bare Au electrode and the SHTPP modified electrode in 1 mM Fe(CN)63-/4- containing 0.1 M KPF6. Inset: the cyclic voltammogramm of a Pt tip in 1 mM Fe(CN)63-/4- and 0.1 M KCl as supporting electrolyte.

3.2. Anion Recognition Properties of Porphyrin Monolayers by Electro-Chemical Methods Using Fe(CN)63-/4- as the Redox Probe. EIS and SECM were employed to anticipate the interfacial anion recognition properties using the redox couple Fe(CN)63-/4- as the reporter ion redox probe. When anions bind the receptors self-assembled on the electrode surfaces, the electron transfer process can be affected between the electrode and the redox couple Fe(CN)63-/4- in electrolyte solutions because of the electrostatic attraction or repulsion between the SAM surface and the soluble redox couple. Figure 4 shows the complex impedance responses of 1 mM Fe(CN)63-/4- at the TCA-ATP SAM-modified gold electrode in the absence and presence of different anions compared to those in the case of the bare Au electrode. The SAM-modified gold electrode exhibited a value of 240 kΩ for the charge transfer resistance (Rct), indicating the very densely packed SAM which can effectively retard electron transfer to the redox probe at the SAM-monolayer/solution interface. The complex impedance mainly reveals a straight line (Warburg impedance) at the frequency range at a bare gold electrode due to a fast electrode reaction in Figure 2a. Upon the addition of 20 mM Cl-, Br-, NO3-, or NO2- to the electrolytes, no obvious effect on Rct was observed, revealing that the SAM receptors cannot bind with these anions, and the surface charge remained unchanged.19,20 However, upon the addition of 20 mM H2PO4-, significant binding was observed in this case by the obvious change of Rct, demonstrating the high affinity for the H2PO4- ion with the SAM-modified gold

Figure 4. Impedance response of Fe(CN)63-/4- at the ATP-TCP monolayer-modified gold electrode in 1 mM Fe(CN)63-/4- containing 0.1 M KPF6 upon the addition of 20 mM NO3-, Cl-, Br-, NO2-, and H2PO4- in water from top to bottom.

Figure 5. Impedance response of Fe(CN)63-/4- at SAMs of SH-TPP in 1 mM Fe(CN)63-/4- containing 0.1 M KPF6 upon the addition of (a) 0 mM H2PO4-, (b) 1 mM H2PO4-, and (c) 1 mM H2PO4- + 10 mM NO3-, Cl-, and Br- in water.

electrode (either TCA-ATP or SH-TPP). In addition, in Figure 5, gold electrodes modified with SH-TPP detect H2PO4- even in the presence of a 10-fold excess of other anions studied, including Cl-, Br-, NO2-, and NO3-. Other anions could not create interference when recognitizing H2PO4-. Furthermore, Figure 6a illustrated the complex impedance responses of 1 mM Fe(CN)63-/4- at SH-TPP SAM-modified gold electrodes in the absence and presence of increasing amounts of H2PO4-. It was obvious that Rct decreased from 47650 Ω in the absence of H2PO4- to a limited value of 19067Ω in the

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Figure 6. Impedance response (a) and variation of Ket (b) of Fe(CN)63-/4- at the porphyrin monolayer-modified gold electrode SH-TPP in the presence of H2PO4- from top to bottom (0, 0.02, 0.1, 0.2, 0.5, 1.0, 3.0, 5.0, 10, 20, 30, and 40 mM).

SCHEME 2: Molecular Structure of the Protonation of meso-Tetraphenylporphyrin

presence of 40 mM H2PO4-. The obvious change of Rct indicated that the redox probe exhibited relatively gradually fast interfacial electron transfer at a SH-TPP SAM-modified electrode. A consistent phenomenon was also shown by CV: with increasing concentrations of H2PO4-, the peak currents were higher than those in the absence of H2PO4- (either TCAATP or SH-TPP). It was possible that protonation of a simple meso-tetraphenylporphyrin occurred in the aqueous system induced by the low concentration of H+ which was ionized from the proton sources of H2PO4- (the structure was shown in Scheme 2). The reversible protonation behavior of SAMs of this receptor was investigated with Fe(CN)64- as the electroactive marker. At relatively low pH, Fe(CN)64- was oxidized more easily than at high pH. This reflected the degree of protonation of the receptor monolayers and thereby the charge density; the marker can access the electrode surface easily at low pH (pH > 4.3), where the receptor SAM was protonated. However, in the presence of H2PO4-, Fe(CN)64- oxidation occurred easily even at high pH (pH < 6), indicating that H2PO4- bound to the protonated porphyrin receptor SAMs. On the basis of the advantages of porphyrin high efficiency, good regioselectivity, compatibility with the reaction environment, and the unique properties of diprotonated porphyrin core, it is easy to participate in hydrogen bonds with the dihydrogenphosphate anion in aqueous medium. This phenomenon allowed the determination of H2PO4- concentrations in aqueous solutions. Hence, the electrostatic attraction between the redox couple and the electron-poor pyrrole resulted in intermolecular charge transfer from Fe(CN)64- to the electron-poor pyrrole, thus enhancing the electron transfer. Also, the pyrrole NH became more acidic and more available for hydrogen bonding, which resulted in increased anion affinities. We presumed that the intramolecular NH hydrogen bonds of the former porphine

SCHEME 3: Schematic Graph of the 1:1 Cationic Inner-Sphere Complex Formed between Protonated Porphyrin and the Dihydrogenphosphate Aniona

a

Selected hydrogen atoms have been omitted for clarity.

nucleus were somewhat decreased by the formation of NHsH2PO4- hydrogen bonds; while interacting with H2PO4-, the central protons were tied up tightly in the nitrogen to oxygen of dihydrogenphosphate anion hydrogen bonds, thereby reducing the ability of the NH groups to act as vibrational energy sinks.48 The formation of the NHsH2PO4hydrogen bonds destroyed the conjugation of the porphine plane.49 Therefore, we proposed a molecular model in which the 1:1 cationic complex formed between dihydrogenphosphate and the diprotonated form of the porphyrin core is shown in Scheme 3 and typified one type of porphyrin/ phosphate interaction, wherein the dihydrogenphosphate anion is coordinated to the protonated porphyrin core via four near-identical helicopter-like hydrogen bonds. In addition, the limiting value reached at [H2PO4-] ) 40 mM reveals that surface complexation sites are saturated. According to the electrochemical impedance frequency spectrum and these equations for the redox reactions, the charge transfer resistance is as follows:

Rct ) RT(nFI0)-1

(1)

According to eq 1, the following relationship is derived:

I0 ) RT(nFRct)-1

(2)

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Figure 7. Probe approach curves for the ATP-TCP modified electrode in 1 mM Fe(CN)63-/4- containing 0.1 M KPF6 in the presence of (a) H2PO4(from bottom to top 0, 0.5, 1.0, 2.0, 4.0, and 8.0 mM) and (b) AcO- (from top to bottom 0, 0.5, 1.0, 2.5, and 5.0 mM).

The charge transfer resistance can be translated into the exchange current under equilibrium, I0 (eq 2). However, the equilibrium current can be represented by

I0 ) nFAKet[c]

(3)

According to eq 3, we obtain

Ket ) I0 /nFA[c]

(4)

where Ket is the heterogeneous electron-transfer rate constant, R is the gas constant, T is the temperature (K), A is the electrode area (cm2), [c] corresponds to the bulk concentration of the redox probe (mol/cm3), and n is the number of transferred electrons per molecule of the redox probe (n ) 1 for the Fe(CN)63-/4- probes).50 According to the above equations, the values of Ket (4.46 × 10-4 cm2/s) at [H2PO4-] ) 40 mM can be obtained at the SH-TPP modified electrode. Compared to the values of Ket (1.78 × 10-4 cm2/s) obtained at [H2PO4-] ) 0 mM, the value was drastically increased, which meant that the redox probe molecule exhibited relatively fast interfacial electron transfer at an SH-TPP SAM-modified electrode because the SH-TPP SAMs intensively interacted with H2PO4-. Figure 6b shows the variation of change in Ket with [H2PO4-]. At low concentrations, the plot was approximately linear but exhibited saturation as the concentration increased. As to the TCA-ATP, the monolayer responded with slight affinity to the presence of AcO- by hydrogen bonding with the amide NH groups.19 AcO- could create some interference when recognizing H2PO4-. SECM has been proven to be a powerful technique for probing a wide range of interfacial processes with high spatial and temporal resolution.51-53 The modified electrode acted as a substrate. A series of approach curves were obtained by SECM on the gold electrode for different stages in 1 mM Fe(CN)63-/4- solution. When a steady-state is established, the normalized current becomes independent of the ratio of diffusion coefficients and depends only on the tip/substrate distance.54 The results were normalized to allow the comparison of different experiments. Figure 7a presented the feedback current decreased on the totally SAM-covered electrode, whereas it gradually increased by approach curves with the increase of concentrations of H2PO4-. The change showed that electron transfer was accelerated at a SAMmodified electrode because of the existence of H2PO4-. However, under the same concentration of Cl-, Br-, NO3-,

or NO2- to the electrolytes, no effect was observed by the approach curve. Of course, the same monolayer responded with slight affinity to the presence of AcO- by hydrogen bonding with the amide NH groups in Figure 7b.19 AcOcould create some interference when sensing H2PO4-. All approach curves were obtained at a distance of 10 µm from the electrode surface. As to SH-TPP, a consistent phenomenon was observed by SECM. It is interesting to design artificial receptors for the H2PO4- ion because of its biological importance; the present work will be of great significance. More importantly, it also establishes the potential of the porphyrin system as an anion recognition moiety and the importance of recognition and the recognition of anions. To prove the protonation of a simple meso-tetraphenylporphyrin in an aqueous system induced by the low concentration of H+ which was ionized from the proton sources of H2PO4-, the anion recognition properties of SAMs of cobalt porphyrin was investigated by observing their blocking effect on the cyclic voltammetric response of the Fe(CN)63-/4- redox couple. With the insertion of Co ions, the CV curve was much different from those of SH-TPP. It was clear that SH-CoTPP amplifies the peak current, and the peak currents were higher than those of the SH-TPP modified electrode but lower than those of the bare Au electrode, indicating that the electron transfer ability of the SAM of SH-CoTPP increased compared to that of the SAM of SH-TPP. The decreased resistance also indicated that the electron transfer ability of the SAM of SH-CoTPP increased compared to that of the SAM of SH-TPP, which was probably due to the fact that (i) the insertion of Co ions changed the molecular structure, and the molecular structure of SH-CoTPP played an important role in electron transfer through the SAM and that (ii) the insertion of Co ions increased the electron tunneling probability through the monolayer.55,56 Figure 8 shows the CV response of the Fe(CN)63-/4- redox couple at a SAMmodified electrode with the increase of H2PO4- concentrations; no obvious change on the redox current of the redox couple was observed, indicating that the SH-CoTPP SAM cannot increase electron transfer with the increase of H2PO4- concentrations. It further demonstrated that the protonation of a simple meso-tetraphenylporphyrin in an aqueous system induced by the low concentration of H+ cannot occur due to the insertion of Co ions. Moreover, the addition of H2PO4- to the electrolyte solution resulted in a weak change to the couple of Co(II)/Co(III) in the Faradaic current. Both cathodic and anodic currents are weakly reduced with the change of H2PO4- concentration. However, the cathodic (0.46 V) and anodic (0.56 V) peaks remained stable. This observation demonstrated that compound

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Figure 8. CVs of SAM-modified gold electrodes with SH-CoTPP in 1 mM Fe(CN)63-/4- containing 0.1 M KPF6 in the presence of H2PO4from bottom to top (0, 10, 20, 30 mM) at a scan rate of 50 mV/s.

Figure 9. Visible spectra of TPPS4 and upon the addition of 10 mM Cl-, Br-, NO2-, NO3-, and H2PO4- in water from top to bottom.

SH-CoTPP can weakly bind H2PO4-. Despite these effects by porphyrin and auxiliary ligands, the weak change seemed to be governed by the nature of a central metal ion in principle. The metal center in a higher oxidation state was suited for the recognition of strongly hydrated anions. This phenomenon indicated that SH-CoTPP had a small selectivity for H2PO4over the other anions. 3.3. Anion Recognition Detected by UV-Vis Spectroscopy. In order to further confirm the protonation of a mesotetraphenylporphyrin induced by H+, the water souble mesotetro-(4-sulfonatophenyl) porphyrin (TPPS4) was synthesized and purified by us. The binding properties of TPPS4 with different anions were also assessed via UV-vis spectroscopy. Figure 9 showed the changes in the visible spectra of TPPS4 at

J. Phys. Chem. C, Vol. 113, No. 30, 2009 13171 a concentration of 8.5 × 10-6 M in aqueous solution upon the addition of different anions. The maximum Soret band for the free TPPS4 solution appears at 413 nm. When the same equivalent amount (10 mM) of Cl-, Br-, NO3-, or NO2- was introduced to the solution of TPPS4, the absorbance almost remained stable, indicating no significant interaction between these anions and TPPS4. However, when H2PO4- was added to the solution of TPPS4, the absorbance at the 413 nm peak drastically decreased with a red shift of the Soret band to 432 nm (Soret band of the protonated monomer). These observations demonstrated that the TPPS4 cannot bind appreciably with Cl-, Br-, NO3-, or NO2-, but bound more strongly to H2PO4-. Of course, after the addition of H2PO4-, the absorbance wavelengths were obviously significantly red-shifted, and the color changed from straw yellow to green. Figure 10a showed the absorption spectra changes of TPPS4 in the presence of different amounts of H2PO4- in the solution of TPPS4. The maximum Soret band of TPPS4 occurs at 413 nm, upon the addition of 0.5 mM of H2PO4-, leading to a slight decrease of the intensity of the absorption band at 413 nm. However, upon the addition of increasing concentrations of H2PO4- to the TPPS4 aqueous solution caused not only a significant hypochromicity of the 413 nm Soret band but also bathochromic shifts of the 432 nm Soret band. The evident changes in visible absorption spectra of TPPS4 after the addition of H2PO4- should be attributed to the interaction of protons with TPPS4, which increased the acidity of the pyrrole NH proton.49 In addition, no absorbance changes at 475 and 725 nm (absorption bands of H4TPPS42+ aggregations57) appeared, suggesting no porphyrin aggregates in the presence of H2PO4-. These spectroscopic changes further indicate that the H2PO4anion acts as an important element interacting with the protonated porphyrin and makes a complex influence on porphyrin protonation. It was beneficial for the recognition process. First, it rendered the pyrrole NHs more acidic and therefore more available for hydrogen bonding (while not being too acidic to undergo deprotonation). Second, the central protons were tied up tightly in the nitrogen to oxygen of dihydrogeyphosphate anion hydrogen bonds responsible for the color change being more effective. Figure 10b shows the variation of change in absorbance with [H2PO4-]. At low concentrations, the plot was approximately linear but exhibited saturation as the concentration increased. 4. Conclusions Three SAMs based on porphyrins were successfully prepared to detect H2PO4- recognition properties by EIS and SECM

Figure 10. (a) Visible spectra of TPPS4 upon the addition of increasing amounts of H2PO4- in water; the arrows indicate the signal changes as increases in H2PO4- concentrations (0, 0.5, 1.0, 2.5, 3.5, 5.0, 7.0, 10.0, 20.0, 30.0, and 40.0 mM). (b) Variation of change in absorbance with the addition of increasing amounts of H2PO4-.

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because of a negatively charged redox couple in the electrolyte conversion which turns the binding event into an electrochemically detectable signal. The experimental results verified that selective binding with H2PO4- occurred on the SAM (either SH-TPP or TCA-ATP) interface among other anions such as Cl-, Br-, NO2-, or NO3-. However, SH-CoTPP had a small selectivity for H2PO4- because the metal center in a higher oxidation state was suited for recognition of strongly hydrated anions. The protonation of the porphyrin core was confirmed in an aqueous system. A molecular model that the 1:1 cationic complex formed between dihydrogenphosphate and the diprotonated form of the porphyrin core by hydrogen bond was proposed. Upon binding H2PO4- anions, SAMs (either SH-TPP or TCA-ATP) showed a monotonically drastic decrease of Rct values with the increase of H2PO4- concentrations. The dependence of Ket on the concentrations of H2PO4- demonstrated that binding of H2PO4- to the monolayer of SH-TPP increases the heterogeneous electron transfer rate constant (Ket) from 1.78 × 10-4 cm2/s to a limiting value of 4.46 × 10-4 cm2/s at concentrations above 30 mM, indicative of surface binding site saturation. It is of particular significance to detect the dihydrogenphosphate anion in an aqueous solution because of its biological importance, including in clinical chemistry, environmental chemistry, biochemistry, pharmacology, and others. The use of electrochemical measurements on SAMs provide an advantage for anion recognition, which can be easily carried out in aqueous media. Acknowledgment. This work was supported by the Natural Science Foundation of China (nos. 20775060 and 20875077), the Natural Science Foundation of Gansu (nos. 0701RJZA109 and 0803RJZA105), and Key Projects of Scientific Research Base of Department of Education (no. 08zx-07), Gansu Province, China. Supporting Information Available: General methods for the synthesis of porphyrins. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Kwok, K. S.; Ellenbogen, J. C. Mater. Today 2002, 5, 28–37. (2) Carroll, R. L.; Gorman, C. B. Angew. Chem., Int. Ed. 2002, 41, 4378–4400. (3) Tour, J. M. Acc. Chem. Res. 2000, 33, 791–804. (4) Zhao, J.; Uosaki, K. Nano Lett. 2002, 2, 137–140. (5) Beulen, M. W. J.; Huisman, B.-H.; van der Heijden, P. A.; van Veggel, F. C. J. M.; Simons, M. G.; Biemond, E. M. E. F.; de Lange, P. J.; Reinhoudt, D. N. Langmuir 1996, 12, 6170–6172. (6) Yang, X.; McBranch, D.; Swanson, B.; Li, D. Angew. Chem., Int. Ed. 1996, 35, 538–540. (7) Roscoe, S. B.; Kakkar, A. K.; Marks, T. J.; Malik, A.; Durbin, M. K.; Lin, W.; Wong, G. K.; Dutta, P. Langmuir 1996, 12, 4218–4223. (8) Doron, A.; Portnoy, M.; Lion-Dagan, M.; Katz, E.; Willner, I. J. Am. Chem. Soc. 1996, 118, 8937–8944. (9) Scho¨nherr, H.; Vancso, G. J.; Huisman, B.-H.; van Veggel, F. C. J. M.; Reinhoudt, D. N. Langmuir 1997, 13, 1567–1570. (10) Huisman, B.-H.; Kooyman, R. P. H.; van Veggel, F. C. J. M.; Reinhoudt, D. N. AdV. Mater. 1996, 8, 561–564. (11) Brandyopadhyay, K.; Liu, H.; Liu, S.-G.; Echegoyen, L. Chem. Commun. 2000, 141–142. (12) Flink, S.; Boukamp, B. A.; Ban den Berg, A.; van Veggel, F. C. J. M.; Reinhoudt, D. N. J. Am. Chem. Soc. 1998, 120, 4652–4657. (13) Zhang, S.; Echegoyen, L. Tetrahedron Lett. 2003, 44, 9079–9082. (14) Zhang, S.; Song, F. Y.; Echegoyen, L. Eur. J. Org. Chem. 2004, 2936–2943. (15) Seel, C.; de Mendoza, J. In ComprehensiVe Supramolecular Chemistry; Atwood, J., Davies, J. E. D., McNichol, D. D., Vo¨gtle, F., Eds.; Elsevier: New York, 1996; Vol. 2, Chapter 17, pp 519-552.

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