Reagentless Polyol Detection by Conductivity Increase in the Course

Nov 2, 2014 - We report on the novel reagentless and label-free detection principle based on electroactive (conducting) polymers considering sensors f...
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Reagentless Polyol Detection by Conductivity Increase in the Course of Self-Doping of Boronate-Substituted Polyaniline Egor A. Andreyev,† Maria A. Komkova,† Vita N. Nikitina,† Nikolay V. Zaryanov,† Oleg G. Voronin,† Elena E. Karyakina,† Anatoly K. Yatsimirsky,‡ and Arkady A. Karyakin*,† †

Chemistry Faculty and LG-MSU Joint Laboratory, M.V. Lomonosov Moscow State University, 119991, Moscow, Russia Facultad de Química, Universidad Nacional Autónoma de México, 04510 México D.F., México

‡

S Supporting Information *

ABSTRACT: We report on the novel reagentless and label-free detection principle based on electroactive (conducting) polymers considering sensors for polyols, particularly, saccharides and hydroxy acids. Unlike the majority of impedimetric and conductometric (bio)sensors, which specific and unspecific signals are directed in the same way (resistance increase), making doubtful their real applications, the response of the reported system results in resistance decrease, which is directed oppositely to the background. The mechanism of the resistance decrease is the polyaniline self-doping, i.e., as an alternative to proton doping, an appearance of the negatively charged aromatic ring substituents in polymer chain. Negative charge “freezing” at the boron atom is indeed a result of complex formation with di- and polyols, specific binding. Changes in Raman spectra of boronate-substituted polyaniline after addition of glucose are similar to those caused by proton doping of the polymer. Thermodynamic data on interaction of the electropolymerized 3-aminophenylboronic acid with saccharides and hydroxy acids also confirm that the observed resistance decrease is due to polymer interaction with polyols. The first reported conductivity increase as a specific signal opens new horizons for reagentless affinity sensors, allowing the discrimination of specific affinity bindings from nonspecific interactions.

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The area of phenylboronic acid based sugar sensors has been extensively reviewed.17−25 For sensing applications the ester formation is accompanied by a signaling event which most often is a change in optical (absorbance,26−28 fluorescence,29−31 or holographic32,33) or electrical properties.34−36 The electrochemical sensors are obviously the most versatile due to low cost, higher sensitivity and selectivity, and practical independence of analyte color and turbidity. Among them, the reagentless ones attract particular attention for analyte monitoring in real objects without sample pretreatment. To elaborate reagentless sensors it is possible to use unique properties of conducting polymers. Indeed, boronate-substituted polyaniline has been synthesized by electropolymerization of 3-aminophenylboronic acid.37 Accordingly, the potentiometric sensor for saccharides has been elaborated.38−40 Unfortunately, the resulting sensor is not of practical importance, because the maximum response recorded is at the level of 1.5−2.0 mV for glucose. This, taken into account continuous baseline drift, is not suitable for sensing of real objects. Impedimetric sensors based on application of the small sine-wave potential perturbation are apparently among the more sensitive ones. Indeed, such sensors on the basis of boronate-substituted polyaniline are also reported.41−44

olyols comprise a very important group of analytes these days. Among them, the saccharides either free or in the form of glycoconjugates (glycoproteins and glycolipids) serve as biomarkers for many diseases, and their detection is required for many medicinal applications.1 Extremely important is monitoring of glucose for diabetes.2 In addition, both saccharides and hydroxy acids inlay the outer surface of the microbial cell walls, serving thus as promising ligands for detection of microorganisms.3−5 Detection of monosaccharides as well as some of hydroxy acids (lactate) is possible amperometriclly on the basis of the corresponding enzymes oxidases.6−10 However, in conjugated forms polyols require specific biochemical (antibodies, aptamers, etc.) or chemical receptors. The latter are attractive due to the much higher stability (both storage and operational) despite lower selectivity. Among chemical receptors the phenylboronic acid is particularly attractive providing selectivity to bind compounds possessing 1,2- or 1,3-diol functions, which are common structural elements of saccharides and hydroxy acids. The underlying chemistry of this process has been thoroughly investigated.11−15 The generally accepted mechanism is shown in Scheme 1. Starting from weak acidic solutions, phenylboronic acid forms a stable ester with polyols with the negative charge allocated at the boron atom.16 © 2014 American Chemical Society

Received: August 9, 2014 Accepted: November 2, 2014 Published: November 2, 2014 11690

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Scheme 1. Scheme of Phenylboronic Acid Interaction with Diols

printed structures (Rusens Ltd., Moscow) with carbon working electrode were used as supports. Electropolymerization of 3-aminophenylboronic acid was carried out from its 0.04−0.05 M solution in 0.1−0.5 M hydrochloric or sulfuric acid containing 0.2 M NaF in cyclic voltammetric regime. Potential range was from 0.0 to 0.9 V (Ag|AgCl). Sweep rate was 0.04 V s−1. Impedance spectra were recorded at a room temperature taking sine-wave voltage amplitude of 5 mV.

However, these sensors, like sensors based on phenylboronic acid,45−48 and like the majority of other impedimetric sensors (see, for example, ref 49), are practically useless. The reason is that commonly the signaling event consists in increase of resistance.50 Similarly, even conductive polymer based sensors respond on enzyme-catalyzed redox reaction51,52 as resistance increase. However, the increase of resistance is also observed in the presence of nonspecific interactions resulting in, e.g., just blocking the conducting surface of the electrode. Hence, it is impossible to discriminate the specific interactions from nonspecific ones. Moreover, degradation of the conducting polymer based transducers also results in resistance increase. Here we report on the first conducting polymer based transducer with the specific signal resulting in conductivity increase. The mechanism is the self-doping of boronatesubstituted polyaniline by freezing negative charges on boronate ring substituents as a result of complexation with polyols. Since both unspecific signals and polymer degradation are resulted in resistance increase (opposite direction), the reported sensor is of practical importance allowing the discrimination of specific affinity interactions from nonspecific binding.

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RESULTS AND DISCUSSION

Electropolymerization of 3-Aminophenylboronic Acid. 3-Aminophenylboronic acid has been electropolymerized from acidic solution containing fluoride ion as previously reported.37 Optimization of polymerization conditions has been carried out to avoid polymer degradation upon synthesis. Since polyaniline degradation occurs at high anodic potentials it is important to decrease the anodic switching potential during synthesis in potentiodynamic conditions. However, too low anodic switching potential cannot provide enough formation of cation radicals required for polymerization. The lowest value of the anodic switching potential providing successful electropolymerization is 0.9 V (Ag|AgCl), which is 200 mV lower than reported in ref 39. This particular anodic switching potential provides synthesis of the conducting polymer, which is noticed from the increasing monomer oxidation current from cycle to cycle (Supporting Information). Optimization of the growing medium has been carried out in order to achieve the maximum rate of the electropolymerization. Similarly to common polyaniline,53 the highest growth rate for poly(3-aminophenylboronic acid) is provided by sulfuric acid. However, whereas for common polyaniline the growth rate is increased with the rise of acid concentration, the electropolymerization of 3-aminophenylboronic acid is faster in diluted H2SO4 (Supporting Information). This unique behavior can be explained as follows. Polyaniline is known to grow according to para-substitution relative to the amino group of the monomer. Boronic acid residue, −B(OH)2, as the electron acceptor meta-substituent blocks this position. To unlock common polyaniline growth it is necessary to convert the metasubstituent to electron donor. For this aim fluoride ion is added to the growing solution (see above). However, at low pH values fluoride ion exists in the form of hydrofluoric acid, and its free form required to convert boronic acid residue to trifluoroborate is available only in weak acidic solutions, considering the pKa value of HF (≈3.1). That is why electropolymerization of 3aminophenylboronic acid is increased as the concentration of sulfuric acid is decreased. However, to provide polyaniline conductivity the doping by protons is required; hence, there is an optimal concentration of sulfuric acid, in which the growth rate reaches its maximum.

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EXPERIMENTAL SECTION Materials. Experiments were carried out with Millipore Milli-Q water (resistivity 18.2 MΩ·cm at a room temperature). Potassium L-lactate and 3-aminophenylboronic acid hydrochloride were purchased from Sigma-Aldrich (Germany). DGlucose, D-fructose, inorganic salts, and acids were obtained of the highest purity from Reachim (Moscow, Russia). Instrumentation. Cyclic voltammetry was carried out using a PalmSens potentiostat/galvanostat (PalmSens Instruments BV, Netherlands) interfaced to a PC. Impedance spectra were recorded using a Solartron 1255 frequency response analyzer (Solartron, U.K.) with a homemade low-noise electrochemical interface. A Renishaw InVia Raman microscope (Renishaw, U.K.) was used for recording Raman spectra. The latter were acquired with 514 nm excitation laser source, 20× objective in the range from 520 to 1800 cm−1. Methods. A three-compartment electrochemical cell that contained a platinum net auxiliary electrode and Ag/AgCl reference electrode in 1 M KCl was used for electrochemical investigations. Glassy carbon disk electrodes (2.0 mm in diameter) were used as working electrodes. Prior to use, the working electrode was mechanically polished with alumina powder (Al2O3, 5 μm) up to mirror finish observed. Impedance measurements were carried out using the electrochemical cell with the common space of working and auxiliary electrodes. The latter was a platinum cylinder encircling working electrode. To record Raman spectra the three-electrode planar screen11691

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Indeed, such maximum appears in the range from 0.1 to 0.2 M H2SO4 concentration (Supporting Information). Cyclic voltammograms of the electropolymerized 3-aminophenylboronic acid are shown in Figure 1. In acidic solution the

Figure 2. Impedance spectra of the electropolymerized 3-aminophenylboronic acid, 0.05 M phosphate pH = 7.0 with 0.1 M KCl, ΔE = 5 mV, Edc = 50 mV: (Δ) initial, (○) in the presence of 20 mM fructose, (■) in buffer after fructose washing out. Inset: the equivalent circuit used.

Figure 1. Cyclic voltammograms of poly(3-APBA)-modified electrodes in () pH = 1.2 (0.1 M HCl), (− ·) pH = 3.0 and (− · ·) pH = 5.0 (both 0.05 M phthalate), (− −) pH = 7.0 (0.05 M phosphate), 0.1 M KCl as supporting electrolyte. Scan rate: 40 mV s−1.

resistance. We note that solution resistance (see inset in Figure 2) evaluated from the impedance spectrum before and after saccharide addition remains nearly unchanged. Figure 2 also illustrates reversibility of the impedimetric response upon fructose addition. The background experiment with polyaniline without boronic substituents causes only resistance increase after addition of saccharides (Supporting Information). The achieved resistance decrease in the course of the specific interaction is extremely important for analytical purposes. In previous reports on boronate-substituted polyanilines,41−44 on phenylboronic acid,45−48 as well as on the majority of other impedimetric sensors, the only resistance increase as a result of specific binding has been registered. However, unspecific interactions obviously cause also resistance increase, because they deteriorate ion transport toward the electrode surface. Hence, it is impossible to discriminate specific and nonspecific interactions, which makes doubtful real analytical applications. On the contrary, in the reported system the responses for specific and nonspecific interactions are directed in the opposite ways, opening new horizons for reagentless affinity sensors. The achieved resistance decrease upon complexation of boronate-substituted polyaniline with polyols has been expected and even desired due to the reasons described below. For this aim the thorough optimization of the polymer synthesis achieving its best electron conductive properties has been carried out (see the previous section). On the Mechanism for Conductivity Increase of Boronate-Substituted Polyaniline upon Complexation. The discussion on the response mechanism is essential for novel principles of chemical measurements. For this aim we first have to consider electroactivity of the electropolymerized 3-aminophenylboronic acid in neutral solutions, which can be explained in terms of polyaniline self-doping. Polyaniline conductivity is known to be induced by proton doping. The term “self-doped”, i.e., electroactivity in the absence of protonic doping, was first proposed for polyaniline synthesized from fuming sulfuric acid and thus contained sulfo groups as aromatic ring substituents.56 Such negatively charged substituents are able to provide polyaniline redox activity in neutral solutions, where proton concentration is not enough.

cyclic voltammogram is very similar to common polyaniline confirming the above conclusion concerning synthesis of conducting polymer. However, the electropolymerized 3aminophenylboronic acid displays redox activity even in neutral solutions, pH = 7.0 (Figure 1), where common polyaniline is electrochemically inactive. Hence, for analytical purposes it is possible to use voltammetry of the electropolymerized 3aminophenylboronic acid even in neutral solutions. Impedance Spectroscopy of Electropolymerized 3Aminophenylboronic Acid. Impedance spectroscopy has been chosen as one of the most sensitive and less destructive electrochemical tools to investigate a response of the boronic acid substituted polyaniline to saccharides. Impedance spectra have been recorded at the polymer redox potential found from cyclic voltammograms. Impedance spectra of boronate-substituted polyaniline are displayed in Figure 2. The constant potential (Edc) has been chosen close to the polyaniline formal potential found from its cyclic voltammograms in the same solution, where polymer conductivity is of its highest value. In order to investigate properties of the conductive polymer, the impedance spectra were fit to apparently the most simple equivalent circuit valid for conductive polymers (Figure 2, inset).54 Here Rs is solution resistance, Rp is film resistance, C is double layer and film capacitance, and Wo is the diffusion impedance with reflective boundary conditions.55 We note that, except for variation of the exponent in Wo, which is possible for fitting of real conductive polymer spectra,54 all other elements of the circuit are real resistances and capacitance rather than constant phase elements. Fitting (solid lines in Figure 2) give satisfactory results; the errors in all variables do not exceed few percentages. In Figure 2 it is seen that after addition of fructose (20 mM final concentration) the diameter of the high-frequency semicircle is significantly decreased. This means that the resistance of boronate-substituted polyaniline is decreased. Fitting experimental data confirms this observation: fructose addition causes 3 times decreased conducting polymer film 11692

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Apparently, the most successful self-doping has been reported by our group by copolymerization of aniline with maminobenzenesulfonic acid.57,58 Electroactivity of the resulting polymer has been prolonged toward alkaline media for 6−7 pH units. Since local pH change in polyelectrolyte cannot exceed 1−1.5 pH units, the observed electroactivity in basic solutions is indeed a result of the negatively charged aromatic ring substituent in the polyaniline chain.57,58 As shown in Scheme 1, in neutral and basic solutions the boronic acid residue can accept a hydroxyl ion and become negatively charged. Despite that equilibrium in neutral solutions is shifted toward hydroxyl release, a certain concentration of the negatively charged ring substituents is present in the polyaniline chain providing its electroactivity in neutral solutions. Scheme 1 also illustrates that complexation of phenylboronic acid with diols results in a shift of equilibrium toward formation of esters with negatively charged boronic substituent. Hence, in the presence of polyols the concentration of negatively charged ring substituents in polymer matrix is significantly increased, which, considering polyaniline self-doping, should increase polymer electroactivity. Accordingly, the mechanism of the observed conductivity increase in the presence of polyols can be referred to as polyaniline self-doping by “freezing” negative charges in ring substituents upon complexation. To prove the above conclusion the electropolymerized 3aminophenylboronic acid has been investigated by means of Raman spectroscopy. In Figure 3 the spectrum of the polymer

Figure 4. Calibration graphs for glucose (●), fructose (▲), galactose (○), and lactate (□); 0.05 M phosphate with 0.1 M KCl: pH = 7.4 for saccharides and pH = 5.8 for lactate.

normalized calibration graphs for impedimetric detection of glucose, fructose, galactose, and lactate. From these plots it is possible to evaluate the corresponding binding constants using eq 1:

Rp =

R p0 + R p∞K appC 1 + K appC

(1)

where C is polyol concentration, Rp is polymer resistance, R0p and R∞ p are resistance at zero and infinite polyol concentration, respectively, and Kapp is an apparent binding constant. As seen in Figure 4, fitting of the experimental data is quite successful, which confirms an applicability of eq 1. The evaluated binding constants with the electropolymerized 3aminophenylboronic acid for glucose, fructose, and galactose at pH = 7.4 are 20 ± 2, 240 ± 10, and 100 ± 15 M−1, respectively. The binding constant with lactate at pH = 5.8 is 45 ± 10 M−1. The evaluated binding constants of saccharides with boronatesubstituted polyaniline are increased in the row glucose < galactose < fructose. This row is similar to the binding ability to the phenylboronic acid.12 Hence, the increased conductivity is indeed due to its complexation of polyols. Considering binding constants it is possible to evaluate analytical performance characteristics of the sensors. Calibration curves are quite reproducible. Indeed, 22 different sensors displayed variation in glucose binding constant of Sr < 0.1. The shelf life of the synthesized boronate-substituted polyaniline is rather good similarly to common polyaniline. No change in the binding constants is observed after a few weeks of storage. Operational stability can be illustrated by less than 10% variation of the binding constant during several hours of impedance spectra recording in neutral solutions. Sensor reversibility is shown in Figure 2. We note that binding constants for both saccharides and hydroxy acids for the electropolymerized 3-aminophenylboronic acid are similar or slightly higher than those for free phenylboronic acid.12 This indicates a minor influence of geometrical changes upon esterification on the conducting properties of polyaniline. The major exception is the binding constant for glucose, which in case of the polymer is higher 4− 5 times12 as compared to phenylboronic acid. The increased binding constant in the case of the polymer can be explained in terms of polydentate binding: when one polyol molecule interacts with several boronic acid residues in the polymer

Figure 3. Raman spectra of poly(3-aminophenylboronic acid) (a) in phosphate buffer, pH = 7.0, (b) in 0.5 M H2SO4, (c) with 50 mM glucose in phosphate buffer, pH = 7.0.

in neutral buffer solution is compared to the spectrum in the presence of glucose. As seen, in the presence of saccharide the peak corresponding to Raman shift of 1475 cm−1 corresponding to the stretching vibration of CN practically disappears while the peak at 1320 cm−1 corresponding to the C−N+ vibration increases. It is important that the polymer spectrum in the absence of glucose, but in acidic medium, displays quite similar shape (Figure 3). Hence, addition of saccharide in neutral solution, which converts neutral boronic acid groups into anionic boronate esters, affects the polymer similarly to polyaniline protonation, which causes its doping. This confirms that complexation with diol results in self-doping of the electropolymerized 3-aminophenylboronic acid. Thermodynamics of Polyol Complexation with Boronate-Substituted Polyaniline. Figure 4 displays the 11693

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chain. Indeed, it is known that binding constants for glucose to diboronic acids are much higher as compared to it for glucose interaction with phenylboronic acid.15,19−21 On the contrary, fructose is known to interact with the only one boronate substituent.22,59 For further understanding the thermodynamics of polyol complexation with boronate-substituted polyaniline, we have investigated its pH dependence. The pH dependence of diols binding constants to phenylboronic acid is known to the determined by ionization constants (pKa) of both components.11,60 Between the pKa values, when the only one of the components is negatively charged, a plateau corresponding to the maximum binding constant appears. At pH values higher than the highest pKa value and lower than the lowest one there is a sharp decrease of affinity. The ionization constant for phenylboronic acid (pKa ≈ 8.961) is in between of it for saccharides and hydroxy acids (for glucose pKa ≈ 12.3, and for lactate pKa ≈ 3.86). Hence, there should be a common threshold point in pH dependences of binding constants for both saccharides and hydroxy acids. However, whereas affinity of the former reaches in this point its maximum plateau region, the lactate binding constant starts to decrease as pH is increased.53 Figure 5 displays the pH dependences of the binding constants of boronate-substituted polyaniline with glucose and

phenylboronic acid can be explained in terms of complicated receptor structure. Deeper analysis of binding thermodynamics requires independent investigation. To confirm an existence of the ionogenic group in the electropolymerized 3-aminophenylboronic acid with the pKa value close to the discovered threshold point we have compared the impedance spectra of the polymer in solutions with different pH values. The dependence of the evaluated polymer resistance on the solution pH in semilogarithmic plots is almost linear. The deviation from linearity in weak acidic solutions is convex downward, and in neutral media, convex upward (Supporting Information). The inflection point found after taking a derivative of the resistance logarithm versus pH plot corresponds to the pH range from 6.2 to 6.6. This range is quite similar to the range of the threshold point under discussion. Thermodynamic data also confirm that the observed conductivity increase as a result of interaction of the electropolymerized 3-aminophenylboronic acid with polyols is indeed a result of complex formations between boronic acid residue and saccharides or hydroxy acids.

Figure 5. Dependences of the apparent binding constants for lactate (○) and glucose (■) on solution pH.

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CONCLUSIONS The reported reagentless sensing principle is unique for analytical chemistry. The great majority of known impedimetric and conductometric (bio)sensors generate their specific signals as the increase in resistance. This, however, is of doubtful practical importance because nonspecific signals (background) are also directed in resistance increase. Thus, one never knows whether low concentration of analyte or high content of interfering compounds is sensed. To overcome this problem it is necessary to wash out interfering compounds by high ionic strength, extreme pH, or detergents. This makes such protocols not reagentless. Accordingly, it is hard to overestimate the true reagentless analytical principle based on the transducer, which specific signal is resulted in resistance decrease, directed oppositely to the background. In the present report for generation of the specific signal the effect of polyaniline self-doping in neutral media is used. This, however, will not limit a search for similar analytical principles, which obviously opens new horizons for analytical chemistry.

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

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+

K app =

ASSOCIATED CONTENT

* Supporting Information

lactate. As seen, indeed for hydroxy acid (lactate) the binding constant displays a plateau at low pH values and starts to decrease as pH is increased. As expected, for the saccharide the dependence is inversed. Moreover, fitting the pH dependences of the binding constants to the equation K ′ + K ″[H ]/K aB 1 + [H+]/K aB

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +74959394675. Notes

where K″ and K′ are fitting parameters, the latter is equal to zero for hydroxy acids, gives similar threshold points: pKBa = 6.1 ± 0.2 for glucose and pKBa = 6.3 ± 0.2 for lactate. The evaluated protonation constant from pH dependence of the fructose binding constant (Supporting Information) is also very close (pKBa = 6.6 ± 0.2). Hence, in pH dependences of the electropolymerized 3-aminophenylboronic acid affinity there is a common threshold point with inversed behavior for saccharides and hydroxy acids. The fact that the discovered threshold point does not coincide with the pKa value for

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support from LG Electronics through LG-MSU joint laboratory is greatly acknowledged.

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dx.doi.org/10.1021/ac5029819 | Anal. Chem. 2014, 86, 11690−11695