Overoxidation and Camphorsulfonic Acid Coinduced Chiral

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Overoxidation and Camphorsulfonic Acid Coinduced Chiral Microenvironment in Polypyrrole for Electrochemical Chiral Sensing Jiapei Yang, Yin Yu, Datong Wu, Yongxin Tao, Linhong Deng, and Yong Kong Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b02269 • Publication Date (Web): 12 Jul 2018 Downloaded from http://pubs.acs.org on July 13, 2018

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Analytical Chemistry

Overoxidation and Camphorsulfonic Acid Coinduced Chiral Microenvironment in Polypyrrole for Electrochemical Chiral Sensing

Jiapei Yang, Yin Yu, Datong Wu, Yongxin Tao, Linhong Deng, and Yong Kong* Advanced Catalysis and Green Manufacturing Collaborative Innovation Center, Changzhou University, Changzhou 213164, China

Email: [email protected] Tel.: 86-519-86330253; fax: 86-519-86330167.

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Analytical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT:

Polypyrrole (PPy) was

synthesized by galvanostatic method using

(1S)-(+)-10-camphorsulfonic acid ((+)-CSA) as the dopant, and the produced PPy was further overoxidized in the solution of (+)-CSA. Chiral microenvironment was successfully formed in the overoxidized PPy (OPPy) due to the synergistic effects of overoxidation and (+)-CSA, resulting in a twisted helical architecture of the OPPy chains. The formation of optical active OPPy was confirmed from the aspects of morphology (SEM and AFM) and circular dichroism (CD) spectra. Finally, an electrochemical chiral sensor was fabricated based on the resultant OPPy, which exhibits excellent biomolecular homochirality in the discrimination of tryptophan (Trp) enantiomers.


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Chirality (handedness) is of significant importance in the living world. In polymer chemistry, although molecularly imprinted polymers (MIPs) have been successfully applied for chiral recognition,1‒3 the design and synthesis of synthetic chiral polymers has received considerable attention in the past decades since they may mimic the exceptional behavior of biological polymers4 and have found potential applications in chiral separations.5‒8 Because of its good chemical stability, relative ease of synthesis and exceptional homochirality, chiral polypyrrole (PPy) has attracted particular attention to date.9‒11 Usually, chiral PPy is synthesized by oxidative polymerization of pyrrole monomer in the presence of a chiral dopant anion. Overoxidation of conducting polymers can lead to the partial destruction of polymeric backbone and generation of oxygen-containing groups,12,13 and overoxidized PPy (OPPy) has been used in electrochemical sensors and MIPs owing to the high permselectivity of OPPy.14‒16 However, as far as we are aware, little or no attention has been paid to the chiral recognition with OPPy directly, which might be attributed to the poor optical activity of OPPy. Herein, we report on the synthesis of optical active OPPy and its applications in chiral sensing. PPy was firstly galvanostatically synthesized using (1S)-(+)-10-camphorsulfonic acid ((+)-CSA) as the dopant, and then the electrodeposited PPy was further overoxidized in the solution of (+)-CSA. The polymeric backbone of PPy was partially disrupted during the overoxidation process, which was accompanied by the formation of H-bonds between OPPy and (+)-CSA in the solution. The synergistic effects of backbone destruction resulted from overoxidation and intermolecular H-bonds between OPPy and (+)-CSA would lead to 3 ACS Paragon Plus Environment


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conformation change of the OPPy and result in a twisted helical architecture of the OPPy chains. Finally, the chiral OPPy was applied successfully in the chiral recognition of tryptophan (Trp) enantiomers. Here, Trp was chosen as the target model because both L- and 17

D-Trp play important roles in natural systems. L-Trp is an essential constituent of proteins

while D-Trp has great influence on biofilm formation and bacteria growth.18,19 Excitingly, the proposed optical active OPPy showed extremely high recognition ability toward the Trp enantiomers, which was significantly superior to those with the polysaccharides-based chiral sensing platforms reported by our group.20‒24 EXPERIMENTAL SECTION Reagents and Apparatus. Pyrrole (99.7%), (1S)-(+)-10-camphorsulfonic acid ((+)-CSA), (1R)-(–)-10-camphorsulfonic acid ((–)-CSA), L-(+)-tartaric acid ((+)-TA), L-tryptophan (L-Trp) and D-tryptophan (D-Trp) were purchased from Aladdin Chemistry Co., Ltd. (Shanghai, China), and pyrrole was distilled under reduced pressure before use. Potassium ferrocyanide (K4[Fe(CN)6]), potassium ferricyanide (K3[Fe(CN)6]), sodium chloride (NaCl), potassium chloride (KCl), sodium dihydrogen phosphate (NaH2PO4) and disodium hydrogen phosphate (Na2HPO4) were received from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All solutions were prepared with ultrapure water (Milli-Q, Millipore). All electrochemical experiments including galvanostatic electrodeposition of polypyrrole (PPy), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and differential pulse voltammetry (DPV) were carried out at room temperature (25 oC) on a CHI-660D electrochemical workstation in a conventional three-electrode system with a glassy carbon electrode (GCE) or modified GCE (PPy/GCE, OPPy/GCE) as the working 4 ACS Paragon Plus Environment

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electrode, a platinum foil (10 × 5 mm) as the auxiliary electrode and a saturated calomel electrode (SCE) as the reference electrode. The morphology and surface topography of PPy and OPPy were characterized with a Supra55 field emission scanning electron microscope (FESEM, Zeiss, Germany) and a JPK NanoWizard3 atomic force microscope (AFM, Germany), respectively. The circular dichroism (CD) spectra were recorded on a Jasco J-810 spectrometer with a scan rate of 60 nm min-1. The measurements of water contact angles on different samples were carried out with a DSA25 water contact angle goniometer (Kruss GmbH, Germany), and 3 μL of water droplet was dropped onto the testing surface for each measurement. Preparation of OPPy Films. The films of PPy doped with (+)-CSA were deposited galvanostatically (0.3 mA) on a GCE (3 mm in diameter) in an aqueous solution containing 0.15 M pyrrole and 50 mM (+)-CSA for 15–180 s. The PPy films were then overoxidized in a 50 mM (+)-CSA aqueous solution, and the overoxidation was carried out by scanning the potential in a range from 0 to +1.2 V at a scan rate of 100 mV s-1 for 20 cycles. For control experiments, several other OPPy films were also synthesized as follows. (1) PPy was firstly galvanostatically prepared without the doping of (+)-CSA, which was then overoxidized in 0.1 M phosphate buffer solution (PBS) of pH 7.0. The resultant OPPy films were abbreviated as OPPy–1. (2) PPy was galvanostatically synthesized with the doping of (+)-CSA, and then it was overoxidized in 0.1 M PBS of pH 7.0 (OPPy–2). (3) PPy films were firstly deposited galvanostatically with the doping of 50 mM (+)-TA, and then the PPy films doped with (+)-TA were overoxidized in a 50 mM (+)-TA solution (OPPy–3). Electrochemical Measurements. Cyclic voltammograms of different OPPy samples 5 ACS Paragon Plus Environment


were recorded over the potential range from –0.9 to 0.5 V in 1.0 M NaCl solution at a scan rate of 100 mV s-1, and the Nyquist plots of different OPPy samples were recorded in 0.1 M KCl solution containing 5 mM [Fe(CN)6]3-/4- in the frequency range from 105 to 0.01 Hz. Chiral Recognition of Trp Enantiomers with OPPy. Chiral recognition of Trp enantiomers was investigated at room temperature by DPV since DPV can easily convert a chiral recognition event into identifiable changes in potential or current signals.3,25 The three-electrode system with the OPPy modified GCE (OPPy/GCE) or other modified electrodes (OPPy–1/GCE, OPPy–2/GCE, and OPPy–3/GCE) as the working electrode was placed into 25 mL 0.1 M PBS (pH = 4.5–7.5) containing 1.0 mM of L- or D-Trp for 90 s, and then the differential pulse voltammograms of L- and D-Trp combined with these OPPy samples were recorded in a range from 0.4 to 1.2 V. Each DPV experiment was repeated in triplicate and the standard deviation was calculated for labeling the error bars. The recognition efficiency of different electrodes toward the Trp enantiomers was evaluated by using the peak current ratio of D-Trp to L-Trp (ID-Trp/IL-Trp) as the indicator. RESULTS AND DISCUSSION Formation of Optical Active OPPy Films. The formation of the OPPy films induced by overoxidation in the (+)-CSA solution could be seen from the SEM images of PPy and OPPy (Figure 1). The PPy films prepared without the doping of (+)-CSA had a cauliflower-like appearance, however, PPy doped with (+)-CSA, a chiral acid dopant, was consisted of interwoven nanofibers with a few scattered cavities. After overoxidation in the (+)-CSA solution, a twisted helical architecture was found for the resultant OPPy. Partial destruction of the polymeric backbone of PPy occurred during the overoxidation process, meanwhile, 6 ACS Paragon Plus Environment

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interactions between the (+)-CSA in the solution and OPPy took place via two types of H-bonds: (1) H-bonds between the hydroxyl groups of (+)-CSA and the oxygen-containing groups on OPPy; (2) H-bonds between the carbonyl groups of (+)-CSA and the imido groups in OPPy. As a result, the OPPy chains were pulled apart by overcoming the interactions among them, generating a twisted helical architecture. The formation of the OPPy films was illustrated as Figure 2. The AFM images of the OPPy films also revealed the existence of amounts of nano-sized cavities and a topography of spiral ribbons (Figure 3), further confirming the formation of optical active OPPy films with twisted helical architecture.

Figure 1. SEM images of PPy without (A, B) and with (C, D) (+)-CSA doping, and OPPy (E, F) with two different resolutions.



Figure 2. Formation mechanisms of OPPy films induced by overoxidation in (+)-CSA solution.

Figure 3. AFM images of OPPy films (A, B) with two different scales.

Circular Dichroism (CD) Spectra of OPPy. Next, the circular dichroism (CD) spectra of OPPy and (+)-CSA were recorded to examine the chirality of the OPPy (Figure 4). For (+)-CSA (0.045 M), a positive CD peak appeared at 289 nm with a magnitude (Δε) of 2.38 M-1 cm-1 at 25 oC, agreeing well with the previous reports.26,27 There was also a positive CD peak at 290 nm for the OPPy with a significantly increased Δε (5.74 M-1 cm-1). Since OPPy does not contain an asymmetric carbon atom in the structure, one might expect that the 8 ACS Paragon Plus Environment

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greatly enhanced Δε would be ascribed to the newly formed helical architecture along the OPPy chains during the overoxidation in the (+)-CSA solution. 125

(+)-CSA OPPy

100

CD / mdeg

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


75 50 25 0 250

300

350

400

450

500

Wavelength / nm

Figure 4. Circular dichroism spectra of (+)-CSA and OPPy.

Electrochemical Properties of OPPy. The electrochemical properties of the OPPy were further studied by CV and EIS. For comparison, PPy without and with (+)-CSA doping were also studied. As shown in Figure 5A, PPy without (+)-CSA doping exhibited good electroactivity with the appearance of a pair of well defined redox peaks (Curve a), and the electroactivity was further enhanced for the PPy doped with (+)-CSA (Curve b). Interestingly, the electroactivity of the OPPy was dramatically suppressed since the area under the CV curve was fairly small (Curve c). The decreased electroactivity of OPPy could be attributed to the fact that overoxidation is regarded as an undesirable degradation process and usually leads to the loss of conductivity of conducting polymers.12



0.4

b

0.3

a

0.2 0.1

c

0.0 -0.1

12 10 8

800

-1.0 -0.8 -0.6 -0.4 -0.2 0.0

0.2

0.4

600

Q

b

400

Rct Wd

200 0

6

0

200

400

600

800

c

1000

Z-real / ohm

4 2 a b

0

0.6

Rs

a

0

-0.2

(B)

1000

Z-imaginary / ohm

14

(A) Z-imaginary / kohm

0.5

I / mA

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2

E vs. SCE / V

4

6

8

10

12

14

Z-real / kohm

Figure 5. (A) Cyclic voltammograms of PPy without (a) and with (b) (+)-CSA doping, and OPPy (c) in 1.0 M NaCl solution at a scan rate of 100 mV s-1. (B) Nyquist plots of PPy without (a) and with (b) (+)-CSA doping, and OPPy (c) in 0.1 M KCl solution containing 5 mM [Fe(CN)6]3-/4- in the frequency range of 105 to 0.01 Hz. Inset of (B): Enlargement of Curves a and b (left) and the corresponding equivalent circuit (right).

EIS of different samples was studied to know the impedance changes at the electrode-solution interfaces. The Nyquist plots of OPPy, PPy without and with the doping of (+)-CSA were shown in Figure 5B, and the diameter of the suppressed semicircle referred to the charge transfer resistance (Rct). The Rct value of PPy without (+)-CSA doping was 121.6 Ω, and it was decreased to 106.8 Ω for the PPy doped with (+)-CSA. However, the value of Rct was significantly increased to 11.73 kΩ after the PPy doped with (+)-CSA was further overoxidized, indicating that overoxidation can really cause the loss of conductivity of OPPy. The EIS result agreed well with that of CV. Chiral Recognition of Trp Enantiomers with OPPy. Recognition of Trp enantiomers with the OPPy and other samples was studied by DPV at room temperature (Figure 6). PPy doped with (+)-CSA showed an obvious oxidation peak on the differential pulse voltammograms (DPVs), interfering in the chiral recognition of Trp enantiomers (Figure 6A). 10 ACS Paragon Plus Environment

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Therefore, PPy was firstly overoxidized before its application in the chiral recognition. That is to say, overoxidation not only brought about chirality, but also circumvented the oxidation peak of PPy on the DPVs. The DPVs of the Trp enantiomers combined with chiral OPPy, OPPy–1 and OPPy–2 were recorded. As expected, the oxidation peaks of L- and D-Trp were almost overlapped at the OPPy–1 (Figure 6B), since (+)-CSA was absent during the synthesis and overoxidation of PPy. Although (+)-CSA did not participate in the overoxidation of PPy for the preparation of OPPy–2, a discernible peak current ratio (ID-Trp/IL-Trp) was observed for the OPPy–2 (Figure 6C), suggesting that L- and D-Trp could be discriminated with the OPPy–2 due to the doping of chiral (+)-CSA during the synthesis of PPy. It was even more exciting to find that the peak current ratio was greatly increased to 10.22 at our OPPy (Figure 6D), indicating that extremely high recognition efficiency could be achieved at the OPPy resulted from the overoxidation of PPy in (+)-CSA solution. Of particular interest, it showed that IL-Trp was significantly lower than ID-Trp, implying that the OPPy possesses excellent homochirality.



5

25 20 15 10

0

0.2

0.4

0.6

0.8

3 2

(C)

0

1.0

0.6

1.0

1.2

(D)

L-Trp D-Trp

10 8

I / µA

6

ID / IL = 5.05

6

2

0

0 0.4

0.6

0.8

1.0

ID / IL = 10.22

4

2

0.4

1.2

0.6

0.8

1.0

1.2

E vs. SCE / V

E vs. SCE / V 12

12

(E)

L-Trp D-Trp

10

(F)

L-Trp D-Trp

10 8

I / µA

8 6 4

0.8

12 L-Trp D-Trp

8

4

0.4

E vs. SCE / V

E vs. SCE / V

12

I / µA

D-Trp

1

5

10

L-Trp

4

I / µA

I / µA

(B)

PPy L-Trp D-Trp

(A)

I / µA

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ID / IL = 3.93

6

2

2

0

0

0.4

0.6

0.8

1.0

IL / ID = 9.67

4

0.4

1.2

0.6

0.8

1.0

1.2

E vs. SCE / V

E vs. SCE / V

Figure 6. Differential pulse voltammograms of L- and D-Trp combined with PPy doped with (+)-CSA (A), OPPy–1 (B), OPPy–2 (C), OPPy (D), (+)-CSA (E) and OPPy prepared using (–)-CSA for doping and overoxidation (F).

To further understand the chiral recognition was caused by overoxidation of PPy in (+)-CSA solution or the doping of (+)-CSA in the OPPy, DPVs of L- and D-Trp bound to the (+)-CSA modified GCE were recorded, in which the (+)-CSA/GCE was prepared by direct electrodeposition of (+)-CSA (50 mM) on the surface of GCE at a constant potential of +1.0 V for 600 s. As shown in Figure 6E, the value of ID-Trp/IL-Trp at the (+)-CSA/GCE was dramatically decreased compared to that at the OPPy/GCE (3.93 versus 10.22), suggesting 12 ACS Paragon Plus Environment

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that actually overoxidation in the (+)-CSA solution played the leading role in the efficient discrimination of the Trp enantiomers. Another experiment was designed to better understand the biomolecular homochirality of the OPPy, in which OPPy was synthesized using (–)-CSA for doping and overoxidation, and the DPVs of L- and D-Trp combined with the resultant OPPy were shown in Figure 6F. High recognition efficiency (IL-Trp/ID-Trp = 9.67) was also achieved with the (–)-CSA-related OPPy, however, the homochirality was completely reversed. In other words, D-Trp combined with the (–)-CSA-related OPPy was actually ignorable compared to L-Trp. In addition, OPPy–3 (using (+)-TA for doping and overoxidation) was also used for the chiral recognition, and the peak current ratio (ID-Trp/IL-Trp) at the OPPy–3 was only 3.36 (Figure 7A). To understand the decreased recognition efficiency with OPPy–3, the SEM image of OPPy–3 was also recorded (Figure 7B). The twisted helical architecture of the OPPy (Figures 1E and F) was not observed in the OPPy–3, which might be responsible for the decreased recognition ability of OPPy–3. To prepare the OPPy–3, PPy was firstly synthesized galvanostatically using (+)-TA as the dopant, and then it was further overoxidized in the solution of (+)-TA. Although (+)-TA also has two chiral carbon atoms as (+)-CSA, the two chiral carbon atoms have the same microenvironments (Figure 8, left). Therefore, one-directional helical growth could not be achieved for the OPPy–3. However, the two chiral carbon atoms in (+)-CSA are located in quite different microenvironments (Figure 8, right), and the consequently formed helical wrapping along the OPPy chains could effectively improve the recognition ability of OPPy.



12

(A)

L-Trp D-Trp

10 8

I / µA

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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6

ID / IL = 3.36

4 2 0 0.4

0.6

0.8

1.0

1.2

E vs. SCE / V

Figure 7. (A) Differential pulse voltammograms of L- and D-Trp combined with OPPy–3. (B) SEM image of OPPy–3. H 3C O HO

*

OH * OH

*

OH

*

CH3

O O

O

O

S OH

(+)-CSA

(+)-TA

Figure 8. Chemical structures of L-(+)-tartaric acid ((+)-TA) and (1S)-(+)-10-camphorsulfonic acid ((+)-CSA).

Comparison of Recognition Ability between OPPy and Other Chiral Trp Sensors. A comparison between the OPPy and the polysaccharides-based chiral sensors reported by our group was made, and the results indicated that the OPPy-based chiral sensor outperformed the previous sensors for the recognition of Trp (Table 1). The greatly enhanced recognition efficiency of the OPPy could be ascribed to the following two reasons. On one hand, electrochemical recognition with the polysaccharides-based sensors was driven by the intrinsic chirality of the polysaccharides (-cyclodextrin, chitosan, Cu2+-modified -cyclodextrin, and so on); however, recognition with the OPPy was achieved by the induced chiral microenvironment by overoxidation and (+)-CSA. On the other hand, polysaccharides are weak-polar substances whereas (+)-CSA is a chiral 14 ACS Paragon Plus Environment

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acid with strong polarity, and the interactions between (+)-CSA and the strong-polar targets (Trp enantiomers) might be more pronounced compared with the weak-polar polysaccharides. Table 1 Comparison of recognition ability between OPPy and polysaccharides-based chiral sensors for the electrochemical recognition of Trp enantiomers. Recognition efficiency Chiral Trp sensor

Reference (peak current ratio)

-CD

2.30

20

sulfonated chitosan

2.38

21

Cu2+-modified -CD

3.28

22

carboxymethyl cellulose-chitosan

4.95

23

Cu2+-modified -CD self-assembled to chitosan

7.26

24

optical active OPPy

10.22

this work

Optimization of Parameters for Chiral Recognition. The parameters influencing the chiral recognition of Trp enantiomers such as the electrodeposition time of PPy, the pH of Trp solution and the duration time before DPV detection were optimized. The films of PPy doped with (+)-CSA were deposited galvanostatically on GCE, and therefore the thickness of the PPy films was closely associated with the deposition time of PPy. As shown in Figure 9, the thickness of the PPy films was increased with increasing deposition time from 30 to 150 s. Since a twisted helical architecture was formed along the OPPy chains during the overoxidation process due to the synergistic effects of polymeric backbone destruction resulted from overoxidation and H-bonds between (+)-CSA and OPPy, excessively thick 15 ACS Paragon Plus Environment


OPPy films were disadvantageous to the formation of such helical architecture owing to the generated extra stresses, leading to a decreased recognition efficiency (Figure S1, see Supporting Information). Noted that the recognition efficiency was also not high enough (ID-Trp/IL-Trp = 7.07) when PPy was galvanostatically deposited for only 15 s, and it might be ascribed to the fact that excessively thin films of PPy were easily destructed during the overoxidation process.

Figure 9. AFM images of OPPy films resulted from the overoxidation of PPy synthesized with different deposition time.

The pH of the Trp solution also influenced the recognition efficiency. As shown in Figure S2 (see Supporting Information), the recognition efficiencies at pH 4.5, 5.0 and 5.5 were significantly higher than those at pH above 6.0. Both L- and D-Trp are positively charged at pH below 5.89 because the isoelectric point of Trp is 5.89, meanwhile, OPPy has a high electronegativity after overoxidation due to the generation of amounts of oxygen-containing groups. Therefore, the electrostatic attractions between the positively charged Trp enantiomers and OPPy would be advantageous to the combination of Trp and OPPy at pH below 5.89. On the contrary, the Trp enantiomers were negatively charged at pH above the 16 ACS Paragon Plus Environment

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isoelectric point, and therefore the electrostatic repulsions between OPPy and the Trp enantiomers would lead to a decreased recognition efficiency. The dependence of recognition efficiency on the duration time was investigated (Figure S3, see Supporting Information). The results indicated that the recognition efficiency experienced a rapid augment with increasing duration time from 10 to 90 s, and a high recognition efficiency (10.22) was obtained at the duration time of 90 s. When the duration time was further increased from 90 to 110 s, only a slight increase in the recognition efficiency was observed (10.45). Higher Affinity of OPPy toward D-Trp. Figure 6D showed that the OPPy exhibited a higher affinity toward D-Trp than its enantiomer. Here, the higher affinity of OPPy toward D-Trp was further demonstrated by wettability measurements. As shown in Figure 10, the

water contact angle on bare GCE was 60.0o. After the enrichment of L- and D-Trp on the bare GCE, the wettability was almost identical for the both samples (56.0o and 55.9o), suggesting that the Trp enantiomers could not be discriminated with bare GCE at all due to the absence of chiral sites. Although the wettability of OPPy–1/GCE was increased a little after the combination with L- and D-Trp, few differences in the water contact angle were found (52.6o and 52.9o) owing to the poor recognition ability of OPPy–1. Interestingly, the wettability of OPPy/GCE was greatly increased (41.4o) compared to bare GCE (60.0o) and OPPy–1/GCE (62.6o), which might be ascribed to the doping of hydrophilic (+)-CSA in PPy and the subsequent overoxidation of PPy in (+)-CSA solution. More importantly, the wettability of OPPy was further increased to 21.6o after the combination of D-Trp, which was more pronounced than that combined with L-Trp (38.0o), and the results clearly indicated that more 17 ACS Paragon Plus Environment


D-Trp were combined with the OPPy than L-Trp. In other words, the OPPy had excellent

homochirality and showed a higher affinity toward D-Trp.

Figure 10. Water droplet (3 μL) images and water contact angle measurements on different samples.

Chiral Specificity, Electrochemical Stability and Reproducibility of OPPy. The chiral specificity of the OPPy was studied. Figures 11A and B showed the DPVs of L-/D-cysteine (Cys) and L-/D-tyrosine (Tyr) combined with the OPPy under the same conditions as L-/D-Trp. As shown in Figure 11C, the ID/IL values for the enantiomers of Cys (1.30) and Tyr

(1.61) were greatly lower than that of Trp (10.22), suggesting that the recognition efficiency of the OPPy for Trp was significantly higher than those for Cys and Tyr. The excellent chiral specificity of the OPPy was mostly likely due to the different chemical structures of the three amino acids. The Trp molecule contains an indole ring, which is consisted of a benzene ring and a five-membered pyrrole ring, and thus the double-aromatic ring structure of Trp was preferably entered into the twisted helical architecture of OPPy. However, Cys and Tyr belong to non-aromatic and mono-aromatic compounds, respectively, and the small sizes of the two chiral amino acids resulted in greatly lower recognition efficiencies than Trp. Noted 18 ACS Paragon Plus Environment

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that the recognition efficiency of Tyr was a little higher than Cys (1.61 versus 1.30), which could be ascribed to the relatively larger size of Tyr possessing a mono-aromatic ring. 12

12 10

L-Tyr D-Tyr

(B)

L-Cys D-Cys

(A)

10 8

8

ID / IL = 1.30

6

I / µA

I / µA

4

6

2

0

0 0.4

0.6

0.8

1.0

ID / IL = 1.61

4

2

0.4

1.2

0.6

0.8

1.0

1.2

E vs. SCE / V

E vs. SCE / V 12

(C) 10 8

ID / IL

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6 4 2 0

Cys

Tyr

Trp

Different amino acids

Figure 11. Differential pulse voltammograms of L-/D-Cys (A) and L-/D-Tyr (B) at OPPy/GCE. (C) Comparison of recognition efficiencies of L-/D-Cys, L-/D-Tyr and L-/D-Trp at OPPy/GCE. Error bars represent standard deviation for three independent measurements.

The electrochemical stability of the chiral sensor was further investigated. Figure 12 showed the cyclic voltammograms of the OPPy/GCE in 0.1 M KCl containing 5 mM [Fe(CN)6] 4−/3− at a scan rate of 100 mV s−1. The increase in the oxidation peak current was only 12% of the initial current after 30 continuous cycles, demonstrating the excellent electrochemical stability of the OPPy. Finally, the reproducibility of the OPPy-based chiral sensing platform was studied, and the results were shown in Figure 13. The ID/IL values at five independently prepared OPPy/GCE were 10.22, 10.03, 11.17, 10.94 and 9.80, 19 ACS Paragon Plus Environment


respectively, with a relative standard deviation (RSD) of 5.7%, showing an acceptable reproducibility of the OPPy-based chiral sensor.

Figure 12. Cyclic voltammograms of OPPy/GCE in 0.1 M KCl containing 5 mM [Fe(CN)6]4−/3− from the 1st to the 30th cycle. 14 12 10

ID / IL

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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11.17 10.22

10.03

1

2

10.94 9.80

8 6 4 2 0

3

4

5

Number of chiral sensors

Figure 13. Recognition efficiencies for the Trp enantiomers at five independently prepared

OPPy-based chiral sensors. CONCLUSIONS To summarize, optical active OPPy was electrochemically synthesized using (+)-CSA as the dopant and conformation inducer. A twisted helical architecture was formed along the OPPy chains due to the synergistic effects of the polymeric backbone destruction resulted from overoxidation and the H-bonds between (+)-CSA and OPPy, leading to the generation of optical active OPPy with excellent homochirality. The resultant OPPy was further applied 20 ACS Paragon Plus Environment

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in the electrochemical chiral recognition of Trp enantiomers, and an extremely high recognition efficiency was achieved (ID-Trp/IL-Trp = 10.22). Moreover, the OPPy-based electrochemical chiral sensing platform exhibited high chiral specificity, excellent electrochemical stability and reproducibility. With more attention paid to the high efficient recognition of chiral compounds, this work might open up a new avenue for the construction of convenient and smart chiral sensing platforms for chiral amino acids and chiral drugs. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This research was supported financially by National Natural Science Foundation of China (21775013, 11532003, 31670950), Natural Science Foundation of Jiangsu Province (BK20171194), and Qing Lan Project of Jiangsu Higher Education Institutions. ASSOCIATED CONTENT Supporting Information Additional figures as noted in the text, the optimization of electrodeposition time of PPy, pH of the Trp solution, and duration time before DPV detection. This material is available free of charge via the Internet at http://pubs.acs.org.



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Overoxidation and Camphorsulfonic Acid Coinduced Chiral

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