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A Highly Sensitive Photoelectrochemical Assay with Donor-AcceptorType Material as Photoactive Material and Polyaniline as Signal Enhancer Tao Hu, Ying-Ning Zheng, Meng-Jie Li, Wen-Bin Liang, Yaqin Chai, and Ruo Yuan Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b00093 • Publication Date (Web): 20 Apr 2018 Downloaded from http://pubs.acs.org on April 20, 2018

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Analytical Chemistry

A Highly Sensitive Photoelectrochemical Assay with Donor-Acceptor-Type Material as Photoactive Material and Polyaniline as Signal Enhancer Tao Hu, Ying-Ning Zheng, Meng-Jie Li, Wen-Bin Liang, Ya-Qin Chai∗, Ruo Yuan∗ Key Laboratory of Luminescent and Real-Time Analytical Chemistry, Ministry of Education, College of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, People’s Republic of China Abstract In this work, a highly sensitive photoelectrochemical (PEC) assay was constructed based on a donor-acceptor (D-A)-type material poly{4,8-bis[5-(2-ethylhexyl) thiophen-2-yl]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophene-4,6-diyl} (PTB7-Th) as photoactive material and polyaniline (PANI) in-situ deposited on the surface of PTB7-Th as signal enhancer. Initially, PTB7-Th that contains an electron-rich unit as donor and an electron-deficient unit as acceptor with the easy separation of electron-hole pairs and intermolecular electron transfer provided an excellent photocurrent response. Subsequently, an input target thrombin (TB) was converted to an output single-stranded DNA by a protein converting strategy. The obtained single-stranded DNA thus triggered a rolling circle amplification (RCA) to form a tandem

* Corresponding author. Tel.: +86-23-68252277; Fax: +86-23-68253172. E-mail address: [email protected], (Y. Q. Chai), [email protected] (R. Yuan)

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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

multi-hairpin DNA nanostructure, which could function as a skeleton for immobilizing manganese porphyrin (MnTMPyP). In the presence of H2O2 and aniline, a PANI layer could be in-situ deposited onto the tandem multi-hairpin DNA nanostructure with use of MnTMPyP as catalyst, leading to a significantly enhanced photocurrent for detection of TB. The proposed PEC assay presented a wide detection range of 100 fM to 10 nM with the limit of detection (LOD) of 34.6 fM. Furthermore, the proposed strategy provides a PEC analysis method based on PTB7-Th that can significantly improve the photoelectric conversion efficiency and opens an intriguing avenue to establish low background, ultra-sensitive and highly stable analytical techniques. Keywords: photoelectrochemical, self-enhanced, donor-acceptor-type, polyaniline, DNA nanostructure Introduction Photoelectrochemical (PEC) assay, which determines the analytes by monitoring the photo-induced current signals, is a vibrantly developing analytical technique with properties of low background, high sensitivity and low cost.1-3 Classical PEC assay commonly requires the addition of electron donors or acceptors as coreactant to assist photoactive materials in producing photocurrent responses.4-8 In these assays, the photocurrent is generated by the intermolecular electron transfer between different materials, which may result in low electron transfer efficiency, poor photoelectric conversion efficiency for poor sensitivity and stability of biosensors.9 Recently, our group has fabricated a self-enhanced PEC assay based on a nanocapsule packaging 2


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the donor-acceptor (D-A)-type material poly{4,8-bis[5-(2-ethylhexyl)thiophen-2-yl] benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-3-fluoro-2-[(2-ethylhexyl)-carbonyl] thieno[3,4-b]thiophene-4,6-diyl} (PTB7-Th) and its signal enhancer fullerene to obtain a remarkably enhanced photocurrent signal.10 PTB7-Th is characterized by an electron-rich unit as donor and an electron-deficient unit as acceptor to improve the photoelectric conversion efficiency.11,

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However, it is difficult for immobilizing

fullerene as signal enhancer on PTB7-Th.10, 13 And moreover, the contact and distance between PTB7-Th and nano-C60 cannot be accurately controlled. Thus the total electron transfer efficiency and enhancement of photocurrent signal were still limited. Therefore, it is highly meaningful but full of challenges to search for a new signal enhancer with easier immobilization ability and much higher signal enhancement ability for construction of PEC assays with high photoelectric conversion efficiency. Polyaniline (PANI) is a p-type semiconductor which characterizes a favorable connection of aromatic rings via nitrogen-containing groups with strong light absorption, low energy optical transition and high electron affinity. 14-16 Recent years, PANI has been widely used in solar cells to enhance the photoelectric conversion efficiency of some photovoltaic materials.17-20 Fortunately, it has been reported that PANI can be readily deposited onto double-stranded DNA using aniline monomers in the presence of H2O2 and peroxidase.21-23 Considering the inherent advantages of PANI and its easy immobilization property, in this work, we firstly immobilized PANI as signal enhancer on the surface of PTB7-Th with a specially designed DNA 3



nanostructure as skeleton and manganese porphyrin (MnTMPyP) as catalyst to enhance the photoelectric conversion efficiency of PTB7-Th. As shown in Scheme 1, a highly sensitive PEC assay with D-A-type PTB7-Th as photoactive material and PANI in-situ deposited as signal enhancer was constructed for sensitive detection of thrombin (TB). Firstly, PTB7-Th was filmed on the electrode to provide an initial photocurrent. Subsequently, by a protein converting strategy, the input target (TB) was converted to the output single-stranded DNA (primer), which could hybridize with the capture DNA on surface of PTB7-Th to trigger a rolling circle amplification (RCA). With the assistance of RCA, the primer could be elongated and self-assembled to form a tandem multi-hairpin DNA nanostructure for the insertion of MnTMPyP. In the presence of H2O2 and aniline, a PANI layer could be largely and steadily in-situ deposited onto the multi-hairpin DNA nanostructure with use of MnTMPyP as catalyst. With the target TB, the DNA nanostructure can be mass-produced to support sufficient PANI to achieve the photocurrent enhancement of PTB7-Th. The detection range of target TB from 100 fM to 10 nM with the limit of detection (LOD) of 34.6 fM was obtained by measuring the photocurrent signal. Impressively, the designed strategy opened a new avenue for simple and effective construction of PEC assays with high photoelectric conversion efficiency.

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Scheme 1. Schematic illustrateon of (A) the conversion process of target TB, (B) the fabrication of PEC aptasensor, (C) the catalysis deposition of PANI layer on a tandem multi-hairpin DNA nanostructure, (D) the electron transfer in PTB7-Th and (E) the electron transfer between PTB7-Th and PANI.

Experimental Section Fabrication of PEC aptasensor. To construct the PEC aptasensor, GCE was burnished entirely using 0.3 µm alumina powder and then rinsed ultrasonically in ethanol and ultrapure water for several times, respectively. Then, 6 µL PTB7-Th was gently dropped and filmed onto the electrode surface. Gold nanoparticles (AuNPs) were electrodeposited onto the PTB7-Th membrane in 1 % HAuCl4 solution for 30 s. Subsequently, 20 µL 2.5 µM capture DNA was immobilized on the electrode for 12 h at 4 °C through Au-N interaction. Thenceforth, the modified electrode was rinsed with ultrapure water and incubated with 10 µL 0.3 µM 6-mercaptohexanol (MCH) for 30 5



min to block nonspecific binding sites. PEC measurement. The rolling circle amplification (RCA) was performed by incubating capture DNA, circular DNA template, deoxynucleotides (dNTPs), phi29 polymerase for 6 h at 37 °C. First, 20 µL TB solution was added into 20 µL magnetic microbeads-aptamer for TB-primer (MB-TBA-primer) complex solution and incubated for 30 min. Then the obtained MB-TBA-TB complex were separated by a magnet and 20 µL the supernatant was incubated with capture DNA, prepared by the section of Fabrication of PEC aptasensor, for 1 h at 37°C. Next, 0.5 µL phi29 DNA polymerase, 4 µL 10× phi29 buffer, 0.5 µL dNTPs and 15 µL circular DNA template were added and incubated for 6 h at 37 °C. Subsequently, 20 µL 50 µM MnTMPyP solution was added and incubated for 30 min at 37 °C. Afterwards, 20 µL aniline solution (20 mM) containing 4 mM H2O2 and 0.1 M KAc-HAc was added and incubated for 20 min to deposit PANI on the obtained DNA nanostructure. The photocurrent signal was measured by an Ivium PEC workstation in 6 mL PBS solution. The excitation light was provided by the LED lamp with a power of 260 mW and switched off-on-off for 10-20-10 s. Results and discussion A mechanism of electron transfer for the aptasensor has been proposed, in which photo-excited free electrons were transferring from the photoactive PTB7-Th to the signal enhancer PANI, leading to an enhanced photocurrent. The validity of this proposal was based on the matching of the energy levels of PTB7-Th and PANI. PTB7-Th owns the lowest unoccupied molecular orbital (LUMO) energy level of 6


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-3.59 eV and the highest occupied molecular orbital (HOMO) energy level of -5.20 eV. Meanwhile, the LUMO and HOMO energy levels of PANI were calculated to be -3.98 eV and -5.37 eV, respectively (page 7, line 3 in SI). The LUMO and HOMO energy levels of PANI are 0.39 eV and 0.17 eV lower than these of PTB7-Th, respectively. This stepped energy level arrangement can effectively promote the transfer of free electrons. When under photo excitation, ground state electrons in PTB7-Th and PANI receive the energy and transition to the excited state, forming electron-hole pairs. The electrons are responsible for the LUMO energy level while the holes are responsible for the HOMO energy level. Then the principal transfer directions of photo-excited electrons are proposed. As shown in Figure 1, the first is the intramolecular electron transfer between the donor unit and the acceptor unit in PTB7-Th (process 1), which provides an initial photocurrent. Subsequently, the intermolecular electron transfer between PTB7-Th and PANI (process 2) and the electron transfer between the external GCE and PTB7-Th (process 3) occurred, leading to an increased photocurrent. Due to the stepped energy level arrangement between PTB7-Th and PANI and the excellent conductive property of PANI, the photo-excited free electrons transfer from the LUMO of PTB7-Th to the LUMO of PANI, and the recombination of electron-hole pairs is suppressed, thereby achieving the improvement of photoelectric conversion efficiency and the enhancement of PEC signal.

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Figure 1. Reaction scheme of proposed PEC progress.

In order to characterize the stepwise modified electrode, the assembly steps of the sensing interface were investigated by CV measurement. As can be seen in Figure 2, a well-defined redox peak of [Fe(CN)6]3-/4- was observed for the bare electrode (curve a). After PTB7-Th was modified on the GCE, the redox peak obviously decreased due to the low conductive property of PTB7-Th (curve b). After AuNPs were deposited, the redox peak obviously increased because AuNPs could promote the electron transfer (curve c). When the capture DNA was introduced, the redox peak decreased (curve d), which could be attributed to the electronic repulsion of negatively charged DNA. After the immobilization of MCH, a decline was exhibited attributed to that MCH can inhibit the electron transfer (curve e). When the primer was immobilized onto the electrode, the redox peak decreased (curve f), and it further decreased after the elongation of the primer (curve g). With the insertion of MnTMPyP, the redox peak increased due to the good electron transfer property (curve h). After the incubation of aniline, the redox peak increased, indicating the successful deposition of polyaniline which could promote the electron transfer due to its non-localized π electron system (curve i). The CVs above evidently indicated the successful assembly 8


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of the sensing interface. Electrochemical impedance spectroscopy (EIS) was also employed to illustrate the stepwise modified electrode (page 8, line 17 in SI).

Figure 2. CV characterizations for the modifications of electrodes: (a) GCE, (b) PTB7-Th/GCE, (c) AuNPs/PTB7-Th/GCE, (d) capture DNA/AuNPs/PTB7-Th/GCE, (e) MCH/capture DNA/ AuNPs/PTB7-Th/GCE, (f) primer/MCH/ capture DNA/AuNPs/PTB7-Th/GCE, (g) RCA/primer/ MCH/capture DNA/AuNPs/PTB7-Th/GCE, (h) MnTMPyP/RCA/primer/MCH/capture DNA/ AuNPs/PTB7-Th/GCE, (i) PANI/MnTMPyP/ RCA/primer/MCH/capture DNA/AuNPs/ PTB7-Th/ GCE in [Fe(CN)6]3-/4- with the potential scanning from -0.2 to 0.6 V at a scan rate of 0.05 V/s.

Optimal Conditions for the PEC aptasensor. The photocurrent of the fabricated PEC aptasensor was related to the volume of PTB7-Th and the incubation time for aniline. As shown in Figure 3A, the PEC signal increased with increasing the volume of PTB7-Th solution and then reached a plateau when PTB7-Th solution increased to 6 µL. Thus, 6 µL PTB7-Th solution was selected as the optimized volume. The PEC signal in Figure 3B increased with the passage of incubation time for aniline from 5 to 20 min, while it decreased with the incubation time from 20 to 50 min. Thus, 20 min was chosen as the optimal incubation time for aniline. 9



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Figure 3. Illustration of (A) the optimization curve for the volume of PTB7-Th; (B) the optimization curve for the incubation time for aniline.

PEC response of different signal amplification states. To investigate the enhancement effect of PANI for PTB7-Th, the PEC responses of different signal amplification states were recorded by the as-prepared aptasensor (MCH/capture DNA/AuNPs/PTB7-Th/GCE) with 1.0 nM TB solution and the results were listed in Figure 4. In Figure 4A, the PEC signal was just about 280 nA when the RCA was introduced due

to

the

electronic

repulsion

of

negatively charged

DNA

(RCA/primer/MCH/capture DNA/AuNPs/PTB7-Th/GCE). After the aptasensor was introduced with MnTMPyP, the PEC signal slightly increased to 450 nA (Figure 4B), which can be attributed that MnTMPyP accelerated the electron transfer on the DNA nanostructure. Comparatively, an obviously increased PEC signal of 1250 nA (Figure 4C) was obtained after the deposition of PANI. This demonstrated that PANI was successfully deposited on the multi-hairpin DNA nanostructure and thus efficiently promoted the electron transfer originated in PTB7-Th.

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Figure 4. PEC responses of different signal amplification states: (A) RCA/primer/MCH/capture DNA/AuNPs/PTB7-Th/GCE; (B) MnTMPyP/RCA/primer/MCH/capture DNA/AuNPs /PTB7-Th/ GCE; (C) PANI/MnTMPyP/RCA/primer/MCH/capture DNA/AuNPs/PTB7-Th/GCE. All of the PEC signals were measured in the presence of 1.0 nM TB in PBS solution.

Analytical Performance of the aptasensor. The analytical performance of the constructed PEC aptasensor was examined using the TB solution at various concentrations under the optimized conditions. As shown in Figure 5A, PEC signals for different concentrations of TB were recorded. With the incremental concentration of TB from 100 fM to 10 nM, PEC signals increased gradually. In Figure 5B, the linear relationship was I = - 2.8130 – 0.1718 lgcTB, where I was the photocurrent signal and c was the concentration of TB. Limits of quantification (LOQ) and limits of detection (LOD) were calculated to be 100 fM and 34.6 fM based on the expression 10σ/S and 3.3σ/S respectively, where σ is assumed to be the standard deviation of the blank, and S is the slope of the calibration curve in low concentrations. The comparison of different biosensors for the detection of TB is shown in Table S3. It is noticeable that the proposed aptasensor exhibited prominent potential for sensitive detection of TB.

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Figure 5. Illustration of (A) PEC signals for different concentrations of TB (100 fM, 200 fM, 400 fM, 500 fM, 700 fM, 1.0 pM, 5.0 pM, 10 pM, 100 pM, 1.0 nM, 10 nM) in 0.1 M PBS solution (pH 7.0) and (B) the linear relationship between PEC signal versus logarithm of TB concentration. The insert of (B) displayed the calibration curve with a low concentration of TB.

Specificity and stability of PEC aptasensor. The specificity of the PEC aptasensor was evaluated by incubating the MB solution with several interferences, including microRNA-141

(miRNA-141),

L-cysteine

(L-cys),

hemoglobin

(Hb),

immunoglobulin G (IgG) and lysozyme. As depicted in Figure 6A, no obvious PEC signals were observed from the interferences (100 nM) while significant PEC signals were obtained from TB (1 nM) and the mixture with TB (1 nM), illustrating that the proposed aptasensor features an excellent specificity, which can be attributed to the excellent affinity of TBA to TB during the target recognition progress. Figure 6B exhibited the stability with the RSD of 2.75% of the as-prepared aptasensor incubated with 10 pM TB under consecutive off-on-off light for 9 cycles. The desirable stability can be attributed to the satisfying photo stability of PTB7-Th and PANI.

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Figure 6. Specificity of the aptasensor (A) toward the blank control, miRNA-141 (100 nM), L-cys (100 nM), Hb (100 nM), IgG (100 nM), lysozyme (100 nM), TB (1.0 nM) and the mixture (1.0 nM TB with 100 nM of microRNA-141, L-Cys, Hb, IgG and lysozyme, respectively) and stability of the aptasensor incubated with 10 pM TB under consecutive off-on-off light for 9 cycles (B).

Preliminary application of PEC aptasensor. To assess the feasibility of the PEC aptasensor, the recovery was calculated by blanking spikes using healthy human sera. Different concentrations (100 fM, 1.0 pM, 10 pM, 100 pM and 1.0 nM) of TB spiked with 40 folds diluted human sera were investigated to measure the corresponding photocurrents. The recoveries were calculated from 92.3% to 106.4% and the RSD varied from 3.1% to 6.6%, which denoted that the PEC aptasensor could achieve feasible results in clinical research.

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Table 1. Determination of TB added in human blood sera. Sera sample

Concentration of TB

Concentration obtained

Recovery/ %

RSD/ %

1

100 fM

106.4 fM

106.4

3.2

2

1.0 pM

0.975 pM

97.5

5.1

3

10 pM

10.32 pM

103.2

5.1

4

100 pM

104.6 pM

104.6

3.1

5

1.0 nM

0.923 nM

92.3

6.6

Conclusion In summary, a highly sensitive PEC assay was constructed based on PTB7-Th as photoactive material and PANI as signal enhancer. PTB7-Th provided a strong photocurrent due to the easy separation of electron-hole pairs and highly efficient intermolecular electron transfer. PANI was in-situ deposited on the surface of PTB7-Th to further enhance the photocurrent of PTB7-Th. The established PEC aptasensor realized sensitive detection of target TB. Moreover, the proposed PEC assay with high photoelectric conversion efficiency could also be applied to detect other biomolecules, such as nucleic acid, microRNA and proteins, providing a promising tool for bioanalysis and diagnosis. Acknowledgements This work was financially supported by the National Natural Science Foundation (NNSF) of China (21775124, 21575116, 21675129) and the Fundamental Research Funds for the Central Universities, China (XDJK2015C099, SWU114079), China Postdoctoral Science Foundation (2015M572427) and Chongqing Postdoctoral 14


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