Semiconducting OrganicâInorganic Nanodots Heterojunctions

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Semiconducting Organic-Inorganic Nanodots Heterojunctions: Novel Platforms for General Photoelectrochemical Bioanalysis Application Qian Wang, Yi-Fan Ruan, Wei-Wei Zhao, Peng Lin, Jing-Juan Xu, and Hong-Yuan Chen Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b03852 • Publication Date (Web): 05 Mar 2018 Downloaded from http://pubs.acs.org on March 6, 2018

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Analytical Chemistry

Semiconducting Organic-Inorganic Nanodots Heterojunctions: Novel Platforms for General Photoelectrochemical Bioanalysis Application Qian Wang,1,† Yi-Fan Ruan, 1,† Wei-Wei Zhao,1,2,* Peng Lin,3,* Jing-Juan Xu1 and Hong-Yuan Chen1 1

State Key Laboratory of Analytical Chemistry for Life Science and Collaborative Innovation Center of

Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, China 2

Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, United

States 3

College of Materials Science and Engineering, Shenzhen University, Shenzhen 518060, China

†These authors contributed to this work equally.

* E-mail: [email protected]; [email protected]

* E-mail: [email protected]

ACS Paragon Plus Environment

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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

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ABSTRACT: In this study, semiconducting organic polymer dots (Pdots) and inorganic quantum dots (Qdots) were firstly utilized to construct the organic-inorganic nanodots heterojunction for the photoelectrochemical (PEC) bioanalysis application. Specifically, n-type CdS

Qdots,

p-type

CdTe

Qdots

and

tetraphenylporphyrin

(TPP)-doped

poly[(9,9-

dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo-{2,1′,3}-thiadazole)] (PFBT) Pdots were fabricated and their energy levels, i.e., their valence band (VB)/conduction band (CB) or lowest unoccupied molecular orbital (LUMO)/highest occupied molecular orbital (HOMO) values, were also determined. Then these nanodots were integrated to construct four types p-n and p-p organicinorganic nanodots heterojunctions, i.e., CdS Qdots/TPP-doped PFBT Pdots, TPP-doped PFBT Pdots/CdS Qdots, CdTe Qdots/TPP-doped PFBT Pdots, TPP-doped PFBT Pdots/CdTe Qdots, on the transparent glass electrode. Upon light irradiation, four heterojunctions exhibited different PEC behaviors, and some with prominent photocurrent enhancement. With the model molecule L-cysteine (L-cys) as target, the proposed PEC sensor exhibited good performances. In brief, this work presents the first semiconducting organic-inorganic nanodots heterojunction for PEC bioanalysis application, which could be easily used as a general platform for future PEC bioanalysis building. Besides, it is expected to inspire more interest in the design, development, and implementation of various organic-inorganic heterojunctions for advanced PEC bioanalysis in the future. KEYWORDS: Photoelectrochemical Bioanalysis, Polymer Dots, Quantum Dots, OrganicInorganic, Heterojunction


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Motivated by the rapid development of advanced electronics, energy conversion and storage, optoelectronic devices, photocatalysis and photovoltaics, nanostructured heterojunctions have been attracting an exponential rise of attention due to their unique properties and promising potentials in future applications.1-7 For semiconductors, they are the multifunctional materials which combine different properties of the individual semiconductors. Significant prevalence of semiconductor heterojunctions over pure ones exist in the flexible arrangement of different n-/ptype semiconductors and thus judicious design of particular hybrid photoanode/photocathode.8-9 Besides, due to the difference in the direction of electron/hole movement across the heterojunction, the semiconductor interface would be formed with varying charges that generate the internal electric field at the heterojunction. Such a potential gradient at the interface may contribute to the photoinduced charge separation in both the semiconductors, inhibit the recombination rate and increase the lifetime of these charge carriers, thereby leading to enhancement in photo conversion efficiency and better performances in specific applications.8 Photoelectrochemical (PEC) bioanalysis is a newly developed sensing technique built upon the light-harvesting capabilities of semiconductors.10-18 Ever since its very beginning, the exploitation of various functional semiconductive materials and their utilization as advanced sensing platforms have been a research focus. Under such efforts, various semiconductor quantum dots (Qdots) and semiconductor oxides, e.g. Cd-/Zn-/Pb-chalcogenide (S, Se, Te), TiO2 and ZnO, have been highly exploited. In the quest for achieving better performance, heterojunctions consisting of two semiconductors are being looked upon as favorite schemes because appropriate alignment of particular semiconductors may produce novel photoelectrodes with fascinating characteristics. To this end, our and other groups have developed many hierarchical heterostructures for PEC bioanalysis applications.19-23 For example, p-n


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heterojunction of BiOI nanoflakes/TiO2 nanotubes and p-p heterojunction of PdS Qdots/NiO nanofilm have been developed for PEC immunoassay24 and PEC enzymatic sensing,25 respectively. However, these hybrids were generally composed of various inorganic semiconductors. Comparing with the state-of-the-art development of inorganic heterojunction in this field, organic semiconducting nanostructures and their based heterojunctions have seldom been studied. Conjugated polymer dots (Pdots), with extraordinary brightness, fast emission rates and good photostability, are a new family of promising nanomaterials for various optoelectronic applications, e.g., light-emitting diodes, field-effect transistors, and photovoltaic devices.26-28 Besides, due to their excellent fluorescent characteristics, Pdots have recently attracted considerable attention for biology and medicine applications.31 Especially, the energy transfer properties of Pdots have led to the development of a variety of chemsensors and biosensors.30-32 Inspired by these optical biosensors, our group then exploited their light-harvesting property and first demonstrated their feasibility for sensitive PEC bioanalysis application.33 In continuation of our work on construction of innovative heterojunction for PEC bioanalysis, this scenario presents the first preparation, characterization, and visible-light-driven organicinorganic p-n and p-p semiconducting nanodots heterojunction that composed of the organic Pdots and inorganic Qdots for potential PEC bioanalysis application. Specifically, the n-type CdS Qdots and p-type CdTe Qdots were fabricated by the solution method,34,35 while the photosensitizer tetraphenylporphyrin (TPP), poly(styrene-co-maleic anhydride) (PSMA), and the conjugated

polymer

poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo-{2,1′,3}-thiadazole)]

(PFBT) were used to prepared the TPP-doped PFBT Pdots via the reprecipitation method.35 Their energy levels, i.e., the valence band (VB)/conduction band (CB) or lowest unoccupied


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molecular orbital (LUMO)/highest occupied molecular orbital (HOMO) values, were then calculated by cyclic voltammetry (CV) and optical measurements. Subsequently, as shown in Scheme 1, four different hybrid heterojunctions (Qdots/Pdots and Pdots/Qdots) were constructed separately on the transparent indium tin oxide (ITO) glass electrodes and then characterized by PEC techniques. Incidenatlly, the as-fabricated hybrid dots-based heterojunctions were not as perfect as those of planar heterojunctions. Finally, the applicabilities of the as-obtained nanoarchitectures in PEC bioanalysis were tested. According to the latest reviews in this area,36,37 such organic-inorganic semiconducting nanostructured heterojunctions have not been reported for PEC bioanalysis. This work manifested their great potential for future PEC bioanalysis, which would underlie elegant platforms for probing numerous biological events. Scheme 1. Schematic Illustration for the Fabrication of Inorganic-Organic Semiconducting Nanodots Heterojunctions with Corresponding Band Positions


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RESULTS AND DISCUSSION

Figure 1. (A1-C1) Typical TEM, (A2-C2) High-resolution TEM images and (A3-C3) normalized fluorescence excitation and emission spectra of the (A) CdS Qdots, (B) CdTe Qdots and (C) TPP-doped PFBT Pdots. Insets in C1-C3: Photograph and fluorescence images of the corresponding particles under a 365 nm UV lamp.

Materials Characterization. The particle sizes and morphologies of the as-synthesized CdS Qdots, CdTe Qdots and TPP-doped PFBT Pdots were characterized by transmission electron microscope (TEM), with the representative and high-resolution images shown in Figure 1 (A1C1) and (A2-C2), respectively. As demonstrated, these particles appeared as quasi-spherical particles with the sizes corresponding to c.a. 5 nm, 5 nm, and 3 nm, respectively. Such uniform morphologies implied their suitability for the construction of homogeneous photoresponsive films and hence production of stable signals. Besides, the high-resolution images of Figure 1 (A2) and (B2) also indicated these Qdots had relatively good crystallinity. Figure 1 (A3-C3) depict the


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normalized absorption and emission spectra of these particles. As shown, for CdS Qdots, optimal excitation wavelength emerged at λex = 484 nm and the respective emission maximum at λem = 620 nm, while the optimal excitation wavelength of CdTe Qdots presented at λex = 380 nm and the respective emission maximum at λem = 560 nm. Especially, for TPP-doped PFBT Pdots, the doublet peaks in the excitation spectrum were consistent with the TPP doping, while the emission spectrum exhibited a red fluorescence (∼660 nm) of the TPP dopant, with the fluorescence of PFBT being greatly quenched. This was because the existence of TPP dopant could greatly quench the donor’s fluorescence due to the combined effects of energy diffusion and transfer.38 Incidentally, the insets in Figure 1 (A3-C3) show the corresponding photograph and fluorescence images of the particles under a 365 nm UV lamp. Figure 2 depicts the UV−Vis absorption or diffuse reflectance spectra of (A) CdS Qdots, Pdots/CdS Qdots, (B) CdTe Qdots, Pdots/CdTe Qdots, (C) Pdots, CdS Qdots/Pdots, and (D) Pdots, CdTe Qdots/Pdots. As shown, after attaching the Pdots, both the absorption of Pdots/CdS Qdots and Pdots/CdTe Qdots electrode possessed obviously growth. On the contrary, both the absorption of CdS Qdots and CdTe Qdots exhibited a slight growth after attaching Qdots. Consistently, these results indicated not only that the fabrication of the designed electrodes was successful but also that the Pdots had a relatively strong absorption as compared to other two inorganic Qdots. The insets in Figure 2 (A-D) recorded the photocurrent responses of these samples at different wavelengths, the results of which conformed well to the corresponding absorption spectra.


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Figure 2. UV−vis absorption or diffuse reflectance spectra of the samples as indicated in the graphs. (A) CdS (black line); Pdots/CdS (red line). (B) CdTe (black line); Pdots/CdTe (red line). (C) Pdots (black line); CdS/Pdots (red line). (D) Pdots (black line); CdTe/Pdots (red line) Insets: the corresponding photocurrent responses of these samples at different wavelengths.

Determination of HOMO/LUMO or VB/CB levels of These Nanodots. The electrochemical properties of these nanodots were further investigated by CV with the results shown in Figure 3. The CB and VB of the semiconductor nanocrystals can be obtained by calculating their electron affinity (EA) and ionization potential (IP)39 by using the following formulas: IP = -(4.80 - E1/2Fc/Fc+ + Eox), EA = IP + Eg; Eg = 1240/λonset (E1/2Fc/Fc+ is the formal potential of Fc/Fc+, Eox is the oxidation initiation potential, and Eg is optical bandgap).40 Similarly, the energy levels of the polymer can be gained by counting its lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) by using the


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formulas: EHOMO = -(4.80 - E1/2Fc/Fc+ + Eox), ELUMO= EHOMO + Eg.41 Potentials were calibrated with the ferrocene/ferrocenium (Fc/Fc+) couple, and the potential of Fc/Fc+ had an absolute energy level of 4.80 eV to vacuum.42 As shown in Figure 3A, with the use of bare FTO electrode and scan rate of 50 mV s-1 in 0.5 mM Fc solution, E1/2Fc/Fc+ was determined as + 0.48 V. From Figure 3A inset, the λonset of the as-prepared CdS Qdots, CdTe Qdots and Pdots were recorded as 495 nm, 577 nm and 529 nm, respectively, corresponding to Eg of 2.50 eV, 2.15 eV and 2.34 eV, respectively. On the other hand, as shown in Figure 3B of the CV curves, the Eox of CdS Qdots, CdTe Qdots and Pdots were disclosed as 0.97 eV, 0.58 eV and 1.38 eV, respectively. Using the aforementioned formulas, as shown in Table S1 in Supporting Information, the CB/VB was thus calculated as -2.78 eV/-5.28 eV for CdS Qdots and -2.75 eV/-4.90 eV for CdTe Qdots, while LUMO/HOMO of the Pdots was determined as -3.36 eV/ -5.70 eV. These values would be useful for understanding the PEC behaviors of the designed heterojunctions.

Figure 3. (A) CVs of the FTO (as working electrodes) in a deoxygenated anhydrous acetonitrile solution of -1

tetrabutylammonium hexafluorophosphate (0.1 M) and ferrocene(0.5 mM) at scan rate of 50 mV s , with a Pt wire counter electrode, an Ag/AgCl reference electrode (with saturated KCl); Insets: UV-vis measurements of Pdots (black line); CdTe (red line); CdS (blue line); (B) CVs of the FTO modified by (a) Pdots; (b) CdTe (c)


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CdS in a deoxygenated anhydrous acetonitrile solution of tetrabutylammonium hexafluorophosphate (0.1 M) at -1

scan rate of 50 mV s ; Eox were calculated from tangents of species’ oxidation peaks.

Figure 4. Photoresponsibility of (A) Pdots/ITO (black); CdS Qdots/Pdots/ITO (red); Pdots: CdS Qdots=5:1 (blue) in 0.1 M PBS (pH=7.1); (B) Pdots/ITO (black); CdTe Qdots/Pdots/ITO (red); Pdots: CdTe

Qdots=5:1 (blue) in 0.1 M PBS (pH=7.1); (C) CdS Qdots/ITO (black); Pdots/CdS Qdots/ITO (red); Pdots: CdS Qdots=1:5 in 0.1 M AA dissolved 0.1 M PBS, (pH=7.1), and (D) CdTe Qdots/ITO (black), and Pdots/CdTe Qdots/ITO (red); Pdots: CdTe Qdots=1:5 in 0.1 M PBS (pH=7.1).

PEC Behaviors. As shown in Figure 4, the visible-light-responsibilities of the as-fabricated heterojunctions were then revealed by the chronoamperometric I−t curves from the stepwise transient photocurrent responses upon the intermittent light irradiation. Figure 4A demonstrates


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the photocurrents of Pdots/ITO electrode (black curve) and CdS Qdots/Pdots/ITO (red curve), while Figure 4B compares the values of Pdots/ITO electrode (black curve) and CdTe Qdots/Pdots/ITO (red curve). As shown, upon irradiation, bare Pdots/ITO electrode only showed a very small cathodic photocurrent, whereas both the CdS Qdots/Pdots/ITO electrode and CdTe Qdots/Pdots/ITO electrode exhibited much enhanced cathodic photocurrent under no biased potential, suggesting the successful formation of p-n junction (CdS Qdots/Pdots) and p-p junction (CdTe Qdots/Pdots) and also the good charge transport within the junction. According to the calculated energy levels, the insets illustrate the corresponding charge transfer pathways with the junctions, note that whether the p-n or p-p junctions were formed, the resulted Pdotsbased heterojunctions exhibited the same cathodic direction of photocurrents.8 Then, CdS and CdTe Qdots were used as the bottom layer to immobilize the Pdots layer. As shown in Figure 4C, bare CdS Qdots/ITO electrode showed a strong anodic photocurrent, but the Pdots/CdS Qdots electrode exhibited a lower anodic photocurrent, the result of which didn’t conform to the matched energy levels as indicated in the inset. This should be attributed to the lower incident photon-to-electron conversion efficiency (IPCE) of Pdots than the Qdots, and these Pdots at the outside layer may reduce the absorption of the Qdots layer at the interior. According to the expression of IPCEሺλሻ =

1240×Iscሺλሻ Iሺλሻ×λ

where Isc(λ) (nA cm-2) is the measured photocurrent density, λ

(nm) is the wavelength of incident light, and I(λ) (µW cm-2) is power density of the incident light, the IPCE of Pdots and CdS Qdots were determined as 11.7% and 41.1%, respectively.24, 33, 43 Figure 4D demonstrated that bare CdTe Qdots/ITO electrode showed a strong cathodic photocurrent, whereas the Pdots/CdTe Qdots/ITO electrode exhibited a much smaller cathodic photocurrent. This was because the presence of as-synthesized Pdots layer could not lead to the formation of proper junction between the two nanodots as indicated in the inset of Figure 4D.


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Obviously, among the four hybrid electrodes, both the Pdots-based organic-inorganic p-n and p-p heterojunctions exhibited good PEC responsibilities. And then we tested the stability of the two electrodes. As shown in Figure S1, over 15 repeated cycles, neither the electrodes have noticeable changes in the current intensity, which implied its qualification for the further employment in PEC bioanalysis. Incidentally, the layer-by-layer fabrication process was also monitored by the scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS), the results and the corresponding discussion were presented with Figure S2 and S3, respectively. Besides, the random mixtures of the Pdots and CdS Qdos or CdTe Qdots were also prepared and tested for photocurrent generation, and the results were included in Figure S4 and S5.

PEC responses toward typical electron acceptor and acceptor. As well known, a typical PEC bioanalysis necessitates the appropriate interaction between the specific PEC transducer and electron donor/acceptor, and the general sensing mechanism is that the specific sensing event could be converted into the corresponding photocurrent signals. Among kinds of electron donor or acceptor species, L-cysteine (L-cys)44-47 and dissolved oxygen (D-O2)25, 35 have demonstrated themselves as the ideal electron donor and acceptor, respectively, in the electrolyte for PEC bioanalysis. Herein we used them to reveal the PEC responses of CdS Qdots/Pdots/ITO and CdTe Qdots/Pdots/ITO to typical electron acceptor and acceptor, and the results demonstrated that both the two electrodes were very sensitive to these species. As shown in Figure 5A and B, with the increase of L-cys concentration, it could gradually inhibit the transfer of the photogenerated electrons to the D-O2 and steer the electron transfer to the electrode. The ejection of the photo-induced electrons to the ITO with the simultaneous transfer of electrons from the Lcys would thus generate the enhanced anodic photocurrent.33 On the other hand, as shown in


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Figure 5C and D, the removal of D-O2 from the electrolyte would decrease the intensities of cathodic photocurrents. If in the presence of 4.0 mM L-cys, the phenomena would be more be more obvious. If the concentration of L-cys was further increased to 50 mM, the elimination of D-O2 would also enhance the anodic photocurrents. All these results have clearly indicated that the junctions were sensitive to the L-cys and D-O2, and L-cys could be easily used to tune the PEC responses of these electrodes.

Figure 5. Photocurrent responses of (A) CdS Qdots/Pdots and (B) CdTe Qdots/Pdots electrodes and in airsaturated PBS solution (pH 7.1) containing different concentrations of L-cys (0 to 80 mM). Photocurrent responses of (C) CdS Qdots/Pdots and (D) CdTe Qdots/Pdots in 0.1 M PBS solution (pH=7.1) in the presence


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of 0 mM, 4 mM and 50 mM L-cys with and without O2 as indicated. The PEC tests were performed with 0.0 V applied voltage and 460 nm excitation light.

PEC bioanalysis application. In fact, as an essential thiol-containing amino acid, L-cys plays a fundamental role in many biological processes and its deficiency leads to many diseases such as slowed growth, depigmentation of hair, loss of muscle and fat, skin lesions, edema, lethargy and liver damage. It has been recognized as an important indicator for disease diagnosis and been addressed by many PEC bioanalysis reports.47-49 Here we also use CdTe Qdots/Pdots electrode and L-cys as the model transducer and target molecule to demonstrate the applicability of the proposed system for PEC bioanalysis. The foregoing experiment has manifested the electrodes were sensitive to the concentration of L-cys, and Figure 6A shows the derived calibration curve of CdTe Qdots/Pdots electrode with the corresponding linear equation obtained by the linear fitting. As to the concentration dependence, it has revealed that the low concentration of L-cys led to a cathodic photocurrent response, whereas the high concentration caused an anodic one. This unique phenomenon should be attributed to the unique electrochemistry between the excitonic Pdots and surrounding species. Specifically, upon light excitation of Pdots, the conjugated polymer could efficiently absorb and transfer the excitation energy to the photosensitizer units to generate the singlet oxygen.29-33 In present case of using electrical signal as readout, the yield of the cathodic photocurrent proved beyond doubt that the existence of ITO electrode has opened a new route of electron transfer for the excitonic responses of Pdots, i.e., the transfer of the photo-generated electrons to the ambient oxygen with simultaneous supply of electrons from the electrode to neutralize the holes. However, with the increase of L-cys concentration, it could gradually inhibit the aforementioned process and steer the electron transfer to the electrode, generating gentle slope at low concentration. As the increase of L-cys concentration, the ejection of the photo-generated electrons to the electrode with the concomitant


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transfer of electrons from the L-cys would dominate and thus produce the anodic photocurrent as well as steep slope in concentration dependence. The selectivity of the system was then assessed by testing some common interfering agents including L-cys, ascorbic acid, L-phenylalanine, Lglutamate, L-lysine, L-histidine, L-GSH, glucose, and tryptophan at the same concentrations. These species are either common amino acids (eg. Phe; Glu; Lys; His; Trp) or important ingredients (eg. AA; L-GSH; Glc) in human being. As shown in Figure 6B, it demonstrated that none of these interferents could cause anodic response except the target L-cys, implying the excellent selectivity.

Figure 6. (A) The linear range of CdTe Qdots/Pdots electrode. (B) The corresponding selectivity of the system in 0.1 M L-1 PBS (pH = 7.4) containing L-cys, ascorbic acid (AA), L-phenylalanine (Phe), L-glutamate (Glu), L-lysine (Lys), L-histidine (His), L-GSH, glucose (Glc), and tryptophan (Trp) at the same concentrations of 40 mM as well as their mixed sample.

CONCLUSIONS To conclude, using CdS Qdots Qdots, CdTe Qdots and TPP-doped PFBT Qdots, this work has demonstrated the construction and characterization of different semiconducting organicinorganic nanodots heterojunctions and some of them as potential platforms for the PEC


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bioanalysis applications for the first time. The structural, optical and PEC characterizations were performed, and their specific energy levels (VB/CB or HOMO/LUMO) were also determined. The ideal coupling effect was observed between Pdots-based electrode and Qdots, and the corresponding good PEC properties were thus achieved. In the detection of model molecules of L-cys, the proposed system also demonstrated high sensitivity and good selectivity. Combined with various recognition elements, this platform could be easily extended for addressing many other targets of interest.50-52 This work displayed the great promise of organic-inorganic heterostructures as innovative platforms for PEC bioanalysis, and is expected to inspire more interest in the design, development, and implementation of numerous other organic-inorganic heterojunctions for advanced PEC bioanalysis in the future.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem… Experimental section, synthesis of CdS Qdots, CdTe Qdots and TPP-doped PFBT Pdots; fabrication

of

Qdots/Pdots/ITO

and

Pdots/Qdots/ITO

electrodes,

determination

of

HOMO/LUMO levels of TPP-doped PFBT Pdots, VB/CB of CdS and CdTe Qdots, stability of electrodes; SEM and XPS characterization of the layer structure, PEC behavior of the random mixtures, and the corresponding discussion (PDF). AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]; [email protected].


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* E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT Financial support from the National Natural Science Foundation of China (grant nos. 21327902 and 21675080), the Natural Science Foundation of Jiangsu Province (grant BK20170073), and the Scientific Research Foundation of the Graduate School of Nanjing University (grant 2016CL06) is appreciated. REFERENCES (1) Zhao, W. W.; Xu, J. J.; Chen, H. Y. Chem. Rev. 2014, 114, 7421-7441. (2) Li, H. N.; Mu, Y. W.; Yan, J. R.; Cui, D. M.; Ou, W. J.; Wan, Y. K.; Liu, S. Q. Anal. Chem. 2015, 87, 2007-2015. (3) Zhuang, J. Y.; Lai, W. Q.; Xu, M. D.; Zhou, Q.; Tang, D. P. ACS Appl. Mater. Interfaces 2015, 7, 8330-8338. (4) Tang, J.; Zhang, Y. Y.; Kong, B.; Wang, Y. C.; Da, P. M.; Li, J.; Elzatahry, A. A.; Zhao, D. Y.; Gong, X. G.; Zheng, G. F. Nano Lett. 2014, 14, 2702-2708. (5) Zayats, M.; Kharitonov, A. B.; Pogorelova, S. P.; Lioubashevski, O.; Katz, E.; Willner, I. J. Am. Chem. Soc. 2003, 125, 16006-16014. (6) Haddour, N.; Chauvin, J.; Gondran, C.; Cosnier, S. J. Am. Chem. Soc. 2006, 128, 9693-9698.


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(7) Wang, L. N.; Ma, W. G.; Gan, S. Y.; Han, D. X.; Zhang, Q. X.; Niu, L. Anal. Chem. 2014, 86, 10171-10178. (8) Surbhi, C.; Sumant, U.; Pushpendra, K.; Nirupama, S.; Vibha, R. S.; Rohir, S.; Sahab, D. Int. J. Hydrogen Energy. 2012, 37, 18713-18730. (9) Liu, Q.; Huan, J.; Hao, N.; Qian, J.; Mao, H, P.; Wang, K. ACS Appl. Mater. Interfaces 2017, 9, 18369-18376. (10) Zhao, W. W.; Yu, P. P.; Shan, Y.; Wang, J.; Xu, J. J.; Chen, H. Y. Anal. Chem. 2012, 84, 5892-5897. (11) Li, R. Y.; Zhang, Y.; Tu, W. W.; Dai, Z. H. ACS Appl. Mater. Interfaces 2017, 9, 2228922297. (12) Li, Z.; Xin, Y.; Zhang, Z. Anal. Chem. 2015, 87, 10491-10497. (13) Zhuang, J. Y.; Lai, W. Q.; Xu, M. D.; Zhou, Q.; Tang, D. P. ACS Appl. Mater. Interfaces 2015, 7, 8330-8338. (14) Chen, D.; Zhang, H.; Li, X.; Li, J. H. Anal. Chem. 2010, 82, 2253-2261. (15) Ma, Z. Y.; Pan, J. B.; Lu, C. Yu.; Zhao, W. W.; Xu, J. J.; Chen, H. Y. Chem. Commun. 2014, 50, 12088-12090. (16) Wang, J.; Liu, Z. H.; Hu, C. G.; Hu, S. S. Anal. Chem. 2015, 87, 9368-9375. (17) Ariel, E.; Omer, Y.; Ran, T. V.; Dorit, M.; Rachel, N.; Itamar, W. ACS Nano 2012, 6, 92589226.


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