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“On-off-on” Photoelectrochemical/Visual Lab-on-Paper Sensing via Signal Amplification of CdS Quantum Dots@Leaf-Shape ZnO and Quenching of Au Modified Prism-Anchored Octahedral CeO2 Nanoparticles Qingkun Kong, Kang Cui, Lina Zhang, Yanhu Wang, Jianli Sun, Shenguang Ge, Yan Zhang, and Jinghua Yu Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b01844 • Publication Date (Web): 20 Aug 2018 Downloaded from http://pubs.acs.org on August 22, 2018

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Analytical Chemistry

“On-off-on” Photoelectrochemical/Visual Lab-on-Paper Sensing via Signal Amplification of CdS Quantum Dots@Leaf-Shape ZnO and Quenching of Au Modified Prism-Anchored Octahedral CeO2 Nanoparticles

Qingkun Kong,†,┴ Kang Cui,†,┴ Lina Zhang,§ Yanhu Wang,† Jianli Sun,† Shenguang Ge,‡ Yan Zhang,*,†,‡,║ and Jinghua Yu*,†

†

School of Chemistry and Chemical Engineering, University of Jinan, Jinan 250022, P.R. China

§

Shandong Provincial Key Laboratory of Preparation and Measurement of Building Materials,

University of Jinan, Jinan 250022, P.R. China ‡

Institute for Advanced Interdisciplinary Research, University of Jinan, Jinan 250022, P.R. China

║

School of Materials Science and Engineering, University of Jinan, Jinan 250022, P.R. China

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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：： An effective “on-off-on” photoelectrochemical (PEC)/visual sensing system based on cleaning-switchable lab-on-paper device was designed to achieve ultrasensitive detection of analytes. The first amplified ‘‘signal on’’ PEC state was gained by CdS quantum dots sensitized leaf-shape ZnO (CdS QDs/leaf-shape ZnO) structure, which was assembled on reduced graphene oxide (rGO) modified paper electrode. Then Au modified prism-anchored octahedral CeO2 nanoparticles (Au@PO-CeO2 NPs), as an efficient signal quencher, were immobilized on the CdS QDs/leaf-shape ZnO with the assistance of DNA hybridization, resulting in a noticeably photocurrent response decrement with the ‘‘signal off’’ PEC state. With the addition of analytes, the quencher Au@PO-CeO2 NPs were immediately released from the sensing surface and robust PEC response was recovered to the “signal on” state again. Meanwhile, the disengaged quencher in electrolyte solution flowed to the colorimetric detection area of lab-on-paper device and catalyzed oxidation of the chromogenic substrate 3,3’,5,5’-tetramethylbenzidine in the presence of H2O2 to form the colored product, making the analytes detection more convincing with the visual discrimination. Under optimal conditions, the proposed PEC/visual lab-on-paper device possessed the detection limits toward adenosine and potassium ion as low as 0.15 nM and 0.06 nM respectively. With ingenious design of actuating conversion process between hydrophilicity and hydrophobicity by slipping paper tab to solve cleaning issue in the assay procedures, the cleaning-switchable lab-on-paper device was constructed for high-performance biosensing applications. It provides an unambiguous simplicity and portable operation for exploring high reliability and sensitivity of novel point-of-care diagnostic tool with dual-signal readout.


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INTRODUCTION Photoelectrochemical (PEC) technique, as a newly emerged approach applied in bioanalysis, have increasingly attracted substantial research scrutiny in recent years owing to their high sensitivity, simple equipment, cost effectiveness and ease of miniaturization.1-3 By using photoirradiation coupled with electrochemical detection, PEC technique can offer an elegant route for probing various biological events at relevant topics, such as the PEC aptasensor, PEC enzymatic biosensors, or PEC immunoassays.4-7 Among these PEC strategy, most of them adopted either ‘‘signal on’’ or ‘‘signal off’’ readout strategies.8-10 However, there are some challenges to overcome the associated drawbacks in the PEC system, such as the false positive signal and the unignorable background signals. In order to address these issues, an alternative design is to establish PEC protocol based on “on-off-on” switch system because the combination of enhanced “switch on” state and quenching rate of the “switch off” state can effectively avoid the false positive signal and reduce the background signal.11-13 Undoubtedly, the outstanding photoactive materials, which have a great effect on PEC process, are important requisite of receiving the desirable initial amplified PEC signal in the PEC assays. ZnO has a unique crystalline structure with high electrons mobility, high melting point and excellent chemical stability, making it significant in sensors, photocatalysts, and photovoltaics.14-16 Yet, the photo-current conversion efficiency of ZnO is still challenged by its wide band gap.17 In order to achieve remarkable photocurrent density, the strategy of ZnO nanostructure sensitized with other small bandgap semiconductor materials such as CdS quantum dots (QDs) is normally used, which can broaden the light absorption range and improve efficiency of photogenerated charge separation.18,19 Apart from the desirable first amplified PEC signal, an efficient quencher also play a critical role in reducing background signals and extending the linear range in ‘‘on-off-on’’ PEC system. So far, several strategies have been applied for quenching the photocurrent when bio-recognition events occur, such as steric-hindrance effects, enzymatic reactions, sensitization effects, and energy transfer.20-22 For example, the protocol based on energy transfer between ACS Paragon Plus Environment


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semiconductor QDs and gold nanoparticles (Au NPs) have been widely utilized in the amplification for “signal off” PEC assays.23,24 However, since the limitation amount of quenchers immobilized on biosensors greatly hinder the quenching efficiency, exploring high sensitive signal amplification methods should be developed. In recent years, cerium dioxide (CeO2) nanomaterial has exhibited potential applications in the fields of photocatalysis, optical limiters and biosensors based on its favorable electrical, optical, and super intrinsic oxidase-like properties.25-27 Specifically, when Au NPs are in intimate contact with CeO2, CeO2 supported Au NPs (Au@CeO2) can effectively harvest photon energy across the visible light spectrum, and catalyze electron donor reagent due to surface plasmon resonance (SPR) of Au NPs and their catalytically active surfaces,28 resulting in change of electrons transfer processes. Evidently, the Au@CeO2 nanocomposites can be utilized potentially as an effective quencher in PEC sensing system to tune current intensity. In order to meet the need of end-user applications, a sensing system with operation-to-easy, rapid, and direct readout strategy is significant for portable analytical devices. Since the Whitesides group reported three-dimensional fluidic devices built on cellulosic paper platforms in 2007, lab-on-paper device based on folding or stacking layers, have aroused much interest due to their integrated multi-functionality while retaining small device size.29,30 In comparison to traditional analytical platform, lab-on-paper device employ hydrophobic materials printed onto paper to control the flow path and further achieve the overall functionality.31-33 With the inherent merits of paper such as low-cost, no instrument requirement, ease of mass production,

disposability,

and

desired

biocompatibility

and

hydrophilicity,

lab-on-paper devices have been applied to revolutionize healthcare diagnostics, environmental monitoring, and food safety inspection.34-36 Generally, multiple individually timed steps of binding, washing, and amplification were required for common assays. Performing these steps manually, however, is time-consuming, labor intensive and even prone to manual operation errors. Therefore, it is necessary to develop a cleaning-switchable lab-on-paper device featured with remarkable simplicity and portable operation for functionalizing the working electrode. ACS Paragon Plus Environment

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Herein, a novel “on-off-on” PEC/visual cleaning-switchable lab-on-paper device was designed based on signal amplification of CdS QDs sensitized leaf-shape ZnO (CdS QDs@leaf-shape ZnO) and signal quenching of Au NPs modified prism-anchored octahedral CeO2 NPs (Au@PO-CeO2 NPs). By actuating conversion process between hydrophilicity and hydrophobicity on the slip tab to achieve cleaning procedure, the integrated ‘‘on-off-on’’ PEC lab-on-paper device with simple, multifunctional and portable operation presented ultrasensitive detection of analytes including adenosine or potassium ion with an important complement of visual signal output. As shown in Scheme S1, CdS QDs/leaf-shape ZnO sensitized structure were firstly assembled on the reduced graphene oxide (rGO) modified paper working electrode via a hydrothermal method, exhibiting strongly enhanced ‘‘signal on’’ PEC performance. And then, the Au@PO-CeO2 hybrid structure, where the morphologies of CeO2 NPs can be tuned by carefully controlling the amount of Na3PO4, assembled capture DNA was immobilized onto the CdS QDs/leaf-shape ZnO part by hybridization with single-stranded DNA (ssDNA), resulting in a noticeably PEC response decrement. Finally, with the addition of analytes, the Au@PO-CeO2 NPs quencher was released from the sensing interface to form quencher-analytes complex, and the ‘‘signal on’’ PEC state was consequently recovered. Meanwhile, the released quencher was able to efficiently catalyze the 3,3’,5,5’-tetramethylbenzidine (TMB) in the presence of H2O2, and a color change of TMB can be directly read out, resulting in a rapid visual detection of analytes. By employing a slipping paper layer to address washing problem in the assay procedure, the “on-off-on” PEC/visual lab-on-paper device with the advantages of both cleaning-switchable platform and dual-signal readout mode is designed to realize the ultrasensitive detection of analytes, which opens a new avenue for designing numerous elegant biosensor with excellent performance.

EXPERIMENTAL SECTION Design of the Lab-on-Paper Device. The lab-on-paper device patterns (Figure S1) were designed by Adobe Illustrator CS6 and printed on Whatman ACS Paragon Plus Environment


Figure 1. The photograph of (A) one side of lab-on-paper device with the RE and CE; (B) reverse side of part A with the WE.

chromatography paper #2 using the wax printer. The paper sheet was placed in an oven at 130 °C for 50 s to melt the wax and form three-dimensional hydrophobic wall. To integrate “on-off-on” PEC performance and colorimetric reaction sensing system on one device, and solve cleaning problem in the assay procedures, a seven layers paper-based device was designed (Figure 1A&B). Layer I contained one circular paper reservoirs, which was utilized to catalyze chromogenic reactions and showed the color with respective intensity. Layer II was comprised of hollow circular zone, hydrophilic circular pattern and channel, in which hydrophilic patterns were employed to gather released quencher from layer IV via hollow circular zone. Layer III as a hydrophobic tab was constructed to prevent solution from contaminating hydrophilic pattern on layer II. Layer IV as detection tab had a circular hydrophilic pattern and hollow rectangular section, and carbon working electrode (WE) was screen-printed on the circular hydrophilic pattern to carry out PEC reaction. It was noted that the bottom edge of the layer IV was utilized as guides in fabricating process. Layer V (the slip tab) comprised of rectangle hydrophilic area and scaleplate was employed to actuate conversion process between hydrophilicity and hydrophobicity in the fabricating process by manually slipping a piece of paper. Layer VI composed of hollow zone and one polygonal sink pad was utilized to collect effluent. Finally, layer VII containing a hydrophilic layer was ready for screen-printing of Ag/AgCl reference electrode (RE), and carbon counter electrode (CE). After the patterns were wax-printed, each individual device was cut from the ACS Paragon Plus Environment

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paper sheet using scissors.

Assay Procedures of the Lab-on-Paper Device. Firstly, a desirable initial amplified PEC ‘‘signal on’’ state was gained by the introduction of a CdS QDs/leaf-shape ZnO/lab-on-paper device (detailed procedures shown in supporting information). Subsequently, 20 µL PBS (pH 7.4) solution consisting of 20 mM N-(3-dimethylaminopropyl)-N’-ethylcarbodiimide and 10 mM N-hydroxysuccinimide was added on the WE for activating the carboxyl groups of CdS QDs. The unbound or physically adsorbed molecules were removed with assist of the washing buffer solution (PBS, pH 7.4, 10 mM). After that, CdS QDs/leaf-shape ZnO modified electrode was incubated with 10 µL of 2 µM ssDNA at room temperature for 1 h. To reduce nonspecific adsorption, the modified electrode was blocked with 10 µL of 0.25% MCH aqueous solution by 40 min treatment. And then, 40 µL of capture DNA/Au@PO-CeO2 NPs (preparation shown in supporting information) was dropped onto WE to obviously reduce the background signal for further improvement of the PEC sensitivity. Next, the analytes with different concentrations were dropped onto the lab-on-paper device at room temperature. Meanwhile, 20 µL of 20 mM TMB, 20 µL of acetic acid (pH 4.5) and 20 µL of 0.5 M H2O2 were pre-loaded on hydrophilic area of the layer I. When the dissociative Au@PO-CeO2 NPs in electrolyte solution flowed to the colorimetric detection area of lab-on-paper device, the Au@PO-CeO2 NPs were employed to catalyze chromogenic reactions from colorless to blue. Finally, after layers III&V were pulled away, photocurrent was collected in the presence of 0.05 M H2O2 by a Xe lamp with a spectral range from 200 to 2500 nm as excitation light source.

RESULTS AND DISCUSSION Description and Assembly of the Lab-on-Paper Device. A multilayered lab-on-paper device was designed to achieve “on-off-on” PEC/visual assay as well as effectively address cleaning problem in the fabricating process. The assembly and folding principles of origami were schematically shown in Figure 2. After the introduction of CdS QDs/leaf-shape ZnO on the layer IV, the lab-on-paper device was ACS Paragon Plus Environment


Figure 2. Assembly and operation illustration of the cleaning-switchable lab-on-paper device during (A) folding, (B) cellulose modification, (C) cellulose cleaning, (D) visual signal output and (E) PEC sensing platform.

folded as depicted in Figure 2A and ready for further use. The initial desirable ‘‘signal on’’ state was gained on the CdS QDs/ZnO electrode to significantly extend linear range. To effectively facilitate the functional group binding and achieve cleaning process on the WE, the cleaning-switchable lab-on-paper device featured with remarkable simplicity and portable operation was designed for construction of analytical platform by actuating a fluidic switch, which was significant for improving multiple individually timed process. Concretely, when red line on the slip tab (layer V) was aligned with the guides on the layer IV by utilizing thumb pad, it can be found that the hollow area on the layer IV was coincided with the wax printed area on the layer V (Figure 2B). The solution containing reagent can restore at paper working electrode due to utility of wax’ hydrophobicity. Similarly, cleaning step was conducted after black line was aligned with the guides on the layer IV by manually slipping a piece of paper. It should be noted that the hollow area on the layer IV coincided with rectangle hydrophilic area on the slip tab, which made buffer solution containing unbound or physically adsorbed molecules flow to polygonal sink pad via capillary action along well-defined channels (Figure 2C). Through changing the state of hydrophilicity and hydrophobicity on the slip layer, different molecules were


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modified on the WE. And then, the ‘‘signal off’’ state was further achieved by exploiting the quenching effects of capture DNA/Au@PO-CeO2 NPs. To reduce the background signal and avoid the false positive signal, the PEC “off” performance was dramatically achieved by utilizing the efficient quenching effects of Au@PO-CeO2 NPs on electrons transfer process. With the addition of analytes, the quencher was released from the sensing interface through the formation of an aptamer complex and the PEC signal was immediately recovered to the second “on” states. Meanwhile, the released quencher can flow to circular area on the layer II via hydrophobic channel in the absence of layer III (Figure 2D), leading to colorimetric reaction. The Au@PO-CeO2 NPs could catalyze TMB chromogenic reactions with color changes from colorless to blue in the presence of H2O2 that was pre-loaded on hydrophilic area of the layer I. Finally, recovered PEC signal was collected after layer V was pulled away (Figure 2E and S3). With remarkable simplicity and portable operation, the proposed platform presented high reliable and sensitive PEC signal combined with visualized output for the determination of analytes.

“On-off-on” Analytical Protocol. Successful fabrication of rGO on the electrode was verified by scanning electron microscopy (SEM). When the rGO solution was added to cellulose paper, the thin and rough rGO layer was formed to enhance the electro-conductibility of paper electrode (Figure S4). To obtain a desirable PEC signal, leaf-shape ZnO nanosheets were prepared on the rGO modified WE via a hydrothermal method. As depicted in Figure 3A, the ZnO assembled cellulose exhibited a good uniformity on the large scale without any crack gaps. It could be clearly observed that the dense nanosheets were composed of leaf-shape thin slices with a layer structure in the region. All smooth nanosheets stood almost vertically on the substrate with the thickness range from 80 to 150 nm and remained highly anisotropic (Figure 3B and C), which were beneficial for the electrolyte diffusion. X-ray diffraction (XRD) patterns (Figure S5) also proved the successful preparation of leaf-shape ZnO/rGO/cellulose. The representative diffraction peaks in the range of 30-60o were assigned to (100), (002), (101) and (110) planes of the ZnO



Figure 3. SEM images of (A, B, C) leaf-shape ZnO/rGO/cellulose at different magnification and (D) CdS QDs/leaf-shape ZnO/rGO/cellulose with corresponding element mapping of (E) O, Zn, Cd, and S, respectively. SEM images of (F) PO-CeO2 NPs, (I) Au@PO-CeO2 NPs; (G, H) HR-TEM images of PO-CeO2 NPs, insets in the Figure 3F and G: different magnification SEM image of PO-CeO2 NPs and TEM image of PO-CeO2 NPs.

(JCPDS 36-1451). To further enhance the light absorption efficiency and facilitate the separation of photogenerated charge carriers, CdS QDs were employed to form CdS QDs/ZnO sensitized structure. After ZnO was coated by CdS QDs, it could be clearly observed that sensitization structure still remained leaf-shape (Figure 3D), yet with dark regions on the surface of the composites. In addition, the emergence of Cd and S elements was presented in the energy dispersive spectroscopy (EDS) patterns compared with the leaf-shape ZnO/rGO/cellulose in Figure S7. The mapping showed that both Cd and S elements distributed uniformly throughout the cellulose paper (Figure 3E), revealing that ZnO was homogeneously coated by CdS QDs. In this study, to obtain preferable property and morphology of quencher in the PEC “signal off” state, CeO2 NPs with a tunable shape were synthesized by a simple solution-based approach using Na3PO4 as a growth modifier. The role of Na3PO4 ACS Paragon Plus Environment

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concentration on the morphology of CeO2 was examined by SEM and high-resolution transmission electron microscopy (HR-TEM). With addition of Na3PO4, it should be noted that the PO-CeO2 NPs exhibited an average size of 150-180 nm and were anchored by nanorod-like arm with the length of ∼200 nm (Figure 3F and G). The nanorods assembled octahedral structure revealed that the nucleated reaction on the surface of the octahedron was occurred and subsequently prism grew out of it. The corresponding TEM image of the PO-CeO2 NPs clearly demonstrated the nanorods-like projects anchored octahedron. The HR-TEM analysis was further performed to analyze the structure of PO-CeO2 NPs. The lattice fringe spacing of building block and prism were determined to 0.328 nm and 0.298 nm, which were identical with the (111) and (200) panels, respectively (Figure 3H). It also demonstrated that the growth rate of (111) panels was higher than that of (200) panels, which was thought to be responsible for the formation of octahedral nanostructure. The architecture was favorable for developing the homojunction facets and further effectively promoting charges transfer. For a comparison, octahedral CeO2 NPs (O-CeO2) were obtained in the absence of Na3PO4. It was quite evident that O-CeO2 NPs demonstrated regular octahedral shape with average size of 180 nm and smooth surface (Figure S9A). TEM image also revealed the good crystallinity of the O-CeO2 NPs (Figure S9B). Besides, the X-ray diffraction (XRD) patterns were clearly displayed in Figure S10A. The results indicated that all the diffraction peaks of CeO2 NPs could be well indexed to the body centered cubic structure of CeO2 (JCPDS no. 34-0394). The UV-vis diffuse reflectance spectra of CeO2 NPs revealed a clear absorption at 330 nm (Figure 4A). By comparing the two samples, it was obvious that PO-CeO2 NPs had a stronger absorption band, which was resulted from the high surface area and unique structure of PO-CeO2 NPs.37 These results exhibited that the CeO2 morphology can be tuned by carefully controlling the amount of Na3PO4. To achieve the efficient signal quenching performance, Au NPs were decorated onto the surface of these two different morphologies of CeO2 to form Au@PO-CeO2 NPs and Au@O-CeO2 NPs. As shown in their SEM images ( Figure 3I and S9C), many tiny spots with homogeneous structure unevenly distributed on both PO-CeO2 ACS Paragon Plus Environment


NPs and O-CeO2 NPs surface, implying that as-prepared Au NPs were successfully adsorbed on CeO2 NPs surface. Meanwhile, the UV-Vis diffuse reflectance spectroscopy was utilized to characterize the formation of Au@CeO2 NPs. The Au@CeO2 nanocomposites exhibited typical absorption in the range of 500-600 nm in the visible region, which was ascribed to the surface plasmon band characteristics of gold. These results further confirmed that the Au NPs had been successfully adsorbed at the surface of CeO2 NPs, and nanocomposites could make full use of the light energy in the entire visible region.

Figure 4. (A) UV−vis diffuse absorption spectra of Au NPs, PO-CeO2 NPs, Au@PO-CeO2 NPs, O-CeO2 NPs and Au@O-CeO2 NPs. (B) Photocurrent responses of (a) rGO/celluloses, (b) leaf-shape ZnO/rGO/celluloses, (c) CdS QDs/leaf-shape ZnO/rGO/celluloses, (d) ssDNA/CdS QDs/leaf-shape ZnO/rGO/celluloses, (e) 20 µL of capture DNA/Au@PO-CeO2 NPs and (e') Au@O-CeO2 NPs conjugated with d after MCH blocking respectively, (f) after incubation with 20 µL of 30 nM adenosine.

The building process of the PEC sensing system could be followed and characterized through PEC signal. As shown in Figure 4B, no photocurrent response was observed in rGO modified working electrode (curve a). After the immobilization of leaf-shape ZnO on lab-on-paper device, the photocurrent increased obviously (curve b). This could be explained that electrons were excited from the valence band to the conduction band of ZnO under light irradiation. And then CdS QDs sensitized leaf-shape ZnO composites corresponded to the highest photocurrent response (curve c), which was ascribed to the movement of photoexcited electrons from CdS QDs/ZnO structure to electrodes and the effective separation of the photogenerated


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charges. After ssDNA was successively added onto the electrode surface, the photocurrent intensity gradually decreased with increase of evident steric hindrance (curve d). Notably, the photocurrent further decreased greatly after the capture DNA/Au@PO-CeO2 DNA immobilization (curve e), which was resulted from the quenching effect of Au@PO-CeO2 on electrons transfer processes combined with increasing steric hindrance. To further provide insight to effects of the properties of Au@CeO2 NPs on PEC signal, different morphologies of CeO2 NPs were employed to quench PEC response. It was observed that PEC response of PO-CeO2 NPs was ~49% lower than that of O-CeO2 NPs (curve e'). This was attributed to that PO-CeO2 NPs were more beneficial for survive time of the electron and exhibited more stable catalytic properties due to unique structure.38 The photoluminescence (PL) technique was exploited to verify the separation efficiency of the photo-induced electrons and holes. It was well-known that the PL intensity with higher value signified higher recombination possibility of electron-hole pairs, whereas weaker intensity indicated lower recombination probability of photoexcited charge carriers.39 Obviously, PO-CeO2 NPs displayed much lower PL intensity compared with O-CeO2 NPs in Figure S10B, which indicated that PO-CeO2 NPs could exhibit the more effective separation of photogenerated charge carriers. Under the light irradiation, H2O2 in the solution was efficiently consumed by the holes in the valence band of PO-CeO2 NPs, resulting in highly carrier recombination opportunity of CdS QDs/leaf-shape ZnO structure. This photocurrent decrement revealed that the properties and structure of PO-CeO2 NPs were favorable for quenching PEC performance. After the as-fabricated lab-on-paper device was incubated with analytes, the photocurrent responses instantly recovered to PEC “signal on” state (curve f). The photocurrent characterization successfully confirmed the development of the proposed cellulose was feasible for constructing paper-based analytical device. In addition, electrochemical impedance spectroscopy was a useful means to characterize interface properties of the electrodes (Figure S11), showing that the sensing interface was effectively constructed.



Scheme 1. Schematic Illustration of (A) Photogenerated Electron-hole Transfer Quenching Mechanism of the Au@PO-CeO2 NPs, and (B) Colorimetric Reaction Mechanism

Photoelectrochemical Mechanism of the Lab-on-Paper Device. The “on-off-on” PEC/visual mechanism of proposed sensing system is demonstrated in Scheme 1. In order to realize the initial amplified “ signal-on ” PEC state, the narrow-band gap CdS QDs assembled ZnO sensitized structure with cascade band-edge levels exhibited the excellent performance due to preferable morphology, which can evidently extend the absorption range, harvest light energy and ultimately promote the photocurrent intensity. Under the light irradiation, the photoinduced charge carriers remained separated due to their intrinsic electrostatic field. The photo-induced electrons in the conduction band of CdS QDs were transferred to ZnO in the absence of quencher. Simultaneously, holes from the valence band of ZnO nanosheets were transferred to CdS QDs, where they were consumed by H2O2. H2O2 as electrons donors tremendously improved separation efficiency of electron-hole carrier and enlarged the initial PEC response. For “signal off” elements, quenching


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effects of Au@PO-CeO2 NPs were employed to effectively decrease the unignorable background signals and further improve sensitivity of analytical strategy. Concisely, three main aspects contributed to quenching mechanism of the Au@PO-CeO2 NPs in the proposed protocol (Scheme 1A) when quencher was immobilized onto the CdS QDs/leaf-shape ZnO part by hybridization with DNA. Firstly, the prepared Au@PO-CeO2 NPs possessing a broad absorption range across the visible spectrum, could competitively absorb the light energy so as to weaken light absorption efficiency for the CdS/ZnO matrix.40-42 Secondly, the fermi energy of the Au NPs was higher than that of CeO2, which led to electrons rapidly transfer from fermi level of Au to conduction band of CeO2 via a SPR mechanism.43,44 Meanwhile, these injected electrons were trapped by oxygen molecules from the decomposition of H2O2 to yield high oxidative species such as superoxide radical anions O2-, and the holes in the valence band of CeO2 were neutralized by H2O2. During this process, the electron-hole recombination efficiency of CdS/ZnO matrix was increased due to the consumption of H2O2 caused by the Au@PO-CeO2 NPs, resulting in the decrement of PEC signal. Thirdly, obvious steric effects arose when the Au@PO-CeO2 NPs quencher bound to sensing interface through DNA hybridization. This can affect electrons transfer processes on the electrode surface, resulting in the decrease of the photocurrent response. In the presence of quencher, the electron-hole recombination process was in the ascendant, while the faster carrier migration was efficiently suppressed by utilizing the quencher. After the device was incubated with analytes, the quencher could be released from the sensing interface due to capture DNA specific recognition for analytes and the originally suppressed electron transfer process could be effectively activated with the increase of analytes concentration, which was responsible for an enhanced photocurrent response again. In addition, Au@PO-CeO2 NPs were released from the sensing interface, and flowed into the colorimetric test area of device to catalyze TMB in the presence of H2O2, leading to color change of TMB which can be visually read out (Scheme 1B). Accordingly, a dual ‘‘on-off-on’’ PEC/visual cleaning-switchable lab-on-paper analytical device was successfully designed, which could be extensively employed as a quantitative ACS Paragon Plus Environment


analytical platform with visual detection for screening of the analytes.

Figure 5. (A) Photocurrent responses of the “on-off-on” PEC lab-on-paper device incubated with various concentrations of adenosine (from a to g: 0.5, 1.5, 5, 30, 300, 10000 and 50000 nM) upon light on/off. Calibration curves of (B) PEC signal and (C) grey intensity vs. logarithm of adenosine concentration.

Photoelectrochemical Detection for Analytes. The photocurrent signals were responsive to the changing concentrations of target (using adenosine as an example). Under the optimum conditions (details in the supporting information), an efficient electrons transfer process occurring between CdS QDs/leaf-shape ZnO and Au@PO-CeO2 NPs quencher. The relationship between the increased current intensity and the concentration of adenosine was demonstrated in Figure 5A and B. As expected, the photocurrent response ∆I (∆I = I – I0, I0 was the current response when analytes concentration was zero; I was the current response of analytes at different concentrations) increased obviously as the concentration of adenosine varied from 0.5 nM to 50 µM with a limit of detection (LOD) of 0.15 nM at a signal-to-noise ratio of 3. Since the CeO2 catalyzed chromogenic reactions to produce a color change from colorless to blue, a complementary visualized detection could be accomplished by measuring color intensity. As the concentration of the analytes increased, more blue products formed and accumulated on the hydrophilic area of layer I. The color change could be discriminated by the naked eye and quantified from its photograph with the assistant of Adobe Photoshop software. The color intensity as reference value could preliminarily judge the accuracy of the experiment. An example of this visualized detection was recorded by Figure 5C. Meanwhile, the specificity, reproducibility, stability (Figure S13) were also investigated. These results revealed that the proposed


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PEC lab-on-paper device could be applied to determine adenosine in real sample (Table S1). In addition, the as-prepared PEC lab-on-paper device was also applied to detect potassium ion (Figure S14). The photocurrent was proportional to the concentration logarithm in the range from 0.1 nM to 50 µM for potassium ion with detection limit as low as 0.06 nM. As was compared with previously reported methodologies, the proposed PEC strategy presented a much lower detection limit and a wider linear range (Table S2 and S3).

CONCLUSIONS In the presented work, a promising “on-off-on”/visual PEC lab-on-paper device was successfully applied for ultrasensitive detection of analytes including adenosine and

potassium

ion.

The

conversion

process

between

hydrophilicity

and

hydrophobicity that can be actuated by manually slipping a piece of paper was successfully adopted to address cleaning problem in the fabricating process of the lab-on-paper device. The as-prepared CdS QDs/leaf-shape ZnO structures on the electrode, as photoactive materials, exhibited outstanding PEC performances. The efficient PEC signal quenching was achieved with the assistance of Au@PO-CeO2 NPs due to competitive absorption of exciting light and effective consumption of H2O2 electron donor as well as evident steric hindrance. Additionally, it is interesting to find the morphology of CeO2 NPs play a critical role in improving sensitivity of analytical strategy through degradation of H2O2. When the quencher Au@PO-CeO2 NPs were released from the sensing interface in the presence of analytes, PEC response was recovered to the desired “on” state successfully. Meanwhile, released Au@PO-CeO2 NPs showed the visual detection with color intensity changes. The proposed strategy might provide an extensive approach featured with simplicity and portable operation for the ultrasensitive detection of various kinds of biomarkers, heavy metal ions with dual-signal readout.



ASSOCIATED CONTENT Supporting Information Preparation of CdS QDs/leaf-shape ZnO nanosheets, CeO2 NPs and Au NPs, figure and tables, optimization of conditions for photoelectrochemical detection, real samples analysis.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. Phone: +86-531-82767040. *E-mail: [email protected]. Phone: +86-531-82767161.

ORCID Yan Zhang: 0000-0002-1936-4619 Jinghua Yu: 0000-0001-5043-0322

Author Contributions ┴Q.K.

and K.C. contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors are thankful for support from the program for Taishan Scholar of Shandong Province (ts201712048), the National Natural Science Foundation of China (21874055, 21775055), National Postdoctoral Program for Innovative Talents of China (BX20180129) and Major Program of Shandong Province Natural Science Foundation (ZR2017ZC0124).

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table of contents graphic


Visual Lab-on-Paper Sensing via

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