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Exciton-Plasmon Interaction between AuNPs/Graphene Nanohybrids and CdS QDs/TiO2 for Photoelectrochemical Aptasensing of Prostate-Specific Antigen Guoneng Cai, Zhengzhong Yu, Rongrong Ren, and Dianping Tang ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.7b00899 • Publication Date (Web): 21 Feb 2018 Downloaded from http://pubs.acs.org on February 23, 2018

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ACS Sensors

Exciton-Plasmon Interaction between AuNPs/Graphene Nanohybrids and CdS QDs/TiO2 for Photoelectrochemical Aptasensing of Prostate-Specific Antigen

Guoneng Cai, Zhengzhong Yu, Rongrong Ren, and Dianping Tang*

Key Laboratory for Analytical Science of Food Safety and Biology (MOE & Fujian Province), State Key Laboratory of Photocatalysis on Energy and Environment, Department of Chemistry, Fuzhou University, Fuzhou 35011168, People's Republic of China

CORRESPONDING AUTHOR INFORMATION Phone: +86-591-2286 6125; fax: +86-591-2286 6135; e-mail: [email protected] (D. Tang)

KEYWORDS: photoelectrochemical aptasensor, competitive-displacement reaction, CdS quantum dots-coated mesoporous TiO2, nanogold-functionalized graphene nanosheets, exciton-plasmon interaction

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ABSTRACT: A competitive-displacement reaction strategy based on target-induced dissociation of gold nanoparticles-coated graphene nanosheet (AuNPs/GN) from CdS quantum dots-functionalized mesoporous titanium dioxide (CdS QDs/TiO2) was designed for the sensitive photoelectrochemical (PEC) aptasensing of prostate-specific antigen (PSA) through the exciton-plasmon interaction (EPI) between CdS QDs and AuNPs. To construct such an aptasensing system, capture DNA was initially conjugated covalently onto CdS QDs/TiO2-modified electrode, and then AuNPs/GN-labeled PSA aptamer was bound onto biofunctionalized CdS QDs/TiO2 via hybridization chain reaction of partial bases with capture DNA. Introduction of AuNPs/GN efficiently quenched the photocurrent of CdS QDs/TiO2 thanks to energy transfer. Upon addition of target PSA, the sandwiched aptamer between CdS QDs/TiO2 and AuNPs/GN reacted with the analyte analyte, thus resulting in the dissociation of AuNPs/GN from the CdS QDs/TiO2 to increase the photocurrent. Under optimum conditions, the aptasensing platform exhibited a high sensitivity for PSA detection within a dynamic linear range of 1.0 pg/mL – 8.0 ng/mL at a low limitation of detection of 0.52 pg/mL. The interparticle distance of exciton-plasmon interaction and contents of AuNPs corresponding to EPI effect in this system were also studied. Good selectivity and high reproducibility were obtained for the analysis of target PSA. Importantly, the accuracy and matrix effect of PEC aptasensor was evaluated for the determination of human serum specimens and new-born calf serum-diluted PSA standards, giving a well-matched result with the referenced PSA ELISA kit.

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Aptamer,

an oligonucleic acid (peptide) molecule that can specifically bind to various proteins

based on the DNA-protein interaction, is getting more and more researchers' concentration.1 Owing to the portability for the synthetic and functional design, the aptamers have been regarded as the biorecognition probes for the application of various biosensing platforms, e.g., enhanced surface plasmon resonance,2,3 electrochemistry,4,5 electrochemiluminescence,6 and colorimetric assay.7 Recently, photoelectrochemical (PEC; an innovative and rapidly developing analysis tool) sensing platform is widely used for the construction of aptasensors because of its low cost and high sensitivity.8 Typically, the sensitivity of PEC aptasensors mainly depends on the efficiency of interfacial charge transfer and the photon-to-current's conversion on the photoactive materials.9,10 Therefore, establishing detective format with low environmental impact and designing the manifold materials with the favorable photocurrent response are the key to improve the sensitivity of PEC aptasensors. Both metals and semiconductors are the material systems that contain the excitations at optical frequencies, and thus their interaction (called as the exciton-plasmon interaction, EPI) is explored continuously in the fields of photoelectric conversion.11 It has been demonstrated that the resonance energy could transfer between CdS quantum dots (QDs) and gold nanoparticles (AuNPs) that were irradiated and placed in close proximity.12 Zhang et al. devised a new signal-on PEC biosensor on the graphene/CdSe QDs nanohybrids which relied on electron transfer of bipyridinium relay and the energy transfer of AuNPs.13 Ma et al. presented an energy-transfer-based PEC probing of DNA-protein interactions by target-induced distortion of duplex DNA-capped AuNPs to adjust the interparticle distance and thus modulate the photocurrent of CdS QDs through electron transfer process between QDs and AuNPs.14 Usually, most PEC sensing strategies depend on the biological reaction to change the photoactive species of the electrode,15,16 or enhance the electron transfer efficiency in liquid phase.17,18 In contrast, EPI-based PEC sensing systems between metal sulfide (e.g., CdS QDs) and noble metal nanoparticles (e.g., AuNPs) can efficiently avoid the unfavorable signal disturbances between the photoactivated sensing surface and ambient environment, because it can adjust the distance between noble metal nanoparticles (the photocurrent quencher) and metal sulfide (the photocurrent producer). Owing to the well-controlled length, the aptamer is an ideal biorecognition to conjugate with noble metal nanoparticles and metal sulfide. Therefore, exploiting

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excellent materials to immobilize the DNA-probe with the stable EPI is advantageous. Nanomaterials (e.g., SnO2, TiO2 and CdS QDs) with good surface plasmon excitations, efficient absorption and emission of light have been extensively applied in PEC biosensors because of their excellent optical properties.19 Usually, the recombination of electron and hole in the single material would reduce the photocurrent. On a contrary, semiconductor hybrids with large or small band gap can efficiently improve the separation of charge.8 For example, CdS QD is an eminent material to sensitize TiO2 for improving photoelectrochemical capability.20-22 Significantly, ordered mesoporous TiO2 provides a robust template for the incorporation of semiconductor sensitizer because of its large surface area and multiple scattering. Li et al. reported an ion-exchange method to synthesize CdS QDs-embedded mesoporous TiO2.23 Two-phase contact between nanostructures promoted the transfer of charge and the separation of electron-hole in the interface.22 Furthermore, the ordered arrangement of CdS QDs in mesoporous TiO2 promoted the valid binding of photoactivated sensing surface and biorecognition probe.

Scheme 1. Schematic illustration of target-induced displacement reaction between biofunctional CdS QDs/TiO2 and gold nanoparticles-functionalized graphene nanosheet (AuNPs/GN) for photoelectrochemical (PEC) detection of prostate-specific antigen (PSA) based on the exciton-plasmon interaction between CdS QDs and AuNPs (CdS QDs/TiO2: CdS quantum dots-functionalized mesoporous TiO2; MCH: 6-mercapto-1-hexanol; C-DNA: capture DNA; AA: ascorbic acid; FTO: fluorine-doped tin oxide).

Herein, we introduce a sensitive signal-on photoelectrochemical sensing platform for detection of prostate-specific antigen (PSA; a glycoprotein enzyme encoded in humans by the kallikrein-3 gene)

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based on exciton-plasmon interactions between CdS QDs-coated mesoporous TiO2 (CdS QDs/TiO2) and AuNPs-functionalized graphene nanosheet (AuNPs/GN) (Scheme 1). Initially, thiolated capture DNA and PSA aptamer are covalently conjugated onto CdS QDs/TiO2 (as the photocurrent emitter; via the Cd-S bond) and AuNPs/GN (as the photocurrent quencher; via the Au-S bond), respectively. In the presence of PSA, the aptamer is prone to form the aptamer-PSA affinity complexes, thereby destroying the sandwiched structure consisting of CdS QDs, aptamer and AuNPs/GN.24,25 In this case, the photocurrent increases relative to the background signal using ascorbic acid (AA) as the electron mediator.

■ EXPERIMENTAL SECTION Synthesis of CdS QDs-Functionalized Mesoporous TiO2. CdS QDs-functionalized mesoporous TiO2 (CdS QDs/TiO2) was prepared on the basis of the ion-exchange method.23 Initially, 1.5 g of poly(alkyleneoxide) block copolymer (Pluronic F-127) and 0.14 g of Cd(NO3)2·4H2O were dissolved into 19 mL of ethanol. Thereafter, titanium tetrachloride (15 mmol) was added into the resulting mixture under vigorous stirring. After incubation for 30 min, the obtained sol was transferred into an open Petri dish at 40 °C in the air for 7 days to form the gel. Following that, the as-prepared gel was calcined at 400 °C for 4 h in air [note: During this process, CdO-embedded mesoporous TiO2 (CdO/TiO2; as the precursor) was obtained]. Subsequently, the as-synthesized CdO/TiO2 (0.3 g) was dispersed into Na2S aqueous solution (50 mL, 0.2 M). After stirring for 6 h at room temperature (RT), the mixture was filtered, washed with ultrapure water for several times, and dried in air. Finally, the obtained CdS QDs/TiO2 was stored in the dark prior to use.

Synthesis of Gold Nanoparticles-Functionalized Graphene Nanosheet (AuNPs/GN). Before functionalization with gold nanoparticles, graphene oxide (GO) was initially synthesized according to the Hummers' method, as described in the Supporting Information. Following that, gold nanoparticles-functionalized graphene nanosheet (AuNPs/GN) was prepared referring to the literature with minor modification.27 125 mg of the as-synthesized GO was put into 500 mL of ultrapure water containing hydrazine solution (350 µL, 25 wt % in water) and ammonia solution (2.0 mL, 25 wt % in water). After being stirred for 5 min under vigorous stirring, 10 mL of isooctane was added to construct the organic layer on the top of aqueous solution. The solution was then heated to 95 °C in an oil bath for 60 min. Subsequently, the resulting graphene colloids kept

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standing for two weeks to remove excess hydrazine. To synthesize the AuNPs/GN, HAuCl4 aqueous solution (100 µL, 50 mM) was added into 10 mL of the above-prepared graphene colloids (0.25 mg /mL). After reaction for 2 h, the obtained nanocomposites were collected by centrifugation and washed several times with ultrapure water. The final sediments (AuNPs/GN) were dried by using the freeze-drying method.

Fabrication of PSA Aptasensing Platform. The fabrication procedure of PSA aptasensing platform is schematically illustrated in Scheme 1. Prior to modification, the fluorine-doped tin oxide (FTO) electrode was sequentially cleaned with 1.0 M NaOH, H2O2 (10%), acetone and ultrapure water, and then sonicated in ethanol and water, in turn, finally dried under an N2 flow. Following that, CdS QDs/TiO2 suspension (20 µL, 1.0 mg/mL, dispersed in ultrapure water) was dropped onto the pretreated FTO electrode with a fixed area of 0.25 cm2. Afterwards, 10 µL of 2.0 µM capture DNA was mixed with 2 µL of 1.0 mM TCEP in a 200-µL PCR tube to incubate for 90 min at RT (note: The aim to reduce the disulfide bond). After that, 10 µL of the reduced capture DNA solution was dropped directly onto electrode for 2 h in the dark to form a self-assembled monolayer via the Cd-S bond. After being washed with pH 7.4 PBS, 10 µL of 10 mM MCH solution was incubated on the electrode at RT for 1 h to block the unmodified site, followed by drying with the help of nitrogen. Subsequently, the hybridization reaction was carried out by dropping PSA aptamer-labeled AuNPs/GN suspension (1.0 mg/mL) at 37 °C for 90 min for the formation of DNA duplex between capture DNA and PSA aptamer (note: PSA aptamer-conjugated AuNPs/GN was prepared by mixing 100 µL of 200 nM PSA aptamer with 900 µL of 1.0 mg/mL AuNPs/GN homogeneous solution). Finally, the as-prepared aptasensor was stored at 4 °C when not in use. After each step, the resulting electrode was washed carefully with 0.1 M PBS (pH 7.4).

Photoelectrochemical Measurement toward Target PSA. To carry out the PEC measurement, the as-prepared aptasensor was incubated with 10 µL target PSA with different concentrations at 37 °C for 90 min, followed by rinsing with 0.1 M PBS, pH 7.4. Afterward, the modified electrode was inserted in 5-mL PBS (0.1 M, pH 7.4) containing 1.0 mM AA as the sacrificial electron donor at RT. Utilizing a 500-W Xe lamp (NBET, Beijing, China) to produce white light with a spectral range of 200−2500 nm as the excitation light, the photocurrent was measured on AutoLab electrochemical workstation (Eco Chemie, Netherlands) at an applied potential 0 V with the light switched on and off at the interval of 10 s. 6


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■ RESULTS AND DISCUSSION Characterization of CdS QDs/TiO2. Figure 1A gives the typical transmission electron microscope (TEM; H-7650, Hitachi Instruments, Tokyo, Japan) image of the as-synthesized CdS QDs/TiO2. The average size was about 20 nm. As seen from high-resolution transmission electron microscope (HRTEM) image of the nanocrystals in Figure S1 in the Supporting Information, their lattice and shape could be obviously observed, and the D(101) values were 0.316 nm and 0.352 nm for CdS and TiO2, respectively. Logically, a puzzling question arises to whether the nanocrystals consisted of CdS QDs and TiO2. To verify this issue, the microscopic structures could be initially confirmed using X-ray photoelectron spectroscope (XPS; Thermo Fisher Scientific, Model Escalab 250 spectrometer). As shown in Figure 1B and Figure S2, the characteristic spin-orbit splits of Ti2p3/2 (Figure S2-A), O1s (Figure S2-B), Cd3d5/2 and Cd3d3/2 peaks (Figure S2-C), and S2p signal (Figure 1B, inset) could clearly appear,23 suggesting the existence of four elements including Ti, Cd, S and O in the hybrid nanostructures. Meanwhile, the wide-angle X-ray diffraction pattern (XRD; PANalytical X'Pet spectrometer) was also employed to investigate the crystalline phase of CdS QDs/TiO2 (Figure 1C). As shown in Figure 1C, the diffraction peaks of the sample were at 2θ of 25.3°, 37.8°, 48.0°, 54.0°, 55.1°, 62.6° and 75.3°, corresponded to anatase-TiO2 (JCPDF 21-1272). Additionally, two diffraction peaks at 2θ of 28.3° and 48.5° which could be assigned to (101) and (110) crystal planes of hexagonal CdS phase (JCPDF 41-1049), respectively. Due to the ultrasmall size of the CdS QDs, these two peaks were rather weak, and the other diffraction peaks of CdS were not clearly exhibited because of the overlap of diffraction peaks came from the anatase TiO2. For comparison, XRD pattern of CdO/TiO2 was also investigated, as shown in Figure S3. No diffraction peaks attributed to CdO phase could be found, perhaps as a result of its ultrasmall size and favorable dispersity in TiO2. Nevertheless, the particle diameter of anatase for CdS QDs/TiO2 was calculated to be 20.1 nm, utilizing the fwhm of the TiO2 (101) peak on the basis of Scherrer’s equation,29 which was in accordance with the result of TEM.

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Figure 1. (A) TEM image, (B) XPS spectra (inset: high-resolution XPS spectra of S element), (C) XRD pattern data, and (D) N2 adsorption-desorption isotherm (inset: the corresponding pore size distribution curve) of CdS QDs/TiO2.

As mentioned above, mesoporous TiO2 nanocrystals were used for preparation of CdS QDs/TiO2. Maybe, another question to be produced was whether the hybrid nanostructures were mesoporous. In this regard, type IV adsorption-desorption isotherms and H1 hysteresis loops of the nanocrystals were investigated, and N2 adsorption–desorption isotherm of nanostructures exhibited an isotherm of type IV isotherm (Figure 1D), corresponding to a characteristic of mesoporous materials. The pore size distribution of desorption branch is depicted in the inset of Figure 1D. It clearly shows a bimodal pore-size distribution for porous nanomaterials. The surface area of CdS QDs/TiO2 was calculated to be 112 m 2/g by using the Brunauer-Emmett-Teller (BET) model and the average pore size of CdS QDs/TiO2 was 4.9 nm with narrow distribution. Therefore, we might make a conclusion that two components were distributed in the heterogeneous junction owing to the intimate contraction with each other in the framework. Such a junction can improve the interelectron transfer between two components and lead to the favorable photocatalytic activity.22

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Characterization of AuNPs/GN and Feasibility Investigation. In this work, the energy transfer is implemented through the exciton-plasmon interaction of the assembled gold nanoparticles on the graphene nanosheets with the immobilized CdS QDs/TiO2 on the electrode under irradiation. As control test, the morphology of graphene nanosheets was characterized by using TEM. As shown in Figure S4, the synthesized graphene oxide was planar sheet-like, indicating that the nanostructures were neither carbon nanotubes nor graphite powder. Significantly, we might observe that many nanoparticles were coated on the nanosheets after formation of AuNPs/GN (Figure 2A). Moreover, most gold nanoparticles (~2.0 nm in diameter) could be dispersed homogeneously on the nanosheets (Figure 2A, inset). Owing to the planning effect, the AuNPs/GN could efficiently connect with the aptamer to produce relatively stable energy transfer and quench the photocurrent.30

Figure 2. (A) TEM image of AuNPs/GN (inset: HRTEM); (B) Raman spectra of AuNPs/GN prepared by using 10 mL of graphene nanosheets (0.25 mg/mL) and 50-mM HAuCl4 with different volumes (GN: 0 uL; AuNPs/GN-1: 50 uL; AuNPs/GN-2: 100 uL; AuNPs/GN-3: 200 uL); (C) UV-vis diffuse reflectance spectroscope (a) and PLS spectrum (b) of CdS QDs/TiO2 (insets: UV-vis absorption spectrum of AuNPs/GN); (D) Photocurrents of the aptasensors prepared by using different quenchers: (a) AuNPs/GN, (b) AuNPs/CNT and (c) AuNP in the absence of target PSA.

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To verify the formation of AuNPs/GN, the as-prepared nanostructures by using different HAuCl4 amounts were characterized on Raman spectrum (Figure 2B) and UV-vis absorption spectroscopy (Figure S5). As seen from Figure 2B, the peaks at D and G bands got a shift in a tiny breadth, and the ratios of ID/IG were 1.25, 1.15, 1.12, and 1.07, respectively. The reason was ascribed to the formation of different-amount AuNPs on the graphene. Compared with graphene oxide alone (Figure 2B, curve 'a'), the Raman intensities of AuNPs/GN at G, D, 2D and D+D' bands increased with the increasing HAuCl4 during the preparation process (Figure 2B, curves 'b-d'), which was attributed to plasmonic effect of AuNPs. Moreover, introduction of AuNPs did not almost change the in-plane sp2 domains of graphene in the size.31 The results were in agreement with those obtained by UV-vis absorption spectroscopy (Figure S5). No absorption peak was obtained at pure graphene nanosheets (curve 'a'). With the increasing HAuCl4, the characteristic peak for gold nanoparticles obviously appeared at ~540 nm (curves 'b-d'). These results revealed that AuNPs/GN could be successfully synthesized by our design. Considering the PEC application of the newly prepared AuNPs/GN, one important factor on this issue lies in the fact that the excitation of EPI should have a sufficient spectral overlap between the emission spectra of exciton band and absorption spectra of plasmon band.12,32 To investigate the interaction between CdS QDs/TiO2 and AuNPs/GN under light radiation, the photoluminescence spectrum (PLS) and UV-visible diffuse reflectance spectroscope (DRS) were used to characterize the photoelectronic states of the CdS QDs/TiO2 (Figure 2C). As seen from curve 'b', PLS emission peak of CdS QDs/TiO2 mainly centered at ~495 nm under illumination at 410 nm within the broad absorption range, which was suitable for employment as photoactive substrate. Moreover, UV-vis absorption spectrum of AuNPs/GN exhibited a maximum peak at 544 nm (Figure 2C, inset), which was beneficial to the subsequent generation of EPI in the system owing to the large spectral overlap between absorption spectra of the AuNPs/GN and PL emission of the CdS QDs/TiO2. Next, the as-prepared CdS QDs/TiO2 and AuNPs/GN were utilized for fabrication of aptasensor. To investigate the feasibility of the aptasensing platform, the photoelectrochemical properties after each step were monitored by using electrochemical impedance spectroscopy (EIS) and photocurrent measurement (Figure S6 in the Supporting Information). As shown in Figure S6, the aptasensing platform could preliminarily utilized for the detection of PSA based on target-induced displacement reaction between aptamer-assembled CdS QDs/TiO2 and AuNPs/GN. As described above, the 10


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change in the photocurrent derived from exciton-plasmon interaction of CdS QDs with the coated AuNPs on graphene nanosheets. Usage of graphene nanosheets aimed to enhance the sensitivity of PEC sensing platform. When one aptamer on the AuNPs/GN reacted with the immobilized C-DNA on the electrode, all the AuNPs on the same graphene nanosheet were carried over, thus resulting in the heavy decreasing of CdS QDs in the photocurrent. Vice versus, the photocurrent increased in the competitive-type assay formation upon addition of target PSA during the subsequent assay. Another important issue of using graphene nanosheet was dependent on its large surface coverage. To clarify the aforementioned concerns, three types of the aptasensors were prepared using different quenched substrates including AuNPs/GN, gold nanoparticle (AuNP, 16 nm in diameter) and AuNPs-coated carbon nanotube (AuNPs/CNT), respectively (note: Gold nanoparticles and AuNPs-coated carbon nanotubes were prepared and described in our previous work33). The quenched effect was studied between AuNPs and CdS QDs in the absence of target PSA. As shown in Figure 2D, utilization of AuNPs/GN could exhibit lower photocurrents than those of AuNPs and AuNPs/CNT. Such a low photocurrent could be restored at most upon target PSA introduction in this system. The results also revealed that exciton-plasmon interaction between CdS QDs and AuNPs/GN could be used for in-situ amplified photocurrent response of our designed strategy toward target PSA.

Figure 3. The effects of (A) concentration of PSA aptamer, (B) incubation time with target PSA and (C) pH of detection solution on the analytical performance of the aptasensing platform.

Optimization of Experimental Conditions. Certainly, some external experimental conditions for design of the aptasensing platform should be studied in order to acquire an optimum detectable signal. Firstly, we investigated the effect of the quencher component (i.e., capture DNA-modified AuNPs/GN) on the photocurrent of the aptasensing system. As indicated from Figure S7 in the Supporting Information, 24-nt capture DNA-functionalized AuNPs/GN-2 (i.e., by adding 100 µL of

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HAuCl4 into 10 mL of 0.25 mg/mL GN solution) exhibited the change of an optimum photocurrent. Thus, 24-nt capture DNA-functionalized AuNPs/GN-2 were used for preparation of the aptasensing platform in this work. Certainly, the amount of the sandwiched aptamer between CdS QDs/TiO2 and AuNPs/GN directly affected the analytical properties of the aptasensor. As seen in Figure 3A, the photocurrent decreased with the increasing PSA aptamer, and reached a maximum quenching effect at up to 200 nmol/L. To reduce the cost, 200 nmol/L of aptamer was used for the preparation of PEC aptasensor. At these conditions, this aptasensor was employed for the detection of target PSA. As seen from Figure 3B, the photocurrent increased with the increasing incubation time, and tended to reach a plateau at 90 min. A longer incubation time did not cause significant increase in the photocurrent. To save the assay time, 90 min was used for the reaction between target PSA and the aptasensor. Figure 3C gives the relationship between pH of detection solution and photocurrent. A lowest photocurrent was achieved at pH 7.4 PBS. A higher or lower pH value would affect the quenching effect of PEC aptasensor.34 So, pH 7.4 PBS (0.1 M) was selected as the supporting electrolyte for PEC measurement. Dose Response of PEC Aptasensing Platform toward Target PSA. By using the experimental conditions optimized above, displacement reaction-based PEC aptasensor was employed for the quantitative detection of target PSA standards with different concentrations. The photocurrent was recorded in PBS (pH 7.4, 0.1 M) containing AA (1.0 mM) at an applied potential of 0 V. Figure 4A gives the photocurrent curves of the aptasensors after incubation with target PSA standards for 90 min at room temperature, and the photocurrents increased gradually with the increment of target PSA concentrations, which was attributed to the detachment of AuNPs/GN quencher induced by the distortion of the aptamer-protein binding. A good linear relationship between the photocurrent and PSA concentration could be fitted within the dynamic range from 0.001 ng/mL to 8.0 ng/mL (Figure 4B). The regression equation was ∆I = 0.2022 × CPSA + 0.02828 (ng/mL) with a correlation coefficient of 0.9969 (r). Furthermore, the limitation of detection was found to be 0.52 pg/mL at the signal-to-noise ratio of 3, which was lower than those of commercial human PSA ELISA kits [e.g., 10 pg/mL for Sigma-Aldrich (Cat#: RAB0331); 9.4 pg/mL for Abcom (Cat#: ab188388); 69 ng/mL for R&D systems (Cat#: DKK300)].

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Figure 4. (A) Photocurrents of PEC aptasensor toward different-concentration PSA standards (0, 0.001, 0.005, 0.01, 0.05, 0.1, 0.05, 1, 2, 4, 8, 9 ng/mL from 'a' to 'l'), (B) the corresponding calibration plots (∆I = I − I0, I0 and I standard for the photocurrent of the aptasensor before and after incubation with target PSA, respectively) (inset: the magnified responses from 0.001 to 0.1 ng/mL for PSA determination), (C) selectivity of the PEC aptasensor toward PSA (0.5 ng/mL) against CEA (5 ng/mL), CA 15-3 (5 U/mL), CA 125(5 U/ml), CA 15-9(5 U/mL), IgG (5 ng/mL) and AFP (5 ng/mL) (note: The mixture contained the above-mentioned analytes), and (D) stability of the PEC aptasensor under repeated light irradiation (4.0 ng/mL PSA used as an example) in the (a) absence and (b) presence of target PSA. All the photocurrents were measured in pH 7.4 PBS (0.1 M) containing AA (1.0 mM) at applied potential of 0 V, whilst these data were obtained by using 24-nt capture DNA-functionalized AuNPs/GN-2 (i.e., by adding 100 µL of HAuCl4 into 10 mL of 0.25 mg/mL GN solution).

Selectivity, Stability and Repeatability. The selectivity and specificity of displacement reaction -based PEC aptasensor were tested toward some non-target analytes, e.g., carcinoembryonic antigen (CEA), alpha-fetoprotein (AFP), cancer antigen 125 (CA 125), immunoglobulin G (IgG), cancer antigen 15-3 (CA 15-3), and cancer antigen 15-9 (CA 15-9). The evaluation was carried out by comparison of the photocurrents relative to the background signal. As indicated in Figure 4C, all the

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non-targets including CEA, IgG, AFP, CA 125, CA 15-9 and CA 15-3 exhibited low photocurrents, which were close to the background signal (i.e., blank sample). More importantly, introduction of non-targets with target PSA did not cause the signal change in the photocurrent relative to target PSA alone. Thus, displacement reaction-based PEC aptasensor could be used for specific detection of target PSA. For the photoactive material-based PEC sensing system, the detectable signal is usually affected through continuous light irradiation during photoelectrochemical measurement. To achieve a good analytical performance, the as-prepared photoactive materials should possess good reproducibility and stability. In this case, the photocurrents of the newly prepared CdS QDs/TiO2-modified FTO electrode were continuously monitored within 12 runs by regularly controlling the switch 'on-off' of light irradiation in the absence and presence of target PSA. As displayed in Figure 4D, the baseline of the aptasensor among these 12 runs were very stable, whereas the photocurrent responses were almost the same. The relative standard deviation (RSD) values toward the photocurrent responses at the 'on' state were 1.7% and 2.4% (n = 12) in the absence and presence of target PSA, respectively. Therefore, displacement reaction-based PEC aptasensor exhibited good stability and reproducibility, and could be repeatedly utilized for the photocurrent measurement. Except for repeated measurement toward the photoactive materials, the stability of the prepared aptasensor during the storage process was also very important. Generally speaking, the immobilized biomolecules with the bioactivity (e.g., capture DNA and aptamer) have certain lifetime. To verify this issue, the newly prepared aptasensors with the same batch were stored at 4 °C when not in use, and measured intermittently (every 3-5 days) for detection of 1.0 ng/mL PSA (used as an example) by using the same assay mode. Experimental results indicated that the photocurrents could maintain 98.1%, 97.2% and 90.2% (n = 3) of the initial response at 1st, 3rd and 6th weak, respectively. Hence, the stability of PEC aptasensor was acceptable. To further investigate the repeatability test, the as-prepared aptasensors with the same batch or different batches were used for quantitative detection of various-concentration target PSA standards. The repeatability was evaluated by calculating the coefficient of variation (CVs) of the intra-assay or inter-assay. Results showed that the CVs of using the same-batch aptasensors were 2.9%, 5.8%, 4.2%, 4.8%, 3.2% and 5.5% (n = 6) at 0.005 ng/mL, 0.01 ng/mL, 0.05 ng/mL, 0.5 ng/mL, 1.0 ng/mL and 4 ng/mL PSA, respectively. In contrast, the CVs of using different-batch aptasensors 14


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were 3.8%, 7.6%, 5.2%, 6.5%, 6.3% and 7.0% (n = 6) for the aforementioned PSA standards, respectively. These results revealed that our designed aptasensors could be used repeatedly, thereby further demonstrated the possibility of the batch-to-batch preparation. Analysis of Human Serum Sample. To meet the need of practical application in future toward a newly developed detection protocol, the accuracy of displacement reaction-based PEC aptasensing platform was investigated, relative to commercial human PSA ELISA kit (Sigma-Aldrich), for the detection of clinical serum specimens. 8 human serum samples containing different-concentration target PSA, which were collected from local hospital of our University, were directly determined by using our developed PEC aptasensors and human PSA ELISA kit (note: The assays by using human PSA ELISA kit toward these samples were finished in the hospital). The results are shown in Table 1 (sample nos: 1-8). Significantly, all texp data toward human serum specimens were < 2.77 (tcrit[0.05,4] = 2.77) on the basis of t-test's results between two methods. To further monitor the matrix effect of the aptasensors as compared to those of using standard PSA solutions in Figure 4, 50 ng/mL PSA standard were directly diluted to 5-fold, 10-fold, 20-fold, 50-fold, 100-fold and 200-fold with blank PSA new-born calf serum. These diluted samples were measured by using PEC aptasensor and PSA ELISA kit, respectively. As seen from sample nos 9-14 in Table 1, all the assayed levels per sample were close to the added concentrations. Meanwhile, the texp values between two methods were also below 2.77. Furthermore, the relative standard deviations toward these 14 samples were less than 13%. These results revealed that displacement reaction-based PEC aptasensing platform could be utilized for determination of target PSA in complex biological system by using exciton-plasmon interaction between CdS QD and AuNP.

Table 1. Comparison of the results obtained on PEC aptasensor modified with 24-nt capture DNA-functionalized AuNPs/GN-2 and human PSA ELISA kit for 8 human serum specimens containing target PSA and 6 diluted PSA standards by using blank new-born calf serum a Method; concentration [mean ± SD (RSD), ng/mL, n = 3]c Matrix

Sample no

PEC aptasensor

PSA ELISA kit

texp

1

1.92 ± 0.13 (6.77%)

2.10 ± 0.11 (5.24%)

1.83

2

35.21 ± 2.82 (8.01%)

33.72 ± 2.32 (6.88%)

0.71

3

0.95 ± 0.02 (2.11%)

1.01 ± 0.04 (3.96%)

2.32

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Human serum

4

13.01 ± 1.07 (8.22%)

12.35 ± 1.18 (9.55%)

0.71

specimens

5

6.23 ± 0.58 (9.31%)

6.42 ± 0.81 (12.61%)

0.29

6

0.094 ± 0.004 (4.26%)

0.100 ± 0.006 (6.00%)

1.44

7

4.61 ± 0.56 (12.15%)

5.06 ± 0.61 (12.06%)

0.94

8

22.09 ± 1.78 (8.06%)

23.78 ± 1.90 (7.99%)

1.12

9

10.21 ± 1.01 (9.89%)

10.11 ± 0.94 (9.30%)

0.13

PSA standard

10

4.89 ± 0.43 (8.79%)

5.03 ± 0.47 (9.34%)

0.46

diluted with

11

2.31 ± 0.24 (10.39%)

2.46 ± 0.21 (8.54%)

0.99

new-born calf

12

1.08 ± 0.11 (10.19%)

0.98 ± 0.07 (7.14%)

1.54

serum b

13

0.48 ± 0.04 (8.33%)

0.53 ± 0.03 (5.66%)

2.14

14

0.24 ± 0.02 (8.33%)

0.26 ± 0.01 (3.85%)

1.73

a

Use of new-born calf serum for the dilution of PSA standards aimed to avoid the interfering of possible PSA in the human

serum on the aptasensor. b

Samples 9-14 were obtained by diluting 50 ng/mL PSA standard to 5-fold, 10-fold, 20-fold, 50-fold, 100-fold and 200-fold

with blank new-born calf serum. c

The high-concentration samples were calculated on the basis of the diluted ratio.

■ CONCLUSION

In this contribution, a new photoelectrochemical aptasensing platform based on the exciton-plasmon interaction was successfully demonstrated for sensitive and specific detection of prostate-specific antigen by coupling with target-induced competitive-displacement reaction. The photocurrent was quantitatively evaluated via exciton-plasmon interaction between CdS QDs/TiO2 and AuNPs/GN. Use of mesoporous TiO2 nanostructures was favorable for the distribution and efficient contact of CdS QDs on the two-phase nanomaterials, thus enhancing the charge transfer and the electron-hole separation in the interface. Compared with traditional PEC immunoassays, highlights of the present work rely on the following issues: (i) the displacement reaction-based PEC aptasensor is highly efficient and sensitive for the detection of low-abundance proteins/biomarkers; (ii) our system can avoid the participation of natural enzymes by using the exciton-plasmon interaction between CdS QDs and AuNPs; and (iii) the competitive-displacement mode can efficiently decrease the labeling process of the biomolecules. Impressively, our strategy opens a new opportunity for detection of other disease-related biomarkers by changing the sequence of the corresponding aptamer.

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■ ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.acssensors.0000. Reagent and Chemical, Synthesis of graphene oxide (GO), HRTEM image of CdS QDs/TiO2 (Figure S1), XPS spectra (Figure S2); XRD patterns (Figure S3), TEM image of graphene oxide (Figure S4), UV-vis absorption spectroscopy and photographs (Figure S5), Characterization of the aptasensing platform by using EIS and PEC measurement (Figure S6), and Component optimization of C-DNA-coated AuNPs/GN (Figure S7) (PDF)

■ AUTHOR INFORMATION Corresponding Author * Phone: +86-591-2286 6125. Fax: +86-591-2286 6135. E-mail: [email protected] (D. Tang)

Notes The authors declare no competing financial interest.

■ ACKNOWLEDGEMENTS We sincerely acknowledge financial supports from the National Natural Science Foundation of China (Grant nos: 21675029 and 21475025), and the Program for Changjiang Scholars and Innovative Research Team in University (Grant no.: IRT15R11). ■ REFERENCES (1) Ren, B.; Robert, F.; Wyrick, J.; Aparicio, O.; Jennings, E.; Simon, I.; Zeitlinger, J.; Schreiber, J.; Hannett, N.; Kanin, E.; Volkert, T.; Wilson, C.; Bell, S.; Young, R. Genome-wide location and function of DNA binding proteins. Science 2000, 290, 2306-2309. (2) Bonham, A.; Braun, G.; Pavel, I.; Moskovits, M.; Reich, N. Detection of sequence-specific protein-DNA interactions via surface enhanced resonance Raman scattering. J. Am. Chem. Soc. 2007, 129, 14572-14573. (3) Kim, S.; Lee, H.J. Gold nanostar enhanced surface plasmon resonance detection of an antibiotic at attomolar concentrations via an aptamer-antibody sandwich assay. Anal. Chem. 2017, 89, 6624-6630. (4) Zhao, W.; Xu, J.; Chen, H. Photoelectrochemical bioanalysis: the state of the art. Chem. Soc. Rev. 2015, 44, 729-741. (5) Zhou, Q.; Lin, Y.; Wei, Q.; Chen, G.; Tang, D. Highly sensitive electrochemical sensing platform for lead ion based on synergetic catalysis of DNAzyme and Au-Pd porous bimetallic nanostructures. Biosens. Bioelectron. 2016, 78, 236-243. (6) Yu, Y.; Zhang, H.; Chai, Y.; Yuan, R.; Zhuo, Y. A sensitive electrochemiluminescent aptasensor based on perylene derivatives as a novel co-reaction accelerator for signal amplification. Biosens. Bioelectron. 2016, 85, 8-15. (7) Jiang, Y.; Shi, M.; Liu, Y.; Wan, S.; Cui, C.; Zhang, L.; Tan, W. Aptamer/AuNP biosensor for colorimetric profiling of exosomal proteins. Angew. Chem., Int. Ed. 2017, 56, 11916-11920.

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