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A sandwich-type photoelectrochemical sensor based on tremella-like graphdiyne as photoelectrochemical platform and graphdiyne oxide nanosheets as signal inhibitor
Sensors & Actuators: B. Chemical xxx (xxxx) xxxx
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
A sandwich-type photoelectrochemical sensor based on tremella-like graphdiyne as photoelectrochemical platform and graphdiyne oxide nanosheets as signal inhibitor Hao Wanga, Keqin Denga,*, Jing Xiaob, Chunxiang Lib, Shaowei Zhanga, Xiaofang Lia,* a
Key Laboratory of Theoretical Organic Chemistry and Function Molecule, Ministry of Education, Hunan University of Science and Technology, Xiangtan 411201, China Hunan Provincial Key Laboratory of Controllable Preparation and Functional Application of Fine Polymers, Hunan Provincial Key Laboratory of Advanced Materials for New Energy Storage and Conversion, School of Chemistry and Chemical Engineering, Hunan University of Science and Technology, Xiangtan 411201, China
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A R T I C LE I N FO
A B S T R A C T
Keywords: Graphdiyne Graphdiyne oxide Photoelectrochemical detection cadmium sulfide quantum dots microRNA-21
Graphdiyne (GDY) was the first reported graphynes to be prepared practically. Owing to its low dispersibility, only very limited GDY-based hybrids were synthesized. Furthermore, no any high quality of GDY-based hybrid with photoelectrochemical performance was synthesized by simple hydrothermal process. Herein, we report acidified tremella-like GDY nanotubes. The dispersibility of GDY was highly improved. The composite of GDY and cadmium sulfide quantum dots (GDY-CdSQDs) was synthesized through one-step hydrothermal method. GDY-CdSQDs exhibited excellent photocurrent response under optical illumination. Small size of GDY oxide (GDYO) nanosheet was obtained through oxidation and size screening. A photocurrent signal inhibitor was thus prepared by covalently binding DNA probe 2 (P2) on GDYO nanosheet (P2-GDYO). With GDY-CdSQDs and DNA capturing probe 1 (P1) acting as photoelectrochemical sensing platform, the target object of microRNA-21 (miR21) was captured by P1 and hybridized with P2-GDYO. Because GDYO exhibited low conductivity, high loading capacity for insulative P2, and surface absorption to light irradiation, it caused a magnified photocurrent change. The changed photocurrent was used to quantitatively detect miR-21. This method avoided expensive bioreagents and labeled target/detection DNA or miRNAs. It was potential for other bioanalysis and clinic diagnosis.
1. Introduction Graphdiyne (GDY) is a kind of a flat material with sp and sp2 hybridized carbon atoms and high degrees of π conjugation [1,2]. GDY film possesses excellent semiconducting property with a naturally bandgap of 0.47 eV calculated by density functional theory [3,4]. Its unique atomic arrangement and electronic structure made it become highly efficient catalyst [5,6]. GDY-based electrodes also displayed excellent electrochemical performance and good conductivity [7]. Moreover, GDY was used as electron-transport material for photodegradation of methylene blue [8]. Its nanomaterials exhibited higher field emission performance than graphite and carbon nanotubes [9,10]. As is well known that graphene, a two-dimensional carbon nanomaterial, has been widely used to develop graphene-based hybrids [11–13]. These hybrids displayed superior properties and some special properties through the combination of graphene with other nanoscale materials. But as for GDY, only a few GDY-based hybrids were reported [6,14–16]. GDY − ZnO nanohybrids was used as an advanced
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photocatalytic material [14]. Cobalt-nitrogen-doped GDY was demonstrated to be as an efficient bifunctional catalyst [15]. Robust CeS bond integrated graphdiyne-MoS2 enhanced lithium storage capability [16]. GDY with 4-mercaptopyridine surface-functionalized CdSe quantum dots as sensitizer was beneficial to the hole transportation and enhancement of the photocurrent performance [7]. Motivated by the graphene-based hybrids, we hope to develop novel and eminent GDYbased nanocomposites. Photoelectrochemical (PEC) sensor has attracted substantial attention owing to its low signal background, inexpensive photoelectric devices, and high sensitivity [17,18]. Many semiconductor materials have been used to fabricate PEC sensors, which can respond to light irradiation. Among them, Cadmium sulfide quantum dots (CdSQDs) is one kind of special semiconductor nanoparticles. But their toxicity influences the activity of biomolecules immobilized on the surface of CdS [19]. In addition, the optical properties of CdSQDs varies with the surface modification groups [20]. Being a low-toxic and natural aminothiol, L-cysteine (Cys) has been used as modifier and stabilizer for
Corresponding author. E-mail addresses: [email protected] (K. Deng), [email protected] (X. Li).
https://doi.org/10.1016/j.snb.2019.127363 Received 21 August 2019; Received in revised form 25 October 2019; Accepted 29 October 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Hao Wang, et al., Sensors & Actuators: B. Chemical, https://doi.org/10.1016/j.snb.2019.127363
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Fig. 1. (A) XPS surveys of different GDY. (B) Corresponding Nyquist plots of electrochemical impedance spectroscopy. (C) FTIR spectra of GDY and GDYO. (D) UV–vis spectrum of GDYO.
mixed with concentrated HNO3 (1.0 mL) and concentrated H2SO4 (3.0 mL). The mixture was heated in an oil bath at 60 °C for 8 h. After being cooled, NaOH was carefully used to adjust pH till 6.0 in the ice bath. The resulting solution was dialyzed for three days to remove metal ion impurities. GDY oxide was named as GDYO. To obtain small size of GDYO, 0.22 um syringe filter was used to filtrate the GDYO solution. The filtrate was dried to give the GDYO powder.
preparation of CdSQDs [21,22]. To the best of our knowledge, only Li reported a GDY loaded with AuNPs for constructing PEC platform [23]. The low dispersibility of GDY in water limited the synthesis of high quality of nanocomposite with photoelectrochemical performance by simple hydrothermal process. In this study, acidification treatment was performed to improve the dispersibility of tremella-like GDY nanotubes. Small size of GDY oxide (GDYO) nanosheets were obtained by oxidation and filtration screening. GDY loaded with low-toxic Cys-coated CdSQDs nanocomposites (GDY-CdSQDs) was prepared through one-step hydrothermal method. The flower-like GDY had large specific surface and loaded large amount of CdSQDs. By applying a significant cancer marker miR-21 as model target, the nanocomposite was used as sensitive photocurrent signal platform. Furthermore, GDYO bonded DNA probe molecules effectively reduced the photocurrent signal by inhibiting electron conduction.
2.2. Synthesis of GDY-CdSQDs and P2-GDYO conjugate GDY2 (2.0 mg) was dispersed into 10 ml Cd(NO3)2 (10.0 mM) and thoroughly mixed with magnetic stirring for 2 h. Then, 18.0 mg of cysteine (Cys) was dissolved in the mixed solution and NaOH was used to adjust pH till 8.0. After Na2S (8.0 mM) was added into the solution, it was transferred into a Teflon-lined stainless-steel autoclave and heated at 80 °C for 12 h. The resulted product was thoroughly washed with ethanol and centrifuged repeatedly for three times. It was denoted as GDY2-CdSQDs. For comparison, GDYO-CdSQDs, GDY1-CdSQDs, and GDY3-CdSQDs were prepared using the similar procedure. For the synthesis of P2-GDYO nanoprobe, GDYO (0.5 mg mL−1) were mixed with EDC (10 mM) and NHS (20 mM). After 15 min, P2 (4.0 μM) was added into the activated GDYO solution. The mixed solution was kept reaction for another 3 h and then dialyzed for three days.
2. Experimental 2.1. Acidification of GDY and synthesis of GDYO nanosheets The tremella-like graphdiyne (GDY) was donated by Prof. Yuliang Li (Institute of Chemistry, Chinese Academy of Sciences, Beijing, China). The preparation process was shown in previous work [24]. GDY was treated under different condition. In brief, GDY was carefully mixed with concentrated H2SO4.The mixture was stirred vigorously for 24 h in an oil bath at 60 °C. The suspension was centrifuged at 12,000 rpm for 10 min and washed till neutral. The resulted GDY was denoted as GDY1. Other two substitutive methods were refluxing GDY in 1:1 or 3:1 concentrated acid of H2SO4/HNO3 for 6 h, respectively. The acidified GDY was named as GDY2 and GDY3. GDYO was prepared by the following procedure [25]. GDY (10.0 mg) and KMnO4 (10.0 mg) were carefully
2.3. Fabrication of sensor 6 μL of GDY2-CdSQDs (1 mg mL−1) in 1% acetic acid solution containing 0.3% CS was decorated on cleaned ITO and dried at 60 °C. Then, 10 μL of 2.0% GLD was covered onto ITO surface and remained for 1 h at room temperature. The exposed geometric area of ITO glass to optical radiation was controlled at 0.25 cm2 by treating with black insulating waterproof tape. After washing away the redundant GLD, 2
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about 15 ∼30 nm (Fig. 2E, 2 F). It was worth mentioning that the monolayer GDY was obtained by ultrasonic treatment of raw GDY dispersed in DMF (spectral purity). It showed an average thickness of about 0.8 nm (Fig. S3A, B).
10 µL of probe 1 (P1) was dropped on ITO for another 6 h. BSA (1 mg mL−1) was then used to block nonspecific sites for 30 min. The resulted P1/GDY2-CdSQDs/ITO was used for hybridization. Hybridization was performed by dropping 8 μL of 1:1 (V:V) mixture of miR-21 and P2-GDYO on the surface of P1/GDY2-CdSQDs/ITO and incubating in a humidor at 45 °C for 1 h. The resulted electrode was rinsed for photocurrent measurement. It was named as P2-GDYO/RNA/ P1/GDY2-CdSQDs/ITO.
3.2. Characterization of GDY-CdSQDs and GDYO-CdSQDs Cadmium sulfide as photoelectrochemical material has achieved many applications due to good properties [31,32]. Many nanocomposites hybridized with CdS showed distinctive functions [19,33,34]. To obtain excellent GDY-CdSQDs composite, the photoelectrochemical performance of GDY1-CdSQDs, GDY2-CdSQDs, GDY3-CdSQDs, GDYOCdSQDs, and CdSQDs was tested and compared (Fig. 3A). All GDYCdSQDs composites exhibited higher photocurrent than CdSQDs, showing the improved photoelectrochemical performance. GDY2CdSQDs displayed higher photocurrent. We deduced that black GDY1 had stronger light adsorption (Fig. 3B), which might partially reduce photon-induced electrons yield from CdSQDs because of the “shielding effect” [35]. Furthermore, GDY3 and GDYO had lower conductivity and higher resistance (Fig. 1B), which hindered the photon-induced electrons transfer. Therefore, GDY2-CdSQDs was chosen as the optimal photoelectrochemical platform. Fig. 3B exhibited the UV–vis diffuse reflectance spectra. GDY1 and GDY2 showed excellent light absorption. By contrast with GDY1, GDY2 displayed a slightly reduced absorption in the visible light region. As for the CdSQDs and GDY2-CdSQDs, GDY2-CdSQDs showed a better absorption in the range of > 365 nm. The bandgap energy of CdSQDs and GDY2-CdSQDs was estimated by the relationship of (αhν)1/2 versus photon energy (Fig. 3D). Their energy gap was 2.85 and 2.72 eV, respectively. It demonstrated that the hybridization of CdSQDs with GDY enhanced light absorption and expanded light absorbing range. As shown the XRD patterns in Fig. 3D, GDY2 showed a peak at 2θ = 22.7°. GDY2-CdSQDs exhibited the characteristic diffraction peaks at 28.5°, 45.4°, 54.1°, matching well with that of cubic CdS phase (JCPDS No. 42-1411). A low characteristic peaks of GDY2 appeared on GDY2-CdSQDs, suggesting the relatively low diffraction intensity of GDY2. The morphology of GDY2-CdSQDs was depicted by TEM image (Fig. 3E). Well-dispersed CdSQDs were attached onto the surface of GDY sheets. Fig. S4A showed the CdSQDs with average diameter of ∼8.7 nm. By compared with Fig. S4B, pure CdSQDs has the size only ∼4.2 nm. It manifested that GDY was favorable for the growth of large size of CdSQDs. Furthermore, the SEM image of GDY2-CdSQDs also indicated that a mass of CdSQDs were uniform and compact on the surface of GDY flakes (Fig. 3F). Compared with Fig. 2A, the morphology of GDY didn’t change after loading CdSQDs. The TEM image of GDY2CdSQDs heated for 4 h in autoclave showed very sparse CdSQDs on GDY sheets (Fig. S5).
3. Results and discussion 3.1. Characterization of GDY and GDYO Surface properties of carbon nanomaterials, such as dispersibility, electrocatalysis, heterogeneous electron transfer, and chemical adsorption, were greatly influenced by the oxygen-containing functional groups [26,27]. The low dispersibility of GDY affected its wide application. In order to obtain well-dispersed GDY, different acidification methods were adopted. X-ray photoelectron spectroscopy (XPS) analysis was applied to analyze the change of surface O/C atom ratio occurred during the acidification of GDY. As shown in Fig. 1A, the O/C ratio of GDY, GDY1, GDY2, GDY3, and GDYO was 16.72%, 23.37%, 29.14%, 40.19%, and 68.43%, respectively. Obviously, the O/C atom ratio became higher and higher, indicating that the acidification treatment virtually led to the generation of oxygen-containing species [26], which was helpful for the improved dispersibility of GDY. The color change of GDY1, GDY2, GDY3, GDYO was also visible from black, to dark brown, and bright brown (Fig. S1). Precipitate was found in GDY1, GDY2, and GDY3 aqueous dispersion after placing about 2 h, 5 h, and 8 h, respectively. GDYO was completely soluble in water. These acidified GDY was individually immobilized on GCE by 0.2% Nafion for electrochemical impedance spectroscopy (EIS) measurement. As shown in Fig. 1B, the Ret semicircle portions displayed a gradual reinforcement from GDY1, GDY2, GDY3, to GDYO. The resistance (Ret) were 138, 196, 475, and 994 Ω, respectively. GDY1 and GDY2 had the lower resistance and higher conductivity. GDYO showed a higher Ret value. It indicated the gradually weakened conductivity and increased hindrance for the access of the redox probe to the electrode surface because of the increasing acidification degree from GDY1 to GDYO. Fig. 1C showed FT-IR spectra of GDY and GDYO. The bands of GDY at around 1633, 1450, and 1091 cm−1 were the stretching motions of C]C (ν(C]C)) as well as some oxygen-containing functional groups [28,29]. The spectrum of GDYO displayed an improved band appeared at 1623 cm−1, which was ascribed to the stretching motion of carbonyl groups (ν(C]O)) [28]. A greatly increased the stretching vibration band of C–O (ν(C–O)) and a new bending vibration band of OeH (β(OeH)) appeared at around 1125 and 619 cm−1, respectively [30]. Besides, the bands at 1048 and 880 cm−1 corresponding to C]C structure lowered dramatically. These results suggested that the acidification turned C]C groups into the oxygen-containing groups, especially including a large amount of eCOOH group, which was helpful for high loading of P2 probe. UV–vis spectrum of GDYO showed a strong absorption at 230 nm (Fig. 1D), which was ascribed to theπ→π* transition of aromatic C]C bond. A weak shoulder peak at about 360 nm was attributed to n→π* transition of C]O bond. Furthermore, GDYO exhibited visible light absorption within < 500 nm. Fig. 2A presented the morphology of raw GDY. The GDY was flowerlike nanotubes with diameter of about 200∼600 nm, which composed of numerous transparent ultrathin nanosheets (Fig. 2B). The morphology provided a very high surface area for loading other nanoparticles. After acidification, GDY1, GDY2, and GDY3 kept the original morphology feature as the raw GDY. But, highly oxidized GDYO changed the feature and showed well-dispersed nanosheets (Fig. S2). The GDYO were not of uniform size from about 50 nm to 5 um (Fig. 2C). After filtration screening, GDYO with the size < 400 nm was obtained (Fig. 2D). The AFM image showed that the thickness of GDYO was
3.3. Characterization of the construction process of PEC sensor The fabrication process of PEC sensor was proved by the electrochemical impedance spectroscopy (EIS) and photocurrent measurement. Fig. 4A exhibited the impedance spectra of different electrodes. Compared with GDY2-CdSQDs/GCE, after P1 immobilizing, the Ret increased because of the insulativity of biomolecules, suggesting capturing probe P1 was immobilized on the ITO surface. Then, after stepwise BSA blocking and hybridizing with miR-21 and P2-GDYO, furtherly increase of interfacial resistance was observed. Fig. 4B showed that the photocurrent intensity gradually reduced after the continuous immobilization of P1, BSA, miR-21 and P2-GDYO signal probe on GDY2-CdSQDs/ITO. It was because the insulated biomolecules hampered electron transfer and led to the recombination of photo-generated electrons and electron-holes. The changing Ret and photocurrent suggested that the PEC sensor was successfully fabricated. 3
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Fig. 2. SEM images of tremella-like GDY nanotubes at low (A) and high (B) magnification, and GDYO nanosheets (C) and GDYO nanosheets after size screening (D). AFM image of small size of GDYO (E) and relative heights (F).
Fig. 3. (A) Photocurrent response of different hybrids on ITO electrode, (B) XRD patterns of GDY and GDY2-CdSQDs nanohybrid, (C) UV–vis diffuse reflectance spectra of different materials, (D) Plots of (Ahν )1/2 versus the energy (hν) for the band gap energy of CdSQDs and GDY2-CdSQDs, (E) TEM image of GDY2-CdSQD, and (F) SEM image of GDY2-CdSQDs.
3.4. Optimization of detection conditions
after hybridization with miR-21 and P1. However, when P2-GDYO was used as a signal inhibitor (Fig. 5A-II), Δi increased as high as 3.48 μA, suggesting that P2-GDYO resulted in a remarkable decrease in the photocurrent intensity. Fig. 5A-III displayed Δi of 5.05 μA under the condition of ascorbic acid (AA) as the sacrificial agent and P2-GDYO as signal inhibitor. In comparison, the change of photocurrent to miR-21 was enhanced by ca. 7 times. Furthermore, Fig. 5A-III also showed the
To prove that P2-GDYO signal probe could improve the sensitivity of PEC sensor, P2 without and with GDYO signal inhibitor was used to hybridize with 1 nM of miR-21. From Fig. 5A-I, the photocurrent decayed fast under light illumination, indicating the photo corrosion. The initial photocurrent intensity of sensor only gave Δi (i0-i) of 0.74 μA 4
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Fig. 4. Nyquist plots (A) and photocurrent responses (B) of different electrodes.
photocurrent reduced. We deduced that overdense CdSQDs on GDY2 partially hindered the electron transfer on electrode interface since CdSQDs was a semiconductor with low conductivity. The quantity of P1 on platform was also very important for miR-21 detection. As shown in Fig. 5D, the photocurrent intensity decreased rapidly. As the concentration reached 500 nM. It exhibited a sluggish decrease for higher concentration. Considering the good photocurrent signal and wide detection range, 500.0 nM of P1 was chosen.
stable photocurrent since AA acted as electron donor [36], which suppressed the photo corrosion. Thus, AA played a great role in photocurrent response. P2 was secondary recognition probe. Owing to the insulativity of DNA, the amount of P2 influenced the Δi for miR-21 detection. To observe the relationship between P2 amount loaded on GDYO and sensitivity, the Δi of the proposed PEC sensor towards 1 nM of miR-21 was investigated in 50 mM of AA solution. As could be seen from Fig. 5B, Δi began to enhance with increasing the amount of P2 in the range of 0.1 to 10.0 μM. It achieved the maximum and then kept stable at 4.0 μM, indicating that P2 probe on GDYO reached saturation. Thus, 4.0 μM of P2 was selected for the preparation of P2-GDYO. In this work, GDY2-CdSQDs was used as photoelectrochemical platform. To obtain GDY2-CdSQDs with excellent photoelectrochemical performance, four types of GDY2-CdSQDs were synthesized by heating at 80 °C for 4, 8, 12, and 15 h, respectively. Fig. 5C showed a gradually rising photocurrent with the prolonging of heating time. GDY2-CdSQDs heated for 12 h displayed higher photocurrent. But for 15 h,
3.5. Possible mechanism for PEC sensor GDY has the electron acceptor nature similar to that of graphene [14]. The conductive GDY makes it a potential candidate for effective transfer of photogenerated electrons to suppress the recombination of free charge carriers [37]. Here, the photo-generated electrons from the valence band (VB) of CdSQDs to conduction band jumped into GDY and then injected into ITO electrode to produce the anodic photocurrent. With the formation of the electron-hole pairs, AA captured the hole Fig. 5. (A) Photocurrent response of PEC sensor hybridized with 500 pM of miR-21 and different signal inhibitor. I, only P2; II, P2GDYO; III, II + AA as the sacrificial agent. (B) the changed photocurrent response Δi (i0-i) with different amount of P2 on GDYO as signal inhibitor. (C) Photocurrent response of GDY2CdSQDs synthesized under different heating time. (D) Photocurrent of GDY2-CdSQDs platform modified with different amount of P1.
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Scheme 1. Illustration of the principle for the determination of miR-21.
methods were compared in Table S2. The proposed detection performance was comparable to some earlier works. The practical applicability of the proposed PEC sensor was estimated by applying serum samples (collected from 4 normal female volunteer after informed consent at Hunan Provincial People's Hospital) without pretreatment process. The recovery test was carried out by spiking miR-21 into human serum. Each sample was then detected with the proposed PEC sensor. The recovery was in the range of 93.7–102.0% with RSD < 9.7% (Table S3). It suggested that GDYCdSQDs was a potential photoelectrochemical platform for target biomolecules detection, such as DNA, RNA, antigen, and antibody, etc. Fig. 7 showed the photocurrent stability of the GDY2-CdSQDs/ITO. The current intensity was quite steady after several on/off irradiation cycles. Suggesting that photo corrosion of GDY2-CdSQs/ITO was negligible during the process of photocurrent detection. The storage stability was also tested. No visible photocurrent change was found after storing for four weeks, indicating the PEC sensor was stable under cold storage. The reproducibility was evaluated by analyzing four independently PEC sensor after being hybridized with 1.0 pM of miRNA21 and P2-GDYO. The relative standard deviation (RSD) was estimated to be 6.3%. The specificity of sensor was investigated by testing the photocurrent produced after hybridizing with 10 pM miRNA-21 mixed with 1.0 nM different RNA fragments (sequences listed in Table S1). Fig. 7 showed that only single-base terminal mismatched sequences decreased 17.2% and 26.0% of photocurrent intensity, respectively. Other fragments displayed insignificant photocurrent change. These results manifested that the proposed PEC sensing system discriminated mismatched sequences well.
effectively and furtherly suppressed the recombination of electron-hole pairs and improved the photocurrent (Scheme 1). After achieving the stable PEC current, the hybridization reaction was conducted and the P2-GDYO was attached on the surface of ITO. The P2-GDYO signal inhibitor sensitively quenched the photocurrent which reflected the concentration of miR-21 indirectly. The reason was as follows: (1) GDYO exhibited a large surface area, low conductivity, and high loading capacity for insulative P2, which limited the electron transfer and led to the recombination of photo-generated electrons and electronholes; (2) a strong electrostatic repulsion existed between electronegative P2-GDYO and AA molecules, which reduced the hole trapping ability of AA; (3) P2-GDYO on ITO surface absorbed a part of visible light irradiation towards GDY2-CdSQDs and lowered the amount of photo induced electrons, and thus decreased the photocurrent. Therefore, it was essential for P2-GDYO to increase the sensitivity of PEC sensor. 3.6. Determination of miR-21 Under optimal conditions, the PEC sensor hybridized with different concentrations of miR-21 and P2-GDYO was performed photocurrent determination. The photocurrent signals displayed a gradual decrease with the increasing of miR-21 (Fig. 6). It was found that Δi (i0-i) was proportional to the logarithm of miR-21 concentration (Inset of Fig. 6). In the linear detection range of 0.1∼10,000 pM, the linear regression equation was Δi (μA) = 1.9202+ +1.2513 lgC (pM) (R2 = 0.993) with a detection limit (LOD) of 0.02 pM. Some different miR-21 detection
4. Conclusions In this work, acidification improved the dispersibility of tremellalike GDY. One-step hydrothermal method synthesized the hybrid of GDY and CdSQDs. High photocurrent response of GDY-CdSQDs to light irradiation made it become a good photoelectrochemical platform. Small size of GDYO nanosheets limited the electron transfer and decreased the photocurrent signal generated from photoelectrochemical platform owing to low conductivity, high loading capacity for insulative DNA probe P2, etc. Through using a significant cancer marker miR-21 as model target, the proposed PEC sensor with GDY-CdSQDs as photoelectrochemical platform and P2-GDYO as photocurrent signal inhibitor was demonstrated to be a potential method for other bioanalysis and clinical diagnosis. Fig. 6. Photocurrent response of PEC sensor in phosphate buffer (pH 6.5) with different concentration of miR-21. From a to h: 0, 0.1, 1, 10, 100, 1000, 10000, 50,000 pM. The inset: the corresponding calibration plot for changed photocurrent responses Δi (i0-i) vs. the logarithm of miR-21 concentration (n = 4).
Declaration of Competing Interest None. 6
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Fig. 7. (A) Stability evaluation of the P2GDYO/miR-21/P1/GDY2-CdSQDs/ITO. Photocurrent measurements were performed in phosphate buffer (pH 6.5) containing 50 mM of AA at the potential of 0 V. (B) The photocurrent responses of 10 pM of miR-21 mixed with 1.0 nM of target sequences. Error bars represented the standard deviation calculated from four independent experiments.
Acknowledgments This work was supported by National Natural Science Foundation of China (Nos. 21671063) and Scientific Research Fund of Hunan Provincial Education Department (16B088).
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Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.snb.2019.127363.
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Hao Wang is currently a master graduate student in school of Chemistry and Chemical Engineering, Hunan University of Science and Technology. He specializes in the preparation of novel nanomaterials and biosensing applications.
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Shaowei Zhang is currently a researcher and associate professor in Key Laboratory of Theoretical Organic Chemistry and Function Molecule, Ministry of Education and Hunan University of Science and Technology. He received his doctor degree from Institute of Chemistry and Chemical Engineering, Nankai University in 2015. His current research interests include functional complex chemistry, polymetallic oxygen cluster chemistry and application.
Keqin Deng is currently a researcher and associate professor in Key Laboratory of Theoretical Organic Chemistry and Function Molecule, Ministry of Education and Hunan University of Science and Technology. He received his doctor degree from Institute of Chemistry and Chemical Engineering, Hunan University in 2010. His current research interests include novel nanomaterials and biosensing applications. Jin Xiao is currently a master graduate student in school of Chemistry and Chemical Engineering, Hunan University of Science and Technology. She specializes in electrochemistry.
Xiaofang Li is currently a professor in Key Laboratory of Theoretical Organic Chemistry and Function Molecule, Ministry of Education and Hunan University of Science and Technology. He received his doctor degree from Institute of Chemistry, Tianjin University in 2004. His current research interests include synthetic chemistry and the design of conjugated molecular materials.
Chunxiang Li is currently a lecturer of Hunan University of Science and Technology. She received her master degree from Institute of Chemistry and Chemical Engineering, Hunan University in 2003. Her research interests focus on electrochemical biosensor and novel nanomaterials.
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Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
A sandwich-type photoelectrochemical sensor based on tremella-like graphdiyne as photoelectrochemical platform and graphdiyne oxide nanosheets as signal inhibitor Hao Wanga, Keqin Denga,*, Jing Xiaob, Chunxiang Lib, Shaowei Zhanga, Xiaofang Lia,* a
Key Laboratory of Theoretical Organic Chemistry and Function Molecule, Ministry of Education, Hunan University of Science and Technology, Xiangtan 411201, China Hunan Provincial Key Laboratory of Controllable Preparation and Functional Application of Fine Polymers, Hunan Provincial Key Laboratory of Advanced Materials for New Energy Storage and Conversion, School of Chemistry and Chemical Engineering, Hunan University of Science and Technology, Xiangtan 411201, China
b
A R T I C LE I N FO
A B S T R A C T
Keywords: Graphdiyne Graphdiyne oxide Photoelectrochemical detection cadmium sulfide quantum dots microRNA-21
Graphdiyne (GDY) was the first reported graphynes to be prepared practically. Owing to its low dispersibility, only very limited GDY-based hybrids were synthesized. Furthermore, no any high quality of GDY-based hybrid with photoelectrochemical performance was synthesized by simple hydrothermal process. Herein, we report acidified tremella-like GDY nanotubes. The dispersibility of GDY was highly improved. The composite of GDY and cadmium sulfide quantum dots (GDY-CdSQDs) was synthesized through one-step hydrothermal method. GDY-CdSQDs exhibited excellent photocurrent response under optical illumination. Small size of GDY oxide (GDYO) nanosheet was obtained through oxidation and size screening. A photocurrent signal inhibitor was thus prepared by covalently binding DNA probe 2 (P2) on GDYO nanosheet (P2-GDYO). With GDY-CdSQDs and DNA capturing probe 1 (P1) acting as photoelectrochemical sensing platform, the target object of microRNA-21 (miR21) was captured by P1 and hybridized with P2-GDYO. Because GDYO exhibited low conductivity, high loading capacity for insulative P2, and surface absorption to light irradiation, it caused a magnified photocurrent change. The changed photocurrent was used to quantitatively detect miR-21. This method avoided expensive bioreagents and labeled target/detection DNA or miRNAs. It was potential for other bioanalysis and clinic diagnosis.
1. Introduction Graphdiyne (GDY) is a kind of a flat material with sp and sp2 hybridized carbon atoms and high degrees of π conjugation [1,2]. GDY film possesses excellent semiconducting property with a naturally bandgap of 0.47 eV calculated by density functional theory [3,4]. Its unique atomic arrangement and electronic structure made it become highly efficient catalyst [5,6]. GDY-based electrodes also displayed excellent electrochemical performance and good conductivity [7]. Moreover, GDY was used as electron-transport material for photodegradation of methylene blue [8]. Its nanomaterials exhibited higher field emission performance than graphite and carbon nanotubes [9,10]. As is well known that graphene, a two-dimensional carbon nanomaterial, has been widely used to develop graphene-based hybrids [11–13]. These hybrids displayed superior properties and some special properties through the combination of graphene with other nanoscale materials. But as for GDY, only a few GDY-based hybrids were reported [6,14–16]. GDY − ZnO nanohybrids was used as an advanced
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photocatalytic material [14]. Cobalt-nitrogen-doped GDY was demonstrated to be as an efficient bifunctional catalyst [15]. Robust CeS bond integrated graphdiyne-MoS2 enhanced lithium storage capability [16]. GDY with 4-mercaptopyridine surface-functionalized CdSe quantum dots as sensitizer was beneficial to the hole transportation and enhancement of the photocurrent performance [7]. Motivated by the graphene-based hybrids, we hope to develop novel and eminent GDYbased nanocomposites. Photoelectrochemical (PEC) sensor has attracted substantial attention owing to its low signal background, inexpensive photoelectric devices, and high sensitivity [17,18]. Many semiconductor materials have been used to fabricate PEC sensors, which can respond to light irradiation. Among them, Cadmium sulfide quantum dots (CdSQDs) is one kind of special semiconductor nanoparticles. But their toxicity influences the activity of biomolecules immobilized on the surface of CdS [19]. In addition, the optical properties of CdSQDs varies with the surface modification groups [20]. Being a low-toxic and natural aminothiol, L-cysteine (Cys) has been used as modifier and stabilizer for
Corresponding author. E-mail addresses: [email protected] (K. Deng), [email protected] (X. Li).
https://doi.org/10.1016/j.snb.2019.127363 Received 21 August 2019; Received in revised form 25 October 2019; Accepted 29 October 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Hao Wang, et al., Sensors & Actuators: B. Chemical, https://doi.org/10.1016/j.snb.2019.127363
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Fig. 1. (A) XPS surveys of different GDY. (B) Corresponding Nyquist plots of electrochemical impedance spectroscopy. (C) FTIR spectra of GDY and GDYO. (D) UV–vis spectrum of GDYO.
mixed with concentrated HNO3 (1.0 mL) and concentrated H2SO4 (3.0 mL). The mixture was heated in an oil bath at 60 °C for 8 h. After being cooled, NaOH was carefully used to adjust pH till 6.0 in the ice bath. The resulting solution was dialyzed for three days to remove metal ion impurities. GDY oxide was named as GDYO. To obtain small size of GDYO, 0.22 um syringe filter was used to filtrate the GDYO solution. The filtrate was dried to give the GDYO powder.
preparation of CdSQDs [21,22]. To the best of our knowledge, only Li reported a GDY loaded with AuNPs for constructing PEC platform [23]. The low dispersibility of GDY in water limited the synthesis of high quality of nanocomposite with photoelectrochemical performance by simple hydrothermal process. In this study, acidification treatment was performed to improve the dispersibility of tremella-like GDY nanotubes. Small size of GDY oxide (GDYO) nanosheets were obtained by oxidation and filtration screening. GDY loaded with low-toxic Cys-coated CdSQDs nanocomposites (GDY-CdSQDs) was prepared through one-step hydrothermal method. The flower-like GDY had large specific surface and loaded large amount of CdSQDs. By applying a significant cancer marker miR-21 as model target, the nanocomposite was used as sensitive photocurrent signal platform. Furthermore, GDYO bonded DNA probe molecules effectively reduced the photocurrent signal by inhibiting electron conduction.
2.2. Synthesis of GDY-CdSQDs and P2-GDYO conjugate GDY2 (2.0 mg) was dispersed into 10 ml Cd(NO3)2 (10.0 mM) and thoroughly mixed with magnetic stirring for 2 h. Then, 18.0 mg of cysteine (Cys) was dissolved in the mixed solution and NaOH was used to adjust pH till 8.0. After Na2S (8.0 mM) was added into the solution, it was transferred into a Teflon-lined stainless-steel autoclave and heated at 80 °C for 12 h. The resulted product was thoroughly washed with ethanol and centrifuged repeatedly for three times. It was denoted as GDY2-CdSQDs. For comparison, GDYO-CdSQDs, GDY1-CdSQDs, and GDY3-CdSQDs were prepared using the similar procedure. For the synthesis of P2-GDYO nanoprobe, GDYO (0.5 mg mL−1) were mixed with EDC (10 mM) and NHS (20 mM). After 15 min, P2 (4.0 μM) was added into the activated GDYO solution. The mixed solution was kept reaction for another 3 h and then dialyzed for three days.
2. Experimental 2.1. Acidification of GDY and synthesis of GDYO nanosheets The tremella-like graphdiyne (GDY) was donated by Prof. Yuliang Li (Institute of Chemistry, Chinese Academy of Sciences, Beijing, China). The preparation process was shown in previous work [24]. GDY was treated under different condition. In brief, GDY was carefully mixed with concentrated H2SO4.The mixture was stirred vigorously for 24 h in an oil bath at 60 °C. The suspension was centrifuged at 12,000 rpm for 10 min and washed till neutral. The resulted GDY was denoted as GDY1. Other two substitutive methods were refluxing GDY in 1:1 or 3:1 concentrated acid of H2SO4/HNO3 for 6 h, respectively. The acidified GDY was named as GDY2 and GDY3. GDYO was prepared by the following procedure [25]. GDY (10.0 mg) and KMnO4 (10.0 mg) were carefully
2.3. Fabrication of sensor 6 μL of GDY2-CdSQDs (1 mg mL−1) in 1% acetic acid solution containing 0.3% CS was decorated on cleaned ITO and dried at 60 °C. Then, 10 μL of 2.0% GLD was covered onto ITO surface and remained for 1 h at room temperature. The exposed geometric area of ITO glass to optical radiation was controlled at 0.25 cm2 by treating with black insulating waterproof tape. After washing away the redundant GLD, 2
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about 15 ∼30 nm (Fig. 2E, 2 F). It was worth mentioning that the monolayer GDY was obtained by ultrasonic treatment of raw GDY dispersed in DMF (spectral purity). It showed an average thickness of about 0.8 nm (Fig. S3A, B).
10 µL of probe 1 (P1) was dropped on ITO for another 6 h. BSA (1 mg mL−1) was then used to block nonspecific sites for 30 min. The resulted P1/GDY2-CdSQDs/ITO was used for hybridization. Hybridization was performed by dropping 8 μL of 1:1 (V:V) mixture of miR-21 and P2-GDYO on the surface of P1/GDY2-CdSQDs/ITO and incubating in a humidor at 45 °C for 1 h. The resulted electrode was rinsed for photocurrent measurement. It was named as P2-GDYO/RNA/ P1/GDY2-CdSQDs/ITO.
3.2. Characterization of GDY-CdSQDs and GDYO-CdSQDs Cadmium sulfide as photoelectrochemical material has achieved many applications due to good properties [31,32]. Many nanocomposites hybridized with CdS showed distinctive functions [19,33,34]. To obtain excellent GDY-CdSQDs composite, the photoelectrochemical performance of GDY1-CdSQDs, GDY2-CdSQDs, GDY3-CdSQDs, GDYOCdSQDs, and CdSQDs was tested and compared (Fig. 3A). All GDYCdSQDs composites exhibited higher photocurrent than CdSQDs, showing the improved photoelectrochemical performance. GDY2CdSQDs displayed higher photocurrent. We deduced that black GDY1 had stronger light adsorption (Fig. 3B), which might partially reduce photon-induced electrons yield from CdSQDs because of the “shielding effect” [35]. Furthermore, GDY3 and GDYO had lower conductivity and higher resistance (Fig. 1B), which hindered the photon-induced electrons transfer. Therefore, GDY2-CdSQDs was chosen as the optimal photoelectrochemical platform. Fig. 3B exhibited the UV–vis diffuse reflectance spectra. GDY1 and GDY2 showed excellent light absorption. By contrast with GDY1, GDY2 displayed a slightly reduced absorption in the visible light region. As for the CdSQDs and GDY2-CdSQDs, GDY2-CdSQDs showed a better absorption in the range of > 365 nm. The bandgap energy of CdSQDs and GDY2-CdSQDs was estimated by the relationship of (αhν)1/2 versus photon energy (Fig. 3D). Their energy gap was 2.85 and 2.72 eV, respectively. It demonstrated that the hybridization of CdSQDs with GDY enhanced light absorption and expanded light absorbing range. As shown the XRD patterns in Fig. 3D, GDY2 showed a peak at 2θ = 22.7°. GDY2-CdSQDs exhibited the characteristic diffraction peaks at 28.5°, 45.4°, 54.1°, matching well with that of cubic CdS phase (JCPDS No. 42-1411). A low characteristic peaks of GDY2 appeared on GDY2-CdSQDs, suggesting the relatively low diffraction intensity of GDY2. The morphology of GDY2-CdSQDs was depicted by TEM image (Fig. 3E). Well-dispersed CdSQDs were attached onto the surface of GDY sheets. Fig. S4A showed the CdSQDs with average diameter of ∼8.7 nm. By compared with Fig. S4B, pure CdSQDs has the size only ∼4.2 nm. It manifested that GDY was favorable for the growth of large size of CdSQDs. Furthermore, the SEM image of GDY2-CdSQDs also indicated that a mass of CdSQDs were uniform and compact on the surface of GDY flakes (Fig. 3F). Compared with Fig. 2A, the morphology of GDY didn’t change after loading CdSQDs. The TEM image of GDY2CdSQDs heated for 4 h in autoclave showed very sparse CdSQDs on GDY sheets (Fig. S5).
3. Results and discussion 3.1. Characterization of GDY and GDYO Surface properties of carbon nanomaterials, such as dispersibility, electrocatalysis, heterogeneous electron transfer, and chemical adsorption, were greatly influenced by the oxygen-containing functional groups [26,27]. The low dispersibility of GDY affected its wide application. In order to obtain well-dispersed GDY, different acidification methods were adopted. X-ray photoelectron spectroscopy (XPS) analysis was applied to analyze the change of surface O/C atom ratio occurred during the acidification of GDY. As shown in Fig. 1A, the O/C ratio of GDY, GDY1, GDY2, GDY3, and GDYO was 16.72%, 23.37%, 29.14%, 40.19%, and 68.43%, respectively. Obviously, the O/C atom ratio became higher and higher, indicating that the acidification treatment virtually led to the generation of oxygen-containing species [26], which was helpful for the improved dispersibility of GDY. The color change of GDY1, GDY2, GDY3, GDYO was also visible from black, to dark brown, and bright brown (Fig. S1). Precipitate was found in GDY1, GDY2, and GDY3 aqueous dispersion after placing about 2 h, 5 h, and 8 h, respectively. GDYO was completely soluble in water. These acidified GDY was individually immobilized on GCE by 0.2% Nafion for electrochemical impedance spectroscopy (EIS) measurement. As shown in Fig. 1B, the Ret semicircle portions displayed a gradual reinforcement from GDY1, GDY2, GDY3, to GDYO. The resistance (Ret) were 138, 196, 475, and 994 Ω, respectively. GDY1 and GDY2 had the lower resistance and higher conductivity. GDYO showed a higher Ret value. It indicated the gradually weakened conductivity and increased hindrance for the access of the redox probe to the electrode surface because of the increasing acidification degree from GDY1 to GDYO. Fig. 1C showed FT-IR spectra of GDY and GDYO. The bands of GDY at around 1633, 1450, and 1091 cm−1 were the stretching motions of C]C (ν(C]C)) as well as some oxygen-containing functional groups [28,29]. The spectrum of GDYO displayed an improved band appeared at 1623 cm−1, which was ascribed to the stretching motion of carbonyl groups (ν(C]O)) [28]. A greatly increased the stretching vibration band of C–O (ν(C–O)) and a new bending vibration band of OeH (β(OeH)) appeared at around 1125 and 619 cm−1, respectively [30]. Besides, the bands at 1048 and 880 cm−1 corresponding to C]C structure lowered dramatically. These results suggested that the acidification turned C]C groups into the oxygen-containing groups, especially including a large amount of eCOOH group, which was helpful for high loading of P2 probe. UV–vis spectrum of GDYO showed a strong absorption at 230 nm (Fig. 1D), which was ascribed to theπ→π* transition of aromatic C]C bond. A weak shoulder peak at about 360 nm was attributed to n→π* transition of C]O bond. Furthermore, GDYO exhibited visible light absorption within < 500 nm. Fig. 2A presented the morphology of raw GDY. The GDY was flowerlike nanotubes with diameter of about 200∼600 nm, which composed of numerous transparent ultrathin nanosheets (Fig. 2B). The morphology provided a very high surface area for loading other nanoparticles. After acidification, GDY1, GDY2, and GDY3 kept the original morphology feature as the raw GDY. But, highly oxidized GDYO changed the feature and showed well-dispersed nanosheets (Fig. S2). The GDYO were not of uniform size from about 50 nm to 5 um (Fig. 2C). After filtration screening, GDYO with the size < 400 nm was obtained (Fig. 2D). The AFM image showed that the thickness of GDYO was
3.3. Characterization of the construction process of PEC sensor The fabrication process of PEC sensor was proved by the electrochemical impedance spectroscopy (EIS) and photocurrent measurement. Fig. 4A exhibited the impedance spectra of different electrodes. Compared with GDY2-CdSQDs/GCE, after P1 immobilizing, the Ret increased because of the insulativity of biomolecules, suggesting capturing probe P1 was immobilized on the ITO surface. Then, after stepwise BSA blocking and hybridizing with miR-21 and P2-GDYO, furtherly increase of interfacial resistance was observed. Fig. 4B showed that the photocurrent intensity gradually reduced after the continuous immobilization of P1, BSA, miR-21 and P2-GDYO signal probe on GDY2-CdSQDs/ITO. It was because the insulated biomolecules hampered electron transfer and led to the recombination of photo-generated electrons and electron-holes. The changing Ret and photocurrent suggested that the PEC sensor was successfully fabricated. 3
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Fig. 2. SEM images of tremella-like GDY nanotubes at low (A) and high (B) magnification, and GDYO nanosheets (C) and GDYO nanosheets after size screening (D). AFM image of small size of GDYO (E) and relative heights (F).
Fig. 3. (A) Photocurrent response of different hybrids on ITO electrode, (B) XRD patterns of GDY and GDY2-CdSQDs nanohybrid, (C) UV–vis diffuse reflectance spectra of different materials, (D) Plots of (Ahν )1/2 versus the energy (hν) for the band gap energy of CdSQDs and GDY2-CdSQDs, (E) TEM image of GDY2-CdSQD, and (F) SEM image of GDY2-CdSQDs.
3.4. Optimization of detection conditions
after hybridization with miR-21 and P1. However, when P2-GDYO was used as a signal inhibitor (Fig. 5A-II), Δi increased as high as 3.48 μA, suggesting that P2-GDYO resulted in a remarkable decrease in the photocurrent intensity. Fig. 5A-III displayed Δi of 5.05 μA under the condition of ascorbic acid (AA) as the sacrificial agent and P2-GDYO as signal inhibitor. In comparison, the change of photocurrent to miR-21 was enhanced by ca. 7 times. Furthermore, Fig. 5A-III also showed the
To prove that P2-GDYO signal probe could improve the sensitivity of PEC sensor, P2 without and with GDYO signal inhibitor was used to hybridize with 1 nM of miR-21. From Fig. 5A-I, the photocurrent decayed fast under light illumination, indicating the photo corrosion. The initial photocurrent intensity of sensor only gave Δi (i0-i) of 0.74 μA 4
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Fig. 4. Nyquist plots (A) and photocurrent responses (B) of different electrodes.
photocurrent reduced. We deduced that overdense CdSQDs on GDY2 partially hindered the electron transfer on electrode interface since CdSQDs was a semiconductor with low conductivity. The quantity of P1 on platform was also very important for miR-21 detection. As shown in Fig. 5D, the photocurrent intensity decreased rapidly. As the concentration reached 500 nM. It exhibited a sluggish decrease for higher concentration. Considering the good photocurrent signal and wide detection range, 500.0 nM of P1 was chosen.
stable photocurrent since AA acted as electron donor [36], which suppressed the photo corrosion. Thus, AA played a great role in photocurrent response. P2 was secondary recognition probe. Owing to the insulativity of DNA, the amount of P2 influenced the Δi for miR-21 detection. To observe the relationship between P2 amount loaded on GDYO and sensitivity, the Δi of the proposed PEC sensor towards 1 nM of miR-21 was investigated in 50 mM of AA solution. As could be seen from Fig. 5B, Δi began to enhance with increasing the amount of P2 in the range of 0.1 to 10.0 μM. It achieved the maximum and then kept stable at 4.0 μM, indicating that P2 probe on GDYO reached saturation. Thus, 4.0 μM of P2 was selected for the preparation of P2-GDYO. In this work, GDY2-CdSQDs was used as photoelectrochemical platform. To obtain GDY2-CdSQDs with excellent photoelectrochemical performance, four types of GDY2-CdSQDs were synthesized by heating at 80 °C for 4, 8, 12, and 15 h, respectively. Fig. 5C showed a gradually rising photocurrent with the prolonging of heating time. GDY2-CdSQDs heated for 12 h displayed higher photocurrent. But for 15 h,
3.5. Possible mechanism for PEC sensor GDY has the electron acceptor nature similar to that of graphene [14]. The conductive GDY makes it a potential candidate for effective transfer of photogenerated electrons to suppress the recombination of free charge carriers [37]. Here, the photo-generated electrons from the valence band (VB) of CdSQDs to conduction band jumped into GDY and then injected into ITO electrode to produce the anodic photocurrent. With the formation of the electron-hole pairs, AA captured the hole Fig. 5. (A) Photocurrent response of PEC sensor hybridized with 500 pM of miR-21 and different signal inhibitor. I, only P2; II, P2GDYO; III, II + AA as the sacrificial agent. (B) the changed photocurrent response Δi (i0-i) with different amount of P2 on GDYO as signal inhibitor. (C) Photocurrent response of GDY2CdSQDs synthesized under different heating time. (D) Photocurrent of GDY2-CdSQDs platform modified with different amount of P1.
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Scheme 1. Illustration of the principle for the determination of miR-21.
methods were compared in Table S2. The proposed detection performance was comparable to some earlier works. The practical applicability of the proposed PEC sensor was estimated by applying serum samples (collected from 4 normal female volunteer after informed consent at Hunan Provincial People's Hospital) without pretreatment process. The recovery test was carried out by spiking miR-21 into human serum. Each sample was then detected with the proposed PEC sensor. The recovery was in the range of 93.7–102.0% with RSD < 9.7% (Table S3). It suggested that GDYCdSQDs was a potential photoelectrochemical platform for target biomolecules detection, such as DNA, RNA, antigen, and antibody, etc. Fig. 7 showed the photocurrent stability of the GDY2-CdSQDs/ITO. The current intensity was quite steady after several on/off irradiation cycles. Suggesting that photo corrosion of GDY2-CdSQs/ITO was negligible during the process of photocurrent detection. The storage stability was also tested. No visible photocurrent change was found after storing for four weeks, indicating the PEC sensor was stable under cold storage. The reproducibility was evaluated by analyzing four independently PEC sensor after being hybridized with 1.0 pM of miRNA21 and P2-GDYO. The relative standard deviation (RSD) was estimated to be 6.3%. The specificity of sensor was investigated by testing the photocurrent produced after hybridizing with 10 pM miRNA-21 mixed with 1.0 nM different RNA fragments (sequences listed in Table S1). Fig. 7 showed that only single-base terminal mismatched sequences decreased 17.2% and 26.0% of photocurrent intensity, respectively. Other fragments displayed insignificant photocurrent change. These results manifested that the proposed PEC sensing system discriminated mismatched sequences well.
effectively and furtherly suppressed the recombination of electron-hole pairs and improved the photocurrent (Scheme 1). After achieving the stable PEC current, the hybridization reaction was conducted and the P2-GDYO was attached on the surface of ITO. The P2-GDYO signal inhibitor sensitively quenched the photocurrent which reflected the concentration of miR-21 indirectly. The reason was as follows: (1) GDYO exhibited a large surface area, low conductivity, and high loading capacity for insulative P2, which limited the electron transfer and led to the recombination of photo-generated electrons and electronholes; (2) a strong electrostatic repulsion existed between electronegative P2-GDYO and AA molecules, which reduced the hole trapping ability of AA; (3) P2-GDYO on ITO surface absorbed a part of visible light irradiation towards GDY2-CdSQDs and lowered the amount of photo induced electrons, and thus decreased the photocurrent. Therefore, it was essential for P2-GDYO to increase the sensitivity of PEC sensor. 3.6. Determination of miR-21 Under optimal conditions, the PEC sensor hybridized with different concentrations of miR-21 and P2-GDYO was performed photocurrent determination. The photocurrent signals displayed a gradual decrease with the increasing of miR-21 (Fig. 6). It was found that Δi (i0-i) was proportional to the logarithm of miR-21 concentration (Inset of Fig. 6). In the linear detection range of 0.1∼10,000 pM, the linear regression equation was Δi (μA) = 1.9202+ +1.2513 lgC (pM) (R2 = 0.993) with a detection limit (LOD) of 0.02 pM. Some different miR-21 detection
4. Conclusions In this work, acidification improved the dispersibility of tremellalike GDY. One-step hydrothermal method synthesized the hybrid of GDY and CdSQDs. High photocurrent response of GDY-CdSQDs to light irradiation made it become a good photoelectrochemical platform. Small size of GDYO nanosheets limited the electron transfer and decreased the photocurrent signal generated from photoelectrochemical platform owing to low conductivity, high loading capacity for insulative DNA probe P2, etc. Through using a significant cancer marker miR-21 as model target, the proposed PEC sensor with GDY-CdSQDs as photoelectrochemical platform and P2-GDYO as photocurrent signal inhibitor was demonstrated to be a potential method for other bioanalysis and clinical diagnosis. Fig. 6. Photocurrent response of PEC sensor in phosphate buffer (pH 6.5) with different concentration of miR-21. From a to h: 0, 0.1, 1, 10, 100, 1000, 10000, 50,000 pM. The inset: the corresponding calibration plot for changed photocurrent responses Δi (i0-i) vs. the logarithm of miR-21 concentration (n = 4).
Declaration of Competing Interest None. 6
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Fig. 7. (A) Stability evaluation of the P2GDYO/miR-21/P1/GDY2-CdSQDs/ITO. Photocurrent measurements were performed in phosphate buffer (pH 6.5) containing 50 mM of AA at the potential of 0 V. (B) The photocurrent responses of 10 pM of miR-21 mixed with 1.0 nM of target sequences. Error bars represented the standard deviation calculated from four independent experiments.
Acknowledgments This work was supported by National Natural Science Foundation of China (Nos. 21671063) and Scientific Research Fund of Hunan Provincial Education Department (16B088).
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Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.snb.2019.127363.
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Hao Wang is currently a master graduate student in school of Chemistry and Chemical Engineering, Hunan University of Science and Technology. He specializes in the preparation of novel nanomaterials and biosensing applications.
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Shaowei Zhang is currently a researcher and associate professor in Key Laboratory of Theoretical Organic Chemistry and Function Molecule, Ministry of Education and Hunan University of Science and Technology. He received his doctor degree from Institute of Chemistry and Chemical Engineering, Nankai University in 2015. His current research interests include functional complex chemistry, polymetallic oxygen cluster chemistry and application.
Keqin Deng is currently a researcher and associate professor in Key Laboratory of Theoretical Organic Chemistry and Function Molecule, Ministry of Education and Hunan University of Science and Technology. He received his doctor degree from Institute of Chemistry and Chemical Engineering, Hunan University in 2010. His current research interests include novel nanomaterials and biosensing applications. Jin Xiao is currently a master graduate student in school of Chemistry and Chemical Engineering, Hunan University of Science and Technology. She specializes in electrochemistry.
Xiaofang Li is currently a professor in Key Laboratory of Theoretical Organic Chemistry and Function Molecule, Ministry of Education and Hunan University of Science and Technology. He received his doctor degree from Institute of Chemistry, Tianjin University in 2004. His current research interests include synthetic chemistry and the design of conjugated molecular materials.
Chunxiang Li is currently a lecturer of Hunan University of Science and Technology. She received her master degree from Institute of Chemistry and Chemical Engineering, Hunan University in 2003. Her research interests focus on electrochemical biosensor and novel nanomaterials.
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