One-Step Synthesis of CuOâCu2O Heterojunction by Flame Spray

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One-Step Synthesis of CuO-Cu2O Heterojunction by Flame Spray Pyrolysis for Cathodic Photoelectrochemical Sensing of L-Cysteine Yuhan Zhu, Zuwei Xu, Kai Yan, Haibo Zhao, and Jingdong Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13020 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 7, 2017

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ACS Applied Materials & Interfaces

One-Step Synthesis of CuO-Cu2O Heterojunction by Flame Spray Pyrolysis for Cathodic Photoelectrochemical Sensing of L-Cysteine Yuhan Zhu,1 Zuwei Xu,2 Kai Yan,1 Haibo Zhao,2 and Jingdong Zhang*,1 1

Key laboratory of Material Chemistry for Energy Conversion and Storage (Ministry

of Education), School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Luoyu Road 1037, Wuhan 430074, P.R. China 2

State Key Laboratory of Coal Combustion, School of Energy and Power Engineering,

Huazhong University of Science and Technology, Luoyu Road 1037,Wuhan 430074, P.R. China KEYWORDS:

CuO-Cu2O

heterojunction;

Flame

spray

pyrolysis;

Photoelectrochemical sensor; L-Cysteine; Cathodic photocurrent ABSTRACT: CuO-Cu2O heterojunction was synthesized via a one-step flame spray pyrolysis (FSP) process and employed as photoactive material in construction of a photoelectrochemical (PEC) sensing device. The surface analysis showed that CuO-Cu2O nanocomposites in the size less than 10 nm were formed and uniformly distributed on the electrode surface. Under visible light irradiation, the CuO-Cu2O coated electrode exhibited admirable cathodic photocurrent response, owing to the favorable property of the CuO-Cu2O heterojunction such as strong absorption in the visible region and effective separation of photogenerated electron-hole pairs. Based on the interaction of L-cysteine (L-Cys) with Cu-containing compounds via the 1

ACS Paragon Plus Environment


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formation of Cu-S bond, the CuO-Cu2O was proposed as a photoelectrochemical (PEC) sensor for L-Cys detection. A declined photocurrent response of CuO-Cu2O to addition of L-Cys was observed. Influence factors including CuO-Cu2O concentration, coating amount of CuO-Cu2O and applied bias potential on the PEC response toward L-Cys were optimized. Under optimum conditions, the photocurrent of the proposed sensor was linearly declined with increasing the concentration of L-Cys from 0.2 µM to 10 µM, with a detection limit (3S/N) of 0.05 µM. Moreover, this PEC sensor displayed high selectivity, reproducibility and stability. The potential applicability of the proposed PEC sensor was assessed in human urine samples. 1. INTRODUCTION CuO and Cu2O are two important p-type semiconductors with narrow band gaps. Due to their superiorities of affordable price, environmentally friendly, abundance in nature, unique optical and electrical properties, both CuO and Cu2O have attracted considerable research interest.1-3 On account of their desirable properties that can complement each other, it is reasonable to believe that coupling CuO with Cu2O may exhibit a better performance. As a matter of fact, the nanocomposites of CuO and Cu2O have been employed in many fields, including lithium-ion battery,4 supercapacitor,5

CO2

reduction,6

photocatalytic

water

photoelectrolysis,7

photocatalytic degradation,8 and electrochemical sensor.3 In particular, as highly efficient photocatalyst in photocatalytic or photoelectrochemical (PEC) device, the heterojunction materials of CuO and Cu2O have been demonstrated to exhibit enhanced absorption in the visible light region and promoted separation of 2


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photogenerated electrons/holes.9,10 Therefore, developing CuO-Cu2O heterojunction possessing admirable catalytic performance using simple preparation strategy is highly desirable. There have been many methods for the synthesis of heterostructured nanocomposites, however, they generally involve time-consuming complex steps such as hydrothermal reaction, further doping, drying and annealing.11,12 By contrast, flame synthesis can quickly produce nanoparticles with homogenous morphology and narrow size distribution in a single-step continuous process.13,14 Moreover, the high temperature in flame synthesis can lead to highly crystalline nanomaterials without the requirement of external heating.15 Thus, flame synthesis has attracted much interest in recent years. In flame synthesis, various flame configurations such as diffusion, premixed, and spray flames in both laminar and turbulent modes have been explored. Among these, flame spray pyrolysis (FSP) offers a greater flexibility for the use of a wide range of precursors, solvents and process conditions, and thus provides a precise control over particle size and composition.16,17 So far, a variety of functional metal and mixed metal oxide nanoparticles have been synthesized by FSP, such as SiO2,18 CuO,19 TiO2,20 Au/TiO2,20 ZnO/CeO2 nanocomposites.21 As a typical thiol-containing amino, L-cysteine (L-Cys) plays a very important role in maintaining regular function of the biological system, and has been widely used in clinical diagnoses, food and pharmaceutical industry.22-25 The abnormal level of L-Cys is associated with many syndromes, such as slow growth of children, hair depigmentation, skin lesions, loss of muscle and fat, edema, lethargy, weakness, liver 3



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damage and hypoglycemic brain damage.26,27 Therefore, it is necessary to develop sensitive and selective methods for the detection of L-Cys in biological fluids and external environment. Various analytical techniques have been established for the determination of L-Cys, including high-performance liquid chromatography (HPLC),28 electrochemiluminescence (ECL),29 colorimetry,30 flow injection analysis (FIA),31 and electrochemical analysis.32,33 PEC analysis based on semiconducting materials modified photoelectrodes, which exhibits higher sensitivity than common electrochemical analysis owing to the separation of light excitation source and photocurrent detection signal, has been developed for L-Cys detection.34,35 Actually,

both

sensitivity

and

selectivity

are

important

performance

characteristics of sensors. However, due to broad photocatalytic reactions on photoelectrodes, many PEC sensors suffer from the poor selectivity. To overcome this problem, recognition elements including aptamers, antibodies, enzymes and molecularly imprinted polymers have been coupled with photoelectrodes in construction of PEC sensors.36 Nevertheless, the incorporation of these recognition elements in sensors requires additional costly materials and time-consuming immobilization processes. Alternatively, selective PEC sensors can also be realized based on the interactions between the analytes and photoelectrode materials without using additional recognition elements. For example, Ma et al. have employed WS2/TiO2 composites to develop a PEC sensing platform for o-diphenol and its derivatives whose adjacent double oxygen atoms could bind with the Ti (IV) surface site by chelate effect and form a five/six-atom ring structure with visible-light 4


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activity.37 Qian et al. have proposed a BiPO4–rGO-based PEC detection of chlorpyrifos based on the formation of a Bi–chlorpyrifos complex on BiPO4 NPs which results in an obvious photocurrent decline for the steric hindrance effect.38 Wen et al. have developed an excitation trapping-based PEC strategy for detecting Hg2+ by the competition interaction of Hg2+ ion toward the dithiol stabilizer on quantum dots with a subsequent cathode photocurrent decrease.39 Wu et al. have designed a triphenylamine-based organic dye, TTA, with an acrylic group to graft TiO2 nanoparticles for selective PEC sensing of cysteine based on the reaction of cysteine molecule with the acrylic moiety of TTA.40 In the present work, we facilely prepared CuO-Cu2O heterojunction with improved PEC activity by using FSP synthesis. Compared with previously reported methods, the present FSP process provided a novel one-step approach to the synthesis of CuO-Cu2O composites with uniform nanoscale size distribution and high crystallinity. Considering that copper-based materials including Cu2+,41 copper nanoparticles,30 and Cu-containing compounds42,43 could interact with L-Cys via the formation of Cu-S bond which were useful for selective detection of L-Cys, we applied the CuO-Cu2O heterojunction to develop a PEC sensor for selective detection of L-Cys. Scheme 1 illustrates the proposed PEC sensor. Under visible-light illumination, CuO-Cu2O heterojunction generated significant cathodic photocurrent. While L-Cys was present in the solution, a decrease of photocurrent response was observed, owing to the binding interaction of L-Cys molecules and nanoparticles. Thus, a novel cathodic “signal-off” PEC sensing strategy for highly sensitive and 5



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selective detection of L-Cys was developed.

Scheme 1. Schematic Illustration of the PEC Sensing Platform for L-Cys Detection

2. EXPERIMENTAL SECTION 2.1. Chemicals Amino acids including L-Cys, L-phenylalanine (L-Phe), L-arginine (L-Arg), L-glutamic acid (L-Glu), L-lysine (L-Lys), glycine (Gly), L-tryptophane (L-Trp), L-tyrosine (L-Tyr), and L-methionine (L-Met) were obtained from Shanghai Experimental Reagent Co., China. Homocysteine (Hcy) and poly(sodium 4-styrene sulfonate) (PSS, average Mw ~200000, 30 wt. % in H2O) were purchased from Sigma-Aldrich. Copper nitrate trihydrate (Cu(NO3)2⋅3H2O), glutathione (GSH) and other reagents of analytical grade were provided by Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China), and used without further purification. Doubly distilled water was used throughout the investigation. 6


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Figure 1. Experimental apparatus for FSP. 2.2. Synthesis of CuO-Cu2O Heterojunction CuO-Cu2O nanomaterial was synthesized using a commercial laboratory-scale FSP system (Tethis NPS10, Italy). The experimental apparatus for FSP is shown in Figure 1. The solution containing precursor and fuel flows was atomized into small droplets using a two-fluid nozzle and then was ignited by a pilot flame. The chemical reaction of a precursor involved the formation of the product particles by nucleation and growth processes in the presence of aerosol. The particles were collected in a glass fibre filtering system placed above the flame. In this work, a typical precursor solution was prepared by dissolving 50 mmol of Cu(NO3)2·3H2O into 100 mL of anhydrous ethanol. The precursor solution was feed into the atomizer by a syringe pump at the rate of 5 mL/min. The flow of oxygen dispersion gas was 5 L/min with 7



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atomization pressure drop of 1.5 bar. The pilot flame employed methane and oxygen premixed at 0.75 L/min and 1.5 L/min, respectively. For comparison, the pure copper oxide sample was prepared by annealing the solid Cu(NO3)2·3H2O in air at 450 °C for 3 hours.

2.3. Modification of Electrode Prior to modification, the bare glass carbon electrode (GCE) surface was polished to a mirror-like smoothness with an aqueous slurry of 0.05 µm alumina powder and then cleaned with ethanol and water successively. After being dried with nitrogen gas, the GCE substrate was immersed into 1% PSS solution containing 0.5 mol·L-1 NaCl for 30 min, followed by thoroughly rinsing with distilled water to remove the loosely adsorbed PSS. Then, the GCE surface with an exposed geometric area of 0.096 cm2 was dip-coated with 5 µL of CuO-Cu2O suspension and dried at 60 °C for further use. On account of the electrostatic attraction in this process, negatively charged PSS was beneficial to the immobilization of positively charged CuO-Cu2O heterojunction on electrode surface, thus generating significant PEC response upon illumination. For comparison, CuO modified electrode was prepared in the same procedure except that calcined CuO was used instead of CuO-Cu2O composite.

2.4. Apparatus and Procedure A Quanta 200 field emission scanning electron microscope (SEM) (FEI, Netherlands) was implemented to observe the sample morphology. A Tecnai G220 transmission electron microscope (TEM) (FEI, The Netherlands) was used for TEM 8


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or high-resolution transmission electron microscopic (HRTEM) characterization. The X-ray diffraction (XRD) patterns were analyzed by a Bruker D8 Advance X-ray diffractometer (Bruker Instruments, Germany) with Cu Kα radiation. The X-ray photoelectron spectra (XPS) were recorded on a 5300 ESCA instrument (Perkin-Elmer PHI Co., USA) using an Al Kα X-ray source at a power of 250 W. The UV-visible absorption spectra were measured with a TU-1900 spectrometer (Beijing Purkinje General Instrument Company, China). The photocurrent curves and electrochemical impedance spectra (EIS) were recorded on a CHI660A electrochemical working station (Shanghai Chenhua Instrument Co. Ltd., China) in a conventional three-electrode system. The fabricated CuO-Cu2O film-modified electrode, a platinum wire, and a saturated calomel electrode (SCE) were employed as the working, auxiliary and reference electrodes, respectively. EIS measurements were performed in 0.5 M KCl solution containing 5.0 mM K3Fe(CN)6/K4Fe(CN)6 by applying a bias potential of 0.2 V (versus SCE) and the frequency range was from 0.1 Hz to 100 kHz. A CEL-S500/350 xenon lamp (CEAULIGHT Co., China) with an optical filter (λ > 420 nm) was used as the irradiation source, and the distance between the light source and working electrode surface was 10 cm. 3. RESULTS AND DISCUSSION

3.1. Characterization of CuO-Cu2O Heterojunction The surface morphology of CuO-Cu2O heterojunction film was investigated by SEM. Figure 2A depicts a relatively rich and uniform mesoporous film structure 9



composed of many CuxO nanoparticles. Meanwhile, TEM characterization was employed to study the microstructures of the sample. As shown in Figure 2B, the shape of the obtained nanoparticles is in good agreement with the SEM result, and the average diameter of the particles is less than 10 nm. Furthermore, the lattice spacing of 0.243 nm in the HRTEM image is assigned to (111) planes of Cu2O while the lattice spacings of 0.237 nm and 0.252 nm are correspond to (111) and (-111) planes of CuO, respectively (Figure 2C). On the other hand, the crystalline nature of the as-synthesized material was investigated by XRD. Figure 2D illustrates that the peaks marked with a rhombus symbol can be indexed to the monoclinic phase of CuO (JCPDS card no. 05-0661), and the other peaks can be attributed to the cubic phase

20

30

♦ (C uO )

50

70

♦ • (311) ♦

• (220) ♦ (113)

60

♦ (311) ♦ (220)

♦ (202)

♦ (202)

• (200)

40

♦ (020)

• (Cu 2 O )

• (111)

♦ (110)

• (110)

♦ (111)

D

♦ (111)

structure of Cu2O (JCPDS card no. 05-0667).

Intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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80

2 θ / degree

Figure 2. (A) SEM image, (B) TEM image, (C) HRTEM image, and (D) XRD pattern of CuO-Cu2O composite.

10


1200

1000

800

Cu 3s Cu 3p3/2

Cu LMM

C 1s

Intensity (a.u.)

Cu 2p1/2 Cu 2p3/2

A

600

400

200

0

Binding Energy (eV)

B

Cu 2p3/2

Intensity/ (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


O 1s

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satellite line CuO Cu 2p3/2

930

940

Cu 2p1/2

satellite line CuO Cu 2p1/2

950

960

Binding energy/ eV

Figure 3. (A) XPS survey and (B) Cu 2p spectrum of CuO-Cu2O composite.

In order to further confirm the chemical composition of the prepared sample, XPS analysis was carried out. Figure 3A shows the survey spectrum of CuO-Cu2O composite, which indicates that only Cu, O and C (from the atmosphere) elements are detected and no other impurities exist. The Cu 2p spectrum of the sample is presented in Figure 3B. The two peaks centered at binding energies of 934.2 and 954.1 eV are attributed to Cu 2p3/2 and Cu 2p1/2 of CuO, respectively. The two smaller fitting peaks at 933.0 and 952.7 eV are ascribed to Cu2O. From the XPS peak area, the content ratio of CuO to Cu2O is estimated to be around 8:1. Moreover, the two detectable 11



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shake-up satellite peaks further manifest the existence of CuO. The results are in accordance with those of XRD, further confirming the heterojunction of CuO and Cu2O has been prepared successfully.

3.2. Optical, Photocurrent and EIS Analysis The optical property of the as-synthesized CuO-Cu2O was studied by UV-visible absorption spectroscopy. As shown incurve a of Figure 4A, the CuO-Cu2O heterojunction presents strong absorption capacity in a wide range from UV to visible light. For comparison, we also prepared pure CuO by annealing the same precursor. Figure S1 depicts the XRD pattern of the calcined CuO, which can be indexed to the monoclinic phase of CuO (JCPDS card no. 05-0661). As can be seen in curve b of Figure 4A, the pure CuO shows relatively weak light absorption, which is unfavorable to photoexcitation. The photocurrent responses of both CuO-Cu2O heterojunction and pure CuO modified electrodes were recorded in 0.1 M Na2SO4 solution at an applied potential of -0.1 V. As shown in curve a in Figure 4B, the CuO-Cu2O/GCE responds sensitively to the irradiation of visible light and exhibits a cathodic photocurrent of ca. 2 µA, showing the photocatalytic activity of CuO-Cu2O heterojunction. However, the photocurrent response of CuO-Cu2O/GCE decreases significantly in deaerated solution under nitrogen atmosphere (curve b in Figure 4B), indicating the vital role of oxygen as sacrificial species in such a photocathodic process. It is considered that

p-type semiconductors are prone to interact with electron acceptors. According to the energy band diagram of CuO-Cu2O (Figure 4C), the photogenerated electrons from 12


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the conduction band (CB) of Cu2O could thermodynamically transfer to the CB of CuO because the position of CuO is more positive than that of Cu2O. Further, the CB position of CuO is more negative than the reduction potential of O2. As a result, the photogenerated electrons could eventually transfer to O2 molecules adsorbed on the electrode surface. In photocathodic process, the dissolved oxygen in the electrolyte serves as acceptor to consume the photogenerated electrons, while the photogenerated holes would finally be captured by the electrode to generate cathodic photocurrent.44 Therefore, all the following PEC experiments were carried out in air-saturated electrolyte. On the other hand, the photocurrent response of CuO-Cu2O/GCE is decreased by ~30% when L-Cys is added in the electrolyte (curve c in Figure 4B). In this case, the interaction of L-Cys with CuO-Cu2O heterojunction results in the occurrence of steric hindrance effect which could effectively block the mass transfer of electron acceptor (i.e. oxygen) to the electrode surface, and thus decreasing the photocurrent.45 In comparison with CuO-Cu2O heterojunction, CuO/GCE exhibits rather low photocurrent response in 0.1 M Na2SO4 solution at an applied potential of -0.1 V (curve a in Figure 4D). This is consistent with the weak optical absorption of CuO as illustrated in Figure 4A. Moreover, the undesired recombination of photogenerated electrons and holes of pure CuO is also not advantageous to the generation of photocurrent response. While L-Cys is added in the electrolyte, the photocurrent of the CuO/GCE also declines (curve b in Figure 4D), confirming the interaction between CuO and L-Cys molecules. Nevertheless, the photocurrent difference (∆PI) 13



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before and after interaction with L-Cys on CuO/GCE is only 0.06 µA, almost 10 times smaller than that on CuO-Cu2O/GCE. Accordingly, the CuO-Cu2O heterojunction with admirable visible light harvesting property and sensitive photocurrent response to L-Cys is the preferable candidate for fabricating the PEC sensor. The interaction between L-Cys and copper-based materials was studied by UV-visible absorption spectroscopy. As shown in Figure S2, the absorption of CuO in the wavelength range above 320 nm decreases after adding L-Cys, consistent with the phenomenon observed for the influence of L-Cys on CuS absorption, which clearly indicates the interaction of L-Cys with Cu.43 A similar phenomenon is also observed for the absorption of CuO-Cu2O (Figure S3). However, the decrease of the absorption of CuO-Cu2O above 320 nm by L-Cys is more remarkable than that of CuO, implying that the CuO-Cu2O composite affects such an interaction, which is beneficial to promoting the PEC response toward L-Cys. Actually, the valence electronic configurations of Cu(I) and Cu(II) are 3d10 and 3d9, respectively; and both Cu(I) and Cu(II) possess unoccupied 4s and 4p orbitals. Since L-Cys has a lone pair of electrons in its thiol group, it can readily interact with Cu(I) and Cu(II) to form Cu-S covalent bond by transferring lone pair electrons from the sulfur atom of thiol group to the vacant orbitals of Cu(I) and Cu(II). On the other hand, in order to monitor the change of interfacial electrochemical behavior during the fabrication of sensor with CuO-Cu2O, EIS analysis was performed in 0.1 M KCl solution using [Fe(CN)6]3−/4− redox probe. As shown in Figure S4, the electron-transfer resistance (Ret) value of bare GCE, estimated from the 14


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semicircle diameter of Nyquist plot at high frequency region, is increased after coating with CuO-Cu2O, due to the low conductivity of metal-oxide semiconductors. After CuO-Cu2O/GCE reacts with L-Cys, the Ret value is further increased. This result indicates that the interaction of L-Cys with CuO-Cu2O inhibits the electron transfer, consistent with the reduced PEC response observed in Figure 4B. The similar inhibition effect and mechanism has also been verified in previous studies.46,47 A

B

0.0

Photocurrent/ µ A

b

Absorbance

a

b

300

400

-0.5

-1.0

-1.5

c

-2.0

500

600

a 0

700

10

20

30

40

50

60

Time / s

Wavelength/ nm

-0.06

Photocurrent/ µ A

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


D

-0.12

b -0.18

a -0.24

0

20

Time / s

40

60

Figure 4. (A) UV-visible absorption spectra of (a) CuO-Cu2O composite and (b) CuO. (B) Photocurrent responses of CuO-Cu2O/GCE recorded in 0.1 M Na2SO4 at −0.1 V: (a) before and (c) after interaction with 5 µM L-Cys; (b) in deaerated electrolyte. (C) Energy band diagram of CuO-Cu2O heterojunction. CB: conduction band; VB: valence band. (D) Photocurrent responses of CuO/GCE recorded in 0.1 M Na2SO4 at −0.1 V (a) before and (b) after interaction with 5 µM L-Cys. 15



A

∆PI/ µ A

0.6

0.4

0.2

0.0 1.0

1.5

2.0

2.5

3.0 -1

Concentration/ g·L

B 0.6

∆ PI/ µ A

0.5 0.4 0.3 0.2 2

4

6

8

10

12

Amount/ µL

0.6

∆ PI/ µ A

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

C

0.4

0.2

0.0 0.00

-0.05

-0.10

-0.15

-0.20

Potential/ V

Figure 5. Influences of (A) CuO-Cu2O concentration, (B) CuO-Cu2O amount, and (C) applied bias potential on the photocurrent response of modified electrodes toward 5 µM L-Cys in 0.1 M Na2SO4 solution. Error bars are derived from the standard deviation of three measurements. 16


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3.3. Optimization of Photoelectrochemical Sensor The influences of CuO-Cu2O concentration, coating amount and applied bias potential on the PEC response toward L-Cys were studied to optimize the proposed sensor. Figure 5A displays the effect of CuO-Cu2O concentration on the photocurrent response of PEC sensor to L-Cys. It is observed that the response is gradually enhanced with increasing the material concentration from 1 to 2 g/L, suggesting that more CuO-Cu2O nanoparticles are involved in the interaction of L-Cys molecules on the electrode surface. However, the PEC response decreases when the material concentration further increases, due to saturation of the amount of nanocomposite on the surface of electrode. Therefore, the optimum concentration of choice for sensor fabrication is 2 g/L. The influence of the coating amount on the PEC response is depicted in Figure 5B. As can be seen, the optimal amount of CuO-Cu2O nanomaterial is 5 uL. The less quantity does not satisfy the needs of photoelectrocatalysis process, while the excess loading of material results in a decreased ∆PI value. This is possibly a result of excessive thickness of the CuO-Cu2O film immobilized on the surface of the electrode, which may turn into hindrance for the electron transfer and arouse the undesired recombination of photogenerated electrons and holes.36 Moreover, the photocurrent response of the as-prepared sensor is studied at different applied bias potential, which serve as an important parameter in the process of PEC sensing. As shown in Figure 5C, the ∆PI value of the sensor increases with changing the bias potential from 0 to -0.1 V, indicating that more negative potential 17



drives more photogenerated electrons to the working electrode for the reduction of electron acceptor. While the applied potential is more negative than -0.1 V, a significant decline in photocurrent difference is observed, possibly due to the response of the sensor reach saturation. Thus, -0.1 V is selected as the optimum potential for the following measurements. 3.4. Photoelectrochemical Sensing of L-Cys

0.0 1.2

-0.5 0.8

-1.0

∆ PI/ µ A

Photocurrent/ µA

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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h

0.4

-1.5

0.0 0

2

4

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8

10

CL-Cys/ µM

-2.0

a 0

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20

30

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60

70

80

90

100

Time / s Figure 6. Photocurrent responses of the fabricated sensor in 0.1 M Na2SO4 solution containing (a) 0, (b) 0.2, (c) 0.5, (d) 0.7, (e) 1, (f) 2, (g) 5, (h) 10 µM L-Cys at −0.1 V. Inset: calibration curve for L-Cys on the PEC sensor. Error bars are derived from the standard deviation of three measurements.

18


Page 19 of 33

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Table 1. Comparison of Different Methods for L-Cys Determination

a

Method

Linear range (µM)

Detection limit (µM)

Reference

ECL

1.3 – 35

0.87

29

colorimetry

0 – 25

0.1

30

FIA

0.4 – 40

0.1

31

Cyclic voltammetry

1.1 – 10

0.31

32

Amperometry

1.3 – 720

0.8

33

Amperometry

1–1000

0.07

48

Amperometry

0.05–100

0.008

49

Amperometry

0.1– 100

0.02

50

DPVa

0.9–12.4 12.4–538.5

0.28

51

DPASVb

0.008 – 5.943

0.002

52

PEC

60– 500

12.6

34

PEC

0.2 – 2.8

0.1

35

PEC

0.2 – 10

0.05

This work

DPV is abbreviated from differential pulse voltammetry, bDPASV is abbreviated

from differential pulse anodic stripping voltammetry.

The proposed PEC sensor was applied to the quantitative determination of L-Cys under optimized experimental condition. Figure 6 illustrates the responses of L-Cys at different amounts on the CuO-Cu2O/GCE in 0.1 M Na2SO4 solution. As can be seen, 19



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Page 20 of 33

the cathodic photocurrent response is found to decrease with increasing the L-Cys concentration, indicating that more L-Cys molecules are captured and participate in the PEC process. Moreover, the difference in photocurrent response is found to be linearly proportional to the concentration of L-Cys ranging from 0.2 µM to 10 µM (inset

of

Figure

6).

The

linear

regression

equation

is

expressed

as

∆PI/µA=0.1C/µM+0.06, with a correlation coefficient R2=0.995. In addition, the limit of detection (3S/N) is estimated to be 0.05 µM, which is lower than many previously reported sensors for L-Cys determination (Table 1). Therefore, the proposed PEC sensor is sensitive and feasible for L-Cys detection. To further evaluate the selectivity of the proposed PEC sensor, interference measurements were investigated by determination of the photocurrent responses of 5 µM L-Cys in Na2SO4 solution under optimum conditions in the existence of 5 µM some other amino acids (L-Phe, L-Arg, L-Glu, L-Lys, Gly, L-Trp, L-Tyr, L-Met and Hcy) and a biologically important thiol compound (GSH). As shown in Figure 7, most of these amino acids such as L-Phe, L-Arg, L-Glu, L-Lys, Gly, L-Trp and L-Tyr show negligible effect on the determination of L-Cys, since there is no –SH group in their molecular

structures

to

interact

with

CuO-Cu2O.

L-Met,

a

typical

sulfur-containing amino acid, also does not shown obvious interference in the determination of L-Cys. This can be ascribed to the fact that the binding ability of –S– in L-Met with Cu is far less than that of –SH in L-Cys. However, GSH and Hcy interfere in L-Cys determination, due to their thiol groups. Especially Hcy has a very similar molecular structure to L-Cys, and its interference in L-Cys determination is 20


Page 21 of 33

pronounced. By contrast, GSH only slightly interferes in L-Cys determination, owing to its steric hindrance which greatly suppresses its binding ability with Cu.43 Therefore, the proposed sensor has good selectivity toward L-Cys against many interferences except thiol compounds. In previously reported electrochemical and PEC methods for L-Cys detection as listed in Table 1, the electrochemical and photoelectrocatalytic oxidation signals of L-Cys are employed, which can be easily influenced by other oxidation reactions occurring at the electrode interface. By contrast, the present PEC sensor can effectively avoid such a problem since it is based on the cathodic photocurrent response of CuO-Cu2O heterojunction and specific interaction between L-Cys and CuO-Cu2O; and thus the selectivity is greatly improved.

1.5

1.0

In

∆ PI / ∆ PIL-Cys

0.5

cy G SH LM e L- t Ph e LA rg LG lu LLy s G l L- y Tr p LTy r

H

ys

0.0 LC

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7. Histogram for ∆PI determined on PEC sensor upon addition of 5 µΜ L-Cys solution containing 5 µM other amino acids and GSH. Error bars are derived from the standard deviation of three measurements. 21



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Page 22 of 33

In addition, the reproducibility of such a PEC sensor was investigated by comparing the responses of five independently prepared CuO-Cu2O/GCE toward 5 µM L-Cys. The relative standard deviation (RSD) obtained for the detection is 2.7 %, implying a favorable reproducibility. At the same time, no obvious change is observed for the response of the CuO-Cu2O/GCE after storage at 4 °C in a refrigerator for 6 weeks, indicating a high stability of the sensor. Furthermore, the stability of the photocurrent response upon “on–off” irradiation cycle was carried out in 0.1 M Na2SO4 solution. As shown in Figure S5, there is no apparent change in photocurrent response after 15-min continuous test.

Table 2. Determination of L-Cys in Human Urine Samples by Proposed Sensor (n=3) Samples

Spiked (µM)

Found (µM)

Recovery (%)

RSD (%)

1

2.00

1.89

94.50

2.62

2

5.00

4.96

99.20

1.80

3

8.00

7.62

95.30

2.45

To confirm the feasibility, the proposed photoelectrochemical sensor was employed to analyze L-Cys in human urine samples, which were collected from healthy volunteers. Prior to analysis, the sample was centrifuged for 10 min at 4000 rpm, and the supernatant liquid was appropriately diluted with 0.1 M Na2SO4. After homogenizing, the diluted sample was filtered by 0.22-µm nylon membrane filters. 22


Page 23 of 33

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The standard addition method was adopted for the determination of L-Cys in urine samples under optimized conditions. As can be seen (Table 2), the recoveries of the proposed method are obtained in the range from 94.5% to 99.2%, indicating the reliability of the proposed PEC sensor for L-Cys determination in urine samples.

4. CONCLUSIONS In this work, CuO-Cu2O heterojunction was successfully synthesized by a simple one-step FSP method, which did not require tedious fabrication processes. The obtained CuO-Cu2O heterojunction possessed high PEC activity and showed sensitive cathodic photocurrent response under visible light irradiation. Due to the interaction of L-Cys with Cu-based material via the formation of Cu-S bond, the photocurrent of CuO-Cu2O electrode was reduced in the electrolyte containing L-Cys. Accordingly, a novel PEC sensor for L-Cys detection was successfully developed, which displayed high sensitivity and selectivity, desirable reproducibility and good stability. Our work demonstrates the feasibility of FSP technique in synthesis of metal oxide nanocomposites and potential application of CuO-Cu2O heterojunction in PEC devices.

ASSOCIATED CONTENT

Supporting Information

23



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Page 24 of 33

XRD spectrum of CuO; UV-visible absorption spectra; EIS of modified electrodes; time-based photocurrent response of PEC sensor. The Supporting Information is available free of charge on the ACS Publications Website at DIO:

AUTHOR INFORMATION

Corresponding Author * Tel: +862787543032. Fax: +862787543632. E-mail: [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 61571198, 51606079) and the opening fund of Hubei Key Laboratory

of

Bioinorganic

Chemistry

&

Materia

Medica

(Grant

No.

BCMM201704). We also thank the Analytical and Testing Center of Huazhong University of Science and Technology for help in materials characterization. REFERENCES (1) Zhang, Q.; Zhang, K.; Xu, D.; Yang, G.; Huang, H.; Nie, F.; Liu, C.; Yang, S. CuO Nanostructures: Synthesis, Characterization, Growth Mechanisms, Fundamental Properties, and Applications. Prog. Mater. Sci. 2014, 60, 208-337. (2) Huang, W. C.; Lyu, L. M.; Yang, Y. C.; Huang, M. H. Synthesis of Cu2O Nanocrystals from Cubic to Rhombic Dodecahedral Structures and Their Comparative Photocatalytic Activity. J. Am. Chem. Soc. 2012, 134 (2), 1261-1267. 24


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Table of Contents Graphics (for TOC only)

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One-Step Synthesis of CuOâCu2O Heterojunction by Flame Spray

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