Article pubs.acs.org/ac
Photochemical Synthesis of Shape-Controlled Nanostructured Gold on Zinc Oxide Nanorods as Photocatalytically Renewable Sensors Jia-Quan Xu, Huan-Huan Duo, Yu-Ge Zhang, Xin-Wei Zhang, Wei Fang, Yan-Ling Liu, Ai-Guo Shen, Ji-Ming Hu, and Wei-Hua Huang* Key Laboratory of Analytical Chemistry for Biology and Medicine, Ministry of Education, College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China S Supporting Information *
ABSTRACT: Biosensors always suffer from passivation that prevents their reutilization. To address this issue, photocatalytically renewable sensors composed of semiconductor photocatalysts and sensing materials have emerged recently. In this work, we developed a robust and versatile method to construct different kinds of renewable biosensors consisting of ZnO nanorods and nanostructured Au. Via a facile and efficient photochemical reduction, various nanostructured Au was obtained successfully on ZnO nanorods. As-prepared sensors concurrently possess excellent sensing capability and desirable photocatalytic cleaning performance. Experimental results demonstrate that dendritic Au/ZnO composite has the strongest surface-enhanced Raman scattering (SERS) enhancement, and dense Au nanoparticles (NPs)/ZnO composite has the highest electrochemical activity, which was successfully used for electrochemical detection of NO release from cells. Furthermore, both of the SERS and electrochemical sensors can be regenerated efficiently for renewable applications via photodegrading adsorbed probe molecules and biomolecules. Our strategy provides an efficient and versatile method to construct various kinds of highly sensitive renewable sensors and might expand the application of the photocatalytically renewable sensor in the biosensing area.
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Among the biosensing materials, Au is an excellent material due to its unique optical, electronic, and thermal properties.23−29 It is worth noting that the morphology of Au significantly affects its properties and applications;25 for example, nanostructured Au with abundant “hot spots” was reported to be beneficial for surface-enhanced Raman scattering (SERS).30,31 However, the morphology of Au in current renewable biosensors was always nanoparticles,17,19,21 which cannot meet the requirement of various biosensing applications. Therefore, it is significant to fabricate composite renewable biosensors with various structures of Au which would have different unique properties and can be used for a diversity of biosensing applications. Recently, photochemical reduction is reported to be a convenient method to generate nanostructured Au and the morphology of Au could be varied by adding different ligands into HAuCl4 solution during photochemical reduction.32−38 This suggests that photochemical reduction has the potential to become a facile method to controllably synthesize diverse nanostructured Au on semiconductor photocatalysts. Herein, we construct several photocatalysis-induced renewable biosensors for different applications by depositing diverse
iosensors are widely used in biomedical detection because of their unique characteristics such as simplicity, rapidity, and moderate cost.1−6 However, there is an inherent drawback of the traditional biosensors that they are not able to be reused easily, since molecules adsorbed on the substrate cannot be efficiently removed.7−10 To solve this issue, a great effort has been made to limit the extent of biofouling. For instance, some fouling-resistant materials (e.g., polypyrrole/PSS,11 poly(3,4ethylenedioxythiophene)/PSS,12 boron-doped diamond,13 and carbon nanotubes14) and surface regeneration methods (e.g., acid washing,15 chemical oxidation or reduction,15 and electrochemical oxidative or reductive cleaning16) have been used to minimize the biofouling of electrochemical sensors. However, using fouling-resistant materials cannot remove the adsorbate, while the current surface regeneration methods usually damage the sensor surface to some extent. Recently, a kind of renewable biosensor based on semiconductor photocatalysts and sensing materials has been developed.17−22 Photocatalysts in the renewable sensors provide efficient renewable properties by photodegrading undesired adsorbed biomolecules with reactive oxygen species produced under UV or visible light in a convenient way, while the sensing materials give the sensors high sensing performance. This strategy can efficiently improve the usage rate of the sensor; this is significant to these sensors and highly integrated devices needing elaborate design and complex fabrication. © XXXX American Chemical Society
Received: December 19, 2015 Accepted: March 1, 2016
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DOI: 10.1021/acs.analchem.5b04810 Anal. Chem. XXXX, XXX, XXX−XXX
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NO was produced by dropwise adding 2 M of sulfuric acid into 4 M sodium nitrite, followed by bubbling through NaOH solution. Finally, the purified NO gas was collected in PBS solution to form saturated NO solution which is reported to be 1.8 mM at room temperature. Fabrication of ZnO NRs. ZnO NRs on ITO conducting glass were prepared via a modified method from literature.40 Briefly, zinc acetate dehydrate solution (10.0 mM) in ethanol was spin-coated onto ITO followed by thermal decomposition at 573 K for 2 h to form ZnO seeds. Then, the ZnO seedscovered ITO was suspended vertically in an aqueous solution of zinc nitrate (0.025 M) and hexamethylenetetramine (0.025 M) at 363 K for 3 h to get vertically arranged ZnO NRs. Fabrication of Au/ZnO Composite. The process of the Au/ZnO preparation consisted of two steps. First, 1.0 mM HAuCl4 aqueous solution was prepared freshly followed by addition of different additives. Second, ZnO NRs substrate was immersed into the aqueous solution and then irradiated directly with UV light (main wavelength, 365 nm, 12 mW/cm2). The same UV light was used for all the experiments unless noted otherwise. In a typical experiment of synthesis of dendritic Au, ZnO NRs substrate (0.5 cm × 1 cm) was immersed in 1.0 mL of 1.0 mM HAuCl4 aqueous solution that contained 50.0 μL of ammonium hydroxide (28%), and then the whole system was placed away from the UV light at a distance of about 5.0 cm for 15 min of irradiation. Then the sample was rinsed thoroughly with ultrapure water and dried with N2 to obtain dendritic golddecorated ZnO NRs. To deposit Au nanoparticles (NPs) on ZnO NRs, 1.0 mM aqueous HAuCl4 with pH of 11 adjusted by NaOH was used as growth solution. To prepare sea-urchin-like Au, 1.0 mM HAuCl4 solution prepared by dissolving HAuCl4 in PBS was used as growth solution. To synthesize dense Au NPs, Au chain, Au sphere, and conical Au, 1.0 mM HAuCl4 aqueous solution that contained saturated melamine (25 °C), 0.15 M ammonium carbonate, 0.15 M p-phenylenediamine, and 0.1 M hexamethylenetetramine was used as growth solution, respectively. Cyclic SERS Spectra Measurement. Rhodamine 6G (R6G) was used as a probe molecular to test the SERS enhancement of Au/ZnO substrate. Au/ZnO substrate was soaked in R6G solution for 12 h to obtain adsorption equilibrium. The substrate was then take out from the solution and washed with ultrapure water. SERS measurements were conducted with Jobin Yvon Raman confocal microscope (HR800, France) equipped with a 632.8 nm excitation laser and a 50L× objective. The collecting parameters of each SERS spectrum are determined as 5 s of exposure time and integrating twice. In a typical cyclic experiment, after being used for R6G SERS detection, the Au/ZnO substrate was immersed in water and irradiated with UV light at room temperature for 1 h to clean the substrate. Then the substrate was rinsed with ultrapure water to remove the residual ions and molecules, followed by applying in SERS detection again. The process was repeated three times to ensure recyclable detection. Cyclic Electrochemical Activity Test. A copper wire was adhered on the ITO of Au/ZnO/ITO with conductive adhesive to fabricate a Au/ZnO/ITO working electrode. A poly(dimethylsiloxane) (PDMS) well was fabricated on the working electrode to define the sensor surface and load the electrolyte. Electrochemical characterization of the electrode was processed on a CHI 660A electrochemical workstation (CH instruments Inc.). A three-electrode system employing Ag/AgCl as
nanostructured Au on ZnO nanorods (NRs) via one-step and one-pot UV photochemical reduction (Scheme 1). By adding Scheme 1. Schematic Diagram of the ZnO NRs Catalyzed Photochemical Synthesis of Nanostructured Au for Construction of Renewable SERS and Electrochemical Sensorsa
a
The photoinduced Au nanostructure is labeled as conical Au, sphere Au, dendritic Au, sea-urchin-like Au, and Au chain from left to right, respectively.
different additives to the HAuCl4 aqueous solution, different nanostructured Au such as dendritic, spherical, sea-urchin-like, and conical structures are obtained on ZnO NRs for the first time. Due to the properties of Au varied by the structures, asprepared nanostructured Au/ZnO composites were then employed to construct renewable SERS and electrochemical sensors. Proof-of-concept experiments indicate that the dendritic Au/ZnO composite has the strongest SERS enhancement, and the dense Au NPs/ZnO composite has the highest electrochemical activity which was successfully used for detection of NO release from human umbilical vein endothelial cells (HUVECs). Both of the SERS and electrochemical sensors possess efficient photocatalysis-induced renewable properties via photodegrading adsorbed probe molecules or biomolecules, respectively. This work represents a first step toward the construction of kinds of photocatalytically renewable biosensors for different applications by using shapecontrolled nanostructured Au on ZnO NRs.
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EXPERIMENTAL SECTION Materials. Indium−tin oxide (ITO) conductive glass (film thickness, 180 nm; conductivity, 10 Ω/sq) was purchased from Crystal Great Technology Co., Ltd. (Shenzhen, China). 5Hydroxytryptamine (5-HT), L-arginine (L-Arg), and NOS inhibitor Nω-nitro-L-arginine methyl ester hydrochloride (LNAME) were purchased from Sigma. The cell culture medium RPMI 1640, L-glutamine, and 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) were purchased from GIBCO. 3′,6′-Di(o-acetyl)-4′,5′-bis[N,N-bis(carboxymethy)-aminomethy] fluorescein, tetraacetoxymethylester (Calcein-AM), and 3,8-diamino-5-[3-(diethylmethylammonio)propyl]-6-phenylphenanthridinium diiodide (PI) for cell staining were obtained from Dojindo Laboratory (U.S.A.). The HUVECs lines were obtained from CHI Scientific, Inc. (Shanghai China). All other chemicals unless specified were reagent grade and were used as received. Phosphate buffer saline (PBS) saturated nitric oxide (NO) solution was prepared according to a previous report.39 Briefly, the whole reaction system was degassed with N2 first. Then, B
DOI: 10.1021/acs.analchem.5b04810 Anal. Chem. XXXX, XXX, XXX−XXX
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Figure 1. (a) SEM image of ZnO NRs. (b) XRD patterns of ZnO NRs and Au/ZnO. (c and d) SEM image (c) and energy-dispersive X-ray (EDX) spectrum (d) of dendritic Au formed on ZnO NRs in NH4OH/HAuCl4 solution by UV irradiation for 15 min (In and Sn are attributed to the indium−tin oxide glass substrate). (e) UV−vis spectra of (I) 1.0 mM HAuCl4 solution, (II) 1.0 mM HAuCl4 solution containing 0.32 M ammonium hydroxide, (III) solution (II) after irradiation with UV light in the absence of ZnO NRs, (IV) solution (II) after irradiation with UV light in the presence of ZnO NRs. Inset: the amplification of lines III and IV in the 400−700 nm region. (f) TEM images of dendritic Au. Inset: magnified view of the part marked by the red square. The boundaries of Au NPs are marked by red arrows.
synthesized on a glass via a typical hydrothermal method (Figure 1a).40 The average diameter of ZnO NRs is about 70 nm. The characteristic diffraction peaks of ZnO (Figure 1b) match well with the standard peak positions of a hexagonal wurtzite ZnO structure (JCPDS 36-1451), and the strong diffraction peak at 34.8° (Figure 1b) indicates that as-grown ZnO NRs are highly oriented with their c-axis. To obtain Au/ZnO composite, ZnO NRs were immersed in 1.0 mM HAuCl4 solution contained 0.32 M ammonium hydroxide and irradiated with UV light. The morphology of the Au formed on ZnO NRs is time-dependent (Figure 1c and Figure S1). After UV irradiation for 15 min, dendritic Au (Figure 1c) was obtained on the ZnO NRs surface which is further confirmed by energy-dispersive X-ray (EDX) results (Figure 1d). The XRD pattern of Au/ZnO shows that the dendritic Au possess (111), (200), (220), and (311) facets corresponding to the face-centered cubic crystalline Au (JCPDS 04-0784) (Figure 1b). Another advantage of this photochemical reduction method is that the Au/ZnO substrate is easy to be patterned (Figure S2). According to the previous reports,41 a possible mechanism was proposed that the dendritic Au may form by an oriented attachment of the Au NPs in the solution phase. First, a complex might be formed between AuCl4− and ammonium hydroxide, which can be indicated by the shift of the absorption peak of HAuCl4 at about 320 nm (Figure 1e, I) to 275 nm (Figure 1e, II).36 Then the complex was reduced to Au NPs in the presence of ZnO NRs with UV light (Figure 1e, III and IV),34 which was verified by the appearance of a broad absorption peak at around 540 nm and the decrease of absorption at 275 nm. After that, as-reduced Au NPs attached each other to form dendritic Au structure, which can be certified by the TEM results (Figure 1f) showing obvious boundary between two Au NPs in dendritic Au. During the oriented attachment of Au NPs process, ammonium hydroxide and Zn2+ play important roles. The formation mechanism of the dendritic Au is further discussed in detail in Supporting Information (Figures S3−S7). Shape-Controlled Synthesis of Nanostructured Au. As the results illustrate above, ammonium hydroxide plays an important role in the formation of dendritic Au by oriented aggregation of Au NPs. Accordingly, the way of oriented
reference electrode and Pt wire as counter electrode was used in the experiment. The renewable performance of the electrochemical sensor was evaluated by applying a potential of +0.6 V for 600 s in 0.1 mM 5-HT solution to passivate the electrode. After passivation, the electrode was regenerated by exposure to UV light at room temperature. K3[Fe(CN)6] was used as probe to test the electrochemical activity of the Au/ZnO/ITO electrode before and after regeneration. The process of passivation and regeneration was also repeated three times to ensure recyclable detection. Cyclic Detection of NO Release from HUVECs. HUVECs were routinely culture using RPMI 1640 culture medium with 4.766 mg/mL HEPES, 0.292 mg/mL Lglutamine, 0.85 mg/mL NaHCO3, 12% fetal bovine serum, penicillin, and streptomycin (100 U) in a culture flask at 37 °C in a humidified incubator (95% air with 5% CO2). For NO release detection, the cells were harvested and suspended in culture medium, followed by dropping 50 μL of cell suspensions on the Au/ZnO/ITO electrode with a 4 mm × 4 mm PDMS well. About 10 min later of the cells suspensions dropping on the substrate, cells were almost deposited on the electrode surface; then amperometric detection was carried out by applying a constant potential of +0.85 V. The NO release was evoked by L-arginine (L-Arg). After detection, the Au/ ZnO/ITO electrode was irradiated with UV light to regenerate, and then used for NO release detection again. Characterization. Scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) images were obtained on a field-emission scanning electron microscope (Zeiss SIGMA) and JEM-2100 transmission electron microscope, respectively. UV−vis spectra were acquired on Shimadzu UV-3600. The powder X-ray diffraction (XRD) was recorded with a Bruker D8-Advance using Cu Kα radiation.
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RESULTS AND DISCUSSION
Fabrication of Dendritic Au Nanostructures on ZnO NRs. The fabrication process of the Au/ZnO substrates contains two steps involving ZnO NRs synthesis and photochemical reduction of Au on ZnO NRs. First, highly oriented ZnO NRs arrays with clear hexagonal structure were C
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Figure 2. SEM images of (a) Au NPs, (b) dense Au NPs, (c) Au chain, (d) sphere Au, (e) sea-urchin-like Au, and (f) conical Au synthesized by UVirradiated HAuCl4 solution containing (a) sodium hydroxide, (b) melamine, (c) ammonium carbonate, (d) p-phenylenediamine, (e) PBS, and (f) hexamethylenetetramine, respectively.
aggregation may change to form different Au shapes if other additives instead of ammonium hydroxide are added into the photoreduction system. On the basis of this concept, several kinds of additives were added into the photochemical reduction system. As illustrated in Figure 2, when NaOH, melamine, ammonium carbonate, p-phenylenediamine, PBS, and hexamethylenetetramine were used as additives, Au NPs, dense Au NPs, Au chain, Au sphere, sea-urchin-like Au, and conical Au could be obtained, respectively. This provides a facile and efficient method to fabricate diverse nanostructured Au by ZnO NRs-assisted and additives-regulated photochemical reduction. Among the nanostructured Au, dense Au NPs/ZnO was obtained by coating dense Au NPs on the ZnO NRs (Figure 2b). As we know, ZnO is easy to be etched by acid, but this substrate prepared in melamine had nice resistance ability to acid, indicating that the Au NPs coating on the ZnO NRs was dense and compact. Au chain (Figure 2c), sphere Au (Figure 2d), sea-urchin-like Au (Figure 2e), and conical Au (Figure 2f) were synthesized on ZnO NRs via photochemical reduction for the first time, although some of them (sphere and sea-urchinlike Au) have been prepared via seed-mediated or electrochemical methods.32,42 Notably, previous reports showed that conical Au was prepared by a complex process involving depositing Au in a conical nanopore and etching away the template.43,44 In this work, conical Au (Figure 2f) was easily synthesized via a template-free method. Recyclable SERS Detection on Au/ZnO Hybrids. As reported previously, due to the abundant gap and high surface area, nanostructured Au such as dendritic Au, Au chain, and sea-urchin-like Au have been widely used as SERS substrates.25 Herein, as-fabricated nanostructured Au/ZnO composites were used as renewable SERS substrates to test their sensing and photocatalytic cleaning ability. As shown in Figure 3a, the SERS intensity of R6G increases successively on these substrate in an order of Au spheres, Au NPs, dense Au NPs, conical Au, seaurchin-like Au, and dendritic Au. The strongest SERS enhancement was obtained on nanostructured dendritic Au maybe due to the hierarchical structure and relative high coverage, which could provide abundant gaps for SERS enhancement.45 Although sea-urchin-like Au possesses fractal structure, relatively weak SERS enhancement was obtained, which may be attributed to the low coverage. For Au NPs, the coverage of the Au structure is high, but weak SERS enhancement was obtained, which maybe result from lacking sufficient gaps for SERS enhancement.
Figure 3. (a) SERS spectra of 10−5 M R6G on (I) Au NPs, (II) dendritic Au, (III) sea-urchin-like Au, (IV) dense Au NPs, (V) conical Au, (VI) sphere Au, and (VII) ZnO NRs. (b) SERS spectra of R6G with (I) 10−5, (II) 10−7, and (III) 10−9 M on dendritic Au. (c) Reversible SERS behavior of dendritic Au with three cycles. The black line, red line, and blue line represent the first, second, and third detection and photocatalytic cleaning process, respectively. (d) Degradation and recovery percentage of Raman peak intensity around 1360 cm−1 of R6G on dendritic Au/ZnO obtained by dividing the intensities of R6G-degraded and R6G-reloaded Au/ZnO by the original detection signals. Error bars represent standard deviations (n = 3).
Figure 3b shows the intensity of SERS spectra of R6G with various concentrations on dendritic Au. The Raman peaks between 1300 and 1400 cm−1 strongly decay in intensity with the concentration of R6G decreasing. Nevertheless, Raman peaks are clearly observable at the concentration as low as 10−9 M, indicating that dendritic Au/ZnO could provide strong SERS signals. A renewable substrate would be therefore obtained if the target molecules absorbing on the substrate could be fully cleaned away by photocatalytic degradation. Thus, dendritic Au/ZnO substrate was chosen to test its recyclable capacity since dendritic Au has the strongest SERS enhancement and ZnO can degrade the target molecules via photodegradation.46 In a typical experiment, dendritic Au/ZnO substrate was immersed in R6G solution for 12 h and characterized by SERS D
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perhaps due to the highest specific surface area. Amperometric response shows that a signal of 10 nM NO can be obviously observed on dense Au NPs/ZnO/ITO (Figure 4b). The detection limit was calculated to be about 5 nM (S/N = 3) with a linear region from 10 to 500 nM (Figure 4b). To assess the renewable performance, the electrochemical sensor was first fouled with 5-HT by applying a constant potential (+0.6 V),22 followed by UV light regeneration. Figure 4c displays the cyclic voltammogram of K3[Fe(CN)6] of the dense Au NPs/ZnO/ITO electrode after passivation and regeneration. Compared to the original electrode, the fouled electrode almost has no response to K3[Fe(CN)6], while the signal can be efficiently recovered after 60 min of UV irradiation (12 mW/cm2), which can be attributed to the photocatalytic removal of the contaminant on the electrode by ZnO NRs.46 The regeneration performance of other nanostructured Au/ZnO/ITO electrodes was also investigated, and the results (Figure 4d) indicated that the electrochemical activity can be fully recovered with more than 85% recovery in three cycles. The loss of the electrochemical signal can be attributed to the incomplete degradation of contaminant, since the photochemical cleaning process only slightly decreases the electrochemical activity by about 3% (Figure S9). To investigate the recyclable electrochemical sensing performance, the dense Au NPs/ZnO/ITO electrode was used to detect NO release from HUVECs. After dropping 50 μL of HUVECs suspension (5 × 105 cells/mL in cell culture medium) onto the electrode (4 mm × 4 mm), the cells deposited onto the electrode, and cells staining results indicated that all the cells were alive (Figure 5a−c). After injected 10 μL of 10 mM L-Arg into the cell suspension, an obvious signal can be observed (black line in Figure 5d), which can be attributed to the NO release from HUVECs.50 To confirm that the signal was evoked by L-Arg stimulation, cells were simultaneously stimulated with L-NAME (a specific NOS inhibitor) and L-Arg, and there was no signal that can be observed (red line in Figure
detection. Then the substrate was immersed in ultrapure water with UV light for about 1 h to remove the adsorbate. Results (Figure 3c) show that the SERS signal of R6G almost disappeared after UV irradiation for 1 h due to the degradation of the target molecules, and the signal can well recover by loading molecule again. A ratio of Raman peak intensity around 1360 cm−1 of R6G was calculated by dividing the intensities of R6G-degraded and R6G-reloaded Au/ZnO by original detection signals (Figure 3d). After photocatalytic cleaning treatment, the dendritic Au/ZnO substrate can be used repeatedly with more than 90% recovery percentage (Figure 3d). Besides dendritic Au/ZnO substrate, other substrates can be also used as recyclable SERS substrate despite their relatively low sensitivity (Figure S8). Recyclable Electrochemical Detection on Au/ZnO Hybrids. Due to the high specific surface of ZnO NRs and excellent electrochemical activity of Au NPs, the Au/ZnO substrates also have the potential to be used as renewable electrochemical sensors with high sensitivity. To test the electrochemical performance of various electrodes with different shapes of nanostructured Au, NO was use as a probe due to its vital roles in the regulation of physiological and pathological processes.47−49 As shown in Figure 4a, compared to that of ZnO/ITO, the electrochemical activity of NO on all nanostructured Au/ZnO/ITO electrodes was greatly improved in terms of amplitude of oxidation current, which can be attributed to the high electrochemical performance of nanostructured Au.33 Among the gold-modified electrodes, dense Au NPs/ZnO/ITO displays the best electrochemical activity
Figure 4. (a) Cyclic voltammograms of dense Au NPs/ZnO/ITO (black line), Au NPs/ZnO/ITO (green line), conical Au/ZnO/ITO (blue line), sea-urchin-like Au/ZnO/ITO (pink line), dendritic Au/ ZnO/ITO (yellow line), and ZnO/ITO (red line) in the presence of 0.1 mM NO in deaerated PBS solution. (b) Amperometric curves of a dense Au NPs/ZnO/ITO electrode to a serial concentration of NO in a stirred deaerated PBS solution. A potential of +0.85 V (vs Ag/AgCl) was applied to the electrode. Inset: calibration curves of dense Au NPs/ZnO/ITO for increasing NO concentration. (c) Cyclic voltammograms of 1.0 mM K3[Fe(CN)6] in 1.0 M KCl on a dense Au NPs/ZnO/ITO electrode before fouling (black), after fouling (blue), and after being renewed by UV light. (d) Recovery efficiency of dendritic Au (black line), sea-urchin-like Au (red line), conical Au (blue line), Au NPs (green line), and dense Au NPs (pink line) obtained from the change of K3[Fe(CN)6] oxidation current with three cycles. Error bars represent standard deviations (n = 3).
Figure 5. Microscopy images of HUVECs on a dense Au NPs/ZnO/ ITO electrode: (a) bright-field, (b) fluorescence (stained by CalceinAM and PI), and (c) merged image of bright-field and fluorescence. (d) Amperometric response of the dense Au NPs/ZnO/ITO electrode toward NO release from HUVECs at different conditions. Inset: cyclic voltammograms of 0.1 mM NO. (e) Amperometric response of a fouled (red line) and renewed dense Au NPs/ZnO/ITO electrode (black line). Inset: cyclic voltammograms of 0.1 mM NO on fouled (red line) and renewed sensors (black line). E
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5d). To further exclude the interference of L-Arg, L-Arg was injected into a cell culture medium without cells and there was no signal produced. During the cell detection, the nonelectrochemical activity biomolecules in cell culture medium were easy absorbed on the electrode to cause passivation.51,52 Compared to the original electrode (inset in Figure 5d), the fouled electrode has poor electrochemical activity on NO (red line in the inset of Figure 5e) and there was almost no signal (red line in Figure 5e) that can be observed on the fouled electrode when it was used to cell detection. After UV irradiation, the electrochemical activity of the fouled electrode can be recovered about 90% (black line in the inset of Figure 5e) by photocatalytically cleaning the contaminant on the fouled electrode. When we used this regenerated electrode for cell detection, obvious signal can be obtained again (black line in Figure 5e), showing excellent photocatalytic cleaning performance for recyclable biosensing.
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (nos. 21375099, 91017013), the Specialized Research Fund for the Doctoral Program of Higher Education (20120141110031), and the Fundamental Research Funds for the Central Universities (2042014kf0192).
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CONCLUSIONS In conclusion, we constructed several renewable SERS and electrochemical sensors by photochemical synthesis of shapecontrolled nanostructured Au on ZnO NRs. By changing the additive in the HAuCl4 solution, several kinds of nanostructured Au have been obtained on ZnO NRs. Due to the high sensitivity of nanostructured Au and efficient photocatalysis of ZnO NRs, these composite structures not only have high sensitivity, but also have photocatalytic regeneration properties. Taking into account the properties of different structures, nanostructured Au/ZnO composites were used as SERS substrates, and the electrochemical sensor demonstrates the composites possess excellent biosensing and photocatalytic cleaning performance. SERS results indicated that the dendritic Au/ZnO composite has the strongest enhancement, and electrochemical results indicated that dense Au NPs/ZnO composite has the highest activity, which was successfully used for electrochemical detection of NO release from HUVECs. The renewal of both the SERS and electrochemical sensors can be easily achieved by photodegrading adsorbed probe molecules or biomolecules on the sensors. Taken together, this work uniquely enables shape-controlled synthesis and direct application of different nanostructured Au on ZnO, indicating great potential in construction of highly sensitive and photocatalytically renewable biosensors.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.5b04810. Nine figures, showing SEM images of dendritic Au prepared with 5 and 10 min of UV irradiation, SEM images of patterned dendritic Au, investigation of dendritic Au formation mechanism, reversible SERS behavior of several Au/ZnO substrates, and effect of UV irradiation on Au/ZnO/ITO electrochemical performance (PDF)
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DOI: 10.1021/acs.analchem.5b04810 Anal. Chem. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.analchem.5b04810 Anal. Chem. XXXX, XXX, XXX−XXX
