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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Robust and High Spatial Resolution Light Addressable Electrochemistry Using Hematite (α-Fe2O3) Photoanodes Daye Seo,† Sung Yul Lim,†,§ Jihye Lee,† Jeongse Yun,† and Taek Dong Chung*,†,‡ †
Department of Chemistry, Seoul National University, Seoul 08826, Korea Advanced Institutes of Convergence Technology, Suwon-si, Gyeonggi-do 16229, Korea
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ACS Appl. Mater. Interfaces Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 09/20/18. For personal use only.
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ABSTRACT: Light addressable/activated electrochemistry (LAE) has recently attracted attention as it can provide spatially resolved electrochemical information without using pre-patterned electrodes whose sizes and positions are unchangeable. Here, we propose hematite (α-Fe2O3) as the photoanode for LAE, which does not require any sort of surface modification for protection or facilitating charge transfer. As experimentally confirmed with various redox species, hematite is stable enough to be used for repetitive electroanalytical measurements. More importantly, it offers exceptionally high spatial resolution so that the “virtual electrode” is exactly as large as the light spot owing to the short diffusion length of the minority carriers. Quantitative analysis of dopamine in this study shows that the hematite-based photoanode is a promising platform for many potential LAE applications including spatially selective detection of oxidizable biomolecules. KEYWORDS: electrochemical imaging, light addressable electrochemistry (LAE), photoelectrochemistry (PEC), hematite, virtual electrode, dopamine
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INTRODUCTION The electrochemical method is an effective way to investigate the chemical properties of samples with good temporal and spatial resolution. Specifically, it allows quantitative measurements of biomolecules such as neurotransmitters and peptides, so that microelectrode arrays (MEAs) and probe-type electrodes have emerged to be the center of research interest in terms of imaging various in vitro and in vivo samples.1−3 An MEA consisting of a large number of electrodes enables simultaneous electrochemical measurements at multiple tiny sites. In spite of substantial advances in the electrode density and wiring process,4−6 the inherent problems with regard to the insulating space between the electrodes, that is, the blind spot for the electrochemical measurement and the adversities in manufacture and measurement remain unsolved yet. On the other hand, probe-type microelectrodes can be placed where desired and used to scan over the sample, if necessary, giving the spatially resolved electrochemical information. However, the experiments in practice require considerable dexterity, patience, and time to sophisticatedly manipulate the probe because the angle and the distance between the electrode and the substrate should be controlled precisely for reliable results.7,8 As a suggestion for these concerns, locally focused light was introduced to the semiconductor electrode.9−12 When the semiconductor absorbs photons of high enough energy, electron−hole pairs are created. Among them, the minority carriers migrate to the semiconductor surface and react with the redox species in the solution. The electrochemical reaction © XXXX American Chemical Society
is expected to occur where illuminated by the light as long as the excitons survive until the faradaic process and do not go far from where they are created. Thus, it is possible to induce a “temporary” electrode on the semiconductor substrate wherever and whenever we want without complicated electrical wiring process or the electrode-positioning procedure. This is called the light addressable or light-activated electrochemistry (LAE) method (Scheme 1). In spite of the fascinating strategy, only couple of studies have been reported mainly because the LAE semiconductor works only when the following conditions are fulfilled.10−12 First, the electrochemical reaction at the semiconductor should be controlled by turning on and off the light. Faster charge transfer between the analytes and the illuminated semiconductor surface is better for a higher signal-to-noise ratio. Second, sluggish lateral diffusion of the minority carrier generated by absorption of photons is critical for high spatial resolution. If the charge carrier easily diffuses away from the spot where it was created, the photoelectrochemical reaction takes place in the significantly wider region than the illuminated area. Third, the substrate must be stable enough to be a reliable electrode for reproducible quantitative analysis. For the purposes of live cell studies and clinical applications, in addition, the semiconductor itself and the operating conditions Received: June 29, 2018 Accepted: September 6, 2018
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DOI: 10.1021/acsami.8b10812 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces Scheme 1. Schematic Illustration of the Concept of LAEa
hole diffusion length in hematite is an attractive characteristic which may lead to high spatial resolution. We investigate how hematite works as the LAE substrate and demonstrate quantitative analysis of dopamine (DA), a representative neurotransmitter, to assess its potential as a new analytical method.
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EXPERIMENTAL SECTION
Photoanode Preparation. Hematite thin films were prepared by electrodeposition as reported previously.24,25 Fluorine-doped tin oxide (FTO)-coated aluminoborosilicate glass substrates (Solaronix) were cleaned by sonication with soap, deionized water, and isopropyl alcohol each for 10 min followed by blow drying with a stream of N2. Cleaned FTO substrates were used as the working electrode, and a platinum coil was used as the counter electrode. Electrodeposition was performed in 0.1 M FeCl2·4H2O at 60 °C by applying 1.2 V versus Ag/AgCl (3 M NaCl, Bioanalytical System, Inc.) reference electrode for 30 min under gentle stirring. The as-deposited films were annealed at 800 °C for 10 min in air to obtain crystalline αFe2O3 electrodes. Photoelectrochemical Measurement. All photoelectrochemical measurements were performed using a homemade Teflon three electrode cell and a CHI 440 electrochemical workstation (CH Instruments, Austin, TX). Hematite electrodes were the working electrodes whereas the platinum coil and Ag/AgCl (3 M NaCl, Bioanalytical System, Inc.) were the counter electrode and the reference electrode, respectively. A portion of hematite was selectively exposed to the solution using an electroplating tape (3M Electroplating Tape 470). The exposed area was 0.0314 cm2 unless otherwise stated. A 150 W Xenon arc lamp equipped with an AM 1.5 solar filter (100 mW cm−2) was used to illuminate the whole area of the exposed electrode. To scale down the light size and illuminate specific area, we used a laser (488 nm He/Ne laser, LASOS Lasertechnik GmbH) as part of a home-built micro-Raman system (Dongwoo Optron Co., Ltd). When the laser was focused on hematite with 20× objective lens, the diameter of the light was about 5 μm. The laser intensity was 2 mW and the location of the illuminated area was manipulated with a nanopositioning system in which a piezoelectric stage was embedded (Nano-LP300, Mad City Laboratories, Inc.). All photoelectrochemical measurements were carried out by shining light on the electrodes through the electrolyte side. K4Fe(CN)6·3H2O, ferrocenecarboxylic acid, 1,1′-ferrocenedicarboxylic acid, and K3IrCl6 were purchased from Sigma-Aldrich and used as received. Solutions were all maintained at pH 7.4 by 0.1 M phosphate buffer. Co−Pi Deposition. Co−Pi was deposited onto hematite by a slightly modified version of the previously published procedure.26 The same three-electrode cell for the photocurrent measurement was used. 30 cycles of potential pulse between 0.4 V (vs Ag/AgCl) for 5 s and 0 V (vs Ag/AgCl) for 5 s were applied to hematite in a solution of 0.5 mM cobalt nitrate (Sigma-Aldrich) in 0.1 M potassium phosphate buffer at pH 7 under 455 nm light-emitting diode (LED) (Mightex, PLS-0455-030-S) illumination. For the generation of optical pattern images, a homemade digital micromirror device (DMD) display module (Uninanotech) equipped with the LED was installed on a BX43 Olympus upright microscope (Scheme S1). The scanning electron microscopy (SEM) image of the patterned Co−Pi was taken using SUPRA 55VP (Carl Zeiss) at an accelerating voltage of 10 kV. DA Quantitative Analysis. DA (3-hydroxytyramine hydrochloride, TCI chemicals) solutions were prepared in 0.1 M phosphate buffer (pH 7.4). The DA concentrations of the solution were from 1 to 100 μM. We employed the same experimental setup with the laser (488 nm) for the photoelectrochemical measurement. In the photocurrent measurement, the light was switched on for 10 s under an external bias of 0 V (vs Ag/AgCl). We sampled the steadystate currents that were obtained at 9 s after turning on the light to construct the calibration curves. Characterization. The top and cross-sectional view of the hematite photoanode were taken using SUPRA 55VP (Carl Zeiss) at an accelerating voltage of 2 kV in National Instrumentation Center
a
An n-type semiconductor can drive oxidation reaction with the aid of light. The region where the photoelectrochemical reaction takes place is determined by the size (d in the scheme) of the focused light and the lateral extent to which the photogenerated holes diffuse in the semiconductor (diffusion length, Lp).
including the wavelength of the light source should be biocompatible. A representative candidate for the LAE substrate is crystalline silicon (c-Si) decorated with a self-assembled monolayer (SAM) to which the electron mediator is attached.11,13 The SAM protects silicon in aqueous solution without an oxide layer that hinders the electron transfer between the silicon substrate and the solution phase. However, extremely long diffusion length of the minority carrier in c-Si is a critical drawback leading to severe discrepancy between the illuminated area and the electrochemically active region.14,15 Another suggestion is the quantum dot (QD)-immobilized electrode.10,16 QDs are semiconductor particles electrically disconnected with each other preventing the minority carrier from diffusing out. QDs also offer a good way to study a multistep electron-transfer cascade from the biological elements such as enzymes to the electrode.17,18 Nonetheless, transition-metal chalcogenide QDs are not free from the issues on stability, biocompatibility, and uniform functionality.19 With regard to this research interest, amorphous silicon (aSi) is an intriguing candidate.12,20,21 Its p−i−n structure generates internal potential gradient, helping with driving the electrochemical process. Owing to its short diffusion length and the facile charge transfer through the thin native oxide, it can serve as a pretty successful photocathode requiring neither surface protection nor functionalization. We need photoanodes that have been rarely reported, however, as many important electroactive biomolecules (e.g. catecholamines, serotonin, and peptides containing tryptophan or tyrosine) are oxidizable. Herein, we propose a hematite thin film as the LAE substrate. Hematite photoanodes have been intensively studied for water splitting because of its earth abundance, non-toxicity, high chemical stability, and narrow bandgap (1.9−2.2 eV).22,23 The advantages of hematite toward water splitting are also valid for LAE. For example, the excellent stability of hematite in the aqueous environment makes it free from the protection layer or surface modification when operating in solution. It is interesting that the short hole diffusion length (Lp, 2−4 nm), which has been considered as a critical weakness in view of water splitting efficiency,22 can be converted into valuable benefit for LAE. While the entire area of the hematite surface exposed to the solution participates in water splitting, LAE selectively exploits a small part of the hematite, which should be as large as the region illuminated. In this sense, the short B
DOI: 10.1021/acsami.8b10812 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces for Environmental Management of Seoul National University. The Xray diffraction (XRD) analysis was carried out using an X-ray diffractometer (New D8 ADVANCE) at the Seoul National University Research Institute of Advanced Materials. Capacitances were measured to Mott−Schottky plots using Gamry Reference 600 (Gamry Instruments, Warminster, PA). The same three-electrode cell for the photocurrent measurement was used with a 0.1 M phosphate buffer (pH 7.4). A sinusoidal modulation of 10 mV in amplitude was applied at frequencies of 0.5 and 1 kHz.
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RESULTS AND DISCUSSION The most essential requirement for LAE is the ability to control the photoelectrochemical reaction with the light. In order to demonstrate that, the photoelectrochemical response of hematite was characterized with cyclic voltammetry in a 0.1 M phosphate buffer (pH 7.4) containing 1 mM of [Fe(CN)6]4− as a hole scavenger. As shown in Figure 1a, oxidative current flowed only when hematite was illuminated by simulated AM 1.5 G irradiation (100 mW cm−2). The presence of a peak around 0.3 V (vs Ag/AgCl) indicates that the oxidation of [Fe(CN)6]4− on the illuminated hematite electrode was diffusion-controlled. Without the hole scavenger, negligible photocurrent passed even when the applied potential was quite positive (∼0.5 V vs Ag/AgCl) because of the sluggish kinetics for the oxygen evolution reaction at the hematite/solution interface (Figure 1a, inset).22,23 This result assures that hematite responds to electrochemically oxidizable species in the solution when illuminated. In contrast to the small and stable background signal, its photoelectrochemical faradaic current is high enough to attempt quantitative analysis in a low concentration range. The light source was replaced by the laser (488 nm) to see if LAE still works when light spot gets smaller to microscale (Figure 1c). A laser beam was focused on the substrate through ×20 objective lens to make the size of the illuminated area less than 5 μm in diameter, which is comparable to the commercially available ultramicroelectrode. The potential of hematite was maintained constant at 0.1 V (vs Ag/AgCl), and the laser beam was chopped periodically (Figure 1b). When the laser was turned on, a sharp photocurrent appeared immediately and decayed to reach a steady-state photocurrent. As confirmed by the j−E curve in Figure 1a, water oxidation cannot take place at this potential. Therefore, [Fe(CN)6]4− oxidation is responsible for the anodic current. Instantly after turning off the light, we observed a slight cathodic current related to the discharge of the surface state, and then the current was restored to the level attained before the light perturbation.27,28 The immediate response to the light shows that it is possible to control the chemical reaction by the light with high temporal resolution. XRD analysis and SEM imaging were carried out for further characterization of hematite employed in this study. The XRD pattern (Figure S1) confirmed that the iron oxide film electrodeposited on FTO was hematite. As shown in the top and the cross-sectional view of SEM images (Figure S2), the hematite film had quite a uniform surface with the thickness of about 50 nm that is comparable to previously reported values.24,25 The flat-band potential (EFB) of hematite calculated from the Mott−Schottky plot (Figure S3) was −0.23 V (vs Ag/AgCl) in pH 7.4 phosphate buffer. On the basis of this value, we determined the potential positive than −0.23 V (vs Ag/AgCl) as the working potential of hematite-based LAE to induce the depletion layer in the semiconductor. All the following experiments were conducted under the condition
Figure 1. (a) CVs of 1 mM [Fe(CN)6]4− in 0.1 M phosphate buffer (pH 7.4) under illumination (red solid line) and in the dark (red dotted line) at the hematite electrode. Inset: Black solid line and black dotted line represent the CVs in the same solution without [Fe(CN)6]4− under illumination and in the dark, respectively. The entire area of the photoanode was illuminated, and the scan rates were 50 mV/s. (b) Transient photocurrent of 1 mM [Fe(CN)6]4−. While hematite was biased to 0.1 V (vs Ag/AgCl), the laser (488 nm) was chopped periodically. The light spot size was about 5 μm in diameter. The arrows show the moments that the laser was on and off. (c) Experimental scheme of hematite-based LAE when using the laser as a light source.
that the hematite was biased positively with respect to the EFB. The band edge positions of hematite estimated based on the measured EFB and well-known bandgap (2.1 eV)22,25 are shown in Scheme S2. Theoretically, any redox species whose reduction potentials exist between the ECB and the EVB can be oxidized by the photo-generated holes in the n-type semiconductor.29 Some redox species other than [Fe(CN)6]4− (ferrocenecarboxylic acid, 1,1′-ferrocenedicarboxylic acid, and iridium(III) chloride) indeed showed photoresponse on illuminated hematite (Figure S3). For all the cases, the photo-responses were similar to that of [Fe(CN)6]4− reflecting the versatility of hematite-based LAE. For LAE developing into more elaborate analytical tool with high spatial resolution, the region where the photoelectrochemical reaction takes place needs to be as close as possible to C
DOI: 10.1021/acsami.8b10812 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 2. (a) CV of hematite in the presence of the cobalt precursor under illumination (red solid line) and in the dark (black solid line). Inset: the current response during the pulse electrodeposition of Co−Pi. The white zone represents the period for oxidation, and gray one shows where opencircuit potential is applied. (b) Comparison between the radii of the light and patterned Co−Pi. The radii of the circular spots were measured using ImageJ software, and the standard deviations were too small to be represented. (c) One of the bitmap images loaded to the DMD (left) and the images of electrodeposited Co−Pi (center: optical, right: SEM). In the SEM image, the red dotted line represents the boundary between the illuminated and the unilluminated regions.
the spot on which the light sheds. The electrochemically active area is called “virtual electrode” in the sense that the electrode does not exist physically but is transiently present during illumination.12,20 To see how close the virtual electrode on hematite is to the area illuminated by the light, we deposited Co−Pi dot patterns with a variety of diameters of the light spots using a 455 nm LED combined with a DMD as a light source. Co−Pi, a well-known metal oxide water oxidation catalyst, can be deposited upon the oxidation of Co2+ to Co3+ in a solution of 0.5 mM cobalt nitrate in a 0.1 M potassium phosphate buffer at pH 7.30 The cyclic voltammograms (CVs) obtained prior to electrochemical patterning show how the hematite works as a photoanode in the cobalt precursor solution (Figure 2a). As the LED illuminated all over the photoanode, large photooxidation current appears, which is consistent with the literature that reported the photo-assisted Co−Pi deposition.26 The photogenerated holes react with the cobalt ions in the solution leaving the cobalt-based catalyst on the hematite surface. Therefore, we assume that the region the Co−Pi deposited indicates where the virtual electrode existed. DMD projects the optical pattern corresponding to that drawn in the computer. Exploiting this device, we can easily modify LED light into the desired shape that is reflected on the hematite surface. Six bitmap images of 4 dots in different radii from 20 to 200 μm were loaded to the DMD. One of the bitmap images is shown in Figure 2c. We patterned 4 dots rather than a single dot to ensure the validity of the measurements. While the patterned LED light was continuously focused on the hematite, potential pulses were applied periodically, that is oxidation potential at 0.4 V (vs Ag/AgCl) for 5 s followed by open circuit potential (0 V vs Ag/AgCl) for 5 s was repeated 30 times. Oxidation of the cobalt ions was
expected to occur during the period at 0.4 V (Figure 2a, inset). As the potential pulses were repeatedly applied, four circles in the bitmap image uploaded to the DMD turned into Co−Pi spots on the hematite surface (Figure 2c, center). The radii of electrodeposited Co−Pi dots in the array estimated from the optical image were averaged and compared with that of the light (Figure 2b). They showed less than 1 μm discrepancy for all the sizes. The slope of the plot in Figure 2b, 1.002, also demonstrated that the cobalt-phosphate catalysts were formed exclusively at the illuminated area. To look into the border line between illuminated and non-illuminated regions more closely, we deposited Co−Pi much thicker by applying 3600 cycles of potential pulse and took the SEM image (Figure 2c, right). Again, the distinct boundary assures that the virtual electrode was exclusively created where the light was irradiated. Such a consistency which is attributed to the short diffusion length of holes in hematite is what has never been obtained from other n-type semiconductors considered for LAE before. In the case of c-Si, the virtual electrode estimated by scanning electrochemical microscopy is 5 times larger than the light pointer.31 The spatial resolution of QDbased LAE has not been investigated experimentally yet. The dots smaller than 40 μm in diameter were unable to be patterned because of the focusing ability of the optical setup in the present work. However, higher resolution, for example, several micrometers, could be easily achieved by using the laser as a light source because the focused light size is solely responsible for the spatial resolution of hematite-based LAE. DA is an important oxidizable neurotransmitter which is related to several neural diseases (e.g. Parkinson’s disease and schizophrenia) and reward mechanism.32−34 To elucidate the pathway responsible for the neural functions, quantitative D
DOI: 10.1021/acsami.8b10812 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
tration. The calibration curve constructed based on the steadystate photocurrent is shown in Figure 3b. The linear dynamic range was from 1 to 100 μM and the limit of detection (LOD) defined as the lowest concentration whose signal is above the three-time standard deviation of the background signal was 0.67 μM. The LOD is low enough to detect extracellular concentration of DA when an electrical or chemical stimulation is applied to the neurons.37−39 Some experimental conditions influencing the analytical performance were investigated. First, the intensity of the laser can alter the calibration curve (Figure S5). The photoelectrochemical reaction is widely assumed to be a first-order reaction of the photoinduced minority charge carriers at the semiconductor and the redox species in the solution.40 Thus, it is rational prediction that the photocurrent will be proportional to the intensity of the light. As expected, higher laser intensity gave rise to the photocurrent. In other words, the stronger light source leads to the more sensitive detection of DA. Otherwise, increasing laser intensity does not improve the LOD because the standard deviation of the background signal also rises as a consequence of the overall photocurrent increase (Table S1). The size of the light spot affects the calibration curve in the same way that the light intensity did. This is not surprising because the photocurrent increases linearly as a function of the spot size (Figure S6). The whole hematite electrode area exposed to the solution is also related to the LAE operation. Figure S7 shows the photocurrent responses from the hematite electrodes of different diameters (2, 4, and 6 mm). Larger hematite suffered from more serious noise which directly hinders the LOD. This result implies that the trade-off between the scope where the virtual electrode could be induced and the LOD needs to be considered when using LAE as a practical analytical tool. Stability is another issue for LAE. As aforementioned, hematite itself has the exceptionally inert surface so as to give quite stable response.22,23 However, stability could be still problematic when a nonconducting film is formed on the electrode as a consequence of the chemical reaction coupled to the heterogeneous electron transfer. When it comes to DA, polymerization of the product of the oxidation reaction leaves a polydopamine film, which hinders the electron transfer and causes decrease of the photocurrent.41 To assess the stability of the hematite virtual electrode during the DA detection, we repeated tens of measurements of 100 μM DA using the same experimental configuration used above. During the successive measurements, the laser was focused on a single spot. The photocurrent from the virtual electrode decreased gradually as
analysis or imaging of DA based on electrochemistry has been performed for decades.35,36 Therefore, we chose DA as the model analyte to prove how well hematite-based LAE works for quantitative analysis of biogenic molecules. The 455 nm laser was focused onto the hematite through ×20 objective lens. The laser spot operated as a virtual electrode of 5 μm in diameter. Figures 3a and S4 show the photocurrent under the
Figure 3. Quantitative analysis of DA. All the DA solutions were prepared in the pH 7.4 phosphate buffer and the laser intensity was 2 mW. (a) Photocurrent from hematite as a function of the concentration of DA. The laser was turned on at 5 s and off at 15 s. (b) Calibration curve for DA. Steady-state photocurrent corrected with the dark current was used to construct the plot. The currents were extracted from Figure 3a and other three measurements. Inset: the low concentration region of the calibration curve.
condition of light pulse for 10 s and constant electrode potential, 0 V (vs Ag/AgCl). Basically, the current response in the presence of various concentrations of DA was similar to that in the presence of [Fe(CN)6]4− and the magnitude of the photocurrent increases in proportional to the DA concen-
Figure 4. Stability of hematite-based LAE. (a) Photocurrent in the presence of 100 μM DA was measured for 50 times, out of which 5 sampled data were overlaid. The location of the laser was fixed at a single spot. (b) Steady-state currents extracted from the photocurrents in (a). E
DOI: 10.1021/acsami.8b10812 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces Notes
the test was repeated. After 50 times of measurements, the magnitude of photocurrent reduced to 80% of the initial value (Figure 4a,b). Moving the light spot to other regions, we performed the same procedure. Five other virtual electrodes, which are five different spots selected randomly showed almost identical photocurrent responses (Figure S8). The degradation of photocurrent at one spot as a result of the continuous reaction exerted negligible influence to the response from other regions. To our best knowledge, this is the first report on the stable and reproducible quantitative analysis of the oxidizable analyte using LAE.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (no. 2017R1E1A1A01074236), and CABMC (Control of Animal Brain using MEMS Chip) funded by Defense Acquisition Program Administration (UD140069ID).
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CONCLUSIONS In this study, we introduced hematite as a semiconductor substrate for the light addressable electrode and evaluated its analytical performance. We confirmed that the physical and chemical characteristics of hematite is appropriate for the reliable photoanode in LAE. Hematite was capable of switching on and off the oxidation reaction of various redox species with light. By virtue of the short diffusion length of minority carrier in hematite, the reaction takes place exclusively where the light illuminates. In addition, the virtual electrode induced on hematite detects DA with the moderate LOD (0.67 μM), along with good stability. Even though only a few previous works have been reported on the hematite photoanode for analytical application,42 the synergistic utilization with LAE will offer an alternative opportunity for hematite as a successful semiconductor substrate for electrochemical imaging. Moreover, hematite allows DA detection at rather low potential (0 V vs Ag/AgCl), potentially favorable for suppressing interference of other oxidizable species. Therefore, LAE based on hematite suggests a new way to analyze the biological specimens.
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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/acsami.8b10812.
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REFERENCES
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Experimental scheme of hematite-based LAE using LED (455 nm) combined with DMD as a light source, XRD spectra of the bare FTO and hematite electrode, SEM images of the bare FTO and hematite, Mott−Schottky plot and CVs of various redox species, band diagram of hematite, photocurrents from hematite electrode for low concentrations of DA, calibration curves for DA as a function of laser intensity, quantitative analysis of DA at a series of laser intensities, photocurrent as a function of the area of the illuminated spot, effect of the exposed hematite area, and reproducibility of hematite-based LAE (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Sung Yul Lim: 0000-0002-2838-6967 Taek Dong Chung: 0000-0003-1092-8550 Present Address
§ Department of Energy Conversion and Storage, Technical University of Denmark, 2800 Lyngby, Denmark.
F
DOI: 10.1021/acsami.8b10812 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
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DOI: 10.1021/acsami.8b10812 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
