Observation of Local Redox Events at Individual Au Nanoparticles

Article pubs.acs.org/JPCC

Observation of Local Redox Events at Individual Au Nanoparticles Using Electrogenerated Chemiluminescence Microscopy Shanlin Pan,* Jia Liu, and Caleb M. Hill Department of Chemistry, The University of Alabama, Tuscaloosa, Alabama 29487-0336, United States S Supporting Information *

ABSTRACT: Au nanoparticles (NPs) are known to be able to enhance the oxidation of tripropylamine (TrPA) and the generation of electrogenerated chemiluminescence (ECL) from Ru(bpy)32+ when TrPA is used as a coreactant. Local redox activities of single Au NPs can therefore be investigated using the combined methods of voltammetry and ECL imaging. Our study shows that ECL generation at individual Au NPs increases with particle size (diameters from 30 to 300 nm) and is affected by the local chemical and charge transfer environment of the NPs. Such an ECL detection scheme can allow one to study the local redox activities of single nanoparticles with improved spatial resolution. ECL at single Au NPs shows slight temporal variations in intensity attributed to the oxidation and reconstruction of small clusters on the Au surface during ECL generation. Quantitative agreement between calculations and experiment concerning the effect of particle size and electrode potential on spatial and transient ECL profiles is presented. detected when they are immobilized onto a microelectrode.13 Recent studies show that ECL can be used to detect single collision events of single nanostructures such as graphene sheets14 and Pt NPs with an ultramicroelectrode (UME).15 These UME-based ECL methods provide a powerful platform for analyzing single redox events at individual catalytic nanostructures, but the nanostructures are analyzed one at a time and in a transient manner (i.e., the duration of the collision event). In order to rapidly measure local redox reactions from individual NPs, combined optical and electrochemical methods are highly desirable, as shown by our recent single molecule and NP spectroelectrochemistry studies.16,17 Our recent work shows that ECL at a Ti or TiO2 electrode can be effectively activated by electrodepositing Au particles onto its surface,18 and ECL collection at single Au particles was attempted but the ECL was collected in a slow scanning confocal configuration and as a result information on the structure−function relationship of single Au NP electrodes and temporal/spatial characteristics of local ECL generation was not obtained. This issue can be addressed through a combination of ECL or fluorescence, scanning electron microscopy (SEM), and dark field scattering (DFS) as recently reported by our laboratory.19 Here, we present a spectroelectrochemical method for the rapid determination of local ECL generation at individual Au NPs in an improved wide-field ECL collection configuration. The ECL-based imaging of local redox reactions and digital simualtions are correlated, demonstrating an effcient method

1. INTRODUCTION Electrogenerated chemiluminescence (ECL) is a process in which light is generated through the relaxation of electrochemically generated excited states.1 ECL has drawn great attention and been vigorously studied since the first ECL experiment was reported in 1964.2 In a prototypical ECL system, Ru(bpy)32+ emits light when a sufficient electrode potential is applied in an aqueous solution in the presence of tripropylamine (TrPA) as a coreactant.3,4 This technique can serve as a highly sensitive detection scheme for measuring molecular binding and recognition events5,6 with good repeatability, high sensitivity, and low background. ECL has recently been applied to study individual nanostructures such as single polymer NPs,7 demonstrating the possibility of detecting small light emitting species at the nanometer scale for more accurate and precise quantitative applications. More commonly, small nanostructures are probed by other optical techniques such as light scattering. For example, the binding of single protein molecules to Au nanorods (NRs) can be detected by measuring shifts in the scattering spectrum of the plasmonic Au NRs.8 Fluorescencebased detection has also been employed for chemical detection at the nanometer scale.9−11 However, both scattering and fluorescence-based methods suffer from high background signals due to the scattering of the incident light or fluorescence from the substrate or solution. Neither of these issues is involved in ECL detection because no external light source is used for ECL generation. Numerous studies suggest that electrode size plays an important role in the temporal and spatial resolution of ECL detection. For example, as many as 40 000 redox molecules can be probed using an microelectrode as shown by Wrighton and co-workers.12 Redox reactions at >1000 individual NPs can be © 2015 American Chemical Society

Received: July 15, 2015 Revised: October 30, 2015 Published: November 3, 2015 27095

DOI: 10.1021/acs.jpcc.5b06829 J. Phys. Chem. C 2015, 119, 27095−27103

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The Journal of Physical Chemistry C

electrode electrochemical system with a transparent ITO working electrode, which supports either presynthesized or electrodeposted Au NPs. This setup also allows us to image single Au NPs via dark field mode as described in our previous study.17,19 In-situ particle size measurements were carried out by tracking the deposition of individual NPs on Al-indexed ITO via dark field scattering. Briefly, individual particle sizes were calculated from the measured dark field scattering intensity and particle size distribution via SEM according to Mie theory. Then, for each individual gold nanoparticle, the calculated particle size and ECL intensity was correlated. Immediately after dark field scattering measurements, an electrolyte solution containing 0.1 M tripropylamine (TrPA), 5 mM Ru(bpy)3Cl2, and 0.1 M phosphate buffer at pH 7.4 was added to the cell, taking care not to move the sample. Cyclic voltammetry (CV) was performed from 0 to 1.2 V vs Ag QRE at a scan rate of 0.1 V/s and the ECL signal generated at the working electrode was collected by an EM-CCD camera using a 100× oil-immersion objective (NA = 1.3) or a 40×, NA = 0.75 objective. The exposure time of EM-CCD was 0.03s. 2.6. Data Extraction. Custom MATLAB programs were used for all data analysis. The dark field scattering and ECL data were input as Tagged-Image File Format (TIFF) image stacks. Every frame of the given TIFF image stack was combined to create a “Raw” image. First and second derivative images were then calculated from the “Raw” image. A “Composite” image was produced by mixing the Raw, first derivative, and second derivative images together visually in a GUI. Active pixels in the composite image were located by setting an appropriate intensity threshold, and adjacent pixels were grouped together into “spots”, each representing a single nanoparticle. ECL trajectories for each single nanoparticle were extracted by summing the pixel intensities within one spot on a frame-by-frame basis (ECL trajectories of self-assembled single Au NPs were extracted by choosing the max ECL intensity of all pixels within one spot). This method was also used for analysis of the scattering images. 2.7. SEM Analysis and Digital Simualtions. Scanning electron microscopy of the samples was carried out after the ECL experiments, using a JEOL 7000 FE-SEM operating at a 20.0 keV accelerating voltage. Digital simualtion of ECL generation at an Au NP electrode was performed with DigiElch 7.0 (ElchSoft, Germany), and 3D ECL profiles of an Au NP were obtained using an electrochemistry module of COMSOL Multiphysics (COMSOL 5.1).

for studying local redox reactions at individual NPs with small light scattering cross sections.

2. EXPERIMENTAL SECTION 2.1. Reagents and Materials. Ru(bpy)3Cl2 and HAuCl4 (∼30% solution by weight in dilute HCl) were purchased from Sigma-Aldrich. Tripropylamine (TrPA) was purchased from Acros Organics. Commercial Au NP solutions (average particle diameters of 15, 50, 150 nm) were purchased from BBI Solutions. Thioglycolic acid was purchased from Alfa Aesar. The phosphate buffer was prepared by mixing NaH2PO4 (Acros Organics, 0.05 M) and Na2HPO4 (Fisher Scientific International, Inc., 0.15 M) in DI water and adjusting pH to 7.4 by with HCl. Three mm Ni binary TEM grids were purchased from SPI Supplies/Structure Probe, Inc. ITO glass slides (0.17 mm thick, 18 × 18 mm2 area) were purchased from SPI Supplies. 2.2. Preparation of Indexed ITO. Indexed TEM grids were taped onto ITO slides. Then, patterned aluminum films were deposited via thermal evaporation (rate of ∼1 Å/s, pressure of 10−6 Torr) through TEM grids onto room temperature ITO substrates suspended roughly 20 cm above the deposition boat. The film thickness, roughly 40 nm, was monitored by a quartz crystal microbalance. 2.3. Electrodeposition of Gold Nanoparticles (Au NPs). ITO coated cover glass slides were sequentially sonicated in 25g/L acetone solution of KOH, DI water, acetone, DI water and was then treated with UV−O3 (UV/Ozone procleaner, Bioforce nanosciences) after drying under a N2 stream. The electrodeposition was carried out with a three electrode system including cleaned ITO glass as the working electrode, platinum wire as the counter electrode and a silver wire as quasireference electrode. A solution containing 1 M NaCl, 0.01 wt % HAuCl4 and 0.1 M phosphate buffer at pH 7.4 was added to the electrochemical cell and potential pulses of −0.05 V or −0.1 V vs Ag quasi reference electrode (EQRE ≈ 0.36 V vs SHE) were applied 5 times to electrodeposit Au NPs onto the ITO substrate. The time span of each potential pulse was 0.1, 0.3, or 0.5 s for different samples to vary the size of the Au NPs. 2.4. Self-Assembly of Au NPs. ITO slides were immersed into an aqueous solution of 1 mM of thioglycolic acid and 2.9 × 10−6 M of 15 nm Au NPs (or 9.3 × 10−8 M of 50 nm Au NPs, or 3.4 × 10−9 M of 150 nm Au NPs) for 8 h. The ITO slides were then thoroughly rinsed with DI water to remove any unassembled Au NPs. 2.5. ECL Experiment Setup. Figure 1 shows a schematic of our ECL generation and imaging setup composed of a three-

3. RESULTS AND DISCUSSION 3.1. ECL Imaging and Particle Size Dependence of Au NPs Electrodeposited onto ITO. To visualize single Au NP ECL with the imaging configuration shown in Figure 1, a threeelectrode system was used to electrodeposit (or self-assemble) Au NPs onto an ITO glass slide. ECL generation was carried out in the same cell with a different electrolyte solution. ECL was detected by an Electron Multiplying Charge-Coupled Device (EM-CCD) camera and analyzed using custom MATLAB programs as described in our previous publications.19,20 The ECL mechanism for Ru(bpy)32+ and TrPA is well documented in the literature.21 Briefly, both Ru(bpy)32+ and TrPA are first oxidized to produce Ru(bpy)33+ and TrPA radicals, respectively, and these two energetic redox species will react to produce excited state of Ru(bpy)32+, which emits light via radiative relaxation to the ground state. The oxidation of TrPA is kinetically favored at Au as compared to ITO, as shown

Figure 1. Schematic of apparatus used to measure ECL generation at single Au NPs with the combined methods of ECL microscopy and electrochemistry in a three-electrode configuration. 27096

DOI: 10.1021/acs.jpcc.5b06829 J. Phys. Chem. C 2015, 119, 27095−27103

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Table 1. Electrodeposition Conditions and Particle Sizes for Different Au NP Samples potential (V) vs SHE

deposition time (s)

0.147 0.120 0.115 0.097

0.1 0.5 0.3 0.5

Au NP diameter (nm) 29 72 83 310

(±2) (±4) (±6) (±30)

ECL intensity (counts/10 ms) 4000 6400 7500 15000

(±1000) (±200) (±300) (±3000)

# of particles analyzed 57 344 68 124

in Figure 2A. No appreciable ECL signal is detected at the bare ITO electrode at potentials 1 μm due to the increase of the diffusion layer thickness and differences in the concentration profiles as well as the short lifetime of radical ions near the (comparatively) large electrode. The simulated ECL

heterogeneities in ECL intensity for electrodeposited, polydisperse Au NPs and presynthesized Au NPs are illustrated in Figure S1 and summarized in Table S1. It should be noted that such ECL heterogeneity caused by extensive heterogeneity in the local conductivity of ITO is expected to be more severe in the case of the self-assembled Au NPs-modified ITOs than electrodeposited Au NPs if monodisperse Au NPs can be obtained electrochemically. This is because electrodeposited Au NPs can be selectively deposited at the more conductive sites. The ECL intensity distribution from individual electrodeposited Au NPs on an indexed ITO substrate does show a narrow ECL distribution as shown by Table 1. Such heterogeneities can be decreased by increasing the NP population sampled. 3.6. Digital Simualtion of Single Au NP ECL. We hypothesize that the total ECL intensity from single Au NPs increases linearly with the particle diameter if the diameter of the particle is less than 1 μm becasue of the spherical mass transfer profile for a small electrode. Digital simulations were obtained by using finite difference methods with three charge 27101

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intensities generally agree with the experimental correlation of ECL peak intensity and particle size for more than 100 Au NPs with particle sizes ranging from 30 to 400 nm as shown in Table 1 and Figure 8. Figure 10 A shows a 3D ECL intensity profile from the center of an 80 nm Au NP with its maximium at the NP surface at a potential of 1.6 V (vs SHE) near the limiting current region as shown by Figure 9A. The calculation was done using COMSOL 5.1 with an electrochemistry module by only considering redox reaction 1 and 2, and following chemical reactions 4, 7, 8, and radiative decay eq 12 as listed in Table 2. Detailed simulation parameters and boundary conditions are included in the SI. Only 1/8 of the spherical ECL profile is shown in order to better illustrate the ECL distribution from the Au NP center. The ECL is localized near the Au NP surface when the ECL starts taking off near around 1.0 V and its radius increases as the potential is linearly scanned at a rate of 100 mV/s. A 1D ECL profile from the center of the particle is shown in Figure 10B with maximium ECL intenesity localized at its surface. This cannot be physically true because of surface luminescence quenching by the metal, but our ECL imaging technique cannot resolve the quenching effect at such short distances. In addition, short-range ECL extinction by a 80 nm Au NP can be negligible in comparison to the mass transfer controlled ECL emission region that extends far from the Au surface up to 500 nm. Therefore, the total ECL signal would still be at the central location of a single Au NP as shown by our data in Figure 7. The calculated ECL spot size for a 80 nm Au NP has a comparable fwhm to that measured in our experiments as shown in Figure 7.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (S.P.). Author Contributions

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 material is based upon work supported by National Science Foundation (NSF) under Award Number 1508192. We thank the Central Analytical Facility (CAF) of the University of Alabama for its major surface characterization facility support. S.P. also acknowledges NSF for supporting his COMSOL simulation effort under Award Number 1539035.

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4. CONCLUSIONS To conclude, the detection of ECL generated at single Au NPs and its correlation with NP size and electrode potential is demonstrated for both electrodeposited and presynthesized Au NPs. ECL at single Au NPs shows slight temporal variations in intensity attributed to the oxidation and reconstruction of small clusters on the Au surface during ECL generation. It should be noted that our ECL imaging technique can be used for large Au particles (Figure S4) and Au nanowires (Figure S5) in order to study their local redox activities. The simulation of the correlation between spherical NP size and ECL intensity is achieved and agrees with experimental data. Insights into the local redox events occurring at Au NPs including heterogeneities caused by differences in size and surface oxidation are presented and discussed. Such measurements at the single NP level could prove invaluable for future studies in ultrasensitive electrochemical sensing, energy conversion, and catalysis.

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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.jpcc.5b06829. Quantitative comparison of heterogeneities in particle size and ECL intensity, effect of chloride ions on the stability of ECL, a high resolution SEM/ECL image of Au NPs, ECL imaging for micron sized Au particles and Au nanowires, and detailed single Au NP ECL simulation report (PDF) 27102

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