Photoinduced Electron Transfer Process Visualized on Single Silver Nanoparticles Gang Lei,† Peng Fei Gao,*,† Tong Yang,† Jun Zhou,† Hong Zhi Zhang,† Shan Shan Sun,‡ Ming Xuan Gao,‡ and Cheng Zhi Huang*,†,‡ †
Key Laboratory of Luminescent and Real-Time Analytical Chemistry (Southwest University), Ministry of Education, College of Pharmaceutical Sciences, Southwest University, Chongqing 400716, P. R. China ‡ College of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, P. R. China S Supporting Information *
ABSTRACT: Understanding the photoinduced electron transfer (PET) mechanism is vital to improving the photoelectric conversion efficiency for solar energy materials and photosensitization systems. Herein, we visually demonstrate the PET process by real-time monitoring the photoinduced chemical transformation of p-aminothiophenol (p-ATP), an important SERS signal molecule, to 4,4′dimercaptoazobenzene on single silver nanoparticles (AgNPs) with a localized surface plasmon resonance (LSPR) spectroscopy coupled dark-field microscopy. The bidirectional LSPR scattering spectral shifts bathochromically at first and hypsochromically then, which are caused by the electron transfer delay of p-ATP, disclose the PET path from p-ATP to O2 through AgNPs during the reaction, and enable us to digitalize the corresponding electron loss and gain on the surface of AgNP at different time periods. This visualized PET process could provide a simple and efficient approach to explore the nature of PET and help to interpret the SERS mechanism in terms of p-ATP. KEYWORDS: dark-field microscopy, localized surface plasmon resonance, p-aminothiophenol, photoinduced electron transfer, single silver nanoparticles
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so, the real PET path from p-ATP to oxygen or other oxidants through MNPs is still not clear. In addition, there are still no reports involving the morphological change of MNPs after the PICT of p-ATP, while disclosure of the morphological chemistry, in fact, might be possible to resolve this longstanding mystery in SERS. Therefore, the MNPs-catalyzed PICT of p-ATP was selected as a model reaction to investigate the PET process in this study. Monitoring the dynamic PET process through lightscattering dark-field microscopy imaging (iDFM) technique could be an interesting alternative to explore the nature of PET, since iDFM technique has been used to effectively analyze chemical reactions at the single nanoparticle level in real time.14−16 Currently, iDFM technique has been widely applied for chemical analysis, real-time monitoring of biological processes, live-cell imaging, and molecular detection.17−23 In the operation of iDFM, MNPs represented by silver nanoparticles (AgNPs) and gold nanoparticles (AuNPs) are
hotoinduced electron transfer (PET), a common form of photoelectric conversion widely applied in solar energy materials,1−4 fluorescent sensors, and fluorescent switches,5 has been extensively investigated in chemistry, physics, and biology due to the fundamental interest in the excited states of organic molecules.6−8 However, visualizing the PET process, which can reveal the reaction mechanisms involving the electron transfer information between reactants and thus improve the photoelectric conversion efficiency, or design more efficient PET sensors and switches, still remains a big challenge. p-Aminothiophenol (p-ATP), one of the most popular probe molecules for surface-enhanced Raman spectroscopy (SERS),9,10 can be oxidized to form 4,4′-dimercaptoazobenzene (DMAB) on the rough silver surface after laser illumination.11 The photoinduced chemical transformation (PICT) of p-ATP to DMAB involves a metal-mediated PET process, and some density functional theory (DFT) calculations have predicted the PET direction, wherein a hot hole in metallic nanoparticles (MNPs) at first captures an electron from the highest occupied molecular orbital (HOMO) of an absorbed p-ATP and then the hot electron is quenched by oxygen or other oxidants.12,13 Even © 2017 American Chemical Society
Received: December 9, 2016 Accepted: January 24, 2017 Published: January 24, 2017 2085
DOI: 10.1021/acsnano.6b08282 ACS Nano 2017, 11, 2085−2093
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Figure 1. Spectra and iDFMs of spherical AgNPs during the PICT of p-ATP. (A) Typical simultaneous spectroscopic measurements of the same spherical AgNP during the reaction; (B) electron exchange velocities of the chosen AgNP at different time periods; (C) dynamic iDFMs of AgNPs in the same area captured at different time points with a latency time of 10 min; cp‑ATP, 1.2 × 10−5 mol L−1; pH, 8.69. (D) Timedependent intensity analysis of corresponding particles in (C); (E and F) time-dependent RGB analysis of particles 1 and 2 in (C). The scale bar is 2 μm for all images.
RESULTS AND DISCUSSION Digitalization and Visualization of PET Process on Single AgNPs. The AgNPs employed as scattering probes were mainly spherical with an average diameter of about 43.16 ± 7.73 nm and presented blue under DFM (for the spectral feature, morphological characterization, and original iDFM, see Figure S1). The continuous LSPR scattering spectra were first scanned to quantify the electron density change of the chosen AgNP as the reaction progressed. With the addition of p-ATP solution into the cell, the scattering spectra of the chosen AgNP initially underwent an 88 nm redshift (from 485 to 573 nm) and then a 13 nm blueshift backward (from 573 to 560 nm) about 80 min later since the start (Figure 1A). Equation 1 was used to calculate the amount of the electron gain or loss of AgNPs (ΔN).33
commonly utilized as the light-scattering probes because of their localized surface plasmon resonance (LSPR) features,24,25 which endow them with excellent optical properties such as strong UV−vis absorption and LSPR scattering bands. The LSPR scattering light of MNPs is very sensitive to the surrounding environment, and that forms the basis of single nanoparticle analysis (SNA) technique.26,27 Until now, utilizing MNPs as scattering probes to monitor chemical reactions has been the most common application of iDFM.28−32 Excitingly, Mulvaney’ group found that the electron density of MNPs could also influence the LSPR scattering band and achieved the quantification of electron exchange in metal catalysis using surface plasmon spectroscopy, which set a precedent for tracking the path of electron transfer and measuring the rates of redox catalysis.33−35 However, investigations about visualizing the PET process through iDFM have not been reported to date. Herein, the PICT of p-ATP was investigated on single AgNPs by using the continuous LSPR scattering spectra of the specific AgNP to measure the real-time electron density and the dynamic iDFMs of AgNPs to visually monitor the PET process under irradiation. This LSPR spectroscopy coupled DFM efficiently demonstrated the real-time changes of the scattering signals from single AgNPs and enabled the comprehensive analysis of the photoreaction. To carry out the monitoring, AgNPs were first deposited on the homemade flow cell with a proper density.27 White light from a 100 W tungsten lamp excited AgNPs to scatter characteristic colored light and served as the light energy to initiate the AgNP-catalyzed transformation of p-ATP.
Δλ = −
⎛1 − L ⎞ ΔN ⎟ε λp ε∞ + ⎜ ⎝ L ⎠ m 2N
(1)
Herein, N is the electron density of the AgNPs, ε∞ is the highfrequency dielectric constant of Ag (5),36 L is the shape factor of the spherical AgNPs (0.33),37 λp is the bulk wavelength of Ag (129 nm), and εm is the dielectric permittivity (1.3332).38,39 N could be derived from the volume of the AgNPs and one Ag atom (0.008992 nm3) and calculated as 4.681 × 106. As a result, we estimated that the chosen AgNP first lost ∼2.177 × 106 electrons in 80 min, corresponding to the redshift of the scattering spectra, followed by ∼3.216 × 105 electrons injected in 30 min, corresponding to the blueshift of the scattering spectra. The electron exchange velocities of the chosen AgNP 2086
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at different time periods were also calculated (Figure 1B) and implied that an electron counter for PET could be developed through our strategy, which could be further applied for a realtime digitalization of the electron transfer. The dynamic iDFMs of AgNPs in the same area intuitively showed that the scattering light color of AgNPs first gradually changed from blue to green or red and then returned from green or red to cyan during the photoreaction (Figure 1C and Movie S1). The light intensity and the RGB proportion of two typical AgNPs over time were analyzed to quantify the scattering light of the AgNPs.40 The light intensity of particles 1−2 increased a little bit at first, then significantly decreased to lower than 10% of their original light intensity, and finally recovered slightly around 15% (Figure 1D). As time passed, the percentages of blue gradually decreased to ∼5% and then increased to ∼20%, while the percentages of red first increased from ∼8% to ∼70% and finally decreased to less than 15%. The percentages of green changed in a fluctuating way and ultimately reached ∼70%, becoming the dominant color (Figure 1E,F). Mechanism of AgNP-Catalyzed PICT of p-ATP. On normal glass slides, after exposure to illumination for 1 h, there was no change on the morphology and the scattering light color of AgNPs in the absence of p-ATP (Figure S2). This outlook is greatly different from that on TiO2, where simply shining light on AgNPs could cause the severe degradation of AgNPs and thus result in the multicolor changes of scattering light.41 Actually, the observed phenomenon in Figure 1 indicated that AgNPs initiatively lost electrons at first, subsequently gaining them during the photoreaction.35,39 In catalytic reactions, silver could accept the electrons of aniline to promote the formation of azo bond and be reduced from Ag+ ion to Ag atom.42,43 By experiment, we found Ag+ ions could oxidize p-ATP to DMAB, which was reduced to amorphous silver (Figure S3). The SERS spectra indicated that Ag+ ions played a decisive role in the PICT of p-ATP to DMAB (Figure 2). Recent advances showed that AgNPs could pass their electrons to the ambient O244 and could be dissolved in aqueous media and release Ag+ ions in the presence of O2 under illumination, although the dissolution process was time consuming.45,46 In such cases, we supposed the AgNP-catalyzed PICT of p-ATP involved in three steps. First, AgNPs suffered a preoxidation before the connection of p-ATP through Ag−S bond.47 At the same time, some electrons on AgNP surface were converted into hot electrons under the
Figure 2. Characterization of the formation of DMAB. (A) Normal Raman spectrum of p-ATP (1.0 × 10−3 mol L−1), (B) SERS spectrum of the mixture of p-ATP (0.5 × 10−3 mol L−1) and AgNPs, and (C) SERS spectrum of the mixture of p-ATP (0.5 × 10−3 mol L−1) and Ag+ ions. Laser wavelength, 532 nm; power, 28 mW; lens, 50× objective; acquisition time, 5 s.
excitation of light.48,49 Afterward, the generated hot electrons were transferred to the O2 in solution rapidly and the Ag+/ Ag2O layer was formed on the surface.50 Finally, the electrons of p-ATP were shifted back to Ag+ ions on the surface of Ag@ Ag+/Ag2O NPs to form DMAB, as described in eqs 2 and 3. The overall reaction of the AgNP-catalyzed PICT of p-ATP is described with eq 4. It is worth mentioning that the normal metal-catalyzed redox reactions could not cause the bidirectional movement of the scattering light of MNPs,51 but the reactions whose electron exchange have a certain time interval could.35 Herein, we proposed an academic term called “electron transfer delay” to explain this phenomenon. Since the electrons of p-ATP were transferred to the Ag@Ag+/Ag2O NPs in batches, the delay of the second batch of the electrons of p-ATP made the electron loss and gain of AgNPs inevitably have an order (Figure 3).52 Initially, AgNPs presented a betatopic state by transferring their electrons to O2 and then accepting the first batch and some of the second batch of electrons from p-ATP, giving rise to the 2087
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Figure 3. Schematic demonstration of the electron transfer from p-ATP to Ag+ ions to form DMAB.
Figure 4. Evidence for the generation of new NPs after the PICT of p-ATP. (A) SEM images of AgNPs with the treatment of p-ATP before (top) and after (bottom) the irradiation of 365 nm ultraviolet lamp with a power of 12 W for 70 min in water; cp‑ATP, 1.2 × 10−5 mol L−1; the whole reaction was performed on the monocrystalline silicon substrate used for SEM, and the scale bar is 100 nm for all images; (B) iDFMs of AgNPs in the same area during the PICT of p-ATP; 1.2 × 10−4 mol L−1; (C) HRTEM images of AgNPs with the treatment of p-ATP after the irradiation of 365 nm ultraviolet lamp with a power of 12 w for 70 min in water; cp‑ATP, 1.2 × 10−4 mol L−1; the scale bar is 50 nm for all images.
Figure 5. Characterization of the chemical composition of reacted AgNPs and new generated NPs. (A) STEM image of AgNPs with the treatment of p-ATP after irradiation of a 365 nm ultraviolet lamp with a power of 12 W for 70 min in water; cp‑ATP, 1.2 × 10−4 mol L−1; (B) EDS spectra for the areas indicated in (A).
gradual redshift of the scattering light. Then on the surface, Ag+ ions produced in the betatopic state reached saturation as the
reaction progressed, followed by the remaining part of the second batch of electrons from p-ATP injected into the Ag@ 2088
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Figure 6. Real-time monitoring of the PICT of p-ATP at low pH value. (A) Dynamic iDFMs of AgNPs in the same area after 120 μL of p-ATP acid solution added into the cell, with a latency time of 5 min; cp‑ATP, 1.2 × 10−5 mol L−1; pH, 1.81; (B) time-dependent RGB analysis of particle 1 in A; (C) time-dependent RGB analysis of particle 2 in A. The scale bar is 2 μm for all images.
Ag+/Ag2O NPs, with the scattering light color returning to cyan. The hysteresis quality of the charge process of AgNPs caused by the electron transfer delay of p-ATP gave birth to the bidirectional movement of scattering light and the unevenness of electron exchange velocities (Figure 1). The scattering light of AgNPs could not return to the original color, which indicated that AgNPs would lose more electrons than gain them after the catalysis. That is because some Ag atoms on AgNP surface were oxidized to Ag2O, and some produced Ag+ ions diffused in solution during the reaction. Release of Ag+ Ions. Scanning electron microscopy (SEM) images of the AgNPs before and after reaction showed new, smaller nanoparticles (NPs) were generated in the vicinity of the parent particles, and an obvious ∼6 nm film formed on the surface of AgNPs (Figure 4A), implying that some of the Ag+ ions generated on the surface of AgNPs were released into water, followed by the formation of new NPs by accepting the electrons of p-ATP in solution. If the concentration of p-ATP was increased to 10−4 mol L−1, the appearance of new NPs would be observed more obviously under DFM (Figure 4B). We easily found that the reaction was more intense since some new generated NPs and large clumps (the product of Ag+ ions and p-ATP) were observed floating in solution and finally depositing on the slide (Movie S2). However, there was no new substance formed when 10−4 mol L−1 of p-ATP solution was added into the cell unmodified with AgNPs (Figure S4). Highresolution transmission electron microscopy (HRTEM) images clearly demonstrated the morphology of the new formed NPs and clumps in the vicinity of parent AgNPs (Figure 4C). Through scanning transmission electron microscopy (STEM), we found the reacted AgNPs and the new generated substances both contained Ag element, which proved the release of Ag+ ions after the reaction (Figure 5). The energy dispersive spectroscopy (EDS) spectrum showed that the content of O
and S of the reacted AgNPs was 78.78% and 8.66%, respectively (Figure S5), providing further evidence for the participation of O2 in the reaction. The particle size distribution showed that AgNPs treated with p-ATP in sunlight had an average diameter of 37.00 ± 3.31 nm, and the average diameter of AgNPs treated with p-ATP under illumination of 365 nm UV light was 23.48 ± 4.77 nm (Figure S6), which indicated that the size of AgNPs decreased after the PICT of p-ATP and decreased more in UVlight than sunlight. The iDFMs of the control area which did not suffer illumination showed that the scattering light of most AgNPs was still blue (Figure S7), proving the scattering light color change during the reaction was mainly due to the electron transfer rather than the refractive index change caused by the connectivity of p-ATP through the Ag−S bond. To study the influence of DMAB (the dimer of p-ATP) on the scattering light of AgNPs, we synthesized the commercially unavailable DMAB (for the 1H and 13C NMR spectra of DMAB, see Figure S8), and the results showed DMAB could hardly influence the scattering light color of AgNPs (Figure S9). pH-Dependency of AgNP-Catalyzed PICT of p-ATP. Since the PICT rate of p-ATP is pH-dependent, p-ATP will bring about distinct influences on the scattering light of AgNPs at different pH values, which could also be demonstrated through iDFM (Figure S10). Equation 3 showed that OH− was the major reactant in the PICT of p-ATP, offering an alternative way to explain the reaction pH dependency. When the p-ATP solution with very low pH value was added into the cell, the scattering light of AgNPs experienced a nonreversible redshift because the protonation of amine group disenabled the formation of DMAB and immensely prevented the second batch of electrons from p-ATP transferred to Ag+ ions (Figure 6). At the end of the reaction, most AgNPs disappeared due to the excessive decrease of the scattering light intensity caused by 2089
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Figure 7. Dynamic iDFMs of AgNPs in the same area after 120 μL of 4-nitrothiophenol solution added into the cell, with a latency time of 1 min; c4‑nitrothiophenol, 1.2 × 10−5 mol L−1; pH, 7. The scale bar is 2 μm for all images.
the incessant diffusion of Ag+ ions produced on the surface of AgNPs in solution. However, due to the nonuniformity of the employed AgNPs, the scattering light intensity of the chosen two AgNPs underwent a flexural change over time (Figure S11). To remove the disturbances on the scattering light of AgNPs caused by the solution pH value, we made a control experiment by placing AgNPs in pure BR buffer solution with different pH values, keeping other conditions the same. The results showed that BR buffer solution with very low pH value could cause a slight intensity decrease of the scattering light of AgNPs, but no color change (Figure S12), and the alkaline BR buffer solution could not influence the scattering light of AgNPs at all (Figure S13). That is because small amount of Ag2O generated on the surface of AgNPs could be dissolved under acidic conditions, and then the produced Ag+ ions would react with PO43− ions in BR buffer solution (pH, 1.81), generating Ag3PO4 precipitation (for the SEM images before and after reaction see Figure S14) and slightly reducing the scattering light intensity of AgNPs. The fact that phosphoric acid is stronger than acetic acid indicated that the generated Ag3PO4 precipitation could exist in BR buffer solution (pH, 1.81). In alkaline conditions, the amount of H+ ions was so low that the above reaction could not proceed, which resulted in the unchanged scattering light of AgNPs. Electron-Withdrawing Inductive Effects of 4-Nitrothiophenol. Interestingly, if 4-nitrothiophenol was connected to the surface of AgNPs, the electron-withdrawing inductive effects of nitro group would be visually presented through iDFM under illumination. With the addition of 4-nitrothiophenol solution into the cell, the scattering light of AgNPs gradually changed from blue to red, with decreasing intensity (Figure 7). Similarly, the reaction could not happen without illumination (Figure S15). SEM images of the AgNPs before and after reaction indicated the reacted AgNPs seemed to suffer corrosion after reaction (Figure S16). The plasmon resonance energy transfer (PRET) between MNPs and compounds could also quench the scattering light of MNPs,53,54 but it could not change the scattering light color. Further investigation of the differences in the quenching
mechanisms between PET and PRET could provide information on their relationship.
CONCLUSIONS In summary, we real-time monitored the PICT of p-ATP to DMAB on single AgNPs and proposed the reaction mechanism. The PET process during the reaction was visually tracked and digitalized through the iDFM technique coupled with LSPR spectroscopy. In other words, we developed a visual method to acquire the electron-transfer information between reactants. The electron-transfer delay of p-ATP made the scattering light of AgNPs produce a bidirectional movement, of which the order enabled us to acquire the PET path during the reaction. In addition, an obvious morphological change of AgNPs was found after the catalysis, verifying the release of Ag+ ions and providing instructive revelation about the improvement of catalytic efficiency by utilizing the morphological instability of AgNPs. These conclusions might help us explore the nature of PET, understand the MNP-catalyzed organic reaction mechanisms, and design systems exhibiting favorable redox chemistry for photosensitization. Besides p-ATP, our strategy could also be utilized to study the photoreactions of other sulfydryl-containing organic molecules involving in electron transfer. MATERIALS AND METHODS Preparation of AgNPs. AgNPs were prepared using the previously reported method. First, a 50 mL glycerol/water mixture (20 vol % glycerol) was stirred (1200 rpm, 2.5 cm stirring bar) in a 100 mL flask and heated to 95 °C. Then 9 mg of silver nitrate was added to the solution, and 1 min later, 1 mL of sodium citrate (3%) was added into the mixture. The reaction mixture was stirred for 1 h at 95 °C. The color change rate of the solution was found to be slightly slower than that of the classical method of Lee and Meisel.55 After the reaction completed, the colloid was stored at 4 °C. Apparatus. The UV−vis absorption and scattering spectra of AgNP colloid were measured with a UV-3600 spectrophotometer (Hitachi, Tokyo, Japan) and an F-4500 fluorescence spectrophotometer (Shimazdu, Japan), respectively. The morphology and size of the synthetic AgNPs were imaged by SEM (S-4800, Hitachi, Tokyo, Japan). To perform the scattering light iDFM technique, a BX51 2090
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ACS Nano optical microscope (Olympus, Japan) equipped with a high numerical dark-field condenser (U-DCW, 1.2−1.4) was used, and the scattering light was collected by a 100× objective lens. The images were acquired using a DP72 single chip true color CCD camera (Olympus, Japan). Image-Pro Plus 6.0 (IPP) software (Media Cybernetics, USA) was employed to analyze the scattering light of AgNPs. The obtained images were all 24-bit truecolor TIFF/BMP picture files. The collection and assembly of the dark-field images was carried out using Image-Pro Plus (IPP) 6.0 software (Media Cybernetics, USA). All scattering spectra of AgNPs in this study were smoothed through Origin 8.0 software. Real-Time Monitoring of the Dynamic PICT Process of p-ATP to DMAB through iDFM. We employed a homemade flow cell by the combination of a glass slide and coverslips as the reaction zone to observe the scattering features of AgNPs as they catalyzed the PICT of p-ATP. First, 10 μL of AgNP colloid with a 100-time dilution by purified water was deposited over the reaction zone, and 30 min later, the flow cell was washed by purified water and dried by N2. Then, the flow cell was placed under the dark-field microscopy for lightscattering imaging. A representative area containing AgNPs was selected for real-time monitoring. After taking the original iDFM of AgNPs before reaction, 120 μL of p-ATP solution with a concentration of 1.2 × 10−5 mol L−1 was added into the cell to initiate the AgNPscatalyzed reaction. With a certain time interval, a series of iDFMs of AgNPs in the same area were recorded during the reaction. The acquired images were orderly assembled to track the PET process from p-ATP to oxygen through AgNPs. The pH dependency of the PICT of p-ATP was also investigated by analyzing the distinct influences of the transformation on the scattering light of AgNPs at different pH values, wherein a BR buffer solution was employed to adjust the solution pH values. Synthesis of DMAB. DMAB, the dimer of p-ATP, was successfully synthesized according to the as-reported work with some modifications: first, 12.5 mL 36% hydrochloric acid, 7.5 mL 36% acetic acid, and 37.5 mL of water were mixed in a 250 mL three-necked flask and placed in an ice−salt bath to cool. After the temperature reached
