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An ultrasensitive photoelectrochemical platform for quantifying photoinduced electron-transfer properties of a single entity


Understanding the photoinduced electron-transfer process is of paramount importance for realizing efficient solar energy conversion. It is rather difficult to clarify the link between the specific properties and the photoelectrochemical performance of an individual component in an ensemble system because data are usually presented as averages because of interplay of the heterogeneity of the bulk system. Here, we report a step-by-step protocol to fabricate an ultrasensitive photoelectrochemical platform for real-time detection of the intrinsic photoelectrochemical behaviors of a single entity with picoampere and sub-millisecond sensitivity. Using a micron-thickness nanoparticulate TiO2-filmed Au ultramicroelectrode (UME) as the electron-transport electrode, photocurrent transients can be observed for each individual dye-tagged oxide semiconductor nanoparticle collision associated with a single-entity photoelectrochemical reaction. This protocol allows researchers to obtain high-resolution photocurrent signals to quantify the photoinduced electron-transfer properties of an individual entity, as well as to precisely process the data obtained. We also include procedures for dynamic light scattering (DLS) analysis, transmission electron microscopy (TEM) imaging and collision frequency–concentration correlation to confirm that the photoelectrochemical collision events occur at an unambiguously single-entity level. The time required for the entire protocol is ~36 h, with a single-entity photoelectrochemical measurement taking <1 h to complete for each independent experiment. This protocol requires basic nanoelectrochemistry and nanotechnology skills, as well as an intermediate-level understanding of photoelectrochemistry.

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Fig. 1: Single-entity photoelectrochemical measurement.
Fig. 2: Equipment setups for electrochemical measurement and single-entity photoelectrochemical measurement.
Fig. 3: Procedure for preparation of TiO2@Au UMEs.
Fig. 4: Characterization of TiO2@Au UMEs.
Fig. 5: Photoelectrochemical response of a single N719@TiO2 entity for a dye-sensitized solar cell system.
Fig. 6: Photoelectrochemical responses of a single N719@ZnO entity for visible light water splitting.

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This research was supported by the National Natural Science Foundation of China (grants 21775043 and 21421004), Shanghai Municipal Natural Science Foundation (19ZR1472100), the Program of Introducing Talents of Discipline to Universities (B16017) and the Innovation Program of the Shanghai Municipal Education Commission (2017-01-07-00-02-E00023).

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Authors and Affiliations



W.M. and Y.-T.L. conceived and designed the research. H.M., Y.-Y.P. and W.M. performed all experiments and analyzed the data. W.M. and H.M. wrote the manuscript. W.M. and Y.-T.L. finalized the preparation of the manuscript. Y.-T.L. and H.T. designed and managed the project. All the authors discussed the results and commented on the manuscript.

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Correspondence to He Tian or Yi-Tao Long.

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The authors declare no competing interests.

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Key references using this protocol

Peng, Y.-Y., Ma, H., Ma, W., Long, Y.-T. & Tian, H. Angew. Chem. Int. Ed. 57, 3758–3762 (2018):

Ma, H. et al. J. Am. Chem. Soc. 140, 5272–5279 (2018):

Integrated supplementary information

Supplementary Figure 1 Control experiment of individual dye-tagged oxide semiconductor entities at a bare Au UME.

Amperometric current−time curves of (a) individual N719@TiO2 entities (174 pM) at a bare Au UME in acetonitrile containing 10 mM LiI and 1 mM I2 under illumination with a Xe lamp (λ > 450 nm) at the applied potentials of +600 mV vs Ag/AgCl and (b) individual N719@ZnO entities (200 pM) at a bare Au UME at the applied potentials of +500 mV vs Ag/AgCl in a DMSO solution water containing 50 mM TBAP under visible light. (Modified with permission from ref. 40 and ref. 41).

Supplementary Figure 2 Spectral composition of the light source after filtering.

Fluorescence spectrum profiles of the light source (xenon lamp) without and with a filter to cut-off the wavelengths of light less than 450 nm. Supplementary Fig. 2 shows the spectral profiles of a light source ranging from 350 nm to 1050 nm without and with a filter to cut-off the wavelengths of light less than 450 nm as measured with a fiber optic spectrometer (Ocean Optics, USB 4000). It is clear that the spectral composition below 450 nm was completely cut off when this filtering was mounted.

Supplementary Figure 3 Optimization of applied potential at TiO2@Au UME.

Linear sweep voltammograms (a) without and (b) with N719 in the dark (black) and under visible light (red); (c) cyclic voltammogram with N719 obtained in acetonitrile containing 0.1 M LiClO4 at a scan rate of 20 mV/s on a TiO2 film-coated Au foil electrode. (Reproduced with permission from ref. 40).

Supplementary Figure 4 Optimization of applied potential at TiO2@Au UME.

Current−time curves of (a) individual N719@TiO2 entities in acetonitrile containing 10 mM LiI and 1 mM I2 and (b) individual N719@ZnO entities in DMSO/water solution containing 50 mM TBAP at a TiO2@Au UME at different bias potentials under visible light. (Modified with permission from ref. 40 and ref. 41).

Supplementary Figure 5 Confirmation of single-entity photoelectrochemical event.

DLS result of the N719@TiO2 entities before (grey) and after (red) the photoelectrochemical measurements. (Modified with permission from ref. 40).

Supplementary Figure 6 Confirmation of single-entity photoelectrochemical event.

TEM images and size distributions of the N719@ZnO entities before (a) and after (b) single entity photoelectrochemical experiments. (Modified with permission from ref. 41).

Supplementary Figure 7 Confirmation of single-entity photoelectrochemical event.

Plots of the collision frequency as a function of N719@ZnO entities suspension concentration for the experimental data (black) and the theoretical calculated results (yellow). (Modified with permission from ref. 41).

Supplementary Figure 8 Photoelectrochemical response of individual bare TiO2 nanoparticles.

Amperometric current−time curves of bare TiO2 nanoparticles solution in I-/I3- redox electrolyte acetonitrile solution at a TiO2@Au UME in the dark and under visible light illumination with a Xe lamp (λ > 450 nm) at +600 mV vs Ag/AgCl. (Modified with permission from ref. 40).

Supplementary Figure 9 Average number of N719 molecules attached to a single TiO2 nanoparticle surface.

(a) A plot of N719 absorbance wavelength at 500 nm as a function of the dye concentration. The red line represents the fitting result. (b) UV-vis spectra of the rinse solutions obtained by rinsing the N719 dye molecules from the N719@TiO2 entities for three independent experiments. (Reproduced with permission from ref. 40). Average number of N719 molecules on a single TiO2 nanoparticle is associated with the surface area of TiO2 nanoparticles and the diameter of N719 molecules. At full coverage condition, the number obtained with these data is calculated to be 785. We also determined the number of N719 per TiO2 nanoparticle by UV-vis absorption spectroscopy. First, a plot of the absorbance as a function of the N719 dye concentration in 0.1 M aqueous NaOH solutions was obtained and fitted as a standard curve (Supplementary Fig. 9a). Sequentially, the number of N719 dye molecules adsorbed on the TiO2 surface can be estimated by observing the absorption spectra of the 0.1 M aqueous NaOH solutions obtained by rinsing the dye molecules from the N719@TiO2 entities (Supplementary Fig. 9b). \(m_{{\mathrm{TiO}}_2} = 0.0543g\quad \rho _{{\mathrm{TiO}}_2} = 3.9g/cm^3\;V_{NaOH} = 45mL\) \(N_{{\mathrm{TiO}}_2} = \frac{{m_{{\mathrm{TiO}}_2}}}{{\rho _{{\mathrm{TiO}}_2}V_{{\mathrm{TiO}}_2}}} = 1.70 \times 10^{15}\) \(A_{N719} = 0.68 \pm 0.03\) \(A_{N719} = 16.8{\mathrm{C}}_{N719} + 0.02\) \(C_{N719} = 0.039 \pm 0.001mM\) \(N_{N719} = C_{N719}V_{NaOH}{\mathrm{N}}_{\mathrm{A}} = \left( {1.06 \pm 0.03} \right) \times 10^{18}\) \(n_{N719} = \frac{{N_{N719}}}{{N_{{\mathrm{TiO}}_2}}} = 623 \pm 12\) We estimated the average occupied number of N719 on a single TiO2 nanoparticle to be approximately 600. It is smaller than theoretically calculated value of 785. This indicates N719 dye molecules are adsorbed on the TiO2 surface with some space between dye molecules due to the twist of N719 configuration or the measurement errors present in our experiment.

Supplementary Figure 10 Photoelectrochemical response of individual N719@ZnO entities for visible-light water splitting.

(a) Current-time curves on a TiO2@Au UME for 200 pM of N719@ZnO nanoparticles in a DMSO solution containing 50 mM TBAP with and without water. The plot in the red rectangle is a further enlarged view and representative time-resolved current pulses of certain sections. (b) Typical gas chromatographic traces of evolved oxygen in the detector cell containing N719@ZnO nanoparticles in DMSO with (red) and without (black) water under visible illumination. (c) Histograms of the transient current (I), duration (T), the integrated charge (Q) for the individual N719@ZnO nanoparticle collision events and the turnover number (TON) of photocatalytic O2 evolution of a single N719 molecule per stochastic collision event. The number of collision events in each histogram is at least 1,000. (Modified with permission from ref. 41).

Supplementary Figure 11 Characterization of N719@TiO2 entities.

(a) UV-vis absorption spectra, (b) FTIR spectra, (c) high-resolution TEM image and (d) DLS result of N719@TiO2 entities. (Modified with permission from ref. 40). Comparative studies of the UV-vis spectra (Supplementary Fig. 11a) and FTIR spectra (Supplementary Fig. 11b) for N719@TiO2 entities indicate that N719 self-assembled on the TiO2 surface. The shift of the C=O peak in the UV-vis spectra to a longer wavelength (547 nm to 570 nm) is due to the close proximity of the functional groups to the surface of the TiO2 nanoparticles and the close packing of surface-anchored COO-. The characterization peaks in the FTIR spectra at 1,230 cm-1 and 1,718 cm-1 can be attributed to the C-O and C=O vibrations, respectively. This indicates N719 molecules surface-anchored on the surface of TiO2 nanoparticles. Furthermore, high-resolution TEM images (Supplementary Fig. 11c) and dynamic light scattering results (Supplementary Fig. 11d) show that the homogeneously dispersed N719@TiO2 nanoparticle suspensions have a narrow size distribution with an average size of 25 nm ± 5 nm.

Supplementary Figure 12 Characterization of the N719@ZnO entities.

(a) UV-vis and (b) FTIR spectra of N719, ZnO nanoparticles and N719@ZnO entities. Inset: Anchoring structure of the N719 dye on the ZnO nanoparticle. TEM images of (c) bare ZnO nanoparticles and (d) N719@ZnO entities. (Reproduced with permission from ref. 41). The UV-vis spectra for the N719@ZnO entities exhibit the superposition of the ZnO contribution below 400 nm and the dye absorption in the visible region at approximately 500-560 nm, which indicates successful modification of dye-molecule N719 on the surface of ZnO nanoparticles (Supplementary Fig. 12a). Furthermore, FTIR spectra of the N719@ZnO entities are shown together with the ZnO nanoparticles and the N719 dye in Supplementary Fig. 12b. The decrease of the characterization peak at 1714.4 cm-1 can be assigned to the C=O vibration in the N719 dye. It suggests that N719 molecules are surface-anchored on ZnO nanoparticles via bidentate carboxylate linkages. Moreover, the high-resolution TEM images indicate the similar monodispersed narrow distribution of the ZnO nanoparticles and N719@ZnO entities with an average size of 4 ± 1 nm in diameter (Supplementary Fig. 12c,d).

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Supplementary Figs. 1–12, Supplementary Table 1, Supplementary Data 1 and Supplementary Manual

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How to fabricate TiO2@Au UMEs (Steps 1–13 of the Procedure)

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Ma, W., Ma, H., Peng, YY. et al. An ultrasensitive photoelectrochemical platform for quantifying photoinduced electron-transfer properties of a single entity. Nat Protoc 14, 2672–2690 (2019).

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