Fluorescence probe dyes, perylene (Pr) of the highest reagent grade from Kanto Chemical Co. (Japan), Di10ASP-PS, was used as received (The fluorescence and excitation spectra of these dyes are showed in Fig. 8-1).
Figure 8-1. The fluorescence (black line) and excitation spectra (red line) of two dyes used in this study, perylene in hexane solution and the excitation wavelength was 401 nm, and Di10ASP-PS in chloroform solution and the excitation wavelength was 521 nm.
129 8.4. Results and Discussion
8.4.1. Fluorescence Intensity change of HD microdroplets in KClO4 solution on Au(1 1 1) electrode
Fig. 8-2-a demonstrate the C-E curve for a Au(1 1 1) electrode with HD microdroplets contaning Perylene (Pr) as probe dye prepared by the touching method. Addition of Pr in the HD microdroplets did not largely alter the C-E curve, indicating that the dye acts actually as a probe. The fluorescence of the probe dye molecules at a close proximity the metal electrode surface is quenched as a result of reduced lifetime of the excited state. When the probe dye molecules have greater distance from the electrode surface, the fluorescence intensity becomes stronger as a monotonical function of the distance [5−14]. If HD stays put on the surface as droplets and its height changes in response to the potential change, the fluorescence intensity should depend on the electrode potential. Squeezing of the bottom area of a constant-volume droplet should increase the height of the center of the droplet.
Even when a HD + dye liquid phase of a nanometer-level thickness covers a large area, potential dependent spreading-retracting (in other words, wetting- dewetting) should be observed as the change of the fluorescence image, especially as the change of intensity map.
Remind the fact that the shorter the distance between a dye molecule and the metal surface, the greater the extent of fluorescence quenching. At potentials at which the HD droplet is of bellshape, it appears as a fluorescent droplet in the image. In contrast, at the potentials at which the HD is spreading to be a much thinner film, the fluorescent area should be greater but the intensity is substantially quenched. The images in Fig. 8-2-b clearly show that HD deposited on electrode surface split into a number of droplets as displayed by the light spots.
All the light spots in the images were smaller than 50 μm in diameter. The number of light spots increased in negative scan by new appearance of relatively small light spots. The weak light spots present at the positive potentials turned to be brighter spots at negative potentials.
In addition, the surroundings of relatively big light spots remained dark even at −0.7 V. This suggested that there was no microdroplets because almost all the HD droplet in the image exhibited spreading-retracting; the scale of the change for each droplet was smaller than 50 μm. A few droplets of HD gathered from surroundings, nevertheless this process never produced a droplet greater than 50 μm in diameter. To sum up, HD does not form a continuous film but splits into many droplets.
130 Fig. 8-2-c shows IFL obtained in the course of potential cycling at a potential sweep rate (v) of 5 mV s-1 for the Au(1 1 1) electrode with HD containing Pr. As described in the experimental section, the background image of IFL was taken in which the difference value was set zero at all the pixels, and each pixel in the image of the sample is quantified 256 shades of gray. The summation of the differences of the shade over the whole image was converted to IFL. The onset of a steep rise of IFL occurred at −0.3 V on the negative scan (Fig.
8-2-c, black line). IFL reached a peaked maximum at −0.55 V after the turning of the potential scan, and decayed until 0.20 V was reached. The results of repeated potential scans to record four more cycles (Fig. 8-2-c) revealed that photo-bleaching was inconsiderable.
Figure 8-2 C-E curve (a) for a Au(1 1 1) electrode with Pr contained in HD on its surface deposited by the touching method in 50 mM KClO4 aqueous solution (The gray lines are the data for a bare Au(1 1 1) electrode). Fluorescence images (b) obtained in the course of in situ potential cycling at v = 5 mVs−1 with Pr, and (c) integrated intensity (IFL)-potential (E) curves with Pr obtained by integrating one screen of the entire fluorescence microscopic images obtained at v = 5 mVs−1. The excitation wavelength range was 355–375 nm for Pr.
The detecting wavelength range was 395–700 nm for Pr. The diameter of circular viewing field in (b) is 1020 mm. In (c), IFL–E curves for continuous five repeated cyclic scans are shown where the order of the lines is black-red-blue-green-gray in color (in gray scale, the first scan is the thickest line and the 5th scan is the thinnest one).
131 In Fig. 8-2-c, the value of IFL attained its maximum at −0.55 V, whereas the capacitance in C-E curve before and after the maximum was rather featureless (Fig. 8-2-a). An important message of this observation is that the fluorescence intensity change is largely due to the droplet height change far beyond the electrochemical double layer thickness region, because the changes in such a far region are hardly reflected in the capacitive current change.
Note that the characteristic thickness of the diffuse layer (i.e. the distance to decay the potential drop to be 1/e based on the Gouy-Chapman model) in the present experimental condition is ca. 1.3 nm.
8.4.2. Discussion on the state of HD droplets on the basis of microscopic fluorescence data I obtained the CV (Fig. 8-3-a) for a Au(1 1 1) electrode with HD newly prepared by touching method obtained in a solution containing Fe(CN)63−. The cathodic peak of the first potential scan was negatively shifted to 0.09 V, and the cathodic peaks potentials in the following scans were at +0.13 V. Fig. 8-3-a indicates that the condition of HD on Au(111) electrode surface prepared by touching method exhibits history-dependent change when sweeping the potential. To observe the state of the HD on the electrode surface, I conducted fluorescence microscopic measurements. Fig. 8-3-b shows the fluorescence image obtained when the electrode with HD + Pr firstly touched to the electrolyte aqueous solution in the electrochemical cell under an open circuit condition. Fig. 8-3-c was obtained when holding the electrode potential at −0.6 V in first potential scan. To obtain these images, each pixel in the image of the sample is quantified 256 shades of color.
The microdroplets observed are bigger than those of in Fig. 8-2-c because of using the objective lens ×40. In Fig. 8-3-b, HD droplet has indefinite shapes. After the electrode potential was held at −0.6 V, the shape of HD droplet, looking down from the top, changed to be in a circle. The area covered by HD just after touching is greater than the area under application of −0.6 V. Initially at an open circuit potential, HD cover large area of the electrode surface, therefore, the reduction of hexacyanoferrate was blocked and the corresponding peak potential was negatively shifted.
Although HD existed on Au(1 1 1) electrode surface, the redox reaction of Fe(CN)63−/4−
was not affected by HD on the electrode surface (Fig. 4-5 in Chapter 4). The HD just after deposited by touching method can affect the redox reaction of Fe(CN)63−/4− in Fig. 8-3-a.
132 When the HD droplet retracts at negative potential, the electrode surface covered with HD exposes to the electrolyte aqueous solution. Fig. 8-3-a suggests that the area covered by HD decreases while the potential sweeping, and the area that appeared after the retraction of large HD droplet has no HD microdroplet.
To consider the state of Pr molecules in HD microdroplets at Au(1 1 1) electrode/aqueous electrolyte solution interface, I conducted in situ fluorescence spectroscopy for Pr in HD microdroplets on Au(1 1 1) electrode. In a m order dimension, chemical process can be different from that in the bulk. For example, the tendency of an excimer formation can be different [15].
Figure 8-3. CV (a) at v = 10 mVs−1 for a Au(1 1 1) electrodes with HD prepared by the touching method (upper panel) for the solid lines obtained in 0.50 mM K3[Fe(CN)6] + 2 mM KHCO3 + 50 mM KClO4 aqueous solution. Fluorescence images (a and b) obtained when the electrode with HD + Pr firstly touched to the electrolyte aqueous solution in the electrochemical cell under an open circuit condition (b), and the image of the electrode under -0.6 V in first potential scan (c).
(a)
(b) (c)
133 In fact, excimer formation was observed in liquid paraffin droplets [15]. The efficiency of the excimer formation is dependent on the size of the microdroplets. When p-terphenyl as surfactant was mixed in liquid paraffin droplet, the decreasing the diameter of the droplet and the rotational relaxation time increased. The decrease in the solubility of monomer in droplets results in the decrease of excimer formation.
Fig. 8-4-a shows the in situ fluorescence spectra of Pr in HD on Au(1 1 1) electrode surface in 50 mM KClO4 aqueous solution obtained by the spectroscopic photo-detector connected to the fluorescence microscope. The spectra were obtained at constant potentials from 0.0 V in steps of 0.1 V to finally reach −0.7 V. The obtained spectrum at each potential agreed with Fig.8-1. The applied potential did not affect the fluorescence spectra of Pr on the electrode surface. It is known that a Pr excimer exhibit, featureless emission peak around 640 nm [16]. The obtained spectra in Fig. 8-4-a did not have such a emission peak. This result shows that Pr molecules exist relatively lower concentration in HD microdroplets. Fig. 8-4-b shows the plot of fluorescence intensity at 441.7 nm of Fig. 8-4-a
Figure 8-4. Fluorescence spectra (a) obtained by in situ fluorescence spectroscopy for Pr contained in HD on Au(1 1 1) electrode in 50 mM KClO4 aqueous solution for stepping the potential from 0.0 V to −0.7 V with an interval of the step potential being 0.1 V. (b) Plot of fluorescence intensity at 441.7 nm from the fluorescence spectra (a) when the electrode potential was stepped negatively and then positively.
(a) (b)
134 when the electrode potential was stepped negatively and then positively. The spectral fluorescence intensity change of Pr in HD on the Au(1 1 1) electrode depending on the applied potential was almost in line with the IFL-E curve in Fig. 8-2-c. Pr molecule has a polarity, but it neither localizes at HD/water interface nor absorbs at HD/electrode interface.
To discuss the state of Pr at the electrode surface, I compared the fluorescence spectrum of high concentration of Pr in HD solutions with that of low concentration in hexane solution.
Fig. 8-5 shows the fluorescence and excitation spectra of Pr in 30 M HD solution and 4 M hexane solution (gray line). The fluorescence spectral intensity was normalized at 471 nm.
The excitation spectra were normalized at 432 nm. Concentrations of Pr in hexane and in HD were nearly equal to their saturation. The excitation spectra for Pr on Au(1 1 1) electrode surface has an emission line on Hg lump < 370 nm in Fig. 8-4, whereas the excitation spectra for Pr in hexane solution in Fig. 8-5 was 386 nm. The difference was not observed between them. This agreement suggested that the concentration in HD microdroplets on Au(1 1 1) surface was nearly 4 M and not 30 M.
Figure 8-5. Fluorescence and excitation spectra of probe dyes, perylene (Pr); gray line, 4 M hexane solution; black line, 30 M HD solution. Solid line represents, fluorescence spectrum with excitation at 386 nm, and dotted line does, absorbance spectrum. The fluorescence spectral intensity was normalized at 471 nm, and the absorbance spectral intensity was normalized at 432 nm.
135 In the fluorescence spectra (Fig. 8-5), the peak at 435 nm was observed in 4 M hexane solution but not in 30 M HD solution. The lack of 435 nm peak is because of the photo reabsorbance process of the Pr molecule in 30 M HD solution. In contrast, this reabsorbance process is not considerable in 4 M hexane solution. The fluorescence spectra obtained at the Au electrode surface (Fig. 8-4) resembled the fluorescence spectra 4 M hexane solution, although Pr 30 M HD solution is used to deposit HD with Pr on the electrode surface (Procedure A: touching method). Even in the high concentration solution of Pr, the reabsorbance process can be neglected by using the cell when the optical path length is 12 m [16]. To sum up, appearance of the strong peak at 441 nm observed in Fig.
8-4-a suggests that the Pr concentration in HD was nearly 4 M or the height of HD microdroplets is nearly m order.
8.4.3. IFL-E curve obtained using surfactant dye
As mentioned in Chapter 4, the microdroplets forms on the Au(1 1 1) electrode surface when HD is deposited by the Procedure A (touching method). I intended to use a the surfactant dye that tends to adsorb at the HD/water interface to capture the profile of the HD microdroplets using the function expressing the fluorescence intensity dependency on the distance between the dye and the electrode surface as a measurement scale. Fig. 8-6-a shows the C-E curve for the Au(1 1 1) electrode with HD containing Di10ASP-PS as a surfactant dye.
The C-E curve did not differ considerably from the curve Fig. 8-2, indicating that dye acted as a probe. Fig. 8-6-b shows in situ IFL-E curve obtained in the course of potential cycling at v
= 5 mV s−1 for the Au(1 1 1) electrode with HD containing Di10ASP-PS as a surfactant dye.
IFL shows the difference from the intensity at +0.7 V.
In Fig. 8-6-b, the onset of a gradual rise of IFL on the negative scan occurred around 0.0 V and steep rise occurred at −0.6 V. IFL reached a peaked maximum at −0.65 V after the turning of the potential scan. IFL decayed until +0.20 V was reached. The result showed that the microdroplets changed its shape in broader potential change than when using Pr.
The potential-dependent shape change of HD microdroplets on a Au(1 1 1) electrode surface using Di10ASP-PS as a surfactant dye is not different from that using Pr. Although the same behavior was observed, the number of bright spots in the obtained fluorescence image was lower than that with Pr and the diameter of each bright spot was greater than that
136 with Pr. These resulted from the fact that the surfactant dye lowered the interfacial tension of the HD/water interface and smaller droplets coalesced and larger droplet produced.
Fluorescence can be observed even more positive potential than pzc when the larger droplet fully spread around pzc. The distance between dye and the electrode surface was far enough to emit fluorescence when the droplet spread around pzc. Fluorescence microscopy with a surfactant dye enables us to obtain IFL-E curve in agreement with -E curve, indicating reasonable potential-dependent shape change.
To confirm whether the fluorescence image captured the fluorescence from the surfactant dye, I conducted the fluorescence spectroscopy in Fig. 8-4-a. The obtained spectrum did not show the spectral curve as the same as Fig. 8-1 (Di10ASP-PS). It was concluded that the observed fluorescence was from uncertain fluorescence molecules contained in HD but not from Di10ASP-PS as surfactant dye. Fluorescence intensity from only Di10ASP-PS was not strong enough to be detected by the fluorescence microscope. Therefore, the fluorescence images with Di10ASP-PS in Fig. 8-6-b are also contained from the fluorescence of the uncertain fluorescence molecules contained in HD.
Figure 8-6. C-E curve (a) for a Au(1 1 1) electrode in 50 mM KClO4 aqueous solution; on the Au(1 1 1) surface, Di10ASP-PS contained in HD was deposited by the touching method (The gray lines are the data for a bare Au(1 1 1) electrode.) Integrated intensity (IFL)-potential (E) curve (b) with Di10ASP-PS obtained by integrating one screen of the entire fluorescence microscopic images obtained at v = 5 mVs−1. The wavelength range for detection was 395–
700 nm
(a) (b)
137 8.4.4. Electrochemical measurements for HD with SDS on Au(1 1 1) electrode
8.4.4.1. IFL-E curve obtained using surfactant dye
I conducted fluorescence microscopic measurements for Di10ASP-PS in HD in 0.5 mM SDS aqueous solution to reveal the potential-dependent behavior of HD microdroplets whether it is the same as the behavior of 1 L droplet or not. Di10ASP-PS and HD were prepared by Procedure A on a Au(1 1 1) electrode surface. In following discussion, the obtained fluorescence images contain the fluorescence of the uncertain fluorescence molecules contained in HD, especially when using Di10ASP-PS.
Fig. 8-7-a shows the IFL-E curve of Di10ASP-PS contained in HD preparing by Procedure A on Au(1 1 1) electrode surface in 0.5 mM SDS + 50 mM KClO4 aqueous solution in comparison with the IFL-E curve of HD without SDS (gray line). IFL was normalized to set the magnitude of the difference between maximum and minimum intensity being unity. The initial potential of the potential scan was +0.7 V. IFL remained nearly constant down to
−0.34 V and began to increase at more negative potentials. After the turning of the potential scan, IFL reached a maximum at −0.6 V and began to decrease at −0.30 V. The IFLdecreased slowly from −0.30 to +0.10 V, and a sudden decrease was recorded at + 0.45 V.
The potentials of marked decrease of IFL, -0.30 V and +0.50 V in Fig. 8-7-a, corresponded to _ and _ peak potentials as the same as the -E curve (Fig. 6-2). The HD microdroplets spread in the positive potential scan, because DS− anions adsorb on the electrode/water interface and change their adlayer structure resulting in lower the interfacial tension of the electrode/water interface. Similar IFL-E curve was also observed when using Pr as fluorescence probe, in case that the larger droplet appeared (Fig. 8-7-c-1). On the other hand, the IFL-E curve obtained by integrated fluorescence intensity from the small droplet was same as the IFL-E curve without SDS (Fig. 8-2-c-2). The IFL-E curve of larger HD microdroplet on a Au(1 1 1) electrode surface in SDS solution is different from that of smaller microdroplet. This resulted from the fact that DS− anions lowered the interfacial tension of HD/water interface and smaller droplets coalesced and produced larger droplet.
Fluorescence can be observed even more positive potential than pzc when the larger droplet spread around pzc. The distance between dye and the electrode surface was far enough to emit fluorescence when the droplet spread around pzc. The larger droplets enable us to obtain IFL-E curve in agreement with the -E curve in SDS aqueous solution, indicating reasonable potential-dependent shape change. The potential dependence of IFL and shows
138 close agreement with each other. At least, the microdroplets showed the shape change depending on the electrode potential as same as that of HD 1 L droplet. I could not know, however, what really happens in the other area except to the fluorescence spots.
Figure 8-7. Integrated intensity (IFL)-potential (E) curves with (a) Di10ASP-PS in HD prepared by the touching method on Au(1 1 1) electrode surface obtained in 0.50 mM SDS + 50 mM KClO4 aqueous solution (The gray lines are the data for Di10ASP-PS in HD prepared by the touching method on Au(1 1 1) electrode in 50 mM KClO4 aqueous solution without SDS.) obtained by integrating one screen of the entire fluorescence microscopic images obtained at v
= 5 mVs−1. (b)The fluorescence image (left) for the HD droplet contained Pr and No. 1 & 2 corresponds to the area integrated obtained IFL. (c) IFL- E curve of Pr in HD on Au(1 1 1) electrode in 0.50 mM SDS + 50 mM KClO4 aqueous solution.
8.4.4.2. IFL-t curve for the shape change of microdroplets
Fig. 8-8-a shows IFL-E curve for Pr in HD prepared by Procedure A on Au(1 1 1) electrode surface in 0.5 mM SDS + 50 mM KClO4 aqueous solution. The curve of Fig.
8-8-a was differed from that of Fig. 8-7-a, because the microdroplet of the former was small.
The curve in Fig. 8-8-a showed a likeness to the IFL-E curve in Fig. 8-7-c-2. I measured the IFL-t curve in response to potential steps and obtained the IFL-t curves in Fig. 8-8-c to evaluate
(b) (a)
Fluorescence IntensityFluorescence Intensity
(c)
139 the time dependence of fluorescence intensity change. Fig. 8-8-c shows the IFL-t curves obtained by the same procedure to obtain a IFL-E curve in which the difference value was set zero at all the pixels and the summation of the differences of the shade was converted to IFL as a function of time. The typical current transient was also shown in Fig. 8-8-b. The current transient decayed in 0.5 s, whereas the IFL continued to change in 60 s.
Figure 8-8. (a) Integrated intensity (IFL)-potential (E) curves with Pr in HD prepared by the touching method on Au(1 1 1) electrode surface in 0.50 mM SDS + 50 mM KClO4 aqueous solution obtained by integrating one screen of the entire fluorescence microscopic images obtained at v = 5 mVs−1. (b) Typical current transient curves as the results of potential step chronoamperometry for a Au(1 1 1) electrode with HD + Pr prepared by touching method in 0.50 mM SDS + 50 mM KClO4 aqueous solution. Initial/final potentials was 0.0 V/−0.60 V.
(c) Typical IFL-t curves as the results of potential step fluorescence microscopy for Pr in HD prepared by the touching method on Au(1 1 1) electrode surface in 0.50 mM SDS + 50 mM KClO4 aqueous solution obtained by integrating one screen of the entire fluorescence microscopic images. Initial/final potentials were 0.0 V/−0.60 V, −0.60 V/0.0 V, 0.0 V/0.4 V and 0.4 V/0.7 V.
140 In each potential step range, the HD microdroplets spread and retract as described above (Fig. 8-7). In the step between -0.6 V and 0.0 V, the droplet retracts and spreads. It means that the IFL also increase and decrease as same as Fig. 8-7-a. In addition, in the step from 0.0 V to 0.4 V and from 0.4 V to 0.7 V, IFL decreased because of the presence of DS- adlayer at the electrode/water interface. These spreading processes are slow because corresponding relaxation time to shape change of the droplet should be long. These IFL-t curves showed that the process of the shape change of HD microdroplet was sluggish compared with double layer charging. The change of IFL likely originated from the transfer of Pr molecules in HD microdroplet. It means that Pr molecules changed their intra-droplet positional distribution as its shape changed.
IFL-t curves need 60 s or longer time to reach constant on Au(1 1 1) electrode surface (Fig.
8-8-c). To confirm whether Pr molecules can transfer in a HD microdroplets on the electrode surface, I also conducted the same experiment on Au(1 1 0) electrode surface, on which HD droplet did not change its shape when potential sweeping. If the microdroplets on Au(1 1 0) electrode surface do not change its shape, the IFL-t curve will be different from the curve in Fig. 8-8-c.
Fig. 8-9-a shows C-E curve for a Au(1 1 0) electrode with HD + Pr prepared by Procedure A obtained in 0.50 mM SDS + 50 mM KClO4. This C-E curve was difference from that in Fig. 8-6, because DS− anions could not form both himi-micelle and interdigitated bilayer on a Au(1 1 0) electrode surface. In addition, the HD droplet did not change its shape at the electrode/water interface as the potential scan. Although the HD 1 L droplet did not change its shape, IFL-E curve in Fig. 8-9-b changed with the electrode potential scan. These results suggested that the HD microdroplets would change its shape or Pr molecules transfer in HD droplet. To explain the reason for the change of IFL, the same measurements of Fig.
8-8 were conducted in the HD microdroplets at Au(1 1 0) electrode surface.
Fig. 8-10-a shows IFL-E curve for Pr in HD prepared by Procedure A on Au(1 1 0) electrode surface obtained in 0.5 mM SDS + 50 mM KClO4 aqueous solution. In this system, I recorded the IFL-t curve in response to potential steps. Fig. 8-10-b shows a typical i-t curve.
The IFL-t curves were also shown in Fig. 8-8-c. The current transient decayed in 0.5 s, and the IFL changed in about 5 s. These IFL-t curves were entirely different from those in Fig.
8-8-c obtained in the system that Pr fluorescence intensity change on a Au(1 1 1) electrode surface. The time-dependent IFL change with potential step on Au(1 1 0) is faster than that
141 on Au(1 1 1). This result suggests the microdroplets on Au(1 1 0) does not change its shape but Pr molecules transfer in HD microdroplets. A Pr molecule is an electron rich and can interact with the electrode potential near the three-phase contact line. At the negative potential, Pr molecules repel and go away from the electrode surface in HD microdroplets, and the IFL increased when Pr molecules go away from the electrode surface. This process is faster than that of shape change of microdroplets.
In conclusion, the change of IFL over 60 s represents that the process of the shape change of HD microdroplet on Au(1 1 1) electrode surface is sluggish compared with double layer charging. In addition, the change of IFL observed in Fig. 8-8 consists also the transfer of Pr molecules to the electrode surface in HD microdroplets.
Figure 8-9. (a) C-E curve with a frequency of 14 Hz at v = 5 mV s-1 for a Au(1 1 0) electrode with HD + Pr prepared by the touching method obtained in 0.50 mM SDS + 50 mM KClO4
(The gray lines are the data for a bare Au(1 1 0) electrode.) (b) Integrated intensity (IFL)-potential (E) curves with Pr in HD prepared by the touching method on Au(1 1 0) electrode surface obtained in 0.50 mM SDS + 50 mM KClO4 aqueous solution obtained by integrating one screen of the entire fluorescence microscopic images obtained at v = 5 mVs−1.
142 Figure 8-10. (a) Integrated intensity (IFL)-potential (E) curves with Pr in HD prepared by the touching method on Au(1 1 0) electrode surface in 0.50 mM SDS + 50 mM KClO4 aqueous solution obtained by integrating one screen of the entire fluorescence microscopic images obtained at v = 5 mVs−1. (b) Typical current transient curves as the results of potential step chronoamperometry for a Au(1 1 0) electrode with HD + Pr prepared by touching method in 0.50 mM SDS + 50 mM KClO4 aqueous solution. Initial/final potentials was 0.0 V/0.70 V. (c) Typical IFL-t curves as the results of potential step fluorescence microscopy for Pr in HD prepared by the touching method on Au(1 1 0) electrode surface in 0.50 mM SDS + 50 mM KClO4 aqueous solution obtained by integrating one screen of the entire fluorescence microscopic images. Initial/final potentials were 0.0 V/−0.60 V, −0.60 V/0.0 V, and 0.0 V/0.7 V.
143 8.4.5. Fluorescence Image Analysis
In this section, we work on the following questions to study the state of the microdroplets on the electrode surface through analyzing the obtained fluorescence images.
1. How many droplets are there on the electrode surface and how is thier size?
2. How does the size of those microdroplets change when the electrode potential is sweep?
3. Is there any relationship between the fluorescence intensity of the brightest spot in a microdroplet and the height calculated from the microdroplet diameter?
4. How much HD on the electrode can be observed by the fluorescence microscopy without quenching? (Is the cover area by HD observed by the fluorescence microscopy equal to that obtained by the electrochemical capacitance measurement?)
First, I counted the number of the bright spots and measured the diameter of the bright spot in the obtained fluorescence images as shown in Figs. 8-11-a, and the data was plotted in a histogram to answer the question 1. The majority of the droplets have a diameter around from 14 m to 16 m in Fig. 8-11-a and from 16 m to 18 m in Fig. 8-12-a. The almost all droplets have a diameter less than 50 m.
minimum of the diameter 8 m
maximum of the diameter 106 m
average of the diameter 19 m
standard deviation 9.5
dispersion 90
Figure 8-11. (a) Fluorescence image I of HD microdroplets with Pr on Au(1 1 1) electrode surface at the potential = -0.7 V. (b) Histogram and a Gaussian distribution fitting curve obtained by the image I. (c) Statistical values.
Number of droplets
Droplet diameter /m
144
minimum of the diameter 8 m
maximum of the diameter 72 m
average of the diameter 17 m
standard deviation 8.0
dispersion 63
Figure 8-12. (a) Fluorescence image II of HD microdroplets with Pr on Au(1 1 1) electrode surface at the potential = -0.7 V. (b) Histogram and a Gaussian distribution fitting curve obtained by the image II. (c) Statistical values.
2. How does the size of those microdroplets change when the electrode potential is sweep?
Fluorescence image III (Fig. 8-13) and IV (Fig. 8-14) were recorded during a potential sweeping at +0.3 V and -0.19 V when using DPH as a probe dye. Fig. 8-15 and 8-16 show the fluorescence image V (0.0 V) and the image VI (-0.7 V) when using Pr as a probe dye.
When using DPH, the number of the fluorescence spot increased because the microdroplet retract at the negative potential and the diameter of the fluorescence spot was mostly less than 16 m. The number of the fluorescence spot with a diameter with 4 m drastically increased.
In addition, the number of the fluorescence spot with a diameter 3 m newly appeared.
When using Pr, the fluorescence spot with 5 m diameter already existed at 0.0 V. After potential sweeping to -0.7 V, the number of droplets with diameter around 6-20 m drastically increased.
Number of droplets
Droplet diameter /m