Langmuir 2000, 16, 9137-9141
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Micro-heterogeneous Oxygen Response in Luminescence Sensor Films Joanne M. Bedlek-Anslow,† J. Paul Hubner,‡ Bruce F. Carroll,‡ and Kirk S. Schanze*,† Department of Chemistry, University of Florida, Gainesville, Florida 32611-7200, and Department of Aerospace Engineering, Mechanics & Engineering Science, University of Florida, Gainesville, Florida 32611-6250 Received August 14, 2000. In Final Form: September 21, 2000 A fluorescence microscopy technique has been developed that allows investigation of the luminescence properties of working film-based O2 sensors with spatial resolution of 5 µs), a feature that would suggest that the emission should be quenched at high Pair.27 However, SV studies of the two sensors using a conventional fluorescence spectrometer indicate that they exhibit dramatically different luminescence intensity responses to varying Pair values (see Supporting Information for SV plots). First, as anticipated, the PtTFPP sensor exhibits an excellent SV response to Pair. The film features a large KSV value of 0.87 psi-1 (r2 ) 0.991), which indicates that the sensor’s luminescence intensity decreases by more than a factor of 10 when Pair increases from 0 to 14.6 psi. By contrast, the Rudpp/PDMS film exhibits a poor SV response: the film has a very low KSV value of 0.016 psi-1
(r2 ) 0.968), which indicates that the luminescence intensity of this film decreases by less than a factor of 2 when Pair increases from 0 to 14.6 psi. In previous studies of oxygen sensor films that contain ruthenium(4,7-diphenyl-1,10-phenanthroline)32+ salts, it has been suggested that the sensor response may be reduced when the metal complex exists in the film in an aggregated (or microcrystalline) state.28 Although we had no direct evidence on this point before initiating the microscope experiments outlined below, we suspected that the dramatic difference in sensor response of the PtTFPP and Rudpp sensors arises because the two dyes have a different ability to disperse or dissolve in the PDMS binder. Specifically, we suspected that the PtTFPP dye is molecularly dispersed in the polymer matrix and the Rudpp dye is not. Clear evidence supporting this hypothesis is presented below. Fluorescence Microscopy of PtTFPP/PDMS Sensor Films. A fluorescence microscope image of a PtTFPP/ PDMS film obtained at Pair ) 0.5 psi with a 10× objective is shown in Figure 1a. The image is undoubtedly featurelesssthe evenness of the gray-tone indicates that the luminescence from PtTFPP is spatially uniform. We conclude that the dye is evenly dispersed within the PMDS binder. To probe the SV response of the PtTFPP sensor with high spatial resolution, a series of quantitative image maps of the sensor’s response to Pair were generated using the 40× objective (see the Experimental Section for the method). Five 111 µm × 88 µm regions of a PtTFPP/PDMS film were interrogated, affording five KSV(x,y) image maps along with the statistical data listed in Table 1. Figure 2a illustrates the luminescence intensity distribution for one representative region (Ic(x,y;Pair)0.2 psi)), and Figure 2b illustrates the KSV image map for the identical region. It is clear from the KSV image map in Figure 2b that the sensor response of the PtTFPP/PDMS film is uniform, even on a length scale < 5 µm. This uniformity is also seen on longer length scales, as demonstrated by the consistency of the KSVavg values for the 5 regions (Table 1). The uniformity within each microscopic region is confirmed by the fact that the standard deviations (σKSV) in KSV are low for every region. Indeed, the σKSV values observed for the PtTFPP/PDMS film likely represent the scatter
(27) Ji, H.-F.; Shen, Y.; Hubner, J. P.; Carroll, B. F.; Schanze, K. S. Appl. Spectrosc. 2000, 54, 856-863.
(28) Draxler, S.; Lippitsch, M. E.; Klimant, I.; Kraus, H.; Wolfbeis, O. S. J. Phys. Chem. 1995, 99, 3162-3167.
ficients are simply used as an indication of Stern-Volmer linearity. Values less than 1 indicate nonlinear response. (3) A matrix of SV constants is then computed from the A and B matrixes according to
KSV[x,y] )
B[x,y] A[x,y]
(6)
(4) Standard deviations in r[x,y] and KSV[x,y] (σr and σKSV, respectively) are computed for each spatial field of view. Each of these operations is carried out for images obtained on five separate microscopic regions of a sensor film. (5) An average SV constant (KSVavg) is computed for each microscopic spatial field of view according to eq 7,
∑∑ K
SV[i,j]
KSV
avg
)
i
j
i×j
(7)
where i × j represents the size of the CCD image (typically 650 pixels × 515 pixels). (6) Finally, the macro allows the user to create false-color images that delineate the pixel-by-pixel values of KSV (and other parameters), where the individual values of KSV are indicated by a color map.
Results and Discussion
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Langmuir, Vol. 16, No. 24, 2000
Letters
Table 1. Stern-Volmer Analysis of Microscopic Regions of Sensor Filmsa PtTFPP/PDMS region 1 2 3 4 5 averaged
Rudpp/PDMS
KSVavg b/psi-1 σKSVc/% KSVavg b/10-2 psi-1 σKSVc/% 0.855 1.54 1.38 1.48 1.63 1.38
7.84 7.01 6.52 7.09 8.04 19.8e
0.8 1.5 1.4 1.2 0.9 1.2
58.6 37.7 34.4 34.9 56.4 25.0e
a Data acquired using a 40× objective lens. Analysis of 111 × 88 µm2 regions (650 pixels × 515 pixels). b Stern-Volmer constant computed for the microscopic area according to eq 7; see text. c Standard deviation in K 2 SV within each 111 × 88 µm microscopic region expressed as % of KSV. d Average of the KSVavg values for each of the five regions. e Standard deviation in the average of the five KSVavg values expressed as %.
Figure 2. Fluorescence intensity image and KSV(x,y) image map of a PtTFPP/PDMS film obtained using a 40× objective. White scale bars are 23.5 µm long (images are 111 µm × 88 µm; 650 × 515 pixels; calibration ) 0.169 µm‚pixel-1). (a) Intensity image obtained at Pair ) 0.2 psi; intensity color scale is shown at left. (b) KSV(x,y) image map for the identical region; KSV color scale is shown at left.
expected due to random noise in the imaging experiments (shot noise, etc). In summary, the PtTFPP/PDMS sensor appears to behave as an ideal O2 sensor on length scales ranging from millimeters to nanometers. This ideal behavior clearly arises because the PtTFPP dye is well dispersed within the PDMS matrix. However, the ideal behavior of the PtTFPP/PDMS sensor also suggests that the PDMS matrix is homogeneous, at least with respect to O2 permeability, on length scales approaching 1 µm. Fluorescence Microscopy of Rudpp/PDMS Sensor Films. A fluorescence microscope image of a typical Rudpp/ PDMS film obtained at Pair ) 0.3 psi with the 10× objective is illustrated in Figure 1b. This image has a striking “starfield appearance”sthere are very bright fluorescent spots with sizes ranging from 1 to 10 µm. The spots are superimposed on a background field that has a varying fluorescence intensity. Quite clearly the Rudpp dye is not evenly dispersed within the PDMS binder. Indeed, it is
possible that the dye is present in some regions as microcrystals or in a strongly aggregated state. The effect of the inhomogeneous distribution of the Rudpp dye in the PDMS binder on the spatial distribution of the SV response was probed by making a series of quantitative image maps of the sensor’s response to Pair using the 40× objective. Thus, five 111 µm × 88 µm regions of a Rudpp/PDMS film were interrogated, affording five KSV(x,y) image maps along with the statistical data listed in Table 1. Figure 3a-c illustrates the luminescence intensity distribution (Ic(x,y;Pair)0.2 psi)) for three representative regions of the film, and Figure 3d-f illustrates the KSV image maps for the identical three regions. The microscopic SV studies on the Rudpp/PDMS sensor reveal a number of significant features. First, on a qualitative level, the KSV images (Figure 3d-f) and the statistical data listed in Table 1 clearly demonstrate that the inhomogeneous distribution of Rudpp in the PDMS causes the film to exhibit a spatially heterogeneous sensor response. Indeed, over the five different 111 µm × 88 µm regions of the film, the SV response varies significantly, reaching a maximum at 0.05 psi-1 and a minimum of
