pubs.acs.org/Langmuir © 2010 American Chemical Society
Plasmon-Enhanced Luminescence from Ultrathin Hybrid Polymer Nanoassemblies for Microscopic Oxygen Sensor Application Masaya Mitsuishi,*,† Hiroyuki Tanaka,† Makoto Obata,‡ and Tokuji Miyashita† †
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan, and ‡Interdisciplinary Graduate School of Medicine and Engineering, University of Yamanashi, 4-4-37 Takeda, Kofu 400-8510, Japan Received August 10, 2010. Revised Manuscript Received September 1, 2010 Plasmon-enhanced luminescence was developed for luminescent oxygen sensor application. Luminescent polymer Langmuir-Blodgett films containing platinum-porphyrin were assembled plane-to-plane with a silver nanoparticle array. The hybrid polymer nanoassemblies allow more than 10-fold luminescence enhancement in air. The luminescence intensity and lifetime measurements as functions of the number of layers revealed that some platinum-porphyrin, which is close to silver nanoparticles, is effectively enhanced. The enhancement enables us to monitor 2D oxygen distribution mapping on the micrometer scale.
Introduction The use of metal nanoparticles provides many fascinating features and possibilities that are strongly related to localized surface plasmons. Myriad studies dealing with fundamental concepts and application-oriented work pertaining to optoelectronic devices, markers, and sensors have been reported.1-3 Indeed, plasmon-coupled sensors have been proposed and have come into practical use, particularly those addressing propagating surface plasmons generated on ultrathin metal film surfaces.4,5 Localized surface plasmons (LSPs),6 which also have attracted much attention because of their ease of handling and design flexibility, present sensing protocol limits in absorbance or transmittance because of their intrinsically strong quenching character.7,8 In terms of fundamental aspects, luminescence enhancement is a very hot topic because LSPs have opened new scientific fields such as nanophotonics.9,10 The advantages of ultrathin sensor systems are their ease of use, ease of remote placement, and high time resolution. Ultrathin films offer a fast response and high sensitivity because of the high surface-to-volume ratio.11 The disadvantages of the idea include a lack of sufficient signal intensity to be monitored because of the small number of luminophores, which suffers from difficulty in gaining a sufficient signal-noise ratio. Plasmon-enhanced luminescence is a promising approach to overcoming this disadvantage. Previously, we reported plasmon-enhanced luminescence from ruthenium *Corresponding author. Tel: þ81-22-217-5638. Fax: þ81-22-217-5638. E-mail:
[email protected].
(1) Maier, S. A. Plasmonics: Fundamentals and Applications; Springer: New York, 2007. (2) Willets, K. A.; Van Duyne, R. P. Annu. Rev. Phys. Chem. 2007, 58, 267–297. (3) Giljohann, D. A.; Seferos, D. S.; Daniel, W. L.; Massich, M. D.; Patel, P. C.; Mirkin, C. A. Angew. Chem., Int. Ed. 2010, 49, 3280–3294. (4) Homola, J.; Yee, S. S.; Gauglitz, G. Sens. Actuators, B 1999, 54, 3–15. (5) Knoll, W. Annu. Rev. Phys. Chem. 1998, 49, 569–638. (6) Hutter, E.; Fendler, J. H. Adv. Mater. 2004, 16, 1685–1706. (7) Anker, J. N.; Hall, W. P.; Lyandres, O.; Shah, N. C.; Zhao, J.; Van Duyne, R. P. Nat. Mater. 2008, 7, 442–453. (8) Stewart, M. E.; Anderton, C. R.; Thompson, L. B.; Maria, J.; Gray, S. K.; Rogers, J. A.; Nuzzo, R. G. Chem. Rev. 2008, 108, 494–521. (9) Schuller, J. A.; Barnard, E. S.; Cai, W. S.; Jun, Y. C.; White, J. S.; Brongersma, M. L. Nat. Mater. 2010, 9, 193–204. (10) Prasad, P. N. Nanophotonics; Wiley-Interscience: Hoboken, NJ, 2004. (11) Chu, B. W. K.; Yam, V. W. W. Langmuir 2006, 22, 7437–7443.
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complexes. Luminescence sensor application for oxygen was demonstrated; however, the hybrid polymer nanoassemblies showed no remarkable sensitivity to oxygen (Figure S9).12,13 The advantage of hybrid polymer nanoassemblies is the flexible design of the structure. Herein we demonstrate a very sensitive, effective oxygen luminescence sensor system based on plasmon-enhanced luminescence using ultrathin hybrid polymer nanoassemblies in which luminophores and metal nanoparticles are assembled plane-to-plane on the nanometer scale.13-15 The enhancement enables us to monitor 2D oxygen distribution mapping on the micrometer scale.
Results and Discussion We chose platinum(II) porphyrin (PtTPP) as a luminophore for oxygen sensing.16,17 Platinum porphyrins have longer luminescence (phosphorescence) lifetimes (∼100 μs),18-20 and the luminescence intensity is well known to depend strongly on the surrounding oxygen concentration. Luminescent polymer (a copolymer of N-dodecyl acrylamide (DDA) and PtTPP (p(DDA/PtTPP), Figure S1) nanosheets were assembled with a silver nanoparticle (AgNP) array using a bottom-up approach consisting of a LangmuirBlodgett technique and an immersion method. Four-layer pDDA nanosheets, n-layer p(DDA/PtTPP) nanosheets (n = 2, 4, and 10, luminescence layer), and two-layer p(DDA/DONH) nanosheets (cationic layer for AgNP adsorption) were transferred onto a quartz substrate in turn by vertical dipping. The substrate was immersed in an aqueous solution of AgNPs (62 nmφ), which was (12) Mitsuishi, M.; Ishifuji, M.; Endo, H.; Tanaka, H.; Miyashita, T. Mol. Cryst. Liq. Cryst. 2007, 471, 11–19. (13) Mitsuishi, M.; Ishifuji, M.; Endo, H.; Tanaka, H.; Miyashita, T. Polym. J. 2007, 39, 411–422. (14) Ishifuji, M.; Mitsuishi, M.; Miyashita, T. J. Am. Chem. Soc. 2009, 131, 4418–4424. (15) Mitsuishi, M.; Matsui, J.; Miyashita, T. J. Mater. Chem. 2009, 19, 325–329. (16) Papkovsky, D. B.; Olah, J.; Troyanovsky, I. V.; Sadovsky, N. A.; Rumyantseva, V. D.; Mironov, A. F.; Yaropolov, A. I.; Savitsky, A. P. Biosens. Bioelectron. 1992, 7, 199–206. (17) Lu, X.; Winnik, M. A. Chem. Mater. 2001, 13, 3449–3463. (18) Stich, M. I. J.; Schaeferling, M.; Wolfbeis, O. S. Adv. Mater. 2009, 21, 2216– 2219. (19) McDonagh, C.; Burke, C. S.; MacCraith, B. D. Chem. Rev. 2008, 108, 400– 422. (20) Amao, Y. Microchim. Acta 2003, 143, 1–12.
Published on Web 09/07/2010
DOI: 10.1021/la103175b
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Figure 1. (top) Layer structure of p(DDA/PtTPP)n and HP-n’s. (a) Extinction spectra and (b) luminescence spectra of p(DDA/PtTPP)4 and HP-4. The samples were excited at 400 nm in air.
Figure 2. (a) Luminescence spectra of HP-4 as a function of oxygen concentration; (b) Stern-Volmer plot of HP-2 (diamonds), HP-4 (triangles), and HP-10 (circles); and (c) time course of the luminescence intensity of HP-4. Solid curves in plot b are fitted ones using the modified two-site model.
prepared using a citrate-reduction method. The averaged surface limiting area of p(DDA/PtTPP) and the mole content of PtTPP in p(DDA/PtTPP) were 0.3 nm2 and 3 mol %, respectively. Consequently, the surface density of PtTPP in its monolayer was determined to be 1.0 105 (μm-2). All polymer nanosheets have 1.7-2 nm monolayer thickness. Control experiments using fourlayer p(DDA/PtTPP) nanosheets (p(DDA/PtTPP)n, n = 2, 4, and 10) and four-layer hybrid polymer nanoassemblies (HP-n, n = 2, 4, and 10) revealed that more than 10-fold luminescence enhancement was achieved by a localized surface plasmon generated from silver nanoparticle arrays (Figure 1b). In our case, p(DDA/PtTPP) nanosheets were 4 nm away from the AgNP 15118 DOI: 10.1021/la103175b
array using cationic polymer nanosheets as a template. In fact, AgNP is distributed randomly with no aggregate formation. The surface density and the surface coverage of AgNPs were, respectively, 148 ( 30 μm-2 and 42 ( 3% (Figure S3). The enhanced luminescence was also very sensitive to molecular oxygen (Figure 2a); the luminescence intensity at 660 nm decreased drastically as the oxygen concentration increased. Figure 2b depicts Stern-Volmer plots of HP-n’s. They show an almost linear relationship between I0/I and [O2], indicating that the HP-n film is a good material for oxygen sensing. (21) Demas, J. N.; Degraff, B. A.; Xu, W. Y. Anal. Chem. 1995, 67, 1377–1380.
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All Stern-Volmer plots were fitted to the two-site model, described as shown below.21 I0 ¼ I
1 f1 f2 þ 1 þ KSV1 ½O2 1 þ KSV2 ½O2
Therein, KSV1 and KSV2 are Stern-Volmer constants, and f1 and f2 are fractions (f1 þ f2 = 1).We assumed that KSV2 is negligible (Table 1) because the first trials gave very small KSV2 values (|KSV2| , 0.005); the second term in the denominator is independent of oxygen concentration. The f2 values of HP-n’s are 10 times larger than those of p(DDA/PtTPP)n’s; the values remain the same regardless of the number of layers. In other words, the f2 component reflects film inhomogeneities and uniform AgNP immobilization (Figure S3). The HP film’s response is quick, reproducible, and reversible; the luminescence intensity changes immediately (Figure 2c) by varying the oxygen concentration within 5 s, which is the rate limit of the injected gas flow. The luminescence from HPs is stable under 400 nm light irradiation. The changes in the intensity at [O2] = 0% was less than 2% after Table 1. Oxygen Quenching Parameters Using the Modified Two-Site Modela sample
I0/I100
KSV1
f1
f2
HP-2i 12 0.124 0.990 0.010 HP-2o 12 0.162 0.973 0.027 HP-4 22 0.275 0.988 0.012 HP-10 25 0.321 0.990 0.010 P(DDA/PtTPP)2 77 1.092 0.997 0.003 P(DDA/PtTPP)4 77 0.949 0.998 0.002 P(DDA/PtTPP)10 74 0.825 0.998 0.002 a Regarding HP-2s, HP-10, and p(DDA/PtTPP)n’s, see Figures S4 and S5 in Supporting Information.
50 min of irradiation. The time response remains in the domain of seconds at the moment because of our less-developed technique using a gas flow system. However, we believe that the time response can eventually be faster than the 1 s range. The Stern-Volmer plot gradient increases as the number of layers increases (Figure 2b). Plots of the luminescence intensity ratio of HP-n’s at different oxygen concentrations to those of respective p(DDA/PtTPP)n’s enable us to consider the effects of LSP-enhanced luminescence on oxygen sensitivity (Figures 3a and S4). The ratios become larger and saturated as the oxygen concentration increases. An interesting phenomenon is that the luminescence enhancement at [O2] = 100% decreases as the number of layers increases: 25 (HP-2), 17 (HP-4), and 11 (HP10), which indicates that the luminescence enhancement is most effective in HP-2. The solid curves in Figure 3a are fitting ones, assuming that the lifetimes of p(DDA/PtTPP)n’s at [O2] = 0 (%) are the same (18.3 μs, see below). From the fitting, we obtained the averaged radiative and nonradiative rates of PtTPP and the enhancement factor: (1.13 ( 0.2) 104 s-1, (4.36 ( 0.2) 104 s-1, 28.0 (HP-2), 18.0 (HP-4), and 12.0 (HP-10), respectively (Table S1). The phosphorescence quantum yield of PtTPP (0.21) shows good agreement with that found in the literature,22 though the value was obtained using a simple model. In terms of the sensitivity (I0/I100, see Table 1), however, a more detailed consideration of the luminescence enhancement is necessary. In the case of HP-2, its sensitivity is independent of the layer deposition order; the sensitivity of outermost p(DDA/PtTPP) nanosheets (HP-2o) was almost identical to that of inner p(DDA/ PtTPP) nanosheets (HP-2i) (Figures S6 and S7). The I0/I100 value of HP-2’s (12) is much smaller than that of p(DDA/PtTPP)2 (77). We conclude that two HP-2’s have a small but significant number of PtTPP components that are emissive even at [O2] = 100%. In other words, some PtTPP’s located near AgNP’s are enhanced by AgNP’s and undergo no luminescence quenching by oxygen,
Figure 3. (a) Luminescence intensity ratios of HP-2 (diamonds), HP-4 (triangles), and HP-10 (circless). (Bottom) Time-resolved luminescence decay curves for (b) HP-4 and (c) p(DDA/PtTPP)4. Solid curves in plot a are fitted ones. See details in the Supporting Information. Langmuir 2010, 26(19), 15117–15120
DOI: 10.1021/la103175b
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Mitsuishi et al. Table 2. Fitting Parameters Obtained from Figure 3b,c
sample
[O2] (%)
τ1 (μs)
P(DDA/PtTPP)4
0 20 100
18.3 2.44 1.01
HP-4
0 20 100
1.08 0.37 0.18
τ2 (μs)
A1
A2
0.078 0.088 0.066 11.1 2.53 1.10
0.106 0.181 0.393
0.101 0.075 0.081
although the detailed mechanism is unknown and should be clarified. Lifetime measurements reveal that the lifetimes of p(DDA/PtTPP)4 can be described by a single-exponential function whereas those of HP-4 can be described by a doubleexponential function (Figure 3b,c and Table 2). The lifetime of p(DDA/PtTPP)4 decreases as [O2] increases. The lifetimes of both the fast and slow components of HP-4 decrease with increasing oxygen concentration, and the fast component is predominant at [O2] = 100%. The longer lifetimes (slow components) of HP-4 as a function of oxygen concentration are identical to those of p(DDA/PtTPP)4. These results imply that part of the PtTPP’s, which are not enhanced by AgNP’s in HP-4, emit luminescence similarly to PtTPP’s in p(DDA/PtTPP)4 and that most of the enhanced PtTPP’s are quenched by molecular oxygen in HP-4. The ratios of the luminescence intensity at [O2] = 0% were 3.8 (HP-2), 4.7 (HP-4), and 3.8 (HP-10). The luminescence enhancement reaches its maximum for HP-4. We can estimate the extent of emissive PtTPPs at [O2] = 100% in HP-n’s to those at [O2] = 0%: 7.1, 3.3, and 2.7% for HP-2, HP-4, and HP-10, respectively (Figure S4). Assuming that the electromagnetic field enhancement at the excitation wavelength (400 nm) is a predominant factor for the luminescence enhancement in HP-n films, these results imply that LSPs decay in strength exponentially on the nanometer scale (d1/e = 1.9 (nm), Figure S8) away from the nanoparticle edge and that they enhance the luminescence from PtTPPs. Considering the extents of emissive PtTPPs and the surface coverage of AgNPs (42%, see above), ∼100 PtTPP molecules per single AgNP are strongly enhanced in HP-n’s, undergoing no oxygen quenching. Regarding the possibility of another quenching process, AgNPs may quench the luminescence from nearby PtTPPs, though it is difficult to distinguish the process under luminescence enhancement conditions. An energy transfer would occur between excited PtTPP and AgNP’s because of the spectral overlap (Figure 1a,b). However, because the luminescence enhancement reaches its maximum at HP-4 at [O2] = 0 (%), it is likely that the energy transfer takes place in HP-n’s. The lowered I0/I100 value described above provides another benefit of the LSP sensor for oxygen sensing. Figure 4b shows microscopic luminescence images of HP-4. A control experiment was also done using p(DDA/PtTPP)4 (Figure 4a). We generated a nitrogen gas stream on the surface via a flow from the bottom right corner. Both ultrathin films offered uniform luminescence images at zero nitrogen flow rate, implying that PtTPPs and AgNPs are uniformly distributed. It is particularly interesting that the nitrogen gas stream was viewed clearly as luminescence images (22) Hanson, K.; Tamayo, A.; Diev, V. V.; Whited, M. T.; Djurovich, P. I.; Thompson, M. E. Inorg. Chem. 2010, 49, 6077–6084.
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Figure 4. Schematic of microscopic luminescence imaging for (a) p(DDA/PtTPP)4 and (b) HP-4 under different nitrogen flow rates. The bright areas reflect the lower oxygen concentration. The white scale bar corresponds to 1 mm.
in HP-4; both the main stream and an expanding substream are visible. The brighter area reflects the lower oxygen concentration. In the case of p(DDA/PtTPP)4, the luminescence intensity at [O2] = 0% decreased to the one-fifth level at [O2] = 4%. Microscopic monitoring of the surface oxygen distribution from 0 to 21% was difficult using p(DDA/PtTPP)4 (Figure 4a). However, HP-4 maintains the luminescence intensity above the onefifth level even at [O2] = 16% (Figure S4b). Consequently, HP-4 enables us to monitor the 2D oxygen distribution in the wider oxygen concentration range of 0-21% (Figure 4b). In conclusion, we have demonstrated that it is possible to obtain an ultrathin sensor system based on plasmon-enhanced luminescence; enhanced PtTPP’s are very sensitive to oxygen, although some of them are insensitive to it and maintain an emissive state even at [O2] = 100%. The present bottom-up approach allows us to construct well-defined polymer nanoassemblies that retain the same p(DDA/PtTPP) nanosheet structure, leading to the quantitative characterization of plasmonenhanced luminescence. Two-dimensional oxygen distribution mapping was demonstrated on the micrometer scale with a much clearer contrast. The ultrathin sensor system is applicable to a narrowly confined area. A luminescence sensor for temperature is also an attractive application of hybrid polymer nanoassemblies. Work toward that goal is now in process. Acknowledgment. This work was supported by grants-in-aid for scientific research ((S) no. 17105006 and (B) no. 20350100) from the Japanese Society for the Promotion of Science (JSPS) and for a priority area (470, no. 21020005) from the Ministry of Education, Culture, Sports, Science and Technology of Japan. Supporting Information Available: Materials and experimental details. This material is available free of charge via the Internet at http://pubs.acs.org.
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