1. Introduction

The Purcell effect has been widely used in the development of photonic bandgap materials [1], plasmonic nanostructures [2], high Q optical cavities [3], cavity quantum electrodynamics [4], and meta-materials [5]. To date, most studies [6–8] have focused on applying the Purcell effect to construct photonic nanostructures with desired photon emission characteristics, such as single photon sources. The Purcell effect has also been used to enhance or quench fluorescence intensity in the vicinity of metal nanostructures [9,10]. In this paper, we take advantage of such phenomena and demonstrate the feasibility of using the Purcell effect to accurately quantify physical displacement with nanoscale accuracy. Specifically, our method is based on the observation that due to the dramatically modified photonic density of states (PDOS), the emission dynamics of fluorophores placed in the vicinity of a plasmonic nanostructure depends critically on the distance between the fluorophores and the nanostructure. Consequently, it is possible to accurately characterize the morphology of the nanostructure through fluorescence lifetime (FL) measurements.

To demonstrate the advantage of the FL-based approach, we apply this method to characterize amine-rich polyelectrolyte multilayer (PEM) films, which is a class of stimulus-responsive polymer films [11,12] with the unusual property that they reversibly expand and shrink by up to 600% in response to changes in ambient pH [13]. These films have found numerous applications including controlling cell proliferation and attachment [14], drug release [15], and hysteretic gating [16]. By incorporating such PEM films into an active plasmonic nanostructure that contains both fluorophores and metal structures, we can both dynamically tune this plasmonic nanostructure as well as monitor its mechanical changes with extremely high accuracy. Since our method is non-contact in nature, it can be used to measure nanoscale changes associated with a monolayer embedded within a film, and could also be applied to the monitoring of intracellular or in vivo processes, where techniques such as atomic force microscopy (AFM) no longer apply. To the best of our knowledge, such a measurement has not previously been demonstrated on stimulus-responsive polymer films. Additionally, we aim to use two classes of examples to demonstrate the possibility of using PEM films to construct active and tunable plasmonic nanodevices. The first structure consist of a swellable PEM film “sandwiched” between a planar gold substrate and colloidal quantum dots (QD). The second structure contains a monolayer of Texas Red (TR) incorporated with the polymer at the bottom of the PEM film, and gold nanospheres adsorbed over the top surface of the PEM film.

2. Sample fabrication and method validation

The first class of samples was fabricated using the procedure illustrated in Fig. 1(a). We immersed a glass slide covered with a 100 nm thick gold film in a 1 mM solution of mercaptohexadecanoic acid (MHDA) to create a negatively charged surface [17]. Then, we built PEM films following the standard procedure. Specifically, we alternately immersed the sample in a positively charged poly(allylamine hydrochloride) PAH solution and a negatively charged poly(styrene sulfonate) (PSS) solution (each with 10 mM concentration on a monomer basis), ending with PAH deposition to create a positively charged surface. Finally, we immersed the sample in a suspension of negatively charged CdSe/ZnS core/shell QDs (NN-LABS, emission peak at 621 nm, mean diameter 5 nm) to deposit QDs onto the PEM surface. Photoluminescence from QDs was measured with time-correlated single-photon counting (TCSPC) (Picoharp 300, PicoQuant), where a pulsed 473 nm laser diode (BDL-473-C, Becker and Hickl) was used for optical excitation.

 

Fig. 1 (a) Schematic illustration of the first class of plasmonic nanostructure. (b) PDOS enhancement as a function of the distance d between a fluorophore and the planar Au substrate when the upper medium is air (solid line) or water (dashed line). The refractive index of the Au is taken from Ref. 16. The index of the PEM film is 1.45, as determined by ellipsometry. (c) Experimental QD photoluminescence produced by four samples with different values of d. (d) Comparison between film thicknesses obtained using FL measurements and ellipsometry. The four points correspond to the four samples in (c).

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The details of the procedure for thickness measurement have been published elsewhere [6]. To summarize, we first calculate the average QD fluorescence lifetime $\tau = {{\displaystyle \int }}_0^{\infty }t I(t)dt/{{\displaystyle \int }}_0^{\infty }I(t)dt$ [18], where I(t) is the measured fluorescence intensity. Near a plasmonic structure, τ depends on the intrinsic fluorescence lifetime τ_o, the quantum yield q, and the modified PDOS, and can be written as:

We first validated our method through direct comparison with ellipsometry. The films used at this juncture were assembled at neutral pH and have a definite thickness that does not vary with pH. Since the QDs are located on top of the PEM film, the value of d is essentially the sum of film thickness and QD radius. In Fig. 1(c), we show QD photoluminescence traces from four samples with different film thicknesses obtained through different numbers of PAH/PSS bilayers. Note that the intensity decay is multi-exponential, due to QD blinking [20,21]. Figure 1(d) compares film thicknesses obtained with FL measurements (d_PDOS) and ellipsometry (d_Ellip) in different samples. The good agreement clearly demonstrates that our approach can measure film thickness with nanoscale resolution.

3. Characterization of swellable PEM films

Next, we investigated the pH-induced swelling and shrinking (or deswelling) in amine-rich PEM films. These films were assembled on gold substrates using the procedure described above, except that all polyelectrolyte solutions were adjusted to pH 9.3 with NaOH. The high pH ensures the film contains a large surplus of weakly cationic amines that are responsible for the swelling/deswelling behaviors, since the film can lose or gain charges as the solution pH changes [13–16]. The tunable PEM films were either a single layer of PAH, or a three-layer structure containing PAH/PSS/PAH. QD Photoluminescence was measured with samples placed in aqueous solutions. The solution pH was adjusted by adding HCl or NaOH. For a swelling cycle, we gradually reduced solution pH from 10 to pH 3. For deswelling, we simply raised solution pH from 3 to 10. At each pH value, samples were immersed in the solution for 10 minutes for stabilization, before FL measurements.

As a control, we measured QD FL as a function of pH. The sample contained QDs adsorbed onto a planar glass substrate that was coated with a single PAH layer. The QD FL was 7.31 ± 0.11 ns at pH 3, 7.30 ± 0.15 ns at pH 6.1, and 7.25 ± 0.11 ns at pH 10.5. This demonstrates that the QDs were unaffected by pH changes in the range of 3 to 10.5.

With tunable PEM films assembled on gold substrates, the FL became highly dependent on the pH. Figure 2 shows several representative examples of photoluminescence produced by QDs assembled on Au/PAH and Au/PAH/PSS/PAH structures and measured at different pH values. Additionally, Fig. 3(a) and 3(b) respectively show the average QD FL for the PAH and PAH/PSS/PAH samples as we vary ambient pH. The hysteresis behavior is obvious; in both samples, the value of τ clearly depends on whether we decrease or increase solution pH values, although the hysteresis is much pronounced in the PAH/PSS/PAH sample. Applying Eq. (1), we can extract film thicknesses from the measured FL, as indicated in Fig. 3(c) and 3(d). We see that the PAH film varies in thickness between 7 and 18 nm, while the PAH/PSS/PAH film varies between 12 nm and 42 nm.

 

Fig. 2 Photoluminescence produced by QDs assembled above Au at different pH values in (a) a PAH single layer film and measured during the swelling cycle and (b) a PAH/PSS/PAH three-layer film and measured during the deswelling cycle.

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Fig. 3 Average QD FL measured using (a) PAH sample and (b) PAH/PSS/PAH sample. Corresponding film thicknesses of (c) PAH sample and (d) PAH/PSS/PAH sample. The red squares and green circles represent data generated during swelling and deswelling cycles, respectively.

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The large hysteresis associated with the three-layer PEM structure in Fig. 3(d) is consistent with previous reports [13–16]. However, to our best knowledge, the much smaller hysteresis observed in the single PAH sample has not been reported previously but is not surprising. The film swelling and deswelling is due to variations in osmotic pressure as excess amines in the film gain or lose charge when the pH is tuned through the amines’ pK_a. As the pH is raised, the amines lose their charge and coalesce into hydrophobic regions [16], and will in this state have a much lower pK_a than in their ionized, hydrophilic state. Consequently, a large lowering of pH is required to reionize them and thus rehydrate the film. This shift in pK_a depends on the nature of the accompanying polyanion, with more hydrophobic polyanions giving rise to larger shifts [22] and greater hysteresis [13]. Although the exact role of the polyanion is not well understood in this mechanism, it is reasonable that hysteresis should be minimum in the complete absence of polyanions in the film as in our single PAH layer.

To further validate the FL-based measurements, we measured the thickness of the tunable PEM films using liquid cell AFM, where the sample was placed in a liquid cell filled with a pH-adjusted aqueous solution. Then, the AFM tip in contact mode and at high pressure was used to remove a section of the film before imaging film height in tapping mode, as shown in Fig. 4(a). The height profile along the diagonal line, plotted in Fig. 4(b), shows a clear step corresponding to the film thickness.

 

Fig. 4 (a) AFM image of a single layer of PAH film in a liquid cell at pH 10.5. The AFM tip was used to remove the film from the central dark square in the image and the excessive material was accumulated into the bright regions on either side. The height profile along the dashed white line is shown in (b). The two dashed lines in (b) give the average height of regions with PEM removed and PEM intact, respectively. The difference between the two is d_AFM. (c) Comparison between PEM film thickness extrapolated using the FL and the AFM measurements. The four data points are obtained using the single layer and the three layer sample, measured at pH 3 and pH 10.5, respectively.

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In Fig. 4(c), the thicknesses d_PDOS obtained through FL measurements are compared to those from AFM (d_AFM). The four data points are obtained using the monolayer PAH sample and the three-layer sample, measured in the swelled state (at pH 3) and the deswelled state (pH 10.5). The good agreement between the two approaches again confirms that the FL-based approach can achieve nanoscale resolution.

4. Characterization of an active and tunable plasmonics nanostructure

We can also use swellable polymer to construct another plasmonic nanostructures that are active and tunable. As a demonstration, we consider a second class of sample shown schematically in Fig. 5(a), where the positions of fluorophores and plasmonic structures are reversed from the first class of structures. The fluorophore was contained in an initial layer of PAH that was conjugated [23] with Texas Red (TR) dye (Sigma-Aldrich, emission peak at 620 nm) and assembled on a glass substrate. This was followed by assembly of five PSS/PAH bilayers of swellable film (assembled at pH 9.3) and capped with small (3 nm diameter) negatively charged gold nanospheres (Purest Colloids).

 

Fig. 5 (a) Schematic illustration of the second type of nanostructure studied. Fluorescence decay produced by the nanostructure during (b) the swelling cycles and (c) the deswelling cycles. The TR fluorescence measured without Au nanoparticle deposition is also shown in (b). (d) FL of the TR dye as a function of the pH. The squares and circles represent data measured during swelling and deswelling cycles, respectively.

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Again, we began by ensuring that the FL of TR is unaffected by changes in pH values. A single layer of TR-PAH was assembled on a negatively charged planar glass substrate, and the measured photoluminescence is shown in Fig. 5(b). The TR’s FL was 4.25 ± 0.05 ns at pH 3, 4.27 ± 0.03 ns at pH 6, and 4.32 ± 0.03 ns at pH 10.5. Again, this result confirms that varying pH between 3 and 10.5 has little impact on TR fluorescence.

Representative photoluminescence generated by the TR/gold nanoparticle samples are shown in Fig. 5. The FL clearly depends significantly on pH, which can be attributed to the modified PDOS caused by mechanical displacement of the gold nanoparticles as the PEM film swells and deswells. The multi-exponential decay is clearly distinct from the emission dynamics of TR dye without any gold nanoparticles in its vicinity. The change in fluorescence dynamics can be explained by the fact that gold nanospheres are randomly positioned across the PEM film. As such, the distance from each dye molecule to the neighboring gold nanosphere varies, which is why the fluorescence emission is composed of a range of lifetimes. It may be possible to extract the distance distribution from the decay function by way of an inverse Laplace transform. Such an analysis, however, is beyond the scope of this paper, and will be left as the subject of future work. Even without this analysis, we can nonetheless reproduce similar hysteretic behaviors observed in the QD decorated samples by plotting the average FL vs pH, as shown in Fig. 5(d).

5. Conclusion

In conclusion, we reported a novel nanoscale measurement method based on the modified FL (or PDOS) in the vicinity of plasmonic nanostructures, i.e., the Purcell effect. Through direct comparison with ellipsometry and AFM measurements, we find the FL-based method approach can measure mechanical displacement with nanometer accuracy. The FL-based approach also enjoys significant advantages such as being noncontact and capable of monitoring nanomaterials embedded with condensed matter structures. As an application of the FL-based approach, we applied it to characterize the swelling/deswelling behavior of PAH and PAH/PSS/PAH films. Our results indicate a significant difference in the hysteresis dynamics between the two cases. Finally, we experimentally characterized a plasmonic nanostructure containing dye molecules and swellable PEM films, and demonstrated a tunable Purcell effect. To the best of our knowledge, we are the first group to combine stimuli-responsive polymers, fluorophores, and plasmonic nanoparticles to form active and tunable plasmonic nanostructures. Such structures may find a myriad of nanophotonic applications that demand tunability and optical amplification or gain.