A.
Steinbrück
a,
J.-W.
Choi
b,
S.
Fasold
a,
C.
Menzel
a,
A.
Sergeyev
a,
T.
Pertsch
a and
R.
Grange
*a
aFriedrich-Schiller-Universität Jena, Institute of Applied Physics, Abbe Center of Photonics, Albert-Einstein Straße 15, 07745 Jena, Germany. E-mail: rachel.grange@uni-jena.de
bSwiss Federal Institute of Technology Lausanne (EPFL) Optics Laboratory, School of Engineering, CH-1015 Lausanne, Switzerland
First published on 12th November 2014
In this work, we show local laser-induced heating in fluids with gold nanodot arrays prepared by electron-beam lithography that cover resonances in the near infrared spectral range from 750 nm to 880 nm. We utilize two approaches to demonstrate thermal effects, solvent evaporation and flow stop, with a thermosensitive polymer solution. We show that with fluences as low as 4 μJ cm−2, significant heating of the nanostructures occurs in their immediate vicinity. We perform power and wavelength dependent measurements to determine the threshold of the thermal effects. Using wavelengths about 20 nm away from the plasmonic resonance peak, the heating drops drastically, and 30 to 40 nm away, there is mostly no thermal effect. Therefore, working close to the threshold laser power offers the possibility of multiplexed reactions or sensing without cross-talk even though a typical full width at half maximum of a plasmonic resonance spectrum can be as broad as 200 nm. Additionally, comparison with theoretical calculations of heat generation show good agreement with the experimentally determined threshold powers.
The recent theoretical works give precise insights about temperature changes and the different mechanisms involved depending on the laser sources or the metallic structures (e.g. spheres or rods).30,32 However, to the best of our knowledge no quantified experimental investigations of heat-induced effects at different wavelengths and incident powers have been performed, especially with pulsed illumination. Here, we combine thermo-plasmonics and optofluidics to accurately determine threshold powers needed to perform heat-induced experiments from gold nanodot arrays into fluids. We use electron-beam lithography to fabricate gold nanostructure arrays on glass very locally instead of depositing colloidal nanoparticles on a large area and to obtain reusable devices by avoiding detachment of the polymer chip that contains the microfluidic channels. By varying the sizes of the dots we cover localized surface plasmon resonances in the near infrared spectral range from 750 to 880 nm. For each nanostructure, we will accurately determine the threshold laser power inducing significant heating of the gold dots thereby heating liquid in a microfluidic channel. Working close to this threshold power offers the possibility of multiplexed reactions or sensing without cross-talk even though plasmonic resonances can be as broad as 200 nm full width at half maximum (FWHM). As a proof of principle for light- or heat-induced reactions, we perform fluidic experiments of solvent evaporation at the liquid–air interface and immobilizing effects with a thermosensitive polymer solution in microfluidic channels. To quantitatively study both effects, we conduct power- and wavelength-dependent measurements on- and off-resonance to determine thresholds and show that close-by resonances can be well used for multiplexing experiments by working close to the laser threshold power to limit cross-talk. To complete our study, we provide simulations of the heat generation based on the absorption of the nanostructures that match the experimental results.
For the future, we envision that reactions that require light or heat could be conducted in a microfluidic device under well-controlled conditions in a closed and continuous flow regime.17 Especially when poisonous and/or expensive reactants are used or created, such an optofluidic device would be advantageous over conventional reactors.33 In this context, the synthesis of several products or the detection of several analytes in parallel (multiplexing) represents a great demand. Therefore, it is interesting to investigate the multiplexing possibilities offered by metallic nanostructures resonating at different wavelengths to perform multiple experiments on the same chip e.g. in different channels of the same microfluidic chip. In this work, we demonstrate full control of the plasmonic resonances and the particle location via electron-beam lithography. Furthermore, we can choose the reaction spot within the microfluidic channel and adapt to its sensitivity with laser illumination by varying the power or tuning the wavelength.
The microfluidic device was assembled by bonding an oxygen plasma-activated PDMS chip to a nanodot array chip carefully positioning the channels with respect to the nanodot array areas (Fig. 1a). For the optofluidic experiments, an inverse optical microscope (IX73, Olympus) was coupled with a tunable Ti:sapphire femtosecond pulsed laser (Fig. 1b).
The second possible case is the liquid flow itself that will be influenced. In our case, we use a pluronic solution. Pluronic is a thermosensitive, water-soluble triblock copolymer that remains liquid at room temperature and undergoes a sol–gel transition with increasing temperature (about 37 °C for the pluronic concentration used here).37 Thus, when the laser light heats the plasmonic carpet (in resonance) also the surrounding medium of the gold dots is heated generating a gel-like area locally (blue circle) that can hinder or stop the liquid flow in the channel (Fig. 3b). We will show experiments for both processes.
For each array investigated, we display three snapshots in Fig. 4 representing solvent evaporation above (left image in each panel), at (middle), and below (right) the threshold fluence. We define the threshold fluence for solvent evaporation as the fluence when it was still possible to produce (a few) droplets. Above the threshold fluence, many (small) droplets can be observed in close proximity to the meniscus in the snap shot pictures whereas below the threshold fluence, no such droplets appear (compare left and right images in each panel in Fig. 4). As the process of laser energy conversion to heat is directly linked to the absorption properties of the nanostructures32,38 we expect similar threshold fluences for all used wavelengths. Indeed, the absorption of the nanostructures (from FMM simulations in Fig. SI1†) is very similar for all the structures (13–17%). Thus, for all samples, we found quite similar threshold fluences spanning from 4 to 35 μJ cm−2. In Table 1 we summarize the average laser power and the corresponding fluences at the threshold for evaporation of the solvent. For comparison, 50 mW average power over a 10 μm laser spot diameter was used in experiments with continuous wave (cw) laser at 532 nm.17 Although the authors don't claim the power as threshold powers in the experiment, pulsed femtosecond (fs) irradiation is more effective for heat generation than cw illumination as we need much less average power (factor 20 to 60) to detect heat-induced effects.
The droplets (solvent condensate) we observe in our experiments are different in nature to so-called nano- or micro-bubbles described in the literature.36,39 Nanobubbles exist at solid–liquid interfaces and cannot be observed by our microscopic setup. Microbubbles do not consist of solvent steam but of air molecules that were dissolved in the liquid (usually water). When removing the laser source, microbubbles shrink slowly whereas solvent evaporation immediately stops. Baffou et al. investigated microbubbles that were generated by cw illumination.36 At high fs laser power, we also observe the formation of such microbubbles within the liquid-filled channel that do not disappear immediately after turning off the laser (not shown). Those bubbles slowly decrease in size and then disappear; they don't even move with the liquid flow. This sort of bubbles will not be discussed further in this paper.
Fig. 5 In-resonance laser experiment to determine fluence threshold for liquid immobilization at wavelengths 774 nm (a), 850 nm (b), and 880 nm (c) displayed as microscopic snapshot images from recorded videos (see videos 1–9 in ESI†). The applied fluence is given at the bottom of each image in μJ cm−2. The affected area is indicated by the black circle. The channel width is 30 μm. |
As described above, we did the experiments starting at high laser power and decreasing it stepwise until we could not observe an influence on the flow anymore including repeated experiment close to the threshold power necessary to still affect the flow. The circles in Fig. 5 indicate the area where beads are immobilized. The size of this area decreases with laser power and disappears below threshold.
When the liquid is flowing rather fast (sample with 775 nm resonance wavelength in Fig. 5a and videos 1–3 in the ESI†), one could see blurred lines of moving latex beads in liquid. When the laser is switched on, we observed an area (black circle) where beads were clearly visible because they were immobilized in the gelled pluronic solution and thus not moving anymore, whereas outside of this area, the liquid was still moving unaffectedly (see blurred lines in periphery in images in Fig. 5a). The size of the gelled area decreased with decreasing applied laser power as expected (compare images in Fig. 5a). Below a certain threshold laser power, no gelation of the pluronic solution was detected as the movement of beads (hence the liquid flow) was not influenced.
Even though the flow was slower for samples with 850 nm and 880 nm resonance wavelength, the immobilization of beads was observed in these experiments too (Fig. 5b with videos 4–6 and 5c with videos 7–9†). For these two samples, the liquid flow was not hindered due to any dirt in the microchannel. We also observed the gelling of an area when the laser was switched on. Its size decreased with decreasing laser power for both samples until the power was so low that it did not influence the flow anymore (compare black circles in images of Fig. 5).
Again, we calculated the fluences at sample position for each sample to simplify comparison between samples. We found quite similar threshold fluences for the three samples in the μJ cm−2 regime spanning from 35 to 89 μJ cm−2 (Table 2). Similar to the process of solvent evaporation, the conversion of laser energy is directly dependent on the absorption properties of the nanostructures.32,38 We found similar absorption behavior for all used nanostructures (Fig. SI1†) resulting in similar threshold fluences for all used wavelengths.
Additionally, negative controls without gold nanodot arrays were performed at all laser wavelengths used in the experiments before. Microscopic snapshots and videos can be found in the ESI, Fig. SI2 and videos 10–15†. In general, on samples without nanostructures solvent evaporation or liquid flow stop was only observed at very high laser fluence (50 times higher than the threshold values in the presence of nanodot arrays).
In Fig. 6 microscopic snapshots from videos recorded during a solvent evaporation experiment are displayed. In this experiment, the liquid was slowly flowing through the microfluidic channel from the top to the bottom. The water phase with the latex beads (top) is again well distinguishable from the air phase (bottom). The videos were once again recorded while moving the sample back and forth so that the liquid meniscus is moved in and out of the laser spot (reddish spot in Fig. 6 still visible for 750 nm). However, we kept the laser fluence constant for this experiment but varied the laser wavelength. As the applied power we chose to use approximately three times the threshold fluence needed to still observe solvent evaporation (here 28 μJ cm−2) at 846 nm resonance wavelength. We decided to work slightly above the threshold power to comfortably observe droplet formation. We expect that droplets originating from the air–water interface will or will not be observed depending on the laser wavelength used. Shifting away (red or blue) from the resonance wavelength should result in less droplet formation since the absorption is not optimal anymore. Accordingly, the heating of the nanodots will decrease and less and less solvent evaporation is expected. In other words, it is expected to need higher laser power at any wavelength that is not the resonant wavelength to achieve the same effect as with threshold laser power at resonance wavelength.
As indicated by the plus signs in Fig. 6, we indeed observed different behavior with regard to droplet formation depending on the laser wavelength. Starting the experiment at resonance wavelength we observed lots of droplets. Also at 820 nm and 800 nm we detected significant solvent evaporation. Moving to 750 nm we could not detect any more droplets. Shifting the wavelength further into the NIR we detected less droplets at 860 nm compared to the experiment at 846 nm and no droplet formation at 880 nm. Hence, our experimental data match well with our previous considerations that at wavelengths shifted from the resonant wavelength we don't observe solvent evaporation (or at least decreased the effect) at laser powers that cause clear heating of nanostructures at resonant wavelength.
Fig. 7 shows results from flow stop experiments using two nanodot arrays. Again, videos were recorded with alternately switching the laser on and off while the liquid was (slowly) flowing through the channel. As in the case of wavelength-dependent experiments for solvent evaporation, the laser power was kept constant and the laser wavelength was changed. For these experiments, we expect a similar trend of the influence of the laser wavelength as seen in the wavelength-dependent solvent evaporation experiments. Changing the laser wavelength should result in less influence of the liquid flow since the absorption (and consequently the heating) is not optimal anymore. We want to discuss results from arrays with 850 nm and 880 nm resonance wavelength (Fig. 7a and b) that were illuminated at the laser threshold fluence measured in the previous experiments. As expected, we observed less influence on the liquid immobilization for both samples when changing the laser wavelength. In particular, for the 850 nm sample (Fig. 7a) we detected flow stop at 850 nm with a laser fluence of 42 μJ cm−2 (red area in Fig. 7a) and could still detect a slight influence on the liquid flow at 830, 870, and 890 nm although the affected area was very small (yellow area in Fig. 7a). In the experiment only very few latex beads were immobilized when the laser was on. For 790 and 810 nm we observed no flow stop as latex beads were moving through the laser spot with the same speed as when the laser was off (green area in Fig. 7a). The second sample with 880 nm resonance wavelength was illuminated with a fluence of 89 μJ cm−2 (Fig. 7b) and we found similar results. We detected an influence on the liquid flow for 880 nm (at resonance, red area in Fig. 7b) and for 860 nm, and 890 nm (yellow area in Fig. 7b). On this array, it is sometimes not a clear flow stop but more like a turbulent flow; especially for 860 nm laser wavelength. At 840 nm and 800 nm the flow is not stopped as well as at 900 and 920 nm as latex beads could be seen that move through the laser spot area (green areas in Fig. 7b).
Therefore, the experimental data confirm our previous expectation that at non-resonant wavelengths we observe less and less influence of laser light (at threshold fluence) due to non-optimal heating of nanostructures.
These examples serve as a good example to demonstrate that one has to carefully choose the laser fluence when heating needs to be confined to a certain area or in potential applications with multiplexing to avoid crosstalk between several wavelengths. Both experiments (solvent evaporation and liquid immobilization) that were performed close to the threshold fluence show that it is possible to (i) confine the heat to a small area and (ii) to limit crosstalk between several wavelengths as long as the peaks are well separated from each other. Indeed, already 20 nm away from the resonance wavelength (Fig. 7), the influence of the laser is much less and 40 nm away, the affected area is nearly gone. Therefore, even though the plasmonic resonances of gold nanostructures tend to be broad with a full width at half maximum up to ∼200 nm (see Fig. 2 or SI1†) a 20 nm laser tuning off the resonance is enough to modify the thermal behavior. Finally, these experiments show that multiplexing is possible even for quite broad bandwidth leading to very versatile lab-on-a-chip applications.
Three different plasmonic structures were simulated according to the measured threshold laser fluences in both experimental approaches (solvent evaporation and liquid immobilization) as shown in Table 3. The absorption data was extracted from theoretical simulations (Fig. SI4†).
Size of dot (by SEM) | Height of dot (by AFM) | Resonance wavelength (n = 1.33) | Laser fluence threshold for solvent evaporation | Laser fluence threshold for liquid immobilization |
---|---|---|---|---|
200 nm | 30 nm | 774 nm | No simulation | 35 μJ cm−2 |
215 nm | 40 nm | 850 nm | 4.3 μJ cm−2 | 42 μJ cm−2 |
235 nm | 35 nm | 880 nm | 13 μJ cm−2 | 89 μJ cm−2 |
The plasmonic structure is replaced by a heat source of a fixed radius, determined by the total amount of absorbed laser power and the laser beam width. The microfluidic structure is similar for each chip, with a PDMS layer, a fluidic layer containing water, and a glass substrate as shown in Fig. 1c and 8a. The measured laser spot diameter for different wavelengths was taken into account for the simulations. For all simulations, heat is not only transferred to the liquid surrounding the nanostructures but also to the PDMS as well as the glass substrate (see insets in Fig. 8 and 9).
First, simulations to cause evaporation of liquid from a meniscus inside a fluidic channel are discussed for two nanodot sizes. As shown in Fig. 8b and c, both simulations show a slight temperature change within the fluidic channel of 2.3 K and 0.1 K for 850 nm and 880 nm, respectively. Besides heating also pressure comes into play in this case. The slight temperature change within a confined space causes a small vapor pressure differential required to cause evaporation at the liquid–air interface. As explained by Baffou et al., due to the very small curvature of the generated bubbles, a higher inner pressure is expected compared to the surroundings.32 Thus, the simulation model is too simple to explain the evaporation of solvent in our experiments with considering only temperature. We believe that more complex processes occur at the liquid–air interface.
Second, simulations to phase change the pluronic solution are discussed for three different arrays according to Table 3. As shown in Fig. 9a–c, all simulations display a significant temperature increase to at least 40 °C. The pluronic solution used in the experiment undergoes phase change at 37 °C.37 Hence, the predicted temperature increase due to nanostructure-induced heating is sufficient to change phase of the pluronic solution and decrease fluidic flow (eventually) immobilizing the liquid as observed in the experiments.
Baffou et al. have described a rather confined temperature regime with fs pulsed illumination for arrays with large interparticle distances (periodicity ∼ four to five times the length of the nanostructure).30 In our case, we expect a more uniform temperature distribution due to the smaller periodicity of the nanostructures used here involving collective effects from (immediate) neighbors. Indeed, we observe quite uniform elevated temperatures across the laser spot from the simulations (Fig. 9).
The two dimensional simulations fit well with the experimental results. Especially, we obtained the expected temperature for stopping the flow upon gelation of the pluronic. Ideally, we should perform three dimensional simulations in future works.
Furthermore, simpler light sources than a Ti:Sa oscillator can be used, since the tuning range can be narrower and the power needed is on the order of a few milliwatts. Such experiments are also possible with continuous wave laser but the threshold powers will be higher. Thus, combining lithography, plasmonics, and optofluidics seems a robust and versatile solution for future lab-on-a-chip devices either for chemical or biological applications. From a more fundamental point of view, this system is also convenient for studying the complicated thermodynamics involved with plasmonic nanostructures which is the current focus of many research groups.
Footnote |
† Electronic supplementary information (ESI) available: Experimental details, microscopic images and videos of control experiments without nanodots. Geometry and simulated absorption properties of the nanodot arrays. Videos for power- and wavelength-dependent flow experiments. See DOI: 10.1039/c4ra13312a |
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