In the present study, we explored plasmonic optical trapping (POT) of nanometer-sized organic crystals, carbocyanine dye aggregates (JC-1). JC-1 dye forms both J- and H- aggregates in aqueous solution. POT behavior was analyzed using fluorescence microspectroscopy. POT of JC-1 aggregates was realized in an increase in their fluorescence intensity from the focus area upon plasmon excitation. Repeating on-and-off plasmonic excitation resulted in POT of JC-1 aggregates in a trap-and-release mode. Such POT of nanometer-sized dye aggregates lying in a Rayleigh scattering regime (< 100 nm) is important toward molecular manipulation. Furthermore, interestingly, we found that the J-aggregates were preferentially trapped than H-aggregates. It possibly indicates semi-selective optical trapping of nanoparticles on the basis of molecular alignments.
© 2017 Optical Society of America
Since the first experimental demonstration by Kishan Dholakia’s group  and a study by Grigorenko et al. following it , plasmonic optical nano-tweezers (or plasmonic optical trapping; POT) have undergone a rapid progress due to their high potential to manipulate nanomaterial in various ways [3–7]. Localized surface plasmon (LSP) of a metallic nanostructure enhances an electromagnetic field (E) of incident light, resulting in making an optical force (grip of nano-tweezer) much intense with a factor over 103-104 in E2. This is an origin of POT. In particular, nanostructures with “nano-gaps” can much enhance E at the nanogap. It is called gap-mode LSP. Metallic nano-dimer geometries such as plasmonic nano-bow-tie or nano-junction structures were frequently been used for plamonic optical nano-tweezers [3–8].
So far, latex beads (polystyrene nanosphere, etc.) [7,9] and metallic (gold or silver) nanoparticles [10,11] have mainly been examined as a target of POT by the Quidant’s group and other groups. In 2010, we also succeeded in stable POT of another nanosphere of hard matter, CdSe quantum dots, and revealed the POT behavior using fluorescence microspectroscopy . In addition to such nanoparticles of hard matters, soft matters such as proteins , artificial polymers , and DNA  should be fascinating targets for POT, since spatial manipulation of these polymeric and bio-molecules is important as basis both of fundamental bio-science and developments of molecular/bio- devices. It should be pointed out here that Pang et al. successfully trap a single protein molecule using a plasmonic double-nanoholes (SIBA trapping) in 2012 . In 2013, we previously demonstrated a switchable plasmonic trapping for λ-DNA . We can permanently fix the DNA on a metallic nanostructured surface using a continuous-wave (CW) laser (λ = 780 nm) for LSP excitation, and switch POT to a trap-and-release mode by replacing the CW laser with a femtosecond pulsed laser (λ = 808 nm). Such unique behavior has never been observed for POT of microspheres/nanospheres of hard materials (metal, semiconductor, and polymer beads). Furthermore, we discovered characteristic micro-patterning and morphological changes of a micro-assembly of artificial polymers during POT . These are characteristic behavior peculiar to POT of soft matters. This also motivates us to develop a further study on POT of organic soft matters. Such molecular manipulation with POT will be a powerful method with a combination of plasmonic applications; for instance, high-sensitive sensors, chemical reactions. However, the pioneering works on POT of soft nanomaterials [13–15] indicates that the POT behaviors are complicated because of photothermal effects (also discuss it in this manuscript). Thus, these demonstrations of POT for molecular materials will provide valuable a guideline for future applications.
Toward plasmonic optical manipulation for molecular materials, molecular aggregates should be an important trapping target, since their size lies in a range between molecules and microspheres used in POT studies and molecular aggregates have various functions. Chemical properties of aggregates might be reflected on trapping behavior. In the present study, we targeted J/H-aggregates of an organic dye, 5,5′-6,6’-Tetrachloro-1,1’,3,3′-tetraethylbenzimidazolocarbocyanine (JC-1), whose chemical structure is displayed in Fig. 1(a). The motivation of selection of the dye is as follows. First, as we previously reported [16,17], JC-1 forms stable J-aggregates in water due to π−π interactions. Second, it also forms H-aggregates in the presence of tetrakis(4-fluorophenyl)borate (TFPB) in solution. Third, both aggregates are fluorescent and J- and H-aggregates are detectable to each other on the basis of their fluorescence spectra. We can separately observe POT behavior of H- and J- aggregate using a confocal fluorescence microscope.
It should be noted that Tanaka and Masuhara et al. investigated conventional optical trapping for J-aggregate of pseudoisocyanine dye [18,19]. They found that J-aggregates formation was accelerated by an optical force with a more highly-order alignment than normal J-aggregates, suggesting that optical trapping of dye aggregates was fruitful. Moreover, nowadays dye aggregates in plasmonic fields attract much attention in nanophotonics since strong coupling (Rabi splitting) is frequently observed due to a large transition moment of J-aggregates [20–22]. We investigated POT behavior of J-/H-aggregate of JC-1, and detected a sign of selective optical trapping: J-aggregates were preferentially trapped than H-aggregates. This behavior is interpreted on the basis of molecular alignment of the aggregates. Characteristics and mechanism of POT of dye aggregates are revealed and discussed using fluorescence data. The effect of molecular orientation is firstly examined for POT in the present work. We believe that such preferential POT of J-aggregates promises an interesting new avenue exploiting the selective optical manipulation of organic molecules based on the polarizability of trapped molecules.
A fluorescence dye, JC-1 (Fig. 1(a)), was synthesized and purified in our previous work. In the presence of TFPB (p-tetrafluorophenylborone) (Fig. 1(b)), JC-1 forms, both J- and H-aggregates. It should be noted that absorption and fluorescence spectral bands of the dye aggregates hardly overlapped with the resonant spectrum of gap-mode plasmon (Fig. 2(a)). The former ranged in λ < 600 nm, while the latter ranged in λ > 600 nm. This means neither the strong coupling nor resonant trapping should be involved in the present POT. In their molecular alignments, The J-aggregate and the H-aggregate take a head-to-tail structure and a stacking structure, respectively [16,17]. They can be distinguished to each other by fluorescence spectroscopy, as indicated in Fig. 2(b). The H-aggregates showed an emission peak maximum at 557 nm, while J-aggregates appeared around 600 nm as a shoulder band. The sizes of J-/H aggregates were roughly controllable by changing the molar ratio of TFPB to JC-1; ρ = [TFPB]/[JC-1], with increasing ρ, the size becomes smaller . In this study, we used an aqueous solution with ρ = 3, where the diameter (d) of the aggregates ranged from 65 to 115 nm, with averaged value of 90 nm.
For LSP generation, gold nano-pyramidal dimer arrays were fabricated on glass substrates using angle-resolved-nanosphere lithography [23–25]. It shows a broad resonant band with a maximum around 800 nm (Fig. 2(a)), which was ascribed to the gap mode LSP. It can enhance electromagnetic fields (E) of incident light with a factor of 103-104 (E2/Eo2) around the gaps of nano-pyramids. These specifications including scanning electron microscopic images, extinction spectrum, and the enhanced E-field distributions around the nanostructures have been already described in previous literatures [12,14]. The sample solution was sandwiched by the plasmonic substrate and a cover slip, and used in the following experiments.
POT observation was carried out using a confocal fluorescence microscope, whose details were previously described elsewhere [11,12,14,15,25]. 50 μl of the sample solution was placed between a glass slide with a hole (maximum depth ∼300 μm) and the plasmonic substrate. Near-infrared (NIR) laser light (λ = 808 nm, Shanghai Laser & Optics Century Co. Ltd., IRM808TB-100SR) and visible laser light (λ = 473 nm, Shanghai Laser & Optics Century Co. Ltd., SDL-473-LN-0005T) were used for excitation of LSP and JC-1 aggregates, respectively. These laser beams were coaxially introduced into a confocal inverted optical microscope (NIKON, Ti-U) to irradiate the sample solution. The NIR laser beam was tightly focused with an oil-immersion objective lens (Nikon, Plan Apo VC, × 100, N.A. = 1.4, irradiation area ≈1 μm2). The light intensity at the focus was varied from 10 to 140 kW/cm2. Fluorescence signals were passed through a pinhole (100 μm). In this confocal arrangement, the observation area was x,y < 500 nm (lateral) and z ~2 μm (transverse). Fluorescence from the dye solution was detected with a photodetector (a spectrometer with a cooled CCD camera supplied by Princeton Instruments). All the experiments were carried out at room temperature under ambient condition. The main experiment was performed in the following manner. The 473 nm laser light was illuminated during the observation period (always on). Simultaneously, 808 nm laser light was illuminated in a repetitive on-and-off mode, and fluorescence from the focal point was monitored continuously. During POT of dye aggregates with 808 nm laser irradiation, the fluorescence intensity became higher because the number of the trapped dye aggregates increased in the observation area.
3. Results and discussion
The sample solution in the cell was irradiated at 808 nm (LSP excitation intensity Iexcitation = 100 kW/cm2) to induce POT of dye aggregates. Under simple optical microscopy observation, however, we never detect any sign of POT. This is natural since the sizes of both J- and H-aggregates are smaller below the diffraction limit of visible light. Then, we explored POT behavior in the sample solution using confocal fluorescence microspectroscopy. Figure 3 shows fluorescence spectra measured at the focal point at the metallic surface before and during LSP excitation. Before LSP excitation (Fig. 3, black line), fluorescence spectrum consisting of bands of J- and H-aggregates was slightly observed and it was consistent with a result of steady-state measurement (Fig. 2). With a quick response to LSP excitation (Fig. 3, red line), fluorescence immediately became brighter. Such fluorescence enhancement has frequently been observed in our previous studies on POT of quantum dots , polymer beads , artificial polymers , and DNA . The LSP-enhanced optical force can optically trap nanoparticles at a LSP excitation area, leading to an increment of the number of nanoparticles in the excitation area. This is the origin of fluorescence enhancements upon LSP excitation. Namely, the fluorescence enhancement seen in Fig. 3 clearly indicates POT of JC-1 aggregates at the focal point. We will discuss the different increment of fluorescence intensity between J- and H-aggregate during plasmon excitation in later section.
Figure 4 shows temporal profiles of the fluorescence intensity modulation in accordance with repetitive on-and-off LSP excitation (blue line in Fig. 4). It was recorded for the center of the surface area of the excitation at 100 kW/cm2 (the same experimental condition as used in Fig. 3), and monitoring wavelength was 557 nm (fluorescence wavelength maximum of the H-aggregate). Note that the ordinate of Fig. 4 indicates “relative fluorescence”. It is a ratio of fluorescence intensity (FI) during 808 nm light irradiation (for LSP excitation) to that without light irradiation; FIon-LSP/FIoff-LSP. As references, similar experiments were carried out replacing the LSP substrate with a glass (Pyrex) substrate or a glass substrate coated with Cr. While the glass substrate should never provide any optical or thermal perturbations to the sample solution, the Cr-coated glass substrate affects the sample solution thermally. For the experiment using the glass substrate (black line in Fig. 4), the relative fluorescence intensity never responded to LSP excitation. It only gave a base line in these experiments. For the experiment using the Cr-coated substrate (red line in Fig. 4), the relative fluorescence intensity slightly decreased upon LSP excitation, and recovered to the original intensity upon stop of it. This is presumably due to thermophoresis driven by a huge temperature gradient generated by local temperature elevation (a photothermal effect). It is discussed in a latter part of discussion. Thermophoresis acts as a repulsive force for a large part of nanoparticles, excluding these nanoparticles from the hot excitation area toward the outer cold side surrounding the excitation area. Such thermophoresis exerts dye aggregates, resulting in the slight decrease in fluorescence intensity.
By contrast, fluorescence intensity obviously showed sensitive responses to LSP excitation (blue line in Fig. 4). Just upon LSP excitation, fluorescence intensity raised rapidly. As previously described, the rapid rise of fluorescence intensity corresponds to POT of dye aggregates around the excitation area. It should be pointed out here that, during LSP excitation, the raised fluorescence intensity was sharply fluctuated and scattered. The origin of such intensity scattering is discussed in a latter part. Upon stop of LSP excitation, fluorescence intensity rapidly dropped off to the original intensity. It corresponds to the escape and dissipation of dye aggregates from the excitation area (whose center was the fluorescence observation area) due to the Brownian motion. Such behavior was well reproducible by repetitive on-and-off LSP excitation.
In Fig. 5, the relative fluorescence intensity in average during LSP excitation (FIaveraged) is plotted as a function of intensity of LSP excitation (Iexcitation). The value of FIaveraged slightly raised when I Iexcitation = 10 kw/cm2, and gradually increased up to FIaveraged = 3 with increasing Iexcitation to 100 kW/cm2. This observation means that the number of trapped JC-1 aggregates increased with increasing Iexcitation. For the stable trapping, the trapping potential Utrap should overcome thermal fluctuation energy (kT);18,19], and E is an electric field vector of incident light. With increasing E2 in accordance with Iexcitation, the area where the relation |Utrap| > kT holds should be expanded, resulting in the increase in the number of trapped JC-1 aggregate. At Iexcitation = 130 kW/cm2, FIaveraged slightly dropped and it is an upper limit of Iexcitation for stable POT. When Iexcitation > 130 kW/cm2, a micro-bubble appeared around the LSP excitation area to block POT. The bubble generation is due to a photothermal effect caused by lattice relaxation of excited electrons in gold, and frequently reported in studies concerning photoexcitation of a metal film in liquid phase. In this way, we can observe reversible POT of JC-1 dye aggregates in a trap-and-release mode.
The threshold value in Iexcitation for POT was evaluated to be 10 kW/cm2. It is about 10 times higher than that in POT of polymer beads (diameter ~500 nm) using the same plasmonic substrate (AR-NSL substrate). According to Eq. (1), Utrap is proportional to α, and hence to a volume of a trapped nanoparticle (α r3, r is the radius of the nanoparticle). Therefore, as a nanoparticle becomes smaller, an optical force should become weaker. The average size of JC-1 aggregates was ca. 80 nm and it is smaller than polymer beads (100 – 500 nm used in relevant studies). Therefore, the threshold Iexcitation for JC-1 aggregates should be higher than that for polymer beads. Since the optical force is weak and dissipation due to diffusion is fast for JC-1 aggregate, aggregates frequently repeat trapping to and escape from the LSP excitation area, leading to the sharply fluctuation and scattering of fluorescence intensity during POT (Fig. 4).
Inspection of fluorescence spectra during POT implies interesting POT behavior. In Fig. 6 (a), fluorescence spectra recorded at the focal point before and during POT are displayed with being normalized at 557 nm (corresponding to emission form H-aggregate). We can obviously realize that fluorescence of J-aggregate around 600 nm is relatively enhanced for the spectrum recorded during POT. As previously described, the fluorescence spectra consist of a superposition of bands of H- and J-aggregates. These were reproducible by a linear combination of two Gaussian functions, as shown in Fig. 6 (b). Thus, we can separately evaluate relative contribution of H- and J- aggregate to the fluorescence spectra. Based on this procedure, we plotted the ratio of each fluorescence intensity FIJ/FIH in Fig. 7. Here, FIJ and FIH are fluorescence intensity of J- and H- aggregates, respectively, that are evaluated using a method of Fig. 6(b). In the sample solution before LSP excitation, FIJ/FIH was 0.22. As clearly seen in Fig. 7, just upon POT at the threshold (Iexcitation = 10 kW/cm2), FIJ/FIH slightly raised to 0.29, and kept a gradual increase with increasing excitation intensity. The value of FIJ/FIH reached 0.4, about two times higher than the original value, when Iexcitation > 100 kW/cm2. The results clearly indicate that the J-aggregates of JC-1 can be preferentially trapped than the H-aggregates.
Here we briefly discuss the origin of this semi-selective trapping. Generally, a J-aggregate takes a head-to-tail molecular alignment (end-to-end stacking), while a H-aggregate takes a side-by-side molecular alignment (plane-to-plane stacking) . In accordance with these molecular alignments, the polarizability α of JC-1 J-aggregate (αJ) is presumably larger than that of JC-1 H aggregate (αH). Although it is rather difficult to precisely evaluate values of a for both of the aggregate due to complicated molecular alignments and the size distribution (α of an aggregate is proportional to the number of dye molecules in itself), it was suggested that αJ is about twice larger than αH for a Cyanine dye . Therefore, based on Eq. (1) and (2), Utrap for JC-1 J aggregate would be deeper than that for JC-1 H aggregate. This leads to the semi-selective POT as seen in Fig. 6.
We investigated POT of nanometer-sized J- and H-aggregates of JC-1 dye molecules by means of fluorescence microspectroscopy. A plasmonic optical tweezer based on a gold nano-pyramidal dimer array can optically trap these dye aggregates in a reversible trap-and-release mode. The threshold value of LSP excitation intensity for POT was 10 kW/cm2 and the POT efficiency increased with increasing the excitation intensity up to 120 kW/cm2, above which a microbubble was generated to block POT. The J-aggregates can be preferentially trapped than the H-aggregates. The origin of this semi-selective POT is relatively larger polarizability α for J-aggregates.
JGC-S Scholarship Foundation; JSPS KAKENHI Grant Numbers JP26288011, JP16K17922, and JP16H06506/JP16H06507 in Scientific Research on Innovative Areas “Nano-Material Manipulation and Structural Order Control with Optical Forces”.
References and links
1. V. Garcés-Chávez, R. Quidant, P. J. Reece, G. Badenes, L. Torner, and K. Dholakia, “Extended organization of colloidal microparticles by surface plasmon polariton excitation,” Phys. Rev. B 73(8), 085417 (2006). [CrossRef]
2. A. N. Grigorenko, N. W. Roberts, M. R. Dickinson, and Y. Zhang, “Nanometric optical tweezers based on nanostructured substrates,” Nat. Photonics 2(6), 365–370 (2008). [CrossRef]
3. K. Wang, E. Schonbrun, P. Steinvurzel, and K. B. Crozier, “Trapping and rotating nanoparticles using a plasmonic nano-tweezer with an integrated heat sink,” Nat. Commun. 2, 469 (2011). [CrossRef] [PubMed]
4. W. Y. Tsai, J.-S. Huang, and C. B. Huang, “Selective trapping or rotation of isotropic dielectric microparticles by optical near field in a plasmonic Archimedes spiral,” Nano Lett. 14(2), 547–552 (2014). [CrossRef] [PubMed]
5. J. Berthelot, S. S. Aćimović, M. L. Juan, M. P. Kreuzer, J. Renger, and R. Quidant, “Three-dimensional manipulation with scanning near-field optical nanotweezers,” Nat. Nanotechnol. 9(4), 295–299 (2014). [CrossRef] [PubMed]
7. M. Juan, M. Righini, and R. Quidant, “Plasmon nano-optical tweezers,” Nat. Photonics 5(6), 349–356 (2011). [CrossRef]
8. B. J. Roxworthy, K. D. Ko, A. Kumar, K. H. Fung, E. K. C. Chow, G. L. Liu, N. X. Fang, and K. C. Toussaint Jr., “Application of plasmonic bowtie nanoantenna arrays for optical trapping, stacking, and sorting,” Nano Lett. 12(2), 796–801 (2012). [CrossRef] [PubMed]
11. T. Shoji, M. Shibata, N. Kitamura, F. Nagasawa, M. Takase, K. Murakoshi, A. Nobuhiro, Y. Mizumoto, H. Ishihara, and Y. Tsuboi, “Reversible photoinduced formation and manipulation of a two-dimensional closely packed assembly of polystyrene nanospheres on a metallic nanostructure,” J. Phys. Chem. C 117(6), 2500–2506 (2013). [CrossRef]
12. Y. Tsuboi, T. Shoji, N. Kitamura, M. Takase, K. Murakoshi, Y. Mizumoto, and H. Ishihara, “Optical trapping of quantum dots based on gap-mode-excitation of localized surface plasmon,” J. Phys. Chem. Lett. 1(15), 2327–2333 (2010). [CrossRef]
14. M. Toshimitsu, Y. Matsumura, T. Shoji, N. Kitamura, M. Takase, K. Murakoshi, H. Yamauchi, S. Ito, H. Miyasaka, A. Nobuhiro, Y. Mizumoto, H. Ishihara, and Y. Tsuboi, “Metallic-nanostructure-enhanced optical trapping of flexible polymer chains in aqueous solution as revealed by confocal fluorescence microspectroscopy,” J. Phys. Chem. C 116(27), 14610–14618 (2012). [CrossRef]
15. T. Shoji, J. Saitoh, N. Kitamura, F. Nagasawa, K. Murakoshi, H. Yamauchi, S. Ito, H. Miyasaka, H. Ishihara, and Y. Tsuboi, “Permanent fixing or reversible trapping and release of DNA micropatterns on a gold nanostructure using continuous-wave or femtosecond-pulsed near-infrared laser light,” J. Am. Chem. Soc. 135(17), 6643–6648 (2013). [CrossRef] [PubMed]
16. T. Enseki and H. Yao, “Controlled formation of fluorescent organic nanoparticles of carbocyanine dye via Ion-association approach,” Chem. Lett. 41(10), 1119–1121 (2012). [CrossRef]
17. H. Yao and K. Ashiba, “Highly fluorescent organic nanoparticles of thiacyanine dye: A synergetic effect of intermolecular H-aggregation and restricted intramolecular rotation,” RSC Advances 1(5), 834–838 (2011). [CrossRef]
18. Y. Tanaka, H. Yoshikawa, and H. Masuhara, “Two-photon fluorescence spectroscopy of individually trapped pseudoisocyanine J-aggregates in aqueous solution,” J. Phys. Chem. B 110(36), 17906–17911 (2006). [CrossRef] [PubMed]
19. Y. Tanaka, H. Yoshikawa, and H. Masuhara, “Laser-induced self-assembly of pseudoisocyanine J-aggregates,” J. Phys. Chem. C 111(50), 18457–18460 (2007). [CrossRef]
20. J. Bellessa, C. Bonnand, J. C. Plenet, and J. Mugnier, “Strong coupling between surface plasmons and excitons in an organic semiconductor,” Phys. Rev. Lett. 93(3), 036404 (2004). [CrossRef] [PubMed]
21. G. Zengin, G. Johansson, P. Johansson, T. J. Antosiewicz, M. Käll, and T. Shegai, “Approaching the strong coupling limit in single plasmonic nanorods interacting with J-aggregates,” Sci. Rep. 3(1), 3074 (2013). [CrossRef] [PubMed]
22. A. Canaguier-Durand, E. Devaux, J. George, Y. Pang, J. A. Hutchison, T. Schwartz, C. Genet, N. Wilhelms, J.-M. Lehn, and T. W. Ebbesen, “Thermodynamics of molecules strongly coupled to the vacuum field,” Angew. Chem. Int. Ed. Engl. 52(40), 10533–10536 (2013). [CrossRef] [PubMed]
23. C. L. Haynes, A. D. McFarland, M. T. Smith, J. C. Hulteen, and R. P. Van Duyne, “Angle-resolved nanosphere lithography: manipulation of nanoparticle size, shape, and interparticle spacing,” J. Phys. Chem. B 106(8), 1898–1902 (2002). [CrossRef]
24. M. Takase, H. Ajiki, Y. Mizumoto, K. Komeda, M. Nara, H. Nabika, S. Yasuda, H. Ishihara, and K. Murakoshi, “Selection-rule breakdown in plasmon-induced electronic excitation of an isolated single-walled carbon nanotube,” Nat. Photonics 7(7), 550–554 (2013). [CrossRef]
25. T. Shoji and Y. Tsuboi, “Plasmonic optical tweezers toward molecular manipulation: tailoring plasmonic nanostructure, light source, and resonant trapping,” J. Phys. Chem. Lett. 5(17), 2957–2967 (2014). [CrossRef] [PubMed]
26. A. Chowdhury, S. Wachsmann-Hogiu, P. R. Bangal, I. Raheem, and L. A. Peteanu, “Characterization of chiral H and J aggregates of cyanine dyes formed by DNA templating using stark and fluorescence spectroscopies,” J. Phys. Chem. B 105(48), 12196–12201 (2001). [CrossRef]