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Abstract
The effect of hybridization of a clay fluorohectorite (FHT) nanosheet with a π-conjugated organic compound, α,β,γ,δ-tetrakis(1-methylpyridinium-4-yl)porphyrin p-toluene-sulfonate (TMPyP), on its optical manipulation is investigated. Although the hybridized FHT is optically trapped essentially in the same manner as that of neat FHT, the hybridization with TMPyP allows for manipulation of FHT with lower laser intensity or a shorter period, or both. This is ascribed to the larger refractive index and polarizability of TMPyP compared with neat FHT.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
An optical manipulation technique based on radiation pressure enables noncontact manipulation and nondestructive operation of micro-sized objects, such as glass beads [1], polystylene spheres [2], and bacteria [3]. This technique, therefore, covers many research fields such as materials science [4–6], biological science [7–10], and mechatronics [11–14]. An increase in the radiation pressure results in a stronger ability to grasp an object for optical manipulation.
Nanosheets are regarded as highly anisotropic two-dimensional functional nanomaterials [15–19]. To apply the co-operative effect of their electronic property and structural thinness to electric and opto-electric devices, it is necessary that they can be transported, arranged, and accumulated at will. Our research group has attempted to exploit the optical manipulation behavior of nanosheets. So far, we have revealed that a single nanosheet dispersed in water is optically trapped, whereby the nanosheet is unidirectionally oriented parallel to the propagation direction and the linear polarization plane of the illuminating light [20–23]. Additionally, large structural assemblies of nanosheets can be constructed outside the focal area by means of the inter-particle interactions in liquid crystalline phases that can occur at higher concentrations of the nanosheets [24,25]. We have also attempted to increase the optical radiation pressure by changing the electrical properties of the nanosheets. Based on the research employing nanosheets with different refractive indices, such as fluorohectrite (FHT), niobate nanosheets, and titania nanosheets, it was concluded that the optical radiation pressure exerted on the nanosheet can be increased by increasing the refractive index of the nanosheet. An increase of the optical radiation pressure relating to a higher refractive index contributes to a quicker movement of the nanosheets and lowers the required intensity of the trapping laser beam [21,22].
In this study, we propose an alternative method to increase the radiation pressure applied to a targeted nanosheet by modifying the electronic property of the nanosheets, that is, by hybridization of the nanosheet with a π-conjugated organic compound. Generally, an organic compound equipped with a large π-conjugated system exhibits a stronger interaction with illuminating light compared with inorganic materials, owing to the larger polarizability [26]. Therefore, the response of optical trapping should be enhanced in a hybridized nanosheet compared with that without hybridization. FHT is hybridized with a cationic porphyrin derivative by electrostatic interaction in an aqueous dispersion. A focused laser beam is incident to a colloidal single layer of the hybridized nanosheet. Quick movement of the nanosheet hybridized with the porphyrin is achieved and an optical orientation of the nanosheet parallel to the propagation and polarization direction is observed, similar to the nanosheet without hybridization. Furthermore, the required laser intensity for the optical orientation is significantly lower than that needed in the case of a nanosheet without hybridization. These observations indicate that hybridization with a π-conjugated organic compound increases the optical radiation pressure exerted on the nanosheet. This increase of the optical radiation pressure is ascribed to the larger refractive index and polarizability of TMPyP than those of FHT.
2. Experimental
2.1 Materials
FHT was obtained by melt synthesis followed by long-term annealing, according to an established procedure [27–29]. The nominal composition of the FHT was [Na0.5]inter[Mg2.5Li0.5]oct[Si4]tetO10F2 and the material featured a cation exchange capacity (CEC) of 1.27 mmol g−1. FHT was dispersed in water to obtain a fully delaminated, all single layer FHT colloid. A porphyrin derivative, α,β,γ,δ-tetrakis(1-methylpyridinium-4-yl)porphyrin p-toluenesulfonate (TMPyP; Fig. 1 (left)) was employed as an organic compound because clay-TMPyP hybrid is dispersed in water as a single nanosheet [30–33]. The FHT with TMPyP was prepared by mixing a water dispersion of FHT with an aqueous solution of TMPyP. TMPyP was adsorbed on the FHT surface at a loading of 37% versus CEC. At the loading, the surface coverage of TMPyP on FHT was calculated to be 10% when the TMPyP was assumed to be adsorbed onto the FHT surface without aggregation. FHT hybridized with TMPyP was prepared at a concentration of 0.01 g L−1.
Fig. 1. Chemical structure of TMPyP (left), and absorption spectrum (right) of neat FHT (dashed line) and FHT hybridized with TMPyP (solid line) dispersed in water.
The absorption spectra of colloids of neat FHT and FHT hybridized with TMPyP, obtained by a JASCO V-670-UV-NIR spectrophotometer (JASCO, Tokyo, Japan), using 10-mm quartz cuvettes, are shown in Fig. 1 (right). As for a blank FHT dispersion without TMPyP, a broad spectrum that showed a monotonic increase with the decrease of the wavelength was observed. This arose from light-scattering of the colloidal dispersion. A similar extent of light-scattering of the dispersion of FHT hybridized with TMPyP was also observed in the spectrum. Considering the light-scattering, the light absorption of FHT hybridized with TMPyP at a wavelength region longer than 700 nm was judged as being negligible.
2.2 Optical configuration for optical trapping
Figure 2 shows a schematic representation of the optical configuration. As described, the light absorption of FHT hybridized with TMPyP at the near infra-red region was negligible. To avoid absorption of the laser beam by the hybrid, a linearly polarized continuous-wave (CW) DPSS laser (LSR1064NL, LASEVER, China) oscillating at 1064 nm in the TEM00 mode was employed. The colloid samples were injected into a 100-µm-thick thin-layer glass cell. The sample cell was set on the stage of an inverted microscope (IX70, Olympus, Japan). The laser beam was focused on the center of the cell by using an objective lens (Apo, 60×, numerical aperture = 1.20, Olympus). The beam diameter was adjusted to the pupil diameter of the objective lens by using a beam expander. The beam waist size at the focal position was calculated to be 0.33 µm. The sample was simultaneously illuminated by a halogen lamp and the image was monitored by using a digital CMOS camera (ORCA-Flash 4.0 V3, Hamamatsu Photonics, Japan). The incident laser beam was completely blocked by a dichroic mirror and a band pass filter inserted before the camera.
3. Results and discussion
A microscopy observation of the FHTs before and after hybridization of the TMPyP was performed to confirm the hybridization. Figure 3 shows that hybridized FHT was opaque by virtue of the fact that the TMPyP exhibited absorption while neat FHT was transparent. Both the FHT with and without TMPyP were single nanosheets at the present concentration. In addition, TMPyP was assumed to be adsorbed on to FHT homogeneously because FHT with TMPyP (Fig. 3(b)) appeared to be uniformly black. These FHT nanosheets moved three-dimensionally with Brownian motion. Thus, the FHT nanosheets were sometimes clearly observed but sometimes were blurred because their motion moved them in and out of focus. Brownian motion was essentially the same as for neat FHT (see Visualization 1 (neat FHT) and Visualization 2 (hybridized FHT)).
The effect of hybridization on the dynamics of optical manipulation behavior was investigated through optical microscopy observations. The optical microscopy images of neat FHT and FHT with TMPyP before and after illumination by a linearly polarized laser beam are shown in Fig. 4, Visualization 3 (neat FHT) and Visualization 4 (hybridized FHT). As described later, the required laser power for manipulating a nanosheet depends on the size of nanosheet. Thus, the sizes of neat FHT and FHT with TMPyP exemplified in Fig. 4 were almost identical with areas of 92 and 89 µm2, respectively. Figure 4(a) and 4(f) shows the microscopy images of neat FHT and FHT with TMPyP before laser illumination. The nanosheets oriented with their in-plane direction parallel to the focal plane. Upon continuous illumination of a 15-mW laser beam, the FHT and FHT with TMPyP started tilting toward the direction perpendicular to the focal plane; that is, parallel to the propagation direction of the laser beam (Fig. 4(d), 4(g)). The final orientation of the nanosheet edge was parallel to the polarization direction of the incident laser beam (Fig. 4(e), 4(h)–4(j)). The behavior of the FHT with TMPyP under laser illumination was essentially the same as for other nanosheet materials [21]. However, FHT with TMPyP could be manipulated more quickly than neat FHT. FHT with TMPyP started tilting only 7 s after switching on the continuous laser illumination (Fig. 4(g)). The orientation of FHT with TMPyP was completed within as little as 10 s (Fig. 4(h)). However, in the case of FHT without TMPyP, 230 s of beam illumination was required for tilting (Fig. 4(d)) and orientation was completed after 240 s (Fig. 4(e)). Thus, the hybridization of FHT with TMPyP drastically decreased the required time for completing the optical manipulation of FHT.
Fig. 4. Bright-field optical microscopy images of (a–e) neat FHT and (f–j) FHT with TMPyP when illuminated by a 15 mW laser beam. These images indicate (a, f) before illumination and after (b, g) 7 s, (c, h) 10 s, (d, i) 230 s and (e, j) 240 s of continuous laser illumination. The white double arrows indicate the polarization direction.
Although the thickness of FHT was only 0.95 nm, the edge of FHT is clearly observed in Fig. 4. The most probable reason for the observation of such a small sized edge was the considerably larger lateral size (5–20 µm) of the trapped FHT compared with the focal depth (0.3 µm) of the objective lens. Because the focal depth was significantly smaller than the lateral size of FHT, both edges of the FHT were located far from the focal depth of the objective lens when the FHT was oriented with its in-plane direction parallel to the propagation direction of the laser beam and the focal depth was set to the center of FHT. Thus, the image of the edges should be blurred. Owing to this defocusing of the edges, the single FHT was observed as if it was a thick object.
Repeatability of the optical manipulation behavior was confirmed through on-off switching of the laser beam. Figure 5 exemplifies the microscopy observations of the optical trapping behavior of hybridized FHT by repeated on-off switching of the laser beam. The trapped FHT with TMPyP was gradually relaxed by Brownian motion when the laser illumination was ceased. The nanosheet trapped at the focal point began to be positionally and orientationally shifted just after stopping the laser illumination. The nanosheet was then gradually moved and tilted in the direction parallel to the focal plane. As shown in Fig. 5(c), a similar orientation of the nanosheet to that before commencing laser illumination was observed 300 s after stopping the laser illumination. The FHT with TMPyP was repeatedly trapped at the focal point by on-off switching of the laser illumination. As shown in Fig. 5(d), the appearance of hybridized FHT after 20 cycles of on-off switching of the laser beam was almost unchanged compared with that before laser illumination. This observation indicated that decomposition of TMPyP arising from the laser illumination had not occurred for at least 20 cycles of on-off switching of the laser illumination.
Fig. 5. Optical microscopy images of the repeatedly trapped FHT hybridized with TMPyP by on-off switching of the laser illumination. Before laser irradiation (a), after 300 s of continuous laser illumination (b), 300 s after laser illumination was ceased (c) and after 20 cycles of on-off switching of the laser illumination (d).
To discuss the dependence of the nanosheets in laser manipulation on the illuminating laser power, Fig. 6 schematically shows the dynamics of the nanosheet under illumination of a focused laser beam. Figure 6(a) corresponds to (a)–(c) and (f) in Fig. 4, Fig. 6(b) to (d) and (g) in Fig. 4, and Fig. 6(c) to (e) and (h)–(j) in Fig. 4. The resultant radiation force of the scattering and the gradient forces were related to the optical manipulation of the nanosheet. The former primarily worked in the transition process from the initial condition shown in Fig. 6(a) to the rotation shown in Fig. 6(b); the latter worked more dominantly with an increase in the area of the nanosheet inside the electric field of the illuminating beam shown from Fig. 6(b) to Fig. 6(c).
The radiation force induced by light scattering is given by [34]
where n1P/c is the incident momentum per second of a ray of power P in a surrounding medium with a refractive index of n1, and c is the speed of light in a vacuum. The quantity Qpr is the efficiency factor for radiation pressure that is the dimensionless constant given by the cross-section Cpr of the scatterer for radiation pressure divided by the geometrical cross-section. The radiation pressure cross-section is given by where Cext, Csca and Cabs are the cross sections of the scatterer for extinction, scattering, and absorption, respectively. g is the mean cosine of the scattering angle θ weighted with the scattering phase function p(θ) and defined byThe dependence of the minimum laser power on the size and refractive index of the scatterer may arise from the fact that the forward momentum of the scattered light is proportional to the magnitude of the isotropic component in scattering, as shown in Eq. (2). Table 1 shows the minimum laser power required to manipulate neat and hybridized FHT within 300 s. We tested 5 samples in each of the particle-size ranges. All tested nanosheets could be manipulated essentially in the same manner as shown in Fig. 4. Owing to a large variation of particle shape, it was found that the minimum laser powers required for manipulations of both the neat FHT and hybridized FHT did not consistently increase with their size. Referring to the numerical calculation based on the Mie theory, the radiation pressure for a nano-sized object increases consistently with the illuminating laser power [35] but does not increases in the same manner for micro-sized object [36]. In fact, the sizes of the nanosheets (in-plane direction) were in the Mie theory region and the sizes were much larger than the spot size of the illuminated laser beam. This meant the absolute value of radiation pressure applied to the nanosheet was constant even though the size of the nanosheet varied. Thus the factor that determined the size dependence in Table 1 was proposed to be the viscous resistance of a solvent. This is because the viscous resistance that operates on nanosheets during transitional and rotational movement of the nanosheet should be increased with the increase in the size of the nanosheets. The minimum laser power required for optical manipulation was clearly smaller for the hybridized FHT than the neat FHT. This was considered to be related to the effect of hybridization because both the neat and hybridized FHT were transparent (i.e., nonabsorbent) at the wavelength of 1064 nm. The scattering cross-section and g-parameter in Eq. (4) depends only on the refractive index of the scatter because the illuminating beam size was much smaller than the size of the nanosheets from the initial condition shown in Fig. 6(a) to the middle of the rotation shown in Fig. 6(b). The typical refractive index of clay minerals is 1.5, as has been reported in the previous works [37,38], and the mutual refractive index of FHT suspended in water is 1.1. In contrast, π-conjugated molecules generally have a higher refractive index than that of clay minerals. In fact, TMPyP has been reported to have a refractive index of 1.7 [39] and, consequently, the mutual refractive index of TMPyP is 1.3. The difference in the mutual refractive index should be a reason why FHT with TMPyP can be manipulated more quickly even by low power laser beam illumination compared with neat FHT.
Table 1. Minimum laser power required to manipulate neat FHT and FHT with TMPyP within 300 s.
We next considered the relationship of another force, i.e. the gradient force, with the polarizability of the scatter. The gradient force is written as the following expression [40–43];
The dependency of the temporal response in optical manipulation on the laser intensity is considered to be related directly with the gradient force. The relationship between the required times for optical trapping and the laser intensity is summarized in Table 2. When illuminating the same samples shown in Fig. 4(a) and 4(f) with a laser beam intensity of 8 mW, 150 s were required for complete orientation of FHT with TMPyP, whereas neat FHT could not be trapped at all even with continuous irradiation for more than 600 s. When the power of the laser beam was lowered further to 5 mW, then neither neat FHT nor FHT with TMPyP could be trapped even with 600 s of continuous laser illumination. The gradient force makes it possible to manipulate the FHT with TMPyP more quickly under a low-power illumination compared with neat FHT. In addition, the gradient force will act on the stable rotation of the nanosheet because it acts on an extremely narrow area of the nanosheet around the focal point from opposite directions, as shown in Fig. 6.
Table 2. Laser power and time required for the optical trapping of 92 µm2 of neat FHT and 89 µm2 of FHT with TMPyP.
4. Conclusions
This study provides a novel technique for the quick and low power on-demand orientation manipulation of nanosheets. By hybridizing with an organic compound of high refractive index and large polarizability, the nanosheet can be manipulated quickly even with a low power laser beam. By applying this methodology to the large variety of combinations of nanosheet materials and organic compounds, much easier orientation manipulation of nanosheets should be realized.
Furthermore, we have demonstrated optical manipulation of a clay nanosheet hybridized with an organic compound. To fully explore the functionality of hybrids, the development of a methodology that enables local and on-demand orientation manipulation of hybrids is strongly required. Manipulation of hybrids, as established in this study, should be a powerful tool to realize local and on-demand orientation manipulation of hybrids.
Unfortunately, a quantitative discussion on the polarizability of the nanosheet cannot be made from the work despite the importance of this parameter. Generally speaking, a polarizability measurement of the nanosheet is quite difficult owing to the chemical and physicochemical complexities of the nanosheet and its aqueous suspension. We would expect a possibility that the polarizability of the nanosheet can be measured by estimating the temporal dynamics of the optical manipulation through this research.
Funding
Japan Society for the Promotion of Science (19H02689, 19H04677, 19K05403).
Acknowledgments
The authors gratefully acknowledge Ms. Misato Suehisa and Mr. Hiroaki Yahara (Yamaguchi University) for their support with the experiments. We thank Edanz Group (www.edanzediting.com/ac) for editing a draft of this manuscript.
Disclosures
The authors declare no conflicts of interest.
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