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Cite this: Chem. Commun., 2013, 49, 11038
Published on 08 October 2013. Downloaded on 06/12/2013 07:38:19.
Received 9th September 2013, Accepted 8th October 2013
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Differential interference contrast microscopy imaging of micrometer-long plasmonic nanowires† Ji Won Ha, Kuangcai Chen and Ning Fang*
DOI: 10.1039/c3cc46871b www.rsc.org/chemcomm
We report polarization- and wavelength-sensitive differential interference contrast (DIC) images and intensities of 2 lm-long gold nanowires. The feasibility and usefulness of the gold nanowires as optical sensors and probes in biological systems are demonstrated through combining with a DIC microscope.
One-dimensional gold (Au) nanowires, despite their relatively large sizes, are promising optical sensors and probes in biological systems.1 However, the use of Au nanowires in live cell studies has not been extensively explored due to the cytotoxicity issue. Lately, with much progress in surface modifications of metallic nanoparticles,2–4 there have been a few reports demonstrating the use of Au nanowires as optical sensors and probes in biological environments. Micrometer-long Au nanowires were utilized as biological and chemical sensors based on surface plasmon resonance (SPR) and surface enhanced Raman scattering (SERS) detection mechanisms.5,6 Micrometer-long multi-segment nanowires were used as barcodes for multiplexing.7 Furthermore, Kuo et al. demonstrated that serum-coated Au nanowires with lengths up to a few micrometers can be readily internalized by cells with low toxicity.8,9 Au nanowire optical probes possess several advantages in biological studies. First, it is possible to directly visualize their translational and rotational motions either on a cell membrane or in the intracellular microenvironment under a light microscope. Second, it is easier to manipulate relatively large Au nanowires along with live cells through external forces than smaller nanoparticles. For example, using AC electric fields, Au nanowires can be precisely manipulated to concentrate onto designated places in biological environments.10,11 So far, the Au nanowires have been studied and visualized using bright field microscopy, dark field microscopy,12 and photothermal heterodyne imaging.13 However, these techniques
Ames Laboratory, U.S. Department of Energy, and Department of Chemistry, Iowa State University, Ames, Iowa 50011, USA. E-mail: [email protected]; Fax: +1 515 294 0105 † Electronic supplementary information (ESI) available: Additional experimental details and results. See DOI: 10.1039/c3cc46871b
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Chem. Commun., 2013, 49, 11038--11040
have limited applicability to biological studies due to the following reasons. Bright field microscopy typically generates low contrast for most biological samples as few absorb to a great extent. Scattering-based dark field microscopy may suffer from the relatively high scattering background caused by intracellular components. Absorption-based photothermal heterodyne imaging is limited by raster scanning of the sample and the heating and probe beams that could damage biological samples. Differential interference contrast (DIC) microscopy, on the other hand, enables us to overcome the limitations of the aforementioned imaging techniques and it is better suited to observe motions of plasmonic nanoparticles in live cells.14–16 The interfering nature of DIC microscopy makes it insensitive to the scattered light from surrounding cellular components and gives it high-throughput capability with high contrast images. It is a prerequisite to elucidate the optical properties of Au nanowires under a DIC microscope for their potential use as optical sensors and probes in biological environments. The present communication reports the wavelength-, polarization-, and orientation-dependent DIC images of Au nanowires for the first time. Au nanowires used here were purchased from Nanopartz (A14-2000, Loveland, CO). Fig. S1A (ESI†) shows a transmission electron microscopy (TEM) image of the Au nanowires. The mean length and diameter are 2 mm and 75 nm, respectively. Fig. S1B (ESI†) shows a UV-Vis spectrum of these Au nanowires dispersed in water. As the length increases, the dipole plasmon mode red-shifts while more extinction peaks appear at shorter wavelengths, which correspond to the longitudinal higherorder modes.17–19 In addition, the transverse mode blue-shifts to slightly shorter wavelengths.17 As shown in Fig. S1B (ESI†), the transverse SPR mode appears at around 520 nm and the prominent higher-order modes are observed in the visible spectrum at around 550 nm and 640 nm. Thus, in the present study, we investigated the optical properties of single Au nanowires at three SPR wavelengths of 520 nm, 550 nm, and 640 nm under a DIC microscope (Fig. S2, ESI†). In particular, we tried to elucidate the effect of polarization of light on the DIC image patterns and intensities of the nanowires at the three chosen wavelengths. This journal is
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The Royal Society of Chemistry 2013
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Published on 08 October 2013. Downloaded on 06/12/2013 07:38:19.
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Fig. 1 Polarization-dependent DIC images and intensities of 2 mm long Au nanowires at the transverse SPR wavelength of 520 nm. (A) DIC image of single Au nanowires. (B) The definition of the orientation angle j in the Cartesian coordinate. DIC images of Au nanowire 1 are shown as a function of orientation angle. (C) Change in the bright and dark DIC intensities of Au nanowire 1 as a function of orientation angle.
We first studied the optical properties of Au nanowires at the transverse SPR mode at 520 nm. Fig. 1A shows a DIC image of the immobilized single Au nanowires on a glass slide measured at 520 nm excitation. The orientation angle j is defined with the nanowire’s long axis and the polarization directions, as shown in Fig. 1B. A 3601 rotational study was carried out by rotating the stage by 101 per step to position Au nanowire 1 in different orientations. In this study, Au nanospheres with a diameter of 80 nm were used as reference to find the focal plane after each 101 rotation of the stage. Interestingly, a totally dark DIC image of Au nanowire 1 highlighted with a white square in Fig. 1A was observed when its long axis was aligned with the dark polarization axis (blue arrow, j = 01). After a 901 rotation of the sample slide, the long axis of the nanowire 1 was aligned with the bright polarization axis (red arrow, j = 901) to generate a totally bright image. Fig. S3 (ESI†) shows the complete set of DIC images of Au nanowire 1 from 01 to 3601 with an interval of 101. It is worthwhile to note that this is consistent with the polarization-dependent DIC images of short AuNRs (25 nm 73 nm, Fig. S4, ESI†). DIC intensities from bright and dark polarization directions at 520 nm are plotted as a function of orientation angle in Fig. 1C. The DIC intensities change periodically in both polarization directions when the stage rotates by 101 per step. The bright and dark intensity curves are anti-correlated, that is, an increase in the bright intensity is accompanied by a decrease in the dark intensity, and vice versa (Fig. 1C and Fig. S5, ESI†). In Fig. 1C, the dark intensity of the nanowire increases toward the lower DIC intensity values in the y-axis. The normalized bright and dark DIC intensities of Au nanowire 1 are well fitted with functions of sin4(j) and cos4(j), respectively (Fig. S5, ESI†). Fig. 1C and Fig. S3 (ESI†) elucidate the polarization-sensitive DIC images and intensities of 2 mm long Au nanowires at 520 nm, which are similar to the trend in short AuNRs. However, we found This journal is
c
The Royal Society of Chemistry 2013
ChemComm that there is a clear difference between short AuNRs and long Au nanowires. A DIC image of short AuNRs aligned with the bright polarization axis (j = 01) and measured at the transverse SPR wavelength shows a totally dark image yielding the lowest intensity in the bright polarization axis.20 However, the DIC image of the Au nanowire aligned with the bright polarization axis (j = 01) and measured at 520 nm shows a totally bright image giving the highest intensity in the bright polarization axis. This result suggests that for the Au nanowire at j = 01, the DIC intensity from multipolar mode (or the bright polarization axis) is greater than that from the transverse mode (or the dark polarization axis) at an excitation wavelength of 520 nm. Besides the transverse SPR mode, we further measured single Au nanowires on a glass slide at multipolar SPR wavelengths of 550 nm and 640 nm. Fig. 2A shows a DIC image of the immobilized single Au nanowires measured at 550 nm. A 3601 rotational study was carried out by rotating the stage by 101 per step. Similar to the DIC images at 520 nm (Fig. 1), a totally bright DIC image of Au nanowire 2 highlighted with a green square in Fig. 2A was observed when its long axis was aligned with the bright polarization axis (Fig. 2B) at both wavelengths. As the orientation angle j was increased from 01 to 901, the bright image turned to a dark image because the long axis got aligned with the dark polarization axis. Fig. S6 and S7 (ESI†) show the complete set of DIC images of Au nanowire 2 from 01 to 3601 with an interval of 101 at both 550 nm (Fig. S6, ESI†) and 640 nm (Fig. S7, ESI†). DIC intensities from bright and dark polarization directions at both wavelengths are plotted as a function of orientation angle in Fig. 2C. It should be noted that the DIC intensities at two different SPR wavelengths are different, and 550 nm illumination provided higher contrast (or intensity) than 640 nm illumination (Fig. 2C). We ensured that the transmission of two band-pass filters at 550 nm and 640 nm used in this study is almost the same (Fig. S8, ESI†). Therefore, the difference in the DIC intensities of the same nanowire for two wavelengths can be explained by the fact that 2 mm long Au nanowires absorb more light at
Fig. 2 Polarization- and wavelength-dependent DIC images and intensities of 2 mm-long Au nanowires at the multipolar SPR wavelengths of 550 nm and 640 nm. (A) DIC image of single Au nanowires at 550 nm. (B) DIC images of Au nanowire 2 as a function of orientation angle. (C) Change in the bright and dark DIC intensities of Au nanowire 2 as a function of orientation angle. (D) Comparison of the modulation depth of Au nanowire 2 at 550 nm (blue-curve) and 640 nm (red-curve).
Chem. Commun., 2013, 49, 11038--11040
11039
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Published on 08 October 2013. Downloaded on 06/12/2013 07:38:19.
Fig. 3 Direct observation of three different conformations of Au nanowires on the membranes. (A–C) 16-successive DIC images of three nanowires as a function of time. (D) Polar graph to show the in-plane orientation of the three nanowires for the successive image frames.
550 nm than at 640 nm as supported in Fig. S1B (ESI†). To quantitatively compare, we further obtained relative modulation depth in the intensity profiles at both wavelengths (Fig. 2D). When the modulation depth at 550 nm was 1, the relative modulation depth at 640 nm was determined to be 0.46 for Au nanowire 2. We further measured 10 more individual nanowires and the average relative modulation depth at 640 nm was found to be 0.45 (Fig. S9, ESI†). Finally, we chose single Au nanowires rotating on synthetic membranes as model systems to demonstrate the feasibility and usefulness of Au nanowires as optical probes in biological studies. Cetyltrimethylammoniumbromide (CTAB)-coated Au nanowires were introduced into a chamber. The initially freely diffusing nanowires were bound to the membrane through non-specific interactions. We recorded movies that show rotational motions of surface-bound Au nanowires under 550 nm illumination at a temporal resolution of 100 ms. The 550 nm excitation is advantageous for biological studies because the higher order modes are less sensitive to the refractive index of the surrounding medium.19 Fig. S10A (ESI†) shows a DIC image of single Au nanowires bound onto the membrane. Movies to show the rotational motions of the three Au nanowires squared in Fig. S10A are provided in ESI.† From the movies, we can directly observe three distinct conformations of Au nanowires on the membrane as depicted in Fig. S10B (ESI†). The Au nanowire 3 has one fixed binding site at around its center, while the nanowire 4 has one fixed binding site at the end. The two nanowires show high freedom of rotation, and it is observed that their DIC intensities in the bright and dark polarization directions are mostly anti-correlated during the dynamic process (Fig. S11, ESI†). In contrast, the nanowire 5 has multiple binding sites on the membrane, leading to much reduced freedom of motion. 16-consecutive frames for each nanowire are displayed in Fig. 3A–C. The polar graph in Fig. 3D shows the rotational track of the in-plane orientation angle j as a function of time for the image sequences of the nanowires. The orientation angle j for
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Communication the image frames was measured visually. As demonstrated in Fig. 3B, clockwise or anti-clockwise rotations of each nanowire can be tracked under a DIC microscope, which can be critical in studies of many biological phenomena, such as endocytosis. It is worthwhile to note that although the orientation of Au nanowires can be determined directly by visual inspection of the micrographs, the analysis of the orientation-dependent DIC images provides a fundamental understanding of the optical response of Au nanowires for future biosensing applications. In conclusion, we described polarization- and wavelengthsensitive DIC images and intensities of 2 mm-long gold nanowires. We found that their bright and dark DIC intensities at SPR wavelengths are anti-correlated and well fitted with sin4(j) and cos4(j), respectively. Furthermore, we demonstrated the feasibility and advantages of using the Au nanowires in biological studies through combining with a DIC microscope. This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division. The Ames Laboratory is operated for the U.S. Department of Energy by Iowa State University under contract no. DE-AC02-07CH11358.
Notes and references 1 R. Yan, J.-H. Park, Y. Choi, C.-J. Heo, S.-M. Yang, L. P. Lee and P. Yang, Nat. Nanotechnol., 2012, 7, 191–196. 2 P. Nativo, I. A. Prior and M. Brust, ACS Nano, 2008, 2, 1639–1644. 3 B. Baruah, C. Craighead and C. Abolarin, Langmuir, 2012, 28, 15168–15176. 4 K. E. Sapsford, W. R. Algar, L. Berti, K. B. Gemmill, B. J. Casey, E. Oh, M. H. Stewart and I. L. Medintz, Chem. Rev., 2013, 113, 1904–2074. 5 X. Hong, D. Wang and Y. Li, Chem. Commun., 2011, 47, 9909–9911. 6 K. C. Bantz, A. F. Meyer, N. J. Wittenberg, H. Im, O. Kurtulus, S. H. Lee, N. C. Lindquist, S.-H. Oh and C. L. Haynes, Phys. Chem. Chem. Phys., 2011, 13, 11551–11567. 7 S. J. Hurst, E. K. Payne, L. Qin and C. A. Mirkin, Angew. Chem., Int. Ed., 2006, 45, 2672–2692. 8 C. W. Kuo, J. J. Lai, K. H. Wei and P. Chen, Adv. Funct. Mater., 2007, 17, 3707–3714. 9 K. Chiung-Wen, L. Jun-Jung, W. Kung Hwa and C. Peilin, Nanotechnology, 2008, 19, 025103. 10 D. L. Fan, F. Q. Zhu, R. C. Cammarata and C. L. Chien, Appl. Phys. Lett., 2004, 85, 4175–4177. 11 D. Fan, Z. Yin, R. Cheong, F. Q. Zhu, R. C. Cammarata, C. L. Chien and A. Levchenko, Nat. Nanotechnol., 2010, 5, 545–551. ¨nnichsen and A. P. Alivisatos, Nano Lett., 2004, 5, 301–304. 12 C. So 13 D. Boyer, P. Tamarat, A. Maali, B. Lounis and M. Orrit, Science, 2002, 297, 1160–1163. 14 W. Sun, G. Wang, N. Fang and E. S. Yeung, Anal. Chem., 2009, 81, 9203–9208. 15 J. W. Ha, W. Sun, G. Wang and N. Fang, Chem. Commun., 2011, 47, 7743–7745. 16 J. W. Ha, W. Sun, A. S. Stender and N. Fang, J. Phys. Chem. C, 2012, 116, 2766–2771. 17 E. K. Payne, K. L. Shuford, S. Park, G. C. Schatz and C. A. Mirkin, J. Phys. Chem. B, 2006, 110, 2150–2154. 18 E. R. Encina and E. A. Coronado, J. Phys. Chem. C, 2007, 111, 16796–16801. 19 B. N. Khlebtsov and N. G. Khlebtsov, J. Phys. Chem. C, 2007, 111, 11516–11527. 20 G. Wang, W. Sun, Y. Luo and N. Fang, J. Am. Chem. Soc., 2010, 132, 16417–16422.
This journal is
c
The Royal Society of Chemistry 2013
COMMUNICATION
Cite this: Chem. Commun., 2013, 49, 11038
Published on 08 October 2013. Downloaded on 06/12/2013 07:38:19.
Received 9th September 2013, Accepted 8th October 2013
View Journal | View Issue
Differential interference contrast microscopy imaging of micrometer-long plasmonic nanowires† Ji Won Ha, Kuangcai Chen and Ning Fang*
DOI: 10.1039/c3cc46871b www.rsc.org/chemcomm
We report polarization- and wavelength-sensitive differential interference contrast (DIC) images and intensities of 2 lm-long gold nanowires. The feasibility and usefulness of the gold nanowires as optical sensors and probes in biological systems are demonstrated through combining with a DIC microscope.
One-dimensional gold (Au) nanowires, despite their relatively large sizes, are promising optical sensors and probes in biological systems.1 However, the use of Au nanowires in live cell studies has not been extensively explored due to the cytotoxicity issue. Lately, with much progress in surface modifications of metallic nanoparticles,2–4 there have been a few reports demonstrating the use of Au nanowires as optical sensors and probes in biological environments. Micrometer-long Au nanowires were utilized as biological and chemical sensors based on surface plasmon resonance (SPR) and surface enhanced Raman scattering (SERS) detection mechanisms.5,6 Micrometer-long multi-segment nanowires were used as barcodes for multiplexing.7 Furthermore, Kuo et al. demonstrated that serum-coated Au nanowires with lengths up to a few micrometers can be readily internalized by cells with low toxicity.8,9 Au nanowire optical probes possess several advantages in biological studies. First, it is possible to directly visualize their translational and rotational motions either on a cell membrane or in the intracellular microenvironment under a light microscope. Second, it is easier to manipulate relatively large Au nanowires along with live cells through external forces than smaller nanoparticles. For example, using AC electric fields, Au nanowires can be precisely manipulated to concentrate onto designated places in biological environments.10,11 So far, the Au nanowires have been studied and visualized using bright field microscopy, dark field microscopy,12 and photothermal heterodyne imaging.13 However, these techniques
Ames Laboratory, U.S. Department of Energy, and Department of Chemistry, Iowa State University, Ames, Iowa 50011, USA. E-mail: [email protected]; Fax: +1 515 294 0105 † Electronic supplementary information (ESI) available: Additional experimental details and results. See DOI: 10.1039/c3cc46871b
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Chem. Commun., 2013, 49, 11038--11040
have limited applicability to biological studies due to the following reasons. Bright field microscopy typically generates low contrast for most biological samples as few absorb to a great extent. Scattering-based dark field microscopy may suffer from the relatively high scattering background caused by intracellular components. Absorption-based photothermal heterodyne imaging is limited by raster scanning of the sample and the heating and probe beams that could damage biological samples. Differential interference contrast (DIC) microscopy, on the other hand, enables us to overcome the limitations of the aforementioned imaging techniques and it is better suited to observe motions of plasmonic nanoparticles in live cells.14–16 The interfering nature of DIC microscopy makes it insensitive to the scattered light from surrounding cellular components and gives it high-throughput capability with high contrast images. It is a prerequisite to elucidate the optical properties of Au nanowires under a DIC microscope for their potential use as optical sensors and probes in biological environments. The present communication reports the wavelength-, polarization-, and orientation-dependent DIC images of Au nanowires for the first time. Au nanowires used here were purchased from Nanopartz (A14-2000, Loveland, CO). Fig. S1A (ESI†) shows a transmission electron microscopy (TEM) image of the Au nanowires. The mean length and diameter are 2 mm and 75 nm, respectively. Fig. S1B (ESI†) shows a UV-Vis spectrum of these Au nanowires dispersed in water. As the length increases, the dipole plasmon mode red-shifts while more extinction peaks appear at shorter wavelengths, which correspond to the longitudinal higherorder modes.17–19 In addition, the transverse mode blue-shifts to slightly shorter wavelengths.17 As shown in Fig. S1B (ESI†), the transverse SPR mode appears at around 520 nm and the prominent higher-order modes are observed in the visible spectrum at around 550 nm and 640 nm. Thus, in the present study, we investigated the optical properties of single Au nanowires at three SPR wavelengths of 520 nm, 550 nm, and 640 nm under a DIC microscope (Fig. S2, ESI†). In particular, we tried to elucidate the effect of polarization of light on the DIC image patterns and intensities of the nanowires at the three chosen wavelengths. This journal is
c
The Royal Society of Chemistry 2013
View Article Online
Published on 08 October 2013. Downloaded on 06/12/2013 07:38:19.
Communication
Fig. 1 Polarization-dependent DIC images and intensities of 2 mm long Au nanowires at the transverse SPR wavelength of 520 nm. (A) DIC image of single Au nanowires. (B) The definition of the orientation angle j in the Cartesian coordinate. DIC images of Au nanowire 1 are shown as a function of orientation angle. (C) Change in the bright and dark DIC intensities of Au nanowire 1 as a function of orientation angle.
We first studied the optical properties of Au nanowires at the transverse SPR mode at 520 nm. Fig. 1A shows a DIC image of the immobilized single Au nanowires on a glass slide measured at 520 nm excitation. The orientation angle j is defined with the nanowire’s long axis and the polarization directions, as shown in Fig. 1B. A 3601 rotational study was carried out by rotating the stage by 101 per step to position Au nanowire 1 in different orientations. In this study, Au nanospheres with a diameter of 80 nm were used as reference to find the focal plane after each 101 rotation of the stage. Interestingly, a totally dark DIC image of Au nanowire 1 highlighted with a white square in Fig. 1A was observed when its long axis was aligned with the dark polarization axis (blue arrow, j = 01). After a 901 rotation of the sample slide, the long axis of the nanowire 1 was aligned with the bright polarization axis (red arrow, j = 901) to generate a totally bright image. Fig. S3 (ESI†) shows the complete set of DIC images of Au nanowire 1 from 01 to 3601 with an interval of 101. It is worthwhile to note that this is consistent with the polarization-dependent DIC images of short AuNRs (25 nm 73 nm, Fig. S4, ESI†). DIC intensities from bright and dark polarization directions at 520 nm are plotted as a function of orientation angle in Fig. 1C. The DIC intensities change periodically in both polarization directions when the stage rotates by 101 per step. The bright and dark intensity curves are anti-correlated, that is, an increase in the bright intensity is accompanied by a decrease in the dark intensity, and vice versa (Fig. 1C and Fig. S5, ESI†). In Fig. 1C, the dark intensity of the nanowire increases toward the lower DIC intensity values in the y-axis. The normalized bright and dark DIC intensities of Au nanowire 1 are well fitted with functions of sin4(j) and cos4(j), respectively (Fig. S5, ESI†). Fig. 1C and Fig. S3 (ESI†) elucidate the polarization-sensitive DIC images and intensities of 2 mm long Au nanowires at 520 nm, which are similar to the trend in short AuNRs. However, we found This journal is
c
The Royal Society of Chemistry 2013
ChemComm that there is a clear difference between short AuNRs and long Au nanowires. A DIC image of short AuNRs aligned with the bright polarization axis (j = 01) and measured at the transverse SPR wavelength shows a totally dark image yielding the lowest intensity in the bright polarization axis.20 However, the DIC image of the Au nanowire aligned with the bright polarization axis (j = 01) and measured at 520 nm shows a totally bright image giving the highest intensity in the bright polarization axis. This result suggests that for the Au nanowire at j = 01, the DIC intensity from multipolar mode (or the bright polarization axis) is greater than that from the transverse mode (or the dark polarization axis) at an excitation wavelength of 520 nm. Besides the transverse SPR mode, we further measured single Au nanowires on a glass slide at multipolar SPR wavelengths of 550 nm and 640 nm. Fig. 2A shows a DIC image of the immobilized single Au nanowires measured at 550 nm. A 3601 rotational study was carried out by rotating the stage by 101 per step. Similar to the DIC images at 520 nm (Fig. 1), a totally bright DIC image of Au nanowire 2 highlighted with a green square in Fig. 2A was observed when its long axis was aligned with the bright polarization axis (Fig. 2B) at both wavelengths. As the orientation angle j was increased from 01 to 901, the bright image turned to a dark image because the long axis got aligned with the dark polarization axis. Fig. S6 and S7 (ESI†) show the complete set of DIC images of Au nanowire 2 from 01 to 3601 with an interval of 101 at both 550 nm (Fig. S6, ESI†) and 640 nm (Fig. S7, ESI†). DIC intensities from bright and dark polarization directions at both wavelengths are plotted as a function of orientation angle in Fig. 2C. It should be noted that the DIC intensities at two different SPR wavelengths are different, and 550 nm illumination provided higher contrast (or intensity) than 640 nm illumination (Fig. 2C). We ensured that the transmission of two band-pass filters at 550 nm and 640 nm used in this study is almost the same (Fig. S8, ESI†). Therefore, the difference in the DIC intensities of the same nanowire for two wavelengths can be explained by the fact that 2 mm long Au nanowires absorb more light at
Fig. 2 Polarization- and wavelength-dependent DIC images and intensities of 2 mm-long Au nanowires at the multipolar SPR wavelengths of 550 nm and 640 nm. (A) DIC image of single Au nanowires at 550 nm. (B) DIC images of Au nanowire 2 as a function of orientation angle. (C) Change in the bright and dark DIC intensities of Au nanowire 2 as a function of orientation angle. (D) Comparison of the modulation depth of Au nanowire 2 at 550 nm (blue-curve) and 640 nm (red-curve).
Chem. Commun., 2013, 49, 11038--11040
11039
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ChemComm
Published on 08 October 2013. Downloaded on 06/12/2013 07:38:19.
Fig. 3 Direct observation of three different conformations of Au nanowires on the membranes. (A–C) 16-successive DIC images of three nanowires as a function of time. (D) Polar graph to show the in-plane orientation of the three nanowires for the successive image frames.
550 nm than at 640 nm as supported in Fig. S1B (ESI†). To quantitatively compare, we further obtained relative modulation depth in the intensity profiles at both wavelengths (Fig. 2D). When the modulation depth at 550 nm was 1, the relative modulation depth at 640 nm was determined to be 0.46 for Au nanowire 2. We further measured 10 more individual nanowires and the average relative modulation depth at 640 nm was found to be 0.45 (Fig. S9, ESI†). Finally, we chose single Au nanowires rotating on synthetic membranes as model systems to demonstrate the feasibility and usefulness of Au nanowires as optical probes in biological studies. Cetyltrimethylammoniumbromide (CTAB)-coated Au nanowires were introduced into a chamber. The initially freely diffusing nanowires were bound to the membrane through non-specific interactions. We recorded movies that show rotational motions of surface-bound Au nanowires under 550 nm illumination at a temporal resolution of 100 ms. The 550 nm excitation is advantageous for biological studies because the higher order modes are less sensitive to the refractive index of the surrounding medium.19 Fig. S10A (ESI†) shows a DIC image of single Au nanowires bound onto the membrane. Movies to show the rotational motions of the three Au nanowires squared in Fig. S10A are provided in ESI.† From the movies, we can directly observe three distinct conformations of Au nanowires on the membrane as depicted in Fig. S10B (ESI†). The Au nanowire 3 has one fixed binding site at around its center, while the nanowire 4 has one fixed binding site at the end. The two nanowires show high freedom of rotation, and it is observed that their DIC intensities in the bright and dark polarization directions are mostly anti-correlated during the dynamic process (Fig. S11, ESI†). In contrast, the nanowire 5 has multiple binding sites on the membrane, leading to much reduced freedom of motion. 16-consecutive frames for each nanowire are displayed in Fig. 3A–C. The polar graph in Fig. 3D shows the rotational track of the in-plane orientation angle j as a function of time for the image sequences of the nanowires. The orientation angle j for
11040
Chem. Commun., 2013, 49, 11038--11040
Communication the image frames was measured visually. As demonstrated in Fig. 3B, clockwise or anti-clockwise rotations of each nanowire can be tracked under a DIC microscope, which can be critical in studies of many biological phenomena, such as endocytosis. It is worthwhile to note that although the orientation of Au nanowires can be determined directly by visual inspection of the micrographs, the analysis of the orientation-dependent DIC images provides a fundamental understanding of the optical response of Au nanowires for future biosensing applications. In conclusion, we described polarization- and wavelengthsensitive DIC images and intensities of 2 mm-long gold nanowires. We found that their bright and dark DIC intensities at SPR wavelengths are anti-correlated and well fitted with sin4(j) and cos4(j), respectively. Furthermore, we demonstrated the feasibility and advantages of using the Au nanowires in biological studies through combining with a DIC microscope. This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division. The Ames Laboratory is operated for the U.S. Department of Energy by Iowa State University under contract no. DE-AC02-07CH11358.
Notes and references 1 R. Yan, J.-H. Park, Y. Choi, C.-J. Heo, S.-M. Yang, L. P. Lee and P. Yang, Nat. Nanotechnol., 2012, 7, 191–196. 2 P. Nativo, I. A. Prior and M. Brust, ACS Nano, 2008, 2, 1639–1644. 3 B. Baruah, C. Craighead and C. Abolarin, Langmuir, 2012, 28, 15168–15176. 4 K. E. Sapsford, W. R. Algar, L. Berti, K. B. Gemmill, B. J. Casey, E. Oh, M. H. Stewart and I. L. Medintz, Chem. Rev., 2013, 113, 1904–2074. 5 X. Hong, D. Wang and Y. Li, Chem. Commun., 2011, 47, 9909–9911. 6 K. C. Bantz, A. F. Meyer, N. J. Wittenberg, H. Im, O. Kurtulus, S. H. Lee, N. C. Lindquist, S.-H. Oh and C. L. Haynes, Phys. Chem. Chem. Phys., 2011, 13, 11551–11567. 7 S. J. Hurst, E. K. Payne, L. Qin and C. A. Mirkin, Angew. Chem., Int. Ed., 2006, 45, 2672–2692. 8 C. W. Kuo, J. J. Lai, K. H. Wei and P. Chen, Adv. Funct. Mater., 2007, 17, 3707–3714. 9 K. Chiung-Wen, L. Jun-Jung, W. Kung Hwa and C. Peilin, Nanotechnology, 2008, 19, 025103. 10 D. L. Fan, F. Q. Zhu, R. C. Cammarata and C. L. Chien, Appl. Phys. Lett., 2004, 85, 4175–4177. 11 D. Fan, Z. Yin, R. Cheong, F. Q. Zhu, R. C. Cammarata, C. L. Chien and A. Levchenko, Nat. Nanotechnol., 2010, 5, 545–551. ¨nnichsen and A. P. Alivisatos, Nano Lett., 2004, 5, 301–304. 12 C. So 13 D. Boyer, P. Tamarat, A. Maali, B. Lounis and M. Orrit, Science, 2002, 297, 1160–1163. 14 W. Sun, G. Wang, N. Fang and E. S. Yeung, Anal. Chem., 2009, 81, 9203–9208. 15 J. W. Ha, W. Sun, G. Wang and N. Fang, Chem. Commun., 2011, 47, 7743–7745. 16 J. W. Ha, W. Sun, A. S. Stender and N. Fang, J. Phys. Chem. C, 2012, 116, 2766–2771. 17 E. K. Payne, K. L. Shuford, S. Park, G. C. Schatz and C. A. Mirkin, J. Phys. Chem. B, 2006, 110, 2150–2154. 18 E. R. Encina and E. A. Coronado, J. Phys. Chem. C, 2007, 111, 16796–16801. 19 B. N. Khlebtsov and N. G. Khlebtsov, J. Phys. Chem. C, 2007, 111, 11516–11527. 20 G. Wang, W. Sun, Y. Luo and N. Fang, J. Am. Chem. Soc., 2010, 132, 16417–16422.
This journal is
c
The Royal Society of Chemistry 2013
