Letter pubs.acs.org/Langmuir
Near Field Guided Chemical Nanopatterning Karl-Heinz Dostert, Marta Á lvarez, Kaloian Koynov, Aránzazu del Campo, Hans-Jürgen Butt, and Maximilian Kreiter* Max-Planck-Institut für Polymerforschung, Ackermannweg 10, 55128 Mainz, Germany S Supporting Information *
ABSTRACT: This article demonstrates the possibility of creating welldefined and functional surface chemical nanopatterns using the optical near field of metal nanostructures and a photosensitive organic layer. The intensity distribution of the near field controlled the site and the extent of the photochemical reaction at the surface. The resulting pattern was used to guide the controlled assembly of colloids with a complementary surface functionality onto the substrate. Gold colloids of 20 nm diameter were covalently bound to the activated nanosites and proved the functionality of the suboptical wavelength structures and enabled direct visualization by means of electron microscopy. Our results prove, for the first time, the possibility of using optical near field to perform chemical reactions and assembly at the nanoscale.
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INTRODUCTION Optical excitation of metal nanoparticles at their resonance frequency induces strong oscillations of the conduction electrons of the metal that lead to very high local charge accumulations and thus to a strong enhancement of the electromagnetic field in the proximity of the particle.1,2 These enhanced optical near fields are spatially confined very close to the particle and extend only few tens of nanometers from the metal surface depending on the structure. Clearly, the optical near field of a metal nanoparticle can strongly affect and interact with molecules or other nanoparticles located in its proximity. That is why the optical near fields have been extensively studied during the last decades with respect to their numerous applications in fluorescence and Raman signal enhancement,3−8 chemo- and biosensing,9−11 and near field lithography.12,13 A number of numerical methods that allow for calculation of plasmon modes and intensity distribution of the near field for nanoparticles of different geometries are available.14−17 Furthermore, high resolution techniques such as scanning near field optical microscopy18,19 and single molecule imaging20 have been used for direct experimental visualization of the enhanced optical near fields (local distribution and intensity) of metal nanostructures. Alternative visualization techniques include imprinting the near field in a photosensitive polymer coating,12,21−28 in the metal nanostructures themselves29,30 or ablative reactions on silicon or glass,31,3233 that cause a topography change that reflects the intensity enhancement and can be imaged with Atomic Force Microscopy (AFM) or Scanning Electron Microscopy (SEM). All these results have shown that the optical near fields of metal particles decay on very small length scales and can be used to modify materials located in the nanometer proximity of the particles. In this manuscript, we demonstrate the possibility of creating well-defined and functional chemical nanopatterns on sub© 2012 American Chemical Society
strates using optical near field and photosensitive organic layers. The intensity distribution of the near field controls the site and the extent of the photochemical reaction at the surface. The resulting chemical nanopattern was used to guide the controlled assembly of colloids with a complementary surface functionality onto the substrate. Our results prove, for the first time, the possibility of using optical near field to perform chemical reactions and assembly at the nanoscale.
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METHODS
The deposition of crescent shaped gold nanoparticles onto glass substrates was made via nanosphere template lithography (see Supporting Information).34 The synthesis of R-TEG-NH-NVOC photosensitive organosilane (Figure S1 in Supporting Information) containing nitroveratryl (NVOC)-caged amine groups was performed as previously reported.35 Glass substrates were modified with the photosensitive silane as specified in the Supporting Information. Two-Photon Photocleavage of the NVOC Groups and Generation of Surface Nanopatterns. The substrates were illuminated with a Ti:Sa laser (Mai Tai, Spectra Physics Inc., USA) coupled to a confocal laser scanning microscope (LSM 510 and Axiovert 200 M, Carl Zeiss, Göttingen, Germany). This laser is tunable from λ = 780 nm to λ = 920 nm and provides ∼100 fs pulses at a repetition rate of 80 MHz. The laser light was tightly focused on the substrate to a spot with a diameter of ∼1 μm using a Plan-Neofluar 20× microscope objective (Carl Zeiss, Jena, Germany) with numerical aperture (NA) of 0.5 and working distance of 2 mm. As the NVOC has its absorption maximum around 350 nm, the lowest available laser wavelength of 780 nm was selected for the two-photon excitation of the NVOC group. The maximum time-averaged laser power in the object plane at this wavelength was about 190 mW, corresponding to pulse energy of roughly 2 nJ. Using the galvanometric mirrors of the LSM 510, the focal spot was continuously raster-scanned (512 lines) Received: January 2, 2012 Revised: February 6, 2012 Published: February 8, 2012 3699
dx.doi.org/10.1021/la300009a | Langmuir 2012, 28, 3699−3703
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Letter
Figure 1. Strategy for near field guided chemical nanopatterning followed by nanoparticle assembly. on the substrate surface, creating squared illuminated patterns with a typical size of 45 × 45 μm2. Both the total exposure time and the applied laser power were varied. Since the progress of a two-photon reaction depends quadratically on the intensity of the applied field I780 and linearly on the time of irradiation t, the dose of irradiation D780 2 was defined as D780 = I780 ·t Regiospecific Assembly of Gold Colloids. Gold colloids (20 nm in diameter) were prepared following reported procedures.36 A 0.25 mM aqueous solution of hydrogen tetrachloroaurate (III) trihydrate was heated to the boiling point and an 85 mM aqueous solution of trisodium citrate dehydrate was added. In order to functionalize the obtained particles with the carboxylic terminated thiol, 5 mL out of the obtained dispersion were added to a dispersion of 0.25 mg 11-mercaptoundecanic acid (95% purity, Aldrich) in 5 mL phosphate buffered solution (PBS, Invitrogen) at pH = 7.4 and stirred for 16 h. 0.40 mg N-hydroxysuccinimide (NHS) and 0.14 mg 1-ethyl3(3-dimethylaminopropyl)carbodiimide (EDC) were added to the 5 mL gold colloid suspension and the NVOC/amine functionalized surfaces were incubated in this suspension for 2 h. The samples were sonicated in water for 4 min and finally dried.
the crescents). Incubation with a suspension of gold nanoparticles functionalized with carboxylic acid groups leads to selective deposition of the nanoparticles onto these sites and creates a near field driven nanopattern. We first studied the light intensity (photon dose) threshold required for the cleavage of the NVOC cage at 780 nm in the absence of gold crescents. In order to determine the threshold, quartz substrates modified with the photosensitive organosilane (see Methods section for details) were illuminated by rasterscanning over the substrate surface with the tightly focused beam of a Ti:Sa laser operating at 780 nm. In this way 45 × 45 μm2 squares were written at various light intensities (I) between 5 and 190 mW and irradiation times (t) between 0.8 to 62 s. The photon dose at each square can be defined as D780 = I2t.38 After exposure, the sample was incubated with COOH functionalized gold colloids under EDC/NHS activation to create an amide bond between the amines at the exposed regions and the carboxylic groups at the surface of the gold nanoparticles (see formulas in Figure 1). The obtained patterns as imaged by SEM are presented in Figure 2. The brightness scale in the image correlates with the surface density of adsorbed gold colloids. From these studies, the threshold intensity for the photocleavage of NVOC was determined to be D780 ≈ 820 μW2·s. In the next step we performed similar experiments on substrates containing both the photocleavable molecule and the Au crescents. We used irradiation doses below the previously identified threshold for NVOC photocleavage in the absence of crescents (i.e.,
