Direct photopatterning of light-activated gold nanoparticles

Article abstract of DOI:10.1039/c1jm11035g

Photoactivatable gold NPs were patterned via photolithography. In this approach, charge reversal of the ligands on NPs upon UV irradiation induces crosslinking to generate stable NP patterns.

Full text of DOI:10.1039/c1jm11035g

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Direct photopatterning of light-activated gold nanoparticles†
Chandramouleeswaran Subramani,a Xi Yu,a Sarit. S. Agasti,a Bradley Duncan,a Serkan Eymur,b Murat Tongaa
a
*
and Vincent M. Rotello
Received 9th March 2011, Accepted 4th May 2011
DOI: 10.1039/c1jm11035g
orthogonally functionalized surfaces through electrostatic deposition
of quantum dots (Scheme 1).
Photoactivatable gold NPs were patterned via photolithography. In
this approach, charge reversal of the ligands on NPs upon UV
irradiation induces crosslinking to generate stable NP patterns.
In our initial studies, we determined the time course of the pho-
toactivation using UV-vis spectroscopy. We investigated the photo-
cleaving behavior of the free photocleavable thiol ligand and the NP
using 365 nm UV radiation. A decrease of absorbance was observed
at 250 nm, along with an increase at 320 nm, indicating the breakage
of the photolabile ester bond and the formation of o-nitro-
benzaldehyde with concomitant charge conversion on the ligand.
These results confirm that the photolytic reaction of the NP proceeds
in an analogous fashion to that of the free photocleavable thiol ligand
Fabrication of functional structures using spatial organization of
nanometre-scale building blocks including nanoparticles, carbon
nanotubes, proteins, organic molecules and polymers is important for
various applications such as electronics,1 optics,2 biological,3 and
sensing systems.4 Recently, ligand protected metallic and semi-
conducting nanoparticles (NPs) have been widely used in the fabri-
cation of functional structures. The tunable size and surface
properties of NPs provide potential applications in nanostructured
conductors, novel resists, and templates for electronics,5 sensors,6 and
biological recognition.7
Templated assembly method and direct patterning are two major
techniques that have been widely used to pattern NPs onto surfaces.
In the templated assembly method, the template is created using
lithographic techniques including photo-passivation,8,9 nanoscale
phase-separated block copolymer,10 electron beam lithography,11
nanoshaving,12
nanoimprinting,13
electro-oxidation
nano-
lithography14 and scanning probe lithographies.15 While NP
patterning through templates is useful for many applications, direct
patterning16 provides a facile way to create features of arbitrary
thickness that are substrate independent. Moreover, direct patterning
of charged NPs provides a platform for the fabrication bio-sensors17
and optoelectronic devices.18
Herein, we present a direct approach to pattern NPs via photoli-
thography using NPs featuring photocleavable ligands.19 Our
strategy differs from previous methods by using partial charge
reversal to effect patterning of charged NPs, providing patterns of
arbitrary thickness, an important criterion for the fabrication of
nanoparticle based devices. Using this strategy, photopatterning of
charge switchable gold NPs provided stable features. NP patterning
on crosslinked polyethyleneimine (PEI) substrates provided access to
Scheme 1 Fabrication of NP pattern via photolithography of photo-
activable gold NPs. (a) Spin-coating NPs onto PEI coated silicon
substrate, (b) light activation of NPs under a mask using UV lamp
365 nm wavelength; zoomed image shows the reversal of charge on NPs,
(c) developing NP patterns using a methanol wash, (d) electrostatic
assembly quantum dots onto PEI surface, (e) chemical structure of NP
before light activation, (f) chemical structure of NP after light activation,
(g) chemical structure of quantum dot, and (h) chemical structure of
polyethyleneimine.
aDepartment of Chemistry, University of Massachusetts Amherst, 710
North Pleasant Street, Amherst, MA, 01003, USA. E-mail: rotello@
chem.umass.edu; Fax: +1 413-5452058
bDepartment of Chemistry, Middle East Technical University, 06531
Ankara, Turkey
† Electronic supplementary information (ESI) available: AFM images,
Synthesis of NPs, and NMR data. See DOI: 10.1039/c1jm11035g
14156 | J. Mater. Chem., 2011, 21, 14156–14158
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(Fig. 1). After 365 nm UV exposure, there is no change in absorbance
at 525 nm in UV-vis spectra (Fig. 1) which clearly demonstrates that
365 nm light activation retains the optical properties of the starting
NPS.
For our patterning studies, the particles were deposited onto
crosslinked PEI. To provide the PEI layer, PEI and 5% wt of
diglycidyl ether of bisphenol-A (DGEBA) were spin-coated onto
a silicon substrate and thermally crosslinked at 100 ꢀC for 5 h. This
process provides a surface with a sufficient amount of amine func-
tional groups still available for subsequent electrostatic interactions
on the surface. A thin layer (ꢁ100 nm) of photo-switchable 6 nm core
NPs was then spin-coated on top of the PEI. Photolithography was
performed for 30 minutes, resulting in transfer of the mask pattern
into the NP layer. Next, the pattern was developed onto the PEI film
by washing it with methanol providing patterned NP features
(Fig. 2a). AFM shows there is no significant change in the thickness
of the film after photo-irradiation and methanol washing (ESI†).
Scanning electron microscope (SEM) was used to image the NP
patterns (Fig. 2b). The magnified SEM image showed the discrete NP
structure in the patterns. We next carried out atomic force micros-
copy measurements to find out the pattern thickness. The feature
height of 100 nm was observed by AFM imaging (Fig. 2c), consistent
with the spin coating thickness, indicating multiple layers of particles
(NP core diameters ꢁ6 nm). No NP deposition was detected in the
trench regions between features, indicating the high specificity of the
particles photocrosslinking.
Fig. 2 (a) Bright field image of NP pattern, (b) SEM image of NP
pattern, inset shows discrete NP structures in a magnified image (scale
bar 100 nm), (c) AFM image of NP pattern and (d) Z-height profile for
the NP pattern.
on the order of 10À11 s. No coulomb blockade-like phenomenon was
observed, which is possibly due to the relatively large size of the gold
NPs we used.21 In comparison, the surface with only a PEI film
showed no obvious current response from À0.5 to 0.5 V, demon-
strating that the conductivity is due to the NP layer.
In summary, we have demonstrated the use of photoactivable NPs
to provide functionalized nanostructures via charge reversal of the
ligands on NPs upon UV irradiation. This structural motif can be
used to generate platforms for photonics, nonvolatile memories,
microfluidics, biosensors, floating gates, and optoelectronics. More-
over, orthogonal functionalization of these NP patterns via non-
covalent (electrostatic) assembly of QDs shows potential applications
to fabricate metal–semiconductor junctions. We are currently
applying this strategy to the creation of functional electronic devices.
We next investigated the possibility of co-assembling metallic and
semiconducting nanoparticles (QDs) onto the substrate.20 To provide
orthogonal assembly, we utilized the electrostatic assembly of nega-
tively charged carboxylate functionalized QDs onto the positively
charged exposed PEI surface. Exposure of the pattern to negatively
charged QDs at a pH of 7.5 for 30 min, followed by thorough
washing resulted in specific deposition of the QDs onto the surface
(Fig. 2). The increase in fluorescence demonstrates the successful
assembly of QDs onto the PEI surface where gold NPs were not
photoswitched (Fig. 3).
Experimental
Synthesis of NPs
To investigate the electronic properties of the crosslinked NP film,
current–voltage (I–V) curves were recorded by applying a DC bias
across the film with a top contact. As shown in Fig. 4, the NP
patterned surfaces showed a linear I–V behavior with a conduction
NPs/QDs and their ligands were prepared as described in previous
reports.16,22
Film preparation
Solutions consisting of 400 mg PEI, 10 mg of diglycidyl ether of
bisphenol-A (DGEBA) and 10 mg of an accelerant 2,4,6-tris(dime-
thylaminomethyl)phenol in 30 mL of 1 : 3 methanol/chloroform were
prepared. The solution was filtered and spin-coated at 3000 rpm for
60 s onto a silicon substrate, yielding a thin film of PEI (ꢁ25 nm).
ꢀ
This film was crosslinked by heating at 100 C for 5 hours in air.
Fig. 3 (a) Fluorescence image of NP pattern after assembling QD via
electrostatic assembly, inset shows before QD assembly (b) fluorescence
profiles of NP pattern before and after assembly of QDs. Scale bar in the
inset is 20 mm.
Fig. 1 Overlaid UV-vis spectral changes upon irradiation of NP with
UV light (365 nm) and inset shows the spectra for ligand.
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This research was supported by the NSF: (CHE-1025889, VR),
MRSEC facilities, and the Center for Hierarchical Manufacturing
facilities (DMI-0531171). S.E. thanks TUBITAK for 2214-Research
fellowship program and the METU-DPT-OYP program on the
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This journal is ª The Royal Society of Chemistry 2011