Lyotropic Optical Diffraction Gratings
-from lipid multilayer
Abstract
Lyotropic optical diffraction gratings—composed of
biofunctional lipid multilayers with controllable heights
between ∼5 and 100 nm—can be fabricated by lipid dip-pen
nanolithography carried out in parallel.
Multiple materials can be simultaneously written into arbitrary
patterns on pre-structured surfaces to generate complex
structures and devices
We also show that fluid and biocompatible lipid multilayer
gratings allow label-free and specific detection of lipid–protein
interactions in solution.
This biosensing capability takes advantage of the adhesion
properties of the phospholipid superstructures and the
changes in the size and shape of the grating elements that take
place in response to analyte binding.
Diffraction gratings with varying Period
Diffracted light was detectd by simple CCD device camera
The different colours observed as a function of grating pitch are described by the
grating equation
where
d is the period of the grating,
Ѳm and Ѳi are the angles of diffraction maxima and incidence, respectively,
In our set-up, we use white incident light and observe the intensity of light at Ѳm = 0o
normal to the grating plane.
The colour observed depends only on the grating period Ѳi and , which are adjusted to
give optimal contrast with a period of 600 nm illuminated at Ѳi = 70o.
Grating Height correlated by AFM to Intensity of light diffracted
Height varied between 2-100 nm by dip pen nanolithography
Grating efficiency increases upto
50nm
Fused grating beyond 60nm and
intensity drops off
Note: Error in each height measurement corresponds to surface roughness
Waveguide grating Couplers
The waveguides were formed on
poly(methyl methacrylate) (PMMA)
surfaces by exposure to deep ultraviolet
light through a chromium mask
Light from a supercontinuum laser source
(Koheras SuperK Versa) with a spectral
emission range of 500–800 nm was
coupled into the waveguide through an
optical fibre.
A lipid grating with a period of 700 nm was defined on top of the UV-induced
PMMA waveguide with the lines
perpendicular to the waveguide.
Supercontinuum laser light of different
wavelengths was decoupled under
different angles by the grating coupler
Multiplexed DPN
2 different waveguides
simultaneously fabricated from tip in
parallel array dipped in inks doped
with rhodamine and fluorescein
Individual elements within a single grating
are composed of alternatiing material
The three effects observed as a result of lipid adhesion with the
substrate and interaction with protein from solution:
1.
2.
3.
Spreading
Dewetting
Intercalation
Green represents the lipid multilayers and red the protein
Lipid Spreading in air after 5 min of exposure to humidity above 40%
Dewetting of smooth lines of biotin-containing gratings under solution to form
droplets after 1 min of exposure to the protein streptavidin
Intercalation of protein into lipid multilayer grating lines of different heights after 1 h
Humidity induced Spreading
Time-lapse fluorescence images of grating
Structures of lipid layer on hydrophobic surface in air and in aqueous
Humidity above 40%
Lipid multilayer gratings permits study
of structural layer on binding of
biological molecules
The tendency for the lipid grating elements to change size and
shape upon protein binding, in combination with their optical properties
and innate biofunctions, opens the possibility of a new, biologically
inspired mechanism for label-free protein detection.
The lipid gratings described differ from the existing grating based Sensors in
two aspects:
1. the biofunctions can be incorporated directly into the phospholipid ink,
eliminating the need for further biofunctionalization steps of the transducer as
in the case of existing solid gratings.
2. the immiscibility of the adherent liquid phospholipid droplets with water
permits studies in biologically relevant aqueous solutions
Optical response of biotinylated gratings upon exposure to
streptavidin protein
Label-free detection of protein
binding by monitoring of the
diffraction from gratings upon
exposure to protein at different
concentrations.
The decrease in diffraction intensity
under these conditions is due to the
dewetting mechanism
The limit of detection of 5 nM protein after 15 min is comparable to that of solid
grating-based sensors, which are typically diffusion limited at concentrations on
the order of 5 nM, but after incubation for 90 min,
we are able to observe significant dewetting of biotinylated gratings,
as compared to the pure DOPC control gratings, at a protein concentration
of 500 pM
Protein detection in a complex mixture
The phospholipid bilayers are highly resistant to non-specific protein binding, and
we were therefore able to carry out the same detection of protein added to fetal
calf serum
The diffraction intensities of a DOPC
control grating and a biotinylated grating
were monitored before and after addition
of 10% fetal calf serum at time = 0
and streptavidin protein (added to yield
a final concentration of 50 nM) at time =
10 minutes
Lipids are highly resistant to non-specific
protein binding .
lipid multilayer gratings for specific detection of
protein in complex mixtures such as serum
Response of the grating- Height Dependence
Gratings of different heights were
fabricated on the same surface and
exposed to streptavidin protein.
The grating height was determined
from the relative diffraction
intensity using the intensity/height
data as a calibration
Phase I indicates the diffraction intensity before addition of the protein
Phase II (<30 s after protein addition) a quick dewetting response resulted in lowered
intensity for the high gratings
Phase III a slower intercalation process resulted in the increase in intensity over about
5 minutes for all gratings
Dewetting occurs on shorter time scales – protein only bind to the surface of the
lipid multilayer to change the interfacial energy
To cause a measurable intercalation effect , protein have to fill the multilayer volume
The technique is useful for high-throughput biophysical analysis
with lipid-based photonic structures and novel photonic
sensing elements capable of label-free biosensing by means of a
dynamic shape change upon analyte binding.
The ability to observe analyte intercalation and desorption
from lipid multilayers provides a new, label-free method of
characterizing loading and release conditions of liposomes for
delivery and nanoscale chemistry applications
Higher gratings that respond to analyte binding by a surfacetension change are found to be suitable for detection of
analytes at low concentrations.
The mechanisms based on intercalation of materials into the
fluid gratings can expand the dynamic range of sensing as well
as provide a new way to probe dynamic biomembrane function.