U.S. patent application number 14/021856 was filed with the patent office on 2014-01-09 for method for fabrication of localized plasmon transducers.
This patent application is currently assigned to YEDA RESEARCH AND DEVELOPMENT CO.LTD.. The applicant listed for this patent is YEDA RESEARCH AND DEVELOPMENT CO.LTD.. Invention is credited to Tatyana Karakouz, Israel Rubinstein, Alexander Vaskevich.
Application Number | 20140009754 14/021856 |
Document ID | / |
Family ID | 40983311 |
Filed Date | 2014-01-09 |
United States Patent
Application |
20140009754 |
Kind Code |
A1 |
Rubinstein; Israel ; et
al. |
January 9, 2014 |
METHOD FOR FABRICATION OF LOCALIZED PLASMON TRANSDUCERS
Abstract
A method is presented for use in fabrication of metal islands on
an oxide substrate. The method comprises: depositing a selected
metal on the oxide substrate by evaporation of said selected metal;
and annealing a film of the selected metal on said substrate at
temperatures including an annealing temperature being less than
50.degree. c lower than a glass transition temperature, thereby
forming the metal islands partially embedded in said substrate.
Inventors: |
Rubinstein; Israel;
(Rehovot, IL) ; Vaskevich; Alexander; (Rehovot,
IL) ; Karakouz; Tatyana; (Rehovot, IL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
YEDA RESEARCH AND DEVELOPMENT CO.LTD. |
Rehovot |
IL |
US |
|
|
Assignee: |
YEDA RESEARCH AND DEVELOPMENT
CO.LTD.
Rehovot
IL
|
Family ID: |
40983311 |
Appl. No.: |
14/021856 |
Filed: |
September 9, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12988067 |
Oct 15, 2010 |
8529988 |
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PCT/IL2009/000404 |
Apr 7, 2009 |
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14021856 |
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61045069 |
Apr 15, 2008 |
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Current U.S.
Class: |
356/128 ;
428/119 |
Current CPC
Class: |
C03C 17/006 20130101;
G01N 21/554 20130101; C03C 2218/151 20130101; G02B 5/008 20130101;
C03C 2218/32 20130101; B82Y 30/00 20130101; C03C 2217/42 20130101;
Y10T 428/24174 20150115; C23C 14/5853 20130101; C23C 14/18
20130101; B82Y 15/00 20130101; B82Y 20/00 20130101; Y10T 428/24413
20150115; G01N 21/41 20130101; C23C 14/5806 20130101 |
Class at
Publication: |
356/128 ;
428/119 |
International
Class: |
G02B 5/00 20060101
G02B005/00; G01N 21/41 20060101 G01N021/41 |
Claims
1. A structure comprising a glass substrate carrying a plurality of
isolated metal islands on a surface of the glass substrate, wherein
said islands are depressed into said substrate being partially
embedded into said glass substrate, said metal islands being
characterized by stabilized morphology and optical properties.
2. A structure according to claim 1, wherein a majority of said
islands are embedded into depressions with a rim higher than 0.5
nm.
3. A structure according to claim 1, wherein a majority of in-plane
shapes of said islands have at least one crystallographic
angle.
4. A structure according to claim 1, wherein said metal islands
directly interface said substrate without an intermediate metal or
organic adhesion layer.
5. A structure according to claim 3, wherein said metal islands
directly interface said substrate without an intermediate metal or
organic adhesion layer.
6. A structure according to claim 1, wherein said metal is
gold.
7. A structure according to claim 1, wherein said plurality of
islands are embedded into depressions with closed-loop glass
nano-sized rims on said substrate, major axis of a majority of said
islands being between 5 nm and 400 nm and heights of a majority of
said depressions being higher than 0.5 nm.
8. A structure according to claim 7, wherein a majority of said
rims enclose depressions with more than 0.5 nm depths, said depths
being measured from main surface of said glass substrate.
9. A structure according to claim 1, wherein said islands have
substantially elongated geometry.
10. A structure according to claim 1, wherein said glass is
borosilicate glass.
11. A structure according to claim 1 being fabricated by a method
comprising: depositing a selected metal on the glass substrate by
evaporation of said selected metal; and annealing a deposited film
of the selected metal on said substrate, said annealing being
carried out for a selected period of time and at temperatures
including an annealing temperature being less than 50.degree. C.
lower than a glass transition temperature of said substrate,
thereby forming the metal islands depressed into said substrate so
that the metal islands become partially embedded within said
substrate.
12. A structure according to claim 11, wherein the annealing
temperature substantially does not exceed the transition
temperature by more than 100.degree. C.
13. A structure according to claim 11, wherein the annealing at
said annealing temperature is performed with access of oxygen to
said film.
14. A structure according to claim 11, wherein said metal is
gold.
15. A structure according to claim 11, wherein said annealing is
done for a time longer than 30 minutes.
16. A structure according to claim 11, wherein said annealing is
done for a time longer than three hours.
17. A structure according to claim 11, wherein said film for more
than 10% consists of islands of said metal.
18. A structure according to claim 11, wherein the film of said
metal directly interfaces with said substrate without any
intermediate adhesive layer between the film and the substrate.
19. A structure according to claim 11, wherein said annealing
temperature is less than 20.degree. C. lower than said transition
temperature.
20. A structure according to claim 11, wherein said glass is a
borosilicate glass.
21. A sensor device comprising the structure of claim 1, the sensor
device being optically responsive to changes in refractive index of
foreign chemical or biological material compositions to which the
structure is exposed.
Description
TECHNICAL FIELD AND BACKGROUND OF THE INVENTION
[0001] The invention is generally in the field of nanostructures
and relates to localized plasmon transducers and methods of their
fabrication, and may be used in the field of chemical and
biological sensors.
[0002] Metal islands present unique optical and electronic
properties, notably different from those of the bulk material. The
special optical phenomena, associated with metal clusters (e.g.,
gold, silver, copper), may be attributed to excitation of localized
surface plasmons (SPs) and observed as strong light scattering,
intense light absorption, and local electromagnetic field
enhancement. The localized surface plasmon resonance (LSPE)
extinction band (shape, amplitude, frequency of maximum extinction)
of an ensemble of Au or Ag nanoislands depends on the particle
size, shape and spacing, as well as on the effective refractive
index of the surrounding medium. The latter forms the basis for
sensing applications based on refractive index change at the
nanoisland-medium interface as a consequence of analyte binding.
Optical transducers with LSPR band may be used in relatively simple
and low-cost optical systems for biological and chemical
sensing.
GENERAL DESCRIPTION
[0003] There is a need in the art for novel nanostructures for
metal island films on glass and novel methods of fabrication of
such films, aimed at obtaining strongly bonded and morphologically
stable metal nanostructured films. This is associated with the
following.
[0004] One of the effects associated with the known metal-island
based LSPR transducers is the instability of such systems toward
changes in the environmental conditions. This instability was
experienced in experiments conducted with Ag as well as Au island
films immersed in various solvents. In particular, exposure to
aqueous solutions (as is a typical biological milieu) can
significantly hinder the adhesion of Ag and Au island films to
glass. Also, solvent immersion and drying of Au (or other noble
metal) island films can cause significant changes in their
morphology and optical properties. The instability can be due to
(i) poor adhesion of noble metals to oxide surfaces such as glass,
commonly used as a transparent substrate for LSPR measurements;
(ii) high sensitivity of the optical response of metal-island based
LSPR systems to small changes in the island morphology.
[0005] Generally the stability of the metal island films could be
improved by relying on solvent preconditioning and avoiding drying.
This technique, however, is not so widely used in applications.
Somewhat more generally, the weak chemical interaction between Au
and oxide substrates can be improved by using metallic (e.g. Cr,
Ni, Ti) or organic (e.g. amino- or mercapto-silane; dendrimer)
coupling layers. However, metal underlayers might introduce optical
and chemical interference, while organic underlayers might not
furnish the necessary stability.
[0006] Concerning preparation processes for Au films on various
substrates, annealing or melting in the presence of oxygen can
improve the bonding strength of Au to fused silica, glass, and
sapphire. For example, melting of Au on silica resulted in the
presence of Au in the silica near the triple gas-metal-oxide
interface, as it was determined by autoradiography after removing
the exposed metal by dissolution in aqua regia.
[0007] The inventors present a novel technique that can be used for
obtaining strongly bonded and morphologically stable metal
nanostructured (nanoisland) films on oxide substrates.
Particularly, the technique of the inventors has been tested to
work with a noble metal, gold (Au), directly deposited on the oxide
substrate (typically a glass substrate), i.e. without any
intermediate adhesion layer.
[0008] The tried deposition processes included gold thermal
evaporation. The deposited Au nanostructures were annealed in the
presence of oxygen at temperatures close to a glass transition
temperature T.sub.g of the glass substrate. The annealing has led
to (partial) embedding of Au islands in the glass substrate and
stabilization of the island morphology and optical properties: upon
annealing in air the Au features settled into depressions, formed
in the glass and encircled by glass rims. The strong adhesion and
stability of ultrasmall gold islands on glass is valuable for
fundamental studies and for applications, including glass/Au
optical sensing based on localized surface plasmon resonance (LSPR)
spectroscopy. The islands are generally of elongated geometry (e.g.
ellipsoidal-like, i.e. with aspect ratio higher than 1).
[0009] The inventors have found that deposition processes based on
the metal (e.g. gold) thermal evaporation followed by annealing at
temperatures close to a transition temperature of the oxide
substrate resulted in the formation of a metal nanostructured film
characterized by extended stability and higher refractive index
sensitivity to foreign materials (chemical and biological
compositions), e.g. 2-3 times higher, as compared to the
conventional techniques of the kind specified.
[0010] In a broad aspect of the invention, there is provided a
method of fabrication of metal islands on an oxide substrate, the
method comprising:
[0011] depositing a selected metal on the oxide substrate by
evaporation of said selected metal; and
[0012] annealing a film of the selected metal on said substrate at
temperatures including an annealing temperature being less than
50.degree. C. lower than a transition temperature of said oxide,
thereby forming the metal islands partially embedded in said
substrate.
[0013] Preferably, the annealing temperature does not exceed said
transition temperature of the oxide by more than 100.degree. C.
[0014] Preferably the oxide substrate is a glass substrate. The
glass may be a borosilicate glass.
[0015] The annealing temperature may be less than 20.degree. C.
lower than the glass transition temperature.
[0016] The film may be discontinuous. The annealing may take longer
than 30 minutes; furthermore, it may take longer than three
hours.
[0017] The annealed film may for more than 10% consist of islands
of the selected metal.
[0018] As noted above, the annealed film may be in direct
interaction (contact) with the substrate (glass) without any
intermediate adhesive layer between the film and the glass.
[0019] In another aspect of the invention, there is provided a
method of fabrication of a metal island film on glass, the method
including annealing a film of a selected metal on glass in presence
of oxygen, the annealing being carried out at temperatures and for
a period of time selected to produce generally elongated islands
partially embedded into said glass.
[0020] A majority of islands with a major axis between 5 and 400 nm
may be depressed into glass for more than 0.5 nm.
[0021] In yet another aspect of the invention, there is provided a
structure including a plurality of metal islands on a glass
substrate, the islands being generally of an elongated geometry and
being partially thermally embedded into the glass substrate.
[0022] In yet another aspect of the invention, there is provided a
structure including a plurality of metal islands partially embedded
into a glass substrate, a majority of the islands being embedded
into depressions with a rim higher than 0.5 nm.
[0023] In yet another aspect of the invention, there is provided a
structure including a plurality of metal islands partially embedded
into a glass substrate, a majority of in-plane shapes of the
islands having at least one crystallographic angle.
[0024] Metal islands may be applied to the substrate without an
intermediate metal or organic adhesion layer. The metal may be gold
(Au).
[0025] In yet another broad aspect of the invention, there is
provided a method of use of the structure in solution
characterization, the method including exposing the structure to a
solution to be characterized without using a protective layer for
the metal islands.
[0026] In yet another broad aspect of the invention, there is
provided a structure including a glass substrate and a plurality of
glass closed-loop nano-sized rims on the substrate, major axis of a
majority of the thus being between 5 and 400 nm and heights of a
majority of the rims being higher than 0.5 nm. A majority of the
rims may enclose depressions with more than 0.5 nm depths, the
depths being measured from main surface of the glass substrate.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] In order to understand the invention and to see how it may
be carried out in practice, a few embodiments of it will now be
described, by way of non-limiting example only, with reference to
accompanying drawings, in which:
[0028] FIGS. 1A-1D are high-resolution scanning electron microscopy
(SEM) images of unannealed and annealed Au island film and of an
affected cover-glass substrate.
[0029] FIG. 1E is a collection of transmission UV-vis spectroscopic
profiles of the unannealed and annealed Au island film and of the
affected substrate;
[0030] FIGS. 2A-2J are AFM images of unannealed and annealed Au
island films and of the affected glass substrates;
[0031] FIG. 3 is a family of X-ray diffraction (XRD) patterns for
Au island films differing in temperature of annealing;
[0032] FIGS. 4A-4D are histograms of measurements of Au island
heights and depression depths by methods of scanning electron
microscopy and atomic force microscopy;
[0033] FIG. 5 is an illustration of the kinetics of island thermal
embedding into glass;
[0034] FIGS. 6A-6B are AFM images of Au island films annealed at
different temperatures on Berliner glass and of the modified glass
substrates;
[0035] FIGS. 7A and 7B present transmission UV-vis extinction
spectra of Au island films differing by annealing temperature,
before and after the film is brought into contact with non-inert
medium;
[0036] FIGS. 7C and 7E show AFM images of the metal island film on
the glass substrates differing by annealing temperature,
corresponding to the film before it is brought into contact with
non-inert medium;
[0037] FIGS. 7D and 7F show AFM images of the metal island film on
the glass substrates differing by annealing temperature,
corresponding to the film after it is brought into contact with
non-inert medium;
[0038] FIGS. 8A-8F show optical responses of Au island films
annealed at different temperatures before and after the films are
treated with various solvents or solutions; and
[0039] FIGS. 8G and 8H show morphology changing occurring during
the sensing sequence of FIGS. 8E and 8F, respectively, showing
photographs of 22.times.9 mm.sup.2 slides taken after the treatment
sequence, presenting additional evidence that thermally embedded Au
island remained stable under the treatments.
DESCRIPTION OF EMBODIMENTS
[0040] Details of some experiments conducted by inventors, are
herein presented. In a first series of experiments, 10 nm (nominal
thickness) Au films were prepared by thermal evaporation on
microscope glass cover slides (hereafter denoted cover-glass, which
constitute an oxide substrate). The films were then annealed at
high temperature (550-600.degree. C.) under ambient conditions. No
additional underlayers or overlayers, including intermediate
adhesion layers between gold and glass, were used.
[0041] In FIGS. 1A-1D, the structure of one of such evaporated, and
then annealed, film is illustrated; FIG. 1E shows a family of
transmission UV-vis spectra characterizing this film.
[0042] More particularly, FIG. 1A shows a scanning electron
microscopy (SEM) image of the as-evaporated film. (The scale bar
drawn in the lower right corner of the image corresponds to 200
nm.) The film has a percolated structure with a network of voids:
it is highly conductive. Moreover, curve 1 in FIG. 1E shows a
transmission UV-vis spectrum of this film, and it is typical of
continuous Au films, with an extinction minimum and the absence of
the surface plasmon band.
[0043] In accordance with what was said before, the evaporated Au
film was annealed for 10 hours at temperature 550.degree. C., which
is near the glass-transition temperature T.sub.g of the cover-glass
substrate (T.sub.g=550.degree. C.). In FIG. 1B, there is shown a
SEM image of the annealed film. It is seen that the film structure
was transformed from its percolated state (FIG. 1A, before
annealing) to discontinuous, thus acquiring large, well-separated,
faceted islands. This major change in island morphology occurred in
less than 10 min annealing. Transformation of the percolated Au
film to isolated islands led to the appearance of a well-defined
surface plasmon band (FIG. 1E, curve 2).
[0044] The inventors studied the sample also by cross-sectional
transmission electron microscopy (TEM). The corresponding TEM image
is illustrated in FIG. 1C, where the dashed line is drawn as a
guide to the eye. FIG. 1C shows that the high-temperature annealed
islands have elongated geometry (ellipsoidal-like or generally
aspect ratio higher than 1), and also might have substantially flat
top surfaces. The TEM image was obtained by the technique discussed
in the article M. Wanunu, R. Popovitz-Biro, H, Cohen, A. Vaskevich,
I. Rubinstein, Journal of the American Chemical Society 2005, 127,
9207. Such elongated metal islands resulted in high sensitivity to
changes in refractive index of foreign materials that can be sensed
by the structure of the present invention.
[0045] The inventors further studied how the gold deposition and
annealing affected the cover-glass. To this end, they dissolved the
annealed island film in aqua regia. FIG. 1D presents yet another
SEM image showing the substrate modified by the deposition and
annealing. As shown, there appeared island footprints. Line 3 in
FIG. 1E corresponds to the transmission spectrum after Au island
dissolution: this spectrum is featureless and similar to that of
bare glass (line 4 in FIG. 1E).
[0046] The inventors also explored the transformations of the gold
island film on cover glass by atomic force microscopy (AFM). Some
of the obtained AFM images are shown in FIGS. 2A-2J. Insets in
these figures show 3D images of single features. Each of the
illustrated AFM scans was performed over a square with one micron
side, however the z-scale differs between the images. In this
respect. FIGS. 2A, 2C, 2E, 2G, and 2I correspond to transducers
after gold deposition and before the gold dissolution; FIGS. 2B 2D,
2F, 2H, and 2J correspond to transducers with gold dissolved in
aqua regia (i.e. to the cover-glass substrate). FIGS. 2A and 2B
were obtained without the annealing step; other scans were taken
after 10-hour annealing: FIGS. 2C and 2D--at temperature
500.degree. C., FIGS. 2E and 2F--at temperature 550.degree. C.,
FIGS. 2G and 2H--at temperature 600.degree. C., and FIGS. 2I and
2J--at temperature 650.degree. C. As it is seen, well-defined
islands with large, substantially flat tops were obtained with
films annealed at 500-650.degree. C., where the top surfaces were
either parallel or showed a small tilt with respect to the
substrate. The AFM measured roughness of the islands' top surface
was ca. 0.1 nm over 100 nm scan, indicating atomically-flat
surfaces.
[0047] As commonly observed with annealed Au films, the islands'
in-plane shapes typically have characteristic hexagonal angles,
which is an indication of the Au {111} planes oriented parallel to
the substrate surface. Accordingly, the X-ray diffraction (XRD)
patterns of the annealed samples show prominent (111) and (222)
peaks.
[0048] In FIG. 3 there are demonstrated XRD patterns obtained for
an unannealed sample (curve C.sub.1) and for samples annealed at
temperatures 500.degree. C., 550.degree. C., 600.degree. C., and
650.degree. C. (curves C.sub.2-C.sub.5). It is seen that in the
latter four patterns the (111) and (222) peaks are the two highest
and they are significantly higher than the same peaks in the
pattern of the unannealed sample.
[0049] It should be noted, as it was mentioned above, that
annealing at elevated temperatures in air resulted in drastic
improvement of the adhesion between Au island films and the glass
substrate. All films evaporated on cover-glass and annealed at a
temperature of 550.degree. C. or higher passed the adhesive tape
test, while films annealed at lower temperatures did not.
[0050] Turning back to FIGS. 2A-2J, they are indicative of the
morphology of the Au island-glass interface. AFM images of
as-evaporated Au film before and after Au dissolution (FIGS. 2A and
2B) are indicative of that the flat glass surface is virtually not
changed under Au evaporation without annealing. On the other hand,
the AFM topography imaging (FIGS. 2D, 2F, 2H, 2J) shows actual
depressions in the glass encircled by a rim. Generation of
depressions and rims can also be derived from FIG. 1D. The
depression depth and rim height increase with the annealing
temperature.
[0051] Quantitative data on the size of the Au and glass features,
obtained using ARM and SEM Imaging for a film annealed at
550.degree. C., are presented in Table 1 presenting mean values of
the major axis of islands and their footprints in the cover-glass
substrate for initially 10 nm Au island samples after annealing at
550.degree. C., measured before and after Au dissolution in aqua
regia.
TABLE-US-00001 TABLE 1 Method SEM AFM Island major axis (nm) 138
.+-. 46 185 .+-. 62 Depression major axis (nm) 117 .+-. 39 114 .+-.
42
[0052] The above mean values of the major axis were calculated
using large statistics (150 measurements of each parameter);
corresponding experimental distribution histograms are presented in
FIGS. 4A-4D. More specifically, in FIGS. 4A and 4B there are
presented histograms obtained from SEM and AFM measurements
performed on the annealed film, and in FIGS. 4C and 4D there are
presented histograms obtained from SEM and AFM measurements
performed on the shallow footprints. The mean lateral dimension of
the Au islands obtained by SEM is notably smaller than the value
obtained from the AFM imaging. The latter is attributed to a tip
convolution, which is rather severe considering the average island
height after annealing of 50.+-.15 nm (as it will be seen from
Table 2 below). The mean lateral dimensions of the shallow
footprints measured after island dissolution using AFM and SEM
imaging are quite similar, in agreement with the much smaller
depression depth (ca. 1.5 nm, again as it will be seen from Table
2) compared to the island height. The difference in the
SEM-measured mean major axis of the Au islands and their footprints
in the glass is attributed to the contact angle larger than
90.degree. between the islands and the glass substrate, as seen in
the cross-sectional TEM image (FIG. 1C).
[0053] In Table 2, there are presented various mean Au island
heights and footprint depression depths, each calculated on the
basis of respective series of fifty AFM measurements of these
parameters, wherein the various series differed either in the type
of glass or in the temperature of the annealing. Except the cover
glass, Berliner glass, denoted high temperature glass, HTG, was
used. The glass transition temperature of HTG was
T.sub.g=662.degree. C., which is 105.degree. C. higher than that of
the cover-glass.
TABLE-US-00002 TABLE 2 Annealing temperature (.degree. C.) 500 550
600 650 Island height cover-glass 50 .+-. 15 49 .+-. 15 41 .+-. 13
26 .+-. 10 (nm) HTG 46 .+-. 16 51 .+-. 14 Depression cover-glass
within 1.5 .+-. 0.5 9 .+-. 2 27 .+-. 6 depth (nm) noise level HTG
within 0.7 .+-. 0.3 noise level
[0054] It is seen from Table 2 and also from the above described
FIGS. 2D, 2F, 2H, 2J, that the depressions in the glass became more
pronounced and hole-like as the annealing temperature was raised
while more glass accumulated in the rim around the island.
Accordingly, the island height (above the glass substrate)
decreased.
[0055] Referring to FIG. 5, it describes the kinetics of an effect
of Au island embedding during annealing. In this example, annealing
was applied to initially 10 nm Au island film on cover-glass and
was carrier out at constant annealing temperature of 600.degree. C.
The graph shown in FIG. 5 presents the average island height above
the glass substrate and the average depression depth as functions
of the annealing time; average island height and the average
depression depth are respectively marked with circles and squares
and denoted C and S. As before, depressions were obtained by Au
dissolution in aqua regia and measurements were done with the use
of AFM. Each mean value was calculated from measurements of 50
islands or depressions. Standard deviations were also calculated
and are shown in FIG. 5 as error bars around the mean values.
[0056] It is seen from FIG. 5 that the rate of embedding decreases
with the annealing time. The sum of the mean island height and
depression depth under all preparation conditions remained
approximately the same at ca. 50 nm. It also should be noted, that
the drastic reshaping of islands of the type caused by annealing
and evident from comparison of FIG. 1A with FIG. 1B was practically
complete in the first few minutes of annealing, while the embedding
process, probably accompanied by subtle morphological changes in
the islands, took hours. The latter phase of the fabrication
process thus can be described as thermal embedding of Au islands in
the glass substrate.
[0057] Turning again to the experiments with Berliner glass (HTG)
(some of their results were presented in Table 2), these
experiments allowed evaluating the effect of glass transition
temperature T.sub.g of the glass substrate on the embedding
process. In particular, in these experiments, the increase in glass
transition temperature had to be accompanied by an increase in the
annealing; temperature. This is because the fabrication process of
the stable and strongly bonded islands included the phase of
thermal embedding. This said, HTG samples annealed in air for 10 h
at temperatures starting from 650.degree. C. yielded highly-stable
films which passed the adhesive tape test. Except to this shift to
higher annealing temperatures, obtained structures are
qualitatively similar to those obtained with the cover-glass.
[0058] For example, AFM images of the annealed Au islands on HTG
and of their footprints after Au dissolution shown in FIGS. 6A-6D
are similar to images shown in FIGS. 2C-2J: islands tend to
gradually be embedded and become surrounded by rims of glass. Here,
FIGS. 6A and 6B correspond to the annealing temperature of
550.degree. C. where no embedding is seen (see Table 2), and FIGS.
6C and 6D correspond to the annealing temperature of 650.degree.
C., where the islands are embedded (see Table 2). The as-evaporated
gold film, which is not shown, was of 10 nm initial thickness. Each
of the AFM scans was performed in 1.times.1 micron.sup.2 scan
field. Insets in FIGS. 6A-6D show 3D images of a single feature
(i.e. island or depression). As before, the vertical (normal)
scales are different for islands annealed at different
temperatures.
[0059] The inventors explored the embedding mechanism with more
experiments. In one experiment, they annealed Au islands on
cover-glass for 10 h at 550.degree. C. with the Au facing up in one
case and facing down in another case. In both cases, the same
embedding pattern was produced. In another experiment, Au island
films evaporated on cover-glass were annealed at 600.degree. C. in
pure N.sub.2 (Inert) atmosphere. These films did not show Au island
embedding, and the islands failed the adhesive tape test. The
evidence on the partial thermal embedding of Au islands into glass,
gathered by the inventors, is indicative of that this process
occurs in the vicinity of the glass transition of the substrate,
that there is no gravitational effect, and that the presence of a
reactive atmosphere, such as atmosphere with oxygen, is
required.
[0060] For application of Au island films as LSPE transducers not
only structural stability (e.g., passing the adhesive tape test),
but also stability of the optical response toward dipping in
solvents and drying, is desired. However, subjecting islands to
solvents and capillary forces can modify or move islands. The
stability of the morphology and optical response of Au island films
evaporated on cover-glass and annealed at high temperatures
(embedded films) and its relevance to sensing applications, were
evaluated and compared with non-embedded films. Both types of
transducers were similarly subjected to a number of treatments
including those common in biosensing. These include washing in
ethanol and in phosphate buffer saline (PBS) solution, the latter
being the most common biological solvent; self-assembly (SA) of an
alkanethiol monolayer; self-assembly of a 5' thiol-modified
oligonucleotide (ss-DNA, 43 bases); and protein recognition
(immunoassay) using IgG proteins. (The ss-DNA was 5'-modified with
a --(CH.sub.2).sub.6--S--S--(CH.sub.2).sub.6--OH group and was used
as purchased.)
[0061] FIGS. 7A and 7B present transmission UV-vis extinction
spectra of 10 nm Au island films on cover glass obtained by Au
evaporation and annealing for 10 hours at 500.degree. C. (FIG. 7A)
and 550.degree. C. (FIG. 7B). Spectra are presented for as-prepared
(film deposition by evaporation and annealing at relatively high
temperatures close to the transition temperature of the substrate)
samples (curves C.sub.1), after ethanol wash and dry (curves
C.sub.2), and then after C.sub.18SH self-assembly (curves C.sub.3).
FIGS. 7C-7E and 7D-7F show AFM images (1.times.1 micron.sup.2scan)
of films corresponding to FIGS. 7A and 7B respectively after
ethanol wash and dry (images I.sub.1) and after C.sub.18SH
self-assembly (images I.sub.2).
[0062] In the case of annealing at 500.degree. C. (i.e., islands
are not embedded), a decrease in the extinction is observed after
washing in ethanol and drying, while a much more pronounced change
in the surface plasmon extinction and the band shape is seen after
self-assembly of 1-octadecanethiol (C.sub.18SH). The AFM images in
FIG. 7A show that the Au island film is somewhat unstable toward
C.sub.18SH self-assembly followed by washing and drying, exhibiting
island displacement on the surface. On the other hand, a similar
island film annealed at 550.degree. C., just below T.sub.g (i.e.,
islands are partially embedded), remains essentially intact after
the above treatments. Small difference between curve C.sub.3 and
other curves in FIG. 7B, i.e. the increase in the extinction alter
C.sub.18SH self-assembly, is attributed to change of the refractive
index near the island surface due to thiol binding. As explained
above, films which are not annealed are unstable and show
substantial spectral changes upon treatment with solvents,
chemicals and drying.
[0063] FIGS. 8A-8F present transmission UV-vis spectra of 10 nm Au
island films on cover glass annealed at 500.degree. C. (FIGS. 8A,
8C, 8E) and at 550.degree. C. (FIGS. 8B, 8D, 8F). In FIGS. 8A and
8B, curves C.sub.1 correspond to initial spectra, while curves
C.sub.2, C.sub.3 correspond to spectra obtained after first and
second ethanol wash and dry, and curves C.sub.4 correspond to
spectra obtained after PBS wash followed by water wash and dry. It
is seen that the film annealed at 500.degree. C. (i.e.
demonstrating no embedding) shows a small decrease in the
extinction after washing in ethanol and drying, while a dramatic
change in the SP extinction and the band shape is seen after
treatment in PBS solution. As noted above, such samples also fail
the adhesive tape test. In marked contrast, a similar island film
annealed at 550.degree. C., just below T.sub.g, and with embedded
islands, remains essentially intact after both treatments,
including PBS.
[0064] Likewise, as seen from FIGS. 8C and 8D, showing UV-vis
spectra of islands after an ethanol wash (curves C.sub.5) and after
a self-assembly of a monolayer of ss-DNA (curves C.sub.6), only Au
islands annealed at 550.degree. C. were stable toward such
self-assembly. The non-embedded Au island film did not withstand
the DNA binding. The difference in extinction between curves
C.sub.5 and C.sub.6 in FIG. 8D is attributed to the change in the
local refractive index as a result of DNA adsorption.
[0065] The actual sensing ability of the thermally embedded films
is demonstrated using protein-protein, in this example IgG
antigen-antibody, recognition, UV-vis spectra C.sub.7-C.sub.10,
obtained in a recognition experiment and presented in FIGS. 8E and
8D, are respectively as follows: C.sub.7 corresponds to spectra of
annealed film after ethanol wash and dry, C.sub.8--spectra of these
films after IgG antigen adsorption from solution, washing and
drying, C.sub.8--spectra after exposure to BSA solution, washing
and drying, and C.sub.9--spectra after antibody-antigen interaction
by exposure to IgG antibody solution, washing and drying. The
response of the Au film annealed at 500.degree. C. is
incomprehensible. This is associated with morphology changing
occurring during the sensing sequence. The changes are evident from
the respective photograph in FIG. 8G showing the 22.times.9
mm.sup.2 slide, taken after the treatment sequence (it should be
noted herein that the upper parts of the samples were not treated).
In contrast, the Au film annealed at 550.degree. C. was stable and
showed high sensitivity to both the IgG antigen binding and the
specific antibody-antigen interaction. The photograph in FIG. 8H
taken after the treatment sequence presents additional evidence
that thermally embedded Au island remained stable under the
treatments.
[0066] The inventors also conducted an experiment similar to the
latter, but in which the bound Rabbit IgG antigen was exposed to
Mouse IgG antibody. This treatment caused no change in the SP band,
attesting to the specificity and high stability of the system.
[0067] Thus, the technique of the present invention shows that
metal (e.g. Au) island films evaporated on untreated oxide
substrates (glass substrates) and subjected to post-deposition
annealing in the presence of oxygen at a temperature T in the
vicinity of the substrate-glass transition T.sub.g undergo partial
embedding in the glass substrate. This embedding is accompanied by
formation of a glass rim around the islands. The island films thus
obtained often show hexagonal shape or angles with atomically-flat
top surfaces characteristic of the (111) crystallographic plane.
The films are stable toward immersion in solvents, drying, binding
of organic and biological molecules, and pass successfully the
adhesive tape test. The adhesion and stability can be obtained
without the use of an intermediate adhesion layer or a protective
overlayer. The stability of the film optical response and the
availability of the simple one-step post-deposition film
preparation procedure make these films highly promising for LSPR
transducers in sensing applications. Since a stabilizing coating is
not used, full sensitivity of the system can be exploited.
[0068] Additional details on experiments are presented below.
[0069] Gold Island Film Preparation and Dissolution:
[0070] Microscope glass cover slides No. 3 (Schott AG borosilicate
glass D263T) with T.sub.g=557.degree. C. supplied by Menzel-Glaser,
and Berliner borosilicate glass (Schott AF45) with
T.sub.g=662.degree. C., were cut to 22.times.9 mm.sup.2 and cleaned
by immersion in freshly-prepared hot piranha solution (1:3
H.sub.2O.sub.2:H.sub.2SO.sub.4) for 1 h followed by rinsing with
triply-distilled water, rinsing in ethanol three times in an
ultrasonic bath (Cole-Parmer 8890), and drying under a steam of
nitrogen. After cleaning, the slides were mounted in a cryo-HV
evaporator (Key High Vacuum) equipped with a Maxtek TM-100
thickness monitor for evaporation of ultrathin Au films.
Homogeneous Au deposition was obtained by moderate rotation of the
substrate plate. Au (nominal thickness, 10 nm) was resistively
evaporated from a tungsten boat at 1-3.times.10.sup.-6 torr at a
deposition rate of 0.005-0.006 nm s.sup.-1 (determined by measuring
the evaporation time of 0.1 nm Au using a stop-watch).
Post-deposition annealing of Au-coated slides was carried out in
air (unless otherwise specified) at 500-650.degree. C. for 10 h in
an oven (Ney Vulcan 3-550). The heating rate was 5.degree. C.
min.sup.-1, and the annealed slides were left to cool to room
temperature in air. Au films were dissolved from glass substrates
by dipping into freshly-prepared aqua regia (1:3 HNO.sub.3:HCl)
followed by rinsing with triply-distilled water.
[0071] Adhesion and Stability Tests:
[0072] The strength of the Au island adhesion to glass substrates
was evaluated qualitatively using the adhesive tape test: A piece
of clear Scotch tape (3M) was pressed against the film and pulled
away. Detachment of poorly adhesive films is clearly seen with the
naked eye. Stability of the optical response was evaluated by
comparing transmission UV-vis spectra before and after the
following treatments: stirring in ethanol for 20 min and drying;
stirring in PBS solution for 20 min, washing in water and drying
under a stream of nitrogen; self-assembly of 1-octadecanethiol
(C.sub.18SH) from 1 mM solution in ethanol for 1 h, followed by
washing and drying; self-assembly of a 5' thiol-modified
oligonucleotide (ss-DNA, 43 bases) (Integrated DNA Technologies,
Inc.) from a 1 .mu.M solution in PBS overnight at room temperature
(22-23.degree. C.), followed by washing in PBS and water and drying
under a stream of nitrogen.
[0073] Sensing of Antigen-Antibody Interaction:
[0074] Antigen: A stock solution of 1 mg mL.sup.-1 immunoglobulin G
(IgG) protein from Rabbit serum (Sigma) was diluted with 0.3 M
acetate buffer, pH=4.6 to a final concentration of 100 .mu.g
mL.sup.-1. Antibody: A stock solution of 1 mg mL.sup.-1 Anti-Rabbit
IgG antibody produced in goat (Sigma) was diluted with PBS to a
final concentration of .about.1.times.10.sup.-6 M. In each step 30
.mu.L of IgG protein were spread on the surface (working
area.about.1 cm.sup.2) and left for 20 min (Rabbit IgG) or 30 min
(Anti Rabbit IgG) in air at room temperature (22-23.degree. C.).
The slides were then washed in PBS solution for 20 min and in water
and dried under a stream of nitrogen. After adsorption of Rabbit
IgG the samples were exposed to Bovine Serum Albumine (BSA) (100
.mu.g mL.sup.-1) in the same manner as described above to minimize
non-specific adsorption of Anti Rabbit IgG.
[0075] Measurements:
[0076] UV-vis spectroscopy was applied and transmission spectra
were obtained with a Varian Cary 50 spectrophotometer using air as
the baseline. The wavelength resolution was 1 nm and the average
acquisition time per point was 0.1 s. The slide-holder was designed
to ensure reproducible position of the sample.
[0077] Atomic-force microscopy (AFM) measurements were carried out
in air at room temperature (22-23.degree. C.) using Molecular
Imaging (MI) PicoScan.TM. instrument operating in the acoustic AC
mode. The cantilevers used were NSC12 and NSC36 series of
ultrasharp silicon (MikroMasch, Estonia) with a resonant frequency
of .about.150 kHz and an average radius of .ltoreq.10 nm.
[0078] High-resolution scanning electron microscopy (HRSEM) images
were obtained with an Ultra 55 FEG Zeiss high-resolution SEM using
the SE detector.
[0079] Cross-sectional transmission electron microscopy
(cross-sectional TEM) imaging was performed with a Philips CM-120
transmission electron microscope operating at 120 kV, equipped with
a CCD camera (2kx2k, Satan Ultxascan 1000). Samples for imaging
were embedded in a phenol-based M-Bond 610 epoxy resin (Ted Pella
Inc., USA) according to a procedure described previously (M.
Wanunu, R. Popovitz-Biro, H. Cohen, A. Vaskevich, I. Rubinstein,
Journal of the American Chemical Society 2005, 127, 9207).
[0080] X-ray diffractometry (XRD) measurements were performed in
the .theta.-2.theta. Bragg configuration using a rotating anode
generator-based TTKAXS III (Rigaku) diffractometer in the parallel
beam (PB) mode.
[0081] Those skilled in the art will readily appreciate that
various modifications and changes can be applied to the embodiments
of the invention without departing from its scope defined in and by
the appended claims.
* * * * *