U.S. patent application number 11/152627 was filed with the patent office on 2006-03-16 for gold thiolate and photochemically functionalized microcantilevers using molecular recognition agents.
Invention is credited to Vassil I. Boiadjiev, Peter V. Bonnesen, Gilbert M. Brown, Gudrun Goretzki, Lal A. Pinnaduwage, Thomas G. Thundat.
Application Number | 20060057026 11/152627 |
Document ID | / |
Family ID | 35638455 |
Filed Date | 2006-03-16 |
United States Patent
Application |
20060057026 |
Kind Code |
A1 |
Boiadjiev; Vassil I. ; et
al. |
March 16, 2006 |
Gold thiolate and photochemically functionalized microcantilevers
using molecular recognition agents
Abstract
Highly sensitive sensor platforms for the detection of specific
reagents, such as chromate, gasoline and biological species, using
microcantilevers and other microelectromechanical systems (MEMS)
whose surfaces have been modified with photochemically attached
organic monolayers, such as self-assembled monolayers (SAM), or
gold-thiol surface linkage are taught. The microcantilever sensors
use photochemical hydrosilylation to modify silicon surfaces and
gold-thiol chemistry to modify metallic surfaces thereby enabling
individual microcantilevers in multicantilever array chips to be
modified separately. Terminal vinyl substituted hydrocarbons with a
variety of molecular recognition sites can be attached to the
surface of silicon via the photochemical hydrosilylation process.
By focusing the activating UV light sequentially on selected
silicon or silicon nitride hydrogen terminated surfaces and soaking
or spotting selected metallic surfaces with organic thiols,
sulfides, or disulfides, the microcantilevers are functionalized.
The device and photochemical method are intended to be integrated
into systems for detecting specific agents including chromate
groundwater contamination, gasoline, and biological species.
Inventors: |
Boiadjiev; Vassil I.;
(Knoxville, TN) ; Brown; Gilbert M.; (Knoxville,
TN) ; Pinnaduwage; Lal A.; (Knoxville, TN) ;
Thundat; Thomas G.; (Knoxville, TN) ; Bonnesen; Peter
V.; (Knoxville, TN) ; Goretzki; Gudrun;
(Nottingham, GB) |
Correspondence
Address: |
UT-Battelle, LLC;Office of Intellectual Property
One Bethal Valley Road
4500N, MS-6258
Oak Ridge
TN
37831
US
|
Family ID: |
35638455 |
Appl. No.: |
11/152627 |
Filed: |
June 14, 2005 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
60609610 |
Sep 14, 2004 |
|
|
|
Current U.S.
Class: |
422/88 ; 427/343;
427/352; 438/49; 73/31.05 |
Current CPC
Class: |
G01N 29/022 20130101;
B82Y 30/00 20130101; G01N 2291/0427 20130101; G01N 2291/0426
20130101; G01N 2291/0423 20130101; B82Y 40/00 20130101; G01N
2291/0256 20130101; B82Y 35/00 20130101 |
Class at
Publication: |
422/088 ;
073/031.05; 427/343; 427/352; 438/049 |
International
Class: |
G01N 31/00 20060101
G01N031/00; G01N 27/416 20060101 G01N027/416; B32B 37/02 20060101
B32B037/02; B05D 5/12 20060101 B05D005/12; B05D 3/00 20060101
B05D003/00 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
[0002] This invention was made with United States Government
support under Contract No. DE-AC05-00OR22725 between the United
States Department of Energy and U. T. Battelle, LLC. The United
States Government has certain rights in this invention.
Claims
1. Functionalized cantilevers comprising: at least one cantilever
mounted on a base, said at least one cantilever having a top
surface and a bottom surface; a coating disposed on said at least
one cantilever, said coating exhibiting a binding interaction with
one or more agents, said coating having been disposed by a method
selected from the group consisting of gold-thiol and photochemical
hydrosilylation; and a means for detecting said binding
interaction.
2. Functionalized cantilevers according to claim 1 wherein said
binding interaction causes a change in surface stress in the
cantilever.
3. Functionalized cantilevers according to claim 1 wherein said
binding interaction is reversible using electrocycling.
4. Functionalized cantilevers according to claim 1 further
comprising at least one metallic coating on said top surface
selected from the group consisting of Au, Pt, Cu, Pd, Al, and
Ti.
5. Functionalized cantilevers according to claim 1 wherein said
coating further comprises an organic monolayer.
6. Functionalized cantilevers according to claim 5 wherein said
organic monolayer is at least one monolayer selected from the group
consisting of 4-mercaptopyridine,
12-mercaptododecyltriethylammonium bromide,
11-undecenyltriethylammonium bromide, thiol-based pyridines and
quaternary ammonias.
7. Functionalized cantilevers according to claim 1 wherein said
means for detecting further comprises at least one method selected
from the group consisting of optical, piezoresistive,
piezoelectric, and capacitive.
8. Functionalized cantilevers according to claim 7 wherein said
means for detecting further comprises a detection threshold of
approximately of 4.times.10.sup.-9 M of chromate.
9. Functionalized cantilevers according to claim 1 wherein said
agent is directly detected in at least one mixture selected from
the group consisting of liquid, neutral aqueous solutions,
acidified aqueous solutions, vapor, and gas.
10. Functionalized cantilevers according to claim 1 wherein said
cantilevers are disposed in an array.
11. Functionalized cantilevers according to claim 10 wherein said
coating is at least one coating selected from the group consisting
of agent selective, partially agent selective, and agent
non-selective.
12. Functionalized cantilevers according to claim 11 wherein said
array further comprises at least one reference microcantilever.
13. A method for modifying the gold surface of at least one
gold-coated microcantilever comprising the steps of: a. cleaning
said microcantilever in a cleaning mixture, b. immersing said
microcantilever in a coating mixture thereby forming a self
assembled monolayer on the gold surface, c. rinsing said
microcantilever with a rinsing mixture.
14. The method of claim 13 wherein said cleaning step further
comprises the sequential steps of: a. rinsing in acetone, b.
rinsing in absolute ethanol, c. rinsing in deionized water, d.
rinsing in piranha solution, e. rinsing in ultrapure deionized
water, and f. rinsing and soaking in absolute ethanol.
15. The method of claim 14 wherein said piranha solution further
comprises a mixture of approximately 7 parts H.sub.2SO.sub.4 (98%)
and approximately 3 parts H.sub.2O.sub.2 (31%).
16. The method of claim 13 wherein said coating mixture further
comprises at least one agent selected from the group consisting of
alkylthiol, arylthiol, and dialkanesulfides.
17. The method of claim 16 wherein said agent further comprises at
least one agent selected from the group consisting of quaternary
ammonias, crown ethers, azacrown compounds, borate esters, ureas,
antibody-antigens, organic acids, organic esters, organic amides,
organic amines, organic aldehydes, phosphonic acids, phosphonic
esters, buckyballs, and hydroxyls.
18. The method of claim 13 wherein said coating mixture further
comprises an aqueous solution of approximately 5.times.10.sup.-3 M
of 4-MPy (95%) in approximately 0.1 N H.sub.2SO.sub.4.
19. The method of claim 13 wherein said coating mixture further
comprises an aqueous solution of approximately 5.times.10.sup.-3 M
of 4-MPy (95%) in absolute ethanol.
20. A method for modifying the silicon surface of at least one
microcantilever comprising the steps of: a. cleaning said at least
one microcantilever silicon surface, b. hydrogen terminating said
at least one microcantilever silicon surface, c. carbon linking a
molecular recognition agent to a selected hydrogen terminated
silicon surface using photochemical hydrosilylation, and d.
repeating steps a. thru c. for selected molecular recognition
agents.
21. The method of claim 20 wherein said cleaning step further
comprises the sequential steps of: a. rinsing in acetone, b.
rinsing in absolute ethanol, c. rinsing in deionized water, d.
rinsing in piranha solution, e. rinsing in ultrapure deionized
water, and f. rinsing in absolute ethanol.
22. The method of claim 20 wherein said hydrogen terminating step
further comprises: a. immersing said silicon surface in
approximately 40% NH.sub.4F argon-purged solution, and b. drying
said silicon surface in argon.
23. The method of claim 20 wherein said carbon linking step further
comprises: a. disposing said hydrogen termination silicon surface
in a molecular recognition agent solution, b. irradiating at least
one microcantilever with ultraviolet light, and c. rinsing said
surface.
24. The method of claim 23 wherein said molecular recognition agent
further comprises at least one agent selected from the group
consisting of alkylthiol, arylthiol, and dialkanesulfides.
25. The method of claim 24 wherein said molecular recognition agent
further comprises at least one agent selected from the group
consisting of quaternary ammonias, crown ethers, azacrown
compounds, borate esters, ureas, biomolecule-selective
antibody-antigens, DNA, proteins, organic acids, organic esters,
organic amides, organic amines, organic aldehydes, phosphonic
acids, phosphonic esters, buckyballs, and hydroxyls.
26. The method of claim 25 wherein said quaternary ammonias further
comprise 11-undecenyltriethylammonium bromide.
27. The method of claim 23 wherein said ultraviolet light is
emitted from a mercury lamp.
28. Functionalized MEMS comprising: at least one MEM having a top
surface and a bottom surface; a coating disposed on said at least
one MEM, said coating exhibiting a binding interaction with one or
more agents, said coating having been disposed by a method selected
from the group consisting of gold-thiol and photochemical
hydrosilylation; and a means for detecting said binding
interaction.
29. Functionalized MEMS according to claim 28 wherein said binding
interaction causes a change in surface stress in the MEM.
30. Functionalized MEMS according to claim 28 wherein said binding
interaction is reversible using electrocycling.
31. Functionalized MEMS according to claim 28 further comprising at
least one metallic coating on said top surface selected from the
group consisting of Au, Pt, Cu, Pd, Al and Ti.
32. Functionalized MEMS according to claim 28 wherein said coating
further comprises an organic monolayer.
33. Functionalized MEMS according to claim 32 wherein said organic
monolayer is at least one monolayer selected from the group
consisting of 4-mercaptopyridine,
12-mercaptododecyltriethylammonium bromide,
11-undecenyltriethylammonium bromide, thiol-based pyridines and
quaternary ammonias.
34. Functionalized MEMS according to claim 28 wherein said means
for detecting further comprises at least one method selected from
the group consisting of optical, piezoresistive, piezoelectric, and
capacitive.
35. Functionalized MEMS according to claim 34 wherein said means
for detecting further comprises a detection threshold of
approximately of 4.times.10.sup.-9 M of chromate.
36. Functionalized MEMS according to claim 28 wherein said agent is
directly detected in at least one mixture selected from the group
consisting of liquid, neutral aqueous solutions, acidified aqueous
solutions, vapor, and gas.
37. Functionalized MEMS according to claim 28 wherein said MEMS are
disposed in an array.
38. Functionalized MEMS according to claim 37 wherein said coating
is at least one coating selected from the group consisting of agent
selective, partially agent selective, and agent non-selective.
39. Functionalized MEMS according to claim 38 wherein said array
further comprises at least one reference MEM.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to U.S. Provisional Patent
Application 60/609,610 filed Sep. 14, 2004, and is related to U.S.
patent application Ser. No. 11/059,170, filed Feb. 16, 2005, both
herein incorporated by reference.
FIELD OF THE INVENTION
[0003] This invention relates to highly sensitive sensor platforms
for the detection of specific reagents, such as chromate, gasoline
and biological species, using microcantilevers and other
microelectromechanical systems (MEMS) whose surfaces have been
modified with photochemically attached organic monolayers, such as
self-assembled monolayers (SAM), or gold-thiol surface linkage. The
microcantilever sensors use photochemical hydrosilylation to modify
silicon surfaces and gold-thiol chemistry to modify metallic
surfaces thereby enabling individual microcantilevers in
multicantilever array chips to be modified separately. By focusing
the activating UV light sequentially on selected silicon or silicon
nitride hydrogen terminated surfaces and soaking or spotting
selected metallic surfaces with organic thiols, sulfides, or
disulfides, the microcantilevers are functionalized. The device and
photochemical method are intended to be integrated into systems for
detecting specific agents including chromate groundwater
contamination, gasoline, and biological species.
BACKGROUND OF THE INVENTION
[0004] Micro-electro-mechanical systems (MEMS) are likely
candidates for extremely sensitive, inexpensive sensors, which can
be mass produced. Microcantilever sensors offer much better
sensitivities compared to other MEMS sensors and have surface areas
of the order 10.sup.-4 cm.sup.2, which is smaller than that of
other miniature devices (such as Surface Acoustic Wave devices,
SAW) by orders of magnitude. They can be mass produced at
relatively low cost using standard semiconductor manufacturing
processes and have demonstrated superior detection sensitivities
for physical, chemical and biological sensing.
Microcantilever-based sensors have been shown to be extremely
sensitive; however silicon or silicon nitride microcantilevers
coated on one surface with gold do not have any particular chemical
selectivity. Chemical selectivity has been achieved by coating the
gold surface of the microcantilevers with a selective film such as
a self-assembled monolayer (SAM) of an alkane thiol having a head
group suitable for molecular recognition. Also, functionalized
films can be attached to hydrogen terminated silicon and silicon
nitride surfaces by photochemical hydrosilylation to achieve more
stable coatings. The main feature distinguishing microcantilevers
from other MEMS is their unique bending response. They have a high
surface-to-volume ratio, and therefore changes in the Gibbs surface
free energy induced by surface-analyte interactions lead to large
surface forces. When such interactions are restricted to one
surface, then the resulting differential stress leads to bending of
the cantilever. This bending detection mode can be used in liquid
phase, as well as in gas phase, which makes cantilever sensors
suitable for both molecular and ionic analytes if selective
adsorption can be achieved on one of their surfaces using
analyte-specific surface functionalities. A preferred approach to
the design of selective sensors is to immobilize agents for
selective molecular recognition in a matrix that mimics the organic
medium in a solvent extraction system. In this manner, the matrix
can enhance both the separation and the achievement of chemical
selectivity. The transduction part of the microcantilever sensor is
based on binding the molecular recognition agent to one surface of
the cantilever so that the adsorption-induced stress change can be
detected via bending of the microcantilever.
[0005] A problem exists with the formation of SAM coatings on gold
coated cantilevers if an array of cantilevers is used. It is
difficult to apply a coating, especially if a long period of time
is required for a tightly packed layer to form, to a single
cantilever in an array without contaminating other cantilevers in
the array. Other approaches to modifying a single surface of a
silicon cantilever involve reaction of silane reagents with the
Si--OH groups on the surface, but again it is problematic to modify
only a single cantilever in an array. The photoactivation method of
this invention provides a solution to this problem wherein
cantilevers are only activated to react with an ethylene
substituted hydrocarbon when irradiated with UV light.
[0006] Arrays of cantilevers can be conveniently prepared with each
cantilever or group of cantilevers having a separate molecular
recognition agent to impart chemical selectivity. Attachment of
molecular recognition agents to the surface with robust Si--C bonds
gives a layer with superior stability. For example, chromium(VI) or
chromate can be selectively detected by the microcantilever of this
invention.
[0007] Chromium is naturally occurring in several different
oxidation states. The most frequently encountered forms are the III
and VI oxidation states. Chromium(III) is an essential trace
element in the human body and plays an important role in the
metabolism of glucose, lipids, and proteins. In contrast, Cr(VI) in
the form of chromate (CrO.sub.4.sup.2-) is considered to be toxic
to animals and humans. Most of the methods used to determine
CrO.sub.4.sup.2- (such as ion exchange, chromatography, and atomic
absorption spectroscopy) are generally time-consuming, have less
than desired accuracy, or are expensive.
[0008] In addition, Cr(VI) is more soluble in groundwater than
Cr(III), and thus has a greater potential of affecting human health
and the environment. Various techniques have been tested for the
direct determination of Cr(VI) in water, but most techniques are
not suitable due to insufficient detection limits and/or matrix
interferences. The method that is widely being used at present
requires selective reaction of Cr(VI) with
1,5-diphenylcarbohydrazide followed by spectrophotometry. The
commercial sensor technique based on the above method and used
widely for Cr(VI) monitoring, has a detection limit of
.apprxeq.1.9.times.10.sup.-7 M, close to the current EPA regulation
level of 2.1.times.10.sup.-7 M. However, this method is not viable
for remote monitoring, and also would not be applicable if
federal/local regulated levels are tightened. Therefore, developing
inexpensive, easily deployable techniques with higher sensitivity
is important for environmental monitoring and remediation. Due to
the possibility of mass deployment at low cost,
microelectromechanical systems (MEMS), especially microcantilevers,
have attracted attention recently due to their high sensitivity of
detection.
BRIEF DESCRIPTION OF THE INVENTION
[0009] Photochemical hydrosilylation of
11-undecenyltriethylammonium bromide with hydrogen-terminated
silicon microcantilever surfaces yielded a robust quaternary
ammonium terminated organic monolayer that is suitable for chromate
detection. Terminal vinyl substituted hydrocarbons with a variety
of molecular recognition sites can be attached to the surface of
silicon via the photochemical hydrosilylation process. Since the
chemicals only react at the surface of Si when irradiated it allows
an array of cantilevers to be sequentially modified by exposing an
array to the derivatization agent but only activating one or a
select group of cantilevers before changing the solution and
activating a different cantilever of group of cantilevers. Another
embodiment of this invention enables the detection of hexavalent
chromium, Cr(VI), in ground water using at least one
microcantilever coated with a self-assembled monolayer of
4-mercaptopyridine. The microcantilever sensors use gold-thiol
attachment approach for 4-mercaptopyridine (4-MPy) and
photochemical hydrosilylation for grafting
11-undecenyltriethylammonium bromide or vinyl pyridine to modify
the microcantilever surface for chromate sensing.
[0010] One embodiment of the device and method enables the
detection of hexavalent chromium, Cr(VI), in ground water using a
single microcantilever sensor coated with a self-assembled
monolayer of 4-mercaptopyridine. The experiments showed that
CrO.sub.4.sup.2- ions can be detected with the microcantilever
sensor in the presence of significant concentrations (>1000
pg/l) of Ca.sup.2+, Cl.sup.-, Mg.sup.2+, NO.sub.3.sup.-, K.sup.+,
Na.sup.+, and SO.sub.4.sup.2- ions and a variety of other ions of
smaller concentrations. The chromate concentrations were also
measured using the Hach spectrophotometric kit, which is widely
used for chromate monitoring. The cantilever measurements are an
order of magnitude more sensitive compared to the
spectrophotometric method currently in use, and are amenable for
remote detection.
[0011] The microcantilever sensor uses gold-thiol or photochemical
hydrosilylation to modify the microcantilever surface. The
photochemical process enables individual microcantilevers in
multicantilever array chips to be modified separately by focusing
the activating UV light sequentially on each particular cantilever.
Grafting of selected gold coated microcantilevers can be achieved
by spotting techniques. The surface functionalities retain their
affinity toward Cr(VI), and the organic monolayer is dense enough
to generate significant surface stress upon subsequent adsorption
of chromate ions from aqueous solutions.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIGS. 1a and 1b are graphs showing (a) Results of the
photometric measurements on the three water samples from the
Hanford site. (b) Comparison of the photometric signal from the
B173R9 sample with those from standard chromate solutions of
concentrations of 2.5.times.10.sup.-7 M and 5.0.times.10.sup.-7
M.
[0013] FIGS. 2a and 2b are graphs showing cantilever bending
signals of 4-Mpy modified cantilever due to the injections of
different amounts of the acidified sample solutions at 10 mL/hr;
(a) different amounts of B173R7 and (b) same amount of B173R8
samples. The relative magnitudes of the cantilever bending signals
(after normalizing to the injection volumes) are in agreement with
the relative concentrations obtained from the photometric
measurements.
[0014] FIG. 3 is a graph showing cantilever bending signal due to
the injection of 2 mL (at pumping speed of 10 mL/hr) of the
acidified B173R9 water sample taken from a well at the Hanford
site. From historical data for this well, the concentration of
hexavalent chromium is expected to be <2.5.times.10.sup.-7 M.
The water from this well contains numerous other contaminants, some
of which have concentrations exceeding 10.sup.-4 M.
[0015] FIG. 4 is the bending response of a gold-coated silicon
microcantilever, modified with 11-undecenyltriethylammonium bromide
by photochemical hydrosilylation on the silicon side, upon
injections of 1 mL of sample chromate solutions: (1)
1.times.10.sup.-4 M CrO.sub.4.sup.2-, left scale; (2)
1.times.10.sup.-9 M CrO.sub.4.sup.2-, right scale.
[0016] FIG. 5 shows photochemical hydrosilylation of alkenes and
alkynes. Figure taken from Buriak, Chem. Rev. 2002, 102 (5),
1271-1308.
[0017] FIG. 6 shows surface structures reported by Buriak, Chem.
Rev. 2002, 102 (5), 1271-1308 and Voicu, R., Langmuir, 2004, 20,
pp. 11713-11720.
[0018] FIG. 7 shows anticipated structure of Si surface modified
with quaternary ammonium coating and bonding mode for chromate. It
is a schematic representation of a cantilever Si surface
functionalized with 11-undecenyltriethylammonium halide using the
photochemical hydrosilylation approach for chromate detection.
X--.dbd.Br-- immediately following the hydrosilylation process.
[0019] FIG. 8 shows photochemical hydrosilylation of
11-undecenyltriethylammonium bromide to hydrogen terminated
microcantilevers.
[0020] FIG. 9 is a photograph of a typical functionalized
microcantilever.
DETAILED DESCRIPTION OF THE INVENTION
[0021] Chromate cantilever sensors using two different types of
chromate-specific surface functionalities bound to gold-coated
cantilever surfaces as thiol-based pyridine and thiol-based
quaternary ammonium terminated self-assembled monolayers (SAMs) are
described. The 4-mercaptopyridine based microcantilever chromate
sensor has exceptional stability and very high selectivity and can
be used for months in acidic media utilizing a single cantilever.
The 12-mercaptododecyltriethylammonium bromide based chromate
sensor, despite its superior initial sensitivity to chromate,
appeared to be unstable and lost its activity within 1 week.
Ethanol solutions of the quaternary ammonium terminated thiol
(.about.1 mM) from the self-assembly process were studied 2 weeks
after the cantilever treatment and indicated significant
degradation of the quaternary ammonium thiol. Some decomposition
was also detected in the solid compound, which was stored in a
closed container for a similar time period. We hypothesize that
nucleophilic attack of the sulfur on the carbon attached directly
to the positive nitrogen analogous to Hofmann degradation may have
occurred.
[0022] Photochemical Hydrosilylation: Direct covalent attachment of
the quaternary ammonium monolayer via robust Si--C bonds (FIG. 4)
improved the stability and robustness of the cantilever sensors.
This is achieved in a two-step process involving hydrogen
termination of the silicon cantilever surface and subsequent
photochemical (UV) hydrosilylation with the unsaturated hydrocarbon
chain of the quaternary ammonium compound, which results in a
stable Si--C surface linkage (FIG. 8).
[0023] 11-Undecenyltriethylammonium bromide was synthesized in a
one-step reaction by stirring 10 g (43 mmol) of 11-bromo-1-undecene
(Aldrich Chemical Co., 95%) and 10 g (0.1 mol) of triethylamine
(Aldrich, 99.5%) in absolute ethanol under argon for 24 h under
reflux. Removal of the volatiles yielded a thick oil, which was
triturated several times with ether to yield a white, hygroscopic
solid (11.4 g, 80% yield) C.sub.17H.sub.36NBr (MW=334.28). About
0.8 g of the white quaternary ammonium salt was then placed in a
specially designed two-compartment quartz vacuum cell, dried and
deoxygenated by prolonged evacuation until a base pressure of
2.1.times.10.sup.-6 Torr was reached. Contact tipless V-shaped
microcantilevers (180 .mu.m long, 1 .mu.m thick, and 25 .mu.m wide,
having a force constant of 0.26 N/m) from Thermomicroscopes, CA,
were cleaned by the following procedure: soaking and washing in
acetone (15 min), absolute ethanol (15 min), ultrapure (MilliQ)
water, piranha (.about.10 s, 1:3 v 30% H.sub.2O.sub.2 and
concentrated H.sub.2SO.sub.4), and ultrapure water and drying under
argon flow. These cantilevers had been coated on one side with gold
(a 30-nm thick gold layer on top of a 3-nm titanium adhesion layer)
by the manufacturer. Hydrogen termination of the silicon surface
was achieved by immersing each cantilever (.about.6 min) in 40%
aqueous NH.sub.4F solution, which had been purged with argon for at
least 30 min to remove dissolved oxygen. The resulting surface
(Si--H) was dried in an argon flow and evacuated to remove any
residual NH.sub.4F. Each hydrogen-terminated silicon
microcantilever was placed in a quartz tube (2 mm i.d.) and
transferred under argon backflow into the second compartment of the
quartz cell. Then all cantilevers were evacuated together with the
quaternary ammonium salt. Anhydrous methylene chloride (Aldrich
Chemical Co., 99.8%, bottled under nitrogen) was additionally dried
with CaH.sub.2, degassed by at least six freeze-pump-thaw cycles,
and then distilled under vacuum into the cooled quartz cell
compartment containing the 11-undecenyltriethylammonium bromide.
Upon reaching room temperature, the quartz cell was backfilled with
high-purity argon and a clear solution with an approximate
concentration between 0.1 and 0.15 M was prepared under magnetic
stirring. This solution was then carefully poured into the other
cell compartment containing the hydrogen-terminated silicon
microcantilevers. The argon-filled quartz cell was closed,
disconnected from the vacuum manifold, and then placed into a
bigger quartz vessel containing water to prevent heating during UV
irradiation. The silicon surface was irradiated using frequencies
emitted by a mercury lamp (100 W, .about.25 cm distance from the
surface) for 7-10 days to ensure sufficient time for dense packing
of the positively charged quaternary ammonium terminated
hydrocarbon chains. This time scale was prompted from our earlier
results in the case of the self-assembly of mercapto-terminated
quaternary ammonium hydrocarbon chains on gold-coated
microcantilever surfaces, which needed about 1 week to yield the
desired sensitivity to chromate ions. Following the photochemical
reaction, the cantilevers were thoroughly rinsed with methylene
chloride and water and then stored under ultrapure water until
their response to chromate ions was measured. Blank tests performed
on clean gold-coated silicon microcantilevers confirmed that
unmodified cantilevers do not bend in response to 1.times.10.sup.-4
M CrO.sub.4.sup.2- nor do they respond even at concentrations as
high as 1.times.10.sup.-3 M, which is in agreement with our earlier
reports. Control experiments show that no appreciable
hydrosilylation takes place on the cantilever surfaces without UV
irradiation at room temperature.
[0024] Cantilever deflection measurements were conducted in the
optical mode using an atomic force microscopy (AFM) head. The
experiments were performed in a flow-through glass fluid cell that
holds the V-shaped microcantilever. The volume of the flow cell was
0.3 cm.sup.3, ensuring fast replacement of the solution. A constant
flow rate of 10 ml/h for the ultrapure (or tap) water carrier was
maintained using a syringe pump. Samples of chromate solutions were
introduced by directing the carrier through a 1-mL reservoir HPLC
loop using a switch valve. This provided about 6 min contact time
of the analyte with the active cantilever surface. The bending of
the cantilever was measured by monitoring the position of a laser
beam reflected off the apex of the cantilever (from the gold-coated
surface) onto a position sensitive detector.
[0025] Using photochemically functionalized silicon
microcantilevers, we were able to obtain significant bending
response upon injection of 1.times.10.sup.-4 M chromate solution in
ultrapure water (FIG. 4, curve 1), a result which is in agreement
with previous testing. In addition, as a result of the robust Si--C
covalent linkage to the cantilever surface, the monolayer stability
has been greatly improved. Well-defined signals were also
registered at concentrations as low as 1.times.10.sup.-9 M
CrO.sub.4.sup.2- in ultrapure water. This is also similar to the
earlier results obtained using the Au-thiolate strategy for
self-assembly of 12-mercaptododecyltriethylammonium bromide. This
result illustrates that similar ultrahigh sensitivity of the
microcantilever sensor to chromate ions can be achieved with a
densely packed quaternary ammonium terminated monolayer derived
from photochemical hydrosilylation directly on Si. Although there
is no direct evidence for self-assembly, our data clearly indicate
that the quaternary ammonium functionalities are readily accessible
and packed closely enough to generate significant surface stress
upon chromate adsorption. It is also clear (FIG. 4) that the
cantilever deflection signal is quite readily reversible at the
lowest concentrations of Cr. (VI) (FIG. 4, curve 2), compared to
the higher (1.times.10.sup.-4 M) concentration (FIG. 4, curve 1),
although the latter case requires a longer time period (30
min).
[0026] Similar magnitude signals were registered upon injection of
a 1.times.10.sup.-4 M chromate solution in tap water, where tap
water served as a background as well as a carrier fluid. It appears
that washing with water recovers significant cantilever activity to
new chromate loading. This effect has been discussed in greater
detail earlier. In this study, tap or groundwater washing seems to
regenerate cantilever activity faster compared to ultrapure water,
most likely due to facilitated surface ion exchange. Signals with
similar magnitude to that shown in FIG. 4, curve 1, were registered
repeatedly using the same cantilever following multiple chromate
injections and subsequent washing with ultrapure and tap water for
a period of 11 days. This illustrates the expected stability of the
functionalized silicon surface and low-level interference from the
ions present in tap water with the chromate detection process under
these experimental conditions. Another advantage of the stable
quaternary ammonium cantilever sensor is its ability to detect
chromate ions directly in liquid, neutral aqueous solutions, vapor,
and gas, thus potentially eliminating the necessity for sample
collection and preparation (such as acidifying to pH 1 in the case
of the 4-mercaptopyridine based sensor). Such a chromate sensor
that incorporates a reference cantilever (inert toward Cr(VI), for
example, a clean Au-coated silicon cantilever) in a multicantilever
array may be suitable for direct monitoring of groundwater and
industrial wastewater. Using reference cantilever(s) in a
multicantilever array can reduce adverse environmental effects such
as gross changes in ionic strength and thermal drifts. Further
optimization of this photochemical surface functionalization
process may allow even better chromate sensitivity and selectivity
with the desired stability and robustness necessary for field
applications.
[0027] In the case of photochemical hydrosilylation, surface
attachment of the quaternary ammonium terminated alkyl chains could
occur via a radical-based mechanism proposed earlier, which
involves homolytic Si--H bond cleavage giving rise to surface
silicon radicals. Such silicon radicals react very rapidly with the
unsaturated carbon-carbon bonds of 11-undecenyltriethylammonium
bromide from the solution, thus restricting the radical chain
reaction only to the surface. This model was strongly supported by
the fact that no polymerization was observed in the solution during
this photochemical reaction. The entire process can be limited by
the access of the double-bond ends of the reactant to the activated
surface, which becomes strongly hindered with increasing surface
coverage. This is in addition to the electrostatic repulsion
between the positively charged quaternary ammonium groups, which
has appeared to significantly slow the self-assembly process of
triethyl-12-mercaptododecylammonium bromide on gold surfaces (to
about 1 week deposition time) compared to self-assembly of normal
1-thiols (a few hours to 1 day). Therefore, based on these earlier
results, we have allowed sufficient time (6-10 days) in order to
ensure dense packing of this particular ion-terminated organic
layer, being fully aware that complete hydrosilylation of normal
aliphatic alkenes such as 1-pentene and 1-octadecene on flat
silicon surfaces has been reported to occur within only .about.2 h.
Indeed, long deposition time periods may not be such a serious
issue for other types of organic layers, which do not contain ionic
functionalities. In support of this argument, the best results in
this study were obtained with cantilevers irradiated for 10
days.
[0028] In view of the long deposition time, thorough deoxygenation
and dehydration of the quaternary ammonium salt and solvent were
essential for this approach in order to avoid competitive oxidation
of the hydrogen-terminated silicon surface under UV irradiation and
to ensure effective surface functionalization. Only densely packed
monolayers, functionalized with active head groups, can yield large
surface stress upon analyte adsorption and thus a high sensitivity
of detection. Once covalently attached to the silicon surface, the
quaternary ammonium terminated hydrocarbon chains and the growing
organic layer appears to be stable upon continued UV irradiation
(for at least up to 10 days) under the conditions of our
experiment. This is demonstrated by the magnitude of the response
to chromate (FIG. 4, curve 1) following a period of prolonged
irradiation.
[0029] Another embodiment of this invention is a method of
modifying the surface of individual cantilevers in an array so that
each cantilever (or a group of cantilevers) can have separate
selectivity for sorption of analytes of interest. Furthermore the
chemical modification involves the formation of robust Si--C bonds
that are more chemically resistant than Si--O--Si bonding or the
thiol SAMs on a gold coated microcantilever. Silicon
microcantilevers having gold on one side are treated with NH.sub.4F
to form Si--H terminated surface on the uncoated silicon side of
the cantilever. This Si--H surface can be photochemically activated
with UV light to react with an olefin. The olefin will not react
with the Si--H bonds at room temperature, thus an array of
cantilevers under a solution containing an olefin will not react
until irradiated with UV light. To functionalize an array of
cantilevers, the solution can be removed from the array of
cantilevers, rinsed with a suitable solvent, and a second olefin
functionalized with another molecular recognition end group can be
added and induced to react with UV irradiation. This process can be
repeated as often as necessary to create an array of cantilever
sensors with different functionalities. This chemistry is
compatible with a variety of molecular recognition agents including
quaternary ammonium and pyridine groups (chromate selective), crown
ethers and azacrown compounds (metal ion selective), borate esters
(sugars), ureas (nitrate and organonitrate compounds),
biomolecule-selective antibody-antigens, DNA, proteins, as well as
organic acids, esters, amides, amines, aldehydes, phosphonic acids
and esters, and related compounds.
[0030] For photochemical hydrosilylation, surface attachment of the
quaternary ammonium terminated alkyl chains occurs via a
radical-based mechanism summarized in Buriak, J. M. Chemical
Reviews 2002, 102 (5), 1271-1308, herein incorporated by reference.
This process involves homolytic Si--H bond cleavage giving rise to
surface silicon radicals. Such silicon radicals react very rapidly
with the unsaturated carbon-carbon bonds of alkenes dissolved in
the solution, thus restricting the radical chain reaction only to
the surface. This model was strongly supported by the fact that no
polymerization was observed in the solution during this
photochemical reaction. The entire process can be completed in as
little as 2-3 hours or longer depending on desired functionality.
The process can also be limited by the access of the double-bond
ends of the reactant to the activated surface, which becomes
strongly hindered with increasing surface coverage.
[0031] It is known in the organic and organometallic literature
that UV irradiation can promote hydrosilylation of unsaturated
compounds due to homolytic cleavage of Si--H bonds [Fleming, I. In
Comprehensive Organic Chemistry; Jones, N., Ed.; Pergamon: New
York, 1979; Vol. 3, p 568.] UV photoinduction takes place at room
temperature and thus provides a way to avoid problems from thermal
input that could be harmful to delicate or small features on a
silicon chip. Minimal input of thermal energy would be preferable
in any IC manufacturing process (thermal budget). Irradiation of a
hydride-terminated Si (111) surface with UV light (185 and 253.7
nm) in the presence of an aliphatic alkene like 1-pentene or
1-octadecene brings about hydrosilylation in 2 h at room
temperature, as shown in FIG. 5.
[0032] 4-Mpy Functionalized Gold-coated Microcantilevers: The
cantilever deflection measurements were conducted in the optical
mode using an Atomic Force Microscopy (AFM) head. The cantilever
bending signal (which depends on the cantilever dimensions as well
as on the electronic circuit details) was converted to a surface
stress in order to express in universal terms. The experiments were
performed in a flow-through glass fluid cell that holds the
V-shaped microcantilever (180 .mu.m long, 1 .mu.m thick, and a
force constant of 0.26 N/m) from Thermomicroscopes, CA. The
manufacturer had coated one side of these cantilevers with gold (a
30-nm thick gold layer on top of a 3-nm titanium adhesion layer).
The metallic coating can be at least one of Au, Pt, Cu, Pd, Al, or
Ti. The volume of the glass cell was 0.3 cm.sup.3, ensuring fast
replacement of the solution. A constant flow rate of 10 ml/hr for
the acidic carrier solution (0.1 N H.sub.2SO.sub.4 in ultrahigh
purity deionized water, pH 1) was used in these experiments; ground
water samples or standard chromium (VI) solutions acidified the
same way were injected into the carrier. The B--B stacking between
the aromatic 4-mercaptopyridine (4-MPy) molecules and their
conformational rigidity within the SAM have the tendency to align
them into polymer-like chains, which can be observed by scanning
tunneling microscopy (STM). These structures expose densely packed
surface sites with high affinity for the chromate analyte, which is
apparent from the selective chromate preconcentration. Ordered
domains with pH-dependent row structure, where the fraction of
protonated molecular rows depends on solution pH, have been
observed on Au (111) modified by 4-MPy SAMs.
[0033] The earlier electrochemical studies of Turyan and Mandler
together with the above STM studies and the high affinity of the
pyridinium species towards chromate and dichromate ions (where
pyridinium chlorochromate (PCC) is widely used as a mild oxidant)
led to exploring this system for cantilever-based chromate
sensors.
[0034] Gold-coated silicon cantilevers were cleaned in acetone, in
absolute ethanol, in deionized water, and (for only 10 s) in
piranha solution (7:3 H.sub.2SO.sub.4 98%/H.sub.2O.sub.2 31%), and
rinsed with ultrapure deionized water (3 times) and absolute
ethanol (2 times). The formation of a 4-MPy SAM on the gold surface
of the cantilever was achieved by immersing the cantilever into an
acidic aqueous solution of 5.times.10.sup.-3 M of 4-MPy (95%, from
Aldrich Chemical Company) in 0.1 NH.sub.2SO.sub.4/de-ionized water
for six days. Upon removal from the solution, the cantilever was
rinsed with de-ionized water and then dried before use in the
experiments. Similar preliminary results were obtained with
cantilevers modified using .apprxeq.5 mM 4-MPy in absolute ethanol.
In both cases, large cantilever bending signals (corresponding to a
cantilever stress of up to .apprxeq.1.3 N/m) were observed when
de-ionized water was replaced by the acidified carrier due to SAM
protonation.
[0035] For comparison, photometric measurements were conducted on
the ground water samples using a sample analytical kit that we
purchased from the Hach company. FIG. 1a shows the photometric data
for the 3 samples. The chromate concentration in the B173R9 sample
is below the detection level of the photometric method, and samples
B173R8 and B173R7 have chromate concentrations of
.apprxeq.2.4.times.10.sup.-6 M and .apprxeq.3.7.times.10.sup.-5 M,
respectively. These numbers agree reasonably well with the
historical data for these wells. We also measured the photometric
signals due to standard chromate solutions of concentrations
5.times.10.sup.-7 M and 2.5.times.10.sup.-7 M and compared these
with the signal due to the B173R9 sample; see FIG. 1b. It is quite
clear that the chromate concentration in the B173R9 sample is well
below 2.5.times.10.sup.-7 M
[0036] The cantilever bending response from acidified ground water
samples B173R7 and B173R8 are shown in FIG. 2. As can be seen from
FIG. 2(a), the bending signal is roughly proportional to the amount
of the sample injected; on the other hand, similar bending signals
are obtained when the same amount of chromate is injected; see FIG.
2(b). The signal level for the B173R8 sample is roughly 10 times
smaller compared to that of the B173R7 sample when normalized to
the injection volumes. Thus the cantilever bending data are in
agreement with the photometric data for those two samples. In the
data of FIG. 2 the cantilever bending is only partially recovered
after the exposure to chromate. Even though this does not affect
the reproducibility--we have done tens of injections--it is
important to note that the cantilever bending can be quickly
recovered back to the original state by electrocycling between 0.5
and -0.15 V at a rate of 50 mV/s in the acidic carrier. This
procedure reduces Cr(VI) to Cr(III), which is repelled from the
intact positively charged pyridinium SAM, thus clearing the surface
for subsequent chromate adsorption. This procedure can be further
simplified, optimized and incorporated into a low-cost
cantilever-based analytical device to achieve the highly desired
quick sensor recovery and fast multiple sample analysis.
[0037] Partially reversible chromate signals may be due to two-step
surface process involving rapid reversible displacement of sulphate
in the vicinity of the positively charged (protonated)
4-mercaptopyridinium layer for chromate (lower hydration energy),
followed by stronger and much less reversible complexation, of
chromate to the SAM surface. When the chromate sample is displaced
from the flow cell by the carrier, fast (minutes) ion exchange in
the diffuse ionic layer may be responsible for the fast initial
recovery followed by much slower (hours) desorption of the strongly
bound chromate.
[0038] The fast ion exchange of sulfate for chromate ions is, in
general, not a selective process. There is a "bias" for large
poorly hydrated anions over smaller hydrated anions. However, the
slower complexation process should be chemically specific. There
are very few anionic species like molybdate (MoO.sub.4.sup.2-),
permanganate (MnO.sub.4.sup.-) and vanadate (VO.sub.4.sup.-), which
can potentially react and interfere after the initial ion exchange
step since their structure and size are similar to that of the
chromate anion. Some cations like Fe.sup.3+, Ag.sup.+, Cu.sup.2+
have also been shown to interfere at concentrations near and above
10.sup.-4 M. In principle, most of these potentially interfering
ions are quite toxic and if they ever reach the interfering level,
their early detection in a routine field test would be as important
as that of the chromate. From the detailed historical data of the
ground wells used for sampling in this study, we could verify, that
the concentration of any of the above potential interferences has
always been at least 3 orders of magnitude below the interfering
level reported by Turyan and Mandler. Therefore we do not
anticipate any significant interference in our ground water
measurements. As mentioned above, some small interference can be
expected in the "fast reversible" ion exchange portion of the
chromate bending signals from common mineral salts dissolved in the
ground water samples. In our experiments this effect is strongly
suppressed by the relatively high 0.1 N H.sub.2SO.sub.4
concentration in the carrier and acidified sample solutions. Small,
usually less than mM concentrations of common mineral anions would
not significantly displace the much more concentrated sulfate ions
from the pyridinium surface layer. The high specificity of the
pyridinium layer towards chromate justifies the use of acidified
ultrapure water carrier to obtain a base-line for the chromate
measurement. Preliminary evaluation of this common "ion exchange"
interference effects was carried out by injecting acidified (0.1 N
H.sub.2SO.sub.4) samples of commercially available Avian brand
natural spring water (Ca.sup.2+ 78 ppm, Mg.sup.2+ 24 ppm, silica 14
ppm, HCO.sub.3.sup.- 357 ppm, SO.sub.4.sup.2- 10 ppm, Cl.sup.- 4
ppm, NO.sub.3.sup.- 1 ppm) into the acidified (0.1 N
H.sub.2SO.sub.4) ultra-pure water carrier. The bending signals were
reversible and well within 0.03 N/m. As expected, their magnitude
did not depend on the sample size and the contact time with the
cantilever sensor. This is in contrast to the reference signals
obtained from low concentration chromate samples (at and below
10.sup.-8 M CrO.sub.4.sup.2-), where the signal kept slowly
increasing in magnitude as the sample size and the contact time
with the cantilever sensor increased.
[0039] Complexation of chromate with pyridinium and
pyridinium-terminated monolayers has been widely attributed to
inter-ionic hydrogen bonding. In a very recent study, Mosier-Boss
and Lieberman [Langmuir 19, 6826 (2003)] used a molecular modeling
calculation to show the importance of the ordered pyridinium
monolayer structure and proposed the existence of specific
microcavities between adjacent pyridinium moieties on the SAM
surface with a three-dimensional structure, complimentary in both
shape and chemical functionality to that of the chromate ion. They
have also discussed in detail, and supported with spectroscopic
evidence, the different nature of interactions between
pyridinium-terminated monolayers and ClO.sub.4.sup.- (perchlorate
ions, mainly electrostatic) as opposed to CrO.sub.4.sup.2-
(chromate, specific hydrogen bonding). The ion-pair constants
calculated from the spectroscopically-obtained Frumkin isotherms
indicate that the interaction of the pyridinium layer with
CrO.sub.4.sup.2- (specifically hydrogen bonding) is three orders of
magnitude stronger than that for perchlorate (mainly
electrostatic). This explains the high chromate selectivity and
sensitivity. Furthermore, the negative value of the Frumkin
parameter for the chromate-pyridinium interaction (g=-2.80.+-.0.18)
obtained in the study indicates a repulsive force between the
adsorbed chromate ions, which becomes very significant at high
surface concentrations. These repulsive chromate-chromate
interactions may be responsible for the extremely large cantilever
bending signals (>1 N/m) at high chromate surface coverage,
reflecting a huge increase of the adsorption-induced surface
stress. In light of this argument, it is important to note that the
cantilever sensitivity significantly increased after a "critical"
surface concentration of pre-adsorbed chromate was achieved by
injection of standard chromate solution. (following the
electrochemical recovery). It apparently remains steady until high
chromate coverage (near 1 N/m of total cantilever bending signal
and higher) is achieved. The data shown in this work are taken
within this sensitivity range. In a real device the sensitivity can
be periodically verified and corrected by injection of small volume
low-concentration chromate standard to determine the exact
sensitivity factor for the following and previous sets of samples.
Overloading of the cantilever sensor can be prevented by using
small sample volumes and short contact times at high chromate
concentrations as seen in FIG. 2. If overloaded, the cantilever
sensor can be quickly recovered electrochemically as described
above. It was also found that following a number (>10) of
chromate injections (significant chromate loads), prolonged wash
(overnight) with acidified ultrapure water (.about.60 ml at slow
pumping speeds of few ml/hr) recovers the cantilever sensor
capacity within the working sensitivity range as during the
previous day and thus, no pre-adsorption of chromate was necessary
in order to obtain high intensity bending signals with magnitude
similar to that of the corresponding samples analyzed during the
previous day. This can be attributed to the slow desorption of the
"overstressing" hydrogen-bonded chromate surface species
facilitated by the repulsive chromate-chromate interactions at high
surface coverage, as well as by the elastic forces of the strongly
bent cantilever and the concentration gradient. The remaining
chromate species will have a low enough surface density, which
assures minimal chromate-chromate electrostatic repulsion. The
sensor is than ready to respond with a strong signal to any, even
minimal, change in the surface stress induced by adsorbing any
chromate ions in closer than this equilibrium proximity. The mobile
nature of any hydrogen-bonding interaction, leads to the removal of
pre-adsorbed chromate species by the clean acidified water
carrier.
[0040] Cantilever response to the acidified B173R9 sample (FIG. 3)
shows that the cantilever is able to detect this low chromate level
clearly. Comparing this signal magnitude with cantilever data for
the B173R7 sample (FIG. 2(a)) and the B173R8 sample (FIG. 2(b))
yielded estimated concentrations of 1.times.10.sup.-7 M and
6.times.10.sup.-8 M respectively for the B173R9 sample. Therefore,
we estimate the concentration of the B173R9 sample to be
.apprxeq.8.times.10.sup.-8 M. At lower chromate concentrations,
much larger sample volumes are used at the same pumping speed, thus
also increasing the contact time between the chromate and the
pyridinium layer.
[0041] An estimate of the detection threshold can be obtained by
using the data of FIG. 3, and estimating the chromate concentration
that would be above the noise level of FIG. 3. By taking the noise
level to be 3 standard deviations of the background fluctuations
(i.e. .apprxeq.3.times.0.002 N/m), we obtained a detection
threshold of 4.times.10.sup.-9 M. When we injected standard
chromate solutions of varying concentrations made out of calibrated
acidic chromate samples, we were able to observe clear signals
above/near 1.times.10.sup.-8 M. Therefore, the detection threshold
is roughly an order of magnitude better than the photometric
method. It may be possible to improve the sensitivity further by
varying the pH, contact time with the chromate sample, and SAM
quality, as well as the cantilever specific parameters and
substrate cleaning procedure; such studies are underway in our
laboratory. Furthermore, we have used a few cantilevers with the
4-mercaptopyridine coating for several months without any apparent
decrease in performance.
[0042] Further details are discussed in the following literature,
herein incorporated by reference:
[0043] 1) Terry, J.; Linford, M. R.; Wigren, C.; Cao, R.; Pianetta,
P.; Chidsey, C. E. D. Appl. Phys. Letter. 1997, 71, 1056-1058.
[0044] 2) Terry, J.; Mo, R.; Wigren, C.; Cao, R.; Mount, G.;
Pianetta, P.; Linford, M. R.; Chidsey, C. E. D. Nucl. Instrum.
Methods Phys. Res., Sect. B 1997, 133, 94.
[0045] 3) Terry, J.; Linford, M. R.; Wigren, C.; Cao, R.; Pianetta,
P.; Chidsey, C. E. D. J. Appl. Phys. 1999, 85, 213.
[0046] 4) Cicero, R. L.; Linford, M. R.; Chidsey, C. E. D.
Langmuir, 2000, 16, 5688.
[0047] 5) L. A. Pinnaduwage V. I. Boiadjiev, G. M. Brown, T.
Thundat, S. W. Petersen, "Detection of Hexavalent Chromium in
Ground Water Using a Single Microcantilever Sensor", Sensor Letters
Vol. 2, No. 1 (2004).
[0048] 6) V. I. Boiadjiev, G. M. Brown, L. A. Pinnaduwage, G.
Goretzki, P. V. Bonnesen, and:T. Thundat, "Photochemical
Hydrosilylation of 11-Undecenyltriethylammonium Bromide with
Hydrogen-Terminated Si Surfaces for the Development of Robust
Microcantilever Sensors for Cr(VI)", Langmuir, ASAP Article
10.1021/la047852n S0743-7463(04)07852-7; Web Release Date: Jan. 20,
2005.
[0049] 7) X. Zhou, M. Ishida, A. Imanisahi, Y. Nakota, "Roles of
Charge Polarization and Steric Hindrance in Determining the
Chemical Reactivity of Surface Si--H and Si--Si Bonds at
H-Terminated Si(100) and -(111)", J. Phys. Chem. B, 2001, 105, pp
156-163.
[0050] 8) A. Arafat, K. Schroen, L. C. P. M. de Smet, E. J. R.
Sudholter, H. Zuilhof, "Tailor-Made Functionalization of Silicon
Nitride Surfaces", J. Am. Chem. Soc., 2004,126, pp 8600-8601.
[0051] 9) R. Voicu, R. Boukherroub, V. Bartzoka, T. Ward, J. T. C.
Wojtyk, D. D. M. Wayner, "Formation, Characterization, and Chemisty
of Undecanoic Acid-Terminated Silicon Surfaces: Patterning and
Immobilization of DNA", Langmuir, 2004, 20, pp. 11713-11720.
[0052] 10) C. M. Yam, J. M. Lopez-Romero, J. Gu, C. Cai,
"Protein-resistant Monolayers Prepared by Hydrosilylation of
.alpha.-Oligo (ethylene glycol)-.omega.-Alkenes on
Hydrogen-Terminated Silicon (111) Surfaces", Chemical
Communications, 2004, pp. 2510-2511.
[0053] A range of alkenes and alkynes were successfully tried,
including 1-octene, 1-octadecene, 1-octyne, styrene, and
phenylacetylene, with the alkenes yielding alkyl monolayers and the
alkynes yielding alkenyl monolayers. Examples of surfaces prepared
and discussed in the literature are shown in FIG. 6.
[0054] According to Buriak, the mechanism proposed is radical
based, with homolytic Si--H bond cleavage initiating the reaction
to form a silicon radical (dangling bond). Because silicon radicals
are known to react very rapidly with unsaturated carbon-carbon
bonds, Si--C bond formation is expected to be a facile step,
forming the surface-bound carbon-based radical on the
.omega.-carbon of the olefin. Abstraction of neighboring hydrogen
completes the hydrosilylation. On the basis of the bond strengths
of the weakest Si--H bond on a silicon surface, the monohydride
Si--H group (.about.3.5 eV), it appears that a minimum of 3.5 eV UV
(<350 nm) is required to efficiently perform Si--H bond
homolysis. Buriak reports that irradiation of the Si(111)-H surface
in air results in fast and efficient loss of hydrides, as observed
by ATR-FTIR, only at wavelengths shorter than 350 nm, again
pointing to the threshold near this wavelength for Si--H bond
activation on this surface
[0055] As an example we attached a quaternary ammonium group to the
surface of a cantilever and demonstrated that we could use this
modified cantilever to sense Cr(VI) in solution. A long reaction
time was chosen because of the electrostatic repulsion between the
positively charged quaternary ammonium groups, which has appeared
to significantly slow down the self assembly process of
triethyl-12-mercaptododecylammonium bromide on gold surfaces (to
about one week deposition time) compared to self-assembly of normal
1-thiols (few hours to 1 day). Therefore, based on these earlier
results, we have allowed sufficient time (6 to 10 days) in order to
assure dense packing of this particular ion-terminated organic
layer, being fully aware that complete hydrosilylation of normal
aliphatic alkenes like 1-pentene and 1-octadecene on flat silicon
surfaces has been reported to occur within only .about.2 hrs.
Indeed, long deposition time periods may not be such a serious
issue for other types of organic layers, which do not contain ionic
functionalities. In support of this argument, best results in this
study were obtained with cantilevers irradiated for 10 days. The
proposed surface structure is shown in FIG. 7.
[0056] The photochemical surface activation strategy can also allow
individual cantilevers in multicantilever array chips to be
modified separately by focusing the activating UV light
sequentially on each particular cantilever. A set of reactant
solutions can be prepared and recycled in a sequential
photochemical treatment procedure where the array will be exposed
to one solution at a time. The entire chip would be washed with
solvent following each UV irradiation before the next solution is
brought in. This will allow specific modification of individual
irradiated cantilevers on the chip, which cannot be achieved using
non-selective thermal activation. Thermal activation would initiate
deposition of the same organic layer on all cantilever surfaces at
the same time. In addition, carrying out the photochemical reaction
at ambient temperature eliminates cantilever deformations due to
bimetallic effects upon heating. Increasing the number of
independently functionalized cantilevers on an array chip would
strongly enhance the recognition power of the sensor device. Such
modified surfaces can also be used as reactive platforms for
further surface functionalization by spotting.
[0057] The Ti/Au coating of the cantilevers did not appear to
interfere with the photochemical silicon functionalization process
when a 40% NH4F solution was used to produce the
hydrogen-terminated silicon surface. It was severely attacked when
an HF treatment was attempted. Nevertheless, a protective Ti/Au
coating may not be required if only one side of the lever can be
selectively modified during the photochemical hydrosilylation step,
while the other hydrogen-terminated silicon side will oxidize back
to its original functionality upon contact with water and air
following the photochemical process.
[0058] Photochemical hydrosilylation of silicon cantilevers is,
suitable for cantilever sensor development if the desired surface
functionality is compatible with the reaction requirements. The
resulting functionalized organic layers can be dense enough to
generate sufficient surface stress upon specific analyte adsorption
at low concentrations. Cantilever sensors prepared using these
strategies have unsurpassed robustness and stability due to the
direct covalent Si--C linkage.
[0059] Although the single cantilever approach seems to work
extremely well in laboratory applications, it is less useful in
real environment applications where many other parameters can
produce signal interference. To avoid this potential problem, it is
necessary to look at the differential response of a set, or array,
of cantilevers. For example, variations in physical parameters such
as temperature, acceleration, and mechanical noises can contribute
to cantilever bending. Differential signals obtained by common mode
rejection can provide highly sensitive data.
[0060] Chemical selectivity can be achieved by arrays consisting of
several microcantilevers, each coated with different selective or
partially selective coatings. The response of a given modified
microcantilever will depend on the concentration of the analyte and
the strength of the coating-analyte interactions (e.g. hydrogen
bonding, dispersion, and dipole-dipole interactions). A unique
response pattern characteristic to a particular analyte can be
obtained from an array where each microcantilever is modified with
a different coating. The higher the number of modified cantilevers,
the greater the uniqueness of the response pattern. Since the
microcantilever response to a given analyte depends on the
functional end-groups of modifying agents, judicious selection of
coatings can lead to significant differences in the response
patterns for different analytes. Using an array consisting of a
large number of microcantilevers, unique response patterns can be
attained for individual analytes, class of analytes, or analytes in
complex mixtures. The results of testing with a large number of
analyte and mixtures are recorded in a look-up table and referenced
routinely when an array is in service.
[0061] Using hydrosilylation procedure, which in certain cases can
be combined with sequential surface reactions, it is possible to
derive coatings containing various molecular recognition groups
such as hydrocarbon chains, esters, metal-containing organic
functionalities, buckyballs (C-60), carboxylic acids, and even
hydroxyls. One example is the use of hydrocarbon layers grafted
directly on the silicon surface for gasoline detection (FIG. 6).
Environmental sampling and process control are areas wherein the
arrays offer advantages of size, simplicity and reliability.
[0062] While there has been shown and described what are at present
considered the preferred embodiments of the invention, it will be
obvious to those skilled in the art that various changes and
modifications can be made therein without departing from the
scope.
* * * * *