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SUPPL
EMENTAL INFORMATION
Vapor Sensing Characteristics of Nanoelectromechanical Chemical Sensors
Functionalized Using Surface-
Initiated Polymerization
Heather C. McCaig, Ed Myers, Nathan S. Lewis, Michael L. Roukes
I. Method Details
II. Vapor Exposure D
ata
III. Calculation
of Partition Coefficients
IV. Ellipsometry of Film Swelling
V. Extended (5000 s) Exposures to Vapors
VI. Sensor to Sensor Repeatability of Responses to Analyte Vapors
I. METHOD DETAILS
A. Materials
Methyl met
hacrylate (10-100 ppm MEH
Q inhibitor
), HQ/MEHQ inhibitor removal column
packing, and poly(methyl methacrylate) (PMMA; Tg=105 °C; MW = 35,000
; ρ = 1.20 g
mL
-1
) were
purchased from Scientific Polymer Products, Inc.
Bis(2
-[2’-bromoisobutyryloxy]ethyl)disulfide
(BiBOEDS) (>90%) was p
urchased from ATRP Solutions.
Methanol (anhydrous, 99.8%), copper
(I)
bromide
(CuBr)
(98%) and 2,2’-bypyridyl (>99%) were purchased from Aldrich.
CuBr
was
purified by stirring with glacial acetic acid for 24 h at room temperature, followed by rinsing wit
h
ethanol and diethyl ether, and then drying
overnight in vacuum
. Purified CuBr was
stored in a
vacuum desiccator until use. Absolute ethanol was purchased from Decon Laboratories, Inc.
Regent grade
hexane, heptane, toluene, ethyl acetate, chloroform,
tetrahydrofuran,
isopropanol
,
and carbon tetrachloride
were purchased from VWR, and were used to produce analyte vapors
.
All chemicals were used as received unless otherwise stated.
Ultrapure,
deionized water
(18 MΩ
cm
resistivity
) was used for all synthes
es.
B. Nanocantilevers
The fabrication of silicon nitride nanocantilever
s with integrated piezoresistive readouts
has been described in detail previously.
1, 2
Briefly, cantilever and bond-pad shapes were
pattered with electron-beam lithography onto a 100 nm thick SiN layer on a silicon substrate,
followed by gold film deposition, and then liftoff. Dry plasma etching was then used to release the
cantilevers.
The gold overlayer served as both an
etch mask during fabrication and later as a
piezoresistive transducer.
3
Nanocantilevers had a typical fundamental resonance frequency of
10-12 MHz, quality (
Q
) factors of
100-
200 in ambient conditions, and a capture area of 1.5 μm
2
.
The r
esonance was actuated thermoelastically.
4
Nanocantilever sensors were
operated with
home-built, LabView controlled, electronics
3
that
tracked
each sensor’s resonance frequency
using parallel, independent phase-locked loops.
C. Initiator SAM Formation
Substrates were cleaned by sequential rinses with
hexane, acetone, tetrahydrofuran,
methanol, and absolute ethanol, followed by a UV/ozone plasma clean (Samco UV
-1 ) for 8 min
with a stage temperature of 65 °C. After rinsing with deionized water and thoroughly drying with
a stream of compressed air,
the
substrates were immersed into a 5 mM solution of initiator in
absolute alcohol
. For a single chip, 2.5 mL of the solution (15 mg initiator), in a 20 mL scintillation
vial was sufficient to fully immerse the substrate. The sealed vials were stored into the dark for
24-48 h to ensure the f
ormation of a dense and ordered self
-assembled monolayer
(SAM)
.
D. Polymerization
Polymerization of MMA was
performed
by water
-accelerated SI
-ATRP
.
5
Neat MMA was
first passed through an inhibitor removal column.
The purified MMA was either used immediately,
or was stored in sealed vials that were placed in a freezer until use.
A two
-necked round bottom
flask was
charged with 7.5 mg of
purified
MMA, 6 mL methanol, and 1.5 mL deionized water.
The flask was sealed with septa and then the solution was
sonicated while sparging
for 45 min
with
N
2
(g)
or
Ar(g).
The catalyst components, 258 mg of CuBr and 114 mg 2-bipy
, were then
added to the solution,
and the solution
was then
simultaneously
sonicated
and sparged
until
the
catalyst dissolved, which generally
took
30-45 min.
Substrates were suspended with a flat
alligator clip in a 20 mL scintillation vial equipped with
a small stir bar and a
sept
um
. The clip was
attached to a wire that was sufficiently long to bend over the rim of the vial, and was held in place
with the septum. The vial was purged with N
2
(g)
or
Ar(g)
for at least 45 min before introduction of
the sol
ution.
To minimize contact with oxygen, t
he reaction solution was transferred via syringe
from the round-bottom flask to the vial
that contained
the suspended substrate. The reaction was
allowed to proceed at room temperature with stirring and
a constant i
nert gas purge. A
t the
desired time,
the substrate was removed and thoroughly
rinsed
sequentially
with tetrahydrofuran,
methanol,
and
absolute ethanol
, respectively
.
E. Polymer Film Characterization
Films were characterized using ellipsometry and scanni
ng-electron microscopy (SEM).
Flat substrates for ellipsometr
ic measurements were prepared by evaporating 3 nm of chromium,
followed by 30 nm of gold
, onto a silicon wafer that had been coated
with
a native oxide
layer.
The c
leaning, SAM formation, and polymerization procedures were identical to those used for the
nanocantilevers.
The
PMMA film thickness was measured
using
a Gaertner L166C ellipsometer,
equipped with
a He-Ne (633 nm wavelength) laser, at a 70°
angle of incidence. The optical
constants of t
he flat gold substrates were measured prior to formation of the initiator SAM.
The
thickness
was determined for
both
the
initiator SAM and
the
polymer film. The refractive index
of
the SAM was assumed to be 1.46, and the refractive index of the PMMA
film
was
assumed to be
1.49
.
6
The thickness of the initiator SAM was
0-2 Å.
SEM (ZEISS 1550 VP FESEM
and
FEI
Sirion
) images were
used to verify the quality of nanocantilever fabrication
as well as
the quality
of the SI
-ATRP grown polymer films.
F. Drop-cast Polymer Films
PMMA solutions for drop-casting were formed by first
makin
g a concentrated solution by
sonicating
~ 100 mg
of
PMMA in 20 mL toluene,
until the polymer beads had
dissolved. This
concentrated solution was diluted
to
5 mM
of PMMA.
A
10 μL
micropipette was used to apply a
1.5 μL droplet of the dilute PMMA solution
to the chip that contained the
nanocantilever sensors,
and the solvent
was
then
allowed to evaporate.
G. Vapor Exposures Experiments
Nanocantilevers were exposed to analyte vapors using an automated vapor delivery
system controlled by LabView.
7
At least three sensors of each type (bare, dropcast PMMA, SI
-
ATRP PMMA) were tested. The analytes (hexane, toluene, heptane, ethyl acetate, chloroform,
tetrahydrofuran, and isopropanol) were delivered at concentrations of 0.005-0.08
P
/
P
o
(partial
pressure divided by saturated vapor pressure), and each exposure consisted of 70 s of pure
carrier gas, 400 s
of analyte vapor exposure, followed by 630 s
of carrier gas to purge the
system.
For single concentration ex
periments, a run consisted of five exposures to each analyte
at
P
/
P
o
= 0.020
. For linearity experiments, five exposures per concentration per analyte were
delivered in the order
P
/
P
o
= 0.030
, 0.010
, 0.0480
, 0.0050
, 0.080
, and 0.02
0, to prevent possible
hysteresis from affecting
the
linearity profile.
The nanocantilevers were housed in a brass
chamber with an internal volume of 100 mL.
Between one and four sensors were tested in each
experimental run, and prior to data collection all
of the sensors were
conditioned
by multiple
exposures to each analyte.
The t
emperature was not controlled, but was stable at 21 ± 1 °C.
H. Data Analysis
The nanocantilever f
requency data
were
corrected for baseline drift prior to extraction of
the
sensor responses. The bas
eline noise was computed as the standard deviation of the
drift
-
corrected baseline f
requency over a period of 10 s
prior to the
sensor response. The s
ignal
-to
-
noise ratio was calculated as the average response divided by three times the baseline noise.
Dat
a analysis was performed
using OriginLab (Version 7.5).
The nanocantilever sensor response
data reported in the
figures and tables were recorded from single, representative sensors. Some
variation was observed between individual sensors of each type, but
the variation
did not distort
the reported trends
.
II. VAPOR EXPOSURE DATA
A. Figures of Cantilever Frequency Measurements for 0.02 P/P
o
Vapor Exposures:
The following figures show baseline-drift
-corrected frequency traces for three cantilever
sensor
s with different surface functionalizations
: bare gold (S1), a
drop-cast PMMA film (S2), and
a SI-ATRP
-grown PMMA film (S3). Each figure depicts the response of a single sensor to a
series of exposures to analyte vapors at a concentration of
P
/
P
o
= 0.020.
For these experiments,
five sequential exposures to each of seven vapors were conducted in the order hexane, toluene,
heptane, ethyl acetate, chloroform, isopropanol, and tetrahydrofuran.
Figure S1: Responses of a bare cantilever to a series of
analyte vapors delivered at
P
/
P
o
= 0.020
.
The dashed red lines denote a
change of analyte.
Figure S2: Responses of a cantilever functionalized with a drop-cast film of PMMA to a series of
analyte vapors delivered at
P
/
P
o
= 0.02
0. Functionaliz
ati on of
the cantilever with a drop-cast
polymer film introduced
selectivity and enhanced
the
sensitivity relative to a bare sensor.
The
dashed red lines denote a
change of analyte.
Figure S3: Responses of a cantilever functionalized with an SI
-ATRP
grown PMMA film to a
series of analyte vapors delivered at
P
/
P
o
= 0.020
. The sensor’s responsivity to polar vapors was
greatly enhanced relative to a cantilever that had instead been functionalized with a drop-cast
PMMA film.
The dashed red lines denote a
change of analyte.
B. Tabulated Responses Data
Table S1: Relative frequency shifts (a) and signal
-to-noise ratio (b) for nanocantilevers with
a
bare gold surface, a
dropcast PMMA film, and
a SI-ATRP PMMA film exposed to 400 s pulses of
various analyt
e vapors.
(a) Δf
max
/f
O
x 10
6
Analyte
Bare
Drop-cast PMMA
SI-ATRP PMMA
Hexane
-56.89
± 3.34
-39.71
± 4.63
-42.37
± 2.62
Toluene
-22.96
± 2.79
-13.73
± 1.87
18.94
± 5.45
Heptane
-33.02
± 2.93
-26.83
± 2.94
-31.67
± 2.39
Ethyl Acetate
-40.45
± 2.43
-52.31
± 7.09
478.36
± 26.47
Chloroform
-60.53
± 9.34
-74.28
± 6.29
219.21
± 39.51
Isopropanol
-33.86
± 9.22
-13.28
± 2.52
149.13
± 10..28
Tetrahydrofuran
-48.22
± 4.55
-28.16
± 1.83
126.52
± 11.27
(
b
)
SNR
Analyte
Bare
Dropcast PMMA
SI-ATRP PMMA
Hexane
9.8
± 0.6
6.9
± 0.8
4.6
± 0.05
Toluene
4.0
± 0.5
2.4
± 0.3
1.1
± 0.1
Heptane
5.7
± 0.5
4.6
± 0.5
3.1
± 0.3
Ethyl Acetate
7.0
± 0.4
9.1
± 1.2
49.6
± 1.1
Chloroform
10.4
± 1.6
12.9
± 1.1
24.2
± 1.1
Isopropanol
5.8
± 1.6
2.3
± 0.4
17.2
± 2.0
Tetrahydrofuran
8.3
± 0.8
4.9
± 0.3
13.5
± 0.6
C.
Equilibrium Response to Polar Vapors
When cantilevers
coated
with PMMA grown using SI
-ATRP were
exposed
to polar
vapors, an
initial period
(~ 15 min)
of rapid in
crease in frequency
was
observed. This
period was
followed
by a transition to a slower rate of increase
that continued
for
> 1 h before the sensor
reached
steady
-state
. Non-Fickian diffusion, characterized by
an initial mass loading that is
followed by a
delayed relaxation of the polymer chains, commonly occurs i
n glassy polymers
such as PMMA, which may explain the shape of the responses to polar vapors
of cantilevers that
had been coated with PMMA grown using SI
-ATRP.
8-13
Figure S4:
A cantilever
coated with PMMA grown using SI
-ATRP
exposed to ethyl acetate vapor
(
P
/
P
O =
0.020
) required
long exposure times to reach equilibrium with polar vapors.
Figure S5: Comparison of
the responses of cantilever
s coated with PMMA grown using SI
-ATRP
to polar vapors exposed for 400 s
and 5000 s
.
Table S2: Equilibrium responses of a nanocantilever coated with PMMA grown using SI
-ATRP
,
along with the percentage increase in response magnitude compared to the sensor response
generated by a 400 s pulse of analyte vapor.
SI
-
ATRP PMMA E
quilibrium Responses
Analyte
Δ
f
max
/f
O
x 10
6
Percent increase in response
magnitude
Ethyl Acetate
714.73
± 13.21
53%
Chloroform
508.05
± 7.85
122%
Isopropanol
307.83
± 61.40
90%
Tetrahydrofuran
305.51
± 59.68
138%
III.
PARTITION COEFFICIENTS FOR DROPCAST AND
SI-AT RP
GRO
WN
PMMA F
ILMS
A. Method for Determining Partition Coefficients
Partition coefficients (
K
eq
) were determined by measur
ement of
the mass uptake of
PMMA films applied to quartz
-crystal microbalances (QCMs). Each QCM was cleaned
by
sequential rinses with
hexane, acetone, and methanol
, before measurement of
the
initial
resonance frequency,
F
O,i
. PMMA films were prepared either by spray
coating the QCM with a
solution of PMMA (Scientific Polymer Products, Inc.) in tetrahydrofuran (
160
mg / 20 mL) using an
air
brush, or by SI
-ATRP, as described above. All PMMA films were stored in a closed, but not
sealed, container for at least 24 h
after film formation,
to aid in the evaporation of any trapped
solvent. Before data collection,
QCMs were conditioned
by
exposur
e to a randomized series of
vapor exposures for 12-18
h.
The
QCMs were exposed to analyte vapors with an automated vapor delivery system.
7
Five
exposures of the seven vapors
at each of the
five
concentrations (
P
/
P
o
= 0.010
, 0.02
0,.
0.040
, 0.060
, 0.080
) were conducted in an order randomized for both analyte
identi
ty and
concentration.
After an initial purge of 500 s
, e
ach exposure was
400
s in duration, with a
700
s
purge between exposures. The change in
resonance frequency due to polymer coating,
ΔF
polymer
,
was
calculated
as the difference between
the resonance frequency before and after coating. The
frequency change due to each vapor exposure,
ΔF
analyte
, w
as calculated as the difference in
frequency between the QCM during exposure relative to the
baseline frequency. The baseline
frequency was calculated as the average frequency during the 20
s prior to the specific vapor
exposure, and the frequency during exposure was calculated as the average frequency between
350
and
398
s after the exposure had begun.
The calculation of the partition coefficient from the QCM frequency shift data has been
described previously.
14
Briefly, a line with a forced zero
was
first
fitted versus
the
data as a
function of analyte
concentration. The slope of this fit was
then converted into a partition
coefficient
using:
atm
polymer
W
eq
P
f
M
RTm
K
=
6
10
*
ρ
where
R
is the ideal gas constant (1 atm mol
-1
K
-1
), ρ is the density (g mL
-1
) of the polymer,
T
is
the temperature (K),
m
is the slope
of
ΔF
analyte
versus concentration (Hz/ppth in air),
M
W
is the
molecular weight (g mol
-1
) of the analyte,
ΔF
polymer
(Hz) is the frequency
shift due to
the
polymer
coating, and
P
atm
is the atmospheric pressure (atm). The density of PMMA used in the
K
eq
calculations was 1.
20
g mL
-1
for QCMs
that had
both spray
-coated and SI
-ATRP
-grown
PMMA
films.
B. Tabulated Partition Coefficients
Table S
3: Calculated partition coefficients for bulk and SI
-ATRP PMMA films.
Partition Coefficients (K
eq
)
Analyte
Spray
-Coated Bulk PMMA
SI-ATRP PMMA
Hexane
65
40
Toluene
540
375
Heptane
175
90
Ethyl Acetate
390
280
Chloroform
245
200
Isopropanol
415
350
Tetrahydrofuran
160
115
IV. ELLIPSOMETRY OF POLYMER FILM SWELLING
A. Method
Substrates coated with either a drop-cast or SI
-ATRP
-grown PMMA film were placed into
a vapor exposure chamber that was situated on the sample stage of the ellipsometer. T
he vapor
exposure chamber consisted of a plastic box with ports for the
laser beam as well as the
vapor
stream input. A glass window made from a microscope slide cover slip was installed in the top of
the chamber to allow for substrate alignment. Saturat
ed analyte vapor was generated by passing
a stream of laboratory air though a bubbler. A manual valve was used to switch the gas stream
flowing through the sensor chamber between laboratory air and saturated analyte vapor.
Substrates for measurement of the swelling of drop-cast and SI
-ATRP
-grown PMMA
film
s were QCMs that had been previously used to determine
K
eq
of the SI
-ATRP grown film.
The
baseline film thickness of each sample was measured after exposure to a flow of laboratory air for
2 min. The flow was then switched to a stream of saturated analyte vapor for 6 min, and the film
thickness was measured again. This procedure was repeated three times for each vapor.
B. Polymer Film Swelling Data
The relative vertical swelling of the SI
-AT
RP
-grown PMMA film
did
not match the
observed trend of enhanced nanocantilever sensor responses to polar vapors.
Table S4: Relative swelling of SI
-ATRP PMMA films exposed to saturated analyte vapors.
Relative Swelling (ΔH/H x 10
5
)
Analyte
SI-ATRP PMMA
Hexane
0.01
Toluene
0.12
Heptane
0.04
Ethyl Acetate
0.20
Chloroform
1.26
Isopropanol
0.18
Tetrahydrofuran
0.34
The ratio
of the relative vertical film swelling to the partition coefficient was
also
determined, and the comparison of vertical film swelling to mass loading did not match the
observed trend of enhanced nanocantilever responses to polar vapors.
Table S5: Ratio of relative swelling to partition coefficient of SI
-ATRP PMMA for variou
s analyte
vapors.
Relative Swelling (ΔH/H)/K
eq
X 100
Analyte
SI-ATRP PMMA
Hexane
2.5
Toluene
3.3
Heptane
4.3
Ethyl Acetate
7.0
Chloroform
63.3
Isopropanol
5.2
Tetrahydrofuran
29.7
V.
EXTENDED (5000 s) EXPOSURES TO VAPORS
The steady
-state response of the
functionalized sensors to polar analyte vapors required
53
-138 % longer than a 400 s exposure. Furthermore, even after 5000 s exposures
to ethyl
acetate and isopropanol
, cantilevers
coated with PMMA grown by Si
-ATRP
exhibited a nonlinear
concentration
response. At higher concentrations, even a 5000 s exposure was insufficient for
the sensor to reach steady
-state. The PMMA film also exhibited a pronounced history effect
when exposed to ethyl acetate vapor, whereby an exposure to ethyl acetate at
P
/
P
o
= 0.030
produced a smaller response than an exposure to ethyl acetate later in the experiment at
P
/
P
o
=
0.020
(Figure S6).
Figure S5: Longer exposure times (5000 s) to polar analyte vapors did not improve linearity
because equilibrium was not reached for
high vapor concentrations.
VI. SENSOR TO SENSOR REPEATABILITY OF RESPONSES TO ANALYTE VAPORS
Figure S6 shows
the
responses of two sensors each for drop-cast PMMA and SI
-ATRP
-
coated nanocantilevers. The cantilevers coated with the
drop-cast PMMA films were from the
same chip. The cantilevers with
the
SI-ATRP films were also from the same chip, and thus were
both coated via the same reaction. The responses data are
for sensors that exhibited
no obvious
defects observed with SEM after
the
vapor ex
posure experiments
had been completed. Poorly
-
performing sensors were not used for vapor exposure experiments and were observed in both
categories, typically due to occasional fabrication defects, nanocantilevers damaged by debris, or
the gluing-down of nanocantilevers upon attempted deposition of drop-cast PMMA films.
Poorly
-
performing sensors exhibited either no resonance due to extreme mechanical failure, or very low
quality factors (Q), such that changes in resonance frequency could not be
readily
measured.
Generally,
the
SI-ATRP coating procedure resulted in a higher yield (no gluing down, and fewer
debris) of nanocantilevers that exhibited resonance and acted as good chemical vapor sensors.
Figure S6: Repeatability of sensor responses to analyte
vapors.
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