Supporting Information
Gold nanospikes based microsensor as a highly accurate mercury emission monitoring system
Ylias M. Sabri1, Samuel J. Ippolito1,*, James Tardio1, Vipul Bansal1, Anthony P. O’Mullane2 and Suresh K.
Bhargava1,*
1
Centre for Advanced Materials and Industrial Chemistry, School of Applied Sciences, RMIT University,
GPO Box 2476V, Melbourne, VIC, Australia, 3001.
2
School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, GPO
Box 2434, Brisbane, Australia.
Email: samuel.ippolito@rmit.edu.au, suresh.bhargava@rmit.edu.au
Phone: +61 3 99252330
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Figure S1. Supporting information showing the dimensions of Au nanospikes in a 40° angular side-view
SEM image of a QCM edge
Figure S2. Supporting information showing the extremely uniform coverage of Au nanospikes obtained
after 600 sec of electrodeposition time, as shown in this low-magnification SEM image.
2
Au M
Si
O
Au L
Au L
a)
0
6
12
18
E (keV)
24
30
Au4f
Pb4f
Intensity (a.u.)
Intensity (a.u.)
Absence of Pb
Au4d
150 145 140 135
Binding Energy (eV)
O1s
C1s
Absence of Pb
b)
1200 900 600 300
0
Binding Energy (eV)
Figure S3. a) EDX spectra of electrodeposited Au nanospikes in the presence of lead (II) acetate. The
spectra confirms the presence of Au as well as Si and O arising from the underlying quartz substrate.
The Pb was not detected in EDX spectra confirming absence of Pb on the electrodeposited surfaces which
was further confirmed with b) XPS that neither Pb2+ ions or metallic Pb are present on the
electrodeposited Au nanospikes.
3
The Hg sensing capability of a nanospike based QCM was tested against that of the Au control QCM
(both with a 100 nm e-beam evaporated Au electrode layer) at five different Hg concentrations (C1 =
1.02, C2 = 1.87, C3 = 3.65, C4 = 5.70, and C5 = 10.55 mg/m3) and an operating temperatures of 28°C
as shown in Figure S4a. It is evident from Figure S4a that the Au nanospikes show significantly higher
response magnitudes towards all the Hg vapor concentrations in comparison with that of the control Au
QCM (solid vs. dashed lines). The Au nanospikes based QCM shows 490% response magnitude (~5
times the sensitivity) of that of the Au-control QCM towards a Hg concentration of 10.55 mg/m3 at 28°C.
When the sensor response magnitudes are normalized for their electrochemically active surface area
(Fig. S4b), it is evident that the nanospikes based QCM still had significantly higher sensitivity towards
Hg0 vapor in comparison to that of an Au-control QCM. This indicates that the higher sensitivity of the Au
nanospikes is due to the additional atomic-scale features (surface defect sites) formed during their
growth process and not just due to their increased surface area.
The three standard deviation of the noise profile related to the sensors’ response towards dry N 2 (blank
sample) over a 1 hour period was used to calculate the limit of detection (LOD). It was found that the
LOD of the Au nanospike sensor was 5 times better (6 µg/m3) over the Au control based QCM
(35 µg/m3).
28 °C
0
ƒ (Hz)
3
1.02 mg/m
-500
C1 1.87 mg/m3
C2
3
3.65 mg/m
-1000
C3
3
5.70 mg/m
C4
-1500
-2000
Au control
Au nanospikes
-2500
0
1
2
3
4 5 6 7
Time (hour)
3
10.55 mg/m
C5
8
9 10
Figure S4a. QCM sensor response of Au surfaces towards Hg at 28 ± 1°C before (dashed curves) and
after Au nanospikes electrodeposition (solid curves). The QCM sensors have been tested at different Hg
concentrations ranging from 1.02 to 10.55 mg/m3.
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Au control
Au nanospikes
Au nanospikes
(surface area)
1600
ƒ (Hz)
1200
28 °C
slope = 155.2
800
slope = 49.2
400
slope = 32.81
0
0
2
4
6
8
3
Hg Concentration, C (mg/m )
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Figure S4b. Analysis of QCM sensor response of Au control and nanospikes based QCMs shown in S4a
above. The blue curves show the response magnitude of the nanospikes based QCM after normalizing
for the electrochemical surface area to be the same as the Au control surface.
Thermal stability of nanospikes
For most of the applications, including Hg sensing in industrial streams, it is extremely important that
the nanostructures grown onto a surface (QCM sensing platform in this case) are thermally stable, which
is generally difficult to achieve with Au nanostructures. Figure S5 shows that the electrodeposited Au
nanospikes QCM have good thermal stability after being exposed to 150°C for 48 hours in air with no
significant change being observed. To compare the thermal stability of Au nanospikes with the regularlyreported dendritic (nanowires-like) Au nanostructures1, Au nanodendrites were electrodeposited onto
QCM crystal using similar electrodeposition parameters as nanospikes, however with 10 times higher
concentrations of both AuCl4- and Pb2+ ions (Fig. S5b). Upon exposure to 150°C for only 10 hours, these
nano-dendrites melted and completely lost their morphology even under non-oxidative N2 atmospheric
conditions (Fig. S5c), indicating their unsuitability for most of the applications requiring moderately high
temperatures (≥150°C). This clearly demonstrates the thermal superiority of Au nanospikes over widelyreported Au nano-dendrites in the literature. Notably, these high surface area Au nano-dendrites could
not be successfully used for Hg sensing even at room temperature, because growth of compact Au
nano-dendrites onto QCM crystal significantly dampened its quality factor, thus making it impossible for
the QCM to oscillate, rendering the sensor unusable.
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Figure S5a,b,c. SEM micrographs of (A) Au nanospikes after heat treatment at 150°C for 48 hours, (B)
Au nanodendrites before, and (C) after heat treatment at 150°C for 10 hours. The scale bar in each
micrograph corresponds to 500 nm.
Figure
S6. Box plots of QCM response magnitudes (desorption data) when exposed towards different Hg
concentrations with/without the presence of interferent gas species over a continuous 127 day testing
period at 101°C, simulating industrial conditions. The n values indicate the number of data points
collected for each Hg concentration.
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Current (mA)
0.06
Au Oxidation
0.04
0.02
0.00
Pb UPD
-0.02
-0.04
-0.06
Au Reduction
-0.5
0.0
0.5
1.0
1.5
Potential (V) vs Ag/AgCl
Figure S7. Cyclic voltammetry profile of the electrolyte solution containing
tetrachloroaurate(III) tri-hydrate (2.718 g/L) and lead(II) acetate tri-hydrate (0.177 g/L).
hydrogen
References
1
van Noort, D., Rani, R. & Mandenius, C.-F. Improving the Sensitivity of a Quartz Crystal
Microbalance for Biosensing by Using Porous Gold. Microchim Acta 136, 49-53 (2001).
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