Combinatorial and High-Throughput Screening of Materials ...

ACS Combinatorial Science
REVIEW
Figure 37. Wireless high-throughput screening of materials properties using thickness shear mode resonators. Evaluation of selectivity of Nafion
sensing films to several vapors after conditioning at different temperatures: (A) 22, (B) 90, and (C) 125 °C. Vapors: H 2 O (water), EtOH (ethanol), and
ACN (acetonitrile). Concentrations of vapors are 0, 0.02, 0.04, 0.07, and 0.10 P/P o . Arrows indicate the increase of concentrations of each vapor. 392
from 0 to 0.1 of the saturated vapor pressure P o as shown in
Figure 37 B D. It was found that conditioning of sensing films at
125 °C compared to room temperature conditioning provided
(1) an improvement in the linearity in response to EtOH and
ACN vapors, (2) an increase in relative response to ACN, and
(3) a 10-fold increase of the contribution to principal component
2. The latter point signifies an improvement in the discrimination
ability between different vapors upon conditioning of the sensing
material at 125 °C. This new knowledge will be critical in
designing sensors for practical applications where the need exists
to preserve sensor response selectivity over the long exploitation
time or when there is a temperature cycling for an accelerated
sensor-film recovery after vapor exposure.
Other materials for sensors based on mechanical energy
transduction that were explored using CHT techniques are
alkane thiol materials. A 2-D multiplexed cantilever array platform
was developed for an elegant combinatorial screening of
vapor responses of alkane thiols with different functional end
groups. 393,394 The cantilever sensor array chip (size 2.5
2.5 cm) had ∼720 cantilevers and was fabricated using surface
and bulk micromachining techniques. The optical readout has
been developed for parallel analysis of deflections from individual
cantilevers. To evaluate the performance of this 2-D sensor array
for screening of sensing materials, nonpolar and polar vapors
such as toluene and water vapor were selected as analytes. The
screening system was tested with three candidate alkane thiol
materials as sensing films with different functional end groups
such as mercaptoundecanoic acid SH (CH 2 ) 10 COOH
(MUA), mercaptoundecanol SH (CH 2 ) 11 OH (MUO), and
dodecanethiol SH (CH 2 ) 11 CH 3 (DOT). Each type of sensing
films had a different chemical and physical property because
COOH group is acidic in nature and can dissociate to give
COO group, OH group does not dissociate easily but can
form hydrogen bonds with polar molecules, and CH3 group
would be inert to polar molecules and the only interactions that it
can have are from van der Waals and hydrophobic effects.
3.4.3. Materials for Sensors Based on Electrical Energy
Transduction. Sensors based on electrical energy transduction
utilize sensing materials that undergo electrically detectable
changes, for example changes in resistance, capacitance as related
to different mechanisms of material-analyte interactions. Typical
devices for these applications include electrochemical and electronic
transducers.
395 399
The simplicity of microfabrication of electrode arrays and their
subsequent application as transducer surfaces makes sensors
based on electrical energy transduction a very attractive platform
for construction of discrete arrays for screening of sensor
materials. The possibility to regulate polymerization on solid
conductive surfaces by application of corresponding electrochemical
potentials suggested a realization of this process in the form
of multiple polymerization regions on multiple electrodes of an
electronic sensor system. 324,400 Arranging such polymerization
electrodes in an array eliminated the need for dispensing systems
and allowed an electrically addressable immobilization. This
approach has been demonstrated on electropolymerization of
aniline that was independently performed on different electrodes
of the array. 324,400 Thin layer polymerization of defined mixtures
of monomers was performed directly on the 96 interdigital
addressed electrodes of an electrode array on an area of less
than 20 mm 20 mm (see Figure 38A). The electrodes had an
interdigital configuration designed for four-point measurements
and fabricated by lithography on an oxidized silicon wafer. Using
this electropolymerization system, numerous copolymers have
been screened. 19 An introduction of nonconductive monomers
into polymer decreased the polymer conductance and therefore
decreased the difference between conductive and insulating
polymer states. This has caused the decrease of the absolute
sensitivity (Figure 38B). Normalization to the polymer conductance
without analyte exposure compensated this effect and
demonstrated that the polymer synthesized from the mixture
of aminobenzoic acid and aniline possessed the highest relative
sensitivity (Figure 38C). This effect may be explained by the
strong dependence of polymer conductance on the defect
number in polymer chains. In comparison with pure polyaniline,
this copolymer had better recovery efficiency but a slower
response time (Figure 38D, E). The developed high-throughput
screening system was capable of reliable ranking of sensing
materials and required only ∼20 min of manual interactions
with the system and ∼14 h of computer controlled combinatorial
screening compared to ∼2 weeks of laboratory work using
traditional electrochemical polymer synthesis and materials
evaluation. 324
Semiconducting metal oxides is another type of sensing
materials that benefits from the combinatorial screening technologies.
Semiconducting metal oxides are typically used as gassensing
materials that change their electrical resistance upon
exposure to oxidizing or reducing gases. While over the years,
significant technological advances have been made that resulted
in practical and commercially available sensors, new materials are
being developed that improve further sensing performance of
these sensors. To enhance the response selectivity and stability,
an accepted approach is to formulate multicomponent materials
614 dx.doi.org/10.1021/co200007w |ACS Comb. Sci. 2011, 13, 579–633