Review of carbon based materials for electronic bio-sensing

Bio-sensing represents one of the most attractive applications of carbon material based electronic devices; nevertheless, the complete integration of bioactive transducing elements still represents a major challenge, particularly in terms of preserving biological function and specificity while maintaining the sensor’s…
Bio-sensing represents one of the most attractive applications of carbon material based electronic devices; nevertheless, the complete integration of bioactive transducing elements still represents a major challenge, particularly in terms of preserving biological function and specificity while maintaining the sensor’s electronic performance. This review highlights recent advances in the realization of field-effect transistor (FET) based sensors that comprise a bio-receptor within the FET channel. A birds-eye view will be provided of the most promising classes of active layers as well as different device architectures and methods of fabrication. Finally, strategies for interfacing bio-components with organic or carbon nano- structured electronic active layers are reported.
Maria D. Angionea, Rosa Pilollia, Serafina Cotronea, Maria Magliuloa, Antonia Mallardib, Gerardo Palazzoa, Luigia Sabbatinia,
Daniel Finec, Ananth Dodabalapurd, Nicola Cioffia*, and Luisa Torsia*
a Department of Chemistry,University of Bari, Via Orabona, 4, I-70126 Bari, Italy
b CNR–IPCF,Istituto per i Processi Chimico-Fisici -Bari, Italy
c Department of NanoMedicine and BioMedical Engineering, The University of Texas Health Science Center at Houston, TX, USA
d Department of Electrical and Computer Engineering Microelectronics Research Center The University of Texas at Austin, TX USA
* E-mail: cioffi@chimica.uniba.it and torsi@chimica.uniba.it
Recently, increasing medical and biological interest in cheap
disposable, analytical, and diagnostic devices has driven research
towards the development and adaptation of low-cost electronic
sensing devices. While a lot of effort has been devoted to silicon
based nanostructured active layers, we focus here on carbonbased
materials. Organic semiconductors can be implemented
in a platform for flexible devices, including those built on
biodegradable and resorbable substrates, while carbon based
nano-structured materials, such as carbon nanotubes and graphene
sheets, offer higher performance in terms of field-effect mobility
and sensitivity. In all cases, the major challenge is to develop
active layer materials that allow for the inclusion of biologically
active bio-receptors while maintaining electronic performance. The
present contribution reviews and compares recent developments
in these related fields.

Organic field-effect transistor bio-sensors
Organic transistor-based devices have previously been demonstrated
for gas, vapor, and liquid sensing1-5, as well as for the detection of
single ions or ensembles of ions in solution6-12. Organic field-effect
transistor (OFET) based sensing technologies take advantage of the
physical or chemical changes taking place in or around the organic
(semi)conducting material when exposed to different analytes in
gaseous or aqueous environments. Field-effect transistors (FETs) for
their multi-parametric13 response and multi-functionality-related
versatility, comprise the primary functional component in an array of
sensing platforms proposed in the field of environmental monitoring8,
military defense9,10,14-16 and preventative medical care11. The
integration of active organic materials into these devices has allowed
for the implementation of electronics with plastic substrates, and thus
low-cost, lightweight, and flexible sensing devices.
Organic thin-film transistors (OTFTs) are three-terminal electronic
devices consisting of a thin organic semiconducting layer, an insulating
(dielectric) layer, and three conductive terminals, the source, drain,
and gate. The source and drain electrodes are fabricated to be directly
in contact with the semiconductor, either on top of the previously
deposited organic semiconductor (top-contact), or on the insulating
layer to be covered with the semiconductor later (bottom-contact).
The “channel” refers to the region in the semiconductor layer between
the defined source and drain electrodes12. The active semiconductor
layer may consist of small molecules17-22 polymers23,24, or organic
nanostructured materials25-27, among others. Contacts are usually
composed of gold; although a vast number of conductive materials
(e.g., doped conjugated polymers and printed metallic nanoparticles)
are actively being used for this purpose28,29. The dielectric layer,
which electrically isolates the semiconductor from the underlying gate
electrode, may be composed of inorganic oxides30, properly tailored
polymers or composites7, or ultrathin self-assembled layers31.
Recent advancements in the field of biotechnology have provided
systems that are able to efficiently transduce biological events using
electronic devices32,33. This progress has led to the improvement
of biological sensing platforms demonstrating the potential to be
applied for the rapid screening of biological samples and point-ofcare
diagnostics. Advancements in biomaterial processing and organic
electronic device fabrication have allowed for the potential integration
of biomolecules as active components in all of the materials employed
in the realization of an organic transistor including the bulk substrate,
the dielectric interface, and even the active semiconducting layers and
electrodes 34.
Other interesting aspects of biomolecule integration in organic
electronic devices have been explored in electrochemical biotransistors35
that have proven to be quite sensitive and selective. Some
limitations persist, however, and are connected to their implementation
in large area sensor arrays as it is not clear if the need for a reference
electrode can be ruled out. Significant effort has also been invested in
recent years to develop new hybrid systems in which living cells have
been integrated in electronic field-effect devices36,37.The technological
impact of these systems is attracting the interest of many research
groups as evidenced by the numerous contributions that can be found
in the literature38. These studies involve major efforts in assessing
biocompatibility and bio-functionality of the materials and the biosystems
involved. These aspects are not dealt with here as they would
require a review of their own. Only recent advances in processing and
integrating natural and synthetic bio-active components into electronic
devices based on both organic or carbon nanostructured materials are
considered (excluding whole cell implementations). This includes new
studies on biodegradable substrates for OFETs as well as bio-species
included in gate dielectric materials or on electronic active layers.

OFETs fabricated on resorbable and biodegradable substrates
Efforts to fabricate flexible electronic circuits have routinely focused on
developing processes that are compatible with flexible plastic substrate
materials such as poly(ethylene terephthalate) (PET)39, poly(imide)40,
poly(ether sulfone)41, cellulose42, silk fibroin, or other functional
fibers for e-textile development43,44. Various organic polymeric
systems composed of biodegradable polymers have demonstrated
their usefulness in important applications like temporary medical
implants45,46, and biodegradable products47. However, the realization
of these systems requires fabrication processes that are compatible
with tight substrate compatibility requirements; in particular, in the
case of PET, a moderate use of aqueous solvent and low-temperature
post processing, must be taken into account as a limiting step in
the fabrication procedure. To realize electronic circuits that are not
only flexible but also foldable, materials such as aluminum foil48 and
paper49 have been explored. Paper films were used as substrates with
pentacene-based active layers. This approach was further expanded
to create complete circuits using foldable paper-based substrates,
motivated by the fact that paper substrates are mechanically flexible
and capable of small bending radii. Roll-to-roll processing has also
been demonstrated by Siegel and co-workers that allows for the
realization of a complete circuit from simple folding and cutting
techniques (Fig. 1)34. The relative fragility of paper-based circuits
could potentially be utilized in security applications. One specific
example is the fabrication of an envelope integrated with a system
that, if compromised via destruction of the envelope, will cease to
function. Paper-based devices also present distinct advantages in
terms of disposability and environmental biodegradability, the latter
being extremely important in the context of medical biomaterials.
The motivation of this nascent research topic is to improve the
compatibility of biomaterials and electronic devices for applications
including restorable electronically active medical implants45,46.
Mechanically robust, water-insoluble, natural proteins have also been
explored as substrates for silicon electronics. A key example is provided
by Kim et al.43 who used silk fibroin as a substrate for traditional
silicon-based transistors. The performance of these devices is only
slightly affected when the substrate is mechanically deformed.
Bao’s group recently investigated materials and fabrication
strategies for the realization of organic thin-film transistors using a
small-molecule semiconductor in combination with a biodegradable 
polymeric substrate and dielectric 50. They demonstrated that these
devices perform stably after exposure to water and, since they are
made of nearly entirely biodegradable materials, are resorbable in a
simulated degradation environment in vitro. A schematic image of the
device is shown in Fig. 2. The strategy for material selection focused on
utilizing materials that are not only biocompatible and biodegradable,
but also exhibit adequate electronic properties and a suitable device
manufacturing process. The semiconducting molecule utilized in this
study, 5,50-bis-(7-dodecyl-9H-fluoren-2-yl)-2,20-bithiophene (DDFTTF),
is a robust small-molecule p-channel semiconductor that exhibits
excellent device performance and is resistant to harsh aqueous
environments. Although the biodegradation of DDFTTF has not been
explicitly studied, degradation mechanisms that decompose melanin51,52,
a conjugated amorphous semiconductor53,54, could potentially act
in a similar manner towards DDFTTF. Poly(vinyl alcohol) was used as
the insulating layer. Devices were fabricated with “non-crosslinked”
PVA (nPVA) and crosslinked PVA (xPVA) in order to demonstrate the
improved performance of the xPVA-based device in terms of leakage
current and surface roughness. The substrate, which composed 99.89 %
of the total mass, consists of poly(L-lactide-co-glycolide) (PLGA), a linear
thermoplastic biodegradable polyester. This polymer contains significant
amounts of lactic acid (PLGA 85:15) and thus allows for processing at
elevated temperatures due to the high glass-transition temperature, Tg.
Thin-film transistors based on p-channel DDFTTF active layers and (PVA)
dielectrics exhibited electron mobilities as high as 0.253 cm2s-1V-1 and
on/off current ratios (Ion/Ioff) of up to 9.4 × 103. These devices maintained
functionality after direct exposure to water and are principally resorbable
and biodegradable. To avoid interaction with ionic species eventually
present in the analyte aqueous solution, an appropriate encapsulation and
packaging of the device was needed. The potential utilization of resorbable
organic electronics could serve as a motivating factor for the development
of high-performance dielectrics and semiconducting molecules with the
additional properties of biocompatibility and biodegradability into nontoxic
and environmentally safe monomers.

OFETs dielectrics modified with biomolecules
Improvements of bio-sensor figures of merit have been realized
through the development of new high performance dielectrics and
device architectures, as well as by tuning device properties via
interface engineering. Precise control over the threshold voltage (Vt)
of pentacene-based organic thin film transistors, desirable for better
integration of OFET devices into electronic circuits, has been achieved
by inserting well characterized genetically engineered peptides for
inorganics (GEPI) at the semiconductor-dielectric interface55 . Further
tuning of the Vt can be achieved by controlling the peptide assembly
conditions, such as the pH of the precursor solutions, as demonstrated
by the group of A. K-Y. Jen who fabricated pentacene based OTFTs on
top of quartz-binding polypeptide (QBP)-modified SiO2/Si substrates
with a 50 nm pentacene active layer. The transfer characteristics for
these devices are shown in Fig. 3, along with the device structure.
A shift in Vt of about 30 V compared to the unmodified SiO2 was
observed upon assembly of the peptide in pH-neutral condition. From
this point, a positive shift in Vt was observed when the GEPI-QBP was
assembled in different concentrations of HCl, while a negative shift
in Vt was observed when QBP was assembled from a KOH solution at
different concentrations. Although the approach is very interesting, the
stability of the system has not been discussed.
OFETs with functional deoxyribonucleic acids (DNAs) as dielectric
material have been pioneered by the Grote and Sariciftci groups56-58.
Specifically, organic-soluble DNA, such as cetyltrimethylammonium
chloride (CTMA)-DNA, have been synthesized and their physical
and electronic properties have been elucidated. Since CTMA-DNA
is insoluble in water but soluble in alcohol, solution processing
methods are suitable for thin-film fabrication. An interesting
field-effect carrier transport phenomenon was observed in these
devices, although an undesired hysteresis could be seen in the
corresponding transfer curves. An additional blocking layer of
5,5’-(9,10-bis((4-hexylphenyl)ethynyl)anthracene-2,6-yl-diyl)
bis(ethyne-2,1-diyl)bis(2-hexylthiophene) (HB-ant-THT) molecules
was introduced between the organic semiconductor layer and the DNA
dielectric, to improve the electrical performance, including a reduction
in leakage current. Y. S. Kim59 further reported on the development
of photo-crosslinkable DNA-based dielectrics detailing the precise
effect of the CTMA units in the copolymer on TFT performance.
Particularly, the TFT device with the CTMA-DNA-co-CcDNA dielectric
(Fig. 4) has a very high field-effect mobility implying that the long alkyl
chains in the CTMA unit help enhance the ordering of the HB-ant-THT
molecules.

Bio-functionalization of the organic semiconductor
Many examples have been proposed for the detection of analyte
vapors using OTFTs, with numerous reports addressing the ability
to identify particular analytes either through the use of a fingerprint
response60,61 or by incorporating selective detection layers4,62.
On the contrary few examples of chemical and bio-detection
in aqueous systems have been demonstrated, the first example
being published in 2002 by Someya and co-workers63. Selective
in situ detection with OTFTs requires a versatile method for the
immobilization of various selective molecular probes within close
proximity to the active transport channel. Bao’s et al. reported in
2010 a real-time, in situ selective detection scheme with short-chain
DNA targets employing organic transistors as the electrical transducer
(Fig. 5) 64 . The device was realized by modifying the OTFT surfaces with
a PECVD (plasma-enhanced chemical vapor deposition)-deposited thin
maleic anhydride (MA) polymer layer65,66. Such a surface pretreatment
allowed for the covalent attachment of the peptide nucleic acid (PNA)
strands, which were then used to selectively detect the target DNA
molecules within limits approaching 1 nM.
Genetic diagnostic tools have been proposed by Subramaniam’s
group that integrate OTFT-based DNA sensors with microfluidic
channels. A novel photolithography-based microfluidics fabrication
method was used to directly enable on-chip hybridization, a significant
advancement in the realization of disposable rapid turn-around tools
for field-deployable genetic diagnosis67. Another example of direct
semiconductor functionalization was provided by Lim’s group where an
organic semiconducting copolymer composed of biotinylatedfluorene
and bithiophene was synthesized through a palladium(0)-mediated
Suzuki coupling polymerization. OTFTs were fabricated using this
p-type polymer and electrically characterized in the presence of avidin
(Fig. 6)68 . The binding of avidin-biotin moieties in the polymer were
correlated to changes in the channel chemoresistivity and to the on/off
characteristics of the OTFTs. Avidin exposure resulted in a lowering
of the drain current (Ids) of nearly five orders of magnitude when the
device was operated at a drain voltage of −40 V. Detection in this
case was done at about 50 ppm, which is quite high considering the
specificity of the interaction.
Recently, our group proposed the inclusion of a photosynthetic
membrane protein, the bacterial reaction center, into a phospholipid
bilayer formed on top of the active layer of an OTFT for herbicide
detection69,70. A pictorial view of this sensing device bearing the
membrane protein as a bioreceptor is shown in Fig. 7a. The lipid
bilayer facilitates a close association of the protein with the carriers
(electrons or holes) accumulated in the channel at the semiconductor/
gate-dielectric interface. Atomic force microscopy (AFM) (Fig. 7b) and
confocal microscopy (Fig. 7c) have been performed to investigate the
structure of the lipid-protein layer deposited on top of the organic
semiconductor and shows a very uniform deposition of biomaterials.
Very little electronic performance degradation (Fig. 7d) is seen as
the bioreceptor containing layer is deposited on top of the channel
material.
Another interesting and very recent architecture that explores organic
semiconductor functionalization has been presented by Blom’s research
group71. Here a dual gate organic field-effect transistor with integrated
sulfate binding proteins (SBP) for sensing of sulfate ions is proposed.
Fluorescence spectroscopy and tapping mode AFM were used to confirm
the covalent coupling of the SBP receptor to the surface of a maleimide
functionalized polystyrene layer. This system guarantees protein stability
without loss of protein selectivity, as demonstrated by the dissociation
constant measurement of SBP after drying and rehydration showing that
the protein remains active even after being dried.

Organic nanomaterials for biosensing by FET transduction.
One promising direction for future transistor technology involves
“nanoelectronics” in which the active part of the device is composed
of nanometer sized materials. In particular the integration of 1D
nanomaterials, such as nanowires and nanotubes, into functional
electronic devices has received considerable attention for application
as highly sensitive tools for biological applications. The comparable
sizes of engineered nanomaterials and natural biological systems
(e.g., antibodies, enzymes, transport proteins, etc.) make this
approach appropriate for creating high throughput sensing probes:
shrinking the dimensions of the materials down to the nanometer
scale maximizes the effect of biochemical changes at the surface of
the device. A single surface binding event will lead to a much larger
change in device conductance than planar FETs as a result of the
accumulation/depletion of carriers through the entire cross section of
the device versus only a thin region near the surface. Nanomaterial
based electronic devices thus provide a unique class of biosensor,
with intrinsic ultra-sensitivity towards changes in their local chemical
environment. These devices behave as both the sensitizing layer and
the transducer allowing for the direct conversion of bio-chemical
information into an electronic signal without labels allowing for realtime
continuous monitoring.

Conducting polymer nanostructures
Given previously reported assessments of three-dimensional conducting
polymers as electronic chemical sensors72, 1D conducting polymer (CP)
nanomaterials present attractive alternatives to carbon nanotubes and
silicon nanowires that merge properties from inorganic, carbonaceous,
and polymeric materials: tunable conductivity, chemical diversity,
mechanical stability, and biocompatibility73-77. Further advantages
include their lightweight, low cost, easy processing/patterning, and
scalable production. A crucial factor that needs to be accounted for
in designing CP-FET biosensors is the sensitivity of the polymeric
nanostructure towards contact resistance variation due to exposure to
the liquid phase. Usually, in order to overcome this issue, a functional
monomer such as a polymeric derivative with carboxylic side-chain
groups78-82 has been integrated into a copolymer scheme. The acid
functionality not only provides sufficient immobilization sites for
biomolecules, but also assists in the covalent binding of 1D CPs on
amine-terminated silanized SiO2 substrates, to form stable contacts
with the metal electrodes, as illustrated in Fig. 882. Thrombin78,79,
glucose80, human serum albumin81 and odorant molecule82 detection
was demonstrated using recognition probes such as aptamers, glucose
oxidase enzyme, antibodies and a human olfactory receptor (hOR),
respectively. The study by Yoon et al. deserves further mention,
since it is the first example of a FET-type bioelectronic nose based
on hOR-conjugated CP nanotubes82. The authors developed a
reliable chemical immobilization strategy for the fabrication of
CPNT-FET devices with quantitative control on the degree of hOR
functionalization. Moreover, the feasibility of specific odorant detection
down to concentrations as low as tens of femtomoles was assessed.

Carbon nanotubes
Since their discovery in 1991 by Ijima83, carbon nanotubes (CNTs)
have been regarded as an important nanostructured material derived
from bottom-up chemical synthesis. As such, a great deal of effort
has been devoted towards understanding their electrical, mechanical,
and chemical properties. In this review we will cover the most
recent applications in electronic sensing proposed in the literature
during the last year. For further details, several previous review
papers covering various aspects of carbon nanotube electronics are
recommended26,84-88.
The first CNT field-effect transistors (CNT-FETs) were independently
obtained in 1998 from both the Dekker group at Delft University89
and the Avouris group at IBM90, with the first biologica l application
of CNT-FET proposed by the Dai group in 2003 while investigating
specific protein–protein interactions91. Recent rapid development of
chemical modification approaches and bio-functionalization methods
have allowed a new class of bioactive carbon nanotubes conjugated
with proteins, carbohydrates, nucleic acids or aptamers26,83-90,92,93.
Single walled CNTs (SWNTs) have usually been preferred because
of the availability of each atom in the SWNTs to the surrounding
environment leading to maximized electrical coupling.
An interesting advance in functionalizing carbon nanotubes to
obtain the detection of protein-receptor interactions is reported
in the paper of Star and co-workers94. The authors presented a
supramolecular approach based on the noncovalent functionalization of
nanotubes, in order to avoid the disadvantage of covalent modification
that impairs physical properties of carbon nanotubes. A PEI/PEG
polymer coating is used both to attach receptor molecules to the
sidewalls of nanotubes and to prevent nonspecific binding of proteins.
The biotin-streptavidin binding has been chosen as a model system to
demonstrate the effectiveness of the device architecture.
As an alternative to biological molecules conjugated directly to
the CNTs, a lipid bilayer-based coating for the CNT channel was
proposed95,96. The coating performed tw o key roles acting both as a
protective impenetrable barrier between the sensing channel and the
surrounding medium, as well as a dispersive matrix for membrane
proteins. Usually these devices incorporated passive biological elements
which transmitted an environmental change to the CNTs. In 2010
Huang et al. assessed the feasibility of using an adenosine triphosphate
(ATP) powered biological pump to control a nanoelectronic circuit96.
The authors reported on a hybrid bionanoelectronic transistor in
which a local Na+/K+-ATPase protein gate, was embedded in the lipid
membrane (Fig. 9). The ion pump modulated the transistor output
current by up to 40 % by shifting the pH of the water layer located
between the lipid bilayer and the nanotube surface.
Moreover, aligned CNT-FETs have also been proposed as innovative
platforms to fabricate biosensors with high sensitivity down to the
picomolar range97,98. Aptamer functionalized SWNT-film arrays were
deposited by dielectrophoresis and the surface tension associated with
a water meniscus; an AC voltage (frequency 1 MHz, amplitude 5 Vp-p)
was applied to align SWNTs in solution, which were then compressed
by the surface tension of the water meniscus after slow, careful drying.
The attachment of the SWNTs to the electrodes was mediated by
Van der Waals forces. The resulting SWNT-film, suspended between
cantilever electrodes, allowed for a highly specific and real-time
detection of thrombin97.

Graphene
Graphene is highly promising for new types of chemical/biological
sensors with excellent sensitivity due to a combination of a high active
surface area (i.e., a 2D material with all the carbon atoms exposed
to the analyte of interest), exceptional electrical properties, and low
noise26,99,100,101. The operational principle of graphene bio-electronic
sensors is based on the change of graphene electrical conductivity (σ)
due to adsorption of molecules on its surface. The graphene-FET, based
on a non-functionalized single-sheet, has been shown to exhibit a
proportional increase in conductance upon protein adsorption at a subnanomolar
level102. Mohanty and Berry developed a graphene-based
bio-sensor capable of single bacterium resolution, investigating for the
first time the interaction between chemically-modified graphene and
bioentities103. Chemical vapor deposition-grown graphene films have
also been used to detect DNA with single-base mismatch sensitivity104,
as well as carbohydrates and neurotransmitters105.
Further steps in the development of robust graphene-living cell
interfaces have been accomplished in only the last few months. A
graphene-FET device detected well-defined extracellular signals while
exposed to embryonic chicken cardiomyocytes106. Moreover, hormonal
catecholamine molecules and their dynamic secretion from living
cells were monitored in real time by biosensors based on a reduced
graphene oxide (rGO) film (Fig. 10)107. Noteworthy in this study, the
authors focused their attention on competitively alternative materials
of graphene, namely rGO, produced by facile and scalable solution
processes to extend the applications of graphene for plastic electronics.
They proposed the fabrication of centimeter-long, ultrathin (1 – 3 nm),
and electrically continuous micropatterns of highly uniform parallel
arrays of rGO films on various substrates by using the “micromolding
in capillary” method. Remarkable sensitivity was achieved that was
shown to be insensitive to substrate binding.

Conclusions and future perspectives 
The incorporation of biomaterials into all of the different structural
components of organic and nanomaterial based electronic devices has
the potential to be applied towards a range of applications in medicine
and point-of-care diagnostics. Future efforts will need to be focused on
the development of new hybrid biomaterials integrated into electronic
devices capable of being processed on flexible substrates. The overall
progress of this research field will have enormous implications for both
fundamental scientific discovery and technological development. In
particular, novel bio-OFETs could be used to study the fundamentals of
electron transfer in naturally occurring biomolecules. The investigation
of the interface of biomaterials and organic electronics could also lead
to the realization of new classes of electronically active medical devices
for use in advancing human health.

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